Changeset 598 for draft-ietf-httpbis


Ignore:
Timestamp:
Jun 13, 2009, 3:14:22 AM (10 years ago)
Author:
julian.reschke@…
Message:

fix Makefile, add RFC 2817/2818, re-gen HTML

Location:
draft-ietf-httpbis/orig
Files:
4 added
10 edited

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  • draft-ietf-httpbis/orig/Makefile

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    343 </style><link rel="Contents" href="#rfc.toc"><link rel="Author" href="#rfc.authors"><link rel="Copyright" href="#rfc.copyright"><link rel="Chapter" title="1 Introduction" href="#rfc.section.1"><link rel="Chapter" title="2 HTTP version numbers" href="#rfc.section.2"><link rel="Chapter" title="3 Security Considerations" href="#rfc.section.3"><link rel="Chapter" href="#rfc.section.4" title="4 References"><link rel="Alternate" title="Authorative ASCII version" href="http://www.ietf.org/rfc/rfc2145.txt"><meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.389, 2008-08-20 14:21:35, XSLT vendor: SAXON 6.5.5 from Michael Kay http://saxon.sf.net/"><meta name="keywords" content="HTTP, hypertext transfer protocol"><link rel="schema.DC" href="http://purl.org/dc/elements/1.1/"><meta name="DC.Creator" content="Mogul, J. C."><meta name="DC.Creator" content="Fielding, R. T."><meta name="DC.Creator" content="Gettys, J."><meta name="DC.Creator" content="Frystyk, H."><meta name="DC.Identifier" content="urn:ietf:rfc:2145"><meta name="DC.Date.Issued" scheme="ISO8601" content="1997-05"><meta name="DC.Description.Abstract" content="HTTP request and response messages include an HTTP protocol version number. Some confusion exists concerning the proper use and interpretation of HTTP version numbers, and concerning interoperability of HTTP implementations of different protocol versions. This document is an attempt to clarify the situation. It is not a modification of the intended meaning of the existing HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention of the authors of those documents, and can be considered definitive when there is any ambiguity in those documents concerning HTTP version numbers, for all versions of HTTP."></head><body><table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1"><tr><td class="header left">Network Working Group</td><td class="header right">J. C. Mogul</td></tr><tr><td class="header left">Request for Comments: 2145</td><td class="header right">DEC</td></tr><tr><td class="header left">Category: Informational</td><td class="header right">R. T. Fielding</td></tr><tr><td class="header left"></td><td class="header right">UC Irvine</td></tr><tr><td class="header left"></td><td class="header right">J. Gettys</td></tr><tr><td class="header left"></td><td class="header right">DEC</td></tr><tr><td class="header left"></td><td class="header right">H. Frystyk</td></tr><tr><td class="header left"></td><td class="header right">MIT/LCS</td></tr><tr><td class="header left"></td><td class="header right">May 1997</td></tr></table><p class="title">Use and Interpretation of HTTP Version Numbers</p><h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1><p>This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.</p><h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1><p>Copyright &copy; The Internet Society (1997). All Rights Reserved.</p><h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> <p>HTTP request and response messages include an HTTP protocol version number. Some confusion exists concerning the proper use and interpretation of HTTP version numbers, and concerning interoperability of HTTP implementations of different protocol versions. This document is an attempt to clarify the situation. It is not a modification of the intended meaning of the existing HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention of the authors of those documents, and can be considered definitive when there is any ambiguity in those documents concerning HTTP version numbers, for all versions of HTTP.</p> <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note</a></h1> <p>Distribution of this document is unlimited. Please send comments to the HTTP working group at &lt;http-wg@cuckoo.hpl.hp.com&gt;. Discussions of the working group are archived at &lt;URL:http://www.ics.uci.edu/pub/ietf/http/&gt;. General discussions about HTTP and the applications which use HTTP should take place on the &lt;www-talk@w3.org&gt; mailing list.</p> <hr class="noprint"><h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1><ul class="toc"><li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Introduction</a><ul class="toc"><li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Robustness Principle</a></li></ul></li><li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">HTTP version numbers</a><ul class="toc"><li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#proxy.behavior">Proxy behavior</a></li><li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.2">Compatibility between minor versions of the same major version</a></li><li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.3">Which version number to send in a message</a></li></ul></li><li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Security Considerations</a></li><li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li><li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.authors">Authors' Addresses</a></li><li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li></ul><hr class="noprint"><h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Introduction</h1><p id="rfc.section.1.p.1">HTTP request and response messages include an HTTP protocol version number. According to section <a href="http://tools.ietf.org/html/rfc2068#section-3.1">3.1</a> of the HTTP/1.1 specification <a href="#RFC2068"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>,</p><blockquote id="rfc.section.1.p.2" cite="http://tools.ietf.org/html/rfc2068#section-3.1"> <p>HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather than the features obtained via that communication. No change is made to the version number for the addition of message components which do not affect communication behavior or which only add to extensible field values. The &lt;minor&gt; number is incremented when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may add to the message semantics and imply additional capabilities of the sender. The &lt;major&gt; number is incremented when the format of a message within the protocol is changed.</p> </blockquote><p id="rfc.section.1.p.3">The same language appears in the description of HTTP/1.0 <a href="#RFC1945"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a>.</p><p id="rfc.section.1.p.4">Many readers of these documents have expressed some confusion about the intended meaning of this policy. Also, some people who wrote HTTP implementations before RFC1945 <a href="#RFC1945"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a> was issued were not aware of the intentions behind the introduction of version numbers in HTTP/1.0. This has led to debate and inconsistency regarding the use and interpretation of HTTP version numbers, and has led to interoperability problems in certain cases.</p><p id="rfc.section.1.p.5">This document is an attempt to clarify the situation. It is not a modification of the intended meaning of the existing HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention of the authors of those documents. In any case where either of those two documents is ambiguous regarding the use and interpretation of HTTP version numbers, this document should be considered the definitive as to the intentions of the designers of HTTP.</p><p id="rfc.section.1.p.6">The specification described in this document is not part of the specification of any individual version of HTTP, such as HTTP/1.0 or HTTP/1.1. Rather, this document describes the use of HTTP version numbers in any version of HTTP (except for HTTP/0.9, which did not include version numbers).</p><p id="rfc.section.1.p.7">No vendor or other provider of an HTTP implementation should claim any compliance with any IETF HTTP specification unless the implementation conditionally complies with the rules in this document.</p><h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Robustness Principle</h2><p id="rfc.section.1.1.p.1">RFC791 <a href="#RFC0791"><cite title="Internet Protocol">[4]</cite></a> defines the "robustness principle" in section <a href="http://tools.ietf.org/html/rfc791#section-3.2">3.2</a>:</p><blockquote id="rfc.section.1.1.p.2" cite="http://tools.ietf.org/html/rfc791#section-3.2"> <p>an implementation must be conservative in its sending behavior, and liberal in its receiving behavior.</p> </blockquote><p id="rfc.section.1.1.p.3">This principle applies to HTTP, as well. It is the fundamental basis for interpreting any part of the HTTP specification that might still be ambiguous. In particular, implementations of HTTP <em class="bcp14">SHOULD NOT</em> reject messages or generate errors unnecessarily.</p><hr class="noprint"><h1 id="rfc.section.2" class="np"><a href="#rfc.section.2">2.</a>&nbsp;HTTP version numbers</h1><p id="rfc.section.2.p.1">We start by restating the language quoted above from section <a href="http://tools.ietf.org/html/rfc2068#section-3.1">3.1</a> of the HTTP/1.1 specification <a href="#RFC2068"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>: </p><dl class="empty"><dd>It is, and has always been, the explicit intent of the HTTP specification that the interpretation of an HTTP message header does not change between minor versions of the same major version.</dd><dd>It is, and has always been, the explicit intent of the HTTP specification that an implementation receiving a message header that it does not understand <em class="bcp14">MUST</em> ignore that header. (The word "ignore" has a special meaning for proxies; see section <a href="#proxy.behavior" title="Proxy behavior">2.1</a> below.)</dd></dl><p id="rfc.section.2.p.2">To make this as clear as possible: The major version sent in a message <em class="bcp14">MAY</em> indicate the interpretation of other header fields. The minor version sent in a message <em class="bcp14">MUST NOT</em> indicate the interpretation of other header fields. This reflects the principle that the minor version labels the capability of the sender, not the interpretation of the message.</p><div class="note"> <p>Note: In a future version of HTTP, we may introduce a mechanism that explicitly requires a receiving implementation to reject a message if it does not understand certain headers. For example, this might be implemented by means of a header that lists a set of other message headers that must be understood by the recipient. Any implementation claiming at least conditional compliance with this future version of HTTP would have to implement this mechanism. However, no implementation claiming compliance with a lower HTTP version (in particular, HTTP/1.1) will have to implement this mechanism.</p>  <p>This future change may be required to support the Protocol Extension Protocol (PEP) <a href="#Kha"><cite title="HTTP/1.2 Extension Protocol (PEP)">[3]</cite></a>.</p> </div><p id="rfc.section.2.p.4">One consequence of these rules is that an HTTP/1.1 message sent to an HTTP/1.0 recipient (or a recipient whose version is unknown) <em class="bcp14">MUST</em> be constructed so that it remains a valid HTTP/1.0 message when all headers not defined in the HTTP/1.0 specification <a href="#RFC1945"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a> are removed.</p><h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="proxy.behavior" href="#proxy.behavior">Proxy behavior</a></h2><p id="rfc.section.2.1.p.1">A proxy <em class="bcp14">MUST</em> forward an unknown header, unless it is protected by a Connection header. A proxy implementing an HTTP version &gt;= 1.1 <em class="bcp14">MUST NOT</em> forward unknown headers that are protected by a Connection header, as described in section <a href="http://tools.ietf.org/html/rfc2068#section-14.10">14.10</a> of the HTTP/1.1 specification <a href="#RFC2068"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>.</p><p id="rfc.section.2.1.p.2">We remind the reader that that HTTP version numbers are hop-by-hop components of HTTP messages, and are not end-to-end. That is, an HTTP proxy never "forwards" an HTTP version number in either a request or response.</p><h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;Compatibility between minor versions of the same major version</h2><p id="rfc.section.2.2.p.1">An implementation of HTTP/x.b sending a message to a recipient whose version is known to be HTTP/x.a, a &lt; b, <em class="bcp14">MAY</em> send a header that is not defined in the specification for HTTP/x.a. For example, an HTTP/1.1 server may send a "Cache-control" header to an HTTP/1.0 client; this may be useful if the immediate recipient is an HTTP/1.0 proxy, but the ultimate recipient is an HTTP/1.1 client.</p><p id="rfc.section.2.2.p.2">An implementation of HTTP/x.b sending a message to a recipient whose version is known to be HTTP/x.a, a &lt; b, <em class="bcp14">MUST NOT</em> depend on the recipient understanding a header not defined in the specification for HTTP/x.a. For example, HTTP/1.0 clients cannot be expected to understand chunked encodings, and so an HTTP/1.1 server must never send "Transfer-Encoding: chunked" in response to an HTTP/1.0 request.</p><h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;Which version number to send in a message</h2><p id="rfc.section.2.3.p.1">The most strenuous debate over the use of HTTP version numbers has centered on the problem of implementations that do not follow the robustness principle, and which fail to produce useful results when they receive a message with an HTTP minor version higher than the minor version they implement. We consider these implementations buggy, but we recognize that the robustness principle also implies that message senders should make concessions to buggy implementations when this is truly necessary for interoperation.</p><p id="rfc.section.2.3.p.2">An HTTP client <em class="bcp14">SHOULD</em> send a request version equal to the highest version for which the client is at least conditionally compliant, and whose major version is no higher than the highest version supported by the server, if this is known. An HTTP client <em class="bcp14">MUST NOT</em> send a version for which it is not at least conditionally compliant.</p><p id="rfc.section.2.3.p.3">An HTTP client <em class="bcp14">MAY</em> send a lower request version, if it is known that the server incorrectly implements the HTTP specification, but only after the client has determined that the server is actually buggy.</p><p id="rfc.section.2.3.p.4">An HTTP server <em class="bcp14">SHOULD</em> send a response version equal to the highest version for which the server is at least conditionally compliant, and whose major version is less than or equal to the one received in the request. An HTTP server <em class="bcp14">MUST NOT</em> send a version for which it is not at least conditionally compliant. A server <em class="bcp14">MAY</em> send a 505 (HTTP Version Not Supported) response if cannot send a response using the major version used in the client's request.</p><p id="rfc.section.2.3.p.5">An HTTP server <em class="bcp14">MAY</em> send a lower response version, if it is known or suspected that the client incorrectly implements the HTTP specification, but this should not be the default, and this <em class="bcp14">SHOULD</em> NOT be done if the request version is HTTP/1.1 or greater.</p><hr class="noprint"><h1 id="rfc.section.3" class="np"><a href="#rfc.section.3">3.</a>&nbsp;Security Considerations</h1><p id="rfc.section.3.p.1">None, except to the extent that security mechanisms introduced in one version of HTTP might depend on the proper interpretation of HTTP version numbers in older implementations.</p><h1 class="np" id="rfc.references"><a href="#rfc.section.4" id="rfc.section.4">4.</a> References</h1><table summary="References"> <tr><td class="reference"><b id="RFC1945">[1]</b></td><td class="top"><a href="mailto:timbl@w3.org" title="MIT, Laboratory for Computer Science">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:frystyk@w3.org" title="W3 Consortium, MIT Laboratory for Computer Science">H.F. Nielsen</a>, &#8220;<a href="http://tools.ietf.org/html/rfc1945">Hypertext Transfer Protocol -- HTTP/1.0</a>&#8221;, RFC&nbsp;1945, May&nbsp;1996.</td></tr>  <tr><td class="reference"><b id="RFC2068">[2]</b></td><td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="MIT Laboratory for Computer Science">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Digital Equipment Corporation, Western Research Laboratory">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Nielsen, H.</a>, and <a href="mailto:timbl@w3.org" title="MIT Laboratory for Computer Science">T. Berners-Lee</a>, &#8220;<a href="http://tools.ietf.org/html/rfc2068">Hypertext Transfer Protocol -- HTTP/1.1</a>&#8221;, RFC&nbsp;2068, January&nbsp;1997.</td></tr>  <tr><td class="reference"><b id="Kha">[3]</b></td><td class="top">Khare, R., &#8220;HTTP/1.2 Extension Protocol (PEP)&#8221;.<br>HTTP Working Group, Work in Progress.</td></tr>  <tr><td class="reference"><b id="RFC0791">[4]</b></td><td class="top">Postel, J., &#8220;<a href="http://tools.ietf.org/html/rfc791">Internet Protocol</a>&#8221;, RFC&nbsp;791, September&nbsp;1981.</td></tr> </table><hr class="noprint"><h1 id="rfc.authors" class="np"><a href="#rfc.section.5" id="rfc.section.5">5.</a> <a href="#rfc.authors">Authors' Addresses</a></h1><address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Western Research Laboratory</span><span class="adr"><span class="street-address vcardline">Digital Equipment Corporation</span><span class="street-address vcardline">250 University Avenue</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">California</span>&nbsp;<span class="postal-code">94305</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:mogul@wrl.dec.com"><span class="email">mogul@wrl.dec.com</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span><span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Department of Information and Computer Science</span><span class="adr"><span class="street-address vcardline">University of California</span><span class="vcardline"><span class="locality">Irvine</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">92717-3425</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1(714)824-4056"><span class="value">+1 (714) 824-4056</span></a></span><span class="vcardline">EMail: <a href="mailto:fielding@ics.uci.edu"><span class="email">fielding@ics.uci.edu</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">MIT Laboratory for Computer Science</span><span class="adr"><span class="street-address vcardline">545 Technology Square</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1(617)2588682"><span class="value">+1 (617) 258 8682</span></a></span><span class="vcardline">EMail: <a href="mailto:jg@w3.org"><span class="email">jg@w3.org</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">W3 Consortium</span><span class="adr"><span class="street-address vcardline">MIT Laboratory for Computer Science</span><span class="street-address vcardline">545 Technology Square</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1(617)2588682"><span class="value">+1 (617) 258 8682</span></a></span><span class="vcardline">EMail: <a href="mailto:frystyk@w3.org"><span class="email">frystyk@w3.org</span></a></span></address><h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1><p>Copyright &copy; The Internet Society (1997). All Rights Reserved.</p><p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.</p><p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees.</p><p>This document and the information contained herein is provided on an &#8220;AS IS&#8221; basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.</p><hr class="noprint"><h1 class="np"><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1><p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat.</p><p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director.</p></body></html>
     308</style><link rel="Contents" href="#rfc.toc">
     309      <link rel="Author" href="#rfc.authors">
     310      <link rel="Copyright" href="#rfc.copyright">
     311      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
     312      <link rel="Chapter" title="2 HTTP version numbers" href="#rfc.section.2">
     313      <link rel="Chapter" title="3 Security Considerations" href="#rfc.section.3">
     314      <link rel="Chapter" href="#rfc.section.4" title="4 References">
     315      <link rel="Alternate" title="Authorative ASCII Version" href="http://www.ietf.org/rfc/rfc2145.txt">
     316      <link rel="Help" title="Additional Information on tools.ietf.org" href="http://tools.ietf.org/html/rfc2145">
     317      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.438, 2009-05-27 13:34:05, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
     318      <meta name="keywords" content="HTTP, hypertext transfer protocol">
     319      <link rel="schema.DC" href="http://purl.org/dc/elements/1.1/">
     320      <meta name="DC.Creator" content="Mogul, J. C.">
     321      <meta name="DC.Creator" content="Fielding, R. T.">
     322      <meta name="DC.Creator" content="Gettys, J.">
     323      <meta name="DC.Creator" content="Frystyk, H.">
     324      <meta name="DC.Identifier" content="urn:ietf:rfc:2145">
     325      <meta name="DC.Date.Issued" scheme="ISO8601" content="1997-05">
     326      <meta name="DC.Description.Abstract" content="HTTP request and response messages include an HTTP protocol version number. Some confusion exists concerning the proper use and interpretation of HTTP version numbers, and concerning interoperability of HTTP implementations of different protocol versions. This document is an attempt to clarify the situation. It is not a modification of the intended meaning of the existing HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention of the authors of those documents, and can be considered definitive when there is any ambiguity in those documents concerning HTTP version numbers, for all versions of HTTP.">
     327      <meta name="DC.isPartOf" content="urn:ISSN:2070-1721">
     328   </head>
     329   <body>
     330      <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
     331         <tr>
     332            <td class="header left">Network Working Group</td>
     333            <td class="header right">J. C. Mogul</td>
     334         </tr>
     335         <tr>
     336            <td class="header left">Request for Comments: 2145</td>
     337            <td class="header right">DEC</td>
     338         </tr>
     339         <tr>
     340            <td class="header left">Category: Informational</td>
     341            <td class="header right">R. T. Fielding</td>
     342         </tr>
     343         <tr>
     344            <td class="header left"></td>
     345            <td class="header right">UC Irvine</td>
     346         </tr>
     347         <tr>
     348            <td class="header left"></td>
     349            <td class="header right">J. Gettys</td>
     350         </tr>
     351         <tr>
     352            <td class="header left"></td>
     353            <td class="header right">DEC</td>
     354         </tr>
     355         <tr>
     356            <td class="header left"></td>
     357            <td class="header right">H. Frystyk</td>
     358         </tr>
     359         <tr>
     360            <td class="header left"></td>
     361            <td class="header right">MIT/LCS</td>
     362         </tr>
     363         <tr>
     364            <td class="header left"></td>
     365            <td class="header right">May 1997</td>
     366         </tr>
     367      </table>
     368      <p class="title">Use and Interpretation of HTTP Version Numbers</p>
     369      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
     370      <p>This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution
     371         of this memo is unlimited.
     372      </p>
     373      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
     374      <p>Copyright © The Internet Society (1997). All Rights Reserved.</p>
     375      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
     376      <p>HTTP request and response messages include an HTTP protocol version number. Some confusion exists concerning the proper use
     377         and interpretation of HTTP version numbers, and concerning interoperability of HTTP implementations of different protocol
     378         versions. This document is an attempt to clarify the situation. It is not a modification of the intended meaning of the existing
     379         HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention of the authors of those documents, and can be considered
     380         definitive when there is any ambiguity in those documents concerning HTTP version numbers, for all versions of HTTP.
     381      </p>
     382      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note</a></h1>
     383      <p>Distribution of this document is unlimited. Please send comments to the HTTP working group at &lt;http-wg@cuckoo.hpl.hp.com&gt;.
     384         Discussions of the working group are archived at &lt;URL:http://www.ics.uci.edu/pub/ietf/http/&gt;. General discussions about HTTP
     385         and the applications which use HTTP should take place on the &lt;www-talk@w3.org&gt; mailing list.
     386      </p>
     387      <hr class="noprint">
     388      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
     389      <ul class="toc">
     390         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Introduction</a><ul class="toc">
     391               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Robustness Principle</a></li>
     392            </ul>
     393         </li>
     394         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">HTTP version numbers</a><ul class="toc">
     395               <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#proxy.behavior">Proxy behavior</a></li>
     396               <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.2">Compatibility between minor versions of the same major version</a></li>
     397               <li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2.3">Which version number to send in a message</a></li>
     398            </ul>
     399         </li>
     400         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Security Considerations</a></li>
     401         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
     402         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.authors">Authors' Addresses</a></li>
     403         <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
     404      </ul>
     405      <hr class="noprint">
     406      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Introduction
     407      </h1>
     408      <p id="rfc.section.1.p.1">HTTP request and response messages include an HTTP protocol version number. According to section <a href="http://tools.ietf.org/html/rfc2068#section-3.1">3.1</a> of the HTTP/1.1 specification <a href="#RFC2068"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>,
     409      </p>
     410      <blockquote id="rfc.section.1.p.2" cite="http://tools.ietf.org/html/rfc2068#section-3.1">
     411         <p>HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended
     412            to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather
     413            than the features obtained via that communication. No change is made to the version number for the addition of message components
     414            which do not affect communication behavior or which only add to extensible field values. The &lt;minor&gt; number is incremented
     415            when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may
     416            add to the message semantics and imply additional capabilities of the sender. The &lt;major&gt; number is incremented when the format
     417            of a message within the protocol is changed.
     418         </p>
     419      </blockquote>
     420      <p id="rfc.section.1.p.3">The same language appears in the description of HTTP/1.0 <a href="#RFC1945"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a>.
     421      </p>
     422      <p id="rfc.section.1.p.4">Many readers of these documents have expressed some confusion about the intended meaning of this policy. Also, some people
     423         who wrote HTTP implementations before RFC1945 <a href="#RFC1945"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a> was issued were not aware of the intentions behind the introduction of version numbers in HTTP/1.0. This has led to debate
     424         and inconsistency regarding the use and interpretation of HTTP version numbers, and has led to interoperability problems in
     425         certain cases.
     426      </p>
     427      <p id="rfc.section.1.p.5">This document is an attempt to clarify the situation. It is not a modification of the intended meaning of the existing HTTP/1.0
     428         and HTTP/1.1 documents, but it does describe the intention of the authors of those documents. In any case where either of
     429         those two documents is ambiguous regarding the use and interpretation of HTTP version numbers, this document should be considered
     430         the definitive as to the intentions of the designers of HTTP.
     431      </p>
     432      <p id="rfc.section.1.p.6">The specification described in this document is not part of the specification of any individual version of HTTP, such as HTTP/1.0
     433         or HTTP/1.1. Rather, this document describes the use of HTTP version numbers in any version of HTTP (except for HTTP/0.9,
     434         which did not include version numbers).
     435      </p>
     436      <p id="rfc.section.1.p.7">No vendor or other provider of an HTTP implementation should claim any compliance with any IETF HTTP specification unless
     437         the implementation conditionally complies with the rules in this document.
     438      </p>
     439      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Robustness Principle
     440      </h2>
     441      <p id="rfc.section.1.1.p.1">RFC791 <a href="#RFC0791"><cite title="Internet Protocol">[4]</cite></a> defines the "robustness principle" in section <a href="http://tools.ietf.org/html/rfc791#section-3.2">3.2</a>:
     442      </p>
     443      <blockquote id="rfc.section.1.1.p.2" cite="http://tools.ietf.org/html/rfc791#section-3.2">
     444         <p>an implementation must be conservative in its sending behavior, and liberal in its receiving behavior.</p>
     445      </blockquote>
     446      <p id="rfc.section.1.1.p.3">This principle applies to HTTP, as well. It is the fundamental basis for interpreting any part of the HTTP specification that
     447         might still be ambiguous. In particular, implementations of HTTP <em class="bcp14">SHOULD NOT</em> reject messages or generate errors unnecessarily.
     448      </p>
     449      <hr class="noprint">
     450      <h1 id="rfc.section.2" class="np"><a href="#rfc.section.2">2.</a>&nbsp;HTTP version numbers
     451      </h1>
     452      <p id="rfc.section.2.p.1">We start by restating the language quoted above from section <a href="http://tools.ietf.org/html/rfc2068#section-3.1">3.1</a> of the HTTP/1.1 specification <a href="#RFC2068"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>:
     453      </p>
     454      <dl class="empty">
     455         <dd>It is, and has always been, the explicit intent of the HTTP specification that the interpretation of an HTTP message header
     456            does not change between minor versions of the same major version.
     457         </dd>
     458         <dd>It is, and has always been, the explicit intent of the HTTP specification that an implementation receiving a message header
     459            that it does not understand <em class="bcp14">MUST</em> ignore that header. (The word "ignore" has a special meaning for proxies; see section <a href="#proxy.behavior" title="Proxy behavior">2.1</a> below.)
     460         </dd>
     461      </dl>
     462      <p id="rfc.section.2.p.2">To make this as clear as possible: The major version sent in a message <em class="bcp14">MAY</em> indicate the interpretation of other header fields. The minor version sent in a message <em class="bcp14">MUST NOT</em> indicate the interpretation of other header fields. This reflects the principle that the minor version labels the capability
     463         of the sender, not the interpretation of the message.
     464      </p>
     465      <div class="note">
     466         <p>Note: In a future version of HTTP, we may introduce a mechanism that explicitly requires a receiving implementation to reject
     467            a message if it does not understand certain headers. For example, this might be implemented by means of a header that lists
     468            a set of other message headers that must be understood by the recipient. Any implementation claiming at least conditional
     469            compliance with this future version of HTTP would have to implement this mechanism. However, no implementation claiming compliance
     470            with a lower HTTP version (in particular, HTTP/1.1) will have to implement this mechanism.
     471         </p> 
     472         <p>This future change may be required to support the Protocol Extension Protocol (PEP) <a href="#Kha"><cite title="HTTP/1.2 Extension Protocol (PEP)">[3]</cite></a>.
     473         </p>
     474      </div>
     475      <p id="rfc.section.2.p.4">One consequence of these rules is that an HTTP/1.1 message sent to an HTTP/1.0 recipient (or a recipient whose version is
     476         unknown) <em class="bcp14">MUST</em> be constructed so that it remains a valid HTTP/1.0 message when all headers not defined in the HTTP/1.0 specification <a href="#RFC1945"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a> are removed.
     477      </p>
     478      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="proxy.behavior" href="#proxy.behavior">Proxy behavior</a></h2>
     479      <p id="rfc.section.2.1.p.1">A proxy <em class="bcp14">MUST</em> forward an unknown header, unless it is protected by a Connection header. A proxy implementing an HTTP version &gt;= 1.1 <em class="bcp14">MUST NOT</em> forward unknown headers that are protected by a Connection header, as described in section <a href="http://tools.ietf.org/html/rfc2068#section-14.10">14.10</a> of the HTTP/1.1 specification <a href="#RFC2068"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>.
     480      </p>
     481      <p id="rfc.section.2.1.p.2">We remind the reader that that HTTP version numbers are hop-by-hop components of HTTP messages, and are not end-to-end. That
     482         is, an HTTP proxy never "forwards" an HTTP version number in either a request or response.
     483      </p>
     484      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;Compatibility between minor versions of the same major version
     485      </h2>
     486      <p id="rfc.section.2.2.p.1">An implementation of HTTP/x.b sending a message to a recipient whose version is known to be HTTP/x.a, a &lt; b, <em class="bcp14">MAY</em> send a header that is not defined in the specification for HTTP/x.a. For example, an HTTP/1.1 server may send a "Cache-control"
     487         header to an HTTP/1.0 client; this may be useful if the immediate recipient is an HTTP/1.0 proxy, but the ultimate recipient
     488         is an HTTP/1.1 client.
     489      </p>
     490      <p id="rfc.section.2.2.p.2">An implementation of HTTP/x.b sending a message to a recipient whose version is known to be HTTP/x.a, a &lt; b, <em class="bcp14">MUST NOT</em> depend on the recipient understanding a header not defined in the specification for HTTP/x.a. For example, HTTP/1.0 clients
     491         cannot be expected to understand chunked encodings, and so an HTTP/1.1 server must never send "Transfer-Encoding: chunked"
     492         in response to an HTTP/1.0 request.
     493      </p>
     494      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;Which version number to send in a message
     495      </h2>
     496      <p id="rfc.section.2.3.p.1">The most strenuous debate over the use of HTTP version numbers has centered on the problem of implementations that do not
     497         follow the robustness principle, and which fail to produce useful results when they receive a message with an HTTP minor version
     498         higher than the minor version they implement. We consider these implementations buggy, but we recognize that the robustness
     499         principle also implies that message senders should make concessions to buggy implementations when this is truly necessary
     500         for interoperation.
     501      </p>
     502      <p id="rfc.section.2.3.p.2">An HTTP client <em class="bcp14">SHOULD</em> send a request version equal to the highest version for which the client is at least conditionally compliant, and whose major
     503         version is no higher than the highest version supported by the server, if this is known. An HTTP client <em class="bcp14">MUST NOT</em> send a version for which it is not at least conditionally compliant.
     504      </p>
     505      <p id="rfc.section.2.3.p.3">An HTTP client <em class="bcp14">MAY</em> send a lower request version, if it is known that the server incorrectly implements the HTTP specification, but only after
     506         the client has determined that the server is actually buggy.
     507      </p>
     508      <p id="rfc.section.2.3.p.4">An HTTP server <em class="bcp14">SHOULD</em> send a response version equal to the highest version for which the server is at least conditionally compliant, and whose major
     509         version is less than or equal to the one received in the request. An HTTP server <em class="bcp14">MUST NOT</em> send a version for which it is not at least conditionally compliant. A server <em class="bcp14">MAY</em> send a 505 (HTTP Version Not Supported) response if cannot send a response using the major version used in the client's request.
     510      </p>
     511      <p id="rfc.section.2.3.p.5">An HTTP server <em class="bcp14">MAY</em> send a lower response version, if it is known or suspected that the client incorrectly implements the HTTP specification,
     512         but this should not be the default, and this <em class="bcp14">SHOULD</em> NOT be done if the request version is HTTP/1.1 or greater.
     513      </p>
     514      <hr class="noprint">
     515      <h1 id="rfc.section.3" class="np"><a href="#rfc.section.3">3.</a>&nbsp;Security Considerations
     516      </h1>
     517      <p id="rfc.section.3.p.1">None, except to the extent that security mechanisms introduced in one version of HTTP might depend on the proper interpretation
     518         of HTTP version numbers in older implementations.
     519      </p>
     520      <h1 class="np" id="rfc.references"><a href="#rfc.section.4" id="rfc.section.4">4.</a> References
     521      </h1>
     522      <table summary="References">
     523         <tr>
     524            <td class="reference"><b id="RFC1945">[1]</b></td>
     525            <td class="top"><a href="mailto:timbl@w3.org" title="MIT, Laboratory for Computer Science">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:frystyk@w3.org" title="W3 Consortium, MIT Laboratory for Computer Science">H.F. Nielsen</a>, “<a href="http://tools.ietf.org/html/rfc1945">Hypertext Transfer Protocol -- HTTP/1.0</a>”, RFC&nbsp;1945, May&nbsp;1996.
     526            </td>
     527         </tr> 
     528         <tr>
     529            <td class="reference"><b id="RFC2068">[2]</b></td>
     530            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="MIT Laboratory for Computer Science">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Digital Equipment Corporation, Western Research Laboratory">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Nielsen, H.</a>, and <a href="mailto:timbl@w3.org" title="MIT Laboratory for Computer Science">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2068">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2068, January&nbsp;1997.
     531            </td>
     532         </tr> 
     533         <tr>
     534            <td class="reference"><b id="Kha">[3]</b></td>
     535            <td class="top">Khare, R., “HTTP/1.2 Extension Protocol (PEP)”.<br>HTTP Working Group, Work in Progress.
     536            </td>
     537         </tr> 
     538         <tr>
     539            <td class="reference"><b id="RFC0791">[4]</b></td>
     540            <td class="top">Postel, J., “<a href="http://tools.ietf.org/html/rfc791">Internet Protocol</a>”, RFC&nbsp;791, September&nbsp;1981.
     541            </td>
     542         </tr>
     543      </table>
     544      <hr class="noprint">
     545      <h1 id="rfc.authors" class="np"><a href="#rfc.section.5" id="rfc.section.5">5.</a> <a href="#rfc.authors">Authors' Addresses</a></h1>
     546      <address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Western Research Laboratory</span><span class="adr"><span class="street-address vcardline">Digital Equipment Corporation</span><span class="street-address vcardline">250 University Avenue</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">California</span>&nbsp;<span class="postal-code">94305</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:mogul@wrl.dec.com"><span class="email">mogul@wrl.dec.com</span></a></span></address>
     547      <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span><span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Department of Information and Computer Science</span><span class="adr"><span class="street-address vcardline">University of California</span><span class="vcardline"><span class="locality">Irvine</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">92717-3425</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1(714)824-4056"><span class="value">+1 (714) 824-4056</span></a></span><span class="vcardline">EMail: <a href="mailto:fielding@ics.uci.edu"><span class="email">fielding@ics.uci.edu</span></a></span></address>
     548      <address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">MIT Laboratory for Computer Science</span><span class="adr"><span class="street-address vcardline">545 Technology Square</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1(617)2588682"><span class="value">+1 (617) 258 8682</span></a></span><span class="vcardline">EMail: <a href="mailto:jg@w3.org"><span class="email">jg@w3.org</span></a></span></address>
     549      <address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">W3 Consortium</span><span class="adr"><span class="street-address vcardline">MIT Laboratory for Computer Science</span><span class="street-address vcardline">545 Technology Square</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1(617)2588682"><span class="value">+1 (617) 258 8682</span></a></span><span class="vcardline">EMail: <a href="mailto:frystyk@w3.org"><span class="email">frystyk@w3.org</span></a></span></address>
     550      <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
     551      <p>Copyright © The Internet Society (1997). All Rights Reserved.</p>
     552      <p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise
     553         explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without
     554         restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative
     555         works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references
     556         to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards
     557         in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to
     558         translate it into languages other than English.
     559      </p>
     560      <p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees.</p>
     561      <p>This document and the information contained herein is provided on an “AS IS” basis and THE INTERNET SOCIETY AND THE INTERNET
     562         ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
     563         OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
     564         PURPOSE.
     565      </p>
     566      <hr class="noprint">
     567      <h1 class="np"><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
     568      <p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed
     569         to pertain to the implementation or use of the technology described in this document or the extent to which any license under
     570         such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights.
     571         Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be
     572         found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available,
     573         or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors
     574         or users of this specification can be obtained from the IETF Secretariat.
     575      </p>
     576      <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
     577         rights which may cover technology that may be required to practice this standard. Please address the information to the IETF
     578         Executive Director.
     579      </p>
     580   </body>
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    354338      <link rel="Appendix" title="19 Appendices" href="#rfc.section.19">
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    54355419            <td class="reference"><b id="ISO-8859">[ISO-8859]</b></td>
    5436             <td class="top">International Organization for Standardization, “Information technology - 8-bit single byte coded graphic - character sets”, 1987-1990.<br>Part 1: Latin alphabet No. 1, ISO-8859-1:1987. Part 2: Latin alphabet No. 2, ISO-8859-2, 1987. Part 3: Latin alphabet No.
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    333 </style><link rel="Contents" href="#rfc.toc"><link rel="Author" href="#rfc.authors"><link rel="Copyright" href="#rfc.copyright"><link rel="Index" href="#rfc.index"><link rel="Chapter" title="1 Introduction" href="#rfc.section.1"><link rel="Chapter" title="2 Notational Conventions and Generic Grammar" href="#rfc.section.2"><link rel="Chapter" title="3 Protocol Parameters" href="#rfc.section.3"><link rel="Chapter" title="4 HTTP Message" href="#rfc.section.4"><link rel="Chapter" title="5 Request" href="#rfc.section.5"><link rel="Chapter" title="6 Response" href="#rfc.section.6"><link rel="Chapter" title="7 Entity" href="#rfc.section.7"><link rel="Chapter" title="8 Connections" href="#rfc.section.8"><link rel="Chapter" title="9 Method Definitions" href="#rfc.section.9"><link rel="Chapter" title="10 Status Code Definitions" href="#rfc.section.10"><link rel="Chapter" title="11 Access Authentication" href="#rfc.section.11"><link rel="Chapter" title="12 Content Negotiation" href="#rfc.section.12"><link rel="Chapter" title="13 Caching in HTTP" href="#rfc.section.13"><link rel="Chapter" title="14 Header Field Definitions" href="#rfc.section.14"><link rel="Chapter" title="15 Security Considerations" href="#rfc.section.15"><link rel="Chapter" title="16 Acknowledgments" href="#rfc.section.16"><link rel="Chapter" href="#rfc.section.17" title="17 References"><link rel="Appendix" title="19 Appendices" href="#rfc.section.19"><link rel="Appendix" title="20 Index" href="#rfc.section.20"><link rel="Alternate" title="Authorative ASCII version" href="http://www.ietf.org/rfc/rfc2616.txt"><meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.389, 2008-08-20 14:21:35, XSLT vendor: SAXON 6.5.5 from Michael Kay http://saxon.sf.net/"><link rel="schema.DC" href="http://purl.org/dc/elements/1.1/"><meta name="DC.Creator" content="Fielding, R."><meta name="DC.Creator" content="Gettys, J."><meta name="DC.Creator" content="Mogul, J."><meta name="DC.Creator" content="Frystyk, H."><meta name="DC.Creator" content="Masinter, L."><meta name="DC.Creator" content="Leach, P."><meta name="DC.Creator" content="Berners-Lee, T."><meta name="DC.Identifier" content="urn:ietf:rfc:2616"><meta name="DC.Date.Issued" scheme="ISO8601" content="1999-06"><meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2068"><meta name="DC.Description.Abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and distributed object management systems, through extension of its request methods, error codes and headers . A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred. HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as &#34;HTTP/1.1&#34;, and is an update to RFC 2068 ."></head><body><table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1"><tr><td class="header left">Network Working Group</td><td class="header right">R. Fielding</td></tr><tr><td class="header left">Request for Comments: 2616</td><td class="header right">UC Irvine</td></tr><tr><td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2068">2068</a></td><td class="header right">J. Gettys</td></tr><tr><td class="header left">Category: Standards Track</td><td class="header right">Compaq/W3C</td></tr><tr><td class="header left"></td><td class="header right">J. Mogul</td></tr><tr><td class="header left"></td><td class="header right">Compaq</td></tr><tr><td class="header left"></td><td class="header right">H. Frystyk</td></tr><tr><td class="header left"></td><td class="header right">W3C/MIT</td></tr><tr><td class="header left"></td><td class="header right">L. Masinter</td></tr><tr><td class="header left"></td><td class="header right">Xerox</td></tr><tr><td class="header left"></td><td class="header right">P. Leach</td></tr><tr><td class="header left"></td><td class="header right">Microsoft</td></tr><tr><td class="header left"></td><td class="header right">T. Berners-Lee</td></tr><tr><td class="header left"></td><td class="header right">W3C/MIT</td></tr><tr><td class="header left"></td><td class="header right">June 1999</td></tr></table><p class="title">Hypertext Transfer Protocol -- HTTP/1.1</p><h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1><p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the &#8220;Internet Official Protocol Standards&#8221; (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.</p><h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1><p>Copyright &copy; The Internet Society (1999). All Rights Reserved.</p><h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and distributed object management systems, through extension of its request methods, error codes and headers <a href="#RFC2324" id="rfc.xref.RFC2324.1"><cite title="Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)">[47]</cite></a>. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.</p>  <p>HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[33]</cite></a>.</p> <hr class="noprint"><h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1><ul class="toc"><li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul class="toc"><li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li><li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li><li class="tocline1">1.3&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li><li class="tocline1">1.4&nbsp;&nbsp;&nbsp;<a href="#intro.overall.operation">Overall Operation</a></li></ul></li><li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#notation">Notational Conventions and Generic Grammar</a><ul class="toc"><li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#notation.abnf">Augmented BNF</a></li><li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#basic.rules">Basic Rules</a></li></ul></li><li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#protocol.parameters">Protocol Parameters</a><ul class="toc"><li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#http.version">HTTP Version</a></li><li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#uri">Uniform Resource Identifiers</a><ul class="toc"><li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#general.syntax">General Syntax</a></li><li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#http.url">http URL</a></li><li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#uri.comparison">URI Comparison</a></li></ul></li><li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#date.time.formats">Date/Time Formats</a><ul class="toc"><li class="tocline1">3.3.1&nbsp;&nbsp;&nbsp;<a href="#full.date">Full Date</a></li><li class="tocline1">3.3.2&nbsp;&nbsp;&nbsp;<a href="#delta.seconds">Delta Seconds</a></li></ul></li><li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#character.sets">Character Sets</a><ul class="toc"><li class="tocline1">3.4.1&nbsp;&nbsp;&nbsp;<a href="#missing.charset">Missing Charset</a></li></ul></li><li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#content.codings">Content Codings</a></li><li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#transfer.codings">Transfer Codings</a><ul class="toc"><li class="tocline1">3.6.1&nbsp;&nbsp;&nbsp;<a href="#chunked.transfer.encoding">Chunked Transfer Coding</a></li></ul></li><li class="tocline1">3.7&nbsp;&nbsp;&nbsp;<a href="#media.types">Media Types</a><ul class="toc"><li class="tocline1">3.7.1&nbsp;&nbsp;&nbsp;<a href="#canonicalization.and.text.defaults">Canonicalization and Text Defaults</a></li><li class="tocline1">3.7.2&nbsp;&nbsp;&nbsp;<a href="#multipart.types">Multipart Types</a></li></ul></li><li class="tocline1">3.8&nbsp;&nbsp;&nbsp;<a href="#product.tokens">Product Tokens</a></li><li class="tocline1">3.9&nbsp;&nbsp;&nbsp;<a href="#quality.values">Quality Values</a></li><li class="tocline1">3.10&nbsp;&nbsp;&nbsp;<a href="#language.tags">Language Tags</a></li><li class="tocline1">3.11&nbsp;&nbsp;&nbsp;<a href="#entity.tags">Entity Tags</a></li><li class="tocline1">3.12&nbsp;&nbsp;&nbsp;<a href="#range.units">Range Units</a></li></ul></li><li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#http.message">HTTP Message</a><ul class="toc"><li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#message.types">Message Types</a></li><li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#message.headers">Message Headers</a></li><li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#message.body">Message Body</a></li><li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#message.length">Message Length</a></li><li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#general.header.fields">General Header Fields</a></li></ul></li><li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#request">Request</a><ul class="toc"><li class="tocline1">5.1&nbsp;&nbsp;&nbsp;<a href="#request-line">Request-Line</a><ul class="toc"><li class="tocline1">5.1.1&nbsp;&nbsp;&nbsp;<a href="#method">Method</a></li><li class="tocline1">5.1.2&nbsp;&nbsp;&nbsp;<a href="#request-uri">Request-URI</a></li></ul></li><li class="tocline1">5.2&nbsp;&nbsp;&nbsp;<a href="#the.resource.identified.by.a.request">The Resource Identified by a Request</a></li><li class="tocline1">5.3&nbsp;&nbsp;&nbsp;<a href="#request.header.fields">Request Header Fields</a></li></ul></li><li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#response">Response</a><ul class="toc"><li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#status-line">Status-Line</a><ul class="toc"><li class="tocline1">6.1.1&nbsp;&nbsp;&nbsp;<a href="#status.code.and.reason.phrase">Status Code and Reason Phrase</a></li></ul></li><li class="tocline1">6.2&nbsp;&nbsp;&nbsp;<a href="#response.header.fields">Response Header Fields</a></li></ul></li><li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#entity">Entity</a><ul class="toc"><li class="tocline1">7.1&nbsp;&nbsp;&nbsp;<a href="#entity.header.fields">Entity Header Fields</a></li><li class="tocline1">7.2&nbsp;&nbsp;&nbsp;<a href="#entity.body">Entity Body</a><ul class="toc"><li class="tocline1">7.2.1&nbsp;&nbsp;&nbsp;<a href="#type">Type</a></li><li class="tocline1">7.2.2&nbsp;&nbsp;&nbsp;<a href="#entity.length">Entity Length</a></li></ul></li></ul></li><li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#connections">Connections</a><ul class="toc"><li class="tocline1">8.1&nbsp;&nbsp;&nbsp;<a href="#persistent.connections">Persistent Connections</a><ul class="toc"><li class="tocline1">8.1.1&nbsp;&nbsp;&nbsp;<a href="#persistent.purpose">Purpose</a></li><li class="tocline1">8.1.2&nbsp;&nbsp;&nbsp;<a href="#persistent.overall">Overall Operation</a><ul class="toc"><li class="tocline1">8.1.2.1&nbsp;&nbsp;&nbsp;<a href="#persistent.negotiation">Negotiation</a></li><li class="tocline1">8.1.2.2&nbsp;&nbsp;&nbsp;<a href="#pipelining">Pipelining</a></li></ul></li><li class="tocline1">8.1.3&nbsp;&nbsp;&nbsp;<a href="#persistent.proxy">Proxy Servers</a></li><li class="tocline1">8.1.4&nbsp;&nbsp;&nbsp;<a href="#persistent.practical">Practical Considerations</a></li></ul></li><li class="tocline1">8.2&nbsp;&nbsp;&nbsp;<a href="#message.transmission.requirements">Message Transmission Requirements</a><ul class="toc"><li class="tocline1">8.2.1&nbsp;&nbsp;&nbsp;<a href="#persistent.flow">Persistent Connections and Flow Control</a></li><li class="tocline1">8.2.2&nbsp;&nbsp;&nbsp;<a href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></li><li class="tocline1">8.2.3&nbsp;&nbsp;&nbsp;<a href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></li><li class="tocline1">8.2.4&nbsp;&nbsp;&nbsp;<a href="#connection.premature">Client Behavior if Server Prematurely Closes Connection</a></li></ul></li></ul></li><li class="tocline0">9.&nbsp;&nbsp;&nbsp;<a href="#method.definitions">Method Definitions</a><ul class="toc"><li class="tocline1">9.1&nbsp;&nbsp;&nbsp;<a href="#safe.and.idempotent">Safe and Idempotent Methods</a><ul class="toc"><li class="tocline1">9.1.1&nbsp;&nbsp;&nbsp;<a href="#safe.methods">Safe Methods</a></li><li class="tocline1">9.1.2&nbsp;&nbsp;&nbsp;<a href="#idempotent.methods">Idempotent Methods</a></li></ul></li><li class="tocline1">9.2&nbsp;&nbsp;&nbsp;<a href="#OPTIONS">OPTIONS</a></li><li class="tocline1">9.3&nbsp;&nbsp;&nbsp;<a href="#GET">GET</a></li><li class="tocline1">9.4&nbsp;&nbsp;&nbsp;<a href="#HEAD">HEAD</a></li><li class="tocline1">9.5&nbsp;&nbsp;&nbsp;<a href="#POST">POST</a></li><li class="tocline1">9.6&nbsp;&nbsp;&nbsp;<a href="#PUT">PUT</a></li><li class="tocline1">9.7&nbsp;&nbsp;&nbsp;<a href="#DELETE">DELETE</a></li><li class="tocline1">9.8&nbsp;&nbsp;&nbsp;<a href="#TRACE">TRACE</a></li><li class="tocline1">9.9&nbsp;&nbsp;&nbsp;<a href="#CONNECT">CONNECT</a></li></ul></li><li class="tocline0">10.&nbsp;&nbsp;&nbsp;<a href="#status.codes">Status Code Definitions</a><ul class="toc"><li class="tocline1">10.1&nbsp;&nbsp;&nbsp;<a href="#status.1xx">Informational 1xx</a><ul class="toc"><li class="tocline1">10.1.1&nbsp;&nbsp;&nbsp;<a href="#status.100">100 Continue</a></li><li class="tocline1">10.1.2&nbsp;&nbsp;&nbsp;<a href="#status.101">101 Switching Protocols</a></li></ul></li><li class="tocline1">10.2&nbsp;&nbsp;&nbsp;<a href="#status.2xx">Successful 2xx</a><ul class="toc"><li class="tocline1">10.2.1&nbsp;&nbsp;&nbsp;<a href="#status.200">200 OK</a></li><li class="tocline1">10.2.2&nbsp;&nbsp;&nbsp;<a href="#status.201">201 Created</a></li><li class="tocline1">10.2.3&nbsp;&nbsp;&nbsp;<a href="#status.202">202 Accepted</a></li><li class="tocline1">10.2.4&nbsp;&nbsp;&nbsp;<a href="#status.203">203 Non-Authoritative Information</a></li><li class="tocline1">10.2.5&nbsp;&nbsp;&nbsp;<a href="#status.204">204 No Content</a></li><li class="tocline1">10.2.6&nbsp;&nbsp;&nbsp;<a href="#status.205">205 Reset Content</a></li><li class="tocline1">10.2.7&nbsp;&nbsp;&nbsp;<a href="#status.206">206 Partial Content</a></li></ul></li><li class="tocline1">10.3&nbsp;&nbsp;&nbsp;<a href="#status.3xx">Redirection 3xx</a><ul class="toc"><li class="tocline1">10.3.1&nbsp;&nbsp;&nbsp;<a href="#status.300">300 Multiple Choices</a></li><li class="tocline1">10.3.2&nbsp;&nbsp;&nbsp;<a href="#status.301">301 Moved Permanently</a></li><li class="tocline1">10.3.3&nbsp;&nbsp;&nbsp;<a href="#status.302">302 Found</a></li><li class="tocline1">10.3.4&nbsp;&nbsp;&nbsp;<a href="#status.303">303 See Other</a></li><li class="tocline1">10.3.5&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li><li class="tocline1">10.3.6&nbsp;&nbsp;&nbsp;<a href="#status.305">305 Use Proxy</a></li><li class="tocline1">10.3.7&nbsp;&nbsp;&nbsp;<a href="#status.306">306 (Unused)</a></li><li class="tocline1">10.3.8&nbsp;&nbsp;&nbsp;<a href="#status.307">307 Temporary Redirect</a></li></ul></li><li class="tocline1">10.4&nbsp;&nbsp;&nbsp;<a href="#status.4xx">Client Error 4xx</a><ul class="toc"><li class="tocline1">10.4.1&nbsp;&nbsp;&nbsp;<a href="#status.400">400 Bad Request</a></li><li class="tocline1">10.4.2&nbsp;&nbsp;&nbsp;<a href="#status.401">401 Unauthorized</a></li><li class="tocline1">10.4.3&nbsp;&nbsp;&nbsp;<a href="#status.402">402 Payment Required</a></li><li class="tocline1">10.4.4&nbsp;&nbsp;&nbsp;<a href="#status.403">403 Forbidden</a></li><li class="tocline1">10.4.5&nbsp;&nbsp;&nbsp;<a href="#status.404">404 Not Found</a></li><li class="tocline1">10.4.6&nbsp;&nbsp;&nbsp;<a href="#status.405">405 Method Not Allowed</a></li><li class="tocline1">10.4.7&nbsp;&nbsp;&nbsp;<a href="#status.406">406 Not Acceptable</a></li><li class="tocline1">10.4.8&nbsp;&nbsp;&nbsp;<a href="#status.407">407 Proxy Authentication Required</a></li><li class="tocline1">10.4.9&nbsp;&nbsp;&nbsp;<a href="#status.408">408 Request Timeout</a></li><li class="tocline1">10.4.10&nbsp;&nbsp;&nbsp;<a href="#status.409">409 Conflict</a></li><li class="tocline1">10.4.11&nbsp;&nbsp;&nbsp;<a href="#status.410">410 Gone</a></li><li class="tocline1">10.4.12&nbsp;&nbsp;&nbsp;<a href="#status.411">411 Length Required</a></li><li class="tocline1">10.4.13&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li><li class="tocline1">10.4.14&nbsp;&nbsp;&nbsp;<a href="#status.413">413 Request Entity Too Large</a></li><li class="tocline1">10.4.15&nbsp;&nbsp;&nbsp;<a href="#status.414">414 Request-URI Too Long</a></li><li class="tocline1">10.4.16&nbsp;&nbsp;&nbsp;<a href="#status.415">415 Unsupported Media Type</a></li><li class="tocline1">10.4.17&nbsp;&nbsp;&nbsp;<a href="#status.416">416 Requested Range Not Satisfiable</a></li><li class="tocline1">10.4.18&nbsp;&nbsp;&nbsp;<a href="#status.417">417 Expectation Failed</a></li></ul></li><li class="tocline1">10.5&nbsp;&nbsp;&nbsp;<a href="#status.5xx">Server Error 5xx</a><ul class="toc"><li class="tocline1">10.5.1&nbsp;&nbsp;&nbsp;<a href="#status.500">500 Internal Server Error</a></li><li class="tocline1">10.5.2&nbsp;&nbsp;&nbsp;<a href="#status.501">501 Not Implemented</a></li><li class="tocline1">10.5.3&nbsp;&nbsp;&nbsp;<a href="#status.502">502 Bad Gateway</a></li><li class="tocline1">10.5.4&nbsp;&nbsp;&nbsp;<a href="#status.503">503 Service Unavailable</a></li><li class="tocline1">10.5.5&nbsp;&nbsp;&nbsp;<a href="#status.504">504 Gateway Timeout</a></li><li class="tocline1">10.5.6&nbsp;&nbsp;&nbsp;<a href="#status.505">505 HTTP Version Not Supported</a></li></ul></li></ul></li><li class="tocline0">11.&nbsp;&nbsp;&nbsp;<a href="#access.authentication">Access Authentication</a></li><li class="tocline0">12.&nbsp;&nbsp;&nbsp;<a href="#content.negotiation">Content Negotiation</a><ul class="toc"><li class="tocline1">12.1&nbsp;&nbsp;&nbsp;<a href="#server-driven.negotiation">Server-driven Negotiation</a></li><li class="tocline1">12.2&nbsp;&nbsp;&nbsp;<a href="#agent-driven.negotiation">Agent-driven Negotiation</a></li><li class="tocline1">12.3&nbsp;&nbsp;&nbsp;<a href="#transparent.negotiation">Transparent Negotiation</a></li></ul></li><li class="tocline0">13.&nbsp;&nbsp;&nbsp;<a href="#caching">Caching in HTTP</a><ul class="toc"><li class="tocline1">13.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.13.1"></a><ul class="toc"><li class="tocline1">13.1.1&nbsp;&nbsp;&nbsp;<a href="#cache.correctness">Cache Correctness</a></li><li class="tocline1">13.1.2&nbsp;&nbsp;&nbsp;<a href="#warnings">Warnings</a></li><li class="tocline1">13.1.3&nbsp;&nbsp;&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></li><li class="tocline1">13.1.4&nbsp;&nbsp;&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></li><li class="tocline1">13.1.5&nbsp;&nbsp;&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></li><li class="tocline1">13.1.6&nbsp;&nbsp;&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></li></ul></li><li class="tocline1">13.2&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Expiration Model</a><ul class="toc"><li class="tocline1">13.2.1&nbsp;&nbsp;&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></li><li class="tocline1">13.2.2&nbsp;&nbsp;&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></li><li class="tocline1">13.2.3&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Age Calculations</a></li><li class="tocline1">13.2.4&nbsp;&nbsp;&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></li><li class="tocline1">13.2.5&nbsp;&nbsp;&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></li><li class="tocline1">13.2.6&nbsp;&nbsp;&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></li></ul></li><li class="tocline1">13.3&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul class="toc"><li class="tocline1">13.3.1&nbsp;&nbsp;&nbsp;<a href="#last-modified.dates">Last-Modified Dates</a></li><li class="tocline1">13.3.2&nbsp;&nbsp;&nbsp;<a href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></li><li class="tocline1">13.3.3&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak and Strong Validators</a></li><li class="tocline1">13.3.4&nbsp;&nbsp;&nbsp;<a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity Tags and Last-Modified Dates</a></li><li class="tocline1">13.3.5&nbsp;&nbsp;&nbsp;<a href="#non-validating.conditionals">Non-validating Conditionals</a></li></ul></li><li class="tocline1">13.4&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li><li class="tocline1">13.5&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a><ul class="toc"><li class="tocline1">13.5.1&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></li><li class="tocline1">13.5.2&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></li><li class="tocline1">13.5.3&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Headers</a></li><li class="tocline1">13.5.4&nbsp;&nbsp;&nbsp;<a href="#combining.byte.ranges">Combining Byte Ranges</a></li></ul></li><li class="tocline1">13.6&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li><li class="tocline1">13.7&nbsp;&nbsp;&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></li><li class="tocline1">13.8&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></li><li class="tocline1">13.9&nbsp;&nbsp;&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></li><li class="tocline1">13.10&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></li><li class="tocline1">13.11&nbsp;&nbsp;&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></li><li class="tocline1">13.12&nbsp;&nbsp;&nbsp;<a href="#cache.replacement">Cache Replacement</a></li><li class="tocline1">13.13&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li></ul></li><li class="tocline0">14.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc"><li class="tocline1">14.1&nbsp;&nbsp;&nbsp;<a href="#header.accept">Accept</a></li><li class="tocline1">14.2&nbsp;&nbsp;&nbsp;<a href="#header.accept-charset">Accept-Charset</a></li><li class="tocline1">14.3&nbsp;&nbsp;&nbsp;<a href="#header.accept-encoding">Accept-Encoding</a></li><li class="tocline1">14.4&nbsp;&nbsp;&nbsp;<a href="#header.accept-language">Accept-Language</a></li><li class="tocline1">14.5&nbsp;&nbsp;&nbsp;<a href="#header.accept-ranges">Accept-Ranges</a></li><li class="tocline1">14.6&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li><li class="tocline1">14.7&nbsp;&nbsp;&nbsp;<a href="#header.allow">Allow</a></li><li class="tocline1">14.8&nbsp;&nbsp;&nbsp;<a href="#header.authorization">Authorization</a></li><li class="tocline1">14.9&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc"><li class="tocline1">14.9.1&nbsp;&nbsp;&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></li><li class="tocline1">14.9.2&nbsp;&nbsp;&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></li><li class="tocline1">14.9.3&nbsp;&nbsp;&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></li><li class="tocline1">14.9.4&nbsp;&nbsp;&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></li><li class="tocline1">14.9.5&nbsp;&nbsp;&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></li><li class="tocline1">14.9.6&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li></ul></li><li class="tocline1">14.10&nbsp;&nbsp;&nbsp;<a href="#header.connection">Connection</a></li><li class="tocline1">14.11&nbsp;&nbsp;&nbsp;<a href="#header.content-encoding">Content-Encoding</a></li><li class="tocline1">14.12&nbsp;&nbsp;&nbsp;<a href="#header.content-language">Content-Language</a></li><li class="tocline1">14.13&nbsp;&nbsp;&nbsp;<a href="#header.content-length">Content-Length</a></li><li class="tocline1">14.14&nbsp;&nbsp;&nbsp;<a href="#header.content-location">Content-Location</a></li><li class="tocline1">14.15&nbsp;&nbsp;&nbsp;<a href="#header.content-md5">Content-MD5</a></li><li class="tocline1">14.16&nbsp;&nbsp;&nbsp;<a href="#header.content-range">Content-Range</a></li><li class="tocline1">14.17&nbsp;&nbsp;&nbsp;<a href="#header.content-type">Content-Type</a></li><li class="tocline1">14.18&nbsp;&nbsp;&nbsp;<a href="#header.date">Date</a><ul class="toc"><li class="tocline1">14.18.1&nbsp;&nbsp;&nbsp;<a href="#clockless.origin.server.operation">Clockless Origin Server Operation</a></li></ul></li><li class="tocline1">14.19&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a></li><li class="tocline1">14.20&nbsp;&nbsp;&nbsp;<a href="#header.expect">Expect</a></li><li class="tocline1">14.21&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li><li class="tocline1">14.22&nbsp;&nbsp;&nbsp;<a href="#header.from">From</a></li><li class="tocline1">14.23&nbsp;&nbsp;&nbsp;<a href="#header.host">Host</a></li><li class="tocline1">14.24&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li><li class="tocline1">14.25&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li><li class="tocline1">14.26&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li><li class="tocline1">14.27&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li><li class="tocline1">14.28&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li><li class="tocline1">14.29&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a></li><li class="tocline1">14.30&nbsp;&nbsp;&nbsp;<a href="#header.location">Location</a></li><li class="tocline1">14.31&nbsp;&nbsp;&nbsp;<a href="#header.max-forwards">Max-Forwards</a></li><li class="tocline1">14.32&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li><li class="tocline1">14.33&nbsp;&nbsp;&nbsp;<a href="#header.proxy-authenticate">Proxy-Authenticate</a></li><li class="tocline1">14.34&nbsp;&nbsp;&nbsp;<a href="#header.proxy-authorization">Proxy-Authorization</a></li><li class="tocline1">14.35&nbsp;&nbsp;&nbsp;<a href="#header.range">Range</a><ul class="toc"><li class="tocline1">14.35.1&nbsp;&nbsp;&nbsp;<a href="#byte.ranges">Byte Ranges</a></li><li class="tocline1">14.35.2&nbsp;&nbsp;&nbsp;<a href="#range.retrieval.requests">Range Retrieval Requests</a></li></ul></li><li class="tocline1">14.36&nbsp;&nbsp;&nbsp;<a href="#header.referer">Referer</a></li><li class="tocline1">14.37&nbsp;&nbsp;&nbsp;<a href="#header.retry-after">Retry-After</a></li><li class="tocline1">14.38&nbsp;&nbsp;&nbsp;<a href="#header.server">Server</a></li><li class="tocline1">14.39&nbsp;&nbsp;&nbsp;<a href="#header.te">TE</a></li><li class="tocline1">14.40&nbsp;&nbsp;&nbsp;<a href="#header.trailer">Trailer</a></li><li class="tocline1">14.41&nbsp;&nbsp;&nbsp;<a href="#header.transfer-encoding">Transfer-Encoding</a></li><li class="tocline1">14.42&nbsp;&nbsp;&nbsp;<a href="#header.upgrade">Upgrade</a></li><li class="tocline1">14.43&nbsp;&nbsp;&nbsp;<a href="#header.user-agent">User-Agent</a></li><li class="tocline1">14.44&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li><li class="tocline1">14.45&nbsp;&nbsp;&nbsp;<a href="#header.via">Via</a></li><li class="tocline1">14.46&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li><li class="tocline1">14.47&nbsp;&nbsp;&nbsp;<a href="#header.www-authenticate">WWW-Authenticate</a></li></ul></li><li class="tocline0">15.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul class="toc"><li class="tocline1">15.1&nbsp;&nbsp;&nbsp;<a href="#personal.information">Personal Information</a><ul class="toc"><li class="tocline1">15.1.1&nbsp;&nbsp;&nbsp;<a href="#abuse.of.server.log.information">Abuse of Server Log Information</a></li><li class="tocline1">15.1.2&nbsp;&nbsp;&nbsp;<a href="#security.sensitive">Transfer of Sensitive Information</a></li><li class="tocline1">15.1.3&nbsp;&nbsp;&nbsp;<a href="#encoding.sensitive.information.in.uris">Encoding Sensitive Information in URI's</a></li><li class="tocline1">15.1.4&nbsp;&nbsp;&nbsp;<a href="#privacy.issues.connected.to.accept.headers">Privacy Issues Connected to Accept Headers</a></li></ul></li><li class="tocline1">15.2&nbsp;&nbsp;&nbsp;<a href="#attack.pathname">Attacks Based On File and Path Names</a></li><li class="tocline1">15.3&nbsp;&nbsp;&nbsp;<a href="#dns.spoofing">DNS Spoofing</a></li><li class="tocline1">15.4&nbsp;&nbsp;&nbsp;<a href="#location.spoofing">Location Headers and Spoofing</a></li><li class="tocline1">15.5&nbsp;&nbsp;&nbsp;<a href="#content-disposition.issues">Content-Disposition Issues</a></li><li class="tocline1">15.6&nbsp;&nbsp;&nbsp;<a href="#auth.credentials.and.idle.clients">Authentication Credentials and Idle Clients</a></li><li class="tocline1">15.7&nbsp;&nbsp;&nbsp;<a href="#attack.proxies">Proxies and Caching</a><ul class="toc"><li class="tocline1">15.7.1&nbsp;&nbsp;&nbsp;<a href="#attack.DoS">Denial of Service Attacks on Proxies</a></li></ul></li></ul></li><li class="tocline0">16.&nbsp;&nbsp;&nbsp;<a href="#acknowledgments">Acknowledgments</a></li><li class="tocline0">17.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li><li class="tocline0">18.&nbsp;&nbsp;&nbsp;<a href="#rfc.authors">Authors' Addresses</a></li><li class="tocline0">19.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.19">Appendices</a><ul class="toc"><li class="tocline1">19.1&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.http">Internet Media Type message/http and application/http</a></li><li class="tocline1">19.2&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.multipart.byteranges">Internet Media Type multipart/byteranges</a></li><li class="tocline1">19.3&nbsp;&nbsp;&nbsp;<a href="#tolerant.applications">Tolerant Applications</a></li><li class="tocline1">19.4&nbsp;&nbsp;&nbsp;<a href="#differences.between.http.entities.and.rfc.2045.entities">Differences Between HTTP Entities and RFC 2045 Entities</a><ul class="toc"><li class="tocline1">19.4.1&nbsp;&nbsp;&nbsp;<a href="#mime-version">MIME-Version</a></li><li class="tocline1">19.4.2&nbsp;&nbsp;&nbsp;<a href="#conversion.to.canonical.form">Conversion to Canonical Form</a></li><li class="tocline1">19.4.3&nbsp;&nbsp;&nbsp;<a href="#conversion.of.date.formats">Conversion of Date Formats</a></li><li class="tocline1">19.4.4&nbsp;&nbsp;&nbsp;<a href="#introduction.of.content-encoding">Introduction of Content-Encoding</a></li><li class="tocline1">19.4.5&nbsp;&nbsp;&nbsp;<a href="#no.content-transfer-encoding">No Content-Transfer-Encoding</a></li><li class="tocline1">19.4.6&nbsp;&nbsp;&nbsp;<a href="#introduction.of.transfer-encoding">Introduction of Transfer-Encoding</a></li><li class="tocline1">19.4.7&nbsp;&nbsp;&nbsp;<a href="#mhtml.line.length">MHTML and Line Length Limitations</a></li></ul></li><li class="tocline1">19.5&nbsp;&nbsp;&nbsp;<a href="#additional.features">Additional Features</a><ul class="toc"><li class="tocline1">19.5.1&nbsp;&nbsp;&nbsp;<a href="#content-disposition">Content-Disposition</a></li></ul></li><li class="tocline1">19.6&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul class="toc"><li class="tocline1">19.6.1&nbsp;&nbsp;&nbsp;<a href="#changes.from.1.0">Changes from HTTP/1.0</a><ul class="toc"><li class="tocline1">19.6.1.1&nbsp;&nbsp;&nbsp;<a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses</a></li></ul></li><li class="tocline1">19.6.2&nbsp;&nbsp;&nbsp;<a href="#compatibility.with.http.1.0.persistent.connections">Compatibility with HTTP/1.0 Persistent Connections</a></li><li class="tocline1">19.6.3&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></li></ul></li></ul></li><li class="tocline0">20.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.20">Index</a></li><li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li><li class="tocline0"><a href="#rfc.index">Index</a></li></ul><hr class="noprint"><h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1><h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.purpose" href="#intro.purpose">Purpose</a></h2><p id="rfc.section.1.1.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by RFC 1945 <a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[6]</cite></a>, improved the protocol by allowing messages to be in the format of MIME-like messages, containing metainformation about the data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration the effects of hierarchical proxies, caching, the need for persistent connections, or virtual hosts. In addition, the proliferation of incompletely-implemented applications calling themselves "HTTP/1.0" has necessitated a protocol version change in order for two communicating applications to determine each other's true capabilities.</p><p id="rfc.section.1.1.p.2">This specification defines the protocol referred to as "HTTP/1.1". This protocol includes more stringent requirements than HTTP/1.0 in order to ensure reliable implementation of its features.</p><p id="rfc.section.1.1.p.3">Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP allows an open-ended set of methods and headers that indicate the purpose of a request <a href="#RFC2324" id="rfc.xref.RFC2324.2"><cite title="Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)">[47]</cite></a>. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) <a href="#RFC1630" id="rfc.xref.RFC1630.1"><cite title="Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web">[3]</cite></a>, as a location (URL) <a href="#RFC1738" id="rfc.xref.RFC1738.1"><cite title="Uniform Resource Locators (URL)">[4]</cite></a> or name (URN) <a href="#RFC1737" id="rfc.xref.RFC1737.1"><cite title="Functional Requirements for Uniform Resource Names">[20]</cite></a>, for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by Internet mail <a href="#RFC822" id="rfc.xref.RFC822.1"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a> as defined by the Multipurpose Internet Mail Extensions (MIME) <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[7]</cite></a>.</p><p id="rfc.section.1.1.p.4">HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems, including those supported by the SMTP <a href="#RFC821" id="rfc.xref.RFC821.1"><cite title="Simple Mail Transfer Protocol">[16]</cite></a>, NNTP <a href="#RFC977" id="rfc.xref.RFC977.1"><cite title="Network News Transfer Protocol">[13]</cite></a>, FTP <a href="#RFC959" id="rfc.xref.RFC959.1"><cite title="File Transfer Protocol">[18]</cite></a>, Gopher <a href="#RFC1436" id="rfc.xref.RFC1436.1"><cite title="The Internet Gopher Protocol (a distributed document search and retrieval protocol)">[2]</cite></a>, and WAIS <a href="#WAIS" id="rfc.xref.WAIS.1"><cite title="WAIS Interface Protocol Prototype Functional Specification (v1.5)">[10]</cite></a> protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications.</p><h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2><p id="rfc.section.1.2.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[34]</cite></a>.</p><p id="rfc.section.1.2.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."</p><h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a id="intro.terminology" href="#intro.terminology">Terminology</a></h2><p id="rfc.section.1.3.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.</p><p id="rfc.section.1.3.p.2"> <span id="rfc.iref.c.1"></span>  <dfn>connection</dfn>  </p><dl class="empty"><dd>A transport layer virtual circuit established between two programs for the purpose of communication.</dd></dl><p id="rfc.section.1.3.p.3"> <span id="rfc.iref.m.1"></span>  <dfn>message</dfn>  </p><dl class="empty"><dd>The basic unit of HTTP communication, consisting of a structured sequence of octets matching the syntax defined in <a href="#http.message" title="HTTP Message">Section&nbsp;4</a> and transmitted via the connection.</dd></dl><p id="rfc.section.1.3.p.4"> <span id="rfc.iref.r.1"></span>  <dfn>request</dfn>  </p><dl class="empty"><dd>An HTTP request message, as defined in <a href="#request" title="Request">Section&nbsp;5</a>.</dd></dl><p id="rfc.section.1.3.p.5"> <span id="rfc.iref.r.2"></span>  <dfn>response</dfn>  </p><dl class="empty"><dd>An HTTP response message, as defined in <a href="#response" title="Response">Section&nbsp;6</a>.</dd></dl><p id="rfc.section.1.3.p.6"> <span id="rfc.iref.r.3"></span>  <dfn>resource</dfn>  </p><dl class="empty"><dd>A network data object or service that can be identified by a URI, as defined in <a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;3.2</a>. Resources may be available in multiple representations (e.g. multiple languages, data formats, size, and resolutions) or vary in other ways.</dd></dl><p id="rfc.section.1.3.p.7"> <span id="rfc.iref.e.1"></span>  <dfn>entity</dfn>  </p><dl class="empty"><dd>The information transferred as the payload of a request or response. An entity consists of metainformation in the form of entity-header fields and content in the form of an entity-body, as described in <a href="#entity" title="Entity">Section&nbsp;7</a>.</dd></dl><p id="rfc.section.1.3.p.8"> <span id="rfc.iref.r.4"></span>  <dfn>representation</dfn>  </p><dl class="empty"><dd>An entity included with a response that is subject to content negotiation, as described in <a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>. There may exist multiple representations associated with a particular response status.</dd></dl><p id="rfc.section.1.3.p.9"> <span id="rfc.iref.c.2"></span>  <dfn>content negotiation</dfn>  </p><dl class="empty"><dd>The mechanism for selecting the appropriate representation when servicing a request, as described in <a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>. The representation of entities in any response can be negotiated (including error responses).</dd></dl><p id="rfc.section.1.3.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>variant</dfn>  </p><dl class="empty"><dd>A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations is termed a `varriant'. Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation.</dd></dl><p id="rfc.section.1.3.p.11"> <span id="rfc.iref.c.3"></span>  <dfn>client</dfn>  </p><dl class="empty"><dd>A program that establishes connections for the purpose of sending requests.</dd></dl><p id="rfc.section.1.3.p.12"> <span id="rfc.iref.u.1"></span>  <dfn>user agent</dfn>  </p><dl class="empty"><dd>The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user tools.</dd></dl><p id="rfc.section.1.3.p.13"> <span id="rfc.iref.s.1"></span>  <dfn>server</dfn>  </p><dl class="empty"><dd>An application program that accepts connections in order to service requests by sending back responses. Any given program may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the program's capabilities in general. Likewise, any server may act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.</dd></dl><p id="rfc.section.1.3.p.14"> <span id="rfc.iref.o.1"></span>  <dfn>origin server</dfn>  </p><dl class="empty"><dd>The server on which a given resource resides or is to be created.</dd></dl><p id="rfc.section.1.3.p.15"> <span id="rfc.iref.p.1"></span>  <dfn>proxy</dfn>  </p><dl class="empty"><dd>An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy <em class="bcp14">MUST</em> implement both the client and server requirements of this specification. A "transparent proxy" is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. A "non-transparent proxy" is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies.</dd></dl><p id="rfc.section.1.3.p.16"> <span id="rfc.iref.g.1"></span>  <dfn>gateway</dfn>  </p><dl class="empty"><dd>A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway.</dd></dl><p id="rfc.section.1.3.p.17"> <span id="rfc.iref.t.1"></span>  <dfn>tunnel</dfn>  </p><dl class="empty"><dd>An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist when both ends of the relayed connections are closed.</dd></dl><p id="rfc.section.1.3.p.18"> <span id="rfc.iref.c.4"></span>  <dfn>cache</dfn>  </p><dl class="empty"><dd>A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.</dd></dl><p id="rfc.section.1.3.p.19"> <span id="rfc.iref.c.5"></span>  <dfn>cacheable</dfn>  </p><dl class="empty"><dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cacheability of HTTP responses are defined in <a href="#caching" title="Caching in HTTP">Section&nbsp;13</a>. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular request.</dd></dl><p id="rfc.section.1.3.p.20"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn>  </p><dl class="empty"><dd>A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more proxies. A response is also first-hand if its validity has just been checked directly with the origin server.</dd></dl><p id="rfc.section.1.3.p.21"> <span id="rfc.iref.e.2"></span>  <dfn>explicit expiration time</dfn>  </p><dl class="empty"><dd>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</dd></dl><p id="rfc.section.1.3.p.22"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn>  </p><dl class="empty"><dd>An expiration time assigned by a cache when no explicit expiration time is available.</dd></dl><p id="rfc.section.1.3.p.23"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn>  </p><dl class="empty"><dd>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</dd></dl><p id="rfc.section.1.3.p.24"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn>  </p><dl class="empty"><dd>The length of time between the generation of a response and its expiration time.</dd></dl><p id="rfc.section.1.3.p.25"> <span id="rfc.iref.f.3"></span>  <dfn>fresh</dfn>  </p><dl class="empty"><dd>A response is fresh if its age has not yet exceeded its freshness lifetime.</dd></dl><p id="rfc.section.1.3.p.26"> <span id="rfc.iref.s.2"></span>  <dfn>stale</dfn>  </p><dl class="empty"><dd>A response is stale if its age has passed its freshness lifetime.</dd></dl><p id="rfc.section.1.3.p.27"> <span id="rfc.iref.s.3"></span>  <dfn>semantically transparent</dfn>  </p><dl class="empty"><dd>A cache behaves in a "semantically transparent" manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except for hop-by-hop headers) that it would have received had its request been handled directly by the origin server.</dd></dl><p id="rfc.section.1.3.p.28"> <span id="rfc.iref.v.2"></span>  <dfn>validator</dfn>  </p><dl class="empty"><dd>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent copy of an entity.</dd></dl><p id="rfc.section.1.3.p.29"> <span id="rfc.iref.u.2"></span>  <span id="rfc.iref.d.1"></span>  <dfn>upstream</dfn>/<dfn>downstream</dfn>  </p><dl class="empty"><dd>Upstream and downstream describe the flow of a message: all messages flow from upstream to downstream.</dd></dl><p id="rfc.section.1.3.p.30"> <span id="rfc.iref.i.1"></span>  <span id="rfc.iref.o.2"></span>  <dfn>inbound</dfn>/<dfn>outbound</dfn>  </p><dl class="empty"><dd>Inbound and outbound refer to the request and response paths for messages: "inbound" means "traveling toward the origin server", and "outbound" means "traveling toward the user agent"</dd></dl><h2 id="rfc.section.1.4"><a href="#rfc.section.1.4">1.4</a>&nbsp;<a id="intro.overall.operation" href="#intro.overall.operation">Overall Operation</a></h2><p id="rfc.section.1.4.p.1">The HTTP protocol is a request/response protocol. A client sends a request to the server in the form of a request method, URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a connection with a server. The server responds with a status line, including the message's protocol version and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible entity-body content. The relationship between HTTP and MIME is described in <a href="#differences.between.http.entities.and.rfc.2045.entities" title="Differences Between HTTP Entities and RFC 2045 Entities">Appendix&nbsp;19.4</a>.</p><p id="rfc.section.1.4.p.2">Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin server (O).</p><div id="rfc.figure.u.1"></div><pre class="drawing">       request chain ------------------------&gt;
     317</style><link rel="Contents" href="#rfc.toc">
     318      <link rel="Author" href="#rfc.authors">
     319      <link rel="Copyright" href="#rfc.copyright">
     320      <link rel="Index" href="#rfc.index">
     321      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
     322      <link rel="Chapter" title="2 Notational Conventions and Generic Grammar" href="#rfc.section.2">
     323      <link rel="Chapter" title="3 Protocol Parameters" href="#rfc.section.3">
     324      <link rel="Chapter" title="4 HTTP Message" href="#rfc.section.4">
     325      <link rel="Chapter" title="5 Request" href="#rfc.section.5">
     326      <link rel="Chapter" title="6 Response" href="#rfc.section.6">
     327      <link rel="Chapter" title="7 Entity" href="#rfc.section.7">
     328      <link rel="Chapter" title="8 Connections" href="#rfc.section.8">
     329      <link rel="Chapter" title="9 Method Definitions" href="#rfc.section.9">
     330      <link rel="Chapter" title="10 Status Code Definitions" href="#rfc.section.10">
     331      <link rel="Chapter" title="11 Access Authentication" href="#rfc.section.11">
     332      <link rel="Chapter" title="12 Content Negotiation" href="#rfc.section.12">
     333      <link rel="Chapter" title="13 Caching in HTTP" href="#rfc.section.13">
     334      <link rel="Chapter" title="14 Header Field Definitions" href="#rfc.section.14">
     335      <link rel="Chapter" title="15 Security Considerations" href="#rfc.section.15">
     336      <link rel="Chapter" title="16 Acknowledgments" href="#rfc.section.16">
     337      <link rel="Chapter" href="#rfc.section.17" title="17 References">
     338      <link rel="Appendix" title="19 Appendices" href="#rfc.section.19">
     339      <link rel="Appendix" title="20 Index" href="#rfc.section.20">
     340      <link rel="Alternate" title="Authorative ASCII Version" href="http://www.ietf.org/rfc/rfc2616.txt">
     341      <link rel="Help" title="Additional Information on tools.ietf.org" href="http://tools.ietf.org/html/rfc2616">
     342      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.438, 2009-05-27 13:34:05, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
     343      <link rel="schema.DC" href="http://purl.org/dc/elements/1.1/">
     344      <meta name="DC.Creator" content="Fielding, R.">
     345      <meta name="DC.Creator" content="Gettys, J.">
     346      <meta name="DC.Creator" content="Mogul, J.">
     347      <meta name="DC.Creator" content="Frystyk, H.">
     348      <meta name="DC.Creator" content="Masinter, L.">
     349      <meta name="DC.Creator" content="Leach, P.">
     350      <meta name="DC.Creator" content="Berners-Lee, T.">
     351      <meta name="DC.Identifier" content="urn:ietf:rfc:2616">
     352      <meta name="DC.Date.Issued" scheme="ISO8601" content="1999-06">
     353      <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2068">
     354      <meta name="DC.Description.Abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name servers and distributed object management systems, through extension of its request methods, error codes and headers . A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred. HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as &#34;HTTP/1.1&#34;, and is an update to RFC 2068 .">
     355      <meta name="DC.isPartOf" content="urn:ISSN:2070-1721">
     356   </head>
     357   <body>
     358      <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
     359         <tr>
     360            <td class="header left">Network Working Group</td>
     361            <td class="header right">R. Fielding</td>
     362         </tr>
     363         <tr>
     364            <td class="header left">Request for Comments: 2616</td>
     365            <td class="header right">UC Irvine</td>
     366         </tr>
     367         <tr>
     368            <td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2068">2068</a></td>
     369            <td class="header right">J. Gettys</td>
     370         </tr>
     371         <tr>
     372            <td class="header left">Category: Standards Track</td>
     373            <td class="header right">Compaq/W3C</td>
     374         </tr>
     375         <tr>
     376            <td class="header left"></td>
     377            <td class="header right">J. Mogul</td>
     378         </tr>
     379         <tr>
     380            <td class="header left"></td>
     381            <td class="header right">Compaq</td>
     382         </tr>
     383         <tr>
     384            <td class="header left"></td>
     385            <td class="header right">H. Frystyk</td>
     386         </tr>
     387         <tr>
     388            <td class="header left"></td>
     389            <td class="header right">W3C/MIT</td>
     390         </tr>
     391         <tr>
     392            <td class="header left"></td>
     393            <td class="header right">L. Masinter</td>
     394         </tr>
     395         <tr>
     396            <td class="header left"></td>
     397            <td class="header right">Xerox</td>
     398         </tr>
     399         <tr>
     400            <td class="header left"></td>
     401            <td class="header right">P. Leach</td>
     402         </tr>
     403         <tr>
     404            <td class="header left"></td>
     405            <td class="header right">Microsoft</td>
     406         </tr>
     407         <tr>
     408            <td class="header left"></td>
     409            <td class="header right">T. Berners-Lee</td>
     410         </tr>
     411         <tr>
     412            <td class="header left"></td>
     413            <td class="header right">W3C/MIT</td>
     414         </tr>
     415         <tr>
     416            <td class="header left"></td>
     417            <td class="header right">June 1999</td>
     418         </tr>
     419      </table>
     420      <p class="title">Hypertext Transfer Protocol -- HTTP/1.1</p>
     421      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
     422      <p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions
     423         for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization
     424         state and status of this protocol. Distribution of this memo is unlimited.
     425      </p>
     426      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
     427      <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
     428      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
     429      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
     430         systems. It is a generic, stateless, protocol which can be used for many tasks beyond its use for hypertext, such as name
     431         servers and distributed object management systems, through extension of its request methods, error codes and headers <a href="#RFC2324" id="rfc.xref.RFC2324.1"><cite title="Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)">[47]</cite></a>. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the
     432         data being transferred.
     433      </p> 
     434      <p>HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol
     435         referred to as "HTTP/1.1", and is an update to RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[33]</cite></a>.
     436      </p>
     437      <hr class="noprint">
     438      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
     439      <ul class="toc">
     440         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul class="toc">
     441               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
     442               <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
     443               <li class="tocline1">1.3&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
     444               <li class="tocline1">1.4&nbsp;&nbsp;&nbsp;<a href="#intro.overall.operation">Overall Operation</a></li>
     445            </ul>
     446         </li>
     447         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#notation">Notational Conventions and Generic Grammar</a><ul class="toc">
     448               <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#notation.abnf">Augmented BNF</a></li>
     449               <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#basic.rules">Basic Rules</a></li>
     450            </ul>
     451         </li>
     452         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#protocol.parameters">Protocol Parameters</a><ul class="toc">
     453               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#http.version">HTTP Version</a></li>
     454               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#uri">Uniform Resource Identifiers</a><ul class="toc">
     455                     <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#general.syntax">General Syntax</a></li>
     456                     <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#http.url">http URL</a></li>
     457                     <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#uri.comparison">URI Comparison</a></li>
     458                  </ul>
     459               </li>
     460               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#date.time.formats">Date/Time Formats</a><ul class="toc">
     461                     <li class="tocline1">3.3.1&nbsp;&nbsp;&nbsp;<a href="#full.date">Full Date</a></li>
     462                     <li class="tocline1">3.3.2&nbsp;&nbsp;&nbsp;<a href="#delta.seconds">Delta Seconds</a></li>
     463                  </ul>
     464               </li>
     465               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#character.sets">Character Sets</a><ul class="toc">
     466                     <li class="tocline1">3.4.1&nbsp;&nbsp;&nbsp;<a href="#missing.charset">Missing Charset</a></li>
     467                  </ul>
     468               </li>
     469               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#content.codings">Content Codings</a></li>
     470               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#transfer.codings">Transfer Codings</a><ul class="toc">
     471                     <li class="tocline1">3.6.1&nbsp;&nbsp;&nbsp;<a href="#chunked.transfer.encoding">Chunked Transfer Coding</a></li>
     472                  </ul>
     473               </li>
     474               <li class="tocline1">3.7&nbsp;&nbsp;&nbsp;<a href="#media.types">Media Types</a><ul class="toc">
     475                     <li class="tocline1">3.7.1&nbsp;&nbsp;&nbsp;<a href="#canonicalization.and.text.defaults">Canonicalization and Text Defaults</a></li>
     476                     <li class="tocline1">3.7.2&nbsp;&nbsp;&nbsp;<a href="#multipart.types">Multipart Types</a></li>
     477                  </ul>
     478               </li>
     479               <li class="tocline1">3.8&nbsp;&nbsp;&nbsp;<a href="#product.tokens">Product Tokens</a></li>
     480               <li class="tocline1">3.9&nbsp;&nbsp;&nbsp;<a href="#quality.values">Quality Values</a></li>
     481               <li class="tocline1">3.10&nbsp;&nbsp;&nbsp;<a href="#language.tags">Language Tags</a></li>
     482               <li class="tocline1">3.11&nbsp;&nbsp;&nbsp;<a href="#entity.tags">Entity Tags</a></li>
     483               <li class="tocline1">3.12&nbsp;&nbsp;&nbsp;<a href="#range.units">Range Units</a></li>
     484            </ul>
     485         </li>
     486         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#http.message">HTTP Message</a><ul class="toc">
     487               <li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#message.types">Message Types</a></li>
     488               <li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#message.headers">Message Headers</a></li>
     489               <li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#message.body">Message Body</a></li>
     490               <li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#message.length">Message Length</a></li>
     491               <li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#general.header.fields">General Header Fields</a></li>
     492            </ul>
     493         </li>
     494         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#request">Request</a><ul class="toc">
     495               <li class="tocline1">5.1&nbsp;&nbsp;&nbsp;<a href="#request-line">Request-Line</a><ul class="toc">
     496                     <li class="tocline1">5.1.1&nbsp;&nbsp;&nbsp;<a href="#method">Method</a></li>
     497                     <li class="tocline1">5.1.2&nbsp;&nbsp;&nbsp;<a href="#request-uri">Request-URI</a></li>
     498                  </ul>
     499               </li>
     500               <li class="tocline1">5.2&nbsp;&nbsp;&nbsp;<a href="#the.resource.identified.by.a.request">The Resource Identified by a Request</a></li>
     501               <li class="tocline1">5.3&nbsp;&nbsp;&nbsp;<a href="#request.header.fields">Request Header Fields</a></li>
     502            </ul>
     503         </li>
     504         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#response">Response</a><ul class="toc">
     505               <li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#status-line">Status-Line</a><ul class="toc">
     506                     <li class="tocline1">6.1.1&nbsp;&nbsp;&nbsp;<a href="#status.code.and.reason.phrase">Status Code and Reason Phrase</a></li>
     507                  </ul>
     508               </li>
     509               <li class="tocline1">6.2&nbsp;&nbsp;&nbsp;<a href="#response.header.fields">Response Header Fields</a></li>
     510            </ul>
     511         </li>
     512         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#entity">Entity</a><ul class="toc">
     513               <li class="tocline1">7.1&nbsp;&nbsp;&nbsp;<a href="#entity.header.fields">Entity Header Fields</a></li>
     514               <li class="tocline1">7.2&nbsp;&nbsp;&nbsp;<a href="#entity.body">Entity Body</a><ul class="toc">
     515                     <li class="tocline1">7.2.1&nbsp;&nbsp;&nbsp;<a href="#type">Type</a></li>
     516                     <li class="tocline1">7.2.2&nbsp;&nbsp;&nbsp;<a href="#entity.length">Entity Length</a></li>
     517                  </ul>
     518               </li>
     519            </ul>
     520         </li>
     521         <li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#connections">Connections</a><ul class="toc">
     522               <li class="tocline1">8.1&nbsp;&nbsp;&nbsp;<a href="#persistent.connections">Persistent Connections</a><ul class="toc">
     523                     <li class="tocline1">8.1.1&nbsp;&nbsp;&nbsp;<a href="#persistent.purpose">Purpose</a></li>
     524                     <li class="tocline1">8.1.2&nbsp;&nbsp;&nbsp;<a href="#persistent.overall">Overall Operation</a><ul class="toc">
     525                           <li class="tocline1">8.1.2.1&nbsp;&nbsp;&nbsp;<a href="#persistent.negotiation">Negotiation</a></li>
     526                           <li class="tocline1">8.1.2.2&nbsp;&nbsp;&nbsp;<a href="#pipelining">Pipelining</a></li>
     527                        </ul>
     528                     </li>
     529                     <li class="tocline1">8.1.3&nbsp;&nbsp;&nbsp;<a href="#persistent.proxy">Proxy Servers</a></li>
     530                     <li class="tocline1">8.1.4&nbsp;&nbsp;&nbsp;<a href="#persistent.practical">Practical Considerations</a></li>
     531                  </ul>
     532               </li>
     533               <li class="tocline1">8.2&nbsp;&nbsp;&nbsp;<a href="#message.transmission.requirements">Message Transmission Requirements</a><ul class="toc">
     534                     <li class="tocline1">8.2.1&nbsp;&nbsp;&nbsp;<a href="#persistent.flow">Persistent Connections and Flow Control</a></li>
     535                     <li class="tocline1">8.2.2&nbsp;&nbsp;&nbsp;<a href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></li>
     536                     <li class="tocline1">8.2.3&nbsp;&nbsp;&nbsp;<a href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></li>
     537                     <li class="tocline1">8.2.4&nbsp;&nbsp;&nbsp;<a href="#connection.premature">Client Behavior if Server Prematurely Closes Connection</a></li>
     538                  </ul>
     539               </li>
     540            </ul>
     541         </li>
     542         <li class="tocline0">9.&nbsp;&nbsp;&nbsp;<a href="#method.definitions">Method Definitions</a><ul class="toc">
     543               <li class="tocline1">9.1&nbsp;&nbsp;&nbsp;<a href="#safe.and.idempotent">Safe and Idempotent Methods</a><ul class="toc">
     544                     <li class="tocline1">9.1.1&nbsp;&nbsp;&nbsp;<a href="#safe.methods">Safe Methods</a></li>
     545                     <li class="tocline1">9.1.2&nbsp;&nbsp;&nbsp;<a href="#idempotent.methods">Idempotent Methods</a></li>
     546                  </ul>
     547               </li>
     548               <li class="tocline1">9.2&nbsp;&nbsp;&nbsp;<a href="#OPTIONS">OPTIONS</a></li>
     549               <li class="tocline1">9.3&nbsp;&nbsp;&nbsp;<a href="#GET">GET</a></li>
     550               <li class="tocline1">9.4&nbsp;&nbsp;&nbsp;<a href="#HEAD">HEAD</a></li>
     551               <li class="tocline1">9.5&nbsp;&nbsp;&nbsp;<a href="#POST">POST</a></li>
     552               <li class="tocline1">9.6&nbsp;&nbsp;&nbsp;<a href="#PUT">PUT</a></li>
     553               <li class="tocline1">9.7&nbsp;&nbsp;&nbsp;<a href="#DELETE">DELETE</a></li>
     554               <li class="tocline1">9.8&nbsp;&nbsp;&nbsp;<a href="#TRACE">TRACE</a></li>
     555               <li class="tocline1">9.9&nbsp;&nbsp;&nbsp;<a href="#CONNECT">CONNECT</a></li>
     556            </ul>
     557         </li>
     558         <li class="tocline0">10.&nbsp;&nbsp;&nbsp;<a href="#status.codes">Status Code Definitions</a><ul class="toc">
     559               <li class="tocline1">10.1&nbsp;&nbsp;&nbsp;<a href="#status.1xx">Informational 1xx</a><ul class="toc">
     560                     <li class="tocline1">10.1.1&nbsp;&nbsp;&nbsp;<a href="#status.100">100 Continue</a></li>
     561                     <li class="tocline1">10.1.2&nbsp;&nbsp;&nbsp;<a href="#status.101">101 Switching Protocols</a></li>
     562                  </ul>
     563               </li>
     564               <li class="tocline1">10.2&nbsp;&nbsp;&nbsp;<a href="#status.2xx">Successful 2xx</a><ul class="toc">
     565                     <li class="tocline1">10.2.1&nbsp;&nbsp;&nbsp;<a href="#status.200">200 OK</a></li>
     566                     <li class="tocline1">10.2.2&nbsp;&nbsp;&nbsp;<a href="#status.201">201 Created</a></li>
     567                     <li class="tocline1">10.2.3&nbsp;&nbsp;&nbsp;<a href="#status.202">202 Accepted</a></li>
     568                     <li class="tocline1">10.2.4&nbsp;&nbsp;&nbsp;<a href="#status.203">203 Non-Authoritative Information</a></li>
     569                     <li class="tocline1">10.2.5&nbsp;&nbsp;&nbsp;<a href="#status.204">204 No Content</a></li>
     570                     <li class="tocline1">10.2.6&nbsp;&nbsp;&nbsp;<a href="#status.205">205 Reset Content</a></li>
     571                     <li class="tocline1">10.2.7&nbsp;&nbsp;&nbsp;<a href="#status.206">206 Partial Content</a></li>
     572                  </ul>
     573               </li>
     574               <li class="tocline1">10.3&nbsp;&nbsp;&nbsp;<a href="#status.3xx">Redirection 3xx</a><ul class="toc">
     575                     <li class="tocline1">10.3.1&nbsp;&nbsp;&nbsp;<a href="#status.300">300 Multiple Choices</a></li>
     576                     <li class="tocline1">10.3.2&nbsp;&nbsp;&nbsp;<a href="#status.301">301 Moved Permanently</a></li>
     577                     <li class="tocline1">10.3.3&nbsp;&nbsp;&nbsp;<a href="#status.302">302 Found</a></li>
     578                     <li class="tocline1">10.3.4&nbsp;&nbsp;&nbsp;<a href="#status.303">303 See Other</a></li>
     579                     <li class="tocline1">10.3.5&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
     580                     <li class="tocline1">10.3.6&nbsp;&nbsp;&nbsp;<a href="#status.305">305 Use Proxy</a></li>
     581                     <li class="tocline1">10.3.7&nbsp;&nbsp;&nbsp;<a href="#status.306">306 (Unused)</a></li>
     582                     <li class="tocline1">10.3.8&nbsp;&nbsp;&nbsp;<a href="#status.307">307 Temporary Redirect</a></li>
     583                  </ul>
     584               </li>
     585               <li class="tocline1">10.4&nbsp;&nbsp;&nbsp;<a href="#status.4xx">Client Error 4xx</a><ul class="toc">
     586                     <li class="tocline1">10.4.1&nbsp;&nbsp;&nbsp;<a href="#status.400">400 Bad Request</a></li>
     587                     <li class="tocline1">10.4.2&nbsp;&nbsp;&nbsp;<a href="#status.401">401 Unauthorized</a></li>
     588                     <li class="tocline1">10.4.3&nbsp;&nbsp;&nbsp;<a href="#status.402">402 Payment Required</a></li>
     589                     <li class="tocline1">10.4.4&nbsp;&nbsp;&nbsp;<a href="#status.403">403 Forbidden</a></li>
     590                     <li class="tocline1">10.4.5&nbsp;&nbsp;&nbsp;<a href="#status.404">404 Not Found</a></li>
     591                     <li class="tocline1">10.4.6&nbsp;&nbsp;&nbsp;<a href="#status.405">405 Method Not Allowed</a></li>
     592                     <li class="tocline1">10.4.7&nbsp;&nbsp;&nbsp;<a href="#status.406">406 Not Acceptable</a></li>
     593                     <li class="tocline1">10.4.8&nbsp;&nbsp;&nbsp;<a href="#status.407">407 Proxy Authentication Required</a></li>
     594                     <li class="tocline1">10.4.9&nbsp;&nbsp;&nbsp;<a href="#status.408">408 Request Timeout</a></li>
     595                     <li class="tocline1">10.4.10&nbsp;&nbsp;&nbsp;<a href="#status.409">409 Conflict</a></li>
     596                     <li class="tocline1">10.4.11&nbsp;&nbsp;&nbsp;<a href="#status.410">410 Gone</a></li>
     597                     <li class="tocline1">10.4.12&nbsp;&nbsp;&nbsp;<a href="#status.411">411 Length Required</a></li>
     598                     <li class="tocline1">10.4.13&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
     599                     <li class="tocline1">10.4.14&nbsp;&nbsp;&nbsp;<a href="#status.413">413 Request Entity Too Large</a></li>
     600                     <li class="tocline1">10.4.15&nbsp;&nbsp;&nbsp;<a href="#status.414">414 Request-URI Too Long</a></li>
     601                     <li class="tocline1">10.4.16&nbsp;&nbsp;&nbsp;<a href="#status.415">415 Unsupported Media Type</a></li>
     602                     <li class="tocline1">10.4.17&nbsp;&nbsp;&nbsp;<a href="#status.416">416 Requested Range Not Satisfiable</a></li>
     603                     <li class="tocline1">10.4.18&nbsp;&nbsp;&nbsp;<a href="#status.417">417 Expectation Failed</a></li>
     604                  </ul>
     605               </li>
     606               <li class="tocline1">10.5&nbsp;&nbsp;&nbsp;<a href="#status.5xx">Server Error 5xx</a><ul class="toc">
     607                     <li class="tocline1">10.5.1&nbsp;&nbsp;&nbsp;<a href="#status.500">500 Internal Server Error</a></li>
     608                     <li class="tocline1">10.5.2&nbsp;&nbsp;&nbsp;<a href="#status.501">501 Not Implemented</a></li>
     609                     <li class="tocline1">10.5.3&nbsp;&nbsp;&nbsp;<a href="#status.502">502 Bad Gateway</a></li>
     610                     <li class="tocline1">10.5.4&nbsp;&nbsp;&nbsp;<a href="#status.503">503 Service Unavailable</a></li>
     611                     <li class="tocline1">10.5.5&nbsp;&nbsp;&nbsp;<a href="#status.504">504 Gateway Timeout</a></li>
     612                     <li class="tocline1">10.5.6&nbsp;&nbsp;&nbsp;<a href="#status.505">505 HTTP Version Not Supported</a></li>
     613                  </ul>
     614               </li>
     615            </ul>
     616         </li>
     617         <li class="tocline0">11.&nbsp;&nbsp;&nbsp;<a href="#access.authentication">Access Authentication</a></li>
     618         <li class="tocline0">12.&nbsp;&nbsp;&nbsp;<a href="#content.negotiation">Content Negotiation</a><ul class="toc">
     619               <li class="tocline1">12.1&nbsp;&nbsp;&nbsp;<a href="#server-driven.negotiation">Server-driven Negotiation</a></li>
     620               <li class="tocline1">12.2&nbsp;&nbsp;&nbsp;<a href="#agent-driven.negotiation">Agent-driven Negotiation</a></li>
     621               <li class="tocline1">12.3&nbsp;&nbsp;&nbsp;<a href="#transparent.negotiation">Transparent Negotiation</a></li>
     622            </ul>
     623         </li>
     624         <li class="tocline0">13.&nbsp;&nbsp;&nbsp;<a href="#caching">Caching in HTTP</a><ul class="toc">
     625               <li class="tocline1">13.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.13.1"></a><ul class="toc">
     626                     <li class="tocline1">13.1.1&nbsp;&nbsp;&nbsp;<a href="#cache.correctness">Cache Correctness</a></li>
     627                     <li class="tocline1">13.1.2&nbsp;&nbsp;&nbsp;<a href="#warnings">Warnings</a></li>
     628                     <li class="tocline1">13.1.3&nbsp;&nbsp;&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></li>
     629                     <li class="tocline1">13.1.4&nbsp;&nbsp;&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></li>
     630                     <li class="tocline1">13.1.5&nbsp;&nbsp;&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></li>
     631                     <li class="tocline1">13.1.6&nbsp;&nbsp;&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></li>
     632                  </ul>
     633               </li>
     634               <li class="tocline1">13.2&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Expiration Model</a><ul class="toc">
     635                     <li class="tocline1">13.2.1&nbsp;&nbsp;&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></li>
     636                     <li class="tocline1">13.2.2&nbsp;&nbsp;&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></li>
     637                     <li class="tocline1">13.2.3&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Age Calculations</a></li>
     638                     <li class="tocline1">13.2.4&nbsp;&nbsp;&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></li>
     639                     <li class="tocline1">13.2.5&nbsp;&nbsp;&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></li>
     640                     <li class="tocline1">13.2.6&nbsp;&nbsp;&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></li>
     641                  </ul>
     642               </li>
     643               <li class="tocline1">13.3&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul class="toc">
     644                     <li class="tocline1">13.3.1&nbsp;&nbsp;&nbsp;<a href="#last-modified.dates">Last-Modified Dates</a></li>
     645                     <li class="tocline1">13.3.2&nbsp;&nbsp;&nbsp;<a href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></li>
     646                     <li class="tocline1">13.3.3&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak and Strong Validators</a></li>
     647                     <li class="tocline1">13.3.4&nbsp;&nbsp;&nbsp;<a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity Tags and Last-Modified Dates</a></li>
     648                     <li class="tocline1">13.3.5&nbsp;&nbsp;&nbsp;<a href="#non-validating.conditionals">Non-validating Conditionals</a></li>
     649                  </ul>
     650               </li>
     651               <li class="tocline1">13.4&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li>
     652               <li class="tocline1">13.5&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a><ul class="toc">
     653                     <li class="tocline1">13.5.1&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></li>
     654                     <li class="tocline1">13.5.2&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></li>
     655                     <li class="tocline1">13.5.3&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Headers</a></li>
     656                     <li class="tocline1">13.5.4&nbsp;&nbsp;&nbsp;<a href="#combining.byte.ranges">Combining Byte Ranges</a></li>
     657                  </ul>
     658               </li>
     659               <li class="tocline1">13.6&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
     660               <li class="tocline1">13.7&nbsp;&nbsp;&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></li>
     661               <li class="tocline1">13.8&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></li>
     662               <li class="tocline1">13.9&nbsp;&nbsp;&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></li>
     663               <li class="tocline1">13.10&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></li>
     664               <li class="tocline1">13.11&nbsp;&nbsp;&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></li>
     665               <li class="tocline1">13.12&nbsp;&nbsp;&nbsp;<a href="#cache.replacement">Cache Replacement</a></li>
     666               <li class="tocline1">13.13&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
     667            </ul>
     668         </li>
     669         <li class="tocline0">14.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
     670               <li class="tocline1">14.1&nbsp;&nbsp;&nbsp;<a href="#header.accept">Accept</a></li>
     671               <li class="tocline1">14.2&nbsp;&nbsp;&nbsp;<a href="#header.accept-charset">Accept-Charset</a></li>
     672               <li class="tocline1">14.3&nbsp;&nbsp;&nbsp;<a href="#header.accept-encoding">Accept-Encoding</a></li>
     673               <li class="tocline1">14.4&nbsp;&nbsp;&nbsp;<a href="#header.accept-language">Accept-Language</a></li>
     674               <li class="tocline1">14.5&nbsp;&nbsp;&nbsp;<a href="#header.accept-ranges">Accept-Ranges</a></li>
     675               <li class="tocline1">14.6&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
     676               <li class="tocline1">14.7&nbsp;&nbsp;&nbsp;<a href="#header.allow">Allow</a></li>
     677               <li class="tocline1">14.8&nbsp;&nbsp;&nbsp;<a href="#header.authorization">Authorization</a></li>
     678               <li class="tocline1">14.9&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
     679                     <li class="tocline1">14.9.1&nbsp;&nbsp;&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></li>
     680                     <li class="tocline1">14.9.2&nbsp;&nbsp;&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></li>
     681                     <li class="tocline1">14.9.3&nbsp;&nbsp;&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></li>
     682                     <li class="tocline1">14.9.4&nbsp;&nbsp;&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></li>
     683                     <li class="tocline1">14.9.5&nbsp;&nbsp;&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></li>
     684                     <li class="tocline1">14.9.6&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
     685                  </ul>
     686               </li>
     687               <li class="tocline1">14.10&nbsp;&nbsp;&nbsp;<a href="#header.connection">Connection</a></li>
     688               <li class="tocline1">14.11&nbsp;&nbsp;&nbsp;<a href="#header.content-encoding">Content-Encoding</a></li>
     689               <li class="tocline1">14.12&nbsp;&nbsp;&nbsp;<a href="#header.content-language">Content-Language</a></li>
     690               <li class="tocline1">14.13&nbsp;&nbsp;&nbsp;<a href="#header.content-length">Content-Length</a></li>
     691               <li class="tocline1">14.14&nbsp;&nbsp;&nbsp;<a href="#header.content-location">Content-Location</a></li>
     692               <li class="tocline1">14.15&nbsp;&nbsp;&nbsp;<a href="#header.content-md5">Content-MD5</a></li>
     693               <li class="tocline1">14.16&nbsp;&nbsp;&nbsp;<a href="#header.content-range">Content-Range</a></li>
     694               <li class="tocline1">14.17&nbsp;&nbsp;&nbsp;<a href="#header.content-type">Content-Type</a></li>
     695               <li class="tocline1">14.18&nbsp;&nbsp;&nbsp;<a href="#header.date">Date</a><ul class="toc">
     696                     <li class="tocline1">14.18.1&nbsp;&nbsp;&nbsp;<a href="#clockless.origin.server.operation">Clockless Origin Server Operation</a></li>
     697                  </ul>
     698               </li>
     699               <li class="tocline1">14.19&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a></li>
     700               <li class="tocline1">14.20&nbsp;&nbsp;&nbsp;<a href="#header.expect">Expect</a></li>
     701               <li class="tocline1">14.21&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
     702               <li class="tocline1">14.22&nbsp;&nbsp;&nbsp;<a href="#header.from">From</a></li>
     703               <li class="tocline1">14.23&nbsp;&nbsp;&nbsp;<a href="#header.host">Host</a></li>
     704               <li class="tocline1">14.24&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
     705               <li class="tocline1">14.25&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
     706               <li class="tocline1">14.26&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
     707               <li class="tocline1">14.27&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
     708               <li class="tocline1">14.28&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
     709               <li class="tocline1">14.29&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a></li>
     710               <li class="tocline1">14.30&nbsp;&nbsp;&nbsp;<a href="#header.location">Location</a></li>
     711               <li class="tocline1">14.31&nbsp;&nbsp;&nbsp;<a href="#header.max-forwards">Max-Forwards</a></li>
     712               <li class="tocline1">14.32&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
     713               <li class="tocline1">14.33&nbsp;&nbsp;&nbsp;<a href="#header.proxy-authenticate">Proxy-Authenticate</a></li>
     714               <li class="tocline1">14.34&nbsp;&nbsp;&nbsp;<a href="#header.proxy-authorization">Proxy-Authorization</a></li>
     715               <li class="tocline1">14.35&nbsp;&nbsp;&nbsp;<a href="#header.range">Range</a><ul class="toc">
     716                     <li class="tocline1">14.35.1&nbsp;&nbsp;&nbsp;<a href="#byte.ranges">Byte Ranges</a></li>
     717                     <li class="tocline1">14.35.2&nbsp;&nbsp;&nbsp;<a href="#range.retrieval.requests">Range Retrieval Requests</a></li>
     718                  </ul>
     719               </li>
     720               <li class="tocline1">14.36&nbsp;&nbsp;&nbsp;<a href="#header.referer">Referer</a></li>
     721               <li class="tocline1">14.37&nbsp;&nbsp;&nbsp;<a href="#header.retry-after">Retry-After</a></li>
     722               <li class="tocline1">14.38&nbsp;&nbsp;&nbsp;<a href="#header.server">Server</a></li>
     723               <li class="tocline1">14.39&nbsp;&nbsp;&nbsp;<a href="#header.te">TE</a></li>
     724               <li class="tocline1">14.40&nbsp;&nbsp;&nbsp;<a href="#header.trailer">Trailer</a></li>
     725               <li class="tocline1">14.41&nbsp;&nbsp;&nbsp;<a href="#header.transfer-encoding">Transfer-Encoding</a></li>
     726               <li class="tocline1">14.42&nbsp;&nbsp;&nbsp;<a href="#header.upgrade">Upgrade</a></li>
     727               <li class="tocline1">14.43&nbsp;&nbsp;&nbsp;<a href="#header.user-agent">User-Agent</a></li>
     728               <li class="tocline1">14.44&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
     729               <li class="tocline1">14.45&nbsp;&nbsp;&nbsp;<a href="#header.via">Via</a></li>
     730               <li class="tocline1">14.46&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
     731               <li class="tocline1">14.47&nbsp;&nbsp;&nbsp;<a href="#header.www-authenticate">WWW-Authenticate</a></li>
     732            </ul>
     733         </li>
     734         <li class="tocline0">15.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul class="toc">
     735               <li class="tocline1">15.1&nbsp;&nbsp;&nbsp;<a href="#personal.information">Personal Information</a><ul class="toc">
     736                     <li class="tocline1">15.1.1&nbsp;&nbsp;&nbsp;<a href="#abuse.of.server.log.information">Abuse of Server Log Information</a></li>
     737                     <li class="tocline1">15.1.2&nbsp;&nbsp;&nbsp;<a href="#security.sensitive">Transfer of Sensitive Information</a></li>
     738                     <li class="tocline1">15.1.3&nbsp;&nbsp;&nbsp;<a href="#encoding.sensitive.information.in.uris">Encoding Sensitive Information in URI's</a></li>
     739                     <li class="tocline1">15.1.4&nbsp;&nbsp;&nbsp;<a href="#privacy.issues.connected.to.accept.headers">Privacy Issues Connected to Accept Headers</a></li>
     740                  </ul>
     741               </li>
     742               <li class="tocline1">15.2&nbsp;&nbsp;&nbsp;<a href="#attack.pathname">Attacks Based On File and Path Names</a></li>
     743               <li class="tocline1">15.3&nbsp;&nbsp;&nbsp;<a href="#dns.spoofing">DNS Spoofing</a></li>
     744               <li class="tocline1">15.4&nbsp;&nbsp;&nbsp;<a href="#location.spoofing">Location Headers and Spoofing</a></li>
     745               <li class="tocline1">15.5&nbsp;&nbsp;&nbsp;<a href="#content-disposition.issues">Content-Disposition Issues</a></li>
     746               <li class="tocline1">15.6&nbsp;&nbsp;&nbsp;<a href="#auth.credentials.and.idle.clients">Authentication Credentials and Idle Clients</a></li>
     747               <li class="tocline1">15.7&nbsp;&nbsp;&nbsp;<a href="#attack.proxies">Proxies and Caching</a><ul class="toc">
     748                     <li class="tocline1">15.7.1&nbsp;&nbsp;&nbsp;<a href="#attack.DoS">Denial of Service Attacks on Proxies</a></li>
     749                  </ul>
     750               </li>
     751            </ul>
     752         </li>
     753         <li class="tocline0">16.&nbsp;&nbsp;&nbsp;<a href="#acknowledgments">Acknowledgments</a></li>
     754         <li class="tocline0">17.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
     755         <li class="tocline0">18.&nbsp;&nbsp;&nbsp;<a href="#rfc.authors">Authors' Addresses</a></li>
     756         <li class="tocline0">19.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.19">Appendices</a><ul class="toc">
     757               <li class="tocline1">19.1&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.http">Internet Media Type message/http and application/http</a></li>
     758               <li class="tocline1">19.2&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.multipart.byteranges">Internet Media Type multipart/byteranges</a></li>
     759               <li class="tocline1">19.3&nbsp;&nbsp;&nbsp;<a href="#tolerant.applications">Tolerant Applications</a></li>
     760               <li class="tocline1">19.4&nbsp;&nbsp;&nbsp;<a href="#differences.between.http.entities.and.rfc.2045.entities">Differences Between HTTP Entities and RFC 2045 Entities</a><ul class="toc">
     761                     <li class="tocline1">19.4.1&nbsp;&nbsp;&nbsp;<a href="#mime-version">MIME-Version</a></li>
     762                     <li class="tocline1">19.4.2&nbsp;&nbsp;&nbsp;<a href="#conversion.to.canonical.form">Conversion to Canonical Form</a></li>
     763                     <li class="tocline1">19.4.3&nbsp;&nbsp;&nbsp;<a href="#conversion.of.date.formats">Conversion of Date Formats</a></li>
     764                     <li class="tocline1">19.4.4&nbsp;&nbsp;&nbsp;<a href="#introduction.of.content-encoding">Introduction of Content-Encoding</a></li>
     765                     <li class="tocline1">19.4.5&nbsp;&nbsp;&nbsp;<a href="#no.content-transfer-encoding">No Content-Transfer-Encoding</a></li>
     766                     <li class="tocline1">19.4.6&nbsp;&nbsp;&nbsp;<a href="#introduction.of.transfer-encoding">Introduction of Transfer-Encoding</a></li>
     767                     <li class="tocline1">19.4.7&nbsp;&nbsp;&nbsp;<a href="#mhtml.line.length">MHTML and Line Length Limitations</a></li>
     768                  </ul>
     769               </li>
     770               <li class="tocline1">19.5&nbsp;&nbsp;&nbsp;<a href="#additional.features">Additional Features</a><ul class="toc">
     771                     <li class="tocline1">19.5.1&nbsp;&nbsp;&nbsp;<a href="#content-disposition">Content-Disposition</a></li>
     772                  </ul>
     773               </li>
     774               <li class="tocline1">19.6&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul class="toc">
     775                     <li class="tocline1">19.6.1&nbsp;&nbsp;&nbsp;<a href="#changes.from.1.0">Changes from HTTP/1.0</a><ul class="toc">
     776                           <li class="tocline1">19.6.1.1&nbsp;&nbsp;&nbsp;<a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses</a></li>
     777                        </ul>
     778                     </li>
     779                     <li class="tocline1">19.6.2&nbsp;&nbsp;&nbsp;<a href="#compatibility.with.http.1.0.persistent.connections">Compatibility with HTTP/1.0 Persistent Connections</a></li>
     780                     <li class="tocline1">19.6.3&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></li>
     781                  </ul>
     782               </li>
     783            </ul>
     784         </li>
     785         <li class="tocline0">20.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.20">Index</a></li>
     786         <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
     787         <li class="tocline0"><a href="#rfc.index">Index</a></li>
     788      </ul>
     789      <hr class="noprint">
     790      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
     791      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.purpose" href="#intro.purpose">Purpose</a></h2>
     792      <p id="rfc.section.1.1.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
     793         systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred
     794         to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by RFC 1945 <a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[6]</cite></a>, improved the protocol by allowing messages to be in the format of MIME-like messages, containing metainformation about the
     795         data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration
     796         the effects of hierarchical proxies, caching, the need for persistent connections, or virtual hosts. In addition, the proliferation
     797         of incompletely-implemented applications calling themselves "HTTP/1.0" has necessitated a protocol version change in order
     798         for two communicating applications to determine each other's true capabilities.
     799      </p>
     800      <p id="rfc.section.1.1.p.2">This specification defines the protocol referred to as "HTTP/1.1". This protocol includes more stringent requirements than
     801         HTTP/1.0 in order to ensure reliable implementation of its features.
     802      </p>
     803      <p id="rfc.section.1.1.p.3">Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation.
     804         HTTP allows an open-ended set of methods and headers that indicate the purpose of a request <a href="#RFC2324" id="rfc.xref.RFC2324.2"><cite title="Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)">[47]</cite></a>. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) <a href="#RFC1630" id="rfc.xref.RFC1630.1"><cite title="Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web">[3]</cite></a>, as a location (URL) <a href="#RFC1738" id="rfc.xref.RFC1738.1"><cite title="Uniform Resource Locators (URL)">[4]</cite></a> or name (URN) <a href="#RFC1737" id="rfc.xref.RFC1737.1"><cite title="Functional Requirements for Uniform Resource Names">[20]</cite></a>, for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by
     805         Internet mail <a href="#RFC822" id="rfc.xref.RFC822.1"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a> as defined by the Multipurpose Internet Mail Extensions (MIME) <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[7]</cite></a>.
     806      </p>
     807      <p id="rfc.section.1.1.p.4">HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems,
     808         including those supported by the SMTP <a href="#RFC821" id="rfc.xref.RFC821.1"><cite title="Simple Mail Transfer Protocol">[16]</cite></a>, NNTP <a href="#RFC977" id="rfc.xref.RFC977.1"><cite title="Network News Transfer Protocol">[13]</cite></a>, FTP <a href="#RFC959" id="rfc.xref.RFC959.1"><cite title="File Transfer Protocol">[18]</cite></a>, Gopher <a href="#RFC1436" id="rfc.xref.RFC1436.1"><cite title="The Internet Gopher Protocol (a distributed document search and retrieval protocol)">[2]</cite></a>, and WAIS <a href="#WAIS" id="rfc.xref.WAIS.1"><cite title="WAIS Interface Protocol Prototype Functional Specification (v1.5)">[10]</cite></a> protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications.
     809      </p>
     810      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
     811      <p id="rfc.section.1.2.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
     812         in this document are to be interpreted as described in RFC 2119 <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[34]</cite></a>.
     813      </p>
     814      <p id="rfc.section.1.2.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
     815      </p>
     816      <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a id="intro.terminology" href="#intro.terminology">Terminology</a></h2>
     817      <p id="rfc.section.1.3.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.</p>
     818      <p id="rfc.section.1.3.p.2"> <span id="rfc.iref.c.1"></span>  <dfn>connection</dfn> 
     819      </p>
     820      <dl class="empty">
     821         <dd>A transport layer virtual circuit established between two programs for the purpose of communication.</dd>
     822      </dl>
     823      <p id="rfc.section.1.3.p.3"> <span id="rfc.iref.m.1"></span>  <dfn>message</dfn> 
     824      </p>
     825      <dl class="empty">
     826         <dd>The basic unit of HTTP communication, consisting of a structured sequence of octets matching the syntax defined in <a href="#http.message" title="HTTP Message">Section&nbsp;4</a> and transmitted via the connection.
     827         </dd>
     828      </dl>
     829      <p id="rfc.section.1.3.p.4"> <span id="rfc.iref.r.1"></span>  <dfn>request</dfn> 
     830      </p>
     831      <dl class="empty">
     832         <dd>An HTTP request message, as defined in <a href="#request" title="Request">Section&nbsp;5</a>.
     833         </dd>
     834      </dl>
     835      <p id="rfc.section.1.3.p.5"> <span id="rfc.iref.r.2"></span>  <dfn>response</dfn> 
     836      </p>
     837      <dl class="empty">
     838         <dd>An HTTP response message, as defined in <a href="#response" title="Response">Section&nbsp;6</a>.
     839         </dd>
     840      </dl>
     841      <p id="rfc.section.1.3.p.6"> <span id="rfc.iref.r.3"></span>  <dfn>resource</dfn> 
     842      </p>
     843      <dl class="empty">
     844         <dd>A network data object or service that can be identified by a URI, as defined in <a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;3.2</a>. Resources may be available in multiple representations (e.g. multiple languages, data formats, size, and resolutions) or
     845            vary in other ways.
     846         </dd>
     847      </dl>
     848      <p id="rfc.section.1.3.p.7"> <span id="rfc.iref.e.1"></span>  <dfn>entity</dfn> 
     849      </p>
     850      <dl class="empty">
     851         <dd>The information transferred as the payload of a request or response. An entity consists of metainformation in the form of
     852            entity-header fields and content in the form of an entity-body, as described in <a href="#entity" title="Entity">Section&nbsp;7</a>.
     853         </dd>
     854      </dl>
     855      <p id="rfc.section.1.3.p.8"> <span id="rfc.iref.r.4"></span>  <dfn>representation</dfn> 
     856      </p>
     857      <dl class="empty">
     858         <dd>An entity included with a response that is subject to content negotiation, as described in <a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>. There may exist multiple representations associated with a particular response status.
     859         </dd>
     860      </dl>
     861      <p id="rfc.section.1.3.p.9"> <span id="rfc.iref.c.2"></span>  <dfn>content negotiation</dfn> 
     862      </p>
     863      <dl class="empty">
     864         <dd>The mechanism for selecting the appropriate representation when servicing a request, as described in <a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>. The representation of entities in any response can be negotiated (including error responses).
     865         </dd>
     866      </dl>
     867      <p id="rfc.section.1.3.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>variant</dfn> 
     868      </p>
     869      <dl class="empty">
     870         <dd>A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations
     871            is termed a `varriant'. Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation.
     872         </dd>
     873      </dl>
     874      <p id="rfc.section.1.3.p.11"> <span id="rfc.iref.c.3"></span>  <dfn>client</dfn> 
     875      </p>
     876      <dl class="empty">
     877         <dd>A program that establishes connections for the purpose of sending requests.</dd>
     878      </dl>
     879      <p id="rfc.section.1.3.p.12"> <span id="rfc.iref.u.1"></span>  <dfn>user agent</dfn> 
     880      </p>
     881      <dl class="empty">
     882         <dd>The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user
     883            tools.
     884         </dd>
     885      </dl>
     886      <p id="rfc.section.1.3.p.13"> <span id="rfc.iref.s.1"></span>  <dfn>server</dfn> 
     887      </p>
     888      <dl class="empty">
     889         <dd>An application program that accepts connections in order to service requests by sending back responses. Any given program
     890            may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the
     891            program for a particular connection, rather than to the program's capabilities in general. Likewise, any server may act as
     892            an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.
     893         </dd>
     894      </dl>
     895      <p id="rfc.section.1.3.p.14"> <span id="rfc.iref.o.1"></span>  <dfn>origin server</dfn> 
     896      </p>
     897      <dl class="empty">
     898         <dd>The server on which a given resource resides or is to be created.</dd>
     899      </dl>
     900      <p id="rfc.section.1.3.p.15"> <span id="rfc.iref.p.1"></span>  <dfn>proxy</dfn> 
     901      </p>
     902      <dl class="empty">
     903         <dd>An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients.
     904            Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy <em class="bcp14">MUST</em> implement both the client and server requirements of this specification. A "transparent proxy" is a proxy that does not modify
     905            the request or response beyond what is required for proxy authentication and identification. A "non-transparent proxy" is
     906            a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation
     907            services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent
     908            behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies.
     909         </dd>
     910      </dl>
     911      <p id="rfc.section.1.3.p.16"> <span id="rfc.iref.g.1"></span>  <dfn>gateway</dfn> 
     912      </p>
     913      <dl class="empty">
     914         <dd>A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the
     915            origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway.
     916         </dd>
     917      </dl>
     918      <p id="rfc.section.1.3.p.17"> <span id="rfc.iref.t.1"></span>  <dfn>tunnel</dfn> 
     919      </p>
     920      <dl class="empty">
     921         <dd>An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered
     922            a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist
     923            when both ends of the relayed connections are closed.
     924         </dd>
     925      </dl>
     926      <p id="rfc.section.1.3.p.18"> <span id="rfc.iref.c.4"></span>  <dfn>cache</dfn> 
     927      </p>
     928      <dl class="empty">
     929         <dd>A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion.
     930            A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
     931            requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
     932         </dd>
     933      </dl>
     934      <p id="rfc.section.1.3.p.19"> <span id="rfc.iref.c.5"></span>  <dfn>cacheable</dfn> 
     935      </p>
     936      <dl class="empty">
     937         <dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
     938            The rules for determining the cacheability of HTTP responses are defined in <a href="#caching" title="Caching in HTTP">Section&nbsp;13</a>. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
     939            request.
     940         </dd>
     941      </dl>
     942      <p id="rfc.section.1.3.p.20"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
     943      </p>
     944      <dl class="empty">
     945         <dd>A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more
     946            proxies. A response is also first-hand if its validity has just been checked directly with the origin server.
     947         </dd>
     948      </dl>
     949      <p id="rfc.section.1.3.p.21"> <span id="rfc.iref.e.2"></span>  <dfn>explicit expiration time</dfn> 
     950      </p>
     951      <dl class="empty">
     952         <dd>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</dd>
     953      </dl>
     954      <p id="rfc.section.1.3.p.22"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
     955      </p>
     956      <dl class="empty">
     957         <dd>An expiration time assigned by a cache when no explicit expiration time is available.</dd>
     958      </dl>
     959      <p id="rfc.section.1.3.p.23"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
     960      </p>
     961      <dl class="empty">
     962         <dd>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</dd>
     963      </dl>
     964      <p id="rfc.section.1.3.p.24"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn> 
     965      </p>
     966      <dl class="empty">
     967         <dd>The length of time between the generation of a response and its expiration time.</dd>
     968      </dl>
     969      <p id="rfc.section.1.3.p.25"> <span id="rfc.iref.f.3"></span>  <dfn>fresh</dfn> 
     970      </p>
     971      <dl class="empty">
     972         <dd>A response is fresh if its age has not yet exceeded its freshness lifetime.</dd>
     973      </dl>
     974      <p id="rfc.section.1.3.p.26"> <span id="rfc.iref.s.2"></span>  <dfn>stale</dfn> 
     975      </p>
     976      <dl class="empty">
     977         <dd>A response is stale if its age has passed its freshness lifetime.</dd>
     978      </dl>
     979      <p id="rfc.section.1.3.p.27"> <span id="rfc.iref.s.3"></span>  <dfn>semantically transparent</dfn> 
     980      </p>
     981      <dl class="empty">
     982         <dd>A cache behaves in a "semantically transparent" manner, with respect to a particular response, when its use affects neither
     983            the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the
     984            client receives exactly the same response (except for hop-by-hop headers) that it would have received had its request been
     985            handled directly by the origin server.
     986         </dd>
     987      </dl>
     988      <p id="rfc.section.1.3.p.28"> <span id="rfc.iref.v.2"></span>  <dfn>validator</dfn> 
     989      </p>
     990      <dl class="empty">
     991         <dd>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent
     992            copy of an entity.
     993         </dd>
     994      </dl>
     995      <p id="rfc.section.1.3.p.29"> <span id="rfc.iref.u.2"></span>  <span id="rfc.iref.d.1"></span>  <dfn>upstream</dfn>/<dfn>downstream</dfn> 
     996      </p>
     997      <dl class="empty">
     998         <dd>Upstream and downstream describe the flow of a message: all messages flow from upstream to downstream.</dd>
     999      </dl>
     1000      <p id="rfc.section.1.3.p.30"> <span id="rfc.iref.i.1"></span>  <span id="rfc.iref.o.2"></span>  <dfn>inbound</dfn>/<dfn>outbound</dfn> 
     1001      </p>
     1002      <dl class="empty">
     1003         <dd>Inbound and outbound refer to the request and response paths for messages: "inbound" means "traveling toward the origin server",
     1004            and "outbound" means "traveling toward the user agent"
     1005         </dd>
     1006      </dl>
     1007      <h2 id="rfc.section.1.4"><a href="#rfc.section.1.4">1.4</a>&nbsp;<a id="intro.overall.operation" href="#intro.overall.operation">Overall Operation</a></h2>
     1008      <p id="rfc.section.1.4.p.1">The HTTP protocol is a request/response protocol. A client sends a request to the server in the form of a request method,
     1009         URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible
     1010         body content over a connection with a server. The server responds with a status line, including the message's protocol version
     1011         and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible
     1012         entity-body content. The relationship between HTTP and MIME is described in <a href="#differences.between.http.entities.and.rfc.2045.entities" title="Differences Between HTTP Entities and RFC 2045 Entities">Appendix&nbsp;19.4</a>.
     1013      </p>
     1014      <p id="rfc.section.1.4.p.2">Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin
     1015         server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin
     1016         server (O).
     1017      </p>
     1018      <div id="rfc.figure.u.1"></div><pre class="drawing">       request chain ------------------------&gt;
    3341019    UA -------------------v------------------- O
    3351020       &lt;----------------------- response chain
    336 </pre><p id="rfc.section.1.4.p.4">A more complicated situation occurs when one or more intermediaries are present in the request/response chain. There are three common forms of intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent, receiving requests for a URI in its absolute form, rewriting all or part of the message, and forwarding the reformatted request toward the server identified by the URI. A gateway is a receiving agent, acting as a layer above some other server(s) and, if necessary, translating the requests to the underlying server's protocol. A tunnel acts as a relay point between two connections without changing the messages; tunnels are used when the communication needs to pass through an intermediary (such as a firewall) even when the intermediary cannot understand the contents of the messages.</p><div id="rfc.figure.u.2"></div><pre class="drawing">       request chain --------------------------------------&gt;
     1021</pre><p id="rfc.section.1.4.p.4">A more complicated situation occurs when one or more intermediaries are present in the request/response chain. There are three
     1022         common forms of intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent, receiving requests for a URI in its
     1023         absolute form, rewriting all or part of the message, and forwarding the reformatted request toward the server identified by
     1024         the URI. A gateway is a receiving agent, acting as a layer above some other server(s) and, if necessary, translating the requests
     1025         to the underlying server's protocol. A tunnel acts as a relay point between two connections without changing the messages;
     1026         tunnels are used when the communication needs to pass through an intermediary (such as a firewall) even when the intermediary
     1027         cannot understand the contents of the messages.
     1028      </p>
     1029      <div id="rfc.figure.u.2"></div><pre class="drawing">       request chain --------------------------------------&gt;
    3371030    UA -----v----- A -----v----- B -----v----- C -----v----- O
    3381031       &lt;------------------------------------- response chain
    339 </pre><p id="rfc.section.1.4.p.6">The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response message that travels the whole chain will pass through four separate connections. This distinction is important because some HTTP communication options may apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along the chain. Although the diagram is linear, each participant may be engaged in multiple, simultaneous communications. For example, B may be receiving requests from many clients other than A, and/or forwarding requests to servers other than C, at the same time that it is handling A's request.</p><p id="rfc.section.1.4.p.7">Any party to the communication which is not acting as a tunnel may employ an internal cache for handling requests. The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from O (via C) for a request which has not been cached by UA or A.</p><div id="rfc.figure.u.3"></div><pre class="drawing">          request chain ----------&gt;
     1032</pre><p id="rfc.section.1.4.p.6">The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response
     1033         message that travels the whole chain will pass through four separate connections. This distinction is important because some
     1034         HTTP communication options may apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points
     1035         of the chain, or to all connections along the chain. Although the diagram is linear, each participant may be engaged in multiple,
     1036         simultaneous communications. For example, B may be receiving requests from many clients other than A, and/or forwarding requests
     1037         to servers other than C, at the same time that it is handling A's request.
     1038      </p>
     1039      <p id="rfc.section.1.4.p.7">Any party to the communication which is not acting as a tunnel may employ an internal cache for handling requests. The effect
     1040         of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response
     1041         applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from
     1042         O (via C) for a request which has not been cached by UA or A.
     1043      </p>
     1044      <div id="rfc.figure.u.3"></div><pre class="drawing">          request chain ----------&gt;
    3401045       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
    3411046          &lt;--------- response chain
    342 </pre><p id="rfc.section.1.4.p.9">Not all responses are usefully cacheable, and some requests may contain modifiers which place special requirements on cache behavior. HTTP requirements for cache behavior and cacheable responses are defined in <a href="#caching" title="Caching in HTTP">Section&nbsp;13</a>.</p><p id="rfc.section.1.4.p.10">In fact, there are a wide variety of architectures and configurations of caches and proxies currently being experimented with or deployed across the World Wide Web. These systems include national hierarchies of proxy caches to save transoceanic bandwidth, systems that broadcast or multicast cache entries, organizations that distribute subsets of cached data via CD-ROM, and so on. HTTP systems are used in corporate intranets over high-bandwidth links, and for access via PDAs with low-power radio links and intermittent connectivity. The goal of HTTP/1.1 is to support the wide diversity of configurations already deployed while introducing protocol constructs that meet the needs of those who build web applications that require high reliability and, failing that, at least reliable indications of failure.</p><p id="rfc.section.1.4.p.11">HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80 <a href="#RFC1700" id="rfc.xref.RFC1700.1"><cite title="Assigned Numbers">[19]</cite></a>, but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification.</p><p id="rfc.section.1.4.p.12">In HTTP/1.0, most implementations used a new connection for each request/response exchange. In HTTP/1.1, a connection may be used for one or more request/response exchanges, although connections may be closed for a variety of reasons (see <a href="#persistent.connections" title="Persistent Connections">Section&nbsp;8.1</a>).</p><hr class="noprint"><h1 id="rfc.section.2" class="np"><a href="#rfc.section.2">2.</a>&nbsp;<a id="notation" href="#notation">Notational Conventions and Generic Grammar</a></h1><h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="notation.abnf" href="#notation.abnf">Augmented BNF</a></h2><p id="rfc.section.2.1.p.1">All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar to that used by RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.2"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a>. Implementors will need to be familiar with the notation in order to understand this specification. The augmented BNF includes the following constructs:</p><p id="rfc.section.2.1.p.2">name = definition </p><dl class="empty"><dd>The name of a rule is simply the name itself (without any enclosing "&lt;" and "&gt;") and is separated from its definition by the equal "=" character. White space is only significant in that indentation of continuation lines is used to indicate a rule definition that spans more than one line. Certain basic rules are in uppercase, such as <a href="#basic.rules" class="smpl" id="rfc.extref.s.1">SP</a>, <a href="#basic.rules.lws" class="smpl" id="rfc.extref.l.1">LWS</a>, <a href="#basic.rules" class="smpl" id="rfc.extref.h.1">HT</a>, <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.1">CRLF</a>, <a href="#basic.rules" class="smpl" id="rfc.extref.d.1">DIGIT</a>, <a href="#basic.rules" class="smpl" id="rfc.extref.a.1">ALPHA</a>, etc. Angle brackets are used within definitions whenever their presence will facilitate discerning the use of rule names.</dd></dl><p id="rfc.section.2.1.p.3">"literal" </p><dl class="empty"><dd>Quotation marks surround literal text. Unless stated otherwise, the text is case-insensitive.</dd></dl><p id="rfc.section.2.1.p.4">rule1 | rule2 </p><dl class="empty"><dd>Elements separated by a bar ("|") are alternatives, e.g., "yes | no" will accept yes or no.</dd></dl><p id="rfc.section.2.1.p.5">(rule1 rule2) </p><dl class="empty"><dd>Elements enclosed in parentheses are treated as a single element. Thus, "(elem (foo | bar) elem)" allows the token sequences "elem foo elem" and "elem bar elem".</dd></dl><p id="rfc.section.2.1.p.6">*rule </p><dl class="empty"><dd>The character "*" preceding an element indicates repetition. The full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and at most &lt;m&gt; occurrences of element. Default values are 0 and infinity so that "*(element)" allows any number, including zero; "1*element" requires at least one; and "1*2element" allows one or two.</dd></dl><p id="rfc.section.2.1.p.7">[rule] </p><dl class="empty"><dd>Square brackets enclose optional elements; "[foo bar]" is equivalent to "*1(foo bar)".</dd></dl><p id="rfc.section.2.1.p.8">N rule </p><dl class="empty"><dd>Specific repetition: "&lt;n&gt;(element)" is equivalent to "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic characters.</dd></dl><p id="rfc.section.2.1.p.9">#rule </p><dl class="empty"><dd>A construct "#" is defined, similar to "*", for defining lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas (",") and <em class="bcp14">OPTIONAL</em> linear white space (LWS). This makes the usual form of lists very easy; a rule such as <div id="rfc.figure.u.4"></div><pre class="text">   ( *LWS element *( *LWS "," *LWS element ))
    343 </pre> </dd><dd>can be shown as <div id="rfc.figure.u.5"></div><pre class="text">   1#element
    344 </pre> </dd><dd>Wherever this construct is used, null elements are allowed, but do not contribute to the count of elements present. That is, "(element), , (element) " is permitted, but counts as only two elements. Therefore, where at least one element is required, at least one non-null element <em class="bcp14">MUST</em> be present. Default values are 0 and infinity so that "#element" allows any number, including zero; "1#element" requires at least one; and "1#2element" allows one or two.</dd></dl><p id="rfc.section.2.1.p.10">; comment </p><dl class="empty"><dd>A semi-colon, set off some distance to the right of rule text, starts a comment that continues to the end of line. This is a simple way of including useful notes in parallel with the specifications.</dd></dl><div id="implied.LWS"><p id="rfc.section.2.1.p.11">implied *LWS </p><dl class="empty"><dd>The grammar described by this specification is word-based. Except where noted otherwise, linear white space (LWS) can be included between any two adjacent words (token or quoted-string), and between adjacent words and separators, without changing the interpretation of a field. At least one delimiter (LWS and/or separators) <em class="bcp14">MUST</em> exist between any two tokens (for the definition of "token" below), since they would otherwise be interpreted as a single token.</dd></dl></div><h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="basic.rules" href="#basic.rules">Basic Rules</a></h2><p id="rfc.section.2.2.p.1">The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII coded character set is defined by ANSI X3.4-1986 <a href="#USASCII" id="rfc.xref.USASCII.1"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[21]</cite></a>.</p><div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span><span id="rfc.iref.g.7"></span><span id="rfc.iref.g.8"></span><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span>    <a href="#basic.rules" class="smpl" id="rfc.extref.o.1">OCTET</a>          = &lt;any 8-bit sequence of data&gt;
     1047</pre><p id="rfc.section.1.4.p.9">Not all responses are usefully cacheable, and some requests may contain modifiers which place special requirements on cache
     1048         behavior. HTTP requirements for cache behavior and cacheable responses are defined in <a href="#caching" title="Caching in HTTP">Section&nbsp;13</a>.
     1049      </p>
     1050      <p id="rfc.section.1.4.p.10">In fact, there are a wide variety of architectures and configurations of caches and proxies currently being experimented with
     1051         or deployed across the World Wide Web. These systems include national hierarchies of proxy caches to save transoceanic bandwidth,
     1052         systems that broadcast or multicast cache entries, organizations that distribute subsets of cached data via CD-ROM, and so
     1053         on. HTTP systems are used in corporate intranets over high-bandwidth links, and for access via PDAs with low-power radio links
     1054         and intermittent connectivity. The goal of HTTP/1.1 is to support the wide diversity of configurations already deployed while
     1055         introducing protocol constructs that meet the needs of those who build web applications that require high reliability and,
     1056         failing that, at least reliable indications of failure.
     1057      </p>
     1058      <p id="rfc.section.1.4.p.11">HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80 <a href="#RFC1700" id="rfc.xref.RFC1700.1"><cite title="Assigned Numbers">[19]</cite></a>, but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet,
     1059         or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the
     1060         mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside
     1061         the scope of this specification.
     1062      </p>
     1063      <p id="rfc.section.1.4.p.12">In HTTP/1.0, most implementations used a new connection for each request/response exchange. In HTTP/1.1, a connection may
     1064         be used for one or more request/response exchanges, although connections may be closed for a variety of reasons (see <a href="#persistent.connections" title="Persistent Connections">Section&nbsp;8.1</a>).
     1065      </p>
     1066      <hr class="noprint">
     1067      <h1 id="rfc.section.2" class="np"><a href="#rfc.section.2">2.</a>&nbsp;<a id="notation" href="#notation">Notational Conventions and Generic Grammar</a></h1>
     1068      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="notation.abnf" href="#notation.abnf">Augmented BNF</a></h2>
     1069      <p id="rfc.section.2.1.p.1">All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar
     1070         to that used by RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.2"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a>. Implementors will need to be familiar with the notation in order to understand this specification. The augmented BNF includes
     1071         the following constructs:
     1072      </p>
     1073      <p id="rfc.section.2.1.p.2">name = definition </p>
     1074      <dl class="empty">
     1075         <dd>The name of a rule is simply the name itself (without any enclosing "&lt;" and "&gt;") and is separated from its definition by the
     1076            equal "=" character. White space is only significant in that indentation of continuation lines is used to indicate a rule
     1077            definition that spans more than one line. Certain basic rules are in uppercase, such as <a href="#basic.rules" class="smpl" id="rfc.extref.s.1">SP</a>, <a href="#basic.rules.lws" class="smpl" id="rfc.extref.l.1">LWS</a>, <a href="#basic.rules" class="smpl" id="rfc.extref.h.1">HT</a>, <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.1">CRLF</a>, <a href="#basic.rules" class="smpl" id="rfc.extref.d.1">DIGIT</a>, <a href="#basic.rules" class="smpl" id="rfc.extref.a.1">ALPHA</a>, etc. Angle brackets are used within definitions whenever their presence will facilitate discerning the use of rule names.
     1078         </dd>
     1079      </dl>
     1080      <p id="rfc.section.2.1.p.3">"literal" </p>
     1081      <dl class="empty">
     1082         <dd>Quotation marks surround literal text. Unless stated otherwise, the text is case-insensitive.</dd>
     1083      </dl>
     1084      <p id="rfc.section.2.1.p.4">rule1 | rule2 </p>
     1085      <dl class="empty">
     1086         <dd>Elements separated by a bar ("|") are alternatives, e.g., "yes | no" will accept yes or no.</dd>
     1087      </dl>
     1088      <p id="rfc.section.2.1.p.5">(rule1 rule2) </p>
     1089      <dl class="empty">
     1090         <dd>Elements enclosed in parentheses are treated as a single element. Thus, "(elem (foo | bar) elem)" allows the token sequences
     1091            "elem foo elem" and "elem bar elem".
     1092         </dd>
     1093      </dl>
     1094      <p id="rfc.section.2.1.p.6">*rule </p>
     1095      <dl class="empty">
     1096         <dd>The character "*" preceding an element indicates repetition. The full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and
     1097            at most &lt;m&gt; occurrences of element. Default values are 0 and infinity so that "*(element)" allows any number, including zero;
     1098            "1*element" requires at least one; and "1*2element" allows one or two.
     1099         </dd>
     1100      </dl>
     1101      <p id="rfc.section.2.1.p.7">[rule] </p>
     1102      <dl class="empty">
     1103         <dd>Square brackets enclose optional elements; "[foo bar]" is equivalent to "*1(foo bar)".</dd>
     1104      </dl>
     1105      <p id="rfc.section.2.1.p.8">N rule </p>
     1106      <dl class="empty">
     1107         <dd>Specific repetition: "&lt;n&gt;(element)" is equivalent to "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element). Thus
     1108            2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic characters.
     1109         </dd>
     1110      </dl>
     1111      <p id="rfc.section.2.1.p.9">#rule </p>
     1112      <dl class="empty">
     1113         <dd>A construct "#" is defined, similar to "*", for defining lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at
     1114            least &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas (",") and <em class="bcp14">OPTIONAL</em> linear white space (LWS). This makes the usual form of lists very easy; a rule such as
     1115            <div id="rfc.figure.u.4"></div><pre class="text">   ( *LWS element *( *LWS "," *LWS element ))
     1116</pre> </dd>
     1117         <dd>can be shown as
     1118            <div id="rfc.figure.u.5"></div><pre class="text">   1#element
     1119</pre> </dd>
     1120         <dd>Wherever this construct is used, null elements are allowed, but do not contribute to the count of elements present. That is,
     1121            "(element), , (element) " is permitted, but counts as only two elements. Therefore, where at least one element is required,
     1122            at least one non-null element <em class="bcp14">MUST</em> be present. Default values are 0 and infinity so that "#element" allows any number, including zero; "1#element" requires at
     1123            least one; and "1#2element" allows one or two.
     1124         </dd>
     1125      </dl>
     1126      <p id="rfc.section.2.1.p.10">; comment </p>
     1127      <dl class="empty">
     1128         <dd>A semi-colon, set off some distance to the right of rule text, starts a comment that continues to the end of line. This is
     1129            a simple way of including useful notes in parallel with the specifications.
     1130         </dd>
     1131      </dl>
     1132      <div id="implied.LWS">
     1133         <p id="rfc.section.2.1.p.11">implied *LWS </p>
     1134         <dl class="empty">
     1135            <dd>The grammar described by this specification is word-based. Except where noted otherwise, linear white space (LWS) can be included
     1136               between any two adjacent words (token or quoted-string), and between adjacent words and separators, without changing the interpretation
     1137               of a field. At least one delimiter (LWS and/or separators) <em class="bcp14">MUST</em> exist between any two tokens (for the definition of "token" below), since they would otherwise be interpreted as a single
     1138               token.
     1139            </dd>
     1140         </dl>
     1141      </div>
     1142      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="basic.rules" href="#basic.rules">Basic Rules</a></h2>
     1143      <p id="rfc.section.2.2.p.1">The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII coded character
     1144         set is defined by ANSI X3.4-1986 <a href="#USASCII" id="rfc.xref.USASCII.1"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[21]</cite></a>.
     1145      </p>
     1146      <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span><span id="rfc.iref.g.7"></span><span id="rfc.iref.g.8"></span><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span>    <a href="#basic.rules" class="smpl" id="rfc.extref.o.1">OCTET</a>          = &lt;any 8-bit sequence of data&gt;
    3451147    <a href="#basic.rules" class="smpl" id="rfc.extref.c.2">CHAR</a>           = &lt;any US-ASCII character (octets 0 - 127)&gt;
    3461148    <a href="#basic.rules" class="smpl" id="rfc.extref.u.1">UPALPHA</a>        = &lt;any US-ASCII uppercase letter "A".."Z"&gt;
     
    3551157    <a href="#basic.rules" class="smpl" id="rfc.extref.h.2">HT</a>             = &lt;US-ASCII HT, horizontal-tab (9)&gt;
    3561158    &lt;"&gt;            = &lt;US-ASCII double-quote mark (34)&gt;
    357 </pre><div id="basic.rules.crlf"><p id="rfc.section.2.2.p.3">  HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all protocol elements except the entity-body (see <a href="#tolerant.applications" title="Tolerant Applications">Appendix&nbsp;19.3</a> for tolerant applications). The end-of-line marker within an entity-body is defined by its associated media type, as described in <a href="#media.types" title="Media Types">Section&nbsp;3.7</a>.</p></div><div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.13"></span>    <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.5">CRLF</a>           = <a href="#basic.rules" class="smpl" id="rfc.extref.c.6">CR</a> <a href="#basic.rules" class="smpl" id="rfc.extref.l.5">LF</a>
    358 </pre><div id="basic.rules.lws"><p id="rfc.section.2.2.p.5">  HTTP/1.1 header field values can be folded onto multiple lines if the continuation line begins with a space or horizontal tab. All linear white space, including folding, has the same semantics as SP. A recipient <em class="bcp14">MAY</em> replace any linear white space with a single SP before interpreting the field value or forwarding the message downstream.</p></div><div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.14"></span>    <a href="#basic.rules.lws" class="smpl" id="rfc.extref.l.6">LWS</a>            = [<a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.7">CRLF</a>] 1*( <a href="#basic.rules" class="smpl" id="rfc.extref.s.3">SP</a> | <a href="#basic.rules" class="smpl" id="rfc.extref.h.3">HT</a> )
    359 </pre><div id="basic.rules.text"><p id="rfc.section.2.2.p.7">  The TEXT rule is only used for descriptive field contents and values that are not intended to be interpreted by the message parser. Words of *TEXT <em class="bcp14">MAY</em> contain characters from character sets other than ISO-8859-1 <a href="#ISO-8859" id="rfc.xref.ISO-8859.1"><cite title="Information technology - 8-bit single byte coded graphic - character sets">[22]</cite></a> only when encoded according to the rules of RFC 2047 <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[14]</cite></a>.</p></div><div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.15"></span>    <a href="#basic.rules.text" class="smpl" id="rfc.extref.t.1">TEXT</a>           = &lt;any <a href="#basic.rules" class="smpl" id="rfc.extref.o.2">OCTET</a> except <a href="#basic.rules" class="smpl" id="rfc.extref.c.8">CTL</a>s,
     1159</pre><div id="basic.rules.crlf">
     1160         <p id="rfc.section.2.2.p.3">  HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all protocol elements except the entity-body (see <a href="#tolerant.applications" title="Tolerant Applications">Appendix&nbsp;19.3</a> for tolerant applications). The end-of-line marker within an entity-body is defined by its associated media type, as described
     1161            in <a href="#media.types" title="Media Types">Section&nbsp;3.7</a>.
     1162         </p>
     1163      </div>
     1164      <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.13"></span>    <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.5">CRLF</a>           = <a href="#basic.rules" class="smpl" id="rfc.extref.c.6">CR</a> <a href="#basic.rules" class="smpl" id="rfc.extref.l.5">LF</a>
     1165</pre><div id="basic.rules.lws">
     1166         <p id="rfc.section.2.2.p.5">  HTTP/1.1 header field values can be folded onto multiple lines if the continuation line begins with a space or horizontal
     1167            tab. All linear white space, including folding, has the same semantics as SP. A recipient <em class="bcp14">MAY</em> replace any linear white space with a single SP before interpreting the field value or forwarding the message downstream.
     1168         </p>
     1169      </div>
     1170      <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.14"></span>    <a href="#basic.rules.lws" class="smpl" id="rfc.extref.l.6">LWS</a>            = [<a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.7">CRLF</a>] 1*( <a href="#basic.rules" class="smpl" id="rfc.extref.s.3">SP</a> | <a href="#basic.rules" class="smpl" id="rfc.extref.h.3">HT</a> )
     1171</pre><div id="basic.rules.text">
     1172         <p id="rfc.section.2.2.p.7">  The TEXT rule is only used for descriptive field contents and values that are not intended to be interpreted by the message
     1173            parser. Words of *TEXT <em class="bcp14">MAY</em> contain characters from character sets other than ISO-8859-1 <a href="#ISO-8859" id="rfc.xref.ISO-8859.1"><cite title="Information technology - 8-bit single byte coded graphic - character sets">[22]</cite></a> only when encoded according to the rules of RFC 2047 <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[14]</cite></a>.
     1174         </p>
     1175      </div>
     1176      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.15"></span>    <a href="#basic.rules.text" class="smpl" id="rfc.extref.t.1">TEXT</a>           = &lt;any <a href="#basic.rules" class="smpl" id="rfc.extref.o.2">OCTET</a> except <a href="#basic.rules" class="smpl" id="rfc.extref.c.8">CTL</a>s,
    3601177                     but including <a href="#basic.rules.lws" class="smpl" id="rfc.extref.l.7">LWS</a>&gt;
    361 </pre><p id="rfc.section.2.2.p.9">A CRLF is allowed in the definition of TEXT only as part of a header field continuation. It is expected that the folding LWS will be replaced with a single SP before interpretation of the TEXT value.</p><div id="basic.rules.hex"><p id="rfc.section.2.2.p.10">  Hexadecimal numeric characters are used in several protocol elements.</p></div><div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.16"></span>    <a href="#basic.rules.hex" class="smpl" id="rfc.extref.h.4">HEX</a>            = "A" | "B" | "C" | "D" | "E" | "F"
     1178</pre><p id="rfc.section.2.2.p.9">A CRLF is allowed in the definition of TEXT only as part of a header field continuation. It is expected that the folding LWS
     1179         will be replaced with a single SP before interpretation of the TEXT value.
     1180      </p>
     1181      <div id="basic.rules.hex">
     1182         <p id="rfc.section.2.2.p.10">  Hexadecimal numeric characters are used in several protocol elements.</p>
     1183      </div>
     1184      <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.16"></span>    <a href="#basic.rules.hex" class="smpl" id="rfc.extref.h.4">HEX</a>            = "A" | "B" | "C" | "D" | "E" | "F"
    3621185                   | "a" | "b" | "c" | "d" | "e" | "f" | <a href="#basic.rules" class="smpl" id="rfc.extref.d.3">DIGIT</a>
    363 </pre><div id="basic.rules.token"><p id="rfc.section.2.2.p.12">    Many HTTP/1.1 header field values consist of words separated by LWS or special characters. These special characters <em class="bcp14">MUST</em> be in a quoted string to be used within a parameter value (as defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>).</p></div><div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span>    <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.2">token</a>          = 1*&lt;any <a href="#basic.rules" class="smpl" id="rfc.extref.c.9">CHAR</a> except <a href="#basic.rules" class="smpl" id="rfc.extref.c.10">CTL</a>s or <a href="#basic.rules.token" class="smpl" id="rfc.extref.s.4">separators</a>&gt;
     1186</pre><div id="basic.rules.token">
     1187         <p id="rfc.section.2.2.p.12">    Many HTTP/1.1 header field values consist of words separated by LWS or special characters. These special characters <em class="bcp14">MUST</em> be in a quoted string to be used within a parameter value (as defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>).
     1188         </p>
     1189      </div>
     1190      <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span>    <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.2">token</a>          = 1*&lt;any <a href="#basic.rules" class="smpl" id="rfc.extref.c.9">CHAR</a> except <a href="#basic.rules" class="smpl" id="rfc.extref.c.10">CTL</a>s or <a href="#basic.rules.token" class="smpl" id="rfc.extref.s.4">separators</a>&gt;
    3641191    <a href="#basic.rules.token" class="smpl" id="rfc.extref.s.5">separators</a>     = "(" | ")" | "&lt;" | "&gt;" | "@"
    3651192                   | "," | ";" | ":" | "\" | &lt;"&gt;
    3661193                   | "/" | "[" | "]" | "?" | "="
    3671194                   | "{" | "}" | <a href="#basic.rules" class="smpl" id="rfc.extref.s.6">SP</a> | <a href="#basic.rules" class="smpl" id="rfc.extref.h.5">HT</a>
    368 </pre><div id="basic.rules.comment"><p id="rfc.section.2.2.p.14">    Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing "comment" as part of their field value definition. In all other fields, parentheses are considered part of the field value.</p></div><div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span>    <a href="#basic.rules.comment" class="smpl" id="rfc.extref.c.11">comment</a>        = "(" *( <a href="#basic.rules.comment" class="smpl" id="rfc.extref.c.12">ctext</a> | <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.1">quoted-pair</a> | <a href="#basic.rules.comment" class="smpl" id="rfc.extref.c.13">comment</a> ) ")"
     1195</pre><div id="basic.rules.comment">
     1196         <p id="rfc.section.2.2.p.14">    Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed
     1197            in fields containing "comment" as part of their field value definition. In all other fields, parentheses are considered part
     1198            of the field value.
     1199         </p>
     1200      </div>
     1201      <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span>    <a href="#basic.rules.comment" class="smpl" id="rfc.extref.c.11">comment</a>        = "(" *( <a href="#basic.rules.comment" class="smpl" id="rfc.extref.c.12">ctext</a> | <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.1">quoted-pair</a> | <a href="#basic.rules.comment" class="smpl" id="rfc.extref.c.13">comment</a> ) ")"
    3691202    <a href="#basic.rules.comment" class="smpl" id="rfc.extref.c.14">ctext</a>          = &lt;any <a href="#basic.rules.text" class="smpl" id="rfc.extref.t.3">TEXT</a> excluding "(" and ")"&gt;
    370 </pre><div id="basic.rules.quoted-string"><p id="rfc.section.2.2.p.16">      A string of text is parsed as a single word if it is quoted using double-quote marks.</p></div><div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span>    <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.2">quoted-string</a>  = ( &lt;"&gt; *(<a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.3">qdtext</a> | <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.4">quoted-pair</a> ) &lt;"&gt; )
     1203</pre><div id="basic.rules.quoted-string">
     1204         <p id="rfc.section.2.2.p.16">      A string of text is parsed as a single word if it is quoted using double-quote marks.</p>
     1205      </div>
     1206      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span>    <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.2">quoted-string</a>  = ( &lt;"&gt; *(<a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.3">qdtext</a> | <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.4">quoted-pair</a> ) &lt;"&gt; )
    3711207    <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.5">qdtext</a>         = &lt;any <a href="#basic.rules.text" class="smpl" id="rfc.extref.t.4">TEXT</a> except &lt;"&gt;&gt;
    372 </pre><p id="rfc.section.2.2.p.18">The backslash character ("\") <em class="bcp14">MAY</em> be used as a single-character quoting mechanism only within quoted-string and comment constructs.</p><div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.23"></span>    <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.6">quoted-pair</a>    = "\" <a href="#basic.rules" class="smpl" id="rfc.extref.c.15">CHAR</a>
    373 </pre><hr class="noprint"><h1 id="rfc.section.3" class="np"><a href="#rfc.section.3">3.</a>&nbsp;<a id="protocol.parameters" href="#protocol.parameters">Protocol Parameters</a></h1><h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="http.version" href="#http.version">HTTP Version</a></h2><p id="rfc.section.3.1.p.1">HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather than the features obtained via that communication. No change is made to the version number for the addition of message components which do not affect communication behavior or which only add to extensible field values. The &lt;minor&gt; number is incremented when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may add to the message semantics and imply additional capabilities of the sender. The &lt;major&gt; number is incremented when the format of a message within the protocol is changed. See RFC 2145 <a href="#RFC2145" id="rfc.xref.RFC2145.1"><cite title="Use and Interpretation of HTTP Version Numbers">[36]</cite></a> for a fuller explanation.</p><p id="rfc.section.3.1.p.2">The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message.</p><div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.24"></span>       <a href="#http.version" class="smpl" id="rfc.extref.h.6">HTTP-Version</a>   = "HTTP" "/" 1*<a href="#basic.rules" class="smpl" id="rfc.extref.d.4">DIGIT</a> "." 1*<a href="#basic.rules" class="smpl" id="rfc.extref.d.5">DIGIT</a>
    374 </pre><p id="rfc.section.3.1.p.4">Note that the major and minor numbers <em class="bcp14">MUST</em> be treated as separate integers and that each <em class="bcp14">MAY</em> be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3. Leading zeros <em class="bcp14">MUST</em> be ignored by recipients and <em class="bcp14">MUST NOT</em> be sent.</p><p id="rfc.section.3.1.p.5">An application that sends a request or response message that includes HTTP-Version of "HTTP/1.1" <em class="bcp14">MUST</em> be at least conditionally compliant with this specification. Applications that are at least conditionally compliant with this specification <em class="bcp14">SHOULD</em> use an HTTP-Version of "HTTP/1.1" in their messages, and <em class="bcp14">MUST</em> do so for any message that is not compatible with HTTP/1.0. For more details on when to send specific HTTP-Version values, see RFC 2145 <a href="#RFC2145" id="rfc.xref.RFC2145.2"><cite title="Use and Interpretation of HTTP Version Numbers">[36]</cite></a>.</p><p id="rfc.section.3.1.p.6">The HTTP version of an application is the highest HTTP version for which the application is at least conditionally compliant.</p><p id="rfc.section.3.1.p.7">Proxy and gateway applications need to be careful when forwarding messages in protocol versions different from that of the application. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway <em class="bcp14">MUST NOT</em> send a message with a version indicator which is greater than its actual version. If a higher version request is received, the proxy/gateway <em class="bcp14">MUST</em> either downgrade the request version, or respond with an error, or switch to tunnel behavior.</p><p id="rfc.section.3.1.p.8">Due to interoperability problems with HTTP/1.0 proxies discovered since the publication of RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[33]</cite></a>, caching proxies <em class="bcp14">MUST</em>, gateways <em class="bcp14">MAY</em>, and tunnels <em class="bcp14">MUST NOT</em> upgrade the request to the highest version they support. The proxy/gateway's response to that request <em class="bcp14">MUST</em> be in the same major version as the request.</p><p id="rfc.section.3.1.p.9"> </p><dl class="empty"><dd> <b>Note:</b> Converting between versions of HTTP may involve modification of header fields required or forbidden by the versions involved.</dd></dl><h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="uri" href="#uri">Uniform Resource Identifiers</a></h2><p id="rfc.section.3.2.p.1">URIs have been known by many names: WWW addresses, Universal Document Identifiers, Universal Resource Identifiers <a href="#RFC1630" id="rfc.xref.RFC1630.2"><cite title="Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web">[3]</cite></a>, and finally the combination of Uniform Resource Locators (URL) <a href="#RFC1738" id="rfc.xref.RFC1738.2"><cite title="Uniform Resource Locators (URL)">[4]</cite></a> and Names (URN) <a href="#RFC1737" id="rfc.xref.RFC1737.2"><cite title="Functional Requirements for Uniform Resource Names">[20]</cite></a>. As far as HTTP is concerned, Uniform Resource Identifiers are simply formatted strings which identify--via name, location, or any other characteristic--a resource.</p><h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="general.syntax" href="#general.syntax">General Syntax</a></h3><p id="rfc.section.3.2.1.p.1">URIs in HTTP can be represented in absolute form or relative to some known base URI <a href="#RFC1808" id="rfc.xref.RFC1808.1"><cite title="Relative Uniform Resource Locators">[11]</cite></a>, depending upon the context of their use. The two forms are differentiated by the fact that absolute URIs always begin with a scheme name followed by a colon. For definitive information on URL syntax and semantics, see "Uniform Resource Identifiers (URI): Generic Syntax and Semantics," RFC 2396 <a href="#RFC2396" id="rfc.xref.RFC2396.1"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[42]</cite></a> (which replaces RFCs 1738 <a href="#RFC1738" id="rfc.xref.RFC1738.3"><cite title="Uniform Resource Locators (URL)">[4]</cite></a> and RFC 1808 <a href="#RFC1808" id="rfc.xref.RFC1808.2"><cite title="Relative Uniform Resource Locators">[11]</cite></a>). This specification adopts the definitions of "URI-reference", "absoluteURI", "relativeURI", "port", "host","abs_path", "rel_path", and "authority" from that specification.</p><p id="rfc.section.3.2.1.p.2">The HTTP protocol does not place any a priori limit on the length of a URI. Servers <em class="bcp14">MUST</em> be able to handle the URI of any resource they serve, and <em class="bcp14">SHOULD</em> be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server <em class="bcp14">SHOULD</em> return <a href="#status.414" class="smpl">414 (Request-URI Too Long)</a> status if a URI is longer than the server can handle (see <a href="#status.414" id="rfc.xref.status.414.1" title="414 Request-URI Too Long">Section&nbsp;10.4.15</a>).</p><p id="rfc.section.3.2.1.p.3"> </p><dl class="empty"><dd> <b>Note:</b> Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations might not properly support these lengths.</dd></dl><div id="rfc.iref.h.2"></div><div id="rfc.iref.u.3"></div><h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="http.url" href="#http.url">http URL</a></h3><p id="rfc.section.3.2.2.p.1">The "http" scheme is used to locate network resources via the HTTP protocol. This section defines the scheme-specific syntax and semantics for http URLs.</p><div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.25"></span>http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
    375 </pre><p id="rfc.section.3.2.2.p.3">If the port is empty or not given, port 80 is assumed. The semantics are that the identified resource is located at the server listening for TCP connections on that port of that host, and the Request-URI for the resource is abs_path (<a href="#request-uri" title="Request-URI">Section&nbsp;5.1.2</a>). The use of IP addresses in URLs <em class="bcp14">SHOULD</em> be avoided whenever possible (see RFC 1900 <a href="#RFC1900" id="rfc.xref.RFC1900.1"><cite title="Renumbering Needs Work">[24]</cite></a>). If the abs_path is not present in the URL, it <em class="bcp14">MUST</em> be given as "/" when used as a Request-URI for a resource (<a href="#request-uri" title="Request-URI">Section&nbsp;5.1.2</a>). If a proxy receives a host name which is not a fully qualified domain name, it <em class="bcp14">MAY</em> add its domain to the host name it received. If a proxy receives a fully qualified domain name, the proxy <em class="bcp14">MUST NOT</em> change the host name.</p><h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="uri.comparison" href="#uri.comparison">URI Comparison</a></h3><p id="rfc.section.3.2.3.p.1">When comparing two URIs to decide if they match or not, a client <em class="bcp14">SHOULD</em> use a case-sensitive octet-by-octet comparison of the entire URIs, with these exceptions: </p><ul><li>A port that is empty or not given is equivalent to the default port for that URI-reference;</li><li>Comparisons of host names <em class="bcp14">MUST</em> be case-insensitive;</li><li>Comparisons of scheme names <em class="bcp14">MUST</em> be case-insensitive;</li><li>An empty abs_path is equivalent to an abs_path of "/".</li></ul><p id="rfc.section.3.2.3.p.2">Characters other than those in the "reserved" and "unsafe" sets (see RFC 2396 <a href="#RFC2396" id="rfc.xref.RFC2396.2"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[42]</cite></a>) are equivalent to their ""%" HEX HEX" encoding.</p><p id="rfc.section.3.2.3.p.3">For example, the following three URIs are equivalent:</p><div id="rfc.figure.u.17"></div><pre class="text">   http://abc.com:80/~smith/home.html
     1208</pre><p id="rfc.section.2.2.p.18">The backslash character ("\") <em class="bcp14">MAY</em> be used as a single-character quoting mechanism only within quoted-string and comment constructs.
     1209      </p>
     1210      <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.23"></span>    <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.6">quoted-pair</a>    = "\" <a href="#basic.rules" class="smpl" id="rfc.extref.c.15">CHAR</a>
     1211</pre><hr class="noprint">
     1212      <h1 id="rfc.section.3" class="np"><a href="#rfc.section.3">3.</a>&nbsp;<a id="protocol.parameters" href="#protocol.parameters">Protocol Parameters</a></h1>
     1213      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="http.version" href="#http.version">HTTP Version</a></h2>
     1214      <p id="rfc.section.3.1.p.1">HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended
     1215         to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather
     1216         than the features obtained via that communication. No change is made to the version number for the addition of message components
     1217         which do not affect communication behavior or which only add to extensible field values. The &lt;minor&gt; number is incremented
     1218         when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may
     1219         add to the message semantics and imply additional capabilities of the sender. The &lt;major&gt; number is incremented when the format
     1220         of a message within the protocol is changed. See RFC 2145 <a href="#RFC2145" id="rfc.xref.RFC2145.1"><cite title="Use and Interpretation of HTTP Version Numbers">[36]</cite></a> for a fuller explanation.
     1221      </p>
     1222      <p id="rfc.section.3.1.p.2">The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message.</p>
     1223      <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.24"></span>       <a href="#http.version" class="smpl" id="rfc.extref.h.6">HTTP-Version</a>   = "HTTP" "/" 1*<a href="#basic.rules" class="smpl" id="rfc.extref.d.4">DIGIT</a> "." 1*<a href="#basic.rules" class="smpl" id="rfc.extref.d.5">DIGIT</a>
     1224</pre><p id="rfc.section.3.1.p.4">Note that the major and minor numbers <em class="bcp14">MUST</em> be treated as separate integers and that each <em class="bcp14">MAY</em> be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3.
     1225         Leading zeros <em class="bcp14">MUST</em> be ignored by recipients and <em class="bcp14">MUST NOT</em> be sent.
     1226      </p>
     1227      <p id="rfc.section.3.1.p.5">An application that sends a request or response message that includes HTTP-Version of "HTTP/1.1" <em class="bcp14">MUST</em> be at least conditionally compliant with this specification. Applications that are at least conditionally compliant with this
     1228         specification <em class="bcp14">SHOULD</em> use an HTTP-Version of "HTTP/1.1" in their messages, and <em class="bcp14">MUST</em> do so for any message that is not compatible with HTTP/1.0. For more details on when to send specific HTTP-Version values,
     1229         see RFC 2145 <a href="#RFC2145" id="rfc.xref.RFC2145.2"><cite title="Use and Interpretation of HTTP Version Numbers">[36]</cite></a>.
     1230      </p>
     1231      <p id="rfc.section.3.1.p.6">The HTTP version of an application is the highest HTTP version for which the application is at least conditionally compliant.</p>
     1232      <p id="rfc.section.3.1.p.7">Proxy and gateway applications need to be careful when forwarding messages in protocol versions different from that of the
     1233         application. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway <em class="bcp14">MUST NOT</em> send a message with a version indicator which is greater than its actual version. If a higher version request is received,
     1234         the proxy/gateway <em class="bcp14">MUST</em> either downgrade the request version, or respond with an error, or switch to tunnel behavior.
     1235      </p>
     1236      <p id="rfc.section.3.1.p.8">Due to interoperability problems with HTTP/1.0 proxies discovered since the publication of RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[33]</cite></a>, caching proxies <em class="bcp14">MUST</em>, gateways <em class="bcp14">MAY</em>, and tunnels <em class="bcp14">MUST NOT</em> upgrade the request to the highest version they support. The proxy/gateway's response to that request <em class="bcp14">MUST</em> be in the same major version as the request.
     1237      </p>
     1238      <p id="rfc.section.3.1.p.9"> </p>
     1239      <dl class="empty">
     1240         <dd> <b>Note:</b> Converting between versions of HTTP may involve modification of header fields required or forbidden by the versions involved.
     1241         </dd>
     1242      </dl>
     1243      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="uri" href="#uri">Uniform Resource Identifiers</a></h2>
     1244      <p id="rfc.section.3.2.p.1">URIs have been known by many names: WWW addresses, Universal Document Identifiers, Universal Resource Identifiers <a href="#RFC1630" id="rfc.xref.RFC1630.2"><cite title="Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web">[3]</cite></a>, and finally the combination of Uniform Resource Locators (URL) <a href="#RFC1738" id="rfc.xref.RFC1738.2"><cite title="Uniform Resource Locators (URL)">[4]</cite></a> and Names (URN) <a href="#RFC1737" id="rfc.xref.RFC1737.2"><cite title="Functional Requirements for Uniform Resource Names">[20]</cite></a>. As far as HTTP is concerned, Uniform Resource Identifiers are simply formatted strings which identify--via name, location,
     1245         or any other characteristic--a resource.
     1246      </p>
     1247      <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="general.syntax" href="#general.syntax">General Syntax</a></h3>
     1248      <p id="rfc.section.3.2.1.p.1">URIs in HTTP can be represented in absolute form or relative to some known base URI <a href="#RFC1808" id="rfc.xref.RFC1808.1"><cite title="Relative Uniform Resource Locators">[11]</cite></a>, depending upon the context of their use. The two forms are differentiated by the fact that absolute URIs always begin with
     1249         a scheme name followed by a colon. For definitive information on URL syntax and semantics, see "Uniform Resource Identifiers
     1250         (URI): Generic Syntax and Semantics," RFC 2396 <a href="#RFC2396" id="rfc.xref.RFC2396.1"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[42]</cite></a> (which replaces RFCs 1738 <a href="#RFC1738" id="rfc.xref.RFC1738.3"><cite title="Uniform Resource Locators (URL)">[4]</cite></a> and RFC 1808 <a href="#RFC1808" id="rfc.xref.RFC1808.2"><cite title="Relative Uniform Resource Locators">[11]</cite></a>). This specification adopts the definitions of "URI-reference", "absoluteURI", "relativeURI", "port", "host","abs_path",
     1251         "rel_path", and "authority" from that specification.
     1252      </p>
     1253      <p id="rfc.section.3.2.1.p.2">The HTTP protocol does not place any a priori limit on the length of a URI. Servers <em class="bcp14">MUST</em> be able to handle the URI of any resource they serve, and <em class="bcp14">SHOULD</em> be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server <em class="bcp14">SHOULD</em> return <a href="#status.414" class="smpl">414 (Request-URI Too Long)</a> status if a URI is longer than the server can handle (see <a href="#status.414" id="rfc.xref.status.414.1" title="414 Request-URI Too Long">Section&nbsp;10.4.15</a>).
     1254      </p>
     1255      <p id="rfc.section.3.2.1.p.3"> </p>
     1256      <dl class="empty">
     1257         <dd> <b>Note:</b> Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations
     1258            might not properly support these lengths.
     1259         </dd>
     1260      </dl>
     1261      <div id="rfc.iref.h.2"></div>
     1262      <div id="rfc.iref.u.3"></div>
     1263      <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="http.url" href="#http.url">http URL</a></h3>
     1264      <p id="rfc.section.3.2.2.p.1">The "http" scheme is used to locate network resources via the HTTP protocol. This section defines the scheme-specific syntax
     1265         and semantics for http URLs.
     1266      </p>
     1267      <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.25"></span>http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
     1268</pre><p id="rfc.section.3.2.2.p.3">If the port is empty or not given, port 80 is assumed. The semantics are that the identified resource is located at the server
     1269         listening for TCP connections on that port of that host, and the Request-URI for the resource is abs_path (<a href="#request-uri" title="Request-URI">Section&nbsp;5.1.2</a>). The use of IP addresses in URLs <em class="bcp14">SHOULD</em> be avoided whenever possible (see RFC 1900 <a href="#RFC1900" id="rfc.xref.RFC1900.1"><cite title="Renumbering Needs Work">[24]</cite></a>). If the abs_path is not present in the URL, it <em class="bcp14">MUST</em> be given as "/" when used as a Request-URI for a resource (<a href="#request-uri" title="Request-URI">Section&nbsp;5.1.2</a>). If a proxy receives a host name which is not a fully qualified domain name, it <em class="bcp14">MAY</em> add its domain to the host name it received. If a proxy receives a fully qualified domain name, the proxy <em class="bcp14">MUST NOT</em> change the host name.
     1270      </p>
     1271      <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="uri.comparison" href="#uri.comparison">URI Comparison</a></h3>
     1272      <p id="rfc.section.3.2.3.p.1">When comparing two URIs to decide if they match or not, a client <em class="bcp14">SHOULD</em> use a case-sensitive octet-by-octet comparison of the entire URIs, with these exceptions:
     1273      </p>
     1274      <ul>
     1275         <li>A port that is empty or not given is equivalent to the default port for that URI-reference;</li>
     1276         <li>Comparisons of host names <em class="bcp14">MUST</em> be case-insensitive;
     1277         </li>
     1278         <li>Comparisons of scheme names <em class="bcp14">MUST</em> be case-insensitive;
     1279         </li>
     1280         <li>An empty abs_path is equivalent to an abs_path of "/".</li>
     1281      </ul>
     1282      <p id="rfc.section.3.2.3.p.2">Characters other than those in the "reserved" and "unsafe" sets (see RFC 2396 <a href="#RFC2396" id="rfc.xref.RFC2396.2"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[42]</cite></a>) are equivalent to their ""%" HEX HEX" encoding.
     1283      </p>
     1284      <p id="rfc.section.3.2.3.p.3">For example, the following three URIs are equivalent:</p>
     1285      <div id="rfc.figure.u.17"></div><pre class="text">   http://abc.com:80/~smith/home.html
    3761286   http://ABC.com/%7Esmith/home.html
    3771287   http://ABC.com:/%7esmith/home.html
    378 </pre><h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="date.time.formats" href="#date.time.formats">Date/Time Formats</a></h2><h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a id="full.date" href="#full.date">Full Date</a></h3><p id="rfc.section.3.3.1.p.1">HTTP applications have historically allowed three different formats for the representation of date/time stamps:</p><div id="rfc.figure.u.18"></div><pre class="text">   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
     1288</pre><h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="date.time.formats" href="#date.time.formats">Date/Time Formats</a></h2>
     1289      <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a id="full.date" href="#full.date">Full Date</a></h3>
     1290      <p id="rfc.section.3.3.1.p.1">HTTP applications have historically allowed three different formats for the representation of date/time stamps:</p>
     1291      <div id="rfc.figure.u.18"></div><pre class="text">   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
    3791292   Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
    3801293   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
    381 </pre><p id="rfc.section.3.3.1.p.3">The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123 <a href="#RFC1123" id="rfc.xref.RFC1123.1"><cite title="Requirements for Internet Hosts - Application and Support">[8]</cite></a> (an update to RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.3"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a>). The second format is in common use, but is based on the obsolete RFC 850 <a href="#RFC1036" id="rfc.xref.RFC1036.1"><cite title="Standard for interchange of USENET messages">[12]</cite></a> date format and lacks a four-digit year. HTTP/1.1 clients and servers that parse the date value <em class="bcp14">MUST</em> accept all three formats (for compatibility with HTTP/1.0), though they <em class="bcp14">MUST</em> only generate the RFC 1123 format for representing HTTP-date values in header fields. See <a href="#tolerant.applications" title="Tolerant Applications">Appendix&nbsp;19.3</a> for further information.</p><dl class="empty"><dd> <b>Note:</b> Recipients of date values are encouraged to be robust in accepting date values that may have been sent by non-HTTP applications, as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.</dd></dl><p id="rfc.section.3.3.1.p.5">All HTTP date/time stamps <em class="bcp14">MUST</em> be represented in Greenwich Mean Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal to UTC (Coordinated Universal Time). This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for time zone, and <em class="bcp14">MUST</em> be assumed when reading the asctime format. HTTP-date is case sensitive and <em class="bcp14">MUST NOT</em> include additional LWS beyond that specifically included as SP in the grammar.</p><div id="rfc.figure.u.19"></div><pre class="inline"><span id="rfc.iref.g.26"></span><span id="rfc.iref.g.27"></span><span id="rfc.iref.g.28"></span><span id="rfc.iref.g.29"></span><span id="rfc.iref.g.30"></span><span id="rfc.iref.g.31"></span><span id="rfc.iref.g.32"></span><span id="rfc.iref.g.33"></span><span id="rfc.iref.g.34"></span><span id="rfc.iref.g.35"></span><span id="rfc.iref.g.36"></span>    <a href="#full.date" class="smpl" id="rfc.extref.h.7">HTTP-date</a>    = rfc1123-date | rfc850-date | asctime-date
     1294</pre><p id="rfc.section.3.3.1.p.3">The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123 <a href="#RFC1123" id="rfc.xref.RFC1123.1"><cite title="Requirements for Internet Hosts - Application and Support">[8]</cite></a> (an update to RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.3"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a>). The second format is in common use, but is based on the obsolete RFC 850 <a href="#RFC1036" id="rfc.xref.RFC1036.1"><cite title="Standard for interchange of USENET messages">[12]</cite></a> date format and lacks a four-digit year. HTTP/1.1 clients and servers that parse the date value <em class="bcp14">MUST</em> accept all three formats (for compatibility with HTTP/1.0), though they <em class="bcp14">MUST</em> only generate the RFC 1123 format for representing HTTP-date values in header fields. See <a href="#tolerant.applications" title="Tolerant Applications">Appendix&nbsp;19.3</a> for further information.
     1295      </p>
     1296      <dl class="empty">
     1297         <dd> <b>Note:</b> Recipients of date values are encouraged to be robust in accepting date values that may have been sent by non-HTTP applications,
     1298            as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.
     1299         </dd>
     1300      </dl>
     1301      <p id="rfc.section.3.3.1.p.5">All HTTP date/time stamps <em class="bcp14">MUST</em> be represented in Greenwich Mean Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal to UTC (Coordinated
     1302         Universal Time). This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for
     1303         time zone, and <em class="bcp14">MUST</em> be assumed when reading the asctime format. HTTP-date is case sensitive and <em class="bcp14">MUST NOT</em> include additional LWS beyond that specifically included as SP in the grammar.
     1304      </p>
     1305      <div id="rfc.figure.u.19"></div><pre class="inline"><span id="rfc.iref.g.26"></span><span id="rfc.iref.g.27"></span><span id="rfc.iref.g.28"></span><span id="rfc.iref.g.29"></span><span id="rfc.iref.g.30"></span><span id="rfc.iref.g.31"></span><span id="rfc.iref.g.32"></span><span id="rfc.iref.g.33"></span><span id="rfc.iref.g.34"></span><span id="rfc.iref.g.35"></span><span id="rfc.iref.g.36"></span>    <a href="#full.date" class="smpl" id="rfc.extref.h.7">HTTP-date</a>    = rfc1123-date | rfc850-date | asctime-date
    3821306    rfc1123-date = wkday "," <a href="#basic.rules" class="smpl" id="rfc.extref.s.7">SP</a> date1 <a href="#basic.rules" class="smpl" id="rfc.extref.s.8">SP</a> time <a href="#basic.rules" class="smpl" id="rfc.extref.s.9">SP</a> "GMT"
    3831307    rfc850-date  = weekday "," <a href="#basic.rules" class="smpl" id="rfc.extref.s.10">SP</a> date2 <a href="#basic.rules" class="smpl" id="rfc.extref.s.11">SP</a> time <a href="#basic.rules" class="smpl" id="rfc.extref.s.12">SP</a> "GMT"
     
    3981322                 | "May" | "Jun" | "Jul" | "Aug"
    3991323                 | "Sep" | "Oct" | "Nov" | "Dec"
    400 </pre><p id="rfc.section.3.3.1.p.7"> <b>Note:</b> HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers are not required to use these formats for user presentation, request logging, etc.</p><h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a id="delta.seconds" href="#delta.seconds">Delta Seconds</a></h3><p id="rfc.section.3.3.2.p.1">Some HTTP header fields allow a time value to be specified as an integer number of seconds, represented in decimal, after the time that the message was received.</p><div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.37"></span>    <a href="#delta.seconds" class="smpl">delta-seconds</a>  = 1*<a href="#basic.rules" class="smpl" id="rfc.extref.d.17">DIGIT</a>
    401 </pre><h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="character.sets" href="#character.sets">Character Sets</a></h2><p id="rfc.section.3.4.p.1">HTTP uses the same definition of the term "character set" as that described for MIME:</p><p id="rfc.section.3.4.p.2">The term "character set" is used in this document to refer to a method used with one or more tables to convert a sequence of octets into a sequence of characters. Note that unconditional conversion in the other direction is not required, in that not all characters may be available in a given character set and a character set may provide more than one sequence of octets to represent a particular character. This definition is intended to allow various kinds of character encoding, from simple single-table mappings such as US-ASCII to complex table switching methods such as those that use ISO-2022's techniques. However, the definition associated with a MIME character set name <em class="bcp14">MUST</em> fully specify the mapping to be performed from octets to characters. In particular, use of external profiling information to determine the exact mapping is not permitted.</p><dl class="empty"><dd> <b>Note:</b> This use of the term "character set" is more commonly referred to as a "character encoding." However, since HTTP and MIME share the same registry, it is important that the terminology also be shared.</dd></dl><div id="charset"><p id="rfc.section.3.4.p.4">  HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character Set registry <a href="#RFC1700" id="rfc.xref.RFC1700.2"><cite title="Assigned Numbers">[19]</cite></a>.</p></div><div id="rfc.figure.u.21"></div><pre class="inline"><span id="rfc.iref.g.38"></span>    <a href="#charset" class="smpl">charset</a> = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.5">token</a>
    402 </pre><p id="rfc.section.3.4.p.6">Although HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA Character Set registry <a href="#RFC1700" id="rfc.xref.RFC1700.3"><cite title="Assigned Numbers">[19]</cite></a>  <em class="bcp14">MUST</em> represent the character set defined by that registry. Applications <em class="bcp14">SHOULD</em> limit their use of character sets to those defined by the IANA registry.</p><p id="rfc.section.3.4.p.7">Implementors should be aware of IETF character set requirements <a href="#RFC2279" id="rfc.xref.RFC2279.1"><cite title="UTF-8, a transformation format of ISO 10646">[38]</cite></a>  <a href="#RFC2277" id="rfc.xref.RFC2277.1"><cite title="IETF Policy on Character Sets and Languages">[41]</cite></a>.</p><h3 id="rfc.section.3.4.1"><a href="#rfc.section.3.4.1">3.4.1</a>&nbsp;<a id="missing.charset" href="#missing.charset">Missing Charset</a></h3><p id="rfc.section.3.4.1.p.1">Some HTTP/1.0 software has interpreted a Content-Type header without charset parameter incorrectly to mean "recipient should guess." Senders wishing to defeat this behavior <em class="bcp14">MAY</em> include a charset parameter even when the charset is ISO-8859-1 and <em class="bcp14">SHOULD</em> do so when it is known that it will not confuse the recipient.</p><p id="rfc.section.3.4.1.p.2">Unfortunately, some older HTTP/1.0 clients did not deal properly with an explicit charset parameter. HTTP/1.1 recipients <em class="bcp14">MUST</em> respect the charset label provided by the sender; and those user agents that have a provision to "guess" a charset <em class="bcp14">MUST</em> use the charset from the content-type field if they support that charset, rather than the recipient's preference, when initially displaying a document. See <a href="#canonicalization.and.text.defaults" title="Canonicalization and Text Defaults">Section&nbsp;3.7.1</a>.</p><h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="content.codings" href="#content.codings">Content Codings</a></h2><div id="rfc.iref.c.6"></div><p id="rfc.section.3.5.p.1">Content coding values indicate an encoding transformation that has been or can be applied to an entity. Content codings are primarily used to allow a document to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Frequently, the entity is stored in coded form, transmitted directly, and only decoded by the recipient.</p><div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.39"></span>    <a href="#content.codings" class="smpl">content-coding</a>   = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.6">token</a>
    403 </pre><p id="rfc.section.3.5.p.3">All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (<a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.1" title="Accept-Encoding">Section&nbsp;14.3</a>) and Content-Encoding (<a href="#header.content-encoding" id="rfc.xref.header.content-encoding.1" title="Content-Encoding">Section&nbsp;14.11</a>) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding mechanism will be required to remove the encoding.</p><p id="rfc.section.3.5.p.4">The Internet Assigned Numbers Authority (IANA) acts as a registry for content-coding value tokens. Initially, the registry contains the following tokens:</p><p id="rfc.section.3.5.p.5">gzip<span id="rfc.iref.g.40"></span><span id="rfc.iref.c.7"></span>  </p><dl class="empty"><dd>An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 <a href="#RFC1952" id="rfc.xref.RFC1952.1"><cite title="GZIP file format specification version 4.3">[25]</cite></a>. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.</dd></dl><p id="rfc.section.3.5.p.6">compress<span id="rfc.iref.c.8"></span><span id="rfc.iref.c.9"></span>  </p><dl class="empty"><dd>The encoding format produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch coding (LZW).</dd><dd>Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings. Their use here is representative of historical practice, not good design. For compatibility with previous implementations of HTTP, applications <em class="bcp14">SHOULD</em> consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.</dd></dl><p id="rfc.section.3.5.p.7">deflate<span id="rfc.iref.d.2"></span><span id="rfc.iref.c.10"></span>  </p><dl class="empty"><dd>The "zlib" format defined in RFC 1950 <a href="#RFC1950" id="rfc.xref.RFC1950.1"><cite title="ZLIB Compressed Data Format Specification version 3.3">[31]</cite></a> in combination with the "deflate" compression mechanism described in RFC 1951 <a href="#RFC1951" id="rfc.xref.RFC1951.1"><cite title="DEFLATE Compressed Data Format Specification version 1.3">[29]</cite></a>.</dd></dl><p id="rfc.section.3.5.p.8">identity<span id="rfc.iref.i.2"></span><span id="rfc.iref.c.11"></span>  </p><dl class="empty"><dd>The default (identity) encoding; the use of no transformation whatsoever. This content-coding is used only in the Accept-Encoding header, and <em class="bcp14">SHOULD NOT</em> be used in the Content-Encoding header.</dd></dl><p id="rfc.section.3.5.p.9">New content-coding value tokens <em class="bcp14">SHOULD</em> be registered; to allow interoperability between clients and servers, specifications of the content coding algorithms needed to implement a new value <em class="bcp14">SHOULD</em> be publicly available and adequate for independent implementation, and conform to the purpose of content coding defined in this section.</p><h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="transfer.codings" href="#transfer.codings">Transfer Codings</a></h2><p id="rfc.section.3.6.p.1">Transfer-coding values are used to indicate an encoding transformation that has been, can be, or may need to be applied to an entity-body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer-coding is a property of the message, not of the original entity.</p><div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.41"></span><span id="rfc.iref.g.42"></span>    <a href="#transfer.codings" class="smpl">transfer-coding</a>         = "chunked" | transfer-extension
     1324</pre><p id="rfc.section.3.3.1.p.7"> <b>Note:</b> HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers
     1325         are not required to use these formats for user presentation, request logging, etc.
     1326      </p>
     1327      <h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a id="delta.seconds" href="#delta.seconds">Delta Seconds</a></h3>
     1328      <p id="rfc.section.3.3.2.p.1">Some HTTP header fields allow a time value to be specified as an integer number of seconds, represented in decimal, after
     1329         the time that the message was received.
     1330      </p>
     1331      <div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.37"></span>    <a href="#delta.seconds" class="smpl">delta-seconds</a>  = 1*<a href="#basic.rules" class="smpl" id="rfc.extref.d.17">DIGIT</a>
     1332</pre><h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="character.sets" href="#character.sets">Character Sets</a></h2>
     1333      <p id="rfc.section.3.4.p.1">HTTP uses the same definition of the term "character set" as that described for MIME:</p>
     1334      <p id="rfc.section.3.4.p.2">The term "character set" is used in this document to refer to a method used with one or more tables to convert a sequence
     1335         of octets into a sequence of characters. Note that unconditional conversion in the other direction is not required, in that
     1336         not all characters may be available in a given character set and a character set may provide more than one sequence of octets
     1337         to represent a particular character. This definition is intended to allow various kinds of character encoding, from simple
     1338         single-table mappings such as US-ASCII to complex table switching methods such as those that use ISO-2022's techniques. However,
     1339         the definition associated with a MIME character set name <em class="bcp14">MUST</em> fully specify the mapping to be performed from octets to characters. In particular, use of external profiling information
     1340         to determine the exact mapping is not permitted.
     1341      </p>
     1342      <dl class="empty">
     1343         <dd> <b>Note:</b> This use of the term "character set" is more commonly referred to as a "character encoding." However, since HTTP and MIME
     1344            share the same registry, it is important that the terminology also be shared.
     1345         </dd>
     1346      </dl>
     1347      <div id="charset">
     1348         <p id="rfc.section.3.4.p.4">  HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character
     1349            Set registry <a href="#RFC1700" id="rfc.xref.RFC1700.2"><cite title="Assigned Numbers">[19]</cite></a>.
     1350         </p>
     1351      </div>
     1352      <div id="rfc.figure.u.21"></div><pre class="inline"><span id="rfc.iref.g.38"></span>    <a href="#charset" class="smpl">charset</a> = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.5">token</a>
     1353</pre><p id="rfc.section.3.4.p.6">Although HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA
     1354         Character Set registry <a href="#RFC1700" id="rfc.xref.RFC1700.3"><cite title="Assigned Numbers">[19]</cite></a>  <em class="bcp14">MUST</em> represent the character set defined by that registry. Applications <em class="bcp14">SHOULD</em> limit their use of character sets to those defined by the IANA registry.
     1355      </p>
     1356      <p id="rfc.section.3.4.p.7">Implementors should be aware of IETF character set requirements <a href="#RFC2279" id="rfc.xref.RFC2279.1"><cite title="UTF-8, a transformation format of ISO 10646">[38]</cite></a>  <a href="#RFC2277" id="rfc.xref.RFC2277.1"><cite title="IETF Policy on Character Sets and Languages">[41]</cite></a>.
     1357      </p>
     1358      <h3 id="rfc.section.3.4.1"><a href="#rfc.section.3.4.1">3.4.1</a>&nbsp;<a id="missing.charset" href="#missing.charset">Missing Charset</a></h3>
     1359      <p id="rfc.section.3.4.1.p.1">Some HTTP/1.0 software has interpreted a Content-Type header without charset parameter incorrectly to mean "recipient should
     1360         guess." Senders wishing to defeat this behavior <em class="bcp14">MAY</em> include a charset parameter even when the charset is ISO-8859-1 and <em class="bcp14">SHOULD</em> do so when it is known that it will not confuse the recipient.
     1361      </p>
     1362      <p id="rfc.section.3.4.1.p.2">Unfortunately, some older HTTP/1.0 clients did not deal properly with an explicit charset parameter. HTTP/1.1 recipients <em class="bcp14">MUST</em> respect the charset label provided by the sender; and those user agents that have a provision to "guess" a charset <em class="bcp14">MUST</em> use the charset from the content-type field if they support that charset, rather than the recipient's preference, when initially
     1363         displaying a document. See <a href="#canonicalization.and.text.defaults" title="Canonicalization and Text Defaults">Section&nbsp;3.7.1</a>.
     1364      </p>
     1365      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="content.codings" href="#content.codings">Content Codings</a></h2>
     1366      <div id="rfc.iref.c.6"></div>
     1367      <p id="rfc.section.3.5.p.1">Content coding values indicate an encoding transformation that has been or can be applied to an entity. Content codings are
     1368         primarily used to allow a document to be compressed or otherwise usefully transformed without losing the identity of its underlying
     1369         media type and without loss of information. Frequently, the entity is stored in coded form, transmitted directly, and only
     1370         decoded by the recipient.
     1371      </p>
     1372      <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.39"></span>    <a href="#content.codings" class="smpl">content-coding</a>   = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.6">token</a>
     1373</pre><p id="rfc.section.3.5.p.3">All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (<a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.1" title="Accept-Encoding">Section&nbsp;14.3</a>) and Content-Encoding (<a href="#header.content-encoding" id="rfc.xref.header.content-encoding.1" title="Content-Encoding">Section&nbsp;14.11</a>) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding
     1374         mechanism will be required to remove the encoding.
     1375      </p>
     1376      <p id="rfc.section.3.5.p.4">The Internet Assigned Numbers Authority (IANA) acts as a registry for content-coding value tokens. Initially, the registry
     1377         contains the following tokens:
     1378      </p>
     1379      <p id="rfc.section.3.5.p.5">gzip<span id="rfc.iref.g.40"></span><span id="rfc.iref.c.7"></span> 
     1380      </p>
     1381      <dl class="empty">
     1382         <dd>An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 <a href="#RFC1952" id="rfc.xref.RFC1952.1"><cite title="GZIP file format specification version 4.3">[25]</cite></a>. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
     1383         </dd>
     1384      </dl>
     1385      <p id="rfc.section.3.5.p.6">compress<span id="rfc.iref.c.8"></span><span id="rfc.iref.c.9"></span> 
     1386      </p>
     1387      <dl class="empty">
     1388         <dd>The encoding format produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch
     1389            coding (LZW).
     1390         </dd>
     1391         <dd>Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings.
     1392            Their use here is representative of historical practice, not good design. For compatibility with previous implementations
     1393            of HTTP, applications <em class="bcp14">SHOULD</em> consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.
     1394         </dd>
     1395      </dl>
     1396      <p id="rfc.section.3.5.p.7">deflate<span id="rfc.iref.d.2"></span><span id="rfc.iref.c.10"></span> 
     1397      </p>
     1398      <dl class="empty">
     1399         <dd>The "zlib" format defined in RFC 1950 <a href="#RFC1950" id="rfc.xref.RFC1950.1"><cite title="ZLIB Compressed Data Format Specification version 3.3">[31]</cite></a> in combination with the "deflate" compression mechanism described in RFC 1951 <a href="#RFC1951" id="rfc.xref.RFC1951.1"><cite title="DEFLATE Compressed Data Format Specification version 1.3">[29]</cite></a>.
     1400         </dd>
     1401      </dl>
     1402      <p id="rfc.section.3.5.p.8">identity<span id="rfc.iref.i.2"></span><span id="rfc.iref.c.11"></span> 
     1403      </p>
     1404      <dl class="empty">
     1405         <dd>The default (identity) encoding; the use of no transformation whatsoever. This content-coding is used only in the Accept-Encoding
     1406            header, and <em class="bcp14">SHOULD NOT</em> be used in the Content-Encoding header.
     1407         </dd>
     1408      </dl>
     1409      <p id="rfc.section.3.5.p.9">New content-coding value tokens <em class="bcp14">SHOULD</em> be registered; to allow interoperability between clients and servers, specifications of the content coding algorithms needed
     1410         to implement a new value <em class="bcp14">SHOULD</em> be publicly available and adequate for independent implementation, and conform to the purpose of content coding defined in
     1411         this section.
     1412      </p>
     1413      <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="transfer.codings" href="#transfer.codings">Transfer Codings</a></h2>
     1414      <p id="rfc.section.3.6.p.1">Transfer-coding values are used to indicate an encoding transformation that has been, can be, or may need to be applied to
     1415         an entity-body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer-coding
     1416         is a property of the message, not of the original entity.
     1417      </p>
     1418      <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.41"></span><span id="rfc.iref.g.42"></span>    <a href="#transfer.codings" class="smpl">transfer-coding</a>         = "chunked" | transfer-extension
    4041419    transfer-extension      = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.8">token</a> *( ";" <a href="#rule.parameter" class="smpl">parameter</a> )
    405 </pre><div id="rule.parameter"><p id="rfc.section.3.6.p.3">  Parameters are in the form of attribute/value pairs.</p></div><div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.43"></span><span id="rfc.iref.g.44"></span><span id="rfc.iref.g.45"></span>    <a href="#rule.parameter" class="smpl">parameter</a>               = attribute "=" value
     1420</pre><div id="rule.parameter">
     1421         <p id="rfc.section.3.6.p.3">  Parameters are in the form of attribute/value pairs.</p>
     1422      </div>
     1423      <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.43"></span><span id="rfc.iref.g.44"></span><span id="rfc.iref.g.45"></span>    <a href="#rule.parameter" class="smpl">parameter</a>               = attribute "=" value
    4061424    attribute               = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.9">token</a>
    4071425    value                   = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.10">token</a> | <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.7">quoted-string</a>
    408 </pre><p id="rfc.section.3.6.p.5">All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer-coding values in the TE header field (<a href="#header.te" id="rfc.xref.header.te.1" title="TE">Section&nbsp;14.39</a>) and in the Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.1" title="Transfer-Encoding">Section&nbsp;14.41</a>).</p><p id="rfc.section.3.6.p.6">Whenever a transfer-coding is applied to a message-body, the set of transfer-codings <em class="bcp14">MUST</em> include "chunked", unless the message is terminated by closing the connection. When the "chunked" transfer-coding is used, it <em class="bcp14">MUST</em> be the last transfer-coding applied to the message-body. The "chunked" transfer-coding <em class="bcp14">MUST NOT</em> be applied more than once to a message-body. These rules allow the recipient to determine the transfer-length of the message (<a href="#message.length" title="Message Length">Section&nbsp;4.4</a>).</p><p id="rfc.section.3.6.p.7">Transfer-codings are analogous to the Content-Transfer-Encoding values of MIME <a href="#RFC2045" id="rfc.xref.RFC2045.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[7]</cite></a>, which were designed to enable safe transport of binary data over a 7-bit transport service. However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message-bodies is the difficulty in determining the exact body length (<a href="#entity.length" title="Entity Length">Section&nbsp;7.2.2</a>), or the desire to encrypt data over a shared transport.</p><p id="rfc.section.3.6.p.8">The Internet Assigned Numbers Authority (IANA) acts as a registry for transfer-coding value tokens. Initially, the registry contains the following tokens: "chunked" (<a href="#chunked.transfer.encoding" title="Chunked Transfer Coding">Section&nbsp;3.6.1</a>), "identity" (section 3.6.2), "gzip" (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>), "compress" (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>), and "deflate" (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>).</p><p id="rfc.section.3.6.p.9">New transfer-coding value tokens <em class="bcp14">SHOULD</em> be registered in the same way as new content-coding value tokens (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>).</p><p id="rfc.section.3.6.p.10">A server which receives an entity-body with a transfer-coding it does not understand <em class="bcp14">SHOULD</em> return <a href="#status.501" class="smpl">501 (Unimplemented)</a>, and close the connection. A server <em class="bcp14">MUST NOT</em> send transfer-codings to an HTTP/1.0 client.</p><h3 id="rfc.section.3.6.1"><a href="#rfc.section.3.6.1">3.6.1</a>&nbsp;<a id="chunked.transfer.encoding" href="#chunked.transfer.encoding">Chunked Transfer Coding</a></h3><p id="rfc.section.3.6.1.p.1">The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an <em class="bcp14">OPTIONAL</em> trailer containing entity-header fields. This allows dynamically produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.</p><div id="rfc.figure.u.25"></div><pre class="inline"><span id="rfc.iref.g.46"></span><span id="rfc.iref.g.47"></span><span id="rfc.iref.g.48"></span><span id="rfc.iref.g.49"></span><span id="rfc.iref.g.50"></span><span id="rfc.iref.g.51"></span><span id="rfc.iref.g.52"></span><span id="rfc.iref.g.53"></span><span id="rfc.iref.g.54"></span>    Chunked-Body   = *chunk
     1426</pre><p id="rfc.section.3.6.p.5">All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer-coding values in the TE header field (<a href="#header.te" id="rfc.xref.header.te.1" title="TE">Section&nbsp;14.39</a>) and in the Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.1" title="Transfer-Encoding">Section&nbsp;14.41</a>).
     1427      </p>
     1428      <p id="rfc.section.3.6.p.6">Whenever a transfer-coding is applied to a message-body, the set of transfer-codings <em class="bcp14">MUST</em> include "chunked", unless the message is terminated by closing the connection. When the "chunked" transfer-coding is used,
     1429         it <em class="bcp14">MUST</em> be the last transfer-coding applied to the message-body. The "chunked" transfer-coding <em class="bcp14">MUST NOT</em> be applied more than once to a message-body. These rules allow the recipient to determine the transfer-length of the message
     1430         (<a href="#message.length" title="Message Length">Section&nbsp;4.4</a>).
     1431      </p>
     1432      <p id="rfc.section.3.6.p.7">Transfer-codings are analogous to the Content-Transfer-Encoding values of MIME <a href="#RFC2045" id="rfc.xref.RFC2045.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[7]</cite></a>, which were designed to enable safe transport of binary data over a 7-bit transport service. However, safe transport has
     1433         a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message-bodies is the difficulty
     1434         in determining the exact body length (<a href="#entity.length" title="Entity Length">Section&nbsp;7.2.2</a>), or the desire to encrypt data over a shared transport.
     1435      </p>
     1436      <p id="rfc.section.3.6.p.8">The Internet Assigned Numbers Authority (IANA) acts as a registry for transfer-coding value tokens. Initially, the registry
     1437         contains the following tokens: "chunked" (<a href="#chunked.transfer.encoding" title="Chunked Transfer Coding">Section&nbsp;3.6.1</a>), "identity" (section 3.6.2), "gzip" (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>), "compress" (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>), and "deflate" (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>).
     1438      </p>
     1439      <p id="rfc.section.3.6.p.9">New transfer-coding value tokens <em class="bcp14">SHOULD</em> be registered in the same way as new content-coding value tokens (<a href="#content.codings" title="Content Codings">Section&nbsp;3.5</a>).
     1440      </p>
     1441      <p id="rfc.section.3.6.p.10">A server which receives an entity-body with a transfer-coding it does not understand <em class="bcp14">SHOULD</em> return <a href="#status.501" class="smpl">501 (Unimplemented)</a>, and close the connection. A server <em class="bcp14">MUST NOT</em> send transfer-codings to an HTTP/1.0 client.
     1442      </p>
     1443      <h3 id="rfc.section.3.6.1"><a href="#rfc.section.3.6.1">3.6.1</a>&nbsp;<a id="chunked.transfer.encoding" href="#chunked.transfer.encoding">Chunked Transfer Coding</a></h3>
     1444      <p id="rfc.section.3.6.1.p.1">The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size
     1445         indicator, followed by an <em class="bcp14">OPTIONAL</em> trailer containing entity-header fields. This allows dynamically produced content to be transferred along with the information
     1446         necessary for the recipient to verify that it has received the full message.
     1447      </p>
     1448      <div id="rfc.figure.u.25"></div><pre class="inline"><span id="rfc.iref.g.46"></span><span id="rfc.iref.g.47"></span><span id="rfc.iref.g.48"></span><span id="rfc.iref.g.49"></span><span id="rfc.iref.g.50"></span><span id="rfc.iref.g.51"></span><span id="rfc.iref.g.52"></span><span id="rfc.iref.g.53"></span><span id="rfc.iref.g.54"></span>    Chunked-Body   = *chunk
    4091449                     last-chunk
    4101450                     trailer
     
    4211461    chunk-data     = chunk-size(<a href="#basic.rules" class="smpl" id="rfc.extref.o.3">OCTET</a>)
    4221462    trailer        = *(<a href="#entity.header.fields" class="smpl">entity-header</a> <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.22">CRLF</a>)
    423 </pre><p id="rfc.section.3.6.1.p.3">The chunk-size field is a string of hex digits indicating the size of the chunk. The chunked encoding is ended by any chunk whose size is zero, followed by the trailer, which is terminated by an empty line.</p><p id="rfc.section.3.6.1.p.4">The trailer allows the sender to include additional HTTP header fields at the end of the message. The Trailer header field can be used to indicate which header fields are included in a trailer (see <a href="#header.trailer" id="rfc.xref.header.trailer.1" title="Trailer">Section&nbsp;14.40</a>).</p><p id="rfc.section.3.6.1.p.5">A server using chunked transfer-coding in a response <em class="bcp14">MUST NOT</em> use the trailer for any header fields unless at least one of the following is true: </p><ol><li>the request included a TE header field that indicates "trailers" is acceptable in the transfer-coding of the response, as described in <a href="#header.te" id="rfc.xref.header.te.2" title="TE">Section&nbsp;14.39</a>; or,</li><li>the server is the origin server for the response, the trailer fields consist entirely of optional metadata, and the recipient could use the message (in a manner acceptable to the origin server) without receiving this metadata. In other words, the origin server is willing to accept the possibility that the trailer fields might be silently discarded along the path to the client.</li></ol><p id="rfc.section.3.6.1.p.6">This requirement prevents an interoperability failure when the message is being received by an HTTP/1.1 (or later) proxy and forwarded to an HTTP/1.0 recipient. It avoids a situation where compliance with the protocol would have necessitated a possibly infinite buffer on the proxy.</p><p id="rfc.section.3.6.1.p.7">An example process for decoding a Chunked-Body is presented in <a href="#introduction.of.transfer-encoding" title="Introduction of Transfer-Encoding">Appendix&nbsp;19.4.6</a>.</p><p id="rfc.section.3.6.1.p.8">All HTTP/1.1 applications <em class="bcp14">MUST</em> be able to receive and decode the "chunked" transfer-coding, and <em class="bcp14">MUST</em> ignore chunk-extension extensions they do not understand.</p><h2 id="rfc.section.3.7"><a href="#rfc.section.3.7">3.7</a>&nbsp;<a id="media.types" href="#media.types">Media Types</a></h2><p id="rfc.section.3.7.p.1">HTTP uses Internet Media Types <a href="#RFC1590" id="rfc.xref.RFC1590.1"><cite title="Media Type Registration Procedure">[17]</cite></a> in the Content-Type (<a href="#header.content-type" id="rfc.xref.header.content-type.1" title="Content-Type">Section&nbsp;14.17</a>) and Accept (<a href="#header.accept" id="rfc.xref.header.accept.1" title="Accept">Section&nbsp;14.1</a>) header fields in order to provide open and extensible data typing and type negotiation.</p><div id="rfc.figure.u.26"></div><pre class="inline"><span id="rfc.iref.g.55"></span><span id="rfc.iref.g.56"></span><span id="rfc.iref.g.57"></span>    <a href="#media.types" class="smpl">media-type</a>     = <a href="#media.types" class="smpl">type</a> "/" <a href="#media.types" class="smpl">subtype</a> *( ";" <a href="#rule.parameter" class="smpl">parameter</a> )
     1463</pre><p id="rfc.section.3.6.1.p.3">The chunk-size field is a string of hex digits indicating the size of the chunk. The chunked encoding is ended by any chunk
     1464         whose size is zero, followed by the trailer, which is terminated by an empty line.
     1465      </p>
     1466      <p id="rfc.section.3.6.1.p.4">The trailer allows the sender to include additional HTTP header fields at the end of the message. The Trailer header field
     1467         can be used to indicate which header fields are included in a trailer (see <a href="#header.trailer" id="rfc.xref.header.trailer.1" title="Trailer">Section&nbsp;14.40</a>).
     1468      </p>
     1469      <p id="rfc.section.3.6.1.p.5">A server using chunked transfer-coding in a response <em class="bcp14">MUST NOT</em> use the trailer for any header fields unless at least one of the following is true:
     1470      </p>
     1471      <ol>
     1472         <li>the request included a TE header field that indicates "trailers" is acceptable in the transfer-coding of the response, as
     1473            described in <a href="#header.te" id="rfc.xref.header.te.2" title="TE">Section&nbsp;14.39</a>; or,
     1474         </li>
     1475         <li>the server is the origin server for the response, the trailer fields consist entirely of optional metadata, and the recipient
     1476            could use the message (in a manner acceptable to the origin server) without receiving this metadata. In other words, the origin
     1477            server is willing to accept the possibility that the trailer fields might be silently discarded along the path to the client.
     1478         </li>
     1479      </ol>
     1480      <p id="rfc.section.3.6.1.p.6">This requirement prevents an interoperability failure when the message is being received by an HTTP/1.1 (or later) proxy and
     1481         forwarded to an HTTP/1.0 recipient. It avoids a situation where compliance with the protocol would have necessitated a possibly
     1482         infinite buffer on the proxy.
     1483      </p>
     1484      <p id="rfc.section.3.6.1.p.7">An example process for decoding a Chunked-Body is presented in <a href="#introduction.of.transfer-encoding" title="Introduction of Transfer-Encoding">Appendix&nbsp;19.4.6</a>.
     1485      </p>
     1486      <p id="rfc.section.3.6.1.p.8">All HTTP/1.1 applications <em class="bcp14">MUST</em> be able to receive and decode the "chunked" transfer-coding, and <em class="bcp14">MUST</em> ignore chunk-extension extensions they do not understand.
     1487      </p>
     1488      <h2 id="rfc.section.3.7"><a href="#rfc.section.3.7">3.7</a>&nbsp;<a id="media.types" href="#media.types">Media Types</a></h2>
     1489      <p id="rfc.section.3.7.p.1">HTTP uses Internet Media Types <a href="#RFC1590" id="rfc.xref.RFC1590.1"><cite title="Media Type Registration Procedure">[17]</cite></a> in the Content-Type (<a href="#header.content-type" id="rfc.xref.header.content-type.1" title="Content-Type">Section&nbsp;14.17</a>) and Accept (<a href="#header.accept" id="rfc.xref.header.accept.1" title="Accept">Section&nbsp;14.1</a>) header fields in order to provide open and extensible data typing and type negotiation.
     1490      </p>
     1491      <div id="rfc.figure.u.26"></div><pre class="inline"><span id="rfc.iref.g.55"></span><span id="rfc.iref.g.56"></span><span id="rfc.iref.g.57"></span>    <a href="#media.types" class="smpl">media-type</a>     = <a href="#media.types" class="smpl">type</a> "/" <a href="#media.types" class="smpl">subtype</a> *( ";" <a href="#rule.parameter" class="smpl">parameter</a> )
    4241492    <a href="#media.types" class="smpl">type</a>           = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.15">token</a>
    4251493    <a href="#media.types" class="smpl">subtype</a>        = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.16">token</a>
    426 </pre><p id="rfc.section.3.7.p.3">Parameters <em class="bcp14">MAY</em> follow the type/subtype in the form of attribute/value pairs (as defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>).</p><p id="rfc.section.3.7.p.4">The type, subtype, and parameter attribute names are case-insensitive. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name. Linear white space (LWS) <em class="bcp14">MUST NOT</em> be used between the type and subtype, nor between an attribute and its value. The presence or absence of a parameter might be significant to the processing of a media-type, depending on its definition within the media type registry.</p><p id="rfc.section.3.7.p.5">Note that some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications, implementations <em class="bcp14">SHOULD</em> only use media type parameters when they are required by that type/subtype definition.</p><p id="rfc.section.3.7.p.6">Media-type values are registered with the Internet Assigned Number Authority (IANA <a href="#RFC1700" id="rfc.xref.RFC1700.4"><cite title="Assigned Numbers">[19]</cite></a>). The media type registration process is outlined in RFC 1590 <a href="#RFC1590" id="rfc.xref.RFC1590.2"><cite title="Media Type Registration Procedure">[17]</cite></a>. Use of non-registered media types is discouraged.</p><h3 id="rfc.section.3.7.1"><a href="#rfc.section.3.7.1">3.7.1</a>&nbsp;<a id="canonicalization.and.text.defaults" href="#canonicalization.and.text.defaults">Canonicalization and Text Defaults</a></h3><p id="rfc.section.3.7.1.p.1">Internet media types are registered with a canonical form. An entity-body transferred via HTTP messages <em class="bcp14">MUST</em> be represented in the appropriate canonical form prior to its transmission except for "text" types, as defined in the next paragraph.</p><p id="rfc.section.3.7.1.p.2">When in canonical form, media subtypes of the "text" type use CRLF as the text line break. HTTP relaxes this requirement and allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an entire entity-body. HTTP applications <em class="bcp14">MUST</em> accept CRLF, bare CR, and bare LF as being representative of a line break in text media received via HTTP. In addition, if the text is represented in a character set that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character sets, HTTP allows the use of whatever octet sequences are defined by that character set to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the entity-body; a bare CR or LF <em class="bcp14">MUST NOT</em> be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries).</p><p id="rfc.section.3.7.1.p.3">If an entity-body is encoded with a content-coding, the underlying data <em class="bcp14">MUST</em> be in a form defined above prior to being encoded.</p><p id="rfc.section.3.7.1.p.4">The "charset" parameter is used with some media types to define the character set (<a href="#character.sets" title="Character Sets">Section&nbsp;3.4</a>) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or its subsets <em class="bcp14">MUST</em> be labeled with an appropriate charset value. See <a href="#missing.charset" title="Missing Charset">Section&nbsp;3.4.1</a> for compatibility problems.</p><h3 id="rfc.section.3.7.2"><a href="#rfc.section.3.7.2">3.7.2</a>&nbsp;<a id="multipart.types" href="#multipart.types">Multipart Types</a></h3><p id="rfc.section.3.7.2.p.1">MIME provides for a number of "multipart" types -- encapsulations of one or more entities within a single message-body. All multipart types share a common syntax, as defined in section <a href="http://tools.ietf.org/html/rfc2046#section-5.1.1" id="rfc.xref.RFC2046.1">5.1.1</a> of RFC 2046 <a href="#RFC2046" id="rfc.xref.RFC2046.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[40]</cite></a>, and <em class="bcp14">MUST</em> include a boundary parameter as part of the media type value. The message body is itself a protocol element and <em class="bcp14">MUST</em> therefore use only CRLF to represent line breaks between body-parts. Unlike in RFC 2046, the epilogue of any multipart message <em class="bcp14">MUST</em> be empty; HTTP applications <em class="bcp14">MUST NOT</em> transmit the epilogue (even if the original multipart contains an epilogue). These restrictions exist in order to preserve the self-delimiting nature of a multipart message-body, wherein the "end" of the message-body is indicated by the ending multipart boundary.</p><p id="rfc.section.3.7.2.p.2">In general, HTTP treats a multipart message-body no differently than any other media type: strictly as payload. The one exception is the "multipart/byteranges" type (<a href="#internet.media.type.multipart.byteranges" title="Internet Media Type multipart/byteranges">Appendix&nbsp;19.2</a>) when it appears in a 206 (Partial Content) response, which will be interpreted by some HTTP caching mechanisms as described in sections <a href="#combining.byte.ranges" title="Combining Byte Ranges">13.5.4</a> and <a href="#header.content-range" id="rfc.xref.header.content-range.1" title="Content-Range">14.16</a>. In all other cases, an HTTP user agent <em class="bcp14">SHOULD</em> follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. The MIME header fields within each body-part of a multipart message-body do not have any significance to HTTP beyond that defined by their MIME semantics.</p><p id="rfc.section.3.7.2.p.3">In general, an HTTP user agent <em class="bcp14">SHOULD</em> follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application <em class="bcp14">MUST</em> treat it as being equivalent to "multipart/mixed".</p><dl class="empty"><dd> <b>Note:</b> The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in RFC 1867 <a href="#RFC1867" id="rfc.xref.RFC1867.1"><cite title="Form-based File Upload in HTML">[15]</cite></a>.</dd></dl><h2 id="rfc.section.3.8"><a href="#rfc.section.3.8">3.8</a>&nbsp;<a id="product.tokens" href="#product.tokens">Product Tokens</a></h2><p id="rfc.section.3.8.p.1">Product tokens are used to allow communicating applications to identify themselves by software name and version. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by white space. By convention, the products are listed in order of their significance for identifying the application.</p><div id="rfc.figure.u.27"></div><pre class="inline"><span id="rfc.iref.g.58"></span><span id="rfc.iref.g.59"></span>    <a href="#product.tokens" class="smpl">product</a>         = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.17">token</a> ["/" product-version]
     1494</pre><p id="rfc.section.3.7.p.3">Parameters <em class="bcp14">MAY</em> follow the type/subtype in the form of attribute/value pairs (as defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>).
     1495      </p>
     1496      <p id="rfc.section.3.7.p.4">The type, subtype, and parameter attribute names are case-insensitive. Parameter values might or might not be case-sensitive,
     1497         depending on the semantics of the parameter name. Linear white space (LWS) <em class="bcp14">MUST NOT</em> be used between the type and subtype, nor between an attribute and its value. The presence or absence of a parameter might
     1498         be significant to the processing of a media-type, depending on its definition within the media type registry.
     1499      </p>
     1500      <p id="rfc.section.3.7.p.5">Note that some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications,
     1501         implementations <em class="bcp14">SHOULD</em> only use media type parameters when they are required by that type/subtype definition.
     1502      </p>
     1503      <p id="rfc.section.3.7.p.6">Media-type values are registered with the Internet Assigned Number Authority (IANA <a href="#RFC1700" id="rfc.xref.RFC1700.4"><cite title="Assigned Numbers">[19]</cite></a>). The media type registration process is outlined in RFC 1590 <a href="#RFC1590" id="rfc.xref.RFC1590.2"><cite title="Media Type Registration Procedure">[17]</cite></a>. Use of non-registered media types is discouraged.
     1504      </p>
     1505      <h3 id="rfc.section.3.7.1"><a href="#rfc.section.3.7.1">3.7.1</a>&nbsp;<a id="canonicalization.and.text.defaults" href="#canonicalization.and.text.defaults">Canonicalization and Text Defaults</a></h3>
     1506      <p id="rfc.section.3.7.1.p.1">Internet media types are registered with a canonical form. An entity-body transferred via HTTP messages <em class="bcp14">MUST</em> be represented in the appropriate canonical form prior to its transmission except for "text" types, as defined in the next
     1507         paragraph.
     1508      </p>
     1509      <p id="rfc.section.3.7.1.p.2">When in canonical form, media subtypes of the "text" type use CRLF as the text line break. HTTP relaxes this requirement and
     1510         allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an
     1511         entire entity-body. HTTP applications <em class="bcp14">MUST</em> accept CRLF, bare CR, and bare LF as being representative of a line break in text media received via HTTP. In addition, if
     1512         the text is represented in a character set that does not use octets 13 and 10 for CR and LF respectively, as is the case for
     1513         some multi-byte character sets, HTTP allows the use of whatever octet sequences are defined by that character set to represent
     1514         the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the entity-body;
     1515         a bare CR or LF <em class="bcp14">MUST NOT</em> be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries).
     1516      </p>
     1517      <p id="rfc.section.3.7.1.p.3">If an entity-body is encoded with a content-coding, the underlying data <em class="bcp14">MUST</em> be in a form defined above prior to being encoded.
     1518      </p>
     1519      <p id="rfc.section.3.7.1.p.4">The "charset" parameter is used with some media types to define the character set (<a href="#character.sets" title="Character Sets">Section&nbsp;3.4</a>) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined
     1520         to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or
     1521         its subsets <em class="bcp14">MUST</em> be labeled with an appropriate charset value. See <a href="#missing.charset" title="Missing Charset">Section&nbsp;3.4.1</a> for compatibility problems.
     1522      </p>
     1523      <h3 id="rfc.section.3.7.2"><a href="#rfc.section.3.7.2">3.7.2</a>&nbsp;<a id="multipart.types" href="#multipart.types">Multipart Types</a></h3>
     1524      <p id="rfc.section.3.7.2.p.1">MIME provides for a number of "multipart" types -- encapsulations of one or more entities within a single message-body. All
     1525         multipart types share a common syntax, as defined in section <a href="http://tools.ietf.org/html/rfc2046#section-5.1.1" id="rfc.xref.RFC2046.1">5.1.1</a> of RFC 2046 <a href="#RFC2046" id="rfc.xref.RFC2046.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[40]</cite></a>, and <em class="bcp14">MUST</em> include a boundary parameter as part of the media type value. The message body is itself a protocol element and <em class="bcp14">MUST</em> therefore use only CRLF to represent line breaks between body-parts. Unlike in RFC 2046, the epilogue of any multipart message <em class="bcp14">MUST</em> be empty; HTTP applications <em class="bcp14">MUST NOT</em> transmit the epilogue (even if the original multipart contains an epilogue). These restrictions exist in order to preserve
     1526         the self-delimiting nature of a multipart message-body, wherein the "end" of the message-body is indicated by the ending multipart
     1527         boundary.
     1528      </p>
     1529      <p id="rfc.section.3.7.2.p.2">In general, HTTP treats a multipart message-body no differently than any other media type: strictly as payload. The one exception
     1530         is the "multipart/byteranges" type (<a href="#internet.media.type.multipart.byteranges" title="Internet Media Type multipart/byteranges">Appendix&nbsp;19.2</a>) when it appears in a 206 (Partial Content) response, which will be interpreted by some HTTP caching mechanisms as described
     1531         in sections <a href="#combining.byte.ranges" title="Combining Byte Ranges">13.5.4</a> and <a href="#header.content-range" id="rfc.xref.header.content-range.1" title="Content-Range">14.16</a>. In all other cases, an HTTP user agent <em class="bcp14">SHOULD</em> follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. The MIME header fields within
     1532         each body-part of a multipart message-body do not have any significance to HTTP beyond that defined by their MIME semantics.
     1533      </p>
     1534      <p id="rfc.section.3.7.2.p.3">In general, an HTTP user agent <em class="bcp14">SHOULD</em> follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives
     1535         an unrecognized multipart subtype, the application <em class="bcp14">MUST</em> treat it as being equivalent to "multipart/mixed".
     1536      </p>
     1537      <dl class="empty">
     1538         <dd> <b>Note:</b> The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request
     1539            method, as described in RFC 1867 <a href="#RFC1867" id="rfc.xref.RFC1867.1"><cite title="Form-based File Upload in HTML">[15]</cite></a>.
     1540         </dd>
     1541      </dl>
     1542      <h2 id="rfc.section.3.8"><a href="#rfc.section.3.8">3.8</a>&nbsp;<a id="product.tokens" href="#product.tokens">Product Tokens</a></h2>
     1543      <p id="rfc.section.3.8.p.1">Product tokens are used to allow communicating applications to identify themselves by software name and version. Most fields
     1544         using product tokens also allow sub-products which form a significant part of the application to be listed, separated by white
     1545         space. By convention, the products are listed in order of their significance for identifying the application.
     1546      </p>
     1547      <div id="rfc.figure.u.27"></div><pre class="inline"><span id="rfc.iref.g.58"></span><span id="rfc.iref.g.59"></span>    <a href="#product.tokens" class="smpl">product</a>         = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.17">token</a> ["/" product-version]
    4271548    product-version = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.18">token</a>
    428 </pre><p id="rfc.section.3.8.p.3">Examples:</p><div id="rfc.figure.u.28"></div><pre class="text">    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
     1549</pre><p id="rfc.section.3.8.p.3">Examples:</p>
     1550      <div id="rfc.figure.u.28"></div><pre class="text">    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
    4291551    Server: Apache/0.8.4
    430 </pre><p id="rfc.section.3.8.p.5">Product tokens <em class="bcp14">SHOULD</em> be short and to the point. They <em class="bcp14">MUST NOT</em> be used for advertising or other non-essential information. Although any token character <em class="bcp14">MAY</em> appear in a product-version, this token <em class="bcp14">SHOULD</em> only be used for a version identifier (i.e., successive versions of the same product <em class="bcp14">SHOULD</em> only differ in the product-version portion of the product value).</p><h2 id="rfc.section.3.9"><a href="#rfc.section.3.9">3.9</a>&nbsp;<a id="quality.values" href="#quality.values">Quality Values</a></h2><p id="rfc.section.3.9.p.1">HTTP content negotiation (<a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>) uses short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. If a parameter has a quality value of 0, then content with this parameter is `not acceptable' for the client. HTTP/1.1 applications <em class="bcp14">MUST NOT</em> generate more than three digits after the decimal point. User configuration of these values <em class="bcp14">SHOULD</em> also be limited in this fashion.</p><div id="rfc.figure.u.29"></div><pre class="inline"><span id="rfc.iref.g.60"></span>    <a href="#quality.values" class="smpl">qvalue</a>         = ( "0" [ "." 0*3<a href="#basic.rules" class="smpl" id="rfc.extref.d.18">DIGIT</a> ] )
     1552</pre><p id="rfc.section.3.8.p.5">Product tokens <em class="bcp14">SHOULD</em> be short and to the point. They <em class="bcp14">MUST NOT</em> be used for advertising or other non-essential information. Although any token character <em class="bcp14">MAY</em> appear in a product-version, this token <em class="bcp14">SHOULD</em> only be used for a version identifier (i.e., successive versions of the same product <em class="bcp14">SHOULD</em> only differ in the product-version portion of the product value).
     1553      </p>
     1554      <h2 id="rfc.section.3.9"><a href="#rfc.section.3.9">3.9</a>&nbsp;<a id="quality.values" href="#quality.values">Quality Values</a></h2>
     1555      <p id="rfc.section.3.9.p.1">HTTP content negotiation (<a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>) uses short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight
     1556         is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. If a parameter has
     1557         a quality value of 0, then content with this parameter is `not acceptable' for the client. HTTP/1.1 applications <em class="bcp14">MUST NOT</em> generate more than three digits after the decimal point. User configuration of these values <em class="bcp14">SHOULD</em> also be limited in this fashion.
     1558      </p>
     1559      <div id="rfc.figure.u.29"></div><pre class="inline"><span id="rfc.iref.g.60"></span>    <a href="#quality.values" class="smpl">qvalue</a>         = ( "0" [ "." 0*3<a href="#basic.rules" class="smpl" id="rfc.extref.d.18">DIGIT</a> ] )
    4311560                   | ( "1" [ "." 0*3("0") ] )
    432 </pre><p id="rfc.section.3.9.p.3">"Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.</p><h2 id="rfc.section.3.10"><a href="#rfc.section.3.10">3.10</a>&nbsp;<a id="language.tags" href="#language.tags">Language Tags</a></h2><p id="rfc.section.3.10.p.1">A language tag identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to other human beings. Computer languages are explicitly excluded. HTTP uses language tags within the Accept-Language and Content-Language fields.</p><p id="rfc.section.3.10.p.2">The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 <a href="#RFC1766" id="rfc.xref.RFC1766.1"><cite title="Tags for the Identification of Languages">[1]</cite></a>. In summary, a language tag is composed of 1 or more parts: A primary language tag and a possibly empty series of subtags:</p><div id="rfc.figure.u.30"></div><pre class="inline"><span id="rfc.iref.g.61"></span><span id="rfc.iref.g.62"></span><span id="rfc.iref.g.63"></span>     <a href="#language.tags" class="smpl">language-tag</a>  = primary-tag *( "-" subtag )
     1561</pre><p id="rfc.section.3.9.p.3">"Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.</p>
     1562      <h2 id="rfc.section.3.10"><a href="#rfc.section.3.10">3.10</a>&nbsp;<a id="language.tags" href="#language.tags">Language Tags</a></h2>
     1563      <p id="rfc.section.3.10.p.1">A language tag identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information
     1564         to other human beings. Computer languages are explicitly excluded. HTTP uses language tags within the Accept-Language and
     1565         Content-Language fields.
     1566      </p>
     1567      <p id="rfc.section.3.10.p.2">The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 <a href="#RFC1766" id="rfc.xref.RFC1766.1"><cite title="Tags for the Identification of Languages">[1]</cite></a>. In summary, a language tag is composed of 1 or more parts: A primary language tag and a possibly empty series of subtags:
     1568      </p>
     1569      <div id="rfc.figure.u.30"></div><pre class="inline"><span id="rfc.iref.g.61"></span><span id="rfc.iref.g.62"></span><span id="rfc.iref.g.63"></span>     <a href="#language.tags" class="smpl">language-tag</a>  = primary-tag *( "-" subtag )
    4331570     primary-tag   = 1*8<a href="#basic.rules" class="smpl" id="rfc.extref.a.3">ALPHA</a>
    4341571     subtag        = 1*8<a href="#basic.rules" class="smpl" id="rfc.extref.a.4">ALPHA</a>
    435 </pre><p id="rfc.section.3.10.p.4">White space is not allowed within the tag and all tags are case-insensitive. The name space of language tags is administered by the IANA. Example tags include:</p><div id="rfc.figure.u.31"></div><pre class="text">    en, en-US, en-cockney, i-cherokee, x-pig-latin
    436 </pre><p id="rfc.section.3.10.p.6">where any two-letter primary-tag is an ISO-639 language abbreviation and any two-letter initial subtag is an ISO-3166 country code. (The last three tags above are not registered tags; all but the last are examples of tags which could be registered in future.)</p><h2 id="rfc.section.3.11"><a href="#rfc.section.3.11">3.11</a>&nbsp;<a id="entity.tags" href="#entity.tags">Entity Tags</a></h2><p id="rfc.section.3.11.p.1">Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the ETag (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;14.19</a>), If-Match (<a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;14.24</a>), If-None-Match (<a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section&nbsp;14.26</a>), and If-Range (<a href="#header.if-range" id="rfc.xref.header.if-range.1" title="If-Range">Section&nbsp;14.27</a>) header fields. The definition of how they are used and compared as cache validators is in <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;13.3.3</a>. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.</p><div id="rfc.figure.u.32"></div><pre class="inline"><span id="rfc.iref.g.64"></span><span id="rfc.iref.g.65"></span><span id="rfc.iref.g.66"></span>   <a href="#entity.tags" class="smpl">entity-tag</a> = [ weak ] opaque-tag
     1572</pre><p id="rfc.section.3.10.p.4">White space is not allowed within the tag and all tags are case-insensitive. The name space of language tags is administered
     1573         by the IANA. Example tags include:
     1574      </p>
     1575      <div id="rfc.figure.u.31"></div><pre class="text">    en, en-US, en-cockney, i-cherokee, x-pig-latin
     1576</pre><p id="rfc.section.3.10.p.6">where any two-letter primary-tag is an ISO-639 language abbreviation and any two-letter initial subtag is an ISO-3166 country
     1577         code. (The last three tags above are not registered tags; all but the last are examples of tags which could be registered
     1578         in future.)
     1579      </p>
     1580      <h2 id="rfc.section.3.11"><a href="#rfc.section.3.11">3.11</a>&nbsp;<a id="entity.tags" href="#entity.tags">Entity Tags</a></h2>
     1581      <p id="rfc.section.3.11.p.1">Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the
     1582         ETag (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;14.19</a>), If-Match (<a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;14.24</a>), If-None-Match (<a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section&nbsp;14.26</a>), and If-Range (<a href="#header.if-range" id="rfc.xref.header.if-range.1" title="If-Range">Section&nbsp;14.27</a>) header fields. The definition of how they are used and compared as cache validators is in <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;13.3.3</a>. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
     1583      </p>
     1584      <div id="rfc.figure.u.32"></div><pre class="inline"><span id="rfc.iref.g.64"></span><span id="rfc.iref.g.65"></span><span id="rfc.iref.g.66"></span>   <a href="#entity.tags" class="smpl">entity-tag</a> = [ weak ] opaque-tag
    4371585   weak       = "W/"
    4381586   opaque-tag = <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.10">quoted-string</a>
    439 </pre><p id="rfc.section.3.11.p.3">A "strong entity tag" <em class="bcp14">MAY</em> be shared by two entities of a resource only if they are equivalent by octet equality.</p><p id="rfc.section.3.11.p.4">A "weak entity tag," indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no significant change in semantics. A weak entity tag can only be used for weak comparison.</p><p id="rfc.section.3.11.p.5">An entity tag <em class="bcp14">MUST</em> be unique across all versions of all entities associated with a particular resource. A given entity tag value <em class="bcp14">MAY</em> be used for entities obtained by requests on different URIs. The use of the same entity tag value in conjunction with entities obtained by requests on different URIs does not imply the equivalence of those entities.</p><h2 id="rfc.section.3.12"><a href="#rfc.section.3.12">3.12</a>&nbsp;<a id="range.units" href="#range.units">Range Units</a></h2><p id="rfc.section.3.12.p.1">HTTP/1.1 allows a client to request that only part (a range of) the response entity be included within the response. HTTP/1.1 uses range units in the Range (<a href="#header.range" id="rfc.xref.header.range.1" title="Range">Section&nbsp;14.35</a>) and Content-Range (<a href="#header.content-range" id="rfc.xref.header.content-range.2" title="Content-Range">Section&nbsp;14.16</a>) header fields. An entity can be broken down into subranges according to various structural units.</p><div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.67"></span><span id="rfc.iref.g.68"></span><span id="rfc.iref.g.69"></span>   <a href="#range.units" class="smpl">range-unit</a>       = bytes-unit | other-range-unit
     1587</pre><p id="rfc.section.3.11.p.3">A "strong entity tag" <em class="bcp14">MAY</em> be shared by two entities of a resource only if they are equivalent by octet equality.
     1588      </p>
     1589      <p id="rfc.section.3.11.p.4">A "weak entity tag," indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no
     1590         significant change in semantics. A weak entity tag can only be used for weak comparison.
     1591      </p>
     1592      <p id="rfc.section.3.11.p.5">An entity tag <em class="bcp14">MUST</em> be unique across all versions of all entities associated with a particular resource. A given entity tag value <em class="bcp14">MAY</em> be used for entities obtained by requests on different URIs. The use of the same entity tag value in conjunction with entities
     1593         obtained by requests on different URIs does not imply the equivalence of those entities.
     1594      </p>
     1595      <h2 id="rfc.section.3.12"><a href="#rfc.section.3.12">3.12</a>&nbsp;<a id="range.units" href="#range.units">Range Units</a></h2>
     1596      <p id="rfc.section.3.12.p.1">HTTP/1.1 allows a client to request that only part (a range of) the response entity be included within the response. HTTP/1.1
     1597         uses range units in the Range (<a href="#header.range" id="rfc.xref.header.range.1" title="Range">Section&nbsp;14.35</a>) and Content-Range (<a href="#header.content-range" id="rfc.xref.header.content-range.2" title="Content-Range">Section&nbsp;14.16</a>) header fields. An entity can be broken down into subranges according to various structural units.
     1598      </p>
     1599      <div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.67"></span><span id="rfc.iref.g.68"></span><span id="rfc.iref.g.69"></span>   <a href="#range.units" class="smpl">range-unit</a>       = bytes-unit | other-range-unit
    4401600   bytes-unit       = "bytes"
    4411601   other-range-unit = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.19">token</a>
    442 </pre><p id="rfc.section.3.12.p.3">The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 implementations <em class="bcp14">MAY</em> ignore ranges specified using other units.</p><p id="rfc.section.3.12.p.4">HTTP/1.1 has been designed to allow implementations of applications that do not depend on knowledge of ranges.</p><hr class="noprint"><h1 id="rfc.section.4" class="np"><a href="#rfc.section.4">4.</a>&nbsp;<a id="http.message" href="#http.message">HTTP Message</a></h1><h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="message.types" href="#message.types">Message Types</a></h2><p id="rfc.section.4.1.p.1">HTTP messages consist of requests from client to server and responses from server to client.</p><div id="rfc.figure.u.34"></div><pre class="inline"><span id="rfc.iref.g.70"></span>    HTTP-message   = <a href="#request" class="smpl">Request</a> | <a href="#response" class="smpl">Response</a>     ; HTTP/1.1 messages
    443 </pre><p id="rfc.section.4.1.p.3">Request (<a href="#request" title="Request">Section&nbsp;5</a>) and Response (<a href="#response" title="Response">Section&nbsp;6</a>) messages use the generic message format of RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.4"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a> for transferring entities (the payload of the message). Both types of message consist of a start-line, zero or more header fields (also known as "headers"), an empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header fields, and possibly a message-body.</p><div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.71"></span><span id="rfc.iref.g.72"></span>     generic-message = start-line
     1602</pre><p id="rfc.section.3.12.p.3">The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 implementations <em class="bcp14">MAY</em> ignore ranges specified using other units.
     1603      </p>
     1604      <p id="rfc.section.3.12.p.4">HTTP/1.1 has been designed to allow implementations of applications that do not depend on knowledge of ranges.</p>
     1605      <hr class="noprint">
     1606      <h1 id="rfc.section.4" class="np"><a href="#rfc.section.4">4.</a>&nbsp;<a id="http.message" href="#http.message">HTTP Message</a></h1>
     1607      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="message.types" href="#message.types">Message Types</a></h2>
     1608      <p id="rfc.section.4.1.p.1">HTTP messages consist of requests from client to server and responses from server to client.</p>
     1609      <div id="rfc.figure.u.34"></div><pre class="inline"><span id="rfc.iref.g.70"></span>    HTTP-message   = <a href="#request" class="smpl">Request</a> | <a href="#response" class="smpl">Response</a>     ; HTTP/1.1 messages
     1610</pre><p id="rfc.section.4.1.p.3">Request (<a href="#request" title="Request">Section&nbsp;5</a>) and Response (<a href="#response" title="Response">Section&nbsp;6</a>) messages use the generic message format of RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.4"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a> for transferring entities (the payload of the message). Both types of message consist of a start-line, zero or more header
     1611         fields (also known as "headers"), an empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header
     1612         fields, and possibly a message-body.
     1613      </p>
     1614      <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.71"></span><span id="rfc.iref.g.72"></span>     generic-message = start-line
    4441615                       *(<a href="#message.headers" class="smpl">message-header</a> <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.23">CRLF</a>)
    4451616                       <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.24">CRLF</a>
    4461617                       [ <a href="#message.body" class="smpl">message-body</a> ]
    4471618     start-line      = <a href="#request-line" class="smpl">Request-Line</a> | <a href="#status-line" class="smpl">Status-Line</a>
    448 </pre><p id="rfc.section.4.1.p.5">In the interest of robustness, servers <em class="bcp14">SHOULD</em> ignore any empty line(s) received where a Request-Line is expected. In other words, if the server is reading the protocol stream at the beginning of a message and receives a CRLF first, it should ignore the CRLF.</p><p id="rfc.section.4.1.p.6">Certain buggy HTTP/1.0 client implementations generate extra CRLF's after a POST request. To restate what is explicitly forbidden by the BNF, an HTTP/1.1 client <em class="bcp14">MUST NOT</em> preface or follow a request with an extra CRLF.</p><h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="message.headers" href="#message.headers">Message Headers</a></h2><p id="rfc.section.4.2.p.1">HTTP header fields, which include general-header (<a href="#general.header.fields" title="General Header Fields">Section&nbsp;4.5</a>), request-header (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5.3</a>), response-header (<a href="#response.header.fields" title="Response Header Fields">Section&nbsp;6.2</a>), and entity-header (<a href="#entity.header.fields" title="Entity Header Fields">Section&nbsp;7.1</a>) fields, follow the same generic format as that given in <a href="http://tools.ietf.org/html/rfc822#section-3.1" id="rfc.xref.RFC822.5">Section 3.1</a> of RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.6"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a>. Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive. The field value <em class="bcp14">MAY</em> be preceded by any amount of LWS, though a single SP is preferred. Header fields can be extended over multiple lines by preceding each extra line with at least one SP or HT. Applications ought to follow "common form", where one is known or indicated, when generating HTTP constructs, since there might exist some implementations that fail to accept anything beyond the common forms.</p><div id="rfc.figure.u.36"></div><pre class="inline"><span id="rfc.iref.g.73"></span><span id="rfc.iref.g.74"></span><span id="rfc.iref.g.75"></span><span id="rfc.iref.g.76"></span>    <a href="#message.headers" class="smpl">message-header</a> = <a href="#message.headers" class="smpl">field-name</a> ":" [ field-value ]
     1619</pre><p id="rfc.section.4.1.p.5">In the interest of robustness, servers <em class="bcp14">SHOULD</em> ignore any empty line(s) received where a Request-Line is expected. In other words, if the server is reading the protocol
     1620         stream at the beginning of a message and receives a CRLF first, it should ignore the CRLF.
     1621      </p>
     1622      <p id="rfc.section.4.1.p.6">Certain buggy HTTP/1.0 client implementations generate extra CRLF's after a POST request. To restate what is explicitly forbidden
     1623         by the BNF, an HTTP/1.1 client <em class="bcp14">MUST NOT</em> preface or follow a request with an extra CRLF.
     1624      </p>
     1625      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="message.headers" href="#message.headers">Message Headers</a></h2>
     1626      <p id="rfc.section.4.2.p.1">HTTP header fields, which include general-header (<a href="#general.header.fields" title="General Header Fields">Section&nbsp;4.5</a>), request-header (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5.3</a>), response-header (<a href="#response.header.fields" title="Response Header Fields">Section&nbsp;6.2</a>), and entity-header (<a href="#entity.header.fields" title="Entity Header Fields">Section&nbsp;7.1</a>) fields, follow the same generic format as that given in <a href="http://tools.ietf.org/html/rfc822#section-3.1" id="rfc.xref.RFC822.5">Section 3.1</a> of RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.6"><cite title="Standard for the format of ARPA Internet text messages">[9]</cite></a>. Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive. The
     1627         field value <em class="bcp14">MAY</em> be preceded by any amount of LWS, though a single SP is preferred. Header fields can be extended over multiple lines by preceding
     1628         each extra line with at least one SP or HT. Applications ought to follow "common form", where one is known or indicated, when
     1629         generating HTTP constructs, since there might exist some implementations that fail to accept anything beyond the common forms.
     1630      </p>
     1631      <div id="rfc.figure.u.36"></div><pre class="inline"><span id="rfc.iref.g.73"></span><span id="rfc.iref.g.74"></span><span id="rfc.iref.g.75"></span><span id="rfc.iref.g.76"></span>    <a href="#message.headers" class="smpl">message-header</a> = <a href="#message.headers" class="smpl">field-name</a> ":" [ field-value ]
    4491632    <a href="#message.headers" class="smpl">field-name</a>     = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.20">token</a>
    4501633    field-value    = *( field-content | <a href="#basic.rules.lws" class="smpl" id="rfc.extref.l.9">LWS</a> )
     
    4521635                     and consisting of either *<a href="#basic.rules.text" class="smpl" id="rfc.extref.t.21">TEXT</a> or combinations
    4531636                     of <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.22">token</a>, <a href="#basic.rules.token" class="smpl" id="rfc.extref.s.23">separators</a>, and <a href="#basic.rules.quoted-string" class="smpl" id="rfc.extref.q.11">quoted-string</a>&gt;
    454 </pre><p id="rfc.section.4.2.p.3">The field-content does not include any leading or trailing LWS: linear white space occurring before the first non-whitespace character of the field-value or after the last non-whitespace character of the field-value. Such leading or trailing LWS <em class="bcp14">MAY</em> be removed without changing the semantics of the field value. Any LWS that occurs between field-content <em class="bcp14">MAY</em> be replaced with a single SP before interpreting the field value or forwarding the message downstream.</p><p id="rfc.section.4.2.p.4">The order in which header fields with differing field names are received is not significant. However, it is "good practice" to send general-header fields first, followed by request-header or response-header fields, and ending with the entity-header fields.</p><p id="rfc.section.4.2.p.5">Multiple message-header fields with the same field-name <em class="bcp14">MAY</em> be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It <em class="bcp14">MUST</em> be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy <em class="bcp14">MUST NOT</em> change the order of these field values when a message is forwarded.</p><h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="message.body" href="#message.body">Message Body</a></h2><p id="rfc.section.4.3.p.1">The message-body (if any) of an HTTP message is used to carry the entity-body associated with the request or response. The message-body differs from the entity-body only when a transfer-coding has been applied, as indicated by the Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.2" title="Transfer-Encoding">Section&nbsp;14.41</a>).</p><div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.77"></span>    <a href="#message.body" class="smpl">message-body</a> = <a href="#entity.body" class="smpl">entity-body</a>
     1637</pre><p id="rfc.section.4.2.p.3">The field-content does not include any leading or trailing LWS: linear white space occurring before the first non-whitespace
     1638         character of the field-value or after the last non-whitespace character of the field-value. Such leading or trailing LWS <em class="bcp14">MAY</em> be removed without changing the semantics of the field value. Any LWS that occurs between field-content <em class="bcp14">MAY</em> be replaced with a single SP before interpreting the field value or forwarding the message downstream.
     1639      </p>
     1640      <p id="rfc.section.4.2.p.4">The order in which header fields with differing field names are received is not significant. However, it is "good practice"
     1641         to send general-header fields first, followed by request-header or response-header fields, and ending with the entity-header
     1642         fields.
     1643      </p>
     1644      <p id="rfc.section.4.2.p.5">Multiple message-header fields with the same field-name <em class="bcp14">MAY</em> be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e.,
     1645         #(values)]. It <em class="bcp14">MUST</em> be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics
     1646         of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header
     1647         fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and
     1648         thus a proxy <em class="bcp14">MUST NOT</em> change the order of these field values when a message is forwarded.
     1649      </p>
     1650      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="message.body" href="#message.body">Message Body</a></h2>
     1651      <p id="rfc.section.4.3.p.1">The message-body (if any) of an HTTP message is used to carry the entity-body associated with the request or response. The
     1652         message-body differs from the entity-body only when a transfer-coding has been applied, as indicated by the Transfer-Encoding
     1653         header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.2" title="Transfer-Encoding">Section&nbsp;14.41</a>).
     1654      </p>
     1655      <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.77"></span>    <a href="#message.body" class="smpl">message-body</a> = <a href="#entity.body" class="smpl">entity-body</a>
    4551656                 | &lt;<a href="#entity.body" class="smpl">entity-body</a> encoded as per Transfer-Encoding&gt;
    456 </pre><p id="rfc.section.4.3.p.3">Transfer-Encoding <em class="bcp14">MUST</em> be used to indicate any transfer-codings applied by an application to ensure safe and proper transfer of the message. Transfer-Encoding is a property of the message, not of the entity, and thus <em class="bcp14">MAY</em> be added or removed by any application along the request/response chain. (However, <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a> places restrictions on when certain transfer-codings may be used.)</p><p id="rfc.section.4.3.p.4">The rules for when a message-body is allowed in a message differ for requests and responses.</p><p id="rfc.section.4.3.p.5">The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field in the request's message-headers. A message-body <em class="bcp14">MUST NOT</em> be included in a request if the specification of the request method (<a href="#method" title="Method">Section&nbsp;5.1.1</a>) does not allow sending an entity-body in requests. A server <em class="bcp14">SHOULD</em> read and forward a message-body on any request; if the request method does not include defined semantics for an entity-body, then the message-body <em class="bcp14">SHOULD</em> be ignored when handling the request.</p><p id="rfc.section.4.3.p.6">For response messages, whether or not a message-body is included with a message is dependent on both the request method and the response status code (<a href="#status.code.and.reason.phrase" title="Status Code and Reason Phrase">Section&nbsp;6.1.1</a>). All responses to the HEAD request method <em class="bcp14">MUST NOT</em> include a message-body, even though the presence of entity-header fields might lead one to believe they do. All 1xx (informational), <a href="#status.204" class="smpl">204 (no content)</a>, and <a href="#status.304" class="smpl">304 (not modified)</a> responses <em class="bcp14">MUST NOT</em> include a message-body. All other responses do include a message-body, although it <em class="bcp14">MAY</em> be of zero length.</p><h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a id="message.length" href="#message.length">Message Length</a></h2><p id="rfc.section.4.4.p.1">The transfer-length of a message is the length of the message-body as it appears in the message; that is, after any transfer-codings have been applied. When a message-body is included with a message, the transfer-length of that body is determined by one of the following (in order of precedence):</p><p id="rfc.section.4.4.p.2"> </p><ol><li><p>Any response message which "<em class="bcp14">MUST NOT</em>" include a message-body (such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated by the first empty line after the header fields, regardless of the entity-header fields present in the message.</p></li><li><p>If a Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.3" title="Transfer-Encoding">Section&nbsp;14.41</a>) is present and has any value other than "identity", then the transfer-length is defined by use of the "chunked" transfer-coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>), unless the message is terminated by closing the connection.</p></li><li><p>If a Content-Length header field (<a href="#header.content-length" id="rfc.xref.header.content-length.1" title="Content-Length">Section&nbsp;14.13</a>) is present, its decimal value in <a href="#basic.rules" class="smpl" id="rfc.extref.o.5">OCTET</a>s represents both the entity-length and the transfer-length. The Content-Length header field <em class="bcp14">MUST NOT</em> be sent if these two lengths are different (i.e., if a Transfer-Encoding header field is present). If a message is received with both a Transfer-Encoding header field and a Content-Length header field, the latter <em class="bcp14">MUST</em> be ignored.</p></li><li><p>If the message uses the media type "multipart/byteranges", and the ransfer-length is not otherwise specified, then this self-elimiting media type defines the transfer-length. This media type UST NOT be used unless the sender knows that the recipient can arse it; the presence in a request of a Range header with ultiple byte-range specifiers from a 1.1 client implies that the lient can parse multipart/byteranges responses. </p><dl class="empty"><dd>A range header might be forwarded by a 1.0 proxy that does not understand multipart/byteranges; in this case the server <em class="bcp14">MUST</em> delimit the message using methods defined in items 1, 3 or 5 of this section.</dd></dl></li><li><p>By the server closing the connection. (Closing the connection cannot be used to indicate the end of a request body, since that would leave no possibility for the server to send back a response.)</p></li></ol><p id="rfc.section.4.4.p.3">For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing a message-body <em class="bcp14">MUST</em> include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. If a request contains a message-body and a Content-Length is not given, the server <em class="bcp14">SHOULD</em> respond with <a href="#status.400" class="smpl">400 (bad request)</a> if it cannot determine the length of the message, or with <a href="#status.411" class="smpl">411 (length required)</a> if it wishes to insist on receiving a valid Content-Length.</p><p id="rfc.section.4.4.p.4">All HTTP/1.1 applications that receive entities <em class="bcp14">MUST</em> accept the "chunked" transfer-coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>), thus allowing this mechanism to be used for messages when the message length cannot be determined in advance.</p><p id="rfc.section.4.4.p.5">Messages <em class="bcp14">MUST NOT</em> include both a Content-Length header field and a non-identity transfer-coding. If the message does include a non-identity transfer-coding, the Content-Length <em class="bcp14">MUST</em> be ignored.</p><p id="rfc.section.4.4.p.6">When a Content-Length is given in a message where a message-body is allowed, its field value <em class="bcp14">MUST</em> exactly match the number of <a href="#basic.rules" class="smpl" id="rfc.extref.o.6">OCTET</a>s in the message-body. HTTP/1.1 user agents <em class="bcp14">MUST</em> notify the user when an invalid length is received and detected.</p><h2 id="rfc.section.4.5"><a href="#rfc.section.4.5">4.5</a>&nbsp;<a id="general.header.fields" href="#general.header.fields">General Header Fields</a></h2><p id="rfc.section.4.5.p.1">There are a few header fields which have general applicability for both request and response messages, but which do not apply to the entity being transferred. These header fields apply only to the message being transmitted.</p><div id="rfc.figure.u.38"></div><pre class="inline"><span id="rfc.iref.g.78"></span>    <a href="#general.header.fields" class="smpl">general-header</a> = Cache-Control            ; <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;14.9</a>
     1657</pre><p id="rfc.section.4.3.p.3">Transfer-Encoding <em class="bcp14">MUST</em> be used to indicate any transfer-codings applied by an application to ensure safe and proper transfer of the message. Transfer-Encoding
     1658         is a property of the message, not of the entity, and thus <em class="bcp14">MAY</em> be added or removed by any application along the request/response chain. (However, <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a> places restrictions on when certain transfer-codings may be used.)
     1659      </p>
     1660      <p id="rfc.section.4.3.p.4">The rules for when a message-body is allowed in a message differ for requests and responses.</p>
     1661      <p id="rfc.section.4.3.p.5">The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field
     1662         in the request's message-headers. A message-body <em class="bcp14">MUST NOT</em> be included in a request if the specification of the request method (<a href="#method" title="Method">Section&nbsp;5.1.1</a>) does not allow sending an entity-body in requests. A server <em class="bcp14">SHOULD</em> read and forward a message-body on any request; if the request method does not include defined semantics for an entity-body,
     1663         then the message-body <em class="bcp14">SHOULD</em> be ignored when handling the request.
     1664      </p>
     1665      <p id="rfc.section.4.3.p.6">For response messages, whether or not a message-body is included with a message is dependent on both the request method and
     1666         the response status code (<a href="#status.code.and.reason.phrase" title="Status Code and Reason Phrase">Section&nbsp;6.1.1</a>). All responses to the HEAD request method <em class="bcp14">MUST NOT</em> include a message-body, even though the presence of entity-header fields might lead one to believe they do. All 1xx (informational), <a href="#status.204" class="smpl">204 (no content)</a>, and <a href="#status.304" class="smpl">304 (not modified)</a> responses <em class="bcp14">MUST NOT</em> include a message-body. All other responses do include a message-body, although it <em class="bcp14">MAY</em> be of zero length.
     1667      </p>
     1668      <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a id="message.length" href="#message.length">Message Length</a></h2>
     1669      <p id="rfc.section.4.4.p.1">The transfer-length of a message is the length of the message-body as it appears in the message; that is, after any transfer-codings
     1670         have been applied. When a message-body is included with a message, the transfer-length of that body is determined by one of
     1671         the following (in order of precedence):
     1672      </p>
     1673      <p id="rfc.section.4.4.p.2"> </p>
     1674      <ol>
     1675         <li>
     1676            <p>Any response message which "<em class="bcp14">MUST NOT</em>" include a message-body (such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated
     1677               by the first empty line after the header fields, regardless of the entity-header fields present in the message.
     1678            </p>
     1679         </li>
     1680         <li>
     1681            <p>If a Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.3" title="Transfer-Encoding">Section&nbsp;14.41</a>) is present and has any value other than "identity", then the transfer-length is defined by use of the "chunked" transfer-coding
     1682               (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>), unless the message is terminated by closing the connection.
     1683            </p>
     1684         </li>
     1685         <li>
     1686            <p>If a Content-Length header field (<a href="#header.content-length" id="rfc.xref.header.content-length.1" title="Content-Length">Section&nbsp;14.13</a>) is present, its decimal value in <a href="#basic.rules" class="smpl" id="rfc.extref.o.5">OCTET</a>s represents both the entity-length and the transfer-length. The Content-Length header field <em class="bcp14">MUST NOT</em> be sent if these two lengths are different (i.e., if a Transfer-Encoding header field is present). If a message is received
     1687               with both a Transfer-Encoding header field and a Content-Length header field, the latter <em class="bcp14">MUST</em> be ignored.
     1688            </p>
     1689         </li>
     1690         <li>
     1691            <p>If the message uses the media type "multipart/byteranges", and the ransfer-length is not otherwise specified, then this self-elimiting
     1692               media type defines the transfer-length. This media type UST NOT be used unless the sender knows that the recipient can arse
     1693               it; the presence in a request of a Range header with ultiple byte-range specifiers from a 1.1 client implies that the lient
     1694               can parse multipart/byteranges responses.
     1695            </p>
     1696            <dl class="empty">
     1697               <dd>A range header might be forwarded by a 1.0 proxy that does not understand multipart/byteranges; in this case the server <em class="bcp14">MUST</em> delimit the message using methods defined in items 1, 3 or 5 of this section.
     1698               </dd>
     1699            </dl>
     1700         </li>
     1701         <li>
     1702            <p>By the server closing the connection. (Closing the connection cannot be used to indicate the end of a request body, since
     1703               that would leave no possibility for the server to send back a response.)
     1704            </p>
     1705         </li>
     1706      </ol>
     1707      <p id="rfc.section.4.4.p.3">For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing a message-body <em class="bcp14">MUST</em> include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. If a request contains a message-body
     1708         and a Content-Length is not given, the server <em class="bcp14">SHOULD</em> respond with <a href="#status.400" class="smpl">400 (bad request)</a> if it cannot determine the length of the message, or with <a href="#status.411" class="smpl">411 (length required)</a> if it wishes to insist on receiving a valid Content-Length.
     1709      </p>
     1710      <p id="rfc.section.4.4.p.4">All HTTP/1.1 applications that receive entities <em class="bcp14">MUST</em> accept the "chunked" transfer-coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>), thus allowing this mechanism to be used for messages when the message length cannot be determined in advance.
     1711      </p>
     1712      <p id="rfc.section.4.4.p.5">Messages <em class="bcp14">MUST NOT</em> include both a Content-Length header field and a non-identity transfer-coding. If the message does include a non-identity
     1713         transfer-coding, the Content-Length <em class="bcp14">MUST</em> be ignored.
     1714      </p>
     1715      <p id="rfc.section.4.4.p.6">When a Content-Length is given in a message where a message-body is allowed, its field value <em class="bcp14">MUST</em> exactly match the number of <a href="#basic.rules" class="smpl" id="rfc.extref.o.6">OCTET</a>s in the message-body. HTTP/1.1 user agents <em class="bcp14">MUST</em> notify the user when an invalid length is received and detected.
     1716      </p>
     1717      <h2 id="rfc.section.4.5"><a href="#rfc.section.4.5">4.5</a>&nbsp;<a id="general.header.fields" href="#general.header.fields">General Header Fields</a></h2>
     1718      <p id="rfc.section.4.5.p.1">There are a few header fields which have general applicability for both request and response messages, but which do not apply
     1719         to the entity being transferred. These header fields apply only to the message being transmitted.
     1720      </p>
     1721      <div id="rfc.figure.u.38"></div><pre class="inline"><span id="rfc.iref.g.78"></span>    <a href="#general.header.fields" class="smpl">general-header</a> = Cache-Control            ; <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;14.9</a>
    4571722                   | Connection               ; <a href="#header.connection" id="rfc.xref.header.connection.1" title="Connection">Section&nbsp;14.10</a>
    4581723                   | Date                     ; <a href="#header.date" id="rfc.xref.header.date.1" title="Date">Section&nbsp;14.18</a>
     
    4631728                   | Via                      ; <a href="#header.via" id="rfc.xref.header.via.1" title="Via">Section&nbsp;14.45</a>
    4641729                   | Warning                  ; <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;14.46</a>
    465 </pre><p id="rfc.section.4.5.p.3">General-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields may be given the semantics of general header fields if all parties in the communication recognize them to be general-header fields. Unrecognized header fields are treated as entity-header fields.</p><hr class="noprint"><h1 id="rfc.section.5" class="np"><a href="#rfc.section.5">5.</a>&nbsp;<a id="request" href="#request">Request</a></h1><p id="rfc.section.5.p.1">A request message from a client to a server includes, within the first line of that message, the method to be applied to the resource, the identifier of the resource, and the protocol version in use.</p><div id="rfc.figure.u.39"></div><pre class="inline"><span id="rfc.iref.g.79"></span>     <a href="#request" class="smpl">Request</a>       = <a href="#request-line" class="smpl">Request-Line</a>              ; <a href="#request-line" title="Request-Line">Section&nbsp;5.1</a>
     1730</pre><p id="rfc.section.4.5.p.3">General-header field names can be extended reliably only in combination with a change in the protocol version. However, new
     1731         or experimental header fields may be given the semantics of general header fields if all parties in the communication recognize
     1732         them to be general-header fields. Unrecognized header fields are treated as entity-header fields.
     1733      </p>
     1734      <hr class="noprint">
     1735      <h1 id="rfc.section.5" class="np"><a href="#rfc.section.5">5.</a>&nbsp;<a id="request" href="#request">Request</a></h1>
     1736      <p id="rfc.section.5.p.1">A request message from a client to a server includes, within the first line of that message, the method to be applied to the
     1737         resource, the identifier of the resource, and the protocol version in use.
     1738      </p>
     1739      <div id="rfc.figure.u.39"></div><pre class="inline"><span id="rfc.iref.g.79"></span>     <a href="#request" class="smpl">Request</a>       = <a href="#request-line" class="smpl">Request-Line</a>              ; <a href="#request-line" title="Request-Line">Section&nbsp;5.1</a>
    4661740                     *(( <a href="#general.header.fields" class="smpl">general-header</a>        ; <a href="#general.header.fields" title="General Header Fields">Section&nbsp;4.5</a>
    4671741                      | <a href="#request.header.fields" class="smpl">request-header</a>         ; <a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5.3</a>
     
    4691743                     <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.26">CRLF</a>
    4701744                     [ <a href="#message.body" class="smpl">message-body</a> ]          ; <a href="#message.body" title="Message Body">Section&nbsp;4.3</a>
    471 </pre><h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="request-line" href="#request-line">Request-Line</a></h2><p id="rfc.section.5.1.p.1">The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending with CRLF. The elements are separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.</p><div id="rfc.figure.u.40"></div><pre class="inline"><span id="rfc.iref.g.80"></span>     <a href="#request-line" class="smpl">Request-Line</a>   = <a href="#method" class="smpl">Method</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.24">SP</a> <a href="#request-uri" class="smpl">Request-URI</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.25">SP</a> <a href="#http.version" class="smpl" id="rfc.extref.h.9">HTTP-Version</a> <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.27">CRLF</a>
    472 </pre><h3 id="rfc.section.5.1.1"><a href="#rfc.section.5.1.1">5.1.1</a>&nbsp;<a id="method" href="#method">Method</a></h3><p id="rfc.section.5.1.1.p.1">The Method token indicates the method to be performed on the resource identified by the Request-URI. The method is case-sensitive.</p><div id="rfc.figure.u.41"></div><pre class="inline"><span id="rfc.iref.g.81"></span><span id="rfc.iref.g.82"></span>    Method         = "OPTIONS"                ; <a href="#OPTIONS" id="rfc.xref.OPTIONS.1" title="OPTIONS">Section&nbsp;9.2</a>
     1745</pre><h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="request-line" href="#request-line">Request-Line</a></h2>
     1746      <p id="rfc.section.5.1.p.1">The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending with CRLF. The
     1747         elements are separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.
     1748      </p>
     1749      <div id="rfc.figure.u.40"></div><pre class="inline"><span id="rfc.iref.g.80"></span>     <a href="#request-line" class="smpl">Request-Line</a>   = <a href="#method" class="smpl">Method</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.24">SP</a> <a href="#request-uri" class="smpl">Request-URI</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.25">SP</a> <a href="#http.version" class="smpl" id="rfc.extref.h.9">HTTP-Version</a> <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.27">CRLF</a>
     1750</pre><h3 id="rfc.section.5.1.1"><a href="#rfc.section.5.1.1">5.1.1</a>&nbsp;<a id="method" href="#method">Method</a></h3>
     1751      <p id="rfc.section.5.1.1.p.1">The Method token indicates the method to be performed on the resource identified by the Request-URI. The method is case-sensitive.</p>
     1752      <div id="rfc.figure.u.41"></div><pre class="inline"><span id="rfc.iref.g.81"></span><span id="rfc.iref.g.82"></span>    Method         = "OPTIONS"                ; <a href="#OPTIONS" id="rfc.xref.OPTIONS.1" title="OPTIONS">Section&nbsp;9.2</a>
    4731753                   | "GET"                    ; <a href="#GET" id="rfc.xref.GET.1" title="GET">Section&nbsp;9.3</a>
    4741754                   | "HEAD"                   ; <a href="#HEAD" id="rfc.xref.HEAD.1" title="HEAD">Section&nbsp;9.4</a>
     
    4801760                   | extension-method
    4811761    extension-method = <a href="#basic.rules.token" class="smpl" id="rfc.extref.t.23">token</a>
    482 </pre><p id="rfc.section.5.1.1.p.3">The list of methods allowed by a resource can be specified in an Allow header field (<a href="#header.allow" id="rfc.xref.header.allow.1" title="Allow">Section&nbsp;14.7</a>). The return code of the response always notifies the client whether a method is currently allowed on a resource, since the set of allowed methods can change dynamically. An origin server <em class="bcp14">SHOULD</em> return the status code <a href="#status.405" class="smpl">405 (Method Not Allowed)</a> if the method is known by the origin server but not allowed for the requested resource, and <a href="#status.501" class="smpl">501 (Not Implemented)</a> if the method is unrecognized or not implemented by the origin server. The methods GET and HEAD <em class="bcp14">MUST</em> be supported by all general-purpose servers. All other methods are <em class="bcp14">OPTIONAL</em>; however, if the above methods are implemented, they <em class="bcp14">MUST</em> be implemented with the same semantics as those specified in <a href="#method.definitions" title="Method Definitions">Section&nbsp;9</a>.</p><h3 id="rfc.section.5.1.2"><a href="#rfc.section.5.1.2">5.1.2</a>&nbsp;<a id="request-uri" href="#request-uri">Request-URI</a></h3><p id="rfc.section.5.1.2.p.1">The Request-URI is a Uniform Resource Identifier (<a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;3.2</a>) and identifies the resource upon which to apply the request.</p><div id="rfc.figure.u.42"></div><pre class="inline"><span id="rfc.iref.g.83"></span>    <a href="#request-uri" class="smpl">Request-URI</a>    = "*" | <a href="#general.syntax" class="smpl">absoluteURI</a> | abs_path | authority
    483 </pre><p id="rfc.section.5.1.2.p.3">The four options for Request-URI are dependent on the nature of the request. The asterisk "*" means that the request does not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily apply to a resource. One example would be</p><div id="rfc.figure.u.43"></div><pre class="text">    OPTIONS * HTTP/1.1
    484 </pre><p id="rfc.section.5.1.2.p.5">The absoluteURI form is <em class="bcp14">REQUIRED</em> when the request is being made to a proxy. The proxy is requested to forward the request or service it from a valid cache, and return the response. Note that the proxy <em class="bcp14">MAY</em> forward the request on to another proxy or directly to the server specified by the absoluteURI. In order to avoid request loops, a proxy <em class="bcp14">MUST</em> be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. An example Request-Line would be:</p><div id="rfc.figure.u.44"></div><pre class="text">    GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
    485 </pre><p id="rfc.section.5.1.2.p.7">To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers <em class="bcp14">MUST</em> accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies.</p><p id="rfc.section.5.1.2.p.8">The authority form is only used by the CONNECT method (<a href="#CONNECT" id="rfc.xref.CONNECT.2" title="CONNECT">Section&nbsp;9.9</a>).</p><p id="rfc.section.5.1.2.p.9">The most common form of Request-URI is that used to identify a resource on an origin server or gateway. In this case the absolute path of the URI <em class="bcp14">MUST</em> be transmitted (see <a href="#general.syntax" title="General Syntax">Section&nbsp;3.2.1</a>, abs_path) as the Request-URI, and the network location of the URI (authority) <em class="bcp14">MUST</em> be transmitted in a Host header field. For example, a client wishing to retrieve the resource above directly from the origin server would create a TCP connection to port 80 of the host "www.w3.org" and send the lines:</p><div id="rfc.figure.u.45"></div><pre class="text">    GET /pub/WWW/TheProject.html HTTP/1.1
     1762</pre><p id="rfc.section.5.1.1.p.3">The list of methods allowed by a resource can be specified in an Allow header field (<a href="#header.allow" id="rfc.xref.header.allow.1" title="Allow">Section&nbsp;14.7</a>). The return code of the response always notifies the client whether a method is currently allowed on a resource, since the
     1763         set of allowed methods can change dynamically. An origin server <em class="bcp14">SHOULD</em> return the status code <a href="#status.405" class="smpl">405 (Method Not Allowed)</a> if the method is known by the origin server but not allowed for the requested resource, and <a href="#status.501" class="smpl">501 (Not Implemented)</a> if the method is unrecognized or not implemented by the origin server. The methods GET and HEAD <em class="bcp14">MUST</em> be supported by all general-purpose servers. All other methods are <em class="bcp14">OPTIONAL</em>; however, if the above methods are implemented, they <em class="bcp14">MUST</em> be implemented with the same semantics as those specified in <a href="#method.definitions" title="Method Definitions">Section&nbsp;9</a>.
     1764      </p>
     1765      <h3 id="rfc.section.5.1.2"><a href="#rfc.section.5.1.2">5.1.2</a>&nbsp;<a id="request-uri" href="#request-uri">Request-URI</a></h3>
     1766      <p id="rfc.section.5.1.2.p.1">The Request-URI is a Uniform Resource Identifier (<a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;3.2</a>) and identifies the resource upon which to apply the request.
     1767      </p>
     1768      <div id="rfc.figure.u.42"></div><pre class="inline"><span id="rfc.iref.g.83"></span>    <a href="#request-uri" class="smpl">Request-URI</a>    = "*" | <a href="#general.syntax" class="smpl">absoluteURI</a> | abs_path | authority
     1769</pre><p id="rfc.section.5.1.2.p.3">The four options for Request-URI are dependent on the nature of the request. The asterisk "*" means that the request does
     1770         not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily
     1771         apply to a resource. One example would be
     1772      </p>
     1773      <div id="rfc.figure.u.43"></div><pre class="text">    OPTIONS * HTTP/1.1
     1774</pre><p id="rfc.section.5.1.2.p.5">The absoluteURI form is <em class="bcp14">REQUIRED</em> when the request is being made to a proxy. The proxy is requested to forward the request or service it from a valid cache,
     1775         and return the response. Note that the proxy <em class="bcp14">MAY</em> forward the request on to another proxy or directly to the server specified by the absoluteURI. In order to avoid request
     1776         loops, a proxy <em class="bcp14">MUST</em> be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. An example
     1777         Request-Line would be:
     1778      </p>
     1779      <div id="rfc.figure.u.44"></div><pre class="text">    GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
     1780</pre><p id="rfc.section.5.1.2.p.7">To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers <em class="bcp14">MUST</em> accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies.
     1781      </p>
     1782      <p id="rfc.section.5.1.2.p.8">The authority form is only used by the CONNECT method (<a href="#CONNECT" id="rfc.xref.CONNECT.2" title="CONNECT">Section&nbsp;9.9</a>).
     1783      </p>
     1784      <p id="rfc.section.5.1.2.p.9">The most common form of Request-URI is that used to identify a resource on an origin server or gateway. In this case the absolute
     1785         path of the URI <em class="bcp14">MUST</em> be transmitted (see <a href="#general.syntax" title="General Syntax">Section&nbsp;3.2.1</a>, abs_path) as the Request-URI, and the network location of the URI (authority) <em class="bcp14">MUST</em> be transmitted in a Host header field. For example, a client wishing to retrieve the resource above directly from the origin
     1786         server would create a TCP connection to port 80 of the host "www.w3.org" and send the lines:
     1787      </p>
     1788      <div id="rfc.figure.u.45"></div><pre class="text">    GET /pub/WWW/TheProject.html HTTP/1.1
    4861789    Host: www.w3.org
    487 </pre><p id="rfc.section.5.1.2.p.11">followed by the remainder of the Request. Note that the absolute path cannot be empty; if none is present in the original URI, it <em class="bcp14">MUST</em> be given as "/" (the server root).</p><p id="rfc.section.5.1.2.p.12">The Request-URI is transmitted in the format specified in <a href="#general.syntax" title="General Syntax">Section&nbsp;3.2.1</a>. If the Request-URI is encoded using the "% HEX HEX" encoding <a href="#RFC2396" id="rfc.xref.RFC2396.3"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[42]</cite></a>, the origin server <em class="bcp14">MUST</em> decode the Request-URI in order to properly interpret the request. Servers <em class="bcp14">SHOULD</em> respond to invalid Request-URIs with an appropriate status code.</p><p id="rfc.section.5.1.2.p.13">A transparent proxy <em class="bcp14">MUST NOT</em> rewrite the "abs_path" part of the received Request-URI when forwarding it to the next inbound server, except as noted above to replace a null abs_path with "/".</p><p id="rfc.section.5.1.2.p.14"> </p><dl class="empty"><dd> <b>Note:</b> The "no rewrite" rule prevents the proxy from changing the meaning of the request when the origin server is improperly using a non-reserved URI character for a reserved purpose. Implementors should be aware that some pre-HTTP/1.1 proxies have been known to rewrite the Request-URI.</dd></dl><h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="the.resource.identified.by.a.request" href="#the.resource.identified.by.a.request">The Resource Identified by a Request</a></h2><p id="rfc.section.5.2.p.1">The exact resource identified by an Internet request is determined by examining both the Request-URI and the Host header field.</p><p id="rfc.section.5.2.p.2">An origin server that does not allow resources to differ by the requested host <em class="bcp14">MAY</em> ignore the Host header field value when determining the resource identified by an HTTP/1.1 request. (But see <a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" title="Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses">Appendix&nbsp;19.6.1.1</a> for other requirements on Host support in HTTP/1.1.)</p><p id="rfc.section.5.2.p.3">An origin server that does differentiate resources based on the host requested (sometimes referred to as virtual hosts or vanity host names) <em class="bcp14">MUST</em> use the following rules for determining the requested resource on an HTTP/1.1 request: </p><ol><li>If Request-URI is an absoluteURI, the host is part of the Request-URI. Any Host header field value in the request <em class="bcp14">MUST</em> be ignored.</li><li>If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host header field value.</li><li>If the host as determined by rule 1 or 2 is not a valid host on the server, the response <em class="bcp14">MUST</em> be a <a href="#status.400" class="smpl">400 (Bad Request)</a> error message.</li></ol><p id="rfc.section.5.2.p.4">Recipients of an HTTP/1.0 request that lacks a Host header field <em class="bcp14">MAY</em> attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine what exact resource is being requested.</p><h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a id="request.header.fields" href="#request.header.fields">Request Header Fields</a></h2><p id="rfc.section.5.3.p.1">The request-header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language method invocation.</p><div id="rfc.figure.u.46"></div><pre class="inline"><span id="rfc.iref.g.84"></span>    <a href="#request.header.fields" class="smpl">request-header</a> = Accept                   ; <a href="#header.accept" id="rfc.xref.header.accept.2" title="Accept">Section&nbsp;14.1</a>
     1790</pre><p id="rfc.section.5.1.2.p.11">followed by the remainder of the Request. Note that the absolute path cannot be empty; if none is present in the original
     1791         URI, it <em class="bcp14">MUST</em> be given as "/" (the server root).
     1792      </p>
     1793      <p id="rfc.section.5.1.2.p.12">The Request-URI is transmitted in the format specified in <a href="#general.syntax" title="General Syntax">Section&nbsp;3.2.1</a>. If the Request-URI is encoded using the "% HEX HEX" encoding <a href="#RFC2396" id="rfc.xref.RFC2396.3"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[42]</cite></a>, the origin server <em class="bcp14">MUST</em> decode the Request-URI in order to properly interpret the request. Servers <em class="bcp14">SHOULD</em> respond to invalid Request-URIs with an appropriate status code.
     1794      </p>
     1795      <p id="rfc.section.5.1.2.p.13">A transparent proxy <em class="bcp14">MUST NOT</em> rewrite the "abs_path" part of the received Request-URI when forwarding it to the next inbound server, except as noted above
     1796         to replace a null abs_path with "/".
     1797      </p>
     1798      <p id="rfc.section.5.1.2.p.14"> </p>
     1799      <dl class="empty">
     1800         <dd> <b>Note:</b> The "no rewrite" rule prevents the proxy from changing the meaning of the request when the origin server is improperly using
     1801            a non-reserved URI character for a reserved purpose. Implementors should be aware that some pre-HTTP/1.1 proxies have been
     1802            known to rewrite the Request-URI.
     1803         </dd>
     1804      </dl>
     1805      <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="the.resource.identified.by.a.request" href="#the.resource.identified.by.a.request">The Resource Identified by a Request</a></h2>
     1806      <p id="rfc.section.5.2.p.1">The exact resource identified by an Internet request is determined by examining both the Request-URI and the Host header field.</p>
     1807      <p id="rfc.section.5.2.p.2">An origin server that does not allow resources to differ by the requested host <em class="bcp14">MAY</em> ignore the Host header field value when determining the resource identified by an HTTP/1.1 request. (But see <a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" title="Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses">Appendix&nbsp;19.6.1.1</a> for other requirements on Host support in HTTP/1.1.)
     1808      </p>
     1809      <p id="rfc.section.5.2.p.3">An origin server that does differentiate resources based on the host requested (sometimes referred to as virtual hosts or
     1810         vanity host names) <em class="bcp14">MUST</em> use the following rules for determining the requested resource on an HTTP/1.1 request:
     1811      </p>
     1812      <ol>
     1813         <li>If Request-URI is an absoluteURI, the host is part of the Request-URI. Any Host header field value in the request <em class="bcp14">MUST</em> be ignored.
     1814         </li>
     1815         <li>If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host
     1816            header field value.
     1817         </li>
     1818         <li>If the host as determined by rule 1 or 2 is not a valid host on the server, the response <em class="bcp14">MUST</em> be a <a href="#status.400" class="smpl">400 (Bad Request)</a> error message.
     1819         </li>
     1820      </ol>
     1821      <p id="rfc.section.5.2.p.4">Recipients of an HTTP/1.0 request that lacks a Host header field <em class="bcp14">MAY</em> attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine
     1822         what exact resource is being requested.
     1823      </p>
     1824      <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a id="request.header.fields" href="#request.header.fields">Request Header Fields</a></h2>
     1825      <p id="rfc.section.5.3.p.1">The request-header fields allow the client to pass additional information about the request, and about the client itself,
     1826         to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language
     1827         method invocation.
     1828      </p>
     1829      <div id="rfc.figure.u.46"></div><pre class="inline"><span id="rfc.iref.g.84"></span>    <a href="#request.header.fields" class="smpl">request-header</a> = Accept                   ; <a href="#header.accept" id="rfc.xref.header.accept.2" title="Accept">Section&nbsp;14.1</a>
    4881830                   | Accept-Charset           ; <a href="#header.accept-charset" id="rfc.xref.header.accept-charset.1" title="Accept-Charset">Section&nbsp;14.2</a>
    4891831                   | Accept-Encoding          ; <a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.2" title="Accept-Encoding">Section&nbsp;14.3</a>
     
    5041846                   | TE                       ; <a href="#header.te" id="rfc.xref.header.te.3" title="TE">Section&nbsp;14.39</a>
    5051847                   | User-Agent               ; <a href="#header.user-agent" id="rfc.xref.header.user-agent.1" title="User-Agent">Section&nbsp;14.43</a>
    506 </pre><p id="rfc.section.5.3.p.3">Request-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields <em class="bcp14">MAY</em> be given the semantics of request-header fields if all parties in the communication recognize them to be request-header fields. Unrecognized header fields are treated as entity-header fields.</p><hr class="noprint"><h1 id="rfc.section.6" class="np"><a href="#rfc.section.6">6.</a>&nbsp;<a id="response" href="#response">Response</a></h1><p id="rfc.section.6.p.1">After receiving and interpreting a request message, a server responds with an HTTP response message.</p><div id="rfc.figure.u.47"></div><pre class="inline"><span id="rfc.iref.g.85"></span>    <a href="#response" class="smpl">Response</a>      = <a href="#status-line" class="smpl">Status-Line</a>               ; <a href="#status-line" title="Status-Line">Section&nbsp;6.1</a>
     1848</pre><p id="rfc.section.5.3.p.3">Request-header field names can be extended reliably only in combination with a change in the protocol version. However, new
     1849         or experimental header fields <em class="bcp14">MAY</em> be given the semantics of request-header fields if all parties in the communication recognize them to be request-header fields.
     1850         Unrecognized header fields are treated as entity-header fields.
     1851      </p>
     1852      <hr class="noprint">
     1853      <h1 id="rfc.section.6" class="np"><a href="#rfc.section.6">6.</a>&nbsp;<a id="response" href="#response">Response</a></h1>
     1854      <p id="rfc.section.6.p.1">After receiving and interpreting a request message, a server responds with an HTTP response message.</p>
     1855      <div id="rfc.figure.u.47"></div><pre class="inline"><span id="rfc.iref.g.85"></span>    <a href="#response" class="smpl">Response</a>      = <a href="#status-line" class="smpl">Status-Line</a>               ; <a href="#status-line" title="Status-Line">Section&nbsp;6.1</a>
    5071856                    *(( <a href="#general.header.fields" class="smpl">general-header</a>        ; <a href="#general.header.fields" title="General Header Fields">Section&nbsp;4.5</a>
    5081857                     | <a href="#response.header.fields" class="smpl">response-header</a>        ; <a href="#response.header.fields" title="Response Header Fields">Section&nbsp;6.2</a>
     
    5101859                    <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.29">CRLF</a>
    5111860                    [ <a href="#message.body" class="smpl">message-body</a> ]          ; <a href="#entity.body" title="Entity Body">Section&nbsp;7.2</a>
    512 </pre><h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="status-line" href="#status-line">Status-Line</a></h2><p id="rfc.section.6.1.p.1">The first line of a Response message is the Status-Line, consisting of the protocol version followed by a numeric status code and its associated textual phrase, with each element separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.</p><div id="rfc.figure.u.48"></div><pre class="inline"><span id="rfc.iref.g.86"></span>    <a href="#status-line" class="smpl">Status-Line</a> = <a href="#http.version" class="smpl" id="rfc.extref.h.10">HTTP-Version</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.28">SP</a> <a href="#status.code.and.reason.phrase.codes" class="smpl">Status-Code</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.30">SP</a> <a href="#status.code.and.reason.phrase.codes" class="smpl">Reason-Phrase</a> <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.30">CRLF</a>
    513 </pre><h3 id="rfc.section.6.1.1"><a href="#rfc.section.6.1.1">6.1.1</a>&nbsp;<a id="status.code.and.reason.phrase" href="#status.code.and.reason.phrase">Status Code and Reason Phrase</a></h3><p id="rfc.section.6.1.1.p.1">The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. These codes are fully defined in <a href="#status.codes" title="Status Code Definitions">Section&nbsp;10</a>. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason-Phrase.</p><p id="rfc.section.6.1.1.p.2">The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit: </p><ul><li>1xx: Informational - Request received, continuing process</li><li>2xx: Success - The action was successfully received, understood, and accepted</li><li>3xx: Redirection - Further action must be taken in order to complete the request</li><li>4xx: Client Error - The request contains bad syntax or cannot be fulfilled</li><li>5xx: Server Error - The server failed to fulfill an apparently valid request</li></ul><div id="status.code.and.reason.phrase.codes"><p id="rfc.section.6.1.1.p.3">    The individual values of the numeric status codes defined for HTTP/1.1, and an example set of corresponding Reason-Phrase's, are presented below. The reason phrases listed here are only recommendations -- they <em class="bcp14">MAY</em> be replaced by local equivalents without affecting the protocol.</p></div><div id="rfc.figure.u.49"></div><pre class="inline"><span id="rfc.iref.g.87"></span><span id="rfc.iref.g.88"></span><span id="rfc.iref.g.89"></span>   <a href="#status.code.and.reason.phrase.codes" class="smpl">Status-Code</a>    =
     1861</pre><h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="status-line" href="#status-line">Status-Line</a></h2>
     1862      <p id="rfc.section.6.1.p.1">The first line of a Response message is the Status-Line, consisting of the protocol version followed by a numeric status code
     1863         and its associated textual phrase, with each element separated by SP characters. No CR or LF is allowed except in the final
     1864         CRLF sequence.
     1865      </p>
     1866      <div id="rfc.figure.u.48"></div><pre class="inline"><span id="rfc.iref.g.86"></span>    <a href="#status-line" class="smpl">Status-Line</a> = <a href="#http.version" class="smpl" id="rfc.extref.h.10">HTTP-Version</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.28">SP</a> <a href="#status.code.and.reason.phrase.codes" class="smpl">Status-Code</a> <a href="#basic.rules" class="smpl" id="rfc.extref.s.30">SP</a> <a href="#status.code.and.reason.phrase.codes" class="smpl">Reason-Phrase</a> <a href="#basic.rules.crlf" class="smpl" id="rfc.extref.c.30">CRLF</a>
     1867</pre><h3 id="rfc.section.6.1.1"><a href="#rfc.section.6.1.1">6.1.1</a>&nbsp;<a id="status.code.and.reason.phrase" href="#status.code.and.reason.phrase">Status Code and Reason Phrase</a></h3>
     1868      <p id="rfc.section.6.1.1.p.1">The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. These codes
     1869         are fully defined in <a href="#status.codes" title="Status Code Definitions">Section&nbsp;10</a>. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use
     1870         by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason-Phrase.
     1871      </p>
     1872      <p id="rfc.section.6.1.1.p.2">The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role.
     1873         There are 5 values for the first digit:
     1874      </p>
     1875      <ul>
     1876         <li>1xx: Informational - Request received, continuing process</li>
     1877         <li>2xx: Success - The action was successfully received, understood, and accepted</li>
     1878         <li>3xx: Redirection - Further action must be taken in order to complete the request</li>
     1879         <li>4xx: Client Error - The request contains bad syntax or cannot be fulfilled</li>
     1880         <li>5xx: Server Error - The server failed to fulfill an apparently valid request</li>
     1881      </ul>
     1882      <div id="status.code.and.reason.phrase.codes">
     1883         <p id="rfc.section.6.1.1.p.3">    The individual values of the numeric status codes defined for HTTP/1.1, and an example set of corresponding Reason-Phrase's,
     1884            are presented below. The reason phrases listed here are only recommendations -- they <em class="bcp14">MAY</em> be replaced by local equivalents without affecting the protocol.
     1885         </p>
     1886      </div>
     1887      <div id="rfc.figure.u.49"></div><pre class="inline"><span id="rfc.iref.g.87"></span><span id="rfc.iref.g.88"></span><span id="rfc.iref.g.89"></span>   <a href="#status.code.and.reason.phrase.codes" class="smpl">Status-Code</a>    =
    5141888         "100"  ; <a href="#status.100" id="rfc.xref.status.100.1" title="100 Continue">Section&nbsp;10.1.1</a>: Continue
    5151889       | "101"  ; <a href="#status.101" id="rfc.xref.status.101.1" title="101 Switching Protocols">Section&nbsp;10.1.2</a>: Switching Protocols
     
    5561930   extension-code = 3<a href="#basic.rules" class="smpl" id="rfc.extref.d.19">DIGIT</a>
    5571931   <a href="#status.code.and.reason.phrase.codes" class="smpl">Reason-Phrase</a>  = *&lt;<a href="#basic.rules.text" class="smpl" id="rfc.extref.t.24">TEXT</a>, excluding <a href="#basic.rules" class="smpl" id="rfc.extref.c.31">CR</a>, <a href="#basic.rules" class="smpl" id="rfc.extref.l.10">LF</a>&gt;
    558 </pre><p id="rfc.section.6.1.1.p.5">HTTP status codes are extensible. HTTP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications <em class="bcp14">MUST</em> understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class, with the exception that an unrecognized response <em class="bcp14">MUST NOT</em> be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents <em class="bcp14">SHOULD</em> present to the user the entity returned with the response, since that entity is likely to include human-readable information which will explain the unusual status.</p><h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="response.header.fields" href="#response.header.fields">Response Header Fields</a></h2><p id="rfc.section.6.2.p.1">The response-header fields allow the server to pass additional information about the response which cannot be placed in the Status-Line. These header fields give information about the server and about further access to the resource identified by the Request-URI.</p><div id="rfc.figure.u.50"></div><pre class="inline"><span id="rfc.iref.g.90"></span>    <a href="#response.header.fields" class="smpl">response-header</a> = Accept-Ranges           ; <a href="#header.accept-ranges" id="rfc.xref.header.accept-ranges.1" title="Accept-Ranges">Section&nbsp;14.5</a>
     1932</pre><p id="rfc.section.6.1.1.p.5">HTTP status codes are extensible. HTTP applications are not required to understand the meaning of all registered status codes,
     1933         though such understanding is obviously desirable. However, applications <em class="bcp14">MUST</em> understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent
     1934         to the x00 status code of that class, with the exception that an unrecognized response <em class="bcp14">MUST NOT</em> be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was
     1935         something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents <em class="bcp14">SHOULD</em> present to the user the entity returned with the response, since that entity is likely to include human-readable information
     1936         which will explain the unusual status.
     1937      </p>
     1938      <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="response.header.fields" href="#response.header.fields">Response Header Fields</a></h2>
     1939      <p id="rfc.section.6.2.p.1">The response-header fields allow the server to pass additional information about the response which cannot be placed in the
     1940         Status-Line. These header fields give information about the server and about further access to the resource identified by
     1941         the Request-URI.
     1942      </p>
     1943      <div id="rfc.figure.u.50"></div><pre class="inline"><span id="rfc.iref.g.90"></span>    <a href="#response.header.fields" class="smpl">response-header</a> = Accept-Ranges           ; <a href="#header.accept-ranges" id="rfc.xref.header.accept-ranges.1" title="Accept-Ranges">Section&nbsp;14.5</a>
    5591944                    | Age                     ; <a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;14.6</a>
    5601945                    | ETag                    ; <a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section&nbsp;14.19</a>
     
    5651950                    | Vary                    ; <a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;14.44</a>
    5661951                    | WWW-Authenticate        ; <a href="#header.www-authenticate" id="rfc.xref.header.www-authenticate.1" title="WWW-Authenticate">Section&nbsp;14.47</a>
    567 </pre><p id="rfc.section.6.2.p.3">Response-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields <em class="bcp14">MAY</em> be given the semantics of response-header fields if all parties in the communication recognize them to be response-header fields. Unrecognized header fields are treated as entity-header fields.</p><hr class="noprint"><h1 id="rfc.section.7" class="np"><a href="#rfc.section.7">7.</a>&nbsp;<a id="entity" href="#entity">Entity</a></h1><p id="rfc.section.7.p.1">Request and Response messages <em class="bcp14">MAY</em> transfer an entity if not otherwise restricted by the request method or response status code. An entity consists of entity-header fields and an entity-body, although some responses will only include the entity-headers.</p><p id="rfc.section.7.p.2">In this section, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity.</p><h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="entity.header.fields" href="#entity.header.fields">Entity Header Fields</a></h2><p id="rfc.section.7.1.p.1">Entity-header fields define metainformation about the entity-body or, if no body is present, about the resource identified by the request. Some of this metainformation is <em class="bcp14">OPTIONAL</em>; some might be <em class="bcp14">REQUIRED</em> by portions of this specification.</p><div id="rfc.figure.u.51"></div><pre class="inline"><span id="rfc.iref.g.91"></span><span id="rfc.iref.g.92"></span>    <a href="#entity.header.fields" class="smpl">entity-header</a>  = Allow                    ; <a href="#header.allow" id="rfc.xref.header.allow.2" title="Allow">Section&nbsp;14.7</a>
     1952</pre><p id="rfc.section.6.2.p.3">Response-header field names can be extended reliably only in combination with a change in the protocol version. However, new
     1953         or experimental header fields <em class="bcp14">MAY</em> be given the semantics of response-header fields if all parties in the communication recognize them to be response-header
     1954         fields. Unrecognized header fields are treated as entity-header fields.
     1955      </p>
     1956      <hr class="noprint">
     1957      <h1 id="rfc.section.7" class="np"><a href="#rfc.section.7">7.</a>&nbsp;<a id="entity" href="#entity">Entity</a></h1>
     1958      <p id="rfc.section.7.p.1">Request and Response messages <em class="bcp14">MAY</em> transfer an entity if not otherwise restricted by the request method or response status code. An entity consists of entity-header
     1959         fields and an entity-body, although some responses will only include the entity-headers.
     1960      </p>
     1961      <p id="rfc.section.7.p.2">In this section, both sender and recipient refer to either the client or the server, depending on who sends and who receives
     1962         the entity.
     1963      </p>
     1964      <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="entity.header.fields" href="#entity.header.fields">Entity Header Fields</a></h2>
     1965      <p id="rfc.section.7.1.p.1">Entity-header fields define metainformation about the entity-body or, if no body is present, about the resource identified
     1966         by the request. Some of this metainformation is <em class="bcp14">OPTIONAL</em>; some might be <em class="bcp14">REQUIRED</em> by portions of this specification.
     1967      </p>
     1968      <div id="rfc.figure.u.51"></div><pre class="inline"><span id="rfc.iref.g.91"></span><span id="rfc.iref.g.92"></span>    <a href="#entity.header.fields" class="smpl">entity-header</a>  = Allow                    ; <a href="#header.allow" id="rfc.xref.header.allow.2" title="Allow">Section&nbsp;14.7</a>
    5681969                   | Content-Encoding         ; <a href="#header.content-encoding" id="rfc.xref.header.content-encoding.2" title="Content-Encoding">Section&nbsp;14.11</a>
    5691970                   | Content-Language         ; <a href="#header.content-language" id="rfc.xref.header.content-language.1" title="Content-Language">Section&nbsp;14.12</a>
     
    5781979
    5791980    extension-header = <a href="#message.headers" class="smpl">message-header</a>
    580 </pre><p id="rfc.section.7.1.p.3">The extension-header mechanism allows additional entity-header fields to be defined without changing the protocol, but these fields cannot be assumed to be recognizable by the recipient. Unrecognized header fields <em class="bcp14">SHOULD</em> be ignored by the recipient and <em class="bcp14">MUST</em> be forwarded by transparent proxies.</p><h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a id="entity.body" href="#entity.body">Entity Body</a></h2><p id="rfc.section.7.2.p.1">The entity-body (if any) sent with an HTTP request or response is in a format and encoding defined by the entity-header fields.</p><div id="rfc.figure.u.52"></div><pre class="inline"><span id="rfc.iref.g.93"></span>    <a href="#entity.body" class="smpl">entity-body</a>    = *<a href="#basic.rules" class="smpl" id="rfc.extref.o.7">OCTET</a>
    581 </pre><p id="rfc.section.7.2.p.3">An entity-body is only present in a message when a message-body is present, as described in <a href="#message.body" title="Message Body">Section&nbsp;4.3</a>. The entity-body is obtained from the message-body by decoding any Transfer-Encoding that might have been applied to ensure safe and proper transfer of the message.</p><h3 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;<a id="type" href="#type">Type</a></h3><p id="rfc.section.7.2.1.p.1">When an entity-body is included with a message, the data type of that body is determined via the header fields Content-Type and Content-Encoding. These define a two-layer, ordered encoding model:</p><div id="rfc.figure.u.53"></div><pre class="text">    entity-body := Content-Encoding( Content-Type( data ) )
    582 </pre><p id="rfc.section.7.2.1.p.3">Content-Type specifies the media type of the underlying data. Content-Encoding may be used to indicate any additional content codings applied to the data, usually for the purpose of data compression, that are a property of the requested resource. There is no default encoding.</p><p id="rfc.section.7.2.1.p.4">Any HTTP/1.1 message containing an entity-body <em class="bcp14">SHOULD</em> include a Content-Type header field defining the media type of that body. If and only if the media type is not given by a Content-Type field, the recipient <em class="bcp14">MAY</em> attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource. If the media type remains unknown, the recipient <em class="bcp14">SHOULD</em> treat it as type "application/octet-stream".</p><h3 id="rfc.section.7.2.2"><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;<a id="entity.length" href="#entity.length">Entity Length</a></h3><p id="rfc.section.7.2.2.p.1">The entity-length of a message is the length of the message-body before any transfer-codings have been applied. <a href="#message.length" title="Message Length">Section&nbsp;4.4</a> defines how the transfer-length of a message-body is determined.</p><hr class="noprint"><h1 id="rfc.section.8" class="np"><a href="#rfc.section.8">8.</a>&nbsp;<a id="connections" href="#connections">Connections</a></h1><h2 id="rfc.section.8.1"><a href="#rfc.section.8.1">8.1</a>&nbsp;<a id="persistent.connections" href="#persistent.connections">Persistent Connections</a></h2><h3 id="rfc.section.8.1.1"><a href="#rfc.section.8.1.1">8.1.1</a>&nbsp;<a id="persistent.purpose" href="#persistent.purpose">Purpose</a></h3><p id="rfc.section.8.1.1.p.1">Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP servers and causing congestion on the Internet. The use of inline images and other associated data often require a client to make multiple requests of the same server in a short amount of time. Analysis of these performance problems and results from a prototype implementation are available <a href="#Pad1995" id="rfc.xref.Pad1995.1"><cite title="Improving HTTP Latency">[26]</cite></a>  <a href="#Spe" id="rfc.xref.Spe.1"><cite title="Analysis of HTTP Performance Problems">[30]</cite></a>. Implementation experience and measurements of actual HTTP/1.1 (RFC 2068) implementations show good results <a href="#Nie1997" id="rfc.xref.Nie1997.1"><cite title="Network Performance Effects of HTTP/1.1, CSS1, and PNG">[39]</cite></a>. Alternatives have also been explored, for example, T/TCP <a href="#Tou1998" id="rfc.xref.Tou1998.1"><cite title="Analysis of HTTP Performance">[27]</cite></a>.</p><p id="rfc.section.8.1.1.p.2">Persistent HTTP connections have a number of advantages: </p><ul><li>By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways, tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.</li><li>HTTP requests and responses can be pipelined on a connection. Pipelining allows a client to make multiple requests without waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.</li><li>Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to determine the congestion state of the network.</li><li>Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.</li><li>HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old semantics after an error is reported.</li></ul><p id="rfc.section.8.1.1.p.3">HTTP implementations <em class="bcp14">SHOULD</em> implement persistent connections.</p><h3 id="rfc.section.8.1.2"><a href="#rfc.section.8.1.2">8.1.2</a>&nbsp;<a id="persistent.overall" href="#persistent.overall">Overall Operation</a></h3><p id="rfc.section.8.1.2.p.1">A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior of any HTTP connection. That is, unless otherwise indicated, the client <em class="bcp14">SHOULD</em> assume that the server will maintain a persistent connection, even after error responses from the server.</p><p id="rfc.section.8.1.2.p.2">Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling takes place using the Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.2" title="Connection">Section&nbsp;14.10</a>). Once a close has been signaled, the client <em class="bcp14">MUST NOT</em> send any more requests on that connection.</p><h4 id="rfc.section.8.1.2.1"><a href="#rfc.section.8.1.2.1">8.1.2.1</a>&nbsp;<a id="persistent.negotiation" href="#persistent.negotiation">Negotiation</a></h4><p id="rfc.section.8.1.2.1.p.1">An HTTP/1.1 server <em class="bcp14">MAY</em> assume that a HTTP/1.1 client intends to maintain a persistent connection unless a Connection header including the connection-token "close" was sent in the request. If the server chooses to close the connection immediately after sending the response, it <em class="bcp14">SHOULD</em> send a Connection header including the connection-token close.</p><p id="rfc.section.8.1.2.1.p.2">An HTTP/1.1 client <em class="bcp14">MAY</em> expect a connection to remain open, but would decide to keep it open based on whether the response from a server contains a Connection header with the connection-token close. In case the client does not want to maintain a connection for more than that request, it <em class="bcp14">SHOULD</em> send a Connection header including the connection-token close.</p><p id="rfc.section.8.1.2.1.p.3">If either the client or the server sends the close token in the Connection header, that request becomes the last one for the connection.</p><p id="rfc.section.8.1.2.1.p.4">Clients and servers <em class="bcp14">SHOULD NOT</em> assume that a persistent connection is maintained for HTTP versions less than 1.1 unless it is explicitly signaled. See <a href="#compatibility.with.http.1.0.persistent.connections" title="Compatibility with HTTP/1.0 Persistent Connections">Appendix&nbsp;19.6.2</a> for more information on backward compatibility with HTTP/1.0 clients.</p><p id="rfc.section.8.1.2.1.p.5">In order to remain persistent, all messages on the connection <em class="bcp14">MUST</em> have a self-defined message length (i.e., one not defined by closure of the connection), as described in <a href="#message.length" title="Message Length">Section&nbsp;4.4</a>.</p><h4 id="rfc.section.8.1.2.2"><a href="#rfc.section.8.1.2.2">8.1.2.2</a>&nbsp;<a id="pipelining" href="#pipelining">Pipelining</a></h4><p id="rfc.section.8.1.2.2.p.1">A client that supports persistent connections <em class="bcp14">MAY</em> "pipeline" its requests (i.e., send multiple requests without waiting for each response). A server <em class="bcp14">MUST</em> send its responses to those requests in the same order that the requests were received.</p><p id="rfc.section.8.1.2.2.p.2">Clients which assume persistent connections and pipeline immediately after connection establishment <em class="bcp14">SHOULD</em> be prepared to retry their connection if the first pipelined attempt fails. If a client does such a retry, it <em class="bcp14">MUST NOT</em> pipeline before it knows the connection is persistent. Clients <em class="bcp14">MUST</em> also be prepared to resend their requests if the server closes the connection before sending all of the corresponding responses.</p><p id="rfc.section.8.1.2.2.p.3">Clients <em class="bcp14">SHOULD NOT</em> pipeline requests using non-idempotent methods or non-idempotent sequences of methods (see <a href="#idempotent.methods" title="Idempotent Methods">Section&nbsp;9.1.2</a>). Otherwise, a premature termination of the transport connection could lead to indeterminate results. A client wishing to send a non-idempotent request <em class="bcp14">SHOULD</em> wait to send that request until it has received the response status for the previous request.</p><h3 id="rfc.section.8.1.3"><a href="#rfc.section.8.1.3">8.1.3</a>&nbsp;<a id="persistent.proxy" href="#persistent.proxy">Proxy Servers</a></h3><p id="rfc.section.8.1.3.p.1">It is especially important that proxies correctly implement the properties of the Connection header field as specified in <a href="#header.connection" id="rfc.xref.header.connection.3" title="Connection">Section&nbsp;14.10</a>.</p><p id="rfc.section.8.1.3.p.2">The proxy server <em class="bcp14">MUST</em> signal persistent connections separately with its clients and the origin servers (or other proxy servers) that it connects to. Each persistent connection applies to only one transport link.</p><p id="rfc.section.8.1.3.p.3">A proxy server <em class="bcp14">MUST NOT</em> establish a HTTP/1.1 persistent connection with an HTTP/1.0 client (but see RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[33]</cite></a> for information and discussion of the problems with the Keep-Alive header implemented by many HTTP/1.0 clients).</p><h3 id="rfc.section.8.1.4"><a href="#rfc.section.8.1.4">8.1.4</a>&nbsp;<a id="persistent.practical" href="#persistent.practical">Practical Considerations</a></h3><p id="rfc.section.8.1.4.p.1">Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers might make this a higher value since it is likely that the client will be making more connections through the same server. The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client or the server.</p><p id="rfc.section.8.1.4.p.2">When a client or server wishes to time-out it <em class="bcp14">SHOULD</em> issue a graceful close on the transport connection. Clients and servers <em class="bcp14">SHOULD</em> both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does not detect the other side's close promptly it could cause unnecessary resource drain on the network.</p><p id="rfc.section.8.1.4.p.3">A client, server, or proxy <em class="bcp14">MAY</em> close the transport connection at any time. For example, a client might have started to send a new request at the same time that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed while it was idle, but from the client's point of view, a request is in progress.</p><p id="rfc.section.8.1.4.p.4">This means that clients, servers, and proxies <em class="bcp14">MUST</em> be able to recover from asynchronous close events. Client software <em class="bcp14">SHOULD</em> reopen the transport connection and retransmit the aborted sequence of requests without user interaction so long as the request sequence is idempotent (see <a href="#idempotent.methods" title="Idempotent Methods">Section&nbsp;9.1.2</a>). Non-idempotent methods or sequences <em class="bcp14">MUST NOT</em> be automatically retried, although user agents <em class="bcp14">MAY</em> offer a human operator the choice of retrying the request(s). Confirmation by user-agent software with semantic understanding of the application <em class="bcp14">MAY</em> substitute for user confirmation. The automatic retry <em class="bcp14">SHOULD NOT</em> be repeated if the second sequence of requests fails.</p><p id="rfc.section.8.1.4.p.5">Servers <em class="bcp14">SHOULD</em> always respond to at least one request per connection, if at all possible. Servers <em class="bcp14">SHOULD NOT</em> close a connection in the middle of transmitting a response, unless a network or client failure is suspected.</p><p id="rfc.section.8.1.4.p.6">Clients that use persistent connections <em class="bcp14">SHOULD</em> limit the number of simultaneous connections that they maintain to a given server. A single-user client <em class="bcp14">SHOULD NOT</em> maintain more than 2 connections with any server or proxy. A proxy <em class="bcp14">SHOULD</em> use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion.</p><h2 id="rfc.section.8.2"><a href="#rfc.section.8.2">8.2</a>&nbsp;<a id="message.transmission.requirements" href="#message.transmission.requirements">Message Transmission Requirements</a></h2><h3 id="rfc.section.8.2.1"><a href="#rfc.section.8.2.1">8.2.1</a>&nbsp;<a id="persistent.flow" href="#persistent.flow">Persistent Connections and Flow Control</a></h3><p id="rfc.section.8.2.1.p.1">HTTP/1.1 servers <em class="bcp14">SHOULD</em> maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.</p><h3 id="rfc.section.8.2.2"><a href="#rfc.section.8.2.2">8.2.2</a>&nbsp;<a id="persistent.monitor" href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></h3><p id="rfc.section.8.2.2.p.1">An HTTP/1.1 (or later) client sending a message-body <em class="bcp14">SHOULD</em> monitor the network connection for an error status while it is transmitting the request. If the client sees an error status, it <em class="bcp14">SHOULD</em> immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>), a zero length chunk and empty trailer <em class="bcp14">MAY</em> be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header, the client <em class="bcp14">MUST</em> close the connection.</p><h3 id="rfc.section.8.2.3"><a href="#rfc.section.8.2.3">8.2.3</a>&nbsp;<a id="use.of.the.100.status" href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></h3><p id="rfc.section.8.2.3.p.1">The purpose of the <a href="#status.100" class="smpl">100 (Continue)</a> status (see <a href="#status.100" id="rfc.xref.status.100.2" title="100 Continue">Section&nbsp;10.1.1</a>) is to allow a client that is sending a request message with a request body to determine if the origin server is willing to accept the request (based on the request headers) before the client sends the request body. In some cases, it might either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking at the body.</p><p id="rfc.section.8.2.3.p.2">Requirements for HTTP/1.1 clients: </p><ul><li>If a client will wait for a <a href="#status.100" class="smpl">100 (Continue)</a> response before sending the request body, it <em class="bcp14">MUST</em> send an Expect request-header field (<a href="#header.expect" id="rfc.xref.header.expect.2" title="Expect">Section&nbsp;14.20</a>) with the "100-continue" expectation.</li><li>A client <em class="bcp14">MUST NOT</em> send an Expect request-header field (<a href="#header.expect" id="rfc.xref.header.expect.3" title="Expect">Section&nbsp;14.20</a>) with the "100-continue" expectation if it does not intend to send a request body.</li></ul><p id="rfc.section.8.2.3.p.3">Because of the presence of older implementations, the protocol allows ambiguous situations in which a client may send "Expect: 100-continue" without receiving either a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status or a <a href="#status.100" class="smpl">100 (Continue)</a> status. Therefore, when a client sends this header field to an origin server (possibly via a proxy) from which it has never seen a <a href="#status.100" class="smpl">100 (Continue)</a> status, the client <em class="bcp14">SHOULD NOT</em> wait for an indefinite period before sending the request body.</p><p id="rfc.section.8.2.3.p.4">Requirements for HTTP/1.1 origin servers: </p><ul><li>Upon receiving a request which includes an Expect request-header field with the "100-continue" expectation, an origin server <em class="bcp14">MUST</em> either respond with <a href="#status.100" class="smpl">100 (Continue)</a> status and continue to read from the input stream, or respond with a final status code. The origin server <em class="bcp14">MUST NOT</em> wait for the request body before sending the <a href="#status.100" class="smpl">100 (Continue)</a> response. If it responds with a final status code, it <em class="bcp14">MAY</em> close the transport connection or it <em class="bcp14">MAY</em> continue to read and discard the rest of the request. It <em class="bcp14">MUST NOT</em> perform the requested method if it returns a final status code.</li><li>An origin server <em class="bcp14">SHOULD NOT</em> send a <a href="#status.100" class="smpl">100 (Continue)</a> response if the request message does not include an Expect request-header field with the "100-continue" expectation, and <em class="bcp14">MUST NOT</em> send a <a href="#status.100" class="smpl">100 (Continue)</a> response if such a request comes from an HTTP/1.0 (or earlier) client. There is an exception to this rule: for compatibility with RFC 2068, a server <em class="bcp14">MAY</em> send a <a href="#status.100" class="smpl">100 (Continue)</a> status in response to an HTTP/1.1 PUT or POST request that does not include an Expect request-header field with the "100-continue" expectation. This exception, the purpose of which is to minimize any client processing delays associated with an undeclared wait for <a href="#status.100" class="smpl">100 (Continue)</a> status, applies only to HTTP/1.1 requests, and not to requests with any other HTTP-version value.</li><li>An origin server <em class="bcp14">MAY</em> omit a <a href="#status.100" class="smpl">100 (Continue)</a> response if it has already received some or all of the request body for the corresponding request.</li><li>An origin server that sends a <a href="#status.100" class="smpl">100 (Continue)</a> response <em class="bcp14">MUST</em> ultimately send a final status code, once the request body is received and processed, unless it terminates the transport connection prematurely.</li><li>If an origin server receives a request that does not include an Expect request-header field with the "100-continue" expectation, the request includes a request body, and the server responds with a final status code before reading the entire request body from the transport connection, then the server <em class="bcp14">SHOULD NOT</em> close the transport connection until it has read the entire request, or until the client closes the connection. Otherwise, the client might not reliably receive the response message. However, this requirement is not be construed as preventing a server from defending itself against denial-of-service attacks, or from badly broken client implementations.</li></ul><p id="rfc.section.8.2.3.p.5">Requirements for HTTP/1.1 proxies: </p><ul><li>If a proxy receives a request that includes an Expect request-header field with the "100-continue" expectation, and the proxy either knows that the next-hop server complies with HTTP/1.1 or higher, or does not know the HTTP version of the next-hop server, it <em class="bcp14">MUST</em> forward the request, including the Expect header field.</li><li>If the proxy knows that the version of the next-hop server is HTTP/1.0 or lower, it <em class="bcp14">MUST NOT</em> forward the request, and it <em class="bcp14">MUST</em> respond with a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status.</li><li>Proxies <em class="bcp14">SHOULD</em> maintain a cache recording the HTTP version numbers received from recently-referenced next-hop servers.</li><li>A proxy <em class="bcp14">MUST NOT</em> forward a <a href="#status.100" class="smpl">100 (Continue)</a> response if the request message was received from an HTTP/1.0 (or earlier) client and did not include an Expect request-header field with the "100-continue" expectation. This requirement overrides the general rule for forwarding of 1xx responses (see <a href="#status.1xx" title="Informational 1xx">Section&nbsp;10.1</a>).</li></ul><h3 id="rfc.section.8.2.4"><a href="#rfc.section.8.2.4">8.2.4</a>&nbsp;<a id="connection.premature" href="#connection.premature">Client Behavior if Server Prematurely Closes Connection</a></h3><p id="rfc.section.8.2.4.p.1">If an HTTP/1.1 client sends a request which includes a request body, but which does not include an Expect request-header field with the "100-continue" expectation, and if the client is not directly connected to an HTTP/1.1 origin server, and if the client sees the connection close before receiving any status from the server, the client <em class="bcp14">SHOULD</em> retry the request. If the client does retry this request, it <em class="bcp14">MAY</em> use the following "binary exponential backoff" algorithm to be assured of obtaining a reliable response: </p><ol><li>Initiate a new connection to the server</li><li>Transmit the request-headers</li><li>Initialize a variable R to the estimated round-trip time to the server (e.g., based on the time it took to establish the connection), or to a constant value of 5 seconds if the round-trip time is not available.</li><li>Compute T = R * (2**N), where N is the number of previous retries of this request.</li><li>Wait either for an error response from the server, or for T seconds (whichever comes first)</li><li>If no error response is received, after T seconds transmit the body of the request.</li><li>If client sees that the connection is closed prematurely, repeat from step 1 until the request is accepted, an error response is received, or the user becomes impatient and terminates the retry process.</li></ol><p id="rfc.section.8.2.4.p.2">If at any point an error status is received, the client </p><ul><li><em class="bcp14">SHOULD NOT</em> continue and</li><li><em class="bcp14">SHOULD</em> close the connection if it has not completed sending the request message.</li></ul><hr class="noprint"><h1 id="rfc.section.9" class="np"><a href="#rfc.section.9">9.</a>&nbsp;<a id="method.definitions" href="#method.definitions">Method Definitions</a></h1><p id="rfc.section.9.p.1">The set of common methods for HTTP/1.1 is defined below. Although this set can be expanded, additional methods cannot be assumed to share the same semantics for separately extended clients and servers.</p><p id="rfc.section.9.p.2">The Host request-header field (<a href="#header.host" id="rfc.xref.header.host.2" title="Host">Section&nbsp;14.23</a>) <em class="bcp14">MUST</em> accompany all HTTP/1.1 requests.</p><h2 id="rfc.section.9.1"><a href="#rfc.section.9.1">9.1</a>&nbsp;<a id="safe.and.idempotent" href="#safe.and.idempotent">Safe and Idempotent Methods</a></h2><h3 id="rfc.section.9.1.1"><a href="#rfc.section.9.1.1">9.1.1</a>&nbsp;<a id="safe.methods" href="#safe.methods">Safe Methods</a></h3><p id="rfc.section.9.1.1.p.1">Implementors should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they might take which may have an unexpected significance to themselves or others.</p><p id="rfc.section.9.1.1.p.2">In particular, the convention has been established that the GET and HEAD methods <em class="bcp14">SHOULD NOT</em> have the significance of taking an action other than retrieval. These methods ought to be considered "safe". This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.</p><p id="rfc.section.9.1.1.p.3">Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.</p><h3 id="rfc.section.9.1.2"><a href="#rfc.section.9.1.2">9.1.2</a>&nbsp;<a id="idempotent.methods" href="#idempotent.methods">Idempotent Methods</a></h3><p id="rfc.section.9.1.2.p.1">Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N &gt; 0 identical requests is the same as for a single request. The methods GET, HEAD, PUT and DELETE share this property. Also, the methods OPTIONS and TRACE <em class="bcp14">SHOULD NOT</em> have side effects, and so are inherently idempotent.</p><p id="rfc.section.9.1.2.p.2">However, it is possible that a sequence of several requests is non-idempotent, even if all of the methods executed in that sequence are idempotent. (A sequence is idempotent if a single execution of the entire sequence always yields a result that is not changed by a reexecution of all, or part, of that sequence.) For example, a sequence is non-idempotent if its result depends on a value that is later modified in the same sequence.</p><p id="rfc.section.9.1.2.p.3">A sequence that never has side effects is idempotent, by definition (provided that no concurrent operations are being executed on the same set of resources).</p><div id="rfc.iref.o.3"></div><div id="rfc.iref.m.2"></div><h2 id="rfc.section.9.2"><a href="#rfc.section.9.2">9.2</a>&nbsp;<a id="OPTIONS" href="#OPTIONS">OPTIONS</a></h2><p id="rfc.section.9.2.p.1">The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI. This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.</p><p id="rfc.section.9.2.p.2">Responses to this method are not cacheable.</p><p id="rfc.section.9.2.p.3">If the OPTIONS request includes an entity-body (as indicated by the presence of Content-Length or Transfer-Encoding), then the media type <em class="bcp14">MUST</em> be indicated by a Content-Type field. Although this specification does not define any use for such a body, future extensions to HTTP might use the OPTIONS body to make more detailed queries on the server. A server that does not support such an extension <em class="bcp14">MAY</em> discard the request body.</p><p id="rfc.section.9.2.p.4">If the Request-URI is an asterisk ("*"), the OPTIONS request is intended to apply to the server in general rather than to a specific resource. Since a server's communication options typically depend on the resource, the "*" request is only useful as a "ping" or "no-op" type of method; it does nothing beyond allowing the client to test the capabilities of the server. For example, this can be used to test a proxy for HTTP/1.1 compliance (or lack thereof).</p><p id="rfc.section.9.2.p.5">If the Request-URI is not an asterisk, the OPTIONS request applies only to the options that are available when communicating with that resource.</p><p id="rfc.section.9.2.p.6">A 200 response <em class="bcp14">SHOULD</em> include any header fields that indicate optional features implemented by the server and applicable to that resource (e.g., Allow), possibly including extensions not defined by this specification. The response body, if any, <em class="bcp14">SHOULD</em> also include information about the communication options. The format for such a body is not defined by this specification, but might be defined by future extensions to HTTP. Content negotiation <em class="bcp14">MAY</em> be used to select the appropriate response format. If no response body is included, the response <em class="bcp14">MUST</em> include a Content-Length field with a field-value of "0".</p><p id="rfc.section.9.2.p.7">The Max-Forwards request-header field <em class="bcp14">MAY</em> be used to target a specific proxy in the request chain. When a proxy receives an OPTIONS request on an absoluteURI for which request forwarding is permitted, the proxy <em class="bcp14">MUST</em> check for a Max-Forwards field. If the Max-Forwards field-value is zero ("0"), the proxy <em class="bcp14">MUST NOT</em> forward the message; instead, the proxy <em class="bcp14">SHOULD</em> respond with its own communication options. If the Max-Forwards field-value is an integer greater than zero, the proxy <em class="bcp14">MUST</em> decrement the field-value when it forwards the request. If no Max-Forwards field is present in the request, then the forwarded request <em class="bcp14">MUST NOT</em> include a Max-Forwards field.</p><div id="rfc.iref.g.94"></div><div id="rfc.iref.m.3"></div><h2 id="rfc.section.9.3"><a href="#rfc.section.9.3">9.3</a>&nbsp;<a id="GET" href="#GET">GET</a></h2><p id="rfc.section.9.3.p.1">The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process.</p><p id="rfc.section.9.3.p.2">The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.</p><p id="rfc.section.9.3.p.3">The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred, as described in <a href="#header.range" id="rfc.xref.header.range.3" title="Range">Section&nbsp;14.35</a>. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client.</p><p id="rfc.section.9.3.p.4">The response to a GET request is cacheable if and only if it meets the requirements for HTTP caching described in <a href="#caching" title="Caching in HTTP">Section&nbsp;13</a>.</p><p id="rfc.section.9.3.p.5">See <a href="#encoding.sensitive.information.in.uris" title="Encoding Sensitive Information in URI's">Section&nbsp;15.1.3</a> for security considerations when used for forms.</p><div id="rfc.iref.h.3"></div><div id="rfc.iref.m.4"></div><h2 id="rfc.section.9.4"><a href="#rfc.section.9.4">9.4</a>&nbsp;<a id="HEAD" href="#HEAD">HEAD</a></h2><p id="rfc.section.9.4.p.1">The HEAD method is identical to GET except that the server <em class="bcp14">MUST NOT</em> return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request <em class="bcp14">SHOULD</em> be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification.</p><p id="rfc.section.9.4.p.2">The response to a HEAD request <em class="bcp14">MAY</em> be cacheable in the sense that the information contained in the response <em class="bcp14">MAY</em> be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the cache <em class="bcp14">MUST</em> treat the cache entry as stale.</p><div id="rfc.iref.p.2"></div><div id="rfc.iref.m.5"></div><h2 id="rfc.section.9.5"><a href="#rfc.section.9.5">9.5</a>&nbsp;<a id="POST" href="#POST">POST</a></h2><p id="rfc.section.9.5.p.1">The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions: </p><ul><li>Annotation of existing resources;</li><li>Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;</li><li>Providing a block of data, such as the result of submitting a form, to a data-handling process;</li><li>Extending a database through an append operation.</li></ul><p id="rfc.section.9.5.p.2">The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.</p><p id="rfc.section.9.5.p.3">The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either <a href="#status.200" class="smpl">200 (OK)</a> or <a href="#status.204" class="smpl">204 (No Content)</a> is the appropriate response status, depending on whether or not the response includes an entity that describes the result.</p><p id="rfc.section.9.5.p.4">If a resource has been created on the origin server, the response <em class="bcp14">SHOULD</em> be <a href="#status.201" class="smpl">201 (Created)</a> and contain an entity which describes the status of the request and refers to the new resource, and a Location header (see <a href="#header.location" id="rfc.xref.header.location.2" title="Location">Section&nbsp;14.30</a>).</p><p id="rfc.section.9.5.p.5">Responses to this method are not cacheable, unless the response includes appropriate Cache-Control or Expires header fields. However, the <a href="#status.303" class="smpl">303 (See Other)</a> response can be used to direct the user agent to retrieve a cacheable resource.</p><p id="rfc.section.9.5.p.6">POST requests <em class="bcp14">MUST</em> obey the message transmission requirements set out in <a href="#message.transmission.requirements" title="Message Transmission Requirements">Section&nbsp;8.2</a>.</p><p id="rfc.section.9.5.p.7">See <a href="#encoding.sensitive.information.in.uris" title="Encoding Sensitive Information in URI's">Section&nbsp;15.1.3</a> for security considerations.</p><div id="rfc.iref.p.3"></div><div id="rfc.iref.m.6"></div><h2 id="rfc.section.9.6"><a href="#rfc.section.9.6">9.6</a>&nbsp;<a id="PUT" href="#PUT">PUT</a></h2><p id="rfc.section.9.6.p.1">The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity <em class="bcp14">SHOULD</em> be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI. If a new resource is created, the origin server <em class="bcp14">MUST</em> inform the user agent via the <a href="#status.201" class="smpl">201 (Created)</a> response. If an existing resource is modified, either the <a href="#status.200" class="smpl">200 (OK)</a> or <a href="#status.204" class="smpl">204 (No Content)</a> response codes <em class="bcp14">SHOULD</em> be sent to indicate successful completion of the request. If the resource could not be created or modified with the Request-URI, an appropriate error response <em class="bcp14">SHOULD</em> be given that reflects the nature of the problem. The recipient of the entity <em class="bcp14">MUST NOT</em> ignore any Content-* (e.g. Content-Range) headers that it does not understand or implement and <em class="bcp14">MUST</em> return a <a href="#status.501" class="smpl">501 (Not Implemented)</a> response in such cases.</p><p id="rfc.section.9.6.p.2">If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries <em class="bcp14">SHOULD</em> be treated as stale. Responses to this method are not cacheable.</p><p id="rfc.section.9.6.p.3">The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource might be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server <em class="bcp14">MUST NOT</em> attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI, it <em class="bcp14">MUST</em> send a <a href="#status.301" class="smpl">301 (Moved Permanently)</a> response; the user agent <em class="bcp14">MAY</em> then make its own decision regarding whether or not to redirect the request.</p><p id="rfc.section.9.6.p.4">A single resource <em class="bcp14">MAY</em> be identified by many different URIs. For example, an article might have a URI for identifying "the current version" which is separate from the URI identifying each particular version. In this case, a PUT request on a general URI might result in several other URIs being defined by the origin server.</p><p id="rfc.section.9.6.p.5">HTTP/1.1 does not define how a PUT method affects the state of an origin server.</p><p id="rfc.section.9.6.p.6">PUT requests <em class="bcp14">MUST</em> obey the message transmission requirements set out in <a href="#message.transmission.requirements" title="Message Transmission Requirements">Section&nbsp;8.2</a>.</p><p id="rfc.section.9.6.p.7">Unless otherwise specified for a particular entity-header, the entity-headers in the PUT request <em class="bcp14">SHOULD</em> be applied to the resource created or modified by the PUT.</p><div id="rfc.iref.d.3"></div><div id="rfc.iref.m.7"></div><h2 id="rfc.section.9.7"><a href="#rfc.section.9.7">9.7</a>&nbsp;<a id="DELETE" href="#DELETE">DELETE</a></h2><p id="rfc.section.9.7.p.1">The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method <em class="bcp14">MAY</em> be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server <em class="bcp14">SHOULD NOT</em> indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.</p><p id="rfc.section.9.7.p.2">A successful response <em class="bcp14">SHOULD</em> be <a href="#status.200" class="smpl">200 (OK)</a> if the response includes an entity describing the status, <a href="#status.202" class="smpl">202 (Accepted)</a> if the action has not yet been enacted, or <a href="#status.204" class="smpl">204 (No Content)</a> if the action has been enacted but the response does not include an entity.</p><p id="rfc.section.9.7.p.3">If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries <em class="bcp14">SHOULD</em> be treated as stale. Responses to this method are not cacheable.</p><div id="rfc.iref.t.2"></div><div id="rfc.iref.m.8"></div><h2 id="rfc.section.9.8"><a href="#rfc.section.9.8">9.8</a>&nbsp;<a id="TRACE" href="#TRACE">TRACE</a></h2><p id="rfc.section.9.8.p.1">The TRACE method is used to invoke a remote, application-layer loop-back of the request message. The final recipient of the request <em class="bcp14">SHOULD</em> reflect the message received back to the client as the entity-body of a <a href="#status.200" class="smpl">200 (OK)</a> response. The final recipient is either the origin server or the first proxy or gateway to receive a Max-Forwards value of zero (0) in the request (see <a href="#header.max-forwards" id="rfc.xref.header.max-forwards.2" title="Max-Forwards">Section&nbsp;14.31</a>). A TRACE request <em class="bcp14">MUST NOT</em> include an entity.</p><p id="rfc.section.9.8.p.2">TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (<a href="#header.via" id="rfc.xref.header.via.2" title="Via">Section&nbsp;14.45</a>) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.</p><p id="rfc.section.9.8.p.3">If the request is valid, the response <em class="bcp14">SHOULD</em> contain the entire request message in the entity-body, with a Content-Type of "message/http". Responses to this method <em class="bcp14">MUST NOT</em> be cached.</p><div id="rfc.iref.c.12"></div><div id="rfc.iref.m.9"></div><h2 id="rfc.section.9.9"><a href="#rfc.section.9.9">9.9</a>&nbsp;<a id="CONNECT" href="#CONNECT">CONNECT</a></h2><p id="rfc.section.9.9.p.1">This specification reserves the method name CONNECT for use with a proxy that can dynamically switch to being a tunnel (e.g. SSL tunneling <a href="#Luo1998" id="rfc.xref.Luo1998.1"><cite title="Tunneling TCP based protocols through Web proxy servers">[44]</cite></a>).</p><hr class="noprint"><h1 id="rfc.section.10" class="np"><a href="#rfc.section.10">10.</a>&nbsp;<a id="status.codes" href="#status.codes">Status Code Definitions</a></h1><p id="rfc.section.10.p.1">Each Status-Code is described below, including a description of which method(s) it can follow and any metainformation required in the response.</p><h2 id="rfc.section.10.1"><a href="#rfc.section.10.1">10.1</a>&nbsp;<a id="status.1xx" href="#status.1xx">Informational 1xx</a></h2><p id="rfc.section.10.1.p.1">This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. There are no required headers for this class of status code. Since HTTP/1.0 did not define any 1xx status codes, servers <em class="bcp14">MUST NOT</em> send a 1xx response to an HTTP/1.0 client except under experimental conditions.</p><p id="rfc.section.10.1.p.2">A client <em class="bcp14">MUST</em> be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a <a href="#status.100" class="smpl">100 (Continue)</a> status message. Unexpected 1xx status responses <em class="bcp14">MAY</em> be ignored by a user agent.</p><p id="rfc.section.10.1.p.3">Proxies <em class="bcp14">MUST</em> forward 1xx responses, unless the connection between the proxy and its client has been closed, or unless the proxy itself requested the generation of the 1xx response. (For example, if a proxy adds a "Expect: 100-continue" field when it forwards a request, then it need not forward the corresponding <a href="#status.100" class="smpl">100 (Continue)</a> response(s).)</p><div id="rfc.iref.149"></div><div id="rfc.iref.s.4"></div><h3 id="rfc.section.10.1.1"><a href="#rfc.section.10.1.1">10.1.1</a>&nbsp;<a id="status.100" href="#status.100">100 Continue</a></h3><p id="rfc.section.10.1.1.p.1">The client <em class="bcp14">SHOULD</em> continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client <em class="bcp14">SHOULD</em> continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server <em class="bcp14">MUST</em> send a final response after the request has been completed. See <a href="#use.of.the.100.status" title="Use of the 100 (Continue) Status">Section&nbsp;8.2.3</a> for detailed discussion of the use and handling of this status code.</p><div id="rfc.iref.150"></div><div id="rfc.iref.s.5"></div><h3 id="rfc.section.10.1.2"><a href="#rfc.section.10.1.2">10.1.2</a>&nbsp;<a id="status.101" href="#status.101">101 Switching Protocols</a></h3><p id="rfc.section.10.1.2.p.1">The server understands and is willing to comply with the client's request, via the Upgrade message header field (<a href="#header.upgrade" id="rfc.xref.header.upgrade.2" title="Upgrade">Section&nbsp;14.42</a>), for a change in the application protocol being used on this connection. The server will switch protocols to those defined by the response's Upgrade header field immediately after the empty line which terminates the 101 response.</p><p id="rfc.section.10.1.2.p.2">The protocol <em class="bcp14">SHOULD</em> be switched only when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol might be advantageous when delivering resources that use such features.</p><h2 id="rfc.section.10.2"><a href="#rfc.section.10.2">10.2</a>&nbsp;<a id="status.2xx" href="#status.2xx">Successful 2xx</a></h2><p id="rfc.section.10.2.p.1">This class of status code indicates that the client's request was successfully received, understood, and accepted.</p><div id="rfc.iref.151"></div><div id="rfc.iref.s.6"></div><h3 id="rfc.section.10.2.1"><a href="#rfc.section.10.2.1">10.2.1</a>&nbsp;<a id="status.200" href="#status.200">200 OK</a></h3><p id="rfc.section.10.2.1.p.1">The request has succeeded. The information returned with the response is dependent on the method used in the request, for example: </p><dl><dt>GET</dt><dd>an entity corresponding to the requested resource is sent in the response;</dd><dt>HEAD</dt><dd>the entity-header fields corresponding to the requested resource are sent in the response without any message-body;</dd><dt>POST</dt><dd>an entity describing or containing the result of the action;</dd><dt>TRACE</dt><dd>an entity containing the request message as received by the end server.</dd></dl><div id="rfc.iref.152"></div><div id="rfc.iref.s.7"></div><h3 id="rfc.section.10.2.2"><a href="#rfc.section.10.2.2">10.2.2</a>&nbsp;<a id="status.201" href="#status.201">201 Created</a></h3><p id="rfc.section.10.2.2.p.1">The request has been fulfilled and resulted in a new resource being created. The newly created resource can be referenced by the URI(s) returned in the entity of the response, with the most specific URI for the resource given by a Location header field. The response <em class="bcp14">SHOULD</em> include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. The origin server <em class="bcp14">MUST</em> create the resource before returning the 201 status code. If the action cannot be carried out immediately, the server <em class="bcp14">SHOULD</em> respond with <a href="#status.202" class="smpl">202 (Accepted)</a> response instead.</p><p id="rfc.section.10.2.2.p.2">A 201 response <em class="bcp14">MAY</em> contain an ETag response header field indicating the current value of the entity tag for the requested variant just created, see <a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section&nbsp;14.19</a>.</p><div id="rfc.iref.153"></div><div id="rfc.iref.s.8"></div><h3 id="rfc.section.10.2.3"><a href="#rfc.section.10.2.3">10.2.3</a>&nbsp;<a id="status.202" href="#status.202">202 Accepted</a></h3><p id="rfc.section.10.2.3.p.1">The request has been accepted for processing, but the processing has not been completed. The request might or might not eventually be acted upon, as it might be disallowed when processing actually takes place. There is no facility for re-sending a status code from an asynchronous operation such as this.</p><p id="rfc.section.10.2.3.p.2">The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The entity returned with this response <em class="bcp14">SHOULD</em> include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled.</p><div id="rfc.iref.154"></div><div id="rfc.iref.s.9"></div><h3 id="rfc.section.10.2.4"><a href="#rfc.section.10.2.4">10.2.4</a>&nbsp;<a id="status.203" href="#status.203">203 Non-Authoritative Information</a></h3><p id="rfc.section.10.2.4.p.1">The returned metainformation in the entity-header is not the definitive set as available from the origin server, but is gathered from a local or a third-party copy. The set presented <em class="bcp14">MAY</em> be a subset or superset of the original version. For example, including local annotation information about the resource might result in a superset of the metainformation known by the origin server. Use of this response code is not required and is only appropriate when the response would otherwise be <a href="#status.200" class="smpl">200 (OK)</a>.</p><div id="rfc.iref.155"></div><div id="rfc.iref.s.10"></div><h3 id="rfc.section.10.2.5"><a href="#rfc.section.10.2.5">10.2.5</a>&nbsp;<a id="status.204" href="#status.204">204 No Content</a></h3><p id="rfc.section.10.2.5.p.1">The server has fulfilled the request but does not need to return an entity-body, and might want to return updated metainformation. The response <em class="bcp14">MAY</em> include new or updated metainformation in the form of entity-headers, which if present <em class="bcp14">SHOULD</em> be associated with the requested variant.</p><p id="rfc.section.10.2.5.p.2">If the client is a user agent, it <em class="bcp14">SHOULD NOT</em> change its document view from that which caused the request to be sent. This response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view, although any new or updated metainformation <em class="bcp14">SHOULD</em> be applied to the document currently in the user agent's active view.</p><p id="rfc.section.10.2.5.p.3">The 204 response <em class="bcp14">MUST NOT</em> include a message-body, and thus is always terminated by the first empty line after the header fields.</p><div id="rfc.iref.156"></div><div id="rfc.iref.s.11"></div><h3 id="rfc.section.10.2.6"><a href="#rfc.section.10.2.6">10.2.6</a>&nbsp;<a id="status.205" href="#status.205">205 Reset Content</a></h3><p id="rfc.section.10.2.6.p.1">The server has fulfilled the request and the user agent <em class="bcp14">SHOULD</em> reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action. The response <em class="bcp14">MUST NOT</em> include an entity.</p><div id="rfc.iref.157"></div><div id="rfc.iref.s.12"></div><h3 id="rfc.section.10.2.7"><a href="#rfc.section.10.2.7">10.2.7</a>&nbsp;<a id="status.206" href="#status.206">206 Partial Content</a></h3><p id="rfc.section.10.2.7.p.1">The server has fulfilled the partial GET request for the resource. The request <em class="bcp14">MUST</em> have included a Range header field (<a href="#header.range" id="rfc.xref.header.range.4" title="Range">Section&nbsp;14.35</a>) indicating the desired range, and <em class="bcp14">MAY</em> have included an If-Range header field (<a href="#header.if-range" id="rfc.xref.header.if-range.3" title="If-Range">Section&nbsp;14.27</a>) to make the request conditional.</p><p id="rfc.section.10.2.7.p.2">The response <em class="bcp14">MUST</em> include the following header fields: </p><ul><li>Either a Content-Range header field (<a href="#header.content-range" id="rfc.xref.header.content-range.4" title="Content-Range">Section&nbsp;14.16</a>) indicating the range included with this response, or a multipart/byteranges Content-Type including Content-Range fields for each part. If a Content-Length header field is present in the response, its value <em class="bcp14">MUST</em> match the actual number of <a href="#basic.rules" class="smpl" id="rfc.extref.o.8">OCTET</a>s transmitted in the message-body.</li><li>Date</li><li>ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request</li><li>Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same variant</li></ul><p id="rfc.section.10.2.7.p.3">If the 206 response is the result of an If-Range request that used a strong cache validator (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;13.3.3</a>), the response <em class="bcp14">SHOULD NOT</em> include other entity-headers. If the response is the result of an If-Range request that used a weak validator, the response <em class="bcp14">MUST NOT</em> include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers. Otherwise, the response <em class="bcp14">MUST</em> include all of the entity-headers that would have been returned with a <a href="#status.200" class="smpl">200 (OK)</a> response to the same request.</p><p id="rfc.section.10.2.7.p.4">A cache <em class="bcp14">MUST NOT</em> combine a 206 response with other previously cached content if the ETag or Last-Modified headers do not match exactly, see <a href="#combining.byte.ranges" title="Combining Byte Ranges">13.5.4</a>.</p><p id="rfc.section.10.2.7.p.5">A cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> cache <a href="#status.206" class="smpl">206 (Partial)</a> responses.</p><h2 id="rfc.section.10.3"><a href="#rfc.section.10.3">10.3</a>&nbsp;<a id="status.3xx" href="#status.3xx">Redirection 3xx</a></h2><p id="rfc.section.10.3.p.1">This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. The action required <em class="bcp14">MAY</em> be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A client <em class="bcp14">SHOULD</em> detect infinite redirection loops, since such loops generate network traffic for each redirection. </p><dl class="empty"><dd> <b>Note:</b> previous versions of this specification recommended a maximum of five redirections. Content developers should be aware that there might be clients that implement such a fixed limitation.</dd></dl><div id="rfc.iref.158"></div><div id="rfc.iref.s.13"></div><h3 id="rfc.section.10.3.1"><a href="#rfc.section.10.3.1">10.3.1</a>&nbsp;<a id="status.300" href="#status.300">300 Multiple Choices</a></h3><p id="rfc.section.10.3.1.p.1">The requested resource corresponds to any one of a set of representations, each with its own specific location, and agent-driven negotiation information (<a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>) is being provided so that the user (or user agent) can select a preferred representation and redirect its request to that location.</p><p id="rfc.section.10.3.1.p.2">Unless it was a HEAD request, the response <em class="bcp14">SHOULD</em> include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice <em class="bcp14">MAY</em> be performed automatically. However, this specification does not define any standard for such automatic selection.</p><p id="rfc.section.10.3.1.p.3">If the server has a preferred choice of representation, it <em class="bcp14">SHOULD</em> include the specific URI for that representation in the Location field; user agents <em class="bcp14">MAY</em> use the Location field value for automatic redirection. This response is cacheable unless indicated otherwise.</p><div id="rfc.iref.159"></div><div id="rfc.iref.s.14"></div><h3 id="rfc.section.10.3.2"><a href="#rfc.section.10.3.2">10.3.2</a>&nbsp;<a id="status.301" href="#status.301">301 Moved Permanently</a></h3><p id="rfc.section.10.3.2.p.1">The requested resource has been assigned a new permanent URI and any future references to this resource <em class="bcp14">SHOULD</em> use one of the returned URIs. Clients with link editing capabilities ought to automatically re-link references to the Request-URI to one or more of the new references returned by the server, where possible. This response is cacheable unless indicated otherwise.</p><p id="rfc.section.10.3.2.p.2">The new permanent URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s).</p><p id="rfc.section.10.3.2.p.3">If the 301 status code is received in response to a request other than GET or HEAD, the user agent <em class="bcp14">MUST NOT</em> automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued. </p><dl class="empty"><dd> <b>Note:</b> When automatically redirecting a POST request after receiving a 301 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request.</dd></dl><div id="rfc.iref.160"></div><div id="rfc.iref.s.15"></div><h3 id="rfc.section.10.3.3"><a href="#rfc.section.10.3.3">10.3.3</a>&nbsp;<a id="status.302" href="#status.302">302 Found</a></h3><p id="rfc.section.10.3.3.p.1">The requested resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client <em class="bcp14">SHOULD</em> continue to use the Request-URI for future requests. This response is only cacheable if indicated by a Cache-Control or Expires header field.</p><p id="rfc.section.10.3.3.p.2">The temporary URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s).</p><p id="rfc.section.10.3.3.p.3">If the 302 status code is received in response to a request other than GET or HEAD, the user agent <em class="bcp14">MUST NOT</em> automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued. </p><dl class="empty"><dd> <b>Note:</b> RFC 1945 and RFC 2068 specify that the client is not allowed to change the method on the redirected request. However, most existing user agent implementations treat 302 as if it were a 303 response, performing a GET on the Location field-value regardless of the original request method. The status codes 303 and 307 have been added for servers that wish to make unambiguously clear which kind of reaction is expected of the client.</dd></dl><div id="rfc.iref.161"></div><div id="rfc.iref.s.16"></div><h3 id="rfc.section.10.3.4"><a href="#rfc.section.10.3.4">10.3.4</a>&nbsp;<a id="status.303" href="#status.303">303 See Other</a></h3><p id="rfc.section.10.3.4.p.1">The response to the request can be found under a different URI and <em class="bcp14">SHOULD</em> be retrieved using a GET method on that resource. This method exists primarily to allow the output of a POST-activated script to redirect the user agent to a selected resource. The new URI is not a substitute reference for the originally requested resource. The 303 response <em class="bcp14">MUST NOT</em> be cached, but the response to the second (redirected) request might be cacheable.</p><p id="rfc.section.10.3.4.p.2">The different URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s). </p><dl class="empty"><dd> <b>Note:</b> Many pre-HTTP/1.1 user agents do not understand the 303 status. When interoperability with such clients is a concern, the 302 status code may be used instead, since most user agents react to a 302 response as described here for 303.</dd></dl><div id="rfc.iref.162"></div><div id="rfc.iref.s.17"></div><h3 id="rfc.section.10.3.5"><a href="#rfc.section.10.3.5">10.3.5</a>&nbsp;<a id="status.304" href="#status.304">304 Not Modified</a></h3><p id="rfc.section.10.3.5.p.1">If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server <em class="bcp14">SHOULD</em> respond with this status code. The 304 response <em class="bcp14">MUST NOT</em> contain a message-body, and thus is always terminated by the first empty line after the header fields.</p><p id="rfc.section.10.3.5.p.2">The response <em class="bcp14">MUST</em> include the following header fields: </p><ul><li>Date, unless its omission is required by <a href="#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section&nbsp;14.18.1</a></li></ul><p id="rfc.section.10.3.5.p.3">If a clockless origin server obeys these rules, and proxies and clients add their own Date to any response received without one (as already specified by [RFC 2068], section <a href="http://tools.ietf.org/html/rfc2068#section-14.19" id="rfc.xref.RFC2068.4">14.19</a>), caches will operate correctly. </p><ul><li>ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request</li><li>Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same variant</li></ul><p id="rfc.section.10.3.5.p.4">If the conditional GET used a strong cache validator (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;13.3.3</a>), the response <em class="bcp14">SHOULD NOT</em> include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response <em class="bcp14">MUST NOT</em> include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.</p><p id="rfc.section.10.3.5.p.5">If a 304 response indicates an entity not currently cached, then the cache <em class="bcp14">MUST</em> disregard the response and repeat the request without the conditional.</p><p id="rfc.section.10.3.5.p.6">If a cache uses a received 304 response to update a cache entry, the cache <em class="bcp14">MUST</em> update the entry to reflect any new field values given in the response.</p><div id="rfc.iref.163"></div><div id="rfc.iref.s.18"></div><h3 id="rfc.section.10.3.6"><a href="#rfc.section.10.3.6">10.3.6</a>&nbsp;<a id="status.305" href="#status.305">305 Use Proxy</a></h3><p id="rfc.section.10.3.6.p.1">The requested resource <em class="bcp14">MUST</em> be accessed through the proxy given by the Location field. The Location field gives the URI of the proxy. The recipient is expected to repeat this single request via the proxy. 305 responses <em class="bcp14">MUST</em> only be generated by origin servers. </p><dl class="empty"><dd> <b>Note:</b> RFC 2068 was not clear that 305 was intended to redirect a single request, and to be generated by origin servers only. Not observing these limitations has significant security consequences.</dd></dl><div id="rfc.iref.164"></div><div id="rfc.iref.s.19"></div><h3 id="rfc.section.10.3.7"><a href="#rfc.section.10.3.7">10.3.7</a>&nbsp;<a id="status.306" href="#status.306">306 (Unused)</a></h3><p id="rfc.section.10.3.7.p.1">The 306 status code was used in a previous version of the specification, is no longer used, and the code is reserved.</p><div id="rfc.iref.165"></div><div id="rfc.iref.s.20"></div><h3 id="rfc.section.10.3.8"><a href="#rfc.section.10.3.8">10.3.8</a>&nbsp;<a id="status.307" href="#status.307">307 Temporary Redirect</a></h3><p id="rfc.section.10.3.8.p.1">The requested resource resides temporarily under a different URI. Since the redirection <em class="bcp14">MAY</em> be altered on occasion, the client <em class="bcp14">SHOULD</em> continue to use the Request-URI for future requests. This response is only cacheable if indicated by a Cache-Control or Expires header field.</p><p id="rfc.section.10.3.8.p.2">The temporary URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s) , since many pre-HTTP/1.1 user agents do not understand the 307 status. Therefore, the note <em class="bcp14">SHOULD</em> contain the information necessary for a user to repeat the original request on the new URI.</p><p id="rfc.section.10.3.8.p.3">If the 307 status code is received in response to a request other than GET or HEAD, the user agent <em class="bcp14">MUST NOT</em> automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.</p><h2 id="rfc.section.10.4"><a href="#rfc.section.10.4">10.4</a>&nbsp;<a id="status.4xx" href="#status.4xx">Client Error 4xx</a></h2><p id="rfc.section.10.4.p.1">The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server <em class="bcp14">SHOULD</em> include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents <em class="bcp14">SHOULD</em> display any included entity to the user.</p><p id="rfc.section.10.4.p.2">If the client is sending data, a server implementation using TCP <em class="bcp14">SHOULD</em> be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.</p><div id="rfc.iref.166"></div><div id="rfc.iref.s.21"></div><h3 id="rfc.section.10.4.1"><a href="#rfc.section.10.4.1">10.4.1</a>&nbsp;<a id="status.400" href="#status.400">400 Bad Request</a></h3><p id="rfc.section.10.4.1.p.1">The request could not be understood by the server due to malformed syntax. The client <em class="bcp14">SHOULD NOT</em> repeat the request without modifications.</p><div id="rfc.iref.167"></div><div id="rfc.iref.s.22"></div><h3 id="rfc.section.10.4.2"><a href="#rfc.section.10.4.2">10.4.2</a>&nbsp;<a id="status.401" href="#status.401">401 Unauthorized</a></h3><p id="rfc.section.10.4.2.p.1">The request requires user authentication. The response <em class="bcp14">MUST</em> include a WWW-Authenticate header field (<a href="#header.www-authenticate" id="rfc.xref.header.www-authenticate.2" title="WWW-Authenticate">Section&nbsp;14.47</a>) containing a challenge applicable to the requested resource. The client <em class="bcp14">MAY</em> repeat the request with a suitable Authorization header field (<a href="#header.authorization" id="rfc.xref.header.authorization.2" title="Authorization">Section&nbsp;14.8</a>). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user <em class="bcp14">SHOULD</em> be presented the entity that was given in the response, since that entity might include relevant diagnostic information. HTTP access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication" <a href="#RFC2617" id="rfc.xref.RFC2617.1"><cite title="HTTP Authentication: Basic and Digest Access Authentication">[43]</cite></a>.</p><div id="rfc.iref.168"></div><div id="rfc.iref.s.23"></div><h3 id="rfc.section.10.4.3"><a href="#rfc.section.10.4.3">10.4.3</a>&nbsp;<a id="status.402" href="#status.402">402 Payment Required</a></h3><p id="rfc.section.10.4.3.p.1">This code is reserved for future use.</p><div id="rfc.iref.169"></div><div id="rfc.iref.s.24"></div><h3 id="rfc.section.10.4.4"><a href="#rfc.section.10.4.4">10.4.4</a>&nbsp;<a id="status.403" href="#status.403">403 Forbidden</a></h3><p id="rfc.section.10.4.4.p.1">The server understood the request, but is refusing to fulfill it. Authorization will not help and the request <em class="bcp14">SHOULD NOT</em> be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it <em class="bcp14">SHOULD</em> describe the reason for the refusal in the entity. If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead.</p><div id="rfc.iref.170"></div><div id="rfc.iref.s.25"></div><h3 id="rfc.section.10.4.5"><a href="#rfc.section.10.4.5">10.4.5</a>&nbsp;<a id="status.404" href="#status.404">404 Not Found</a></h3><p id="rfc.section.10.4.5.p.1">The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. The <a href="#status.410" class="smpl">410 (Gone)</a> status code <em class="bcp14">SHOULD</em> be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.</p><div id="rfc.iref.171"></div><div id="rfc.iref.s.26"></div><h3 id="rfc.section.10.4.6"><a href="#rfc.section.10.4.6">10.4.6</a>&nbsp;<a id="status.405" href="#status.405">405 Method Not Allowed</a></h3><p id="rfc.section.10.4.6.p.1">The method specified in the Request-Line is not allowed for the resource identified by the Request-URI. The response <em class="bcp14">MUST</em> include an Allow header containing a list of valid methods for the requested resource.</p><div id="rfc.iref.172"></div><div id="rfc.iref.s.27"></div><h3 id="rfc.section.10.4.7"><a href="#rfc.section.10.4.7">10.4.7</a>&nbsp;<a id="status.406" href="#status.406">406 Not Acceptable</a></h3><p id="rfc.section.10.4.7.p.1">The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.</p><p id="rfc.section.10.4.7.p.2">Unless it was a HEAD request, the response <em class="bcp14">SHOULD</em> include an entity containing a list of available entity characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice <em class="bcp14">MAY</em> be performed automatically. However, this specification does not define any standard for such automatic selection. </p><dl class="empty"><dd> <b>Note:</b> HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept headers sent in the request. In some cases, this may even be preferable to sending a 406 response. User agents are encouraged to inspect the headers of an incoming response to determine if it is acceptable.</dd></dl><p id="rfc.section.10.4.7.p.3">If the response could be unacceptable, a user agent <em class="bcp14">SHOULD</em> temporarily stop receipt of more data and query the user for a decision on further actions.</p><div id="rfc.iref.173"></div><div id="rfc.iref.s.28"></div><h3 id="rfc.section.10.4.8"><a href="#rfc.section.10.4.8">10.4.8</a>&nbsp;<a id="status.407" href="#status.407">407 Proxy Authentication Required</a></h3><p id="rfc.section.10.4.8.p.1">This code is similar to <a href="#status.401" class="smpl">401 (Unauthorized)</a>, but indicates that the client must first authenticate itself with the proxy. The proxy <em class="bcp14">MUST</em> return a Proxy-Authenticate header field (<a href="#header.proxy-authenticate" id="rfc.xref.header.proxy-authenticate.2" title="Proxy-Authenticate">Section&nbsp;14.33</a>) containing a challenge applicable to the proxy for the requested resource. The client <em class="bcp14">MAY</em> repeat the request with a suitable Proxy-Authorization header field (<a href="#header.proxy-authorization" id="rfc.xref.header.proxy-authorization.2" title="Proxy-Authorization">Section&nbsp;14.34</a>). HTTP access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication" <a href="#RFC2617" id="rfc.xref.RFC2617.2"><cite title="HTTP Authentication: Basic and Digest Access Authentication">[43]</cite></a>.</p><div id="rfc.iref.174"></div><div id="rfc.iref.s.29"></div><h3 id="rfc.section.10.4.9"><a href="#rfc.section.10.4.9">10.4.9</a>&nbsp;<a id="status.408" href="#status.408">408 Request Timeout</a></h3><p id="rfc.section.10.4.9.p.1">The client did not produce a request within the time that the server was prepared to wait. The client <em class="bcp14">MAY</em> repeat the request without modifications at any later time.</p><div id="rfc.iref.175"></div><div id="rfc.iref.s.30"></div><h3 id="rfc.section.10.4.10"><a href="#rfc.section.10.4.10">10.4.10</a>&nbsp;<a id="status.409" href="#status.409">409 Conflict</a></h3><p id="rfc.section.10.4.10.p.1">The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body <em class="bcp14">SHOULD</em> include enough information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user agent to fix the problem; however, that might not be possible and is not required.</p><p id="rfc.section.10.4.10.p.2">Conflicts are most likely to occur in response to a PUT request. For example, if versioning were being used and the entity being PUT included changes to a resource which conflict with those made by an earlier (third-party) request, the server might use the 409 response to indicate that it can't complete the request. In this case, the response entity would likely contain a list of the differences between the two versions in a format defined by the response Content-Type.</p><div id="rfc.iref.176"></div><div id="rfc.iref.s.31"></div><h3 id="rfc.section.10.4.11"><a href="#rfc.section.10.4.11">10.4.11</a>&nbsp;<a id="status.410" href="#status.410">410 Gone</a></h3><p id="rfc.section.10.4.11.p.1">The requested resource is no longer available at the server and no forwarding address is known. This condition is expected to be considered permanent. Clients with link editing capabilities <em class="bcp14">SHOULD</em> delete references to the Request-URI after user approval. If the server does not know, or has no facility to determine, whether or not the condition is permanent, the status code <a href="#status.404" class="smpl">404 (Not Found)</a>  <em class="bcp14">SHOULD</em> be used instead. This response is cacheable unless indicated otherwise.</p><p id="rfc.section.10.4.11.p.2">The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for any length of time -- that is left to the discretion of the server owner.</p><div id="rfc.iref.177"></div><div id="rfc.iref.s.32"></div><h3 id="rfc.section.10.4.12"><a href="#rfc.section.10.4.12">10.4.12</a>&nbsp;<a id="status.411" href="#status.411">411 Length Required</a></h3><p id="rfc.section.10.4.12.p.1">The server refuses to accept the request without a defined Content-Length. The client <em class="bcp14">MAY</em> repeat the request if it adds a valid Content-Length header field containing the length of the message-body in the request message.</p><div id="rfc.iref.178"></div><div id="rfc.iref.s.33"></div><h3 id="rfc.section.10.4.13"><a href="#rfc.section.10.4.13">10.4.13</a>&nbsp;<a id="status.412" href="#status.412">412 Precondition Failed</a></h3><p id="rfc.section.10.4.13.p.1">The precondition given in one or more of the request-header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource metainformation (header field data) and thus prevent the requested method from being applied to a resource other than the one intended.</p><div id="rfc.iref.179"></div><div id="rfc.iref.s.34"></div><h3 id="rfc.section.10.4.14"><a href="#rfc.section.10.4.14">10.4.14</a>&nbsp;<a id="status.413" href="#status.413">413 Request Entity Too Large</a></h3><p id="rfc.section.10.4.14.p.1">The server is refusing to process a request because the request entity is larger than the server is willing or able to process. The server <em class="bcp14">MAY</em> close the connection to prevent the client from continuing the request.</p><p id="rfc.section.10.4.14.p.2">If the condition is temporary, the server <em class="bcp14">SHOULD</em> include a Retry-After header field to indicate that it is temporary and after what time the client <em class="bcp14">MAY</em> try again.</p><div id="rfc.iref.180"></div><div id="rfc.iref.s.35"></div><h3 id="rfc.section.10.4.15"><a href="#rfc.section.10.4.15">10.4.15</a>&nbsp;<a id="status.414" href="#status.414">414 Request-URI Too Long</a></h3><p id="rfc.section.10.4.15.p.1">The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a URI "black hole" of redirection (e.g., a redirected URI prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the Request-URI.</p><div id="rfc.iref.181"></div><div id="rfc.iref.s.36"></div><h3 id="rfc.section.10.4.16"><a href="#rfc.section.10.4.16">10.4.16</a>&nbsp;<a id="status.415" href="#status.415">415 Unsupported Media Type</a></h3><p id="rfc.section.10.4.16.p.1">The server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method.</p><div id="rfc.iref.182"></div><div id="rfc.iref.s.37"></div><h3 id="rfc.section.10.4.17"><a href="#rfc.section.10.4.17">10.4.17</a>&nbsp;<a id="status.416" href="#status.416">416 Requested Range Not Satisfiable</a></h3><p id="rfc.section.10.4.17.p.1">A server <em class="bcp14">SHOULD</em> return a response with this status code if a request included a Range request-header field (<a href="#header.range" id="rfc.xref.header.range.5" title="Range">Section&nbsp;14.35</a>), and none of the range-specifier values in this field overlap the current extent of the selected resource, and the request did not include an If-Range request-header field. (For byte-ranges, this means that the first-byte-pos of all of the byte-range-spec values were greater than the current length of the selected resource.)</p><p id="rfc.section.10.4.17.p.2">When this status code is returned for a byte-range request, the response <em class="bcp14">SHOULD</em> include a Content-Range entity-header field specifying the current length of the selected resource (see <a href="#header.content-range" id="rfc.xref.header.content-range.5" title="Content-Range">Section&nbsp;14.16</a>). This response <em class="bcp14">MUST NOT</em> use the multipart/byteranges content-type.</p><div id="rfc.iref.183"></div><div id="rfc.iref.s.38"></div><h3 id="rfc.section.10.4.18"><a href="#rfc.section.10.4.18">10.4.18</a>&nbsp;<a id="status.417" href="#status.417">417 Expectation Failed</a></h3><p id="rfc.section.10.4.18.p.1">The expectation given in an Expect request-header field (see <a href="#header.expect" id="rfc.xref.header.expect.4" title="Expect">Section&nbsp;14.20</a>) could not be met by this server, or, if the server is a proxy, the server has unambiguous evidence that the request could not be met by the next-hop server.</p><h2 id="rfc.section.10.5"><a href="#rfc.section.10.5">10.5</a>&nbsp;<a id="status.5xx" href="#status.5xx">Server Error 5xx</a></h2><p id="rfc.section.10.5.p.1">Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has erred or is incapable of performing the request. Except when responding to a HEAD request, the server <em class="bcp14">SHOULD</em> include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. User agents <em class="bcp14">SHOULD</em> display any included entity to the user. These response codes are applicable to any request method.</p><div id="rfc.iref.184"></div><div id="rfc.iref.s.39"></div><h3 id="rfc.section.10.5.1"><a href="#rfc.section.10.5.1">10.5.1</a>&nbsp;<a id="status.500" href="#status.500">500 Internal Server Error</a></h3><p id="rfc.section.10.5.1.p.1">The server encountered an unexpected condition which prevented it from fulfilling the request.</p><div id="rfc.iref.185"></div><div id="rfc.iref.s.40"></div><h3 id="rfc.section.10.5.2"><a href="#rfc.section.10.5.2">10.5.2</a>&nbsp;<a id="status.501" href="#status.501">501 Not Implemented</a></h3><p id="rfc.section.10.5.2.p.1">The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.</p><div id="rfc.iref.186"></div><div id="rfc.iref.s.41"></div><h3 id="rfc.section.10.5.3"><a href="#rfc.section.10.5.3">10.5.3</a>&nbsp;<a id="status.502" href="#status.502">502 Bad Gateway</a></h3><p id="rfc.section.10.5.3.p.1">The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.</p><div id="rfc.iref.187"></div><div id="rfc.iref.s.42"></div><h3 id="rfc.section.10.5.4"><a href="#rfc.section.10.5.4">10.5.4</a>&nbsp;<a id="status.503" href="#status.503">503 Service Unavailable</a></h3><p id="rfc.section.10.5.4.p.1">The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay <em class="bcp14">MAY</em> be indicated in a Retry-After header. If no Retry-After is given, the client <em class="bcp14">SHOULD</em> handle the response as it would for a 500 response. </p><dl class="empty"><dd> <b>Note:</b> The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.</dd></dl><div id="rfc.iref.188"></div><div id="rfc.iref.s.43"></div><h3 id="rfc.section.10.5.5"><a href="#rfc.section.10.5.5">10.5.5</a>&nbsp;<a id="status.504" href="#status.504">504 Gateway Timeout</a></h3><p id="rfc.section.10.5.5.p.1">The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server specified by the URI (e.g. HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed to access in attempting to complete the request. </p><dl class="empty"><dd> <b>Note:</b> Note to implementors: some deployed proxies are known to return 400 or 500 when DNS lookups time out.</dd></dl><div id="rfc.iref.189"></div><div id="rfc.iref.s.44"></div><h3 id="rfc.section.10.5.6"><a href="#rfc.section.10.5.6">10.5.6</a>&nbsp;<a id="status.505" href="#status.505">505 HTTP Version Not Supported</a></h3><p id="rfc.section.10.5.6.p.1">The server does not support, or refuses to support, the HTTP protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client, as described in <a href="#http.version" title="HTTP Version">Section&nbsp;3.1</a>, other than with this error message. The response <em class="bcp14">SHOULD</em> contain an entity describing why that version is not supported and what other protocols are supported by that server.</p><hr class="noprint"><h1 id="rfc.section.11" class="np"><a href="#rfc.section.11">11.</a>&nbsp;<a id="access.authentication" href="#access.authentication">Access Authentication</a></h1><p id="rfc.section.11.p.1">HTTP provides several <em class="bcp14">OPTIONAL</em> challenge-response authentication mechanisms which can be used by a server to challenge a client request and by a client to provide authentication information. The general framework for access authentication, and the specification of "basic" and "digest" authentication, are specified in "HTTP Authentication: Basic and Digest Access Authentication" <a href="#RFC2617" id="rfc.xref.RFC2617.3"><cite title="HTTP Authentication: Basic and Digest Access Authentication">[43]</cite></a>. This specification adopts the definitions of "challenge" and "credentials" from that specification.</p><hr class="noprint"><h1 id="rfc.section.12" class="np"><a href="#rfc.section.12">12.</a>&nbsp;<a id="content.negotiation" href="#content.negotiation">Content Negotiation</a></h1><p id="rfc.section.12.p.1">Most HTTP responses include an entity which contains information for interpretation by a human user. Naturally, it is desirable to supply the user with the "best available" entity corresponding to the request. Unfortunately for servers and caches, not all users have the same preferences for what is "best," and not all user agents are equally capable of rendering all entity types. For that reason, HTTP has provisions for several mechanisms for "content negotiation" -- the process of selecting the best representation for a given response when there are multiple representations available. </p><dl class="empty"><dd> <b>Note:</b> This is not called "format negotiation" because the alternate representations may be of the same media type, but use different capabilities of that type, be in different languages, etc.</dd></dl><p id="rfc.section.12.p.2">Any response containing an entity-body <em class="bcp14">MAY</em> be subject to negotiation, including error responses.</p><p id="rfc.section.12.p.3">There are two kinds of content negotiation which are possible in HTTP: server-driven and agent-driven negotiation. These two kinds of negotiation are orthogonal and thus may be used separately or in combination. One method of combination, referred to as transparent negotiation, occurs when a cache uses the agent-driven negotiation information provided by the origin server in order to provide server-driven negotiation for subsequent requests.</p><h2 id="rfc.section.12.1"><a href="#rfc.section.12.1">12.1</a>&nbsp;<a id="server-driven.negotiation" href="#server-driven.negotiation">Server-driven Negotiation</a></h2><p id="rfc.section.12.1.p.1">If the selection of the best representation for a response is made by an algorithm located at the server, it is called server-driven negotiation. Selection is based on the available representations of the response (the dimensions over which it can vary; e.g. language, content-coding, etc.) and the contents of particular header fields in the request message or on other information pertaining to the request (such as the network address of the client).</p><p id="rfc.section.12.1.p.2">Server-driven negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to the user agent, or when the server desires to send its "best guess" to the client along with the first response (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to improve the server's guess, the user agent <em class="bcp14">MAY</em> include request header fields (Accept, Accept-Language, Accept-Encoding, etc.) which describe its preferences for such a response.</p><p id="rfc.section.12.1.p.3">Server-driven negotiation has disadvantages: </p><ol><li>It is impossible for the server to accurately determine what might be "best" for any given user, since that would require complete knowledge of both the capabilities of the user agent and the intended use for the response (e.g., does the user want to view it on screen or print it on paper?).</li><li>Having the user agent describe its capabilities in every request can be both very inefficient (given that only a small percentage of responses have multiple representations) and a potential violation of the user's privacy.</li><li>It complicates the implementation of an origin server and the algorithms for generating responses to a request.</li><li>It may limit a public cache's ability to use the same response for multiple user's requests.</li></ol><p id="rfc.section.12.1.p.4">HTTP/1.1 includes the following request-header fields for enabling server-driven negotiation through description of user agent capabilities and user preferences: Accept (<a href="#header.accept" id="rfc.xref.header.accept.3" title="Accept">Section&nbsp;14.1</a>), Accept-Charset (<a href="#header.accept-charset" id="rfc.xref.header.accept-charset.2" title="Accept-Charset">Section&nbsp;14.2</a>), Accept-Encoding (<a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.3" title="Accept-Encoding">Section&nbsp;14.3</a>), Accept-Language (<a href="#header.accept-language" id="rfc.xref.header.accept-language.2" title="Accept-Language">Section&nbsp;14.4</a>), and User-Agent (<a href="#header.user-agent" id="rfc.xref.header.user-agent.2" title="User-Agent">Section&nbsp;14.43</a>). However, an origin server is not limited to these dimensions and <em class="bcp14">MAY</em> vary the response based on any aspect of the request, including information outside the request-header fields or within extension header fields not defined by this specification.</p><p id="rfc.section.12.1.p.5">The Vary header field can be used to express the parameters the server uses to select a representation that is subject to server-driven negotiation. See <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;13.6</a> for use of the Vary header field by caches and <a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section&nbsp;14.44</a> for use of the Vary header field by servers.</p><h2 id="rfc.section.12.2"><a href="#rfc.section.12.2">12.2</a>&nbsp;<a id="agent-driven.negotiation" href="#agent-driven.negotiation">Agent-driven Negotiation</a></h2><p id="rfc.section.12.2.p.1">With agent-driven negotiation, selection of the best representation for a response is performed by the user agent after receiving an initial response from the origin server. Selection is based on a list of the available representations of the response included within the header fields or entity-body of the initial response, with each representation identified by its own URI. Selection from among the representations may be performed automatically (if the user agent is capable of doing so) or manually by the user selecting from a generated (possibly hypertext) menu.</p><p id="rfc.section.12.2.p.2">Agent-driven negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally when public caches are used to distribute server load and reduce network usage.</p><p id="rfc.section.12.2.p.3">Agent-driven negotiation suffers from the disadvantage of needing a second request to obtain the best alternate representation. This second request is only efficient when caching is used. In addition, this specification does not define any mechanism for supporting automatic selection, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1.</p><p id="rfc.section.12.2.p.4">HTTP/1.1 defines the <a href="#status.300" class="smpl">300 (Multiple Choices)</a> and <a href="#status.406" class="smpl">406 (Not Acceptable)</a> status codes for enabling agent-driven negotiation when the server is unwilling or unable to provide a varying response using server-driven negotiation.</p><h2 id="rfc.section.12.3"><a href="#rfc.section.12.3">12.3</a>&nbsp;<a id="transparent.negotiation" href="#transparent.negotiation">Transparent Negotiation</a></h2><p id="rfc.section.12.3.p.1">Transparent negotiation is a combination of both server-driven and agent-driven negotiation. When a cache is supplied with a form of the list of available representations of the response (as in agent-driven negotiation) and the dimensions of variance are completely understood by the cache, then the cache becomes capable of performing server-driven negotiation on behalf of the origin server for subsequent requests on that resource.</p><p id="rfc.section.12.3.p.2">Transparent negotiation has the advantage of distributing the negotiation work that would otherwise be required of the origin server and also removing the second request delay of agent-driven negotiation when the cache is able to correctly guess the right response.</p><p id="rfc.section.12.3.p.3">This specification does not define any mechanism for transparent negotiation, though it also does not prevent any such mechanism from being developed as an extension that could be used within HTTP/1.1.</p><hr class="noprint"><h1 id="rfc.section.13" class="np"><a href="#rfc.section.13">13.</a>&nbsp;<a id="caching" href="#caching">Caching in HTTP</a></h1><p id="rfc.section.13.p.1">HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches. The HTTP/1.1 protocol includes a number of elements intended to make caching work as well as possible. Because these elements are inextricable from other aspects of the protocol, and because they interact with each other, it is useful to describe the basic caching design of HTTP separately from the detailed descriptions of methods, headers, response codes, etc.</p><p id="rfc.section.13.p.2">Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former reduces the number of network round-trips required for many operations; we use an "expiration" mechanism for this purpose (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;13.2</a>). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;13.3</a>).</p><p id="rfc.section.13.p.3">Requirements for performance, availability, and disconnected operation require us to be able to relax the goal of semantic transparency. The HTTP/1.1 protocol allows origin servers, caches, and clients to explicitly reduce transparency when necessary. However, because non-transparent operation may confuse non-expert users, and might be incompatible with certain server applications (such as those for ordering merchandise), the protocol requires that transparency be relaxed </p><ul><li>only by an explicit protocol-level request when relaxed by client or origin server</li><li>only with an explicit warning to the end user when relaxed by cache or client</li></ul><p id="rfc.section.13.p.4">Therefore, the HTTP/1.1 protocol provides these important elements: </p><ol><li>Protocol features that provide full semantic transparency when this is required by all parties.</li><li>Protocol features that allow an origin server or user agent to explicitly request and control non-transparent operation.</li><li>Protocol features that allow a cache to attach warnings to responses that do not preserve the requested approximation of semantic transparency.</li></ol><p id="rfc.section.13.p.5">A basic principle is that it must be possible for the clients to detect any potential relaxation of semantic transparency. </p><dl class="empty"><dd> <b>Note:</b> The server, cache, or client implementor might be faced with design decisions not explicitly discussed in this specification. If a decision might affect semantic transparency, the implementor ought to err on the side of maintaining transparency unless a careful and complete analysis shows significant benefits in breaking transparency.</dd></dl><h2 id="rfc.section.13.1"><a href="#rfc.section.13.1">13.1</a>&nbsp;</h2><h3 id="rfc.section.13.1.1"><a href="#rfc.section.13.1.1">13.1.1</a>&nbsp;<a id="cache.correctness" href="#cache.correctness">Cache Correctness</a></h3><p id="rfc.section.13.1.1.p.1">A correct cache <em class="bcp14">MUST</em> respond to a request with the most up-to-date response held by the cache that is appropriate to the request (see sections <a href="#disambiguating.expiration.values" title="Disambiguating Expiration Values">13.2.5</a>, <a href="#disambiguating.multiple.responses" title="Disambiguating Multiple Responses">13.2.6</a>, and <a href="#cache.replacement" title="Cache Replacement">13.12</a>) which meets one of the following conditions: </p><ol><li>It has been checked for equivalence with what the origin server would have returned by revalidating the response with the origin server (<a href="#validation.model" title="Validation Model">Section&nbsp;13.3</a>);</li><li>It is "fresh enough" (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;13.2</a>). In the default case, this means it meets the least restrictive freshness requirement of the client, origin server, and cache (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;14.9</a>); if the origin server so specifies, it is the freshness requirement of the origin server alone. If a stored response is not "fresh enough" by the most restrictive freshness requirement of both the client and the origin server, in carefully considered circumstances the cache <em class="bcp14">MAY</em> still return the response with the appropriate Warning header (see section <a href="#exceptions.to.the.rules.and.warnings" title="Exceptions to the Rules and Warnings">13.1.5</a> and <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">14.46</a>), unless such a response is prohibited (e.g., by a "no-store" cache-directive, or by a "no-cache" cache-request-directive; see <a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;14.9</a>).</li><li>It is an appropriate <a href="#status.304" class="smpl">304 (Not Modified)</a>, <a href="#status.305" class="smpl">305 (Proxy Redirect)</a>, or error (4xx or 5xx) response message.</li></ol><p id="rfc.section.13.1.1.p.2">If the cache can not communicate with the origin server, then a correct cache <em class="bcp14">SHOULD</em> respond as above if the response can be correctly served from the cache; if not it <em class="bcp14">MUST</em> return an error or warning indicating that there was a communication failure.</p><p id="rfc.section.13.1.1.p.3">If a cache receives a response (either an entire response, or a 304 (Not Modified) response) that it would normally forward to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">SHOULD</em> forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache <em class="bcp14">SHOULD NOT</em> attempt to revalidate a response simply because that response became stale in transit; this might lead to an infinite loop. A user agent that receives a stale response without a Warning <em class="bcp14">MAY</em> display a warning indication to the user.</p><h3 id="rfc.section.13.1.2"><a href="#rfc.section.13.1.2">13.1.2</a>&nbsp;<a id="warnings" href="#warnings">Warnings</a></h3><p id="rfc.section.13.1.2.p.1">Whenever a cache returns a response that is neither first-hand nor "fresh enough" (in the sense of condition 2 in <a href="#cache.correctness" title="Cache Correctness">Section&nbsp;13.1.1</a>), it <em class="bcp14">MUST</em> attach a warning to that effect, using a Warning general-header. The Warning header and the currently defined warnings are described in <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;14.46</a>. The warning allows clients to take appropriate action.</p><p id="rfc.section.13.1.2.p.2">Warnings <em class="bcp14">MAY</em> be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish these responses from true failures.</p><p id="rfc.section.13.1.2.p.3">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning <em class="bcp14">MUST</em> or <em class="bcp14">MUST NOT</em> be deleted from a stored cache entry after a successful revalidation:</p><p id="rfc.section.13.1.2.p.4"> </p><dl><dt>1xx</dt><dd>Warnings that describe the freshness or revalidation status of the response, and so <em class="bcp14">MUST</em> be deleted after a successful revalidation. 1XX warn-codes <em class="bcp14">MAY</em> be generated by a cache only when validating a cached entry. It <em class="bcp14">MUST NOT</em> be generated by clients.</dd><dt>2xx</dt><dd>Warnings that describe some aspect of the entity body or entity headers that is not rectified by a revalidation (for example, a lossy compression of the entity bodies) and which <em class="bcp14">MUST NOT</em> be deleted after a successful revalidation.</dd></dl><p id="rfc.section.13.1.2.p.5">See <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;14.46</a> for the definitions of the codes themselves.</p><p id="rfc.section.13.1.2.p.6">HTTP/1.0 caches will cache all Warnings in responses, without deleting the ones in the first category. Warnings in responses that are passed to HTTP/1.0 caches carry an extra warning-date field, which prevents a future HTTP/1.1 recipient from believing an erroneously cached Warning.</p><p id="rfc.section.13.1.2.p.7">Warnings also carry a warning text. The text <em class="bcp14">MAY</em> be in any appropriate natural language (perhaps based on the client's Accept headers), and include an <em class="bcp14">OPTIONAL</em> indication of what character set is used.</p><p id="rfc.section.13.1.2.p.8">Multiple warnings <em class="bcp14">MAY</em> be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number. For example, a server might provide the same warning with texts in both English and Basque.</p><p id="rfc.section.13.1.2.p.9">When multiple warnings are attached to a response, it might not be practical or reasonable to display all of them to the user. This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but does suggest some heuristics.</p><h3 id="rfc.section.13.1.3"><a href="#rfc.section.13.1.3">13.1.3</a>&nbsp;<a id="cache-control.mechanisms" href="#cache-control.mechanisms">Cache-control Mechanisms</a></h3><p id="rfc.section.13.1.3.p.1">The basic cache mechanisms in HTTP/1.1 (server-specified expiration times and validators) are implicit directives to caches. In some cases, a server or client might need to provide explicit directives to the HTTP caches. We use the Cache-Control header for this purpose.</p><p id="rfc.section.13.1.3.p.2">The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between header values, the most restrictive interpretation is applied (that is, the one that is most likely to preserve semantic transparency). However, in some cases, cache-control directives are explicitly specified as weakening the approximation of semantic transparency (for example, "max-stale" or "public").</p><p id="rfc.section.13.1.3.p.3">The cache-control directives are described in detail in <a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;14.9</a>.</p><h3 id="rfc.section.13.1.4"><a href="#rfc.section.13.1.4">13.1.4</a>&nbsp;<a id="explicit.ua.warnings" href="#explicit.ua.warnings">Explicit User Agent Warnings</a></h3><p id="rfc.section.13.1.4.p.1">Many user agents make it possible for users to override the basic caching mechanisms. For example, the user agent might allow the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually add "Cache-Control: max-stale=3600" to every request. The user agent <em class="bcp14">SHOULD NOT</em> default to either non-transparent behavior, or behavior that results in abnormally ineffective caching, but <em class="bcp14">MAY</em> be explicitly configured to do so by an explicit action of the user.</p><p id="rfc.section.13.1.4.p.2">If the user has overridden the basic caching mechanisms, the user agent <em class="bcp14">SHOULD</em> explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other indicator.</p><p id="rfc.section.13.1.4.p.3">If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user agent <em class="bcp14">SHOULD</em> continually indicate this state to the user (for example, by a display of a picture of currency in flames) so that the user does not inadvertently consume excess resources or suffer from excessive latency.</p><h3 id="rfc.section.13.1.5"><a href="#rfc.section.13.1.5">13.1.5</a>&nbsp;<a id="exceptions.to.the.rules.and.warnings" href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></h3><p id="rfc.section.13.1.5.p.1">In some cases, the operator of a cache <em class="bcp14">MAY</em> choose to configure it to return stale responses even when not requested by clients. This decision ought not be made lightly, but may be necessary for reasons of availability or performance, especially when the cache is poorly connected to the origin server. Whenever a cache returns a stale response, it <em class="bcp14">MUST</em> mark it as such (using a Warning header) enabling the client software to alert the user that there might be a potential problem.</p><p id="rfc.section.13.1.5.p.2">It also allows the user agent to take steps to obtain a first-hand or fresh response. For this reason, a cache <em class="bcp14">SHOULD NOT</em> return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply for technical or policy reasons.</p><h3 id="rfc.section.13.1.6"><a href="#rfc.section.13.1.6">13.1.6</a>&nbsp;<a id="client-controlled.behavior" href="#client-controlled.behavior">Client-controlled Behavior</a></h3><p id="rfc.section.13.1.6.p.1">While the origin server (and to a lesser extent, intermediate caches, by their contribution to the age of a response) are the primary source of expiration information, in some cases the client might need to control a cache's decision about whether to return a cached response without validating it. Clients do this using several directives of the Cache-Control header.</p><p id="rfc.section.13.1.6.p.2">A client's request <em class="bcp14">MAY</em> specify the maximum age it is willing to accept of an unvalidated response; specifying a value of zero forces the cache(s) to revalidate all responses. A client <em class="bcp14">MAY</em> also specify the minimum time remaining before a response expires. Both of these options increase constraints on the behavior of caches, and so cannot further relax the cache's approximation of semantic transparency.</p><p id="rfc.section.13.1.6.p.3">A client <em class="bcp14">MAY</em> also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on the caches, and so might violate the origin server's specified constraints on semantic transparency, but might be necessary to support disconnected operation, or high availability in the face of poor connectivity.</p><h2 id="rfc.section.13.2"><a href="#rfc.section.13.2">13.2</a>&nbsp;<a id="expiration.model" href="#expiration.model">Expiration Model</a></h2><h3 id="rfc.section.13.2.1"><a href="#rfc.section.13.2.1">13.2.1</a>&nbsp;<a id="server-specified.expiration" href="#server-specified.expiration">Server-Specified Expiration</a></h3><p id="rfc.section.13.2.1.p.1">HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding requests is for an origin server to provide an explicit expiration time in the future, indicating that a response <em class="bcp14">MAY</em> be used to satisfy subsequent requests. In other words, a cache can return a fresh response without first contacting the server.</p><p id="rfc.section.13.2.1.p.2">Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic transparency, as long as the server's expiration times are carefully chosen.</p><p id="rfc.section.13.2.1.p.3">The expiration mechanism applies only to responses taken from a cache and not to first-hand responses forwarded immediately to the requesting client.</p><p id="rfc.section.13.2.1.p.4">If an origin server wishes to force a semantically transparent cache to validate every request, it <em class="bcp14">MAY</em> assign an explicit expiration time in the past. This means that the response is always stale, and so the cache <em class="bcp14">SHOULD</em> validate it before using it for subsequent requests. See <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;14.9.4</a> for a more restrictive way to force revalidation.</p><p id="rfc.section.13.2.1.p.5">If an origin server wishes to force any HTTP/1.1 cache, no matter how it is configured, to validate every request, it <em class="bcp14">SHOULD</em> use the "must-revalidate" cache-control directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.5" title="Cache-Control">Section&nbsp;14.9</a>).</p><p id="rfc.section.13.2.1.p.6">Servers specify explicit expiration times using either the Expires header, or the max-age directive of the Cache-Control header.</p><p id="rfc.section.13.2.1.p.7">An expiration time cannot be used to force a user agent to refresh its display or reload a resource; its semantics apply only to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource is initiated. See <a href="#history.lists" title="History Lists">Section&nbsp;13.13</a> for an explanation of the difference between caches and history mechanisms.</p><h3 id="rfc.section.13.2.2"><a href="#rfc.section.13.2.2">13.2.2</a>&nbsp;<a id="heuristic.expiration" href="#heuristic.expiration">Heuristic Expiration</a></h3><p id="rfc.section.13.2.2.p.1">Since origin servers do not always provide explicit expiration times, HTTP caches typically assign heuristic expiration times, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results. Since heuristic expiration times might compromise semantic transparency, they ought to used cautiously, and we encourage origin servers to provide explicit expiration times as much as possible.</p><h3 id="rfc.section.13.2.3"><a href="#rfc.section.13.2.3">13.2.3</a>&nbsp;<a id="age.calculations" href="#age.calculations">Age Calculations</a></h3><p id="rfc.section.13.2.3.p.1">In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how to calculate the latter in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;13.2.4</a>; this section describes how to calculate the age of a response or cache entry.</p><p id="rfc.section.13.2.3.p.2">In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially hosts running origin servers and caches, <em class="bcp14">SHOULD</em> use NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[28]</cite></a> or some similar protocol to synchronize their clocks to a globally accurate time standard.</p><p id="rfc.section.13.2.3.p.3">HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response was generated (see <a href="#header.date" id="rfc.xref.header.date.2" title="Date">Section&nbsp;14.18</a>). We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.</p><p id="rfc.section.13.2.3.p.4">HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The Age field value is the cache's estimate of the amount of time since the response was generated or revalidated by the origin server.</p><p id="rfc.section.13.2.3.p.5">In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths.</p><p id="rfc.section.13.2.3.p.6">We use the term "age_value" to denote the value of the Age header, in a form appropriate for arithmetic operations.</p><p id="rfc.section.13.2.3.p.7">A response's age can be calculated in two entirely independent ways: </p><ol><li>now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative, the result is replaced by zero.</li><li>age_value, if all of the caches along the response path implement HTTP/1.1.</li></ol><p id="rfc.section.13.2.3.p.8">Given that we have two independent ways to compute the age of a response when it is received, we can combine these as</p><div id="rfc.figure.u.54"></div><pre class="text">    corrected_received_age = max(now - date_value, age_value)
    583 </pre><p id="rfc.section.13.2.3.p.10">and as long as we have either nearly synchronized clocks or all-HTTP/1.1 paths, one gets a reliable (conservative) result.</p><p id="rfc.section.13.2.3.p.11">Because of network-imposed delays, some significant interval might pass between the time that a server generates a response and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low ages.</p><p id="rfc.section.13.2.3.p.12">Because the request that resulted in the returned Age value must have been initiated prior to that Age value's generation, we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age value is received, it <em class="bcp14">MUST</em> be interpreted relative to the time the request was initiated, not the time that the response was received. This algorithm results in conservative behavior no matter how much delay is experienced. So, we compute:</p><div id="rfc.figure.u.55"></div><pre class="text">   corrected_initial_age = corrected_received_age
     1981</pre><p id="rfc.section.7.1.p.3">The extension-header mechanism allows additional entity-header fields to be defined without changing the protocol, but these
     1982         fields cannot be assumed to be recognizable by the recipient. Unrecognized header fields <em class="bcp14">SHOULD</em> be ignored by the recipient and <em class="bcp14">MUST</em> be forwarded by transparent proxies.
     1983      </p>
     1984      <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a id="entity.body" href="#entity.body">Entity Body</a></h2>
     1985      <p id="rfc.section.7.2.p.1">The entity-body (if any) sent with an HTTP request or response is in a format and encoding defined by the entity-header fields.</p>
     1986      <div id="rfc.figure.u.52"></div><pre class="inline"><span id="rfc.iref.g.93"></span>    <a href="#entity.body" class="smpl">entity-body</a>    = *<a href="#basic.rules" class="smpl" id="rfc.extref.o.7">OCTET</a>
     1987</pre><p id="rfc.section.7.2.p.3">An entity-body is only present in a message when a message-body is present, as described in <a href="#message.body" title="Message Body">Section&nbsp;4.3</a>. The entity-body is obtained from the message-body by decoding any Transfer-Encoding that might have been applied to ensure
     1988         safe and proper transfer of the message.
     1989      </p>
     1990      <h3 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;<a id="type" href="#type">Type</a></h3>
     1991      <p id="rfc.section.7.2.1.p.1">When an entity-body is included with a message, the data type of that body is determined via the header fields Content-Type
     1992         and Content-Encoding. These define a two-layer, ordered encoding model:
     1993      </p>
     1994      <div id="rfc.figure.u.53"></div><pre class="text">    entity-body := Content-Encoding( Content-Type( data ) )
     1995</pre><p id="rfc.section.7.2.1.p.3">Content-Type specifies the media type of the underlying data. Content-Encoding may be used to indicate any additional content
     1996         codings applied to the data, usually for the purpose of data compression, that are a property of the requested resource. There
     1997         is no default encoding.
     1998      </p>
     1999      <p id="rfc.section.7.2.1.p.4">Any HTTP/1.1 message containing an entity-body <em class="bcp14">SHOULD</em> include a Content-Type header field defining the media type of that body. If and only if the media type is not given by a
     2000         Content-Type field, the recipient <em class="bcp14">MAY</em> attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the
     2001         resource. If the media type remains unknown, the recipient <em class="bcp14">SHOULD</em> treat it as type "application/octet-stream".
     2002      </p>
     2003      <h3 id="rfc.section.7.2.2"><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;<a id="entity.length" href="#entity.length">Entity Length</a></h3>
     2004      <p id="rfc.section.7.2.2.p.1">The entity-length of a message is the length of the message-body before any transfer-codings have been applied. <a href="#message.length" title="Message Length">Section&nbsp;4.4</a> defines how the transfer-length of a message-body is determined.
     2005      </p>
     2006      <hr class="noprint">
     2007      <h1 id="rfc.section.8" class="np"><a href="#rfc.section.8">8.</a>&nbsp;<a id="connections" href="#connections">Connections</a></h1>
     2008      <h2 id="rfc.section.8.1"><a href="#rfc.section.8.1">8.1</a>&nbsp;<a id="persistent.connections" href="#persistent.connections">Persistent Connections</a></h2>
     2009      <h3 id="rfc.section.8.1.1"><a href="#rfc.section.8.1.1">8.1.1</a>&nbsp;<a id="persistent.purpose" href="#persistent.purpose">Purpose</a></h3>
     2010      <p id="rfc.section.8.1.1.p.1">Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP
     2011         servers and causing congestion on the Internet. The use of inline images and other associated data often require a client
     2012         to make multiple requests of the same server in a short amount of time. Analysis of these performance problems and results
     2013         from a prototype implementation are available <a href="#Pad1995" id="rfc.xref.Pad1995.1"><cite title="Improving HTTP Latency">[26]</cite></a>  <a href="#Spe" id="rfc.xref.Spe.1"><cite title="Analysis of HTTP Performance Problems">[30]</cite></a>. Implementation experience and measurements of actual HTTP/1.1 (RFC 2068) implementations show good results <a href="#Nie1997" id="rfc.xref.Nie1997.1"><cite title="Network Performance Effects of HTTP/1.1, CSS1, and PNG">[39]</cite></a>. Alternatives have also been explored, for example, T/TCP <a href="#Tou1998" id="rfc.xref.Tou1998.1"><cite title="Analysis of HTTP Performance">[27]</cite></a>.
     2014      </p>
     2015      <p id="rfc.section.8.1.1.p.2">Persistent HTTP connections have a number of advantages: </p>
     2016      <ul>
     2017         <li>By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways,
     2018            tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.
     2019         </li>
     2020         <li>HTTP requests and responses can be pipelined on a connection. Pipelining allows a client to make multiple requests without
     2021            waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.
     2022         </li>
     2023         <li>Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to
     2024            determine the congestion state of the network.
     2025         </li>
     2026         <li>Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.</li>
     2027         <li>HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using
     2028            future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old
     2029            semantics after an error is reported.
     2030         </li>
     2031      </ul>
     2032      <p id="rfc.section.8.1.1.p.3">HTTP implementations <em class="bcp14">SHOULD</em> implement persistent connections.
     2033      </p>
     2034      <h3 id="rfc.section.8.1.2"><a href="#rfc.section.8.1.2">8.1.2</a>&nbsp;<a id="persistent.overall" href="#persistent.overall">Overall Operation</a></h3>
     2035      <p id="rfc.section.8.1.2.p.1">A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior
     2036         of any HTTP connection. That is, unless otherwise indicated, the client <em class="bcp14">SHOULD</em> assume that the server will maintain a persistent connection, even after error responses from the server.
     2037      </p>
     2038      <p id="rfc.section.8.1.2.p.2">Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling
     2039         takes place using the Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.2" title="Connection">Section&nbsp;14.10</a>). Once a close has been signaled, the client <em class="bcp14">MUST NOT</em> send any more requests on that connection.
     2040      </p>
     2041      <h4 id="rfc.section.8.1.2.1"><a href="#rfc.section.8.1.2.1">8.1.2.1</a>&nbsp;<a id="persistent.negotiation" href="#persistent.negotiation">Negotiation</a></h4>
     2042      <p id="rfc.section.8.1.2.1.p.1">An HTTP/1.1 server <em class="bcp14">MAY</em> assume that a HTTP/1.1 client intends to maintain a persistent connection unless a Connection header including the connection-token
     2043         "close" was sent in the request. If the server chooses to close the connection immediately after sending the response, it <em class="bcp14">SHOULD</em> send a Connection header including the connection-token close.
     2044      </p>
     2045      <p id="rfc.section.8.1.2.1.p.2">An HTTP/1.1 client <em class="bcp14">MAY</em> expect a connection to remain open, but would decide to keep it open based on whether the response from a server contains
     2046         a Connection header with the connection-token close. In case the client does not want to maintain a connection for more than
     2047         that request, it <em class="bcp14">SHOULD</em> send a Connection header including the connection-token close.
     2048      </p>
     2049      <p id="rfc.section.8.1.2.1.p.3">If either the client or the server sends the close token in the Connection header, that request becomes the last one for the
     2050         connection.
     2051      </p>
     2052      <p id="rfc.section.8.1.2.1.p.4">Clients and servers <em class="bcp14">SHOULD NOT</em> assume that a persistent connection is maintained for HTTP versions less than 1.1 unless it is explicitly signaled. See <a href="#compatibility.with.http.1.0.persistent.connections" title="Compatibility with HTTP/1.0 Persistent Connections">Appendix&nbsp;19.6.2</a> for more information on backward compatibility with HTTP/1.0 clients.
     2053      </p>
     2054      <p id="rfc.section.8.1.2.1.p.5">In order to remain persistent, all messages on the connection <em class="bcp14">MUST</em> have a self-defined message length (i.e., one not defined by closure of the connection), as described in <a href="#message.length" title="Message Length">Section&nbsp;4.4</a>.
     2055      </p>
     2056      <h4 id="rfc.section.8.1.2.2"><a href="#rfc.section.8.1.2.2">8.1.2.2</a>&nbsp;<a id="pipelining" href="#pipelining">Pipelining</a></h4>
     2057      <p id="rfc.section.8.1.2.2.p.1">A client that supports persistent connections <em class="bcp14">MAY</em> "pipeline" its requests (i.e., send multiple requests without waiting for each response). A server <em class="bcp14">MUST</em> send its responses to those requests in the same order that the requests were received.
     2058      </p>
     2059      <p id="rfc.section.8.1.2.2.p.2">Clients which assume persistent connections and pipeline immediately after connection establishment <em class="bcp14">SHOULD</em> be prepared to retry their connection if the first pipelined attempt fails. If a client does such a retry, it <em class="bcp14">MUST NOT</em> pipeline before it knows the connection is persistent. Clients <em class="bcp14">MUST</em> also be prepared to resend their requests if the server closes the connection before sending all of the corresponding responses.
     2060      </p>
     2061      <p id="rfc.section.8.1.2.2.p.3">Clients <em class="bcp14">SHOULD NOT</em> pipeline requests using non-idempotent methods or non-idempotent sequences of methods (see <a href="#idempotent.methods" title="Idempotent Methods">Section&nbsp;9.1.2</a>). Otherwise, a premature termination of the transport connection could lead to indeterminate results. A client wishing to
     2062         send a non-idempotent request <em class="bcp14">SHOULD</em> wait to send that request until it has received the response status for the previous request.
     2063      </p>
     2064      <h3 id="rfc.section.8.1.3"><a href="#rfc.section.8.1.3">8.1.3</a>&nbsp;<a id="persistent.proxy" href="#persistent.proxy">Proxy Servers</a></h3>
     2065      <p id="rfc.section.8.1.3.p.1">It is especially important that proxies correctly implement the properties of the Connection header field as specified in <a href="#header.connection" id="rfc.xref.header.connection.3" title="Connection">Section&nbsp;14.10</a>.
     2066      </p>
     2067      <p id="rfc.section.8.1.3.p.2">The proxy server <em class="bcp14">MUST</em> signal persistent connections separately with its clients and the origin servers (or other proxy servers) that it connects
     2068         to. Each persistent connection applies to only one transport link.
     2069      </p>
     2070      <p id="rfc.section.8.1.3.p.3">A proxy server <em class="bcp14">MUST NOT</em> establish a HTTP/1.1 persistent connection with an HTTP/1.0 client (but see RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[33]</cite></a> for information and discussion of the problems with the Keep-Alive header implemented by many HTTP/1.0 clients).
     2071      </p>
     2072      <h3 id="rfc.section.8.1.4"><a href="#rfc.section.8.1.4">8.1.4</a>&nbsp;<a id="persistent.practical" href="#persistent.practical">Practical Considerations</a></h3>
     2073      <p id="rfc.section.8.1.4.p.1">Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers
     2074         might make this a higher value since it is likely that the client will be making more connections through the same server.
     2075         The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client
     2076         or the server.
     2077      </p>
     2078      <p id="rfc.section.8.1.4.p.2">When a client or server wishes to time-out it <em class="bcp14">SHOULD</em> issue a graceful close on the transport connection. Clients and servers <em class="bcp14">SHOULD</em> both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does
     2079         not detect the other side's close promptly it could cause unnecessary resource drain on the network.
     2080      </p>
     2081      <p id="rfc.section.8.1.4.p.3">A client, server, or proxy <em class="bcp14">MAY</em> close the transport connection at any time. For example, a client might have started to send a new request at the same time
     2082         that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed
     2083         while it was idle, but from the client's point of view, a request is in progress.
     2084      </p>
     2085      <p id="rfc.section.8.1.4.p.4">This means that clients, servers, and proxies <em class="bcp14">MUST</em> be able to recover from asynchronous close events. Client software <em class="bcp14">SHOULD</em> reopen the transport connection and retransmit the aborted sequence of requests without user interaction so long as the request
     2086         sequence is idempotent (see <a href="#idempotent.methods" title="Idempotent Methods">Section&nbsp;9.1.2</a>). Non-idempotent methods or sequences <em class="bcp14">MUST NOT</em> be automatically retried, although user agents <em class="bcp14">MAY</em> offer a human operator the choice of retrying the request(s). Confirmation by user-agent software with semantic understanding
     2087         of the application <em class="bcp14">MAY</em> substitute for user confirmation. The automatic retry <em class="bcp14">SHOULD NOT</em> be repeated if the second sequence of requests fails.
     2088      </p>
     2089      <p id="rfc.section.8.1.4.p.5">Servers <em class="bcp14">SHOULD</em> always respond to at least one request per connection, if at all possible. Servers <em class="bcp14">SHOULD NOT</em> close a connection in the middle of transmitting a response, unless a network or client failure is suspected.
     2090      </p>
     2091      <p id="rfc.section.8.1.4.p.6">Clients that use persistent connections <em class="bcp14">SHOULD</em> limit the number of simultaneous connections that they maintain to a given server. A single-user client <em class="bcp14">SHOULD NOT</em> maintain more than 2 connections with any server or proxy. A proxy <em class="bcp14">SHOULD</em> use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines
     2092         are intended to improve HTTP response times and avoid congestion.
     2093      </p>
     2094      <h2 id="rfc.section.8.2"><a href="#rfc.section.8.2">8.2</a>&nbsp;<a id="message.transmission.requirements" href="#message.transmission.requirements">Message Transmission Requirements</a></h2>
     2095      <h3 id="rfc.section.8.2.1"><a href="#rfc.section.8.2.1">8.2.1</a>&nbsp;<a id="persistent.flow" href="#persistent.flow">Persistent Connections and Flow Control</a></h3>
     2096      <p id="rfc.section.8.2.1.p.1">HTTP/1.1 servers <em class="bcp14">SHOULD</em> maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating
     2097         connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.
     2098      </p>
     2099      <h3 id="rfc.section.8.2.2"><a href="#rfc.section.8.2.2">8.2.2</a>&nbsp;<a id="persistent.monitor" href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></h3>
     2100      <p id="rfc.section.8.2.2.p.1">An HTTP/1.1 (or later) client sending a message-body <em class="bcp14">SHOULD</em> monitor the network connection for an error status while it is transmitting the request. If the client sees an error status,
     2101         it <em class="bcp14">SHOULD</em> immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.6</a>), a zero length chunk and empty trailer <em class="bcp14">MAY</em> be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header, the client <em class="bcp14">MUST</em> close the connection.
     2102      </p>
     2103      <h3 id="rfc.section.8.2.3"><a href="#rfc.section.8.2.3">8.2.3</a>&nbsp;<a id="use.of.the.100.status" href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></h3>
     2104      <p id="rfc.section.8.2.3.p.1">The purpose of the <a href="#status.100" class="smpl">100 (Continue)</a> status (see <a href="#status.100" id="rfc.xref.status.100.2" title="100 Continue">Section&nbsp;10.1.1</a>) is to allow a client that is sending a request message with a request body to determine if the origin server is willing
     2105         to accept the request (based on the request headers) before the client sends the request body. In some cases, it might either
     2106         be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking
     2107         at the body.
     2108      </p>
     2109      <p id="rfc.section.8.2.3.p.2">Requirements for HTTP/1.1 clients: </p>
     2110      <ul>
     2111         <li>If a client will wait for a <a href="#status.100" class="smpl">100 (Continue)</a> response before sending the request body, it <em class="bcp14">MUST</em> send an Expect request-header field (<a href="#header.expect" id="rfc.xref.header.expect.2" title="Expect">Section&nbsp;14.20</a>) with the "100-continue" expectation.
     2112         </li>
     2113         <li>A client <em class="bcp14">MUST NOT</em> send an Expect request-header field (<a href="#header.expect" id="rfc.xref.header.expect.3" title="Expect">Section&nbsp;14.20</a>) with the "100-continue" expectation if it does not intend to send a request body.
     2114         </li>
     2115      </ul>
     2116      <p id="rfc.section.8.2.3.p.3">Because of the presence of older implementations, the protocol allows ambiguous situations in which a client may send "Expect:
     2117         100-continue" without receiving either a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status or a <a href="#status.100" class="smpl">100 (Continue)</a> status. Therefore, when a client sends this header field to an origin server (possibly via a proxy) from which it has never
     2118         seen a <a href="#status.100" class="smpl">100 (Continue)</a> status, the client <em class="bcp14">SHOULD NOT</em> wait for an indefinite period before sending the request body.
     2119      </p>
     2120      <p id="rfc.section.8.2.3.p.4">Requirements for HTTP/1.1 origin servers: </p>
     2121      <ul>
     2122         <li>Upon receiving a request which includes an Expect request-header field with the "100-continue" expectation, an origin server <em class="bcp14">MUST</em> either respond with <a href="#status.100" class="smpl">100 (Continue)</a> status and continue to read from the input stream, or respond with a final status code. The origin server <em class="bcp14">MUST NOT</em> wait for the request body before sending the <a href="#status.100" class="smpl">100 (Continue)</a> response. If it responds with a final status code, it <em class="bcp14">MAY</em> close the transport connection or it <em class="bcp14">MAY</em> continue to read and discard the rest of the request. It <em class="bcp14">MUST NOT</em> perform the requested method if it returns a final status code.
     2123         </li>
     2124         <li>An origin server <em class="bcp14">SHOULD NOT</em> send a <a href="#status.100" class="smpl">100 (Continue)</a> response if the request message does not include an Expect request-header field with the "100-continue" expectation, and <em class="bcp14">MUST NOT</em> send a <a href="#status.100" class="smpl">100 (Continue)</a> response if such a request comes from an HTTP/1.0 (or earlier) client. There is an exception to this rule: for compatibility
     2125            with RFC 2068, a server <em class="bcp14">MAY</em> send a <a href="#status.100" class="smpl">100 (Continue)</a> status in response to an HTTP/1.1 PUT or POST request that does not include an Expect request-header field with the "100-continue"
     2126            expectation. This exception, the purpose of which is to minimize any client processing delays associated with an undeclared
     2127            wait for <a href="#status.100" class="smpl">100 (Continue)</a> status, applies only to HTTP/1.1 requests, and not to requests with any other HTTP-version value.
     2128         </li>
     2129         <li>An origin server <em class="bcp14">MAY</em> omit a <a href="#status.100" class="smpl">100 (Continue)</a> response if it has already received some or all of the request body for the corresponding request.
     2130         </li>
     2131         <li>An origin server that sends a <a href="#status.100" class="smpl">100 (Continue)</a> response <em class="bcp14">MUST</em> ultimately send a final status code, once the request body is received and processed, unless it terminates the transport connection
     2132            prematurely.
     2133         </li>
     2134         <li>If an origin server receives a request that does not include an Expect request-header field with the "100-continue" expectation,
     2135            the request includes a request body, and the server responds with a final status code before reading the entire request body
     2136            from the transport connection, then the server <em class="bcp14">SHOULD NOT</em> close the transport connection until it has read the entire request, or until the client closes the connection. Otherwise,
     2137            the client might not reliably receive the response message. However, this requirement is not be construed as preventing a
     2138            server from defending itself against denial-of-service attacks, or from badly broken client implementations.
     2139         </li>
     2140      </ul>
     2141      <p id="rfc.section.8.2.3.p.5">Requirements for HTTP/1.1 proxies: </p>
     2142      <ul>
     2143         <li>If a proxy receives a request that includes an Expect request-header field with the "100-continue" expectation, and the proxy
     2144            either knows that the next-hop server complies with HTTP/1.1 or higher, or does not know the HTTP version of the next-hop
     2145            server, it <em class="bcp14">MUST</em> forward the request, including the Expect header field.
     2146         </li>
     2147         <li>If the proxy knows that the version of the next-hop server is HTTP/1.0 or lower, it <em class="bcp14">MUST NOT</em> forward the request, and it <em class="bcp14">MUST</em> respond with a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status.
     2148         </li>
     2149         <li>Proxies <em class="bcp14">SHOULD</em> maintain a cache recording the HTTP version numbers received from recently-referenced next-hop servers.
     2150         </li>
     2151         <li>A proxy <em class="bcp14">MUST NOT</em> forward a <a href="#status.100" class="smpl">100 (Continue)</a> response if the request message was received from an HTTP/1.0 (or earlier) client and did not include an Expect request-header
     2152            field with the "100-continue" expectation. This requirement overrides the general rule for forwarding of 1xx responses (see <a href="#status.1xx" title="Informational 1xx">Section&nbsp;10.1</a>).
     2153         </li>
     2154      </ul>
     2155      <h3 id="rfc.section.8.2.4"><a href="#rfc.section.8.2.4">8.2.4</a>&nbsp;<a id="connection.premature" href="#connection.premature">Client Behavior if Server Prematurely Closes Connection</a></h3>
     2156      <p id="rfc.section.8.2.4.p.1">If an HTTP/1.1 client sends a request which includes a request body, but which does not include an Expect request-header field
     2157         with the "100-continue" expectation, and if the client is not directly connected to an HTTP/1.1 origin server, and if the
     2158         client sees the connection close before receiving any status from the server, the client <em class="bcp14">SHOULD</em> retry the request. If the client does retry this request, it <em class="bcp14">MAY</em> use the following "binary exponential backoff" algorithm to be assured of obtaining a reliable response:
     2159      </p>
     2160      <ol>
     2161         <li>Initiate a new connection to the server</li>
     2162         <li>Transmit the request-headers</li>
     2163         <li>Initialize a variable R to the estimated round-trip time to the server (e.g., based on the time it took to establish the connection),
     2164            or to a constant value of 5 seconds if the round-trip time is not available.
     2165         </li>
     2166         <li>Compute T = R * (2**N), where N is the number of previous retries of this request.</li>
     2167         <li>Wait either for an error response from the server, or for T seconds (whichever comes first)</li>
     2168         <li>If no error response is received, after T seconds transmit the body of the request.</li>
     2169         <li>If client sees that the connection is closed prematurely, repeat from step 1 until the request is accepted, an error response
     2170            is received, or the user becomes impatient and terminates the retry process.
     2171         </li>
     2172      </ol>
     2173      <p id="rfc.section.8.2.4.p.2">If at any point an error status is received, the client </p>
     2174      <ul>
     2175         <li><em class="bcp14">SHOULD NOT</em> continue and
     2176         </li>
     2177         <li><em class="bcp14">SHOULD</em> close the connection if it has not completed sending the request message.
     2178         </li>
     2179      </ul>
     2180      <hr class="noprint">
     2181      <h1 id="rfc.section.9" class="np"><a href="#rfc.section.9">9.</a>&nbsp;<a id="method.definitions" href="#method.definitions">Method Definitions</a></h1>
     2182      <p id="rfc.section.9.p.1">The set of common methods for HTTP/1.1 is defined below. Although this set can be expanded, additional methods cannot be assumed
     2183         to share the same semantics for separately extended clients and servers.
     2184      </p>
     2185      <p id="rfc.section.9.p.2">The Host request-header field (<a href="#header.host" id="rfc.xref.header.host.2" title="Host">Section&nbsp;14.23</a>) <em class="bcp14">MUST</em> accompany all HTTP/1.1 requests.
     2186      </p>
     2187      <h2 id="rfc.section.9.1"><a href="#rfc.section.9.1">9.1</a>&nbsp;<a id="safe.and.idempotent" href="#safe.and.idempotent">Safe and Idempotent Methods</a></h2>
     2188      <h3 id="rfc.section.9.1.1"><a href="#rfc.section.9.1.1">9.1.1</a>&nbsp;<a id="safe.methods" href="#safe.methods">Safe Methods</a></h3>
     2189      <p id="rfc.section.9.1.1.p.1">Implementors should be aware that the software represents the user in their interactions over the Internet, and should be
     2190         careful to allow the user to be aware of any actions they might take which may have an unexpected significance to themselves
     2191         or others.
     2192      </p>
     2193      <p id="rfc.section.9.1.1.p.2">In particular, the convention has been established that the GET and HEAD methods <em class="bcp14">SHOULD NOT</em> have the significance of taking an action other than retrieval. These methods ought to be considered "safe". This allows user
     2194         agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact
     2195         that a possibly unsafe action is being requested.
     2196      </p>
     2197      <p id="rfc.section.9.1.1.p.3">Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request;
     2198         in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the
     2199         side-effects, so therefore cannot be held accountable for them.
     2200      </p>
     2201      <h3 id="rfc.section.9.1.2"><a href="#rfc.section.9.1.2">9.1.2</a>&nbsp;<a id="idempotent.methods" href="#idempotent.methods">Idempotent Methods</a></h3>
     2202      <p id="rfc.section.9.1.2.p.1">Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N
     2203         &gt; 0 identical requests is the same as for a single request. The methods GET, HEAD, PUT and DELETE share this property. Also,
     2204         the methods OPTIONS and TRACE <em class="bcp14">SHOULD NOT</em> have side effects, and so are inherently idempotent.
     2205      </p>
     2206      <p id="rfc.section.9.1.2.p.2">However, it is possible that a sequence of several requests is non-idempotent, even if all of the methods executed in that
     2207         sequence are idempotent. (A sequence is idempotent if a single execution of the entire sequence always yields a result that
     2208         is not changed by a reexecution of all, or part, of that sequence.) For example, a sequence is non-idempotent if its result
     2209         depends on a value that is later modified in the same sequence.
     2210      </p>
     2211      <p id="rfc.section.9.1.2.p.3">A sequence that never has side effects is idempotent, by definition (provided that no concurrent operations are being executed
     2212         on the same set of resources).
     2213      </p>
     2214      <div id="rfc.iref.o.3"></div>
     2215      <div id="rfc.iref.m.2"></div>
     2216      <h2 id="rfc.section.9.2"><a href="#rfc.section.9.2">9.2</a>&nbsp;<a id="OPTIONS" href="#OPTIONS">OPTIONS</a></h2>
     2217      <p id="rfc.section.9.2.p.1">The OPTIONS method represents a request for information about the communication options available on the request/response
     2218         chain identified by the Request-URI. This method allows the client to determine the options and/or requirements associated
     2219         with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.
     2220      </p>
     2221      <p id="rfc.section.9.2.p.2">Responses to this method are not cacheable.</p>
     2222      <p id="rfc.section.9.2.p.3">If the OPTIONS request includes an entity-body (as indicated by the presence of Content-Length or Transfer-Encoding), then
     2223         the media type <em class="bcp14">MUST</em> be indicated by a Content-Type field. Although this specification does not define any use for such a body, future extensions
     2224         to HTTP might use the OPTIONS body to make more detailed queries on the server. A server that does not support such an extension <em class="bcp14">MAY</em> discard the request body.
     2225      </p>
     2226      <p id="rfc.section.9.2.p.4">If the Request-URI is an asterisk ("*"), the OPTIONS request is intended to apply to the server in general rather than to
     2227         a specific resource. Since a server's communication options typically depend on the resource, the "*" request is only useful
     2228         as a "ping" or "no-op" type of method; it does nothing beyond allowing the client to test the capabilities of the server.
     2229         For example, this can be used to test a proxy for HTTP/1.1 compliance (or lack thereof).
     2230      </p>
     2231      <p id="rfc.section.9.2.p.5">If the Request-URI is not an asterisk, the OPTIONS request applies only to the options that are available when communicating
     2232         with that resource.
     2233      </p>
     2234      <p id="rfc.section.9.2.p.6">A 200 response <em class="bcp14">SHOULD</em> include any header fields that indicate optional features implemented by the server and applicable to that resource (e.g.,
     2235         Allow), possibly including extensions not defined by this specification. The response body, if any, <em class="bcp14">SHOULD</em> also include information about the communication options. The format for such a body is not defined by this specification,
     2236         but might be defined by future extensions to HTTP. Content negotiation <em class="bcp14">MAY</em> be used to select the appropriate response format. If no response body is included, the response <em class="bcp14">MUST</em> include a Content-Length field with a field-value of "0".
     2237      </p>
     2238      <p id="rfc.section.9.2.p.7">The Max-Forwards request-header field <em class="bcp14">MAY</em> be used to target a specific proxy in the request chain. When a proxy receives an OPTIONS request on an absoluteURI for which
     2239         request forwarding is permitted, the proxy <em class="bcp14">MUST</em> check for a Max-Forwards field. If the Max-Forwards field-value is zero ("0"), the proxy <em class="bcp14">MUST NOT</em> forward the message; instead, the proxy <em class="bcp14">SHOULD</em> respond with its own communication options. If the Max-Forwards field-value is an integer greater than zero, the proxy <em class="bcp14">MUST</em> decrement the field-value when it forwards the request. If no Max-Forwards field is present in the request, then the forwarded
     2240         request <em class="bcp14">MUST NOT</em> include a Max-Forwards field.
     2241      </p>
     2242      <div id="rfc.iref.g.94"></div>
     2243      <div id="rfc.iref.m.3"></div>
     2244      <h2 id="rfc.section.9.3"><a href="#rfc.section.9.3">9.3</a>&nbsp;<a id="GET" href="#GET">GET</a></h2>
     2245      <p id="rfc.section.9.3.p.1">The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI
     2246         refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not
     2247         the source text of the process, unless that text happens to be the output of the process.
     2248      </p>
     2249      <p id="rfc.section.9.3.p.2">The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since,
     2250         If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under
     2251         the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary
     2252         network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already
     2253         held by the client.
     2254      </p>
     2255      <p id="rfc.section.9.3.p.3">The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial
     2256         GET requests that only part of the entity be transferred, as described in <a href="#header.range" id="rfc.xref.header.range.3" title="Range">Section&nbsp;14.35</a>. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed
     2257         without transferring data already held by the client.
     2258      </p>
     2259      <p id="rfc.section.9.3.p.4">The response to a GET request is cacheable if and only if it meets the requirements for HTTP caching described in <a href="#caching" title="Caching in HTTP">Section&nbsp;13</a>.
     2260      </p>
     2261      <p id="rfc.section.9.3.p.5">See <a href="#encoding.sensitive.information.in.uris" title="Encoding Sensitive Information in URI's">Section&nbsp;15.1.3</a> for security considerations when used for forms.
     2262      </p>
     2263      <div id="rfc.iref.h.3"></div>
     2264      <div id="rfc.iref.m.4"></div>
     2265      <h2 id="rfc.section.9.4"><a href="#rfc.section.9.4">9.4</a>&nbsp;<a id="HEAD" href="#HEAD">HEAD</a></h2>
     2266      <p id="rfc.section.9.4.p.1">The HEAD method is identical to GET except that the server <em class="bcp14">MUST NOT</em> return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request <em class="bcp14">SHOULD</em> be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about
     2267         the entity implied by the request without transferring the entity-body itself. This method is often used for testing hypertext
     2268         links for validity, accessibility, and recent modification.
     2269      </p>
     2270      <p id="rfc.section.9.4.p.2">The response to a HEAD request <em class="bcp14">MAY</em> be cacheable in the sense that the information contained in the response <em class="bcp14">MAY</em> be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs
     2271         from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the
     2272         cache <em class="bcp14">MUST</em> treat the cache entry as stale.
     2273      </p>
     2274      <div id="rfc.iref.p.2"></div>
     2275      <div id="rfc.iref.m.5"></div>
     2276      <h2 id="rfc.section.9.5"><a href="#rfc.section.9.5">9.5</a>&nbsp;<a id="POST" href="#POST">POST</a></h2>
     2277      <p id="rfc.section.9.5.p.1">The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of
     2278         the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following
     2279         functions:
     2280      </p>
     2281      <ul>
     2282         <li>Annotation of existing resources;</li>
     2283         <li>Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;</li>
     2284         <li>Providing a block of data, such as the result of submitting a form, to a data-handling process;</li>
     2285         <li>Extending a database through an append operation.</li>
     2286      </ul>
     2287      <p id="rfc.section.9.5.p.2">The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI.
     2288         The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news
     2289         article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.
     2290      </p>
     2291      <p id="rfc.section.9.5.p.3">The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either <a href="#status.200" class="smpl">200 (OK)</a> or <a href="#status.204" class="smpl">204 (No Content)</a> is the appropriate response status, depending on whether or not the response includes an entity that describes the result.
     2292      </p>
     2293      <p id="rfc.section.9.5.p.4">If a resource has been created on the origin server, the response <em class="bcp14">SHOULD</em> be <a href="#status.201" class="smpl">201 (Created)</a> and contain an entity which describes the status of the request and refers to the new resource, and a Location header (see <a href="#header.location" id="rfc.xref.header.location.2" title="Location">Section&nbsp;14.30</a>).
     2294      </p>
     2295      <p id="rfc.section.9.5.p.5">Responses to this method are not cacheable, unless the response includes appropriate Cache-Control or Expires header fields.
     2296         However, the <a href="#status.303" class="smpl">303 (See Other)</a> response can be used to direct the user agent to retrieve a cacheable resource.
     2297      </p>
     2298      <p id="rfc.section.9.5.p.6">POST requests <em class="bcp14">MUST</em> obey the message transmission requirements set out in <a href="#message.transmission.requirements" title="Message Transmission Requirements">Section&nbsp;8.2</a>.
     2299      </p>
     2300      <p id="rfc.section.9.5.p.7">See <a href="#encoding.sensitive.information.in.uris" title="Encoding Sensitive Information in URI's">Section&nbsp;15.1.3</a> for security considerations.
     2301      </p>
     2302      <div id="rfc.iref.p.3"></div>
     2303      <div id="rfc.iref.m.6"></div>
     2304      <h2 id="rfc.section.9.6"><a href="#rfc.section.9.6">9.6</a>&nbsp;<a id="PUT" href="#PUT">PUT</a></h2>
     2305      <p id="rfc.section.9.6.p.1">The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an
     2306         already existing resource, the enclosed entity <em class="bcp14">SHOULD</em> be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing
     2307         resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create
     2308         the resource with that URI. If a new resource is created, the origin server <em class="bcp14">MUST</em> inform the user agent via the <a href="#status.201" class="smpl">201 (Created)</a> response. If an existing resource is modified, either the <a href="#status.200" class="smpl">200 (OK)</a> or <a href="#status.204" class="smpl">204 (No Content)</a> response codes <em class="bcp14">SHOULD</em> be sent to indicate successful completion of the request. If the resource could not be created or modified with the Request-URI,
     2309         an appropriate error response <em class="bcp14">SHOULD</em> be given that reflects the nature of the problem. The recipient of the entity <em class="bcp14">MUST NOT</em> ignore any Content-* (e.g. Content-Range) headers that it does not understand or implement and <em class="bcp14">MUST</em> return a <a href="#status.501" class="smpl">501 (Not Implemented)</a> response in such cases.
     2310      </p>
     2311      <p id="rfc.section.9.6.p.2">If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries <em class="bcp14">SHOULD</em> be treated as stale. Responses to this method are not cacheable.
     2312      </p>
     2313      <p id="rfc.section.9.6.p.3">The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The
     2314         URI in a POST request identifies the resource that will handle the enclosed entity. That resource might be a data-accepting
     2315         process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request
     2316         identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server <em class="bcp14">MUST NOT</em> attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI,
     2317         it <em class="bcp14">MUST</em> send a <a href="#status.301" class="smpl">301 (Moved Permanently)</a> response; the user agent <em class="bcp14">MAY</em> then make its own decision regarding whether or not to redirect the request.
     2318      </p>
     2319      <p id="rfc.section.9.6.p.4">A single resource <em class="bcp14">MAY</em> be identified by many different URIs. For example, an article might have a URI for identifying "the current version" which
     2320         is separate from the URI identifying each particular version. In this case, a PUT request on a general URI might result in
     2321         several other URIs being defined by the origin server.
     2322      </p>
     2323      <p id="rfc.section.9.6.p.5">HTTP/1.1 does not define how a PUT method affects the state of an origin server.</p>
     2324      <p id="rfc.section.9.6.p.6">PUT requests <em class="bcp14">MUST</em> obey the message transmission requirements set out in <a href="#message.transmission.requirements" title="Message Transmission Requirements">Section&nbsp;8.2</a>.
     2325      </p>
     2326      <p id="rfc.section.9.6.p.7">Unless otherwise specified for a particular entity-header, the entity-headers in the PUT request <em class="bcp14">SHOULD</em> be applied to the resource created or modified by the PUT.
     2327      </p>
     2328      <div id="rfc.iref.d.3"></div>
     2329      <div id="rfc.iref.m.7"></div>
     2330      <h2 id="rfc.section.9.7"><a href="#rfc.section.9.7">9.7</a>&nbsp;<a id="DELETE" href="#DELETE">DELETE</a></h2>
     2331      <p id="rfc.section.9.7.p.1">The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method <em class="bcp14">MAY</em> be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation
     2332         has been carried out, even if the status code returned from the origin server indicates that the action has been completed
     2333         successfully. However, the server <em class="bcp14">SHOULD NOT</em> indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible
     2334         location.
     2335      </p>
     2336      <p id="rfc.section.9.7.p.2">A successful response <em class="bcp14">SHOULD</em> be <a href="#status.200" class="smpl">200 (OK)</a> if the response includes an entity describing the status, <a href="#status.202" class="smpl">202 (Accepted)</a> if the action has not yet been enacted, or <a href="#status.204" class="smpl">204 (No Content)</a> if the action has been enacted but the response does not include an entity.
     2337      </p>
     2338      <p id="rfc.section.9.7.p.3">If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries <em class="bcp14">SHOULD</em> be treated as stale. Responses to this method are not cacheable.
     2339      </p>
     2340      <div id="rfc.iref.t.2"></div>
     2341      <div id="rfc.iref.m.8"></div>
     2342      <h2 id="rfc.section.9.8"><a href="#rfc.section.9.8">9.8</a>&nbsp;<a id="TRACE" href="#TRACE">TRACE</a></h2>
     2343      <p id="rfc.section.9.8.p.1">The TRACE method is used to invoke a remote, application-layer loop-back of the request message. The final recipient of the
     2344         request <em class="bcp14">SHOULD</em> reflect the message received back to the client as the entity-body of a <a href="#status.200" class="smpl">200 (OK)</a> response. The final recipient is either the origin server or the first proxy or gateway to receive a Max-Forwards value of
     2345         zero (0) in the request (see <a href="#header.max-forwards" id="rfc.xref.header.max-forwards.2" title="Max-Forwards">Section&nbsp;14.31</a>). A TRACE request <em class="bcp14">MUST NOT</em> include an entity.
     2346      </p>
     2347      <p id="rfc.section.9.8.p.2">TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing
     2348         or diagnostic information. The value of the Via header field (<a href="#header.via" id="rfc.xref.header.via.2" title="Via">Section&nbsp;14.45</a>) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the
     2349         client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an
     2350         infinite loop.
     2351      </p>
     2352      <p id="rfc.section.9.8.p.3">If the request is valid, the response <em class="bcp14">SHOULD</em> contain the entire request message in the entity-body, with a Content-Type of "message/http". Responses to this method <em class="bcp14">MUST NOT</em> be cached.
     2353      </p>
     2354      <div id="rfc.iref.c.12"></div>
     2355      <div id="rfc.iref.m.9"></div>
     2356      <h2 id="rfc.section.9.9"><a href="#rfc.section.9.9">9.9</a>&nbsp;<a id="CONNECT" href="#CONNECT">CONNECT</a></h2>
     2357      <p id="rfc.section.9.9.p.1">This specification reserves the method name CONNECT for use with a proxy that can dynamically switch to being a tunnel (e.g.
     2358         SSL tunneling <a href="#Luo1998" id="rfc.xref.Luo1998.1"><cite title="Tunneling TCP based protocols through Web proxy servers">[44]</cite></a>).
     2359      </p>
     2360      <hr class="noprint">
     2361      <h1 id="rfc.section.10" class="np"><a href="#rfc.section.10">10.</a>&nbsp;<a id="status.codes" href="#status.codes">Status Code Definitions</a></h1>
     2362      <p id="rfc.section.10.p.1">Each Status-Code is described below, including a description of which method(s) it can follow and any metainformation required
     2363         in the response.
     2364      </p>
     2365      <h2 id="rfc.section.10.1"><a href="#rfc.section.10.1">10.1</a>&nbsp;<a id="status.1xx" href="#status.1xx">Informational 1xx</a></h2>
     2366      <p id="rfc.section.10.1.p.1">This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is
     2367         terminated by an empty line. There are no required headers for this class of status code. Since HTTP/1.0 did not define any
     2368         1xx status codes, servers <em class="bcp14">MUST NOT</em> send a 1xx response to an HTTP/1.0 client except under experimental conditions.
     2369      </p>
     2370      <p id="rfc.section.10.1.p.2">A client <em class="bcp14">MUST</em> be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a <a href="#status.100" class="smpl">100 (Continue)</a> status message. Unexpected 1xx status responses <em class="bcp14">MAY</em> be ignored by a user agent.
     2371      </p>
     2372      <p id="rfc.section.10.1.p.3">Proxies <em class="bcp14">MUST</em> forward 1xx responses, unless the connection between the proxy and its client has been closed, or unless the proxy itself
     2373         requested the generation of the 1xx response. (For example, if a proxy adds a "Expect: 100-continue" field when it forwards
     2374         a request, then it need not forward the corresponding <a href="#status.100" class="smpl">100 (Continue)</a> response(s).)
     2375      </p>
     2376      <div id="rfc.iref.149"></div>
     2377      <div id="rfc.iref.s.4"></div>
     2378      <h3 id="rfc.section.10.1.1"><a href="#rfc.section.10.1.1">10.1.1</a>&nbsp;<a id="status.100" href="#status.100">100 Continue</a></h3>
     2379      <p id="rfc.section.10.1.1.p.1">The client <em class="bcp14">SHOULD</em> continue with its request. This interim response is used to inform the client that the initial part of the request has been
     2380         received and has not yet been rejected by the server. The client <em class="bcp14">SHOULD</em> continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The
     2381         server <em class="bcp14">MUST</em> send a final response after the request has been completed. See <a href="#use.of.the.100.status" title="Use of the 100 (Continue) Status">Section&nbsp;8.2.3</a> for detailed discussion of the use and handling of this status code.
     2382      </p>
     2383      <div id="rfc.iref.150"></div>
     2384      <div id="rfc.iref.s.5"></div>
     2385      <h3 id="rfc.section.10.1.2"><a href="#rfc.section.10.1.2">10.1.2</a>&nbsp;<a id="status.101" href="#status.101">101 Switching Protocols</a></h3>
     2386      <p id="rfc.section.10.1.2.p.1">The server understands and is willing to comply with the client's request, via the Upgrade message header field (<a href="#header.upgrade" id="rfc.xref.header.upgrade.2" title="Upgrade">Section&nbsp;14.42</a>), for a change in the application protocol being used on this connection. The server will switch protocols to those defined
     2387         by the response's Upgrade header field immediately after the empty line which terminates the 101 response.
     2388      </p>
     2389      <p id="rfc.section.10.1.2.p.2">The protocol <em class="bcp14">SHOULD</em> be switched only when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over
     2390         older versions, and switching to a real-time, synchronous protocol might be advantageous when delivering resources that use
     2391         such features.
     2392      </p>
     2393      <h2 id="rfc.section.10.2"><a href="#rfc.section.10.2">10.2</a>&nbsp;<a id="status.2xx" href="#status.2xx">Successful 2xx</a></h2>
     2394      <p id="rfc.section.10.2.p.1">This class of status code indicates that the client's request was successfully received, understood, and accepted.</p>
     2395      <div id="rfc.iref.151"></div>
     2396      <div id="rfc.iref.s.6"></div>
     2397      <h3 id="rfc.section.10.2.1"><a href="#rfc.section.10.2.1">10.2.1</a>&nbsp;<a id="status.200" href="#status.200">200 OK</a></h3>
     2398      <p id="rfc.section.10.2.1.p.1">The request has succeeded. The information returned with the response is dependent on the method used in the request, for
     2399         example:
     2400      </p>
     2401      <dl>
     2402         <dt>GET</dt>
     2403         <dd>an entity corresponding to the requested resource is sent in the response;</dd>
     2404         <dt>HEAD</dt>
     2405         <dd>the entity-header fields corresponding to the requested resource are sent in the response without any message-body;</dd>
     2406         <dt>POST</dt>
     2407         <dd>an entity describing or containing the result of the action;</dd>
     2408         <dt>TRACE</dt>
     2409         <dd>an entity containing the request message as received by the end server.</dd>
     2410      </dl>
     2411      <div id="rfc.iref.152"></div>
     2412      <div id="rfc.iref.s.7"></div>
     2413      <h3 id="rfc.section.10.2.2"><a href="#rfc.section.10.2.2">10.2.2</a>&nbsp;<a id="status.201" href="#status.201">201 Created</a></h3>
     2414      <p id="rfc.section.10.2.2.p.1">The request has been fulfilled and resulted in a new resource being created. The newly created resource can be referenced
     2415         by the URI(s) returned in the entity of the response, with the most specific URI for the resource given by a Location header
     2416         field. The response <em class="bcp14">SHOULD</em> include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose
     2417         the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. The origin
     2418         server <em class="bcp14">MUST</em> create the resource before returning the 201 status code. If the action cannot be carried out immediately, the server <em class="bcp14">SHOULD</em> respond with <a href="#status.202" class="smpl">202 (Accepted)</a> response instead.
     2419      </p>
     2420      <p id="rfc.section.10.2.2.p.2">A 201 response <em class="bcp14">MAY</em> contain an ETag response header field indicating the current value of the entity tag for the requested variant just created,
     2421         see <a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section&nbsp;14.19</a>.
     2422      </p>
     2423      <div id="rfc.iref.153"></div>
     2424      <div id="rfc.iref.s.8"></div>
     2425      <h3 id="rfc.section.10.2.3"><a href="#rfc.section.10.2.3">10.2.3</a>&nbsp;<a id="status.202" href="#status.202">202 Accepted</a></h3>
     2426      <p id="rfc.section.10.2.3.p.1">The request has been accepted for processing, but the processing has not been completed. The request might or might not eventually
     2427         be acted upon, as it might be disallowed when processing actually takes place. There is no facility for re-sending a status
     2428         code from an asynchronous operation such as this.
     2429      </p>
     2430      <p id="rfc.section.10.2.3.p.2">The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process
     2431         (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the
     2432         server persist until the process is completed. The entity returned with this response <em class="bcp14">SHOULD</em> include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the
     2433         user can expect the request to be fulfilled.
     2434      </p>
     2435      <div id="rfc.iref.154"></div>
     2436      <div id="rfc.iref.s.9"></div>
     2437      <h3 id="rfc.section.10.2.4"><a href="#rfc.section.10.2.4">10.2.4</a>&nbsp;<a id="status.203" href="#status.203">203 Non-Authoritative Information</a></h3>
     2438      <p id="rfc.section.10.2.4.p.1">The returned metainformation in the entity-header is not the definitive set as available from the origin server, but is gathered
     2439         from a local or a third-party copy. The set presented <em class="bcp14">MAY</em> be a subset or superset of the original version. For example, including local annotation information about the resource might
     2440         result in a superset of the metainformation known by the origin server. Use of this response code is not required and is only
     2441         appropriate when the response would otherwise be <a href="#status.200" class="smpl">200 (OK)</a>.
     2442      </p>
     2443      <div id="rfc.iref.155"></div>
     2444      <div id="rfc.iref.s.10"></div>
     2445      <h3 id="rfc.section.10.2.5"><a href="#rfc.section.10.2.5">10.2.5</a>&nbsp;<a id="status.204" href="#status.204">204 No Content</a></h3>
     2446      <p id="rfc.section.10.2.5.p.1">The server has fulfilled the request but does not need to return an entity-body, and might want to return updated metainformation.
     2447         The response <em class="bcp14">MAY</em> include new or updated metainformation in the form of entity-headers, which if present <em class="bcp14">SHOULD</em> be associated with the requested variant.
     2448      </p>
     2449      <p id="rfc.section.10.2.5.p.2">If the client is a user agent, it <em class="bcp14">SHOULD NOT</em> change its document view from that which caused the request to be sent. This response is primarily intended to allow input
     2450         for actions to take place without causing a change to the user agent's active document view, although any new or updated metainformation <em class="bcp14">SHOULD</em> be applied to the document currently in the user agent's active view.
     2451      </p>
     2452      <p id="rfc.section.10.2.5.p.3">The 204 response <em class="bcp14">MUST NOT</em> include a message-body, and thus is always terminated by the first empty line after the header fields.
     2453      </p>
     2454      <div id="rfc.iref.156"></div>
     2455      <div id="rfc.iref.s.11"></div>
     2456      <h3 id="rfc.section.10.2.6"><a href="#rfc.section.10.2.6">10.2.6</a>&nbsp;<a id="status.205" href="#status.205">205 Reset Content</a></h3>
     2457      <p id="rfc.section.10.2.6.p.1">The server has fulfilled the request and the user agent <em class="bcp14">SHOULD</em> reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions
     2458         to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate
     2459         another input action. The response <em class="bcp14">MUST NOT</em> include an entity.
     2460      </p>
     2461      <div id="rfc.iref.157"></div>
     2462      <div id="rfc.iref.s.12"></div>
     2463      <h3 id="rfc.section.10.2.7"><a href="#rfc.section.10.2.7">10.2.7</a>&nbsp;<a id="status.206" href="#status.206">206 Partial Content</a></h3>
     2464      <p id="rfc.section.10.2.7.p.1">The server has fulfilled the partial GET request for the resource. The request <em class="bcp14">MUST</em> have included a Range header field (<a href="#header.range" id="rfc.xref.header.range.4" title="Range">Section&nbsp;14.35</a>) indicating the desired range, and <em class="bcp14">MAY</em> have included an If-Range header field (<a href="#header.if-range" id="rfc.xref.header.if-range.3" title="If-Range">Section&nbsp;14.27</a>) to make the request conditional.
     2465      </p>
     2466      <p id="rfc.section.10.2.7.p.2">The response <em class="bcp14">MUST</em> include the following header fields:
     2467      </p>
     2468      <ul>
     2469         <li>Either a Content-Range header field (<a href="#header.content-range" id="rfc.xref.header.content-range.4" title="Content-Range">Section&nbsp;14.16</a>) indicating the range included with this response, or a multipart/byteranges Content-Type including Content-Range fields
     2470            for each part. If a Content-Length header field is present in the response, its value <em class="bcp14">MUST</em> match the actual number of <a href="#basic.rules" class="smpl" id="rfc.extref.o.8">OCTET</a>s transmitted in the message-body.
     2471         </li>
     2472         <li>Date</li>
     2473         <li>ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request</li>
     2474         <li>Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same
     2475            variant
     2476         </li>
     2477      </ul>
     2478      <p id="rfc.section.10.2.7.p.3">If the 206 response is the result of an If-Range request that used a strong cache validator (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;13.3.3</a>), the response <em class="bcp14">SHOULD NOT</em> include other entity-headers. If the response is the result of an If-Range request that used a weak validator, the response <em class="bcp14">MUST NOT</em> include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers. Otherwise, the
     2479         response <em class="bcp14">MUST</em> include all of the entity-headers that would have been returned with a <a href="#status.200" class="smpl">200 (OK)</a> response to the same request.
     2480      </p>
     2481      <p id="rfc.section.10.2.7.p.4">A cache <em class="bcp14">MUST NOT</em> combine a 206 response with other previously cached content if the ETag or Last-Modified headers do not match exactly, see <a href="#combining.byte.ranges" title="Combining Byte Ranges">13.5.4</a>.
     2482      </p>
     2483      <p id="rfc.section.10.2.7.p.5">A cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> cache <a href="#status.206" class="smpl">206 (Partial)</a> responses.
     2484      </p>
     2485      <h2 id="rfc.section.10.3"><a href="#rfc.section.10.3">10.3</a>&nbsp;<a id="status.3xx" href="#status.3xx">Redirection 3xx</a></h2>
     2486      <p id="rfc.section.10.3.p.1">This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request.
     2487         The action required <em class="bcp14">MAY</em> be carried out by the user agent without interaction with the user if and only if the method used in the second request is
     2488         GET or HEAD. A client <em class="bcp14">SHOULD</em> detect infinite redirection loops, since such loops generate network traffic for each redirection.
     2489      </p>
     2490      <dl class="empty">
     2491         <dd> <b>Note:</b> previous versions of this specification recommended a maximum of five redirections. Content developers should be aware that
     2492            there might be clients that implement such a fixed limitation.
     2493         </dd>
     2494      </dl>
     2495      <div id="rfc.iref.158"></div>
     2496      <div id="rfc.iref.s.13"></div>
     2497      <h3 id="rfc.section.10.3.1"><a href="#rfc.section.10.3.1">10.3.1</a>&nbsp;<a id="status.300" href="#status.300">300 Multiple Choices</a></h3>
     2498      <p id="rfc.section.10.3.1.p.1">The requested resource corresponds to any one of a set of representations, each with its own specific location, and agent-driven
     2499         negotiation information (<a href="#content.negotiation" title="Content Negotiation">Section&nbsp;12</a>) is being provided so that the user (or user agent) can select a preferred representation and redirect its request to that
     2500         location.
     2501      </p>
     2502      <p id="rfc.section.10.3.1.p.2">Unless it was a HEAD request, the response <em class="bcp14">SHOULD</em> include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose
     2503         the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending
     2504         upon the format and the capabilities of the user agent, selection of the most appropriate choice <em class="bcp14">MAY</em> be performed automatically. However, this specification does not define any standard for such automatic selection.
     2505      </p>
     2506      <p id="rfc.section.10.3.1.p.3">If the server has a preferred choice of representation, it <em class="bcp14">SHOULD</em> include the specific URI for that representation in the Location field; user agents <em class="bcp14">MAY</em> use the Location field value for automatic redirection. This response is cacheable unless indicated otherwise.
     2507      </p>
     2508      <div id="rfc.iref.159"></div>
     2509      <div id="rfc.iref.s.14"></div>
     2510      <h3 id="rfc.section.10.3.2"><a href="#rfc.section.10.3.2">10.3.2</a>&nbsp;<a id="status.301" href="#status.301">301 Moved Permanently</a></h3>
     2511      <p id="rfc.section.10.3.2.p.1">The requested resource has been assigned a new permanent URI and any future references to this resource <em class="bcp14">SHOULD</em> use one of the returned URIs. Clients with link editing capabilities ought to automatically re-link references to the Request-URI
     2512         to one or more of the new references returned by the server, where possible. This response is cacheable unless indicated otherwise.
     2513      </p>
     2514      <p id="rfc.section.10.3.2.p.2">The new permanent URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s).
     2515      </p>
     2516      <p id="rfc.section.10.3.2.p.3">If the 301 status code is received in response to a request other than GET or HEAD, the user agent <em class="bcp14">MUST NOT</em> automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which
     2517         the request was issued.
     2518      </p>
     2519      <dl class="empty">
     2520         <dd> <b>Note:</b> When automatically redirecting a POST request after receiving a 301 status code, some existing HTTP/1.0 user agents will erroneously
     2521            change it into a GET request.
     2522         </dd>
     2523      </dl>
     2524      <div id="rfc.iref.160"></div>
     2525      <div id="rfc.iref.s.15"></div>
     2526      <h3 id="rfc.section.10.3.3"><a href="#rfc.section.10.3.3">10.3.3</a>&nbsp;<a id="status.302" href="#status.302">302 Found</a></h3>
     2527      <p id="rfc.section.10.3.3.p.1">The requested resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the
     2528         client <em class="bcp14">SHOULD</em> continue to use the Request-URI for future requests. This response is only cacheable if indicated by a Cache-Control or Expires
     2529         header field.
     2530      </p>
     2531      <p id="rfc.section.10.3.3.p.2">The temporary URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s).
     2532      </p>
     2533      <p id="rfc.section.10.3.3.p.3">If the 302 status code is received in response to a request other than GET or HEAD, the user agent <em class="bcp14">MUST NOT</em> automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which
     2534         the request was issued.
     2535      </p>
     2536      <dl class="empty">
     2537         <dd> <b>Note:</b> RFC 1945 and RFC 2068 specify that the client is not allowed to change the method on the redirected request. However, most
     2538            existing user agent implementations treat 302 as if it were a 303 response, performing a GET on the Location field-value regardless
     2539            of the original request method. The status codes 303 and 307 have been added for servers that wish to make unambiguously clear
     2540            which kind of reaction is expected of the client.
     2541         </dd>
     2542      </dl>
     2543      <div id="rfc.iref.161"></div>
     2544      <div id="rfc.iref.s.16"></div>
     2545      <h3 id="rfc.section.10.3.4"><a href="#rfc.section.10.3.4">10.3.4</a>&nbsp;<a id="status.303" href="#status.303">303 See Other</a></h3>
     2546      <p id="rfc.section.10.3.4.p.1">The response to the request can be found under a different URI and <em class="bcp14">SHOULD</em> be retrieved using a GET method on that resource. This method exists primarily to allow the output of a POST-activated script
     2547         to redirect the user agent to a selected resource. The new URI is not a substitute reference for the originally requested
     2548         resource. The 303 response <em class="bcp14">MUST NOT</em> be cached, but the response to the second (redirected) request might be cacheable.
     2549      </p>
     2550      <p id="rfc.section.10.3.4.p.2">The different URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s).
     2551      </p>
     2552      <dl class="empty">
     2553         <dd> <b>Note:</b> Many pre-HTTP/1.1 user agents do not understand the 303 status. When interoperability with such clients is a concern, the
     2554            302 status code may be used instead, since most user agents react to a 302 response as described here for 303.
     2555         </dd>
     2556      </dl>
     2557      <div id="rfc.iref.162"></div>
     2558      <div id="rfc.iref.s.17"></div>
     2559      <h3 id="rfc.section.10.3.5"><a href="#rfc.section.10.3.5">10.3.5</a>&nbsp;<a id="status.304" href="#status.304">304 Not Modified</a></h3>
     2560      <p id="rfc.section.10.3.5.p.1">If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server <em class="bcp14">SHOULD</em> respond with this status code. The 304 response <em class="bcp14">MUST NOT</em> contain a message-body, and thus is always terminated by the first empty line after the header fields.
     2561      </p>
     2562      <p id="rfc.section.10.3.5.p.2">The response <em class="bcp14">MUST</em> include the following header fields:
     2563      </p>
     2564      <ul>
     2565         <li>Date, unless its omission is required by <a href="#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section&nbsp;14.18.1</a></li>
     2566      </ul>
     2567      <p id="rfc.section.10.3.5.p.3">If a clockless origin server obeys these rules, and proxies and clients add their own Date to any response received without
     2568         one (as already specified by [RFC 2068], section <a href="http://tools.ietf.org/html/rfc2068#section-14.19" id="rfc.xref.RFC2068.4">14.19</a>), caches will operate correctly.
     2569      </p>
     2570      <ul>
     2571         <li>ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request</li>
     2572         <li>Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same
     2573            variant
     2574         </li>
     2575      </ul>
     2576      <p id="rfc.section.10.3.5.p.4">If the conditional GET used a strong cache validator (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;13.3.3</a>), the response <em class="bcp14">SHOULD NOT</em> include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response <em class="bcp14">MUST NOT</em> include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.
     2577      </p>
     2578      <p id="rfc.section.10.3.5.p.5">If a 304 response indicates an entity not currently cached, then the cache <em class="bcp14">MUST</em> disregard the response and repeat the request without the conditional.
     2579      </p>
     2580      <p id="rfc.section.10.3.5.p.6">If a cache uses a received 304 response to update a cache entry, the cache <em class="bcp14">MUST</em> update the entry to reflect any new field values given in the response.
     2581      </p>
     2582      <div id="rfc.iref.163"></div>
     2583      <div id="rfc.iref.s.18"></div>
     2584      <h3 id="rfc.section.10.3.6"><a href="#rfc.section.10.3.6">10.3.6</a>&nbsp;<a id="status.305" href="#status.305">305 Use Proxy</a></h3>
     2585      <p id="rfc.section.10.3.6.p.1">The requested resource <em class="bcp14">MUST</em> be accessed through the proxy given by the Location field. The Location field gives the URI of the proxy. The recipient is
     2586         expected to repeat this single request via the proxy. 305 responses <em class="bcp14">MUST</em> only be generated by origin servers.
     2587      </p>
     2588      <dl class="empty">
     2589         <dd> <b>Note:</b> RFC 2068 was not clear that 305 was intended to redirect a single request, and to be generated by origin servers only. Not
     2590            observing these limitations has significant security consequences.
     2591         </dd>
     2592      </dl>
     2593      <div id="rfc.iref.164"></div>
     2594      <div id="rfc.iref.s.19"></div>
     2595      <h3 id="rfc.section.10.3.7"><a href="#rfc.section.10.3.7">10.3.7</a>&nbsp;<a id="status.306" href="#status.306">306 (Unused)</a></h3>
     2596      <p id="rfc.section.10.3.7.p.1">The 306 status code was used in a previous version of the specification, is no longer used, and the code is reserved.</p>
     2597      <div id="rfc.iref.165"></div>
     2598      <div id="rfc.iref.s.20"></div>
     2599      <h3 id="rfc.section.10.3.8"><a href="#rfc.section.10.3.8">10.3.8</a>&nbsp;<a id="status.307" href="#status.307">307 Temporary Redirect</a></h3>
     2600      <p id="rfc.section.10.3.8.p.1">The requested resource resides temporarily under a different URI. Since the redirection <em class="bcp14">MAY</em> be altered on occasion, the client <em class="bcp14">SHOULD</em> continue to use the Request-URI for future requests. This response is only cacheable if indicated by a Cache-Control or Expires
     2601         header field.
     2602      </p>
     2603      <p id="rfc.section.10.3.8.p.2">The temporary URI <em class="bcp14">SHOULD</em> be given by the Location field in the response. Unless the request method was HEAD, the entity of the response <em class="bcp14">SHOULD</em> contain a short hypertext note with a hyperlink to the new URI(s) , since many pre-HTTP/1.1 user agents do not understand
     2604         the 307 status. Therefore, the note <em class="bcp14">SHOULD</em> contain the information necessary for a user to repeat the original request on the new URI.
     2605      </p>
     2606      <p id="rfc.section.10.3.8.p.3">If the 307 status code is received in response to a request other than GET or HEAD, the user agent <em class="bcp14">MUST NOT</em> automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which
     2607         the request was issued.
     2608      </p>
     2609      <h2 id="rfc.section.10.4"><a href="#rfc.section.10.4">10.4</a>&nbsp;<a id="status.4xx" href="#status.4xx">Client Error 4xx</a></h2>
     2610      <p id="rfc.section.10.4.p.1">The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD
     2611         request, the server <em class="bcp14">SHOULD</em> include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition.
     2612         These status codes are applicable to any request method. User agents <em class="bcp14">SHOULD</em> display any included entity to the user.
     2613      </p>
     2614      <p id="rfc.section.10.4.p.2">If the client is sending data, a server implementation using TCP <em class="bcp14">SHOULD</em> be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes
     2615         the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send
     2616         a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted
     2617         by the HTTP application.
     2618      </p>
     2619      <div id="rfc.iref.166"></div>
     2620      <div id="rfc.iref.s.21"></div>
     2621      <h3 id="rfc.section.10.4.1"><a href="#rfc.section.10.4.1">10.4.1</a>&nbsp;<a id="status.400" href="#status.400">400 Bad Request</a></h3>
     2622      <p id="rfc.section.10.4.1.p.1">The request could not be understood by the server due to malformed syntax. The client <em class="bcp14">SHOULD NOT</em> repeat the request without modifications.
     2623      </p>
     2624      <div id="rfc.iref.167"></div>
     2625      <div id="rfc.iref.s.22"></div>
     2626      <h3 id="rfc.section.10.4.2"><a href="#rfc.section.10.4.2">10.4.2</a>&nbsp;<a id="status.401" href="#status.401">401 Unauthorized</a></h3>
     2627      <p id="rfc.section.10.4.2.p.1">The request requires user authentication. The response <em class="bcp14">MUST</em> include a WWW-Authenticate header field (<a href="#header.www-authenticate" id="rfc.xref.header.www-authenticate.2" title="WWW-Authenticate">Section&nbsp;14.47</a>) containing a challenge applicable to the requested resource. The client <em class="bcp14">MAY</em> repeat the request with a suitable Authorization header field (<a href="#header.authorization" id="rfc.xref.header.authorization.2" title="Authorization">Section&nbsp;14.8</a>). If the request already included Authorization credentials, then the 401 response indicates that authorization has been
     2628         refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has
     2629         already attempted authentication at least once, then the user <em class="bcp14">SHOULD</em> be presented the entity that was given in the response, since that entity might include relevant diagnostic information. HTTP
     2630         access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication" <a href="#RFC2617" id="rfc.xref.RFC