Changeset 2726 for draft-ietf-httpbis/22


Ignore:
Timestamp:
14/06/14 11:20:37 (9 years ago)
Author:
julian.reschke@…
Message:

update to latest version of rfc2629.xslt, regen all HTML

Location:
draft-ietf-httpbis/22
Files:
7 edited

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  • draft-ietf-httpbis/22/p0-introduction.html

    r2189 r2726  
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    389        content: "HTTP/1.1 Introduction"; 
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    419418      <link rel="Chapter" href="#rfc.section.3" title="3 References">
    420419      <link href="p1-messaging.html" rel="next">
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    422421      <link rel="schema.dct" href="http://purl.org/dc/terms/">
    423422      <meta name="dct.creator" content="Fielding, R.">
     
    426425      <meta name="dct.creator" content="Reschke, J. F.">
    427426      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p0-introduction-latest">
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    429428      <meta name="dct.abstract" content="This document is the first in a series that, collectively, define the HyperText Transfer Protocol, version 1.1; otherwise known as HTTP/1.1.">
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    434433         <tbody>
     
    446445            </tr>
    447446            <tr>
    448                <td class="left">Expires: August 27, 2013</td>
     447               <td class="left">Expires: August 2013</td>
    449448               <td class="right">W3C</td>
    450449            </tr>
     
    467466            <tr>
    468467               <td class="left"></td>
    469                <td class="right">February 23, 2013</td>
     468               <td class="right">February 2013</td>
    470469            </tr>
    471470         </tbody>
    472471      </table>
    473472      <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Introduction<br><span class="filename">draft-ietf-httpbis-p0-introduction-latest</span></p>
    474       <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
     473      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    475474      <p>This document is the first in a series that, collectively, define the HyperText Transfer Protocol, version 1.1; otherwise
    476475         known as HTTP/1.1.
    477       </p> 
    478       <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
     476      </p>
     477      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
    479478      <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
    480       </p> 
     479      </p>
    481480      <p>The current issues list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
    482       </p>   
    483       <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
    484       <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
    485       <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
    486          working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
    487481      </p>
    488       <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
    489          documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
    490          in progress”.
    491       </p>
    492       <p>This Internet-Draft will expire on August 27, 2013.</p>
    493       <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    494       <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
    495       <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
    496          and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
    497          text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
    498          BSD License.
    499       </p>
    500       <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
    501          10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
    502          allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
    503          controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
    504          works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
    505          it into languages other than English.
    506       </p>
     482      <div id="rfc.status">
     483         <h1><a href="#rfc.status">Status of This Memo</a></h1>
     484         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
     485         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
     486            working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
     487         </p>
     488         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
     489            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
     490            in progress”.
     491         </p>
     492         <p>This Internet-Draft will expire in August 2013.</p>
     493      </div>
     494      <div id="rfc.copyrightnotice">
     495         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
     496         <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
     497         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
     498            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
     499            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
     500            BSD License.
     501         </p>
     502         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
     503            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
     504            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
     505            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
     506            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
     507            it into languages other than English.
     508         </p>
     509      </div>
    507510      <hr class="noprint">
    508511      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
     
    515518            </ul>
    516519         </li>
     520         <li><a href="#rfc.index">Index</a></li>
    517521         <li><a href="#rfc.authors">Authors' Addresses</a></li>
    518          <li><a href="#rfc.index">Index</a></li>
    519522      </ul>
    520       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Introduction to the HTTP Document Series
    521       </h1>
    522       <p id="rfc.section.1.p.1">This document is the first in a series that, collectively, define the HyperText Transfer Protocol, version 1.1; otherwise
    523          known as HTTP/1.1.
    524       </p>
    525       <p id="rfc.section.1.p.2">The document series is organized as follows:</p>
    526       <ul>
    527          <li>HTTP/1.1 Introduction - this document</li>
    528          <li><a href="#Part1" id="rfc.xref.Part1.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a> HTTP/1.1 Message Routing and Syntax - How to parse a HTTP/1.1 (or below) message, and layer it onto connection-oriented protocols.
    529             Also includes the HTTP and HTTPS URI schemes.
    530          </li>
    531          <li><a href="#Part2" id="rfc.xref.Part2.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> HTTP/1.1 Semantics and Payloads - Protocol elements such as methods, status codes, and payload-specific header fields. Also
    532             includes content negotiation mechanisms.
    533          </li>
    534          <li><a href="#Part4" id="rfc.xref.Part4.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a> HTTP/1.1 Conditional Requests - An extension to make requests contingent upon their current state.
    535          </li>
    536          <li><a href="#Part5" id="rfc.xref.Part5.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a> HTTP/1.1 Range Requests - An extension to request that only a portion of a response be sent back.
    537          </li>
    538          <li><a href="#Part6" id="rfc.xref.Part6.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a> HTTP/1.1 Caching - An extension to allow storage and reuse of responses.
    539          </li>
    540          <li><a href="#Part7" id="rfc.xref.Part7.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a> HTTP/1.1 Authentication Framework - extension enabling client authentication to proxy and origin servers
    541          </li>
    542       </ul>
    543       <p id="rfc.section.1.p.4">The "core" of HTTP/1.1 is defined by the first two specifications. The remaining specifications in the series are generally
    544          not mandatory for implementations, but might be required in some implementation or deployment scenarios; when this is the
    545          case, it will be noted.
    546       </p>
    547       <p id="rfc.section.1.p.5">Collectively, these documents obsolete <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>. Note that many other specifications extend and refine the use of HTTP (generally, as protocol extensions, where allowed
    548          by these specifications); they are not considered part of this series, but they are still "part of HTTP".
    549       </p>
    550       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="wat" href="#wat">What is HTTP?</a></h1>
    551       <p id="rfc.section.2.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level request/response protocol that uses extensible semantics and
    552          MIME-like message payloads for flexible interaction with network-based hypertext information systems. HTTP relies upon the
    553          Uniform Resource Identifier (URI) standard <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> to indicate the target resource and relationships between resources.
    554       </p>
    555       <p id="rfc.section.2.p.2">HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented
    556          by presenting a uniform interface to clients that is independent of the types of resources provided. Likewise, servers do
    557          not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated
    558          with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used
    559          effectively in many different contexts and for which implementations can evolve independently over time.
    560       </p>
    561       <p id="rfc.section.2.p.3">HTTP is also designed for use as an intermediation protocol for translating communication to and from non-HTTP information
    562          systems. HTTP proxies and gateways can provide access to alternative information services by translating their diverse protocols
    563          into a hypertext format that can be viewed and manipulated by clients in the same way as HTTP services.
    564       </p>
    565       <p id="rfc.section.2.p.4">One consequence of HTTP flexibility is that the protocol cannot be defined in terms of what occurs behind the interface. Instead,
    566          we are limited to defining the syntax of communication, the intent of received communication, and the expected behavior of
    567          recipients. If the communication is considered in isolation, then successful actions ought to be reflected in corresponding
    568          changes to the observable interface provided by servers. However, since multiple clients might act in parallel and perhaps
    569          at cross-purposes, we cannot require that such changes be observable beyond the scope of a single response.
    570       </p>
    571       <p id="rfc.section.2.p.5"> <span class="comment" id="rfc.comment.1">[<a href="#rfc.comment.1" class="smpl">rfc.comment.1</a>: TODO: remove corresponding text from p1 Introduction.]</span>
    572       </p>
     523      <div>
     524         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Introduction to the HTTP Document Series
     525         </h1>
     526         <p id="rfc.section.1.p.1">This document is the first in a series that, collectively, define the HyperText Transfer Protocol, version 1.1; otherwise
     527            known as HTTP/1.1.
     528         </p>
     529         <p id="rfc.section.1.p.2">The document series is organized as follows:</p>
     530         <ul>
     531            <li>HTTP/1.1 Introduction - this document</li>
     532            <li><a href="#Part1" id="rfc.xref.Part1.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a> HTTP/1.1 Message Routing and Syntax - How to parse a HTTP/1.1 (or below) message, and layer it onto connection-oriented protocols.
     533               Also includes the HTTP and HTTPS URI schemes.
     534            </li>
     535            <li><a href="#Part2" id="rfc.xref.Part2.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> HTTP/1.1 Semantics and Payloads - Protocol elements such as methods, status codes, and payload-specific header fields. Also
     536               includes content negotiation mechanisms.
     537            </li>
     538            <li><a href="#Part4" id="rfc.xref.Part4.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a> HTTP/1.1 Conditional Requests - An extension to make requests contingent upon their current state.
     539            </li>
     540            <li><a href="#Part5" id="rfc.xref.Part5.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a> HTTP/1.1 Range Requests - An extension to request that only a portion of a response be sent back.
     541            </li>
     542            <li><a href="#Part6" id="rfc.xref.Part6.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a> HTTP/1.1 Caching - An extension to allow storage and reuse of responses.
     543            </li>
     544            <li><a href="#Part7" id="rfc.xref.Part7.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a> HTTP/1.1 Authentication Framework - extension enabling client authentication to proxy and origin servers
     545            </li>
     546         </ul>
     547         <p id="rfc.section.1.p.4">The "core" of HTTP/1.1 is defined by the first two specifications. The remaining specifications in the series are generally
     548            not mandatory for implementations, but might be required in some implementation or deployment scenarios; when this is the
     549            case, it will be noted.
     550         </p>
     551         <p id="rfc.section.1.p.5">Collectively, these documents obsolete <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>. Note that many other specifications extend and refine the use of HTTP (generally, as protocol extensions, where allowed
     552            by these specifications); they are not considered part of this series, but they are still "part of HTTP".
     553         </p>
     554      </div>
     555      <div id="wat">
     556         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#wat">What is HTTP?</a></h1>
     557         <p id="rfc.section.2.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level request/response protocol that uses extensible semantics and
     558            MIME-like message payloads for flexible interaction with network-based hypertext information systems. HTTP relies upon the
     559            Uniform Resource Identifier (URI) standard <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> to indicate the target resource and relationships between resources.
     560         </p>
     561         <p id="rfc.section.2.p.2">HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented
     562            by presenting a uniform interface to clients that is independent of the types of resources provided. Likewise, servers do
     563            not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated
     564            with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used
     565            effectively in many different contexts and for which implementations can evolve independently over time.
     566         </p>
     567         <p id="rfc.section.2.p.3">HTTP is also designed for use as an intermediation protocol for translating communication to and from non-HTTP information
     568            systems. HTTP proxies and gateways can provide access to alternative information services by translating their diverse protocols
     569            into a hypertext format that can be viewed and manipulated by clients in the same way as HTTP services.
     570         </p>
     571         <p id="rfc.section.2.p.4">One consequence of HTTP flexibility is that the protocol cannot be defined in terms of what occurs behind the interface. Instead,
     572            we are limited to defining the syntax of communication, the intent of received communication, and the expected behavior of
     573            recipients. If the communication is considered in isolation, then successful actions ought to be reflected in corresponding
     574            changes to the observable interface provided by servers. However, since multiple clients might act in parallel and perhaps
     575            at cross-purposes, we cannot require that such changes be observable beyond the scope of a single response.
     576         </p>
     577         <p id="rfc.section.2.p.5"><span class="comment" id="rfc.comment.1">[<a href="#rfc.comment.1" class="smpl">rfc.comment.1</a>: TODO: remove corresponding text from p1 Introduction.]</span>
     578         </p>
     579      </div>
    573580      <h1 id="rfc.references"><a id="rfc.section.3" href="#rfc.section.3">3.</a> References
    574581      </h1>
    575582      <h2 id="rfc.references.1"><a href="#rfc.section.3.1" id="rfc.section.3.1">3.1</a> Normative References
    576583      </h2>
    577       <table>             
     584      <table>
    578585         <tr>
    579586            <td class="reference"><b id="Part1">[Part1]</b></td>
    580             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), February&nbsp;2013.
     587            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), February&nbsp;2013.
    581588            </td>
    582589         </tr>
    583590         <tr>
    584591            <td class="reference"><b id="Part2">[Part2]</b></td>
    585             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), February&nbsp;2013.
     592            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), February&nbsp;2013.
    586593            </td>
    587594         </tr>
    588595         <tr>
    589596            <td class="reference"><b id="Part4">[Part4]</b></td>
    590             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-latest">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-latest (work in progress), February&nbsp;2013.
     597            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-latest">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-latest (work in progress), February&nbsp;2013.
    591598            </td>
    592599         </tr>
    593600         <tr>
    594601            <td class="reference"><b id="Part5">[Part5]</b></td>
    595             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), February&nbsp;2013.
     602            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), February&nbsp;2013.
    596603            </td>
    597604         </tr>
    598605         <tr>
    599606            <td class="reference"><b id="Part6">[Part6]</b></td>
    600             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Akamai">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">Hypertext Transfer Protocol (HTTP/1.1): Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), February&nbsp;2013.
     607            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Akamai">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">Hypertext Transfer Protocol (HTTP/1.1): Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), February&nbsp;2013.
    601608            </td>
    602609         </tr>
    603610         <tr>
    604611            <td class="reference"><b id="Part7">[Part7]</b></td>
    605             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-latest">Hypertext Transfer Protocol (HTTP/1.1): Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-latest (work in progress), February&nbsp;2013.
     612            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-latest">Hypertext Transfer Protocol (HTTP/1.1): Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-latest (work in progress), February&nbsp;2013.
    606613            </td>
    607614         </tr>
    608615         <tr>
    609616            <td class="reference"><b id="RFC3986">[RFC3986]</b></td>
    610             <td class="top"><a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R.</a>, and <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">L. Masinter</a>, “<a href="http://tools.ietf.org/html/rfc3986">Uniform Resource Identifier (URI): Generic Syntax</a>”, STD&nbsp;66, RFC&nbsp;3986, January&nbsp;2005.
     617            <td class="top"><a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R.</a>, and <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">L. Masinter</a>, “<a href="https://tools.ietf.org/html/rfc3986">Uniform Resource Identifier (URI): Generic Syntax</a>”, STD&nbsp;66, RFC&nbsp;3986, January&nbsp;2005.
    611618            </td>
    612619         </tr>
     
    614621      <h2 id="rfc.references.2"><a href="#rfc.section.3.2" id="rfc.section.3.2">3.2</a> Informative References
    615622      </h2>
    616       <table> 
     623      <table>
    617624         <tr>
    618625            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
    619             <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
     626            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
    620627            </td>
    621628         </tr>
    622629      </table>
    623       <div class="avoidbreak">
    624          <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
    625          <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
    626                (editor)
    627                <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:fielding@gbiv.com"><span class="email">fielding@gbiv.com</span></a></span><span class="vcardline">URI: <a href="http://roy.gbiv.com/" class="url">http://roy.gbiv.com/</a></span></address>
    628          <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
    629                (editor)
    630                <span class="n hidden"><span class="family-name">Lafon</span><span class="given-name">Yves</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">W3C / ERCIM</span><span class="street-address vcardline">2004, rte des Lucioles</span><span class="vcardline"><span class="locality">Sophia-Antipolis</span>, <span class="region">AM</span>&nbsp;<span class="postal-code">06902</span></span><span class="country-name vcardline">France</span></span><span class="vcardline">Email: <a href="mailto:ylafon@w3.org"><span class="email">ylafon@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.raubacapeu.net/people/yves/" class="url">http://www.raubacapeu.net/people/yves/</a></span></address>
    631          <address class="vcard"><span class="vcardline"><span class="fn">Mark Nottingham</span>
    632                (editor)
    633                <span class="n hidden"><span class="family-name">Nottingham</span><span class="given-name">Mark</span></span></span><span class="org vcardline">Akamai</span><span class="vcardline">Email: <a href="mailto:mnot@mnot.net"><span class="email">mnot@mnot.net</span></a></span><span class="vcardline">URI: <a href="http://www.mnot.net/" class="url">http://www.mnot.net/</a></span></address>
    634          <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
    635                (editor)
    636                <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline">Email: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address>
    637       </div>
    638630      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
    639631      <p class="noprint"><a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a>
     
    657649         </ul>
    658650      </div>
     651      <div class="avoidbreak">
     652         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
     653         <p><b>Roy T. Fielding</b>
     654            (editor)
     655            <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: <a href="mailto:fielding@gbiv.com">fielding@gbiv.com</a><br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p>
     656         <p><b>Yves Lafon</b>
     657            (editor)
     658            <br>World Wide Web Consortium<br>W3C / ERCIM<br>2004, rte des Lucioles<br>Sophia-Antipolis, AM&nbsp;06902<br>France<br>Email: <a href="mailto:ylafon@w3.org">ylafon@w3.org</a><br>URI: <a href="http://www.raubacapeu.net/people/yves/">http://www.raubacapeu.net/people/yves/</a></p>
     659         <p><b>Mark Nottingham</b>
     660            (editor)
     661            <br>Akamai<br>Email: <a href="mailto:mnot@mnot.net">mnot@mnot.net</a><br>URI: <a href="http://www.mnot.net/">http://www.mnot.net/</a></p>
     662         <p><b>Julian F. Reschke</b>
     663            (editor)
     664            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Email: <a href="mailto:julian.reschke@greenbytes.de">julian.reschke@greenbytes.de</a><br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p>
     665      </div>
    659666   </body>
    660667</html>
  • draft-ietf-httpbis/22/p1-messaging.html

    r2190 r2726  
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    33<html lang="en">
    4    <head profile="http://www.w3.org/2006/03/hcard http://dublincore.org/documents/2008/08/04/dc-html/">
     4   <head profile="http://dublincore.org/documents/2008/08/04/dc-html/">
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    442        content: "February 2013"; 
    443   } 
     456       content: "February 2013";
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    445        content: "HTTP/1.1 Message Syntax and Routing"; 
    446   } 
     459       content: "HTTP/1.1 Message Syntax and Routing";
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    447461  @bottom-left {
    448        content: "Fielding & Reschke"; 
    449   } 
     462       content: "Fielding & Reschke";
     463  }
    450464  @bottom-center {
    451        content: "Expires August 27, 2013"; 
    452   } 
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    486500      <link rel="Appendix" title="D Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.D">
    487501      <link href="p2-semantics.html" rel="next">
    488       <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.590, 2013/01/23 17:59:36, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
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    489503      <link rel="schema.dct" href="http://purl.org/dc/terms/">
    490504      <meta name="dct.creator" content="Fielding, R.">
     
    497511      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document provides an overview of HTTP architecture and its associated terminology, defines the &#34;http&#34; and &#34;https&#34; Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes general security concerns for implementations.">
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    501515         <tbody>
     
    509523            </tr>
    510524            <tr>
    511                <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2145">2145</a>, <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
     525               <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2145">2145</a>, <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved)
    512526               </td>
    513527               <td class="right">J. Reschke, Editor</td>
    514528            </tr>
    515529            <tr>
    516                <td class="left">Updates: <a href="http://tools.ietf.org/html/rfc2817">2817</a>, <a href="http://tools.ietf.org/html/rfc2818">2818</a> (if approved)
     530               <td class="left">Updates: <a href="https://tools.ietf.org/html/rfc2817">2817</a>, <a href="https://tools.ietf.org/html/rfc2818">2818</a> (if approved)
    517531               </td>
    518532               <td class="right">greenbytes</td>
     
    529543      </table>
    530544      <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing<br><span class="filename">draft-ietf-httpbis-p1-messaging-22</span></p>
    531       <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
     545      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    532546      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
    533547         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document provides an overview
    534548         of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes,
    535549         defines the HTTP/1.1 message syntax and parsing requirements, and describes general security concerns for implementations.
    536       </p> 
    537       <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
     550      </p>
     551      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
    538552      <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
    539       </p> 
     553      </p>
    540554      <p>The current issues list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
    541       </p> 
     555      </p>
    542556      <p>The changes in this draft are summarized in <a href="#changes.since.21" title="Since draft-ietf-httpbis-p1-messaging-21">Appendix&nbsp;D.2</a>.
    543       </p>
    544       <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
    545       <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
    546       <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
    547          working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
    548557      </p>
    549       <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
    550          documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
    551          in progress”.
    552       </p>
    553       <p>This Internet-Draft will expire on August 27, 2013.</p>
    554       <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    555       <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
    556       <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
    557          and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
    558          text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
    559          BSD License.
    560       </p>
    561       <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
    562          10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
    563          allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
    564          controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
    565          works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
    566          it into languages other than English.
    567       </p>
     558      <div id="rfc.status">
     559         <h1><a href="#rfc.status">Status of This Memo</a></h1>
     560         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
     561         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
     562            working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
     563         </p>
     564         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
     565            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
     566            in progress”.
     567         </p>
     568         <p>This Internet-Draft will expire on August 27, 2013.</p>
     569      </div>
     570      <div id="rfc.copyrightnotice">
     571         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
     572         <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
     573         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
     574            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
     575            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
     576            BSD License.
     577         </p>
     578         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
     579            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
     580            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
     581            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
     582            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
     583            it into languages other than English.
     584         </p>
     585      </div>
    568586      <hr class="noprint">
    569587      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
     
    685703            </ul>
    686704         </li>
    687          <li><a href="#rfc.authors">Authors' Addresses</a></li>
    688705         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#compatibility">HTTP Version History</a><ul>
    689706               <li><a href="#rfc.section.A.1">A.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.1.0">Changes from HTTP/1.0</a><ul>
     
    704721         </li>
    705722         <li><a href="#rfc.index">Index</a></li>
     723         <li><a href="#rfc.authors">Authors' Addresses</a></li>
    706724      </ul>
    707       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
    708       <p id="rfc.section.1.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level request/response protocol that uses extensible semantics and
    709          self-descriptive message payloads for flexible interaction with network-based hypertext information systems. This document
    710          is the first in a series of documents that collectively form the HTTP/1.1 specification:
    711       </p>
    712       <ul class="empty">
    713          <li>RFC xxx1: Message Syntax and Routing</li>
    714          <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content" id="rfc.xref.Part2.1">RFC xxx2</cite>: Semantics and Content
    715          </li>
    716          <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests" id="rfc.xref.Part4.1">RFC xxx3</cite>: Conditional Requests
    717          </li>
    718          <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests" id="rfc.xref.Part5.1">RFC xxx4</cite>: Range Requests
    719          </li>
    720          <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching" id="rfc.xref.Part6.1">RFC xxx5</cite>: Caching
    721          </li>
    722          <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication" id="rfc.xref.Part7.1">RFC xxx6</cite>: Authentication
    723          </li>
    724       </ul>
    725       <p id="rfc.section.1.p.2">This HTTP/1.1 specification obsoletes and moves to historic status <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2616.1">RFC 2616</cite>, its predecessor <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2068.1">RFC 2068</cite>, and <cite title="Use and Interpretation of HTTP Version Numbers" id="rfc.xref.RFC2145.1">RFC 2145</cite> (on HTTP versioning). This specification also updates the use of CONNECT to establish a tunnel, previously defined in <cite title="Upgrading to TLS Within HTTP/1.1" id="rfc.xref.RFC2817.1">RFC 2817</cite>, and defines the "https" URI scheme that was described informally in <cite title="HTTP Over TLS" id="rfc.xref.RFC2818.1">RFC 2818</cite>.
    726       </p>
    727       <p id="rfc.section.1.p.3">HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented
    728          by presenting a uniform interface to clients that is independent of the types of resources provided. Likewise, servers do
    729          not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated
    730          with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used
    731          effectively in many different contexts and for which implementations can evolve independently over time.
    732       </p>
    733       <p id="rfc.section.1.p.4">HTTP is also designed for use as an intermediation protocol for translating communication to and from non-HTTP information
    734          systems. HTTP proxies and gateways can provide access to alternative information services by translating their diverse protocols
    735          into a hypertext format that can be viewed and manipulated by clients in the same way as HTTP services.
    736       </p>
    737       <p id="rfc.section.1.p.5">One consequence of this flexibility is that the protocol cannot be defined in terms of what occurs behind the interface. Instead,
    738          we are limited to defining the syntax of communication, the intent of received communication, and the expected behavior of
    739          recipients. If the communication is considered in isolation, then successful actions ought to be reflected in corresponding
    740          changes to the observable interface provided by servers. However, since multiple clients might act in parallel and perhaps
    741          at cross-purposes, we cannot require that such changes be observable beyond the scope of a single response.
    742       </p>
    743       <p id="rfc.section.1.p.6">This document describes the architectural elements that are used or referred to in HTTP, defines the "http" and "https" URI
    744          schemes, describes overall network operation and connection management, and defines HTTP message framing and forwarding requirements.
    745          Our goal is to define all of the mechanisms necessary for HTTP message handling that are independent of message semantics,
    746          thereby defining the complete set of requirements for message parsers and message-forwarding intermediaries.
    747       </p>
    748       <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirement Notation</a></h2>
    749       <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
    750          in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
    751       </p>
    752       <p id="rfc.section.1.1.p.2">Conformance criteria and considerations regarding error handling are defined in <a href="#conformance" title="Conformance and Error Handling">Section&nbsp;2.5</a>.
    753       </p>
    754       <div id="rfc.iref.g.1"></div>
    755       <div id="rfc.iref.g.2"></div>
    756       <div id="rfc.iref.g.3"></div>
    757       <div id="rfc.iref.g.4"></div>
    758       <div id="rfc.iref.g.5"></div>
    759       <div id="rfc.iref.g.6"></div>
    760       <div id="rfc.iref.g.7"></div>
    761       <div id="rfc.iref.g.8"></div>
    762       <div id="rfc.iref.g.9"></div>
    763       <div id="rfc.iref.g.10"></div>
    764       <div id="rfc.iref.g.11"></div>
    765       <div id="rfc.iref.g.12"></div>
    766       <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="notation" href="#notation">Syntax Notation</a></h2>
    767       <p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="#abnf.extension" title="ABNF list extension: #rule">Appendix&nbsp;B</a>. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;C</a> shows the collected ABNF with the list rule expanded.
    768       </p>
    769       <div id="core.rules">
    770          <p id="rfc.section.1.2.p.2">                        The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="http://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
    771             (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR
    772             (any visible <a href="#USASCII" id="rfc.xref.USASCII.1"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a> character).
     725      <div id="introduction">
     726         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#introduction">Introduction</a></h1>
     727         <p id="rfc.section.1.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level request/response protocol that uses extensible semantics and
     728            self-descriptive message payloads for flexible interaction with network-based hypertext information systems. This document
     729            is the first in a series of documents that collectively form the HTTP/1.1 specification:
    773730         </p>
     731         <ul class="empty">
     732            <li>RFC xxx1: Message Syntax and Routing</li>
     733            <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content" id="rfc.xref.Part2.1">RFC xxx2</cite>: Semantics and Content
     734            </li>
     735            <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests" id="rfc.xref.Part4.1">RFC xxx3</cite>: Conditional Requests
     736            </li>
     737            <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests" id="rfc.xref.Part5.1">RFC xxx4</cite>: Range Requests
     738            </li>
     739            <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching" id="rfc.xref.Part6.1">RFC xxx5</cite>: Caching
     740            </li>
     741            <li><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication" id="rfc.xref.Part7.1">RFC xxx6</cite>: Authentication
     742            </li>
     743         </ul>
     744         <p id="rfc.section.1.p.2">This HTTP/1.1 specification obsoletes and moves to historic status <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2616.1">RFC 2616</cite>, its predecessor <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2068.1">RFC 2068</cite>, and <cite title="Use and Interpretation of HTTP Version Numbers" id="rfc.xref.RFC2145.1">RFC 2145</cite> (on HTTP versioning). This specification also updates the use of CONNECT to establish a tunnel, previously defined in <cite title="Upgrading to TLS Within HTTP/1.1" id="rfc.xref.RFC2817.1">RFC 2817</cite>, and defines the "https" URI scheme that was described informally in <cite title="HTTP Over TLS" id="rfc.xref.RFC2818.1">RFC 2818</cite>.
     745         </p>
     746         <p id="rfc.section.1.p.3">HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented
     747            by presenting a uniform interface to clients that is independent of the types of resources provided. Likewise, servers do
     748            not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated
     749            with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used
     750            effectively in many different contexts and for which implementations can evolve independently over time.
     751         </p>
     752         <p id="rfc.section.1.p.4">HTTP is also designed for use as an intermediation protocol for translating communication to and from non-HTTP information
     753            systems. HTTP proxies and gateways can provide access to alternative information services by translating their diverse protocols
     754            into a hypertext format that can be viewed and manipulated by clients in the same way as HTTP services.
     755         </p>
     756         <p id="rfc.section.1.p.5">One consequence of this flexibility is that the protocol cannot be defined in terms of what occurs behind the interface. Instead,
     757            we are limited to defining the syntax of communication, the intent of received communication, and the expected behavior of
     758            recipients. If the communication is considered in isolation, then successful actions ought to be reflected in corresponding
     759            changes to the observable interface provided by servers. However, since multiple clients might act in parallel and perhaps
     760            at cross-purposes, we cannot require that such changes be observable beyond the scope of a single response.
     761         </p>
     762         <p id="rfc.section.1.p.6">This document describes the architectural elements that are used or referred to in HTTP, defines the "http" and "https" URI
     763            schemes, describes overall network operation and connection management, and defines HTTP message framing and forwarding requirements.
     764            Our goal is to define all of the mechanisms necessary for HTTP message handling that are independent of message semantics,
     765            thereby defining the complete set of requirements for message parsers and message-forwarding intermediaries.
     766         </p>
     767         <div id="intro.requirements">
     768            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#intro.requirements">Requirement Notation</a></h2>
     769            <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
     770               in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
     771            </p>
     772            <p id="rfc.section.1.1.p.2">Conformance criteria and considerations regarding error handling are defined in <a href="#conformance" title="Conformance and Error Handling">Section&nbsp;2.5</a>.
     773            </p>
     774         </div>
     775         <div id="notation">
     776            <div id="rfc.iref.g.1"></div>
     777            <div id="rfc.iref.g.2"></div>
     778            <div id="rfc.iref.g.3"></div>
     779            <div id="rfc.iref.g.4"></div>
     780            <div id="rfc.iref.g.5"></div>
     781            <div id="rfc.iref.g.6"></div>
     782            <div id="rfc.iref.g.7"></div>
     783            <div id="rfc.iref.g.8"></div>
     784            <div id="rfc.iref.g.9"></div>
     785            <div id="rfc.iref.g.10"></div>
     786            <div id="rfc.iref.g.11"></div>
     787            <div id="rfc.iref.g.12"></div>
     788            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
     789            <p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="#abnf.extension" title="ABNF list extension: #rule">Appendix&nbsp;B</a>. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;C</a> shows the collected ABNF with the list rule expanded.
     790            </p>
     791            <div id="core.rules">
     792               <p id="rfc.section.1.2.p.2">            The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
     793                  (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR
     794                  (any visible <a href="#USASCII" id="rfc.xref.USASCII.1"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a> character).
     795               </p>
     796            </div>
     797            <p id="rfc.section.1.2.p.3">As a convention, ABNF rule names prefixed with "obs-" denote "obsolete" grammar rules that appear for historical reasons.</p>
     798         </div>
    774799      </div>
    775       <p id="rfc.section.1.2.p.3">As a convention, ABNF rule names prefixed with "obs-" denote "obsolete" grammar rules that appear for historical reasons.</p>
    776       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="architecture" href="#architecture">Architecture</a></h1>
    777       <p id="rfc.section.2.p.1">HTTP was created for the World Wide Web architecture and has evolved over time to support the scalability needs of a worldwide
    778          hypertext system. Much of that architecture is reflected in the terminology and syntax productions used to define HTTP.
    779       </p>
    780       <div id="rfc.iref.c.1"></div>
    781       <div id="rfc.iref.s.1"></div>
    782       <div id="rfc.iref.c.2"></div>
    783       <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="operation" href="#operation">Client/Server Messaging</a></h2>
    784       <p id="rfc.section.2.1.p.1">HTTP is a stateless request/response protocol that operates by exchanging <dfn>messages</dfn> (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) across a reliable transport or session-layer "<dfn>connection</dfn>" (<a href="#connection.management" title="Connection Management">Section&nbsp;6</a>). An HTTP "<dfn>client</dfn>" is a program that establishes a connection to a server for the purpose of sending one or more HTTP requests. An HTTP "<dfn>server</dfn>" is a program that accepts connections in order to service HTTP requests by sending HTTP responses.
    785       </p>
    786       <div id="rfc.iref.u.1"></div>
    787       <div id="rfc.iref.o.1"></div>
    788       <div id="rfc.iref.b.1"></div>
    789       <div id="rfc.iref.s.2"></div>
    790       <div id="rfc.iref.s.3"></div>
    791       <div id="rfc.iref.r.1"></div>
    792       <p id="rfc.section.2.1.p.2">The terms client and server refer only to the roles that these programs perform for a particular connection. The same program
    793          might act as a client on some connections and a server on others. We use the term "<dfn>user agent</dfn>" to refer to any of the various client programs that initiate a request, including (but not limited to) browsers, spiders
    794          (web-based robots), command-line tools, native applications, and mobile apps. The term "<dfn>origin server</dfn>" is used to refer to the program that can originate authoritative responses to a request. For general requirements, we use
    795          the terms "<dfn>sender</dfn>" and "<dfn>recipient</dfn>" to refer to any component that sends or receives, respectively, a given message.
    796       </p>
    797       <p id="rfc.section.2.1.p.3">HTTP relies upon the Uniform Resource Identifier (URI) standard <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> to indicate the target resource (<a href="#target-resource" title="Identifying a Target Resource">Section&nbsp;5.1</a>) and relationships between resources. Messages are passed in a format similar to that used by Internet mail <a href="#RFC5322" id="rfc.xref.RFC5322.1"><cite title="Internet Message Format">[RFC5322]</cite></a> and 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">[RFC2045]</cite></a> (see <a href="p2-semantics.html#differences.between.http.and.mime" title="Differences between HTTP and MIME">Appendix A</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> for the differences between HTTP and MIME messages).
    798       </p>
    799       <p id="rfc.section.2.1.p.4">Most HTTP communication consists of a retrieval request (GET) for a representation of some resource identified by a URI. In
    800          the simplest case, this might be accomplished via a single bidirectional connection (===) between the user agent (UA) and
    801          the origin server (O).
    802       </p>
    803       <div id="rfc.figure.u.1"></div><pre class="drawing">         request   &gt;
     800      <div id="architecture">
     801         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#architecture">Architecture</a></h1>
     802         <p id="rfc.section.2.p.1">HTTP was created for the World Wide Web architecture and has evolved over time to support the scalability needs of a worldwide
     803            hypertext system. Much of that architecture is reflected in the terminology and syntax productions used to define HTTP.
     804         </p>
     805         <div id="operation">
     806            <div id="rfc.iref.c.1"></div>
     807            <div id="rfc.iref.s.1"></div>
     808            <div id="rfc.iref.c.2"></div>
     809            <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a href="#operation">Client/Server Messaging</a></h2>
     810            <p id="rfc.section.2.1.p.1">HTTP is a stateless request/response protocol that operates by exchanging <dfn>messages</dfn> (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) across a reliable transport or session-layer "<dfn>connection</dfn>" (<a href="#connection.management" title="Connection Management">Section&nbsp;6</a>). An HTTP "<dfn>client</dfn>" is a program that establishes a connection to a server for the purpose of sending one or more HTTP requests. An HTTP "<dfn>server</dfn>" is a program that accepts connections in order to service HTTP requests by sending HTTP responses.
     811            </p>
     812            <div id="rfc.iref.u.1"></div>
     813            <div id="rfc.iref.o.1"></div>
     814            <div id="rfc.iref.b.1"></div>
     815            <div id="rfc.iref.s.2"></div>
     816            <div id="rfc.iref.s.3"></div>
     817            <div id="rfc.iref.r.1"></div>
     818            <p id="rfc.section.2.1.p.2">The terms client and server refer only to the roles that these programs perform for a particular connection. The same program
     819               might act as a client on some connections and a server on others. We use the term "<dfn>user agent</dfn>" to refer to any of the various client programs that initiate a request, including (but not limited to) browsers, spiders
     820               (web-based robots), command-line tools, native applications, and mobile apps. The term "<dfn>origin server</dfn>" is used to refer to the program that can originate authoritative responses to a request. For general requirements, we use
     821               the terms "<dfn>sender</dfn>" and "<dfn>recipient</dfn>" to refer to any component that sends or receives, respectively, a given message.
     822            </p>
     823            <p id="rfc.section.2.1.p.3">HTTP relies upon the Uniform Resource Identifier (URI) standard <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> to indicate the target resource (<a href="#target-resource" title="Identifying a Target Resource">Section&nbsp;5.1</a>) and relationships between resources. Messages are passed in a format similar to that used by Internet mail <a href="#RFC5322" id="rfc.xref.RFC5322.1"><cite title="Internet Message Format">[RFC5322]</cite></a> and 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">[RFC2045]</cite></a> (see <a href="p2-semantics.html#differences.between.http.and.mime" title="Differences between HTTP and MIME">Appendix A</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> for the differences between HTTP and MIME messages).
     824            </p>
     825            <p id="rfc.section.2.1.p.4">Most HTTP communication consists of a retrieval request (GET) for a representation of some resource identified by a URI. In
     826               the simplest case, this might be accomplished via a single bidirectional connection (===) between the user agent (UA) and
     827               the origin server (O).
     828            </p>
     829            <div id="rfc.figure.u.1"></div><pre class="drawing">         request   &gt;
    804830    <b>UA</b> ======================================= <b>O</b>
    805831                                &lt;   response
    806832</pre><div id="rfc.iref.m.1"></div>
    807       <div id="rfc.iref.r.2"></div>
    808       <div id="rfc.iref.r.3"></div>
    809       <p id="rfc.section.2.1.p.6">A client sends an HTTP request to a server in the form of a <dfn>request</dfn> message, beginning with a request-line that includes a method, URI, and protocol version (<a href="#request.line" title="Request Line">Section&nbsp;3.1.1</a>), followed by header fields containing request modifiers, client information, and representation metadata (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
    810       </p>
    811       <p id="rfc.section.2.1.p.7">A server responds to a client's request by sending one or more HTTP <dfn>response</dfn> messages, each beginning with a status line that includes the protocol version, a success or error code, and textual reason
    812          phrase (<a href="#status.line" title="Status Line">Section&nbsp;3.1.2</a>), possibly followed by header fields containing server information, resource metadata, and representation metadata (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
    813       </p>
    814       <p id="rfc.section.2.1.p.8">A connection might be used for multiple request/response exchanges, as defined in <a href="#persistent.connections" title="Persistence">Section&nbsp;6.3</a>.
    815       </p>
    816       <p id="rfc.section.2.1.p.9">The following example illustrates a typical message exchange for a GET request on the URI "http://www.example.com/hello.txt":</p>
    817       <div id="rfc.figure.u.2"></div>
    818       <p>client request:</p><pre class="text2">GET /hello.txt HTTP/1.1
     833            <div id="rfc.iref.r.2"></div>
     834            <div id="rfc.iref.r.3"></div>
     835            <p id="rfc.section.2.1.p.6">A client sends an HTTP request to a server in the form of a <dfn>request</dfn> message, beginning with a request-line that includes a method, URI, and protocol version (<a href="#request.line" title="Request Line">Section&nbsp;3.1.1</a>), followed by header fields containing request modifiers, client information, and representation metadata (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
     836            </p>
     837            <p id="rfc.section.2.1.p.7">A server responds to a client's request by sending one or more HTTP <dfn>response</dfn> messages, each beginning with a status line that includes the protocol version, a success or error code, and textual reason
     838               phrase (<a href="#status.line" title="Status Line">Section&nbsp;3.1.2</a>), possibly followed by header fields containing server information, resource metadata, and representation metadata (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
     839            </p>
     840            <p id="rfc.section.2.1.p.8">A connection might be used for multiple request/response exchanges, as defined in <a href="#persistent.connections" title="Persistence">Section&nbsp;6.3</a>.
     841            </p>
     842            <p id="rfc.section.2.1.p.9">The following example illustrates a typical message exchange for a GET request on the URI "http://www.example.com/hello.txt":</p>
     843            <div id="rfc.figure.u.2"></div>
     844            <p>client request:</p><pre class="text2">GET /hello.txt HTTP/1.1
    819845User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
    820846Host: www.example.com
     
    822848
    823849</pre><div id="rfc.figure.u.3"></div>
    824       <p>server response:</p><pre class="text">HTTP/1.1 200 OK
     850            <p>server response:</p><pre class="text">HTTP/1.1 200 OK
    825851Date: Mon, 27 Jul 2009 12:28:53 GMT
    826852Server: Apache
     
    833859
    834860<span id="exbody">Hello World!
    835 </span></pre>  <p>(Note that the content length includes the trailing CR/LF sequence of the body text)</p>
    836       <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="implementation-diversity" href="#implementation-diversity">Implementation Diversity</a></h2>
    837       <p id="rfc.section.2.2.p.1">When considering the design of HTTP, it is easy to fall into a trap of thinking that all user agents are general-purpose browsers
    838          and all origin servers are large public websites. That is not the case in practice. Common HTTP user agents include household
    839          appliances, stereos, scales, firmware update scripts, command-line programs, mobile apps, and communication devices in a multitude
    840          of shapes and sizes. Likewise, common HTTP origin servers include home automation units, configurable networking components,
    841          office machines, autonomous robots, news feeds, traffic cameras, ad selectors, and video delivery platforms.
    842       </p>
    843       <p id="rfc.section.2.2.p.2">The term "user agent" does not imply that there is a human user directly interacting with the software agent at the time of
    844          a request. In many cases, a user agent is installed or configured to run in the background and save its results for later
    845          inspection (or save only a subset of those results that might be interesting or erroneous). Spiders, for example, are typically
    846          given a start URI and configured to follow certain behavior while crawling the Web as a hypertext graph.
    847       </p>
    848       <p id="rfc.section.2.2.p.3">The implementation diversity of HTTP means that we cannot assume the user agent can make interactive suggestions to a user
    849          or provide adequate warning for security or privacy options. In the few cases where this specification requires reporting
    850          of errors to the user, it is acceptable for such reporting to only be observable in an error console or log file. Likewise,
    851          requirements that an automated action be confirmed by the user before proceeding can be met via advance configuration choices,
    852          run-time options, or simply not proceeding with the unsafe action.
    853       </p>
    854       <div id="rfc.iref.i.1"></div>
    855       <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="intermediaries" href="#intermediaries">Intermediaries</a></h2>
    856       <p id="rfc.section.2.3.p.1">HTTP enables the use of intermediaries to satisfy requests through a chain of connections. There are three common forms of
    857          HTTP <dfn>intermediary</dfn>: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an origin server, proxy, gateway, or tunnel,
    858          switching behavior based on the nature of each request.
    859       </p>
    860       <div id="rfc.figure.u.4"></div><pre class="drawing">         &gt;             &gt;             &gt;             &gt;
     861</span></pre><p>(Note that the content length includes the trailing CR/LF sequence of the body text)</p>
     862         </div>
     863         <div id="implementation-diversity">
     864            <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a href="#implementation-diversity">Implementation Diversity</a></h2>
     865            <p id="rfc.section.2.2.p.1">When considering the design of HTTP, it is easy to fall into a trap of thinking that all user agents are general-purpose browsers
     866               and all origin servers are large public websites. That is not the case in practice. Common HTTP user agents include household
     867               appliances, stereos, scales, firmware update scripts, command-line programs, mobile apps, and communication devices in a multitude
     868               of shapes and sizes. Likewise, common HTTP origin servers include home automation units, configurable networking components,
     869               office machines, autonomous robots, news feeds, traffic cameras, ad selectors, and video delivery platforms.
     870            </p>
     871            <p id="rfc.section.2.2.p.2">The term "user agent" does not imply that there is a human user directly interacting with the software agent at the time of
     872               a request. In many cases, a user agent is installed or configured to run in the background and save its results for later
     873               inspection (or save only a subset of those results that might be interesting or erroneous). Spiders, for example, are typically
     874               given a start URI and configured to follow certain behavior while crawling the Web as a hypertext graph.
     875            </p>
     876            <p id="rfc.section.2.2.p.3">The implementation diversity of HTTP means that we cannot assume the user agent can make interactive suggestions to a user
     877               or provide adequate warning for security or privacy options. In the few cases where this specification requires reporting
     878               of errors to the user, it is acceptable for such reporting to only be observable in an error console or log file. Likewise,
     879               requirements that an automated action be confirmed by the user before proceeding can be met via advance configuration choices,
     880               run-time options, or simply not proceeding with the unsafe action.
     881            </p>
     882         </div>
     883         <div id="intermediaries">
     884            <div id="rfc.iref.i.1"></div>
     885            <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a href="#intermediaries">Intermediaries</a></h2>
     886            <p id="rfc.section.2.3.p.1">HTTP enables the use of intermediaries to satisfy requests through a chain of connections. There are three common forms of
     887               HTTP <dfn>intermediary</dfn>: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an origin server, proxy, gateway, or tunnel,
     888               switching behavior based on the nature of each request.
     889            </p>
     890            <div id="rfc.figure.u.4"></div><pre class="drawing">         &gt;             &gt;             &gt;             &gt;
    861891    <b>UA</b> =========== <b>A</b> =========== <b>B</b> =========== <b>C</b> =========== <b>O</b>
    862892               &lt;             &lt;             &lt;             &lt;
    863893</pre><p id="rfc.section.2.3.p.3">The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response
    864          message that travels the whole chain will pass through four separate connections. Some HTTP communication options might apply
    865          only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along
    866          the chain. Although the diagram is linear, each participant might be engaged in multiple, simultaneous communications. For
    867          example, B might be receiving requests from many clients other than A, and/or forwarding requests to servers other than C,
    868          at the same time that it is handling A's request.
    869       </p>
    870       <p id="rfc.section.2.3.p.4"> <span id="rfc.iref.u.2"></span><span id="rfc.iref.d.1"></span>  <span id="rfc.iref.i.2"></span><span id="rfc.iref.o.2"></span> We use the terms "<dfn>upstream</dfn>" and "<dfn>downstream</dfn>" to describe various requirements in relation to the directional flow of a message: all messages flow from upstream to downstream.
    871          Likewise, we use the terms inbound and outbound to refer to directions in relation to the request path: "<dfn>inbound</dfn>" means toward the origin server and "<dfn>outbound</dfn>" means toward the user agent.
    872       </p>
    873       <p id="rfc.section.2.3.p.5"><span id="rfc.iref.p.1"></span> A "<dfn>proxy</dfn>" is a message forwarding agent that is selected by the client, usually via local configuration rules, to receive requests
    874          for some type(s) of absolute URI and attempt to satisfy those requests via translation through the HTTP interface. Some translations
    875          are minimal, such as for proxy requests for "http" URIs, whereas other requests might require translation to and from entirely
    876          different application-level protocols. Proxies are often used to group an organization's HTTP requests through a common intermediary
    877          for the sake of security, annotation services, or shared caching.
    878       </p>
    879       <p id="rfc.section.2.3.p.6"> <span id="rfc.iref.t.1"></span>  <span id="rfc.iref.n.1"></span> An HTTP-to-HTTP proxy is called a "<dfn>transforming proxy</dfn>" if it is designed or configured to modify request or response messages in a semantically meaningful way (i.e., modifications,
    880          beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original
    881          sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared
    882          annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder,
    883          or an intranet-to-Internet privacy filter. Such transformations are presumed to be desired by the client (or client organization)
    884          that selected the proxy and are beyond the scope of this specification. However, when a proxy is not intended to transform
    885          a given message, we use the term "<dfn>non-transforming proxy</dfn>" to target requirements that preserve HTTP message semantics. See <a href="p2-semantics.html#status.203" title="203 Non-Authoritative Information">Section 6.3.4</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> and <a href="p6-cache.html#header.warning" title="Warning">Section 7.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a> for status and warning codes related to transformations.
    886       </p>
    887       <p id="rfc.section.2.3.p.7"><span id="rfc.iref.g.13"></span><span id="rfc.iref.r.4"></span>  <span id="rfc.iref.a.1"></span> A "<dfn>gateway</dfn>" (a.k.a., "<dfn>reverse proxy</dfn>") is a receiving agent that acts as a layer above some other server(s) and translates the received requests to the underlying
    888          server's protocol. Gateways are often used to encapsulate legacy or untrusted information services, to improve server performance
    889          through "<dfn>accelerator</dfn>" caching, and to enable partitioning or load-balancing of HTTP services across multiple machines.
    890       </p>
    891       <p id="rfc.section.2.3.p.8">A gateway behaves as an origin server on its outbound connection and as a user agent on its inbound connection. All HTTP requirements
    892          applicable to an origin server also apply to the outbound communication of a gateway. A gateway communicates with inbound
    893          servers using any protocol that it desires, including private extensions to HTTP that are outside the scope of this specification.
    894          However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party HTTP servers <em class="bcp14">MUST</em> conform to HTTP user agent requirements on the gateway's inbound connection and <em class="bcp14">MUST</em> implement the <a href="#header.connection" class="smpl">Connection</a> (<a href="#header.connection" id="rfc.xref.header.connection.1" title="Connection">Section&nbsp;6.1</a>) and <a href="#header.via" class="smpl">Via</a> (<a href="#header.via" id="rfc.xref.header.via.1" title="Via">Section&nbsp;5.7.1</a>) header fields for both connections.
    895       </p>
    896       <p id="rfc.section.2.3.p.9"><span id="rfc.iref.t.2"></span> A "<dfn>tunnel</dfn>" acts as a blind relay between two connections without changing the messages. Once active, a tunnel is not considered a party
    897          to the HTTP communication, though the tunnel might have been initiated by an HTTP request. A tunnel ceases to exist when both
    898          ends of the relayed connection are closed. Tunnels are used to extend a virtual connection through an intermediary, such as
    899          when Transport Layer Security (TLS, <a href="#RFC5246" id="rfc.xref.RFC5246.1"><cite title="The Transport Layer Security (TLS) Protocol Version 1.2">[RFC5246]</cite></a>) is used to establish confidential communication through a shared firewall proxy.
    900       </p>
    901       <p id="rfc.section.2.3.p.10"><span id="rfc.iref.i.3"></span>  <span id="rfc.iref.t.3"></span>  <span id="rfc.iref.c.3"></span> The above categories for intermediary only consider those acting as participants in the HTTP communication. There are also
    902          intermediaries that can act on lower layers of the network protocol stack, filtering or redirecting HTTP traffic without the
    903          knowledge or permission of message senders. Network intermediaries often introduce security flaws or interoperability problems
    904          by violating HTTP semantics. For example, an "<dfn>interception proxy</dfn>" <a href="#RFC3040" id="rfc.xref.RFC3040.1"><cite title="Internet Web Replication and Caching Taxonomy">[RFC3040]</cite></a> (also commonly known as a "<dfn>transparent proxy</dfn>" <a href="#RFC1919" id="rfc.xref.RFC1919.1"><cite title="Classical versus Transparent IP Proxies">[RFC1919]</cite></a> or "<dfn>captive portal</dfn>") differs from an HTTP proxy because it is not selected by the client. Instead, an interception proxy filters or redirects
    905          outgoing TCP port 80 packets (and occasionally other common port traffic). Interception proxies are commonly found on public
    906          network access points, as a means of enforcing account subscription prior to allowing use of non-local Internet services,
    907          and within corporate firewalls to enforce network usage policies. They are indistinguishable from a man-in-the-middle attack.
    908       </p>
    909       <p id="rfc.section.2.3.p.11">HTTP is defined as a stateless protocol, meaning that each request message can be understood in isolation. Many implementations
    910          depend on HTTP's stateless design in order to reuse proxied connections or dynamically load-balance requests across multiple
    911          servers. Hence, servers <em class="bcp14">MUST NOT</em> assume that two requests on the same connection are from the same user agent unless the connection is secured and specific
    912          to that agent. Some non-standard HTTP extensions (e.g., <a href="#RFC4559" id="rfc.xref.RFC4559.1"><cite title="SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows">[RFC4559]</cite></a>) have been known to violate this requirement, resulting in security and interoperability problems.
    913       </p>
    914       <div id="rfc.iref.c.4"></div>
    915       <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="caches" href="#caches">Caches</a></h2>
    916       <p id="rfc.section.2.4.p.1">A "<dfn>cache</dfn>" is a local store of previous response messages and the subsystem that controls its message storage, retrieval, and deletion.
    917          A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
    918          requests. Any client or server <em class="bcp14">MAY</em> employ a cache, though a cache cannot be used by a server while it is acting as a tunnel.
    919       </p>
    920       <p id="rfc.section.2.4.p.2">The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached
    921          response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response
    922          from O (via C) for a request that has not been cached by UA or A.
    923       </p>
    924       <div id="rfc.figure.u.5"></div><pre class="drawing">            &gt;             &gt;
     894               message that travels the whole chain will pass through four separate connections. Some HTTP communication options might apply
     895               only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along
     896               the chain. Although the diagram is linear, each participant might be engaged in multiple, simultaneous communications. For
     897               example, B might be receiving requests from many clients other than A, and/or forwarding requests to servers other than C,
     898               at the same time that it is handling A's request.
     899            </p>
     900            <p id="rfc.section.2.3.p.4"><span id="rfc.iref.u.2"></span><span id="rfc.iref.d.1"></span> <span id="rfc.iref.i.2"></span><span id="rfc.iref.o.2"></span> We use the terms "<dfn>upstream</dfn>" and "<dfn>downstream</dfn>" to describe various requirements in relation to the directional flow of a message: all messages flow from upstream to downstream.
     901               Likewise, we use the terms inbound and outbound to refer to directions in relation to the request path: "<dfn>inbound</dfn>" means toward the origin server and "<dfn>outbound</dfn>" means toward the user agent.
     902            </p>
     903            <p id="rfc.section.2.3.p.5"><span id="rfc.iref.p.1"></span> A "<dfn>proxy</dfn>" is a message forwarding agent that is selected by the client, usually via local configuration rules, to receive requests
     904               for some type(s) of absolute URI and attempt to satisfy those requests via translation through the HTTP interface. Some translations
     905               are minimal, such as for proxy requests for "http" URIs, whereas other requests might require translation to and from entirely
     906               different application-level protocols. Proxies are often used to group an organization's HTTP requests through a common intermediary
     907               for the sake of security, annotation services, or shared caching.
     908            </p>
     909            <p id="rfc.section.2.3.p.6"><span id="rfc.iref.t.1"></span> <span id="rfc.iref.n.1"></span> An HTTP-to-HTTP proxy is called a "<dfn>transforming proxy</dfn>" if it is designed or configured to modify request or response messages in a semantically meaningful way (i.e., modifications,
     910               beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original
     911               sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared
     912               annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder,
     913               or an intranet-to-Internet privacy filter. Such transformations are presumed to be desired by the client (or client organization)
     914               that selected the proxy and are beyond the scope of this specification. However, when a proxy is not intended to transform
     915               a given message, we use the term "<dfn>non-transforming proxy</dfn>" to target requirements that preserve HTTP message semantics. See <a href="p2-semantics.html#status.203" title="203 Non-Authoritative Information">Section 6.3.4</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> and <a href="p6-cache.html#header.warning" title="Warning">Section 7.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a> for status and warning codes related to transformations.
     916            </p>
     917            <p id="rfc.section.2.3.p.7"><span id="rfc.iref.g.13"></span><span id="rfc.iref.r.4"></span> <span id="rfc.iref.a.1"></span> A "<dfn>gateway</dfn>" (a.k.a., "<dfn>reverse proxy</dfn>") is a receiving agent that acts as a layer above some other server(s) and translates the received requests to the underlying
     918               server's protocol. Gateways are often used to encapsulate legacy or untrusted information services, to improve server performance
     919               through "<dfn>accelerator</dfn>" caching, and to enable partitioning or load-balancing of HTTP services across multiple machines.
     920            </p>
     921            <p id="rfc.section.2.3.p.8">A gateway behaves as an origin server on its outbound connection and as a user agent on its inbound connection. All HTTP requirements
     922               applicable to an origin server also apply to the outbound communication of a gateway. A gateway communicates with inbound
     923               servers using any protocol that it desires, including private extensions to HTTP that are outside the scope of this specification.
     924               However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party HTTP servers <em class="bcp14">MUST</em> conform to HTTP user agent requirements on the gateway's inbound connection and <em class="bcp14">MUST</em> implement the <a href="#header.connection" class="smpl">Connection</a> (<a href="#header.connection" id="rfc.xref.header.connection.1" title="Connection">Section&nbsp;6.1</a>) and <a href="#header.via" class="smpl">Via</a> (<a href="#header.via" id="rfc.xref.header.via.1" title="Via">Section&nbsp;5.7.1</a>) header fields for both connections.
     925            </p>
     926            <p id="rfc.section.2.3.p.9"><span id="rfc.iref.t.2"></span> A "<dfn>tunnel</dfn>" acts as a blind relay between two connections without changing the messages. Once active, a tunnel is not considered a party
     927               to the HTTP communication, though the tunnel might have been initiated by an HTTP request. A tunnel ceases to exist when both
     928               ends of the relayed connection are closed. Tunnels are used to extend a virtual connection through an intermediary, such as
     929               when Transport Layer Security (TLS, <a href="#RFC5246" id="rfc.xref.RFC5246.1"><cite title="The Transport Layer Security (TLS) Protocol Version 1.2">[RFC5246]</cite></a>) is used to establish confidential communication through a shared firewall proxy.
     930            </p>
     931            <p id="rfc.section.2.3.p.10"><span id="rfc.iref.i.3"></span> <span id="rfc.iref.t.3"></span> <span id="rfc.iref.c.3"></span> The above categories for intermediary only consider those acting as participants in the HTTP communication. There are also
     932               intermediaries that can act on lower layers of the network protocol stack, filtering or redirecting HTTP traffic without the
     933               knowledge or permission of message senders. Network intermediaries often introduce security flaws or interoperability problems
     934               by violating HTTP semantics. For example, an "<dfn>interception proxy</dfn>" <a href="#RFC3040" id="rfc.xref.RFC3040.1"><cite title="Internet Web Replication and Caching Taxonomy">[RFC3040]</cite></a> (also commonly known as a "<dfn>transparent proxy</dfn>" <a href="#RFC1919" id="rfc.xref.RFC1919.1"><cite title="Classical versus Transparent IP Proxies">[RFC1919]</cite></a> or "<dfn>captive portal</dfn>") differs from an HTTP proxy because it is not selected by the client. Instead, an interception proxy filters or redirects
     935               outgoing TCP port 80 packets (and occasionally other common port traffic). Interception proxies are commonly found on public
     936               network access points, as a means of enforcing account subscription prior to allowing use of non-local Internet services,
     937               and within corporate firewalls to enforce network usage policies. They are indistinguishable from a man-in-the-middle attack.
     938            </p>
     939            <p id="rfc.section.2.3.p.11">HTTP is defined as a stateless protocol, meaning that each request message can be understood in isolation. Many implementations
     940               depend on HTTP's stateless design in order to reuse proxied connections or dynamically load-balance requests across multiple
     941               servers. Hence, servers <em class="bcp14">MUST NOT</em> assume that two requests on the same connection are from the same user agent unless the connection is secured and specific
     942               to that agent. Some non-standard HTTP extensions (e.g., <a href="#RFC4559" id="rfc.xref.RFC4559.1"><cite title="SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows">[RFC4559]</cite></a>) have been known to violate this requirement, resulting in security and interoperability problems.
     943            </p>
     944         </div>
     945         <div id="caches">
     946            <div id="rfc.iref.c.4"></div>
     947            <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a href="#caches">Caches</a></h2>
     948            <p id="rfc.section.2.4.p.1">A "<dfn>cache</dfn>" is a local store of previous response messages and the subsystem that controls its message storage, retrieval, and deletion.
     949               A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
     950               requests. Any client or server <em class="bcp14">MAY</em> employ a cache, though a cache cannot be used by a server while it is acting as a tunnel.
     951            </p>
     952            <p id="rfc.section.2.4.p.2">The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached
     953               response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response
     954               from O (via C) for a request that has not been cached by UA or A.
     955            </p>
     956            <div id="rfc.figure.u.5"></div><pre class="drawing">            &gt;             &gt;
    925957       <b>UA</b> =========== <b>A</b> =========== <b>B</b> - - - - - - <b>C</b> - - - - - - <b>O</b>
    926958                  &lt;             &lt;
    927959</pre><p id="rfc.section.2.4.p.4"><span id="rfc.iref.c.5"></span> A response is "<dfn>cacheable</dfn>" if a cache is allowed to store a copy of the response message for use in answering subsequent requests. Even when a response
    928          is cacheable, there might be additional constraints placed by the client or by the origin server on when that cached response
    929          can be used for a particular request. HTTP requirements for cache behavior and cacheable responses are defined in <a href="p6-cache.html#caching.overview" title="Overview of Cache Operation">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
    930       </p>
    931       <p id="rfc.section.2.4.p.5">There are a wide variety of architectures and configurations of caches deployed across the World Wide Web and inside large
    932          organizations. These include national hierarchies of proxy caches to save transoceanic bandwidth, collaborative systems that
    933          broadcast or multicast cache entries, archives of pre-fetched cache entries for use in off-line or high-latency environments,
    934          and so on.
    935       </p>
    936       <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a id="conformance" href="#conformance">Conformance and Error Handling</a></h2>
    937       <p id="rfc.section.2.5.p.1">This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP
    938          requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways,
    939          or caches, depending on what behavior is being constrained by the requirement. Additional (social) requirements are placed
    940          on implementations, resource owners, and protocol element registrations when they apply beyond the scope of a single communication.
    941       </p>
    942       <p id="rfc.section.2.5.p.2">The verb "generate" is used instead of "send" where a requirement differentiates between creating a protocol element and merely
    943          forwarding a received element downstream.
    944       </p>
    945       <p id="rfc.section.2.5.p.3">An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes
    946          in HTTP. Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
    947       </p>
    948       <p id="rfc.section.2.5.p.4">Conformance applies to both the syntax and semantics of HTTP protocol elements. A sender <em class="bcp14">MUST NOT</em> generate protocol elements that convey a meaning that is known by that sender to be false. A sender <em class="bcp14">MUST NOT</em> generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements that are applicable
    949          to the sender's role. If a received protocol element is processed, the recipient <em class="bcp14">MUST</em> be able to parse any value that would match the ABNF rules for that protocol element, excluding only those rules not applicable
    950          to the recipient's role.
    951       </p>
    952       <p id="rfc.section.2.5.p.5">Unless noted otherwise, a recipient <em class="bcp14">MAY</em> attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms
    953          except when they have a direct impact on security, since different applications of the protocol require different error handling
    954          strategies. For example, a Web browser might wish to transparently recover from a response where the <a href="p2-semantics.html#header.location" class="smpl">Location</a> header field doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery
    955          to be dangerous.
    956       </p>
    957       <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a id="http.version" href="#http.version">Protocol Versioning</a></h2>
    958       <p id="rfc.section.2.6.p.1">HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. This specification defines version "1.1".
    959          The protocol version as a whole indicates the sender's conformance with the set of requirements laid out in that version's
    960          corresponding specification of HTTP.
    961       </p>
    962       <p id="rfc.section.2.6.p.2">The version of an HTTP message is indicated by an HTTP-version field in the first line of the message. HTTP-version is case-sensitive.</p>
    963       <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span>  <a href="#http.version" class="smpl">HTTP-version</a>  = <a href="#http.version" class="smpl">HTTP-name</a> "/" <a href="#core.rules" class="smpl">DIGIT</a> "." <a href="#core.rules" class="smpl">DIGIT</a>
     960               is cacheable, there might be additional constraints placed by the client or by the origin server on when that cached response
     961               can be used for a particular request. HTTP requirements for cache behavior and cacheable responses are defined in <a href="p6-cache.html#caching.overview" title="Overview of Cache Operation">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
     962            </p>
     963            <p id="rfc.section.2.4.p.5">There are a wide variety of architectures and configurations of caches deployed across the World Wide Web and inside large
     964               organizations. These include national hierarchies of proxy caches to save transoceanic bandwidth, collaborative systems that
     965               broadcast or multicast cache entries, archives of pre-fetched cache entries for use in off-line or high-latency environments,
     966               and so on.
     967            </p>
     968         </div>
     969         <div id="conformance">
     970            <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a href="#conformance">Conformance and Error Handling</a></h2>
     971            <p id="rfc.section.2.5.p.1">This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP
     972               requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways,
     973               or caches, depending on what behavior is being constrained by the requirement. Additional (social) requirements are placed
     974               on implementations, resource owners, and protocol element registrations when they apply beyond the scope of a single communication.
     975            </p>
     976            <p id="rfc.section.2.5.p.2">The verb "generate" is used instead of "send" where a requirement differentiates between creating a protocol element and merely
     977               forwarding a received element downstream.
     978            </p>
     979            <p id="rfc.section.2.5.p.3">An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes
     980               in HTTP. Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
     981            </p>
     982            <p id="rfc.section.2.5.p.4">Conformance applies to both the syntax and semantics of HTTP protocol elements. A sender <em class="bcp14">MUST NOT</em> generate protocol elements that convey a meaning that is known by that sender to be false. A sender <em class="bcp14">MUST NOT</em> generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements that are applicable
     983               to the sender's role. If a received protocol element is processed, the recipient <em class="bcp14">MUST</em> be able to parse any value that would match the ABNF rules for that protocol element, excluding only those rules not applicable
     984               to the recipient's role.
     985            </p>
     986            <p id="rfc.section.2.5.p.5">Unless noted otherwise, a recipient <em class="bcp14">MAY</em> attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms
     987               except when they have a direct impact on security, since different applications of the protocol require different error handling
     988               strategies. For example, a Web browser might wish to transparently recover from a response where the <a href="p2-semantics.html#header.location" class="smpl">Location</a> header field doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery
     989               to be dangerous.
     990            </p>
     991         </div>
     992         <div id="http.version">
     993            <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a href="#http.version">Protocol Versioning</a></h2>
     994            <p id="rfc.section.2.6.p.1">HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. This specification defines version "1.1".
     995               The protocol version as a whole indicates the sender's conformance with the set of requirements laid out in that version's
     996               corresponding specification of HTTP.
     997            </p>
     998            <p id="rfc.section.2.6.p.2">The version of an HTTP message is indicated by an HTTP-version field in the first line of the message. HTTP-version is case-sensitive.</p>
     999            <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span>  <a href="#http.version" class="smpl">HTTP-version</a>  = <a href="#http.version" class="smpl">HTTP-name</a> "/" <a href="#core.rules" class="smpl">DIGIT</a> "." <a href="#core.rules" class="smpl">DIGIT</a>
    9641000  <a href="#http.version" class="smpl">HTTP-name</a>     = %x48.54.54.50 ; "HTTP", case-sensitive
    9651001</pre><p id="rfc.section.2.6.p.4">The HTTP version number consists of two decimal digits separated by a "." (period or decimal point). The first digit ("major
    966          version") indicates the HTTP messaging syntax, whereas the second digit ("minor version") indicates the highest minor version
    967          to which the sender is conformant and able to understand for future communication. The minor version advertises the sender's
    968          communication capabilities even when the sender is only using a backwards-compatible subset of the protocol, thereby letting
    969          the recipient know that more advanced features can be used in response (by servers) or in future requests (by clients).
    970       </p>
    971       <p id="rfc.section.2.6.p.5">When an HTTP/1.1 message is sent to an HTTP/1.0 recipient <a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[RFC1945]</cite></a> or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0
    972          message if all of the newer features are ignored. This specification places recipient-version requirements on some new features
    973          so that a conformant sender will only use compatible features until it has determined, through configuration or the receipt
    974          of a message, that the recipient supports HTTP/1.1.
    975       </p>
    976       <p id="rfc.section.2.6.p.6">The interpretation of a header field does not change between minor versions of the same major HTTP version, though the default
    977          behavior of a recipient in the absence of such a field can change. Unless specified otherwise, header fields defined in HTTP/1.1
    978          are defined for all versions of HTTP/1.x. In particular, the <a href="#header.host" class="smpl">Host</a> and <a href="#header.connection" class="smpl">Connection</a> header fields ought to be implemented by all HTTP/1.x implementations whether or not they advertise conformance with HTTP/1.1.
    979       </p>
    980       <p id="rfc.section.2.6.p.7">New header fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation
    981          of previously defined header fields. When an implementation receives an unrecognized header field, the recipient <em class="bcp14">MUST</em> ignore that header field for local processing regardless of the message's HTTP version. An unrecognized header field received
    982          by a proxy <em class="bcp14">MUST</em> be forwarded downstream unless the header field's field-name is listed in the message's <a href="#header.connection" class="smpl">Connection</a> header field (see <a href="#header.connection" id="rfc.xref.header.connection.2" title="Connection">Section&nbsp;6.1</a>). These requirements allow HTTP's functionality to be enhanced without requiring prior update of deployed intermediaries.
    983       </p>
    984       <p id="rfc.section.2.6.p.8">Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as tunnels) <em class="bcp14">MUST</em> send their own HTTP-version in forwarded messages. In other words, they <em class="bcp14">MUST NOT</em> blindly forward the first line of an HTTP message without ensuring that the protocol version in that message matches a version
    985          to which that intermediary is conformant for both the receiving and sending of messages. Forwarding an HTTP message without
    986          rewriting the HTTP-version might result in communication errors when downstream recipients use the message sender's version
    987          to determine what features are safe to use for later communication with that sender.
    988       </p>
    989       <p id="rfc.section.2.6.p.9">An HTTP client <em class="bcp14">SHOULD</em> send a request version equal to the highest version to which the client is conformant and whose major version is no higher
    990          than the highest version supported by the server, if this is known. An HTTP client <em class="bcp14">MUST NOT</em> send a version to which it is not conformant.
    991       </p>
    992       <p id="rfc.section.2.6.p.10">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
    993          the client has attempted at least one normal request and determined from the response status or header fields (e.g., <a href="p2-semantics.html#header.server" class="smpl">Server</a>) that the server improperly handles higher request versions.
    994       </p>
    995       <p id="rfc.section.2.6.p.11">An HTTP server <em class="bcp14">SHOULD</em> send a response version equal to the highest version to which the server is conformant and whose major version is less than
    996          or equal to the one received in the request. An HTTP server <em class="bcp14">MUST NOT</em> send a version to which it is not conformant. A server <em class="bcp14">MAY</em> send a <a href="p2-semantics.html#status.505" class="smpl">505 (HTTP Version Not
    997             Supported)</a> response if it cannot send a response using the major version used in the client's request.
    998       </p>
    999       <p id="rfc.section.2.6.p.12">An HTTP server <em class="bcp14">MAY</em> send an HTTP/1.0 response to an HTTP/1.0 request if it is known or suspected that the client incorrectly implements the HTTP
    1000          specification and is incapable of correctly processing later version responses, such as when a client fails to parse the version
    1001          number correctly or when an intermediary is known to blindly forward the HTTP-version even when it doesn't conform to the
    1002          given minor version of the protocol. Such protocol downgrades <em class="bcp14">SHOULD NOT</em> be performed unless triggered by specific client attributes, such as when one or more of the request header fields (e.g., <a href="p2-semantics.html#header.user-agent" class="smpl">User-Agent</a>) uniquely match the values sent by a client known to be in error.
    1003       </p>
    1004       <p id="rfc.section.2.6.p.13">The intention of HTTP's versioning design is that the major number will only be incremented if an incompatible message syntax
    1005          is introduced, and that the minor number will only be incremented when changes made to the protocol have the effect of adding
    1006          to the message semantics or implying additional capabilities of the sender. However, the minor version was not incremented
    1007          for the changes introduced between <a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> and <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, and this revision has specifically avoiding any such changes to the protocol.
    1008       </p>
    1009       <div id="rfc.iref.r.5"></div>
    1010       <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a id="uri" href="#uri">Uniform Resource Identifiers</a></h2>
    1011       <p id="rfc.section.2.7.p.1">Uniform Resource Identifiers (URIs) <a href="#RFC3986" id="rfc.xref.RFC3986.2"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> are used throughout HTTP as the means for identifying resources (<a href="p2-semantics.html#resources" title="Resources">Section 2</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). URI references are used to target requests, indicate redirects, and define relationships.
    1012       </p>
    1013       <p id="rfc.section.2.7.p.2">This specification adopts the definitions of "URI-reference", "absolute-URI", "relative-part", "port", "host", "path-abempty",
    1014          "query", "segment", and "authority" from the URI generic syntax. In addition, we define an "absolute-path" rule (that differs
    1015          from RFC 3986's "path-absolute" in that it allows a leading "//") and a "partial-URI" rule for protocol elements that allow
    1016          a relative URI but not a fragment.
    1017       </p>
    1018       <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span><span id="rfc.iref.g.24"></span>  <a href="#uri" class="smpl">URI-reference</a> = &lt;URI-reference, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.3"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-4.1">Section 4.1</a>&gt;
    1019   <a href="#uri" class="smpl">absolute-URI</a>  = &lt;absolute-URI, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.4"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-4.3">Section 4.3</a>&gt;
    1020   <a href="#uri" class="smpl">relative-part</a> = &lt;relative-part, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.5"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-4.2">Section 4.2</a>&gt;
    1021   <a href="#uri" class="smpl">authority</a>     = &lt;authority, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.6"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2">Section 3.2</a>&gt;
    1022   <a href="#uri" class="smpl">path-abempty</a>  = &lt;path-abempty, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.7"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
    1023   <a href="#uri" class="smpl">port</a>          = &lt;port, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.8"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.3">Section 3.2.3</a>&gt;
    1024   <a href="#uri" class="smpl">query</a>         = &lt;query, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.9"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.4">Section 3.4</a>&gt;
    1025   <a href="#uri" class="smpl">segment</a>       = &lt;segment, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.10"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
    1026   <a href="#uri" class="smpl">uri-host</a>      = &lt;host, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.11"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>&gt;
     1002               version") indicates the HTTP messaging syntax, whereas the second digit ("minor version") indicates the highest minor version
     1003               to which the sender is conformant and able to understand for future communication. The minor version advertises the sender's
     1004               communication capabilities even when the sender is only using a backwards-compatible subset of the protocol, thereby letting
     1005               the recipient know that more advanced features can be used in response (by servers) or in future requests (by clients).
     1006            </p>
     1007            <p id="rfc.section.2.6.p.5">When an HTTP/1.1 message is sent to an HTTP/1.0 recipient <a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[RFC1945]</cite></a> or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0
     1008               message if all of the newer features are ignored. This specification places recipient-version requirements on some new features
     1009               so that a conformant sender will only use compatible features until it has determined, through configuration or the receipt
     1010               of a message, that the recipient supports HTTP/1.1.
     1011            </p>
     1012            <p id="rfc.section.2.6.p.6">The interpretation of a header field does not change between minor versions of the same major HTTP version, though the default
     1013               behavior of a recipient in the absence of such a field can change. Unless specified otherwise, header fields defined in HTTP/1.1
     1014               are defined for all versions of HTTP/1.x. In particular, the <a href="#header.host" class="smpl">Host</a> and <a href="#header.connection" class="smpl">Connection</a> header fields ought to be implemented by all HTTP/1.x implementations whether or not they advertise conformance with HTTP/1.1.
     1015            </p>
     1016            <p id="rfc.section.2.6.p.7">New header fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation
     1017               of previously defined header fields. When an implementation receives an unrecognized header field, the recipient <em class="bcp14">MUST</em> ignore that header field for local processing regardless of the message's HTTP version. An unrecognized header field received
     1018               by a proxy <em class="bcp14">MUST</em> be forwarded downstream unless the header field's field-name is listed in the message's <a href="#header.connection" class="smpl">Connection</a> header field (see <a href="#header.connection" id="rfc.xref.header.connection.2" title="Connection">Section&nbsp;6.1</a>). These requirements allow HTTP's functionality to be enhanced without requiring prior update of deployed intermediaries.
     1019            </p>
     1020            <p id="rfc.section.2.6.p.8">Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as tunnels) <em class="bcp14">MUST</em> send their own HTTP-version in forwarded messages. In other words, they <em class="bcp14">MUST NOT</em> blindly forward the first line of an HTTP message without ensuring that the protocol version in that message matches a version
     1021               to which that intermediary is conformant for both the receiving and sending of messages. Forwarding an HTTP message without
     1022               rewriting the HTTP-version might result in communication errors when downstream recipients use the message sender's version
     1023               to determine what features are safe to use for later communication with that sender.
     1024            </p>
     1025            <p id="rfc.section.2.6.p.9">An HTTP client <em class="bcp14">SHOULD</em> send a request version equal to the highest version to which the client is conformant and whose major version is no higher
     1026               than the highest version supported by the server, if this is known. An HTTP client <em class="bcp14">MUST NOT</em> send a version to which it is not conformant.
     1027            </p>
     1028            <p id="rfc.section.2.6.p.10">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
     1029               the client has attempted at least one normal request and determined from the response status or header fields (e.g., <a href="p2-semantics.html#header.server" class="smpl">Server</a>) that the server improperly handles higher request versions.
     1030            </p>
     1031            <p id="rfc.section.2.6.p.11">An HTTP server <em class="bcp14">SHOULD</em> send a response version equal to the highest version to which the server is conformant and whose major version is less than
     1032               or equal to the one received in the request. An HTTP server <em class="bcp14">MUST NOT</em> send a version to which it is not conformant. A server <em class="bcp14">MAY</em> send a <a href="p2-semantics.html#status.505" class="smpl">505 (HTTP Version Not
     1033                  Supported)</a> response if it cannot send a response using the major version used in the client's request.
     1034            </p>
     1035            <p id="rfc.section.2.6.p.12">An HTTP server <em class="bcp14">MAY</em> send an HTTP/1.0 response to an HTTP/1.0 request if it is known or suspected that the client incorrectly implements the HTTP
     1036               specification and is incapable of correctly processing later version responses, such as when a client fails to parse the version
     1037               number correctly or when an intermediary is known to blindly forward the HTTP-version even when it doesn't conform to the
     1038               given minor version of the protocol. Such protocol downgrades <em class="bcp14">SHOULD NOT</em> be performed unless triggered by specific client attributes, such as when one or more of the request header fields (e.g., <a href="p2-semantics.html#header.user-agent" class="smpl">User-Agent</a>) uniquely match the values sent by a client known to be in error.
     1039            </p>
     1040            <p id="rfc.section.2.6.p.13">The intention of HTTP's versioning design is that the major number will only be incremented if an incompatible message syntax
     1041               is introduced, and that the minor number will only be incremented when changes made to the protocol have the effect of adding
     1042               to the message semantics or implying additional capabilities of the sender. However, the minor version was not incremented
     1043               for the changes introduced between <a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> and <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, and this revision has specifically avoiding any such changes to the protocol.
     1044            </p>
     1045         </div>
     1046         <div id="uri">
     1047            <div id="rfc.iref.r.5"></div>
     1048            <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a href="#uri">Uniform Resource Identifiers</a></h2>
     1049            <p id="rfc.section.2.7.p.1">Uniform Resource Identifiers (URIs) <a href="#RFC3986" id="rfc.xref.RFC3986.2"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> are used throughout HTTP as the means for identifying resources (<a href="p2-semantics.html#resources" title="Resources">Section 2</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). URI references are used to target requests, indicate redirects, and define relationships.
     1050            </p>
     1051            <p id="rfc.section.2.7.p.2">This specification adopts the definitions of "URI-reference", "absolute-URI", "relative-part", "port", "host", "path-abempty",
     1052               "query", "segment", and "authority" from the URI generic syntax. In addition, we define an "absolute-path" rule (that differs
     1053               from RFC 3986's "path-absolute" in that it allows a leading "//") and a "partial-URI" rule for protocol elements that allow
     1054               a relative URI but not a fragment.
     1055            </p>
     1056            <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span><span id="rfc.iref.g.24"></span>  <a href="#uri" class="smpl">URI-reference</a> = &lt;URI-reference, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.3"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-4.1">Section 4.1</a>&gt;
     1057  <a href="#uri" class="smpl">absolute-URI</a>  = &lt;absolute-URI, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.4"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-4.3">Section 4.3</a>&gt;
     1058  <a href="#uri" class="smpl">relative-part</a> = &lt;relative-part, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.5"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-4.2">Section 4.2</a>&gt;
     1059  <a href="#uri" class="smpl">authority</a>     = &lt;authority, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.6"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2">Section 3.2</a>&gt;
     1060  <a href="#uri" class="smpl">path-abempty</a>  = &lt;path-abempty, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.7"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
     1061  <a href="#uri" class="smpl">port</a>          = &lt;port, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.8"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.3">Section 3.2.3</a>&gt;
     1062  <a href="#uri" class="smpl">query</a>         = &lt;query, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.9"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.4">Section 3.4</a>&gt;
     1063  <a href="#uri" class="smpl">segment</a>       = &lt;segment, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.10"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
     1064  <a href="#uri" class="smpl">uri-host</a>      = &lt;host, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.11"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>&gt;
    10271065 
    10281066  <a href="#uri" class="smpl">absolute-path</a> = 1*( "/" segment )
    10291067  <a href="#uri" class="smpl">partial-URI</a>   = relative-part [ "?" query ]
    10301068</pre><p id="rfc.section.2.7.p.4">Each protocol element in HTTP that allows a URI reference will indicate in its ABNF production whether the element allows
    1031          any form of reference (URI-reference), only a URI in absolute form (absolute-URI), only the path and optional query components,
    1032          or some combination of the above. Unless otherwise indicated, URI references are parsed relative to the effective request
    1033          URI (<a href="#effective.request.uri" title="Effective Request URI">Section&nbsp;5.5</a>).
    1034       </p>
    1035       <h3 id="rfc.section.2.7.1"><a href="#rfc.section.2.7.1">2.7.1</a>&nbsp;<a id="http.uri" href="#http.uri">http URI scheme</a></h3>
    1036       <div id="rfc.iref.h.1"></div>
    1037       <div id="rfc.iref.u.3"></div>
    1038       <p id="rfc.section.2.7.1.p.1">The "http" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
    1039          namespace governed by a potential HTTP origin server listening for TCP connections on a given port.
    1040       </p>
    1041       <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.25"></span>  <a href="#http.uri" class="smpl">http-URI</a> = "http:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
    1042 </pre><p id="rfc.section.2.7.1.p.3">The HTTP origin server is identified by the generic syntax's <a href="#uri" class="smpl">authority</a> component, which includes a host identifier and optional TCP port (<a href="#RFC3986" id="rfc.xref.RFC3986.12"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>). The remainder of the URI, consisting of both the hierarchical path component and optional query component, serves as an
    1043          identifier for a potential resource within that origin server's name space.
    1044       </p>
    1045       <p id="rfc.section.2.7.1.p.4">If the host identifier is provided as an IP address, then the origin server is any listener on the indicated TCP port at that
    1046          IP address. If host is a registered name, then that name is considered an indirect identifier and the recipient might use
    1047          a name resolution service, such as DNS, to find the address of a listener for that host. The host <em class="bcp14">MUST NOT</em> be empty; if an "http" URI is received with an empty host, then it <em class="bcp14">MUST</em> be rejected as invalid. If the port subcomponent is empty or not given, then TCP port 80 is assumed (the default reserved
    1048          port for WWW services).
    1049       </p>
    1050       <p id="rfc.section.2.7.1.p.5">Regardless of the form of host identifier, access to that host is not implied by the mere presence of its name or address.
    1051          The host might or might not exist and, even when it does exist, might or might not be running an HTTP server or listening
    1052          to the indicated port. The "http" URI scheme makes use of the delegated nature of Internet names and addresses to establish
    1053          a naming authority (whatever entity has the ability to place an HTTP server at that Internet name or address) and allows that
    1054          authority to determine which names are valid and how they might be used.
    1055       </p>
    1056       <p id="rfc.section.2.7.1.p.6">When an "http" URI is used within a context that calls for access to the indicated resource, a client <em class="bcp14">MAY</em> attempt access by resolving the host to an IP address, establishing a TCP connection to that address on the indicated port,
    1057          and sending an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) containing the URI's identifying data (<a href="#message.routing" title="Message Routing">Section&nbsp;5</a>) to the server. If the server responds to that request with a non-interim HTTP response message, as described in <a href="p2-semantics.html#status.codes" title="Response Status Codes">Section 6</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, then that response is considered an authoritative answer to the client's request.
    1058       </p>
    1059       <p id="rfc.section.2.7.1.p.7">Although HTTP is independent of the transport protocol, the "http" scheme is specific to TCP-based services because the name
    1060          delegation process depends on TCP for establishing authority. An HTTP service based on some other underlying connection protocol
    1061          would presumably be identified using a different URI scheme, just as the "https" scheme (below) is used for resources that
    1062          require an end-to-end secured connection. Other protocols might also be used to provide access to "http" identified resources
    1063          — it is only the authoritative interface used for mapping the namespace that is specific to TCP.
    1064       </p>
    1065       <p id="rfc.section.2.7.1.p.8">The URI generic syntax for authority also includes a deprecated userinfo subcomponent (<a href="#RFC3986" id="rfc.xref.RFC3986.13"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.1">Section 3.2.1</a>) for including user authentication information in the URI. Some implementations make use of the userinfo component for internal
    1066          configuration of authentication information, such as within command invocation options, configuration files, or bookmark lists,
    1067          even though such usage might expose a user identifier or password. Senders <em class="bcp14">MUST</em> exclude the userinfo subcomponent (and its "@" delimiter) when an "http" URI is transmitted within a message as a request
    1068          target or header field value. Recipients of an "http" URI reference <em class="bcp14">SHOULD</em> parse for userinfo and treat its presence as an error, since it is likely being used to obscure the authority for the sake
    1069          of phishing attacks.
    1070       </p>
    1071       <h3 id="rfc.section.2.7.2"><a href="#rfc.section.2.7.2">2.7.2</a>&nbsp;<a id="https.uri" href="#https.uri">https URI scheme</a></h3>
    1072       <div id="rfc.iref.h.2"></div>
    1073       <div id="rfc.iref.u.4"></div>
    1074       <p id="rfc.section.2.7.2.p.1">The "https" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
    1075          namespace governed by a potential HTTP origin server listening to a given TCP port for TLS-secured connections <a href="#RFC5246" id="rfc.xref.RFC5246.2"><cite title="The Transport Layer Security (TLS) Protocol Version 1.2">[RFC5246]</cite></a>.
    1076       </p>
    1077       <p id="rfc.section.2.7.2.p.2">All of the requirements listed above for the "http" scheme are also requirements for the "https" scheme, except that a default
    1078          TCP port of 443 is assumed if the port subcomponent is empty or not given, and the TCP connection <em class="bcp14">MUST</em> be secured, end-to-end, through the use of strong encryption prior to sending the first HTTP request.
    1079       </p>
    1080       <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.26"></span>  <a href="#https.uri" class="smpl">https-URI</a> = "https:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
     1069               any form of reference (URI-reference), only a URI in absolute form (absolute-URI), only the path and optional query components,
     1070               or some combination of the above. Unless otherwise indicated, URI references are parsed relative to the effective request
     1071               URI (<a href="#effective.request.uri" title="Effective Request URI">Section&nbsp;5.5</a>).
     1072            </p>
     1073            <div id="http.uri">
     1074               <h3 id="rfc.section.2.7.1"><a href="#rfc.section.2.7.1">2.7.1</a>&nbsp;<a href="#http.uri">http URI scheme</a></h3>
     1075               <div id="rfc.iref.h.1"></div>
     1076               <div id="rfc.iref.u.3"></div>
     1077               <p id="rfc.section.2.7.1.p.1">The "http" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
     1078                  namespace governed by a potential HTTP origin server listening for TCP connections on a given port.
     1079               </p>
     1080               <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.25"></span>  <a href="#http.uri" class="smpl">http-URI</a> = "http:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
     1081</pre><p id="rfc.section.2.7.1.p.3">The HTTP origin server is identified by the generic syntax's <a href="#uri" class="smpl">authority</a> component, which includes a host identifier and optional TCP port (<a href="#RFC3986" id="rfc.xref.RFC3986.12"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>). The remainder of the URI, consisting of both the hierarchical path component and optional query component, serves as an
     1082                  identifier for a potential resource within that origin server's name space.
     1083               </p>
     1084               <p id="rfc.section.2.7.1.p.4">If the host identifier is provided as an IP address, then the origin server is any listener on the indicated TCP port at that
     1085                  IP address. If host is a registered name, then that name is considered an indirect identifier and the recipient might use
     1086                  a name resolution service, such as DNS, to find the address of a listener for that host. The host <em class="bcp14">MUST NOT</em> be empty; if an "http" URI is received with an empty host, then it <em class="bcp14">MUST</em> be rejected as invalid. If the port subcomponent is empty or not given, then TCP port 80 is assumed (the default reserved
     1087                  port for WWW services).
     1088               </p>
     1089               <p id="rfc.section.2.7.1.p.5">Regardless of the form of host identifier, access to that host is not implied by the mere presence of its name or address.
     1090                  The host might or might not exist and, even when it does exist, might or might not be running an HTTP server or listening
     1091                  to the indicated port. The "http" URI scheme makes use of the delegated nature of Internet names and addresses to establish
     1092                  a naming authority (whatever entity has the ability to place an HTTP server at that Internet name or address) and allows that
     1093                  authority to determine which names are valid and how they might be used.
     1094               </p>
     1095               <p id="rfc.section.2.7.1.p.6">When an "http" URI is used within a context that calls for access to the indicated resource, a client <em class="bcp14">MAY</em> attempt access by resolving the host to an IP address, establishing a TCP connection to that address on the indicated port,
     1096                  and sending an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) containing the URI's identifying data (<a href="#message.routing" title="Message Routing">Section&nbsp;5</a>) to the server. If the server responds to that request with a non-interim HTTP response message, as described in <a href="p2-semantics.html#status.codes" title="Response Status Codes">Section 6</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, then that response is considered an authoritative answer to the client's request.
     1097               </p>
     1098               <p id="rfc.section.2.7.1.p.7">Although HTTP is independent of the transport protocol, the "http" scheme is specific to TCP-based services because the name
     1099                  delegation process depends on TCP for establishing authority. An HTTP service based on some other underlying connection protocol
     1100                  would presumably be identified using a different URI scheme, just as the "https" scheme (below) is used for resources that
     1101                  require an end-to-end secured connection. Other protocols might also be used to provide access to "http" identified resources
     1102                  — it is only the authoritative interface used for mapping the namespace that is specific to TCP.
     1103               </p>
     1104               <p id="rfc.section.2.7.1.p.8">The URI generic syntax for authority also includes a deprecated userinfo subcomponent (<a href="#RFC3986" id="rfc.xref.RFC3986.13"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.1">Section 3.2.1</a>) for including user authentication information in the URI. Some implementations make use of the userinfo component for internal
     1105                  configuration of authentication information, such as within command invocation options, configuration files, or bookmark lists,
     1106                  even though such usage might expose a user identifier or password. Senders <em class="bcp14">MUST</em> exclude the userinfo subcomponent (and its "@" delimiter) when an "http" URI is transmitted within a message as a request
     1107                  target or header field value. Recipients of an "http" URI reference <em class="bcp14">SHOULD</em> parse for userinfo and treat its presence as an error, since it is likely being used to obscure the authority for the sake
     1108                  of phishing attacks.
     1109               </p>
     1110            </div>
     1111            <div id="https.uri">
     1112               <h3 id="rfc.section.2.7.2"><a href="#rfc.section.2.7.2">2.7.2</a>&nbsp;<a href="#https.uri">https URI scheme</a></h3>
     1113               <div id="rfc.iref.h.2"></div>
     1114               <div id="rfc.iref.u.4"></div>
     1115               <p id="rfc.section.2.7.2.p.1">The "https" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
     1116                  namespace governed by a potential HTTP origin server listening to a given TCP port for TLS-secured connections <a href="#RFC5246" id="rfc.xref.RFC5246.2"><cite title="The Transport Layer Security (TLS) Protocol Version 1.2">[RFC5246]</cite></a>.
     1117               </p>
     1118               <p id="rfc.section.2.7.2.p.2">All of the requirements listed above for the "http" scheme are also requirements for the "https" scheme, except that a default
     1119                  TCP port of 443 is assumed if the port subcomponent is empty or not given, and the TCP connection <em class="bcp14">MUST</em> be secured, end-to-end, through the use of strong encryption prior to sending the first HTTP request.
     1120               </p>
     1121               <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.26"></span>  <a href="#https.uri" class="smpl">https-URI</a> = "https:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
    10811122</pre><p id="rfc.section.2.7.2.p.4">Resources made available via the "https" scheme have no shared identity with the "http" scheme even if their resource identifiers
    1082          indicate the same authority (the same host listening to the same TCP port). They are distinct name spaces and are considered
    1083          to be distinct origin servers. However, an extension to HTTP that is defined to apply to entire host domains, such as the
    1084          Cookie protocol <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>, can allow information set by one service to impact communication with other services within a matching group of host domains.
    1085       </p>
    1086       <p id="rfc.section.2.7.2.p.5">The process for authoritative access to an "https" identified resource is defined in <a href="#RFC2818" id="rfc.xref.RFC2818.2"><cite title="HTTP Over TLS">[RFC2818]</cite></a>.
    1087       </p>
    1088       <h3 id="rfc.section.2.7.3"><a href="#rfc.section.2.7.3">2.7.3</a>&nbsp;<a id="uri.comparison" href="#uri.comparison">http and https URI Normalization and Comparison</a></h3>
    1089       <p id="rfc.section.2.7.3.p.1">Since the "http" and "https" schemes conform to the URI generic syntax, such URIs are normalized and compared according to
    1090          the algorithm defined in <a href="#RFC3986" id="rfc.xref.RFC3986.14"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-6">Section 6</a>, using the defaults described above for each scheme.
    1091       </p>
    1092       <p id="rfc.section.2.7.3.p.2">If the port is equal to the default port for a scheme, the normal form is to elide the port subcomponent. When not being used
    1093          in absolute form as the request target of an OPTIONS request, an empty path component is equivalent to an absolute path of
    1094          "/", so the normal form is to provide a path of "/" instead. The scheme and host are case-insensitive and normally provided
    1095          in lowercase; all other components are compared in a case-sensitive manner. Characters other than those in the "reserved"
    1096          set are equivalent to their percent-encoded octets (see <a href="#RFC3986" id="rfc.xref.RFC3986.15"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-2.1">Section 2.1</a>): the normal form is to not encode them.
    1097       </p>
    1098       <p id="rfc.section.2.7.3.p.3">For example, the following three URIs are equivalent:</p>
    1099       <div id="rfc.figure.u.10"></div><pre class="text">   http://example.com:80/~smith/home.html
     1123                  indicate the same authority (the same host listening to the same TCP port). They are distinct name spaces and are considered
     1124                  to be distinct origin servers. However, an extension to HTTP that is defined to apply to entire host domains, such as the
     1125                  Cookie protocol <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>, can allow information set by one service to impact communication with other services within a matching group of host domains.
     1126               </p>
     1127               <p id="rfc.section.2.7.2.p.5">The process for authoritative access to an "https" identified resource is defined in <a href="#RFC2818" id="rfc.xref.RFC2818.2"><cite title="HTTP Over TLS">[RFC2818]</cite></a>.
     1128               </p>
     1129            </div>
     1130            <div id="uri.comparison">
     1131               <h3 id="rfc.section.2.7.3"><a href="#rfc.section.2.7.3">2.7.3</a>&nbsp;<a href="#uri.comparison">http and https URI Normalization and Comparison</a></h3>
     1132               <p id="rfc.section.2.7.3.p.1">Since the "http" and "https" schemes conform to the URI generic syntax, such URIs are normalized and compared according to
     1133                  the algorithm defined in <a href="#RFC3986" id="rfc.xref.RFC3986.14"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-6">Section 6</a>, using the defaults described above for each scheme.
     1134               </p>
     1135               <p id="rfc.section.2.7.3.p.2">If the port is equal to the default port for a scheme, the normal form is to elide the port subcomponent. When not being used
     1136                  in absolute form as the request target of an OPTIONS request, an empty path component is equivalent to an absolute path of
     1137                  "/", so the normal form is to provide a path of "/" instead. The scheme and host are case-insensitive and normally provided
     1138                  in lowercase; all other components are compared in a case-sensitive manner. Characters other than those in the "reserved"
     1139                  set are equivalent to their percent-encoded octets (see <a href="#RFC3986" id="rfc.xref.RFC3986.15"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-2.1">Section 2.1</a>): the normal form is to not encode them.
     1140               </p>
     1141               <p id="rfc.section.2.7.3.p.3">For example, the following three URIs are equivalent:</p>
     1142               <div id="rfc.figure.u.10"></div><pre class="text">   http://example.com:80/~smith/home.html
    11001143   http://EXAMPLE.com/%7Esmith/home.html
    11011144   http://EXAMPLE.com:/%7esmith/home.html
    1102 </pre><h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="http.message" href="#http.message">Message Format</a></h1>
    1103       <div id="rfc.iref.h.3"></div>
    1104       <div id="rfc.iref.h.4"></div>
    1105       <div id="rfc.iref.h.5"></div>
    1106       <p id="rfc.section.3.p.1">All HTTP/1.1 messages consist of a start-line followed by a sequence of octets in a format similar to the Internet Message
    1107          Format <a href="#RFC5322" id="rfc.xref.RFC5322.2"><cite title="Internet Message Format">[RFC5322]</cite></a>: zero or more header fields (collectively referred to as the "headers" or the "header section"), an empty line indicating
    1108          the end of the header section, and an optional message body.
    1109       </p>
    1110       <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.27"></span>  <a href="#http.message" class="smpl">HTTP-message</a>   = <a href="#http.message" class="smpl">start-line</a>
     1145</pre></div>
     1146         </div>
     1147      </div>
     1148      <div id="http.message">
     1149         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#http.message">Message Format</a></h1>
     1150         <div id="rfc.iref.h.3"></div>
     1151         <div id="rfc.iref.h.4"></div>
     1152         <div id="rfc.iref.h.5"></div>
     1153         <p id="rfc.section.3.p.1">All HTTP/1.1 messages consist of a start-line followed by a sequence of octets in a format similar to the Internet Message
     1154            Format <a href="#RFC5322" id="rfc.xref.RFC5322.2"><cite title="Internet Message Format">[RFC5322]</cite></a>: zero or more header fields (collectively referred to as the "headers" or the "header section"), an empty line indicating
     1155            the end of the header section, and an optional message body.
     1156         </p>
     1157         <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.27"></span>  <a href="#http.message" class="smpl">HTTP-message</a>   = <a href="#http.message" class="smpl">start-line</a>
    11111158                   *( <a href="#header.fields" class="smpl">header-field</a> <a href="#core.rules" class="smpl">CRLF</a> )
    11121159                   <a href="#core.rules" class="smpl">CRLF</a>
    11131160                   [ <a href="#message.body" class="smpl">message-body</a> ]
    11141161</pre><p id="rfc.section.3.p.3">The normal procedure for parsing an HTTP message is to read the start-line into a structure, read each header field into a
    1115          hash table by field name until the empty line, and then use the parsed data to determine if a message body is expected. If
    1116          a message body has been indicated, then it is read as a stream until an amount of octets equal to the message body length
    1117          is read or the connection is closed.
    1118       </p>
    1119       <p id="rfc.section.3.p.4">Recipients <em class="bcp14">MUST</em> parse an HTTP message as a sequence of octets in an encoding that is a superset of US-ASCII <a href="#USASCII" id="rfc.xref.USASCII.2"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Parsing an HTTP message as a stream of Unicode characters, without regard for the specific encoding, creates security vulnerabilities
    1120          due to the varying ways that string processing libraries handle invalid multibyte character sequences that contain the octet
    1121          LF (%x0A). String-based parsers can only be safely used within protocol elements after the element has been extracted from
    1122          the message, such as within a header field-value after message parsing has delineated the individual fields.
    1123       </p>
    1124       <p id="rfc.section.3.p.5">An HTTP message can be parsed as a stream for incremental processing or forwarding downstream. However, recipients cannot
    1125          rely on incremental delivery of partial messages, since some implementations will buffer or delay message forwarding for the
    1126          sake of network efficiency, security checks, or payload transformations.
    1127       </p>
    1128       <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="start.line" href="#start.line">Start Line</a></h2>
    1129       <p id="rfc.section.3.1.p.1">An HTTP message can either be a request from client to server or a response from server to client. Syntactically, the two
    1130          types of message differ only in the start-line, which is either a request-line (for requests) or a status-line (for responses),
    1131          and in the algorithm for determining the length of the message body (<a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
    1132       </p>
    1133       <p id="rfc.section.3.1.p.2">In theory, a client could receive requests and a server could receive responses, distinguishing them by their different start-line
    1134          formats, but in practice servers are implemented to only expect a request (a response is interpreted as an unknown or invalid
    1135          request method) and clients are implemented to only expect a response.
    1136       </p>
    1137       <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.28"></span>  <a href="#http.message" class="smpl">start-line</a>     = <a href="#request.line" class="smpl">request-line</a> / <a href="#status.line" class="smpl">status-line</a>
     1162            hash table by field name until the empty line, and then use the parsed data to determine if a message body is expected. If
     1163            a message body has been indicated, then it is read as a stream until an amount of octets equal to the message body length
     1164            is read or the connection is closed.
     1165         </p>
     1166         <p id="rfc.section.3.p.4">Recipients <em class="bcp14">MUST</em> parse an HTTP message as a sequence of octets in an encoding that is a superset of US-ASCII <a href="#USASCII" id="rfc.xref.USASCII.2"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Parsing an HTTP message as a stream of Unicode characters, without regard for the specific encoding, creates security vulnerabilities
     1167            due to the varying ways that string processing libraries handle invalid multibyte character sequences that contain the octet
     1168            LF (%x0A). String-based parsers can only be safely used within protocol elements after the element has been extracted from
     1169            the message, such as within a header field-value after message parsing has delineated the individual fields.
     1170         </p>
     1171         <p id="rfc.section.3.p.5">An HTTP message can be parsed as a stream for incremental processing or forwarding downstream. However, recipients cannot
     1172            rely on incremental delivery of partial messages, since some implementations will buffer or delay message forwarding for the
     1173            sake of network efficiency, security checks, or payload transformations.
     1174         </p>
     1175         <div id="start.line">
     1176            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#start.line">Start Line</a></h2>
     1177            <p id="rfc.section.3.1.p.1">An HTTP message can either be a request from client to server or a response from server to client. Syntactically, the two
     1178               types of message differ only in the start-line, which is either a request-line (for requests) or a status-line (for responses),
     1179               and in the algorithm for determining the length of the message body (<a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
     1180            </p>
     1181            <p id="rfc.section.3.1.p.2">In theory, a client could receive requests and a server could receive responses, distinguishing them by their different start-line
     1182               formats, but in practice servers are implemented to only expect a request (a response is interpreted as an unknown or invalid
     1183               request method) and clients are implemented to only expect a response.
     1184            </p>
     1185            <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.28"></span>  <a href="#http.message" class="smpl">start-line</a>     = <a href="#request.line" class="smpl">request-line</a> / <a href="#status.line" class="smpl">status-line</a>
    11381186</pre><p id="rfc.section.3.1.p.4">A sender <em class="bcp14">MUST NOT</em> send whitespace between the start-line and the first header field. The presence of such whitespace in a request might be an
    1139          attempt to trick a server into ignoring that field or processing the line after it as a new request, either of which might
    1140          result in a security vulnerability if other implementations within the request chain interpret the same message differently.
    1141          Likewise, the presence of such whitespace in a response might be ignored by some clients or cause others to cease parsing.
    1142       </p>
    1143       <p id="rfc.section.3.1.p.5">A recipient that receives whitespace between the start-line and the first header field <em class="bcp14">MUST</em> either reject the message as invalid or consume each whitespace-preceded line without further processing of it (i.e., ignore
    1144          the entire line, along with any subsequent lines preceded by whitespace, until a properly formed header field is received
    1145          or the header block is terminated).
    1146       </p>
    1147       <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;<a id="request.line" href="#request.line">Request Line</a></h3>
    1148       <p id="rfc.section.3.1.1.p.1">A request-line begins with a method token, followed by a single space (SP), the request-target, another single space (SP),
    1149          the protocol version, and ending with CRLF.
    1150       </p>
    1151       <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.29"></span>  <a href="#request.line" class="smpl">request-line</a>   = <a href="#method" class="smpl">method</a> <a href="#core.rules" class="smpl">SP</a> <a href="#request-target" class="smpl">request-target</a> <a href="#core.rules" class="smpl">SP</a> <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">CRLF</a>
     1187               attempt to trick a server into ignoring that field or processing the line after it as a new request, either of which might
     1188               result in a security vulnerability if other implementations within the request chain interpret the same message differently.
     1189               Likewise, the presence of such whitespace in a response might be ignored by some clients or cause others to cease parsing.
     1190            </p>
     1191            <p id="rfc.section.3.1.p.5">A recipient that receives whitespace between the start-line and the first header field <em class="bcp14">MUST</em> either reject the message as invalid or consume each whitespace-preceded line without further processing of it (i.e., ignore
     1192               the entire line, along with any subsequent lines preceded by whitespace, until a properly formed header field is received
     1193               or the header block is terminated).
     1194            </p>
     1195            <div id="request.line">
     1196               <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;<a href="#request.line">Request Line</a></h3>
     1197               <p id="rfc.section.3.1.1.p.1">A request-line begins with a method token, followed by a single space (SP), the request-target, another single space (SP),
     1198                  the protocol version, and ending with CRLF.
     1199               </p>
     1200               <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.29"></span>  <a href="#request.line" class="smpl">request-line</a>   = <a href="#method" class="smpl">method</a> <a href="#core.rules" class="smpl">SP</a> <a href="#request-target" class="smpl">request-target</a> <a href="#core.rules" class="smpl">SP</a> <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">CRLF</a>
    11521201</pre><div id="rfc.iref.m.2"></div>
    1153       <div id="method">
    1154          <p id="rfc.section.3.1.1.p.3">The method token indicates the request method to be performed on the target resource. The request method is case-sensitive.</p>
    1155       </div>
    1156       <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.30"></span>  <a href="#method" class="smpl">method</a>         = <a href="#rule.token.separators" class="smpl">token</a>
     1202               <div id="method">
     1203                  <p id="rfc.section.3.1.1.p.3">The method token indicates the request method to be performed on the target resource. The request method is case-sensitive.</p>
     1204               </div>
     1205               <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.30"></span>  <a href="#method" class="smpl">method</a>         = <a href="#rule.token.separators" class="smpl">token</a>
    11571206</pre><p id="rfc.section.3.1.1.p.5">The methods defined by this specification can be found in <a href="p2-semantics.html#methods" title="Request Methods">Section 4</a> of <a href="#Part2" id="rfc.xref.Part2.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, along with information regarding the HTTP method registry and considerations for defining new methods.
    1158       </p>
    1159       <div id="rfc.iref.r.6"></div>
    1160       <p id="rfc.section.3.1.1.p.6">The request-target identifies the target resource upon which to apply the request, as defined in <a href="#request-target" title="Request Target">Section&nbsp;5.3</a>.
    1161       </p>
    1162       <p id="rfc.section.3.1.1.p.7">No whitespace is allowed inside the method, request-target, and protocol version. Hence, recipients typically parse the request-line
    1163          into its component parts by splitting on whitespace (see <a href="#message.robustness" title="Message Parsing Robustness">Section&nbsp;3.5</a>).
    1164       </p>
    1165       <p id="rfc.section.3.1.1.p.8">Unfortunately, some user agents fail to properly encode hypertext references that have embedded whitespace, sending the characters
    1166          directly instead of properly encoding or excluding the disallowed characters. Recipients of an invalid request-line <em class="bcp14">SHOULD</em> respond with either a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> error or a <a href="p2-semantics.html#status.301" class="smpl">301 (Moved Permanently)</a> redirect with the request-target properly encoded. Recipients <em class="bcp14">SHOULD NOT</em> attempt to autocorrect and then process the request without a redirect, since the invalid request-line might be deliberately
    1167          crafted to bypass security filters along the request chain.
    1168       </p>
    1169       <p id="rfc.section.3.1.1.p.9">HTTP does not place a pre-defined limit on the length of a request-line. A server that receives a method longer than any that
    1170          it implements <em class="bcp14">SHOULD</em> respond with a <a href="p2-semantics.html#status.501" class="smpl">501 (Not Implemented)</a> status code. A server <em class="bcp14">MUST</em> be prepared to receive URIs of unbounded length and respond with the <a href="p2-semantics.html#status.414" class="smpl">414 (URI Too Long)</a> status code if the received request-target would be longer than the server wishes to handle (see <a href="p2-semantics.html#status.414" title="414 URI Too Long">Section 6.5.12</a> of <a href="#Part2" id="rfc.xref.Part2.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>).
    1171       </p>
    1172       <p id="rfc.section.3.1.1.p.10">Various ad-hoc limitations on request-line length are found in practice. It is <em class="bcp14">RECOMMENDED</em> that all HTTP senders and recipients support, at a minimum, request-line lengths of 8000 octets.
    1173       </p>
    1174       <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;<a id="status.line" href="#status.line">Status Line</a></h3>
    1175       <p id="rfc.section.3.1.2.p.1">The first line of a response message is the status-line, consisting of the protocol version, a space (SP), the status code,
    1176          another space, a possibly-empty textual phrase describing the status code, and ending with CRLF.
    1177       </p>
    1178       <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.31"></span>  <a href="#status.line" class="smpl">status-line</a> = <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">SP</a> <a href="#status.line" class="smpl">status-code</a> <a href="#core.rules" class="smpl">SP</a> <a href="#status.line" class="smpl">reason-phrase</a> <a href="#core.rules" class="smpl">CRLF</a>
     1207               </p>
     1208               <div id="rfc.iref.r.6"></div>
     1209               <p id="rfc.section.3.1.1.p.6">The request-target identifies the target resource upon which to apply the request, as defined in <a href="#request-target" title="Request Target">Section&nbsp;5.3</a>.
     1210               </p>
     1211               <p id="rfc.section.3.1.1.p.7">No whitespace is allowed inside the method, request-target, and protocol version. Hence, recipients typically parse the request-line
     1212                  into its component parts by splitting on whitespace (see <a href="#message.robustness" title="Message Parsing Robustness">Section&nbsp;3.5</a>).
     1213               </p>
     1214               <p id="rfc.section.3.1.1.p.8">Unfortunately, some user agents fail to properly encode hypertext references that have embedded whitespace, sending the characters
     1215                  directly instead of properly encoding or excluding the disallowed characters. Recipients of an invalid request-line <em class="bcp14">SHOULD</em> respond with either a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> error or a <a href="p2-semantics.html#status.301" class="smpl">301 (Moved Permanently)</a> redirect with the request-target properly encoded. Recipients <em class="bcp14">SHOULD NOT</em> attempt to autocorrect and then process the request without a redirect, since the invalid request-line might be deliberately
     1216                  crafted to bypass security filters along the request chain.
     1217               </p>
     1218               <p id="rfc.section.3.1.1.p.9">HTTP does not place a pre-defined limit on the length of a request-line. A server that receives a method longer than any that
     1219                  it implements <em class="bcp14">SHOULD</em> respond with a <a href="p2-semantics.html#status.501" class="smpl">501 (Not Implemented)</a> status code. A server <em class="bcp14">MUST</em> be prepared to receive URIs of unbounded length and respond with the <a href="p2-semantics.html#status.414" class="smpl">414 (URI Too Long)</a> status code if the received request-target would be longer than the server wishes to handle (see <a href="p2-semantics.html#status.414" title="414 URI Too Long">Section 6.5.12</a> of <a href="#Part2" id="rfc.xref.Part2.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>).
     1220               </p>
     1221               <p id="rfc.section.3.1.1.p.10">Various ad-hoc limitations on request-line length are found in practice. It is <em class="bcp14">RECOMMENDED</em> that all HTTP senders and recipients support, at a minimum, request-line lengths of 8000 octets.
     1222               </p>
     1223            </div>
     1224            <div id="status.line">
     1225               <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;<a href="#status.line">Status Line</a></h3>
     1226               <p id="rfc.section.3.1.2.p.1">The first line of a response message is the status-line, consisting of the protocol version, a space (SP), the status code,
     1227                  another space, a possibly-empty textual phrase describing the status code, and ending with CRLF.
     1228               </p>
     1229               <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.31"></span>  <a href="#status.line" class="smpl">status-line</a> = <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">SP</a> <a href="#status.line" class="smpl">status-code</a> <a href="#core.rules" class="smpl">SP</a> <a href="#status.line" class="smpl">reason-phrase</a> <a href="#core.rules" class="smpl">CRLF</a>
    11791230</pre><p id="rfc.section.3.1.2.p.3">The status-code element is a 3-digit integer code describing the result of the server's attempt to understand and satisfy
    1180          the client's corresponding request. The rest of the response message is to be interpreted in light of the semantics defined
    1181          for that status code. See <a href="p2-semantics.html#status.codes" title="Response Status Codes">Section 6</a> of <a href="#Part2" id="rfc.xref.Part2.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> for information about the semantics of status codes, including the classes of status code (indicated by the first digit),
    1182          the status codes defined by this specification, considerations for the definition of new status codes, and the IANA registry.
    1183       </p>
    1184       <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.32"></span>  <a href="#status.line" class="smpl">status-code</a>    = 3<a href="#core.rules" class="smpl">DIGIT</a>
     1231                  the client's corresponding request. The rest of the response message is to be interpreted in light of the semantics defined
     1232                  for that status code. See <a href="p2-semantics.html#status.codes" title="Response Status Codes">Section 6</a> of <a href="#Part2" id="rfc.xref.Part2.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> for information about the semantics of status codes, including the classes of status code (indicated by the first digit),
     1233                  the status codes defined by this specification, considerations for the definition of new status codes, and the IANA registry.
     1234               </p>
     1235               <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.32"></span>  <a href="#status.line" class="smpl">status-code</a>    = 3<a href="#core.rules" class="smpl">DIGIT</a>
    11851236</pre><p id="rfc.section.3.1.2.p.5">The reason-phrase element exists for the sole purpose of providing a textual description associated with the numeric status
    1186          code, mostly out of deference to earlier Internet application protocols that were more frequently used with interactive text
    1187          clients. A client <em class="bcp14">SHOULD</em> ignore the reason-phrase content.
    1188       </p>
    1189       <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.33"></span>  <a href="#status.line" class="smpl">reason-phrase</a>  = *( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
    1190 </pre><h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.fields" href="#header.fields">Header Fields</a></h2>
    1191       <p id="rfc.section.3.2.p.1">Each HTTP header field consists of a case-insensitive field name followed by a colon (":"), optional whitespace, and the field
    1192          value.
    1193       </p>
    1194       <div id="rfc.figure.u.18"></div><pre class="inline"><span id="rfc.iref.g.34"></span><span id="rfc.iref.g.35"></span><span id="rfc.iref.g.36"></span><span id="rfc.iref.g.37"></span><span id="rfc.iref.g.38"></span>  <a href="#header.fields" class="smpl">header-field</a>   = <a href="#header.fields" class="smpl">field-name</a> ":" <a href="#rule.whitespace" class="smpl">OWS</a> <a href="#header.fields" class="smpl">field-value</a> <a href="#rule.whitespace" class="smpl">BWS</a>
     1237                  code, mostly out of deference to earlier Internet application protocols that were more frequently used with interactive text
     1238                  clients. A client <em class="bcp14">SHOULD</em> ignore the reason-phrase content.
     1239               </p>
     1240               <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.33"></span>  <a href="#status.line" class="smpl">reason-phrase</a>  = *( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
     1241</pre></div>
     1242         </div>
     1243         <div id="header.fields">
     1244            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#header.fields">Header Fields</a></h2>
     1245            <p id="rfc.section.3.2.p.1">Each HTTP header field consists of a case-insensitive field name followed by a colon (":"), optional whitespace, and the field
     1246               value.
     1247            </p>
     1248            <div id="rfc.figure.u.18"></div><pre class="inline"><span id="rfc.iref.g.34"></span><span id="rfc.iref.g.35"></span><span id="rfc.iref.g.36"></span><span id="rfc.iref.g.37"></span><span id="rfc.iref.g.38"></span>  <a href="#header.fields" class="smpl">header-field</a>   = <a href="#header.fields" class="smpl">field-name</a> ":" <a href="#rule.whitespace" class="smpl">OWS</a> <a href="#header.fields" class="smpl">field-value</a> <a href="#rule.whitespace" class="smpl">BWS</a>
    11951249  <a href="#header.fields" class="smpl">field-name</a>     = <a href="#rule.token.separators" class="smpl">token</a>
    11961250  <a href="#header.fields" class="smpl">field-value</a>    = *( <a href="#header.fields" class="smpl">field-content</a> / <a href="#header.fields" class="smpl">obs-fold</a> )
     
    12001254                 ; see <a href="#field.parsing" title="Field Parsing">Section&nbsp;3.2.4</a>
    12011255</pre><p id="rfc.section.3.2.p.3">The field-name token labels the corresponding field-value as having the semantics defined by that header field. For example,
    1202          the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field is defined in <a href="p2-semantics.html#header.date" title="Date">Section 7.1.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> as containing the origination timestamp for the message in which it appears.
    1203       </p>
    1204       <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="field.extensibility" href="#field.extensibility">Field Extensibility</a></h3>
    1205       <p id="rfc.section.3.2.1.p.1">HTTP header fields are fully extensible: there is no limit on the introduction of new field names, each presumably defining
    1206          new semantics, nor on the number of header fields used in a given message. Existing fields are defined in each part of this
    1207          specification and in many other specifications outside the core standard. New header fields can be introduced without changing
    1208          the protocol version if their defined semantics allow them to be safely ignored by recipients that do not recognize them.
    1209       </p>
    1210       <p id="rfc.section.3.2.1.p.2">New HTTP header fields ought to be be registered with IANA in the Message Header Field Registry, as described in <a href="p2-semantics.html#header.field.registry" title="Header Field Registry">Section 8.3</a> of <a href="#Part2" id="rfc.xref.Part2.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>. A proxy <em class="bcp14">MUST</em> forward unrecognized header fields unless the field-name is listed in the <a href="#header.connection" class="smpl">Connection</a> header field (<a href="#header.connection" id="rfc.xref.header.connection.3" title="Connection">Section&nbsp;6.1</a>) or the proxy is specifically configured to block, or otherwise transform, such fields. Other recipients <em class="bcp14">SHOULD</em> ignore unrecognized header fields.
    1211       </p>
    1212       <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="field.order" href="#field.order">Field Order</a></h3>
    1213       <p id="rfc.section.3.2.2.p.1">The order in which header fields with differing field names are received is not significant. However, it is "good practice"
    1214          to send header fields that contain control data first, such as <a href="#header.host" class="smpl">Host</a> on requests and <a href="p2-semantics.html#header.date" class="smpl">Date</a> on responses, so that implementations can decide when not to handle a message as early as possible. A server <em class="bcp14">MUST</em> wait until the entire header section is received before interpreting a request message, since later header fields might include
    1215          conditionals, authentication credentials, or deliberately misleading duplicate header fields that would impact request processing.
    1216       </p>
    1217       <p id="rfc.section.3.2.2.p.2">A sender <em class="bcp14">MUST NOT</em> generate multiple header fields with the same field name in a message unless either the entire field value for that header
    1218          field is defined as a comma-separated list [i.e., #(values)] or the header field is a well-known exception (as noted below).
    1219       </p>
    1220       <p id="rfc.section.3.2.2.p.3">Multiple header fields with the same field name can be combined into one "field-name: field-value" pair, without changing
    1221          the semantics of the message, by appending each subsequent field value to the combined field value in order, separated by
    1222          a comma. The order in which header fields with the same field name are received is therefore significant to the interpretation
    1223          of the combined field value; a proxy <em class="bcp14">MUST NOT</em> change the order of these field values when forwarding a message.
    1224       </p>
    1225       <div class="note" id="rfc.section.3.2.2.p.4">
    1226          <p> <b>Note:</b> In practice, the "Set-Cookie" header field (<a href="#RFC6265" id="rfc.xref.RFC6265.2"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>) often appears multiple times in a response message and does not use the list syntax, violating the above requirements on
    1227             multiple header fields with the same name. Since it cannot be combined into a single field-value, recipients ought to handle
    1228             "Set-Cookie" as a special case while processing header fields. (See Appendix A.2.3 of <a href="#Kri2001" id="rfc.xref.Kri2001.1"><cite title="HTTP Cookies: Standards, Privacy, and Politics">[Kri2001]</cite></a> for details.)
    1229          </p>
    1230       </div>
    1231       <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="whitespace" href="#whitespace">Whitespace</a></h3>
    1232       <div id="rule.LWS">
    1233          <p id="rfc.section.3.2.3.p.1">This specification uses three rules to denote the use of linear whitespace: OWS (optional whitespace), RWS (required whitespace),
    1234             and BWS ("bad" whitespace).
    1235          </p>
    1236       </div>
    1237       <div id="rule.OWS">
    1238          <p id="rfc.section.3.2.3.p.2">The OWS rule is used where zero or more linear whitespace octets might appear. OWS <em class="bcp14">SHOULD</em> either not be generated or be generated as a single SP. Multiple OWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets (each octet other than SP replaced with SP) before interpreting
    1239             the field value or forwarding the message downstream.
    1240          </p>
    1241       </div>
    1242       <div id="rule.RWS">
    1243          <p id="rfc.section.3.2.3.p.3">RWS is used when at least one linear whitespace octet is required to separate field tokens. RWS <em class="bcp14">SHOULD</em> be generated as a single SP. Multiple RWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets before interpreting the field value or forwarding the
    1244             message downstream.
    1245          </p>
    1246       </div>
    1247       <div id="rule.BWS">
    1248          <p id="rfc.section.3.2.3.p.4">BWS is used where the grammar allows optional whitespace, for historical reasons, but senders <em class="bcp14">SHOULD NOT</em> generate it in messages; recipients <em class="bcp14">MUST</em> accept such bad optional whitespace and remove it before interpreting the field value or forwarding the message downstream.
    1249          </p>
    1250       </div>
    1251       <div id="rule.whitespace">
    1252          <p id="rfc.section.3.2.3.p.5">      </p>
    1253       </div>
    1254       <div id="rfc.figure.u.19"></div><pre class="inline"><span id="rfc.iref.g.39"></span><span id="rfc.iref.g.40"></span><span id="rfc.iref.g.41"></span>  <a href="#rule.whitespace" class="smpl">OWS</a>            = *( <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">HTAB</a> )
     1256               the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field is defined in <a href="p2-semantics.html#header.date" title="Date">Section 7.1.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> as containing the origination timestamp for the message in which it appears.
     1257            </p>
     1258            <div id="field.extensibility">
     1259               <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a href="#field.extensibility">Field Extensibility</a></h3>
     1260               <p id="rfc.section.3.2.1.p.1">HTTP header fields are fully extensible: there is no limit on the introduction of new field names, each presumably defining
     1261                  new semantics, nor on the number of header fields used in a given message. Existing fields are defined in each part of this
     1262                  specification and in many other specifications outside the core standard. New header fields can be introduced without changing
     1263                  the protocol version if their defined semantics allow them to be safely ignored by recipients that do not recognize them.
     1264               </p>
     1265               <p id="rfc.section.3.2.1.p.2">New HTTP header fields ought to be be registered with IANA in the Message Header Field Registry, as described in <a href="p2-semantics.html#header.field.registry" title="Header Field Registry">Section 8.3</a> of <a href="#Part2" id="rfc.xref.Part2.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>. A proxy <em class="bcp14">MUST</em> forward unrecognized header fields unless the field-name is listed in the <a href="#header.connection" class="smpl">Connection</a> header field (<a href="#header.connection" id="rfc.xref.header.connection.3" title="Connection">Section&nbsp;6.1</a>) or the proxy is specifically configured to block, or otherwise transform, such fields. Other recipients <em class="bcp14">SHOULD</em> ignore unrecognized header fields.
     1266               </p>
     1267            </div>
     1268            <div id="field.order">
     1269               <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a href="#field.order">Field Order</a></h3>
     1270               <p id="rfc.section.3.2.2.p.1">The order in which header fields with differing field names are received is not significant. However, it is "good practice"
     1271                  to send header fields that contain control data first, such as <a href="#header.host" class="smpl">Host</a> on requests and <a href="p2-semantics.html#header.date" class="smpl">Date</a> on responses, so that implementations can decide when not to handle a message as early as possible. A server <em class="bcp14">MUST</em> wait until the entire header section is received before interpreting a request message, since later header fields might include
     1272                  conditionals, authentication credentials, or deliberately misleading duplicate header fields that would impact request processing.
     1273               </p>
     1274               <p id="rfc.section.3.2.2.p.2">A sender <em class="bcp14">MUST NOT</em> generate multiple header fields with the same field name in a message unless either the entire field value for that header
     1275                  field is defined as a comma-separated list [i.e., #(values)] or the header field is a well-known exception (as noted below).
     1276               </p>
     1277               <p id="rfc.section.3.2.2.p.3">Multiple header fields with the same field name can be combined into one "field-name: field-value" pair, without changing
     1278                  the semantics of the message, by appending each subsequent field value to the combined field value in order, separated by
     1279                  a comma. The order in which header fields with the same field name are received is therefore significant to the interpretation
     1280                  of the combined field value; a proxy <em class="bcp14">MUST NOT</em> change the order of these field values when forwarding a message.
     1281               </p>
     1282               <div class="note" id="rfc.section.3.2.2.p.4">
     1283                  <p><b>Note:</b> In practice, the "Set-Cookie" header field (<a href="#RFC6265" id="rfc.xref.RFC6265.2"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>) often appears multiple times in a response message and does not use the list syntax, violating the above requirements on
     1284                     multiple header fields with the same name. Since it cannot be combined into a single field-value, recipients ought to handle
     1285                     "Set-Cookie" as a special case while processing header fields. (See Appendix A.2.3 of <a href="#Kri2001" id="rfc.xref.Kri2001.1"><cite title="HTTP Cookies: Standards, Privacy, and Politics">[Kri2001]</cite></a> for details.)
     1286                  </p>
     1287               </div>
     1288            </div>
     1289            <div id="whitespace">
     1290               <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a href="#whitespace">Whitespace</a></h3>
     1291               <div id="rule.LWS">
     1292                  <p id="rfc.section.3.2.3.p.1">This specification uses three rules to denote the use of linear whitespace: OWS (optional whitespace), RWS (required whitespace),
     1293                     and BWS ("bad" whitespace).
     1294                  </p>
     1295               </div>
     1296               <div id="rule.OWS">
     1297                  <p id="rfc.section.3.2.3.p.2">The OWS rule is used where zero or more linear whitespace octets might appear. OWS <em class="bcp14">SHOULD</em> either not be generated or be generated as a single SP. Multiple OWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets (each octet other than SP replaced with SP) before interpreting
     1298                     the field value or forwarding the message downstream.
     1299                  </p>
     1300               </div>
     1301               <div id="rule.RWS">
     1302                  <p id="rfc.section.3.2.3.p.3">RWS is used when at least one linear whitespace octet is required to separate field tokens. RWS <em class="bcp14">SHOULD</em> be generated as a single SP. Multiple RWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets before interpreting the field value or forwarding the
     1303                     message downstream.
     1304                  </p>
     1305               </div>
     1306               <div id="rule.BWS">
     1307                  <p id="rfc.section.3.2.3.p.4">BWS is used where the grammar allows optional whitespace, for historical reasons, but senders <em class="bcp14">SHOULD NOT</em> generate it in messages; recipients <em class="bcp14">MUST</em> accept such bad optional whitespace and remove it before interpreting the field value or forwarding the message downstream.
     1308                  </p>
     1309               </div>
     1310               <div id="rule.whitespace">
     1311                  <p id="rfc.section.3.2.3.p.5">   </p>
     1312               </div>
     1313               <div id="rfc.figure.u.19"></div><pre class="inline"><span id="rfc.iref.g.39"></span><span id="rfc.iref.g.40"></span><span id="rfc.iref.g.41"></span>  <a href="#rule.whitespace" class="smpl">OWS</a>            = *( <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">HTAB</a> )
    12551314                 ; optional whitespace
    12561315  <a href="#rule.whitespace" class="smpl">RWS</a>            = 1*( <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">HTAB</a> )
     
    12581317  <a href="#rule.whitespace" class="smpl">BWS</a>            = <a href="#rule.whitespace" class="smpl">OWS</a>
    12591318                 ; "bad" whitespace
    1260 </pre><h3 id="rfc.section.3.2.4"><a href="#rfc.section.3.2.4">3.2.4</a>&nbsp;<a id="field.parsing" href="#field.parsing">Field Parsing</a></h3>
    1261       <p id="rfc.section.3.2.4.p.1">No whitespace is allowed between the header field-name and colon. In the past, differences in the handling of such whitespace
    1262          have led to security vulnerabilities in request routing and response handling. A server <em class="bcp14">MUST</em> reject any received request message that contains whitespace between a header field-name and colon with a response code of <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a>. A proxy <em class="bcp14">MUST</em> remove any such whitespace from a response message before forwarding the message downstream.
    1263       </p>
    1264       <p id="rfc.section.3.2.4.p.2">A field value is preceded by optional whitespace (OWS); a single SP is preferred. The field value does not include any leading
    1265          or trailing white space: OWS occurring before the first non-whitespace octet of the field value or after the last non-whitespace
    1266          octet of the field value is ignored and <em class="bcp14">SHOULD</em> be removed before further processing (as this does not change the meaning of the header field).
    1267       </p>
    1268       <p id="rfc.section.3.2.4.p.3">Historically, HTTP header field values could be extended over multiple lines by preceding each extra line with at least one
    1269          space or horizontal tab (obs-fold). This specification deprecates such line folding except within the message/http media type
    1270          (<a href="#internet.media.type.message.http" title="Internet Media Type message/http">Section&nbsp;7.3.1</a>). Senders <em class="bcp14">MUST NOT</em> generate messages that include line folding (i.e., that contain any field-value that contains a match to the <a href="#header.fields" class="smpl">obs-fold</a> rule) unless the message is intended for packaging within the message/http media type. When an <a href="#header.fields" class="smpl">obs-fold</a> is received in a message, recipients <em class="bcp14">MUST</em> do one of:
    1271       </p>
    1272       <ul>
    1273          <li>accept the message and replace any embedded <a href="#header.fields" class="smpl">obs-fold</a> whitespace with either a single <a href="#core.rules" class="smpl">SP</a> or a matching number of <a href="#core.rules" class="smpl">SP</a> octets (to avoid buffer copying) prior to interpreting the field value or forwarding the message downstream;
    1274          </li>
    1275          <li>if it is a request, reject the message by sending a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> response with a representation explaining that obsolete line folding is unacceptable; or,
    1276          </li>
    1277          <li>if it is a response, discard the message and generate a <a href="p2-semantics.html#status.502" class="smpl">502 (Bad Gateway)</a> response with a representation explaining that unacceptable line folding was received.
    1278          </li>
    1279       </ul>
    1280       <p> Recipients that choose not to implement <a href="#header.fields" class="smpl">obs-fold</a> processing (as described above) <em class="bcp14">MUST NOT</em> accept messages containing header fields with leading whitespace, as this can expose them to attacks that exploit this difference
    1281          in processing.
    1282       </p>
    1283       <p id="rfc.section.3.2.4.p.4">Historically, HTTP has allowed field content with text in the ISO-8859-1 <a href="#ISO-8859-1" id="rfc.xref.ISO-8859-1.1"><cite title="Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1">[ISO-8859-1]</cite></a> charset, supporting other charsets only through use of <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a> encoding. In practice, most HTTP header field values use only a subset of the US-ASCII charset <a href="#USASCII" id="rfc.xref.USASCII.3"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Newly defined header fields <em class="bcp14">SHOULD</em> limit their field values to US-ASCII octets. Recipients <em class="bcp14">SHOULD</em> treat other octets in field content (obs-text) as opaque data.
    1284       </p>
    1285       <h3 id="rfc.section.3.2.5"><a href="#rfc.section.3.2.5">3.2.5</a>&nbsp;<a id="field.limits" href="#field.limits">Field Limits</a></h3>
    1286       <p id="rfc.section.3.2.5.p.1">HTTP does not place a pre-defined limit on the length of each header field or on the length of the header block as a whole.
    1287          Various ad-hoc limitations on individual header field length are found in practice, often depending on the specific field
    1288          semantics.
    1289       </p>
    1290       <p id="rfc.section.3.2.5.p.2">A server <em class="bcp14">MUST</em> be prepared to receive request header fields of unbounded length and respond with an appropriate <a href="p2-semantics.html#status.4xx" class="smpl">4xx (Client Error)</a> status code if the received header field(s) are larger than the server wishes to process.
    1291       </p>
    1292       <p id="rfc.section.3.2.5.p.3">A client <em class="bcp14">MUST</em> be prepared to receive response header fields of unbounded length. A client <em class="bcp14">MAY</em> discard or truncate received header fields that are larger than the client wishes to process if the field semantics are such
    1293          that the dropped value(s) can be safely ignored without changing the response semantics.
    1294       </p>
    1295       <h3 id="rfc.section.3.2.6"><a href="#rfc.section.3.2.6">3.2.6</a>&nbsp;<a id="field.components" href="#field.components">Field value components</a></h3>
    1296       <div id="rule.token.separators">
    1297          <p id="rfc.section.3.2.6.p.1">        Many HTTP header field values consist of words (token or quoted-string) separated by whitespace or special characters. These
    1298             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;4</a>).
    1299          </p>
    1300       </div>
    1301       <div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.42"></span><span id="rfc.iref.g.43"></span><span id="rfc.iref.g.44"></span><span id="rfc.iref.g.45"></span>  <a href="#rule.token.separators" class="smpl">word</a>           = <a href="#rule.token.separators" class="smpl">token</a> / <a href="#rule.quoted-string" class="smpl">quoted-string</a>
     1319</pre></div>
     1320            <div id="field.parsing">
     1321               <h3 id="rfc.section.3.2.4"><a href="#rfc.section.3.2.4">3.2.4</a>&nbsp;<a href="#field.parsing">Field Parsing</a></h3>
     1322               <p id="rfc.section.3.2.4.p.1">No whitespace is allowed between the header field-name and colon. In the past, differences in the handling of such whitespace
     1323                  have led to security vulnerabilities in request routing and response handling. A server <em class="bcp14">MUST</em> reject any received request message that contains whitespace between a header field-name and colon with a response code of <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a>. A proxy <em class="bcp14">MUST</em> remove any such whitespace from a response message before forwarding the message downstream.
     1324               </p>
     1325               <p id="rfc.section.3.2.4.p.2">A field value is preceded by optional whitespace (OWS); a single SP is preferred. The field value does not include any leading
     1326                  or trailing white space: OWS occurring before the first non-whitespace octet of the field value or after the last non-whitespace
     1327                  octet of the field value is ignored and <em class="bcp14">SHOULD</em> be removed before further processing (as this does not change the meaning of the header field).
     1328               </p>
     1329               <p id="rfc.section.3.2.4.p.3">Historically, HTTP header field values could be extended over multiple lines by preceding each extra line with at least one
     1330                  space or horizontal tab (obs-fold). This specification deprecates such line folding except within the message/http media type
     1331                  (<a href="#internet.media.type.message.http" title="Internet Media Type message/http">Section&nbsp;7.3.1</a>). Senders <em class="bcp14">MUST NOT</em> generate messages that include line folding (i.e., that contain any field-value that contains a match to the <a href="#header.fields" class="smpl">obs-fold</a> rule) unless the message is intended for packaging within the message/http media type. When an <a href="#header.fields" class="smpl">obs-fold</a> is received in a message, recipients <em class="bcp14">MUST</em> do one of:
     1332               </p>
     1333               <ul>
     1334                  <li>accept the message and replace any embedded <a href="#header.fields" class="smpl">obs-fold</a> whitespace with either a single <a href="#core.rules" class="smpl">SP</a> or a matching number of <a href="#core.rules" class="smpl">SP</a> octets (to avoid buffer copying) prior to interpreting the field value or forwarding the message downstream;
     1335                  </li>
     1336                  <li>if it is a request, reject the message by sending a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> response with a representation explaining that obsolete line folding is unacceptable; or,
     1337                  </li>
     1338                  <li>if it is a response, discard the message and generate a <a href="p2-semantics.html#status.502" class="smpl">502 (Bad Gateway)</a> response with a representation explaining that unacceptable line folding was received.
     1339                  </li>
     1340               </ul>
     1341               <p> Recipients that choose not to implement <a href="#header.fields" class="smpl">obs-fold</a> processing (as described above) <em class="bcp14">MUST NOT</em> accept messages containing header fields with leading whitespace, as this can expose them to attacks that exploit this difference
     1342                  in processing.
     1343               </p>
     1344               <p id="rfc.section.3.2.4.p.4">Historically, HTTP has allowed field content with text in the ISO-8859-1 <a href="#ISO-8859-1" id="rfc.xref.ISO-8859-1.1"><cite title="Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1">[ISO-8859-1]</cite></a> charset, supporting other charsets only through use of <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a> encoding. In practice, most HTTP header field values use only a subset of the US-ASCII charset <a href="#USASCII" id="rfc.xref.USASCII.3"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Newly defined header fields <em class="bcp14">SHOULD</em> limit their field values to US-ASCII octets. Recipients <em class="bcp14">SHOULD</em> treat other octets in field content (obs-text) as opaque data.
     1345               </p>
     1346            </div>
     1347            <div id="field.limits">
     1348               <h3 id="rfc.section.3.2.5"><a href="#rfc.section.3.2.5">3.2.5</a>&nbsp;<a href="#field.limits">Field Limits</a></h3>
     1349               <p id="rfc.section.3.2.5.p.1">HTTP does not place a pre-defined limit on the length of each header field or on the length of the header block as a whole.
     1350                  Various ad-hoc limitations on individual header field length are found in practice, often depending on the specific field
     1351                  semantics.
     1352               </p>
     1353               <p id="rfc.section.3.2.5.p.2">A server <em class="bcp14">MUST</em> be prepared to receive request header fields of unbounded length and respond with an appropriate <a href="p2-semantics.html#status.4xx" class="smpl">4xx (Client Error)</a> status code if the received header field(s) are larger than the server wishes to process.
     1354               </p>
     1355               <p id="rfc.section.3.2.5.p.3">A client <em class="bcp14">MUST</em> be prepared to receive response header fields of unbounded length. A client <em class="bcp14">MAY</em> discard or truncate received header fields that are larger than the client wishes to process if the field semantics are such
     1356                  that the dropped value(s) can be safely ignored without changing the response semantics.
     1357               </p>
     1358            </div>
     1359            <div id="field.components">
     1360               <h3 id="rfc.section.3.2.6"><a href="#rfc.section.3.2.6">3.2.6</a>&nbsp;<a href="#field.components">Field value components</a></h3>
     1361               <div id="rule.token.separators">
     1362                  <p id="rfc.section.3.2.6.p.1">    Many HTTP header field values consist of words (token or quoted-string) separated by whitespace or special characters. These
     1363                     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;4</a>).
     1364                  </p>
     1365               </div>
     1366               <div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.42"></span><span id="rfc.iref.g.43"></span><span id="rfc.iref.g.44"></span><span id="rfc.iref.g.45"></span>  <a href="#rule.token.separators" class="smpl">word</a>           = <a href="#rule.token.separators" class="smpl">token</a> / <a href="#rule.quoted-string" class="smpl">quoted-string</a>
    13021367
    13031368  <a href="#rule.token.separators" class="smpl">token</a>          = 1*<a href="#rule.token.separators" class="smpl">tchar</a>
     
    13121377                 / "]" / "?" / "=" / "{" / "}"
    13131378</pre><div id="rule.quoted-string">
    1314          <p id="rfc.section.3.2.6.p.3">      A string of text is parsed as a single word if it is quoted using double-quote marks.</p>
    1315       </div>
    1316       <div id="rfc.figure.u.21"></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>  <a href="#rule.quoted-string" class="smpl">quoted-string</a>  = <a href="#core.rules" class="smpl">DQUOTE</a> *( <a href="#rule.quoted-string" class="smpl">qdtext</a> / <a href="#rule.quoted-pair" class="smpl">quoted-pair</a> ) <a href="#core.rules" class="smpl">DQUOTE</a>
     1379                  <p id="rfc.section.3.2.6.p.3">   A string of text is parsed as a single word if it is quoted using double-quote marks.</p>
     1380               </div>
     1381               <div id="rfc.figure.u.21"></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>  <a href="#rule.quoted-string" class="smpl">quoted-string</a>  = <a href="#core.rules" class="smpl">DQUOTE</a> *( <a href="#rule.quoted-string" class="smpl">qdtext</a> / <a href="#rule.quoted-pair" class="smpl">quoted-pair</a> ) <a href="#core.rules" class="smpl">DQUOTE</a>
    13171382  <a href="#rule.quoted-string" class="smpl">qdtext</a>         = <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> /%x21 / %x23-5B / %x5D-7E / <a href="#rule.quoted-string" class="smpl">obs-text</a>
    13181383  <a href="#rule.quoted-string" class="smpl">obs-text</a>       = %x80-FF
    13191384</pre><div id="rule.quoted-pair">
    1320          <p id="rfc.section.3.2.6.p.5"> The backslash octet ("\") can be used as a single-octet quoting mechanism within quoted-string constructs:</p>
    1321       </div>
    1322       <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.49"></span>  <a href="#rule.quoted-pair" class="smpl">quoted-pair</a>    = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
     1385                  <p id="rfc.section.3.2.6.p.5"> The backslash octet ("\") can be used as a single-octet quoting mechanism within quoted-string constructs:</p>
     1386               </div>
     1387               <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.49"></span>  <a href="#rule.quoted-pair" class="smpl">quoted-pair</a>    = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
    13231388</pre><p id="rfc.section.3.2.6.p.7">Recipients that process the value of a quoted-string <em class="bcp14">MUST</em> handle a quoted-pair as if it were replaced by the octet following the backslash.
    1324       </p>
    1325       <p id="rfc.section.3.2.6.p.8">Senders <em class="bcp14">SHOULD NOT</em> generate a quoted-pair in a quoted-string except where necessary to quote DQUOTE and backslash octets occurring within that
    1326          string.
    1327       </p>
    1328       <div id="rule.comment">
    1329          <p id="rfc.section.3.2.6.p.9">    Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed
    1330             in fields containing "comment" as part of their field value definition.
    1331          </p>
    1332       </div>
    1333       <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.50"></span><span id="rfc.iref.g.51"></span>  <a href="#rule.comment" class="smpl">comment</a>        = "(" *( <a href="#rule.comment" class="smpl">ctext</a> / <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a> / <a href="#rule.comment" class="smpl">comment</a> ) ")"
     1389               </p>
     1390               <p id="rfc.section.3.2.6.p.8">Senders <em class="bcp14">SHOULD NOT</em> generate a quoted-pair in a quoted-string except where necessary to quote DQUOTE and backslash octets occurring within that
     1391                  string.
     1392               </p>
     1393               <div id="rule.comment">
     1394                  <p id="rfc.section.3.2.6.p.9">  Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed
     1395                     in fields containing "comment" as part of their field value definition.
     1396                  </p>
     1397               </div>
     1398               <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.50"></span><span id="rfc.iref.g.51"></span>  <a href="#rule.comment" class="smpl">comment</a>        = "(" *( <a href="#rule.comment" class="smpl">ctext</a> / <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a> / <a href="#rule.comment" class="smpl">comment</a> ) ")"
    13341399  <a href="#rule.comment" class="smpl">ctext</a>          = <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / %x21-27 / %x2A-5B / %x5D-7E / <a href="#rule.quoted-string" class="smpl">obs-text</a>
    13351400</pre><div id="rule.quoted-cpair">
    1336          <p id="rfc.section.3.2.6.p.11">  The backslash octet ("\") can be used as a single-octet quoting mechanism within comment constructs:</p>
     1401                  <p id="rfc.section.3.2.6.p.11"> The backslash octet ("\") can be used as a single-octet quoting mechanism within comment constructs:</p>
     1402               </div>
     1403               <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.52"></span>  <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a>   = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
     1404</pre><p id="rfc.section.3.2.6.p.13">Senders <em class="bcp14">SHOULD NOT</em> escape octets in comments that do not require escaping (i.e., other than the backslash octet "\" and the parentheses "(" and
     1405                  ")").
     1406               </p>
     1407            </div>
     1408         </div>
     1409         <div id="message.body">
     1410            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#message.body">Message Body</a></h2>
     1411            <p id="rfc.section.3.3.p.1">The message body (if any) of an HTTP message is used to carry the payload body of that request or response. The message body
     1412               is identical to the payload body unless a transfer coding has been applied, as described in <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.1" title="Transfer-Encoding">Section&nbsp;3.3.1</a>.
     1413            </p>
     1414            <div id="rfc.figure.u.25"></div><pre class="inline"><span id="rfc.iref.g.53"></span>  <a href="#message.body" class="smpl">message-body</a> = *OCTET
     1415</pre><p id="rfc.section.3.3.p.3">The rules for when a message body is allowed in a message differ for requests and responses.</p>
     1416            <p id="rfc.section.3.3.p.4">The presence of a message body in a request is signaled by a <a href="#header.content-length" class="smpl">Content-Length</a> or <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field. Request message framing is independent of method semantics, even if the method does not define any use for a
     1417               message body.
     1418            </p>
     1419            <p id="rfc.section.3.3.p.5">The presence of a message body in a response depends on both the request method to which it is responding and the response
     1420               status code (<a href="#status.line" title="Status Line">Section&nbsp;3.1.2</a>). Responses to the HEAD request method never include a message body because the associated response header fields (e.g., <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a>, <a href="#header.content-length" class="smpl">Content-Length</a>, etc.), if present, indicate only what their values would have been if the request method had been GET (<a href="p2-semantics.html#HEAD" title="HEAD">Section 4.3.2</a> of <a href="#Part2" id="rfc.xref.Part2.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> responses to CONNECT switch to tunnel mode instead of having a message body (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 4.3.6</a> of <a href="#Part2" id="rfc.xref.Part2.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). All <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a>, <a href="p2-semantics.html#status.204" class="smpl">204 (No Content)</a>, and <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> responses do not include a message body. All other responses do include a message body, although the body might be of zero
     1421               length.
     1422            </p>
     1423            <div id="header.transfer-encoding">
     1424               <div id="rfc.iref.t.4"></div>
     1425               <div id="rfc.iref.c.6"></div>
     1426               <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a href="#header.transfer-encoding">Transfer-Encoding</a></h3>
     1427               <p id="rfc.section.3.3.1.p.1">The Transfer-Encoding header field lists the transfer coding names corresponding to the sequence of transfer codings that
     1428                  have been (or will be) applied to the payload body in order to form the message body. Transfer codings are defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>.
     1429               </p>
     1430               <div id="rfc.figure.u.26"></div><pre class="inline"><span id="rfc.iref.g.54"></span>  <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> = 1#<a href="#transfer.codings" class="smpl">transfer-coding</a>
     1431</pre><p id="rfc.section.3.3.1.p.3">Transfer-Encoding is analogous to the Content-Transfer-Encoding field of MIME, which was designed to enable safe transport
     1432                  of binary data over a 7-bit transport service (<a href="#RFC2045" id="rfc.xref.RFC2045.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>, <a href="https://tools.ietf.org/html/rfc2045#section-6">Section 6</a>). However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP's case, Transfer-Encoding is
     1433                  primarily intended to accurately delimit a dynamically generated payload and to distinguish payload encodings that are only
     1434                  applied for transport efficiency or security from those that are characteristics of the selected resource.
     1435               </p>
     1436               <p id="rfc.section.3.3.1.p.4">All HTTP/1.1 recipients <em class="bcp14">MUST</em> implement the chunked transfer coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) because it plays a crucial role in framing messages when the payload body size is not known in advance. If chunked is applied
     1437                  to a payload body, the sender <em class="bcp14">MUST NOT</em> apply chunked more than once (i.e., chunking an already chunked message is not allowed). If any transfer coding is applied
     1438                  to a request payload body, the sender <em class="bcp14">MUST</em> apply chunked as the final transfer coding to ensure that the message is properly framed. If any transfer coding is applied
     1439                  to a response payload body, the sender <em class="bcp14">MUST</em> either apply chunked as the final transfer coding or terminate the message by closing the connection.
     1440               </p>
     1441               <div id="rfc.figure.u.27"></div>
     1442               <p>For example,</p><pre class="text">  Transfer-Encoding: gzip, chunked
     1443</pre><p>indicates that the payload body has been compressed using the gzip coding and then chunked using the chunked coding while
     1444                  forming the message body.
     1445               </p>
     1446               <p id="rfc.section.3.3.1.p.6">Unlike <a href="p2-semantics.html#header.content-encoding" class="smpl">Content-Encoding</a> (<a href="p2-semantics.html#content.codings" title="Content Codings">Section 3.1.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.13"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>), Transfer-Encoding is a property of the message, not of the representation, and any recipient along the request/response
     1447                  chain <em class="bcp14">MAY</em> decode the received transfer coding(s) or apply additional transfer coding(s) to the message body, assuming that corresponding
     1448                  changes are made to the Transfer-Encoding field-value. Additional information about the encoding parameters <em class="bcp14">MAY</em> be provided by other header fields not defined by this specification.
     1449               </p>
     1450               <p id="rfc.section.3.3.1.p.7">Transfer-Encoding <em class="bcp14">MAY</em> be sent in a response to a HEAD request or in a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response (<a href="p4-conditional.html#status.304" title="304 Not Modified">Section 4.1</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>) to a GET request, neither of which includes a message body, to indicate that the origin server would have applied a transfer
     1451                  coding to the message body if the request had been an unconditional GET. This indication is not required, however, because
     1452                  any recipient on the response chain (including the origin server) can remove transfer codings when they are not needed.
     1453               </p>
     1454               <p id="rfc.section.3.3.1.p.8">Transfer-Encoding was added in HTTP/1.1. It is generally assumed that implementations advertising only HTTP/1.0 support will
     1455                  not understand how to process a transfer-encoded payload. A client <em class="bcp14">MUST NOT</em> send a request containing Transfer-Encoding unless it knows the server will handle HTTP/1.1 (or later) requests; such knowledge
     1456                  might be in the form of specific user configuration or by remembering the version of a prior received response. A server <em class="bcp14">MUST NOT</em> send a response containing Transfer-Encoding unless the corresponding request indicates HTTP/1.1 (or later).
     1457               </p>
     1458               <p id="rfc.section.3.3.1.p.9">A server that receives a request message with a transfer coding it does not understand <em class="bcp14">SHOULD</em> respond with <a href="p2-semantics.html#status.501" class="smpl">501 (Not Implemented)</a>.
     1459               </p>
     1460            </div>
     1461            <div id="header.content-length">
     1462               <div id="rfc.iref.c.7"></div>
     1463               <h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a href="#header.content-length">Content-Length</a></h3>
     1464               <p id="rfc.section.3.3.2.p.1">When a message does not have a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field, a Content-Length header field can provide the anticipated size, as a decimal number of octets, for a potential
     1465                  payload body. For messages that do include a payload body, the Content-Length field-value provides the framing information
     1466                  necessary for determining where the body (and message) ends. For messages that do not include a payload body, the Content-Length
     1467                  indicates the size of the selected representation (<a href="p2-semantics.html#representations" title="Representations">Section 3</a> of <a href="#Part2" id="rfc.xref.Part2.14"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>).
     1468               </p>
     1469               <div id="rfc.figure.u.28"></div><pre class="inline"><span id="rfc.iref.g.55"></span>  <a href="#header.content-length" class="smpl">Content-Length</a> = 1*<a href="#core.rules" class="smpl">DIGIT</a>
     1470</pre><p id="rfc.section.3.3.2.p.3">An example is</p>
     1471               <div id="rfc.figure.u.29"></div><pre class="text">  Content-Length: 3495
     1472</pre><p id="rfc.section.3.3.2.p.5">A sender <em class="bcp14">MUST NOT</em> send a Content-Length header field in any message that contains a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field.
     1473               </p>
     1474               <p id="rfc.section.3.3.2.p.6">A user agent <em class="bcp14">SHOULD</em> send a Content-Length in a request message when no <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> is sent and the request method defines a meaning for an enclosed payload body. For example, a Content-Length header field
     1475                  is normally sent in a POST request even when the value is 0 (indicating an empty payload body). A user agent <em class="bcp14">SHOULD NOT</em> send a Content-Length header field when the request message does not contain a payload body and the method semantics do not
     1476                  anticipate such a body.
     1477               </p>
     1478               <p id="rfc.section.3.3.2.p.7">A server <em class="bcp14">MAY</em> send a Content-Length header field in a response to a HEAD request (<a href="p2-semantics.html#HEAD" title="HEAD">Section 4.3.2</a> of <a href="#Part2" id="rfc.xref.Part2.15"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>); a server <em class="bcp14">MUST NOT</em> send Content-Length in such a response unless its field-value equals the decimal number of octets that would have been sent
     1479                  in the payload body of a response if the same request had used the GET method.
     1480               </p>
     1481               <p id="rfc.section.3.3.2.p.8">A server <em class="bcp14">MAY</em> send a Content-Length header field in a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response to a conditional GET request (<a href="p4-conditional.html#status.304" title="304 Not Modified">Section 4.1</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>); a server <em class="bcp14">MUST NOT</em> send Content-Length in such a response unless its field-value equals the decimal number of octets that would have been sent
     1482                  in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to the same request.
     1483               </p>
     1484               <p id="rfc.section.3.3.2.p.9">A server <em class="bcp14">MUST NOT</em> send a Content-Length header field in any response with a status code of <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a> or <a href="p2-semantics.html#status.204" class="smpl">204 (No Content)</a>. A server <em class="bcp14">SHOULD NOT</em> send a Content-Length header field in any <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> response to a CONNECT request (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 4.3.6</a> of <a href="#Part2" id="rfc.xref.Part2.16"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>).
     1485               </p>
     1486               <p id="rfc.section.3.3.2.p.10">Aside from the cases defined above, in the absence of Transfer-Encoding, an origin server <em class="bcp14">SHOULD</em> send a Content-Length header field when the payload body size is known prior to sending the complete header block. This will
     1487                  allow downstream recipients to measure transfer progress, know when a received message is complete, and potentially reuse
     1488                  the connection for additional requests.
     1489               </p>
     1490               <p id="rfc.section.3.3.2.p.11">Any Content-Length field value greater than or equal to zero is valid. Since there is no predefined limit to the length of
     1491                  a payload, recipients <em class="bcp14">SHOULD</em> anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows (<a href="#attack.protocol.element.size.overflows" title="Buffer Overflows">Section&nbsp;8.3</a>).
     1492               </p>
     1493               <p id="rfc.section.3.3.2.p.12">If a message is received that has multiple Content-Length header fields with field-values consisting of the same decimal value,
     1494                  or a single Content-Length header field with a field value containing a list of identical decimal values (e.g., "Content-Length:
     1495                  42, 42"), indicating that duplicate Content-Length header fields have been generated or combined by an upstream message processor,
     1496                  then the recipient <em class="bcp14">MUST</em> either reject the message as invalid or replace the duplicated field-values with a single valid Content-Length field containing
     1497                  that decimal value prior to determining the message body length.
     1498               </p>
     1499               <div class="note" id="rfc.section.3.3.2.p.13">
     1500                  <p><b>Note:</b> HTTP's use of Content-Length for message framing differs significantly from the same field's use in MIME, where it is an optional
     1501                     field used only within the "message/external-body" media-type.
     1502                  </p>
     1503               </div>
     1504            </div>
     1505            <div id="message.body.length">
     1506               <div id="rfc.iref.c.8"></div>
     1507               <h3 id="rfc.section.3.3.3"><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;<a href="#message.body.length">Message Body Length</a></h3>
     1508               <p id="rfc.section.3.3.3.p.1">The length of a message body is determined by one of the following (in order of precedence):</p>
     1509               <p id="rfc.section.3.3.3.p.2"></p>
     1510               <ol>
     1511                  <li>
     1512                     <p>Any response to a HEAD request and any response with a <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a>, <a href="p2-semantics.html#status.204" class="smpl">204 (No Content)</a>, or <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> status code is always terminated by the first empty line after the header fields, regardless of the header fields present
     1513                        in the message, and thus cannot contain a message body.
     1514                     </p>
     1515                  </li>
     1516                  <li>
     1517                     <p>Any <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> response to a CONNECT request implies that the connection will become a tunnel immediately after the empty line that concludes
     1518                        the header fields. A client <em class="bcp14">MUST</em> ignore any <a href="#header.content-length" class="smpl">Content-Length</a> or <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header fields received in such a message.
     1519                     </p>
     1520                  </li>
     1521                  <li>
     1522                     <p>If a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field is present and the chunked transfer coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) is the final encoding, the message body length is determined by reading and decoding the chunked data until the transfer
     1523                        coding indicates the data is complete.
     1524                     </p>
     1525                     <p>If a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field is present in a response and the chunked transfer coding is not the final encoding, the message body length is
     1526                        determined by reading the connection until it is closed by the server. If a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field is present in a request and the chunked transfer coding is not the final encoding, the message body length cannot
     1527                        be determined reliably; the server <em class="bcp14">MUST</em> respond with the <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> status code and then close the connection.
     1528                     </p>
     1529                     <p>If a message is received with both a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> and a <a href="#header.content-length" class="smpl">Content-Length</a> header field, the Transfer-Encoding overrides the Content-Length. Such a message might indicate an attempt to perform request
     1530                        or response smuggling (bypass of security-related checks on message routing or content) and thus ought to be handled as an
     1531                        error. A sender <em class="bcp14">MUST</em> remove the received Content-Length field prior to forwarding such a message downstream.
     1532                     </p>
     1533                  </li>
     1534                  <li>
     1535                     <p>If a message is received without <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> and with either multiple <a href="#header.content-length" class="smpl">Content-Length</a> header fields having differing field-values or a single Content-Length header field having an invalid value, then the message
     1536                        framing is invalid and <em class="bcp14">MUST</em> be treated as an error to prevent request or response smuggling. If this is a request message, the server <em class="bcp14">MUST</em> respond with a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> status code and then close the connection. If this is a response message received by a proxy, the proxy <em class="bcp14">MUST</em> discard the received response, send a <a href="p2-semantics.html#status.502" class="smpl">502 (Bad Gateway)</a> status code as its downstream response, and then close the connection. If this is a response message received by a user agent,
     1537                        it <em class="bcp14">MUST</em> be treated as an error by discarding the message and closing the connection.
     1538                     </p>
     1539                  </li>
     1540                  <li>
     1541                     <p>If a valid <a href="#header.content-length" class="smpl">Content-Length</a> header field is present without <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a>, its decimal value defines the expected message body length in octets. If the sender closes the connection or the recipient
     1542                        times out before the indicated number of octets are received, the recipient <em class="bcp14">MUST</em> consider the message to be incomplete and close the connection.
     1543                     </p>
     1544                  </li>
     1545                  <li>
     1546                     <p>If this is a request message and none of the above are true, then the message body length is zero (no message body is present).</p>
     1547                  </li>
     1548                  <li>
     1549                     <p>Otherwise, this is a response message without a declared message body length, so the message body length is determined by
     1550                        the number of octets received prior to the server closing the connection.
     1551                     </p>
     1552                  </li>
     1553               </ol>
     1554               <p id="rfc.section.3.3.3.p.3">Since there is no way to distinguish a successfully completed, close-delimited message from a partially-received message interrupted
     1555                  by network failure, a server <em class="bcp14">SHOULD</em> use encoding or length-delimited messages whenever possible. The close-delimiting feature exists primarily for backwards compatibility
     1556                  with HTTP/1.0.
     1557               </p>
     1558               <p id="rfc.section.3.3.3.p.4">A server <em class="bcp14">MAY</em> reject a request that contains a message body but not a <a href="#header.content-length" class="smpl">Content-Length</a> by responding with <a href="p2-semantics.html#status.411" class="smpl">411 (Length Required)</a>.
     1559               </p>
     1560               <p id="rfc.section.3.3.3.p.5">Unless a transfer coding other than chunked has been applied, a client that sends a request containing a message body <em class="bcp14">SHOULD</em> use a valid <a href="#header.content-length" class="smpl">Content-Length</a> header field if the message body length is known in advance, rather than the chunked transfer coding, since some existing
     1561                  services respond to chunked with a <a href="p2-semantics.html#status.411" class="smpl">411 (Length Required)</a> status code even though they understand the chunked transfer coding. This is typically because such services are implemented
     1562                  via a gateway that requires a content-length in advance of being called and the server is unable or unwilling to buffer the
     1563                  entire request before processing.
     1564               </p>
     1565               <p id="rfc.section.3.3.3.p.6">A user agent that sends a request containing a message body <em class="bcp14">MUST</em> send a valid <a href="#header.content-length" class="smpl">Content-Length</a> header field if it does not know the server will handle HTTP/1.1 (or later) requests; such knowledge can be in the form of
     1566                  specific user configuration or by remembering the version of a prior received response.
     1567               </p>
     1568               <p id="rfc.section.3.3.3.p.7">If the final response to the last request on a connection has been completely received and there remains additional data to
     1569                  read, a user agent <em class="bcp14">MAY</em> discard the remaining data or attempt to determine if that data belongs as part of the prior response body, which might be
     1570                  the case if the prior message's Content-Length value is incorrect. A client <em class="bcp14">MUST NOT</em> process, cache, or forward such extra data as a separate response, since such behavior would be vulnerable to cache poisoning.
     1571               </p>
     1572            </div>
     1573         </div>
     1574         <div id="incomplete.messages">
     1575            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a href="#incomplete.messages">Handling Incomplete Messages</a></h2>
     1576            <p id="rfc.section.3.4.p.1">A server that receives an incomplete request message, usually due to a canceled request or a triggered time-out exception, <em class="bcp14">MAY</em> send an error response prior to closing the connection.
     1577            </p>
     1578            <p id="rfc.section.3.4.p.2">A client that receives an incomplete response message, which can occur when a connection is closed prematurely or when decoding
     1579               a supposedly chunked transfer coding fails, <em class="bcp14">MUST</em> record the message as incomplete. Cache requirements for incomplete responses are defined in <a href="p6-cache.html#response.cacheability" title="Storing Responses in Caches">Section 3</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
     1580            </p>
     1581            <p id="rfc.section.3.4.p.3">If a response terminates in the middle of the header block (before the empty line is received) and the status code might rely
     1582               on header fields to convey the full meaning of the response, then the client cannot assume that meaning has been conveyed;
     1583               the client might need to repeat the request in order to determine what action to take next.
     1584            </p>
     1585            <p id="rfc.section.3.4.p.4">A message body that uses the chunked transfer coding is incomplete if the zero-sized chunk that terminates the encoding has
     1586               not been received. A message that uses a valid <a href="#header.content-length" class="smpl">Content-Length</a> is incomplete if the size of the message body received (in octets) is less than the value given by Content-Length. A response
     1587               that has neither chunked transfer coding nor Content-Length is terminated by closure of the connection, and thus is considered
     1588               complete regardless of the number of message body octets received, provided that the header block was received intact.
     1589            </p>
     1590         </div>
     1591         <div id="message.robustness">
     1592            <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a href="#message.robustness">Message Parsing Robustness</a></h2>
     1593            <p id="rfc.section.3.5.p.1">Older HTTP/1.0 user agent implementations might send an extra CRLF after a POST request as a lame workaround for some early
     1594               server applications that failed to read message body content that was not terminated by a line-ending. An HTTP/1.1 user agent <em class="bcp14">MUST NOT</em> preface or follow a request with an extra CRLF. If terminating the request message body with a line-ending is desired, then
     1595               the user agent <em class="bcp14">MUST</em> count the terminating CRLF octets as part of the message body length.
     1596            </p>
     1597            <p id="rfc.section.3.5.p.2">In the interest of robustness, servers <em class="bcp14">SHOULD</em> ignore at least one empty line received where a request-line is expected. In other words, if a server is reading the protocol
     1598               stream at the beginning of a message and receives a CRLF first, the server <em class="bcp14">SHOULD</em> ignore the CRLF.
     1599            </p>
     1600            <p id="rfc.section.3.5.p.3">Although the line terminator for the start-line and header fields is the sequence CRLF, recipients <em class="bcp14">MAY</em> recognize a single LF as a line terminator and ignore any preceding CR.
     1601            </p>
     1602            <p id="rfc.section.3.5.p.4">Although the request-line and status-line grammar rules require that each of the component elements be separated by a single
     1603               SP octet, recipients <em class="bcp14">MAY</em> instead parse on whitespace-delimited word boundaries and, aside from the CRLF terminator, treat any form of whitespace as
     1604               the SP separator while ignoring preceding or trailing whitespace; such whitespace includes one or more of the following octets:
     1605               SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
     1606            </p>
     1607            <p id="rfc.section.3.5.p.5">When a server listening only for HTTP request messages, or processing what appears from the start-line to be an HTTP request
     1608               message, receives a sequence of octets that does not match the HTTP-message grammar aside from the robustness exceptions listed
     1609               above, the server <em class="bcp14">SHOULD</em> respond with a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> response.
     1610            </p>
     1611         </div>
    13371612      </div>
    1338       <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.52"></span>  <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a>   = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
    1339 </pre><p id="rfc.section.3.2.6.p.13">Senders <em class="bcp14">SHOULD NOT</em> escape octets in comments that do not require escaping (i.e., other than the backslash octet "\" and the parentheses "(" and
    1340          ")").
    1341       </p>
    1342       <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="message.body" href="#message.body">Message Body</a></h2>
    1343       <p id="rfc.section.3.3.p.1">The message body (if any) of an HTTP message is used to carry the payload body of that request or response. The message body
    1344          is identical to the payload body unless a transfer coding has been applied, as described in <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.1" title="Transfer-Encoding">Section&nbsp;3.3.1</a>.
    1345       </p>
    1346       <div id="rfc.figure.u.25"></div><pre class="inline"><span id="rfc.iref.g.53"></span>  <a href="#message.body" class="smpl">message-body</a> = *OCTET
    1347 </pre><p id="rfc.section.3.3.p.3">The rules for when a message body is allowed in a message differ for requests and responses.</p>
    1348       <p id="rfc.section.3.3.p.4">The presence of a message body in a request is signaled by a <a href="#header.content-length" class="smpl">Content-Length</a> or <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field. Request message framing is independent of method semantics, even if the method does not define any use for a
    1349          message body.
    1350       </p>
    1351       <p id="rfc.section.3.3.p.5">The presence of a message body in a response depends on both the request method to which it is responding and the response
    1352          status code (<a href="#status.line" title="Status Line">Section&nbsp;3.1.2</a>). Responses to the HEAD request method never include a message body because the associated response header fields (e.g., <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a>, <a href="#header.content-length" class="smpl">Content-Length</a>, etc.), if present, indicate only what their values would have been if the request method had been GET (<a href="p2-semantics.html#HEAD" title="HEAD">Section 4.3.2</a> of <a href="#Part2" id="rfc.xref.Part2.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> responses to CONNECT switch to tunnel mode instead of having a message body (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 4.3.6</a> of <a href="#Part2" id="rfc.xref.Part2.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). All <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a>, <a href="p2-semantics.html#status.204" class="smpl">204 (No Content)</a>, and <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> responses do not include a message body. All other responses do include a message body, although the body might be of zero
    1353          length.
    1354       </p>
    1355       <div id="rfc.iref.t.4"></div>
    1356       <div id="rfc.iref.c.6"></div>
    1357       <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a id="header.transfer-encoding" href="#header.transfer-encoding">Transfer-Encoding</a></h3>
    1358       <p id="rfc.section.3.3.1.p.1">The Transfer-Encoding header field lists the transfer coding names corresponding to the sequence of transfer codings that
    1359          have been (or will be) applied to the payload body in order to form the message body. Transfer codings are defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>.
    1360       </p>
    1361       <div id="rfc.figure.u.26"></div><pre class="inline"><span id="rfc.iref.g.54"></span>  <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> = 1#<a href="#transfer.codings" class="smpl">transfer-coding</a>
    1362 </pre><p id="rfc.section.3.3.1.p.3">Transfer-Encoding is analogous to the Content-Transfer-Encoding field of MIME, which was designed to enable safe transport
    1363          of binary data over a 7-bit transport service (<a href="#RFC2045" id="rfc.xref.RFC2045.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>, <a href="http://tools.ietf.org/html/rfc2045#section-6">Section 6</a>). However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP's case, Transfer-Encoding is
    1364          primarily intended to accurately delimit a dynamically generated payload and to distinguish payload encodings that are only
    1365          applied for transport efficiency or security from those that are characteristics of the selected resource.
    1366       </p>
    1367       <p id="rfc.section.3.3.1.p.4">All HTTP/1.1 recipients <em class="bcp14">MUST</em> implement the chunked transfer coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) because it plays a crucial role in framing messages when the payload body size is not known in advance. If chunked is applied
    1368          to a payload body, the sender <em class="bcp14">MUST NOT</em> apply chunked more than once (i.e., chunking an already chunked message is not allowed). If any transfer coding is applied
    1369          to a request payload body, the sender <em class="bcp14">MUST</em> apply chunked as the final transfer coding to ensure that the message is properly framed. If any transfer coding is applied
    1370          to a response payload body, the sender <em class="bcp14">MUST</em> either apply chunked as the final transfer coding or terminate the message by closing the connection.
    1371       </p>
    1372       <div id="rfc.figure.u.27"></div>
    1373       <p>For example,</p><pre class="text">  Transfer-Encoding: gzip, chunked
    1374 </pre><p>indicates that the payload body has been compressed using the gzip coding and then chunked using the chunked coding while
    1375          forming the message body.
    1376       </p>
    1377       <p id="rfc.section.3.3.1.p.6">Unlike <a href="p2-semantics.html#header.content-encoding" class="smpl">Content-Encoding</a> (<a href="p2-semantics.html#content.codings" title="Content Codings">Section 3.1.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.13"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>), Transfer-Encoding is a property of the message, not of the representation, and any recipient along the request/response
    1378          chain <em class="bcp14">MAY</em> decode the received transfer coding(s) or apply additional transfer coding(s) to the message body, assuming that corresponding
    1379          changes are made to the Transfer-Encoding field-value. Additional information about the encoding parameters <em class="bcp14">MAY</em> be provided by other header fields not defined by this specification.
    1380       </p>
    1381       <p id="rfc.section.3.3.1.p.7">Transfer-Encoding <em class="bcp14">MAY</em> be sent in a response to a HEAD request or in a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response (<a href="p4-conditional.html#status.304" title="304 Not Modified">Section 4.1</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>) to a GET request, neither of which includes a message body, to indicate that the origin server would have applied a transfer
    1382          coding to the message body if the request had been an unconditional GET. This indication is not required, however, because
    1383          any recipient on the response chain (including the origin server) can remove transfer codings when they are not needed.
    1384       </p>
    1385       <p id="rfc.section.3.3.1.p.8">Transfer-Encoding was added in HTTP/1.1. It is generally assumed that implementations advertising only HTTP/1.0 support will
    1386          not understand how to process a transfer-encoded payload. A client <em class="bcp14">MUST NOT</em> send a request containing Transfer-Encoding unless it knows the server will handle HTTP/1.1 (or later) requests; such knowledge
    1387          might be in the form of specific user configuration or by remembering the version of a prior received response. A server <em class="bcp14">MUST NOT</em> send a response containing Transfer-Encoding unless the corresponding request indicates HTTP/1.1 (or later).
    1388       </p>
    1389       <p id="rfc.section.3.3.1.p.9">A server that receives a request message with a transfer coding it does not understand <em class="bcp14">SHOULD</em> respond with <a href="p2-semantics.html#status.501" class="smpl">501 (Not Implemented)</a>.
    1390       </p>
    1391       <div id="rfc.iref.c.7"></div>
    1392       <h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a id="header.content-length" href="#header.content-length">Content-Length</a></h3>
    1393       <p id="rfc.section.3.3.2.p.1">When a message does not have a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field, a Content-Length header field can provide the anticipated size, as a decimal number of octets, for a potential
    1394          payload body. For messages that do include a payload body, the Content-Length field-value provides the framing information
    1395          necessary for determining where the body (and message) ends. For messages that do not include a payload body, the Content-Length
    1396          indicates the size of the selected representation (<a href="p2-semantics.html#representations" title="Representations">Section 3</a> of <a href="#Part2" id="rfc.xref.Part2.14"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>).
    1397       </p>
    1398       <div id="rfc.figure.u.28"></div><pre class="inline"><span id="rfc.iref.g.55"></span>  <a href="#header.content-length" class="smpl">Content-Length</a> = 1*<a href="#core.rules" class="smpl">DIGIT</a>
    1399 </pre><p id="rfc.section.3.3.2.p.3">An example is</p>
    1400       <div id="rfc.figure.u.29"></div><pre class="text">  Content-Length: 3495
    1401 </pre><p id="rfc.section.3.3.2.p.5">A sender <em class="bcp14">MUST NOT</em> send a Content-Length header field in any message that contains a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field.
    1402       </p>
    1403       <p id="rfc.section.3.3.2.p.6">A user agent <em class="bcp14">SHOULD</em> send a Content-Length in a request message when no <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> is sent and the request method defines a meaning for an enclosed payload body. For example, a Content-Length header field
    1404          is normally sent in a POST request even when the value is 0 (indicating an empty payload body). A user agent <em class="bcp14">SHOULD NOT</em> send a Content-Length header field when the request message does not contain a payload body and the method semantics do not
    1405          anticipate such a body.
    1406       </p>
    1407       <p id="rfc.section.3.3.2.p.7">A server <em class="bcp14">MAY</em> send a Content-Length header field in a response to a HEAD request (<a href="p2-semantics.html#HEAD" title="HEAD">Section 4.3.2</a> of <a href="#Part2" id="rfc.xref.Part2.15"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>); a server <em class="bcp14">MUST NOT</em> send Content-Length in such a response unless its field-value equals the decimal number of octets that would have been sent
    1408          in the payload body of a response if the same request had used the GET method.
    1409       </p>
    1410       <p id="rfc.section.3.3.2.p.8">A server <em class="bcp14">MAY</em> send a Content-Length header field in a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response to a conditional GET request (<a href="p4-conditional.html#status.304" title="304 Not Modified">Section 4.1</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>); a server <em class="bcp14">MUST NOT</em> send Content-Length in such a response unless its field-value equals the decimal number of octets that would have been sent
    1411          in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to the same request.
    1412       </p>
    1413       <p id="rfc.section.3.3.2.p.9">A server <em class="bcp14">MUST NOT</em> send a Content-Length header field in any response with a status code of <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a> or <a href="p2-semantics.html#status.204" class="smpl">204 (No Content)</a>. A server <em class="bcp14">SHOULD NOT</em> send a Content-Length header field in any <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> response to a CONNECT request (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 4.3.6</a> of <a href="#Part2" id="rfc.xref.Part2.16"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>).
    1414       </p>
    1415       <p id="rfc.section.3.3.2.p.10">Aside from the cases defined above, in the absence of Transfer-Encoding, an origin server <em class="bcp14">SHOULD</em> send a Content-Length header field when the payload body size is known prior to sending the complete header block. This will
    1416          allow downstream recipients to measure transfer progress, know when a received message is complete, and potentially reuse
    1417          the connection for additional requests.
    1418       </p>
    1419       <p id="rfc.section.3.3.2.p.11">Any Content-Length field value greater than or equal to zero is valid. Since there is no predefined limit to the length of
    1420          a payload, recipients <em class="bcp14">SHOULD</em> anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows (<a href="#attack.protocol.element.size.overflows" title="Buffer Overflows">Section&nbsp;8.3</a>).
    1421       </p>
    1422       <p id="rfc.section.3.3.2.p.12">If a message is received that has multiple Content-Length header fields with field-values consisting of the same decimal value,
    1423          or a single Content-Length header field with a field value containing a list of identical decimal values (e.g., "Content-Length:
    1424          42, 42"), indicating that duplicate Content-Length header fields have been generated or combined by an upstream message processor,
    1425          then the recipient <em class="bcp14">MUST</em> either reject the message as invalid or replace the duplicated field-values with a single valid Content-Length field containing
    1426          that decimal value prior to determining the message body length.
    1427       </p>
    1428       <div class="note" id="rfc.section.3.3.2.p.13">
    1429          <p> <b>Note:</b> HTTP's use of Content-Length for message framing differs significantly from the same field's use in MIME, where it is an optional
    1430             field used only within the "message/external-body" media-type.
    1431          </p>
    1432       </div>
    1433       <div id="rfc.iref.c.8"></div>
    1434       <h3 id="rfc.section.3.3.3"><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;<a id="message.body.length" href="#message.body.length">Message Body Length</a></h3>
    1435       <p id="rfc.section.3.3.3.p.1">The length of a message body is determined by one of the following (in order of precedence):</p>
    1436       <p id="rfc.section.3.3.3.p.2"> </p>
    1437       <ol>
    1438          <li>
    1439             <p>Any response to a HEAD request and any response with a <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a>, <a href="p2-semantics.html#status.204" class="smpl">204 (No Content)</a>, or <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> status code is always terminated by the first empty line after the header fields, regardless of the header fields present
    1440                in the message, and thus cannot contain a message body.
    1441             </p>
    1442          </li>
    1443          <li>
    1444             <p>Any <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> response to a CONNECT request implies that the connection will become a tunnel immediately after the empty line that concludes
    1445                the header fields. A client <em class="bcp14">MUST</em> ignore any <a href="#header.content-length" class="smpl">Content-Length</a> or <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header fields received in such a message.
    1446             </p>
    1447          </li>
    1448          <li>
    1449             <p>If a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field is present and the chunked transfer coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) is the final encoding, the message body length is determined by reading and decoding the chunked data until the transfer
    1450                coding indicates the data is complete.
    1451             </p>
    1452             <p>If a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field is present in a response and the chunked transfer coding is not the final encoding, the message body length is
    1453                determined by reading the connection until it is closed by the server. If a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field is present in a request and the chunked transfer coding is not the final encoding, the message body length cannot
    1454                be determined reliably; the server <em class="bcp14">MUST</em> respond with the <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> status code and then close the connection.
    1455             </p>
    1456             <p>If a message is received with both a <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> and a <a href="#header.content-length" class="smpl">Content-Length</a> header field, the Transfer-Encoding overrides the Content-Length. Such a message might indicate an attempt to perform request
    1457                or response smuggling (bypass of security-related checks on message routing or content) and thus ought to be handled as an
    1458                error. A sender <em class="bcp14">MUST</em> remove the received Content-Length field prior to forwarding such a message downstream.
    1459             </p>
    1460          </li>
    1461          <li>
    1462             <p>If a message is received without <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> and with either multiple <a href="#header.content-length" class="smpl">Content-Length</a> header fields having differing field-values or a single Content-Length header field having an invalid value, then the message
    1463                framing is invalid and <em class="bcp14">MUST</em> be treated as an error to prevent request or response smuggling. If this is a request message, the server <em class="bcp14">MUST</em> respond with a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> status code and then close the connection. If this is a response message received by a proxy, the proxy <em class="bcp14">MUST</em> discard the received response, send a <a href="p2-semantics.html#status.502" class="smpl">502 (Bad Gateway)</a> status code as its downstream response, and then close the connection. If this is a response message received by a user agent,
    1464                it <em class="bcp14">MUST</em> be treated as an error by discarding the message and closing the connection.
    1465             </p>
    1466          </li>
    1467          <li>
    1468             <p>If a valid <a href="#header.content-length" class="smpl">Content-Length</a> header field is present without <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a>, its decimal value defines the expected message body length in octets. If the sender closes the connection or the recipient
    1469                times out before the indicated number of octets are received, the recipient <em class="bcp14">MUST</em> consider the message to be incomplete and close the connection.
    1470             </p>
    1471          </li>
    1472          <li>
    1473             <p>If this is a request message and none of the above are true, then the message body length is zero (no message body is present).</p>
    1474          </li>
    1475          <li>
    1476             <p>Otherwise, this is a response message without a declared message body length, so the message body length is determined by
    1477                the number of octets received prior to the server closing the connection.
    1478             </p>
    1479          </li>
    1480       </ol>
    1481       <p id="rfc.section.3.3.3.p.3">Since there is no way to distinguish a successfully completed, close-delimited message from a partially-received message interrupted
    1482          by network failure, a server <em class="bcp14">SHOULD</em> use encoding or length-delimited messages whenever possible. The close-delimiting feature exists primarily for backwards compatibility
    1483          with HTTP/1.0.
    1484       </p>
    1485       <p id="rfc.section.3.3.3.p.4">A server <em class="bcp14">MAY</em> reject a request that contains a message body but not a <a href="#header.content-length" class="smpl">Content-Length</a> by responding with <a href="p2-semantics.html#status.411" class="smpl">411 (Length Required)</a>.
    1486       </p>
    1487       <p id="rfc.section.3.3.3.p.5">Unless a transfer coding other than chunked has been applied, a client that sends a request containing a message body <em class="bcp14">SHOULD</em> use a valid <a href="#header.content-length" class="smpl">Content-Length</a> header field if the message body length is known in advance, rather than the chunked transfer coding, since some existing
    1488          services respond to chunked with a <a href="p2-semantics.html#status.411" class="smpl">411 (Length Required)</a> status code even though they understand the chunked transfer coding. This is typically because such services are implemented
    1489          via a gateway that requires a content-length in advance of being called and the server is unable or unwilling to buffer the
    1490          entire request before processing.
    1491       </p>
    1492       <p id="rfc.section.3.3.3.p.6">A user agent that sends a request containing a message body <em class="bcp14">MUST</em> send a valid <a href="#header.content-length" class="smpl">Content-Length</a> header field if it does not know the server will handle HTTP/1.1 (or later) requests; such knowledge can be in the form of
    1493          specific user configuration or by remembering the version of a prior received response.
    1494       </p>
    1495       <p id="rfc.section.3.3.3.p.7">If the final response to the last request on a connection has been completely received and there remains additional data to
    1496          read, a user agent <em class="bcp14">MAY</em> discard the remaining data or attempt to determine if that data belongs as part of the prior response body, which might be
    1497          the case if the prior message's Content-Length value is incorrect. A client <em class="bcp14">MUST NOT</em> process, cache, or forward such extra data as a separate response, since such behavior would be vulnerable to cache poisoning.
    1498       </p>
    1499       <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="incomplete.messages" href="#incomplete.messages">Handling Incomplete Messages</a></h2>
    1500       <p id="rfc.section.3.4.p.1">A server that receives an incomplete request message, usually due to a canceled request or a triggered time-out exception, <em class="bcp14">MAY</em> send an error response prior to closing the connection.
    1501       </p>
    1502       <p id="rfc.section.3.4.p.2">A client that receives an incomplete response message, which can occur when a connection is closed prematurely or when decoding
    1503          a supposedly chunked transfer coding fails, <em class="bcp14">MUST</em> record the message as incomplete. Cache requirements for incomplete responses are defined in <a href="p6-cache.html#response.cacheability" title="Storing Responses in Caches">Section 3</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
    1504       </p>
    1505       <p id="rfc.section.3.4.p.3">If a response terminates in the middle of the header block (before the empty line is received) and the status code might rely
    1506          on header fields to convey the full meaning of the response, then the client cannot assume that meaning has been conveyed;
    1507          the client might need to repeat the request in order to determine what action to take next.
    1508       </p>
    1509       <p id="rfc.section.3.4.p.4">A message body that uses the chunked transfer coding is incomplete if the zero-sized chunk that terminates the encoding has
    1510          not been received. A message that uses a valid <a href="#header.content-length" class="smpl">Content-Length</a> is incomplete if the size of the message body received (in octets) is less than the value given by Content-Length. A response
    1511          that has neither chunked transfer coding nor Content-Length is terminated by closure of the connection, and thus is considered
    1512          complete regardless of the number of message body octets received, provided that the header block was received intact.
    1513       </p>
    1514       <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="message.robustness" href="#message.robustness">Message Parsing Robustness</a></h2>
    1515       <p id="rfc.section.3.5.p.1">Older HTTP/1.0 user agent implementations might send an extra CRLF after a POST request as a lame workaround for some early
    1516          server applications that failed to read message body content that was not terminated by a line-ending. An HTTP/1.1 user agent <em class="bcp14">MUST NOT</em> preface or follow a request with an extra CRLF. If terminating the request message body with a line-ending is desired, then
    1517          the user agent <em class="bcp14">MUST</em> count the terminating CRLF octets as part of the message body length.
    1518       </p>
    1519       <p id="rfc.section.3.5.p.2">In the interest of robustness, servers <em class="bcp14">SHOULD</em> ignore at least one empty line received where a request-line is expected. In other words, if a server is reading the protocol
    1520          stream at the beginning of a message and receives a CRLF first, the server <em class="bcp14">SHOULD</em> ignore the CRLF.
    1521       </p>
    1522       <p id="rfc.section.3.5.p.3">Although the line terminator for the start-line and header fields is the sequence CRLF, recipients <em class="bcp14">MAY</em> recognize a single LF as a line terminator and ignore any preceding CR.
    1523       </p>
    1524       <p id="rfc.section.3.5.p.4">Although the request-line and status-line grammar rules require that each of the component elements be separated by a single
    1525          SP octet, recipients <em class="bcp14">MAY</em> instead parse on whitespace-delimited word boundaries and, aside from the CRLF terminator, treat any form of whitespace as
    1526          the SP separator while ignoring preceding or trailing whitespace; such whitespace includes one or more of the following octets:
    1527          SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
    1528       </p>
    1529       <p id="rfc.section.3.5.p.5">When a server listening only for HTTP request messages, or processing what appears from the start-line to be an HTTP request
    1530          message, receives a sequence of octets that does not match the HTTP-message grammar aside from the robustness exceptions listed
    1531          above, the server <em class="bcp14">SHOULD</em> respond with a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> response.
    1532       </p>
    1533       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="transfer.codings" href="#transfer.codings">Transfer Codings</a></h1>
    1534       <p id="rfc.section.4.p.1">Transfer coding names are used to indicate an encoding transformation that has been, can be, or might need to be applied to
    1535          a payload body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer
    1536          coding is a property of the message rather than a property of the representation that is being transferred.
    1537       </p>
    1538       <div id="rfc.figure.u.30"></div><pre class="inline"><span id="rfc.iref.g.56"></span><span id="rfc.iref.g.57"></span>  <a href="#transfer.codings" class="smpl">transfer-coding</a>    = "chunked" ; <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>
     1613      <div id="transfer.codings">
     1614         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#transfer.codings">Transfer Codings</a></h1>
     1615         <p id="rfc.section.4.p.1">Transfer coding names are used to indicate an encoding transformation that has been, can be, or might need to be applied to
     1616            a payload body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer
     1617            coding is a property of the message rather than a property of the representation that is being transferred.
     1618         </p>
     1619         <div id="rfc.figure.u.30"></div><pre class="inline"><span id="rfc.iref.g.56"></span><span id="rfc.iref.g.57"></span>  <a href="#transfer.codings" class="smpl">transfer-coding</a>    = "chunked" ; <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>
    15391620                     / "compress" ; <a href="#compress.coding" title="Compress Coding">Section&nbsp;4.2.1</a>
    15401621                     / "deflate" ; <a href="#deflate.coding" title="Deflate Coding">Section&nbsp;4.2.2</a>
     
    15431624  <a href="#transfer.codings" class="smpl">transfer-extension</a> = <a href="#rule.token.separators" class="smpl">token</a> *( <a href="#rule.whitespace" class="smpl">OWS</a> ";" <a href="#rule.whitespace" class="smpl">OWS</a> <a href="#rule.parameter" class="smpl">transfer-parameter</a> )
    15441625</pre><div id="rule.parameter">
    1545          <p id="rfc.section.4.p.3">      Parameters are in the form of attribute/value pairs.</p>
    1546       </div>
    1547       <div id="rfc.figure.u.31"></div><pre class="inline"><span id="rfc.iref.g.58"></span><span id="rfc.iref.g.59"></span><span id="rfc.iref.g.60"></span><span id="rfc.iref.g.61"></span><span id="rfc.iref.g.62"></span>  <a href="#rule.parameter" class="smpl">transfer-parameter</a> = <a href="#rule.parameter" class="smpl">attribute</a> <a href="#rule.whitespace" class="smpl">BWS</a> "=" <a href="#rule.whitespace" class="smpl">BWS</a> <a href="#rule.parameter" class="smpl">value</a>
     1626            <p id="rfc.section.4.p.3">   Parameters are in the form of attribute/value pairs.</p>
     1627         </div>
     1628         <div id="rfc.figure.u.31"></div><pre class="inline"><span id="rfc.iref.g.58"></span><span id="rfc.iref.g.59"></span><span id="rfc.iref.g.60"></span><span id="rfc.iref.g.61"></span><span id="rfc.iref.g.62"></span>  <a href="#rule.parameter" class="smpl">transfer-parameter</a> = <a href="#rule.parameter" class="smpl">attribute</a> <a href="#rule.whitespace" class="smpl">BWS</a> "=" <a href="#rule.whitespace" class="smpl">BWS</a> <a href="#rule.parameter" class="smpl">value</a>
    15481629  <a href="#rule.parameter" class="smpl">attribute</a>          = <a href="#rule.token.separators" class="smpl">token</a>
    15491630  <a href="#rule.parameter" class="smpl">value</a>              = <a href="#rule.token.separators" class="smpl">word</a>
    15501631</pre><p id="rfc.section.4.p.5">All transfer-coding names are case-insensitive and ought to be registered within the HTTP Transfer Coding registry, as defined
    1551          in <a href="#transfer.coding.registry" title="Transfer Coding Registry">Section&nbsp;7.4</a>. They are used in the <a href="#header.te" class="smpl">TE</a> (<a href="#header.te" id="rfc.xref.header.te.1" title="TE">Section&nbsp;4.3</a>) and <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.2" title="Transfer-Encoding">Section&nbsp;3.3.1</a>) header fields.
    1552       </p>
    1553       <div id="rfc.iref.c.9"></div>
    1554       <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="chunked.encoding" href="#chunked.encoding">Chunked Transfer Coding</a></h2>
    1555       <p id="rfc.section.4.1.p.1">The chunked transfer coding modifies the body of a message in order to transfer it as a series of chunks, each with its own
    1556          size indicator, followed by an <em class="bcp14">OPTIONAL</em> trailer containing header fields. This allows dynamically generated content to be transferred along with the information necessary
    1557          for the recipient to verify that it has received the full message.
    1558       </p>
    1559       <div id="rfc.figure.u.32"></div><pre class="inline"><span id="rfc.iref.g.63"></span><span id="rfc.iref.g.64"></span><span id="rfc.iref.g.65"></span><span id="rfc.iref.g.66"></span><span id="rfc.iref.g.67"></span><span id="rfc.iref.g.68"></span><span id="rfc.iref.g.69"></span><span id="rfc.iref.g.70"></span><span id="rfc.iref.g.71"></span><span id="rfc.iref.g.72"></span><span id="rfc.iref.g.73"></span>  <a href="#chunked.encoding" class="smpl">chunked-body</a>   = *<a href="#chunked.encoding" class="smpl">chunk</a>
     1632            in <a href="#transfer.coding.registry" title="Transfer Coding Registry">Section&nbsp;7.4</a>. They are used in the <a href="#header.te" class="smpl">TE</a> (<a href="#header.te" id="rfc.xref.header.te.1" title="TE">Section&nbsp;4.3</a>) and <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.2" title="Transfer-Encoding">Section&nbsp;3.3.1</a>) header fields.
     1633         </p>
     1634         <div id="chunked.encoding">
     1635            <div id="rfc.iref.c.9"></div>
     1636            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#chunked.encoding">Chunked Transfer Coding</a></h2>
     1637            <p id="rfc.section.4.1.p.1">The chunked transfer coding modifies the body of a message in order to transfer it as a series of chunks, each with its own
     1638               size indicator, followed by an <em class="bcp14">OPTIONAL</em> trailer containing header fields. This allows dynamically generated content to be transferred along with the information necessary
     1639               for the recipient to verify that it has received the full message.
     1640            </p>
     1641            <div id="rfc.figure.u.32"></div><pre class="inline"><span id="rfc.iref.g.63"></span><span id="rfc.iref.g.64"></span><span id="rfc.iref.g.65"></span><span id="rfc.iref.g.66"></span><span id="rfc.iref.g.67"></span><span id="rfc.iref.g.68"></span><span id="rfc.iref.g.69"></span><span id="rfc.iref.g.70"></span><span id="rfc.iref.g.71"></span><span id="rfc.iref.g.72"></span><span id="rfc.iref.g.73"></span>  <a href="#chunked.encoding" class="smpl">chunked-body</a>   = *<a href="#chunked.encoding" class="smpl">chunk</a>
    15601642                   <a href="#chunked.encoding" class="smpl">last-chunk</a>
    15611643                   <a href="#chunked.encoding" class="smpl">trailer-part</a>
     
    15771659  <a href="#chunked.encoding" class="smpl">qdtext-nf</a>      = <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / %x21 / %x23-5B / %x5D-7E / <a href="#rule.quoted-string" class="smpl">obs-text</a>
    15781660</pre><p id="rfc.section.4.1.p.3">Chunk extensions within the chunked transfer coding are deprecated. Senders <em class="bcp14">SHOULD NOT</em> send chunk-ext. Definition of new chunk extensions is discouraged.
    1579       </p>
    1580       <p id="rfc.section.4.1.p.4">The chunk-size field is a string of hex digits indicating the size of the chunk-data in octets. The chunked transfer coding
    1581          is complete when a chunk with a chunk-size of zero is received, possibly followed by a trailer, and finally terminated by
    1582          an empty line.
    1583       </p>
    1584       <div id="rfc.iref.t.5"></div>
    1585       <h3 id="rfc.section.4.1.1"><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;<a id="header.trailer" href="#header.trailer">Trailer</a></h3>
    1586       <p id="rfc.section.4.1.1.p.1">A trailer allows the sender to include additional fields at the end of a chunked message in order to supply metadata that
    1587          might be dynamically generated while the message body is sent, such as a message integrity check, digital signature, or post-processing
    1588          status. The trailer <em class="bcp14">MUST NOT</em> contain fields that need to be known before a recipient processes the body, such as <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a>, <a href="#header.content-length" class="smpl">Content-Length</a>, and <a href="#header.trailer" class="smpl">Trailer</a>.
    1589       </p>
    1590       <p id="rfc.section.4.1.1.p.2">When a message includes a message body encoded with the chunked transfer coding and the sender desires to send metadata in
    1591          the form of trailer fields at the end of the message, the sender <em class="bcp14">SHOULD</em> send a <a href="#header.trailer" class="smpl">Trailer</a> header field before the message body to indicate which fields will be present in the trailers. This allows the recipient to
    1592          prepare for receipt of that metadata before it starts processing the body, which is useful if the message is being streamed
    1593          and the recipient wishes to confirm an integrity check on the fly.
    1594       </p>
    1595       <div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.74"></span>  <a href="#header.trailer" class="smpl">Trailer</a> = 1#<a href="#header.fields" class="smpl">field-name</a>
     1661            </p>
     1662            <p id="rfc.section.4.1.p.4">The chunk-size field is a string of hex digits indicating the size of the chunk-data in octets. The chunked transfer coding
     1663               is complete when a chunk with a chunk-size of zero is received, possibly followed by a trailer, and finally terminated by
     1664               an empty line.
     1665            </p>
     1666            <div id="header.trailer">
     1667               <div id="rfc.iref.t.5"></div>
     1668               <h3 id="rfc.section.4.1.1"><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;<a href="#header.trailer">Trailer</a></h3>
     1669               <p id="rfc.section.4.1.1.p.1">A trailer allows the sender to include additional fields at the end of a chunked message in order to supply metadata that
     1670                  might be dynamically generated while the message body is sent, such as a message integrity check, digital signature, or post-processing
     1671                  status. The trailer <em class="bcp14">MUST NOT</em> contain fields that need to be known before a recipient processes the body, such as <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a>, <a href="#header.content-length" class="smpl">Content-Length</a>, and <a href="#header.trailer" class="smpl">Trailer</a>.
     1672               </p>
     1673               <p id="rfc.section.4.1.1.p.2">When a message includes a message body encoded with the chunked transfer coding and the sender desires to send metadata in
     1674                  the form of trailer fields at the end of the message, the sender <em class="bcp14">SHOULD</em> send a <a href="#header.trailer" class="smpl">Trailer</a> header field before the message body to indicate which fields will be present in the trailers. This allows the recipient to
     1675                  prepare for receipt of that metadata before it starts processing the body, which is useful if the message is being streamed
     1676                  and the recipient wishes to confirm an integrity check on the fly.
     1677               </p>
     1678               <div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.74"></span>  <a href="#header.trailer" class="smpl">Trailer</a> = 1#<a href="#header.fields" class="smpl">field-name</a>
    15961679</pre><p id="rfc.section.4.1.1.p.4">If no <a href="#header.trailer" class="smpl">Trailer</a> header field is present, the sender of a chunked message body <em class="bcp14">SHOULD</em> send an empty trailer.
    1597       </p>
    1598       <p id="rfc.section.4.1.1.p.5">A server <em class="bcp14">MUST</em> send an empty trailer with the chunked transfer coding unless at least one of the following is true:
    1599       </p>
    1600       <ol>
    1601          <li>the request included a <a href="#header.te" class="smpl">TE</a> 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;4.3</a>; or,
    1602          </li>
    1603          <li>the trailer fields consist entirely of optional metadata and the recipient could use the message (in a manner acceptable to
    1604             the server where the field originated) without receiving that metadata. In other words, the server that generated the header
    1605             field is willing to accept the possibility that the trailer fields might be silently discarded along the path to the client.
    1606          </li>
    1607       </ol>
    1608       <p id="rfc.section.4.1.1.p.6">The above requirement prevents the need for an infinite buffer when a message is being received by an HTTP/1.1 (or later)
    1609          proxy and forwarded to an HTTP/1.0 recipient.
    1610       </p>
    1611       <h3 id="rfc.section.4.1.2"><a href="#rfc.section.4.1.2">4.1.2</a>&nbsp;<a id="decoding.chunked" href="#decoding.chunked">Decoding chunked</a></h3>
    1612       <p id="rfc.section.4.1.2.p.1">A process for decoding the chunked transfer coding can be represented in pseudo-code as:</p>
    1613       <div id="rfc.figure.u.34"></div><pre class="text">  length := 0
     1680               </p>
     1681               <p id="rfc.section.4.1.1.p.5">A server <em class="bcp14">MUST</em> send an empty trailer with the chunked transfer coding unless at least one of the following is true:
     1682               </p>
     1683               <ol>
     1684                  <li>the request included a <a href="#header.te" class="smpl">TE</a> 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;4.3</a>; or,
     1685                  </li>
     1686                  <li>the trailer fields consist entirely of optional metadata and the recipient could use the message (in a manner acceptable to
     1687                     the server where the field originated) without receiving that metadata. In other words, the server that generated the header
     1688                     field is willing to accept the possibility that the trailer fields might be silently discarded along the path to the client.
     1689                  </li>
     1690               </ol>
     1691               <p id="rfc.section.4.1.1.p.6">The above requirement prevents the need for an infinite buffer when a message is being received by an HTTP/1.1 (or later)
     1692                  proxy and forwarded to an HTTP/1.0 recipient.
     1693               </p>
     1694            </div>
     1695            <div id="decoding.chunked">
     1696               <h3 id="rfc.section.4.1.2"><a href="#rfc.section.4.1.2">4.1.2</a>&nbsp;<a href="#decoding.chunked">Decoding chunked</a></h3>
     1697               <p id="rfc.section.4.1.2.p.1">A process for decoding the chunked transfer coding can be represented in pseudo-code as:</p>
     1698               <div id="rfc.figure.u.34"></div><pre class="text">  length := 0
    16141699  read chunk-size, chunk-ext (if any) and CRLF
    16151700  while (chunk-size &gt; 0) {
     
    16281713  Remove Trailer from existing header fields
    16291714</pre><p id="rfc.section.4.1.2.p.3">All recipients <em class="bcp14">MUST</em> be able to receive and decode the chunked transfer coding and <em class="bcp14">MUST</em> ignore chunk-ext extensions they do not understand.
    1630       </p>
    1631       <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="compression.codings" href="#compression.codings">Compression Codings</a></h2>
    1632       <p id="rfc.section.4.2.p.1">The codings defined below can be used to compress the payload of a message.</p>
    1633       <div id="rfc.iref.c.10"></div>
    1634       <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a id="compress.coding" href="#compress.coding">Compress Coding</a></h3>
    1635       <p id="rfc.section.4.2.1.p.1">The "compress" format is produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch
    1636          coding (LZW). Recipients <em class="bcp14">SHOULD</em> consider "x-compress" to be equivalent to "compress".
    1637       </p>
    1638       <div id="rfc.iref.d.2"></div>
    1639       <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a id="deflate.coding" href="#deflate.coding">Deflate Coding</a></h3>
    1640       <p id="rfc.section.4.2.2.p.1">The "deflate" format is defined as the "deflate" compression mechanism (described in <a href="#RFC1951" id="rfc.xref.RFC1951.1"><cite title="DEFLATE Compressed Data Format Specification version 1.3">[RFC1951]</cite></a>) used inside the "zlib" data format (<a href="#RFC1950" id="rfc.xref.RFC1950.1"><cite title="ZLIB Compressed Data Format Specification version 3.3">[RFC1950]</cite></a>).
    1641       </p>
    1642       <div class="note" id="rfc.section.4.2.2.p.2">
    1643          <p> <b>Note:</b> Some incorrect implementations send the "deflate" compressed data without the zlib wrapper.
    1644          </p>
    1645       </div>
    1646       <div id="rfc.iref.g.75"></div>
    1647       <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a id="gzip.coding" href="#gzip.coding">Gzip Coding</a></h3>
    1648       <p id="rfc.section.4.2.3.p.1">The "gzip" format is produced by the file compression program "gzip" (GNU zip), as described in <a href="#RFC1952" id="rfc.xref.RFC1952.1"><cite title="GZIP file format specification version 4.3">[RFC1952]</cite></a>. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC. Recipients <em class="bcp14">SHOULD</em> consider "x-gzip" to be equivalent to "gzip".
    1649       </p>
    1650       <div id="rfc.iref.t.6"></div>
    1651       <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="header.te" href="#header.te">TE</a></h2>
    1652       <p id="rfc.section.4.3.p.1">The "TE" header field in a request indicates what transfer codings, besides chunked, the client is willing to accept in response,
    1653          and whether or not the client is willing to accept trailer fields in a chunked transfer coding.
    1654       </p>
    1655       <p id="rfc.section.4.3.p.2">The TE field-value consists of a comma-separated list of transfer coding names, each allowing for optional parameters (as
    1656          described in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>), and/or the keyword "trailers". Clients <em class="bcp14">MUST NOT</em> send the chunked transfer coding name in TE; chunked is always acceptable for HTTP/1.1 recipients.
    1657       </p>
    1658       <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.76"></span><span id="rfc.iref.g.77"></span><span id="rfc.iref.g.78"></span><span id="rfc.iref.g.79"></span>  <a href="#header.te" class="smpl">TE</a>        = #<a href="#header.te" class="smpl">t-codings</a>
     1715               </p>
     1716            </div>
     1717         </div>
     1718         <div id="compression.codings">
     1719            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#compression.codings">Compression Codings</a></h2>
     1720            <p id="rfc.section.4.2.p.1">The codings defined below can be used to compress the payload of a message.</p>
     1721            <div id="compress.coding">
     1722               <div id="rfc.iref.c.10"></div>
     1723               <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a href="#compress.coding">Compress Coding</a></h3>
     1724               <p id="rfc.section.4.2.1.p.1">The "compress" format is produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch
     1725                  coding (LZW). Recipients <em class="bcp14">SHOULD</em> consider "x-compress" to be equivalent to "compress".
     1726               </p>
     1727            </div>
     1728            <div id="deflate.coding">
     1729               <div id="rfc.iref.d.2"></div>
     1730               <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a href="#deflate.coding">Deflate Coding</a></h3>
     1731               <p id="rfc.section.4.2.2.p.1">The "deflate" format is defined as the "deflate" compression mechanism (described in <a href="#RFC1951" id="rfc.xref.RFC1951.1"><cite title="DEFLATE Compressed Data Format Specification version 1.3">[RFC1951]</cite></a>) used inside the "zlib" data format (<a href="#RFC1950" id="rfc.xref.RFC1950.1"><cite title="ZLIB Compressed Data Format Specification version 3.3">[RFC1950]</cite></a>).
     1732               </p>
     1733               <div class="note" id="rfc.section.4.2.2.p.2">
     1734                  <p><b>Note:</b> Some incorrect implementations send the "deflate" compressed data without the zlib wrapper.
     1735                  </p>
     1736               </div>
     1737            </div>
     1738            <div id="gzip.coding">
     1739               <div id="rfc.iref.g.75"></div>
     1740               <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a href="#gzip.coding">Gzip Coding</a></h3>
     1741               <p id="rfc.section.4.2.3.p.1">The "gzip" format is produced by the file compression program "gzip" (GNU zip), as described in <a href="#RFC1952" id="rfc.xref.RFC1952.1"><cite title="GZIP file format specification version 4.3">[RFC1952]</cite></a>. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC. Recipients <em class="bcp14">SHOULD</em> consider "x-gzip" to be equivalent to "gzip".
     1742               </p>
     1743            </div>
     1744         </div>
     1745         <div id="header.te">
     1746            <div id="rfc.iref.t.6"></div>
     1747            <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a href="#header.te">TE</a></h2>
     1748            <p id="rfc.section.4.3.p.1">The "TE" header field in a request indicates what transfer codings, besides chunked, the client is willing to accept in response,
     1749               and whether or not the client is willing to accept trailer fields in a chunked transfer coding.
     1750            </p>
     1751            <p id="rfc.section.4.3.p.2">The TE field-value consists of a comma-separated list of transfer coding names, each allowing for optional parameters (as
     1752               described in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>), and/or the keyword "trailers". Clients <em class="bcp14">MUST NOT</em> send the chunked transfer coding name in TE; chunked is always acceptable for HTTP/1.1 recipients.
     1753            </p>
     1754            <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.76"></span><span id="rfc.iref.g.77"></span><span id="rfc.iref.g.78"></span><span id="rfc.iref.g.79"></span>  <a href="#header.te" class="smpl">TE</a>        = #<a href="#header.te" class="smpl">t-codings</a>
    16591755  <a href="#header.te" class="smpl">t-codings</a> = "trailers" / ( <a href="#transfer.codings" class="smpl">transfer-coding</a> [ <a href="#header.te" class="smpl">t-ranking</a> ] )
    16601756  <a href="#header.te" class="smpl">t-ranking</a> = <a href="#rule.whitespace" class="smpl">OWS</a> ";" <a href="#rule.whitespace" class="smpl">OWS</a> "q=" <a href="#header.te" class="smpl">rank</a>
     
    16621758             / ( "1" [ "." 0*3("0") ] )
    16631759</pre><p id="rfc.section.4.3.p.4">Three examples of TE use are below.</p>
    1664       <div id="rfc.figure.u.36"></div><pre class="text">  TE: deflate
     1760            <div id="rfc.figure.u.36"></div><pre class="text">  TE: deflate
    16651761  TE:
    16661762  TE: trailers, deflate;q=0.5
    16671763</pre><p id="rfc.section.4.3.p.6">The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer
    1668          coding, as defined in <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>, on behalf of itself and any downstream clients. For chained requests, this implies that either: (a) all downstream clients
    1669          are willing to accept trailer fields in the forwarded response; or, (b) the client will attempt to buffer the response on
    1670          behalf of downstream recipients. Note that HTTP/1.1 does not define any means to limit the size of a chunked response such
    1671          that a client can be assured of buffering the entire response.
    1672       </p>
    1673       <p id="rfc.section.4.3.p.7">When multiple transfer codings are acceptable, the client <em class="bcp14">MAY</em> rank the codings by preference using a case-insensitive "q" parameter (similar to the qvalues used in content negotiation
    1674          fields, <a href="p2-semantics.html#quality.values" title="Quality Values">Section 5.3.1</a> of <a href="#Part2" id="rfc.xref.Part2.17"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). The rank value is a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred;
    1675          a value of 0 means "not acceptable".
    1676       </p>
    1677       <p id="rfc.section.4.3.p.8">If the TE field-value is empty or if no TE field is present, the only acceptable transfer coding is chunked. A message with
    1678          no transfer coding is always acceptable.
    1679       </p>
    1680       <p id="rfc.section.4.3.p.9">Since the TE header field only applies to the immediate connection, a sender of TE <em class="bcp14">MUST</em> also send a "TE" connection option within the <a href="#header.connection" class="smpl">Connection</a> header field (<a href="#header.connection" id="rfc.xref.header.connection.4" title="Connection">Section&nbsp;6.1</a>) in order to prevent the TE field from being forwarded by intermediaries that do not support its semantics.
    1681       </p>
    1682       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="message.routing" href="#message.routing">Message Routing</a></h1>
    1683       <p id="rfc.section.5.p.1">HTTP request message routing is determined by each client based on the target resource, the client's proxy configuration,
    1684          and establishment or reuse of an inbound connection. The corresponding response routing follows the same connection chain
    1685          back to the client.
    1686       </p>
    1687       <div id="rfc.iref.t.7"></div>
    1688       <div id="rfc.iref.t.8"></div>
    1689       <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="target-resource" href="#target-resource">Identifying a Target Resource</a></h2>
    1690       <p id="rfc.section.5.1.p.1">HTTP is used in a wide variety of applications, ranging from general-purpose computers to home appliances. In some cases,
    1691          communication options are hard-coded in a client's configuration. However, most HTTP clients rely on the same resource identification
    1692          mechanism and configuration techniques as general-purpose Web browsers.
    1693       </p>
    1694       <p id="rfc.section.5.1.p.2">HTTP communication is initiated by a user agent for some purpose. The purpose is a combination of request semantics, which
    1695          are defined in <a href="#Part2" id="rfc.xref.Part2.18"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, and a target resource upon which to apply those semantics. A URI reference (<a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;2.7</a>) is typically used as an identifier for the "<dfn>target resource</dfn>", which a user agent would resolve to its absolute form in order to obtain the "<dfn>target URI</dfn>". The target URI excludes the reference's fragment identifier component, if any, since fragment identifiers are reserved
    1696          for client-side processing (<a href="#RFC3986" id="rfc.xref.RFC3986.16"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.5">Section 3.5</a>).
    1697       </p>
    1698       <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="connecting.inbound" href="#connecting.inbound">Connecting Inbound</a></h2>
    1699       <p id="rfc.section.5.2.p.1">Once the target URI is determined, a client needs to decide whether a network request is necessary to accomplish the desired
    1700          semantics and, if so, where that request is to be directed.
    1701       </p>
    1702       <p id="rfc.section.5.2.p.2">If the client has a response cache and the request semantics can be satisfied by a cache (<a href="#Part6" id="rfc.xref.Part6.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>), then the request is usually directed to the cache first.
    1703       </p>
    1704       <p id="rfc.section.5.2.p.3">If the request is not satisfied by a cache, then a typical client will check its configuration to determine whether a proxy
    1705          is to be used to satisfy the request. Proxy configuration is implementation-dependent, but is often based on URI prefix matching,
    1706          selective authority matching, or both, and the proxy itself is usually identified by an "http" or "https" URI. If a proxy
    1707          is applicable, the client connects inbound by establishing (or reusing) a connection to that proxy.
    1708       </p>
    1709       <p id="rfc.section.5.2.p.4">If no proxy is applicable, a typical client will invoke a handler routine, usually specific to the target URI's scheme, to
    1710          connect directly to an authority for the target resource. How that is accomplished is dependent on the target URI scheme and
    1711          defined by its associated specification, similar to how this specification defines origin server access for resolution of
    1712          the "http" (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) and "https" (<a href="#https.uri" title="https URI scheme">Section&nbsp;2.7.2</a>) schemes.
    1713       </p>
    1714       <p id="rfc.section.5.2.p.5">HTTP requirements regarding connection management are defined in <a href="#connection.management" title="Connection Management">Section&nbsp;6</a>.
    1715       </p>
    1716       <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a id="request-target" href="#request-target">Request Target</a></h2>
    1717       <p id="rfc.section.5.3.p.1">Once an inbound connection is obtained, the client sends an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) with a request-target derived from the target URI. There are four distinct formats for the request-target, depending on
    1718          both the method being requested and whether the request is to a proxy.
    1719       </p>
    1720       <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.80"></span><span id="rfc.iref.g.81"></span><span id="rfc.iref.g.82"></span><span id="rfc.iref.g.83"></span><span id="rfc.iref.g.84"></span>  <a href="#request-target" class="smpl">request-target</a> = <a href="#origin-form" class="smpl">origin-form</a>
     1764               coding, as defined in <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>, on behalf of itself and any downstream clients. For chained requests, this implies that either: (a) all downstream clients
     1765               are willing to accept trailer fields in the forwarded response; or, (b) the client will attempt to buffer the response on
     1766               behalf of downstream recipients. Note that HTTP/1.1 does not define any means to limit the size of a chunked response such
     1767               that a client can be assured of buffering the entire response.
     1768            </p>
     1769            <p id="rfc.section.4.3.p.7">When multiple transfer codings are acceptable, the client <em class="bcp14">MAY</em> rank the codings by preference using a case-insensitive "q" parameter (similar to the qvalues used in content negotiation
     1770               fields, <a href="p2-semantics.html#quality.values" title="Quality Values">Section 5.3.1</a> of <a href="#Part2" id="rfc.xref.Part2.17"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). The rank value is a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred;
     1771               a value of 0 means "not acceptable".
     1772            </p>
     1773            <p id="rfc.section.4.3.p.8">If the TE field-value is empty or if no TE field is present, the only acceptable transfer coding is chunked. A message with
     1774               no transfer coding is always acceptable.
     1775            </p>
     1776            <p id="rfc.section.4.3.p.9">Since the TE header field only applies to the immediate connection, a sender of TE <em class="bcp14">MUST</em> also send a "TE" connection option within the <a href="#header.connection" class="smpl">Connection</a> header field (<a href="#header.connection" id="rfc.xref.header.connection.4" title="Connection">Section&nbsp;6.1</a>) in order to prevent the TE field from being forwarded by intermediaries that do not support its semantics.
     1777            </p>
     1778         </div>
     1779      </div>
     1780      <div id="message.routing">
     1781         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#message.routing">Message Routing</a></h1>
     1782         <p id="rfc.section.5.p.1">HTTP request message routing is determined by each client based on the target resource, the client's proxy configuration,
     1783            and establishment or reuse of an inbound connection. The corresponding response routing follows the same connection chain
     1784            back to the client.
     1785         </p>
     1786         <div id="target-resource">
     1787            <div id="rfc.iref.t.7"></div>
     1788            <div id="rfc.iref.t.8"></div>
     1789            <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a href="#target-resource">Identifying a Target Resource</a></h2>
     1790            <p id="rfc.section.5.1.p.1">HTTP is used in a wide variety of applications, ranging from general-purpose computers to home appliances. In some cases,
     1791               communication options are hard-coded in a client's configuration. However, most HTTP clients rely on the same resource identification
     1792               mechanism and configuration techniques as general-purpose Web browsers.
     1793            </p>
     1794            <p id="rfc.section.5.1.p.2">HTTP communication is initiated by a user agent for some purpose. The purpose is a combination of request semantics, which
     1795               are defined in <a href="#Part2" id="rfc.xref.Part2.18"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, and a target resource upon which to apply those semantics. A URI reference (<a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;2.7</a>) is typically used as an identifier for the "<dfn>target resource</dfn>", which a user agent would resolve to its absolute form in order to obtain the "<dfn>target URI</dfn>". The target URI excludes the reference's fragment identifier component, if any, since fragment identifiers are reserved
     1796               for client-side processing (<a href="#RFC3986" id="rfc.xref.RFC3986.16"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.5">Section 3.5</a>).
     1797            </p>
     1798         </div>
     1799         <div id="connecting.inbound">
     1800            <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a href="#connecting.inbound">Connecting Inbound</a></h2>
     1801            <p id="rfc.section.5.2.p.1">Once the target URI is determined, a client needs to decide whether a network request is necessary to accomplish the desired
     1802               semantics and, if so, where that request is to be directed.
     1803            </p>
     1804            <p id="rfc.section.5.2.p.2">If the client has a response cache and the request semantics can be satisfied by a cache (<a href="#Part6" id="rfc.xref.Part6.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>), then the request is usually directed to the cache first.
     1805            </p>
     1806            <p id="rfc.section.5.2.p.3">If the request is not satisfied by a cache, then a typical client will check its configuration to determine whether a proxy
     1807               is to be used to satisfy the request. Proxy configuration is implementation-dependent, but is often based on URI prefix matching,
     1808               selective authority matching, or both, and the proxy itself is usually identified by an "http" or "https" URI. If a proxy
     1809               is applicable, the client connects inbound by establishing (or reusing) a connection to that proxy.
     1810            </p>
     1811            <p id="rfc.section.5.2.p.4">If no proxy is applicable, a typical client will invoke a handler routine, usually specific to the target URI's scheme, to
     1812               connect directly to an authority for the target resource. How that is accomplished is dependent on the target URI scheme and
     1813               defined by its associated specification, similar to how this specification defines origin server access for resolution of
     1814               the "http" (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) and "https" (<a href="#https.uri" title="https URI scheme">Section&nbsp;2.7.2</a>) schemes.
     1815            </p>
     1816            <p id="rfc.section.5.2.p.5">HTTP requirements regarding connection management are defined in <a href="#connection.management" title="Connection Management">Section&nbsp;6</a>.
     1817            </p>
     1818         </div>
     1819         <div id="request-target">
     1820            <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a href="#request-target">Request Target</a></h2>
     1821            <p id="rfc.section.5.3.p.1">Once an inbound connection is obtained, the client sends an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) with a request-target derived from the target URI. There are four distinct formats for the request-target, depending on
     1822               both the method being requested and whether the request is to a proxy.
     1823            </p>
     1824            <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.80"></span><span id="rfc.iref.g.81"></span><span id="rfc.iref.g.82"></span><span id="rfc.iref.g.83"></span><span id="rfc.iref.g.84"></span>  <a href="#request-target" class="smpl">request-target</a> = <a href="#origin-form" class="smpl">origin-form</a>
    17211825                 / <a href="#absolute-form" class="smpl">absolute-form</a>
    17221826                 / <a href="#authority-form" class="smpl">authority-form</a>
     
    17281832  <a href="#asterisk-form" class="smpl">asterisk-form</a>  = "*"
    17291833</pre><div id="origin-form">
    1730          <p id="rfc.section.5.3.p.3"><span id="rfc.iref.o.3"></span> <b>origin-form</b>
    1731          </p>
    1732       </div>
    1733       <p id="rfc.section.5.3.p.4">The most common form of request-target is the <dfn>origin-form</dfn>. When making a request directly to an origin server, other than a CONNECT or server-wide OPTIONS request (as detailed below),
    1734          a client <em class="bcp14">MUST</em> send only the absolute path and query components of the target URI as the request-target. If the target URI's path component
    1735          is empty, then the client <em class="bcp14">MUST</em> send "/" as the path within the origin-form of request-target. A <a href="#header.host" class="smpl">Host</a> header field is also sent, as defined in <a href="#header.host" id="rfc.xref.header.host.1" title="Host">Section&nbsp;5.4</a>, containing the target URI's authority component (excluding any userinfo).
    1736       </p>
    1737       <p id="rfc.section.5.3.p.5">For example, a client wishing to retrieve a representation of the resource identified as</p>
    1738       <div id="rfc.figure.u.38"></div><pre class="text">http://www.example.org/where?q=now
     1834               <p id="rfc.section.5.3.p.3"><span id="rfc.iref.o.3"></span> <b>origin-form</b>
     1835               </p>
     1836            </div>
     1837            <p id="rfc.section.5.3.p.4">The most common form of request-target is the <dfn>origin-form</dfn>. When making a request directly to an origin server, other than a CONNECT or server-wide OPTIONS request (as detailed below),
     1838               a client <em class="bcp14">MUST</em> send only the absolute path and query components of the target URI as the request-target. If the target URI's path component
     1839               is empty, then the client <em class="bcp14">MUST</em> send "/" as the path within the origin-form of request-target. A <a href="#header.host" class="smpl">Host</a> header field is also sent, as defined in <a href="#header.host" id="rfc.xref.header.host.1" title="Host">Section&nbsp;5.4</a>, containing the target URI's authority component (excluding any userinfo).
     1840            </p>
     1841            <p id="rfc.section.5.3.p.5">For example, a client wishing to retrieve a representation of the resource identified as</p>
     1842            <div id="rfc.figure.u.38"></div><pre class="text">http://www.example.org/where?q=now
    17391843</pre><p id="rfc.section.5.3.p.7">directly from the origin server would open (or reuse) a TCP connection to port 80 of the host "www.example.org" and send the
    1740          lines:
    1741       </p>
    1742       <div id="rfc.figure.u.39"></div><pre class="text2">GET /where?q=now HTTP/1.1
     1844               lines:
     1845            </p>
     1846            <div id="rfc.figure.u.39"></div><pre class="text2">GET /where?q=now HTTP/1.1
    17431847Host: www.example.org
    17441848</pre><p id="rfc.section.5.3.p.9">followed by the remainder of the request message.</p>
    1745       <div id="absolute-form">
    1746          <p id="rfc.section.5.3.p.10"><span id="rfc.iref.a.2"></span> <b>absolute-form</b>
    1747          </p>
    1748       </div>
    1749       <p id="rfc.section.5.3.p.11">When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client <em class="bcp14">MUST</em> send the target URI in <dfn>absolute-form</dfn> as the request-target. The proxy is requested to either service that request from a valid cache, if possible, or make the
    1750          same request on the client's behalf to either the next inbound proxy server or directly to the origin server indicated by
    1751          the request-target. Requirements on such "forwarding" of messages are defined in <a href="#message.forwarding" title="Message Forwarding">Section&nbsp;5.7</a>.
    1752       </p>
    1753       <p id="rfc.section.5.3.p.12">An example absolute-form of request-line would be:</p>
    1754       <div id="rfc.figure.u.40"></div><pre class="text2">GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
     1849            <div id="absolute-form">
     1850               <p id="rfc.section.5.3.p.10"><span id="rfc.iref.a.2"></span> <b>absolute-form</b>
     1851               </p>
     1852            </div>
     1853            <p id="rfc.section.5.3.p.11">When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client <em class="bcp14">MUST</em> send the target URI in <dfn>absolute-form</dfn> as the request-target. The proxy is requested to either service that request from a valid cache, if possible, or make the
     1854               same request on the client's behalf to either the next inbound proxy server or directly to the origin server indicated by
     1855               the request-target. Requirements on such "forwarding" of messages are defined in <a href="#message.forwarding" title="Message Forwarding">Section&nbsp;5.7</a>.
     1856            </p>
     1857            <p id="rfc.section.5.3.p.12">An example absolute-form of request-line would be:</p>
     1858            <div id="rfc.figure.u.40"></div><pre class="text2">GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
    17551859</pre><p id="rfc.section.5.3.p.14">To allow for transition to the absolute-form for all requests in some future version of HTTP, HTTP/1.1 servers <em class="bcp14">MUST</em> accept the absolute-form in requests, even though HTTP/1.1 clients will only send them in requests to proxies.
    1756       </p>
    1757       <div id="authority-form">
    1758          <p id="rfc.section.5.3.p.15"><span id="rfc.iref.a.3"></span> <b>authority-form</b>
    1759          </p>
    1760       </div>
    1761       <p id="rfc.section.5.3.p.16">The <dfn>authority-form</dfn> of request-target is only used for CONNECT requests (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 4.3.6</a> of <a href="#Part2" id="rfc.xref.Part2.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). When making a CONNECT request to establish a tunnel through one or more proxies, a client <em class="bcp14">MUST</em> send only the target URI's authority component (excluding any userinfo) as the request-target. For example,
    1762       </p>
    1763       <div id="rfc.figure.u.41"></div><pre class="text2">CONNECT www.example.com:80 HTTP/1.1
     1860            </p>
     1861            <div id="authority-form">
     1862               <p id="rfc.section.5.3.p.15"><span id="rfc.iref.a.3"></span> <b>authority-form</b>
     1863               </p>
     1864            </div>
     1865            <p id="rfc.section.5.3.p.16">The <dfn>authority-form</dfn> of request-target is only used for CONNECT requests (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 4.3.6</a> of <a href="#Part2" id="rfc.xref.Part2.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). When making a CONNECT request to establish a tunnel through one or more proxies, a client <em class="bcp14">MUST</em> send only the target URI's authority component (excluding any userinfo) as the request-target. For example,
     1866            </p>
     1867            <div id="rfc.figure.u.41"></div><pre class="text2">CONNECT www.example.com:80 HTTP/1.1
    17641868</pre><div id="asterisk-form">
    1765          <p id="rfc.section.5.3.p.18"><span id="rfc.iref.a.4"></span> <b>asterisk-form</b>
    1766          </p>
    1767       </div>
    1768       <p id="rfc.section.5.3.p.19">The <dfn>asterisk-form</dfn> of request-target is only used for a server-wide OPTIONS request (<a href="p2-semantics.html#OPTIONS" title="OPTIONS">Section 4.3.7</a> of <a href="#Part2" id="rfc.xref.Part2.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server,
    1769          the client <em class="bcp14">MUST</em> send only "*" (%x2A) as the request-target. For example,
    1770       </p>
    1771       <div id="rfc.figure.u.42"></div><pre class="text2">OPTIONS * HTTP/1.1
     1869               <p id="rfc.section.5.3.p.18"><span id="rfc.iref.a.4"></span> <b>asterisk-form</b>
     1870               </p>
     1871            </div>
     1872            <p id="rfc.section.5.3.p.19">The <dfn>asterisk-form</dfn> of request-target is only used for a server-wide OPTIONS request (<a href="p2-semantics.html#OPTIONS" title="OPTIONS">Section 4.3.7</a> of <a href="#Part2" id="rfc.xref.Part2.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server,
     1873               the client <em class="bcp14">MUST</em> send only "*" (%x2A) as the request-target. For example,
     1874            </p>
     1875            <div id="rfc.figure.u.42"></div><pre class="text2">OPTIONS * HTTP/1.1
    17721876</pre><p id="rfc.section.5.3.p.21">If a proxy receives an OPTIONS request with an absolute-form of request-target in which the URI has an empty path and no query
    1773          component, then the last proxy on the request chain <em class="bcp14">MUST</em> send a request-target of "*" when it forwards the request to the indicated origin server.
    1774       </p>
    1775       <div id="rfc.figure.u.43"></div>
    1776       <p>For example, the request</p><pre class="text2">OPTIONS http://www.example.org:8001 HTTP/1.1
     1877               component, then the last proxy on the request chain <em class="bcp14">MUST</em> send a request-target of "*" when it forwards the request to the indicated origin server.
     1878            </p>
     1879            <div id="rfc.figure.u.43"></div>
     1880            <p>For example, the request</p><pre class="text2">OPTIONS http://www.example.org:8001 HTTP/1.1
    17771881</pre><div id="rfc.figure.u.44"></div>
    1778       <p>would be forwarded by the final proxy as</p><pre class="text2">OPTIONS * HTTP/1.1
     1882            <p>would be forwarded by the final proxy as</p><pre class="text2">OPTIONS * HTTP/1.1
    17791883Host: www.example.org:8001
    1780 </pre>  <p>after connecting to port 8001 of host "www.example.org".</p>
    1781       <div id="rfc.iref.h.6"></div>
    1782       <h2 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;<a id="header.host" href="#header.host">Host</a></h2>
    1783       <p id="rfc.section.5.4.p.1">The "Host" header field in a request provides the host and port information from the target URI, enabling the origin server
    1784          to distinguish among resources while servicing requests for multiple host names on a single IP address. Since the Host field-value
    1785          is critical information for handling a request, it <em class="bcp14">SHOULD</em> be sent as the first header field following the request-line.
    1786       </p>
    1787       <div id="rfc.figure.u.45"></div><pre class="inline"><span id="rfc.iref.g.85"></span>  <a href="#header.host" class="smpl">Host</a> = <a href="#uri" class="smpl">uri-host</a> [ ":" <a href="#uri" class="smpl">port</a> ] ; <a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>
     1884</pre><p>after connecting to port 8001 of host "www.example.org".</p>
     1885         </div>
     1886         <div id="header.host">
     1887            <div id="rfc.iref.h.6"></div>
     1888            <h2 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;<a href="#header.host">Host</a></h2>
     1889            <p id="rfc.section.5.4.p.1">The "Host" header field in a request provides the host and port information from the target URI, enabling the origin server
     1890               to distinguish among resources while servicing requests for multiple host names on a single IP address. Since the Host field-value
     1891               is critical information for handling a request, it <em class="bcp14">SHOULD</em> be sent as the first header field following the request-line.
     1892            </p>
     1893            <div id="rfc.figure.u.45"></div><pre class="inline"><span id="rfc.iref.g.85"></span>  <a href="#header.host" class="smpl">Host</a> = <a href="#uri" class="smpl">uri-host</a> [ ":" <a href="#uri" class="smpl">port</a> ] ; <a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>
    17881894</pre><p id="rfc.section.5.4.p.3">A client <em class="bcp14">MUST</em> send a Host header field in all HTTP/1.1 request messages. If the target URI includes an authority component, then the Host
    1789          field-value <em class="bcp14">MUST</em> be identical to that authority component after excluding any userinfo (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>). If the authority component is missing or undefined for the target URI, then the Host header field <em class="bcp14">MUST</em> be sent with an empty field-value.
    1790       </p>
    1791       <p id="rfc.section.5.4.p.4">For example, a GET request to the origin server for &lt;http://www.example.org/pub/WWW/&gt; would begin with:</p>
    1792       <div id="rfc.figure.u.46"></div><pre class="text2">GET /pub/WWW/ HTTP/1.1
     1895               field-value <em class="bcp14">MUST</em> be identical to that authority component after excluding any userinfo (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>). If the authority component is missing or undefined for the target URI, then the Host header field <em class="bcp14">MUST</em> be sent with an empty field-value.
     1896            </p>
     1897            <p id="rfc.section.5.4.p.4">For example, a GET request to the origin server for &lt;http://www.example.org/pub/WWW/&gt; would begin with:</p>
     1898            <div id="rfc.figure.u.46"></div><pre class="text2">GET /pub/WWW/ HTTP/1.1
    17931899Host: www.example.org
    17941900</pre><p id="rfc.section.5.4.p.6">The Host header field <em class="bcp14">MUST</em> be sent in an HTTP/1.1 request even if the request-target is in the absolute-form, since this allows the Host information
    1795          to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.
    1796       </p>
    1797       <p id="rfc.section.5.4.p.7">When a proxy receives a request with an absolute-form of request-target, the proxy <em class="bcp14">MUST</em> ignore the received Host header field (if any) and instead replace it with the host information of the request-target. If
    1798          the proxy forwards the request, it <em class="bcp14">MUST</em> generate a new Host field-value based on the received request-target rather than forward the received Host field-value.
    1799       </p>
    1800       <p id="rfc.section.5.4.p.8">Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to
    1801          poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it
    1802          relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache,
    1803          without first verifying that the intercepted connection is targeting a valid IP address for that host.
    1804       </p>
    1805       <p id="rfc.section.5.4.p.9">A server <em class="bcp14">MUST</em> respond with a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than
    1806          one Host header field or a Host header field with an invalid field-value.
    1807       </p>
    1808       <div id="rfc.iref.e.1"></div>
    1809       <h2 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;<a id="effective.request.uri" href="#effective.request.uri">Effective Request URI</a></h2>
    1810       <p id="rfc.section.5.5.p.1">A server that receives an HTTP request message <em class="bcp14">MUST</em> reconstruct the user agent's original target URI, based on the pieces of information learned from the request-target, <a href="#header.host" class="smpl">Host</a> header field, and connection context, in order to identify the intended target resource and properly service the request.
    1811          The URI derived from this reconstruction process is referred to as the "<dfn>effective request URI</dfn>".
    1812       </p>
    1813       <p id="rfc.section.5.5.p.2">For a user agent, the effective request URI is the target URI.</p>
    1814       <p id="rfc.section.5.5.p.3">If the request-target is in absolute-form, then the effective request URI is the same as the request-target. Otherwise, the
    1815          effective request URI is constructed as follows.
    1816       </p>
    1817       <p id="rfc.section.5.5.p.4">If the request is received over a TLS-secured TCP connection, then the effective request URI's scheme is "https"; otherwise,
    1818          the scheme is "http".
    1819       </p>
    1820       <p id="rfc.section.5.5.p.5">If the request-target is in authority-form, then the effective request URI's authority component is the same as the request-target.
    1821          Otherwise, if a <a href="#header.host" class="smpl">Host</a> header field is supplied with a non-empty field-value, then the authority component is the same as the Host field-value. Otherwise,
    1822          the authority component is the concatenation of the default host name configured for the server, a colon (":"), and the connection's
    1823          incoming TCP port number in decimal form.
    1824       </p>
    1825       <p id="rfc.section.5.5.p.6">If the request-target is in authority-form or asterisk-form, then the effective request URI's combined path and query component
    1826          is empty. Otherwise, the combined path and query component is the same as the request-target.
    1827       </p>
    1828       <p id="rfc.section.5.5.p.7">The components of the effective request URI, once determined as above, can be combined into absolute-URI form by concatenating
    1829          the scheme, "://", authority, and combined path and query component.
    1830       </p>
    1831       <div id="rfc.figure.u.47"></div>
    1832       <p>Example 1: the following message received over an insecure TCP connection</p>  <pre class="text">GET /pub/WWW/TheProject.html HTTP/1.1
     1901               to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.
     1902            </p>
     1903            <p id="rfc.section.5.4.p.7">When a proxy receives a request with an absolute-form of request-target, the proxy <em class="bcp14">MUST</em> ignore the received Host header field (if any) and instead replace it with the host information of the request-target. If
     1904               the proxy forwards the request, it <em class="bcp14">MUST</em> generate a new Host field-value based on the received request-target rather than forward the received Host field-value.
     1905            </p>
     1906            <p id="rfc.section.5.4.p.8">Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to
     1907               poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it
     1908               relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache,
     1909               without first verifying that the intercepted connection is targeting a valid IP address for that host.
     1910            </p>
     1911            <p id="rfc.section.5.4.p.9">A server <em class="bcp14">MUST</em> respond with a <a href="p2-semantics.html#status.400" class="smpl">400 (Bad Request)</a> status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than
     1912               one Host header field or a Host header field with an invalid field-value.
     1913            </p>
     1914         </div>
     1915         <div id="effective.request.uri">
     1916            <div id="rfc.iref.e.1"></div>
     1917            <h2 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;<a href="#effective.request.uri">Effective Request URI</a></h2>
     1918            <p id="rfc.section.5.5.p.1">A server that receives an HTTP request message <em class="bcp14">MUST</em> reconstruct the user agent's original target URI, based on the pieces of information learned from the request-target, <a href="#header.host" class="smpl">Host</a> header field, and connection context, in order to identify the intended target resource and properly service the request.
     1919               The URI derived from this reconstruction process is referred to as the "<dfn>effective request URI</dfn>".
     1920            </p>
     1921            <p id="rfc.section.5.5.p.2">For a user agent, the effective request URI is the target URI.</p>
     1922            <p id="rfc.section.5.5.p.3">If the request-target is in absolute-form, then the effective request URI is the same as the request-target. Otherwise, the
     1923               effective request URI is constructed as follows.
     1924            </p>
     1925            <p id="rfc.section.5.5.p.4">If the request is received over a TLS-secured TCP connection, then the effective request URI's scheme is "https"; otherwise,
     1926               the scheme is "http".
     1927            </p>
     1928            <p id="rfc.section.5.5.p.5">If the request-target is in authority-form, then the effective request URI's authority component is the same as the request-target.
     1929               Otherwise, if a <a href="#header.host" class="smpl">Host</a> header field is supplied with a non-empty field-value, then the authority component is the same as the Host field-value. Otherwise,
     1930               the authority component is the concatenation of the default host name configured for the server, a colon (":"), and the connection's
     1931               incoming TCP port number in decimal form.
     1932            </p>
     1933            <p id="rfc.section.5.5.p.6">If the request-target is in authority-form or asterisk-form, then the effective request URI's combined path and query component
     1934               is empty. Otherwise, the combined path and query component is the same as the request-target.
     1935            </p>
     1936            <p id="rfc.section.5.5.p.7">The components of the effective request URI, once determined as above, can be combined into absolute-URI form by concatenating
     1937               the scheme, "://", authority, and combined path and query component.
     1938            </p>
     1939            <div id="rfc.figure.u.47"></div>
     1940            <p>Example 1: the following message received over an insecure TCP connection</p><pre class="text">GET /pub/WWW/TheProject.html HTTP/1.1
    18331941Host: www.example.org:8080
    1834 </pre> <div id="rfc.figure.u.48"></div>
    1835       <p>has an effective request URI of</p>  <pre class="text">http://www.example.org:8080/pub/WWW/TheProject.html
    1836 </pre> <div id="rfc.figure.u.49"></div>
    1837       <p>Example 2: the following message received over a TLS-secured TCP connection</p>  <pre class="text">OPTIONS * HTTP/1.1
     1942</pre><div id="rfc.figure.u.48"></div>
     1943            <p>has an effective request URI of</p><pre class="text">http://www.example.org:8080/pub/WWW/TheProject.html
     1944</pre><div id="rfc.figure.u.49"></div>
     1945            <p>Example 2: the following message received over a TLS-secured TCP connection</p><pre class="text">OPTIONS * HTTP/1.1
    18381946Host: www.example.org
    1839 </pre> <div id="rfc.figure.u.50"></div>
    1840       <p>has an effective request URI of</p>  <pre class="text">https://www.example.org
    1841 </pre> <p id="rfc.section.5.5.p.12">An origin server that does not allow resources to differ by requested host <em class="bcp14">MAY</em> ignore the <a href="#header.host" class="smpl">Host</a> field-value and instead replace it with a configured server name when constructing the effective request URI.
    1842       </p>
    1843       <p id="rfc.section.5.5.p.13">Recipients of an HTTP/1.0 request that lacks a <a href="#header.host" class="smpl">Host</a> 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 guess
    1844          the effective request URI's authority component.
    1845       </p>
    1846       <h2 id="rfc.section.5.6"><a href="#rfc.section.5.6">5.6</a>&nbsp;<a id="associating.response.to.request" href="#associating.response.to.request">Associating a Response to a Request</a></h2>
    1847       <p id="rfc.section.5.6.p.1">HTTP does not include a request identifier for associating a given request message with its corresponding one or more response
    1848          messages. Hence, it relies on the order of response arrival to correspond exactly to the order in which requests are made
    1849          on the same connection. More than one response message per request only occurs when one or more informational responses (<a href="p2-semantics.html#status.1xx" class="smpl">1xx</a>, see <a href="p2-semantics.html#status.1xx" title="Informational 1xx">Section 6.2</a> of <a href="#Part2" id="rfc.xref.Part2.21"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) precede a final response to the same request.
    1850       </p>
    1851       <p id="rfc.section.5.6.p.2">A client that has more than one outstanding request on a connection <em class="bcp14">MUST</em> maintain a list of outstanding requests in the order sent and <em class="bcp14">MUST</em> associate each received response message on that connection to the highest ordered request that has not yet received a final
    1852          (non-<a href="p2-semantics.html#status.1xx" class="smpl">1xx</a>) response.
    1853       </p>
    1854       <h2 id="rfc.section.5.7"><a href="#rfc.section.5.7">5.7</a>&nbsp;<a id="message.forwarding" href="#message.forwarding">Message Forwarding</a></h2>
    1855       <p id="rfc.section.5.7.p.1">As described in <a href="#intermediaries" title="Intermediaries">Section&nbsp;2.3</a>, intermediaries can serve a variety of roles in the processing of HTTP requests and responses. Some intermediaries are used
    1856          to improve performance or availability. Others are used for access control or to filter content. Since an HTTP stream has
    1857          characteristics similar to a pipe-and-filter architecture, there are no inherent limits to the extent an intermediary can
    1858          enhance (or interfere) with either direction of the stream.
    1859       </p>
    1860       <p id="rfc.section.5.7.p.2">Intermediaries that forward a message <em class="bcp14">MUST</em> implement the <a href="#header.connection" class="smpl">Connection</a> header field, as specified in <a href="#header.connection" id="rfc.xref.header.connection.5" title="Connection">Section&nbsp;6.1</a>, to exclude fields that are only intended for the incoming connection.
    1861       </p>
    1862       <p id="rfc.section.5.7.p.3">In order to avoid request loops, a proxy that forwards requests to other proxies <em class="bcp14">MUST</em> be able to recognize and exclude all of its own server names, including any aliases, local variations, or literal IP addresses.
    1863       </p>
    1864       <div id="rfc.iref.v.1"></div>
    1865       <h3 id="rfc.section.5.7.1"><a href="#rfc.section.5.7.1">5.7.1</a>&nbsp;<a id="header.via" href="#header.via">Via</a></h3>
    1866       <p id="rfc.section.5.7.1.p.1">The "Via" header field <em class="bcp14">MUST</em> be sent by a proxy or gateway in forwarded messages to indicate the intermediate protocols and recipients between the user
    1867          agent and the server on requests, and between the origin server and the client on responses. It is analogous to the "Received"
    1868          field used by email systems (<a href="http://tools.ietf.org/html/rfc5322#section-3.6.7">Section 3.6.7</a> of <a href="#RFC5322" id="rfc.xref.RFC5322.3"><cite title="Internet Message Format">[RFC5322]</cite></a>). Via is used in HTTP for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of
    1869          all senders along the request/response chain.
    1870       </p>
    1871       <div id="rfc.figure.u.51"></div><pre class="inline"><span id="rfc.iref.g.86"></span><span id="rfc.iref.g.87"></span><span id="rfc.iref.g.88"></span><span id="rfc.iref.g.89"></span><span id="rfc.iref.g.90"></span><span id="rfc.iref.g.91"></span>  <a href="#header.via" class="smpl">Via</a>               = 1#( <a href="#header.via" class="smpl">received-protocol</a> <a href="#rule.whitespace" class="smpl">RWS</a> <a href="#header.via" class="smpl">received-by</a>
     1947</pre><div id="rfc.figure.u.50"></div>
     1948            <p>has an effective request URI of</p><pre class="text">https://www.example.org
     1949</pre><p id="rfc.section.5.5.p.12">An origin server that does not allow resources to differ by requested host <em class="bcp14">MAY</em> ignore the <a href="#header.host" class="smpl">Host</a> field-value and instead replace it with a configured server name when constructing the effective request URI.
     1950            </p>
     1951            <p id="rfc.section.5.5.p.13">Recipients of an HTTP/1.0 request that lacks a <a href="#header.host" class="smpl">Host</a> 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 guess
     1952               the effective request URI's authority component.
     1953            </p>
     1954         </div>
     1955         <div id="associating.response.to.request">
     1956            <h2 id="rfc.section.5.6"><a href="#rfc.section.5.6">5.6</a>&nbsp;<a href="#associating.response.to.request">Associating a Response to a Request</a></h2>
     1957            <p id="rfc.section.5.6.p.1">HTTP does not include a request identifier for associating a given request message with its corresponding one or more response
     1958               messages. Hence, it relies on the order of response arrival to correspond exactly to the order in which requests are made
     1959               on the same connection. More than one response message per request only occurs when one or more informational responses (<a href="p2-semantics.html#status.1xx" class="smpl">1xx</a>, see <a href="p2-semantics.html#status.1xx" title="Informational 1xx">Section 6.2</a> of <a href="#Part2" id="rfc.xref.Part2.21"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) precede a final response to the same request.
     1960            </p>
     1961            <p id="rfc.section.5.6.p.2">A client that has more than one outstanding request on a connection <em class="bcp14">MUST</em> maintain a list of outstanding requests in the order sent and <em class="bcp14">MUST</em> associate each received response message on that connection to the highest ordered request that has not yet received a final
     1962               (non-<a href="p2-semantics.html#status.1xx" class="smpl">1xx</a>) response.
     1963            </p>
     1964         </div>
     1965         <div id="message.forwarding">
     1966            <h2 id="rfc.section.5.7"><a href="#rfc.section.5.7">5.7</a>&nbsp;<a href="#message.forwarding">Message Forwarding</a></h2>
     1967            <p id="rfc.section.5.7.p.1">As described in <a href="#intermediaries" title="Intermediaries">Section&nbsp;2.3</a>, intermediaries can serve a variety of roles in the processing of HTTP requests and responses. Some intermediaries are used
     1968               to improve performance or availability. Others are used for access control or to filter content. Since an HTTP stream has
     1969               characteristics similar to a pipe-and-filter architecture, there are no inherent limits to the extent an intermediary can
     1970               enhance (or interfere) with either direction of the stream.
     1971            </p>
     1972            <p id="rfc.section.5.7.p.2">Intermediaries that forward a message <em class="bcp14">MUST</em> implement the <a href="#header.connection" class="smpl">Connection</a> header field, as specified in <a href="#header.connection" id="rfc.xref.header.connection.5" title="Connection">Section&nbsp;6.1</a>, to exclude fields that are only intended for the incoming connection.
     1973            </p>
     1974            <p id="rfc.section.5.7.p.3">In order to avoid request loops, a proxy that forwards requests to other proxies <em class="bcp14">MUST</em> be able to recognize and exclude all of its own server names, including any aliases, local variations, or literal IP addresses.
     1975            </p>
     1976            <div id="header.via">
     1977               <div id="rfc.iref.v.1"></div>
     1978               <h3 id="rfc.section.5.7.1"><a href="#rfc.section.5.7.1">5.7.1</a>&nbsp;<a href="#header.via">Via</a></h3>
     1979               <p id="rfc.section.5.7.1.p.1">The "Via" header field <em class="bcp14">MUST</em> be sent by a proxy or gateway in forwarded messages to indicate the intermediate protocols and recipients between the user
     1980                  agent and the server on requests, and between the origin server and the client on responses. It is analogous to the "Received"
     1981                  field used by email systems (<a href="https://tools.ietf.org/html/rfc5322#section-3.6.7">Section 3.6.7</a> of <a href="#RFC5322" id="rfc.xref.RFC5322.3"><cite title="Internet Message Format">[RFC5322]</cite></a>). Via is used in HTTP for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of
     1982                  all senders along the request/response chain.
     1983               </p>
     1984               <div id="rfc.figure.u.51"></div><pre class="inline"><span id="rfc.iref.g.86"></span><span id="rfc.iref.g.87"></span><span id="rfc.iref.g.88"></span><span id="rfc.iref.g.89"></span><span id="rfc.iref.g.90"></span><span id="rfc.iref.g.91"></span>  <a href="#header.via" class="smpl">Via</a>               = 1#( <a href="#header.via" class="smpl">received-protocol</a> <a href="#rule.whitespace" class="smpl">RWS</a> <a href="#header.via" class="smpl">received-by</a>
    18721985                          [ <a href="#rule.whitespace" class="smpl">RWS</a> <a href="#rule.comment" class="smpl">comment</a> ] )
    18731986  <a href="#header.via" class="smpl">received-protocol</a> = [ <a href="#header.upgrade" class="smpl">protocol-name</a> "/" ] <a href="#header.upgrade" class="smpl">protocol-version</a>
     
    18761989  <a href="#header.via" class="smpl">pseudonym</a>         = <a href="#rule.token.separators" class="smpl">token</a>
    18771990</pre><p id="rfc.section.5.7.1.p.3">The received-protocol indicates the protocol version of the message received by the server or client along each segment of
    1878          the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded
    1879          so that information about the protocol capabilities of upstream applications remains visible to all recipients.
    1880       </p>
    1881       <p id="rfc.section.5.7.1.p.4">The protocol-name is excluded if and only if it would be "HTTP". The received-by field is normally the host and optional port
    1882          number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to
    1883          be sensitive information, it <em class="bcp14">MAY</em> be replaced by a pseudonym. If the port is not given, it <em class="bcp14">MAY</em> be assumed to be the default port of the received-protocol.
    1884       </p>
    1885       <p id="rfc.section.5.7.1.p.5">Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient <em class="bcp14">MUST</em> append its information such that the end result is ordered according to the sequence of forwarding applications.
    1886       </p>
    1887       <p id="rfc.section.5.7.1.p.6">Comments <em class="bcp14">MAY</em> be used in the Via header field to identify the software of each recipient, analogous to the <a href="p2-semantics.html#header.user-agent" class="smpl">User-Agent</a> and <a href="p2-semantics.html#header.server" class="smpl">Server</a> header fields. However, all comments in the Via field are optional and <em class="bcp14">MAY</em> be removed by any recipient prior to forwarding the message.
    1888       </p>
    1889       <p id="rfc.section.5.7.1.p.7">For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses
    1890          HTTP/1.1 to forward the request to a public proxy at p.example.net, which completes the request by forwarding it to the origin
    1891          server at www.example.com. The request received by www.example.com would then have the following Via header field:
    1892       </p>
    1893       <div id="rfc.figure.u.52"></div><pre class="text">  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
     1991                  the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded
     1992                  so that information about the protocol capabilities of upstream applications remains visible to all recipients.
     1993               </p>
     1994               <p id="rfc.section.5.7.1.p.4">The protocol-name is excluded if and only if it would be "HTTP". The received-by field is normally the host and optional port
     1995                  number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to
     1996                  be sensitive information, it <em class="bcp14">MAY</em> be replaced by a pseudonym. If the port is not given, it <em class="bcp14">MAY</em> be assumed to be the default port of the received-protocol.
     1997               </p>
     1998               <p id="rfc.section.5.7.1.p.5">Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient <em class="bcp14">MUST</em> append its information such that the end result is ordered according to the sequence of forwarding applications.
     1999               </p>
     2000               <p id="rfc.section.5.7.1.p.6">Comments <em class="bcp14">MAY</em> be used in the Via header field to identify the software of each recipient, analogous to the <a href="p2-semantics.html#header.user-agent" class="smpl">User-Agent</a> and <a href="p2-semantics.html#header.server" class="smpl">Server</a> header fields. However, all comments in the Via field are optional and <em class="bcp14">MAY</em> be removed by any recipient prior to forwarding the message.
     2001               </p>
     2002               <p id="rfc.section.5.7.1.p.7">For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses
     2003                  HTTP/1.1 to forward the request to a public proxy at p.example.net, which completes the request by forwarding it to the origin
     2004                  server at www.example.com. The request received by www.example.com would then have the following Via header field:
     2005               </p>
     2006               <div id="rfc.figure.u.52"></div><pre class="text">  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
    18942007</pre><p id="rfc.section.5.7.1.p.9">A proxy or gateway used as a portal through a network firewall <em class="bcp14">SHOULD NOT</em> forward the names and ports of hosts within the firewall region unless it is explicitly enabled to do so. If not enabled,
    1895          the received-by host of any host behind the firewall <em class="bcp14">SHOULD</em> be replaced by an appropriate pseudonym for that host.
    1896       </p>
    1897       <p id="rfc.section.5.7.1.p.10">A proxy or gateway <em class="bcp14">MAY</em> combine an ordered subsequence of Via header field entries into a single such entry if the entries have identical received-protocol
    1898          values. For example,
    1899       </p>
    1900       <div id="rfc.figure.u.53"></div><pre class="text">  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
     2008                  the received-by host of any host behind the firewall <em class="bcp14">SHOULD</em> be replaced by an appropriate pseudonym for that host.
     2009               </p>
     2010               <p id="rfc.section.5.7.1.p.10">A proxy or gateway <em class="bcp14">MAY</em> combine an ordered subsequence of Via header field entries into a single such entry if the entries have identical received-protocol
     2011                  values. For example,
     2012               </p>
     2013               <div id="rfc.figure.u.53"></div><pre class="text">  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
    19012014</pre><p id="rfc.section.5.7.1.p.12">could be collapsed to</p>
    1902       <div id="rfc.figure.u.54"></div><pre class="text">  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
     2015               <div id="rfc.figure.u.54"></div><pre class="text">  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
    19032016</pre><p id="rfc.section.5.7.1.p.14">Senders <em class="bcp14">SHOULD NOT</em> combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced
    1904          by pseudonyms. Senders <em class="bcp14">MUST NOT</em> combine entries that have different received-protocol values.
    1905       </p>
    1906       <h3 id="rfc.section.5.7.2"><a href="#rfc.section.5.7.2">5.7.2</a>&nbsp;<a id="message.transformations" href="#message.transformations">Transformations</a></h3>
    1907       <p id="rfc.section.5.7.2.p.1">Some intermediaries include features for transforming messages and their payloads. A transforming proxy might, for example,
    1908          convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. However, operational
    1909          problems might occur when these transformations are applied to payloads intended for critical applications, such as medical
    1910          imaging or scientific data analysis, particularly when integrity checks or digital signatures are used to ensure that the
    1911          payload received is identical to the original.
    1912       </p>
    1913       <p id="rfc.section.5.7.2.p.2">If a proxy receives a request-target with a host name that is not a fully qualified domain name, it <em class="bcp14">MAY</em> add its own domain to the host name it received when forwarding the request. A proxy <em class="bcp14">MUST NOT</em> change the host name if it is a fully qualified domain name.
    1914       </p>
    1915       <p id="rfc.section.5.7.2.p.3">A proxy <em class="bcp14">MUST NOT</em> modify the "absolute-path" and "query" parts of the received request-target when forwarding it to the next inbound server,
    1916          except as noted above to replace an empty path with "/" or "*".
    1917       </p>
    1918       <p id="rfc.section.5.7.2.p.4">A proxy <em class="bcp14">MUST NOT</em> modify header fields that provide information about the end points of the communication chain, the resource state, or the
    1919          selected representation. A proxy <em class="bcp14">MAY</em> change the message body through application or removal of a transfer coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>).
    1920       </p>
    1921       <p id="rfc.section.5.7.2.p.5">A non-transforming proxy <em class="bcp14">MUST</em> preserve the message payload (<a href="p2-semantics.html#payload" title="Payload Semantics">Section 3.3</a> of <a href="#Part2" id="rfc.xref.Part2.22"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). A transforming proxy <em class="bcp14">MUST</em> preserve the payload of a message that contains the no-transform cache-control directive.
    1922       </p>
    1923       <p id="rfc.section.5.7.2.p.6">A transforming proxy <em class="bcp14">MAY</em> transform the payload of a message that does not contain the no-transform cache-control directive; if the payload is transformed,
    1924          the transforming proxy <em class="bcp14">MUST</em> add a Warning 214 (Transformation applied) header field if one does not already appear in the message (see <a href="p6-cache.html#header.warning" title="Warning">Section 7.5</a> of <a href="#Part6" id="rfc.xref.Part6.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    1925       </p>
    1926       <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="connection.management" href="#connection.management">Connection Management</a></h1>
    1927       <p id="rfc.section.6.p.1">HTTP messaging is independent of the underlying transport or session-layer connection protocol(s). HTTP only presumes a reliable
    1928          transport with in-order delivery of requests and the corresponding in-order delivery of responses. The mapping of HTTP request
    1929          and response structures onto the data units of an underlying transport protocol is outside the scope of this specification.
    1930       </p>
    1931       <p id="rfc.section.6.p.2">As described in <a href="#connecting.inbound" title="Connecting Inbound">Section&nbsp;5.2</a>, the specific connection protocols to be used for an HTTP interaction are determined by client configuration and the <a href="#target-resource" class="smpl">target URI</a>. For example, the "http" URI scheme (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) indicates a default connection of TCP over IP, with a default TCP port of 80, but the client might be configured to use
    1932          a proxy via some other connection, port, or protocol.
    1933       </p>
    1934       <p id="rfc.section.6.p.3">HTTP implementations are expected to engage in connection management, which includes maintaining the state of current connections,
    1935          establishing a new connection or reusing an existing connection, processing messages received on a connection, detecting connection
    1936          failures, and closing each connection. Most clients maintain multiple connections in parallel, including more than one connection
    1937          per server endpoint. Most servers are designed to maintain thousands of concurrent connections, while controlling request
    1938          queues to enable fair use and detect denial of service attacks.
    1939       </p>
    1940       <div id="rfc.iref.c.11"></div>
    1941       <div id="rfc.iref.c.12"></div>
    1942       <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="header.connection" href="#header.connection">Connection</a></h2>
    1943       <p id="rfc.section.6.1.p.1">The "Connection" header field allows the sender to indicate desired control options for the current connection. In order to
    1944          avoid confusing downstream recipients, a proxy or gateway <em class="bcp14">MUST</em> remove or replace any received connection options before forwarding the message.
    1945       </p>
    1946       <p id="rfc.section.6.1.p.2">When a header field aside from Connection is used to supply control information for or about the current connection, the sender <em class="bcp14">MUST</em> list the corresponding field-name within the "Connection" header field. A proxy or gateway <em class="bcp14">MUST</em> parse a received Connection header field before a message is forwarded and, for each connection-option in this field, remove
    1947          any header field(s) from the message with the same name as the connection-option, and then remove the Connection header field
    1948          itself (or replace it with the intermediary's own connection options for the forwarded message).
    1949       </p>
    1950       <p id="rfc.section.6.1.p.3">Hence, the Connection header field provides a declarative way of distinguishing header fields that are only intended for the
    1951          immediate recipient ("hop-by-hop") from those fields that are intended for all recipients on the chain ("end-to-end"), enabling
    1952          the message to be self-descriptive and allowing future connection-specific extensions to be deployed without fear that they
    1953          will be blindly forwarded by older intermediaries.
    1954       </p>
    1955       <p id="rfc.section.6.1.p.4">The Connection header field's value has the following grammar:</p>
    1956       <div id="rfc.figure.u.55"></div><pre class="inline"><span id="rfc.iref.g.92"></span><span id="rfc.iref.g.93"></span>  <a href="#header.connection" class="smpl">Connection</a>        = 1#<a href="#header.connection" class="smpl">connection-option</a>
     2017                  by pseudonyms. Senders <em class="bcp14">MUST NOT</em> combine entries that have different received-protocol values.
     2018               </p>
     2019            </div>
     2020            <div id="message.transformations">
     2021               <h3 id="rfc.section.5.7.2"><a href="#rfc.section.5.7.2">5.7.2</a>&nbsp;<a href="#message.transformations">Transformations</a></h3>
     2022               <p id="rfc.section.5.7.2.p.1">Some intermediaries include features for transforming messages and their payloads. A transforming proxy might, for example,
     2023                  convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. However, operational
     2024                  problems might occur when these transformations are applied to payloads intended for critical applications, such as medical
     2025                  imaging or scientific data analysis, particularly when integrity checks or digital signatures are used to ensure that the
     2026                  payload received is identical to the original.
     2027               </p>
     2028               <p id="rfc.section.5.7.2.p.2">If a proxy receives a request-target with a host name that is not a fully qualified domain name, it <em class="bcp14">MAY</em> add its own domain to the host name it received when forwarding the request. A proxy <em class="bcp14">MUST NOT</em> change the host name if it is a fully qualified domain name.
     2029               </p>
     2030               <p id="rfc.section.5.7.2.p.3">A proxy <em class="bcp14">MUST NOT</em> modify the "absolute-path" and "query" parts of the received request-target when forwarding it to the next inbound server,
     2031                  except as noted above to replace an empty path with "/" or "*".
     2032               </p>
     2033               <p id="rfc.section.5.7.2.p.4">A proxy <em class="bcp14">MUST NOT</em> modify header fields that provide information about the end points of the communication chain, the resource state, or the
     2034                  selected representation. A proxy <em class="bcp14">MAY</em> change the message body through application or removal of a transfer coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>).
     2035               </p>
     2036               <p id="rfc.section.5.7.2.p.5">A non-transforming proxy <em class="bcp14">MUST</em> preserve the message payload (<a href="p2-semantics.html#payload" title="Payload Semantics">Section 3.3</a> of <a href="#Part2" id="rfc.xref.Part2.22"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). A transforming proxy <em class="bcp14">MUST</em> preserve the payload of a message that contains the no-transform cache-control directive.
     2037               </p>
     2038               <p id="rfc.section.5.7.2.p.6">A transforming proxy <em class="bcp14">MAY</em> transform the payload of a message that does not contain the no-transform cache-control directive; if the payload is transformed,
     2039                  the transforming proxy <em class="bcp14">MUST</em> add a Warning 214 (Transformation applied) header field if one does not already appear in the message (see <a href="p6-cache.html#header.warning" title="Warning">Section 7.5</a> of <a href="#Part6" id="rfc.xref.Part6.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
     2040               </p>
     2041            </div>
     2042         </div>
     2043      </div>
     2044      <div id="connection.management">
     2045         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#connection.management">Connection Management</a></h1>
     2046         <p id="rfc.section.6.p.1">HTTP messaging is independent of the underlying transport or session-layer connection protocol(s). HTTP only presumes a reliable
     2047            transport with in-order delivery of requests and the corresponding in-order delivery of responses. The mapping of HTTP request
     2048            and response structures onto the data units of an underlying transport protocol is outside the scope of this specification.
     2049         </p>
     2050         <p id="rfc.section.6.p.2">As described in <a href="#connecting.inbound" title="Connecting Inbound">Section&nbsp;5.2</a>, the specific connection protocols to be used for an HTTP interaction are determined by client configuration and the <a href="#target-resource" class="smpl">target URI</a>. For example, the "http" URI scheme (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) indicates a default connection of TCP over IP, with a default TCP port of 80, but the client might be configured to use
     2051            a proxy via some other connection, port, or protocol.
     2052         </p>
     2053         <p id="rfc.section.6.p.3">HTTP implementations are expected to engage in connection management, which includes maintaining the state of current connections,
     2054            establishing a new connection or reusing an existing connection, processing messages received on a connection, detecting connection
     2055            failures, and closing each connection. Most clients maintain multiple connections in parallel, including more than one connection
     2056            per server endpoint. Most servers are designed to maintain thousands of concurrent connections, while controlling request
     2057            queues to enable fair use and detect denial of service attacks.
     2058         </p>
     2059         <div id="header.connection">
     2060            <div id="rfc.iref.c.11"></div>
     2061            <div id="rfc.iref.c.12"></div>
     2062            <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a href="#header.connection">Connection</a></h2>
     2063            <p id="rfc.section.6.1.p.1">The "Connection" header field allows the sender to indicate desired control options for the current connection. In order to
     2064               avoid confusing downstream recipients, a proxy or gateway <em class="bcp14">MUST</em> remove or replace any received connection options before forwarding the message.
     2065            </p>
     2066            <p id="rfc.section.6.1.p.2">When a header field aside from Connection is used to supply control information for or about the current connection, the sender <em class="bcp14">MUST</em> list the corresponding field-name within the "Connection" header field. A proxy or gateway <em class="bcp14">MUST</em> parse a received Connection header field before a message is forwarded and, for each connection-option in this field, remove
     2067               any header field(s) from the message with the same name as the connection-option, and then remove the Connection header field
     2068               itself (or replace it with the intermediary's own connection options for the forwarded message).
     2069            </p>
     2070            <p id="rfc.section.6.1.p.3">Hence, the Connection header field provides a declarative way of distinguishing header fields that are only intended for the
     2071               immediate recipient ("hop-by-hop") from those fields that are intended for all recipients on the chain ("end-to-end"), enabling
     2072               the message to be self-descriptive and allowing future connection-specific extensions to be deployed without fear that they
     2073               will be blindly forwarded by older intermediaries.
     2074            </p>
     2075            <p id="rfc.section.6.1.p.4">The Connection header field's value has the following grammar:</p>
     2076            <div id="rfc.figure.u.55"></div><pre class="inline"><span id="rfc.iref.g.92"></span><span id="rfc.iref.g.93"></span>  <a href="#header.connection" class="smpl">Connection</a>        = 1#<a href="#header.connection" class="smpl">connection-option</a>
    19572077  <a href="#header.connection" class="smpl">connection-option</a> = <a href="#rule.token.separators" class="smpl">token</a>
    19582078</pre><p id="rfc.section.6.1.p.6">Connection options are case-insensitive.</p>
    1959       <p id="rfc.section.6.1.p.7">A sender <em class="bcp14">MUST NOT</em> send a connection option corresponding to a header field that is intended for all recipients of the payload. For example, <a href="p6-cache.html#header.cache-control" class="smpl">Cache-Control</a> is never appropriate as a connection option (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 7.2</a> of <a href="#Part6" id="rfc.xref.Part6.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    1960       </p>
    1961       <p id="rfc.section.6.1.p.8">The connection options do not have to correspond to a header field present in the message, since a connection-specific header
    1962          field might not be needed if there are no parameters associated with that connection option. Recipients that trigger certain
    1963          connection behavior based on the presence of connection options <em class="bcp14">MUST</em> do so based on the presence of the connection-option rather than only the presence of the optional header field. In other
    1964          words, if the connection option is received as a header field but not indicated within the Connection field-value, then the
    1965          recipient <em class="bcp14">MUST</em> ignore the connection-specific header field because it has likely been forwarded by an intermediary that is only partially
    1966          conformant.
    1967       </p>
    1968       <p id="rfc.section.6.1.p.9">When defining new connection options, specifications ought to carefully consider existing deployed header fields and ensure
    1969          that the new connection option does not share the same name as an unrelated header field that might already be deployed. Defining
    1970          a new connection option essentially reserves that potential field-name for carrying additional information related to the
    1971          connection option, since it would be unwise for senders to use that field-name for anything else.
    1972       </p>
    1973       <p id="rfc.section.6.1.p.10">The "<dfn>close</dfn>" connection option is defined for a sender to signal that this connection will be closed after completion of the response.
    1974          For example,
    1975       </p>
    1976       <div id="rfc.figure.u.56"></div><pre class="text">  Connection: close
     2079            <p id="rfc.section.6.1.p.7">A sender <em class="bcp14">MUST NOT</em> send a connection option corresponding to a header field that is intended for all recipients of the payload. For example, <a href="p6-cache.html#header.cache-control" class="smpl">Cache-Control</a> is never appropriate as a connection option (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 7.2</a> of <a href="#Part6" id="rfc.xref.Part6.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
     2080            </p>
     2081            <p id="rfc.section.6.1.p.8">The connection options do not have to correspond to a header field present in the message, since a connection-specific header
     2082               field might not be needed if there are no parameters associated with that connection option. Recipients that trigger certain
     2083               connection behavior based on the presence of connection options <em class="bcp14">MUST</em> do so based on the presence of the connection-option rather than only the presence of the optional header field. In other
     2084               words, if the connection option is received as a header field but not indicated within the Connection field-value, then the
     2085               recipient <em class="bcp14">MUST</em> ignore the connection-specific header field because it has likely been forwarded by an intermediary that is only partially
     2086               conformant.
     2087            </p>
     2088            <p id="rfc.section.6.1.p.9">When defining new connection options, specifications ought to carefully consider existing deployed header fields and ensure
     2089               that the new connection option does not share the same name as an unrelated header field that might already be deployed. Defining
     2090               a new connection option essentially reserves that potential field-name for carrying additional information related to the
     2091               connection option, since it would be unwise for senders to use that field-name for anything else.
     2092            </p>
     2093            <p id="rfc.section.6.1.p.10">The "<dfn>close</dfn>" connection option is defined for a sender to signal that this connection will be closed after completion of the response.
     2094               For example,
     2095            </p>
     2096            <div id="rfc.figure.u.56"></div><pre class="text">  Connection: close
    19772097</pre><p id="rfc.section.6.1.p.12">in either the request or the response header fields indicates that the connection <em class="bcp14">MUST</em> be closed after the current request/response is complete (<a href="#persistent.tear-down" id="rfc.xref.persistent.tear-down.1" title="Tear-down">Section&nbsp;6.6</a>).
    1978       </p>
    1979       <p id="rfc.section.6.1.p.13">A client that does not support <a href="#persistent.connections" class="smpl">persistent connections</a>  <em class="bcp14">MUST</em> send the "close" connection option in every request message.
    1980       </p>
    1981       <p id="rfc.section.6.1.p.14">A server that does not support <a href="#persistent.connections" class="smpl">persistent connections</a>  <em class="bcp14">MUST</em> send the "close" connection option in every response message that does not have a <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a> status code.
    1982       </p>
    1983       <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="persistent.establishment" href="#persistent.establishment">Establishment</a></h2>
    1984       <p id="rfc.section.6.2.p.1">It is beyond the scope of this specification to describe how connections are established via various transport or session-layer
    1985          protocols. Each connection applies to only one transport link.
    1986       </p>
    1987       <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a id="persistent.connections" href="#persistent.connections">Persistence</a></h2>
    1988       <p id="rfc.section.6.3.p.1">HTTP/1.1 defaults to the use of "<dfn>persistent connections</dfn>", allowing multiple requests and responses to be carried over a single connection. The "<a href="#header.connection" class="smpl">close</a>" connection-option is used to signal that a connection will not persist after the current request/response. HTTP implementations <em class="bcp14">SHOULD</em> support persistent connections.
    1989       </p>
    1990       <p id="rfc.section.6.3.p.2">A recipient determines whether a connection is persistent or not based on the most recently received message's protocol version
    1991          and <a href="#header.connection" class="smpl">Connection</a> header field (if any):
    1992       </p>
    1993       <ul>
    1994          <li>If the <a href="#header.connection" class="smpl">close</a> connection option is present, the connection will not persist after the current response; else,
    1995          </li>
    1996          <li>If the received protocol is HTTP/1.1 (or later), the connection will persist after the current response; else,</li>
    1997          <li>If the received protocol is HTTP/1.0, the "keep-alive" connection option is present, the recipient is not a proxy, and the
    1998             recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism, the connection will persist after the current response; otherwise,
    1999          </li>
    2000          <li>The connection will close after the current response.</li>
    2001       </ul>
    2002       <p id="rfc.section.6.3.p.3">A server <em class="bcp14">MAY</em> assume that an HTTP/1.1 client intends to maintain a persistent connection until a <a href="#header.connection" class="smpl">close</a> connection option is received in a request.
    2003       </p>
    2004       <p id="rfc.section.6.3.p.4">A client <em class="bcp14">MAY</em> reuse a persistent connection until it sends or receives a <a href="#header.connection" class="smpl">close</a> connection option or receives an HTTP/1.0 response without a "keep-alive" connection option.
    2005       </p>
    2006       <p id="rfc.section.6.3.p.5">In order to remain persistent, all messages on a 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.body" title="Message Body">Section&nbsp;3.3</a>. A server <em class="bcp14">MUST</em> read the entire request message body or close the connection after sending its response, since otherwise the remaining data
    2007          on a persistent connection would be misinterpreted as the next request. Likewise, a client <em class="bcp14">MUST</em> read the entire response message body if it intends to reuse the same connection for a subsequent request.
    2008       </p>
    2009       <p id="rfc.section.6.3.p.6">A proxy server <em class="bcp14">MUST NOT</em> maintain a persistent connection with an HTTP/1.0 client (see <a href="http://tools.ietf.org/html/rfc2068#section-19.7.1">Section 19.7.1</a> of <a href="#RFC2068" id="rfc.xref.RFC2068.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> for information and discussion of the problems with the Keep-Alive header field implemented by many HTTP/1.0 clients).
    2010       </p>
    2011       <p id="rfc.section.6.3.p.7">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="Keep-Alive Connections">Appendix&nbsp;A.1.2</a> for more information on backward compatibility with HTTP/1.0 clients.
    2012       </p>
    2013       <h3 id="rfc.section.6.3.1"><a href="#rfc.section.6.3.1">6.3.1</a>&nbsp;<a id="persistent.retrying.requests" href="#persistent.retrying.requests">Retrying Requests</a></h3>
    2014       <p id="rfc.section.6.3.1.p.1">Connections can be closed at any time, with or without intention. Implementations ought to anticipate the need to recover
    2015          from asynchronous close events.
    2016       </p>
    2017       <p id="rfc.section.6.3.1.p.2">When an inbound connection is closed prematurely, a client <em class="bcp14">MAY</em> open a new connection and automatically retransmit an aborted sequence of requests if all of those requests have idempotent
    2018          methods (<a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 4.2.2</a> of <a href="#Part2" id="rfc.xref.Part2.23"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). A proxy <em class="bcp14">MUST NOT</em> automatically retry non-idempotent requests.
    2019       </p>
    2020       <p id="rfc.section.6.3.1.p.3">A user agent <em class="bcp14">MUST NOT</em> automatically retry a request with a non-idempotent method unless it has some means to know that the request semantics are
    2021          actually idempotent, regardless of the method, or some means to detect that the original request was never applied. For example,
    2022          a user agent that knows (through design or configuration) that a POST request to a given resource is safe can repeat that
    2023          request automatically. Likewise, a user agent designed specifically to operate on a version control repository might be able
    2024          to recover from partial failure conditions by checking the target resource revision(s) after a failed connection, reverting
    2025          or fixing any changes that were partially applied, and then automatically retrying the requests that failed.
    2026       </p>
    2027       <p id="rfc.section.6.3.1.p.4">An automatic retry <em class="bcp14">SHOULD NOT</em> be repeated if it fails.
    2028       </p>
    2029       <h3 id="rfc.section.6.3.2"><a href="#rfc.section.6.3.2">6.3.2</a>&nbsp;<a id="pipelining" href="#pipelining">Pipelining</a></h3>
    2030       <p id="rfc.section.6.3.2.p.1">A client that supports persistent connections <em class="bcp14">MAY</em> "<dfn>pipeline</dfn>" its requests (i.e., send multiple requests without waiting for each response). A server <em class="bcp14">MAY</em> process a sequence of pipelined requests in parallel if they all have safe methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.24"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>), but <em class="bcp14">MUST</em> send the corresponding responses in the same order that the requests were received.
    2031       </p>
    2032       <p id="rfc.section.6.3.2.p.2">A client that pipelines requests <em class="bcp14">MUST</em> be prepared to retry those requests if the connection closes before it receives all of the corresponding responses. A client
    2033          that assumes a persistent connection and pipelines immediately after connection establishment <em class="bcp14">MUST NOT</em> pipeline on a retry connection until it knows the connection is persistent.
    2034       </p>
    2035       <p id="rfc.section.6.3.2.p.3">Idempotent methods (<a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 4.2.2</a> of <a href="#Part2" id="rfc.xref.Part2.25"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) are significant to pipelining because they can be automatically retried after a connection failure. A user agent <em class="bcp14">SHOULD NOT</em> pipeline requests after a non-idempotent method until the final response status code for that method has been received, unless
    2036          the user agent has a means to detect and recover from partial failure conditions involving the pipelined sequence.
    2037       </p>
    2038       <p id="rfc.section.6.3.2.p.4">An intermediary that receives pipelined requests <em class="bcp14">MAY</em> pipeline those requests when forwarding them inbound, since it can rely on the outbound user agent(s) to determine what requests
    2039          can be safely pipelined. If the inbound connection fails before receiving a response, the pipelining intermediary <em class="bcp14">MAY</em> attempt to retry a sequence of requests that have yet to receive a response if the requests all have idempotent methods; otherwise,
    2040          the pipelining intermediary <em class="bcp14">SHOULD</em> forward any received responses and then close the corresponding outbound connection(s) so that the outbound user agent(s)
    2041          can recover accordingly.
    2042       </p>
    2043       <h2 id="rfc.section.6.4"><a href="#rfc.section.6.4">6.4</a>&nbsp;<a id="persistent.concurrency" href="#persistent.concurrency">Concurrency</a></h2>
    2044       <p id="rfc.section.6.4.p.1">Clients <em class="bcp14">SHOULD</em> limit the number of simultaneous connections that they maintain to a given server.
    2045       </p>
    2046       <p id="rfc.section.6.4.p.2">Previous revisions of HTTP gave a specific number of connections as a ceiling, but this was found to be impractical for many
    2047          applications. As a result, this specification does not mandate a particular maximum number of connections, but instead encourages
    2048          clients to be conservative when opening multiple connections.
    2049       </p>
    2050       <p id="rfc.section.6.4.p.3">Multiple connections are typically used to avoid the "head-of-line blocking" problem, wherein a request that takes significant
    2051          server-side processing and/or has a large payload blocks subsequent requests on the same connection. However, each connection
    2052          consumes server resources. Furthermore, using multiple connections can cause undesirable side effects in congested networks.
    2053       </p>
    2054       <p id="rfc.section.6.4.p.4">Note that servers might reject traffic that they deem abusive, including an excessive number of connections from a client.</p>
    2055       <h2 id="rfc.section.6.5"><a href="#rfc.section.6.5">6.5</a>&nbsp;<a id="persistent.failures" href="#persistent.failures">Failures and Time-outs</a></h2>
    2056       <p id="rfc.section.6.5.p.1">Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers
    2057          might make this a higher value since it is likely that the client will be making more connections through the same server.
    2058          The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client
    2059          or the server.
    2060       </p>
    2061       <p id="rfc.section.6.5.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
    2062          not detect the other side's close promptly it could cause unnecessary resource drain on the network.
    2063       </p>
    2064       <p id="rfc.section.6.5.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
    2065          that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed
    2066          while it was idle, but from the client's point of view, a request is in progress.
    2067       </p>
    2068       <p id="rfc.section.6.5.p.4">Servers <em class="bcp14">SHOULD</em> maintain persistent connections and allow the underlying transport's flow control mechanisms to resolve temporary overloads,
    2069          rather than terminate connections with the expectation that clients will retry. The latter technique can exacerbate network
    2070          congestion.
    2071       </p>
    2072       <p id="rfc.section.6.5.p.5">A client sending a message body <em class="bcp14">SHOULD</em> monitor the network connection for an error status code while it is transmitting the request. If the client sees an error
    2073          status code, it <em class="bcp14">SHOULD</em> immediately cease transmitting the body and close the connection.
    2074       </p>
    2075       <div id="rfc.iref.c.13"></div>
    2076       <div id="rfc.iref.c.14"></div>
    2077       <h2 id="rfc.section.6.6"><a href="#rfc.section.6.6">6.6</a>&nbsp;<a id="persistent.tear-down" href="#persistent.tear-down">Tear-down</a></h2>
    2078       <p id="rfc.section.6.6.p.1">The <a href="#header.connection" class="smpl">Connection</a> header field (<a href="#header.connection" id="rfc.xref.header.connection.6" title="Connection">Section&nbsp;6.1</a>) provides a "<a href="#header.connection" class="smpl">close</a>" connection option that a sender <em class="bcp14">SHOULD</em> send when it wishes to close the connection after the current request/response pair.
    2079       </p>
    2080       <p id="rfc.section.6.6.p.2">A client that sends a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST NOT</em> send further requests on that connection (after the one containing <a href="#header.connection" class="smpl">close</a>) and <em class="bcp14">MUST</em> close the connection after reading the final response message corresponding to this request.
    2081       </p>
    2082       <p id="rfc.section.6.6.p.3">A server that receives a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST</em> initiate a lingering close (see below) of the connection after it sends the final response to the request that contained <a href="#header.connection" class="smpl">close</a>. The server <em class="bcp14">SHOULD</em> send a <a href="#header.connection" class="smpl">close</a> connection option in its final response on that connection. The server <em class="bcp14">MUST NOT</em> process any further requests received on that connection.
    2083       </p>
    2084       <p id="rfc.section.6.6.p.4">A server that sends a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST</em> initiate a lingering close of the connection after it sends the response containing <a href="#header.connection" class="smpl">close</a>. The server <em class="bcp14">MUST NOT</em> process any further requests received on that connection.
    2085       </p>
    2086       <p id="rfc.section.6.6.p.5">A client that receives a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST</em> cease sending requests on that connection and close the connection after reading the response message containing the close;
    2087          if additional pipelined requests had been sent on the connection, the client <em class="bcp14">SHOULD</em> assume that they will not be processed by the server.
    2088       </p>
    2089       <p id="rfc.section.6.6.p.6">If a server performs an immediate close of a TCP connection, there is a significant risk that the client will not be able
    2090          to read the last HTTP response. If the server receives additional data from the client on a fully-closed connection, such
    2091          as another request that was sent by the client before receiving the server's response, the server's TCP stack will send a
    2092          reset packet to the client; unfortunately, the reset packet might erase the client's unacknowledged input buffers before they
    2093          can be read and interpreted by the client's HTTP parser.
    2094       </p>
    2095       <p id="rfc.section.6.6.p.7">To avoid the TCP reset problem, a server can perform a lingering close on a connection by closing only the write side of the
    2096          read/write connection (a half-close) and continuing to read from the connection until the connection is closed by the client
    2097          or the server is reasonably certain that its own TCP stack has received the client's acknowledgement of the packet(s) containing
    2098          the server's last response. It is then safe for the server to fully close the connection.
    2099       </p>
    2100       <p id="rfc.section.6.6.p.8">It is unknown whether the reset problem is exclusive to TCP or might also be found in other transport connection protocols.</p>
    2101       <div id="rfc.iref.u.5"></div>
    2102       <h2 id="rfc.section.6.7"><a href="#rfc.section.6.7">6.7</a>&nbsp;<a id="header.upgrade" href="#header.upgrade">Upgrade</a></h2>
    2103       <p id="rfc.section.6.7.p.1">The "Upgrade" header field is intended to provide a simple mechanism for transitioning from HTTP/1.1 to some other protocol
    2104          on the same connection. A client <em class="bcp14">MAY</em> send a list of protocols in the Upgrade header field of a request to invite the server to switch to one or more of those protocols
    2105          before sending the final response. A server <em class="bcp14">MUST</em> send an Upgrade header field in <a href="p2-semantics.html#status.101" class="smpl">101 (Switching
    2106             Protocols)</a> responses to indicate which protocol(s) are being switched to, and <em class="bcp14">MUST</em> send it in <a href="p2-semantics.html#status.426" class="smpl">426 (Upgrade Required)</a> responses to indicate acceptable protocols. A server <em class="bcp14">MAY</em> send an Upgrade header field in any other response to indicate that they might be willing to upgrade to one of the specified
    2107          protocols for a future request.
    2108       </p>
    2109       <div id="rfc.figure.u.57"></div><pre class="inline"><span id="rfc.iref.g.94"></span>  <a href="#header.upgrade" class="smpl">Upgrade</a>          = 1#<a href="#header.upgrade" class="smpl">protocol</a>
     2098            </p>
     2099            <p id="rfc.section.6.1.p.13">A client that does not support <a href="#persistent.connections" class="smpl">persistent connections</a> <em class="bcp14">MUST</em> send the "close" connection option in every request message.
     2100            </p>
     2101            <p id="rfc.section.6.1.p.14">A server that does not support <a href="#persistent.connections" class="smpl">persistent connections</a> <em class="bcp14">MUST</em> send the "close" connection option in every response message that does not have a <a href="p2-semantics.html#status.1xx" class="smpl">1xx (Informational)</a> status code.
     2102            </p>
     2103         </div>
     2104         <div id="persistent.establishment">
     2105            <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a href="#persistent.establishment">Establishment</a></h2>
     2106            <p id="rfc.section.6.2.p.1">It is beyond the scope of this specification to describe how connections are established via various transport or session-layer
     2107               protocols. Each connection applies to only one transport link.
     2108            </p>
     2109         </div>
     2110         <div id="persistent.connections">
     2111            <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a href="#persistent.connections">Persistence</a></h2>
     2112            <p id="rfc.section.6.3.p.1">HTTP/1.1 defaults to the use of "<dfn>persistent connections</dfn>", allowing multiple requests and responses to be carried over a single connection. The "<a href="#header.connection" class="smpl">close</a>" connection-option is used to signal that a connection will not persist after the current request/response. HTTP implementations <em class="bcp14">SHOULD</em> support persistent connections.
     2113            </p>
     2114            <p id="rfc.section.6.3.p.2">A recipient determines whether a connection is persistent or not based on the most recently received message's protocol version
     2115               and <a href="#header.connection" class="smpl">Connection</a> header field (if any):
     2116            </p>
     2117            <ul>
     2118               <li>If the <a href="#header.connection" class="smpl">close</a> connection option is present, the connection will not persist after the current response; else,
     2119               </li>
     2120               <li>If the received protocol is HTTP/1.1 (or later), the connection will persist after the current response; else,</li>
     2121               <li>If the received protocol is HTTP/1.0, the "keep-alive" connection option is present, the recipient is not a proxy, and the
     2122                  recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism, the connection will persist after the current response; otherwise,
     2123               </li>
     2124               <li>The connection will close after the current response.</li>
     2125            </ul>
     2126            <p id="rfc.section.6.3.p.3">A server <em class="bcp14">MAY</em> assume that an HTTP/1.1 client intends to maintain a persistent connection until a <a href="#header.connection" class="smpl">close</a> connection option is received in a request.
     2127            </p>
     2128            <p id="rfc.section.6.3.p.4">A client <em class="bcp14">MAY</em> reuse a persistent connection until it sends or receives a <a href="#header.connection" class="smpl">close</a> connection option or receives an HTTP/1.0 response without a "keep-alive" connection option.
     2129            </p>
     2130            <p id="rfc.section.6.3.p.5">In order to remain persistent, all messages on a 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.body" title="Message Body">Section&nbsp;3.3</a>. A server <em class="bcp14">MUST</em> read the entire request message body or close the connection after sending its response, since otherwise the remaining data
     2131               on a persistent connection would be misinterpreted as the next request. Likewise, a client <em class="bcp14">MUST</em> read the entire response message body if it intends to reuse the same connection for a subsequent request.
     2132            </p>
     2133            <p id="rfc.section.6.3.p.6">A proxy server <em class="bcp14">MUST NOT</em> maintain a persistent connection with an HTTP/1.0 client (see <a href="https://tools.ietf.org/html/rfc2068#section-19.7.1">Section 19.7.1</a> of <a href="#RFC2068" id="rfc.xref.RFC2068.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> for information and discussion of the problems with the Keep-Alive header field implemented by many HTTP/1.0 clients).
     2134            </p>
     2135            <p id="rfc.section.6.3.p.7">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="Keep-Alive Connections">Appendix&nbsp;A.1.2</a> for more information on backward compatibility with HTTP/1.0 clients.
     2136            </p>
     2137            <div id="persistent.retrying.requests">
     2138               <h3 id="rfc.section.6.3.1"><a href="#rfc.section.6.3.1">6.3.1</a>&nbsp;<a href="#persistent.retrying.requests">Retrying Requests</a></h3>
     2139               <p id="rfc.section.6.3.1.p.1">Connections can be closed at any time, with or without intention. Implementations ought to anticipate the need to recover
     2140                  from asynchronous close events.
     2141               </p>
     2142               <p id="rfc.section.6.3.1.p.2">When an inbound connection is closed prematurely, a client <em class="bcp14">MAY</em> open a new connection and automatically retransmit an aborted sequence of requests if all of those requests have idempotent
     2143                  methods (<a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 4.2.2</a> of <a href="#Part2" id="rfc.xref.Part2.23"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). A proxy <em class="bcp14">MUST NOT</em> automatically retry non-idempotent requests.
     2144               </p>
     2145               <p id="rfc.section.6.3.1.p.3">A user agent <em class="bcp14">MUST NOT</em> automatically retry a request with a non-idempotent method unless it has some means to know that the request semantics are
     2146                  actually idempotent, regardless of the method, or some means to detect that the original request was never applied. For example,
     2147                  a user agent that knows (through design or configuration) that a POST request to a given resource is safe can repeat that
     2148                  request automatically. Likewise, a user agent designed specifically to operate on a version control repository might be able
     2149                  to recover from partial failure conditions by checking the target resource revision(s) after a failed connection, reverting
     2150                  or fixing any changes that were partially applied, and then automatically retrying the requests that failed.
     2151               </p>
     2152               <p id="rfc.section.6.3.1.p.4">An automatic retry <em class="bcp14">SHOULD NOT</em> be repeated if it fails.
     2153               </p>
     2154            </div>
     2155            <div id="pipelining">
     2156               <h3 id="rfc.section.6.3.2"><a href="#rfc.section.6.3.2">6.3.2</a>&nbsp;<a href="#pipelining">Pipelining</a></h3>
     2157               <p id="rfc.section.6.3.2.p.1">A client that supports persistent connections <em class="bcp14">MAY</em> "<dfn>pipeline</dfn>" its requests (i.e., send multiple requests without waiting for each response). A server <em class="bcp14">MAY</em> process a sequence of pipelined requests in parallel if they all have safe methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.24"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>), but <em class="bcp14">MUST</em> send the corresponding responses in the same order that the requests were received.
     2158               </p>
     2159               <p id="rfc.section.6.3.2.p.2">A client that pipelines requests <em class="bcp14">MUST</em> be prepared to retry those requests if the connection closes before it receives all of the corresponding responses. A client
     2160                  that assumes a persistent connection and pipelines immediately after connection establishment <em class="bcp14">MUST NOT</em> pipeline on a retry connection until it knows the connection is persistent.
     2161               </p>
     2162               <p id="rfc.section.6.3.2.p.3">Idempotent methods (<a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 4.2.2</a> of <a href="#Part2" id="rfc.xref.Part2.25"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) are significant to pipelining because they can be automatically retried after a connection failure. A user agent <em class="bcp14">SHOULD NOT</em> pipeline requests after a non-idempotent method until the final response status code for that method has been received, unless
     2163                  the user agent has a means to detect and recover from partial failure conditions involving the pipelined sequence.
     2164               </p>
     2165               <p id="rfc.section.6.3.2.p.4">An intermediary that receives pipelined requests <em class="bcp14">MAY</em> pipeline those requests when forwarding them inbound, since it can rely on the outbound user agent(s) to determine what requests
     2166                  can be safely pipelined. If the inbound connection fails before receiving a response, the pipelining intermediary <em class="bcp14">MAY</em> attempt to retry a sequence of requests that have yet to receive a response if the requests all have idempotent methods; otherwise,
     2167                  the pipelining intermediary <em class="bcp14">SHOULD</em> forward any received responses and then close the corresponding outbound connection(s) so that the outbound user agent(s)
     2168                  can recover accordingly.
     2169               </p>
     2170            </div>
     2171         </div>
     2172         <div id="persistent.concurrency">
     2173            <h2 id="rfc.section.6.4"><a href="#rfc.section.6.4">6.4</a>&nbsp;<a href="#persistent.concurrency">Concurrency</a></h2>
     2174            <p id="rfc.section.6.4.p.1">Clients <em class="bcp14">SHOULD</em> limit the number of simultaneous connections that they maintain to a given server.
     2175            </p>
     2176            <p id="rfc.section.6.4.p.2">Previous revisions of HTTP gave a specific number of connections as a ceiling, but this was found to be impractical for many
     2177               applications. As a result, this specification does not mandate a particular maximum number of connections, but instead encourages
     2178               clients to be conservative when opening multiple connections.
     2179            </p>
     2180            <p id="rfc.section.6.4.p.3">Multiple connections are typically used to avoid the "head-of-line blocking" problem, wherein a request that takes significant
     2181               server-side processing and/or has a large payload blocks subsequent requests on the same connection. However, each connection
     2182               consumes server resources. Furthermore, using multiple connections can cause undesirable side effects in congested networks.
     2183            </p>
     2184            <p id="rfc.section.6.4.p.4">Note that servers might reject traffic that they deem abusive, including an excessive number of connections from a client.</p>
     2185         </div>
     2186         <div id="persistent.failures">
     2187            <h2 id="rfc.section.6.5"><a href="#rfc.section.6.5">6.5</a>&nbsp;<a href="#persistent.failures">Failures and Time-outs</a></h2>
     2188            <p id="rfc.section.6.5.p.1">Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers
     2189               might make this a higher value since it is likely that the client will be making more connections through the same server.
     2190               The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client
     2191               or the server.
     2192            </p>
     2193            <p id="rfc.section.6.5.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
     2194               not detect the other side's close promptly it could cause unnecessary resource drain on the network.
     2195            </p>
     2196            <p id="rfc.section.6.5.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
     2197               that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed
     2198               while it was idle, but from the client's point of view, a request is in progress.
     2199            </p>
     2200            <p id="rfc.section.6.5.p.4">Servers <em class="bcp14">SHOULD</em> maintain persistent connections and allow the underlying transport's flow control mechanisms to resolve temporary overloads,
     2201               rather than terminate connections with the expectation that clients will retry. The latter technique can exacerbate network
     2202               congestion.
     2203            </p>
     2204            <p id="rfc.section.6.5.p.5">A client sending a message body <em class="bcp14">SHOULD</em> monitor the network connection for an error status code while it is transmitting the request. If the client sees an error
     2205               status code, it <em class="bcp14">SHOULD</em> immediately cease transmitting the body and close the connection.
     2206            </p>
     2207         </div>
     2208         <div id="persistent.tear-down">
     2209            <div id="rfc.iref.c.13"></div>
     2210            <div id="rfc.iref.c.14"></div>
     2211            <h2 id="rfc.section.6.6"><a href="#rfc.section.6.6">6.6</a>&nbsp;<a href="#persistent.tear-down">Tear-down</a></h2>
     2212            <p id="rfc.section.6.6.p.1">The <a href="#header.connection" class="smpl">Connection</a> header field (<a href="#header.connection" id="rfc.xref.header.connection.6" title="Connection">Section&nbsp;6.1</a>) provides a "<a href="#header.connection" class="smpl">close</a>" connection option that a sender <em class="bcp14">SHOULD</em> send when it wishes to close the connection after the current request/response pair.
     2213            </p>
     2214            <p id="rfc.section.6.6.p.2">A client that sends a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST NOT</em> send further requests on that connection (after the one containing <a href="#header.connection" class="smpl">close</a>) and <em class="bcp14">MUST</em> close the connection after reading the final response message corresponding to this request.
     2215            </p>
     2216            <p id="rfc.section.6.6.p.3">A server that receives a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST</em> initiate a lingering close (see below) of the connection after it sends the final response to the request that contained <a href="#header.connection" class="smpl">close</a>. The server <em class="bcp14">SHOULD</em> send a <a href="#header.connection" class="smpl">close</a> connection option in its final response on that connection. The server <em class="bcp14">MUST NOT</em> process any further requests received on that connection.
     2217            </p>
     2218            <p id="rfc.section.6.6.p.4">A server that sends a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST</em> initiate a lingering close of the connection after it sends the response containing <a href="#header.connection" class="smpl">close</a>. The server <em class="bcp14">MUST NOT</em> process any further requests received on that connection.
     2219            </p>
     2220            <p id="rfc.section.6.6.p.5">A client that receives a <a href="#header.connection" class="smpl">close</a> connection option <em class="bcp14">MUST</em> cease sending requests on that connection and close the connection after reading the response message containing the close;
     2221               if additional pipelined requests had been sent on the connection, the client <em class="bcp14">SHOULD</em> assume that they will not be processed by the server.
     2222            </p>
     2223            <p id="rfc.section.6.6.p.6">If a server performs an immediate close of a TCP connection, there is a significant risk that the client will not be able
     2224               to read the last HTTP response. If the server receives additional data from the client on a fully-closed connection, such
     2225               as another request that was sent by the client before receiving the server's response, the server's TCP stack will send a
     2226               reset packet to the client; unfortunately, the reset packet might erase the client's unacknowledged input buffers before they
     2227               can be read and interpreted by the client's HTTP parser.
     2228            </p>
     2229            <p id="rfc.section.6.6.p.7">To avoid the TCP reset problem, a server can perform a lingering close on a connection by closing only the write side of the
     2230               read/write connection (a half-close) and continuing to read from the connection until the connection is closed by the client
     2231               or the server is reasonably certain that its own TCP stack has received the client's acknowledgement of the packet(s) containing
     2232               the server's last response. It is then safe for the server to fully close the connection.
     2233            </p>
     2234            <p id="rfc.section.6.6.p.8">It is unknown whether the reset problem is exclusive to TCP or might also be found in other transport connection protocols.</p>
     2235         </div>
     2236         <div id="header.upgrade">
     2237            <div id="rfc.iref.u.5"></div>
     2238            <h2 id="rfc.section.6.7"><a href="#rfc.section.6.7">6.7</a>&nbsp;<a href="#header.upgrade">Upgrade</a></h2>
     2239            <p id="rfc.section.6.7.p.1">The "Upgrade" header field is intended to provide a simple mechanism for transitioning from HTTP/1.1 to some other protocol
     2240               on the same connection. A client <em class="bcp14">MAY</em> send a list of protocols in the Upgrade header field of a request to invite the server to switch to one or more of those protocols
     2241               before sending the final response. A server <em class="bcp14">MUST</em> send an Upgrade header field in <a href="p2-semantics.html#status.101" class="smpl">101 (Switching
     2242                  Protocols)</a> responses to indicate which protocol(s) are being switched to, and <em class="bcp14">MUST</em> send it in <a href="p2-semantics.html#status.426" class="smpl">426 (Upgrade Required)</a> responses to indicate acceptable protocols. A server <em class="bcp14">MAY</em> send an Upgrade header field in any other response to indicate that they might be willing to upgrade to one of the specified
     2243               protocols for a future request.
     2244            </p>
     2245            <div id="rfc.figure.u.57"></div><pre class="inline"><span id="rfc.iref.g.94"></span>  <a href="#header.upgrade" class="smpl">Upgrade</a>          = 1#<a href="#header.upgrade" class="smpl">protocol</a>
    21102246
    21112247  <a href="#header.upgrade" class="smpl">protocol</a>         = <a href="#header.upgrade" class="smpl">protocol-name</a> ["/" <a href="#header.upgrade" class="smpl">protocol-version</a>]
     
    21132249  <a href="#header.upgrade" class="smpl">protocol-version</a> = <a href="#rule.token.separators" class="smpl">token</a>
    21142250</pre><p id="rfc.section.6.7.p.3">For example,</p>
    2115       <div id="rfc.figure.u.58"></div><pre class="text">  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
     2251            <div id="rfc.figure.u.58"></div><pre class="text">  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
    21162252</pre><p id="rfc.section.6.7.p.5">Upgrade eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the
    2117          more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if available
    2118          (where "better" is determined by the server, possibly according to the nature of the request method or target resource).
    2119       </p>
    2120       <p id="rfc.section.6.7.p.6">Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities
    2121          and nature of the application-level communication after the protocol change is entirely dependent upon the new protocol chosen,
    2122          although the first action after changing the protocol <em class="bcp14">MUST</em> be a response to the initial HTTP request that contained the Upgrade header field.
    2123       </p>
    2124       <p id="rfc.section.6.7.p.7">For example, if the Upgrade header field is received in a GET request and the server decides to switch protocols, then it
    2125          first responds with a <a href="p2-semantics.html#status.101" class="smpl">101 (Switching Protocols)</a> message in HTTP/1.1 and then immediately follows that with the new protocol's equivalent of a response to a GET on the target
    2126          resource. This allows a connection to be upgraded to protocols with the same semantics as HTTP without the latency cost of
    2127          an additional round-trip. A server <em class="bcp14">MUST NOT</em> switch protocols unless the received message semantics can be honored by the new protocol; an OPTIONS request can be honored
    2128          by any protocol.
    2129       </p>
    2130       <p id="rfc.section.6.7.p.8">When Upgrade is sent, a sender <em class="bcp14">MUST</em> also send a <a href="#header.connection" class="smpl">Connection</a> header field (<a href="#header.connection" id="rfc.xref.header.connection.7" title="Connection">Section&nbsp;6.1</a>) that contains the "upgrade" connection option, in order to prevent Upgrade from being accidentally forwarded by intermediaries
    2131          that might not implement the listed protocols. A server <em class="bcp14">MUST</em> ignore an Upgrade header field that is received in an HTTP/1.0 request.
    2132       </p>
    2133       <p id="rfc.section.6.7.p.9">The Upgrade header field only applies to switching application-level protocols on the existing connection; it cannot be used
    2134          to switch to a protocol on a different connection. For that purpose, it is more appropriate to use a <a href="p2-semantics.html#status.3xx" class="smpl">3xx (Redirection)</a> response (<a href="p2-semantics.html#status.3xx" title="Redirection 3xx">Section 6.4</a> of <a href="#Part2" id="rfc.xref.Part2.26"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>).
    2135       </p>
    2136       <p id="rfc.section.6.7.p.10">This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined
    2137          by the HTTP version rules of <a href="#http.version" title="Protocol Versioning">Section&nbsp;2.6</a> and future updates to this specification. Additional tokens ought to be registered with IANA using the registration procedure
    2138          defined in <a href="#upgrade.token.registry" title="Upgrade Token Registry">Section&nbsp;7.6</a>.
    2139       </p>
    2140       <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
    2141       <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
    2142       <p id="rfc.section.7.1.p.1">HTTP header fields are registered within the Message Header Field Registry <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a> maintained by IANA at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt;.
    2143       </p>
    2144       <p id="rfc.section.7.1.p.2">This document defines the following HTTP header fields, so their associated registry entries shall be updated according to
    2145          the permanent registrations below:
    2146       </p>
    2147       <div id="rfc.table.1">
    2148          <div id="iana.header.registration.table"></div>
    2149          <table class="tt full left" cellpadding="3" cellspacing="0">
    2150             <thead>
    2151                <tr>
    2152                   <th>Header Field Name</th>
    2153                   <th>Protocol</th>
    2154                   <th>Status</th>
    2155                   <th>Reference</th>
    2156                </tr>
    2157             </thead>
    2158             <tbody>
    2159                <tr>
    2160                   <td class="left">Connection</td>
    2161                   <td class="left">http</td>
    2162                   <td class="left">standard</td>
    2163                   <td class="left"> <a href="#header.connection" id="rfc.xref.header.connection.8" title="Connection">Section&nbsp;6.1</a>
    2164                   </td>
    2165                </tr>
    2166                <tr>
    2167                   <td class="left">Content-Length</td>
    2168                   <td class="left">http</td>
    2169                   <td class="left">standard</td>
    2170                   <td class="left"> <a href="#header.content-length" id="rfc.xref.header.content-length.1" title="Content-Length">Section&nbsp;3.3.2</a>
    2171                   </td>
    2172                </tr>
    2173                <tr>
    2174                   <td class="left">Host</td>
    2175                   <td class="left">http</td>
    2176                   <td class="left">standard</td>
    2177                   <td class="left"> <a href="#header.host" id="rfc.xref.header.host.2" title="Host">Section&nbsp;5.4</a>
    2178                   </td>
    2179                </tr>
    2180                <tr>
    2181                   <td class="left">TE</td>
    2182                   <td class="left">http</td>
    2183                   <td class="left">standard</td>
    2184                   <td class="left"> <a href="#header.te" id="rfc.xref.header.te.3" title="TE">Section&nbsp;4.3</a>
    2185                   </td>
    2186                </tr>
    2187                <tr>
    2188                   <td class="left">Trailer</td>
    2189                   <td class="left">http</td>
    2190                   <td class="left">standard</td>
    2191                   <td class="left"> <a href="#header.trailer" id="rfc.xref.header.trailer.1" title="Trailer">Section&nbsp;4.1.1</a>
    2192                   </td>
    2193                </tr>
    2194                <tr>
    2195                   <td class="left">Transfer-Encoding</td>
    2196                   <td class="left">http</td>
    2197                   <td class="left">standard</td>
    2198                   <td class="left"> <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.3" title="Transfer-Encoding">Section&nbsp;3.3.1</a>
    2199                   </td>
    2200                </tr>
    2201                <tr>
    2202                   <td class="left">Upgrade</td>
    2203                   <td class="left">http</td>
    2204                   <td class="left">standard</td>
    2205                   <td class="left"> <a href="#header.upgrade" id="rfc.xref.header.upgrade.2" title="Upgrade">Section&nbsp;6.7</a>
    2206                   </td>
    2207                </tr>
    2208                <tr>
    2209                   <td class="left">Via</td>
    2210                   <td class="left">http</td>
    2211                   <td class="left">standard</td>
    2212                   <td class="left"> <a href="#header.via" id="rfc.xref.header.via.2" title="Via">Section&nbsp;5.7.1</a>
    2213                   </td>
    2214                </tr>
    2215             </tbody>
    2216          </table>
     2253               more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if available
     2254               (where "better" is determined by the server, possibly according to the nature of the request method or target resource).
     2255            </p>
     2256            <p id="rfc.section.6.7.p.6">Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities
     2257               and nature of the application-level communication after the protocol change is entirely dependent upon the new protocol chosen,
     2258               although the first action after changing the protocol <em class="bcp14">MUST</em> be a response to the initial HTTP request that contained the Upgrade header field.
     2259            </p>
     2260            <p id="rfc.section.6.7.p.7">For example, if the Upgrade header field is received in a GET request and the server decides to switch protocols, then it
     2261               first responds with a <a href="p2-semantics.html#status.101" class="smpl">101 (Switching Protocols)</a> message in HTTP/1.1 and then immediately follows that with the new protocol's equivalent of a response to a GET on the target
     2262               resource. This allows a connection to be upgraded to protocols with the same semantics as HTTP without the latency cost of
     2263