source: draft-ietf-httpbis/latest/p1-messaging.html @ 95

Last change on this file since 95 was 95, checked in by julian.reschke@…, 12 years ago

List Yves Lafon & Julian Reschke as Editors everywhere, remove specific ack for Julian.

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330</style><link rel="Contents" href="#rfc.toc">
331      <link rel="Author" href="#rfc.authors">
332      <link rel="Copyright" href="#rfc.copyright">
333      <link rel="Index" href="#rfc.index">
334      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
335      <link rel="Chapter" title="2 Notational Conventions and Generic Grammar" href="#rfc.section.2">
336      <link rel="Chapter" title="3 Protocol Parameters" href="#rfc.section.3">
337      <link rel="Chapter" title="4 HTTP Message" href="#rfc.section.4">
338      <link rel="Chapter" title="5 Request" href="#rfc.section.5">
339      <link rel="Chapter" title="6 Response" href="#rfc.section.6">
340      <link rel="Chapter" title="7 Connections" href="#rfc.section.7">
341      <link rel="Chapter" title="8 Header Field Definitions" href="#rfc.section.8">
342      <link rel="Chapter" title="9 IANA Considerations" href="#rfc.section.9">
343      <link rel="Chapter" title="10 Security Considerations" href="#rfc.section.10">
344      <link rel="Chapter" title="11 Acknowledgments" href="#rfc.section.11">
345      <link rel="Chapter" href="#rfc.section.12" title="12 References">
346      <link rel="Appendix" title="A Internet Media Type message/http and application/http" href="#rfc.section.A">
347      <link rel="Appendix" title="B Tolerant Applications" href="#rfc.section.B">
348      <link rel="Appendix" title="C Conversion of Date Formats" href="#rfc.section.C">
349      <link rel="Appendix" title="D Compatibility with Previous Versions" href="#rfc.section.D">
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351      <link rel="schema.DC" href="http://purl.org/dc/elements/1.1/">
352      <meta name="DC.Creator" content="Fielding, R.">
353      <meta name="DC.Creator" content="Gettys, J.">
354      <meta name="DC.Creator" content="Mogul, J.">
355      <meta name="DC.Creator" content="Frystyk, H.">
356      <meta name="DC.Creator" content="Masinter, L.">
357      <meta name="DC.Creator" content="Leach, P.">
358      <meta name="DC.Creator" content="Berners-Lee, T.">
359      <meta name="DC.Creator" content="Lafon, Y.">
360      <meta name="DC.Creator" content="Reschke, J. F.">
361      <meta name="DC.Identifier" content="urn:ietf:id:draft-ietf-httpbis-p1-messaging-latest">
362      <meta name="DC.Date.Issued" scheme="ISO8601" content="2007-12">
363      <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2068">
364      <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2616">
365      <meta name="DC.Description.Abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 1 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 1 provides an overview of HTTP and its associated terminology, defines the &#34;http&#34; and &#34;https&#34; Uniform Resource Identifier (URI) schemes, defines the generic message syntax and parsing requirements for HTTP message frames, and describes general security concerns for implementations.">
366   </head>
367   <body>
368      <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
369         <tr>
370            <td class="header left">Network Working Group</td>
371            <td class="header right">R. Fielding, Editor</td>
372         </tr>
373         <tr>
374            <td class="header left">Internet Draft</td>
375            <td class="header right">Day Software</td>
376         </tr>
377         <tr>
378            <td class="header left">
379               &lt;draft-ietf-httpbis-p1-messaging-latest&gt;
380               
381            </td>
382            <td class="header right">J. Gettys</td>
383         </tr>
384         <tr>
385            <td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2068">2068</a>,
386               <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
387            </td>
388            <td class="header right">One Laptop per Child</td>
389         </tr>
390         <tr>
391            <td class="header left">Intended status: Standards Track</td>
392            <td class="header right">J. Mogul</td>
393         </tr>
394         <tr>
395            <td class="header left">Expires: June 2008</td>
396            <td class="header right">HP</td>
397         </tr>
398         <tr>
399            <td class="header left"></td>
400            <td class="header right">H. Frystyk</td>
401         </tr>
402         <tr>
403            <td class="header left"></td>
404            <td class="header right">Microsoft</td>
405         </tr>
406         <tr>
407            <td class="header left"></td>
408            <td class="header right">L. Masinter</td>
409         </tr>
410         <tr>
411            <td class="header left"></td>
412            <td class="header right">Adobe Systems</td>
413         </tr>
414         <tr>
415            <td class="header left"></td>
416            <td class="header right">P. Leach</td>
417         </tr>
418         <tr>
419            <td class="header left"></td>
420            <td class="header right">Microsoft</td>
421         </tr>
422         <tr>
423            <td class="header left"></td>
424            <td class="header right">T. Berners-Lee</td>
425         </tr>
426         <tr>
427            <td class="header left"></td>
428            <td class="header right">W3C/MIT</td>
429         </tr>
430         <tr>
431            <td class="header left"></td>
432            <td class="header right">Y. Lafon, Editor</td>
433         </tr>
434         <tr>
435            <td class="header left"></td>
436            <td class="header right">W3C</td>
437         </tr>
438         <tr>
439            <td class="header left"></td>
440            <td class="header right">J. F. Reschke, Editor</td>
441         </tr>
442         <tr>
443            <td class="header left"></td>
444            <td class="header right">greenbytes</td>
445         </tr>
446         <tr>
447            <td class="header left"></td>
448            <td class="header right">December 2007</td>
449         </tr>
450      </table>
451      <p class="title">HTTP/1.1, part 1: URIs, Connections, and Message Parsing<br><span class="filename">draft-ietf-httpbis-p1-messaging-latest</span></p>
452      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
453      <p>By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she
454         is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section
455         6 of BCP 79.
456      </p>
457      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note
458         that other groups may also distribute working documents as Internet-Drafts.
459      </p>
460      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
461         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
462         in progress”.
463      </p>
464      <p>The list of current Internet-Drafts can be accessed at &lt;<a href="http://www.ietf.org/ietf/1id-abstracts.txt">http://www.ietf.org/ietf/1id-abstracts.txt</a>&gt;.
465      </p>
466      <p>The list of Internet-Draft Shadow Directories can be accessed at &lt;<a href="http://www.ietf.org/shadow.html">http://www.ietf.org/shadow.html</a>&gt;.
467      </p>
468      <p>This Internet-Draft will expire in June 2008.</p>
469      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
470      <p>Copyright © The IETF Trust (2007). All Rights Reserved.</p>
471      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
472      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
473         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 1 of the
474         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part
475         1 provides an overview of HTTP and its associated terminology, defines the "http" and "https" Uniform Resource Identifier
476         (URI) schemes, defines the generic message syntax and parsing requirements for HTTP message frames, and describes general
477         security concerns for implementations.
478      </p> 
479      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
480      <p>This version of the HTTP specification contains only minimal editorial changes from <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a> (abstract, introductory paragraph, and authors' addresses). All other changes are due to partitioning the original into seven
481         mostly independent parts. The intent is for readers of future drafts to able to use draft 00 as the basis for comparison when
482         the WG makes later changes to the specification text. This draft will shortly be followed by draft 01 (containing the first
483         round of changes that have already been agreed to on the mailing list). There is no point in reviewing this draft other than
484         to verify that the partitioning has been done correctly. Roy T. Fielding, Yves Lafon, and Julian Reschke will be the editors
485         after draft 00 is submitted.
486      </p> 
487      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues
488         list is at &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/report/11">http://www3.tools.ietf.org/wg/httpbis/trac/report/11</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/">http://www3.tools.ietf.org/wg/httpbis/</a>&gt;.
489      </p> 
490      <hr class="noprint">
491      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
492      <ul class="toc">
493         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul class="toc">
494               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
495               <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
496               <li class="tocline1">1.3&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
497               <li class="tocline1">1.4&nbsp;&nbsp;&nbsp;<a href="#intro.overall.operation">Overall Operation</a></li>
498            </ul>
499         </li>
500         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#notation">Notational Conventions and Generic Grammar</a><ul class="toc">
501               <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#notation.abnf">Augmented BNF</a></li>
502               <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#basic.rules">Basic Rules</a></li>
503            </ul>
504         </li>
505         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#protocol.parameters">Protocol Parameters</a><ul class="toc">
506               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#http.version">HTTP Version</a></li>
507               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#uri">Uniform Resource Identifiers</a><ul class="toc">
508                     <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#general.syntax">General Syntax</a></li>
509                     <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#http.url">http URL</a></li>
510                     <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#uri.comparison">URI Comparison</a></li>
511                  </ul>
512               </li>
513               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#date.time.formats">Date/Time Formats</a><ul class="toc">
514                     <li class="tocline1">3.3.1&nbsp;&nbsp;&nbsp;<a href="#full.date">Full Date</a></li>
515                  </ul>
516               </li>
517               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#transfer.codings">Transfer Codings</a><ul class="toc">
518                     <li class="tocline1">3.4.1&nbsp;&nbsp;&nbsp;<a href="#chunked.transfer.encoding">Chunked Transfer Coding</a></li>
519                  </ul>
520               </li>
521            </ul>
522         </li>
523         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#http.message">HTTP Message</a><ul class="toc">
524               <li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#message.types">Message Types</a></li>
525               <li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#message.headers">Message Headers</a></li>
526               <li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#message.body">Message Body</a></li>
527               <li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#message.length">Message Length</a></li>
528               <li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#general.header.fields">General Header Fields</a></li>
529            </ul>
530         </li>
531         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#request">Request</a><ul class="toc">
532               <li class="tocline1">5.1&nbsp;&nbsp;&nbsp;<a href="#request-line">Request-Line</a><ul class="toc">
533                     <li class="tocline1">5.1.1&nbsp;&nbsp;&nbsp;<a href="#method">Method</a></li>
534                     <li class="tocline1">5.1.2&nbsp;&nbsp;&nbsp;<a href="#request-uri">Request-URI</a></li>
535                  </ul>
536               </li>
537               <li class="tocline1">5.2&nbsp;&nbsp;&nbsp;<a href="#the.resource.identified.by.a.request">The Resource Identified by a Request</a></li>
538            </ul>
539         </li>
540         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#response">Response</a><ul class="toc">
541               <li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#status-line">Status-Line</a><ul class="toc">
542                     <li class="tocline1">6.1.1&nbsp;&nbsp;&nbsp;<a href="#status.code.and.reason.phrase">Status Code and Reason Phrase</a></li>
543                  </ul>
544               </li>
545            </ul>
546         </li>
547         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#connections">Connections</a><ul class="toc">
548               <li class="tocline1">7.1&nbsp;&nbsp;&nbsp;<a href="#persistent.connections">Persistent Connections</a><ul class="toc">
549                     <li class="tocline1">7.1.1&nbsp;&nbsp;&nbsp;<a href="#persistent.purpose">Purpose</a></li>
550                     <li class="tocline1">7.1.2&nbsp;&nbsp;&nbsp;<a href="#persistent.overall">Overall Operation</a><ul class="toc">
551                           <li class="tocline1">7.1.2.1&nbsp;&nbsp;&nbsp;<a href="#persistent.negotiation">Negotiation</a></li>
552                           <li class="tocline1">7.1.2.2&nbsp;&nbsp;&nbsp;<a href="#pipelining">Pipelining</a></li>
553                        </ul>
554                     </li>
555                     <li class="tocline1">7.1.3&nbsp;&nbsp;&nbsp;<a href="#persistent.proxy">Proxy Servers</a></li>
556                     <li class="tocline1">7.1.4&nbsp;&nbsp;&nbsp;<a href="#persistent.practical">Practical Considerations</a></li>
557                  </ul>
558               </li>
559               <li class="tocline1">7.2&nbsp;&nbsp;&nbsp;<a href="#message.transmission.requirements">Message Transmission Requirements</a><ul class="toc">
560                     <li class="tocline1">7.2.1&nbsp;&nbsp;&nbsp;<a href="#persistent.flow">Persistent Connections and Flow Control</a></li>
561                     <li class="tocline1">7.2.2&nbsp;&nbsp;&nbsp;<a href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></li>
562                     <li class="tocline1">7.2.3&nbsp;&nbsp;&nbsp;<a href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></li>
563                     <li class="tocline1">7.2.4&nbsp;&nbsp;&nbsp;<a href="#connection.premature">Client Behavior if Server Prematurely Closes Connection</a></li>
564                  </ul>
565               </li>
566            </ul>
567         </li>
568         <li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
569               <li class="tocline1">8.1&nbsp;&nbsp;&nbsp;<a href="#header.connection">Connection</a></li>
570               <li class="tocline1">8.2&nbsp;&nbsp;&nbsp;<a href="#header.content-length">Content-Length</a></li>
571               <li class="tocline1">8.3&nbsp;&nbsp;&nbsp;<a href="#header.date">Date</a><ul class="toc">
572                     <li class="tocline1">8.3.1&nbsp;&nbsp;&nbsp;<a href="#clockless.origin.server.operation">Clockless Origin Server Operation</a></li>
573                  </ul>
574               </li>
575               <li class="tocline1">8.4&nbsp;&nbsp;&nbsp;<a href="#header.host">Host</a></li>
576               <li class="tocline1">8.5&nbsp;&nbsp;&nbsp;<a href="#header.te">TE</a></li>
577               <li class="tocline1">8.6&nbsp;&nbsp;&nbsp;<a href="#header.trailer">Trailer</a></li>
578               <li class="tocline1">8.7&nbsp;&nbsp;&nbsp;<a href="#header.transfer-encoding">Transfer-Encoding</a></li>
579               <li class="tocline1">8.8&nbsp;&nbsp;&nbsp;<a href="#header.upgrade">Upgrade</a></li>
580               <li class="tocline1">8.9&nbsp;&nbsp;&nbsp;<a href="#header.via">Via</a></li>
581            </ul>
582         </li>
583         <li class="tocline0">9.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a></li>
584         <li class="tocline0">10.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul class="toc">
585               <li class="tocline1">10.1&nbsp;&nbsp;&nbsp;<a href="#personal.information">Personal Information</a></li>
586               <li class="tocline1">10.2&nbsp;&nbsp;&nbsp;<a href="#abuse.of.server.log.information">Abuse of Server Log Information</a></li>
587               <li class="tocline1">10.3&nbsp;&nbsp;&nbsp;<a href="#attack.pathname">Attacks Based On File and Path Names</a></li>
588               <li class="tocline1">10.4&nbsp;&nbsp;&nbsp;<a href="#dns.spoofing">DNS Spoofing</a></li>
589               <li class="tocline1">10.5&nbsp;&nbsp;&nbsp;<a href="#attack.proxies">Proxies and Caching</a></li>
590               <li class="tocline1">10.6&nbsp;&nbsp;&nbsp;<a href="#attack.DoS">Denial of Service Attacks on Proxies</a></li>
591            </ul>
592         </li>
593         <li class="tocline0">11.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
594         <li class="tocline0">12.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
595         <li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li>
596         <li class="tocline0">A.&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.http">Internet Media Type message/http and application/http</a></li>
597         <li class="tocline0">B.&nbsp;&nbsp;&nbsp;<a href="#tolerant.applications">Tolerant Applications</a></li>
598         <li class="tocline0">C.&nbsp;&nbsp;&nbsp;<a href="#conversion.of.date.formats">Conversion of Date Formats</a></li>
599         <li class="tocline0">D.&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul class="toc">
600               <li class="tocline1">D.1&nbsp;&nbsp;&nbsp;<a href="#changes.from.1.0">Changes from HTTP/1.0</a><ul class="toc">
601                     <li class="tocline1">D.1.1&nbsp;&nbsp;&nbsp;<a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses</a></li>
602                  </ul>
603               </li>
604               <li class="tocline1">D.2&nbsp;&nbsp;&nbsp;<a href="#compatibility.with.http.1.0.persistent.connections">Compatibility with HTTP/1.0 Persistent Connections</a></li>
605               <li class="tocline1">D.3&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></li>
606            </ul>
607         </li>
608         <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
609         <li class="tocline0"><a href="#rfc.index">Index</a></li>
610      </ul>
611      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
612      <p id="rfc.section.1.p.1">This document will define aspects of HTTP related to overall network operation, message framing, interaction with transport
613         protocols, and URI schemes. Right now it only includes the extracted relevant sections of <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
614      </p>
615      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.purpose" href="#intro.purpose">Purpose</a></h2>
616      <p id="rfc.section.1.1.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
617         systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred
618         to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by RFC 1945 <a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[RFC1945]</cite></a>, improved the protocol by allowing messages to be in the format of MIME-like messages, containing metainformation about the
619         data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration
620         the effects of hierarchical proxies, caching, the need for persistent connections, or virtual hosts. In addition, the proliferation
621         of incompletely-implemented applications calling themselves "HTTP/1.0" has necessitated a protocol version change in order
622         for two communicating applications to determine each other's true capabilities.
623      </p>
624      <p id="rfc.section.1.1.p.2">This specification defines the protocol referred to as "HTTP/1.1". This protocol includes more stringent requirements than
625         HTTP/1.0 in order to ensure reliable implementation of its features.
626      </p>
627      <p id="rfc.section.1.1.p.3">Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation.
628         HTTP allows an open-ended set of methods and headers that indicate the purpose of a request <a href="#RFC2324" id="rfc.xref.RFC2324.1"><cite title="Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)">[RFC2324]</cite></a>. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) <a href="#RFC1630" id="rfc.xref.RFC1630.1"><cite title="Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web">[RFC1630]</cite></a>, as a location (URL) <a href="#RFC1738" id="rfc.xref.RFC1738.1"><cite title="Uniform Resource Locators (URL)">[RFC1738]</cite></a> or name (URN) <a href="#RFC1737" id="rfc.xref.RFC1737.1"><cite title="Functional Requirements for Uniform Resource Names">[RFC1737]</cite></a>, for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by
629         Internet mail <a href="#RFC822" id="rfc.xref.RFC822.1"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a> as defined by the Multipurpose Internet Mail Extensions (MIME) <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>.
630      </p>
631      <p id="rfc.section.1.1.p.4">HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems,
632         including those supported by the SMTP <a href="#RFC821" id="rfc.xref.RFC821.1"><cite title="Simple Mail Transfer Protocol">[RFC821]</cite></a>, NNTP <a href="#RFC3977" id="rfc.xref.RFC3977.1"><cite title="Network News Transfer Protocol (NNTP)">[RFC3977]</cite></a>, FTP <a href="#RFC959" id="rfc.xref.RFC959.1"><cite title="File Transfer Protocol">[RFC959]</cite></a>, Gopher <a href="#RFC1436" id="rfc.xref.RFC1436.1"><cite title="The Internet Gopher Protocol (a distributed document search and retrieval protocol)">[RFC1436]</cite></a>, and WAIS <a href="#WAIS" id="rfc.xref.WAIS.1"><cite title="WAIS Interface Protocol Prototype Functional Specification (v1.5)">[WAIS]</cite></a> protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications.
633      </p>
634      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
635      <p id="rfc.section.1.2.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
636         in this document are to be interpreted as described in RFC 2119 <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
637      </p>
638      <p id="rfc.section.1.2.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
639      </p>
640      <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a id="intro.terminology" href="#intro.terminology">Terminology</a></h2>
641      <p id="rfc.section.1.3.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.</p>
642      <p id="rfc.section.1.3.p.2"> <span id="rfc.iref.c.1"></span>  <dfn>connection</dfn> 
643      </p>
644      <dl class="empty">
645         <dd>A transport layer virtual circuit established between two programs for the purpose of communication.</dd>
646      </dl>
647      <p id="rfc.section.1.3.p.3"> <span id="rfc.iref.m.1"></span>  <dfn>message</dfn> 
648      </p>
649      <dl class="empty">
650         <dd>The basic unit of HTTP communication, consisting of a structured sequence of octets matching the syntax defined in <a href="#http.message" title="HTTP Message">Section&nbsp;4</a> and transmitted via the connection.
651         </dd>
652      </dl>
653      <p id="rfc.section.1.3.p.4"> <span id="rfc.iref.r.1"></span>  <dfn>request</dfn> 
654      </p>
655      <dl class="empty">
656         <dd>An HTTP request message, as defined in <a href="#request" title="Request">Section&nbsp;5</a>.
657         </dd>
658      </dl>
659      <p id="rfc.section.1.3.p.5"> <span id="rfc.iref.r.2"></span>  <dfn>response</dfn> 
660      </p>
661      <dl class="empty">
662         <dd>An HTTP response message, as defined in <a href="#response" title="Response">Section&nbsp;6</a>.
663         </dd>
664      </dl>
665      <p id="rfc.section.1.3.p.6"> <span id="rfc.iref.r.3"></span>  <dfn>resource</dfn> 
666      </p>
667      <dl class="empty">
668         <dd>A network data object or service that can be identified by a URI, as defined in <a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;3.2</a>. Resources may be available in multiple representations (e.g. multiple languages, data formats, size, and resolutions) or
669            vary in other ways.
670         </dd>
671      </dl>
672      <p id="rfc.section.1.3.p.7"> <span id="rfc.iref.e.1"></span>  <dfn>entity</dfn> 
673      </p>
674      <dl class="empty">
675         <dd>The information transferred as the payload of a request or response. An entity consists of metainformation in the form of
676            entity-header fields and content in the form of an entity-body, as described in <a href="p3-payload.html#entity" title="Entity">Section 3</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>.
677         </dd>
678      </dl>
679      <p id="rfc.section.1.3.p.8"> <span id="rfc.iref.r.4"></span>  <dfn>representation</dfn> 
680      </p>
681      <dl class="empty">
682         <dd>An entity included with a response that is subject to content negotiation, as described in <a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 4</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>. There may exist multiple representations associated with a particular response status.
683         </dd>
684      </dl>
685      <p id="rfc.section.1.3.p.9"> <span id="rfc.iref.c.2"></span>  <dfn>content negotiation</dfn> 
686      </p>
687      <dl class="empty">
688         <dd>The mechanism for selecting the appropriate representation when servicing a request, as described in <a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 4</a> of <a href="#Part3" id="rfc.xref.Part3.3"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>. The representation of entities in any response can be negotiated (including error responses).
689         </dd>
690      </dl>
691      <p id="rfc.section.1.3.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>variant</dfn> 
692      </p>
693      <dl class="empty">
694         <dd>A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations
695            is termed a `variant'. Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation.
696         </dd>
697      </dl>
698      <p id="rfc.section.1.3.p.11"> <span id="rfc.iref.c.3"></span>  <dfn>client</dfn> 
699      </p>
700      <dl class="empty">
701         <dd>A program that establishes connections for the purpose of sending requests.</dd>
702      </dl>
703      <p id="rfc.section.1.3.p.12"> <span id="rfc.iref.u.1"></span>  <dfn>user agent</dfn> 
704      </p>
705      <dl class="empty">
706         <dd>The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user
707            tools.
708         </dd>
709      </dl>
710      <p id="rfc.section.1.3.p.13"> <span id="rfc.iref.s.1"></span>  <dfn>server</dfn> 
711      </p>
712      <dl class="empty">
713         <dd>An application program that accepts connections in order to service requests by sending back responses. Any given program
714            may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the
715            program for a particular connection, rather than to the program's capabilities in general. Likewise, any server may act as
716            an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.
717         </dd>
718      </dl>
719      <p id="rfc.section.1.3.p.14"> <span id="rfc.iref.o.1"></span>  <dfn>origin server</dfn> 
720      </p>
721      <dl class="empty">
722         <dd>The server on which a given resource resides or is to be created.</dd>
723      </dl>
724      <p id="rfc.section.1.3.p.15"> <span id="rfc.iref.p.1"></span>  <dfn>proxy</dfn> 
725      </p>
726      <dl class="empty">
727         <dd>An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients.
728            Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy <em class="bcp14">MUST</em> implement both the client and server requirements of this specification. A "transparent proxy" is a proxy that does not modify
729            the request or response beyond what is required for proxy authentication and identification. A "non-transparent proxy" is
730            a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation
731            services, media type transformation, protocol reduction, or anonymity filtering. Except where either transparent or non-transparent
732            behavior is explicitly stated, the HTTP proxy requirements apply to both types of proxies.
733         </dd>
734      </dl>
735      <p id="rfc.section.1.3.p.16"> <span id="rfc.iref.g.1"></span>  <dfn>gateway</dfn> 
736      </p>
737      <dl class="empty">
738         <dd>A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the
739            origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway.
740         </dd>
741      </dl>
742      <p id="rfc.section.1.3.p.17"> <span id="rfc.iref.t.1"></span>  <dfn>tunnel</dfn> 
743      </p>
744      <dl class="empty">
745         <dd>An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered
746            a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist
747            when both ends of the relayed connections are closed.
748         </dd>
749      </dl>
750      <p id="rfc.section.1.3.p.18"> <span id="rfc.iref.c.4"></span>  <dfn>cache</dfn> 
751      </p>
752      <dl class="empty">
753         <dd>A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion.
754            A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
755            requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
756         </dd>
757      </dl>
758      <p id="rfc.section.1.3.p.19"> <span id="rfc.iref.c.5"></span>  <dfn>cacheable</dfn> 
759      </p>
760      <dl class="empty">
761         <dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
762            The rules for determining the cacheability of HTTP responses are defined in <a href="p6-cache.html#caching" title="Caching in HTTP">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
763            request.
764         </dd>
765      </dl>
766      <p id="rfc.section.1.3.p.20"> <span id="rfc.iref.u.2"></span>  <span id="rfc.iref.d.1"></span>  <dfn>upstream</dfn>/<dfn>downstream</dfn> 
767      </p>
768      <dl class="empty">
769         <dd>Upstream and downstream describe the flow of a message: all messages flow from upstream to downstream.</dd>
770      </dl>
771      <p id="rfc.section.1.3.p.21"> <span id="rfc.iref.i.1"></span>  <span id="rfc.iref.o.2"></span>  <dfn>inbound</dfn>/<dfn>outbound</dfn> 
772      </p>
773      <dl class="empty">
774         <dd>Inbound and outbound refer to the request and response paths for messages: "inbound" means "traveling toward the origin server",
775            and "outbound" means "traveling toward the user agent"
776         </dd>
777      </dl>
778      <h2 id="rfc.section.1.4"><a href="#rfc.section.1.4">1.4</a>&nbsp;<a id="intro.overall.operation" href="#intro.overall.operation">Overall Operation</a></h2>
779      <p id="rfc.section.1.4.p.1">The HTTP protocol is a request/response protocol. A client sends a request to the server in the form of a request method,
780         URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible
781         body content over a connection with a server. The server responds with a status line, including the message's protocol version
782         and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible
783         entity-body content. The relationship between HTTP and MIME is described in <a href="p3-payload.html#differences.between.http.entities.and.rfc.2045.entities" title="Differences Between HTTP Entities and RFC 2045 Entities">Appendix A</a> of <a href="#Part3" id="rfc.xref.Part3.4"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>.
784      </p>
785      <p id="rfc.section.1.4.p.2">Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin
786         server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin
787         server (O).
788      </p>
789      <div id="rfc.figure.u.1"></div><pre class="drawing">       request chain ------------------------&gt;
790    UA -------------------v------------------- O
791       &lt;----------------------- response chain
792</pre><p id="rfc.section.1.4.p.4">A more complicated situation occurs when one or more intermediaries are present in the request/response chain. There are three
793         common forms of intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent, receiving requests for a URI in its
794         absolute form, rewriting all or part of the message, and forwarding the reformatted request toward the server identified by
795         the URI. A gateway is a receiving agent, acting as a layer above some other server(s) and, if necessary, translating the requests
796         to the underlying server's protocol. A tunnel acts as a relay point between two connections without changing the messages;
797         tunnels are used when the communication needs to pass through an intermediary (such as a firewall) even when the intermediary
798         cannot understand the contents of the messages.
799      </p>
800      <div id="rfc.figure.u.2"></div><pre class="drawing">       request chain --------------------------------------&gt;
801    UA -----v----- A -----v----- B -----v----- C -----v----- O
802       &lt;------------------------------------- response chain
803</pre><p id="rfc.section.1.4.p.6">The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response
804         message that travels the whole chain will pass through four separate connections. This distinction is important because some
805         HTTP communication options may apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points
806         of the chain, or to all connections along the chain. Although the diagram is linear, each participant may be engaged in multiple,
807         simultaneous communications. For example, B may be receiving requests from many clients other than A, and/or forwarding requests
808         to servers other than C, at the same time that it is handling A's request.
809      </p>
810      <p id="rfc.section.1.4.p.7">Any party to the communication which is not acting as a tunnel may employ an internal cache for handling requests. The effect
811         of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response
812         applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from
813         O (via C) for a request which has not been cached by UA or A.
814      </p>
815      <div id="rfc.figure.u.3"></div><pre class="drawing">          request chain ----------&gt;
816       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
817          &lt;--------- response chain
818</pre><p id="rfc.section.1.4.p.9">Not all responses are usefully cacheable, and some requests may contain modifiers which place special requirements on cache
819         behavior. HTTP requirements for cache behavior and cacheable responses are defined in <a href="p6-cache.html#caching" title="Caching in HTTP">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
820      </p>
821      <p id="rfc.section.1.4.p.10">In fact, there are a wide variety of architectures and configurations of caches and proxies currently being experimented with
822         or deployed across the World Wide Web. These systems include national hierarchies of proxy caches to save transoceanic bandwidth,
823         systems that broadcast or multicast cache entries, organizations that distribute subsets of cached data via CD-ROM, and so
824         on. HTTP systems are used in corporate intranets over high-bandwidth links, and for access via PDAs with low-power radio links
825         and intermittent connectivity. The goal of HTTP/1.1 is to support the wide diversity of configurations already deployed while
826         introducing protocol constructs that meet the needs of those who build web applications that require high reliability and,
827         failing that, at least reliable indications of failure.
828      </p>
829      <p id="rfc.section.1.4.p.11">HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80 (&lt;<a href="http://www.iana.org/assignments/port-numbers">http://www.iana.org/assignments/port-numbers</a>&gt;), but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet,
830         or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the
831         mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside
832         the scope of this specification.
833      </p>
834      <p id="rfc.section.1.4.p.12">In HTTP/1.0, most implementations used a new connection for each request/response exchange. In HTTP/1.1, a connection may
835         be used for one or more request/response exchanges, although connections may be closed for a variety of reasons (see <a href="#persistent.connections" title="Persistent Connections">Section&nbsp;7.1</a>).
836      </p>
837      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="notation" href="#notation">Notational Conventions and Generic Grammar</a></h1>
838      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="notation.abnf" href="#notation.abnf">Augmented BNF</a></h2>
839      <p id="rfc.section.2.1.p.1">All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar
840         to that used by RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.2"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a>. Implementors will need to be familiar with the notation in order to understand this specification. The augmented BNF includes
841         the following constructs:
842      </p>
843      <p id="rfc.section.2.1.p.2">name = definition </p>
844      <dl class="empty">
845         <dd>The name of a rule is simply the name itself (without any enclosing "&lt;" and "&gt;") and is separated from its definition by the
846            equal "=" character. White space is only significant in that indentation of continuation lines is used to indicate a rule
847            definition that spans more than one line. Certain basic rules are in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc.
848            Angle brackets are used within definitions whenever their presence will facilitate discerning the use of rule names.
849         </dd>
850      </dl>
851      <p id="rfc.section.2.1.p.3">"literal" </p>
852      <dl class="empty">
853         <dd>Quotation marks surround literal text. Unless stated otherwise, the text is case-insensitive.</dd>
854      </dl>
855      <p id="rfc.section.2.1.p.4">rule1 | rule2 </p>
856      <dl class="empty">
857         <dd>Elements separated by a bar ("|") are alternatives, e.g., "yes | no" will accept yes or no.</dd>
858      </dl>
859      <p id="rfc.section.2.1.p.5">(rule1 rule2) </p>
860      <dl class="empty">
861         <dd>Elements enclosed in parentheses are treated as a single element. Thus, "(elem (foo | bar) elem)" allows the token sequences
862            "elem foo elem" and "elem bar elem".
863         </dd>
864      </dl>
865      <p id="rfc.section.2.1.p.6">*rule </p>
866      <dl class="empty">
867         <dd>The character "*" preceding an element indicates repetition. The full form is "&lt;n&gt;*&lt;m&gt;element" indicating at least &lt;n&gt; and
868            at most &lt;m&gt; occurrences of element. Default values are 0 and infinity so that "*(element)" allows any number, including zero;
869            "1*element" requires at least one; and "1*2element" allows one or two.
870         </dd>
871      </dl>
872      <p id="rfc.section.2.1.p.7">[rule] </p>
873      <dl class="empty">
874         <dd>Square brackets enclose optional elements; "[foo bar]" is equivalent to "*1(foo bar)".</dd>
875      </dl>
876      <p id="rfc.section.2.1.p.8">N rule </p>
877      <dl class="empty">
878         <dd>Specific repetition: "&lt;n&gt;(element)" is equivalent to "&lt;n&gt;*&lt;n&gt;(element)"; that is, exactly &lt;n&gt; occurrences of (element). Thus
879            2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic characters.
880         </dd>
881      </dl>
882      <p id="rfc.section.2.1.p.9">#rule </p>
883      <dl class="empty">
884         <dd>A construct "#" is defined, similar to "*", for defining lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element" indicating at
885            least &lt;n&gt; and at most &lt;m&gt; elements, each separated by one or more commas (",") and <em class="bcp14">OPTIONAL</em> linear white space (LWS). This makes the usual form of lists very easy; a rule such as
886         </dd>
887         <dd>( *LWS element *( *LWS "," *LWS element ))</dd>
888         <dd>can be shown as</dd>
889         <dd>1#element</dd>
890         <dd>Wherever this construct is used, null elements are allowed, but do not contribute to the count of elements present. That is,
891            "(element), , (element) " is permitted, but counts as only two elements. Therefore, where at least one element is required,
892            at least one non-null element <em class="bcp14">MUST</em> be present. Default values are 0 and infinity so that "#element" allows any number, including zero; "1#element" requires at
893            least one; and "1#2element" allows one or two.
894         </dd>
895      </dl>
896      <p id="rfc.section.2.1.p.10">; comment </p>
897      <dl class="empty">
898         <dd>A semi-colon, set off some distance to the right of rule text, starts a comment that continues to the end of line. This is
899            a simple way of including useful notes in parallel with the specifications.
900         </dd>
901      </dl>
902      <p id="rfc.section.2.1.p.11">implied *LWS </p>
903      <dl class="empty">
904         <dd>The grammar described by this specification is word-based. Except where noted otherwise, linear white space (LWS) can be included
905            between any two adjacent words (token or quoted-string), and between adjacent words and separators, without changing the interpretation
906            of a field. At least one delimiter (LWS and/or separators) <em class="bcp14">MUST</em> exist between any two tokens (for the definition of "token" below), since they would otherwise be interpreted as a single
907            token.
908         </dd>
909      </dl>
910      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="basic.rules" href="#basic.rules">Basic Rules</a></h2>
911      <p id="rfc.section.2.2.p.1">The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII coded character
912         set is defined by ANSI X3.4-1986 <a href="#USASCII" id="rfc.xref.USASCII.1"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>.
913      </p>
914      <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span><span id="rfc.iref.g.7"></span><span id="rfc.iref.g.8"></span><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span>    OCTET          = &lt;any 8-bit sequence of data&gt;
915    CHAR           = &lt;any US-ASCII character (octets 0 - 127)&gt;
916    UPALPHA        = &lt;any US-ASCII uppercase letter "A".."Z"&gt;
917    LOALPHA        = &lt;any US-ASCII lowercase letter "a".."z"&gt;
918    ALPHA          = UPALPHA | LOALPHA
919    DIGIT          = &lt;any US-ASCII digit "0".."9"&gt;
920    CTL            = &lt;any US-ASCII control character
921                     (octets 0 - 31) and DEL (127)&gt;
922    CR             = &lt;US-ASCII CR, carriage return (13)&gt;
923    LF             = &lt;US-ASCII LF, linefeed (10)&gt;
924    SP             = &lt;US-ASCII SP, space (32)&gt;
925    HT             = &lt;US-ASCII HT, horizontal-tab (9)&gt;
926    &lt;"&gt;            = &lt;US-ASCII double-quote mark (34)&gt;
927</pre><p id="rfc.section.2.2.p.3">HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all protocol elements except the entity-body (see <a href="#tolerant.applications" title="Tolerant Applications">Appendix&nbsp;B</a> for tolerant applications). The end-of-line marker within an entity-body is defined by its associated media type, as described
928         in <a href="#Part3" id="rfc.xref.Part3.5"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>.
929      </p>
930      <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.g.13"></span>    CRLF           = CR LF
931</pre><p id="rfc.section.2.2.p.5">HTTP/1.1 header field values can be folded onto multiple lines if the continuation line begins with a space or horizontal
932         tab. All linear white space, including folding, has the same semantics as SP. A recipient <em class="bcp14">MAY</em> replace any linear white space with a single SP before interpreting the field value or forwarding the message downstream.
933      </p>
934      <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.14"></span>    LWS            = [CRLF] 1*( SP | HT )
935</pre><p id="rfc.section.2.2.p.7">The TEXT rule is only used for descriptive field contents and values that are not intended to be interpreted by the message
936         parser. Words of *TEXT <em class="bcp14">MAY</em> contain characters from character sets other than ISO-8859-1 <a href="#ISO-8859" id="rfc.xref.ISO-8859.1"><cite title="Information technology - 8-bit single byte coded graphic - character sets">[ISO-8859]</cite></a> only when encoded according to the rules of RFC 2047 <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a>.
937      </p>
938      <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.15"></span>    TEXT           = &lt;any OCTET except CTLs,
939                     but including LWS&gt;
940</pre><p id="rfc.section.2.2.p.9">A CRLF is allowed in the definition of TEXT only as part of a header field continuation. It is expected that the folding LWS
941         will be replaced with a single SP before interpretation of the TEXT value.
942      </p>
943      <p id="rfc.section.2.2.p.10">Hexadecimal numeric characters are used in several protocol elements.</p>
944      <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.16"></span>    HEX            = "A" | "B" | "C" | "D" | "E" | "F"
945                   | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
946</pre><p id="rfc.section.2.2.p.12">Many HTTP/1.1 header field values consist of words separated by LWS or special characters. These special characters <em class="bcp14">MUST</em> be in a quoted string to be used within a parameter value (as defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.4</a>).
947      </p>
948      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span>    token          = 1*&lt;any CHAR except CTLs or separators&gt;
949    separators     = "(" | ")" | "&lt;" | "&gt;" | "@"
950                   | "," | ";" | ":" | "\" | &lt;"&gt;
951                   | "/" | "[" | "]" | "?" | "="
952                   | "{" | "}" | SP | HT
953</pre><p id="rfc.section.2.2.p.14">Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed
954         in fields containing "comment" as part of their field value definition. In all other fields, parentheses are considered part
955         of the field value.
956      </p>
957      <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span>    comment        = "(" *( ctext | quoted-pair | comment ) ")"
958    ctext          = &lt;any TEXT excluding "(" and ")"&gt;
959</pre><p id="rfc.section.2.2.p.16">A string of text is parsed as a single word if it is quoted using double-quote marks.</p>
960      <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span>    quoted-string  = ( &lt;"&gt; *(qdtext | quoted-pair ) &lt;"&gt; )
961    qdtext         = &lt;any TEXT except &lt;"&gt;&gt;
962</pre><p id="rfc.section.2.2.p.18">The backslash character ("\") <em class="bcp14">MAY</em> be used as a single-character quoting mechanism only within quoted-string and comment constructs.
963      </p>
964      <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.23"></span>    quoted-pair    = "\" CHAR
965</pre><h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="protocol.parameters" href="#protocol.parameters">Protocol Parameters</a></h1>
966      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="http.version" href="#http.version">HTTP Version</a></h2>
967      <p id="rfc.section.3.1.p.1">HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended
968         to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather
969         than the features obtained via that communication. No change is made to the version number for the addition of message components
970         which do not affect communication behavior or which only add to extensible field values. The &lt;minor&gt; number is incremented
971         when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may
972         add to the message semantics and imply additional capabilities of the sender. The &lt;major&gt; number is incremented when the format
973         of a message within the protocol is changed. See RFC 2145 <a href="#RFC2145" id="rfc.xref.RFC2145.1"><cite title="Use and Interpretation of HTTP Version Numbers">[RFC2145]</cite></a> for a fuller explanation.
974      </p>
975      <p id="rfc.section.3.1.p.2">The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message. HTTP-Version is case-sensitive.</p>
976      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.24"></span>       HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
977</pre><p id="rfc.section.3.1.p.4">Note that the major and minor numbers <em class="bcp14">MUST</em> be treated as separate integers and that each <em class="bcp14">MAY</em> be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3.
978         Leading zeros <em class="bcp14">MUST</em> be ignored by recipients and <em class="bcp14">MUST NOT</em> be sent.
979      </p>
980      <p id="rfc.section.3.1.p.5">An application that sends a request or response message that includes HTTP-Version of "HTTP/1.1" <em class="bcp14">MUST</em> be at least conditionally compliant with this specification. Applications that are at least conditionally compliant with this
981         specification <em class="bcp14">SHOULD</em> use an HTTP-Version of "HTTP/1.1" in their messages, and <em class="bcp14">MUST</em> do so for any message that is not compatible with HTTP/1.0. For more details on when to send specific HTTP-Version values,
982         see RFC 2145 <a href="#RFC2145" id="rfc.xref.RFC2145.2"><cite title="Use and Interpretation of HTTP Version Numbers">[RFC2145]</cite></a>.
983      </p>
984      <p id="rfc.section.3.1.p.6">The HTTP version of an application is the highest HTTP version for which the application is at least conditionally compliant.</p>
985      <p id="rfc.section.3.1.p.7">Proxy and gateway applications need to be careful when forwarding messages in protocol versions different from that of the
986         application. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway <em class="bcp14">MUST NOT</em> send a message with a version indicator which is greater than its actual version. If a higher version request is received,
987         the proxy/gateway <em class="bcp14">MUST</em> either downgrade the request version, or respond with an error, or switch to tunnel behavior.
988      </p>
989      <p id="rfc.section.3.1.p.8">Due to interoperability problems with HTTP/1.0 proxies discovered since the publication of RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a>, caching proxies MUST, gateways <em class="bcp14">MAY</em>, and tunnels <em class="bcp14">MUST NOT</em> upgrade the request to the highest version they support. The proxy/gateway's response to that request <em class="bcp14">MUST</em> be in the same major version as the request.
990      </p>
991      <p id="rfc.section.3.1.p.9"> </p>
992      <dl class="empty">
993         <dd> <b>Note:</b> Converting between versions of HTTP may involve modification of header fields required or forbidden by the versions involved.
994         </dd>
995      </dl>
996      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="uri" href="#uri">Uniform Resource Identifiers</a></h2>
997      <p id="rfc.section.3.2.p.1">URIs have been known by many names: WWW addresses, Universal Document Identifiers, Universal Resource Identifiers <a href="#RFC1630" id="rfc.xref.RFC1630.2"><cite title="Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web">[RFC1630]</cite></a>, and finally the combination of Uniform Resource Locators (URL) <a href="#RFC1738" id="rfc.xref.RFC1738.2"><cite title="Uniform Resource Locators (URL)">[RFC1738]</cite></a> and Names (URN) <a href="#RFC1737" id="rfc.xref.RFC1737.2"><cite title="Functional Requirements for Uniform Resource Names">[RFC1737]</cite></a>. As far as HTTP is concerned, Uniform Resource Identifiers are simply formatted strings which identify--via name, location,
998         or any other characteristic--a resource.
999      </p>
1000      <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="general.syntax" href="#general.syntax">General Syntax</a></h3>
1001      <p id="rfc.section.3.2.1.p.1">URIs in HTTP can be represented in absolute form or relative to some known base URI <a href="#RFC1808" id="rfc.xref.RFC1808.1"><cite title="Relative Uniform Resource Locators">[RFC1808]</cite></a>, depending upon the context of their use. The two forms are differentiated by the fact that absolute URIs always begin with
1002         a scheme name followed by a colon. For definitive information on URL syntax and semantics, see "Uniform Resource Identifiers
1003         (URI): Generic Syntax and Semantics," RFC 2396 <a href="#RFC2396" id="rfc.xref.RFC2396.1"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[RFC2396]</cite></a> (which replaces RFCs 1738 <a href="#RFC1738" id="rfc.xref.RFC1738.3"><cite title="Uniform Resource Locators (URL)">[RFC1738]</cite></a> and RFC 1808 <a href="#RFC1808" id="rfc.xref.RFC1808.2"><cite title="Relative Uniform Resource Locators">[RFC1808]</cite></a>). This specification adopts the definitions of "URI-reference", "absoluteURI", "relativeURI", "port", "host","abs_path",
1004         "rel_path", and "authority" from that specification.
1005      </p>
1006      <p id="rfc.section.3.2.1.p.2">The HTTP protocol does not place any a priori limit on the length of a URI. Servers <em class="bcp14">MUST</em> be able to handle the URI of any resource they serve, and <em class="bcp14">SHOULD</em> be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server <em class="bcp14">SHOULD</em> return 414 (Request-URI Too Long) status if a URI is longer than the server can handle (see <a href="p2-semantics.html#status.414" title="414 Request-URI Too Long">Section 9.4.15</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
1007      </p>
1008      <p id="rfc.section.3.2.1.p.3"> </p>
1009      <dl class="empty">
1010         <dd> <b>Note:</b> Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations
1011            might not properly support these lengths.
1012         </dd>
1013      </dl>
1014      <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="http.url" href="#http.url">http URL</a></h3>
1015      <p id="rfc.section.3.2.2.p.1">The "http" scheme is used to locate network resources via the HTTP protocol. This section defines the scheme-specific syntax
1016         and semantics for http URLs.
1017      </p>
1018      <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.25"></span>http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
1019</pre><p id="rfc.section.3.2.2.p.3">If the port is empty or not given, port 80 is assumed. The semantics are that the identified resource is located at the server
1020         listening for TCP connections on that port of that host, and the Request-URI for the resource is abs_path (<a href="#request-uri" title="Request-URI">Section&nbsp;5.1.2</a>). The use of IP addresses in URLs <em class="bcp14">SHOULD</em> be avoided whenever possible (see RFC 1900 <a href="#RFC1900" id="rfc.xref.RFC1900.1"><cite title="Renumbering Needs Work">[RFC1900]</cite></a>). If the abs_path is not present in the URL, it <em class="bcp14">MUST</em> be given as "/" when used as a Request-URI for a resource (<a href="#request-uri" title="Request-URI">Section&nbsp;5.1.2</a>). If a proxy receives a host name which is not a fully qualified domain name, it <em class="bcp14">MAY</em> add its domain to the host name it received. If a proxy receives a fully qualified domain name, the proxy <em class="bcp14">MUST NOT</em> change the host name.
1021      </p>
1022      <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="uri.comparison" href="#uri.comparison">URI Comparison</a></h3>
1023      <p id="rfc.section.3.2.3.p.1">When comparing two URIs to decide if they match or not, a client <em class="bcp14">SHOULD</em> use a case-sensitive octet-by-octet comparison of the entire URIs, with these exceptions:
1024      </p>
1025      <ul>
1026         <li>A port that is empty or not given is equivalent to the default port for that URI-reference;</li>
1027         <li>Comparisons of host names <em class="bcp14">MUST</em> be case-insensitive;
1028         </li>
1029         <li>Comparisons of scheme names <em class="bcp14">MUST</em> be case-insensitive;
1030         </li>
1031         <li>An empty abs_path is equivalent to an abs_path of "/".</li>
1032      </ul>
1033      <p id="rfc.section.3.2.3.p.2">Characters other than those in the "reserved" set (see RFC 2396 <a href="#RFC2396" id="rfc.xref.RFC2396.2"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[RFC2396]</cite></a>) are equivalent to their ""%" HEX HEX" encoding.
1034      </p>
1035      <p id="rfc.section.3.2.3.p.3">For example, the following three URIs are equivalent:</p>
1036      <div id="rfc.figure.u.15"></div><pre class="text">   http://example.com:80/~smith/home.html
1037   http://EXAMPLE.com/%7Esmith/home.html
1038   http://EXAMPLE.com:/%7esmith/home.html
1039</pre><h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="date.time.formats" href="#date.time.formats">Date/Time Formats</a></h2>
1040      <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a id="full.date" href="#full.date">Full Date</a></h3>
1041      <p id="rfc.section.3.3.1.p.1">HTTP applications have historically allowed three different formats for the representation of date/time stamps:</p>
1042      <div id="rfc.figure.u.16"></div><pre class="text">   Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
1043   Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
1044   Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
1045</pre><p id="rfc.section.3.3.1.p.3">The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123 <a href="#RFC1123" id="rfc.xref.RFC1123.1"><cite title="Requirements for Internet Hosts - Application and Support">[RFC1123]</cite></a> (an update to RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.3"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a>). The other formats are described here only for compatibility with obsolete implementations. HTTP/1.1 clients and servers
1046         that parse the date value <em class="bcp14">MUST</em> accept all three formats (for compatibility with HTTP/1.0), though they <em class="bcp14">MUST</em> only generate the RFC 1123 format for representing HTTP-date values in header fields. See <a href="#tolerant.applications" title="Tolerant Applications">Appendix&nbsp;B</a> for further information.
1047      </p>
1048      <dl class="empty">
1049         <dd> <b>Note:</b> Recipients of date values are encouraged to be robust in accepting date values that may have been sent by non-HTTP applications,
1050            as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.
1051         </dd>
1052      </dl>
1053      <p id="rfc.section.3.3.1.p.5">All HTTP date/time stamps <em class="bcp14">MUST</em> be represented in Greenwich Mean Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal to UTC (Coordinated
1054         Universal Time). This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for
1055         time zone, and <em class="bcp14">MUST</em> be assumed when reading the asctime format. HTTP-date is case sensitive and <em class="bcp14">MUST NOT</em> include additional LWS beyond that specifically included as SP in the grammar.
1056      </p>
1057      <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.26"></span><span id="rfc.iref.g.27"></span><span id="rfc.iref.g.28"></span><span id="rfc.iref.g.29"></span><span id="rfc.iref.g.30"></span><span id="rfc.iref.g.31"></span><span id="rfc.iref.g.32"></span><span id="rfc.iref.g.33"></span><span id="rfc.iref.g.34"></span><span id="rfc.iref.g.35"></span><span id="rfc.iref.g.36"></span>    HTTP-date    = rfc1123-date | rfc850-date | asctime-date
1058    rfc1123-date = wkday "," SP date1 SP time SP "GMT"
1059    rfc850-date  = weekday "," SP date2 SP time SP "GMT"
1060    asctime-date = wkday SP date3 SP time SP 4DIGIT
1061    date1        = 2DIGIT SP month SP 4DIGIT
1062                   ; day month year (e.g., 02 Jun 1982)
1063    date2        = 2DIGIT "-" month "-" 2DIGIT
1064                   ; day-month-year (e.g., 02-Jun-82)
1065    date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
1066                   ; month day (e.g., Jun  2)
1067    time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
1068                   ; 00:00:00 - 23:59:59
1069    wkday        = "Mon" | "Tue" | "Wed"
1070                 | "Thu" | "Fri" | "Sat" | "Sun"
1071    weekday      = "Monday" | "Tuesday" | "Wednesday"
1072                 | "Thursday" | "Friday" | "Saturday" | "Sunday"
1073    month        = "Jan" | "Feb" | "Mar" | "Apr"
1074                 | "May" | "Jun" | "Jul" | "Aug"
1075                 | "Sep" | "Oct" | "Nov" | "Dec"
1076</pre><p id="rfc.section.3.3.1.p.7"> <b>Note:</b> HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers
1077         are not required to use these formats for user presentation, request logging, etc.
1078      </p>
1079      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="transfer.codings" href="#transfer.codings">Transfer Codings</a></h2>
1080      <p id="rfc.section.3.4.p.1">Transfer-coding values are used to indicate an encoding transformation that has been, can be, or may need to be applied to
1081         an entity-body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer-coding
1082         is a property of the message, not of the original entity.
1083      </p>
1084      <div id="rfc.figure.u.18"></div><pre class="inline"><span id="rfc.iref.g.37"></span><span id="rfc.iref.g.38"></span>    transfer-coding         = "chunked" | transfer-extension
1085    transfer-extension      = token *( ";" parameter )
1086</pre><p id="rfc.section.3.4.p.3">Parameters are in the form of attribute/value pairs.</p>
1087      <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>    parameter               = attribute "=" value
1088    attribute               = token
1089    value                   = token | quoted-string
1090</pre><p id="rfc.section.3.4.p.5">All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer-coding values in the TE header field (<a href="#header.te" id="rfc.xref.header.te.1" title="TE">Section&nbsp;8.5</a>) and in the Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.1" title="Transfer-Encoding">Section&nbsp;8.7</a>).
1091      </p>
1092      <p id="rfc.section.3.4.p.6">Whenever a transfer-coding is applied to a message-body, the set of transfer-codings <em class="bcp14">MUST</em> include "chunked", unless the message is terminated by closing the connection. When the "chunked" transfer-coding is used,
1093         it <em class="bcp14">MUST</em> be the last transfer-coding applied to the message-body. The "chunked" transfer-coding <em class="bcp14">MUST NOT</em> be applied more than once to a message-body. These rules allow the recipient to determine the transfer-length of the message
1094         (<a href="#message.length" title="Message Length">Section&nbsp;4.4</a>).
1095      </p>
1096      <p id="rfc.section.3.4.p.7">Transfer-codings are analogous to the Content-Transfer-Encoding values of MIME <a href="#RFC2045" id="rfc.xref.RFC2045.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>, which were designed to enable safe transport of binary data over a 7-bit transport service. However, safe transport has
1097         a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message-bodies is the difficulty
1098         in determining the exact body length (<a href="#message.length" title="Message Length">Section&nbsp;4.4</a>), or the desire to encrypt data over a shared transport.
1099      </p>
1100      <p id="rfc.section.3.4.p.8">The Internet Assigned Numbers Authority (IANA) acts as a registry for transfer-coding value tokens. Initially, the registry
1101         contains the following tokens: "chunked" (<a href="#chunked.transfer.encoding" title="Chunked Transfer Coding">Section&nbsp;3.4.1</a>), "gzip" (<a href="#Part3" id="rfc.xref.Part3.6"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), "compress" (<a href="#Part3" id="rfc.xref.Part3.7"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), and "deflate" (<a href="#Part3" id="rfc.xref.Part3.8"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>).
1102      </p>
1103      <p id="rfc.section.3.4.p.9">New transfer-coding value tokens <em class="bcp14">SHOULD</em> be registered in the same way as new content-coding value tokens (<a href="#Part3" id="rfc.xref.Part3.9"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>).
1104      </p>
1105      <p id="rfc.section.3.4.p.10">A server which receives an entity-body with a transfer-coding it does not understand <em class="bcp14">SHOULD</em> return 501 (Unimplemented), and close the connection. A server <em class="bcp14">MUST NOT</em> send transfer-codings to an HTTP/1.0 client.
1106      </p>
1107      <h3 id="rfc.section.3.4.1"><a href="#rfc.section.3.4.1">3.4.1</a>&nbsp;<a id="chunked.transfer.encoding" href="#chunked.transfer.encoding">Chunked Transfer Coding</a></h3>
1108      <p id="rfc.section.3.4.1.p.1">The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size
1109         indicator, followed by an <em class="bcp14">OPTIONAL</em> trailer containing entity-header fields. This allows dynamically produced content to be transferred along with the information
1110         necessary for the recipient to verify that it has received the full message.
1111      </p>
1112      <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><span id="rfc.iref.g.46"></span><span id="rfc.iref.g.47"></span><span id="rfc.iref.g.48"></span><span id="rfc.iref.g.49"></span><span id="rfc.iref.g.50"></span>    Chunked-Body   = *chunk
1113                     last-chunk
1114                     trailer
1115                     CRLF
1116
1117    chunk          = chunk-size [ chunk-extension ] CRLF
1118                     chunk-data CRLF
1119    chunk-size     = 1*HEX
1120    last-chunk     = 1*("0") [ chunk-extension ] CRLF
1121
1122    chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
1123    chunk-ext-name = token
1124    chunk-ext-val  = token | quoted-string
1125    chunk-data     = chunk-size(OCTET)
1126    trailer        = *(entity-header CRLF)
1127</pre><p id="rfc.section.3.4.1.p.3">The chunk-size field is a string of hex digits indicating the size of the chunk-data in octets. The chunked encoding is ended
1128         by any chunk whose size is zero, followed by the trailer, which is terminated by an empty line.
1129      </p>
1130      <p id="rfc.section.3.4.1.p.4">The trailer allows the sender to include additional HTTP header fields at the end of the message. The Trailer header field
1131         can be used to indicate which header fields are included in a trailer (see <a href="#header.trailer" id="rfc.xref.header.trailer.1" title="Trailer">Section&nbsp;8.6</a>).
1132      </p>
1133      <p id="rfc.section.3.4.1.p.5">A server using chunked transfer-coding in a response <em class="bcp14">MUST NOT</em> use the trailer for any header fields unless at least one of the following is true:
1134      </p>
1135      <ol>
1136         <li>the request included a TE header field that indicates "trailers" is acceptable in the transfer-coding of the response, as
1137            described in <a href="#header.te" id="rfc.xref.header.te.2" title="TE">Section&nbsp;8.5</a>; or,
1138         </li>
1139         <li>the server is the origin server for the response, the trailer fields consist entirely of optional metadata, and the recipient
1140            could use the message (in a manner acceptable to the origin server) without receiving this metadata. In other words, the origin
1141            server is willing to accept the possibility that the trailer fields might be silently discarded along the path to the client.
1142         </li>
1143      </ol>
1144      <p id="rfc.section.3.4.1.p.6">This requirement prevents an interoperability failure when the message is being received by an HTTP/1.1 (or later) proxy and
1145         forwarded to an HTTP/1.0 recipient. It avoids a situation where compliance with the protocol would have necessitated a possibly
1146         infinite buffer on the proxy.
1147      </p>
1148      <p id="rfc.section.3.4.1.p.7">A process for decoding the "chunked" transfer-coding can be represented in pseudo-code as:</p>
1149      <div id="rfc.figure.u.21"></div><pre class="text">    length := 0
1150    read chunk-size, chunk-extension (if any) and CRLF
1151    while (chunk-size &gt; 0) {
1152       read chunk-data and CRLF
1153       append chunk-data to entity-body
1154       length := length + chunk-size
1155       read chunk-size and CRLF
1156    }
1157    read entity-header
1158    while (entity-header not empty) {
1159       append entity-header to existing header fields
1160       read entity-header
1161    }
1162    Content-Length := length
1163    Remove "chunked" from Transfer-Encoding
1164</pre><p id="rfc.section.3.4.1.p.9">All HTTP/1.1 applications <em class="bcp14">MUST</em> be able to receive and decode the "chunked" transfer-coding, and <em class="bcp14">MUST</em> ignore chunk-extension extensions they do not understand.
1165      </p>
1166      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="http.message" href="#http.message">HTTP Message</a></h1>
1167      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="message.types" href="#message.types">Message Types</a></h2>
1168      <p id="rfc.section.4.1.p.1">HTTP messages consist of requests from client to server and responses from server to client.</p>
1169      <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.51"></span>    HTTP-message   = Request | Response     ; HTTP/1.1 messages
1170</pre><p id="rfc.section.4.1.p.3">Request (<a href="#request" title="Request">Section&nbsp;5</a>) and Response (<a href="#response" title="Response">Section&nbsp;6</a>) messages use the generic message format of RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.4"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a> for transferring entities (the payload of the message). Both types of message consist of a start-line, zero or more header
1171         fields (also known as "headers"), an empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header
1172         fields, and possibly a message-body.
1173      </p>
1174      <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.52"></span><span id="rfc.iref.g.53"></span>     generic-message = start-line
1175                       *(message-header CRLF)
1176                       CRLF
1177                       [ message-body ]
1178     start-line      = Request-Line | Status-Line
1179</pre><p id="rfc.section.4.1.p.5">In the interest of robustness, servers <em class="bcp14">SHOULD</em> ignore any empty line(s) received where a Request-Line is expected. In other words, if the server is reading the protocol
1180         stream at the beginning of a message and receives a CRLF first, it should ignore the CRLF.
1181      </p>
1182      <p id="rfc.section.4.1.p.6">Certain buggy HTTP/1.0 client implementations generate extra CRLF's after a POST request. To restate what is explicitly forbidden
1183         by the BNF, an HTTP/1.1 client <em class="bcp14">MUST NOT</em> preface or follow a request with an extra CRLF.
1184      </p>
1185      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="message.headers" href="#message.headers">Message Headers</a></h2>
1186      <p id="rfc.section.4.2.p.1">HTTP header fields, which include general-header (<a href="#general.header.fields" title="General Header Fields">Section&nbsp;4.5</a>), request-header (<a href="p2-semantics.html#request.header.fields" title="Request Header Fields">Section 4</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>), response-header (<a href="p2-semantics.html#response.header.fields" title="Response Header Fields">Section 6</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>), and entity-header (<a href="p3-payload.html#entity.header.fields" title="Entity Header Fields">Section 3.1</a> of <a href="#Part3" id="rfc.xref.Part3.10"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>) fields, follow the same generic format as that given in <a href="http://tools.ietf.org/html/rfc822#section-3.1" id="rfc.xref.RFC822.5">Section 3.1</a> of RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.6"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a>. Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive. The
1187         field value <em class="bcp14">MAY</em> be preceded by any amount of LWS, though a single SP is preferred. Header fields can be extended over multiple lines by preceding
1188         each extra line with at least one SP or HT. Applications ought to follow "common form", where one is known or indicated, when
1189         generating HTTP constructs, since there might exist some implementations that fail to accept anything beyond the common forms.
1190      </p>
1191      <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.54"></span><span id="rfc.iref.g.55"></span><span id="rfc.iref.g.56"></span><span id="rfc.iref.g.57"></span>    message-header = field-name ":" [ field-value ]
1192    field-name     = token
1193    field-value    = *( field-content | LWS )
1194    field-content  = &lt;the OCTETs making up the field-value
1195                     and consisting of either *TEXT or combinations
1196                     of token, separators, and quoted-string&gt;
1197</pre><p id="rfc.section.4.2.p.3">The field-content does not include any leading or trailing LWS: linear white space occurring before the first non-whitespace
1198         character of the field-value or after the last non-whitespace character of the field-value. Such leading or trailing LWS <em class="bcp14">MAY</em> be removed without changing the semantics of the field value. Any LWS that occurs between field-content <em class="bcp14">MAY</em> be replaced with a single SP before interpreting the field value or forwarding the message downstream.
1199      </p>
1200      <p id="rfc.section.4.2.p.4">The order in which header fields with differing field names are received is not significant. However, it is "good practice"
1201         to send general-header fields first, followed by request-header or response-header fields, and ending with the entity-header
1202         fields.
1203      </p>
1204      <p id="rfc.section.4.2.p.5">Multiple message-header fields with the same field-name <em class="bcp14">MAY</em> be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e.,
1205         #(values)]. It <em class="bcp14">MUST</em> be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics
1206         of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header
1207         fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and
1208         thus a proxy <em class="bcp14">MUST NOT</em> change the order of these field values when a message is forwarded.
1209      </p>
1210      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="message.body" href="#message.body">Message Body</a></h2>
1211      <p id="rfc.section.4.3.p.1">The message-body (if any) of an HTTP message is used to carry the entity-body associated with the request or response. The
1212         message-body differs from the entity-body only when a transfer-coding has been applied, as indicated by the Transfer-Encoding
1213         header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.2" title="Transfer-Encoding">Section&nbsp;8.7</a>).
1214      </p>
1215      <div id="rfc.figure.u.25"></div><pre class="inline"><span id="rfc.iref.g.58"></span>    message-body = entity-body
1216                 | &lt;entity-body encoded as per Transfer-Encoding&gt;
1217</pre><p id="rfc.section.4.3.p.3">Transfer-Encoding <em class="bcp14">MUST</em> be used to indicate any transfer-codings applied by an application to ensure safe and proper transfer of the message. Transfer-Encoding
1218         is a property of the message, not of the entity, and thus <em class="bcp14">MAY</em> be added or removed by any application along the request/response chain. (However, <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.4</a> places restrictions on when certain transfer-codings may be used.)
1219      </p>
1220      <p id="rfc.section.4.3.p.4">The rules for when a message-body is allowed in a message differ for requests and responses.</p>
1221      <p id="rfc.section.4.3.p.5">The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field
1222         in the request's message-headers. A message-body <em class="bcp14">MUST NOT</em> be included in a request if the specification of the request method (<a href="p2-semantics.html#method" title="Method">Section 3</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) does not allow sending an entity-body in requests. A server <em class="bcp14">SHOULD</em> read and forward a message-body on any request; if the request method does not include defined semantics for an entity-body,
1223         then the message-body <em class="bcp14">SHOULD</em> be ignored when handling the request.
1224      </p>
1225      <p id="rfc.section.4.3.p.6">For response messages, whether or not a message-body is included with a message is dependent on both the request method and
1226         the response status code (<a href="#status.code.and.reason.phrase" title="Status Code and Reason Phrase">Section&nbsp;6.1.1</a>). All responses to the HEAD request method <em class="bcp14">MUST NOT</em> include a message-body, even though the presence of entity-header fields might lead one to believe they do. All 1xx (informational),
1227         204 (no content), and 304 (not modified) responses <em class="bcp14">MUST NOT</em> include a message-body. All other responses do include a message-body, although it <em class="bcp14">MAY</em> be of zero length.
1228      </p>
1229      <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a id="message.length" href="#message.length">Message Length</a></h2>
1230      <p id="rfc.section.4.4.p.1">The transfer-length of a message is the length of the message-body as it appears in the message; that is, after any transfer-codings
1231         have been applied. When a message-body is included with a message, the transfer-length of that body is determined by one of
1232         the following (in order of precedence):
1233      </p>
1234      <p id="rfc.section.4.4.p.2"> </p>
1235      <ol>
1236         <li>
1237            <p>Any response message which "<em class="bcp14">MUST NOT</em>" include a message-body (such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated
1238               by the first empty line after the header fields, regardless of the entity-header fields present in the message.
1239            </p>
1240         </li>
1241         <li>
1242            <p>If a Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.3" title="Transfer-Encoding">Section&nbsp;8.7</a>) is present, then the transfer-length is defined by use of the "chunked" transfer-coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.4</a>), unless the message is terminated by closing the connection.
1243            </p>
1244         </li>
1245         <li>
1246            <p>If a Content-Length header field (<a href="#header.content-length" id="rfc.xref.header.content-length.1" title="Content-Length">Section&nbsp;8.2</a>) is present, its decimal value in OCTETs represents both the entity-length and the transfer-length. The Content-Length header
1247               field <em class="bcp14">MUST NOT</em> be sent if these two lengths are different (i.e., if a Transfer-Encoding header field is present). If a message is received
1248               with both a Transfer-Encoding header field and a Content-Length header field, the latter <em class="bcp14">MUST</em> be ignored.
1249            </p>
1250         </li>
1251         <li>
1252            <p>If the message uses the media type "multipart/byteranges", and the transfer-length is not otherwise specified, then this self-delimiting
1253               media type defines the transfer-length. This media type <em class="bcp14">MUST NOT</em> be used unless the sender knows that the recipient can parse it; the presence in a request of a Range header with multiple
1254               byte-range specifiers from a 1.1 client implies that the client can parse multipart/byteranges responses.
1255            </p>
1256            <dl class="empty">
1257               <dd>A range header might be forwarded by a 1.0 proxy that does not understand multipart/byteranges; in this case the server <em class="bcp14">MUST</em> delimit the message using methods defined in items 1, 3 or 5 of this section.
1258               </dd>
1259            </dl>
1260         </li>
1261         <li>
1262            <p>By the server closing the connection. (Closing the connection cannot be used to indicate the end of a request body, since
1263               that would leave no possibility for the server to send back a response.)
1264            </p>
1265         </li>
1266      </ol>
1267      <p id="rfc.section.4.4.p.3">For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing a message-body <em class="bcp14">MUST</em> include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. If a request contains a message-body
1268         and a Content-Length is not given, the server <em class="bcp14">SHOULD</em> respond with 400 (bad request) if it cannot determine the length of the message, or with 411 (length required) if it wishes
1269         to insist on receiving a valid Content-Length.
1270      </p>
1271      <p id="rfc.section.4.4.p.4">All HTTP/1.1 applications that receive entities <em class="bcp14">MUST</em> accept the "chunked" transfer-coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.4</a>), thus allowing this mechanism to be used for messages when the message length cannot be determined in advance.
1272      </p>
1273      <p id="rfc.section.4.4.p.5">Messages <em class="bcp14">MUST NOT</em> include both a Content-Length header field and a transfer-coding. If the message does include a transfer-coding, the Content-Length <em class="bcp14">MUST</em> be ignored.
1274      </p>
1275      <p id="rfc.section.4.4.p.6">When a Content-Length is given in a message where a message-body is allowed, its field value <em class="bcp14">MUST</em> exactly match the number of OCTETs in the message-body. HTTP/1.1 user agents <em class="bcp14">MUST</em> notify the user when an invalid length is received and detected.
1276      </p>
1277      <h2 id="rfc.section.4.5"><a href="#rfc.section.4.5">4.5</a>&nbsp;<a id="general.header.fields" href="#general.header.fields">General Header Fields</a></h2>
1278      <p id="rfc.section.4.5.p.1">There are a few header fields which have general applicability for both request and response messages, but which do not apply
1279         to the entity being transferred. These header fields apply only to the message being transmitted.
1280      </p>
1281      <div id="rfc.figure.u.26"></div><pre class="inline"><span id="rfc.iref.g.59"></span>    general-header = Cache-Control            ; <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 3.2</a>
1282                   | Connection               ; <a href="#header.connection" id="rfc.xref.header.connection.1" title="Connection">Section&nbsp;8.1</a>
1283                   | Date                     ; <a href="#header.date" id="rfc.xref.header.date.1" title="Date">Section&nbsp;8.3</a>
1284                   | Pragma                   ; <a href="#Part6" id="rfc.xref.Part6.4"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.pragma" title="Pragma">Section 3.4</a>
1285                   | Trailer                  ; <a href="#header.trailer" id="rfc.xref.header.trailer.2" title="Trailer">Section&nbsp;8.6</a>
1286                   | Transfer-Encoding        ; <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.4" title="Transfer-Encoding">Section&nbsp;8.7</a>
1287                   | Upgrade                  ; <a href="#header.upgrade" id="rfc.xref.header.upgrade.1" title="Upgrade">Section&nbsp;8.8</a>
1288                   | Via                      ; <a href="#header.via" id="rfc.xref.header.via.1" title="Via">Section&nbsp;8.9</a>
1289                   | Warning                  ; <a href="#Part6" id="rfc.xref.Part6.5"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>, <a href="p6-cache.html#header.warning" title="Warning">Section 3.6</a>
1290</pre><p id="rfc.section.4.5.p.3">General-header field names can be extended reliably only in combination with a change in the protocol version. However, new
1291         or experimental header fields may be given the semantics of general header fields if all parties in the communication recognize
1292         them to be general-header fields. Unrecognized header fields are treated as entity-header fields.
1293      </p>
1294      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="request" href="#request">Request</a></h1>
1295      <p id="rfc.section.5.p.1">A request message from a client to a server includes, within the first line of that message, the method to be applied to the
1296         resource, the identifier of the resource, and the protocol version in use.
1297      </p>
1298      <div id="rfc.figure.u.27"></div><pre class="inline"><span id="rfc.iref.g.60"></span>     Request       = Request-Line              ; <a href="#request-line" title="Request-Line">Section&nbsp;5.1</a>
1299                     *(( general-header        ; <a href="#general.header.fields" title="General Header Fields">Section&nbsp;4.5</a>
1300                      | request-header         ; <a href="#Part2" id="rfc.xref.Part2.5"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, <a href="p2-semantics.html#request.header.fields" title="Request Header Fields">Section 4</a>
1301                      | entity-header ) CRLF)  ; <a href="#Part3" id="rfc.xref.Part3.11"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>, <a href="p3-payload.html#entity.header.fields" title="Entity Header Fields">Section 3.1</a>
1302                     CRLF
1303                     [ message-body ]          ; <a href="#message.body" title="Message Body">Section&nbsp;4.3</a>
1304</pre><h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="request-line" href="#request-line">Request-Line</a></h2>
1305      <p id="rfc.section.5.1.p.1">The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending with CRLF. The
1306         elements are separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.
1307      </p>
1308      <div id="rfc.figure.u.28"></div><pre class="inline"><span id="rfc.iref.g.61"></span>     Request-Line   = Method SP Request-URI SP HTTP-Version CRLF
1309</pre><h3 id="rfc.section.5.1.1"><a href="#rfc.section.5.1.1">5.1.1</a>&nbsp;<a id="method" href="#method">Method</a></h3>
1310      <p id="rfc.section.5.1.1.p.1">The Method token indicates the method to be performed on the resource identified by the Request-URI. The method is case-sensitive.</p>
1311      <div id="rfc.figure.u.29"></div><pre class="inline"><span id="rfc.iref.g.62"></span><span id="rfc.iref.g.63"></span>    Method         = token
1312</pre><h3 id="rfc.section.5.1.2"><a href="#rfc.section.5.1.2">5.1.2</a>&nbsp;<a id="request-uri" href="#request-uri">Request-URI</a></h3>
1313      <p id="rfc.section.5.1.2.p.1">The Request-URI is a Uniform Resource Identifier (<a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;3.2</a>) and identifies the resource upon which to apply the request.
1314      </p>
1315      <div id="rfc.figure.u.30"></div><pre class="inline"><span id="rfc.iref.g.64"></span>    Request-URI    = "*"
1316                   | absoluteURI
1317                   | ( abs_path [ "?" query ] )
1318                   | authority
1319</pre><p id="rfc.section.5.1.2.p.3">The four options for Request-URI are dependent on the nature of the request. The asterisk "*" means that the request does
1320         not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily
1321         apply to a resource. One example would be
1322      </p>
1323      <div id="rfc.figure.u.31"></div><pre class="text">    OPTIONS * HTTP/1.1
1324</pre><p id="rfc.section.5.1.2.p.5">The absoluteURI form is <em class="bcp14">REQUIRED</em> when the request is being made to a proxy. The proxy is requested to forward the request or service it from a valid cache,
1325         and return the response. Note that the proxy <em class="bcp14">MAY</em> forward the request on to another proxy or directly to the server specified by the absoluteURI. In order to avoid request
1326         loops, a proxy <em class="bcp14">MUST</em> be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. An example
1327         Request-Line would be:
1328      </p>
1329      <div id="rfc.figure.u.32"></div><pre class="text">    GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
1330</pre><p id="rfc.section.5.1.2.p.7">To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers <em class="bcp14">MUST</em> accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies.
1331      </p>
1332      <p id="rfc.section.5.1.2.p.8">The authority form is only used by the CONNECT method (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 8.9</a> of <a href="#Part2" id="rfc.xref.Part2.6"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
1333      </p>
1334      <p id="rfc.section.5.1.2.p.9">The most common form of Request-URI is that used to identify a resource on an origin server or gateway. In this case the absolute
1335         path of the URI <em class="bcp14">MUST</em> be transmitted (see <a href="#general.syntax" title="General Syntax">Section&nbsp;3.2.1</a>, abs_path) as the Request-URI, and the network location of the URI (authority) <em class="bcp14">MUST</em> be transmitted in a Host header field. For example, a client wishing to retrieve the resource above directly from the origin
1336         server would create a TCP connection to port 80 of the host "www.example.org" and send the lines:
1337      </p>
1338      <div id="rfc.figure.u.33"></div><pre class="text">    GET /pub/WWW/TheProject.html HTTP/1.1
1339    Host: www.example.org
1340</pre><p id="rfc.section.5.1.2.p.11">followed by the remainder of the Request. Note that the absolute path cannot be empty; if none is present in the original
1341         URI, it <em class="bcp14">MUST</em> be given as "/" (the server root).
1342      </p>
1343      <p id="rfc.section.5.1.2.p.12">The Request-URI is transmitted in the format specified in <a href="#general.syntax" title="General Syntax">Section&nbsp;3.2.1</a>. If the Request-URI is encoded using the "% HEX HEX" encoding <a href="#RFC2396" id="rfc.xref.RFC2396.3"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[RFC2396]</cite></a>, the origin server <em class="bcp14">MUST</em> decode the Request-URI in order to properly interpret the request. Servers <em class="bcp14">SHOULD</em> respond to invalid Request-URIs with an appropriate status code.
1344      </p>
1345      <p id="rfc.section.5.1.2.p.13">A transparent proxy <em class="bcp14">MUST NOT</em> rewrite the "abs_path" part of the received Request-URI when forwarding it to the next inbound server, except as noted above
1346         to replace a null abs_path with "/".
1347      </p>
1348      <p id="rfc.section.5.1.2.p.14"> </p>
1349      <dl class="empty">
1350         <dd> <b>Note:</b> The "no rewrite" rule prevents the proxy from changing the meaning of the request when the origin server is improperly using
1351            a non-reserved URI character for a reserved purpose. Implementors should be aware that some pre-HTTP/1.1 proxies have been
1352            known to rewrite the Request-URI.
1353         </dd>
1354      </dl>
1355      <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="the.resource.identified.by.a.request" href="#the.resource.identified.by.a.request">The Resource Identified by a Request</a></h2>
1356      <p id="rfc.section.5.2.p.1">The exact resource identified by an Internet request is determined by examining both the Request-URI and the Host header field.</p>
1357      <p id="rfc.section.5.2.p.2">An origin server that does not allow resources to differ by the requested host <em class="bcp14">MAY</em> ignore the Host header field value when determining the resource identified by an HTTP/1.1 request. (But see <a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" title="Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses">Appendix&nbsp;D.1.1</a> for other requirements on Host support in HTTP/1.1.)
1358      </p>
1359      <p id="rfc.section.5.2.p.3">An origin server that does differentiate resources based on the host requested (sometimes referred to as virtual hosts or
1360         vanity host names) <em class="bcp14">MUST</em> use the following rules for determining the requested resource on an HTTP/1.1 request:
1361      </p>
1362      <ol>
1363         <li>If Request-URI is an absoluteURI, the host is part of the Request-URI. Any Host header field value in the request <em class="bcp14">MUST</em> be ignored.
1364         </li>
1365         <li>If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host
1366            header field value.
1367         </li>
1368         <li>If the host as determined by rule 1 or 2 is not a valid host on the server, the response <em class="bcp14">MUST</em> be a 400 (Bad Request) error message.
1369         </li>
1370      </ol>
1371      <p id="rfc.section.5.2.p.4">Recipients of an HTTP/1.0 request that lacks a Host header field <em class="bcp14">MAY</em> attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine
1372         what exact resource is being requested.
1373      </p>
1374      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="response" href="#response">Response</a></h1>
1375      <p id="rfc.section.6.p.1">After receiving and interpreting a request message, a server responds with an HTTP response message.</p>
1376      <div id="rfc.figure.u.34"></div><pre class="inline"><span id="rfc.iref.g.65"></span>    Response      = Status-Line               ; <a href="#status-line" title="Status-Line">Section&nbsp;6.1</a>
1377                    *(( general-header        ; <a href="#general.header.fields" title="General Header Fields">Section&nbsp;4.5</a>
1378                     | response-header        ; <a href="#Part2" id="rfc.xref.Part2.7"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, <a href="p2-semantics.html#response.header.fields" title="Response Header Fields">Section 6</a>
1379                     | entity-header ) CRLF)  ; <a href="#Part3" id="rfc.xref.Part3.12"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>, <a href="p3-payload.html#entity.header.fields" title="Entity Header Fields">Section 3.1</a>
1380                    CRLF
1381                    [ message-body ]          ; <a href="#message.body" title="Message Body">Section&nbsp;4.3</a>
1382</pre><h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="status-line" href="#status-line">Status-Line</a></h2>
1383      <p id="rfc.section.6.1.p.1">The first line of a Response message is the Status-Line, consisting of the protocol version followed by a numeric status code
1384         and its associated textual phrase, with each element separated by SP characters. No CR or LF is allowed except in the final
1385         CRLF sequence.
1386      </p>
1387      <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.66"></span>    Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
1388</pre><h3 id="rfc.section.6.1.1"><a href="#rfc.section.6.1.1">6.1.1</a>&nbsp;<a id="status.code.and.reason.phrase" href="#status.code.and.reason.phrase">Status Code and Reason Phrase</a></h3>
1389      <p id="rfc.section.6.1.1.p.1">The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. These codes
1390         are fully defined in <a href="p2-semantics.html#status.codes" title="Status Code Definitions">Section 9</a> of <a href="#Part2" id="rfc.xref.Part2.8"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use
1391         by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason-Phrase.
1392      </p>
1393      <p id="rfc.section.6.1.1.p.2">The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role.
1394         There are 5 values for the first digit:
1395      </p>
1396      <ul>
1397         <li>1xx: Informational - Request received, continuing process</li>
1398         <li>2xx: Success - The action was successfully received, understood, and accepted</li>
1399         <li>3xx: Redirection - Further action must be taken in order to complete the request</li>
1400         <li>4xx: Client Error - The request contains bad syntax or cannot be fulfilled</li>
1401         <li>5xx: Server Error - The server failed to fulfill an apparently valid request</li>
1402      </ul>
1403      <div id="rfc.figure.u.36"></div><pre class="inline"><span id="rfc.iref.g.67"></span><span id="rfc.iref.g.68"></span><span id="rfc.iref.g.69"></span>   Status-Code    = 3DIGIT
1404   Reason-Phrase  = *&lt;TEXT, excluding CR, LF&gt;
1405</pre><h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="connections" href="#connections">Connections</a></h1>
1406      <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="persistent.connections" href="#persistent.connections">Persistent Connections</a></h2>
1407      <h3 id="rfc.section.7.1.1"><a href="#rfc.section.7.1.1">7.1.1</a>&nbsp;<a id="persistent.purpose" href="#persistent.purpose">Purpose</a></h3>
1408      <p id="rfc.section.7.1.1.p.1">Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP
1409         servers and causing congestion on the Internet. The use of inline images and other associated data often require a client
1410         to make multiple requests of the same server in a short amount of time. Analysis of these performance problems and results
1411         from a prototype implementation are available <a href="#Pad1995" id="rfc.xref.Pad1995.1"><cite title="Improving HTTP Latency">[Pad1995]</cite></a>  <a href="#Spe" id="rfc.xref.Spe.1"><cite title="Analysis of HTTP Performance Problems">[Spe]</cite></a>. Implementation experience and measurements of actual HTTP/1.1 (RFC 2068) implementations show good results <a href="#Nie1997" id="rfc.xref.Nie1997.1"><cite title="Network Performance Effects of HTTP/1.1, CSS1, and PNG">[Nie1997]</cite></a>. Alternatives have also been explored, for example, T/TCP <a href="#Tou1998" id="rfc.xref.Tou1998.1"><cite title="Analysis of HTTP Performance">[Tou1998]</cite></a>.
1412      </p>
1413      <p id="rfc.section.7.1.1.p.2">Persistent HTTP connections have a number of advantages: </p>
1414      <ul>
1415         <li>By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways,
1416            tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.
1417         </li>
1418         <li>HTTP requests and responses can be pipelined on a connection. Pipelining allows a client to make multiple requests without
1419            waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.
1420         </li>
1421         <li>Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to
1422            determine the congestion state of the network.
1423         </li>
1424         <li>Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.</li>
1425         <li>HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using
1426            future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old
1427            semantics after an error is reported.
1428         </li>
1429      </ul>
1430      <p id="rfc.section.7.1.1.p.3">HTTP implementations <em class="bcp14">SHOULD</em> implement persistent connections.
1431      </p>
1432      <h3 id="rfc.section.7.1.2"><a href="#rfc.section.7.1.2">7.1.2</a>&nbsp;<a id="persistent.overall" href="#persistent.overall">Overall Operation</a></h3>
1433      <p id="rfc.section.7.1.2.p.1">A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior
1434         of any HTTP connection. That is, unless otherwise indicated, the client <em class="bcp14">SHOULD</em> assume that the server will maintain a persistent connection, even after error responses from the server.
1435      </p>
1436      <p id="rfc.section.7.1.2.p.2">Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling
1437         takes place using the Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.2" title="Connection">Section&nbsp;8.1</a>). Once a close has been signaled, the client <em class="bcp14">MUST NOT</em> send any more requests on that connection.
1438      </p>
1439      <h4 id="rfc.section.7.1.2.1"><a href="#rfc.section.7.1.2.1">7.1.2.1</a>&nbsp;<a id="persistent.negotiation" href="#persistent.negotiation">Negotiation</a></h4>
1440      <p id="rfc.section.7.1.2.1.p.1">An HTTP/1.1 server <em class="bcp14">MAY</em> assume that a HTTP/1.1 client intends to maintain a persistent connection unless a Connection header including the connection-token
1441         "close" was sent in the request. If the server chooses to close the connection immediately after sending the response, it <em class="bcp14">SHOULD</em> send a Connection header including the connection-token close.
1442      </p>
1443      <p id="rfc.section.7.1.2.1.p.2">An HTTP/1.1 client <em class="bcp14">MAY</em> expect a connection to remain open, but would decide to keep it open based on whether the response from a server contains
1444         a Connection header with the connection-token close. In case the client does not want to maintain a connection for more than
1445         that request, it <em class="bcp14">SHOULD</em> send a Connection header including the connection-token close.
1446      </p>
1447      <p id="rfc.section.7.1.2.1.p.3">If either the client or the server sends the close token in the Connection header, that request becomes the last one for the
1448         connection.
1449      </p>
1450      <p id="rfc.section.7.1.2.1.p.4">Clients and servers <em class="bcp14">SHOULD NOT</em> assume that a persistent connection is maintained for HTTP versions less than 1.1 unless it is explicitly signaled. See <a href="#compatibility.with.http.1.0.persistent.connections" title="Compatibility with HTTP/1.0 Persistent Connections">Appendix&nbsp;D.2</a> for more information on backward compatibility with HTTP/1.0 clients.
1451      </p>
1452      <p id="rfc.section.7.1.2.1.p.5">In order to remain persistent, all messages on the connection <em class="bcp14">MUST</em> have a self-defined message length (i.e., one not defined by closure of the connection), as described in <a href="#message.length" title="Message Length">Section&nbsp;4.4</a>.
1453      </p>
1454      <h4 id="rfc.section.7.1.2.2"><a href="#rfc.section.7.1.2.2">7.1.2.2</a>&nbsp;<a id="pipelining" href="#pipelining">Pipelining</a></h4>
1455      <p id="rfc.section.7.1.2.2.p.1">A client that supports persistent connections <em class="bcp14">MAY</em> "pipeline" its requests (i.e., send multiple requests without waiting for each response). A server <em class="bcp14">MUST</em> send its responses to those requests in the same order that the requests were received.
1456      </p>
1457      <p id="rfc.section.7.1.2.2.p.2">Clients which assume persistent connections and pipeline immediately after connection establishment <em class="bcp14">SHOULD</em> be prepared to retry their connection if the first pipelined attempt fails. If a client does such a retry, it <em class="bcp14">MUST NOT</em> pipeline before it knows the connection is persistent. Clients <em class="bcp14">MUST</em> also be prepared to resend their requests if the server closes the connection before sending all of the corresponding responses.
1458      </p>
1459      <p id="rfc.section.7.1.2.2.p.3">Clients <em class="bcp14">SHOULD NOT</em> pipeline requests using non-idempotent methods or non-idempotent sequences of methods (see <a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 8.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.9"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). Otherwise, a premature termination of the transport connection could lead to indeterminate results. A client wishing to
1460         send a non-idempotent request <em class="bcp14">SHOULD</em> wait to send that request until it has received the response status for the previous request.
1461      </p>
1462      <h3 id="rfc.section.7.1.3"><a href="#rfc.section.7.1.3">7.1.3</a>&nbsp;<a id="persistent.proxy" href="#persistent.proxy">Proxy Servers</a></h3>
1463      <p id="rfc.section.7.1.3.p.1">It is especially important that proxies correctly implement the properties of the Connection header field as specified in <a href="#header.connection" id="rfc.xref.header.connection.3" title="Connection">Section&nbsp;8.1</a>.
1464      </p>
1465      <p id="rfc.section.7.1.3.p.2">The proxy server <em class="bcp14">MUST</em> signal persistent connections separately with its clients and the origin servers (or other proxy servers) that it connects
1466         to. Each persistent connection applies to only one transport link.
1467      </p>
1468      <p id="rfc.section.7.1.3.p.3">A proxy server <em class="bcp14">MUST NOT</em> establish a HTTP/1.1 persistent connection with an HTTP/1.0 client (but see RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> for information and discussion of the problems with the Keep-Alive header implemented by many HTTP/1.0 clients).
1469      </p>
1470      <h3 id="rfc.section.7.1.4"><a href="#rfc.section.7.1.4">7.1.4</a>&nbsp;<a id="persistent.practical" href="#persistent.practical">Practical Considerations</a></h3>
1471      <p id="rfc.section.7.1.4.p.1">Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers
1472         might make this a higher value since it is likely that the client will be making more connections through the same server.
1473         The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client
1474         or the server.
1475      </p>
1476      <p id="rfc.section.7.1.4.p.2">When a client or server wishes to time-out it <em class="bcp14">SHOULD</em> issue a graceful close on the transport connection. Clients and servers <em class="bcp14">SHOULD</em> both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does
1477         not detect the other side's close promptly it could cause unnecessary resource drain on the network.
1478      </p>
1479      <p id="rfc.section.7.1.4.p.3">A client, server, or proxy <em class="bcp14">MAY</em> close the transport connection at any time. For example, a client might have started to send a new request at the same time
1480         that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed
1481         while it was idle, but from the client's point of view, a request is in progress.
1482      </p>
1483      <p id="rfc.section.7.1.4.p.4">This means that clients, servers, and proxies <em class="bcp14">MUST</em> be able to recover from asynchronous close events. Client software <em class="bcp14">SHOULD</em> reopen the transport connection and retransmit the aborted sequence of requests without user interaction so long as the request
1484         sequence is idempotent (see <a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 8.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.10"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). Non-idempotent methods or sequences <em class="bcp14">MUST NOT</em> be automatically retried, although user agents <em class="bcp14">MAY</em> offer a human operator the choice of retrying the request(s). Confirmation by user-agent software with semantic understanding
1485         of the application <em class="bcp14">MAY</em> substitute for user confirmation. The automatic retry <em class="bcp14">SHOULD NOT</em> be repeated if the second sequence of requests fails.
1486      </p>
1487      <p id="rfc.section.7.1.4.p.5">Servers <em class="bcp14">SHOULD</em> always respond to at least one request per connection, if at all possible. Servers <em class="bcp14">SHOULD NOT</em> close a connection in the middle of transmitting a response, unless a network or client failure is suspected.
1488      </p>
1489      <p id="rfc.section.7.1.4.p.6">Clients that use persistent connections <em class="bcp14">SHOULD</em> limit the number of simultaneous connections that they maintain to a given server. A single-user client <em class="bcp14">SHOULD NOT</em> maintain more than 2 connections with any server or proxy. A proxy <em class="bcp14">SHOULD</em> use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines
1490         are intended to improve HTTP response times and avoid congestion.
1491      </p>
1492      <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a id="message.transmission.requirements" href="#message.transmission.requirements">Message Transmission Requirements</a></h2>
1493      <h3 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;<a id="persistent.flow" href="#persistent.flow">Persistent Connections and Flow Control</a></h3>
1494      <p id="rfc.section.7.2.1.p.1">HTTP/1.1 servers <em class="bcp14">SHOULD</em> maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating
1495         connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.
1496      </p>
1497      <h3 id="rfc.section.7.2.2"><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;<a id="persistent.monitor" href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></h3>
1498      <p id="rfc.section.7.2.2.p.1">An HTTP/1.1 (or later) client sending a message-body <em class="bcp14">SHOULD</em> monitor the network connection for an error status while it is transmitting the request. If the client sees an error status,
1499         it <em class="bcp14">SHOULD</em> immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.4</a>), a zero length chunk and empty trailer <em class="bcp14">MAY</em> be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header, the client <em class="bcp14">MUST</em> close the connection.
1500      </p>
1501      <h3 id="rfc.section.7.2.3"><a href="#rfc.section.7.2.3">7.2.3</a>&nbsp;<a id="use.of.the.100.status" href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></h3>
1502      <p id="rfc.section.7.2.3.p.1">The purpose of the 100 (Continue) status (see <a href="p2-semantics.html#status.100" title="100 Continue">Section 9.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.11"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) is to allow a client that is sending a request message with a request body to determine if the origin server is willing
1503         to accept the request (based on the request headers) before the client sends the request body. In some cases, it might either
1504         be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking
1505         at the body.
1506      </p>
1507      <p id="rfc.section.7.2.3.p.2">Requirements for HTTP/1.1 clients: </p>
1508      <ul>
1509         <li>If a client will wait for a 100 (Continue) response before sending the request body, it <em class="bcp14">MUST</em> send an Expect request-header field (<a href="p2-semantics.html#header.expect" title="Expect">Section 10.2</a> of <a href="#Part2" id="rfc.xref.Part2.12"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) with the "100-continue" expectation.
1510         </li>
1511         <li>A client <em class="bcp14">MUST NOT</em> send an Expect request-header field (<a href="p2-semantics.html#header.expect" title="Expect">Section 10.2</a> of <a href="#Part2" id="rfc.xref.Part2.13"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) with the "100-continue" expectation if it does not intend to send a request body.
1512         </li>
1513      </ul>
1514      <p id="rfc.section.7.2.3.p.3">Because of the presence of older implementations, the protocol allows ambiguous situations in which a client may send "Expect:
1515         100-continue" without receiving either a 417 (Expectation Failed) status or a 100 (Continue) status. Therefore, when a client
1516         sends this header field to an origin server (possibly via a proxy) from which it has never seen a 100 (Continue) status, the
1517         client <em class="bcp14">SHOULD NOT</em> wait for an indefinite period before sending the request body.
1518      </p>
1519      <p id="rfc.section.7.2.3.p.4">Requirements for HTTP/1.1 origin servers: </p>
1520      <ul>
1521         <li>Upon receiving a request which includes an Expect request-header field with the "100-continue" expectation, an origin server <em class="bcp14">MUST</em> either respond with 100 (Continue) status and continue to read from the input stream, or respond with a final status code.
1522            The origin server <em class="bcp14">MUST NOT</em> wait for the request body before sending the 100 (Continue) response. If it responds with a final status code, it <em class="bcp14">MAY</em> close the transport connection or it <em class="bcp14">MAY</em> continue to read and discard the rest of the request. It <em class="bcp14">MUST NOT</em> perform the requested method if it returns a final status code.
1523         </li>
1524         <li>An origin server <em class="bcp14">SHOULD NOT</em> send a 100 (Continue) response if the request message does not include an Expect request-header field with the "100-continue"
1525            expectation, and <em class="bcp14">MUST NOT</em> send a 100 (Continue) response if such a request comes from an HTTP/1.0 (or earlier) client. There is an exception to this
1526            rule: for compatibility with RFC 2068, a server <em class="bcp14">MAY</em> send a 100 (Continue) status in response to an HTTP/1.1 PUT or POST request that does not include an Expect request-header
1527            field with the "100-continue" expectation. This exception, the purpose of which is to minimize any client processing delays
1528            associated with an undeclared wait for 100 (Continue) status, applies only to HTTP/1.1 requests, and not to requests with
1529            any other HTTP-version value.
1530         </li>
1531         <li>An origin server <em class="bcp14">MAY</em> omit a 100 (Continue) response if it has already received some or all of the request body for the corresponding request.
1532         </li>
1533         <li>An origin server that sends a 100 (Continue) response <em class="bcp14">MUST</em> ultimately send a final status code, once the request body is received and processed, unless it terminates the transport connection
1534            prematurely.
1535         </li>
1536         <li>If an origin server receives a request that does not include an Expect request-header field with the "100-continue" expectation,
1537            the request includes a request body, and the server responds with a final status code before reading the entire request body
1538            from the transport connection, then the server <em class="bcp14">SHOULD NOT</em> close the transport connection until it has read the entire request, or until the client closes the connection. Otherwise,
1539            the client might not reliably receive the response message. However, this requirement is not be construed as preventing a
1540            server from defending itself against denial-of-service attacks, or from badly broken client implementations.
1541         </li>
1542      </ul>
1543      <p id="rfc.section.7.2.3.p.5">Requirements for HTTP/1.1 proxies: </p>
1544      <ul>
1545         <li>If a proxy receives a request that includes an Expect request-header field with the "100-continue" expectation, and the proxy
1546            either knows that the next-hop server complies with HTTP/1.1 or higher, or does not know the HTTP version of the next-hop
1547            server, it <em class="bcp14">MUST</em> forward the request, including the Expect header field.
1548         </li>
1549         <li>If the proxy knows that the version of the next-hop server is HTTP/1.0 or lower, it <em class="bcp14">MUST NOT</em> forward the request, and it <em class="bcp14">MUST</em> respond with a 417 (Expectation Failed) status.
1550         </li>
1551         <li>Proxies <em class="bcp14">SHOULD</em> maintain a cache recording the HTTP version numbers received from recently-referenced next-hop servers.
1552         </li>
1553         <li>A proxy <em class="bcp14">MUST NOT</em> forward a 100 (Continue) response if the request message was received from an HTTP/1.0 (or earlier) client and did not include
1554            an Expect request-header field with the "100-continue" expectation. This requirement overrides the general rule for forwarding
1555            of 1xx responses (see <a href="p2-semantics.html#status.1xx" title="Informational 1xx">Section 9.1</a> of <a href="#Part2" id="rfc.xref.Part2.14"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
1556         </li>
1557      </ul>
1558      <h3 id="rfc.section.7.2.4"><a href="#rfc.section.7.2.4">7.2.4</a>&nbsp;<a id="connection.premature" href="#connection.premature">Client Behavior if Server Prematurely Closes Connection</a></h3>
1559      <p id="rfc.section.7.2.4.p.1">If an HTTP/1.1 client sends a request which includes a request body, but which does not include an Expect request-header field
1560         with the "100-continue" expectation, and if the client is not directly connected to an HTTP/1.1 origin server, and if the
1561         client sees the connection close before receiving any status from the server, the client <em class="bcp14">SHOULD</em> retry the request. If the client does retry this request, it <em class="bcp14">MAY</em> use the following "binary exponential backoff" algorithm to be assured of obtaining a reliable response:
1562      </p>
1563      <ol>
1564         <li>Initiate a new connection to the server</li>
1565         <li>Transmit the request-headers</li>
1566         <li>Initialize a variable R to the estimated round-trip time to the server (e.g., based on the time it took to establish the connection),
1567            or to a constant value of 5 seconds if the round-trip time is not available.
1568         </li>
1569         <li>Compute T = R * (2**N), where N is the number of previous retries of this request.</li>
1570         <li>Wait either for an error response from the server, or for T seconds (whichever comes first)</li>
1571         <li>If no error response is received, after T seconds transmit the body of the request.</li>
1572         <li>If client sees that the connection is closed prematurely, repeat from step 1 until the request is accepted, an error response
1573            is received, or the user becomes impatient and terminates the retry process.
1574         </li>
1575      </ol>
1576      <p id="rfc.section.7.2.4.p.2">If at any point an error status is received, the client </p>
1577      <ul>
1578         <li><em class="bcp14">SHOULD NOT</em> continue and
1579         </li>
1580         <li><em class="bcp14">SHOULD</em> close the connection if it has not completed sending the request message.
1581         </li>
1582      </ul>
1583      <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
1584      <p id="rfc.section.8.p.1">This section defines the syntax and semantics of all standard HTTP/1.1 header fields. For entity-header fields, both sender
1585         and recipient refer to either the client or the server, depending on who sends and who receives the entity.
1586      </p>
1587      <div id="rfc.iref.c.6"></div>
1588      <div id="rfc.iref.h.1"></div>
1589      <h2 id="rfc.section.8.1"><a href="#rfc.section.8.1">8.1</a>&nbsp;<a id="header.connection" href="#header.connection">Connection</a></h2>
1590      <p id="rfc.section.8.1.p.1">The Connection general-header field allows the sender to specify options that are desired for that particular connection and <em class="bcp14">MUST NOT</em> be communicated by proxies over further connections.
1591      </p>
1592      <p id="rfc.section.8.1.p.2">The Connection header has the following grammar:</p>
1593      <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.70"></span><span id="rfc.iref.g.71"></span>    Connection = "Connection" ":" 1#(connection-token)
1594    connection-token  = token
1595</pre><p id="rfc.section.8.1.p.4">HTTP/1.1 proxies <em class="bcp14">MUST</em> parse the Connection header field before a message is forwarded and, for each connection-token in this field, remove any header
1596         field(s) from the message with the same name as the connection-token. Connection options are signaled by the presence of a
1597         connection-token in the Connection header field, not by any corresponding additional header field(s), since the additional
1598         header field may not be sent if there are no parameters associated with that connection option.
1599      </p>
1600      <p id="rfc.section.8.1.p.5">Message headers listed in the Connection header <em class="bcp14">MUST NOT</em> include end-to-end headers, such as Cache-Control.
1601      </p>
1602      <p id="rfc.section.8.1.p.6">HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion
1603         of the response. For example,
1604      </p>
1605      <div id="rfc.figure.u.38"></div><pre class="text">    Connection: close
1606</pre><p id="rfc.section.8.1.p.8">in either the request or the response header fields indicates that the connection <em class="bcp14">SHOULD NOT</em> be considered `persistent' (<a href="#persistent.connections" title="Persistent Connections">Section&nbsp;7.1</a>) after the current request/response is complete.
1607      </p>
1608      <p id="rfc.section.8.1.p.9">An HTTP/1.1 client that does not support persistent connections <em class="bcp14">MUST</em> include the "close" connection option in every request message.
1609      </p>
1610      <p id="rfc.section.8.1.p.10">An HTTP/1.1 server that does not support persistent connections <em class="bcp14">MUST</em> include the "close" connection option in every response message that does not have a 1xx (informational) status code.
1611      </p>
1612      <p id="rfc.section.8.1.p.11">A system receiving an HTTP/1.0 (or lower-version) message that includes a Connection header MUST, for each connection-token
1613         in this field, remove and ignore any header field(s) from the message with the same name as the connection-token. This protects
1614         against mistaken forwarding of such header fields by pre-HTTP/1.1 proxies. See <a href="#compatibility.with.http.1.0.persistent.connections" title="Compatibility with HTTP/1.0 Persistent Connections">Appendix&nbsp;D.2</a>.
1615      </p>
1616      <div id="rfc.iref.c.7"></div>
1617      <div id="rfc.iref.h.2"></div>
1618      <h2 id="rfc.section.8.2"><a href="#rfc.section.8.2">8.2</a>&nbsp;<a id="header.content-length" href="#header.content-length">Content-Length</a></h2>
1619      <p id="rfc.section.8.2.p.1">The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient
1620         or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.
1621      </p>
1622      <div id="rfc.figure.u.39"></div><pre class="inline"><span id="rfc.iref.g.72"></span>    Content-Length    = "Content-Length" ":" 1*DIGIT
1623</pre><p id="rfc.section.8.2.p.3">An example is</p>
1624      <div id="rfc.figure.u.40"></div><pre class="text">    Content-Length: 3495
1625</pre><p id="rfc.section.8.2.p.5">Applications <em class="bcp14">SHOULD</em> use this field to indicate the transfer-length of the message-body, unless this is prohibited by the rules in <a href="#message.length" title="Message Length">Section&nbsp;4.4</a>.
1626      </p>
1627      <p id="rfc.section.8.2.p.6">Any Content-Length greater than or equal to zero is a valid value. <a href="#message.length" title="Message Length">Section&nbsp;4.4</a> describes how to determine the length of a message-body if a Content-Length is not given.
1628      </p>
1629      <p id="rfc.section.8.2.p.7">Note that the meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional
1630         field used within the "message/external-body" content-type. In HTTP, it <em class="bcp14">SHOULD</em> be sent whenever the message's length can be determined prior to being transferred, unless this is prohibited by the rules
1631         in <a href="#message.length" title="Message Length">Section&nbsp;4.4</a>.
1632      </p>
1633      <div id="rfc.iref.d.2"></div>
1634      <div id="rfc.iref.h.3"></div>
1635      <h2 id="rfc.section.8.3"><a href="#rfc.section.8.3">8.3</a>&nbsp;<a id="header.date" href="#header.date">Date</a></h2>
1636      <p id="rfc.section.8.3.p.1">The Date general-header field represents the date and time at which the message was originated, having the same semantics
1637         as orig-date in RFC 822. The field value is an HTTP-date, as described in <a href="#full.date" title="Full Date">Section&nbsp;3.3.1</a>; it <em class="bcp14">MUST</em> be sent in rfc1123-date format.
1638      </p>
1639      <div id="rfc.figure.u.41"></div><pre class="inline"><span id="rfc.iref.g.73"></span>    Date  = "Date" ":" HTTP-date
1640</pre><p id="rfc.section.8.3.p.3">An example is</p>
1641      <div id="rfc.figure.u.42"></div><pre class="text">    Date: Tue, 15 Nov 1994 08:12:31 GMT
1642</pre><p id="rfc.section.8.3.p.5">Origin servers <em class="bcp14">MUST</em> include a Date header field in all responses, except in these cases:
1643      </p>
1644      <ol>
1645         <li>If the response status code is 100 (Continue) or 101 (Switching Protocols), the response <em class="bcp14">MAY</em> include a Date header field, at the server's option.
1646         </li>
1647         <li>If the response status code conveys a server error, e.g. 500 (Internal Server Error) or 503 (Service Unavailable), and it
1648            is inconvenient or impossible to generate a valid Date.
1649         </li>
1650         <li>If the server does not have a clock that can provide a reasonable approximation of the current time, its responses <em class="bcp14">MUST NOT</em> include a Date header field. In this case, the rules in <a href="#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section&nbsp;8.3.1</a>  <em class="bcp14">MUST</em> be followed.
1651         </li>
1652      </ol>
1653      <p id="rfc.section.8.3.p.6">A received message that does not have a Date header field <em class="bcp14">MUST</em> be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires
1654         a Date. An HTTP implementation without a clock <em class="bcp14">MUST NOT</em> cache responses without revalidating them on every use. An HTTP cache, especially a shared cache, <em class="bcp14">SHOULD</em> use a mechanism, such as NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>, to synchronize its clock with a reliable external standard.
1655      </p>
1656      <p id="rfc.section.8.3.p.7">Clients <em class="bcp14">SHOULD</em> only send a Date header field in messages that include an entity-body, as in the case of the PUT and POST requests, and even
1657         then it is optional. A client without a clock <em class="bcp14">MUST NOT</em> send a Date header field in a request.
1658      </p>
1659      <p id="rfc.section.8.3.p.8">The HTTP-date sent in a Date header <em class="bcp14">SHOULD NOT</em> represent a date and time subsequent to the generation of the message. It <em class="bcp14">SHOULD</em> represent the best available approximation of the date and time of message generation, unless the implementation has no means
1660         of generating a reasonably accurate date and time. In theory, the date ought to represent the moment just before the entity
1661         is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic
1662         value.
1663      </p>
1664      <h3 id="rfc.section.8.3.1"><a href="#rfc.section.8.3.1">8.3.1</a>&nbsp;<a id="clockless.origin.server.operation" href="#clockless.origin.server.operation">Clockless Origin Server Operation</a></h3>
1665      <p id="rfc.section.8.3.1.p.1">Some origin server implementations might not have a clock available. An origin server without a clock <em class="bcp14">MUST NOT</em> assign Expires or Last-Modified values to a response, unless these values were associated with the resource by a system or
1666         user with a reliable clock. It <em class="bcp14">MAY</em> assign an Expires value that is known, at or before server configuration time, to be in the past (this allows "pre-expiration"
1667         of responses without storing separate Expires values for each resource).
1668      </p>
1669      <div id="rfc.iref.h.4"></div>
1670      <div id="rfc.iref.h.5"></div>
1671      <h2 id="rfc.section.8.4"><a href="#rfc.section.8.4">8.4</a>&nbsp;<a id="header.host" href="#header.host">Host</a></h2>
1672      <p id="rfc.section.8.4.p.1">The Host request-header field specifies the Internet host and port number of the resource being requested, as obtained from
1673         the original URI given by the user or referring resource (generally an HTTP URL, as described in <a href="#http.url" title="http URL">Section&nbsp;3.2.2</a>). The Host field value <em class="bcp14">MUST</em> represent the naming authority of the origin server or gateway given by the original URL. This allows the origin server or
1674         gateway to differentiate between internally-ambiguous URLs, such as the root "/" URL of a server for multiple host names on
1675         a single IP address.
1676      </p>
1677      <div id="rfc.figure.u.43"></div><pre class="inline"><span id="rfc.iref.g.74"></span>    Host = "Host" ":" host [ ":" port ] ; <a href="#http.url" title="http URL">Section&nbsp;3.2.2</a>
1678</pre><p id="rfc.section.8.4.p.3">A "host" without any trailing port information implies the default port for the service requested (e.g., "80" for an HTTP
1679         URL). For example, a request on the origin server for &lt;http://www.example.org/pub/WWW/&gt; would properly include:
1680      </p>
1681      <div id="rfc.figure.u.44"></div><pre class="text">    GET /pub/WWW/ HTTP/1.1
1682    Host: www.example.org
1683</pre><p id="rfc.section.8.4.p.5">A client <em class="bcp14">MUST</em> include a Host header field in all HTTP/1.1 request messages . If the requested URI does not include an Internet host name
1684         for the service being requested, then the Host header field <em class="bcp14">MUST</em> be given with an empty value. An HTTP/1.1 proxy <em class="bcp14">MUST</em> ensure that any request message it forwards does contain an appropriate Host header field that identifies the service being
1685         requested by the proxy. All Internet-based HTTP/1.1 servers <em class="bcp14">MUST</em> respond with a 400 (Bad Request) status code to any HTTP/1.1 request message which lacks a Host header field.
1686      </p>
1687      <p id="rfc.section.8.4.p.6">See sections <a href="#the.resource.identified.by.a.request" title="The Resource Identified by a Request">5.2</a> and <a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" title="Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses">D.1.1</a> for other requirements relating to Host.
1688      </p>
1689      <div id="rfc.iref.t.2"></div>
1690      <div id="rfc.iref.h.6"></div>
1691      <h2 id="rfc.section.8.5"><a href="#rfc.section.8.5">8.5</a>&nbsp;<a id="header.te" href="#header.te">TE</a></h2>
1692      <p id="rfc.section.8.5.p.1">The TE request-header field indicates what extension transfer-codings it is willing to accept in the response and whether
1693         or not it is willing to accept trailer fields in a chunked transfer-coding. Its value may consist of the keyword "trailers"
1694         and/or a comma-separated list of extension transfer-coding names with optional accept parameters (as described in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.4</a>).
1695      </p>
1696      <div id="rfc.figure.u.45"></div><pre class="inline"><span id="rfc.iref.g.75"></span><span id="rfc.iref.g.76"></span>    TE        = "TE" ":" #( t-codings )
1697    t-codings = "trailers" | ( transfer-extension [ accept-params ] )
1698</pre><p id="rfc.section.8.5.p.3">The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer-coding,
1699         as defined in <a href="#chunked.transfer.encoding" title="Chunked Transfer Coding">Section&nbsp;3.4.1</a>. This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding.
1700      </p>
1701      <p id="rfc.section.8.5.p.4">Examples of its use are:</p>
1702      <div id="rfc.figure.u.46"></div><pre class="text">    TE: deflate
1703    TE:
1704    TE: trailers, deflate;q=0.5
1705</pre><p id="rfc.section.8.5.p.6">The TE header field only applies to the immediate connection. Therefore, the keyword <em class="bcp14">MUST</em> be supplied within a Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.4" title="Connection">Section&nbsp;8.1</a>) whenever TE is present in an HTTP/1.1 message.
1706      </p>
1707      <p id="rfc.section.8.5.p.7">A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules: </p>
1708      <ol>
1709         <li>
1710            <p>The "chunked" transfer-coding is always acceptable. If the keyword "trailers" is listed, the client indicates that it is willing
1711               to accept trailer fields in the chunked response on behalf of itself and any downstream clients. The implication is that,
1712               if given, the client is stating that either all downstream clients are willing to accept trailer fields in the forwarded response,
1713               or that it will attempt to buffer the response on behalf of downstream recipients.
1714            </p>
1715            <p> <b>Note:</b> HTTP/1.1 does not define any means to limit the size of a chunked response such that a client can be assured of buffering
1716               the entire response.
1717            </p>
1718         </li>
1719         <li>
1720            <p>If the transfer-coding being tested is one of the transfer-codings listed in the TE field, then it is acceptable unless it
1721               is accompanied by a qvalue of 0. (As defined in <a href="p3-payload.html#quality.values" title="Quality Values">Section 2.4</a> of <a href="#Part3" id="rfc.xref.Part3.13"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>, a qvalue of 0 means "not acceptable.")
1722            </p>
1723         </li>
1724         <li>
1725            <p>If multiple transfer-codings are acceptable, then the acceptable transfer-coding with the highest non-zero qvalue is preferred.
1726               The "chunked" transfer-coding always has a qvalue of 1.
1727            </p>
1728         </li>
1729      </ol>
1730      <p id="rfc.section.8.5.p.8">If the TE field-value is empty or if no TE field is present, the only transfer-coding is "chunked". A message with no transfer-coding
1731         is always acceptable.
1732      </p>
1733      <div id="rfc.iref.t.3"></div>
1734      <div id="rfc.iref.h.7"></div>
1735      <h2 id="rfc.section.8.6"><a href="#rfc.section.8.6">8.6</a>&nbsp;<a id="header.trailer" href="#header.trailer">Trailer</a></h2>
1736      <p id="rfc.section.8.6.p.1">The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded
1737         with chunked transfer-coding.
1738      </p>
1739      <div id="rfc.figure.u.47"></div><pre class="inline"><span id="rfc.iref.g.77"></span>    Trailer  = "Trailer" ":" 1#field-name
1740</pre><p id="rfc.section.8.6.p.3">An HTTP/1.1 message <em class="bcp14">SHOULD</em> include a Trailer header field in a message using chunked transfer-coding with a non-empty trailer. Doing so allows the recipient
1741         to know which header fields to expect in the trailer.
1742      </p>
1743      <p id="rfc.section.8.6.p.4">If no Trailer header field is present, the trailer <em class="bcp14">SHOULD NOT</em> include any header fields. See <a href="#chunked.transfer.encoding" title="Chunked Transfer Coding">Section&nbsp;3.4.1</a> for restrictions on the use of trailer fields in a "chunked" transfer-coding.
1744      </p>
1745      <p id="rfc.section.8.6.p.5">Message header fields listed in the Trailer header field <em class="bcp14">MUST NOT</em> include the following header fields:
1746      </p>
1747      <ul>
1748         <li>Transfer-Encoding</li>
1749         <li>Content-Length</li>
1750         <li>Trailer</li>
1751      </ul>
1752      <div id="rfc.iref.t.4"></div>
1753      <div id="rfc.iref.h.8"></div>
1754      <h2 id="rfc.section.8.7"><a href="#rfc.section.8.7">8.7</a>&nbsp;<a id="header.transfer-encoding" href="#header.transfer-encoding">Transfer-Encoding</a></h2>
1755      <p id="rfc.section.8.7.p.1">The Transfer-Encoding general-header field indicates what (if any) type of transformation has been applied to the message
1756         body in order to safely transfer it between the sender and the recipient. This differs from the content-coding in that the
1757         transfer-coding is a property of the message, not of the entity.
1758      </p>
1759      <div id="rfc.figure.u.48"></div><pre class="inline"><span id="rfc.iref.g.78"></span>  Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-coding
1760</pre><p id="rfc.section.8.7.p.3">Transfer-codings are defined in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;3.4</a>. An example is:
1761      </p>
1762      <div id="rfc.figure.u.49"></div><pre class="text">  Transfer-Encoding: chunked
1763</pre><p id="rfc.section.8.7.p.5">If multiple encodings have been applied to an entity, the transfer-codings <em class="bcp14">MUST</em> be listed in the order in which they were applied. Additional information about the encoding parameters <em class="bcp14">MAY</em> be provided by other entity-header fields not defined by this specification.
1764      </p>
1765      <p id="rfc.section.8.7.p.6">Many older HTTP/1.0 applications do not understand the Transfer-Encoding header.</p>
1766      <div id="rfc.iref.u.3"></div>
1767      <div id="rfc.iref.h.9"></div>
1768      <h2 id="rfc.section.8.8"><a href="#rfc.section.8.8">8.8</a>&nbsp;<a id="header.upgrade" href="#header.upgrade">Upgrade</a></h2>
1769      <p id="rfc.section.8.8.p.1">The Upgrade general-header allows the client to specify what additional communication protocols it supports and would like
1770         to use if the server finds it appropriate to switch protocols. The server <em class="bcp14">MUST</em> use the Upgrade header field within a 101 (Switching Protocols) response to indicate which protocol(s) are being switched.
1771      </p>
1772      <div id="rfc.figure.u.50"></div><pre class="inline"><span id="rfc.iref.g.79"></span>    Upgrade        = "Upgrade" ":" 1#product
1773</pre><p id="rfc.section.8.8.p.3">For example,</p>
1774      <div id="rfc.figure.u.51"></div><pre class="text">    Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
1775</pre><p id="rfc.section.8.8.p.5">The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible
1776         protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP
1777         with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult
1778         transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol
1779         while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by
1780         the server, possibly according to the nature of the method and/or resource being requested).
1781      </p>
1782      <p id="rfc.section.8.8.p.6">The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection.
1783         Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities
1784         and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen,
1785         although the first action after changing the protocol <em class="bcp14">MUST</em> be a response to the initial HTTP request containing the Upgrade header field.
1786      </p>
1787      <p id="rfc.section.8.8.p.7">The Upgrade header field only applies to the immediate connection. Therefore, the upgrade keyword <em class="bcp14">MUST</em> be supplied within a Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.5" title="Connection">Section&nbsp;8.1</a>) whenever Upgrade is present in an HTTP/1.1 message.
1788      </p>
1789      <p id="rfc.section.8.8.p.8">The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it
1790         is more appropriate to use a 301, 302, 303, or 305 redirection response.
1791      </p>
1792      <p id="rfc.section.8.8.p.9">This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined
1793         by the HTTP version rules of <a href="#http.version" title="HTTP Version">Section&nbsp;3.1</a> and future updates to this specification. Any token can be used as a protocol name; however, it will only be useful if both
1794         the client and server associate the name with the same protocol.
1795      </p>
1796      <div id="rfc.iref.v.2"></div>
1797      <div id="rfc.iref.h.10"></div>
1798      <h2 id="rfc.section.8.9"><a href="#rfc.section.8.9">8.9</a>&nbsp;<a id="header.via" href="#header.via">Via</a></h2>
1799      <p id="rfc.section.8.9.p.1">The Via general-header field <em class="bcp14">MUST</em> be used by gateways and proxies to indicate the intermediate protocols and recipients between the user agent and the server
1800         on requests, and between the origin server and the client on responses. It is analogous to the "Received" field of RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.7"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a> and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities
1801         of all senders along the request/response chain.
1802      </p>
1803      <div id="rfc.figure.u.52"></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><span id="rfc.iref.g.85"></span>   Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
1804   received-protocol = [ protocol-name "/" ] protocol-version
1805   protocol-name     = token
1806   protocol-version  = token
1807   received-by       = ( host [ ":" port ] ) | pseudonym
1808   pseudonym         = token
1809</pre><p id="rfc.section.8.9.p.3">The received-protocol indicates the protocol version of the message received by the server or client along each segment of
1810         the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded
1811         so that information about the protocol capabilities of upstream applications remains visible to all recipients.
1812      </p>
1813      <p id="rfc.section.8.9.p.4">The protocol-name is optional if and only if it would be "HTTP". The received-by field is normally the host and optional port
1814         number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to
1815         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.
1816      </p>
1817      <p id="rfc.section.8.9.p.5">Multiple Via field values represents 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.
1818      </p>
1819      <p id="rfc.section.8.9.p.6">Comments <em class="bcp14">MAY</em> be used in the Via header field to identify the software of the recipient proxy or gateway, analogous to the User-Agent and
1820         Server 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.
1821      </p>
1822      <p id="rfc.section.8.9.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
1823         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
1824         server at www.example.com. The request received by www.example.com would then have the following Via header field:
1825      </p>
1826      <div id="rfc.figure.u.53"></div><pre class="text">    Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
1827</pre><p id="rfc.section.8.9.p.9">Proxies and gateways used as a portal through a network firewall <em class="bcp14">SHOULD NOT</em>, by default, forward the names and ports of hosts within the firewall region. This information <em class="bcp14">SHOULD</em> only be propagated if explicitly enabled. If not enabled, the received-by host of any host behind the firewall <em class="bcp14">SHOULD</em> be replaced by an appropriate pseudonym for that host.
1828      </p>
1829      <p id="rfc.section.8.9.p.10">For organizations that have strong privacy requirements for hiding internal structures, a proxy <em class="bcp14">MAY</em> combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry.
1830         For example,
1831      </p>
1832      <div id="rfc.figure.u.54"></div><pre class="text">    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
1833</pre><p id="rfc.section.8.9.p.12">could be collapsed to</p>
1834      <div id="rfc.figure.u.55"></div><pre class="text">    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
1835</pre><p id="rfc.section.8.9.p.14">Applications <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
1836         by pseudonyms. Applications <em class="bcp14">MUST NOT</em> combine entries which have different received-protocol values.
1837      </p>
1838      <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
1839      <p id="rfc.section.9.p.1">TBD.</p>
1840      <h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1841      <p id="rfc.section.10.p.1">This section is meant to inform application developers, information providers, and users of the security limitations in HTTP/1.1
1842         as described by this document. The discussion does not include definitive solutions to the problems revealed, though it does
1843         make some suggestions for reducing security risks.
1844      </p>
1845      <h2 id="rfc.section.10.1"><a href="#rfc.section.10.1">10.1</a>&nbsp;<a id="personal.information" href="#personal.information">Personal Information</a></h2>
1846      <p id="rfc.section.10.1.p.1">HTTP clients are often privy to large amounts of personal information (e.g. the user's name, location, mail address, passwords,
1847         encryption keys, etc.), and <em class="bcp14">SHOULD</em> be very careful to prevent unintentional leakage of this information via the HTTP protocol to other sources. We very strongly
1848         recommend that a convenient interface be provided for the user to control dissemination of such information, and that designers
1849         and implementors be particularly careful in this area. History shows that errors in this area often create serious security
1850         and/or privacy problems and generate highly adverse publicity for the implementor's company.
1851      </p>
1852      <h2 id="rfc.section.10.2"><a href="#rfc.section.10.2">10.2</a>&nbsp;<a id="abuse.of.server.log.information" href="#abuse.of.server.log.information">Abuse of Server Log Information</a></h2>
1853      <p id="rfc.section.10.2.p.1">A server is in the position to save personal data about a user's requests which might identify their reading patterns or subjects
1854         of interest. This information is clearly confidential in nature and its handling can be constrained by law in certain countries.
1855         People using the HTTP protocol to provide data are responsible for ensuring that such material is not distributed without
1856         the permission of any individuals that are identifiable by the published results.
1857      </p>
1858      <h2 id="rfc.section.10.3"><a href="#rfc.section.10.3">10.3</a>&nbsp;<a id="attack.pathname" href="#attack.pathname">Attacks Based On File and Path Names</a></h2>
1859      <p id="rfc.section.10.3.p.1">Implementations of HTTP origin servers <em class="bcp14">SHOULD</em> be careful to restrict the documents returned by HTTP requests to be only those that were intended by the server administrators.
1860         If an HTTP server translates HTTP URIs directly into file system calls, the server <em class="bcp14">MUST</em> take special care not to serve files that were not intended to be delivered to HTTP clients. For example, UNIX, Microsoft
1861         Windows, and other operating systems use ".." as a path component to indicate a directory level above the current one. On
1862         such a system, an HTTP server <em class="bcp14">MUST</em> disallow any such construct in the Request-URI if it would otherwise allow access to a resource outside those intended to
1863         be accessible via the HTTP server. Similarly, files intended for reference only internally to the server (such as access control
1864         files, configuration files, and script code) <em class="bcp14">MUST</em> be protected from inappropriate retrieval, since they might contain sensitive information. Experience has shown that minor
1865         bugs in such HTTP server implementations have turned into security risks.
1866      </p>
1867      <h2 id="rfc.section.10.4"><a href="#rfc.section.10.4">10.4</a>&nbsp;<a id="dns.spoofing" href="#dns.spoofing">DNS Spoofing</a></h2>
1868      <p id="rfc.section.10.4.p.1">Clients using HTTP rely heavily on the Domain Name Service, and are thus generally prone to security attacks based on the
1869         deliberate mis-association of IP addresses and DNS names. Clients need to be cautious in assuming the continuing validity
1870         of an IP number/DNS name association.
1871      </p>
1872      <p id="rfc.section.10.4.p.2">In particular, HTTP clients <em class="bcp14">SHOULD</em> rely on their name resolver for confirmation of an IP number/DNS name association, rather than caching the result of previous
1873         host name lookups. Many platforms already can cache host name lookups locally when appropriate, and they <em class="bcp14">SHOULD</em> be configured to do so. It is proper for these lookups to be cached, however, only when the TTL (Time To Live) information
1874         reported by the name server makes it likely that the cached information will remain useful.
1875      </p>
1876      <p id="rfc.section.10.4.p.3">If HTTP clients cache the results of host name lookups in order to achieve a performance improvement, they <em class="bcp14">MUST</em> observe the TTL information reported by DNS.
1877      </p>
1878      <p id="rfc.section.10.4.p.4">If HTTP clients do not observe this rule, they could be spoofed when a previously-accessed server's IP address changes. As
1879         network renumbering is expected to become increasingly common <a href="#RFC1900" id="rfc.xref.RFC1900.2"><cite title="Renumbering Needs Work">[RFC1900]</cite></a>, the possibility of this form of attack will grow. Observing this requirement thus reduces this potential security vulnerability.
1880      </p>
1881      <p id="rfc.section.10.4.p.5">This requirement also improves the load-balancing behavior of clients for replicated servers using the same DNS name and reduces
1882         the likelihood of a user's experiencing failure in accessing sites which use that strategy.
1883      </p>
1884      <h2 id="rfc.section.10.5"><a href="#rfc.section.10.5">10.5</a>&nbsp;<a id="attack.proxies" href="#attack.proxies">Proxies and Caching</a></h2>
1885      <p id="rfc.section.10.5.p.1">By their very nature, HTTP proxies are men-in-the-middle, and represent an opportunity for man-in-the-middle attacks. Compromise
1886         of the systems on which the proxies run can result in serious security and privacy problems. Proxies have access to security-related
1887         information, personal information about individual users and organizations, and proprietary information belonging to users
1888         and content providers. A compromised proxy, or a proxy implemented or configured without regard to security and privacy considerations,
1889         might be used in the commission of a wide range of potential attacks.
1890      </p>
1891      <p id="rfc.section.10.5.p.2">Proxy operators should protect the systems on which proxies run as they would protect any system that contains or transports
1892         sensitive information. In particular, log information gathered at proxies often contains highly sensitive personal information,
1893         and/or information about organizations. Log information should be carefully guarded, and appropriate guidelines for use developed
1894         and followed. (<a href="#abuse.of.server.log.information" title="Abuse of Server Log Information">Section&nbsp;10.2</a>).
1895      </p>
1896      <p id="rfc.section.10.5.p.3">Proxy implementors should consider the privacy and security implications of their design and coding decisions, and of the
1897         configuration options they provide to proxy operators (especially the default configuration).
1898      </p>
1899      <p id="rfc.section.10.5.p.4">Users of a proxy need to be aware that they are no trustworthier than the people who run the proxy; HTTP itself cannot solve
1900         this problem.
1901      </p>
1902      <p id="rfc.section.10.5.p.5">The judicious use of cryptography, when appropriate, may suffice to protect against a broad range of security and privacy
1903         attacks. Such cryptography is beyond the scope of the HTTP/1.1 specification.
1904      </p>
1905      <h2 id="rfc.section.10.6"><a href="#rfc.section.10.6">10.6</a>&nbsp;<a id="attack.DoS" href="#attack.DoS">Denial of Service Attacks on Proxies</a></h2>
1906      <p id="rfc.section.10.6.p.1">They exist. They are hard to defend against. Research continues. Beware.</p>
1907      <h1 id="rfc.section.11"><a href="#rfc.section.11">11.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
1908      <p id="rfc.section.11.p.1">This specification makes heavy use of the augmented BNF and generic constructs defined by David H. Crocker for RFC 822 <a href="#RFC822" id="rfc.xref.RFC822.8"><cite title="Standard for the format of ARPA Internet text messages">[RFC822]</cite></a>. Similarly, it reuses many of the definitions provided by Nathaniel Borenstein and Ned Freed for MIME <a href="#RFC2045" id="rfc.xref.RFC2045.3"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>. We hope that their inclusion in this specification will help reduce past confusion over the relationship between HTTP and
1909         Internet mail message formats.
1910      </p>
1911      <p id="rfc.section.11.p.2">The HTTP protocol has evolved considerably over the years. It has benefited from a large and active developer community--the
1912         many people who have participated on the www-talk mailing list--and it is that community which has been most responsible for
1913         the success of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau, Daniel W. Connolly, Bob Denny,
1914         John Franks, Jean-Francois Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli, Dave Raggett,
1915         Tony Sanders, and Marc VanHeyningen deserve special recognition for their efforts in defining early aspects of the protocol.
1916      </p>
1917      <p id="rfc.section.11.p.3">This document has benefited greatly from the comments of all those participating in the HTTP-WG. In addition to those already
1918         mentioned, the following individuals have contributed to this specification:
1919      </p>
1920      <div id="rfc.figure.u.56"></div><pre class="text">    Gary Adams                  Ross Patterson
1921    Harald Tveit Alvestrand     Albert Lunde
1922    Keith Ball                  John C. Mallery
1923    Brian Behlendorf            Jean-Philippe Martin-Flatin
1924    Paul Burchard               Mitra
1925    Maurizio Codogno            David Morris
1926    Mike Cowlishaw              Gavin Nicol
1927    Roman Czyborra              Bill Perry
1928    Michael A. Dolan            Jeffrey Perry
1929    David J. Fiander            Scott Powers
1930    Alan Freier                 Owen Rees
1931    Marc Hedlund                Luigi Rizzo
1932    Greg Herlihy                David Robinson
1933    Koen Holtman                Marc Salomon
1934    Alex Hopmann                Rich Salz
1935    Bob Jernigan                Allan M. Schiffman
1936    Shel Kaphan                 Jim Seidman
1937    Rohit Khare                 Chuck Shotton
1938    John Klensin                Eric W. Sink
1939    Martijn Koster              Simon E. Spero
1940    Alexei Kosut                Richard N. Taylor
1941    David M. Kristol            Robert S. Thau
1942    Daniel LaLiberte            Bill (BearHeart) Weinman
1943    Ben Laurie                  Francois Yergeau
1944    Paul J. Leach               Mary Ellen Zurko
1945    Daniel DuBois               Josh Cohen
1946</pre><p id="rfc.section.11.p.5">Thanks to the "cave men" of Palo Alto. You know who you are.</p>
1947      <p id="rfc.section.11.p.6">Jim Gettys (the current editor of this document) wishes particularly to thank Roy Fielding, the previous editor of this document,
1948         along with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott
1949         Lawrence, and Larry Masinter for their help. And thanks go particularly to Jeff Mogul and Scott Lawrence for performing the
1950         "MUST/MAY/SHOULD" audit.
1951      </p>
1952      <p id="rfc.section.11.p.7">The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik Frystyk implemented RFC 2068 early, and we wish to thank
1953         them for the discovery of many of the problems that this document attempts to rectify.
1954      </p>
1955      <h1 id="rfc.references"><a href="#rfc.section.12" id="rfc.section.12">12.</a> References
1956      </h1>
1957      <table summary="References">                                                               
1958         <tr>
1959            <td class="reference"><b id="ISO-8859">[ISO-8859]</b></td>
1960            <td class="top">International Organization for Standardization, “Information technology - 8-bit single byte coded graphic - character sets”, 1987-1990.<br>Part 1: Latin alphabet No. 1, ISO-8859-1:1987. Part 2: Latin alphabet No. 2, ISO-8859-2, 1987. Part 3: Latin alphabet No.
1961               3, ISO-8859-3, 1988. Part 4: Latin alphabet No. 4, ISO-8859-4, 1988. Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988. Part
1962               6: Latin/Arabic alphabet, ISO-8859-6, 1987. Part 7: Latin/Greek alphabet, ISO-8859-7, 1987. Part 8: Latin/Hebrew alphabet,
1963               ISO-8859-8, 1988. Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
1964            </td>
1965         </tr>
1966         <tr>
1967            <td class="reference"><b id="Nie1997">[Nie1997]</b></td>
1968            <td class="top">Nielsen, H.F.., Gettys, J., Prud'hommeaux, E., Lie, H., and C. Lilley, “Network Performance Effects of HTTP/1.1, CSS1, and PNG”, Proceedings of ACM SIGCOMM '97, Cannes France, Sep&nbsp;1997.</td>
1969         </tr>
1970         <tr>
1971            <td class="reference"><b id="Pad1995">[Pad1995]</b></td>
1972            <td class="top">Padmanabhan, V.N. and J.C. Mogul, “Improving HTTP Latency”, Computer Networks and ISDN Systems&nbsp;v. 28, pp. 25-35, Dec&nbsp;1995.<br>Slightly revised version of paper in Proc. 2nd International WWW Conference '94: Mosaic and the Web, Oct. 1994, which is available
1973               at &lt;<a href="http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html">http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.html</a>&gt;.
1974            </td>
1975         </tr>
1976         <tr>
1977            <td class="reference"><b id="Part2">[Part2]</b></td>
1978            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">HTTP/1.1, part 2: Message Semantics</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), December&nbsp;2007.
1979            </td>
1980         </tr>
1981         <tr>
1982            <td class="reference"><b id="Part3">[Part3]</b></td>
1983            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-latest">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p3-payload-latest (work in progress), December&nbsp;2007.
1984            </td>
1985         </tr>
1986         <tr>
1987            <td class="reference"><b id="Part6">[Part6]</b></td>
1988            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">HTTP/1.1, part 6: Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), December&nbsp;2007.
1989            </td>
1990         </tr>
1991         <tr>
1992            <td class="reference"><b id="RFC1123">[RFC1123]</b></td>
1993            <td class="top"><a title="University of Southern California (USC), Information Sciences Institute">Braden, R.</a>, “<a href="http://tools.ietf.org/html/rfc1123">Requirements for Internet Hosts - Application and Support</a>”, STD&nbsp;3, RFC&nbsp;1123, October&nbsp;1989.
1994            </td>
1995         </tr>
1996         <tr>
1997            <td class="reference"><b id="RFC1305">[RFC1305]</b></td>
1998            <td class="top"><a title="University of Delaware, Electrical Engineering Department">Mills, D.</a>, “<a href="http://tools.ietf.org/html/rfc1305">Network Time Protocol (Version 3) Specification, Implementation</a>”, RFC&nbsp;1305, March&nbsp;1992.
1999            </td>
2000         </tr>
2001         <tr>
2002            <td class="reference"><b id="RFC1436">[RFC1436]</b></td>
2003            <td class="top"><a title="University of Minnesota, Computer and Information Services">Anklesaria, F.</a>, <a title="University of Minnesota, Computer and Information Services">McCahill, M.</a>, <a title="University of Minnesota, Computer and Information Services">Lindner, P.</a>, <a title="University of Minnesota, Computer and Information Services">Johnson, D.</a>, <a title="University of Minnesota, Computer and Information Services">Torrey, D.</a>, and <a title="University of Minnesota, Computer and Information Services">B. Alberti</a>, “<a href="http://tools.ietf.org/html/rfc1436">The Internet Gopher Protocol (a distributed document search and retrieval protocol)</a>”, RFC&nbsp;1436, March&nbsp;1993.
2004            </td>
2005         </tr>
2006         <tr>
2007            <td class="reference"><b id="RFC1630">[RFC1630]</b></td>
2008            <td class="top"><a title="CERN, World-Wide Web project">Berners-Lee, T.</a>, “<a href="http://tools.ietf.org/html/rfc1630">Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network
2009                  as used in the World-Wide Web</a>”, RFC&nbsp;1630, June&nbsp;1994.
2010            </td>
2011         </tr>
2012         <tr>
2013            <td class="reference"><b id="RFC1737">[RFC1737]</b></td>
2014            <td class="top"><a title="Xerox Palo Alto Research Center">Masinter, L.</a> and <a title="MIT Laboratory for Computer Science">K. Sollins</a>, “<a href="http://tools.ietf.org/html/rfc1737">Functional Requirements for Uniform Resource Names</a>”, RFC&nbsp;1737, December&nbsp;1994.
2015            </td>
2016         </tr>
2017         <tr>
2018            <td class="reference"><b id="RFC1738">[RFC1738]</b></td>
2019            <td class="top"><a title="CERN, World-Wide Web project">Berners-Lee, T.</a>, <a title="Xerox PARC">Masinter, L.</a>, and <a title="University of Minnesota, Computer and Information Services">M. McCahill</a>, “<a href="http://tools.ietf.org/html/rfc1738">Uniform Resource Locators (URL)</a>”, RFC&nbsp;1738, December&nbsp;1994.
2020            </td>
2021         </tr>
2022         <tr>
2023            <td class="reference"><b id="RFC1808">[RFC1808]</b></td>
2024            <td class="top"><a title="University of California Irvine, Department of Information and Computer Science">Fielding, R.</a>, “<a href="http://tools.ietf.org/html/rfc1808">Relative Uniform Resource Locators</a>”, RFC&nbsp;1808, June&nbsp;1995.
2025            </td>
2026         </tr>
2027         <tr>
2028            <td class="reference"><b id="RFC1900">[RFC1900]</b></td>
2029            <td class="top"><a title="CERN, Computing and Networks Division">Carpenter, B.</a> and <a title="cisco Systems">Y. Rekhter</a>, “<a href="http://tools.ietf.org/html/rfc1900">Renumbering Needs Work</a>”, RFC&nbsp;1900, February&nbsp;1996.
2030            </td>
2031         </tr>
2032         <tr>
2033            <td class="reference"><b id="RFC1945">[RFC1945]</b></td>
2034            <td class="top"><a title="MIT, Laboratory for Computer Science">Berners-Lee, T.</a>, <a title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.T.</a>, and <a title="W3 Consortium, MIT Laboratory for Computer Science">H.F. Nielsen</a>, “<a href="http://tools.ietf.org/html/rfc1945">Hypertext Transfer Protocol -- HTTP/1.0</a>”, RFC&nbsp;1945, May&nbsp;1996.
2035            </td>
2036         </tr>
2037         <tr>
2038            <td class="reference"><b id="RFC2045">[RFC2045]</b></td>
2039            <td class="top"><a title="Innosoft International, Inc.">Freed, N.</a> and <a title="First Virtual Holdings">N.S. Borenstein</a>, “<a href="http://tools.ietf.org/html/rfc2045">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</a>”, RFC&nbsp;2045, November&nbsp;1996.
2040            </td>
2041         </tr>
2042         <tr>
2043            <td class="reference"><b id="RFC2047">[RFC2047]</b></td>
2044            <td class="top"><a title="University of Tennessee">Moore, K.</a>, “<a href="http://tools.ietf.org/html/rfc2047">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</a>”, RFC&nbsp;2047, November&nbsp;1996.
2045            </td>
2046         </tr>
2047         <tr>
2048            <td class="reference"><b id="RFC2068">[RFC2068]</b></td>
2049            <td class="top"><a title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.</a>, <a title="MIT Laboratory for Computer Science">Gettys, J.</a>, <a title="Digital Equipment Corporation, Western Research Laboratory">Mogul, J.</a>, <a title="MIT Laboratory for Computer Science">Nielsen, H.</a>, and <a title="MIT Laboratory for Computer Science">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2068">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2068, January&nbsp;1997.
2050            </td>
2051         </tr>
2052         <tr>
2053            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
2054            <td class="top"><a title="Harvard University">Bradner, S.</a>, “<a href="http://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
2055            </td>
2056         </tr>
2057         <tr>
2058            <td class="reference"><b id="RFC2145">[RFC2145]</b></td>
2059            <td class="top"><a title="Western Research Laboratory">Mogul, J.C.</a>, <a title="Department of Information and Computer Science">Fielding, R.T.</a>, <a title="MIT Laboratory for Computer Science">Gettys, J.</a>, and <a title="W3 Consortium">H.F. Nielsen</a>, “<a href="http://tools.ietf.org/html/rfc2145">Use and Interpretation of HTTP Version Numbers</a>”, RFC&nbsp;2145, May&nbsp;1997.
2060            </td>
2061         </tr>
2062         <tr>
2063            <td class="reference"><b id="RFC2324">[RFC2324]</b></td>
2064            <td class="top"><a title="Xerox Palo Alto Research Center">Masinter, L.</a>, “<a href="http://tools.ietf.org/html/rfc2324">Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0)</a>”, RFC&nbsp;2324, April&nbsp;1998.
2065            </td>
2066         </tr>
2067         <tr>
2068            <td class="reference"><b id="RFC2396">[RFC2396]</b></td>
2069            <td class="top"><a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="Department of Information and Computer Science">Fielding, R.T.</a>, and <a title="Xerox PARC">L. Masinter</a>, “<a href="http://tools.ietf.org/html/rfc2396">Uniform Resource Identifiers (URI): Generic Syntax</a>”, RFC&nbsp;2396, August&nbsp;1998.
2070            </td>
2071         </tr>
2072         <tr>
2073            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
2074            <td class="top"><a title="University of California, Irvine">Fielding, R.</a>, <a title="W3C">Gettys, J.</a>, <a title="Compaq Computer Corporation">Mogul, J.</a>, <a title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a title="Xerox Corporation">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, and <a 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.
2075            </td>
2076         </tr>
2077         <tr>
2078            <td class="reference"><b id="RFC3977">[RFC3977]</b></td>
2079            <td class="top"><a title="THUS plc">Feather, C.</a>, “Network News Transfer Protocol (NNTP)”, October&nbsp;2006.
2080            </td>
2081         </tr>
2082         <tr>
2083            <td class="reference"><b id="RFC4288">[RFC4288]</b></td>
2084            <td class="top"><a title="Sun Microsystems">Freed, N.</a> and <a>J. Klensin</a>, “<a href="http://tools.ietf.org/html/rfc4288">Media Type Specifications and Registration Procedures</a>”, BCP&nbsp;13, RFC&nbsp;4288, December&nbsp;2005.
2085            </td>
2086         </tr>
2087         <tr>
2088            <td class="reference"><b id="RFC821">[RFC821]</b></td>
2089            <td class="top">Postel, J.B., “<a href="http://tools.ietf.org/html/rfc821">Simple Mail Transfer Protocol</a>”, STD&nbsp;10, RFC&nbsp;821, August&nbsp;1982.
2090            </td>
2091         </tr>
2092         <tr>
2093            <td class="reference"><b id="RFC822">[RFC822]</b></td>
2094            <td class="top"><a title="University of Delaware, Dept. of Electrical Engineering">Crocker, D.H.</a>, “<a href="http://tools.ietf.org/html/rfc822">Standard for the format of ARPA Internet text messages</a>”, STD&nbsp;11, RFC&nbsp;822, August&nbsp;1982.
2095            </td>
2096         </tr>
2097         <tr>
2098            <td class="reference"><b id="RFC959">[RFC959]</b></td>
2099            <td class="top">Postel, J. and J. Reynolds, “<a href="http://tools.ietf.org/html/rfc959">File Transfer Protocol</a>”, STD&nbsp;9, RFC&nbsp;959, October&nbsp;1985.
2100            </td>
2101         </tr>
2102         <tr>
2103            <td class="reference"><b id="Spe">[Spe]</b></td>
2104            <td class="top">Spero, S., “<a href="http://sunsite.unc.edu/mdma-release/http-prob.html">Analysis of HTTP Performance Problems</a>”, &lt;<a href="http://sunsite.unc.edu/mdma-release/http-prob.html">http://sunsite.unc.edu/mdma-release/http-prob.html</a>&gt;.
2105            </td>
2106         </tr>
2107         <tr>
2108            <td class="reference"><b id="Tou1998">[Tou1998]</b></td>
2109            <td class="top"><a title="USC/Information Sciences Institute">Touch, J.</a>, <a title="USC/Information Sciences Institute">Heidemann, J.</a>, and <a title="USC/Information Sciences Institute">K. Obraczka</a>, “<a href="http://www.isi.edu/touch/pubs/http-perf96/">Analysis of HTTP Performance</a>”, ISI Research Report&nbsp;ISI/RR-98-463 (original report dated Aug.1996), Aug&nbsp;1998, &lt;<a href="http://www.isi.edu/touch/pubs/http-perf96/">http://www.isi.edu/touch/pubs/http-perf96/</a>&gt;.
2110            </td>
2111         </tr>
2112         <tr>
2113            <td class="reference"><b id="USASCII">[USASCII]</b></td>
2114            <td class="top">American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI&nbsp;X3.4, 1986.</td>
2115         </tr>
2116         <tr>
2117            <td class="reference"><b id="WAIS">[WAIS]</b></td>
2118            <td class="top">Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R., Sui, J., and M. Grinbaum, “WAIS Interface Protocol Prototype Functional Specification (v1.5)”, Thinking Machines Corporation, April&nbsp;1990.</td>
2119         </tr>
2120      </table>
2121      <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
2122      <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
2123            (editor)
2124            <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Day Software</span><span class="adr"><span class="street-address vcardline">23 Corporate Plaza DR, Suite 280</span><span class="vcardline"><span class="locality">Newport Beach</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">92660</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel">Phone: <a href="tel:+1-949-706-5300"><span class="value">+1-949-706-5300</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1-949-706-5305"><span class="value">+1-949-706-5305</span></a></span><span class="vcardline">EMail: <a><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>
2125      <address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">One Laptop per Child</span><span class="adr"><span class="street-address vcardline">21 Oak Knoll Road</span><span class="vcardline"><span class="locality">Carlisle</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01741</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a><span class="email">jg@laptop.org</span></a></span><span class="vcardline">URI: <a href="http://www.laptop.org/" class="url">http://www.laptop.org/</a></span></address>
2126      <address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Hewlett-Packard Company</span><span class="adr"><span class="street-address vcardline">HP Labs, Large Scale Systems Group</span><span class="street-address vcardline">1501 Page Mill Road, MS 1177</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94304</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a><span class="email">JeffMogul@acm.org</span></a></span></address>
2127      <address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a><span class="email">henrikn@microsoft.com</span></a></span></address>
2128      <address class="vcard"><span class="vcardline"><span class="fn">Larry Masinter</span><span class="n hidden"><span class="family-name">Masinter</span><span class="given-name">Larry</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><span class="email">LMM@acm.org</span></a></span><span class="vcardline">URI: <a href="http://larry.masinter.net/" class="url">http://larry.masinter.net/</a></span></address>
2129      <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span></span><span class="vcardline">EMail: <a><span class="email">paulle@microsoft.com</span></a></span></address>
2130      <address class="vcard"><span class="vcardline"><span class="fn">Tim Berners-Lee</span><span class="n hidden"><span class="family-name">Berners-Lee</span><span class="given-name">Tim</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">MIT Computer Science and Artificial Intelligence Laboratory</span><span class="street-address vcardline">The Stata Center, Building 32</span><span class="street-address vcardline">32 Vassar Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a><span class="email">timbl@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.w3.org/People/Berners-Lee/" class="url">http://www.w3.org/People/Berners-Lee/</a></span></address>
2131      <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
2132            (editor)
2133            <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><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>
2134      <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
2135            (editor)
2136            <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 tel">Phone: <a href="tel:+492512807760"><span class="value">+49 251 2807760</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+492512807761"><span class="value">+49 251 2807761</span></a></span><span class="vcardline">EMail: <a><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>
2137      <div id="rfc.iref.m.2"></div>
2138      <div id="rfc.iref.m.3"></div>
2139      <div id="rfc.iref.m.4"></div>
2140      <div id="rfc.iref.a.1"></div>
2141      <h1 id="rfc.section.A"><a href="#rfc.section.A">A.</a>&nbsp;<a id="internet.media.type.http" href="#internet.media.type.http">Internet Media Type message/http and application/http</a></h1>
2142      <p id="rfc.section.A.p.1">In addition to defining the HTTP/1.1 protocol, this document serves as the specification for the Internet media type "message/http"
2143         and "application/http". The message/http type can be used to enclose a single HTTP request or response message, provided that
2144         it obeys the MIME restrictions for all "message" types regarding line length and encodings. The application/http type can
2145         be used to enclose a pipeline of one or more HTTP request or response messages (not intermixed). The following is to be registered
2146         with IANA <a href="#RFC4288" id="rfc.xref.RFC4288.1"><cite title="Media Type Specifications and Registration Procedures">[RFC4288]</cite></a>.
2147      </p>
2148      <p id="rfc.section.A.p.2"> </p>
2149      <dl>
2150         <dt>Media Type name:</dt>
2151         <dd>message</dd>
2152         <dt>Media subtype name:</dt>
2153         <dd>http</dd>
2154         <dt>Required parameters:</dt>
2155         <dd>none</dd>
2156         <dt>Optional parameters:</dt>
2157         <dd>version, msgtype
2158            <dl>
2159               <dt>version:</dt>
2160               <dd>The HTTP-Version number of the enclosed message (e.g., "1.1"). If not present, the version can be determined from the first
2161                  line of the body.
2162               </dd>
2163               <dt>msgtype:</dt>
2164               <dd>The message type -- "request" or "response". If not present, the type can be determined from the first line of the body.</dd>
2165            </dl> 
2166         </dd>
2167         <dt>Encoding considerations:</dt>
2168         <dd>only "7bit", "8bit", or "binary" are permitted</dd>
2169         <dt>Security considerations:</dt>
2170         <dd>none</dd>
2171      </dl>
2172      <p id="rfc.section.A.p.3"> </p>
2173      <dl>
2174         <dt>Media Type name:</dt>
2175         <dd>application</dd>
2176         <dt>Media subtype name:</dt>
2177         <dd>http</dd>
2178         <dt>Required parameters:</dt>
2179         <dd>none</dd>
2180         <dt>Optional parameters:</dt>
2181         <dd>version, msgtype
2182            <dl>
2183               <dt>version:</dt>
2184               <dd>The HTTP-Version number of the enclosed messages (e.g., "1.1"). If not present, the version can be determined from the first
2185                  line of the body.
2186               </dd>
2187               <dt>msgtype:</dt>
2188               <dd>The message type -- "request" or "response". If not present, the type can be determined from the first line of the body.</dd>
2189            </dl> 
2190         </dd>
2191         <dt>Encoding considerations:</dt>
2192         <dd>HTTP messages enclosed by this type are in "binary" format; use of an appropriate Content-Transfer-Encoding is required when
2193            transmitted via E-mail.
2194         </dd>
2195         <dt>Security considerations:</dt>
2196         <dd>none</dd>
2197      </dl>
2198      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="tolerant.applications" href="#tolerant.applications">Tolerant Applications</a></h1>
2199      <p id="rfc.section.B.p.1">Although this document specifies the requirements for the generation of HTTP/1.1 messages, not all applications will be correct
2200         in their implementation. We therefore recommend that operational applications be tolerant of deviations whenever those deviations
2201         can be interpreted unambiguously.
2202      </p>
2203      <p id="rfc.section.B.p.2">Clients <em class="bcp14">SHOULD</em> be tolerant in parsing the Status-Line and servers tolerant when parsing the Request-Line. In particular, they <em class="bcp14">SHOULD</em> accept any amount of SP or HT characters between fields, even though only a single SP is required.
2204      </p>
2205      <p id="rfc.section.B.p.3">The line terminator for message-header fields is the sequence CRLF. However, we recommend that applications, when parsing
2206         such headers, recognize a single LF as a line terminator and ignore the leading CR.
2207      </p>
2208      <p id="rfc.section.B.p.4">The character set of an entity-body <em class="bcp14">SHOULD</em> be labeled as the lowest common denominator of the character codes used within that body, with the exception that not labeling
2209         the entity is preferred over labeling the entity with the labels US-ASCII or ISO-8859-1. See <a href="#Part3" id="rfc.xref.Part3.14"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>.
2210      </p>
2211      <p id="rfc.section.B.p.5">Additional rules for requirements on parsing and encoding of dates and other potential problems with date encodings include:</p>
2212      <p id="rfc.section.B.p.6"> </p>
2213      <ul>
2214         <li>HTTP/1.1 clients and caches <em class="bcp14">SHOULD</em> assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve
2215            the "year 2000" problem).
2216         </li>
2217         <li>An HTTP/1.1 implementation <em class="bcp14">MAY</em> internally represent a parsed Expires date as earlier than the proper value, but <em class="bcp14">MUST NOT</em> internally represent a parsed Expires date as later than the proper value.
2218         </li>
2219         <li>All expiration-related calculations <em class="bcp14">MUST</em> be done in GMT. The local time zone <em class="bcp14">MUST NOT</em> influence the calculation or comparison of an age or expiration time.
2220         </li>
2221         <li>If an HTTP header incorrectly carries a date value with a time zone other than GMT, it <em class="bcp14">MUST</em> be converted into GMT using the most conservative possible conversion.
2222         </li>
2223      </ul>
2224      <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a id="conversion.of.date.formats" href="#conversion.of.date.formats">Conversion of Date Formats</a></h1>
2225      <p id="rfc.section.C.p.1">HTTP/1.1 uses a restricted set of date formats (<a href="#full.date" title="Full Date">Section&nbsp;3.3.1</a>) to simplify the process of date comparison. Proxies and gateways from other protocols <em class="bcp14">SHOULD</em> ensure that any Date header field present in a message conforms to one of the HTTP/1.1 formats and rewrite the date if necessary.
2226      </p>
2227      <h1 id="rfc.section.D"><a href="#rfc.section.D">D.</a>&nbsp;<a id="compatibility" href="#compatibility">Compatibility with Previous Versions</a></h1>
2228      <p id="rfc.section.D.p.1">It is beyond the scope of a protocol specification to mandate compliance with previous versions. HTTP/1.1 was deliberately
2229         designed, however, to make supporting previous versions easy. It is worth noting that, at the time of composing this specification
2230         (1996), we would expect commercial HTTP/1.1 servers to:
2231      </p>
2232      <ul>
2233         <li>recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 requests;</li>
2234         <li>understand any valid request in the format of HTTP/0.9, 1.0, or 1.1;</li>
2235         <li>respond appropriately with a message in the same major version used by the client.</li>
2236      </ul>
2237      <p id="rfc.section.D.p.2">And we would expect HTTP/1.1 clients to: </p>
2238      <ul>
2239         <li>recognize the format of the Status-Line for HTTP/1.0 and 1.1 responses;</li>
2240         <li>understand any valid response in the format of HTTP/0.9, 1.0, or 1.1.</li>
2241      </ul>
2242      <p id="rfc.section.D.p.3">For most implementations of HTTP/1.0, each connection is established by the client prior to the request and closed by the
2243         server after sending the response. Some implementations implement the Keep-Alive version of persistent connections described
2244         in <a href="http://tools.ietf.org/html/rfc2068#section-19.7.1" id="rfc.xref.RFC2068.3">Section 19.7.1</a> of RFC 2068 <a href="#RFC2068" id="rfc.xref.RFC2068.4"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a>.
2245      </p>
2246      <h2 id="rfc.section.D.1"><a href="#rfc.section.D.1">D.1</a>&nbsp;<a id="changes.from.1.0" href="#changes.from.1.0">Changes from HTTP/1.0</a></h2>
2247      <p id="rfc.section.D.1.p.1">This section summarizes major differences between versions HTTP/1.0 and HTTP/1.1.</p>
2248      <h3 id="rfc.section.D.1.1"><a href="#rfc.section.D.1.1">D.1.1</a>&nbsp;<a id="changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses" href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses</a></h3>
2249      <p id="rfc.section.D.1.1.p.1">The requirements that clients and servers support the Host request-header, report an error if the Host request-header (<a href="#header.host" id="rfc.xref.header.host.1" title="Host">Section&nbsp;8.4</a>) is missing from an HTTP/1.1 request, and accept absolute URIs (<a href="#request-uri" title="Request-URI">Section&nbsp;5.1.2</a>) are among the most important changes defined by this specification.
2250      </p>
2251      <p id="rfc.section.D.1.1.p.2">Older HTTP/1.0 clients assumed a one-to-one relationship of IP addresses and servers; there was no other established mechanism
2252         for distinguishing the intended server of a request than the IP address to which that request was directed. The changes outlined
2253         above will allow the Internet, once older HTTP clients are no longer common, to support multiple Web sites from a single IP
2254         address, greatly simplifying large operational Web servers, where allocation of many IP addresses to a single host has created
2255         serious problems. The Internet will also be able to recover the IP addresses that have been allocated for the sole purpose
2256         of allowing special-purpose domain names to be used in root-level HTTP URLs. Given the rate of growth of the Web, and the
2257         number of servers already deployed, it is extremely important that all implementations of HTTP (including updates to existing
2258         HTTP/1.0 applications) correctly implement these requirements:
2259      </p>
2260      <ul>
2261         <li>Both clients and servers <em class="bcp14">MUST</em> support the Host request-header.
2262         </li>
2263         <li>A client that sends an HTTP/1.1 request <em class="bcp14">MUST</em> send a Host header.
2264         </li>
2265         <li>Servers <em class="bcp14">MUST</em> report a 400 (Bad Request) error if an HTTP/1.1 request does not include a Host request-header.
2266         </li>
2267         <li>Servers <em class="bcp14">MUST</em> accept absolute URIs.
2268         </li>
2269      </ul>
2270      <h2 id="rfc.section.D.2"><a href="#rfc.section.D.2">D.2</a>&nbsp;<a id="compatibility.with.http.1.0.persistent.connections" href="#compatibility.with.http.1.0.persistent.connections">Compatibility with HTTP/1.0 Persistent Connections</a></h2>
2271      <p id="rfc.section.D.2.p.1">Some clients and servers might wish to be compatible with some previous implementations of persistent connections in HTTP/1.0
2272         clients and servers. Persistent connections in HTTP/1.0 are explicitly negotiated as they are not the default behavior. HTTP/1.0
2273         experimental implementations of persistent connections are faulty, and the new facilities in HTTP/1.1 are designed to rectify
2274         these problems. The problem was that some existing 1.0 clients may be sending Keep-Alive to a proxy server that doesn't understand
2275         Connection, which would then erroneously forward it to the next inbound server, which would establish the Keep-Alive connection
2276         and result in a hung HTTP/1.0 proxy waiting for the close on the response. The result is that HTTP/1.0 clients must be prevented
2277         from using Keep-Alive when talking to proxies.
2278      </p>
2279      <p id="rfc.section.D.2.p.2">However, talking to proxies is the most important use of persistent connections, so that prohibition is clearly unacceptable.
2280         Therefore, we need some other mechanism for indicating a persistent connection is desired, which is safe to use even when
2281         talking to an old proxy that ignores Connection. Persistent connections are the default for HTTP/1.1 messages; we introduce
2282         a new keyword (Connection: close) for declaring non-persistence. See <a href="#header.connection" id="rfc.xref.header.connection.6" title="Connection">Section&nbsp;8.1</a>.
2283      </p>
2284      <p id="rfc.section.D.2.p.3">The original HTTP/1.0 form of persistent connections (the Connection: Keep-Alive and Keep-Alive header) is documented in RFC
2285         2068. <a href="#RFC2068" id="rfc.xref.RFC2068.5"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> 
2286      </p>
2287      <h2 id="rfc.section.D.3"><a href="#rfc.section.D.3">D.3</a>&nbsp;<a id="changes.from.rfc.2068" href="#changes.from.rfc.2068">Changes from RFC 2068</a></h2>
2288      <p id="rfc.section.D.3.p.1">This specification has been carefully audited to correct and disambiguate key word usage; RFC 2068 had many problems in respect
2289         to the conventions laid out in RFC 2119 <a href="#RFC2119" id="rfc.xref.RFC2119.2"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
2290      </p>
2291      <p id="rfc.section.D.3.p.2">Transfer-coding and message lengths all interact in ways that required fixing exactly when chunked encoding is used (to allow
2292         for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are
2293         computed.
2294      </p>
2295      <p id="rfc.section.D.3.p.3">The use and interpretation of HTTP version numbers has been clarified by RFC 2145. Require proxies to upgrade requests to
2296         highest protocol version they support to deal with problems discovered in HTTP/1.0 implementations (<a href="#http.version" title="HTTP Version">Section&nbsp;3.1</a>)
2297      </p>
2298      <p id="rfc.section.D.3.p.4">Proxies should be able to add Content-Length when appropriate.</p>
2299      <p id="rfc.section.D.3.p.5">Transfer-coding had significant problems, particularly with interactions with chunked encoding. The solution is that transfer-codings
2300         become as full fledged as content-codings. This involves adding an IANA registry for transfer-codings (separate from content
2301         codings), a new header field (TE) and enabling trailer headers in the future. Transfer encoding is a major performance benefit,
2302         so it was worth fixing <a href="#Nie1997" id="rfc.xref.Nie1997.2"><cite title="Network Performance Effects of HTTP/1.1, CSS1, and PNG">[Nie1997]</cite></a>. TE also solves another, obscure, downward interoperability problem that could have occurred due to interactions between
2303         authentication trailers, chunked encoding and HTTP/1.0 clients.(Section <a href="#transfer.codings" title="Transfer Codings">3.4</a>, <a href="#chunked.transfer.encoding" title="Chunked Transfer Coding">3.4.1</a>, and <a href="#header.te" id="rfc.xref.header.te.3" title="TE">8.5</a>)
2304      </p>
2305      <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
2306      <p>Copyright © The IETF Trust (2007).</p>
2307      <p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the
2308         authors retain all their rights.
2309      </p>
2310      <p>This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION
2311         HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE
2312         DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
2313         WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
2314      </p>
2315      <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
2316      <p>The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might
2317         be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any
2318         license under such rights might or might not be available; nor does it represent that it has made any independent effort to
2319         identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and
2320         BCP 79.
2321      </p>
2322      <p>Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result
2323         of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users
2324         of this specification can be obtained from the IETF on-line IPR repository at &lt;<a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>&gt;.
2325      </p>
2326      <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
2327         rights that may cover technology that may be required to implement this standard. Please address the information to the IETF
2328         at <a href="mailto:ietf-ipr@ietf.org">ietf-ipr@ietf.org</a>.
2329      </p>
2330      <h1>Acknowledgement</h1>
2331      <p>Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).</p>
2332      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
2333      <p class="noprint"><a href="#rfc.index.A">A</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.D">D</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.I">I</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.N">N</a> <a href="#rfc.index.O">O</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.T">T</a> <a href="#rfc.index.U">U</a> <a href="#rfc.index.V">V</a> <a href="#rfc.index.W">W</a> 
2334      </p>
2335      <div class="print2col">
2336         <ul class="ind">
2337            <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
2338                  <li class="indline1">application/http Media Type&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1"><b>A</b></a></li>
2339               </ul>
2340            </li>
2341            <li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind">
2342                  <li class="indline1">cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4">1.3</a></li>
2343                  <li class="indline1">cacheable&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5">1.3</a></li>
2344                  <li class="indline1">client&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.3">1.3</a></li>
2345                  <li class="indline1">connection&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1">1.3</a></li>
2346                  <li class="indline1">Connection header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.connection.1">4.5</a>, <a class="iref" href="#rfc.xref.header.connection.2">7.1.2</a>, <a class="iref" href="#rfc.xref.header.connection.3">7.1.3</a>, <a class="iref" href="#rfc.iref.c.6"><b>8.1</b></a>, <a class="iref" href="#rfc.xref.header.connection.4">8.5</a>, <a class="iref" href="#rfc.xref.header.connection.5">8.8</a>, <a class="iref" href="#rfc.xref.header.connection.6">D.2</a></li>
2347                  <li class="indline1">content negotiation&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2">1.3</a></li>
2348                  <li class="indline1">Content-Length header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.content-length.1">4.4</a>, <a class="iref" href="#rfc.iref.c.7"><b>8.2</b></a></li>
2349               </ul>
2350            </li>
2351            <li class="indline0"><a id="rfc.index.D" href="#rfc.index.D"><b>D</b></a><ul class="ind">
2352                  <li class="indline1">Date header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.date.1">4.5</a>, <a class="iref" href="#rfc.iref.d.2"><b>8.3</b></a></li>
2353                  <li class="indline1">downstream&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.1">1.3</a></li>
2354               </ul>
2355            </li>
2356            <li class="indline0"><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul class="ind">
2357                  <li class="indline1">entity&nbsp;&nbsp;<a class="iref" href="#rfc.iref.e.1">1.3</a></li>
2358               </ul>
2359            </li>
2360            <li class="indline0"><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul class="ind">
2361                  <li class="indline1">gateway&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1">1.3</a></li>
2362                  <li class="indline1"><tt>Grammar</tt>&nbsp;&nbsp;
2363                     <ul class="ind">
2364                        <li class="indline1"><tt>ALPHA</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>2.2</b></a></li>
2365                        <li class="indline1"><tt>asctime-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.29"><b>3.3.1</b></a></li>
2366                        <li class="indline1"><tt>attribute</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.40"><b>3.4</b></a></li>
2367                        <li class="indline1"><tt>CHAR</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>2.2</b></a></li>
2368                        <li class="indline1"><tt>chunk</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.43"><b>3.4.1</b></a></li>
2369                        <li class="indline1"><tt>chunk-data</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.49"><b>3.4.1</b></a></li>
2370                        <li class="indline1"><tt>chunk-ext-name</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.47"><b>3.4.1</b></a></li>
2371                        <li class="indline1"><tt>chunk-ext-val</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.48"><b>3.4.1</b></a></li>
2372                        <li class="indline1"><tt>chunk-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.46"><b>3.4.1</b></a></li>
2373                        <li class="indline1"><tt>chunk-size</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.44"><b>3.4.1</b></a></li>
2374                        <li class="indline1"><tt>Chunked-Body</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.42"><b>3.4.1</b></a></li>
2375                        <li class="indline1"><tt>comment</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>2.2</b></a></li>
2376                        <li class="indline1"><tt>Connection</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.70"><b>8.1</b></a></li>
2377                        <li class="indline1"><tt>connection-token</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.71"><b>8.1</b></a></li>
2378                        <li class="indline1"><tt>Content-Length</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.72"><b>8.2</b></a></li>
2379                        <li class="indline1"><tt>CR</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>2.2</b></a></li>
2380                        <li class="indline1"><tt>CRLF</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>2.2</b></a></li>
2381                        <li class="indline1"><tt>ctext</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.20"><b>2.2</b></a></li>
2382                        <li class="indline1"><tt>CTL</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>2.2</b></a></li>
2383                        <li class="indline1"><tt>Date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.73"><b>8.3</b></a></li>
2384                        <li class="indline1"><tt>date1</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.30"><b>3.3.1</b></a></li>
2385                        <li class="indline1"><tt>date2</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.31"><b>3.3.1</b></a></li>
2386                        <li class="indline1"><tt>date3</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.32"><b>3.3.1</b></a></li>
2387                        <li class="indline1"><tt>DIGIT</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>2.2</b></a></li>
2388                        <li class="indline1"><tt>extension-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.68"><b>6.1.1</b></a></li>
2389                        <li class="indline1"><tt>extension-method</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.63"><b>5.1.1</b></a></li>
2390                        <li class="indline1"><tt>field-content</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.57"><b>4.2</b></a></li>
2391                        <li class="indline1"><tt>field-name</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.55"><b>4.2</b></a></li>
2392                        <li class="indline1"><tt>field-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.56"><b>4.2</b></a></li>
2393                        <li class="indline1"><tt>general-header</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.59"><b>4.5</b></a></li>
2394                        <li class="indline1"><tt>generic-message</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.52"><b>4.1</b></a></li>
2395                        <li class="indline1"><tt>HEX</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>2.2</b></a></li>
2396                        <li class="indline1"><tt>Host</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.74"><b>8.4</b></a></li>
2397                        <li class="indline1"><tt>HT</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>2.2</b></a></li>
2398                        <li class="indline1"><tt>HTTP-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.26"><b>3.3.1</b></a></li>
2399                        <li class="indline1"><tt>HTTP-message</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.51"><b>4.1</b></a></li>
2400                        <li class="indline1"><tt>HTTP-Version</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.24"><b>3.1</b></a></li>
2401                        <li class="indline1"><tt>http_URL</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.25"><b>3.2.2</b></a></li>
2402                        <li class="indline1"><tt>last-chunk</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.45"><b>3.4.1</b></a></li>
2403                        <li class="indline1"><tt>LF</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>2.2</b></a></li>
2404                        <li class="indline1"><tt>LOALPHA</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>2.2</b></a></li>
2405                        <li class="indline1"><tt>LWS</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>2.2</b></a></li>
2406                        <li class="indline1"><tt>message-body</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.58"><b>4.3</b></a></li>
2407                        <li class="indline1"><tt>message-header</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.54"><b>4.2</b></a></li>
2408                        <li class="indline1"><tt>Method</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.62"><b>5.1.1</b></a></li>
2409                        <li class="indline1"><tt>month</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.36"><b>3.3.1</b></a></li>
2410                        <li class="indline1"><tt>OCTET</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>2.2</b></a></li>
2411                        <li class="indline1"><tt>parameter</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.39"><b>3.4</b></a></li>
2412                        <li class="indline1"><tt>protocol-name</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.82"><b>8.9</b></a></li>
2413                        <li class="indline1"><tt>protocol-version</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.83"><b>8.9</b></a></li>
2414                        <li class="indline1"><tt>pseudonym</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.85"><b>8.9</b></a></li>
2415                        <li class="indline1"><tt>qdtext</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.22"><b>2.2</b></a></li>
2416                        <li class="indline1"><tt>quoted-pair</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.23"><b>2.2</b></a></li>
2417                        <li class="indline1"><tt>quoted-string</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.21"><b>2.2</b></a></li>
2418                        <li class="indline1"><tt>Reason-Phrase</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.69"><b>6.1.1</b></a></li>
2419                        <li class="indline1"><tt>received-by</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.84"><b>8.9</b></a></li>
2420                        <li class="indline1"><tt>received-protocol</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.81"><b>8.9</b></a></li>
2421                        <li class="indline1"><tt>Request</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.60"><b>5</b></a></li>
2422                        <li class="indline1"><tt>Request-Line</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.61"><b>5.1</b></a></li>
2423                        <li class="indline1"><tt>Request-URI</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.64"><b>5.1.2</b></a></li>
2424                        <li class="indline1"><tt>Response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.65"><b>6</b></a></li>
2425                        <li class="indline1"><tt>rfc1123-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.27"><b>3.3.1</b></a></li>
2426                        <li class="indline1"><tt>rfc850-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.28"><b>3.3.1</b></a></li>
2427                        <li class="indline1"><tt>separators</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>2.2</b></a></li>
2428                        <li class="indline1"><tt>SP</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>2.2</b></a></li>
2429                        <li class="indline1"><tt>start-line</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.53"><b>4.1</b></a></li>
2430                        <li class="indline1"><tt>Status-Code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.67"><b>6.1.1</b></a></li>
2431                        <li class="indline1"><tt>Status-Line</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.66"><b>6.1</b></a></li>
2432                        <li class="indline1"><tt>t-codings</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.76"><b>8.5</b></a></li>
2433                        <li class="indline1"><tt>TE</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.75"><b>8.5</b></a></li>
2434                        <li class="indline1"><tt>TEXT</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>2.2</b></a></li>
2435                        <li class="indline1"><tt>time</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.33"><b>3.3.1</b></a></li>
2436                        <li class="indline1"><tt>token</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>2.2</b></a></li>
2437                        <li class="indline1"><tt>trailer</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.50"><b>3.4.1</b></a></li>
2438                        <li class="indline1"><tt>Trailer</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.77"><b>8.6</b></a></li>
2439                        <li class="indline1"><tt>transfer-coding</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.37"><b>3.4</b></a></li>
2440                        <li class="indline1"><tt>Transfer-Encoding</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.78"><b>8.7</b></a></li>
2441                        <li class="indline1"><tt>transfer-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.38"><b>3.4</b></a></li>
2442                        <li class="indline1"><tt>UPALPHA</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>2.2</b></a></li>
2443                        <li class="indline1"><tt>Upgrade</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.79"><b>8.8</b></a></li>
2444                        <li class="indline1"><tt>value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.41"><b>3.4</b></a></li>
2445                        <li class="indline1"><tt>Via</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.80"><b>8.9</b></a></li>
2446                        <li class="indline1"><tt>weekday</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.35"><b>3.3.1</b></a></li>
2447                        <li class="indline1"><tt>wkday</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.34"><b>3.3.1</b></a></li>
2448                     </ul>
2449                  </li>
2450               </ul>
2451            </li>
2452            <li class="indline0"><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul class="ind">
2453                  <li class="indline1">Headers&nbsp;&nbsp;
2454                     <ul class="ind">
2455                        <li class="indline1">Connection&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.connection.1">4.5</a>, <a class="iref" href="#rfc.xref.header.connection.2">7.1.2</a>, <a class="iref" href="#rfc.xref.header.connection.3">7.1.3</a>, <a class="iref" href="#rfc.iref.h.1"><b>8.1</b></a>, <a class="iref" href="#rfc.xref.header.connection.4">8.5</a>, <a class="iref" href="#rfc.xref.header.connection.5">8.8</a>, <a class="iref" href="#rfc.xref.header.connection.6">D.2</a></li>
2456                        <li class="indline1">Content-Length&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.content-length.1">4.4</a>, <a class="iref" href="#rfc.iref.h.2"><b>8.2</b></a></li>
2457                        <li class="indline1">Date&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.date.1">4.5</a>, <a class="iref" href="#rfc.iref.h.3"><b>8.3</b></a></li>
2458                        <li class="indline1">Host&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.5"><b>8.4</b></a>, <a class="iref" href="#rfc.xref.header.host.1">D.1.1</a></li>
2459                        <li class="indline1">TE&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.te.1">3.4</a>, <a class="iref" href="#rfc.xref.header.te.2">3.4.1</a>, <a class="iref" href="#rfc.iref.h.6"><b>8.5</b></a>, <a class="iref" href="#rfc.xref.header.te.3">D.3</a></li>
2460                        <li class="indline1">Trailer&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.trailer.1">3.4.1</a>, <a class="iref" href="#rfc.xref.header.trailer.2">4.5</a>, <a class="iref" href="#rfc.iref.h.7"><b>8.6</b></a></li>
2461                        <li class="indline1">Transfer-Encoding&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.transfer-encoding.1">3.4</a>, <a class="iref" href="#rfc.xref.header.transfer-encoding.2">4.3</a>, <a class="iref" href="#rfc.xref.header.transfer-encoding.3">4.4</a>, <a class="iref" href="#rfc.xref.header.transfer-encoding.4">4.5</a>, <a class="iref" href="#rfc.iref.h.8"><b>8.7</b></a></li>
2462                        <li class="indline1">Upgrade&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.upgrade.1">4.5</a>, <a class="iref" href="#rfc.iref.h.9"><b>8.8</b></a></li>
2463                        <li class="indline1">Via&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.via.1">4.5</a>, <a class="iref" href="#rfc.iref.h.10"><b>8.9</b></a></li>
2464                     </ul>
2465                  </li>
2466                  <li class="indline1">Host header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.4"><b>8.4</b></a>, <a class="iref" href="#rfc.xref.header.host.1">D.1.1</a></li>
2467               </ul>
2468            </li>
2469            <li class="indline0"><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul class="ind">
2470                  <li class="indline1">inbound&nbsp;&nbsp;<a class="iref" href="#rfc.iref.i.1">1.3</a></li>
2471                  <li class="indline1"><em>ISO-8859</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ISO-8859.1">2.2</a>, <a class="iref" href="#ISO-8859"><b>12</b></a></li>
2472               </ul>
2473            </li>
2474            <li class="indline0"><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul class="ind">
2475                  <li class="indline1">Media Type&nbsp;&nbsp;
2476                     <ul class="ind">
2477                        <li class="indline1">application/http&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.4"><b>A</b></a></li>
2478                        <li class="indline1">message/http&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>A</b></a></li>
2479                     </ul>
2480                  </li>
2481                  <li class="indline1">message&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1">1.3</a></li>
2482                  <li class="indline1">message/http Media Type&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>A</b></a></li>
2483               </ul>
2484            </li>
2485            <li class="indline0"><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul class="ind">
2486                  <li class="indline1"><em>Nie1997</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Nie1997.1">7.1.1</a>, <a class="iref" href="#Nie1997"><b>12</b></a>, <a class="iref" href="#rfc.xref.Nie1997.2">D.3</a></li>
2487               </ul>
2488            </li>
2489            <li class="indline0"><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul class="ind">
2490                  <li class="indline1">origin server&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1">1.3</a></li>
2491                  <li class="indline1">outbound&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.2">1.3</a></li>
2492               </ul>
2493            </li>
2494            <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
2495                  <li class="indline1"><em>Pad1995</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Pad1995.1">7.1.1</a>, <a class="iref" href="#Pad1995"><b>12</b></a></li>
2496                  <li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">3.2.1</a>, <a class="iref" href="#rfc.xref.Part2.2">4.2</a>, <a class="iref" href="#rfc.xref.Part2.3">4.2</a>, <a class="iref" href="#rfc.xref.Part2.4">4.3</a>, <a class="iref" href="#rfc.xref.Part2.5">5</a>, <a class="iref" href="#rfc.xref.Part2.6">5.1.2</a>, <a class="iref" href="#rfc.xref.Part2.7">6</a>, <a class="iref" href="#rfc.xref.Part2.8">6.1.1</a>, <a class="iref" href="#rfc.xref.Part2.9">7.1.2.2</a>, <a class="iref" href="#rfc.xref.Part2.10">7.1.4</a>, <a class="iref" href="#rfc.xref.Part2.11">7.2.3</a>, <a class="iref" href="#rfc.xref.Part2.12">7.2.3</a>, <a class="iref" href="#rfc.xref.Part2.13">7.2.3</a>, <a class="iref" href="#rfc.xref.Part2.14">7.2.3</a>, <a class="iref" href="#Part2"><b>12</b></a><ul class="ind">
2497                        <li class="indline1"><em>Section 3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.4">4.3</a></li>
2498                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.2">4.2</a>, <a class="iref" href="#rfc.xref.Part2.5">5</a></li>
2499                        <li class="indline1"><em>Section 6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.3">4.2</a>, <a class="iref" href="#rfc.xref.Part2.7">6</a></li>
2500                        <li class="indline1"><em>Section 8.1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.9">7.1.2.2</a>, <a class="iref" href="#rfc.xref.Part2.10">7.1.4</a></li>
2501                        <li class="indline1"><em>Section 8.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.6">5.1.2</a></li>
2502                        <li class="indline1"><em>Section 9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.8">6.1.1</a></li>
2503                        <li class="indline1"><em>Section 9.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.11">7.2.3</a></li>
2504                        <li class="indline1"><em>Section 9.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.14">7.2.3</a></li>
2505                        <li class="indline1"><em>Section 9.4.15</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">3.2.1</a></li>
2506                        <li class="indline1"><em>Section 10.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.12">7.2.3</a>, <a class="iref" href="#rfc.xref.Part2.13">7.2.3</a></li>
2507                     </ul>
2508                  </li>
2509                  <li class="indline1"><em>Part3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">1.3</a>, <a class="iref" href="#rfc.xref.Part3.2">1.3</a>, <a class="iref" href="#rfc.xref.Part3.3">1.3</a>, <a class="iref" href="#rfc.xref.Part3.4">1.4</a>, <a class="iref" href="#rfc.xref.Part3.5">2.2</a>, <a class="iref" href="#rfc.xref.Part3.6">3.4</a>, <a class="iref" href="#rfc.xref.Part3.7">3.4</a>, <a class="iref" href="#rfc.xref.Part3.8">3.4</a>, <a class="iref" href="#rfc.xref.Part3.9">3.4</a>, <a class="iref" href="#rfc.xref.Part3.10">4.2</a>, <a class="iref" href="#rfc.xref.Part3.11">5</a>, <a class="iref" href="#rfc.xref.Part3.12">6</a>, <a class="iref" href="#rfc.xref.Part3.13">8.5</a>, <a class="iref" href="#Part3"><b>12</b></a>, <a class="iref" href="#rfc.xref.Part3.14">B</a><ul class="ind">
2510                        <li class="indline1"><em>Section 2.4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.13">8.5</a></li>
2511                        <li class="indline1"><em>Section 3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">1.3</a></li>
2512                        <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.10">4.2</a>, <a class="iref" href="#rfc.xref.Part3.11">5</a>, <a class="iref" href="#rfc.xref.Part3.12">6</a></li>
2513                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.2">1.3</a>, <a class="iref" href="#rfc.xref.Part3.3">1.3</a></li>
2514                        <li class="indline1"><em>Section A</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.4">1.4</a></li>
2515                     </ul>
2516                  </li>
2517                  <li class="indline1"><em>Part6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">1.3</a>, <a class="iref" href="#rfc.xref.Part6.2">1.4</a>, <a class="iref" href="#rfc.xref.Part6.3">4.5</a>, <a class="iref" href="#rfc.xref.Part6.4">4.5</a>, <a class="iref" href="#rfc.xref.Part6.5">4.5</a>, <a class="iref" href="#Part6"><b>12</b></a><ul class="ind">
2518                        <li class="indline1"><em>Section 2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part6.1">1.3</a>, <a class=