Changeset 2726 for draft-ietf-httpbis/19


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

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

Location:
draft-ietf-httpbis/19
Files:
7 edited

Legend:

Unmodified
Added
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  • draft-ietf-httpbis/19/p1-messaging.html

    r1592 r2726  
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    33<html lang="en">
    4    <head profile="http://www.w3.org/2006/03/hcard http://dublincore.org/documents/2008/08/04/dc-html/">
     4   <head profile="http://dublincore.org/documents/2008/08/04/dc-html/">
    55      <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    66      <title>HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title><script>
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    … …  
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    2525  buttonsAdded = !buttonsAdded;
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    … …  
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    … …  
    5858      ref += "#" + id;
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    60    
     60
    6161    // docname
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    … …  
    6565    var section = node.textContent;
    6666    section = section.replace("\u00a0", " ");
    67    
     67
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    … …  
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    … …  
    448451@page {
    449452  @top-left {
    450        content: "Internet-Draft"; 
    451   } 
     453       content: "Internet-Draft";
     454  }
    452455  @top-right {
    453        content: "March 2012"; 
    454   } 
     456       content: "March 2012";
     457  }
    455458  @top-center {
    456        content: "HTTP/1.1, Part 1"; 
    457   } 
     459       content: "HTTP/1.1, Part 1";
     460  }
    458461  @bottom-left {
    459        content: "Fielding, et al."; 
    460   } 
     462       content: "Fielding, et al.";
     463  }
    461464  @bottom-center {
    462        content: "Expires September 13, 2012"; 
    463   } 
     465       content: "Expires September 13, 2012";
     466  }
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    465        content: "[Page " counter(page) "]"; 
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    468 
    469 @page:first { 
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     470}
     471
     472@page:first {
    470473    @top-left {
    471474      content: normal;
    … …  
    496499      <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C">
    497500      <link href="p2-semantics.html" rel="next">
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    499502      <link rel="schema.dct" href="http://purl.org/dc/terms/">
    500503      <meta name="dct.creator" content="Fielding, R.">
    … …  
    508511      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document 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 and moves it to historic status, along with its predecessor RFC 2068. 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. This part also obsoletes RFCs 2145 (on HTTP version numbers) and 2817 (on using CONNECT for TLS upgrades) and moves them to historic status.">
    509512   </head>
    510    <body onload="init();">
     513   <body onload="initFeedback();">
    511514      <table class="header">
    512515         <tbody>
    … …  
    520523            </tr>
    521524            <tr>
    522                <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2145">2145</a>, <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
     525               <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2145">2145</a>, <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved)
    523526               </td>
    524527               <td class="right">Y. Lafon, Editor</td>
    525528            </tr>
    526529            <tr>
    527                <td class="left">Updates: <a href="http://tools.ietf.org/html/rfc2817">2817</a> (if approved)
     530               <td class="left">Updates: <a href="https://tools.ietf.org/html/rfc2817">2817</a> (if approved)
    528531               </td>
    529532               <td class="right">W3C</td>
    … …  
    544547      </table>
    545548      <p class="title">HTTP/1.1, part 1: URIs, Connections, and Message Parsing<br><span class="filename">draft-ietf-httpbis-p1-messaging-19</span></p>
    546       <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
     549      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    547550      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
    548551         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 1 of the
    549552         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2616.1">RFC 2616</cite> and moves it to historic status, along with its predecessor <cite title="Hypertext Transfer Protocol -- HTTP/1.1" id="rfc.xref.RFC2068.1">RFC 2068</cite>.
    550       </p> 
     553      </p>
    551554      <p>Part 1 provides an overview of HTTP and its associated terminology, defines the "http" and "https" Uniform Resource Identifier
    552555         (URI) schemes, defines the generic message syntax and parsing requirements for HTTP message frames, and describes general
    553556         security concerns for implementations.
    554       </p> 
     557      </p>
    555558      <p>This part also obsoletes RFCs <cite title="Use and Interpretation of HTTP Version Numbers" id="rfc.xref.RFC2145.1">2145</cite> (on HTTP version numbers) and <cite title="Upgrading to TLS Within HTTP/1.1" id="rfc.xref.RFC2817.1">2817</cite> (on using CONNECT for TLS upgrades) and moves them to historic status.
    556       </p> 
    557       <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
     559      </p>
     560      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
    558561      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived
    559562         at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
    560       </p> 
     563      </p>
    561564      <p>The current issues list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
    562       </p> 
     565      </p>
    563566      <p>The changes in this draft are summarized in <a href="#changes.since.18" title="Since draft-ietf-httpbis-p1-messaging-18">Appendix&nbsp;C.20</a>.
    564       </p>
    565       <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
    566       <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
    567       <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
    568          working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
    569567      </p>
    570       <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
    571          documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
    572          in progress”.
    573       </p>
    574       <p>This Internet-Draft will expire on September 13, 2012.</p>
    575       <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    576       <p>Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
    577       <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
    578          and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
    579          text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
    580          BSD License.
    581       </p>
    582       <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
    583          10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
    584          allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
    585          controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
    586          works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
    587          it into languages other than English.
    588       </p>
     568      <div id="rfc.status">
     569         <h1><a href="#rfc.status">Status of This Memo</a></h1>
     570         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
     571         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
     572            working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
     573         </p>
     574         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
     575            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
     576            in progress”.
     577         </p>
     578         <p>This Internet-Draft will expire on September 13, 2012.</p>
     579      </div>
     580      <div id="rfc.copyrightnotice">
     581         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
     582         <p>Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
     583         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
     584            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
     585            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
     586            BSD License.
     587         </p>
     588         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
     589            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
     590            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
     591            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
     592            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
     593            it into languages other than English.
     594         </p>
     595      </div>
    589596      <hr class="noprint">
    590597      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
    591598      <ul class="toc">
    592          <li>1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
    593                <li>1.1&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirement Notation</a></li>
    594                <li>1.2&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
     599         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
     600               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirement Notation</a></li>
     601               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
    595602            </ul>
    596603         </li>
    597          <li>2.&nbsp;&nbsp;&nbsp;<a href="#architecture">Architecture</a><ul>
    598                <li>2.1&nbsp;&nbsp;&nbsp;<a href="#operation">Client/Server Messaging</a></li>
    599                <li>2.2&nbsp;&nbsp;&nbsp;<a href="#transport-independence">Connections and Transport Independence</a></li>
    600                <li>2.3&nbsp;&nbsp;&nbsp;<a href="#intermediaries">Intermediaries</a></li>
    601                <li>2.4&nbsp;&nbsp;&nbsp;<a href="#caches">Caches</a></li>
    602                <li>2.5&nbsp;&nbsp;&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></li>
    603                <li>2.6&nbsp;&nbsp;&nbsp;<a href="#http.version">Protocol Versioning</a></li>
    604                <li>2.7&nbsp;&nbsp;&nbsp;<a href="#uri">Uniform Resource Identifiers</a><ul>
    605                      <li>2.7.1&nbsp;&nbsp;&nbsp;<a href="#http.uri">http URI scheme</a></li>
    606                      <li>2.7.2&nbsp;&nbsp;&nbsp;<a href="#https.uri">https URI scheme</a></li>
    607                      <li>2.7.3&nbsp;&nbsp;&nbsp;<a href="#uri.comparison">http and https URI Normalization and Comparison</a></li>
     604         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#architecture">Architecture</a><ul>
     605               <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#operation">Client/Server Messaging</a></li>
     606               <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#transport-independence">Connections and Transport Independence</a></li>
     607               <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#intermediaries">Intermediaries</a></li>
     608               <li><a href="#rfc.section.2.4">2.4</a>&nbsp;&nbsp;&nbsp;<a href="#caches">Caches</a></li>
     609               <li><a href="#rfc.section.2.5">2.5</a>&nbsp;&nbsp;&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></li>
     610               <li><a href="#rfc.section.2.6">2.6</a>&nbsp;&nbsp;&nbsp;<a href="#http.version">Protocol Versioning</a></li>
     611               <li><a href="#rfc.section.2.7">2.7</a>&nbsp;&nbsp;&nbsp;<a href="#uri">Uniform Resource Identifiers</a><ul>
     612                     <li><a href="#rfc.section.2.7.1">2.7.1</a>&nbsp;&nbsp;&nbsp;<a href="#http.uri">http URI scheme</a></li>
     613                     <li><a href="#rfc.section.2.7.2">2.7.2</a>&nbsp;&nbsp;&nbsp;<a href="#https.uri">https URI scheme</a></li>
     614                     <li><a href="#rfc.section.2.7.3">2.7.3</a>&nbsp;&nbsp;&nbsp;<a href="#uri.comparison">http and https URI Normalization and Comparison</a></li>
    608615                  </ul>
    609616               </li>
    610617            </ul>
    611618         </li>
    612          <li>3.&nbsp;&nbsp;&nbsp;<a href="#http.message">Message Format</a><ul>
    613                <li>3.1&nbsp;&nbsp;&nbsp;<a href="#start.line">Start Line</a><ul>
    614                      <li>3.1.1&nbsp;&nbsp;&nbsp;<a href="#request.line">Request Line</a></li>
    615                      <li>3.1.2&nbsp;&nbsp;&nbsp;<a href="#status.line">Status Line</a></li>
     619         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#http.message">Message Format</a><ul>
     620               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#start.line">Start Line</a><ul>
     621                     <li><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;&nbsp;&nbsp;<a href="#request.line">Request Line</a></li>
     622                     <li><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;&nbsp;&nbsp;<a href="#status.line">Status Line</a></li>
    616623                  </ul>
    617624               </li>
    618                <li>3.2&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Fields</a><ul>
    619                      <li>3.2.1&nbsp;&nbsp;&nbsp;<a href="#whitespace">Whitespace</a></li>
    620                      <li>3.2.2&nbsp;&nbsp;&nbsp;<a href="#field.parsing">Field Parsing</a></li>
    621                      <li>3.2.3&nbsp;&nbsp;&nbsp;<a href="#field.length">Field Length</a></li>
    622                      <li>3.2.4&nbsp;&nbsp;&nbsp;<a href="#field.components">Field value components</a></li>
    623                      <li>3.2.5&nbsp;&nbsp;&nbsp;<a href="#abnf.extension">ABNF list extension: #rule</a></li>
     625               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Fields</a><ul>
     626                     <li><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#whitespace">Whitespace</a></li>
     627                     <li><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#field.parsing">Field Parsing</a></li>
     628                     <li><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#field.length">Field Length</a></li>
     629                     <li><a href="#rfc.section.3.2.4">3.2.4</a>&nbsp;&nbsp;&nbsp;<a href="#field.components">Field value components</a></li>
     630                     <li><a href="#rfc.section.3.2.5">3.2.5</a>&nbsp;&nbsp;&nbsp;<a href="#abnf.extension">ABNF list extension: #rule</a></li>
    624631                  </ul>
    625632               </li>
    626                <li>3.3&nbsp;&nbsp;&nbsp;<a href="#message.body">Message Body</a><ul>
    627                      <li>3.3.1&nbsp;&nbsp;&nbsp;<a href="#header.transfer-encoding">Transfer-Encoding</a></li>
    628                      <li>3.3.2&nbsp;&nbsp;&nbsp;<a href="#header.content-length">Content-Length</a></li>
    629                      <li>3.3.3&nbsp;&nbsp;&nbsp;<a href="#message.body.length">Message Body Length</a></li>
     633               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#message.body">Message Body</a><ul>
     634                     <li><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.transfer-encoding">Transfer-Encoding</a></li>
     635                     <li><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.content-length">Content-Length</a></li>
     636                     <li><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#message.body.length">Message Body Length</a></li>
    630637                  </ul>
    631638               </li>
    632                <li>3.4&nbsp;&nbsp;&nbsp;<a href="#incomplete.messages">Handling Incomplete Messages</a></li>
    633                <li>3.5&nbsp;&nbsp;&nbsp;<a href="#message.robustness">Message Parsing Robustness</a></li>
     639               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#incomplete.messages">Handling Incomplete Messages</a></li>
     640               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#message.robustness">Message Parsing Robustness</a></li>
    634641            </ul>
    635642         </li>
    636          <li>4.&nbsp;&nbsp;&nbsp;<a href="#transfer.codings">Transfer Codings</a><ul>
    637                <li>4.1&nbsp;&nbsp;&nbsp;<a href="#chunked.encoding">Chunked Transfer Coding</a></li>
    638                <li>4.2&nbsp;&nbsp;&nbsp;<a href="#compression.codings">Compression Codings</a><ul>
    639                      <li>4.2.1&nbsp;&nbsp;&nbsp;<a href="#compress.coding">Compress Coding</a></li>
    640                      <li>4.2.2&nbsp;&nbsp;&nbsp;<a href="#deflate.coding">Deflate Coding</a></li>
    641                      <li>4.2.3&nbsp;&nbsp;&nbsp;<a href="#gzip.coding">Gzip Coding</a></li>
     643         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#transfer.codings">Transfer Codings</a><ul>
     644               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#chunked.encoding">Chunked Transfer Coding</a></li>
     645               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#compression.codings">Compression Codings</a><ul>
     646                     <li><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#compress.coding">Compress Coding</a></li>
     647                     <li><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#deflate.coding">Deflate Coding</a></li>
     648                     <li><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#gzip.coding">Gzip Coding</a></li>
    642649                  </ul>
    643650               </li>
    644                <li>4.3&nbsp;&nbsp;&nbsp;<a href="#header.te">TE</a><ul>
    645                      <li>4.3.1&nbsp;&nbsp;&nbsp;<a href="#quality.values">Quality Values</a></li>
     651               <li><a href="#rfc.section.4.3">4.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.te">TE</a><ul>
     652                     <li><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#quality.values">Quality Values</a></li>
    646653                  </ul>
    647654               </li>
    648                <li>4.4&nbsp;&nbsp;&nbsp;<a href="#header.trailer">Trailer</a></li>
     655               <li><a href="#rfc.section.4.4">4.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.trailer">Trailer</a></li>
    649656            </ul>
    650657         </li>
    651          <li>5.&nbsp;&nbsp;&nbsp;<a href="#message.routing">Message Routing</a><ul>
    652                <li>5.1&nbsp;&nbsp;&nbsp;<a href="#target-resource">Identifying a Target Resource</a></li>
    653                <li>5.2&nbsp;&nbsp;&nbsp;<a href="#connecting.inbound">Connecting Inbound</a></li>
    654                <li>5.3&nbsp;&nbsp;&nbsp;<a href="#request-target">Request Target</a></li>
    655                <li>5.4&nbsp;&nbsp;&nbsp;<a href="#header.host">Host</a></li>
    656                <li>5.5&nbsp;&nbsp;&nbsp;<a href="#effective.request.uri">Effective Request URI</a></li>
    657                <li>5.6&nbsp;&nbsp;&nbsp;<a href="#intermediary.forwarding">Intermediary Forwarding</a><ul>
    658                      <li>5.6.1&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.header-fields">End-to-end and Hop-by-hop Header Fields</a></li>
    659                      <li>5.6.2&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.header-fields">Non-modifiable Header Fields</a></li>
     658         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#message.routing">Message Routing</a><ul>
     659               <li><a href="#rfc.section.5.1">5.1</a>&nbsp;&nbsp;&nbsp;<a href="#target-resource">Identifying a Target Resource</a></li>
     660               <li><a href="#rfc.section.5.2">5.2</a>&nbsp;&nbsp;&nbsp;<a href="#connecting.inbound">Connecting Inbound</a></li>
     661               <li><a href="#rfc.section.5.3">5.3</a>&nbsp;&nbsp;&nbsp;<a href="#request-target">Request Target</a></li>
     662               <li><a href="#rfc.section.5.4">5.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.host">Host</a></li>
     663               <li><a href="#rfc.section.5.5">5.5</a>&nbsp;&nbsp;&nbsp;<a href="#effective.request.uri">Effective Request URI</a></li>
     664               <li><a href="#rfc.section.5.6">5.6</a>&nbsp;&nbsp;&nbsp;<a href="#intermediary.forwarding">Intermediary Forwarding</a><ul>
     665                     <li><a href="#rfc.section.5.6.1">5.6.1</a>&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.header-fields">End-to-end and Hop-by-hop Header Fields</a></li>
     666                     <li><a href="#rfc.section.5.6.2">5.6.2</a>&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.header-fields">Non-modifiable Header Fields</a></li>
    660667                  </ul>
    661668               </li>
    662                <li>5.7&nbsp;&nbsp;&nbsp;<a href="#associating.response.to.request">Associating a Response to a Request</a></li>
     669               <li><a href="#rfc.section.5.7">5.7</a>&nbsp;&nbsp;&nbsp;<a href="#associating.response.to.request">Associating a Response to a Request</a></li>
    663670            </ul>
    664671         </li>
    665          <li>6.&nbsp;&nbsp;&nbsp;<a href="#connection.management">Connection Management</a><ul>
    666                <li>6.1&nbsp;&nbsp;&nbsp;<a href="#header.connection">Connection</a></li>
    667                <li>6.2&nbsp;&nbsp;&nbsp;<a href="#header.via">Via</a></li>
    668                <li>6.3&nbsp;&nbsp;&nbsp;<a href="#persistent.connections">Persistent Connections</a><ul>
    669                      <li>6.3.1&nbsp;&nbsp;&nbsp;<a href="#persistent.purpose">Purpose</a></li>
    670                      <li>6.3.2&nbsp;&nbsp;&nbsp;<a href="#persistent.overall">Overall Operation</a><ul>
    671                            <li>6.3.2.1&nbsp;&nbsp;&nbsp;<a href="#persistent.negotiation">Negotiation</a></li>
    672                            <li>6.3.2.2&nbsp;&nbsp;&nbsp;<a href="#pipelining">Pipelining</a></li>
    673                         </ul>
    674                      </li>
    675                      <li>6.3.3&nbsp;&nbsp;&nbsp;<a href="#persistent.practical">Practical Considerations</a></li>
    676                      <li>6.3.4&nbsp;&nbsp;&nbsp;<a href="#persistent.retrying.requests">Retrying Requests</a></li>
     672         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#connection.management">Connection Management</a><ul>
     673               <li><a href="#rfc.section.6.1">6.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.connection">Connection</a></li>
     674               <li><a href="#rfc.section.6.2">6.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.via">Via</a></li>
     675               <li><a href="#rfc.section.6.3">6.3</a>&nbsp;&nbsp;&nbsp;<a href="#persistent.connections">Persistent Connections</a><ul>
     676                     <li><a href="#rfc.section.6.3.1">6.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#persistent.purpose">Purpose</a></li>
     677                     <li><a href="#rfc.section.6.3.2">6.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#persistent.overall">Overall Operation</a></li>
     678                     <li><a href="#rfc.section.6.3.3">6.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#persistent.practical">Practical Considerations</a></li>
     679                     <li><a href="#rfc.section.6.3.4">6.3.4</a>&nbsp;&nbsp;&nbsp;<a href="#persistent.retrying.requests">Retrying Requests</a></li>
    677680                  </ul>
    678681               </li>
    679                <li>6.4&nbsp;&nbsp;&nbsp;<a href="#message.transmission.requirements">Message Transmission Requirements</a><ul>
    680                      <li>6.4.1&nbsp;&nbsp;&nbsp;<a href="#persistent.flow">Persistent Connections and Flow Control</a></li>
    681                      <li>6.4.2&nbsp;&nbsp;&nbsp;<a href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></li>
    682                      <li>6.4.3&nbsp;&nbsp;&nbsp;<a href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></li>
    683                      <li>6.4.4&nbsp;&nbsp;&nbsp;<a href="#closing.connections.on.error">Closing Connections on Error</a></li>
     682               <li><a href="#rfc.section.6.4">6.4</a>&nbsp;&nbsp;&nbsp;<a href="#message.transmission.requirements">Message Transmission Requirements</a><ul>
     683                     <li><a href="#rfc.section.6.4.1">6.4.1</a>&nbsp;&nbsp;&nbsp;<a href="#persistent.flow">Persistent Connections and Flow Control</a></li>
     684                     <li><a href="#rfc.section.6.4.2">6.4.2</a>&nbsp;&nbsp;&nbsp;<a href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></li>
     685                     <li><a href="#rfc.section.6.4.3">6.4.3</a>&nbsp;&nbsp;&nbsp;<a href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></li>
     686                     <li><a href="#rfc.section.6.4.4">6.4.4</a>&nbsp;&nbsp;&nbsp;<a href="#closing.connections.on.error">Closing Connections on Error</a></li>
    684687                  </ul>
    685688               </li>
    686                <li>6.5&nbsp;&nbsp;&nbsp;<a href="#header.upgrade">Upgrade</a></li>
     689               <li><a href="#rfc.section.6.5">6.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.upgrade">Upgrade</a></li>
    687690            </ul>
    688691         </li>
    689          <li>7.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
    690                <li>7.1&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
    691                <li>7.2&nbsp;&nbsp;&nbsp;<a href="#uri.scheme.registration">URI Scheme Registration</a></li>
    692                <li>7.3&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.http">Internet Media Type Registrations</a><ul>
    693                      <li>7.3.1&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.message.http">Internet Media Type message/http</a></li>
    694                      <li>7.3.2&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.application.http">Internet Media Type application/http</a></li>
     692         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
     693               <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
     694               <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#uri.scheme.registration">URI Scheme Registration</a></li>
     695               <li><a href="#rfc.section.7.3">7.3</a>&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.http">Internet Media Type Registrations</a><ul>
     696                     <li><a href="#rfc.section.7.3.1">7.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.message.http">Internet Media Type message/http</a></li>
     697                     <li><a href="#rfc.section.7.3.2">7.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#internet.media.type.application.http">Internet Media Type application/http</a></li>
    695698                  </ul>
    696699               </li>
    697                <li>7.4&nbsp;&nbsp;&nbsp;<a href="#transfer.coding.registry">Transfer Coding Registry</a></li>
    698                <li>7.5&nbsp;&nbsp;&nbsp;<a href="#transfer.coding.registration">Transfer Coding Registrations</a></li>
    699                <li>7.6&nbsp;&nbsp;&nbsp;<a href="#upgrade.token.registry">Upgrade Token Registry</a></li>
    700                <li>7.7&nbsp;&nbsp;&nbsp;<a href="#upgrade.token.registration">Upgrade Token Registration</a></li>
     700               <li><a href="#rfc.section.7.4">7.4</a>&nbsp;&nbsp;&nbsp;<a href="#transfer.coding.registry">Transfer Coding Registry</a></li>
     701               <li><a href="#rfc.section.7.5">7.5</a>&nbsp;&nbsp;&nbsp;<a href="#transfer.coding.registration">Transfer Coding Registrations</a></li>
     702               <li><a href="#rfc.section.7.6">7.6</a>&nbsp;&nbsp;&nbsp;<a href="#upgrade.token.registry">Upgrade Token Registry</a></li>
     703               <li><a href="#rfc.section.7.7">7.7</a>&nbsp;&nbsp;&nbsp;<a href="#upgrade.token.registration">Upgrade Token Registration</a></li>
    701704            </ul>
    702705         </li>
    703          <li>8.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul>
    704                <li>8.1&nbsp;&nbsp;&nbsp;<a href="#personal.information">Personal Information</a></li>
    705                <li>8.2&nbsp;&nbsp;&nbsp;<a href="#abuse.of.server.log.information">Abuse of Server Log Information</a></li>
    706                <li>8.3&nbsp;&nbsp;&nbsp;<a href="#attack.pathname">Attacks Based On File and Path Names</a></li>
    707                <li>8.4&nbsp;&nbsp;&nbsp;<a href="#dns.related.attacks">DNS-related Attacks</a></li>
    708                <li>8.5&nbsp;&nbsp;&nbsp;<a href="#attack.intermediaries">Intermediaries and Caching</a></li>
    709                <li>8.6&nbsp;&nbsp;&nbsp;<a href="#attack.protocol.element.size.overflows">Protocol Element Size Overflows</a></li>
     706         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul>
     707               <li><a href="#rfc.section.8.1">8.1</a>&nbsp;&nbsp;&nbsp;<a href="#personal.information">Personal Information</a></li>
     708               <li><a href="#rfc.section.8.2">8.2</a>&nbsp;&nbsp;&nbsp;<a href="#abuse.of.server.log.information">Abuse of Server Log Information</a></li>
     709               <li><a href="#rfc.section.8.3">8.3</a>&nbsp;&nbsp;&nbsp;<a href="#attack.pathname">Attacks Based On File and Path Names</a></li>
     710               <li><a href="#rfc.section.8.4">8.4</a>&nbsp;&nbsp;&nbsp;<a href="#dns.related.attacks">DNS-related Attacks</a></li>
     711               <li><a href="#rfc.section.8.5">8.5</a>&nbsp;&nbsp;&nbsp;<a href="#attack.intermediaries">Intermediaries and Caching</a></li>
     712               <li><a href="#rfc.section.8.6">8.6</a>&nbsp;&nbsp;&nbsp;<a href="#attack.protocol.element.size.overflows">Protocol Element Size Overflows</a></li>
    710713            </ul>
    711714         </li>
    712          <li>9.&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
    713          <li>10.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
    714                <li>10.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
    715                <li>10.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
     715         <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
     716         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
     717               <li><a href="#rfc.section.10.1">10.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
     718               <li><a href="#rfc.section.10.2">10.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
    716719            </ul>
    717720         </li>
    718          <li><a href="#rfc.authors">Authors' Addresses</a></li>
    719          <li>A.&nbsp;&nbsp;&nbsp;<a href="#compatibility">HTTP Version History</a><ul>
    720                <li>A.1&nbsp;&nbsp;&nbsp;<a href="#changes.from.1.0">Changes from HTTP/1.0</a><ul>
    721                      <li>A.1.1&nbsp;&nbsp;&nbsp;<a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">Multi-homed Web Servers</a></li>
    722                      <li>A.1.2&nbsp;&nbsp;&nbsp;<a href="#compatibility.with.http.1.0.persistent.connections">Keep-Alive Connections</a></li>
     721         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#compatibility">HTTP Version History</a><ul>
     722               <li><a href="#rfc.section.A.1">A.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.1.0">Changes from HTTP/1.0</a><ul>
     723                     <li><a href="#rfc.section.A.1.1">A.1.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.to.simplify.multi-homed.web.servers.and.conserve.ip.addresses">Multi-homed Web Servers</a></li>
     724                     <li><a href="#rfc.section.A.1.2">A.1.2</a>&nbsp;&nbsp;&nbsp;<a href="#compatibility.with.http.1.0.persistent.connections">Keep-Alive Connections</a></li>
    723725                  </ul>
    724726               </li>
    725                <li>A.2&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
    726                <li>A.3&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2817">Changes from RFC 2817</a></li>
     727               <li><a href="#rfc.section.A.2">A.2</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
     728               <li><a href="#rfc.section.A.3">A.3</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2817">Changes from RFC 2817</a></li>
    727729            </ul>
    728730         </li>
    729          <li>B.&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
    730          <li>C.&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
    731                <li>C.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC 2616</a></li>
    732                <li>C.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p1-messaging-00</a></li>
    733                <li>C.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p1-messaging-01</a></li>
    734                <li>C.4&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p1-messaging-02</a></li>
    735                <li>C.5&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p1-messaging-03</a></li>
    736                <li>C.6&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p1-messaging-04</a></li>
    737                <li>C.7&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p1-messaging-05</a></li>
    738                <li>C.8&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p1-messaging-06</a></li>
    739                <li>C.9&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p1-messaging-07</a></li>
    740                <li>C.10&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p1-messaging-08</a></li>
    741                <li>C.11&nbsp;&nbsp;&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p1-messaging-09</a></li>
    742                <li>C.12&nbsp;&nbsp;&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p1-messaging-10</a></li>
    743                <li>C.13&nbsp;&nbsp;&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p1-messaging-11</a></li>
    744                <li>C.14&nbsp;&nbsp;&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p1-messaging-12</a></li>
    745                <li>C.15&nbsp;&nbsp;&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p1-messaging-13</a></li>
    746                <li>C.16&nbsp;&nbsp;&nbsp;<a href="#changes.since.14">Since draft-ietf-httpbis-p1-messaging-14</a></li>
    747                <li>C.17&nbsp;&nbsp;&nbsp;<a href="#changes.since.15">Since draft-ietf-httpbis-p1-messaging-15</a></li>
    748                <li>C.18&nbsp;&nbsp;&nbsp;<a href="#changes.since.16">Since draft-ietf-httpbis-p1-messaging-16</a></li>
    749                <li>C.19&nbsp;&nbsp;&nbsp;<a href="#changes.since.17">Since draft-ietf-httpbis-p1-messaging-17</a></li>
    750                <li>C.20&nbsp;&nbsp;&nbsp;<a href="#changes.since.18">Since draft-ietf-httpbis-p1-messaging-18</a></li>
     731         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
     732         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
     733               <li><a href="#rfc.section.C.1">C.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC 2616</a></li>
     734               <li><a href="#rfc.section.C.2">C.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p1-messaging-00</a></li>
     735               <li><a href="#rfc.section.C.3">C.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p1-messaging-01</a></li>
     736               <li><a href="#rfc.section.C.4">C.4</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p1-messaging-02</a></li>
     737               <li><a href="#rfc.section.C.5">C.5</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p1-messaging-03</a></li>
     738               <li><a href="#rfc.section.C.6">C.6</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p1-messaging-04</a></li>
     739               <li><a href="#rfc.section.C.7">C.7</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p1-messaging-05</a></li>
     740               <li><a href="#rfc.section.C.8">C.8</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p1-messaging-06</a></li>
     741               <li><a href="#rfc.section.C.9">C.9</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p1-messaging-07</a></li>
     742               <li><a href="#rfc.section.C.10">C.10</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p1-messaging-08</a></li>
     743               <li><a href="#rfc.section.C.11">C.11</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p1-messaging-09</a></li>
     744               <li><a href="#rfc.section.C.12">C.12</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p1-messaging-10</a></li>
     745               <li><a href="#rfc.section.C.13">C.13</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p1-messaging-11</a></li>
     746               <li><a href="#rfc.section.C.14">C.14</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p1-messaging-12</a></li>
     747               <li><a href="#rfc.section.C.15">C.15</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p1-messaging-13</a></li>
     748               <li><a href="#rfc.section.C.16">C.16</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.14">Since draft-ietf-httpbis-p1-messaging-14</a></li>
     749               <li><a href="#rfc.section.C.17">C.17</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.15">Since draft-ietf-httpbis-p1-messaging-15</a></li>
     750               <li><a href="#rfc.section.C.18">C.18</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.16">Since draft-ietf-httpbis-p1-messaging-16</a></li>
     751               <li><a href="#rfc.section.C.19">C.19</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.17">Since draft-ietf-httpbis-p1-messaging-17</a></li>
     752               <li><a href="#rfc.section.C.20">C.20</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.18">Since draft-ietf-httpbis-p1-messaging-18</a></li>
    751753            </ul>
    752754         </li>
    753755         <li><a href="#rfc.index">Index</a></li>
     756         <li><a href="#rfc.authors">Authors' Addresses</a></li>
    754757      </ul>
    755       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
    756       <p id="rfc.section.1.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level request/response protocol that uses extensible semantics and
    757          MIME-like message payloads for flexible interaction with network-based hypertext information systems. HTTP relies upon the
    758          Uniform Resource Identifier (URI) standard <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> to indicate the target resource (<a href="#target-resource" title="Identifying a Target Resource">Section&nbsp;5.1</a>) and relationships between resources. Messages are passed in a format similar to that used by Internet mail <a href="#RFC5322" id="rfc.xref.RFC5322.1"><cite title="Internet Message Format">[RFC5322]</cite></a> and the Multipurpose Internet Mail Extensions (MIME) <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a> (see <a href="p3-payload.html#differences.between.http.and.mime" title="Differences between HTTP and MIME">Appendix A</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> for the differences between HTTP and MIME messages).
    759       </p>
    760       <p id="rfc.section.1.p.2">HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented
    761          by presenting a uniform interface to clients that is independent of the types of resources provided. Likewise, servers do
    762          not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated
    763          with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used
    764          effectively in many different contexts and for which implementations can evolve independently over time.
    765       </p>
    766       <p id="rfc.section.1.p.3">HTTP is also designed for use as an intermediation protocol for translating communication to and from non-HTTP information
    767          systems. HTTP proxies and gateways can provide access to alternative information services by translating their diverse protocols
    768          into a hypertext format that can be viewed and manipulated by clients in the same way as HTTP services.
    769       </p>
    770       <p id="rfc.section.1.p.4">One consequence of HTTP flexibility is that the protocol cannot be defined in terms of what occurs behind the interface. Instead,
    771          we are limited to defining the syntax of communication, the intent of received communication, and the expected behavior of
    772          recipients. If the communication is considered in isolation, then successful actions ought to be reflected in corresponding
    773          changes to the observable interface provided by servers. However, since multiple clients might act in parallel and perhaps
    774          at cross-purposes, we cannot require that such changes be observable beyond the scope of a single response.
    775       </p>
    776       <p id="rfc.section.1.p.5">This document is Part 1 of the seven-part specification of HTTP, defining the protocol referred to as "HTTP/1.1", obsoleting <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a> and <a href="#RFC2145" id="rfc.xref.RFC2145.2"><cite title="Use and Interpretation of HTTP Version Numbers">[RFC2145]</cite></a>. Part 1 describes the architectural elements that are used or referred to in HTTP, defines the "http" and "https" URI schemes,
    777          describes overall network operation and connection management, and defines HTTP message framing and forwarding requirements.
    778          Our goal is to define all of the mechanisms necessary for HTTP message handling that are independent of message semantics,
    779          thereby defining the complete set of requirements for message parsers and message-forwarding intermediaries.
    780       </p>
    781       <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirement Notation</a></h2>
    782       <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
    783          in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
    784       </p>
    785       <div id="rfc.iref.g.1"></div>
    786       <div id="rfc.iref.g.2"></div>
    787       <div id="rfc.iref.g.3"></div>
    788       <div id="rfc.iref.g.4"></div>
    789       <div id="rfc.iref.g.5"></div>
    790       <div id="rfc.iref.g.6"></div>
    791       <div id="rfc.iref.g.7"></div>
    792       <div id="rfc.iref.g.8"></div>
    793       <div id="rfc.iref.g.9"></div>
    794       <div id="rfc.iref.g.10"></div>
    795       <div id="rfc.iref.g.11"></div>
    796       <div id="rfc.iref.g.12"></div>
    797       <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="notation" href="#notation">Syntax Notation</a></h2>
    798       <p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="#abnf.extension" title="ABNF list extension: #rule">Section&nbsp;3.2.5</a>. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF with the list rule expanded.
    799       </p>
    800       <div id="core.rules">
    801          <p id="rfc.section.1.2.p.2">                        The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="http://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
    802             (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR
    803             (any visible <a href="#USASCII" id="rfc.xref.USASCII.1"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a> character).
     758      <div id="introduction">
     759         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#introduction">Introduction</a></h1>
     760         <p id="rfc.section.1.p.1">The Hypertext Transfer Protocol (HTTP) is an application-level request/response protocol that uses extensible semantics and
     761            MIME-like message payloads for flexible interaction with network-based hypertext information systems. HTTP relies upon the
     762            Uniform Resource Identifier (URI) standard <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> to indicate the target resource (<a href="#target-resource" title="Identifying a Target Resource">Section&nbsp;5.1</a>) and relationships between resources. Messages are passed in a format similar to that used by Internet mail <a href="#RFC5322" id="rfc.xref.RFC5322.1"><cite title="Internet Message Format">[RFC5322]</cite></a> and the Multipurpose Internet Mail Extensions (MIME) <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a> (see <a href="p3-payload.html#differences.between.http.and.mime" title="Differences between HTTP and MIME">Appendix A</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> for the differences between HTTP and MIME messages).
    804763         </p>
     764         <p id="rfc.section.1.p.2">HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented
     765            by presenting a uniform interface to clients that is independent of the types of resources provided. Likewise, servers do
     766            not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated
     767            with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used
     768            effectively in many different contexts and for which implementations can evolve independently over time.
     769         </p>
     770         <p id="rfc.section.1.p.3">HTTP is also designed for use as an intermediation protocol for translating communication to and from non-HTTP information
     771            systems. HTTP proxies and gateways can provide access to alternative information services by translating their diverse protocols
     772            into a hypertext format that can be viewed and manipulated by clients in the same way as HTTP services.
     773         </p>
     774         <p id="rfc.section.1.p.4">One consequence of HTTP flexibility is that the protocol cannot be defined in terms of what occurs behind the interface. Instead,
     775            we are limited to defining the syntax of communication, the intent of received communication, and the expected behavior of
     776            recipients. If the communication is considered in isolation, then successful actions ought to be reflected in corresponding
     777            changes to the observable interface provided by servers. However, since multiple clients might act in parallel and perhaps
     778            at cross-purposes, we cannot require that such changes be observable beyond the scope of a single response.
     779         </p>
     780         <p id="rfc.section.1.p.5">This document is Part 1 of the seven-part specification of HTTP, defining the protocol referred to as "HTTP/1.1", obsoleting <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a> and <a href="#RFC2145" id="rfc.xref.RFC2145.2"><cite title="Use and Interpretation of HTTP Version Numbers">[RFC2145]</cite></a>. Part 1 describes the architectural elements that are used or referred to in HTTP, defines the "http" and "https" URI schemes,
     781            describes overall network operation and connection management, and defines HTTP message framing and forwarding requirements.
     782            Our goal is to define all of the mechanisms necessary for HTTP message handling that are independent of message semantics,
     783            thereby defining the complete set of requirements for message parsers and message-forwarding intermediaries.
     784         </p>
     785         <div id="intro.requirements">
     786            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#intro.requirements">Requirement Notation</a></h2>
     787            <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
     788               in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
     789            </p>
     790         </div>
     791         <div id="notation">
     792            <div id="rfc.iref.g.1"></div>
     793            <div id="rfc.iref.g.2"></div>
     794            <div id="rfc.iref.g.3"></div>
     795            <div id="rfc.iref.g.4"></div>
     796            <div id="rfc.iref.g.5"></div>
     797            <div id="rfc.iref.g.6"></div>
     798            <div id="rfc.iref.g.7"></div>
     799            <div id="rfc.iref.g.8"></div>
     800            <div id="rfc.iref.g.9"></div>
     801            <div id="rfc.iref.g.10"></div>
     802            <div id="rfc.iref.g.11"></div>
     803            <div id="rfc.iref.g.12"></div>
     804            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
     805            <p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="#abnf.extension" title="ABNF list extension: #rule">Section&nbsp;3.2.5</a>. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF with the list rule expanded.
     806            </p>
     807            <div id="core.rules">
     808               <p id="rfc.section.1.2.p.2">            The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
     809                  (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR
     810                  (any visible <a href="#USASCII" id="rfc.xref.USASCII.1"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a> character).
     811               </p>
     812            </div>
     813            <p id="rfc.section.1.2.p.3">As a convention, ABNF rule names prefixed with "obs-" denote "obsolete" grammar rules that appear for historical reasons.</p>
     814         </div>
    805815      </div>
    806       <p id="rfc.section.1.2.p.3">As a convention, ABNF rule names prefixed with "obs-" denote "obsolete" grammar rules that appear for historical reasons.</p>
    807       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="architecture" href="#architecture">Architecture</a></h1>
    808       <p id="rfc.section.2.p.1">HTTP was created for the World Wide Web architecture and has evolved over time to support the scalability needs of a worldwide
    809          hypertext system. Much of that architecture is reflected in the terminology and syntax productions used to define HTTP.
    810       </p>
    811       <div id="rfc.iref.c.1"></div>
    812       <div id="rfc.iref.s.1"></div>
    813       <div id="rfc.iref.c.2"></div>
    814       <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="operation" href="#operation">Client/Server Messaging</a></h2>
    815       <p id="rfc.section.2.1.p.1">HTTP is a stateless request/response protocol that operates by exchanging <dfn>messages</dfn> (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) across a reliable transport or session-layer "<dfn>connection</dfn>". An HTTP "<dfn>client</dfn>" is a program that establishes a connection to a server for the purpose of sending one or more HTTP requests. An HTTP "<dfn>server</dfn>" is a program that accepts connections in order to service HTTP requests by sending HTTP responses.
    816       </p>
    817       <div id="rfc.iref.u.1"></div>
    818       <div id="rfc.iref.o.1"></div>
    819       <div id="rfc.iref.b.1"></div>
    820       <div id="rfc.iref.s.2"></div>
    821       <div id="rfc.iref.s.3"></div>
    822       <div id="rfc.iref.r.1"></div>
    823       <p id="rfc.section.2.1.p.2">Note that the terms client and server refer only to the roles that these programs perform for a particular connection. The
    824          same program might act as a client on some connections and a server on others. We use the term "<dfn>user agent</dfn>" to refer to the program that initiates a request, such as a WWW browser, editor, or spider (web-traversing robot), and the
    825          term "<dfn>origin server</dfn>" to refer to the program that can originate authoritative responses to a request. For general requirements, we use the term
    826          "<dfn>sender</dfn>" to refer to whichever component sent a given message and the term "<dfn>recipient</dfn>" to refer to any component that receives the message.
    827       </p>
    828       <div class="note" id="rfc.section.2.1.p.3">
    829          <p> <b>Note:</b> The term 'user agent' covers both those situations where there is a user (human) interacting with the software agent (and
    830             for which user interface or interactive suggestions might be made, e.g., warning the user or given the user an option in the
    831             case of security or privacy options) and also those where the software agent may act autonomously.
    832          </p>
    833       </div>
    834       <p id="rfc.section.2.1.p.4">Most HTTP communication consists of a retrieval request (GET) for a representation of some resource identified by a URI. In
    835          the simplest case, this might be accomplished via a single bidirectional connection (===) between the user agent (UA) and
    836          the origin server (O).
    837       </p>
    838       <div id="rfc.figure.u.1"></div><pre class="drawing">         request   &gt;
     816      <div id="architecture">
     817         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#architecture">Architecture</a></h1>
     818         <p id="rfc.section.2.p.1">HTTP was created for the World Wide Web architecture and has evolved over time to support the scalability needs of a worldwide
     819            hypertext system. Much of that architecture is reflected in the terminology and syntax productions used to define HTTP.
     820         </p>
     821         <div id="operation">
     822            <div id="rfc.iref.c.1"></div>
     823            <div id="rfc.iref.s.1"></div>
     824            <div id="rfc.iref.c.2"></div>
     825            <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a href="#operation">Client/Server Messaging</a></h2>
     826            <p id="rfc.section.2.1.p.1">HTTP is a stateless request/response protocol that operates by exchanging <dfn>messages</dfn> (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) across a reliable transport or session-layer "<dfn>connection</dfn>". An HTTP "<dfn>client</dfn>" is a program that establishes a connection to a server for the purpose of sending one or more HTTP requests. An HTTP "<dfn>server</dfn>" is a program that accepts connections in order to service HTTP requests by sending HTTP responses.
     827            </p>
     828            <div id="rfc.iref.u.1"></div>
     829            <div id="rfc.iref.o.1"></div>
     830            <div id="rfc.iref.b.1"></div>
     831            <div id="rfc.iref.s.2"></div>
     832            <div id="rfc.iref.s.3"></div>
     833            <div id="rfc.iref.r.1"></div>
     834            <p id="rfc.section.2.1.p.2">Note that the terms client and server refer only to the roles that these programs perform for a particular connection. The
     835               same program might act as a client on some connections and a server on others. We use the term "<dfn>user agent</dfn>" to refer to the program that initiates a request, such as a WWW browser, editor, or spider (web-traversing robot), and the
     836               term "<dfn>origin server</dfn>" to refer to the program that can originate authoritative responses to a request. For general requirements, we use the term
     837               "<dfn>sender</dfn>" to refer to whichever component sent a given message and the term "<dfn>recipient</dfn>" to refer to any component that receives the message.
     838            </p>
     839            <div class="note" id="rfc.section.2.1.p.3">
     840               <p><b>Note:</b> The term 'user agent' covers both those situations where there is a user (human) interacting with the software agent (and
     841                  for which user interface or interactive suggestions might be made, e.g., warning the user or given the user an option in the
     842                  case of security or privacy options) and also those where the software agent may act autonomously.
     843               </p>
     844            </div>
     845            <p id="rfc.section.2.1.p.4">Most HTTP communication consists of a retrieval request (GET) for a representation of some resource identified by a URI. In
     846               the simplest case, this might be accomplished via a single bidirectional connection (===) between the user agent (UA) and
     847               the origin server (O).
     848            </p>
     849            <div id="rfc.figure.u.1"></div><pre class="drawing">         request   &gt;
    839850    UA ======================================= O
    840851                                &lt;   response
    841852</pre><div id="rfc.iref.m.1"></div>
    842       <div id="rfc.iref.r.2"></div>
    843       <div id="rfc.iref.r.3"></div>
    844       <p id="rfc.section.2.1.p.6">A client sends an HTTP request to the server in the form of a <dfn>request</dfn> message, beginning with a request-line that includes a method, URI, and protocol version (<a href="#request.line" title="Request Line">Section&nbsp;3.1.1</a>), followed by MIME-like header fields containing request modifiers, client information, and representation metadata (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
    845       </p>
    846       <p id="rfc.section.2.1.p.7">A server responds to the client's request by sending one or more HTTP <dfn>response</dfn> messages, each beginning with a status line that includes the protocol version, a success or error code, and textual reason
    847          phrase (<a href="#status.line" title="Status Line">Section&nbsp;3.1.2</a>), possibly followed by MIME-like header fields containing server information, resource metadata, and representation metadata
    848          (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
    849       </p>
    850       <p id="rfc.section.2.1.p.8">The following example illustrates a typical message exchange for a GET request on the URI "http://www.example.com/hello.txt":</p>
    851       <div id="rfc.figure.u.2"></div>
    852       <p>client request:</p><pre class="text2">GET /hello.txt HTTP/1.1
     853            <div id="rfc.iref.r.2"></div>
     854            <div id="rfc.iref.r.3"></div>
     855            <p id="rfc.section.2.1.p.6">A client sends an HTTP request to the server in the form of a <dfn>request</dfn> message, beginning with a request-line that includes a method, URI, and protocol version (<a href="#request.line" title="Request Line">Section&nbsp;3.1.1</a>), followed by MIME-like header fields containing request modifiers, client information, and representation metadata (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
     856            </p>
     857            <p id="rfc.section.2.1.p.7">A server responds to the client's request by sending one or more HTTP <dfn>response</dfn> messages, each beginning with a status line that includes the protocol version, a success or error code, and textual reason
     858               phrase (<a href="#status.line" title="Status Line">Section&nbsp;3.1.2</a>), possibly followed by MIME-like header fields containing server information, resource metadata, and representation metadata
     859               (<a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>), an empty line to indicate the end of the header section, and finally a message body containing the payload body (if any, <a href="#message.body" title="Message Body">Section&nbsp;3.3</a>).
     860            </p>
     861            <p id="rfc.section.2.1.p.8">The following example illustrates a typical message exchange for a GET request on the URI "http://www.example.com/hello.txt":</p>
     862            <div id="rfc.figure.u.2"></div>
     863            <p>client request:</p><pre class="text2">GET /hello.txt HTTP/1.1
    853864User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
    854865Host: www.example.com
    … …  
    856867
    857868</pre><div id="rfc.figure.u.3"></div>
    858       <p>server response:</p><pre class="text">HTTP/1.1 200 OK
     869            <p>server response:</p><pre class="text">HTTP/1.1 200 OK
    859870Date: Mon, 27 Jul 2009 12:28:53 GMT
    860871Server: Apache
    … …  
    867878
    868879<span id="exbody">Hello World!
    869 </span></pre><h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="transport-independence" href="#transport-independence">Connections and Transport Independence</a></h2>
    870       <p id="rfc.section.2.2.p.1">HTTP messaging is independent of the underlying transport or session-layer connection protocol(s). HTTP only presumes a reliable
    871          transport with in-order delivery of requests and the corresponding in-order delivery of responses. The mapping of HTTP request
    872          and response structures onto the data units of the underlying transport protocol is outside the scope of this specification.
    873       </p>
    874       <p id="rfc.section.2.2.p.2">The specific connection protocols to be used for an interaction are determined by client configuration and the target URI
    875          (<a href="#target-resource" title="Identifying a Target Resource">Section&nbsp;5.1</a>). For example, the "http" URI scheme (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) indicates a default connection of TCP over IP, with a default TCP port of 80, but the client might be configured to use
    876          a proxy via some other connection port or protocol instead of using the defaults.
    877       </p>
    878       <p id="rfc.section.2.2.p.3">A connection might be used for multiple HTTP request/response exchanges, as defined in <a href="#persistent.connections" title="Persistent Connections">Section&nbsp;6.3</a>.
    879       </p>
    880       <div id="rfc.iref.i.1"></div>
    881       <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="intermediaries" href="#intermediaries">Intermediaries</a></h2>
    882       <p id="rfc.section.2.3.p.1">HTTP enables the use of intermediaries to satisfy requests through a chain of connections. There are three common forms of
    883          HTTP <dfn>intermediary</dfn>: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an origin server, proxy, gateway, or tunnel,
    884          switching behavior based on the nature of each request.
    885       </p>
    886       <div id="rfc.figure.u.4"></div><pre class="drawing">         &gt;             &gt;             &gt;             &gt;
     880</span></pre></div>
     881         <div id="transport-independence">
     882            <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a href="#transport-independence">Connections and Transport Independence</a></h2>
     883            <p id="rfc.section.2.2.p.1">HTTP messaging is independent of the underlying transport or session-layer connection protocol(s). HTTP only presumes a reliable
     884               transport with in-order delivery of requests and the corresponding in-order delivery of responses. The mapping of HTTP request
     885               and response structures onto the data units of the underlying transport protocol is outside the scope of this specification.
     886            </p>
     887            <p id="rfc.section.2.2.p.2">The specific connection protocols to be used for an interaction are determined by client configuration and the target URI
     888               (<a href="#target-resource" title="Identifying a Target Resource">Section&nbsp;5.1</a>). For example, the "http" URI scheme (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) indicates a default connection of TCP over IP, with a default TCP port of 80, but the client might be configured to use
     889               a proxy via some other connection port or protocol instead of using the defaults.
     890            </p>
     891            <p id="rfc.section.2.2.p.3">A connection might be used for multiple HTTP request/response exchanges, as defined in <a href="#persistent.connections" title="Persistent Connections">Section&nbsp;6.3</a>.
     892            </p>
     893         </div>
     894         <div id="intermediaries">
     895            <div id="rfc.iref.i.1"></div>
     896            <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a href="#intermediaries">Intermediaries</a></h2>
     897            <p id="rfc.section.2.3.p.1">HTTP enables the use of intermediaries to satisfy requests through a chain of connections. There are three common forms of
     898               HTTP <dfn>intermediary</dfn>: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an origin server, proxy, gateway, or tunnel,
     899               switching behavior based on the nature of each request.
     900            </p>
     901            <div id="rfc.figure.u.4"></div><pre class="drawing">         &gt;             &gt;             &gt;             &gt;
    887902    <b>UA</b> =========== <b>A</b> =========== <b>B</b> =========== <b>C</b> =========== <b>O</b>
    888903               &lt;             &lt;             &lt;             &lt;
    889904</pre><p id="rfc.section.2.3.p.3">The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response
    890          message that travels the whole chain will pass through four separate connections. Some HTTP communication options might apply
    891          only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along
    892          the chain. Although the diagram is linear, each participant might be engaged in multiple, simultaneous communications. For
    893          example, B might be receiving requests from many clients other than A, and/or forwarding requests to servers other than C,
    894          at the same time that it is handling A's request.
    895       </p>
    896       <p id="rfc.section.2.3.p.4"> <span id="rfc.iref.u.2"></span><span id="rfc.iref.d.1"></span>  <span id="rfc.iref.i.2"></span><span id="rfc.iref.o.2"></span> We use the terms "<dfn>upstream</dfn>" and "<dfn>downstream</dfn>" to describe various requirements in relation to the directional flow of a message: all messages flow from upstream to downstream.
    897          Likewise, we use the terms inbound and outbound to refer to directions in relation to the request path: "<dfn>inbound</dfn>" means toward the origin server and "<dfn>outbound</dfn>" means toward the user agent.
    898       </p>
    899       <p id="rfc.section.2.3.p.5"><span id="rfc.iref.p.1"></span> A "<dfn>proxy</dfn>" is a message forwarding agent that is selected by the client, usually via local configuration rules, to receive requests
    900          for some type(s) of absolute URI and attempt to satisfy those requests via translation through the HTTP interface. Some translations
    901          are minimal, such as for proxy requests for "http" URIs, whereas other requests might require translation to and from entirely
    902          different application-layer protocols. Proxies are often used to group an organization's HTTP requests through a common intermediary
    903          for the sake of security, annotation services, or shared caching.
    904       </p>
    905       <p id="rfc.section.2.3.p.6"> <span id="rfc.iref.t.1"></span>  <span id="rfc.iref.n.1"></span> An HTTP-to-HTTP proxy is called a "<dfn>transforming proxy</dfn>" if it is designed or configured to modify request or response messages in a semantically meaningful way (i.e., modifications,
    906          beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original
    907          sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared
    908          annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder,
    909          or an intranet-to-Internet privacy filter. Such transformations are presumed to be desired by the client (or client organization)
    910          that selected the proxy and are beyond the scope of this specification. However, when a proxy is not intended to transform
    911          a given message, we use the term "<dfn>non-transforming proxy</dfn>" to target requirements that preserve HTTP message semantics. See <a href="p2-semantics.html#status.203" title="203 Non-Authoritative Information">Section 7.2.4</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> and <a href="p6-cache.html#header.warning" title="Warning">Section 3.6</a> of <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a> for status and warning codes related to transformations.
    912       </p>
    913       <p id="rfc.section.2.3.p.7"><span id="rfc.iref.g.13"></span><span id="rfc.iref.r.4"></span>  <span id="rfc.iref.a.1"></span> A "<dfn>gateway</dfn>" (a.k.a., "<dfn>reverse proxy</dfn>") is a receiving agent that acts as a layer above some other server(s) and translates the received requests to the underlying
    914          server's protocol. Gateways are often used to encapsulate legacy or untrusted information services, to improve server performance
    915          through "<dfn>accelerator</dfn>" caching, and to enable partitioning or load-balancing of HTTP services across multiple machines.
    916       </p>
    917       <p id="rfc.section.2.3.p.8">A gateway behaves as an origin server on its outbound connection and as a user agent on its inbound connection. All HTTP requirements
    918          applicable to an origin server also apply to the outbound communication of a gateway. A gateway communicates with inbound
    919          servers using any protocol that it desires, including private extensions to HTTP that are outside the scope of this specification.
    920          However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party HTTP servers <em class="bcp14">MUST</em> conform to HTTP user agent requirements on the gateway's inbound connection and <em class="bcp14">MUST</em> implement the Connection (<a href="#header.connection" id="rfc.xref.header.connection.1" title="Connection">Section&nbsp;6.1</a>) and Via (<a href="#header.via" id="rfc.xref.header.via.1" title="Via">Section&nbsp;6.2</a>) header fields for both connections.
    921       </p>
    922       <p id="rfc.section.2.3.p.9"><span id="rfc.iref.t.2"></span> A "<dfn>tunnel</dfn>" acts as a blind relay between two connections without changing the messages. Once active, a tunnel is not considered a party
    923          to the HTTP communication, though the tunnel might have been initiated by an HTTP request. A tunnel ceases to exist when both
    924          ends of the relayed connection are closed. Tunnels are used to extend a virtual connection through an intermediary, such as
    925          when transport-layer security is used to establish private communication through a shared firewall proxy.
    926       </p>
    927       <p id="rfc.section.2.3.p.10"><span id="rfc.iref.i.3"></span><span id="rfc.iref.t.3"></span>  <span id="rfc.iref.c.3"></span> In addition, there may exist network intermediaries that are not considered part of the HTTP communication but nevertheless
    928          act as filters or redirecting agents (usually violating HTTP semantics, causing security problems, and otherwise making a
    929          mess of things). Such a network intermediary, often referred to as an "<dfn>interception proxy</dfn>" <a href="#RFC3040" id="rfc.xref.RFC3040.1"><cite title="Internet Web Replication and Caching Taxonomy">[RFC3040]</cite></a>, "<dfn>transparent proxy</dfn>" <a href="#RFC1919" id="rfc.xref.RFC1919.1"><cite title="Classical versus Transparent IP Proxies">[RFC1919]</cite></a>, or "<dfn>captive portal</dfn>", differs from an HTTP proxy because it has not been selected by the client. Instead, the network intermediary redirects
    930          outgoing TCP port 80 packets (and occasionally other common port traffic) to an internal HTTP server. Interception proxies
    931          are commonly found on public network access points, as a means of enforcing account subscription prior to allowing use of
    932          non-local Internet services, and within corporate firewalls to enforce network usage policies. They are indistinguishable
    933          from a man-in-the-middle attack.
    934       </p>
    935       <p id="rfc.section.2.3.p.11">HTTP is defined as a stateless protocol, meaning that each request message can be understood in isolation. Many implementations
    936          depend on HTTP's stateless design in order to reuse proxied connections or dynamically load balance requests across multiple
    937          servers. Hence, servers <em class="bcp14">MUST NOT</em> assume that two requests on the same connection are from the same user agent unless the connection is secured and specific
    938          to that agent. Some non-standard HTTP extensions (e.g., <a href="#RFC4559" id="rfc.xref.RFC4559.1"><cite title="SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows">[RFC4559]</cite></a>) have been known to violate this requirement, resulting in security and interoperability problems.
    939       </p>
    940       <div id="rfc.iref.c.4"></div>
    941       <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="caches" href="#caches">Caches</a></h2>
    942       <p id="rfc.section.2.4.p.1">A "<dfn>cache</dfn>" is a local store of previous response messages and the subsystem that controls its message storage, retrieval, and deletion.
    943          A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
    944          requests. Any client or server <em class="bcp14">MAY</em> employ a cache, though a cache cannot be used by a server while it is acting as a tunnel.
    945       </p>
    946       <p id="rfc.section.2.4.p.2">The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached
    947          response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response
    948          from O (via C) for a request which has not been cached by UA or A.
    949       </p>
    950       <div id="rfc.figure.u.5"></div><pre class="drawing">            &gt;             &gt;
     905               message that travels the whole chain will pass through four separate connections. Some HTTP communication options might apply
     906               only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along
     907               the chain. Although the diagram is linear, each participant might be engaged in multiple, simultaneous communications. For
     908               example, B might be receiving requests from many clients other than A, and/or forwarding requests to servers other than C,
     909               at the same time that it is handling A's request.
     910            </p>
     911            <p id="rfc.section.2.3.p.4"><span id="rfc.iref.u.2"></span><span id="rfc.iref.d.1"></span> <span id="rfc.iref.i.2"></span><span id="rfc.iref.o.2"></span> We use the terms "<dfn>upstream</dfn>" and "<dfn>downstream</dfn>" to describe various requirements in relation to the directional flow of a message: all messages flow from upstream to downstream.
     912               Likewise, we use the terms inbound and outbound to refer to directions in relation to the request path: "<dfn>inbound</dfn>" means toward the origin server and "<dfn>outbound</dfn>" means toward the user agent.
     913            </p>
     914            <p id="rfc.section.2.3.p.5"><span id="rfc.iref.p.1"></span> A "<dfn>proxy</dfn>" is a message forwarding agent that is selected by the client, usually via local configuration rules, to receive requests
     915               for some type(s) of absolute URI and attempt to satisfy those requests via translation through the HTTP interface. Some translations
     916               are minimal, such as for proxy requests for "http" URIs, whereas other requests might require translation to and from entirely
     917               different application-layer protocols. Proxies are often used to group an organization's HTTP requests through a common intermediary
     918               for the sake of security, annotation services, or shared caching.
     919            </p>
     920            <p id="rfc.section.2.3.p.6"><span id="rfc.iref.t.1"></span> <span id="rfc.iref.n.1"></span> An HTTP-to-HTTP proxy is called a "<dfn>transforming proxy</dfn>" if it is designed or configured to modify request or response messages in a semantically meaningful way (i.e., modifications,
     921               beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original
     922               sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared
     923               annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder,
     924               or an intranet-to-Internet privacy filter. Such transformations are presumed to be desired by the client (or client organization)
     925               that selected the proxy and are beyond the scope of this specification. However, when a proxy is not intended to transform
     926               a given message, we use the term "<dfn>non-transforming proxy</dfn>" to target requirements that preserve HTTP message semantics. See <a href="p2-semantics.html#status.203" title="203 Non-Authoritative Information">Section 7.2.4</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> and <a href="p6-cache.html#header.warning" title="Warning">Section 3.6</a> of <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a> for status and warning codes related to transformations.
     927            </p>
     928            <p id="rfc.section.2.3.p.7"><span id="rfc.iref.g.13"></span><span id="rfc.iref.r.4"></span> <span id="rfc.iref.a.1"></span> A "<dfn>gateway</dfn>" (a.k.a., "<dfn>reverse proxy</dfn>") is a receiving agent that acts as a layer above some other server(s) and translates the received requests to the underlying
     929               server's protocol. Gateways are often used to encapsulate legacy or untrusted information services, to improve server performance
     930               through "<dfn>accelerator</dfn>" caching, and to enable partitioning or load-balancing of HTTP services across multiple machines.
     931            </p>
     932            <p id="rfc.section.2.3.p.8">A gateway behaves as an origin server on its outbound connection and as a user agent on its inbound connection. All HTTP requirements
     933               applicable to an origin server also apply to the outbound communication of a gateway. A gateway communicates with inbound
     934               servers using any protocol that it desires, including private extensions to HTTP that are outside the scope of this specification.
     935               However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party HTTP servers <em class="bcp14">MUST</em> conform to HTTP user agent requirements on the gateway's inbound connection and <em class="bcp14">MUST</em> implement the Connection (<a href="#header.connection" id="rfc.xref.header.connection.1" title="Connection">Section&nbsp;6.1</a>) and Via (<a href="#header.via" id="rfc.xref.header.via.1" title="Via">Section&nbsp;6.2</a>) header fields for both connections.
     936            </p>
     937            <p id="rfc.section.2.3.p.9"><span id="rfc.iref.t.2"></span> A "<dfn>tunnel</dfn>" acts as a blind relay between two connections without changing the messages. Once active, a tunnel is not considered a party
     938               to the HTTP communication, though the tunnel might have been initiated by an HTTP request. A tunnel ceases to exist when both
     939               ends of the relayed connection are closed. Tunnels are used to extend a virtual connection through an intermediary, such as
     940               when transport-layer security is used to establish private communication through a shared firewall proxy.
     941            </p>
     942            <p id="rfc.section.2.3.p.10"><span id="rfc.iref.i.3"></span><span id="rfc.iref.t.3"></span> <span id="rfc.iref.c.3"></span> In addition, there may exist network intermediaries that are not considered part of the HTTP communication but nevertheless
     943               act as filters or redirecting agents (usually violating HTTP semantics, causing security problems, and otherwise making a
     944               mess of things). Such a network intermediary, often referred to as an "<dfn>interception proxy</dfn>" <a href="#RFC3040" id="rfc.xref.RFC3040.1"><cite title="Internet Web Replication and Caching Taxonomy">[RFC3040]</cite></a>, "<dfn>transparent proxy</dfn>" <a href="#RFC1919" id="rfc.xref.RFC1919.1"><cite title="Classical versus Transparent IP Proxies">[RFC1919]</cite></a>, or "<dfn>captive portal</dfn>", differs from an HTTP proxy because it has not been selected by the client. Instead, the network intermediary redirects
     945               outgoing TCP port 80 packets (and occasionally other common port traffic) to an internal HTTP server. Interception proxies
     946               are commonly found on public network access points, as a means of enforcing account subscription prior to allowing use of
     947               non-local Internet services, and within corporate firewalls to enforce network usage policies. They are indistinguishable
     948               from a man-in-the-middle attack.
     949            </p>
     950            <p id="rfc.section.2.3.p.11">HTTP is defined as a stateless protocol, meaning that each request message can be understood in isolation. Many implementations
     951               depend on HTTP's stateless design in order to reuse proxied connections or dynamically load balance requests across multiple
     952               servers. Hence, servers <em class="bcp14">MUST NOT</em> assume that two requests on the same connection are from the same user agent unless the connection is secured and specific
     953               to that agent. Some non-standard HTTP extensions (e.g., <a href="#RFC4559" id="rfc.xref.RFC4559.1"><cite title="SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows">[RFC4559]</cite></a>) have been known to violate this requirement, resulting in security and interoperability problems.
     954            </p>
     955         </div>
     956         <div id="caches">
     957            <div id="rfc.iref.c.4"></div>
     958            <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a href="#caches">Caches</a></h2>
     959            <p id="rfc.section.2.4.p.1">A "<dfn>cache</dfn>" is a local store of previous response messages and the subsystem that controls its message storage, retrieval, and deletion.
     960               A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
     961               requests. Any client or server <em class="bcp14">MAY</em> employ a cache, though a cache cannot be used by a server while it is acting as a tunnel.
     962            </p>
     963            <p id="rfc.section.2.4.p.2">The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached
     964               response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response
     965               from O (via C) for a request which has not been cached by UA or A.
     966            </p>
     967            <div id="rfc.figure.u.5"></div><pre class="drawing">            &gt;             &gt;
    951968       UA =========== A =========== B - - - - - - C - - - - - - O
    952969                  &lt;             &lt;
    953970</pre><p id="rfc.section.2.4.p.4"><span id="rfc.iref.c.5"></span> A response is "<dfn>cacheable</dfn>" if a cache is allowed to store a copy of the response message for use in answering subsequent requests. Even when a response
    954          is cacheable, there might be additional constraints placed by the client or by the origin server on when that cached response
    955          can be used for a particular request. HTTP requirements for cache behavior and cacheable responses are defined in <a href="p6-cache.html#caching.overview" title="Cache Operation">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
    956       </p>
    957       <p id="rfc.section.2.4.p.5">There are a wide variety of architectures and configurations of caches and proxies deployed across the World Wide Web and
    958          inside large organizations. These systems include national hierarchies of proxy caches to save transoceanic bandwidth, systems
    959          that broadcast or multicast cache entries, organizations that distribute subsets of cached data via optical media, and so
    960          on.
    961       </p>
    962       <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a id="intro.conformance.and.error.handling" href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></h2>
    963       <p id="rfc.section.2.5.p.1">This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP
    964          requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways,
    965          or caches, depending on what behavior is being constrained by the requirement.
    966       </p>
    967       <p id="rfc.section.2.5.p.2">An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes
    968          in HTTP.
    969       </p>
    970       <p id="rfc.section.2.5.p.3">Senders <em class="bcp14">MUST NOT</em> generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements.
    971       </p>
    972       <p id="rfc.section.2.5.p.4">Unless otherwise noted, recipients <em class="bcp14">MAY</em> attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms
    973          except when they have a direct impact on security, since different applications of the protocol require different error handling
    974          strategies. For example, a Web browser might wish to transparently recover from a response where the Location header field
    975          doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery to be dangerous.
    976       </p>
    977       <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a id="http.version" href="#http.version">Protocol Versioning</a></h2>
    978       <p id="rfc.section.2.6.p.1">HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. This specification defines version "1.1".
    979          The protocol version as a whole indicates the sender's conformance with the set of requirements laid out in that version's
    980          corresponding specification of HTTP.
    981       </p>
    982       <p id="rfc.section.2.6.p.2">The version of an HTTP message is indicated by an HTTP-version field in the first line of the message. HTTP-version is case-sensitive.</p>
    983       <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span>  <a href="#http.version" class="smpl">HTTP-version</a>  = <a href="#http.version" class="smpl">HTTP-name</a> "/" <a href="#core.rules" class="smpl">DIGIT</a> "." <a href="#core.rules" class="smpl">DIGIT</a>
     971               is cacheable, there might be additional constraints placed by the client or by the origin server on when that cached response
     972               can be used for a particular request. HTTP requirements for cache behavior and cacheable responses are defined in <a href="p6-cache.html#caching.overview" title="Cache Operation">Section 2</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
     973            </p>
     974            <p id="rfc.section.2.4.p.5">There are a wide variety of architectures and configurations of caches and proxies deployed across the World Wide Web and
     975               inside large organizations. These systems include national hierarchies of proxy caches to save transoceanic bandwidth, systems
     976               that broadcast or multicast cache entries, organizations that distribute subsets of cached data via optical media, and so
     977               on.
     978            </p>
     979         </div>
     980         <div id="intro.conformance.and.error.handling">
     981            <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></h2>
     982            <p id="rfc.section.2.5.p.1">This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP
     983               requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways,
     984               or caches, depending on what behavior is being constrained by the requirement.
     985            </p>
     986            <p id="rfc.section.2.5.p.2">An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes
     987               in HTTP.
     988            </p>
     989            <p id="rfc.section.2.5.p.3">Senders <em class="bcp14">MUST NOT</em> generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements.
     990            </p>
     991            <p id="rfc.section.2.5.p.4">Unless otherwise noted, recipients <em class="bcp14">MAY</em> attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms
     992               except when they have a direct impact on security, since different applications of the protocol require different error handling
     993               strategies. For example, a Web browser might wish to transparently recover from a response where the Location header field
     994               doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery to be dangerous.
     995            </p>
     996         </div>
     997         <div id="http.version">
     998            <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a href="#http.version">Protocol Versioning</a></h2>
     999            <p id="rfc.section.2.6.p.1">HTTP uses a "&lt;major&gt;.&lt;minor&gt;" numbering scheme to indicate versions of the protocol. This specification defines version "1.1".
     1000               The protocol version as a whole indicates the sender's conformance with the set of requirements laid out in that version's
     1001               corresponding specification of HTTP.
     1002            </p>
     1003            <p id="rfc.section.2.6.p.2">The version of an HTTP message is indicated by an HTTP-version field in the first line of the message. HTTP-version is case-sensitive.</p>
     1004            <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span>  <a href="#http.version" class="smpl">HTTP-version</a>  = <a href="#http.version" class="smpl">HTTP-name</a> "/" <a href="#core.rules" class="smpl">DIGIT</a> "." <a href="#core.rules" class="smpl">DIGIT</a>
    9841005  <a href="#http.version" class="smpl">HTTP-name</a>     = %x48.54.54.50 ; "HTTP", case-sensitive
    9851006</pre><p id="rfc.section.2.6.p.4">The HTTP version number consists of two decimal digits separated by a "." (period or decimal point). The first digit ("major
    986          version") indicates the HTTP messaging syntax, whereas the second digit ("minor version") indicates the highest minor version
    987          to which the sender is conformant and able to understand for future communication. The minor version advertises the sender's
    988          communication capabilities even when the sender is only using a backwards-compatible subset of the protocol, thereby letting
    989          the recipient know that more advanced features can be used in response (by servers) or in future requests (by clients).
    990       </p>
    991       <p id="rfc.section.2.6.p.5">When an HTTP/1.1 message is sent to an HTTP/1.0 recipient <a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[RFC1945]</cite></a> or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0
    992          message if all of the newer features are ignored. This specification places recipient-version requirements on some new features
    993          so that a conformant sender will only use compatible features until it has determined, through configuration or the receipt
    994          of a message, that the recipient supports HTTP/1.1.
    995       </p>
    996       <p id="rfc.section.2.6.p.6">The interpretation of a header field does not change between minor versions of the same major HTTP version, though the default
    997          behavior of a recipient in the absence of such a field can change. Unless specified otherwise, header fields defined in HTTP/1.1
    998          are defined for all versions of HTTP/1.x. In particular, the Host and Connection header fields ought to be implemented by
    999          all HTTP/1.x implementations whether or not they advertise conformance with HTTP/1.1.
    1000       </p>
    1001       <p id="rfc.section.2.6.p.7">New header fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation
    1002          of previously defined header fields. When an implementation receives an unrecognized header field, the recipient <em class="bcp14">MUST</em> ignore that header field for local processing regardless of the message's HTTP version. An unrecognized header field received
    1003          by a proxy <em class="bcp14">MUST</em> be forwarded downstream unless the header field's field-name is listed in the message's Connection header-field (see <a href="#header.connection" id="rfc.xref.header.connection.2" title="Connection">Section&nbsp;6.1</a>). These requirements allow HTTP's functionality to be enhanced without requiring prior update of deployed intermediaries.
    1004       </p>
    1005       <p id="rfc.section.2.6.p.8">Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as tunnels) <em class="bcp14">MUST</em> send their own HTTP-version in forwarded messages. In other words, they <em class="bcp14">MUST NOT</em> blindly forward the first line of an HTTP message without ensuring that the protocol version in that message matches a version
    1006          to which that intermediary is conformant for both the receiving and sending of messages. Forwarding an HTTP message without
    1007          rewriting the HTTP-version might result in communication errors when downstream recipients use the message sender's version
    1008          to determine what features are safe to use for later communication with that sender.
    1009       </p>
    1010       <p id="rfc.section.2.6.p.9">An HTTP client <em class="bcp14">SHOULD</em> send a request version equal to the highest version to which the client is conformant and whose major version is no higher
    1011          than the highest version supported by the server, if this is known. An HTTP client <em class="bcp14">MUST NOT</em> send a version to which it is not conformant.
    1012       </p>
    1013       <p id="rfc.section.2.6.p.10">An HTTP client <em class="bcp14">MAY</em> send a lower request version if it is known that the server incorrectly implements the HTTP specification, but only after
    1014          the client has attempted at least one normal request and determined from the response status or header fields (e.g., Server)
    1015          that the server improperly handles higher request versions.
    1016       </p>
    1017       <p id="rfc.section.2.6.p.11">An HTTP server <em class="bcp14">SHOULD</em> send a response version equal to the highest version to which the server is conformant and whose major version is less than
    1018          or equal to the one received in the request. An HTTP server <em class="bcp14">MUST NOT</em> send a version to which it is not conformant. A server <em class="bcp14">MAY</em> send a 505 (HTTP Version Not Supported) response if it cannot send a response using the major version used in the client's
    1019          request.
    1020       </p>
    1021       <p id="rfc.section.2.6.p.12">An HTTP server <em class="bcp14">MAY</em> send an HTTP/1.0 response to an HTTP/1.0 request if it is known or suspected that the client incorrectly implements the HTTP
    1022          specification and is incapable of correctly processing later version responses, such as when a client fails to parse the version
    1023          number correctly or when an intermediary is known to blindly forward the HTTP-version even when it doesn't conform to the
    1024          given minor version of the protocol. Such protocol downgrades <em class="bcp14">SHOULD NOT</em> be performed unless triggered by specific client attributes, such as when one or more of the request header fields (e.g.,
    1025          User-Agent) uniquely match the values sent by a client known to be in error.
    1026       </p>
    1027       <p id="rfc.section.2.6.p.13">The intention of HTTP's versioning design is that the major number will only be incremented if an incompatible message syntax
    1028          is introduced, and that the minor number will only be incremented when changes made to the protocol have the effect of adding
    1029          to the message semantics or implying additional capabilities of the sender. However, the minor version was not incremented
    1030          for the changes introduced between <a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> and <a href="#RFC2616" id="rfc.xref.RFC2616.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, and this revision is specifically avoiding any such changes to the protocol.
    1031       </p>
    1032       <div id="rfc.iref.r.5"></div>
    1033       <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a id="uri" href="#uri">Uniform Resource Identifiers</a></h2>
    1034       <p id="rfc.section.2.7.p.1">Uniform Resource Identifiers (URIs) <a href="#RFC3986" id="rfc.xref.RFC3986.2"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> are used throughout HTTP as the means for identifying resources. URI references are used to target requests, indicate redirects,
    1035          and define relationships. HTTP does not limit what a resource might be; it merely defines an interface that can be used to
    1036          interact with a resource via HTTP. More information on the scope of URIs and resources can be found in <a href="#RFC3986" id="rfc.xref.RFC3986.3"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>.
    1037       </p>
    1038       <p id="rfc.section.2.7.p.2">This specification adopts the definitions of "URI-reference", "absolute-URI", "relative-part", "port", "host", "path-abempty",
    1039          "path-absolute", "query", and "authority" from the URI generic syntax <a href="#RFC3986" id="rfc.xref.RFC3986.4"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>. In addition, we define a partial-URI rule for protocol elements that allow a relative URI but not a fragment.
    1040       </p>
    1041       <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span>  <a href="#uri" class="smpl">URI-reference</a> = &lt;URI-reference, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.5"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-4.1">Section 4.1</a>&gt;
    1042   <a href="#uri" class="smpl">absolute-URI</a>  = &lt;absolute-URI, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.6"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-4.3">Section 4.3</a>&gt;
    1043   <a href="#uri" class="smpl">relative-part</a> = &lt;relative-part, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.7"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-4.2">Section 4.2</a>&gt;
    1044   <a href="#uri" class="smpl">authority</a>     = &lt;authority, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.8"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2">Section 3.2</a>&gt;
    1045   <a href="#uri" class="smpl">path-abempty</a>  = &lt;path-abempty, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.9"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
    1046   <a href="#uri" class="smpl">path-absolute</a> = &lt;path-absolute, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.10"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
    1047   <a href="#uri" class="smpl">port</a>          = &lt;port, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.11"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.3">Section 3.2.3</a>&gt;
    1048   <a href="#uri" class="smpl">query</a>         = &lt;query, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.12"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.4">Section 3.4</a>&gt;
    1049   <a href="#uri" class="smpl">uri-host</a>      = &lt;host, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.13"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>&gt;
     1007               version") indicates the HTTP messaging syntax, whereas the second digit ("minor version") indicates the highest minor version
     1008               to which the sender is conformant and able to understand for future communication. The minor version advertises the sender's
     1009               communication capabilities even when the sender is only using a backwards-compatible subset of the protocol, thereby letting
     1010               the recipient know that more advanced features can be used in response (by servers) or in future requests (by clients).
     1011            </p>
     1012            <p id="rfc.section.2.6.p.5">When an HTTP/1.1 message is sent to an HTTP/1.0 recipient <a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[RFC1945]</cite></a> or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0
     1013               message if all of the newer features are ignored. This specification places recipient-version requirements on some new features
     1014               so that a conformant sender will only use compatible features until it has determined, through configuration or the receipt
     1015               of a message, that the recipient supports HTTP/1.1.
     1016            </p>
     1017            <p id="rfc.section.2.6.p.6">The interpretation of a header field does not change between minor versions of the same major HTTP version, though the default
     1018               behavior of a recipient in the absence of such a field can change. Unless specified otherwise, header fields defined in HTTP/1.1
     1019               are defined for all versions of HTTP/1.x. In particular, the Host and Connection header fields ought to be implemented by
     1020               all HTTP/1.x implementations whether or not they advertise conformance with HTTP/1.1.
     1021            </p>
     1022            <p id="rfc.section.2.6.p.7">New header fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation
     1023               of previously defined header fields. When an implementation receives an unrecognized header field, the recipient <em class="bcp14">MUST</em> ignore that header field for local processing regardless of the message's HTTP version. An unrecognized header field received
     1024               by a proxy <em class="bcp14">MUST</em> be forwarded downstream unless the header field's field-name is listed in the message's Connection header-field (see <a href="#header.connection" id="rfc.xref.header.connection.2" title="Connection">Section&nbsp;6.1</a>). These requirements allow HTTP's functionality to be enhanced without requiring prior update of deployed intermediaries.
     1025            </p>
     1026            <p id="rfc.section.2.6.p.8">Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as tunnels) <em class="bcp14">MUST</em> send their own HTTP-version in forwarded messages. In other words, they <em class="bcp14">MUST NOT</em> blindly forward the first line of an HTTP message without ensuring that the protocol version in that message matches a version
     1027               to which that intermediary is conformant for both the receiving and sending of messages. Forwarding an HTTP message without
     1028               rewriting the HTTP-version might result in communication errors when downstream recipients use the message sender's version
     1029               to determine what features are safe to use for later communication with that sender.
     1030            </p>
     1031            <p id="rfc.section.2.6.p.9">An HTTP client <em class="bcp14">SHOULD</em> send a request version equal to the highest version to which the client is conformant and whose major version is no higher
     1032               than the highest version supported by the server, if this is known. An HTTP client <em class="bcp14">MUST NOT</em> send a version to which it is not conformant.
     1033            </p>
     1034            <p id="rfc.section.2.6.p.10">An HTTP client <em class="bcp14">MAY</em> send a lower request version if it is known that the server incorrectly implements the HTTP specification, but only after
     1035               the client has attempted at least one normal request and determined from the response status or header fields (e.g., Server)
     1036               that the server improperly handles higher request versions.
     1037            </p>
     1038            <p id="rfc.section.2.6.p.11">An HTTP server <em class="bcp14">SHOULD</em> send a response version equal to the highest version to which the server is conformant and whose major version is less than
     1039               or equal to the one received in the request. An HTTP server <em class="bcp14">MUST NOT</em> send a version to which it is not conformant. A server <em class="bcp14">MAY</em> send a 505 (HTTP Version Not Supported) response if it cannot send a response using the major version used in the client's
     1040               request.
     1041            </p>
     1042            <p id="rfc.section.2.6.p.12">An HTTP server <em class="bcp14">MAY</em> send an HTTP/1.0 response to an HTTP/1.0 request if it is known or suspected that the client incorrectly implements the HTTP
     1043               specification and is incapable of correctly processing later version responses, such as when a client fails to parse the version
     1044               number correctly or when an intermediary is known to blindly forward the HTTP-version even when it doesn't conform to the
     1045               given minor version of the protocol. Such protocol downgrades <em class="bcp14">SHOULD NOT</em> be performed unless triggered by specific client attributes, such as when one or more of the request header fields (e.g.,
     1046               User-Agent) uniquely match the values sent by a client known to be in error.
     1047            </p>
     1048            <p id="rfc.section.2.6.p.13">The intention of HTTP's versioning design is that the major number will only be incremented if an incompatible message syntax
     1049               is introduced, and that the minor number will only be incremented when changes made to the protocol have the effect of adding
     1050               to the message semantics or implying additional capabilities of the sender. However, the minor version was not incremented
     1051               for the changes introduced between <a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> and <a href="#RFC2616" id="rfc.xref.RFC2616.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, and this revision is specifically avoiding any such changes to the protocol.
     1052            </p>
     1053         </div>
     1054         <div id="uri">
     1055            <div id="rfc.iref.r.5"></div>
     1056            <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a href="#uri">Uniform Resource Identifiers</a></h2>
     1057            <p id="rfc.section.2.7.p.1">Uniform Resource Identifiers (URIs) <a href="#RFC3986" id="rfc.xref.RFC3986.2"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a> are used throughout HTTP as the means for identifying resources. URI references are used to target requests, indicate redirects,
     1058               and define relationships. HTTP does not limit what a resource might be; it merely defines an interface that can be used to
     1059               interact with a resource via HTTP. More information on the scope of URIs and resources can be found in <a href="#RFC3986" id="rfc.xref.RFC3986.3"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>.
     1060            </p>
     1061            <p id="rfc.section.2.7.p.2">This specification adopts the definitions of "URI-reference", "absolute-URI", "relative-part", "port", "host", "path-abempty",
     1062               "path-absolute", "query", and "authority" from the URI generic syntax <a href="#RFC3986" id="rfc.xref.RFC3986.4"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>. In addition, we define a partial-URI rule for protocol elements that allow a relative URI but not a fragment.
     1063            </p>
     1064            <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span>  <a href="#uri" class="smpl">URI-reference</a> = &lt;URI-reference, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.5"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-4.1">Section 4.1</a>&gt;
     1065  <a href="#uri" class="smpl">absolute-URI</a>  = &lt;absolute-URI, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.6"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-4.3">Section 4.3</a>&gt;
     1066  <a href="#uri" class="smpl">relative-part</a> = &lt;relative-part, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.7"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-4.2">Section 4.2</a>&gt;
     1067  <a href="#uri" class="smpl">authority</a>     = &lt;authority, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.8"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2">Section 3.2</a>&gt;
     1068  <a href="#uri" class="smpl">path-abempty</a>  = &lt;path-abempty, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.9"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
     1069  <a href="#uri" class="smpl">path-absolute</a> = &lt;path-absolute, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.10"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.3">Section 3.3</a>&gt;
     1070  <a href="#uri" class="smpl">port</a>          = &lt;port, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.11"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.3">Section 3.2.3</a>&gt;
     1071  <a href="#uri" class="smpl">query</a>         = &lt;query, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.12"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.4">Section 3.4</a>&gt;
     1072  <a href="#uri" class="smpl">uri-host</a>      = &lt;host, defined in <a href="#RFC3986" id="rfc.xref.RFC3986.13"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>&gt;
    10501073 
    10511074  <a href="#uri" class="smpl">partial-URI</a>   = relative-part [ "?" query ]
    10521075</pre><p id="rfc.section.2.7.p.4">Each protocol element in HTTP that allows a URI reference will indicate in its ABNF production whether the element allows
    1053          any form of reference (URI-reference), only a URI in absolute form (absolute-URI), only the path and optional query components,
    1054          or some combination of the above. Unless otherwise indicated, URI references are parsed relative to the effective request
    1055          URI (<a href="#effective.request.uri" title="Effective Request URI">Section&nbsp;5.5</a>).
    1056       </p>
    1057       <h3 id="rfc.section.2.7.1"><a href="#rfc.section.2.7.1">2.7.1</a>&nbsp;<a id="http.uri" href="#http.uri">http URI scheme</a></h3>
    1058       <div id="rfc.iref.h.1"></div>
    1059       <div id="rfc.iref.u.3"></div>
    1060       <p id="rfc.section.2.7.1.p.1">The "http" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
    1061          namespace governed by a potential HTTP origin server listening for TCP connections on a given port.
    1062       </p>
    1063       <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.23"></span>  <a href="#http.uri" class="smpl">http-URI</a> = "http:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
    1064 </pre><p id="rfc.section.2.7.1.p.3">The HTTP origin server is identified by the generic syntax's <a href="#uri" class="smpl">authority</a> component, which includes a host identifier and optional TCP port (<a href="#RFC3986" id="rfc.xref.RFC3986.14"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>). The remainder of the URI, consisting of both the hierarchical path component and optional query component, serves as an
    1065          identifier for a potential resource within that origin server's name space.
    1066       </p>
    1067       <p id="rfc.section.2.7.1.p.4">If the host identifier is provided as an IP literal or IPv4 address, then the origin server is any listener on the indicated
    1068          TCP port at that IP address. If host is a registered name, then that name is considered an indirect identifier and the recipient
    1069          might use a name resolution service, such as DNS, to find the address of a listener for that host. The host <em class="bcp14">MUST NOT</em> be empty; if an "http" URI is received with an empty host, then it <em class="bcp14">MUST</em> be rejected as invalid. If the port subcomponent is empty or not given, then TCP port 80 is assumed (the default reserved
    1070          port for WWW services).
    1071       </p>
    1072       <p id="rfc.section.2.7.1.p.5">Regardless of the form of host identifier, access to that host is not implied by the mere presence of its name or address.
    1073          The host might or might not exist and, even when it does exist, might or might not be running an HTTP server or listening
    1074          to the indicated port. The "http" URI scheme makes use of the delegated nature of Internet names and addresses to establish
    1075          a naming authority (whatever entity has the ability to place an HTTP server at that Internet name or address) and allows that
    1076          authority to determine which names are valid and how they might be used.
    1077       </p>
    1078       <p id="rfc.section.2.7.1.p.6">When an "http" URI is used within a context that calls for access to the indicated resource, a client <em class="bcp14">MAY</em> attempt access by resolving the host to an IP address, establishing a TCP connection to that address on the indicated port,
    1079          and sending an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) containing the URI's identifying data (<a href="#message.routing" title="Message Routing">Section&nbsp;5</a>) to the server. If the server responds to that request with a non-interim HTTP response message, as described in <a href="p2-semantics.html#status.code.and.reason.phrase" title="Status Code and Reason Phrase">Section 4</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, then that response is considered an authoritative answer to the client's request.
    1080       </p>
    1081       <p id="rfc.section.2.7.1.p.7">Although HTTP is independent of the transport protocol, the "http" scheme is specific to TCP-based services because the name
    1082          delegation process depends on TCP for establishing authority. An HTTP service based on some other underlying connection protocol
    1083          would presumably be identified using a different URI scheme, just as the "https" scheme (below) is used for servers that require
    1084          an SSL/TLS transport layer on a connection. Other protocols might also be used to provide access to "http" identified resources
    1085          — it is only the authoritative interface used for mapping the namespace that is specific to TCP.
    1086       </p>
    1087       <p id="rfc.section.2.7.1.p.8">The URI generic syntax for authority also includes a deprecated userinfo subcomponent (<a href="#RFC3986" id="rfc.xref.RFC3986.15"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.2.1">Section 3.2.1</a>) for including user authentication information in the URI. Some implementations make use of the userinfo component for internal
    1088          configuration of authentication information, such as within command invocation options, configuration files, or bookmark lists,
    1089          even though such usage might expose a user identifier or password. Senders <em class="bcp14">MUST NOT</em> include a userinfo subcomponent (and its "@" delimiter) when transmitting an "http" URI in a message. Recipients of HTTP messages
    1090          that contain a URI reference <em class="bcp14">SHOULD</em> parse for the existence of userinfo and treat its presence as an error, likely indicating that the deprecated subcomponent
    1091          is being used to obscure the authority for the sake of phishing attacks.
    1092       </p>
    1093       <h3 id="rfc.section.2.7.2"><a href="#rfc.section.2.7.2">2.7.2</a>&nbsp;<a id="https.uri" href="#https.uri">https URI scheme</a></h3>
    1094       <div id="rfc.iref.h.2"></div>
    1095       <div id="rfc.iref.u.4"></div>
    1096       <p id="rfc.section.2.7.2.p.1">The "https" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
    1097          namespace governed by a potential HTTP origin server listening for SSL/TLS-secured connections on a given TCP port.
    1098       </p>
    1099       <p id="rfc.section.2.7.2.p.2">All of the requirements listed above for the "http" scheme are also requirements for the "https" scheme, except that a default
    1100          TCP port of 443 is assumed if the port subcomponent is empty or not given, and the TCP connection <em class="bcp14">MUST</em> be secured for privacy through the use of strong encryption prior to sending the first HTTP request.
    1101       </p>
    1102       <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.24"></span>  <a href="#https.uri" class="smpl">https-URI</a> = "https:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
     1076               any form of reference (URI-reference), only a URI in absolute form (absolute-URI), only the path and optional query components,
     1077               or some combination of the above. Unless otherwise indicated, URI references are parsed relative to the effective request
     1078               URI (<a href="#effective.request.uri" title="Effective Request URI">Section&nbsp;5.5</a>).
     1079            </p>
     1080            <div id="http.uri">
     1081               <h3 id="rfc.section.2.7.1"><a href="#rfc.section.2.7.1">2.7.1</a>&nbsp;<a href="#http.uri">http URI scheme</a></h3>
     1082               <div id="rfc.iref.h.1"></div>
     1083               <div id="rfc.iref.u.3"></div>
     1084               <p id="rfc.section.2.7.1.p.1">The "http" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
     1085                  namespace governed by a potential HTTP origin server listening for TCP connections on a given port.
     1086               </p>
     1087               <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.23"></span>  <a href="#http.uri" class="smpl">http-URI</a> = "http:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
     1088</pre><p id="rfc.section.2.7.1.p.3">The HTTP origin server is identified by the generic syntax's <a href="#uri" class="smpl">authority</a> component, which includes a host identifier and optional TCP port (<a href="#RFC3986" id="rfc.xref.RFC3986.14"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.2">Section 3.2.2</a>). The remainder of the URI, consisting of both the hierarchical path component and optional query component, serves as an
     1089                  identifier for a potential resource within that origin server's name space.
     1090               </p>
     1091               <p id="rfc.section.2.7.1.p.4">If the host identifier is provided as an IP literal or IPv4 address, then the origin server is any listener on the indicated
     1092                  TCP port at that IP address. If host is a registered name, then that name is considered an indirect identifier and the recipient
     1093                  might use a name resolution service, such as DNS, to find the address of a listener for that host. The host <em class="bcp14">MUST NOT</em> be empty; if an "http" URI is received with an empty host, then it <em class="bcp14">MUST</em> be rejected as invalid. If the port subcomponent is empty or not given, then TCP port 80 is assumed (the default reserved
     1094                  port for WWW services).
     1095               </p>
     1096               <p id="rfc.section.2.7.1.p.5">Regardless of the form of host identifier, access to that host is not implied by the mere presence of its name or address.
     1097                  The host might or might not exist and, even when it does exist, might or might not be running an HTTP server or listening
     1098                  to the indicated port. The "http" URI scheme makes use of the delegated nature of Internet names and addresses to establish
     1099                  a naming authority (whatever entity has the ability to place an HTTP server at that Internet name or address) and allows that
     1100                  authority to determine which names are valid and how they might be used.
     1101               </p>
     1102               <p id="rfc.section.2.7.1.p.6">When an "http" URI is used within a context that calls for access to the indicated resource, a client <em class="bcp14">MAY</em> attempt access by resolving the host to an IP address, establishing a TCP connection to that address on the indicated port,
     1103                  and sending an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) containing the URI's identifying data (<a href="#message.routing" title="Message Routing">Section&nbsp;5</a>) to the server. If the server responds to that request with a non-interim HTTP response message, as described in <a href="p2-semantics.html#status.code.and.reason.phrase" title="Status Code and Reason Phrase">Section 4</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, then that response is considered an authoritative answer to the client's request.
     1104               </p>
     1105               <p id="rfc.section.2.7.1.p.7">Although HTTP is independent of the transport protocol, the "http" scheme is specific to TCP-based services because the name
     1106                  delegation process depends on TCP for establishing authority. An HTTP service based on some other underlying connection protocol
     1107                  would presumably be identified using a different URI scheme, just as the "https" scheme (below) is used for servers that require
     1108                  an SSL/TLS transport layer on a connection. Other protocols might also be used to provide access to "http" identified resources
     1109                  — it is only the authoritative interface used for mapping the namespace that is specific to TCP.
     1110               </p>
     1111               <p id="rfc.section.2.7.1.p.8">The URI generic syntax for authority also includes a deprecated userinfo subcomponent (<a href="#RFC3986" id="rfc.xref.RFC3986.15"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.2.1">Section 3.2.1</a>) for including user authentication information in the URI. Some implementations make use of the userinfo component for internal
     1112                  configuration of authentication information, such as within command invocation options, configuration files, or bookmark lists,
     1113                  even though such usage might expose a user identifier or password. Senders <em class="bcp14">MUST NOT</em> include a userinfo subcomponent (and its "@" delimiter) when transmitting an "http" URI in a message. Recipients of HTTP messages
     1114                  that contain a URI reference <em class="bcp14">SHOULD</em> parse for the existence of userinfo and treat its presence as an error, likely indicating that the deprecated subcomponent
     1115                  is being used to obscure the authority for the sake of phishing attacks.
     1116               </p>
     1117            </div>
     1118            <div id="https.uri">
     1119               <h3 id="rfc.section.2.7.2"><a href="#rfc.section.2.7.2">2.7.2</a>&nbsp;<a href="#https.uri">https URI scheme</a></h3>
     1120               <div id="rfc.iref.h.2"></div>
     1121               <div id="rfc.iref.u.4"></div>
     1122               <p id="rfc.section.2.7.2.p.1">The "https" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical
     1123                  namespace governed by a potential HTTP origin server listening for SSL/TLS-secured connections on a given TCP port.
     1124               </p>
     1125               <p id="rfc.section.2.7.2.p.2">All of the requirements listed above for the "http" scheme are also requirements for the "https" scheme, except that a default
     1126                  TCP port of 443 is assumed if the port subcomponent is empty or not given, and the TCP connection <em class="bcp14">MUST</em> be secured for privacy through the use of strong encryption prior to sending the first HTTP request.
     1127               </p>
     1128               <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.24"></span>  <a href="#https.uri" class="smpl">https-URI</a> = "https:" "//" <a href="#uri" class="smpl">authority</a> <a href="#uri" class="smpl">path-abempty</a> [ "?" <a href="#uri" class="smpl">query</a> ]
    11031129</pre><p id="rfc.section.2.7.2.p.4">Unlike the "http" scheme, responses to "https" identified requests are never "public" and thus <em class="bcp14">MUST NOT</em> be reused for shared caching. They can, however, be reused in a private cache if the message is cacheable by default in HTTP
    1104          or specifically indicated as such by the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 3.2</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>).
    1105       </p>
    1106       <p id="rfc.section.2.7.2.p.5">Resources made available via the "https" scheme have no shared identity with the "http" scheme even if their resource identifiers
    1107          indicate the same authority (the same host listening to the same TCP port). They are distinct name spaces and are considered
    1108          to be distinct origin servers. However, an extension to HTTP that is defined to apply to entire host domains, such as the
    1109          Cookie protocol <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>, can allow information set by one service to impact communication with other services within a matching group of host domains.
    1110       </p>
    1111       <p id="rfc.section.2.7.2.p.6">The process for authoritative access to an "https" identified resource is defined in <a href="#RFC2818" id="rfc.xref.RFC2818.1"><cite title="HTTP Over TLS">[RFC2818]</cite></a>.
    1112       </p>
    1113       <h3 id="rfc.section.2.7.3"><a href="#rfc.section.2.7.3">2.7.3</a>&nbsp;<a id="uri.comparison" href="#uri.comparison">http and https URI Normalization and Comparison</a></h3>
    1114       <p id="rfc.section.2.7.3.p.1">Since the "http" and "https" schemes conform to the URI generic syntax, such URIs are normalized and compared according to
    1115          the algorithm defined in <a href="#RFC3986" id="rfc.xref.RFC3986.16"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-6">Section 6</a>, using the defaults described above for each scheme.
    1116       </p>
    1117       <p id="rfc.section.2.7.3.p.2">If the port is equal to the default port for a scheme, the normal form is to elide the port subcomponent. Likewise, an empty
    1118          path component is equivalent to an absolute path of "/", so the normal form is to provide a path of "/" instead. The scheme
    1119          and host are case-insensitive and normally provided in lowercase; all other components are compared in a case-sensitive manner.
    1120          Characters other than those in the "reserved" set are equivalent to their percent-encoded octets (see <a href="#RFC3986" id="rfc.xref.RFC3986.17"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-2.1">Section 2.1</a>): the normal form is to not encode them.
    1121       </p>
    1122       <p id="rfc.section.2.7.3.p.3">For example, the following three URIs are equivalent:</p>
    1123       <div id="rfc.figure.u.10"></div><pre class="text">   http://example.com:80/~smith/home.html
     1130                  or specifically indicated as such by the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 3.2</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>).
     1131               </p>
     1132               <p id="rfc.section.2.7.2.p.5">Resources made available via the "https" scheme have no shared identity with the "http" scheme even if their resource identifiers
     1133                  indicate the same authority (the same host listening to the same TCP port). They are distinct name spaces and are considered
     1134                  to be distinct origin servers. However, an extension to HTTP that is defined to apply to entire host domains, such as the
     1135                  Cookie protocol <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>, can allow information set by one service to impact communication with other services within a matching group of host domains.
     1136               </p>
     1137               <p id="rfc.section.2.7.2.p.6">The process for authoritative access to an "https" identified resource is defined in <a href="#RFC2818" id="rfc.xref.RFC2818.1"><cite title="HTTP Over TLS">[RFC2818]</cite></a>.
     1138               </p>
     1139            </div>
     1140            <div id="uri.comparison">
     1141               <h3 id="rfc.section.2.7.3"><a href="#rfc.section.2.7.3">2.7.3</a>&nbsp;<a href="#uri.comparison">http and https URI Normalization and Comparison</a></h3>
     1142               <p id="rfc.section.2.7.3.p.1">Since the "http" and "https" schemes conform to the URI generic syntax, such URIs are normalized and compared according to
     1143                  the algorithm defined in <a href="#RFC3986" id="rfc.xref.RFC3986.16"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-6">Section 6</a>, using the defaults described above for each scheme.
     1144               </p>
     1145               <p id="rfc.section.2.7.3.p.2">If the port is equal to the default port for a scheme, the normal form is to elide the port subcomponent. Likewise, an empty
     1146                  path component is equivalent to an absolute path of "/", so the normal form is to provide a path of "/" instead. The scheme
     1147                  and host are case-insensitive and normally provided in lowercase; all other components are compared in a case-sensitive manner.
     1148                  Characters other than those in the "reserved" set are equivalent to their percent-encoded octets (see <a href="#RFC3986" id="rfc.xref.RFC3986.17"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-2.1">Section 2.1</a>): the normal form is to not encode them.
     1149               </p>
     1150               <p id="rfc.section.2.7.3.p.3">For example, the following three URIs are equivalent:</p>
     1151               <div id="rfc.figure.u.10"></div><pre class="text">   http://example.com:80/~smith/home.html
    11241152   http://EXAMPLE.com/%7Esmith/home.html
    11251153   http://EXAMPLE.com:/%7esmith/home.html
    1126 </pre><h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="http.message" href="#http.message">Message Format</a></h1>
    1127       <div id="rfc.iref.h.3"></div>
    1128       <div id="rfc.iref.h.4"></div>
    1129       <div id="rfc.iref.h.5"></div>
    1130       <p id="rfc.section.3.p.1">All HTTP/1.1 messages consist of a start-line followed by a sequence of octets in a format similar to the Internet Message
    1131          Format <a href="#RFC5322" id="rfc.xref.RFC5322.2"><cite title="Internet Message Format">[RFC5322]</cite></a>: zero or more header fields (collectively referred to as the "headers" or the "header section"), an empty line indicating
    1132          the end of the header section, and an optional message body.
    1133       </p>
    1134       <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.25"></span>  <a href="#http.message" class="smpl">HTTP-message</a>   = <a href="#http.message" class="smpl">start-line</a>
     1154</pre></div>
     1155         </div>
     1156      </div>
     1157      <div id="http.message">
     1158         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#http.message">Message Format</a></h1>
     1159         <div id="rfc.iref.h.3"></div>
     1160         <div id="rfc.iref.h.4"></div>
     1161         <div id="rfc.iref.h.5"></div>
     1162         <p id="rfc.section.3.p.1">All HTTP/1.1 messages consist of a start-line followed by a sequence of octets in a format similar to the Internet Message
     1163            Format <a href="#RFC5322" id="rfc.xref.RFC5322.2"><cite title="Internet Message Format">[RFC5322]</cite></a>: zero or more header fields (collectively referred to as the "headers" or the "header section"), an empty line indicating
     1164            the end of the header section, and an optional message body.
     1165         </p>
     1166         <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.25"></span>  <a href="#http.message" class="smpl">HTTP-message</a>   = <a href="#http.message" class="smpl">start-line</a>
    11351167                   *( <a href="#header.fields" class="smpl">header-field</a> <a href="#core.rules" class="smpl">CRLF</a> )
    11361168                   <a href="#core.rules" class="smpl">CRLF</a>
    11371169                   [ <a href="#message.body" class="smpl">message-body</a> ]
    11381170</pre><p id="rfc.section.3.p.3">The normal procedure for parsing an HTTP message is to read the start-line into a structure, read each header field into a
    1139          hash table by field name until the empty line, and then use the parsed data to determine if a message body is expected. If
    1140          a message body has been indicated, then it is read as a stream until an amount of octets equal to the message body length
    1141          is read or the connection is closed.
    1142       </p>
    1143       <p id="rfc.section.3.p.4">Recipients <em class="bcp14">MUST</em> parse an HTTP message as a sequence of octets in an encoding that is a superset of US-ASCII <a href="#USASCII" id="rfc.xref.USASCII.2"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Parsing an HTTP message as a stream of Unicode characters, without regard for the specific encoding, creates security vulnerabilities
    1144          due to the varying ways that string processing libraries handle invalid multibyte character sequences that contain the octet
    1145          LF (%x0A). String-based parsers can only be safely used within protocol elements after the element has been extracted from
    1146          the message, such as within a header field-value after message parsing has delineated the individual fields.
    1147       </p>
    1148       <p id="rfc.section.3.p.5">An HTTP message can be parsed as a stream for incremental processing or forwarding downstream. However, recipients cannot
    1149          rely on incremental delivery of partial messages, since some implementations will buffer or delay message forwarding for the
    1150          sake of network efficiency, security checks, or payload transformations.
    1151       </p>
    1152       <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="start.line" href="#start.line">Start Line</a></h2>
    1153       <p id="rfc.section.3.1.p.1">An HTTP message can either be a request from client to server or a response from server to client. Syntactically, the two
    1154          types of message differ only in the start-line, which is either a request-line (for requests) or a status-line (for responses),
    1155          and in the algorithm for determining the length of the message body (<a href="#message.body" title="Message Body">Section&nbsp;3.3</a>). In theory, a client could receive requests and a server could receive responses, distinguishing them by their different
    1156          start-line formats, but in practice servers are implemented to only expect a request (a response is interpreted as an unknown
    1157          or invalid request method) and clients are implemented to only expect a response.
    1158       </p>
    1159       <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.26"></span>  <a href="#http.message" class="smpl">start-line</a>     = <a href="#request.line" class="smpl">request-line</a> / <a href="#status.line" class="smpl">status-line</a>
     1171            hash table by field name until the empty line, and then use the parsed data to determine if a message body is expected. If
     1172            a message body has been indicated, then it is read as a stream until an amount of octets equal to the message body length
     1173            is read or the connection is closed.
     1174         </p>
     1175         <p id="rfc.section.3.p.4">Recipients <em class="bcp14">MUST</em> parse an HTTP message as a sequence of octets in an encoding that is a superset of US-ASCII <a href="#USASCII" id="rfc.xref.USASCII.2"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Parsing an HTTP message as a stream of Unicode characters, without regard for the specific encoding, creates security vulnerabilities
     1176            due to the varying ways that string processing libraries handle invalid multibyte character sequences that contain the octet
     1177            LF (%x0A). String-based parsers can only be safely used within protocol elements after the element has been extracted from
     1178            the message, such as within a header field-value after message parsing has delineated the individual fields.
     1179         </p>
     1180         <p id="rfc.section.3.p.5">An HTTP message can be parsed as a stream for incremental processing or forwarding downstream. However, recipients cannot
     1181            rely on incremental delivery of partial messages, since some implementations will buffer or delay message forwarding for the
     1182            sake of network efficiency, security checks, or payload transformations.
     1183         </p>
     1184         <div id="start.line">
     1185            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#start.line">Start Line</a></h2>
     1186            <p id="rfc.section.3.1.p.1">An HTTP message can either be a request from client to server or a response from server to client. Syntactically, the two
     1187               types of message differ only in the start-line, which is either a request-line (for requests) or a status-line (for responses),
     1188               and in the algorithm for determining the length of the message body (<a href="#message.body" title="Message Body">Section&nbsp;3.3</a>). In theory, a client could receive requests and a server could receive responses, distinguishing them by their different
     1189               start-line formats, but in practice servers are implemented to only expect a request (a response is interpreted as an unknown
     1190               or invalid request method) and clients are implemented to only expect a response.
     1191            </p>
     1192            <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.26"></span>  <a href="#http.message" class="smpl">start-line</a>     = <a href="#request.line" class="smpl">request-line</a> / <a href="#status.line" class="smpl">status-line</a>
    11601193</pre><p id="rfc.section.3.1.p.4">Implementations <em class="bcp14">MUST NOT</em> send whitespace between the start-line and the first header field. The presence of such whitespace in a request might be an
    1161          attempt to trick a server into ignoring that field or processing the line after it as a new request, either of which might
    1162          result in a security vulnerability if other implementations within the request chain interpret the same message differently.
    1163          Likewise, the presence of such whitespace in a response might be ignored by some clients or cause others to cease parsing.
    1164       </p>
    1165       <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;<a id="request.line" href="#request.line">Request Line</a></h3>
    1166       <p id="rfc.section.3.1.1.p.1">A request-line begins with a method token, followed by a single space (SP), the request-target, another single space (SP),
    1167          the protocol version, and ending with CRLF.
    1168       </p>
    1169       <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.27"></span>  <a href="#request.line" class="smpl">request-line</a>   = <a href="#method" class="smpl">method</a> <a href="#core.rules" class="smpl">SP</a> <a href="#request-target" class="smpl">request-target</a> <a href="#core.rules" class="smpl">SP</a> <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">CRLF</a>
     1194               attempt to trick a server into ignoring that field or processing the line after it as a new request, either of which might
     1195               result in a security vulnerability if other implementations within the request chain interpret the same message differently.
     1196               Likewise, the presence of such whitespace in a response might be ignored by some clients or cause others to cease parsing.
     1197            </p>
     1198            <div id="request.line">
     1199               <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;<a href="#request.line">Request Line</a></h3>
     1200               <p id="rfc.section.3.1.1.p.1">A request-line begins with a method token, followed by a single space (SP), the request-target, another single space (SP),
     1201                  the protocol version, and ending with CRLF.
     1202               </p>
     1203               <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.27"></span>  <a href="#request.line" class="smpl">request-line</a>   = <a href="#method" class="smpl">method</a> <a href="#core.rules" class="smpl">SP</a> <a href="#request-target" class="smpl">request-target</a> <a href="#core.rules" class="smpl">SP</a> <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">CRLF</a>
    11701204</pre><div id="rfc.iref.m.2"></div>
    1171       <div id="method">
    1172          <p id="rfc.section.3.1.1.p.3">The method token indicates the request method to be performed on the target resource. The request method is case-sensitive.</p>
    1173       </div>
    1174       <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.28"></span>  <a href="#method" class="smpl">method</a>         = <a href="#rule.token.separators" class="smpl">token</a>
     1205               <div id="method">
     1206                  <p id="rfc.section.3.1.1.p.3">The method token indicates the request method to be performed on the target resource. The request method is case-sensitive.</p>
     1207               </div>
     1208               <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.28"></span>  <a href="#method" class="smpl">method</a>         = <a href="#rule.token.separators" class="smpl">token</a>
    11751209</pre><p id="rfc.section.3.1.1.p.5">The methods defined by this specification can be found in <a href="p2-semantics.html#method" title="Method">Section 2</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, along with information regarding the HTTP method registry and considerations for defining new methods.
    1176       </p>
    1177       <div id="rfc.iref.r.6"></div>
    1178       <p id="rfc.section.3.1.1.p.6">The request-target identifies the target resource upon which to apply the request, as defined in <a href="#request-target" title="Request Target">Section&nbsp;5.3</a>.
    1179       </p>
    1180       <p id="rfc.section.3.1.1.p.7">No whitespace is allowed inside the method, request-target, and protocol version. Hence, recipients typically parse the request-line
    1181          into its component parts by splitting on the SP characters.
    1182       </p>
    1183       <p id="rfc.section.3.1.1.p.8">Unfortunately, some user agents fail to properly encode hypertext references that have embedded whitespace, sending the characters
    1184          directly instead of properly percent-encoding the disallowed characters. Recipients of an invalid request-line <em class="bcp14">SHOULD</em> respond with either a 400 (Bad Request) error or a 301 (Moved Permanently) redirect with the request-target properly encoded.
    1185          Recipients <em class="bcp14">SHOULD NOT</em> attempt to autocorrect and then process the request without a redirect, since the invalid request-line might be deliberately
    1186          crafted to bypass security filters along the request chain.
    1187       </p>
    1188       <p id="rfc.section.3.1.1.p.9">HTTP does not place a pre-defined limit on the length of a request-line. A server that receives a method longer than any that
    1189          it implements <em class="bcp14">SHOULD</em> respond with either a 404 (Not Allowed), if it is an origin server, or a 501 (Not Implemented) status code. A server <em class="bcp14">MUST</em> be prepared to receive URIs of unbounded length and respond with the 414 (URI Too Long) status code if the received request-target
    1190          would be longer than the server wishes to handle (see <a href="p2-semantics.html#status.414" title="414 URI Too Long">Section 7.4.12</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
    1191       </p>
    1192       <p id="rfc.section.3.1.1.p.10">Various ad-hoc limitations on request-line length are found in practice. It is <em class="bcp14">RECOMMENDED</em> that all HTTP senders and recipients support, at a minimum, request-line lengths of up to 8000 octets.
    1193       </p>
    1194       <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;<a id="status.line" href="#status.line">Status Line</a></h3>
    1195       <p id="rfc.section.3.1.2.p.1">The first line of a response message is the status-line, consisting of the protocol version, a space (SP), the status code,
    1196          another space, a possibly-empty textual phrase describing the status code, and ending with CRLF.
    1197       </p>
    1198       <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.29"></span>  <a href="#status.line" class="smpl">status-line</a> = <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">SP</a> <a href="#status-code" class="smpl">status-code</a> <a href="#core.rules" class="smpl">SP</a> <a href="#reason-phrase" class="smpl">reason-phrase</a> <a href="#core.rules" class="smpl">CRLF</a>
     1210               </p>
     1211               <div id="rfc.iref.r.6"></div>
     1212               <p id="rfc.section.3.1.1.p.6">The request-target identifies the target resource upon which to apply the request, as defined in <a href="#request-target" title="Request Target">Section&nbsp;5.3</a>.
     1213               </p>
     1214               <p id="rfc.section.3.1.1.p.7">No whitespace is allowed inside the method, request-target, and protocol version. Hence, recipients typically parse the request-line
     1215                  into its component parts by splitting on the SP characters.
     1216               </p>
     1217               <p id="rfc.section.3.1.1.p.8">Unfortunately, some user agents fail to properly encode hypertext references that have embedded whitespace, sending the characters
     1218                  directly instead of properly percent-encoding the disallowed characters. Recipients of an invalid request-line <em class="bcp14">SHOULD</em> respond with either a 400 (Bad Request) error or a 301 (Moved Permanently) redirect with the request-target properly encoded.
     1219                  Recipients <em class="bcp14">SHOULD NOT</em> attempt to autocorrect and then process the request without a redirect, since the invalid request-line might be deliberately
     1220                  crafted to bypass security filters along the request chain.
     1221               </p>
     1222               <p id="rfc.section.3.1.1.p.9">HTTP does not place a pre-defined limit on the length of a request-line. A server that receives a method longer than any that
     1223                  it implements <em class="bcp14">SHOULD</em> respond with either a 404 (Not Allowed), if it is an origin server, or a 501 (Not Implemented) status code. A server <em class="bcp14">MUST</em> be prepared to receive URIs of unbounded length and respond with the 414 (URI Too Long) status code if the received request-target
     1224                  would be longer than the server wishes to handle (see <a href="p2-semantics.html#status.414" title="414 URI Too Long">Section 7.4.12</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
     1225               </p>
     1226               <p id="rfc.section.3.1.1.p.10">Various ad-hoc limitations on request-line length are found in practice. It is <em class="bcp14">RECOMMENDED</em> that all HTTP senders and recipients support, at a minimum, request-line lengths of up to 8000 octets.
     1227               </p>
     1228            </div>
     1229            <div id="status.line">
     1230               <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;<a href="#status.line">Status Line</a></h3>
     1231               <p id="rfc.section.3.1.2.p.1">The first line of a response message is the status-line, consisting of the protocol version, a space (SP), the status code,
     1232                  another space, a possibly-empty textual phrase describing the status code, and ending with CRLF.
     1233               </p>
     1234               <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.29"></span>  <a href="#status.line" class="smpl">status-line</a> = <a href="#http.version" class="smpl">HTTP-version</a> <a href="#core.rules" class="smpl">SP</a> <a href="#status-code" class="smpl">status-code</a> <a href="#core.rules" class="smpl">SP</a> <a href="#reason-phrase" class="smpl">reason-phrase</a> <a href="#core.rules" class="smpl">CRLF</a>
    11991235</pre><div id="status-code">
    1200          <p id="rfc.section.3.1.2.p.3">The status-code element is a 3-digit integer result code of the attempt to understand and satisfy the request. See <a href="p2-semantics.html#status.code.and.reason.phrase" title="Status Code and Reason Phrase">Section 4</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> for further information, such as the list of status codes defined by this specification, the IANA registry, and considerations
    1201             for new status codes.
    1202          </p>
    1203       </div>
    1204       <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.30"></span>  <a href="#status-code" class="smpl">status-code</a>    = 3<a href="#core.rules" class="smpl">DIGIT</a>
     1236                  <p id="rfc.section.3.1.2.p.3">The status-code element is a 3-digit integer result code of the attempt to understand and satisfy the request. See <a href="p2-semantics.html#status.code.and.reason.phrase" title="Status Code and Reason Phrase">Section 4</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> for further information, such as the list of status codes defined by this specification, the IANA registry, and considerations
     1237                     for new status codes.
     1238                  </p>
     1239               </div>
     1240               <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.30"></span>  <a href="#status-code" class="smpl">status-code</a>    = 3<a href="#core.rules" class="smpl">DIGIT</a>
    12051241</pre><div id="reason-phrase">
    1206          <p id="rfc.section.3.1.2.p.5">The reason-phrase element exists for the sole purpose of providing a textual description associated with the numeric status
    1207             code, mostly out of deference to earlier Internet application protocols that were more frequently used with interactive text
    1208             clients. A client <em class="bcp14">SHOULD</em> ignore the reason-phrase content.
    1209          </p>
    1210       </div>
    1211       <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.31"></span>  <a href="#reason-phrase" class="smpl">reason-phrase</a>  = *( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
    1212 </pre><h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.fields" href="#header.fields">Header Fields</a></h2>
    1213       <p id="rfc.section.3.2.p.1">Each HTTP header field consists of a case-insensitive field name followed by a colon (":"), optional whitespace, and the field
    1214          value.
    1215       </p>
    1216       <div id="rfc.figure.u.18"></div><pre class="inline"><span id="rfc.iref.g.32"></span><span id="rfc.iref.g.33"></span><span id="rfc.iref.g.34"></span><span id="rfc.iref.g.35"></span><span id="rfc.iref.g.36"></span>  <a href="#header.fields" class="smpl">header-field</a>   = <a href="#header.fields" class="smpl">field-name</a> ":" <a href="#rule.whitespace" class="smpl">OWS</a> <a href="#header.fields" class="smpl">field-value</a> <a href="#rule.whitespace" class="smpl">BWS</a>
     1242                  <p id="rfc.section.3.1.2.p.5">The reason-phrase element exists for the sole purpose of providing a textual description associated with the numeric status
     1243                     code, mostly out of deference to earlier Internet application protocols that were more frequently used with interactive text
     1244                     clients. A client <em class="bcp14">SHOULD</em> ignore the reason-phrase content.
     1245                  </p>
     1246               </div>
     1247               <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.31"></span>  <a href="#reason-phrase" class="smpl">reason-phrase</a>  = *( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
     1248</pre></div>
     1249         </div>
     1250         <div id="header.fields">
     1251            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#header.fields">Header Fields</a></h2>
     1252            <p id="rfc.section.3.2.p.1">Each HTTP header field consists of a case-insensitive field name followed by a colon (":"), optional whitespace, and the field
     1253               value.
     1254            </p>
     1255            <div id="rfc.figure.u.18"></div><pre class="inline"><span id="rfc.iref.g.32"></span><span id="rfc.iref.g.33"></span><span id="rfc.iref.g.34"></span><span id="rfc.iref.g.35"></span><span id="rfc.iref.g.36"></span>  <a href="#header.fields" class="smpl">header-field</a>   = <a href="#header.fields" class="smpl">field-name</a> ":" <a href="#rule.whitespace" class="smpl">OWS</a> <a href="#header.fields" class="smpl">field-value</a> <a href="#rule.whitespace" class="smpl">BWS</a>
    12171256  <a href="#header.fields" class="smpl">field-name</a>     = <a href="#rule.token.separators" class="smpl">token</a>
    12181257  <a href="#header.fields" class="smpl">field-value</a>    = *( <a href="#header.fields" class="smpl">field-content</a> / <a href="#header.fields" class="smpl">obs-fold</a> )
    … …  
    12221261                 ; see <a href="#field.parsing" title="Field Parsing">Section&nbsp;3.2.2</a>
    12231262</pre><p id="rfc.section.3.2.p.3">The field-name token labels the corresponding field-value as having the semantics defined by that header field. For example,
    1224          the Date header field is defined in <a href="p2-semantics.html#header.date" title="Date">Section 10.2</a> of <a href="#Part2" id="rfc.xref.Part2.6"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> as containing the origination timestamp for the message in which it appears.
    1225       </p>
    1226       <p id="rfc.section.3.2.p.4">HTTP header fields are fully extensible: there is no limit on the introduction of new field names, each presumably defining
    1227          new semantics, or on the number of header fields used in a given message. Existing fields are defined in each part of this
    1228          specification and in many other specifications outside the standards process. New header fields can be introduced without
    1229          changing the protocol version if their defined semantics allow them to be safely ignored by recipients that do not recognize
    1230          them.
    1231       </p>
    1232       <p id="rfc.section.3.2.p.5">New HTTP header fields <em class="bcp14">SHOULD</em> be registered with IANA according to the procedures in <a href="p2-semantics.html#considerations.for.creating.header.fields" title="Considerations for Creating Header Fields">Section 3.1</a> of <a href="#Part2" id="rfc.xref.Part2.7"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>. Unrecognized header fields <em class="bcp14">MUST</em> be forwarded by a proxy unless the field-name is listed in the Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.3" title="Connection">Section&nbsp;6.1</a>) or the proxy is specifically configured to block or otherwise transform such fields. Unrecognized header fields <em class="bcp14">SHOULD</em> be ignored by other recipients.
    1233       </p>
    1234       <p id="rfc.section.3.2.p.6">The order in which header fields with differing field names are received is not significant. However, it is "good practice"
    1235          to send header fields that contain control data first, such as Host on requests and Date on responses, so that implementations
    1236          can decide when not to handle a message as early as possible. A server <em class="bcp14">MUST</em> wait until the entire header section is received before interpreting a request message, since later header fields might include
    1237          conditionals, authentication credentials, or deliberately misleading duplicate header fields that would impact request processing.
    1238       </p>
    1239       <p id="rfc.section.3.2.p.7">Multiple header fields with the same field name <em class="bcp14">MUST NOT</em> be sent in a message unless the entire field value for that header field is defined as a comma-separated list [i.e., #(values)].
    1240          Multiple header fields with the same field name can be combined into one "field-name: field-value" pair, without changing
    1241          the semantics of the message, by appending each subsequent field value to the combined field value in order, separated by
    1242          a comma. The order in which header fields with the same field name are received is therefore significant to the interpretation
    1243          of the combined field value; a proxy <em class="bcp14">MUST NOT</em> change the order of these field values when forwarding a message.
    1244       </p>
    1245       <div class="note" id="rfc.section.3.2.p.8">
    1246          <p> <b>Note:</b> The "Set-Cookie" header field as implemented in practice can occur multiple times, but does not use the list syntax, and thus
    1247             cannot be combined into a single line (<a href="#RFC6265" id="rfc.xref.RFC6265.2"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>). (See Appendix A.2.3 of <a href="#Kri2001" id="rfc.xref.Kri2001.1"><cite title="HTTP Cookies: Standards, Privacy, and Politics">[Kri2001]</cite></a> for details.) Also note that the Set-Cookie2 header field specified in <a href="#RFC2965" id="rfc.xref.RFC2965.1"><cite title="HTTP State Management Mechanism">[RFC2965]</cite></a> does not share this problem.
    1248          </p>
    1249       </div>
    1250       <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="whitespace" href="#whitespace">Whitespace</a></h3>
    1251       <div id="rule.LWS">
    1252          <p id="rfc.section.3.2.1.p.1">This specification uses three rules to denote the use of linear whitespace: OWS (optional whitespace), RWS (required whitespace),
    1253             and BWS ("bad" whitespace).
    1254          </p>
    1255       </div>
    1256       <div id="rule.OWS">
    1257          <p id="rfc.section.3.2.1.p.2">The OWS rule is used where zero or more linear whitespace octets might appear. OWS <em class="bcp14">SHOULD</em> either not be produced or be produced as a single SP. Multiple OWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets (each octet other than SP replaced with SP) before interpreting
    1258             the field value or forwarding the message downstream.
    1259          </p>
    1260       </div>
    1261       <div id="rule.RWS">
    1262          <p id="rfc.section.3.2.1.p.3">RWS is used when at least one linear whitespace octet is required to separate field tokens. RWS <em class="bcp14">SHOULD</em> be produced as a single SP. Multiple RWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets before interpreting the field value or forwarding the
    1263             message downstream.
    1264          </p>
    1265       </div>
    1266       <div id="rule.BWS">
    1267          <p id="rfc.section.3.2.1.p.4">BWS is used where the grammar allows optional whitespace for historical reasons but senders <em class="bcp14">SHOULD NOT</em> produce it in messages. HTTP/1.1 recipients <em class="bcp14">MUST</em> accept such bad optional whitespace and remove it before interpreting the field value or forwarding the message downstream.
    1268          </p>
    1269       </div>
    1270       <div id="rule.whitespace">
    1271          <p id="rfc.section.3.2.1.p.5">      </p>
    1272       </div>
    1273       <div id="rfc.figure.u.19"></div><pre class="inline"><span id="rfc.iref.g.37"></span><span id="rfc.iref.g.38"></span><span id="rfc.iref.g.39"></span>  <a href="#rule.whitespace" class="smpl">OWS</a>            = *( <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">HTAB</a> )
     1263               the Date header field is defined in <a href="p2-semantics.html#header.date" title="Date">Section 10.2</a> of <a href="#Part2" id="rfc.xref.Part2.6"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> as containing the origination timestamp for the message in which it appears.
     1264            </p>
     1265            <p id="rfc.section.3.2.p.4">HTTP header fields are fully extensible: there is no limit on the introduction of new field names, each presumably defining
     1266               new semantics, or on the number of header fields used in a given message. Existing fields are defined in each part of this
     1267               specification and in many other specifications outside the standards process. New header fields can be introduced without
     1268               changing the protocol version if their defined semantics allow them to be safely ignored by recipients that do not recognize
     1269               them.
     1270            </p>
     1271            <p id="rfc.section.3.2.p.5">New HTTP header fields <em class="bcp14">SHOULD</em> be registered with IANA according to the procedures in <a href="p2-semantics.html#considerations.for.creating.header.fields" title="Considerations for Creating Header Fields">Section 3.1</a> of <a href="#Part2" id="rfc.xref.Part2.7"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>. Unrecognized header fields <em class="bcp14">MUST</em> be forwarded by a proxy unless the field-name is listed in the Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.3" title="Connection">Section&nbsp;6.1</a>) or the proxy is specifically configured to block or otherwise transform such fields. Unrecognized header fields <em class="bcp14">SHOULD</em> be ignored by other recipients.
     1272            </p>
     1273            <p id="rfc.section.3.2.p.6">The order in which header fields with differing field names are received is not significant. However, it is "good practice"
     1274               to send header fields that contain control data first, such as Host on requests and Date on responses, so that implementations
     1275               can decide when not to handle a message as early as possible. A server <em class="bcp14">MUST</em> wait until the entire header section is received before interpreting a request message, since later header fields might include
     1276               conditionals, authentication credentials, or deliberately misleading duplicate header fields that would impact request processing.
     1277            </p>
     1278            <p id="rfc.section.3.2.p.7">Multiple header fields with the same field name <em class="bcp14">MUST NOT</em> be sent in a message unless the entire field value for that header field is defined as a comma-separated list [i.e., #(values)].
     1279               Multiple header fields with the same field name can be combined into one "field-name: field-value" pair, without changing
     1280               the semantics of the message, by appending each subsequent field value to the combined field value in order, separated by
     1281               a comma. The order in which header fields with the same field name are received is therefore significant to the interpretation
     1282               of the combined field value; a proxy <em class="bcp14">MUST NOT</em> change the order of these field values when forwarding a message.
     1283            </p>
     1284            <div class="note" id="rfc.section.3.2.p.8">
     1285               <p><b>Note:</b> The "Set-Cookie" header field as implemented in practice can occur multiple times, but does not use the list syntax, and thus
     1286                  cannot be combined into a single line (<a href="#RFC6265" id="rfc.xref.RFC6265.2"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>). (See Appendix A.2.3 of <a href="#Kri2001" id="rfc.xref.Kri2001.1"><cite title="HTTP Cookies: Standards, Privacy, and Politics">[Kri2001]</cite></a> for details.) Also note that the Set-Cookie2 header field specified in <a href="#RFC2965" id="rfc.xref.RFC2965.1"><cite title="HTTP State Management Mechanism">[RFC2965]</cite></a> does not share this problem.
     1287               </p>
     1288            </div>
     1289            <div id="whitespace">
     1290               <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a href="#whitespace">Whitespace</a></h3>
     1291               <div id="rule.LWS">
     1292                  <p id="rfc.section.3.2.1.p.1">This specification uses three rules to denote the use of linear whitespace: OWS (optional whitespace), RWS (required whitespace),
     1293                     and BWS ("bad" whitespace).
     1294                  </p>
     1295               </div>
     1296               <div id="rule.OWS">
     1297                  <p id="rfc.section.3.2.1.p.2">The OWS rule is used where zero or more linear whitespace octets might appear. OWS <em class="bcp14">SHOULD</em> either not be produced or be produced as a single SP. Multiple OWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets (each octet other than SP replaced with SP) before interpreting
     1298                     the field value or forwarding the message downstream.
     1299                  </p>
     1300               </div>
     1301               <div id="rule.RWS">
     1302                  <p id="rfc.section.3.2.1.p.3">RWS is used when at least one linear whitespace octet is required to separate field tokens. RWS <em class="bcp14">SHOULD</em> be produced as a single SP. Multiple RWS octets that occur within field-content <em class="bcp14">SHOULD</em> either be replaced with a single SP or transformed to all SP octets before interpreting the field value or forwarding the
     1303                     message downstream.
     1304                  </p>
     1305               </div>
     1306               <div id="rule.BWS">
     1307                  <p id="rfc.section.3.2.1.p.4">BWS is used where the grammar allows optional whitespace for historical reasons but senders <em class="bcp14">SHOULD NOT</em> produce it in messages. HTTP/1.1 recipients <em class="bcp14">MUST</em> accept such bad optional whitespace and remove it before interpreting the field value or forwarding the message downstream.
     1308                  </p>
     1309               </div>
     1310               <div id="rule.whitespace">
     1311                  <p id="rfc.section.3.2.1.p.5">   </p>
     1312               </div>
     1313               <div id="rfc.figure.u.19"></div><pre class="inline"><span id="rfc.iref.g.37"></span><span id="rfc.iref.g.38"></span><span id="rfc.iref.g.39"></span>  <a href="#rule.whitespace" class="smpl">OWS</a>            = *( <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">HTAB</a> )
    12741314                 ; "optional" whitespace
    12751315  <a href="#rule.whitespace" class="smpl">RWS</a>            = 1*( <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">HTAB</a> )
    … …  
    12771317  <a href="#rule.whitespace" class="smpl">BWS</a>            = <a href="#rule.whitespace" class="smpl">OWS</a>
    12781318                 ; "bad" whitespace
    1279 </pre><h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="field.parsing" href="#field.parsing">Field Parsing</a></h3>
    1280       <p id="rfc.section.3.2.2.p.1">No whitespace is allowed between the header field-name and colon. In the past, differences in the handling of such whitespace
    1281          have led to security vulnerabilities in request routing and response handling. Any received request message that contains
    1282          whitespace between a header field-name and colon <em class="bcp14">MUST</em> be rejected with a response code of 400 (Bad Request). A proxy <em class="bcp14">MUST</em> remove any such whitespace from a response message before forwarding the message downstream.
    1283       </p>
    1284       <p id="rfc.section.3.2.2.p.2">A field value <em class="bcp14">MAY</em> be preceded by optional whitespace (OWS); a single SP is preferred. The field value does not include any leading or trailing
    1285          white space: OWS occurring before the first non-whitespace octet of the field value or after the last non-whitespace octet
    1286          of the field value is ignored and <em class="bcp14">SHOULD</em> be removed before further processing (as this does not change the meaning of the header field).
    1287       </p>
    1288       <p id="rfc.section.3.2.2.p.3">Historically, HTTP header field values could be extended over multiple lines by preceding each extra line with at least one
    1289          space or horizontal tab (obs-fold). This specification deprecates such line folding except within the message/http media type
    1290          (<a href="#internet.media.type.message.http" title="Internet Media Type message/http">Section&nbsp;7.3.1</a>). HTTP senders <em class="bcp14">MUST NOT</em> produce messages that include line folding (i.e., that contain any field-value that matches the obs-fold rule) unless the
    1291          message is intended for packaging within the message/http media type. HTTP recipients <em class="bcp14">SHOULD</em> accept line folding and replace any embedded obs-fold whitespace with either a single SP or a matching number of SP octets
    1292          (to avoid buffer copying) prior to interpreting the field value or forwarding the message downstream.
    1293       </p>
    1294       <p id="rfc.section.3.2.2.p.4">Historically, HTTP has allowed field content with text in the ISO-8859-1 <a href="#ISO-8859-1" id="rfc.xref.ISO-8859-1.1"><cite title="Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1">[ISO-8859-1]</cite></a> character encoding and supported other character sets only through use of <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a> encoding. In practice, most HTTP header field values use only a subset of the US-ASCII character encoding <a href="#USASCII" id="rfc.xref.USASCII.3"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Newly defined header fields <em class="bcp14">SHOULD</em> limit their field values to US-ASCII octets. Recipients <em class="bcp14">SHOULD</em> treat other (obs-text) octets in field content as opaque data.
    1295       </p>
    1296       <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="field.length" href="#field.length">Field Length</a></h3>
    1297       <p id="rfc.section.3.2.3.p.1">HTTP does not place a pre-defined limit on the length of header fields, either in isolation or as a set. A server <em class="bcp14">MUST</em> be prepared to receive request header fields of unbounded length and respond with a 4xx status code if the received header
    1298          field(s) would be longer than the server wishes to handle.
    1299       </p>
    1300       <p id="rfc.section.3.2.3.p.2">A client that receives response headers that are longer than it wishes to handle can only treat it as a server error.</p>
    1301       <p id="rfc.section.3.2.3.p.3">Various ad-hoc limitations on header length are found in practice. It is <em class="bcp14">RECOMMENDED</em> that all HTTP senders and recipients support messages whose combined header fields have 4000 or more octets.
    1302       </p>
    1303       <h3 id="rfc.section.3.2.4"><a href="#rfc.section.3.2.4">3.2.4</a>&nbsp;<a id="field.components" href="#field.components">Field value components</a></h3>
    1304       <div id="rule.token.separators">
    1305          <p id="rfc.section.3.2.4.p.1">        Many HTTP/1.1 header field values consist of words (token or quoted-string) separated by whitespace or special characters.
    1306             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;4</a>).
    1307          </p>
    1308       </div>
    1309       <div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.40"></span><span id="rfc.iref.g.41"></span><span id="rfc.iref.g.42"></span><span id="rfc.iref.g.43"></span>  <a href="#rule.token.separators" class="smpl">word</a>           = <a href="#rule.token.separators" class="smpl">token</a> / <a href="#rule.quoted-string" class="smpl">quoted-string</a>
     1319</pre></div>
     1320            <div id="field.parsing">
     1321               <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a href="#field.parsing">Field Parsing</a></h3>
     1322               <p id="rfc.section.3.2.2.p.1">No whitespace is allowed between the header field-name and colon. In the past, differences in the handling of such whitespace
     1323                  have led to security vulnerabilities in request routing and response handling. Any received request message that contains
     1324                  whitespace between a header field-name and colon <em class="bcp14">MUST</em> be rejected with a response code of 400 (Bad Request). A proxy <em class="bcp14">MUST</em> remove any such whitespace from a response message before forwarding the message downstream.
     1325               </p>
     1326               <p id="rfc.section.3.2.2.p.2">A field value <em class="bcp14">MAY</em> be preceded by optional whitespace (OWS); a single SP is preferred. The field value does not include any leading or trailing
     1327                  white space: OWS occurring before the first non-whitespace octet of the field value or after the last non-whitespace octet
     1328                  of the field value is ignored and <em class="bcp14">SHOULD</em> be removed before further processing (as this does not change the meaning of the header field).
     1329               </p>
     1330               <p id="rfc.section.3.2.2.p.3">Historically, HTTP header field values could be extended over multiple lines by preceding each extra line with at least one
     1331                  space or horizontal tab (obs-fold). This specification deprecates such line folding except within the message/http media type
     1332                  (<a href="#internet.media.type.message.http" title="Internet Media Type message/http">Section&nbsp;7.3.1</a>). HTTP senders <em class="bcp14">MUST NOT</em> produce messages that include line folding (i.e., that contain any field-value that matches the obs-fold rule) unless the
     1333                  message is intended for packaging within the message/http media type. HTTP recipients <em class="bcp14">SHOULD</em> accept line folding and replace any embedded obs-fold whitespace with either a single SP or a matching number of SP octets
     1334                  (to avoid buffer copying) prior to interpreting the field value or forwarding the message downstream.
     1335               </p>
     1336               <p id="rfc.section.3.2.2.p.4">Historically, HTTP has allowed field content with text in the ISO-8859-1 <a href="#ISO-8859-1" id="rfc.xref.ISO-8859-1.1"><cite title="Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1">[ISO-8859-1]</cite></a> character encoding and supported other character sets only through use of <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a> encoding. In practice, most HTTP header field values use only a subset of the US-ASCII character encoding <a href="#USASCII" id="rfc.xref.USASCII.3"><cite title="Coded Character Set -- 7-bit American Standard Code for Information Interchange">[USASCII]</cite></a>. Newly defined header fields <em class="bcp14">SHOULD</em> limit their field values to US-ASCII octets. Recipients <em class="bcp14">SHOULD</em> treat other (obs-text) octets in field content as opaque data.
     1337               </p>
     1338            </div>
     1339            <div id="field.length">
     1340               <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a href="#field.length">Field Length</a></h3>
     1341               <p id="rfc.section.3.2.3.p.1">HTTP does not place a pre-defined limit on the length of header fields, either in isolation or as a set. A server <em class="bcp14">MUST</em> be prepared to receive request header fields of unbounded length and respond with a 4xx status code if the received header
     1342                  field(s) would be longer than the server wishes to handle.
     1343               </p>
     1344               <p id="rfc.section.3.2.3.p.2">A client that receives response headers that are longer than it wishes to handle can only treat it as a server error.</p>
     1345               <p id="rfc.section.3.2.3.p.3">Various ad-hoc limitations on header length are found in practice. It is <em class="bcp14">RECOMMENDED</em> that all HTTP senders and recipients support messages whose combined header fields have 4000 or more octets.
     1346               </p>
     1347            </div>
     1348            <div id="field.components">
     1349               <h3 id="rfc.section.3.2.4"><a href="#rfc.section.3.2.4">3.2.4</a>&nbsp;<a href="#field.components">Field value components</a></h3>
     1350               <div id="rule.token.separators">
     1351                  <p id="rfc.section.3.2.4.p.1">    Many HTTP/1.1 header field values consist of words (token or quoted-string) separated by whitespace or special characters.
     1352                     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;4</a>).
     1353                  </p>
     1354               </div>
     1355               <div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.40"></span><span id="rfc.iref.g.41"></span><span id="rfc.iref.g.42"></span><span id="rfc.iref.g.43"></span>  <a href="#rule.token.separators" class="smpl">word</a>           = <a href="#rule.token.separators" class="smpl">token</a> / <a href="#rule.quoted-string" class="smpl">quoted-string</a>
    13101356
    13111357  <a href="#rule.token.separators" class="smpl">token</a>          = 1*<a href="#rule.token.separators" class="smpl">tchar</a>
    … …  
    13201366                 / "]" / "?" / "=" / "{" / "}"
    13211367</pre><div id="rule.quoted-string">
    1322          <p id="rfc.section.3.2.4.p.3">      A string of text is parsed as a single word if it is quoted using double-quote marks.</p>
    1323       </div>
    1324       <div id="rfc.figure.u.21"></div><pre class="inline"><span id="rfc.iref.g.44"></span><span id="rfc.iref.g.45"></span><span id="rfc.iref.g.46"></span>  <a href="#rule.quoted-string" class="smpl">quoted-string</a>  = <a href="#core.rules" class="smpl">DQUOTE</a> *( <a href="#rule.quoted-string" class="smpl">qdtext</a> / <a href="#rule.quoted-pair" class="smpl">quoted-pair</a> ) <a href="#core.rules" class="smpl">DQUOTE</a>
     1368                  <p id="rfc.section.3.2.4.p.3">   A string of text is parsed as a single word if it is quoted using double-quote marks.</p>
     1369               </div>
     1370               <div id="rfc.figure.u.21"></div><pre class="inline"><span id="rfc.iref.g.44"></span><span id="rfc.iref.g.45"></span><span id="rfc.iref.g.46"></span>  <a href="#rule.quoted-string" class="smpl">quoted-string</a>  = <a href="#core.rules" class="smpl">DQUOTE</a> *( <a href="#rule.quoted-string" class="smpl">qdtext</a> / <a href="#rule.quoted-pair" class="smpl">quoted-pair</a> ) <a href="#core.rules" class="smpl">DQUOTE</a>
    13251371  <a href="#rule.quoted-string" class="smpl">qdtext</a>         = <a href="#rule.whitespace" class="smpl">OWS</a> / %x21 / %x23-5B / %x5D-7E / <a href="#rule.quoted-string" class="smpl">obs-text</a>
    13261372  <a href="#rule.quoted-string" class="smpl">obs-text</a>       = %x80-FF
    13271373</pre><div id="rule.quoted-pair">
    1328          <p id="rfc.section.3.2.4.p.5"> The backslash octet ("\") can be used as a single-octet quoting mechanism within quoted-string constructs:</p>
    1329       </div>
    1330       <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.47"></span>  <a href="#rule.quoted-pair" class="smpl">quoted-pair</a>    = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
     1374                  <p id="rfc.section.3.2.4.p.5"> The backslash octet ("\") can be used as a single-octet quoting mechanism within quoted-string constructs:</p>
     1375               </div>
     1376               <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.47"></span>  <a href="#rule.quoted-pair" class="smpl">quoted-pair</a>    = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
    13311377</pre><p id="rfc.section.3.2.4.p.7">Recipients that process the value of the quoted-string <em class="bcp14">MUST</em> handle a quoted-pair as if it were replaced by the octet following the backslash.
    1332       </p>
    1333       <p id="rfc.section.3.2.4.p.8">Senders <em class="bcp14">SHOULD NOT</em> escape octets in quoted-strings that do not require escaping (i.e., other than DQUOTE and the backslash octet).
    1334       </p>
    1335       <div id="rule.comment">
    1336          <p id="rfc.section.3.2.4.p.9">    Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed
    1337             in fields containing "comment" as part of their field value definition.
    1338          </p>
    1339       </div>
    1340       <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.48"></span><span id="rfc.iref.g.49"></span>  <a href="#rule.comment" class="smpl">comment</a>        = "(" *( <a href="#rule.comment" class="smpl">ctext</a> / <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a> / <a href="#rule.comment" class="smpl">comment</a> ) ")"
     1378               </p>
     1379               <p id="rfc.section.3.2.4.p.8">Senders <em class="bcp14">SHOULD NOT</em> escape octets in quoted-strings that do not require escaping (i.e., other than DQUOTE and the backslash octet).
     1380               </p>
     1381               <div id="rule.comment">
     1382                  <p id="rfc.section.3.2.4.p.9">  Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed
     1383                     in fields containing "comment" as part of their field value definition.
     1384                  </p>
     1385               </div>
     1386               <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.48"></span><span id="rfc.iref.g.49"></span>  <a href="#rule.comment" class="smpl">comment</a>        = "(" *( <a href="#rule.comment" class="smpl">ctext</a> / <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a> / <a href="#rule.comment" class="smpl">comment</a> ) ")"
    13411387  <a href="#rule.comment" class="smpl">ctext</a>          = <a href="#rule.whitespace" class="smpl">OWS</a> / %x21-27 / %x2A-5B / %x5D-7E / <a href="#rule.quoted-string" class="smpl">obs-text</a>
    13421388</pre><div id="rule.quoted-cpair">
    1343          <p id="rfc.section.3.2.4.p.11"> The backslash octet ("\") can be used as a single-octet quoting mechanism within comment constructs:</p>
    1344       </div>
    1345       <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.50"></span>  <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a>   = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
     1389                  <p id="rfc.section.3.2.4.p.11"> The backslash octet ("\") can be used as a single-octet quoting mechanism within comment constructs:</p>
     1390               </div>
     1391               <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.50"></span>  <a href="#rule.quoted-cpair" class="smpl">quoted-cpair</a>   = "\" ( <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / <a href="#core.rules" class="smpl">VCHAR</a> / <a href="#rule.quoted-string" class="smpl">obs-text</a> )
    13461392</pre><p id="rfc.section.3.2.4.p.13">Senders <em class="bcp14">SHOULD NOT</em> escape octets in comments that do not require escaping (i.e., other than the backslash octet "\" and the parentheses "(" and
    1347          ")").
    1348       </p>
    1349       <h3 id="rfc.section.3.2.5"><a href="#rfc.section.3.2.5">3.2.5</a>&nbsp;<a id="abnf.extension" href="#abnf.extension">ABNF list extension: #rule</a></h3>
    1350       <p id="rfc.section.3.2.5.p.1">A #rule extension to the ABNF rules of <a href="#RFC5234" id="rfc.xref.RFC5234.3"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> is used to improve readability in the definitions of some header field values.
    1351       </p>
    1352       <p id="rfc.section.3.2.5.p.2">A construct "#" is defined, similar to "*", for defining comma-delimited lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element"
    1353          indicating at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single comma (",") and optional whitespace (OWS).
    1354       </p>
    1355       <div id="rfc.figure.u.25"></div>
    1356       <p>Thus,</p><pre class="text">  1#element =&gt; element *( OWS "," OWS element )
     1393                  ")").
     1394               </p>
     1395            </div>
     1396            <div id="abnf.extension">
     1397               <h3 id="rfc.section.3.2.5"><a href="#rfc.section.3.2.5">3.2.5</a>&nbsp;<a href="#abnf.extension">ABNF list extension: #rule</a></h3>
     1398               <p id="rfc.section.3.2.5.p.1">A #rule extension to the ABNF rules of <a href="#RFC5234" id="rfc.xref.RFC5234.3"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> is used to improve readability in the definitions of some header field values.
     1399               </p>
     1400               <p id="rfc.section.3.2.5.p.2">A construct "#" is defined, similar to "*", for defining comma-delimited lists of elements. The full form is "&lt;n&gt;#&lt;m&gt;element"
     1401                  indicating at least &lt;n&gt; and at most &lt;m&gt; elements, each separated by a single comma (",") and optional whitespace (OWS).
     1402               </p>
     1403               <div id="rfc.figure.u.25"></div>
     1404               <p>Thus,</p><pre class="text">  1#element =&gt; element *( OWS "," OWS element )
    13571405</pre><div id="rfc.figure.u.26"></div>
    1358       <p>and:</p><pre class="text">  #element =&gt; [ 1#element ]
     1406               <p>and:</p><pre class="text">  #element =&gt; [ 1#element ]
    13591407</pre><div id="rfc.figure.u.27"></div>
    1360       <p>and for n &gt;= 1 and m &gt; 1:</p><pre class="text">  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
     1408               <p>and for n &gt;= 1 and m &gt; 1:</p><pre class="text">  &lt;n&gt;#&lt;m&gt;element =&gt; element &lt;n-1&gt;*&lt;m-1&gt;( OWS "," OWS element )
    13611409</pre><p id="rfc.section.3.2.5.p.6">For compatibility with legacy list rules, recipients <em class="bcp14">SHOULD</em> accept empty list elements. In other words, consumers would follow the list productions:
    1362       </p>
    1363       <div id="rfc.figure.u.28"></div><pre class="text">  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
     1410               </p>
     1411               <div id="rfc.figure.u.28"></div><pre class="text">  #element =&gt; [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
    13641412 
    13651413  1#element =&gt; *( "," OWS ) element *( OWS "," [ OWS element ] )
    13661414</pre><p id="rfc.section.3.2.5.p.8">Note that empty elements do not contribute to the count of elements present, though.</p>
    1367       <p id="rfc.section.3.2.5.p.9">For example, given these ABNF productions:</p>
    1368       <div id="rfc.figure.u.29"></div><pre class="text">  example-list      = 1#example-list-elmt
     1415               <p id="rfc.section.3.2.5.p.9">For example, given these ABNF productions:</p>
     1416               <div id="rfc.figure.u.29"></div><pre class="text">  example-list      = 1#example-list-elmt
    13691417  example-list-elmt = token ; see <a href="#field.components" title="Field value components">Section&nbsp;3.2.4</a>
    13701418</pre><p id="rfc.section.3.2.5.p.11">Then these are valid values for example-list (not including the double quotes, which are present for delimitation only):</p>
    1371       <div id="rfc.figure.u.30"></div><pre class="text">  "foo,bar"
     1419               <div id="rfc.figure.u.30"></div><pre class="text">  "foo,bar"
    13721420  "foo ,bar,"
    13731421  "foo , ,bar,charlie   "
    13741422</pre><p id="rfc.section.3.2.5.p.13">But these values would be invalid, as at least one non-empty element is required:</p>
    1375       <div id="rfc.figure.u.31"></div><pre class="text">  ""
     1423               <div id="rfc.figure.u.31"></div><pre class="text">  ""
    13761424  ","
    13771425  ",   ,"
    1378 </pre><p id="rfc.section.3.2.5.p.15"> <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF, with the list rules expanded as explained above.
    1379       </p>
    1380       <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="message.body" href="#message.body">Message Body</a></h2>
    1381       <p id="rfc.section.3.3.p.1">The message body (if any) of an HTTP message is used to carry the payload body of that request or response. The message body
    1382          is identical to the payload body unless a transfer coding has been applied, as described in <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.1" title="Transfer-Encoding">Section&nbsp;3.3.1</a>.
    1383       </p>
    1384       <div id="rfc.figure.u.32"></div><pre class="inline"><span id="rfc.iref.g.51"></span>  <a href="#message.body" class="smpl">message-body</a> = *OCTET
     1426</pre><p id="rfc.section.3.2.5.p.15"><a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF, with the list rules expanded as explained above.
     1427               </p>
     1428            </div>
     1429         </div>
     1430         <div id="message.body">
     1431            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#message.body">Message Body</a></h2>
     1432            <p id="rfc.section.3.3.p.1">The message body (if any) of an HTTP message is used to carry the payload body of that request or response. The message body
     1433               is identical to the payload body unless a transfer coding has been applied, as described in <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.1" title="Transfer-Encoding">Section&nbsp;3.3.1</a>.
     1434            </p>
     1435            <div id="rfc.figure.u.32"></div><pre class="inline"><span id="rfc.iref.g.51"></span>  <a href="#message.body" class="smpl">message-body</a> = *OCTET
    13851436</pre><p id="rfc.section.3.3.p.3">The rules for when a message body is allowed in a message differ for requests and responses.</p>
    1386       <p id="rfc.section.3.3.p.4">The presence of a message body in a request is signaled by a a Content-Length or Transfer-Encoding header field. Request message
    1387          framing is independent of method semantics, even if the method does not define any use for a message body.
    1388       </p>
    1389       <p id="rfc.section.3.3.p.5">The presence of a message body in a response depends on both the request method to which it is responding and the response
    1390          status code (<a href="#status-code">Paragraph&nbsp;3</a>). Responses to the HEAD request method never include a message body because the associated response header fields (e.g.,
    1391          Transfer-Encoding, Content-Length, etc.) only indicate what their values would have been if the request method had been GET.
    1392          Successful (2xx) responses to CONNECT switch to tunnel mode instead of having a message body. All 1xx (Informational), 204
    1393          (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 the body <em class="bcp14">MAY</em> be of zero length.
    1394       </p>
    1395       <div id="rfc.iref.t.4"></div>
    1396       <div id="rfc.iref.h.6"></div>
    1397       <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a id="header.transfer-encoding" href="#header.transfer-encoding">Transfer-Encoding</a></h3>
    1398       <p id="rfc.section.3.3.1.p.1">When one or more transfer codings are applied to a payload body in order to form the message body, a Transfer-Encoding header
    1399          field <em class="bcp14">MUST</em> be sent in the message and <em class="bcp14">MUST</em> contain the list of corresponding transfer-coding names in the same order that they were applied. Transfer codings are defined
    1400          in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>.
    1401       </p>
    1402       <div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.52"></span>  <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> = 1#<a href="#transfer.codings" class="smpl">transfer-coding</a>
     1437            <p id="rfc.section.3.3.p.4">The presence of a message body in a request is signaled by a a Content-Length or Transfer-Encoding header field. Request message
     1438               framing is independent of method semantics, even if the method does not define any use for a message body.
     1439            </p>
     1440            <p id="rfc.section.3.3.p.5">The presence of a message body in a response depends on both the request method to which it is responding and the response
     1441               status code (<a href="#status-code">Paragraph&nbsp;3</a>). Responses to the HEAD request method never include a message body because the associated response header fields (e.g.,
     1442               Transfer-Encoding, Content-Length, etc.) only indicate what their values would have been if the request method had been GET.
     1443               Successful (2xx) responses to CONNECT switch to tunnel mode instead of having a message body. All 1xx (Informational), 204
     1444               (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 the body <em class="bcp14">MAY</em> be of zero length.
     1445            </p>
     1446            <div id="header.transfer-encoding">
     1447               <div id="rfc.iref.t.4"></div>
     1448               <div id="rfc.iref.h.6"></div>
     1449               <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a href="#header.transfer-encoding">Transfer-Encoding</a></h3>
     1450               <p id="rfc.section.3.3.1.p.1">When one or more transfer codings are applied to a payload body in order to form the message body, a Transfer-Encoding header
     1451                  field <em class="bcp14">MUST</em> be sent in the message and <em class="bcp14">MUST</em> contain the list of corresponding transfer-coding names in the same order that they were applied. Transfer codings are defined
     1452                  in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>.
     1453               </p>
     1454               <div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.52"></span>  <a href="#header.transfer-encoding" class="smpl">Transfer-Encoding</a> = 1#<a href="#transfer.codings" class="smpl">transfer-coding</a>
    14031455</pre><p id="rfc.section.3.3.1.p.3">Transfer-Encoding is analogous to the Content-Transfer-Encoding field of MIME, which was designed to enable safe transport
    1404          of binary data over a 7-bit transport service (<a href="#RFC2045" id="rfc.xref.RFC2045.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>, <a href="http://tools.ietf.org/html/rfc2045#section-6">Section 6</a>). However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP's case, Transfer-Encoding is
    1405          primarily intended to accurately delimit a dynamically generated payload and to distinguish payload encodings that are only
    1406          applied for transport efficiency or security from those that are characteristics of the target resource.
    1407       </p>
    1408       <p id="rfc.section.3.3.1.p.4">The "chunked" transfer-coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) <em class="bcp14">MUST</em> be implemented by all HTTP/1.1 recipients because it plays a crucial role in delimiting messages when the payload body size
    1409          is not known in advance. When the "chunked" transfer-coding is used, it <em class="bcp14">MUST</em> be the last transfer-coding applied to form the message body and <em class="bcp14">MUST NOT</em> be applied more than once in a message body. If any transfer-coding is applied to a request payload body, the final transfer-coding
    1410          applied <em class="bcp14">MUST</em> be "chunked". If any transfer-coding is applied to a response payload body, then either the final transfer-coding applied <em class="bcp14">MUST</em> be "chunked" or the message <em class="bcp14">MUST</em> be terminated by closing the connection.
    1411       </p>
    1412       <div id="rfc.figure.u.34"></div>
    1413       <p>For example,</p><pre class="text">  Transfer-Encoding: gzip, chunked
     1456                  of binary data over a 7-bit transport service (<a href="#RFC2045" id="rfc.xref.RFC2045.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>, <a href="https://tools.ietf.org/html/rfc2045#section-6">Section 6</a>). However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP's case, Transfer-Encoding is
     1457                  primarily intended to accurately delimit a dynamically generated payload and to distinguish payload encodings that are only
     1458                  applied for transport efficiency or security from those that are characteristics of the target resource.
     1459               </p>
     1460               <p id="rfc.section.3.3.1.p.4">The "chunked" transfer-coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) <em class="bcp14">MUST</em> be implemented by all HTTP/1.1 recipients because it plays a crucial role in delimiting messages when the payload body size
     1461                  is not known in advance. When the "chunked" transfer-coding is used, it <em class="bcp14">MUST</em> be the last transfer-coding applied to form the message body and <em class="bcp14">MUST NOT</em> be applied more than once in a message body. If any transfer-coding is applied to a request payload body, the final transfer-coding
     1462                  applied <em class="bcp14">MUST</em> be "chunked". If any transfer-coding is applied to a response payload body, then either the final transfer-coding applied <em class="bcp14">MUST</em> be "chunked" or the message <em class="bcp14">MUST</em> be terminated by closing the connection.
     1463               </p>
     1464               <div id="rfc.figure.u.34"></div>
     1465               <p>For example,</p><pre class="text">  Transfer-Encoding: gzip, chunked
    14141466</pre><p>indicates that the payload body has been compressed using the gzip coding and then chunked using the chunked coding while
    1415          forming the message body.
    1416       </p>
    1417       <p id="rfc.section.3.3.1.p.6">If more than one Transfer-Encoding header field is present in a message, the multiple field-values <em class="bcp14">MUST</em> be combined into one field-value, according to the algorithm defined in <a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>, before determining the message body length.
    1418       </p>
    1419       <p id="rfc.section.3.3.1.p.7">Unlike Content-Encoding (<a href="p3-payload.html#content.codings" title="Content Codings">Section 2.2</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>), Transfer-Encoding is a property of the message, not of the payload, and thus <em class="bcp14">MAY</em> be added or removed by any implementation along the request/response chain. Additional information about the encoding parameters <em class="bcp14">MAY</em> be provided by other header fields not defined by this specification.
    1420       </p>
    1421       <p id="rfc.section.3.3.1.p.8">Transfer-Encoding <em class="bcp14">MAY</em> be sent in a response to a HEAD request or in a 304 response to a GET request, neither of which includes a message body, to
    1422          indicate that the origin server would have applied a transfer coding to the message body if the request had been an unconditional
    1423          GET. This indication is not required, however, because any recipient on the response chain (including the origin server) can
    1424          remove transfer codings when they are not needed.
    1425       </p>
    1426       <p id="rfc.section.3.3.1.p.9">Transfer-Encoding was added in HTTP/1.1. It is generally assumed that implementations advertising only HTTP/1.0 support will
    1427          not understand how to process a transfer-encoded payload. A client <em class="bcp14">MUST NOT</em> send a request containing Transfer-Encoding unless it knows the server will handle HTTP/1.1 (or later) requests; such knowledge
    1428          might be in the form of specific user configuration or by remembering the version of a prior received response. A server <em class="bcp14">MUST NOT</em> send a response containing Transfer-Encoding unless the corresponding request indicates HTTP/1.1 (or later).
    1429       </p>
    1430       <p id="rfc.section.3.3.1.p.10">A server that receives a request message with a transfer-coding it does not understand <em class="bcp14">SHOULD</em> respond with 501 (Not Implemented) and then close the connection.
    1431       </p>
    1432       <div id="rfc.iref.c.6"></div>
    1433       <div id="rfc.iref.h.7"></div>
    1434       <h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a id="header.content-length" href="#header.content-length">Content-Length</a></h3>
    1435       <p id="rfc.section.3.3.2.p.1">When a message does not have a Transfer-Encoding header field and the payload body length can be determined prior to being
    1436          transferred, a Content-Length header field <em class="bcp14">SHOULD</em> be sent to indicate the length of the payload body that is either present as the message body, for requests and non-HEAD responses
    1437          other than 304, or would have been present had the request been an unconditional GET. The length is expressed as a decimal
    1438          number of octets.
    1439       </p>
    1440       <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.53"></span>  <a href="#header.content-length" class="smpl">Content-Length</a> = 1*<a href="#core.rules" class="smpl">DIGIT</a>
     1467                  forming the message body.
     1468               </p>
     1469               <p id="rfc.section.3.3.1.p.6">If more than one Transfer-Encoding header field is present in a message, the multiple field-values <em class="bcp14">MUST</em> be combined into one field-value, according to the algorithm defined in <a href="#header.fields" title="Header Fields">Section&nbsp;3.2</a>, before determining the message body length.
     1470               </p>
     1471               <p id="rfc.section.3.3.1.p.7">Unlike Content-Encoding (<a href="p3-payload.html#content.codings" title="Content Codings">Section 2.2</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>), Transfer-Encoding is a property of the message, not of the payload, and thus <em class="bcp14">MAY</em> be added or removed by any implementation along the request/response chain. Additional information about the encoding parameters <em class="bcp14">MAY</em> be provided by other header fields not defined by this specification.
     1472               </p>
     1473               <p id="rfc.section.3.3.1.p.8">Transfer-Encoding <em class="bcp14">MAY</em> be sent in a response to a HEAD request or in a 304 response to a GET request, neither of which includes a message body, to
     1474                  indicate that the origin server would have applied a transfer coding to the message body if the request had been an unconditional
     1475                  GET. This indication is not required, however, because any recipient on the response chain (including the origin server) can
     1476                  remove transfer codings when they are not needed.
     1477               </p>
     1478               <p id="rfc.section.3.3.1.p.9">Transfer-Encoding was added in HTTP/1.1. It is generally assumed that implementations advertising only HTTP/1.0 support will
     1479                  not understand how to process a transfer-encoded payload. A client <em class="bcp14">MUST NOT</em> send a request containing Transfer-Encoding unless it knows the server will handle HTTP/1.1 (or later) requests; such knowledge
     1480                  might be in the form of specific user configuration or by remembering the version of a prior received response. A server <em class="bcp14">MUST NOT</em> send a response containing Transfer-Encoding unless the corresponding request indicates HTTP/1.1 (or later).
     1481               </p>
     1482               <p id="rfc.section.3.3.1.p.10">A server that receives a request message with a transfer-coding it does not understand <em class="bcp14">SHOULD</em> respond with 501 (Not Implemented) and then close the connection.
     1483               </p>
     1484            </div>
     1485            <div id="header.content-length">
     1486               <div id="rfc.iref.c.6"></div>
     1487               <div id="rfc.iref.h.7"></div>
     1488               <h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a href="#header.content-length">Content-Length</a></h3>
     1489               <p id="rfc.section.3.3.2.p.1">When a message does not have a Transfer-Encoding header field and the payload body length can be determined prior to being
     1490                  transferred, a Content-Length header field <em class="bcp14">SHOULD</em> be sent to indicate the length of the payload body that is either present as the message body, for requests and non-HEAD responses
     1491                  other than 304, or would have been present had the request been an unconditional GET. The length is expressed as a decimal
     1492                  number of octets.
     1493               </p>
     1494               <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.53"></span>  <a href="#header.content-length" class="smpl">Content-Length</a> = 1*<a href="#core.rules" class="smpl">DIGIT</a>
    14411495</pre><p id="rfc.section.3.3.2.p.3">An example is</p>
    1442       <div id="rfc.figure.u.36"></div><pre class="text">  Content-Length: 3495
     1496               <div id="rfc.figure.u.36"></div><pre class="text">  Content-Length: 3495
    14431497</pre><p id="rfc.section.3.3.2.p.5">In the case of a response to a HEAD request, Content-Length indicates the size of the payload body (without any potential
    1444          transfer-coding) that would have been sent had the request been a GET. In the case of a 304 (Not Modified) response to a GET
    1445          request, Content-Length indicates the size of the payload body (without any potential transfer-coding) that would have been
    1446          sent in a 200 (OK) response.
    1447       </p>
    1448       <p id="rfc.section.3.3.2.p.6">HTTP's use of Content-Length is significantly different from how it is used in MIME, where it is an optional field used only
    1449          within the "message/external-body" media-type.
    1450       </p>
    1451       <p id="rfc.section.3.3.2.p.7">Any Content-Length field value greater than or equal to zero is valid. Since there is no predefined limit to the length of
    1452          an HTTP payload, recipients <em class="bcp14">SHOULD</em> anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows (<a href="#attack.protocol.element.size.overflows" title="Protocol Element Size Overflows">Section&nbsp;8.6</a>).
    1453       </p>
    1454       <p id="rfc.section.3.3.2.p.8">If a message is received that has multiple Content-Length header fields (<a href="#header.content-length" id="rfc.xref.header.content-length.1" title="Content-Length">Section&nbsp;3.3.2</a>) with field-values consisting of the same decimal value, or a single Content-Length header field with a field value containing
    1455          a list of identical decimal values (e.g., "Content-Length: 42, 42"), indicating that duplicate Content-Length header fields
    1456          have been generated or combined by an upstream message processor, then the recipient <em class="bcp14">MUST</em> either reject the message as invalid or replace the duplicated field-values with a single valid Content-Length field containing
    1457          that decimal value prior to determining the message body length.
    1458       </p>
    1459       <h3 id="rfc.section.3.3.3"><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;<a id="message.body.length" href="#message.body.length">Message Body Length</a></h3>
    1460       <p id="rfc.section.3.3.3.p.1">The length of a message body is determined by one of the following (in order of precedence):</p>
    1461       <p id="rfc.section.3.3.3.p.2"> </p>
    1462       <ol>
    1463          <li>
    1464             <p>Any response to a HEAD request and any response with a status code of 100-199, 204, or 304 is always terminated by the first
    1465                empty line after the header fields, regardless of the header fields present in the message, and thus cannot contain a message
    1466                body.
    1467             </p>
    1468          </li>
    1469          <li>
    1470             <p>Any successful (2xx) response to a CONNECT request implies that the connection will become a tunnel immediately after the
    1471                empty line that concludes the header fields. A client <em class="bcp14">MUST</em> ignore any Content-Length or Transfer-Encoding header fields received in such a message.
    1472             </p>
    1473          </li>
    1474          <li>
    1475             <p>If a Transfer-Encoding header field is present and the "chunked" transfer-coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) is the final encoding, the message body length is determined by reading and decoding the chunked data until the transfer-coding
    1476                indicates the data is complete.
    1477             </p>
    1478             <p>If a Transfer-Encoding header field is present in a response and the "chunked" transfer-coding is not the final encoding,
    1479                the message body length is determined by reading the connection until it is closed by the server. If a Transfer-Encoding header
    1480                field is present in a request and the "chunked" transfer-coding is not the final encoding, the message body length cannot
    1481                be determined reliably; the server <em class="bcp14">MUST</em> respond with the 400 (Bad Request) status code and then close the connection.
    1482             </p>
    1483             <p>If a message is received with both a Transfer-Encoding header field and a Content-Length header field, the Transfer-Encoding
    1484                overrides the Content-Length. Such a message might indicate an attempt to perform request or response smuggling (bypass of
    1485                security-related checks on message routing or content) and thus ought to be handled as an error. The provided Content-Length <em class="bcp14">MUST</em> be removed, prior to forwarding the message downstream, or replaced with the real message body length after the transfer-coding
    1486                is decoded.
    1487             </p>
    1488          </li>
    1489          <li>
    1490             <p>If a message is received without Transfer-Encoding and with either multiple Content-Length header fields having differing
    1491                field-values or a single Content-Length header field having an invalid value, then the message framing is invalid and <em class="bcp14">MUST</em> be treated as an error to prevent request or response smuggling. If this is a request message, the server <em class="bcp14">MUST</em> respond with a 400 (Bad Request) status code and then close the connection. If this is a response message received by a proxy,
    1492                the proxy <em class="bcp14">MUST</em> discard the received response, send a 502 (Bad Gateway) status code as its downstream response, and then close the connection.
    1493                If this is a response message received by a user-agent, it <em class="bcp14">MUST</em> be treated as an error by discarding the message and closing the connection.
    1494             </p>
    1495          </li>
    1496          <li>
    1497             <p>If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the message body length
    1498                in octets. If the actual number of octets sent in the message is less than the indicated Content-Length, the recipient <em class="bcp14">MUST</em> consider the message to be incomplete and treat the connection as no longer usable. If the actual number of octets sent in
    1499                the message is more than the indicated Content-Length, the recipient <em class="bcp14">MUST</em> only process the message body up to the field value's number of octets; the remainder of the message <em class="bcp14">MUST</em> either be discarded or treated as the next message in a pipeline. For the sake of robustness, a user-agent <em class="bcp14">MAY</em> attempt to detect and correct such an error in message framing if it is parsing the response to the last request on a connection
    1500                and the connection has been closed by the server.
    1501             </p>
    1502          </li>
    1503          <li>
    1504             <p>If this is a request message and none of the above are true, then the message body length is zero (no message body is present).</p>
    1505          </li>
    1506          <li>
    1507             <p>Otherwise, this is a response message without a declared message body length, so the message body length is determined by
    1508                the number of octets received prior to the server closing the connection.
    1509             </p>
    1510          </li>
    1511       </ol>
    1512       <p id="rfc.section.3.3.3.p.3">Since there is no way to distinguish a successfully completed, close-delimited message from a partially-received message interrupted
    1513          by network failure, implementations <em class="bcp14">SHOULD</em> use encoding or length-delimited messages whenever possible. The close-delimiting feature exists primarily for backwards compatibility
    1514          with HTTP/1.0.
    1515       </p>
    1516       <p id="rfc.section.3.3.3.p.4">A server <em class="bcp14">MAY</em> reject a request that contains a message body but not a Content-Length by responding with 411 (Length Required).
    1517       </p>
    1518       <p id="rfc.section.3.3.3.p.5">Unless a transfer-coding other than "chunked" has been applied, a client that sends a request containing a message body <em class="bcp14">SHOULD</em> use a valid Content-Length header field if the message body length is known in advance, rather than the "chunked" encoding,
    1519          since some existing services respond to "chunked" with a 411 (Length Required) status code even though they understand the
    1520          chunked encoding. This is typically because such services are implemented via a gateway that requires a content-length in
    1521          advance of being called and the server is unable or unwilling to buffer the entire request before processing.
    1522       </p>
    1523       <p id="rfc.section.3.3.3.p.6">A client that sends a request containing a message body <em class="bcp14">MUST</em> include a valid Content-Length header field if it does not know the server will handle HTTP/1.1 (or later) requests; such
    1524          knowledge can be in the form of specific user configuration or by remembering the version of a prior received response.
    1525       </p>
    1526       <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="incomplete.messages" href="#incomplete.messages">Handling Incomplete Messages</a></h2>
    1527       <p id="rfc.section.3.4.p.1">Request messages that are prematurely terminated, possibly due to a cancelled connection or a server-imposed time-out exception, <em class="bcp14">MUST</em> result in closure of the connection; sending an HTTP/1.1 error response prior to closing the connection is <em class="bcp14">OPTIONAL</em>.
    1528       </p>
    1529       <p id="rfc.section.3.4.p.2">Response messages that are prematurely terminated, usually by closure of the connection prior to receiving the expected number
    1530          of octets or by failure to decode a transfer-encoded message body, <em class="bcp14">MUST</em> be recorded as incomplete. A response that terminates in the middle of the header block (before the empty line is received)
    1531          cannot be assumed to convey the full semantics of the response and <em class="bcp14">MUST</em> be treated as an error.
    1532       </p>
    1533       <p id="rfc.section.3.4.p.3">A message body that uses the chunked transfer encoding is incomplete if the zero-sized chunk that terminates the encoding
    1534          has not been received. A message that uses a valid Content-Length is incomplete if the size of the message body received (in
    1535          octets) is less than the value given by Content-Length. A response that has neither chunked transfer encoding nor Content-Length
    1536          is terminated by closure of the connection, and thus is considered complete regardless of the number of message body octets
    1537          received, provided that the header block was received intact.
    1538       </p>
    1539       <p id="rfc.section.3.4.p.4">A user agent <em class="bcp14">MUST NOT</em> render an incomplete response message body as if it were complete (i.e., some indication must be given to the user that an
    1540          error occurred). Cache requirements for incomplete responses are defined in <a href="p6-cache.html#response.cacheability" title="Response Cacheability">Section 2.1</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
    1541       </p>
    1542       <p id="rfc.section.3.4.p.5">A server <em class="bcp14">MUST</em> read the entire request message body or close the connection after sending its response, since otherwise the remaining data
    1543          on a persistent connection would be misinterpreted as the next request. Likewise, a client <em class="bcp14">MUST</em> read the entire response message body if it intends to reuse the same connection for a subsequent request. Pipelining multiple
    1544          requests on a connection is described in <a href="#pipelining" title="Pipelining">Section&nbsp;6.3.2.2</a>.
    1545       </p>
    1546       <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="message.robustness" href="#message.robustness">Message Parsing Robustness</a></h2>
    1547       <p id="rfc.section.3.5.p.1">Older HTTP/1.0 client implementations might send an extra CRLF after a POST request as a lame workaround for some early server
    1548          applications that failed to read message body content that was not terminated by a line-ending. An HTTP/1.1 client <em class="bcp14">MUST NOT</em> preface or follow a request with an extra CRLF. If terminating the request message body with a line-ending is desired, then
    1549          the client <em class="bcp14">MUST</em> include the terminating CRLF octets as part of the message body length.
    1550       </p>
    1551       <p id="rfc.section.3.5.p.2">In the interest of robustness, servers <em class="bcp14">SHOULD</em> ignore at least one empty line received where a request-line is expected. In other words, if the server is reading the protocol
    1552          stream at the beginning of a message and receives a CRLF first, it <em class="bcp14">SHOULD</em> ignore the CRLF. Likewise, although the line terminator for the start-line and header fields is the sequence CRLF, we recommend
    1553          that recipients recognize a single LF as a line terminator and ignore any CR.
    1554       </p>
    1555       <p id="rfc.section.3.5.p.3">When a server listening only for HTTP request messages, or processing what appears from the start-line to be an HTTP request
    1556          message, receives a sequence of octets that does not match the HTTP-message grammar aside from the robustness exceptions listed
    1557          above, the server <em class="bcp14">MUST</em> respond with an HTTP/1.1 400 (Bad Request) response.
    1558       </p>
    1559       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="transfer.codings" href="#transfer.codings">Transfer Codings</a></h1>
    1560       <p id="rfc.section.4.p.1">Transfer-coding values are used to indicate an encoding transformation that has been, can be, or might need to be applied
    1561          to a payload body in order to ensure "safe transport" through the network. This differs from a content coding in that the
    1562          transfer-coding is a property of the message rather than a property of the representation that is being transferred.
    1563       </p>
    1564       <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.54"></span><span id="rfc.iref.g.55"></span>  <a href="#transfer.codings" class="smpl">transfer-coding</a>    = "chunked" ; <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>
     1498                  transfer-coding) that would have been sent had the request been a GET. In the case of a 304 (Not Modified) response to a GET
     1499                  request, Content-Length indicates the size of the payload body (without any potential transfer-coding) that would have been
     1500                  sent in a 200 (OK) response.
     1501               </p>
     1502               <p id="rfc.section.3.3.2.p.6">HTTP's use of Content-Length is significantly different from how it is used in MIME, where it is an optional field used only
     1503                  within the "message/external-body" media-type.
     1504               </p>
     1505               <p id="rfc.section.3.3.2.p.7">Any Content-Length field value greater than or equal to zero is valid. Since there is no predefined limit to the length of
     1506                  an HTTP payload, recipients <em class="bcp14">SHOULD</em> anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows (<a href="#attack.protocol.element.size.overflows" title="Protocol Element Size Overflows">Section&nbsp;8.6</a>).
     1507               </p>
     1508               <p id="rfc.section.3.3.2.p.8">If a message is received that has multiple Content-Length header fields (<a href="#header.content-length" id="rfc.xref.header.content-length.1" title="Content-Length">Section&nbsp;3.3.2</a>) with field-values consisting of the same decimal value, or a single Content-Length header field with a field value containing
     1509                  a list of identical decimal values (e.g., "Content-Length: 42, 42"), indicating that duplicate Content-Length header fields
     1510                  have been generated or combined by an upstream message processor, then the recipient <em class="bcp14">MUST</em> either reject the message as invalid or replace the duplicated field-values with a single valid Content-Length field containing
     1511                  that decimal value prior to determining the message body length.
     1512               </p>
     1513            </div>
     1514            <div id="message.body.length">
     1515               <h3 id="rfc.section.3.3.3"><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;<a href="#message.body.length">Message Body Length</a></h3>
     1516               <p id="rfc.section.3.3.3.p.1">The length of a message body is determined by one of the following (in order of precedence):</p>
     1517               <p id="rfc.section.3.3.3.p.2"></p>
     1518               <ol>
     1519                  <li>
     1520                     <p>Any response to a HEAD request and any response with a status code of 100-199, 204, or 304 is always terminated by the first
     1521                        empty line after the header fields, regardless of the header fields present in the message, and thus cannot contain a message
     1522                        body.
     1523                     </p>
     1524                  </li>
     1525                  <li>
     1526                     <p>Any successful (2xx) response to a CONNECT request implies that the connection will become a tunnel immediately after the
     1527                        empty line that concludes the header fields. A client <em class="bcp14">MUST</em> ignore any Content-Length or Transfer-Encoding header fields received in such a message.
     1528                     </p>
     1529                  </li>
     1530                  <li>
     1531                     <p>If a Transfer-Encoding header field is present and the "chunked" transfer-coding (<a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>) is the final encoding, the message body length is determined by reading and decoding the chunked data until the transfer-coding
     1532                        indicates the data is complete.
     1533                     </p>
     1534                     <p>If a Transfer-Encoding header field is present in a response and the "chunked" transfer-coding is not the final encoding,
     1535                        the message body length is determined by reading the connection until it is closed by the server. If a Transfer-Encoding header
     1536                        field is present in a request and the "chunked" transfer-coding is not the final encoding, the message body length cannot
     1537                        be determined reliably; the server <em class="bcp14">MUST</em> respond with the 400 (Bad Request) status code and then close the connection.
     1538                     </p>
     1539                     <p>If a message is received with both a Transfer-Encoding header field and a Content-Length header field, the Transfer-Encoding
     1540                        overrides the Content-Length. Such a message might indicate an attempt to perform request or response smuggling (bypass of
     1541                        security-related checks on message routing or content) and thus ought to be handled as an error. The provided Content-Length <em class="bcp14">MUST</em> be removed, prior to forwarding the message downstream, or replaced with the real message body length after the transfer-coding
     1542                        is decoded.
     1543                     </p>
     1544                  </li>
     1545                  <li>
     1546                     <p>If a message is received without Transfer-Encoding and with either multiple Content-Length header fields having differing
     1547                        field-values or a single Content-Length header field having an invalid value, then the message framing is invalid and <em class="bcp14">MUST</em> be treated as an error to prevent request or response smuggling. If this is a request message, the server <em class="bcp14">MUST</em> respond with a 400 (Bad Request) status code and then close the connection. If this is a response message received by a proxy,
     1548                        the proxy <em class="bcp14">MUST</em> discard the received response, send a 502 (Bad Gateway) status code as its downstream response, and then close the connection.
     1549                        If this is a response message received by a user-agent, it <em class="bcp14">MUST</em> be treated as an error by discarding the message and closing the connection.
     1550                     </p>
     1551                  </li>
     1552                  <li>
     1553                     <p>If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the message body length
     1554                        in octets. If the actual number of octets sent in the message is less than the indicated Content-Length, the recipient <em class="bcp14">MUST</em> consider the message to be incomplete and treat the connection as no longer usable. If the actual number of octets sent in
     1555                        the message is more than the indicated Content-Length, the recipient <em class="bcp14">MUST</em> only process the message body up to the field value's number of octets; the remainder of the message <em class="bcp14">MUST</em> either be discarded or treated as the next message in a pipeline. For the sake of robustness, a user-agent <em class="bcp14">MAY</em> attempt to detect and correct such an error in message framing if it is parsing the response to the last request on a connection
     1556                        and the connection has been closed by the server.
     1557                     </p>
     1558                  </li>
     1559                  <li>
     1560                     <p>If this is a request message and none of the above are true, then the message body length is zero (no message body is present).</p>
     1561                  </li>
     1562                  <li>
     1563                     <p>Otherwise, this is a response message without a declared message body length, so the message body length is determined by
     1564                        the number of octets received prior to the server closing the connection.
     1565                     </p>
     1566                  </li>
     1567               </ol>
     1568               <p id="rfc.section.3.3.3.p.3">Since there is no way to distinguish a successfully completed, close-delimited message from a partially-received message interrupted
     1569                  by network failure, implementations <em class="bcp14">SHOULD</em> use encoding or length-delimited messages whenever possible. The close-delimiting feature exists primarily for backwards compatibility
     1570                  with HTTP/1.0.
     1571               </p>
     1572               <p id="rfc.section.3.3.3.p.4">A server <em class="bcp14">MAY</em> reject a request that contains a message body but not a Content-Length by responding with 411 (Length Required).
     1573               </p>
     1574               <p id="rfc.section.3.3.3.p.5">Unless a transfer-coding other than "chunked" has been applied, a client that sends a request containing a message body <em class="bcp14">SHOULD</em> use a valid Content-Length header field if the message body length is known in advance, rather than the "chunked" encoding,
     1575                  since some existing services respond to "chunked" with a 411 (Length Required) status code even though they understand the
     1576                  chunked encoding. This is typically because such services are implemented via a gateway that requires a content-length in
     1577                  advance of being called and the server is unable or unwilling to buffer the entire request before processing.
     1578               </p>
     1579               <p id="rfc.section.3.3.3.p.6">A client that sends a request containing a message body <em class="bcp14">MUST</em> include a valid Content-Length header field if it does not know the server will handle HTTP/1.1 (or later) requests; such
     1580                  knowledge can be in the form of specific user configuration or by remembering the version of a prior received response.
     1581               </p>
     1582            </div>
     1583         </div>
     1584         <div id="incomplete.messages">
     1585            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a href="#incomplete.messages">Handling Incomplete Messages</a></h2>
     1586            <p id="rfc.section.3.4.p.1">Request messages that are prematurely terminated, possibly due to a cancelled connection or a server-imposed time-out exception, <em class="bcp14">MUST</em> result in closure of the connection; sending an HTTP/1.1 error response prior to closing the connection is <em class="bcp14">OPTIONAL</em>.
     1587            </p>
     1588            <p id="rfc.section.3.4.p.2">Response messages that are prematurely terminated, usually by closure of the connection prior to receiving the expected number
     1589               of octets or by failure to decode a transfer-encoded message body, <em class="bcp14">MUST</em> be recorded as incomplete. A response that terminates in the middle of the header block (before the empty line is received)
     1590               cannot be assumed to convey the full semantics of the response and <em class="bcp14">MUST</em> be treated as an error.
     1591            </p>
     1592            <p id="rfc.section.3.4.p.3">A message body that uses the chunked transfer encoding is incomplete if the zero-sized chunk that terminates the encoding
     1593               has not been received. A message that uses a valid Content-Length is incomplete if the size of the message body received (in
     1594               octets) is less than the value given by Content-Length. A response that has neither chunked transfer encoding nor Content-Length
     1595               is terminated by closure of the connection, and thus is considered complete regardless of the number of message body octets
     1596               received, provided that the header block was received intact.
     1597            </p>
     1598            <p id="rfc.section.3.4.p.4">A user agent <em class="bcp14">MUST NOT</em> render an incomplete response message body as if it were complete (i.e., some indication must be given to the user that an
     1599               error occurred). Cache requirements for incomplete responses are defined in <a href="p6-cache.html#response.cacheability" title="Response Cacheability">Section 2.1</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>.
     1600            </p>
     1601            <p id="rfc.section.3.4.p.5">A server <em class="bcp14">MUST</em> read the entire request message body or close the connection after sending its response, since otherwise the remaining data
     1602               on a persistent connection would be misinterpreted as the next request. Likewise, a client <em class="bcp14">MUST</em> read the entire response message body if it intends to reuse the same connection for a subsequent request. Pipelining multiple
     1603               requests on a connection is described in <a href="#pipelining" title="Pipelining">Section&nbsp;6.3.2.2</a>.
     1604            </p>
     1605         </div>
     1606         <div id="message.robustness">
     1607            <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a href="#message.robustness">Message Parsing Robustness</a></h2>
     1608            <p id="rfc.section.3.5.p.1">Older HTTP/1.0 client implementations might send an extra CRLF after a POST request as a lame workaround for some early server
     1609               applications that failed to read message body content that was not terminated by a line-ending. An HTTP/1.1 client <em class="bcp14">MUST NOT</em> preface or follow a request with an extra CRLF. If terminating the request message body with a line-ending is desired, then
     1610               the client <em class="bcp14">MUST</em> include the terminating CRLF octets as part of the message body length.
     1611            </p>
     1612            <p id="rfc.section.3.5.p.2">In the interest of robustness, servers <em class="bcp14">SHOULD</em> ignore at least one empty line received where a request-line is expected. In other words, if the server is reading the protocol
     1613               stream at the beginning of a message and receives a CRLF first, it <em class="bcp14">SHOULD</em> ignore the CRLF. Likewise, although the line terminator for the start-line and header fields is the sequence CRLF, we recommend
     1614               that recipients recognize a single LF as a line terminator and ignore any CR.
     1615            </p>
     1616            <p id="rfc.section.3.5.p.3">When a server listening only for HTTP request messages, or processing what appears from the start-line to be an HTTP request
     1617               message, receives a sequence of octets that does not match the HTTP-message grammar aside from the robustness exceptions listed
     1618               above, the server <em class="bcp14">MUST</em> respond with an HTTP/1.1 400 (Bad Request) response.
     1619            </p>
     1620         </div>
     1621      </div>
     1622      <div id="transfer.codings">
     1623         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#transfer.codings">Transfer Codings</a></h1>
     1624         <p id="rfc.section.4.p.1">Transfer-coding values are used to indicate an encoding transformation that has been, can be, or might need to be applied
     1625            to a payload body in order to ensure "safe transport" through the network. This differs from a content coding in that the
     1626            transfer-coding is a property of the message rather than a property of the representation that is being transferred.
     1627         </p>
     1628         <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.54"></span><span id="rfc.iref.g.55"></span>  <a href="#transfer.codings" class="smpl">transfer-coding</a>    = "chunked" ; <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>
    15651629                     / "compress" ; <a href="#compress.coding" title="Compress Coding">Section&nbsp;4.2.1</a>
    15661630                     / "deflate" ; <a href="#deflate.coding" title="Deflate Coding">Section&nbsp;4.2.2</a>
    … …  
    15691633  <a href="#transfer.codings" class="smpl">transfer-extension</a> = <a href="#rule.token.separators" class="smpl">token</a> *( <a href="#rule.whitespace" class="smpl">OWS</a> ";" <a href="#rule.whitespace" class="smpl">OWS</a> <a href="#rule.parameter" class="smpl">transfer-parameter</a> )
    15701634</pre><div id="rule.parameter">
    1571          <p id="rfc.section.4.p.3">      Parameters are in the form of attribute/value pairs.</p>
    1572       </div>
    1573       <div id="rfc.figure.u.38"></div><pre class="inline"><span id="rfc.iref.g.56"></span><span id="rfc.iref.g.57"></span><span id="rfc.iref.g.58"></span><span id="rfc.iref.g.59"></span><span id="rfc.iref.g.60"></span>  <a href="#rule.parameter" class="smpl">transfer-parameter</a> = <a href="#rule.parameter" class="smpl">attribute</a> <a href="#rule.whitespace" class="smpl">BWS</a> "=" <a href="#rule.whitespace" class="smpl">BWS</a> <a href="#rule.parameter" class="smpl">value</a>
     1635            <p id="rfc.section.4.p.3">   Parameters are in the form of attribute/value pairs.</p>
     1636         </div>
     1637         <div id="rfc.figure.u.38"></div><pre class="inline"><span id="rfc.iref.g.56"></span><span id="rfc.iref.g.57"></span><span id="rfc.iref.g.58"></span><span id="rfc.iref.g.59"></span><span id="rfc.iref.g.60"></span>  <a href="#rule.parameter" class="smpl">transfer-parameter</a> = <a href="#rule.parameter" class="smpl">attribute</a> <a href="#rule.whitespace" class="smpl">BWS</a> "=" <a href="#rule.whitespace" class="smpl">BWS</a> <a href="#rule.parameter" class="smpl">value</a>
    15741638  <a href="#rule.parameter" class="smpl">attribute</a>          = <a href="#rule.token.separators" class="smpl">token</a>
    15751639  <a href="#rule.parameter" class="smpl">value</a>              = <a href="#rule.token.separators" class="smpl">word</a>
    15761640</pre><p id="rfc.section.4.p.5">All transfer-coding values are case-insensitive. The HTTP Transfer Coding registry is defined in <a href="#transfer.coding.registry" title="Transfer Coding Registry">Section&nbsp;7.4</a>. 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;4.3</a>) and in the Transfer-Encoding header field (<a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.2" title="Transfer-Encoding">Section&nbsp;3.3.1</a>).
    1577       </p>
    1578       <div id="rfc.iref.c.7"></div>
    1579       <div id="rfc.iref.c.8"></div>
    1580       <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="chunked.encoding" href="#chunked.encoding">Chunked Transfer Coding</a></h2>
    1581       <p id="rfc.section.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
    1582          indicator, followed by an <em class="bcp14">OPTIONAL</em> trailer containing header fields. This allows dynamically produced content to be transferred along with the information necessary
    1583          for the recipient to verify that it has received the full message.
    1584       </p>
    1585       <div id="rfc.figure.u.39"></div><pre class="inline"><span id="rfc.iref.g.61"></span><span id="rfc.iref.g.62"></span><span id="rfc.iref.g.63"></span><span id="rfc.iref.g.64"></span><span id="rfc.iref.g.65"></span><span id="rfc.iref.g.66"></span><span id="rfc.iref.g.67"></span><span id="rfc.iref.g.68"></span><span id="rfc.iref.g.69"></span><span id="rfc.iref.g.70"></span><span id="rfc.iref.g.71"></span>  <a href="#chunked.encoding" class="smpl">chunked-body</a>   = *<a href="#chunked.encoding" class="smpl">chunk</a>
     1641         </p>
     1642         <div id="chunked.encoding">
     1643            <div id="rfc.iref.c.7"></div>
     1644            <div id="rfc.iref.c.8"></div>
     1645            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#chunked.encoding">Chunked Transfer Coding</a></h2>
     1646            <p id="rfc.section.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
     1647               indicator, followed by an <em class="bcp14">OPTIONAL</em> trailer containing header fields. This allows dynamically produced content to be transferred along with the information necessary
     1648               for the recipient to verify that it has received the full message.
     1649            </p>
     1650            <div id="rfc.figure.u.39"></div><pre class="inline"><span id="rfc.iref.g.61"></span><span id="rfc.iref.g.62"></span><span id="rfc.iref.g.63"></span><span id="rfc.iref.g.64"></span><span id="rfc.iref.g.65"></span><span id="rfc.iref.g.66"></span><span id="rfc.iref.g.67"></span><span id="rfc.iref.g.68"></span><span id="rfc.iref.g.69"></span><span id="rfc.iref.g.70"></span><span id="rfc.iref.g.71"></span>  <a href="#chunked.encoding" class="smpl">chunked-body</a>   = *<a href="#chunked.encoding" class="smpl">chunk</a>
    15861651                   <a href="#chunked.encoding" class="smpl">last-chunk</a>
    15871652                   <a href="#chunked.encoding" class="smpl">trailer-part</a>
    … …  
    16031668  <a href="#chunked.encoding" class="smpl">qdtext-nf</a>      = <a href="#core.rules" class="smpl">HTAB</a> / <a href="#core.rules" class="smpl">SP</a> / %x21 / %x23-5B / %x5D-7E / <a href="#rule.quoted-string" class="smpl">obs-text</a>
    16041669</pre><p id="rfc.section.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
    1605          by any chunk whose size is zero, followed by the trailer, which is terminated by an empty line.
    1606       </p>
    1607       <p id="rfc.section.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
    1608          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;4.4</a>).
    1609       </p>
    1610       <p id="rfc.section.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:
    1611       </p>
    1612       <ol>
    1613          <li>the request included a TE header field that indicates "trailers" is acceptable in the transfer-coding of the response, as
    1614             described in <a href="#header.te" id="rfc.xref.header.te.2" title="TE">Section&nbsp;4.3</a>; or,
    1615          </li>
    1616          <li>the trailer fields consist entirely of optional metadata, and the recipient could use the message (in a manner acceptable
    1617             to the server where the field originated) without receiving it. In other words, the server that generated the header (often
    1618             but not always the origin server) is willing to accept the possibility that the trailer fields might be silently discarded
    1619             along the path to the client.
    1620          </li>
    1621       </ol>
    1622       <p id="rfc.section.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
    1623          forwarded to an HTTP/1.0 recipient. It avoids a situation where conformance with the protocol would have necessitated a possibly
    1624          infinite buffer on the proxy.
    1625       </p>
    1626       <p id="rfc.section.4.1.p.7">A process for decoding the "chunked" transfer-coding can be represented in pseudo-code as:</p>
    1627       <div id="rfc.figure.u.40"></div><pre class="text">  length := 0
     1670               by any chunk whose size is zero, followed by the trailer, which is terminated by an empty line.
     1671            </p>
     1672            <p id="rfc.section.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
     1673               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;4.4</a>).
     1674            </p>
     1675            <p id="rfc.section.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:
     1676            </p>
     1677            <ol>
     1678               <li>the request included a TE header field that indicates "trailers" is acceptable in the transfer-coding of the response, as
     1679                  described in <a href="#header.te" id="rfc.xref.header.te.2" title="TE">Section&nbsp;4.3</a>; or,
     1680               </li>
     1681               <li>the trailer fields consist entirely of optional metadata, and the recipient could use the message (in a manner acceptable
     1682                  to the server where the field originated) without receiving it. In other words, the server that generated the header (often
     1683                  but not always the origin server) is willing to accept the possibility that the trailer fields might be silently discarded
     1684                  along the path to the client.
     1685               </li>
     1686            </ol>
     1687            <p id="rfc.section.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
     1688               forwarded to an HTTP/1.0 recipient. It avoids a situation where conformance with the protocol would have necessitated a possibly
     1689               infinite buffer on the proxy.
     1690            </p>
     1691            <p id="rfc.section.4.1.p.7">A process for decoding the "chunked" transfer-coding can be represented in pseudo-code as:</p>
     1692            <div id="rfc.figure.u.40"></div><pre class="text">  length := 0
    16281693  read chunk-size, chunk-ext (if any) and CRLF
    16291694  while (chunk-size &gt; 0) {
    … …  
    16411706  Remove "chunked" from Transfer-Encoding
    16421707</pre><p id="rfc.section.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-ext extensions they do not understand.
    1643       </p>
    1644       <p id="rfc.section.4.1.p.10">Use of chunk-ext extensions by senders is deprecated; they <em class="bcp14">SHOULD NOT</em> be sent and definition of new chunk-extensions is discouraged.
    1645       </p>
    1646       <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="compression.codings" href="#compression.codings">Compression Codings</a></h2>
    1647       <p id="rfc.section.4.2.p.1">The codings defined below can be used to compress the payload of a message.</p>
    1648       <div class="note" id="rfc.section.4.2.p.2">
    1649          <p> <b>Note:</b> Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings.
    1650             Their use here is representative of historical practice, not good design.
    1651          </p>
    1652       </div>
    1653       <div class="note" id="rfc.section.4.2.p.3">
    1654          <p> <b>Note:</b> For compatibility with previous implementations of HTTP, applications <em class="bcp14">SHOULD</em> consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.
    1655          </p>
    1656       </div>
    1657       <div id="rfc.iref.c.9"></div>
    1658       <div id="rfc.iref.c.10"></div>
    1659       <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a id="compress.coding" href="#compress.coding">Compress Coding</a></h3>
    1660       <p id="rfc.section.4.2.1.p.1">The "compress" format is produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch
    1661          coding (LZW).
    1662       </p>
    1663       <div id="rfc.iref.d.2"></div>
    1664       <div id="rfc.iref.c.11"></div>
    1665       <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a id="deflate.coding" href="#deflate.coding">Deflate Coding</a></h3>
    1666       <p id="rfc.section.4.2.2.p.1">The "deflate" format is defined as the "deflate" compression mechanism (described in <a href="#RFC1951" id="rfc.xref.RFC1951.1"><cite title="DEFLATE Compressed Data Format Specification version 1.3">[RFC1951]</cite></a>) used inside the "zlib" data format (<a href="#RFC1950" id="rfc.xref.RFC1950.1"><cite title="ZLIB Compressed Data Format Specification version 3.3">[RFC1950]</cite></a>).
    1667       </p>
    1668       <div class="note" id="rfc.section.4.2.2.p.2">
    1669          <p> <b>Note:</b> Some incorrect implementations send the "deflate" compressed data without the zlib wrapper.
    1670          </p>
    1671       </div>
    1672       <div id="rfc.iref.g.72"></div>
    1673       <div id="rfc.iref.c.12"></div>
    1674       <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a id="gzip.coding" href="#gzip.coding">Gzip Coding</a></h3>
    1675       <p id="rfc.section.4.2.3.p.1">The "gzip" format is produced by the file compression program "gzip" (GNU zip), as described in <a href="#RFC1952" id="rfc.xref.RFC1952.1"><cite title="GZIP file format specification version 4.3">[RFC1952]</cite></a>. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
    1676       </p>
    1677       <div id="rfc.iref.t.5"></div>
    1678       <div id="rfc.iref.h.8"></div>
    1679       <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="header.te" href="#header.te">TE</a></h2>
    1680       <p id="rfc.section.4.3.p.1">The "TE" header field indicates what extension transfer-codings the client is willing to accept in the response, and whether
    1681          or not it is willing to accept trailer fields in a chunked transfer-coding.
    1682       </p>
    1683       <p id="rfc.section.4.3.p.2">Its value consists of the keyword "trailers" and/or a comma-separated list of extension transfer-coding names with optional
    1684          accept parameters (as described in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>).
    1685       </p>
    1686       <div id="rfc.figure.u.41"></div><pre class="inline"><span id="rfc.iref.g.73"></span><span id="rfc.iref.g.74"></span><span id="rfc.iref.g.75"></span><span id="rfc.iref.g.76"></span>  <a href="#header.te" class="smpl">TE</a>        = #<a href="#header.te" class="smpl">t-codings</a>
     1708            </p>
     1709            <p id="rfc.section.4.1.p.10">Use of chunk-ext extensions by senders is deprecated; they <em class="bcp14">SHOULD NOT</em> be sent and definition of new chunk-extensions is discouraged.
     1710            </p>
     1711         </div>
     1712         <div id="compression.codings">
     1713            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#compression.codings">Compression Codings</a></h2>
     1714            <p id="rfc.section.4.2.p.1">The codings defined below can be used to compress the payload of a message.</p>
     1715            <div class="note" id="rfc.section.4.2.p.2">
     1716               <p><b>Note:</b> Use of program names for the identification of encoding formats is not desirable and is discouraged for future encodings.
     1717                  Their use here is representative of historical practice, not good design.
     1718               </p>
     1719            </div>
     1720            <div class="note" id="rfc.section.4.2.p.3">
     1721               <p><b>Note:</b> For compatibility with previous implementations of HTTP, applications <em class="bcp14">SHOULD</em> consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.
     1722               </p>
     1723            </div>
     1724            <div id="compress.coding">
     1725               <div id="rfc.iref.c.9"></div>
     1726               <div id="rfc.iref.c.10"></div>
     1727               <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a href="#compress.coding">Compress Coding</a></h3>
     1728               <p id="rfc.section.4.2.1.p.1">The "compress" format is produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch
     1729                  coding (LZW).
     1730               </p>
     1731            </div>
     1732            <div id="deflate.coding">
     1733               <div id="rfc.iref.d.2"></div>
     1734               <div id="rfc.iref.c.11"></div>
     1735               <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a href="#deflate.coding">Deflate Coding</a></h3>
     1736               <p id="rfc.section.4.2.2.p.1">The "deflate" format is defined as the "deflate" compression mechanism (described in <a href="#RFC1951" id="rfc.xref.RFC1951.1"><cite title="DEFLATE Compressed Data Format Specification version 1.3">[RFC1951]</cite></a>) used inside the "zlib" data format (<a href="#RFC1950" id="rfc.xref.RFC1950.1"><cite title="ZLIB Compressed Data Format Specification version 3.3">[RFC1950]</cite></a>).
     1737               </p>
     1738               <div class="note" id="rfc.section.4.2.2.p.2">
     1739                  <p><b>Note:</b> Some incorrect implementations send the "deflate" compressed data without the zlib wrapper.
     1740                  </p>
     1741               </div>
     1742            </div>
     1743            <div id="gzip.coding">
     1744               <div id="rfc.iref.g.72"></div>
     1745               <div id="rfc.iref.c.12"></div>
     1746               <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a href="#gzip.coding">Gzip Coding</a></h3>
     1747               <p id="rfc.section.4.2.3.p.1">The "gzip" format is produced by the file compression program "gzip" (GNU zip), as described in <a href="#RFC1952" id="rfc.xref.RFC1952.1"><cite title="GZIP file format specification version 4.3">[RFC1952]</cite></a>. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
     1748               </p>
     1749            </div>
     1750         </div>
     1751         <div id="header.te">
     1752            <div id="rfc.iref.t.5"></div>
     1753            <div id="rfc.iref.h.8"></div>
     1754            <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a href="#header.te">TE</a></h2>
     1755            <p id="rfc.section.4.3.p.1">The "TE" header field indicates what extension transfer-codings the client is willing to accept in the response, and whether
     1756               or not it is willing to accept trailer fields in a chunked transfer-coding.
     1757            </p>
     1758            <p id="rfc.section.4.3.p.2">Its value consists of the keyword "trailers" and/or a comma-separated list of extension transfer-coding names with optional
     1759               accept parameters (as described in <a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>).
     1760            </p>
     1761            <div id="rfc.figure.u.41"></div><pre class="inline"><span id="rfc.iref.g.73"></span><span id="rfc.iref.g.74"></span><span id="rfc.iref.g.75"></span><span id="rfc.iref.g.76"></span>  <a href="#header.te" class="smpl">TE</a>        = #<a href="#header.te" class="smpl">t-codings</a>
    16871762  <a href="#header.te" class="smpl">t-codings</a> = "trailers" / ( <a href="#transfer.codings" class="smpl">transfer-extension</a> [ <a href="#header.te" class="smpl">te-params</a> ] )
    16881763  <a href="#header.te" class="smpl">te-params</a> = <a href="#rule.whitespace" class="smpl">OWS</a> ";" <a href="#rule.whitespace" class="smpl">OWS</a> "q=" <a href="#quality.values" class="smpl">qvalue</a> *( <a href="#header.te" class="smpl">te-ext</a> )
    16891764  <a href="#header.te" class="smpl">te-ext</a>    = <a href="#rule.whitespace" class="smpl">OWS</a> ";" <a href="#rule.whitespace" class="smpl">OWS</a> <a href="#rule.token.separators" class="smpl">token</a> [ "=" <a href="#rule.token.separators" class="smpl">word</a> ]
    16901765</pre><p id="rfc.section.4.3.p.4">The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer-coding,
    1691          as defined in <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>. This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding.
    1692       </p>
    1693       <p id="rfc.section.4.3.p.5">Examples of its use are:</p>
    1694       <div id="rfc.figure.u.42"></div><pre class="text">  TE: deflate
     1766               as defined in <a href="#chunked.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a>. This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding.
     1767            </p>
     1768            <p id="rfc.section.4.3.p.5">Examples of its use are:</p>
     1769            <div id="rfc.figure.u.42"></div><pre class="text">  TE: deflate
    16951770  TE:
    16961771  TE: trailers, deflate;q=0.5
    16971772</pre><p id="rfc.section.4.3.p.7">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;6.1</a>) whenever TE is present in an HTTP/1.1 message.
    1698       </p>
    1699       <p id="rfc.section.4.3.p.8">A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules: </p>
    1700       <ol>
    1701          <li>
    1702             <p>The "chunked" transfer-coding is always acceptable. If the keyword "trailers" is listed, the client indicates that it is willing
    1703                to accept trailer fields in the chunked response on behalf of itself and any downstream clients. The implication is that,
    1704                if given, the client is stating that either all downstream clients are willing to accept trailer fields in the forwarded response,
    1705                or that it will attempt to buffer the response on behalf of downstream recipients.
    1706             </p>
    1707             <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
    1708                the entire response.
    1709             </p>
    1710          </li>
    1711          <li>
    1712             <p>If the transfer-coding being tested is one of the transfer-codings listed in the TE field, then it is acceptable unless it
    1713                is accompanied by a qvalue of 0. (As defined in <a href="#quality.values" title="Quality Values">Section&nbsp;4.3.1</a>, a qvalue of 0 means "not acceptable".)
    1714             </p>
    1715          </li>
    1716          <li>
    1717             <p>If multiple transfer-codings are acceptable, then the acceptable transfer-coding with the highest non-zero qvalue is preferred.
    1718                The "chunked" transfer-coding always has a qvalue of 1.
    1719             </p>
    1720          </li>
    1721       </ol>
    1722       <p id="rfc.section.4.3.p.9">If the TE field-value is empty or if no TE field is present, the only acceptable transfer-coding is "chunked". A message with
    1723          no transfer-coding is always acceptable.
    1724       </p>
    1725       <h3 id="rfc.section.4.3.1"><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;<a id="quality.values" href="#quality.values">Quality Values</a></h3>
    1726       <p id="rfc.section.4.3.1.p.1">Both transfer codings (TE request header field, <a href="#header.te" id="rfc.xref.header.te.3" title="TE">Section&nbsp;4.3</a>) and content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</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>) use short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight
    1727          is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. If a parameter has
    1728          a quality value of 0, then content with this parameter is "not acceptable" for the client. HTTP/1.1 applications <em class="bcp14">MUST NOT</em> generate more than three digits after the decimal point. User configuration of these values <em class="bcp14">SHOULD</em> also be limited in this fashion.
    1729       </p>
    1730       <div id="rfc.figure.u.43"></div><pre class="inline"><span id="rfc.iref.g.77"></span>  <a href="#quality.values" class="smpl">qvalue</a>         = ( "0" [ "." 0*3<a href="#core.rules" class="smpl">DIGIT</a> ] )
     1773            </p>
     1774            <p id="rfc.section.4.3.p.8">A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules: </p>
     1775            <ol>
     1776               <li>
     1777                  <p>The "chunked" transfer-coding is always acceptable. If the keyword "trailers" is listed, the client indicates that it is willing
     1778                     to accept trailer fields in the chunked response on behalf of itself and any downstream clients. The implication is that,
     1779                     if given, the client is stating that either all downstream clients are willing to accept trailer fields in the forwarded response,
     1780                     or that it will attempt to buffer the response on behalf of downstream recipients.
     1781                  </p>
     1782                  <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
     1783                     the entire response.
     1784                  </p>
     1785               </li>
     1786               <li>
     1787                  <p>If the transfer-coding being tested is one of the transfer-codings listed in the TE field, then it is acceptable unless it
     1788                     is accompanied by a qvalue of 0. (As defined in <a href="#quality.values" title="Quality Values">Section&nbsp;4.3.1</a>, a qvalue of 0 means "not acceptable".)
     1789                  </p>
     1790               </li>
     1791               <li>
     1792                  <p>If multiple transfer-codings are acceptable, then the acceptable transfer-coding with the highest non-zero qvalue is preferred.
     1793                     The "chunked" transfer-coding always has a qvalue of 1.
     1794                  </p>
     1795               </li>
     1796            </ol>
     1797            <p id="rfc.section.4.3.p.9">If the TE field-value is empty or if no TE field is present, the only acceptable transfer-coding is "chunked". A message with
     1798               no transfer-coding is always acceptable.
     1799            </p>
     1800            <div id="quality.values">
     1801               <h3 id="rfc.section.4.3.1"><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;<a href="#quality.values">Quality Values</a></h3>
     1802               <p id="rfc.section.4.3.1.p.1">Both transfer codings (TE request header field, <a href="#header.te" id="rfc.xref.header.te.3" title="TE">Section&nbsp;4.3</a>) and content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</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>) use short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight
     1803                  is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. If a parameter has
     1804                  a quality value of 0, then content with this parameter is "not acceptable" for the client. HTTP/1.1 applications <em class="bcp14">MUST NOT</em> generate more than three digits after the decimal point. User configuration of these values <em class="bcp14">SHOULD</em> also be limited in this fashion.
     1805               </p>
     1806               <div id="rfc.figure.u.43"></div><pre class="inline"><span id="rfc.iref.g.77"></span>  <a href="#quality.values" class="smpl">qvalue</a>         = ( "0" [ "." 0*3<a href="#core.rules" class="smpl">DIGIT</a> ] )
    17311807                 / ( "1" [ "." 0*3("0") ] )
    1732 </pre><div class="note" id="rfc.section.4.3.1.p.3">
    1733          <p> <b>Note:</b> "Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.
    1734          </p>
     1808</pre><div class="note" id="rfc.section.4.3.1.p.3">
     1809                  <p><b>Note:</b> "Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.
     1810                  </p>
     1811               </div>
     1812            </div>
     1813         </div>
     1814         <div id="header.trailer">
     1815            <div id="rfc.iref.t.6"></div>
     1816            <div id="rfc.iref.h.9"></div>
     1817            <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a href="#header.trailer">Trailer</a></h2>
     1818            <p id="rfc.section.4.4.p.1">The "Trailer" header field indicates that the given set of header fields is present in the trailer of a message encoded with
     1819               chunked transfer-coding.
     1820            </p>
     1821            <div id="rfc.figure.u.44"></div><pre class="inline"><span id="rfc.iref.g.78"></span>  <a href="#header.trailer" class="smpl">Trailer</a> = 1#<a href="#header.fields" class="smpl">field-name</a>
     1822</pre><p id="rfc.section.4.4.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
     1823               to know which header fields to expect in the trailer.
     1824            </p>
     1825            <p id="rfc.section.4.4.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.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a> for restrictions on the use of trailer fields in a "chunked" transfer-coding.
     1826            </p>
     1827            <p id="rfc.section.4.4.p.5">Message header fields listed in the Trailer header field <em class="bcp14">MUST NOT</em> include the following header fields:
     1828            </p>
     1829            <ul>
     1830               <li>Transfer-Encoding</li>
     1831               <li>Content-Length</li>
     1832               <li>Trailer</li>
     1833            </ul>
     1834         </div>
    17351835      </div>
    1736       <div id="rfc.iref.t.6"></div>
    1737       <div id="rfc.iref.h.9"></div>
    1738       <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a id="header.trailer" href="#header.trailer">Trailer</a></h2>
    1739       <p id="rfc.section.4.4.p.1">The "Trailer" header field indicates that the given set of header fields is present in the trailer of a message encoded with
    1740          chunked transfer-coding.
    1741       </p>
    1742       <div id="rfc.figure.u.44"></div><pre class="inline"><span id="rfc.iref.g.78"></span>  <a href="#header.trailer" class="smpl">Trailer</a> = 1#<a href="#header.fields" class="smpl">field-name</a>
    1743 </pre><p id="rfc.section.4.4.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
    1744          to know which header fields to expect in the trailer.
    1745       </p>
    1746       <p id="rfc.section.4.4.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.encoding" title="Chunked Transfer Coding">Section&nbsp;4.1</a> for restrictions on the use of trailer fields in a "chunked" transfer-coding.
    1747       </p>
    1748       <p id="rfc.section.4.4.p.5">Message header fields listed in the Trailer header field <em class="bcp14">MUST NOT</em> include the following header fields:
    1749       </p>
    1750       <ul>
    1751          <li>Transfer-Encoding</li>
    1752          <li>Content-Length</li>
    1753          <li>Trailer</li>
    1754       </ul>
    1755       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="message.routing" href="#message.routing">Message Routing</a></h1>
    1756       <p id="rfc.section.5.p.1">HTTP request message routing is determined by each client based on the target resource, the client's proxy configuration,
    1757          and establishment or reuse of an inbound connection. The corresponding response routing follows the same connection chain
    1758          back to the client.
    1759       </p>
    1760       <div id="rfc.iref.t.7"></div>
    1761       <div id="rfc.iref.t.8"></div>
    1762       <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="target-resource" href="#target-resource">Identifying a Target Resource</a></h2>
    1763       <p id="rfc.section.5.1.p.1">HTTP is used in a wide variety of applications, ranging from general-purpose computers to home appliances. In some cases,
    1764          communication options are hard-coded in a client's configuration. However, most HTTP clients rely on the same resource identification
    1765          mechanism and configuration techniques as general-purpose Web browsers.
    1766       </p>
    1767       <p id="rfc.section.5.1.p.2">HTTP communication is initiated by a user agent for some purpose. The purpose is a combination of request semantics, which
    1768          are defined in <a href="#Part2" id="rfc.xref.Part2.8"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, and a target resource upon which to apply those semantics. A URI reference (<a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;2.7</a>) is typically used as an identifier for the "target resource", which a user agent would resolve to its absolute form in order
    1769          to obtain the "target URI". The target URI excludes the reference's fragment identifier component, if any, since fragment
    1770          identifiers are reserved for client-side processing (<a href="#RFC3986" id="rfc.xref.RFC3986.18"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="http://tools.ietf.org/html/rfc3986#section-3.5">Section 3.5</a>).
    1771       </p>
    1772       <p id="rfc.section.5.1.p.3">HTTP intermediaries obtain the request semantics and target URI from the request-line of an incoming request message.</p>
    1773       <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="connecting.inbound" href="#connecting.inbound">Connecting Inbound</a></h2>
    1774       <p id="rfc.section.5.2.p.1">Once the target URI is determined, a client needs to decide whether a network request is necessary to accomplish the desired
    1775          semantics and, if so, where that request is to be directed.
    1776       </p>
    1777       <p id="rfc.section.5.2.p.2">If the client has a response cache and the request semantics can be satisfied by a cache (<a href="#Part6" id="rfc.xref.Part6.5"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>), then the request is usually directed to the cache first.
    1778       </p>
    1779       <p id="rfc.section.5.2.p.3">If the request is not satisfied by a cache, then a typical client will check its configuration to determine whether a proxy
    1780          is to be used to satisfy the request. Proxy configuration is implementation-dependent, but is often based on URI prefix matching,
    1781          selective authority matching, or both, and the proxy itself is usually identified by an "http" or "https" URI. If a proxy
    1782          is applicable, the client connects inbound by establishing (or reusing) a connection to that proxy.
    1783       </p>
    1784       <p id="rfc.section.5.2.p.4">If no proxy is applicable, a typical client will invoke a handler routine, usually specific to the target URI's scheme, to
    1785          connect directly to an authority for the target resource. How that is accomplished is dependent on the target URI scheme and
    1786          defined by its associated specification, similar to how this specification defines origin server access for resolution of
    1787          the "http" (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) and "https" (<a href="#https.uri" title="https URI scheme">Section&nbsp;2.7.2</a>) schemes.
    1788       </p>
    1789       <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a id="request-target" href="#request-target">Request Target</a></h2>
    1790       <p id="rfc.section.5.3.p.1">Once an inbound connection is obtained (<a href="#connection.management" title="Connection Management">Section&nbsp;6</a>), the client sends an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) with a request-target derived from the target URI. There are four distinct formats for the request-target, depending on
    1791          both the method being requested and whether the request is to a proxy.
    1792       </p>
    1793       <div id="rfc.figure.u.45"></div><pre class="inline"><span id="rfc.iref.g.79"></span><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>  <a href="#request-target" class="smpl">request-target</a> = <a href="#origin-form" class="smpl">origin-form</a>
     1836      <div id="message.routing">
     1837         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#message.routing">Message Routing</a></h1>
     1838         <p id="rfc.section.5.p.1">HTTP request message routing is determined by each client based on the target resource, the client's proxy configuration,
     1839            and establishment or reuse of an inbound connection. The corresponding response routing follows the same connection chain
     1840            back to the client.
     1841         </p>
     1842         <div id="target-resource">
     1843            <div id="rfc.iref.t.7"></div>
     1844            <div id="rfc.iref.t.8"></div>
     1845            <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a href="#target-resource">Identifying a Target Resource</a></h2>
     1846            <p id="rfc.section.5.1.p.1">HTTP is used in a wide variety of applications, ranging from general-purpose computers to home appliances. In some cases,
     1847               communication options are hard-coded in a client's configuration. However, most HTTP clients rely on the same resource identification
     1848               mechanism and configuration techniques as general-purpose Web browsers.
     1849            </p>
     1850            <p id="rfc.section.5.1.p.2">HTTP communication is initiated by a user agent for some purpose. The purpose is a combination of request semantics, which
     1851               are defined in <a href="#Part2" id="rfc.xref.Part2.8"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, and a target resource upon which to apply those semantics. A URI reference (<a href="#uri" title="Uniform Resource Identifiers">Section&nbsp;2.7</a>) is typically used as an identifier for the "target resource", which a user agent would resolve to its absolute form in order
     1852               to obtain the "target URI". The target URI excludes the reference's fragment identifier component, if any, since fragment
     1853               identifiers are reserved for client-side processing (<a href="#RFC3986" id="rfc.xref.RFC3986.18"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, <a href="https://tools.ietf.org/html/rfc3986#section-3.5">Section 3.5</a>).
     1854            </p>
     1855            <p id="rfc.section.5.1.p.3">HTTP intermediaries obtain the request semantics and target URI from the request-line of an incoming request message.</p>
     1856         </div>
     1857         <div id="connecting.inbound">
     1858            <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a href="#connecting.inbound">Connecting Inbound</a></h2>
     1859            <p id="rfc.section.5.2.p.1">Once the target URI is determined, a client needs to decide whether a network request is necessary to accomplish the desired
     1860               semantics and, if so, where that request is to be directed.
     1861            </p>
     1862            <p id="rfc.section.5.2.p.2">If the client has a response cache and the request semantics can be satisfied by a cache (<a href="#Part6" id="rfc.xref.Part6.5"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>), then the request is usually directed to the cache first.
     1863            </p>
     1864            <p id="rfc.section.5.2.p.3">If the request is not satisfied by a cache, then a typical client will check its configuration to determine whether a proxy
     1865               is to be used to satisfy the request. Proxy configuration is implementation-dependent, but is often based on URI prefix matching,
     1866               selective authority matching, or both, and the proxy itself is usually identified by an "http" or "https" URI. If a proxy
     1867               is applicable, the client connects inbound by establishing (or reusing) a connection to that proxy.
     1868            </p>
     1869            <p id="rfc.section.5.2.p.4">If no proxy is applicable, a typical client will invoke a handler routine, usually specific to the target URI's scheme, to
     1870               connect directly to an authority for the target resource. How that is accomplished is dependent on the target URI scheme and
     1871               defined by its associated specification, similar to how this specification defines origin server access for resolution of
     1872               the "http" (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>) and "https" (<a href="#https.uri" title="https URI scheme">Section&nbsp;2.7.2</a>) schemes.
     1873            </p>
     1874         </div>
     1875         <div id="request-target">
     1876            <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a href="#request-target">Request Target</a></h2>
     1877            <p id="rfc.section.5.3.p.1">Once an inbound connection is obtained (<a href="#connection.management" title="Connection Management">Section&nbsp;6</a>), the client sends an HTTP request message (<a href="#http.message" title="Message Format">Section&nbsp;3</a>) with a request-target derived from the target URI. There are four distinct formats for the request-target, depending on
     1878               both the method being requested and whether the request is to a proxy.
     1879            </p>
     1880            <div id="rfc.figure.u.45"></div><pre class="inline"><span id="rfc.iref.g.79"></span><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>  <a href="#request-target" class="smpl">request-target</a> = <a href="#origin-form" class="smpl">origin-form</a>
    17941881                 / <a href="#absolute-form" class="smpl">absolute-form</a>
    17951882                 / <a href="#authority-form" class="smpl">authority-form</a>
    … …  
    18011888  <a href="#asterisk-form" class="smpl">asterisk-form</a>  = "*"
    18021889</pre><div id="origin-form">
    1803          <p id="rfc.section.5.3.p.3"><span id="rfc.iref.o.3"></span> The most common form of request-target is the origin-form. When making a request directly to an origin server, other than
    1804             a CONNECT or server-wide OPTIONS request (as detailed below), a client <em class="bcp14">MUST</em> send only the absolute path and query components of the target URI as the request-target. If the target URI's path component
    1805             is empty, then the client <em class="bcp14">MUST</em> send "/" as the path within the origin-form of request-target. A Host header field is also sent, as defined in <a href="#header.host" id="rfc.xref.header.host.1" title="Host">Section&nbsp;5.4</a>, containing the target URI's authority component (excluding any userinfo).
    1806          </p>
    1807       </div>
    1808       <p id="rfc.section.5.3.p.4">For example, a client wishing to retrieve a representation of the resource identified as</p>
    1809       <div id="rfc.figure.u.46"></div><pre>http://www.example.org/where?q=now
     1890               <p id="rfc.section.5.3.p.3"><span id="rfc.iref.o.3"></span> The most common form of request-target is the origin-form. When making a request directly to an origin server, other than
     1891                  a CONNECT or server-wide OPTIONS request (as detailed below), a client <em class="bcp14">MUST</em> send only the absolute path and query components of the target URI as the request-target. If the target URI's path component
     1892                  is empty, then the client <em class="bcp14">MUST</em> send "/" as the path within the origin-form of request-target. A Host header field is also sent, as defined in <a href="#header.host" id="rfc.xref.header.host.1" title="Host">Section&nbsp;5.4</a>, containing the target URI's authority component (excluding any userinfo).
     1893               </p>
     1894            </div>
     1895            <p id="rfc.section.5.3.p.4">For example, a client wishing to retrieve a representation of the resource identified as</p>
     1896            <div id="rfc.figure.u.46"></div><pre>http://www.example.org/where?q=now
    18101897</pre><p id="rfc.section.5.3.p.6">directly from the origin server would open (or reuse) a TCP connection to port 80 of the host "www.example.org" and send the
    1811          lines:
    1812       </p>
    1813       <div id="rfc.figure.u.47"></div><pre class="text2">GET /where?q=now HTTP/1.1
     1898               lines:
     1899            </p>
     1900            <div id="rfc.figure.u.47"></div><pre class="text2">GET /where?q=now HTTP/1.1
    18141901Host: www.example.org
    18151902</pre><p id="rfc.section.5.3.p.8">followed by the remainder of the request message.</p>
    1816       <div id="absolute-form">
    1817          <p id="rfc.section.5.3.p.9"><span id="rfc.iref.a.2"></span> When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client <em class="bcp14">MUST</em> send the target URI in absolute-form as the request-target. The proxy is requested to either service that request from a valid
    1818             cache, if possible, or make the same request on the client's behalf to either the next inbound proxy server or directly to
    1819             the origin server indicated by the request-target. Requirements on such "forwarding" of messages are defined in <a href="#intermediary.forwarding" title="Intermediary Forwarding">Section&nbsp;5.6</a>.
    1820          </p>
    1821       </div>
    1822       <p id="rfc.section.5.3.p.10">An example absolute-form of request-line would be:</p>
    1823       <div id="rfc.figure.u.48"></div><pre class="text2">GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
     1903            <div id="absolute-form">
     1904               <p id="rfc.section.5.3.p.9"><span id="rfc.iref.a.2"></span> When making a request to a proxy, other than a CONNECT or server-wide OPTIONS request (as detailed below), a client <em class="bcp14">MUST</em> send the target URI in absolute-form as the request-target. The proxy is requested to either service that request from a valid
     1905                  cache, if possible, or make the same request on the client's behalf to either the next inbound proxy server or directly to
     1906                  the origin server indicated by the request-target. Requirements on such "forwarding" of messages are defined in <a href="#intermediary.forwarding" title="Intermediary Forwarding">Section&nbsp;5.6</a>.
     1907               </p>
     1908            </div>
     1909            <p id="rfc.section.5.3.p.10">An example absolute-form of request-line would be:</p>
     1910            <div id="rfc.figure.u.48"></div><pre class="text2">GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
    18241911</pre><p id="rfc.section.5.3.p.12">To allow for transition to the absolute-form for all requests in some future version of HTTP, HTTP/1.1 servers <em class="bcp14">MUST</em> accept the absolute-form in requests, even though HTTP/1.1 clients will only send them in requests to proxies.
    1825       </p>
    1826       <div id="authority-form">
    1827          <p id="rfc.section.5.3.p.13"><span id="rfc.iref.a.3"></span> The authority-form of request-target is only used for CONNECT requests (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 6.9</a> of <a href="#Part2" id="rfc.xref.Part2.9"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). When making a CONNECT request to establish a tunnel through one or more proxies, a client <em class="bcp14">MUST</em> send only the target URI's authority component (excluding any userinfo) as the request-target. For example,
    1828          </p>
    1829       </div>
    1830       <div id="rfc.figure.u.49"></div><pre class="text2">CONNECT www.example.com:80 HTTP/1.1
     1912            </p>
     1913            <div id="authority-form">
     1914               <p id="rfc.section.5.3.p.13"><span id="rfc.iref.a.3"></span> The authority-form of request-target is only used for CONNECT requests (<a href="p2-semantics.html#CONNECT" title="CONNECT">Section 6.9</a> of <a href="#Part2" id="rfc.xref.Part2.9"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). When making a CONNECT request to establish a tunnel through one or more proxies, a client <em class="bcp14">MUST</em> send only the target URI's authority component (excluding any userinfo) as the request-target. For example,
     1915               </p>
     1916            </div>
     1917            <div id="rfc.figure.u.49"></div><pre class="text2">CONNECT www.example.com:80 HTTP/1.1
    18311918</pre><div id="asterisk-form">
    1832          <p id="rfc.section.5.3.p.15"><span id="rfc.iref.a.4"></span> The asterisk-form of request-target is only used for a server-wide OPTIONS request (<a href="p2-semantics.html#OPTIONS" title="OPTIONS">Section 6.2</a> of <a href="#Part2" id="rfc.xref.Part2.10"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server,
    1833             the client <em class="bcp14">MUST</em> send only "*" (%x2A) as the request-target. For example,
    1834          </p>
    1835       </div>
    1836       <div id="rfc.figure.u.50"></div><pre class="text2">OPTIONS * HTTP/1.1
     1919               <p id="rfc.section.5.3.p.15"><span id="rfc.iref.a.4"></span> The asterisk-form of request-target is only used for a server-wide OPTIONS request (<a href="p2-semantics.html#OPTIONS" title="OPTIONS">Section 6.2</a> of <a href="#Part2" id="rfc.xref.Part2.10"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server,
     1920                  the client <em class="bcp14">MUST</em> send only "*" (%x2A) as the request-target. For example,
     1921               </p>
     1922            </div>
     1923            <div id="rfc.figure.u.50"></div><pre class="text2">OPTIONS * HTTP/1.1
    18371924</pre><p id="rfc.section.5.3.p.17">If a proxy receives an OPTIONS request with an absolute-form of request-target in which the URI has an empty path and no query
    1838          component, then the last proxy on the request chain <em class="bcp14">MUST</em> send a request-target of "*" when it forwards the request to the indicated origin server.
    1839       </p>
    1840       <div id="rfc.figure.u.51"></div>
    1841       <p>For example, the request</p><pre class="text2">OPTIONS http://www.example.org:8001 HTTP/1.1
     1925               component, then the last proxy on the request chain <em class="bcp14">MUST</em> send a request-target of "*" when it forwards the request to the indicated origin server.
     1926            </p>
     1927            <div id="rfc.figure.u.51"></div>
     1928            <p>For example, the request</p><pre class="text2">OPTIONS http://www.example.org:8001 HTTP/1.1
    18421929</pre><div id="rfc.figure.u.52"></div>
    1843       <p>would be forwarded by the final proxy as</p><pre class="text2">OPTIONS * HTTP/1.1
     1930            <p>would be forwarded by the final proxy as</p><pre class="text2">OPTIONS * HTTP/1.1
    18441931Host: www.example.org:8001
    1845 </pre>  <p>after connecting to port 8001 of host "www.example.org".</p>
    1846       <div id="rfc.iref.h.10"></div>
    1847       <div id="rfc.iref.h.11"></div>
    1848       <h2 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;<a id="header.host" href="#header.host">Host</a></h2>
    1849       <p id="rfc.section.5.4.p.1">The "Host" header field in a request provides the host and port information from the target URI, enabling the origin server
    1850          to distinguish among resources while servicing requests for multiple host names on a single IP address. Since the Host field-value
    1851          is critical information for handling a request, it <em class="bcp14">SHOULD</em> be sent as the first header field following the request-line.
    1852       </p>
    1853       <div id="rfc.figure.u.53"></div><pre class="inline"><span id="rfc.iref.g.84"></span>  <a href="#header.host" class="smpl">Host</a> = <a href="#uri" class="smpl">uri-host</a> [ ":" <a href="#uri" class="smpl">port</a> ] ; <a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>
     1932</pre><p>after connecting to port 8001 of host "www.example.org".</p>
     1933         </div>
     1934         <div id="header.host">
     1935            <div id="rfc.iref.h.10"></div>
     1936            <div id="rfc.iref.h.11"></div>
     1937            <h2 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;<a href="#header.host">Host</a></h2>
     1938            <p id="rfc.section.5.4.p.1">The "Host" header field in a request provides the host and port information from the target URI, enabling the origin server
     1939               to distinguish among resources while servicing requests for multiple host names on a single IP address. Since the Host field-value
     1940               is critical information for handling a request, it <em class="bcp14">SHOULD</em> be sent as the first header field following the request-line.
     1941            </p>
     1942            <div id="rfc.figure.u.53"></div><pre class="inline"><span id="rfc.iref.g.84"></span>  <a href="#header.host" class="smpl">Host</a> = <a href="#uri" class="smpl">uri-host</a> [ ":" <a href="#uri" class="smpl">port</a> ] ; <a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>
    18541943</pre><p id="rfc.section.5.4.p.3">A client <em class="bcp14">MUST</em> send a Host header field in all HTTP/1.1 request messages. If the target URI includes an authority component, then the Host
    1855          field-value <em class="bcp14">MUST</em> be identical to that authority component after excluding any userinfo (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>). If the authority component is missing or undefined for the target URI, then the Host header field <em class="bcp14">MUST</em> be sent with an empty field-value.
    1856       </p>
    1857       <p id="rfc.section.5.4.p.4">For example, a GET request to the origin server for &lt;http://www.example.org/pub/WWW/&gt; would begin with:</p>
    1858       <div id="rfc.figure.u.54"></div><pre class="text2">GET /pub/WWW/ HTTP/1.1
     1944               field-value <em class="bcp14">MUST</em> be identical to that authority component after excluding any userinfo (<a href="#http.uri" title="http URI scheme">Section&nbsp;2.7.1</a>). If the authority component is missing or undefined for the target URI, then the Host header field <em class="bcp14">MUST</em> be sent with an empty field-value.
     1945            </p>
     1946            <p id="rfc.section.5.4.p.4">For example, a GET request to the origin server for &lt;http://www.example.org/pub/WWW/&gt; would begin with:</p>
     1947            <div id="rfc.figure.u.54"></div><pre class="text2">GET /pub/WWW/ HTTP/1.1
    18591948Host: www.example.org
    18601949</pre><p id="rfc.section.5.4.p.6">The Host header field <em class="bcp14">MUST</em> be sent in an HTTP/1.1 request even if the request-target is in the absolute-form, since this allows the Host information
    1861          to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.
    1862       </p>
    1863       <p id="rfc.section.5.4.p.7">When an HTTP/1.1 proxy receives a request with an absolute-form of request-target, the proxy <em class="bcp14">MUST</em> ignore the received Host header field (if any) and instead replace it with the host information of the request-target. If
    1864          the proxy forwards the request, it <em class="bcp14">MUST</em> generate a new Host field-value based on the received request-target rather than forward the received Host field-value.
    1865       </p>
    1866       <p id="rfc.section.5.4.p.8">Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to
    1867          poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it
    1868          relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache,
    1869          without first verifying that the intercepted connection is targeting a valid IP address for that host.
    1870       </p>
    1871       <p id="rfc.section.5.4.p.9">A server <em class="bcp14">MUST</em> respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request
    1872          message that contains more than one Host header field or a Host header field with an invalid field-value.
    1873       </p>
    1874       <div id="rfc.iref.e.1"></div>
    1875       <h2 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;<a id="effective.request.uri" href="#effective.request.uri">Effective Request URI</a></h2>
    1876       <p id="rfc.section.5.5.p.1">A server that receives an HTTP request message <em class="bcp14">MUST</em> reconstruct the user agent's original target URI, based on the pieces of information learned from the request-target, Host,
    1877          and connection context, in order to identify the intended target resource and properly service the request. The URI derived
    1878          from this reconstruction process is referred to as the "effective request URI".
    1879       </p>
    1880       <p id="rfc.section.5.5.p.2">For a user agent, the effective request URI is the target URI.</p>
    1881       <p id="rfc.section.5.5.p.3">If the request-target is in absolute-form, then the effective request URI is the same as the request-target. Otherwise, the
    1882          effective request URI is constructed as follows.
    1883       </p>
    1884       <p id="rfc.section.5.5.p.4">If the request is received over an SSL/TLS-secured TCP connection, then the effective request URI's scheme is "https"; otherwise,
    1885          the scheme is "http".
    1886       </p>
    1887       <p id="rfc.section.5.5.p.5">If the request-target is in authority-form, then the effective request URI's authority component is the same as the request-target.
    1888          Otherwise, if a Host header field is supplied with a non-empty field-value, then the authority component is the same as the
    1889          Host field-value. Otherwise, the authority component is the concatenation of the default hostname configured for the server,
    1890          a colon (":"), and the connection's incoming TCP port number in decimal form.
    1891       </p>
    1892       <p id="rfc.section.5.5.p.6">If the request-target is in authority-form or asterisk-form, then the effective request URI's combined path and query component
    1893          is empty. Otherwise, the combined path and query component is the same as the request-target.
    1894       </p>
    1895       <p id="rfc.section.5.5.p.7">The components of the effective request URI, once determined as above, can be combined into absolute-URI form by concatenating
    1896          the scheme, "://", authority, and combined path and query component.
    1897       </p>
    1898       <div id="rfc.figure.u.55"></div>
    1899       <p>Example 1: the following message received over an insecure TCP connection</p>  <pre class="text">GET /pub/WWW/TheProject.html HTTP/1.1
     1950               to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.
     1951            </p>
     1952            <p id="rfc.section.5.4.p.7">When an HTTP/1.1 proxy receives a request with an absolute-form of request-target, the proxy <em class="bcp14">MUST</em> ignore the received Host header field (if any) and instead replace it with the host information of the request-target. If
     1953               the proxy forwards the request, it <em class="bcp14">MUST</em> generate a new Host field-value based on the received request-target rather than forward the received Host field-value.
     1954            </p>
     1955            <p id="rfc.section.5.4.p.8">Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to
     1956               poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it
     1957               relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache,
     1958               without first verifying that the intercepted connection is targeting a valid IP address for that host.
     1959            </p>
     1960            <p id="rfc.section.5.4.p.9">A server <em class="bcp14">MUST</em> respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request
     1961               message that contains more than one Host header field or a Host header field with an invalid field-value.
     1962            </p>
     1963         </div>
     1964         <div id="effective.request.uri">
     1965            <div id="rfc.iref.e.1"></div>
     1966            <h2 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;<a href="#effective.request.uri">Effective Request URI</a></h2>
     1967            <p id="rfc.section.5.5.p.1">A server that receives an HTTP request message <em class="bcp14">MUST</em> reconstruct the user agent's original target URI, based on the pieces of information learned from the request-target, Host,
     1968               and connection context, in order to identify the intended target resource and properly service the request. The URI derived
     1969               from this reconstruction process is referred to as the "effective request URI".
     1970            </p>
     1971            <p id="rfc.section.5.5.p.2">For a user agent, the effective request URI is the target URI.</p>
     1972            <p id="rfc.section.5.5.p.3">If the request-target is in absolute-form, then the effective request URI is the same as the request-target. Otherwise, the
     1973               effective request URI is constructed as follows.
     1974            </p>
     1975            <p id="rfc.section.5.5.p.4">If the request is received over an SSL/TLS-secured TCP connection, then the effective request URI's scheme is "https"; otherwise,
     1976               the scheme is "http".
     1977            </p>
     1978            <p id="rfc.section.5.5.p.5">If the request-target is in authority-form, then the effective request URI's authority component is the same as the request-target.
     1979               Otherwise, if a Host header field is supplied with a non-empty field-value, then the authority component is the same as the
     1980               Host field-value. Otherwise, the authority component is the concatenation of the default hostname configured for the server,
     1981               a colon (":"), and the connection's incoming TCP port number in decimal form.
     1982            </p>
     1983            <p id="rfc.section.5.5.p.6">If the request-target is in authority-form or asterisk-form, then the effective request URI's combined path and query component
     1984               is empty. Otherwise, the combined path and query component is the same as the request-target.
     1985            </p>
     1986            <p id="rfc.section.5.5.p.7">The components of the effective request URI, once determined as above, can be combined into absolute-URI form by concatenating
     1987               the scheme, "://", authority, and combined path and query component.
     1988            </p>
     1989            <div id="rfc.figure.u.55"></div>
     1990            <p>Example 1: the following message received over an insecure TCP connection</p><pre class="text">GET /pub/WWW/TheProject.html HTTP/1.1
    19001991Host: www.example.org:8080
    1901 </pre> <div id="rfc.figure.u.56"></div>
    1902       <p>has an effective request URI of</p>  <pre class="text">http://www.example.org:8080/pub/WWW/TheProject.html
    1903 </pre> <div id="rfc.figure.u.57"></div>
    1904       <p>Example 2: the following message received over an SSL/TLS-secured TCP connection</p>  <pre class="text">OPTIONS * HTTP/1.1
     1992</pre><div id="rfc.figure.u.56"></div>
     1993            <p>has an effective request URI of</p><pre class="text">http://www.example.org:8080/pub/WWW/TheProject.html
     1994</pre><div id="rfc.figure.u.57"></div>
     1995            <p>Example 2: the following message received over an SSL/TLS-secured TCP connection</p><pre class="text">OPTIONS * HTTP/1.1
    19051996Host: www.example.org
    1906 </pre> <div id="rfc.figure.u.58"></div>
    1907       <p>has an effective request URI of</p>  <pre class="text">https://www.example.org
    1908 </pre> <p id="rfc.section.5.5.p.12">An origin server that does not allow resources to differ by requested host <em class="bcp14">MAY</em> ignore the Host field-value and instead replace it with a configured server name when constructing the effective request URI.
    1909       </p>
    1910       <p id="rfc.section.5.5.p.13">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 guess
    1911          the effective request URI's authority component.
    1912       </p>
    1913       <h2 id="rfc.section.5.6"><a href="#rfc.section.5.6">5.6</a>&nbsp;<a id="intermediary.forwarding" href="#intermediary.forwarding">Intermediary Forwarding</a></h2>
    1914       <p id="rfc.section.5.6.p.1">As described in <a href="#intermediaries" title="Intermediaries">Section&nbsp;2.3</a>, intermediaries can serve a variety of roles in the processing of HTTP requests and responses. Some intermediaries are used
    1915          to improve performance or availability. Others are used for access control or to filter content. Since an HTTP stream has
    1916          characteristics similar to a pipe-and-filter architecture, there are no inherent limits to the extent an intermediary can
    1917          enhance (or interfere) with either direction of the stream.
    1918       </p>
    1919       <p id="rfc.section.5.6.p.2">In order to avoid request loops, a proxy that forwards requests to other proxies <em class="bcp14">MUST</em> be able to recognize and exclude all of its own server names, including any aliases, local variations, or literal IP addresses.
    1920       </p>
    1921       <p id="rfc.section.5.6.p.3">If a proxy receives a request-target with a host name that is not a fully qualified domain name, it <em class="bcp14">MAY</em> add its domain to the host name it received when forwarding the request. A proxy <em class="bcp14">MUST NOT</em> change the host name if it is a fully qualified domain name.
    1922       </p>
    1923       <p id="rfc.section.5.6.p.4">A non-transforming proxy <em class="bcp14">MUST NOT</em> rewrite the "path-absolute" and "query" parts of the received request-target when forwarding it to the next inbound server,
    1924          except as noted above to replace an empty path with "/" or "*".
    1925       </p>
    1926       <p id="rfc.section.5.6.p.5">Intermediaries that forward a message <em class="bcp14">MUST</em> implement the Connection header field as specified in <a href="#header.connection" id="rfc.xref.header.connection.5" title="Connection">Section&nbsp;6.1</a>.
    1927       </p>
    1928       <h3 id="rfc.section.5.6.1"><a href="#rfc.section.5.6.1">5.6.1</a>&nbsp;<a id="end-to-end.and.hop-by-hop.header-fields" href="#end-to-end.and.hop-by-hop.header-fields">End-to-end and Hop-by-hop Header Fields</a></h3>
    1929       <p id="rfc.section.5.6.1.p.1">For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP header fields into two categories: </p>
    1930       <ul>
    1931          <li>End-to-end header fields, which are transmitted to the ultimate recipient of a request or response. End-to-end header fields
    1932             in responses <em class="bcp14">MUST</em> be stored as part of a cache entry and <em class="bcp14">MUST</em> be transmitted in any response formed from a cache entry.
    1933          </li>
    1934          <li>Hop-by-hop header fields, which are meaningful only for a single transport-level connection, and are not stored by caches
    1935             or forwarded by proxies.
    1936          </li>
    1937       </ul>
    1938       <p id="rfc.section.5.6.1.p.2">The following HTTP/1.1 header fields are hop-by-hop header fields: </p>
    1939       <ul>
    1940          <li>Connection</li>
    1941          <li>Keep-Alive</li>
    1942          <li>Proxy-Authenticate</li>
    1943          <li>Proxy-Authorization</li>
    1944          <li>TE</li>
    1945          <li>Trailer</li>
    1946          <li>Transfer-Encoding</li>
    1947          <li>Upgrade</li>
    1948       </ul>
    1949       <p id="rfc.section.5.6.1.p.3">All other header fields defined by HTTP/1.1 are end-to-end header fields.</p>
    1950       <p id="rfc.section.5.6.1.p.4">Other hop-by-hop header fields <em class="bcp14">MUST</em> be listed in a Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.6" title="Connection">Section&nbsp;6.1</a>).
    1951       </p>
    1952       <h3 id="rfc.section.5.6.2"><a href="#rfc.section.5.6.2">5.6.2</a>&nbsp;<a id="non-modifiable.header-fields" href="#non-modifiable.header-fields">Non-modifiable Header Fields</a></h3>
    1953       <p id="rfc.section.5.6.2.p.1">Some features of HTTP/1.1, such as Digest Authentication, depend on the value of certain end-to-end header fields. A non-transforming
    1954          proxy <em class="bcp14">SHOULD NOT</em> modify an end-to-end header field unless the definition of that header field requires or specifically allows that.
    1955       </p>
    1956       <p id="rfc.section.5.6.2.p.2">A non-transforming proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a request or response, and it <em class="bcp14">MUST NOT</em> add any of these fields if not already present:
    1957       </p>
    1958       <ul>
    1959          <li>Allow</li>
    1960          <li>Content-Location</li>
    1961          <li>Content-MD5</li>
    1962          <li>ETag</li>
    1963          <li>Last-Modified</li>
    1964          <li>Server</li>
    1965       </ul>
    1966       <p id="rfc.section.5.6.2.p.3">A non-transforming proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a response:
    1967       </p>
    1968       <ul>
    1969          <li>Expires</li>
    1970       </ul>
    1971       <p id="rfc.section.5.6.2.p.4">but it <em class="bcp14">MAY</em> add any of these fields if not already present. If an Expires header field is added, it <em class="bcp14">MUST</em> be given a field-value identical to that of the Date header field in that response.
    1972       </p>
    1973       <p id="rfc.section.5.6.2.p.5">A proxy <em class="bcp14">MUST NOT</em> modify or add any of the following fields in a message that contains the no-transform cache-control directive, or in any request:
    1974       </p>
    1975       <ul>
    1976          <li>Content-Encoding</li>
    1977          <li>Content-Range</li>
    1978          <li>Content-Type</li>
    1979       </ul>
    1980       <p id="rfc.section.5.6.2.p.6">A transforming proxy <em class="bcp14">MAY</em> modify or add these fields to a message that does not include no-transform, but if it does so, it <em class="bcp14">MUST</em> add a Warning 214 (Transformation applied) if one does not already appear in the message (see <a href="p6-cache.html#header.warning" title="Warning">Section 3.6</a> of <a href="#Part6" id="rfc.xref.Part6.6"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>).
    1981       </p>
    1982       <div class="note" id="rfc.section.5.6.2.p.7">
    1983          <p> <b>Warning:</b> Unnecessary modification of end-to-end header fields might cause authentication failures if stronger authentication mechanisms
    1984             are introduced in later versions of HTTP. Such authentication mechanisms <em class="bcp14">MAY</em> rely on the values of header fields not listed here.
    1985          </p>
     1997</pre><div id="rfc.figure.u.58"></div>
     1998            <p>has an effective request URI of</p><pre class="text">https://www.example.org
     1999</pre><p id="rfc.section.5.5.p.12">An origin server that does not allow resources to differ by requested host <em class="bcp14">MAY</em> ignore the Host field-value and instead replace it with a configured server name when constructing the effective request URI.
     2000            </p>
     2001            <p id="rfc.section.5.5.p.13">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 guess
     2002               the effective request URI's authority component.
     2003            </p>
     2004         </div>
     2005         <div id="intermediary.forwarding">
     2006            <h2 id="rfc.section.5.6"><a href="#rfc.section.5.6">5.6</a>&nbsp;<a href="#intermediary.forwarding">Intermediary Forwarding</a></h2>
     2007            <p id="rfc.section.5.6.p.1">As described in <a href="#intermediaries" title="Intermediaries">Section&nbsp;2.3</a>, intermediaries can serve a variety of roles in the processing of HTTP requests and responses. Some intermediaries are used
     2008               to improve performance or availability. Others are used for access control or to filter content. Since an HTTP stream has
     2009               characteristics similar to a pipe-and-filter architecture, there are no inherent limits to the extent an intermediary can
     2010               enhance (or interfere) with either direction of the stream.
     2011            </p>
     2012            <p id="rfc.section.5.6.p.2">In order to avoid request loops, a proxy that forwards requests to other proxies <em class="bcp14">MUST</em> be able to recognize and exclude all of its own server names, including any aliases, local variations, or literal IP addresses.
     2013            </p>
     2014            <p id="rfc.section.5.6.p.3">If a proxy receives a request-target with a host name that is not a fully qualified domain name, it <em class="bcp14">MAY</em> add its domain to the host name it received when forwarding the request. A proxy <em class="bcp14">MUST NOT</em> change the host name if it is a fully qualified domain name.
     2015            </p>
     2016            <p id="rfc.section.5.6.p.4">A non-transforming proxy <em class="bcp14">MUST NOT</em> rewrite the "path-absolute" and "query" parts of the received request-target when forwarding it to the next inbound server,
     2017               except as noted above to replace an empty path with "/" or "*".
     2018            </p>
     2019            <p id="rfc.section.5.6.p.5">Intermediaries that forward a message <em class="bcp14">MUST</em> implement the Connection header field as specified in <a href="#header.connection" id="rfc.xref.header.connection.5" title="Connection">Section&nbsp;6.1</a>.
     2020            </p>
     2021            <div id="end-to-end.and.hop-by-hop.header-fields">
     2022               <h3 id="rfc.section.5.6.1"><a href="#rfc.section.5.6.1">5.6.1</a>&nbsp;<a href="#end-to-end.and.hop-by-hop.header-fields">End-to-end and Hop-by-hop Header Fields</a></h3>
     2023               <p id="rfc.section.5.6.1.p.1">For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP header fields into two categories: </p>
     2024               <ul>
     2025                  <li>End-to-end header fields, which are transmitted to the ultimate recipient of a request or response. End-to-end header fields
     2026                     in responses <em class="bcp14">MUST</em> be stored as part of a cache entry and <em class="bcp14">MUST</em> be transmitted in any response formed from a cache entry.
     2027                  </li>
     2028                  <li>Hop-by-hop header fields, which are meaningful only for a single transport-level connection, and are not stored by caches
     2029                     or forwarded by proxies.
     2030                  </li>
     2031               </ul>
     2032               <p id="rfc.section.5.6.1.p.2">The following HTTP/1.1 header fields are hop-by-hop header fields: </p>
     2033               <ul>
     2034                  <li>Connection</li>
     2035                  <li>Keep-Alive</li>
     2036                  <li>Proxy-Authenticate</li>
     2037                  <li>Proxy-Authorization</li>
     2038                  <li>TE</li>
     2039                  <li>Trailer</li>
     2040                  <li>Transfer-Encoding</li>
     2041                  <li>Upgrade</li>
     2042               </ul>
     2043               <p id="rfc.section.5.6.1.p.3">All other header fields defined by HTTP/1.1 are end-to-end header fields.</p>
     2044               <p id="rfc.section.5.6.1.p.4">Other hop-by-hop header fields <em class="bcp14">MUST</em> be listed in a Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.6" title="Connection">Section&nbsp;6.1</a>).
     2045               </p>
     2046            </div>
     2047            <div id="non-modifiable.header-fields">
     2048               <h3 id="rfc.section.5.6.2"><a href="#rfc.section.5.6.2">5.6.2</a>&nbsp;<a href="#non-modifiable.header-fields">Non-modifiable Header Fields</a></h3>
     2049               <p id="rfc.section.5.6.2.p.1">Some features of HTTP/1.1, such as Digest Authentication, depend on the value of certain end-to-end header fields. A non-transforming
     2050                  proxy <em class="bcp14">SHOULD NOT</em> modify an end-to-end header field unless the definition of that header field requires or specifically allows that.
     2051               </p>
     2052               <p id="rfc.section.5.6.2.p.2">A non-transforming proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a request or response, and it <em class="bcp14">MUST NOT</em> add any of these fields if not already present:
     2053               </p>
     2054               <ul>
     2055                  <li>Allow</li>
     2056                  <li>Content-Location</li>
     2057                  <li>Content-MD5</li>
     2058                  <li>ETag</li>
     2059                  <li>Last-Modified</li>
     2060                  <li>Server</li>
     2061               </ul>
     2062               <p id="rfc.section.5.6.2.p.3">A non-transforming proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a response:
     2063               </p>
     2064               <ul>
     2065                  <li>Expires</li>
     2066               </ul>
     2067               <p id="rfc.section.5.6.2.p.4">but it <em class="bcp14">MAY</em> add any of these fields if not already present. If an Expires header field is added, it <em class="bcp14">MUST</em> be given a field-value identical to that of the Date header field in that response.
     2068               </p>
     2069               <p id="rfc.section.5.6.2.p.5">A proxy <em class="bcp14">MUST NOT</em> modify or add any of the following fields in a message that contains the no-transform cache-control directive, or in any request:
     2070               </p>
     2071               <ul>
     2072                  <li>Content-Encoding</li>
     2073                  <li>Content-Range</li>
     2074                  <li>Content-Type</li>
     2075               </ul>
     2076               <p id="rfc.section.5.6.2.p.6">A transforming proxy <em class="bcp14">MAY</em> modify or add these fields to a message that does not include no-transform, but if it does so, it <em class="bcp14">MUST</em> add a Warning 214 (Transformation applied) if one does not already appear in the message (see <a href="p6-cache.html#header.warning" title="Warning">Section 3.6</a> of <a href="#Part6" id="rfc.xref.Part6.6"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>).
     2077               </p>
     2078               <div class="note" id="rfc.section.5.6.2.p.7">
     2079                  <p><b>Warning:</b> Unnecessary modification of end-to-end header fields might cause authentication failures if stronger authentication mechanisms
     2080                     are introduced in later versions of HTTP. Such authentication mechanisms <em class="bcp14">MAY</em> rely on the values of header fields not listed here.
     2081                  </p>
     2082               </div>
     2083               <p id="rfc.section.5.6.2.p.8">A non-transforming proxy <em class="bcp14">MUST</em> preserve the message payload (<a href="#Part3" id="rfc.xref.Part3.4"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), though it <em class="bcp14">MAY</em> change the message body through application or removal of a transfer-coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>).
     2084               </p>
     2085            </div>
     2086         </div>
     2087         <div id="associating.response.to.request">
     2088            <h2 id="rfc.section.5.7"><a href="#rfc.section.5.7">5.7</a>&nbsp;<a href="#associating.response.to.request">Associating a Response to a Request</a></h2>
     2089            <p id="rfc.section.5.7.p.1">HTTP does not include a request identifier for associating a given request message with its corresponding one or more response
     2090               messages. Hence, it relies on the order of response arrival to correspond exactly to the order in which requests are made
     2091               on the same connection. More than one response message per request only occurs when one or more informational responses (1xx,
     2092               see <a href="p2-semantics.html#status.1xx" title="Informational 1xx">Section 7.1</a> of <a href="#Part2" id="rfc.xref.Part2.11"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) precede a final response to the same request.
     2093            </p>
     2094            <p id="rfc.section.5.7.p.2">A client that uses persistent connections and sends more than one request per connection <em class="bcp14">MUST</em> maintain a list of outstanding requests in the order sent on that connection and <em class="bcp14">MUST</em> associate each received response message to the highest ordered request that has not yet received a final (non-1xx) response.
     2095            </p>
     2096         </div>
    19862097      </div>
    1987       <p id="rfc.section.5.6.2.p.8">A non-transforming proxy <em class="bcp14">MUST</em> preserve the message payload (<a href="#Part3" id="rfc.xref.Part3.4"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), though it <em class="bcp14">MAY</em> change the message body through application or removal of a transfer-coding (<a href="#transfer.codings" title="Transfer Codings">Section&nbsp;4</a>).
    1988       </p>
    1989       <h2 id="rfc.section.5.7"><a href="#rfc.section.5.7">5.7</a>&nbsp;<a id="associating.response.to.request" href="#associating.response.to.request">Associating a Response to a Request</a></h2>
    1990       <p id="rfc.section.5.7.p.1">HTTP does not include a request identifier for associating a given request message with its corresponding one or more response
    1991          messages. Hence, it relies on the order of response arrival to correspond exactly to the order in which requests are made
    1992          on the same connection. More than one response message per request only occurs when one or more informational responses (1xx,
    1993          see <a href="p2-semantics.html#status.1xx" title="Informational 1xx">Section 7.1</a> of <a href="#Part2" id="rfc.xref.Part2.11"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) precede a final response to the same request.
    1994       </p>
    1995       <p id="rfc.section.5.7.p.2">A client that uses persistent connections and sends more than one request per connection <em class="bcp14">MUST</em> maintain a list of outstanding requests in the order sent on that connection and <em class="bcp14">MUST</em> associate each received response message to the highest ordered request that has not yet received a final (non-1xx) response.
    1996       </p>
    1997       <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="connection.management" href="#connection.management">Connection Management</a></h1>
    1998       <div id="rfc.iref.c.13"></div>
    1999       <div id="rfc.iref.h.12"></div>
    2000       <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="header.connection" href="#header.connection">Connection</a></h2>
    2001       <p id="rfc.section.6.1.p.1">The "Connection" header field allows the sender to specify options that are desired only for that particular connection. Such
    2002          connection options <em class="bcp14">MUST</em> be removed or replaced before the message can be forwarded downstream by a proxy or gateway. This mechanism also allows the
    2003          sender to indicate which HTTP header fields used in the message are only intended for the immediate recipient ("hop-by-hop"),
    2004          as opposed to all recipients on the chain ("end-to-end"), enabling the message to be self-descriptive and allowing future
    2005          connection-specific extensions to be deployed in HTTP without fear that they will be blindly forwarded by previously deployed
    2006          intermediaries.
    2007       </p>
    2008       <p id="rfc.section.6.1.p.2">The Connection header field's value has the following grammar:</p>
    2009       <div id="rfc.figure.u.59"></div><pre class="inline"><span id="rfc.iref.g.85"></span><span id="rfc.iref.g.86"></span>  <a href="#header.connection" class="smpl">Connection</a>       = 1#<a href="#header.connection" class="smpl">connection-token</a>
     2098      <div id="connection.management">
     2099         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#connection.management">Connection Management</a></h1>
     2100         <div id="header.connection">
     2101            <div id="rfc.iref.c.13"></div>
     2102            <div id="rfc.iref.h.12"></div>
     2103            <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a href="#header.connection">Connection</a></h2>
     2104            <p id="rfc.section.6.1.p.1">The "Connection" header field allows the sender to specify options that are desired only for that particular connection. Such
     2105               connection options <em class="bcp14">MUST</em> be removed or replaced before the message can be forwarded downstream by a proxy or gateway. This mechanism also allows the
     2106               sender to indicate which HTTP header fields used in the message are only intended for the immediate recipient ("hop-by-hop"),
     2107               as opposed to all recipients on the chain ("end-to-end"), enabling the message to be self-descriptive and allowing future
     2108               connection-specific extensions to be deployed in HTTP without fear that they will be blindly forwarded by previously deployed
     2109               intermediaries.
     2110            </p>
     2111            <p id="rfc.section.6.1.p.2">The Connection header field's value has the following grammar:</p>
     2112            <div id="rfc.figure.u.59"></div><pre class="inline"><span id="rfc.iref.g.85"></span><span id="rfc.iref.g.86"></span>  <a href="#header.connection" class="smpl">Connection</a>       = 1#<a href="#header.connection" class="smpl">connection-token</a>
    20102113  <a href="#header.connection" class="smpl">connection-token</a> = <a href="#rule.token.separators" class="smpl">token</a>
    20112114</pre><p id="rfc.section.6.1.p.4">A proxy or gateway <em class="bcp14">MUST</em> parse a received Connection header field before a message is forwarded and, for each connection-token in this field, remove
    2012          any header field(s) from the message with the same name as the connection-token, and then remove the Connection header field
    2013          itself or replace it with the sender's own connection options for the forwarded message.
    2014       </p>
    2015       <p id="rfc.section.6.1.p.5">A sender <em class="bcp14">MUST NOT</em> include field-names in the Connection header field-value for fields that are defined as expressing constraints for all recipients
    2016          in the request or response chain, such as the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 3.2</a> of <a href="#Part6" id="rfc.xref.Part6.7"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>).
    2017       </p>
    2018       <p id="rfc.section.6.1.p.6">The connection options do not have to correspond to a header field present in the message, since a connection-specific header
    2019          field might not be needed if there are no parameters associated with that connection option. Recipients that trigger certain
    2020          connection behavior based on the presence of connection options <em class="bcp14">MUST</em> do so based on the presence of the connection-token rather than only the presence of the optional header field. In other words,
    2021          if the connection option is received as a header field but not indicated within the Connection field-value, then the recipient <em class="bcp14">MUST</em> ignore the connection-specific header field because it has likely been forwarded by an intermediary that is only partially
    2022          conformant.
    2023       </p>
    2024       <p id="rfc.section.6.1.p.7">When defining new connection options, specifications ought to carefully consider existing deployed header fields and ensure
    2025          that the new connection-token does not share the same name as an unrelated header field that might already be deployed. Defining
    2026          a new connection-token essentially reserves that potential field-name for carrying additional information related to the connection
    2027          option, since it would be unwise for senders to use that field-name for anything else.
    2028       </p>
    2029       <p id="rfc.section.6.1.p.8">HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion
    2030          of the response. For example,
    2031       </p>
    2032       <div id="rfc.figure.u.60"></div><pre class="text">  Connection: close
     2115               any header field(s) from the message with the same name as the connection-token, and then remove the Connection header field
     2116               itself or replace it with the sender's own connection options for the forwarded message.
     2117            </p>
     2118            <p id="rfc.section.6.1.p.5">A sender <em class="bcp14">MUST NOT</em> include field-names in the Connection header field-value for fields that are defined as expressing constraints for all recipients
     2119               in the request or response chain, such as the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 3.2</a> of <a href="#Part6" id="rfc.xref.Part6.7"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>).
     2120            </p>
     2121            <p id="rfc.section.6.1.p.6">The connection options do not have to correspond to a header field present in the message, since a connection-specific header
     2122               field might not be needed if there are no parameters associated with that connection option. Recipients that trigger certain
     2123               connection behavior based on the presence of connection options <em class="bcp14">MUST</em> do so based on the presence of the connection-token rather than only the presence of the optional header field. In other words,
     2124               if the connection option is received as a header field but not indicated within the Connection field-value, then the recipient <em class="bcp14">MUST</em> ignore the connection-specific header field because it has likely been forwarded by an intermediary that is only partially
     2125               conformant.
     2126            </p>
     2127            <p id="rfc.section.6.1.p.7">When defining new connection options, specifications ought to carefully consider existing deployed header fields and ensure
     2128               that the new connection-token does not share the same name as an unrelated header field that might already be deployed. Defining
     2129               a new connection-token essentially reserves that potential field-name for carrying additional information related to the connection
     2130               option, since it would be unwise for senders to use that field-name for anything else.
     2131            </p>
     2132            <p id="rfc.section.6.1.p.8">HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion
     2133               of the response. For example,
     2134            </p>
     2135            <div id="rfc.figure.u.60"></div><pre class="text">  Connection: close
    20332136</pre><p id="rfc.section.6.1.p.10">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;6.3</a>) after the current request/response is complete.
    2034       </p>
    2035       <p id="rfc.section.6.1.p.11">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.
    2036       </p>
    2037       <p id="rfc.section.6.1.p.12">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.
    2038       </p>
    2039       <div id="rfc.iref.v.1"></div>
    2040       <div id="rfc.iref.h.13"></div>
    2041       <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="header.via" href="#header.via">Via</a></h2>
    2042       <p id="rfc.section.6.2.p.1">The "Via" header field <em class="bcp14">MUST</em> be sent by a proxy or gateway to indicate the intermediate protocols and recipients between the user agent and the server
    2043          on requests, and between the origin server and the client on responses. It is analogous to the "Received" field used by email
    2044          systems (<a href="http://tools.ietf.org/html/rfc5322#section-3.6.7">Section 3.6.7</a> of <a href="#RFC5322" id="rfc.xref.RFC5322.3"><cite title="Internet Message Format">[RFC5322]</cite></a>) and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities
    2045          of all senders along the request/response chain.
    2046       </p>
    2047       <div id="rfc.figure.u.61"></div><pre class="inline"><span id="rfc.iref.g.87"></span><span id="rfc.iref.g.88"></span><span id="rfc.iref.g.89"></span><span id="rfc.iref.g.90"></span><span id="rfc.iref.g.91"></span><span id="rfc.iref.g.92"></span>  <a href="#header.via" class="smpl">Via</a>               = 1#( <a href="#header.via" class="smpl">received-protocol</a> <a href="#rule.whitespace" class="smpl">RWS</a> <a href="#header.via" class="smpl">received-by</a>
     2137            </p>
     2138            <p id="rfc.section.6.1.p.11">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.
     2139            </p>
     2140            <p id="rfc.section.6.1.p.12">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.
     2141            </p>
     2142         </div>
     2143         <div id="header.via">
     2144            <div id="rfc.iref.v.1"></div>
     2145            <div id="rfc.iref.h.13"></div>
     2146            <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a href="#header.via">Via</a></h2>
     2147            <p id="rfc.section.6.2.p.1">The "Via" header field <em class="bcp14">MUST</em> be sent by a proxy or gateway to indicate the intermediate protocols and recipients between the user agent and the server
     2148               on requests, and between the origin server and the client on responses. It is analogous to the "Received" field used by email
     2149               systems (<a href="https://tools.ietf.org/html/rfc5322#section-3.6.7">Section 3.6.7</a> of <a href="#RFC5322" id="rfc.xref.RFC5322.3"><cite title="Internet Message Format">[RFC5322]</cite></a>) and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities
     2150               of all senders along the request/response chain.
     2151            </p>
     2152            <div id="rfc.figure.u.61"></div><pre class="inline"><span id="rfc.iref.g.87"></span><span id="rfc.iref.g.88"></span><span id="rfc.iref.g.89"></span><span id="rfc.iref.g.90"></span><span id="rfc.iref.g.91"></span><span id="rfc.iref.g.92"></span>  <a href="#header.via" class="smpl">Via</a>               = 1#( <a href="#header.via" class="smpl">received-protocol</a> <a href="#rule.whitespace" class="smpl">RWS</a> <a href="#header.via" class="smpl">received-by</a>
    20482153                          [ <a href="#rule.whitespace" class="smpl">RWS</a> <a href="#rule.comment" class="smpl">comment</a> ] )
    20492154  <a href="#header.via" class="smpl">received-protocol</a> = [ <a href="#header.upgrade" class="smpl">protocol-name</a> "/" ] <a href="#header.upgrade" class="smpl">protocol-version</a>
    … …  
    20512156  <a href="#header.via" class="smpl">pseudonym</a>         = <a href="#rule.token.separators" class="smpl">token</a>
    20522157</pre><p id="rfc.section.6.2.p.3">The received-protocol indicates the protocol version of the message received by the server or client along each segment of
    2053          the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded
    2054          so that information about the protocol capabilities of upstream applications remains visible to all recipients.
    2055       </p>
    2056       <p id="rfc.section.6.2.p.4">The protocol-name is excluded if and only if it would be "HTTP". The received-by field is normally the host and optional port
    2057          number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to
    2058          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.
    2059       </p>
    2060       <p id="rfc.section.6.2.p.5">Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient <em class="bcp14">MUST</em> append its information such that the end result is ordered according to the sequence of forwarding applications.
    2061       </p>
    2062       <p id="rfc.section.6.2.p.6">Comments <em class="bcp14">MAY</em> be used in the Via header field to identify the software of each recipient, analogous to the User-Agent and Server header
    2063          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.
    2064       </p>
    2065       <p id="rfc.section.6.2.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
    2066          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
    2067          server at www.example.com. The request received by www.example.com would then have the following Via header field:
    2068       </p>
    2069       <div id="rfc.figure.u.62"></div><pre class="text">  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
     2158               the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded
     2159               so that information about the protocol capabilities of upstream applications remains visible to all recipients.
     2160            </p>
     2161            <p id="rfc.section.6.2.p.4">The protocol-name is excluded if and only if it would be "HTTP". The received-by field is normally the host and optional port
     2162               number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to
     2163               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.
     2164            </p>
     2165            <p id="rfc.section.6.2.p.5">Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient <em class="bcp14">MUST</em> append its information such that the end result is ordered according to the sequence of forwarding applications.
     2166            </p>
     2167            <p id="rfc.section.6.2.p.6">Comments <em class="bcp14">MAY</em> be used in the Via header field to identify the software of each recipient, analogous to the User-Agent and Server header
     2168               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.
     2169            </p>
     2170            <p id="rfc.section.6.2.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
     2171               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
     2172               server at www.example.com. The request received by www.example.com would then have the following Via header field:
     2173            </p>
     2174            <div id="rfc.figure.u.62"></div><pre class="text">  Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
    20702175</pre><p id="rfc.section.6.2.p.9">A proxy or gateway used as a portal through a network firewall <em class="bcp14">SHOULD NOT</em> forward the names and ports of hosts within the firewall region unless it is explicitly enabled to do so. If not enabled,
    2071          the received-by host of any host behind the firewall <em class="bcp14">SHOULD</em> be replaced by an appropriate pseudonym for that host.
    2072       </p>
    2073       <p id="rfc.section.6.2.p.10">For organizations that have strong privacy requirements for hiding internal structures, a proxy or gateway <em class="bcp14">MAY</em> combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry.
    2074          For example,
    2075       </p>
    2076       <div id="rfc.figure.u.63"></div><pre class="text">  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
     2176               the received-by host of any host behind the firewall <em class="bcp14">SHOULD</em> be replaced by an appropriate pseudonym for that host.
     2177            </p>
     2178            <p id="rfc.section.6.2.p.10">For organizations that have strong privacy requirements for hiding internal structures, a proxy or gateway <em class="bcp14">MAY</em> combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry.
     2179               For example,
     2180            </p>
     2181            <div id="rfc.figure.u.63"></div><pre class="text">  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
    20772182</pre><p id="rfc.section.6.2.p.12">could be collapsed to</p>
    2078       <div id="rfc.figure.u.64"></div><pre class="text">  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
     2183            <div id="rfc.figure.u.64"></div><pre class="text">  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
    20792184</pre><p id="rfc.section.6.2.p.14">Senders <em class="bcp14">SHOULD NOT</em> combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced
    2080          by pseudonyms. Senders <em class="bcp14">MUST NOT</em> combine entries which have different received-protocol values.
    2081       </p>
    2082       <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a id="persistent.connections" href="#persistent.connections">Persistent Connections</a></h2>
    2083       <h3 id="rfc.section.6.3.1"><a href="#rfc.section.6.3.1">6.3.1</a>&nbsp;<a id="persistent.purpose" href="#persistent.purpose">Purpose</a></h3>
    2084       <p id="rfc.section.6.3.1.p.1">Prior to persistent connections, a separate TCP connection was established for each request, increasing the load on HTTP servers
    2085          and causing congestion on the Internet. The use of inline images and other associated data often requires a client to make
    2086          multiple requests of the same server in a short amount of time. Analysis of these performance problems and results from a
    2087          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 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>.
    2088       </p>
    2089       <p id="rfc.section.6.3.1.p.2">Persistent HTTP connections have a number of advantages: </p>
    2090       <ul>
    2091          <li>By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways,
    2092             tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.
    2093          </li>
    2094          <li>HTTP requests and responses can be pipelined on a connection. Pipelining allows a client to make multiple requests without
    2095             waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.
    2096          </li>
    2097          <li>Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to
    2098             determine the congestion state of the network.
    2099          </li>
    2100          <li>Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.</li>
    2101          <li>HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using
    2102             future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old
    2103             semantics after an error is reported.
    2104          </li>
    2105       </ul>
    2106       <p id="rfc.section.6.3.1.p.3">HTTP implementations <em class="bcp14">SHOULD</em> implement persistent connections.
    2107       </p>
    2108       <h3 id="rfc.section.6.3.2"><a href="#rfc.section.6.3.2">6.3.2</a>&nbsp;<a id="persistent.overall" href="#persistent.overall">Overall Operation</a></h3>
    2109       <p id="rfc.section.6.3.2.p.1">A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior
    2110          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.
    2111       </p>
    2112       <p id="rfc.section.6.3.2.p.2">Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling
    2113          takes place using the Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.7" title="Connection">Section&nbsp;6.1</a>). Once a close has been signaled, the client <em class="bcp14">MUST NOT</em> send any more requests on that connection.
    2114       </p>
    2115       <h4 id="rfc.section.6.3.2.1"><a href="#rfc.section.6.3.2.1">6.3.2.1</a>&nbsp;<a id="persistent.negotiation" href="#persistent.negotiation">Negotiation</a></h4>
    2116       <p id="rfc.section.6.3.2.1.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 field including the connection-token
    2117          "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 field including the connection-token "close".
    2118       </p>
    2119       <p id="rfc.section.6.3.2.1.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
    2120          a Connection header field with the connection-token close. In case the client does not want to maintain a connection for more
    2121          than that request, it <em class="bcp14">SHOULD</em> send a Connection header field including the connection-token close.
    2122       </p>
    2123       <p id="rfc.section.6.3.2.1.p.3">If either the client or the server sends the close token in the Connection header field, that request becomes the last one
    2124          for the connection.
    2125       </p>
    2126       <p id="rfc.section.6.3.2.1.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="Keep-Alive Connections">Appendix&nbsp;A.1.2</a> for more information on backward compatibility with HTTP/1.0 clients.
    2127       </p>
    2128       <p id="rfc.section.6.3.2.1.p.5">Each persistent connection applies to only one transport link.</p>
    2129       <p id="rfc.section.6.3.2.1.p.6">A proxy server <em class="bcp14">MUST NOT</em> establish a HTTP/1.1 persistent connection with an HTTP/1.0 client (but see <a href="http://tools.ietf.org/html/rfc2068#section-19.7.1">Section 19.7.1</a> of <a href="#RFC2068" id="rfc.xref.RFC2068.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> for information and discussion of the problems with the Keep-Alive header field implemented by many HTTP/1.0 clients).
    2130       </p>
    2131       <p id="rfc.section.6.3.2.1.p.7">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.body" title="Message Body">Section&nbsp;3.3</a>.
    2132       </p>
    2133       <h4 id="rfc.section.6.3.2.2"><a href="#rfc.section.6.3.2.2">6.3.2.2</a>&nbsp;<a id="pipelining" href="#pipelining">Pipelining</a></h4>
    2134       <p id="rfc.section.6.3.2.2.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.
    2135       </p>
    2136       <p id="rfc.section.6.3.2.2.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.
    2137       </p>
    2138       <p id="rfc.section.6.3.2.2.p.3">Clients <em class="bcp14">SHOULD NOT</em> pipeline requests using non-idempotent request methods or non-idempotent sequences of request methods (see <a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 6.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.12"><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
    2139          send a non-idempotent request <em class="bcp14">SHOULD</em> wait to send that request until it has received the response status line for the previous request.
    2140       </p>
    2141       <h3 id="rfc.section.6.3.3"><a href="#rfc.section.6.3.3">6.3.3</a>&nbsp;<a id="persistent.practical" href="#persistent.practical">Practical Considerations</a></h3>
    2142       <p id="rfc.section.6.3.3.p.1">Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers
    2143          might make this a higher value since it is likely that the client will be making more connections through the same server.
    2144          The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client
    2145          or the server.
    2146       </p>
    2147       <p id="rfc.section.6.3.3.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
    2148          not detect the other side's close promptly it could cause unnecessary resource drain on the network.
    2149       </p>
    2150       <p id="rfc.section.6.3.3.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
    2151          that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed
    2152          while it was idle, but from the client's point of view, a request is in progress.
    2153       </p>
    2154       <p id="rfc.section.6.3.3.p.4">Clients (including proxies) <em class="bcp14">SHOULD</em> limit the number of simultaneous connections that they maintain to a given server (including proxies).
    2155       </p>
    2156       <p id="rfc.section.6.3.3.p.5">Previous revisions of HTTP gave a specific number of connections as a ceiling, but this was found to be impractical for many
    2157          applications. As a result, this specification does not mandate a particular maximum number of connections, but instead encourages
    2158          clients to be conservative when opening multiple connections.
    2159       </p>
    2160       <p id="rfc.section.6.3.3.p.6">In particular, while using multiple connections avoids the "head-of-line blocking" problem (whereby a request that takes significant
    2161          server-side processing and/or has a large payload can block subsequent requests on the same connection), each connection used
    2162          consumes server resources (sometimes significantly), and furthermore using multiple connections can cause undesirable side
    2163          effects in congested networks.
    2164       </p>
    2165       <p id="rfc.section.6.3.3.p.7">Note that servers might reject traffic that they deem abusive, including an excessive number of connections from a client.</p>
    2166       <h3 id="rfc.section.6.3.4"><a href="#rfc.section.6.3.4">6.3.4</a>&nbsp;<a id="persistent.retrying.requests" href="#persistent.retrying.requests">Retrying Requests</a></h3>
    2167       <p id="rfc.section.6.3.4.p.1">Senders can close the transport connection at any time. Therefore, clients, servers, and proxies <em class="bcp14">MUST</em> be able to recover from asynchronous close events. Client software <em class="bcp14">MAY</em> reopen the transport connection and retransmit the aborted sequence of requests without user interaction so long as the request
    2168          sequence is idempotent (see <a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 6.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.13"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). Non-idempotent request 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
    2169          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.
    2170       </p>
    2171       <h2 id="rfc.section.6.4"><a href="#rfc.section.6.4">6.4</a>&nbsp;<a id="message.transmission.requirements" href="#message.transmission.requirements">Message Transmission Requirements</a></h2>
    2172       <h3 id="rfc.section.6.4.1"><a href="#rfc.section.6.4.1">6.4.1</a>&nbsp;<a id="persistent.flow" href="#persistent.flow">Persistent Connections and Flow Control</a></h3>
    2173       <p id="rfc.section.6.4.1.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
    2174          connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.
    2175       </p>
    2176       <h3 id="rfc.section.6.4.2"><a href="#rfc.section.6.4.2">6.4.2</a>&nbsp;<a id="persistent.monitor" href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></h3>
    2177       <p id="rfc.section.6.4.2.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 code while it is transmitting the request. If the client sees an error
    2178          status code, 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;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 field, the client <em class="bcp14">MUST</em> close the connection.
    2179       </p>
    2180       <h3 id="rfc.section.6.4.3"><a href="#rfc.section.6.4.3">6.4.3</a>&nbsp;<a id="use.of.the.100.status" href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></h3>
    2181       <p id="rfc.section.6.4.3.p.1">The purpose of the 100 (Continue) status code (see <a href="p2-semantics.html#status.100" title="100 Continue">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.14"><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
    2182          to accept the request (based on the request header fields) before the client sends the request body. In some cases, it might
    2183          either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without
    2184          looking at the body.
    2185       </p>
    2186       <p id="rfc.section.6.4.3.p.2">Requirements for HTTP/1.1 clients: </p>
    2187       <ul>
    2188          <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 header field (<a href="p2-semantics.html#header.expect" title="Expect">Section 10.3</a> of <a href="#Part2" id="rfc.xref.Part2.15"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) with the "100-continue" expectation.
    2189          </li>
    2190          <li>A client <em class="bcp14">MUST NOT</em> send an Expect header field (<a href="p2-semantics.html#header.expect" title="Expect">Section 10.3</a> of <a href="#Part2" id="rfc.xref.Part2.16"><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.
    2191          </li>
    2192       </ul>
    2193       <p id="rfc.section.6.4.3.p.3">Because of the presence of older implementations, the protocol allows ambiguous situations in which a client might send "Expect:
    2194          100-continue" without receiving either a 417 (Expectation Failed) or a 100 (Continue) status code. Therefore, when a client
    2195          sends this header field to an origin server (possibly via a proxy) from which it has never seen a 100 (Continue) status code,
    2196          the client <em class="bcp14">SHOULD NOT</em> wait for an indefinite period before sending the request body.
    2197       </p>
    2198       <p id="rfc.section.6.4.3.p.4">Requirements for HTTP/1.1 origin servers: </p>
    2199       <ul>
    2200          <li>Upon receiving a request which includes an Expect header field with the "100-continue" expectation, an origin server <em class="bcp14">MUST</em> either respond with 100 (Continue) status code and continue to read from the input stream, or respond with a final status
    2201             code. 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 request method if it returns a final status code.
    2202          </li>
    2203          <li>An origin server <em class="bcp14">SHOULD NOT</em> send a 100 (Continue) response if the request message does not include an Expect header field with the "100-continue" expectation,
    2204             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
    2205             rule: for compatibility with <a href="#RFC2068" id="rfc.xref.RFC2068.4"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a>, a server <em class="bcp14">MAY</em> send a 100 (Continue) status code in response to an HTTP/1.1 PUT or POST request that does not include an Expect header field
    2206             with the "100-continue" expectation. This exception, the purpose of which is to minimize any client processing delays associated
    2207             with an undeclared wait for 100 (Continue) status code, applies only to HTTP/1.1 requests, and not to requests with any other
    2208             HTTP-version value.
    2209          </li>
    2210          <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.
    2211          </li>
    2212          <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
    2213             prematurely.
    2214          </li>
    2215          <li>If an origin server receives a request that does not include an Expect header field with the "100-continue" expectation, the
    2216             request includes a request body, and the server responds with a final status code before reading the entire request body from
    2217             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,
    2218             the client might not reliably receive the response message. However, this requirement ought not be construed as preventing
    2219             a server from defending itself against denial-of-service attacks, or from badly broken client implementations.
    2220          </li>
    2221       </ul>
    2222       <p id="rfc.section.6.4.3.p.5">Requirements for HTTP/1.1 proxies: </p>
    2223       <ul>
    2224          <li>If a proxy receives a request that includes an Expect header field with the "100-continue" expectation, and the proxy either
    2225             knows that the next-hop server complies with HTTP/1.1 or higher, or does not know the HTTP version of the next-hop server,
    2226             it <em class="bcp14">MUST</em> forward the request, including the Expect header field.
    2227          </li>
    2228          <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 code.
    2229          </li>
    2230          <li>Proxies <em class="bcp14">SHOULD</em> maintain a record of the HTTP version numbers received from recently-referenced next-hop servers.
    2231          </li>
    2232          <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
    2233             an Expect header field with the "100-continue" expectation. This requirement overrides the general rule for forwarding of
    2234             1xx responses (see <a href="p2-semantics.html#status.1xx" title="Informational 1xx">Section 7.1</a> of <a href="#Part2" id="rfc.xref.Part2.17"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
    2235          </li>
    2236       </ul>
    2237       <h3 id="rfc.section.6.4.4"><a href="#rfc.section.6.4.4">6.4.4</a>&nbsp;<a id="closing.connections.on.error" href="#closing.connections.on.error">Closing Connections on Error</a></h3>
    2238       <p id="rfc.section.6.4.4.p.1">If the client is sending data, a server implementation using TCP <em class="bcp14">SHOULD</em> be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes
    2239          the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send
    2240          a reset packet to the client, which might erase the client's unacknowledged input buffers before they can be read and interpreted
    2241          by the HTTP application.
    2242       </p>
    2243       <div id="rfc.iref.u.5"></div>
    2244       <div id="rfc.iref.h.14"></div>
    2245       <h2 id="rfc.section.6.5"><a href="#rfc.section.6.5">6.5</a>&nbsp;<a id="header.upgrade" href="#header.upgrade">Upgrade</a></h2>
    2246       <p id="rfc.section.6.5.p.1">The "Upgrade" header field allows the client to specify what additional communication protocols it would like to use, if the
    2247          server chooses to switch protocols. Servers can use it to indicate what protocols they are willing to switch to.
    2248       </p>
    2249       <div id="rfc.figure.u.65"></div><pre class="inline"><span id="rfc.iref.g.93"></span>  <a href="#header.upgrade" class="smpl">Upgrade</a>          = 1#<a href="#header.upgrade" class="smpl">protocol</a>
     2185               by pseudonyms. Senders <em class="bcp14">MUST NOT</em> combine entries which have different received-protocol values.
     2186            </p>
     2187         </div>
     2188         <div id="persistent.connections">
     2189            <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a href="#persistent.connections">Persistent Connections</a></h2>
     2190            <div id="persistent.purpose">
     2191               <h3 id="rfc.section.6.3.1"><a href="#rfc.section.6.3.1">6.3.1</a>&nbsp;<a href="#persistent.purpose">Purpose</a></h3>
     2192               <p id="rfc.section.6.3.1.p.1">Prior to persistent connections, a separate TCP connection was established for each request, increasing the load on HTTP servers
     2193                  and causing congestion on the Internet. The use of inline images and other associated data often requires a client to make
     2194                  multiple requests of the same server in a short amount of time. Analysis of these performance problems and results from a
     2195                  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 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>.
     2196               </p>
     2197               <p id="rfc.section.6.3.1.p.2">Persistent HTTP connections have a number of advantages: </p>
     2198               <ul>
     2199                  <li>By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways,
     2200                     tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.
     2201                  </li>
     2202                  <li>HTTP requests and responses can be pipelined on a connection. Pipelining allows a client to make multiple requests without
     2203                     waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.
     2204                  </li>
     2205                  <li>Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to
     2206                     determine the congestion state of the network.
     2207                  </li>
     2208                  <li>Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.</li>
     2209                  <li>HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using
     2210                     future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old
     2211                     semantics after an error is reported.
     2212                  </li>
     2213               </ul>
     2214               <p id="rfc.section.6.3.1.p.3">HTTP implementations <em class="bcp14">SHOULD</em> implement persistent connections.
     2215               </p>
     2216            </div>
     2217            <div id="persistent.overall">
     2218               <h3 id="rfc.section.6.3.2"><a href="#rfc.section.6.3.2">6.3.2</a>&nbsp;<a href="#persistent.overall">Overall Operation</a></h3>
     2219               <p id="rfc.section.6.3.2.p.1">A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior
     2220                  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.
     2221               </p>
     2222               <p id="rfc.section.6.3.2.p.2">Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling
     2223                  takes place using the Connection header field (<a href="#header.connection" id="rfc.xref.header.connection.7" title="Connection">Section&nbsp;6.1</a>). Once a close has been signaled, the client <em class="bcp14">MUST NOT</em> send any more requests on that connection.
     2224               </p>
     2225               <div id="persistent.negotiation">
     2226                  <h4 id="rfc.section.6.3.2.1"><a href="#rfc.section.6.3.2.1">6.3.2.1</a>&nbsp;<a href="#persistent.negotiation">Negotiation</a></h4>
     2227                  <p id="rfc.section.6.3.2.1.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 field including the connection-token
     2228                     "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 field including the connection-token "close".
     2229                  </p>
     2230                  <p id="rfc.section.6.3.2.1.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
     2231                     a Connection header field with the connection-token close. In case the client does not want to maintain a connection for more
     2232                     than that request, it <em class="bcp14">SHOULD</em> send a Connection header field including the connection-token close.
     2233                  </p>
     2234                  <p id="rfc.section.6.3.2.1.p.3">If either the client or the server sends the close token in the Connection header field, that request becomes the last one
     2235                     for the connection.
     2236                  </p>
     2237                  <p id="rfc.section.6.3.2.1.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="Keep-Alive Connections">Appendix&nbsp;A.1.2</a> for more information on backward compatibility with HTTP/1.0 clients.
     2238                  </p>
     2239                  <p id="rfc.section.6.3.2.1.p.5">Each persistent connection applies to only one transport link.</p>
     2240                  <p id="rfc.section.6.3.2.1.p.6">A proxy server <em class="bcp14">MUST NOT</em> establish a HTTP/1.1 persistent connection with an HTTP/1.0 client (but see <a href="https://tools.ietf.org/html/rfc2068#section-19.7.1">Section 19.7.1</a> of <a href="#RFC2068" id="rfc.xref.RFC2068.3"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> for information and discussion of the problems with the Keep-Alive header field implemented by many HTTP/1.0 clients).
     2241                  </p>
     2242                  <p id="rfc.section.6.3.2.1.p.7">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.body" title="Message Body">Section&nbsp;3.3</a>.
     2243                  </p>
     2244               </div>
     2245               <div id="pipelining">
     2246                  <h4 id="rfc.section.6.3.2.2"><a href="#rfc.section.6.3.2.2">6.3.2.2</a>&nbsp;<a href="#pipelining">Pipelining</a></h4>
     2247                  <p id="rfc.section.6.3.2.2.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.
     2248                  </p>
     2249                  <p id="rfc.section.6.3.2.2.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.
     2250                  </p>
     2251                  <p id="rfc.section.6.3.2.2.p.3">Clients <em class="bcp14">SHOULD NOT</em> pipeline requests using non-idempotent request methods or non-idempotent sequences of request methods (see <a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 6.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.12"><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
     2252                     send a non-idempotent request <em class="bcp14">SHOULD</em> wait to send that request until it has received the response status line for the previous request.
     2253                  </p>
     2254               </div>
     2255            </div>
     2256            <div id="persistent.practical">
     2257               <h3 id="rfc.section.6.3.3"><a href="#rfc.section.6.3.3">6.3.3</a>&nbsp;<a href="#persistent.practical">Practical Considerations</a></h3>
     2258               <p id="rfc.section.6.3.3.p.1">Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers
     2259                  might make this a higher value since it is likely that the client will be making more connections through the same server.
     2260                  The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client
     2261                  or the server.
     2262               </p>
     2263               <p id="rfc.section.6.3.3.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
     2264                  not detect the other side's close promptly it could cause unnecessary resource drain on the network.
     2265               </p>
     2266               <p id="rfc.section.6.3.3.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
     2267                  that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed
     2268                  while it was idle, but from the client's point of view, a request is in progress.
     2269               </p>
     2270               <p id="rfc.section.6.3.3.p.4">Clients (including proxies) <em class="bcp14">SHOULD</em> limit the number of simultaneous connections that they maintain to a given server (including proxies).
     2271               </p>
     2272               <p id="rfc.section.6.3.3.p.5">Previous revisions of HTTP gave a specific number of connections as a ceiling, but this was found to be impractical for many
     2273                  applications. As a result, this specification does not mandate a particular maximum number of connections, but instead encourages
     2274                  clients to be conservative when opening multiple connections.
     2275               </p>
     2276               <p id="rfc.section.6.3.3.p.6">In particular, while using multiple connections avoids the "head-of-line blocking" problem (whereby a request that takes significant
     2277                  server-side processing and/or has a large payload can block subsequent requests on the same connection), each connection used
     2278                  consumes server resources (sometimes significantly), and furthermore using multiple connections can cause undesirable side
     2279                  effects in congested networks.
     2280               </p>
     2281               <p id="rfc.section.6.3.3.p.7">Note that servers might reject traffic that they deem abusive, including an excessive number of connections from a client.</p>
     2282            </div>
     2283            <div id="persistent.retrying.requests">
     2284               <h3 id="rfc.section.6.3.4"><a href="#rfc.section.6.3.4">6.3.4</a>&nbsp;<a href="#persistent.retrying.requests">Retrying Requests</a></h3>
     2285               <p id="rfc.section.6.3.4.p.1">Senders can close the transport connection at any time. Therefore, clients, servers, and proxies <em class="bcp14">MUST</em> be able to recover from asynchronous close events. Client software <em class="bcp14">MAY</em> reopen the transport connection and retransmit the aborted sequence of requests without user interaction so long as the request
     2286                  sequence is idempotent (see <a href="p2-semantics.html#idempotent.methods" title="Idempotent Methods">Section 6.1.2</a> of <a href="#Part2" id="rfc.xref.Part2.13"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>). Non-idempotent request 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
     2287                  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.
     2288               </p>
     2289            </div>
     2290         </div>
     2291         <div id="message.transmission.requirements">
     2292            <h2 id="rfc.section.6.4"><a href="#rfc.section.6.4">6.4</a>&nbsp;<a href="#message.transmission.requirements">Message Transmission Requirements</a></h2>
     2293            <div id="persistent.flow">
     2294               <h3 id="rfc.section.6.4.1"><a href="#rfc.section.6.4.1">6.4.1</a>&nbsp;<a href="#persistent.flow">Persistent Connections and Flow Control</a></h3>
     2295               <p id="rfc.section.6.4.1.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
     2296                  connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.
     2297               </p>
     2298            </div>
     2299            <div id="persistent.monitor">
     2300               <h3 id="rfc.section.6.4.2"><a href="#rfc.section.6.4.2">6.4.2</a>&nbsp;<a href="#persistent.monitor">Monitoring Connections for Error Status Messages</a></h3>
     2301               <p id="rfc.section.6.4.2.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 code while it is transmitting the request. If the client sees an error
     2302                  status code, 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;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 field, the client <em class="bcp14">MUST</em> close the connection.
     2303               </p>
     2304            </div>
     2305            <div id="use.of.the.100.status">
     2306               <h3 id="rfc.section.6.4.3"><a href="#rfc.section.6.4.3">6.4.3</a>&nbsp;<a href="#use.of.the.100.status">Use of the 100 (Continue) Status</a></h3>
     2307               <p id="rfc.section.6.4.3.p.1">The purpose of the 100 (Continue) status code (see <a href="p2-semantics.html#status.100" title="100 Continue">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.14"><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
     2308                  to accept the request (based on the request header fields) before the client sends the request body. In some cases, it might
     2309                  either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without
     2310                  looking at the body.
     2311               </p>
     2312               <p id="rfc.section.6.4.3.p.2">Requirements for HTTP/1.1 clients: </p>
     2313               <ul>
     2314                  <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 header field (<a href="p2-semantics.html#header.expect" title="Expect">Section 10.3</a> of <a href="#Part2" id="rfc.xref.Part2.15"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) with the "100-continue" expectation.
     2315                  </li>
     2316                  <li>A client <em class="bcp14">MUST NOT</em> send an Expect header field (<a href="p2-semantics.html#header.expect" title="Expect">Section 10.3</a> of <a href="#Part2" id="rfc.xref.Part2.16"><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.
     2317                  </li>
     2318               </ul>
     2319               <p id="rfc.section.6.4.3.p.3">Because of the presence of older implementations, the protocol allows ambiguous situations in which a client might send "Expect:
     2320                  100-continue" without receiving either a 417 (Expectation Failed) or a 100 (Continue) status code. Therefore, when a client
     2321                  sends this header field to an origin server (possibly via a proxy) from which it has never seen a 100 (Continue) status code,
     2322                  the client <em class="bcp14">SHOULD NOT</em> wait for an indefinite period before sending the request body.
     2323               </p>
     2324               <p id="rfc.section.6.4.3.p.4">Requirements for HTTP/1.1 origin servers: </p>
     2325               <ul>
     2326                  <li>Upon receiving a request which includes an Expect header field with the "100-continue" expectation, an origin server <em class="bcp14">MUST</em> either respond with 100 (Continue) status code and continue to read from the input stream, or respond with a final status
     2327                     code. 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 request method if it returns a final status code.
     2328                  </li>
     2329                  <li>An origin server <em class="bcp14">SHOULD NOT</em> send a 100 (Continue) response if the request message does not include an Expect header field with the "100-continue" expectation,
     2330                     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
     2331                     rule: for compatibility with <a href="#RFC2068" id="rfc.xref.RFC2068.4"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a>, a server <em class="bcp14">MAY</em> send a 100 (Continue) status code in response to an HTTP/1.1 PUT or POST request that does not include an Expect header field
     2332                     with the "100-continue" expectation. This exception, the purpose of which is to minimize any client processing delays associated
     2333                     with an undeclared wait for 100 (Continue) status code, applies only to HTTP/1.1 requests, and not to requests with any other
     2334                     HTTP-version value.
     2335                  </li>
     2336                  <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.
     2337                  </li>
     2338                  <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
     2339                     prematurely.
     2340                  </li>
     2341                  <li>If an origin server receives a request that does not include an Expect header field with the "100-continue" expectation, the
     2342                     request includes a request body, and the server responds with a final status code before reading the entire request body from
     2343                     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,
     2344                     the client might not reliably receive the response message. However, this requirement ought not be construed as preventing
     2345                     a server from defending itself against denial-of-service attacks, or from badly broken client implementations.
     2346                  </li>
     2347               </ul>
     2348               <p id="rfc.section.6.4.3.p.5">Requirements for HTTP/1.1 proxies: </p>
     2349               <ul>
     2350                  <li>If a proxy receives a request that includes an Expect header field with the "100-continue" expectation, and the proxy either
     2351                     knows that the next-hop server complies with HTTP/1.1 or higher, or does not know the HTTP version of the next-hop server,
     2352                     it <em class="bcp14">MUST</em> forward the request, including the Expect header field.
     2353                  </li>
     2354                  <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 code.
     2355                  </li>
     2356                  <li>Proxies <em class="bcp14">SHOULD</em> maintain a record of the HTTP version numbers received from recently-referenced next-hop servers.
     2357                  </li>
     2358                  <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
     2359                     an Expect header field with the "100-continue" expectation. This requirement overrides the general rule for forwarding of
     2360                     1xx responses (see <a href="p2-semantics.html#status.1xx" title="Informational 1xx">Section 7.1</a> of <a href="#Part2" id="rfc.xref.Part2.17"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
     2361                  </li>
     2362               </ul>
     2363            </div>
     2364            <div id="closing.connections.on.error">
     2365               <h3 id="rfc.section.6.4.4"><a href="#rfc.section.6.4.4">6.4.4</a>&nbsp;<a href="#closing.connections.on.error">Closing Connections on Error</a></h3>
     2366               <p id="rfc.section.6.4.4.p.1">If the client is sending data, a server implementation using TCP <em class="bcp14">SHOULD</em> be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes
     2367                  the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send
     2368                  a reset packet to the client, which might erase the client's unacknowledged input buffers before they can be read and interpreted
     2369                  by the HTTP application.
     2370               </p>
     2371            </div>
     2372         </div>
     2373         <div id="header.upgrade">
     2374            <div id="rfc.iref.u.5"></div>
     2375            <div id="rfc.iref.h.14"></div>
     2376            <h2 id="rfc.section.6.5"><a href="#rfc.section.6.5">6.5</a>&nbsp;<a href="#header.upgrade">Upgrade</a></h2>
     2377            <p id="rfc.section.6.5.p.1">The "Upgrade" header field allows the client to specify what additional communication protocols it would like to use, if the
     2378               server chooses to switch protocols. Servers can use it to indicate what protocols they are willing to switch to.
     2379            </p>
     2380            <div id="rfc.figure.u.65"></div><pre class="inline"><span id="rfc.iref.g.93"></span>  <a href="#header.upgrade" class="smpl">Upgrade</a>          = 1#<a href="#header.upgrade" class="smpl">protocol</a>
    22502381
    22512382  <a href="#header.upgrade" class="smpl">protocol</a>         = <a href="#header.upgrade" class="smpl">protocol-name</a> ["/" <a href="#header.upgrade" class="smpl">protocol-version</a>]
    … …  
    22532384  <a href="#header.upgrade" class="smpl">protocol-version</a> = <a href="#rule.token.separators" class="smpl">token</a>
    22542385</pre><p id="rfc.section.6.5.p.3">For example,</p>
    2255       <div id="rfc.figure.u.66"></div><pre class="text">  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
     2386            <div id="rfc.figure.u.66"></div><pre class="text">  Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
    22562387</pre><p id="rfc.section.6.5.p.5">The Upgrade header field is intended to provide a simple mechanism for transitioning from HTTP/1.1 to some other, incompatible
    2257          protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP
    2258          with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult
    2259          transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol
    2260          while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by
    2261          the server, possibly according to the nature of the request method or target resource).
    2262       </p>
    2263       <p id="rfc.section.6.5.p.6">The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection.
    2264          Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities
    2265          and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen,
    2266          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.
    2267       </p>
    2268       <p id="rfc.section.6.5.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.8" title="Connection">Section&nbsp;6.1</a>) whenever Upgrade is present in an HTTP/1.1 message.
    2269       </p>
    2270       <p id="rfc.section.6.5.p.8">The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it
    2271          is more appropriate to use a 3xx redirection response (<a href="p2-semantics.html#status.3xx" title="Redirection 3xx">Section 7.3</a> of <a href="#Part2" id="rfc.xref.Part2.18"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
    2272       </p>
    2273       <p id="rfc.section.6.5.p.9">Servers <em class="bcp14">MUST</em> include the "Upgrade" header field in 101 (Switching Protocols) responses to indicate which protocol(s) are being switched
    2274          to, and <em class="bcp14">MUST</em> include it in 426 (Upgrade Required) responses to indicate acceptable protocols to upgrade to. Servers <em class="bcp14">MAY</em> include it in any other response to indicate that they are willing to upgrade to one of the specified protocols.
    2275       </p>
    2276       <p id="rfc.section.6.5.p.10">This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined
    2277          by the HTTP version rules of <a href="#http.version" title="Protocol Versioning">Section&nbsp;2.6</a> and future updates to this specification. Additional tokens can be registered with IANA using the registration procedure defined
    2278          in <a href="#upgrade.token.registry" title="Upgrade Token Registry">Section&nbsp;7.6</a>.
    2279       </p>
    2280       <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
    2281       <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
    2282       <p id="rfc.section.7.1.p.1">HTTP header fields are registered within the Message Header Field Registry <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a> maintained by IANA at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt;.
    2283       </p>
    2284       <p id="rfc.section.7.1.p.2">This document defines the following HTTP header fields, so their associated registry entries shall be updated according to
    2285          the permanent registrations below:
    2286       </p>
    2287       <div id="rfc.table.1">
    2288          <div id="iana.header.registration.table"></div>
    2289          <table class="tt full left" cellpadding="3" cellspacing="0">
    2290             <thead>
    2291                <tr>
    2292                   <th>Header Field Name</th>
    2293                   <th>Protocol</th>
    2294                   <th>Status</th>
    2295                   <th>Reference</th>
    2296                </tr>
    2297             </thead>
    2298             <tbody>
    2299                <tr>
    2300                   <td class="left">Connection</td>
    2301                   <td class="left">http</td>
    2302                   <td class="left">standard</td>
    2303                   <td class="left"> <a href="#header.connection" id="rfc.xref.header.connection.9" title="Connection">Section&nbsp;6.1</a>
    2304                   </td>
    2305                </tr>
    2306                <tr>
    2307                   <td class="left">Content-Length</td>
    2308                   <td class="left">http</td>
    2309                   <td class="left">standard</td>
    2310                   <td class="left"> <a href="#header.content-length" id="rfc.xref.header.content-length.2" title="Content-Length">Section&nbsp;3.3.2</a>
    2311                   </td>
    2312                </tr>
    2313                <tr>
    2314                   <td class="left">Host</td>
    2315                   <td class="left">http</td>
    2316                   <td class="left">standard</td>
    2317                   <td class="left"> <a href="#header.host" id="rfc.xref.header.host.2" title="Host">Section&nbsp;5.4</a>
    2318                   </td>
    2319                </tr>
    2320                <tr>
    2321                   <td class="left">TE</td>
    2322                   <td class="left">http</td>
    2323                   <td class="left">standard</td>
    2324                   <td class="left"> <a href="#header.te" id="rfc.xref.header.te.4" title="TE">Section&nbsp;4.3</a>
    2325                   </td>
    2326                </tr>
    2327                <tr>
    2328                   <td class="left">Trailer</td>
    2329                   <td class="left">http</td>
    2330                   <td class="left">standard</td>
    2331                   <td class="left"> <a href="#header.trailer" id="rfc.xref.header.trailer.2" title="Trailer">Section&nbsp;4.4</a>
    2332                   </td>
    2333                </tr>
    2334                <tr>
    2335                   <td class="left">Transfer-Encoding</td>
    2336                   <td class="left">http</td>
    2337                   <td class="left">standard</td>
    2338                   <td class="left"> <a href="#header.transfer-encoding" id="rfc.xref.header.transfer-encoding.3" title="Transfer-Encoding">Section&nbsp;3.3.1</a>
    2339                   </td>
    2340                </tr>
    2341                <tr>
    2342                   <td class="left">Upgrade</td>
    2343                   <td class="left">http</td>
    2344                   <td class="left">standard</td>
    2345                   <td class="left"> <a href="#header.upgrade" id="rfc.xref.header.upgrade.1" title="Upgrade">Section&nbsp;6.5</a>
    2346                   </td>
    2347                </tr>
    2348                <tr>
    2349                   <td class="left">Via</td>
    2350                   <td class="left">http</td>
    2351                   <td class="left">standard</td>
    2352                   <td class="left"> <a href="#header.via" id="rfc.xref.header.via.2" title="Via">Section&nbsp;6.2</a>
    2353                   </td>
    2354                </tr>
    2355             </tbody>
    2356          </table>
     2388               protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP
     2389               with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult
     2390               transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol
     2391               while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by
     2392               the server, possibly according to the nature of the request method or target resource).
     2393            </p>
     2394            <p id="rfc.section.6.5.p.6">The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection.
     2395               Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities
     2396               and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen,
     2397               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.
     2398            </p>
     2399            <p id="rfc.section.6.5.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.8" title="Connection">Section&nbsp;6.1</a>) whenever Upgrade is present in an HTTP/1.1 message.
     2400            </p>
     2401            <p id="rfc.section.6.5.p.8">The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it
     2402               is more appropriate to use a 3xx redirection response (<a href="p2-semantics.html#status.3xx" title="Redirection 3xx">Section 7.3</a> of <a href="#Part2" id="rfc.xref.Part2.18"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>).
     2403            </p>
     2404            <p id="rfc.section.6.5.p.9">Servers <em class="bcp14">MUST</em> include the "Upgrade" header field in 101 (Switching Protocols) responses to indicate which protocol(s) are being switched
     2405               to, and <em class="bcp14">MUST</em> include it in 426 (Upgrade Required) responses to indicate acceptable protocols to upgrade to. Servers <em class="bcp14">MAY</em> include it in any other response to indicate that they are willing to upgrade to one of the specified protocols.
     2406            </p>
     2407            <p id="rfc.section.6.5.p.10">This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined
     2408               by the HTTP version rules of <a href="#http.version" title="Protocol Versioning">Section&nbsp;2.6</a> and future updates to this specification. Additional tokens can be registered with IANA using the registration procedure defined
     2409               in <a href="#upgrade.token.registry" title="Upgrade Token Registry">Section&nbsp;7.6</a>.
     2410            </p>
     2411         </div>
    23572412      </div>
    2358       <p id="rfc.section.7.1.p.3">Furthermore, the header field-name "Close" shall be registered as "reserved", since using that name as an HTTP header field
    2359          might conflict with the "close" connection option of the "Connection" header field (<a href="#header.connection" id="rfc.xref.header.connection.10" title="Connection">Section&nbsp;6.1</a>).
    2360       </p>
    2361       <div id="rfc.table.u.1">
    2362          <table class="tt full left" cellpadding="3" cellspacing="0">
    2363             <thead>
    2364                <tr>
    2365                   <th>Header Field Name</th>
    2366                   <th>Protocol</th>
    2367                   <th>Status</th>
    2368                   <th>Reference</th>
    2369                </tr>
    2370             </thead>
    2371             <tbody>
    2372                <tr>
    2373                   <td class="left">Close</td>
    2374                   <td class="left">http</td>
    2375                   <td class="left">reserved</td>
    2376                   <td class="left"> <a href="#header.field.registration" title="Header Field Registration">Section&nbsp;7.1</a>
    2377                   </td>
    2378                </tr>
    2379             </tbody>
    2380          </table>
     2413      <div id="IANA.considerations">
     2414         <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a href="#IANA.considerations">IANA Considerations</a></h1>
     2415         <div id="header.field.registration">
     2416            <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a href="#header.field.registration">Header Field Registration</a></h2>
     2417            <p id="rfc.section.7.1.p.1">HTTP header fields are registered within the Message Header Field Registry <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a> maintained by IANA at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt;.
     2418            </p>
     2419            <p id="rfc.section.7.1.p.2">This document defines the following HTTP header fields, so their associated registry entries shall be updated according to
     2420               the permanent registrations below:
     2421            </p>
     2422            <div id="rfc.table.1">
     2423               <div id="iana.header.registration.table"></div>
     2424               <table class="tt full left" cellpadding="3" cellspacing="0">
     2425                  <thead>
     2426                     <tr>
     2427