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Timestamp:
25/09/13 13:01:17 (8 years ago)
Author:
julian.reschke@…
Message:

Update to latest version of rfc2629.xslt

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  • draft-ietf-httpbis/latest/p2-semantics.html

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    448        content: "Expires March 28, 2014";
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    486485      <link href="p1-messaging.html" rel="prev">
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    489488      <link rel="schema.dct" href="http://purl.org/dc/terms/">
    490489      <meta name="dct.creator" content="Fielding, R.">
    491490      <meta name="dct.creator" content="Reschke, J. F.">
    492491      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p2-semantics-latest">
    493       <meta name="dct.issued" scheme="ISO8601" content="2013-09-24">
     492      <meta name="dct.issued" scheme="ISO8601" content="2013-09-25">
    494493      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
    495494      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation.">
     
    519518            <tr>
    520519               <td class="left">Intended status: Standards Track</td>
    521                <td class="right">September 24, 2013</td>
     520               <td class="right">September 25, 2013</td>
    522521            </tr>
    523522            <tr>
    524                <td class="left">Expires: March 28, 2014</td>
     523               <td class="left">Expires: March 29, 2014</td>
    525524               <td class="right"></td>
    526525            </tr>
     
    541540      <p>The changes in this draft are summarized in <a href="#changes.since.24" title="Since draft-ietf-httpbis-p2-semantics-24">Appendix&nbsp;E.5</a>.
    542541      </p>
    543       <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
    544       <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
    545       <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
    546          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>.
    547       </p>
    548       <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
    549          documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
    550          in progress”.
    551       </p>
    552       <p>This Internet-Draft will expire on March 28, 2014.</p>
    553       <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    554       <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
    555       <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
    556          and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
    557          text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
    558          BSD License.
    559       </p>
    560       <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
    561          10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
    562          allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
    563          controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
    564          works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
    565          it into languages other than English.
    566       </p>
     542      <div id="rfc.status">
     543         <h1><a href="#rfc.status">Status of This Memo</a></h1>
     544         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
     545         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
     546            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>.
     547         </p>
     548         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
     549            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
     550            in progress”.
     551         </p>
     552         <p>This Internet-Draft will expire on March 29, 2014.</p>
     553      </div>
     554      <div id="rfc.copyrightnotice">
     555         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
     556         <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
     557         <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
     558            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
     559            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
     560            BSD License.
     561         </p>
     562         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
     563            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
     564            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
     565            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
     566            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
     567            it into languages other than English.
     568         </p>
     569      </div>
    567570      <hr class="noprint">
    568571      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
     
    794797         <li><a href="#rfc.index">Index</a></li>
    795798      </ul>
    796       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
    797       <p id="rfc.section.1.p.1">Each Hypertext Transfer Protocol (HTTP) message is either a request or a response. A server listens on a connection for a
    798          request, parses each message received, interprets the message semantics in relation to the identified request target, and
    799          responds to that request with one or more response messages. A client constructs request messages to communicate specific
    800          intentions, and examines received responses to see if the intentions were carried out and determine how to interpret the results.
    801          This document defines HTTP/1.1 request and response semantics in terms of the architecture defined in <a href="#Part1" id="rfc.xref.Part1.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
    802       </p>
    803       <p id="rfc.section.1.p.2">HTTP provides a uniform interface for interacting with a resource (<a href="#resources" title="Resources">Section&nbsp;2</a>), regardless of its type, nature, or implementation, via the manipulation and transfer of representations (<a href="#representations" title="Representations">Section&nbsp;3</a>).
    804       </p>
    805       <p id="rfc.section.1.p.3">HTTP semantics include the intentions defined by each request method (<a href="#methods" title="Request Methods">Section&nbsp;4</a>), extensions to those semantics that might be described in request header fields (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5</a>), the meaning of status codes to indicate a machine-readable response (<a href="#status.codes" title="Response Status Codes">Section&nbsp;6</a>), and the meaning of other control data and resource metadata that might be given in response header fields (<a href="#response.header.fields" title="Response Header Fields">Section&nbsp;7</a>).
    806       </p>
    807       <p id="rfc.section.1.p.4"><span id="rfc.iref.c.1"></span> This document also defines representation metadata that describe how a payload is intended to be interpreted by a recipient,
    808          the request header fields that might influence content selection, and the various selection algorithms that are collectively
    809          referred to as "<dfn>content negotiation</dfn>" (<a href="#content.negotiation" title="Content Negotiation">Section&nbsp;3.4</a>).
    810       </p>
    811       <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="conformance" href="#conformance">Conformance and Error Handling</a></h2>
    812       <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"
    813          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>.
    814       </p>
    815       <p id="rfc.section.1.1.p.2">Conformance criteria and considerations regarding error handling are defined in <a href="p1-messaging.html#conformance" title="Conformance and Error Handling">Section 2.5</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
    816       </p>
    817       <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>
    818       <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="p1-messaging.html#abnf.extension" title="ABNF list extension: #rule">Section 7</a> of <a href="#Part1" id="rfc.xref.Part1.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. <a href="#imported.abnf" title="Imported ABNF">Appendix&nbsp;C</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;D</a> shows the collected ABNF with the list rule expanded.
    819       </p>
    820       <p id="rfc.section.1.2.p.2">This specification uses the terms "character", "character encoding scheme", "charset", and "protocol element" as they are
    821          defined in <a href="#RFC6365" id="rfc.xref.RFC6365.1"><cite title="Terminology Used in Internationalization in the IETF">[RFC6365]</cite></a>.
    822       </p>
    823       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="resources" href="#resources">Resources</a></h1>
    824       <p id="rfc.section.2.p.1">The target of an HTTP request is called a resource. HTTP does not limit the nature of a resource; it merely defines an interface
    825          that might be used to interact with resources. Each resource is identified by a Uniform Resource Identifier (URI), as described
    826          in <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.7</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
    827       </p>
    828       <p id="rfc.section.2.p.2">When a client constructs an HTTP/1.1 request message, it sends the <a href="p1-messaging.html#target-resource" class="smpl">target URI</a> in one of various forms, as defined in (<a href="p1-messaging.html#request-target" title="Request Target">Section 5.3</a> of <a href="#Part1" id="rfc.xref.Part1.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). When a request is received, the server reconstructs an <a href="p1-messaging.html#effective.request.uri" class="smpl">effective request URI</a> for the target resource (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>).
    829       </p>
    830       <p id="rfc.section.2.p.3">One design goal of HTTP is to separate resource identification from request semantics, which is made possible by vesting the
    831          request semantics in the request method (<a href="#methods" title="Request Methods">Section&nbsp;4</a>) and a few request-modifying header fields (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5</a>). Resource owners <em class="bcp14">SHOULD NOT</em> include request semantics within a URI, such as by specifying an action to invoke within the path or query components of the
    832          effective request URI, unless those semantics are disabled when they are inconsistent with the request method.
    833       </p>
    834       <div id="rfc.iref.r.1"></div>
    835       <div id="rfc.iref.s.1"></div>
    836       <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="representations" href="#representations">Representations</a></h1>
    837       <p id="rfc.section.3.p.1">If we consider that a resource could be anything, and that the uniform interface provided by HTTP is similar to a window through
    838          which one can observe and act upon such a thing only through the communication of messages to some independent actor on the
    839          other side, then we need an abstraction to represent ("take the place of") the current or desired state of that thing in our
    840          communications. We call that abstraction a representation <a href="#REST" id="rfc.xref.REST.1"><cite title="Architectural Styles and the Design of Network-based Software Architectures">[REST]</cite></a>.
    841       </p>
    842       <p id="rfc.section.3.p.2">For the purposes of HTTP, a "<dfn>representation</dfn>" is information that is intended to reflect a past, current, or desired state of a given resource, in a format that can be
    843          readily communicated via the protocol, and that consists of a set of representation metadata and a potentially unbounded stream
    844          of representation data.
    845       </p>
    846       <p id="rfc.section.3.p.3">An origin server might be provided with, or capable of generating, multiple representations that are each intended to reflect
    847          the current state of a <a href="#resources" class="smpl">target resource</a>. In such cases, some algorithm is used by the origin server to select one of those representations as most applicable to
    848          a given request, usually based on <a href="#content.negotiation" class="smpl">content negotiation</a>. We refer to that one representation as the "<dfn>selected representation</dfn>" and use its particular data and metadata for evaluating conditional requests <a href="#Part4" id="rfc.xref.Part4.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a> and constructing the payload for <a href="#status.200" class="smpl">200 (OK)</a> and <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> responses to GET (<a href="#GET" id="rfc.xref.GET.1" title="GET">Section&nbsp;4.3.1</a>).
    849       </p>
    850       <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="representation.metadata" href="#representation.metadata">Representation Metadata</a></h2>
    851       <p id="rfc.section.3.1.p.1">Representation header fields provide metadata about the representation. When a message includes a payload body, the representation
    852          header fields describe how to interpret the representation data enclosed in the payload body. In a response to a HEAD request,
    853          the representation header fields describe the representation data that would have been enclosed in the payload body if the
    854          same request had been a GET.
    855       </p>
    856       <p id="rfc.section.3.1.p.2">The following header fields convey representation metadata:</p>
    857       <div id="rfc.table.u.1">
    858          <table class="tt full left" cellpadding="3" cellspacing="0">
    859             <thead>
    860                <tr>
    861                   <th>Header Field Name</th>
    862                   <th>Defined in...</th>
    863                </tr>
    864             </thead>
    865             <tbody>
    866                <tr>
    867                   <td class="left">Content-Type</td>
    868                   <td class="left"><a href="#header.content-type" id="rfc.xref.header.content-type.1" title="Content-Type">Section&nbsp;3.1.1.5</a></td>
    869                </tr>
    870                <tr>
    871                   <td class="left">Content-Encoding</td>
    872                   <td class="left"><a href="#header.content-encoding" id="rfc.xref.header.content-encoding.1" title="Content-Encoding">Section&nbsp;3.1.2.2</a></td>
    873                </tr>
    874                <tr>
    875                   <td class="left">Content-Language</td>
    876                   <td class="left"><a href="#header.content-language" id="rfc.xref.header.content-language.1" title="Content-Language">Section&nbsp;3.1.3.2</a></td>
    877                </tr>
    878                <tr>
    879                   <td class="left">Content-Location</td>
    880                   <td class="left"><a href="#header.content-location" id="rfc.xref.header.content-location.1" title="Content-Location">Section&nbsp;3.1.4.2</a></td>
    881                </tr>
    882             </tbody>
    883          </table>
     799      <div id="introduction">
     800         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#introduction">Introduction</a></h1>
     801         <p id="rfc.section.1.p.1">Each Hypertext Transfer Protocol (HTTP) message is either a request or a response. A server listens on a connection for a
     802            request, parses each message received, interprets the message semantics in relation to the identified request target, and
     803            responds to that request with one or more response messages. A client constructs request messages to communicate specific
     804            intentions, and examines received responses to see if the intentions were carried out and determine how to interpret the results.
     805            This document defines HTTP/1.1 request and response semantics in terms of the architecture defined in <a href="#Part1" id="rfc.xref.Part1.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
     806         </p>
     807         <p id="rfc.section.1.p.2">HTTP provides a uniform interface for interacting with a resource (<a href="#resources" title="Resources">Section&nbsp;2</a>), regardless of its type, nature, or implementation, via the manipulation and transfer of representations (<a href="#representations" title="Representations">Section&nbsp;3</a>).
     808         </p>
     809         <p id="rfc.section.1.p.3">HTTP semantics include the intentions defined by each request method (<a href="#methods" title="Request Methods">Section&nbsp;4</a>), extensions to those semantics that might be described in request header fields (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5</a>), the meaning of status codes to indicate a machine-readable response (<a href="#status.codes" title="Response Status Codes">Section&nbsp;6</a>), and the meaning of other control data and resource metadata that might be given in response header fields (<a href="#response.header.fields" title="Response Header Fields">Section&nbsp;7</a>).
     810         </p>
     811         <p id="rfc.section.1.p.4"><span id="rfc.iref.c.1"></span> This document also defines representation metadata that describe how a payload is intended to be interpreted by a recipient,
     812            the request header fields that might influence content selection, and the various selection algorithms that are collectively
     813            referred to as "<dfn>content negotiation</dfn>" (<a href="#content.negotiation" title="Content Negotiation">Section&nbsp;3.4</a>).
     814         </p>
     815         <div id="conformance">
     816            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#conformance">Conformance and Error Handling</a></h2>
     817            <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"
     818               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>.
     819            </p>
     820            <p id="rfc.section.1.1.p.2">Conformance criteria and considerations regarding error handling are defined in <a href="p1-messaging.html#conformance" title="Conformance and Error Handling">Section 2.5</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
     821            </p>
     822         </div>
     823         <div id="notation">
     824            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
     825            <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="p1-messaging.html#abnf.extension" title="ABNF list extension: #rule">Section 7</a> of <a href="#Part1" id="rfc.xref.Part1.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. <a href="#imported.abnf" title="Imported ABNF">Appendix&nbsp;C</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;D</a> shows the collected ABNF with the list rule expanded.
     826            </p>
     827            <p id="rfc.section.1.2.p.2">This specification uses the terms "character", "character encoding scheme", "charset", and "protocol element" as they are
     828               defined in <a href="#RFC6365" id="rfc.xref.RFC6365.1"><cite title="Terminology Used in Internationalization in the IETF">[RFC6365]</cite></a>.
     829            </p>
     830         </div>
    884831      </div>
    885       <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;<a id="data.type" href="#data.type">Processing Representation Data</a></h3>
    886       <h4 id="rfc.section.3.1.1.1"><a href="#rfc.section.3.1.1.1">3.1.1.1</a>&nbsp;<a id="media.type" href="#media.type">Media Type</a></h4>
    887       <p id="rfc.section.3.1.1.1.p.1">HTTP uses Internet Media Types <a href="#RFC2046" id="rfc.xref.RFC2046.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[RFC2046]</cite></a> in the <a href="#header.content-type" class="smpl">Content-Type</a> (<a href="#header.content-type" id="rfc.xref.header.content-type.2" title="Content-Type">Section&nbsp;3.1.1.5</a>) and <a href="#header.accept" class="smpl">Accept</a> (<a href="#header.accept" id="rfc.xref.header.accept.1" title="Accept">Section&nbsp;5.3.2</a>) header fields in order to provide open and extensible data typing and type negotiation. Media types define both a data format
    888          and various processing models: how to process that data in accordance with each context in which it is received.
    889       </p>
    890       <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span>  <a href="#media.type" class="smpl">media-type</a> = <a href="#media.type" class="smpl">type</a> "/" <a href="#media.type" class="smpl">subtype</a> *( <a href="#imported.abnf" class="smpl">OWS</a> ";" <a href="#imported.abnf" class="smpl">OWS</a> <a href="#rule.parameter" class="smpl">parameter</a> )
     832      <div id="resources">
     833         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#resources">Resources</a></h1>
     834         <p id="rfc.section.2.p.1">The target of an HTTP request is called a resource. HTTP does not limit the nature of a resource; it merely defines an interface
     835            that might be used to interact with resources. Each resource is identified by a Uniform Resource Identifier (URI), as described
     836            in <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.7</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
     837         </p>
     838         <p id="rfc.section.2.p.2">When a client constructs an HTTP/1.1 request message, it sends the <a href="p1-messaging.html#target-resource" class="smpl">target URI</a> in one of various forms, as defined in (<a href="p1-messaging.html#request-target" title="Request Target">Section 5.3</a> of <a href="#Part1" id="rfc.xref.Part1.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). When a request is received, the server reconstructs an <a href="p1-messaging.html#effective.request.uri" class="smpl">effective request URI</a> for the target resource (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>).
     839         </p>
     840         <p id="rfc.section.2.p.3">One design goal of HTTP is to separate resource identification from request semantics, which is made possible by vesting the
     841            request semantics in the request method (<a href="#methods" title="Request Methods">Section&nbsp;4</a>) and a few request-modifying header fields (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5</a>). Resource owners <em class="bcp14">SHOULD NOT</em> include request semantics within a URI, such as by specifying an action to invoke within the path or query components of the
     842            effective request URI, unless those semantics are disabled when they are inconsistent with the request method.
     843         </p>
     844      </div>
     845      <div id="representations">
     846         <div id="rfc.iref.r.1"></div>
     847         <div id="rfc.iref.s.1"></div>
     848         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#representations">Representations</a></h1>
     849         <p id="rfc.section.3.p.1">If we consider that a resource could be anything, and that the uniform interface provided by HTTP is similar to a window through
     850            which one can observe and act upon such a thing only through the communication of messages to some independent actor on the
     851            other side, then we need an abstraction to represent ("take the place of") the current or desired state of that thing in our
     852            communications. We call that abstraction a representation <a href="#REST" id="rfc.xref.REST.1"><cite title="Architectural Styles and the Design of Network-based Software Architectures">[REST]</cite></a>.
     853         </p>
     854         <p id="rfc.section.3.p.2">For the purposes of HTTP, a "<dfn>representation</dfn>" is information that is intended to reflect a past, current, or desired state of a given resource, in a format that can be
     855            readily communicated via the protocol, and that consists of a set of representation metadata and a potentially unbounded stream
     856            of representation data.
     857         </p>
     858         <p id="rfc.section.3.p.3">An origin server might be provided with, or capable of generating, multiple representations that are each intended to reflect
     859            the current state of a <a href="#resources" class="smpl">target resource</a>. In such cases, some algorithm is used by the origin server to select one of those representations as most applicable to
     860            a given request, usually based on <a href="#content.negotiation" class="smpl">content negotiation</a>. We refer to that one representation as the "<dfn>selected representation</dfn>" and use its particular data and metadata for evaluating conditional requests <a href="#Part4" id="rfc.xref.Part4.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a> and constructing the payload for <a href="#status.200" class="smpl">200 (OK)</a> and <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> responses to GET (<a href="#GET" id="rfc.xref.GET.1" title="GET">Section&nbsp;4.3.1</a>).
     861         </p>
     862         <div id="representation.metadata">
     863            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#representation.metadata">Representation Metadata</a></h2>
     864            <p id="rfc.section.3.1.p.1">Representation header fields provide metadata about the representation. When a message includes a payload body, the representation
     865               header fields describe how to interpret the representation data enclosed in the payload body. In a response to a HEAD request,
     866               the representation header fields describe the representation data that would have been enclosed in the payload body if the
     867               same request had been a GET.
     868            </p>
     869            <p id="rfc.section.3.1.p.2">The following header fields convey representation metadata:</p>
     870            <div id="rfc.table.u.1">
     871               <table class="tt full left" cellpadding="3" cellspacing="0">
     872                  <thead>
     873                     <tr>
     874                        <th>Header Field Name</th>
     875                        <th>Defined in...</th>
     876                     </tr>
     877                  </thead>
     878                  <tbody>
     879                     <tr>
     880                        <td class="left">Content-Type</td>
     881                        <td class="left"><a href="#header.content-type" id="rfc.xref.header.content-type.1" title="Content-Type">Section&nbsp;3.1.1.5</a></td>
     882                     </tr>
     883                     <tr>
     884                        <td class="left">Content-Encoding</td>
     885                        <td class="left"><a href="#header.content-encoding" id="rfc.xref.header.content-encoding.1" title="Content-Encoding">Section&nbsp;3.1.2.2</a></td>
     886                     </tr>
     887                     <tr>
     888                        <td class="left">Content-Language</td>
     889                        <td class="left"><a href="#header.content-language" id="rfc.xref.header.content-language.1" title="Content-Language">Section&nbsp;3.1.3.2</a></td>
     890                     </tr>
     891                     <tr>
     892                        <td class="left">Content-Location</td>
     893                        <td class="left"><a href="#header.content-location" id="rfc.xref.header.content-location.1" title="Content-Location">Section&nbsp;3.1.4.2</a></td>
     894                     </tr>
     895                  </tbody>
     896               </table>
     897            </div>
     898            <div id="data.type">
     899               <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;<a href="#data.type">Processing Representation Data</a></h3>
     900               <div id="media.type">
     901                  <h4 id="rfc.section.3.1.1.1"><a href="#rfc.section.3.1.1.1">3.1.1.1</a>&nbsp;<a href="#media.type">Media Type</a></h4>
     902                  <p id="rfc.section.3.1.1.1.p.1">HTTP uses Internet Media Types <a href="#RFC2046" id="rfc.xref.RFC2046.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[RFC2046]</cite></a> in the <a href="#header.content-type" class="smpl">Content-Type</a> (<a href="#header.content-type" id="rfc.xref.header.content-type.2" title="Content-Type">Section&nbsp;3.1.1.5</a>) and <a href="#header.accept" class="smpl">Accept</a> (<a href="#header.accept" id="rfc.xref.header.accept.1" title="Accept">Section&nbsp;5.3.2</a>) header fields in order to provide open and extensible data typing and type negotiation. Media types define both a data format
     903                     and various processing models: how to process that data in accordance with each context in which it is received.
     904                  </p>
     905                  <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span>  <a href="#media.type" class="smpl">media-type</a> = <a href="#media.type" class="smpl">type</a> "/" <a href="#media.type" class="smpl">subtype</a> *( <a href="#imported.abnf" class="smpl">OWS</a> ";" <a href="#imported.abnf" class="smpl">OWS</a> <a href="#rule.parameter" class="smpl">parameter</a> )
    891906  <a href="#media.type" class="smpl">type</a>       = <a href="#imported.abnf" class="smpl">token</a>
    892907  <a href="#media.type" class="smpl">subtype</a>    = <a href="#imported.abnf" class="smpl">token</a>
    893908</pre><div id="rule.parameter">
    894          <p id="rfc.section.3.1.1.1.p.3">   The type/subtype <em class="bcp14">MAY</em> be followed by parameters in the form of attribute/value pairs.
    895          </p>
    896       </div>
    897       <div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#rule.parameter" class="smpl">parameter</a>      = <a href="#rule.parameter" class="smpl">attribute</a> "=" <a href="#rule.parameter" class="smpl">value</a>
     909                     <p id="rfc.section.3.1.1.1.p.3">   The type/subtype <em class="bcp14">MAY</em> be followed by parameters in the form of attribute/value pairs.
     910                     </p>
     911                  </div>
     912                  <div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#rule.parameter" class="smpl">parameter</a>      = <a href="#rule.parameter" class="smpl">attribute</a> "=" <a href="#rule.parameter" class="smpl">value</a>
    898913  <a href="#rule.parameter" class="smpl">attribute</a>      = <a href="#imported.abnf" class="smpl">token</a>
    899914  <a href="#rule.parameter" class="smpl">value</a>          = <a href="#imported.abnf" class="smpl">word</a>
    900915</pre><p id="rfc.section.3.1.1.1.p.5">The type, subtype, and parameter attribute names are case-insensitive. Parameter values might or might not be case-sensitive,
    901          depending on the semantics of the parameter name. The presence or absence of a parameter might be significant to the processing
    902          of a media-type, depending on its definition within the media type registry.
    903       </p>
    904       <p id="rfc.section.3.1.1.1.p.6">A parameter value that matches the <a href="#imported.abnf" class="smpl">token</a> production can be transmitted as either a token or within a quoted-string. The quoted and unquoted values are equivalent.
    905          For example, the following examples are all equivalent, but the first is preferred for consistency:
    906       </p>
    907       <div id="rfc.figure.u.3"></div><pre class="text">  text/html;charset=utf-8
     916                     depending on the semantics of the parameter name. The presence or absence of a parameter might be significant to the processing
     917                     of a media-type, depending on its definition within the media type registry.
     918                  </p>
     919                  <p id="rfc.section.3.1.1.1.p.6">A parameter value that matches the <a href="#imported.abnf" class="smpl">token</a> production can be transmitted as either a token or within a quoted-string. The quoted and unquoted values are equivalent.
     920                     For example, the following examples are all equivalent, but the first is preferred for consistency:
     921                  </p>
     922                  <div id="rfc.figure.u.3"></div><pre class="text">  text/html;charset=utf-8
    908923  text/html;charset=UTF-8
    909924  Text/HTML;Charset="utf-8"
    910925  text/html; charset="utf-8"
    911926</pre><p id="rfc.section.3.1.1.1.p.8">Internet media types ought to be registered with IANA according to the procedures defined in <a href="#BCP13" id="rfc.xref.BCP13.1"><cite title="Media Type Specifications and Registration Procedures">[BCP13]</cite></a>.
    912       </p>
    913       <div class="note" id="rfc.section.3.1.1.1.p.9">
    914          <p><b>Note:</b> Unlike some similar constructs in other header fields, media type parameters do not allow whitespace (even "bad" whitespace)
    915             around the "=" character.
    916          </p>
     927                  </p>
     928                  <div class="note" id="rfc.section.3.1.1.1.p.9">
     929                     <p><b>Note:</b> Unlike some similar constructs in other header fields, media type parameters do not allow whitespace (even "bad" whitespace)
     930                        around the "=" character.
     931                     </p>
     932                  </div>
     933               </div>
     934               <div id="charset">
     935                  <h4 id="rfc.section.3.1.1.2"><a href="#rfc.section.3.1.1.2">3.1.1.2</a>&nbsp;<a href="#charset">Charset</a></h4>
     936                  <p id="rfc.section.3.1.1.2.p.1">HTTP uses <dfn>charset</dfn> names to indicate or negotiate the character encoding scheme of a textual representation <a href="#RFC6365" id="rfc.xref.RFC6365.2"><cite title="Terminology Used in Internationalization in the IETF">[RFC6365]</cite></a>. A charset is identified by a case-insensitive token.
     937                  </p>
     938                  <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#charset" class="smpl">charset</a> = <a href="#imported.abnf" class="smpl">token</a>
     939</pre><p id="rfc.section.3.1.1.2.p.3">Charset names ought to be registered in IANA Character Set registry (&lt;<a href="http://www.iana.org/assignments/character-sets">http://www.iana.org/assignments/character-sets</a>&gt;) according to the procedures defined in <a href="#RFC2978" id="rfc.xref.RFC2978.1"><cite title="IANA Charset Registration Procedures">[RFC2978]</cite></a>.
     940                  </p>
     941               </div>
     942               <div id="canonicalization.and.text.defaults">
     943                  <h4 id="rfc.section.3.1.1.3"><a href="#rfc.section.3.1.1.3">3.1.1.3</a>&nbsp;<a href="#canonicalization.and.text.defaults">Canonicalization and Text Defaults</a></h4>
     944                  <p id="rfc.section.3.1.1.3.p.1">Internet media types are registered with a canonical form in order to be interoperable among systems with varying native encoding
     945                     formats. Representations selected or transferred via HTTP ought to be in canonical form, for many of the same reasons described
     946                     by the Multipurpose Internet Mail Extensions (MIME) <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>. However, the performance characteristics of email deployments (i.e., store and forward messages to peers) are significantly
     947                     different from those common to HTTP and the Web (server-based information services). Furthermore, MIME's constraints for the
     948                     sake of compatibility with older mail transfer protocols do not apply to HTTP (see <a href="#differences.between.http.and.mime" title="Differences between HTTP and MIME">Appendix&nbsp;A</a>).
     949                  </p>
     950                  <p id="rfc.section.3.1.1.3.p.2">MIME's canonical form requires that media subtypes of the "text" type use CRLF as the text line break. HTTP allows the transfer
     951                     of text media with plain CR or LF alone representing a line break, when such line breaks are consistent for an entire representation.
     952                     An HTTP sender <em class="bcp14">MAY</em> generate, and a recipient <em class="bcp14">MUST</em> be able to parse, line breaks in text media that consist of CRLF, bare CR, or bare LF. In addition, text media in HTTP is
     953                     not limited to charsets that use octets 13 and 10 for CR and LF, respectively. This flexibility regarding line breaks applies
     954                     only to text within a representation that has been assigned a "text" media type; it does not apply to "multipart" types or
     955                     HTTP elements outside the payload body (e.g., header fields).
     956                  </p>
     957                  <p id="rfc.section.3.1.1.3.p.3">If a representation is encoded with a content-coding, the underlying data ought to be in a form defined above prior to being
     958                     encoded.
     959                  </p>
     960               </div>
     961               <div id="multipart.types">
     962                  <h4 id="rfc.section.3.1.1.4"><a href="#rfc.section.3.1.1.4">3.1.1.4</a>&nbsp;<a href="#multipart.types">Multipart Types</a></h4>
     963                  <p id="rfc.section.3.1.1.4.p.1">MIME provides for a number of "multipart" types — encapsulations of one or more representations within a single message body.
     964                     All multipart types share a common syntax, as defined in <a href="http://tools.ietf.org/html/rfc2046#section-5.1.1">Section 5.1.1</a> of <a href="#RFC2046" id="rfc.xref.RFC2046.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[RFC2046]</cite></a>, and include a boundary parameter as part of the media type value. The message body is itself a protocol element; a sender <em class="bcp14">MUST</em> generate only CRLF to represent line breaks between body parts.
     965                  </p>
     966                  <p id="rfc.section.3.1.1.4.p.2">HTTP message framing does not use the multipart boundary as an indicator of message body length, though it might be used by
     967                     implementations that generate or process the payload. For example, the "multipart/form-data" type is often used for carrying
     968                     form data in a request, as described in <a href="#RFC2388" id="rfc.xref.RFC2388.1"><cite title="Returning Values from Forms: multipart/form-data">[RFC2388]</cite></a>, and the "multipart/byteranges" type is defined by this specification for use in some <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> responses <a href="#Part5" id="rfc.xref.Part5.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>.
     969                  </p>
     970               </div>
     971               <div id="header.content-type">
     972                  <div id="rfc.iref.c.2"></div>
     973                  <h4 id="rfc.section.3.1.1.5"><a href="#rfc.section.3.1.1.5">3.1.1.5</a>&nbsp;<a href="#header.content-type">Content-Type</a></h4>
     974                  <p id="rfc.section.3.1.1.5.p.1">The "Content-Type" header field indicates the media type of the associated representation: either the representation enclosed
     975                     in the message payload or the <a href="#representations" class="smpl">selected representation</a>, as determined by the message semantics. The indicated media type defines both the data format and how that data is intended
     976                     to be processed by a recipient, within the scope of the received message semantics, after any content codings indicated by <a href="#header.content-encoding" class="smpl">Content-Encoding</a> are decoded.
     977                  </p>
     978                  <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.content-type" class="smpl">Content-Type</a> = <a href="#media.type" class="smpl">media-type</a>
     979</pre><p id="rfc.section.3.1.1.5.p.3">Media types are defined in <a href="#media.type" title="Media Type">Section&nbsp;3.1.1.1</a>. An example of the field is
     980                  </p>
     981                  <div id="rfc.figure.u.6"></div><pre class="text">  Content-Type: text/html; charset=ISO-8859-4
     982</pre><p id="rfc.section.3.1.1.5.p.5">A sender that generates a message containing a payload body <em class="bcp14">SHOULD</em> generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown
     983                     to the sender. If a Content-Type header field is not present, the recipient <em class="bcp14">MAY</em> either assume a media type of "application/octet-stream" (<a href="#RFC2046" id="rfc.xref.RFC2046.3"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[RFC2046]</cite></a>, <a href="http://tools.ietf.org/html/rfc2046#section-4.5.1">Section 4.5.1</a>) or examine the data to determine its type.
     984                  </p>
     985                  <p id="rfc.section.3.1.1.5.p.6">In practice, resource owners do not always properly configure their origin server to provide the correct Content-Type for
     986                     a given representation, with the result that some clients will examine a payload's content and override the specified type.
     987                     Clients that do so risk drawing incorrect conclusions, which might expose additional security risks (e.g., "privilege escalation").
     988                     Furthermore, it is impossible to determine the sender's intent by examining the data format: many data formats match multiple
     989                     media types that differ only in processing semantics. Implementers are encouraged to provide a means of disabling such "content
     990                     sniffing" when it is used.
     991                  </p>
     992               </div>
     993            </div>
     994            <div id="data.encoding">
     995               <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;<a href="#data.encoding">Encoding for Compression or Integrity</a></h3>
     996               <div id="content.codings">
     997                  <div id="rfc.iref.c.3"></div>
     998                  <div id="rfc.iref.c.4"></div>
     999                  <div id="rfc.iref.x.1"></div>
     1000                  <div id="rfc.iref.d.1"></div>
     1001                  <div id="rfc.iref.g.9"></div>
     1002                  <div id="rfc.iref.x.2"></div>
     1003                  <h4 id="rfc.section.3.1.2.1"><a href="#rfc.section.3.1.2.1">3.1.2.1</a>&nbsp;<a href="#content.codings">Content Codings</a></h4>
     1004                  <p id="rfc.section.3.1.2.1.p.1">Content coding values indicate an encoding transformation that has been or can be applied to a representation. Content codings
     1005                     are primarily used to allow a representation to be compressed or otherwise usefully transformed without losing the identity
     1006                     of its underlying media type and without loss of information. Frequently, the representation is stored in coded form, transmitted
     1007                     directly, and only decoded by the final recipient.
     1008                  </p>
     1009                  <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.10"></span>  <a href="#content.codings" class="smpl">content-coding</a>   = <a href="#imported.abnf" class="smpl">token</a>
     1010</pre><p id="rfc.section.3.1.2.1.p.3">All content-coding values are case-insensitive and ought to be registered within the HTTP Content Coding registry, as defined
     1011                     in <a href="#content.coding.registry" title="Content Coding Registry">Section&nbsp;8.4</a>. They are used in the <a href="#header.accept-encoding" class="smpl">Accept-Encoding</a> (<a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.1" title="Accept-Encoding">Section&nbsp;5.3.4</a>) and <a href="#header.content-encoding" class="smpl">Content-Encoding</a> (<a href="#header.content-encoding" id="rfc.xref.header.content-encoding.2" title="Content-Encoding">Section&nbsp;3.1.2.2</a>) header fields.
     1012                  </p>
     1013                  <p id="rfc.section.3.1.2.1.p.4">The following content-coding values are defined by this specification: </p>
     1014                  <ul class="empty">
     1015                     <li>compress (and x-compress): See <a href="p1-messaging.html#compress.coding" title="Compress Coding">Section 4.2.1</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
     1016                     </li>
     1017                     <li>deflate: See <a href="p1-messaging.html#deflate.coding" title="Deflate Coding">Section 4.2.2</a> of <a href="#Part1" id="rfc.xref.Part1.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
     1018                     </li>
     1019                     <li>gzip (and x-gzip): See <a href="p1-messaging.html#gzip.coding" title="Gzip Coding">Section 4.2.3</a> of <a href="#Part1" id="rfc.xref.Part1.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
     1020                     </li>
     1021                  </ul>
     1022               </div>
     1023               <div id="header.content-encoding">
     1024                  <div id="rfc.iref.c.5"></div>
     1025                  <h4 id="rfc.section.3.1.2.2"><a href="#rfc.section.3.1.2.2">3.1.2.2</a>&nbsp;<a href="#header.content-encoding">Content-Encoding</a></h4>
     1026                  <p id="rfc.section.3.1.2.2.p.1">The "Content-Encoding" header field indicates what content codings have been applied to the representation, beyond those inherent
     1027                     in the media type, and thus what decoding mechanisms have to be applied in order to obtain data in the media type referenced
     1028                     by the <a href="#header.content-type" class="smpl">Content-Type</a> header field. Content-Encoding is primarily used to allow a representation's data to be compressed without losing the identity
     1029                     of its underlying media type.
     1030                  </p>
     1031                  <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.11"></span>  <a href="#header.content-encoding" class="smpl">Content-Encoding</a> = 1#<a href="#content.codings" class="smpl">content-coding</a>
     1032</pre><p id="rfc.section.3.1.2.2.p.3">An example of its use is</p>
     1033                  <div id="rfc.figure.u.9"></div><pre class="text">  Content-Encoding: gzip
     1034</pre><p id="rfc.section.3.1.2.2.p.5">If one or more encodings have been applied to a representation, the sender that applied the encodings <em class="bcp14">MUST</em> generate a Content-Encoding header field that lists the content codings in the order in which they were applied. Additional
     1035                     information about the encoding parameters <em class="bcp14">MAY</em> be provided by other header fields not defined by this specification.
     1036                  </p>
     1037                  <p id="rfc.section.3.1.2.2.p.6">Unlike Transfer-Encoding (<a href="p1-messaging.html#header.transfer-encoding" title="Transfer-Encoding">Section 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>), the codings listed in Content-Encoding are a characteristic of the representation; the representation is defined in terms
     1038                     of the coded form, and all other metadata about the representation is about the coded form unless otherwise noted in the metadata
     1039                     definition. Typically, the representation is only decoded just prior to rendering or analogous usage.
     1040                  </p>
     1041                  <p id="rfc.section.3.1.2.2.p.7">If the media type includes an inherent encoding, such as a data format that is always compressed, then that encoding would
     1042                     not be restated in Content-Encoding even if it happens to be the same algorithm as one of the content codings. Such a content
     1043                     coding would only be listed if, for some bizarre reason, it is applied a second time to form the representation. Likewise,
     1044                     an origin server might choose to publish the same data as multiple representations that differ only in whether the coding
     1045                     is defined as part of <a href="#header.content-type" class="smpl">Content-Type</a> or Content-Encoding, since some user agents will behave differently in their handling of each response (e.g., open a "Save
     1046                     as ..." dialog instead of automatic decompression and rendering of content).
     1047                  </p>
     1048                  <p id="rfc.section.3.1.2.2.p.8">An origin server <em class="bcp14">MAY</em> respond with a status code of <a href="#status.415" class="smpl">415 (Unsupported Media Type)</a> if a representation in the request message has a content coding that is not acceptable.
     1049                  </p>
     1050               </div>
     1051            </div>
     1052            <div id="audience.language">
     1053               <h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;<a href="#audience.language">Audience Language</a></h3>
     1054               <div id="language.tags">
     1055                  <h4 id="rfc.section.3.1.3.1"><a href="#rfc.section.3.1.3.1">3.1.3.1</a>&nbsp;<a href="#language.tags">Language Tags</a></h4>
     1056                  <p id="rfc.section.3.1.3.1.p.1">A language tag, as defined in <a href="#RFC5646" id="rfc.xref.RFC5646.1"><cite title="Tags for Identifying Languages">[RFC5646]</cite></a>, identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to
     1057                     other human beings. Computer languages are explicitly excluded.
     1058                  </p>
     1059                  <p id="rfc.section.3.1.3.1.p.2">HTTP uses language tags within the <a href="#header.accept-language" class="smpl">Accept-Language</a> and <a href="#header.content-language" class="smpl">Content-Language</a> header fields. <a href="#header.accept-language" class="smpl">Accept-Language</a> uses the broader language-range production defined in <a href="#header.accept-language" id="rfc.xref.header.accept-language.1" title="Accept-Language">Section&nbsp;5.3.5</a>, whereas <a href="#header.content-language" class="smpl">Content-Language</a> uses the language-tag production defined below.
     1060                  </p>
     1061                  <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.12"></span>  <a href="#language.tags" class="smpl">language-tag</a> = &lt;Language-Tag, defined in <a href="#RFC5646" id="rfc.xref.RFC5646.2"><cite title="Tags for Identifying Languages">[RFC5646]</cite></a>, <a href="http://tools.ietf.org/html/rfc5646#section-2.1">Section 2.1</a>&gt;
     1062</pre><p id="rfc.section.3.1.3.1.p.4">A language tag is a sequence of one or more case-insensitive subtags, each separated by a hyphen character ("-", %x2D). In
     1063                     most cases, a language tag consists of a primary language subtag that identifies a broad family of related languages (e.g.,
     1064                     "en" = English) which is optionally followed by a series of subtags that refine or narrow that language's range (e.g., "en-CA"
     1065                     = the variety of English as communicated in Canada). Whitespace is not allowed within a language tag. Example tags include:
     1066                  </p>
     1067                  <div id="rfc.figure.u.11"></div><pre class="text">  fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
     1068</pre><p id="rfc.section.3.1.3.1.p.6">See <a href="#RFC5646" id="rfc.xref.RFC5646.3"><cite title="Tags for Identifying Languages">[RFC5646]</cite></a> for further information.
     1069                  </p>
     1070               </div>
     1071               <div id="header.content-language">
     1072                  <div id="rfc.iref.c.6"></div>
     1073                  <h4 id="rfc.section.3.1.3.2"><a href="#rfc.section.3.1.3.2">3.1.3.2</a>&nbsp;<a href="#header.content-language">Content-Language</a></h4>
     1074                  <p id="rfc.section.3.1.3.2.p.1">The "Content-Language" header field describes the natural language(s) of the intended audience for the representation. Note
     1075                     that this might not be equivalent to all the languages used within the representation.
     1076                  </p>
     1077                  <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.13"></span>  <a href="#header.content-language" class="smpl">Content-Language</a> = 1#<a href="#language.tags" class="smpl">language-tag</a>
     1078</pre><p id="rfc.section.3.1.3.2.p.3">Language tags are defined in <a href="#language.tags" title="Language Tags">Section&nbsp;3.1.3.1</a>. The primary purpose of Content-Language is to allow a user to identify and differentiate representations according to the
     1079                     users' own preferred language. Thus, if the content is intended only for a Danish-literate audience, the appropriate field
     1080                     is
     1081                  </p>
     1082                  <div id="rfc.figure.u.13"></div><pre class="text">  Content-Language: da
     1083</pre><p id="rfc.section.3.1.3.2.p.5">If no Content-Language is specified, the default is that the content is intended for all language audiences. This might mean
     1084                     that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language
     1085                     it is intended.
     1086                  </p>
     1087                  <p id="rfc.section.3.1.3.2.p.6">Multiple languages <em class="bcp14">MAY</em> be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi", presented
     1088                     simultaneously in the original Maori and English versions, would call for
     1089                  </p>
     1090                  <div id="rfc.figure.u.14"></div><pre class="text">  Content-Language: mi, en
     1091</pre><p id="rfc.section.3.1.3.2.p.8">However, just because multiple languages are present within a representation does not mean that it is intended for multiple
     1092                     linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin", which is clearly
     1093                     intended to be used by an English-literate audience. In this case, the Content-Language would properly only include "en".
     1094                  </p>
     1095                  <p id="rfc.section.3.1.3.2.p.9">Content-Language <em class="bcp14">MAY</em> be applied to any media type — it is not limited to textual documents.
     1096                  </p>
     1097               </div>
     1098            </div>
     1099            <div id="identification">
     1100               <h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;<a href="#identification">Identification</a></h3>
     1101               <div id="identifying.payload">
     1102                  <h4 id="rfc.section.3.1.4.1"><a href="#rfc.section.3.1.4.1">3.1.4.1</a>&nbsp;<a href="#identifying.payload">Identifying a Representation</a></h4>
     1103                  <p id="rfc.section.3.1.4.1.p.1">When a complete or partial representation is transferred in a message payload, it is often desirable for the sender to supply,
     1104                     or the recipient to determine, an identifier for a resource corresponding to that representation.
     1105                  </p>
     1106                  <p id="rfc.section.3.1.4.1.p.2">For a request message: </p>
     1107                  <ul>
     1108                     <li>If the request has a <a href="#header.content-location" class="smpl">Content-Location</a> header field, then the sender asserts that the payload is a representation of the resource identified by the Content-Location
     1109                        field-value. However, such an assertion cannot be trusted unless it can be verified by other means (not defined by this specification).
     1110                        The information might still be useful for revision history links.
     1111                     </li>
     1112                     <li>Otherwise, the payload is unidentified.</li>
     1113                  </ul>
     1114                  <p id="rfc.section.3.1.4.1.p.3">For a response message, the following rules are applied in order until a match is found: </p>
     1115                  <ol>
     1116                     <li>If the request is GET or HEAD and the response status code is <a href="#status.200" class="smpl">200 (OK)</a>, <a href="#status.204" class="smpl">204 (No Content)</a>, <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a>, or <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a>, the payload is a representation of the resource identified by the effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>).
     1117                     </li>
     1118                     <li>If the request is GET or HEAD and the response status code is <a href="#status.203" class="smpl">203 (Non-Authoritative Information)</a>, the payload is a potentially modified or enhanced representation of the <a href="#resources" class="smpl">target resource</a> as provided by an intermediary.
     1119                     </li>
     1120                     <li>If the response has a <a href="#header.content-location" class="smpl">Content-Location</a> header field and its field-value is a reference to the same URI as the effective request URI, the payload is a representation
     1121                        of the resource identified by the effective request URI.
     1122                     </li>
     1123                     <li>If the response has a <a href="#header.content-location" class="smpl">Content-Location</a> header field and its field-value is a reference to a URI different from the effective request URI, then the sender asserts
     1124                        that the payload is a representation of the resource identified by the Content-Location field-value. However, such an assertion
     1125                        cannot be trusted unless it can be verified by other means (not defined by this specification).
     1126                     </li>
     1127                     <li>Otherwise, the payload is unidentified.</li>
     1128                  </ol>
     1129               </div>
     1130               <div id="header.content-location">
     1131                  <div id="rfc.iref.c.7"></div>
     1132                  <h4 id="rfc.section.3.1.4.2"><a href="#rfc.section.3.1.4.2">3.1.4.2</a>&nbsp;<a href="#header.content-location">Content-Location</a></h4>
     1133                  <p id="rfc.section.3.1.4.2.p.1">The "Content-Location" header field references a URI that can be used as an identifier for a specific resource corresponding
     1134                     to the representation in this message's payload. In other words, if one were to perform a GET request on this URI at the time
     1135                     of this message's generation, then a <a href="#status.200" class="smpl">200 (OK)</a> response would contain the same representation that is enclosed as payload in this message.
     1136                  </p>
     1137                  <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.14"></span>  <a href="#header.content-location" class="smpl">Content-Location</a> = <a href="#imported.abnf" class="smpl">absolute-URI</a> / <a href="#imported.abnf" class="smpl">partial-URI</a>
     1138</pre><p id="rfc.section.3.1.4.2.p.3">The Content-Location value is not a replacement for the effective Request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). It is representation metadata. It has the same syntax and semantics as the header field of the same name defined for MIME
     1139                     body parts in <a href="http://tools.ietf.org/html/rfc2557#section-4">Section 4</a> of <a href="#RFC2557" id="rfc.xref.RFC2557.1"><cite title="MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)">[RFC2557]</cite></a>. However, its appearance in an HTTP message has some special implications for HTTP recipients.
     1140                  </p>
     1141                  <p id="rfc.section.3.1.4.2.p.4">If Content-Location is included in a <a href="#status.2xx" class="smpl">2xx (Successful)</a> response message and its value refers (after conversion to absolute form) to a URI that is the same as the effective request
     1142                     URI, then the recipient <em class="bcp14">MAY</em> consider the payload to be a current representation of that resource at the time indicated by the message origination date.
     1143                     For a GET or HEAD request, this is the same as the default semantics when no Content-Location is provided by the server. For
     1144                     a state-changing request like PUT or POST, it implies that the server's response contains the new representation of that resource,
     1145                     thereby distinguishing it from representations that might only report about the action (e.g., "It worked!"). This allows authoring
     1146                     applications to update their local copies without the need for a subsequent GET request.
     1147                  </p>
     1148                  <p id="rfc.section.3.1.4.2.p.5">If Content-Location is included in a <a href="#status.2xx" class="smpl">2xx (Successful)</a> response message and its field-value refers to a URI that differs from the effective request URI, then the origin server claims
     1149                     that the URI is an identifier for a different resource corresponding to the enclosed representation. Such a claim can only
     1150                     be trusted if both identifiers share the same resource owner, which cannot be programmatically determined via HTTP.
     1151                  </p>
     1152                  <ul>
     1153                     <li>For a response to a GET or HEAD request, this is an indication that the effective request URI refers to a resource that is
     1154                        subject to content negotiation and the Content-Location field-value is a more specific identifier for the <a href="#representations" class="smpl">selected representation</a>.
     1155                     </li>
     1156                     <li>For a <a href="#status.201" class="smpl">201 (Created)</a> response to a state-changing method, a Content-Location field-value that is identical to the <a href="#header.location" class="smpl">Location</a> field-value indicates that this payload is a current representation of the newly created resource.
     1157                     </li>
     1158                     <li>Otherwise, such a Content-Location indicates that this payload is a representation reporting on the requested action's status
     1159                        and that the same report is available (for future access with GET) at the given URI. For example, a purchase transaction made
     1160                        via a POST request might include a receipt document as the payload of the <a href="#status.200" class="smpl">200 (OK)</a> response; the Content-Location field-value provides an identifier for retrieving a copy of that same receipt in the future.
     1161                     </li>
     1162                  </ul>
     1163                  <p id="rfc.section.3.1.4.2.p.6">A user agent that sends Content-Location in a request message is stating that its value refers to where the user agent originally
     1164                     obtained the content of the enclosed representation (prior to any modifications made by that user agent). In other words,
     1165                     the user agent is providing a back link to the source of the original representation.
     1166                  </p>
     1167                  <p id="rfc.section.3.1.4.2.p.7">An origin server that receives a Content-Location field in a request message <em class="bcp14">MUST</em> treat the information as transitory request context rather than as metadata to be saved verbatim as part of the representation.
     1168                     An origin server <em class="bcp14">MAY</em> use that context to guide in processing the request or to save it for other uses, such as within source links or versioning
     1169                     metadata. However, an origin server <em class="bcp14">MUST NOT</em> use such context information to alter the request semantics.
     1170                  </p>
     1171                  <p id="rfc.section.3.1.4.2.p.8">For example, if a client makes a PUT request on a negotiated resource and the origin server accepts that PUT (without redirection),
     1172                     then the new state of that resource is expected to be consistent with the one representation supplied in that PUT; the Content-Location
     1173                     cannot be used as a form of reverse content selection identifier to update only one of the negotiated representations. If
     1174                     the user agent had wanted the latter semantics, it would have applied the PUT directly to the Content-Location URI.
     1175                  </p>
     1176               </div>
     1177            </div>
     1178         </div>
     1179         <div id="representation.data">
     1180            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#representation.data">Representation Data</a></h2>
     1181            <p id="rfc.section.3.2.p.1">The representation data associated with an HTTP message is either provided as the payload body of the message or referred
     1182               to by the message semantics and the effective request URI. The representation data is in a format and encoding defined by
     1183               the representation metadata header fields.
     1184            </p>
     1185            <p id="rfc.section.3.2.p.2">The data type of the representation data is determined via the header fields <a href="#header.content-type" class="smpl">Content-Type</a> and <a href="#header.content-encoding" class="smpl">Content-Encoding</a>. These define a two-layer, ordered encoding model:
     1186            </p>
     1187            <div id="rfc.figure.u.16"></div><pre class="text">  representation-data := Content-Encoding( Content-Type( bits ) )
     1188</pre></div>
     1189         <div id="payload">
     1190            <div id="rfc.iref.p.1"></div>
     1191            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#payload">Payload Semantics</a></h2>
     1192            <p id="rfc.section.3.3.p.1">Some HTTP messages transfer a complete or partial representation as the message "<dfn>payload</dfn>". In some cases, a payload might contain only the associated representation's header fields (e.g., responses to HEAD) or
     1193               only some part(s) of the representation data (e.g., the <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> status code).
     1194            </p>
     1195            <p id="rfc.section.3.3.p.2">The purpose of a payload in a request is defined by the method semantics. For example, a representation in the payload of
     1196               a PUT request (<a href="#PUT" id="rfc.xref.PUT.1" title="PUT">Section&nbsp;4.3.4</a>) represents the desired state of the <a href="#resources" class="smpl">target resource</a> if the request is successfully applied, whereas a representation in the payload of a POST request (<a href="#POST" id="rfc.xref.POST.1" title="POST">Section&nbsp;4.3.3</a>) represents an anonymous resource for providing data to be processed, such as the information that a user entered within
     1197               an HTML form.
     1198            </p>
     1199            <p id="rfc.section.3.3.p.3">In a response, the payload's purpose is defined by both the request method and the response status code. For example, the
     1200               payload of a <a href="#status.200" class="smpl">200 (OK)</a> response to GET (<a href="#GET" id="rfc.xref.GET.2" title="GET">Section&nbsp;4.3.1</a>) represents the current state of the <a href="#resources" class="smpl">target resource</a>, as observed at the time of the message origination date (<a href="#header.date" id="rfc.xref.header.date.1" title="Date">Section&nbsp;7.1.1.2</a>), whereas the payload of the same status code in a response to POST might represent either the processing result or the new
     1201               state of the target resource after applying the processing. Response messages with an error status code usually contain a
     1202               payload that represents the error condition, such that it describes the error state and what next steps are suggested for
     1203               resolving it.
     1204            </p>
     1205            <p id="rfc.section.3.3.p.4">Header fields that specifically describe the payload, rather than the associated representation, are referred to as "payload
     1206               header fields". Payload header fields are defined in other parts of this specification, due to their impact on message parsing.
     1207            </p>
     1208            <div id="rfc.table.u.2">
     1209               <table class="tt full left" cellpadding="3" cellspacing="0">
     1210                  <thead>
     1211                     <tr>
     1212                        <th>Header Field Name</th>
     1213                        <th>Defined in...</th>
     1214                     </tr>
     1215                  </thead>
     1216                  <tbody>
     1217                     <tr>
     1218                        <td class="left">Content-Length</td>
     1219                        <td class="left"><a href="p1-messaging.html#header.content-length" title="Content-Length">Section 3.3.2</a> of <a href="#Part1" id="rfc.xref.Part1.13"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
     1220                     </tr>
     1221                     <tr>
     1222                        <td class="left">Content-Range</td>
     1223                        <td class="left"><a href="p5-range.html#header.content-range" title="Content-Range">Section 4.2</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
     1224                     </tr>
     1225                     <tr>
     1226                        <td class="left">Transfer-Encoding</td>
     1227                        <td class="left"><a href="p1-messaging.html#header.transfer-encoding" title="Transfer-Encoding">Section 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.14"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
     1228                     </tr>
     1229                  </tbody>
     1230               </table>
     1231            </div>
     1232         </div>
     1233         <div id="content.negotiation">
     1234            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a href="#content.negotiation">Content Negotiation</a></h2>
     1235            <p id="rfc.section.3.4.p.1">When responses convey payload information, whether indicating a success or an error, the origin server often has different
     1236               ways of representing that information; for example, in different formats, languages, or encodings. Likewise, different users
     1237               or user agents might have differing capabilities, characteristics, or preferences that could influence which representation,
     1238               among those available, would be best to deliver. For this reason, HTTP provides mechanisms for <a href="#content.negotiation" class="smpl">content negotiation</a>.
     1239            </p>
     1240            <p id="rfc.section.3.4.p.2">This specification defines two patterns of content negotiation that can be made visible within the protocol: "proactive",
     1241               where the server selects the representation based upon the user agent's stated preferences, and "reactive" negotiation, where
     1242               the server provides a list of representations for the user agent to choose from. Other patterns of content negotiation include
     1243               "conditional content", where the representation consists of multiple parts that are selectively rendered based on user agent
     1244               parameters, "active content", where the representation contains a script that makes additional (more specific) requests based
     1245               on the user agent characteristics, and "Transparent Content Negotiation" (<a href="#RFC2295" id="rfc.xref.RFC2295.1"><cite title="Transparent Content Negotiation in HTTP">[RFC2295]</cite></a>), where content selection is performed by an intermediary. These patterns are not mutually exclusive, and each has trade-offs
     1246               in applicability and practicality.
     1247            </p>
     1248            <p id="rfc.section.3.4.p.3">Note that, in all cases, HTTP is not aware of the resource semantics. The consistency with which an origin server responds
     1249               to requests, over time and over the varying dimensions of content negotiation, and thus the "sameness" of a resource's observed
     1250               representations over time, is determined entirely by whatever entity or algorithm selects or generates those responses. HTTP
     1251               pays no attention to the man behind the curtain.
     1252            </p>
     1253            <div id="proactive.negotiation">
     1254               <h3 id="rfc.section.3.4.1"><a href="#rfc.section.3.4.1">3.4.1</a>&nbsp;<a href="#proactive.negotiation">Proactive Negotiation</a></h3>
     1255               <p id="rfc.section.3.4.1.p.1">When content negotiation preferences are sent by the user agent in a request to encourage an algorithm located at the server
     1256                  to select the preferred representation, it is called <dfn>proactive negotiation</dfn> (a.k.a., <dfn>server-driven negotiation</dfn>). Selection is based on the available representations for a response (the dimensions over which it might vary, such as language,
     1257                  content-coding, etc.) compared to various information supplied in the request, including both the explicit negotiation fields
     1258                  of <a href="#request.conneg" title="Content Negotiation">Section&nbsp;5.3</a> and implicit characteristics, such as the client's network address or parts of the <a href="#header.user-agent" class="smpl">User-Agent</a> field.
     1259               </p>
     1260               <p id="rfc.section.3.4.1.p.2">Proactive negotiation is advantageous when the algorithm for selecting from among the available representations is difficult
     1261                  to describe to a user agent, or when the server desires to send its "best guess" to the user agent along with the first response
     1262                  (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to
     1263                  improve the server's guess, a user agent <em class="bcp14">MAY</em> send request header fields that describe its preferences.
     1264               </p>
     1265               <p id="rfc.section.3.4.1.p.3">Proactive negotiation has serious disadvantages: </p>
     1266               <ul>
     1267                  <li>It is impossible for the server to accurately determine what might be "best" for any given user, since that would require
     1268                     complete knowledge of both the capabilities of the user agent and the intended use for the response (e.g., does the user want
     1269                     to view it on screen or print it on paper?);
     1270                  </li>
     1271                  <li>Having the user agent describe its capabilities in every request can be both very inefficient (given that only a small percentage
     1272                     of responses have multiple representations) and a potential risk to the user's privacy;
     1273                  </li>
     1274                  <li>It complicates the implementation of an origin server and the algorithms for generating responses to a request; and,</li>
     1275                  <li>It limits the reusability of responses for shared caching.</li>
     1276               </ul>
     1277               <p id="rfc.section.3.4.1.p.4">A user agent cannot rely on proactive negotiation preferences being consistently honored, since the origin server might not
     1278                  implement proactive negotiation for the requested resource or might decide that sending a response that doesn't conform to
     1279                  the user agent's preferences is better than sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response.
     1280               </p>
     1281               <p id="rfc.section.3.4.1.p.5">An origin server <em class="bcp14">MAY</em> generate a <a href="#header.vary" class="smpl">Vary</a> header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;7.1.4</a>) in responses that are subject to proactive negotiation to indicate what parameters of request information might be used
     1282                  in its selection algorithm, thereby providing a means for recipients to determine the reusability of that same response for
     1283                  user agents with differing request information.
     1284               </p>
     1285            </div>
     1286            <div id="reactive.negotiation">
     1287               <h3 id="rfc.section.3.4.2"><a href="#rfc.section.3.4.2">3.4.2</a>&nbsp;<a href="#reactive.negotiation">Reactive Negotiation</a></h3>
     1288               <p id="rfc.section.3.4.2.p.1">With <dfn>reactive negotiation</dfn> (a.k.a., <dfn>agent-driven negotiation</dfn>), selection of the best response representation (regardless of the status code) is performed by the user agent after receiving
     1289                  an initial response from the origin server that contains a list of resources for alternative representations. If the user
     1290                  agent is not satisfied by the initial response representation, it can perform a GET request on one or more of the alternative
     1291                  resources, selected based on metadata included in the list, to obtain a different form of representation for that response.
     1292                  Selection of alternatives might be performed automatically by the user agent or manually by the user selecting from a generated
     1293                  (possibly hypertext) menu.
     1294               </p>
     1295               <p id="rfc.section.3.4.2.p.2">Note that the above refers to representations of the response, in general, not representations of the resource. The alternative
     1296                  representations are only considered representations of the target resource if the response in which those alternatives are
     1297                  provided has the semantics of being a representation of the target resource (e.g., a <a href="#status.200" class="smpl">200 (OK)</a> response to a GET request) or has the semantics of providing links to alternative representations for the target resource
     1298                  (e.g., a <a href="#status.300" class="smpl">300 (Multiple Choices)</a> response to a GET request).
     1299               </p>
     1300               <p id="rfc.section.3.4.2.p.3">A server might choose not to send an initial representation, other than the list of alternatives, and thereby indicate that
     1301                  reactive negotiation by the user agent is preferred. For example, the alternatives listed in responses with the <a href="#status.300" class="smpl">300 (Multiple Choices)</a> and <a href="#status.406" class="smpl">406 (Not Acceptable)</a> status codes include information about the available representations so that the user or user agent can react by making a
     1302                  selection.
     1303               </p>
     1304               <p id="rfc.section.3.4.2.p.4">Reactive negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or
     1305                  encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally
     1306                  when public caches are used to distribute server load and reduce network usage.
     1307               </p>
     1308               <p id="rfc.section.3.4.2.p.5">Reactive negotiation suffers from the disadvantages of transmitting a list of alternatives to the user agent, which degrades
     1309                  user-perceived latency if transmitted in the header section, and needing a second request to obtain an alternate representation.
     1310                  Furthermore, this specification does not define a mechanism for supporting automatic selection, though it does not prevent
     1311                  such a mechanism from being developed as an extension.
     1312               </p>
     1313            </div>
     1314         </div>
    9171315      </div>
    918       <h4 id="rfc.section.3.1.1.2"><a href="#rfc.section.3.1.1.2">3.1.1.2</a>&nbsp;<a id="charset" href="#charset">Charset</a></h4>
    919       <p id="rfc.section.3.1.1.2.p.1">HTTP uses <dfn>charset</dfn> names to indicate or negotiate the character encoding scheme of a textual representation <a href="#RFC6365" id="rfc.xref.RFC6365.2"><cite title="Terminology Used in Internationalization in the IETF">[RFC6365]</cite></a>. A charset is identified by a case-insensitive token.
    920       </p>
    921       <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#charset" class="smpl">charset</a> = <a href="#imported.abnf" class="smpl">token</a>
    922 </pre><p id="rfc.section.3.1.1.2.p.3">Charset names ought to be registered in IANA Character Set registry (&lt;<a href="http://www.iana.org/assignments/character-sets">http://www.iana.org/assignments/character-sets</a>&gt;) according to the procedures defined in <a href="#RFC2978" id="rfc.xref.RFC2978.1"><cite title="IANA Charset Registration Procedures">[RFC2978]</cite></a>.
    923       </p>
    924       <h4 id="rfc.section.3.1.1.3"><a href="#rfc.section.3.1.1.3">3.1.1.3</a>&nbsp;<a id="canonicalization.and.text.defaults" href="#canonicalization.and.text.defaults">Canonicalization and Text Defaults</a></h4>
    925       <p id="rfc.section.3.1.1.3.p.1">Internet media types are registered with a canonical form in order to be interoperable among systems with varying native encoding
    926          formats. Representations selected or transferred via HTTP ought to be in canonical form, for many of the same reasons described
    927          by the Multipurpose Internet Mail Extensions (MIME) <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[RFC2045]</cite></a>. However, the performance characteristics of email deployments (i.e., store and forward messages to peers) are significantly
    928          different from those common to HTTP and the Web (server-based information services). Furthermore, MIME's constraints for the
    929          sake of compatibility with older mail transfer protocols do not apply to HTTP (see <a href="#differences.between.http.and.mime" title="Differences between HTTP and MIME">Appendix&nbsp;A</a>).
    930       </p>
    931       <p id="rfc.section.3.1.1.3.p.2">MIME's canonical form requires that media subtypes of the "text" type use CRLF as the text line break. HTTP allows the transfer
    932          of text media with plain CR or LF alone representing a line break, when such line breaks are consistent for an entire representation.
    933          An HTTP sender <em class="bcp14">MAY</em> generate, and a recipient <em class="bcp14">MUST</em> be able to parse, line breaks in text media that consist of CRLF, bare CR, or bare LF. In addition, text media in HTTP is
    934          not limited to charsets that use octets 13 and 10 for CR and LF, respectively. This flexibility regarding line breaks applies
    935          only to text within a representation that has been assigned a "text" media type; it does not apply to "multipart" types or
    936          HTTP elements outside the payload body (e.g., header fields).
    937       </p>
    938       <p id="rfc.section.3.1.1.3.p.3">If a representation is encoded with a content-coding, the underlying data ought to be in a form defined above prior to being
    939          encoded.
    940       </p>
    941       <h4 id="rfc.section.3.1.1.4"><a href="#rfc.section.3.1.1.4">3.1.1.4</a>&nbsp;<a id="multipart.types" href="#multipart.types">Multipart Types</a></h4>
    942       <p id="rfc.section.3.1.1.4.p.1">MIME provides for a number of "multipart" types — encapsulations of one or more representations within a single message body.
    943          All multipart types share a common syntax, as defined in <a href="http://tools.ietf.org/html/rfc2046#section-5.1.1">Section 5.1.1</a> of <a href="#RFC2046" id="rfc.xref.RFC2046.2"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[RFC2046]</cite></a>, and include a boundary parameter as part of the media type value. The message body is itself a protocol element; a sender <em class="bcp14">MUST</em> generate only CRLF to represent line breaks between body parts.
    944       </p>
    945       <p id="rfc.section.3.1.1.4.p.2">HTTP message framing does not use the multipart boundary as an indicator of message body length, though it might be used by
    946          implementations that generate or process the payload. For example, the "multipart/form-data" type is often used for carrying
    947          form data in a request, as described in <a href="#RFC2388" id="rfc.xref.RFC2388.1"><cite title="Returning Values from Forms: multipart/form-data">[RFC2388]</cite></a>, and the "multipart/byteranges" type is defined by this specification for use in some <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> responses <a href="#Part5" id="rfc.xref.Part5.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>.
    948       </p>
    949       <div id="rfc.iref.c.2"></div>
    950       <h4 id="rfc.section.3.1.1.5"><a href="#rfc.section.3.1.1.5">3.1.1.5</a>&nbsp;<a id="header.content-type" href="#header.content-type">Content-Type</a></h4>
    951       <p id="rfc.section.3.1.1.5.p.1">The "Content-Type" header field indicates the media type of the associated representation: either the representation enclosed
    952          in the message payload or the <a href="#representations" class="smpl">selected representation</a>, as determined by the message semantics. The indicated media type defines both the data format and how that data is intended
    953          to be processed by a recipient, within the scope of the received message semantics, after any content codings indicated by <a href="#header.content-encoding" class="smpl">Content-Encoding</a> are decoded.
    954       </p>
    955       <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.content-type" class="smpl">Content-Type</a> = <a href="#media.type" class="smpl">media-type</a>
    956 </pre><p id="rfc.section.3.1.1.5.p.3">Media types are defined in <a href="#media.type" title="Media Type">Section&nbsp;3.1.1.1</a>. An example of the field is
    957       </p>
    958       <div id="rfc.figure.u.6"></div><pre class="text">  Content-Type: text/html; charset=ISO-8859-4
    959 </pre><p id="rfc.section.3.1.1.5.p.5">A sender that generates a message containing a payload body <em class="bcp14">SHOULD</em> generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown
    960          to the sender. If a Content-Type header field is not present, the recipient <em class="bcp14">MAY</em> either assume a media type of "application/octet-stream" (<a href="#RFC2046" id="rfc.xref.RFC2046.3"><cite title="Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types">[RFC2046]</cite></a>, <a href="http://tools.ietf.org/html/rfc2046#section-4.5.1">Section 4.5.1</a>) or examine the data to determine its type.
    961       </p>
    962       <p id="rfc.section.3.1.1.5.p.6">In practice, resource owners do not always properly configure their origin server to provide the correct Content-Type for
    963          a given representation, with the result that some clients will examine a payload's content and override the specified type.
    964          Clients that do so risk drawing incorrect conclusions, which might expose additional security risks (e.g., "privilege escalation").
    965          Furthermore, it is impossible to determine the sender's intent by examining the data format: many data formats match multiple
    966          media types that differ only in processing semantics. Implementers are encouraged to provide a means of disabling such "content
    967          sniffing" when it is used.
    968       </p>
    969       <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;<a id="data.encoding" href="#data.encoding">Encoding for Compression or Integrity</a></h3>
    970       <div id="rfc.iref.c.3"></div>
    971       <div id="rfc.iref.c.4"></div>
    972       <div id="rfc.iref.x.1"></div>
    973       <div id="rfc.iref.d.1"></div>
    974       <div id="rfc.iref.g.9"></div>
    975       <div id="rfc.iref.x.2"></div>
    976       <h4 id="rfc.section.3.1.2.1"><a href="#rfc.section.3.1.2.1">3.1.2.1</a>&nbsp;<a id="content.codings" href="#content.codings">Content Codings</a></h4>
    977       <p id="rfc.section.3.1.2.1.p.1">Content coding values indicate an encoding transformation that has been or can be applied to a representation. Content codings
    978          are primarily used to allow a representation to be compressed or otherwise usefully transformed without losing the identity
    979          of its underlying media type and without loss of information. Frequently, the representation is stored in coded form, transmitted
    980          directly, and only decoded by the final recipient.
    981       </p>
    982       <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.10"></span>  <a href="#content.codings" class="smpl">content-coding</a>   = <a href="#imported.abnf" class="smpl">token</a>
    983 </pre><p id="rfc.section.3.1.2.1.p.3">All content-coding values are case-insensitive and ought to be registered within the HTTP Content Coding registry, as defined
    984          in <a href="#content.coding.registry" title="Content Coding Registry">Section&nbsp;8.4</a>. They are used in the <a href="#header.accept-encoding" class="smpl">Accept-Encoding</a> (<a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.1" title="Accept-Encoding">Section&nbsp;5.3.4</a>) and <a href="#header.content-encoding" class="smpl">Content-Encoding</a> (<a href="#header.content-encoding" id="rfc.xref.header.content-encoding.2" title="Content-Encoding">Section&nbsp;3.1.2.2</a>) header fields.
    985       </p>
    986       <p id="rfc.section.3.1.2.1.p.4">The following content-coding values are defined by this specification: </p>
    987       <ul class="empty">
    988          <li>compress (and x-compress): See <a href="p1-messaging.html#compress.coding" title="Compress Coding">Section 4.2.1</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
    989          </li>
    990          <li>deflate: See <a href="p1-messaging.html#deflate.coding" title="Deflate Coding">Section 4.2.2</a> of <a href="#Part1" id="rfc.xref.Part1.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
    991          </li>
    992          <li>gzip (and x-gzip): See <a href="p1-messaging.html#gzip.coding" title="Gzip Coding">Section 4.2.3</a> of <a href="#Part1" id="rfc.xref.Part1.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
    993          </li>
    994       </ul>
    995       <div id="rfc.iref.c.5"></div>
    996       <h4 id="rfc.section.3.1.2.2"><a href="#rfc.section.3.1.2.2">3.1.2.2</a>&nbsp;<a id="header.content-encoding" href="#header.content-encoding">Content-Encoding</a></h4>
    997       <p id="rfc.section.3.1.2.2.p.1">The "Content-Encoding" header field indicates what content codings have been applied to the representation, beyond those inherent
    998          in the media type, and thus what decoding mechanisms have to be applied in order to obtain data in the media type referenced
    999          by the <a href="#header.content-type" class="smpl">Content-Type</a> header field. Content-Encoding is primarily used to allow a representation's data to be compressed without losing the identity
    1000          of its underlying media type.
    1001       </p>
    1002       <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.11"></span>  <a href="#header.content-encoding" class="smpl">Content-Encoding</a> = 1#<a href="#content.codings" class="smpl">content-coding</a>
    1003 </pre><p id="rfc.section.3.1.2.2.p.3">An example of its use is</p>
    1004       <div id="rfc.figure.u.9"></div><pre class="text">  Content-Encoding: gzip
    1005 </pre><p id="rfc.section.3.1.2.2.p.5">If one or more encodings have been applied to a representation, the sender that applied the encodings <em class="bcp14">MUST</em> generate a Content-Encoding header field that lists the content codings in the order in which they were applied. Additional
    1006          information about the encoding parameters <em class="bcp14">MAY</em> be provided by other header fields not defined by this specification.
    1007       </p>
    1008       <p id="rfc.section.3.1.2.2.p.6">Unlike Transfer-Encoding (<a href="p1-messaging.html#header.transfer-encoding" title="Transfer-Encoding">Section 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>), the codings listed in Content-Encoding are a characteristic of the representation; the representation is defined in terms
    1009          of the coded form, and all other metadata about the representation is about the coded form unless otherwise noted in the metadata
    1010          definition. Typically, the representation is only decoded just prior to rendering or analogous usage.
    1011       </p>
    1012       <p id="rfc.section.3.1.2.2.p.7">If the media type includes an inherent encoding, such as a data format that is always compressed, then that encoding would
    1013          not be restated in Content-Encoding even if it happens to be the same algorithm as one of the content codings. Such a content
    1014          coding would only be listed if, for some bizarre reason, it is applied a second time to form the representation. Likewise,
    1015          an origin server might choose to publish the same data as multiple representations that differ only in whether the coding
    1016          is defined as part of <a href="#header.content-type" class="smpl">Content-Type</a> or Content-Encoding, since some user agents will behave differently in their handling of each response (e.g., open a "Save
    1017          as ..." dialog instead of automatic decompression and rendering of content).
    1018       </p>
    1019       <p id="rfc.section.3.1.2.2.p.8">An origin server <em class="bcp14">MAY</em> respond with a status code of <a href="#status.415" class="smpl">415 (Unsupported Media Type)</a> if a representation in the request message has a content coding that is not acceptable.
    1020       </p>
    1021       <h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;<a id="audience.language" href="#audience.language">Audience Language</a></h3>
    1022       <h4 id="rfc.section.3.1.3.1"><a href="#rfc.section.3.1.3.1">3.1.3.1</a>&nbsp;<a id="language.tags" href="#language.tags">Language Tags</a></h4>
    1023       <p id="rfc.section.3.1.3.1.p.1">A language tag, as defined in <a href="#RFC5646" id="rfc.xref.RFC5646.1"><cite title="Tags for Identifying Languages">[RFC5646]</cite></a>, identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to
    1024          other human beings. Computer languages are explicitly excluded.
    1025       </p>
    1026       <p id="rfc.section.3.1.3.1.p.2">HTTP uses language tags within the <a href="#header.accept-language" class="smpl">Accept-Language</a> and <a href="#header.content-language" class="smpl">Content-Language</a> header fields. <a href="#header.accept-language" class="smpl">Accept-Language</a> uses the broader language-range production defined in <a href="#header.accept-language" id="rfc.xref.header.accept-language.1" title="Accept-Language">Section&nbsp;5.3.5</a>, whereas <a href="#header.content-language" class="smpl">Content-Language</a> uses the language-tag production defined below.
    1027       </p>
    1028       <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.12"></span>  <a href="#language.tags" class="smpl">language-tag</a> = &lt;Language-Tag, defined in <a href="#RFC5646" id="rfc.xref.RFC5646.2"><cite title="Tags for Identifying Languages">[RFC5646]</cite></a>, <a href="http://tools.ietf.org/html/rfc5646#section-2.1">Section 2.1</a>&gt;
    1029 </pre><p id="rfc.section.3.1.3.1.p.4">A language tag is a sequence of one or more case-insensitive subtags, each separated by a hyphen character ("-", %x2D). In
    1030          most cases, a language tag consists of a primary language subtag that identifies a broad family of related languages (e.g.,
    1031          "en" = English) which is optionally followed by a series of subtags that refine or narrow that language's range (e.g., "en-CA"
    1032          = the variety of English as communicated in Canada). Whitespace is not allowed within a language tag. Example tags include:
    1033       </p>
    1034       <div id="rfc.figure.u.11"></div><pre class="text">  fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
    1035 </pre><p id="rfc.section.3.1.3.1.p.6">See <a href="#RFC5646" id="rfc.xref.RFC5646.3"><cite title="Tags for Identifying Languages">[RFC5646]</cite></a> for further information.
    1036       </p>
    1037       <div id="rfc.iref.c.6"></div>
    1038       <h4 id="rfc.section.3.1.3.2"><a href="#rfc.section.3.1.3.2">3.1.3.2</a>&nbsp;<a id="header.content-language" href="#header.content-language">Content-Language</a></h4>
    1039       <p id="rfc.section.3.1.3.2.p.1">The "Content-Language" header field describes the natural language(s) of the intended audience for the representation. Note
    1040          that this might not be equivalent to all the languages used within the representation.
    1041       </p>
    1042       <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.13"></span>  <a href="#header.content-language" class="smpl">Content-Language</a> = 1#<a href="#language.tags" class="smpl">language-tag</a>
    1043 </pre><p id="rfc.section.3.1.3.2.p.3">Language tags are defined in <a href="#language.tags" title="Language Tags">Section&nbsp;3.1.3.1</a>. The primary purpose of Content-Language is to allow a user to identify and differentiate representations according to the
    1044          users' own preferred language. Thus, if the content is intended only for a Danish-literate audience, the appropriate field
    1045          is
    1046       </p>
    1047       <div id="rfc.figure.u.13"></div><pre class="text">  Content-Language: da
    1048 </pre><p id="rfc.section.3.1.3.2.p.5">If no Content-Language is specified, the default is that the content is intended for all language audiences. This might mean
    1049          that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language
    1050          it is intended.
    1051       </p>
    1052       <p id="rfc.section.3.1.3.2.p.6">Multiple languages <em class="bcp14">MAY</em> be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi", presented
    1053          simultaneously in the original Maori and English versions, would call for
    1054       </p>
    1055       <div id="rfc.figure.u.14"></div><pre class="text">  Content-Language: mi, en
    1056 </pre><p id="rfc.section.3.1.3.2.p.8">However, just because multiple languages are present within a representation does not mean that it is intended for multiple
    1057          linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin", which is clearly
    1058          intended to be used by an English-literate audience. In this case, the Content-Language would properly only include "en".
    1059       </p>
    1060       <p id="rfc.section.3.1.3.2.p.9">Content-Language <em class="bcp14">MAY</em> be applied to any media type — it is not limited to textual documents.
    1061       </p>
    1062       <h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;<a id="identification" href="#identification">Identification</a></h3>
    1063       <h4 id="rfc.section.3.1.4.1"><a href="#rfc.section.3.1.4.1">3.1.4.1</a>&nbsp;<a id="identifying.payload" href="#identifying.payload">Identifying a Representation</a></h4>
    1064       <p id="rfc.section.3.1.4.1.p.1">When a complete or partial representation is transferred in a message payload, it is often desirable for the sender to supply,
    1065          or the recipient to determine, an identifier for a resource corresponding to that representation.
    1066       </p>
    1067       <p id="rfc.section.3.1.4.1.p.2">For a request message: </p>
    1068       <ul>
    1069          <li>If the request has a <a href="#header.content-location" class="smpl">Content-Location</a> header field, then the sender asserts that the payload is a representation of the resource identified by the Content-Location
    1070             field-value. However, such an assertion cannot be trusted unless it can be verified by other means (not defined by this specification).
    1071             The information might still be useful for revision history links.
    1072          </li>
    1073          <li>Otherwise, the payload is unidentified.</li>
    1074       </ul>
    1075       <p id="rfc.section.3.1.4.1.p.3">For a response message, the following rules are applied in order until a match is found: </p>
    1076       <ol>
    1077          <li>If the request is GET or HEAD and the response status code is <a href="#status.200" class="smpl">200 (OK)</a>, <a href="#status.204" class="smpl">204 (No Content)</a>, <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a>, or <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a>, the payload is a representation of the resource identified by the effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>).
    1078          </li>
    1079          <li>If the request is GET or HEAD and the response status code is <a href="#status.203" class="smpl">203 (Non-Authoritative Information)</a>, the payload is a potentially modified or enhanced representation of the <a href="#resources" class="smpl">target resource</a> as provided by an intermediary.
    1080          </li>
    1081          <li>If the response has a <a href="#header.content-location" class="smpl">Content-Location</a> header field and its field-value is a reference to the same URI as the effective request URI, the payload is a representation
    1082             of the resource identified by the effective request URI.
    1083          </li>
    1084          <li>If the response has a <a href="#header.content-location" class="smpl">Content-Location</a> header field and its field-value is a reference to a URI different from the effective request URI, then the sender asserts
    1085             that the payload is a representation of the resource identified by the Content-Location field-value. However, such an assertion
    1086             cannot be trusted unless it can be verified by other means (not defined by this specification).
    1087          </li>
    1088          <li>Otherwise, the payload is unidentified.</li>
    1089       </ol>
    1090       <div id="rfc.iref.c.7"></div>
    1091       <h4 id="rfc.section.3.1.4.2"><a href="#rfc.section.3.1.4.2">3.1.4.2</a>&nbsp;<a id="header.content-location" href="#header.content-location">Content-Location</a></h4>
    1092       <p id="rfc.section.3.1.4.2.p.1">The "Content-Location" header field references a URI that can be used as an identifier for a specific resource corresponding
    1093          to the representation in this message's payload. In other words, if one were to perform a GET request on this URI at the time
    1094          of this message's generation, then a <a href="#status.200" class="smpl">200 (OK)</a> response would contain the same representation that is enclosed as payload in this message.
    1095       </p>
    1096       <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.14"></span>  <a href="#header.content-location" class="smpl">Content-Location</a> = <a href="#imported.abnf" class="smpl">absolute-URI</a> / <a href="#imported.abnf" class="smpl">partial-URI</a>
    1097 </pre><p id="rfc.section.3.1.4.2.p.3">The Content-Location value is not a replacement for the effective Request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). It is representation metadata. It has the same syntax and semantics as the header field of the same name defined for MIME
    1098          body parts in <a href="http://tools.ietf.org/html/rfc2557#section-4">Section 4</a> of <a href="#RFC2557" id="rfc.xref.RFC2557.1"><cite title="MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)">[RFC2557]</cite></a>. However, its appearance in an HTTP message has some special implications for HTTP recipients.
    1099       </p>
    1100       <p id="rfc.section.3.1.4.2.p.4">If Content-Location is included in a <a href="#status.2xx" class="smpl">2xx (Successful)</a> response message and its value refers (after conversion to absolute form) to a URI that is the same as the effective request
    1101          URI, then the recipient <em class="bcp14">MAY</em> consider the payload to be a current representation of that resource at the time indicated by the message origination date.
    1102          For a GET or HEAD request, this is the same as the default semantics when no Content-Location is provided by the server. For
    1103          a state-changing request like PUT or POST, it implies that the server's response contains the new representation of that resource,
    1104          thereby distinguishing it from representations that might only report about the action (e.g., "It worked!"). This allows authoring
    1105          applications to update their local copies without the need for a subsequent GET request.
    1106       </p>
    1107       <p id="rfc.section.3.1.4.2.p.5">If Content-Location is included in a <a href="#status.2xx" class="smpl">2xx (Successful)</a> response message and its field-value refers to a URI that differs from the effective request URI, then the origin server claims
    1108          that the URI is an identifier for a different resource corresponding to the enclosed representation. Such a claim can only
    1109          be trusted if both identifiers share the same resource owner, which cannot be programmatically determined via HTTP.
    1110       </p>
    1111       <ul>
    1112          <li>For a response to a GET or HEAD request, this is an indication that the effective request URI refers to a resource that is
    1113             subject to content negotiation and the Content-Location field-value is a more specific identifier for the <a href="#representations" class="smpl">selected representation</a>.
    1114          </li>
    1115          <li>For a <a href="#status.201" class="smpl">201 (Created)</a> response to a state-changing method, a Content-Location field-value that is identical to the <a href="#header.location" class="smpl">Location</a> field-value indicates that this payload is a current representation of the newly created resource.
    1116          </li>
    1117          <li>Otherwise, such a Content-Location indicates that this payload is a representation reporting on the requested action's status
    1118             and that the same report is available (for future access with GET) at the given URI. For example, a purchase transaction made
    1119             via a POST request might include a receipt document as the payload of the <a href="#status.200" class="smpl">200 (OK)</a> response; the Content-Location field-value provides an identifier for retrieving a copy of that same receipt in the future.
    1120          </li>
    1121       </ul>
    1122       <p id="rfc.section.3.1.4.2.p.6">A user agent that sends Content-Location in a request message is stating that its value refers to where the user agent originally
    1123          obtained the content of the enclosed representation (prior to any modifications made by that user agent). In other words,
    1124          the user agent is providing a back link to the source of the original representation.
    1125       </p>
    1126       <p id="rfc.section.3.1.4.2.p.7">An origin server that receives a Content-Location field in a request message <em class="bcp14">MUST</em> treat the information as transitory request context rather than as metadata to be saved verbatim as part of the representation.
    1127          An origin server <em class="bcp14">MAY</em> use that context to guide in processing the request or to save it for other uses, such as within source links or versioning
    1128          metadata. However, an origin server <em class="bcp14">MUST NOT</em> use such context information to alter the request semantics.
    1129       </p>
    1130       <p id="rfc.section.3.1.4.2.p.8">For example, if a client makes a PUT request on a negotiated resource and the origin server accepts that PUT (without redirection),
    1131          then the new state of that resource is expected to be consistent with the one representation supplied in that PUT; the Content-Location
    1132          cannot be used as a form of reverse content selection identifier to update only one of the negotiated representations. If
    1133          the user agent had wanted the latter semantics, it would have applied the PUT directly to the Content-Location URI.
    1134       </p>
    1135       <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="representation.data" href="#representation.data">Representation Data</a></h2>
    1136       <p id="rfc.section.3.2.p.1">The representation data associated with an HTTP message is either provided as the payload body of the message or referred
    1137          to by the message semantics and the effective request URI. The representation data is in a format and encoding defined by
    1138          the representation metadata header fields.
    1139       </p>
    1140       <p id="rfc.section.3.2.p.2">The data type of the representation data is determined via the header fields <a href="#header.content-type" class="smpl">Content-Type</a> and <a href="#header.content-encoding" class="smpl">Content-Encoding</a>. These define a two-layer, ordered encoding model:
    1141       </p>
    1142       <div id="rfc.figure.u.16"></div><pre class="text">  representation-data := Content-Encoding( Content-Type( bits ) )
    1143 </pre><div id="rfc.iref.p.1"></div>
    1144       <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="payload" href="#payload">Payload Semantics</a></h2>
    1145       <p id="rfc.section.3.3.p.1">Some HTTP messages transfer a complete or partial representation as the message "<dfn>payload</dfn>". In some cases, a payload might contain only the associated representation's header fields (e.g., responses to HEAD) or
    1146          only some part(s) of the representation data (e.g., the <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> status code).
    1147       </p>
    1148       <p id="rfc.section.3.3.p.2">The purpose of a payload in a request is defined by the method semantics. For example, a representation in the payload of
    1149          a PUT request (<a href="#PUT" id="rfc.xref.PUT.1" title="PUT">Section&nbsp;4.3.4</a>) represents the desired state of the <a href="#resources" class="smpl">target resource</a> if the request is successfully applied, whereas a representation in the payload of a POST request (<a href="#POST" id="rfc.xref.POST.1" title="POST">Section&nbsp;4.3.3</a>) represents an anonymous resource for providing data to be processed, such as the information that a user entered within
    1150          an HTML form.
    1151       </p>
    1152       <p id="rfc.section.3.3.p.3">In a response, the payload's purpose is defined by both the request method and the response status code. For example, the
    1153          payload of a <a href="#status.200" class="smpl">200 (OK)</a> response to GET (<a href="#GET" id="rfc.xref.GET.2" title="GET">Section&nbsp;4.3.1</a>) represents the current state of the <a href="#resources" class="smpl">target resource</a>, as observed at the time of the message origination date (<a href="#header.date" id="rfc.xref.header.date.1" title="Date">Section&nbsp;7.1.1.2</a>), whereas the payload of the same status code in a response to POST might represent either the processing result or the new
    1154          state of the target resource after applying the processing. Response messages with an error status code usually contain a
    1155          payload that represents the error condition, such that it describes the error state and what next steps are suggested for
    1156          resolving it.
    1157       </p>
    1158       <p id="rfc.section.3.3.p.4">Header fields that specifically describe the payload, rather than the associated representation, are referred to as "payload
    1159          header fields". Payload header fields are defined in other parts of this specification, due to their impact on message parsing.
    1160       </p>
    1161       <div id="rfc.table.u.2">
    1162          <table class="tt full left" cellpadding="3" cellspacing="0">
    1163             <thead>
    1164                <tr>
    1165                   <th>Header Field Name</th>
    1166                   <th>Defined in...</th>
    1167                </tr>
    1168             </thead>
    1169             <tbody>
    1170                <tr>
    1171                   <td class="left">Content-Length</td>
    1172                   <td class="left"><a href="p1-messaging.html#header.content-length" title="Content-Length">Section 3.3.2</a> of <a href="#Part1" id="rfc.xref.Part1.13"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
    1173                </tr>
    1174                <tr>
    1175                   <td class="left">Content-Range</td>
    1176                   <td class="left"><a href="p5-range.html#header.content-range" title="Content-Range">Section 4.2</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
    1177                </tr>
    1178                <tr>
    1179                   <td class="left">Transfer-Encoding</td>
    1180                   <td class="left"><a href="p1-messaging.html#header.transfer-encoding" title="Transfer-Encoding">Section 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.14"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
    1181                </tr>
    1182             </tbody>
    1183          </table>
    1184       </div>
    1185       <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="content.negotiation" href="#content.negotiation">Content Negotiation</a></h2>
    1186       <p id="rfc.section.3.4.p.1">When responses convey payload information, whether indicating a success or an error, the origin server often has different
    1187          ways of representing that information; for example, in different formats, languages, or encodings. Likewise, different users
    1188          or user agents might have differing capabilities, characteristics, or preferences that could influence which representation,
    1189          among those available, would be best to deliver. For this reason, HTTP provides mechanisms for <a href="#content.negotiation" class="smpl">content negotiation</a>.
    1190       </p>
    1191       <p id="rfc.section.3.4.p.2">This specification defines two patterns of content negotiation that can be made visible within the protocol: "proactive",
    1192          where the server selects the representation based upon the user agent's stated preferences, and "reactive" negotiation, where
    1193          the server provides a list of representations for the user agent to choose from. Other patterns of content negotiation include
    1194          "conditional content", where the representation consists of multiple parts that are selectively rendered based on user agent
    1195          parameters, "active content", where the representation contains a script that makes additional (more specific) requests based
    1196          on the user agent characteristics, and "Transparent Content Negotiation" (<a href="#RFC2295" id="rfc.xref.RFC2295.1"><cite title="Transparent Content Negotiation in HTTP">[RFC2295]</cite></a>), where content selection is performed by an intermediary. These patterns are not mutually exclusive, and each has trade-offs
    1197          in applicability and practicality.
    1198       </p>
    1199       <p id="rfc.section.3.4.p.3">Note that, in all cases, HTTP is not aware of the resource semantics. The consistency with which an origin server responds
    1200          to requests, over time and over the varying dimensions of content negotiation, and thus the "sameness" of a resource's observed
    1201          representations over time, is determined entirely by whatever entity or algorithm selects or generates those responses. HTTP
    1202          pays no attention to the man behind the curtain.
    1203       </p>
    1204       <h3 id="rfc.section.3.4.1"><a href="#rfc.section.3.4.1">3.4.1</a>&nbsp;<a id="proactive.negotiation" href="#proactive.negotiation">Proactive Negotiation</a></h3>
    1205       <p id="rfc.section.3.4.1.p.1">When content negotiation preferences are sent by the user agent in a request to encourage an algorithm located at the server
    1206          to select the preferred representation, it is called <dfn>proactive negotiation</dfn> (a.k.a., <dfn>server-driven negotiation</dfn>). Selection is based on the available representations for a response (the dimensions over which it might vary, such as language,
    1207          content-coding, etc.) compared to various information supplied in the request, including both the explicit negotiation fields
    1208          of <a href="#request.conneg" title="Content Negotiation">Section&nbsp;5.3</a> and implicit characteristics, such as the client's network address or parts of the <a href="#header.user-agent" class="smpl">User-Agent</a> field.
    1209       </p>
    1210       <p id="rfc.section.3.4.1.p.2">Proactive negotiation is advantageous when the algorithm for selecting from among the available representations is difficult
    1211          to describe to a user agent, or when the server desires to send its "best guess" to the user agent along with the first response
    1212          (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to
    1213          improve the server's guess, a user agent <em class="bcp14">MAY</em> send request header fields that describe its preferences.
    1214       </p>
    1215       <p id="rfc.section.3.4.1.p.3">Proactive negotiation has serious disadvantages: </p>
    1216       <ul>
    1217          <li>It is impossible for the server to accurately determine what might be "best" for any given user, since that would require
    1218             complete knowledge of both the capabilities of the user agent and the intended use for the response (e.g., does the user want
    1219             to view it on screen or print it on paper?);
    1220          </li>
    1221          <li>Having the user agent describe its capabilities in every request can be both very inefficient (given that only a small percentage
    1222             of responses have multiple representations) and a potential risk to the user's privacy;
    1223          </li>
    1224          <li>It complicates the implementation of an origin server and the algorithms for generating responses to a request; and,</li>
    1225          <li>It limits the reusability of responses for shared caching.</li>
    1226       </ul>
    1227       <p id="rfc.section.3.4.1.p.4">A user agent cannot rely on proactive negotiation preferences being consistently honored, since the origin server might not
    1228          implement proactive negotiation for the requested resource or might decide that sending a response that doesn't conform to
    1229          the user agent's preferences is better than sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response.
    1230       </p>
    1231       <p id="rfc.section.3.4.1.p.5">An origin server <em class="bcp14">MAY</em> generate a <a href="#header.vary" class="smpl">Vary</a> header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;7.1.4</a>) in responses that are subject to proactive negotiation to indicate what parameters of request information might be used
    1232          in its selection algorithm, thereby providing a means for recipients to determine the reusability of that same response for
    1233          user agents with differing request information.
    1234       </p>
    1235       <h3 id="rfc.section.3.4.2"><a href="#rfc.section.3.4.2">3.4.2</a>&nbsp;<a id="reactive.negotiation" href="#reactive.negotiation">Reactive Negotiation</a></h3>
    1236       <p id="rfc.section.3.4.2.p.1">With <dfn>reactive negotiation</dfn> (a.k.a., <dfn>agent-driven negotiation</dfn>), selection of the best response representation (regardless of the status code) is performed by the user agent after receiving
    1237          an initial response from the origin server that contains a list of resources for alternative representations. If the user
    1238          agent is not satisfied by the initial response representation, it can perform a GET request on one or more of the alternative
    1239          resources, selected based on metadata included in the list, to obtain a different form of representation for that response.
    1240          Selection of alternatives might be performed automatically by the user agent or manually by the user selecting from a generated
    1241          (possibly hypertext) menu.
    1242       </p>
    1243       <p id="rfc.section.3.4.2.p.2">Note that the above refers to representations of the response, in general, not representations of the resource. The alternative
    1244          representations are only considered representations of the target resource if the response in which those alternatives are
    1245          provided has the semantics of being a representation of the target resource (e.g., a <a href="#status.200" class="smpl">200 (OK)</a> response to a GET request) or has the semantics of providing links to alternative representations for the target resource
    1246          (e.g., a <a href="#status.300" class="smpl">300 (Multiple Choices)</a> response to a GET request).
    1247       </p>
    1248       <p id="rfc.section.3.4.2.p.3">A server might choose not to send an initial representation, other than the list of alternatives, and thereby indicate that
    1249          reactive negotiation by the user agent is preferred. For example, the alternatives listed in responses with the <a href="#status.300" class="smpl">300 (Multiple Choices)</a> and <a href="#status.406" class="smpl">406 (Not Acceptable)</a> status codes include information about the available representations so that the user or user agent can react by making a
    1250          selection.
    1251       </p>
    1252       <p id="rfc.section.3.4.2.p.4">Reactive negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or
    1253          encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally
    1254          when public caches are used to distribute server load and reduce network usage.
    1255       </p>
    1256       <p id="rfc.section.3.4.2.p.5">Reactive negotiation suffers from the disadvantages of transmitting a list of alternatives to the user agent, which degrades
    1257          user-perceived latency if transmitted in the header section, and needing a second request to obtain an alternate representation.
    1258          Furthermore, this specification does not define a mechanism for supporting automatic selection, though it does not prevent
    1259          such a mechanism from being developed as an extension.
    1260       </p>
    1261       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="methods" href="#methods">Request Methods</a></h1>
    1262       <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="method.overview" href="#method.overview">Overview</a></h2>
    1263       <p id="rfc.section.4.1.p.1">The request method token is the primary source of request semantics; it indicates the purpose for which the client has made
    1264          this request and what is expected by the client as a successful result.
    1265       </p>
    1266       <p id="rfc.section.4.1.p.2">The request method's semantics might be further specialized by the semantics of some header fields when present in a request
    1267          (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5</a>) if those additional semantics do not conflict with the method. For example, a client can send conditional request header
    1268          fields (<a href="#request.conditionals" title="Conditionals">Section&nbsp;5.2</a>) to make the requested action conditional on the current state of the target resource (<a href="#Part4" id="rfc.xref.Part4.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>).
    1269       </p>
    1270       <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.15"></span>  <a href="#method.overview" class="smpl">method</a> = <a href="#imported.abnf" class="smpl">token</a>
     1316      <div id="methods">
     1317         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#methods">Request Methods</a></h1>
     1318         <div id="method.overview">
     1319            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#method.overview">Overview</a></h2>
     1320            <p id="rfc.section.4.1.p.1">The request method token is the primary source of request semantics; it indicates the purpose for which the client has made
     1321               this request and what is expected by the client as a successful result.
     1322            </p>
     1323            <p id="rfc.section.4.1.p.2">The request method's semantics might be further specialized by the semantics of some header fields when present in a request
     1324               (<a href="#request.header.fields" title="Request Header Fields">Section&nbsp;5</a>) if those additional semantics do not conflict with the method. For example, a client can send conditional request header
     1325               fields (<a href="#request.conditionals" title="Conditionals">Section&nbsp;5.2</a>) to make the requested action conditional on the current state of the target resource (<a href="#Part4" id="rfc.xref.Part4.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>).
     1326            </p>
     1327            <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.15"></span>  <a href="#method.overview" class="smpl">method</a> = <a href="#imported.abnf" class="smpl">token</a>
    12711328</pre><p id="rfc.section.4.1.p.4">HTTP was originally designed to be usable as an interface to distributed object systems. The request method was envisioned
    1272          as applying semantics to a <a href="#resources" class="smpl">target resource</a> in much the same way as invoking a defined method on an identified object would apply semantics. The method token is case-sensitive
    1273          because it might be used as a gateway to object-based systems with case-sensitive method names.
    1274       </p>
    1275       <p id="rfc.section.4.1.p.5">Unlike distributed objects, the standardized request methods in HTTP are not resource-specific, since uniform interfaces provide
    1276          for better visibility and reuse in network-based systems <a href="#REST" id="rfc.xref.REST.2"><cite title="Architectural Styles and the Design of Network-based Software Architectures">[REST]</cite></a>. Once defined, a standardized method ought to have the same semantics when applied to any resource, though each resource
    1277          determines for itself whether those semantics are implemented or allowed.
    1278       </p>
    1279       <p id="rfc.section.4.1.p.6">This specification defines a number of standardized methods that are commonly used in HTTP, as outlined by the following table.
    1280          By convention, standardized methods are defined in all-uppercase ASCII letters.
    1281       </p>
    1282       <div id="rfc.table.1">
    1283          <div id="table.of.methods"></div>
    1284          <table class="tt full left" cellpadding="3" cellspacing="0">
    1285             <thead>
    1286                <tr>
    1287                   <th>Method</th>
    1288                   <th>Description</th>
    1289                   <th>Sec.</th>
    1290                </tr>
    1291             </thead>
    1292             <tbody>
    1293                <tr>
    1294                   <td class="left">GET</td>
    1295                   <td class="left">Transfer a current representation of the target resource.</td>
    1296                   <td class="left"><a href="#GET" id="rfc.xref.GET.3" title="GET">4.3.1</a></td>
    1297                </tr>
    1298                <tr>
    1299                   <td class="left">HEAD</td>
    1300                   <td class="left">Same as GET, but only transfer the status line and header section.</td>
    1301                   <td class="left"><a href="#HEAD" id="rfc.xref.HEAD.1" title="HEAD">4.3.2</a></td>
    1302                </tr>
    1303                <tr>
    1304                   <td class="left">POST</td>
    1305                   <td class="left">Perform resource-specific processing on the request payload.</td>
    1306                   <td class="left"><a href="#POST" id="rfc.xref.POST.2" title="POST">4.3.3</a></td>
    1307                </tr>
    1308                <tr>
    1309                   <td class="left">PUT</td>
    1310                   <td class="left">Replace all current representations of the target resource with the request payload.</td>
    1311                   <td class="left"><a href="#PUT" id="rfc.xref.PUT.2" title="PUT">4.3.4</a></td>
    1312                </tr>
    1313                <tr>
    1314                   <td class="left">DELETE</td>
    1315                   <td class="left">Remove all current representations of the target resource.</td>
    1316                   <td class="left"><a href="#DELETE" id="rfc.xref.DELETE.1" title="DELETE">4.3.5</a></td>
    1317                </tr>
    1318                <tr>
    1319                   <td class="left">CONNECT</td>
    1320                   <td class="left">Establish a tunnel to the server identified by the target resource.</td>
    1321                   <td class="left"><a href="#CONNECT" id="rfc.xref.CONNECT.1" title="CONNECT">4.3.6</a></td>
    1322                </tr>
    1323                <tr>
    1324                   <td class="left">OPTIONS</td>
    1325                   <td class="left">Describe the communication options for the target resource.</td>
    1326                   <td class="left"><a href="#OPTIONS" id="rfc.xref.OPTIONS.1" title="OPTIONS">4.3.7</a></td>
    1327                </tr>
    1328                <tr>
    1329                   <td class="left">TRACE</td>
    1330                   <td class="left">Perform a message loop-back test along the path to the target resource.</td>
    1331                   <td class="left"><a href="#TRACE" id="rfc.xref.TRACE.1" title="TRACE">4.3.8</a></td>
    1332                </tr>
    1333             </tbody>
    1334          </table>
    1335       </div>
    1336       <p id="rfc.section.4.1.p.7">All general-purpose servers <em class="bcp14">MUST</em> support the methods GET and HEAD. All other methods are <em class="bcp14">OPTIONAL</em>; when implemented, a server <em class="bcp14">MUST</em> implement the above methods according to the semantics defined for them in <a href="#method.definitions" title="Method Definitions">Section&nbsp;4.3</a>.
    1337       </p>
    1338       <p id="rfc.section.4.1.p.8">Additional methods, outside the scope of this specification, have been standardized for use in HTTP. All such methods ought
    1339          to be registered within the HTTP Method Registry maintained by IANA, as defined in <a href="#method.registry" title="Method Registry">Section&nbsp;8.1</a>.
    1340       </p>
    1341       <p id="rfc.section.4.1.p.9">The set of methods allowed by a target resource can be listed in an <a href="#header.allow" class="smpl">Allow</a> header field (<a href="#header.allow" id="rfc.xref.header.allow.1" title="Allow">Section&nbsp;7.4.1</a>). However, the set of allowed methods can change dynamically. When a request method is received that is unrecognized or not
    1342          implemented by an origin server, the origin server <em class="bcp14">SHOULD</em> respond with the <a href="#status.501" class="smpl">501 (Not Implemented)</a> status code. When a request method is received that is known by an origin server but not allowed for the target resource,
    1343          the origin server <em class="bcp14">SHOULD</em> respond with the <a href="#status.405" class="smpl">405 (Method Not Allowed)</a> status code.
    1344       </p>
    1345       <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="method.properties" href="#method.properties">Common Method Properties</a></h2>
    1346       <div id="rfc.iref.s.2"></div>
    1347       <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a id="safe.methods" href="#safe.methods">Safe Methods</a></h3>
    1348       <p id="rfc.section.4.2.1.p.1">Request methods are considered "<dfn>safe</dfn>" if their defined semantics are essentially read-only; i.e., the client does not request, and does not expect, any state
    1349          change on the origin server as a result of applying a safe method to a target resource. Likewise, reasonable use of a safe
    1350          method is not expected to cause any harm, loss of property, or unusual burden on the origin server.
    1351       </p>
    1352       <p id="rfc.section.4.2.1.p.2">This definition of safe methods does not prevent an implementation from including behavior that is potentially harmful, not
    1353          entirely read-only, or which causes side-effects while invoking a safe method. What is important, however, is that the client
    1354          did not request that additional behavior and cannot be held accountable for it. For example, most servers append request information
    1355          to access log files at the completion of every response, regardless of the method, and that is considered safe even though
    1356          the log storage might become full and crash the server. Likewise, a safe request initiated by selecting an advertisement on
    1357          the Web will often have the side-effect of charging an advertising account.
    1358       </p>
    1359       <p id="rfc.section.4.2.1.p.3">Of the request methods defined by this specification, the GET, HEAD, OPTIONS, and TRACE methods are defined to be safe.</p>
    1360       <p id="rfc.section.4.2.1.p.4">The purpose of distinguishing between safe and unsafe methods is to allow automated retrieval processes (spiders) and cache
    1361          performance optimization (pre-fetching) to work without fear of causing harm. In addition, it allows a user agent to apply
    1362          appropriate constraints on the automated use of unsafe methods when processing potentially untrusted content.
    1363       </p>
    1364       <p id="rfc.section.4.2.1.p.5">A user agent <em class="bcp14">SHOULD</em> distinguish between safe and unsafe methods when presenting potential actions to a user, such that the user can be made aware
    1365          of an unsafe action before it is requested.
    1366       </p>
    1367       <p id="rfc.section.4.2.1.p.6">When a resource is constructed such that parameters within the effective request URI have the effect of selecting an action,
    1368          it is the resource owner's responsibility to ensure that the action is consistent with the request method semantics. For example,
    1369          it is common for Web-based content editing software to use actions within query parameters, such as "page?do=delete". If the
    1370          purpose of such a resource is to perform an unsafe action, then the resource owner <em class="bcp14">MUST</em> disable or disallow that action when it is accessed using a safe request method. Failure to do so will result in unfortunate
    1371          side-effects when automated processes perform a GET on every URI reference for the sake of link maintenance, pre-fetching,
    1372          building a search index, etc.
    1373       </p>
    1374       <div id="rfc.iref.i.1"></div>
    1375       <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a id="idempotent.methods" href="#idempotent.methods">Idempotent Methods</a></h3>
    1376       <p id="rfc.section.4.2.2.p.1">Request methods are considered "<dfn id="idempotent">idempotent</dfn>" if the intended effect of multiple identical requests is the same as for a single request. Of the request methods defined
    1377          by this specification, the PUT, DELETE, and safe request methods are idempotent.
    1378       </p>
    1379       <p id="rfc.section.4.2.2.p.2">Like the definition of safe, the idempotent property only applies to what has been requested by the user; a server is free
    1380          to log each request separately, retain a revision control history, or implement other non-idempotent side-effects for each
    1381          idempotent request.
    1382       </p>
    1383       <p id="rfc.section.4.2.2.p.3">Idempotent methods are distinguished because the request can be repeated automatically if a communication failure occurs before
    1384          the client is able to read the server's response. For example, if a client sends a PUT request and the underlying connection
    1385          is closed before any response is received, then it can establish a new connection and retry the idempotent request because
    1386          it knows that repeating the request will have the same effect even if the original request succeeded. Note, however, that
    1387          repeated failures would indicate a problem within the server.
    1388       </p>
    1389       <div id="rfc.iref.c.8"></div>
    1390       <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a id="cacheable.methods" href="#cacheable.methods">Cacheable Methods</a></h3>
    1391       <p id="rfc.section.4.2.3.p.1">Request methods can be defined as "<dfn id="cacheable">cacheable</dfn>" to indicate that responses to them are allowed to be stored for future reuse; for specific requirements see <a href="#Part6" id="rfc.xref.Part6.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>. In general, safe methods that do not depend on a current or authoritative response are defined as cacheable; this specification
    1392          defines GET, HEAD and POST as cacheable, although the overwhelming majority of cache implementations only support GET and
    1393          HEAD.
    1394       </p>
    1395       <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="method.definitions" href="#method.definitions">Method Definitions</a></h2>
    1396       <h3 id="rfc.section.4.3.1"><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;<a id="GET" href="#GET">GET</a></h3>
    1397       <div id="rfc.iref.g.16"></div>
    1398       <p id="rfc.section.4.3.1.p.1">The GET method requests transfer of a current selected representation for the <a href="#resources" class="smpl">target resource</a>. GET is the primary mechanism of information retrieval and the focus of almost all performance optimizations. Hence, when
    1399          people speak of retrieving some identifiable information via HTTP, they are generally referring to making a GET request.
    1400       </p>
    1401       <p id="rfc.section.4.3.1.p.2">It is tempting to think of resource identifiers as remote filesystem pathnames, and of representations as being a copy of
    1402          the contents of such files. In fact, that is how many resources are implemented (see <a href="#attack.pathname" title="Attacks Based On File and Path Names">Section&nbsp;9.1</a> for related security considerations). However, there are no such limitations in practice. The HTTP interface for a resource
    1403          is just as likely to be implemented as a tree of content objects, a programmatic view on various database records, or a gateway
    1404          to other information systems. Even when the URI mapping mechanism is tied to a filesystem, an origin server might be configured
    1405          to execute the files with the request as input and send the output as the representation, rather than transfer the files directly.
    1406          Regardless, only the origin server needs to know how each of its resource identifiers corresponds to an implementation, and
    1407          how each implementation manages to select and send a current representation of the target resource in a response to GET.
    1408       </p>
    1409       <p id="rfc.section.4.3.1.p.3">A client can alter the semantics of GET to be a "range request", requesting transfer of only some part(s) of the selected
    1410          representation, by sending a <a href="p5-range.html#header.range" class="smpl">Range</a> header field in the request (<a href="#Part5" id="rfc.xref.Part5.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>).
    1411       </p>
    1412       <p id="rfc.section.4.3.1.p.4">A payload within a GET request message has no defined semantics; sending a payload body on a GET request might cause some
    1413          existing implementations to reject the request.
    1414       </p>
    1415       <p id="rfc.section.4.3.1.p.5">The response to a GET request is cacheable; a cache <em class="bcp14">MAY</em> use it to satisfy subsequent GET and HEAD requests unless otherwise indicated by the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 5.2</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    1416       </p>
    1417       <h3 id="rfc.section.4.3.2"><a href="#rfc.section.4.3.2">4.3.2</a>&nbsp;<a id="HEAD" href="#HEAD">HEAD</a></h3>
    1418       <div id="rfc.iref.h.1"></div>
    1419       <p id="rfc.section.4.3.2.p.1">The HEAD method is identical to GET except that the server <em class="bcp14">MUST NOT</em> send a message body in the response (i.e., the response terminates at the end of the header section). The server <em class="bcp14">SHOULD</em> send the same header fields in response to a HEAD request as it would have sent if the request had been a GET, except that
    1420          the payload header fields (<a href="#payload" title="Payload Semantics">Section&nbsp;3.3</a>) <em class="bcp14">MAY</em> be omitted. This method can be used for obtaining metadata about the selected representation without transferring the representation
    1421          data and is often used for testing hypertext links for validity, accessibility, and recent modification.
    1422       </p>
    1423       <p id="rfc.section.4.3.2.p.2">A payload within a HEAD request message has no defined semantics; sending a payload body on a HEAD request might cause some
    1424          existing implementations to reject the request.
    1425       </p>
    1426       <p id="rfc.section.4.3.2.p.3">The response to a HEAD request is cacheable; a cache <em class="bcp14">MAY</em> use it to satisfy subsequent HEAD requests unless otherwise indicated by the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 5.2</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). A HEAD response might also have an effect on previously cached responses to GET; see <a href="p6-cache.html#head.effects" title="Freshening Responses via HEAD">Section 4.3.5</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
    1427       </p>
    1428       <div id="rfc.iref.p.2"></div>
    1429       <h3 id="rfc.section.4.3.3"><a href="#rfc.section.4.3.3">4.3.3</a>&nbsp;<a id="POST" href="#POST">POST</a></h3>
    1430       <p id="rfc.section.4.3.3.p.1">The POST method requests that the <a href="#resources" class="smpl">target resource</a> process the representation enclosed in the request according to the resource's own specific semantics. For example, POST is
    1431          used for the following functions (among others):
    1432       </p>
    1433       <ul>
    1434          <li>Providing a block of data, such as the fields entered into an HTML form, to a data-handling process;</li>
    1435          <li>Posting a message to a bulletin board, newsgroup, mailing list, blog, or similar group of articles;</li>
    1436          <li>Creating a new resource that has yet to be identified by the origin server; and</li>
    1437          <li>Appending data to a resource's existing representation(s).</li>
    1438       </ul>
    1439       <p id="rfc.section.4.3.3.p.2">An origin server indicates response semantics by choosing an appropriate status code depending on the result of processing
    1440          the POST request; almost all of the status codes defined by this specification might be received in a response to POST (the
    1441          exceptions being 206, 304, and 416).
    1442       </p>
    1443       <p id="rfc.section.4.3.3.p.3">If one or more resources has been created on the origin server as a result of successfully processing a POST request, the
    1444          origin server <em class="bcp14">SHOULD</em> send a <a href="#status.201" class="smpl">201 (Created)</a> response containing a <a href="#header.location" class="smpl">Location</a> header field that provides an identifier for the primary resource created (<a href="#header.location" id="rfc.xref.header.location.1" title="Location">Section&nbsp;7.1.2</a>) and a representation that describes the status of the request while referring to the new resource(s).
    1445       </p>
    1446       <p id="rfc.section.4.3.3.p.4">Responses to POST requests are only cacheable when they include explicit freshness information (see <a href="p6-cache.html#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section 4.2.1</a> of <a href="#Part6" id="rfc.xref.Part6.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). However, POST caching is not widely implemented. For cases where an origin server wishes the client to be able to cache
    1447          the result of a POST in a way that can be reused by a later GET, the origin server <em class="bcp14">MAY</em> send a <a href="#status.200" class="smpl">200 (OK)</a> response containing the result and a <a href="#header.content-location" class="smpl">Content-Location</a> header field that has the same value as the POST's effective request URI (<a href="#header.content-location" id="rfc.xref.header.content-location.2" title="Content-Location">Section&nbsp;3.1.4.2</a>).
    1448       </p>
    1449       <p id="rfc.section.4.3.3.p.5">If the result of processing a POST would be equivalent to a representation of an existing resource, an origin server <em class="bcp14">MAY</em> redirect the user agent to that resource by sending a <a href="#status.303" class="smpl">303 (See Other)</a> response with the existing resource's identifier in the <a href="#header.location" class="smpl">Location</a> field. This has the benefits of providing the user agent a resource identifier and transferring the representation via a method
    1450          more amenable to shared caching, though at the cost of an extra request if the user agent does not already have the representation
    1451          cached.
    1452       </p>
    1453       <h3 id="rfc.section.4.3.4"><a href="#rfc.section.4.3.4">4.3.4</a>&nbsp;<a id="PUT" href="#PUT">PUT</a></h3>
    1454       <div id="rfc.iref.p.3"></div>
    1455       <p id="rfc.section.4.3.4.p.1">The PUT method requests that the state of the <a href="#resources" class="smpl">target resource</a> be created or replaced with the state defined by the representation enclosed in the request message payload. A successful
    1456          PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent
    1457          representation being sent in a <a href="#status.200" class="smpl">200 (OK)</a> response. However, there is no guarantee that such a state change will be observable, since the target resource might be acted
    1458          upon by other user agents in parallel, or might be subject to dynamic processing by the origin server, before any subsequent
    1459          GET is received. A successful response only implies that the user agent's intent was achieved at the time of its processing
    1460          by the origin server.
    1461       </p>
    1462       <p id="rfc.section.4.3.4.p.2">If the target resource does not have a current representation and the PUT successfully creates one, then the origin server <em class="bcp14">MUST</em> inform the user agent by sending a <a href="#status.201" class="smpl">201 (Created)</a> response. If the target resource does have a current representation and that representation is successfully modified in accordance
    1463          with the state of the enclosed representation, then the origin server <em class="bcp14">MUST</em> send either a <a href="#status.200" class="smpl">200 (OK)</a> or a <a href="#status.204" class="smpl">204 (No Content)</a> response to indicate successful completion of the request.
    1464       </p>
    1465       <p id="rfc.section.4.3.4.p.3">An origin server <em class="bcp14">SHOULD</em> ignore unrecognized header fields received in a PUT request (i.e., do not save them as part of the resource state).
    1466       </p>
    1467       <p id="rfc.section.4.3.4.p.4">An origin server <em class="bcp14">SHOULD</em> verify that the PUT representation is consistent with any constraints the server has for the target resource that cannot or
    1468          will not be changed by the PUT. This is particularly important when the origin server uses internal configuration information
    1469          related to the URI in order to set the values for representation metadata on GET responses. When a PUT representation is inconsistent
    1470          with the target resource, the origin server <em class="bcp14">SHOULD</em> either make them consistent, by transforming the representation or changing the resource configuration, or respond with an
    1471          appropriate error message containing sufficient information to explain why the representation is unsuitable. The <a href="#status.409" class="smpl">409 (Conflict)</a> or <a href="#status.415" class="smpl">415 (Unsupported Media Type)</a> status codes are suggested, with the latter being specific to constraints on <a href="#header.content-type" class="smpl">Content-Type</a> values.
    1472       </p>
    1473       <p id="rfc.section.4.3.4.p.5">For example, if the target resource is configured to always have a <a href="#header.content-type" class="smpl">Content-Type</a> of "text/html" and the representation being PUT has a Content-Type of "image/jpeg", the origin server ought to do one of:
    1474       </p>
    1475       <ol class="la">
    1476          <li>reconfigure the target resource to reflect the new media type;</li>
    1477          <li>transform the PUT representation to a format consistent with that of the resource before saving it as the new resource state;
    1478             or,
    1479          </li>
    1480          <li>reject the request with a <a href="#status.415" class="smpl">415 (Unsupported Media Type)</a> response indicating that the target resource is limited to "text/html", perhaps including a link to a different resource that
    1481             would be a suitable target for the new representation.
    1482          </li>
    1483       </ol>
    1484       <p id="rfc.section.4.3.4.p.6">HTTP does not define exactly how a PUT method affects the state of an origin server beyond what can be expressed by the intent
    1485          of the user agent request and the semantics of the origin server response. It does not define what a resource might be, in
    1486          any sense of that word, beyond the interface provided via HTTP. It does not define how resource state is "stored", nor how
    1487          such storage might change as a result of a change in resource state, nor how the origin server translates resource state into
    1488          representations. Generally speaking, all implementation details behind the resource interface are intentionally hidden by
    1489          the server.
    1490       </p>
    1491       <p id="rfc.section.4.3.4.p.7">An origin server <em class="bcp14">MUST NOT</em> send a validator header field (<a href="#response.validator" title="Validator Header Fields">Section&nbsp;7.2</a>), such as an <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> or <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> field, in a successful response to PUT unless the request's representation data was saved without any transformation applied
    1492          to the body (i.e., the resource's new representation data is identical to the representation data received in the PUT request)
    1493          and the validator field value reflects the new representation. This requirement allows a user agent to know when the representation
    1494          body it has in memory remains current as a result of the PUT, thus not in need of retrieving again from the origin server,
    1495          and that the new validator(s) received in the response can be used for future conditional requests in order to prevent accidental
    1496          overwrites (<a href="#request.conditionals" title="Conditionals">Section&nbsp;5.2</a>).
    1497       </p>
    1498       <p id="rfc.section.4.3.4.p.8">The fundamental difference between the POST and PUT methods is highlighted by the different intent for the enclosed representation.
    1499          The target resource in a POST request is intended to handle the enclosed representation according to the resource's own semantics,
    1500          whereas the enclosed representation in a PUT request is defined as replacing the state of the target resource. Hence, the
    1501          intent of PUT is idempotent and visible to intermediaries, even though the exact effect is only known by the origin server.
    1502       </p>
    1503       <p id="rfc.section.4.3.4.p.9">Proper interpretation of a PUT request presumes that the user agent knows which target resource is desired. A service that
    1504          selects a proper URI on behalf of the client, after receiving a state-changing request, <em class="bcp14">SHOULD</em> be implemented using the POST method rather than PUT. If the origin server will not make the requested PUT state change to
    1505          the target resource and instead wishes to have it applied to a different resource, such as when the resource has been moved
    1506          to a different URI, then the origin server <em class="bcp14">MUST</em> send an appropriate <a href="#status.3xx" class="smpl">3xx (Redirection)</a> response; the user agent <em class="bcp14">MAY</em> then make its own decision regarding whether or not to redirect the request.
    1507       </p>
    1508       <p id="rfc.section.4.3.4.p.10">A PUT request applied to the target resource can have side-effects on other resources. For example, an article might have
    1509          a URI for identifying "the current version" (a resource) that is separate from the URIs identifying each particular version
    1510          (different resources that at one point shared the same state as the current version resource). A successful PUT request on
    1511          "the current version" URI might therefore create a new version resource in addition to changing the state of the target resource,
    1512          and might also cause links to be added between the related resources.
    1513       </p>
    1514       <p id="rfc.section.4.3.4.p.11">An origin server that allows PUT on a given target resource <em class="bcp14">MUST</em> send a <a href="#status.400" class="smpl">400 (Bad Request)</a> response to a PUT request that contains a <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> header field (<a href="p5-range.html#header.content-range" title="Content-Range">Section 4.2</a> of <a href="#Part5" id="rfc.xref.Part5.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>), since the payload is likely to be partial content that has been mistakenly PUT as a full representation. Partial content
    1515          updates are possible by targeting a separately identified resource with state that overlaps a portion of the larger resource,
    1516          or by using a different method that has been specifically defined for partial updates (for example, the PATCH method defined
    1517          in <a href="#RFC5789" id="rfc.xref.RFC5789.1"><cite title="PATCH Method for HTTP">[RFC5789]</cite></a>).
    1518       </p>
    1519       <p id="rfc.section.4.3.4.p.12">Responses to the PUT method are not cacheable. If a successful PUT request passes through a cache that has one or more stored
    1520          responses for the effective request URI, those stored responses will be invalidated (see <a href="p6-cache.html#invalidation" title="Invalidation">Section 4.4</a> of <a href="#Part6" id="rfc.xref.Part6.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    1521       </p>
    1522       <h3 id="rfc.section.4.3.5"><a href="#rfc.section.4.3.5">4.3.5</a>&nbsp;<a id="DELETE" href="#DELETE">DELETE</a></h3>
    1523       <div id="rfc.iref.d.2"></div>
    1524       <p id="rfc.section.4.3.5.p.1">The DELETE method requests that the origin server remove the association between the <a href="#resources" class="smpl">target resource</a> and its current functionality. In effect, this method is similar to the rm command in UNIX: it expresses a deletion operation
    1525          on the URI mapping of the origin server, rather than an expectation that the previously associated information be deleted.
    1526       </p>
    1527       <p id="rfc.section.4.3.5.p.2">If the target resource has one or more current representations, they might or might not be destroyed by the origin server,
    1528          and the associated storage might or might not be reclaimed, depending entirely on the nature of the resource and its implementation
    1529          by the origin server (which are beyond the scope of this specification). Likewise, other implementation aspects of a resource
    1530          might need to be deactivated or archived as a result of a DELETE, such as database or gateway connections. In general, it
    1531          is assumed that the origin server will only allow DELETE on resources for which it has a prescribed mechanism for accomplishing
    1532          the deletion.
    1533       </p>
    1534       <p id="rfc.section.4.3.5.p.3">Relatively few resources allow the DELETE method — its primary use is for remote authoring environments, where the user has
    1535          some direction regarding its effect. For example, a resource that was previously created using a PUT request, or identified
    1536          via the Location header field after a <a href="#status.201" class="smpl">201 (Created)</a> response to a POST request, might allow a corresponding DELETE request to undo those actions. Similarly, custom user agent
    1537          implementations that implement an authoring function, such as revision control clients using HTTP for remote operations, might
    1538          use DELETE based on an assumption that the server's URI space has been crafted to correspond to a version repository.
    1539       </p>
    1540       <p id="rfc.section.4.3.5.p.4">If a DELETE method is successfully applied, the origin server <em class="bcp14">SHOULD</em> send a <a href="#status.202" class="smpl">202 (Accepted)</a> status code if the action will likely succeed but has not yet been enacted, a <a href="#status.204" class="smpl">204 (No Content)</a> status code if the action has been enacted and no further information is to be supplied, or a <a href="#status.200" class="smpl">200 (OK)</a> status code if the action has been enacted and the response message includes a representation describing the status.
    1541       </p>
    1542       <p id="rfc.section.4.3.5.p.5">A payload within a DELETE request message has no defined semantics; sending a payload body on a DELETE request might cause
    1543          some existing implementations to reject the request.
    1544       </p>
    1545       <p id="rfc.section.4.3.5.p.6">Responses to the DELETE method are not cacheable. If a DELETE request passes through a cache that has one or more stored responses
    1546          for the effective request URI, those stored responses will be invalidated (see <a href="p6-cache.html#invalidation" title="Invalidation">Section 4.4</a> of <a href="#Part6" id="rfc.xref.Part6.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    1547       </p>
    1548       <div id="rfc.iref.c.9"></div>
    1549       <h3 id="rfc.section.4.3.6"><a href="#rfc.section.4.3.6">4.3.6</a>&nbsp;<a id="CONNECT" href="#CONNECT">CONNECT</a></h3>
    1550       <p id="rfc.section.4.3.6.p.1">The CONNECT method requests that the recipient establish a tunnel to the destination origin server identified by the request-target
    1551          and, if successful, thereafter restrict its behavior to blind forwarding of packets, in both directions, until the tunnel
    1552          is closed.
    1553       </p>
    1554       <p id="rfc.section.4.3.6.p.2">CONNECT is intended only for use in requests to a proxy. An origin server that receives a CONNECT request for itself <em class="bcp14">MAY</em> respond with a <a href="#status.2xx" class="smpl">2xx</a> status code to indicate that a connection is established. However, most origin servers do not implement CONNECT.
    1555       </p>
    1556       <p id="rfc.section.4.3.6.p.3">A client sending a CONNECT request <em class="bcp14">MUST</em> send the authority form of request-target (<a href="p1-messaging.html#request-target" title="Request Target">Section 5.3</a> of <a href="#Part1" id="rfc.xref.Part1.15"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>); i.e., the request-target consists of only the host name and port number of the tunnel destination, separated by a colon.
    1557          For example,
    1558       </p>
    1559       <div id="rfc.figure.u.18"></div><pre class="text2">CONNECT server.example.com:80 HTTP/1.1
     1329               as applying semantics to a <a href="#resources" class="smpl">target resource</a> in much the same way as invoking a defined method on an identified object would apply semantics. The method token is case-sensitive
     1330               because it might be used as a gateway to object-based systems with case-sensitive method names.
     1331            </p>
     1332            <p id="rfc.section.4.1.p.5">Unlike distributed objects, the standardized request methods in HTTP are not resource-specific, since uniform interfaces provide
     1333               for better visibility and reuse in network-based systems <a href="#REST" id="rfc.xref.REST.2"><cite title="Architectural Styles and the Design of Network-based Software Architectures">[REST]</cite></a>. Once defined, a standardized method ought to have the same semantics when applied to any resource, though each resource
     1334               determines for itself whether those semantics are implemented or allowed.
     1335            </p>
     1336            <p id="rfc.section.4.1.p.6">This specification defines a number of standardized methods that are commonly used in HTTP, as outlined by the following table.
     1337               By convention, standardized methods are defined in all-uppercase ASCII letters.
     1338            </p>
     1339            <div id="rfc.table.1">
     1340               <div id="table.of.methods"></div>
     1341               <table class="tt full left" cellpadding="3" cellspacing="0">
     1342                  <thead>
     1343                     <tr>
     1344                        <th>Method</th>
     1345                        <th>Description</th>
     1346                        <th>Sec.</th>
     1347                     </tr>
     1348                  </thead>
     1349                  <tbody>
     1350                     <tr>
     1351                        <td class="left">GET</td>
     1352                        <td class="left">Transfer a current representation of the target resource.</td>
     1353                        <td class="left"><a href="#GET" id="rfc.xref.GET.3" title="GET">4.3.1</a></td>
     1354                     </tr>
     1355                     <tr>
     1356                        <td class="left">HEAD</td>
     1357                        <td class="left">Same as GET, but only transfer the status line and header section.</td>
     1358                        <td class="left"><a href="#HEAD" id="rfc.xref.HEAD.1" title="HEAD">4.3.2</a></td>
     1359                     </tr>
     1360                     <tr>
     1361                        <td class="left">POST</td>
     1362                        <td class="left">Perform resource-specific processing on the request payload.</td>
     1363                        <td class="left"><a href="#POST" id="rfc.xref.POST.2" title="POST">4.3.3</a></td>
     1364                     </tr>
     1365                     <tr>
     1366                        <td class="left">PUT</td>
     1367                        <td class="left">Replace all current representations of the target resource with the request payload.</td>
     1368                        <td class="left"><a href="#PUT" id="rfc.xref.PUT.2" title="PUT">4.3.4</a></td>
     1369                     </tr>
     1370                     <tr>
     1371                        <td class="left">DELETE</td>
     1372                        <td class="left">Remove all current representations of the target resource.</td>
     1373                        <td class="left"><a href="#DELETE" id="rfc.xref.DELETE.1" title="DELETE">4.3.5</a></td>
     1374                     </tr>
     1375                     <tr>
     1376                        <td class="left">CONNECT</td>
     1377                        <td class="left">Establish a tunnel to the server identified by the target resource.</td>
     1378                        <td class="left"><a href="#CONNECT" id="rfc.xref.CONNECT.1" title="CONNECT">4.3.6</a></td>
     1379                     </tr>
     1380                     <tr>
     1381                        <td class="left">OPTIONS</td>
     1382                        <td class="left">Describe the communication options for the target resource.</td>
     1383                        <td class="left"><a href="#OPTIONS" id="rfc.xref.OPTIONS.1" title="OPTIONS">4.3.7</a></td>
     1384                     </tr>
     1385                     <tr>
     1386                        <td class="left">TRACE</td>
     1387                        <td class="left">Perform a message loop-back test along the path to the target resource.</td>
     1388                        <td class="left"><a href="#TRACE" id="rfc.xref.TRACE.1" title="TRACE">4.3.8</a></td>
     1389                     </tr>
     1390                  </tbody>
     1391               </table>
     1392            </div>
     1393            <p id="rfc.section.4.1.p.7">All general-purpose servers <em class="bcp14">MUST</em> support the methods GET and HEAD. All other methods are <em class="bcp14">OPTIONAL</em>; when implemented, a server <em class="bcp14">MUST</em> implement the above methods according to the semantics defined for them in <a href="#method.definitions" title="Method Definitions">Section&nbsp;4.3</a>.
     1394            </p>
     1395            <p id="rfc.section.4.1.p.8">Additional methods, outside the scope of this specification, have been standardized for use in HTTP. All such methods ought
     1396               to be registered within the HTTP Method Registry maintained by IANA, as defined in <a href="#method.registry" title="Method Registry">Section&nbsp;8.1</a>.
     1397            </p>
     1398            <p id="rfc.section.4.1.p.9">The set of methods allowed by a target resource can be listed in an <a href="#header.allow" class="smpl">Allow</a> header field (<a href="#header.allow" id="rfc.xref.header.allow.1" title="Allow">Section&nbsp;7.4.1</a>). However, the set of allowed methods can change dynamically. When a request method is received that is unrecognized or not
     1399               implemented by an origin server, the origin server <em class="bcp14">SHOULD</em> respond with the <a href="#status.501" class="smpl">501 (Not Implemented)</a> status code. When a request method is received that is known by an origin server but not allowed for the target resource,
     1400               the origin server <em class="bcp14">SHOULD</em> respond with the <a href="#status.405" class="smpl">405 (Method Not Allowed)</a> status code.
     1401            </p>
     1402         </div>
     1403         <div id="method.properties">
     1404            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#method.properties">Common Method Properties</a></h2>
     1405            <div id="safe.methods">
     1406               <div id="rfc.iref.s.2"></div>
     1407               <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a href="#safe.methods">Safe Methods</a></h3>
     1408               <p id="rfc.section.4.2.1.p.1">Request methods are considered "<dfn>safe</dfn>" if their defined semantics are essentially read-only; i.e., the client does not request, and does not expect, any state
     1409                  change on the origin server as a result of applying a safe method to a target resource. Likewise, reasonable use of a safe
     1410                  method is not expected to cause any harm, loss of property, or unusual burden on the origin server.
     1411               </p>
     1412               <p id="rfc.section.4.2.1.p.2">This definition of safe methods does not prevent an implementation from including behavior that is potentially harmful, not
     1413                  entirely read-only, or which causes side-effects while invoking a safe method. What is important, however, is that the client
     1414                  did not request that additional behavior and cannot be held accountable for it. For example, most servers append request information
     1415                  to access log files at the completion of every response, regardless of the method, and that is considered safe even though
     1416                  the log storage might become full and crash the server. Likewise, a safe request initiated by selecting an advertisement on
     1417                  the Web will often have the side-effect of charging an advertising account.
     1418               </p>
     1419               <p id="rfc.section.4.2.1.p.3">Of the request methods defined by this specification, the GET, HEAD, OPTIONS, and TRACE methods are defined to be safe.</p>
     1420               <p id="rfc.section.4.2.1.p.4">The purpose of distinguishing between safe and unsafe methods is to allow automated retrieval processes (spiders) and cache
     1421                  performance optimization (pre-fetching) to work without fear of causing harm. In addition, it allows a user agent to apply
     1422                  appropriate constraints on the automated use of unsafe methods when processing potentially untrusted content.
     1423               </p>
     1424               <p id="rfc.section.4.2.1.p.5">A user agent <em class="bcp14">SHOULD</em> distinguish between safe and unsafe methods when presenting potential actions to a user, such that the user can be made aware
     1425                  of an unsafe action before it is requested.
     1426               </p>
     1427               <p id="rfc.section.4.2.1.p.6">When a resource is constructed such that parameters within the effective request URI have the effect of selecting an action,
     1428                  it is the resource owner's responsibility to ensure that the action is consistent with the request method semantics. For example,
     1429                  it is common for Web-based content editing software to use actions within query parameters, such as "page?do=delete". If the
     1430                  purpose of such a resource is to perform an unsafe action, then the resource owner <em class="bcp14">MUST</em> disable or disallow that action when it is accessed using a safe request method. Failure to do so will result in unfortunate
     1431                  side-effects when automated processes perform a GET on every URI reference for the sake of link maintenance, pre-fetching,
     1432                  building a search index, etc.
     1433               </p>
     1434            </div>
     1435            <div id="idempotent.methods">
     1436               <div id="rfc.iref.i.1"></div>
     1437               <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a href="#idempotent.methods">Idempotent Methods</a></h3>
     1438               <p id="rfc.section.4.2.2.p.1">Request methods are considered "<dfn id="idempotent">idempotent</dfn>" if the intended effect of multiple identical requests is the same as for a single request. Of the request methods defined
     1439                  by this specification, the PUT, DELETE, and safe request methods are idempotent.
     1440               </p>
     1441               <p id="rfc.section.4.2.2.p.2">Like the definition of safe, the idempotent property only applies to what has been requested by the user; a server is free
     1442                  to log each request separately, retain a revision control history, or implement other non-idempotent side-effects for each
     1443                  idempotent request.
     1444               </p>
     1445               <p id="rfc.section.4.2.2.p.3">Idempotent methods are distinguished because the request can be repeated automatically if a communication failure occurs before
     1446                  the client is able to read the server's response. For example, if a client sends a PUT request and the underlying connection
     1447                  is closed before any response is received, then it can establish a new connection and retry the idempotent request because
     1448                  it knows that repeating the request will have the same effect even if the original request succeeded. Note, however, that
     1449                  repeated failures would indicate a problem within the server.
     1450               </p>
     1451            </div>
     1452            <div id="cacheable.methods">
     1453               <div id="rfc.iref.c.8"></div>
     1454               <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a href="#cacheable.methods">Cacheable Methods</a></h3>
     1455               <p id="rfc.section.4.2.3.p.1">Request methods can be defined as "<dfn id="cacheable">cacheable</dfn>" to indicate that responses to them are allowed to be stored for future reuse; for specific requirements see <a href="#Part6" id="rfc.xref.Part6.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>. In general, safe methods that do not depend on a current or authoritative response are defined as cacheable; this specification
     1456                  defines GET, HEAD and POST as cacheable, although the overwhelming majority of cache implementations only support GET and
     1457                  HEAD.
     1458               </p>
     1459            </div>
     1460         </div>
     1461         <div id="method.definitions">
     1462            <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a href="#method.definitions">Method Definitions</a></h2>
     1463            <div id="GET">
     1464               <h3 id="rfc.section.4.3.1"><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;<a href="#GET">GET</a></h3>
     1465               <div id="rfc.iref.g.16"></div>
     1466               <p id="rfc.section.4.3.1.p.1">The GET method requests transfer of a current selected representation for the <a href="#resources" class="smpl">target resource</a>. GET is the primary mechanism of information retrieval and the focus of almost all performance optimizations. Hence, when
     1467                  people speak of retrieving some identifiable information via HTTP, they are generally referring to making a GET request.
     1468               </p>
     1469               <p id="rfc.section.4.3.1.p.2">It is tempting to think of resource identifiers as remote filesystem pathnames, and of representations as being a copy of
     1470                  the contents of such files. In fact, that is how many resources are implemented (see <a href="#attack.pathname" title="Attacks Based On File and Path Names">Section&nbsp;9.1</a> for related security considerations). However, there are no such limitations in practice. The HTTP interface for a resource
     1471                  is just as likely to be implemented as a tree of content objects, a programmatic view on various database records, or a gateway
     1472                  to other information systems. Even when the URI mapping mechanism is tied to a filesystem, an origin server might be configured
     1473                  to execute the files with the request as input and send the output as the representation, rather than transfer the files directly.
     1474                  Regardless, only the origin server needs to know how each of its resource identifiers corresponds to an implementation, and
     1475                  how each implementation manages to select and send a current representation of the target resource in a response to GET.
     1476               </p>
     1477               <p id="rfc.section.4.3.1.p.3">A client can alter the semantics of GET to be a "range request", requesting transfer of only some part(s) of the selected
     1478                  representation, by sending a <a href="p5-range.html#header.range" class="smpl">Range</a> header field in the request (<a href="#Part5" id="rfc.xref.Part5.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>).
     1479               </p>
     1480               <p id="rfc.section.4.3.1.p.4">A payload within a GET request message has no defined semantics; sending a payload body on a GET request might cause some
     1481                  existing implementations to reject the request.
     1482               </p>
     1483               <p id="rfc.section.4.3.1.p.5">The response to a GET request is cacheable; a cache <em class="bcp14">MAY</em> use it to satisfy subsequent GET and HEAD requests unless otherwise indicated by the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 5.2</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
     1484               </p>
     1485            </div>
     1486            <div id="HEAD">
     1487               <h3 id="rfc.section.4.3.2"><a href="#rfc.section.4.3.2">4.3.2</a>&nbsp;<a href="#HEAD">HEAD</a></h3>
     1488               <div id="rfc.iref.h.1"></div>
     1489               <p id="rfc.section.4.3.2.p.1">The HEAD method is identical to GET except that the server <em class="bcp14">MUST NOT</em> send a message body in the response (i.e., the response terminates at the end of the header section). The server <em class="bcp14">SHOULD</em> send the same header fields in response to a HEAD request as it would have sent if the request had been a GET, except that
     1490                  the payload header fields (<a href="#payload" title="Payload Semantics">Section&nbsp;3.3</a>) <em class="bcp14">MAY</em> be omitted. This method can be used for obtaining metadata about the selected representation without transferring the representation
     1491                  data and is often used for testing hypertext links for validity, accessibility, and recent modification.
     1492               </p>
     1493               <p id="rfc.section.4.3.2.p.2">A payload within a HEAD request message has no defined semantics; sending a payload body on a HEAD request might cause some
     1494                  existing implementations to reject the request.
     1495               </p>
     1496               <p id="rfc.section.4.3.2.p.3">The response to a HEAD request is cacheable; a cache <em class="bcp14">MAY</em> use it to satisfy subsequent HEAD requests unless otherwise indicated by the Cache-Control header field (<a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 5.2</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). A HEAD response might also have an effect on previously cached responses to GET; see <a href="p6-cache.html#head.effects" title="Freshening Responses via HEAD">Section 4.3.5</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
     1497               </p>
     1498            </div>
     1499            <div id="POST">
     1500               <div id="rfc.iref.p.2"></div>
     1501               <h3 id="rfc.section.4.3.3"><a href="#rfc.section.4.3.3">4.3.3</a>&nbsp;<a href="#POST">POST</a></h3>
     1502               <p id="rfc.section.4.3.3.p.1">The POST method requests that the <a href="#resources" class="smpl">target resource</a> process the representation enclosed in the request according to the resource's own specific semantics. For example, POST is
     1503                  used for the following functions (among others):
     1504               </p>
     1505               <ul>
     1506                  <li>Providing a block of data, such as the fields entered into an HTML form, to a data-handling process;</li>
     1507                  <li>Posting a message to a bulletin board, newsgroup, mailing list, blog, or similar group of articles;</li>
     1508                  <li>Creating a new resource that has yet to be identified by the origin server; and</li>
     1509                  <li>Appending data to a resource's existing representation(s).</li>
     1510               </ul>
     1511               <p id="rfc.section.4.3.3.p.2">An origin server indicates response semantics by choosing an appropriate status code depending on the result of processing
     1512                  the POST request; almost all of the status codes defined by this specification might be received in a response to POST (the
     1513                  exceptions being 206, 304, and 416).
     1514               </p>
     1515               <p id="rfc.section.4.3.3.p.3">If one or more resources has been created on the origin server as a result of successfully processing a POST request, the
     1516                  origin server <em class="bcp14">SHOULD</em> send a <a href="#status.201" class="smpl">201 (Created)</a> response containing a <a href="#header.location" class="smpl">Location</a> header field that provides an identifier for the primary resource created (<a href="#header.location" id="rfc.xref.header.location.1" title="Location">Section&nbsp;7.1.2</a>) and a representation that describes the status of the request while referring to the new resource(s).
     1517               </p>
     1518               <p id="rfc.section.4.3.3.p.4">Responses to POST requests are only cacheable when they include explicit freshness information (see <a href="p6-cache.html#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section 4.2.1</a> of <a href="#Part6" id="rfc.xref.Part6.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). However, POST caching is not widely implemented. For cases where an origin server wishes the client to be able to cache
     1519                  the result of a POST in a way that can be reused by a later GET, the origin server <em class="bcp14">MAY</em> send a <a href="#status.200" class="smpl">200 (OK)</a> response containing the result and a <a href="#header.content-location" class="smpl">Content-Location</a> header field that has the same value as the POST's effective request URI (<a href="#header.content-location" id="rfc.xref.header.content-location.2" title="Content-Location">Section&nbsp;3.1.4.2</a>).
     1520               </p>
     1521               <p id="rfc.section.4.3.3.p.5">If the result of processing a POST would be equivalent to a representation of an existing resource, an origin server <em class="bcp14">MAY</em> redirect the user agent to that resource by sending a <a href="#status.303" class="smpl">303 (See Other)</a> response with the existing resource's identifier in the <a href="#header.location" class="smpl">Location</a> field. This has the benefits of providing the user agent a resource identifier and transferring the representation via a method
     1522                  more amenable to shared caching, though at the cost of an extra request if the user agent does not already have the representation
     1523                  cached.
     1524               </p>
     1525            </div>
     1526            <div id="PUT">
     1527               <h3 id="rfc.section.4.3.4"><a href="#rfc.section.4.3.4">4.3.4</a>&nbsp;<a href="#PUT">PUT</a></h3>
     1528               <div id="rfc.iref.p.3"></div>
     1529               <p id="rfc.section.4.3.4.p.1">The PUT method requests that the state of the <a href="#resources" class="smpl">target resource</a> be created or replaced with the state defined by the representation enclosed in the request message payload. A successful
     1530                  PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent
     1531                  representation being sent in a <a href="#status.200" class="smpl">200 (OK)</a> response. However, there is no guarantee that such a state change will be observable, since the target resource might be acted
     1532                  upon by other user agents in parallel, or might be subject to dynamic processing by the origin server, before any subsequent
     1533                  GET is received. A successful response only implies that the user agent's intent was achieved at the time of its processing
     1534                  by the origin server.
     1535               </p>
     1536               <p id="rfc.section.4.3.4.p.2">If the target resource does not have a current representation and the PUT successfully creates one, then the origin server <em class="bcp14">MUST</em> inform the user agent by sending a <a href="#status.201" class="smpl">201 (Created)</a> response. If the target resource does have a current representation and that representation is successfully modified in accordance
     1537                  with the state of the enclosed representation, then the origin server <em class="bcp14">MUST</em> send either a <a href="#status.200" class="smpl">200 (OK)</a> or a <a href="#status.204" class="smpl">204 (No Content)</a> response to indicate successful completion of the request.
     1538               </p>
     1539               <p id="rfc.section.4.3.4.p.3">An origin server <em class="bcp14">SHOULD</em> ignore unrecognized header fields received in a PUT request (i.e., do not save them as part of the resource state).
     1540               </p>
     1541               <p id="rfc.section.4.3.4.p.4">An origin server <em class="bcp14">SHOULD</em> verify that the PUT representation is consistent with any constraints the server has for the target resource that cannot or
     1542                  will not be changed by the PUT. This is particularly important when the origin server uses internal configuration information
     1543                  related to the URI in order to set the values for representation metadata on GET responses. When a PUT representation is inconsistent
     1544                  with the target resource, the origin server <em class="bcp14">SHOULD</em> either make them consistent, by transforming the representation or changing the resource configuration, or respond with an
     1545                  appropriate error message containing sufficient information to explain why the representation is unsuitable. The <a href="#status.409" class="smpl">409 (Conflict)</a> or <a href="#status.415" class="smpl">415 (Unsupported Media Type)</a> status codes are suggested, with the latter being specific to constraints on <a href="#header.content-type" class="smpl">Content-Type</a> values.
     1546               </p>
     1547               <p id="rfc.section.4.3.4.p.5">For example, if the target resource is configured to always have a <a href="#header.content-type" class="smpl">Content-Type</a> of "text/html" and the representation being PUT has a Content-Type of "image/jpeg", the origin server ought to do one of:
     1548               </p>
     1549               <ol class="la">
     1550                  <li>reconfigure the target resource to reflect the new media type;</li>
     1551                  <li>transform the PUT representation to a format consistent with that of the resource before saving it as the new resource state;
     1552                     or,
     1553                  </li>
     1554                  <li>reject the request with a <a href="#status.415" class="smpl">415 (Unsupported Media Type)</a> response indicating that the target resource is limited to "text/html", perhaps including a link to a different resource that
     1555                     would be a suitable target for the new representation.
     1556                  </li>
     1557               </ol>
     1558               <p id="rfc.section.4.3.4.p.6">HTTP does not define exactly how a PUT method affects the state of an origin server beyond what can be expressed by the intent
     1559                  of the user agent request and the semantics of the origin server response. It does not define what a resource might be, in
     1560                  any sense of that word, beyond the interface provided via HTTP. It does not define how resource state is "stored", nor how
     1561                  such storage might change as a result of a change in resource state, nor how the origin server translates resource state into
     1562                  representations. Generally speaking, all implementation details behind the resource interface are intentionally hidden by
     1563                  the server.
     1564               </p>
     1565               <p id="rfc.section.4.3.4.p.7">An origin server <em class="bcp14">MUST NOT</em> send a validator header field (<a href="#response.validator" title="Validator Header Fields">Section&nbsp;7.2</a>), such as an <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> or <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> field, in a successful response to PUT unless the request's representation data was saved without any transformation applied
     1566                  to the body (i.e., the resource's new representation data is identical to the representation data received in the PUT request)
     1567                  and the validator field value reflects the new representation. This requirement allows a user agent to know when the representation
     1568                  body it has in memory remains current as a result of the PUT, thus not in need of retrieving again from the origin server,
     1569                  and that the new validator(s) received in the response can be used for future conditional requests in order to prevent accidental
     1570                  overwrites (<a href="#request.conditionals" title="Conditionals">Section&nbsp;5.2</a>).
     1571               </p>
     1572               <p id="rfc.section.4.3.4.p.8">The fundamental difference between the POST and PUT methods is highlighted by the different intent for the enclosed representation.
     1573                  The target resource in a POST request is intended to handle the enclosed representation according to the resource's own semantics,
     1574                  whereas the enclosed representation in a PUT request is defined as replacing the state of the target resource. Hence, the
     1575                  intent of PUT is idempotent and visible to intermediaries, even though the exact effect is only known by the origin server.
     1576               </p>
     1577               <p id="rfc.section.4.3.4.p.9">Proper interpretation of a PUT request presumes that the user agent knows which target resource is desired. A service that
     1578                  selects a proper URI on behalf of the client, after receiving a state-changing request, <em class="bcp14">SHOULD</em> be implemented using the POST method rather than PUT. If the origin server will not make the requested PUT state change to
     1579                  the target resource and instead wishes to have it applied to a different resource, such as when the resource has been moved
     1580                  to a different URI, then the origin server <em class="bcp14">MUST</em> send an appropriate <a href="#status.3xx" class="smpl">3xx (Redirection)</a> response; the user agent <em class="bcp14">MAY</em> then make its own decision regarding whether or not to redirect the request.
     1581               </p>
     1582               <p id="rfc.section.4.3.4.p.10">A PUT request applied to the target resource can have side-effects on other resources. For example, an article might have
     1583                  a URI for identifying "the current version" (a resource) that is separate from the URIs identifying each particular version
     1584                  (different resources that at one point shared the same state as the current version resource). A successful PUT request on
     1585                  "the current version" URI might therefore create a new version resource in addition to changing the state of the target resource,
     1586                  and might also cause links to be added between the related resources.
     1587               </p>
     1588               <p id="rfc.section.4.3.4.p.11">An origin server that allows PUT on a given target resource <em class="bcp14">MUST</em> send a <a href="#status.400" class="smpl">400 (Bad Request)</a> response to a PUT request that contains a <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> header field (<a href="p5-range.html#header.content-range" title="Content-Range">Section 4.2</a> of <a href="#Part5" id="rfc.xref.Part5.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>), since the payload is likely to be partial content that has been mistakenly PUT as a full representation. Partial content
     1589                  updates are possible by targeting a separately identified resource with state that overlaps a portion of the larger resource,
     1590                  or by using a different method that has been specifically defined for partial updates (for example, the PATCH method defined
     1591                  in <a href="#RFC5789" id="rfc.xref.RFC5789.1"><cite title="PATCH Method for HTTP">[RFC5789]</cite></a>).
     1592               </p>
     1593               <p id="rfc.section.4.3.4.p.12">Responses to the PUT method are not cacheable. If a successful PUT request passes through a cache that has one or more stored
     1594                  responses for the effective request URI, those stored responses will be invalidated (see <a href="p6-cache.html#invalidation" title="Invalidation">Section 4.4</a> of <a href="#Part6" id="rfc.xref.Part6.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
     1595               </p>
     1596            </div>
     1597            <div id="DELETE">
     1598               <h3 id="rfc.section.4.3.5"><a href="#rfc.section.4.3.5">4.3.5</a>&nbsp;<a href="#DELETE">DELETE</a></h3>
     1599               <div id="rfc.iref.d.2"></div>
     1600               <p id="rfc.section.4.3.5.p.1">The DELETE method requests that the origin server remove the association between the <a href="#resources" class="smpl">target resource</a> and its current functionality. In effect, this method is similar to the rm command in UNIX: it expresses a deletion operation
     1601                  on the URI mapping of the origin server, rather than an expectation that the previously associated information be deleted.
     1602               </p>
     1603               <p id="rfc.section.4.3.5.p.2">If the target resource has one or more current representations, they might or might not be destroyed by the origin server,
     1604                  and the associated storage might or might not be reclaimed, depending entirely on the nature of the resource and its implementation
     1605                  by the origin server (which are beyond the scope of this specification). Likewise, other implementation aspects of a resource
     1606                  might need to be deactivated or archived as a result of a DELETE, such as database or gateway connections. In general, it
     1607                  is assumed that the origin server will only allow DELETE on resources for which it has a prescribed mechanism for accomplishing
     1608                  the deletion.
     1609               </p>
     1610               <p id="rfc.section.4.3.5.p.3">Relatively few resources allow the DELETE method — its primary use is for remote authoring environments, where the user has
     1611                  some direction regarding its effect. For example, a resource that was previously created using a PUT request, or identified
     1612                  via the Location header field after a <a href="#status.201" class="smpl">201 (Created)</a> response to a POST request, might allow a corresponding DELETE request to undo those actions. Similarly, custom user agent
     1613                  implementations that implement an authoring function, such as revision control clients using HTTP for remote operations, might
     1614                  use DELETE based on an assumption that the server's URI space has been crafted to correspond to a version repository.
     1615               </p>
     1616               <p id="rfc.section.4.3.5.p.4">If a DELETE method is successfully applied, the origin server <em class="bcp14">SHOULD</em> send a <a href="#status.202" class="smpl">202 (Accepted)</a> status code if the action will likely succeed but has not yet been enacted, a <a href="#status.204" class="smpl">204 (No Content)</a> status code if the action has been enacted and no further information is to be supplied, or a <a href="#status.200" class="smpl">200 (OK)</a> status code if the action has been enacted and the response message includes a representation describing the status.
     1617               </p>
     1618               <p id="rfc.section.4.3.5.p.5">A payload within a DELETE request message has no defined semantics; sending a payload body on a DELETE request might cause
     1619                  some existing implementations to reject the request.
     1620               </p>
     1621               <p id="rfc.section.4.3.5.p.6">Responses to the DELETE method are not cacheable. If a DELETE request passes through a cache that has one or more stored responses
     1622                  for the effective request URI, those stored responses will be invalidated (see <a href="p6-cache.html#invalidation" title="Invalidation">Section 4.4</a> of <a href="#Part6" id="rfc.xref.Part6.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
     1623               </p>
     1624            </div>
     1625            <div id="CONNECT">
     1626               <div id="rfc.iref.c.9"></div>
     1627               <h3 id="rfc.section.4.3.6"><a href="#rfc.section.4.3.6">4.3.6</a>&nbsp;<a href="#CONNECT">CONNECT</a></h3>
     1628               <p id="rfc.section.4.3.6.p.1">The CONNECT method requests that the recipient establish a tunnel to the destination origin server identified by the request-target
     1629                  and, if successful, thereafter restrict its behavior to blind forwarding of packets, in both directions, until the tunnel
     1630                  is closed.
     1631               </p>
     1632               <p id="rfc.section.4.3.6.p.2">CONNECT is intended only for use in requests to a proxy. An origin server that receives a CONNECT request for itself <em class="bcp14">MAY</em> respond with a <a href="#status.2xx" class="smpl">2xx</a> status code to indicate that a connection is established. However, most origin servers do not implement CONNECT.
     1633               </p>
     1634               <p id="rfc.section.4.3.6.p.3">A client sending a CONNECT request <em class="bcp14">MUST</em> send the authority form of request-target (<a href="p1-messaging.html#request-target" title="Request Target">Section 5.3</a> of <a href="#Part1" id="rfc.xref.Part1.15"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>); i.e., the request-target consists of only the host name and port number of the tunnel destination, separated by a colon.
     1635                  For example,
     1636               </p>
     1637               <div id="rfc.figure.u.18"></div><pre class="text2">CONNECT server.example.com:80 HTTP/1.1
    15601638Host: server.example.com:80
    15611639
    15621640</pre><p id="rfc.section.4.3.6.p.5">The recipient proxy can establish a tunnel either by directly connecting to the request-target or, if configured to use another
    1563          proxy, by forwarding the CONNECT request to the next inbound proxy. Any <a href="#status.2xx" class="smpl">2xx (Successful)</a> response indicates that the sender (and all inbound proxies) will switch to tunnel mode immediately after the blank line that
    1564          concludes the successful response's header section; data received after that blank line is from the server identified by the
    1565          request-target. Any response other than a successful response indicates that the tunnel has not yet been formed and that the
    1566          connection remains governed by HTTP.
    1567       </p>
    1568       <p id="rfc.section.4.3.6.p.6">A tunnel is closed when a tunnel intermediary detects that either side has closed its connection: the intermediary <em class="bcp14">MUST</em> attempt to send any outstanding data that came from the closed side to the other side, close both connections, and then discard
    1569          any remaining data left undelivered.
    1570       </p>
    1571       <p id="rfc.section.4.3.6.p.7">Proxy authentication might be used to establish the authority to create a tunnel. For example,</p>
    1572       <div id="rfc.figure.u.19"></div><pre class="text2">CONNECT server.example.com:80 HTTP/1.1
     1641                  proxy, by forwarding the CONNECT request to the next inbound proxy. Any <a href="#status.2xx" class="smpl">2xx (Successful)</a> response indicates that the sender (and all inbound proxies) will switch to tunnel mode immediately after the blank line that
     1642                  concludes the successful response's header section; data received after that blank line is from the server identified by the
     1643                  request-target. Any response other than a successful response indicates that the tunnel has not yet been formed and that the
     1644                  connection remains governed by HTTP.
     1645               </p>
     1646               <p id="rfc.section.4.3.6.p.6">A tunnel is closed when a tunnel intermediary detects that either side has closed its connection: the intermediary <em class="bcp14">MUST</em> attempt to send any outstanding data that came from the closed side to the other side, close both connections, and then discard
     1647                  any remaining data left undelivered.
     1648               </p>
     1649               <p id="rfc.section.4.3.6.p.7">Proxy authentication might be used to establish the authority to create a tunnel. For example,</p>
     1650               <div id="rfc.figure.u.19"></div><pre class="text2">CONNECT server.example.com:80 HTTP/1.1
    15731651Host: server.example.com:80
    15741652Proxy-Authorization: basic aGVsbG86d29ybGQ=
    15751653
    15761654</pre><p id="rfc.section.4.3.6.p.9">There are significant risks in establishing a tunnel to arbitrary servers, particularly when the destination is a well-known
    1577          or reserved TCP port that is not intended for Web traffic. For example, a CONNECT to a request-target of "example.com:25"
    1578          would suggest that the proxy connect to the reserved port for SMTP traffic; if allowed, that could trick the proxy into relaying
    1579          spam email. Proxies that support CONNECT <em class="bcp14">SHOULD</em> restrict its use to a limited set of known ports or a configurable whitelist of safe request targets.
    1580       </p>
    1581       <p id="rfc.section.4.3.6.p.10">A server <em class="bcp14">MUST NOT</em> send any <a href="p1-messaging.html#header.transfer-encoding" class="smpl">Transfer-Encoding</a> or <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> header fields in a <a href="#status.2xx" class="smpl">2xx (Successful)</a> response to CONNECT. A client <em class="bcp14">MUST</em> ignore any Content-Length or Transfer-Encoding header fields received in a successful response to CONNECT.
    1582       </p>
    1583       <p id="rfc.section.4.3.6.p.11">A payload within a CONNECT request message has no defined semantics; sending a payload body on a CONNECT request might cause
    1584          some existing implementations to reject the request.
    1585       </p>
    1586       <p id="rfc.section.4.3.6.p.12">Responses to the CONNECT method are not cacheable.</p>
    1587       <h3 id="rfc.section.4.3.7"><a href="#rfc.section.4.3.7">4.3.7</a>&nbsp;<a id="OPTIONS" href="#OPTIONS">OPTIONS</a></h3>
    1588       <div id="rfc.iref.o.1"></div>
    1589       <p id="rfc.section.4.3.7.p.1">The OPTIONS method requests information about the communication options available for the target resource, either at the origin
    1590          server or an intervening intermediary. This method allows a client to determine the options and/or requirements associated
    1591          with a resource, or the capabilities of a server, without implying a resource action.
    1592       </p>
    1593       <p id="rfc.section.4.3.7.p.2">An OPTIONS request with an asterisk ("*") as the request-target (<a href="p1-messaging.html#request-target" title="Request Target">Section 5.3</a> of <a href="#Part1" id="rfc.xref.Part1.16"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) applies to the server in general rather than to a specific resource. Since a server's communication options typically depend
    1594          on the resource, the "*" request is only useful as a "ping" or "no-op" type of method; it does nothing beyond allowing the
    1595          client to test the capabilities of the server. For example, this can be used to test a proxy for HTTP/1.1 conformance (or
    1596          lack thereof).
    1597       </p>
    1598       <p id="rfc.section.4.3.7.p.3">If the request-target is not an asterisk, the OPTIONS request applies to the options that are available when communicating
    1599          with the target resource.
    1600       </p>
    1601       <p id="rfc.section.4.3.7.p.4">A server generating a successful response to OPTIONS <em class="bcp14">SHOULD</em> send any header fields that might indicate optional features implemented by the server and applicable to the target resource
    1602          (e.g., <a href="#header.allow" class="smpl">Allow</a>), including potential extensions not defined by this specification. The response payload, if any, might also describe the
    1603          communication options in a machine or human-readable representation. A standard format for such a representation is not defined
    1604          by this specification, but might be defined by future extensions to HTTP. A server <em class="bcp14">MUST</em> generate a <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> field with a value of "0" if no payload body is to be sent in the response.
    1605       </p>
    1606       <p id="rfc.section.4.3.7.p.5">A client <em class="bcp14">MAY</em> send a <a href="#header.max-forwards" class="smpl">Max-Forwards</a> header field in an OPTIONS request to target a specific recipient in the request chain (see <a href="#header.max-forwards" id="rfc.xref.header.max-forwards.1" title="Max-Forwards">Section&nbsp;5.1.2</a>). A proxy <em class="bcp14">MUST NOT</em> generate a Max-Forwards header field while forwarding a request unless that request was received with a Max-Forwards field.
    1607       </p>
    1608       <p id="rfc.section.4.3.7.p.6">A client that generates an OPTIONS request containing a payload body <em class="bcp14">MUST</em> send a valid <a href="#header.content-type" class="smpl">Content-Type</a> header field describing the representation media type. Although this specification does not define any use for such a payload,
    1609          future extensions to HTTP might use the OPTIONS body to make more detailed queries about the target resource.
    1610       </p>
    1611       <p id="rfc.section.4.3.7.p.7">Responses to the OPTIONS method are not cacheable.</p>
    1612       <h3 id="rfc.section.4.3.8"><a href="#rfc.section.4.3.8">4.3.8</a>&nbsp;<a id="TRACE" href="#TRACE">TRACE</a></h3>
    1613       <div id="rfc.iref.t.1"></div>
    1614       <p id="rfc.section.4.3.8.p.1">The TRACE method requests a remote, application-level loop-back of the request message. The final recipient of the request <em class="bcp14">SHOULD</em> reflect the message received, excluding some fields described below, back to the client as the message body of a <a href="#status.200" class="smpl">200 (OK)</a> response with a <a href="#header.content-type" class="smpl">Content-Type</a> of "message/http" (<a href="p1-messaging.html#internet.media.type.message.http" title="Internet Media Type message/http">Section 8.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.17"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). The final recipient is either the origin server or the first server to receive a <a href="#header.max-forwards" class="smpl">Max-Forwards</a> value of zero (0) in the request (<a href="#header.max-forwards" id="rfc.xref.header.max-forwards.2" title="Max-Forwards">Section&nbsp;5.1.2</a>).
    1615       </p>
    1616       <p id="rfc.section.4.3.8.p.2">A client <em class="bcp14">MUST NOT</em> generate header fields in a TRACE request containing sensitive data that might be disclosed by the response. For example,
    1617          it would be foolish for a user agent to send stored user credentials <a href="#Part7" id="rfc.xref.Part7.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a> or cookies <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a> in a TRACE request. The final recipient of the request <em class="bcp14">SHOULD</em> exclude any request header fields that are likely to contain sensitive data when that recipient generates the response body.
    1618       </p>
    1619       <p id="rfc.section.4.3.8.p.3">TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing
    1620          or diagnostic information. The value of the <a href="p1-messaging.html#header.via" class="smpl">Via</a> header field (<a href="p1-messaging.html#header.via" title="Via">Section 5.7.1</a> of <a href="#Part1" id="rfc.xref.Part1.18"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) is of particular interest, since it acts as a trace of the request chain. Use of the <a href="#header.max-forwards" class="smpl">Max-Forwards</a> header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding
    1621          messages in an infinite loop.
    1622       </p>
    1623       <p id="rfc.section.4.3.8.p.4">A client <em class="bcp14">MUST NOT</em> send a message body in a TRACE request.
    1624       </p>
    1625       <p id="rfc.section.4.3.8.p.5">Responses to the TRACE method are not cacheable.</p>
    1626       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="request.header.fields" href="#request.header.fields">Request Header Fields</a></h1>
    1627       <p id="rfc.section.5.p.1">A client sends request header fields to provide more information about the request context, make the request conditional based
    1628          on the target resource state, suggest preferred formats for the response, supply authentication credentials, or modify the
    1629          expected request processing. These fields act as request modifiers, similar to the parameters on a programming language method
    1630          invocation.
    1631       </p>
    1632       <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="request.controls" href="#request.controls">Controls</a></h2>
    1633       <p id="rfc.section.5.1.p.1">Controls are request header fields that direct specific handling of the request.</p>
    1634       <div id="rfc.table.u.3">
    1635          <table class="tt full left" cellpadding="3" cellspacing="0">
    1636             <thead>
    1637                <tr>
    1638                   <th>Header Field Name</th>
    1639                   <th>Defined in...</th>
    1640                </tr>
    1641             </thead>
    1642             <tbody>
    1643                <tr>
    1644                   <td class="left">Cache-Control</td>
    1645                   <td class="left"><a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 5.2</a> of <a href="#Part6" id="rfc.xref.Part6.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a></td>
    1646                </tr>
    1647                <tr>
    1648                   <td class="left">Expect</td>
    1649                   <td class="left"><a href="#header.expect" id="rfc.xref.header.expect.1" title="Expect">Section&nbsp;5.1.1</a></td>
    1650                </tr>
    1651                <tr>
    1652                   <td class="left">Host</td>
    1653                   <td class="left"><a href="p1-messaging.html#header.host" title="Host">Section 5.4</a> of <a href="#Part1" id="rfc.xref.Part1.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
    1654                </tr>
    1655                <tr>
    1656                   <td class="left">Max-Forwards</td>
    1657                   <td class="left"><a href="#header.max-forwards" id="rfc.xref.header.max-forwards.3" title="Max-Forwards">Section&nbsp;5.1.2</a></td>
    1658                </tr>
    1659                <tr>
    1660                   <td class="left">Pragma</td>
    1661                   <td class="left"><a href="p6-cache.html#header.pragma" title="Pragma">Section 5.4</a> of <a href="#Part6" id="rfc.xref.Part6.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a></td>
    1662                </tr>
    1663                <tr>
    1664                   <td class="left">Range</td>
    1665                   <td class="left"><a href="p5-range.html#header.range" title="Range">Section 3.1</a> of <a href="#Part5" id="rfc.xref.Part5.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
    1666                </tr>
    1667                <tr>
    1668                   <td class="left">TE</td>
    1669                   <td class="left"><a href="p1-messaging.html#header.te" title="TE">Section 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
    1670                </tr>
    1671             </tbody>
    1672          </table>
     1655                  or reserved TCP port that is not intended for Web traffic. For example, a CONNECT to a request-target of "example.com:25"
     1656                  would suggest that the proxy connect to the reserved port for SMTP traffic; if allowed, that could trick the proxy into relaying
     1657                  spam email. Proxies that support CONNECT <em class="bcp14">SHOULD</em> restrict its use to a limited set of known ports or a configurable whitelist of safe request targets.
     1658               </p>
     1659               <p id="rfc.section.4.3.6.p.10">A server <em class="bcp14">MUST NOT</em> send any <a href="p1-messaging.html#header.transfer-encoding" class="smpl">Transfer-Encoding</a> or <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> header fields in a <a href="#status.2xx" class="smpl">2xx (Successful)</a> response to CONNECT. A client <em class="bcp14">MUST</em> ignore any Content-Length or Transfer-Encoding header fields received in a successful response to CONNECT.
     1660               </p>
     1661               <p id="rfc.section.4.3.6.p.11">A payload within a CONNECT request message has no defined semantics; sending a payload body on a CONNECT request might cause
     1662                  some existing implementations to reject the request.
     1663               </p>
     1664               <p id="rfc.section.4.3.6.p.12">Responses to the CONNECT method are not cacheable.</p>
     1665            </div>
     1666            <div id="OPTIONS">
     1667               <h3 id="rfc.section.4.3.7"><a href="#rfc.section.4.3.7">4.3.7</a>&nbsp;<a href="#OPTIONS">OPTIONS</a></h3>
     1668               <div id="rfc.iref.o.1"></div>
     1669               <p id="rfc.section.4.3.7.p.1">The OPTIONS method requests information about the communication options available for the target resource, either at the origin
     1670                  server or an intervening intermediary. This method allows a client to determine the options and/or requirements associated
     1671                  with a resource, or the capabilities of a server, without implying a resource action.
     1672               </p>
     1673               <p id="rfc.section.4.3.7.p.2">An OPTIONS request with an asterisk ("*") as the request-target (<a href="p1-messaging.html#request-target" title="Request Target">Section 5.3</a> of <a href="#Part1" id="rfc.xref.Part1.16"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) applies to the server in general rather than to a specific resource. Since a server's communication options typically depend
     1674                  on the resource, the "*" request is only useful as a "ping" or "no-op" type of method; it does nothing beyond allowing the
     1675                  client to test the capabilities of the server. For example, this can be used to test a proxy for HTTP/1.1 conformance (or
     1676                  lack thereof).
     1677               </p>
     1678               <p id="rfc.section.4.3.7.p.3">If the request-target is not an asterisk, the OPTIONS request applies to the options that are available when communicating
     1679                  with the target resource.
     1680               </p>
     1681               <p id="rfc.section.4.3.7.p.4">A server generating a successful response to OPTIONS <em class="bcp14">SHOULD</em> send any header fields that might indicate optional features implemented by the server and applicable to the target resource
     1682                  (e.g., <a href="#header.allow" class="smpl">Allow</a>), including potential extensions not defined by this specification. The response payload, if any, might also describe the
     1683                  communication options in a machine or human-readable representation. A standard format for such a representation is not defined
     1684                  by this specification, but might be defined by future extensions to HTTP. A server <em class="bcp14">MUST</em> generate a <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> field with a value of "0" if no payload body is to be sent in the response.
     1685               </p>
     1686               <p id="rfc.section.4.3.7.p.5">A client <em class="bcp14">MAY</em> send a <a href="#header.max-forwards" class="smpl">Max-Forwards</a> header field in an OPTIONS request to target a specific recipient in the request chain (see <a href="#header.max-forwards" id="rfc.xref.header.max-forwards.1" title="Max-Forwards">Section&nbsp;5.1.2</a>). A proxy <em class="bcp14">MUST NOT</em> generate a Max-Forwards header field while forwarding a request unless that request was received with a Max-Forwards field.
     1687               </p>
     1688               <p id="rfc.section.4.3.7.p.6">A client that generates an OPTIONS request containing a payload body <em class="bcp14">MUST</em> send a valid <a href="#header.content-type" class="smpl">Content-Type</a> header field describing the representation media type. Although this specification does not define any use for such a payload,
     1689                  future extensions to HTTP might use the OPTIONS body to make more detailed queries about the target resource.
     1690               </p>
     1691               <p id="rfc.section.4.3.7.p.7">Responses to the OPTIONS method are not cacheable.</p>
     1692            </div>
     1693            <div id="TRACE">
     1694               <h3 id="rfc.section.4.3.8"><a href="#rfc.section.4.3.8">4.3.8</a>&nbsp;<a href="#TRACE">TRACE</a></h3>
     1695               <div id="rfc.iref.t.1"></div>
     1696               <p id="rfc.section.4.3.8.p.1">The TRACE method requests a remote, application-level loop-back of the request message. The final recipient of the request <em class="bcp14">SHOULD</em> reflect the message received, excluding some fields described below, back to the client as the message body of a <a href="#status.200" class="smpl">200 (OK)</a> response with a <a href="#header.content-type" class="smpl">Content-Type</a> of "message/http" (<a href="p1-messaging.html#internet.media.type.message.http" title="Internet Media Type message/http">Section 8.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.17"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). The final recipient is either the origin server or the first server to receive a <a href="#header.max-forwards" class="smpl">Max-Forwards</a> value of zero (0) in the request (<a href="#header.max-forwards" id="rfc.xref.header.max-forwards.2" title="Max-Forwards">Section&nbsp;5.1.2</a>).
     1697               </p>
     1698               <p id="rfc.section.4.3.8.p.2">A client <em class="bcp14">MUST NOT</em> generate header fields in a TRACE request containing sensitive data that might be disclosed by the response. For example,
     1699                  it would be foolish for a user agent to send stored user credentials <a href="#Part7" id="rfc.xref.Part7.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a> or cookies <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a> in a TRACE request. The final recipient of the request <em class="bcp14">SHOULD</em> exclude any request header fields that are likely to contain sensitive data when that recipient generates the response body.
     1700               </p>
     1701               <p id="rfc.section.4.3.8.p.3">TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing
     1702                  or diagnostic information. The value of the <a href="p1-messaging.html#header.via" class="smpl">Via</a> header field (<a href="p1-messaging.html#header.via" title="Via">Section 5.7.1</a> of <a href="#Part1" id="rfc.xref.Part1.18"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) is of particular interest, since it acts as a trace of the request chain. Use of the <a href="#header.max-forwards" class="smpl">Max-Forwards</a> header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding
     1703                  messages in an infinite loop.
     1704               </p>
     1705               <p id="rfc.section.4.3.8.p.4">A client <em class="bcp14">MUST NOT</em> send a message body in a TRACE request.
     1706               </p>
     1707               <p id="rfc.section.4.3.8.p.5">Responses to the TRACE method are not cacheable.</p>
     1708            </div>
     1709         </div>
    16731710      </div>
    1674       <div id="rfc.iref.e.1"></div>
    1675       <div id="rfc.iref.40"></div>
    1676       <h3 id="rfc.section.5.1.1"><a href="#rfc.section.5.1.1">5.1.1</a>&nbsp;<a id="header.expect" href="#header.expect">Expect</a></h3>
    1677       <p id="rfc.section.5.1.1.p.1">The "Expect" header field in a request indicates a certain set of behaviors (expectations) that need to be supported by the
    1678          server in order to properly handle this request. The only such expectation defined by this specification is <a href="#header.expect" class="smpl">100-continue</a>.
    1679       </p>
    1680       <div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.17"></span>  <a href="#header.expect" class="smpl">Expect</a>  = "100-continue"
     1711      <div id="request.header.fields">
     1712         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#request.header.fields">Request Header Fields</a></h1>
     1713         <p id="rfc.section.5.p.1">A client sends request header fields to provide more information about the request context, make the request conditional based
     1714            on the target resource state, suggest preferred formats for the response, supply authentication credentials, or modify the
     1715            expected request processing. These fields act as request modifiers, similar to the parameters on a programming language method
     1716            invocation.
     1717         </p>
     1718         <div id="request.controls">
     1719            <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a href="#request.controls">Controls</a></h2>
     1720            <p id="rfc.section.5.1.p.1">Controls are request header fields that direct specific handling of the request.</p>
     1721            <div id="rfc.table.u.3">
     1722               <table class="tt full left" cellpadding="3" cellspacing="0">
     1723                  <thead>
     1724                     <tr>
     1725                        <th>Header Field Name</th>
     1726                        <th>Defined in...</th>
     1727                     </tr>
     1728                  </thead>
     1729                  <tbody>
     1730                     <tr>
     1731                        <td class="left">Cache-Control</td>
     1732                        <td class="left"><a href="p6-cache.html#header.cache-control" title="Cache-Control">Section 5.2</a> of <a href="#Part6" id="rfc.xref.Part6.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a></td>
     1733                     </tr>
     1734                     <tr>
     1735                        <td class="left">Expect</td>
     1736                        <td class="left"><a href="#header.expect" id="rfc.xref.header.expect.1" title="Expect">Section&nbsp;5.1.1</a></td>
     1737                     </tr>
     1738                     <tr>
     1739                        <td class="left">Host</td>
     1740                        <td class="left"><a href="p1-messaging.html#header.host" title="Host">Section 5.4</a> of <a href="#Part1" id="rfc.xref.Part1.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
     1741                     </tr>
     1742                     <tr>
     1743                        <td class="left">Max-Forwards</td>
     1744                        <td class="left"><a href="#header.max-forwards" id="rfc.xref.header.max-forwards.3" title="Max-Forwards">Section&nbsp;5.1.2</a></td>
     1745                     </tr>
     1746                     <tr>
     1747                        <td class="left">Pragma</td>
     1748                        <td class="left"><a href="p6-cache.html#header.pragma" title="Pragma">Section 5.4</a> of <a href="#Part6" id="rfc.xref.Part6.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a></td>
     1749                     </tr>
     1750                     <tr>
     1751                        <td class="left">Range</td>
     1752                        <td class="left"><a href="p5-range.html#header.range" title="Range">Section 3.1</a> of <a href="#Part5" id="rfc.xref.Part5.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
     1753                     </tr>
     1754                     <tr>
     1755                        <td class="left">TE</td>
     1756                        <td class="left"><a href="p1-messaging.html#header.te" title="TE">Section 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a></td>
     1757                     </tr>
     1758                  </tbody>
     1759               </table>
     1760            </div>
     1761            <div id="header.expect">
     1762               <div id="rfc.iref.e.1"></div>
     1763               <div id="rfc.iref.40"></div>
     1764               <h3 id="rfc.section.5.1.1"><a href="#rfc.section.5.1.1">5.1.1</a>&nbsp;<a href="#header.expect">Expect</a></h3>
     1765               <p id="rfc.section.5.1.1.p.1">The "Expect" header field in a request indicates a certain set of behaviors (expectations) that need to be supported by the
     1766                  server in order to properly handle this request. The only such expectation defined by this specification is <a href="#header.expect" class="smpl">100-continue</a>.
     1767               </p>
     1768               <div id="rfc.figure.u.20"></div><pre class="inline"><span id="rfc.iref.g.17"></span>  <a href="#header.expect" class="smpl">Expect</a>  = "100-continue"
    16811769</pre><p id="rfc.section.5.1.1.p.3">The Expect field-value is case-insensitive.</p>
    1682       <p id="rfc.section.5.1.1.p.4">A server that receives an Expect field-value other than <a href="#header.expect" class="smpl">100-continue</a> <em class="bcp14">MAY</em> respond with a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status code to indicate that the unexpected expectation cannot be met.
    1683       </p>
    1684       <p id="rfc.section.5.1.1.p.5">A <dfn>100-continue</dfn> expectation informs recipients that the client is about to send a (presumably large) message body in this request and wishes
    1685          to receive a <a href="#status.100" class="smpl">100 (Continue)</a> interim response if the request-line and header fields are not sufficient to cause an immediate success, redirect, or error
    1686          response. This allows the client to wait for an indication that it is worthwhile to send the message body before actually
    1687          doing so, which can improve efficiency when the message body is huge or when the client anticipates that an error is likely
    1688          (e.g., when sending a state-changing method, for the first time, without previously verified authentication credentials).
    1689       </p>
    1690       <p id="rfc.section.5.1.1.p.6">For example, a request that begins with</p>
    1691       <div id="rfc.figure.u.21"></div><pre class="text2">PUT /somewhere/fun HTTP/1.1
     1770               <p id="rfc.section.5.1.1.p.4">A server that receives an Expect field-value other than <a href="#header.expect" class="smpl">100-continue</a> <em class="bcp14">MAY</em> respond with a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status code to indicate that the unexpected expectation cannot be met.
     1771               </p>
     1772               <p id="rfc.section.5.1.1.p.5">A <dfn>100-continue</dfn> expectation informs recipients that the client is about to send a (presumably large) message body in this request and wishes
     1773                  to receive a <a href="#status.100" class="smpl">100 (Continue)</a> interim response if the request-line and header fields are not sufficient to cause an immediate success, redirect, or error
     1774                  response. This allows the client to wait for an indication that it is worthwhile to send the message body before actually
     1775                  doing so, which can improve efficiency when the message body is huge or when the client anticipates that an error is likely
     1776                  (e.g., when sending a state-changing method, for the first time, without previously verified authentication credentials).
     1777               </p>
     1778               <p id="rfc.section.5.1.1.p.6">For example, a request that begins with</p>
     1779               <div id="rfc.figure.u.21"></div><pre class="text2">PUT /somewhere/fun HTTP/1.1
    16921780Host: origin.example.com
    16931781Content-Type: video/h264
     
    16961784
    16971785</pre><p id="rfc.section.5.1.1.p.8">allows the origin server to immediately respond with an error message, such as <a href="p7-auth.html#status.401" class="smpl">401 (Unauthorized)</a> or <a href="#status.405" class="smpl">405 (Method Not Allowed)</a>, before the client starts filling the pipes with an unnecessary data transfer.
    1698       </p>
    1699       <p id="rfc.section.5.1.1.p.9">Requirements for clients: </p>
    1700       <ul>
    1701          <li>A client <em class="bcp14">MUST NOT</em> generate a 100-continue expectation in a request that does not include a message body.
    1702          </li>
    1703          <li>A client that will wait for a <a href="#status.100" class="smpl">100 (Continue)</a> response before sending the request message body <em class="bcp14">MUST</em> send an <a href="#header.expect" class="smpl">Expect</a> header field containing a 100-continue expectation.
    1704          </li>
    1705          <li>A client that sends a 100-continue expectation is not required to wait for any specific length of time; such a client <em class="bcp14">MAY</em> proceed to send the message body even if it has not yet received a response. Furthermore, since <a href="#status.100" class="smpl">100 (Continue)</a> responses cannot be sent through an HTTP/1.0 intermediary, such a client <em class="bcp14">SHOULD NOT</em> wait for an indefinite period before sending the message body.
    1706          </li>
    1707          <li>A client that receives a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status code in response to a request containing a 100-continue expectation <em class="bcp14">SHOULD</em> repeat that request without a 100-continue expectation, since the 417 response merely indicates that the response chain does
    1708             not support expectations (e.g., it passes through an HTTP/1.0 server).
    1709          </li>
    1710       </ul>
    1711       <p id="rfc.section.5.1.1.p.10">Requirements for servers: </p>
    1712       <ul>
    1713          <li>A server that receives a 100-continue expectation in an HTTP/1.0 request <em class="bcp14">MUST</em> ignore that expectation.
    1714          </li>
    1715          <li>A server <em class="bcp14">MAY</em> omit sending a <a href="#status.100" class="smpl">100 (Continue)</a> response if it has already received some or all of the message body for the corresponding request, or if the framing indicates
    1716             that there is no message body.
    1717          </li>
    1718          <li>A server that sends a <a href="#status.100" class="smpl">100 (Continue)</a> response <em class="bcp14">MUST</em> ultimately send a final status code, once the message body is received and processed, unless the connection is closed prematurely.
    1719          </li>
    1720          <li>A server that responds with a final status code before reading the entire message body <em class="bcp14">SHOULD</em> indicate in that response whether it intends to close the connection or continue reading and discarding the request message
    1721             (see <a href="p1-messaging.html#persistent.tear-down" title="Tear-down">Section 6.6</a> of <a href="#Part1" id="rfc.xref.Part1.21"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>).
    1722          </li>
    1723       </ul>
    1724       <p id="rfc.section.5.1.1.p.11">An origin server <em class="bcp14">MUST</em>, upon receiving an HTTP/1.1 (or later) request-line and a complete header section that contains a 100-continue expectation
    1725          and indicates a request message body will follow, either send an immediate response with a final status code, if that status
    1726          can be determined by examining just the request-line and header fields, or send an immediate <a href="#status.100" class="smpl">100 (Continue)</a> response to encourage the client to send the request's message body. The origin server <em class="bcp14">MUST NOT</em> wait for the message body before sending the <a href="#status.100" class="smpl">100 (Continue)</a> response.
    1727       </p>
    1728       <p id="rfc.section.5.1.1.p.12">A proxy <em class="bcp14">MUST</em>, upon receiving an HTTP/1.1 (or later) request-line and a complete header section that contains a 100-continue expectation
    1729          and indicates a request message body will follow, either send an immediate response with a final status code, if that status
    1730          can be determined by examining just the request-line and header fields, or begin forwarding the request toward the origin
    1731          server by sending a corresponding request-line and header section to the next inbound server. If the proxy believes (from
    1732          configuration or past interaction) that the next inbound server only supports HTTP/1.0, the proxy <em class="bcp14">MAY</em> generate an immediate <a href="#status.100" class="smpl">100 (Continue)</a> response to encourage the client to begin sending the message body.
    1733       </p>
    1734       <div class="note" id="rfc.section.5.1.1.p.13">
    1735          <p><b>Note:</b> The Expect header field was added after the original publication of HTTP/1.1 <a href="#RFC2068" id="rfc.xref.RFC2068.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> as both the means to request an interim 100 response and the general mechanism for indicating must-understand extensions.
    1736             However, the extension mechanism has not been used by clients and the must-understand requirements have not been implemented
    1737             by many servers, rendering the extension mechanism useless. This specification has removed the extension mechanism in order
    1738             to simplify the definition and processing of 100-continue.
    1739          </p>
    1740       </div>
    1741       <div id="rfc.iref.m.1"></div>
    1742       <h3 id="rfc.section.5.1.2"><a href="#rfc.section.5.1.2">5.1.2</a>&nbsp;<a id="header.max-forwards" href="#header.max-forwards">Max-Forwards</a></h3>
    1743       <p id="rfc.section.5.1.2.p.1">The "Max-Forwards" header field provides a mechanism with the TRACE (<a href="#TRACE" id="rfc.xref.TRACE.2" title="TRACE">Section&nbsp;4.3.8</a>) and OPTIONS (<a href="#OPTIONS" id="rfc.xref.OPTIONS.2" title="OPTIONS">Section&nbsp;4.3.7</a>) request methods to limit the number of times that the request is forwarded by proxies. This can be useful when the client
    1744          is attempting to trace a request that appears to be failing or looping mid-chain.
    1745       </p>
    1746       <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.18"></span>  <a href="#header.max-forwards" class="smpl">Max-Forwards</a> = 1*<a href="#imported.abnf" class="smpl">DIGIT</a>
     1786               </p>
     1787               <p id="rfc.section.5.1.1.p.9">Requirements for clients: </p>
     1788               <ul>
     1789                  <li>A client <em class="bcp14">MUST NOT</em> generate a 100-continue expectation in a request that does not include a message body.
     1790                  </li>
     1791                  <li>A client that will wait for a <a href="#status.100" class="smpl">100 (Continue)</a> response before sending the request message body <em class="bcp14">MUST</em> send an <a href="#header.expect" class="smpl">Expect</a> header field containing a 100-continue expectation.
     1792                  </li>
     1793                  <li>A client that sends a 100-continue expectation is not required to wait for any specific length of time; such a client <em class="bcp14">MAY</em> proceed to send the message body even if it has not yet received a response. Furthermore, since <a href="#status.100" class="smpl">100 (Continue)</a> responses cannot be sent through an HTTP/1.0 intermediary, such a client <em class="bcp14">SHOULD NOT</em> wait for an indefinite period before sending the message body.
     1794                  </li>
     1795                  <li>A client that receives a <a href="#status.417" class="smpl">417 (Expectation Failed)</a> status code in response to a request containing a 100-continue expectation <em class="bcp14">SHOULD</em> repeat that request without a 100-continue expectation, since the 417 response merely indicates that the response chain does
     1796                     not support expectations (e.g., it passes through an HTTP/1.0 server).
     1797                  </li>
     1798               </ul>
     1799               <p id="rfc.section.5.1.1.p.10">Requirements for servers: </p>
     1800               <ul>
     1801                  <li>A server that receives a 100-continue expectation in an HTTP/1.0 request <em class="bcp14">MUST</em> ignore that expectation.
     1802                  </li>
     1803                  <li>A server <em class="bcp14">MAY</em> omit sending a <a href="#status.100" class="smpl">100 (Continue)</a> response if it has already received some or all of the message body for the corresponding request, or if the framing indicates
     1804                     that there is no message body.
     1805                  </li>
     1806                  <li>A server that sends a <a href="#status.100" class="smpl">100 (Continue)</a> response <em class="bcp14">MUST</em> ultimately send a final status code, once the message body is received and processed, unless the connection is closed prematurely.
     1807                  </li>
     1808                  <li>A server that responds with a final status code before reading the entire message body <em class="bcp14">SHOULD</em> indicate in that response whether it intends to close the connection or continue reading and discarding the request message
     1809                     (see <a href="p1-messaging.html#persistent.tear-down" title="Tear-down">Section 6.6</a> of <a href="#Part1" id="rfc.xref.Part1.21"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>).
     1810                  </li>
     1811               </ul>
     1812               <p id="rfc.section.5.1.1.p.11">An origin server <em class="bcp14">MUST</em>, upon receiving an HTTP/1.1 (or later) request-line and a complete header section that contains a 100-continue expectation
     1813                  and indicates a request message body will follow, either send an immediate response with a final status code, if that status
     1814                  can be determined by examining just the request-line and header fields, or send an immediate <a href="#status.100" class="smpl">100 (Continue)</a> response to encourage the client to send the request's message body. The origin server <em class="bcp14">MUST NOT</em> wait for the message body before sending the <a href="#status.100" class="smpl">100 (Continue)</a> response.
     1815               </p>
     1816               <p id="rfc.section.5.1.1.p.12">A proxy <em class="bcp14">MUST</em>, upon receiving an HTTP/1.1 (or later) request-line and a complete header section that contains a 100-continue expectation
     1817                  and indicates a request message body will follow, either send an immediate response with a final status code, if that status
     1818                  can be determined by examining just the request-line and header fields, or begin forwarding the request toward the origin
     1819                  server by sending a corresponding request-line and header section to the next inbound server. If the proxy believes (from
     1820                  configuration or past interaction) that the next inbound server only supports HTTP/1.0, the proxy <em class="bcp14">MAY</em> generate an immediate <a href="#status.100" class="smpl">100 (Continue)</a> response to encourage the client to begin sending the message body.
     1821               </p>
     1822               <div class="note" id="rfc.section.5.1.1.p.13">
     1823                  <p><b>Note:</b> The Expect header field was added after the original publication of HTTP/1.1 <a href="#RFC2068" id="rfc.xref.RFC2068.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a> as both the means to request an interim 100 response and the general mechanism for indicating must-understand extensions.
     1824                     However, the extension mechanism has not been used by clients and the must-understand requirements have not been implemented
     1825                     by many servers, rendering the extension mechanism useless. This specification has removed the extension mechanism in order
     1826                     to simplify the definition and processing of 100-continue.
     1827                  </p>
     1828               </div>
     1829            </div>
     1830            <div id="header.max-forwards">
     1831               <div id="rfc.iref.m.1"></div>
     1832               <h3 id="rfc.section.5.1.2"><a href="#rfc.section.5.1.2">5.1.2</a>&nbsp;<a href="#header.max-forwards">Max-Forwards</a></h3>
     1833               <p id="rfc.section.5.1.2.p.1">The "Max-Forwards" header field provides a mechanism with the TRACE (<a href="#TRACE" id="rfc.xref.TRACE.2" title="TRACE">Section&nbsp;4.3.8</a>) and OPTIONS (<a href="#OPTIONS" id="rfc.xref.OPTIONS.2" title="OPTIONS">Section&nbsp;4.3.7</a>) request methods to limit the number of times that the request is forwarded by proxies. This can be useful when the client
     1834                  is attempting to trace a request that appears to be failing or looping mid-chain.
     1835               </p>
     1836               <div id="rfc.figure.u.22"></div><pre class="inline"><span id="rfc.iref.g.18"></span>  <a href="#header.max-forwards" class="smpl">Max-Forwards</a> = 1*<a href="#imported.abnf" class="smpl">DIGIT</a>
    17471837</pre><p id="rfc.section.5.1.2.p.3">The Max-Forwards value is a decimal integer indicating the remaining number of times this request message can be forwarded.</p>
    1748       <p id="rfc.section.5.1.2.p.4">Each intermediary that receives a TRACE or OPTIONS request containing a Max-Forwards header field <em class="bcp14">MUST</em> check and update its value prior to forwarding the request. If the received value is zero (0), the intermediary <em class="bcp14">MUST NOT</em> forward the request; instead, the intermediary <em class="bcp14">MUST</em> respond as the final recipient. If the received Max-Forwards value is greater than zero, the intermediary <em class="bcp14">MUST</em> generate an updated Max-Forwards field in the forwarded message with a field-value that is the lesser of: a) the received
    1749          value decremented by one (1), or b) the recipient's maximum supported value for Max-Forwards.
    1750       </p>
    1751       <p id="rfc.section.5.1.2.p.5">A recipient <em class="bcp14">MAY</em> ignore a Max-Forwards header field received with any other request methods.
    1752       </p>
    1753       <div id="rfc.iref.c.10"></div>
    1754       <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="request.conditionals" href="#request.conditionals">Conditionals</a></h2>
    1755       <p id="rfc.section.5.2.p.1">The HTTP conditional request header fields <a href="#Part4" id="rfc.xref.Part4.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a> allow a client to place a precondition on the state of the target resource, so that the action corresponding to the method
    1756          semantics will not be applied if the precondition evaluates to false. Each precondition defined by this specification consists
    1757          of a comparison between a set of validators obtained from prior representations of the target resource to the current state
    1758          of validators for the <a href="#representations" class="smpl">selected representation</a> (<a href="#response.validator" title="Validator Header Fields">Section&nbsp;7.2</a>). Hence, these preconditions evaluate whether the state of the target resource has changed since a given state known by the
    1759          client. The effect of such an evaluation depends on the method semantics and choice of conditional, as defined in <a href="p4-conditional.html#evaluation" title="Evaluation">Section 5</a> of <a href="#Part4" id="rfc.xref.Part4.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>.
    1760       </p>
    1761       <div id="rfc.table.u.4">
    1762          <table class="tt full left" cellpadding="3" cellspacing="0">
    1763             <thead>
    1764                <tr>
    1765                   <th>Header Field Name</th>
    1766                   <th>Defined in...</th>
    1767                </tr>
    1768             </thead>
    1769             <tbody>
    1770                <tr>
    1771                   <td class="left">If-Match</td>
    1772                   <td class="left"><a href="p4-conditional.html#header.if-match" title="If-Match">Section 3.1</a> of <a href="#Part4" id="rfc.xref.Part4.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
    1773                </tr>
    1774                <tr>
    1775                   <td class="left">If-None-Match</td>
    1776                   <td class="left"><a href="p4-conditional.html#header.if-none-match" title="If-None-Match">Section 3.2</a> of <a href="#Part4" id="rfc.xref.Part4.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
    1777                </tr>
    1778                <tr>
    1779                   <td class="left">If-Modified-Since</td>
    1780                   <td class="left"><a href="p4-conditional.html#header.if-modified-since" title="If-Modified-Since">Section 3.3</a> of <a href="#Part4" id="rfc.xref.Part4.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
    1781                </tr>
    1782                <tr>
    1783                   <td class="left">If-Unmodified-Since</td>
    1784                   <td class="left"><a href="p4-conditional.html#header.if-unmodified-since" title="If-Unmodified-Since">Section 3.4</a> of <a href="#Part4" id="rfc.xref.Part4.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
    1785                </tr>
    1786                <tr>
    1787                   <td class="left">If-Range</td>
    1788                   <td class="left"><a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#Part5" id="rfc.xref.Part5.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
    1789                </tr>
    1790             </tbody>
    1791          </table>
    1792       </div>
    1793       <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a id="request.conneg" href="#request.conneg">Content Negotiation</a></h2>
    1794       <p id="rfc.section.5.3.p.1">The following request header fields are sent by a user agent to engage in <a href="#proactive.negotiation" class="smpl">proactive negotiation</a> of the response content, as defined in <a href="#proactive.negotiation" title="Proactive Negotiation">Section&nbsp;3.4.1</a>. The preferences sent in these fields apply to any content in the response, including representations of the target resource,
    1795          representations of error or processing status, and potentially even the miscellaneous text strings that might appear within
    1796          the protocol.
    1797       </p>
    1798       <div id="rfc.table.u.5">
    1799          <table class="tt full left" cellpadding="3" cellspacing="0">
    1800             <thead>
    1801                <tr>
    1802                   <th>Header Field Name</th>
    1803                   <th>Defined in...</th>
    1804                </tr>
    1805             </thead>
    1806             <tbody>
    1807                <tr>
    1808                   <td class="left">Accept</td>
    1809                   <td class="left"><a href="#header.accept" id="rfc.xref.header.accept.2" title="Accept">Section&nbsp;5.3.2</a></td>
    1810                </tr>
    1811                <tr>
    1812                   <td class="left">Accept-Charset</td>
    1813                   <td class="left"><a href="#header.accept-charset" id="rfc.xref.header.accept-charset.1" title="Accept-Charset">Section&nbsp;5.3.3</a></td>
    1814                </tr>
    1815                <tr>
    1816                   <td class="left">Accept-Encoding</td>
    1817                   <td class="left"><a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.2" title="Accept-Encoding">Section&nbsp;5.3.4</a></td>
    1818                </tr>
    1819                <tr>
    1820                   <td class="left">Accept-Language</td>
    1821                   <td class="left"><a href="#header.accept-language" id="rfc.xref.header.accept-language.2" title="Accept-Language">Section&nbsp;5.3.5</a></td>
    1822                </tr>
    1823             </tbody>
    1824          </table>
    1825       </div>
    1826       <h3 id="rfc.section.5.3.1"><a href="#rfc.section.5.3.1">5.3.1</a>&nbsp;<a id="quality.values" href="#quality.values">Quality Values</a></h3>
    1827       <p id="rfc.section.5.3.1.p.1">Many of the request header fields for <a href="#proactive.negotiation" class="smpl">proactive negotiation</a> use a common parameter, named "q" (case-insensitive), to assign a relative "weight" to the preference for that associated
    1828          kind of content. This weight is referred to as a "quality value" (or "qvalue") because the same parameter name is often used
    1829          within server configurations to assign a weight to the relative quality of the various representations that can be selected
    1830          for a resource.
    1831       </p>
    1832       <p id="rfc.section.5.3.1.p.2">The weight is normalized to a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred;
    1833          a value of 0 means "not acceptable". If no "q" parameter is present, the default weight is 1.
    1834       </p>
    1835       <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span>  <a href="#quality.values" class="smpl">weight</a> = <a href="#imported.abnf" class="smpl">OWS</a> ";" <a href="#imported.abnf" class="smpl">OWS</a> "q=" <a href="#quality.values" class="smpl">qvalue</a>
     1838               <p id="rfc.section.5.1.2.p.4">Each intermediary that receives a TRACE or OPTIONS request containing a Max-Forwards header field <em class="bcp14">MUST</em> check and update its value prior to forwarding the request. If the received value is zero (0), the intermediary <em class="bcp14">MUST NOT</em> forward the request; instead, the intermediary <em class="bcp14">MUST</em> respond as the final recipient. If the received Max-Forwards value is greater than zero, the intermediary <em class="bcp14">MUST</em> generate an updated Max-Forwards field in the forwarded message with a field-value that is the lesser of: a) the received
     1839                  value decremented by one (1), or b) the recipient's maximum supported value for Max-Forwards.
     1840               </p>
     1841               <p id="rfc.section.5.1.2.p.5">A recipient <em class="bcp14">MAY</em> ignore a Max-Forwards header field received with any other request methods.
     1842               </p>
     1843            </div>
     1844         </div>
     1845         <div id="request.conditionals">
     1846            <div id="rfc.iref.c.10"></div>
     1847            <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a href="#request.conditionals">Conditionals</a></h2>
     1848            <p id="rfc.section.5.2.p.1">The HTTP conditional request header fields <a href="#Part4" id="rfc.xref.Part4.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a> allow a client to place a precondition on the state of the target resource, so that the action corresponding to the method
     1849               semantics will not be applied if the precondition evaluates to false. Each precondition defined by this specification consists
     1850               of a comparison between a set of validators obtained from prior representations of the target resource to the current state
     1851               of validators for the <a href="#representations" class="smpl">selected representation</a> (<a href="#response.validator" title="Validator Header Fields">Section&nbsp;7.2</a>). Hence, these preconditions evaluate whether the state of the target resource has changed since a given state known by the
     1852               client. The effect of such an evaluation depends on the method semantics and choice of conditional, as defined in <a href="p4-conditional.html#evaluation" title="Evaluation">Section 5</a> of <a href="#Part4" id="rfc.xref.Part4.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>.
     1853            </p>
     1854            <div id="rfc.table.u.4">
     1855               <table class="tt full left" cellpadding="3" cellspacing="0">
     1856                  <thead>
     1857                     <tr>
     1858                        <th>Header Field Name</th>
     1859                        <th>Defined in...</th>
     1860                     </tr>
     1861                  </thead>
     1862                  <tbody>
     1863                     <tr>
     1864                        <td class="left">If-Match</td>
     1865                        <td class="left"><a href="p4-conditional.html#header.if-match" title="If-Match">Section 3.1</a> of <a href="#Part4" id="rfc.xref.Part4.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
     1866                     </tr>
     1867                     <tr>
     1868                        <td class="left">If-None-Match</td>
     1869                        <td class="left"><a href="p4-conditional.html#header.if-none-match" title="If-None-Match">Section 3.2</a> of <a href="#Part4" id="rfc.xref.Part4.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
     1870                     </tr>
     1871                     <tr>
     1872                        <td class="left">If-Modified-Since</td>
     1873                        <td class="left"><a href="p4-conditional.html#header.if-modified-since" title="If-Modified-Since">Section 3.3</a> of <a href="#Part4" id="rfc.xref.Part4.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
     1874                     </tr>
     1875                     <tr>
     1876                        <td class="left">If-Unmodified-Since</td>
     1877                        <td class="left"><a href="p4-conditional.html#header.if-unmodified-since" title="If-Unmodified-Since">Section 3.4</a> of <a href="#Part4" id="rfc.xref.Part4.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
     1878                     </tr>
     1879                     <tr>
     1880                        <td class="left">If-Range</td>
     1881                        <td class="left"><a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#Part5" id="rfc.xref.Part5.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
     1882                     </tr>
     1883                  </tbody>
     1884               </table>
     1885            </div>
     1886         </div>
     1887         <div id="request.conneg">
     1888            <h2 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a href="#request.conneg">Content Negotiation</a></h2>
     1889            <p id="rfc.section.5.3.p.1">The following request header fields are sent by a user agent to engage in <a href="#proactive.negotiation" class="smpl">proactive negotiation</a> of the response content, as defined in <a href="#proactive.negotiation" title="Proactive Negotiation">Section&nbsp;3.4.1</a>. The preferences sent in these fields apply to any content in the response, including representations of the target resource,
     1890               representations of error or processing status, and potentially even the miscellaneous text strings that might appear within
     1891               the protocol.
     1892            </p>
     1893            <div id="rfc.table.u.5">
     1894               <table class="tt full left" cellpadding="3" cellspacing="0">
     1895                  <thead>
     1896                     <tr>
     1897                        <th>Header Field Name</th>
     1898                        <th>Defined in...</th>
     1899                     </tr>
     1900                  </thead>
     1901                  <tbody>
     1902                     <tr>
     1903                        <td class="left">Accept</td>
     1904                        <td class="left"><a href="#header.accept" id="rfc.xref.header.accept.2" title="Accept">Section&nbsp;5.3.2</a></td>
     1905                     </tr>
     1906                     <tr>
     1907                        <td class="left">Accept-Charset</td>
     1908                        <td class="left"><a href="#header.accept-charset" id="rfc.xref.header.accept-charset.1" title="Accept-Charset">Section&nbsp;5.3.3</a></td>
     1909                     </tr>
     1910                     <tr>
     1911                        <td class="left">Accept-Encoding</td>
     1912                        <td class="left"><a href="#header.accept-encoding" id="rfc.xref.header.accept-encoding.2" title="Accept-Encoding">Section&nbsp;5.3.4</a></td>
     1913                     </tr>
     1914                     <tr>
     1915                        <td class="left">Accept-Language</td>
     1916                        <td class="left"><a href="#header.accept-language" id="rfc.xref.header.accept-language.2" title="Accept-Language">Section&nbsp;5.3.5</a></td>
     1917                     </tr>
     1918                  </tbody>
     1919               </table>
     1920            </div>
     1921            <div id="quality.values">
     1922               <h3 id="rfc.section.5.3.1"><a href="#rfc.section.5.3.1">5.3.1</a>&nbsp;<a href="#quality.values">Quality Values</a></h3>
     1923               <p id="rfc.section.5.3.1.p.1">Many of the request header fields for <a href="#proactive.negotiation" class="smpl">proactive negotiation</a> use a common parameter, named "q" (case-insensitive), to assign a relative "weight" to the preference for that associated
     1924                  kind of content. This weight is referred to as a "quality value" (or "qvalue") because the same parameter name is often used
     1925                  within server configurations to assign a weight to the relative quality of the various representations that can be selected
     1926                  for a resource.
     1927               </p>
     1928               <p id="rfc.section.5.3.1.p.2">The weight is normalized to a real number in the range 0 through 1, where 0.001 is the least preferred and 1 is the most preferred;
     1929                  a value of 0 means "not acceptable". If no "q" parameter is present, the default weight is 1.
     1930               </p>
     1931               <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span>  <a href="#quality.values" class="smpl">weight</a> = <a href="#imported.abnf" class="smpl">OWS</a> ";" <a href="#imported.abnf" class="smpl">OWS</a> "q=" <a href="#quality.values" class="smpl">qvalue</a>
    18361932  <a href="#quality.values" class="smpl">qvalue</a> = ( "0" [ "." 0*3<a href="#imported.abnf" class="smpl">DIGIT</a> ] )
    18371933         / ( "1" [ "." 0*3("0") ] )
    18381934</pre><p id="rfc.section.5.3.1.p.4">A sender of qvalue <em class="bcp14">MUST NOT</em> generate more than three digits after the decimal point. User configuration of these values ought to be limited in the same
    1839          fashion.
    1840       </p>
    1841       <div id="rfc.iref.a.1"></div>
    1842       <h3 id="rfc.section.5.3.2"><a href="#rfc.section.5.3.2">5.3.2</a>&nbsp;<a id="header.accept" href="#header.accept">Accept</a></h3>
    1843       <p id="rfc.section.5.3.2.p.1">The "Accept" header field can be used by user agents to specify response media types that are acceptable. Accept header fields
    1844          can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request
    1845          for an in-line image.
    1846       </p>
    1847       <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span><span id="rfc.iref.g.24"></span>  <a href="#header.accept" class="smpl">Accept</a> = #( <a href="#header.accept" class="smpl">media-range</a> [ <a href="#header.accept" class="smpl">accept-params</a> ] )
     1935                  fashion.
     1936               </p>
     1937            </div>
     1938            <div id="header.accept">
     1939               <div id="rfc.iref.a.1"></div>
     1940               <h3 id="rfc.section.5.3.2"><a href="#rfc.section.5.3.2">5.3.2</a>&nbsp;<a href="#header.accept">Accept</a></h3>
     1941               <p id="rfc.section.5.3.2.p.1">The "Accept" header field can be used by user agents to specify response media types that are acceptable. Accept header fields
     1942                  can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request
     1943                  for an in-line image.
     1944               </p>
     1945               <div id="rfc.figure.u.24"></div><pre class="inline"><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span><span id="rfc.iref.g.24"></span>  <a href="#header.accept" class="smpl">Accept</a> = #( <a href="#header.accept" class="smpl">media-range</a> [ <a href="#header.accept" class="smpl">accept-params</a> ] )
    18481946 
    18491947  <a href="#header.accept" class="smpl">media-range</a>    = ( "*/*"
     
    18541952  <a href="#header.accept" class="smpl">accept-ext</a>     = <a href="#imported.abnf" class="smpl">OWS</a> ";" <a href="#imported.abnf" class="smpl">OWS</a> <a href="#imported.abnf" class="smpl">token</a> [ "=" <a href="#imported.abnf" class="smpl">word</a> ]
    18551953</pre><p id="rfc.section.5.3.2.p.3">The asterisk "*" character is used to group media types into ranges, with "*/*" indicating all media types and "type/*" indicating
    1856          all subtypes of that type. The media-range can include media type parameters that are applicable to that range.
    1857       </p>
    1858       <p id="rfc.section.5.3.2.p.4">Each media-range might be followed by zero or more applicable media type parameters (e.g., <a href="#charset" class="smpl">charset</a>), an optional "q" parameter for indicating a relative weight (<a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>), and then zero or more extension parameters. The "q" parameter is necessary if any extensions (accept-ext) are present,
    1859          since it acts as a separator between the two parameter sets.
    1860       </p>
    1861       <div class="note" id="rfc.section.5.3.2.p.5">
    1862          <p><b>Note:</b> Use of the "q" parameter name to separate media type parameters from Accept extension parameters is due to historical practice.
    1863             Although this prevents any media type parameter named "q" from being used with a media range, such an event is believed to
    1864             be unlikely given the lack of any "q" parameters in the IANA media type registry and the rare usage of any media type parameters
    1865             in Accept. Future media types are discouraged from registering any parameter named "q".
    1866          </p>
    1867       </div>
    1868       <p id="rfc.section.5.3.2.p.6">The example</p>
    1869       <div id="rfc.figure.u.25"></div><pre class="text">  Accept: audio/*; q=0.2, audio/basic
     1954                  all subtypes of that type. The media-range can include media type parameters that are applicable to that range.
     1955               </p>
     1956               <p id="rfc.section.5.3.2.p.4">Each media-range might be followed by zero or more applicable media type parameters (e.g., <a href="#charset" class="smpl">charset</a>), an optional "q" parameter for indicating a relative weight (<a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>), and then zero or more extension parameters. The "q" parameter is necessary if any extensions (accept-ext) are present,
     1957                  since it acts as a separator between the two parameter sets.
     1958               </p>
     1959               <div class="note" id="rfc.section.5.3.2.p.5">
     1960                  <p><b>Note:</b> Use of the "q" parameter name to separate media type parameters from Accept extension parameters is due to historical practice.
     1961                     Although this prevents any media type parameter named "q" from being used with a media range, such an event is believed to
     1962                     be unlikely given the lack of any "q" parameters in the IANA media type registry and the rare usage of any media type parameters
     1963                     in Accept. Future media types are discouraged from registering any parameter named "q".
     1964                  </p>
     1965               </div>
     1966               <p id="rfc.section.5.3.2.p.6">The example</p>
     1967               <div id="rfc.figure.u.25"></div><pre class="text">  Accept: audio/*; q=0.2, audio/basic
    18701968</pre><p id="rfc.section.5.3.2.p.8">is interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in
    1871          quality".
    1872       </p>
    1873       <p id="rfc.section.5.3.2.p.9">A request without any Accept header field implies that the user agent will accept any media type in response. If the header
    1874          field is present in a request and none of the available representations for the response have a media type that is listed
    1875          as acceptable, the origin server can either honor the header field by sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response or disregard the header field by treating the response as if it is not subject to content negotiation.
    1876       </p>
    1877       <p id="rfc.section.5.3.2.p.10">A more elaborate example is</p>
    1878       <div id="rfc.figure.u.26"></div><pre class="text">  Accept: text/plain; q=0.5, text/html,
     1969                  quality".
     1970               </p>
     1971               <p id="rfc.section.5.3.2.p.9">A request without any Accept header field implies that the user agent will accept any media type in response. If the header
     1972                  field is present in a request and none of the available representations for the response have a media type that is listed
     1973                  as acceptable, the origin server can either honor the header field by sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response or disregard the header field by treating the response as if it is not subject to content negotiation.
     1974               </p>
     1975               <p id="rfc.section.5.3.2.p.10">A more elaborate example is</p>
     1976               <div id="rfc.figure.u.26"></div><pre class="text">  Accept: text/plain; q=0.5, text/html,
    18791977          text/x-dvi; q=0.8, text/x-c
    18801978</pre><p id="rfc.section.5.3.2.p.12">Verbally, this would be interpreted as "text/html and text/x-c are the equally preferred media types, but if they do not exist,
    1881          then send the text/x-dvi representation, and if that does not exist, send the text/plain representation".
    1882       </p>
    1883       <p id="rfc.section.5.3.2.p.13">Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies
    1884          to a given type, the most specific reference has precedence. For example,
    1885       </p>
    1886       <div id="rfc.figure.u.27"></div><pre class="text">  Accept: text/*, text/plain, text/plain;format=flowed, */*
     1979                  then send the text/x-dvi representation, and if that does not exist, send the text/plain representation".
     1980               </p>
     1981               <p id="rfc.section.5.3.2.p.13">Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies
     1982                  to a given type, the most specific reference has precedence. For example,
     1983               </p>
     1984               <div id="rfc.figure.u.27"></div><pre class="text">  Accept: text/*, text/plain, text/plain;format=flowed, */*
    18871985</pre><p id="rfc.section.5.3.2.p.15">have the following precedence: </p>
    1888       <ol>
    1889          <li>text/plain;format=flowed</li>
    1890          <li>text/plain</li>
    1891          <li>text/*</li>
    1892          <li>*/*</li>
    1893       </ol>
    1894       <p id="rfc.section.5.3.2.p.16">The media type quality factor associated with a given type is determined by finding the media range with the highest precedence
    1895          that matches the type. For example,
    1896       </p>
    1897       <div id="rfc.figure.u.28"></div><pre class="text">  Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
     1986               <ol>
     1987                  <li>text/plain;format=flowed</li>
     1988                  <li>text/plain</li>
     1989                  <li>text/*</li>
     1990                  <li>*/*</li>
     1991               </ol>
     1992               <p id="rfc.section.5.3.2.p.16">The media type quality factor associated with a given type is determined by finding the media range with the highest precedence
     1993                  that matches the type. For example,
     1994               </p>
     1995               <div id="rfc.figure.u.28"></div><pre class="text">  Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
    18981996          text/html;level=2;q=0.4, */*;q=0.5
    18991997</pre><p id="rfc.section.5.3.2.p.18">would cause the following values to be associated:</p>
    1900       <div id="rfc.table.u.6">
    1901          <table class="tt full left" cellpadding="3" cellspacing="0">
    1902             <thead>
    1903                <tr>
    1904                   <th>Media Type</th>
    1905                   <th>Quality Value</th>
    1906                </tr>
    1907             </thead>
    1908             <tbody>
    1909                <tr>
    1910                   <td class="left">text/html;level=1</td>
    1911                   <td class="left">1</td>
    1912                </tr>
    1913                <tr>
    1914                   <td class="left">text/html</td>
    1915                   <td class="left">0.7</td>
    1916                </tr>
    1917                <tr>
    1918                   <td class="left">text/plain</td>
    1919                   <td class="left">0.3</td>
    1920                </tr>
    1921                <tr>
    1922                   <td class="left">image/jpeg</td>
    1923                   <td class="left">0.5</td>
    1924                </tr>
    1925                <tr>
    1926                   <td class="left">text/html;level=2</td>
    1927                   <td class="left">0.4</td>
    1928                </tr>
    1929                <tr>
    1930                   <td class="left">text/html;level=3</td>
    1931                   <td class="left">0.7</td>
    1932                </tr>
    1933             </tbody>
    1934          </table>
    1935       </div>
    1936       <p id="rfc.section.5.3.2.p.19"><b>Note:</b> A user agent might be provided with a default set of quality values for certain media ranges. However, unless the user agent
    1937          is a closed system that cannot interact with other rendering agents, this default set ought to be configurable by the user.
    1938       </p>
    1939       <div id="rfc.iref.a.2"></div>
    1940       <h3 id="rfc.section.5.3.3"><a href="#rfc.section.5.3.3">5.3.3</a>&nbsp;<a id="header.accept-charset" href="#header.accept-charset">Accept-Charset</a></h3>
    1941       <p id="rfc.section.5.3.3.p.1">The "Accept-Charset" header field can be sent by a user agent to indicate what charsets are acceptable in textual response
    1942          content. This field allows user agents capable of understanding more comprehensive or special-purpose charsets to signal that
    1943          capability to an origin server that is capable of representing information in those charsets.
    1944       </p>
    1945       <div id="rfc.figure.u.29"></div><pre class="inline"><span id="rfc.iref.g.25"></span>  <a href="#header.accept-charset" class="smpl">Accept-Charset</a> = 1#( ( <a href="#charset" class="smpl">charset</a> / "*" ) [ <a href="#quality.values" class="smpl">weight</a> ] )
     1998               <div id="rfc.table.u.6">
     1999                  <table class="tt full left" cellpadding="3" cellspacing="0">
     2000                     <thead>
     2001                        <tr>
     2002                           <th>Media Type</th>
     2003                           <th>Quality Value</th>
     2004                        </tr>
     2005                     </thead>
     2006                     <tbody>
     2007                        <tr>
     2008                           <td class="left">text/html;level=1</td>
     2009                           <td class="left">1</td>
     2010                        </tr>
     2011                        <tr>
     2012                           <td class="left">text/html</td>
     2013                           <td class="left">0.7</td>
     2014                        </tr>
     2015                        <tr>
     2016                           <td class="left">text/plain</td>
     2017                           <td class="left">0.3</td>
     2018                        </tr>
     2019                        <tr>
     2020                           <td class="left">image/jpeg</td>
     2021                           <td class="left">0.5</td>
     2022                        </tr>
     2023                        <tr>
     2024                           <td class="left">text/html;level=2</td>
     2025                           <td class="left">0.4</td>
     2026                        </tr>
     2027                        <tr>
     2028                           <td class="left">text/html;level=3</td>
     2029                           <td class="left">0.7</td>
     2030                        </tr>
     2031                     </tbody>
     2032                  </table>
     2033               </div>
     2034               <p id="rfc.section.5.3.2.p.19"><b>Note:</b> A user agent might be provided with a default set of quality values for certain media ranges. However, unless the user agent
     2035                  is a closed system that cannot interact with other rendering agents, this default set ought to be configurable by the user.
     2036               </p>
     2037            </div>
     2038            <div id="header.accept-charset">
     2039               <div id="rfc.iref.a.2"></div>
     2040               <h3 id="rfc.section.5.3.3"><a href="#rfc.section.5.3.3">5.3.3</a>&nbsp;<a href="#header.accept-charset">Accept-Charset</a></h3>
     2041               <p id="rfc.section.5.3.3.p.1">The "Accept-Charset" header field can be sent by a user agent to indicate what charsets are acceptable in textual response
     2042                  content. This field allows user agents capable of understanding more comprehensive or special-purpose charsets to signal that
     2043                  capability to an origin server that is capable of representing information in those charsets.
     2044               </p>
     2045               <div id="rfc.figure.u.29"></div><pre class="inline"><span id="rfc.iref.g.25"></span>  <a href="#header.accept-charset" class="smpl">Accept-Charset</a> = 1#( ( <a href="#charset" class="smpl">charset</a> / "*" ) [ <a href="#quality.values" class="smpl">weight</a> ] )
    19462046</pre><p id="rfc.section.5.3.3.p.3">Charset names are defined in <a href="#charset" title="Charset">Section&nbsp;3.1.1.2</a>. A user agent <em class="bcp14">MAY</em> associate a quality value with each charset to indicate the user's relative preference for that charset, as defined in <a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>. An example is
    1947       </p>
    1948       <div id="rfc.figure.u.30"></div><pre class="text">  Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
     2047               </p>
     2048               <div id="rfc.figure.u.30"></div><pre class="text">  Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
    19492049</pre><p id="rfc.section.5.3.3.p.5">The special value "*", if present in the Accept-Charset field, matches every charset that is not mentioned elsewhere in the
    1950          Accept-Charset field. If no "*" is present in an Accept-Charset field, then any charsets not explicitly mentioned in the field
    1951          are considered "not acceptable" to the client.
    1952       </p>
    1953       <p id="rfc.section.5.3.3.p.6">A request without any Accept-Charset header field implies that the user agent will accept any charset in response. Most general-purpose
    1954          user agents do not send Accept-Charset, unless specifically configured to do so, because a detailed list of supported charsets
    1955          makes it easier for a server to identify an individual by virtue of the user agent's request characteristics (<a href="#fingerprinting" title="Browser Fingerprinting">Section&nbsp;9.6</a>).
    1956       </p>
    1957       <p id="rfc.section.5.3.3.p.7">If an Accept-Charset header field is present in a request and none of the available representations for the response has a
    1958          charset that is listed as acceptable, the origin server can either honor the header field, by sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response, or disregard the header field by treating the resource as if it is not subject to content negotiation.
    1959       </p>
    1960       <div id="rfc.iref.a.3"></div>
    1961       <h3 id="rfc.section.5.3.4"><a href="#rfc.section.5.3.4">5.3.4</a>&nbsp;<a id="header.accept-encoding" href="#header.accept-encoding">Accept-Encoding</a></h3>
    1962       <p id="rfc.section.5.3.4.p.1">The "Accept-Encoding" header field can be used by user agents to indicate what response content-codings (<a href="#content.codings" title="Content Codings">Section&nbsp;3.1.2.1</a>) are acceptable in the response. An "identity" token is used as a synonym for "no encoding" in order to communicate when
    1963          no encoding is preferred.
    1964       </p>
    1965       <div id="rfc.figure.u.31"></div><pre class="inline"><span id="rfc.iref.g.26"></span><span id="rfc.iref.g.27"></span>  <a href="#header.accept-encoding" class="smpl">Accept-Encoding</a>  = #( <a href="#header.accept-encoding" class="smpl">codings</a> [ <a href="#quality.values" class="smpl">weight</a> ] )
     2050                  Accept-Charset field. If no "*" is present in an Accept-Charset field, then any charsets not explicitly mentioned in the field
     2051                  are considered "not acceptable" to the client.
     2052               </p>
     2053               <p id="rfc.section.5.3.3.p.6">A request without any Accept-Charset header field implies that the user agent will accept any charset in response. Most general-purpose
     2054                  user agents do not send Accept-Charset, unless specifically configured to do so, because a detailed list of supported charsets
     2055                  makes it easier for a server to identify an individual by virtue of the user agent's request characteristics (<a href="#fingerprinting" title="Browser Fingerprinting">Section&nbsp;9.6</a>).
     2056               </p>
     2057               <p id="rfc.section.5.3.3.p.7">If an Accept-Charset header field is present in a request and none of the available representations for the response has a
     2058                  charset that is listed as acceptable, the origin server can either honor the header field, by sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response, or disregard the header field by treating the resource as if it is not subject to content negotiation.
     2059               </p>
     2060            </div>
     2061            <div id="header.accept-encoding">
     2062               <div id="rfc.iref.a.3"></div>
     2063               <h3 id="rfc.section.5.3.4"><a href="#rfc.section.5.3.4">5.3.4</a>&nbsp;<a href="#header.accept-encoding">Accept-Encoding</a></h3>
     2064               <p id="rfc.section.5.3.4.p.1">The "Accept-Encoding" header field can be used by user agents to indicate what response content-codings (<a href="#content.codings" title="Content Codings">Section&nbsp;3.1.2.1</a>) are acceptable in the response. An "identity" token is used as a synonym for "no encoding" in order to communicate when
     2065                  no encoding is preferred.
     2066               </p>
     2067               <div id="rfc.figure.u.31"></div><pre class="inline"><span id="rfc.iref.g.26"></span><span id="rfc.iref.g.27"></span>  <a href="#header.accept-encoding" class="smpl">Accept-Encoding</a>  = #( <a href="#header.accept-encoding" class="smpl">codings</a> [ <a href="#quality.values" class="smpl">weight</a> ] )
    19662068  <a href="#header.accept-encoding" class="smpl">codings</a>          = <a href="#content.codings" class="smpl">content-coding</a> / "identity" / "*"
    19672069</pre><p id="rfc.section.5.3.4.p.3">Each codings value <em class="bcp14">MAY</em> be given an associated quality value representing the preference for that encoding, as defined in <a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>. The asterisk "*" symbol in an Accept-Encoding field matches any available content-coding not explicitly listed in the header
    1968          field.
    1969       </p>
    1970       <p id="rfc.section.5.3.4.p.4">For example,</p>
    1971       <div id="rfc.figure.u.32"></div><pre class="text">  Accept-Encoding: compress, gzip
     2070                  field.
     2071               </p>
     2072               <p id="rfc.section.5.3.4.p.4">For example,</p>
     2073               <div id="rfc.figure.u.32"></div><pre class="text">  Accept-Encoding: compress, gzip
    19722074  Accept-Encoding:
    19732075  Accept-Encoding: *
     
    19752077  Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
    19762078</pre><p id="rfc.section.5.3.4.p.6">A request without an Accept-Encoding header field implies that the user agent has no preferences regarding content-codings.
    1977          Although this allows the server to use any content-coding in a response, it does not imply that the user agent will be able
    1978          to correctly process all encodings.
    1979       </p>
    1980       <p id="rfc.section.5.3.4.p.7">A server tests whether a content-coding for a given representation is acceptable using these rules: </p>
    1981       <ol>
    1982          <li>If no Accept-Encoding field is in the request, any content-coding is considered acceptable by the user agent.</li>
    1983          <li>If the representation has no content-coding, then it is acceptable by default unless specifically excluded by the Accept-Encoding
    1984             field stating either "identity;q=0" or "*;q=0" without a more specific entry for "identity".
    1985          </li>
    1986          <li>If the representation's content-coding is one of the content-codings listed in the Accept-Encoding field, then it is acceptable
    1987             unless it is accompanied by a qvalue of 0. (As defined in <a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>, a qvalue of 0 means "not acceptable".)
    1988          </li>
    1989          <li>If multiple content-codings are acceptable, then the acceptable content-coding with the highest non-zero qvalue is preferred.</li>
    1990       </ol>
    1991       <p id="rfc.section.5.3.4.p.8">An Accept-Encoding header field with a combined field-value that is empty implies that the user agent does not want any content-coding
    1992          in response. If an Accept-Encoding header field is present in a request and none of the available representations for the
    1993          response have a content-coding that is listed as acceptable, the origin server <em class="bcp14">SHOULD</em> send a response without any content-coding.
    1994       </p>
    1995       <div class="note" id="rfc.section.5.3.4.p.9">
    1996          <p><b>Note:</b> Most HTTP/1.0 applications do not recognize or obey qvalues associated with content-codings. This means that qvalues might
    1997             not work and are not permitted with x-gzip or x-compress.
    1998          </p>
    1999       </div>
    2000       <div id="rfc.iref.a.4"></div>
    2001       <h3 id="rfc.section.5.3.5"><a href="#rfc.section.5.3.5">5.3.5</a>&nbsp;<a id="header.accept-language" href="#header.accept-language">Accept-Language</a></h3>
    2002       <p id="rfc.section.5.3.5.p.1">The "Accept-Language" header field can be used by user agents to indicate the set of natural languages that are preferred
    2003          in the response. Language tags are defined in <a href="#language.tags" title="Language Tags">Section&nbsp;3.1.3.1</a>.
    2004       </p>
    2005       <div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.28"></span><span id="rfc.iref.g.29"></span>  <a href="#header.accept-language" class="smpl">Accept-Language</a> = 1#( <a href="#header.accept-language" class="smpl">language-range</a> [ <a href="#quality.values" class="smpl">weight</a> ] )
     2079                  Although this allows the server to use any content-coding in a response, it does not imply that the user agent will be able
     2080                  to correctly process all encodings.
     2081               </p>
     2082               <p id="rfc.section.5.3.4.p.7">A server tests whether a content-coding for a given representation is acceptable using these rules: </p>
     2083               <ol>
     2084                  <li>If no Accept-Encoding field is in the request, any content-coding is considered acceptable by the user agent.</li>
     2085                  <li>If the representation has no content-coding, then it is acceptable by default unless specifically excluded by the Accept-Encoding
     2086                     field stating either "identity;q=0" or "*;q=0" without a more specific entry for "identity".
     2087                  </li>
     2088                  <li>If the representation's content-coding is one of the content-codings listed in the Accept-Encoding field, then it is acceptable
     2089                     unless it is accompanied by a qvalue of 0. (As defined in <a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>, a qvalue of 0 means "not acceptable".)
     2090                  </li>
     2091                  <li>If multiple content-codings are acceptable, then the acceptable content-coding with the highest non-zero qvalue is preferred.</li>
     2092               </ol>
     2093               <p id="rfc.section.5.3.4.p.8">An Accept-Encoding header field with a combined field-value that is empty implies that the user agent does not want any content-coding
     2094                  in response. If an Accept-Encoding header field is present in a request and none of the available representations for the
     2095                  response have a content-coding that is listed as acceptable, the origin server <em class="bcp14">SHOULD</em> send a response without any content-coding.
     2096               </p>
     2097               <div class="note" id="rfc.section.5.3.4.p.9">
     2098                  <p><b>Note:</b> Most HTTP/1.0 applications do not recognize or obey qvalues associated with content-codings. This means that qvalues might
     2099                     not work and are not permitted with x-gzip or x-compress.
     2100                  </p>
     2101               </div>
     2102            </div>
     2103            <div id="header.accept-language">
     2104               <div id="rfc.iref.a.4"></div>
     2105               <h3 id="rfc.section.5.3.5"><a href="#rfc.section.5.3.5">5.3.5</a>&nbsp;<a href="#header.accept-language">Accept-Language</a></h3>
     2106               <p id="rfc.section.5.3.5.p.1">The "Accept-Language" header field can be used by user agents to indicate the set of natural languages that are preferred
     2107                  in the response. Language tags are defined in <a href="#language.tags" title="Language Tags">Section&nbsp;3.1.3.1</a>.
     2108               </p>
     2109               <div id="rfc.figure.u.33"></div><pre class="inline"><span id="rfc.iref.g.28"></span><span id="rfc.iref.g.29"></span>  <a href="#header.accept-language" class="smpl">Accept-Language</a> = 1#( <a href="#header.accept-language" class="smpl">language-range</a> [ <a href="#quality.values" class="smpl">weight</a> ] )
    20062110  <a href="#header.accept-language" class="smpl">language-range</a>  =
    20072111            &lt;language-range, defined in <a href="#RFC4647" id="rfc.xref.RFC4647.1"><cite title="Matching of Language Tags">[RFC4647]</cite></a>, <a href="http://tools.ietf.org/html/rfc4647#section-2.1">Section 2.1</a>&gt;
    20082112</pre><p id="rfc.section.5.3.5.p.3">Each language-range can be given an associated quality value representing an estimate of the user's preference for the languages
    2009          specified by that range, as defined in <a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>. For example,
    2010       </p>
    2011       <div id="rfc.figure.u.34"></div><pre class="text">  Accept-Language: da, en-gb;q=0.8, en;q=0.7
     2113                  specified by that range, as defined in <a href="#quality.values" title="Quality Values">Section&nbsp;5.3.1</a>. For example,
     2114               </p>
     2115               <div id="rfc.figure.u.34"></div><pre class="text">  Accept-Language: da, en-gb;q=0.8, en;q=0.7
    20122116</pre><p id="rfc.section.5.3.5.p.5">would mean: "I prefer Danish, but will accept British English and other types of English".</p>
    2013       <p id="rfc.section.5.3.5.p.6">A request without any Accept-Language header field implies that the user agent will accept any language in response. If the
    2014          header field is present in a request and none of the available representations for the response have a matching language tag,
    2015          the origin server can either disregard the header field by treating the response as if it is not subject to content negotiation,
    2016          or honor the header field by sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response. However, the latter is not encouraged, as doing so can prevent users from accessing content that they might be able
    2017          to use (with translation software, for example).
    2018       </p>
    2019       <p id="rfc.section.5.3.5.p.7">Note that some recipients treat the order in which language tags are listed as an indication of descending priority, particularly
    2020          for tags that are assigned equal quality values (no value is the same as q=1). However, this behavior cannot be relied upon.
    2021          For consistency and to maximize interoperability, many user agents assign each language tag a unique quality value while also
    2022          listing them in order of decreasing quality. Additional discussion of language priority lists can be found in <a href="http://tools.ietf.org/html/rfc4647#section-2.3">Section 2.3</a> of <a href="#RFC4647" id="rfc.xref.RFC4647.2"><cite title="Matching of Language Tags">[RFC4647]</cite></a>.
    2023       </p>
    2024       <p id="rfc.section.5.3.5.p.8">For matching, <a href="http://tools.ietf.org/html/rfc4647#section-3">Section 3</a> of <a href="#RFC4647" id="rfc.xref.RFC4647.3"><cite title="Matching of Language Tags">[RFC4647]</cite></a> defines several matching schemes. Implementations can offer the most appropriate matching scheme for their requirements. The
    2025          "Basic Filtering" scheme (<a href="#RFC4647" id="rfc.xref.RFC4647.4"><cite title="Matching of Language Tags">[RFC4647]</cite></a>, <a href="http://tools.ietf.org/html/rfc4647#section-3.3.1">Section 3.3.1</a>) is identical to the matching scheme that was previously defined for HTTP in <a href="http://tools.ietf.org/html/rfc2616#section-14.4">Section 14.4</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
    2026       </p>
    2027       <p id="rfc.section.5.3.5.p.9">It might be contrary to the privacy expectations of the user to send an Accept-Language header field with the complete linguistic
    2028          preferences of the user in every request (<a href="#fingerprinting" title="Browser Fingerprinting">Section&nbsp;9.6</a>).
    2029       </p>
    2030       <p id="rfc.section.5.3.5.p.10">Since intelligibility is highly dependent on the individual user, user agents need to allow user control over the linguistic
    2031          preference. A user agent that does not provide such control to the user <em class="bcp14">MUST NOT</em> send an Accept-Language header field.
    2032       </p>
    2033       <div class="note" id="rfc.section.5.3.5.p.11">
    2034          <p><b>Note:</b> User agents ought to provide guidance to users when setting a preference, since users are rarely familiar with the details
    2035             of language matching as described above. For example, users might assume that on selecting "en-gb", they will be served any
    2036             kind of English document if British English is not available. A user agent might suggest, in such a case, to add "en" to the
    2037             list for better matching behavior.
    2038          </p>
    2039       </div>
    2040       <h2 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;<a id="request.auth" href="#request.auth">Authentication Credentials</a></h2>
    2041       <p id="rfc.section.5.4.p.1">Two header fields are used for carrying authentication credentials, as defined in <a href="#Part7" id="rfc.xref.Part7.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a>. Note that various custom mechanisms for user authentication use the Cookie header field for this purpose, as defined in <a href="#RFC6265" id="rfc.xref.RFC6265.2"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>.
    2042       </p>
    2043       <div id="rfc.table.u.7">
    2044          <table class="tt full left" cellpadding="3" cellspacing="0">
    2045             <thead>
    2046                <tr>
    2047                   <th>Header Field Name</th>
    2048                   <th>Defined in...</th>
    2049                </tr>
    2050             </thead>
    2051             <tbody>
    2052                <tr>
    2053                   <td class="left">Authorization</td>
    2054                   <td class="left"><a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</a> of <a href="#Part7" id="rfc.xref.Part7.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
    2055                </tr>
    2056                <tr>
    2057                   <td class="left">Proxy-Authorization</td>
    2058                   <td class="left"><a href="p7-auth.html#header.proxy-authorization" title="Proxy-Authorization">Section 4.3</a> of <a href="#Part7" id="rfc.xref.Part7.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
    2059                </tr>
    2060             </tbody>
    2061          </table>
    2062       </div>
    2063       <h2 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;<a id="request.context" href="#request.context">Request Context</a></h2>
    2064       <p id="rfc.section.5.5.p.1">The following request header fields provide additional information about the request context, including information about
    2065          the user, user agent, and resource behind the request.
    2066       </p>
    2067       <div id="rfc.table.u.8">
    2068          <table class="tt full left" cellpadding="3" cellspacing="0">
    2069             <thead>
    2070                <tr>
    2071                   <th>Header Field Name</th>
    2072                   <th>Defined in...</th>
    2073                </tr>
    2074             </thead>
    2075             <tbody>
    2076                <tr>
    2077                   <td class="left">From</td>
    2078                   <td class="left"><a href="#header.from" id="rfc.xref.header.from.1" title="From">Section&nbsp;5.5.1</a></td>
    2079                </tr>
    2080                <tr>
    2081                   <td class="left">Referer</td>
    2082                   <td class="left"><a href="#header.referer" id="rfc.xref.header.referer.1" title="Referer">Section&nbsp;5.5.2</a></td>
    2083                </tr>
    2084                <tr>
    2085                   <td class="left">User-Agent</td>
    2086                   <td class="left"><a href="#header.user-agent" id="rfc.xref.header.user-agent.1" title="User-Agent">Section&nbsp;5.5.3</a></td>
    2087                </tr>
    2088             </tbody>
    2089          </table>
    2090       </div>
    2091       <div id="rfc.iref.f.1"></div>
    2092       <h3 id="rfc.section.5.5.1"><a href="#rfc.section.5.5.1">5.5.1</a>&nbsp;<a id="header.from" href="#header.from">From</a></h3>
    2093       <p id="rfc.section.5.5.1.p.1">The "From" header field contains an Internet email address for a human user who controls the requesting user agent. The address
    2094          ought to be machine-usable, as defined by "mailbox" in <a href="http://tools.ietf.org/html/rfc5322#section-3.4">Section 3.4</a> of <a href="#RFC5322" id="rfc.xref.RFC5322.1"><cite title="Internet Message Format">[RFC5322]</cite></a>:
    2095       </p>
    2096       <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.30"></span>  <a href="#header.from" class="smpl">From</a>    = <a href="#header.from" class="smpl">mailbox</a>
     2117               <p id="rfc.section.5.3.5.p.6">A request without any Accept-Language header field implies that the user agent will accept any language in response. If the
     2118                  header field is present in a request and none of the available representations for the response have a matching language tag,
     2119                  the origin server can either disregard the header field by treating the response as if it is not subject to content negotiation,
     2120                  or honor the header field by sending a <a href="#status.406" class="smpl">406 (Not Acceptable)</a> response. However, the latter is not encouraged, as doing so can prevent users from accessing content that they might be able
     2121                  to use (with translation software, for example).
     2122               </p>
     2123               <p id="rfc.section.5.3.5.p.7">Note that some recipients treat the order in which language tags are listed as an indication of descending priority, particularly
     2124                  for tags that are assigned equal quality values (no value is the same as q=1). However, this behavior cannot be relied upon.
     2125                  For consistency and to maximize interoperability, many user agents assign each language tag a unique quality value while also
     2126                  listing them in order of decreasing quality. Additional discussion of language priority lists can be found in <a href="http://tools.ietf.org/html/rfc4647#section-2.3">Section 2.3</a> of <a href="#RFC4647" id="rfc.xref.RFC4647.2"><cite title="Matching of Language Tags">[RFC4647]</cite></a>.
     2127               </p>
     2128               <p id="rfc.section.5.3.5.p.8">For matching, <a href="http://tools.ietf.org/html/rfc4647#section-3">Section 3</a> of <a href="#RFC4647" id="rfc.xref.RFC4647.3"><cite title="Matching of Language Tags">[RFC4647]</cite></a> defines several matching schemes. Implementations can offer the most appropriate matching scheme for their requirements. The
     2129                  "Basic Filtering" scheme (<a href="#RFC4647" id="rfc.xref.RFC4647.4"><cite title="Matching of Language Tags">[RFC4647]</cite></a>, <a href="http://tools.ietf.org/html/rfc4647#section-3.3.1">Section 3.3.1</a>) is identical to the matching scheme that was previously defined for HTTP in <a href="http://tools.ietf.org/html/rfc2616#section-14.4">Section 14.4</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
     2130               </p>
     2131               <p id="rfc.section.5.3.5.p.9">It might be contrary to the privacy expectations of the user to send an Accept-Language header field with the complete linguistic
     2132                  preferences of the user in every request (<a href="#fingerprinting" title="Browser Fingerprinting">Section&nbsp;9.6</a>).
     2133               </p>
     2134               <p id="rfc.section.5.3.5.p.10">Since intelligibility is highly dependent on the individual user, user agents need to allow user control over the linguistic
     2135                  preference. A user agent that does not provide such control to the user <em class="bcp14">MUST NOT</em> send an Accept-Language header field.
     2136               </p>
     2137               <div class="note" id="rfc.section.5.3.5.p.11">
     2138                  <p><b>Note:</b> User agents ought to provide guidance to users when setting a preference, since users are rarely familiar with the details
     2139                     of language matching as described above. For example, users might assume that on selecting "en-gb", they will be served any
     2140                     kind of English document if British English is not available. A user agent might suggest, in such a case, to add "en" to the
     2141                     list for better matching behavior.
     2142                  </p>
     2143               </div>
     2144            </div>
     2145         </div>
     2146         <div id="request.auth">
     2147            <h2 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;<a href="#request.auth">Authentication Credentials</a></h2>
     2148            <p id="rfc.section.5.4.p.1">Two header fields are used for carrying authentication credentials, as defined in <a href="#Part7" id="rfc.xref.Part7.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a>. Note that various custom mechanisms for user authentication use the Cookie header field for this purpose, as defined in <a href="#RFC6265" id="rfc.xref.RFC6265.2"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a>.
     2149            </p>
     2150            <div id="rfc.table.u.7">
     2151               <table class="tt full left" cellpadding="3" cellspacing="0">
     2152                  <thead>
     2153                     <tr>
     2154                        <th>Header Field Name</th>
     2155                        <th>Defined in...</th>
     2156                     </tr>
     2157                  </thead>
     2158                  <tbody>
     2159                     <tr>
     2160                        <td class="left">Authorization</td>
     2161                        <td class="left"><a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</a> of <a href="#Part7" id="rfc.xref.Part7.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
     2162                     </tr>
     2163                     <tr>
     2164                        <td class="left">Proxy-Authorization</td>
     2165                        <td class="left"><a href="p7-auth.html#header.proxy-authorization" title="Proxy-Authorization">Section 4.3</a> of <a href="#Part7" id="rfc.xref.Part7.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
     2166                     </tr>
     2167                  </tbody>
     2168               </table>
     2169            </div>
     2170         </div>
     2171         <div id="request.context">
     2172            <h2 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;<a href="#request.context">Request Context</a></h2>
     2173            <p id="rfc.section.5.5.p.1">The following request header fields provide additional information about the request context, including information about
     2174               the user, user agent, and resource behind the request.
     2175            </p>
     2176            <div id="rfc.table.u.8">
     2177               <table class="tt full left" cellpadding="3" cellspacing="0">
     2178                  <thead>
     2179                     <tr>
     2180                        <th>Header Field Name</th>
     2181                        <th>Defined in...</th>
     2182                     </tr>
     2183                  </thead>
     2184                  <tbody>
     2185                     <tr>
     2186                        <td class="left">From</td>
     2187                        <td class="left"><a href="#header.from" id="rfc.xref.header.from.1" title="From">Section&nbsp;5.5.1</a></td>
     2188                     </tr>
     2189                     <tr>
     2190                        <td class="left">Referer</td>
     2191                        <td class="left"><a href="#header.referer" id="rfc.xref.header.referer.1" title="Referer">Section&nbsp;5.5.2</a></td>
     2192                     </tr>
     2193                     <tr>
     2194                        <td class="left">User-Agent</td>
     2195                        <td class="left"><a href="#header.user-agent" id="rfc.xref.header.user-agent.1" title="User-Agent">Section&nbsp;5.5.3</a></td>
     2196                     </tr>
     2197                  </tbody>
     2198               </table>
     2199            </div>
     2200            <div id="header.from">
     2201               <div id="rfc.iref.f.1"></div>
     2202               <h3 id="rfc.section.5.5.1"><a href="#rfc.section.5.5.1">5.5.1</a>&nbsp;<a href="#header.from">From</a></h3>
     2203               <p id="rfc.section.5.5.1.p.1">The "From" header field contains an Internet email address for a human user who controls the requesting user agent. The address
     2204                  ought to be machine-usable, as defined by "mailbox" in <a href="http://tools.ietf.org/html/rfc5322#section-3.4">Section 3.4</a> of <a href="#RFC5322" id="rfc.xref.RFC5322.1"><cite title="Internet Message Format">[RFC5322]</cite></a>:
     2205               </p>
     2206               <div id="rfc.figure.u.35"></div><pre class="inline"><span id="rfc.iref.g.30"></span>  <a href="#header.from" class="smpl">From</a>    = <a href="#header.from" class="smpl">mailbox</a>
    20972207 
    20982208  <a href="#header.from" class="smpl">mailbox</a> = &lt;mailbox, defined in <a href="#RFC5322" id="rfc.xref.RFC5322.2"><cite title="Internet Message Format">[RFC5322]</cite></a>, <a href="http://tools.ietf.org/html/rfc5322#section-3.4">Section 3.4</a>&gt;
    20992209</pre><p id="rfc.section.5.5.1.p.3">An example is:</p>
    2100       <div id="rfc.figure.u.36"></div><pre class="text">  From: webmaster@example.org
     2210               <div id="rfc.figure.u.36"></div><pre class="text">  From: webmaster@example.org
    21012211</pre><p id="rfc.section.5.5.1.p.5">The From header field is rarely sent by non-robotic user agents. A user agent <em class="bcp14">SHOULD NOT</em> send a From header field without explicit configuration by the user, since that might conflict with the user's privacy interests
    2102          or their site's security policy.
    2103       </p>
    2104       <p id="rfc.section.5.5.1.p.6">A robotic user agent <em class="bcp14">SHOULD</em> send a valid From header field so that the person responsible for running the robot can be contacted if problems occur on
    2105          servers, such as if the robot is sending excessive, unwanted, or invalid requests.
    2106       </p>
    2107       <p id="rfc.section.5.5.1.p.7">A server <em class="bcp14">SHOULD NOT</em> use the From header field for access control or authentication, since most recipients will assume that the field value is
    2108          public information.
    2109       </p>
    2110       <div id="rfc.iref.r.2"></div>
    2111       <h3 id="rfc.section.5.5.2"><a href="#rfc.section.5.5.2">5.5.2</a>&nbsp;<a id="header.referer" href="#header.referer">Referer</a></h3>
    2112       <p id="rfc.section.5.5.2.p.1">The "Referer" [sic] header field allows the user agent to specify a URI reference for the resource from which the <a href="p1-messaging.html#target-resource" class="smpl">target URI</a> was obtained (i.e., the "referrer", though the field name is misspelled). A user agent <em class="bcp14">MUST NOT</em> include the fragment and userinfo components of the URI reference <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, if any, when generating the Referer field value.
    2113       </p>
    2114       <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.31"></span>  <a href="#header.referer" class="smpl">Referer</a> = <a href="#imported.abnf" class="smpl">absolute-URI</a> / <a href="#imported.abnf" class="smpl">partial-URI</a>
     2212                  or their site's security policy.
     2213               </p>
     2214               <p id="rfc.section.5.5.1.p.6">A robotic user agent <em class="bcp14">SHOULD</em> send a valid From header field so that the person responsible for running the robot can be contacted if problems occur on
     2215                  servers, such as if the robot is sending excessive, unwanted, or invalid requests.
     2216               </p>
     2217               <p id="rfc.section.5.5.1.p.7">A server <em class="bcp14">SHOULD NOT</em> use the From header field for access control or authentication, since most recipients will assume that the field value is
     2218                  public information.
     2219               </p>
     2220            </div>
     2221            <div id="header.referer">
     2222               <div id="rfc.iref.r.2"></div>
     2223               <h3 id="rfc.section.5.5.2"><a href="#rfc.section.5.5.2">5.5.2</a>&nbsp;<a href="#header.referer">Referer</a></h3>
     2224               <p id="rfc.section.5.5.2.p.1">The "Referer" [sic] header field allows the user agent to specify a URI reference for the resource from which the <a href="p1-messaging.html#target-resource" class="smpl">target URI</a> was obtained (i.e., the "referrer", though the field name is misspelled). A user agent <em class="bcp14">MUST NOT</em> include the fragment and userinfo components of the URI reference <a href="#RFC3986" id="rfc.xref.RFC3986.1"><cite title="Uniform Resource Identifier (URI): Generic Syntax">[RFC3986]</cite></a>, if any, when generating the Referer field value.
     2225               </p>
     2226               <div id="rfc.figure.u.37"></div><pre class="inline"><span id="rfc.iref.g.31"></span>  <a href="#header.referer" class="smpl">Referer</a> = <a href="#imported.abnf" class="smpl">absolute-URI</a> / <a href="#imported.abnf" class="smpl">partial-URI</a>
    21152227</pre><p id="rfc.section.5.5.2.p.3">Referer allows servers to generate back-links to other resources for simple analytics, logging, optimized caching, etc. It
    2116          also allows obsolete or mistyped links to be found for maintenance. Some servers use Referer as a means of denying links from
    2117          other sites (so-called "deep linking") or restricting cross-site request forgery (CSRF), but not all requests contain a Referer
    2118          header field.
    2119       </p>
    2120       <p id="rfc.section.5.5.2.p.4">Example:</p>
    2121       <div id="rfc.figure.u.38"></div><pre class="text">  Referer: http://www.example.org/hypertext/Overview.html
     2228                  also allows obsolete or mistyped links to be found for maintenance. Some servers use Referer as a means of denying links from
     2229                  other sites (so-called "deep linking") or restricting cross-site request forgery (CSRF), but not all requests contain a Referer
     2230                  header field.
     2231               </p>
     2232               <p id="rfc.section.5.5.2.p.4">Example:</p>
     2233               <div id="rfc.figure.u.38"></div><pre class="text">  Referer: http://www.example.org/hypertext/Overview.html
    21222234</pre><p id="rfc.section.5.5.2.p.6">If the target URI was obtained from a source that does not have its own URI (e.g., input from the user keyboard, or an entry
    2123          within the user's bookmarks/favorites), the user agent <em class="bcp14">MUST</em> either exclude Referer or send it with a value of "about:blank".
    2124       </p>
    2125       <p id="rfc.section.5.5.2.p.7">The Referer field has the potential to reveal information about the request context or browsing history of the user, which
    2126          is a privacy concern if the referring resource's identifier reveals personal information (such as an account name) or a resource
    2127          that is supposed to be confidential (such as behind a firewall or internal to a secured service). Most general-purpose user
    2128          agents do not send the Referer header field when the referring resource is a local "file" or "data" URI. A user agent <em class="bcp14">MUST NOT</em> send a <a href="#header.referer" class="smpl">Referer</a> header field in an unsecured HTTP request if the referring page was received with a secure protocol. See <a href="#sensitive.information.in.uris" title="Sensitive Information in URIs">Section&nbsp;9.3</a> for additional security considerations.
    2129       </p>
    2130       <p id="rfc.section.5.5.2.p.8">Some intermediaries have been known to indiscriminately remove Referer header fields from outgoing requests. This has the
    2131          unfortunate side-effect of interfering with protection against CSRF attacks, which can be far more harmful to their users.
    2132          Intermediaries and user agent extensions that wish to limit information disclosure in Referer ought to restrict their changes
    2133          to specific edits, such as replacing internal domain names with pseudonyms or truncating the query and/or path components.
    2134          An intermediary <em class="bcp14">SHOULD NOT</em> modify or delete the Referer header field when the field value shares the same scheme and host as the request target.
    2135       </p>
    2136       <div id="rfc.iref.u.1"></div>
    2137       <h3 id="rfc.section.5.5.3"><a href="#rfc.section.5.5.3">5.5.3</a>&nbsp;<a id="header.user-agent" href="#header.user-agent">User-Agent</a></h3>
    2138       <p id="rfc.section.5.5.3.p.1">The "User-Agent" header field contains information about the user agent originating the request, which is often used by servers
    2139          to help identify the scope of reported interoperability problems, to work around or tailor responses to avoid particular user
    2140          agent limitations, and for analytics regarding browser or operating system use. A user agent <em class="bcp14">SHOULD</em> send a User-Agent field in each request unless specifically configured not to do so.
    2141       </p>
    2142       <div id="rfc.figure.u.39"></div><pre class="inline"><span id="rfc.iref.g.32"></span>  <a href="#header.user-agent" class="smpl">User-Agent</a> = <a href="#header.user-agent" class="smpl">product</a> *( <a href="#imported.abnf" class="smpl">RWS</a> ( <a href="#header.user-agent" class="smpl">product</a> / <a href="#imported.abnf" class="smpl">comment</a> ) )
     2235                  within the user's bookmarks/favorites), the user agent <em class="bcp14">MUST</em> either exclude Referer or send it with a value of "about:blank".
     2236               </p>
     2237               <p id="rfc.section.5.5.2.p.7">The Referer field has the potential to reveal information about the request context or browsing history of the user, which
     2238                  is a privacy concern if the referring resource's identifier reveals personal information (such as an account name) or a resource
     2239                  that is supposed to be confidential (such as behind a firewall or internal to a secured service). Most general-purpose user
     2240                  agents do not send the Referer header field when the referring resource is a local "file" or "data" URI. A user agent <em class="bcp14">MUST NOT</em> send a <a href="#header.referer" class="smpl">Referer</a> header field in an unsecured HTTP request if the referring page was received with a secure protocol. See <a href="#sensitive.information.in.uris" title="Sensitive Information in URIs">Section&nbsp;9.3</a> for additional security considerations.
     2241               </p>
     2242               <p id="rfc.section.5.5.2.p.8">Some intermediaries have been known to indiscriminately remove Referer header fields from outgoing requests. This has the
     2243                  unfortunate side-effect of interfering with protection against CSRF attacks, which can be far more harmful to their users.
     2244                  Intermediaries and user agent extensions that wish to limit information disclosure in Referer ought to restrict their changes
     2245                  to specific edits, such as replacing internal domain names with pseudonyms or truncating the query and/or path components.
     2246                  An intermediary <em class="bcp14">SHOULD NOT</em> modify or delete the Referer header field when the field value shares the same scheme and host as the request target.
     2247               </p>
     2248            </div>
     2249            <div id="header.user-agent">
     2250               <div id="rfc.iref.u.1"></div>
     2251               <h3 id="rfc.section.5.5.3"><a href="#rfc.section.5.5.3">5.5.3</a>&nbsp;<a href="#header.user-agent">User-Agent</a></h3>
     2252               <p id="rfc.section.5.5.3.p.1">The "User-Agent" header field contains information about the user agent originating the request, which is often used by servers
     2253                  to help identify the scope of reported interoperability problems, to work around or tailor responses to avoid particular user
     2254                  agent limitations, and for analytics regarding browser or operating system use. A user agent <em class="bcp14">SHOULD</em> send a User-Agent field in each request unless specifically configured not to do so.
     2255               </p>
     2256               <div id="rfc.figure.u.39"></div><pre class="inline"><span id="rfc.iref.g.32"></span>  <a href="#header.user-agent" class="smpl">User-Agent</a> = <a href="#header.user-agent" class="smpl">product</a> *( <a href="#imported.abnf" class="smpl">RWS</a> ( <a href="#header.user-agent" class="smpl">product</a> / <a href="#imported.abnf" class="smpl">comment</a> ) )
    21432257</pre><p id="rfc.section.5.5.3.p.3">The User-Agent field-value consists of one or more product identifiers, each followed by zero or more comments (<a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a> of <a href="#Part1" id="rfc.xref.Part1.22"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>), which together identify the user agent software and its significant subproducts. By convention, the product identifiers
    2144          are listed in decreasing order of their significance for identifying the user agent software. Each product identifier consists
    2145          of a name and optional version.
    2146       </p>
    2147       <div id="rfc.figure.u.40"></div><pre class="inline"><span id="rfc.iref.g.33"></span><span id="rfc.iref.g.34"></span>  <a href="#header.user-agent" class="smpl">product</a>         = <a href="#imported.abnf" class="smpl">token</a> ["/" <a href="#header.user-agent" class="smpl">product-version</a>]
     2258                  are listed in decreasing order of their significance for identifying the user agent software. Each product identifier consists
     2259                  of a name and optional version.
     2260               </p>
     2261               <div id="rfc.figure.u.40"></div><pre class="inline"><span id="rfc.iref.g.33"></span><span id="rfc.iref.g.34"></span>  <a href="#header.user-agent" class="smpl">product</a>         = <a href="#imported.abnf" class="smpl">token</a> ["/" <a href="#header.user-agent" class="smpl">product-version</a>]
    21482262  <a href="#header.user-agent" class="smpl">product-version</a> = <a href="#imported.abnf" class="smpl">token</a>
    21492263</pre><p id="rfc.section.5.5.3.p.5">A sender <em class="bcp14">SHOULD</em> limit generated product identifiers to what is necessary to identify the product; a sender <em class="bcp14">MUST NOT</em> generate advertising or other non-essential information within the product identifier. A sender <em class="bcp14">SHOULD NOT</em> generate information in <a href="#header.user-agent" class="smpl">product-version</a> that is not a version identifier (i.e., successive versions of the same product name ought to only differ in the product-version
    2150          portion of the product identifier).
    2151       </p>
    2152       <p id="rfc.section.5.5.3.p.6">Example:</p>
    2153       <div id="rfc.figure.u.41"></div><pre class="text">  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
     2264                  portion of the product identifier).
     2265               </p>
     2266               <p id="rfc.section.5.5.3.p.6">Example:</p>
     2267               <div id="rfc.figure.u.41"></div><pre class="text">  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
    21542268</pre><p id="rfc.section.5.5.3.p.8">A user agent <em class="bcp14">SHOULD NOT</em> generate a User-Agent field containing needlessly fine-grained detail and <em class="bcp14">SHOULD</em> limit the addition of subproducts by third parties. Overly long and detailed User-Agent field values increase request latency
    2155          and the risk of a user being identified against their wishes ("fingerprinting").
    2156       </p>
    2157       <p id="rfc.section.5.5.3.p.9">Likewise, implementations are encouraged not to use the product tokens of other implementations in order to declare compatibility
    2158          with them, as this circumvents the purpose of the field. If a user agent masquerades as a different user agent, recipients
    2159          can assume that the user intentionally desires to see responses tailored for that identified user agent, even if they might
    2160          not work as well for the actual user agent being used.
    2161       </p>
    2162       <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="status.codes" href="#status.codes">Response Status Codes</a></h1>
    2163       <p id="rfc.section.6.p.1">The status-code element is a 3-digit integer code giving the result of the attempt to understand and satisfy the request.</p>
    2164       <p id="rfc.section.6.p.2">HTTP status codes are extensible. HTTP clients are not required to understand the meaning of all registered status codes,
    2165          though such understanding is obviously desirable. However, a client <em class="bcp14">MUST</em> understand the class of any status code, as indicated by the first digit, and treat an unrecognized status code as being equivalent
    2166          to the x00 status code of that class, with the exception that a recipient <em class="bcp14">MUST NOT</em> cache a response with an unrecognized status code.
    2167       </p>
    2168       <p id="rfc.section.6.p.3">For example, if an unrecognized status code of 471 is received by a client, the client can assume that there was something
    2169          wrong with its request and treat the response as if it had received a 400 status code. The response message will usually contain
    2170          a representation that explains the status.
    2171       </p>
    2172       <p id="rfc.section.6.p.4">The first digit of the status-code defines the class of response. The last two digits do not have any categorization role.
    2173          There are 5 values for the first digit:
    2174       </p>
    2175       <ul>
    2176          <li><a href="#status.1xx" class="smpl">1xx (Informational)</a>: The request was received, continuing process
    2177          </li>
    2178          <li><a href="#status.2xx" class="smpl">2xx (Successful)</a>: The request was successfully received, understood, and accepted
    2179          </li>
    2180          <li><a href="#status.3xx" class="smpl">3xx (Redirection)</a>: Further action needs to be taken in order to complete the request
    2181          </li>
    2182          <li><a href="#status.4xx" class="smpl">4xx (Client Error)</a>: The request contains bad syntax or cannot be fulfilled
    2183          </li>
    2184          <li><a href="#status.5xx" class="smpl">5xx (Server Error)</a>: The server failed to fulfill an apparently valid request
    2185          </li>
    2186       </ul>
    2187       <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="overview.of.status.codes" href="#overview.of.status.codes">Overview of Status Codes</a></h2>
    2188       <p id="rfc.section.6.1.p.1">The status codes listed below are defined in this specification, <a href="p4-conditional.html#status.code.definitions" title="Status Code Definitions">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>, <a href="p5-range.html#range.response" title="Responses to a Range Request">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>, and <a href="p7-auth.html#status.code.definitions" title="Status Code Definitions">Section 3</a> of <a href="#Part7" id="rfc.xref.Part7.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a>. The reason phrases listed here are only recommendations — they can be replaced by local equivalents without affecting the
    2189          protocol.
    2190       </p>
    2191       <p id="rfc.section.6.1.p.2">Responses with status codes that are defined as cacheable by default (e.g., 200, 203, 206, 300, 301, and 410 in this specification)
    2192          can be reused by a cache with heuristic expiration unless otherwise indicated by the method definition or explicit cache controls <a href="#Part6" id="rfc.xref.Part6.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>; all other status codes are not cacheable by default.
    2193       </p>
    2194       <div id="rfc.table.u.9">
    2195          <table class="tt full left" cellpadding="3" cellspacing="0">
    2196             <thead>
    2197                <tr>
    2198                   <th>code</th>
    2199                   <th>reason-phrase</th>
    2200                   <th>Defined in...</th>
    2201                </tr>
    2202             </thead>
    2203             <tbody>
    2204                <tr>
    2205                   <td class="left">100</td>
    2206                   <td class="left">Continue</td>
    2207                   <td class="left"><a href="#status.100" id="rfc.xref.status.100.1" title="100 Continue">Section&nbsp;6.2.1</a></td>
    2208                </tr>
    2209                <tr>
    2210                   <td class="left">101</td>
    2211                   <td class="left">Switching Protocols</td>
    2212                   <td class="left"><a href="#status.101" id="rfc.xref.status.101.1" title="101 Switching Protocols">Section&nbsp;6.2.2</a></td>
    2213                </tr>
    2214                <tr>
    2215                   <td class="left">200</td>
    2216                   <td class="left">OK</td>
    2217                   <td class="left"><a href="#status.200" id="rfc.xref.status.200.1" title="200 OK">Section&nbsp;6.3.1</a></td>
    2218                </tr>
    2219                <tr>
    2220                   <td class="left">201</td>
    2221                   <td class="left">Created</td>
    2222                   <td class="left"><a href="#status.201" id="rfc.xref.status.201.1" title="201 Created">Section&nbsp;6.3.2</a></td>
    2223                </tr>
    2224                <tr>
    2225                   <td class="left">202</td>
    2226                   <td class="left">Accepted</td>
    2227                   <td class="left"><a href="#status.202" id="rfc.xref.status.202.1" title="202 Accepted">Section&nbsp;6.3.3</a></td>
    2228                </tr>
    2229                <tr>
    2230                   <td class="left">203</td>
    2231                   <td class="left">Non-Authoritative Information</td>
    2232                   <td class="left"><a href="#status.203" id="rfc.xref.status.203.1" title="203 Non-Authoritative Information">Section&nbsp;6.3.4</a></td>
    2233                </tr>
    2234                <tr>
    2235                   <td class="left">204</td>
    2236                   <td class="left">No Content</td>
    2237                   <td class="left"><a href="#status.204" id="rfc.xref.status.204.1" title="204 No Content">Section&nbsp;6.3.5</a></td>
    2238                </tr>
    2239                <tr>
    2240                   <td class="left">205</td>
    2241                   <td class="left">Reset Content</td>
    2242                   <td class="left"><a href="#status.205" id="rfc.xref.status.205.1" title="205 Reset Content">Section&nbsp;6.3.6</a></td>
    2243                </tr>
    2244                <tr>
    2245                   <td class="left">206</td>
    2246                   <td class="left">Partial Content</td>
    2247                   <td id="status.206" class="left"><a href="p5-range.html#status.206" title="206 Partial Content">Section 4.1</a> of <a href="#Part5" id="rfc.xref.Part5.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
    2248                </tr>
    2249                <tr>
    2250                   <td class="left">300</td>
    2251                   <td class="left">Multiple Choices</td>
    2252                   <td class="left"><a href="#status.300" id="rfc.xref.status.300.1" title="300 Multiple Choices">Section&nbsp;6.4.1</a></td>
    2253                </tr>
    2254                <tr>
    2255                   <td class="left">301</td>
    2256                   <td class="left">Moved Permanently</td>
    2257                   <td class="left"><a href="#status.301" id="rfc.xref.status.301.1" title="301 Moved Permanently">Section&nbsp;6.4.2</a></td>
    2258                </tr>
    2259                <tr>
    2260                   <td class="left">302</td>
    2261                   <td class="left">Found</td>
    2262                   <td class="left"><a href="#status.302" id="rfc.xref.status.302.1" title="302 Found">Section&nbsp;6.4.3</a></td>
    2263                </tr>
    2264                <tr>
    2265                   <td class="left">303</td>
    2266                   <td class="left">See Other</td>
    2267                   <td class="left"><a href="#status.303" id="rfc.xref.status.303.1" title="303 See Other">Section&nbsp;6.4.4</a></td>
    2268                </tr>
    2269                <tr>
    2270                   <td class="left">304</td>
    2271                   <td class="left">Not Modified</td>
    2272                   <td id="status.304" class="left"><a href="p4-conditional.html#status.304" title="304 Not Modified">Section 4.1</a> of <a href="#Part4" id="rfc.xref.Part4.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
    2273                </tr>
    2274                <tr>
    2275                   <td class="left">305</td>
    2276                   <td class="left">Use Proxy</td>
    2277                   <td class="left"><a href="#status.305" id="rfc.xref.status.305.1" title="305 Use Proxy">Section&nbsp;6.4.5</a></td>
    2278                </tr>
    2279                <tr>
    2280                   <td class="left">307</td>
    2281                   <td class="left">Temporary Redirect</td>
    2282                   <td class="left"><a href="#status.307" id="rfc.xref.status.307.1" title="307 Temporary Redirect">Section&nbsp;6.4.7</a></td>
    2283                </tr>
    2284                <tr>
    2285                   <td class="left">400</td>
    2286                   <td class="left">Bad Request</td>
    2287                   <td class="left"><a href="#status.400" id="rfc.xref.status.400.1" title="400 Bad Request">Section&nbsp;6.5.1</a></td>
    2288                </tr>
    2289                <tr>
    2290                   <td class="left">401</td>
    2291                   <td class="left">Unauthorized</td>
    2292                   <td id="status.401" class="left"><a href="p7-auth.html#status.401" title="401 Unauthorized">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
    2293                </tr>
    2294                <tr>
    2295                   <td class="left">402</td>
    2296                   <td class="left">Payment Required</td>
    2297                   <td class="left"><a href="#status.402" id="rfc.xref.status.402.1" title="402 Payment Required">Section&nbsp;6.5.2</a></td>
    2298                </tr>
    2299                <tr>
    2300                   <td class="left">403</td>
    2301                   <td class="left">Forbidden</td>
    2302                   <td class="left"><a href="#status.403" id="rfc.xref.status.403.1" title="403 Forbidden">Section&nbsp;6.5.3</a></td>
    2303                </tr>
    2304                <tr>
    2305                   <td class="left">404</td>
    2306                   <td class="left">Not Found</td>
    2307                   <td class="left"><a href="#status.404" id="rfc.xref.status.404.1" title="404 Not Found">Section&nbsp;6.5.4</a></td>
    2308                </tr>
    2309                <tr>
    2310                   <td class="left">405</td>
    2311                   <td class="left">Method Not Allowed</td>
    2312                   <td class="left"><a href="#status.405" id="rfc.xref.status.405.1" title="405 Method Not Allowed">Section&nbsp;6.5.5</a></td>
    2313                </tr>
    2314                <tr>
    2315                   <td class="left">406</td>
    2316                   <td class="left">Not Acceptable</td>
    2317                   <td class="left"><a href="#status.406" id="rfc.xref.status.406.1" title="406 Not Acceptable">Section&nbsp;6.5.6</a></td>
    2318                </tr>
    2319                <tr>
    2320                   <td class="left">407</td>
    2321                   <td class="left">Proxy Authentication Required</td>
    2322                   <td id="status.407" class="left"><a href="p7-auth.html#status.407" title="407 Proxy Authentication Required">Section 3.2</a> of <a href="#Part7" id="rfc.xref.Part7.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
    2323                </tr>
    2324                <tr>
    2325                   <td class="left">408</td>
    2326                   <td class="left">Request Time-out</td>
    2327                   <td class="left"><a href="#status.408" id="rfc.xref.status.408.1" title="408 Request Timeout">Section&nbsp;6.5.7</a></td>
    2328                </tr>
    2329                <tr>
    2330                   <td class="left">409</td>
    2331                   <td class="left">Conflict</td>
    2332                   <td class="left"><a href="#status.409" id="rfc.xref.status.409.1" title="409 Conflict">Section&nbsp;6.5.8</a></td>
    2333                </tr>
    2334                <tr>
    2335                   <td class="left">410</td>
    2336                   <td class="left">Gone</td>
    2337                   <td class="left"><a href="#status.410" id="rfc.xref.status.410.1" title="410 Gone">Section&nbsp;6.5.9</a></td>
    2338                </tr>
    2339                <tr>
    2340                   <td class="left">411</td>
    2341                   <td class="left">Length Required</td>
    2342                   <td class="left"><a href="#status.411" id="rfc.xref.status.411.1" title="411 Length Required">Section&nbsp;6.5.10</a></td>
    2343                </tr>
    2344                <tr>
    2345                   <td class="left">412</td>
    2346                   <td class="left">Precondition Failed</td>
    2347                   <td id="status.412" class="left"><a href="p4-conditional.html#status.412" title="412 Precondition Failed">Section 4.2</a> of <a href="#Part4" id="rfc.xref.Part4.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
    2348                </tr>
    2349                <tr>
    2350                   <td class="left">413</td>
    2351                   <td class="left">Payload Too Large</td>
    2352                   <td class="left"><a href="#status.413" id="rfc.xref.status.413.1" title="413 Payload Too Large">Section&nbsp;6.5.11</a></td>
    2353                </tr>
    2354                <tr>
    2355                   <td class="left">414</td>
    2356                   <td class="left">URI Too Long</td>
    2357                   <td class="left"><a href="#status.414" id="rfc.xref.status.414.1" title="414 URI Too Long">Section&nbsp;6.5.12</a></td>
    2358                </tr>
    2359                <tr>
    2360                   <td class="left">415</td>
    2361                   <td class="left">Unsupported Media Type</td>
    2362                   <td class="left"><a href="#status.415" id="rfc.xref.status.415.1" title="415 Unsupported Media Type">Section&nbsp;6.5.13</a></td>
    2363                </tr>
    2364                <tr>
    2365                   <td class="left">416</td>
    2366                   <td class="left">Range Not Satisfiable</td>
    2367                   <td id="status.416" class="left"><a href="p5-range.html#status.416" title="416 Range Not Satisfiable">Section 4.4</a> of <a href="#Part5" id="rfc.xref.Part5.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
    2368                </tr>
    2369                <tr>
    2370                   <td class="left">417</td>
    2371                   <td class="left">Expectation Failed</td>
    2372                   <td class="left"><a href="#status.417" id="rfc.xref.status.417.1" title="417 Expectation Failed">Section&nbsp;6.5.14</a></td>
    2373                </tr>
    2374                <tr>
    2375                   <td class="left">426</td>
    2376                   <td class="left">Upgrade Required</td>
    2377                   <td class="left"><a href="#status.426" id="rfc.xref.status.426.1" title="426 Upgrade Required">Section&nbsp;6.5.15</a></td>
    2378                </tr>
    2379                <tr>
    2380                   <td class="left">500</td>
    2381                   <td class="left">Internal Server Error</td>
    2382                   <td class="left"><a href="#status.500" id="rfc.xref.status.500.1" title="500 Internal Server Error">Section&nbsp;6.6.1</a></td>
    2383                </tr>
    2384                <tr>
    2385                   <td class="left">501</td>
    2386                   <td class="left">Not Implemented</td>
    2387                   <td class="left"><a href="#status.501" id="rfc.xref.status.501.1" title="501 Not Implemented">Section&nbsp;6.6.2</a></td>
    2388                </tr>
    2389                <tr>
    2390                   <td class="left">502</td>
    2391                   <td class="left">Bad Gateway</td>
    2392                   <td class="left"><a href="#status.502" id="rfc.xref.status.502.1" title="502 Bad Gateway">Section&nbsp;6.6.3</a></td>
    2393                </tr>
    2394                <tr>
    2395                   <td class="left">503</td>
    2396                   <td class="left">Service Unavailable</td>
    2397                   <td class="left"><a href="#status.503" id="rfc.xref.status.503.1" title="503 Service Unavailable">Section&nbsp;6.6.4</a></td>
    2398                </tr>
    2399                <tr>
    2400                   <td class="left">504</td>
    2401                   <td class="left">Gateway Time-out</td>
    2402                   <td class="left"><a href="#status.504" id="rfc.xref.status.504.1" title="504 Gateway Timeout">Section&nbsp;6.6.5</a></td>
    2403                </tr>
    2404                <tr>
    2405                   <td class="left">505</td>
    2406                   <td class="left">HTTP Version Not Supported</td>
    2407                   <td class="left"><a href="#status.505" id="rfc.xref.status.505.1" title="505 HTTP Version Not Supported">Section&nbsp;6.6.6</a></td>
    2408                </tr>
    2409             </tbody>
    2410          </table>
     2269                  and the risk of a user being identified against their wishes ("fingerprinting").
     2270               </p>
     2271               <p id="rfc.section.5.5.3.p.9">Likewise, implementations are encouraged not to use the product tokens of other implementations in order to declare compatibility
     2272                  with them, as this circumvents the purpose of the field. If a user agent masquerades as a different user agent, recipients
     2273                  can assume that the user intentionally desires to see responses tailored for that identified user agent, even if they might
     2274                  not work as well for the actual user agent being used.
     2275               </p>
     2276            </div>
     2277         </div>
    24112278      </div>
    2412       <p id="rfc.section.6.1.p.3">Note that this list is not exhaustive — it does not include extension status codes defined in other specifications. The complete
    2413          list of status codes is maintained by IANA. See <a href="#status.code.registry" title="Status Code Registry">Section&nbsp;8.2</a> for details.
    2414       </p>
    2415       <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="status.1xx" href="#status.1xx">Informational 1xx</a></h2>
    2416       <div id="rfc.iref.67"></div>
    2417       <div id="rfc.iref.s.3"></div>
    2418       <p id="rfc.section.6.2.p.1">The <dfn>1xx (Informational)</dfn> class of status code indicates an interim response for communicating connection status or request progress prior to completing
    2419          the requested action and sending a final response. All 1xx responses consist of only the status-line and optional header fields,
    2420          and thus are terminated by the empty line at the end of the header section. Since HTTP/1.0 did not define any 1xx status codes,
    2421          a server <em class="bcp14">MUST NOT</em> send a 1xx response to an HTTP/1.0 client.
    2422       </p>
    2423       <p id="rfc.section.6.2.p.2">A client <em class="bcp14">MUST</em> be able to parse one or more 1xx responses received prior to a final response, even if the client does not expect one. A user
    2424          agent <em class="bcp14">MAY</em> ignore unexpected 1xx responses.
    2425       </p>
    2426       <p id="rfc.section.6.2.p.3">A proxy <em class="bcp14">MUST</em> forward 1xx responses unless the proxy itself requested the generation of the 1xx response. For example, if a proxy adds an
    2427          "Expect: 100-continue" field when it forwards a request, then it need not forward the corresponding <a href="#status.100" class="smpl">100 (Continue)</a> response(s).
    2428       </p>
    2429       <div id="rfc.iref.68"></div>
    2430       <h3 id="rfc.section.6.2.1"><a href="#rfc.section.6.2.1">6.2.1</a>&nbsp;<a id="status.100" href="#status.100">100 Continue</a></h3>
    2431       <p id="rfc.section.6.2.1.p.1">The <dfn>100 (Continue)</dfn> status code indicates that the initial part of a request has been received and has not yet been rejected by the server. The
    2432          server intends to send a final response after the request has been fully received and acted upon.
    2433       </p>
    2434       <p id="rfc.section.6.2.1.p.2">When the request contains an <a href="#header.expect" class="smpl">Expect</a> header field that includes a <a href="#header.expect" class="smpl">100-continue</a> expectation, the 100 response indicates that the server wishes to receive the request payload body, as described in <a href="#header.expect" id="rfc.xref.header.expect.2" title="Expect">Section&nbsp;5.1.1</a>. The client ought to continue sending the request and discard the 100 response.
    2435       </p>
    2436       <p id="rfc.section.6.2.1.p.3">If the request did not contain an <a href="#header.expect" class="smpl">Expect</a> header field containing the <a href="#header.expect" class="smpl">100-continue</a> expectation, the client can simply discard this interim response.
    2437       </p>
    2438       <div id="rfc.iref.68"></div>
    2439       <h3 id="rfc.section.6.2.2"><a href="#rfc.section.6.2.2">6.2.2</a>&nbsp;<a id="status.101" href="#status.101">101 Switching Protocols</a></h3>
    2440       <p id="rfc.section.6.2.2.p.1">The <dfn>101 (Switching Protocols)</dfn> status code indicates that the server understands and is willing to comply with the client's request, via the <a href="p1-messaging.html#header.upgrade" class="smpl">Upgrade</a> header field (<a href="p1-messaging.html#header.upgrade" title="Upgrade">Section 6.7</a> of <a href="#Part1" id="rfc.xref.Part1.23"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>), for a change in the application protocol being used on this connection. The server <em class="bcp14">MUST</em> generate an Upgrade header field in the response that indicates which protocol(s) will be switched to immediately after the
    2441          empty line that terminates the 101 response.
    2442       </p>
    2443       <p id="rfc.section.6.2.2.p.2">It is assumed that the server will only agree to switch protocols when it is advantageous to do so. For example, switching
    2444          to a newer version of HTTP might be advantageous over older versions, and switching to a real-time, synchronous protocol might
    2445          be advantageous when delivering resources that use such features.
    2446       </p>
    2447       <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a id="status.2xx" href="#status.2xx">Successful 2xx</a></h2>
    2448       <div id="rfc.iref.68"></div>
    2449       <div id="rfc.iref.s.4"></div>
    2450       <p id="rfc.section.6.3.p.1">The <dfn>2xx (Successful)</dfn> class of status code indicates that the client's request was successfully received, understood, and accepted.
    2451       </p>
    2452       <div id="rfc.iref.69"></div>
    2453       <h3 id="rfc.section.6.3.1"><a href="#rfc.section.6.3.1">6.3.1</a>&nbsp;<a id="status.200" href="#status.200">200 OK</a></h3>
    2454       <p id="rfc.section.6.3.1.p.1">The <dfn>200 (OK)</dfn> status code indicates that the request has succeeded. The payload sent in a 200 response depends on the request method. For
    2455          the methods defined by this specification, the intended meaning of the payload can be summarized as:
    2456       </p>
    2457       <dl>
    2458          <dt>GET</dt>
    2459          <dd>a representation of the <a href="#resources" class="smpl">target resource</a>;
    2460          </dd>
    2461          <dt>HEAD</dt>
    2462          <dd>the same representation as GET, but without the representation data;</dd>
    2463          <dt>POST</dt>
    2464          <dd>a representation of the status of, or results obtained from, the action;</dd>
    2465          <dt>PUT, DELETE</dt>
    2466          <dd>a representation of the status of the action;</dd>
    2467          <dt>OPTIONS</dt>
    2468          <dd>a representation of the communications options;</dd>
    2469          <dt>TRACE</dt>
    2470          <dd>a representation of the request message as received by the end server.</dd>
    2471       </dl>
    2472       <p id="rfc.section.6.3.1.p.2">Aside from responses to CONNECT, a 200 response always has a payload, though an origin server <em class="bcp14">MAY</em> generate a payload body of zero length. If no payload is desired, an origin server ought to send <dfn>204 (No Content)</dfn> instead. For CONNECT, no payload is allowed because the successful result is a tunnel, which begins immediately after the
    2473          200 response header section.
    2474       </p>
    2475       <p id="rfc.section.6.3.1.p.3">A 200 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2476       </p>
    2477       <div id="rfc.iref.69"></div>
    2478       <h3 id="rfc.section.6.3.2"><a href="#rfc.section.6.3.2">6.3.2</a>&nbsp;<a id="status.201" href="#status.201">201 Created</a></h3>
    2479       <p id="rfc.section.6.3.2.p.1">The <dfn>201 (Created)</dfn> status code indicates that the request has been fulfilled and has resulted in one or more new resources being created. The
    2480          primary resource created by the request is identified by either a <a href="#header.location" class="smpl">Location</a> header field in the response or, if no <a href="#header.location" class="smpl">Location</a> field is received, by the effective request URI.
    2481       </p>
    2482       <p id="rfc.section.6.3.2.p.2">The 201 response payload typically describes and links to the resource(s) created. See <a href="#response.validator" title="Validator Header Fields">Section&nbsp;7.2</a> for a discussion of the meaning and purpose of validator header fields, such as <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> and <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a>, in a 201 response.
    2483       </p>
    2484       <div id="rfc.iref.69"></div>
    2485       <h3 id="rfc.section.6.3.3"><a href="#rfc.section.6.3.3">6.3.3</a>&nbsp;<a id="status.202" href="#status.202">202 Accepted</a></h3>
    2486       <p id="rfc.section.6.3.3.p.1">The <dfn>202 (Accepted)</dfn> status code indicates that the request has been accepted for processing, but the processing has not been completed. The request
    2487          might or might not eventually be acted upon, as it might be disallowed when processing actually takes place. There is no facility
    2488          in HTTP for re-sending a status code from an asynchronous operation.
    2489       </p>
    2490       <p id="rfc.section.6.3.3.p.2">The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process
    2491          (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the
    2492          server persist until the process is completed. The representation sent with this response ought to describe the request's
    2493          current status and point to (or embed) a status monitor that can provide the user with an estimate of when the request will
    2494          be fulfilled.
    2495       </p>
    2496       <div id="rfc.iref.69"></div>
    2497       <h3 id="rfc.section.6.3.4"><a href="#rfc.section.6.3.4">6.3.4</a>&nbsp;<a id="status.203" href="#status.203">203 Non-Authoritative Information</a></h3>
    2498       <p id="rfc.section.6.3.4.p.1">The <dfn>203 (Non-Authoritative Information)</dfn> status code indicates that the request was successful but the enclosed payload has been modified from that of the origin server's <a href="#status.200" class="smpl">200 (OK)</a> response by a transforming proxy (<a href="p1-messaging.html#message.transformations" title="Transformations">Section 5.7.2</a> of <a href="#Part1" id="rfc.xref.Part1.24"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). This status code allows the proxy to notify recipients when a transformation has been applied, since that knowledge might
    2499          impact later decisions regarding the content. For example, future cache validation requests for the content might only be
    2500          applicable along the same request path (through the same proxies).
    2501       </p>
    2502       <p id="rfc.section.6.3.4.p.2">The 203 response is similar to the Warning code of 214 Transformation Applied (<a href="p6-cache.html#header.warning" title="Warning">Section 5.5</a> of <a href="#Part6" id="rfc.xref.Part6.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>), which has the advantage of being applicable to responses with any status code.
    2503       </p>
    2504       <p id="rfc.section.6.3.4.p.3">A 203 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.13"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2505       </p>
    2506       <div id="rfc.iref.69"></div>
    2507       <h3 id="rfc.section.6.3.5"><a href="#rfc.section.6.3.5">6.3.5</a>&nbsp;<a id="status.204" href="#status.204">204 No Content</a></h3>
    2508       <p id="rfc.section.6.3.5.p.1">The <dfn>204 (No Content)</dfn> status code indicates that the server has successfully fulfilled the request and that there is no additional content to send
    2509          in the response payload body. Metadata in the response header fields refer to the <a href="#resources" class="smpl">target resource</a> and its <a href="#representations" class="smpl">selected representation</a> after the requested action was applied.
    2510       </p>
    2511       <p id="rfc.section.6.3.5.p.2">For example, if a 204 status code is received in response to a PUT request and the response contains an <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> header field, then the PUT was successful and the ETag field-value contains the entity-tag for the new representation of that
    2512          target resource.
    2513       </p>
    2514       <p id="rfc.section.6.3.5.p.3">The 204 response allows a server to indicate that the action has been successfully applied to the target resource, while implying
    2515          that the user agent does not need to traverse away from its current "document view" (if any). The server assumes that the
    2516          user agent will provide some indication of the success to its user, in accord with its own interface, and apply any new or
    2517          updated metadata in the response to its active representation.
    2518       </p>
    2519       <p id="rfc.section.6.3.5.p.4">For example, a 204 status code is commonly used with document editing interfaces corresponding to a "save" action, such that
    2520          the document being saved remains available to the user for editing. It is also frequently used with interfaces that expect
    2521          automated data transfers to be prevalent, such as within distributed version control systems.
    2522       </p>
    2523       <p id="rfc.section.6.3.5.p.5">A 204 response is terminated by the first empty line after the header fields because it cannot contain a message body.</p>
    2524       <p id="rfc.section.6.3.5.p.6">A 204 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.14"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2525       </p>
    2526       <div id="rfc.iref.69"></div>
    2527       <h3 id="rfc.section.6.3.6"><a href="#rfc.section.6.3.6">6.3.6</a>&nbsp;<a id="status.205" href="#status.205">205 Reset Content</a></h3>
    2528       <p id="rfc.section.6.3.6.p.1">The <dfn>205 (Reset Content)</dfn> status code indicates that the server has fulfilled the request and desires that the user agent reset the "document view",
    2529          which caused the request to be sent, to its original state as received from the origin server.
    2530       </p>
    2531       <p id="rfc.section.6.3.6.p.2">This response is intended to support a common data entry use case where the user receives content that supports data entry
    2532          (a form, notepad, canvas, etc.), enters or manipulates data in that space, causes the entered data to be submitted in a request,
    2533          and then the data entry mechanism is reset for the next entry so that the user can easily initiate another input action.
    2534       </p>
    2535       <p id="rfc.section.6.3.6.p.3">Since the 205 status code implies that no additional content will be provided, a server <em class="bcp14">MUST NOT</em> generate a payload in a 205 response. In other words, a server <em class="bcp14">MUST</em> do one of the following for a 205 response: a) indicate a zero-length body for the response by including a <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> header field with a value of 0; b) indicate a zero-length payload for the response by including a <a href="p1-messaging.html#header.transfer-encoding" class="smpl">Transfer-Encoding</a> header field with a value of chunked and a message body consisting of a single chunk of zero-length; or, c) close the connection
    2536          immediately after sending the blank line terminating the header section.
    2537       </p>
    2538       <h2 id="rfc.section.6.4"><a href="#rfc.section.6.4">6.4</a>&nbsp;<a id="status.3xx" href="#status.3xx">Redirection 3xx</a></h2>
    2539       <div id="rfc.iref.69"></div>
    2540       <div id="rfc.iref.s.5"></div>
    2541       <p id="rfc.section.6.4.p.1">The <dfn>3xx (Redirection)</dfn> class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. If
    2542          a <a href="#header.location" class="smpl">Location</a> header field (<a href="#header.location" id="rfc.xref.header.location.2" title="Location">Section&nbsp;7.1.2</a>) is provided, the user agent <em class="bcp14">MAY</em> automatically redirect its request to the URI referenced by the Location field value, even if the specific status code is
    2543          not understood. Automatic redirection needs to done with care for methods not known to be <a href="#safe.methods" class="smpl">safe</a>, as defined in <a href="#safe.methods" title="Safe Methods">Section&nbsp;4.2.1</a>, since the user might not wish to redirect an unsafe request.
    2544       </p>
    2545       <p id="rfc.section.6.4.p.2">There are several types of redirects: </p>
    2546       <ol>
    2547          <li>
    2548             <p>Redirects that indicate the resource might be available at a different URI, as provided by the <a href="#header.location" class="smpl">Location</a> field, as in the status codes <a href="#status.301" class="smpl">301 (Moved Permanently)</a>, <a href="#status.302" class="smpl">302 (Found)</a>, and <a href="#status.307" class="smpl">307 (Temporary Redirect)</a>.
    2549             </p>
    2550          </li>
    2551          <li>
    2552             <p>Redirection that offers a choice of matching resources, each capable of representing the original request target, as in the <a href="#status.300" class="smpl">300 (Multiple Choices)</a> status code.
    2553             </p>
    2554          </li>
    2555          <li>
    2556             <p>Redirection to a different resource, identified by the <a href="#header.location" class="smpl">Location</a> field, that can represent an indirect response to the request, as in the <a href="#status.303" class="smpl">303 (See Other)</a> status code.
    2557             </p>
    2558          </li>
    2559          <li>
    2560             <p>Redirection to a previously cached result, as in the <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> status code.
    2561             </p>
    2562          </li>
    2563       </ol>
    2564       <div class="note" id="rfc.section.6.4.p.3">
    2565          <p><b>Note:</b> In HTTP/1.0, the status codes <a href="#status.301" class="smpl">301 (Moved Permanently)</a> and <a href="#status.302" class="smpl">302 (Found)</a> were defined for the first type of redirect (<a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[RFC1945]</cite></a>, <a href="http://tools.ietf.org/html/rfc1945#section-9.3">Section 9.3</a>). Early user agents split on whether the method applied to the redirect target would be the same as the original request
    2566             or would be rewritten as GET. Although HTTP originally defined the former semantics for <a href="#status.301" class="smpl">301</a> and <a href="#status.302" class="smpl">302</a> (to match its original implementation at CERN), and defined <a href="#status.303" class="smpl">303 (See Other)</a> to match the latter semantics, prevailing practice gradually converged on the latter semantics for <a href="#status.301" class="smpl">301</a> and <a href="#status.302" class="smpl">302</a> as well. The first revision of HTTP/1.1 added <a href="#status.307" class="smpl">307 (Temporary Redirect)</a> to indicate the former semantics without being impacted by divergent practice. Over 10 years later, most user agents still
    2567             do method rewriting for <a href="#status.301" class="smpl">301</a> and <a href="#status.302" class="smpl">302</a>; therefore, this specification makes that behavior conformant when the original request is POST.
    2568          </p>
    2569       </div>
    2570       <p id="rfc.section.6.4.p.4">A client <em class="bcp14">SHOULD</em> detect and intervene in cyclical redirections (i.e., "infinite" redirection loops).
    2571       </p>
    2572       <div class="note" id="rfc.section.6.4.p.5">
    2573          <p><b>Note:</b> An earlier version of this specification recommended a maximum of five redirections (<a href="#RFC2068" id="rfc.xref.RFC2068.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a>, <a href="http://tools.ietf.org/html/rfc2068#section-10.3">Section 10.3</a>). Content developers need to be aware that some clients might implement such a fixed limitation.
    2574          </p>
    2575       </div>
    2576       <div id="rfc.iref.70"></div>
    2577       <h3 id="rfc.section.6.4.1"><a href="#rfc.section.6.4.1">6.4.1</a>&nbsp;<a id="status.300" href="#status.300">300 Multiple Choices</a></h3>
    2578       <p id="rfc.section.6.4.1.p.1">The <dfn>300 (Multiple Choices)</dfn> status code indicates that the <a href="#resources" class="smpl">target resource</a> has more than one representation, each with its own more specific identifier, and information about the alternatives is being
    2579          provided so that the user (or user agent) can select a preferred representation by redirecting its request to one or more
    2580          of those identifiers. In other words, the server desires that the user agent engage in reactive negotiation to select the
    2581          most appropriate representation(s) for its needs (<a href="#content.negotiation" title="Content Negotiation">Section&nbsp;3.4</a>).
    2582       </p>
    2583       <p id="rfc.section.6.4.1.p.2">If the server has a preferred choice, the server <em class="bcp14">SHOULD</em> generate a <a href="#header.location" class="smpl">Location</a> header field containing a preferred choice's URI reference. The user agent <em class="bcp14">MAY</em> use the Location field value for automatic redirection.
    2584       </p>
    2585       <p id="rfc.section.6.4.1.p.3">For request methods other than HEAD, the server <em class="bcp14">SHOULD</em> generate a payload in the 300 response containing a list of representation metadata and URI reference(s) from which the user
    2586          or user agent can choose the one most preferred. The user agent <em class="bcp14">MAY</em> make a selection from that list automatically, depending upon the list format, but this specification does not define a standard
    2587          for such automatic selection.
    2588       </p>
    2589       <p id="rfc.section.6.4.1.p.4">A 300 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.15"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2590       </p>
    2591       <div class="note" id="rfc.section.6.4.1.p.5">
    2592          <p><b>Note:</b> The original proposal for 300 defined the URI header field as providing a list of alternative representations, such that it
    2593             would be usable for 200, 300, and 406 responses and be transferred in responses to the HEAD method. However, lack of deployment
    2594             and disagreement over syntax led to both URI and Alternates (a subsequent proposal) being dropped from this specification.
    2595             It is possible to communicate the list using a set of Link header fields <a href="#RFC5988" id="rfc.xref.RFC5988.1"><cite title="Web Linking">[RFC5988]</cite></a>, each with a relationship of "alternate", though deployment is a chicken-and-egg problem.
    2596          </p>
    2597       </div>
    2598       <div id="rfc.iref.70"></div>
    2599       <h3 id="rfc.section.6.4.2"><a href="#rfc.section.6.4.2">6.4.2</a>&nbsp;<a id="status.301" href="#status.301">301 Moved Permanently</a></h3>
    2600       <p id="rfc.section.6.4.2.p.1">The <dfn>301 (Moved Permanently)</dfn> status code indicates that the <a href="#resources" class="smpl">target resource</a> has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs. Clients
    2601          with link editing capabilities ought to automatically re-link references to the effective request URI to one or more of the
    2602          new references sent by the server, where possible.
    2603       </p>
    2604       <p id="rfc.section.6.4.2.p.2">The server <em class="bcp14">SHOULD</em> generate a <a href="#header.location" class="smpl">Location</a> header field in the response containing a preferred URI reference for the new permanent URI. The user agent <em class="bcp14">MAY</em> use the Location field value for automatic redirection. The server's response payload usually contains a short hypertext note
    2605          with a hyperlink to the new URI(s).
    2606       </p>
    2607       <div class="note" id="rfc.section.6.4.2.p.3">
    2608          <p><b>Note:</b> For historic reasons, a user agent <em class="bcp14">MAY</em> change the request method from POST to GET for the subsequent request. If this behavior is undesired, the <a href="#status.307" class="smpl">307 (Temporary Redirect)</a> status code can be used instead.
    2609          </p>
    2610       </div>
    2611       <p id="rfc.section.6.4.2.p.4">A 301 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.16"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2612       </p>
    2613       <div id="rfc.iref.70"></div>
    2614       <h3 id="rfc.section.6.4.3"><a href="#rfc.section.6.4.3">6.4.3</a>&nbsp;<a id="status.302" href="#status.302">302 Found</a></h3>
    2615       <p id="rfc.section.6.4.3.p.1">The <dfn>302 (Found)</dfn> status code indicates that the target resource resides temporarily under a different URI. Since the redirection might be altered
    2616          on occasion, the client ought to continue to use the effective request URI for future requests.
    2617       </p>
    2618       <p id="rfc.section.6.4.3.p.2">The server <em class="bcp14">SHOULD</em> generate a <a href="#header.location" class="smpl">Location</a> header field in the response containing a URI reference for the different URI. The user agent <em class="bcp14">MAY</em> use the Location field value for automatic redirection. The server's response payload usually contains a short hypertext note
    2619          with a hyperlink to the different URI(s).
    2620       </p>
    2621       <div class="note" id="rfc.section.6.4.3.p.3">
    2622          <p><b>Note:</b> For historic reasons, a user agent <em class="bcp14">MAY</em> change the request method from POST to GET for the subsequent request. If this behavior is undesired, the <a href="#status.307" class="smpl">307 (Temporary Redirect)</a> status code can be used instead.
    2623          </p>
    2624       </div>
    2625       <div id="rfc.iref.70"></div>
    2626       <h3 id="rfc.section.6.4.4"><a href="#rfc.section.6.4.4">6.4.4</a>&nbsp;<a id="status.303" href="#status.303">303 See Other</a></h3>
    2627       <p id="rfc.section.6.4.4.p.1">The <dfn>303 (See Other)</dfn> status code indicates that the server is redirecting the user agent to a different resource, as indicated by a URI in the <a href="#header.location" class="smpl">Location</a> header field, that is intended to provide an indirect response to the original request. In order to satisfy the original request,
    2628          a user agent ought to perform a retrieval request using the Location URI (a GET or HEAD request if using HTTP), which can
    2629          itself be redirected further, and present the eventual result as an answer to the original request. Note that the new URI
    2630          in the Location header field is not considered equivalent to the effective request URI.
    2631       </p>
    2632       <p id="rfc.section.6.4.4.p.2">This status code is applicable to any HTTP method. It is primarily used to allow the output of a POST action to redirect the
    2633          user agent to a selected resource, since doing so provides the information corresponding to the POST response in a form that
    2634          can be separately identified, bookmarked, and cached independent of the original request.
    2635       </p>
    2636       <p id="rfc.section.6.4.4.p.3">A 303 response to a GET request indicates that the origin server does not have a representation of the <a href="#resources" class="smpl">target resource</a> that can be transferred by the server over HTTP. However, the <a href="#header.location" class="smpl">Location</a> field value refers to a resource that is descriptive of the target resource, such that making a retrieval request on that
    2637          other resource might result in a representation that is useful to recipients without implying that it represents the original
    2638          target resource. Note that answers to the questions of what can be represented, what representations are adequate, and what
    2639          might be a useful description are outside the scope of HTTP.
    2640       </p>
    2641       <p id="rfc.section.6.4.4.p.4">Except for responses to a HEAD request, the representation of a 303 response ought to contain a short hypertext note with
    2642          a hyperlink to the same URI reference provided in the <a href="#header.location" class="smpl">Location</a> header field.
    2643       </p>
    2644       <div id="rfc.iref.70"></div>
    2645       <h3 id="rfc.section.6.4.5"><a href="#rfc.section.6.4.5">6.4.5</a>&nbsp;<a id="status.305" href="#status.305">305 Use Proxy</a></h3>
    2646       <p id="rfc.section.6.4.5.p.1">The <dfn>305 (Use Proxy)</dfn> status code was defined in a previous version of this specification and is now deprecated (<a href="#changes.from.rfc.2616" title="Changes from RFC 2616">Appendix&nbsp;B</a>).
    2647       </p>
    2648       <div id="rfc.iref.70"></div>
    2649       <h3 id="rfc.section.6.4.6"><a href="#rfc.section.6.4.6">6.4.6</a>&nbsp;<a id="status.306" href="#status.306">306 (Unused)</a></h3>
    2650       <p id="rfc.section.6.4.6.p.1">The 306 status code was defined in a previous version of this specification, is no longer used, and the code is reserved.</p>
    2651       <div id="rfc.iref.70"></div>
    2652       <h3 id="rfc.section.6.4.7"><a href="#rfc.section.6.4.7">6.4.7</a>&nbsp;<a id="status.307" href="#status.307">307 Temporary Redirect</a></h3>
    2653       <p id="rfc.section.6.4.7.p.1">The <dfn>307 (Temporary Redirect)</dfn> status code indicates that the <a href="#resources" class="smpl">target resource</a> resides temporarily under a different URI and the user agent <em class="bcp14">MUST NOT</em> change the request method if it performs an automatic redirection to that URI. Since the redirection can change over time,
    2654          the client ought to continue using the original effective request URI for future requests.
    2655       </p>
    2656       <p id="rfc.section.6.4.7.p.2">The server <em class="bcp14">SHOULD</em> generate a <a href="#header.location" class="smpl">Location</a> header field in the response containing a URI reference for the different URI. The user agent <em class="bcp14">MAY</em> use the Location field value for automatic redirection. The server's response payload usually contains a short hypertext note
    2657          with a hyperlink to the different URI(s).
    2658       </p>
    2659       <div class="note" id="rfc.section.6.4.7.p.3">
    2660          <p><b>Note:</b> This status code is similar to <a href="#status.302" class="smpl">302 (Found)</a>, except that it does not allow changing the request method from POST to GET. This specification defines no equivalent counterpart
    2661             for <a href="#status.301" class="smpl">301 (Moved
    2662                Permanently)</a> (<a href="#status-308" id="rfc.xref.status-308.1"><cite title="The Hypertext Transfer Protocol (HTTP) Status Code 308 (Permanent Redirect)">[status-308]</cite></a>, however, defines the status code 308 (Permanent Redirect) for this purpose).
    2663          </p>
    2664       </div>
    2665       <h2 id="rfc.section.6.5"><a href="#rfc.section.6.5">6.5</a>&nbsp;<a id="status.4xx" href="#status.4xx">Client Error 4xx</a></h2>
    2666       <div id="rfc.iref.70"></div>
    2667       <div id="rfc.iref.s.6"></div>
    2668       <p id="rfc.section.6.5.p.1">The <dfn>4xx (Client Error)</dfn> class of status code indicates that the client seems to have erred. Except when responding to a HEAD request, the server <em class="bcp14">SHOULD</em> send a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition.
    2669          These status codes are applicable to any request method. User agents <em class="bcp14">SHOULD</em> display any included representation to the user.
    2670       </p>
    2671       <div id="rfc.iref.71"></div>
    2672       <h3 id="rfc.section.6.5.1"><a href="#rfc.section.6.5.1">6.5.1</a>&nbsp;<a id="status.400" href="#status.400">400 Bad Request</a></h3>
    2673       <p id="rfc.section.6.5.1.p.1">The <dfn>400 (Bad Request)</dfn> status code indicates that the server cannot or will not process the request due to something which is perceived to be a client
    2674          error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).
    2675       </p>
    2676       <div id="rfc.iref.71"></div>
    2677       <h3 id="rfc.section.6.5.2"><a href="#rfc.section.6.5.2">6.5.2</a>&nbsp;<a id="status.402" href="#status.402">402 Payment Required</a></h3>
    2678       <p id="rfc.section.6.5.2.p.1">The <dfn>402 (Payment Required)</dfn> status code is reserved for future use.
    2679       </p>
    2680       <div id="rfc.iref.71"></div>
    2681       <h3 id="rfc.section.6.5.3"><a href="#rfc.section.6.5.3">6.5.3</a>&nbsp;<a id="status.403" href="#status.403">403 Forbidden</a></h3>
    2682       <p id="rfc.section.6.5.3.p.1">The <dfn>403 (Forbidden)</dfn> status code indicates that the server understood the request but refuses to authorize it. A server that wishes to make public
    2683          why the request has been forbidden can describe that reason in the response payload (if any).
    2684       </p>
    2685       <p id="rfc.section.6.5.3.p.2">If authentication credentials were provided in the request, the server considers them insufficient to grant access. The client <em class="bcp14">SHOULD NOT</em> automatically repeat the request with the same credentials. The client <em class="bcp14">MAY</em> repeat the request with new or different credentials. However, a request might be forbidden for reasons unrelated to the credentials.
    2686       </p>
    2687       <p id="rfc.section.6.5.3.p.3">An origin server that wishes to "hide" the current existence of a forbidden <a href="#resources" class="smpl">target resource</a> <em class="bcp14">MAY</em> instead respond with a status code of <a href="#status.404" class="smpl">404 (Not Found)</a>.
    2688       </p>
    2689       <div id="rfc.iref.71"></div>
    2690       <h3 id="rfc.section.6.5.4"><a href="#rfc.section.6.5.4">6.5.4</a>&nbsp;<a id="status.404" href="#status.404">404 Not Found</a></h3>
    2691       <p id="rfc.section.6.5.4.p.1">The <dfn>404 (Not Found)</dfn> status code indicates that the origin server did not find a current representation for the <a href="#resources" class="smpl">target resource</a> or is not willing to disclose that one exists. A 404 status code does not indicate whether this lack of representation is
    2692          temporary or permanent; the <a href="#status.410" class="smpl">410 (Gone)</a> status code is preferred over 404 if the origin server knows, presumably through some configurable means, that the condition
    2693          is likely to be permanent.
    2694       </p>
    2695       <p id="rfc.section.6.5.4.p.2">A 404 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.17"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2696       </p>
    2697       <div id="rfc.iref.71"></div>
    2698       <h3 id="rfc.section.6.5.5"><a href="#rfc.section.6.5.5">6.5.5</a>&nbsp;<a id="status.405" href="#status.405">405 Method Not Allowed</a></h3>
    2699       <p id="rfc.section.6.5.5.p.1">The <dfn>405 (Method Not Allowed)</dfn> status code indicates that the method received in the request-line is known by the origin server but not supported by the <a href="#resources" class="smpl">target resource</a>. The origin server <em class="bcp14">MUST</em> generate an <a href="#header.allow" class="smpl">Allow</a> header field in a 405 response containing a list of the target resource's currently supported methods.
    2700       </p>
    2701       <p id="rfc.section.6.5.5.p.2">A 405 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.18"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2702       </p>
    2703       <div id="rfc.iref.71"></div>
    2704       <h3 id="rfc.section.6.5.6"><a href="#rfc.section.6.5.6">6.5.6</a>&nbsp;<a id="status.406" href="#status.406">406 Not Acceptable</a></h3>
    2705       <p id="rfc.section.6.5.6.p.1">The <dfn>406 (Not Acceptable)</dfn> status code indicates that the <a href="#resources" class="smpl">target resource</a> does not have a current representation that would be acceptable to the user agent, according to the <a href="#proactive.negotiation" class="smpl">proactive negotiation</a> header fields received in the request (<a href="#request.conneg" title="Content Negotiation">Section&nbsp;5.3</a>), and the server is unwilling to supply a default representation.
    2706       </p>
    2707       <p id="rfc.section.6.5.6.p.2">The server <em class="bcp14">SHOULD</em> generate a payload containing a list of available representation characteristics and corresponding resource identifiers from
    2708          which the user or user agent can choose the one most appropriate. A user agent <em class="bcp14">MAY</em> automatically select the most appropriate choice from that list. However, this specification does not define any standard
    2709          for such automatic selection, as described in <a href="#status.300" id="rfc.xref.status.300.2" title="300 Multiple Choices">Section&nbsp;6.4.1</a>.
    2710       </p>
    2711       <div id="rfc.iref.71"></div>
    2712       <h3 id="rfc.section.6.5.7"><a href="#rfc.section.6.5.7">6.5.7</a>&nbsp;<a id="status.408" href="#status.408">408 Request Timeout</a></h3>
    2713       <p id="rfc.section.6.5.7.p.1">The <dfn>408 (Request Timeout)</dfn> status code indicates that the server did not receive a complete request message within the time that it was prepared to wait.
    2714          A server <em class="bcp14">SHOULD</em> send the <a href="p1-messaging.html#header.connection" class="smpl">close</a> connection option (<a href="p1-messaging.html#header.connection" title="Connection">Section 6.1</a> of <a href="#Part1" id="rfc.xref.Part1.25"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) in the response, since 408 implies that the server has decided to close the connection rather than continue waiting. If
    2715          the client has an outstanding request in transit, the client <em class="bcp14">MAY</em> repeat that request on a new connection.
    2716       </p>
    2717       <div id="rfc.iref.71"></div>
    2718       <h3 id="rfc.section.6.5.8"><a href="#rfc.section.6.5.8">6.5.8</a>&nbsp;<a id="status.409" href="#status.409">409 Conflict</a></h3>
    2719       <p id="rfc.section.6.5.8.p.1">The <dfn>409 (Conflict)</dfn> status code indicates that the request could not be completed due to a conflict with the current state of the target resource.
    2720          This code is used in situations where the user might be able to resolve the conflict and resubmit the request. The server <em class="bcp14">SHOULD</em> generate a payload that includes enough information for a user to recognize the source of the conflict.
    2721       </p>
    2722       <p id="rfc.section.6.5.8.p.2">Conflicts are most likely to occur in response to a PUT request. For example, if versioning were being used and the representation
    2723          being PUT included changes to a resource that conflict with those made by an earlier (third-party) request, the origin server
    2724          might use a 409 response to indicate that it can't complete the request. In this case, the response representation would likely
    2725          contain information useful for merging the differences based on the revision history.
    2726       </p>
    2727       <div id="rfc.iref.71"></div>
    2728       <h3 id="rfc.section.6.5.9"><a href="#rfc.section.6.5.9">6.5.9</a>&nbsp;<a id="status.410" href="#status.410">410 Gone</a></h3>
    2729       <p id="rfc.section.6.5.9.p.1">The <dfn>410 (Gone)</dfn> status code indicates that access to the <a href="#resources" class="smpl">target resource</a> is no longer available at the origin server and that this condition is likely to be permanent. If the origin server does not
    2730          know, or has no facility to determine, whether or not the condition is permanent, the status code <a href="#status.404" class="smpl">404 (Not Found)</a> ought to be used instead.
    2731       </p>
    2732       <p id="rfc.section.6.5.9.p.2">The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource
    2733          is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event
    2734          is common for limited-time, promotional services and for resources belonging to individuals no longer associated with the
    2735          origin server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for
    2736          any length of time — that is left to the discretion of the server owner.
    2737       </p>
    2738       <p id="rfc.section.6.5.9.p.3">A 410 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2739       </p>
    2740       <div id="rfc.iref.71"></div>
    2741       <h3 id="rfc.section.6.5.10"><a href="#rfc.section.6.5.10">6.5.10</a>&nbsp;<a id="status.411" href="#status.411">411 Length Required</a></h3>
    2742       <p id="rfc.section.6.5.10.p.1">The <dfn>411 (Length Required)</dfn> status code indicates that the server refuses to accept the request without a defined <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> (<a href="p1-messaging.html#header.content-length" title="Content-Length">Section 3.3.2</a> of <a href="#Part1" id="rfc.xref.Part1.26"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). The client <em class="bcp14">MAY</em> repeat the request if it adds a valid Content-Length header field containing the length of the message body in the request
    2743          message.
    2744       </p>
    2745       <div id="rfc.iref.71"></div>
    2746       <h3 id="rfc.section.6.5.11"><a href="#rfc.section.6.5.11">6.5.11</a>&nbsp;<a id="status.413" href="#status.413">413 Payload Too Large</a></h3>
    2747       <p id="rfc.section.6.5.11.p.1">The <dfn>413 (Payload Too Large)</dfn> status code indicates that the server is refusing to process a request because the request payload is larger than the server
    2748          is willing or able to process. The server <em class="bcp14">MAY</em> close the connection to prevent the client from continuing the request.
    2749       </p>
    2750       <p id="rfc.section.6.5.11.p.2">If the condition is temporary, the server <em class="bcp14">SHOULD</em> generate a <a href="#header.retry-after" class="smpl">Retry-After</a> header field to indicate that it is temporary and after what time the client <em class="bcp14">MAY</em> try again.
    2751       </p>
    2752       <div id="rfc.iref.71"></div>
    2753       <h3 id="rfc.section.6.5.12"><a href="#rfc.section.6.5.12">6.5.12</a>&nbsp;<a id="status.414" href="#status.414">414 URI Too Long</a></h3>
    2754       <p id="rfc.section.6.5.12.p.1">The <dfn>414 (URI Too Long)</dfn> status code indicates that the server is refusing to service the request because the request-target (<a href="p1-messaging.html#request-target" title="Request Target">Section 5.3</a> of <a href="#Part1" id="rfc.xref.Part1.27"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly
    2755          converted a POST request to a GET request with long query information, when the client has descended into a "black hole" of
    2756          redirection (e.g., a redirected URI prefix that points to a suffix of itself), or when the server is under attack by a client
    2757          attempting to exploit potential security holes.
    2758       </p>
    2759       <p id="rfc.section.6.5.12.p.2">A 414 response is cacheable unless otherwise indicated by the method definition or explicit cache controls (see <a href="p6-cache.html#heuristic.freshness" title="Calculating Heuristic Freshness">Section 4.2.2</a> of <a href="#Part6" id="rfc.xref.Part6.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>).
    2760       </p>
    2761       <div id="rfc.iref.71"></div>
    2762       <h3 id="rfc.section.6.5.13"><a href="#rfc.section.6.5.13">6.5.13</a>&nbsp;<a id="status.415" href="#status.415">415 Unsupported Media Type</a></h3>
    2763       <p id="rfc.section.6.5.13.p.1">The <dfn>415 (Unsupported Media Type)</dfn> status code indicates that the origin server is refusing to service the request because the payload is in a format not supported
    2764          by this method on the <a href="#resources" class="smpl">target resource</a>. The format problem might be due to the request's indicated <a href="#header.content-type" class="smpl">Content-Type</a> or <a href="#header.content-encoding" class="smpl">Content-Encoding</a>, or as a result of inspecting the data directly.
    2765       </p>
    2766       <div id="rfc.iref.71"></div>
    2767       <h3 id="rfc.section.6.5.14"><a href="#rfc.section.6.5.14">6.5.14</a>&nbsp;<a id="status.417" href="#status.417">417 Expectation Failed</a></h3>
    2768       <p id="rfc.section.6.5.14.p.1">The <dfn>417 (Expectation Failed)</dfn> status code indicates that the expectation given in the request's <a href="#header.expect" class="smpl">Expect</a> header field (<a href="#header.expect" id="rfc.xref.header.expect.3" title="Expect">Section&nbsp;5.1.1</a>) could not be met by at least one of the inbound servers.
    2769       </p>
    2770       <div id="rfc.iref.71"></div>
    2771       <h3 id="rfc.section.6.5.15"><a href="#rfc.section.6.5.15">6.5.15</a>&nbsp;<a id="status.426" href="#status.426">426 Upgrade Required</a></h3>
    2772       <p id="rfc.section.6.5.15.p.1">The <dfn>426 (Upgrade Required)</dfn> status code indicates that the server refuses to perform the request using the current protocol but might be willing to do
    2773          so after the client upgrades to a different protocol. The server <em class="bcp14">MUST</em> send an <a href="p1-messaging.html#header.upgrade" class="smpl">Upgrade</a> header field in a 426 response to indicate the required protocol(s) (<a href="p1-messaging.html#header.upgrade" title="Upgrade">Section 6.7</a> of <a href="#Part1" id="rfc.xref.Part1.28"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>).
    2774       </p>
    2775       <div id="rfc.figure.u.42"></div>
    2776       <p>Example:</p><pre class="text">HTTP/1.1 426 Upgrade Required
     2279      <div id="status.codes">
     2280         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#status.codes">Response Status Codes</a></h1>
     2281         <p id="rfc.section.6.p.1">The status-code element is a 3-digit integer code giving the result of the attempt to understand and satisfy the request.</p>
     2282         <p id="rfc.section.6.p.2">HTTP status codes are extensible. HTTP clients are not required to understand the meaning of all registered status codes,
     2283            though such understanding is obviously desirable. However, a client <em class="bcp14">MUST</em> understand the class of any status code, as indicated by the first digit, and treat an unrecognized status code as being equivalent
     2284            to the x00 status code of that class, with the exception that a recipient <em class="bcp14">MUST NOT</em> cache a response with an unrecognized status code.
     2285         </p>
     2286         <p id="rfc.section.6.p.3">For example, if an unrecognized status code of 471 is received by a client, the client can assume that there was something
     2287            wrong with its request and treat the response as if it had received a 400 status code. The response message will usually contain
     2288            a representation that explains the status.
     2289         </p>
     2290         <p id="rfc.section.6.p.4">The first digit of the status-code defines the class of response. The last two digits do not have any categorization role.
     2291            There are 5 values for the first digit:
     2292         </p>
     2293         <ul>
     2294            <li><a href="#status.1xx" class="smpl">1xx (Informational)</a>: The request was received, continuing process
     2295            </li>
     2296            <li><a href="#status.2xx" class="smpl">2xx (Successful)</a>: The request was successfully received, understood, and accepted
     2297            </li>
     2298            <li><a href="#status.3xx" class="smpl">3xx (Redirection)</a>: Further action needs to be taken in order to complete the request
     2299            </li>
     2300            <li><a href="#status.4xx" class="smpl">4xx (Client Error)</a>: The request contains bad syntax or cannot be fulfilled
     2301            </li>
     2302            <li><a href="#status.5xx" class="smpl">5xx (Server Error)</a>: The server failed to fulfill an apparently valid request
     2303            </li>
     2304         </ul>
     2305         <div id="overview.of.status.codes">
     2306            <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a href="#overview.of.status.codes">Overview of Status Codes</a></h2>
     2307            <p id="rfc.section.6.1.p.1">The status codes listed below are defined in this specification, <a href="p4-conditional.html#status.code.definitions" title="Status Code Definitions">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>, <a href="p5-range.html#range.response" title="Responses to a Range Request">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>, and <a href="p7-auth.html#status.code.definitions" title="Status Code Definitions">Section 3</a> of <a href="#Part7" id="rfc.xref.Part7.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a>. The reason phrases listed here are only recommendations — they can be replaced by local equivalents without affecting the
     2308               protocol.
     2309            </p>
     2310            <p id="rfc.section.6.1.p.2">Responses with status codes that are defined as cacheable by default (e.g., 200, 203, 206, 300, 301, and 410 in this specification)
     2311               can be reused by a cache with heuristic expiration unless otherwise indicated by the method definition or explicit cache controls <a href="#Part6" id="rfc.xref.Part6.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>; all other status codes are not cacheable by default.
     2312            </p>
     2313            <div id="rfc.table.u.9">
     2314               <table class="tt full left" cellpadding="3" cellspacing="0">
     2315                  <thead>
     2316                     <tr>
     2317                        <th>code</th>
     2318                        <th>reason-phrase</th>
     2319                        <th>Defined in...</th>
     2320                     </tr>
     2321                  </thead>
     2322                  <tbody>
     2323                     <tr>
     2324                        <td class="left">100</td>
     2325                        <td class="left">Continue</td>
     2326                        <td class="left"><a href="#status.100" id="rfc.xref.status.100.1" title="100 Continue">Section&nbsp;6.2.1</a></td>
     2327                     </tr>
     2328                     <tr>
     2329                        <td class="left">101</td>
     2330                        <td class="left">Switching Protocols</td>
     2331                        <td class="left"><a href="#status.101" id="rfc.xref.status.101.1" title="101 Switching Protocols">Section&nbsp;6.2.2</a></td>
     2332                     </tr>
     2333                     <tr>
     2334                        <td class="left">200</td>
     2335                        <td class="left">OK</td>
     2336                        <td class="left"><a href="#status.200" id="rfc.xref.status.200.1" title="200 OK">Section&nbsp;6.3.1</a></td>
     2337                     </tr>
     2338                     <tr>
     2339                        <td class="left">201</td>
     2340                        <td class="left">Created</td>
     2341                        <td class="left"><a href="#status.201" id="rfc.xref.status.201.1" title="201 Created">Section&nbsp;6.3.2</a></td>
     2342                     </tr>
     2343                     <tr>
     2344                        <td class="left">202</td>
     2345                        <td class="left">Accepted</td>
     2346                        <td class="left"><a href="#status.202" id="rfc.xref.status.202.1" title="202 Accepted">Section&nbsp;6.3.3</a></td>
     2347                     </tr>
     2348                     <tr>
     2349                        <td class="left">203</td>
     2350                        <td class="left">Non-Authoritative Information</td>
     2351                        <td class="left"><a href="#status.203" id="rfc.xref.status.203.1" title="203 Non-Authoritative Information">Section&nbsp;6.3.4</a></td>
     2352                     </tr>
     2353                     <tr>
     2354                        <td class="left">204</td>
     2355                        <td class="left">No Content</td>
     2356                        <td class="left"><a href="#status.204" id="rfc.xref.status.204.1" title="204 No Content">Section&nbsp;6.3.5</a></td>
     2357                     </tr>
     2358                     <tr>
     2359                        <td class="left">205</td>
     2360                        <td class="left">Reset Content</td>
     2361                        <td class="left"><a href="#status.205" id="rfc.xref.status.205.1" title="205 Reset Content">Section&nbsp;6.3.6</a></td>
     2362                     </tr>
     2363                     <tr>
     2364                        <td class="left">206</td>
     2365                        <td class="left">Partial Content</td>
     2366                        <td id="status.206" class="left"><a href="p5-range.html#status.206" title="206 Partial Content">Section 4.1</a> of <a href="#Part5" id="rfc.xref.Part5.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
     2367                     </tr>
     2368                     <tr>
     2369                        <td class="left">300</td>
     2370                        <td class="left">Multiple Choices</td>
     2371                        <td class="left"><a href="#status.300" id="rfc.xref.status.300.1" title="300 Multiple Choices">Section&nbsp;6.4.1</a></td>
     2372                     </tr>
     2373                     <tr>
     2374                        <td class="left">301</td>
     2375                        <td class="left">Moved Permanently</td>
     2376                        <td class="left"><a href="#status.301" id="rfc.xref.status.301.1" title="301 Moved Permanently">Section&nbsp;6.4.2</a></td>
     2377                     </tr>
     2378                     <tr>
     2379                        <td class="left">302</td>
     2380                        <td class="left">Found</td>
     2381                        <td class="left"><a href="#status.302" id="rfc.xref.status.302.1" title="302 Found">Section&nbsp;6.4.3</a></td>
     2382                     </tr>
     2383                     <tr>
     2384                        <td class="left">303</td>
     2385                        <td class="left">See Other</td>
     2386                        <td class="left"><a href="#status.303" id="rfc.xref.status.303.1" title="303 See Other">Section&nbsp;6.4.4</a></td>
     2387                     </tr>
     2388                     <tr>
     2389                        <td class="left">304</td>
     2390                        <td class="left">Not Modified</td>
     2391                        <td id="status.304" class="left"><a href="p4-conditional.html#status.304" title="304 Not Modified">Section 4.1</a> of <a href="#Part4" id="rfc.xref.Part4.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
     2392                     </tr>
     2393                     <tr>
     2394                        <td class="left">305</td>
     2395                        <td class="left">Use Proxy</td>
     2396                        <td class="left"><a href="#status.305" id="rfc.xref.status.305.1" title="305 Use Proxy">Section&nbsp;6.4.5</a></td>
     2397                     </tr>
     2398                     <tr>
     2399                        <td class="left">307</td>
     2400                        <td class="left">Temporary Redirect</td>
     2401                        <td class="left"><a href="#status.307" id="rfc.xref.status.307.1" title="307 Temporary Redirect">Section&nbsp;6.4.7</a></td>
     2402                     </tr>
     2403                     <tr>
     2404                        <td class="left">400</td>
     2405                        <td class="left">Bad Request</td>
     2406                        <td class="left"><a href="#status.400" id="rfc.xref.status.400.1" title="400 Bad Request">Section&nbsp;6.5.1</a></td>
     2407                     </tr>
     2408                     <tr>
     2409                        <td class="left">401</td>
     2410                        <td class="left">Unauthorized</td>
     2411                        <td id="status.401" class="left"><a href="p7-auth.html#status.401" title="401 Unauthorized">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
     2412                     </tr>
     2413                     <tr>
     2414                        <td class="left">402</td>
     2415                        <td class="left">Payment Required</td>
     2416                        <td class="left"><a href="#status.402" id="rfc.xref.status.402.1" title="402 Payment Required">Section&nbsp;6.5.2</a></td>
     2417                     </tr>
     2418                     <tr>
     2419                        <td class="left">403</td>
     2420                        <td class="left">Forbidden</td>
     2421                        <td class="left"><a href="#status.403" id="rfc.xref.status.403.1" title="403 Forbidden">Section&nbsp;6.5.3</a></td>
     2422                     </tr>
     2423                     <tr>
     2424                        <td class="left">404</td>
     2425                        <td class="left">Not Found</td>
     2426                        <td class="left"><a href="#status.404" id="rfc.xref.status.404.1" title="404 Not Found">Section&nbsp;6.5.4</a></td>
     2427                     </tr>
     2428                     <tr>
     2429                        <td class="left">405</td>
     2430                        <td class="left">Method Not Allowed</td>
     2431                        <td class="left"><a href="#status.405" id="rfc.xref.status.405.1" title="405 Method Not Allowed">Section&nbsp;6.5.5</a></td>
     2432                     </tr>
     2433                     <tr>
     2434                        <td class="left">406</td>
     2435                        <td class="left">Not Acceptable</td>
     2436                        <td class="left"><a href="#status.406" id="rfc.xref.status.406.1" title="406 Not Acceptable">Section&nbsp;6.5.6</a></td>
     2437                     </tr>
     2438                     <tr>
     2439                        <td class="left">407</td>
     2440                        <td class="left">Proxy Authentication Required</td>
     2441                        <td id="status.407" class="left"><a href="p7-auth.html#status.407" title="407 Proxy Authentication Required">Section 3.2</a> of <a href="#Part7" id="rfc.xref.Part7.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[Part7]</cite></a></td>
     2442                     </tr>
     2443                     <tr>
     2444                        <td class="left">408</td>
     2445                        <td class="left">Request Time-out</td>
     2446                        <td class="left"><a href="#status.408" id="rfc.xref.status.408.1" title="408 Request Timeout">Section&nbsp;6.5.7</a></td>
     2447                     </tr>
     2448                     <tr>
     2449                        <td class="left">409</td>
     2450                        <td class="left">Conflict</td>
     2451                        <td class="left"><a href="#status.409" id="rfc.xref.status.409.1" title="409 Conflict">Section&nbsp;6.5.8</a></td>
     2452                     </tr>
     2453                     <tr>
     2454                        <td class="left">410</td>
     2455                        <td class="left">Gone</td>
     2456                        <td class="left"><a href="#status.410" id="rfc.xref.status.410.1" title="410 Gone">Section&nbsp;6.5.9</a></td>
     2457                     </tr>
     2458                     <tr>
     2459                        <td class="left">411</td>
     2460                        <td class="left">Length Required</td>
     2461                        <td class="left"><a href="#status.411" id="rfc.xref.status.411.1" title="411 Length Required">Section&nbsp;6.5.10</a></td>
     2462                     </tr>
     2463                     <tr>
     2464                        <td class="left">412</td>
     2465                        <td class="left">Precondition Failed</td>
     2466                        <td id="status.412" class="left"><a href="p4-conditional.html#status.412" title="412 Precondition Failed">Section 4.2</a> of <a href="#Part4" id="rfc.xref.Part4.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a></td>
     2467                     </tr>
     2468                     <tr>
     2469                        <td class="left">413</td>
     2470                        <td class="left">Payload Too Large</td>
     2471                        <td class="left"><a href="#status.413" id="rfc.xref.status.413.1" title="413 Payload Too Large">Section&nbsp;6.5.11</a></td>
     2472                     </tr>
     2473                     <tr>
     2474                        <td class="left">414</td>
     2475                        <td class="left">URI Too Long</td>
     2476                        <td class="left"><a href="#status.414" id="rfc.xref.status.414.1" title="414 URI Too Long">Section&nbsp;6.5.12</a></td>
     2477                     </tr>
     2478                     <tr>
     2479                        <td class="left">415</td>
     2480                        <td class="left">Unsupported Media Type</td>
     2481                        <td class="left"><a href="#status.415" id="rfc.xref.status.415.1" title="415 Unsupported Media Type">Section&nbsp;6.5.13</a></td>
     2482                     </tr>
     2483                     <tr>
     2484                        <td class="left">416</td>
     2485                        <td class="left">Range Not Satisfiable</td>
     2486                        <td id="status.416" class="left"><a href="p5-range.html#status.416" title="416 Range Not Satisfiable">Section 4.4</a> of <a href="#Part5" id="rfc.xref.Part5.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></td>
     2487                     </tr>
     2488                     <tr>
     2489                        <td class="left">417</td>
     2490                        <td class="left">Expectation Failed</td>
     2491                        <td class="left"><a href="#status.417" id="rfc.xref.status.417.1" title="417 Expectation Failed">Section&nbsp;6.5.14</a></td>
     2492                     </tr>
     2493                     <tr>
     2494                        <td class="left">426</td>
     2495                        <td class="left">Upgrade Required</td>
     2496                        <td class="left"><a href="#status.426" id="rfc.xref.status.426.1" title="426 Upgrade Required">Section&nbsp;6.5.15</a></td>
     2497                     </tr>
     2498                     <tr>
     2499                        <td class="left">500</td>
     2500                        <td class="left">Internal Server Error</td>
     2501                        <td class="left"><a href="#status.500" id="rfc.xref.status.500.1" title="500 Internal Server Error">Section&nbsp;6.6.1</a></td>
     2502                     </tr>
     2503                     <tr>
     2504                        <td class="left">501</td>
     2505                        <td class="left">Not Implemented</td>
     2506                        <td class="left"><a href="#status.501" id="rfc.xref.status.501.1" title="501 Not Implemented">Section&nbsp;6.6.2</a></td>
     2507                     </tr>
     2508                     <tr>
     2509                        <td class="left">502</td>
     2510                        <td class="left">Bad Gateway</td>
     2511                        <td class="left"><a href="#status.502" id="rfc.xref.status.502.1" title="502 Bad Gateway">Section&nbsp;6.6.3</a></td>
     2512                     </tr>
     2513                     <tr>
     2514                        <td class="left">503</td>
     2515                        <td class="left">Service Unavailable</td>
     2516                        <td class="left"><a href="#status.503" id="rfc.xref.status.503.1" title="503 Service Unavailable">Section&nbsp;6.6.4</a></td>
     2517                     </tr>
     2518                     <tr>
     2519                        <td class="left">504</td>
     2520                        <td class="left">Gateway Time-out</td>
     2521                        <td class="left"><a href="#status.504" id="rfc.xref.status.504.1" title="504 Gateway Timeout">Section&nbsp;6.6.5</a></td>
     2522                     </tr>
     2523                     <tr>
     2524                        <td class="left">505</td>
     2525                        <td class="left">HTTP Version Not Supported</td>
     2526                        <td class="left"><a href="#status.505" id="rfc.xref.status.505.1" title="505 HTTP Version Not Supported">Section&nbsp;6.6.6</a></td>
     2527                     </tr>
     2528                  </tbody>
     2529               </table>
     2530            </div>
     2531            <p id="rfc.section.6.1.p.3">Note that this list is not exhaustive — it does not include extension status codes defined in other specifications. The complete
     2532               list of status codes is maintained by IANA. See <a href="#status.code.registry" title="Status Code Registry">Section&nbsp;8.2</a> for details.
     2533            </p>
     2534         </div>
     2535         <div id="status.1xx">
     2536            <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a href="#status.1xx">Informational 1xx</a></h2>
     2537            <div id="rfc.iref.67"></div>
     2538            <div id="rfc.iref.s.3"></div>
     2539            <p id="rfc.section.6.2.p.1">The <dfn>1xx (Informational)</dfn> class of status code indicates an interim response for communicating connection status or request progress prior to completing
     2540               the requested action and sending a final response. All 1xx responses consist of only the status-line and optional header fields,
     2541               and thus are terminated by the empty line at the end of the header section. Since HTTP/1.0 did not define any 1xx status codes,
     2542               a server <em class="bcp14">MUST NOT</em> send a 1xx response to an HTTP/1.0 client.
     2543            </p>
     2544            <p id="rfc.section.6.2.p.2">A client <em class="bcp14">MUST</em> be able to parse one or more 1xx responses received prior to a final response, even if the client does not expect one. A user
     2545               agent <em class="bcp14">MAY</em> ignore unexpected 1xx responses.
     2546            </p>
     2547            <p id="rfc.section.6.2.p.3">A proxy <em class="bcp14">MUST</em> forward 1xx responses unless the proxy itself requested the generation of the 1xx response. For example, if a proxy adds an
     2548               "Expect: 100-continue" field when it forwards a request, then it need not forward the corresponding <a href="#status.100" class="smpl">100 (Continue)</a> response(s).
     2549            </p>
     2550            <div id="status.100">
     2551               <div id="rfc.iref.68"></div>
     2552               <h3 id="rfc.section.6.2.1"><a href="#rfc.section.6.2.1">6.2.1</a>&nbsp;<a href="#status.100">100 Continue</a></h3>
     2553               <p id="rfc.section.6.2.1.p.1">The <dfn>100 (Continue)</dfn> status code indicates that the initial part of a request has been received and has not yet been rejected by the server. The
     2554                  server intends to send a final response after the request has been fully received and acted upon.
     2555               </p>
     2556               <p id="rfc.section.6.2.1.p.2">When the request contains an <a href="#header.expect" class="smpl">Expect</a> header field that includes a <a href="#header.expect" class="smpl">100-continue</a> expectation, the 100 response indicates that the server wishes to receive the request payload body, as described in <a href="#header.expect" id="rfc.xref.header.expect.2" title="Expect">Section&nbsp;5.1.1</a>. The client ought to continue sending the request and discard the 100 response.
     2557               </p>
     2558               <p id="rfc.section.6.2.1.p.3">If the request did not contain an <a href="#header.expect" class="smpl">Expect</a> header field containing the <a href="#header.expect" class="smpl">100-continue</a> expectation, the client can simply discard this interim response.
     2559               </p>
     2560            </div>
     2561            <div id="status.101">
     2562               <div id="rfc.iref.68"></div>
     2563               <h3 id="rfc.section.6.2.2"><a href="#rfc.section.6.2.2">6.2.2</a>&nbsp;<a href="#status.101">101 Switching Protocols</a></h3>
     2564               <p id="rfc.section.6.2.2.p.1">The <dfn>101 (Switching Protocols)</dfn> status code indicates that the server understands and is willing to comply with the client's request, via the <a href="p1-messaging.html#header.upgrade" class="smpl">Upgrade</a> header field (<a href="p1-messaging.html#header.upgrade" title="Upgrade">Section 6.7</a> of <a href="#Part1" id="rfc.xref.Part1.23"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>), for a change in the application protocol being used on this connection. The server <em class="bcp14">MUST</em> generate an Upgrade header field in the response that indicates which protocol(s) will be switched to immediately after the
     2565                  empty line that terminates the 101 response.
     2566               </p>
     2567               <p id="rfc.section.6.2.2.p.2">It is assumed that the server will only agree to switch protocols when it is advantageous to do so. For example, switching
     2568                  to a newer version of HTTP might be advantageous over older versions, and switching to a real-time, synchronous protocol might
     2569                  be advantageous when delivering resources that use such features.
     2570               </p>
     2571            </div>
     2572         </div>
     2573         <div id="status.2xx">
     2574            <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a href="#status.2xx">Successful 2xx</a></h2>
     2575            <div id="rfc.iref.68"></div>
     2576            <div id="rfc.iref.s.4"></div>
     2577            <p id="rfc.section.6.3.p.1">The <dfn>2xx (Successful)</dfn> class of status code indicates that the client's request was successfully received, understood, and accepted.
     2578            </p>
     2579            <div id="status.200">
     2580               <div id="rfc.iref.69"></div>
     2581               <h3 id="rfc.section.6.3.1"><a href="#rfc.section.6.3.1">6.3.1</a>&nbsp;<a href="#status.200">200 OK</a></h3>
     2582               <p id="rfc.section.6.3.1.p.1">The <dfn>200 (OK)</dfn> status code indicates that the request has succeeded. The payload sent in a 200 response depends on the request method. For
     2583                  the methods defined by this specification, the intended meaning of the payload can be summarized as:
     2584               </p>
     2585               <dl>
     2586                  <dt>GET</dt>
     2587                  <dd>a representation of the <a href="#resources" class="smpl">target resource</a>;
     2588                  </dd>
     2589      &nb