Ignore:
Timestamp:
17/12/14 14:46:09 (6 years ago)
Author:
julian.reschke@…
Message:

update XSLTs, switch to Saxon 9.6 HE in Makefile, regen specs

File:
1 edited

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  • draft-ietf-httpbis/latest/p4-conditional.html

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    465        content: "Expires December 16, 2014";
     493       content: "Expires December 2014";
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    501529      <link href="p2-semantics.html" rel="prev">
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    504532      <meta name="keywords" content="Hypertext Transfer Protocol, HTTP, HTTP conditional requests">
    505533      <link rel="schema.dct" href="http://purl.org/dc/terms/">
     
    507535      <meta name="dct.creator" content="Reschke, J. F.">
    508536      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest">
    509       <meta name="dct.issued" scheme="ISO8601" content="2014-06-14">
     537      <meta name="dct.issued" scheme="ISO8601" content="2014-06">
    510538      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
    511539      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.">
     
    513541   </head>
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    515       <table class="header">
     543      <table class="header" id="rfc.headerblock">
    516544         <tbody>
    517545            <tr>
     
    533561            </tr>
    534562            <tr>
    535                <td class="left">Expires: December 16, 2014</td>
    536                <td class="right">June 14, 2014</td>
     563               <td class="left">Expires: December 2014</td>
     564               <td class="right">June 2014</td>
    537565            </tr>
    538566         </tbody>
    539567      </table>
    540       <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p>
     568      <p class="title" id="rfc.title">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p>
    541569      <p style="color: green; text-align: center; font-size: 14pt; background-color: yellow;"><b>Note:</b> a later version of this document has been published as <a href="http://svn.tools.ietf.org/svn/wg/httpbis/specs/rfc7232.html">RFC7232</a>.
    542570         
    543571      </p>
    544572      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    545       <p>The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext
    546          information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating
    547          state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a
    548          conditional request when one or more preconditions evaluate to false.
     573      <p>The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for
     574         distributed, collaborative, hypertext information systems. This document defines HTTP/1.1
     575         conditional requests, including metadata header fields for indicating state changes,
     576         request header fields for making preconditions on such state, and rules for constructing
     577         the responses to a conditional request when one or more preconditions evaluate to
     578         false.
    549579      </p>
    550580      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
    551       <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
     581      <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org),
     582         which is archived at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
    552583      </p>
    553584      <p>The current issues list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
    554585      </p>
    555       <p><em>This is a temporary document for the purpose of tracking the editorial changes made during the AUTH48 (RFC publication) phase.</em>
     586      <p><em>This is a temporary document for the purpose of tracking the editorial changes made
     587            during the AUTH48 (RFC publication) phase.</em>
    556588      </p>
    557589      <div id="rfc.status">
    558590         <h1><a href="#rfc.status">Status of This Memo</a></h1>
    559          <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
    560          <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
    561             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>.
     591         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78
     592            and BCP 79.
    562593         </p>
    563          <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
    564             documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
    565             in progress”.
     594         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF).
     595            Note that other groups may also distribute working documents as Internet-Drafts. The
     596            list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
    566597         </p>
    567          <p>This Internet-Draft will expire on December 16, 2014.</p>
     598         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated,
     599            replaced, or obsoleted by other documents at any time. It is inappropriate to use
     600            Internet-Drafts as reference material or to cite them other than as “work in progress”.
     601         </p>
     602         <p>This Internet-Draft will expire in December 2014.</p>
    568603      </div>
    569604      <div id="rfc.copyrightnotice">
    570605         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    571          <p>Copyright © 2014 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
    572          <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
    573             and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
    574             text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
    575             BSD License.
     606         <p>Copyright © 2014 IETF Trust and the persons identified as the document authors. All
     607            rights reserved.
    576608         </p>
    577          <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
    578             10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
    579             allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
    580             controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
    581             works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
    582             it into languages other than English.
     609         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating
     610            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
     611            carefully, as they describe your rights and restrictions with respect to this document.
     612            Code Components extracted from this document must include Simplified BSD License text
     613            as described in Section 4.e of the Trust Legal Provisions and are provided without
     614            warranty as described in the Simplified BSD License.
     615         </p>
     616         <p>This document may contain material from IETF Documents or IETF Contributions published
     617            or made publicly available before November 10, 2008. The person(s) controlling the
     618            copyright in some of this material may not have granted the IETF Trust the right to
     619            allow modifications of such material outside the IETF Standards Process. Without obtaining
     620            an adequate license from the person(s) controlling the copyright in such materials,
     621            this document may not be modified outside the IETF Standards Process, and derivative
     622            works of it may not be created outside the IETF Standards Process, except to format
     623            it for publication as an RFC or to translate it into languages other than English.
    583624         </p>
    584625      </div>
    585626      <hr class="noprint">
    586       <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
    587       <ul class="toc">
    588          <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
    589                <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#conformance">Conformance and Error Handling</a></li>
    590                <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
    591             </ul>
    592          </li>
    593          <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#validators">Validators</a><ul>
    594                <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
    595                <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
    596                      <li><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
    597                      <li><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
    598                   </ul>
    599                </li>
    600                <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
    601                      <li><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
    602                      <li><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
    603                      <li><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-Tags Varying on Content-Negotiated Resources</a></li>
    604                   </ul>
    605                </li>
    606                <li><a href="#rfc.section.2.4">2.4</a>&nbsp;&nbsp;&nbsp;<a href="#when.to.use.entity.tags.and.last-modified.dates">When to Use Entity-Tags and Last-Modified Dates</a></li>
    607             </ul>
    608          </li>
    609          <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#preconditions">Precondition Header Fields</a><ul>
    610                <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
    611                <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
    612                <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
    613                <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
    614                <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
    615             </ul>
    616          </li>
    617          <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
    618                <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
    619                <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
    620             </ul>
    621          </li>
    622          <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#evaluation">Evaluation</a></li>
    623          <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#precedence">Precedence</a></li>
    624          <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
    625                <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
    626                <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
    627             </ul>
    628          </li>
    629          <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
    630          <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
    631          <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
    632                <li><a href="#rfc.section.10.1">10.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
    633                <li><a href="#rfc.section.10.2">10.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
    634             </ul>
    635          </li>
    636          <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
    637          <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
    638          <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
    639          <li><a href="#rfc.index">Index</a></li>
    640          <li><a href="#rfc.authors">Authors' Addresses</a></li>
    641       </ul>
     627      <div id="rfc.toc">
     628         <h1 class="np"><a href="#rfc.toc">Table of Contents</a></h1>
     629         <ul class="toc">
     630            <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
     631                  <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#conformance">Conformance and Error Handling</a></li>
     632                  <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
     633               </ul>
     634            </li>
     635            <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#validators">Validators</a><ul>
     636                  <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
     637                  <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
     638                        <li><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
     639                        <li><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
     640                     </ul>
     641                  </li>
     642                  <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
     643                        <li><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
     644                        <li><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
     645                        <li><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-Tags Varying on Content-Negotiated Resources</a></li>
     646                     </ul>
     647                  </li>
     648                  <li><a href="#rfc.section.2.4">2.4</a>&nbsp;&nbsp;&nbsp;<a href="#when.to.use.entity.tags.and.last-modified.dates">When to Use Entity-Tags and Last-Modified Dates</a></li>
     649               </ul>
     650            </li>
     651            <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#preconditions">Precondition Header Fields</a><ul>
     652                  <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
     653                  <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
     654                  <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
     655                  <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
     656                  <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
     657               </ul>
     658            </li>
     659            <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
     660                  <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
     661                  <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
     662               </ul>
     663            </li>
     664            <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#evaluation">Evaluation</a></li>
     665            <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#precedence">Precedence</a></li>
     666            <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
     667                  <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
     668                  <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
     669               </ul>
     670            </li>
     671            <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
     672            <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
     673            <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
     674                  <li><a href="#rfc.section.10.1">10.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
     675                  <li><a href="#rfc.section.10.2">10.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
     676               </ul>
     677            </li>
     678            <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
     679            <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
     680            <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
     681            <li><a href="#rfc.index">Index</a></li>
     682            <li><a href="#rfc.authors">Authors' Addresses</a></li>
     683         </ul>
     684      </div>
    642685      <div id="introduction">
    643686         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#introduction">Introduction</a></h1>
    644          <p id="rfc.section.1.p.1">Conditional requests are HTTP requests <a href="#RFC7231" id="rfc.xref.RFC7231.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a> that include one or more header fields indicating a precondition to be tested before applying the method semantics to the
    645             target resource. This document defines the HTTP/1.1 conditional request mechanisms in terms of the architecture, syntax notation,
    646             and conformance criteria defined in <a href="#RFC7230" id="rfc.xref.RFC7230.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
    647          </p>
    648          <p id="rfc.section.1.p.2">Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#RFC7234" id="rfc.xref.RFC7234.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem:
    649             one client accidentally overwriting the work of another client that has been acting in parallel.
    650          </p>
    651          <p id="rfc.section.1.p.3"><span id="rfc.iref.s.1"></span> Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the
    652             state as observed in a previously obtained representation (one value in that set). A resource might have multiple current
    653             representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests
    654             to a "selected representation" (<a href="p2-semantics.html#representations" title="Representations">Section 3</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>) will be consistent over time if the server intends to take advantage of conditionals. Regardless, if the mapping is inconsistent
    655             and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates
    656             to false.
    657          </p>
    658          <p id="rfc.section.1.p.4">The conditional request preconditions defined by this specification (<a href="#preconditions" title="Precondition Header Fields">Section&nbsp;3</a>) are evaluated when applicable to the recipient (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>) according to their order of precedence (<a href="#precedence" title="Precedence">Section&nbsp;6</a>).
    659          </p>
     687         <div id="rfc.section.1.p.1">
     688            <p>Conditional requests are HTTP requests <a href="#RFC7231" id="rfc.xref.RFC7231.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a> that include one or more header fields indicating a precondition to be tested before
     689               applying the method semantics to the target resource. This document defines the HTTP/1.1
     690               conditional request mechanisms in terms of the architecture, syntax notation, and
     691               conformance criteria defined in <a href="#RFC7230" id="rfc.xref.RFC7230.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
     692            </p>
     693         </div>
     694         <div id="rfc.section.1.p.2">
     695            <p>Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#RFC7234" id="rfc.xref.RFC7234.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE,
     696               to prevent the "lost update" problem: one client accidentally overwriting the work
     697               of another client that has been acting in parallel.
     698            </p>
     699         </div>
     700         <div id="rfc.section.1.p.3">
     701            <p><span id="rfc.iref.s.1"></span> Conditional request preconditions are based on the state of the target resource as
     702               a whole (its current value set) or the state as observed in a previously obtained
     703               representation (one value in that set). A resource might have multiple current representations,
     704               each with its own observable state. The conditional request mechanisms assume that
     705               the mapping of requests to a "selected representation" (<a href="p2-semantics.html#representations" title="Representations">Section 3</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>) will be consistent over time if the server intends to take advantage of conditionals.
     706               Regardless, if the mapping is inconsistent and the server is unable to select the
     707               appropriate representation, then no harm will result when the precondition evaluates
     708               to false.
     709            </p>
     710         </div>
     711         <div id="rfc.section.1.p.4">
     712            <p>The conditional request preconditions defined by this specification (<a href="#preconditions" title="Precondition Header Fields">Section&nbsp;3</a>) are evaluated when applicable to the recipient (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>) according to their order of precedence (<a href="#precedence" title="Precedence">Section&nbsp;6</a>).
     713            </p>
     714         </div>
    660715         <div id="conformance">
    661716            <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>
    662             <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"
    663                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>.
    664             </p>
    665             <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="#RFC7230" id="rfc.xref.RFC7230.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
    666             </p>
     717            <div id="rfc.section.1.1.p.1">
     718               <p>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
     719                  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted
     720                  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>.
     721               </p>
     722            </div>
     723            <div id="rfc.section.1.1.p.2">
     724               <p>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="#RFC7230" id="rfc.xref.RFC7230.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
     725               </p>
     726            </div>
    667727         </div>
    668728         <div id="notation">
    669729            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
    670             <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 a list extension, defined in <a href="p1-messaging.html#abnf.extension" title="ABNF List Extension: #rule">Section 7</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates
    671                repetition). <a href="#imported.abnf" title="Imported ABNF">Appendix&nbsp;B</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;C</a> shows the collected grammar with all list operators expanded to standard ABNF notation.
    672             </p>
     730            <div id="rfc.section.1.2.p.1">
     731               <p>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 a list extension, defined in <a href="p1-messaging.html#abnf.extension" title="ABNF List Extension: #rule">Section 7</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, that allows for compact definition of comma-separated lists using a '#' operator
     732                  (similar to how the '*' operator indicates repetition). <a href="#imported.abnf" title="Imported ABNF">Appendix&nbsp;B</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;C</a> shows the collected grammar with all list operators expanded to standard ABNF notation.
     733               </p>
     734            </div>
    673735         </div>
    674736      </div>
    675737      <div id="validators">
    676          <div id="rfc.iref.m.1"></div>
    677          <div id="rfc.iref.v.1"></div>
    678738         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#validators">Validators</a></h1>
    679          <p id="rfc.section.2.p.1">This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions:
    680             modification dates (<a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;2.2</a>) and opaque entity tags (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;2.3</a>). Additional metadata that reflects resource state has been defined by various extensions of HTTP, such as Web Distributed
    681             Authoring and Versioning (WebDAV, <a href="#RFC4918" id="rfc.xref.RFC4918.1"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>), that are beyond the scope of this specification. A resource metadata value is referred to as a "<dfn>validator</dfn>" when it is used within a precondition.
    682          </p>
     739         <div id="rfc.section.2.p.1">
     740            <p>This specification defines two forms of metadata that are commonly used to observe
     741               resource state and test for preconditions: modification dates (<a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;2.2</a>) and opaque entity tags (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;2.3</a>). Additional metadata that reflects resource state has been defined by various extensions
     742               of HTTP, such as Web Distributed Authoring and Versioning (WebDAV, <a href="#RFC4918" id="rfc.xref.RFC4918.1"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>), that are beyond the scope of this specification. A resource metadata value is referred
     743               to as a "<dfn>validator</dfn>" when it is used within a precondition.
     744            </p>
     745         </div>
    683746         <div id="weak.and.strong.validators">
    684             <div id="rfc.iref.v.2"></div>
    685             <div id="rfc.iref.v.3"></div>
    686747            <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></h2>
    687             <p id="rfc.section.2.1.p.1">Validators come in two flavors: strong or weak. Weak validators are easy to generate but are far less useful for comparisons.
    688                Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently.
    689                Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator
    690                in use and imposes restrictions on when weak validators can be used as preconditions.
    691             </p>
    692             <p id="rfc.section.2.1.p.2">A "strong validator" is representation metadata that changes value whenever a change occurs to the representation data that
    693                would be observable in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET.
    694             </p>
    695             <p id="rfc.section.2.1.p.3">A strong validator might change for reasons other than a change to the representation data, such as when a semantically significant
    696                part of the representation metadata is changed (e.g., <a href="p2-semantics.html#header.content-type" class="smpl">Content-Type</a>), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate the stored
    697                responses held by remote caches and authoring tools.
    698             </p>
    699             <p id="rfc.section.2.1.p.4">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate
    700                an entry using a validator that it obtained in the distant past. A strong validator is unique across all versions of all representations
    701                associated with a particular resource over time. However, there is no implication of uniqueness across representations of
    702                different resources (i.e., the same strong validator might be in use for representations of multiple resources at the same
    703                time and does not imply that those representations are equivalent).
    704             </p>
    705             <p id="rfc.section.2.1.p.5">There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change
    706                to a representation always results in a unique node name and revision identifier being assigned before the representation
    707                is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the
    708                data is available prior to the response header fields being sent and the digest does not need to be recalculated every time
    709                a validation request is received. However, if a resource has distinct representations that differ only in their metadata,
    710                such as might occur with content negotiation over media types that happen to share the same data format, then the origin server
    711                needs to incorporate additional information in the validator to distinguish those representations.
    712             </p>
    713             <p id="rfc.section.2.1.p.6">In contrast, a "weak validator" is representation metadata that might not change for every change to the representation data.
    714                This weakness might be due to limitations in how the value is calculated, such as clock resolution, an inability to ensure
    715                uniqueness for all possible representations of the resource, or a desire of the resource owner to group representations by
    716                some self-determined set of equivalency rather than unique sequences of data. An origin server <em class="bcp14">SHOULD</em> change a weak entity-tag whenever it considers prior representations to be unacceptable as a substitute for the current representation.
    717                In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses.
    718             </p>
    719             <p id="rfc.section.2.1.p.7">For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might
    720                be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in
    721                order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server
    722                load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might
    723                be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between
    724                those modifications.
    725             </p>
    726             <p id="rfc.section.2.1.p.8">Likewise, a validator is weak if it is shared by two or more representations of a given resource at the same time, unless
    727                those representations have identical representation data. For example, if the origin server sends the same validator for a
    728                representation with a gzip content coding applied as it does for a representation with no content coding, then that validator
    729                is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation
    730                metadata, such as when two different media types are available for the same representation data.
    731             </p>
    732             <p id="rfc.section.2.1.p.9">Strong validators are usable for all conditional requests, including cache validation, partial content ranges, and "lost update"
    733                avoidance. Weak validators are only usable when the client does not require exact equality with previously obtained representation
    734                data, such as when validating a cache entry or limiting a web traversal to recent changes.
    735             </p>
     748            <div id="rfc.section.2.1.p.1">
     749               <p>Validators come in two flavors: strong or weak. Weak validators are easy to generate
     750                  but are far less useful for comparisons. Strong validators are ideal for comparisons
     751                  but can be very difficult (and occasionally impossible) to generate efficiently. Rather
     752                  than impose that all forms of resource adhere to the same strength of validator, HTTP
     753                  exposes the type of validator in use and imposes restrictions on when weak validators
     754                  can be used as preconditions.
     755               </p>
     756            </div>
     757            <div id="rfc.section.2.1.p.2">
     758               <p>A "strong validator" is representation metadata that changes value whenever a change
     759                  occurs to the representation data that would be observable in the payload body of
     760                  a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET.
     761               </p>
     762            </div>
     763            <div id="rfc.section.2.1.p.3">
     764               <p>A strong validator might change for reasons other than a change to the representation
     765                  data, such as when a semantically significant part of the representation metadata
     766                  is changed (e.g., <a href="p2-semantics.html#header.content-type" class="smpl">Content-Type</a>), but it is in the best interests of the origin server to only change the value when
     767                  it is necessary to invalidate the stored responses held by remote caches and authoring
     768                  tools.
     769               </p>
     770            </div>
     771            <div id="rfc.section.2.1.p.4">
     772               <p>Cache entries might persist for arbitrarily long periods, regardless of expiration
     773                  times. Thus, a cache might attempt to validate an entry using a validator that it
     774                  obtained in the distant past. A strong validator is unique across all versions of
     775                  all representations associated with a particular resource over time. However, there
     776                  is no implication of uniqueness across representations of different resources (i.e.,
     777                  the same strong validator might be in use for representations of multiple resources
     778                  at the same time and does not imply that those representations are equivalent).
     779               </p>
     780            </div>
     781            <div id="rfc.section.2.1.p.5">
     782               <p>There are a variety of strong validators used in practice. The best are based on strict
     783                  revision control, wherein each change to a representation always results in a unique
     784                  node name and revision identifier being assigned before the representation is made
     785                  accessible to GET. A collision-resistant hash function applied to the representation
     786                  data is also sufficient if the data is available prior to the response header fields
     787                  being sent and the digest does not need to be recalculated every time a validation
     788                  request is received. However, if a resource has distinct representations that differ
     789                  only in their metadata, such as might occur with content negotiation over media types
     790                  that happen to share the same data format, then the origin server needs to incorporate
     791                  additional information in the validator to distinguish those representations.
     792               </p>
     793            </div>
     794            <div id="rfc.section.2.1.p.6">
     795               <p>In contrast, a "weak validator" is representation metadata that might not change for
     796                  every change to the representation data. This weakness might be due to limitations
     797                  in how the value is calculated, such as clock resolution, an inability to ensure uniqueness
     798                  for all possible representations of the resource, or a desire of the resource owner
     799                  to group representations by some self-determined set of equivalency rather than unique
     800                  sequences of data. An origin server <em class="bcp14">SHOULD</em> change a weak entity-tag whenever it considers prior representations to be unacceptable
     801                  as a substitute for the current representation. In other words, a weak entity-tag
     802                  ought to change whenever the origin server wants caches to invalidate old responses.
     803               </p>
     804            </div>
     805            <div id="rfc.section.2.1.p.7">
     806               <p>For example, the representation of a weather report that changes in content every
     807                  second, based on dynamic measurements, might be grouped into sets of equivalent representations
     808                  (from the origin server's perspective) with the same weak validator in order to allow
     809                  cached representations to be valid for a reasonable period of time (perhaps adjusted
     810                  dynamically based on server load or weather quality). Likewise, a representation's
     811                  modification time, if defined with only one-second resolution, might be a weak validator
     812                  if it is possible for the representation to be modified twice during a single second
     813                  and retrieved between those modifications.
     814               </p>
     815            </div>
     816            <div id="rfc.section.2.1.p.8">
     817               <p>Likewise, a validator is weak if it is shared by two or more representations of a
     818                  given resource at the same time, unless those representations have identical representation
     819                  data. For example, if the origin server sends the same validator for a representation
     820                  with a gzip content coding applied as it does for a representation with no content
     821                  coding, then that validator is weak. However, two simultaneous representations might
     822                  share the same strong validator if they differ only in the representation metadata,
     823                  such as when two different media types are available for the same representation data.
     824               </p>
     825            </div>
     826            <div id="rfc.section.2.1.p.9">
     827               <p>Strong validators are usable for all conditional requests, including cache validation,
     828                  partial content ranges, and "lost update" avoidance. Weak validators are only usable
     829                  when the client does not require exact equality with previously obtained representation
     830                  data, such as when validating a cache entry or limiting a web traversal to recent
     831                  changes.
     832               </p>
     833            </div>
    736834         </div>
    737835         <div id="header.last-modified">
    738             <div id="rfc.iref.l.1"></div>
    739836            <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a href="#header.last-modified">Last-Modified</a></h2>
    740             <p id="rfc.section.2.2.p.1">The "Last-Modified" header field in a response provides a timestamp indicating the date and time at which the origin server
    741                believes the selected representation was last modified, as determined at the conclusion of handling the request.
    742             </p>
    743             <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.g.1"></span>  <a href="#header.last-modified" class="smpl">Last-Modified</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
    744 </pre><p id="rfc.section.2.2.p.3">An example of its use is</p>
    745             <div id="rfc.figure.u.2"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
    746 </pre><div id="lastmod.generation">
     837            <div id="rfc.section.2.2.p.1">
     838               <p>The "Last-Modified" header field in a response provides a timestamp indicating the
     839                  date and time at which the origin server believes the selected representation was
     840                  last modified, as determined at the conclusion of handling the request.
     841               </p>
     842            </div>
     843            <div id="rfc.figure.u.1"><pre class="inline"><span id="rfc.iref.g.1"></span>  <a href="#header.last-modified" class="smpl">Last-Modified</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
     844</pre></div>
     845            <div id="rfc.section.2.2.p.2">
     846               <p>An example of its use is</p>
     847            </div>
     848            <div id="rfc.figure.u.2"><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
     849</pre></div>
     850            <div id="lastmod.generation">
    747851               <h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a href="#lastmod.generation">Generation</a></h3>
    748                <p id="rfc.section.2.2.1.p.1">An origin server <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined,
    749                   since its use in conditional requests and evaluating cache freshness (<a href="#RFC7234" id="rfc.xref.RFC7234.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service
    750                   scalability and reliability.
    751                </p>
    752                <p id="rfc.section.2.2.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be
    753                   the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
    754                   detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
    755                   use its value to make conditional requests and test the validity of locally cached responses.
    756                </p>
    757                <p id="rfc.section.2.2.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the <a href="p2-semantics.html#header.date" class="smpl">Date</a> field value for its response. This allows a recipient to make an accurate assessment of the representation's modification
    758                   time, especially if the representation changes near the time that the response is generated.
    759                </p>
    760                <p id="rfc.section.2.2.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (<a href="p2-semantics.html#header.date" class="smpl">Date</a>). If the last modification time is derived from implementation-specific metadata that evaluates to some time in the future,
    761                   according to the origin server's clock, then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification date from having an adverse impact
    762                   on cache validation.
    763                </p>
    764                <p id="rfc.section.2.2.1.p.5">An origin server without a clock <em class="bcp14">MUST NOT</em> assign Last-Modified values to a response unless these values were associated with the resource by some other system or user
    765                   with a reliable clock.
    766                </p>
     852               <div id="rfc.section.2.2.1.p.1">
     853                  <p>An origin server <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date
     854                     can be reasonably and consistently determined, since its use in conditional requests
     855                     and evaluating cache freshness (<a href="#RFC7234" id="rfc.xref.RFC7234.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a
     856                     significant factor in improving service scalability and reliability.
     857                  </p>
     858               </div>
     859               <div id="rfc.section.2.2.1.p.2">
     860                  <p>A representation is typically the sum of many parts behind the resource interface.
     861                     The last-modified time would usually be the most recent time that any of those parts
     862                     were changed. How that value is determined for any given resource is an implementation
     863                     detail beyond the scope of this specification. What matters to HTTP is how recipients
     864                     of the Last-Modified header field can use its value to make conditional requests and
     865                     test the validity of locally cached responses.
     866                  </p>
     867               </div>
     868               <div id="rfc.section.2.2.1.p.3">
     869                  <p>An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time
     870                     that it generates the <a href="p2-semantics.html#header.date" class="smpl">Date</a> field value for its response. This allows a recipient to make an accurate assessment
     871                     of the representation's modification time, especially if the representation changes
     872                     near the time that the response is generated.
     873                  </p>
     874               </div>
     875               <div id="rfc.section.2.2.1.p.4">
     876                  <p>An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination
     877                     (<a href="p2-semantics.html#header.date" class="smpl">Date</a>). If the last modification time is derived from implementation-specific metadata
     878                     that evaluates to some time in the future, according to the origin server's clock,
     879                     then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification
     880                     date from having an adverse impact on cache validation.
     881                  </p>
     882               </div>
     883               <div id="rfc.section.2.2.1.p.5">
     884                  <p>An origin server without a clock <em class="bcp14">MUST NOT</em> assign Last-Modified values to a response unless these values were associated with
     885                     the resource by some other system or user with a reliable clock.
     886                  </p>
     887               </div>
    767888            </div>
    768889            <div id="lastmod.comparison">
    769890               <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a href="#lastmod.comparison">Comparison</a></h3>
    770                <p id="rfc.section.2.2.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is
    771                   strong, using the following rules:
    772                </p>
    773                <ul>
    774                   <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
    775                   <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
    776                      presented validator.
    777                   </li>
    778                </ul>
    779                <p id="rfc.section.2.2.2.p.2">or </p>
    780                <ul>
    781                   <li>The validator is about to be used by a client in an <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a>, <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a>, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field, because the client has a cache entry for the associated representation, and
    782                   </li>
    783                   <li>That cache entry includes a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value, which gives the time when the origin server sent the original response, and
    784                   </li>
    785                   <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
    786                </ul>
    787                <p id="rfc.section.2.2.2.p.3">or </p>
    788                <ul>
    789                   <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
    790                      and
    791                   </li>
    792                   <li>That cache entry includes a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value, which gives the time when the origin server sent the original response, and
    793                   </li>
    794                   <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
    795                </ul>
    796                <p id="rfc.section.2.2.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but
    797                   both had the same Last-Modified time, then at least one of those responses would have a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value equal to its Last-Modified time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified
    798                   values are generated from different clocks or at somewhat different times during the preparation of the response. An implementation <em class="bcp14">MAY</em> use a value larger than 60 seconds, if it is believed that 60 seconds is too short.
    799                </p>
     891               <div id="rfc.section.2.2.2.p.1">
     892                  <p>A Last-Modified time, when used as a validator in a request, is implicitly weak unless
     893                     it is possible to deduce that it is strong, using the following rules:
     894                  </p>
     895                  <ul>
     896                     <li>The validator is being compared by an origin server to the actual current validator
     897                        for the representation and,
     898                     </li>
     899                     <li>That origin server reliably knows that the associated representation did not change
     900                        twice during the second covered by the presented validator.
     901                     </li>
     902                  </ul>
     903               </div>
     904               <div id="rfc.section.2.2.2.p.2">
     905                  <p>or </p>
     906                  <ul>
     907                     <li>The validator is about to be used by a client in an <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a>, <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a>, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field, because the client has a cache entry for the associated representation,
     908                        and
     909                     </li>
     910                     <li>That cache entry includes a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value, which gives the time when the origin server sent the original response, and
     911                     </li>
     912                     <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
     913                  </ul>
     914               </div>
     915               <div id="rfc.section.2.2.2.p.3">
     916                  <p>or </p>
     917                  <ul>
     918                     <li>The validator is being compared by an intermediate cache to the validator stored in
     919                        its cache entry for the representation, and
     920                     </li>
     921                     <li>That cache entry includes a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value, which gives the time when the origin server sent the original response, and
     922                     </li>
     923                     <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
     924                  </ul>
     925               </div>
     926               <div id="rfc.section.2.2.2.p.4">
     927                  <p>This method relies on the fact that if two different responses were sent by the origin
     928                     server during the same second, but both had the same Last-Modified time, then at least
     929                     one of those responses would have a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value equal to its Last-Modified time. The arbitrary 60-second limit guards against
     930                     the possibility that the Date and Last-Modified values are generated from different
     931                     clocks or at somewhat different times during the preparation of the response. An implementation <em class="bcp14">MAY</em> use a value larger than 60 seconds, if it is believed that 60 seconds is too short.
     932                  </p>
     933               </div>
    800934            </div>
    801935         </div>
    802936         <div id="header.etag">
    803             <div id="rfc.iref.e.1"></div>
    804937            <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a href="#header.etag">ETag</a></h2>
    805             <p id="rfc.section.2.3.p.1">The "ETag" header field in a response provides the current entity-tag for the selected representation, as determined at the
    806                conclusion of handling the request. An entity-tag is an opaque validator for differentiating between multiple representations
    807                of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content
    808                negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque
    809                quoted string, possibly prefixed by a weakness indicator.
    810             </p>
    811             <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#header.etag" class="smpl">ETag</a>       = <a href="#header.etag" class="smpl">entity-tag</a>
     938            <div id="rfc.section.2.3.p.1">
     939               <p>The "ETag" header field in a response provides the current entity-tag for the selected
     940                  representation, as determined at the conclusion of handling the request. An entity-tag
     941                  is an opaque validator for differentiating between multiple representations of the
     942                  same resource, regardless of whether those multiple representations are due to resource
     943                  state changes over time, content negotiation resulting in multiple representations
     944                  being valid at the same time, or both. An entity-tag consists of an opaque quoted
     945                  string, possibly prefixed by a weakness indicator.
     946               </p>
     947            </div>
     948            <div id="rfc.figure.u.3"><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#header.etag" class="smpl">ETag</a>       = <a href="#header.etag" class="smpl">entity-tag</a>
    812949
    813950  <a href="#header.etag" class="smpl">entity-tag</a> = [ <a href="#header.etag" class="smpl">weak</a> ] <a href="#header.etag" class="smpl">opaque-tag</a>
     
    816953  <a href="#header.etag" class="smpl">etagc</a>      = %x21 / %x23-7E / <a href="#imported.abnf" class="smpl">obs-text</a>
    817954             ; <a href="#imported.abnf" class="smpl">VCHAR</a> except double quotes, plus obs-text
    818 </pre><div class="note" id="rfc.section.2.3.p.3">
    819                <p><b>Note:</b> Previously, opaque-tag was defined to be a quoted-string (<a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, <a href="https://tools.ietf.org/html/rfc2616#section-3.11">Section 3.11</a>); thus, some recipients might perform backslash unescaping. Servers therefore ought to avoid backslash characters in entity
    820                   tags.
    821                </p>
    822             </div>
    823             <p id="rfc.section.2.3.p.4">An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store
    824                modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
    825                not consistently maintained.
    826             </p>
    827             <div id="rfc.figure.u.4"></div>
    828             <p>Examples:</p><pre class="text">  ETag: "xyzzy"
     955</pre></div>
     956            <div class="note">
     957               <div id="rfc.section.2.3.p.2">
     958                  <p><b>Note:</b> Previously, opaque-tag was defined to be a quoted-string (<a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, <a href="https://tools.ietf.org/html/rfc2616#section-3.11">Section 3.11</a>); thus, some recipients might perform backslash unescaping. Servers therefore ought
     959                     to avoid backslash characters in entity tags.
     960                  </p>
     961               </div>
     962            </div>
     963            <div id="rfc.section.2.3.p.3">
     964               <p>An entity-tag can be more reliable for validation than a modification date in situations
     965                  where it is inconvenient to store modification dates, where the one-second resolution
     966                  of HTTP date values is not sufficient, or where modification dates are not consistently
     967                  maintained.
     968               </p>
     969            </div>
     970            <div id="rfc.figure.u.4">
     971               <p>Examples:</p><pre class="text">  ETag: "xyzzy"
    829972  ETag: W/"xyzzy"
    830973  ETag: ""
    831 </pre><p id="rfc.section.2.3.p.6">An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag
    832                for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator
    833                (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section&nbsp;2.1</a>), then the origin server <em class="bcp14">MUST</em> mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive).
    834             </p>
     974</pre></div>
     975            <div id="rfc.section.2.3.p.4">
     976               <p>An entity-tag can be either a weak or strong validator, with strong being the default.
     977                  If an origin server provides an entity-tag for a representation and the generation
     978                  of that entity-tag does not satisfy all of the characteristics of a strong validator
     979                  (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section&nbsp;2.1</a>), then the origin server <em class="bcp14">MUST</em> mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive).
     980               </p>
     981            </div>
    835982            <div id="entity.tag.generation">
    836983               <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a href="#entity.tag.generation">Generation</a></h3>
    837                <p id="rfc.section.2.3.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select
    838                   the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
    839                   sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
    840                   entity-tag is constructed.
    841                </p>
    842                <p id="rfc.section.2.3.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision
    843                   number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
    844                   Other implementations might use a collision-resistant hash of representation content, a combination of various file attributes,
    845                   or a modification timestamp that has sub-second resolution.
    846                </p>
    847                <p id="rfc.section.2.3.1.p.3">An origin server <em class="bcp14">SHOULD</em> send an ETag for any selected representation for which detection of changes can be reasonably and consistently determined,
    848                   since the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#RFC7234" id="rfc.xref.RFC7234.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability
    849                   and reliability.
    850                </p>
     984               <div id="rfc.section.2.3.1.p.1">
     985                  <p>The principle behind entity-tags is that only the service author knows the implementation
     986                     of a resource well enough to select the most accurate and efficient validation mechanism
     987                     for that resource, and that any such mechanism can be mapped to a simple sequence
     988                     of octets for easy comparison. Since the value is opaque, there is no need for the
     989                     client to be aware of how each entity-tag is constructed.
     990                  </p>
     991               </div>
     992               <div id="rfc.section.2.3.1.p.2">
     993                  <p>For example, a resource that has implementation-specific versioning applied to all
     994                     changes might use an internal revision number, perhaps combined with a variance identifier
     995                     for content negotiation, to accurately differentiate between representations. Other
     996                     implementations might use a collision-resistant hash of representation content, a
     997                     combination of various file attributes, or a modification timestamp that has sub-second
     998                     resolution.
     999                  </p>
     1000               </div>
     1001               <div id="rfc.section.2.3.1.p.3">
     1002                  <p>An origin server <em class="bcp14">SHOULD</em> send an ETag for any selected representation for which detection of changes can be
     1003                     reasonably and consistently determined, since the entity-tag's use in conditional
     1004                     requests and evaluating cache freshness (<a href="#RFC7234" id="rfc.xref.RFC7234.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant
     1005                     factor in improving service scalability and reliability.
     1006                  </p>
     1007               </div>
    8511008            </div>
    8521009            <div id="entity.tag.comparison">
    8531010               <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a href="#entity.tag.comparison">Comparison</a></h3>
    854                <p id="rfc.section.2.3.2.p.1">There are two entity-tag comparison functions, depending on whether or not the comparison context allows the use of weak validators: </p>
    855                <ul>
    856                   <li><dfn>Strong comparison</dfn>: two entity-tags are equivalent if both are not weak and their opaque-tags match character-by-character.
    857                   </li>
    858                   <li><dfn>Weak comparison</dfn>: two entity-tags are equivalent if their opaque-tags match character-by-character, regardless of either or both being tagged
    859                      as "weak".
    860                   </li>
    861                </ul>
    862                <p id="rfc.section.2.3.2.p.2">The example below shows the results for a set of entity-tag pairs and both the weak and strong comparison function results:</p>
     1011               <div id="rfc.section.2.3.2.p.1">
     1012                  <p>There are two entity-tag comparison functions, depending on whether or not the comparison
     1013                     context allows the use of weak validators:
     1014                  </p>
     1015                  <ul>
     1016                     <li><dfn>Strong comparison</dfn>: two entity-tags are equivalent if both are not weak and their opaque-tags match
     1017                        character-by-character.
     1018                     </li>
     1019                     <li><dfn>Weak comparison</dfn>: two entity-tags are equivalent if their opaque-tags match character-by-character,
     1020                        regardless of either or both being tagged as "weak".
     1021                     </li>
     1022                  </ul>
     1023               </div>
     1024               <div id="rfc.section.2.3.2.p.2">
     1025                  <p>The example below shows the results for a set of entity-tag pairs and both the weak
     1026                     and strong comparison function results:
     1027                  </p>
     1028               </div>
    8631029               <div id="rfc.table.u.1">
    8641030                  <table class="tt full left" cellpadding="3" cellspacing="0">
     
    9021068            <div id="example.entity.tag.vs.conneg">
    9031069               <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-Tags Varying on Content-Negotiated Resources</a></h3>
    904                <p id="rfc.section.2.3.3.p.1">Consider a resource that is subject to content negotiation (<a href="p2-semantics.html#content.negotiation" title="Content Negotiation">Section 3.4</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>), and where the representations sent in response to a GET request vary based on the <a href="p2-semantics.html#header.accept-encoding" class="smpl">Accept-Encoding</a> request header field (<a href="p2-semantics.html#header.accept-encoding" title="Accept-Encoding">Section 5.3.4</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>):
    905                </p>
    906                <div id="rfc.figure.u.5"></div>
    907                <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
     1070               <div id="rfc.section.2.3.3.p.1">
     1071                  <p>Consider a resource that is subject to content negotiation (<a href="p2-semantics.html#content.negotiation" title="Content Negotiation">Section 3.4</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>), and where the representations sent in response to a GET request vary based on the <a href="p2-semantics.html#header.accept-encoding" class="smpl">Accept-Encoding</a> request header field (<a href="p2-semantics.html#header.accept-encoding" title="Accept-Encoding">Section 5.3.4</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>):
     1072                  </p>
     1073               </div>
     1074               <div id="rfc.figure.u.5">
     1075                  <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
    9081076Host: www.example.com
    9091077Accept-Encoding: gzip
    9101078
    911 </pre><p id="rfc.section.2.3.3.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p>
    912                <div id="rfc.figure.u.6"></div>
    913                <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
     1079</pre></div>
     1080               <div id="rfc.section.2.3.3.p.2">
     1081                  <p>In this case, the response might or might not use the gzip content coding. If it does
     1082                     not, the response might look like:
     1083                  </p>
     1084               </div>
     1085               <div id="rfc.figure.u.6">
     1086                  <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
    9141087Date: Fri, 26 Mar 2010 00:05:00 GMT
    9151088ETag: "123-a"
     
    9231096Hello World!
    9241097Hello World!
    925 </span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p>
    926                <div id="rfc.figure.u.7"></div>
    927                <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
     1098</span></pre></div>
     1099               <div id="rfc.section.2.3.3.p.3">
     1100                  <p>An alternative representation that does use gzip content coding would be:</p>
     1101               </div>
     1102               <div id="rfc.figure.u.7">
     1103                  <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
    9281104Date: Fri, 26 Mar 2010 00:05:00 GMT
    9291105ETag: "123-b"
     
    9331109Content-Encoding: gzip
    9341110
    935 <em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7">
    936                   <p><b>Note:</b> Content codings are a property of the representation data, so a strong entity-tag for a content-encoded representation has
    937                      to be distinct from the entity tag of an unencoded representation to prevent potential conflicts during cache updates and
    938                      range requests. In contrast, transfer codings (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 4</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>) apply only during message transfer and do not result in distinct entity-tags.
    939                   </p>
     1111<em>...binary data...</em></pre></div>
     1112               <div class="note">
     1113                  <div id="rfc.section.2.3.3.p.4">
     1114                     <p><b>Note:</b> Content codings are a property of the representation data, so a strong entity-tag
     1115                        for a content-encoded representation has to be distinct from the entity tag of an
     1116                        unencoded representation to prevent potential conflicts during cache updates and range
     1117                        requests. In contrast, transfer codings (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 4</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>) apply only during message transfer and do not result in distinct entity-tags.
     1118                     </p>
     1119                  </div>
    9401120               </div>
    9411121            </div>
     
    9431123         <div id="when.to.use.entity.tags.and.last-modified.dates">
    9441124            <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a href="#when.to.use.entity.tags.and.last-modified.dates">When to Use Entity-Tags and Last-Modified Dates</a></h2>
    945             <p id="rfc.section.2.4.p.1">In <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> responses to GET or HEAD, an origin server:
    946             </p>
    947             <ul>
    948                <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
    949                </li>
    950                <li><em class="bcp14">MAY</em> send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags,
    951                   or if it is unfeasible to send a strong entity-tag.
    952                </li>
    953                <li><em class="bcp14">SHOULD</em> send a <a href="#header.last-modified" class="smpl">Last-Modified</a> value if it is feasible to send one.
    954                </li>
    955             </ul>
    956             <p id="rfc.section.2.4.p.2">In other words, the preferred behavior for an origin server is to send both a strong entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value in successful responses to a retrieval request.
    957             </p>
    958             <p id="rfc.section.2.4.p.3">A client: </p>
    959             <ul>
    960                <li><em class="bcp14">MUST</em> send that entity-tag in any cache validation request (using <a href="#header.if-match" class="smpl">If-Match</a> or <a href="#header.if-none-match" class="smpl">If-None-Match</a>) if an entity-tag has been provided by the origin server.
    961                </li>
    962                <li><em class="bcp14">SHOULD</em> send the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in non-subrange cache validation requests (using <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a>) if only a Last-Modified value has been provided by the origin server.
    963                </li>
    964                <li><em class="bcp14">MAY</em> send the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in subrange cache validation requests (using <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a>) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty.
    965                </li>
    966                <li><em class="bcp14">SHOULD</em> send both validators in cache validation requests if both an entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
    967                </li>
    968             </ul>
     1125            <div id="rfc.section.2.4.p.1">
     1126               <p>In <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> responses to GET or HEAD, an origin server:
     1127               </p>
     1128               <ul>
     1129                  <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
     1130                  </li>
     1131                  <li><em class="bcp14">MAY</em> send a weak entity-tag instead of a strong entity-tag, if performance considerations
     1132                     support the use of weak entity-tags, or if it is unfeasible to send a strong entity-tag.
     1133                  </li>
     1134                  <li><em class="bcp14">SHOULD</em> send a <a href="#header.last-modified" class="smpl">Last-Modified</a> value if it is feasible to send one.
     1135                  </li>
     1136               </ul>
     1137            </div>
     1138            <div id="rfc.section.2.4.p.2">
     1139               <p>In other words, the preferred behavior for an origin server is to send both a strong
     1140                  entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value in successful responses to a retrieval request.
     1141               </p>
     1142            </div>
     1143            <div id="rfc.section.2.4.p.3">
     1144               <p>A client: </p>
     1145               <ul>
     1146                  <li><em class="bcp14">MUST</em> send that entity-tag in any cache validation request (using <a href="#header.if-match" class="smpl">If-Match</a> or <a href="#header.if-none-match" class="smpl">If-None-Match</a>) if an entity-tag has been provided by the origin server.
     1147                  </li>
     1148                  <li><em class="bcp14">SHOULD</em> send the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in non-subrange cache validation requests (using <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a>) if only a Last-Modified value has been provided by the origin server.
     1149                  </li>
     1150                  <li><em class="bcp14">MAY</em> send the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in subrange cache validation requests (using <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a>) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The
     1151                     user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty.
     1152                  </li>
     1153                  <li><em class="bcp14">SHOULD</em> send both validators in cache validation requests if both an entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1
     1154                     caches to respond appropriately.
     1155                  </li>
     1156               </ul>
     1157            </div>
    9691158         </div>
    9701159      </div>
    9711160      <div id="preconditions">
    9721161         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#preconditions">Precondition Header Fields</a></h1>
    973          <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests. <a href="#evaluation" title="Evaluation">Section&nbsp;5</a> defines when the preconditions are applied. <a href="#precedence" title="Precedence">Section&nbsp;6</a> defines the order of evaluation when more than one precondition is present.
    974          </p>
     1162         <div id="rfc.section.3.p.1">
     1163            <p>This section defines the syntax and semantics of HTTP/1.1 header fields for applying
     1164               preconditions on requests. <a href="#evaluation" title="Evaluation">Section&nbsp;5</a> defines when the preconditions are applied. <a href="#precedence" title="Precedence">Section&nbsp;6</a> defines the order of evaluation when more than one precondition is present.
     1165            </p>
     1166         </div>
    9751167         <div id="header.if-match">
    976             <div id="rfc.iref.i.1"></div>
    9771168            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#header.if-match">If-Match</a></h2>
    978             <p id="rfc.section.3.1.p.1">The "If-Match" header field makes the request method conditional on the recipient origin server either having at least one
    979                current representation of the target resource, when the field-value is "*", or having a current representation of the target
    980                resource that has an entity-tag matching a member of the list of entity-tags provided in the field-value.
    981             </p>
    982             <p id="rfc.section.3.1.p.2">An origin server <em class="bcp14">MUST</em> use the strong comparison function when comparing entity-tags for If-Match (<a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>), since the client intends this precondition to prevent the method from being applied if there have been any changes to the
    983                representation data.
    984             </p>
    985             <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#header.if-match" class="smpl">If-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
    986 </pre><p id="rfc.section.3.1.p.4">Examples:</p>
    987             <div id="rfc.figure.u.9"></div><pre class="text">  If-Match: "xyzzy"
     1169            <div id="rfc.section.3.1.p.1">
     1170               <p>The "If-Match" header field makes the request method conditional on the recipient
     1171                  origin server either having at least one current representation of the target resource,
     1172                  when the field-value is "*", or having a current representation of the target resource
     1173                  that has an entity-tag matching a member of the list of entity-tags provided in the
     1174                  field-value.
     1175               </p>
     1176            </div>
     1177            <div id="rfc.section.3.1.p.2">
     1178               <p>An origin server <em class="bcp14">MUST</em> use the strong comparison function when comparing entity-tags for If-Match (<a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>), since the client intends this precondition to prevent the method from being applied
     1179                  if there have been any changes to the representation data.
     1180               </p>
     1181            </div>
     1182            <div id="rfc.figure.u.8"><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#header.if-match" class="smpl">If-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
     1183</pre></div>
     1184            <div id="rfc.section.3.1.p.3">
     1185               <p>Examples:</p>
     1186            </div>
     1187            <div id="rfc.figure.u.9"><pre class="text">  If-Match: "xyzzy"
    9881188  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
    9891189  If-Match: *
    990 </pre><p id="rfc.section.3.1.p.6">If-Match is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple
    991                user agents might be acting in parallel on the same resource (i.e., to prevent the "lost update" problem). It can also be
    992                used with safe methods to abort a request if the <a href="p2-semantics.html#representations" class="smpl">selected representation</a> does not match one already stored (or partially stored) from a prior request.
    993             </p>
    994             <p id="rfc.section.3.1.p.7">An origin server that receives an If-Match header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). If the field-value is "*", the condition is false if the origin server does not have a current representation for the target
    995                resource. If the field-value is a list of entity-tags, the condition is false if none of the listed tags match the entity-tag
    996                of the selected representation.
    997             </p>
    998             <p id="rfc.section.3.1.p.8">An origin server <em class="bcp14">MUST NOT</em> perform the requested method if a received If-Match condition evaluates to false; instead, the origin server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code or b) one of the <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> status codes if the origin server has verified that a state change is being requested and the final state is already reflected
    999                in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user
    1000                agent might not be aware of it, perhaps because the prior response was lost or a compatible change was made by some other
    1001                user agent). In the latter case, the origin server <em class="bcp14">MUST NOT</em> send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior
    1002                change made by the same user agent.
    1003             </p>
    1004             <p id="rfc.section.3.1.p.9">The If-Match header field can be ignored by caches and intermediaries because it is not applicable to a stored response.</p>
     1190</pre></div>
     1191            <div id="rfc.section.3.1.p.4">
     1192               <p>If-Match is most often used with state-changing methods (e.g., POST, PUT, DELETE)
     1193                  to prevent accidental overwrites when multiple user agents might be acting in parallel
     1194                  on the same resource (i.e., to prevent the "lost update" problem). It can also be
     1195                  used with safe methods to abort a request if the <a href="p2-semantics.html#representations" class="smpl">selected representation</a> does not match one already stored (or partially stored) from a prior request.
     1196               </p>
     1197            </div>
     1198            <div id="rfc.section.3.1.p.5">
     1199               <p>An origin server that receives an If-Match header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). If the field-value is "*", the condition is false if the origin server does not
     1200                  have a current representation for the target resource. If the field-value is a list
     1201                  of entity-tags, the condition is false if none of the listed tags match the entity-tag
     1202                  of the selected representation.
     1203               </p>
     1204            </div>
     1205            <div id="rfc.section.3.1.p.6">
     1206               <p>An origin server <em class="bcp14">MUST NOT</em> perform the requested method if a received If-Match condition evaluates to false;
     1207                  instead, the origin server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code or b) one of the <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> status codes if the origin server has verified that a state change is being requested
     1208                  and the final state is already reflected in the current state of the target resource
     1209                  (i.e., the change requested by the user agent has already succeeded, but the user
     1210                  agent might not be aware of it, perhaps because the prior response was lost or a compatible
     1211                  change was made by some other user agent). In the latter case, the origin server <em class="bcp14">MUST NOT</em> send a validator header field in the response unless it can verify that the request
     1212                  is a duplicate of an immediately prior change made by the same user agent.
     1213               </p>
     1214            </div>
     1215            <div id="rfc.section.3.1.p.7">
     1216               <p>The If-Match header field can be ignored by caches and intermediaries because it is
     1217                  not applicable to a stored response.
     1218               </p>
     1219            </div>
    10051220         </div>
    10061221         <div id="header.if-none-match">
    1007             <div id="rfc.iref.i.2"></div>
    10081222            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#header.if-none-match">If-None-Match</a></h2>
    1009             <p id="rfc.section.3.2.p.1">The "If-None-Match" header field makes the request method conditional on a recipient cache or origin server either not having
    1010                any current representation of the target resource, when the field-value is "*", or having a selected representation with an
    1011                entity-tag that does not match any of those listed in the field-value.
    1012             </p>
    1013             <p id="rfc.section.3.2.p.2">A recipient <em class="bcp14">MUST</em> use the weak comparison function when comparing entity-tags for If-None-Match (<a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>), since weak entity-tags can be used for cache validation even if there have been changes to the representation data.
    1014             </p>
    1015             <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
    1016 </pre><p id="rfc.section.3.2.p.4">Examples:</p>
    1017             <div id="rfc.figure.u.11"></div><pre class="text">  If-None-Match: "xyzzy"
     1223            <div id="rfc.section.3.2.p.1">
     1224               <p>The "If-None-Match" header field makes the request method conditional on a recipient
     1225                  cache or origin server either not having any current representation of the target
     1226                  resource, when the field-value is "*", or having a selected representation with an
     1227                  entity-tag that does not match any of those listed in the field-value.
     1228               </p>
     1229            </div>
     1230            <div id="rfc.section.3.2.p.2">
     1231               <p>A recipient <em class="bcp14">MUST</em> use the weak comparison function when comparing entity-tags for If-None-Match (<a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>), since weak entity-tags can be used for cache validation even if there have been
     1232                  changes to the representation data.
     1233               </p>
     1234            </div>
     1235            <div id="rfc.figure.u.10"><pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
     1236</pre></div>
     1237            <div id="rfc.section.3.2.p.3">
     1238               <p>Examples:</p>
     1239            </div>
     1240            <div id="rfc.figure.u.11"><pre class="text">  If-None-Match: "xyzzy"
    10181241  If-None-Match: W/"xyzzy"
    10191242  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
    10201243  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
    10211244  If-None-Match: *
    1022 </pre><p id="rfc.section.3.2.p.6">If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum
    1023                amount of transaction overhead. When a client desires to update one or more stored responses that have entity-tags, the client <em class="bcp14">SHOULD</em> generate an If-None-Match header field containing a list of those entity-tags when making a GET request; this allows recipient
    1024                servers to send a <a href="#status.304" class="smpl">304 (Not Modified)</a> response to indicate when one of those stored responses matches the selected representation.
    1025             </p>
    1026             <p id="rfc.section.3.2.p.7">If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying
    1027                an existing representation of the target resource when the client believes that the resource does not have a current representation
    1028                (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>). This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial
    1029                representation for the target resource.
    1030             </p>
    1031             <p id="rfc.section.3.2.p.8">An origin server that receives an If-None-Match header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). If the field-value is "*", the condition is false if the origin server has a current representation for the target resource.
    1032                If the field-value is a list of entity-tags, the condition is false if one of the listed tags match the entity-tag of the
    1033                selected representation.
    1034             </p>
    1035             <p id="rfc.section.3.2.p.9">An origin server <em class="bcp14">MUST NOT</em> perform the requested method if the condition evaluates to false; instead, the origin server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.304" class="smpl">304 (Not Modified)</a> status code if the request method is GET or HEAD or b) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code for all other request methods.
    1036             </p>
    1037             <p id="rfc.section.3.2.p.10">Requirements on cache handling of a received If-None-Match header field are defined in <a href="p6-cache.html#validation.received" title="Handling a Received Validation Request">Section 4.3.2</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>.
    1038             </p>
     1245</pre></div>
     1246            <div id="rfc.section.3.2.p.4">
     1247               <p>If-None-Match is primarily used in conditional GET requests to enable efficient updates
     1248                  of cached information with a minimum amount of transaction overhead. When a client
     1249                  desires to update one or more stored responses that have entity-tags, the client <em class="bcp14">SHOULD</em> generate an If-None-Match header field containing a list of those entity-tags when
     1250                  making a GET request; this allows recipient servers to send a <a href="#status.304" class="smpl">304 (Not Modified)</a> response to indicate when one of those stored responses matches the selected representation.
     1251               </p>
     1252            </div>
     1253            <div id="rfc.section.3.2.p.5">
     1254               <p>If-None-Match can also be used with a value of "*" to prevent an unsafe request method
     1255                  (e.g., PUT) from inadvertently modifying an existing representation of the target
     1256                  resource when the client believes that the resource does not have a current representation
     1257                  (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>). This is a variation on the "lost update" problem that might arise if more than
     1258                  one client attempts to create an initial representation for the target resource.
     1259               </p>
     1260            </div>
     1261            <div id="rfc.section.3.2.p.6">
     1262               <p>An origin server that receives an If-None-Match header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). If the field-value is "*", the condition is false if the origin server has a current
     1263                  representation for the target resource. If the field-value is a list of entity-tags,
     1264                  the condition is false if one of the listed tags match the entity-tag of the selected
     1265                  representation.
     1266               </p>
     1267            </div>
     1268            <div id="rfc.section.3.2.p.7">
     1269               <p>An origin server <em class="bcp14">MUST NOT</em> perform the requested method if the condition evaluates to false; instead, the origin
     1270                  server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.304" class="smpl">304 (Not Modified)</a> status code if the request method is GET or HEAD or b) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code for all other request methods.
     1271               </p>
     1272            </div>
     1273            <div id="rfc.section.3.2.p.8">
     1274               <p>Requirements on cache handling of a received If-None-Match header field are defined
     1275                  in <a href="p6-cache.html#validation.received" title="Handling a Received Validation Request">Section 4.3.2</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>.
     1276               </p>
     1277            </div>
    10391278         </div>
    10401279         <div id="header.if-modified-since">
    1041             <div id="rfc.iref.i.3"></div>
    10421280            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></h2>
    1043             <p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field makes a GET or HEAD request method conditional on the selected representation's modification
    1044                date being more recent than the date provided in the field-value. Transfer of the selected representation's data is avoided
    1045                if that data has not changed.
    1046             </p>
    1047             <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.9"></span>  <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
    1048 </pre><p id="rfc.section.3.3.p.3">An example of the field is:</p>
    1049             <div id="rfc.figure.u.13"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
    1050 </pre><p id="rfc.section.3.3.p.5">A recipient <em class="bcp14">MUST</em> ignore If-Modified-Since if the request contains an <a href="#header.if-none-match" class="smpl">If-None-Match</a> header field; the condition in <a href="#header.if-none-match" class="smpl">If-None-Match</a> is considered to be a more accurate replacement for the condition in If-Modified-Since, and the two are only combined for
    1051                the sake of interoperating with older intermediaries that might not implement <a href="#header.if-none-match" class="smpl">If-None-Match</a>.
    1052             </p>
    1053             <p id="rfc.section.3.3.p.6">A recipient <em class="bcp14">MUST</em> ignore the If-Modified-Since header field if the received field-value is not a valid HTTP-date, or if the request method is
    1054                neither GET nor HEAD.
    1055             </p>
    1056             <p id="rfc.section.3.3.p.7">A recipient <em class="bcp14">MUST</em> interpret an If-Modified-Since field-value's timestamp in terms of the origin server's clock.
    1057             </p>
    1058             <p id="rfc.section.3.3.p.8">If-Modified-Since is typically used for two distinct purposes: 1) to allow efficient updates of a cached representation that
    1059                does not have an entity-tag and 2) to limit the scope of a web traversal to resources that have recently changed.
    1060             </p>
    1061             <p id="rfc.section.3.3.p.9">When used for cache updates, a cache will typically use the value of the cached message's <a href="#header.last-modified" class="smpl">Last-Modified</a> field to generate the field value of If-Modified-Since. This behavior is most interoperable for cases where clocks are poorly
    1062                synchronized or when the server has chosen to only honor exact timestamp matches (due to a problem with Last-Modified dates
    1063                that appear to go "back in time" when the origin server's clock is corrected or a representation is restored from an archived
    1064                backup). However, caches occasionally generate the field value based on other data, such as the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field of the cached message or the local clock time that the message was received, particularly when the cached message
    1065                does not contain a <a href="#header.last-modified" class="smpl">Last-Modified</a> field.
    1066             </p>
    1067             <p id="rfc.section.3.3.p.10">When used for limiting the scope of retrieval to a recent time window, a user agent will generate an If-Modified-Since field
    1068                value based on either its own local clock or a <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field received from the server in a prior response. Origin servers that choose an exact timestamp match based on the
    1069                selected representation's <a href="#header.last-modified" class="smpl">Last-Modified</a> field will not be able to help the user agent limit its data transfers to only those changed during the specified window.
    1070             </p>
    1071             <p id="rfc.section.3.3.p.11">An origin server that receives an If-Modified-Since header field <em class="bcp14">SHOULD</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). The origin server <em class="bcp14">SHOULD NOT</em> perform the requested method if the selected representation's last modification date is earlier than or equal to the date
    1072                provided in the field-value; instead, the origin server <em class="bcp14">SHOULD</em> generate a <a href="#status.304" class="smpl">304 (Not Modified)</a> response, including only those metadata that are useful for identifying or updating a previously cached response.
    1073             </p>
    1074             <p id="rfc.section.3.3.p.12">Requirements on cache handling of a received If-Modified-Since header field are defined in <a href="p6-cache.html#validation.received" title="Handling a Received Validation Request">Section 4.3.2</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>.
    1075             </p>
     1281            <div id="rfc.section.3.3.p.1">
     1282               <p>The "If-Modified-Since" header field makes a GET or HEAD request method conditional
     1283                  on the selected representation's modification date being more recent than the date
     1284                  provided in the field-value. Transfer of the selected representation's data is avoided
     1285                  if that data has not changed.
     1286               </p>
     1287            </div>
     1288            <div id="rfc.figure.u.12"><pre class="inline"><span id="rfc.iref.g.9"></span>  <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
     1289</pre></div>
     1290            <div id="rfc.section.3.3.p.2">
     1291               <p>An example of the field is:</p>
     1292            </div>
     1293            <div id="rfc.figure.u.13"><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
     1294</pre></div>
     1295            <div id="rfc.section.3.3.p.3">
     1296               <p>A recipient <em class="bcp14">MUST</em> ignore If-Modified-Since if the request contains an <a href="#header.if-none-match" class="smpl">If-None-Match</a> header field; the condition in <a href="#header.if-none-match" class="smpl">If-None-Match</a> is considered to be a more accurate replacement for the condition in If-Modified-Since,
     1297                  and the two are only combined for the sake of interoperating with older intermediaries
     1298                  that might not implement <a href="#header.if-none-match" class="smpl">If-None-Match</a>.
     1299               </p>
     1300            </div>
     1301            <div id="rfc.section.3.3.p.4">
     1302               <p>A recipient <em class="bcp14">MUST</em> ignore the If-Modified-Since header field if the received field-value is not a valid
     1303                  HTTP-date, or if the request method is neither GET nor HEAD.
     1304               </p>
     1305            </div>
     1306            <div id="rfc.section.3.3.p.5">
     1307               <p>A recipient <em class="bcp14">MUST</em> interpret an If-Modified-Since field-value's timestamp in terms of the origin server's
     1308                  clock.
     1309               </p>
     1310            </div>
     1311            <div id="rfc.section.3.3.p.6">
     1312               <p>If-Modified-Since is typically used for two distinct purposes: 1) to allow efficient
     1313                  updates of a cached representation that does not have an entity-tag and 2) to limit
     1314                  the scope of a web traversal to resources that have recently changed.
     1315               </p>
     1316            </div>
     1317            <div id="rfc.section.3.3.p.7">
     1318               <p>When used for cache updates, a cache will typically use the value of the cached message's <a href="#header.last-modified" class="smpl">Last-Modified</a> field to generate the field value of If-Modified-Since. This behavior is most interoperable
     1319                  for cases where clocks are poorly synchronized or when the server has chosen to only
     1320                  honor exact timestamp matches (due to a problem with Last-Modified dates that appear
     1321                  to go "back in time" when the origin server's clock is corrected or a representation
     1322                  is restored from an archived backup). However, caches occasionally generate the field
     1323                  value based on other data, such as the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field of the cached message or the local clock time that the message was received,
     1324                  particularly when the cached message does not contain a <a href="#header.last-modified" class="smpl">Last-Modified</a> field.
     1325               </p>
     1326            </div>
     1327            <div id="rfc.section.3.3.p.8">
     1328               <p>When used for limiting the scope of retrieval to a recent time window, a user agent
     1329                  will generate an If-Modified-Since field value based on either its own local clock
     1330                  or a <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field received from the server in a prior response. Origin servers that choose
     1331                  an exact timestamp match based on the selected representation's <a href="#header.last-modified" class="smpl">Last-Modified</a> field will not be able to help the user agent limit its data transfers to only those
     1332                  changed during the specified window.
     1333               </p>
     1334            </div>
     1335            <div id="rfc.section.3.3.p.9">
     1336               <p>An origin server that receives an If-Modified-Since header field <em class="bcp14">SHOULD</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). The origin server <em class="bcp14">SHOULD NOT</em> perform the requested method if the selected representation's last modification date
     1337                  is earlier than or equal to the date provided in the field-value; instead, the origin
     1338                  server <em class="bcp14">SHOULD</em> generate a <a href="#status.304" class="smpl">304 (Not Modified)</a> response, including only those metadata that are useful for identifying or updating
     1339                  a previously cached response.
     1340               </p>
     1341            </div>
     1342            <div id="rfc.section.3.3.p.10">
     1343               <p>Requirements on cache handling of a received If-Modified-Since header field are defined
     1344                  in <a href="p6-cache.html#validation.received" title="Handling a Received Validation Request">Section 4.3.2</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>.
     1345               </p>
     1346            </div>
    10761347         </div>
    10771348         <div id="header.if-unmodified-since">
    1078             <div id="rfc.iref.i.4"></div>
    10791349            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
    1080             <p id="rfc.section.3.4.p.1">The "If-Unmodified-Since" header field makes the request method conditional on the selected representation's last modification
    1081                date being earlier than or equal to the date provided in the field-value. This field accomplishes the same purpose as <a href="#header.if-match" class="smpl">If-Match</a> for cases where the user agent does not have an entity-tag for the representation.
    1082             </p>
    1083             <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.10"></span>  <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
    1084 </pre><p id="rfc.section.3.4.p.3">An example of the field is:</p>
    1085             <div id="rfc.figure.u.15"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
    1086 </pre><p id="rfc.section.3.4.p.5">A recipient <em class="bcp14">MUST</em> ignore If-Unmodified-Since if the request contains an <a href="#header.if-match" class="smpl">If-Match</a> header field; the condition in <a href="#header.if-match" class="smpl">If-Match</a> is considered to be a more accurate replacement for the condition in If-Unmodified-Since, and the two are only combined for
    1087                the sake of interoperating with older intermediaries that might not implement <a href="#header.if-match" class="smpl">If-Match</a>.
    1088             </p>
    1089             <p id="rfc.section.3.4.p.6">A recipient <em class="bcp14">MUST</em> ignore the If-Unmodified-Since header field if the received field-value is not a valid HTTP-date.
    1090             </p>
    1091             <p id="rfc.section.3.4.p.7">A recipient <em class="bcp14">MUST</em> interpret an If-Unmodified-Since field-value's timestamp in terms of the origin server's clock.
    1092             </p>
    1093             <p id="rfc.section.3.4.p.8">If-Unmodified-Since is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites
    1094                when multiple user agents might be acting in parallel on a resource that does not supply entity-tags with its representations
    1095                (i.e., to prevent the "lost update" problem). It can also be used with safe methods to abort a request if the <a href="p2-semantics.html#representations" class="smpl">selected representation</a> does not match one already stored (or partially stored) from a prior request.
    1096             </p>
    1097             <p id="rfc.section.3.4.p.9">An origin server that receives an If-Unmodified-Since header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). The origin server <em class="bcp14">MUST NOT</em> perform the requested method if the selected representation's last modification date is more recent than the date provided
    1098                in the field-value; instead the origin server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code or b) one of the <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> status codes if the origin server has verified that a state change is being requested and the final state is already reflected
    1099                in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user
    1100                agent might not be aware of that because the prior response message was lost or a compatible change was made by some other
    1101                user agent). In the latter case, the origin server <em class="bcp14">MUST NOT</em> send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior
    1102                change made by the same user agent.
    1103             </p>
    1104             <p id="rfc.section.3.4.p.10">The If-Unmodified-Since header field can be ignored by caches and intermediaries because it is not applicable to a stored
    1105                response.
    1106             </p>
     1350            <div id="rfc.section.3.4.p.1">
     1351               <p>The "If-Unmodified-Since" header field makes the request method conditional on the
     1352                  selected representation's last modification date being earlier than or equal to the
     1353                  date provided in the field-value. This field accomplishes the same purpose as <a href="#header.if-match" class="smpl">If-Match</a> for cases where the user agent does not have an entity-tag for the representation.
     1354               </p>
     1355            </div>
     1356            <div id="rfc.figure.u.14"><pre class="inline"><span id="rfc.iref.g.10"></span>  <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
     1357</pre></div>
     1358            <div id="rfc.section.3.4.p.2">
     1359               <p>An example of the field is:</p>
     1360            </div>
     1361            <div id="rfc.figure.u.15"><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
     1362</pre></div>
     1363            <div id="rfc.section.3.4.p.3">
     1364               <p>A recipient <em class="bcp14">MUST</em> ignore If-Unmodified-Since if the request contains an <a href="#header.if-match" class="smpl">If-Match</a> header field; the condition in <a href="#header.if-match" class="smpl">If-Match</a> is considered to be a more accurate replacement for the condition in If-Unmodified-Since,
     1365                  and the two are only combined for the sake of interoperating with older intermediaries
     1366                  that might not implement <a href="#header.if-match" class="smpl">If-Match</a>.
     1367               </p>
     1368            </div>
     1369            <div id="rfc.section.3.4.p.4">
     1370               <p>A recipient <em class="bcp14">MUST</em> ignore the If-Unmodified-Since header field if the received field-value is not a valid
     1371                  HTTP-date.
     1372               </p>
     1373            </div>
     1374            <div id="rfc.section.3.4.p.5">
     1375               <p>A recipient <em class="bcp14">MUST</em> interpret an If-Unmodified-Since field-value's timestamp in terms of the origin server's
     1376                  clock.
     1377               </p>
     1378            </div>
     1379            <div id="rfc.section.3.4.p.6">
     1380               <p>If-Unmodified-Since is most often used with state-changing methods (e.g., POST, PUT,
     1381                  DELETE) to prevent accidental overwrites when multiple user agents might be acting
     1382                  in parallel on a resource that does not supply entity-tags with its representations
     1383                  (i.e., to prevent the "lost update" problem). It can also be used with safe methods
     1384                  to abort a request if the <a href="p2-semantics.html#representations" class="smpl">selected representation</a> does not match one already stored (or partially stored) from a prior request.
     1385               </p>
     1386            </div>
     1387            <div id="rfc.section.3.4.p.7">
     1388               <p>An origin server that receives an If-Unmodified-Since header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section&nbsp;5</a>). The origin server <em class="bcp14">MUST NOT</em> perform the requested method if the selected representation's last modification date
     1389                  is more recent than the date provided in the field-value; instead the origin server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code or b) one of the <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> status codes if the origin server has verified that a state change is being requested
     1390                  and the final state is already reflected in the current state of the target resource
     1391                  (i.e., the change requested by the user agent has already succeeded, but the user
     1392                  agent might not be aware of that because the prior response message was lost or a
     1393                  compatible change was made by some other user agent). In the latter case, the origin
     1394                  server <em class="bcp14">MUST NOT</em> send a validator header field in the response unless it can verify that the request
     1395                  is a duplicate of an immediately prior change made by the same user agent.
     1396               </p>
     1397            </div>
     1398            <div id="rfc.section.3.4.p.8">
     1399               <p>The If-Unmodified-Since header field can be ignored by caches and intermediaries because
     1400                  it is not applicable to a stored response.
     1401               </p>
     1402            </div>
    11071403         </div>
    11081404         <div id="header.if-range">
    11091405            <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a href="#header.if-range">If-Range</a></h2>
    1110             <p id="rfc.section.3.5.p.1">The "If-Range" header field provides a special conditional request mechanism that is similar to the <a href="#header.if-match" class="smpl">If-Match</a> and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> header fields but that instructs the recipient to ignore the <a href="p5-range.html#header.range" class="smpl">Range</a> header field if the validator doesn't match, resulting in transfer of the new selected representation instead of a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#RFC7233" id="rfc.xref.RFC7233.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>.
    1111             </p>
     1406            <div id="rfc.section.3.5.p.1">
     1407               <p>The "If-Range" header field provides a special conditional request mechanism that
     1408                  is similar to the <a href="#header.if-match" class="smpl">If-Match</a> and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> header fields but that instructs the recipient to ignore the <a href="p5-range.html#header.range" class="smpl">Range</a> header field if the validator doesn't match, resulting in transfer of the new selected
     1409                  representation instead of a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#RFC7233" id="rfc.xref.RFC7233.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>.
     1410               </p>
     1411            </div>
    11121412         </div>
    11131413      </div>
     
    11151415         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#status.code.definitions">Status Code Definitions</a></h1>
    11161416         <div id="status.304">
    1117             <div id="rfc.iref.3.1"></div>
    11181417            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#status.304">304 Not Modified</a></h2>
    1119             <p id="rfc.section.4.1.p.1">The <dfn>304 (Not Modified)</dfn> status code indicates that a conditional GET or HEAD request has been received and would have resulted in a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response if it were not for the fact that the condition evaluated to false. In other words, there is no need for the server
    1120                to transfer a representation of the target resource because the request indicates that the client, which made the request
    1121                conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored
    1122                representation as if it were the payload of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response.
    1123             </p>
    1124             <p id="rfc.section.4.1.p.2">The server generating a 304 response <em class="bcp14">MUST</em> generate any of the following header fields that would have been sent in a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to the same request: <a href="p6-cache.html#header.cache-control" class="smpl">Cache-Control</a>, <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a>, <a href="p2-semantics.html#header.date" class="smpl">Date</a>, <a href="#header.etag" class="smpl">ETag</a>, <a href="p6-cache.html#header.expires" class="smpl">Expires</a>, and <a href="p2-semantics.html#header.vary" class="smpl">Vary</a>.
    1125             </p>
    1126             <p id="rfc.section.4.1.p.3">Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations,
    1127                a sender <em class="bcp14">SHOULD NOT</em> generate representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding
    1128                cache updates (e.g., <a href="#header.last-modified" class="smpl">Last-Modified</a> might be useful if the response does not have an <a href="#header.etag" class="smpl">ETag</a> field).
    1129             </p>
    1130             <p id="rfc.section.4.1.p.4">Requirements on a cache that receives a 304 response are defined in <a href="p6-cache.html#freshening.responses" title="Freshening Stored Responses upon Validation">Section 4.3.4</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. If the conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional
    1131                GET to a shared proxy, then the proxy <em class="bcp14">SHOULD</em> forward the 304 response to that client.
    1132             </p>
    1133             <p id="rfc.section.4.1.p.5">A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields.</p>
     1418            <div id="rfc.section.4.1.p.1">
     1419               <p>The <dfn>304 (Not Modified)</dfn> status code indicates that a conditional GET or HEAD request has been received and
     1420                  would have resulted in a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response if it were not for the fact that the condition evaluated to false. In other
     1421                  words, there is no need for the server to transfer a representation of the target
     1422                  resource because the request indicates that the client, which made the request conditional,
     1423                  already has a valid representation; the server is therefore redirecting the client
     1424                  to make use of that stored representation as if it were the payload of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response.
     1425               </p>
     1426            </div>
     1427            <div id="rfc.section.4.1.p.2">
     1428               <p>The server generating a 304 response <em class="bcp14">MUST</em> generate any of the following header fields that would have been sent in a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to the same request: <a href="p6-cache.html#header.cache-control" class="smpl">Cache-Control</a>, <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a>, <a href="p2-semantics.html#header.date" class="smpl">Date</a>, <a href="#header.etag" class="smpl">ETag</a>, <a href="p6-cache.html#header.expires" class="smpl">Expires</a>, and <a href="p2-semantics.html#header.vary" class="smpl">Vary</a>.
     1429               </p>
     1430            </div>
     1431            <div id="rfc.section.4.1.p.3">
     1432               <p>Since the goal of a 304 response is to minimize information transfer when the recipient
     1433                  already has one or more cached representations, a sender <em class="bcp14">SHOULD NOT</em> generate representation metadata other than the above listed fields unless said metadata
     1434                  exists for the purpose of guiding cache updates (e.g., <a href="#header.last-modified" class="smpl">Last-Modified</a> might be useful if the response does not have an <a href="#header.etag" class="smpl">ETag</a> field).
     1435               </p>
     1436            </div>
     1437            <div id="rfc.section.4.1.p.4">
     1438               <p>Requirements on a cache that receives a 304 response are defined in <a href="p6-cache.html#freshening.responses" title="Freshening Stored Responses upon Validation">Section 4.3.4</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. If the conditional request originated with an outbound client, such as a user agent
     1439                  with its own cache sending a conditional GET to a shared proxy, then the proxy <em class="bcp14">SHOULD</em> forward the 304 response to that client.
     1440               </p>
     1441            </div>
     1442            <div id="rfc.section.4.1.p.5">
     1443               <p>A 304 response cannot contain a message-body; it is always terminated by the first
     1444                  empty line after the header fields.
     1445               </p>
     1446            </div>
    11341447         </div>
    11351448         <div id="status.412">
    1136             <div id="rfc.iref.4.1"></div>
    11371449            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#status.412">412 Precondition Failed</a></h2>
    1138             <p id="rfc.section.4.2.p.1">The <dfn>412 (Precondition Failed)</dfn> status code indicates that one or more conditions given in the request header fields evaluated to false when tested on the
    1139                server. This response code allows the client to place preconditions on the current resource state (its current representations
    1140                and metadata) and, thus, prevent the request method from being applied if the target resource is in an unexpected state.
    1141             </p>
     1450            <div id="rfc.section.4.2.p.1">
     1451               <p>The <dfn>412 (Precondition Failed)</dfn> status code indicates that one or more conditions given in the request header fields
     1452                  evaluated to false when tested on the server. This response code allows the client
     1453                  to place preconditions on the current resource state (its current representations
     1454                  and metadata) and, thus, prevent the request method from being applied if the target
     1455                  resource is in an unexpected state.
     1456               </p>
     1457            </div>
    11421458         </div>
    11431459      </div>
    11441460      <div id="evaluation">
    11451461         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#evaluation">Evaluation</a></h1>
    1146          <p id="rfc.section.5.p.1">Except when excluded below, a recipient cache or origin server <em class="bcp14">MUST</em> evaluate received request preconditions after it has successfully performed its normal request checks and just before it would
    1147             perform the action associated with the request method. A server <em class="bcp14">MUST</em> ignore all received preconditions if its response to the same request without those conditions would have been a status code
    1148             other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.412" class="smpl">412 (Precondition Failed)</a>. In other words, redirects and failures take precedence over the evaluation of preconditions in conditional requests.
    1149          </p>
    1150          <p id="rfc.section.5.p.2">A server that is not the origin server for the target resource and cannot act as a cache for requests on the target resource <em class="bcp14">MUST NOT</em> evaluate the conditional request header fields defined by this specification, and it <em class="bcp14">MUST</em> forward them if the request is forwarded, since the generating client intends that they be evaluated by a server that can
    1151             provide a current representation. Likewise, a server <em class="bcp14">MUST</em> ignore the conditional request header fields defined by this specification when received with a request method that does not
    1152             involve the selection or modification of a <a href="p2-semantics.html#representations" class="smpl">selected representation</a>, such as CONNECT, OPTIONS, or TRACE.
    1153          </p>
    1154          <p id="rfc.section.5.p.3">Conditional request header fields that are defined by extensions to HTTP might place conditions on all recipients, on the
    1155             state of the target resource in general, or on a group of resources. For instance, the "If" header field in WebDAV can make
    1156             a request conditional on various aspects of multiple resources, such as locks, if the recipient understands and implements
    1157             that field (<a href="#RFC4918" id="rfc.xref.RFC4918.2"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, <a href="https://tools.ietf.org/html/rfc4918#section-10.4">Section 10.4</a>).
    1158          </p>
    1159          <p id="rfc.section.5.p.4">Although conditional request header fields are defined as being usable with the HEAD method (to keep HEAD's semantics consistent
    1160             with those of GET), there is no point in sending a conditional HEAD because a successful response is around the same size
    1161             as a <a href="#status.304" class="smpl">304 (Not Modified)</a> response and more useful than a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response.
    1162          </p>
     1462         <div id="rfc.section.5.p.1">
     1463            <p>Except when excluded below, a recipient cache or origin server <em class="bcp14">MUST</em> evaluate received request preconditions after it has successfully performed its normal
     1464               request checks and just before it would perform the action associated with the request
     1465               method. A server <em class="bcp14">MUST</em> ignore all received preconditions if its response to the same request without those
     1466               conditions would have been a status code other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.412" class="smpl">412 (Precondition Failed)</a>. In other words, redirects and failures take precedence over the evaluation of preconditions
     1467               in conditional requests.
     1468            </p>
     1469         </div>
     1470         <div id="rfc.section.5.p.2">
     1471            <p>A server that is not the origin server for the target resource and cannot act as a
     1472               cache for requests on the target resource <em class="bcp14">MUST NOT</em> evaluate the conditional request header fields defined by this specification, and
     1473               it <em class="bcp14">MUST</em> forward them if the request is forwarded, since the generating client intends that
     1474               they be evaluated by a server that can provide a current representation. Likewise,
     1475               a server <em class="bcp14">MUST</em> ignore the conditional request header fields defined by this specification when received
     1476               with a request method that does not involve the selection or modification of a <a href="p2-semantics.html#representations" class="smpl">selected representation</a>, such as CONNECT, OPTIONS, or TRACE.
     1477            </p>
     1478         </div>
     1479         <div id="rfc.section.5.p.3">
     1480            <p>Conditional request header fields that are defined by extensions to HTTP might place
     1481               conditions on all recipients, on the state of the target resource in general, or on
     1482               a group of resources. For instance, the "If" header field in WebDAV can make a request
     1483               conditional on various aspects of multiple resources, such as locks, if the recipient
     1484               understands and implements that field (<a href="#RFC4918" id="rfc.xref.RFC4918.2"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, <a href="https://tools.ietf.org/html/rfc4918#section-10.4">Section 10.4</a>).
     1485            </p>
     1486         </div>
     1487         <div id="rfc.section.5.p.4">
     1488            <p>Although conditional request header fields are defined as being usable with the HEAD
     1489               method (to keep HEAD's semantics consistent with those of GET), there is no point
     1490               in sending a conditional HEAD because a successful response is around the same size
     1491               as a <a href="#status.304" class="smpl">304 (Not Modified)</a> response and more useful than a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response.
     1492            </p>
     1493         </div>
    11631494      </div>
    11641495      <div id="precedence">
    11651496         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#precedence">Precedence</a></h1>
    1166          <p id="rfc.section.6.p.1">When more than one conditional request header field is present in a request, the order in which the fields are evaluated becomes
    1167             important. In practice, the fields defined in this document are consistently implemented in a single, logical order, since
    1168             "lost update" preconditions have more strict requirements than cache validation, a validated cache is more efficient than
    1169             a partial response, and entity tags are presumed to be more accurate than date validators.
    1170          </p>
    1171          <p id="rfc.section.6.p.2">A recipient cache or origin server <em class="bcp14">MUST</em> evaluate the request preconditions defined by this specification in the following order:
    1172          </p>
    1173          <ol>
    1174             <li id="precedence1">When recipient is the origin server and <a href="#header.if-match" class="smpl">If-Match</a> is present, evaluate the <a href="#header.if-match" class="smpl">If-Match</a> precondition:
    1175                <ul>
    1176                   <li>if true, continue to step <a href="#precedence3">3</a></li>
    1177                   <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a> unless it can be determined that the state-changing request has already succeeded (see <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;3.1</a>)
    1178                   </li>
    1179                </ul>
    1180             </li>
    1181             <li id="precedence2">When recipient is the origin server, <a href="#header.if-match" class="smpl">If-Match</a> is not present, and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> is present, evaluate the <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> precondition:
    1182                <ul>
    1183                   <li>if true, continue to step <a href="#precedence3">3</a></li>
    1184                   <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a> unless it can be determined that the state-changing request has already succeeded (see <a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section&nbsp;3.4</a>)
    1185                   </li>
    1186                </ul>
    1187             </li>
    1188             <li id="precedence3">When <a href="#header.if-none-match" class="smpl">If-None-Match</a> is present, evaluate the <a href="#header.if-none-match" class="smpl">If-None-Match</a> precondition:
    1189                <ul>
    1190                   <li>if true, continue to step <a href="#precedence5">5</a></li>
    1191                   <li>if false for GET/HEAD, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
    1192                   <li>if false for other methods, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li>
    1193                </ul>
    1194             </li>
    1195             <li id="precedence4">When the method is GET or HEAD, <a href="#header.if-none-match" class="smpl">If-None-Match</a> is not present, and <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> is present, evaluate the <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> precondition:
    1196                <ul>
    1197                   <li>if true, continue to step <a href="#precedence5">5</a></li>
    1198                   <li>if false, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
    1199                </ul>
    1200             </li>
    1201             <li id="precedence5">When the method is GET and both <a href="p5-range.html#header.range" class="smpl">Range</a> and <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> are present, evaluate the <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> precondition:
    1202                <ul>
    1203                   <li>if the validator matches and the Range specification is applicable to the selected representation, respond <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> <a href="#RFC7233" id="rfc.xref.RFC7233.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a></li>
    1204                </ul>
    1205             </li>
    1206             <li id="precedencelast">Otherwise,
    1207                <ul>
    1208                   <li>all conditions are met, so perform the requested action and respond according to its success or failure.</li>
    1209                </ul>
    1210             </li>
    1211          </ol>
    1212          <p id="rfc.section.6.p.3">Any extension to HTTP/1.1 that defines additional conditional request header fields ought to define its own expectations regarding
    1213             the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found
    1214             in practice.
    1215          </p>
     1497         <div id="rfc.section.6.p.1">
     1498            <p>When more than one conditional request header field is present in a request, the order
     1499               in which the fields are evaluated becomes important. In practice, the fields defined
     1500               in this document are consistently implemented in a single, logical order, since "lost
     1501               update" preconditions have more strict requirements than cache validation, a validated
     1502               cache is more efficient than a partial response, and entity tags are presumed to be
     1503               more accurate than date validators.
     1504            </p>
     1505         </div>
     1506         <div id="rfc.section.6.p.2">
     1507            <p>A recipient cache or origin server <em class="bcp14">MUST</em> evaluate the request preconditions defined by this specification in the following
     1508               order:
     1509            </p>
     1510            <ol>
     1511               <li id="precedence1">When recipient is the origin server and <a href="#header.if-match" class="smpl">If-Match</a> is present, evaluate the <a href="#header.if-match" class="smpl">If-Match</a> precondition:
     1512                  <ul>
     1513                     <li>if true, continue to step <a href="#precedence3">3</a></li>
     1514                     <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a> unless it can be determined that the state-changing request has already succeeded
     1515                        (see <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;3.1</a>)
     1516                     </li>
     1517                  </ul>
     1518               </li>
     1519               <li id="precedence2">When recipient is the origin server, <a href="#header.if-match" class="smpl">If-Match</a> is not present, and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> is present, evaluate the <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> precondition:
     1520                  <ul>
     1521                     <li>if true, continue to step <a href="#precedence3">3</a></li>
     1522                     <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a> unless it can be determined that the state-changing request has already succeeded
     1523                        (see <a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section&nbsp;3.4</a>)
     1524                     </li>
     1525                  </ul>
     1526               </li>
     1527               <li id="precedence3">When <a href="#header.if-none-match" class="smpl">If-None-Match</a> is present, evaluate the <a href="#header.if-none-match" class="smpl">If-None-Match</a> precondition:
     1528                  <ul>
     1529                     <li>if true, continue to step <a href="#precedence5">5</a></li>
     1530                     <li>if false for GET/HEAD, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
     1531                     <li>if false for other methods, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li>
     1532                  </ul>
     1533               </li>
     1534               <li id="precedence4">When the method is GET or HEAD, <a href="#header.if-none-match" class="smpl">If-None-Match</a> is not present, and <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> is present, evaluate the <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> precondition:
     1535                  <ul>
     1536                     <li>if true, continue to step <a href="#precedence5">5</a></li>
     1537                     <li>if false, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
     1538                  </ul>
     1539               </li>
     1540               <li id="precedence5">When the method is GET and both <a href="p5-range.html#header.range" class="smpl">Range</a> and <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> are present, evaluate the <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> precondition:
     1541                  <ul>
     1542                     <li>if the validator matches and the Range specification is applicable to the selected
     1543                        representation, respond <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> <a href="#RFC7233" id="rfc.xref.RFC7233.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a></li>
     1544                  </ul>
     1545               </li>
     1546               <li id="precedencelast">Otherwise,
     1547                  <ul>
     1548                     <li>all conditions are met, so perform the requested action and respond according to its
     1549                        success or failure.
     1550                     </li>
     1551                  </ul>
     1552               </li>
     1553            </ol>
     1554         </div>
     1555         <div id="rfc.section.6.p.3">
     1556            <p>Any extension to HTTP/1.1 that defines additional conditional request header fields
     1557               ought to define its own expectations regarding the order for evaluating such fields
     1558               in relation to those defined in this document and other conditionals that might be
     1559               found in practice.
     1560            </p>
     1561         </div>
    12161562      </div>
    12171563      <div id="IANA.considerations">
     
    12191565         <div id="status.code.registration">
    12201566            <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a href="#status.code.registration">Status Code Registration</a></h2>
    1221             <p id="rfc.section.7.1.p.1">The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located at &lt;<a href="http://www.iana.org/assignments/http-status-codes">http://www.iana.org/assignments/http-status-codes</a>&gt; has been updated with the registrations below:
    1222             </p>
     1567            <div id="rfc.section.7.1.p.1">
     1568               <p>The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located at &lt;<a href="http://www.iana.org/assignments/http-status-codes">http://www.iana.org/assignments/http-status-codes</a>&gt; has been updated with the registrations below:
     1569               </p>
     1570            </div>
    12231571            <div id="rfc.table.1">
    12241572               <div id="iana.status.code.registration.table"></div>
     
    12501598         <div id="header.field.registration">
    12511599            <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a href="#header.field.registration">Header Field Registration</a></h2>
    1252             <p id="rfc.section.7.2.p.1">HTTP header fields are registered within the "Message Headers" registry maintained at &lt;<a href="http://www.iana.org/assignments/message-headers/">http://www.iana.org/assignments/message-headers/</a>&gt;.
    1253             </p>
    1254             <p id="rfc.section.7.2.p.2">This document defines the following HTTP header fields, so their associated registry entries have been updated according to
    1255                the permanent registrations below (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>):
    1256             </p>
     1600            <div id="rfc.section.7.2.p.1">
     1601               <p>HTTP header fields are registered within the "Message Headers" registry maintained
     1602                  at &lt;<a href="http://www.iana.org/assignments/message-headers/">http://www.iana.org/assignments/message-headers/</a>&gt;.
     1603               </p>
     1604            </div>
     1605            <div id="rfc.section.7.2.p.2">
     1606               <p>This document defines the following HTTP header fields, so their associated registry
     1607                  entries have been updated according to the permanent registrations below (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>):
     1608               </p>
     1609            </div>
    12571610            <div id="rfc.table.2">
    12581611               <div id="iana.header.registration.table"></div>
     
    13121665               </table>
    13131666            </div>
    1314             <p id="rfc.section.7.2.p.3">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
     1667            <div id="rfc.section.7.2.p.3">
     1668               <p>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
     1669            </div>
    13151670         </div>
    13161671      </div>
    13171672      <div id="security.considerations">
    13181673         <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
    1319          <p id="rfc.section.8.p.1">This section is meant to inform developers, information providers, and users of known security concerns specific to the HTTP
    1320             conditional request mechanisms. More general security considerations are addressed in HTTP "Message Syntax and Routing" <a href="#RFC7230" id="rfc.xref.RFC7230.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a> and "Semantics and Content" <a href="#RFC7231" id="rfc.xref.RFC7231.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>.
    1321          </p>
    1322          <p id="rfc.section.8.p.2">The validators defined by this specification are not intended to ensure the validity of a representation, guard against malicious
    1323             changes, or detect man-in-the-middle attacks. At best, they enable more efficient cache updates and optimistic concurrent
    1324             writes when all participants are behaving nicely. At worst, the conditions will fail and the client will receive a response
    1325             that is no more harmful than an HTTP exchange without conditional requests.
    1326          </p>
    1327          <p id="rfc.section.8.p.3">An entity-tag can be abused in ways that create privacy risks. For example, a site might deliberately construct a semantically
    1328             invalid entity-tag that is unique to the user or user agent, send it in a cacheable response with a long freshness time, and
    1329             then read that entity-tag in later conditional requests as a means of re-identifying that user or user agent. Such an identifying
    1330             tag would become a persistent identifier for as long as the user agent retained the original cache entry. User agents that
    1331             cache representations ought to ensure that the cache is cleared or replaced whenever the user performs privacy-maintaining
    1332             actions, such as clearing stored cookies or changing to a private browsing mode.
    1333          </p>
     1674         <div id="rfc.section.8.p.1">
     1675            <p>This section is meant to inform developers, information providers, and users of known
     1676               security concerns specific to the HTTP conditional request mechanisms. More general
     1677               security considerations are addressed in HTTP "Message Syntax and Routing" <a href="#RFC7230" id="rfc.xref.RFC7230.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a> and "Semantics and Content" <a href="#RFC7231" id="rfc.xref.RFC7231.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>.
     1678            </p>
     1679         </div>
     1680         <div id="rfc.section.8.p.2">
     1681            <p>The validators defined by this specification are not intended to ensure the validity
     1682               of a representation, guard against malicious changes, or detect man-in-the-middle
     1683               attacks. At best, they enable more efficient cache updates and optimistic concurrent
     1684               writes when all participants are behaving nicely. At worst, the conditions will fail
     1685               and the client will receive a response that is no more harmful than an HTTP exchange
     1686               without conditional requests.
     1687            </p>
     1688         </div>
     1689         <div id="rfc.section.8.p.3">
     1690            <p>An entity-tag can be abused in ways that create privacy risks. For example, a site
     1691               might deliberately construct a semantically invalid entity-tag that is unique to the
     1692               user or user agent, send it in a cacheable response with a long freshness time, and
     1693               then read that entity-tag in later conditional requests as a means of re-identifying
     1694               that user or user agent. Such an identifying tag would become a persistent identifier
     1695               for as long as the user agent retained the original cache entry. User agents that
     1696               cache representations ought to ensure that the cache is cleared or replaced whenever
     1697               the user performs privacy-maintaining actions, such as clearing stored cookies or
     1698               changing to a private browsing mode.
     1699            </p>
     1700         </div>
    13341701      </div>
    13351702      <div id="acks">
    13361703         <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a href="#acks">Acknowledgments</a></h1>
    1337          <p id="rfc.section.9.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 10</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
    1338          </p>
     1704         <div id="rfc.section.9.p.1">
     1705            <p>See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 10</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
     1706            </p>
     1707         </div>
    13391708      </div>
    13401709      <h1 id="rfc.references"><a id="rfc.section.10" href="#rfc.section.10">10.</a> References
     
    13451714         <tr>
    13461715            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
    1347             <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
     1716            <td class="top">Bradner, S., “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
    13481717            </td>
    13491718         </tr>
    13501719         <tr>
    13511720            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
    1352             <td class="top"><a href="mailto:dcrocker@bbiw.net" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="mailto:paul.overell@thus.net" title="THUS plc.">P. Overell</a>, “<a href="https://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
     1721            <td class="top">Crocker, D., Ed. and P. Overell, “<a href="https://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
    13531722            </td>
    13541723         </tr>
    13551724         <tr>
    13561725            <td class="reference"><b id="RFC7230">[RFC7230]</b></td>
    1357             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), June&nbsp;2014.
     1726            <td class="top">Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), June&nbsp;2014.
    13581727            </td>
    13591728         </tr>
    13601729         <tr>
    13611730            <td class="reference"><b id="RFC7231">[RFC7231]</b></td>
    1362             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), June&nbsp;2014.
     1731            <td class="top">Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), June&nbsp;2014.
    13631732            </td>
    13641733         </tr>
    13651734         <tr>
    13661735            <td class="reference"><b id="RFC7233">[RFC7233]</b></td>
    1367             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), June&nbsp;2014.
     1736            <td class="top">Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), June&nbsp;2014.
    13681737            </td>
    13691738         </tr>
    13701739         <tr>
    13711740            <td class="reference"><b id="RFC7234">[RFC7234]</b></td>
    1372             <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Akamai">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">Hypertext Transfer Protocol (HTTP/1.1): Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), June&nbsp;2014.
     1741            <td class="top">Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">Hypertext Transfer Protocol (HTTP/1.1): Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), June&nbsp;2014.
    13731742            </td>
    13741743         </tr>
     
    13791748         <tr>
    13801749            <td class="reference"><b id="BCP90">[BCP90]</b></td>
    1381             <td class="top"><a href="mailto:GK-IETF@ninebynine.org" title="Nine by Nine">Klyne, G.</a>, <a href="mailto:mnot@pobox.com" title="BEA Systems">Nottingham, M.</a>, and <a href="mailto:JeffMogul@acm.org" title="HP Labs">J. Mogul</a>, “<a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
     1750            <td class="top">Klyne, G., Nottingham, M., and J. Mogul, “<a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
    13821751            </td>
    13831752         </tr>
    13841753         <tr>
    13851754            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
    1386             <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
     1755            <td class="top">Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
    13871756            </td>
    13881757         </tr>
    13891758         <tr>
    13901759            <td class="reference"><b id="RFC4918">[RFC4918]</b></td>
    1391             <td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="https://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC&nbsp;4918, June&nbsp;2007.
     1760            <td class="top">Dusseault, L., Ed., “<a href="https://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC&nbsp;4918, June&nbsp;2007.
    13921761            </td>
    13931762         </tr>
     
    13951764      <div id="changes.from.rfc.2616">
    13961765         <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
    1397          <p id="rfc.section.A.p.1">The definition of validator weakness has been expanded and clarified. (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section&nbsp;2.1</a>)
    1398          </p>
    1399          <p id="rfc.section.A.p.2">Weak entity-tags are now allowed in all requests except range requests. (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.1</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>)
    1400          </p>
    1401          <p id="rfc.section.A.p.3">The <a href="#header.etag" class="smpl">ETag</a> header field ABNF has been changed to not use quoted-string, thus avoiding escaping issues. (<a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section&nbsp;2.3</a>)
    1402          </p>
    1403          <p id="rfc.section.A.p.4">ETag is defined to provide an entity tag for the selected representation, thereby clarifying what it applies to in various
    1404             situations (such as a PUT response). (<a href="#header.etag" id="rfc.xref.header.etag.4" title="ETag">Section&nbsp;2.3</a>)
    1405          </p>
    1406          <p id="rfc.section.A.p.5">The precedence for evaluation of conditional requests has been defined. (<a href="#precedence" title="Precedence">Section&nbsp;6</a>)
    1407          </p>
     1766         <div id="rfc.section.A.p.1">
     1767            <p>The definition of validator weakness has been expanded and clarified. (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section&nbsp;2.1</a>)
     1768            </p>
     1769         </div>
     1770         <div id="rfc.section.A.p.2">
     1771            <p>Weak entity-tags are now allowed in all requests except range requests. (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.1</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>)
     1772            </p>
     1773         </div>
     1774         <div id="rfc.section.A.p.3">
     1775            <p>The <a href="#header.etag" class="smpl">ETag</a> header field ABNF has been changed to not use quoted-string, thus avoiding escaping
     1776               issues. (<a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section&nbsp;2.3</a>)
     1777            </p>
     1778         </div>
     1779         <div id="rfc.section.A.p.4">
     1780            <p>ETag is defined to provide an entity tag for the selected representation, thereby
     1781               clarifying what it applies to in various situations (such as a PUT response). (<a href="#header.etag" id="rfc.xref.header.etag.4" title="ETag">Section&nbsp;2.3</a>)
     1782            </p>
     1783         </div>
     1784         <div id="rfc.section.A.p.5">
     1785            <p>The precedence for evaluation of conditional requests has been defined. (<a href="#precedence" title="Precedence">Section&nbsp;6</a>)
     1786            </p>
     1787         </div>
    14081788      </div>
    14091789      <div id="imported.abnf">
    14101790         <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a href="#imported.abnf">Imported ABNF</a></h1>
    1411          <p id="rfc.section.B.p.1">The following core rules are included by reference, as defined in <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a> of <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
    1412             (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
    1413             character).
    1414          </p>
    1415          <p id="rfc.section.B.p.2">The rules below are defined in <a href="#RFC7230" id="rfc.xref.RFC7230.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>:
    1416          </p>
    1417          <div id="rfc.figure.u.16"></div><pre class="inline">  <a href="#imported.abnf" class="smpl">OWS</a>           = &lt;OWS, see <a href="#RFC7230" id="rfc.xref.RFC7230.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.3</a>&gt;
     1791         <div id="rfc.section.B.p.1">
     1792            <p>The following core rules are included by reference, as defined in <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a> of <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal
     1793               0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET
     1794               (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
     1795            </p>
     1796         </div>
     1797         <div id="rfc.section.B.p.2">
     1798            <p>The rules below are defined in <a href="#RFC7230" id="rfc.xref.RFC7230.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>:
     1799            </p>
     1800         </div>
     1801         <div id="rfc.figure.u.16"><pre class="inline">  <a href="#imported.abnf" class="smpl">OWS</a>           = &lt;OWS, see <a href="#RFC7230" id="rfc.xref.RFC7230.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.3</a>&gt;
    14181802  <a href="#imported.abnf" class="smpl">obs-text</a>      = &lt;obs-text, see <a href="#RFC7230" id="rfc.xref.RFC7230.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#field.components" title="Field Value Components">Section 3.2.6</a>&gt;
    1419 </pre><p id="rfc.section.B.p.4">The rules below are defined in other parts:</p>
    1420          <div id="rfc.figure.u.17"></div><pre class="inline">  <a href="#imported.abnf" class="smpl">HTTP-date</a>     = &lt;HTTP-date, see <a href="#RFC7231" id="rfc.xref.RFC7231.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 7.1.1.1</a>&gt;
    14211803</pre></div>
     1804         <div id="rfc.section.B.p.3">
     1805            <p>The rules below are defined in other parts:</p>
     1806         </div>
     1807         <div id="rfc.figure.u.17"><pre class="inline">  <a href="#imported.abnf" class="smpl">HTTP-date</a>     = &lt;HTTP-date, see <a href="#RFC7231" id="rfc.xref.RFC7231.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 7.1.1.1</a>&gt;
     1808</pre></div>
     1809      </div>
    14221810      <div id="collected.abnf">
    14231811         <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a href="#collected.abnf">Collected ABNF</a></h1>
    1424          <p id="rfc.section.C.p.1">In the collected ABNF below, list rules are expanded as per <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
    1425          </p>
    1426          <div id="rfc.figure.u.18"></div><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
     1812         <div id="rfc.section.C.p.1">
     1813            <p>In the collected ABNF below, list rules are expanded as per <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
     1814            </p>
     1815         </div>
     1816         <div id="rfc.figure.u.18"><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
    14271817
    14281818<a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, see [RFC7231], Section 7.1.1.1&gt;
     
    14481838<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/
    14491839</pre></div>
     1840      </div>
    14501841      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
    14511842      <p class="noprint"><a href="#rfc.index.3">3</a> <a href="#rfc.index.4">4</a> <a href="#rfc.index.B">B</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.I">I</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a>
     
    14541845         <ul class="ind">
    14551846            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
    1456                   <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.3.1"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">7.1</a></li>
     1847                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.section.4.1"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">7.1</a></li>
    14571848               </ul>
    14581849            </li>
    14591850            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
    1460                   <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.4.1"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">7.1</a></li>
     1851                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.section.4.2"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">7.1</a></li>
    14611852               </ul>
    14621853            </li>
     
    14661857            </li>
    14671858            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
    1468                   <li>ETag header field&nbsp;&nbsp;<a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.iref.e.1"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.2">7.2</a>, <a href="#rfc.xref.header.etag.3">A</a>, <a href="#rfc.xref.header.etag.4">A</a></li>
     1859                  <li>ETag header field&nbsp;&nbsp;<a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.section.2.3"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.2">7.2</a>, <a href="#rfc.xref.header.etag.3">A</a>, <a href="#rfc.xref.header.etag.4">A</a></li>
    14691860               </ul>
    14701861            </li>
     
    14871878            </li>
    14881879            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
    1489                   <li>If-Match header field&nbsp;&nbsp;<a href="#rfc.iref.i.1"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">6</a>, <a href="#rfc.xref.header.if-match.2">7.2</a></li>
    1490                   <li>If-Modified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.3"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">7.2</a></li>
    1491                   <li>If-None-Match header field&nbsp;&nbsp;<a href="#rfc.iref.i.2"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">7.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li>
    1492                   <li>If-Unmodified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.4"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">6</a>, <a href="#rfc.xref.header.if-unmodified-since.2">7.2</a></li>
     1880                  <li>If-Match header field&nbsp;&nbsp;<a href="#rfc.section.3.1"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">6</a>, <a href="#rfc.xref.header.if-match.2">7.2</a></li>
     1881                  <li>If-Modified-Since header field&nbsp;&nbsp;<a href="#rfc.section.3.3"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">7.2</a></li>
     1882                  <li>If-None-Match header field&nbsp;&nbsp;<a href="#rfc.section.3.2"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">7.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li>
     1883                  <li>If-Unmodified-Since header field&nbsp;&nbsp;<a href="#rfc.section.3.4"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">6</a>, <a href="#rfc.xref.header.if-unmodified-since.2">7.2</a></li>
    14931884               </ul>
    14941885            </li>
    14951886            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
    1496                   <li>Last-Modified header field&nbsp;&nbsp;<a href="#rfc.xref.header.last-modified.1">2</a>, <a href="#rfc.iref.l.1"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.2">7.2</a></li>
     1887                  <li>Last-Modified header field&nbsp;&nbsp;<a href="#rfc.xref.header.last-modified.1">2</a>, <a href="#rfc.section.2.2"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.2">7.2</a></li>
    14971888               </ul>
    14981889            </li>
    14991890            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
    1500                   <li>metadata&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>2</b></a></li>
     1891                  <li>metadata&nbsp;&nbsp;<a href="#rfc.section.2"><b>2</b></a></li>
    15011892               </ul>
    15021893            </li>
     
    15491940            </li>
    15501941            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
    1551                   <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a><ul>
    1552                         <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.3"><b>2.1</b></a></li>
    1553                         <li>weak&nbsp;&nbsp;<a href="#rfc.iref.v.2"><b>2.1</b></a></li>
     1942                  <li>validator&nbsp;&nbsp;<a href="#rfc.section.2"><b>2</b></a><ul>
     1943                        <li>strong&nbsp;&nbsp;<a href="#rfc.section.2.1"><b>2.1</b></a></li>
     1944                        <li>weak&nbsp;&nbsp;<a href="#rfc.section.2.1"><b>2.1</b></a></li>
    15541945                     </ul>
    15551946                  </li>
     
    15581949         </ul>
    15591950      </div>
    1560       <div class="avoidbreak">
     1951      <div class="avoidbreakinside">
    15611952         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
    15621953         <p><b>Roy T. Fielding</b>
    15631954            (editor)
    1564             <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: <a href="mailto:fielding@gbiv.com">fielding@gbiv.com</a><br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p>
     1955            <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: fielding@gbiv.com<br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p>
    15651956         <p><b>Julian F. Reschke</b>
    15661957            (editor)
    1567             <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Email: <a href="mailto:julian.reschke@greenbytes.de">julian.reschke@greenbytes.de</a><br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p>
     1958            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Email: julian.reschke@greenbytes.de<br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p>
    15681959      </div>
    15691960   </body>
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