source: draft-ietf-httpbis/latest/p4-conditional.html @ 2576

Last change on this file since 2576 was 2569, checked in by fielding@…, 7 years ago

(editorial) OWASP only provides useful additional info for web application semantics and authentication; see #520 and #549

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444       content: "HTTP/1.1 Conditional Requests";
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447       content: "Fielding & Reschke";
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468</style><link rel="Contents" href="#rfc.toc">
469      <link rel="Author" href="#rfc.authors">
470      <link rel="Copyright" href="#rfc.copyrightnotice">
471      <link rel="Index" href="#rfc.index">
472      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
473      <link rel="Chapter" title="2 Validators" href="#rfc.section.2">
474      <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3">
475      <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4">
476      <link rel="Chapter" title="5 Evaluation" href="#rfc.section.5">
477      <link rel="Chapter" title="6 Precedence" href="#rfc.section.6">
478      <link rel="Chapter" title="7 IANA Considerations" href="#rfc.section.7">
479      <link rel="Chapter" title="8 Security Considerations" href="#rfc.section.8">
480      <link rel="Chapter" title="9 Acknowledgments" href="#rfc.section.9">
481      <link rel="Chapter" href="#rfc.section.10" title="10 References">
482      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
483      <link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B">
484      <link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C">
485      <link rel="Appendix" title="D Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.D">
486      <link href="p2-semantics.html" rel="prev">
487      <link href="p5-range.html" rel="next">
488      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.611, 2013/11/27 12:23:51, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
489      <link rel="schema.dct" href="http://purl.org/dc/terms/">
490      <meta name="dct.creator" content="Fielding, R.">
491      <meta name="dct.creator" content="Reschke, J. F.">
492      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest">
493      <meta name="dct.issued" scheme="ISO8601" content="2014-01-23">
494      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
495      <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.">
496      <meta name="description" 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.">
497   </head>
498   <body onload="init();">
499      <table class="header">
500         <tbody>
501            <tr>
502               <td class="left">HTTPbis Working Group</td>
503               <td class="right">R. Fielding, Editor</td>
504            </tr>
505            <tr>
506               <td class="left">Internet-Draft</td>
507               <td class="right">Adobe</td>
508            </tr>
509            <tr>
510               <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
511               </td>
512               <td class="right">J. Reschke, Editor</td>
513            </tr>
514            <tr>
515               <td class="left">Intended status: Standards Track</td>
516               <td class="right">greenbytes</td>
517            </tr>
518            <tr>
519               <td class="left">Expires: July 27, 2014</td>
520               <td class="right">January 23, 2014</td>
521            </tr>
522         </tbody>
523      </table>
524      <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p>
525      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
526      <p>The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext
527         information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating
528         state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a
529         conditional request when one or more preconditions evaluate to false.
530      </p>
531      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
532      <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;.
533      </p>
534      <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;.
535      </p>
536      <p>The changes in this draft are summarized in <a href="#changes.since.25" title="Since draft-ietf-httpbis-p4-conditional-25">Appendix&nbsp;D.2</a>.
537      </p>
538      <div id="rfc.status">
539         <h1><a href="#rfc.status">Status of This Memo</a></h1>
540         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
541         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
542            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>.
543         </p>
544         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
545            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
546            in progress”.
547         </p>
548         <p>This Internet-Draft will expire on July 27, 2014.</p>
549      </div>
550      <div id="rfc.copyrightnotice">
551         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
552         <p>Copyright © 2014 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
553         <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
554            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
555            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
556            BSD License.
557         </p>
558         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
559            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
560            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
561            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
562            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
563            it into languages other than English.
564         </p>
565      </div>
566      <hr class="noprint">
567      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
568      <ul class="toc">
569         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
570               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#conformance">Conformance and Error Handling</a></li>
571               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
572            </ul>
573         </li>
574         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#validators">Validators</a><ul>
575               <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
576               <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
577                     <li><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
578                     <li><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
579                  </ul>
580               </li>
581               <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
582                     <li><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
583                     <li><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
584                     <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>
585                  </ul>
586               </li>
587               <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>
588            </ul>
589         </li>
590         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#preconditions">Precondition Header Fields</a><ul>
591               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
592               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
593               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
594               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
595               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
596            </ul>
597         </li>
598         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
599               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
600               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
601            </ul>
602         </li>
603         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#evaluation">Evaluation</a></li>
604         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#precedence">Precedence</a></li>
605         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
606               <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
607               <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
608            </ul>
609         </li>
610         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
611         <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
612         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
613               <li><a href="#rfc.section.10.1">10.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
614               <li><a href="#rfc.section.10.2">10.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
615            </ul>
616         </li>
617         <li><a href="#rfc.authors">Authors' Addresses</a></li>
618         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
619         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
620         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
621         <li><a href="#rfc.section.D">D.</a>&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
622               <li><a href="#rfc.section.D.1">D.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.24">Since draft-ietf-httpbis-p4-conditional-24</a></li>
623               <li><a href="#rfc.section.D.2">D.2</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.25">Since draft-ietf-httpbis-p4-conditional-25</a></li>
624            </ul>
625         </li>
626         <li><a href="#rfc.index">Index</a></li>
627      </ul>
628      <div id="introduction">
629         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#introduction">Introduction</a></h1>
630         <p id="rfc.section.1.p.1">Conditional requests are HTTP requests <a href="#Part2" id="rfc.xref.Part2.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> that include one or more header fields indicating a precondition to be tested before applying the method semantics to the
631            target resource. This document defines the HTTP/1.1 conditional request mechanisms in terms of the architecture, syntax notation,
632            and conformance criteria defined in <a href="#Part1" id="rfc.xref.Part1.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
633         </p>
634         <p id="rfc.section.1.p.2">Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem:
635            one client accidentally overwriting the work of another client that has been acting in parallel.
636         </p>
637         <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
638            state as observed in a previously obtained representation (one value in that set). A resource might have multiple current
639            representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests
640            to a "selected representation" (<a href="p2-semantics.html#representations" title="Representations">Section 3</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) will be consistent over time if the server intends to take advantage of conditionals. Regardless, if the mapping is inconsistent
641            and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates
642            to false.
643         </p>
644         <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>).
645         </p>
646         <div id="conformance">
647            <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>
648            <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"
649               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>.
650            </p>
651            <p id="rfc.section.1.1.p.2">Conformance criteria and considerations regarding error handling are defined in <a href="p1-messaging.html#conformance" title="Conformance and Error Handling">Section 2.5</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
652            </p>
653         </div>
654         <div id="notation">
655            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
656            <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="#Part1" id="rfc.xref.Part1.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates
657               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.
658            </p>
659         </div>
660      </div>
661      <div id="validators">
662         <div id="rfc.iref.m.1"></div>
663         <div id="rfc.iref.v.1"></div>
664         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#validators">Validators</a></h1>
665         <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:
666            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 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.
667         </p>
668         <div id="weak.and.strong.validators">
669            <div id="rfc.iref.v.2"></div>
670            <div id="rfc.iref.v.3"></div>
671            <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>
672            <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.
673               Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently.
674               Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator
675               in use and imposes restrictions on when weak validators can be used as preconditions.
676            </p>
677            <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
678               would be observable in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET.
679            </p>
680            <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
681               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
682               responses held by remote caches and authoring tools.
683            </p>
684            <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
685               an entry using a validator that it obtained in the distant past. A strong validator is unique across all versions of all representations
686               associated with a particular resource over time. However, there is no implication of uniqueness across representations of
687               different resources (i.e., the same strong validator might be in use for representations of multiple resources at the same
688               time and does not imply that those representations are equivalent).
689            </p>
690            <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
691               to a representation always results in a unique node name and revision identifier being assigned before the representation
692               is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the
693               data is available prior to the response header fields being sent and the digest does not need to be recalculated every time
694               a validation request is received. However, if a resource has distinct representations that differ only in their metadata,
695               such as might occur with content negotiation over media types that happen to share the same data format, then the origin server
696               needs to incorporate additional information in the validator to distinguish those representations.
697            </p>
698            <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.
699               This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to ensure
700               uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations
701               by 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.
702               In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses.
703            </p>
704            <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
705               be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in
706               order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server
707               load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might
708               be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between
709               those modifications.
710            </p>
711            <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
712               those representations have identical representation data. For example, if the origin server sends the same validator for a
713               representation with a gzip content coding applied as it does for a representation with no content coding, then that validator
714               is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation
715               metadata, such as when two different media types are available for the same representation data.
716            </p>
717            <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"
718               avoidance. Weak validators are only usable when the client does not require exact equality with previously obtained representation
719               data, such as when validating a cache entry or limiting a web traversal to recent changes.
720            </p>
721         </div>
722         <div id="header.last-modified">
723            <div id="rfc.iref.l.1"></div>
724            <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>
725            <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
726               believes the selected representation was last modified, as determined at the conclusion of handling the request.
727            </p>
728            <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>
729</pre><p id="rfc.section.2.2.p.3">An example of its use is</p>
730            <div id="rfc.figure.u.2"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
731</pre><div id="lastmod.generation">
732               <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>
733               <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,
734                  since its use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service
735                  scalability and reliability.
736               </p>
737               <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
738                  the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
739                  detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
740                  use its value to make conditional requests and test the validity of locally cached responses.
741               </p>
742               <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
743                  time, especially if the representation changes near the time that the response is generated.
744               </p>
745               <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,
746                  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
747                  on cache validation.
748               </p>
749               <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
750                  with a reliable clock.
751               </p>
752            </div>
753            <div id="lastmod.comparison">
754               <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>
755               <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
756                  strong, using the following rules:
757               </p>
758               <ul>
759                  <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
760                  <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
761                     presented validator.
762                  </li>
763               </ul>
764               <p id="rfc.section.2.2.2.p.2">or </p>
765               <ul>
766                  <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> header field, because the client has a cache entry, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> for the associated representation, and
767                  </li>
768                  <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
769                  </li>
770                  <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
771               </ul>
772               <p id="rfc.section.2.2.2.p.3">or </p>
773               <ul>
774                  <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
775                     and
776                  </li>
777                  <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
778                  </li>
779                  <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
780               </ul>
781               <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
782                  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
783                  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.
784               </p>
785            </div>
786         </div>
787         <div id="header.etag">
788            <div id="rfc.iref.e.1"></div>
789            <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a href="#header.etag">ETag</a></h2>
790            <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
791               conclusion of handling the request. An entity-tag is an opaque validator for differentiating between multiple representations
792               of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content
793               negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque
794               quoted string, possibly prefixed by a weakness indicator.
795            </p>
796            <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>
797
798  <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>
799  <a href="#header.etag" class="smpl">weak</a>       = %x57.2F ; "W/", case-sensitive
800  <a href="#header.etag" class="smpl">opaque-tag</a> = <a href="#imported.abnf" class="smpl">DQUOTE</a> *<a href="#header.etag" class="smpl">etagc</a> <a href="#imported.abnf" class="smpl">DQUOTE</a>
801  <a href="#header.etag" class="smpl">etagc</a>      = %x21 / %x23-7E / <a href="#imported.abnf" class="smpl">obs-text</a>
802             ; <a href="#imported.abnf" class="smpl">VCHAR</a> except double quotes, plus obs-text
803</pre><div class="note" id="rfc.section.2.3.p.3">
804               <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="http://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
805                  tags.
806               </p>
807            </div>
808            <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
809               modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
810               not consistently maintained.
811            </p>
812            <div id="rfc.figure.u.4"></div>
813            <p>Examples:</p><pre class="text">  ETag: "xyzzy"
814  ETag: W/"xyzzy"
815  ETag: ""
816</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
817               for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator
818               (<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).
819            </p>
820            <div id="entity.tag.generation">
821               <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>
822               <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
823                  the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
824                  sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
825                  entity-tag is constructed.
826               </p>
827               <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
828                  number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
829                  Other implementations might use a collision-resistant hash of representation content, a combination of various file attributes,
830                  or a modification timestamp that has sub-second resolution.
831               </p>
832               <p id="rfc.section.2.3.1.p.3">An origin server <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since
833                  the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability
834                  and reliability.
835               </p>
836            </div>
837            <div id="entity.tag.comparison">
838               <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>
839               <p id="rfc.section.2.3.2.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators
840                  or not:
841               </p>
842               <ul>
843                  <li><dfn>Strong comparison</dfn>: two entity-tags are equivalent if both are not weak and their opaque-tags match character-by-character.
844                  </li>
845                  <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
846                     as "weak".
847                  </li>
848               </ul>
849               <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>
850               <div id="rfc.table.u.1">
851                  <table class="tt full left" cellpadding="3" cellspacing="0">
852                     <thead>
853                        <tr>
854                           <th>ETag 1</th>
855                           <th>ETag 2</th>
856                           <th>Strong Comparison</th>
857                           <th>Weak Comparison</th>
858                        </tr>
859                     </thead>
860                     <tbody>
861                        <tr>
862                           <td class="left">W/"1"</td>
863                           <td class="left">W/"1"</td>
864                           <td class="left">no match</td>
865                           <td class="left">match</td>
866                        </tr>
867                        <tr>
868                           <td class="left">W/"1"</td>
869                           <td class="left">W/"2"</td>
870                           <td class="left">no match</td>
871                           <td class="left">no match</td>
872                        </tr>
873                        <tr>
874                           <td class="left">W/"1"</td>
875                           <td class="left">"1"</td>
876                           <td class="left">no match</td>
877                           <td class="left">match</td>
878                        </tr>
879                        <tr>
880                           <td class="left">"1"</td>
881                           <td class="left">"1"</td>
882                           <td class="left">match</td>
883                           <td class="left">match</td>
884                        </tr>
885                     </tbody>
886                  </table>
887               </div>
888            </div>
889            <div id="example.entity.tag.vs.conneg">
890               <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>
891               <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="#Part2" id="rfc.xref.Part2.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</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="#Part2" id="rfc.xref.Part2.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>):
892               </p>
893               <div id="rfc.figure.u.5"></div>
894               <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
895Host: www.example.com
896Accept-Encoding: gzip
897
898</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>
899               <div id="rfc.figure.u.6"></div>
900               <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
901Date: Fri, 26 Mar 2010 00:05:00 GMT
902ETag: "123-a"
903Content-Length: 70
904Vary: Accept-Encoding
905Content-Type: text/plain
906
907<span id="exbody">Hello World!
908Hello World!
909Hello World!
910Hello World!
911Hello World!
912</span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p>
913               <div id="rfc.figure.u.7"></div>
914               <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
915Date: Fri, 26 Mar 2010 00:05:00 GMT
916ETag: "123-b"
917Content-Length: 43
918Vary: Accept-Encoding
919Content-Type: text/plain
920Content-Encoding: gzip
921
922<em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7">
923                  <p><b>Note:</b> Content codings are a property of the representation data, so a strong entity-tag for a content-encoded representation has
924                     to be distinct from the entity tag of an unencoded representation to prevent potential conflicts during cache updates and
925                     range requests. In contrast, transfer codings (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 4</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) apply only during message transfer and do not result in distinct entity-tags.
926                  </p>
927               </div>
928            </div>
929         </div>
930         <div id="when.to.use.entity.tags.and.last-modified.dates">
931            <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>
932            <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:
933            </p>
934            <ul>
935               <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
936               </li>
937               <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,
938                  or if it is unfeasible to send a strong entity-tag.
939               </li>
940               <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.
941               </li>
942            </ul>
943            <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.
944            </p>
945            <p id="rfc.section.2.4.p.3">A client: </p>
946            <ul>
947               <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.
948               </li>
949               <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.
950               </li>
951               <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.
952               </li>
953               <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.
954               </li>
955            </ul>
956         </div>
957      </div>
958      <div id="preconditions">
959         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#preconditions">Precondition Header Fields</a></h1>
960         <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.
961         </p>
962         <div id="header.if-match">
963            <div id="rfc.iref.i.1"></div>
964            <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>
965            <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
966               current representation of the target resource, when the field-value is "*", or having a current representation of the target
967               resource that has an entity-tag matching a member of the list of entity-tags provided in the field-value.
968            </p>
969            <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
970               representation data.
971            </p>
972            <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>
973</pre><p id="rfc.section.3.1.p.4">Examples:</p>
974            <div id="rfc.figure.u.9"></div><pre class="text">  If-Match: "xyzzy"
975  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
976  If-Match: *
977</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
978               user agents might be acting in parallel on the same resource (i.e., to prevent the "lost update" problem). It can also be
979               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.
980            </p>
981            <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
982               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
983               of the selected representation.
984            </p>
985            <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
986               in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user
987               agent might not be aware of it, perhaps because the prior response was lost or a compatible change was made by some other
988               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
989               change made by the same user agent.
990            </p>
991            <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>
992         </div>
993         <div id="header.if-none-match">
994            <div id="rfc.iref.i.2"></div>
995            <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>
996            <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
997               any current representation of the target resource, when the field-value is "*", or having a selected representation with an
998               entity-tag that does not match any of those listed in the field-value.
999            </p>
1000            <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.
1001            </p>
1002            <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>
1003</pre><p id="rfc.section.3.2.p.4">Examples:</p>
1004            <div id="rfc.figure.u.11"></div><pre class="text">  If-None-Match: "xyzzy"
1005  If-None-Match: W/"xyzzy"
1006  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1007  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1008  If-None-Match: *
1009</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
1010               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
1011               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.
1012            </p>
1013            <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
1014               an existing representation of the target resource when the client believes that the resource does not have a current representation
1015               (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial
1016               representation for the target resource.
1017            </p>
1018            <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.
1019               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
1020               selected representation.
1021            </p>
1022            <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.
1023            </p>
1024            <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="#Part6" id="rfc.xref.Part6.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
1025            </p>
1026         </div>
1027         <div id="header.if-modified-since">
1028            <div id="rfc.iref.i.3"></div>
1029            <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>
1030            <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
1031               date being more recent than the date provided in the field-value. Transfer of the selected representation's data is avoided
1032               if that data has not changed.
1033            </p>
1034            <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>
1035</pre><p id="rfc.section.3.3.p.3">An example of the field is:</p>
1036            <div id="rfc.figure.u.13"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1037</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 the
1038               sake of interoperating with older intermediaries that might not implement <a href="#header.if-none-match" class="smpl">If-None-Match</a>.
1039            </p>
1040            <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
1041               neither GET nor HEAD.
1042            </p>
1043            <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.
1044            </p>
1045            <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
1046               does not have an entity-tag; and, 2) to limit the scope of a web traversal to resources that have recently changed.
1047            </p>
1048            <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
1049               synchronized or when the server has chosen to only honor exact timestamp matches (due to a problem with Last-Modified dates
1050               that appear to go "back in time" when the origin server's clock is corrected or a representation is restored from an archived
1051               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
1052               does not contain a <a href="#header.last-modified" class="smpl">Last-Modified</a> field.
1053            </p>
1054            <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
1055               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
1056               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.
1057            </p>
1058            <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
1059               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.
1060            </p>
1061            <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="#Part6" id="rfc.xref.Part6.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>.
1062            </p>
1063         </div>
1064         <div id="header.if-unmodified-since">
1065            <div id="rfc.iref.i.4"></div>
1066            <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>
1067            <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
1068               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.
1069            </p>
1070            <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>
1071</pre><p id="rfc.section.3.4.p.3">An example of the field is:</p>
1072            <div id="rfc.figure.u.15"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1073</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
1074               the sake of interoperating with older intermediaries that might not implement <a href="#header.if-match" class="smpl">If-Match</a>.
1075            </p>
1076            <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.
1077            </p>
1078            <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.
1079            </p>
1080            <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
1081               when multiple user agents might be acting in parallel on a resource that does not supply entity-tags with its representations
1082               (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.
1083            </p>
1084            <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
1085               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
1086               in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user
1087               agent might not be aware of that because the prior response message was lost or a compatible change was made by some other
1088               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
1089               change made by the same user agent.
1090            </p>
1091            <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
1092               response.
1093            </p>
1094         </div>
1095         <div id="header.if-range">
1096            <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>
1097            <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 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 412 response.
1098               If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>.
1099            </p>
1100         </div>
1101      </div>
1102      <div id="status.code.definitions">
1103         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#status.code.definitions">Status Code Definitions</a></h1>
1104         <div id="status.304">
1105            <div id="rfc.iref.21"></div>
1106            <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>
1107            <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 has evaluated to false. In other words, there is no need for the server
1108               to transfer a representation of the target resource because the request indicates that the client, which made the request
1109               conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored
1110               representation as if it were the payload of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response.
1111            </p>
1112            <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>.
1113            </p>
1114            <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,
1115               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
1116               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).
1117            </p>
1118            <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="#Part6" id="rfc.xref.Part6.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>. If the conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional
1119               GET to a shared proxy, then the proxy <em class="bcp14">SHOULD</em> forward the 304 response to that client.
1120            </p>
1121            <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>
1122         </div>
1123         <div id="status.412">
1124            <div id="rfc.iref.21"></div>
1125            <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>
1126            <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
1127               server. This response code allows the client to place preconditions on the current resource state (its current representations
1128               and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
1129            </p>
1130         </div>
1131      </div>
1132      <div id="evaluation">
1133         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#evaluation">Evaluation</a></h1>
1134         <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
1135            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
1136            other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx</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.
1137         </p>
1138         <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 <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
1139            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
1140            involve the selection or modification of a <a href="p2-semantics.html#representations" class="smpl">selected representation</a>, such as CONNECT, OPTIONS, or TRACE.
1141         </p>
1142         <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
1143            state of the target resource in general, or on a group of resources. For instance, the "If" header field in WebDAV can make
1144            a request conditional on various aspects of multiple resources, such as locks, if the recipient understands and implements
1145            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="http://tools.ietf.org/html/rfc4918#section-10.4">Section 10.4</a>).
1146         </p>
1147         <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
1148            with those of GET), there is no point in sending a conditional HEAD because a successful response is around the same size
1149            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.
1150         </p>
1151      </div>
1152      <div id="precedence">
1153         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#precedence">Precedence</a></h1>
1154         <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
1155            important. In practice, the fields defined in this document are consistently implemented in a single, logical order, since
1156            "lost update" preconditions have more strict requirements than cache validation, a validated cache is more efficient than
1157            a partial response, and entity tags are presumed to be more accurate than date validators.
1158         </p>
1159         <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:
1160         </p>
1161         <ol>
1162            <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:
1163               <ul>
1164                  <li>if true, continue to step <a href="#precedence3">3</a></li>
1165                  <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>)
1166                  </li>
1167               </ul>
1168            </li>
1169            <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:
1170               <ul>
1171                  <li>if true, continue to step <a href="#precedence3">3</a></li>
1172                  <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>)
1173                  </li>
1174               </ul>
1175            </li>
1176            <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:
1177               <ul>
1178                  <li>if true, continue to step <a href="#precedence5">5</a></li>
1179                  <li>if false for GET/HEAD, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
1180                  <li>if false for other methods, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li>
1181               </ul>
1182            </li>
1183            <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:
1184               <ul>
1185                  <li>if true, continue to step <a href="#precedence5">5</a></li>
1186                  <li>if false, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
1187               </ul>
1188            </li>
1189            <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:
1190               <ul>
1191                  <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="#Part5" id="rfc.xref.Part5.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></li>
1192               </ul>
1193            </li>
1194            <li id="precedencelast">Otherwise,
1195               <ul>
1196                  <li>all conditions are met, so perform the requested action and respond according to its success or failure.</li>
1197               </ul>
1198            </li>
1199         </ol>
1200         <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
1201            the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found
1202            in practice.
1203         </p>
1204      </div>
1205      <div id="IANA.considerations">
1206         <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a href="#IANA.considerations">IANA Considerations</a></h1>
1207         <div id="status.code.registration">
1208            <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>
1209            <p id="rfc.section.7.1.p.1">The 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; shall be updated with the registrations below:
1210            </p>
1211            <div id="rfc.table.1">
1212               <div id="iana.status.code.registration.table"></div>
1213               <table class="tt full left" cellpadding="3" cellspacing="0">
1214                  <thead>
1215                     <tr>
1216                        <th>Value</th>
1217                        <th>Description</th>
1218                        <th>Reference</th>
1219                     </tr>
1220                  </thead>
1221                  <tbody>
1222                     <tr>
1223                        <td class="left">304</td>
1224                        <td class="left">Not Modified</td>
1225                        <td class="left"><a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section&nbsp;4.1</a>
1226                        </td>
1227                     </tr>
1228                     <tr>
1229                        <td class="left">412</td>
1230                        <td class="left">Precondition Failed</td>
1231                        <td class="left"><a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section&nbsp;4.2</a>
1232                        </td>
1233                     </tr>
1234                  </tbody>
1235               </table>
1236            </div>
1237         </div>
1238         <div id="header.field.registration">
1239            <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>
1240            <p id="rfc.section.7.2.p.1">HTTP header fields are registered within the Message Header Field Registry maintained at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt;.
1241            </p>
1242            <p id="rfc.section.7.2.p.2">This document defines the following HTTP header fields, so their associated registry entries shall be updated according to
1243               the permanent registrations below (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>):
1244            </p>
1245            <div id="rfc.table.2">
1246               <div id="iana.header.registration.table"></div>
1247               <table class="tt full left" cellpadding="3" cellspacing="0">
1248                  <thead>
1249                     <tr>
1250                        <th>Header Field Name</th>
1251                        <th>Protocol</th>
1252                        <th>Status</th>
1253                        <th>Reference</th>
1254                     </tr>
1255                  </thead>
1256                  <tbody>
1257                     <tr>
1258                        <td class="left">ETag</td>
1259                        <td class="left">http</td>
1260                        <td class="left">standard</td>
1261                        <td class="left"><a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section&nbsp;2.3</a>
1262                        </td>
1263                     </tr>
1264                     <tr>
1265                        <td class="left">If-Match</td>
1266                        <td class="left">http</td>
1267                        <td class="left">standard</td>
1268                        <td class="left"><a href="#header.if-match" id="rfc.xref.header.if-match.2" title="If-Match">Section&nbsp;3.1</a>
1269                        </td>
1270                     </tr>
1271                     <tr>
1272                        <td class="left">If-Modified-Since</td>
1273                        <td class="left">http</td>
1274                        <td class="left">standard</td>
1275                        <td class="left"><a href="#header.if-modified-since" id="rfc.xref.header.if-modified-since.1" title="If-Modified-Since">Section&nbsp;3.3</a>
1276                        </td>
1277                     </tr>
1278                     <tr>
1279                        <td class="left">If-None-Match</td>
1280                        <td class="left">http</td>
1281                        <td class="left">standard</td>
1282                        <td class="left"><a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section&nbsp;3.2</a>
1283                        </td>
1284                     </tr>
1285                     <tr>
1286                        <td class="left">If-Unmodified-Since</td>
1287                        <td class="left">http</td>
1288                        <td class="left">standard</td>
1289                        <td class="left"><a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.2" title="If-Unmodified-Since">Section&nbsp;3.4</a>
1290                        </td>
1291                     </tr>
1292                     <tr>
1293                        <td class="left">Last-Modified</td>
1294                        <td class="left">http</td>
1295                        <td class="left">standard</td>
1296                        <td class="left"><a href="#header.last-modified" id="rfc.xref.header.last-modified.2" title="Last-Modified">Section&nbsp;2.2</a>
1297                        </td>
1298                     </tr>
1299                  </tbody>
1300               </table>
1301            </div>
1302            <p id="rfc.section.7.2.p.3">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1303         </div>
1304      </div>
1305      <div id="security.considerations">
1306         <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
1307         <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
1308            conditional request mechanisms. More general security considerations are addressed in HTTP messaging <a href="#Part1" id="rfc.xref.Part1.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a> and semantics <a href="#Part2" id="rfc.xref.Part2.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>.
1309         </p>
1310         <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
1311            changes, or detect man-in-the-middle attacks. At best, they enable more efficient cache updates and optimistic concurrent
1312            writes when all participants are behaving nicely. At worst, the conditions will fail and the client will receive a response
1313            that is no more harmful than an HTTP exchange without conditional requests.
1314         </p>
1315         <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
1316            invalid entity-tag that is unique to the user or user agent, send it in a cacheable response with a long freshness time, and
1317            then read that entity-tag in later conditional requests as a means of re-identifying that user or user agent. Such an identifying
1318            tag would become a persistent identifier for as long as the user agent retained the original cache entry. User agents that
1319            cache representations ought to ensure that the cache is cleared or replaced whenever the user performs privacy-maintaining
1320            actions, such as clearing stored cookies or changing to a private browsing mode.
1321         </p>
1322      </div>
1323      <div id="acks">
1324         <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a href="#acks">Acknowledgments</a></h1>
1325         <p id="rfc.section.9.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 10</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
1326         </p>
1327      </div>
1328      <h1 id="rfc.references"><a id="rfc.section.10" href="#rfc.section.10">10.</a> References
1329      </h1>
1330      <h2 id="rfc.references.1"><a href="#rfc.section.10.1" id="rfc.section.10.1">10.1</a> Normative References
1331      </h2>
1332      <table>
1333         <tr>
1334            <td class="reference"><b id="Part1">[Part1]</b></td>
1335            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), January&nbsp;2014.
1336            </td>
1337         </tr>
1338         <tr>
1339            <td class="reference"><b id="Part2">[Part2]</b></td>
1340            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), January&nbsp;2014.
1341            </td>
1342         </tr>
1343         <tr>
1344            <td class="reference"><b id="Part5">[Part5]</b></td>
1345            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), January&nbsp;2014.
1346            </td>
1347         </tr>
1348         <tr>
1349            <td class="reference"><b id="Part6">[Part6]</b></td>
1350            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Akamai">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">Hypertext Transfer Protocol (HTTP/1.1): Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), January&nbsp;2014.
1351            </td>
1352         </tr>
1353         <tr>
1354            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1355            <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="http://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
1356            </td>
1357         </tr>
1358         <tr>
1359            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1360            <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="http://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
1361            </td>
1362         </tr>
1363      </table>
1364      <h2 id="rfc.references.2"><a href="#rfc.section.10.2" id="rfc.section.10.2">10.2</a> Informative References
1365      </h2>
1366      <table>
1367         <tr>
1368            <td class="reference"><b id="BCP90">[BCP90]</b></td>
1369            <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="http://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
1370            </td>
1371         </tr>
1372         <tr>
1373            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1374            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1375            </td>
1376         </tr>
1377         <tr>
1378            <td class="reference"><b id="RFC4918">[RFC4918]</b></td>
1379            <td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="http://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC&nbsp;4918, June&nbsp;2007.
1380            </td>
1381         </tr>
1382      </table>
1383      <div class="avoidbreak">
1384         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1385         <p><b>Roy T. Fielding</b>
1386            (editor)
1387            <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>
1388         <p><b>Julian F. Reschke</b>
1389            (editor)
1390            <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>
1391      </div>
1392      <div id="changes.from.rfc.2616">
1393         <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>
1394         <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>)
1395         </p>
1396         <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>)
1397         </p>
1398         <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>)
1399         </p>
1400         <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
1401            situations (such as a PUT response). (<a href="#header.etag" id="rfc.xref.header.etag.4" title="ETag">Section&nbsp;2.3</a>)
1402         </p>
1403         <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>)
1404         </p>
1405      </div>
1406      <div id="imported.abnf">
1407         <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a href="#imported.abnf">Imported ABNF</a></h1>
1408         <p id="rfc.section.B.p.1">The following core rules are included by reference, as defined in <a href="http://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
1409            (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
1410            character).
1411         </p>
1412         <p id="rfc.section.B.p.2">The rules below are defined in <a href="#Part1" id="rfc.xref.Part1.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>:
1413         </p>
1414         <div id="rfc.figure.u.16"></div><pre class="inline">  <a href="#imported.abnf" class="smpl">OWS</a>           = &lt;OWS, defined in <a href="#Part1" id="rfc.xref.Part1.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.3</a>&gt;
1415  <a href="#imported.abnf" class="smpl">obs-text</a>      = &lt;obs-text, defined in <a href="#Part1" id="rfc.xref.Part1.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.6</a>&gt;
1416</pre><p id="rfc.section.B.p.4">The rules below are defined in other parts:</p>
1417         <div id="rfc.figure.u.17"></div><pre class="inline">  <a href="#imported.abnf" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part2" id="rfc.xref.Part2.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 7.1.1.1</a>&gt;
1418</pre></div>
1419      <div id="collected.abnf">
1420         <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a href="#collected.abnf">Collected ABNF</a></h1>
1421         <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="#Part1" id="rfc.xref.Part1.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
1422         </p>
1423         <div id="rfc.figure.u.18"></div><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
1424
1425<a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
1426
1427<a href="#header.if-match" class="smpl">If-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1428 entity-tag ] ) )
1429<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date
1430<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1431 entity-tag ] ) )
1432<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date
1433
1434<a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date
1435
1436<a href="#imported.abnf" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
1437
1438<a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag
1439<a href="#header.etag" class="smpl">etagc</a> = "!" / %x23-7E ; '#'-'~'
1440 / obs-text
1441
1442<a href="#imported.abnf" class="smpl">obs-text</a> = &lt;obs-text, defined in [Part1], Section 3.2.6&gt;
1443<a href="#header.etag" class="smpl">opaque-tag</a> = DQUOTE *etagc DQUOTE
1444
1445<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/
1446</pre></div>
1447      <div id="change.log">
1448         <h1 id="rfc.section.D"><a href="#rfc.section.D">D.</a>&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1449         <p id="rfc.section.D.p.1">Changes up to the IETF Last Call draft are summarized in &lt;<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-24#appendix-D">http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-24#appendix-D</a>&gt;.
1450         </p>
1451         <div id="changes.since.24">
1452            <h2 id="rfc.section.D.1"><a href="#rfc.section.D.1">D.1</a>&nbsp;<a href="#changes.since.24">Since draft-ietf-httpbis-p4-conditional-24</a></h2>
1453            <p id="rfc.section.D.1.p.1">Closed issues: </p>
1454            <ul>
1455               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/518">http://tools.ietf.org/wg/httpbis/trac/ticket/518</a>&gt;: "APPSDIR review of draft-ietf-httpbis-p4-conditional-24"
1456               </li>
1457            </ul>
1458         </div>
1459         <div id="changes.since.25">
1460            <h2 id="rfc.section.D.2"><a href="#rfc.section.D.2">D.2</a>&nbsp;<a href="#changes.since.25">Since draft-ietf-httpbis-p4-conditional-25</a></h2>
1461            <p id="rfc.section.D.2.p.1">Closed issues: </p>
1462            <ul>
1463               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/538">http://tools.ietf.org/wg/httpbis/trac/ticket/538</a>&gt;: "add 'stateless' to Abstract"
1464               </li>
1465               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/542">http://tools.ietf.org/wg/httpbis/trac/ticket/542</a>&gt;: "improve introduction of list rule"
1466               </li>
1467               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/549">http://tools.ietf.org/wg/httpbis/trac/ticket/549</a>&gt;: "augment security considerations with pointers to current research"
1468               </li>
1469            </ul>
1470         </div>
1471      </div>
1472      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1473      <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.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a>
1474      </p>
1475      <div class="print2col">
1476         <ul class="ind">
1477            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
1478                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.21"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">7.1</a></li>
1479               </ul>
1480            </li>
1481            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
1482                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.21"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">7.1</a></li>
1483               </ul>
1484            </li>
1485            <li><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul>
1486                  <li><em>BCP90</em>&nbsp;&nbsp;<a href="#rfc.xref.BCP90.1">7.2</a>, <a href="#BCP90"><b>10.2</b></a></li>
1487               </ul>
1488            </li>
1489            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
1490                  <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>
1491               </ul>
1492            </li>
1493            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
1494                  <li><tt>Grammar</tt>&nbsp;&nbsp;
1495                     <ul>
1496                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2.3</b></a></li>
1497                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2.3</b></a></li>
1498                        <li><tt>etagc</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>2.3</b></a></li>
1499                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>3.1</b></a></li>
1500                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>3.3</b></a></li>
1501                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>3.2</b></a></li>
1502                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>3.4</b></a></li>
1503                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2.2</b></a></li>
1504                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>2.3</b></a></li>
1505                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>2.3</b></a></li>
1506                     </ul>
1507                  </li>
1508               </ul>
1509            </li>
1510            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
1511                  <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>
1512                  <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>
1513                  <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>
1514                  <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>
1515               </ul>
1516            </li>
1517            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
1518                  <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>
1519               </ul>
1520            </li>
1521            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
1522                  <li>metadata&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>2</b></a></li>
1523               </ul>
1524            </li>
1525            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
1526                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1</a>, <a href="#rfc.xref.Part1.2">1.1</a>, <a href="#rfc.xref.Part1.3">1.2</a>, <a href="#rfc.xref.Part1.4">2.3.3</a>, <a href="#rfc.xref.Part1.5">8</a>, <a href="#rfc.xref.Part1.6">9</a>, <a href="#Part1"><b>10.1</b></a>, <a href="#rfc.xref.Part1.7">B</a>, <a href="#rfc.xref.Part1.8">B</a>, <a href="#rfc.xref.Part1.9">B</a>, <a href="#rfc.xref.Part1.10">C</a><ul>
1527                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.10">C</a></li>
1528                        <li><em>Section 2.5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.1</a></li>
1529                        <li><em>Section 3.2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.8">B</a></li>
1530                        <li><em>Section 3.2.6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.9">B</a></li>
1531                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">2.3.3</a></li>
1532                        <li><em>Section 7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.3">1.2</a></li>
1533                        <li><em>Section 10</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">9</a></li>
1534                     </ul>
1535                  </li>
1536                  <li><em>Part2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">1</a>, <a href="#rfc.xref.Part2.2">1</a>, <a href="#rfc.xref.Part2.3">2.3.3</a>, <a href="#rfc.xref.Part2.4">2.3.3</a>, <a href="#rfc.xref.Part2.5">3.2</a>, <a href="#rfc.xref.Part2.6">8</a>, <a href="#Part2"><b>10.1</b></a>, <a href="#rfc.xref.Part2.7">B</a><ul>
1537                        <li><em>Section 3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.2">1</a></li>
1538                        <li><em>Section 3.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">2.3.3</a></li>
1539                        <li><em>Section 4.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.5">3.2</a></li>
1540                        <li><em>Section 5.3.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.4">2.3.3</a></li>
1541                        <li><em>Section 7.1.1.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.7">B</a></li>
1542                     </ul>
1543                  </li>
1544                  <li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.5</a>, <a href="#rfc.xref.Part5.2">6</a>, <a href="#Part5"><b>10.1</b></a><ul>
1545                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.5</a></li>
1546                     </ul>
1547                  </li>
1548                  <li><em>Part6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">2.2.1</a>, <a href="#rfc.xref.Part6.3">2.3.1</a>, <a href="#rfc.xref.Part6.4">3.2</a>, <a href="#rfc.xref.Part6.5">3.3</a>, <a href="#rfc.xref.Part6.6">4.1</a>, <a href="#Part6"><b>10.1</b></a><ul>
1549                        <li><em>Section 4.3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.4">3.2</a>, <a href="#rfc.xref.Part6.5">3.3</a></li>
1550                        <li><em>Section 4.3.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.6">4.1</a></li>
1551                     </ul>
1552                  </li>
1553               </ul>
1554            </li>
1555            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
1556                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>10.1</b></a></li>
1557                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">2.3</a>, <a href="#RFC2616"><b>10.2</b></a><ul>
1558                        <li><em>Section 3.11</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">2.3</a></li>
1559                     </ul>
1560                  </li>
1561                  <li><em>RFC4918</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC4918.1">2</a>, <a href="#rfc.xref.RFC4918.2">5</a>, <a href="#RFC4918"><b>10.2</b></a><ul>
1562                        <li><em>Section 10.4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC4918.2">5</a></li>
1563                     </ul>
1564                  </li>
1565                  <li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.2</a>, <a href="#RFC5234"><b>10.1</b></a>, <a href="#rfc.xref.RFC5234.2">B</a><ul>
1566                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">B</a></li>
1567                     </ul>
1568                  </li>
1569               </ul>
1570            </li>
1571            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
1572                  <li>selected representation&nbsp;&nbsp;<a href="#rfc.iref.s.1"><b>1</b></a></li>
1573               </ul>
1574            </li>
1575            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
1576                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a><ul>
1577                        <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.3"><b>2.1</b></a></li>
1578                        <li>weak&nbsp;&nbsp;<a href="#rfc.iref.v.2"><b>2.1</b></a></li>
1579                     </ul>
1580                  </li>
1581               </ul>
1582            </li>
1583         </ul>
1584      </div>
1585   </body>
1586</html>
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