source: draft-ietf-httpbis/latest/p4-conditional.html @ 2362

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445       content: "HTTP/1.1 Conditional Requests";
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469</style><link rel="Contents" href="#rfc.toc">
470      <link rel="Author" href="#rfc.authors">
471      <link rel="Copyright" href="#rfc.copyrightnotice">
472      <link rel="Index" href="#rfc.index">
473      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
474      <link rel="Chapter" title="2 Validators" href="#rfc.section.2">
475      <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3">
476      <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4">
477      <link rel="Chapter" title="5 Evaluation" href="#rfc.section.5">
478      <link rel="Chapter" title="6 Precedence" href="#rfc.section.6">
479      <link rel="Chapter" title="7 IANA Considerations" href="#rfc.section.7">
480      <link rel="Chapter" title="8 Security Considerations" href="#rfc.section.8">
481      <link rel="Chapter" title="9 Acknowledgments" href="#rfc.section.9">
482      <link rel="Chapter" href="#rfc.section.10" title="10 References">
483      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
484      <link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B">
485      <link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C">
486      <link rel="Appendix" title="D Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.D">
487      <link href="p2-semantics.html" rel="prev">
488      <link href="p5-range.html" rel="next">
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490      <link rel="schema.dct" href="http://purl.org/dc/terms/">
491      <meta name="dct.creator" content="Fielding, R.">
492      <meta name="dct.creator" content="Reschke, J. F.">
493      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest">
494      <meta name="dct.issued" scheme="ISO8601" content="2013-08-26">
495      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
496      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an 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      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an 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.">
498   </head>
499   <body onload="init();">
500      <table class="header">
501         <tbody>
502            <tr>
503               <td class="left">HTTPbis Working Group</td>
504               <td class="right">R. Fielding, Editor</td>
505            </tr>
506            <tr>
507               <td class="left">Internet-Draft</td>
508               <td class="right">Adobe</td>
509            </tr>
510            <tr>
511               <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
512               </td>
513               <td class="right">J. Reschke, Editor</td>
514            </tr>
515            <tr>
516               <td class="left">Intended status: Standards Track</td>
517               <td class="right">greenbytes</td>
518            </tr>
519            <tr>
520               <td class="left">Expires: February 27, 2014</td>
521               <td class="right">August 26, 2013</td>
522            </tr>
523         </tbody>
524      </table>
525      <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p>
526      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
527      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
528         systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes,
529         request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request
530         when one or more preconditions evaluate to false.
531      </p>
532      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
533      <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;.
534      </p>
535      <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;.
536      </p>
537      <p>The changes in this draft are summarized in <a href="#changes.since.23" title="Since draft-ietf-httpbis-p4-conditional-23">Appendix&nbsp;D.5</a>.
538      </p>
539      <h1><a id="rfc.status" 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 February 27, 2014.</p>
549      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
550      <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
551      <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
552         and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
553         text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
554         BSD License.
555      </p>
556      <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
557         10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
558         allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
559         controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
560         works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
561         it into languages other than English.
562      </p>
563      <hr class="noprint">
564      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
565      <ul class="toc">
566         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
567               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#conformance">Conformance and Error Handling</a></li>
568               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
569            </ul>
570         </li>
571         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#validators">Validators</a><ul>
572               <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
573               <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
574                     <li><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
575                     <li><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
576                  </ul>
577               </li>
578               <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
579                     <li><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
580                     <li><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
581                     <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>
582                  </ul>
583               </li>
584               <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>
585            </ul>
586         </li>
587         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#preconditions">Precondition Header Fields</a><ul>
588               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
589               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
590               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
591               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
592               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
593            </ul>
594         </li>
595         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
596               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
597               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
598            </ul>
599         </li>
600         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#evaluation">Evaluation</a></li>
601         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#precedence">Precedence</a></li>
602         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
603               <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
604               <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
605            </ul>
606         </li>
607         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
608         <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
609         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
610               <li><a href="#rfc.section.10.1">10.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
611               <li><a href="#rfc.section.10.2">10.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
612            </ul>
613         </li>
614         <li><a href="#rfc.authors">Authors' Addresses</a></li>
615         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
616         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
617         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
618         <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>
619               <li><a href="#rfc.section.D.1">D.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.19">Since draft-ietf-httpbis-p4-conditional-19</a></li>
620               <li><a href="#rfc.section.D.2">D.2</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.20">Since draft-ietf-httpbis-p4-conditional-20</a></li>
621               <li><a href="#rfc.section.D.3">D.3</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.21">Since draft-ietf-httpbis-p4-conditional-21</a></li>
622               <li><a href="#rfc.section.D.4">D.4</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.22">Since draft-ietf-httpbis-p4-conditional-22</a></li>
623               <li><a href="#rfc.section.D.5">D.5</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.23">Since draft-ietf-httpbis-p4-conditional-23</a></li>
624            </ul>
625         </li>
626         <li><a href="#rfc.index">Index</a></li>
627      </ul>
628      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
629      <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
630         target resource. This document defines the HTTP/1.1 conditional request mechanisms in terms of the architecture, syntax notation,
631         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>.
632      </p>
633      <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:
634         one client accidentally overwriting the work of another client that has been acting in parallel.
635      </p>
636      <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
637         state as observed in a previously obtained representation (one value in that set). A resource might have multiple current
638         representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests
639         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
640         and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates
641         to false.
642      </p>
643      <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>).
644      </p>
645      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="conformance" href="#conformance">Conformance and Error Handling</a></h2>
646      <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"
647         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>.
648      </p>
649      <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>.
650      </p>
651      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="notation" href="#notation">Syntax Notation</a></h2>
652      <p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="p1-messaging.html#abnf.extension" title="ABNF list extension: #rule">Section 7</a> of <a href="#Part1" id="rfc.xref.Part1.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. <a href="#imported.abnf" title="Imported ABNF">Appendix&nbsp;B</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;C</a> shows the collected ABNF with the list rule expanded.
653      </p>
654      <div id="rfc.iref.m.1"></div>
655      <div id="rfc.iref.v.1"></div>
656      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="validators" href="#validators">Validators</a></h1>
657      <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:
658         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.
659      </p>
660      <div id="rfc.iref.v.2"></div>
661      <div id="rfc.iref.v.3"></div>
662      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak versus Strong</a></h2>
663      <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.
664         Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently.
665         Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator
666         in use and imposes restrictions on when weak validators can be used as preconditions.
667      </p>
668      <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
669         would be observable in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET.
670      </p>
671      <p id="rfc.section.2.1.p.3">A strong validator might change for other reasons, such as when a semantically significant part of the representation metadata
672         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
673         responses held by remote caches and authoring tools. A strong validator is unique across all representations of a given resource,
674         such that no two representations of that resource can share the same validator unless their representation data is identical.
675      </p>
676      <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
677         an entry using a validator that it obtained in the distant past. A strong validator is unique across all versions of all representations
678         associated with a particular resource over time. However, there is no implication of uniqueness across representations of
679         different resources (i.e., the same strong validator might be in use for representations of multiple resources at the same
680         time and does not imply that those representations are equivalent).
681      </p>
682      <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
683         to a representation always results in a unique node name and revision identifier being assigned before the representation
684         is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the
685         data is available prior to the response header fields being sent and the digest does not need to be recalculated every time
686         a validation request is received. However, if a resource has distinct representations that differ only in their metadata,
687         such as might occur with content negotiation over media types that happen to share the same data format, then the origin server <em class="bcp14">SHOULD</em> incorporate additional information in the validator to distinguish those representations.
688      </p>
689      <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.
690         This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to ensure
691         uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations
692         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.
693         In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses.
694      </p>
695      <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
696         be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in
697         order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server
698         load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might
699         be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between
700         those modifications.
701      </p>
702      <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
703         those representations have identical representation data. For example, if the origin server sends the same validator for a
704         representation with a gzip content coding applied as it does for a representation with no content coding, then that validator
705         is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation
706         metadata, such as when two different media types are available for the same representation data.
707      </p>
708      <p id="rfc.section.2.1.p.9">A "use" of a validator occurs when either a client generates a request and includes the validator in a precondition or when
709         a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of the
710         representation data. Strong validators are usable and preferred for all conditional requests, including cache validation,
711         partial content ranges, and "lost update" avoidance.
712      </p>
713      <div id="rfc.iref.l.1"></div>
714      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2>
715      <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
716         believes the selected representation was last modified, as determined at the conclusion of handling the request.
717      </p>
718      <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>
719</pre><p id="rfc.section.2.2.p.3">An example of its use is</p>
720      <div id="rfc.figure.u.2"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
721</pre><h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a id="lastmod.generation" href="#lastmod.generation">Generation</a></h3>
722      <p id="rfc.section.2.2.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined,
723         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
724         scalability and reliability.
725      </p>
726      <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
727         the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
728         detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
729         use its value to make conditional requests and test the validity of locally cached responses.
730      </p>
731      <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
732         time, especially if the representation changes near the time that the response is generated.
733      </p>
734      <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,
735         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
736         on cache validation.
737      </p>
738      <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
739         with a reliable clock.
740      </p>
741      <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a id="lastmod.comparison" href="#lastmod.comparison">Comparison</a></h3>
742      <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
743         strong, using the following rules:
744      </p>
745      <ul>
746         <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
747         <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
748            presented validator.
749         </li>
750      </ul>
751      <p id="rfc.section.2.2.2.p.2">or </p>
752      <ul>
753         <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
754         </li>
755         <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
756         </li>
757         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
758      </ul>
759      <p id="rfc.section.2.2.2.p.3">or </p>
760      <ul>
761         <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
762            and
763         </li>
764         <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
765         </li>
766         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
767      </ul>
768      <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
769         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
770         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.
771      </p>
772      <div id="rfc.iref.e.1"></div>
773      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="header.etag" href="#header.etag">ETag</a></h2>
774      <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
775         conclusion of handling the request. An entity-tag is an opaque validator for differentiating between multiple representations
776         of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content
777         negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque
778         quoted string, possibly prefixed by a weakness indicator.
779      </p>
780      <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>
781
782  <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>
783  <a href="#header.etag" class="smpl">weak</a>       = %x57.2F ; "W/", case-sensitive
784  <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>
785  <a href="#header.etag" class="smpl">etagc</a>      = %x21 / %x23-7E / <a href="#imported.abnf" class="smpl">obs-text</a>
786             ; <a href="#imported.abnf" class="smpl">VCHAR</a> except double quotes, plus obs-text
787</pre><div class="note" id="rfc.section.2.3.p.3">
788         <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
789            tags.
790         </p> 
791      </div>
792      <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
793         modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
794         not consistently maintained.
795      </p>
796      <div id="rfc.figure.u.4"></div>
797      <p>Examples:</p><pre class="text">  ETag: "xyzzy"
798  ETag: W/"xyzzy"
799  ETag: ""
800</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
801         for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator
802         (<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).
803      </p>
804      <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a id="entity.tag.generation" href="#entity.tag.generation">Generation</a></h3>
805      <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
806         the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
807         sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
808         entity-tag is constructed.
809      </p>
810      <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
811         number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
812         Other implementations might use a collision-resistant hash of representation content, a combination of various filesystem
813         attributes, or a modification timestamp that has sub-second resolution.
814      </p>
815      <p id="rfc.section.2.3.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since
816         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
817         and reliability.
818      </p>
819      <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a id="entity.tag.comparison" href="#entity.tag.comparison">Comparison</a></h3>
820      <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
821         or not:
822      </p>
823      <ul>
824         <li><dfn>Strong comparison</dfn>: two entity-tags are equivalent if both are not weak and their opaque-tags match character-by-character.
825         </li>
826         <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
827            as "weak".
828         </li>
829      </ul>
830      <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>
831      <div id="rfc.table.u.1">
832         <table class="tt full left" cellpadding="3" cellspacing="0">
833            <thead>
834               <tr>
835                  <th>ETag 1</th>
836                  <th>ETag 2</th>
837                  <th>Strong Comparison</th>
838                  <th>Weak Comparison</th>
839               </tr>
840            </thead>
841            <tbody>
842               <tr>
843                  <td class="left">W/"1"</td>
844                  <td class="left">W/"1"</td>
845                  <td class="left">no match</td>
846                  <td class="left">match</td>
847               </tr>
848               <tr>
849                  <td class="left">W/"1"</td>
850                  <td class="left">W/"2"</td>
851                  <td class="left">no match</td>
852                  <td class="left">no match</td>
853               </tr>
854               <tr>
855                  <td class="left">W/"1"</td>
856                  <td class="left">"1"</td>
857                  <td class="left">no match</td>
858                  <td class="left">match</td>
859               </tr>
860               <tr>
861                  <td class="left">"1"</td>
862                  <td class="left">"1"</td>
863                  <td class="left">match</td>
864                  <td class="left">match</td>
865               </tr>
866            </tbody>
867         </table>
868      </div>
869      <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags Varying on Content-Negotiated Resources</a></h3>
870      <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>):
871      </p>
872      <div id="rfc.figure.u.5"></div>
873      <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
874Host: www.example.com
875Accept-Encoding: gzip
876
877</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>
878      <div id="rfc.figure.u.6"></div>
879      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
880Date: Fri, 26 Mar 2010 00:05:00 GMT
881ETag: "123-a"
882Content-Length: 70
883Vary: Accept-Encoding
884Content-Type: text/plain
885
886<span id="exbody">Hello World!
887Hello World!
888Hello World!
889Hello World!
890Hello World!
891</span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p>
892      <div id="rfc.figure.u.7"></div>
893      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
894Date: Fri, 26 Mar 2010 00:05:00 GMT
895ETag: "123-b"
896Content-Length: 43
897Vary: Accept-Encoding
898Content-Type: text/plain
899Content-Encoding: gzip
900
901<em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7">
902         <p><b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation has to be distinct
903            from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings
904            (<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 require distinct entity-tags.
905         </p> 
906      </div>
907      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="when.to.use.entity.tags.and.last-modified.dates" href="#when.to.use.entity.tags.and.last-modified.dates">When to Use Entity-tags and Last-Modified Dates</a></h2>
908      <p id="rfc.section.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types
909         ought to be used, and for what purposes.
910      </p>
911      <p id="rfc.section.2.4.p.2">In <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> responses to GET or HEAD, an origin server:
912      </p>
913      <ul>
914         <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
915         </li>
916         <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,
917            or if it is unfeasible to send a strong entity-tag.
918         </li>
919         <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.
920         </li>
921      </ul>
922      <p id="rfc.section.2.4.p.3">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.
923      </p>
924      <p id="rfc.section.2.4.p.4">A client: </p>
925      <ul>
926         <li><em class="bcp14">MUST</em> use that entity-tag in any cache-conditional 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.
927         </li>
928         <li><em class="bcp14">SHOULD</em> use the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in non-subrange cache-conditional 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.
929         </li>
930         <li><em class="bcp14">MAY</em> use the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in subrange cache-conditional 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.
931         </li>
932         <li><em class="bcp14">SHOULD</em> use both validators in cache-conditional 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.
933         </li>
934      </ul>
935      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="preconditions" href="#preconditions">Precondition Header Fields</a></h1>
936      <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.
937      </p>
938      <div id="rfc.iref.i.1"></div>
939      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="header.if-match" href="#header.if-match">If-Match</a></h2>
940      <p id="rfc.section.3.1.p.1">The "If-Match" header field makes the request method conditional on the recipient cache or origin server either having at
941         least one current representation of the target resource, when the field-value is "*", or having a current representation of
942         the target resource that has an entity-tag matching a member of the list of entity-tags provided in the field-value. A recipient <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>).
943      </p>
944      <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>
945</pre><p id="rfc.section.3.1.p.3">Examples:</p>
946      <div id="rfc.figure.u.9"></div><pre class="text">  If-Match: "xyzzy"
947  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
948  If-Match: *
949</pre><p id="rfc.section.3.1.p.5">If-Match is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple
950         user agents might be acting in parallel on the same resource (i.e., to prevent the "lost update" problem). It can also be
951         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.
952      </p>
953      <p id="rfc.section.3.1.p.6">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
954         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
955         of the selected representation.
956      </p>
957      <p id="rfc.section.3.1.p.7">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
958         in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user
959         agent might not be aware of it, perhaps because the prior response was lost or a compatible change was made by some other
960         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
961         change made by the same user agent.
962      </p>
963      <p id="rfc.section.3.1.p.8">If the request semantics can be satisfied with a cached response, a recipient cache <em class="bcp14">MUST</em> evaluate the condition as part of its selection process for determining a suitable response for that primary cache key (see <a href="p6-cache.html#constructing.responses.from.caches" title="Constructing Responses from Caches">Section 4</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). The cache <em class="bcp14">MUST</em> generate a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response if the field-value is "*" and no suitable response is stored, or if the field-value is a list of entity-tags and
964         none of them match the entity-tag of a suitable stored response. Otherwise, the cache <em class="bcp14">MUST</em> generate a successful <a href="p2-semantics.html#status.2xx" class="smpl">2xx</a> response that reuses the most recent of its matching stored responses to satisfy the request.
965      </p>
966      <div id="rfc.iref.i.2"></div>
967      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.if-none-match" href="#header.if-none-match">If-None-Match</a></h2>
968      <p id="rfc.section.3.2.p.1">The "If-None-Match" header field makes the request method conditional on the recipient cache or origin server either not having
969         a current representation for the target resource, when the field-value is "*", or not having a current representation that
970         has an entity-tag matching a member of the list of entity-tags provided in the field-value. 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>).
971      </p>
972      <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>
973</pre><p id="rfc.section.3.2.p.3">Examples:</p>
974      <div id="rfc.figure.u.11"></div><pre class="text">  If-None-Match: "xyzzy"
975  If-None-Match: W/"xyzzy"
976  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
977  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
978  If-None-Match: *
979</pre><p id="rfc.section.3.2.p.5">If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum
980         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
981         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.
982      </p>
983      <p id="rfc.section.3.2.p.6">If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying
984         an existing representation of the target resource when the client believes that the resource does not have a current representation
985         (<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
986         representation for the target resource.
987      </p>
988      <p id="rfc.section.3.2.p.7">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.
989         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
990         selected representation.
991      </p>
992      <p id="rfc.section.3.2.p.8">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.
993      </p>
994      <p id="rfc.section.3.2.p.9">If the request semantics can be satisfied with a cached response, a recipient cache <em class="bcp14">SHOULD</em> evaluate the condition as part of its selection process for determining a suitable response for that primary cache key (see <a href="p6-cache.html#constructing.responses.from.caches" title="Constructing Responses from Caches">Section 4</a> of <a href="#Part6" id="rfc.xref.Part6.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). If the field-value is "*" and a suitable response is stored, or the field-value is a list of entity-tags and at least one
995         of them match the entity-tag of a suitable stored response, the cache <em class="bcp14">SHOULD</em> generate a <a href="#status.304" class="smpl">304 (Not Modified)</a> response using the most suitable of those matching responses as the selected representation. Otherwise, if the cache has one
996         or more suitable stored responses that do not match the condition, the cache <em class="bcp14">SHOULD</em> generate a successful <a href="p2-semantics.html#status.2xx" class="smpl">2xx</a> response that reuses the most suitable of those stored responses to satisfy the request. Finally, if the cache has no suitable
997         stored responses, the cache <em class="bcp14">SHOULD</em> forward the conditional request toward the origin server; if the received condition contains a list of entity-tags and the
998         cache has its own set of stored responses for that primary cache key, the cache <em class="bcp14">SHOULD</em> take the union of the received set with the set of entity-tags for its own stored set of responses (fresh or stale) and generate
999         an <a href="#header.if-none-match" class="smpl">If-None-Match</a> header field containing that union when it forwards the request toward the origin server.
1000      </p>
1001      <div id="rfc.iref.i.3"></div>
1002      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2>
1003      <p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field can be used with GET or HEAD to make the method conditional by modification date: if
1004         the selected representation has not been modified since the time specified in this field, then respond with a <a href="#status.304" class="smpl">304 (Not Modified)</a> status code and only send metadata that might be useful for identifying or updating a cached representation. Hence, transfer
1005         of the selected representation's data is avoided if that data has not changed.
1006      </p>
1007      <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>
1008</pre><p id="rfc.section.3.3.p.3">An example of the field is:</p>
1009      <div id="rfc.figure.u.13"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1010</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
1011         sake of interoperating with older intermediaries that might not implement <a href="#header.if-none-match" class="smpl">If-None-Match</a>.
1012      </p>
1013      <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
1014         neither GET nor HEAD.
1015      </p>
1016      <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. A recipient cache that evaluates
1017         this precondition <em class="bcp14">SHOULD</em> make its comparisons with respect to a stored <a href="#header.last-modified" class="smpl">Last-Modified</a> or <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field, when available in a prior response from that origin server, rather than comparing it to a timestamp generated
1018         by its own clock.
1019      </p>
1020      <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
1021         does not have an entity-tag; and, 2) to limit the scope of a web traversal to resources that have recently changed.
1022      </p>
1023      <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
1024         synchronized or when the server has chosen to only honor exact timestamp matches (due to a problem with Last-Modified dates
1025         that appear to go "back in time" when the origin server's clock is corrected or a representation is restored from an archived
1026         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
1027         does not contain a <a href="#header.last-modified" class="smpl">Last-Modified</a> field.
1028      </p>
1029      <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
1030         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 during a past run. Origin servers that choose an exact timestamp match based on the
1031         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.
1032      </p>
1033      <p id="rfc.section.3.3.p.11">If the request semantics can be satisfied with a cached response, a recipient cache <em class="bcp14">SHOULD</em> evaluate the condition as part of its selection process for determining a suitable response for that primary cache key (see <a href="p6-cache.html#constructing.responses.from.caches" title="Constructing Responses from Caches">Section 4</a> of <a href="#Part6" id="rfc.xref.Part6.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). The cache <em class="bcp14">SHOULD</em> forward the conditional request toward the origin server if it does not have any suitable stored responses. The cache <em class="bcp14">SHOULD</em> generate a successful <a href="p2-semantics.html#status.2xx" class="smpl">2xx</a> response, reusing the most recent of its stored responses to satisfy the request, if one of the following is true: 1) a stored
1034         response has a <a href="#header.last-modified" class="smpl">Last-Modified</a> field-value that is more recent than the conditional timestamp; 2) no <a href="#header.last-modified" class="smpl">Last-Modified</a> field is present, but a stored response has a <a href="p2-semantics.html#header.date" class="smpl">Date</a> field-value that is more recent than the conditional timestamp; or, 3) neither <a href="#header.last-modified" class="smpl">Last-Modified</a> nor <a href="p2-semantics.html#header.date" class="smpl">Date</a> is present, but the cache recorded a stored response as having been received at a time more recent than the conditional timestamp.
1035         Otherwise, the cache <em class="bcp14">SHOULD</em> generate a <a href="#status.304" class="smpl">304 (Not Modified)</a> response using the most recent of the stored responses as the selected representation.
1036      </p>
1037      <div id="rfc.iref.i.4"></div>
1038      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="header.if-unmodified-since" href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
1039      <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
1040         date being earlier 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.
1041      </p>
1042      <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>
1043</pre><p id="rfc.section.3.4.p.3">An example of the field is:</p>
1044      <div id="rfc.figure.u.15"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1045</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
1046         the sake of interoperating with older intermediaries that might not implement <a href="#header.if-match" class="smpl">If-Match</a>.
1047      </p>
1048      <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.
1049      </p>
1050      <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.
1051      </p>
1052      <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
1053         when multiple user agents might be acting in parallel on a resource that resource does not supply entity-tags with its representations
1054         (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.
1055      </p>
1056      <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 condition is false if the selected representation has been modified since the time specified in this field.
1057      </p>
1058      <p id="rfc.section.3.4.p.10">An origin server <em class="bcp14">MUST NOT</em> perform the requested method if a received If-Unmodified-Since 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
1059         in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user
1060         agent might not be aware of that because the prior response message was lost or a compatible change was made by some other
1061         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
1062         change made by the same user agent.
1063      </p>
1064      <p id="rfc.section.3.4.p.11">If the request semantics can be satisfied with a cached response, a recipient cache <em class="bcp14">MUST</em> evaluate the condition as part of its selection process for determining a suitable response for that primary cache key (see <a href="p6-cache.html#constructing.responses.from.caches" title="Constructing Responses from Caches">Section 4</a> of <a href="#Part6" id="rfc.xref.Part6.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>). The cache <em class="bcp14">MUST</em> forward the conditional request toward the origin server if it does not have any suitable stored responses. The cache <em class="bcp14">MUST</em> generate a successful <a href="p2-semantics.html#status.2xx" class="smpl">2xx</a> response that reuses the most recent of its suitable stored responses to satisfy the request if that stored response has a <a href="#header.last-modified" class="smpl">Last-Modified</a> date that is equal to or earlier than the date given in If-Unmodified-Since. Otherwise, the cache <em class="bcp14">MUST</em> respond with a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code.
1065      </p>
1066      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="header.if-range" href="#header.if-range">If-Range</a></h2>
1067      <p id="rfc.section.3.5.p.1">The "If-Range" header field provides a special conditional request mechanism that is similar the <a href="#header.if-match" class="smpl">If-Match</a> and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> 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.
1068         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>.
1069      </p>
1070      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="status.code.definitions" href="#status.code.definitions">Status Code Definitions</a></h1>
1071      <div id="rfc.iref.21"></div>
1072      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="status.304" href="#status.304">304 Not Modified</a></h2>
1073      <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
1074         to transfer a representation of the target resource because the request indicates that the client, which made the request
1075         conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored
1076         representation as if it were the payload of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response.
1077      </p>
1078      <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="#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>.
1079      </p>
1080      <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,
1081         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
1082         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).
1083      </p>
1084      <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 4.2.1</a> of <a href="#Part6" id="rfc.xref.Part6.8"><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
1085         GET to a shared proxy, then the proxy <em class="bcp14">SHOULD</em> forward the 304 response to that client.
1086      </p>
1087      <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>
1088      <div id="rfc.iref.21"></div>
1089      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="status.412" href="#status.412">412 Precondition Failed</a></h2>
1090      <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
1091         server. This response code allows the client to place preconditions on the current resource state (its current representations
1092         and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
1093      </p>
1094      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="evaluation" href="#evaluation">Evaluation</a></h1>
1095      <p id="rfc.section.5.p.1">Except when excluded below, a server <em class="bcp14">MUST</em> evaluate received request preconditions after it has successfully performed its normal request checks and just before it would
1096         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
1097         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.
1098      </p>
1099      <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
1100         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
1101         involve the selection or modification of a <a href="p2-semantics.html#representations" class="smpl">selected representation</a>, such as CONNECT, OPTIONS, or TRACE.
1102      </p>
1103      <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
1104         state of the target resource in general, or on a group of resources. For instance, the "If" header field in WebDAV can make
1105         a request conditional on various aspects of multiple resources, such as locks, if the recipient understands and implements
1106         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>).
1107      </p>
1108      <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
1109         with those of GET), there is no point in sending a conditional HEAD because a successful response is around the same size
1110         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.
1111      </p>
1112      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="precedence" href="#precedence">Precedence</a></h1>
1113      <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
1114         important. In practice, the fields defined in this document are consistently implemented in a single, logical order, since
1115         "lost update" preconditions have more strict requirements than cache validation, a validated cache is more efficient than
1116         a partial response, and entity tags are presumed to be more accurate than date validators.
1117      </p>
1118      <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:
1119      </p>
1120      <ol>
1121         <li id="precedence1">When <a href="#header.if-match" class="smpl">If-Match</a> is present, evaluate it:
1122            <ul>
1123               <li>if true, continue to step <a href="#precedence3">3</a></li>
1124               <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>)
1125               </li>
1126            </ul> 
1127         </li>
1128         <li id="precedence2">When <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 it:
1129            <ul>
1130               <li>if true, continue to step <a href="#precedence3">3</a></li>
1131               <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>)
1132               </li>
1133            </ul> 
1134         </li>
1135         <li id="precedence3">When <a href="#header.if-none-match" class="smpl">If-None-Match</a> is present, evaluate it:
1136            <ul>
1137               <li>if true, continue to step <a href="#precedence5">5</a></li>
1138               <li>if false for GET/HEAD, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
1139               <li>if false for other methods, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li>
1140            </ul> 
1141         </li>
1142         <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 it:
1143            <ul>
1144               <li>if true, continue to step <a href="#precedence5">5</a></li>
1145               <li>if false, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
1146            </ul> 
1147         </li>
1148         <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 If-Range:
1149            <ul>
1150               <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>
1151            </ul> 
1152         </li>
1153         <li id="precedencelast">Otherwise,
1154            <ul>
1155               <li>all conditions are met, so perform the requested action and respond according to its success or failure.</li>
1156            </ul> 
1157         </li>
1158      </ol>
1159      <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
1160         the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found
1161         in practice.
1162      </p>
1163      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
1164      <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="status.code.registration" href="#status.code.registration">Status Code Registration</a></h2>
1165      <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:
1166      </p>
1167      <div id="rfc.table.1">
1168         <div id="iana.status.code.registration.table"></div>
1169         <table class="tt full left" cellpadding="3" cellspacing="0">
1170            <thead>
1171               <tr>
1172                  <th>Value</th>
1173                  <th>Description</th>
1174                  <th>Reference</th>
1175               </tr>
1176            </thead>
1177            <tbody>
1178               <tr>
1179                  <td class="left">304</td>
1180                  <td class="left">Not Modified</td>
1181                  <td class="left"><a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section&nbsp;4.1</a> 
1182                  </td>
1183               </tr>
1184               <tr>
1185                  <td class="left">412</td>
1186                  <td class="left">Precondition Failed</td>
1187                  <td class="left"><a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section&nbsp;4.2</a> 
1188                  </td>
1189               </tr>
1190            </tbody>
1191         </table>
1192      </div>
1193      <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
1194      <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;.
1195      </p>
1196      <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
1197         the permanent registrations below (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>):
1198      </p>
1199      <div id="rfc.table.2">
1200         <div id="iana.header.registration.table"></div>
1201         <table class="tt full left" cellpadding="3" cellspacing="0">
1202            <thead>
1203               <tr>
1204                  <th>Header Field Name</th>
1205                  <th>Protocol</th>
1206                  <th>Status</th>
1207                  <th>Reference</th>
1208               </tr>
1209            </thead>
1210            <tbody>
1211               <tr>
1212                  <td class="left">ETag</td>
1213                  <td class="left">http</td>
1214                  <td class="left">standard</td>
1215                  <td class="left"><a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section&nbsp;2.3</a> 
1216                  </td>
1217               </tr>
1218               <tr>
1219                  <td class="left">If-Match</td>
1220                  <td class="left">http</td>
1221                  <td class="left">standard</td>
1222                  <td class="left"><a href="#header.if-match" id="rfc.xref.header.if-match.2" title="If-Match">Section&nbsp;3.1</a> 
1223                  </td>
1224               </tr>
1225               <tr>
1226                  <td class="left">If-Modified-Since</td>
1227                  <td class="left">http</td>
1228                  <td class="left">standard</td>
1229                  <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> 
1230                  </td>
1231               </tr>
1232               <tr>
1233                  <td class="left">If-None-Match</td>
1234                  <td class="left">http</td>
1235                  <td class="left">standard</td>
1236                  <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> 
1237                  </td>
1238               </tr>
1239               <tr>
1240                  <td class="left">If-Unmodified-Since</td>
1241                  <td class="left">http</td>
1242                  <td class="left">standard</td>
1243                  <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> 
1244                  </td>
1245               </tr>
1246               <tr>
1247                  <td class="left">Last-Modified</td>
1248                  <td class="left">http</td>
1249                  <td class="left">standard</td>
1250                  <td class="left"><a href="#header.last-modified" id="rfc.xref.header.last-modified.2" title="Last-Modified">Section&nbsp;2.2</a> 
1251                  </td>
1252               </tr>
1253            </tbody>
1254         </table>
1255      </div>
1256      <p id="rfc.section.7.2.p.3">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1257      <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1258      <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/1.1
1259         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>.
1260      </p>
1261      <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
1262         changes, or detect man-in-the-middle attacks. At best, they enable more efficient cache updates and optimistic concurrent
1263         writes when all participants are behaving nicely. At worst, the conditions will fail and the client will receive a response
1264         that is no more harmful than an HTTP exchange without conditional requests.
1265      </p>
1266      <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
1267         invalid entity-tag that is unique to the user or user agent, send it in a cacheable response with a long freshness time, and
1268         then read that entity-tag in later conditional requests as a means of re-identifying that user or user agent. Such an identifying
1269         tag would become a persistent identifier for as long as the user agent retained the original cache entry. User agents that
1270         cache representations ought to ensure that the cache is cleared or replaced whenever the user performs privacy-maintaining
1271         actions, such as clearing stored cookies or changing to a private browsing mode.
1272      </p>
1273      <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a id="acks" href="#acks">Acknowledgments</a></h1>
1274      <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>.
1275      </p>
1276      <h1 id="rfc.references"><a id="rfc.section.10" href="#rfc.section.10">10.</a> References
1277      </h1>
1278      <h2 id="rfc.references.1"><a href="#rfc.section.10.1" id="rfc.section.10.1">10.1</a> Normative References
1279      </h2>
1280      <table>
1281         <tr>
1282            <td class="reference"><b id="Part1">[Part1]</b></td>
1283            <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), August&nbsp;2013.
1284            </td>
1285         </tr>
1286         <tr>
1287            <td class="reference"><b id="Part2">[Part2]</b></td>
1288            <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), August&nbsp;2013.
1289            </td>
1290         </tr>
1291         <tr>
1292            <td class="reference"><b id="Part5">[Part5]</b></td>
1293            <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), August&nbsp;2013.
1294            </td>
1295         </tr>
1296         <tr>
1297            <td class="reference"><b id="Part6">[Part6]</b></td>
1298            <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), August&nbsp;2013.
1299            </td>
1300         </tr>
1301         <tr>
1302            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1303            <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.
1304            </td>
1305         </tr>
1306         <tr>
1307            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1308            <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.
1309            </td>
1310         </tr>
1311      </table>
1312      <h2 id="rfc.references.2"><a href="#rfc.section.10.2" id="rfc.section.10.2">10.2</a> Informative References
1313      </h2>
1314      <table>
1315         <tr>
1316            <td class="reference"><b id="BCP90">[BCP90]</b></td>
1317            <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.
1318            </td>
1319         </tr>
1320         <tr>
1321            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1322            <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.
1323            </td>
1324         </tr>
1325         <tr>
1326            <td class="reference"><b id="RFC4918">[RFC4918]</b></td>
1327            <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.
1328            </td>
1329         </tr>
1330      </table>
1331      <div class="avoidbreak">
1332         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1333         <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
1334               (editor)
1335               <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:fielding@gbiv.com"><span class="email">fielding@gbiv.com</span></a></span><span class="vcardline">URI: <a href="http://roy.gbiv.com/" class="url">http://roy.gbiv.com/</a></span></address>
1336         <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
1337               (editor)
1338               <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline">Email: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address>
1339      </div>
1340      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
1341      <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>)
1342      </p>
1343      <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>).
1344      </p>
1345      <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>)
1346      </p>
1347      <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
1348         situations (such as a PUT response). (<a href="#header.etag" id="rfc.xref.header.etag.4" title="ETag">Section&nbsp;2.3</a>)
1349      </p>
1350      <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>)
1351      </p>
1352      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="imported.abnf" href="#imported.abnf">Imported ABNF</a></h1>
1353      <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
1354         (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
1355         character).
1356      </p>
1357      <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>:
1358      </p>
1359      <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;
1360  <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;
1361</pre><p id="rfc.section.B.p.4">The rules below are defined in other parts:</p>
1362      <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;
1363</pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
1364      <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>.
1365      </p>
1366      <div id="rfc.figure.u.18"></div><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
1367
1368<a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
1369
1370<a href="#header.if-match" class="smpl">If-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1371 entity-tag ] ) )
1372<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date
1373<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1374 entity-tag ] ) )
1375<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date
1376
1377<a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date
1378
1379<a href="#imported.abnf" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
1380
1381<a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag
1382<a href="#header.etag" class="smpl">etagc</a> = "!" / %x23-7E ; '#'-'~'
1383 / obs-text
1384
1385<a href="#imported.abnf" class="smpl">obs-text</a> = &lt;obs-text, defined in [Part1], Section 3.2.6&gt;
1386<a href="#header.etag" class="smpl">opaque-tag</a> = DQUOTE *etagc DQUOTE
1387
1388<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/
1389</pre><h1 id="rfc.section.D"><a href="#rfc.section.D">D.</a>&nbsp;<a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1390      <p id="rfc.section.D.p.1">Changes up to the first Working Group Last Call draft are summarized in &lt;<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C">http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C</a>&gt;.
1391      </p>
1392      <h2 id="rfc.section.D.1"><a href="#rfc.section.D.1">D.1</a>&nbsp;<a id="changes.since.19" href="#changes.since.19">Since draft-ietf-httpbis-p4-conditional-19</a></h2>
1393      <p id="rfc.section.D.1.p.1">Closed issues: </p>
1394      <ul>
1395         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/241">http://tools.ietf.org/wg/httpbis/trac/ticket/241</a>&gt;: "Need to clarify eval order/interaction of conditional headers"
1396         </li>
1397         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/345">http://tools.ietf.org/wg/httpbis/trac/ticket/345</a>&gt;: "Required headers on 304 and 206"
1398         </li>
1399         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/350">http://tools.ietf.org/wg/httpbis/trac/ticket/350</a>&gt;: "Optionality of Conditional Request Support"
1400         </li>
1401         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/354">http://tools.ietf.org/wg/httpbis/trac/ticket/354</a>&gt;: "ETags and Conditional Requests"
1402         </li>
1403         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/361">http://tools.ietf.org/wg/httpbis/trac/ticket/361</a>&gt;: "ABNF requirements for recipients"
1404         </li>
1405         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/363">http://tools.ietf.org/wg/httpbis/trac/ticket/363</a>&gt;: "Rare cases"
1406         </li>
1407         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/365">http://tools.ietf.org/wg/httpbis/trac/ticket/365</a>&gt;: "Conditional Request Security Considerations"
1408         </li>
1409         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/371">http://tools.ietf.org/wg/httpbis/trac/ticket/371</a>&gt;: "If-Modified-Since lacks definition for method != GET"
1410         </li>
1411         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/372">http://tools.ietf.org/wg/httpbis/trac/ticket/372</a>&gt;: "refactor conditional header field descriptions"
1412         </li>
1413      </ul>
1414      <h2 id="rfc.section.D.2"><a href="#rfc.section.D.2">D.2</a>&nbsp;<a id="changes.since.20" href="#changes.since.20">Since draft-ietf-httpbis-p4-conditional-20</a></h2>
1415      <p id="rfc.section.D.2.p.1"></p>
1416      <ul>
1417         <li>Conformance criteria and considerations regarding error handling are now defined in Part 1.</li>
1418      </ul>
1419      <h2 id="rfc.section.D.3"><a href="#rfc.section.D.3">D.3</a>&nbsp;<a id="changes.since.21" href="#changes.since.21">Since draft-ietf-httpbis-p4-conditional-21</a></h2>
1420      <p id="rfc.section.D.3.p.1">Closed issues: </p>
1421      <ul>
1422         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/96">http://tools.ietf.org/wg/httpbis/trac/ticket/96</a>&gt;: "Conditional GET text"
1423         </li>
1424         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/350">http://tools.ietf.org/wg/httpbis/trac/ticket/350</a>&gt;: "Optionality of Conditional Request Support"
1425         </li>
1426         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/384">http://tools.ietf.org/wg/httpbis/trac/ticket/384</a>&gt;: "unclear prose in definition of 304"
1427         </li>
1428         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/401">http://tools.ietf.org/wg/httpbis/trac/ticket/401</a>&gt;: "ETags and Conneg"
1429         </li>
1430         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/402">http://tools.ietf.org/wg/httpbis/trac/ticket/402</a>&gt;: "Comparison function for If-Match and If-None-Match"
1431         </li>
1432         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/406">http://tools.ietf.org/wg/httpbis/trac/ticket/406</a>&gt;: "304 without validator"
1433         </li>
1434         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/427">http://tools.ietf.org/wg/httpbis/trac/ticket/427</a>&gt;: "If-Match and 428"
1435         </li>
1436      </ul>
1437      <h2 id="rfc.section.D.4"><a href="#rfc.section.D.4">D.4</a>&nbsp;<a id="changes.since.22" href="#changes.since.22">Since draft-ietf-httpbis-p4-conditional-22</a></h2>
1438      <p id="rfc.section.D.4.p.1">Closed issues: </p>
1439      <ul>
1440         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/436">http://tools.ietf.org/wg/httpbis/trac/ticket/436</a>&gt;: "explain list expansion in ABNF appendices"
1441         </li>
1442         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/437">http://tools.ietf.org/wg/httpbis/trac/ticket/437</a>&gt;: "incorrect example dates"
1443         </li>
1444      </ul>
1445      <p id="rfc.section.D.4.p.2">Partly resolved issues: </p>
1446      <ul>
1447         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/461">http://tools.ietf.org/wg/httpbis/trac/ticket/461</a>&gt;: "Editorial suggestions"
1448         </li>
1449      </ul>
1450      <h2 id="rfc.section.D.5"><a href="#rfc.section.D.5">D.5</a>&nbsp;<a id="changes.since.23" href="#changes.since.23">Since draft-ietf-httpbis-p4-conditional-23</a></h2>
1451      <p id="rfc.section.D.5.p.1">Closed issues: </p>
1452      <ul>
1453         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/455">http://tools.ietf.org/wg/httpbis/trac/ticket/455</a>&gt;: "PUT + If-Match over-constrained?"
1454         </li>
1455         <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/479">http://tools.ietf.org/wg/httpbis/trac/ticket/479</a>&gt;: MUSTs and other feedback"
1456         </li>
1457      </ul>
1458      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1459      <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> 
1460      </p>
1461      <div class="print2col">
1462         <ul class="ind">
1463            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
1464                  <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>
1465               </ul>
1466            </li>
1467            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
1468                  <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>
1469               </ul>
1470            </li>
1471            <li><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul>
1472                  <li><em>BCP90</em>&nbsp;&nbsp;<a href="#rfc.xref.BCP90.1">7.2</a>, <a href="#BCP90"><b>10.2</b></a></li>
1473               </ul>
1474            </li>
1475            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
1476                  <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>
1477               </ul>
1478            </li>
1479            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
1480                  <li><tt>Grammar</tt>&nbsp;&nbsp;
1481                     <ul>
1482                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2.3</b></a></li>
1483                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2.3</b></a></li>
1484                        <li><tt>etagc</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>2.3</b></a></li>
1485                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>3.1</b></a></li>
1486                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>3.3</b></a></li>
1487                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>3.2</b></a></li>
1488                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>3.4</b></a></li>
1489                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2.2</b></a></li>
1490                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>2.3</b></a></li>
1491                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>2.3</b></a></li>
1492                     </ul>
1493                  </li>
1494               </ul>
1495            </li>
1496            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
1497                  <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>
1498                  <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>
1499                  <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>
1500                  <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>
1501               </ul>
1502            </li>
1503            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
1504                  <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>
1505               </ul>
1506            </li>
1507            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
1508                  <li>metadata&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>2</b></a></li>
1509               </ul>
1510            </li>
1511            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
1512                  <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>
1513                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.10">C</a></li>
1514                        <li><em>Section 2.5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.1</a></li>
1515                        <li><em>Section 3.2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.8">B</a></li>
1516                        <li><em>Section 3.2.6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.9">B</a></li>
1517                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">2.3.3</a></li>
1518                        <li><em>Section 7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.3">1.2</a></li>
1519                        <li><em>Section 10</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">9</a></li>
1520                     </ul>
1521                  </li>
1522                  <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>
1523                        <li><em>Section 3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.2">1</a></li>
1524                        <li><em>Section 3.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">2.3.3</a></li>
1525                        <li><em>Section 4.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.5">3.2</a></li>
1526                        <li><em>Section 5.3.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.4">2.3.3</a></li>
1527                        <li><em>Section 7.1.1.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.7">B</a></li>
1528                     </ul>
1529                  </li>
1530                  <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>
1531                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.5</a></li>
1532                     </ul>
1533                  </li>
1534                  <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">3.1</a>, <a href="#rfc.xref.Part6.5">3.2</a>, <a href="#rfc.xref.Part6.6">3.3</a>, <a href="#rfc.xref.Part6.7">3.4</a>, <a href="#rfc.xref.Part6.8">4.1</a>, <a href="#Part6"><b>10.1</b></a><ul>
1535                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.4">3.1</a>, <a href="#rfc.xref.Part6.5">3.2</a>, <a href="#rfc.xref.Part6.6">3.3</a>, <a href="#rfc.xref.Part6.7">3.4</a></li>
1536                        <li><em>Section 4.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.8">4.1</a></li>
1537                     </ul>
1538                  </li>
1539               </ul>
1540            </li>
1541            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
1542                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>10.1</b></a></li>
1543                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">2.3</a>, <a href="#RFC2616"><b>10.2</b></a><ul>
1544                        <li><em>Section 3.11</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">2.3</a></li>
1545                     </ul>
1546                  </li>
1547                  <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>
1548                        <li><em>Section 10.4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC4918.2">5</a></li>
1549                     </ul>
1550                  </li>
1551                  <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>
1552                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">B</a></li>
1553                     </ul>
1554                  </li>
1555               </ul>
1556            </li>
1557            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
1558                  <li>selected representation&nbsp;&nbsp;<a href="#rfc.iref.s.1"><b>1</b></a></li>
1559               </ul>
1560            </li>
1561            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
1562                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a><ul>
1563                        <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.3"><b>2.1</b></a></li>
1564                        <li>weak&nbsp;&nbsp;<a href="#rfc.iref.v.2"><b>2.1</b></a></li>
1565                     </ul>
1566                  </li>
1567               </ul>
1568            </li>
1569         </ul>
1570      </div>
1571   </body>
1572</html>
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