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355       content: "HTTP/1.1, Part 4";
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358       content: "Fielding, et al.";
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379</style><link rel="Contents" href="#rfc.toc">
380      <link rel="Author" href="#rfc.authors">
381      <link rel="Copyright" href="#rfc.copyrightnotice">
382      <link rel="Index" href="#rfc.index">
383      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
384      <link rel="Chapter" title="2 Entity Tags" href="#rfc.section.2">
385      <link rel="Chapter" title="3 Status Code Definitions" href="#rfc.section.3">
386      <link rel="Chapter" title="4 Weak and Strong Validators" href="#rfc.section.4">
387      <link rel="Chapter" title="5 Rules for When to Use Entity Tags and Last-Modified Dates" href="#rfc.section.5">
388      <link rel="Chapter" title="6 Header Field Definitions" href="#rfc.section.6">
389      <link rel="Chapter" title="7 IANA Considerations" href="#rfc.section.7">
390      <link rel="Chapter" title="8 Security Considerations" href="#rfc.section.8">
391      <link rel="Chapter" title="9 Acknowledgments" href="#rfc.section.9">
392      <link rel="Chapter" href="#rfc.section.10" title="10 References">
393      <link rel="Appendix" title="A Compatibility with Previous Versions" href="#rfc.section.A">
394      <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B">
395      <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C">
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397      <link rel="schema.dct" href="http://purl.org/dc/terms/">
398      <meta name="dct.creator" content="Fielding, R.">
399      <meta name="dct.creator" content="Gettys, J.">
400      <meta name="dct.creator" content="Mogul, J.">
401      <meta name="dct.creator" content="Frystyk, H.">
402      <meta name="dct.creator" content="Masinter, L.">
403      <meta name="dct.creator" content="Leach, P.">
404      <meta name="dct.creator" content="Berners-Lee, T.">
405      <meta name="dct.creator" content="Lafon, Y.">
406      <meta name="dct.creator" content="Reschke, J. F.">
407      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-06">
408      <meta name="dct.issued" scheme="ISO8601" content="2009-03-09">
409      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
410      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.">
411      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.">
412   </head>
413   <body>
414      <table class="header">
415         <tbody>
416            <tr>
417               <td class="left">HTTPbis Working Group</td>
418               <td class="right">R. Fielding, Editor</td>
419            </tr>
420            <tr>
421               <td class="left">Internet-Draft</td>
422               <td class="right">Day Software</td>
423            </tr>
424            <tr>
425               <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved)
426               </td>
427               <td class="right">J. Gettys</td>
428            </tr>
429            <tr>
430               <td class="left">Intended status: Standards Track</td>
431               <td class="right">One Laptop per Child</td>
432            </tr>
433            <tr>
434               <td class="left">Expires: September 10, 2009</td>
435               <td class="right">J. Mogul</td>
436            </tr>
437            <tr>
438               <td class="left"></td>
439               <td class="right">HP</td>
440            </tr>
441            <tr>
442               <td class="left"></td>
443               <td class="right">H. Frystyk</td>
444            </tr>
445            <tr>
446               <td class="left"></td>
447               <td class="right">Microsoft</td>
448            </tr>
449            <tr>
450               <td class="left"></td>
451               <td class="right">L. Masinter</td>
452            </tr>
453            <tr>
454               <td class="left"></td>
455               <td class="right">Adobe Systems</td>
456            </tr>
457            <tr>
458               <td class="left"></td>
459               <td class="right">P. Leach</td>
460            </tr>
461            <tr>
462               <td class="left"></td>
463               <td class="right">Microsoft</td>
464            </tr>
465            <tr>
466               <td class="left"></td>
467               <td class="right">T. Berners-Lee</td>
468            </tr>
469            <tr>
470               <td class="left"></td>
471               <td class="right">W3C/MIT</td>
472            </tr>
473            <tr>
474               <td class="left"></td>
475               <td class="right">Y. Lafon, Editor</td>
476            </tr>
477            <tr>
478               <td class="left"></td>
479               <td class="right">W3C</td>
480            </tr>
481            <tr>
482               <td class="left"></td>
483               <td class="right">J. Reschke, Editor</td>
484            </tr>
485            <tr>
486               <td class="left"></td>
487               <td class="right">greenbytes</td>
488            </tr>
489            <tr>
490               <td class="left"></td>
491               <td class="right">March 9, 2009</td>
492            </tr>
493         </tbody>
494      </table>
495      <p class="title">HTTP/1.1, part 4: Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-06</span></p>
496      <div id="rfc.status">
497         <h1><a href="#rfc.status">Status of this Memo</a></h1>
498         <p>This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain
499            material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s)
500            controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of
501            such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the
502            copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of
503            it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it
504            into languages other than English.
505         </p>
506         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note
507            that other groups may also distribute working documents as Internet-Drafts.
508         </p>
509         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
510            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
511            in progress”.
512         </p>
513         <p>The list of current Internet-Drafts can be accessed at <a href="http://www.ietf.org/ietf/1id-abstracts.txt">http://www.ietf.org/ietf/1id-abstracts.txt</a>.
514         </p>
515         <p>The list of Internet-Draft Shadow Directories can be accessed at <a href="http://www.ietf.org/shadow.html">http://www.ietf.org/shadow.html</a>.
516         </p>
517         <p>This Internet-Draft will expire on September 10, 2009.</p>
518      </div>
519      <div id="rfc.copyrightnotice">
520         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
521         <p>Copyright © 2009 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
522         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date
523            of publication of this document (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>). Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
524         </p>
525      </div>
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, hypermedia information
528         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the
529         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part
530         4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.
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 should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues
534         list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/11">http://tools.ietf.org/wg/httpbis/trac/report/11</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
535      </p>
536      <p>The changes in this draft are summarized in <a href="#changes.since.05" title="Since draft-ietf-httpbis-p4-conditional-05">Appendix&nbsp;C.7</a>.
537      </p>
538      <hr class="noprint">
539      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
540      <ul class="toc">
541         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
542               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
543               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul>
544                     <li><a href="#rfc.section.1.2.1">1.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#core.rules">Core Rules</a></li>
545                     <li><a href="#rfc.section.1.2.2">1.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></li>
546                  </ul>
547               </li>
548            </ul>
549         </li>
550         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tags">Entity Tags</a></li>
551         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Status Code Definitions</a><ul>
552               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
553               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
554            </ul>
555         </li>
556         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak and Strong Validators</a></li>
557         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity Tags and Last-Modified Dates</a></li>
558         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul>
559               <li><a href="#rfc.section.6.1">6.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a></li>
560               <li><a href="#rfc.section.6.2">6.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
561               <li><a href="#rfc.section.6.3">6.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
562               <li><a href="#rfc.section.6.4">6.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
563               <li><a href="#rfc.section.6.5">6.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
564               <li><a href="#rfc.section.6.6">6.6</a>&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a></li>
565            </ul>
566         </li>
567         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
568               <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#message.header.registration">Message Header Registration</a></li>
569            </ul>
570         </li>
571         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
572         <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
573         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
574               <li><a href="#rfc.section.10.1">10.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
575               <li><a href="#rfc.section.10.2">10.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
576            </ul>
577         </li>
578         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul>
579               <li><a href="#rfc.section.A.1">A.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
580            </ul>
581         </li>
582         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
583         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
584               <li><a href="#rfc.section.C.1">C.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC2616</a></li>
585               <li><a href="#rfc.section.C.2">C.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p4-conditional-00</a></li>
586               <li><a href="#rfc.section.C.3">C.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p4-conditional-01</a></li>
587               <li><a href="#rfc.section.C.4">C.4</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></li>
588               <li><a href="#rfc.section.C.5">C.5</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></li>
589               <li><a href="#rfc.section.C.6">C.6</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></li>
590               <li><a href="#rfc.section.C.7">C.7</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></li>
591            </ul>
592         </li>
593         <li><a href="#rfc.index">Index</a></li>
594         <li><a href="#rfc.authors">Authors' Addresses</a></li>
595      </ul>
596      <div id="introduction">
597         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#introduction">Introduction</a></h1>
598         <p id="rfc.section.1.p.1">This document defines HTTP/1.1 response metadata for indicating potential changes to payload content, including modification
599            time stamps and opaque entity-tags, and the HTTP conditional request mechanisms that allow preconditions to be placed on a
600            request method. Conditional GET requests allow for efficient cache updates. Other conditional request methods are used to
601            protect against overwriting or misunderstanding the state of a resource that has been changed unbeknownst to the requesting
602            client.
603         </p>
604         <p id="rfc.section.1.p.2">This document is currently disorganized in order to minimize the changes between drafts and enable reviewers to see the smaller
605            errata changes. The next draft will reorganize the sections to better reflect the content. In particular, the sections on
606            resource metadata will be discussed first and then followed by each conditional request-header, concluding with a definition
607            of precedence and the expectation of ordering strong validator checks before weak validator checks. It is likely that more
608            content from <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a> will migrate to this part, where appropriate. The current mess reflects how widely dispersed these topics and associated requirements
609            had become in <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
610         </p>
611         <div id="intro.requirements">
612            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#intro.requirements">Requirements</a></h2>
613            <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"
614               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>.
615            </p>
616            <p id="rfc.section.1.1.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
617            </p>
618         </div>
619         <div id="notation">
620            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
621            <p id="rfc.section.1.2.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> (which extends the syntax defined in <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with a list rule). <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF, with the list rule expanded.
622            </p>
623            <p id="rfc.section.1.2.p.2">The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
624               (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character),
625               and WSP (whitespace).
626            </p>
627            <div id="core.rules">
628               <h3 id="rfc.section.1.2.1"><a href="#rfc.section.1.2.1">1.2.1</a>&nbsp;<a href="#core.rules">Core Rules</a></h3>
629               <p id="rfc.section.1.2.1.p.1">The core rules below are defined in <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>:
630               </p>
631               <div id="rfc.figure.u.1"></div><pre class="inline">  <a href="#core.rules" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
632  <a href="#core.rules" class="smpl">OWS</a>           = &lt;OWS, defined in <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
633</pre></div>
634            <div id="abnf.dependencies">
635               <h3 id="rfc.section.1.2.2"><a href="#rfc.section.1.2.2">1.2.2</a>&nbsp;<a href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></h3>
636               <p id="rfc.section.1.2.2.p.1">The ABNF rules below are defined in other parts:</p>
637               <div id="rfc.figure.u.2"></div><pre class="inline">  <a href="#abnf.dependencies" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#full.date" title="Full Date">Section 3.2.1</a>&gt;
638</pre></div>
639         </div>
640      </div>
641      <div id="entity.tags">
642         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#entity.tags">Entity Tags</a></h1>
643         <p id="rfc.section.2.p.1">Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the
644            ETag (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;6.1</a>), If-Match (<a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;6.2</a>), If-None-Match (<a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section&nbsp;6.4</a>), and If-Range (<a href="p5-range.html#header.if-range" title="If-Range">Section 5.3</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>) header fields. The definition of how they are used and compared as cache validators is in <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;4</a>. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
645         </p>
646         <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span>  <a href="#entity.tags" class="smpl">entity-tag</a> = [ <a href="#entity.tags" class="smpl">weak</a> ] <a href="#entity.tags" class="smpl">opaque-tag</a>
647  <a href="#entity.tags" class="smpl">weak</a>       = "W/"
648  <a href="#entity.tags" class="smpl">opaque-tag</a> = <a href="#core.rules" class="smpl">quoted-string</a>
649</pre><p id="rfc.section.2.p.3">A "strong entity tag" <em class="bcp14">MAY</em> be shared by two entities of a resource only if they are equivalent by octet equality.
650         </p>
651         <p id="rfc.section.2.p.4">A "weak entity tag," indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no
652            significant change in semantics. A weak entity tag can only be used for weak comparison.
653         </p>
654         <p id="rfc.section.2.p.5">An entity tag <em class="bcp14">MUST</em> be unique across all versions of all entities associated with a particular resource. A given entity tag value <em class="bcp14">MAY</em> be used for entities obtained by requests on different URIs. The use of the same entity tag value in conjunction with entities
655            obtained by requests on different URIs does not imply the equivalence of those entities.
656         </p>
657      </div>
658      <div>
659         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;Status Code Definitions
660         </h1>
661         <div id="status.304">
662            <div id="rfc.iref.3.1"></div>
663            <div id="rfc.iref.s.1"></div>
664            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#status.304">304 Not Modified</a></h2>
665            <p id="rfc.section.3.1.p.1">If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server <em class="bcp14">SHOULD</em> respond with this status code. The 304 response <em class="bcp14">MUST NOT</em> contain a message-body, and thus is always terminated by the first empty line after the header fields.
666            </p>
667            <p id="rfc.section.3.1.p.2">The response <em class="bcp14">MUST</em> include the following header fields:
668            </p>
669            <ul>
670               <li>
671                  <p>Date, unless its omission is required by <a href="p1-messaging.html#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section 8.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
672                  </p>
673                  <p>If a clockless origin server obeys these rules, and proxies and clients add their own Date to any response received without
674                     one (as already specified by <a href="p1-messaging.html#header.date" title="Date">Section 8.3</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, caches will operate correctly.
675                  </p>
676               </li>
677               <li>
678                  <p>ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request.</p>
679               </li>
680               <li>
681                  <p>Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same
682                     variant.
683                  </p>
684               </li>
685            </ul>
686            <p id="rfc.section.3.1.p.3">If the conditional GET used a strong cache validator (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;4</a>), the response <em class="bcp14">SHOULD NOT</em> include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response <em class="bcp14">MUST NOT</em> include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.
687            </p>
688            <p id="rfc.section.3.1.p.4">If a 304 response indicates an entity not currently cached, then the cache <em class="bcp14">MUST</em> disregard the response and repeat the request without the conditional.
689            </p>
690            <p id="rfc.section.3.1.p.5">If a cache uses a received 304 response to update a cache entry, the cache <em class="bcp14">MUST</em> update the entry to reflect any new field values given in the response.
691            </p>
692         </div>
693         <div id="status.412">
694            <div id="rfc.iref.4.1"></div>
695            <div id="rfc.iref.s.2"></div>
696            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#status.412">412 Precondition Failed</a></h2>
697            <p id="rfc.section.3.2.p.1">The precondition given in one or more of the request-header fields evaluated to false when it was tested on the server. This
698               response code allows the client to place preconditions on the current resource metainformation (header field data) and thus
699               prevent the requested method from being applied to a resource other than the one intended.
700            </p>
701         </div>
702      </div>
703      <div id="weak.and.strong.validators">
704         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#weak.and.strong.validators">Weak and Strong Validators</a></h1>
705         <p id="rfc.section.4.p.1">Since both origin servers and caches will compare two validators to decide if they represent the same or different entities,
706            one normally would expect that if the entity (the entity-body or any entity-headers) changes in any way, then the associated
707            validator would change as well. If this is true, then we call this validator a "strong validator."
708         </p>
709         <p id="rfc.section.4.p.2">However, there might be cases when a server prefers to change the validator only on semantically significant changes, and
710            not when insignificant aspects of the entity change. A validator that does not always change when the resource changes is
711            a "weak validator."
712         </p>
713         <p id="rfc.section.4.p.3">Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak." One can
714            think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever
715            the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific
716            entity, while a weak validator is part of an identifier for a set of semantically equivalent entities.
717         </p>
718         <ul class="empty">
719            <li><b>Note:</b> One example of a strong validator is an integer that is incremented in stable storage every time an entity is changed.
720            </li>
721            <li>An entity's modification time, if represented with one-second resolution, could be a weak validator, since it is possible
722               that the resource might be modified twice during a single second.
723            </li>
724            <li>Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects;
725               for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during
726               that period is likely "good enough" to be equivalent.
727            </li>
728         </ul>
729         <p id="rfc.section.4.p.4">A "use" of a validator is either when a client generates a request and includes the validator in a validating header field,
730            or when a server compares two validators.
731         </p>
732         <p id="rfc.section.4.p.5">Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality
733            of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator
734            is usable for a sub-range retrieval, since otherwise the client might end up with an internally inconsistent entity.
735         </p>
736         <p id="rfc.section.4.p.6">Clients <em class="bcp14">MUST NOT</em> use weak validators in range requests (<a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>).
737         </p>
738         <p id="rfc.section.4.p.7">The only function that HTTP/1.1 defines on validators is comparison. There are two validator comparison functions, depending
739            on whether the comparison context allows the use of weak validators or not:
740         </p>
741         <ul>
742            <li>The strong comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, and both <em class="bcp14">MUST NOT</em> be weak.
743            </li>
744            <li>The weak comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character.
745            </li>
746         </ul>
747         <p id="rfc.section.4.p.8">The example below shows the results for a set of entity tag pairs, and both the weak and strong comparison function results:</p>
748         <div id="rfc.table.u.1">
749            <table class="tt full left" cellpadding="3" cellspacing="0">
750               <thead>
751                  <tr>
752                     <th>ETag 1</th>
753                     <th>ETag 2</th>
754                     <th>Strong Comparison</th>
755                     <th>Weak Comparison</th>
756                  </tr>
757               </thead>
758               <tbody>
759                  <tr>
760                     <td class="left">W/"1"</td>
761                     <td class="left">W/"1"</td>
762                     <td class="left">no match</td>
763                     <td class="left">match</td>
764                  </tr>
765                  <tr>
766                     <td class="left">W/"1"</td>
767                     <td class="left">W/"2"</td>
768                     <td class="left">no match</td>
769                     <td class="left">no match</td>
770                  </tr>
771                  <tr>
772                     <td class="left">W/"1"</td>
773                     <td class="left">"1"</td>
774                     <td class="left">no match</td>
775                     <td class="left">match</td>
776                  </tr>
777                  <tr>
778                     <td class="left">"1"</td>
779                     <td class="left">"1"</td>
780                     <td class="left">match</td>
781                     <td class="left">match</td>
782                  </tr>
783               </tbody>
784            </table>
785         </div>
786         <p id="rfc.section.4.p.9">An entity tag is strong unless it is explicitly tagged as weak. <a href="#entity.tags" title="Entity Tags">Section&nbsp;2</a> gives the syntax for entity tags.
787         </p>
788         <p id="rfc.section.4.p.10">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is
789            strong, using the following rules:
790         </p>
791         <ul>
792            <li>The validator is being compared by an origin server to the actual current validator for the entity and,</li>
793            <li>That origin server reliably knows that the associated entity did not change twice during the second covered by the presented
794               validator.
795            </li>
796         </ul>
797         <p id="rfc.section.4.p.11">or </p>
798         <ul>
799            <li>The validator is about to be used by a client in an If-Modified-Since or If-Unmodified-Since header, because the client has
800               a cache entry for the associated entity, and
801            </li>
802            <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
803            <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
804         </ul>
805         <p id="rfc.section.4.p.12">or </p>
806         <ul>
807            <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the entity, and</li>
808            <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
809            <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
810         </ul>
811         <p id="rfc.section.4.p.13">This method relies on the fact that if two different responses were sent by the origin server during the same second, but
812            both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified
813            time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from
814            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.
815         </p>
816         <p id="rfc.section.4.p.14">If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator,
817            it <em class="bcp14">MAY</em> do this only if the Last-Modified time is strong in the sense described here.
818         </p>
819         <p id="rfc.section.4.p.15">A cache or origin server receiving a conditional range request (<a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>) <em class="bcp14">MUST</em> use the strong comparison function to evaluate the condition.
820         </p>
821         <p id="rfc.section.4.p.16">These rules allow HTTP/1.1 caches and clients to safely perform sub-range retrievals on values that have been obtained from
822            HTTP/1.0 servers.
823         </p>
824      </div>
825      <div id="rules.for.when.to.use.entity.tags.and.last-modified.dates">
826         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity Tags and Last-Modified Dates</a></h1>
827         <p id="rfc.section.5.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types
828            ought to be used, and for what purposes.
829         </p>
830         <p id="rfc.section.5.p.2">HTTP/1.1 origin servers: </p>
831         <ul>
832            <li><em class="bcp14">SHOULD</em> send an entity tag validator unless it is not feasible to generate one.
833            </li>
834            <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,
835               or if it is unfeasible to send a strong entity tag.
836            </li>
837            <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one, unless the risk of a breakdown in semantic transparency that could
838               result from using this date in an If-Modified-Since header would lead to serious problems.
839            </li>
840         </ul>
841         <p id="rfc.section.5.p.3">In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity tag and a Last-Modified
842            value.
843         </p>
844         <p id="rfc.section.5.p.4">In order to be legal, a strong entity tag <em class="bcp14">MUST</em> change whenever the associated entity changes in any way. A weak entity tag <em class="bcp14">SHOULD</em> change whenever the associated entity changes in a semantically significant way.
845         </p>
846         <ul class="empty">
847            <li><b>Note:</b> in order to provide semantically transparent caching, an origin server must avoid reusing a specific strong entity tag value
848               for two different entities, or reusing a specific weak entity tag value for two semantically different entities. Cache entries
849               might persist for arbitrarily long periods, regardless of expiration times, so it might be inappropriate to expect that a
850               cache will never again attempt to validate an entry using a validator that it obtained at some point in the past.
851            </li>
852         </ul>
853         <p id="rfc.section.5.p.5">HTTP/1.1 clients: </p>
854         <ul>
855            <li>If an entity tag has been provided by the origin server, <em class="bcp14">MUST</em> use that entity tag in any cache-conditional request (using If-Match or If-None-Match).
856            </li>
857            <li>If only a Last-Modified value has been provided by the origin server, <em class="bcp14">SHOULD</em> use that value in non-subrange cache-conditional requests (using If-Modified-Since).
858            </li>
859            <li>If only a Last-Modified value has been provided by an HTTP/1.0 origin server, <em class="bcp14">MAY</em> use that value in subrange cache-conditional requests (using If-Unmodified-Since:). The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty.
860            </li>
861            <li>If both an entity tag and a Last-Modified value have been provided by the origin server, <em class="bcp14">SHOULD</em> use both validators in cache-conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
862            </li>
863         </ul>
864         <p id="rfc.section.5.p.6">An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since
865            or If-Unmodified-Since header field) and one or more entity tags (e.g., in an If-Match, If-None-Match, or If-Range header
866            field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields in
867            the request.
868         </p>
869         <p id="rfc.section.5.p.7">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity
870            tags as cache validators, <em class="bcp14">MUST NOT</em> return a locally cached response to the client unless that cached response is consistent with all of the conditional header
871            fields in the request.
872         </p>
873         <ul class="empty">
874            <li><b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much non-redundant information
875               as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
876               assumptions about the validators they receive.
877            </li>
878            <li>HTTP/1.0 clients and caches will ignore entity tags. Generally, last-modified values received or used by these systems will
879               support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare
880               cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then
881               HTTP/1.1 origin servers should not provide one.
882            </li>
883         </ul>
884      </div>
885      <div id="header.fields">
886         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#header.fields">Header Field Definitions</a></h1>
887         <p id="rfc.section.6.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to conditional requests.</p>
888         <p id="rfc.section.6.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
889            receives the entity.
890         </p>
891         <div id="header.etag">
892            <div id="rfc.iref.e.1"></div>
893            <div id="rfc.iref.h.1"></div>
894            <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a href="#header.etag">ETag</a></h2>
895            <p id="rfc.section.6.1.p.1">The response-header field "ETag" provides the current value of the entity tag (see <a href="#entity.tags" title="Entity Tags">Section&nbsp;2</a>) for the requested variant. The headers used with entity tags are described in Sections <a href="#header.if-match" id="rfc.xref.header.if-match.2" title="If-Match">6.2</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">6.4</a> of this document, and in <a href="p5-range.html#header.if-range" title="If-Range">Section 5.3</a> of <a href="#Part5" id="rfc.xref.Part5.4"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>. The entity tag <em class="bcp14">MAY</em> be used for comparison with other entities from the same resource (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;4</a>).
896            </p>
897            <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span>  <a href="#header.etag" class="smpl">ETag</a>   = "ETag" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.etag" class="smpl">ETag-v</a>
898  <a href="#header.etag" class="smpl">ETag-v</a> = <a href="#entity.tags" class="smpl">entity-tag</a>
899</pre><div id="rfc.figure.u.5"></div>
900            <p>Examples:</p><pre class="text">  ETag: "xyzzy"
901  ETag: W/"xyzzy"
902  ETag: ""
903</pre><p id="rfc.section.6.1.p.4">The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable
904               validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date
905               values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification
906               dates.
907            </p>
908            <p id="rfc.section.6.1.p.5">The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an
909               appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality
910               would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0)
911               are never used for purposes of validating a cache entry.
912            </p>
913         </div>
914         <div id="header.if-match">
915            <div id="rfc.iref.i.1"></div>
916            <div id="rfc.iref.h.2"></div>
917            <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a href="#header.if-match">If-Match</a></h2>
918            <p id="rfc.section.6.2.p.1">The request-header field "If-Match" is used with a method to make it conditional. A client that has one or more entities previously
919               obtained from the resource can verify that one of those entities is current by including a list of their associated entity
920               tags in the If-Match header field. Entity tags are defined in <a href="#entity.tags" title="Entity Tags">Section&nbsp;2</a>. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.
921               It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. As a special
922               case, the value "*" matches any current entity of the resource.
923            </p>
924            <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.6"></span><span id="rfc.iref.g.7"></span>  <a href="#header.if-match" class="smpl">If-Match</a>   = "If-Match" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.if-match" class="smpl">If-Match-v</a>
925  <a href="#header.if-match" class="smpl">If-Match-v</a> = "*" / 1#<a href="#entity.tags" class="smpl">entity-tag</a>
926</pre><p id="rfc.section.6.2.p.3">If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET
927               request (without the If-Match header) on that resource, or if "*" is given and any current entity exists for that resource,
928               then the server <em class="bcp14">MAY</em> perform the requested method as if the If-Match header field did not exist.
929            </p>
930            <p id="rfc.section.6.2.p.4">A server <em class="bcp14">MUST</em> use the strong comparison function (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;4</a>) to compare the entity tags in If-Match.
931            </p>
932            <p id="rfc.section.6.2.p.5">If none of the entity tags match, or if "*" is given and no current entity exists, the server <em class="bcp14">MUST NOT</em> perform the requested method, and <em class="bcp14">MUST</em> return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method,
933               such as PUT, from modifying a resource that has changed since the client last retrieved it.
934            </p>
935            <p id="rfc.section.6.2.p.6">If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status, then the If-Match
936               header <em class="bcp14">MUST</em> be ignored.
937            </p>
938            <p id="rfc.section.6.2.p.7">The meaning of "If-Match: *" is that the method <em class="bcp14">SHOULD</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 3.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">MUST NOT</em> be performed if the representation does not exist.
939            </p>
940            <p id="rfc.section.6.2.p.8">A request intended to update a resource (e.g., a PUT) <em class="bcp14">MAY</em> include an If-Match header field to signal that the request method <em class="bcp14">MUST NOT</em> be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource.
941               This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without
942               their knowledge. Examples:
943            </p>
944            <div id="rfc.figure.u.7"></div><pre class="text">  If-Match: "xyzzy"
945  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
946  If-Match: *
947</pre><p id="rfc.section.6.2.p.10">The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields
948               is undefined by this specification.
949            </p>
950         </div>
951         <div id="header.if-modified-since">
952            <div id="rfc.iref.i.2"></div>
953            <div id="rfc.iref.h.3"></div>
954            <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></h2>
955            <p id="rfc.section.6.3.p.1">The request-header field "If-Modified-Since" is used with a method to make it conditional: if the requested variant has not
956               been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (Not
957               Modified) response will be returned without any message-body.
958            </p>
959            <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.8"></span><span id="rfc.iref.g.9"></span>  <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a>   = "If-Modified-Since" ":" <a href="#core.rules" class="smpl">OWS</a>
960                        <a href="#header.if-modified-since" class="smpl">If-Modified-Since-v</a>
961  <a href="#header.if-modified-since" class="smpl">If-Modified-Since-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
962</pre><p id="rfc.section.6.3.p.3">An example of the field is:</p>
963            <div id="rfc.figure.u.9"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
964</pre><p id="rfc.section.6.3.p.5">A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only
965               if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes
966               the following cases:
967            </p>
968            <ol>
969               <li>If the request would normally result in anything other than a 200 (OK) status, or if the passed If-Modified-Since date is
970                  invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.
971               </li>
972               <li>If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.</li>
973               <li>If the variant has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> return a 304 (Not Modified) response.
974               </li>
975            </ol>
976            <p id="rfc.section.6.3.p.6">The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. </p>
977            <ul class="empty">
978               <li><b>Note:</b> The Range request-header field modifies the meaning of If-Modified-Since; see <a href="p5-range.html#header.range" title="Range">Section 5.4</a> of <a href="#Part5" id="rfc.xref.Part5.5"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a> for full details.
979               </li>
980               <li><b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.
981               </li>
982               <li><b>Note:</b> When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than
983                  function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since
984                  header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header
985                  field whenever possible.
986               </li>
987               <li><b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for
988                  the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time.
989                  The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between the
990                  client and server. This includes the possibility of race conditions if the document has changed between the time it was first
991                  requested and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the
992                  If-Modified-Since date is derived from the client's clock without correction to the server's clock. Corrections for different
993                  time bases between client and server are at best approximate due to network latency.
994               </li>
995            </ul>
996            <p id="rfc.section.6.3.p.7">The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header
997               fields is undefined by this specification.
998            </p>
999         </div>
1000         <div id="header.if-none-match">
1001            <div id="rfc.iref.i.3"></div>
1002            <div id="rfc.iref.h.4"></div>
1003            <h2 id="rfc.section.6.4"><a href="#rfc.section.6.4">6.4</a>&nbsp;<a href="#header.if-none-match">If-None-Match</a></h2>
1004            <p id="rfc.section.6.4.p.1">The request-header field "If-None-Match" is used with a method to make it conditional. A client that has one or more entities
1005               previously obtained from the resource can verify that none of those entities is current by including a list of their associated
1006               entity tags in the If-None-Match header field. The purpose of this feature is to allow efficient updates of cached information
1007               with a minimum amount of transaction overhead. It is also used to prevent a method (e.g. PUT) from inadvertently modifying
1008               an existing resource when the client believes that the resource does not exist.
1009            </p>
1010            <p id="rfc.section.6.4.p.2">As a special case, the value "*" matches any current entity of the resource.</p>
1011            <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span>  <a href="#header.if-none-match" class="smpl">If-None-Match</a>   = "If-None-Match" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.if-none-match" class="smpl">If-None-Match-v</a>
1012  <a href="#header.if-none-match" class="smpl">If-None-Match-v</a> = "*" / 1#<a href="#entity.tags" class="smpl">entity-tag</a>
1013</pre><p id="rfc.section.6.4.p.4">If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET
1014               request (without the If-None-Match header) on that resource, or if "*" is given and any current entity exists for that resource,
1015               then the server <em class="bcp14">MUST NOT</em> perform the requested method, unless required to do so because the resource's modification date fails to match that supplied
1016               in an If-Modified-Since header field in the request. Instead, if the request method was GET or HEAD, the server <em class="bcp14">SHOULD</em> respond with a 304 (Not Modified) response, including the cache-related header fields (particularly ETag) of one of the entities
1017               that matched. For all other request methods, the server <em class="bcp14">MUST</em> respond with a status of 412 (Precondition Failed).
1018            </p>
1019            <p id="rfc.section.6.4.p.5">See <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;4</a> for rules on how to determine if two entity tags match.
1020            </p>
1021            <p id="rfc.section.6.4.p.6">If none of the entity tags match, then the server <em class="bcp14">MAY</em> perform the requested method as if the If-None-Match header field did not exist, but <em class="bcp14">MUST</em> also ignore any If-Modified-Since header field(s) in the request. That is, if no entity tags match, then the server <em class="bcp14">MUST NOT</em> return a 304 (Not Modified) response.
1022            </p>
1023            <p id="rfc.section.6.4.p.7">If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status, then the
1024               If-None-Match header <em class="bcp14">MUST</em> be ignored. (See <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates" title="Rules for When to Use Entity Tags and Last-Modified Dates">Section&nbsp;5</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
1025            </p>
1026            <p id="rfc.section.6.4.p.8">The meaning of "If-None-Match: *" is that the method <em class="bcp14">MUST NOT</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 3.5</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">SHOULD</em> be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.
1027            </p>
1028            <p id="rfc.section.6.4.p.9">Examples:</p>
1029            <div id="rfc.figure.u.11"></div><pre class="text">  If-None-Match: "xyzzy"
1030  If-None-Match: W/"xyzzy"
1031  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1032  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1033  If-None-Match: *
1034</pre><p id="rfc.section.6.4.p.11">The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header
1035               fields is undefined by this specification.
1036            </p>
1037         </div>
1038         <div id="header.if-unmodified-since">
1039            <div id="rfc.iref.i.4"></div>
1040            <div id="rfc.iref.h.5"></div>
1041            <h2 id="rfc.section.6.5"><a href="#rfc.section.6.5">6.5</a>&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
1042            <p id="rfc.section.6.5.p.1">The request-header field "If-Unmodified-Since" is used with a method to make it conditional. If the requested resource has
1043               not been modified since the time specified in this field, the server <em class="bcp14">SHOULD</em> perform the requested operation as if the If-Unmodified-Since header were not present.
1044            </p>
1045            <p id="rfc.section.6.5.p.2">If the requested variant has been modified since the specified time, the server <em class="bcp14">MUST NOT</em> perform the requested operation, and <em class="bcp14">MUST</em> return a 412 (Precondition Failed).
1046            </p>
1047            <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.12"></span><span id="rfc.iref.g.13"></span>  <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a>   = "If-Unmodified-Since" ":" <a href="#core.rules" class="smpl">OWS</a>
1048                          <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since-v</a>
1049  <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
1050</pre><p id="rfc.section.6.5.p.4">An example of the field is:</p>
1051            <div id="rfc.figure.u.13"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1052</pre><p id="rfc.section.6.5.p.6">If the request normally (i.e., without the If-Unmodified-Since header) would result in anything other than a 2xx or 412 status,
1053               the If-Unmodified-Since header <em class="bcp14">SHOULD</em> be ignored.
1054            </p>
1055            <p id="rfc.section.6.5.p.7">If the specified date is invalid, the header is ignored.</p>
1056            <p id="rfc.section.6.5.p.8">The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since
1057               header fields is undefined by this specification.
1058            </p>
1059         </div>
1060         <div id="header.last-modified">
1061            <div id="rfc.iref.l.1"></div>
1062            <div id="rfc.iref.h.6"></div>
1063            <h2 id="rfc.section.6.6"><a href="#rfc.section.6.6">6.6</a>&nbsp;<a href="#header.last-modified">Last-Modified</a></h2>
1064            <p id="rfc.section.6.6.p.1">The entity-header field "Last-Modified" indicates the date and time at which the origin server believes the variant was last
1065               modified.
1066            </p>
1067            <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span>  <a href="#header.last-modified" class="smpl">Last-Modified</a>   = "Last-Modified" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.last-modified" class="smpl">Last-Modified-v</a>
1068  <a href="#header.last-modified" class="smpl">Last-Modified-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
1069</pre><p id="rfc.section.6.6.p.3">An example of its use is</p>
1070            <div id="rfc.figure.u.15"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1071</pre><p id="rfc.section.6.6.p.5">The exact meaning of this header field depends on the implementation of the origin server and the nature of the original resource.
1072               For files, it may be just the file system last-modified time. For entities with dynamically included parts, it may be the
1073               most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update time
1074               stamp of the record. For virtual objects, it may be the last time the internal state changed.
1075            </p>
1076            <p id="rfc.section.6.6.p.6">An origin server <em class="bcp14">MUST NOT</em> send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's
1077               last modification would indicate some time in the future, the server <em class="bcp14">MUST</em> replace that date with the message origination date.
1078            </p>
1079            <p id="rfc.section.6.6.p.7">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the entity as close as possible to the time that it generates the Date value of its response.
1080               This allows a recipient to make an accurate assessment of the entity's modification time, especially if the entity changes
1081               near the time that the response is generated.
1082            </p>
1083            <p id="rfc.section.6.6.p.8">HTTP/1.1 servers <em class="bcp14">SHOULD</em> send Last-Modified whenever feasible.
1084            </p>
1085            <p id="rfc.section.6.6.p.9">The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered
1086               to be valid if the entity has not been modified since the Last-Modified value.
1087            </p>
1088         </div>
1089      </div>
1090      <div id="IANA.considerations">
1091         <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a href="#IANA.considerations">IANA Considerations</a></h1>
1092         <div id="message.header.registration">
1093            <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a href="#message.header.registration">Message Header Registration</a></h2>
1094            <p id="rfc.section.7.1.p.1">The Message Header Registry located 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; should be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
1095            </p>
1096            <div id="rfc.table.1">
1097               <div id="iana.header.registration.table"></div>
1098               <table class="tt full left" cellpadding="3" cellspacing="0">
1099                  <thead>
1100                     <tr>
1101                        <th>Header Field Name</th>
1102                        <th>Protocol</th>
1103                        <th>Status</th>
1104                        <th>Reference</th>
1105                     </tr>
1106                  </thead>
1107                  <tbody>
1108                     <tr>
1109                        <td class="left">ETag</td>
1110                        <td class="left">http</td>
1111                        <td class="left">standard</td>
1112                        <td class="left"><a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section&nbsp;6.1</a> 
1113                        </td>
1114                     </tr>
1115                     <tr>
1116                        <td class="left">If-Match</td>
1117                        <td class="left">http</td>
1118                        <td class="left">standard</td>
1119                        <td class="left"><a href="#header.if-match" id="rfc.xref.header.if-match.3" title="If-Match">Section&nbsp;6.2</a> 
1120                        </td>
1121                     </tr>
1122                     <tr>
1123                        <td class="left">If-Modified-Since</td>
1124                        <td class="left">http</td>
1125                        <td class="left">standard</td>
1126                        <td class="left"><a href="#header.if-modified-since" id="rfc.xref.header.if-modified-since.1" title="If-Modified-Since">Section&nbsp;6.3</a> 
1127                        </td>
1128                     </tr>
1129                     <tr>
1130                        <td class="left">If-None-Match</td>
1131                        <td class="left">http</td>
1132                        <td class="left">standard</td>
1133                        <td class="left"><a href="#header.if-none-match" id="rfc.xref.header.if-none-match.3" title="If-None-Match">Section&nbsp;6.4</a> 
1134                        </td>
1135                     </tr>
1136                     <tr>
1137                        <td class="left">If-Unmodified-Since</td>
1138                        <td class="left">http</td>
1139                        <td class="left">standard</td>
1140                        <td class="left"><a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section&nbsp;6.5</a> 
1141                        </td>
1142                     </tr>
1143                     <tr>
1144                        <td class="left">Last-Modified</td>
1145                        <td class="left">http</td>
1146                        <td class="left">standard</td>
1147                        <td class="left"><a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;6.6</a> 
1148                        </td>
1149                     </tr>
1150                  </tbody>
1151               </table>
1152            </div>
1153            <p id="rfc.section.7.1.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1154         </div>
1155      </div>
1156      <div id="security.considerations">
1157         <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
1158         <p id="rfc.section.8.p.1">No additional security considerations have been identified beyond those applicable to HTTP in general <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
1159         </p>
1160      </div>
1161      <div id="ack">
1162         <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a href="#ack">Acknowledgments</a></h1>
1163      </div>
1164      <h1 id="rfc.references"><a id="rfc.section.10" href="#rfc.section.10">10.</a> References
1165      </h1>
1166      <h2 id="rfc.references.1"><a href="#rfc.section.10.1" id="rfc.section.10.1">10.1</a> Normative References
1167      </h2>
1168      <table>
1169         <tr>
1170            <td class="reference"><b id="Part1">[Part1]</b></td>
1171            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-06">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-06 (work in progress), March&nbsp;2009.
1172            </td>
1173         </tr>
1174         <tr>
1175            <td class="reference"><b id="Part5">[Part5]</b></td>
1176            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-06">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-06 (work in progress), March&nbsp;2009.
1177            </td>
1178         </tr>
1179         <tr>
1180            <td class="reference"><b id="Part6">[Part6]</b></td>
1181            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-06">HTTP/1.1, part 6: Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-06 (work in progress), March&nbsp;2009.
1182            </td>
1183         </tr>
1184         <tr>
1185            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1186            <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
1187            </td>
1188         </tr>
1189         <tr>
1190            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1191            <td class="top"><a href="mailto:dcrocker@bbiw.net" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="mailto:paul.overell@thus.net" title="THUS plc.">P. Overell</a>, “<a href="https://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
1192            </td>
1193         </tr>
1194      </table>
1195      <h2 id="rfc.references.2"><a href="#rfc.section.10.2" id="rfc.section.10.2">10.2</a> Informative References
1196      </h2>
1197      <table>
1198         <tr>
1199            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1200            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1201            </td>
1202         </tr>
1203         <tr>
1204            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1205            <td class="top"><a href="mailto:GK-IETF@ninebynine.org" title="Nine by Nine">Klyne, G.</a>, <a href="mailto:mnot@pobox.com" title="BEA Systems">Nottingham, M.</a>, and <a href="mailto:JeffMogul@acm.org" title="HP Labs">J. Mogul</a>, “<a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
1206            </td>
1207         </tr>
1208      </table>
1209      <div id="compatibility">
1210         <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a></h1>
1211         <div id="changes.from.rfc.2616">
1212            <h2 id="rfc.section.A.1"><a href="#rfc.section.A.1">A.1</a>&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h2>
1213            <p id="rfc.section.A.1.p.1">Allow weak entity tags in all requests except range requests (Sections <a href="#weak.and.strong.validators" title="Weak and Strong Validators">4</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.4" title="If-None-Match">6.4</a>).
1214            </p>
1215         </div>
1216      </div>
1217      <div id="collected.abnf">
1218         <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a href="#collected.abnf">Collected ABNF</a></h1>
1219         <div id="rfc.figure.u.16"></div><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = "ETag:" OWS ETag-v
1220<a href="#header.etag" class="smpl">ETag-v</a> = entity-tag
1221
1222<a href="#abnf.dependencies" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
1223
1224<a href="#header.if-match" class="smpl">If-Match</a> = "If-Match:" OWS If-Match-v
1225<a href="#header.if-match" class="smpl">If-Match-v</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1226 entity-tag ] ) )
1227<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = "If-Modified-Since:" OWS If-Modified-Since-v
1228<a href="#header.if-modified-since" class="smpl">If-Modified-Since-v</a> = HTTP-date
1229<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "If-None-Match:" OWS If-None-Match-v
1230<a href="#header.if-none-match" class="smpl">If-None-Match-v</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1231 entity-tag ] ) )
1232<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = "If-Unmodified-Since:" OWS
1233 If-Unmodified-Since-v
1234<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since-v</a> = HTTP-date
1235
1236<a href="#header.last-modified" class="smpl">Last-Modified</a> = "Last-Modified:" OWS Last-Modified-v
1237<a href="#header.last-modified" class="smpl">Last-Modified-v</a> = HTTP-date
1238
1239<a href="#core.rules" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1240
1241<a href="#entity.tags" class="smpl">entity-tag</a> = [ weak ] opaque-tag
1242
1243<a href="#entity.tags" class="smpl">opaque-tag</a> = quoted-string
1244
1245<a href="#core.rules" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1246
1247<a href="#entity.tags" class="smpl">weak</a> = "W/"
1248
1249
1250</pre><div id="rfc.figure.u.17"></div>
1251         <p>ABNF diagnostics:</p><pre class="inline">; ETag defined but not used
1252; If-Match defined but not used
1253; If-Modified-Since defined but not used
1254; If-None-Match defined but not used
1255; If-Unmodified-Since defined but not used
1256; Last-Modified defined but not used
1257</pre></div>
1258      <div id="change.log">
1259         <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1260         <div>
1261            <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a>&nbsp;Since RFC2616
1262            </h2>
1263            <p id="rfc.section.C.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
1264            </p>
1265         </div>
1266         <div>
1267            <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-00
1268            </h2>
1269            <p id="rfc.section.C.2.p.1">Closed issues: </p>
1270            <ul>
1271               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/35">http://tools.ietf.org/wg/httpbis/trac/ticket/35</a>&gt;: "Normative and Informative references"
1272               </li>
1273            </ul>
1274            <p id="rfc.section.C.2.p.2">Other changes: </p>
1275            <ul>
1276               <li>Move definitions of 304 and 412 condition codes from Part2.</li>
1277            </ul>
1278         </div>
1279         <div>
1280            <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-01
1281            </h2>
1282            <p id="rfc.section.C.3.p.1">Ongoing work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1283            </p>
1284            <ul>
1285               <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li>
1286            </ul>
1287         </div>
1288         <div id="changes.since.02">
1289            <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a>&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></h2>
1290            <p id="rfc.section.C.4.p.1">Closed issues: </p>
1291            <ul>
1292               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/116">http://tools.ietf.org/wg/httpbis/trac/ticket/116</a>&gt;: "Weak ETags on non-GET requests"
1293               </li>
1294            </ul>
1295            <p id="rfc.section.C.4.p.2">Ongoing work on IANA Message Header Registration (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/40">http://tools.ietf.org/wg/httpbis/trac/ticket/40</a>&gt;):
1296            </p>
1297            <ul>
1298               <li>Reference RFC 3984, and update header registrations for headers defined in this document.</li>
1299            </ul>
1300         </div>
1301         <div id="changes.since.03">
1302            <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a>&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></h2>
1303            <p id="rfc.section.C.5.p.1">Closed issues: </p>
1304            <ul>
1305               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/71">http://tools.ietf.org/wg/httpbis/trac/ticket/71</a>&gt;: "Examples for ETag matching"
1306               </li>
1307               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/124">http://tools.ietf.org/wg/httpbis/trac/ticket/124</a>&gt;: "'entity value' undefined"
1308               </li>
1309               <li>&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/126">http://tools.ietf.org/wg/httpbis/trac/ticket/126</a>&gt;: "bogus 2068 Date header reference"
1310               </li>
1311            </ul>
1312         </div>
1313         <div id="changes.since.04">
1314            <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a>&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></h2>
1315            <p id="rfc.section.C.6.p.1">Ongoing work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1316            </p>
1317            <ul>
1318               <li>Use "/" instead of "|" for alternatives.</li>
1319               <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li>
1320               <li>Rewrite ABNFs to spell out whitespace rules, factor out header value format definitions.</li>
1321            </ul>
1322         </div>
1323         <div id="changes.since.05">
1324            <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a>&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></h2>
1325            <p id="rfc.section.C.7.p.1">Final work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1326            </p>
1327            <ul>
1328               <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li>
1329            </ul>
1330         </div>
1331      </div>
1332      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1333      <p class="noprint"><a href="#rfc.index.3">3</a> <a href="#rfc.index.4">4</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.I">I</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> 
1334      </p>
1335      <div class="print2col">
1336         <ul class="ind">
1337            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
1338                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.3.1"><b>3.1</b></a></li>
1339               </ul>
1340            </li>
1341            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
1342                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.4.1"><b>3.2</b></a></li>
1343               </ul>
1344            </li>
1345            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
1346                  <li>ETag header&nbsp;&nbsp;<a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.iref.e.1"><b>6.1</b></a>, <a href="#rfc.xref.header.etag.2">7.1</a></li>
1347               </ul>
1348            </li>
1349            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
1350                  <li><tt>Grammar</tt>&nbsp;&nbsp;
1351                     <ul>
1352                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2</b></a></li>
1353                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>6.1</b></a></li>
1354                        <li><tt>ETag-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>6.1</b></a></li>
1355                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>6.2</b></a></li>
1356                        <li><tt>If-Match-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>6.2</b></a></li>
1357                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>6.3</b></a></li>
1358                        <li><tt>If-Modified-Since-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>6.3</b></a></li>
1359                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>6.4</b></a></li>
1360                        <li><tt>If-None-Match-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.11"><b>6.4</b></a></li>
1361                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.12"><b>6.5</b></a></li>
1362                        <li><tt>If-Unmodified-Since-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.13"><b>6.5</b></a></li>
1363                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.14"><b>6.6</b></a></li>
1364                        <li><tt>Last-Modified-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.15"><b>6.6</b></a></li>
1365                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2</b></a></li>
1366                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2</b></a></li>
1367                     </ul>
1368                  </li>
1369               </ul>
1370            </li>
1371            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
1372                  <li>Headers&nbsp;&nbsp;
1373                     <ul>
1374                        <li>ETag&nbsp;&nbsp;<a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.iref.h.1"><b>6.1</b></a>, <a href="#rfc.xref.header.etag.2">7.1</a></li>
1375                        <li>If-Match&nbsp;&nbsp;<a href="#rfc.xref.header.if-match.1">2</a>, <a href="#rfc.xref.header.if-match.2">6.1</a>, <a href="#rfc.iref.h.2"><b>6.2</b></a>, <a href="#rfc.xref.header.if-match.3">7.1</a></li>
1376                        <li>If-Modified-Since&nbsp;&nbsp;<a href="#rfc.iref.h.3"><b>6.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">7.1</a></li>
1377                        <li>If-None-Match&nbsp;&nbsp;<a href="#rfc.xref.header.if-none-match.1">2</a>, <a href="#rfc.xref.header.if-none-match.2">6.1</a>, <a href="#rfc.iref.h.4"><b>6.4</b></a>, <a href="#rfc.xref.header.if-none-match.3">7.1</a>, <a href="#rfc.xref.header.if-none-match.4">A.1</a></li>
1378                        <li>If-Unmodified-Since&nbsp;&nbsp;<a href="#rfc.iref.h.5"><b>6.5</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">7.1</a></li>
1379                        <li>Last-Modified&nbsp;&nbsp;<a href="#rfc.iref.h.6"><b>6.6</b></a>, <a href="#rfc.xref.header.last-modified.1">7.1</a></li>
1380                     </ul>
1381                  </li>
1382               </ul>
1383            </li>
1384            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
1385                  <li>If-Match header&nbsp;&nbsp;<a href="#rfc.xref.header.if-match.1">2</a>, <a href="#rfc.xref.header.if-match.2">6.1</a>, <a href="#rfc.iref.i.1"><b>6.2</b></a>, <a href="#rfc.xref.header.if-match.3">7.1</a></li>
1386                  <li>If-Modified-Since header&nbsp;&nbsp;<a href="#rfc.iref.i.2"><b>6.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">7.1</a></li>
1387                  <li>If-None-Match header&nbsp;&nbsp;<a href="#rfc.xref.header.if-none-match.1">2</a>, <a href="#rfc.xref.header.if-none-match.2">6.1</a>, <a href="#rfc.iref.i.3"><b>6.4</b></a>, <a href="#rfc.xref.header.if-none-match.3">7.1</a>, <a href="#rfc.xref.header.if-none-match.4">A.1</a></li>
1388                  <li>If-Unmodified-Since header&nbsp;&nbsp;<a href="#rfc.iref.i.4"><b>6.5</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">7.1</a></li>
1389               </ul>
1390            </li>
1391            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
1392                  <li>Last-Modified header&nbsp;&nbsp;<a href="#rfc.iref.l.1"><b>6.6</b></a>, <a href="#rfc.xref.header.last-modified.1">7.1</a></li>
1393               </ul>
1394            </li>
1395            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
1396                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a>, <a href="#rfc.xref.Part1.2">1.2.1</a>, <a href="#rfc.xref.Part1.3">1.2.1</a>, <a href="#rfc.xref.Part1.4">1.2.1</a>, <a href="#rfc.xref.Part1.5">1.2.2</a>, <a href="#rfc.xref.Part1.6">3.1</a>, <a href="#rfc.xref.Part1.7">3.1</a>, <a href="#rfc.xref.Part1.8">8</a>, <a href="#Part1"><b>10.1</b></a><ul>
1397                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a></li>
1398                        <li><em>Section 1.2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.2.1</a>, <a href="#rfc.xref.Part1.3">1.2.1</a>, <a href="#rfc.xref.Part1.4">1.2.1</a></li>
1399                        <li><em>Section 3.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">1.2.2</a></li>
1400                        <li><em>Section 8.3.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">3.1</a></li>
1401                        <li><em>Section 8.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.7">3.1</a></li>
1402                     </ul>
1403                  </li>
1404                  <li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">2</a>, <a href="#rfc.xref.Part5.2">4</a>, <a href="#rfc.xref.Part5.3">4</a>, <a href="#rfc.xref.Part5.4">6.1</a>, <a href="#rfc.xref.Part5.5">6.3</a>, <a href="#Part5"><b>10.1</b></a><ul>
1405                        <li><em>Section 5.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">2</a>, <a href="#rfc.xref.Part5.4">6.1</a></li>
1406                        <li><em>Section 5.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.5">6.3</a></li>
1407                     </ul>
1408                  </li>
1409                  <li><em>Part6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">6.2</a>, <a href="#rfc.xref.Part6.3">6.4</a>, <a href="#Part6"><b>10.1</b></a><ul>
1410                        <li><em>Section 3.5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.2">6.2</a>, <a href="#rfc.xref.Part6.3">6.4</a></li>
1411                     </ul>
1412                  </li>
1413               </ul>
1414            </li>
1415            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
1416                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>10.1</b></a></li>
1417                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">1</a>, <a href="#RFC2616"><b>10.2</b></a>, <a href="#rfc.xref.RFC2616.2">C.1</a></li>
1418                  <li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">7.1</a>, <a href="#RFC3864"><b>10.2</b></a></li>
1419                  <li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.2</a>, <a href="#rfc.xref.RFC5234.2">1.2</a>, <a href="#RFC5234"><b>10.1</b></a><ul>
1420                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">1.2</a></li>
1421                     </ul>
1422                  </li>
1423               </ul>
1424            </li>
1425            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
1426                  <li>Status Codes&nbsp;&nbsp;
1427                     <ul>
1428                        <li>304 Not Modified&nbsp;&nbsp;<a href="#rfc.iref.s.1"><b>3.1</b></a></li>
1429                        <li>412 Precondition Failed&nbsp;&nbsp;<a href="#rfc.iref.s.2"><b>3.2</b></a></li>
1430                     </ul>
1431                  </li>
1432               </ul>
1433            </li>
1434         </ul>
1435      </div>
1436      <div class="avoidbreak">
1437         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1438         <p><b>Roy T. Fielding</b>
1439            (editor)
1440            <br>Day Software<br>23 Corporate Plaza DR, Suite 280<br>Newport Beach, CA&nbsp;92660<br>USA<br>Phone: <a href="tel:+1-949-706-5300">+1-949-706-5300</a><br>Fax: <a href="fax:+1-949-706-5305">+1-949-706-5305</a><br>EMail: <a href="mailto:fielding@gbiv.com">fielding@gbiv.com</a><br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p>
1441         <p><b>Jim Gettys</b><br>One Laptop per Child<br>21 Oak Knoll Road<br>Carlisle, MA&nbsp;01741<br>USA<br>EMail: <a href="mailto:jg@laptop.org">jg@laptop.org</a><br>URI: <a href="http://www.laptop.org/">http://www.laptop.org/</a></p>
1442         <p><b>Jeffrey C. Mogul</b><br>Hewlett-Packard Company<br>HP Labs, Large Scale Systems Group<br>1501 Page Mill Road, MS 1177<br>Palo Alto, CA&nbsp;94304<br>USA<br>EMail: <a href="mailto:JeffMogul@acm.org">JeffMogul@acm.org</a></p>
1443         <p><b>Henrik Frystyk Nielsen</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>USA<br>EMail: <a href="mailto:henrikn@microsoft.com">henrikn@microsoft.com</a></p>
1444         <p><b>Larry Masinter</b><br>Adobe Systems, Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>EMail: <a href="mailto:LMM@acm.org">LMM@acm.org</a><br>URI: <a href="http://larry.masinter.net/">http://larry.masinter.net/</a></p>
1445         <p><b>Paul J. Leach</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>EMail: <a href="mailto:paulle@microsoft.com">paulle@microsoft.com</a></p>
1446         <p><b>Tim Berners-Lee</b><br>World Wide Web Consortium<br>MIT Computer Science and Artificial Intelligence Laboratory<br>The Stata Center, Building 32<br>32 Vassar Street<br>Cambridge, MA&nbsp;02139<br>USA<br>EMail: <a href="mailto:timbl@w3.org">timbl@w3.org</a><br>URI: <a href="http://www.w3.org/People/Berners-Lee/">http://www.w3.org/People/Berners-Lee/</a></p>
1447         <p><b>Yves Lafon</b>
1448            (editor)
1449            <br>World Wide Web Consortium<br>W3C / ERCIM<br>2004, rte des Lucioles<br>Sophia-Antipolis, AM&nbsp;06902<br>France<br>EMail: <a href="mailto:ylafon@w3.org">ylafon@w3.org</a><br>URI: <a href="http://www.raubacapeu.net/people/yves/">http://www.raubacapeu.net/people/yves/</a></p>
1450         <p><b>Julian F. Reschke</b>
1451            (editor)
1452            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Phone: <a href="tel:+492512807760">+49 251 2807760</a><br>Fax: <a href="fax:+492512807761">+49 251 2807761</a><br>EMail: <a href="mailto:julian.reschke@greenbytes.de">julian.reschke@greenbytes.de</a><br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p>
1453      </div>
1454   </body>
1455</html>
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