source: draft-ietf-httpbis/latest/p4-conditional.html @ 1223

Last change on this file since 1223 was 1223, checked in by fielding@…, 9 years ago

Define "selected representation" as a shorthand for targeting
conditional requests, effectively replacing the old notion
of resource variants. Addresses #89.

Remove the nonsense about semantic equivalence and replace it
with the mechanical reason that origin servers might not want
to change a weak entity-tag on every update. Addresses #101.

Rewrite the condtional evaluation text accordingly and reduce
some duplication of requirements.

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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 Changes from RFC 2616" 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">
396      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.543, 2011-02-18 21:03:40, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
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-latest">
408      <meta name="dct.issued" scheme="ISO8601" content="2011-03-28">
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">Adobe</td>
423            </tr>
424            <tr>
425               <td class="left">Obsoletes: <a href="http://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">Alcatel-Lucent</td>
432            </tr>
433            <tr>
434               <td class="left">Expires: September 29, 2011</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</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 28, 2011</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-latest</span></p>
496      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
497      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
498         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the
499         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part
500         4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.
501      </p> 
502      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
503      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues
504         list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
505      </p> 
506      <p>The changes in this draft are summarized in <a href="#changes.since.13" title="Since draft-ietf-httpbis-p4-conditional-13">Appendix&nbsp;C.15</a>.
507      </p> 
508      <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
509      <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
510      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
511         working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
512      </p>
513      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
514         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
515         in progress”.
516      </p>
517      <p>This Internet-Draft will expire on September 29, 2011.</p>
518      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
519      <p>Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
520      <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
521         and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
522         text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
523         BSD License.
524      </p>
525      <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
526         10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
527         allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
528         controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
529         works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
530         it into languages other than English.
531      </p>
532      <hr class="noprint">
533      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
534      <ul class="toc">
535         <li>1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
536               <li>1.1&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
537               <li>1.2&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul>
538                     <li>1.2.1&nbsp;&nbsp;&nbsp;<a href="#core.rules">Core Rules</a></li>
539                     <li>1.2.2&nbsp;&nbsp;&nbsp;<a href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></li>
540                  </ul>
541               </li>
542            </ul>
543         </li>
544         <li>2.&nbsp;&nbsp;&nbsp;<a href="#entity.tags">Entity-Tags</a><ul>
545               <li>2.1&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>
546            </ul>
547         </li>
548         <li>3.&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
549               <li>3.1&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
550               <li>3.2&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
551            </ul>
552         </li>
553         <li>4.&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak and Strong Validators</a></li>
554         <li>5.&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>
555         <li>6.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul>
556               <li>6.1&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a></li>
557               <li>6.2&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
558               <li>6.3&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
559               <li>6.4&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
560               <li>6.5&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
561               <li>6.6&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a></li>
562            </ul>
563         </li>
564         <li>7.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
565               <li>7.1&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
566               <li>7.2&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
567            </ul>
568         </li>
569         <li>8.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
570         <li>9.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
571         <li>10.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
572               <li>10.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
573               <li>10.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
574            </ul>
575         </li>
576         <li><a href="#rfc.authors">Authors' Addresses</a></li>
577         <li>A.&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
578         <li>B.&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
579         <li>C.&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
580               <li>C.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC 2616</a></li>
581               <li>C.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p4-conditional-00</a></li>
582               <li>C.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p4-conditional-01</a></li>
583               <li>C.4&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></li>
584               <li>C.5&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></li>
585               <li>C.6&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></li>
586               <li>C.7&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></li>
587               <li>C.8&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></li>
588               <li>C.9&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></li>
589               <li>C.10&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></li>
590               <li>C.11&nbsp;&nbsp;&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></li>
591               <li>C.12&nbsp;&nbsp;&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></li>
592               <li>C.13&nbsp;&nbsp;&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></li>
593               <li>C.14&nbsp;&nbsp;&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></li>
594               <li>C.15&nbsp;&nbsp;&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></li>
595            </ul>
596         </li>
597         <li><a href="#rfc.index">Index</a></li>
598      </ul>
599      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
600      <p id="rfc.section.1.p.1">This document defines HTTP/1.1 response metadata for indicating potential changes to payload content, including modification
601         time stamps and opaque entity-tags, and the HTTP conditional request mechanisms that allow preconditions to be placed on a
602         request method. Conditional GET requests allow for efficient cache updates. Other conditional request methods are used to
603         protect against overwriting or misunderstanding the state of a resource that has been changed unbeknownst to the requesting
604         client.
605      </p>
606      <p id="rfc.section.1.p.2">We use the term "selected representation" to refer to the current representation of the target resource that would have been
607         selected and sent as payload in a 200 (OK) response if the same request had used the method GET and had excluded all of the
608         conditional request header fields. The conditions found within conditional requests are evaluated on the basis of either the
609         state of the target resource as a whole or the state of the target resource's selected representation.
610      </p>
611      <p id="rfc.section.1.p.3">This document is currently disorganized in order to minimize the changes between drafts and enable reviewers to see the smaller
612         errata changes. A future draft will reorganize the sections to better reflect the content. In particular, the sections on
613         resource metadata will be discussed first and then followed by each conditional request header field, concluding with a definition
614         of precedence and the expectation of ordering strong validator checks before weak validator checks. It is likely that more
615         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
616         had become in <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
617      </p>
618      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
619      <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"
620         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>.
621      </p>
622      <p id="rfc.section.1.1.p.2">An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the
623         protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements
624         for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not
625         all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant".
626      </p>
627      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="notation" href="#notation">Syntax Notation</a></h2>
628      <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.
629      </p>
630      <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="http://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
631         (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character),
632         and WSP (whitespace).
633      </p>
634      <h3 id="rfc.section.1.2.1"><a href="#rfc.section.1.2.1">1.2.1</a>&nbsp;<a id="core.rules" href="#core.rules">Core Rules</a></h3>
635      <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>:
636      </p>
637      <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;
638  <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;
639</pre><h3 id="rfc.section.1.2.2"><a href="#rfc.section.1.2.2">1.2.2</a>&nbsp;<a id="abnf.dependencies" href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></h3>
640      <p id="rfc.section.1.2.2.p.1">The ABNF rules below are defined in other parts:</p>
641      <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#date.time.formats.full.date" title="Date/Time Formats: Full Date">Section 6.1</a>&gt;
642</pre><h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="entity.tags" href="#entity.tags">Entity-Tags</a></h1>
643      <p id="rfc.section.2.p.1">Entity-tags are used for comparing two or more representations of the same resource. HTTP/1.1 uses entity-tags in the ETag
644         (<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>       = %x57.2F ; "W/", case-sensitive
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 representations 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 representations of a resource. A weak entity-tag can only be used for weak comparison.
652      </p>
653      <p id="rfc.section.2.p.5">Cache entries might persist for arbitrarily long periods, regardless of expiration times, so it is inappropriate to expect
654         that a cache will never again attempt to validate an entry using a validator that it obtained at some point in the past. A
655         strong entity-tag <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no
656         implication of uniqueness across entity-tags of different resources (i.e., the same entity-tag value might be in use for representations
657         of multiple resources at the same time and does not imply that those representations are equivalent).
658      </p>
659      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h2>
660      <p id="rfc.section.2.1.p.1">Consider a resource that is subject to content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (<a href="p3-payload.html#header.accept-encoding" title="Accept-Encoding">Section 6.3</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>):
661      </p>
662      <div id="rfc.figure.u.4"></div>
663      <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
664Host: www.example.com
665Accept-Encoding: gzip
666
667</pre><p id="rfc.section.2.1.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p>
668      <div id="rfc.figure.u.5"></div>
669      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
670Date: Thu, 26 Mar 2010 00:05:00 GMT
671ETag: "123-a"
672Content-Length: 70
673Vary: Accept-Encoding
674Content-Type: text/plain
675
676<span id="exbody">Hello World!
677Hello World!
678Hello World!
679Hello World!
680Hello World!
681</span></pre><p id="rfc.section.2.1.p.5">An alternative representation that does use gzip content coding would be:</p>
682      <div id="rfc.figure.u.6"></div>
683      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
684Date: Thu, 26 Mar 2010 00:05:00 GMT
685ETag: "123-b"
686Content-Length: 43
687Vary: Accept-Encoding
688Content-Type: text/plain
689Content-Encoding: gzip
690
691<em>...binary data...</em></pre><div class="note" id="rfc.section.2.1.p.7"> 
692         <p> <b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation must be distinct
693            from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings
694            (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 6.2</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>) apply only during message transfer and do not require distinct entity-tags.
695         </p> 
696      </div>
697      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="status.code.definitions" href="#status.code.definitions">Status Code Definitions</a></h1>
698      <div id="rfc.iref.3"></div>
699      <div id="rfc.iref.s.1"></div>
700      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="status.304" href="#status.304">304 Not Modified</a></h2>
701      <p id="rfc.section.3.1.p.1">The 304 status code indicates that a conditional GET request has been received and would have resulted in a 200 (OK) response
702         if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server to
703         transfer a representation of the target resource because the client's request indicates that it already has a valid representation,
704         as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation
705         as if it were the payload of a 200 response. 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.
706      </p>
707      <p id="rfc.section.3.1.p.2">A 304 response <em class="bcp14">MUST</em> include a Date header field (<a href="p1-messaging.html#header.date" title="Date">Section 9.3</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>) unless its omission is required by <a href="p1-messaging.html#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section 9.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag,
708         Expires, Last-Modified, or Vary, then those same header fields <em class="bcp14">MUST</em> be sent in a 304 response.
709      </p>
710      <p id="rfc.section.3.1.p.3">Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations,
711         the response <em class="bcp14">SHOULD NOT</em> include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding
712         cache updates (e.g., future HTTP extensions).
713      </p>
714      <p id="rfc.section.3.1.p.4">If the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the
715         304 response, then the recipient <em class="bcp14">MUST NOT</em> use the 304 to update its own cache. If this conditional request originated with an outbound client, such as a user agent
716         with its own cache sending a conditional GET to a shared proxy, then the 304 response <em class="bcp14">MAY</em> be forwarded to the outbound client. Otherwise, the recipient <em class="bcp14">MUST</em> disregard the 304 response and repeat the request without any preconditions.
717      </p>
718      <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.
719      </p>
720      <div id="rfc.iref.4"></div>
721      <div id="rfc.iref.s.2"></div>
722      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="status.412" href="#status.412">412 Precondition Failed</a></h2>
723      <p id="rfc.section.3.2.p.1">The 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested
724         on the server. This response code allows the client to place preconditions on the current resource state (its current representations
725         and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
726      </p>
727      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak and Strong Validators</a></h1>
728      <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 representations,
729         one normally would expect that if the representation (including both representation header fields and representation body)
730         changes in any way, then the associated validator would change as well. If this is true, then we call this validator a "strong
731         validator". One example of a strong validator is an integer that is incremented in stable storage every time a representation
732         is changed.
733      </p>
734      <p id="rfc.section.4.p.2">However, there might be cases when a server prefers to change the validator only when it desires cached representations to
735         be invalidated. For example, the representation of a weather report that changes in content every second, based on dynamic
736         measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) in order to
737         allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load
738         or weather quality). A validator that does not always change when the representation changes is a "weak validator".
739      </p>
740      <p id="rfc.section.4.p.3">An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". One can
741         think of a strong validator as part of an identifier for a specific representation, whereas a weak validator is part of an
742         identifier for a set of equivalent representations (where this notion of equivalence is entirely governed by the origin server
743         and beyond the scope of this specification).
744      </p>
745      <ul class="empty">
746         <li>A representation's modification time, if defined with only one-second resolution, could be a weak validator, since it is possible
747            that the representation might be modified twice during a single second.
748         </li>
749         <li>Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects;
750            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
751            that period is likely "good enough" to be equivalent.
752         </li>
753      </ul>
754      <p id="rfc.section.4.p.4">A strong entity-tag <em class="bcp14">MUST</em> change whenever the associated representation changes in any way. A weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation.
755         In other words, a weak entity tag <em class="bcp14">SHOULD</em> change whenever the origin server wants caches to invalidate old responses.
756      </p>
757      <p id="rfc.section.4.p.5">A "use" of a validator is either when a client generates a request and includes the validator in a validating header field,
758         or when a server compares two validators.
759      </p>
760      <p id="rfc.section.4.p.6">Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality
761         of a representation. For example, either kind is usable for a normal conditional GET. However, only a strong validator is
762         usable for range retrieval (<a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>), since otherwise the client might end up with an internally inconsistent representation. Clients <em class="bcp14">MUST NOT</em> use weak validators in range requests.
763      </p>
764      <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
765         on whether the comparison context allows the use of weak validators or not:
766      </p>
767      <ul>
768         <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.
769         </li>
770         <li>The weak comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, but either or both of them <em class="bcp14">MAY</em> be tagged as "weak" without affecting the result.
771         </li>
772      </ul>
773      <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>
774      <div id="rfc.table.u.1">
775         <table class="tt full left" cellpadding="3" cellspacing="0">
776            <thead>
777               <tr>
778                  <th>ETag 1</th>
779                  <th>ETag 2</th>
780                  <th>Strong Comparison</th>
781                  <th>Weak Comparison</th>
782               </tr>
783            </thead>
784            <tbody>
785               <tr>
786                  <td class="left">W/"1"</td>
787                  <td class="left">W/"1"</td>
788                  <td class="left">no match</td>
789                  <td class="left">match</td>
790               </tr>
791               <tr>
792                  <td class="left">W/"1"</td>
793                  <td class="left">W/"2"</td>
794                  <td class="left">no match</td>
795                  <td class="left">no match</td>
796               </tr>
797               <tr>
798                  <td class="left">W/"1"</td>
799                  <td class="left">"1"</td>
800                  <td class="left">no match</td>
801                  <td class="left">match</td>
802               </tr>
803               <tr>
804                  <td class="left">"1"</td>
805                  <td class="left">"1"</td>
806                  <td class="left">match</td>
807                  <td class="left">match</td>
808               </tr>
809            </tbody>
810         </table>
811      </div>
812      <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.
813      </p>
814      <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
815         strong, using the following rules:
816      </p>
817      <ul>
818         <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
819         <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
820            presented validator.
821         </li>
822      </ul>
823      <p id="rfc.section.4.p.11">or </p>
824      <ul>
825         <li>The validator is about to be used by a client in an If-Modified-Since or If-Unmodified-Since header field, because the client
826            has a cache entry for the associated representation, and
827         </li>
828         <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
829         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
830      </ul>
831      <p id="rfc.section.4.p.12">or </p>
832      <ul>
833         <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
834            and
835         </li>
836         <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
837         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
838      </ul>
839      <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
840         both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified
841         time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from
842         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.
843      </p>
844      <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,
845         it <em class="bcp14">MAY</em> do this only if the Last-Modified time is strong in the sense described here.
846      </p>
847      <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.
848      </p>
849      <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
850         HTTP/1.0 servers.
851      </p>
852      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="rules.for.when.to.use.entity.tags.and.last-modified.dates" 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>
853      <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
854         ought to be used, and for what purposes.
855      </p>
856      <p id="rfc.section.5.p.2">HTTP/1.1 origin servers: </p>
857      <ul>
858         <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
859         </li>
860         <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,
861            or if it is unfeasible to send a strong entity-tag.
862         </li>
863         <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one.
864         </li>
865      </ul>
866      <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
867         value.
868      </p>
869      <p id="rfc.section.5.p.4">HTTP/1.1 clients: </p>
870      <ul>
871         <li><em class="bcp14">MUST</em> use that entity-tag in any cache-conditional request (using If-Match or If-None-Match) if an entity-tag has been provided
872            by the origin server.
873         </li>
874         <li><em class="bcp14">SHOULD</em> use the Last-Modified value in non-subrange cache-conditional requests (using If-Modified-Since) if only a Last-Modified value
875            has been provided by the origin server.
876         </li>
877         <li><em class="bcp14">MAY</em> use the Last-Modified value in subrange cache-conditional requests (using If-Unmodified-Since) if only a Last-Modified value
878            has been provided by an HTTP/1.0 origin server. The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty.
879         </li>
880         <li><em class="bcp14">SHOULD</em> use both validators in cache-conditional requests if both an entity-tag and a Last-Modified value have been provided by the
881            origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
882         </li>
883      </ul>
884      <p id="rfc.section.5.p.5">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
885         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
886         field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status code of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields
887         in the request.
888      </p>
889      <p id="rfc.section.5.p.6">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags
890         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
891         fields in the request.
892      </p>
893      <ul class="empty">
894         <li> <b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients ought to transmit as much non-redundant information
895            as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
896            assumptions about the validators they receive.
897         </li>
898         <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will
899            support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare
900            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
901            HTTP/1.1 origin servers should not provide one.
902         </li>
903      </ul>
904      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
905      <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>
906      <div id="rfc.iref.e.1"></div>
907      <div id="rfc.iref.h.1"></div>
908      <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="header.etag" href="#header.etag">ETag</a></h2>
909      <p id="rfc.section.6.1.p.1">The "ETag" header field provides the current value of the entity-tag (see <a href="#entity.tags" title="Entity-Tags">Section&nbsp;2</a>) for one representation of the target resource. An entity-tag is intended for use as a resource-local identifier for differentiating
910         between representations of the same resource that vary over time or via content negotiation (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section&nbsp;4</a>).
911      </p>
912      <div id="rfc.figure.u.7"></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>
913  <a href="#header.etag" class="smpl">ETag-v</a> = <a href="#entity.tags" class="smpl">entity-tag</a>
914</pre><div id="rfc.figure.u.8"></div>
915      <p>Examples:</p>  <pre class="text">  ETag: "xyzzy"
916  ETag: W/"xyzzy"
917  ETag: ""
918</pre><p id="rfc.section.6.1.p.4">An entity-tag provides an "opaque" cache validator that allows for more reliable validation than modification dates in situations
919         where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient,
920         or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification dates.
921      </p>
922      <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
923         appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality
924         would open up a can of worms. Thus, comparisons of any other header fields (except Last-Modified, for compatibility with HTTP/1.0)
925         are never used for purposes of validating a cache entry.
926      </p>
927      <div id="rfc.iref.i.1"></div>
928      <div id="rfc.iref.h.2"></div>
929      <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="header.if-match" href="#header.if-match">If-Match</a></h2>
930      <p id="rfc.section.6.2.p.1">The "If-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on the current existence or value of an entity-tag for one or more representations
931         of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting
932         against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem).
933         An If-Match field-value of "*" places the precondition on the existence of any current representation for the target resource.
934      </p>
935      <div id="rfc.figure.u.9"></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>
936  <a href="#header.if-match" class="smpl">If-Match-v</a> = "*" / 1#<a href="#entity.tags" class="smpl">entity-tag</a>
937</pre><p id="rfc.section.6.2.p.3">If any of the entity-tags listed in the If-Match field value match the entity-tag of the selected representation for the target
938         resource, or if "*" is given and any current representation exists for the target resource, then the server <em class="bcp14">MAY</em> perform the request method as if the If-Match header field was not present.
939      </p>
940      <p id="rfc.section.6.2.p.4">If none of the entity-tags match, or if "*" is given and no current representation exists, the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, the server <em class="bcp14">MUST</em> respond with the 412 (Precondition Failed) status code.
941      </p>
942      <p id="rfc.section.6.2.p.5">If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status code, then the
943         If-Match header field <em class="bcp14">MUST</em> be ignored.
944      </p>
945      <p id="rfc.section.6.2.p.6">Examples:</p>
946      <div id="rfc.figure.u.10"></div><pre class="text">  If-Match: "xyzzy"
947  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
948  If-Match: *
949</pre><p id="rfc.section.6.2.p.8">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
950         is undefined by this specification.
951      </p>
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 id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2>
955      <p id="rfc.section.6.3.p.1">The "If-Modified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has not been modified since
956         the time specified in this field, then do not perform the request method; instead, respond as detailed below.
957      </p>
958      <div id="rfc.figure.u.11"></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>
959                        <a href="#header.if-modified-since" class="smpl">If-Modified-Since-v</a>
960  <a href="#header.if-modified-since" class="smpl">If-Modified-Since-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
961</pre><p id="rfc.section.6.3.p.3">An example of the field is:</p>
962      <div id="rfc.figure.u.12"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
963</pre><p id="rfc.section.6.3.p.5">A GET method with an If-Modified-Since header field and no Range header field requests that the selected representation be
964         transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining
965         this includes the following cases:
966      </p>
967      <ol>
968         <li>If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date
969            is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is
970            invalid.
971         </li>
972         <li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for
973            a normal GET.
974         </li>
975         <li>If the selected representation has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> return a 304 (Not Modified) response.
976         </li>
977      </ol>
978      <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>
979      <ul class="empty">
980         <li> <b>Note:</b> The Range 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.Part5.4"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a> for full details.
981         </li>
982         <li> <b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.
983         </li>
984         <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
985            function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since
986            header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header
987            field whenever possible.
988         </li>
989         <li> <b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from the Last-Modified header
990            field for the same request, the client needs to be aware that this date is interpreted in the server's understanding of time.
991            Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server, are concerns.
992            This includes the possibility of race conditions if the document has changed between the time it was first requested and the
993            If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since
994            date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between
995            client and server are at best approximate due to network latency.
996         </li>
997      </ul>
998      <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
999         fields is undefined by this specification.
1000      </p>
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 id="header.if-none-match" href="#header.if-none-match">If-None-Match</a></h2>
1004      <p id="rfc.section.6.4.p.1">The "If-None-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on not matching any of the current entity-tag values for representations of the
1005         target resource. If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information
1006         with a minimum amount of transaction overhead. A client that has one or more representations previously obtained from the
1007         target resource can send If-None-Match with a list of the associated entity-tags in the hope of receiving a 304 response if
1008         at least one of those representations matches the selected representation.
1009      </p>
1010      <p id="rfc.section.6.4.p.2">If-None-Match MAY also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying
1011         an existing representation of the target resource when the client believes that the resource does not have a current representation.
1012         This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation
1013         for the target resource.
1014      </p>
1015      <div id="rfc.figure.u.13"></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>
1016  <a href="#header.if-none-match" class="smpl">If-None-Match-v</a> = "*" / 1#<a href="#entity.tags" class="smpl">entity-tag</a>
1017</pre><p id="rfc.section.6.4.p.4">If any of the entity-tags listed in the If-None-Match field-value match the entity-tag of the selected representation, or
1018         if "*" is given and any current representation exists for that resource, then the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, if the request method was GET or HEAD, the server <em class="bcp14">SHOULD</em> respond with a 304 (Not Modified) status code, including the cache-related header fields (particularly ETag) of the selected
1019         representation that has a matching entity-tag. For all other request methods, the server <em class="bcp14">MUST</em> respond with a 412 (Precondition Failed) status code.
1020      </p>
1021      <p id="rfc.section.6.4.p.5">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.6">If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status code, then
1024         the If-None-Match header field <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.7">Examples:</p>
1027      <div id="rfc.figure.u.14"></div><pre class="text">  If-None-Match: "xyzzy"
1028  If-None-Match: W/"xyzzy"
1029  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1030  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1031  If-None-Match: *
1032</pre><p id="rfc.section.6.4.p.9">The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header
1033         fields is undefined by this specification.
1034      </p>
1035      <div id="rfc.iref.i.4"></div>
1036      <div id="rfc.iref.h.5"></div>
1037      <h2 id="rfc.section.6.5"><a href="#rfc.section.6.5">6.5</a>&nbsp;<a id="header.if-unmodified-since" href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
1038      <p id="rfc.section.6.5.p.1">The "If-Unmodified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has been modified since
1039         the time specified in this field, then the server <em class="bcp14">MUST NOT</em> perform the requested operation and <em class="bcp14">MUST</em> instead respond with the 412 (Precondition Failed) status code. If the selected representation has not been modified since
1040         the time specified in this field, the server <em class="bcp14">SHOULD</em> perform the request method as if the If-Unmodified-Since header field were not present.
1041      </p>
1042      <div id="rfc.figure.u.15"></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>
1043                          <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since-v</a>
1044  <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
1045</pre><p id="rfc.section.6.5.p.3">An example of the field is:</p>
1046      <div id="rfc.figure.u.16"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1047</pre><p id="rfc.section.6.5.p.5">If the request normally (i.e., without the If-Unmodified-Since header field) would result in anything other than a 2xx or
1048         412 status code, the If-Unmodified-Since header field <em class="bcp14">SHOULD</em> be ignored.
1049      </p>
1050      <p id="rfc.section.6.5.p.6">If the specified date is invalid, the header field <em class="bcp14">MUST</em> be ignored.
1051      </p>
1052      <p id="rfc.section.6.5.p.7">The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since
1053         header fields is undefined by this specification.
1054      </p>
1055      <div id="rfc.iref.l.1"></div>
1056      <div id="rfc.iref.h.6"></div>
1057      <h2 id="rfc.section.6.6"><a href="#rfc.section.6.6">6.6</a>&nbsp;<a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2>
1058      <p id="rfc.section.6.6.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the representation was last
1059         modified.
1060      </p>
1061      <div id="rfc.figure.u.17"></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>
1062  <a href="#header.last-modified" class="smpl">Last-Modified-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
1063</pre><p id="rfc.section.6.6.p.3">An example of its use is</p>
1064      <div id="rfc.figure.u.18"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1065</pre><p id="rfc.section.6.6.p.5">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be
1066         the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
1067         detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
1068         use its value to make conditional requests and test the validity of locally cached responses.
1069      </p>
1070      <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
1071         last modification would indicate some time in the future, the server <em class="bcp14">MUST</em> replace that date with the message origination date.
1072      </p>
1073      <p id="rfc.section.6.6.p.7">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date value of
1074         its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially
1075         if the representation changes near the time that the response is generated.
1076      </p>
1077      <p id="rfc.section.6.6.p.8">HTTP/1.1 servers <em class="bcp14">SHOULD</em> send Last-Modified whenever feasible.
1078      </p>
1079      <p id="rfc.section.6.6.p.9">The Last-Modified header field value is often used as a cache validator. In simple terms, a cache entry is considered to be
1080         valid if the representation has not been modified since the Last-Modified value.
1081      </p>
1082      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
1083      <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="status.code.registration" href="#status.code.registration">Status Code Registration</a></h2>
1084      <p id="rfc.section.7.1.p.1">The HTTP Status Code Registry located at &lt;<a href="http://www.iana.org/assignments/http-status-codes">http://www.iana.org/assignments/http-status-codes</a>&gt; shall be updated with the registrations below:
1085      </p>
1086      <div id="rfc.table.1">
1087         <div id="iana.status.code.registration.table"></div>
1088         <table class="tt full left" cellpadding="3" cellspacing="0">
1089            <thead>
1090               <tr>
1091                  <th>Value</th>
1092                  <th>Description</th>
1093                  <th>Reference</th>
1094               </tr>
1095            </thead>
1096            <tbody>
1097               <tr>
1098                  <td class="left">304</td>
1099                  <td class="left">Not Modified</td>
1100                  <td class="left"> <a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section&nbsp;3.1</a> 
1101                  </td>
1102               </tr>
1103               <tr>
1104                  <td class="left">412</td>
1105                  <td class="left">Precondition Failed</td>
1106                  <td class="left"> <a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section&nbsp;3.2</a> 
1107                  </td>
1108               </tr>
1109            </tbody>
1110         </table>
1111      </div>
1112      <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
1113      <p id="rfc.section.7.2.p.1">The Message Header Field 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; shall 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>):
1114      </p>
1115      <div id="rfc.table.2">
1116         <div id="iana.header.registration.table"></div>
1117         <table class="tt full left" cellpadding="3" cellspacing="0">
1118            <thead>
1119               <tr>
1120                  <th>Header Field Name</th>
1121                  <th>Protocol</th>
1122                  <th>Status</th>
1123                  <th>Reference</th>
1124               </tr>
1125            </thead>
1126            <tbody>
1127               <tr>
1128                  <td class="left">ETag</td>
1129                  <td class="left">http</td>
1130                  <td class="left">standard</td>
1131                  <td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section&nbsp;6.1</a> 
1132                  </td>
1133               </tr>
1134               <tr>
1135                  <td class="left">If-Match</td>
1136                  <td class="left">http</td>
1137                  <td class="left">standard</td>
1138                  <td class="left"> <a href="#header.if-match" id="rfc.xref.header.if-match.2" title="If-Match">Section&nbsp;6.2</a> 
1139                  </td>
1140               </tr>
1141               <tr>
1142                  <td class="left">If-Modified-Since</td>
1143                  <td class="left">http</td>
1144                  <td class="left">standard</td>
1145                  <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> 
1146                  </td>
1147               </tr>
1148               <tr>
1149                  <td class="left">If-None-Match</td>
1150                  <td class="left">http</td>
1151                  <td class="left">standard</td>
1152                  <td class="left"> <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">Section&nbsp;6.4</a> 
1153                  </td>
1154               </tr>
1155               <tr>
1156                  <td class="left">If-Unmodified-Since</td>
1157                  <td class="left">http</td>
1158                  <td class="left">standard</td>
1159                  <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> 
1160                  </td>
1161               </tr>
1162               <tr>
1163                  <td class="left">Last-Modified</td>
1164                  <td class="left">http</td>
1165                  <td class="left">standard</td>
1166                  <td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;6.6</a> 
1167                  </td>
1168               </tr>
1169            </tbody>
1170         </table>
1171      </div>
1172      <p id="rfc.section.7.2.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1173      <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1174      <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.Part1.9"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
1175      </p>
1176      <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
1177      <h1 id="rfc.references"><a id="rfc.section.10" href="#rfc.section.10">10.</a> References
1178      </h1>
1179      <h2 id="rfc.references.1"><a href="#rfc.section.10.1" id="rfc.section.10.1">10.1</a> Normative References
1180      </h2>
1181      <table>           
1182         <tr>
1183            <td class="reference"><b id="Part1">[Part1]</b></td>
1184            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">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="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), March&nbsp;2011.
1185            </td>
1186         </tr>
1187         <tr>
1188            <td class="reference"><b id="Part3">[Part3]</b></td>
1189            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">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="http://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-latest">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p3-payload-latest (work in progress), March&nbsp;2011.
1190            </td>
1191         </tr>
1192         <tr>
1193            <td class="reference"><b id="Part5">[Part5]</b></td>
1194            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">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="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), March&nbsp;2011.
1195            </td>
1196         </tr>
1197         <tr>
1198            <td class="reference"><b id="Part6">[Part6]</b></td>
1199            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">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>, <a href="mailto:mnot@mnot.net">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">HTTP/1.1, part 6: Caching</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p6-cache-latest (work in progress), March&nbsp;2011.
1200            </td>
1201         </tr>
1202         <tr>
1203            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1204            <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="http://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
1205            </td>
1206         </tr>
1207         <tr>
1208            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1209            <td class="top"><a href="mailto:dcrocker@bbiw.net" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="mailto:paul.overell@thus.net" title="THUS plc.">P. Overell</a>, “<a href="http://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
1210            </td>
1211         </tr>
1212      </table>
1213      <h2 id="rfc.references.2"><a href="#rfc.section.10.2" id="rfc.section.10.2">10.2</a> Informative References
1214      </h2>
1215      <table>   
1216         <tr>
1217            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1218            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1219            </td>
1220         </tr>
1221         <tr>
1222            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1223            <td class="top"><a href="mailto:GK-IETF@ninebynine.org" title="Nine by Nine">Klyne, G.</a>, <a href="mailto:mnot@pobox.com" title="BEA Systems">Nottingham, M.</a>, and <a href="mailto:JeffMogul@acm.org" title="HP Labs">J. Mogul</a>, “<a href="http://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
1224            </td>
1225         </tr>
1226      </table>
1227      <div class="avoidbreak">
1228         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1229         <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
1230               (editor)
1231               <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:fielding@gbiv.com"><span class="email">fielding@gbiv.com</span></a></span><span class="vcardline">URI: <a href="http://roy.gbiv.com/" class="url">http://roy.gbiv.com/</a></span></address>
1232         <address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">Alcatel-Lucent Bell Labs</span><span class="adr"><span class="street-address vcardline">21 Oak Knoll Road</span><span class="vcardline"><span class="locality">Carlisle</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01741</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:jg@freedesktop.org"><span class="email">jg@freedesktop.org</span></a></span><span class="vcardline">URI: <a href="http://gettys.wordpress.com/" class="url">http://gettys.wordpress.com/</a></span></address>
1233         <address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Hewlett-Packard Company</span><span class="adr"><span class="street-address vcardline">HP Labs, Large Scale Systems Group</span><span class="street-address vcardline">1501 Page Mill Road, MS 1177</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94304</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:JeffMogul@acm.org"><span class="email">JeffMogul@acm.org</span></a></span></address>
1234         <address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:henrikn@microsoft.com"><span class="email">henrikn@microsoft.com</span></a></span></address>
1235         <address class="vcard"><span class="vcardline"><span class="fn">Larry Masinter</span><span class="n hidden"><span class="family-name">Masinter</span><span class="given-name">Larry</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:LMM@acm.org"><span class="email">LMM@acm.org</span></a></span><span class="vcardline">URI: <a href="http://larry.masinter.net/" class="url">http://larry.masinter.net/</a></span></address>
1236         <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span></span><span class="vcardline">Email: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address>
1237         <address class="vcard"><span class="vcardline"><span class="fn">Tim Berners-Lee</span><span class="n hidden"><span class="family-name">Berners-Lee</span><span class="given-name">Tim</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">MIT Computer Science and Artificial Intelligence Laboratory</span><span class="street-address vcardline">The Stata Center, Building 32</span><span class="street-address vcardline">32 Vassar Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:timbl@w3.org"><span class="email">timbl@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.w3.org/People/Berners-Lee/" class="url">http://www.w3.org/People/Berners-Lee/</a></span></address>
1238         <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
1239               (editor)
1240               <span class="n hidden"><span class="family-name">Lafon</span><span class="given-name">Yves</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">W3C / ERCIM</span><span class="street-address vcardline">2004, rte des Lucioles</span><span class="vcardline"><span class="locality">Sophia-Antipolis</span>, <span class="region">AM</span>&nbsp;<span class="postal-code">06902</span></span><span class="country-name vcardline">France</span></span><span class="vcardline">Email: <a href="mailto:ylafon@w3.org"><span class="email">ylafon@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.raubacapeu.net/people/yves/" class="url">http://www.raubacapeu.net/people/yves/</a></span></address>
1241         <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
1242               (editor)
1243               <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline tel">Phone: <a href="tel:+492512807760"><span class="value">+49 251 2807760</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+492512807761"><span class="value">+49 251 2807761</span></a></span><span class="vcardline">Email: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address>
1244      </div>
1245      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
1246      <p id="rfc.section.A.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.3" title="If-None-Match">6.4</a>).
1247      </p>
1248      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
1249      <div id="rfc.figure.u.19"></div> <pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = "ETag:" OWS ETag-v
1250<a href="#header.etag" class="smpl">ETag-v</a> = entity-tag
1251
1252<a href="#abnf.dependencies" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1253
1254<a href="#header.if-match" class="smpl">If-Match</a> = "If-Match:" OWS If-Match-v
1255<a href="#header.if-match" class="smpl">If-Match-v</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1256 entity-tag ] ) )
1257<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = "If-Modified-Since:" OWS If-Modified-Since-v
1258<a href="#header.if-modified-since" class="smpl">If-Modified-Since-v</a> = HTTP-date
1259<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "If-None-Match:" OWS If-None-Match-v
1260<a href="#header.if-none-match" class="smpl">If-None-Match-v</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1261 entity-tag ] ) )
1262<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = "If-Unmodified-Since:" OWS
1263 If-Unmodified-Since-v
1264<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since-v</a> = HTTP-date
1265
1266<a href="#header.last-modified" class="smpl">Last-Modified</a> = "Last-Modified:" OWS Last-Modified-v
1267<a href="#header.last-modified" class="smpl">Last-Modified-v</a> = HTTP-date
1268
1269<a href="#core.rules" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1270
1271<a href="#entity.tags" class="smpl">entity-tag</a> = [ weak ] opaque-tag
1272
1273<a href="#entity.tags" class="smpl">opaque-tag</a> = quoted-string
1274
1275<a href="#core.rules" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1276
1277<a href="#entity.tags" class="smpl">weak</a> = %x57.2F ; W/
1278</pre> <div id="rfc.figure.u.20"></div>
1279      <p>ABNF diagnostics:</p><pre class="inline">; ETag defined but not used
1280; If-Match defined but not used
1281; If-Modified-Since defined but not used
1282; If-None-Match defined but not used
1283; If-Unmodified-Since defined but not used
1284; Last-Modified defined but not used
1285</pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1286      <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a>&nbsp;Since RFC 2616
1287      </h2>
1288      <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>.
1289      </p>
1290      <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-00
1291      </h2>
1292      <p id="rfc.section.C.2.p.1">Closed issues: </p>
1293      <ul>
1294         <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"
1295         </li>
1296      </ul>
1297      <p id="rfc.section.C.2.p.2">Other changes: </p>
1298      <ul>
1299         <li>Move definitions of 304 and 412 condition codes from Part2.</li>
1300      </ul>
1301      <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a>&nbsp;Since draft-ietf-httpbis-p4-conditional-01
1302      </h2>
1303      <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;):
1304      </p>
1305      <ul>
1306         <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li>
1307      </ul>
1308      <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a>&nbsp;<a id="changes.since.02" href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></h2>
1309      <p id="rfc.section.C.4.p.1">Closed issues: </p>
1310      <ul>
1311         <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"
1312         </li>
1313      </ul>
1314      <p id="rfc.section.C.4.p.2">Ongoing work on IANA Message Header Field Registration (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/40">http://tools.ietf.org/wg/httpbis/trac/ticket/40</a>&gt;):
1315      </p>
1316      <ul>
1317         <li>Reference RFC 3984, and update header field registrations for header fields defined in this document.</li>
1318      </ul>
1319      <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a>&nbsp;<a id="changes.since.03" href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></h2>
1320      <p id="rfc.section.C.5.p.1">Closed issues: </p>
1321      <ul>
1322         <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"
1323         </li>
1324         <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"
1325         </li>
1326         <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"
1327         </li>
1328      </ul>
1329      <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a>&nbsp;<a id="changes.since.04" href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></h2>
1330      <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;):
1331      </p>
1332      <ul>
1333         <li>Use "/" instead of "|" for alternatives.</li>
1334         <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li>
1335         <li>Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.</li>
1336      </ul>
1337      <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a>&nbsp;<a id="changes.since.05" href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></h2>
1338      <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;):
1339      </p>
1340      <ul>
1341         <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li>
1342      </ul>
1343      <h2 id="rfc.section.C.8"><a href="#rfc.section.C.8">C.8</a>&nbsp;<a id="changes.since.06" href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></h2>
1344      <p id="rfc.section.C.8.p.1">Closed issues: </p>
1345      <ul>
1346         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/153">http://tools.ietf.org/wg/httpbis/trac/ticket/153</a>&gt;: "case-sensitivity of etag weakness indicator"
1347         </li>
1348      </ul>
1349      <h2 id="rfc.section.C.9"><a href="#rfc.section.C.9">C.9</a>&nbsp;<a id="changes.since.07" href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></h2>
1350      <p id="rfc.section.C.9.p.1">Closed issues: </p>
1351      <ul>
1352         <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" (If-Match still was defined to require strong matching)
1353         </li>
1354         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/198">http://tools.ietf.org/wg/httpbis/trac/ticket/198</a>&gt;: "move IANA registrations for optional status codes"
1355         </li>
1356      </ul>
1357      <h2 id="rfc.section.C.10"><a href="#rfc.section.C.10">C.10</a>&nbsp;<a id="changes.since.08" href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></h2>
1358      <p id="rfc.section.C.10.p.1">No significant changes.</p>
1359      <h2 id="rfc.section.C.11"><a href="#rfc.section.C.11">C.11</a>&nbsp;<a id="changes.since.09" href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></h2>
1360      <p id="rfc.section.C.11.p.1">No significant changes.</p>
1361      <h2 id="rfc.section.C.12"><a href="#rfc.section.C.12">C.12</a>&nbsp;<a id="changes.since.10" href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></h2>
1362      <p id="rfc.section.C.12.p.1">Closed issues: </p>
1363      <ul>
1364         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/69">http://tools.ietf.org/wg/httpbis/trac/ticket/69</a>&gt;: "Clarify 'Requested Variant'"
1365         </li>
1366         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/109">http://tools.ietf.org/wg/httpbis/trac/ticket/109</a>&gt;: "Clarify entity / representation / variant terminology"
1367         </li>
1368         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/220">http://tools.ietf.org/wg/httpbis/trac/ticket/220</a>&gt;: "consider removing the 'changes from 2068' sections"
1369         </li>
1370      </ul>
1371      <h2 id="rfc.section.C.13"><a href="#rfc.section.C.13">C.13</a>&nbsp;<a id="changes.since.11" href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></h2>
1372      <p id="rfc.section.C.13.p.1">None.</p>
1373      <h2 id="rfc.section.C.14"><a href="#rfc.section.C.14">C.14</a>&nbsp;<a id="changes.since.12" href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></h2>
1374      <p id="rfc.section.C.14.p.1">Closed issues: </p>
1375      <ul>
1376         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/224">http://tools.ietf.org/wg/httpbis/trac/ticket/224</a>&gt;: "Header Classification"
1377         </li>
1378      </ul>
1379      <h2 id="rfc.section.C.15"><a href="#rfc.section.C.15">C.15</a>&nbsp;<a id="changes.since.13" href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></h2>
1380      <p id="rfc.section.C.15.p.1">None yet.</p>
1381      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1382      <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> 
1383      </p>
1384      <div class="print2col">
1385         <ul class="ind">
1386            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
1387                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.3"><b>3.1</b></a>, <a href="#rfc.xref.status.304.1">7.1</a></li>
1388               </ul>
1389            </li>
1390            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
1391                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.4"><b>3.2</b></a>, <a href="#rfc.xref.status.412.1">7.1</a></li>
1392               </ul>
1393            </li>
1394            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
1395                  <li>ETag header field&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">7.2</a></li>
1396               </ul>
1397            </li>
1398            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
1399                  <li><tt>Grammar</tt>&nbsp;&nbsp;
1400                     <ul>
1401                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2</b></a></li>
1402                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>6.1</b></a></li>
1403                        <li><tt>ETag-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>6.1</b></a></li>
1404                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>6.2</b></a></li>
1405                        <li><tt>If-Match-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>6.2</b></a></li>
1406                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>6.3</b></a></li>
1407                        <li><tt>If-Modified-Since-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>6.3</b></a></li>
1408                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>6.4</b></a></li>
1409                        <li><tt>If-None-Match-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.11"><b>6.4</b></a></li>
1410                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.12"><b>6.5</b></a></li>
1411                        <li><tt>If-Unmodified-Since-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.13"><b>6.5</b></a></li>
1412                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.14"><b>6.6</b></a></li>
1413                        <li><tt>Last-Modified-v</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.15"><b>6.6</b></a></li>
1414                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2</b></a></li>
1415                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2</b></a></li>
1416                     </ul>
1417                  </li>
1418               </ul>
1419            </li>
1420            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
1421                  <li>Header Fields&nbsp;&nbsp;
1422                     <ul>
1423                        <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">7.2</a></li>
1424                        <li>If-Match&nbsp;&nbsp;<a href="#rfc.xref.header.if-match.1">2</a>, <a href="#rfc.iref.h.2"><b>6.2</b></a>, <a href="#rfc.xref.header.if-match.2">7.2</a></li>
1425                        <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">7.2</a></li>
1426                        <li>If-None-Match&nbsp;&nbsp;<a href="#rfc.xref.header.if-none-match.1">2</a>, <a href="#rfc.iref.h.4"><b>6.4</b></a>, <a href="#rfc.xref.header.if-none-match.2">7.2</a>, <a href="#rfc.xref.header.if-none-match.3">A</a></li>
1427                        <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">7.2</a></li>
1428                        <li>Last-Modified&nbsp;&nbsp;<a href="#rfc.iref.h.6"><b>6.6</b></a>, <a href="#rfc.xref.header.last-modified.1">7.2</a></li>
1429                     </ul>
1430                  </li>
1431               </ul>
1432            </li>
1433            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
1434                  <li>If-Match header field&nbsp;&nbsp;<a href="#rfc.xref.header.if-match.1">2</a>, <a href="#rfc.iref.i.1"><b>6.2</b></a>, <a href="#rfc.xref.header.if-match.2">7.2</a></li>
1435                  <li>If-Modified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.2"><b>6.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">7.2</a></li>
1436                  <li>If-None-Match header field&nbsp;&nbsp;<a href="#rfc.xref.header.if-none-match.1">2</a>, <a href="#rfc.iref.i.3"><b>6.4</b></a>, <a href="#rfc.xref.header.if-none-match.2">7.2</a>, <a href="#rfc.xref.header.if-none-match.3">A</a></li>
1437                  <li>If-Unmodified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.4"><b>6.5</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">7.2</a></li>
1438               </ul>
1439            </li>
1440            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
1441                  <li>Last-Modified header field&nbsp;&nbsp;<a href="#rfc.iref.l.1"><b>6.6</b></a>, <a href="#rfc.xref.header.last-modified.1">7.2</a></li>
1442               </ul>
1443            </li>
1444            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
1445                  <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">2.1</a>, <a href="#rfc.xref.Part1.7">3.1</a>, <a href="#rfc.xref.Part1.8">3.1</a>, <a href="#rfc.xref.Part1.9">8</a>, <a href="#Part1"><b>10.1</b></a><ul>
1446                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.2</a></li>
1447                        <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>
1448                        <li><em>Section 6.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">1.2.2</a></li>
1449                        <li><em>Section 6.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">2.1</a></li>
1450                        <li><em>Section 9.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.7">3.1</a></li>
1451                        <li><em>Section 9.3.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.8">3.1</a></li>
1452                     </ul>
1453                  </li>
1454                  <li><em>Part3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.1</a>, <a href="#rfc.xref.Part3.2">2.1</a>, <a href="#Part3"><b>10.1</b></a><ul>
1455                        <li><em>Section 5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.1">2.1</a></li>
1456                        <li><em>Section 6.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part3.2">2.1</a></li>
1457                     </ul>
1458                  </li>
1459                  <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">6.3</a>, <a href="#Part5"><b>10.1</b></a><ul>
1460                        <li><em>Section 5.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">2</a></li>
1461                        <li><em>Section 5.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.4">6.3</a></li>
1462                     </ul>
1463                  </li>
1464                  <li><em>Part6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.1">1</a>, <a href="#Part6"><b>10.1</b></a></li>
1465               </ul>
1466            </li>
1467            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
1468                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>10.1</b></a></li>
1469                  <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>
1470                  <li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">7.2</a>, <a href="#RFC3864"><b>10.2</b></a></li>
1471                  <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>
1472                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">1.2</a></li>
1473                     </ul>
1474                  </li>
1475               </ul>
1476            </li>
1477            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
1478                  <li>Status Codes&nbsp;&nbsp;
1479                     <ul>
1480                        <li>304 Not Modified&nbsp;&nbsp;<a href="#rfc.iref.s.1"><b>3.1</b></a>, <a href="#rfc.xref.status.304.1">7.1</a></li>
1481                        <li>412 Precondition Failed&nbsp;&nbsp;<a href="#rfc.iref.s.2"><b>3.2</b></a>, <a href="#rfc.xref.status.412.1">7.1</a></li>
1482                     </ul>
1483                  </li>
1484               </ul>
1485            </li>
1486         </ul>
1487      </div>
1488   </body>
1489</html>
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