source: draft-ietf-httpbis/latest/p4-conditional.html @ 1738

Last change on this file since 1738 was 1738, checked in by julian.reschke@…, 7 years ago

Work-in-progress: hyperlink header field definitions(P5)

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437@page {
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445       content: "HTTP/1.1, Part 4"; 
446  } 
447  @bottom-left {
448       content: "Fielding, et al."; 
449  } 
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451       content: "Expires January 9, 2013"; 
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469</style><link rel="Contents" href="#rfc.toc">
470      <link rel="Author" href="#rfc.authors">
471      <link rel="Copyright" href="#rfc.copyrightnotice">
472      <link rel="Index" href="#rfc.index">
473      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
474      <link rel="Chapter" title="2 Validators" href="#rfc.section.2">
475      <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3">
476      <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4">
477      <link rel="Chapter" title="5 IANA Considerations" href="#rfc.section.5">
478      <link rel="Chapter" title="6 Security Considerations" href="#rfc.section.6">
479      <link rel="Chapter" title="7 Acknowledgments" href="#rfc.section.7">
480      <link rel="Chapter" href="#rfc.section.8" title="8 References">
481      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
482      <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B">
483      <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C">
484      <link href="p3-payload.html" rel="prev">
485      <link href="p5-range.html" rel="next">
486      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.584, 2012-06-24 15:59:38, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
487      <link rel="schema.dct" href="http://purl.org/dc/terms/">
488      <meta name="dct.creator" content="Fielding, R.">
489      <meta name="dct.creator" content="Lafon, Y.">
490      <meta name="dct.creator" content="Reschke, J. F.">
491      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest">
492      <meta name="dct.issued" scheme="ISO8601" content="2012-07-08">
493      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
494      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext 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.">
495      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext 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.">
496   </head>
497   <body onload="init();">
498      <table class="header">
499         <tbody>
500            <tr>
501               <td class="left">HTTPbis Working Group</td>
502               <td class="right">R. Fielding, Editor</td>
503            </tr>
504            <tr>
505               <td class="left">Internet-Draft</td>
506               <td class="right">Adobe</td>
507            </tr>
508            <tr>
509               <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
510               </td>
511               <td class="right">Y. Lafon, Editor</td>
512            </tr>
513            <tr>
514               <td class="left">Intended status: Standards Track</td>
515               <td class="right">W3C</td>
516            </tr>
517            <tr>
518               <td class="left">Expires: January 9, 2013</td>
519               <td class="right">J. Reschke, Editor</td>
520            </tr>
521            <tr>
522               <td class="left"></td>
523               <td class="right">greenbytes</td>
524            </tr>
525            <tr>
526               <td class="left"></td>
527               <td class="right">July 8, 2012</td>
528            </tr>
529         </tbody>
530      </table>
531      <p class="title">HTTP/1.1, part 4: Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p>
532      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
533      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
534         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the
535         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
536      </p> 
537      <p>Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those
538         requests.
539      </p> 
540      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
541      <p>Discussion of this draft ought to take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived
542         at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
543      </p> 
544      <p>The current issues list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
545      </p> 
546      <p>The changes in this draft are summarized in <a href="#changes.since.19" title="Since draft-ietf-httpbis-p4-conditional-19">Appendix&nbsp;C.1</a>.
547      </p> 
548      <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
549      <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
550      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
551         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>.
552      </p>
553      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
554         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
555         in progress”.
556      </p>
557      <p>This Internet-Draft will expire on January 9, 2013.</p>
558      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
559      <p>Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
560      <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
561         and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
562         text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
563         BSD License.
564      </p>
565      <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
566         10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
567         allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
568         controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
569         works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
570         it into languages other than English.
571      </p>
572      <hr class="noprint">
573      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
574      <ul class="toc">
575         <li>1.&nbsp;&nbsp;&nbsp;<a href="#introduction">Introduction</a><ul>
576               <li>1.1&nbsp;&nbsp;&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></li>
577               <li>1.2&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a></li>
578            </ul>
579         </li>
580         <li>2.&nbsp;&nbsp;&nbsp;<a href="#validators">Validators</a><ul>
581               <li>2.1&nbsp;&nbsp;&nbsp;<a href="#weak.and.strong.validators">Weak versus Strong</a></li>
582               <li>2.2&nbsp;&nbsp;&nbsp;<a href="#header.last-modified">Last-Modified</a><ul>
583                     <li>2.2.1&nbsp;&nbsp;&nbsp;<a href="#lastmod.generation">Generation</a></li>
584                     <li>2.2.2&nbsp;&nbsp;&nbsp;<a href="#lastmod.comparison">Comparison</a></li>
585                  </ul>
586               </li>
587               <li>2.3&nbsp;&nbsp;&nbsp;<a href="#header.etag">ETag</a><ul>
588                     <li>2.3.1&nbsp;&nbsp;&nbsp;<a href="#entity.tag.generation">Generation</a></li>
589                     <li>2.3.2&nbsp;&nbsp;&nbsp;<a href="#entity.tag.comparison">Comparison</a></li>
590                     <li>2.3.3&nbsp;&nbsp;&nbsp;<a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>
591                  </ul>
592               </li>
593               <li>2.4&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>
594            </ul>
595         </li>
596         <li>3.&nbsp;&nbsp;&nbsp;<a href="#header.field.definitions">Precondition Header Fields</a><ul>
597               <li>3.1&nbsp;&nbsp;&nbsp;<a href="#header.if-match">If-Match</a></li>
598               <li>3.2&nbsp;&nbsp;&nbsp;<a href="#header.if-none-match">If-None-Match</a></li>
599               <li>3.3&nbsp;&nbsp;&nbsp;<a href="#header.if-modified-since">If-Modified-Since</a></li>
600               <li>3.4&nbsp;&nbsp;&nbsp;<a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
601               <li>3.5&nbsp;&nbsp;&nbsp;<a href="#header.if-range">If-Range</a></li>
602            </ul>
603         </li>
604         <li>4.&nbsp;&nbsp;&nbsp;<a href="#status.code.definitions">Status Code Definitions</a><ul>
605               <li>4.1&nbsp;&nbsp;&nbsp;<a href="#status.304">304 Not Modified</a></li>
606               <li>4.2&nbsp;&nbsp;&nbsp;<a href="#status.412">412 Precondition Failed</a></li>
607            </ul>
608         </li>
609         <li>5.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
610               <li>5.1&nbsp;&nbsp;&nbsp;<a href="#status.code.registration">Status Code Registration</a></li>
611               <li>5.2&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
612            </ul>
613         </li>
614         <li>6.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
615         <li>7.&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
616         <li>8.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
617               <li>8.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
618               <li>8.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
619            </ul>
620         </li>
621         <li><a href="#rfc.authors">Authors' Addresses</a></li>
622         <li>A.&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
623         <li>B.&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
624         <li>C.&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
625               <li>C.1&nbsp;&nbsp;&nbsp;<a href="#changes.since.19">Since draft-ietf-httpbis-p4-conditional-19</a></li>
626            </ul>
627         </li>
628         <li><a href="#rfc.index">Index</a></li>
629      </ul>
630      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="introduction" href="#introduction">Introduction</a></h1>
631      <p id="rfc.section.1.p.1">This document defines the HTTP/1.1 conditional request mechanisms, including both metadata for indicating/observing changes
632         in resource representations and request header fields that specify preconditions on that metadata; to be checked before performing
633         the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem:
634         one client accidentally overwriting the work of another client that has been acting in parallel.
635      </p>
636      <p id="rfc.section.1.p.2">Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the
637         state as observed in a previously obtained representation (one value in that set). A resource might have multiple current
638         representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests
639         to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless,
640         if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result
641         when the precondition evaluates to false.
642      </p>
643      <p id="rfc.section.1.p.3"><span id="rfc.iref.s.1"></span> We use the term "<dfn>selected representation</dfn>" to refer to the current representation of the target resource that would have been selected in a successful response if
644         the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request
645         preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the
646         selected representation.
647      </p>
648      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.conformance.and.error.handling" href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></h2>
649      <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"
650         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>.
651      </p>
652      <p id="rfc.section.1.1.p.2">This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients,
653         Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See <a href="p1-messaging.html#architecture" title="Architecture">Section 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> for definitions of these terms.
654      </p>
655      <p id="rfc.section.1.1.p.3">An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that
656         SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
657      </p>
658      <p id="rfc.section.1.1.p.4">This document also uses ABNF to define valid protocol elements (<a href="#notation" title="Syntax Notation">Section&nbsp;1.2</a>). In addition to the prose requirements placed upon them, Senders <em class="bcp14">MUST NOT</em> generate protocol elements that are invalid.
659      </p>
660      <p id="rfc.section.1.1.p.5">Unless noted otherwise, Recipients <em class="bcp14">MUST</em> be able to parse all protocol elements matching the ABNF rules defined for them and <em class="bcp14">MAY</em> take steps to recover a usable protocol element from an invalid construct. However, HTTP does not define specific error handling
661         mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require
662         different error handling strategies; for example, a Web browser might wish to transparently recover from a response where
663         the Location header field doesn't parse according to the ABNF, whereby in a systems control protocol using HTTP, this type
664         of error recovery could lead to dangerous consequences.
665      </p>
666      <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>
667      <p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</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>. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF with the list rule expanded.
668      </p>
669      <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
670         (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
671         character).
672      </p>
673      <p id="rfc.section.1.2.p.3">The ABNF rules below are 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> and <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation">[Part2]</cite></a>:
674      </p>
675      <div id="rfc.figure.u.1"></div><pre class="inline">  <a href="#notation" 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#whitespace" title="Whitespace">Section 3.2.1</a>&gt;
676  <a href="#notation" class="smpl">obs-text</a>      = &lt;obs-text, 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#field.components" title="Field value components">Section 3.2.4</a>&gt;
677  <a href="#notation" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation">[Part2]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 5.1</a>&gt;
678</pre><div id="rfc.iref.m.1"></div>
679      <div id="rfc.iref.v.1"></div>
680      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="validators" href="#validators">Validators</a></h1>
681      <p id="rfc.section.2.p.1">This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions:
682         modification dates (<a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section&nbsp;2.2</a>) and opaque entity tags (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section&nbsp;2.3</a>). Additional metadata that reflects resource state has been defined by various extensions of HTTP, such as WebDAV <a href="#RFC4918" id="rfc.xref.RFC4918.1"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, that are beyond the scope of this specification. A resource metadata value is referred to as a "<dfn>validator</dfn>" when it is used within a precondition.
683      </p>
684      <div id="rfc.iref.v.2"></div>
685      <div id="rfc.iref.v.3"></div>
686      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak versus Strong</a></h2>
687      <p id="rfc.section.2.1.p.1">Validators come in two flavors: strong or weak. Weak validators are easy to generate but are far less useful for comparisons.
688         Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently.
689         Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator
690         in use and imposes restrictions on when weak validators can be used as preconditions.
691      </p>
692      <p id="rfc.section.2.1.p.2">A "strong validator" is a representation metadata value that <em class="bcp14">MUST</em> be changed to a new, previously unused or guaranteed unique, value whenever a change occurs to the representation data such
693         that a change would be observable in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET. A strong validator <em class="bcp14">MAY</em> be changed for other reasons, such as when a semantically significant part of the representation metadata is changed (e.g.,
694         Content-Type), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate
695         the stored responses held by remote caches and authoring tools. A strong validator <em class="bcp14">MUST</em> be unique across all representations of a given resource, such that no two representations of that resource share the same
696         validator unless their payload body would be identical.
697      </p>
698      <p id="rfc.section.2.1.p.3">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate
699         an entry using a validator that it obtained in the distant past. A strong validator <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no
700         implication of uniqueness across representations of different resources (i.e., the same strong validator might be in use for
701         representations of multiple resources at the same time and does not imply that those representations are equivalent).
702      </p>
703      <p id="rfc.section.2.1.p.4">There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change
704         to a representation always results in a unique node name and revision identifier being assigned before the representation
705         is made accessible to GET. A cryptographic hash function applied to the representation data is also sufficient if the data
706         is available prior to the response header fields being sent and the digest does not need to be recalculated every time a validation
707         request is received. However, if a resource has distinct representations that differ only in their metadata, such as might
708         occur with content negotiation over media types that happen to share the same data format, then the origin server <em class="bcp14">SHOULD</em> incorporate additional information in the validator to distinguish those representations and avoid confusing cache behavior.
709      </p>
710      <p id="rfc.section.2.1.p.5">In contrast, a "weak validator" is a representation metadata value that might not be changed for every change to the representation
711         data. This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to
712         ensure uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations
713         by some self-determined set of equivalency rather than unique sequences of data. An origin server <em class="bcp14">SHOULD</em> change a weak entity-tag whenever it considers prior representations to be unacceptable as a substitute for the current representation.
714         In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses.
715      </p>
716      <p id="rfc.section.2.1.p.6">For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might
717         be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in
718         order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server
719         load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might
720         be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between
721         those modifications.
722      </p>
723      <p id="rfc.section.2.1.p.7">A "use" of a validator occurs when either a client generates a request and includes the validator in a precondition or when
724         a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of a representation's
725         payload body. Strong validators are usable and preferred for all conditional requests, including cache validation, partial
726         content ranges, and "lost update" avoidance.
727      </p>
728      <div id="rfc.iref.l.1"></div>
729      <div id="rfc.iref.h.1"></div>
730      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2>
731      <p id="rfc.section.2.2.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation
732         was last modified.
733      </p>
734      <div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.g.1"></span>  <a href="#header.last-modified" class="smpl">Last-Modified</a> = <a href="#notation" class="smpl">HTTP-date</a>
735</pre><p id="rfc.section.2.2.p.3">An example of its use is</p>
736      <div id="rfc.figure.u.3"></div><pre class="text">  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
737</pre><h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a>&nbsp;<a id="lastmod.generation" href="#lastmod.generation">Generation</a></h3>
738      <p id="rfc.section.2.2.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined,
739         since its use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service
740         scalability and reliability.
741      </p>
742      <p id="rfc.section.2.2.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be
743         the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
744         detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
745         use its value to make conditional requests and test the validity of locally cached responses.
746      </p>
747      <p id="rfc.section.2.2.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value
748         for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially
749         if the representation changes near the time that the response is generated.
750      </p>
751      <p id="rfc.section.2.2.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time
752         is derived from implementation-specific metadata that evaluates to some time in the future, according to the origin server's
753         clock, then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification date from having an adverse impact
754         on cache validation.
755      </p>
756      <p id="rfc.section.2.2.1.p.5">An origin server without a clock <em class="bcp14">MUST NOT</em> assign Last-Modified values to a response unless these values were associated with the resource by some other system or user
757         with a reliable clock.
758      </p>
759      <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a>&nbsp;<a id="lastmod.comparison" href="#lastmod.comparison">Comparison</a></h3>
760      <p id="rfc.section.2.2.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is
761         strong, using the following rules:
762      </p>
763      <ul>
764         <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
765         <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
766            presented validator.
767         </li>
768      </ul>
769      <p id="rfc.section.2.2.2.p.2">or </p>
770      <ul>
771         <li>The validator is about to be used by a client in an If-Modified-Since, If-Unmodified-Since header field, because the client
772            has a cache entry, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> for the associated representation, and
773         </li>
774         <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
775         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
776      </ul>
777      <p id="rfc.section.2.2.2.p.3">or </p>
778      <ul>
779         <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
780            and
781         </li>
782         <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li>
783         <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
784      </ul>
785      <p id="rfc.section.2.2.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but
786         both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified
787         time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from
788         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.
789      </p>
790      <div id="rfc.iref.e.1"></div>
791      <div id="rfc.iref.h.2"></div>
792      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="header.etag" href="#header.etag">ETag</a></h2>
793      <p id="rfc.section.2.3.p.1">The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator
794         for differentiating between multiple representations of the same resource, regardless of whether those multiple representations
795         are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the
796         same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
797      </p>
798      <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#header.etag" class="smpl">ETag</a>       = <a href="#header.etag" class="smpl">entity-tag</a>
799
800  <a href="#header.etag" class="smpl">entity-tag</a> = [ <a href="#header.etag" class="smpl">weak</a> ] <a href="#header.etag" class="smpl">opaque-tag</a>
801  <a href="#header.etag" class="smpl">weak</a>       = %x57.2F ; "W/", case-sensitive
802  <a href="#header.etag" class="smpl">opaque-tag</a> = <a href="#notation" class="smpl">DQUOTE</a> *<a href="#header.etag" class="smpl">etagc</a> <a href="#notation" class="smpl">DQUOTE</a>
803  <a href="#header.etag" class="smpl">etagc</a>      = %x21 / %x23-7E / <a href="#notation" class="smpl">obs-text</a>
804             ; <a href="#notation" class="smpl">VCHAR</a> except double quotes, plus obs-text
805</pre><div class="note" id="rfc.section.2.3.p.3"> 
806         <p> <b>Note:</b> Previously, opaque-tag was defined to be a quoted-string (<a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, <a href="http://tools.ietf.org/html/rfc2616#section-3.11">Section 3.11</a>), thus some recipients might perform backslash unescaping. Servers therefore ought to avoid backslash characters in entity
807            tags.
808         </p> 
809      </div>
810      <p id="rfc.section.2.3.p.4">An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store
811         modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
812         not consistently maintained.
813      </p>
814      <div id="rfc.figure.u.5"></div>
815      <p>Examples:</p>  <pre class="text">  ETag: "xyzzy"
816  ETag: W/"xyzzy"
817  ETag: ""
818</pre><p id="rfc.section.2.3.p.6">An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag
819         for a representation and the generation of that entity-tag does not satisfy the requirements for a strong validator (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section&nbsp;2.1</a>), then that entity-tag <em class="bcp14">MUST</em> be marked as weak by prefixing its opaque value with "W/" (case-sensitive).
820      </p>
821      <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a id="entity.tag.generation" href="#entity.tag.generation">Generation</a></h3>
822      <p id="rfc.section.2.3.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select
823         the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
824         sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
825         entity-tag is constructed.
826      </p>
827      <p id="rfc.section.2.3.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision
828         number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
829         Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or
830         a modification timestamp that has sub-second resolution.
831      </p>
832      <p id="rfc.section.2.3.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since
833         the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability
834         and reliability.
835      </p>
836      <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a id="entity.tag.comparison" href="#entity.tag.comparison">Comparison</a></h3>
837      <p id="rfc.section.2.3.2.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators
838         or not:
839      </p>
840      <ul>
841         <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.
842         </li>
843         <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.
844         </li>
845      </ul>
846      <p id="rfc.section.2.3.2.p.2">The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:</p>
847      <div id="rfc.table.u.1">
848         <table class="tt full left" cellpadding="3" cellspacing="0">
849            <thead>
850               <tr>
851                  <th>ETag 1</th>
852                  <th>ETag 2</th>
853                  <th>Strong Comparison</th>
854                  <th>Weak Comparison</th>
855               </tr>
856            </thead>
857            <tbody>
858               <tr>
859                  <td class="left">W/"1"</td>
860                  <td class="left">W/"1"</td>
861                  <td class="left">no match</td>
862                  <td class="left">match</td>
863               </tr>
864               <tr>
865                  <td class="left">W/"1"</td>
866                  <td class="left">W/"2"</td>
867                  <td class="left">no match</td>
868                  <td class="left">no match</td>
869               </tr>
870               <tr>
871                  <td class="left">W/"1"</td>
872                  <td class="left">"1"</td>
873                  <td class="left">no match</td>
874                  <td class="left">match</td>
875               </tr>
876               <tr>
877                  <td class="left">"1"</td>
878                  <td class="left">"1"</td>
879                  <td class="left">match</td>
880                  <td class="left">match</td>
881               </tr>
882            </tbody>
883         </table>
884      </div>
885      <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3>
886      <p id="rfc.section.2.3.3.p.1">Consider a resource that is subject to content negotiation (<a href="p2-semantics.html#content.negotiation" title="Content Negotiation">Section 8</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation">[Part2]</cite></a>), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (<a href="p2-semantics.html#header.accept-encoding" title="Accept-Encoding">Section 9.3</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation">[Part2]</cite></a>):
887      </p>
888      <div id="rfc.figure.u.6"></div>
889      <p>&gt;&gt; Request:</p><pre class="text2">GET /index HTTP/1.1
890Host: www.example.com
891Accept-Encoding: gzip
892
893</pre><p id="rfc.section.2.3.3.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p>
894      <div id="rfc.figure.u.7"></div>
895      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
896Date: Thu, 26 Mar 2010 00:05:00 GMT
897ETag: "123-a"
898Content-Length: 70
899Vary: Accept-Encoding
900Content-Type: text/plain
901
902<span id="exbody">Hello World!
903Hello World!
904Hello World!
905Hello World!
906Hello World!
907</span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p>
908      <div id="rfc.figure.u.8"></div>
909      <p>&gt;&gt; Response:</p><pre class="text">HTTP/1.1 200 OK
910Date: Thu, 26 Mar 2010 00:05:00 GMT
911ETag: "123-b"
912Content-Length: 43
913Vary: Accept-Encoding
914Content-Type: text/plain
915Content-Encoding: gzip
916
917<em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7"> 
918         <p> <b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation has to be distinct
919            from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings
920            (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 4</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.
921         </p> 
922      </div>
923      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</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></h2>
924      <p id="rfc.section.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types
925         ought to be used, and for what purposes.
926      </p>
927      <p id="rfc.section.2.4.p.2">HTTP/1.1 origin servers: </p>
928      <ul>
929         <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
930         </li>
931         <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,
932            or if it is unfeasible to send a strong entity-tag.
933         </li>
934         <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one.
935         </li>
936      </ul>
937      <p id="rfc.section.2.4.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
938         value.
939      </p>
940      <p id="rfc.section.2.4.p.4">HTTP/1.1 clients: </p>
941      <ul>
942         <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
943            by the origin server.
944         </li>
945         <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
946            has been provided by the origin server.
947         </li>
948         <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
949            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.
950         </li>
951         <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
952            origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
953         </li>
954      </ul>
955      <p id="rfc.section.2.4.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
956         or If-Unmodified-Since header field) and one or more entity-tags (e.g., in an If-Match, If-None-Match, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status code of <a href="#status.304" class="smpl">304 (Not Modified)</a> unless doing so is consistent with all of the conditional header fields in the request.
957      </p>
958      <p id="rfc.section.2.4.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
959         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
960         fields in the request.
961      </p>
962      <ul class="empty">
963         <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
964            as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
965            assumptions about the validators they receive.
966         </li>
967         <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will
968            support transparent and efficient caching, and so HTTP/1.1 origin servers still ought to provide Last-Modified values.
969         </li>
970      </ul>
971      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="header.field.definitions" href="#header.field.definitions">Precondition Header Fields</a></h1>
972      <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests.</p>
973      <div id="rfc.iref.i.1"></div>
974      <div id="rfc.iref.h.3"></div>
975      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="header.if-match" href="#header.if-match">If-Match</a></h2>
976      <p id="rfc.section.3.1.p.1">The "If-Match" header field can be used to make a request method conditional on the current existence or value of an entity-tag
977         for one or more representations of the target resource. If-Match is generally useful for resource update requests, such as
978         PUT requests, as a means for protecting against accidental overwrites when multiple clients are acting in parallel on the
979         same resource (i.e., the "lost update" problem). An If-Match field-value of "*" places the precondition on the existence of
980         any current representation for the target resource.
981      </p>
982      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#header.if-match" class="smpl">If-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
983</pre><p id="rfc.section.3.1.p.3">If any of the entity-tags listed in the If-Match field value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation
984         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.
985      </p>
986      <p id="rfc.section.3.1.p.4">Origin servers <em class="bcp14">MUST NOT</em> perform the requested method if none of the entity-tags match, or if "*" is given and no current representation exists; instead
987         they <em class="bcp14">MUST</em> respond with the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code.
988      </p>
989      <p id="rfc.section.3.1.p.5">Proxy servers using a cached response as the selected representation <em class="bcp14">MUST NOT</em> perform the requested method if none of the entity-tags match, or if "*" is given and no current representation exists; instead,
990         they <em class="bcp14">MUST</em> forward the request towards the origin server.
991      </p>
992      <p id="rfc.section.3.1.p.6">If the request would, without the If-Match header field, result in anything other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code, then the If-Match header field <em class="bcp14">MUST</em> be ignored.
993      </p>
994      <p id="rfc.section.3.1.p.7">Examples:</p>
995      <div id="rfc.figure.u.10"></div><pre class="text">  If-Match: "xyzzy"
996  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
997  If-Match: *
998</pre><p id="rfc.section.3.1.p.9">The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header field
999         is undefined by this specification.
1000      </p>
1001      <div id="rfc.iref.i.2"></div>
1002      <div id="rfc.iref.h.4"></div>
1003      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.if-none-match" href="#header.if-none-match">If-None-Match</a></h2>
1004      <p id="rfc.section.3.2.p.1">The "If-None-Match" header field can be used to make a request method conditional on not matching any of the current entity-tag
1005         values for representations of the target resource. If-None-Match is primarily used in conditional GET requests to enable efficient
1006         updates of cached information with a minimum amount of transaction overhead. A client that has one or more representations
1007         previously obtained from the target resource can send If-None-Match with a list of the associated entity-tags in the hope
1008         of receiving a <a href="#status.304" class="smpl">304 (Not Modified)</a> response if at least one of those representations matches the selected representation.
1009      </p>
1010      <p id="rfc.section.3.2.p.2">If-None-Match can 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.11"></div><pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
1016</pre><p id="rfc.section.3.2.p.4">If any of the entity-tags listed in the If-None-Match field-value match (as per <a href="#entity.tag.comparison" title="Comparison">Section&nbsp;2.3.2</a>) the entity-tag of the selected representation, or if "*" is given and any current representation exists for that resource,
1017         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 <a href="#status.304" class="smpl">304 (Not Modified)</a> status code, including the cache-related header fields (particularly ETag) of the selected representation that has a matching
1018         entity-tag. For all other request methods, the server <em class="bcp14">MUST</em> respond with a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code.
1019      </p>
1020      <p id="rfc.section.3.2.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 <a href="#status.304" class="smpl">304 (Not Modified)</a> response.
1021      </p>
1022      <p id="rfc.section.3.2.p.6">If the request would, without the If-None-Match header field, result in anything other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.304" class="smpl">304 (Not Modified)</a> status code, then 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;2.4</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)
1023      </p>
1024      <p id="rfc.section.3.2.p.7">Examples:</p>
1025      <div id="rfc.figure.u.12"></div><pre class="text">  If-None-Match: "xyzzy"
1026  If-None-Match: W/"xyzzy"
1027  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1028  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1029  If-None-Match: *
1030</pre><p id="rfc.section.3.2.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
1031         field is undefined by this specification.
1032      </p>
1033      <div id="rfc.iref.i.3"></div>
1034      <div id="rfc.iref.h.5"></div>
1035      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2>
1036      <p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field can be used to make a request method conditional by modification date: if the selected
1037         representation has not been modified since the time specified in this field, then do not perform the request method; instead,
1038         respond as detailed below.
1039      </p>
1040      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span>  <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a>
1041</pre><p id="rfc.section.3.3.p.3">An example of the field is:</p>
1042      <div id="rfc.figure.u.14"></div><pre class="text">  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1043</pre><p id="rfc.section.3.3.p.5">A GET method with an If-Modified-Since header field and no <a href="p5-range.html#range.retrieval.requests" class="smpl">Range</a> header field requests that the selected representation be transferred only if it has been modified since the date given by
1044         the If-Modified-Since header field. The algorithm for determining this includes the following cases:
1045      </p>
1046      <ol>
1047         <li>If the request would normally result in anything other than a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> status code, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date
1048            which is later than the server's current time is invalid.
1049         </li>
1050         <li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for
1051            a normal GET.
1052         </li>
1053         <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 <a href="#status.304" class="smpl">304 (Not Modified)</a> response.
1054         </li>
1055      </ol>
1056      <p id="rfc.section.3.3.p.6">The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. </p>
1057      <ul class="empty">
1058         <li> <b>Note:</b> The <a href="p5-range.html#range.retrieval.requests" class="smpl">Range</a> 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.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a> for full details.
1059         </li>
1060         <li> <b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.
1061         </li>
1062         <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
1063            function, for deciding whether to send a <a href="#status.304" class="smpl">304 (Not Modified)</a> response. To get best results when sending an If-Modified-Since header field for cache validation, clients are advised to
1064            use the exact date string received in a previous Last-Modified header field whenever possible.
1065         </li>
1066         <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
1067            field for the same request, the client needs to be aware that this date is interpreted in the server's understanding of time.
1068            Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server, are concerns.
1069            This includes the possibility of race conditions if the document has changed between the time it was first requested and the
1070            If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since
1071            date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between
1072            client and server are at best approximate due to network latency.
1073         </li>
1074      </ul>
1075      <p id="rfc.section.3.3.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
1076         field is undefined by this specification.
1077      </p>
1078      <div id="rfc.iref.i.4"></div>
1079      <div id="rfc.iref.h.6"></div>
1080      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="header.if-unmodified-since" href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
1081      <p id="rfc.section.3.4.p.1">The "If-Unmodified-Since" header field can be used to make a request method conditional by modification date: if the selected
1082         representation has been modified since 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 <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code. If the selected representation has not been modified since 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.
1083      </p>
1084      <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.10"></span>  <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a>
1085</pre><p id="rfc.section.3.4.p.3">An example of the field is:</p>
1086      <div id="rfc.figure.u.16"></div><pre class="text">  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1087</pre><p id="rfc.section.3.4.p.5">If a request normally (i.e., in absence of the If-Unmodified-Since header field) would result in anything other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code, the If-Unmodified-Since header field <em class="bcp14">SHOULD</em> be ignored.
1088      </p>
1089      <p id="rfc.section.3.4.p.6">If the specified date is invalid, the header field <em class="bcp14">MUST</em> be ignored.
1090      </p>
1091      <p id="rfc.section.3.4.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
1092         header field is undefined by this specification.
1093      </p>
1094      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="header.if-range" href="#header.if-range">If-Range</a></h2>
1095      <p id="rfc.section.3.5.p.1">The "If-Range" header field provides a special conditional request mechanism that is similar to If-Match and If-Unmodified-Since
1096         but specific to HTTP range requests. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 5.3</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>.
1097      </p>
1098      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="status.code.definitions" href="#status.code.definitions">Status Code Definitions</a></h1>
1099      <div id="rfc.iref.27"></div>
1100      <div id="rfc.iref.s.2"></div>
1101      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="status.304" href="#status.304">304 Not Modified</a></h2>
1102      <p id="rfc.section.4.1.p.1">The 304 status code indicates that a conditional GET request has been received and would have resulted in a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server
1103         to transfer a representation of the target resource because the client's request indicates that it already has a valid representation,
1104         as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation
1105         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.
1106      </p>
1107      <p id="rfc.section.4.1.p.2">A 304 response <em class="bcp14">MUST</em> include a Date header field (<a href="p2-semantics.html#header.date" title="Date">Section 9.10</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation">[Part2]</cite></a>) unless the origin server does not have a clock that can provide a reasonable approximation of the current time. If a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, <a href="p6-cache.html#header.expires" class="smpl">Expires</a>, or <a href="p6-cache.html#header.vary" class="smpl">Vary</a>, then those same header fields <em class="bcp14">MUST</em> be sent in a 304 response.
1108      </p>
1109      <p id="rfc.section.4.1.p.3">Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations,
1110         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
1111         cache updates (e.g., future HTTP extensions).
1112      </p>
1113      <p id="rfc.section.4.1.p.4">If the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the
1114         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
1115         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.
1116      </p>
1117      <p id="rfc.section.4.1.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.
1118      </p>
1119      <div id="rfc.iref.28"></div>
1120      <div id="rfc.iref.s.3"></div>
1121      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="status.412" href="#status.412">412 Precondition Failed</a></h2>
1122      <p id="rfc.section.4.2.p.1">The 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested
1123         on the server. This response code allows the client to place preconditions on the current resource state (its current representations
1124         and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
1125      </p>
1126      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
1127      <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="status.code.registration" href="#status.code.registration">Status Code Registration</a></h2>
1128      <p id="rfc.section.5.1.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:
1129      </p>
1130      <div id="rfc.table.1">
1131         <div id="iana.status.code.registration.table"></div>
1132         <table class="tt full left" cellpadding="3" cellspacing="0">
1133            <thead>
1134               <tr>
1135                  <th>Value</th>
1136                  <th>Description</th>
1137                  <th>Reference</th>
1138               </tr>
1139            </thead>
1140            <tbody>
1141               <tr>
1142                  <td class="left">304</td>
1143                  <td class="left">Not Modified</td>
1144                  <td class="left"> <a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section&nbsp;4.1</a> 
1145                  </td>
1146               </tr>
1147               <tr>
1148                  <td class="left">412</td>
1149                  <td class="left">Precondition Failed</td>
1150                  <td class="left"> <a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section&nbsp;4.2</a> 
1151                  </td>
1152               </tr>
1153            </tbody>
1154         </table>
1155      </div>
1156      <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
1157      <p id="rfc.section.5.2.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>):
1158      </p>
1159      <div id="rfc.table.2">
1160         <div id="iana.header.registration.table"></div>
1161         <table class="tt full left" cellpadding="3" cellspacing="0">
1162            <thead>
1163               <tr>
1164                  <th>Header Field Name</th>
1165                  <th>Protocol</th>
1166                  <th>Status</th>
1167                  <th>Reference</th>
1168               </tr>
1169            </thead>
1170            <tbody>
1171               <tr>
1172                  <td class="left">ETag</td>
1173                  <td class="left">http</td>
1174                  <td class="left">standard</td>
1175                  <td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section&nbsp;2.3</a> 
1176                  </td>
1177               </tr>
1178               <tr>
1179                  <td class="left">If-Match</td>
1180                  <td class="left">http</td>
1181                  <td class="left">standard</td>
1182                  <td class="left"> <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section&nbsp;3.1</a> 
1183                  </td>
1184               </tr>
1185               <tr>
1186                  <td class="left">If-Modified-Since</td>
1187                  <td class="left">http</td>
1188                  <td class="left">standard</td>
1189                  <td class="left"> <a href="#header.if-modified-since" id="rfc.xref.header.if-modified-since.1" title="If-Modified-Since">Section&nbsp;3.3</a> 
1190                  </td>
1191               </tr>
1192               <tr>
1193                  <td class="left">If-None-Match</td>
1194                  <td class="left">http</td>
1195                  <td class="left">standard</td>
1196                  <td class="left"> <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section&nbsp;3.2</a> 
1197                  </td>
1198               </tr>
1199               <tr>
1200                  <td class="left">If-Unmodified-Since</td>
1201                  <td class="left">http</td>
1202                  <td class="left">standard</td>
1203                  <td class="left"> <a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section&nbsp;3.4</a> 
1204                  </td>
1205               </tr>
1206               <tr>
1207                  <td class="left">Last-Modified</td>
1208                  <td class="left">http</td>
1209                  <td class="left">standard</td>
1210                  <td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.2" title="Last-Modified">Section&nbsp;2.2</a> 
1211                  </td>
1212               </tr>
1213            </tbody>
1214         </table>
1215      </div>
1216      <p id="rfc.section.5.2.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1217      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1218      <p id="rfc.section.6.p.1">No additional security considerations have been identified beyond those applicable to HTTP in general <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
1219      </p>
1220      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="acks" href="#acks">Acknowledgments</a></h1>
1221      <p id="rfc.section.7.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 9</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>.
1222      </p>
1223      <h1 id="rfc.references"><a id="rfc.section.8" href="#rfc.section.8">8.</a> References
1224      </h1>
1225      <h2 id="rfc.references.1"><a href="#rfc.section.8.1" id="rfc.section.8.1">8.1</a> Normative References
1226      </h2>
1227      <table>           
1228         <tr>
1229            <td class="reference"><b id="Part1">[Part1]</b></td>
1230            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-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), July&nbsp;2012.
1231            </td>
1232         </tr>
1233         <tr>
1234            <td class="reference"><b id="Part2">[Part2]</b></td>
1235            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">HTTP/1.1, part 2: Message Semantics, Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), July&nbsp;2012.
1236            </td>
1237         </tr>
1238         <tr>
1239            <td class="reference"><b id="Part5">[Part5]</b></td>
1240            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), July&nbsp;2012.
1241            </td>
1242         </tr>
1243         <tr>
1244            <td class="reference"><b id="Part6">[Part6]</b></td>
1245            <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Rackspace">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), July&nbsp;2012.
1246            </td>
1247         </tr>
1248         <tr>
1249            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1250            <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.
1251            </td>
1252         </tr>
1253         <tr>
1254            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1255            <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.
1256            </td>
1257         </tr>
1258      </table>
1259      <h2 id="rfc.references.2"><a href="#rfc.section.8.2" id="rfc.section.8.2">8.2</a> Informative References
1260      </h2>
1261      <table>     
1262         <tr>
1263            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1264            <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.
1265            </td>
1266         </tr>
1267         <tr>
1268            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1269            <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.
1270            </td>
1271         </tr>
1272         <tr>
1273            <td class="reference"><b id="RFC4918">[RFC4918]</b></td>
1274            <td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="http://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC&nbsp;4918, June&nbsp;2007.
1275            </td>
1276         </tr>
1277      </table>
1278      <div class="avoidbreak">
1279         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1280         <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
1281               (editor)
1282               <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>
1283         <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
1284               (editor)
1285               <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>
1286         <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
1287               (editor)
1288               <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline">Email: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address>
1289      </div>
1290      <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>
1291      <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 versus Strong">2.1</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>).
1292      </p>
1293      <p id="rfc.section.A.p.2">Change ETag header field ABNF not to use quoted-string, thus avoiding escaping issues. (<a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section&nbsp;2.3</a>)
1294      </p>
1295      <p id="rfc.section.A.p.3">Change ABNF productions for header fields to only define the field value. (<a href="#header.field.definitions" title="Precondition Header Fields">Section&nbsp;3</a>)
1296      </p>
1297      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
1298      <div id="rfc.figure.u.17"></div> <pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
1299
1300<a href="#notation" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
1301
1302<a href="#header.if-match" class="smpl">If-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1303 entity-tag ] ) )
1304<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date
1305<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1306 entity-tag ] ) )
1307<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date
1308
1309<a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date
1310
1311<a href="#notation" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
1312
1313<a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag
1314<a href="#header.etag" class="smpl">etagc</a> = "!" / %x23-7E ; '#'-'~'
1315 / obs-text
1316
1317<a href="#notation" class="smpl">obs-text</a> = &lt;obs-text, defined in [Part1], Section 3.2.4&gt;
1318<a href="#header.etag" class="smpl">opaque-tag</a> = DQUOTE *etagc DQUOTE
1319
1320<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/
1321</pre> <div id="rfc.figure.u.18"></div>
1322      <p>ABNF diagnostics:</p><pre class="inline">; ETag defined but not used
1323; If-Match defined but not used
1324; If-Modified-Since defined but not used
1325; If-None-Match defined but not used
1326; If-Unmodified-Since defined but not used
1327; Last-Modified defined but not used
1328</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>
1329      <p id="rfc.section.C.p.1">Changes up to the first Working Group Last Call draft are summarized in &lt;<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C">http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C</a>&gt;.
1330      </p>
1331      <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a>&nbsp;<a id="changes.since.19" href="#changes.since.19">Since draft-ietf-httpbis-p4-conditional-19</a></h2>
1332      <p id="rfc.section.C.1.p.1">Closed issues: </p>
1333      <ul>
1334         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/354">http://tools.ietf.org/wg/httpbis/trac/ticket/354</a>&gt;: "ETags and Conditional Requests"
1335         </li>
1336         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/361">http://tools.ietf.org/wg/httpbis/trac/ticket/361</a>&gt;: "ABNF requirements for recipients"
1337         </li>
1338         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/363">http://tools.ietf.org/wg/httpbis/trac/ticket/363</a>&gt;: "Rare cases"
1339         </li>
1340      </ul>
1341      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1342      <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.M">M</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a> 
1343      </p>
1344      <div class="print2col">
1345         <ul class="ind">
1346            <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
1347                  <li>304 Not Modified (status code)&nbsp;&nbsp;<a href="#rfc.iref.27"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>
1348               </ul>
1349            </li>
1350            <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
1351                  <li>412 Precondition Failed (status code)&nbsp;&nbsp;<a href="#rfc.iref.28"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>
1352               </ul>
1353            </li>
1354            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
1355                  <li>ETag header field&nbsp;&nbsp;<a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.iref.e.1"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.2">5.2</a>, <a href="#rfc.xref.header.etag.3">A</a></li>
1356               </ul>
1357            </li>
1358            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
1359                  <li><tt>Grammar</tt>&nbsp;&nbsp;
1360                     <ul>
1361                        <li><tt>entity-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>2.3</b></a></li>
1362                        <li><tt>ETag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>2.3</b></a></li>
1363                        <li><tt>etagc</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>2.3</b></a></li>
1364                        <li><tt>If-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>3.1</b></a></li>
1365                        <li><tt>If-Modified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>3.3</b></a></li>
1366                        <li><tt>If-None-Match</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>3.2</b></a></li>
1367                        <li><tt>If-Unmodified-Since</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>3.4</b></a></li>
1368                        <li><tt>Last-Modified</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>2.2</b></a></li>
1369                        <li><tt>opaque-tag</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>2.3</b></a></li>
1370                        <li><tt>weak</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>2.3</b></a></li>
1371                     </ul>
1372                  </li>
1373               </ul>
1374            </li>
1375            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
1376                  <li>Header Fields&nbsp;&nbsp;
1377                     <ul>
1378                        <li>ETag&nbsp;&nbsp;<a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.iref.h.2"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.2">5.2</a>, <a href="#rfc.xref.header.etag.3">A</a></li>
1379                        <li>If-Match&nbsp;&nbsp;<a href="#rfc.iref.h.3"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">5.2</a></li>
1380                        <li>If-Modified-Since&nbsp;&nbsp;<a href="#rfc.iref.h.5"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">5.2</a></li>
1381                        <li>If-None-Match&nbsp;&nbsp;<a href="#rfc.iref.h.4"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">5.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li>
1382                        <li>If-Unmodified-Since&nbsp;&nbsp;<a href="#rfc.iref.h.6"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">5.2</a></li>
1383                        <li>Last-Modified&nbsp;&nbsp;<a href="#rfc.xref.header.last-modified.1">2</a>, <a href="#rfc.iref.h.1"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.2">5.2</a></li>
1384                     </ul>
1385                  </li>
1386               </ul>
1387            </li>
1388            <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
1389                  <li>If-Match header field&nbsp;&nbsp;<a href="#rfc.iref.i.1"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">5.2</a></li>
1390                  <li>If-Modified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.3"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">5.2</a></li>
1391                  <li>If-None-Match header field&nbsp;&nbsp;<a href="#rfc.iref.i.2"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">5.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li>
1392                  <li>If-Unmodified-Since header field&nbsp;&nbsp;<a href="#rfc.iref.i.4"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">5.2</a></li>
1393               </ul>
1394            </li>
1395            <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
1396                  <li>Last-Modified header field&nbsp;&nbsp;<a href="#rfc.xref.header.last-modified.1">2</a>, <a href="#rfc.iref.l.1"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.2">5.2</a></li>
1397               </ul>
1398            </li>
1399            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
1400                  <li>metadata&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>2</b></a></li>
1401               </ul>
1402            </li>
1403            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
1404                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.1</a>, <a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.3">1.2</a>, <a href="#rfc.xref.Part1.4">1.2</a>, <a href="#rfc.xref.Part1.5">1.2</a>, <a href="#rfc.xref.Part1.6">2.3.3</a>, <a href="#rfc.xref.Part1.7">6</a>, <a href="#rfc.xref.Part1.8">7</a>, <a href="#Part1"><b>8.1</b></a><ul>
1405                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.2</a></li>
1406                        <li><em>Section 2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.1</a></li>
1407                        <li><em>Section 3.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">1.2</a></li>
1408                        <li><em>Section 3.2.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">1.2</a></li>
1409                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.6">2.3.3</a></li>
1410                        <li><em>Section 9</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.8">7</a></li>
1411                     </ul>
1412                  </li>
1413                  <li><em>Part2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">1.2</a>, <a href="#rfc.xref.Part2.2">1.2</a>, <a href="#rfc.xref.Part2.3">2.3.3</a>, <a href="#rfc.xref.Part2.4">2.3.3</a>, <a href="#rfc.xref.Part2.5">4.1</a>, <a href="#Part2"><b>8.1</b></a><ul>
1414                        <li><em>Section 5.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.2">1.2</a></li>
1415                        <li><em>Section 8</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">2.3.3</a></li>
1416                        <li><em>Section 9.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.4">2.3.3</a></li>
1417                        <li><em>Section 9.10</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.5">4.1</a></li>
1418                     </ul>
1419                  </li>
1420                  <li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.3</a>, <a href="#rfc.xref.Part5.2">3.5</a>, <a href="#Part5"><b>8.1</b></a><ul>
1421                        <li><em>Section 5.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.2">3.5</a></li>
1422                        <li><em>Section 5.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.3</a></li>
1423                     </ul>
1424                  </li>
1425                  <li><em>Part6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">2.2.1</a>, <a href="#rfc.xref.Part6.3">2.3.1</a>, <a href="#Part6"><b>8.1</b></a></li>
1426               </ul>
1427            </li>
1428            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
1429                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>8.1</b></a></li>
1430                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">2.3</a>, <a href="#RFC2616"><b>8.2</b></a><ul>
1431                        <li><em>Section 3.11</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">2.3</a></li>
1432                     </ul>
1433                  </li>
1434                  <li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">5.2</a>, <a href="#RFC3864"><b>8.2</b></a></li>
1435                  <li><em>RFC4918</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC4918.1">2</a>, <a href="#RFC4918"><b>8.2</b></a></li>
1436                  <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>8.1</b></a><ul>
1437                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">1.2</a></li>
1438                     </ul>
1439                  </li>
1440               </ul>
1441            </li>
1442            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
1443                  <li>selected representation&nbsp;&nbsp;<a href="#rfc.iref.s.1"><b>1</b></a></li>
1444                  <li>Status Codes&nbsp;&nbsp;
1445                     <ul>
1446                        <li>304 Not Modified&nbsp;&nbsp;<a href="#rfc.iref.s.2"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>
1447                        <li>412 Precondition Failed&nbsp;&nbsp;<a href="#rfc.iref.s.3"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>
1448                     </ul>
1449                  </li>
1450               </ul>
1451            </li>
1452            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
1453                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1"><b>2</b></a><ul>
1454                        <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.3"><b>2.1</b></a></li>
1455                        <li>weak&nbsp;&nbsp;<a href="#rfc.iref.v.2"><b>2.1</b></a></li>
1456                     </ul>
1457                  </li>
1458               </ul>
1459            </li>
1460         </ul>
1461      </div>
1462   </body>
1463</html>
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