source: draft-ietf-httpbis/09/p6-cache.html @ 847

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376</style><link rel="Contents" href="#rfc.toc">
377      <link rel="Author" href="#rfc.authors">
378      <link rel="Copyright" href="#rfc.copyrightnotice">
379      <link rel="Index" href="#rfc.index">
380      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
381      <link rel="Chapter" title="2 Cache Operation" href="#rfc.section.2">
382      <link rel="Chapter" title="3 Header Field Definitions" href="#rfc.section.3">
383      <link rel="Chapter" title="4 History Lists" href="#rfc.section.4">
384      <link rel="Chapter" title="5 IANA Considerations" href="#rfc.section.5">
385      <link rel="Chapter" title="6 Security Considerations" href="#rfc.section.6">
386      <link rel="Chapter" title="7 Acknowledgments" href="#rfc.section.7">
387      <link rel="Chapter" href="#rfc.section.8" title="8 References">
388      <link rel="Appendix" title="A Compatibility with Previous Versions" href="#rfc.section.A">
389      <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B">
390      <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C">
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392      <link rel="schema.dct" href="http://purl.org/dc/terms/">
393      <meta name="dct.creator" content="Fielding, R.">
394      <meta name="dct.creator" content="Gettys, J.">
395      <meta name="dct.creator" content="Mogul, J.">
396      <meta name="dct.creator" content="Frystyk, H.">
397      <meta name="dct.creator" content="Masinter, L.">
398      <meta name="dct.creator" content="Leach, P.">
399      <meta name="dct.creator" content="Berners-Lee, T.">
400      <meta name="dct.creator" content="Lafon, Y.">
401      <meta name="dct.creator" content="Nottingham, M.">
402      <meta name="dct.creator" content="Reschke, J. F.">
403      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p6-cache-09">
404      <meta name="dct.issued" scheme="ISO8601" content="2010-03-08">
405      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
406      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
407      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
408   </head>
409   <body>
410      <table class="header">
411         <tbody>
412            <tr>
413               <td class="left">HTTPbis Working Group</td>
414               <td class="right">R. Fielding, Editor</td>
415            </tr>
416            <tr>
417               <td class="left">Internet-Draft</td>
418               <td class="right">Day Software</td>
419            </tr>
420            <tr>
421               <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
422               </td>
423               <td class="right">J. Gettys</td>
424            </tr>
425            <tr>
426               <td class="left">Intended status: Standards Track</td>
427               <td class="right">One Laptop per Child</td>
428            </tr>
429            <tr>
430               <td class="left">Expires: September 9, 2010</td>
431               <td class="right">J. Mogul</td>
432            </tr>
433            <tr>
434               <td class="left"></td>
435               <td class="right">HP</td>
436            </tr>
437            <tr>
438               <td class="left"></td>
439               <td class="right">H. Frystyk</td>
440            </tr>
441            <tr>
442               <td class="left"></td>
443               <td class="right">Microsoft</td>
444            </tr>
445            <tr>
446               <td class="left"></td>
447               <td class="right">L. Masinter</td>
448            </tr>
449            <tr>
450               <td class="left"></td>
451               <td class="right">Adobe Systems</td>
452            </tr>
453            <tr>
454               <td class="left"></td>
455               <td class="right">P. Leach</td>
456            </tr>
457            <tr>
458               <td class="left"></td>
459               <td class="right">Microsoft</td>
460            </tr>
461            <tr>
462               <td class="left"></td>
463               <td class="right">T. Berners-Lee</td>
464            </tr>
465            <tr>
466               <td class="left"></td>
467               <td class="right">W3C/MIT</td>
468            </tr>
469            <tr>
470               <td class="left"></td>
471               <td class="right">Y. Lafon, Editor</td>
472            </tr>
473            <tr>
474               <td class="left"></td>
475               <td class="right">W3C</td>
476            </tr>
477            <tr>
478               <td class="left"></td>
479               <td class="right">M. Nottingham, Editor</td>
480            </tr>
481            <tr>
482               <td class="left"></td>
483               <td class="right">J. F. Reschke, Editor</td>
484            </tr>
485            <tr>
486               <td class="left"></td>
487               <td class="right">greenbytes</td>
488            </tr>
489            <tr>
490               <td class="left"></td>
491               <td class="right">March 8, 2010</td>
492            </tr>
493         </tbody>
494      </table>
495      <p class="title">HTTP/1.1, part 6: Caching<br><span class="filename">draft-ietf-httpbis-p6-cache-09</span></p>
496      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
497      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
498         systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and,
499         taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control
500         cache behavior or indicate cacheable response messages.
501      </p> 
502      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
503      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues
504         list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/11">http://tools.ietf.org/wg/httpbis/trac/report/11</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
505      </p> 
506      <p>The changes in this draft are summarized in <a href="#changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">Appendix&nbsp;C.10</a>.
507      </p> 
508      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
509      <p>This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.</p>
510      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note
511         that other groups may also distribute working documents as Internet-Drafts.
512      </p>
513      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
514         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
515         in progress”.
516      </p>
517      <p>The list of current Internet-Drafts can be accessed at <a href="http://www.ietf.org/ietf/1id-abstracts.txt">http://www.ietf.org/ietf/1id-abstracts.txt</a>.
518      </p>
519      <p>The list of Internet-Draft Shadow Directories can be accessed at <a href="http://www.ietf.org/shadow.html">http://www.ietf.org/shadow.html</a>.
520      </p>
521      <p>This Internet-Draft will expire in September 9, 2010.</p>
522      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
523      <p>Copyright © 2010 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
524      <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
525         and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
526         text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License.
527      </p>
528      <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
529         10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
530         allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
531         controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
532         works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
533         it into languages other than English.
534      </p>
535      <hr class="noprint">
536      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
537      <ul class="toc">
538         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul class="toc">
539               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
540               <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
541               <li class="tocline1">1.3&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
542               <li class="tocline1">1.4&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul class="toc">
543                     <li class="tocline1">1.4.1&nbsp;&nbsp;&nbsp;<a href="#core.rules">Core Rules</a></li>
544                     <li class="tocline1">1.4.2&nbsp;&nbsp;&nbsp;<a href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></li>
545                  </ul>
546               </li>
547            </ul>
548         </li>
549         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Cache Operation</a><ul class="toc">
550               <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a><ul class="toc">
551                     <li class="tocline1">2.1.1&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Storing Partial and Incomplete Responses</a></li>
552                  </ul>
553               </li>
554               <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></li>
555               <li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness Model</a><ul class="toc">
556                     <li class="tocline1">2.3.1&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a><ul class="toc">
557                           <li class="tocline1">2.3.1.1&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></li>
558                        </ul>
559                     </li>
560                     <li class="tocline1">2.3.2&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
561                     <li class="tocline1">2.3.3&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
562                  </ul>
563               </li>
564               <li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a></li>
565               <li class="tocline1">2.5&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
566               <li class="tocline1">2.6&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
567               <li class="tocline1">2.7&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Responses</a></li>
568            </ul>
569         </li>
570         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
571               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
572               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
573                     <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></li>
574                     <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></li>
575                     <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
576                  </ul>
577               </li>
578               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
579               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
580               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
581               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
582            </ul>
583         </li>
584         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
585         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul class="toc">
586               <li class="tocline1">5.1&nbsp;&nbsp;&nbsp;<a href="#message.header.registration">Message Header Registration</a></li>
587            </ul>
588         </li>
589         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
590         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
591         <li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc">
592               <li class="tocline1">8.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
593               <li class="tocline1">8.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
594            </ul>
595         </li>
596         <li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li>
597         <li class="tocline0">A.&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul class="toc">
598               <li class="tocline1">A.1&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></li>
599               <li class="tocline1">A.2&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
600            </ul>
601         </li>
602         <li class="tocline0">B.&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
603         <li class="tocline0">C.&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul class="toc">
604               <li class="tocline1">C.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC2616</a></li>
605               <li class="tocline1">C.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p6-cache-00</a></li>
606               <li class="tocline1">C.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p6-cache-01</a></li>
607               <li class="tocline1">C.4&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p6-cache-02</a></li>
608               <li class="tocline1">C.5&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p6-cache-03</a></li>
609               <li class="tocline1">C.6&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p6-cache-04</a></li>
610               <li class="tocline1">C.7&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></li>
611               <li class="tocline1">C.8&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p6-cache-06</a></li>
612               <li class="tocline1">C.9&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p6-cache-07</a></li>
613               <li class="tocline1">C.10&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p6-cache-08</a></li>
614            </ul>
615         </li>
616         <li class="tocline0"><a href="#rfc.index">Index</a></li>
617      </ul>
618      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="caching" href="#caching">Introduction</a></h1>
619      <p id="rfc.section.1.p.1">HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches.
620         This document defines aspects of HTTP/1.1 related to caching and reusing response messages.
621      </p>
622      <div id="rfc.iref.c.1"></div>
623      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.purpose" href="#intro.purpose">Purpose</a></h2>
624      <p id="rfc.section.1.1.p.1">An HTTP <dfn>cache</dfn> is a local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache
625         stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests.
626         Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
627      </p>
628      <p id="rfc.section.1.1.p.2">Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to reuse a prior
629         response message to satisfy a current request. In some cases, a stored response can be reused without the need for a network
630         request, reducing latency and network round-trips; a "freshness" mechanism is used for this purpose (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>). Even when a new request is required, it is often possible to reuse all or parts of the payload of a prior response to satisfy
631         the request, thereby reducing network bandwidth usage; a "validation" mechanism is used for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>).
632      </p>
633      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="intro.terminology" href="#intro.terminology">Terminology</a></h2>
634      <p id="rfc.section.1.2.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, HTTP caching.</p>
635      <p id="rfc.section.1.2.p.2"> <span id="rfc.iref.c.2"></span>  <dfn>cacheable</dfn> 
636      </p>
637      <ul class="empty">
638         <li>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
639            Even when a response is cacheable, there may be additional constraints on whether a cache can use the cached copy to satisfy
640            a particular request.
641         </li>
642      </ul>
643      <p id="rfc.section.1.2.p.3"> <span id="rfc.iref.e.1"></span>  <dfn>explicit expiration time</dfn> 
644      </p>
645      <ul class="empty">
646         <li>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</li>
647      </ul>
648      <p id="rfc.section.1.2.p.4"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
649      </p>
650      <ul class="empty">
651         <li>An expiration time assigned by a cache when no explicit expiration time is available.</li>
652      </ul>
653      <p id="rfc.section.1.2.p.5"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
654      </p>
655      <ul class="empty">
656         <li>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</li>
657      </ul>
658      <p id="rfc.section.1.2.p.6"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
659      </p>
660      <ul class="empty">
661         <li>A response is first-hand if the freshness model is not in use; i.e., its age is 0.</li>
662      </ul>
663      <p id="rfc.section.1.2.p.7"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn> 
664      </p>
665      <ul class="empty">
666         <li>The length of time between the generation of a response and its expiration time.</li>
667      </ul>
668      <p id="rfc.section.1.2.p.8"> <span id="rfc.iref.f.3"></span>  <dfn>fresh</dfn> 
669      </p>
670      <ul class="empty">
671         <li>A response is fresh if its age has not yet exceeded its freshness lifetime.</li>
672      </ul>
673      <p id="rfc.section.1.2.p.9"> <span id="rfc.iref.s.1"></span>  <dfn>stale</dfn> 
674      </p>
675      <ul class="empty">
676         <li>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</li>
677      </ul>
678      <p id="rfc.section.1.2.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>validator</dfn> 
679      </p>
680      <ul class="empty">
681         <li>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a stored response is an
682            equivalent copy of an entity.
683         </li>
684      </ul>
685      <div id="shared.and.non-shared.caches">
686         <p id="rfc.section.1.2.p.11"> <span id="rfc.iref.v.2"></span>  <dfn>shared cache</dfn> 
687         </p>
688         <ul class="empty">
689            <li>A cache that is accessible to more than one user. A non-shared cache is dedicated to a single user.</li>
690         </ul>
691      </div>
692      <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
693      <p id="rfc.section.1.3.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
694         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>.
695      </p>
696      <p id="rfc.section.1.3.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
697      </p>
698      <h2 id="rfc.section.1.4"><a href="#rfc.section.1.4">1.4</a>&nbsp;<a id="notation" href="#notation">Syntax Notation</a></h2>
699      <p id="rfc.section.1.4.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> (which extends the syntax defined in <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with a list rule). <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF, with the list rule expanded.
700      </p>
701      <p id="rfc.section.1.4.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
702         (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character),
703         and WSP (whitespace).
704      </p>
705      <h3 id="rfc.section.1.4.1"><a href="#rfc.section.1.4.1">1.4.1</a>&nbsp;<a id="core.rules" href="#core.rules">Core Rules</a></h3>
706      <p id="rfc.section.1.4.1.p.1">The core rules below are defined in <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>:
707      </p>
708      <div id="rfc.figure.u.1"></div><pre class="inline">  <a href="#core.rules" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
709  <a href="#core.rules" class="smpl">token</a>         = &lt;token, defined in <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
710  <a href="#core.rules" class="smpl">OWS</a>           = &lt;OWS, 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#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
711</pre><h3 id="rfc.section.1.4.2"><a href="#rfc.section.1.4.2">1.4.2</a>&nbsp;<a id="abnf.dependencies" href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></h3>
712      <p id="rfc.section.1.4.2.p.1">The ABNF rules below are defined in other parts:</p>
713      <div id="rfc.figure.u.2"></div><pre class="inline">  <a href="#abnf.dependencies" class="smpl">field-name</a>    = &lt;field-name, defined in <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a>&gt;
714  <a href="#abnf.dependencies" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#date.time.formats.full.date" title="Date/Time Formats: Full Date">Section 6.1</a>&gt;
715  <a href="#abnf.dependencies" class="smpl">port</a>          = &lt;port, defined in <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.6</a>&gt;
716  <a href="#abnf.dependencies" class="smpl">pseudonym</a>     = &lt;pseudonym, defined in <a href="#Part1" id="rfc.xref.Part1.9"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#header.via" title="Via">Section 9.9</a>&gt; 
717  <a href="#abnf.dependencies" class="smpl">uri-host</a>      = &lt;uri-host, defined in <a href="#Part1" id="rfc.xref.Part1.10"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.6</a>&gt;
718</pre><h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Cache Operation</a></h1>
719      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Response Cacheability</a></h2>
720      <p id="rfc.section.2.1.p.1">A cache <em class="bcp14">MUST NOT</em> store a response to any request, unless:
721      </p>
722      <ul>
723         <li>The request method is understood by the cache and defined as being cacheable, and</li>
724         <li>the response status code is understood by the cache, and</li>
725         <li>the "no-store" cache directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;3.2</a>) does not appear in request or response headers, and
726         </li>
727         <li>the "private" cache response directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a> does not appear in the response, if the cache is shared, and
728         </li>
729         <li>the "Authorization" header (see <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.1"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>) does not appear in the request, if the cache is shared (unless the "public" directive is present; see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;3.2</a>), and
730         </li>
731         <li>the response either:
732            <ul>
733               <li>contains an Expires header (see <a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;3.3</a>), or
734               </li>
735               <li>contains a max-age response cache directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>), or
736               </li>
737               <li>contains a s-maxage response cache directive and the cache is shared, or</li>
738               <li>contains a Cache Control Extension (see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;3.2.3</a>) that allows it to be cached, or
739               </li>
740               <li>has a status code that can be served with heuristic freshness (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;2.3.1.1</a>).
741               </li>
742            </ul> 
743         </li>
744      </ul>
745      <p id="rfc.section.2.1.p.2">In this context, a cache has "understood" a request method or a response status code if it recognises it and implements any
746         cache-specific behaviour. In particular, 206 Partial Content responses cannot be cached by an implementation that does not
747         handle partial content (see <a href="#errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">Section&nbsp;2.1.1</a>).
748      </p>
749      <p id="rfc.section.2.1.p.3">Note that in normal operation, most caches will not store a response that has neither a cache validator nor an explicit expiration
750         time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.
751      </p>
752      <h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a>&nbsp;<a id="errors.or.incomplete.response.cache.behavior" href="#errors.or.incomplete.response.cache.behavior">Storing Partial and Incomplete Responses</a></h3>
753      <p id="rfc.section.2.1.1.p.1">A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header)
754         can store the response, but <em class="bcp14">MUST</em> treat it as a partial response <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>. Partial responses can be combined as described in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</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>; the result might be a full response or might still be partial. A cache <em class="bcp14">MUST NOT</em> return a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.
755      </p>
756      <p id="rfc.section.2.1.1.p.2">A cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> store incomplete or partial responses.
757      </p>
758      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h2>
759      <p id="rfc.section.2.2.p.1">For a presented request, a cache <em class="bcp14">MUST NOT</em> return a stored response, unless:
760      </p>
761      <ul>
762         <li>The presented Request-URI and that of the stored response match (<span class="comment" id="TODO-Request-URI">[<a href="#TODO-Request-URI" class="smpl">TODO-Request-URI</a>: Need to find a new term for this, as Part 1 doesn't define Request-URI anymore; the new term request-target does not work
763               for this. (see &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/196">http://tools.ietf.org/wg/httpbis/trac/ticket/196</a>&gt;)]</span>), and
764         </li>
765         <li>the request method associated with the stored response allows it to be used for the presented request, and</li>
766         <li>selecting request-headers nominated by the stored response (if any) match those presented (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a>), and
767         </li>
768         <li>the presented request and stored response are free from directives that would prevent its use (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;3.2</a> and <a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;3.4</a>), and
769         </li>
770         <li>the stored response is either:
771            <ul>
772               <li>fresh (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>), or
773               </li>
774               <li>allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>), or
775               </li>
776               <li>successfully validated (see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>).
777               </li>
778            </ul> 
779         </li>
780      </ul>
781      <p id="rfc.section.2.2.p.2"> <span class="comment" id="TODO-method-cacheability">[<a href="#TODO-method-cacheability" class="smpl">TODO-method-cacheability</a>: define method cacheability for GET, HEAD and POST in p2-semantics.]</span> 
782      </p>
783      <p id="rfc.section.2.2.p.3">When a stored response is used to satisfy a request, caches <em class="bcp14">MUST</em> include a single Age header field (<a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;3.1</a>) in the response with a value equal to the stored response's current_age; see <a href="#age.calculations" title="Calculating Age">Section&nbsp;2.3.2</a>. <span class="comment" id="DISCUSS-includes-validated">[<a href="#DISCUSS-includes-validated" class="smpl">DISCUSS-includes-validated</a>: this currently includes successfully validated responses.]</span> 
784      </p>
785      <p id="rfc.section.2.2.p.4">Requests with methods that are unsafe (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) <em class="bcp14">MUST</em> be written through the cache to the origin server; i.e., a cache must not reply to such a request before having forwarded
786         the request and having received a corresponding response.
787      </p>
788      <p id="rfc.section.2.2.p.5">Also, note that unsafe requests might invalidate already stored responses; see <a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;2.5</a>.
789      </p>
790      <p id="rfc.section.2.2.p.6">Caches <em class="bcp14">MUST</em> use the most recent response (as determined by the Date header) when more than one suitable response is stored. They can also
791         forward a request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use.
792      </p>
793      <p id="rfc.section.2.2.p.7"> <span class="comment" id="TODO-header-properties">[<a href="#TODO-header-properties" class="smpl">TODO-header-properties</a>: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1]</span> 
794      </p>
795      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="expiration.model" href="#expiration.model">Freshness Model</a></h2>
796      <p id="rfc.section.2.3.p.1">When a response is "fresh" in the cache, it can be used to satisfy subsequent requests without contacting the origin server,
797         thereby improving efficiency.
798      </p>
799      <p id="rfc.section.2.3.p.2">The primary mechanism for determining freshness is for an origin server to provide an explicit expiration time in the future,
800         using either the Expires header (<a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;3.3</a>) or the max-age response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>). Generally, origin servers will assign future explicit expiration times to responses in the belief that the entity is not
801         likely to change in a semantically significant way before the expiration time is reached.
802      </p>
803      <p id="rfc.section.2.3.p.3">If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past.
804         This means that the response is always stale, so that caches should validate it before using it for subsequent requests. <span class="comment" id="TODO-response-stale">[<a href="#TODO-response-stale" class="smpl">TODO-response-stale</a>: This wording may cause confusion, because the response may still be served stale.]</span> 
805      </p>
806      <p id="rfc.section.2.3.p.4">Since origin servers do not always provide explicit expiration times, HTTP caches may also assign heuristic expiration times
807         when they are not specified, employing algorithms that use other header values (such as the Last-Modified time) to estimate
808         a plausible expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints
809         on their results.
810      </p>
811      <div id="rfc.figure.u.3"></div> 
812      <p>The calculation to determine if a response is fresh is:</p>  <pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
813</pre> <p id="rfc.section.2.3.p.6">The freshness_lifetime is defined in <a href="#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section&nbsp;2.3.1</a>; the current_age is defined in <a href="#age.calculations" title="Calculating Age">Section&nbsp;2.3.2</a>.
814      </p>
815      <p id="rfc.section.2.3.p.7">Additionally, clients may need to influence freshness calculation. They can do this using several request cache directives,
816         with the effect of either increasing or loosening constraints on freshness. See <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>.
817      </p>
818      <p id="rfc.section.2.3.p.8"> <span class="comment" id="ISSUE-no-req-for-directives">[<a href="#ISSUE-no-req-for-directives" class="smpl">ISSUE-no-req-for-directives</a>: there are not requirements directly applying to cache-request-directives and freshness.]</span> 
819      </p>
820      <p id="rfc.section.2.3.p.9">Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload
821         a resource. See <a href="#history.lists" title="History Lists">Section&nbsp;4</a> for an explanation of the difference between caches and history mechanisms.
822      </p>
823      <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a id="calculating.freshness.lifetime" href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h3>
824      <p id="rfc.section.2.3.1.p.1">A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of: </p>
825      <ul>
826         <li>If the cache is shared and the s-maxage response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) is present, use its value, or
827         </li>
828         <li>If the max-age response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) is present, use its value, or
829         </li>
830         <li>If the Expires response header (<a href="#header.expires" id="rfc.xref.header.expires.3" title="Expires">Section&nbsp;3.3</a>) is present, use its value minus the value of the Date response header, or
831         </li>
832         <li>Otherwise, no explicit expiration time is present in the response, but a heuristic may be used; see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;2.3.1.1</a>.
833         </li>
834      </ul>
835      <p id="rfc.section.2.3.1.p.2">Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server.</p>
836      <h4 id="rfc.section.2.3.1.1"><a href="#rfc.section.2.3.1.1">2.3.1.1</a>&nbsp;<a id="heuristic.freshness" href="#heuristic.freshness">Calculating Heuristic Freshness</a></h4>
837      <p id="rfc.section.2.3.1.1.p.1">If no explicit expiration time is present in a stored response that has a status code of 200, 203, 206, 300, 301 or 410, a
838         heuristic expiration time can be calculated. Heuristics <em class="bcp14">MUST NOT</em> be used for other response status codes.
839      </p>
840      <p id="rfc.section.2.3.1.1.p.2">When a heuristic is used to calculate freshness lifetime, the cache <em class="bcp14">SHOULD</em> attach a Warning header with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning is
841         not already present.
842      </p>
843      <p id="rfc.section.2.3.1.1.p.3">Also, if the response has a Last-Modified header (<a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 6.6</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), the heuristic expiration value <em class="bcp14">SHOULD</em> be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
844      </p>
845      <p id="rfc.section.2.3.1.1.p.4"> <span class="comment" id="REVIEW-query-string-heuristics">[<a href="#REVIEW-query-string-heuristics" class="smpl">REVIEW-query-string-heuristics</a>: took away HTTP/1.0 query string heuristic uncacheability.]</span> 
846      </p>
847      <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a id="age.calculations" href="#age.calculations">Calculating Age</a></h3>
848      <p id="rfc.section.2.3.2.p.1">HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The
849         Age field value is the cache's estimate of the amount of time since the response was generated or validated by the origin
850         server. In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the
851         path from the origin server, plus the amount of time it has been in transit along network paths.
852      </p>
853      <p id="rfc.section.2.3.2.p.2">The term "age_value" denotes the value of the Age header, in a form appropriate for arithmetic operations.</p>
854      <p id="rfc.section.2.3.2.p.3">HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response
855         was generated (see <a href="p1-messaging.html#header.date" title="Date">Section 9.3</a> of <a href="#Part1" id="rfc.xref.Part1.11"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). The term "date_value" denotes the value of the Date header, in a form appropriate for arithmetic operations.
856      </p>
857      <p id="rfc.section.2.3.2.p.4">The term "now" means "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially
858         hosts running origin servers and caches, <em class="bcp14">SHOULD</em> use NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a> or some similar protocol to synchronize their clocks to a globally accurate time standard.
859      </p>
860      <p id="rfc.section.2.3.2.p.5">A response's age can be calculated in two entirely independent ways: </p>
861      <ol>
862         <li>now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative,
863            the result is replaced by zero.
864         </li>
865         <li>age_value, if all of the caches along the response path implement HTTP/1.1.</li>
866      </ol>
867      <div id="rfc.figure.u.4"></div> 
868      <p>These are combined as</p>  <pre class="text">    corrected_received_age = max(now - date_value, age_value)
869</pre><p id="rfc.section.2.3.2.p.7">When an Age value is received, it <em class="bcp14">MUST</em> be interpreted relative to the time the request was initiated, not the time that the response was received.
870      </p>
871      <div id="rfc.figure.u.5"></div><pre class="text">   corrected_initial_age = corrected_received_age
872                         + (now - request_time)
873</pre><p id="rfc.section.2.3.2.p.9">where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.</p>
874      <p id="rfc.section.2.3.2.p.10">The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response
875         was last validated by the origin server to the corrected_initial_age.
876      </p>
877      <p id="rfc.section.2.3.2.p.11">In summary:</p>
878      <div id="rfc.figure.u.6"></div><pre class="text">  age_value     - Age header field-value received with the response
879  date_value    - Date header field-value received with the response
880  request_time  - local time when the cache made the request
881                 resulting in the stored response
882  response_time - local time when the cache received the response
883  now           - current local time
884 
885  apparent_age = max(0, response_time - date_value);
886  corrected_received_age = max(apparent_age, age_value);
887  response_delay = response_time - request_time;
888  corrected_initial_age = corrected_received_age + response_delay;
889  resident_time = now - response_time;
890  current_age   = corrected_initial_age + resident_time;
891</pre><h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a id="serving.stale.responses" href="#serving.stale.responses">Serving Stale Responses</a></h3>
892      <p id="rfc.section.2.3.3.p.1">A "stale" response is one that either has explicit expiry information, or is allowed to have heuristic expiry calculated,
893         but is not fresh according to the calculations in <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>.
894      </p>
895      <p id="rfc.section.2.3.3.p.2">Caches <em class="bcp14">MUST NOT</em> return a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
896         directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
897         see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>).
898      </p>
899      <p id="rfc.section.2.3.3.p.3">Caches <em class="bcp14">SHOULD NOT</em> return stale responses unless they are disconnected (i.e., it cannot contact the origin server or otherwise find a forward
900         path) or otherwise explicitly allowed (e.g., the max-stale request directive; see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>).
901      </p>
902      <p id="rfc.section.2.3.3.p.4">Stale responses <em class="bcp14">SHOULD</em> have a Warning header with the 110 warn-code (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;3.6</a>). Likewise, the 112 warn-code <em class="bcp14">SHOULD</em> be sent on stale responses if the cache is disconnected.
903      </p>
904      <p id="rfc.section.2.3.3.p.5">If a cache receives a first-hand response (either an entire response, or a 304 (Not Modified) response) that it would normally
905         forward to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">SHOULD</em> forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache <em class="bcp14">SHOULD NOT</em> attempt to validate a response simply because that response became stale in transit.
906      </p>
907      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h2>
908      <p id="rfc.section.2.4.p.1">When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not
909         fresh, or one cannot be selected; see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a>), it can use the conditional request mechanism <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a> in the forwarded request to give the origin server an opportunity to both select a valid stored response to be used, and to
910         update it. This process is known as "validating" or "revalidating" the stored response.
911      </p>
912      <p id="rfc.section.2.4.p.2">When sending such a conditional request, the cache <em class="bcp14">SHOULD</em> add an If-Modified-Since header whose value is that of the Last-Modified header from the selected (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a>) stored response, if available.
913      </p>
914      <p id="rfc.section.2.4.p.3">Additionally, the cache <em class="bcp14">SHOULD</em> add an If-None-Match header whose value is that of the ETag header(s) from all responses stored for the requested URI, if
915         present. However, if any of the stored responses contains only partial content, its entity-tag <em class="bcp14">SHOULD NOT</em> be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that stored
916         response.
917      </p>
918      <p id="rfc.section.2.4.p.4">A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see <a href="#combining.headers" title="Combining Responses">Section&nbsp;2.7</a>.
919      </p>
920      <p id="rfc.section.2.4.p.5">A full response (i.e., one with a response body) indicates that none of the stored responses nominated in the conditional
921         request is suitable. Instead, the full response is used both to satisfy the request and replace the stored response. <span class="comment" id="TODO-req-missing">[<a href="#TODO-req-missing" class="smpl">TODO-req-missing</a>: Should there be a requirement here?]</span> 
922      </p>
923      <p id="rfc.section.2.4.p.6">If a cache receives a 5xx response while attempting to validate a response, it <em class="bcp14">MAY</em> either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it <em class="bcp14">MAY</em> return a previously stored response (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>).
924      </p>
925      <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></h2>
926      <p id="rfc.section.2.5.p.1">Because unsafe methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) have the potential for changing state on the origin server, intervening caches can use them to keep their contents up-to-date.
927      </p>
928      <p id="rfc.section.2.5.p.2">The following HTTP methods <em class="bcp14">MUST</em> cause a cache to invalidate the Request-URI as well as the URI(s) in the Location and Content-Location headers (if present):
929      </p>
930      <ul>
931         <li>PUT</li>
932         <li>DELETE</li>
933         <li>POST</li>
934      </ul>
935      <p id="rfc.section.2.5.p.3">An invalidation based on a URI from a Location or Content-Location header <em class="bcp14">MUST NOT</em> be performed if the host part of that URI differs from the host part in the Request-URI. This helps prevent denial of service
936         attacks.
937      </p>
938      <p id="rfc.section.2.5.p.4"> <span class="comment" id="TODO-def-host-part">[<a href="#TODO-def-host-part" class="smpl">TODO-def-host-part</a>: "host part" needs to be specified better.]</span> 
939      </p>
940      <p id="rfc.section.2.5.p.5">A cache that passes through requests for methods it does not understand <em class="bcp14">SHOULD</em> invalidate the Request-URI.
941      </p>
942      <p id="rfc.section.2.5.p.6">Here, "invalidate" means that the cache will either remove all stored responses related to the Request-URI, or will mark these
943         as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent request.
944      </p>
945      <p id="rfc.section.2.5.p.7">Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the
946         change at the origin server might not have gone through the cache where a response is stored.
947      </p>
948      <p id="rfc.section.2.5.p.8"> <span class="comment" id="TODO-spec-success-invalidate">[<a href="#TODO-spec-success-invalidate" class="smpl">TODO-spec-success-invalidate</a>: specify that only successful (2xx, 3xx?) responses invalidate.]</span> 
949      </p>
950      <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Caching Negotiated Responses</a></h2>
951      <p id="rfc.section.2.6.p.1">When a cache receives a request that can be satisfied by a stored response that has a Vary header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;3.5</a>), it <em class="bcp14">MUST NOT</em> use that response unless all of the selecting request-headers nominated by the Vary header match in both the original request
952         (i.e., that associated with the stored response), and the presented request.
953      </p>
954      <p id="rfc.section.2.6.p.2">The selecting request-headers from two requests are defined to match if and only if those in the first request can be transformed
955         to those in the second request by applying any of the following:
956      </p>
957      <ul>
958         <li>adding or removing whitespace, where allowed in the header's syntax</li>
959         <li>combining multiple message-header fields with the same field name (see <a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a> of <a href="#Part1" id="rfc.xref.Part1.12"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>)
960         </li>
961         <li>normalizing both header values in a way that is known to have identical semantics, according to the header's specification
962            (e.g., re-ordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive)
963         </li>
964      </ul>
965      <p id="rfc.section.2.6.p.3">If (after any normalisation that may take place) a header field is absent from a request, it can only match another request
966         if it is also absent there.
967      </p>
968      <p id="rfc.section.2.6.p.4">A Vary header field-value of "*" always fails to match, and subsequent requests to that resource can only be properly interpreted
969         by the origin server.
970      </p>
971      <p id="rfc.section.2.6.p.5">The stored response with matching selecting request-headers is known as the selected response.</p>
972      <p id="rfc.section.2.6.p.6">If no selected response is available, the cache <em class="bcp14">MAY</em> forward the presented request to the origin server in a conditional request; see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>.
973      </p>
974      <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a id="combining.headers" href="#combining.headers">Combining Responses</a></h2>
975      <p id="rfc.section.2.7.p.1">When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response (in this section, the "new" response"),
976         it needs to created an updated response by combining the stored response with the new one, so that the updated response can
977         be used to satisfy the request.
978      </p>
979      <p id="rfc.section.2.7.p.2">If the new response contains an ETag, it identifies the stored response to use. <span class="comment" id="TODO-mention-CL">[<a href="#TODO-mention-CL" class="smpl">TODO-mention-CL</a>: may need language about Content-Location here]</span><span class="comment" id="TODO-inm-mult-etags">[<a href="#TODO-inm-mult-etags" class="smpl">TODO-inm-mult-etags</a>: cover case where INM with multiple etags was sent]</span> 
980      </p>
981      <p id="rfc.section.2.7.p.3">If the status code is 206 (partial content), both the stored and new responses <em class="bcp14">MUST</em> have validators, and those validators <em class="bcp14">MUST</em> match using the strong comparison function (see <a href="p4-conditional.html#weak.and.strong.validators" title="Weak and Strong Validators">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>). Otherwise, the responses <em class="bcp14">MUST NOT</em> be combined.
982      </p>
983      <p id="rfc.section.2.7.p.4">The stored response headers are used as those of the updated response, except that </p>
984      <ul>
985         <li>any stored Warning headers with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;3.6</a>) <em class="bcp14">MUST</em> be deleted from the stored response and the updated response.
986         </li>
987         <li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the stored response and the updated response.
988         </li>
989         <li>any headers provided in the new response <em class="bcp14">MUST</em> replace the corresponding headers from the stored response.
990         </li>
991      </ul>
992      <p id="rfc.section.2.7.p.5">If a header field-name in the new response matches more than one header in the stored response, all such stored headers <em class="bcp14">MUST</em> be replaced.
993      </p>
994      <p id="rfc.section.2.7.p.6">The updated response can <span class="comment" id="TODO-is-req">[<a href="#TODO-is-req" class="smpl">TODO-is-req</a>: requirement?]</span> be used to replace the stored response in cache. In the case of a 206 response, the combined entity-body <em class="bcp14">MAY</em> be stored.
995      </p>
996      <p id="rfc.section.2.7.p.7"> <span class="comment" id="ISSUE-how-head">[<a href="#ISSUE-how-head" class="smpl">ISSUE-how-head</a>: discuss how to handle HEAD updates]</span> 
997      </p>
998      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
999      <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
1000      <p id="rfc.section.3.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
1001         receives the entity.
1002      </p>
1003      <div id="rfc.iref.a.2"></div>
1004      <div id="rfc.iref.h.2"></div>
1005      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
1006      <p id="rfc.section.3.1.p.1">The "Age" response-header field conveys the sender's estimate of the amount of time since the response was generated or successfully
1007         validated at the origin server. Age values are calculated as specified in <a href="#age.calculations" title="Calculating Age">Section&nbsp;2.3.2</a>.
1008      </p>
1009      <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span>  <a href="#header.age" class="smpl">Age</a>   = "Age" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.age" class="smpl">Age-v</a>
1010  <a href="#header.age" class="smpl">Age-v</a> = <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1011</pre><div id="rule.delta-seconds">
1012         <p id="rfc.section.3.1.p.3">  Age field-values are non-negative integers, representing time in seconds.</p>
1013      </div>
1014      <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.3"></span>  <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>  = 1*<a href="#notation" class="smpl">DIGIT</a>
1015</pre><p id="rfc.section.3.1.p.5">If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows,
1016         it <em class="bcp14">MUST</em> transmit an Age header with a field-value of 2147483648 (2<sup>31</sup>). Caches <em class="bcp14">SHOULD</em> use an arithmetic type of at least 31 bits of range.
1017      </p>
1018      <p id="rfc.section.3.1.p.6">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
1019         true, since HTTP/1.0 caches may not implement the Age header field.
1020      </p>
1021      <div id="rfc.iref.c.3"></div>
1022      <div id="rfc.iref.h.3"></div>
1023      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
1024      <p id="rfc.section.3.2.p.1">The "Cache-Control" general-header field is used to specify directives that <em class="bcp14">MUST</em> be obeyed by all caches along the request/response chain. Such cache directives are unidirectional in that the presence of
1025         a directive in a request does not imply that the same directive is to be given in the response.
1026      </p>
1027      <div class="note" id="rfc.section.3.2.p.2"> 
1028         <p>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;3.4</a>).
1029         </p> 
1030      </div>
1031      <p id="rfc.section.3.2.p.3">Cache directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
1032         might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific
1033         cache.
1034      </p>
1035      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#header.cache-control" class="smpl">Cache-Control</a>   = "Cache-Control" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.cache-control" class="smpl">Cache-Control-v</a>
1036  <a href="#header.cache-control" class="smpl">Cache-Control-v</a> = 1#<a href="#header.cache-control" class="smpl">cache-directive</a>
1037
1038  <a href="#header.cache-control" class="smpl">cache-directive</a> = <a href="#header.cache-control" class="smpl">cache-request-directive</a>
1039     / <a href="#header.cache-control" class="smpl">cache-response-directive</a>
1040
1041  <a href="#header.cache-control" class="smpl">cache-extension</a> = <a href="#core.rules" class="smpl">token</a> [ "=" ( <a href="#core.rules" class="smpl">token</a> / <a href="#core.rules" class="smpl">quoted-string</a> ) ]
1042</pre><h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="cache-request-directive" href="#cache-request-directive">Request Cache-Control Directives</a></h3>
1043      <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#header.cache-control" class="smpl">cache-request-directive</a> =
1044       "no-cache"
1045     / "no-store"
1046     / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1047     / "max-stale" [ "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a> ]
1048     / "min-fresh" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1049     / "no-transform"
1050     / "only-if-cached"
1051     / <a href="#header.cache-control" class="smpl">cache-extension</a>
1052</pre><p id="rfc.section.3.2.1.p.2"> <span id="rfc.iref.c.4"></span>  <span id="rfc.iref.n.1"></span> no-cache
1053      </p>
1054      <ul class="empty">
1055         <li>The no-cache request directive indicates that a stored response <em class="bcp14">MUST NOT</em> be used to satisfy the request without successful validation on the origin server.
1056         </li>
1057      </ul>
1058      <p id="rfc.section.3.2.1.p.3"> <span id="rfc.iref.c.5"></span>  <span id="rfc.iref.n.2"></span> no-store
1059      </p>
1060      <ul class="empty">
1061         <li>The no-store request directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either this request or any response to it. This directive applies to both non-shared and shared caches.
1062            "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
1063         </li>
1064         <li>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1065            might not recognize or obey this directive, and communications networks may be vulnerable to eavesdropping.
1066         </li>
1067      </ul>
1068      <p id="rfc.section.3.2.1.p.4"> <span id="rfc.iref.c.6"></span>  <span id="rfc.iref.m.1"></span> max-age
1069      </p>
1070      <ul class="empty">
1071         <li>The max-age request directive indicates that the client is willing to accept a response whose age is no greater than the specified
1072            time in seconds. Unless max-stale directive is also included, the client is not willing to accept a stale response.
1073         </li>
1074      </ul>
1075      <p id="rfc.section.3.2.1.p.5"> <span id="rfc.iref.c.7"></span>  <span id="rfc.iref.m.2"></span> max-stale
1076      </p>
1077      <ul class="empty">
1078         <li>The max-stale request directive indicates that the client is willing to accept a response that has exceeded its expiration
1079            time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time
1080            by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept
1081            a stale response of any age. <span class="comment" id="TODO-staleness">[<a href="#TODO-staleness" class="smpl">TODO-staleness</a>: of any staleness? --mnot]</span></li>
1082      </ul>
1083      <p id="rfc.section.3.2.1.p.6"> <span id="rfc.iref.c.8"></span>  <span id="rfc.iref.m.3"></span> min-fresh
1084      </p>
1085      <ul class="empty">
1086         <li>The min-fresh request directive indicates that the client is willing to accept a response whose freshness lifetime is no less
1087            than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for
1088            at least the specified number of seconds.
1089         </li>
1090      </ul>
1091      <p id="rfc.section.3.2.1.p.7"> <span id="rfc.iref.c.9"></span>  <span id="rfc.iref.n.3"></span> no-transform
1092      </p>
1093      <ul class="empty">
1094         <li>The no-transform request directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type request headers, nor the request entity-body.
1095         </li>
1096      </ul>
1097      <p id="rfc.section.3.2.1.p.8"> <span id="rfc.iref.c.10"></span>  <span id="rfc.iref.o.1"></span> only-if-cached
1098      </p>
1099      <ul class="empty">
1100         <li>The only-if-cached request directive indicates that the client only wishes to return a stored response. If it receives this
1101            directive, a cache <em class="bcp14">SHOULD</em> either respond using a stored response that is consistent with the other constraints of the request, or respond with a 504
1102            (Gateway Timeout) status. If a group of caches is being operated as a unified system with good internal connectivity, such
1103            a request <em class="bcp14">MAY</em> be forwarded within that group of caches.
1104         </li>
1105      </ul>
1106      <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="cache-response-directive" href="#cache-response-directive">Response Cache-Control Directives</a></h3>
1107      <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.cache-control" class="smpl">cache-response-directive</a> =
1108       "public"
1109     / "private" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ]
1110     / "no-cache" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ]
1111     / "no-store"
1112     / "no-transform"
1113     / "must-revalidate"
1114     / "proxy-revalidate"
1115     / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1116     / "s-maxage" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1117     / <a href="#header.cache-control" class="smpl">cache-extension</a>
1118</pre><p id="rfc.section.3.2.2.p.2"> <span id="rfc.iref.c.11"></span>  <span id="rfc.iref.p.1"></span> public
1119      </p>
1120      <ul class="empty">
1121         <li>The public response directive indicates that the response <em class="bcp14">MAY</em> be cached, even if it would normally be non-cacheable or cacheable only within a non-shared cache. (See also Authorization, <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.2"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>, for additional details.)
1122         </li>
1123      </ul>
1124      <p id="rfc.section.3.2.2.p.3"> <span id="rfc.iref.c.12"></span>  <span id="rfc.iref.p.2"></span> private
1125      </p>
1126      <ul class="empty">
1127         <li>The private response directive indicates that the response message is intended for a single user and <em class="bcp14">MUST NOT</em> be stored by a shared cache. A private (non-shared) cache <em class="bcp14">MAY</em> store the response.
1128         </li>
1129         <li>If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated
1130            with the listed response headers. That is, the specified field-names(s) <em class="bcp14">MUST NOT</em> be stored by a shared cache, whereas the remainder of the response message <em class="bcp14">MAY</em> be.
1131         </li>
1132         <li> <b>Note:</b> This usage of the word private only controls where the response may be stored, and cannot ensure the privacy of the message
1133            content. Also, private response directives with field-names are often handled by implementations as if an unqualified private
1134            directive was received; i.e., the special handling for the qualified form is not widely implemented.
1135         </li>
1136      </ul>
1137      <p id="rfc.section.3.2.2.p.4"> <span id="rfc.iref.c.13"></span>  <span id="rfc.iref.n.4"></span> no-cache
1138      </p>
1139      <ul class="empty">
1140         <li>The no-cache response directive indicates that the response <em class="bcp14">MUST NOT</em> be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to
1141            prevent caching even by caches that have been configured to return stale responses.
1142         </li>
1143         <li>If the no-cache response directive specifies one or more field-names, this requirement is limited to the field-values associated
1144            with the listed response headers. That is, the specified field-name(s) <em class="bcp14">MUST NOT</em> be sent in the response to a subsequent request without successful validation on the origin server. This allows an origin
1145            server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
1146         </li>
1147         <li> <b>Note:</b> Most HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache response directives with field-names are often
1148            handled by implementations as if an unqualified no-cache directive was received; i.e., the special handling for the qualified
1149            form is not widely implemented.
1150         </li>
1151      </ul>
1152      <p id="rfc.section.3.2.2.p.5"> <span id="rfc.iref.c.14"></span>  <span id="rfc.iref.n.5"></span> no-store
1153      </p>
1154      <ul class="empty">
1155         <li>The no-store response directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either the immediate request or response. This directive applies to both non-shared and shared caches. "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
1156         </li>
1157         <li>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1158            might not recognize or obey this directive, and communications networks may be vulnerable to eavesdropping.
1159         </li>
1160      </ul>
1161      <p id="rfc.section.3.2.2.p.6"> <span id="rfc.iref.c.15"></span>  <span id="rfc.iref.m.4"></span> must-revalidate
1162      </p>
1163      <ul class="empty">
1164         <li>The must-revalidate response directive indicates that once it has become stale, the response <em class="bcp14">MUST NOT</em> be used to satisfy subsequent requests without successful validation on the origin server.
1165         </li>
1166         <li>The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
1167            an HTTP/1.1 cache <em class="bcp14">MUST</em> obey the must-revalidate directive; in particular, if the cache cannot reach the origin server for any reason, it <em class="bcp14">MUST</em> generate a 504 (Gateway Timeout) response.
1168         </li>
1169         <li>Servers <em class="bcp14">SHOULD</em> send the must-revalidate directive if and only if failure to validate a request on the entity could result in incorrect operation,
1170            such as a silently unexecuted financial transaction.
1171         </li>
1172      </ul>
1173      <p id="rfc.section.3.2.2.p.7"> <span id="rfc.iref.c.16"></span>  <span id="rfc.iref.p.3"></span> proxy-revalidate
1174      </p>
1175      <ul class="empty">
1176         <li>The proxy-revalidate response directive has the same meaning as the must-revalidate response directive, except that it does
1177            not apply to non-shared caches.
1178         </li>
1179      </ul>
1180      <p id="rfc.section.3.2.2.p.8"> <span id="rfc.iref.c.17"></span>  <span id="rfc.iref.m.5"></span> max-age
1181      </p>
1182      <ul class="empty">
1183         <li>The max-age response directive indicates that response is to be considered stale after its age is greater than the specified
1184            number of seconds.
1185         </li>
1186      </ul>
1187      <p id="rfc.section.3.2.2.p.9"> <span id="rfc.iref.c.18"></span>  <span id="rfc.iref.s.2"></span> s-maxage
1188      </p>
1189      <ul class="empty">
1190         <li>The s-maxage response directive indicates that, in shared caches, the maximum age specified by this directive overrides the
1191            maximum age specified by either the max-age directive or the Expires header. The s-maxage directive also implies the semantics
1192            of the proxy-revalidate response directive.
1193         </li>
1194      </ul>
1195      <p id="rfc.section.3.2.2.p.10"> <span id="rfc.iref.c.19"></span>  <span id="rfc.iref.n.6"></span> no-transform
1196      </p>
1197      <ul class="empty">
1198         <li>The no-transform response directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type response headers, nor the response entity-body.
1199         </li>
1200      </ul>
1201      <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
1202      <p id="rfc.section.3.2.3.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
1203         value. Informational extensions (those that do not require a change in cache behavior) can be added without changing the semantics
1204         of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives.
1205         Both the new directive and the standard directive are supplied, such that applications that do not understand the new directive
1206         will default to the behavior specified by the standard directive, and those that understand the new directive will recognize
1207         it as modifying the requirements associated with the standard directive. In this way, extensions to the cache-control directives
1208         can be made without requiring changes to the base protocol.
1209      </p>
1210      <p id="rfc.section.3.2.3.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
1211         obeying certain extensions, and ignoring all directives that it does not understand.
1212      </p>
1213      <p id="rfc.section.3.2.3.p.3">For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive.
1214         We define this new directive to mean that, in addition to any non-shared cache, any cache that is shared only by members of
1215         the community named within its value may cache the response. An origin server wishing to allow the UCI community to use an
1216         otherwise private response in their shared cache(s) could do so by including
1217      </p>
1218      <div id="rfc.figure.u.12"></div><pre class="text">  Cache-Control: private, community="UCI"
1219</pre><p id="rfc.section.3.2.3.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
1220         it will also see and understand the private directive and thus default to the safe behavior.
1221      </p>
1222      <p id="rfc.section.3.2.3.p.6">Unrecognized cache directives <em class="bcp14">MUST</em> be ignored; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1223         directives (or the response's default cacheability) such that the cache behavior will remain minimally correct even if the
1224         cache does not understand the extension(s).
1225      </p>
1226      <div id="rfc.iref.e.2"></div>
1227      <div id="rfc.iref.h.4"></div>
1228      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
1229      <p id="rfc.section.3.3.p.1">The "Expires" entity-header field gives the date/time after which the response is considered stale. See <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a> for further discussion of the freshness model.
1230      </p>
1231      <p id="rfc.section.3.3.p.2">The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after
1232         that time.
1233      </p>
1234      <p id="rfc.section.3.3.p.3">The field-value is an absolute date and time as defined by HTTP-date in <a href="p1-messaging.html#date.time.formats.full.date" title="Date/Time Formats: Full Date">Section 6.1</a> of <a href="#Part1" id="rfc.xref.Part1.13"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>; it <em class="bcp14">MUST</em> be sent in rfc1123-date format.
1235      </p>
1236      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span>  <a href="#header.expires" class="smpl">Expires</a>   = "Expires" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.expires" class="smpl">Expires-v</a>
1237  <a href="#header.expires" class="smpl">Expires-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
1238</pre><div id="rfc.figure.u.14"></div> 
1239      <p>For example</p>  <pre class="text">  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1240</pre><div class="note" id="rfc.section.3.3.p.6"> 
1241         <p> <b>Note:</b> If a response includes a Cache-Control field with the max-age directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>), that directive overrides the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1242         </p> 
1243      </div>
1244      <p id="rfc.section.3.3.p.7">HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
1245      </p>
1246      <p id="rfc.section.3.3.p.8">HTTP/1.1 clients and caches <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
1247      </p>
1248      <div id="rfc.iref.p.4"></div>
1249      <div id="rfc.iref.h.5"></div>
1250      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
1251      <p id="rfc.section.3.4.p.1">The "Pragma" general-header field is used to include implementation-specific directives that might apply to any recipient
1252         along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however,
1253         some systems <em class="bcp14">MAY</em> require that behavior be consistent with the directives.
1254      </p>
1255      <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span><span id="rfc.iref.g.13"></span><span id="rfc.iref.g.14"></span>  <a href="#header.pragma" class="smpl">Pragma</a>            = "Pragma" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.pragma" class="smpl">Pragma-v</a>
1256  <a href="#header.pragma" class="smpl">Pragma-v</a>          = 1#<a href="#header.pragma" class="smpl">pragma-directive</a>
1257  <a href="#header.pragma" class="smpl">pragma-directive</a>  = "no-cache" / <a href="#header.pragma" class="smpl">extension-pragma</a>
1258  <a href="#header.pragma" class="smpl">extension-pragma</a>  = <a href="#core.rules" class="smpl">token</a> [ "=" ( <a href="#core.rules" class="smpl">token</a> / <a href="#core.rules" class="smpl">quoted-string</a> ) ]
1259</pre><p id="rfc.section.3.4.p.3">When the no-cache directive is present in a request message, an application <em class="bcp14">SHOULD</em> forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive
1260         has the same semantics as the no-cache response directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) and is defined here for backward compatibility with HTTP/1.0. Clients <em class="bcp14">SHOULD</em> include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches <em class="bcp14">SHOULD</em> treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1261      </p>
1262      <div class="note" id="rfc.section.3.4.p.4"> 
1263         <p> <b>Note:</b> Because the meaning of "Pragma: no-cache" as a response-header field is not actually specified, it does not provide a reliable
1264            replacement for "Cache-Control: no-cache" in a response.
1265         </p> 
1266      </div>
1267      <p id="rfc.section.3.4.p.5">This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</p>
1268      <div id="rfc.iref.v.3"></div>
1269      <div id="rfc.iref.h.6"></div>
1270      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="header.vary" href="#header.vary">Vary</a></h2>
1271      <p id="rfc.section.3.5.p.1">The "Vary" response-header field conveys the set of request-header fields that were used to select the representation.</p>
1272      <p id="rfc.section.3.5.p.2">Caches use this information, in part, to determine whether a stored response can be used to satisfy a given request; see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a>. determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request
1273         without validation; see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a>.
1274      </p>
1275      <p id="rfc.section.3.5.p.3">In uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select
1276         the representation.
1277      </p>
1278      <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.15"></span><span id="rfc.iref.g.16"></span>  <a href="#header.vary" class="smpl">Vary</a>   = "Vary" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.vary" class="smpl">Vary-v</a>
1279  <a href="#header.vary" class="smpl">Vary-v</a> = "*" / 1#<a href="#abnf.dependencies" class="smpl">field-name</a>
1280</pre><p id="rfc.section.3.5.p.5">The set of header fields named by the Vary field value is known as the selecting request-headers.</p>
1281      <p id="rfc.section.3.5.p.6">Servers <em class="bcp14">SHOULD</em> include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache
1282         to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that
1283         resource. A server <em class="bcp14">MAY</em> include a Vary header field with a non-cacheable response that is subject to server-driven negotiation, since this might provide
1284         the user agent with useful information about the dimensions over which the response varies at the time of the response.
1285      </p>
1286      <p id="rfc.section.3.5.p.7">A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address
1287         of the client), play a role in the selection of the response representation; therefore, a cache cannot determine whether this
1288         response is appropriate. The "*" value <em class="bcp14">MUST NOT</em> be generated by a proxy server; it may only be generated by an origin server.
1289      </p>
1290      <p id="rfc.section.3.5.p.8">The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names
1291         are case-insensitive.
1292      </p>
1293      <div id="rfc.iref.w.1"></div>
1294      <div id="rfc.iref.h.7"></div>
1295      <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
1296      <p id="rfc.section.3.6.p.1">The "Warning" general-header field is used to carry additional information about the status or transformation of a message
1297         that might not be reflected in the message. This information is typically used to warn about possible incorrectness introduced
1298         by caching operations or transformations applied to the entity body of the message.
1299      </p>
1300      <p id="rfc.section.3.6.p.2">Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status
1301         code, distinguishes these responses from true failures.
1302      </p>
1303      <p id="rfc.section.3.6.p.3">Warning headers can in general be applied to any message, however some warn-codes are specific to caches and can only be applied
1304         to response messages.
1305      </p>
1306      <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span>  <a href="#header.warning" class="smpl">Warning</a>    = "Warning" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.warning" class="smpl">Warning-v</a>
1307  <a href="#header.warning" class="smpl">Warning-v</a>  = 1#<a href="#header.warning" class="smpl">warning-value</a>
1308 
1309  <a href="#header.warning" class="smpl">warning-value</a> = <a href="#header.warning" class="smpl">warn-code</a> <a href="#notation" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-agent</a> <a href="#notation" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-text</a>
1310                                        [<a href="#notation" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-date</a>]
1311 
1312  <a href="#header.warning" class="smpl">warn-code</a>  = 3<a href="#notation" class="smpl">DIGIT</a>
1313  <a href="#header.warning" class="smpl">warn-agent</a> = ( <a href="#abnf.dependencies" class="smpl">uri-host</a> [ ":" <a href="#abnf.dependencies" class="smpl">port</a> ] ) / <a href="#abnf.dependencies" class="smpl">pseudonym</a>
1314                  ; the name or pseudonym of the server adding
1315                  ; the Warning header, for use in debugging
1316  <a href="#header.warning" class="smpl">warn-text</a>  = <a href="#core.rules" class="smpl">quoted-string</a>
1317  <a href="#header.warning" class="smpl">warn-date</a>  = <a href="#notation" class="smpl">DQUOTE</a> <a href="#abnf.dependencies" class="smpl">HTTP-date</a> <a href="#notation" class="smpl">DQUOTE</a>
1318</pre><p id="rfc.section.3.6.p.5">Multiple warnings can be attached to a response (either by the origin server or by a cache), including multiple warnings with
1319         the same code number, only differing in warn-text.
1320      </p>
1321      <p id="rfc.section.3.6.p.6">When this occurs, the user agent <em class="bcp14">SHOULD</em> inform the user of as many of them as possible, in the order that they appear in the response.
1322      </p>
1323      <p id="rfc.section.3.6.p.7">Systems that generate multiple Warning headers <em class="bcp14">SHOULD</em> order them with this user agent behavior in mind. New Warning headers <em class="bcp14">SHOULD</em> be added after any existing Warning headers.
1324      </p>
1325      <p id="rfc.section.3.6.p.8">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning is required to be deleted from
1326         a stored response after validation:
1327      </p>
1328      <ul>
1329         <li>1xx Warnings describe the freshness or validation status of the response, and so <em class="bcp14">MUST</em> be deleted by caches after validation. They can only be generated by a cache when validating a cached entry, and <em class="bcp14">MUST NOT</em> be generated in any other situation.
1330         </li>
1331         <li>2xx Warnings describe some aspect of the entity body or entity headers that is not rectified by a validation (for example,
1332            a lossy compression of the entity bodies) and <em class="bcp14">MUST NOT</em> be deleted by caches after validation, unless a full response is returned, in which case they <em class="bcp14">MUST</em> be.
1333         </li>
1334      </ul>
1335      <p id="rfc.section.3.6.p.9">If an implementation sends a message with one or more Warning headers to a receiver whose version is HTTP/1.0 or lower, then
1336         the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the Date header in the message.
1337      </p>
1338      <p id="rfc.section.3.6.p.10">If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from
1339         the Date value in the response, then that warning-value <em class="bcp14">MUST</em> be deleted from the message before storing, forwarding, or using it. (preventing the consequences of naive caching of Warning
1340         header fields.) If all of the warning-values are deleted for this reason, the Warning header <em class="bcp14">MUST</em> be deleted as well.
1341      </p>
1342      <p id="rfc.section.3.6.p.11">The following warn-codes are defined by this specification, each with a recommended warn-text in English, and a description
1343         of its meaning.
1344      </p>
1345      <p id="rfc.section.3.6.p.12"> 110 Response is stale </p>
1346      <ul class="empty">
1347         <li><em class="bcp14">SHOULD</em> be included whenever the returned response is stale.
1348         </li>
1349      </ul>
1350      <p id="rfc.section.3.6.p.13"> 111 Revalidation failed </p>
1351      <ul class="empty">
1352         <li><em class="bcp14">SHOULD</em> be included if a cache returns a stale response because an attempt to validate the response failed, due to an inability to
1353            reach the server.
1354         </li>
1355      </ul>
1356      <p id="rfc.section.3.6.p.14"> 112 Disconnected operation </p>
1357      <ul class="empty">
1358         <li><em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
1359         </li>
1360      </ul>
1361      <p id="rfc.section.3.6.p.15"> 113 Heuristic expiration </p>
1362      <ul class="empty">
1363         <li><em class="bcp14">SHOULD</em> be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater
1364            than 24 hours.
1365         </li>
1366      </ul>
1367      <p id="rfc.section.3.6.p.16"> 199 Miscellaneous warning </p>
1368      <ul class="empty">
1369         <li>The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action, besides presenting the warning to the user.
1370         </li>
1371      </ul>
1372      <p id="rfc.section.3.6.p.17"> 214 Transformation applied </p>
1373      <ul class="empty">
1374         <li><em class="bcp14">MUST</em> be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the
1375            Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, or the entity-body of the
1376            response, unless this Warning code already appears in the response.
1377         </li>
1378      </ul>
1379      <p id="rfc.section.3.6.p.18"> 299 Miscellaneous persistent warning </p>
1380      <ul class="empty">
1381         <li>The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action.
1382         </li>
1383      </ul>
1384      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
1385      <p id="rfc.section.4.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, that can be used to redisplay an entity
1386         retrieved earlier in a session.
1387      </p>
1388      <p id="rfc.section.4.p.2">The freshness model (<a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>) does not necessarily apply to history mechanisms. I.e., a history mechanism can display a previous representation even if
1389         it has expired.
1390      </p>
1391      <p id="rfc.section.4.p.3">This does not prohibit the history mechanism from telling the user that a view might be stale, or from honoring cache directives
1392         (e.g., Cache-Control: no-store).
1393      </p>
1394      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
1395      <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="message.header.registration" href="#message.header.registration">Message Header Registration</a></h2>
1396      <p id="rfc.section.5.1.p.1">The Message Header Registry located at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt; should be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
1397      </p>
1398      <div id="rfc.table.1">
1399         <div id="iana.header.registration.table"></div>
1400         <table class="tt full left" cellpadding="3" cellspacing="0">
1401            <thead>
1402               <tr>
1403                  <th>Header Field Name</th>
1404                  <th>Protocol</th>
1405                  <th>Status</th>
1406                  <th>Reference</th>
1407               </tr>
1408            </thead>
1409            <tbody>
1410               <tr>
1411                  <td class="left">Age</td>
1412                  <td class="left">http</td>
1413                  <td class="left">standard</td>
1414                  <td class="left"> <a href="#header.age" id="rfc.xref.header.age.2" title="Age">Section&nbsp;3.1</a> 
1415                  </td>
1416               </tr>
1417               <tr>
1418                  <td class="left">Cache-Control</td>
1419                  <td class="left">http</td>
1420                  <td class="left">standard</td>
1421                  <td class="left"> <a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;3.2</a> 
1422                  </td>
1423               </tr>
1424               <tr>
1425                  <td class="left">Expires</td>
1426                  <td class="left">http</td>
1427                  <td class="left">standard</td>
1428                  <td class="left"> <a href="#header.expires" id="rfc.xref.header.expires.4" title="Expires">Section&nbsp;3.3</a> 
1429                  </td>
1430               </tr>
1431               <tr>
1432                  <td class="left">Pragma</td>
1433                  <td class="left">http</td>
1434                  <td class="left">standard</td>
1435                  <td class="left"> <a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;3.4</a> 
1436                  </td>
1437               </tr>
1438               <tr>
1439                  <td class="left">Vary</td>
1440                  <td class="left">http</td>
1441                  <td class="left">standard</td>
1442                  <td class="left"> <a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section&nbsp;3.5</a> 
1443                  </td>
1444               </tr>
1445               <tr>
1446                  <td class="left">Warning</td>
1447                  <td class="left">http</td>
1448                  <td class="left">standard</td>
1449                  <td class="left"> <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;3.6</a> 
1450                  </td>
1451               </tr>
1452            </tbody>
1453         </table>
1454      </div>
1455      <p id="rfc.section.5.1.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1456      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1457      <p id="rfc.section.6.p.1">Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious
1458         exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal information
1459         long after a user believes that the information has been removed from the network. Therefore, cache contents should be protected
1460         as sensitive information.
1461      </p>
1462      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
1463      <p id="rfc.section.7.p.1">Much of the content and presentation of the caching design is due to suggestions and comments from individuals including:
1464         Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
1465      </p>
1466      <h1 id="rfc.references"><a id="rfc.section.8" href="#rfc.section.8">8.</a> References
1467      </h1>
1468      <h2 id="rfc.references.1"><a href="#rfc.section.8.1" id="rfc.section.8.1">8.1</a> Normative References
1469      </h2>
1470      <table>             
1471         <tr>
1472            <td class="reference"><b id="Part1">[Part1]</b></td>
1473            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-09">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-09 (work in progress), March&nbsp;2010.
1474            </td>
1475         </tr>
1476         <tr>
1477            <td class="reference"><b id="Part2">[Part2]</b></td>
1478            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-09">HTTP/1.1, part 2: Message Semantics</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-09 (work in progress), March&nbsp;2010.
1479            </td>
1480         </tr>
1481         <tr>
1482            <td class="reference"><b id="Part4">[Part4]</b></td>
1483            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-09">HTTP/1.1, part 4: Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-09 (work in progress), March&nbsp;2010.
1484            </td>
1485         </tr>
1486         <tr>
1487            <td class="reference"><b id="Part5">[Part5]</b></td>
1488            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-09">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-09 (work in progress), March&nbsp;2010.
1489            </td>
1490         </tr>
1491         <tr>
1492            <td class="reference"><b id="Part7">[Part7]</b></td>
1493            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-09">HTTP/1.1, part 7: Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-09 (work in progress), March&nbsp;2010.
1494            </td>
1495         </tr>
1496         <tr>
1497            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1498            <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.
1499            </td>
1500         </tr>
1501         <tr>
1502            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1503            <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.
1504            </td>
1505         </tr>
1506      </table>
1507      <h2 id="rfc.references.2"><a href="#rfc.section.8.2" id="rfc.section.8.2">8.2</a> Informative References
1508      </h2>
1509      <table>     
1510         <tr>
1511            <td class="reference"><b id="RFC1305">[RFC1305]</b></td>
1512            <td class="top"><a href="mailto:mills@udel.edu" title="University of Delaware, Electrical Engineering Department">Mills, D.</a>, “<a href="http://tools.ietf.org/html/rfc1305">Network Time Protocol (Version 3) Specification, Implementation</a>”, RFC&nbsp;1305, March&nbsp;1992.
1513            </td>
1514         </tr>
1515         <tr>
1516            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1517            <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.
1518            </td>
1519         </tr>
1520         <tr>
1521            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1522            <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.
1523            </td>
1524         </tr>
1525      </table>
1526      <div class="avoidbreak">
1527         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1528         <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
1529               (editor)
1530               <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Day Software</span><span class="adr"><span class="street-address vcardline">23 Corporate Plaza DR, Suite 280</span><span class="vcardline"><span class="locality">Newport Beach</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">92660</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel">Phone: <a href="tel:+1-949-706-5300"><span class="value">+1-949-706-5300</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1-949-706-5305"><span class="value">+1-949-706-5305</span></a></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>
1531         <address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">One Laptop per Child</span><span class="adr"><span class="street-address vcardline">21 Oak Knoll Road</span><span class="vcardline"><span class="locality">Carlisle</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01741</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:jg@laptop.org"><span class="email">jg@laptop.org</span></a></span><span class="vcardline">URI: <a href="http://www.laptop.org/" class="url">http://www.laptop.org/</a></span></address>
1532         <address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Hewlett-Packard Company</span><span class="adr"><span class="street-address vcardline">HP Labs, Large Scale Systems Group</span><span class="street-address vcardline">1501 Page Mill Road, MS 1177</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94304</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:JeffMogul@acm.org"><span class="email">JeffMogul@acm.org</span></a></span></address>
1533         <address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:henrikn@microsoft.com"><span class="email">henrikn@microsoft.com</span></a></span></address>
1534         <address class="vcard"><span class="vcardline"><span class="fn">Larry Masinter</span><span class="n hidden"><span class="family-name">Masinter</span><span class="given-name">Larry</span></span></span><span class="org vcardline">Adobe Systems, Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:LMM@acm.org"><span class="email">LMM@acm.org</span></a></span><span class="vcardline">URI: <a href="http://larry.masinter.net/" class="url">http://larry.masinter.net/</a></span></address>
1535         <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span></span><span class="vcardline">EMail: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address>
1536         <address class="vcard"><span class="vcardline"><span class="fn">Tim Berners-Lee</span><span class="n hidden"><span class="family-name">Berners-Lee</span><span class="given-name">Tim</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">MIT Computer Science and Artificial Intelligence Laboratory</span><span class="street-address vcardline">The Stata Center, Building 32</span><span class="street-address vcardline">32 Vassar Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:timbl@w3.org"><span class="email">timbl@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.w3.org/People/Berners-Lee/" class="url">http://www.w3.org/People/Berners-Lee/</a></span></address>
1537         <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
1538               (editor)
1539               <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>
1540         <address class="vcard"><span class="vcardline"><span class="fn">Mark Nottingham</span>
1541               (editor)
1542               <span class="n hidden"><span class="family-name">Nottingham</span><span class="given-name">Mark</span></span></span><span class="vcardline">EMail: <a href="mailto:mnot@mnot.net"><span class="email">mnot@mnot.net</span></a></span><span class="vcardline">URI: <a href="http://www.mnot.net/" class="url">http://www.mnot.net/</a></span></address>
1543         <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
1544               (editor)
1545               <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline tel">Phone: <a href="tel:+492512807760"><span class="value">+49 251 2807760</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+492512807761"><span class="value">+49 251 2807761</span></a></span><span class="vcardline">EMail: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address>
1546      </div>
1547      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="compatibility" href="#compatibility">Compatibility with Previous Versions</a></h1>
1548      <h2 id="rfc.section.A.1"><a href="#rfc.section.A.1">A.1</a>&nbsp;<a id="changes.from.rfc.2068" href="#changes.from.rfc.2068">Changes from RFC 2068</a></h2>
1549      <p id="rfc.section.A.1.p.1">A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add this missing case. (Sections <a href="#response.cacheability" title="Response Cacheability">2.1</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.5" title="Cache-Control">3.2</a>).
1550      </p>
1551      <p id="rfc.section.A.1.p.2">Range request responses would become very verbose if all meta-data were always returned; by allowing the server to only send
1552         needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Responses">Section&nbsp;2.7</a>)
1553      </p>
1554      <p id="rfc.section.A.1.p.3">The Cache-Control: max-age directive was not properly defined for responses. (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>)
1555      </p>
1556      <p id="rfc.section.A.1.p.4">Warnings could be cached incorrectly, or not updated appropriately. (Section <a href="#expiration.model" title="Freshness Model">2.3</a>, <a href="#combining.headers" title="Combining Responses">2.7</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.6" title="Cache-Control">3.2</a>, and <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">3.6</a>) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.
1557      </p>
1558      <h2 id="rfc.section.A.2"><a href="#rfc.section.A.2">A.2</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h2>
1559      <p id="rfc.section.A.2.p.1">Remove requirement to consider Content-Location in successful responses in order to determine the appropriate response to
1560         use. (<a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>)
1561      </p>
1562      <p id="rfc.section.A.2.p.2">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;2.5</a>)
1563      </p>
1564      <p id="rfc.section.A.2.p.3">Do not mention RFC 2047 encoding and multiple languages in Warning headers anymore, as these aspects never were implemented.
1565         (<a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;3.6</a>)
1566      </p>
1567      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
1568      <div id="rfc.figure.u.18"></div> <pre class="inline"><a href="#header.age" class="smpl">Age</a> = "Age:" OWS Age-v
1569<a href="#header.age" class="smpl">Age-v</a> = delta-seconds
1570
1571<a href="#header.cache-control" class="smpl">Cache-Control</a> = "Cache-Control:" OWS Cache-Control-v
1572<a href="#header.cache-control" class="smpl">Cache-Control-v</a> = *( "," OWS ) cache-directive *( OWS "," [ OWS
1573 cache-directive ] )
1574
1575<a href="#header.expires" class="smpl">Expires</a> = "Expires:" OWS Expires-v
1576<a href="#header.expires" class="smpl">Expires-v</a> = HTTP-date
1577
1578<a href="#abnf.dependencies" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1579
1580<a href="#core.rules" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1581
1582<a href="#header.pragma" class="smpl">Pragma</a> = "Pragma:" OWS Pragma-v
1583<a href="#header.pragma" class="smpl">Pragma-v</a> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1584 pragma-directive ] )
1585
1586<a href="#header.vary" class="smpl">Vary</a> = "Vary:" OWS Vary-v
1587<a href="#header.vary" class="smpl">Vary-v</a> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1588 ] ) )
1589
1590<a href="#header.warning" class="smpl">Warning</a> = "Warning:" OWS Warning-v
1591<a href="#header.warning" class="smpl">Warning-v</a> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1592 ] )
1593
1594<a href="#header.cache-control" class="smpl">cache-directive</a> = cache-request-directive / cache-response-directive
1595<a href="#header.cache-control" class="smpl">cache-extension</a> = token [ "=" ( token / quoted-string ) ]
1596<a href="#header.cache-control" class="smpl">cache-request-directive</a> = "no-cache" / "no-store" / ( "max-age="
1597 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1598 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1599 cache-extension
1600<a href="#header.cache-control" class="smpl">cache-response-directive</a> = "public" / ( "private" [ "=" DQUOTE *( ","
1601 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1602 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1603 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1604 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1605 ) / ( "s-maxage=" delta-seconds ) / cache-extension
1606
1607<a href="#rule.delta-seconds" class="smpl">delta-seconds</a> = 1*DIGIT
1608
1609<a href="#header.pragma" class="smpl">extension-pragma</a> = token [ "=" ( token / quoted-string ) ]
1610
1611<a href="#abnf.dependencies" class="smpl">field-name</a> = &lt;field-name, defined in [Part1], Section 3.2&gt;
1612
1613<a href="#abnf.dependencies" class="smpl">port</a> = &lt;port, defined in [Part1], Section 2.6&gt;
1614<a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / extension-pragma
1615<a href="#abnf.dependencies" class="smpl">pseudonym</a> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
1616
1617<a href="#core.rules" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1618
1619<a href="#core.rules" class="smpl">token</a> = &lt;token, defined in [Part1], Section 1.2.2&gt;
1620
1621<a href="#abnf.dependencies" class="smpl">uri-host</a> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
1622
1623<a href="#header.warning" class="smpl">warn-agent</a> = ( uri-host [ ":" port ] ) / pseudonym
1624<a href="#header.warning" class="smpl">warn-code</a> = 3DIGIT
1625<a href="#header.warning" class="smpl">warn-date</a> = DQUOTE HTTP-date DQUOTE
1626<a href="#header.warning" class="smpl">warn-text</a> = quoted-string
1627<a href="#header.warning" class="smpl">warning-value</a> = warn-code SP warn-agent SP warn-text [ SP warn-date
1628 ]
1629</pre> <div id="rfc.figure.u.19"></div>
1630      <p>ABNF diagnostics:</p><pre class="inline">; Age defined but not used
1631; Cache-Control defined but not used
1632; Expires defined but not used
1633; Pragma defined but not used
1634; Vary defined but not used
1635; Warning defined but not used
1636</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>
1637      <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a>&nbsp;Since RFC2616
1638      </h2>
1639      <p id="rfc.section.C.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
1640      </p>
1641      <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a>&nbsp;Since draft-ietf-httpbis-p6-cache-00
1642      </h2>
1643      <p id="rfc.section.C.2.p.1">Closed issues: </p>
1644      <ul>
1645         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/9">http://tools.ietf.org/wg/httpbis/trac/ticket/9</a>&gt;: "Trailer" (&lt;<a href="http://purl.org/NET/http-errata#trailer-hop">http://purl.org/NET/http-errata#trailer-hop</a>&gt;)
1646         </li>
1647         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/12">http://tools.ietf.org/wg/httpbis/trac/ticket/12</a>&gt;: "Invalidation after Update or Delete" (&lt;<a href="http://purl.org/NET/http-errata#invalidupd">http://purl.org/NET/http-errata#invalidupd</a>&gt;)
1648         </li>
1649         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/35">http://tools.ietf.org/wg/httpbis/trac/ticket/35</a>&gt;: "Normative and Informative references"
1650         </li>
1651         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/48">http://tools.ietf.org/wg/httpbis/trac/ticket/48</a>&gt;: "Date reference typo"
1652         </li>
1653         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/49">http://tools.ietf.org/wg/httpbis/trac/ticket/49</a>&gt;: "Connection header text"
1654         </li>
1655         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/65">http://tools.ietf.org/wg/httpbis/trac/ticket/65</a>&gt;: "Informative references"
1656         </li>
1657         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/66">http://tools.ietf.org/wg/httpbis/trac/ticket/66</a>&gt;: "ISO-8859-1 Reference"
1658         </li>
1659         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/86">http://tools.ietf.org/wg/httpbis/trac/ticket/86</a>&gt;: "Normative up-to-date references"
1660         </li>
1661         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/87">http://tools.ietf.org/wg/httpbis/trac/ticket/87</a>&gt;: "typo in 13.2.2"
1662         </li>
1663      </ul>
1664      <p id="rfc.section.C.2.p.2">Other changes: </p>
1665      <ul>
1666         <li>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;)
1667         </li>
1668      </ul>
1669      <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a>&nbsp;Since draft-ietf-httpbis-p6-cache-01
1670      </h2>
1671      <p id="rfc.section.C.3.p.1">Closed issues: </p>
1672      <ul>
1673         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/82">http://tools.ietf.org/wg/httpbis/trac/ticket/82</a>&gt;: "rel_path not used"
1674         </li>
1675      </ul>
1676      <p id="rfc.section.C.3.p.2">Other changes: </p>
1677      <ul>
1678         <li>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress on &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;)
1679         </li>
1680         <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li>
1681      </ul>
1682      <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a>&nbsp;<a id="changes.since.02" href="#changes.since.02">Since draft-ietf-httpbis-p6-cache-02</a></h2>
1683      <p id="rfc.section.C.4.p.1">Ongoing work on IANA Message Header Registration (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/40">http://tools.ietf.org/wg/httpbis/trac/ticket/40</a>&gt;):
1684      </p>
1685      <ul>
1686         <li>Reference RFC 3984, and update header registrations for headers defined in this document.</li>
1687      </ul>
1688      <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a>&nbsp;<a id="changes.since.03" href="#changes.since.03">Since draft-ietf-httpbis-p6-cache-03</a></h2>
1689      <p id="rfc.section.C.5.p.1">Closed issues: </p>
1690      <ul>
1691         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/106">http://tools.ietf.org/wg/httpbis/trac/ticket/106</a>&gt;: "Vary header classification"
1692         </li>
1693      </ul>
1694      <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a>&nbsp;<a id="changes.since.04" href="#changes.since.04">Since draft-ietf-httpbis-p6-cache-04</a></h2>
1695      <p id="rfc.section.C.6.p.1">Ongoing work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1696      </p>
1697      <ul>
1698         <li>Use "/" instead of "|" for alternatives.</li>
1699         <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li>
1700         <li>Rewrite ABNFs to spell out whitespace rules, factor out header value format definitions.</li>
1701      </ul>
1702      <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a>&nbsp;<a id="changes.since.05" href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></h2>
1703      <p id="rfc.section.C.7.p.1">This is a total rewrite of this part of the specification.</p>
1704      <p id="rfc.section.C.7.p.2">Affected issues: </p>
1705      <ul>
1706         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/54">http://tools.ietf.org/wg/httpbis/trac/ticket/54</a>&gt;: "Definition of 1xx Warn-Codes"
1707         </li>
1708         <li> &lt;<a href="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60">http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60</a>&gt;: "Placement of 13.5.1 and 13.5.2"
1709         </li>
1710         <li> &lt;<a href="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138">http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138</a>&gt;: "The role of Warning and Semantic Transparency in Caching"
1711         </li>
1712         <li> &lt;<a href="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139">http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139</a>&gt;: "Methods and Caching"
1713         </li>
1714      </ul>
1715      <p id="rfc.section.C.7.p.3">In addition: Final work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
1716      </p>
1717      <ul>
1718         <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li>
1719      </ul>
1720      <h2 id="rfc.section.C.8"><a href="#rfc.section.C.8">C.8</a>&nbsp;<a id="changes.since.06" href="#changes.since.06">Since draft-ietf-httpbis-p6-cache-06</a></h2>
1721      <p id="rfc.section.C.8.p.1">Closed issues: </p>
1722      <ul>
1723         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/161">http://tools.ietf.org/wg/httpbis/trac/ticket/161</a>&gt;: "base for numeric protocol elements"
1724         </li>
1725      </ul>
1726      <p id="rfc.section.C.8.p.2">Affected issues: </p>
1727      <ul>
1728         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/37">http://tools.ietf.org/wg/httpbis/trac/ticket/37</a>&gt;: Vary and non-existant headers
1729         </li>
1730      </ul>
1731      <h2 id="rfc.section.C.9"><a href="#rfc.section.C.9">C.9</a>&nbsp;<a id="changes.since.07" href="#changes.since.07">Since draft-ietf-httpbis-p6-cache-07</a></h2>
1732      <p id="rfc.section.C.9.p.1">Closed issues: </p>
1733      <ul>
1734         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/54">http://tools.ietf.org/wg/httpbis/trac/ticket/54</a>&gt;: "Definition of 1xx Warn-Codes"
1735         </li>
1736         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/167">http://tools.ietf.org/wg/httpbis/trac/ticket/167</a>&gt;: "Content-Location on 304 responses"
1737         </li>
1738         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/169">http://tools.ietf.org/wg/httpbis/trac/ticket/169</a>&gt;: "private and no-cache CC directives with headers"
1739         </li>
1740         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/187">http://tools.ietf.org/wg/httpbis/trac/ticket/187</a>&gt;: "RFC2047 and warn-text"
1741         </li>
1742      </ul>
1743      <h2 id="rfc.section.C.10"><a href="#rfc.section.C.10">C.10</a>&nbsp;<a id="changes.since.08" href="#changes.since.08">Since draft-ietf-httpbis-p6-cache-08</a></h2>
1744      <p id="rfc.section.C.10.p.1">Closed issues: </p>
1745      <ul>
1746         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/147">http://tools.ietf.org/wg/httpbis/trac/ticket/147</a>&gt;: "serving negotiated responses from cache: header-specific canonicalization"
1747         </li>
1748         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/197">http://tools.ietf.org/wg/httpbis/trac/ticket/197</a>&gt;: "Effect of CC directives on history lists"
1749         </li>
1750      </ul>
1751      <p id="rfc.section.C.10.p.2">Affected issues: </p>
1752      <ul>
1753         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/199">http://tools.ietf.org/wg/httpbis/trac/ticket/199</a>&gt;: Status codes and caching
1754         </li>
1755      </ul>
1756      <p id="rfc.section.C.10.p.3">Partly resolved issues: </p>
1757      <ul>
1758         <li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/60">http://tools.ietf.org/wg/httpbis/trac/ticket/60</a>&gt;: "Placement of 13.5.1 and 13.5.2"
1759         </li>
1760      </ul>
1761      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1762      <p class="noprint"><a href="#rfc.index.A">A</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.F">F</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.N">N</a> <a href="#rfc.index.O">O</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> <a href="#rfc.index.W">W</a> 
1763      </p>
1764      <div class="print2col">
1765         <ul class="ind">
1766            <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
1767                  <li class="indline1">age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1">1.2</a></li>
1768                  <li class="indline1">Age header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">2.2</a>, <a class="iref" href="#rfc.iref.a.2"><b>3.1</b></a>, <a class="iref" href="#rfc.xref.header.age.2">5.1</a></li>
1769               </ul>
1770            </li>
1771            <li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind">
1772                  <li class="indline1">cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1">1.1</a></li>
1773                  <li class="indline1">Cache Directives&nbsp;&nbsp;
1774                     <ul class="ind">
1775                        <li class="indline1">max-age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.6"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.17"><b>3.2.2</b></a></li>
1776                        <li class="indline1">max-stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.1</b></a></li>
1777                        <li class="indline1">min-fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.1</b></a></li>
1778                        <li class="indline1">must-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.15"><b>3.2.2</b></a></li>
1779                        <li class="indline1">no-cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.13"><b>3.2.2</b></a></li>
1780                        <li class="indline1">no-store&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.14"><b>3.2.2</b></a></li>
1781                        <li class="indline1">no-transform&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.9"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.19"><b>3.2.2</b></a></li>
1782                        <li class="indline1">only-if-cached&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.10"><b>3.2.1</b></a></li>
1783                        <li class="indline1">private&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.12"><b>3.2.2</b></a></li>
1784                        <li class="indline1">proxy-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.16"><b>3.2.2</b></a></li>
1785                        <li class="indline1">public&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.11"><b>3.2.2</b></a></li>
1786                        <li class="indline1">s-maxage&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.18"><b>3.2.2</b></a></li>
1787                     </ul>
1788                  </li>
1789                  <li class="indline1">Cache-Control header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.2</a>, <a class="iref" href="#rfc.iref.c.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.4">5.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">A.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">A.1</a></li>
1790                  <li class="indline1">cacheable&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2">1.2</a></li>
1791               </ul>
1792            </li>
1793            <li class="indline0"><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul class="ind">
1794                  <li class="indline1">Expires header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.1</a>, <a class="iref" href="#rfc.xref.header.expires.2">2.3</a>, <a class="iref" href="#rfc.xref.header.expires.3">2.3.1</a>, <a class="iref" href="#rfc.iref.e.2"><b>3.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.4">5.1</a></li>
1795                  <li class="indline1">explicit expiration time&nbsp;&nbsp;<a class="iref" href="#rfc.iref.e.1">1.2</a></li>
1796               </ul>
1797            </li>
1798            <li class="indline0"><a id="rfc.index.F" href="#rfc.index.F"><b>F</b></a><ul class="ind">
1799                  <li class="indline1">first-hand&nbsp;&nbsp;<a class="iref" href="#rfc.iref.f.1">1.2</a></li>
1800                  <li class="indline1">fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.f.3">1.2</a></li>
1801                  <li class="indline1">freshness lifetime&nbsp;&nbsp;<a class="iref" href="#rfc.iref.f.2">1.2</a></li>
1802               </ul>
1803            </li>
1804            <li class="indline0"><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul class="ind">
1805                  <li class="indline1"><tt>Grammar</tt>&nbsp;&nbsp;
1806                     <ul class="ind">
1807                        <li class="indline1"><tt>Age</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1"><b>3.1</b></a></li>
1808                        <li class="indline1"><tt>Age-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>3.1</b></a></li>
1809                        <li class="indline1"><tt>Cache-Control</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>3.2</b></a></li>
1810                        <li class="indline1"><tt>Cache-Control-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>3.2</b></a></li>
1811                        <li class="indline1"><tt>cache-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>3.2</b></a></li>
1812                        <li class="indline1"><tt>cache-request-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>3.2.1</b></a></li>
1813                        <li class="indline1"><tt>cache-response-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>3.2.2</b></a></li>
1814                        <li class="indline1"><tt>delta-seconds</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>3.1</b></a></li>
1815                        <li class="indline1"><tt>Expires</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>3.3</b></a></li>
1816                        <li class="indline1"><tt>Expires-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>3.3</b></a></li>
1817                        <li class="indline1"><tt>extension-pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>3.4</b></a></li>
1818                        <li class="indline1"><tt>Pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>3.4</b></a></li>
1819                        <li class="indline1"><tt>pragma-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>3.4</b></a></li>
1820                        <li class="indline1"><tt>Pragma-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>3.4</b></a></li>
1821                        <li class="indline1"><tt>Vary</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>3.5</b></a></li>
1822                        <li class="indline1"><tt>Vary-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>3.5</b></a></li>
1823                        <li class="indline1"><tt>warn-agent</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.21"><b>3.6</b></a></li>
1824                        <li class="indline1"><tt>warn-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.20"><b>3.6</b></a></li>
1825                        <li class="indline1"><tt>warn-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.23"><b>3.6</b></a></li>
1826                        <li class="indline1"><tt>warn-text</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.22"><b>3.6</b></a></li>
1827                        <li class="indline1"><tt>Warning</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>3.6</b></a></li>
1828                        <li class="indline1"><tt>Warning-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>3.6</b></a></li>
1829                        <li class="indline1"><tt>warning-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>3.6</b></a></li>
1830                     </ul>
1831                  </li>
1832               </ul>
1833            </li>
1834            <li class="indline0"><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul class="ind">
1835                  <li class="indline1">Headers&nbsp;&nbsp;
1836                     <ul class="ind">
1837                        <li class="indline1">Age&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">2.2</a>, <a class="iref" href="#rfc.iref.h.2"><b>3.1</b></a>, <a class="iref" href="#rfc.xref.header.age.2">5.1</a></li>
1838                        <li class="indline1">Cache-Control&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.2</a>, <a class="iref" href="#rfc.iref.h.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.4">5.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">A.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">A.1</a></li>
1839                        <li class="indline1">Expires&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.1</a>, <a class="iref" href="#rfc.xref.header.expires.2">2.3</a>, <a class="iref" href="#rfc.xref.header.expires.3">2.3.1</a>, <a class="iref" href="#rfc.iref.h.4"><b>3.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.4">5.1</a></li>
1840                        <li class="indline1">Pragma&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">2.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">3.2</a>, <a class="iref" href="#rfc.iref.h.5"><b>3.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">5.1</a></li>
1841                        <li class="indline1">Vary&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.6</a>, <a class="iref" href="#rfc.iref.h.6"><b>3.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">5.1</a></li>
1842                        <li class="indline1">Warning&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.7</a>, <a class="iref" href="#rfc.iref.h.7"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.3">5.1</a>, <a class="iref" href="#rfc.xref.header.warning.4">A.1</a>, <a class="iref" href="#rfc.xref.header.warning.5">A.2</a></li>
1843                     </ul>
1844                  </li>
1845                  <li class="indline1">heuristic expiration time&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.1">1.2</a></li>
1846               </ul>
1847            </li>
1848            <li class="indline0"><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul class="ind">
1849                  <li class="indline1">max-age&nbsp;&nbsp;
1850                     <ul class="ind">
1851                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.m.5"><b>3.2.2</b></a></li>
1852                     </ul>
1853                  </li>
1854                  <li class="indline1">max-stale&nbsp;&nbsp;
1855                     <ul class="ind">
1856                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>3.2.1</b></a></li>
1857                     </ul>
1858                  </li>
1859                  <li class="indline1">min-fresh&nbsp;&nbsp;
1860                     <ul class="ind">
1861                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>3.2.1</b></a></li>
1862                     </ul>
1863                  </li>
1864                  <li class="indline1">must-revalidate&nbsp;&nbsp;
1865                     <ul class="ind">
1866                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.4"><b>3.2.2</b></a></li>
1867                     </ul>
1868                  </li>
1869               </ul>
1870            </li>
1871            <li class="indline0"><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul class="ind">
1872                  <li class="indline1">no-cache&nbsp;&nbsp;
1873                     <ul class="ind">
1874                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.4"><b>3.2.2</b></a></li>
1875                     </ul>
1876                  </li>
1877                  <li class="indline1">no-store&nbsp;&nbsp;
1878                     <ul class="ind">
1879                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.5"><b>3.2.2</b></a></li>
1880                     </ul>
1881                  </li>
1882                  <li class="indline1">no-transform&nbsp;&nbsp;
1883                     <ul class="ind">
1884                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.6"><b>3.2.2</b></a></li>
1885                     </ul>
1886                  </li>
1887               </ul>
1888            </li>
1889            <li class="indline0"><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul class="ind">
1890                  <li class="indline1">only-if-cached&nbsp;&nbsp;
1891                     <ul class="ind">
1892                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.1</b></a></li>
1893                     </ul>
1894                  </li>
1895               </ul>
1896            </li>
1897            <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
1898                  <li class="indline1"><em>Part1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">1.4</a>, <a class="iref" href="#rfc.xref.Part1.2">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.3">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.4">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.5">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.8">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.9">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.10">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.11">2.3.2</a>, <a class="iref" href="#rfc.xref.Part1.12">2.6</a>, <a class="iref" href="#rfc.xref.Part1.13">3.3</a>, <a class="iref" href="#Part1"><b>8.1</b></a><ul class="ind">
1899                        <li class="indline1"><em>Section 1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">1.4</a></li>
1900                        <li class="indline1"><em>Section 1.2.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.2">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.3">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.4">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.5">1.4.1</a></li>
1901                        <li class="indline1"><em>Section 2.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.8">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.10">1.4.2</a></li>
1902                        <li class="indline1"><em>Section 3.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.12">2.6</a></li>
1903                        <li class="indline1"><em>Section 6.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.13">3.3</a></li>
1904                        <li class="indline1"><em>Section 9.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.11">2.3.2</a></li>
1905                        <li class="indline1"><em>Section 9.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.9">1.4.2</a></li>
1906                     </ul>
1907                  </li>
1908                  <li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.2</a>, <a class="iref" href="#rfc.xref.Part2.2">2.5</a>, <a class="iref" href="#Part2"><b>8.1</b></a><ul class="ind">
1909                        <li class="indline1"><em>Section 7.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.2</a>, <a class="iref" href="#rfc.xref.Part2.2">2.5</a></li>
1910                     </ul>
1911                  </li>
1912                  <li class="indline1"><em>Part4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3.1.1</a>, <a class="iref" href="#rfc.xref.Part4.2">2.4</a>, <a class="iref" href="#rfc.xref.Part4.3">2.7</a>, <a class="iref" href="#Part4"><b>8.1</b></a><ul class="ind">
1913                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.3">2.7</a></li>
1914                        <li class="indline1"><em>Section 6.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3.1.1</a></li>
1915                     </ul>
1916                  </li>
1917                  <li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">2.1.1</a>, <a class="iref" href="#rfc.xref.Part5.2">2.1.1</a>, <a class="iref" href="#Part5"><b>8.1</b></a><ul class="ind">
1918                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.2">2.1.1</a></li>
1919                     </ul>
1920                  </li>
1921                  <li class="indline1"><em>Part7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.1</a>, <a class="iref" href="#rfc.xref.Part7.2">3.2.2</a>, <a class="iref" href="#Part7"><b>8.1</b></a><ul class="ind">
1922                        <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.1</a>, <a class="iref" href="#rfc.xref.Part7.2">3.2.2</a></li>
1923                     </ul>
1924                  </li>
1925                  <li class="indline1">Pragma header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">2.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">3.2</a>, <a class="iref" href="#rfc.iref.p.4"><b>3.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">5.1</a></li>
1926                  <li class="indline1">private&nbsp;&nbsp;
1927                     <ul class="ind">
1928                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.2"><b>3.2.2</b></a></li>
1929                     </ul>
1930                  </li>
1931                  <li class="indline1">proxy-revalidate&nbsp;&nbsp;
1932                     <ul class="ind">
1933                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.3"><b>3.2.2</b></a></li>
1934                     </ul>
1935                  </li>
1936                  <li class="indline1">public&nbsp;&nbsp;
1937                     <ul class="ind">
1938                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>3.2.2</b></a></li>
1939                     </ul>
1940                  </li>
1941               </ul>
1942            </li>
1943            <li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind">
1944                  <li class="indline1"><em>RFC1305</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1305.1">2.3.2</a>, <a class="iref" href="#RFC1305"><b>8.2</b></a></li>
1945                  <li class="indline1"><em>RFC2119</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2119.1">1.3</a>, <a class="iref" href="#RFC2119"><b>8.1</b></a></li>
1946                  <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#RFC2616"><b>8.2</b></a>, <a class="iref" href="#rfc.xref.RFC2616.1">C.1</a></li>
1947                  <li class="indline1"><em>RFC3864</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC3864.1">5.1</a>, <a class="iref" href="#RFC3864"><b>8.2</b></a></li>
1948                  <li class="indline1"><em>RFC5234</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5234.1">1.4</a>, <a class="iref" href="#rfc.xref.RFC5234.2">1.4</a>, <a class="iref" href="#RFC5234"><b>8.1</b></a><ul class="ind">
1949                        <li class="indline1"><em>Appendix B.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5234.2">1.4</a></li>
1950                     </ul>
1951                  </li>
1952               </ul>
1953            </li>
1954            <li class="indline0"><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul class="ind">
1955                  <li class="indline1">s-maxage&nbsp;&nbsp;
1956                     <ul class="ind">
1957                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.2"><b>3.2.2</b></a></li>
1958                     </ul>
1959                  </li>
1960                  <li class="indline1">stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1">1.2</a></li>
1961               </ul>
1962            </li>
1963            <li class="indline0"><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul class="ind">
1964                  <li class="indline1">validator&nbsp;&nbsp;<a class="iref" href="#rfc.iref.v.1">1.2</a>, <a class="iref" href="#rfc.iref.v.2">1.2</a></li>
1965                  <li class="indline1">Vary header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.6</a>, <a class="iref" href="#rfc.iref.v.3"><b>3.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">5.1</a></li>
1966               </ul>
1967            </li>
1968            <li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind">
1969                  <li class="indline1">Warning header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.7</a>, <a class="iref" href="#rfc.iref.w.1"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.3">5.1</a>, <a class="iref" href="#rfc.xref.header.warning.4">A.1</a>, <a class="iref" href="#rfc.xref.header.warning.5">A.2</a></li>
1970               </ul>
1971            </li>
1972         </ul>
1973      </div>
1974   </body>
1975</html>
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