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385</style><link rel="Contents" href="#rfc.toc">
386      <link rel="Author" href="#rfc.authors">
387      <link rel="Copyright" href="#rfc.copyrightnotice">
388      <link rel="Index" href="#rfc.index">
389      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
390      <link rel="Chapter" title="2 Cache Operation" href="#rfc.section.2">
391      <link rel="Chapter" title="3 Header Field Definitions" href="#rfc.section.3">
392      <link rel="Chapter" title="4 Security Considerations" href="#rfc.section.4">
393      <link rel="Chapter" title="5 Acknowledgments" href="#rfc.section.5">
394      <link rel="Chapter" href="#rfc.section.6" title="6 References">
395      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
396      <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B">
397      <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C">
398      <link href="p5-range.html" rel="prev">
399      <link href="p7-auth.html" rel="next">
400      <meta name="generator" content=", Revision 1.640, 2014/06/13 12:42:58, XSLT vendor: SAXON 8.9 from Saxonica">
401      <link rel="schema.dct" href="">
402      <meta name="dct.creator" content="Fielding, R.">
403      <meta name="dct.creator" content="Gettys, J.">
404      <meta name="dct.creator" content="Mogul, J.">
405      <meta name="dct.creator" content="Frystyk, H.">
406      <meta name="dct.creator" content="Masinter, L.">
407      <meta name="dct.creator" content="Leach, P.">
408      <meta name="dct.creator" content="Berners-Lee, T.">
409      <meta name="dct.creator" content="Lafon, Y.">
410      <meta name="dct.creator" content="Nottingham, M.">
411      <meta name="dct.creator" content="Reschke, J. F.">
412      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p6-cache-18">
413      <meta name="dct.issued" scheme="ISO8601" content="2012-01-04">
414      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
415      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 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.">
416      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 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.">
417   </head>
418   <body>
419      <table class="header">
420         <tbody>
421            <tr>
422               <td class="left">HTTPbis Working Group</td>
423               <td class="right">R. Fielding, Editor</td>
424            </tr>
425            <tr>
426               <td class="left">Internet-Draft</td>
427               <td class="right">Adobe</td>
428            </tr>
429            <tr>
430               <td class="left">Obsoletes: <a href="">2616</a> (if approved)
431               </td>
432               <td class="right">J. Gettys</td>
433            </tr>
434            <tr>
435               <td class="left">Intended status: Standards Track</td>
436               <td class="right">Alcatel-Lucent</td>
437            </tr>
438            <tr>
439               <td class="left">Expires: July 7, 2012</td>
440               <td class="right">J. Mogul</td>
441            </tr>
442            <tr>
443               <td class="left"></td>
444               <td class="right">HP</td>
445            </tr>
446            <tr>
447               <td class="left"></td>
448               <td class="right">H. Frystyk</td>
449            </tr>
450            <tr>
451               <td class="left"></td>
452               <td class="right">Microsoft</td>
453            </tr>
454            <tr>
455               <td class="left"></td>
456               <td class="right">L. Masinter</td>
457            </tr>
458            <tr>
459               <td class="left"></td>
460               <td class="right">Adobe</td>
461            </tr>
462            <tr>
463               <td class="left"></td>
464               <td class="right">P. Leach</td>
465            </tr>
466            <tr>
467               <td class="left"></td>
468               <td class="right">Microsoft</td>
469            </tr>
470            <tr>
471               <td class="left"></td>
472               <td class="right">T. Berners-Lee</td>
473            </tr>
474            <tr>
475               <td class="left"></td>
476               <td class="right">W3C/MIT</td>
477            </tr>
478            <tr>
479               <td class="left"></td>
480               <td class="right">Y. Lafon, Editor</td>
481            </tr>
482            <tr>
483               <td class="left"></td>
484               <td class="right">W3C</td>
485            </tr>
486            <tr>
487               <td class="left"></td>
488               <td class="right">M. Nottingham, Editor</td>
489            </tr>
490            <tr>
491               <td class="left"></td>
492               <td class="right">Rackspace</td>
493            </tr>
494            <tr>
495               <td class="left"></td>
496               <td class="right">J. Reschke, Editor</td>
497            </tr>
498            <tr>
499               <td class="left"></td>
500               <td class="right">greenbytes</td>
501            </tr>
502            <tr>
503               <td class="left"></td>
504               <td class="right">January 4, 2012</td>
505            </tr>
506         </tbody>
507      </table>
508      <p class="title">HTTP/1.1, part 6: Caching<br><span class="filename">draft-ietf-httpbis-p6-cache-18</span></p>
509      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
510      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
511         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 6 of the
512         seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
513      </p>
514      <p>Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable
515         response messages.
516      </p>
517      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
518      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (, which is archived
519         at &lt;<a href=""></a>&gt;.
520      </p>
521      <p>The current issues list is at &lt;<a href=""></a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href=""></a>&gt;.
522      </p>
523      <p>The changes in this draft are summarized in <a href="#changes.since.17" title="Since draft-ietf-httpbis-p6-cache-17">Appendix&nbsp;C.19</a>.
524      </p>
525      <div id="rfc.status">
526         <h1><a href="#rfc.status">Status of This Memo</a></h1>
527         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
528         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
529            working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href=""></a>.
530         </p>
531         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
532            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
533            in progress”.
534         </p>
535         <p>This Internet-Draft will expire on July 7, 2012.</p>
536      </div>
537      <div id="rfc.copyrightnotice">
538         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
539         <p>Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
540         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href=""></a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
541            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
542            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
543            BSD License.
544         </p>
545         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
546            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
547            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
548            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
549            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
550            it into languages other than English.
551         </p>
552      </div>
553      <hr class="noprint">
554      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
555      <ul class="toc">
556         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul>
557               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
558               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
559               <li><a href="#rfc.section.1.3">1.3</a>&nbsp;&nbsp;&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></li>
560               <li><a href="#rfc.section.1.4">1.4</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul>
561                     <li><a href="#rfc.section.1.4.1">1.4.1</a>&nbsp;&nbsp;&nbsp;<a href="#core.rules">Core Rules</a></li>
562                     <li><a href="#rfc.section.1.4.2">1.4.2</a>&nbsp;&nbsp;&nbsp;<a href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></li>
563                  </ul>
564               </li>
565               <li><a href="#rfc.section.1.5">1.5</a>&nbsp;&nbsp;&nbsp;<a href="#delta-seconds">Delta Seconds</a></li>
566            </ul>
567         </li>
568         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Cache Operation</a><ul>
569               <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li>
570               <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></li>
571               <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness Model</a><ul>
572                     <li><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></li>
573                     <li><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
574                     <li><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
575                  </ul>
576               </li>
577               <li><a href="#rfc.section.2.4">2.4</a>&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul>
578                     <li><a href="#rfc.section.2.4.1">2.4.1</a>&nbsp;&nbsp;&nbsp;<a href="#freshening.responses">Freshening Responses</a></li>
579                  </ul>
580               </li>
581               <li><a href="#rfc.section.2.5">2.5</a>&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
582               <li><a href="#rfc.section.2.6">2.6</a>&nbsp;&nbsp;&nbsp;<a href="#caching.authenticated.responses">Shared Caching of Authenticated Responses</a></li>
583               <li><a href="#rfc.section.2.7">2.7</a>&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
584               <li><a href="#rfc.section.2.8">2.8</a>&nbsp;&nbsp;&nbsp;<a href="#combining.responses">Combining Partial Content</a></li>
585            </ul>
586         </li>
587         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.definitions">Header Field Definitions</a><ul>
588               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
589               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul>
590                     <li><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></li>
591                     <li><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></li>
592                     <li><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
593                  </ul>
594               </li>
595               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
596               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
597               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
598               <li><a href="#rfc.section.3.6">3.6</a>&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a><ul>
599                     <li><a href="#rfc.section.3.6.1">3.6.1</a>&nbsp;&nbsp;&nbsp;<a href="#warn.110">110 Response is Stale</a></li>
600                     <li><a href="#rfc.section.3.6.2">3.6.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.111">111 Revalidation Failed</a></li>
601                     <li><a href="#rfc.section.3.6.3">3.6.3</a>&nbsp;&nbsp;&nbsp;<a href="#warn.112">112 Disconnected Operation</a></li>
602                     <li><a href="#rfc.section.3.6.4">3.6.4</a>&nbsp;&nbsp;&nbsp;<a href="#warn.113">113 Heuristic Expiration</a></li>
603                     <li><a href="#rfc.section.3.6.5">3.6.5</a>&nbsp;&nbsp;&nbsp;<a href="#warn.199">199 Miscellaneous Warning</a></li>
604                     <li><a href="#rfc.section.3.6.6">3.6.6</a>&nbsp;&nbsp;&nbsp;<a href="#warn.214">214 Transformation Applied</a></li>
605                     <li><a href="#rfc.section.3.6.7">3.6.7</a>&nbsp;&nbsp;&nbsp;<a href="#warn.299">299 Miscellaneous Persistent Warning</a></li>
606                     <li><a href="#rfc.section.3.6.8">3.6.8</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.extensions">Warn Code Extensions</a></li>
607                  </ul>
608               </li>
609               <li><a href="#rfc.section.3.7">3.7</a>&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
610               <li><a href="#rfc.section.3.8">3.8</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
611                     <li><a href="#rfc.section.3.8.1">3.8.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registration">Cache Directive Registry</a></li>
612                     <li><a href="#rfc.section.3.8.2">3.8.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registration">Warn Code Registry</a></li>
613                  </ul>
614               </li>
615               <li><a href="#rfc.section.3.9">3.9</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
616            </ul>
617         </li>
618         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
619         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
620         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
621               <li><a href="#rfc.section.6.1">6.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
622               <li><a href="#rfc.section.6.2">6.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
623            </ul>
624         </li>
625         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
626         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
627         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
628               <li><a href="#rfc.section.C.1">C.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC 2616</a></li>
629               <li><a href="#rfc.section.C.2">C.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p6-cache-00</a></li>
630               <li><a href="#rfc.section.C.3">C.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p6-cache-01</a></li>
631               <li><a href="#rfc.section.C.4">C.4</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p6-cache-02</a></li>
632               <li><a href="#rfc.section.C.5">C.5</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p6-cache-03</a></li>
633               <li><a href="#rfc.section.C.6">C.6</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p6-cache-04</a></li>
634               <li><a href="#rfc.section.C.7">C.7</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></li>
635               <li><a href="#rfc.section.C.8">C.8</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p6-cache-06</a></li>
636               <li><a href="#rfc.section.C.9">C.9</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p6-cache-07</a></li>
637               <li><a href="#rfc.section.C.10">C.10</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p6-cache-08</a></li>
638               <li><a href="#rfc.section.C.11">C.11</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p6-cache-09</a></li>
639               <li><a href="#rfc.section.C.12">C.12</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p6-cache-10</a></li>
640               <li><a href="#rfc.section.C.13">C.13</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p6-cache-11</a></li>
641               <li><a href="#rfc.section.C.14">C.14</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p6-cache-12</a></li>
642               <li><a href="#rfc.section.C.15">C.15</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p6-cache-13</a></li>
643               <li><a href="#rfc.section.C.16">C.16</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.14">Since draft-ietf-httpbis-p6-cache-14</a></li>
644               <li><a href="#rfc.section.C.17">C.17</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.15">Since draft-ietf-httpbis-p6-cache-15</a></li>
645               <li><a href="#rfc.section.C.18">C.18</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.16">Since draft-ietf-httpbis-p6-cache-16</a></li>
646               <li><a href="#rfc.section.C.19">C.19</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.17">Since draft-ietf-httpbis-p6-cache-17</a></li>
647            </ul>
648         </li>
649         <li><a href="#rfc.index">Index</a></li>
650         <li><a href="#rfc.authors">Authors' Addresses</a></li>
651      </ul>
652      <div id="caching">
653         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#caching">Introduction</a></h1>
654         <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.
655            This document defines aspects of HTTP/1.1 related to caching and reusing response messages.
656         </p>
657         <div id="intro.purpose">
658            <div id="rfc.iref.c.1"></div>
659            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#intro.purpose">Purpose</a></h2>
660            <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
661               stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests.
662               Any client or server <em class="bcp14">MAY</em> employ a cache, though a cache cannot be used by a server that is acting as a tunnel.
663            </p>
664            <p id="rfc.section.1.1.p.2">The goal of caching in HTTP/1.1 is to significantly improve performance by reusing a prior response message to satisfy a current
665               request. A stored response is considered "fresh", as defined in <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>, if the response can be reused without "validation" (checking with the origin server to see if the cached response remains
666               valid for this request). A fresh cache response can therefore reduce both latency and network transfers each time it is reused.
667               When a cached response is not fresh, it might still be reusable if it can be freshened by validation (<a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>) or if the origin is unavailable.
668            </p>
669         </div>
670         <div id="intro.terminology">
671            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#intro.terminology">Terminology</a></h2>
672            <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>
673            <p id="rfc.section.1.2.p.2"><span id="rfc.iref.c.2"></span> <dfn>cache</dfn>
674            </p>
675            <ul class="empty">
676               <li>A conformant implementation of a HTTP cache. Note that this implies an HTTP/1.1 cache; this specification does not define
677                  conformance for HTTP/1.0 caches.
678               </li>
679            </ul>
680            <div id="shared.and.non-shared.caches">
681               <p id="rfc.section.1.2.p.3"><span id="rfc.iref.s.1"></span> <dfn>shared cache</dfn>
682               </p>
683               <ul class="empty">
684                  <li>A cache that stores responses to be reused by more than one user; usually (but not always) deployed as part of an intermediary.</li>
685               </ul>
686            </div>
687            <p id="rfc.section.1.2.p.4"><span id="rfc.iref.p.1"></span> <dfn>private cache</dfn>
688            </p>
689            <ul class="empty">
690               <li>A cache that is dedicated to a single user.</li>
691            </ul>
692            <p id="rfc.section.1.2.p.5"><span id="rfc.iref.c.3"></span> <dfn>cacheable</dfn>
693            </p>
694            <ul class="empty">
695               <li>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
696                  Even when a response is cacheable, there might be additional constraints on whether a cache can use the stored copy to satisfy
697                  a particular request.
698               </li>
699            </ul>
700            <p id="rfc.section.1.2.p.6"><span id="rfc.iref.e.1"></span> <dfn>explicit expiration time</dfn>
701            </p>
702            <ul class="empty">
703               <li>The time at which the origin server intends that a representation no longer be returned by a cache without further validation.</li>
704            </ul>
705            <p id="rfc.section.1.2.p.7"><span id="rfc.iref.h.1"></span> <dfn>heuristic expiration time</dfn>
706            </p>
707            <ul class="empty">
708               <li>An expiration time assigned by a cache when no explicit expiration time is available.</li>
709            </ul>
710            <p id="rfc.section.1.2.p.8"><span id="rfc.iref.a.1"></span> <dfn>age</dfn>
711            </p>
712            <ul class="empty">
713               <li>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</li>
714            </ul>
715            <p id="rfc.section.1.2.p.9"><span id="rfc.iref.f.1"></span> <dfn>first-hand</dfn>
716            </p>
717            <ul class="empty">
718               <li>A response is first-hand if the freshness model is not in use; i.e., its age is 0.</li>
719            </ul>
720            <p id="rfc.section.1.2.p.10"><span id="rfc.iref.f.2"></span> <dfn>freshness lifetime</dfn>
721            </p>
722            <ul class="empty">
723               <li>The length of time between the generation of a response and its expiration time.</li>
724            </ul>
725            <p id="rfc.section.1.2.p.11"><span id="rfc.iref.f.3"></span> <dfn>fresh</dfn>
726            </p>
727            <ul class="empty">
728               <li>A response is fresh if its age has not yet exceeded its freshness lifetime.</li>
729            </ul>
730            <p id="rfc.section.1.2.p.12"><span id="rfc.iref.s.2"></span> <dfn>stale</dfn>
731            </p>
732            <ul class="empty">
733               <li>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</li>
734            </ul>
735            <p id="rfc.section.1.2.p.13"><span id="rfc.iref.v.1"></span> <dfn>validator</dfn>
736            </p>
737            <ul class="empty">
738               <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
739                  equivalent copy of a representation. See <a href="p4-conditional.html#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
740               </li>
741            </ul>
742            <p id="rfc.section.1.2.p.14"><span id="rfc.iref.s.3"></span> <span id="rfc.iref.v.2"></span> <dfn>strong validator</dfn>
743            </p>
744            <ul class="empty">
745               <li>A validator that is defined by the origin server such that its current value will change if the representation body changes;
746                  i.e., an entity-tag that is not marked as weak (<a href="p4-conditional.html#header.etag" title="ETag">Section 2.3</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>) or, if no entity-tag is provided, a Last-Modified value that is strong in the sense defined by <a href="p4-conditional.html#lastmod.comparison" title="Comparison">Section 2.2.2</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
747               </li>
748            </ul>
749         </div>
750         <div id="intro.conformance.and.error.handling">
751            <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></h2>
752            <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"
753               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>.
754            </p>
755            <p id="rfc.section.1.3.p.2">This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients,
756               Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See <a href="p1-messaging.html#architecture" title="Architecture">Section 2</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> for definitions of these terms.
757            </p>
758            <p id="rfc.section.1.3.p.3">An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that
759               SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
760            </p>
761            <p id="rfc.section.1.3.p.4">This document also uses ABNF to define valid protocol elements (<a href="#notation" title="Syntax Notation">Section&nbsp;1.4</a>). In addition to the prose requirements placed upon them, Senders <em class="bcp14">MUST NOT</em> generate protocol elements that are invalid.
762            </p>
763            <p id="rfc.section.1.3.p.5">Unless noted otherwise, Recipients <em class="bcp14">MAY</em> take steps to recover a usable protocol element from an invalid construct. However, HTTP does not define specific error handling
764               mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require
765               different error handling strategies; for example, a Web browser may wish to transparently recover from a response where the
766               Location header field doesn't parse according to the ABNF, whereby in a systems control protocol using HTTP, this type of
767               error recovery could lead to dangerous consequences.
768            </p>
769         </div>
770         <div id="notation">
771            <h2 id="rfc.section.1.4"><a href="#rfc.section.1.4">1.4</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
772            <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.Part1.2"><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.
773            </p>
774            <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="">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
775               (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
776               character).
777            </p>
778            <div id="core.rules">
779               <h3 id="rfc.section.1.4.1"><a href="#rfc.section.1.4.1">1.4.1</a>&nbsp;<a href="#core.rules">Core Rules</a></h3>
780               <p id="rfc.section.1.4.1.p.1">The core rules below are defined in <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>:
781               </p>
782               <div id="rfc.figure.u.1"></div><pre class="inline">  <a href="#core.rules" class="smpl">OWS</a>           = &lt;OWS, defined in <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
783  <a href="#core.rules" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#field.rules" title="Common Field ABNF Rules">Section 3.2.3</a>&gt;
784  <a href="#core.rules" class="smpl">token</a>         = &lt;token, 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#field.rules" title="Common Field ABNF Rules">Section 3.2.3</a>&gt;
786            <div id="abnf.dependencies">
787               <h3 id="rfc.section.1.4.2"><a href="#rfc.section.1.4.2">1.4.2</a>&nbsp;<a href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></h3>
788               <p id="rfc.section.1.4.2.p.1">The ABNF rules below are defined in other parts:</p>
789               <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.Part1.7"><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;
790  <a href="#abnf.dependencies" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>, <a href="" title="Date/Time Formats">Section 8</a>&gt;
791  <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 2.7</a>&gt;
792  <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 8.8</a>&gt;
793  <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 2.7</a>&gt;
795         </div>
796         <div id="delta-seconds">
797            <h2 id="rfc.section.1.5"><a href="#rfc.section.1.5">1.5</a>&nbsp;<a href="#delta-seconds">Delta Seconds</a></h2>
798            <p id="rfc.section.1.5.p.1">The delta-seconds rule specifies a non-negative integer, representing time in seconds.</p>
799            <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.g.1"></span>  <a href="#delta-seconds" class="smpl">delta-seconds</a>  = 1*<a href="#notation" class="smpl">DIGIT</a>
800</pre><p id="rfc.section.1.5.p.3">If an implementation receives a delta-seconds value larger than the largest positive integer it can represent, or if any of
801               its subsequent calculations overflows, it <em class="bcp14">MUST</em> consider the value to be 2147483648 (2<sup>31</sup>). Recipients parsing a delta-seconds value <em class="bcp14">MUST</em> use an arithmetic type of at least 31 bits of range, and senders <em class="bcp14">MUST NOT</em> send delta-seconds with a value greater than 2147483648.
802            </p>
803         </div>
804      </div>
805      <div id="caching.overview">
806         <div id="rfc.iref.c.4"></div>
807         <div id="rfc.iref.c.5"></div>
808         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#caching.overview">Cache Operation</a></h1>
809         <p id="rfc.section.2.p.1">Proper cache operation preserves the semantics of HTTP transfers (<a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) while eliminating the transfer of information already held in the cache. Although caching is an entirely <em class="bcp14">OPTIONAL</em> feature of HTTP, we assume that reusing the cached response is desirable and that such reuse is the default behavior when
810            no requirement or locally-desired configuration prevents it. Therefore, HTTP cache requirements are focused on preventing
811            a cache from either storing a non-reusable response or reusing a stored response inappropriately.
812         </p>
813         <p id="rfc.section.2.p.2">Each <dfn>cache entry</dfn> consists of a cache key and one or more HTTP responses corresponding to prior requests that used the same key. The most common
814            form of cache entry is a successful result of a retrieval request: i.e., a 200 (OK) response containing a representation of
815            the resource identified by the request target. However, it is also possible to cache negative results (e.g., 404 not found),
816            incomplete results (e.g., 206 partial content), and responses to safe methods other than GET if the method's definition allows
817            such caching and defines something suitable for use as a cache key.
818         </p>
819         <p id="rfc.section.2.p.3">The default <dfn>cache key</dfn> consists of the request method and target URI. However, since HTTP caches in common use today are typically limited to caching
820            responses to GET, most implementations simply decline other methods and use only the URI as the key.
821         </p>
822         <p id="rfc.section.2.p.4">If a request target is subject to content negotiation, its cache entry might consist of multiple stored responses, each differentiated
823            by a secondary key for the values of the original request's selecting header fields (<a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>).
824         </p>
825         <div id="response.cacheability">
826            <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a href="#response.cacheability">Response Cacheability</a></h2>
827            <p id="rfc.section.2.1.p.1">A cache <em class="bcp14">MUST NOT</em> store a response to any request, unless:
828            </p>
829            <ul>
830               <li>The request method is understood by the cache and defined as being cacheable, and</li>
831               <li>the response status code is understood by the cache, and</li>
832               <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 header fields, and
833               </li>
834               <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
835               </li>
836               <li>the "Authorization" header field (see <a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</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 response explicitly allows it (see <a href="#caching.authenticated.responses" title="Shared Caching of Authenticated Responses">Section&nbsp;2.6</a>), and
837               </li>
838               <li>the response either:
839                  <ul>
840                     <li>contains an Expires header field (see <a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;3.3</a>), or
841                     </li>
842                     <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
843                     </li>
844                     <li>contains a s-maxage response cache directive and the cache is shared, or</li>
845                     <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
846                     </li>
847                     <li>has a status code that can be served with heuristic freshness (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;</a>).
848                     </li>
849                  </ul>
850               </li>
851            </ul>
852            <p id="rfc.section.2.1.p.2">Note that any of the requirements listed above can be overridden by a cache-control extension; see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;3.2.3</a>.
853            </p>
854            <p id="rfc.section.2.1.p.3">In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements any
855               cache-specific behavior.
856            </p>
857            <p id="rfc.section.2.1.p.4">Note that, in normal operation, most caches will not store a response that has neither a cache validator nor an explicit expiration
858               time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.
859            </p>
860            <p id="rfc.section.2.1.p.5">A response message is considered complete when all of the octets indicated by the message framing (<a href="#Part1" id="rfc.xref.Part1.11"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) are received prior to the connection being closed. If the request is GET, the response status is 200 (OK), and the entire
861               response header block has been received, a cache <em class="bcp14">MAY</em> store an incomplete response message-body if the cache entry is recorded as incomplete. Likewise, a 206 (Partial Content)
862               response <em class="bcp14">MAY</em> be stored as if it were an incomplete 200 (OK) cache entry. However, a cache <em class="bcp14">MUST NOT</em> store incomplete or partial content responses if it does not support the Range and Content-Range header fields or if it does
863               not understand the range units used in those fields.
864            </p>
865            <p id="rfc.section.2.1.p.6">A cache <em class="bcp14">MAY</em> complete a stored incomplete response by making a subsequent range request (<a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>) and combining the successful response with the stored entry, as defined in <a href="#combining.responses" title="Combining Partial Content">Section&nbsp;2.8</a>. A cache <em class="bcp14">MUST NOT</em> use an incomplete response to answer requests unless the response has been made complete or the request is partial and specifies
866               a range that is wholly within the incomplete response. A cache <em class="bcp14">MUST NOT</em> send a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.
867            </p>
868         </div>
869         <div id="constructing.responses.from.caches">
870            <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h2>
871            <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:
872            </p>
873            <ul>
874               <li>The presented effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 4.3</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>) and that of the stored response match, and
875               </li>
876               <li>the request method associated with the stored response allows it to be used for the presented request, and</li>
877               <li>selecting header fields nominated by the stored response (if any) match those presented (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>), and
878               </li>
879               <li>the presented request does not contain the no-cache pragma (<a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;3.4</a>), nor the no-cache cache directive (<a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>), unless the stored response is successfully validated (<a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>), and
880               </li>
881               <li>the stored response does not contain the no-cache cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>), unless it is successfully validated (<a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>), and
882               </li>
883               <li>the stored response is either:
884                  <ul>
885                     <li>fresh (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>), or
886                     </li>
887                     <li>allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>), or
888                     </li>
889                     <li>successfully validated (see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>).
890                     </li>
891                  </ul>
892               </li>
893            </ul>
894            <p id="rfc.section.2.2.p.2">Note that any of the requirements listed above can be overridden by a cache-control extension; see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;3.2.3</a>.
895            </p>
896            <p id="rfc.section.2.2.p.3">When a stored response is used to satisfy a request without validation, a cache <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>.
897            </p>
898            <p id="rfc.section.2.2.p.4">A cache <em class="bcp14">MUST</em> write through requests with methods that are unsafe (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 6.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) to the origin server; i.e., a cache must not generate a reply to such a request before having forwarded the request and
899               having received a corresponding response.
900            </p>
901            <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>.
902            </p>
903            <p id="rfc.section.2.2.p.6">When more than one suitable response is stored, a cache <em class="bcp14">MUST</em> use the most recent response (as determined by the Date header field). It can also forward a request with "Cache-Control:
904               max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use.
905            </p>
906            <p id="rfc.section.2.2.p.7">A cache that does not have a clock available <em class="bcp14">MUST NOT</em> use stored responses without revalidating them on every use. A cache, especially a shared cache, <em class="bcp14">SHOULD</em> use a mechanism, such as NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>, to synchronize its clock with a reliable external standard.
907            </p>
908         </div>
909         <div id="expiration.model">
910            <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a href="#expiration.model">Freshness Model</a></h2>
911            <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,
912               thereby improving efficiency.
913            </p>
914            <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,
915               using either the Expires header field (<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 representation
916               is not likely to change in a semantically significant way before the expiration time is reached.
917            </p>
918            <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
919               to indicate that the response is already stale. Compliant caches will normally validate the cached response before reusing
920               it for subsequent requests (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>).
921            </p>
922            <p id="rfc.section.2.3.p.4">Since origin servers do not always provide explicit expiration times, a cache <em class="bcp14">MAY</em> assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field
923               values (such as the Last-Modified time) to estimate a plausible expiration time. This specification does not provide specific
924               algorithms, but does impose worst-case constraints on their results.
925            </p>
926            <div id="rfc.figure.u.4"></div>
927            <p>The calculation to determine if a response is fresh is:</p><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
928</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>.
929            </p>
930            <p id="rfc.section.2.3.p.7">Additionally, clients can influence freshness calculation -- either constraining it relaxing it -- by using the max-age and
931               min-fresh request cache directives. See <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a> for details.
932            </p>
933            <p id="rfc.section.2.3.p.8">Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload
934               a resource. See <a href="#history.lists" title="History Lists">Section&nbsp;3.7</a> for an explanation of the difference between caches and history mechanisms.
935            </p>
936            <div id="calculating.freshness.lifetime">
937               <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h3>
938               <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>
939               <ul>
940                  <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
941                  </li>
942                  <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
943                  </li>
944                  <li>If the Expires response header field (<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 field, or
945                  </li>
946                  <li>Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;</a>.
947                  </li>
948               </ul>
949               <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>
950               <div id="heuristic.freshness">
951                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></h4>
952                  <p id="rfc.section.">If no explicit expiration time is present in a stored response that has a status code whose definition allows heuristic freshness
953                     to be used (including the following in <a href="" title="Status Code Definitions">Section 7</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>: 200, 203, 206, 300, 301 and 410), a cache <em class="bcp14">MAY</em> calculate a heuristic expiration time. A cache <em class="bcp14">MUST NOT</em> use heuristics to determine freshness for responses with status codes that do not explicitly allow it.
954                  </p>
955                  <p id="rfc.section.">When a heuristic is used to calculate freshness lifetime, a cache <em class="bcp14">SHOULD</em> attach a Warning header field with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning
956                     is not already present.
957                  </p>
958                  <p id="rfc.section.">Also, if the response has a Last-Modified header field (<a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 2.2</a> of <a href="#Part4" id="rfc.xref.Part4.4"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that
959                     time. A typical setting of this fraction might be 10%.
960                  </p>
961                  <div class="note" id="rfc.section.">
962                     <p><b>Note:</b> RFC 2616 (<a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, <a href="">Section 13.9</a>) required that caches do not calculate heuristic freshness for URIs with query components (i.e., those containing '?'). In
963                        practice, this has not been widely implemented. Therefore, servers are encouraged to send explicit directives (e.g., Cache-Control:
964                        no-cache) if they wish to preclude caching.
965                     </p>
966                  </div>
967               </div>
968            </div>
969            <div id="age.calculations">
970               <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a href="#age.calculations">Calculating Age</a></h3>
971               <p id="rfc.section.2.3.2.p.1">HTTP/1.1 uses the Age header field to convey the estimated age of the response message when obtained from a cache. The Age
972                  field value is the cache's estimate of the amount of time since the response was generated or validated by the origin server.
973                  In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path
974                  from the origin server, plus the amount of time it has been in transit along network paths.
975               </p>
976               <p id="rfc.section.2.3.2.p.2">The following data is used for the age calculation:</p>
977               <p id="rfc.section.2.3.2.p.3"><dfn>age_value</dfn>
978               </p>
979               <ul class="empty">
980                  <li>The term "age_value" denotes the value of the Age header field (<a href="#header.age" id="rfc.xref.header.age.2" title="Age">Section&nbsp;3.1</a>), in a form appropriate for arithmetic operation; or 0, if not available.
981                  </li>
982               </ul>
983               <p id="rfc.section.2.3.2.p.4"><dfn>date_value</dfn>
984               </p>
985               <ul class="empty">
986                  <li>HTTP/1.1 requires origin servers to send a Date header field, if possible, with every response, giving the time at which the
987                     response was generated. The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic
988                     operations. See <a href="" title="Date">Section 9.2</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> for the definition of the Date header field, and for requirements regarding responses without it.
989                  </li>
990               </ul>
991               <p id="rfc.section.2.3.2.p.5"><dfn>now</dfn>
992               </p>
993               <ul class="empty">
994                  <li>The term "now" means "the current value of the clock at the host performing the calculation". A cache <em class="bcp14">SHOULD</em> use NTP (<a href="#RFC1305" id="rfc.xref.RFC1305.2"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>) or some similar protocol to synchronize its clocks to a globally accurate time standard.
995                  </li>
996               </ul>
997               <p id="rfc.section.2.3.2.p.6"><dfn>request_time</dfn>
998               </p>
999               <ul class="empty">
1000                  <li>The current value of the clock at the host at the time the request resulting in the stored response was made.</li>
1001               </ul>
1002               <p id="rfc.section.2.3.2.p.7"><dfn>response_time</dfn>
1003               </p>
1004               <ul class="empty">
1005                  <li>The current value of the clock at the host at the time the response was received.</li>
1006               </ul>
1007               <p id="rfc.section.2.3.2.p.8">A response's age can be calculated in two entirely independent ways: </p>
1008               <ol>
1009                  <li>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the origin server's
1010                     clock. If the result is negative, the result is replaced by zero.
1011                  </li>
1012                  <li>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1. A cache <em class="bcp14">MUST</em> interpret this value relative to the time the request was initiated, not the time that the response was received.
1013                  </li>
1014               </ol>
1015               <div id="rfc.figure.u.5"></div><pre class="text">  apparent_age = max(0, response_time - date_value);
1017  response_delay = response_time - request_time;
1018  corrected_age_value = age_value + response_delay; 
1019</pre><div id="rfc.figure.u.6"></div>
1020               <p>These <em class="bcp14">SHOULD</em> be combined as
1021               </p><pre class="text">  corrected_initial_age = max(apparent_age, corrected_age_value);
1022</pre><p id="rfc.section.2.3.2.p.11">unless the cache is confident in the value of the Age header (e.g., because there are no HTTP/1.0 hops in the Via header),
1023                  in which case the corrected_age_value <em class="bcp14">MAY</em> be used as the corrected_initial_age.
1024               </p>
1025               <p id="rfc.section.2.3.2.p.12">The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response
1026                  was last validated by the origin server to the corrected_initial_age.
1027               </p>
1028               <div id="rfc.figure.u.7"></div><pre class="text">  resident_time = now - response_time;
1029  current_age = corrected_initial_age + resident_time;
1030</pre><p id="rfc.section.2.3.2.p.14">Additionally, to avoid common problems in date parsing:</p>
1031               <p id="rfc.section.2.3.2.p.15"></p>
1032               <ul>
1033                  <li>HTTP/1.1 clients and caches <em class="bcp14">SHOULD</em> assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve
1034                     the "year 2000" problem).
1035                  </li>
1036                  <li>Although all date formats are specified to be case-sensitive, recipients <em class="bcp14">SHOULD</em> match day, week and timezone names case-insensitively.
1037                  </li>
1038                  <li>An HTTP/1.1 implementation <em class="bcp14">MAY</em> internally represent a parsed Expires date as earlier than the proper value, but <em class="bcp14">MUST NOT</em> internally represent a parsed Expires date as later than the proper value.
1039                  </li>
1040                  <li>All expiration-related calculations <em class="bcp14">MUST</em> be done in GMT. The local time zone <em class="bcp14">MUST NOT</em> influence the calculation or comparison of an age or expiration time.
1041                  </li>
1042                  <li>If an HTTP header field incorrectly carries a date value with a time zone other than GMT, it <em class="bcp14">MUST</em> be converted into GMT using the most conservative possible conversion.
1043                  </li>
1044               </ul>
1045            </div>
1046            <div id="serving.stale.responses">
1047               <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></h3>
1048               <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, but
1049                  is not fresh according to the calculations in <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>.
1050               </p>
1051               <p id="rfc.section.2.3.3.p.2">A cache <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
1052                  directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
1053                  see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>).
1054               </p>
1055               <p id="rfc.section.2.3.3.p.3">A cache <em class="bcp14">MUST NOT</em> return stale responses unless it is disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
1056                  or doing so is explicitly allowed (e.g., by the max-stale request directive; see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>).
1057               </p>
1058               <p id="rfc.section.2.3.3.p.4">A cache <em class="bcp14">SHOULD</em> append a Warning header field with the 110 warn-code (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;3.6</a>) to stale responses. Likewise, a cache <em class="bcp14">SHOULD</em> add the 112 warn-code to stale responses if the cache is disconnected.
1059               </p>
1060               <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
1061                  forward to the requesting client, and the received response is no longer fresh, the cache can forward it to the requesting
1062                  client without adding a new Warning (but without removing any existing Warning header fields). A cache shouldn't attempt to
1063                  validate a response simply because that response became stale in transit.
1064               </p>
1065            </div>
1066         </div>
1067         <div id="validation.model">
1068            <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a href="#validation.model">Validation Model</a></h2>
1069            <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
1070               fresh, or one cannot be selected; see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>), it can use the conditional request mechanism <a href="#Part4" id="rfc.xref.Part4.5"><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
1071               update it. This process is known as "validating" or "revalidating" the stored response.
1072            </p>
1073            <p id="rfc.section.2.4.p.2">When sending such a conditional request, a cache adds an If-Modified-Since header field whose value is that of the Last-Modified
1074               header field from the selected (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>) stored response, if available.
1075            </p>
1076            <p id="rfc.section.2.4.p.3">Additionally, a cache can add an If-None-Match header field whose value is that of the ETag header field(s) from all responses
1077               stored for the requested URI, if present. However, if any of the stored responses contains only partial content, the cache
1078               shouldn't include its entity-tag in the If-None-Match header field unless the request is for a range that would be fully satisfied
1079               by that stored response.
1080            </p>
1081            <p id="rfc.section.2.4.p.4">Cache handling of a response to a conditional request is dependent upon its status code:</p>
1082            <p id="rfc.section.2.4.p.5"></p>
1083            <ul>
1084               <li>A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see <a href="#freshening.responses" title="Freshening Responses">Section&nbsp;2.4.1</a>.
1085               </li>
1086               <li>A full response (i.e., one with a response body) indicates that none of the stored responses nominated in the conditional
1087                  request is suitable. Instead, the cache can use the full response to satisfy the request and <em class="bcp14">MAY</em> replace the stored response(s).
1088               </li>
1089               <li>However, if a cache receives a 5xx response while attempting to validate a response, it can either forward this response to
1090                  the requesting client, or act as if the server failed to respond. In the latter case, it can return a previously stored response
1091                  (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>).
1092               </li>
1093            </ul>
1094            <div id="freshening.responses">
1095               <h3 id="rfc.section.2.4.1"><a href="#rfc.section.2.4.1">2.4.1</a>&nbsp;<a href="#freshening.responses">Freshening Responses</a></h3>
1096               <p id="rfc.section.2.4.1.p.1">When a cache receives a 304 (Not Modified) response and already has one or more stored 200 (OK) responses for the same cache
1097                  key, the cache needs to identify which of the stored responses are updated by this new response and then update the stored
1098                  response(s) with the new information provided in the 304 response.
1099               </p>
1100               <ul>
1101                  <li>If the new response contains a strong validator, then that strong validator identifies the selected representation. All of
1102                     the stored responses with the same strong validator are selected. If none of the stored responses contain the same strong
1103                     validator, then this new response corresponds to a new selected representation and <em class="bcp14">MUST NOT</em> update the existing stored responses.
1104                  </li>
1105                  <li>If the new response contains a weak validator and that validator corresponds to one of the cache's stored responses, then
1106                     the most recent of those matching stored responses is selected.
1107                  </li>
1108                  <li>If the new response does not include any form of validator, there is only one stored response, and that stored response also
1109                     lacks a validator, then that stored response is selected.
1110                  </li>
1111               </ul>
1112               <p id="rfc.section.2.4.1.p.2">If a stored response is selected for update, the cache <em class="bcp14">MUST</em>:
1113               </p>
1114               <ul>
1115                  <li>delete any Warning header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;3.6</a>);
1116                  </li>
1117                  <li>retain any Warning header fields in the stored response with warn-code 2xx; and,</li>
1118                  <li>use other header fields provided in the 304 response to replace all instances of the corresponding header fields in the stored
1119                     response.
1120                  </li>
1121               </ul>
1122            </div>
1123         </div>
1124         <div id="invalidation.after.updates.or.deletions">
1125            <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></h2>
1126            <p id="rfc.section.2.5.p.1">Because unsafe request methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 6.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.6"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) such as PUT, POST or DELETE have the potential for changing state on the origin server, intervening caches can use them
1127               to keep their contents up-to-date.
1128            </p>
1129            <p id="rfc.section.2.5.p.2">A cache <em class="bcp14">MUST</em> invalidate the effective Request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 4.3</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>) as well as the URI(s) in the Location and Content-Location header fields (if present) when a non-error response to a request
1130               with an unsafe method is received.
1131            </p>
1132            <p id="rfc.section.2.5.p.3">However, a cache <em class="bcp14">MUST NOT</em> invalidate a URI from a Location or Content-Location header field if the host part of that URI differs from the host part
1133               in the effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.14"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). This helps prevent denial of service attacks.
1134            </p>
1135            <p id="rfc.section.2.5.p.4">A cache <em class="bcp14">MUST</em> invalidate the effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.15"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) when it receives a non-error response to a request with a method whose safety is unknown.
1136            </p>
1137            <p id="rfc.section.2.5.p.5">Here, a "non-error response" is one with a 2xx or 3xx status code. "Invalidate" means that the cache will either remove all
1138               stored responses related to the effective request URI, or will mark these as "invalid" and in need of a mandatory validation
1139               before they can be returned in response to a subsequent request.
1140            </p>
1141            <p id="rfc.section.2.5.p.6">Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the
1142               change at the origin server might not have gone through the cache where a response is stored.
1143            </p>
1144         </div>
1145         <div id="caching.authenticated.responses">
1146            <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a href="#caching.authenticated.responses">Shared Caching of Authenticated Responses</a></h2>
1147            <p id="rfc.section.2.6.p.1">A shared cache <em class="bcp14">MUST NOT</em> use a cached response to a request with an Authorization header field (<a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</a> of <a href="#Part7" id="rfc.xref.Part7.2"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.
1148            </p>
1149            <p id="rfc.section.2.6.p.2">In this specification, the following Cache-Control response directives (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) have such an effect: must-revalidate, public, s-maxage.
1150            </p>
1151            <p id="rfc.section.2.6.p.3">Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be
1152               served stale (<a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy
1153               a subsequent request without revalidating it on the origin server.
1154            </p>
1155         </div>
1156         <div id="caching.negotiated.responses">
1157            <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></h2>
1158            <p id="rfc.section.2.7.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 header fields nominated by the Vary header field match in both the original
1159               request (i.e., that associated with the stored response), and the presented request.
1160            </p>
1161            <p id="rfc.section.2.7.p.2">The selecting header fields from two requests are defined to match if and only if those in the first request can be transformed
1162               to those in the second request by applying any of the following:
1163            </p>
1164            <ul>
1165               <li>adding or removing whitespace, where allowed in the header field's syntax</li>
1166               <li>combining multiple 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.Part1.16"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>)
1167               </li>
1168               <li>normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification
1169                  (e.g., re-ordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive)
1170               </li>
1171            </ul>
1172            <p id="rfc.section.2.7.p.3">If (after any normalization that might take place) a header field is absent from a request, it can only match another request
1173               if it is also absent there.
1174            </p>
1175            <p id="rfc.section.2.7.p.4">A Vary header field-value of "*" always fails to match, and subsequent requests to that resource can only be properly interpreted
1176               by the origin server.
1177            </p>
1178            <p id="rfc.section.2.7.p.5">The stored response with matching selecting header fields is known as the selected response.</p>
1179            <p id="rfc.section.2.7.p.6">If multiple selected responses are available, the most recent response (as determined by the Date header field) is used; see <a href="#constructing.responses.from.caches" title="Constructing Responses from Caches">Section&nbsp;2.2</a>.
1180            </p>
1181            <p id="rfc.section.2.7.p.7">If no selected response is available, the cache can forward the presented request to the origin server in a conditional request;
1182               see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>.
1183            </p>
1184         </div>
1185         <div id="combining.responses">
1186            <h2 id="rfc.section.2.8"><a href="#rfc.section.2.8">2.8</a>&nbsp;<a href="#combining.responses">Combining Partial Content</a></h2>
1187            <p id="rfc.section.2.8.p.1">A response might transfer only a partial representation if the connection closed prematurely or if the request used one or
1188               more Range specifiers (<a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>). After several such transfers, a cache might have received several ranges of the same representation. A cache <em class="bcp14">MAY</em> combine these ranges into a single stored response, and reuse that response to satisfy later requests, if they all share the
1189               same strong validator and the cache complies with the client requirements in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>.
1190            </p>
1191            <p id="rfc.section.2.8.p.2">When combining the new response with one or more stored responses, a cache <em class="bcp14">MUST</em>:
1192            </p>
1193            <ul>
1194               <li>delete any Warning header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;3.6</a>);
1195               </li>
1196               <li>retain any Warning header fields in the stored response with warn-code 2xx; and,</li>
1197               <li>use other header fields provided in the new response, aside from Content-Range, to replace all instances of the corresponding
1198                  header fields in the stored response.
1199               </li>
1200            </ul>
1201         </div>
1202      </div>
1203      <div id="header.field.definitions">
1204         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#header.field.definitions">Header Field Definitions</a></h1>
1205         <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
1206         <div id="header.age">
1207            <div id="rfc.iref.a.2"></div>
1208            <div id="rfc.iref.h.2"></div>
1209            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#header.age">Age</a></h2>
1210            <p id="rfc.section.3.1.p.1">The "Age" header field conveys the sender's estimate of the amount of time since the response was generated or successfully
1211               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>.
1212            </p>
1213            <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.2"></span>  <a href="#header.age" class="smpl">Age</a> = <a href="#delta-seconds" class="smpl">delta-seconds</a>
1214</pre><p id="rfc.section.3.1.p.3">Age field-values are non-negative integers, representing time in seconds (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.5</a>).
1215            </p>
1216            <p id="rfc.section.3.1.p.4">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
1217               true, since HTTP/1.0 caches might not implement the Age header field.
1218            </p>
1219         </div>
1220         <div id="header.cache-control">
1221            <div id="rfc.iref.c.6"></div>
1222            <div id="rfc.iref.h.3"></div>
1223            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#header.cache-control">Cache-Control</a></h2>
1224            <p id="rfc.section.3.2.p.1">The "Cache-Control" header field is used to specify directives for caches along the request/response chain. Such cache directives
1225               are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given
1226               in the response.
1227            </p>
1228            <p id="rfc.section.3.2.p.2">A cache <em class="bcp14">MUST</em> obey the requirements of the Cache-Control directives defined in this section. See <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;3.2.3</a> for information about how Cache-Control directives defined elsewhere are handled.
1229            </p>
1230            <div class="note" id="rfc.section.3.2.p.3">
1231               <p><b>Note:</b> 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>).
1232               </p>
1233            </div>
1234            <p id="rfc.section.3.2.p.4">A proxy, whether or not it implements a cache, <em class="bcp14">MUST</em> pass cache directives through in forwarded messages, regardless of their significance to that application, since the directives
1235               might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific
1236               cache.
1237            </p>
1238            <p id="rfc.section.3.2.p.5">Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.</p>
1239            <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span>  <a href="#header.cache-control" class="smpl">Cache-Control</a>   = 1#<a href="#header.cache-control" class="smpl">cache-directive</a>
1241  <a href="#header.cache-control" class="smpl">cache-directive</a> = <a href="#header.cache-control" class="smpl">cache-request-directive</a>
1242     / <a href="#header.cache-control" class="smpl">cache-response-directive</a>
1244  <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> ) ]
1245</pre><div id="cache-request-directive">
1246               <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></h3>
1247               <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.5"></span>  <a href="#header.cache-control" class="smpl">cache-request-directive</a> =
1248       "no-cache"
1249     / "no-store"
1250     / "max-age" "=" <a href="#delta-seconds" class="smpl">delta-seconds</a>
1251     / "max-stale" [ "=" <a href="#delta-seconds" class="smpl">delta-seconds</a> ]
1252     / "min-fresh" "=" <a href="#delta-seconds" class="smpl">delta-seconds</a>
1253     / "no-transform"
1254     / "only-if-cached"
1255     / <a href="#header.cache-control" class="smpl">cache-extension</a>
1256</pre><p id="rfc.section.3.2.1.p.2"><dfn>no-cache</dfn> <span id="rfc.iref.c.7"></span> <span id="rfc.iref.n.1"></span>
1257               </p>
1258               <ul class="empty">
1259                  <li>The no-cache request directive indicates that a cache <em class="bcp14">MUST NOT</em> use a stored response to satisfy the request without successful validation on the origin server.
1260                  </li>
1261               </ul>
1262               <p id="rfc.section.3.2.1.p.3"><dfn>no-store</dfn> <span id="rfc.iref.c.8"></span> <span id="rfc.iref.n.2"></span>
1263               </p>
1264               <ul class="empty">
1265                  <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 private 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.
1266                  </li>
1267                  <li>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1268                     might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
1269                  </li>
1270                  <li>Note that if a request containing this directive is satisfied from a cache, the no-store request directive does not apply
1271                     to the already stored response.
1272                  </li>
1273               </ul>
1274               <p id="rfc.section.3.2.1.p.4"><dfn>max-age</dfn> <span id="rfc.iref.c.9"></span> <span id="rfc.iref.m.1"></span>
1275               </p>
1276               <ul class="empty">
1277                  <li>The max-age request directive indicates that the client is unwilling to accept a response whose age is greater than the specified
1278                     number of seconds. Unless the max-stale request directive is also present, the client is not willing to accept a stale response.
1279                  </li>
1280               </ul>
1281               <p id="rfc.section.3.2.1.p.5"><dfn>max-stale</dfn> <span id="rfc.iref.c.10"></span> <span id="rfc.iref.m.2"></span>
1282               </p>
1283               <ul class="empty">
1284                  <li>The max-stale request directive indicates that the client is willing to accept a response that has exceeded its expiration
1285                     time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time
1286                     by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept
1287                     a stale response of any age.
1288                  </li>
1289               </ul>
1290               <p id="rfc.section.3.2.1.p.6"><dfn>min-fresh</dfn> <span id="rfc.iref.c.11"></span> <span id="rfc.iref.m.3"></span>
1291               </p>
1292               <ul class="empty">
1293                  <li>The min-fresh request directive indicates that the client is willing to accept a response whose freshness lifetime is no less
1294                     than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for
1295                     at least the specified number of seconds.
1296                  </li>
1297               </ul>
1298               <p id="rfc.section.3.2.1.p.7"><dfn>no-transform</dfn> <span id="rfc.iref.c.12"></span> <span id="rfc.iref.n.3"></span>
1299               </p>
1300               <ul class="empty">
1301                  <li>The no-transform request directive indicates that an intermediary (whether or not it implements a cache) <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type request header fields, nor the request representation.
1302                  </li>
1303               </ul>
1304               <p id="rfc.section.3.2.1.p.8"><dfn>only-if-cached</dfn> <span id="rfc.iref.c.13"></span> <span id="rfc.iref.o.1"></span>
1305               </p>
1306               <ul class="empty">
1307                  <li>The only-if-cached request directive indicates that the client only wishes to obtain a stored response. If it receives this
1308                     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
1309                     (Gateway Timeout) status code. If a group of caches is being operated as a unified system with good internal connectivity,
1310                     a member cache <em class="bcp14">MAY</em> forward such a request within that group of caches.
1311                  </li>
1312               </ul>
1313            </div>
1314            <div id="cache-response-directive">
1315               <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></h3>
1316               <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.6"></span>  <a href="#header.cache-control" class="smpl">cache-response-directive</a> =
1317       "public"
1318     / "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> ]
1319     / "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> ]
1320     / "no-store"
1321     / "no-transform"
1322     / "must-revalidate"
1323     / "proxy-revalidate"
1324     / "max-age" "=" <a href="#delta-seconds" class="smpl">delta-seconds</a>
1325     / "s-maxage" "=" <a href="#delta-seconds" class="smpl">delta-seconds</a>
1326     / <a href="#header.cache-control" class="smpl">cache-extension</a>
1327</pre><p id="rfc.section.3.2.2.p.2"><dfn>public</dfn> <span id="rfc.iref.c.14"></span> <span id="rfc.iref.p.2"></span>
1328               </p>
1329               <ul class="empty">
1330                  <li>The public response directive indicates that a response whose associated request contains an 'Authentication' header <em class="bcp14">MAY</em> be stored (see <a href="#caching.authenticated.responses" title="Shared Caching of Authenticated Responses">Section&nbsp;2.6</a>).
1331                  </li>
1332               </ul>
1333               <p id="rfc.section.3.2.2.p.3"><dfn>private</dfn> <span id="rfc.iref.c.15"></span> <span id="rfc.iref.p.3"></span>
1334               </p>
1335               <ul class="empty">
1336                  <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 cache <em class="bcp14">MAY</em> store the response.
1337                  </li>
1338                  <li>If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated
1339                     with the listed response header fields. That is, a shared cache <em class="bcp14">MUST NOT</em> store the specified field-names(s), whereas it <em class="bcp14">MAY</em> store the remainder of the response message.
1340                  </li>
1341                  <li><b>Note:</b> This usage of the word private only controls where the response can be stored; it cannot ensure the privacy of the message
1342                     content. Also, private response directives with field-names are often handled by implementations as if an unqualified private
1343                     directive was received; i.e., the special handling for the qualified form is not widely implemented.
1344                  </li>
1345               </ul>
1346               <p id="rfc.section.3.2.2.p.4"><dfn>no-cache</dfn> <span id="rfc.iref.c.16"></span> <span id="rfc.iref.n.4"></span>
1347               </p>
1348               <ul class="empty">
1349                  <li>The no-cache response directive indicates that the response MUST NOT be used to satisfy a subsequent request without successful
1350                     validation on the origin server. This allows an origin server to prevent a cache from using it to satisfy a request without
1351                     contacting it, even by caches that have been configured to return stale responses.
1352                  </li>
1353                  <li>If the no-cache response directive specifies one or more field-names, this requirement is limited to the field-values associated
1354                     with the listed response header fields. That is, a cache <em class="bcp14">MUST NOT</em> send the specified field-name(s) in the response to a subsequent request without successful validation on the origin server.
1355                     This allows an origin server to prevent the re-use of certain header fields in a response, while still allowing caching of
1356                     the rest of the response.
1357                  </li>
1358                  <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
1359                     handled by implementations as if an unqualified no-cache directive was received; i.e., the special handling for the qualified
1360                     form is not widely implemented.
1361                  </li>
1362               </ul>
1363               <p id="rfc.section.3.2.2.p.5"><dfn>no-store</dfn> <span id="rfc.iref.c.17"></span> <span id="rfc.iref.n.5"></span>
1364               </p>
1365               <ul class="empty">
1366                  <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 private 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.
1367                  </li>
1368                  <li>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1369                     might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
1370                  </li>
1371               </ul>
1372               <p id="rfc.section.3.2.2.p.6"><dfn>must-revalidate</dfn> <span id="rfc.iref.c.18"></span> <span id="rfc.iref.m.4"></span>
1373               </p>
1374               <ul class="empty">
1375                  <li>The must-revalidate response directive indicates that once it has become stale, a cache <em class="bcp14">MUST NOT</em> use the response to satisfy subsequent requests without successful validation on the origin server.
1376                  </li>
1377                  <li>The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
1378                     a cache <em class="bcp14">MUST</em> obey the must-revalidate directive; in particular, if a cache cannot reach the origin server for any reason, it <em class="bcp14">MUST</em> generate a 504 (Gateway Timeout) response.
1379                  </li>
1380                  <li>The must-revalidate directive ought to be used by servers if and only if failure to validate a request on the representation
1381                     could result in incorrect operation, such as a silently unexecuted financial transaction.
1382                  </li>
1383               </ul>
1384               <p id="rfc.section.3.2.2.p.7"><dfn>proxy-revalidate</dfn> <span id="rfc.iref.c.19"></span> <span id="rfc.iref.p.4"></span>
1385               </p>
1386               <ul class="empty">
1387                  <li>The proxy-revalidate response directive has the same meaning as the must-revalidate response directive, except that it does
1388                     not apply to private caches.
1389                  </li>
1390               </ul>
1391               <p id="rfc.section.3.2.2.p.8"><dfn>max-age</dfn> <span id="rfc.iref.c.20"></span> <span id="rfc.iref.m.5"></span>
1392               </p>
1393               <ul class="empty">
1394                  <li>The max-age response directive indicates that the response is to be considered stale after its age is greater than the specified
1395                     number of seconds.
1396                  </li>
1397               </ul>
1398               <p id="rfc.section.3.2.2.p.9"><dfn>s-maxage</dfn> <span id="rfc.iref.c.21"></span> <span id="rfc.iref.s.4"></span>
1399               </p>
1400               <ul class="empty">
1401                  <li>The s-maxage response directive indicates that, in shared caches, the maximum age specified by this directive overrides the
1402                     maximum age specified by either the max-age directive or the Expires header field. The s-maxage directive also implies the
1403                     semantics of the proxy-revalidate response directive.
1404                  </li>
1405               </ul>
1406               <p id="rfc.section.3.2.2.p.10"><dfn>no-transform</dfn> <span id="rfc.iref.c.22"></span> <span id="rfc.iref.n.6"></span>
1407               </p>
1408               <ul class="empty">
1409                  <li>The no-transform response directive indicates that an intermediary (regardless of whether it implements a cache) <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type response header fields, nor the response representation.
1410                  </li>
1411               </ul>
1412            </div>
1413            <div id="cache.control.extensions">
1414               <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></h3>
1415               <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
1416                  value. Informational extensions (those that do not require a change in cache behavior) can be added without changing the semantics
1417                  of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives.
1418                  Both the new directive and the standard directive are supplied, such that applications that do not understand the new directive
1419                  will default to the behavior specified by the standard directive, and those that understand the new directive will recognize
1420                  it as modifying the requirements associated with the standard directive. In this way, extensions to the cache-control directives
1421                  can be made without requiring changes to the base protocol.
1422               </p>
1423               <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,
1424                  obeying certain extensions, and ignoring all directives that it does not understand.
1425               </p>
1426               <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.
1427                  We define this new directive to mean that, in addition to any private cache, any cache that is shared only by members of the
1428                  community named within its value may cache the response. An origin server wishing to allow the UCI community to use an otherwise
1429                  private response in their shared cache(s) could do so by including
1430               </p>
1431               <div id="rfc.figure.u.12"></div><pre class="text">  Cache-Control: private, community="UCI"
1432</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
1433                  it will also see and understand the private directive and thus default to the safe behavior.
1434               </p>
1435               <p id="rfc.section.3.2.3.p.6">A cache <em class="bcp14">MUST</em> ignore unrecognized cache directives; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache
1436                  will be combined with standard directives (or the response's default cacheability) such that the cache behavior will remain
1437                  minimally correct even if the cache does not understand the extension(s).
1438               </p>
1439               <p id="rfc.section.3.2.3.p.7">The HTTP Cache Directive Registry defines the name space for the cache directives.</p>
1440               <p id="rfc.section.3.2.3.p.8">A registration <em class="bcp14">MUST</em> include the following fields:
1441               </p>
1442               <ul>
1443                  <li>Cache Directive Name</li>
1444                  <li>Pointer to specification text</li>
1445               </ul>
1446               <p id="rfc.section.3.2.3.p.9">Values to be added to this name space are subject to IETF review (<a href="#RFC5226" id="rfc.xref.RFC5226.1"><cite title="Guidelines for Writing an IANA Considerations Section in RFCs">[RFC5226]</cite></a>, <a href="">Section 4.1</a>).
1447               </p>
1448               <p id="rfc.section.3.2.3.p.10">The registry itself is maintained at &lt;<a href=""></a>&gt;.
1449               </p>
1450            </div>
1451         </div>
1452         <div id="header.expires">
1453            <div id="rfc.iref.e.2"></div>
1454            <div id="rfc.iref.h.4"></div>
1455            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#header.expires">Expires</a></h2>
1456            <p id="rfc.section.3.3.p.1">The "Expires" 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.
1457            </p>
1458            <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
1459               that time.
1460            </p>
1461            <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="" title="Date/Time Formats">Section 8</a> of <a href="#Part2" id="rfc.xref.Part2.7"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>; a sender <em class="bcp14">MUST</em> use the rfc1123-date format.
1462            </p>
1463            <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#header.expires" class="smpl">Expires</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
1464</pre><div id="rfc.figure.u.14"></div>
1465            <p>For example</p><pre class="text">  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1466</pre><p id="rfc.section.3.3.p.6">A cache <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
1467            </p>
1468            <div class="note" id="rfc.section.3.3.p.7">
1469               <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.
1470               </p>
1471            </div>
1472            <p id="rfc.section.3.3.p.8">Historically, HTTP required the Expires field-value to be no more than a year in the future. While longer freshness lifetimes
1473               are no longer prohibited, extremely large values have been demonstrated to cause problems (e.g., clock overflows due to use
1474               of 32-bit integers for time values), and most caches will evict a response far sooner than that. Therefore, senders ought
1475               not produce them.
1476            </p>
1477            <p id="rfc.section.3.3.p.9">An origin server without a clock <em class="bcp14">MUST NOT</em> assign Expires values to a response unless these values were associated with the resource by a system or user with a reliable
1478               clock. It <em class="bcp14">MAY</em> assign an Expires value that is known, at or before server configuration time, to be in the past (this allows "pre-expiration"
1479               of responses without storing separate Expires values for each resource).
1480            </p>
1481         </div>
1482         <div id="header.pragma">
1483            <div id="rfc.iref.p.5"></div>
1484            <div id="rfc.iref.h.5"></div>
1485            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a href="#header.pragma">Pragma</a></h2>
1486            <p id="rfc.section.3.4.p.1">The "Pragma" header field allows backwards compatibility with HTTP/1.0 caches, so that clients can specify a "no-cache" request
1487               that they will understand (as Cache-Control was not defined until HTTP/1.1). When the Cache-Control header is also present
1488               and understood in a request, Pragma is ignored.
1489            </p>
1490            <p id="rfc.section.3.4.p.2">In HTTP/1.0, Pragma was defined as an extensible field for implementation-specified directives for recipients. This specification
1491               deprecates such extensions to improve interoperability.
1492            </p>
1493            <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.8"></span><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span>  <a href="#header.pragma" class="smpl">Pragma</a>           = 1#<a href="#header.pragma" class="smpl">pragma-directive</a>
1494  <a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / <a href="#header.pragma" class="smpl">extension-pragma</a>
1495  <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> ) ]
1496</pre><p id="rfc.section.3.4.p.4">When the Cache-Control header is not present in a request, the no-cache request pragma-directive <em class="bcp14">MUST</em> have the same effect on caches as if "Cache-Control: no-cache" were present (see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>).
1497            </p>
1498            <p id="rfc.section.3.4.p.5">When sending a no-cache request, a client ought to include both the pragma and cache-control directives, unless Cache-Control:
1499               no-cache is purposefully omitted to target other Cache-Control response directives at HTTP/1.1 caches. For example:
1500            </p>
1501            <div id="rfc.figure.u.16"></div><pre class="text">GET / HTTP/1.1
1503Cache-Control: max-age=30
1504Pragma: no-cache
1506</pre><p id="rfc.section.3.4.p.7">will constrain HTTP/1.1 caches to serve a response no older than 30 seconds, while precluding implementations that do not
1507               understand Cache-Control from serving a cached response.
1508            </p>
1509            <div class="note" id="rfc.section.3.4.p.8">
1510               <p><b>Note:</b> Because the meaning of "Pragma: no-cache" in responses is not specified, it does not provide a reliable replacement for "Cache-Control:
1511                  no-cache" in them.
1512               </p>
1513            </div>
1514         </div>
1515         <div id="header.vary">
1516            <div id="rfc.iref.v.3"></div>
1517            <div id="rfc.iref.h.6"></div>
1518            <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a href="#header.vary">Vary</a></h2>
1519            <p id="rfc.section.3.5.p.1">The "Vary" header field conveys the set of header fields that were used to select the representation.</p>
1520            <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;2.7</a>. determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request
1521               without validation; see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>.
1522            </p>
1523            <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
1524               the representation.
1525            </p>
1526            <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.11"></span>  <a href="#header.vary" class="smpl">Vary</a> = "*" / 1#<a href="#abnf.dependencies" class="smpl">field-name</a>
1527</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 header fields.</p>
1528            <p id="rfc.section.3.5.p.6">A server <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
1529               to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that
1530               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
1531               the user agent with useful information about the dimensions over which the response varies at the time of the response.
1532            </p>
1533            <p id="rfc.section.3.5.p.7">A Vary field value of "*" signals that unspecified parameters not limited to the header fields (e.g., the network address
1534               of the client), play a role in the selection of the response representation; therefore, a cache cannot determine whether this
1535               response is appropriate. A proxy <em class="bcp14">MUST NOT</em> generate the "*" value.
1536            </p>
1537            <p id="rfc.section.3.5.p.8">The field-names given are not limited to the set of standard header fields defined by this specification. Field names are
1538               case-insensitive.
1539            </p>
1540         </div>
1541         <div id="header.warning">
1542            <div id="rfc.iref.w.1"></div>
1543            <div id="rfc.iref.h.7"></div>
1544            <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a href="#header.warning">Warning</a></h2>
1545            <p id="rfc.section.3.6.p.1">The "Warning" header field is used to carry additional information about the status or transformation of a message that might
1546               not be reflected in the message. This information is typically used to warn about possible incorrectness introduced by caching
1547               operations or transformations applied to the payload of the message.
1548            </p>
1549            <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
1550               code, distinguishes these responses from true failures.
1551            </p>
1552            <p id="rfc.section.3.6.p.3">Warning header fields can in general be applied to any message, however some warn-codes are specific to caches and can only
1553               be applied to response messages.
1554            </p>
1555            <div id="rfc.figure.u.18"></div><pre class="inline"><span id="rfc.iref.g.12"></span><span id="rfc.iref.g.13"></span><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span>  <a href="#header.warning" class="smpl">Warning</a>       = 1#<a href="#header.warning" class="smpl">warning-value</a>
1557  <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>
1558                                        [<a href="#notation" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-date</a>]
1560  <a href="#header.warning" class="smpl">warn-code</a>  = 3<a href="#notation" class="smpl">DIGIT</a>
1561  <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>
1562                  ; the name or pseudonym of the server adding
1563                  ; the Warning header field, for use in debugging
1564  <a href="#header.warning" class="smpl">warn-text</a>  = <a href="#core.rules" class="smpl">quoted-string</a>
1565  <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>
1566</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
1567               the same code number, only differing in warn-text.
1568            </p>
1569            <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.
1570            </p>
1571            <p id="rfc.section.3.6.p.7">Systems that generate multiple Warning header fields are encouraged to order them with this user agent behavior in mind. New
1572               Warning header fields are added after any existing Warning headers fields.
1573            </p>
1574            <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
1575               a stored response after validation:
1576            </p>
1577            <ul>
1578               <li>1xx Warnings describe the freshness or validation status of the response, and so <em class="bcp14">MUST</em> be deleted by a cache 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.
1579               </li>
1580               <li>2xx Warnings describe some aspect of the representation that is not rectified by a validation (for example, a lossy compression
1581                  of the representation) and <em class="bcp14">MUST NOT</em> be deleted by a cache after validation, unless a full response is returned, in which case they <em class="bcp14">MUST</em> be.
1582               </li>
1583            </ul>
1584            <p id="rfc.section.3.6.p.9">If an implementation sends a message with one or more Warning header fields to a receiver whose version is HTTP/1.0 or lower,
1585               then the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the Date header field in the message.
1586            </p>
1587            <p id="rfc.section.3.6.p.10">If a system receives a message with a warning-value that includes a warn-date, and that warn-date is different from the Date
1588               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
1589               header fields.) If all of the warning-values are deleted for this reason, the Warning header field <em class="bcp14">MUST</em> be deleted as well.
1590            </p>
1591            <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
1592               of its meaning.
1593            </p>
1594            <div id="warn.110">
1595               <div id="rfc.iref.1.1"></div>
1596               <div id="rfc.iref.w.2"></div>
1597               <h3 id="rfc.section.3.6.1"><a href="#rfc.section.3.6.1">3.6.1</a>&nbsp;<a href="#warn.110">110 Response is Stale</a></h3>
1598               <p id="rfc.section.3.6.1.p.1">A cache <em class="bcp14">SHOULD</em> include this whenever the returned response is stale.
1599               </p>
1600            </div>
1601            <div id="warn.111">
1602               <div id="rfc.iref.1.2"></div>
1603               <div id="rfc.iref.w.3"></div>
1604               <h3 id="rfc.section.3.6.2"><a href="#rfc.section.3.6.2">3.6.2</a>&nbsp;<a href="#warn.111">111 Revalidation Failed</a></h3>
1605               <p id="rfc.section.3.6.2.p.1">A cache <em class="bcp14">SHOULD</em> include this when returning a stale response because an attempt to validate the response failed, due to an inability to reach
1606                  the server.
1607               </p>
1608            </div>
1609            <div id="warn.112">
1610               <div id="rfc.iref.1.3"></div>
1611               <div id="rfc.iref.w.4"></div>
1612               <h3 id="rfc.section.3.6.3"><a href="#rfc.section.3.6.3">3.6.3</a>&nbsp;<a href="#warn.112">112 Disconnected Operation</a></h3>
1613               <p id="rfc.section.3.6.3.p.1">A cache <em class="bcp14">SHOULD</em> include this if it is intentionally disconnected from the rest of the network for a period of time.
1614               </p>
1615            </div>
1616            <div id="warn.113">
1617               <div id="rfc.iref.1.4"></div>
1618               <div id="rfc.iref.w.5"></div>
1619               <h3 id="rfc.section.3.6.4"><a href="#rfc.section.3.6.4">3.6.4</a>&nbsp;<a href="#warn.113">113 Heuristic Expiration</a></h3>
1620               <p id="rfc.section.3.6.4.p.1">A cache <em class="bcp14">SHOULD</em> include this if it heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24
1621                  hours.
1622               </p>
1623            </div>
1624            <div id="warn.199">
1625               <div id="rfc.iref.1.5"></div>
1626               <div id="rfc.iref.w.6"></div>
1627               <h3 id="rfc.section.3.6.5"><a href="#rfc.section.3.6.5">3.6.5</a>&nbsp;<a href="#warn.199">199 Miscellaneous Warning</a></h3>
1628               <p id="rfc.section.3.6.5.p.1">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.
1629               </p>
1630            </div>
1631            <div id="warn.214">
1632               <div id="rfc.iref.2.1"></div>
1633               <div id="rfc.iref.w.7"></div>
1634               <h3 id="rfc.section.3.6.6"><a href="#rfc.section.3.6.6">3.6.6</a>&nbsp;<a href="#warn.214">214 Transformation Applied</a></h3>
1635               <p id="rfc.section.3.6.6.p.1"><em class="bcp14">MUST</em> be added by a proxy if it applies any transformation to the representation, such as changing the content-coding, media-type,
1636                  or modifying the representation data, unless this Warning code already appears in the response.
1637               </p>
1638            </div>
1639            <div id="warn.299">
1640               <div id="rfc.iref.2.2"></div>
1641               <div id="rfc.iref.w.8"></div>
1642               <h3 id="rfc.section.3.6.7"><a href="#rfc.section.3.6.7">3.6.7</a>&nbsp;<a href="#warn.299">299 Miscellaneous Persistent Warning</a></h3>
1643               <p id="rfc.section.3.6.7.p.1">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.
1644               </p>
1645            </div>
1646            <div id="warn.code.extensions">
1647               <h3 id="rfc.section.3.6.8"><a href="#rfc.section.3.6.8">3.6.8</a>&nbsp;<a href="#warn.code.extensions">Warn Code Extensions</a></h3>
1648               <p id="rfc.section.3.6.8.p.1">The HTTP Warn Code Registry defines the name space for warn codes.</p>
1649               <p id="rfc.section.3.6.8.p.2">A registration <em class="bcp14">MUST</em> include the following fields:
1650               </p>
1651               <ul>
1652                  <li>Warn Code (3 digits)</li>
1653                  <li>Short Description</li>
1654                  <li>Pointer to specification text</li>
1655               </ul>
1656               <p id="rfc.section.3.6.8.p.3">Values to be added to this name space are subject to IETF review (<a href="#RFC5226" id="rfc.xref.RFC5226.2"><cite title="Guidelines for Writing an IANA Considerations Section in RFCs">[RFC5226]</cite></a>, <a href="">Section 4.1</a>).
1657               </p>
1658               <p id="rfc.section.3.6.8.p.4">The registry itself is maintained at &lt;<a href=""></a>&gt;.
1659               </p>
1660            </div>
1661         </div>
1662         <div id="history.lists">
1663            <h2 id="rfc.section.3.7"><a href="#rfc.section.3.7">3.7</a>&nbsp;<a href="#history.lists">History Lists</a></h2>
1664            <p id="rfc.section.3.7.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, that can be used to redisplay a representation
1665               retrieved earlier in a session.
1666            </p>
1667            <p id="rfc.section.3.7.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
1668               it has expired.
1669            </p>
1670            <p id="rfc.section.3.7.p.3">This does not prohibit the history mechanism from telling the user that a view might be stale, or from honoring cache directives
1671               (e.g., Cache-Control: no-store).
1672            </p>
1673         </div>
1674         <div id="IANA.considerations">
1675            <h2 id="rfc.section.3.8"><a href="#rfc.section.3.8">3.8</a>&nbsp;<a href="#IANA.considerations">IANA Considerations</a></h2>
1676            <div id="cache.directive.registration">
1677               <h3 id="rfc.section.3.8.1"><a href="#rfc.section.3.8.1">3.8.1</a>&nbsp;<a href="#cache.directive.registration">Cache Directive Registry</a></h3>
1678               <p id="rfc.section.3.8.1.p.1">The registration procedure for HTTP Cache Directives is defined by <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;3.2.3</a> of this document.
1679               </p>
1680               <p id="rfc.section.3.8.1.p.2">The HTTP Cache Directive Registry shall be created at &lt;<a href=""></a>&gt; and be populated with the registrations below:
1681               </p>
1682               <div id="rfc.table.1">
1683                  <div id="iana.cache.directive.registration.table"></div>
1684                  <table class="tt full left" cellpadding="3" cellspacing="0">
1685                     <thead>
1686                        <tr>
1687                           <th>Cache Directive</th>
1688                           <th>Reference</th>
1689                        </tr>
1690                     </thead>
1691                     <tbody>
1692                        <tr>
1693                           <td class="left">max-age</td>
1694                           <td class="left"><a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>, <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1695                           </td>
1696                        </tr>
1697                        <tr>
1698                           <td class="left">max-stale</td>
1699                           <td class="left"><a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>
1700                           </td>
1701                        </tr>
1702                        <tr>
1703                           <td class="left">min-fresh</td>
1704                           <td class="left"><a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>
1705                           </td>
1706                        </tr>
1707                        <tr>
1708                           <td class="left">must-revalidate</td>
1709                           <td class="left"><a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1710                           </td>
1711                        </tr>
1712                        <tr>
1713                           <td class="left">no-cache</td>
1714                           <td class="left"><a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>, <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1715                           </td>
1716                        </tr>
1717                        <tr>
1718                           <td class="left">no-store</td>
1719                           <td class="left"><a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>, <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1720                           </td>
1721                        </tr>
1722                        <tr>
1723                           <td class="left">no-transform</td>
1724                           <td class="left"><a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>, <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1725                           </td>
1726                        </tr>
1727                        <tr>
1728                           <td class="left">only-if-cached</td>
1729                           <td class="left"><a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>
1730                           </td>
1731                        </tr>
1732                        <tr>
1733                           <td class="left">private</td>
1734                           <td class="left"><a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1735                           </td>
1736                        </tr>
1737                        <tr>
1738                           <td class="left">proxy-revalidate</td>
1739                           <td class="left"><a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1740                           </td>
1741                        </tr>
1742                        <tr>
1743                           <td class="left">public</td>
1744                           <td class="left"><a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1745                           </td>
1746                        </tr>
1747                        <tr>
1748                           <td class="left">s-maxage</td>
1749                           <td class="left"><a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>
1750                           </td>
1751                        </tr>
1752                        <tr>
1753                           <td class="left">stale-if-error</td>
1754                           <td class="left"><a href="#RFC5861" id="rfc.xref.RFC5861.1"><cite title="HTTP Cache-Control Extensions for Stale Content">[RFC5861]</cite></a>, <a href="">Section 4</a>
1755                           </td>
1756                        </tr>
1757                        <tr>
1758                           <td class="left">stale-while-revalidate</td>
1759                           <td class="left"><a href="#RFC5861" id="rfc.xref.RFC5861.2"><cite title="HTTP Cache-Control Extensions for Stale Content">[RFC5861]</cite></a>, <a href="">Section 3</a>
1760                           </td>
1761                        </tr>
1762                     </tbody>
1763                  </table>
1764               </div>
1765            </div>
1766            <div id="warn.code.registration">
1767               <h3 id="rfc.section.3.8.2"><a href="#rfc.section.3.8.2">3.8.2</a>&nbsp;<a href="#warn.code.registration">Warn Code Registry</a></h3>
1768               <p id="rfc.section.3.8.2.p.1">The registration procedure for HTTP Warn Codes is defined by <a href="#warn.code.extensions" title="Warn Code Extensions">Section&nbsp;3.6.8</a> of this document.
1769               </p>
1770               <p id="rfc.section.3.8.2.p.2">The HTTP Warn Code Registry shall be created at &lt;<a href=""></a>&gt; and be populated with the registrations below:
1771               </p>
1772               <div id="rfc.table.2">
1773                  <div id="iana.warn.code.registration.table"></div>
1774                  <table class="tt full left" cellpadding="3" cellspacing="0">
1775                     <thead>
1776                        <tr>
1777                           <th>Warn Code</th>
1778                           <th>Short Description</th>
1779                           <th>Reference</th>
1780                        </tr>
1781                     </thead>
1782                     <tbody>
1783                        <tr>
1784                           <td class="left">110</td>
1785                           <td class="left">Response is Stale</td>
1786                           <td class="left"><a href="#warn.110" id="rfc.xref.warn.110.1" title="110 Response is Stale">Section&nbsp;3.6.1</a>
1787                           </td>
1788                        </tr>
1789                        <tr>
1790                           <td class="left">111</td>
1791                           <td class="left">Revalidation Failed</td>
1792                           <td class="left"><a href="#warn.111" id="rfc.xref.warn.111.1" title="111 Revalidation Failed">Section&nbsp;3.6.2</a>
1793                           </td>
1794                        </tr>
1795                        <tr>
1796                           <td class="left">112</td>
1797                           <td class="left">Disconnected Operation</td>
1798                           <td class="left"><a href="#warn.112" id="rfc.xref.warn.112.1" title="112 Disconnected Operation">Section&nbsp;3.6.3</a>
1799                           </td>
1800                        </tr>
1801                        <tr>
1802                           <td class="left">113</td>
1803                           <td class="left">Heuristic Expiration</td>
1804                           <td class="left"><a href="#warn.113" id="rfc.xref.warn.113.1" title="113 Heuristic Expiration">Section&nbsp;3.6.4</a>
1805                           </td>
1806                        </tr>
1807                        <tr>
1808                           <td class="left">199</td>
1809                           <td class="left">Miscellaneous Warning</td>
1810                           <td class="left"><a href="#warn.199" id="rfc.xref.warn.199.1" title="199 Miscellaneous Warning">Section&nbsp;3.6.5</a>
1811                           </td>
1812                        </tr>
1813                        <tr>
1814                           <td class="left">214</td>
1815                           <td class="left">Transformation Applied</td>
1816                           <td class="left"><a href="#warn.214" id="rfc.xref.warn.214.1" title="214 Transformation Applied">Section&nbsp;3.6.6</a>
1817                           </td>
1818                        </tr>
1819                        <tr>
1820                           <td class="left">299</td>
1821                           <td class="left">Miscellaneous Persistent Warning</td>
1822                           <td class="left"><a href="#warn.299" id="rfc.xref.warn.299.1" title="299 Miscellaneous Persistent Warning">Section&nbsp;3.6.7</a>
1823                           </td>
1824                        </tr>
1825                     </tbody>
1826                  </table>
1827               </div>
1828            </div>
1829         </div>
1830         <div id="header.field.registration">
1831            <h2 id="rfc.section.3.9"><a href="#rfc.section.3.9">3.9</a>&nbsp;<a href="#header.field.registration">Header Field Registration</a></h2>
1832            <p id="rfc.section.3.9.p.1">The Message Header Field Registry located at &lt;<a href=""></a>&gt; shall be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
1833            </p>
1834            <div id="rfc.table.3">
1835               <div id="iana.header.registration.table"></div>
1836               <table class="tt full left" cellpadding="3" cellspacing="0">
1837                  <thead>
1838                     <tr>
1839                        <th>Header Field Name</th>
1840                        <th>Protocol</th>
1841                        <th>Status</th>
1842                        <th>Reference</th>
1843                     </tr>
1844                  </thead>
1845                  <tbody>
1846                     <tr>
1847                        <td class="left">Age</td>
1848                        <td class="left">http</td>
1849                        <td class="left">standard</td>
1850                        <td class="left"><a href="#header.age" id="rfc.xref.header.age.3" title="Age">Section&nbsp;3.1</a>
1851                        </td>
1852                     </tr>
1853                     <tr>
1854                        <td class="left">Cache-Control</td>
1855                        <td class="left">http</td>
1856                        <td class="left">standard</td>
1857                        <td class="left"><a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;3.2</a>
1858                        </td>
1859                     </tr>
1860                     <tr>
1861                        <td class="left">Expires</td>
1862                        <td class="left">http</td>
1863                        <td class="left">standard</td>
1864                        <td class="left"><a href="#header.expires" id="rfc.xref.header.expires.4" title="Expires">Section&nbsp;3.3</a>
1865                        </td>
1866                     </tr>
1867                     <tr>
1868                        <td class="left">Pragma</td>
1869                        <td class="left">http</td>
1870                        <td class="left">standard</td>
1871                        <td class="left"><a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;3.4</a>
1872                        </td>
1873                     </tr>
1874                     <tr>
1875                        <td class="left">Vary</td>
1876                        <td class="left">http</td>
1877                        <td class="left">standard</td>
1878                        <td class="left"><a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section&nbsp;3.5</a>
1879                        </td>
1880                     </tr>
1881                     <tr>
1882                        <td class="left">Warning</td>
1883                        <td class="left">http</td>
1884                        <td class="left">standard</td>
1885                        <td class="left"><a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;3.6</a>
1886                        </td>
1887                     </tr>
1888                  </tbody>
1889               </table>
1890            </div>
1891            <p id="rfc.section.3.9.p.2">The change controller is: "IETF ( - Internet Engineering Task Force".</p>
1892         </div>
1893      </div>
1894      <div id="security.considerations">
1895         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
1896         <p id="rfc.section.4.p.1">Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious
1897            exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal information
1898            long after a user believes that the information has been removed from the network. Therefore, cache contents need to be protected
1899            as sensitive information.
1900         </p>
1901      </div>
1902      <div id="acks">
1903         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#acks">Acknowledgments</a></h1>
1904         <p id="rfc.section.5.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 11</a> of <a href="#Part1" id="rfc.xref.Part1.17"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
1905         </p>
1906      </div>
1907      <h1 id="rfc.references"><a id="rfc.section.6" href="#rfc.section.6">6.</a> References
1908      </h1>
1909      <h2 id="rfc.references.1"><a href="#rfc.section.6.1" id="rfc.section.6.1">6.1</a> Normative References
1910      </h2>
1911      <table>
1912         <tr>
1913            <td class="reference"><b id="Part1">[Part1]</b></td>
1914            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="" title="Microsoft Corporation">Frystyk, H.</a>, <a href="" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, <a href="" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-18 (work in progress), January&nbsp;2012.
1915            </td>
1916         </tr>
1917         <tr>
1918            <td class="reference"><b id="Part2">[Part2]</b></td>
1919            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="" title="Microsoft Corporation">Frystyk, H.</a>, <a href="" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, <a href="" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">HTTP/1.1, part 2: Message Semantics</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-18 (work in progress), January&nbsp;2012.
1920            </td>
1921         </tr>
1922         <tr>
1923            <td class="reference"><b id="Part4">[Part4]</b></td>
1924            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="" title="Microsoft Corporation">Frystyk, H.</a>, <a href="" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, <a href="" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">HTTP/1.1, part 4: Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-18 (work in progress), January&nbsp;2012.
1925            </td>
1926         </tr>
1927         <tr>
1928            <td class="reference"><b id="Part5">[Part5]</b></td>
1929            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="" title="Microsoft Corporation">Frystyk, H.</a>, <a href="" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, <a href="" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-18 (work in progress), January&nbsp;2012.
1930            </td>
1931         </tr>
1932         <tr>
1933            <td class="reference"><b id="Part7">[Part7]</b></td>
1934            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="" title="Microsoft Corporation">Frystyk, H.</a>, <a href="" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, <a href="" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">HTTP/1.1, part 7: Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-18 (work in progress), January&nbsp;2012.
1935            </td>
1936         </tr>
1937         <tr>
1938            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1939            <td class="top"><a href="" title="Harvard University">Bradner, S.</a>, “<a href="">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
1940            </td>
1941         </tr>
1942         <tr>
1943            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1944            <td class="top"><a href="" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="" title="THUS plc.">P. Overell</a>, “<a href="">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
1945            </td>
1946         </tr>
1947      </table>
1948      <h2 id="rfc.references.2"><a href="#rfc.section.6.2" id="rfc.section.6.2">6.2</a> Informative References
1949      </h2>
1950      <table>
1951         <tr>
1952            <td class="reference"><b id="RFC1305">[RFC1305]</b></td>
1953            <td class="top"><a href="" title="University of Delaware, Electrical Engineering Department">Mills, D.</a>, “<a href="">Network Time Protocol (Version 3) Specification, Implementation</a>”, RFC&nbsp;1305, March&nbsp;1992.
1954            </td>
1955         </tr>
1956         <tr>
1957            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1958            <td class="top"><a href="" title="University of California, Irvine">Fielding, R.</a>, <a href="" title="W3C">Gettys, J.</a>, <a href="" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="" title="Xerox Corporation">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, and <a href="" title="W3C">T. Berners-Lee</a>, “<a href="">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1959            </td>
1960         </tr>
1961         <tr>
1962            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1963            <td class="top"><a href="" title="Nine by Nine">Klyne, G.</a>, <a href="" title="BEA Systems">Nottingham, M.</a>, and <a href="" title="HP Labs">J. Mogul</a>, “<a href="">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
1964            </td>
1965         </tr>
1966         <tr>
1967            <td class="reference"><b id="RFC5226">[RFC5226]</b></td>
1968            <td class="top"><a href="" title="IBM">Narten, T.</a> and <a href="" title="Google">H. Alvestrand</a>, “<a href="">Guidelines for Writing an IANA Considerations Section in RFCs</a>”, BCP&nbsp;26, RFC&nbsp;5226, May&nbsp;2008.
1969            </td>
1970         </tr>
1971         <tr>
1972            <td class="reference"><b id="RFC5861">[RFC5861]</b></td>
1973            <td class="top"><a href="" title="Yahoo! Inc.">Nottingham, M.</a>, “<a href="">HTTP Cache-Control Extensions for Stale Content</a>”, RFC&nbsp;5861, April&nbsp;2010.
1974            </td>
1975         </tr>
1976      </table>
1977      <div id="changes.from.rfc.2616">
1978         <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
1979         <p id="rfc.section.A.p.1">Make the specified age calculation algorithm less conservative. (<a href="#age.calculations" title="Calculating Age">Section&nbsp;2.3.2</a>)
1980         </p>
1981         <p id="rfc.section.A.p.2">Remove requirement to consider Content-Location in successful responses in order to determine the appropriate response to
1982            use. (<a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>)
1983         </p>
1984         <p id="rfc.section.A.p.3">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;2.5</a>)
1985         </p>
1986         <p id="rfc.section.A.p.4">Change ABNF productions for header fields to only define the field value. (<a href="#header.field.definitions" title="Header Field Definitions">Section&nbsp;3</a>)
1987         </p>
1988         <p id="rfc.section.A.p.5">Do not mention RFC 2047 encoding and multiple languages in Warning header fields anymore, as these aspects never were implemented.
1989            (<a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;3.6</a>)
1990         </p>
1991      </div>
1992      <div id="collected.abnf">
1993         <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a href="#collected.abnf">Collected ABNF</a></h1>
1994         <div id="rfc.figure.u.19"></div><pre class="inline"><a href="#header.age" class="smpl">Age</a> = delta-seconds
1996<a href="#header.cache-control" class="smpl">Cache-Control</a> = *( "," OWS ) cache-directive *( OWS "," [ OWS
1997 cache-directive ] )
1999<a href="#header.expires" class="smpl">Expires</a> = HTTP-date
2001<a href="#abnf.dependencies" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
2003<a href="#core.rules" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2005<a href="#header.pragma" class="smpl">Pragma</a> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2006 pragma-directive ] )
2008<a href="#header.vary" class="smpl">Vary</a> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2009 ) )
2011<a href="#header.warning" class="smpl">Warning</a> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2012 )
2014<a href="#header.cache-control" class="smpl">cache-directive</a> = cache-request-directive / cache-response-directive
2015<a href="#header.cache-control" class="smpl">cache-extension</a> = token [ "=" ( token / quoted-string ) ]
2016<a href="#header.cache-control" class="smpl">cache-request-directive</a> = "no-cache" / "no-store" / ( "max-age="
2017 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2018 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2019 cache-extension
2020<a href="#header.cache-control" class="smpl">cache-response-directive</a> = "public" / ( "private" [ "=" DQUOTE *( ","
2021 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2022 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2023 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2024 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2025 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2027<a href="#delta-seconds" class="smpl">delta-seconds</a> = 1*DIGIT
2029<a href="#header.pragma" class="smpl">extension-pragma</a> = token [ "=" ( token / quoted-string ) ]
2031<a href="#abnf.dependencies" class="smpl">field-name</a> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2033<a href="#abnf.dependencies" class="smpl">port</a> = &lt;port, defined in [Part1], Section 2.7&gt;
2034<a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / extension-pragma
2035<a href="#abnf.dependencies" class="smpl">pseudonym</a> = &lt;pseudonym, defined in [Part1], Section 8.8&gt;
2037<a href="#core.rules" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
2039<a href="#core.rules" class="smpl">token</a> = &lt;token, defined in [Part1], Section 3.2.3&gt;
2041<a href="#abnf.dependencies" class="smpl">uri-host</a> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2043<a href="#header.warning" class="smpl">warn-agent</a> = ( uri-host [ ":" port ] ) / pseudonym
2044<a href="#header.warning" class="smpl">warn-code</a> = 3DIGIT
2045<a href="#header.warning" class="smpl">warn-date</a> = DQUOTE HTTP-date DQUOTE
2046<a href="#header.warning" class="smpl">warn-text</a> = quoted-string
2047<a href="#header.warning" class="smpl">warning-value</a> = warn-code SP warn-agent SP warn-text [ SP warn-date
2048 ]
2049</pre><div id="rfc.figure.u.20"></div>
2050         <p>ABNF diagnostics:</p><pre class="inline">; Age defined but not used
2051; Cache-Control defined but not used
2052; Expires defined but not used
2053; Pragma defined but not used
2054; Vary defined but not used
2055; Warning defined but not used
2057      <div id="change.log">
2058         <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
2059         <div>
2060            <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a>&nbsp;Since RFC 2616
2061            </h2>
2062            <p id="rfc.section.C.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
2063            </p>
2064         </div>
2065         <div>
2066            <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a>&nbsp;Since draft-ietf-httpbis-p6-cache-00
2067            </h2>
2068            <p id="rfc.section.C.2.p.1">Closed issues: </p>
2069            <ul>
2070               <li>&lt;<a href=""></a>&gt;: "Trailer" (&lt;<a href=""></a>&gt;)
2071               </li>
2072               <li>&lt;<a href=""></a>&gt;: "Invalidation after Update or Delete" (&lt;<a href=""></a>&gt;)
2073               </li>
2074               <li>&lt;<a href=""></a>&gt;: "Normative and Informative references"
2075               </li>
2076               <li>&lt;<a href=""></a>&gt;: "Date reference typo"
2077               </li>
2078               <li>&lt;<a href=""></a>&gt;: "Connection header text"
2079               </li>
2080               <li>&lt;<a href=""></a>&gt;: "Informative references"
2081               </li>
2082               <li>&lt;<a href=""></a>&gt;: "ISO-8859-1 Reference"
2083               </li>
2084               <li>&lt;<a href=""></a>&gt;: "Normative up-to-date references"
2085               </li>
2086               <li>&lt;<a href=""></a>&gt;: "typo in 13.2.2"
2087               </li>
2088            </ul>
2089            <p id="rfc.section.C.2.p.2">Other changes: </p>
2090            <ul>
2091               <li>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on &lt;<a href=""></a>&gt;)
2092               </li>
2093            </ul>
2094         </div>
2095         <div>
2096            <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a>&nbsp;Since draft-ietf-httpbis-p6-cache-01
2097            </h2>
2098            <p id="rfc.section.C.3.p.1">Closed issues: </p>
2099            <ul>
2100               <li>&lt;<a href=""></a>&gt;: "rel_path not used"
2101               </li>
2102            </ul>
2103            <p id="rfc.section.C.3.p.2">Other changes: </p>
2104            <ul>
2105               <li>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress on &lt;<a href=""></a>&gt;)
2106               </li>
2107               <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li>
2108            </ul>
2109         </div>
2110         <div id="changes.since.02">
2111            <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a>&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p6-cache-02</a></h2>
2112            <p id="rfc.section.C.4.p.1">Ongoing work on IANA Message Header Field Registration (&lt;<a href=""></a>&gt;):
2113            </p>
2114            <ul>
2115               <li>Reference RFC 3984, and update header field registrations for header fields defined in this document.</li>
2116            </ul>
2117         </div>
2118         <div id="changes.since.03">
2119            <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a>&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p6-cache-03</a></h2>
2120            <p id="rfc.section.C.5.p.1">Closed issues: </p>
2121            <ul>
2122               <li>&lt;<a href=""></a>&gt;: "Vary header classification"
2123               </li>
2124            </ul>
2125         </div>
2126         <div id="changes.since.04">
2127            <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a>&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p6-cache-04</a></h2>
2128            <p id="rfc.section.C.6.p.1">Ongoing work on ABNF conversion (&lt;<a href=""></a>&gt;):
2129            </p>
2130            <ul>
2131               <li>Use "/" instead of "|" for alternatives.</li>
2132               <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li>
2133               <li>Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.</li>
2134            </ul>
2135         </div>
2136         <div id="changes.since.05">
2137            <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a>&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></h2>
2138            <p id="rfc.section.C.7.p.1">This is a total rewrite of this part of the specification.</p>
2139            <p id="rfc.section.C.7.p.2">Affected issues: </p>
2140            <ul>
2141               <li>&lt;<a href=""></a>&gt;: "Definition of 1xx Warn-Codes"
2142               </li>
2143               <li>&lt;<a href=""></a>&gt;: "Placement of 13.5.1 and 13.5.2"
2144               </li>
2145               <li>&lt;<a href=""></a>&gt;: "The role of Warning and Semantic Transparency in Caching"
2146               </li>
2147               <li>&lt;<a href=""></a>&gt;: "Methods and Caching"
2148               </li>
2149            </ul>
2150            <p id="rfc.section.C.7.p.3">In addition: Final work on ABNF conversion (&lt;<a href=""></a>&gt;):
2151            </p>
2152            <ul>
2153               <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li>
2154            </ul>
2155         </div>
2156         <div id="changes.since.06">
2157            <h2 id="rfc.section.C.8"><a href="#rfc.section.C.8">C.8</a>&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p6-cache-06</a></h2>
2158            <p id="rfc.section.C.8.p.1">Closed issues: </p>
2159            <ul>
2160               <li>&lt;<a href=""></a>&gt;: "base for numeric protocol elements"
2161               </li>
2162            </ul>
2163            <p id="rfc.section.C.8.p.2">Affected issues: </p>
2164            <ul>
2165               <li>&lt;<a href=""></a>&gt;: "Vary and non-existant headers"
2166               </li>
2167            </ul>
2168         </div>
2169         <div id="changes.since.07">
2170            <h2 id="rfc.section.C.9"><a href="#rfc.section.C.9">C.9</a>&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p6-cache-07</a></h2>
2171            <p id="rfc.section.C.9.p.1">Closed issues: </p>
2172            <ul>
2173               <li>&lt;<a href=""></a>&gt;: "Definition of 1xx Warn-Codes"
2174               </li>
2175               <li>&lt;<a href=""></a>&gt;: "Content-Location on 304 responses"
2176               </li>
2177               <li>&lt;<a href=""></a>&gt;: "private and no-cache CC directives with headers"
2178               </li>
2179               <li>&lt;<a href=""></a>&gt;: "RFC2047 and warn-text"
2180               </li>
2181            </ul>
2182         </div>
2183         <div id="changes.since.08">
2184            <h2 id="rfc.section.C.10"><a href="#rfc.section.C.10">C.10</a>&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p6-cache-08</a></h2>
2185            <p id="rfc.section.C.10.p.1">Closed issues: </p>
2186            <ul>
2187               <li>&lt;<a href=""></a>&gt;: "serving negotiated responses from cache: header-specific canonicalization"
2188               </li>
2189               <li>&lt;<a href=""></a>&gt;: "Effect of CC directives on history lists"
2190               </li>
2191               <li>&lt;<a href=""></a>&gt;: "Cache Extensions can override no-store, etc."
2192               </li>
2193            </ul>
2194            <p id="rfc.section.C.10.p.2">Affected issues: </p>
2195            <ul>
2196               <li>&lt;<a href=""></a>&gt;: Status codes and caching
2197               </li>
2198            </ul>
2199            <p id="rfc.section.C.10.p.3">Partly resolved issues: </p>
2200            <ul>
2201               <li>&lt;<a href=""></a>&gt;: "Placement of 13.5.1 and 13.5.2"
2202               </li>
2203            </ul>
2204         </div>
2205         <div id="changes.since.09">
2206            <h2 id="rfc.section.C.11"><a href="#rfc.section.C.11">C.11</a>&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p6-cache-09</a></h2>
2207            <p id="rfc.section.C.11.p.1">Closed issues: </p>
2208            <ul>
2209               <li>&lt;<a href=""></a>&gt;: "Age calculation"
2210               </li>
2211               <li>&lt;<a href=""></a>&gt;: "Clarify differences between / requirements for request and response CC directives"
2212               </li>
2213               <li>&lt;<a href=""></a>&gt;: "Caching authenticated responses"
2214               </li>
2215               <li>&lt;<a href=""></a>&gt;: "IANA registry for cache-control directives"
2216               </li>
2217               <li>&lt;<a href=""></a>&gt;: "Heuristic caching of URLs with query components"
2218               </li>
2219            </ul>
2220            <p id="rfc.section.C.11.p.2">Partly resolved issues: </p>
2221            <ul>
2222               <li>&lt;<a href=""></a>&gt;: "Term for the requested resource's URI"
2223               </li>
2224            </ul>
2225         </div>
2226         <div id="changes.since.10">
2227            <h2 id="rfc.section.C.12"><a href="#rfc.section.C.12">C.12</a>&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p6-cache-10</a></h2>
2228            <p id="rfc.section.C.12.p.1">Closed issues: </p>
2229            <ul>
2230               <li>&lt;<a href=""></a>&gt;: "Clarify entity / representation / variant terminology"
2231               </li>
2232               <li>&lt;<a href=""></a>&gt;: "consider removing the 'changes from 2068' sections"
2233               </li>
2234               <li>&lt;<a href=""></a>&gt;: "Allowing heuristic caching for new status codes"
2235               </li>
2236               <li>Clean up TODOs and prose in "Combining Responses."</li>
2237            </ul>
2238         </div>
2239         <div id="changes.since.11">
2240            <h2 id="rfc.section.C.13"><a href="#rfc.section.C.13">C.13</a>&nbsp;<a href="#changes.since.11">Since draft-ietf-httpbis-p6-cache-11</a></h2>
2241            <p id="rfc.section.C.13.p.1">Closed issues: </p>
2242            <ul>
2243               <li>&lt;<a href=""></a>&gt;: "Text about clock requirement for caches belongs in p6"
2244               </li>
2245            </ul>
2246         </div>
2247         <div id="changes.since.12">
2248            <h2 id="rfc.section.C.14"><a href="#rfc.section.C.14">C.14</a>&nbsp;<a href="#changes.since.12">Since draft-ietf-httpbis-p6-cache-12</a></h2>
2249            <p id="rfc.section.C.14.p.1">Closed issues: </p>
2250            <ul>
2251               <li>&lt;<a href=""></a>&gt;: "Header Classification"
2252               </li>
2253               <li>&lt;<a href=""></a>&gt;: "Clarify 'public'"
2254               </li>
2255            </ul>
2256         </div>
2257         <div id="changes.since.13">
2258            <h2 id="rfc.section.C.15"><a href="#rfc.section.C.15">C.15</a>&nbsp;<a href="#changes.since.13">Since draft-ietf-httpbis-p6-cache-13</a></h2>
2259            <p id="rfc.section.C.15.p.1">Closed issues: </p>
2260            <ul>
2261               <li>&lt;<a href=""></a>&gt;: "untangle ABNFs for header fields"
2262               </li>
2263            </ul>
2264         </div>
2265         <div id="changes.since.14">
2266            <h2 id="rfc.section.C.16"><a href="#rfc.section.C.16">C.16</a>&nbsp;<a href="#changes.since.14">Since draft-ietf-httpbis-p6-cache-14</a></h2>
2267            <p id="rfc.section.C.16.p.1">Closed issues: </p>
2268            <ul>
2269               <li>&lt;<a href=""></a>&gt;: "Mismatch Vary"
2270               </li>
2271               <li>&lt;<a href=""></a>&gt;: "Cache Invalidation only happens upon successful responses"
2272               </li>
2273               <li>&lt;<a href=""></a>&gt;: "Recommend minimum sizes for protocol elements"
2274               </li>
2275               <li>&lt;<a href=""></a>&gt;: "Proxies don't 'understand' methods"
2276               </li>
2277               <li>&lt;<a href=""></a>&gt;: "Cache Extensions can override no-store, etc."
2278               </li>
2279               <li>&lt;<a href=""></a>&gt;: "Pragma"
2280               </li>
2281            </ul>
2282         </div>
2283         <div id="changes.since.15">
2284            <h2 id="rfc.section.C.17"><a href="#rfc.section.C.17">C.17</a>&nbsp;<a href="#changes.since.15">Since draft-ietf-httpbis-p6-cache-15</a></h2>
2285            <p id="rfc.section.C.17.p.1">Closed issues: </p>
2286            <ul>
2287               <li>&lt;<a href=""></a>&gt;: "Motivate one-year limit for Expires"
2288               </li>
2289            </ul>
2290         </div>
2291         <div id="changes.since.16">
2292            <h2 id="rfc.section.C.18"><a href="#rfc.section.C.18">C.18</a>&nbsp;<a href="#changes.since.16">Since draft-ietf-httpbis-p6-cache-16</a></h2>
2293            <p id="rfc.section.C.18.p.1">Closed issues: </p>
2294            <ul>
2295               <li>&lt;<a href=""></a>&gt;: "Document HTTP's error-handling philosophy"
2296               </li>
2297               <li>&lt;<a href=""></a>&gt;: "Cache-Control directive case sensitivity"
2298               </li>
2299            </ul>
2300         </div>
2301         <div id="changes.since.17">
2302            <h2 id="rfc.section.C.19"><a href="#rfc.section.C.19">C.19</a>&nbsp;<a href="#changes.since.17">Since draft-ietf-httpbis-p6-cache-17</a></h2>
2303            <p id="rfc.section.C.19.p.1">Closed issues: </p>
2304            <ul>
2305               <li>&lt;<a href=""></a>&gt;: "Interaction of request and response Cache-Control"
2306               </li>
2307               <li>&lt;<a href=""></a>&gt;: "Refining age for 1.1 proxy chains"
2308               </li>
2309               <li>&lt;<a href=""></a>&gt;: "warn-code registry"
2310               </li>
2311            </ul>
2312         </div>
2313      </div>
2314      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
2315      <p class="noprint"><a href="#rfc.index.1">1</a> <a href="#rfc.index.2">2</a> <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>
2316      </p>
2317      <div class="print2col">
2318         <ul class="ind">
2319            <li><a id="rfc.index.1" href="#rfc.index.1"><b>1</b></a><ul>
2320                  <li>110 Response is Stale (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.1"><b>3.6.1</b></a>, <a href="#rfc.xref.warn.110.1">3.8.2</a></li>
2321                  <li>111 Revalidation Failed (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.2"><b>3.6.2</b></a>, <a href="#rfc.xref.warn.111.1">3.8.2</a></li>
2322                  <li>112 Disconnected Operation (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.3"><b>3.6.3</b></a>, <a href="#rfc.xref.warn.112.1">3.8.2</a></li>
2323                  <li>113 Heuristic Expiration (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.4"><b>3.6.4</b></a>, <a href="#rfc.xref.warn.113.1">3.8.2</a></li>
2324                  <li>199 Miscellaneous Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.5"><b>3.6.5</b></a>, <a href="#rfc.xref.warn.199.1">3.8.2</a></li>
2325               </ul>
2326            </li>
2327            <li><a id="rfc.index.2" href="#rfc.index.2"><b>2</b></a><ul>
2328                  <li>214 Transformation Applied (warn code)&nbsp;&nbsp;<a href="#rfc.iref.2.1"><b>3.6.6</b></a>, <a href="#rfc.xref.warn.214.1">3.8.2</a></li>
2329                  <li>299 Miscellaneous Persistent Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.2.2"><b>3.6.7</b></a>, <a href="#rfc.xref.warn.299.1">3.8.2</a></li>
2330               </ul>
2331            </li>
2332            <li><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul>
2333                  <li>age&nbsp;&nbsp;<a href="#rfc.iref.a.1">1.2</a></li>
2334                  <li>Age header field&nbsp;&nbsp;<a href="#rfc.xref.header.age.1">2.2</a>, <a href="#rfc.xref.header.age.2">2.3.2</a>, <a href="#rfc.iref.a.2"><b>3.1</b></a>, <a href="#rfc.xref.header.age.3">3.9</a></li>
2335               </ul>
2336            </li>
2337            <li><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul>
2338                  <li>cache&nbsp;&nbsp;<a href="#rfc.iref.c.1">1.1</a>, <a href="#rfc.iref.c.2">1.2</a></li>
2339                  <li>Cache Directives&nbsp;&nbsp;
2340                     <ul>
2341                        <li>max-age&nbsp;&nbsp;<a href="#rfc.iref.c.9"><b>3.2.1</b></a>, <a href="#rfc.iref.c.20"><b>3.2.2</b></a></li>
2342                        <li>max-stale&nbsp;&nbsp;<a href="#rfc.iref.c.10"><b>3.2.1</b></a></li>
2343                        <li>min-fresh&nbsp;&nbsp;<a href="#rfc.iref.c.11"><b>3.2.1</b></a></li>
2344                        <li>must-revalidate&nbsp;&nbsp;<a href="#rfc.iref.c.18"><b>3.2.2</b></a></li>
2345                        <li>no-cache&nbsp;&nbsp;<a href="#rfc.iref.c.7"><b>3.2.1</b></a>, <a href="#rfc.iref.c.16"><b>3.2.2</b></a></li>
2346                        <li>no-store&nbsp;&nbsp;<a href="#rfc.iref.c.8"><b>3.2.1</b></a>, <a href="#rfc.iref.c.17"><b>3.2.2</b></a></li>
2347                        <li>no-transform&nbsp;&nbsp;<a href="#rfc.iref.c.12"><b>3.2.1</b></a>, <a href="#rfc.iref.c.22"><b>3.2.2</b></a></li>
2348                        <li>only-if-cached&nbsp;&nbsp;<a href="#rfc.iref.c.13"><b>3.2.1</b></a></li>
2349                        <li>private&nbsp;&nbsp;<a href="#rfc.iref.c.15"><b>3.2.2</b></a></li>
2350                        <li>proxy-revalidate&nbsp;&nbsp;<a href="#rfc.iref.c.19"><b>3.2.2</b></a></li>
2351                        <li>public&nbsp;&nbsp;<a href="#rfc.iref.c.14"><b>3.2.2</b></a></li>
2352                        <li>s-maxage&nbsp;&nbsp;<a href="#rfc.iref.c.21"><b>3.2.2</b></a></li>
2353                     </ul>
2354                  </li>
2355                  <li>cache entry&nbsp;&nbsp;<a href="#rfc.iref.c.4">2</a></li>
2356                  <li>cache key&nbsp;&nbsp;<a href="#rfc.iref.c.5">2</a></li>
2357                  <li>Cache-Control header field&nbsp;&nbsp;<a href="#rfc.xref.header.cache-control.1">2.1</a>, <a href="#rfc.iref.c.6"><b>3.2</b></a>, <a href="#rfc.xref.header.cache-control.2">3.9</a></li>
2358                  <li>cacheable&nbsp;&nbsp;<a href="#rfc.iref.c.3">1.2</a></li>
2359               </ul>
2360            </li>
2361            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
2362                  <li>Expires header field&nbsp;&nbsp;<a href="#rfc.xref.header.expires.1">2.1</a>, <a href="#rfc.xref.header.expires.2">2.3</a>, <a href="#rfc.xref.header.expires.3">2.3.1</a>, <a href="#rfc.iref.e.2"><b>3.3</b></a>, <a href="#rfc.xref.header.expires.4">3.9</a></li>
2363                  <li>explicit expiration time&nbsp;&nbsp;<a href="#rfc.iref.e.1">1.2</a></li>
2364               </ul>
2365            </li>
2366            <li><a id="rfc.index.F" href="#rfc.index.F"><b>F</b></a><ul>
2367                  <li>first-hand&nbsp;&nbsp;<a href="#rfc.iref.f.1">1.2</a></li>
2368                  <li>fresh&nbsp;&nbsp;<a href="#rfc.iref.f.3">1.2</a></li>
2369                  <li>freshness lifetime&nbsp;&nbsp;<a href="#rfc.iref.f.2">1.2</a></li>
2370               </ul>
2371            </li>
2372            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
2373                  <li><tt>Grammar</tt>&nbsp;&nbsp;
2374                     <ul>
2375                        <li><tt>Age</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>3.1</b></a></li>
2376                        <li><tt>Cache-Control</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>3.2</b></a></li>
2377                        <li><tt>cache-extension</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>3.2</b></a></li>
2378                        <li><tt>cache-request-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>3.2.1</b></a></li>
2379                        <li><tt>cache-response-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>3.2.2</b></a></li>
2380                        <li><tt>delta-seconds</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>1.5</b></a></li>
2381                        <li><tt>Expires</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>3.3</b></a></li>
2382                        <li><tt>extension-pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>3.4</b></a></li>
2383                        <li><tt>Pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>3.4</b></a></li>
2384                        <li><tt>pragma-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>3.4</b></a></li>
2385                        <li><tt>Vary</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.11"><b>3.5</b></a></li>
2386                        <li><tt>warn-agent</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.15"><b>3.6</b></a></li>
2387                        <li><tt>warn-code</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.14"><b>3.6</b></a></li>
2388                        <li><tt>warn-date</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.17"><b>3.6</b></a></li>
2389                        <li><tt>warn-text</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.16"><b>3.6</b></a></li>
2390                        <li><tt>Warning</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.12"><b>3.6</b></a></li>
2391                        <li><tt>warning-value</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.13"><b>3.6</b></a></li>
2392                     </ul>
2393                  </li>
2394               </ul>
2395            </li>
2396            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
2397                  <li>Header Fields&nbsp;&nbsp;
2398                     <ul>
2399                        <li>Age&nbsp;&nbsp;<a href="#rfc.xref.header.age.1">2.2</a>, <a href="#rfc.xref.header.age.2">2.3.2</a>, <a href="#rfc.iref.h.2"><b>3.1</b></a>, <a href="#rfc.xref.header.age.3">3.9</a></li>
2400                        <li>Cache-Control&nbsp;&nbsp;<a href="#rfc.xref.header.cache-control.1">2.1</a>, <a href="#rfc.iref.h.3"><b>3.2</b></a>, <a href="#rfc.xref.header.cache-control.2">3.9</a></li>
2401                        <li>Expires&nbsp;&nbsp;<a href="#rfc.xref.header.expires.1">2.1</a>, <a href="#rfc.xref.header.expires.2">2.3</a>, <a href="#rfc.xref.header.expires.3">2.3.1</a>, <a href="#rfc.iref.h.4"><b>3.3</b></a>, <a href="#rfc.xref.header.expires.4">3.9</a></li>
2402                        <li>Pragma&nbsp;&nbsp;<a href="#rfc.xref.header.pragma.1">2.2</a>, <a href="#rfc.xref.header.pragma.2">3.2</a>, <a href="#rfc.iref.h.5"><b>3.4</b></a>, <a href="#rfc.xref.header.pragma.3">3.9</a></li>
2403                        <li>Vary&nbsp;&nbsp;<a href="#rfc.xref.header.vary.1">2.7</a>, <a href="#rfc.iref.h.6"><b>3.5</b></a>, <a href="#rfc.xref.header.vary.2">3.9</a></li>
2404                        <li>Warning&nbsp;&nbsp;<a href="#rfc.xref.header.warning.1">2.3.3</a>, <a href="#rfc.xref.header.warning.2">2.4.1</a>, <a href="#rfc.xref.header.warning.3">2.8</a>, <a href="#rfc.iref.h.7"><b>3.6</b></a>, <a href="#rfc.xref.header.warning.4">3.9</a>, <a href="#rfc.xref.header.warning.5">A</a></li>
2405                     </ul>
2406                  </li>
2407                  <li>heuristic expiration time&nbsp;&nbsp;<a href="#rfc.iref.h.1">1.2</a></li>
2408               </ul>
2409            </li>
2410            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
2411                  <li>max-age&nbsp;&nbsp;
2412                     <ul>
2413                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>3.2.1</b></a>, <a href="#rfc.iref.m.5"><b>3.2.2</b></a></li>
2414                     </ul>
2415                  </li>
2416                  <li>max-stale&nbsp;&nbsp;
2417                     <ul>
2418                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.2"><b>3.2.1</b></a></li>
2419                     </ul>
2420                  </li>
2421                  <li>min-fresh&nbsp;&nbsp;
2422                     <ul>
2423                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.3"><b>3.2.1</b></a></li>
2424                     </ul>
2425                  </li>
2426                  <li>must-revalidate&nbsp;&nbsp;
2427                     <ul>
2428                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.4"><b>3.2.2</b></a></li>
2429                     </ul>
2430                  </li>
2431               </ul>
2432            </li>
2433            <li><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul>
2434                  <li>no-cache&nbsp;&nbsp;
2435                     <ul>
2436                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.n.1"><b>3.2.1</b></a>, <a href="#rfc.iref.n.4"><b>3.2.2</b></a></li>
2437                     </ul>
2438                  </li>
2439                  <li>no-store&nbsp;&nbsp;
2440                     <ul>
2441                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.n.2"><b>3.2.1</b></a>, <a href="#rfc.iref.n.5"><b>3.2.2</b></a></li>
2442                     </ul>
2443                  </li>
2444                  <li>no-transform&nbsp;&nbsp;
2445                     <ul>
2446                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.n.3"><b>3.2.1</b></a>, <a href="#rfc.iref.n.6"><b>3.2.2</b></a></li>
2447                     </ul>
2448                  </li>
2449               </ul>
2450            </li>
2451            <li><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul>
2452                  <li>only-if-cached&nbsp;&nbsp;
2453                     <ul>
2454                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.o.1"><b>3.2.1</b></a></li>
2455                     </ul>
2456                  </li>
2457               </ul>
2458            </li>
2459            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
2460                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.3</a>, <a href="#rfc.xref.Part1.2">1.4</a>, <a href="#rfc.xref.Part1.3">1.4.1</a>, <a href="#rfc.xref.Part1.4">1.4.1</a>, <a href="#rfc.xref.Part1.5">1.4.1</a>, <a href="#rfc.xref.Part1.6">1.4.1</a>, <a href="#rfc.xref.Part1.7">1.4.2</a>, <a href="#rfc.xref.Part1.8">1.4.2</a>, <a href="#rfc.xref.Part1.9">1.4.2</a>, <a href="#rfc.xref.Part1.10">1.4.2</a>, <a href="#rfc.xref.Part1.11">2.1</a>, <a href="#rfc.xref.Part1.12">2.2</a>, <a href="#rfc.xref.Part1.13">2.5</a>, <a href="#rfc.xref.Part1.14">2.5</a>, <a href="#rfc.xref.Part1.15">2.5</a>, <a href="#rfc.xref.Part1.16">2.7</a>, <a href="#rfc.xref.Part1.17">5</a>, <a href="#Part1"><b>6.1</b></a><ul>
2461                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.4</a></li>
2462                        <li><em>Section 1.2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">1.4.1</a></li>
2463                        <li><em>Section 2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.3</a></li>
2464                        <li><em>Section 2.7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.8">1.4.2</a>, <a href="#rfc.xref.Part1.10">1.4.2</a></li>
2465                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.7">1.4.2</a>, <a href="#rfc.xref.Part1.16">2.7</a></li>
2466                        <li><em>Section 3.2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">1.4.1</a>, <a href="#rfc.xref.Part1.6">1.4.1</a></li>
2467                        <li><em>Section 4.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.12">2.2</a>, <a href="#rfc.xref.Part1.13">2.5</a>, <a href="#rfc.xref.Part1.14">2.5</a>, <a href="#rfc.xref.Part1.15">2.5</a></li>
2468                        <li><em>Section 8.8</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.9">1.4.2</a></li>
2469                        <li><em>Section 11</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.17">5</a></li>
2470                     </ul>
2471                  </li>
2472                  <li><em>Part2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">1.4.2</a>, <a href="#rfc.xref.Part2.2">2</a>, <a href="#rfc.xref.Part2.3">2.2</a>, <a href="#rfc.xref.Part2.4"></a>, <a href="#rfc.xref.Part2.5">2.3.2</a>, <a href="#rfc.xref.Part2.6">2.5</a>, <a href="#rfc.xref.Part2.7">3.3</a>, <a href="#Part2"><b>6.1</b></a><ul>
2473                        <li><em>Section 6.1.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">2.2</a>, <a href="#rfc.xref.Part2.6">2.5</a></li>
2474                        <li><em>Section 7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.4"></a></li>
2475                        <li><em>Section 8</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">1.4.2</a>, <a href="#rfc.xref.Part2.7">3.3</a></li>
2476                        <li><em>Section 9.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.5">2.3.2</a></li>
2477                     </ul>
2478                  </li>
2479                  <li><em>Part4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.1">1.2</a>, <a href="#rfc.xref.Part4.2">1.2</a>, <a href="#rfc.xref.Part4.3">1.2</a>, <a href="#rfc.xref.Part4.4"></a>, <a href="#rfc.xref.Part4.5">2.4</a>, <a href="#Part4"><b>6.1</b></a><ul>
2480                        <li><em>Section 2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.1">1.2</a></li>
2481                        <li><em>Section 2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.4"></a></li>
2482                        <li><em>Section 2.2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.3">1.2</a></li>
2483                        <li><em>Section 2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.2">1.2</a></li>
2484                     </ul>
2485                  </li>
2486                  <li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">2.1</a>, <a href="#rfc.xref.Part5.2">2.8</a>, <a href="#rfc.xref.Part5.3">2.8</a>, <a href="#Part5"><b>6.1</b></a><ul>
2487                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.3">2.8</a></li>
2488                     </ul>
2489                  </li>
2490                  <li><em>Part7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part7.1">2.1</a>, <a href="#rfc.xref.Part7.2">2.6</a>, <a href="#Part7"><b>6.1</b></a><ul>
2491                        <li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part7.1">2.1</a>, <a href="#rfc.xref.Part7.2">2.6</a></li>
2492                     </ul>
2493                  </li>
2494                  <li>Pragma header field&nbsp;&nbsp;<a href="#rfc.xref.header.pragma.1">2.2</a>, <a href="#rfc.xref.header.pragma.2">3.2</a>, <a href="#rfc.iref.p.5"><b>3.4</b></a>, <a href="#rfc.xref.header.pragma.3">3.9</a></li>
2495                  <li>private&nbsp;&nbsp;
2496                     <ul>
2497                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.p.3"><b>3.2.2</b></a></li>
2498                     </ul>
2499                  </li>
2500                  <li>private cache&nbsp;&nbsp;<a href="#rfc.iref.p.1">1.2</a></li>
2501                  <li>proxy-revalidate&nbsp;&nbsp;
2502                     <ul>
2503                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.p.4"><b>3.2.2</b></a></li>
2504                     </ul>
2505                  </li>
2506                  <li>public&nbsp;&nbsp;
2507                     <ul>
2508                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.p.2"><b>3.2.2</b></a></li>
2509                     </ul>
2510                  </li>
2511               </ul>
2512            </li>
2513            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
2514                  <li><em>RFC1305</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC1305.1">2.2</a>, <a href="#rfc.xref.RFC1305.2">2.3.2</a>, <a href="#RFC1305"><b>6.2</b></a></li>
2515                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.3</a>, <a href="#RFC2119"><b>6.1</b></a></li>
2516                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1"></a>, <a href="#RFC2616"><b>6.2</b></a>, <a href="#rfc.xref.RFC2616.2">C.1</a><ul>
2517                        <li><em>Section 13.9</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1"></a></li>
2518                     </ul>
2519                  </li>
2520                  <li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">3.9</a>, <a href="#RFC3864"><b>6.2</b></a></li>
2521                  <li><em>RFC5226</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">3.2.3</a>, <a href="#rfc.xref.RFC5226.2">3.6.8</a>, <a href="#RFC5226"><b>6.2</b></a><ul>
2522                        <li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">3.2.3</a>, <a href="#rfc.xref.RFC5226.2">3.6.8</a></li>
2523                     </ul>
2524                  </li>
2525                  <li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.4</a>, <a href="#rfc.xref.RFC5234.2">1.4</a>, <a href="#RFC5234"><b>6.1</b></a><ul>
2526                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">1.4</a></li>
2527                     </ul>
2528                  </li>
2529                  <li><em>RFC5861</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">3.8.1</a>, <a href="#rfc.xref.RFC5861.2">3.8.1</a>, <a href="#RFC5861"><b>6.2</b></a><ul>
2530                        <li><em>Section 3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.2">3.8.1</a></li>
2531                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">3.8.1</a></li>
2532                     </ul>
2533                  </li>
2534               </ul>
2535            </li>
2536            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
2537                  <li>s-maxage&nbsp;&nbsp;
2538                     <ul>
2539                        <li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.s.4"><b>3.2.2</b></a></li>
2540                     </ul>
2541                  </li>
2542                  <li>shared cache&nbsp;&nbsp;<a href="#rfc.iref.s.1">1.2</a></li>
2543                  <li>stale&nbsp;&nbsp;<a href="#rfc.iref.s.2">1.2</a></li>
2544                  <li>strong validator&nbsp;&nbsp;<a href="#rfc.iref.s.3">1.2</a></li>
2545               </ul>
2546            </li>
2547            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
2548                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1">1.2</a><ul>
2549                        <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.2">1.2</a></li>
2550                     </ul>
2551                  </li>
2552                  <li>Vary header field&nbsp;&nbsp;<a href="#rfc.xref.header.vary.1">2.7</a>, <a href="#rfc.iref.v.3"><b>3.5</b></a>, <a href="#rfc.xref.header.vary.2">3.9</a></li>
2553               </ul>
2554            </li>
2555            <li><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul>
2556                  <li>Warn Codes&nbsp;&nbsp;
2557                     <ul>
2558                        <li>110 Response is Stale&nbsp;&nbsp;<a href="#rfc.iref.w.2"><b>3.6.1</b></a>, <a href="#rfc.xref.warn.110.1">3.8.2</a></li>
2559                        <li>111 Revalidation Failed&nbsp;&nbsp;<a href="#rfc.iref.w.3"><b>3.6.2</b></a>, <a href="#rfc.xref.warn.111.1">3.8.2</a></li>
2560                        <li>112 Disconnected Operation&nbsp;&nbsp;<a href="#rfc.iref.w.4"><b>3.6.3</b></a>, <a href="#rfc.xref.warn.112.1">3.8.2</a></li>
2561                        <li>113 Heuristic Expiration&nbsp;&nbsp;<a href="#rfc.iref.w.5"><b>3.6.4</b></a>, <a href="#rfc.xref.warn.113.1">3.8.2</a></li>
2562                        <li>199 Miscellaneous Warning&nbsp;&nbsp;<a href="#rfc.iref.w.6"><b>3.6.5</b></a>, <a href="#rfc.xref.warn.199.1">3.8.2</a></li>
2563                        <li>214 Transformation Applied&nbsp;&nbsp;<a href="#rfc.iref.w.7"><b>3.6.6</b></a>, <a href="#rfc.xref.warn.214.1">3.8.2</a></li>
2564                        <li>299 Miscellaneous Persistent Warning&nbsp;&nbsp;<a href="#rfc.iref.w.8"><b>3.6.7</b></a>, <a href="#rfc.xref.warn.299.1">3.8.2</a></li>
2565                     </ul>
2566                  </li>
2567                  <li>Warning header field&nbsp;&nbsp;<a href="#rfc.xref.header.warning.1">2.3.3</a>, <a href="#rfc.xref.header.warning.2">2.4.1</a>, <a href="#rfc.xref.header.warning.3">2.8</a>, <a href="#rfc.iref.w.1"><b>3.6</b></a>, <a href="#rfc.xref.header.warning.4">3.9</a>, <a href="#rfc.xref.header.warning.5">A</a></li>
2568               </ul>
2569            </li>
2570         </ul>
2571      </div>
2572      <div class="avoidbreak">
2573         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
2574         <p><b>Roy T. Fielding</b>
2575            (editor)
2576            <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2577         <p><b>Jim Gettys</b><br>Alcatel-Lucent Bell Labs<br>21 Oak Knoll Road<br>Carlisle, MA&nbsp;01741<br>USA<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2578         <p><b>Jeffrey C. Mogul</b><br>Hewlett-Packard Company<br>HP Labs, Large Scale Systems Group<br>1501 Page Mill Road, MS 1177<br>Palo Alto, CA&nbsp;94304<br>USA<br>Email: <a href=""></a></p>
2579         <p><b>Henrik Frystyk Nielsen</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>USA<br>Email: <a href=""></a></p>
2580         <p><b>Larry Masinter</b><br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2581         <p><b>Paul J. Leach</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>Email: <a href=""></a></p>
2582         <p><b>Tim Berners-Lee</b><br>World Wide Web Consortium<br>MIT Computer Science and Artificial Intelligence Laboratory<br>The Stata Center, Building 32<br>32 Vassar Street<br>Cambridge, MA&nbsp;02139<br>USA<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2583         <p><b>Yves Lafon</b>
2584            (editor)
2585            <br>World Wide Web Consortium<br>W3C / ERCIM<br>2004, rte des Lucioles<br>Sophia-Antipolis, AM&nbsp;06902<br>France<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2586         <p><b>Mark Nottingham</b>
2587            (editor)
2588            <br>Rackspace<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2589         <p><b>Julian F. Reschke</b>
2590            (editor)
2591            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Phone: <a href="tel:+492512807760">+49 251 2807760</a><br>Fax: <a href="fax:+492512807761">+49 251 2807761</a><br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2592      </div>
2593   </body>
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