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472</style><link rel="Contents" href="#rfc.toc">
473      <link rel="Author" href="#rfc.authors">
474      <link rel="Copyright" href="#rfc.copyrightnotice">
475      <link rel="Index" href="#rfc.index">
476      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
477      <link rel="Chapter" title="2 Overview of Cache Operation" href="#rfc.section.2">
478      <link rel="Chapter" title="3 Storing Responses in Caches" href="#rfc.section.3">
479      <link rel="Chapter" title="4 Constructing Responses from Caches" href="#rfc.section.4">
480      <link rel="Chapter" title="5 Updating Caches with HEAD Responses" href="#rfc.section.5">
481      <link rel="Chapter" title="6 Request Methods that Invalidate" href="#rfc.section.6">
482      <link rel="Chapter" title="7 Header Field Definitions" href="#rfc.section.7">
483      <link rel="Chapter" title="8 History Lists" href="#rfc.section.8">
484      <link rel="Chapter" title="9 IANA Considerations" href="#rfc.section.9">
485      <link rel="Chapter" title="10 Security Considerations" href="#rfc.section.10">
486      <link rel="Chapter" title="11 Acknowledgments" href="#rfc.section.11">
487      <link rel="Chapter" href="#rfc.section.12" title="12 References">
488      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
489      <link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B">
490      <link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C">
491      <link rel="Appendix" title="D Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.D">
492      <link href="p5-range.html" rel="prev">
493      <link href="p7-auth.html" rel="next">
494      <meta name="generator" content=", Revision 1.598, 2013/06/23 14:11:10, XSLT vendor: SAXON 8.9 from Saxonica">
495      <link rel="schema.dct" href="">
496      <meta name="dct.creator" content="Fielding, R.">
497      <meta name="dct.creator" content="Nottingham, M.">
498      <meta name="dct.creator" content="Reschke, J. F.">
499      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p6-cache-latest">
500      <meta name="dct.issued" scheme="ISO8601" content="2013-07-01">
501      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
502      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
503      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
504   </head>
505   <body onload="init();">
506      <table class="header">
507         <tbody>
508            <tr>
509               <td class="left">HTTPbis Working Group</td>
510               <td class="right">R. Fielding, Editor</td>
511            </tr>
512            <tr>
513               <td class="left">Internet-Draft</td>
514               <td class="right">Adobe</td>
515            </tr>
516            <tr>
517               <td class="left">Obsoletes: <a href="">2616</a> (if approved)
518               </td>
519               <td class="right">M. Nottingham, Editor</td>
520            </tr>
521            <tr>
522               <td class="left">Intended status: Standards Track</td>
523               <td class="right">Akamai</td>
524            </tr>
525            <tr>
526               <td class="left">Expires: January 2, 2014</td>
527               <td class="right">J. Reschke, Editor</td>
528            </tr>
529            <tr>
530               <td class="left"></td>
531               <td class="right">greenbytes</td>
532            </tr>
533            <tr>
534               <td class="left"></td>
535               <td class="right">July 1, 2013</td>
536            </tr>
537         </tbody>
538      </table>
539      <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Caching<br><span class="filename">draft-ietf-httpbis-p6-cache-latest</span></p>
540      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
541      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
542         systems. This document defines requirements on HTTP caches and the associated header fields that control cache behavior or
543         indicate cacheable response messages.
544      </p>
545      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
546      <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (, which is archived at &lt;<a href=""></a>&gt;.
547      </p>
548      <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;.
549      </p>
550      <p>The changes in this draft are summarized in <a href="#changes.since.22" title="Since draft-ietf-httpbis-p6-cache-22">Appendix&nbsp;D.4</a>.
551      </p>
552      <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
553      <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
554      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
555         working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href=""></a>.
556      </p>
557      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
558         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
559         in progress”.
560      </p>
561      <p>This Internet-Draft will expire on January 2, 2014.</p>
562      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
563      <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
564      <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
565         and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
566         text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
567         BSD License.
568      </p>
569      <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
570         10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
571         allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
572         controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
573         works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
574         it into languages other than English.
575      </p>
576      <hr class="noprint">
577      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
578      <ul class="toc">
579         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul>
580               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#conformance">Conformance and Error Handling</a></li>
581               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul>
582                     <li><a href="#rfc.section.1.2.1">1.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#delta-seconds">Delta Seconds</a></li>
583                  </ul>
584               </li>
585            </ul>
586         </li>
587         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview of Cache Operation</a></li>
588         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Storing Responses in Caches</a><ul>
589               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#incomplete.responses">Storing Incomplete Responses</a></li>
590               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#caching.authenticated.responses">Storing Responses to Authenticated Requests</a></li>
591               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#combining.responses">Combining Partial Content</a></li>
592            </ul>
593         </li>
594         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a><ul>
595               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness</a><ul>
596                     <li><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></li>
597                     <li><a href="#rfc.section.4.1.2">4.1.2</a>&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></li>
598                     <li><a href="#rfc.section.4.1.3">4.1.3</a>&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
599                     <li><a href="#rfc.section.4.1.4">4.1.4</a>&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
600                  </ul>
601               </li>
602               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation</a><ul>
603                     <li><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#freshening.responses">Freshening Stored Responses upon Validation</a></li>
604                  </ul>
605               </li>
606               <li><a href="#rfc.section.4.3">4.3</a>&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Calculating Secondary Keys with Vary</a></li>
607            </ul>
608         </li>
609         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#head.effects">Updating Caches with HEAD Responses</a></li>
610         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
611         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.definitions">Header Field Definitions</a><ul>
612               <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
613               <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul>
614                     <li><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a><ul>
615                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.max-age">max-age</a></li>
616                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.max-stale">max-stale</a></li>
617                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.min-fresh">min-fresh</a></li>
618                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="">no-cache</a></li>
619                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="">no-store</a></li>
620                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="">no-transform</a></li>
621                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.only-if-cached">only-if-cached</a></li>
622                        </ul>
623                     </li>
624                     <li><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a><ul>
625                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.must-revalidate">must-revalidate</a></li>
626                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="">no-cache</a></li>
627                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="">no-store</a></li>
628                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="">no-transform</a></li>
629                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.public">public</a></li>
630                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.private">private</a></li>
631                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.proxy-revalidate">proxy-revalidate</a></li>
632                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.max-age">max-age</a></li>
633                           <li><a href="#rfc.section."></a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.s-maxage">s-maxage</a></li>
634                        </ul>
635                     </li>
636                     <li><a href="#rfc.section.7.2.3">7.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
637                  </ul>
638               </li>
639               <li><a href="#rfc.section.7.3">7.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
640               <li><a href="#rfc.section.7.4">7.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
641               <li><a href="#rfc.section.7.5">7.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a><ul>
642                     <li><a href="#rfc.section.7.5.1">7.5.1</a>&nbsp;&nbsp;&nbsp;<a href="#warn.110">110 Response is Stale</a></li>
643                     <li><a href="#rfc.section.7.5.2">7.5.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.111">111 Revalidation Failed</a></li>
644                     <li><a href="#rfc.section.7.5.3">7.5.3</a>&nbsp;&nbsp;&nbsp;<a href="#warn.112">112 Disconnected Operation</a></li>
645                     <li><a href="#rfc.section.7.5.4">7.5.4</a>&nbsp;&nbsp;&nbsp;<a href="#warn.113">113 Heuristic Expiration</a></li>
646                     <li><a href="#rfc.section.7.5.5">7.5.5</a>&nbsp;&nbsp;&nbsp;<a href="#warn.199">199 Miscellaneous Warning</a></li>
647                     <li><a href="#rfc.section.7.5.6">7.5.6</a>&nbsp;&nbsp;&nbsp;<a href="#warn.214">214 Transformation Applied</a></li>
648                     <li><a href="#rfc.section.7.5.7">7.5.7</a>&nbsp;&nbsp;&nbsp;<a href="#warn.299">299 Miscellaneous Persistent Warning</a></li>
649                     <li><a href="#rfc.section.7.5.8">7.5.8</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.extensions">Warn Code Extensions</a></li>
650                  </ul>
651               </li>
652            </ul>
653         </li>
654         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
655         <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#iana.considerations">IANA Considerations</a><ul>
656               <li><a href="#rfc.section.9.1">9.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registry">Cache Directive Registry</a><ul>
657                     <li><a href="#rfc.section.9.1.1">9.1.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registry.procedure">Procedure</a></li>
658                     <li><a href="#rfc.section.9.1.2">9.1.2</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.considerations">Considerations for New Cache Control Directives</a></li>
659                     <li><a href="#rfc.section.9.1.3">9.1.3</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registration">Registrations</a></li>
660                  </ul>
661               </li>
662               <li><a href="#rfc.section.9.2">9.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registry">Warn Code Registry</a><ul>
663                     <li><a href="#rfc.section.9.2.1">9.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registry.procedure">Procedure</a></li>
664                     <li><a href="#rfc.section.9.2.2">9.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registration">Registrations</a></li>
665                  </ul>
666               </li>
667               <li><a href="#rfc.section.9.3">9.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
668            </ul>
669         </li>
670         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
671         <li><a href="#rfc.section.11">11.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
672         <li><a href="#rfc.section.12">12.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
673               <li><a href="#rfc.section.12.1">12.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
674               <li><a href="#rfc.section.12.2">12.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
675            </ul>
676         </li>
677         <li><a href="#rfc.authors">Authors' Addresses</a></li>
678         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
679         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
680         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
681         <li><a href="#rfc.section.D">D.</a>&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
682               <li><a href="#rfc.section.D.1">D.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.19">Since draft-ietf-httpbis-p6-cache-19</a></li>
683               <li><a href="#rfc.section.D.2">D.2</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.20">Since draft-ietf-httpbis-p6-cache-20</a></li>
684               <li><a href="#rfc.section.D.3">D.3</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.21">Since draft-ietf-httpbis-p6-cache-21</a></li>
685               <li><a href="#rfc.section.D.4">D.4</a>&nbsp;&nbsp;&nbsp;<a href="#changes.since.22">Since draft-ietf-httpbis-p6-cache-22</a></li>
686            </ul>
687         </li>
688         <li><a href="#rfc.index">Index</a></li>
689      </ul>
690      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="caching" href="#caching">Introduction</a></h1>
691      <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.
692         This document defines aspects of HTTP/1.1 related to caching and reusing response messages.
693      </p>
694      <div id="rfc.iref.c.1"></div>
695      <p id="rfc.section.1.p.2">An HTTP <dfn>cache</dfn> is a local store of response messages and the subsystem that controls storage, retrieval, and deletion of messages in it.
696         A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent
697         requests. 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.
698      </p>
699      <div id="rfc.iref.s.1"></div>
700      <div id="rfc.iref.p.1"></div>
701      <div id="shared.and.non-shared.caches">
702         <p id="rfc.section.1.p.3">A <dfn>shared cache</dfn> is a cache that stores responses to be reused by more than one user; shared caches are usually (but not always) deployed as
703            a part of an intermediary. A <dfn>private cache</dfn>, in contrast, is dedicated to a single user.
704         </p>
705      </div>
706      <p id="rfc.section.1.p.4">The goal of caching in HTTP/1.1 is to significantly improve performance by reusing a prior response message to satisfy a current
707         request. A stored response is considered "fresh", as defined in <a href="#expiration.model" title="Freshness">Section&nbsp;4.1</a>, if the response can be reused without "validation" (checking with the origin server to see if the cached response remains
708         valid for this request). A fresh response can therefore reduce both latency and network overhead each time it is reused. When
709         a cached response is not fresh, it might still be reusable if it can be freshened by validation (<a href="#validation.model" title="Validation">Section&nbsp;4.2</a>) or if the origin is unavailable (<a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>).
710      </p>
711      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="conformance" href="#conformance">Conformance and Error Handling</a></h2>
712      <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
713         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>.
714      </p>
715      <p id="rfc.section.1.1.p.2">Conformance criteria and considerations regarding error handling are defined in <a href="p1-messaging.html#conformance" title="Conformance and Error Handling">Section 2.5</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
716      </p>
717      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="notation" href="#notation">Syntax Notation</a></h2>
718      <p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. <a href="#imported.abnf" title="Imported ABNF">Appendix&nbsp;B</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;C</a> shows the collected ABNF with the list rule expanded.
719      </p>
720      <h3 id="rfc.section.1.2.1"><a href="#rfc.section.1.2.1">1.2.1</a>&nbsp;<a id="delta-seconds" href="#delta-seconds">Delta Seconds</a></h3>
721      <p id="rfc.section.1.2.1.p.1">The delta-seconds rule specifies a non-negative integer, representing time in seconds.</p>
722      <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.g.1"></span>  <a href="#delta-seconds" class="smpl">delta-seconds</a>  = 1*<a href="#imported.abnf" class="smpl">DIGIT</a>
723</pre><p id="rfc.section.1.2.1.p.3">If a cache receives a delta-seconds value larger than the largest positive integer it can represent, or if any of its subsequent
724         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> generate delta-seconds with a value greater than 2147483648.
725      </p>
726      <div id="rfc.iref.c.2"></div>
727      <div id="rfc.iref.c.3"></div>
728      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Overview of Cache Operation</a></h1>
729      <p id="rfc.section.2.p.1">Proper cache operation preserves the semantics of HTTP transfers (<a href="#Part2" id="rfc.xref.Part2.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[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
730         no requirement or local configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from
731         either storing a non-reusable response or reusing a stored response inappropriately, rather than mandating that caches always
732         store and reuse particular responses.
733      </p>
734      <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
735         form of cache entry is a successful result of a retrieval request: i.e., a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to a GET request, which contains a representation of the resource identified by the request target (<a href="p2-semantics.html#GET" title="GET">Section 4.3.1</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). However, it is also possible to cache permanent redirects, negative results (e.g., <a href="p2-semantics.html#status.404" class="smpl">404 (Not Found)</a>), incomplete results (e.g., <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a>), and responses to methods other than GET if the method's definition allows such caching and defines something suitable for
736         use as a cache key.
737      </p>
738      <div id="rfc.iref.c.4"></div>
739      <p id="rfc.section.2.p.3">The primary <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
740         responses to GET, many caches simply decline other methods and use only the URI as the primary cache key.
741      </p>
742      <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
743         by a secondary key for the values of the original request's selecting header fields (<a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.3</a>).
744      </p>
745      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Storing Responses in Caches</a></h1>
746      <p id="rfc.section.3.p.1">A cache <em class="bcp14">MUST NOT</em> store a response to any request, unless:
747      </p>
748      <ul>
749         <li>The request method is understood by the cache and defined as being cacheable, and</li>
750         <li>the response status code is understood by the cache, and</li>
751         <li>the "no-store" cache directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;7.2</a>) does not appear in request or response header fields, and
752         </li>
753         <li>the "private" cache response directive (see <a href="#cache-response-directive.private" title="private">Section&nbsp;</a>) does not appear in the response, if the cache is shared, and
754         </li>
755         <li>the <a href="p7-auth.html#header.authorization" class="smpl">Authorization</a> 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="Hypertext Transfer Protocol (HTTP/1.1): 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="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a>), and
756         </li>
757         <li>the response either:
758            <ul>
759               <li>contains an <a href="#header.expires" class="smpl">Expires</a> header field (see <a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;7.3</a>), or
760               </li>
761               <li>contains a max-age response cache directive (see <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;</a>), or
762               </li>
763               <li>contains a s-maxage response cache directive (see <a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;</a>) and the cache is shared, or
764               </li>
765               <li>contains a Cache Control Extension (see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a>) that allows it to be cached, or
766               </li>
767               <li>has a status code that is defined as cacheable (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.1.2</a>), or
768               </li>
769               <li>contains a public response cache directive (see <a href="#cache-response-directive.public" title="public">Section&nbsp;</a>).
770               </li>
771            </ul>
772         </li>
773      </ul>
774      <p id="rfc.section.3.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;7.2.3</a>.
775      </p>
776      <p id="rfc.section.3.p.3">In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements all
777         specified caching-related behavior.
778      </p>
779      <p id="rfc.section.3.p.4">Note that, in normal operation, some caches will not store a response that has neither a cache validator nor an explicit expiration
780         time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.
781      </p>
782      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="incomplete.responses" href="#incomplete.responses">Storing Incomplete Responses</a></h2>
783      <p id="rfc.section.3.1.p.1">A response message is considered complete when all of the octets indicated by the message framing (<a href="#Part1" id="rfc.xref.Part1.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) are received prior to the connection being closed. If the request is GET, the response status is <a href="p2-semantics.html#status.200" class="smpl">200
784            (OK)</a>, and the entire 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 <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> response <em class="bcp14">MAY</em> be stored as if it were an incomplete <a href="p2-semantics.html#status.200" class="smpl">200
785            (OK)</a> cache entry. However, a cache <em class="bcp14">MUST NOT</em> store incomplete or partial content responses if it does not support the <a href="p5-range.html#header.range" class="smpl">Range</a> and <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> header fields or if it does not understand the range units used in those fields.
786      </p>
787      <p id="rfc.section.3.1.p.2">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="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[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;3.3</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
788         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 <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> status code.
789      </p>
790      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="caching.authenticated.responses" href="#caching.authenticated.responses">Storing Responses to Authenticated Requests</a></h2>
791      <p id="rfc.section.3.2.p.1">A shared cache <em class="bcp14">MUST NOT</em> use a cached response to a request with an <a href="p7-auth.html#header.authorization" class="smpl">Authorization</a> 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="Hypertext Transfer Protocol (HTTP/1.1): 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.
792      </p>
793      <p id="rfc.section.3.2.p.2">In this specification, the following <a href="#header.cache-control" class="smpl">Cache-Control</a> response directives (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;7.2.2</a>) have such an effect: must-revalidate, public, s-maxage.
794      </p>
795      <p id="rfc.section.3.2.p.3">Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be
796         served stale (<a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy
797         a subsequent request without revalidating it on the origin server.
798      </p>
799      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="combining.responses" href="#combining.responses">Combining Partial Content</a></h2>
800      <p id="rfc.section.3.3.p.1">A response might transfer only a partial representation if the connection closed prematurely or if the request used one or
801         more Range specifiers (<a href="#Part5" id="rfc.xref.Part5.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[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
802         same strong validator and the cache complies with the client requirements in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4.3</a> of <a href="#Part5" id="rfc.xref.Part5.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>.
803      </p>
804      <p id="rfc.section.3.3.p.2">When combining the new response with one or more stored responses, a cache <em class="bcp14">MUST</em>:
805      </p>
806      <ul>
807         <li>delete any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;7.5</a>);
808         </li>
809         <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
810         </li>
811         <li>use other header fields provided in the new response, aside from <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a>, to replace all instances of the corresponding header fields in the stored response.
812         </li>
813      </ul>
814      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h1>
815      <p id="rfc.section.4.p.1">When presented with a request, a cache <em class="bcp14">MUST NOT</em> reuse a stored response, unless:
816      </p>
817      <ul>
818         <li>The presented effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) and that of the stored response match, and
819         </li>
820         <li>the request method associated with the stored response allows it to be used for the presented request, and</li>
821         <li>selecting header fields nominated by the stored response (if any) match those presented (see <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.3</a>), and
822         </li>
823         <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;7.4</a>), nor the no-cache cache directive (<a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a>), unless the stored response is successfully validated (<a href="#validation.model" title="Validation">Section&nbsp;4.2</a>), and
824         </li>
825         <li>the stored response does not contain the no-cache cache directive (<a href="" title="no-cache">Section&nbsp;</a>), unless it is successfully validated (<a href="#validation.model" title="Validation">Section&nbsp;4.2</a>), and
826         </li>
827         <li>the stored response is either:
828            <ul>
829               <li>fresh (see <a href="#expiration.model" title="Freshness">Section&nbsp;4.1</a>), or
830               </li>
831               <li>allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>), or
832               </li>
833               <li>successfully validated (see <a href="#validation.model" title="Validation">Section&nbsp;4.2</a>).
834               </li>
835            </ul>
836         </li>
837      </ul>
838      <p id="rfc.section.4.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;7.2.3</a>.
839      </p>
840      <p id="rfc.section.4.p.3">When a stored response is used to satisfy a request without validation, a cache <em class="bcp14">MUST</em> generate an <a href="#header.age" class="smpl">Age</a> header field (<a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;7.1</a>), replacing any present in the response with a value equal to the stored response's current_age; see <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>.
841      </p>
842      <p id="rfc.section.4.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 4.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) to the origin server; i.e., a cache is not allowed to generate a reply to such a request before having forwarded the request
843         and having received a corresponding response.
844      </p>
845      <p id="rfc.section.4.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;6</a>.
846      </p>
847      <p id="rfc.section.4.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 <a href="" class="smpl">Date</a> header field). It can also forward the request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate
848         which response to use.
849      </p>
850      <p id="rfc.section.4.p.7">A cache that does not have a clock available <em class="bcp14">MUST NOT</em> use stored responses without revalidating them upon every use.
851      </p>
852      <div id="rfc.iref.f.1"></div>
853      <div id="rfc.iref.s.2"></div>
854      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="expiration.model" href="#expiration.model">Freshness</a></h2>
855      <p id="rfc.section.4.1.p.1">A <dfn>fresh</dfn> response is one whose age has not yet exceeded its freshness lifetime. Conversely, a <dfn>stale</dfn> response is one where it has.
856      </p>
857      <div id="rfc.iref.f.2"></div>
858      <div id="rfc.iref.e.1"></div>
859      <div id="rfc.iref.h.1"></div>
860      <p id="rfc.section.4.1.p.2">A response's <dfn>freshness lifetime</dfn> is the length of time between its generation by the origin server and its expiration time. An <dfn>explicit expiration time</dfn> is the time at which the origin server intends that a stored response can no longer be used by a cache without further validation,
861         whereas a <dfn>heuristic expiration time</dfn> is assigned by a cache when no explicit expiriation time is available.
862      </p>
863      <div id="rfc.iref.a.1"></div>
864      <p id="rfc.section.4.1.p.3">A response's <dfn>age</dfn> is the time that has passed since it was generated by, or successfully validated with, the origin server.
865      </p>
866      <p id="rfc.section.4.1.p.4">When a response is "fresh" in the cache, it can be used to satisfy subsequent requests without contacting the origin server,
867         thereby improving efficiency.
868      </p>
869      <p id="rfc.section.4.1.p.5">The primary mechanism for determining freshness is for an origin server to provide an explicit expiration time in the future,
870         using either the <a href="#header.expires" class="smpl">Expires</a> header field (<a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;7.3</a>) or the max-age response cache directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;</a>). Generally, origin servers will assign future explicit expiration times to responses in the belief that the representation
871         is not likely to change in a semantically significant way before the expiration time is reached.
872      </p>
873      <p id="rfc.section.4.1.p.6">If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past
874         to indicate that the response is already stale. Compliant caches will normally validate a stale cached response before reusing
875         it for subsequent requests (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>).
876      </p>
877      <p id="rfc.section.4.1.p.7">Since origin servers do not always provide explicit expiration times, caches are also allowed to use a heuristic to determine
878         an expiration time under certain circumstances (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.1.2</a>).
879      </p>
880      <div id="rfc.figure.u.2"></div>
881      <p>The calculation to determine if a response is fresh is:</p><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
882</pre><p id="rfc.section.4.1.p.9">freshness_lifetime is defined in <a href="#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section&nbsp;4.1.1</a>; current_age is defined in <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>.
883      </p>
884      <p id="rfc.section.4.1.p.10">Clients can send the max-age or min-fresh cache directives in a request to constrain or relax freshness calculations for the
885         corresponding response (<a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a>).
886      </p>
887      <p id="rfc.section.4.1.p.11">When calculating freshness, to avoid common problems in date parsing:</p>
888      <p id="rfc.section.4.1.p.12"></p>
889      <ul>
890         <li>Although all date formats are specified to be case-sensitive, cache recipients <em class="bcp14">SHOULD</em> match day, week and timezone names case-insensitively.
891         </li>
892         <li>If a cache recipient's internal implementation of time has less resolution than the value of an HTTP-date, the recipient <em class="bcp14">MUST</em> internally represent a parsed <a href="#header.expires" class="smpl">Expires</a> date as the nearest time equal to or earlier than the received value.
893         </li>
894         <li>Cache recipients <em class="bcp14">MUST NOT</em> allow local time zones to influence the calculation or comparison of an age or expiration time.
895         </li>
896         <li>Cache recipients <em class="bcp14">SHOULD</em> consider a date with a zone abbreviation other than "GMT" to be invalid for calculating expiration.
897         </li>
898      </ul>
899      <p id="rfc.section.4.1.p.13">Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload
900         a resource. See <a href="#history.lists" title="History Lists">Section&nbsp;8</a> for an explanation of the difference between caches and history mechanisms.
901      </p>
902      <h3 id="rfc.section.4.1.1"><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;<a id="calculating.freshness.lifetime" href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h3>
903      <p id="rfc.section.4.1.1.p.1">A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of: </p>
904      <ul>
905         <li>If the cache is shared and the s-maxage response cache directive (<a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;</a>) is present, use its value, or
906         </li>
907         <li>If the max-age response cache directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;</a>) is present, use its value, or
908         </li>
909         <li>If the <a href="#header.expires" class="smpl">Expires</a> response header field (<a href="#header.expires" id="rfc.xref.header.expires.3" title="Expires">Section&nbsp;7.3</a>) is present, use its value minus the value of the <a href="" class="smpl">Date</a> response header field, or
910         </li>
911         <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;4.1.2</a>.
912         </li>
913      </ul>
914      <p id="rfc.section.4.1.1.p.2">Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server.</p>
915      <p id="rfc.section.4.1.1.p.3">When there is more than one value present for a given directive (e.g., two <a href="#header.expires" class="smpl">Expires</a> header fields, multiple Cache-Control: max-age directives), the directive's value is considered invalid. Caches are encouraged
916         to consider responses that have invalid freshness information to be stale.
917      </p>
918      <h3 id="rfc.section.4.1.2"><a href="#rfc.section.4.1.2">4.1.2</a>&nbsp;<a id="heuristic.freshness" href="#heuristic.freshness">Calculating Heuristic Freshness</a></h3>
919      <p id="rfc.section.4.1.2.p.1">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
920         values (such as the <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> time) to estimate a plausible expiration time. This specification does not provide specific algorithms, but does impose worst-case
921         constraints on their results.
922      </p>
923      <p id="rfc.section.4.1.2.p.2">A cache <em class="bcp14">MUST NOT</em> use heuristics to determine freshness when an explicit expiration time is present in the stored response. Because of the requirements
924         in <a href="#response.cacheability" title="Storing Responses in Caches">Section&nbsp;3</a>, this means that, effectively, heuristics can only be used on responses without explicit freshness whose status codes are
925         defined as cacheable, and responses without explicit freshness that have been marked as explicitly cacheable (e.g., with a
926         "public" response cache directive).
927      </p>
928      <p id="rfc.section.4.1.2.p.3">If the response has a <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> header field (<a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 2.2</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): 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
929         time. A typical setting of this fraction might be 10%.
930      </p>
931      <p id="rfc.section.4.1.2.p.4">When a heuristic is used to calculate freshness lifetime, a cache <em class="bcp14">SHOULD</em> attach a <a href="#header.warning" class="smpl">Warning</a> header field with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning is not already
932         present.
933      </p>
934      <div class="note" id="rfc.section.4.1.2.p.5">
935         <p><b>Note:</b> <a href="">Section 13.9</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a> prohibited caches from calculating heuristic freshness for URIs with query components (i.e., those containing '?'). In practice,
936            this has not been widely implemented. Therefore, origin servers are encouraged to send explicit directives (e.g., Cache-Control:
937            no-cache) if they wish to preclude caching.
938         </p>
939      </div>
940      <h3 id="rfc.section.4.1.3"><a href="#rfc.section.4.1.3">4.1.3</a>&nbsp;<a id="age.calculations" href="#age.calculations">Calculating Age</a></h3>
941      <p id="rfc.section.4.1.3.p.1">The <a href="#header.age" class="smpl">Age</a> header field is used to convey an estimated age of the response message when obtained from a cache. The Age field value is
942         the cache's estimate of the number of seconds since the response was generated or validated by the origin server. In essence,
943         the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin
944         server, plus the amount of time it has been in transit along network paths.
945      </p>
946      <p id="rfc.section.4.1.3.p.2">The following data is used for the age calculation:</p>
947      <p id="rfc.section.4.1.3.p.3"><dfn>age_value</dfn>
948      </p>
949      <ul class="empty">
950         <li>The term "age_value" denotes the value of the <a href="#header.age" class="smpl">Age</a> header field (<a href="#header.age" id="rfc.xref.header.age.2" title="Age">Section&nbsp;7.1</a>), in a form appropriate for arithmetic operation; or 0, if not available.
951         </li>
952      </ul>
953      <p id="rfc.section.4.1.3.p.4"><dfn>date_value</dfn>
954      </p>
955      <ul class="empty">
956         <li>The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic operations. See <a href="" title="Date">Section</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> for the definition of the Date header field, and for requirements regarding responses without it.
957         </li>
958      </ul>
959      <p id="rfc.section.4.1.3.p.5"><dfn>now</dfn>
960      </p>
961      <ul class="empty">
962         <li>The term "now" means "the current value of the clock at the host performing the calculation". A host ought to use NTP (<a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>) or some similar protocol to synchronize its clocks to Coordinated Universal Time.
963         </li>
964      </ul>
965      <p id="rfc.section.4.1.3.p.6"><dfn>request_time</dfn>
966      </p>
967      <ul class="empty">
968         <li>The current value of the clock at the host at the time the request resulting in the stored response was made.</li>
969      </ul>
970      <p id="rfc.section.4.1.3.p.7"><dfn>response_time</dfn>
971      </p>
972      <ul class="empty">
973         <li>The current value of the clock at the host at the time the response was received.</li>
974      </ul>
975      <p id="rfc.section.4.1.3.p.8">A response's age can be calculated in two entirely independent ways: </p>
976      <ol>
977         <li>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the origin server's
978            clock. If the result is negative, the result is replaced by zero.
979         </li>
980         <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.
981         </li>
982      </ol>
983      <div id="rfc.figure.u.3"></div><pre class="text">  apparent_age = max(0, response_time - date_value);
985  response_delay = response_time - request_time;
986  corrected_age_value = age_value + response_delay; 
987</pre><div id="rfc.figure.u.4"></div>
988      <p>These are combined as</p><pre class="text">  corrected_initial_age = max(apparent_age, corrected_age_value);
989</pre><p id="rfc.section.4.1.3.p.11">unless the cache is confident in the value of the <a href="#header.age" class="smpl">Age</a> header field (e.g., because there are no HTTP/1.0 hops in the <a href="p1-messaging.html#header.via" class="smpl">Via</a> header field), in which case the corrected_age_value <em class="bcp14">MAY</em> be used as the corrected_initial_age.
990      </p>
991      <p id="rfc.section.4.1.3.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
992         was last validated by the origin server to the corrected_initial_age.
993      </p>
994      <div id="rfc.figure.u.5"></div><pre class="text">  resident_time = now - response_time;
995  current_age = corrected_initial_age + resident_time;
996</pre><h3 id="rfc.section.4.1.4"><a href="#rfc.section.4.1.4">4.1.4</a>&nbsp;<a id="serving.stale.responses" href="#serving.stale.responses">Serving Stale Responses</a></h3>
997      <p id="rfc.section.4.1.4.p.1">A "stale" response is one that either has explicit expiry information or is allowed to have heuristic expiry calculated, but
998         is not fresh according to the calculations in <a href="#expiration.model" title="Freshness">Section&nbsp;4.1</a>.
999      </p>
1000      <p id="rfc.section.4.1.4.p.2">A cache <em class="bcp14">MUST NOT</em> generate a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
1001         directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
1002         see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;7.2.2</a>).
1003      </p>
1004      <p id="rfc.section.4.1.4.p.3">A cache <em class="bcp14">MUST NOT</em> send stale responses unless it is disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
1005         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;7.2.1</a>).
1006      </p>
1007      <p id="rfc.section.4.1.4.p.4">A cache <em class="bcp14">SHOULD</em> append a <a href="#header.warning" class="smpl">Warning</a> header field with the 110 warn-code (see <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;7.5</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.
1008      </p>
1009      <div id="rfc.iref.f.3"></div>
1010      <p id="rfc.section.4.1.4.p.5">Note that if a cache receives a <dfn>first-hand</dfn> response (one where the freshness model is not in use; i.e., its age is 0, whether it is an entire response, or a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response) that it would normally forward to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">MAY</em> forward it to the requesting client without adding a new <a href="#header.warning" class="smpl">Warning</a> (but without removing any existing Warning header fields). A cache ought not attempt to validate a response simply because
1011         that response became stale in transit.
1012      </p>
1013      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="validation.model" href="#validation.model">Validation</a></h2>
1014      <p id="rfc.section.4.2.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
1015         fresh, or one cannot be selected; see <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.3</a>), it can use the conditional request mechanism <a href="#Part4" id="rfc.xref.Part4.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): 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
1016         update it. This process is known as "validating" or "revalidating" the stored response.
1017      </p>
1018      <div id="rfc.iref.v.1"></div>
1019      <p id="rfc.section.4.2.p.2">When sending such a conditional request, a cache adds a <dfn>validator</dfn> (or more than one), that is used to find out whether a stored response is an equivalent copy of a current representation of
1020         the resource.
1021      </p>
1022      <p id="rfc.section.4.2.p.3">One such validator is the <a href="p4-conditional.html#header.if-modified-since" class="smpl">If-Modified-Since</a> header field, whose value is that of the <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> header field from the selected (see <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.3</a>) stored response, if available.
1023      </p>
1024      <p id="rfc.section.4.2.p.4">Another is the <a href="p4-conditional.html#header.if-none-match" class="smpl">If-None-Match</a> header field, whose value is that of the <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> header field(s) from relevant responses stored for the primary cache key, if present. However, if any of the stored responses
1025         contains only partial content, the cache ought not include its entity-tag in the If-None-Match header field unless the request
1026         is for a range that would be fully satisfied by that stored response.
1027      </p>
1028      <p id="rfc.section.4.2.p.5">Cache handling of a response to a conditional request is dependent upon its status code:</p>
1029      <p id="rfc.section.4.2.p.6"></p>
1030      <ul>
1031         <li>A <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response status code indicates that the stored response can be updated and reused; see <a href="#freshening.responses" title="Freshening Stored Responses upon Validation">Section&nbsp;4.2.1</a>.
1032         </li>
1033         <li>A full response (i.e., one with a payload body) indicates that none of the stored responses nominated in the conditional request
1034            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).
1035         </li>
1036         <li>However, if a cache receives a <a href="p2-semantics.html#status.5xx" class="smpl">5xx (Server Error)</a> response while attempting to validate a response, it can either forward this response to the requesting client, or act as
1037            if the server failed to respond. In the latter case, it can send a previously stored response (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>).
1038         </li>
1039      </ul>
1040      <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a id="freshening.responses" href="#freshening.responses">Freshening Stored Responses upon Validation</a></h3>
1041      <p id="rfc.section.4.2.1.p.1">When a cache receives a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response and already has one or more stored <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> responses for the same cache key, the cache needs to identify which of the stored responses are updated by this new response
1042         and then update the stored response(s) with the new information provided in the <a href="p4-conditional.html#status.304" class="smpl">304</a> response.
1043      </p>
1044      <div id="rfc.iref.s.3"></div>
1045      <p id="rfc.section.4.2.1.p.2">The stored response to update is identified by using the first match (if any) of: </p>
1046      <ul>
1047         <li>If the new response contains a <dfn>strong validator</dfn> (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.Part4.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>), then that strong validator identifies the selected representation for update. All of the stored responses with the same
1048            strong validator are selected. If none of the stored responses contain the same strong validator, then the cache <em class="bcp14">MUST NOT</em> use the new response to update any stored responses.
1049         </li>
1050         <li>If the new response contains a weak validator and that validator corresponds to one of the cache's stored responses, then
1051            the most recent of those matching stored responses is selected for update.
1052         </li>
1053         <li>If the new response does not include any form of validator (such as in the case where a client generates an If-Modified-Since
1054            request from a source other than the Last-Modified response header field), and there is only one stored response, and that
1055            stored response also lacks a validator, then that stored response is selected for update.
1056         </li>
1057      </ul>
1058      <p id="rfc.section.4.2.1.p.3">If a stored response is selected for update, the cache <em class="bcp14">MUST</em>:
1059      </p>
1060      <ul>
1061         <li>delete any <a href="#header.warning" class="smpl">Warning</a> 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;7.5</a>);
1062         </li>
1063         <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
1064         </li>
1065         <li>use other header fields provided in the <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response to replace all instances of the corresponding header fields in the stored response.
1066         </li>
1067      </ul>
1068      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Calculating Secondary Keys with Vary</a></h2>
1069      <p id="rfc.section.4.3.p.1">When a cache receives a request that can be satisfied by a stored response that has a <a href="p2-semantics.html#header.vary" class="smpl">Vary</a> header field (<a href="p2-semantics.html#header.vary" title="Vary">Section 7.1.4</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></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
1070         request (i.e., that associated with the stored response), and the presented request.
1071      </p>
1072      <p id="rfc.section.4.3.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
1073         to those in the second request by applying any of the following:
1074      </p>
1075      <ul>
1076         <li>adding or removing whitespace, where allowed in the header field's syntax</li>
1077         <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.Part1.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>)
1078         </li>
1079         <li>normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification
1080            (e.g., re-ordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive)
1081         </li>
1082      </ul>
1083      <p id="rfc.section.4.3.p.3">If (after any normalization that might take place) a header field is absent from a request, it can only match another request
1084         if it is also absent there.
1085      </p>
1086      <p id="rfc.section.4.3.p.4">A <a href="p2-semantics.html#header.vary" class="smpl">Vary</a> header field-value of "*" always fails to match.
1087      </p>
1088      <p id="rfc.section.4.3.p.5">The stored response with matching selecting header fields is known as the selected response.</p>
1089      <p id="rfc.section.4.3.p.6">If multiple selected responses are available (potentially including responses without a Vary header field), the cache will
1090         need to choose one to use. When a selecting header field has a known mechanism for doing so (e.g., qvalues on <a href="p2-semantics.html#header.accept" class="smpl">Accept</a> and similar request header fields), that mechanism <em class="bcp14">MAY</em> be used to select preferred responses; of the remainder, the most recent response (as determined by the <a href="" class="smpl">Date</a> header field) is used, as per <a href="#constructing.responses.from.caches" title="Constructing Responses from Caches">Section&nbsp;4</a>.
1091      </p>
1092      <p id="rfc.section.4.3.p.7">If no selected response is available, the cache cannot satisfy the presented request. Typically, it is forwarded to the origin
1093         server in a (possibly conditional; see <a href="#validation.model" title="Validation">Section&nbsp;4.2</a>) request.
1094      </p>
1095      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="head.effects" href="#head.effects">Updating Caches with HEAD Responses</a></h1>
1096      <p id="rfc.section.5.p.1">A response to the HEAD method is identical to what an equivalent request made with a GET would have been, except it lacks
1097         a body. This property of HEAD responses is used to both invalidate and update cached GET responses.
1098      </p>
1099      <p id="rfc.section.5.p.2">If one or more stored GET responses can be selected (as per <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.3</a>) for a HEAD request, and the <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a>, <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> or <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> value of a HEAD response differs from that in a selected GET response, the cache <em class="bcp14">MUST</em> consider that selected response to be stale.
1100      </p>
1101      <p id="rfc.section.5.p.3">If the <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a>, <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> and <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> values of a HEAD response (when present) are the same as that in a selected GET response (as per <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.3</a>), the cache <em class="bcp14">SHOULD</em> update the remaining header fields in the stored response using the following rules:
1102      </p>
1103      <ul>
1104         <li>delete any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;7.5</a>);
1105         </li>
1106         <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
1107         </li>
1108         <li>use other header fields provided in the response to replace all instances of the corresponding header fields in the stored
1109            response.
1110         </li>
1111      </ul>
1112      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></h1>
1113      <p id="rfc.section.6.p.1">Because unsafe request methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) such as PUT, POST or DELETE have the potential for changing state on the origin server, intervening caches can use them
1114         to keep their contents up-to-date.
1115      </p>
1116      <p id="rfc.section.6.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 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) as well as the URI(s) in the <a href="p2-semantics.html#header.location" class="smpl">Location</a> and <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a> response header fields (if present) when a non-error response to a request with an unsafe method is received.
1117      </p>
1118      <p id="rfc.section.6.p.3">However, a cache <em class="bcp14">MUST NOT</em> invalidate a URI from a <a href="p2-semantics.html#header.location" class="smpl">Location</a> or <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a> response header field if the host part of that URI differs from the host part in the effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>). This helps prevent denial of service attacks.
1119      </p>
1120      <p id="rfc.section.6.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 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) when it receives a non-error response to a request with a method whose safety is unknown.
1121      </p>
1122      <p id="rfc.section.6.p.5">Here, a "non-error response" is one with a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="p2-semantics.html#status.3xx" class="smpl">3xx (Redirection)</a> status code. "Invalidate" means that the cache will either remove all stored responses related to the effective request URI,
1123         or will mark these as "invalid" and in need of a mandatory validation before they can be sent in response to a subsequent
1124         request.
1125      </p>
1126      <p id="rfc.section.6.p.6">Note that this does not guarantee that all appropriate responses are invalidated. For example, a state-changing request might
1127         invalidate responses in the caches it travels through, but relevant responses still might be stored in other caches that it
1128         has not.
1129      </p>
1130      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="header.field.definitions" href="#header.field.definitions">Header Field Definitions</a></h1>
1131      <p id="rfc.section.7.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
1132      <div id="rfc.iref.a.2"></div>
1133      <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
1134      <p id="rfc.section.7.1.p.1">The "Age" header field conveys the sender's estimate of the amount of time since the response was generated or successfully
1135         validated at the origin server. Age values are calculated as specified in <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>.
1136      </p>
1137      <div id="rfc.figure.u.6"></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>
1138</pre><p id="rfc.section.7.1.p.3">Age field-values are non-negative integers, representing time in seconds (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>).
1139      </p>
1140      <p id="rfc.section.7.1.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
1141         true, since HTTP/1.0 caches might not implement the Age header field.
1142      </p>
1143      <div id="rfc.iref.c.5"></div>
1144      <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
1145      <p id="rfc.section.7.2.p.1">The "Cache-Control" header field is used to specify directives for caches along the request/response chain. Such cache directives
1146         are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given
1147         in the response.
1148      </p>
1149      <p id="rfc.section.7.2.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;7.2.3</a> for information about how Cache-Control directives defined elsewhere are handled.
1150      </p>
1151      <div class="note" id="rfc.section.7.2.p.3">
1152         <p><b>Note:</b> Some HTTP/1.0 caches might not implement Cache-Control.
1153         </p>
1154      </div>
1155      <p id="rfc.section.7.2.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
1156         might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific
1157         cache.
1158      </p>
1159      <p id="rfc.section.7.2.p.5">Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument, that can use
1160         both token and quoted-string syntax. For the directives defined below that define arguments, recipients ought to accept both
1161         forms, even if one is documented to be preferred. For any directive not defined by this specification, recipients <em class="bcp14">MUST</em> accept both forms.
1162      </p>
1163      <div id="rfc.figure.u.7"></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>
1165  <a href="#header.cache-control" class="smpl">cache-directive</a> = <a href="#imported.abnf" class="smpl">token</a> [ "=" ( <a href="#imported.abnf" class="smpl">token</a> / <a href="#imported.abnf" class="smpl">quoted-string</a> ) ]
1166</pre><p id="rfc.section.7.2.p.7">For the cache directives defined below, no argument is defined (nor allowed) unless stated otherwise.</p>
1167      <h3 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;<a id="cache-request-directive" href="#cache-request-directive">Request Cache-Control Directives</a></h3>
1168      <div id="rfc.iref.m.1"></div>
1169      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-request-directive.max-age" href="#cache-request-directive.max-age">max-age</a></h4>
1170      <p id="rfc.section.">Argument syntax: </p>
1171      <ul class="empty">
1172         <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1173         </li>
1174      </ul>
1175      <p id="rfc.section.">The "max-age" request directive indicates that the client is unwilling to accept a response whose age is greater than the
1176         specified number of seconds. Unless the max-stale request directive is also present, the client is not willing to accept a
1177         stale response.
1178      </p>
1179      <p id="rfc.section."><b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'max-age=5', not 'max-age="5"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
1180      </p>
1181      <div id="rfc.iref.m.2"></div>
1182      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-request-directive.max-stale" href="#cache-request-directive.max-stale">max-stale</a></h4>
1183      <p id="rfc.section.">Argument syntax: </p>
1184      <ul class="empty">
1185         <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1186         </li>
1187      </ul>
1188      <p id="rfc.section.">The "max-stale" request directive indicates that the client is willing to accept a response that has exceeded its freshness
1189         lifetime. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its freshness
1190         lifetime by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing
1191         to accept a stale response of any age.
1192      </p>
1193      <p id="rfc.section."><b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'max-stale=10', not 'max-stale="10"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
1194      </p>
1195      <div id="rfc.iref.m.3"></div>
1196      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-request-directive.min-fresh" href="#cache-request-directive.min-fresh">min-fresh</a></h4>
1197      <p id="rfc.section.">Argument syntax: </p>
1198      <ul class="empty">
1199         <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1200         </li>
1201      </ul>
1202      <p id="rfc.section.">The "min-fresh" request directive indicates that the client is willing to accept a response whose freshness lifetime is no
1203         less than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh
1204         for at least the specified number of seconds.
1205      </p>
1206      <p id="rfc.section."><b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
1207      </p>
1208      <div id="rfc.iref.n.1"></div>
1209      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="" href="">no-cache</a></h4>
1210      <p id="rfc.section.">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.
1211      </p>
1212      <div id="rfc.iref.n.2"></div>
1213      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="" href="">no-store</a></h4>
1214      <p id="rfc.section.">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.
1215      </p>
1216      <p id="rfc.section.">This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1217         might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
1218      </p>
1219      <p id="rfc.section.">Note that if a request containing this directive is satisfied from a cache, the no-store request directive does not apply
1220         to the already stored response.
1221      </p>
1222      <div id="rfc.iref.n.3"></div>
1223      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="" href="">no-transform</a></h4>
1224      <p id="rfc.section.">The "no-transform" request directive indicates that an intermediary (whether or not it implements a cache) <em class="bcp14">MUST NOT</em> transform the payload, as defined in <a href="p1-messaging.html#message.transformations" title="Transformations">Section 5.7.2</a> of <a href="#Part1" id="rfc.xref.Part1.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
1225      </p>
1226      <div id="rfc.iref.o.1"></div>
1227      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-request-directive.only-if-cached" href="#cache-request-directive.only-if-cached">only-if-cached</a></h4>
1228      <p id="rfc.section.">The "only-if-cached" request directive indicates that the client only wishes to obtain a stored response. If it receives this
1229         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 <a href="p2-semantics.html#status.504" class="smpl">504 (Gateway
1230            Timeout)</a> status code. If a group of caches is being operated as a unified system with good internal connectivity, a member cache <em class="bcp14">MAY</em> forward such a request within that group of caches.
1231      </p>
1232      <h3 id="rfc.section.7.2.2"><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;<a id="cache-response-directive" href="#cache-response-directive">Response Cache-Control Directives</a></h3>
1233      <div id="rfc.iref.m.4"></div>
1234      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-response-directive.must-revalidate" href="#cache-response-directive.must-revalidate">must-revalidate</a></h4>
1235      <p id="rfc.section.">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.
1236      </p>
1237      <p id="rfc.section.">The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
1238         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 <a href="p2-semantics.html#status.504" class="smpl">504 (Gateway Timeout)</a> response.
1239      </p>
1240      <p id="rfc.section.">The must-revalidate directive ought to be used by servers if and only if failure to validate a request on the representation
1241         could result in incorrect operation, such as a silently unexecuted financial transaction.
1242      </p>
1243      <div id="rfc.iref.n.4"></div>
1244      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="" href="">no-cache</a></h4>
1245      <p id="rfc.section.">Argument syntax: </p>
1246      <ul class="empty">
1247         <li>#<a href="#imported.abnf" class="smpl">field-name</a>
1248         </li>
1249      </ul>
1250      <p id="rfc.section.">The "no-cache" response directive indicates that the response <em class="bcp14">MUST NOT</em> be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to
1251         prevent a cache from using it to satisfy a request without contacting it, even by caches that have been configured to send
1252         stale responses.
1253      </p>
1254      <p id="rfc.section.">If the no-cache response directive specifies one or more field-names, then a cache <em class="bcp14">MAY</em> use the response to satisfy a subsequent request, subject to any other restrictions on caching. However, any header fields
1255         in the response that have the field-name(s) listed <em class="bcp14">MUST NOT</em> be sent in the response to a subsequent request without successful revalidation with the origin server. This allows an origin
1256         server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
1257      </p>
1258      <p id="rfc.section.">The field-names given are not limited to the set of header fields defined by this specification. Field names are case-insensitive.</p>
1259      <p id="rfc.section."><b>Note:</b> Although it has been back-ported to many implementations, some HTTP/1.0 caches will not recognize or obey this directive.
1260         Also, no-cache response directives with field-names are often handled by caches as if an unqualified no-cache directive was
1261         received; i.e., the special handling for the qualified form is not widely implemented.
1262      </p>
1263      <p id="rfc.section."><b>Note:</b> This directive uses the quoted-string form of the argument syntax. Senders <em class="bcp14">SHOULD NOT</em> use the token form (even if quoting appears not to be needed for single-entry lists).
1264      </p>
1265      <div id="rfc.iref.n.5"></div>
1266      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="" href="">no-store</a></h4>
1267      <p id="rfc.section.">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.
1268      </p>
1269      <p id="rfc.section.">This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1270         might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
1271      </p>
1272      <div id="rfc.iref.n.6"></div>
1273      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="" href="">no-transform</a></h4>
1274      <p id="rfc.section.">The "no-transform" response directive indicates that an intermediary (regardless of whether it implements a cache) <em class="bcp14">MUST NOT</em> transform the payload, as defined in <a href="p1-messaging.html#message.transformations" title="Transformations">Section 5.7.2</a> of <a href="#Part1" id="rfc.xref.Part1.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
1275      </p>
1276      <div id="rfc.iref.p.2"></div>
1277      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-response-directive.public" href="#cache-response-directive.public">public</a></h4>
1278      <p id="rfc.section.">The "public" response directive indicates that any cache <em class="bcp14">MAY</em> store the response, even if the response would normally be non-cacheable or cacheable only within a non-shared cache. (See <a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a> for additional details related to the use of public in response to a request containing <a href="p7-auth.html#header.authorization" class="smpl">Authorization</a>, and <a href="#response.cacheability" title="Storing Responses in Caches">Section&nbsp;3</a> for details of how public affects responses that would normally not be stored, due to their status codes not being defined
1279         as cacheable.)
1280      </p>
1281      <div id="rfc.iref.p.3"></div>
1282      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-response-directive.private" href="#cache-response-directive.private">private</a></h4>
1283      <p id="rfc.section.">Argument syntax: </p>
1284      <ul class="empty">
1285         <li>#<a href="#imported.abnf" class="smpl">field-name</a>
1286         </li>
1287      </ul>
1288      <p id="rfc.section.">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 and reuse it for later requests, even if the response would normally be non-cacheable.
1289      </p>
1290      <p id="rfc.section.">If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated
1291         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.
1292      </p>
1293      <p id="rfc.section.">The field-names given are not limited to the set of header fields defined by this specification. Field names are case-insensitive.</p>
1294      <p id="rfc.section."><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
1295         content. Also, private response directives with field-names are often handled by caches as if an unqualified private directive
1296         was received; i.e., the special handling for the qualified form is not widely implemented.
1297      </p>
1298      <p id="rfc.section."><b>Note:</b> This directive uses the quoted-string form of the argument syntax. Senders <em class="bcp14">SHOULD NOT</em> use the token form (even if quoting appears not to be needed for single-entry lists).
1299      </p>
1300      <div id="rfc.iref.p.4"></div>
1301      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-response-directive.proxy-revalidate" href="#cache-response-directive.proxy-revalidate">proxy-revalidate</a></h4>
1302      <p id="rfc.section.">The "proxy-revalidate" response directive has the same meaning as the must-revalidate response directive, except that it does
1303         not apply to private caches.
1304      </p>
1305      <div id="rfc.iref.m.5"></div>
1306      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-response-directive.max-age" href="#cache-response-directive.max-age">max-age</a></h4>
1307      <p id="rfc.section.">Argument syntax: </p>
1308      <ul class="empty">
1309         <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1310         </li>
1311      </ul>
1312      <p id="rfc.section.">The "max-age" response directive indicates that the response is to be considered stale after its age is greater than the specified
1313         number of seconds.
1314      </p>
1315      <p id="rfc.section."><b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'max-age=5', not 'max-age="5"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
1316      </p>
1317      <div id="rfc.iref.s.4"></div>
1318      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="cache-response-directive.s-maxage" href="#cache-response-directive.s-maxage">s-maxage</a></h4>
1319      <p id="rfc.section.">Argument syntax: </p>
1320      <ul class="empty">
1321         <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1322         </li>
1323      </ul>
1324      <p id="rfc.section.">The "s-maxage" response directive indicates that, in shared caches, the maximum age specified by this directive overrides
1325         the maximum age specified by either the max-age directive or the <a href="#header.expires" class="smpl">Expires</a> header field. The s-maxage directive also implies the semantics of the proxy-revalidate response directive.
1326      </p>
1327      <p id="rfc.section."><b>Note:</b> This directive uses the token form of the argument syntax; e.g., 's-maxage=10', not 's-maxage="10"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
1328      </p>
1329      <h3 id="rfc.section.7.2.3"><a href="#rfc.section.7.2.3">7.2.3</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
1330      <p id="rfc.section.7.2.3.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
1331         value.
1332      </p>
1333      <p id="rfc.section.7.2.3.p.2">Informational extensions (those that do not require a change in cache behavior) can be added without changing the semantics
1334         of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives.
1335      </p>
1336      <p id="rfc.section.7.2.3.p.3">Both the new directive and the standard directive are supplied, such that applications that do not understand the new directive
1337         will default to the behavior specified by the standard directive, and those that understand the new directive will recognize
1338         it as modifying the requirements associated with the standard directive. In this way, extensions to the cache-control directives
1339         can be made without requiring changes to the base protocol.
1340      </p>
1341      <p id="rfc.section.7.2.3.p.4">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
1342         obeying certain extensions, and ignoring all directives that it does not understand.
1343      </p>
1344      <p id="rfc.section.7.2.3.p.5">For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive.
1345         We define this new directive to mean that, in addition to any private cache, any cache that is shared only by members of the
1346         community named within its value is allowed to cache the response. An origin server wishing to allow the UCI community to
1347         use an otherwise private response in their shared cache(s) could do so by including
1348      </p>
1349      <div id="rfc.figure.u.8"></div><pre class="text">  Cache-Control: private, community="UCI"
1350</pre><p id="rfc.section.7.2.3.p.7">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
1351         it will also see and understand the private directive and thus default to the safe behavior.
1352      </p>
1353      <p id="rfc.section.7.2.3.p.8">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
1354         will be combined with standard directives (or the response's default cacheability) such that the cache behavior will remain
1355         minimally correct even if the cache does not understand the extension(s).
1356      </p>
1357      <div id="rfc.iref.e.2"></div>
1358      <h2 id="rfc.section.7.3"><a href="#rfc.section.7.3">7.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
1359      <p id="rfc.section.7.3.p.1">The "Expires" header field gives the date/time after which the response is considered stale. See <a href="#expiration.model" title="Freshness">Section&nbsp;4.1</a> for further discussion of the freshness model.
1360      </p>
1361      <p id="rfc.section.7.3.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
1362         that time.
1363      </p>
1364      <p id="rfc.section.7.3.p.3">The Expires value is an HTTP-date timestamp, as defined in <a href="" title="Date/Time Formats">Section</a> of <a href="#Part2" id="rfc.xref.Part2.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>.
1365      </p>
1366      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.5"></span>  <a href="#header.expires" class="smpl">Expires</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
1367</pre><div id="rfc.figure.u.10"></div>
1368      <p>For example</p><pre class="text">  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1369</pre><p id="rfc.section.7.3.p.6">A cache recipient <em class="bcp14">MUST</em> interpret invalid date formats, especially the value "0", as representing a time in the past (i.e., "already expired").
1370      </p>
1371      <p id="rfc.section.7.3.p.7">If a response includes a <a href="#header.cache-control" class="smpl">Cache-Control</a> field with the max-age directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;</a>), a recipient <em class="bcp14">MUST</em> ignore the Expires field. Likewise, if a response includes the s-maxage directive (<a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;</a>), a shared cache recipient <em class="bcp14">MUST</em> ignore the Expires field. In both these cases, the value in Expires is only intended for recipients that have not yet implemented
1372         the Cache-Control field.
1373      </p>
1374      <p id="rfc.section.7.3.p.8">An origin server without a clock <em class="bcp14">MUST NOT</em> generate an Expires field unless its value represents a fixed time in the past (always expired) or its value has been associated
1375         with the resource by a system or user with a reliable clock.
1376      </p>
1377      <p id="rfc.section.7.3.p.9">Historically, HTTP required the Expires field-value to be no more than a year in the future. While longer freshness lifetimes
1378         are no longer prohibited, extremely large values have been demonstrated to cause problems (e.g., clock overflows due to use
1379         of 32-bit integers for time values), and many caches will evict a response far sooner than that.
1380      </p>
1381      <div id="rfc.iref.p.5"></div>
1382      <h2 id="rfc.section.7.4"><a href="#rfc.section.7.4">7.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
1383      <p id="rfc.section.7.4.p.1">The "Pragma" header field allows backwards compatibility with HTTP/1.0 caches, so that clients can specify a "no-cache" request
1384         that they will understand (as <a href="#header.cache-control" class="smpl">Cache-Control</a> was not defined until HTTP/1.1). When the Cache-Control header field is also present and understood in a request, Pragma is
1385         ignored.
1386      </p>
1387      <p id="rfc.section.7.4.p.2">In HTTP/1.0, Pragma was defined as an extensible field for implementation-specified directives for recipients. This specification
1388         deprecates such extensions to improve interoperability.
1389      </p>
1390      <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.6"></span><span id="rfc.iref.g.7"></span><span id="rfc.iref.g.8"></span>  <a href="#header.pragma" class="smpl">Pragma</a>           = 1#<a href="#header.pragma" class="smpl">pragma-directive</a>
1391  <a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / <a href="#header.pragma" class="smpl">extension-pragma</a>
1392  <a href="#header.pragma" class="smpl">extension-pragma</a> = <a href="#imported.abnf" class="smpl">token</a> [ "=" ( <a href="#imported.abnf" class="smpl">token</a> / <a href="#imported.abnf" class="smpl">quoted-string</a> ) ]
1393</pre><p id="rfc.section.7.4.p.4">When the <a href="#header.cache-control" class="smpl">Cache-Control</a> header field is not present in a request, caches <em class="bcp14">MUST</em> consider the no-cache request pragma-directive as having the same effect as if "Cache-Control: no-cache" were present (see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a>).
1394      </p>
1395      <p id="rfc.section.7.4.p.5">When sending a no-cache request, a client ought to include both the pragma and cache-control directives, unless Cache-Control:
1396         no-cache is purposefully omitted to target other <a href="#header.cache-control" class="smpl">Cache-Control</a> response directives at HTTP/1.1 caches. For example:
1397      </p>
1398      <div id="rfc.figure.u.12"></div><pre class="text">GET / HTTP/1.1
1400Cache-Control: max-age=30
1401Pragma: no-cache
1403</pre><p id="rfc.section.7.4.p.7">will constrain HTTP/1.1 caches to serve a response no older than 30 seconds, while precluding implementations that do not
1404         understand <a href="#header.cache-control" class="smpl">Cache-Control</a> from serving a cached response.
1405      </p>
1406      <div class="note" id="rfc.section.7.4.p.8">
1407         <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:
1408            no-cache" in them.
1409         </p>
1410      </div>
1411      <div id="rfc.iref.w.1"></div>
1412      <h2 id="rfc.section.7.5"><a href="#rfc.section.7.5">7.5</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
1413      <p id="rfc.section.7.5.p.1">The "Warning" header field is used to carry additional information about the status or transformation of a message that might
1414         not be reflected in the message. This information is typically used to warn about possible incorrectness introduced by caching
1415         operations or transformations applied to the payload of the message.
1416      </p>
1417      <p id="rfc.section.7.5.p.2">Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status
1418         code, distinguishes these responses from true failures.
1419      </p>
1420      <p id="rfc.section.7.5.p.3">Warning header fields can in general be applied to any message, however some warn-codes are specific to caches and can only
1421         be applied to response messages.
1422      </p>
1423      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span><span id="rfc.iref.g.13"></span><span id="rfc.iref.g.14"></span>  <a href="#header.warning" class="smpl">Warning</a>       = 1#<a href="#header.warning" class="smpl">warning-value</a>
1425  <a href="#header.warning" class="smpl">warning-value</a> = <a href="#header.warning" class="smpl">warn-code</a> <a href="#imported.abnf" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-agent</a> <a href="#imported.abnf" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-text</a>
1426                                        [<a href="#imported.abnf" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-date</a>]
1428  <a href="#header.warning" class="smpl">warn-code</a>  = 3<a href="#imported.abnf" class="smpl">DIGIT</a>
1429  <a href="#header.warning" class="smpl">warn-agent</a> = ( <a href="#imported.abnf" class="smpl">uri-host</a> [ ":" <a href="#imported.abnf" class="smpl">port</a> ] ) / <a href="#imported.abnf" class="smpl">pseudonym</a>
1430                  ; the name or pseudonym of the server adding
1431                  ; the Warning header field, for use in debugging
1432  <a href="#header.warning" class="smpl">warn-text</a>  = <a href="#imported.abnf" class="smpl">quoted-string</a>
1433  <a href="#header.warning" class="smpl">warn-date</a>  = <a href="#imported.abnf" class="smpl">DQUOTE</a> <a href="#imported.abnf" class="smpl">HTTP-date</a> <a href="#imported.abnf" class="smpl">DQUOTE</a>
1434</pre><p id="rfc.section.7.5.p.5">Multiple warnings can be attached to a response (either by the origin server or by a cache), including multiple warnings with
1435         the same code number, only differing in warn-text.
1436      </p>
1437      <p id="rfc.section.7.5.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.
1438      </p>
1439      <p id="rfc.section.7.5.p.7">Systems that generate multiple Warning header fields are encouraged to order them with this user agent behavior in mind. New
1440         Warning header fields are added after any existing Warning header fields.
1441      </p>
1442      <p id="rfc.section.7.5.p.8">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning is required to be deleted from
1443         a stored response after validation:
1444      </p>
1445      <ul>
1446         <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.
1447         </li>
1448         <li>2xx Warnings describe some aspect of the representation that is not rectified by a validation (for example, a lossy compression
1449            of the representation) and <em class="bcp14">MUST NOT</em> be deleted by a cache after validation, unless a full response is sent, in which case they <em class="bcp14">MUST</em> be.
1450         </li>
1451      </ul>
1452      <p id="rfc.section.7.5.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,
1453         then the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the <a href="" class="smpl">Date</a> header field in the message.
1454      </p>
1455      <p id="rfc.section.7.5.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 <a href="" class="smpl">Date</a> 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
1456         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.
1457      </p>
1458      <p id="rfc.section.7.5.p.11">The following warn-codes are defined by this specification, each with a recommended warn-text in English, and a description
1459         of its meaning.
1460      </p>
1461      <div id="rfc.iref.50"></div>
1462      <h3 id="rfc.section.7.5.1"><a href="#rfc.section.7.5.1">7.5.1</a>&nbsp;<a id="warn.110" href="#warn.110">110 Response is Stale</a></h3>
1463      <p id="rfc.section.7.5.1.p.1">A cache <em class="bcp14">SHOULD</em> generate this whenever the sent response is stale.
1464      </p>
1465      <div id="rfc.iref.50"></div>
1466      <h3 id="rfc.section.7.5.2"><a href="#rfc.section.7.5.2">7.5.2</a>&nbsp;<a id="warn.111" href="#warn.111">111 Revalidation Failed</a></h3>
1467      <p id="rfc.section.7.5.2.p.1">A cache <em class="bcp14">SHOULD</em> generate this when sending a stale response because an attempt to validate the response failed, due to an inability to reach
1468         the server.
1469      </p>
1470      <div id="rfc.iref.50"></div>
1471      <h3 id="rfc.section.7.5.3"><a href="#rfc.section.7.5.3">7.5.3</a>&nbsp;<a id="warn.112" href="#warn.112">112 Disconnected Operation</a></h3>
1472      <p id="rfc.section.7.5.3.p.1">A cache <em class="bcp14">SHOULD</em> generate this if it is intentionally disconnected from the rest of the network for a period of time.
1473      </p>
1474      <div id="rfc.iref.50"></div>
1475      <h3 id="rfc.section.7.5.4"><a href="#rfc.section.7.5.4">7.5.4</a>&nbsp;<a id="warn.113" href="#warn.113">113 Heuristic Expiration</a></h3>
1476      <p id="rfc.section.7.5.4.p.1">A cache <em class="bcp14">SHOULD</em> generate this if it heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than
1477         24 hours.
1478      </p>
1479      <div id="rfc.iref.50"></div>
1480      <h3 id="rfc.section.7.5.5"><a href="#rfc.section.7.5.5">7.5.5</a>&nbsp;<a id="warn.199" href="#warn.199">199 Miscellaneous Warning</a></h3>
1481      <p id="rfc.section.7.5.5.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.
1482      </p>
1483      <div id="rfc.iref.50"></div>
1484      <h3 id="rfc.section.7.5.6"><a href="#rfc.section.7.5.6">7.5.6</a>&nbsp;<a id="warn.214" href="#warn.214">214 Transformation Applied</a></h3>
1485      <p id="rfc.section.7.5.6.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,
1486         or modifying the representation data, unless this Warning code already appears in the response.
1487      </p>
1488      <div id="rfc.iref.50"></div>
1489      <h3 id="rfc.section.7.5.7"><a href="#rfc.section.7.5.7">7.5.7</a>&nbsp;<a id="warn.299" href="#warn.299">299 Miscellaneous Persistent Warning</a></h3>
1490      <p id="rfc.section.7.5.7.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.
1491      </p>
1492      <h3 id="rfc.section.7.5.8"><a href="#rfc.section.7.5.8">7.5.8</a>&nbsp;<a id="warn.code.extensions" href="#warn.code.extensions">Warn Code Extensions</a></h3>
1493      <p id="rfc.section.7.5.8.p.1">Extension warn codes can be defined; see <a href="#warn.code.registry.procedure" title="Procedure">Section&nbsp;9.2.1</a> for details.
1494      </p>
1495      <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
1496      <p id="rfc.section.8.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, that can be used to redisplay a representation
1497         retrieved earlier in a session.
1498      </p>
1499      <p id="rfc.section.8.p.2">The freshness model (<a href="#expiration.model" title="Freshness">Section&nbsp;4.1</a>) does not necessarily apply to history mechanisms. I.e., a history mechanism can display a previous representation even if
1500         it has expired.
1501      </p>
1502      <p id="rfc.section.8.p.3">This does not prohibit the history mechanism from telling the user that a view might be stale, or from honoring cache directives
1503         (e.g., Cache-Control: no-store).
1504      </p>
1505      <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a id="iana.considerations" href="#iana.considerations">IANA Considerations</a></h1>
1506      <h2 id="rfc.section.9.1"><a href="#rfc.section.9.1">9.1</a>&nbsp;<a id="cache.directive.registry" href="#cache.directive.registry">Cache Directive Registry</a></h2>
1507      <p id="rfc.section.9.1.p.1">The HTTP Cache Directive Registry defines the name space for the cache directives. It will be created and maintained at &lt;<a href=""></a>&gt;.
1508      </p>
1509      <h3 id="rfc.section.9.1.1"><a href="#rfc.section.9.1.1">9.1.1</a>&nbsp;<a id="cache.directive.registry.procedure" href="#cache.directive.registry.procedure">Procedure</a></h3>
1510      <p id="rfc.section.9.1.1.p.1">A registration <em class="bcp14">MUST</em> include the following fields:
1511      </p>
1512      <ul>
1513         <li>Cache Directive Name</li>
1514         <li>Pointer to specification text</li>
1515      </ul>
1516      <p id="rfc.section.9.1.1.p.2">Values to be added to this name space require IETF Review (see <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>).
1517      </p>
1518      <h3 id="rfc.section.9.1.2"><a href="#rfc.section.9.1.2">9.1.2</a>&nbsp;<a id="cache.directive.considerations" href="#cache.directive.considerations">Considerations for New Cache Control Directives</a></h3>
1519      <p id="rfc.section.9.1.2.p.1">New extension directives ought to consider defining:</p>
1520      <p id="rfc.section.9.1.2.p.2"></p>
1521      <ul>
1522         <li>What it means for a directive to be specified multiple times,</li>
1523         <li>When the directive does not take an argument, what it means when an argument is present,</li>
1524         <li>When the directive requires an argument, what it means when it is missing,</li>
1525         <li>Whether the directive is specific to requests, responses, or able to be used in either.</li>
1526      </ul>
1527      <p id="rfc.section.9.1.2.p.3">See also <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a>.
1528      </p>
1529      <h3 id="rfc.section.9.1.3"><a href="#rfc.section.9.1.3">9.1.3</a>&nbsp;<a id="cache.directive.registration" href="#cache.directive.registration">Registrations</a></h3>
1530      <p id="rfc.section.9.1.3.p.1">The HTTP Cache Directive Registry shall be populated with the registrations below:</p>
1531      <div id="rfc.table.1">
1532         <div id="iana.cache.directive.registration.table"></div>
1533         <table class="tt full left" cellpadding="3" cellspacing="0">
1534            <thead>
1535               <tr>
1536                  <th>Cache Directive</th>
1537                  <th>Reference</th>
1538               </tr>
1539            </thead>
1540            <tbody>
1541               <tr>
1542                  <td class="left">max-age</td>
1543                  <td class="left"><a href="#cache-request-directive.max-age" title="max-age">Section&nbsp;</a>, <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;</a>
1544                  </td>
1545               </tr>
1546               <tr>
1547                  <td class="left">max-stale</td>
1548                  <td class="left"><a href="#cache-request-directive.max-stale" title="max-stale">Section&nbsp;</a>
1549                  </td>
1550               </tr>
1551               <tr>
1552                  <td class="left">min-fresh</td>
1553                  <td class="left"><a href="#cache-request-directive.min-fresh" title="min-fresh">Section&nbsp;</a>
1554                  </td>
1555               </tr>
1556               <tr>
1557                  <td class="left">must-revalidate</td>
1558                  <td class="left"><a href="#cache-response-directive.must-revalidate" title="must-revalidate">Section&nbsp;</a>
1559                  </td>
1560               </tr>
1561               <tr>
1562                  <td class="left">no-cache</td>
1563                  <td class="left"><a href="" title="no-cache">Section&nbsp;</a>, <a href="" title="no-cache">Section&nbsp;</a>
1564                  </td>
1565               </tr>
1566               <tr>
1567                  <td class="left">no-store</td>
1568                  <td class="left"><a href="" title="no-store">Section&nbsp;</a>, <a href="" title="no-store">Section&nbsp;</a>
1569                  </td>
1570               </tr>
1571               <tr>
1572                  <td class="left">no-transform</td>
1573                  <td class="left"><a href="" title="no-transform">Section&nbsp;</a>, <a href="" title="no-transform">Section&nbsp;</a>
1574                  </td>
1575               </tr>
1576               <tr>
1577                  <td class="left">only-if-cached</td>
1578                  <td class="left"><a href="#cache-request-directive.only-if-cached" title="only-if-cached">Section&nbsp;</a>
1579                  </td>
1580               </tr>
1581               <tr>
1582                  <td class="left">private</td>
1583                  <td class="left"><a href="#cache-response-directive.private" title="private">Section&nbsp;</a>
1584                  </td>
1585               </tr>
1586               <tr>
1587                  <td class="left">proxy-revalidate</td>
1588                  <td class="left"><a href="#cache-response-directive.proxy-revalidate" title="proxy-revalidate">Section&nbsp;</a>
1589                  </td>
1590               </tr>
1591               <tr>
1592                  <td class="left">public</td>
1593                  <td class="left"><a href="#cache-response-directive.public" title="public">Section&nbsp;</a>
1594                  </td>
1595               </tr>
1596               <tr>
1597                  <td class="left">s-maxage</td>
1598                  <td class="left"><a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;</a>
1599                  </td>
1600               </tr>
1601               <tr>
1602                  <td class="left">stale-if-error</td>
1603                  <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>
1604                  </td>
1605               </tr>
1606               <tr>
1607                  <td class="left">stale-while-revalidate</td>
1608                  <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>
1609                  </td>
1610               </tr>
1611            </tbody>
1612         </table>
1613      </div>
1614      <h2 id="rfc.section.9.2"><a href="#rfc.section.9.2">9.2</a>&nbsp;<a id="warn.code.registry" href="#warn.code.registry">Warn Code Registry</a></h2>
1615      <p id="rfc.section.9.2.p.1">The HTTP Warn Code Registry defines the name space for warn codes. It will be created and maintained at &lt;<a href=""></a>&gt;.
1616      </p>
1617      <h3 id="rfc.section.9.2.1"><a href="#rfc.section.9.2.1">9.2.1</a>&nbsp;<a id="warn.code.registry.procedure" href="#warn.code.registry.procedure">Procedure</a></h3>
1618      <p id="rfc.section.9.2.1.p.1">A registration <em class="bcp14">MUST</em> include the following fields:
1619      </p>
1620      <ul>
1621         <li>Warn Code (3 digits)</li>
1622         <li>Short Description</li>
1623         <li>Pointer to specification text</li>
1624      </ul>
1625      <p id="rfc.section.9.2.1.p.2">Values to be added to this name space require IETF Review (see <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>).
1626      </p>
1627      <h3 id="rfc.section.9.2.2"><a href="#rfc.section.9.2.2">9.2.2</a>&nbsp;<a id="warn.code.registration" href="#warn.code.registration">Registrations</a></h3>
1628      <p id="rfc.section.9.2.2.p.1">The HTTP Warn Code Registry shall be populated with the registrations below:</p>
1629      <div id="rfc.table.2">
1630         <div id="iana.warn.code.registration.table"></div>
1631         <table class="tt full left" cellpadding="3" cellspacing="0">
1632            <thead>
1633               <tr>
1634                  <th>Warn Code</th>
1635                  <th>Short Description</th>
1636                  <th>Reference</th>
1637               </tr>
1638            </thead>
1639            <tbody>
1640               <tr>
1641                  <td class="left">110</td>
1642                  <td class="left">Response is Stale</td>
1643                  <td class="left"><a href="#warn.110" id="rfc.xref.warn.110.1" title="110 Response is Stale">Section&nbsp;7.5.1</a>
1644                  </td>
1645               </tr>
1646               <tr>
1647                  <td class="left">111</td>
1648                  <td class="left">Revalidation Failed</td>
1649                  <td class="left"><a href="#warn.111" id="rfc.xref.warn.111.1" title="111 Revalidation Failed">Section&nbsp;7.5.2</a>
1650                  </td>
1651               </tr>
1652               <tr>
1653                  <td class="left">112</td>
1654                  <td class="left">Disconnected Operation</td>
1655                  <td class="left"><a href="#warn.112" id="rfc.xref.warn.112.1" title="112 Disconnected Operation">Section&nbsp;7.5.3</a>
1656                  </td>
1657               </tr>
1658               <tr>
1659                  <td class="left">113</td>
1660                  <td class="left">Heuristic Expiration</td>
1661                  <td class="left"><a href="#warn.113" id="rfc.xref.warn.113.1" title="113 Heuristic Expiration">Section&nbsp;7.5.4</a>
1662                  </td>
1663               </tr>
1664               <tr>
1665                  <td class="left">199</td>
1666                  <td class="left">Miscellaneous Warning</td>
1667                  <td class="left"><a href="#warn.199" id="rfc.xref.warn.199.1" title="199 Miscellaneous Warning">Section&nbsp;7.5.5</a>
1668                  </td>
1669               </tr>
1670               <tr>
1671                  <td class="left">214</td>
1672                  <td class="left">Transformation Applied</td>
1673                  <td class="left"><a href="#warn.214" id="rfc.xref.warn.214.1" title="214 Transformation Applied">Section&nbsp;7.5.6</a>
1674                  </td>
1675               </tr>
1676               <tr>
1677                  <td class="left">299</td>
1678                  <td class="left">Miscellaneous Persistent Warning</td>
1679                  <td class="left"><a href="#warn.299" id="rfc.xref.warn.299.1" title="299 Miscellaneous Persistent Warning">Section&nbsp;7.5.7</a>
1680                  </td>
1681               </tr>
1682            </tbody>
1683         </table>
1684      </div>
1685      <h2 id="rfc.section.9.3"><a href="#rfc.section.9.3">9.3</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
1686      <p id="rfc.section.9.3.p.1">HTTP header fields are registered within the Message Header Field Registry maintained at &lt;<a href=""></a>&gt;.
1687      </p>
1688      <p id="rfc.section.9.3.p.2">This document defines the following HTTP header fields, so their associated registry entries shall be updated according to
1689         the permanent registrations below (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>):
1690      </p>
1691      <div id="rfc.table.3">
1692         <div id="iana.header.registration.table"></div>
1693         <table class="tt full left" cellpadding="3" cellspacing="0">
1694            <thead>
1695               <tr>
1696                  <th>Header Field Name</th>
1697                  <th>Protocol</th>
1698                  <th>Status</th>
1699                  <th>Reference</th>
1700               </tr>
1701            </thead>
1702            <tbody>
1703               <tr>
1704                  <td class="left">Age</td>
1705                  <td class="left">http</td>
1706                  <td class="left">standard</td>
1707                  <td class="left"><a href="#header.age" id="rfc.xref.header.age.3" title="Age">Section&nbsp;7.1</a>
1708                  </td>
1709               </tr>
1710               <tr>
1711                  <td class="left">Cache-Control</td>
1712                  <td class="left">http</td>
1713                  <td class="left">standard</td>
1714                  <td class="left"><a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;7.2</a>
1715                  </td>
1716               </tr>
1717               <tr>
1718                  <td class="left">Expires</td>
1719                  <td class="left">http</td>
1720                  <td class="left">standard</td>
1721                  <td class="left"><a href="#header.expires" id="rfc.xref.header.expires.4" title="Expires">Section&nbsp;7.3</a>
1722                  </td>
1723               </tr>
1724               <tr>
1725                  <td class="left">Pragma</td>
1726                  <td class="left">http</td>
1727                  <td class="left">standard</td>
1728                  <td class="left"><a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;7.4</a>
1729                  </td>
1730               </tr>
1731               <tr>
1732                  <td class="left">Warning</td>
1733                  <td class="left">http</td>
1734                  <td class="left">standard</td>
1735                  <td class="left"><a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;7.5</a>
1736                  </td>
1737               </tr>
1738            </tbody>
1739         </table>
1740      </div>
1741      <p id="rfc.section.9.3.p.3">The change controller is: "IETF ( - Internet Engineering Task Force".</p>
1742      <h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1743      <p id="rfc.section.10.p.1">This section is meant to inform developers, information providers, and users of known security concerns specific to HTTP/1.1
1744         caching. More general security considerations are addressed in HTTP messaging <a href="#Part1" id="rfc.xref.Part1.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a> and semantics <a href="#Part2" id="rfc.xref.Part2.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>.
1745      </p>
1746      <p id="rfc.section.10.p.2">Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious
1747         exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal information
1748         long after a user believes that the information has been removed from the network. Therefore, cache contents need to be protected
1749         as sensitive information.
1750      </p>
1751      <p id="rfc.section.10.p.3">Furthermore, the very use of a cache can bring about privacy concerns. For example, if two users share a cache, and the first
1752         one browses to a site, the second may be able to detect that the other has been to that site, because the resources from it
1753         load more quickly, thanks to the cache.
1754      </p>
1755      <p id="rfc.section.10.p.4">Implementation flaws might allow attackers to insert content into a cache ("cache poisoning"), leading to compromise of clients
1756         that trust that content. Because of their nature, these attacks are difficult to mitigate.
1757      </p>
1758      <p id="rfc.section.10.p.5">Likewise, implementation flaws (as well as misunderstanding of cache operation) might lead to caching of sensitive information
1759         (e.g., authentication credentials) that is thought to be private, exposing it to unauthorized parties.
1760      </p>
1761      <p id="rfc.section.10.p.6">Note that the Set-Cookie response header field <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a> does not inhibit caching; a cacheable response with a Set-Cookie header field can be (and often is) used to satisfy subsequent
1762         requests to caches. Servers who wish to control caching of these responses are encouraged to emit appropriate Cache-Control
1763         response header fields.
1764      </p>
1765      <h1 id="rfc.section.11"><a href="#rfc.section.11">11.</a>&nbsp;<a id="acks" href="#acks">Acknowledgments</a></h1>
1766      <p id="rfc.section.11.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 9</a> of <a href="#Part1" id="rfc.xref.Part1.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
1767      </p>
1768      <h1 id="rfc.references"><a id="rfc.section.12" href="#rfc.section.12">12.</a> References
1769      </h1>
1770      <h2 id="rfc.references.1"><a href="#rfc.section.12.1" id="rfc.section.12.1">12.1</a> Normative References
1771      </h2>
1772      <table>
1773         <tr>
1774            <td class="reference"><b id="Part1">[Part1]</b></td>
1775            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), July&nbsp;2013.
1776            </td>
1777         </tr>
1778         <tr>
1779            <td class="reference"><b id="Part2">[Part2]</b></td>
1780            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), July&nbsp;2013.
1781            </td>
1782         </tr>
1783         <tr>
1784            <td class="reference"><b id="Part4">[Part4]</b></td>
1785            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-latest (work in progress), July&nbsp;2013.
1786            </td>
1787         </tr>
1788         <tr>
1789            <td class="reference"><b id="Part5">[Part5]</b></td>
1790            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), July&nbsp;2013.
1791            </td>
1792         </tr>
1793         <tr>
1794            <td class="reference"><b id="Part7">[Part7]</b></td>
1795            <td class="top"><a href="" title="Adobe Systems Incorporated">Fielding, R., Ed.</a> and <a href="" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="">Hypertext Transfer Protocol (HTTP/1.1): Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-latest (work in progress), July&nbsp;2013.
1796            </td>
1797         </tr>
1798         <tr>
1799            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1800            <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.
1801            </td>
1802         </tr>
1803         <tr>
1804            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1805            <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.
1806            </td>
1807         </tr>
1808      </table>
1809      <h2 id="rfc.references.2"><a href="#rfc.section.12.2" id="rfc.section.12.2">12.2</a> Informative References
1810      </h2>
1811      <table>
1812         <tr>
1813            <td class="reference"><b id="BCP90">[BCP90]</b></td>
1814            <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.
1815            </td>
1816         </tr>
1817         <tr>
1818            <td class="reference"><b id="RFC1305">[RFC1305]</b></td>
1819            <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.
1820            </td>
1821         </tr>
1822         <tr>
1823            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1824            <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.
1825            </td>
1826         </tr>
1827         <tr>
1828            <td class="reference"><b id="RFC5226">[RFC5226]</b></td>
1829            <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.
1830            </td>
1831         </tr>
1832         <tr>
1833            <td class="reference"><b id="RFC5861">[RFC5861]</b></td>
1834            <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.
1835            </td>
1836         </tr>
1837         <tr>
1838            <td class="reference"><b id="RFC6265">[RFC6265]</b></td>
1839            <td class="top"><a href="" title="&#xA;          University of California, Berkeley&#xA;        ">Barth, A.</a>, “<a href="">HTTP State Management Mechanism</a>”, RFC&nbsp;6265, April&nbsp;2011.
1840            </td>
1841         </tr>
1842      </table>
1843      <div class="avoidbreak">
1844         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1845         <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
1846               (editor)
1847               <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href=""><span class="email"></span></a></span><span class="vcardline">URI: <a href="" class="url"></a></span></address>
1848         <address class="vcard"><span class="vcardline"><span class="fn">Mark Nottingham</span>
1849               (editor)
1850               <span class="n hidden"><span class="family-name">Nottingham</span><span class="given-name">Mark</span></span></span><span class="org vcardline">Akamai</span><span class="vcardline">Email: <a href=""><span class="email"></span></a></span><span class="vcardline">URI: <a href="" class="url"></a></span></address>
1851         <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
1852               (editor)
1853               <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline">Email: <a href=""><span class="email"></span></a></span><span class="vcardline">URI: <a href="" class="url"></a></span></address>
1854      </div>
1855      <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
1856      <p id="rfc.section.A.p.1">Caching-related text has been substantially rewritten for clarity.</p>
1857      <p id="rfc.section.A.p.2">The algorithm for calculating age is now less conservative. (<a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>)
1858      </p>
1859      <p id="rfc.section.A.p.3">Caches are now required to handle dates with timezones as if they're invalid, because it's not possible to accurately guess.
1860         (<a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>)
1861      </p>
1862      <p id="rfc.section.A.p.4">The <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a> response header field is no longer used to determine the appropriate response to use when validating. (<a href="#validation.model" title="Validation">Section&nbsp;4.2</a>)
1863      </p>
1864      <p id="rfc.section.A.p.5">The algorithm for selecting a cached negotiated response to use has been clarified in several ways. In particular, it now
1865         explicitly allows header-specific canonicalization when processing selecting header fields. (<a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.3</a>)
1866      </p>
1867      <p id="rfc.section.A.p.6">Requirements regarding denial of service attack avoidance when performing invalidation have been clarified. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;6</a>)
1868      </p>
1869      <p id="rfc.section.A.p.7">Cache invalidation only occurs when a successful response is received. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;6</a>)
1870      </p>
1871      <p id="rfc.section.A.p.8">The conditions under which an authenticated response can be cached have been clarified. (<a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a>)
1872      </p>
1873      <p id="rfc.section.A.p.9">The one-year limit on Expires header field values has been removed; instead, the reasoning for using a sensible value is given.
1874         (<a href="#header.expires" id="rfc.xref.header.expires.5" title="Expires">Section&nbsp;7.3</a>)
1875      </p>
1876      <p id="rfc.section.A.p.10">The Pragma header field is now only defined for backwards compatibility; future pragmas are deprecated. (<a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;7.4</a>)
1877      </p>
1878      <p id="rfc.section.A.p.11">Cache directives are explicitly defined to be case-insensitive. (<a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;7.2</a>)
1879      </p>
1880      <p id="rfc.section.A.p.12">Handling of multiple instances of cache directives when only one is expected is now defined. (<a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;7.2</a>)
1881      </p>
1882      <p id="rfc.section.A.p.13">The qualified forms of the private and no-cache cache directives are noted to not be widely implemented; e.g., "private=foo"
1883         is interpreted by many caches as simply "private". Additionally, the meaning of the qualified form of no-cache has been clarified.
1884         (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;7.2.2</a>)
1885      </p>
1886      <p id="rfc.section.A.p.14">The "no-store" cache request directive doesn't apply to responses; i.e., a cache can satisfy a request with no-store on it,
1887         and does not invalidate it. (<a href="" title="no-store">Section&nbsp;</a>)
1888      </p>
1889      <p id="rfc.section.A.p.15">The "no-cache" response cache directive's meaning has been clarified. (<a href="" title="no-cache">Section&nbsp;</a>)
1890      </p>
1891      <p id="rfc.section.A.p.16">New status codes can now define that caches are allowed to use heuristic freshness with them. (<a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.1.2</a>)
1892      </p>
1893      <p id="rfc.section.A.p.17">Caches are now allow to calculate heuristic freshness for URLs with query components. (<a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.1.2</a>)
1894      </p>
1895      <p id="rfc.section.A.p.18">Some requirements regarding production of the <a href="#header.warning" class="smpl">Warning</a> header fields have been relaxed, as it is not widely implemented. Furthermore, the <a href="#header.warning" class="smpl">Warning</a> header field no longer uses RFC 2047 encoding, nor allows multiple languages, as these aspects were not implemented. (<a href="#header.warning" id="rfc.xref.header.warning.6" title="Warning">Section&nbsp;7.5</a>)
1896      </p>
1897      <p id="rfc.section.A.p.19">This specification introduces the Cache Directive and Warn Code Registries, and defines considerations for new cache directives.
1898         (<a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a> and <a href="#warn.code.extensions" title="Warn Code Extensions">Section&nbsp;7.5.8</a>)
1899      </p>
1900      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="imported.abnf" href="#imported.abnf">Imported ABNF</a></h1>
1901      <p id="rfc.section.B.p.1">The following core rules are included by reference, as defined in <a href="">Appendix B.1</a> of <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
1902         (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
1903         character).
1904      </p>
1905      <p id="rfc.section.B.p.2">The rules below are defined in <a href="#Part1" id="rfc.xref.Part1.13"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>:
1906      </p>
1907      <div id="rfc.figure.u.14"></div><pre class="inline">  <a href="#imported.abnf" class="smpl">OWS</a>           = &lt;OWS, defined in <a href="#Part1" id="rfc.xref.Part1.14"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.3</a>&gt;
1908  <a href="#imported.abnf" class="smpl">field-name</a>    = &lt;field-name, defined in <a href="#Part1" id="rfc.xref.Part1.15"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a>&gt;
1909  <a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.16"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.6</a>&gt;
1910  <a href="#imported.abnf" class="smpl">token</a>         = &lt;token, defined in <a href="#Part1" id="rfc.xref.Part1.17"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.6</a>&gt;
1912  <a href="#imported.abnf" class="smpl">port</a>          = &lt;port, defined in <a href="#Part1" id="rfc.xref.Part1.18"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.7</a>&gt;
1913  <a href="#imported.abnf" class="smpl">pseudonym</a>     = &lt;pseudonym, defined in <a href="#Part1" id="rfc.xref.Part1.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#header.via" title="Via">Section 5.7.1</a>&gt;
1914  <a href="#imported.abnf" class="smpl">uri-host</a>      = &lt;uri-host, defined in <a href="#Part1" id="rfc.xref.Part1.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.7</a>&gt;
1915</pre><p id="rfc.section.B.p.4">The rules below are defined in other parts:</p>
1916      <div id="rfc.figure.u.15"></div><pre class="inline">  <a href="#imported.abnf" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part2" id="rfc.xref.Part2.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, <a href="" title="Date/Time Formats">Section</a>&gt;
1917</pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
1918      <p id="rfc.section.C.p.1">In the collected ABNF below, list rules are expanded as per <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.21"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
1919      </p>
1920      <div id="rfc.figure.u.16"></div><pre class="inline"><a href="#header.age" class="smpl">Age</a> = delta-seconds
1922<a href="#header.cache-control" class="smpl">Cache-Control</a> = *( "," OWS ) cache-directive *( OWS "," [ OWS
1923 cache-directive ] )
1925<a href="#header.expires" class="smpl">Expires</a> = HTTP-date
1927<a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section;
1929<a href="#imported.abnf" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
1931<a href="#header.pragma" class="smpl">Pragma</a> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1932 pragma-directive ] )
1934<a href="#header.warning" class="smpl">Warning</a> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
1935 )
1937<a href="#header.cache-control" class="smpl">cache-directive</a> = token [ "=" ( token / quoted-string ) ]
1939<a href="#delta-seconds" class="smpl">delta-seconds</a> = 1*DIGIT
1941<a href="#header.pragma" class="smpl">extension-pragma</a> = token [ "=" ( token / quoted-string ) ]
1943<a href="#imported.abnf" class="smpl">field-name</a> = &lt;field-name, defined in [Part1], Section 3.2&gt;
1945<a href="#imported.abnf" class="smpl">port</a> = &lt;port, defined in [Part1], Section 2.7&gt;
1946<a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / extension-pragma
1947<a href="#imported.abnf" class="smpl">pseudonym</a> = &lt;pseudonym, defined in [Part1], Section 5.7.1&gt;
1949<a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 3.2.6&gt;
1951<a href="#imported.abnf" class="smpl">token</a> = &lt;token, defined in [Part1], Section 3.2.6&gt;
1953<a href="#imported.abnf" class="smpl">uri-host</a> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
1955<a href="#header.warning" class="smpl">warn-agent</a> = ( uri-host [ ":" port ] ) / pseudonym
1956<a href="#header.warning" class="smpl">warn-code</a> = 3DIGIT
1957<a href="#header.warning" class="smpl">warn-date</a> = DQUOTE HTTP-date DQUOTE
1958<a href="#header.warning" class="smpl">warn-text</a> = quoted-string
1959<a href="#header.warning" class="smpl">warning-value</a> = warn-code SP warn-agent SP warn-text [ SP warn-date
1960 ]
1961</pre><h1 id="rfc.section.D"><a href="#rfc.section.D">D.</a>&nbsp;<a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1962      <p id="rfc.section.D.p.1">Changes up to the first Working Group Last Call draft are summarized in &lt;<a href=""></a>&gt;.
1963      </p>
1964      <h2 id="rfc.section.D.1"><a href="#rfc.section.D.1">D.1</a>&nbsp;<a id="changes.since.19" href="#changes.since.19">Since draft-ietf-httpbis-p6-cache-19</a></h2>
1965      <p id="rfc.section.D.1.p.1">Closed issues: </p>
1966      <ul>
1967         <li>&lt;<a href=""></a>&gt;: "untangle Cache-Control ABNF"
1968         </li>
1969         <li>&lt;<a href=""></a>&gt;: "Multiple values in Cache-Control header fields"
1970         </li>
1971         <li>&lt;<a href=""></a>&gt;: "Case sensitivity of header fields in CC values"
1972         </li>
1973         <li>&lt;<a href=""></a>&gt;: "Spurious 'MAYs'"
1974         </li>
1975         <li>&lt;<a href=""></a>&gt;: "enhance considerations for new cache control directives"
1976         </li>
1977         <li>&lt;<a href=""></a>&gt;: "ABNF requirements for recipients"
1978         </li>
1979         <li>&lt;<a href=""></a>&gt;: "note introduction of new IANA registries as normative changes"
1980         </li>
1981         <li>&lt;<a href=""></a>&gt;: "broken prose in description of 'Vary'"
1982         </li>
1983      </ul>
1984      <h2 id="rfc.section.D.2"><a href="#rfc.section.D.2">D.2</a>&nbsp;<a id="changes.since.20" href="#changes.since.20">Since draft-ietf-httpbis-p6-cache-20</a></h2>
1985      <p id="rfc.section.D.2.p.1">Closed issues: </p>
1986      <ul>
1987         <li>&lt;<a href=""></a>&gt;: "'Most Conservative'"
1988         </li>
1989      </ul>
1990      <p id="rfc.section.D.2.p.2">Other changes: </p>
1991      <ul>
1992         <li>Conformance criteria and considerations regarding error handling are now defined in Part 1.</li>
1993         <li>Move definition of "Vary" header field into Part 2.</li>
1994         <li>Add security considerations with respect to cache poisoning and the "Set-Cookie" header field.</li>
1995      </ul>
1996      <h2 id="rfc.section.D.3"><a href="#rfc.section.D.3">D.3</a>&nbsp;<a id="changes.since.21" href="#changes.since.21">Since draft-ietf-httpbis-p6-cache-21</a></h2>
1997      <p id="rfc.section.D.3.p.1">Closed issues: </p>
1998      <ul>
1999         <li>&lt;<a href=""></a>&gt;: "Allowing heuristic caching for new status codes"
2000         </li>
2001         <li>&lt;<a href=""></a>&gt;: "304 without validator"
2002         </li>
2003         <li>&lt;<a href=""></a>&gt;: "No-Transform"
2004         </li>
2005         <li>&lt;<a href=""></a>&gt;: "Revert prior change to the meaning of the public cache response directive.
2006         </li>
2007      </ul>
2008      <h2 id="rfc.section.D.4"><a href="#rfc.section.D.4">D.4</a>&nbsp;<a id="changes.since.22" href="#changes.since.22">Since draft-ietf-httpbis-p6-cache-22</a></h2>
2009      <p id="rfc.section.D.4.p.1">Closed issues: </p>
2010      <ul>
2011         <li>&lt;<a href=""></a>&gt;: "explain list expansion in ABNF appendices"
2012         </li>
2013         <li>&lt;<a href=""></a>&gt;: "Returning the freshest response"
2014         </li>
2015         <li>&lt;<a href=""></a>&gt;: "placement of extension point considerations"
2016         </li>
2017         <li>&lt;<a href=""></a>&gt;: "Editorial notes for p6"
2018         </li>
2019         <li>&lt;<a href=""></a>&gt;: "Vary and future requests"
2020         </li>
2021      </ul>
2022      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
2023      <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.B">B</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>
2024      </p>
2025      <div class="print2col">
2026         <ul class="ind">
2027            <li><a id="rfc.index.1" href="#rfc.index.1"><b>1</b></a><ul>
2028                  <li>110 Response is Stale (warn code)&nbsp;&nbsp;<a href="#rfc.iref.50"><b>7.5.1</b></a>, <a href="#rfc.xref.warn.110.1">9.2.2</a></li>
2029                  <li>111 Revalidation Failed (warn code)&nbsp;&nbsp;<a href="#rfc.iref.50"><b>7.5.2</b></a>, <a href="#rfc.xref.warn.111.1">9.2.2</a></li>
2030                  <li>112 Disconnected Operation (warn code)&nbsp;&nbsp;<a href="#rfc.iref.50"><b>7.5.3</b></a>, <a href="#rfc.xref.warn.112.1">9.2.2</a></li>
2031                  <li>113 Heuristic Expiration (warn code)&nbsp;&nbsp;<a href="#rfc.iref.50"><b>7.5.4</b></a>, <a href="#rfc.xref.warn.113.1">9.2.2</a></li>
2032                  <li>199 Miscellaneous Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.50"><b>7.5.5</b></a>, <a href="#rfc.xref.warn.199.1">9.2.2</a></li>
2033               </ul>
2034            </li>
2035            <li><a id="rfc.index.2" href="#rfc.index.2"><b>2</b></a><ul>
2036                  <li>214 Transformation Applied (warn code)&nbsp;&nbsp;<a href="#rfc.iref.50"><b>7.5.6</b></a>, <a href="#rfc.xref.warn.214.1">9.2.2</a></li>
2037                  <li>299 Miscellaneous Persistent Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.50"><b>7.5.7</b></a>, <a href="#rfc.xref.warn.299.1">9.2.2</a></li>
2038               </ul>
2039            </li>
2040            <li><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul>
2041                  <li>age&nbsp;&nbsp;<a href="#rfc.iref.a.1">4.1</a></li>
2042                  <li>Age header field&nbsp;&nbsp;<a href="#rfc.xref.header.age.1">4</a>, <a href="#rfc.xref.header.age.2">4.1.3</a>, <a href="#rfc.iref.a.2"><b>7.1</b></a>, <a href="#rfc.xref.header.age.3">9.3</a></li>
2043               </ul>
2044            </li>
2045            <li><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul>
2046                  <li><em>BCP90</em>&nbsp;&nbsp;<a href="#rfc.xref.BCP90.1">9.3</a>, <a href="#BCP90"><b>12.2</b></a></li>
2047               </ul>
2048            </li>
2049            <li><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul>
2050                  <li>cache&nbsp;&nbsp;<a href="#rfc.iref.c.1">1</a></li>
2051                  <li>cache entry&nbsp;&nbsp;<a href="#rfc.iref.c.2">2</a></li>
2052                  <li>cache key&nbsp;&nbsp;<a href="#rfc.iref.c.3">2</a>, <a href="#rfc.iref.c.4">2</a></li>
2053                  <li>Cache-Control header field&nbsp;&nbsp;<a href="#rfc.xref.header.cache-control.1">3</a>, <a href="#rfc.iref.c.5"><b>7.2</b></a>, <a href="#rfc.xref.header.cache-control.2">9.3</a>, <a href="#rfc.xref.header.cache-control.3">A</a>, <a href="#rfc.xref.header.cache-control.4">A</a></li>
2054               </ul>
2055            </li>
2056            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
2057                  <li>Expires header field&nbsp;&nbsp;<a href="#rfc.xref.header.expires.1">3</a>, <a href="#rfc.xref.header.expires.2">4.1</a>, <a href="#rfc.xref.header.expires.3">4.1.1</a>, <a href="#rfc.iref.e.2"><b>7.3</b></a>, <a href="#rfc.xref.header.expires.4">9.3</a>, <a href="#rfc.xref.header.expires.5">A</a></li>
2058                  <li>explicit expiration time&nbsp;&nbsp;<a href="#rfc.iref.e.1">4.1</a></li>
2059               </ul>
2060            </li>
2061            <li><a id="rfc.index.F" href="#rfc.index.F"><b>F</b></a><ul>
2062                  <li>first-hand&nbsp;&nbsp;<a href="#rfc.iref.f.3">4.1.4</a></li>
2063                  <li>fresh&nbsp;&nbsp;<a href="#rfc.iref.f.1">4.1</a></li>
2064                  <li>freshness lifetime&nbsp;&nbsp;<a href="#rfc.iref.f.2">4.1</a></li>
2065               </ul>
2066            </li>
2067            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
2068                  <li><tt>Grammar</tt>&nbsp;&nbsp;
2069                     <ul>
2070                        <li><tt>Age</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>7.1</b></a></li>
2071                        <li><tt>Cache-Control</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>7.2</b></a></li>
2072                        <li><tt>cache-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>7.2</b></a></li>
2073                        <li><tt>delta-seconds</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>1.2.1</b></a></li>
2074                        <li><tt>Expires</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>7.3</b></a></li>
2075                        <li><tt>extension-pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>7.4</b></a></li>
2076                        <li><tt>Pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>7.4</b></a></li>
2077                        <li><tt>pragma-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>7.4</b></a></li>
2078                        <li><tt>warn-agent</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.12"><b>7.5</b></a></li>
2079                        <li><tt>warn-code</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.11"><b>7.5</b></a></li>
2080                        <li><tt>warn-date</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.14"><b>7.5</b></a></li>
2081                        <li><tt>warn-text</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.13"><b>7.5</b></a></li>
2082                        <li><tt>Warning</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>7.5</b></a></li>
2083                        <li><tt>warning-value</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>7.5</b></a></li>
2084                     </ul>
2085                  </li>
2086               </ul>
2087            </li>
2088            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
2089                  <li>heuristic expiration time&nbsp;&nbsp;<a href="#rfc.iref.h.1">4.1</a></li>
2090               </ul>
2091            </li>
2092            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
2093                  <li>max-age (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b></b></a>, <a href="#rfc.iref.m.5"><b></b></a></li>
2094                  <li>max-stale (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.m.2"><b></b></a></li>
2095                  <li>min-fresh (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.m.3"><b></b></a></li>
2096                  <li>must-revalidate (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.m.4"><b></b></a></li>
2097               </ul>
2098            </li>
2099            <li><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul>
2100                  <li>no-cache (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.n.1"><b></b></a>, <a href="#rfc.iref.n.4"><b></b></a></li>
2101                  <li>no-store (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.n.2"><b></b></a>, <a href="#rfc.iref.n.5"><b></b></a></li>
2102                  <li>no-transform (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.n.3"><b></b></a>, <a href="#rfc.iref.n.6"><b></b></a></li>
2103               </ul>
2104            </li>
2105            <li><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul>
2106                  <li>only-if-cached (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.o.1"><b></b></a></li>
2107               </ul>
2108            </li>
2109            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
2110                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.1</a>, <a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.3">3.1</a>, <a href="#rfc.xref.Part1.4">4</a>, <a href="#rfc.xref.Part1.5">4.3</a>, <a href="#rfc.xref.Part1.6">6</a>, <a href="#rfc.xref.Part1.7">6</a>, <a href="#rfc.xref.Part1.8">6</a>, <a href="#rfc.xref.Part1.9"></a>, <a href="#rfc.xref.Part1.10"></a>, <a href="#rfc.xref.Part1.11">10</a>, <a href="#rfc.xref.Part1.12">11</a>, <a href="#Part1"><b>12.1</b></a>, <a href="#rfc.xref.Part1.13">B</a>, <a href="#rfc.xref.Part1.14">B</a>, <a href="#rfc.xref.Part1.15">B</a>, <a href="#rfc.xref.Part1.16">B</a>, <a href="#rfc.xref.Part1.17">B</a>, <a href="#rfc.xref.Part1.18">B</a>, <a href="#rfc.xref.Part1.19">B</a>, <a href="#rfc.xref.Part1.20">B</a>, <a href="#rfc.xref.Part1.21">C</a><ul>
2111                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.2</a>, <a href="#rfc.xref.Part1.21">C</a></li>
2112                        <li><em>Section 2.5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.1</a></li>
2113                        <li><em>Section 2.7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.18">B</a>, <a href="#rfc.xref.Part1.20">B</a></li>
2114                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">4.3</a>, <a href="#rfc.xref.Part1.15">B</a></li>
2115                        <li><em>Section 3.2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.14">B</a></li>
2116                        <li><em>Section 3.2.6</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.16">B</a>, <a href="#rfc.xref.Part1.17">B</a></li>
2117                        <li><em>Section 5.5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">4</a>, <a href="#rfc.xref.Part1.6">6</a>, <a href="#rfc.xref.Part1.7">6</a>, <a href="#rfc.xref.Part1.8">6</a></li>
2118                        <li><em>Section 5.7.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.19">B</a></li>
2119                        <li><em>Section 5.7.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.9"></a>, <a href="#rfc.xref.Part1.10"></a></li>
2120                        <li><em>Section 9</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.12">11</a></li>
2121                     </ul>
2122                  </li>
2123                  <li><em>Part2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">2</a>, <a href="#rfc.xref.Part2.2">2</a>, <a href="#rfc.xref.Part2.3">4</a>, <a href="#rfc.xref.Part2.4">4.1.3</a>, <a href="#rfc.xref.Part2.5">4.3</a>, <a href="#rfc.xref.Part2.6">6</a>, <a href="#rfc.xref.Part2.7">7.3</a>, <a href="#rfc.xref.Part2.8">10</a>, <a href="#Part2"><b>12.1</b></a>, <a href="#rfc.xref.Part2.9">B</a><ul>
2124                        <li><em>Section 4.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">4</a>, <a href="#rfc.xref.Part2.6">6</a></li>
2125                        <li><em>Section 4.3.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.2">2</a></li>
2126                        <li><em>Section</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.7">7.3</a>, <a href="#rfc.xref.Part2.9">B</a></li>
2127                        <li><em>Section</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.4">4.1.3</a></li>
2128                        <li><em>Section 7.1.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.5">4.3</a></li>
2129                     </ul>
2130                  </li>
2131                  <li><em>Part4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.1">4.1.2</a>, <a href="#rfc.xref.Part4.2">4.2</a>, <a href="#rfc.xref.Part4.3">4.2.1</a>, <a href="#Part4"><b>12.1</b></a><ul>
2132                        <li><em>Section 2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.3">4.2.1</a></li>
2133                        <li><em>Section 2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.1">4.1.2</a></li>
2134                     </ul>
2135                  </li>
2136                  <li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.1</a>, <a href="#rfc.xref.Part5.2">3.3</a>, <a href="#rfc.xref.Part5.3">3.3</a>, <a href="#Part5"><b>12.1</b></a><ul>
2137                        <li><em>Section 4.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.3">3.3</a></li>
2138                     </ul>
2139                  </li>
2140                  <li><em>Part7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part7.1">3</a>, <a href="#rfc.xref.Part7.2">3.2</a>, <a href="#Part7"><b>12.1</b></a><ul>
2141                        <li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part7.1">3</a>, <a href="#rfc.xref.Part7.2">3.2</a></li>
2142                     </ul>
2143                  </li>
2144                  <li>Pragma header field&nbsp;&nbsp;<a href="#rfc.xref.header.pragma.1">4</a>, <a href="#rfc.iref.p.5"><b>7.4</b></a>, <a href="#rfc.xref.header.pragma.2">9.3</a>, <a href="#rfc.xref.header.pragma.3">A</a></li>
2145                  <li>private (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.p.3"><b></b></a></li>
2146                  <li>private cache&nbsp;&nbsp;<a href="#rfc.iref.p.1">1</a></li>
2147                  <li>proxy-revalidate (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.p.4"><b></b></a></li>
2148                  <li>public (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.p.2"><b></b></a></li>
2149               </ul>
2150            </li>
2151            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
2152                  <li><em>RFC1305</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC1305.1">4.1.3</a>, <a href="#RFC1305"><b>12.2</b></a></li>
2153                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>12.1</b></a></li>
2154                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4.1.2</a>, <a href="#RFC2616"><b>12.2</b></a><ul>
2155                        <li><em>Section 13.9</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4.1.2</a></li>
2156                     </ul>
2157                  </li>
2158                  <li><em>RFC5226</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">9.1.1</a>, <a href="#rfc.xref.RFC5226.2">9.2.1</a>, <a href="#RFC5226"><b>12.2</b></a><ul>
2159                        <li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">9.1.1</a>, <a href="#rfc.xref.RFC5226.2">9.2.1</a></li>
2160                     </ul>
2161                  </li>
2162                  <li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.2</a>, <a href="#RFC5234"><b>12.1</b></a>, <a href="#rfc.xref.RFC5234.2">B</a><ul>
2163                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">B</a></li>
2164                     </ul>
2165                  </li>
2166                  <li><em>RFC5861</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">9.1.3</a>, <a href="#rfc.xref.RFC5861.2">9.1.3</a>, <a href="#RFC5861"><b>12.2</b></a><ul>
2167                        <li><em>Section 3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.2">9.1.3</a></li>
2168                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">9.1.3</a></li>
2169                     </ul>
2170                  </li>
2171                  <li><em>RFC6265</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC6265.1">10</a>, <a href="#RFC6265"><b>12.2</b></a></li>
2172               </ul>
2173            </li>
2174            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
2175                  <li>s-maxage (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.s.4"><b></b></a></li>
2176                  <li>shared cache&nbsp;&nbsp;<a href="#rfc.iref.s.1">1</a></li>
2177                  <li>stale&nbsp;&nbsp;<a href="#rfc.iref.s.2">4.1</a></li>
2178                  <li>strong validator&nbsp;&nbsp;<a href="#rfc.iref.s.3">4.2.1</a></li>
2179               </ul>
2180            </li>
2181            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
2182                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1">4.2</a></li>
2183               </ul>
2184            </li>
2185            <li><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul>
2186                  <li>Warning header field&nbsp;&nbsp;<a href="#rfc.xref.header.warning.1">3.3</a>, <a href="#rfc.xref.header.warning.2">4.1.4</a>, <a href="#rfc.xref.header.warning.3">4.2.1</a>, <a href="#rfc.xref.header.warning.4">5</a>, <a href="#rfc.iref.w.1"><b>7.5</b></a>, <a href="#rfc.xref.header.warning.5">9.3</a>, <a href="#rfc.xref.header.warning.6">A</a></li>
2187               </ul>
2188            </li>
2189         </ul>
2190      </div>
2191   </body>
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