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486</style><link rel="Contents" href="#rfc.toc">
487      <link rel="Author" href="#rfc.authors">
488      <link rel="Copyright" href="#rfc.copyrightnotice">
489      <link rel="Index" href="#rfc.index">
490      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
491      <link rel="Chapter" title="2 Overview of Cache Operation" href="#rfc.section.2">
492      <link rel="Chapter" title="3 Storing Responses in Caches" href="#rfc.section.3">
493      <link rel="Chapter" title="4 Constructing Responses from Caches" href="#rfc.section.4">
494      <link rel="Chapter" title="5 Updating Caches with HEAD Responses" href="#rfc.section.5">
495      <link rel="Chapter" title="6 Request Methods that Invalidate" href="#rfc.section.6">
496      <link rel="Chapter" title="7 Header Field Definitions" href="#rfc.section.7">
497      <link rel="Chapter" title="8 History Lists" href="#rfc.section.8">
498      <link rel="Chapter" title="9 IANA Considerations" href="#rfc.section.9">
499      <link rel="Chapter" title="10 Security Considerations" href="#rfc.section.10">
500      <link rel="Chapter" title="11 Acknowledgments" href="#rfc.section.11">
501      <link rel="Chapter" href="#rfc.section.12" title="12 References">
502      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
503      <link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B">
504      <link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C">
505      <link rel="Appendix" title="D Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.D">
506      <link href="p5-range.html" rel="prev">
507      <link href="p7-auth.html" rel="next">
508      <meta name="generator" content=", Revision 1.640, 2014/06/13 12:42:58, XSLT vendor: SAXON 8.9 from Saxonica">
509      <link rel="schema.dct" href="">
510      <meta name="dct.creator" content="Fielding, R.">
511      <meta name="dct.creator" content="Nottingham, M.">
512      <meta name="dct.creator" content="Reschke, J. F.">
513      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p6-cache-21">
514      <meta name="dct.issued" scheme="ISO8601" content="2012-10-04">
515      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
516      <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.">
517      <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.">
518   </head>
519   <body onload="initFeedback();">
520      <table class="header">
521         <tbody>
522            <tr>
523               <td class="left">HTTPbis Working Group</td>
524               <td class="right">R. Fielding, Editor</td>
525            </tr>
526            <tr>
527               <td class="left">Internet-Draft</td>
528               <td class="right">Adobe</td>
529            </tr>
530            <tr>
531               <td class="left">Obsoletes: <a href="">2616</a> (if approved)
532               </td>
533               <td class="right">M. Nottingham, Editor</td>
534            </tr>
535            <tr>
536               <td class="left">Intended status: Standards Track</td>
537               <td class="right">Akamai</td>
538            </tr>
539            <tr>
540               <td class="left">Expires: April 7, 2013</td>
541               <td class="right">J. Reschke, Editor</td>
542            </tr>
543            <tr>
544               <td class="left"></td>
545               <td class="right">greenbytes</td>
546            </tr>
547            <tr>
548               <td class="left"></td>
549               <td class="right">October 4, 2012</td>
550            </tr>
551         </tbody>
552      </table>
553      <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Caching<br><span class="filename">draft-ietf-httpbis-p6-cache-21</span></p>
554      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
555      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
556         systems. This document defines requirements on HTTP caches and the associated header fields that control cache behavior or
557         indicate cacheable response messages.
558      </p>
559      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
560      <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (, which is archived at &lt;<a href=""></a>&gt;.
561      </p>
562      <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;.
563      </p>
564      <p>The changes in this draft are summarized in <a href="#changes.since.20" title="Since draft-ietf-httpbis-p6-cache-20">Appendix&nbsp;D.2</a>.
565      </p>
566      <div id="rfc.status">
567         <h1><a href="#rfc.status">Status of This Memo</a></h1>
568         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
569         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
570            working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href=""></a>.
571         </p>
572         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
573            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
574            in progress”.
575         </p>
576         <p>This Internet-Draft will expire on April 7, 2013.</p>
577      </div>
578      <div id="rfc.copyrightnotice">
579         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
580         <p>Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
581         <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
582            and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
583            text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
584            BSD License.
585         </p>
586         <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
587            10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
588            allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
589            controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
590            works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
591            it into languages other than English.
592         </p>
593      </div>
594      <hr class="noprint">
595      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
596      <ul class="toc">
597         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul>
598               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
599               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
600               <li><a href="#rfc.section.1.3">1.3</a>&nbsp;&nbsp;&nbsp;<a href="#conformance">Conformance and Error Handling</a></li>
601               <li><a href="#rfc.section.1.4">1.4</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul>
602                     <li><a href="#rfc.section.1.4.1">1.4.1</a>&nbsp;&nbsp;&nbsp;<a href="#delta-seconds">Delta Seconds</a></li>
603                  </ul>
604               </li>
605            </ul>
606         </li>
607         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview of Cache Operation</a></li>
608         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Storing Responses in Caches</a><ul>
609               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#incomplete.responses">Storing Incomplete Responses</a></li>
610               <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>
611            </ul>
612         </li>
613         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a><ul>
614               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness Model</a><ul>
615                     <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>
616                     <li><a href="#rfc.section.4.1.2">4.1.2</a>&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></li>
617                     <li><a href="#rfc.section.4.1.3">4.1.3</a>&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
618                     <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>
619                  </ul>
620               </li>
621               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul>
622                     <li><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#freshening.responses">Freshening Responses with 304 Not Modified</a></li>
623                  </ul>
624               </li>
625               <li><a href="#rfc.section.4.3">4.3</a>&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Using Negotiated Responses</a></li>
626               <li><a href="#rfc.section.4.4">4.4</a>&nbsp;&nbsp;&nbsp;<a href="#combining.responses">Combining Partial Content</a></li>
627            </ul>
628         </li>
629         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#head.effects">Updating Caches with HEAD Responses</a></li>
630         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
631         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.definitions">Header Field Definitions</a><ul>
632               <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
633               <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul>
634                     <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></li>
635                     <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></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.registration">Cache Directive Registry</a></li>
657               <li><a href="#rfc.section.9.2">9.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registration">Warn Code Registry</a></li>
658               <li><a href="#rfc.section.9.3">9.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
659            </ul>
660         </li>
661         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
662         <li><a href="#rfc.section.11">11.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
663         <li><a href="#rfc.section.12">12.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
664               <li><a href="#rfc.section.12.1">12.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
665               <li><a href="#rfc.section.12.2">12.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
666            </ul>
667         </li>
668         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
669         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
670         <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
671         <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>
672               <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>
673               <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>
674            </ul>
675         </li>
676         <li><a href="#rfc.index">Index</a></li>
677         <li><a href="#rfc.authors">Authors' Addresses</a></li>
678      </ul>
679      <div id="caching">
680         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#caching">Introduction</a></h1>
681         <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.
682            This document defines aspects of HTTP/1.1 related to caching and reusing response messages.
683         </p>
684         <div id="intro.purpose">
685            <div id="rfc.iref.c.1"></div>
686            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#intro.purpose">Purpose</a></h2>
687            <p id="rfc.section.1.1.p.1">An HTTP <dfn>cache</dfn> is a local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache
688               stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests.
689               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.
690            </p>
691            <p id="rfc.section.1.1.p.2">The goal of caching in HTTP/1.1 is to significantly improve performance by reusing a prior response message to satisfy a current
692               request. A stored response is considered "fresh", as defined in <a href="#expiration.model" title="Freshness Model">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
693               valid for this request). A fresh cache response can therefore reduce both latency and network transfers each time it is reused.
694               When a cached response is not fresh, it might still be reusable if it can be freshened by validation (<a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>) or if the origin is unavailable.
695            </p>
696         </div>
697         <div id="intro.terminology">
698            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#intro.terminology">Terminology</a></h2>
699            <p id="rfc.section.1.2.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, HTTP caching.</p>
700            <p id="rfc.section.1.2.p.2"><span id="rfc.iref.c.2"></span> <dfn>cache</dfn>
701            </p>
702            <ul class="empty">
703               <li>A conformant implementation of a HTTP cache. Note that this implies an HTTP/1.1 cache; this specification does not define
704                  conformance for HTTP/1.0 caches.
705               </li>
706            </ul>
707            <div id="shared.and.non-shared.caches">
708               <p id="rfc.section.1.2.p.3"><span id="rfc.iref.s.1"></span> <dfn>shared cache</dfn>
709               </p>
710               <ul class="empty">
711                  <li>A cache that stores responses to be reused by more than one user; usually (but not always) deployed as part of an intermediary.</li>
712               </ul>
713            </div>
714            <p id="rfc.section.1.2.p.4"><span id="rfc.iref.p.1"></span> <dfn>private cache</dfn>
715            </p>
716            <ul class="empty">
717               <li>A cache that is dedicated to a single user.</li>
718            </ul>
719            <p id="rfc.section.1.2.p.5"><span id="rfc.iref.c.3"></span> <dfn>cacheable</dfn>
720            </p>
721            <ul class="empty">
722               <li>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
723                  Even when a response is cacheable, there might be additional constraints on whether a cache can use the stored copy to satisfy
724                  a particular request.
725               </li>
726            </ul>
727            <p id="rfc.section.1.2.p.6"><span id="rfc.iref.e.1"></span> <dfn>explicit expiration time</dfn>
728            </p>
729            <ul class="empty">
730               <li>The time at which the origin server intends that a representation no longer be returned by a cache without further validation.</li>
731            </ul>
732            <p id="rfc.section.1.2.p.7"><span id="rfc.iref.h.1"></span> <dfn>heuristic expiration time</dfn>
733            </p>
734            <ul class="empty">
735               <li>An expiration time assigned by a cache when no explicit expiration time is available.</li>
736            </ul>
737            <p id="rfc.section.1.2.p.8"><span id="rfc.iref.a.1"></span> <dfn>age</dfn>
738            </p>
739            <ul class="empty">
740               <li>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</li>
741            </ul>
742            <p id="rfc.section.1.2.p.9"><span id="rfc.iref.f.1"></span> <dfn>first-hand</dfn>
743            </p>
744            <ul class="empty">
745               <li>A response is first-hand if the freshness model is not in use; i.e., its age is 0.</li>
746            </ul>
747            <p id="rfc.section.1.2.p.10"><span id="rfc.iref.f.2"></span> <dfn>freshness lifetime</dfn>
748            </p>
749            <ul class="empty">
750               <li>The length of time between the generation of a response and its expiration time.</li>
751            </ul>
752            <p id="rfc.section.1.2.p.11"><span id="rfc.iref.f.3"></span> <dfn>fresh</dfn>
753            </p>
754            <ul class="empty">
755               <li>A response is fresh if its age has not yet exceeded its freshness lifetime.</li>
756            </ul>
757            <p id="rfc.section.1.2.p.12"><span id="rfc.iref.s.2"></span> <dfn>stale</dfn>
758            </p>
759            <ul class="empty">
760               <li>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</li>
761            </ul>
762            <p id="rfc.section.1.2.p.13"><span id="rfc.iref.v.1"></span> <dfn>validator</dfn>
763            </p>
764            <ul class="empty">
765               <li>A protocol element (e.g., an entity-tag or a <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> time) that is used to find out whether a stored response is an equivalent copy of a representation. See <a href="p4-conditional.html#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>.
766               </li>
767            </ul>
768            <p id="rfc.section.1.2.p.14"><span id="rfc.iref.s.3"></span> <span id="rfc.iref.v.2"></span> <dfn>strong validator</dfn>
769            </p>
770            <ul class="empty">
771               <li>A validator that is defined by the origin server such that its current value will change if the representation data changes;
772                  i.e., an entity-tag that is not marked as weak (<a href="p4-conditional.html#header.etag" title="ETag">Section 2.3</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>) or, if no entity-tag is provided, a <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> value that is strong in the sense defined by <a href="p4-conditional.html#lastmod.comparison" title="Comparison">Section 2.2.2</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[Part4]</cite></a>.
773               </li>
774            </ul>
775         </div>
776         <div id="conformance">
777            <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a href="#conformance">Conformance and Error Handling</a></h2>
778            <p id="rfc.section.1.3.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
779               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>.
780            </p>
781            <p id="rfc.section.1.3.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>.
782            </p>
783         </div>
784         <div id="notation">
785            <h2 id="rfc.section.1.4"><a href="#rfc.section.1.4">1.4</a>&nbsp;<a href="#notation">Syntax Notation</a></h2>
786            <p id="rfc.section.1.4.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.
787            </p>
788            <div id="delta-seconds">
789               <h3 id="rfc.section.1.4.1"><a href="#rfc.section.1.4.1">1.4.1</a>&nbsp;<a href="#delta-seconds">Delta Seconds</a></h3>
790               <p id="rfc.section.1.4.1.p.1">The delta-seconds rule specifies a non-negative integer, representing time in seconds.</p>
791               <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>
792</pre><p id="rfc.section.1.4.1.p.3">If an implementation receives a delta-seconds value larger than the largest positive integer it can represent, or if any of
793                  its subsequent calculations overflows, it <em class="bcp14">MUST</em> consider the value to be 2147483648 (2<sup>31</sup>). Recipients parsing a delta-seconds value <em class="bcp14">MUST</em> use an arithmetic type of at least 31 bits of range, and senders <em class="bcp14">MUST NOT</em> send delta-seconds with a value greater than 2147483648.
794               </p>
795            </div>
796         </div>
797      </div>
798      <div id="caching.overview">
799         <div id="rfc.iref.c.4"></div>
800         <div id="rfc.iref.c.5"></div>
801         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#caching.overview">Overview of Cache Operation</a></h1>
802         <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
803            no requirement or locally-desired configuration prevents it. Therefore, HTTP cache requirements are focused on preventing
804            a cache from either storing a non-reusable response or reusing a stored response inappropriately.
805         </p>
806         <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
807            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 containing a representation of the resource identified by the request target. However, it is also possible to cache
808            negative results (e.g., <a href="p2-semantics.html#status.404" class="smpl">404 (Not
809               Found)</a>, incomplete results (e.g., <a href="p5-range.html#status.206" class="smpl">206 (Partial
810               Content)</a>), and responses to methods other than GET if the method's definition allows such caching and defines something suitable for
811            use as a cache key.
812         </p>
813         <p id="rfc.section.2.p.3">The default <dfn>cache key</dfn> consists of the request method and target URI. However, since HTTP caches in common use today are typically limited to caching
814            responses to GET, many implementations simply decline other methods and use only the URI as the key.
815         </p>
816         <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
817            by a secondary key for the values of the original request's selecting header fields (<a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>).
818         </p>
819      </div>
820      <div id="response.cacheability">
821         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#response.cacheability">Storing Responses in Caches</a></h1>
822         <p id="rfc.section.3.p.1">A cache <em class="bcp14">MUST NOT</em> store a response to any request, unless:
823         </p>
824         <ul>
825            <li>The request method is understood by the cache and defined as being cacheable, and</li>
826            <li>the response status code is understood by the cache, and</li>
827            <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
828            </li>
829            <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
830            </li>
831            <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
832            </li>
833            <li>the response either:
834               <ul>
835                  <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
836                  </li>
837                  <li>contains a max-age response cache directive (see <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;</a>), or
838                  </li>
839                  <li>contains a s-maxage response cache directive and the cache is shared, or</li>
840                  <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
841                  </li>
842                  <li>has a status code that can be served with heuristic freshness (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.1.2</a>).
843                  </li>
844               </ul>
845            </li>
846         </ul>
847         <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>.
848         </p>
849         <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 any
850            cache-specific behavior.
851         </p>
852         <p id="rfc.section.3.p.4">Note that, in normal operation, many caches will not store a response that has neither a cache validator nor an explicit expiration
853            time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.
854         </p>
855         <div id="incomplete.responses">
856            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#incomplete.responses">Storing Incomplete Responses</a></h2>
857            <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
858                  (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
859                  (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#range.retrieval.requests" 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.
860            </p>
861            <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;4.4</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
862               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.
863            </p>
864         </div>
865         <div id="caching.authenticated.responses">
866            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#caching.authenticated.responses">Storing Responses to Authenticated Requests</a></h2>
867            <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.
868            </p>
869            <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.
870            </p>
871            <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
872               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
873               a subsequent request without revalidating it on the origin server.
874            </p>
875         </div>
876      </div>
877      <div id="constructing.responses.from.caches">
878         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h1>
879         <p id="rfc.section.4.p.1">For a presented request, a cache <em class="bcp14">MUST NOT</em> return a stored response, unless:
880         </p>
881         <ul>
882            <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
883            </li>
884            <li>the request method associated with the stored response allows it to be used for the presented request, and</li>
885            <li>selecting header fields nominated by the stored response (if any) match those presented (see <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>), and
886            </li>
887            <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 Model">Section&nbsp;4.2</a>), and
888            </li>
889            <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 Model">Section&nbsp;4.2</a>), and
890            </li>
891            <li>the stored response is either:
892               <ul>
893                  <li>fresh (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a>), or
894                  </li>
895                  <li>allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>), or
896                  </li>
897                  <li>successfully validated (see <a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>).
898                  </li>
899               </ul>
900            </li>
901         </ul>
902         <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>.
903         </p>
904         <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> include a single <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>) 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>.
905         </p>
906         <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 5.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) to the origin server; i.e., a cache is not allowed to generate a reply to such a request before having forwarded the request
907            and having received a corresponding response.
908         </p>
909         <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>.
910         </p>
911         <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 a request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate
912            which response to use.
913         </p>
914         <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 on every use. A cache, especially a shared cache, <em class="bcp14">SHOULD</em> use a mechanism, such as NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>, to synchronize its clock with a reliable external standard.
915         </p>
916         <div id="expiration.model">
917            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#expiration.model">Freshness Model</a></h2>
918            <p id="rfc.section.4.1.p.1">When a response is "fresh" in the cache, it can be used to satisfy subsequent requests without contacting the origin server,
919               thereby improving efficiency.
920            </p>
921            <p id="rfc.section.4.1.p.2">The primary mechanism for determining freshness is for an origin server to provide an explicit expiration time in the future,
922               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
923               is not likely to change in a semantically significant way before the expiration time is reached.
924            </p>
925            <p id="rfc.section.4.1.p.3">If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past
926               to indicate that the response is already stale. Compliant caches will normally validate the cached response before reusing
927               it for subsequent requests (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>).
928            </p>
929            <p id="rfc.section.4.1.p.4">Since origin servers do not always provide explicit expiration times, a cache <em class="bcp14">MAY</em> assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field
930               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
931               constraints on their results.
932            </p>
933            <div id="rfc.figure.u.2"></div>
934            <p>The calculation to determine if a response is fresh is:</p><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
935</pre><p id="rfc.section.4.1.p.6">The freshness_lifetime is defined in <a href="#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section&nbsp;4.1.1</a>; the current_age is defined in <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>.
936            </p>
937            <p id="rfc.section.4.1.p.7">Additionally, clients can influence freshness calculation — either constraining it relaxing it — by using the max-age and
938               min-fresh request cache directives. See <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a> for details.
939            </p>
940            <p id="rfc.section.4.1.p.8">Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload
941               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.
942            </p>
943            <div id="calculating.freshness.lifetime">
944               <h3 id="rfc.section.4.1.1"><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h3>
945               <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>
946               <ul>
947                  <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
948                  </li>
949                  <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
950                  </li>
951                  <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
952                  </li>
953                  <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>.
954                  </li>
955               </ul>
956               <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>
957               <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), it is considered invalid. Caches are encouraged to consider responses
958                  that have invalid freshness information to be stale.
959               </p>
960            </div>
961            <div id="heuristic.freshness">
962               <h3 id="rfc.section.4.1.2"><a href="#rfc.section.4.1.2">4.1.2</a>&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></h3>
963               <p id="rfc.section.4.1.2.p.1">If no explicit expiration time is present in a stored response that has a status code whose definition allows heuristic freshness
964                  to be used (including the following in <a href="" title="Response Status Codes">Section 7</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>: <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a>, <a href="p2-semantics.html#status.203" class="smpl">203 (Non-Authoritative Information)</a>, <a href="p5-range.html#status.206" class="smpl">206 (Partial
965                     Content)</a>, <a href="p2-semantics.html#status.300" class="smpl">300 (Multiple Choices)</a>, <a href="p2-semantics.html#status.301" class="smpl">301 (Moved
966                     Permanently)</a> and <a href="p2-semantics.html#status.410" class="smpl">410 (Gone)</a>), a cache <em class="bcp14">MAY</em> calculate a heuristic expiration time. A cache <em class="bcp14">MUST NOT</em> use heuristics to determine freshness for responses with status codes that do not explicitly allow it.
967               </p>
968               <p id="rfc.section.4.1.2.p.2">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
969                  present.
970               </p>
971               <p id="rfc.section.4.1.2.p.3">Also, 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.Part4.4"><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
972                  time. A typical setting of this fraction might be 10%.
973               </p>
974               <div class="note" id="rfc.section.4.1.2.p.4">
975                  <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,
976                     this has not been widely implemented. Therefore, servers are encouraged to send explicit directives (e.g., Cache-Control:
977                     no-cache) if they wish to preclude caching.
978                  </p>
979               </div>
980            </div>
981            <div id="age.calculations">
982               <h3 id="rfc.section.4.1.3"><a href="#rfc.section.4.1.3">4.1.3</a>&nbsp;<a href="#age.calculations">Calculating Age</a></h3>
983               <p id="rfc.section.4.1.3.p.1">HTTP/1.1 uses the <a href="#header.age" class="smpl">Age</a> header field to convey the estimated age of the response message when obtained from a cache. The Age field value is the cache's
984                  estimate of the amount of time since the response was generated or validated by the origin server. In essence, the Age value
985                  is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus
986                  the amount of time it has been in transit along network paths.
987               </p>
988               <p id="rfc.section.4.1.3.p.2">The following data is used for the age calculation:</p>
989               <p id="rfc.section.4.1.3.p.3"><dfn>age_value</dfn>
990               </p>
991               <ul class="empty">
992                  <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.
993                  </li>
994               </ul>
995               <p id="rfc.section.4.1.3.p.4"><dfn>date_value</dfn>
996               </p>
997               <ul class="empty">
998                  <li>HTTP/1.1 requires origin servers to send a <a href="" class="smpl">Date</a> header field, if possible, with every response, giving the time at which the response was generated. The term "date_value"
999                     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.
1000                  </li>
1001               </ul>
1002               <p id="rfc.section.4.1.3.p.5"><dfn>now</dfn>
1003               </p>
1004               <ul class="empty">
1005                  <li>The term "now" means "the current value of the clock at the host performing the calculation". A cache <em class="bcp14">SHOULD</em> use NTP (<a href="#RFC1305" id="rfc.xref.RFC1305.2"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>) or some similar protocol to synchronize its clocks to a globally accurate time standard.
1006                  </li>
1007               </ul>
1008               <p id="rfc.section.4.1.3.p.6"><dfn>request_time</dfn>
1009               </p>
1010               <ul class="empty">
1011                  <li>The current value of the clock at the host at the time the request resulting in the stored response was made.</li>
1012               </ul>
1013               <p id="rfc.section.4.1.3.p.7"><dfn>response_time</dfn>
1014               </p>
1015               <ul class="empty">
1016                  <li>The current value of the clock at the host at the time the response was received.</li>
1017               </ul>
1018               <p id="rfc.section.4.1.3.p.8">A response's age can be calculated in two entirely independent ways: </p>
1019               <ol>
1020                  <li>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the origin server's
1021                     clock. If the result is negative, the result is replaced by zero.
1022                  </li>
1023                  <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.
1024                  </li>
1025               </ol>
1026               <div id="rfc.figure.u.3"></div><pre class="text">  apparent_age = max(0, response_time - date_value);
1028  response_delay = response_time - request_time;
1029  corrected_age_value = age_value + response_delay; 
1030</pre><div id="rfc.figure.u.4"></div>
1031               <p>These <em class="bcp14">SHOULD</em> be combined as
1032               </p><pre class="text">  corrected_initial_age = max(apparent_age, corrected_age_value);
1033</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.
1034               </p>
1035               <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
1036                  was last validated by the origin server to the corrected_initial_age.
1037               </p>
1038               <div id="rfc.figure.u.5"></div><pre class="text">  resident_time = now - response_time;
1039  current_age = corrected_initial_age + resident_time;
1040</pre><p id="rfc.section.4.1.3.p.14">Additionally, to avoid common problems in date parsing:</p>
1041               <p id="rfc.section.4.1.3.p.15"></p>
1042               <ul>
1043                  <li>Recipients <em class="bcp14">SHOULD</em> assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve
1044                     the "year 2000" problem).
1045                  </li>
1046                  <li>Although all date formats are specified to be case-sensitive, recipients <em class="bcp14">SHOULD</em> match day, week and timezone names case-insensitively.
1047                  </li>
1048                  <li>An implementation <em class="bcp14">MAY</em> internally represent a parsed <a href="#header.expires" class="smpl">Expires</a> date as earlier than the proper value, but <em class="bcp14">MUST NOT</em> internally represent a parsed Expires date as later than the proper value.
1049                  </li>
1050                  <li>Recipients <em class="bcp14">MUST</em> perform all expiration-related calculations in GMT. The local time zone <em class="bcp14">MUST NOT</em> influence the calculation or comparison of an age or expiration time.
1051                  </li>
1052                  <li>Caches <em class="bcp14">SHOULD</em> consider dates with time zones other than "GMT" invalid.
1053                  </li>
1054               </ul>
1055            </div>
1056            <div id="serving.stale.responses">
1057               <h3 id="rfc.section.4.1.4"><a href="#rfc.section.4.1.4">4.1.4</a>&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></h3>
1058               <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
1059                  is not fresh according to the calculations in <a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a>.
1060               </p>
1061               <p id="rfc.section.4.1.4.p.2">A cache <em class="bcp14">MUST NOT</em> return a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
1062                  directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
1063                  see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;7.2.2</a>).
1064               </p>
1065               <p id="rfc.section.4.1.4.p.3">A cache <em class="bcp14">MUST NOT</em> return stale responses unless it is disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
1066                  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>).
1067               </p>
1068               <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.1" 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.
1069               </p>
1070               <p id="rfc.section.4.1.4.p.5">If a cache receives a first-hand response (either 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
1071                  can 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 shouldn't attempt to validate a response simply because
1072                  that response became stale in transit.
1073               </p>
1074            </div>
1075         </div>
1076         <div id="validation.model">
1077            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#validation.model">Validation Model</a></h2>
1078            <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
1079               fresh, or one cannot be selected; see <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>), it can use the conditional request mechanism <a href="#Part4" id="rfc.xref.Part4.5"><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
1080               update it. This process is known as "validating" or "revalidating" the stored response.
1081            </p>
1082            <p id="rfc.section.4.2.p.2">When sending such a conditional request, a cache adds an <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="Using Negotiated Responses">Section&nbsp;4.3</a>) stored response, if available.
1083            </p>
1084            <p id="rfc.section.4.2.p.3">Additionally, a cache can add an <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 all responses stored for the requested URI, if present. However, if any of the stored responses contains
1085               only partial content, the cache shouldn't include its entity-tag in the If-None-Match header field unless the request is for
1086               a range that would be fully satisfied by that stored response.
1087            </p>
1088            <p id="rfc.section.4.2.p.4">Cache handling of a response to a conditional request is dependent upon its status code:</p>
1089            <p id="rfc.section.4.2.p.5"></p>
1090            <ul>
1091               <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 Responses with 304 Not Modified">Section&nbsp;4.2.1</a>.
1092               </li>
1093               <li>A full response (i.e., one with a payload body) indicates that none of the stored responses nominated in the conditional request
1094                  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).
1095               </li>
1096               <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
1097                  if the server failed to respond. In the latter case, it can return a previously stored response (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>).
1098               </li>
1099            </ul>
1100            <div id="freshening.responses">
1101               <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a href="#freshening.responses">Freshening Responses with 304 Not Modified</a></h3>
1102               <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
1103                  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.
1104               </p>
1105               <ul>
1106                  <li>If the new response contains a strong validator, then that strong validator identifies the selected representation. All of
1107                     the stored responses with the same strong validator are selected. If none of the stored responses contain the same strong
1108                     validator, then this new response corresponds to a new selected representation and <em class="bcp14">MUST NOT</em> update the existing stored responses.
1109                  </li>
1110                  <li>If the new response contains a weak validator and that validator corresponds to one of the cache's stored responses, then
1111                     the most recent of those matching stored responses is selected.
1112                  </li>
1113                  <li>If the new response does not include any form of validator, there is only one stored response, and that stored response also
1114                     lacks a validator, then that stored response is selected.
1115                  </li>
1116               </ul>
1117               <p id="rfc.section.4.2.1.p.2">If a stored response is selected for update, the cache <em class="bcp14">MUST</em>:
1118               </p>
1119               <ul>
1120                  <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.2" title="Warning">Section&nbsp;7.5</a>);
1121                  </li>
1122                  <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
1123                  </li>
1124                  <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.
1125                  </li>
1126               </ul>
1127            </div>
1128         </div>
1129         <div id="caching.negotiated.responses">
1130            <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a href="#caching.negotiated.responses">Using Negotiated Responses</a></h2>
1131            <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 8.2.1</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
1132               request (i.e., that associated with the stored response), and the presented request.
1133            </p>
1134            <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
1135               to those in the second request by applying any of the following:
1136            </p>
1137            <ul>
1138               <li>adding or removing whitespace, where allowed in the header field's syntax</li>
1139               <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>)
1140               </li>
1141               <li>normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification
1142                  (e.g., re-ordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive)
1143               </li>
1144            </ul>
1145            <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
1146               if it is also absent there.
1147            </p>
1148            <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, and subsequent requests to that resource can only be properly interpreted
1149               by the origin server.
1150            </p>
1151            <p id="rfc.section.4.3.p.5">The stored response with matching selecting header fields is known as the selected response.</p>
1152            <p id="rfc.section.4.3.p.6">If multiple selected responses are available, the most recent response (as determined by the <a href="" class="smpl">Date</a> header field) is used; see <a href="#constructing.responses.from.caches" title="Constructing Responses from Caches">Section&nbsp;4</a>.
1153            </p>
1154            <p id="rfc.section.4.3.p.7">If no selected response is available, the cache can forward the presented request to the origin server in a conditional request;
1155               see <a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>.
1156            </p>
1157         </div>
1158         <div id="combining.responses">
1159            <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a href="#combining.responses">Combining Partial Content</a></h2>
1160            <p id="rfc.section.4.4.p.1">A response might transfer only a partial representation if the connection closed prematurely or if the request used one or
1161               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
1162               same strong validator and the cache complies with the client requirements in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4.2</a> of <a href="#Part5" id="rfc.xref.Part5.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>.
1163            </p>
1164            <p id="rfc.section.4.4.p.2">When combining the new response with one or more stored responses, a cache <em class="bcp14">MUST</em>:
1165            </p>
1166            <ul>
1167               <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>);
1168               </li>
1169               <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
1170               </li>
1171               <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.
1172               </li>
1173            </ul>
1174         </div>
1175      </div>
1176      <div id="head.effects">
1177         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#head.effects">Updating Caches with HEAD Responses</a></h1>
1178         <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
1179            a body. This property of HEAD responses is used to both invalidate and update cached GET responses.
1180         </p>
1181         <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="Using Negotiated Responses">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.
1182         </p>
1183         <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="Using Negotiated Responses">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:
1184         </p>
1185         <ul>
1186            <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>);
1187            </li>
1188            <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
1189            </li>
1190            <li>use other header fields provided in the response to replace all instances of the corresponding header fields in the stored
1191               response.
1192            </li>
1193         </ul>
1194      </div>
1195      <div id="invalidation.after.updates.or.deletions">
1196         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></h1>
1197         <p id="rfc.section.6.p.1">Because unsafe request methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 5.2.1</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
1198            to keep their contents up-to-date.
1199         </p>
1200         <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.
1201         </p>
1202         <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.
1203         </p>
1204         <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.
1205         </p>
1206         <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,
1207            or will mark these as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent
1208            request.
1209         </p>
1210         <p id="rfc.section.6.p.6">Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the
1211            change at the origin server might not have gone through the cache where a response is stored.
1212         </p>
1213      </div>
1214      <div id="header.field.definitions">
1215         <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a href="#header.field.definitions">Header Field Definitions</a></h1>
1216         <p id="rfc.section.7.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
1217         <div id="header.age">
1218            <div id="rfc.iref.a.2"></div>
1219            <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a href="#header.age">Age</a></h2>
1220            <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
1221               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>.
1222            </p>
1223            <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>
1224</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;1.4.1</a>).
1225            </p>
1226            <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
1227               true, since HTTP/1.0 caches might not implement the Age header field.
1228            </p>
1229         </div>
1230         <div id="header.cache-control">
1231            <div id="rfc.iref.c.6"></div>
1232            <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a href="#header.cache-control">Cache-Control</a></h2>
1233            <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
1234               are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given
1235               in the response.
1236            </p>
1237            <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.
1238            </p>
1239            <div class="note" id="rfc.section.7.2.p.3">
1240               <p><b>Note:</b> HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;7.4</a>).
1241               </p>
1242            </div>
1243            <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
1244               might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific
1245               cache.
1246            </p>
1247            <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
1248               both token and quoted-string syntax. For the directives defined below that define arguments, recipients ought to accept both
1249               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.
1250            </p>
1251            <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>
1253  <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> ) ]
1254</pre><p id="rfc.section.7.2.p.7">For the cache directives defined below, no argument is defined (nor allowed) otherwise stated otherwise.</p>
1255            <div id="cache-request-directive">
1256               <h3 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></h3>
1257               <div id="">
1258                  <div id="rfc.iref.n.1"></div>
1259                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="">no-cache</a></h4>
1260                  <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.
1261                  </p>
1262               </div>
1263               <div id="">
1264                  <div id="rfc.iref.n.2"></div>
1265                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="">no-store</a></h4>
1266                  <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.
1267                  </p>
1268                  <p id="rfc.section.">This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1269                     might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
1270                  </p>
1271                  <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
1272                     to the already stored response.
1273                  </p>
1274               </div>
1275               <div id="cache-request-directive.max-age">
1276                  <div id="rfc.iref.m.1"></div>
1277                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-request-directive.max-age">max-age</a></h4>
1278                  <p id="rfc.section.">Argument syntax: </p>
1279                  <ul class="empty">
1280                     <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.4.1</a>)
1281                     </li>
1282                  </ul>
1283                  <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
1284                     specified number of seconds. Unless the max-stale request directive is also present, the client is not willing to accept a
1285                     stale response.
1286                  </p>
1287                  <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.
1288                  </p>
1289               </div>
1290               <div id="cache-request-directive.max-stale">
1291                  <div id="rfc.iref.m.2"></div>
1292                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-request-directive.max-stale">max-stale</a></h4>
1293                  <p id="rfc.section.">Argument syntax: </p>
1294                  <ul class="empty">
1295                     <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.4.1</a>)
1296                     </li>
1297                  </ul>
1298                  <p id="rfc.section.">The "max-stale" request directive indicates that the client is willing to accept a response that has exceeded its expiration
1299                     time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time
1300                     by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept
1301                     a stale response of any age.
1302                  </p>
1303                  <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.
1304                  </p>
1305               </div>
1306               <div id="cache-request-directive.min-fresh">
1307                  <div id="rfc.iref.m.3"></div>
1308                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-request-directive.min-fresh">min-fresh</a></h4>
1309                  <p id="rfc.section.">Argument syntax: </p>
1310                  <ul class="empty">
1311                     <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.4.1</a>)
1312                     </li>
1313                  </ul>
1314                  <p id="rfc.section.">The "min-fresh" request directive indicates that the client is willing to accept a response whose freshness lifetime is no
1315                     less than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh
1316                     for at least the specified number of seconds.
1317                  </p>
1318                  <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.
1319                  </p>
1320               </div>
1321               <div id="">
1322                  <div id="rfc.iref.n.3"></div>
1323                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="">no-transform</a></h4>
1324                  <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> change the <a href="p2-semantics.html#header.content-encoding" class="smpl">Content-Encoding</a>, <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> or <a href="p2-semantics.html#header.content-type" class="smpl">Content-Type</a> request header fields, nor the request representation.
1325                  </p>
1326               </div>
1327               <div id="cache-request-directive.only-if-cached">
1328                  <div id="rfc.iref.o.1"></div>
1329                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-request-directive.only-if-cached">only-if-cached</a></h4>
1330                  <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
1331                     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
1332                        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.
1333                  </p>
1334               </div>
1335            </div>
1336            <div id="cache-response-directive">
1337               <h3 id="rfc.section.7.2.2"><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></h3>
1338               <div id="cache-response-directive.only-if-cached">
1339                  <div id="rfc.iref.p.2"></div>
1340                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-response-directive.only-if-cached">public</a></h4>
1341                  <p id="rfc.section.">The "public" response directive indicates that a response whose associated request contains an 'Authentication' header <em class="bcp14">MAY</em> be stored (see <a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a>).
1342                  </p>
1343               </div>
1344               <div id="cache-response-directive.private">
1345                  <div id="rfc.iref.p.3"></div>
1346                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-response-directive.private">private</a></h4>
1347                  <p id="rfc.section.">Argument syntax: </p>
1348                  <ul class="empty">
1349                     <li>#<a href="#imported.abnf" class="smpl">field-name</a>
1350                     </li>
1351                  </ul>
1352                  <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.
1353                  </p>
1354                  <p id="rfc.section.">If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated
1355                     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.
1356                  </p>
1357                  <p id="rfc.section.">The field-names given are not limited to the set of standard header fields defined by this specification. Field names are
1358                     case-insensitive.
1359                  </p>
1360                  <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
1361                     content. Also, private response directives with field-names are often handled by implementations as if an unqualified private
1362                     directive was received; i.e., the special handling for the qualified form is not widely implemented.
1363                  </p>
1364                  <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).
1365                  </p>
1366               </div>
1367               <div id="">
1368                  <div id="rfc.iref.n.4"></div>
1369                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="">no-cache</a></h4>
1370                  <p id="rfc.section.">Argument syntax: </p>
1371                  <ul class="empty">
1372                     <li>#<a href="#imported.abnf" class="smpl">field-name</a>
1373                     </li>
1374                  </ul>
1375                  <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
1376                     prevent a cache from using it to satisfy a request without contacting it, even by caches that have been configured to return
1377                     stale responses.
1378                  </p>
1379                  <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
1380                     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
1381                     server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
1382                  </p>
1383                  <p id="rfc.section.">The field-names given are not limited to the set of standard header fields defined by this specification. Field names are
1384                     case-insensitive.
1385                  </p>
1386                  <p id="rfc.section."><b>Note:</b> Many HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache response directives with field-names are often
1387                     handled by implementations as if an unqualified no-cache directive was received; i.e., the special handling for the qualified
1388                     form is not widely implemented.
1389                  </p>
1390                  <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).
1391                  </p>
1392               </div>
1393               <div id="">
1394                  <div id="rfc.iref.n.5"></div>
1395                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="">no-store</a></h4>
1396                  <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.
1397                  </p>
1398                  <p id="rfc.section.">This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1399                     might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
1400                  </p>
1401               </div>
1402               <div id="cache-response-directive.must-revalidate">
1403                  <div id="rfc.iref.m.4"></div>
1404                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-response-directive.must-revalidate">must-revalidate</a></h4>
1405                  <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.
1406                  </p>
1407                  <p id="rfc.section.">The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
1408                     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.
1409                  </p>
1410                  <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
1411                     could result in incorrect operation, such as a silently unexecuted financial transaction.
1412                  </p>
1413               </div>
1414               <div id="cache-response-directive.proxy-revalidate">
1415                  <div id="rfc.iref.p.4"></div>
1416                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-response-directive.proxy-revalidate">proxy-revalidate</a></h4>
1417                  <p id="rfc.section.">The "proxy-revalidate" response directive has the same meaning as the must-revalidate response directive, except that it does
1418                     not apply to private caches.
1419                  </p>
1420               </div>
1421               <div id="cache-response-directive.max-age">
1422                  <div id="rfc.iref.m.5"></div>
1423                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-response-directive.max-age">max-age</a></h4>
1424                  <p id="rfc.section.">Argument syntax: </p>
1425                  <ul class="empty">
1426                     <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.4.1</a>)
1427                     </li>
1428                  </ul>
1429                  <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
1430                     number of seconds.
1431                  </p>
1432                  <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.
1433                  </p>
1434               </div>
1435               <div id="cache-response-directive.s-maxage">
1436                  <div id="rfc.iref.s.4"></div>
1437                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="#cache-response-directive.s-maxage">s-maxage</a></h4>
1438                  <p id="rfc.section.">Argument syntax: </p>
1439                  <ul class="empty">
1440                     <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.4.1</a>)
1441                     </li>
1442                  </ul>
1443                  <p id="rfc.section.">The "s-maxage" response directive indicates that, in shared caches, the maximum age specified by this directive overrides
1444                     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.
1445                  </p>
1446                  <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.
1447                  </p>
1448               </div>
1449               <div id="">
1450                  <div id="rfc.iref.n.6"></div>
1451                  <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a href="">no-transform</a></h4>
1452                  <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> change the <a href="p2-semantics.html#header.content-encoding" class="smpl">Content-Encoding</a>, <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> or <a href="p2-semantics.html#header.content-type" class="smpl">Content-Type</a> response header fields, nor the response representation.
1453                  </p>
1454               </div>
1455            </div>
1456            <div id="cache.control.extensions">
1457               <h3 id="rfc.section.7.2.3"><a href="#rfc.section.7.2.3">7.2.3</a>&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></h3>
1458               <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
1459                  value. Informational extensions (those that do not require a change in cache behavior) can be added without changing the semantics
1460                  of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives.
1461                  Both the new directive and the standard directive are supplied, such that applications that do not understand the new directive
1462                  will default to the behavior specified by the standard directive, and those that understand the new directive will recognize
1463                  it as modifying the requirements associated with the standard directive. In this way, extensions to the cache-control directives
1464                  can be made without requiring changes to the base protocol.
1465               </p>
1466               <p id="rfc.section.7.2.3.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
1467                  obeying certain extensions, and ignoring all directives that it does not understand.
1468               </p>
1469               <p id="rfc.section.7.2.3.p.3">For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive.
1470                  We define this new directive to mean that, in addition to any private cache, any cache that is shared only by members of the
1471                  community named within its value is allowed to cache the response. An origin server wishing to allow the UCI community to
1472                  use an otherwise private response in their shared cache(s) could do so by including
1473               </p>
1474               <div id="rfc.figure.u.8"></div><pre class="text">  Cache-Control: private, community="UCI"
1475</pre><p id="rfc.section.7.2.3.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
1476                  it will also see and understand the private directive and thus default to the safe behavior.
1477               </p>
1478               <p id="rfc.section.7.2.3.p.6">A cache <em class="bcp14">MUST</em> ignore unrecognized cache directives; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache
1479                  will be combined with standard directives (or the response's default cacheability) such that the cache behavior will remain
1480                  minimally correct even if the cache does not understand the extension(s).
1481               </p>
1482               <p id="rfc.section.7.2.3.p.7">New extension directives ought to consider defining:</p>
1483               <p id="rfc.section.7.2.3.p.8"></p>
1484               <ul>
1485                  <li>What it means for a directive to be specified multiple times,</li>
1486                  <li>When the directive does not take an argument, what it means when an argument is present,</li>
1487                  <li>When the directive requires an argument, what it means when it is missing.</li>
1488               </ul>
1489               <p id="rfc.section.7.2.3.p.9">The HTTP Cache Directive Registry defines the name space for the cache directives.</p>
1490               <p id="rfc.section.7.2.3.p.10">A registration <em class="bcp14">MUST</em> include the following fields:
1491               </p>
1492               <ul>
1493                  <li>Cache Directive Name</li>
1494                  <li>Pointer to specification text</li>
1495               </ul>
1496               <p id="rfc.section.7.2.3.p.11">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>).
1497               </p>
1498               <p id="rfc.section.7.2.3.p.12">The registry itself is maintained at &lt;<a href=""></a>&gt;.
1499               </p>
1500            </div>
1501         </div>
1502         <div id="header.expires">
1503            <div id="rfc.iref.e.2"></div>
1504            <h2 id="rfc.section.7.3"><a href="#rfc.section.7.3">7.3</a>&nbsp;<a href="#header.expires">Expires</a></h2>
1505            <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 Model">Section&nbsp;4.1</a> for further discussion of the freshness model.
1506            </p>
1507            <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
1508               that time.
1509            </p>
1510            <p id="rfc.section.7.3.p.3">The field-value is an absolute date and time as defined by HTTP-date in <a href="" title="Date/Time Formats">Section</a> of <a href="#Part2" id="rfc.xref.Part2.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>; a sender <em class="bcp14">MUST</em> use the rfc1123-date format.
1511            </p>
1512            <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>
1513</pre><div id="rfc.figure.u.10"></div>
1514            <p>For example</p><pre class="text">  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1515</pre><p id="rfc.section.7.3.p.6">A cache <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
1516            </p>
1517            <div class="note" id="rfc.section.7.3.p.7">
1518               <p><b>Note:</b> If a response includes a <a href="#header.cache-control" class="smpl">Cache-Control</a> field with the max-age directive (see <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;</a>), that directive overrides the Expires field. Likewise, the s-maxage directive (<a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;</a>) overrides the <a href="#header.expires" class="smpl">Expires</a> header fieldin shared caches.
1519               </p>
1520            </div>
1521            <p id="rfc.section.7.3.p.8">Historically, HTTP required the Expires field-value to be no more than a year in the future. While longer freshness lifetimes
1522               are no longer prohibited, extremely large values have been demonstrated to cause problems (e.g., clock overflows due to use
1523               of 32-bit integers for time values), and many caches will evict a response far sooner than that. Therefore, senders ought
1524               not produce them.
1525            </p>
1526            <p id="rfc.section.7.3.p.9">An origin server without a clock <em class="bcp14">MUST NOT</em> assign Expires values to a response unless these values were associated with the resource by a system or user with a reliable
1527               clock. It <em class="bcp14">MAY</em> assign an Expires value that is known, at or before server configuration time, to be in the past (this allows "pre-expiration"
1528               of responses without storing separate Expires values for each resource).
1529            </p>
1530         </div>
1531         <div id="header.pragma">
1532            <div id="rfc.iref.p.5"></div>
1533            <h2 id="rfc.section.7.4"><a href="#rfc.section.7.4">7.4</a>&nbsp;<a href="#header.pragma">Pragma</a></h2>
1534            <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
1535               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
1536               ignored.
1537            </p>
1538            <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
1539               deprecates such extensions to improve interoperability.
1540            </p>
1541            <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>
1542  <a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / <a href="#header.pragma" class="smpl">extension-pragma</a>
1543  <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> ) ]
1544</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, the no-cache request pragma-directive <em class="bcp14">MUST</em> have the same effect on caches as if "Cache-Control: no-cache" were present (see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a>).
1545            </p>
1546            <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:
1547               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:
1548            </p>
1549            <div id="rfc.figure.u.12"></div><pre class="text">GET / HTTP/1.1
1551Cache-Control: max-age=30
1552Pragma: no-cache
1554</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
1555               understand <a href="#header.cache-control" class="smpl">Cache-Control</a> from serving a cached response.
1556            </p>
1557            <div class="note" id="rfc.section.7.4.p.8">
1558               <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:
1559                  no-cache" in them.
1560               </p>
1561            </div>
1562         </div>
1563         <div id="header.warning">
1564            <div id="rfc.iref.w.1"></div>
1565            <h2 id="rfc.section.7.5"><a href="#rfc.section.7.5">7.5</a>&nbsp;<a href="#header.warning">Warning</a></h2>
1566            <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
1567               not be reflected in the message. This information is typically used to warn about possible incorrectness introduced by caching
1568               operations or transformations applied to the payload of the message.
1569            </p>
1570            <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
1571               code, distinguishes these responses from true failures.
1572            </p>
1573            <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
1574               be applied to response messages.
1575            </p>
1576            <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>
1578  <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>
1579                                        [<a href="#imported.abnf" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-date</a>]
1581  <a href="#header.warning" class="smpl">warn-code</a>  = 3<a href="#imported.abnf" class="smpl">DIGIT</a>
1582  <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>
1583                  ; the name or pseudonym of the server adding
1584                  ; the Warning header field, for use in debugging
1585  <a href="#header.warning" class="smpl">warn-text</a>  = <a href="#imported.abnf" class="smpl">quoted-string</a>
1586  <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>
1587</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
1588               the same code number, only differing in warn-text.
1589            </p>
1590            <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.
1591            </p>
1592            <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
1593               Warning header fields are added after any existing Warning header fields.
1594            </p>
1595            <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
1596               a stored response after validation:
1597            </p>
1598            <ul>
1599               <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.
1600               </li>
1601               <li>2xx Warnings describe some aspect of the representation that is not rectified by a validation (for example, a lossy compression
1602                  of the representation) and <em class="bcp14">MUST NOT</em> be deleted by a cache after validation, unless a full response is returned, in which case they <em class="bcp14">MUST</em> be.
1603               </li>
1604            </ul>
1605            <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,
1606               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.
1607            </p>
1608            <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
1609               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.
1610            </p>
1611            <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
1612               of its meaning.
1613            </p>
1614            <div id="warn.110">
1615               <div id="rfc.iref.1.1"></div>
1616               <h3 id="rfc.section.7.5.1"><a href="#rfc.section.7.5.1">7.5.1</a>&nbsp;<a href="#warn.110">110 Response is Stale</a></h3>
1617               <p id="rfc.section.7.5.1.p.1">A cache <em class="bcp14">SHOULD</em> include this whenever the returned response is stale.
1618               </p>
1619            </div>
1620            <div id="warn.111">
1621               <div id="rfc.iref.1.2"></div>
1622               <h3 id="rfc.section.7.5.2"><a href="#rfc.section.7.5.2">7.5.2</a>&nbsp;<a href="#warn.111">111 Revalidation Failed</a></h3>
1623               <p id="rfc.section.7.5.2.p.1">A cache <em class="bcp14">SHOULD</em> include this when returning a stale response because an attempt to validate the response failed, due to an inability to reach
1624                  the server.
1625               </p>
1626            </div>
1627            <div id="warn.112">
1628               <div id="rfc.iref.1.3"></div>
1629               <h3 id="rfc.section.7.5.3"><a href="#rfc.section.7.5.3">7.5.3</a>&nbsp;<a href="#warn.112">112 Disconnected Operation</a></h3>
1630               <p id="rfc.section.7.5.3.p.1">A cache <em class="bcp14">SHOULD</em> include this if it is intentionally disconnected from the rest of the network for a period of time.
1631               </p>
1632            </div>
1633            <div id="warn.113">
1634               <div id="rfc.iref.1.4"></div>
1635               <h3 id="rfc.section.7.5.4"><a href="#rfc.section.7.5.4">7.5.4</a>&nbsp;<a href="#warn.113">113 Heuristic Expiration</a></h3>
1636               <p id="rfc.section.7.5.4.p.1">A cache <em class="bcp14">SHOULD</em> include this if it heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24
1637                  hours.
1638               </p>
1639            </div>
1640            <div id="warn.199">
1641               <div id="rfc.iref.1.5"></div>
1642               <h3 id="rfc.section.7.5.5"><a href="#rfc.section.7.5.5">7.5.5</a>&nbsp;<a href="#warn.199">199 Miscellaneous Warning</a></h3>
1643               <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.
1644               </p>
1645            </div>
1646            <div id="warn.214">
1647               <div id="rfc.iref.2.1"></div>
1648               <h3 id="rfc.section.7.5.6"><a href="#rfc.section.7.5.6">7.5.6</a>&nbsp;<a href="#warn.214">214 Transformation Applied</a></h3>
1649               <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,
1650                  or modifying the representation data, unless this Warning code already appears in the response.
1651               </p>
1652            </div>
1653            <div id="warn.299">
1654               <div id="rfc.iref.2.2"></div>
1655               <h3 id="rfc.section.7.5.7"><a href="#rfc.section.7.5.7">7.5.7</a>&nbsp;<a href="#warn.299">299 Miscellaneous Persistent Warning</a></h3>
1656               <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.
1657               </p>
1658            </div>
1659            <div id="warn.code.extensions">
1660               <h3 id="rfc.section.7.5.8"><a href="#rfc.section.7.5.8">7.5.8</a>&nbsp;<a href="#warn.code.extensions">Warn Code Extensions</a></h3>
1661               <p id="rfc.section.7.5.8.p.1">The HTTP Warn Code Registry defines the name space for warn codes.</p>
1662               <p id="rfc.section.7.5.8.p.2">A registration <em class="bcp14">MUST</em> include the following fields:
1663               </p>
1664               <ul>
1665                  <li>Warn Code (3 digits)</li>
1666                  <li>Short Description</li>
1667                  <li>Pointer to specification text</li>
1668               </ul>
1669               <p id="rfc.section.7.5.8.p.3">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>).
1670               </p>
1671               <p id="rfc.section.7.5.8.p.4">The registry itself is maintained at &lt;<a href=""></a>&gt;.
1672               </p>
1673            </div>
1674         </div>
1675      </div>
1676      <div id="history.lists">
1677         <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a href="#history.lists">History Lists</a></h1>
1678         <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
1679            retrieved earlier in a session.
1680         </p>
1681         <p id="rfc.section.8.p.2">The freshness model (<a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a>) does not necessarily apply to history mechanisms. I.e., a history mechanism can display a previous representation even if
1682            it has expired.
1683         </p>
1684         <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
1685            (e.g., Cache-Control: no-store).
1686         </p>
1687      </div>
1688      <div id="IANA.considerations">
1689         <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a href="#IANA.considerations">IANA Considerations</a></h1>
1690         <div id="cache.directive.registration">
1691            <h2 id="rfc.section.9.1"><a href="#rfc.section.9.1">9.1</a>&nbsp;<a href="#cache.directive.registration">Cache Directive Registry</a></h2>
1692            <p id="rfc.section.9.1.p.1">The registration procedure for HTTP Cache Directives is defined by <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a> of this document.
1693            </p>
1694            <p id="rfc.section.9.1.p.2">The HTTP Cache Directive Registry shall be created at &lt;<a href=""></a>&gt; and be populated with the registrations below:
1695            </p>
1696            <div id="rfc.table.1">
1697               <div id="iana.cache.directive.registration.table"></div>
1698               <table class="tt full left" cellpadding="3" cellspacing="0">
1699                  <thead>
1700                     <tr>
1701                        <th>Cache Directive</th>
1702                        <th>Reference</th>
1703                     </tr>
1704                  </thead>
1705                  <tbody>
1706                     <tr>
1707                        <td class="left">max-age</td>
1708                        <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>
1709                        </td>
1710                     </tr>
1711                     <tr>
1712                        <td class="left">max-stale</td>
1713                        <td class="left"><a href="#cache-request-directive.max-stale" title="max-stale">Section&nbsp;</a>
1714                        </td>
1715                     </tr>
1716                     <tr>
1717                        <td class="left">min-fresh</td>
1718                        <td class="left"><a href="#cache-request-directive.min-fresh" title="min-fresh">Section&nbsp;</a>
1719                        </td>
1720                     </tr>
1721                     <tr>
1722                        <td class="left">must-revalidate</td>
1723                        <td class="left"><a href="#cache-response-directive.must-revalidate" title="must-revalidate">Section&nbsp;</a>
1724                        </td>
1725                     </tr>
1726                     <tr>
1727                        <td class="left">no-cache</td>
1728                        <td class="left"><a href="" title="no-cache">Section&nbsp;</a>, <a href="" title="no-cache">Section&nbsp;</a>
1729                        </td>
1730                     </tr>
1731                     <tr>
1732                        <td class="left">no-store</td>
1733                        <td class="left"><a href="" title="no-store">Section&nbsp;</a>, <a href="" title="no-store">Section&nbsp;</a>
1734                        </td>
1735                     </tr>
1736                     <tr>
1737                        <td class="left">no-transform</td>
1738                        <td class="left"><a href="" title="no-transform">Section&nbsp;</a>, <a href="" title="no-transform">Section&nbsp;</a>
1739                        </td>
1740                     </tr>
1741                     <tr>
1742                        <td class="left">only-if-cached</td>
1743                        <td class="left"><a href="#cache-request-directive.only-if-cached" title="only-if-cached">Section&nbsp;</a>
1744                        </td>
1745                     </tr>
1746                     <tr>
1747                        <td class="left">private</td>
1748                        <td class="left"><a href="#cache-response-directive.private" title="private">Section&nbsp;</a>
1749                        </td>
1750                     </tr>
1751                     <tr>
1752                        <td class="left">proxy-revalidate</td>
1753                        <td class="left"><a href="#cache-response-directive.proxy-revalidate" title="proxy-revalidate">Section&nbsp;</a>
1754                        </td>
1755                     </tr>
1756                     <tr>
1757                        <td class="left">public</td>
1758                        <td class="left"><a href="#cache-response-directive.only-if-cached" title="public">Section&nbsp;</a>
1759                        </td>
1760                     </tr>
1761                     <tr>
1762                        <td class="left">s-maxage</td>
1763                        <td class="left"><a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;</a>
1764                        </td>
1765                     </tr>
1766                     <tr>
1767                        <td class="left">stale-if-error</td>
1768                        <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>
1769                        </td>
1770                     </tr>
1771                     <tr>
1772                        <td class="left">stale-while-revalidate</td>
1773                        <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>
1774                        </td>
1775                     </tr>
1776                  </tbody>
1777               </table>
1778            </div>
1779         </div>
1780         <div id="warn.code.registration">
1781            <h2 id="rfc.section.9.2"><a href="#rfc.section.9.2">9.2</a>&nbsp;<a href="#warn.code.registration">Warn Code Registry</a></h2>
1782            <p id="rfc.section.9.2.p.1">The registration procedure for HTTP Warn Codes is defined by <a href="#warn.code.extensions" title="Warn Code Extensions">Section&nbsp;7.5.8</a> of this document.
1783            </p>
1784            <p id="rfc.section.9.2.p.2">The HTTP Warn Code Registry shall be created at &lt;<a href=""></a>&gt; and be populated with the registrations below:
1785            </p>
1786            <div id="rfc.table.2">
1787               <div id="iana.warn.code.registration.table"></div>
1788               <table class="tt full left" cellpadding="3" cellspacing="0">
1789                  <thead>
1790                     <tr>
1791                        <th>Warn Code</th>
1792                        <th>Short Description</th>
1793                        <th>Reference</th>
1794                     </tr>
1795                  </thead>
1796                  <tbody>
1797                     <tr>
1798                        <td class="left">110</td>
1799                        <td class="left">Response is Stale</td>
1800                        <td class="left"><a href="#warn.110" id="rfc.xref.warn.110.1" title="110 Response is Stale">Section&nbsp;7.5.1</a>
1801                        </td>
1802                     </tr>
1803                     <tr>
1804                        <td class="left">111</td>
1805                        <td class="left">Revalidation Failed</td>
1806                        <td class="left"><a href="#warn.111" id="rfc.xref.warn.111.1" title="111 Revalidation Failed">Section&nbsp;7.5.2</a>
1807                        </td>
1808                     </tr>
1809                     <tr>
1810                        <td class="left">112</td>
1811                        <td class="left">Disconnected Operation</td>
1812                        <td class="left"><a href="#warn.112" id="rfc.xref.warn.112.1" title="112 Disconnected Operation">Section&nbsp;7.5.3</a>
1813                        </td>
1814                     </tr>
1815                     <tr>
1816                        <td class="left">113</td>
1817                        <td class="left">Heuristic Expiration</td>
1818                        <td class="left"><a href="#warn.113" id="rfc.xref.warn.113.1" title="113 Heuristic Expiration">Section&nbsp;7.5.4</a>
1819                        </td>
1820                     </tr>
1821                     <tr>
1822                        <td class="left">199</td>
1823                        <td class="left">Miscellaneous Warning</td>
1824                        <td class="left"><a href="#warn.199" id="rfc.xref.warn.199.1" title="199 Miscellaneous Warning">Section&nbsp;7.5.5</a>
1825                        </td>
1826                     </tr>
1827                     <tr>
1828                        <td class="left">214</td>
1829                        <td class="left">Transformation Applied</td>
1830                        <td class="left"><a href="#warn.214" id="rfc.xref.warn.214.1" title="214 Transformation Applied">Section&nbsp;7.5.6</a>
1831                        </td>
1832                     </tr>
1833                     <tr>
1834                        <td class="left">299</td>
1835                        <td class="left">Miscellaneous Persistent Warning</td>
1836                        <td class="left"><a href="#warn.299" id="rfc.xref.warn.299.1" title="299 Miscellaneous Persistent Warning">Section&nbsp;7.5.7</a>
1837                        </td>
1838                     </tr>
1839                  </tbody>
1840               </table>
1841            </div>
1842         </div>
1843         <div id="header.field.registration">
1844            <h2 id="rfc.section.9.3"><a href="#rfc.section.9.3">9.3</a>&nbsp;<a href="#header.field.registration">Header Field Registration</a></h2>
1845            <p id="rfc.section.9.3.p.1">The Message Header Field Registry located at &lt;<a href=""></a>&gt; shall be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
1846            </p>
1847            <div id="rfc.table.3">
1848               <div id="iana.header.registration.table"></div>
1849               <table class="tt full left" cellpadding="3" cellspacing="0">
1850                  <thead>
1851                     <tr>
1852                        <th>Header Field Name</th>
1853                        <th>Protocol</th>
1854                        <th>Status</th>
1855                        <th>Reference</th>
1856                     </tr>
1857                  </thead>
1858                  <tbody>
1859                     <tr>
1860                        <td class="left">Age</td>
1861                        <td class="left">http</td>
1862                        <td class="left">standard</td>
1863                        <td class="left"><a href="#header.age" id="rfc.xref.header.age.3" title="Age">Section&nbsp;7.1</a>
1864                        </td>
1865                     </tr>
1866                     <tr>
1867                        <td class="left">Cache-Control</td>
1868                        <td class="left">http</td>
1869                        <td class="left">standard</td>
1870                        <td class="left"><a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;7.2</a>
1871                        </td>
1872                     </tr>
1873                     <tr>
1874                        <td class="left">Expires</td>
1875                        <td class="left">http</td>
1876                        <td class="left">standard</td>
1877                        <td class="left"><a href="#header.expires" id="rfc.xref.header.expires.4" title="Expires">Section&nbsp;7.3</a>
1878                        </td>
1879                     </tr>
1880                     <tr>
1881                        <td class="left">Pragma</td>
1882                        <td class="left">http</td>
1883                        <td class="left">standard</td>
1884                        <td class="left"><a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;7.4</a>
1885                        </td>
1886                     </tr>
1887                     <tr>
1888                        <td class="left">Warning</td>
1889                        <td class="left">http</td>
1890                        <td class="left">standard</td>
1891                        <td class="left"><a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;7.5</a>
1892                        </td>
1893                     </tr>
1894                  </tbody>
1895               </table>
1896            </div>
1897            <p id="rfc.section.9.3.p.2">The change controller is: "IETF ( - Internet Engineering Task Force".</p>
1898         </div>
1899      </div>
1900      <div id="security.considerations">
1901         <h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
1902         <p id="rfc.section.10.p.1">Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious
1903            exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal information
1904            long after a user believes that the information has been removed from the network. Therefore, cache contents need to be protected
1905            as sensitive information.
1906         </p>
1907         <p id="rfc.section.10.p.2">Implementation flaws might allow attackers to insert content into a cache ("cache poisoning"), leading to compromise of clients
1908            that trust that content. Because of their nature, these attacks are difficult to mitigate.
1909         </p>
1910         <p id="rfc.section.10.p.3">Likewise, implementation flaws (as well as misunderstanding of cache operation) might lead to caching of sensitive information
1911            (e.g., authentication credentials) that is thought to be private, exposing it to unauthorised parties.
1912         </p>
1913         <p id="rfc.section.10.p.4">Note that the Set-Cookie response header <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 can be (and often is) used to satisfy subsequent requests
1914            to caches. Servers who wish to control caching of these responses are encouraged to emit appropriate Cache-Control response
1915            headers.
1916         </p>
1917      </div>
1918      <div id="acks">
1919         <h1 id="rfc.section.11"><a href="#rfc.section.11">11.</a>&nbsp;<a href="#acks">Acknowledgments</a></h1>
1920         <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.Part1.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>.
1921         </p>
1922      </div>
1923      <h1 id="rfc.references"><a id="rfc.section.12" href="#rfc.section.12">12.</a> References
1924      </h1>
1925      <h2 id="rfc.references.1"><a href="#rfc.section.12.1" id="rfc.section.12.1">12.1</a> Normative References
1926      </h2>
1927      <table>
1928         <tr>
1929            <td class="reference"><b id="Part1">[Part1]</b></td>
1930            <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-21 (work in progress), October&nbsp;2012.
1931            </td>
1932         </tr>
1933         <tr>
1934            <td class="reference"><b id="Part2">[Part2]</b></td>
1935            <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-21 (work in progress), October&nbsp;2012.
1936            </td>
1937         </tr>
1938         <tr>
1939            <td class="reference"><b id="Part4">[Part4]</b></td>
1940            <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-21 (work in progress), October&nbsp;2012.
1941            </td>
1942         </tr>
1943         <tr>
1944            <td class="reference"><b id="Part5">[Part5]</b></td>
1945            <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-21 (work in progress), October&nbsp;2012.
1946            </td>
1947         </tr>
1948         <tr>
1949            <td class="reference"><b id="Part7">[Part7]</b></td>
1950            <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-21 (work in progress), October&nbsp;2012.
1951            </td>
1952         </tr>
1953         <tr>
1954            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1955            <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.
1956            </td>
1957         </tr>
1958         <tr>
1959            <td class="reference"><b id="RFC5234">[RFC5234]</b></td>
1960            <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.
1961            </td>
1962         </tr>
1963      </table>
1964      <h2 id="rfc.references.2"><a href="#rfc.section.12.2" id="rfc.section.12.2">12.2</a> Informative References
1965      </h2>
1966      <table>
1967         <tr>
1968            <td class="reference"><b id="RFC1305">[RFC1305]</b></td>
1969            <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.
1970            </td>
1971         </tr>
1972         <tr>
1973            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1974            <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.
1975            </td>
1976         </tr>
1977         <tr>
1978            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1979            <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.
1980            </td>
1981         </tr>
1982         <tr>
1983            <td class="reference"><b id="RFC5226">[RFC5226]</b></td>
1984            <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.
1985            </td>
1986         </tr>
1987         <tr>
1988            <td class="reference"><b id="RFC5861">[RFC5861]</b></td>
1989            <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.
1990            </td>
1991         </tr>
1992         <tr>
1993            <td class="reference"><b id="RFC6265">[RFC6265]</b></td>
1994            <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.
1995            </td>
1996         </tr>
1997      </table>
1998      <div id="changes.from.rfc.2616">
1999         <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
2000         <p id="rfc.section.A.p.1">Make the specified age calculation algorithm less conservative. (<a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>)
2001         </p>
2002         <p id="rfc.section.A.p.2">Remove requirement to consider "<a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a>" in successful responses in order to determine the appropriate response to use. (<a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>)
2003         </p>
2004         <p id="rfc.section.A.p.3">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;6</a>)
2005         </p>
2006         <p id="rfc.section.A.p.4">Do not mention RFC 2047 encoding and multiple languages in "<a href="#header.warning" class="smpl">Warning</a>" header fields anymore, as these aspects never were implemented. (<a href="#header.warning" id="rfc.xref.header.warning.6" title="Warning">Section&nbsp;7.5</a>)
2007         </p>
2008         <p id="rfc.section.A.p.5">Introduce Cache Directive and Warn Code Registries. (<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>)
2009         </p>
2010      </div>
2011      <div id="imported.abnf">
2012         <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a href="#imported.abnf">Imported ABNF</a></h1>
2013         <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
2014            (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
2015            character).
2016         </p>
2017         <p id="rfc.section.B.p.2">The rules below are defined in <a href="#Part1" id="rfc.xref.Part1.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>:
2018         </p>
2019         <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.Part1.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.1</a>&gt;
2020  <a href="#imported.abnf" class="smpl">field-name</a>    = &lt;field-name, defined in <a href="#Part1" id="rfc.xref.Part1.12"><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;
2021  <a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, 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>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.4</a>&gt;
2022  <a href="#imported.abnf" class="smpl">token</a>         = &lt;token, 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#field.components" title="Field value components">Section 3.2.4</a>&gt;
2024  <a href="#imported.abnf" class="smpl">port</a>          = &lt;port, 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#uri" title="Uniform Resource Identifiers">Section 2.7</a>&gt;
2025  <a href="#imported.abnf" class="smpl">pseudonym</a>     = &lt;pseudonym, 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#header.via" title="Via">Section 5.7</a>&gt;
2026  <a href="#imported.abnf" class="smpl">uri-host</a>      = &lt;uri-host, 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#uri" title="Uniform Resource Identifiers">Section 2.7</a>&gt;
2027</pre><p id="rfc.section.B.p.4">The rules below are defined in other parts:</p>
2028         <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.Part2.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, <a href="" title="Date/Time Formats">Section</a>&gt;
2030      <div id="collected.abnf">
2031         <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a href="#collected.abnf">Collected ABNF</a></h1>
2032         <div id="rfc.figure.u.16"></div><pre class="inline"><a href="#header.age" class="smpl">Age</a> = delta-seconds
2034<a href="#header.cache-control" class="smpl">Cache-Control</a> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2035 cache-directive ] )
2037<a href="#header.expires" class="smpl">Expires</a> = HTTP-date
2039<a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section;
2041<a href="#imported.abnf" class="smpl">OWS</a> = &lt;OWS, defined in [Part1], Section 3.2.1&gt;
2043<a href="#header.pragma" class="smpl">Pragma</a> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2044 pragma-directive ] )
2046<a href="#header.warning" class="smpl">Warning</a> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2047 )
2049<a href="#header.cache-control" class="smpl">cache-directive</a> = token [ "=" ( token / quoted-string ) ]
2051<a href="#delta-seconds" class="smpl">delta-seconds</a> = 1*DIGIT
2053<a href="#header.pragma" class="smpl">extension-pragma</a> = token [ "=" ( token / quoted-string ) ]
2055<a href="#imported.abnf" class="smpl">field-name</a> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2057<a href="#imported.abnf" class="smpl">port</a> = &lt;port, defined in [Part1], Section 2.7&gt;
2058<a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / extension-pragma
2059<a href="#imported.abnf" class="smpl">pseudonym</a> = &lt;pseudonym, defined in [Part1], Section 5.7&gt;
2061<a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 3.2.4&gt;
2063<a href="#imported.abnf" class="smpl">token</a> = &lt;token, defined in [Part1], Section 3.2.4&gt;
2065<a href="#imported.abnf" class="smpl">uri-host</a> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2067<a href="#header.warning" class="smpl">warn-agent</a> = ( uri-host [ ":" port ] ) / pseudonym
2068<a href="#header.warning" class="smpl">warn-code</a> = 3DIGIT
2069<a href="#header.warning" class="smpl">warn-date</a> = DQUOTE HTTP-date DQUOTE
2070<a href="#header.warning" class="smpl">warn-text</a> = quoted-string
2071<a href="#header.warning" class="smpl">warning-value</a> = warn-code SP warn-agent SP warn-text [ SP warn-date
2072 ]
2074      <div id="change.log">
2075         <h1 id="rfc.section.D"><a href="#rfc.section.D">D.</a>&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
2076         <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;.
2077         </p>
2078         <div id="changes.since.19">
2079            <h2 id="rfc.section.D.1"><a href="#rfc.section.D.1">D.1</a>&nbsp;<a href="#changes.since.19">Since draft-ietf-httpbis-p6-cache-19</a></h2>
2080            <p id="rfc.section.D.1.p.1">Closed issues: </p>
2081            <ul>
2082               <li>&lt;<a href=""></a>&gt;: "untangle Cache-Control ABNF"
2083               </li>
2084               <li>&lt;<a href=""></a>&gt;: "Multiple values in Cache-Control header fields"
2085               </li>
2086               <li>&lt;<a href=""></a>&gt;: "Case sensitivity of header fields in CC values"
2087               </li>
2088               <li>&lt;<a href=""></a>&gt;: "Spurious 'MAYs'"
2089               </li>
2090               <li>&lt;<a href=""></a>&gt;: "enhance considerations for new cache control directives"
2091               </li>
2092               <li>&lt;<a href=""></a>&gt;: "ABNF requirements for recipients"
2093               </li>
2094               <li>&lt;<a href=""></a>&gt;: "note introduction of new IANA registries as normative changes"
2095               </li>
2096               <li>&lt;<a href=""></a>&gt;: "broken prose in description of 'Vary'"
2097               </li>
2098            </ul>
2099         </div>
2100         <div id="changes.since.20">
2101            <h2 id="rfc.section.D.2"><a href="#rfc.section.D.2">D.2</a>&nbsp;<a href="#changes.since.20">Since draft-ietf-httpbis-p6-cache-20</a></h2>
2102            <p id="rfc.section.D.2.p.1">Closed issues: </p>
2103            <ul>
2104               <li>&lt;<a href=""></a>&gt;: "'Most Conservative'"
2105               </li>
2106            </ul>
2107            <p id="rfc.section.D.2.p.2">Other changes: </p>
2108            <ul>
2109               <li>Conformance criteria and considerations regarding error handling are now defined in Part 1.</li>
2110               <li>Move definition of "Vary" header field into Part 2.</li>
2111               <li>Add security considerations with respect to cache poisoning and the "Set-Cookie" header field.</li>
2112            </ul>
2113         </div>
2114      </div>
2115      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
2116      <p class="noprint"><a href="#rfc.index.1">1</a> <a href="#rfc.index.2">2</a> <a href="#rfc.index.A">A</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.F">F</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.N">N</a> <a href="#rfc.index.O">O</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a> <a href="#rfc.index.W">W</a>
2117      </p>
2118      <div class="print2col">
2119         <ul class="ind">
2120            <li><a id="rfc.index.1" href="#rfc.index.1"><b>1</b></a><ul>
2121                  <li>110 Response is Stale (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.1"><b>7.5.1</b></a>, <a href="#rfc.xref.warn.110.1">9.2</a></li>
2122                  <li>111 Revalidation Failed (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.2"><b>7.5.2</b></a>, <a href="#rfc.xref.warn.111.1">9.2</a></li>
2123                  <li>112 Disconnected Operation (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.3"><b>7.5.3</b></a>, <a href="#rfc.xref.warn.112.1">9.2</a></li>
2124                  <li>113 Heuristic Expiration (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.4"><b>7.5.4</b></a>, <a href="#rfc.xref.warn.113.1">9.2</a></li>
2125                  <li>199 Miscellaneous Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.1.5"><b>7.5.5</b></a>, <a href="#rfc.xref.warn.199.1">9.2</a></li>
2126               </ul>
2127            </li>
2128            <li><a id="rfc.index.2" href="#rfc.index.2"><b>2</b></a><ul>
2129                  <li>214 Transformation Applied (warn code)&nbsp;&nbsp;<a href="#rfc.iref.2.1"><b>7.5.6</b></a>, <a href="#rfc.xref.warn.214.1">9.2</a></li>
2130                  <li>299 Miscellaneous Persistent Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.2.2"><b>7.5.7</b></a>, <a href="#rfc.xref.warn.299.1">9.2</a></li>
2131               </ul>
2132            </li>
2133            <li><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul>
2134                  <li>age&nbsp;&nbsp;<a href="#rfc.iref.a.1">1.2</a></li>
2135                  <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>
2136               </ul>
2137            </li>
2138            <li><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul>
2139                  <li>cache&nbsp;&nbsp;<a href="#rfc.iref.c.1">1.1</a>, <a href="#rfc.iref.c.2">1.2</a></li>
2140                  <li>cache entry&nbsp;&nbsp;<a href="#rfc.iref.c.4">2</a></li>
2141                  <li>cache key&nbsp;&nbsp;<a href="#rfc.iref.c.5">2</a></li>
2142                  <li>Cache-Control header field&nbsp;&nbsp;<a href="#rfc.xref.header.cache-control.1">3</a>, <a href="#rfc.iref.c.6"><b>7.2</b></a>, <a href="#rfc.xref.header.cache-control.2">9.3</a></li>
2143                  <li>cacheable&nbsp;&nbsp;<a href="#rfc.iref.c.3">1.2</a></li>
2144               </ul>
2145            </li>
2146            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
2147                  <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></li>
2148                  <li>explicit expiration time&nbsp;&nbsp;<a href="#rfc.iref.e.1">1.2</a></li>
2149               </ul>
2150            </li>
2151            <li><a id="rfc.index.F" href="#rfc.index.F"><b>F</b></a><ul>
2152                  <li>first-hand&nbsp;&nbsp;<a href="#rfc.iref.f.1">1.2</a></li>
2153                  <li>fresh&nbsp;&nbsp;<a href="#rfc.iref.f.3">1.2</a></li>
2154                  <li>freshness lifetime&nbsp;&nbsp;<a href="#rfc.iref.f.2">1.2</a></li>
2155               </ul>
2156            </li>
2157            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
2158                  <li><tt>Grammar</tt>&nbsp;&nbsp;
2159                     <ul>
2160                        <li><tt>Age</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>7.1</b></a></li>
2161                        <li><tt>Cache-Control</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>7.2</b></a></li>
2162                        <li><tt>cache-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>7.2</b></a></li>
2163                        <li><tt>delta-seconds</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>1.4.1</b></a></li>
2164                        <li><tt>Expires</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>7.3</b></a></li>
2165                        <li><tt>extension-pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>7.4</b></a></li>
2166                        <li><tt>Pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>7.4</b></a></li>
2167                        <li><tt>pragma-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>7.4</b></a></li>
2168                        <li><tt>warn-agent</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.12"><b>7.5</b></a></li>
2169                        <li><tt>warn-code</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.11"><b>7.5</b></a></li>
2170                        <li><tt>warn-date</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.14"><b>7.5</b></a></li>
2171                        <li><tt>warn-text</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.13"><b>7.5</b></a></li>
2172                        <li><tt>Warning</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>7.5</b></a></li>
2173                        <li><tt>warning-value</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>7.5</b></a></li>
2174                     </ul>
2175                  </li>
2176               </ul>
2177            </li>
2178            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
2179                  <li>heuristic expiration time&nbsp;&nbsp;<a href="#rfc.iref.h.1">1.2</a></li>
2180               </ul>
2181            </li>
2182            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
2183                  <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>
2184                  <li>max-stale (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.m.2"><b></b></a></li>
2185                  <li>min-fresh (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.m.3"><b></b></a></li>
2186                  <li>must-revalidate (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.m.4"><b></b></a></li>
2187               </ul>
2188            </li>
2189            <li><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul>
2190                  <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>
2191                  <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>
2192                  <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>
2193               </ul>
2194            </li>
2195            <li><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul>
2196                  <li>only-if-cached (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.o.1"><b></b></a></li>
2197               </ul>
2198            </li>
2199            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
2200                  <li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.3</a>, <a href="#rfc.xref.Part1.2">1.4</a>, <a href="#rfc.xref.Part1.3">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">11</a>, <a href="#Part1"><b>12.1</b></a>, <a href="#rfc.xref.Part1.10">B</a>, <a href="#rfc.xref.Part1.11">B</a>, <a href="#rfc.xref.Part1.12">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><ul>
2201                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.2">1.4</a></li>
2202                        <li><em>Section 2.5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.3</a></li>
2203                        <li><em>Section 2.7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.15">B</a>, <a href="#rfc.xref.Part1.17">B</a></li>
2204                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">4.3</a>, <a href="#rfc.xref.Part1.12">B</a></li>
2205                        <li><em>Section 3.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.11">B</a></li>
2206                        <li><em>Section 3.2.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.13">B</a>, <a href="#rfc.xref.Part1.14">B</a></li>
2207                        <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>
2208                        <li><em>Section 5.7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.16">B</a></li>
2209                        <li><em>Section 9</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.9">11</a></li>
2210                     </ul>
2211                  </li>
2212                  <li><em>Part2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">2</a>, <a href="#rfc.xref.Part2.2">4</a>, <a href="#rfc.xref.Part2.3">4.1.2</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="#Part2"><b>12.1</b></a>, <a href="#rfc.xref.Part2.8">B</a><ul>
2213                        <li><em>Section 5.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.2">4</a>, <a href="#rfc.xref.Part2.6">6</a></li>
2214                        <li><em>Section 7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">4.1.2</a></li>
2215                        <li><em>Section</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.7">7.3</a>, <a href="#rfc.xref.Part2.8">B</a></li>
2216                        <li><em>Section</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.4">4.1.3</a></li>
2217                        <li><em>Section 8.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.5">4.3</a></li>
2218                     </ul>
2219                  </li>
2220                  <li><em>Part4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.1">1.2</a>, <a href="#rfc.xref.Part4.2">1.2</a>, <a href="#rfc.xref.Part4.3">1.2</a>, <a href="#rfc.xref.Part4.4">4.1.2</a>, <a href="#rfc.xref.Part4.5">4.2</a>, <a href="#Part4"><b>12.1</b></a><ul>
2221                        <li><em>Section 2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.1">1.2</a></li>
2222                        <li><em>Section 2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.4">4.1.2</a></li>
2223                        <li><em>Section 2.2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.3">1.2</a></li>
2224                        <li><em>Section 2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.2">1.2</a></li>
2225                     </ul>
2226                  </li>
2227                  <li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.1</a>, <a href="#rfc.xref.Part5.2">4.4</a>, <a href="#rfc.xref.Part5.3">4.4</a>, <a href="#Part5"><b>12.1</b></a><ul>
2228                        <li><em>Section 4.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.3">4.4</a></li>
2229                     </ul>
2230                  </li>
2231                  <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>
2232                        <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>
2233                     </ul>
2234                  </li>
2235                  <li>Pragma header field&nbsp;&nbsp;<a href="#rfc.xref.header.pragma.1">4</a>, <a href="#rfc.xref.header.pragma.2">7.2</a>, <a href="#rfc.iref.p.5"><b>7.4</b></a>, <a href="#rfc.xref.header.pragma.3">9.3</a></li>
2236                  <li>private (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.p.3"><b></b></a></li>
2237                  <li>private cache&nbsp;&nbsp;<a href="#rfc.iref.p.1">1.2</a></li>
2238                  <li>proxy-revalidate (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.p.4"><b></b></a></li>
2239                  <li>public (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.p.2"><b></b></a></li>
2240               </ul>
2241            </li>
2242            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
2243                  <li><em>RFC1305</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC1305.1">4</a>, <a href="#rfc.xref.RFC1305.2">4.1.3</a>, <a href="#RFC1305"><b>12.2</b></a></li>
2244                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.3</a>, <a href="#RFC2119"><b>12.1</b></a></li>
2245                  <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>
2246                        <li><em>Section 13.9</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4.1.2</a></li>
2247                     </ul>
2248                  </li>
2249                  <li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">9.3</a>, <a href="#RFC3864"><b>12.2</b></a></li>
2250                  <li><em>RFC5226</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">7.2.3</a>, <a href="#rfc.xref.RFC5226.2">7.5.8</a>, <a href="#RFC5226"><b>12.2</b></a><ul>
2251                        <li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">7.2.3</a>, <a href="#rfc.xref.RFC5226.2">7.5.8</a></li>
2252                     </ul>
2253                  </li>
2254                  <li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.4</a>, <a href="#RFC5234"><b>12.1</b></a>, <a href="#rfc.xref.RFC5234.2">B</a><ul>
2255                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">B</a></li>
2256                     </ul>
2257                  </li>
2258                  <li><em>RFC5861</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">9.1</a>, <a href="#rfc.xref.RFC5861.2">9.1</a>, <a href="#RFC5861"><b>12.2</b></a><ul>
2259                        <li><em>Section 3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.2">9.1</a></li>
2260                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">9.1</a></li>
2261                     </ul>
2262                  </li>
2263                  <li><em>RFC6265</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC6265.1">10</a>, <a href="#RFC6265"><b>12.2</b></a></li>
2264               </ul>
2265            </li>
2266            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
2267                  <li>s-maxage (cache directive)&nbsp;&nbsp;<a href="#rfc.iref.s.4"><b></b></a></li>
2268                  <li>shared cache&nbsp;&nbsp;<a href="#rfc.iref.s.1">1.2</a></li>
2269                  <li>stale&nbsp;&nbsp;<a href="#rfc.iref.s.2">1.2</a></li>
2270                  <li>strong validator&nbsp;&nbsp;<a href="#rfc.iref.s.3">1.2</a></li>
2271               </ul>
2272            </li>
2273            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
2274                  <li>validator&nbsp;&nbsp;<a href="#rfc.iref.v.1">1.2</a><ul>
2275                        <li>strong&nbsp;&nbsp;<a href="#rfc.iref.v.2">1.2</a></li>
2276                     </ul>
2277                  </li>
2278               </ul>
2279            </li>
2280            <li><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul>
2281                  <li>Warning header field&nbsp;&nbsp;<a href="#rfc.xref.header.warning.1">4.1.4</a>, <a href="#rfc.xref.header.warning.2">4.2.1</a>, <a href="#rfc.xref.header.warning.3">4.4</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>
2282               </ul>
2283            </li>
2284         </ul>
2285      </div>
2286      <div class="avoidbreak">
2287         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
2288         <p><b>Roy T. Fielding</b>
2289            (editor)
2290            <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2291         <p><b>Mark Nottingham</b>
2292            (editor)
2293            <br>Akamai<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2294         <p><b>Julian F. Reschke</b>
2295            (editor)
2296            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Email: <a href=""></a><br>URI: <a href=""></a></p>
2297      </div>
2298   </body>
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