source: draft-ietf-httpbis/latest/p6-cache.html

Last change on this file was 2748, checked in by julian.reschke@…, 4 years ago

Update XSLTs, fix Makefile wrt sorting of ABNF, regen HTML

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File size: 202.5 KB
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493       content: "Internet-Draft";
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496       content: "June 2014";
497  }
498  @top-center {
499       content: "HTTP/1.1 Caching";
500  }
501  @bottom-left {
502       content: "Fielding, et al.";
503  }
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505       content: "Expires December 2014";
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523</style><link rel="Contents" href="#rfc.toc">
524      <link rel="Author" href="#rfc.authors">
525      <link rel="Copyright" href="#rfc.copyrightnotice">
526      <link rel="Index" href="#rfc.index">
527      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
528      <link rel="Chapter" title="2 Overview of Cache Operation" href="#rfc.section.2">
529      <link rel="Chapter" title="3 Storing Responses in Caches" href="#rfc.section.3">
530      <link rel="Chapter" title="4 Constructing Responses from Caches" href="#rfc.section.4">
531      <link rel="Chapter" title="5 Header Field Definitions" href="#rfc.section.5">
532      <link rel="Chapter" title="6 History Lists" href="#rfc.section.6">
533      <link rel="Chapter" title="7 IANA Considerations" href="#rfc.section.7">
534      <link rel="Chapter" title="8 Security Considerations" href="#rfc.section.8">
535      <link rel="Chapter" title="9 Acknowledgments" href="#rfc.section.9">
536      <link rel="Chapter" href="#rfc.section.10" title="10 References">
537      <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
538      <link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B">
539      <link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C">
540      <link rel="Alternate" title="RFC7234" href="http://svn.tools.ietf.org/svn/wg/httpbis/specs/rfc7234.html">
541      <link href="p5-range.html" rel="prev">
542      <link href="p7-auth.html" rel="next">
543      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.756, 2015/12/31 17:59:21, XSLT vendor: Saxonica http://www.saxonica.com/">
544      <meta name="keywords" content="Hypertext Transfer Protocol, HTTP, HTTP Caching">
545      <link rel="schema.dct" href="http://purl.org/dc/terms/">
546      <meta name="dct.creator" content="Fielding, R.">
547      <meta name="dct.creator" content="Nottingham, M.">
548      <meta name="dct.creator" content="Reschke, J. F.">
549      <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p6-cache-latest">
550      <meta name="dct.issued" scheme="ISO8601" content="2014-06">
551      <meta name="dct.replaces" content="urn:ietf:rfc:2616">
552      <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
553      <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
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556      <table class="header" id="rfc.headerblock">
557         <tbody>
558            <tr>
559               <td class="left">HTTPbis Working Group</td>
560               <td class="right">R. Fielding, Editor</td>
561            </tr>
562            <tr>
563               <td class="left">Internet-Draft</td>
564               <td class="right">Adobe</td>
565            </tr>
566            <tr>
567               <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved)
568               </td>
569               <td class="right">M. Nottingham, Editor</td>
570            </tr>
571            <tr>
572               <td class="left">Intended status: Standards Track</td>
573               <td class="right">Akamai</td>
574            </tr>
575            <tr>
576               <td class="left">Expires: December 2014</td>
577               <td class="right">J. Reschke, Editor</td>
578            </tr>
579            <tr>
580               <td class="left"></td>
581               <td class="right">greenbytes</td>
582            </tr>
583            <tr>
584               <td class="left"></td>
585               <td class="right">June 2014</td>
586            </tr>
587         </tbody>
588      </table>
589      <div id="rfc.title">
590         <h1>Hypertext Transfer Protocol (HTTP/1.1): Caching</h1>
591         <div class="filename">draft-ietf-httpbis-p6-cache-latest</div>
592      </div>
593      <p class="publishedasrfc"><b>Note:</b> a later version of this document has been published as <a href="http://svn.tools.ietf.org/svn/wg/httpbis/specs/rfc7234.html">RFC7234</a>.
594         
595      </p>
596      <h2 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h2>
597      <p>The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for
598         distributed, collaborative, hypertext information systems. This document defines HTTP
599         caches and the associated header fields that control cache behavior or indicate cacheable
600         response messages.
601      </p>
602      <h2 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h2>
603      <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org),
604         which is archived at &lt;<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>&gt;.
605      </p>
606      <p>The current issues list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
607      </p>
608      <p><em>This is a temporary document for the purpose of tracking the editorial changes made
609            during the AUTH48 (RFC publication) phase.</em> 
610      </p>
611      <div id="rfc.status">
612         <h2><a href="#rfc.status">Status of This Memo</a></h2>
613         <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78
614            and BCP 79.
615         </p>
616         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF).
617            Note that other groups may also distribute working documents as Internet-Drafts. The
618            list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
619         </p>
620         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated,
621            replaced, or obsoleted by other documents at any time. It is inappropriate to use
622            Internet-Drafts as reference material or to cite them other than as “work in progress”.
623         </p>
624         <p>This Internet-Draft will expire in December 2014.</p>
625      </div>
626      <div id="rfc.copyrightnotice">
627         <h2><a href="#rfc.copyrightnotice">Copyright Notice</a></h2>
628         <p>Copyright © 2014 IETF Trust and the persons identified as the document authors. All
629            rights reserved.
630         </p>
631         <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating
632            to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents
633            carefully, as they describe your rights and restrictions with respect to this document.
634            Code Components extracted from this document must include Simplified BSD License text
635            as described in Section 4.e of the Trust Legal Provisions and are provided without
636            warranty as described in the Simplified BSD License.
637         </p>
638         <p>This document may contain material from IETF Documents or IETF Contributions published
639            or made publicly available before November 10, 2008. The person(s) controlling the
640            copyright in some of this material may not have granted the IETF Trust the right to
641            allow modifications of such material outside the IETF Standards Process. Without obtaining
642            an adequate license from the person(s) controlling the copyright in such materials,
643            this document may not be modified outside the IETF Standards Process, and derivative
644            works of it may not be created outside the IETF Standards Process, except to format
645            it for publication as an RFC or to translate it into languages other than English.
646         </p>
647      </div>
648      <hr class="noprint">
649      <div id="rfc.toc">
650         <h2 class="np"><a href="#rfc.toc">Table of Contents</a></h2>
651         <ul class="toc">
652            <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul>
653                  <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#conformance">Conformance and Error Handling</a></li>
654                  <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul>
655                        <li><a href="#rfc.section.1.2.1">1.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#delta-seconds">Delta Seconds</a></li>
656                     </ul>
657                  </li>
658               </ul>
659            </li>
660            <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview of Cache Operation</a></li>
661            <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Storing Responses in Caches</a><ul>
662                  <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#incomplete.responses">Storing Incomplete Responses</a></li>
663                  <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>
664                  <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#combining.responses">Combining Partial Content</a></li>
665               </ul>
666            </li>
667            <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a><ul>
668                  <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Calculating Secondary Keys with Vary</a></li>
669                  <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness</a><ul>
670                        <li><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></li>
671                        <li><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></li>
672                        <li><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
673                        <li><a href="#rfc.section.4.2.4">4.2.4</a>&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
674                     </ul>
675                  </li>
676                  <li><a href="#rfc.section.4.3">4.3</a>&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation</a><ul>
677                        <li><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;&nbsp;&nbsp;<a href="#validation.sent">Sending a Validation Request</a></li>
678                        <li><a href="#rfc.section.4.3.2">4.3.2</a>&nbsp;&nbsp;&nbsp;<a href="#validation.received">Handling a Received Validation Request</a></li>
679                        <li><a href="#rfc.section.4.3.3">4.3.3</a>&nbsp;&nbsp;&nbsp;<a href="#validation.response">Handling a Validation Response</a></li>
680                        <li><a href="#rfc.section.4.3.4">4.3.4</a>&nbsp;&nbsp;&nbsp;<a href="#freshening.responses">Freshening Stored Responses upon Validation</a></li>
681                        <li><a href="#rfc.section.4.3.5">4.3.5</a>&nbsp;&nbsp;&nbsp;<a href="#head.effects">Freshening Responses via HEAD</a></li>
682                     </ul>
683                  </li>
684                  <li><a href="#rfc.section.4.4">4.4</a>&nbsp;&nbsp;&nbsp;<a href="#invalidation">Invalidation</a></li>
685               </ul>
686            </li>
687            <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.definitions">Header Field Definitions</a><ul>
688                  <li><a href="#rfc.section.5.1">5.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
689                  <li><a href="#rfc.section.5.2">5.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul>
690                        <li><a href="#rfc.section.5.2.1">5.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></li>
691                        <li><a href="#rfc.section.5.2.2">5.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></li>
692                        <li><a href="#rfc.section.5.2.3">5.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
693                     </ul>
694                  </li>
695                  <li><a href="#rfc.section.5.3">5.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
696                  <li><a href="#rfc.section.5.4">5.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
697                  <li><a href="#rfc.section.5.5">5.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a><ul>
698                        <li><a href="#rfc.section.5.5.1">5.5.1</a>&nbsp;&nbsp;&nbsp;<a href="#warn.110">Warning: 110 - "Response is Stale"</a></li>
699                        <li><a href="#rfc.section.5.5.2">5.5.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.111">Warning: 111 - "Revalidation Failed"</a></li>
700                        <li><a href="#rfc.section.5.5.3">5.5.3</a>&nbsp;&nbsp;&nbsp;<a href="#warn.112">Warning: 112 - "Disconnected Operation"</a></li>
701                        <li><a href="#rfc.section.5.5.4">5.5.4</a>&nbsp;&nbsp;&nbsp;<a href="#warn.113">Warning: 113 - "Heuristic Expiration"</a></li>
702                        <li><a href="#rfc.section.5.5.5">5.5.5</a>&nbsp;&nbsp;&nbsp;<a href="#warn.199">Warning: 199 - "Miscellaneous Warning"</a></li>
703                        <li><a href="#rfc.section.5.5.6">5.5.6</a>&nbsp;&nbsp;&nbsp;<a href="#warn.214">Warning: 214 - "Transformation Applied"</a></li>
704                        <li><a href="#rfc.section.5.5.7">5.5.7</a>&nbsp;&nbsp;&nbsp;<a href="#warn.299">Warning: 299 - "Miscellaneous Persistent Warning"</a></li>
705                     </ul>
706                  </li>
707               </ul>
708            </li>
709            <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
710            <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#iana.considerations">IANA Considerations</a><ul>
711                  <li><a href="#rfc.section.7.1">7.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registry">Cache Directive Registry</a><ul>
712                        <li><a href="#rfc.section.7.1.1">7.1.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registry.procedure">Procedure</a></li>
713                        <li><a href="#rfc.section.7.1.2">7.1.2</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.considerations">Considerations for New Cache Control Directives</a></li>
714                        <li><a href="#rfc.section.7.1.3">7.1.3</a>&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registration">Registrations</a></li>
715                     </ul>
716                  </li>
717                  <li><a href="#rfc.section.7.2">7.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registry">Warn Code Registry</a><ul>
718                        <li><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registry.procedure">Procedure</a></li>
719                        <li><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#warn.code.registration">Registrations</a></li>
720                     </ul>
721                  </li>
722                  <li><a href="#rfc.section.7.3">7.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
723               </ul>
724            </li>
725            <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
726            <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
727            <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
728                  <li><a href="#rfc.section.10.1">10.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
729                  <li><a href="#rfc.section.10.2">10.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
730               </ul>
731            </li>
732            <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
733            <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
734            <li><a href="#rfc.section.C">C.</a>&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
735            <li><a href="#rfc.index">Index</a></li>
736            <li><a href="#rfc.authors">Authors' Addresses</a></li>
737         </ul>
738      </div>
739      <div id="caching">
740         <h2 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#caching">Introduction</a></h2>
741         <div id="rfc.section.1.p.1">
742            <p>HTTP is typically used for distributed information systems, where performance can
743               be improved by the use of response caches. This document defines aspects of HTTP/1.1
744               related to caching and reusing response messages.
745            </p>
746         </div>
747         <div id="rfc.iref.c.1"></div>
748         <div id="rfc.section.1.p.2">
749            <p>An HTTP <dfn>cache</dfn> is a local store of response messages and the subsystem that controls storage, retrieval,
750               and deletion of messages in it. A cache stores cacheable responses in order to reduce
751               the response time and network bandwidth consumption on future, equivalent requests.
752               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.
753            </p>
754         </div>
755         <div id="rfc.iref.s.1"></div>
756         <div id="rfc.iref.p.1"></div>
757         <div id="shared.and.private.caches">
758            <div id="rfc.section.1.p.3">
759               <p>A <dfn>shared cache</dfn> is a cache that stores responses to be reused by more than one user; shared caches
760                  are usually (but not always) deployed as a part of an intermediary. A <dfn>private cache</dfn>, in contrast, is dedicated to a single user; often, they are deployed as a component
761                  of a user agent.
762               </p>
763            </div>
764         </div>
765         <div id="rfc.section.1.p.4">
766            <p>The goal of caching in HTTP/1.1 is to significantly improve performance by reusing
767               a prior response message to satisfy a current request. A stored response is considered
768               "fresh", as defined in <a href="#expiration.model" title="Freshness">Section&nbsp;4.2</a>, if the response can be reused without "validation" (checking with the origin server
769               to see if the cached response remains valid for this request). A fresh response can
770               therefore reduce both latency and network overhead each time it is reused. When a
771               cached response is not fresh, it might still be reusable if it can be freshened by
772               validation (<a href="#validation.model" title="Validation">Section&nbsp;4.3</a>) or if the origin is unavailable (<a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.2.4</a>).
773            </p>
774         </div>
775         <div id="conformance">
776            <h3 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#conformance">Conformance and Error Handling</a></h3>
777            <div id="rfc.section.1.1.p.1">
778               <p>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
779                  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted
780                  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>.
781               </p>
782            </div>
783            <div id="rfc.section.1.1.p.2">
784               <p>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="#RFC7230" id="rfc.xref.RFC7230.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
785               </p>
786            </div>
787         </div>
788         <div id="notation">
789            <h3 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#notation">Syntax Notation</a></h3>
790            <div id="rfc.section.1.2.p.1">
791               <p>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 a list extension, defined in <a href="p1-messaging.html#abnf.extension" title="ABNF List Extension: #rule">Section 7</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, that allows for compact definition of comma-separated lists using a '#' operator
792                  (similar to how the '*' operator indicates repetition). <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 grammar with all list operators expanded to standard ABNF notation.
793               </p>
794            </div>
795            <div id="delta-seconds">
796               <h4 id="rfc.section.1.2.1"><a href="#rfc.section.1.2.1">1.2.1</a>&nbsp;<a href="#delta-seconds">Delta Seconds</a></h4>
797               <div id="rfc.section.1.2.1.p.1">
798                  <p>The delta-seconds rule specifies a non-negative integer, representing time in seconds.</p>
799               </div>
800               <div id="rfc.figure.u.1"><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>
801</pre></div>
802               <div id="rfc.section.1.2.1.p.2">
803                  <p>A recipient parsing a delta-seconds value and converting it to binary form ought to
804                     use an arithmetic type of at least 31 bits of non-negative integer range. If a cache
805                     receives a delta-seconds value greater than the greatest integer it can represent,
806                     or if any of its subsequent calculations overflows, the cache <em class="bcp14">MUST</em> consider the value to be either 2147483648 (2<sup>31</sup>) or the greatest positive integer it can conveniently represent.
807                  </p>
808               </div>
809               <div class="note">
810                  <div id="rfc.section.1.2.1.p.3">
811                     <p><b>Note:</b> The value 2147483648 is here for historical reasons, effectively represents infinity
812                        (over 68 years), and does not need to be stored in binary form; an implementation
813                        could produce it as a canned string if any overflow occurs, even if the calculations
814                        are performed with an arithmetic type incapable of directly representing that number.
815                        What matters here is that an overflow be detected and not treated as a negative value
816                        in later calculations.
817                     </p>
818                  </div>
819               </div>
820            </div>
821         </div>
822      </div>
823      <div id="caching.overview">
824         <h2 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#caching.overview">Overview of Cache Operation</a></h2>
825         <div id="rfc.section.2.p.1">
826            <p>Proper cache operation preserves the semantics of HTTP transfers (<a href="#RFC7231" id="rfc.xref.RFC7231.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>) while eliminating the transfer of information already held in the cache. Although
827               caching is an entirely <em class="bcp14">OPTIONAL</em> feature of HTTP, it can be assumed that reusing a cached response is desirable and
828               that such reuse is the default behavior when no requirement or local configuration
829               prevents it. Therefore, HTTP cache requirements are focused on preventing a cache
830               from either storing a non-reusable response or reusing a stored response inappropriately,
831               rather than mandating that caches always store and reuse particular responses.
832            </p>
833         </div>
834         <div id="rfc.section.2.p.2">
835            <p>Each <dfn>cache entry</dfn> consists of a cache key and one or more HTTP responses corresponding to prior requests
836               that used the same key. The most common form of cache entry is a successful result
837               of a retrieval request: i.e., a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to a GET request, which contains a representation of the resource identified
838               by the request target (<a href="p2-semantics.html#GET" title="GET">Section 4.3.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>). However, it is also possible to cache permanent redirects, negative results (e.g., <a href="p2-semantics.html#status.404" class="smpl">404 (Not Found)</a>), incomplete results (e.g., <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a>), and responses to methods other than GET if the method's definition allows such
839               caching and defines something suitable for use as a cache key.
840            </p>
841         </div>
842         <div id="rfc.iref.c.2"></div>
843         <div id="rfc.section.2.p.3">
844            <p>The primary <dfn>cache key</dfn> consists of the request method and target URI. However, since HTTP caches in common
845               use today are typically limited to caching responses to GET, many caches simply decline
846               other methods and use only the URI as the primary cache key.
847            </p>
848         </div>
849         <div id="rfc.section.2.p.4">
850            <p>If a request target is subject to content negotiation, its cache entry might consist
851               of multiple stored responses, each differentiated by a secondary key for the values
852               of the original request's selecting header fields (<a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.1</a>).
853            </p>
854         </div>
855      </div>
856      <div id="response.cacheability">
857         <h2 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#response.cacheability">Storing Responses in Caches</a></h2>
858         <div id="rfc.section.3.p.1">
859            <p>A cache <em class="bcp14">MUST NOT</em> store a response to any request, unless:
860            </p>
861            <ul>
862               <li>The request method is understood by the cache and defined as being cacheable, and</li>
863               <li>the response status code is understood by the cache, and</li>
864               <li>the "no-store" cache directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;5.2</a>) does not appear in request or response header fields, and
865               </li>
866               <li>the "private" response directive (see <a href="#cache-response-directive.private" title="private">Section&nbsp;5.2.2.6</a>) does not appear in the response, if the cache is shared, and
867               </li>
868               <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 4.2</a> of <a href="#RFC7235" id="rfc.xref.RFC7235.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[RFC7235]</cite></a>) does not appear in the request, if the cache is shared, unless the response explicitly
869                  allows it (see <a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a>), and
870               </li>
871               <li>the response either:
872                  <ul>
873                     <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;5.3</a>), or
874                     </li>
875                     <li>contains a max-age response directive (see <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;5.2.2.8</a>), or
876                     </li>
877                     <li>contains a s-maxage response directive (see <a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;5.2.2.9</a>) and the cache is shared, or
878                     </li>
879                     <li>contains a Cache Control Extension (see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;5.2.3</a>) that allows it to be cached, or
880                     </li>
881                     <li>has a status code that is defined as cacheable by default (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.2.2</a>), or
882                     </li>
883                     <li>contains a public response directive (see <a href="#cache-response-directive.public" title="public">Section&nbsp;5.2.2.5</a>).
884                     </li>
885                  </ul> 
886               </li>
887            </ul>
888         </div>
889         <div id="rfc.section.3.p.2">
890            <p>Note that any of the requirements listed above can be overridden by a cache-control
891               extension; see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;5.2.3</a>.
892            </p>
893         </div>
894         <div id="rfc.section.3.p.3">
895            <p>In this context, a cache has "understood" a request method or a response status code
896               if it recognizes it and implements all specified caching-related behavior.
897            </p>
898         </div>
899         <div id="rfc.section.3.p.4">
900            <p>Note that, in normal operation, some caches will not store a response that has neither
901               a cache validator nor an explicit expiration time, as such responses are not usually
902               useful to store. However, caches are not prohibited from storing such responses.
903            </p>
904         </div>
905         <div id="incomplete.responses">
906            <h3 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#incomplete.responses">Storing Incomplete Responses</a></h3>
907            <div id="rfc.section.3.1.p.1">
908               <p>A response message is considered complete when all of the octets indicated by the
909                  message framing (<a href="#RFC7230" id="rfc.xref.RFC7230.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>) are received prior to the connection being closed. If the request method is GET,
910                  the response status code is <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a>, and the entire response header section has been received, a cache <em class="bcp14">MAY</em> store an incomplete response message body if the cache entry is recorded as incomplete.
911                  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
912                     (OK)</a> cache entry. However, a cache <em class="bcp14">MUST NOT</em> store incomplete or partial-content responses if it does not support the <a href="p5-range.html#header.range" class="smpl">Range</a> and <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> header fields or if it does not understand the range units used in those fields.
913               </p>
914            </div>
915            <div id="rfc.section.3.1.p.2">
916               <p>A cache <em class="bcp14">MAY</em> complete a stored incomplete response by making a subsequent range request (<a href="#RFC7233" id="rfc.xref.RFC7233.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>) and combining the successful response with the stored entry, as defined in <a href="#combining.responses" title="Combining Partial Content">Section&nbsp;3.3</a>. A cache <em class="bcp14">MUST NOT</em> use an incomplete response to answer requests unless the response has been made complete
917                  or the request is partial and specifies a range that is wholly within the incomplete
918                  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.
919               </p>
920            </div>
921         </div>
922         <div id="caching.authenticated.responses">
923            <h3 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></h3>
924            <div id="rfc.section.3.2.p.1">
925               <p>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 4.2</a> of <a href="#RFC7235" id="rfc.xref.RFC7235.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Authentication">[RFC7235]</cite></a>) to satisfy any subsequent request unless a cache directive that allows such responses
926                  to be stored is present in the response.
927               </p>
928            </div>
929            <div id="rfc.section.3.2.p.2">
930               <p>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;5.2.2</a>) have such an effect: must-revalidate, public, and s-maxage.
931               </p>
932            </div>
933            <div id="rfc.section.3.2.p.3">
934               <p>Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response
935                  directives are not allowed to be served stale (<a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.2.4</a>) by shared caches. In particular, a response with either "max-age=0, must-revalidate"
936                  or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating
937                  it on the origin server.
938               </p>
939            </div>
940         </div>
941         <div id="combining.responses">
942            <h3 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#combining.responses">Combining Partial Content</a></h3>
943            <div id="rfc.section.3.3.p.1">
944               <p>A response might transfer only a partial representation if the connection closed prematurely
945                  or if the request used one or more Range specifiers (<a href="#RFC7233" id="rfc.xref.RFC7233.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>). After several such transfers, a cache might have received several ranges of the
946                  same representation. A cache <em class="bcp14">MAY</em> combine these ranges into a single stored response, and reuse that response to satisfy
947                  later requests, if they all share the same strong validator and the cache complies
948                  with the client requirements in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4.3</a> of <a href="#RFC7233" id="rfc.xref.RFC7233.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>.
949               </p>
950            </div>
951            <div id="rfc.section.3.3.p.2">
952               <p>When combining the new response with one or more stored responses, a cache <em class="bcp14">MUST</em>:
953               </p>
954               <ul>
955                  <li>delete any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;5.5</a>);
956                  </li>
957                  <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
958                  </li>
959                  <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.
960                  </li>
961               </ul>
962            </div>
963         </div>
964      </div>
965      <div id="constructing.responses.from.caches">
966         <h2 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h2>
967         <div id="rfc.section.4.p.1">
968            <p>When presented with a request, a cache <em class="bcp14">MUST NOT</em> reuse a stored response, unless:
969            </p>
970            <ul>
971               <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="#RFC7230" id="rfc.xref.RFC7230.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>) and that of the stored response match, and
972               </li>
973               <li>the request method associated with the stored response allows it to be used for the
974                  presented request, and
975               </li>
976               <li>selecting header fields nominated by the stored response (if any) match those presented
977                  (see <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.1</a>), and
978               </li>
979               <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;5.4</a>), nor the no-cache cache directive (<a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;5.2.1</a>), unless the stored response is successfully validated (<a href="#validation.model" title="Validation">Section&nbsp;4.3</a>), and
980               </li>
981               <li>the stored response does not contain the no-cache cache directive (<a href="#cache-response-directive.no-cache" title="no-cache">Section&nbsp;5.2.2.2</a>), unless it is successfully validated (<a href="#validation.model" title="Validation">Section&nbsp;4.3</a>), and
982               </li>
983               <li>the stored response is either:
984                  <ul>
985                     <li>fresh (see <a href="#expiration.model" title="Freshness">Section&nbsp;4.2</a>), or
986                     </li>
987                     <li>allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.2.4</a>), or
988                     </li>
989                     <li>successfully validated (see <a href="#validation.model" title="Validation">Section&nbsp;4.3</a>).
990                     </li>
991                  </ul> 
992               </li>
993            </ul>
994         </div>
995         <div id="rfc.section.4.p.2">
996            <p>Note that any of the requirements listed above can be overridden by a cache-control
997               extension; see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;5.2.3</a>.
998            </p>
999         </div>
1000         <div id="rfc.section.4.p.3">
1001            <p>When a stored response is used to satisfy a request without validation, a cache <em class="bcp14">MUST</em> generate an <a href="#header.age" class="smpl">Age</a> header field (<a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;5.1</a>), replacing any present in the response with a value equal to the stored response's
1002               current_age; see <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.2.3</a>.
1003            </p>
1004         </div>
1005         <div id="rfc.section.4.p.4">
1006            <p>A cache <em class="bcp14">MUST</em> write through requests with methods that are unsafe (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>) to the origin server; i.e., a cache is not allowed to generate a reply to such a
1007               request before having forwarded the request and having received a corresponding response.
1008            </p>
1009         </div>
1010         <div id="rfc.section.4.p.5">
1011            <p>Also, note that unsafe requests might invalidate already-stored responses; see <a href="#invalidation" title="Invalidation">Section&nbsp;4.4</a>.
1012            </p>
1013         </div>
1014         <div id="rfc.section.4.p.6">
1015            <p>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="p2-semantics.html#header.date" class="smpl">Date</a> header field). It can also forward the request with "Cache-Control: max-age=0" or
1016               "Cache-Control: no-cache" to disambiguate which response to use.
1017            </p>
1018         </div>
1019         <div id="rfc.section.4.p.7">
1020            <p>A cache that does not have a clock available <em class="bcp14">MUST NOT</em> use stored responses without revalidating them upon every use.
1021            </p>
1022         </div>
1023         <div id="caching.negotiated.responses">
1024            <h3 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#caching.negotiated.responses">Calculating Secondary Keys with Vary</a></h3>
1025            <div id="rfc.section.4.1.p.1">
1026               <p>When a cache receives a request that can be satisfied by a stored response that has
1027                  a <a href="p2-semantics.html#header.vary" class="smpl">Vary</a> header field (<a href="p2-semantics.html#header.vary" title="Vary">Section 7.1.4</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>), it <em class="bcp14">MUST NOT</em> use that response unless all of the selecting header fields nominated by the Vary
1028                  header field match in both the original request (i.e., that associated with the stored
1029                  response), and the presented request.
1030               </p>
1031            </div>
1032            <div id="rfc.section.4.1.p.2">
1033               <p>The selecting header fields from two requests are defined to match if and only if
1034                  those in the first request can be transformed to those in the second request by applying
1035                  any of the following:
1036               </p>
1037               <ul>
1038                  <li>adding or removing whitespace, where allowed in the header field's syntax</li>
1039                  <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="#RFC7230" id="rfc.xref.RFC7230.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>)
1040                  </li>
1041                  <li>normalizing both header field values in a way that is known to have identical semantics,
1042                     according to the header field's specification (e.g., reordering field values when
1043                     order is not significant; case-normalization, where values are defined to be case-insensitive)
1044                  </li>
1045               </ul>
1046            </div>
1047            <div id="rfc.section.4.1.p.3">
1048               <p>If (after any normalization that might take place) a header field is absent from a
1049                  request, it can only match another request if it is also absent there.
1050               </p>
1051            </div>
1052            <div id="rfc.section.4.1.p.4">
1053               <p>A <a href="p2-semantics.html#header.vary" class="smpl">Vary</a> header field-value of "*" always fails to match.
1054               </p>
1055            </div>
1056            <div id="rfc.section.4.1.p.5">
1057               <p>The stored response with matching selecting header fields is known as the selected
1058                  response.
1059               </p>
1060            </div>
1061            <div id="rfc.section.4.1.p.6">
1062               <p>If multiple selected responses are available (potentially including responses without
1063                  a Vary header field), the cache will need to choose one to use. When a selecting header
1064                  field has a known mechanism for doing so (e.g., qvalues on <a href="p2-semantics.html#header.accept" class="smpl">Accept</a> and similar request header fields), that mechanism <em class="bcp14">MAY</em> be used to select preferred responses; of the remainder, the most recent response
1065                  (as determined by the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field) is used, as per <a href="#constructing.responses.from.caches" title="Constructing Responses from Caches">Section&nbsp;4</a>.
1066               </p>
1067            </div>
1068            <div id="rfc.section.4.1.p.7">
1069               <p>If no selected response is available, the cache cannot satisfy the presented request.
1070                  Typically, it is forwarded to the origin server in a (possibly conditional; see <a href="#validation.model" title="Validation">Section&nbsp;4.3</a>) request.
1071               </p>
1072            </div>
1073         </div>
1074         <div id="expiration.model">
1075            <h3 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#expiration.model">Freshness</a></h3>
1076            <div id="rfc.section.4.2.p.1">
1077               <p>A <dfn>fresh</dfn> response is one whose age has not yet exceeded its freshness lifetime. Conversely,
1078                  a <dfn>stale</dfn> response is one where it has.
1079               </p>
1080            </div>
1081            <div id="rfc.iref.f.1"></div>
1082            <div id="rfc.iref.e.1"></div>
1083            <div id="rfc.iref.h.1"></div>
1084            <div id="rfc.section.4.2.p.2">
1085               <p>A response's <dfn>freshness lifetime</dfn> is the length of time between its generation by the origin server and its expiration
1086                  time. An <dfn>explicit expiration time</dfn> is the time at which the origin server intends that a stored response can no longer
1087                  be used by a cache without further validation, whereas a <dfn>heuristic expiration time</dfn> is assigned by a cache when no explicit expiration time is available.
1088               </p>
1089            </div>
1090            <div id="rfc.iref.a.1"></div>
1091            <div id="rfc.section.4.2.p.3">
1092               <p>A response's <dfn>age</dfn> is the time that has passed since it was generated by, or successfully validated with,
1093                  the origin server.
1094               </p>
1095            </div>
1096            <div id="rfc.section.4.2.p.4">
1097               <p>When a response is "fresh" in the cache, it can be used to satisfy subsequent requests
1098                  without contacting the origin server, thereby improving efficiency.
1099               </p>
1100            </div>
1101            <div id="rfc.section.4.2.p.5">
1102               <p>The primary mechanism for determining freshness is for an origin server to provide
1103                  an explicit expiration time in the future, 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;5.3</a>) or the max-age response directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;5.2.2.8</a>). Generally, origin servers will assign future explicit expiration times to responses
1104                  in the belief that the representation is not likely to change in a semantically significant
1105                  way before the expiration time is reached.
1106               </p>
1107            </div>
1108            <div id="rfc.section.4.2.p.6">
1109               <p>If an origin server wishes to force a cache to validate every request, it can assign
1110                  an explicit expiration time in the past to indicate that the response is already stale.
1111                  Compliant caches will normally validate a stale cached response before reusing it
1112                  for subsequent requests (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.2.4</a>).
1113               </p>
1114            </div>
1115            <div id="rfc.section.4.2.p.7">
1116               <p>Since origin servers do not always provide explicit expiration times, caches are also
1117                  allowed to use a heuristic to determine an expiration time under certain circumstances
1118                  (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.2.2</a>).
1119               </p>
1120            </div>
1121            <div id="rfc.figure.u.2">
1122               <p>The calculation to determine if a response is fresh is:</p><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
1123</pre></div>
1124            <div id="rfc.section.4.2.p.8">
1125               <p>freshness_lifetime is defined in <a href="#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section&nbsp;4.2.1</a>; current_age is defined in <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.2.3</a>.
1126               </p>
1127            </div>
1128            <div id="rfc.section.4.2.p.9">
1129               <p>Clients can send the max-age or min-fresh cache directives in a request to constrain
1130                  or relax freshness calculations for the corresponding response (<a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;5.2.1</a>).
1131               </p>
1132            </div>
1133            <div id="rfc.section.4.2.p.10" class="avoidbreakafter">
1134               <p>When calculating freshness, to avoid common problems in date parsing:</p>
1135            </div>
1136            <div id="rfc.section.4.2.p.11">
1137               <ul>
1138                  <li>Although all date formats are specified to be case-sensitive, a cache recipient <em class="bcp14">SHOULD</em> match day, week, and time-zone names case-insensitively.
1139                  </li>
1140                  <li>If a cache recipient's internal implementation of time has less resolution than the
1141                     value of an HTTP-date, the recipient <em class="bcp14">MUST</em> internally represent a parsed <a href="#header.expires" class="smpl">Expires</a> date as the nearest time equal to or earlier than the received value.
1142                  </li>
1143                  <li>A cache recipient <em class="bcp14">MUST NOT</em> allow local time zones to influence the calculation or comparison of an age or expiration
1144                     time.
1145                  </li>
1146                  <li>A cache recipient <em class="bcp14">SHOULD</em> consider a date with a zone abbreviation other than GMT or UTC to be invalid for calculating
1147                     expiration.
1148                  </li>
1149               </ul>
1150            </div>
1151            <div id="rfc.section.4.2.p.12">
1152               <p>Note that freshness applies only to cache operation; it cannot be used to force a
1153                  user agent to refresh its display or reload a resource. See <a href="#history.lists" title="History Lists">Section&nbsp;6</a> for an explanation of the difference between caches and history mechanisms.
1154               </p>
1155            </div>
1156            <div id="calculating.freshness.lifetime">
1157               <h4 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h4>
1158               <div id="rfc.section.4.2.1.p.1">
1159                  <p>A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
1160                     response by using the first match of the following:
1161                  </p>
1162                  <ul>
1163                     <li>If the cache is shared and the s-maxage response directive (<a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;5.2.2.9</a>) is present, use its value, or
1164                     </li>
1165                     <li>If the max-age response directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;5.2.2.8</a>) is present, use its value, or
1166                     </li>
1167                     <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;5.3</a>) is present, use its value minus the value of the <a href="p2-semantics.html#header.date" class="smpl">Date</a> response header field, or
1168                     </li>
1169                     <li>Otherwise, no explicit expiration time is present in the response. A heuristic freshness
1170                        lifetime might be applicable; see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.2.2</a>.
1171                     </li>
1172                  </ul>
1173               </div>
1174               <div id="rfc.section.4.2.1.p.2">
1175                  <p>Note that this calculation is not vulnerable to clock skew, since all of the information
1176                     comes from the origin server.
1177                  </p>
1178               </div>
1179               <div id="rfc.section.4.2.1.p.3">
1180                  <p>When there is more than one value present for a given directive (e.g., two <a href="#header.expires" class="smpl">Expires</a> header fields, multiple Cache-Control: max-age directives), the directive's value
1181                     is considered invalid. Caches are encouraged to consider responses that have invalid
1182                     freshness information to be stale.
1183                  </p>
1184               </div>
1185            </div>
1186            <div id="heuristic.freshness">
1187               <h4 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></h4>
1188               <div id="rfc.section.4.2.2.p.1">
1189                  <p>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
1190                     algorithms that use other header field 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
1191                     specific algorithms, but does impose worst-case constraints on their results.
1192                  </p>
1193               </div>
1194               <div id="rfc.section.4.2.2.p.2">
1195                  <p>A cache <em class="bcp14">MUST NOT</em> use heuristics to determine freshness when an explicit expiration time is present
1196                     in the stored response. Because of the requirements in <a href="#response.cacheability" title="Storing Responses in Caches">Section&nbsp;3</a>, this means that, effectively, heuristics can only be used on responses without explicit
1197                     freshness whose status codes are defined as cacheable by default (see <a href="p2-semantics.html#overview.of.status.codes" title="Overview of Status Codes">Section 6.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>), and those responses without explicit freshness that have been marked as explicitly
1198                     cacheable (e.g., with a "public" response directive).
1199                  </p>
1200               </div>
1201               <div id="rfc.section.4.2.2.p.3">
1202                  <p>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="#RFC7232" id="rfc.xref.RFC7232.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a>), caches are encouraged to use a heuristic expiration value that is no more than
1203                     some fraction of the interval since that time. A typical setting of this fraction
1204                     might be 10%.
1205                  </p>
1206               </div>
1207               <div id="rfc.section.4.2.2.p.4">
1208                  <p>When a heuristic is used to calculate freshness lifetime, a cache <em class="bcp14">SHOULD</em> generate a <a href="#header.warning" class="smpl">Warning</a> header field with a 113 warn-code (see <a href="#warn.113" id="rfc.xref.warn.113.1" title="Warning: 113 - &#34;Heuristic Expiration&#34;">Section&nbsp;5.5.4</a>) in the response if its current_age is more than 24 hours and such a warning is not
1209                     already present.
1210                  </p>
1211               </div>
1212               <div class="note">
1213                  <div id="rfc.section.4.2.2.p.5">
1214                     <p><b>Note:</b> <a href="https://tools.ietf.org/html/rfc2616#section-13.9">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
1215                        (i.e., those containing '?'). In practice, this has not been widely implemented. Therefore,
1216                        origin servers are encouraged to send explicit directives (e.g., Cache-Control: no-cache)
1217                        if they wish to preclude caching.
1218                     </p>
1219                  </div>
1220               </div>
1221            </div>
1222            <div id="age.calculations">
1223               <h4 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a href="#age.calculations">Calculating Age</a></h4>
1224               <div id="rfc.section.4.2.3.p.1">
1225                  <p>The <a href="#header.age" class="smpl">Age</a> header field is used to convey an estimated age of the response message when obtained
1226                     from a cache. The Age field value is the cache's estimate of the number of seconds
1227                     since the response was generated or validated by the origin server. In essence, the
1228                     Age value is the sum of the time that the response has been resident in each of the
1229                     caches along the path from the origin server, plus the amount of time it has been
1230                     in transit along network paths.
1231                  </p>
1232               </div>
1233               <div id="rfc.section.4.2.3.p.2" class="avoidbreakafter">
1234                  <p>The following data is used for the age calculation:</p>
1235               </div>
1236               <div id="rfc.section.4.2.3.p.3">
1237                  <p><dfn>age_value</dfn> 
1238                  </p>
1239                  <ul class="empty">
1240                     <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;5.1</a>), in a form appropriate for arithmetic operation; or 0, if not available.
1241                     </li>
1242                  </ul>
1243               </div>
1244               <div id="rfc.section.4.2.3.p.4">
1245                  <p><dfn>date_value</dfn> 
1246                  </p>
1247                  <ul class="empty">
1248                     <li>The term "date_value" denotes the value of the Date header field, in a form appropriate
1249                        for arithmetic operations. See <a href="p2-semantics.html#header.date" title="Date">Section 7.1.1.2</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a> for the definition of the Date header field, and for requirements regarding responses
1250                        without it.
1251                     </li>
1252                  </ul>
1253               </div>
1254               <div id="rfc.section.4.2.3.p.5">
1255                  <p><dfn>now</dfn> 
1256                  </p>
1257                  <ul class="empty">
1258                     <li>The term "now" means "the current value of the clock at the host performing the calculation".
1259                        A host ought to use NTP (<a href="#RFC5905" id="rfc.xref.RFC5905.1"><cite title="Network Time Protocol Version 4: Protocol and Algorithms Specification">[RFC5905]</cite></a>) or some similar protocol to synchronize its clocks to Coordinated Universal Time.
1260                     </li>
1261                  </ul>
1262               </div>
1263               <div id="rfc.section.4.2.3.p.6">
1264                  <p><dfn>request_time</dfn> 
1265                  </p>
1266                  <ul class="empty">
1267                     <li>The current value of the clock at the host at the time the request resulting in the
1268                        stored response was made.
1269                     </li>
1270                  </ul>
1271               </div>
1272               <div id="rfc.section.4.2.3.p.7">
1273                  <p><dfn>response_time</dfn> 
1274                  </p>
1275                  <ul class="empty">
1276                     <li>The current value of the clock at the host at the time the response was received.</li>
1277                  </ul>
1278               </div>
1279               <div id="rfc.section.4.2.3.p.8">
1280                  <p>A response's age can be calculated in two entirely independent ways: </p>
1281                  <ol>
1282                     <li>the "apparent_age": response_time minus date_value, if the local clock is reasonably
1283                        well synchronized to the origin server's clock. If the result is negative, the result
1284                        is replaced by zero.
1285                     </li>
1286                     <li>the "corrected_age_value", if all of the caches along the response path implement
1287                        HTTP/1.1. A cache <em class="bcp14">MUST</em> interpret this value relative to the time the request was initiated, not the time
1288                        that the response was received.
1289                     </li>
1290                  </ol>
1291               </div>
1292               <div id="rfc.figure.u.3"><pre class="text">  apparent_age = max(0, response_time - date_value);
1293
1294  response_delay = response_time - request_time;
1295  corrected_age_value = age_value + response_delay; 
1296</pre></div>
1297               <div id="rfc.figure.u.4">
1298                  <p>These are combined as</p><pre class="text">  corrected_initial_age = max(apparent_age, corrected_age_value);
1299</pre></div>
1300               <div id="rfc.section.4.2.3.p.9">
1301                  <p>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.
1302                  </p>
1303               </div>
1304               <div id="rfc.section.4.2.3.p.10">
1305                  <p>The current_age of a stored response can then be calculated by adding the amount of
1306                     time (in seconds) since the stored response was last validated by the origin server
1307                     to the corrected_initial_age.
1308                  </p>
1309               </div>
1310               <div id="rfc.figure.u.5"><pre class="text">  resident_time = now - response_time;
1311  current_age = corrected_initial_age + resident_time;
1312</pre></div>
1313            </div>
1314            <div id="serving.stale.responses">
1315               <h4 id="rfc.section.4.2.4"><a href="#rfc.section.4.2.4">4.2.4</a>&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></h4>
1316               <div id="rfc.section.4.2.4.p.1">
1317                  <p>A "stale" response is one that either has explicit expiry information or is allowed
1318                     to have heuristic expiry calculated, but is not fresh according to the calculations
1319                     in <a href="#expiration.model" title="Freshness">Section&nbsp;4.2</a>.
1320                  </p>
1321               </div>
1322               <div id="rfc.section.4.2.4.p.2">
1323                  <p>A cache <em class="bcp14">MUST NOT</em> generate a stale response if it is prohibited by an explicit in-protocol directive
1324                     (e.g., by a "no-store" or "no-cache" cache directive, a "must-revalidate" cache-response-directive,
1325                     or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive; see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;5.2.2</a>).
1326                  </p>
1327               </div>
1328               <div id="rfc.section.4.2.4.p.3">
1329                  <p>A cache <em class="bcp14">MUST NOT</em> send stale responses unless it is disconnected (i.e., it cannot contact the origin
1330                     server or otherwise find a forward path) or doing so is explicitly allowed (e.g.,
1331                     by the max-stale request directive; see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;5.2.1</a>).
1332                  </p>
1333               </div>
1334               <div id="rfc.section.4.2.4.p.4">
1335                  <p>A cache <em class="bcp14">SHOULD</em> generate a <a href="#header.warning" class="smpl">Warning</a> header field with the 110 warn-code (see <a href="#warn.110" id="rfc.xref.warn.110.1" title="Warning: 110 - &#34;Response is Stale&#34;">Section&nbsp;5.5.1</a>) in stale responses. Likewise, a cache <em class="bcp14">SHOULD</em> generate a 112 warn-code (see <a href="#warn.112" id="rfc.xref.warn.112.1" title="Warning: 112 - &#34;Disconnected Operation&#34;">Section&nbsp;5.5.3</a>) in stale responses if the cache is disconnected.
1336                  </p>
1337               </div>
1338               <div id="rfc.section.4.2.4.p.5">
1339                  <p>A cache <em class="bcp14">SHOULD NOT</em> generate a new <a href="#header.warning" class="smpl">Warning</a> header field when forwarding a response that does not have an <a href="#header.age" class="smpl">Age</a> header field, even if the response is already stale. A cache need not validate a response
1340                     that merely became stale in transit.
1341                  </p>
1342               </div>
1343            </div>
1344         </div>
1345         <div id="validation.model">
1346            <h3 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a href="#validation.model">Validation</a></h3>
1347            <div id="rfc.section.4.3.p.1">
1348               <p>When a cache has one or more stored responses for a requested URI, but cannot serve
1349                  any of them (e.g., because they are not fresh, or one cannot be selected; see <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.1</a>), it can use the conditional request mechanism <a href="#RFC7232" id="rfc.xref.RFC7232.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a> in the forwarded request to give the next inbound server an opportunity to select
1350                  a valid stored response to use, updating the stored metadata in the process, or to
1351                  replace the stored response(s) with a new response. This process is known as "validating"
1352                  or "revalidating" the stored response.
1353               </p>
1354            </div>
1355            <div id="validation.sent">
1356               <h4 id="rfc.section.4.3.1"><a href="#rfc.section.4.3.1">4.3.1</a>&nbsp;<a href="#validation.sent">Sending a Validation Request</a></h4>
1357               <div id="rfc.section.4.3.1.p.1">
1358                  <p>When sending a conditional request for cache validation, a cache sends one or more
1359                     precondition header fields containing <dfn>validator</dfn> metadata from its stored response(s), which is then compared by recipients to determine
1360                     whether a stored response is equivalent to a current representation of the resource.
1361                  </p>
1362               </div>
1363               <div id="rfc.section.4.3.1.p.2">
1364                  <p>One such validator is the timestamp given in 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="#RFC7232" id="rfc.xref.RFC7232.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a>), which can be used in an <a href="p4-conditional.html#header.if-modified-since" class="smpl">If-Modified-Since</a> header field for response validation, or in an <a href="p4-conditional.html#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field for representation selection (i.e., the client is referring specifically
1365                     to a previously obtained representation with that timestamp).
1366                  </p>
1367               </div>
1368               <div id="rfc.section.4.3.1.p.3">
1369                  <p>Another validator is the entity-tag given in an <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> header field (<a href="p4-conditional.html#header.etag" title="ETag">Section 2.3</a> of <a href="#RFC7232" id="rfc.xref.RFC7232.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a>). One or more entity-tags, indicating one or more stored responses, can be used in
1370                     an <a href="p4-conditional.html#header.if-none-match" class="smpl">If-None-Match</a> header field for response validation, or in an <a href="p4-conditional.html#header.if-match" class="smpl">If-Match</a> or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field for representation selection (i.e., the client is referring specifically
1371                     to one or more previously obtained representations with the listed entity-tags).
1372                  </p>
1373               </div>
1374            </div>
1375            <div id="validation.received">
1376               <h4 id="rfc.section.4.3.2"><a href="#rfc.section.4.3.2">4.3.2</a>&nbsp;<a href="#validation.received">Handling a Received Validation Request</a></h4>
1377               <div id="rfc.section.4.3.2.p.1">
1378                  <p>Each client in the request chain may have its own cache, so it is common for a cache
1379                     at an intermediary to receive conditional requests from other (outbound) caches. Likewise,
1380                     some user agents make use of conditional requests to limit data transfers to recently
1381                     modified representations or to complete the transfer of a partially retrieved representation.
1382                  </p>
1383               </div>
1384               <div id="rfc.section.4.3.2.p.2">
1385                  <p>If a cache receives a request that can be satisfied by reusing one of its stored <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> or <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> responses, the cache <em class="bcp14">SHOULD</em> evaluate any applicable conditional header field preconditions received in that request
1386                     with respect to the corresponding validators contained within the selected response.
1387                     A cache <em class="bcp14">MUST NOT</em> evaluate conditional header fields that are only applicable to an origin server, found
1388                     in a request with semantics that cannot be satisfied with a cached response, or applied
1389                     to a target resource for which it has no stored responses; such preconditions are
1390                     likely intended for some other (inbound) server.
1391                  </p>
1392               </div>
1393               <div id="rfc.section.4.3.2.p.3">
1394                  <p>The proper evaluation of conditional requests by a cache depends on the received precondition
1395                     header fields and their precedence, as defined in <a href="p4-conditional.html#precedence" title="Precedence">Section 6</a> of <a href="#RFC7232" id="rfc.xref.RFC7232.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a>. The <a href="p4-conditional.html#header.if-match" class="smpl">If-Match</a> and <a href="p4-conditional.html#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> conditional header fields are not applicable to a cache.
1396                  </p>
1397               </div>
1398               <div id="rfc.section.4.3.2.p.4">
1399                  <p>A request containing an <a href="p4-conditional.html#header.if-none-match" class="smpl">If-None-Match</a> header field (<a href="p4-conditional.html#header.if-none-match" title="If-None-Match">Section 3.2</a> of <a href="#RFC7232" id="rfc.xref.RFC7232.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a>) indicates that the client wants to validate one or more of its own stored responses
1400                     in comparison to whichever stored response is selected by the cache. If the field-value
1401                     is "*", or if the field-value is a list of entity-tags and at least one of them matches
1402                     the entity-tag of the selected stored response, a cache recipient <em class="bcp14">SHOULD</em> generate a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response (using the metadata of the selected stored response) instead of sending that
1403                     stored response.
1404                  </p>
1405               </div>
1406               <div id="rfc.section.4.3.2.p.5">
1407                  <p>When a cache decides to revalidate its own stored responses for a request that contains
1408                     an <a href="p4-conditional.html#header.if-none-match" class="smpl">If-None-Match</a> list of entity-tags, the cache <em class="bcp14">MAY</em> combine the received list with a list of entity-tags from its own stored set of responses
1409                     (fresh or stale) and send the union of the two lists as a replacement <a href="p4-conditional.html#header.if-none-match" class="smpl">If-None-Match</a> header field value in the forwarded request. If a stored response contains only partial
1410                     content, the cache <em class="bcp14">MUST NOT</em> include its entity-tag in the union unless the request is for a range that would be
1411                     fully satisfied by that partial stored response. If the response to the forwarded
1412                     request is <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> and has an ETag header field value with an entity-tag that is not in the client's
1413                     list, the cache <em class="bcp14">MUST</em> generate a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response for the client by reusing its corresponding stored response, as updated by
1414                     the 304 response metadata (<a href="#freshening.responses" title="Freshening Stored Responses upon Validation">Section&nbsp;4.3.4</a>).
1415                  </p>
1416               </div>
1417               <div id="rfc.section.4.3.2.p.6">
1418                  <p>If an <a href="p4-conditional.html#header.if-none-match" class="smpl">If-None-Match</a> header field is not present, a request containing an <a href="p4-conditional.html#header.if-modified-since" class="smpl">If-Modified-Since</a> header field (<a href="p4-conditional.html#header.if-modified-since" title="If-Modified-Since">Section 3.3</a> of <a href="#RFC7232" id="rfc.xref.RFC7232.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a>) indicates that the client wants to validate one or more of its own stored responses
1419                     by modification date. A cache recipient <em class="bcp14">SHOULD</em> generate a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response (using the metadata of the selected stored response) if one of the following
1420                     cases is true: 1) the selected stored response has a <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> field-value that is earlier than or equal to the conditional timestamp; 2) no <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> field is present in the selected stored response, but it has a <a href="p2-semantics.html#header.date" class="smpl">Date</a> field-value that is earlier than or equal to the conditional timestamp; or, 3) neither <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> nor <a href="p2-semantics.html#header.date" class="smpl">Date</a> is present in the selected stored response, but the cache recorded it as having been
1421                     received at a time earlier than or equal to the conditional timestamp.
1422                  </p>
1423               </div>
1424               <div id="rfc.section.4.3.2.p.7">
1425                  <p>A cache that implements partial responses to range requests, as defined in <a href="#RFC7233" id="rfc.xref.RFC7233.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>, also needs to evaluate a received <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field (<a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#RFC7233" id="rfc.xref.RFC7233.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>) with respect to its selected stored response.
1426                  </p>
1427               </div>
1428            </div>
1429            <div id="validation.response">
1430               <h4 id="rfc.section.4.3.3"><a href="#rfc.section.4.3.3">4.3.3</a>&nbsp;<a href="#validation.response">Handling a Validation Response</a></h4>
1431               <div id="rfc.section.4.3.3.p.1" class="avoidbreakafter">
1432                  <p>Cache handling of a response to a conditional request is dependent upon its status
1433                     code:
1434                  </p>
1435               </div>
1436               <div id="rfc.section.4.3.3.p.2">
1437                  <ul>
1438                     <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;
1439                        see <a href="#freshening.responses" title="Freshening Stored Responses upon Validation">Section&nbsp;4.3.4</a>.
1440                     </li>
1441                     <li>A full response (i.e., one with a payload body) indicates that none of the stored
1442                        responses nominated in the conditional request is suitable. Instead, the cache <em class="bcp14">MUST</em> use the full response to satisfy the request and <em class="bcp14">MAY</em> replace the stored response(s).
1443                     </li>
1444                     <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
1445                        to the requesting client, or act as if the server failed to respond. In the latter
1446                        case, the cache <em class="bcp14">MAY</em> send a previously stored response (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.2.4</a>).
1447                     </li>
1448                  </ul>
1449               </div>
1450            </div>
1451            <div id="freshening.responses">
1452               <h4 id="rfc.section.4.3.4"><a href="#rfc.section.4.3.4">4.3.4</a>&nbsp;<a href="#freshening.responses">Freshening Stored Responses upon Validation</a></h4>
1453               <div id="rfc.section.4.3.4.p.1">
1454                  <p>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
1455                     responses are updated by this new response and then update the stored response(s)
1456                     with the new information provided in the <a href="p4-conditional.html#status.304" class="smpl">304</a> response.
1457                  </p>
1458               </div>
1459               <div id="rfc.iref.s.2"></div>
1460               <div id="rfc.section.4.3.4.p.2">
1461                  <p>The stored response to update is identified by using the first match (if any) of the
1462                     following:
1463                  </p>
1464                  <ul>
1465                     <li>If the new response contains a <dfn>strong validator</dfn> (see <a href="p4-conditional.html#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a> of <a href="#RFC7232" id="rfc.xref.RFC7232.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests">[RFC7232]</cite></a>), then that strong validator identifies the selected representation for update. All
1466                        of the stored responses with the same strong validator are selected. If none of the
1467                        stored responses contain the same strong validator, then the cache <em class="bcp14">MUST NOT</em> use the new response to update any stored responses.
1468                     </li>
1469                     <li>If the new response contains a weak validator and that validator corresponds to one
1470                        of the cache's stored responses, then the most recent of those matching stored responses
1471                        is selected for update.
1472                     </li>
1473                     <li>If the new response does not include any form of validator (such as in the case where
1474                        a client generates an If-Modified-Since request from a source other than the Last-Modified
1475                        response header field), and there is only one stored response, and that stored response
1476                        also lacks a validator, then that stored response is selected for update.
1477                     </li>
1478                  </ul>
1479               </div>
1480               <div id="rfc.section.4.3.4.p.3">
1481                  <p>If a stored response is selected for update, the cache <em class="bcp14">MUST</em>:
1482                  </p>
1483                  <ul>
1484                     <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;5.5</a>);
1485                     </li>
1486                     <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
1487                     </li>
1488                     <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
1489                        response.
1490                     </li>
1491                  </ul>
1492               </div>
1493            </div>
1494            <div id="head.effects">
1495               <h4 id="rfc.section.4.3.5"><a href="#rfc.section.4.3.5">4.3.5</a>&nbsp;<a href="#head.effects">Freshening Responses via HEAD</a></h4>
1496               <div id="rfc.section.4.3.5.p.1">
1497                  <p>A response to the HEAD method is identical to what an equivalent request made with
1498                     a GET would have been, except it lacks a body. This property of HEAD responses can
1499                     be used to invalidate or update a cached GET response if the more efficient conditional
1500                     GET request mechanism is not available (due to no validators being present in the
1501                     stored response) or if transmission of the representation body is not desired even
1502                     if it has changed.
1503                  </p>
1504               </div>
1505               <div id="rfc.section.4.3.5.p.2">
1506                  <p>When a cache makes an inbound HEAD request for a given request target and receives
1507                     a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response, the cache <em class="bcp14">SHOULD</em> update or invalidate each of its stored GET responses that could have been selected
1508                     for that request (see <a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.1</a>).
1509                  </p>
1510               </div>
1511               <div id="rfc.section.4.3.5.p.3">
1512                  <p>For each of the stored responses that could have been selected, if the stored response
1513                     and HEAD response have matching values for any received validator fields (<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>) and, if the HEAD response has a <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> header field, the value of <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a> matches that of the stored response, the cache <em class="bcp14">SHOULD</em> update the stored response as described below; otherwise, the cache <em class="bcp14">SHOULD</em> consider the stored response to be stale.
1514                  </p>
1515               </div>
1516               <div id="rfc.section.4.3.5.p.4">
1517                  <p>If a cache updates a stored response with the metadata provided in a HEAD response,
1518                     the cache <em class="bcp14">MUST</em>:
1519                  </p>
1520                  <ul>
1521                     <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;5.5</a>);
1522                     </li>
1523                     <li>retain any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 2xx; and,
1524                     </li>
1525                     <li>use other header fields provided in the HEAD response to replace all instances of
1526                        the corresponding header fields in the stored response and append new header fields
1527                        to the stored response's header section unless otherwise restricted by the <a href="#header.cache-control" class="smpl">Cache-Control</a> header field.
1528                     </li>
1529                  </ul>
1530               </div>
1531            </div>
1532         </div>
1533         <div id="invalidation">
1534            <h3 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a href="#invalidation">Invalidation</a></h3>
1535            <div id="rfc.section.4.4.p.1">
1536               <p>Because unsafe request methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>) such as PUT, POST or DELETE have the potential for changing state on the origin
1537                  server, intervening caches can use them to keep their contents up to date.
1538               </p>
1539            </div>
1540            <div id="rfc.section.4.4.p.2">
1541               <p>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="#RFC7230" id="rfc.xref.RFC7230.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</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 status code is received in response
1542                  to an unsafe request method.
1543               </p>
1544            </div>
1545            <div id="rfc.section.4.4.p.3">
1546               <p>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
1547                  effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>). This helps prevent denial-of-service attacks.
1548               </p>
1549            </div>
1550            <div id="rfc.section.4.4.p.4">
1551               <p>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="#RFC7230" id="rfc.xref.RFC7230.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>) when it receives a non-error response to a request with a method whose safety is
1552                  unknown.
1553               </p>
1554            </div>
1555            <div id="rfc.section.4.4.p.5">
1556               <p>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
1557                  related to the effective request URI or will mark these as "invalid" and in need of
1558                  a mandatory validation before they can be sent in response to a subsequent request.
1559               </p>
1560            </div>
1561            <div id="rfc.section.4.4.p.6">
1562               <p>Note that this does not guarantee that all appropriate responses are invalidated.
1563                  For example, a state-changing request might invalidate responses in the caches it
1564                  travels through, but relevant responses still might be stored in other caches that
1565                  it has not.
1566               </p>
1567            </div>
1568         </div>
1569      </div>
1570      <div id="header.field.definitions">
1571         <h2 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#header.field.definitions">Header Field Definitions</a></h2>
1572         <div id="rfc.section.5.p.1">
1573            <p>This section defines the syntax and semantics of HTTP/1.1 header fields related to
1574               caching.
1575            </p>
1576         </div>
1577         <div id="header.age">
1578            <h3 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a href="#header.age">Age</a></h3>
1579            <div id="rfc.section.5.1.p.1">
1580               <p>The "Age" header field conveys the sender's estimate of the amount of time since the
1581                  response was generated or successfully validated at the origin server. Age values
1582                  are calculated as specified in <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.2.3</a>.
1583               </p>
1584            </div>
1585            <div id="rfc.figure.u.6"><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>
1586</pre></div>
1587            <div id="rfc.section.5.1.p.2">
1588               <p>The Age field-value is a non-negative integer, representing time in seconds (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>).
1589               </p>
1590            </div>
1591            <div id="rfc.section.5.1.p.3">
1592               <p>The presence of an Age header field implies that the response was not generated or
1593                  validated by the origin server for this request. However, lack of an Age header field
1594                  does not imply the origin was contacted, since the response might have been received
1595                  from an HTTP/1.0 cache that does not implement Age.
1596               </p>
1597            </div>
1598         </div>
1599         <div id="header.cache-control">
1600            <h3 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a>&nbsp;<a href="#header.cache-control">Cache-Control</a></h3>
1601            <div id="rfc.section.5.2.p.1">
1602               <p>The "Cache-Control" header field is used to specify directives for caches along the
1603                  request/response chain. Such cache directives are unidirectional in that the presence
1604                  of a directive in a request does not imply that the same directive is to be given
1605                  in the response.
1606               </p>
1607            </div>
1608            <div id="rfc.section.5.2.p.2">
1609               <p>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;5.2.3</a> for information about how Cache-Control directives defined elsewhere are handled.
1610               </p>
1611            </div>
1612            <div class="note">
1613               <div id="rfc.section.5.2.p.3">
1614                  <p><b>Note:</b> Some HTTP/1.0 caches might not implement Cache-Control.
1615                  </p>
1616               </div>
1617            </div>
1618            <div id="rfc.section.5.2.p.4">
1619               <p>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
1620                  to that application, since the directives might be applicable to all recipients along
1621                  the request/response chain. It is not possible to target a directive to a specific
1622                  cache.
1623               </p>
1624            </div>
1625            <div id="rfc.section.5.2.p.5">
1626               <p>Cache directives are identified by a token, to be compared case-insensitively, and
1627                  have an optional argument, that can use both token and quoted-string syntax. For the
1628                  directives defined below that define arguments, recipients ought to accept both forms,
1629                  even if one is documented to be preferred. For any directive not defined by this specification,
1630                  a recipient <em class="bcp14">MUST</em> accept both forms.
1631               </p>
1632            </div>
1633            <div id="rfc.figure.u.7"><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>
1634
1635  <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> ) ]
1636</pre></div>
1637            <div id="rfc.section.5.2.p.6">
1638               <p>For the cache directives defined below, no argument is defined (nor allowed) unless
1639                  stated otherwise.
1640               </p>
1641            </div>
1642            <div id="cache-request-directive">
1643               <h4 id="rfc.section.5.2.1"><a href="#rfc.section.5.2.1">5.2.1</a>&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></h4>
1644               <div id="cache-request-directive.max-age">
1645                  <h5 id="rfc.section.5.2.1.1"><a href="#rfc.section.5.2.1.1">5.2.1.1</a>&nbsp;<a href="#cache-request-directive.max-age">max-age</a></h5>
1646                  <div id="rfc.section.5.2.1.1.p.1">
1647                     <p>Argument syntax: </p>
1648                     <ul class="empty">
1649                        <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1650                        </li>
1651                     </ul>
1652                  </div>
1653                  <div id="rfc.section.5.2.1.1.p.2">
1654                     <p>The "max-age" request directive indicates that the client is unwilling to accept a
1655                        response whose age is greater than the specified number of seconds. Unless the max-stale
1656                        request directive is also present, the client is not willing to accept a stale response.
1657                     </p>
1658                  </div>
1659                  <div id="rfc.section.5.2.1.1.p.3">
1660                     <p>This directive uses the token form of the argument syntax: e.g., 'max-age=5' not 'max-age="5"'.
1661                        A sender <em class="bcp14">SHOULD NOT</em> generate the quoted-string form.
1662                     </p>
1663                  </div>
1664               </div>
1665               <div id="cache-request-directive.max-stale">
1666                  <h5 id="rfc.section.5.2.1.2"><a href="#rfc.section.5.2.1.2">5.2.1.2</a>&nbsp;<a href="#cache-request-directive.max-stale">max-stale</a></h5>
1667                  <div id="rfc.section.5.2.1.2.p.1">
1668                     <p>Argument syntax: </p>
1669                     <ul class="empty">
1670                        <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1671                        </li>
1672                     </ul>
1673                  </div>
1674                  <div id="rfc.section.5.2.1.2.p.2">
1675                     <p>The "max-stale" request directive indicates that the client is willing to accept a
1676                        response that has exceeded its freshness lifetime. If max-stale is assigned a value,
1677                        then the client is willing to accept a response that has exceeded its freshness lifetime
1678                        by no more than the specified number of seconds. If no value is assigned to max-stale,
1679                        then the client is willing to accept a stale response of any age.
1680                     </p>
1681                  </div>
1682                  <div id="rfc.section.5.2.1.2.p.3">
1683                     <p>This directive uses the token form of the argument syntax: e.g., 'max-stale=10' not
1684                        'max-stale="10"'. A sender <em class="bcp14">SHOULD NOT</em> generate the quoted-string form.
1685                     </p>
1686                  </div>
1687               </div>
1688               <div id="cache-request-directive.min-fresh">
1689                  <h5 id="rfc.section.5.2.1.3"><a href="#rfc.section.5.2.1.3">5.2.1.3</a>&nbsp;<a href="#cache-request-directive.min-fresh">min-fresh</a></h5>
1690                  <div id="rfc.section.5.2.1.3.p.1">
1691                     <p>Argument syntax: </p>
1692                     <ul class="empty">
1693                        <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1694                        </li>
1695                     </ul>
1696                  </div>
1697                  <div id="rfc.section.5.2.1.3.p.2">
1698                     <p>The "min-fresh" request directive indicates that the client is willing to accept a
1699                        response whose freshness lifetime is no less than its current age plus the specified
1700                        time in seconds. That is, the client wants a response that will still be fresh for
1701                        at least the specified number of seconds.
1702                     </p>
1703                  </div>
1704                  <div id="rfc.section.5.2.1.3.p.3">
1705                     <p>This directive uses the token form of the argument syntax: e.g., 'min-fresh=20' not
1706                        'min-fresh="20"'. A sender <em class="bcp14">SHOULD NOT</em> generate the quoted-string form.
1707                     </p>
1708                  </div>
1709               </div>
1710               <div id="cache-request-directive.no-cache">
1711                  <h5 id="rfc.section.5.2.1.4"><a href="#rfc.section.5.2.1.4">5.2.1.4</a>&nbsp;<a href="#cache-request-directive.no-cache">no-cache</a></h5>
1712                  <div id="rfc.section.5.2.1.4.p.1">
1713                     <p>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
1714                        origin server.
1715                     </p>
1716                  </div>
1717               </div>
1718               <div id="cache-request-directive.no-store">
1719                  <h5 id="rfc.section.5.2.1.5"><a href="#rfc.section.5.2.1.5">5.2.1.5</a>&nbsp;<a href="#cache-request-directive.no-store">no-store</a></h5>
1720                  <div id="rfc.section.5.2.1.5.p.1">
1721                     <p>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
1722                        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
1723                        as possible after forwarding it.
1724                     </p>
1725                  </div>
1726                  <div id="rfc.section.5.2.1.5.p.2">
1727                     <p>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1728                        particular, malicious or compromised caches might not recognize or obey this directive,
1729                        and communications networks might be vulnerable to eavesdropping.
1730                     </p>
1731                  </div>
1732                  <div id="rfc.section.5.2.1.5.p.3">
1733                     <p>Note that if a request containing this directive is satisfied from a cache, the no-store
1734                        request directive does not apply to the already stored response.
1735                     </p>
1736                  </div>
1737               </div>
1738               <div id="cache-request-directive.no-transform">
1739                  <h5 id="rfc.section.5.2.1.6"><a href="#rfc.section.5.2.1.6">5.2.1.6</a>&nbsp;<a href="#cache-request-directive.no-transform">no-transform</a></h5>
1740                  <div id="rfc.section.5.2.1.6.p.1">
1741                     <p>The "no-transform" request directive indicates that an intermediary (whether or not
1742                        it implements a cache) <em class="bcp14">MUST NOT</em> transform the payload, as defined in <a href="p1-messaging.html#message.transformations" title="Transformations">Section 5.7.2</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
1743                     </p>
1744                  </div>
1745               </div>
1746               <div id="cache-request-directive.only-if-cached">
1747                  <h5 id="rfc.section.5.2.1.7"><a href="#rfc.section.5.2.1.7">5.2.1.7</a>&nbsp;<a href="#cache-request-directive.only-if-cached">only-if-cached</a></h5>
1748                  <div id="rfc.section.5.2.1.7.p.1">
1749                     <p>The "only-if-cached" request directive indicates that the client only wishes to obtain
1750                        a stored response. If it receives this directive, a cache <em class="bcp14">SHOULD</em> either respond using a stored response that is consistent with the other constraints
1751                        of the request, or respond with a <a href="p2-semantics.html#status.504" class="smpl">504 (Gateway
1752                           Timeout)</a> status code. If a group of caches is being operated as a unified system with good
1753                        internal connectivity, a member cache <em class="bcp14">MAY</em> forward such a request within that group of caches.
1754                     </p>
1755                  </div>
1756               </div>
1757            </div>
1758            <div id="cache-response-directive">
1759               <h4 id="rfc.section.5.2.2"><a href="#rfc.section.5.2.2">5.2.2</a>&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></h4>
1760               <div id="cache-response-directive.must-revalidate">
1761                  <h5 id="rfc.section.5.2.2.1"><a href="#rfc.section.5.2.2.1">5.2.2.1</a>&nbsp;<a href="#cache-response-directive.must-revalidate">must-revalidate</a></h5>
1762                  <div id="rfc.section.5.2.2.1.p.1">
1763                     <p>The "must-revalidate" response directive indicates that once it has become stale,
1764                        a cache <em class="bcp14">MUST NOT</em> use the response to satisfy subsequent requests without successful validation on the
1765                        origin server.
1766                     </p>
1767                  </div>
1768                  <div id="rfc.section.5.2.2.1.p.2">
1769                     <p>The must-revalidate directive is necessary to support reliable operation for certain
1770                        protocol features. In all circumstances a cache <em class="bcp14">MUST</em> obey the must-revalidate directive; in particular, if a cache cannot reach the origin
1771                        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.
1772                     </p>
1773                  </div>
1774                  <div id="rfc.section.5.2.2.1.p.3">
1775                     <p>The must-revalidate directive ought to be used by servers if and only if failure to
1776                        validate a request on the representation could result in incorrect operation, such
1777                        as a silently unexecuted financial transaction.
1778                     </p>
1779                  </div>
1780               </div>
1781               <div id="cache-response-directive.no-cache">
1782                  <h5 id="rfc.section.5.2.2.2"><a href="#rfc.section.5.2.2.2">5.2.2.2</a>&nbsp;<a href="#cache-response-directive.no-cache">no-cache</a></h5>
1783                  <div id="rfc.section.5.2.2.2.p.1">
1784                     <p>Argument syntax: </p>
1785                     <ul class="empty">
1786                        <li>#<a href="#imported.abnf" class="smpl">field-name</a> 
1787                        </li>
1788                     </ul>
1789                  </div>
1790                  <div id="rfc.section.5.2.2.2.p.2">
1791                     <p>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
1792                        server. This allows an origin server to prevent a cache from using it to satisfy a
1793                        request without contacting it, even by caches that have been configured to send stale
1794                        responses.
1795                     </p>
1796                  </div>
1797                  <div id="rfc.section.5.2.2.2.p.3">
1798                     <p>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
1799                        on caching. However, any header fields in the response that have the field-name(s)
1800                        listed <em class="bcp14">MUST NOT</em> be sent in the response to a subsequent request without successful revalidation with
1801                        the origin server. This allows an origin server to prevent the re-use of certain header
1802                        fields in a response, while still allowing caching of the rest of the response.
1803                     </p>
1804                  </div>
1805                  <div id="rfc.section.5.2.2.2.p.4">
1806                     <p>The field-names given are not limited to the set of header fields defined by this
1807                        specification. Field names are case-insensitive.
1808                     </p>
1809                  </div>
1810                  <div id="rfc.section.5.2.2.2.p.5">
1811                     <p>This directive uses the quoted-string form of the argument syntax. A sender <em class="bcp14">SHOULD NOT</em> generate the token form (even if quoting appears not to be needed for single-entry
1812                        lists).
1813                     </p>
1814                  </div>
1815                  <div id="rfc.section.5.2.2.2.p.6">
1816                     <p><b>Note:</b> Although it has been back-ported to many implementations, some HTTP/1.0 caches will
1817                        not recognize or obey this directive. Also, no-cache response directives with field-names
1818                        are often handled by caches as if an unqualified no-cache directive was received;
1819                        i.e., the special handling for the qualified form is not widely implemented.
1820                     </p>
1821                  </div>
1822               </div>
1823               <div id="cache-response-directive.no-store">
1824                  <h5 id="rfc.section.5.2.2.3"><a href="#rfc.section.5.2.2.3">5.2.2.3</a>&nbsp;<a href="#cache-response-directive.no-store">no-store</a></h5>
1825                  <div id="rfc.section.5.2.2.3.p.1">
1826                     <p>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
1827                        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
1828                        as possible after forwarding it.
1829                     </p>
1830                  </div>
1831                  <div id="rfc.section.5.2.2.3.p.2">
1832                     <p>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1833                        particular, malicious or compromised caches might not recognize or obey this directive,
1834                        and communications networks might be vulnerable to eavesdropping.
1835                     </p>
1836                  </div>
1837               </div>
1838               <div id="cache-response-directive.no-transform">
1839                  <h5 id="rfc.section.5.2.2.4"><a href="#rfc.section.5.2.2.4">5.2.2.4</a>&nbsp;<a href="#cache-response-directive.no-transform">no-transform</a></h5>
1840                  <div id="rfc.section.5.2.2.4.p.1">
1841                     <p>The "no-transform" response directive indicates that an intermediary (regardless of
1842                        whether it implements a cache) <em class="bcp14">MUST NOT</em> transform the payload, as defined in <a href="p1-messaging.html#message.transformations" title="Transformations">Section 5.7.2</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
1843                     </p>
1844                  </div>
1845               </div>
1846               <div id="cache-response-directive.public">
1847                  <h5 id="rfc.section.5.2.2.5"><a href="#rfc.section.5.2.2.5">5.2.2.5</a>&nbsp;<a href="#cache-response-directive.public">public</a></h5>
1848                  <div id="rfc.section.5.2.2.5.p.1">
1849                     <p>The "public" response directive indicates that any cache <em class="bcp14">MAY</em> store the response, even if the response would normally be non-cacheable or cacheable
1850                        only within a private cache. (See <a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a> for additional details related to the use of public in response to a request containing <a href="p7-auth.html#header.authorization" class="smpl">Authorization</a>, and <a href="#response.cacheability" title="Storing Responses in Caches">Section&nbsp;3</a> for details of how public affects responses that would normally not be stored, due
1851                        to their status codes not being defined as cacheable by default; see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.2.2</a>.)
1852                     </p>
1853                  </div>
1854               </div>
1855               <div id="cache-response-directive.private">
1856                  <h5 id="rfc.section.5.2.2.6"><a href="#rfc.section.5.2.2.6">5.2.2.6</a>&nbsp;<a href="#cache-response-directive.private">private</a></h5>
1857                  <div id="rfc.section.5.2.2.6.p.1">
1858                     <p>Argument syntax: </p>
1859                     <ul class="empty">
1860                        <li>#<a href="#imported.abnf" class="smpl">field-name</a> 
1861                        </li>
1862                     </ul>
1863                  </div>
1864                  <div id="rfc.section.5.2.2.6.p.2">
1865                     <p>The "private" response directive indicates that the response message is intended for
1866                        a single user and <em class="bcp14">MUST NOT</em> be stored by a shared cache. A private cache <em class="bcp14">MAY</em> store the response and reuse it for later requests, even if the response would normally
1867                        be non-cacheable.
1868                     </p>
1869                  </div>
1870                  <div id="rfc.section.5.2.2.6.p.3">
1871                     <p>If the private response directive specifies one or more field-names, this requirement
1872                        is limited to the field-values associated with the listed response header fields.
1873                        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.
1874                     </p>
1875                  </div>
1876                  <div id="rfc.section.5.2.2.6.p.4">
1877                     <p>The field-names given are not limited to the set of header fields defined by this
1878                        specification. Field names are case-insensitive.
1879                     </p>
1880                  </div>
1881                  <div id="rfc.section.5.2.2.6.p.5">
1882                     <p>This directive uses the quoted-string form of the argument syntax. A sender <em class="bcp14">SHOULD NOT</em> generate the token form (even if quoting appears not to be needed for single-entry
1883                        lists).
1884                     </p>
1885                  </div>
1886                  <div id="rfc.section.5.2.2.6.p.6">
1887                     <p><b>Note:</b> This usage of the word "private" only controls where the response can be stored; it
1888                        cannot ensure the privacy of the message content. Also, private response directives
1889                        with field-names are often handled by caches as if an unqualified private directive
1890                        was received; i.e., the special handling for the qualified form is not widely implemented.
1891                     </p>
1892                  </div>
1893               </div>
1894               <div id="cache-response-directive.proxy-revalidate">
1895                  <h5 id="rfc.section.5.2.2.7"><a href="#rfc.section.5.2.2.7">5.2.2.7</a>&nbsp;<a href="#cache-response-directive.proxy-revalidate">proxy-revalidate</a></h5>
1896                  <div id="rfc.section.5.2.2.7.p.1">
1897                     <p>The "proxy-revalidate" response directive has the same meaning as the must-revalidate
1898                        response directive, except that it does not apply to private caches.
1899                     </p>
1900                  </div>
1901               </div>
1902               <div id="cache-response-directive.max-age">
1903                  <h5 id="rfc.section.5.2.2.8"><a href="#rfc.section.5.2.2.8">5.2.2.8</a>&nbsp;<a href="#cache-response-directive.max-age">max-age</a></h5>
1904                  <div id="rfc.section.5.2.2.8.p.1">
1905                     <p>Argument syntax: </p>
1906                     <ul class="empty">
1907                        <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1908                        </li>
1909                     </ul>
1910                  </div>
1911                  <div id="rfc.section.5.2.2.8.p.2">
1912                     <p>The "max-age" response directive indicates that the response is to be considered stale
1913                        after its age is greater than the specified number of seconds.
1914                     </p>
1915                  </div>
1916                  <div id="rfc.section.5.2.2.8.p.3">
1917                     <p>This directive uses the token form of the argument syntax: e.g., 'max-age=5' not 'max-age="5"'.
1918                        A sender <em class="bcp14">SHOULD NOT</em> generate the quoted-string form.
1919                     </p>
1920                  </div>
1921               </div>
1922               <div id="cache-response-directive.s-maxage">
1923                  <h5 id="rfc.section.5.2.2.9"><a href="#rfc.section.5.2.2.9">5.2.2.9</a>&nbsp;<a href="#cache-response-directive.s-maxage">s-maxage</a></h5>
1924                  <div id="rfc.section.5.2.2.9.p.1">
1925                     <p>Argument syntax: </p>
1926                     <ul class="empty">
1927                        <li><a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.2.1</a>)
1928                        </li>
1929                     </ul>
1930                  </div>
1931                  <div id="rfc.section.5.2.2.9.p.2">
1932                     <p>The "s-maxage" response directive indicates that, in shared caches, the maximum age
1933                        specified by this directive overrides the maximum age specified by either the max-age
1934                        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
1935                        response directive.
1936                     </p>
1937                  </div>
1938                  <div id="rfc.section.5.2.2.9.p.3">
1939                     <p>This directive uses the token form of the argument syntax: e.g., 's-maxage=10' not
1940                        's-maxage="10"'. A sender <em class="bcp14">SHOULD NOT</em> generate the quoted-string form.
1941                     </p>
1942                  </div>
1943               </div>
1944            </div>
1945            <div id="cache.control.extensions">
1946               <h4 id="rfc.section.5.2.3"><a href="#rfc.section.5.2.3">5.2.3</a>&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></h4>
1947               <div id="rfc.section.5.2.3.p.1">
1948                  <p>The Cache-Control header field can be extended through the use of one or more cache-extension
1949                     tokens, each with an optional value. A cache <em class="bcp14">MUST</em> ignore unrecognized cache directives.
1950                  </p>
1951               </div>
1952               <div id="rfc.section.5.2.3.p.2">
1953                  <p>Informational extensions (those that do not require a change in cache behavior) can
1954                     be added without changing the semantics of other directives.
1955                  </p>
1956               </div>
1957               <div id="rfc.section.5.2.3.p.3">
1958                  <p>Behavioral extensions are designed to work by acting as modifiers to the existing
1959                     base of cache directives. Both the new directive and the old directive are supplied,
1960                     such that applications that do not understand the new directive will default to the
1961                     behavior specified by the old directive, and those that understand the new directive
1962                     will recognize it as modifying the requirements associated with the old directive.
1963                     In this way, extensions to the existing cache-control directives can be made without
1964                     breaking deployed caches.
1965                  </p>
1966               </div>
1967               <div id="rfc.section.5.2.3.p.4">
1968                  <p>For example, consider a hypothetical new response directive called "community" that
1969                     acts as a modifier to the private directive: in addition to private caches, any cache
1970                     that is shared only by members of the named community is allowed to cache the response.
1971                     An origin server wishing to allow the UCI community to use an otherwise private response
1972                     in their shared cache(s) could do so by including
1973                  </p>
1974               </div>
1975               <div id="rfc.figure.u.8"><pre class="text">  Cache-Control: private, community="UCI"
1976</pre></div>
1977               <div id="rfc.section.5.2.3.p.5">
1978                  <p>A cache that recognizes such a community cache-extension could broaden its behavior
1979                     in accordance with that extension. A cache that does not recognize the community cache-extension
1980                     would ignore it and adhere to the private directive.
1981                  </p>
1982               </div>
1983            </div>
1984         </div>
1985         <div id="header.expires">
1986            <h3 id="rfc.section.5.3"><a href="#rfc.section.5.3">5.3</a>&nbsp;<a href="#header.expires">Expires</a></h3>
1987            <div id="rfc.section.5.3.p.1">
1988               <p>The "Expires" header field gives the date/time after which the response is considered
1989                  stale. See <a href="#expiration.model" title="Freshness">Section&nbsp;4.2</a> for further discussion of the freshness model.
1990               </p>
1991            </div>
1992            <div id="rfc.section.5.3.p.2">
1993               <p>The presence of an Expires field does not imply that the original resource will change
1994                  or cease to exist at, before, or after that time.
1995               </p>
1996            </div>
1997            <div id="rfc.section.5.3.p.3">
1998               <p>The Expires value is an HTTP-date timestamp, as defined in <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 7.1.1.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>.
1999               </p>
2000            </div>
2001            <div id="rfc.figure.u.9"><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>
2002</pre></div>
2003            <div id="rfc.figure.u.10">
2004               <p>For example</p><pre class="text">  Expires: Thu, 01 Dec 1994 16:00:00 GMT
2005</pre></div>
2006            <div id="rfc.section.5.3.p.4">
2007               <p>A cache recipient <em class="bcp14">MUST</em> interpret invalid date formats, especially the value "0", as representing a time in
2008                  the past (i.e., "already expired").
2009               </p>
2010            </div>
2011            <div id="rfc.section.5.3.p.5">
2012               <p>If a response includes a <a href="#header.cache-control" class="smpl">Cache-Control</a> field with the max-age directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;5.2.2.8</a>), a recipient <em class="bcp14">MUST</em> ignore the Expires field. Likewise, if a response includes the s-maxage directive
2013                  (<a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;5.2.2.9</a>), a shared cache recipient <em class="bcp14">MUST</em> ignore the Expires field. In both these cases, the value in Expires is only intended
2014                  for recipients that have not yet implemented the Cache-Control field.
2015               </p>
2016            </div>
2017            <div id="rfc.section.5.3.p.6">
2018               <p>An origin server without a clock <em class="bcp14">MUST NOT</em> generate an Expires field unless its value represents a fixed time in the past (always
2019                  expired) or its value has been associated with the resource by a system or user with
2020                  a reliable clock.
2021               </p>
2022            </div>
2023            <div id="rfc.section.5.3.p.7">
2024               <p>Historically, HTTP required the Expires field-value to be no more than a year in the
2025                  future. While longer freshness lifetimes are no longer prohibited, extremely large
2026                  values have been demonstrated to cause problems (e.g., clock overflows due to use
2027                  of 32-bit integers for time values), and many caches will evict a response far sooner
2028                  than that.
2029               </p>
2030            </div>
2031         </div>
2032         <div id="header.pragma">
2033            <h3 id="rfc.section.5.4"><a href="#rfc.section.5.4">5.4</a>&nbsp;<a href="#header.pragma">Pragma</a></h3>
2034            <div id="rfc.section.5.4.p.1">
2035               <p>The "Pragma" header field allows backwards compatibility with HTTP/1.0 caches, so
2036                  that clients can specify a "no-cache" request 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
2037                  and understood in a request, Pragma is ignored.
2038               </p>
2039            </div>
2040            <div id="rfc.section.5.4.p.2">
2041               <p>In HTTP/1.0, Pragma was defined as an extensible field for implementation-specified
2042                  directives for recipients. This specification deprecates such extensions to improve
2043                  interoperability.
2044               </p>
2045            </div>
2046            <div id="rfc.figure.u.11"><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>
2047  <a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / <a href="#header.pragma" class="smpl">extension-pragma</a>
2048  <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> ) ]
2049</pre></div>
2050            <div id="rfc.section.5.4.p.3">
2051               <p>When the <a href="#header.cache-control" class="smpl">Cache-Control</a> header field is not present in a request, caches <em class="bcp14">MUST</em> consider the no-cache request pragma-directive as having the same effect as if "Cache-Control:
2052                  no-cache" were present (see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;5.2.1</a>).
2053               </p>
2054            </div>
2055            <div id="rfc.section.5.4.p.4" class="avoidbreakafter">
2056               <p>When sending a no-cache request, a client ought to include both the pragma and cache-control
2057                  directives, unless Cache-Control: 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:
2058               </p>
2059            </div>
2060            <div id="rfc.figure.u.12"><pre class="text">GET / HTTP/1.1
2061Host: www.example.com
2062Cache-Control: max-age=30
2063Pragma: no-cache
2064
2065</pre></div>
2066            <div id="rfc.section.5.4.p.5">
2067               <p>will constrain HTTP/1.1 caches to serve a response no older than 30 seconds, while
2068                  precluding implementations that do not understand <a href="#header.cache-control" class="smpl">Cache-Control</a> from serving a cached response.
2069               </p>
2070            </div>
2071            <div class="note">
2072               <div id="rfc.section.5.4.p.6">
2073                  <p><b>Note:</b> Because the meaning of "Pragma: no-cache" in responses is not specified, it does not
2074                     provide a reliable replacement for "Cache-Control: no-cache" in them.
2075                  </p>
2076               </div>
2077            </div>
2078         </div>
2079         <div id="header.warning">
2080            <h3 id="rfc.section.5.5"><a href="#rfc.section.5.5">5.5</a>&nbsp;<a href="#header.warning">Warning</a></h3>
2081            <div id="rfc.section.5.5.p.1">
2082               <p>The "Warning" header field is used to carry additional information about the status
2083                  or transformation of a message that might not be reflected in the status code. This
2084                  information is typically used to warn about possible incorrectness introduced by caching
2085                  operations or transformations applied to the payload of the message.
2086               </p>
2087            </div>
2088            <div id="rfc.section.5.5.p.2">
2089               <p>Warnings can be used for other purposes, both cache-related and otherwise. The use
2090                  of a warning, rather than an error status code, distinguishes these responses from
2091                  true failures.
2092               </p>
2093            </div>
2094            <div id="rfc.section.5.5.p.3">
2095               <p>Warning header fields can in general be applied to any message, however some warn-codes
2096                  are specific to caches and can only be applied to response messages.
2097               </p>
2098            </div>
2099            <div id="rfc.figure.u.13"><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>
2100 
2101  <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>
2102                                        [ <a href="#imported.abnf" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-date</a> ]
2103 
2104  <a href="#header.warning" class="smpl">warn-code</a>  = 3<a href="#imported.abnf" class="smpl">DIGIT</a>
2105  <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>
2106                  ; the name or pseudonym of the server adding
2107                  ; the Warning header field, for use in debugging
2108                  ; a single "-" is recommended when agent unknown
2109  <a href="#header.warning" class="smpl">warn-text</a>  = <a href="#imported.abnf" class="smpl">quoted-string</a>
2110  <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>
2111</pre></div>
2112            <div id="rfc.section.5.5.p.4">
2113               <p>Multiple warnings can be generated in a response (either by the origin server or by
2114                  a cache), including multiple warnings with the same warn-code number that only differ
2115                  in warn-text.
2116               </p>
2117            </div>
2118            <div id="rfc.section.5.5.p.5">
2119               <p>A user agent that receives one or more Warning header fields <em class="bcp14">SHOULD</em> inform the user of as many of them as possible, in the order that they appear in the
2120                  response. Senders that generate multiple Warning header fields are encouraged to order
2121                  them with this user agent behavior in mind. A sender that generates new Warning header
2122                  fields <em class="bcp14">MUST</em> append them after any existing Warning header fields.
2123               </p>
2124            </div>
2125            <div id="rfc.section.5.5.p.6">
2126               <p>Warnings are assigned three digit warn-codes. The first digit indicates whether the
2127                  Warning is required to be deleted from a stored response after validation:
2128               </p>
2129               <ul>
2130                  <li>1xx warn-codes describe the freshness or validation status of the response, and so
2131                     they <em class="bcp14">MUST</em> be deleted by a cache after validation. They can only be generated by a cache when
2132                     validating a cached entry, and <em class="bcp14">MUST NOT</em> be generated in any other situation.
2133                  </li>
2134                  <li>2xx warn-codes describe some aspect of the representation that is not rectified by
2135                     a validation (for example, a lossy compression of the representation) and they <em class="bcp14">MUST NOT</em> be deleted by a cache after validation, unless a full response is sent, in which case
2136                     they <em class="bcp14">MUST</em> be.
2137                  </li>
2138               </ul>
2139            </div>
2140            <div id="rfc.section.5.5.p.7" class="avoidbreakafter">
2141               <p>If a sender generates one or more 1xx warn-codes in a message to be sent to a recipient
2142                  known to implement only HTTP/1.0, the sender <em class="bcp14">MUST</em> include in each corresponding warning-value a warn-date that matches the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field in the message. For example:
2143               </p>
2144            </div>
2145            <div id="rfc.figure.u.14"><pre class="text">HTTP/1.1 200 OK
2146Date: Sat, 25 Aug 2012 23:34:45 GMT
2147Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"
2148
2149</pre></div>
2150            <div id="rfc.section.5.5.p.8">
2151               <p>Warnings have accompanying warn-text that describes the error, e.g., for logging.
2152                  It is advisory only, and its content does not affect interpretation of the warn-code.
2153               </p>
2154            </div>
2155            <div id="rfc.section.5.5.p.9">
2156               <p>If a recipient that uses, evaluates, or displays Warning header fields receives a
2157                  warn-date that is different from the <a href="p2-semantics.html#header.date" class="smpl">Date</a> value in the same message, the recipient <em class="bcp14">MUST</em> exclude the warning-value containing that warn-date before storing, forwarding, or
2158                  using the message. This allows recipients to exclude warning-values that were improperly
2159                  retained after a cache validation. If all of the warning-values are excluded, the
2160                  recipient <em class="bcp14">MUST</em> exclude the Warning header field as well.
2161               </p>
2162            </div>
2163            <div id="rfc.section.5.5.p.10">
2164               <p>The following warn-codes are defined by this specification, each with a recommended
2165                  warn-text in English, and a description of its meaning. The procedure for defining
2166                  additional warn codes is described in <a href="#warn.code.registry.procedure" title="Procedure">Section&nbsp;7.2.1</a>.
2167               </p>
2168            </div>
2169            <div id="warn.110">
2170               <h4 id="rfc.section.5.5.1"><a href="#rfc.section.5.5.1">5.5.1</a>&nbsp;<a href="#warn.110">Warning: 110 - "Response is Stale"</a></h4>
2171               <div id="rfc.section.5.5.1.p.1">
2172                  <p>A cache <em class="bcp14">SHOULD</em> generate this whenever the sent response is stale.
2173                  </p>
2174               </div>
2175            </div>
2176            <div id="warn.111">
2177               <h4 id="rfc.section.5.5.2"><a href="#rfc.section.5.5.2">5.5.2</a>&nbsp;<a href="#warn.111">Warning: 111 - "Revalidation Failed"</a></h4>
2178               <div id="rfc.section.5.5.2.p.1">
2179                  <p>A cache <em class="bcp14">SHOULD</em> generate this when sending a stale response because an attempt to validate the response
2180                     failed, due to an inability to reach the server.
2181                  </p>
2182               </div>
2183            </div>
2184            <div id="warn.112">
2185               <h4 id="rfc.section.5.5.3"><a href="#rfc.section.5.5.3">5.5.3</a>&nbsp;<a href="#warn.112">Warning: 112 - "Disconnected Operation"</a></h4>
2186               <div id="rfc.section.5.5.3.p.1">
2187                  <p>A cache <em class="bcp14">SHOULD</em> generate this if it is intentionally disconnected from the rest of the network for
2188                     a period of time.
2189                  </p>
2190               </div>
2191            </div>
2192            <div id="warn.113">
2193               <h4 id="rfc.section.5.5.4"><a href="#rfc.section.5.5.4">5.5.4</a>&nbsp;<a href="#warn.113">Warning: 113 - "Heuristic Expiration"</a></h4>
2194               <div id="rfc.section.5.5.4.p.1">
2195                  <p>A cache <em class="bcp14">SHOULD</em> generate this if it heuristically chose a freshness lifetime greater than 24 hours
2196                     and the response's age is greater than 24 hours.
2197                  </p>
2198               </div>
2199            </div>
2200            <div id="warn.199">
2201               <h4 id="rfc.section.5.5.5"><a href="#rfc.section.5.5.5">5.5.5</a>&nbsp;<a href="#warn.199">Warning: 199 - "Miscellaneous Warning"</a></h4>
2202               <div id="rfc.section.5.5.5.p.1">
2203                  <p>The warning text can include arbitrary information to be presented to a human user
2204                     or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action, besides presenting the warning to the user.
2205                  </p>
2206               </div>
2207            </div>
2208            <div id="warn.214">
2209               <h4 id="rfc.section.5.5.6"><a href="#rfc.section.5.5.6">5.5.6</a>&nbsp;<a href="#warn.214">Warning: 214 - "Transformation Applied"</a></h4>
2210               <div id="rfc.section.5.5.6.p.1">
2211                  <p>This Warning code <em class="bcp14">MUST</em> be added by a proxy if it applies any transformation to the representation, such as
2212                     changing the content-coding, media-type, or modifying the representation data, unless
2213                     this Warning code already appears in the response.
2214                  </p>
2215               </div>
2216            </div>
2217            <div id="warn.299">
2218               <h4 id="rfc.section.5.5.7"><a href="#rfc.section.5.5.7">5.5.7</a>&nbsp;<a href="#warn.299">Warning: 299 - "Miscellaneous Persistent Warning"</a></h4>
2219               <div id="rfc.section.5.5.7.p.1">
2220                  <p>The warning text can include arbitrary information to be presented to a human user
2221                     or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action.
2222                  </p>
2223               </div>
2224            </div>
2225         </div>
2226      </div>
2227      <div id="history.lists">
2228         <h2 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#history.lists">History Lists</a></h2>
2229         <div id="rfc.section.6.p.1">
2230            <p>User agents often have history mechanisms, such as "Back" buttons and history lists,
2231               that can be used to redisplay a representation retrieved earlier in a session.
2232            </p>
2233         </div>
2234         <div id="rfc.section.6.p.2">
2235            <p>The freshness model (<a href="#expiration.model" title="Freshness">Section&nbsp;4.2</a>) does not necessarily apply to history mechanisms. That is, a history mechanism can
2236               display a previous representation even if it has expired.
2237            </p>
2238         </div>
2239         <div id="rfc.section.6.p.3">
2240            <p>This does not prohibit the history mechanism from telling the user that a view might
2241               be stale or from honoring cache directives (e.g., Cache-Control: no-store).
2242            </p>
2243         </div>
2244      </div>
2245      <div id="iana.considerations">
2246         <h2 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a href="#iana.considerations">IANA Considerations</a></h2>
2247         <div id="cache.directive.registry">
2248            <h3 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a href="#cache.directive.registry">Cache Directive Registry</a></h3>
2249            <div id="rfc.section.7.1.p.1">
2250               <p>The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" defines the namespace
2251                  for the cache directives. It has been created and is now maintained at &lt;<a href="http://www.iana.org/assignments/http-cache-directives">http://www.iana.org/assignments/http-cache-directives</a>&gt;.
2252               </p>
2253            </div>
2254            <div id="cache.directive.registry.procedure">
2255               <h4 id="rfc.section.7.1.1"><a href="#rfc.section.7.1.1">7.1.1</a>&nbsp;<a href="#cache.directive.registry.procedure">Procedure</a></h4>
2256               <div id="rfc.section.7.1.1.p.1">
2257                  <p>A registration <em class="bcp14">MUST</em> include the following fields:
2258                  </p>
2259                  <ul>
2260                     <li>Cache Directive Name</li>
2261                     <li>Pointer to specification text</li>
2262                  </ul>
2263               </div>
2264               <div id="rfc.section.7.1.1.p.2">
2265                  <p>Values to be added to this namespace 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="https://tools.ietf.org/html/rfc5226#section-4.1">Section 4.1</a>).
2266                  </p>
2267               </div>
2268            </div>
2269            <div id="cache.directive.considerations">
2270               <h4 id="rfc.section.7.1.2"><a href="#rfc.section.7.1.2">7.1.2</a>&nbsp;<a href="#cache.directive.considerations">Considerations for New Cache Control Directives</a></h4>
2271               <div id="rfc.section.7.1.2.p.1" class="avoidbreakafter">
2272                  <p>New extension directives ought to consider defining:</p>
2273               </div>
2274               <div id="rfc.section.7.1.2.p.2">
2275                  <ul>
2276                     <li>What it means for a directive to be specified multiple times,</li>
2277                     <li>When the directive does not take an argument, what it means when an argument is present,</li>
2278                     <li>When the directive requires an argument, what it means when it is missing,</li>
2279                     <li>Whether the directive is specific to requests, responses, or able to be used in either.</li>
2280                  </ul>
2281               </div>
2282               <div id="rfc.section.7.1.2.p.3">
2283                  <p>See also <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;5.2.3</a>.
2284                  </p>
2285               </div>
2286            </div>
2287            <div id="cache.directive.registration">
2288               <h4 id="rfc.section.7.1.3"><a href="#rfc.section.7.1.3">7.1.3</a>&nbsp;<a href="#cache.directive.registration">Registrations</a></h4>
2289               <div id="rfc.section.7.1.3.p.1" class="avoidbreakafter">
2290                  <p>The registry has been populated with the registrations below:</p>
2291               </div>
2292               <div id="rfc.table.1">
2293                  <div id="iana.cache.directive.registration.table"></div>
2294                  <table class="tt full tleft" cellpadding="3" cellspacing="0">
2295                     <thead>
2296                        <tr>
2297                           <th>Cache Directive</th>
2298                           <th>Reference</th>
2299                        </tr>
2300                     </thead>
2301                     <tbody>
2302                        <tr>
2303                           <td class="left">max-age</td>
2304                           <td class="left"><a href="#cache-request-directive.max-age" title="max-age">Section&nbsp;5.2.1.1</a>, <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;5.2.2.8</a> 
2305                           </td>
2306                        </tr>
2307                        <tr>
2308                           <td class="left">max-stale</td>
2309                           <td class="left"><a href="#cache-request-directive.max-stale" title="max-stale">Section&nbsp;5.2.1.2</a> 
2310                           </td>
2311                        </tr>
2312                        <tr>
2313                           <td class="left">min-fresh</td>
2314                           <td class="left"><a href="#cache-request-directive.min-fresh" title="min-fresh">Section&nbsp;5.2.1.3</a> 
2315                           </td>
2316                        </tr>
2317                        <tr>
2318                           <td class="left">must-revalidate</td>
2319                           <td class="left"><a href="#cache-response-directive.must-revalidate" title="must-revalidate">Section&nbsp;5.2.2.1</a> 
2320                           </td>
2321                        </tr>
2322                        <tr>
2323                           <td class="left">no-cache</td>
2324                           <td class="left"><a href="#cache-request-directive.no-cache" title="no-cache">Section&nbsp;5.2.1.4</a>, <a href="#cache-response-directive.no-cache" title="no-cache">Section&nbsp;5.2.2.2</a> 
2325                           </td>
2326                        </tr>
2327                        <tr>
2328                           <td class="left">no-store</td>
2329                           <td class="left"><a href="#cache-request-directive.no-store" title="no-store">Section&nbsp;5.2.1.5</a>, <a href="#cache-response-directive.no-store" title="no-store">Section&nbsp;5.2.2.3</a> 
2330                           </td>
2331                        </tr>
2332                        <tr>
2333                           <td class="left">no-transform</td>
2334                           <td class="left"><a href="#cache-request-directive.no-transform" title="no-transform">Section&nbsp;5.2.1.6</a>, <a href="#cache-response-directive.no-transform" title="no-transform">Section&nbsp;5.2.2.4</a> 
2335                           </td>
2336                        </tr>
2337                        <tr>
2338                           <td class="left">only-if-cached</td>
2339                           <td class="left"><a href="#cache-request-directive.only-if-cached" title="only-if-cached">Section&nbsp;5.2.1.7</a> 
2340                           </td>
2341                        </tr>
2342                        <tr>
2343                           <td class="left">private</td>
2344                           <td class="left"><a href="#cache-response-directive.private" title="private">Section&nbsp;5.2.2.6</a> 
2345                           </td>
2346                        </tr>
2347                        <tr>
2348                           <td class="left">proxy-revalidate</td>
2349                           <td class="left"><a href="#cache-response-directive.proxy-revalidate" title="proxy-revalidate">Section&nbsp;5.2.2.7</a> 
2350                           </td>
2351                        </tr>
2352                        <tr>
2353                           <td class="left">public</td>
2354                           <td class="left"><a href="#cache-response-directive.public" title="public">Section&nbsp;5.2.2.5</a> 
2355                           </td>
2356                        </tr>
2357                        <tr>
2358                           <td class="left">s-maxage</td>
2359                           <td class="left"><a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;5.2.2.9</a> 
2360                           </td>
2361                        </tr>
2362                        <tr>
2363                           <td class="left">stale-if-error</td>
2364                           <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="https://tools.ietf.org/html/rfc5861#section-4">Section 4</a> 
2365                           </td>
2366                        </tr>
2367                        <tr>
2368                           <td class="left">stale-while-revalidate</td>
2369                           <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="https://tools.ietf.org/html/rfc5861#section-3">Section 3</a> 
2370                           </td>
2371                        </tr>
2372                     </tbody>
2373                  </table>
2374               </div>
2375            </div>
2376         </div>
2377         <div id="warn.code.registry">
2378            <h3 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a href="#warn.code.registry">Warn Code Registry</a></h3>
2379            <div id="rfc.section.7.2.p.1">
2380               <p>The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines the namespace
2381                  for warn codes. It has been created and is now maintained at &lt;<a href="http://www.iana.org/assignments/http-warn-codes">http://www.iana.org/assignments/http-warn-codes</a>&gt;.
2382               </p>
2383            </div>
2384            <div id="warn.code.registry.procedure">
2385               <h4 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;<a href="#warn.code.registry.procedure">Procedure</a></h4>
2386               <div id="rfc.section.7.2.1.p.1">
2387                  <p>A registration <em class="bcp14">MUST</em> include the following fields:
2388                  </p>
2389                  <ul>
2390                     <li>Warn Code (3 digits)</li>
2391                     <li>Short Description</li>
2392                     <li>Pointer to specification text</li>
2393                  </ul>
2394               </div>
2395               <div id="rfc.section.7.2.1.p.2">
2396                  <p>Values to be added to this namespace 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="https://tools.ietf.org/html/rfc5226#section-4.1">Section 4.1</a>).
2397                  </p>
2398               </div>
2399            </div>
2400            <div id="warn.code.registration">
2401               <h4 id="rfc.section.7.2.2"><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;<a href="#warn.code.registration">Registrations</a></h4>
2402               <div id="rfc.section.7.2.2.p.1" class="avoidbreakafter">
2403                  <p>The registry has been populated with the registrations below:</p>
2404               </div>
2405               <div id="rfc.table.2">
2406                  <div id="iana.warn.code.registration.table"></div>
2407                  <table class="tt full tleft" cellpadding="3" cellspacing="0">
2408                     <thead>
2409                        <tr>
2410                           <th>Warn Code</th>
2411                           <th>Short Description</th>
2412                           <th>Reference</th>
2413                        </tr>
2414                     </thead>
2415                     <tbody>
2416                        <tr>
2417                           <td class="left">110</td>
2418                           <td class="left">Response is Stale</td>
2419                           <td class="left"><a href="#warn.110" id="rfc.xref.warn.110.2" title="Warning: 110 - &#34;Response is Stale&#34;">Section&nbsp;5.5.1</a> 
2420                           </td>
2421                        </tr>
2422                        <tr>
2423                           <td class="left">111</td>
2424                           <td class="left">Revalidation Failed</td>
2425                           <td class="left"><a href="#warn.111" id="rfc.xref.warn.111.1" title="Warning: 111 - &#34;Revalidation Failed&#34;">Section&nbsp;5.5.2</a> 
2426                           </td>
2427                        </tr>
2428                        <tr>
2429                           <td class="left">112</td>
2430                           <td class="left">Disconnected Operation</td>
2431                           <td class="left"><a href="#warn.112" id="rfc.xref.warn.112.2" title="Warning: 112 - &#34;Disconnected Operation&#34;">Section&nbsp;5.5.3</a> 
2432                           </td>
2433                        </tr>
2434                        <tr>
2435                           <td class="left">113</td>
2436                           <td class="left">Heuristic Expiration</td>
2437                           <td class="left"><a href="#warn.113" id="rfc.xref.warn.113.2" title="Warning: 113 - &#34;Heuristic Expiration&#34;">Section&nbsp;5.5.4</a> 
2438                           </td>
2439                        </tr>
2440                        <tr>
2441                           <td class="left">199</td>
2442                           <td class="left">Miscellaneous Warning</td>
2443                           <td class="left"><a href="#warn.199" id="rfc.xref.warn.199.1" title="Warning: 199 - &#34;Miscellaneous Warning&#34;">Section&nbsp;5.5.5</a> 
2444                           </td>
2445                        </tr>
2446                        <tr>
2447                           <td class="left">214</td>
2448                           <td class="left">Transformation Applied</td>
2449                           <td class="left"><a href="#warn.214" id="rfc.xref.warn.214.1" title="Warning: 214 - &#34;Transformation Applied&#34;">Section&nbsp;5.5.6</a> 
2450                           </td>
2451                        </tr>
2452                        <tr>
2453                           <td class="left">299</td>
2454                           <td class="left">Miscellaneous Persistent Warning</td>
2455                           <td class="left"><a href="#warn.299" id="rfc.xref.warn.299.1" title="Warning: 299 - &#34;Miscellaneous Persistent Warning&#34;">Section&nbsp;5.5.7</a> 
2456                           </td>
2457                        </tr>
2458                     </tbody>
2459                  </table>
2460               </div>
2461            </div>
2462         </div>
2463         <div id="header.field.registration">
2464            <h3 id="rfc.section.7.3"><a href="#rfc.section.7.3">7.3</a>&nbsp;<a href="#header.field.registration">Header Field Registration</a></h3>
2465            <div id="rfc.section.7.3.p.1">
2466               <p>HTTP header fields are registered within the "Message Headers" registry maintained
2467                  at &lt;<a href="http://www.iana.org/assignments/message-headers/">http://www.iana.org/assignments/message-headers/</a>&gt;.
2468               </p>
2469            </div>
2470            <div id="rfc.section.7.3.p.2">
2471               <p>This document defines the following HTTP header fields, so the "Permanent Message
2472                  Header Field Names" registry has been updated accordingly (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>).
2473               </p>
2474            </div>
2475            <div id="rfc.table.3">
2476               <div id="iana.header.registration.table"></div>
2477               <table class="tt full tleft" cellpadding="3" cellspacing="0">
2478                  <thead>
2479                     <tr>
2480                        <th>Header Field Name</th>
2481                        <th>Protocol</th>
2482                        <th>Status</th>
2483                        <th>Reference</th>
2484                     </tr>
2485                  </thead>
2486                  <tbody>
2487                     <tr>
2488                        <td class="left">Age</td>
2489                        <td class="left">http</td>
2490                        <td class="left">standard</td>
2491                        <td class="left"><a href="#header.age" id="rfc.xref.header.age.3" title="Age">Section&nbsp;5.1</a> 
2492                        </td>
2493                     </tr>
2494                     <tr>
2495                        <td class="left">Cache-Control</td>
2496                        <td class="left">http</td>
2497                        <td class="left">standard</td>
2498                        <td class="left"><a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;5.2</a> 
2499                        </td>
2500                     </tr>
2501                     <tr>
2502                        <td class="left">Expires</td>
2503                        <td class="left">http</td>
2504                        <td class="left">standard</td>
2505                        <td class="left"><a href="#header.expires" id="rfc.xref.header.expires.4" title="Expires">Section&nbsp;5.3</a> 
2506                        </td>
2507                     </tr>
2508                     <tr>
2509                        <td class="left">Pragma</td>
2510                        <td class="left">http</td>
2511                        <td class="left">standard</td>
2512                        <td class="left"><a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;5.4</a> 
2513                        </td>
2514                     </tr>
2515                     <tr>
2516                        <td class="left">Warning</td>
2517                        <td class="left">http</td>
2518                        <td class="left">standard</td>
2519                        <td class="left"><a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;5.5</a> 
2520                        </td>
2521                     </tr>
2522                  </tbody>
2523               </table>
2524            </div>
2525            <div id="rfc.section.7.3.p.3">
2526               <p>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
2527            </div>
2528         </div>
2529      </div>
2530      <div id="security.considerations">
2531         <h2 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h2>
2532         <div id="rfc.section.8.p.1">
2533            <p>This section is meant to inform developers, information providers, and users of known
2534               security concerns specific to HTTP caching. More general security considerations are
2535               addressed in HTTP messaging <a href="#RFC7230" id="rfc.xref.RFC7230.11"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a> and semantics <a href="#RFC7231" id="rfc.xref.RFC7231.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>.
2536            </p>
2537         </div>
2538         <div id="rfc.section.8.p.2">
2539            <p>Caches expose additional potential vulnerabilities, since the contents of the cache
2540               represent an attractive target for malicious exploitation. Because cache contents
2541               persist after an HTTP request is complete, an attack on the cache can reveal information
2542               long after a user believes that the information has been removed from the network.
2543               Therefore, cache contents need to be protected as sensitive information.
2544            </p>
2545         </div>
2546         <div id="rfc.section.8.p.3">
2547            <p>In particular, various attacks might be amplified by being stored in a shared cache;
2548               such "cache poisoning" attacks use the cache to distribute a malicious payload to
2549               many clients, and are especially effective when an attacker can use implementation
2550               flaws, elevated privileges, or other techniques to insert such a response into a cache.
2551               One common attack vector for cache poisoning is to exploit differences in message
2552               parsing on proxies and in user agents; see <a href="p1-messaging.html#message.body.length" title="Message Body Length">Section 3.3.3</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.12"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a> for the relevant requirements.
2553            </p>
2554         </div>
2555         <div id="rfc.section.8.p.4">
2556            <p>Likewise, implementation flaws (as well as misunderstanding of cache operation) might
2557               lead to caching of sensitive information (e.g., authentication credentials) that is
2558               thought to be private, exposing it to unauthorized parties.
2559            </p>
2560         </div>
2561         <div id="rfc.section.8.p.5">
2562            <p>Furthermore, the very use of a cache can bring about privacy concerns. For example,
2563               if two users share a cache, and the first one browses to a site, the second may be
2564               able to detect that the other has been to that site, because the resources from it
2565               load more quickly, thanks to the cache.
2566            </p>
2567         </div>
2568         <div id="rfc.section.8.p.6">
2569            <p>Note that the Set-Cookie response header field <a href="#RFC6265" id="rfc.xref.RFC6265.1"><cite title="HTTP State Management Mechanism">[RFC6265]</cite></a> does not inhibit caching; a cacheable response with a Set-Cookie header field can
2570               be (and often is) used to satisfy subsequent requests to caches. Servers who wish
2571               to control caching of these responses are encouraged to emit appropriate Cache-Control
2572               response header fields.
2573            </p>
2574         </div>
2575      </div>
2576      <div id="acks">
2577         <h2 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a href="#acks">Acknowledgments</a></h2>
2578         <div id="rfc.section.9.p.1">
2579            <p>See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 10</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.13"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
2580            </p>
2581         </div>
2582      </div>
2583      <h2 id="rfc.references"><a id="rfc.section.10" href="#rfc.section.10">10.</a> References
2584      </h2>
2585      <h3 id="rfc.references.1"><a href="#rfc.section.10.1" id="rfc.section.10.1">10.1</a> Normative References
2586      </h3>
2587      <dl class="reference">
2588         <dt id="RFC2119">[RFC2119]</dt>
2589         <dd>Bradner, S., “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
2590         </dd>
2591         <dt id="RFC5234">[RFC5234]</dt>
2592         <dd>Crocker, D., Ed. and P. Overell, “<a href="https://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD&nbsp;68, RFC&nbsp;5234, January&nbsp;2008.
2593         </dd>
2594         <dt id="RFC7230">[RFC7230]</dt>
2595         <dd>Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/rfc7230">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, RFC&nbsp;7230, June&nbsp;2014.
2596         </dd>
2597         <dt id="RFC7231">[RFC7231]</dt>
2598         <dd>Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), June&nbsp;2014.
2599         </dd>
2600         <dt id="RFC7232">[RFC7232]</dt>
2601         <dd>Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-latest">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-latest (work in progress), June&nbsp;2014.
2602         </dd>
2603         <dt id="RFC7233">[RFC7233]</dt>
2604         <dd>Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), June&nbsp;2014.
2605         </dd>
2606         <dt id="RFC7235">[RFC7235]</dt>
2607         <dd>Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-latest">Hypertext Transfer Protocol (HTTP/1.1): Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-latest (work in progress), June&nbsp;2014.
2608         </dd>
2609      </dl>
2610      <h3 id="rfc.references.2"><a href="#rfc.section.10.2" id="rfc.section.10.2">10.2</a> Informative References
2611      </h3>
2612      <dl class="reference">
2613         <dt id="BCP90">[BCP90]</dt>
2614         <dd>Klyne, G., Nottingham, M., and J. Mogul, “<a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
2615         </dd>
2616         <dt id="RFC2616">[RFC2616]</dt>
2617         <dd>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
2618         </dd>
2619         <dt id="RFC5226">[RFC5226]</dt>
2620         <dd>Narten, T. and H. Alvestrand, “<a href="https://tools.ietf.org/html/rfc5226">Guidelines for Writing an IANA Considerations Section in RFCs</a>”, BCP&nbsp;26, RFC&nbsp;5226, May&nbsp;2008.
2621         </dd>
2622         <dt id="RFC5861">[RFC5861]</dt>
2623         <dd>Nottingham, M., “<a href="https://tools.ietf.org/html/rfc5861">HTTP Cache-Control Extensions for Stale Content</a>”, RFC&nbsp;5861, April&nbsp;2010.
2624         </dd>
2625         <dt id="RFC5905">[RFC5905]</dt>
2626         <dd>Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, “<a href="https://tools.ietf.org/html/rfc5905">Network Time Protocol Version 4: Protocol and Algorithms Specification</a>”, RFC&nbsp;5905, June&nbsp;2010.
2627         </dd>
2628         <dt id="RFC6265">[RFC6265]</dt>
2629         <dd>Barth, A., “<a href="https://tools.ietf.org/html/rfc6265">HTTP State Management Mechanism</a>”, RFC&nbsp;6265, April&nbsp;2011.
2630         </dd>
2631      </dl>
2632      <div id="changes.from.rfc.2616">
2633         <h2 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></h2>
2634         <div id="rfc.section.A.p.1">
2635            <p>The specification has been substantially rewritten for clarity.</p>
2636         </div>
2637         <div id="rfc.section.A.p.2">
2638            <p>The conditions under which an authenticated response can be cached have been clarified.
2639               (<a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a>)
2640            </p>
2641         </div>
2642         <div id="rfc.section.A.p.3">
2643            <p>New status codes can now define that caches are allowed to use heuristic freshness
2644               with them. Caches are now allowed to calculate heuristic freshness for URIs with query
2645               components. (<a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.2.2</a>)
2646            </p>
2647         </div>
2648         <div id="rfc.section.A.p.4">
2649            <p>The algorithm for calculating age is now less conservative. Caches are now required
2650               to handle dates with time zones as if they're invalid, because it's not possible to
2651               accurately guess. (<a href="#age.calculations" title="Calculating Age">Section&nbsp;4.2.3</a>)
2652            </p>
2653         </div>
2654         <div id="rfc.section.A.p.5">
2655            <p>The <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a> response header field is no longer used to determine the appropriate response to use
2656               when validating. (<a href="#validation.model" title="Validation">Section&nbsp;4.3</a>)
2657            </p>
2658         </div>
2659         <div id="rfc.section.A.p.6">
2660            <p>The algorithm for selecting a cached negotiated response to use has been clarified
2661               in several ways. In particular, it now explicitly allows header-specific canonicalization
2662               when processing selecting header fields. (<a href="#caching.negotiated.responses" title="Calculating Secondary Keys with Vary">Section&nbsp;4.1</a>)
2663            </p>
2664         </div>
2665         <div id="rfc.section.A.p.7">
2666            <p>Requirements regarding denial-of-service attack avoidance when performing invalidation
2667               have been clarified. (<a href="#invalidation" title="Invalidation">Section&nbsp;4.4</a>)
2668            </p>
2669         </div>
2670         <div id="rfc.section.A.p.8">
2671            <p>Cache invalidation only occurs when a successful response is received. (<a href="#invalidation" title="Invalidation">Section&nbsp;4.4</a>)
2672            </p>
2673         </div>
2674         <div id="rfc.section.A.p.9">
2675            <p>Cache directives are explicitly defined to be case-insensitive. Handling of multiple
2676               instances of cache directives when only one is expected is now defined. (<a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;5.2</a>)
2677            </p>
2678         </div>
2679         <div id="rfc.section.A.p.10">
2680            <p>The "no-store" request directive doesn't apply to responses; i.e., a cache can satisfy
2681               a request with no-store on it and does not invalidate it. (<a href="#cache-request-directive.no-store" title="no-store">Section&nbsp;5.2.1.5</a>)
2682            </p>
2683         </div>
2684         <div id="rfc.section.A.p.11">
2685            <p>The qualified forms of the private and no-cache cache directives are noted to not
2686               be widely implemented; for example, "private=foo" is interpreted by many caches as
2687               simply "private". Additionally, the meaning of the qualified form of no-cache has
2688               been clarified. (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;5.2.2</a>)
2689            </p>
2690         </div>
2691         <div id="rfc.section.A.p.12">
2692            <p>The "no-cache" response directive's meaning has been clarified. (<a href="#cache-response-directive.no-cache" title="no-cache">Section&nbsp;5.2.2.2</a>)
2693            </p>
2694         </div>
2695         <div id="rfc.section.A.p.13">
2696            <p>The one-year limit on <a href="#header.expires" class="smpl">Expires</a> header field values has been removed; instead, the reasoning for using a sensible
2697               value is given. (<a href="#header.expires" id="rfc.xref.header.expires.5" title="Expires">Section&nbsp;5.3</a>)
2698            </p>
2699         </div>
2700         <div id="rfc.section.A.p.14">
2701            <p>The <a href="#header.pragma" class="smpl">Pragma</a> header field is now only defined for backwards compatibility; future pragmas are deprecated.
2702               (<a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;5.4</a>)
2703            </p>
2704         </div>
2705         <div id="rfc.section.A.p.15">
2706            <p>Some requirements regarding production and processing of the <a href="#header.warning" class="smpl">Warning</a> header fields have been relaxed, as it is not widely implemented. Furthermore, the <a href="#header.warning" class="smpl">Warning</a> header field no longer uses RFC 2047 encoding, nor does it allow multiple languages,
2707               as these aspects were not implemented. (<a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;5.5</a>)
2708            </p>
2709         </div>
2710         <div id="rfc.section.A.p.16">
2711            <p>This specification introduces the Cache Directive and Warn Code Registries, and defines
2712               considerations for new cache directives. (<a href="#cache.directive.registry" title="Cache Directive Registry">Section&nbsp;7.1</a> and <a href="#warn.code.registry" title="Warn Code Registry">Section&nbsp;7.2</a>)
2713            </p>
2714         </div>
2715      </div>
2716      <div id="imported.abnf">
2717         <h2 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a href="#imported.abnf">Imported ABNF</a></h2>
2718         <div id="rfc.section.B.p.1">
2719            <p>The following core rules are included by reference, as defined in <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">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
2720               0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET
2721               (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
2722            </p>
2723         </div>
2724         <div id="rfc.section.B.p.2" class="avoidbreakafter">
2725            <p>The rules below are defined in <a href="#RFC7230" id="rfc.xref.RFC7230.14"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>:
2726            </p>
2727         </div>
2728         <div id="rfc.figure.u.15"><pre class="inline">  <a href="#imported.abnf" class="smpl">OWS</a>           = &lt;OWS, see <a href="#RFC7230" id="rfc.xref.RFC7230.15"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.3</a>&gt;
2729  <a href="#imported.abnf" class="smpl">field-name</a>    = &lt;field-name, see <a href="#RFC7230" id="rfc.xref.RFC7230.16"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a>&gt;
2730  <a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, see <a href="#RFC7230" id="rfc.xref.RFC7230.17"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#field.components" title="Field Value Components">Section 3.2.6</a>&gt;
2731  <a href="#imported.abnf" class="smpl">token</a>         = &lt;token, see <a href="#RFC7230" id="rfc.xref.RFC7230.18"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#field.components" title="Field Value Components">Section 3.2.6</a>&gt;
2732
2733  <a href="#imported.abnf" class="smpl">port</a>          = &lt;port, see <a href="#RFC7230" id="rfc.xref.RFC7230.19"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.7</a>&gt;
2734  <a href="#imported.abnf" class="smpl">pseudonym</a>     = &lt;pseudonym, see <a href="#RFC7230" id="rfc.xref.RFC7230.20"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#header.via" title="Via">Section 5.7.1</a>&gt; 
2735  <a href="#imported.abnf" class="smpl">uri-host</a>      = &lt;uri-host, see <a href="#RFC7230" id="rfc.xref.RFC7230.21"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.7</a>&gt;
2736</pre></div>
2737         <div id="rfc.section.B.p.3" class="avoidbreakafter">
2738            <p>The rules below are defined in other parts:</p>
2739         </div>
2740         <div id="rfc.figure.u.16"><pre class="inline">  <a href="#imported.abnf" class="smpl">HTTP-date</a>     = &lt;HTTP-date, see <a href="#RFC7231" id="rfc.xref.RFC7231.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 7.1.1.1</a>&gt;
2741</pre></div>
2742      </div>
2743      <div id="collected.abnf">
2744         <h2 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a href="#collected.abnf">Collected ABNF</a></h2>
2745         <div id="rfc.section.C.p.1">
2746            <p>In the collected ABNF below, list rules are expanded as per <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.22"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>.
2747            </p>
2748         </div>
2749         <div id="rfc.figure.u.17"><pre class="inline"><a href="#header.age" class="smpl">Age</a> = delta-seconds
2750
2751<a href="#header.cache-control" class="smpl">Cache-Control</a> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2752 cache-directive ] )
2753
2754<a href="#header.expires" class="smpl">Expires</a> = HTTP-date
2755
2756<a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, see [RFC7231], Section 7.1.1.1&gt;
2757
2758<a href="#imported.abnf" class="smpl">OWS</a> = &lt;OWS, see [RFC7230], Section 3.2.3&gt;
2759
2760<a href="#header.pragma" class="smpl">Pragma</a> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2761 pragma-directive ] )
2762
2763<a href="#header.warning" class="smpl">Warning</a> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2764 )
2765
2766<a href="#header.cache-control" class="smpl">cache-directive</a> = token [ "=" ( token / quoted-string ) ]
2767
2768<a href="#delta-seconds" class="smpl">delta-seconds</a> = 1*DIGIT
2769
2770<a href="#header.pragma" class="smpl">extension-pragma</a> = token [ "=" ( token / quoted-string ) ]
2771
2772<a href="#imported.abnf" class="smpl">field-name</a> = &lt;field-name, see [RFC7230], Section 3.2&gt;
2773
2774<a href="#imported.abnf" class="smpl">port</a> = &lt;port, see [RFC7230], Section 2.7&gt;
2775<a href="#header.pragma" class="smpl">pragma-directive</a> = "no-cache" / extension-pragma
2776<a href="#imported.abnf" class="smpl">pseudonym</a> = &lt;pseudonym, see [RFC7230], Section 5.7.1&gt;
2777
2778<a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, see [RFC7230], Section 3.2.6&gt;
2779
2780<a href="#imported.abnf" class="smpl">token</a> = &lt;token, see [RFC7230], Section 3.2.6&gt;
2781
2782<a href="#imported.abnf" class="smpl">uri-host</a> = &lt;uri-host, see [RFC7230], Section 2.7&gt;
2783
2784<a href="#header.warning" class="smpl">warn-agent</a> = ( uri-host [ ":" port ] ) / pseudonym
2785<a href="#header.warning" class="smpl">warn-code</a> = 3DIGIT
2786<a href="#header.warning" class="smpl">warn-date</a> = DQUOTE HTTP-date DQUOTE
2787<a href="#header.warning" class="smpl">warn-text</a> = quoted-string
2788<a href="#header.warning" class="smpl">warning-value</a> = warn-code SP warn-agent SP warn-text [ SP warn-date
2789 ]
2790</pre></div>
2791      </div>
2792      <h2 id="rfc.index"><a href="#rfc.index">Index</a></h2>
2793      <p class="noprint"><a href="#rfc.index.1">1</a> <a href="#rfc.index.2">2</a> <a href="#rfc.index.A">A</a> <a href="#rfc.index.B">B</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.D">D</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.T">T</a> <a href="#rfc.index.V">V</a> <a href="#rfc.index.W">W</a> 
2794      </p>
2795      <div class="print2col">
2796         <ul class="ind">
2797            <li><a id="rfc.index.1" href="#rfc.index.1"><b>1</b></a><ul>
2798                  <li>110 (warn-code)&nbsp;&nbsp;<a href="#rfc.xref.warn.110.1">4.2.4</a>, <a href="#rfc.section.5.5.1"><b>5.5.1</b></a>, <a href="#rfc.xref.warn.110.2">7.2.2</a></li>
2799                  <li>111 (warn-code)&nbsp;&nbsp;<a href="#rfc.section.5.5.2"><b>5.5.2</b></a>, <a href="#rfc.xref.warn.111.1">7.2.2</a></li>
2800                  <li>112 (warn-code)&nbsp;&nbsp;<a href="#rfc.xref.warn.112.1">4.2.4</a>, <a href="#rfc.section.5.5.3"><b>5.5.3</b></a>, <a href="#rfc.xref.warn.112.2">7.2.2</a></li>
2801                  <li>113 (warn-code)&nbsp;&nbsp;<a href="#rfc.xref.warn.113.1">4.2.2</a>, <a href="#rfc.section.5.5.4"><b>5.5.4</b></a>, <a href="#rfc.xref.warn.113.2">7.2.2</a></li>
2802                  <li>199 (warn-code)&nbsp;&nbsp;<a href="#rfc.section.5.5.5"><b>5.5.5</b></a>, <a href="#rfc.xref.warn.199.1">7.2.2</a></li>
2803               </ul>
2804            </li>
2805            <li><a id="rfc.index.2" href="#rfc.index.2"><b>2</b></a><ul>
2806                  <li>214 (warn-code)&nbsp;&nbsp;<a href="#rfc.section.5.5.6"><b>5.5.6</b></a>, <a href="#rfc.xref.warn.214.1">7.2.2</a></li>
2807                  <li>299 (warn-code)&nbsp;&nbsp;<a href="#rfc.section.5.5.7"><b>5.5.7</b></a>, <a href="#rfc.xref.warn.299.1">7.2.2</a></li>
2808               </ul>
2809            </li>
2810            <li><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul>
2811                  <li>age&nbsp;&nbsp;<a href="#rfc.iref.a.1">4.2</a></li>
2812                  <li>Age header field&nbsp;&nbsp;<a href="#rfc.xref.header.age.1">4</a>, <a href="#rfc.xref.header.age.2">4.2.3</a>, <a href="#rfc.section.5.1"><b>5.1</b></a>, <a href="#rfc.xref.header.age.3">7.3</a></li>
2813               </ul>
2814            </li>
2815            <li><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul>
2816                  <li><em>BCP90</em>&nbsp;&nbsp;<a href="#rfc.xref.BCP90.1">7.3</a>, <a href="#BCP90"><b>10.2</b></a></li>
2817               </ul>
2818            </li>
2819            <li><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul>
2820                  <li>cache&nbsp;&nbsp;<a href="#rfc.iref.c.1">1</a></li>
2821                  <li>cache entry&nbsp;&nbsp;<a href="#rfc.section.2">2</a></li>
2822                  <li>cache key&nbsp;&nbsp;<a href="#rfc.section.2">2</a>, <a href="#rfc.iref.c.2">2</a></li>
2823                  <li>Cache-Control header field&nbsp;&nbsp;<a href="#rfc.xref.header.cache-control.1">3</a>, <a href="#rfc.section.5.2"><b>5.2</b></a>, <a href="#rfc.xref.header.cache-control.2">7.3</a>, <a href="#rfc.xref.header.cache-control.3">A</a></li>
2824               </ul>
2825            </li>
2826            <li><a id="rfc.index.D" href="#rfc.index.D"><b>D</b></a><ul>
2827                  <li>Disconnected Operation (warn-text)&nbsp;&nbsp;<a href="#rfc.xref.warn.112.1">4.2.4</a>, <a href="#rfc.section.5.5.3"><b>5.5.3</b></a>, <a href="#rfc.xref.warn.112.2">7.2.2</a></li>
2828               </ul>
2829            </li>
2830            <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
2831                  <li>Expires header field&nbsp;&nbsp;<a href="#rfc.xref.header.expires.1">3</a>, <a href="#rfc.xref.header.expires.2">4.2</a>, <a href="#rfc.xref.header.expires.3">4.2.1</a>, <a href="#rfc.section.5.3"><b>5.3</b></a>, <a href="#rfc.xref.header.expires.4">7.3</a>, <a href="#rfc.xref.header.expires.5">A</a></li>
2832                  <li>explicit expiration time&nbsp;&nbsp;<a href="#rfc.iref.e.1">4.2</a></li>
2833               </ul>
2834            </li>
2835            <li><a id="rfc.index.F" href="#rfc.index.F"><b>F</b></a><ul>
2836                  <li>fresh&nbsp;&nbsp;<a href="#rfc.section.4.2">4.2</a></li>
2837                  <li>freshness lifetime&nbsp;&nbsp;<a href="#rfc.iref.f.1">4.2</a></li>
2838               </ul>
2839            </li>
2840            <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
2841                  <li><tt>Grammar</tt>&nbsp;&nbsp;
2842                     <ul>
2843                        <li><tt>Age</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>5.1</b></a></li>
2844                        <li><tt>Cache-Control</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>5.2</b></a></li>
2845                        <li><tt>cache-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>5.2</b></a></li>
2846                        <li><tt>delta-seconds</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>1.2.1</b></a></li>
2847                        <li><tt>Expires</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>5.3</b></a></li>
2848                        <li><tt>extension-pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>5.4</b></a></li>
2849                        <li><tt>Pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>5.4</b></a></li>
2850                        <li><tt>pragma-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>5.4</b></a></li>
2851                        <li><tt>warn-agent</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.12"><b>5.5</b></a></li>
2852                        <li><tt>warn-code</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.11"><b>5.5</b></a></li>
2853                        <li><tt>warn-date</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.14"><b>5.5</b></a></li>
2854                        <li><tt>warn-text</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.13"><b>5.5</b></a></li>
2855                        <li><tt>Warning</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>5.5</b></a></li>
2856                        <li><tt>warning-value</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>5.5</b></a></li>
2857                     </ul>
2858                  </li>
2859               </ul>
2860            </li>
2861            <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
2862                  <li>Heuristic Expiration (warn-text)&nbsp;&nbsp;<a href="#rfc.xref.warn.113.1">4.2.2</a>, <a href="#rfc.section.5.5.4"><b>5.5.4</b></a>, <a href="#rfc.xref.warn.113.2">7.2.2</a></li>
2863                  <li>heuristic expiration time&nbsp;&nbsp;<a href="#rfc.iref.h.1">4.2</a></li>
2864               </ul>
2865            </li>
2866            <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
2867                  <li>max-age (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.1.1"><b>5.2.1.1</b></a>, <a href="#rfc.section.5.2.2.8"><b>5.2.2.8</b></a></li>
2868                  <li>max-stale (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.1.2"><b>5.2.1.2</b></a></li>
2869                  <li>min-fresh (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.1.3"><b>5.2.1.3</b></a></li>
2870                  <li>Miscellaneous Persistent Warning (warn-text)&nbsp;&nbsp;<a href="#rfc.section.5.5.7"><b>5.5.7</b></a>, <a href="#rfc.xref.warn.299.1">7.2.2</a></li>
2871                  <li>Miscellaneous Warning (warn-text)&nbsp;&nbsp;<a href="#rfc.section.5.5.5"><b>5.5.5</b></a>, <a href="#rfc.xref.warn.199.1">7.2.2</a></li>
2872                  <li>must-revalidate (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.2.1"><b>5.2.2.1</b></a></li>
2873               </ul>
2874            </li>
2875            <li><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul>
2876                  <li>no-cache (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.1.4"><b>5.2.1.4</b></a>, <a href="#rfc.section.5.2.2.2"><b>5.2.2.2</b></a></li>
2877                  <li>no-store (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.1.5"><b>5.2.1.5</b></a>, <a href="#rfc.section.5.2.2.3"><b>5.2.2.3</b></a></li>
2878                  <li>no-transform (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.1.6"><b>5.2.1.6</b></a>, <a href="#rfc.section.5.2.2.4"><b>5.2.2.4</b></a></li>
2879               </ul>
2880            </li>
2881            <li><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul>
2882                  <li>only-if-cached (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.1.7"><b>5.2.1.7</b></a></li>
2883               </ul>
2884            </li>
2885            <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
2886                  <li>Pragma header field&nbsp;&nbsp;<a href="#rfc.xref.header.pragma.1">4</a>, <a href="#rfc.section.5.4"><b>5.4</b></a>, <a href="#rfc.xref.header.pragma.2">7.3</a>, <a href="#rfc.xref.header.pragma.3">A</a></li>
2887                  <li>private (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.2.6"><b>5.2.2.6</b></a></li>
2888                  <li>private cache&nbsp;&nbsp;<a href="#rfc.iref.p.1">1</a></li>
2889                  <li>proxy-revalidate (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.2.7"><b>5.2.2.7</b></a></li>
2890                  <li>public (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.2.5"><b>5.2.2.5</b></a></li>
2891               </ul>
2892            </li>
2893            <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
2894                  <li>Response is Stale (warn-text)&nbsp;&nbsp;<a href="#rfc.xref.warn.110.1">4.2.4</a>, <a href="#rfc.section.5.5.1"><b>5.5.1</b></a>, <a href="#rfc.xref.warn.110.2">7.2.2</a></li>
2895                  <li>Revalidation Failed (warn-text)&nbsp;&nbsp;<a href="#rfc.section.5.5.2"><b>5.5.2</b></a>, <a href="#rfc.xref.warn.111.1">7.2.2</a></li>
2896                  <li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>10.1</b></a></li>
2897                  <li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4.2.2</a>, <a href="#RFC2616"><b>10.2</b></a><ul>
2898                        <li><em>Section 13.9</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4.2.2</a></li>
2899                     </ul>
2900                  </li>
2901                  <li><em>RFC5226</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">7.1.1</a>, <a href="#rfc.xref.RFC5226.2">7.2.1</a>, <a href="#RFC5226"><b>10.2</b></a><ul>
2902                        <li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">7.1.1</a>, <a href="#rfc.xref.RFC5226.2">7.2.1</a></li>
2903                     </ul>
2904                  </li>
2905                  <li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.2</a>, <a href="#RFC5234"><b>10.1</b></a>, <a href="#rfc.xref.RFC5234.2">B</a><ul>
2906                        <li><em>Appendix B.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.2">B</a></li>
2907                     </ul>
2908                  </li>
2909                  <li><em>RFC5861</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">7.1.3</a>, <a href="#rfc.xref.RFC5861.2">7.1.3</a>, <a href="#RFC5861"><b>10.2</b></a><ul>
2910                        <li><em>Section 3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.2">7.1.3</a></li>
2911                        <li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">7.1.3</a></li>
2912                     </ul>
2913                  </li>
2914                  <li><em>RFC5905</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5905.1">4.2.3</a>, <a href="#RFC5905"><b>10.2</b></a></li>
2915                  <li><em>RFC6265</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC6265.1">8</a>, <a href="#RFC6265"><b>10.2</b></a></li>
2916                  <li><em>RFC7230</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.1">1.1</a>, <a href="#rfc.xref.RFC7230.2">1.2</a>, <a href="#rfc.xref.RFC7230.3">3.1</a>, <a href="#rfc.xref.RFC7230.4">4</a>, <a href="#rfc.xref.RFC7230.5">4.1</a>, <a href="#rfc.xref.RFC7230.6">4.4</a>, <a href="#rfc.xref.RFC7230.7">4.4</a>, <a href="#rfc.xref.RFC7230.8">4.4</a>, <a href="#rfc.xref.RFC7230.9">5.2.1.6</a>, <a href="#rfc.xref.RFC7230.10">5.2.2.4</a>, <a href="#rfc.xref.RFC7230.11">8</a>, <a href="#rfc.xref.RFC7230.12">8</a>, <a href="#rfc.xref.RFC7230.13">9</a>, <a href="#RFC7230"><b>10.1</b></a>, <a href="#rfc.xref.RFC7230.14">B</a>, <a href="#rfc.xref.RFC7230.15">B</a>, <a href="#rfc.xref.RFC7230.16">B</a>, <a href="#rfc.xref.RFC7230.17">B</a>, <a href="#rfc.xref.RFC7230.18">B</a>, <a href="#rfc.xref.RFC7230.19">B</a>, <a href="#rfc.xref.RFC7230.20">B</a>, <a href="#rfc.xref.RFC7230.21">B</a>, <a href="#rfc.xref.RFC7230.22">C</a><ul>
2917                        <li><em>Section 1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.22">C</a></li>
2918                        <li><em>Section 2.5</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.1">1.1</a></li>
2919                        <li><em>Section 2.7</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.19">B</a>, <a href="#rfc.xref.RFC7230.21">B</a></li>
2920                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.5">4.1</a>, <a href="#rfc.xref.RFC7230.16">B</a></li>
2921                        <li><em>Section 3.2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.15">B</a></li>
2922                        <li><em>Section 3.2.6</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.17">B</a>, <a href="#rfc.xref.RFC7230.18">B</a></li>
2923                        <li><em>Section 3.3.3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.12">8</a></li>
2924                        <li><em>Section 5.5</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.4">4</a>, <a href="#rfc.xref.RFC7230.6">4.4</a>, <a href="#rfc.xref.RFC7230.7">4.4</a>, <a href="#rfc.xref.RFC7230.8">4.4</a></li>
2925                        <li><em>Section 5.7.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.20">B</a></li>
2926                        <li><em>Section 5.7.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.9">5.2.1.6</a>, <a href="#rfc.xref.RFC7230.10">5.2.2.4</a></li>
2927                        <li><em>Section 7</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.2">1.2</a></li>
2928                        <li><em>Section 10</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7230.13">9</a></li>
2929                     </ul>
2930                  </li>
2931                  <li><em>RFC7231</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7231.1">2</a>, <a href="#rfc.xref.RFC7231.2">2</a>, <a href="#rfc.xref.RFC7231.3">4</a>, <a href="#rfc.xref.RFC7231.4">4.1</a>, <a href="#rfc.xref.RFC7231.5">4.2.2</a>, <a href="#rfc.xref.RFC7231.6">4.2.3</a>, <a href="#rfc.xref.RFC7231.7">4.4</a>, <a href="#rfc.xref.RFC7231.8">5.3</a>, <a href="#rfc.xref.RFC7231.9">8</a>, <a href="#RFC7231"><b>10.1</b></a>, <a href="#rfc.xref.RFC7231.10">B</a><ul>
2932                        <li><em>Section 4.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7231.3">4</a>, <a href="#rfc.xref.RFC7231.7">4.4</a></li>
2933                        <li><em>Section 4.3.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7231.2">2</a></li>
2934                        <li><em>Section 6.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7231.5">4.2.2</a></li>
2935                        <li><em>Section 7.1.1.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7231.8">5.3</a>, <a href="#rfc.xref.RFC7231.10">B</a></li>
2936                        <li><em>Section 7.1.1.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7231.6">4.2.3</a></li>
2937                        <li><em>Section 7.1.4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7231.4">4.1</a></li>
2938                     </ul>
2939                  </li>
2940                  <li><em>RFC7232</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7232.1">4.2.2</a>, <a href="#rfc.xref.RFC7232.2">4.3</a>, <a href="#rfc.xref.RFC7232.3">4.3.1</a>, <a href="#rfc.xref.RFC7232.4">4.3.1</a>, <a href="#rfc.xref.RFC7232.5">4.3.2</a>, <a href="#rfc.xref.RFC7232.6">4.3.2</a>, <a href="#rfc.xref.RFC7232.7">4.3.2</a>, <a href="#rfc.xref.RFC7232.8">4.3.4</a>, <a href="#RFC7232"><b>10.1</b></a><ul>
2941                        <li><em>Section 2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7232.8">4.3.4</a></li>
2942                        <li><em>Section 2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7232.1">4.2.2</a>, <a href="#rfc.xref.RFC7232.3">4.3.1</a></li>
2943                        <li><em>Section 2.3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7232.4">4.3.1</a></li>
2944                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7232.6">4.3.2</a></li>
2945                        <li><em>Section 3.3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7232.7">4.3.2</a></li>
2946                        <li><em>Section 6</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7232.5">4.3.2</a></li>
2947                     </ul>
2948                  </li>
2949                  <li><em>RFC7233</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7233.1">3.1</a>, <a href="#rfc.xref.RFC7233.2">3.3</a>, <a href="#rfc.xref.RFC7233.3">3.3</a>, <a href="#rfc.xref.RFC7233.4">4.3.2</a>, <a href="#rfc.xref.RFC7233.5">4.3.2</a>, <a href="#RFC7233"><b>10.1</b></a><ul>
2950                        <li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7233.5">4.3.2</a></li>
2951                        <li><em>Section 4.3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7233.3">3.3</a></li>
2952                     </ul>
2953                  </li>
2954                  <li><em>RFC7235</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7235.1">3</a>, <a href="#rfc.xref.RFC7235.2">3.2</a>, <a href="#RFC7235"><b>10.1</b></a><ul>
2955                        <li><em>Section 4.2</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC7235.1">3</a>, <a href="#rfc.xref.RFC7235.2">3.2</a></li>
2956                     </ul>
2957                  </li>
2958               </ul>
2959            </li>
2960            <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
2961                  <li>s-maxage (cache directive)&nbsp;&nbsp;<a href="#rfc.section.5.2.2.9"><b>5.2.2.9</b></a></li>
2962                  <li>shared cache&nbsp;&nbsp;<a href="#rfc.iref.s.1">1</a></li>
2963                  <li>stale&nbsp;&nbsp;<a href="#rfc.section.4.2">4.2</a></li>
2964                  <li>strong validator&nbsp;&nbsp;<a href="#rfc.iref.s.2">4.3.4</a></li>
2965               </ul>
2966            </li>
2967            <li><a id="rfc.index.T" href="#rfc.index.T"><b>T</b></a><ul>
2968                  <li>Transformation Applied (warn-text)&nbsp;&nbsp;<a href="#rfc.section.5.5.6"><b>5.5.6</b></a>, <a href="#rfc.xref.warn.214.1">7.2.2</a></li>
2969               </ul>
2970            </li>
2971            <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
2972                  <li>validator&nbsp;&nbsp;<a href="#rfc.section.4.3.1">4.3.1</a></li>
2973               </ul>
2974            </li>
2975            <li><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul>
2976                  <li>Warning header field&nbsp;&nbsp;<a href="#rfc.xref.header.warning.1">3.3</a>, <a href="#rfc.xref.header.warning.2">4.3.4</a>, <a href="#rfc.xref.header.warning.3">4.3.5</a>, <a href="#rfc.section.5.5"><b>5.5</b></a>, <a href="#rfc.xref.header.warning.4">7.3</a>, <a href="#rfc.xref.header.warning.5">A</a></li>
2977               </ul>
2978            </li>
2979         </ul>
2980      </div>
2981      <div class="avoidbreakinside">
2982         <h2 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h2>
2983         <p><b>Roy T. Fielding</b>
2984            (editor)
2985            <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>Email: fielding@gbiv.com<br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p>
2986         <p><b>Mark Nottingham</b>
2987            (editor)
2988            <br>Akamai<br>Email: mnot@mnot.net<br>URI: <a href="http://www.mnot.net/">http://www.mnot.net/</a></p>
2989         <p><b>Julian F. Reschke</b>
2990            (editor)
2991            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Email: julian.reschke@greenbytes.de<br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p>
2992      </div>
2993   </body>
2994</html>
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