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Timestamp:
Mar 5, 2009, 5:51:26 AM (11 years ago)
Author:
julian.reschke@…
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Move P6 from ./latest-roy to ./latest

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  • draft-ietf-httpbis/latest/p6-cache.html

    r532 r538  
    359359      <link rel="Index" href="#rfc.index">
    360360      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
    361       <link rel="Chapter" title="2 Overview" href="#rfc.section.2">
    362       <link rel="Chapter" title="3 Expiration Model" href="#rfc.section.3">
    363       <link rel="Chapter" title="4 Validation Model" href="#rfc.section.4">
    364       <link rel="Chapter" title="5 Response Cacheability" href="#rfc.section.5">
    365       <link rel="Chapter" title="6 Constructing Responses From Caches" href="#rfc.section.6">
    366       <link rel="Chapter" title="7 Caching Negotiated Responses" href="#rfc.section.7">
    367       <link rel="Chapter" title="8 Shared and Non-Shared Caches" href="#rfc.section.8">
    368       <link rel="Chapter" title="9 Errors or Incomplete Response Cache Behavior" href="#rfc.section.9">
    369       <link rel="Chapter" title="10 Side Effects of GET and HEAD" href="#rfc.section.10">
    370       <link rel="Chapter" title="11 Invalidation After Updates or Deletions" href="#rfc.section.11">
    371       <link rel="Chapter" title="12 Write-Through Mandatory" href="#rfc.section.12">
    372       <link rel="Chapter" title="13 Cache Replacement" href="#rfc.section.13">
    373       <link rel="Chapter" title="14 History Lists" href="#rfc.section.14">
    374       <link rel="Chapter" title="15 Header Field Definitions" href="#rfc.section.15">
    375       <link rel="Chapter" title="16 IANA Considerations" href="#rfc.section.16">
    376       <link rel="Chapter" title="17 Security Considerations" href="#rfc.section.17">
    377       <link rel="Chapter" title="18 Acknowledgments" href="#rfc.section.18">
    378       <link rel="Chapter" href="#rfc.section.19" title="19 References">
     361      <link rel="Chapter" title="2 Cache Operation" href="#rfc.section.2">
     362      <link rel="Chapter" title="3 Header Field Definitions" href="#rfc.section.3">
     363      <link rel="Chapter" title="4 History Lists" href="#rfc.section.4">
     364      <link rel="Chapter" title="5 IANA Considerations" href="#rfc.section.5">
     365      <link rel="Chapter" title="6 Security Considerations" href="#rfc.section.6">
     366      <link rel="Chapter" title="7 Acknowledgments" href="#rfc.section.7">
     367      <link rel="Chapter" href="#rfc.section.8" title="8 References">
    379368      <link rel="Appendix" title="A Compatibility with Previous Versions" href="#rfc.section.A">
    380369      <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B">
     
    394383      <meta name="DC.Date.Issued" scheme="ISO8601" content="2009-03">
    395384      <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2616">
    396       <meta name="DC.Description.Abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 6 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
     385      <meta name="DC.Description.Abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
    397386   </head>
    398387   <body>
     
    508497      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    509498      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
    510          systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 6 of the
    511          seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part
    512          6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response
    513          messages.
     499         systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and,
     500         taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control
     501         cache behavior or indicate cacheable response messages.
    514502      </p>
    515503      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
     
    533521            </ul>
    534522         </li>
    535          <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview</a><ul class="toc">
    536                <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#cache.correctness">Cache Correctness</a></li>
    537                <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#warnings">Warnings</a></li>
    538                <li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></li>
    539                <li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></li>
    540                <li class="tocline1">2.5&nbsp;&nbsp;&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></li>
    541                <li class="tocline1">2.6&nbsp;&nbsp;&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></li>
    542             </ul>
    543          </li>
    544          <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Expiration Model</a><ul class="toc">
    545                <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></li>
    546                <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></li>
    547                <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Age Calculations</a></li>
    548                <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></li>
    549                <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></li>
    550                <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></li>
    551             </ul>
    552          </li>
    553          <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a></li>
    554          <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li>
    555          <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a><ul class="toc">
    556                <li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></li>
    557                <li class="tocline1">6.2&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></li>
    558                <li class="tocline1">6.3&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Headers</a></li>
    559             </ul>
    560          </li>
    561          <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
    562          <li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></li>
    563          <li class="tocline0">9.&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></li>
    564          <li class="tocline0">10.&nbsp;&nbsp;&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></li>
    565          <li class="tocline0">11.&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></li>
    566          <li class="tocline0">12.&nbsp;&nbsp;&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></li>
    567          <li class="tocline0">13.&nbsp;&nbsp;&nbsp;<a href="#cache.replacement">Cache Replacement</a></li>
    568          <li class="tocline0">14.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
    569          <li class="tocline0">15.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
    570                <li class="tocline1">15.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
    571                <li class="tocline1">15.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
    572                      <li class="tocline1">15.2.1&nbsp;&nbsp;&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></li>
    573                      <li class="tocline1">15.2.2&nbsp;&nbsp;&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></li>
    574                      <li class="tocline1">15.2.3&nbsp;&nbsp;&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></li>
    575                      <li class="tocline1">15.2.4&nbsp;&nbsp;&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></li>
    576                      <li class="tocline1">15.2.5&nbsp;&nbsp;&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></li>
    577                      <li class="tocline1">15.2.6&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
     523         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Cache Operation</a><ul class="toc">
     524               <li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a><ul class="toc">
     525                     <li class="tocline1">2.1.1&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Storing Partial and Incomplete Responses</a></li>
    578526                  </ul>
    579527               </li>
    580                <li class="tocline1">15.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
    581                <li class="tocline1">15.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
    582                <li class="tocline1">15.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
    583                <li class="tocline1">15.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
     528               <li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></li>
     529               <li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness Model</a><ul class="toc">
     530                     <li class="tocline1">2.3.1&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a><ul class="toc">
     531                           <li class="tocline1">2.3.1.1&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></li>
     532                        </ul>
     533                     </li>
     534                     <li class="tocline1">2.3.2&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
     535                     <li class="tocline1">2.3.3&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
     536                  </ul>
     537               </li>
     538               <li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a></li>
     539               <li class="tocline1">2.5&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
     540               <li class="tocline1">2.6&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
     541               <li class="tocline1">2.7&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Responses</a></li>
    584542            </ul>
    585543         </li>
    586          <li class="tocline0">16.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul class="toc">
    587                <li class="tocline1">16.1&nbsp;&nbsp;&nbsp;<a href="#message.header.registration">Message Header Registration</a></li>
     544         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
     545               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
     546               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
     547                     <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></li>
     548                     <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></li>
     549                     <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
     550                  </ul>
     551               </li>
     552               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
     553               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
     554               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
     555               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
    588556            </ul>
    589557         </li>
    590          <li class="tocline0">17.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
    591          <li class="tocline0">18.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
    592          <li class="tocline0">19.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc">
    593                <li class="tocline1">19.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
    594                <li class="tocline1">19.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
     558         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
     559         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul class="toc">
     560               <li class="tocline1">5.1&nbsp;&nbsp;&nbsp;<a href="#message.header.registration">Message Header Registration</a></li>
     561            </ul>
     562         </li>
     563         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
     564         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
     565         <li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc">
     566               <li class="tocline1">8.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
     567               <li class="tocline1">8.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
    595568            </ul>
    596569         </li>
     
    615588      </ul>
    616589      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="caching" href="#caching">Introduction</a></h1>
    617       <p id="rfc.section.1.p.1">HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches,
    618          and includes a number of elements intended to make caching work as well as possible. Because these elements interact with
    619          each other, it is useful to describe the caching design of HTTP separately. This document defines aspects of HTTP/1.1 related
    620          to caching and reusing response messages.
     590      <p id="rfc.section.1.p.1">HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches.
     591         This document defines aspects of HTTP/1.1 related to caching and reusing response messages.
    621592      </p>
    622593      <div id="rfc.iref.c.1"></div>
     
    627598      </p>
    628599      <p id="rfc.section.1.1.p.2">Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to reuse a prior
    629          response message to satisfy a current request. In some cases, the existing response can be reused without the need for a network
    630          request, reducing latency and network round-trips; we use an "expiration" mechanism for this purpose (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a>). Even when a new request is required, it is often possible to reuse all or parts of the payload of a prior response to satisfy
    631          the request, thereby reducing network bandwidth usage; we use a "validation" mechanism for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;4</a>).
    632       </p>
    633       <div id="rfc.iref.s.1"></div>
    634       <p id="rfc.section.1.1.p.3">A cache behaves in a "<dfn>semantically transparent</dfn>" manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server,
    635          except to improve performance. When a cache is semantically transparent, the client receives exactly the same response status
    636          and payload that it would have received had its request been handled directly by the origin server.
    637       </p>
    638       <p id="rfc.section.1.1.p.4">In an ideal world, all interactions with an HTTP cache would be semantically transparent. However, for some resources, semantic
    639          transparency is not always necessary and can be effectively traded for the sake of bandwidth scaling, disconnected operation,
    640          and high availability. HTTP/1.1 allows origin servers, caches, and clients to explicitly reduce transparency when necessary.
    641          However, because non-transparent operation may confuse non-expert users and might be incompatible with certain server applications
    642          (such as those for ordering merchandise), the protocol requires that transparency be relaxed
    643       </p>
    644       <ul>
    645          <li>only by an explicit protocol-level request when relaxed by client or origin server</li>
    646          <li>only with an explicit warning to the end user when relaxed by cache or client</li>
    647       </ul>
    648       <p id="rfc.section.1.1.p.5">Therefore, HTTP/1.1 provides these important elements: </p>
    649       <ol>
    650          <li>Protocol features that provide full semantic transparency when this is required by all parties.</li>
    651          <li>Protocol features that allow an origin server or user agent to explicitly request and control non-transparent operation.</li>
    652          <li>Protocol features that allow a cache to attach warnings to responses that do not preserve the requested approximation of semantic
    653             transparency.
    654          </li>
    655       </ol>
    656       <p id="rfc.section.1.1.p.6">A basic principle is that it must be possible for the clients to detect any potential relaxation of semantic transparency. </p>
    657       <dl class="empty">
    658          <dd> <b>Note:</b> The server, cache, or client implementor might be faced with design decisions not explicitly discussed in this specification.
    659             If a decision might affect semantic transparency, the implementor ought to err on the side of maintaining transparency unless
    660             a careful and complete analysis shows significant benefits in breaking transparency.
    661          </dd>
    662       </dl>
     600         response message to satisfy a current request. In some cases, a stored response can be reused without the need for a network
     601         request, reducing latency and network round-trips; a "freshness" mechanism is used for this purpose (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>). Even when a new request is required, it is often possible to reuse all or parts of the payload of a prior response to satisfy
     602         the request, thereby reducing network bandwidth usage; a "validation" mechanism is used for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>).
     603      </p>
    663604      <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="intro.terminology" href="#intro.terminology">Terminology</a></h2>
    664605      <p id="rfc.section.1.2.p.1">This specification uses a number of terms to refer to the roles played by participants in, and objects of, HTTP caching.</p>
     
    667608      <dl class="empty">
    668609         <dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
    669             Even when a response is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
    670             request.
    671          </dd>
    672       </dl>
    673       <p id="rfc.section.1.2.p.3"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
    674       </p>
    675       <dl class="empty">
    676          <dd>A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more
    677             proxies. A response is also first-hand if its validity has just been checked directly with the origin server.
    678          </dd>
    679       </dl>
    680       <p id="rfc.section.1.2.p.4"> <span id="rfc.iref.e.1"></span>  <dfn>explicit expiration time</dfn> 
     610            Even when a response is cacheable, there may be additional constraints on whether a cache can use the cached copy to satisfy
     611            a particular request.
     612         </dd>
     613      </dl>
     614      <p id="rfc.section.1.2.p.3"> <span id="rfc.iref.e.1"></span>  <dfn>explicit expiration time</dfn> 
    681615      </p>
    682616      <dl class="empty">
    683617         <dd>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</dd>
    684618      </dl>
    685       <p id="rfc.section.1.2.p.5"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
     619      <p id="rfc.section.1.2.p.4"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
    686620      </p>
    687621      <dl class="empty">
    688622         <dd>An expiration time assigned by a cache when no explicit expiration time is available.</dd>
    689623      </dl>
    690       <p id="rfc.section.1.2.p.6"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
     624      <p id="rfc.section.1.2.p.5"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
    691625      </p>
    692626      <dl class="empty">
    693627         <dd>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</dd>
    694628      </dl>
     629      <p id="rfc.section.1.2.p.6"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
     630      </p>
     631      <dl class="empty">
     632         <dd>A response is first-hand if the freshness model is not in use; i.e., its age is 0.</dd>
     633      </dl>
    695634      <p id="rfc.section.1.2.p.7"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn> 
    696635      </p>
     
    703642         <dd>A response is fresh if its age has not yet exceeded its freshness lifetime.</dd>
    704643      </dl>
    705       <p id="rfc.section.1.2.p.9"> <span id="rfc.iref.s.2"></span>  <dfn>stale</dfn> 
    706       </p>
    707       <dl class="empty">
    708          <dd>A response is stale if its age has passed its freshness lifetime.</dd>
     644      <p id="rfc.section.1.2.p.9"> <span id="rfc.iref.s.1"></span>  <dfn>stale</dfn> 
     645      </p>
     646      <dl class="empty">
     647         <dd>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</dd>
    709648      </dl>
    710649      <p id="rfc.section.1.2.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>validator</dfn> 
    711650      </p>
    712651      <dl class="empty">
    713          <dd>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent
    714             copy of an entity.
    715          </dd>
    716       </dl>
     652         <dd>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a stored response is an
     653            equivalent copy of an entity.
     654         </dd>
     655      </dl>
     656      <div id="shared.and.non-shared.caches">
     657         <p id="rfc.section.1.2.p.11"> <span id="rfc.iref.v.2"></span>  <dfn>shared cache</dfn> 
     658         </p>
     659         <dl class="empty">
     660            <dd>A cache that is accessible to more than one user. A non-shared cache is dedicated to a single user.</dd>
     661         </dl>
     662      </div>
    717663      <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a id="intro.requirements" href="#intro.requirements">Requirements</a></h2>
    718664      <p id="rfc.section.1.3.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
     
    739685  <a href="#abnf.dependencies" class="smpl">pseudonym</a>     = &lt;pseudonym, defined in <a href="#Part1" id="rfc.xref.Part1.9"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#header.via" title="Via">Section 8.9</a>&gt;
    740686  <a href="#abnf.dependencies" class="smpl">uri-host</a>      = &lt;uri-host, defined in <a href="#Part1" id="rfc.xref.Part1.10"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.1</a>&gt;
    741 </pre><h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Overview</a></h1>
    742       <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="cache.correctness" href="#cache.correctness">Cache Correctness</a></h2>
    743       <p id="rfc.section.2.1.p.1">A correct cache <em class="bcp14">MUST</em> respond to a request with the most up-to-date response held by the cache that is appropriate to the request (see Sections <a href="#disambiguating.expiration.values" title="Disambiguating Expiration Values">3.5</a>, <a href="#disambiguating.multiple.responses" title="Disambiguating Multiple Responses">3.6</a>, and <a href="#cache.replacement" title="Cache Replacement">13</a>) which meets one of the following conditions:
    744       </p>
    745       <ol>
    746          <li>It has been checked for equivalence with what the origin server would have returned by revalidating the response with the
    747             origin server (<a href="#validation.model" title="Validation Model">Section&nbsp;4</a>);
    748          </li>
    749          <li>It is "fresh enough" (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a>). In the default case, this means it meets the least restrictive freshness requirement of the client, origin server, and
    750             cache (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;15.2</a>); if the origin server so specifies, it is the freshness requirement of the origin server alone. If a stored response is
    751             not "fresh enough" by the most restrictive freshness requirement of both the client and the origin server, in carefully considered
    752             circumstances the cache <em class="bcp14">MAY</em> still return the response with the appropriate Warning header (see Sections <a href="#exceptions.to.the.rules.and.warnings" title="Exceptions to the Rules and Warnings">2.5</a> and <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">15.6</a>), unless such a response is prohibited (e.g., by a "no-store" cache-directive, or by a "no-cache" cache-request-directive;
    753             see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;15.2</a>).
    754          </li>
    755          <li>It is an appropriate 304 (Not Modified), 305 (Use Proxy), or error (4xx or 5xx) response message.</li>
    756       </ol>
    757       <p id="rfc.section.2.1.p.2">If the cache can not communicate with the origin server, then a correct cache <em class="bcp14">SHOULD</em> respond as above if the response can be correctly served from the cache; if not it <em class="bcp14">MUST</em> return an error or warning indicating that there was a communication failure.
    758       </p>
    759       <p id="rfc.section.2.1.p.3">If a cache receives a response (either an entire response, or a 304 (Not Modified) response) that it would normally forward
    760          to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">SHOULD</em> forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache <em class="bcp14">SHOULD NOT</em> attempt to revalidate a response simply because that response became stale in transit; this might lead to an infinite loop.
    761          A user agent that receives a stale response without a Warning <em class="bcp14">MAY</em> display a warning indication to the user.
    762       </p>
    763       <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="warnings" href="#warnings">Warnings</a></h2>
    764       <p id="rfc.section.2.2.p.1">Whenever a cache returns a response that is neither first-hand nor "fresh enough" (in the sense of condition 2 in <a href="#cache.correctness" title="Cache Correctness">Section&nbsp;2.1</a>), it <em class="bcp14">MUST</em> attach a warning to that effect, using a Warning general-header. The Warning header and the currently defined warnings are
    765          described in <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;15.6</a>. The warning allows clients to take appropriate action.
    766       </p>
    767       <p id="rfc.section.2.2.p.2">Warnings <em class="bcp14">MAY</em> be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish
    768          these responses from true failures.
    769       </p>
    770       <p id="rfc.section.2.2.p.3">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning <em class="bcp14">MUST</em> or <em class="bcp14">MUST NOT</em> be deleted from a stored cache entry after a successful revalidation:
    771       </p>
    772       <p id="rfc.section.2.2.p.4"> </p>
    773       <dl>
    774          <dt>1xx</dt>
    775          <dd>Warnings that describe the freshness or revalidation status of the response, and so <em class="bcp14">MUST</em> be deleted after a successful revalidation. 1xx warn-codes <em class="bcp14">MAY</em> be generated by a cache only when validating a cached entry. It <em class="bcp14">MUST NOT</em> be generated by clients.
    776          </dd>
    777          <dt>2xx</dt>
    778          <dd>Warnings that describe some aspect of the entity body or entity headers that is not rectified by a revalidation (for example,
    779             a lossy compression of the entity bodies) and which <em class="bcp14">MUST NOT</em> be deleted after a successful revalidation.
    780          </dd>
    781       </dl>
    782       <p id="rfc.section.2.2.p.5">See <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;15.6</a> for the definitions of the codes themselves.
    783       </p>
    784       <p id="rfc.section.2.2.p.6">HTTP/1.0 caches will cache all Warnings in responses, without deleting the ones in the first category. Warnings in responses
    785          that are passed to HTTP/1.0 caches carry an extra warning-date field, which prevents a future HTTP/1.1 recipient from believing
    786          an erroneously cached Warning.
    787       </p>
    788       <p id="rfc.section.2.2.p.7">Warnings also carry a warning text. The text <em class="bcp14">MAY</em> be in any appropriate natural language (perhaps based on the client's Accept headers), and include an <em class="bcp14">OPTIONAL</em> indication of what character set is used.
    789       </p>
    790       <p id="rfc.section.2.2.p.8">Multiple warnings <em class="bcp14">MAY</em> be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number.
    791          For example, a server might provide the same warning with texts in both English and Basque.
    792       </p>
    793       <p id="rfc.section.2.2.p.9">When multiple warnings are attached to a response, it might not be practical or reasonable to display all of them to the user.
    794          This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but
    795          does suggest some heuristics.
    796       </p>
    797       <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="cache-control.mechanisms" href="#cache-control.mechanisms">Cache-control Mechanisms</a></h2>
    798       <p id="rfc.section.2.3.p.1">The basic cache mechanisms in HTTP/1.1 (server-specified expiration times and validators) are implicit directives to caches.
    799          In some cases, a server or client might need to provide explicit directives to the HTTP caches. We use the Cache-Control header
    800          for this purpose.
    801       </p>
    802       <p id="rfc.section.2.3.p.2">The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These
    803          directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between
    804          header values, the most restrictive interpretation is applied (that is, the one that is most likely to preserve semantic transparency).
    805          However, in some cases, cache-control directives are explicitly specified as weakening the approximation of semantic transparency
    806          (for example, "max-stale" or "public").
    807       </p>
    808       <p id="rfc.section.2.3.p.3">The cache-control directives are described in detail in <a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;15.2</a>.
    809       </p>
    810       <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="explicit.ua.warnings" href="#explicit.ua.warnings">Explicit User Agent Warnings</a></h2>
    811       <p id="rfc.section.2.4.p.1">Many user agents make it possible for users to override the basic caching mechanisms. For example, the user agent might allow
    812          the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually
    813          add "Cache-Control: max-stale=3600" to every request. The user agent <em class="bcp14">SHOULD NOT</em> default to either non-transparent behavior, or behavior that results in abnormally ineffective caching, but <em class="bcp14">MAY</em> be explicitly configured to do so by an explicit action of the user.
    814       </p>
    815       <p id="rfc.section.2.4.p.2">If the user has overridden the basic caching mechanisms, the user agent <em class="bcp14">SHOULD</em> explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency
    816          requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent
    817          to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication
    818          need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other indicator.
    819       </p>
    820       <p id="rfc.section.2.4.p.3">If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user
    821          agent <em class="bcp14">SHOULD</em> continually indicate this state to the user (for example, by a display of a picture of currency in flames) so that the user
    822          does not inadvertently consume excess resources or suffer from excessive latency.
    823       </p>
    824       <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a id="exceptions.to.the.rules.and.warnings" href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></h2>
    825       <p id="rfc.section.2.5.p.1">In some cases, the operator of a cache <em class="bcp14">MAY</em> choose to configure it to return stale responses even when not requested by clients. This decision ought not be made lightly,
    826          but may be necessary for reasons of availability or performance, especially when the cache is poorly connected to the origin
    827          server. Whenever a cache returns a stale response, it <em class="bcp14">MUST</em> mark it as such (using a Warning header) enabling the client software to alert the user that there might be a potential problem.
    828       </p>
    829       <p id="rfc.section.2.5.p.2">It also allows the user agent to take steps to obtain a first-hand or fresh response. For this reason, a cache <em class="bcp14">SHOULD NOT</em> return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply for
    830          technical or policy reasons.
    831       </p>
    832       <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a id="client-controlled.behavior" href="#client-controlled.behavior">Client-controlled Behavior</a></h2>
    833       <p id="rfc.section.2.6.p.1">While the origin server (and to a lesser extent, intermediate caches, by their contribution to the age of a response) are
    834          the primary source of expiration information, in some cases the client might need to control a cache's decision about whether
    835          to return a cached response without validating it. Clients do this using several directives of the Cache-Control header.
    836       </p>
    837       <p id="rfc.section.2.6.p.2">A client's request <em class="bcp14">MAY</em> specify the maximum age it is willing to accept of an unvalidated response; specifying a value of zero forces the cache(s)
    838          to revalidate all responses. A client <em class="bcp14">MAY</em> also specify the minimum time remaining before a response expires. Both of these options increase constraints on the behavior
    839          of caches, and so cannot further relax the cache's approximation of semantic transparency.
    840       </p>
    841       <p id="rfc.section.2.6.p.3">A client <em class="bcp14">MAY</em> also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on
    842          the caches, and so might violate the origin server's specified constraints on semantic transparency, but might be necessary
    843          to support disconnected operation, or high availability in the face of poor connectivity.
    844       </p>
    845       <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="expiration.model" href="#expiration.model">Expiration Model</a></h1>
    846       <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="server-specified.expiration" href="#server-specified.expiration">Server-Specified Expiration</a></h2>
    847       <p id="rfc.section.3.1.p.1">HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding
    848          requests is for an origin server to provide an explicit expiration time in the future, indicating that a response <em class="bcp14">MAY</em> be used to satisfy subsequent requests. In other words, a cache can return a fresh response without first contacting the server.
    849       </p>
    850       <p id="rfc.section.3.1.p.2">Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is
    851          not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic
    852          transparency, as long as the server's expiration times are carefully chosen.
    853       </p>
    854       <p id="rfc.section.3.1.p.3">The expiration mechanism applies only to responses taken from a cache and not to first-hand responses forwarded immediately
    855          to the requesting client.
    856       </p>
    857       <p id="rfc.section.3.1.p.4">If an origin server wishes to force a semantically transparent cache to validate every request, it <em class="bcp14">MAY</em> assign an explicit expiration time in the past. This means that the response is always stale, and so the cache <em class="bcp14">SHOULD</em> validate it before using it for subsequent requests. See <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a> for a more restrictive way to force revalidation.
    858       </p>
    859       <p id="rfc.section.3.1.p.5">If an origin server wishes to force any HTTP/1.1 cache, no matter how it is configured, to validate every request, it <em class="bcp14">SHOULD</em> use the "must-revalidate" cache-control directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;15.2</a>).
    860       </p>
    861       <p id="rfc.section.3.1.p.6">Servers specify explicit expiration times using either the Expires header, or the max-age directive of the Cache-Control header.</p>
    862       <p id="rfc.section.3.1.p.7">An expiration time cannot be used to force a user agent to refresh its display or reload a resource; its semantics apply only
    863          to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource
    864          is initiated. See <a href="#history.lists" title="History Lists">Section&nbsp;14</a> for an explanation of the difference between caches and history mechanisms.
    865       </p>
    866       <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="heuristic.expiration" href="#heuristic.expiration">Heuristic Expiration</a></h2>
    867       <p id="rfc.section.3.2.p.1">Since origin servers do not always provide explicit expiration times, HTTP caches typically assign heuristic expiration times,
    868          employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time.
    869          The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results.
    870          Since heuristic expiration times might compromise semantic transparency, they ought to be used cautiously, and we encourage
    871          origin servers to provide explicit expiration times as much as possible.
    872       </p>
    873       <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="age.calculations" href="#age.calculations">Age Calculations</a></h2>
    874       <p id="rfc.section.3.3.p.1">In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how
    875          to calculate the latter in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;3.4</a>; this section describes how to calculate the age of a response or cache entry.
    876       </p>
    877       <p id="rfc.section.3.3.p.2">In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation."
    878          Hosts that use HTTP, but especially hosts running origin servers and caches, <em class="bcp14">SHOULD</em> use NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a> or some similar protocol to synchronize their clocks to a globally accurate time standard.
    879       </p>
    880       <p id="rfc.section.3.3.p.3">HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response
    881          was generated (see <a href="p1-messaging.html#header.date" title="Date">Section 8.3</a> of <a href="#Part1" id="rfc.xref.Part1.11"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.
    882       </p>
    883       <p id="rfc.section.3.3.p.4">HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The
    884          Age field value is the cache's estimate of the amount of time since the response was generated or revalidated by the origin
    885          server.
    886       </p>
    887       <p id="rfc.section.3.3.p.5">In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path
    888          from the origin server, plus the amount of time it has been in transit along network paths.
    889       </p>
    890       <p id="rfc.section.3.3.p.6">We use the term "age_value" to denote the value of the Age header, in a form appropriate for arithmetic operations.</p>
    891       <p id="rfc.section.3.3.p.7">A response's age can be calculated in two entirely independent ways: </p>
     687</pre><h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Cache Operation</a></h1>
     688      <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Response Cacheability</a></h2>
     689      <p id="rfc.section.2.1.p.1">A cache <em class="bcp14">MUST NOT</em> store a response to any request, unless:
     690      </p>
     691      <ul>
     692         <li>The request method is defined as being cacheable, and</li>
     693         <li>the "no-store" cache directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;3.2</a>) does not appear in request or response headers, and
     694         </li>
     695         <li>the "private" cache response directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;3.2</a> does not appear in the response, if the cache is shared, and
     696         </li>
     697         <li>the "Authorization" header (see <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.1"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>) does not appear in the request, if the cache is shared (unless the "public" directive is present; see <a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;3.2</a>), and
     698         </li>
     699         <li>the cache understands partial responses, if the response is partial or incomplete (see <a href="#errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">Section&nbsp;2.1.1</a>).
     700         </li>
     701      </ul>
     702      <p id="rfc.section.2.1.p.2">Note that in normal operation, most caches will not store a response that has neither a cache validator nor an explicit expiration
     703         time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.
     704      </p>
     705      <h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a>&nbsp;<a id="errors.or.incomplete.response.cache.behavior" href="#errors.or.incomplete.response.cache.behavior">Storing Partial and Incomplete Responses</a></h3>
     706      <p id="rfc.section.2.1.1.p.1">A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header)
     707         can store the response, but <em class="bcp14">MUST</em> treat it as a partial response <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>. Partial responses can be combined as described in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>; the result might be a full response or might still be partial. A cache <em class="bcp14">MUST NOT</em> return a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.
     708      </p>
     709      <p id="rfc.section.2.1.1.p.2">A cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> store incomplete or partial responses.
     710      </p>
     711      <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h2>
     712      <p id="rfc.section.2.2.p.1">For a presented request, a cache <em class="bcp14">MUST NOT</em> return a stored response, unless:
     713      </p>
     714      <ul>
     715         <li>The presented Request-URI and that of the stored response match (see <span class="comment">[rfc.comment.1: TBD]</span>), and
     716         </li>
     717         <li>the request method associated with the stored response allows it to be used for the presented request, and</li>
     718         <li>selecting request-headers nominated by the stored response (if any) match those presented (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a>), and
     719         </li>
     720         <li>the presented request and stored response are free from directives that would prevent its use (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;3.2</a> and <a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;3.4</a>), and
     721         </li>
     722         <li>the stored response is either:
     723            <ul>
     724               <li>fresh (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>), or
     725               </li>
     726               <li>allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>), or
     727               </li>
     728               <li>successfully validated (see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>).
     729               </li>
     730            </ul>
     731         </li>
     732      </ul>
     733      <p id="rfc.section.2.2.p.2"><span class="comment">[rfc.comment.2: TODO: define method cacheability for GET, HEAD and POST in p2-semantics.]</span></p>
     734      <p id="rfc.section.2.2.p.3">When a stored response is used to satisfy a request, caches <em class="bcp14">MUST</em> include a single Age header field <a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;3.1</a> in the response with a value equal to the stored response's current_age; see <a href="#age.calculations" title="Calculating Age">Section&nbsp;2.3.2</a>. <span class="comment">[rfc.comment.3: DISCUSS: this currently includes successfully validated responses.]</span></p>
     735      <p id="rfc.section.2.2.p.4">Requests with methods that are unsafe (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) <em class="bcp14">MUST</em> be written through the cache to the origin server; i.e., A cache must not reply to such a request before having forwarded
     736         the request and having received a corresponding response.
     737      </p>
     738      <p id="rfc.section.2.2.p.5">Also, note that unsafe requests might invalidate already stored responses; see <a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;2.5</a>.
     739      </p>
     740      <p id="rfc.section.2.2.p.6">Caches <em class="bcp14">MUST</em> use the most recent response (as determined by the Date header) when more than one suitable response is stored. They can also
     741         forward a request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use.
     742      </p>
     743      <p id="rfc.section.2.2.p.7"> <span class="comment">[rfc.comment.4: TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1]</span>
     744      </p>
     745      <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a id="expiration.model" href="#expiration.model">Freshness Model</a></h2>
     746      <p id="rfc.section.2.3.p.1">When a response is "fresh" in the cache, it can be used to satisfy subsequent requests without contacting the origin server,
     747         thereby improving efficiency.
     748      </p>
     749      <p id="rfc.section.2.3.p.2">The primary mechanism for determining freshness is for an origin server to provide an explicit expiration time in the future,
     750         using either the Expires header (<a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;3.3</a>) or the max-age response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>). Generally, origin servers will assign future explicit expiration times to responses in the belief that the entity is not
     751         likely to change in a semantically significant way before the expiration time is reached.
     752      </p>
     753      <p id="rfc.section.2.3.p.3">If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past.
     754         This means that the response is always stale, so that caches should validate it before using it for subsequent requests. <span class="comment">[rfc.comment.5: This wording may cause confusion, because the response may still be served stale.]</span></p>
     755      <p id="rfc.section.2.3.p.4">Since origin servers do not always provide explicit expiration times, HTTP caches may also assign heuristic expiration times
     756         when they are not specified, employing algorithms that use other header values (such as the Last-Modified time) to estimate
     757         a plausible expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints
     758         on their results.
     759      </p>
     760      <p id="rfc.section.2.3.p.5">The calculation to determine if a response is fresh is:</p>
     761      <div id="rfc.figure.u.3"></div> <pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
     762</pre> <p id="rfc.section.2.3.p.7">The freshness_lifetime is defined in <a href="#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section&nbsp;2.3.1</a>; the current_age is defined in <a href="#age.calculations" title="Calculating Age">Section&nbsp;2.3.2</a>.
     763      </p>
     764      <p id="rfc.section.2.3.p.8">Additionally, clients may need to influence freshness calculation. They can do this using several request cache directives,
     765         with the effect of either increasing or loosening constraints on freshness. See <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>.
     766      </p>
     767      <p id="rfc.section.2.3.p.9"> <span class="comment">[rfc.comment.6: ISSUE: there are not requirements directly applying to cache-request-directives and freshness.]</span>
     768      </p>
     769      <p id="rfc.section.2.3.p.10">Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload
     770         a resource. See <a href="#history.lists" title="History Lists">Section&nbsp;4</a> for an explanation of the difference between caches and history mechanisms.
     771      </p>
     772      <h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a>&nbsp;<a id="calculating.freshness.lifetime" href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h3>
     773      <p id="rfc.section.2.3.1.p.1">A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of: </p>
     774      <ul>
     775         <li>If the cache is shared and the s-maxage response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) is present, use its value, or
     776         </li>
     777         <li>If the max-age response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) is present, use its value, or
     778         </li>
     779         <li>If the Expires response header (<a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;3.3</a>) is present, use its value minus the value of the Date response header, or
     780         </li>
     781         <li>Otherwise, no explicit expiration time is present in the response, but a heuristic may be used; see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;2.3.1.1</a>.
     782         </li>
     783      </ul>
     784      <p id="rfc.section.2.3.1.p.2">Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server.</p>
     785      <h4 id="rfc.section.2.3.1.1"><a href="#rfc.section.2.3.1.1">2.3.1.1</a>&nbsp;<a id="heuristic.freshness" href="#heuristic.freshness">Calculating Heuristic Freshness</a></h4>
     786      <p id="rfc.section.2.3.1.1.p.1">If no explicit expiration time is present in a stored response that has a status code of 200, 203, 206, 300, 301 or 410, a
     787         heuristic expiration time can be calculated. Heuristics <em class="bcp14">MUST NOT</em> be used for other response status codes.
     788      </p>
     789      <p id="rfc.section.2.3.1.1.p.2">When a heuristic is used to calculate freshness lifetime, the cache <em class="bcp14">SHOULD</em> attach a Warning header with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning is
     790         not already present.
     791      </p>
     792      <p id="rfc.section.2.3.1.1.p.3">Also, if the response has a Last-Modified header (<a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 6.6</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), the heuristic expiration value <em class="bcp14">SHOULD</em> be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
     793      </p>
     794      <p id="rfc.section.2.3.1.1.p.4"> <span class="comment">[rfc.comment.7: REVIEW: took away HTTP/1.0 query string heuristic uncacheability.]</span>
     795      </p>
     796      <h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a>&nbsp;<a id="age.calculations" href="#age.calculations">Calculating Age</a></h3>
     797      <p id="rfc.section.2.3.2.p.1">HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The
     798         Age field value is the cache's estimate of the amount of time since the response was generated or validated by the origin
     799         server. In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the
     800         path from the origin server, plus the amount of time it has been in transit along network paths.
     801      </p>
     802      <p id="rfc.section.2.3.2.p.2">The term "age_value" denotes the value of the Age header, in a form appropriate for arithmetic operations.</p>
     803      <p id="rfc.section.2.3.2.p.3">HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response
     804         was generated (see <a href="p1-messaging.html#header.date" title="Date">Section 8.3</a> of <a href="#Part1" id="rfc.xref.Part1.11"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). The term "date_value" denotes the value of the Date header, in a form appropriate for arithmetic operations.
     805      </p>
     806      <p id="rfc.section.2.3.2.p.4">The term "now" means "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially
     807         hosts running origin servers and caches, <em class="bcp14">SHOULD</em> use NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a> or some similar protocol to synchronize their clocks to a globally accurate time standard.
     808      </p>
     809      <p id="rfc.section.2.3.2.p.5">A response's age can be calculated in two entirely independent ways: </p>
    892810      <ol>
    893811         <li>now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative,
     
    896814         <li>age_value, if all of the caches along the response path implement HTTP/1.1.</li>
    897815      </ol>
    898       <p id="rfc.section.3.3.p.8">Given that we have two independent ways to compute the age of a response when it is received, we can combine these as</p>
    899       <div id="rfc.figure.u.3"></div><pre class="text">    corrected_received_age = max(now - date_value, age_value)
    900 </pre><p id="rfc.section.3.3.p.10">and as long as we have either nearly synchronized clocks or all-HTTP/1.1 paths, one gets a reliable (conservative) result.</p>
    901       <p id="rfc.section.3.3.p.11">Because of network-imposed delays, some significant interval might pass between the time that a server generates a response
    902          and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low
    903          ages.
    904       </p>
    905       <p id="rfc.section.3.3.p.12">Because the request that resulted in the returned Age value must have been initiated prior to that Age value's generation,
    906          we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age
    907          value is received, it <em class="bcp14">MUST</em> be interpreted relative to the time the request was initiated, not the time that the response was received. This algorithm
    908          results in conservative behavior no matter how much delay is experienced. So, we compute:
    909       </p>
    910       <div id="rfc.figure.u.4"></div><pre class="text">   corrected_initial_age = corrected_received_age
     816      <p id="rfc.section.2.3.2.p.6">These are combined as</p>
     817      <div id="rfc.figure.u.4"></div> <pre class="text">    corrected_received_age = max(now - date_value, age_value)
     818</pre> <p id="rfc.section.2.3.2.p.8">When an Age value is received, it <em class="bcp14">MUST</em> be interpreted relative to the time the request was initiated, not the time that the response was received.
     819      </p>
     820      <div id="rfc.figure.u.5"></div> <pre class="text">   corrected_initial_age = corrected_received_age
    911821                         + (now - request_time)
    912 </pre><p id="rfc.section.3.3.p.14">where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.</p>
    913       <p id="rfc.section.3.3.p.15">Summary of age calculation algorithm, when a cache receives a response:</p>
    914       <div id="rfc.figure.u.5"></div><pre class="text">   /*
    915     * age_value
    916     *      is the value of Age: header received by the cache with
    917     *              this response.
    918     * date_value
    919     *      is the value of the origin server's Date: header
    920     * request_time
    921     *      is the (local) time when the cache made the request
    922     *              that resulted in this cached response
    923     * response_time
    924     *      is the (local) time when the cache received the
    925     *              response
    926     * now
    927     *      is the current (local) time
    928     */
     822</pre> <p id="rfc.section.2.3.2.p.10">where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.</p>
     823      <p id="rfc.section.2.3.2.p.11">The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response
     824         was last validated by the origin server to the corrected_initial_age.
     825      </p>
     826      <p id="rfc.section.2.3.2.p.12">In summary:</p>
     827      <div id="rfc.figure.u.6"></div> <pre class="text">   age_value     - Age header field-value received with the response
     828   date_value    - Date header field-value received with the response
     829   request_time  - local time when the cache made the request
     830                   resulting in the stored response
     831   response_time - local time when the cache received the response
     832   now           - current local time
    929833
    930834   apparent_age = max(0, response_time - date_value);
     
    934838   resident_time = now - response_time;
    935839   current_age   = corrected_initial_age + resident_time;
    936 </pre><p id="rfc.section.3.3.p.17">The current_age of a cache entry is calculated by adding the amount of time (in seconds) since the cache entry was last validated
    937          by the origin server to the corrected_initial_age. When a response is generated from a cache entry, the cache <em class="bcp14">MUST</em> include a single Age header field in the response with a value equal to the cache entry's current_age.
    938       </p>
    939       <p id="rfc.section.3.3.p.18">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
    940          true, since the lack of an Age header field in a response does not imply that the response is first-hand unless all caches
    941          along the request path are compliant with HTTP/1.1 (i.e., older HTTP caches did not implement the Age header field).
    942       </p>
    943       <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="expiration.calculations" href="#expiration.calculations">Expiration Calculations</a></h2>
    944       <p id="rfc.section.3.4.p.1">In order to decide whether a response is fresh or stale, we need to compare its freshness lifetime to its age. The age is
    945          calculated as described in <a href="#age.calculations" title="Age Calculations">Section&nbsp;3.3</a>; this section describes how to calculate the freshness lifetime, and to determine if a response has expired. In the discussion
    946          below, the values can be represented in any form appropriate for arithmetic operations.
    947       </p>
    948       <p id="rfc.section.3.4.p.2">We use the term "expires_value" to denote the value of the Expires header. We use the term "max_age_value" to denote an appropriate
    949          value of the number of seconds carried by the "max-age" directive of the Cache-Control header in a response (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>).
    950       </p>
    951       <p id="rfc.section.3.4.p.3">The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:</p>
    952       <div id="rfc.figure.u.6"></div><pre class="text">   freshness_lifetime = max_age_value
    953 </pre><p id="rfc.section.3.4.p.5">Otherwise, if Expires is present in the response, the calculation is:</p>
    954       <div id="rfc.figure.u.7"></div><pre class="text">   freshness_lifetime = expires_value - date_value
    955 </pre><p id="rfc.section.3.4.p.7">Note that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server.</p>
    956       <p id="rfc.section.3.4.p.8">If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>) appears in the response, and the response does not include other restrictions on caching, the cache <em class="bcp14">MAY</em> compute a freshness lifetime using a heuristic. The cache <em class="bcp14">MUST</em> attach Warning 113 to any response whose age is more than 24 hours if such warning has not already been added.
    957       </p>
    958       <p id="rfc.section.3.4.p.9">Also, if the response does have a Last-Modified time, the heuristic expiration value <em class="bcp14">SHOULD</em> be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
    959       </p>
    960       <p id="rfc.section.3.4.p.10">The calculation to determine if a response has expired is quite simple:</p>
    961       <div id="rfc.figure.u.8"></div><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
    962 </pre><h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="disambiguating.expiration.values" href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></h2>
    963       <p id="rfc.section.3.5.p.1">Because expiration values are assigned optimistically, it is possible for two caches to contain fresh values for the same
    964          resource that are different.
    965       </p>
    966       <p id="rfc.section.3.5.p.2">If a client performing a retrieval receives a non-first-hand response for a request that was already fresh in its own cache,
    967          and the Date header in its existing cache entry is newer than the Date on the new response, then the client <em class="bcp14">MAY</em> ignore the response. If so, it <em class="bcp14">MAY</em> retry the request with a "Cache-Control: max-age=0" directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.5" title="Cache-Control">Section&nbsp;15.2</a>), to force a check with the origin server.
    968       </p>
    969       <p id="rfc.section.3.5.p.3">If a cache has two fresh responses for the same representation with different validators, it <em class="bcp14">MUST</em> use the one with the more recent Date header. This situation might arise because the cache is pooling responses from other
    970          caches, or because a client has asked for a reload or a revalidation of an apparently fresh cache entry.
    971       </p>
    972       <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="disambiguating.multiple.responses" href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></h2>
    973       <p id="rfc.section.3.6.p.1">Because a client might be receiving responses via multiple paths, so that some responses flow through one set of caches and
    974          other responses flow through a different set of caches, a client might receive responses in an order different from that in
    975          which the origin server sent them. We would like the client to use the most recently generated response, even if older responses
    976          are still apparently fresh.
    977       </p>
    978       <p id="rfc.section.3.6.p.2">Neither the entity tag nor the expiration value can impose an ordering on responses, since it is possible that a later response
    979          intentionally carries an earlier expiration time. The Date values are ordered to a granularity of one second.
    980       </p>
    981       <p id="rfc.section.3.6.p.3">When a client tries to revalidate a cache entry, and the response it receives contains a Date header that appears to be older
    982          than the one for the existing entry, then the client <em class="bcp14">SHOULD</em> repeat the request unconditionally, and include
    983       </p>
    984       <div id="rfc.figure.u.9"></div><pre class="text">  Cache-Control: max-age=0
    985 </pre><p id="rfc.section.3.6.p.5">to force any intermediate caches to validate their copies directly with the origin server, or</p>
    986       <div id="rfc.figure.u.10"></div><pre class="text">  Cache-Control: no-cache
    987 </pre><p id="rfc.section.3.6.p.7">to force any intermediate caches to obtain a new copy from the origin server.</p>
    988       <p id="rfc.section.3.6.p.8">If the Date values are equal, then the client <em class="bcp14">MAY</em> use either response (or <em class="bcp14">MAY</em>, if it is being extremely prudent, request a new response). Servers <em class="bcp14">MUST NOT</em> depend on clients being able to choose deterministically between responses generated during the same second, if their expiration
    989          times overlap.
    990       </p>
    991       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h1>
    992       <p id="rfc.section.4.p.1">When a cache has a stale entry that it would like to use as a response to a client's request, it first has to check with the
    993          origin server (or possibly an intermediate cache with a fresh response) to see if its cached entry is still usable. We call
    994          this "validating" the cache entry.
    995       </p>
    996       <p id="rfc.section.4.p.2">HTTP's conditional request mechanism, defined in <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>, is used to avoid retransmitting the response payload when the cached entry is valid. When a cached response includes one
    997          or more "cache validators," such as the field values of an ETag or Last-Modified header field, then a validating GET request <em class="bcp14">SHOULD</em> be made conditional to those field values. The server checks the conditional request's validator against the current state
    998          of the requested resource and, if they match, the server responds with a 304 (Not Modified) status code to indicate that the
    999          cached response can be refreshed and reused without retransmitting the response payload. If the validator does not match the
    1000          current state of the requested resource, then the server returns a full response, including payload, so that the request can
    1001          be satisfied and the cache entry supplanted without the need for an additional network round-trip.
    1002       </p>
    1003       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Response Cacheability</a></h1>
    1004       <p id="rfc.section.5.p.1">Unless specifically constrained by a cache-control (<a href="#header.cache-control" id="rfc.xref.header.cache-control.6" title="Cache-Control">Section&nbsp;15.2</a>) directive, a caching system <em class="bcp14">MAY</em> always store a successful response (see <a href="#errors.or.incomplete.response.cache.behavior" title="Errors or Incomplete Response Cache Behavior">Section&nbsp;9</a>) as a cache entry, <em class="bcp14">MAY</em> return it without validation if it is fresh, and <em class="bcp14">MAY</em> return it after successful validation. If there is neither a cache validator nor an explicit expiration time associated with
    1005          a response, we do not expect it to be cached, but certain caches <em class="bcp14">MAY</em> violate this expectation (for example, when little or no network connectivity is available). A client can usually detect that
    1006          such a response was taken from a cache by comparing the Date header to the current time.
    1007       </p>
    1008       <dl class="empty">
    1009          <dd> <b>Note:</b> some HTTP/1.0 caches are known to violate this expectation without providing any Warning.
    1010          </dd>
    1011       </dl>
    1012       <p id="rfc.section.5.p.2">However, in some cases it might be inappropriate for a cache to retain an entity, or to return it in response to a subsequent
    1013          request. This might be because absolute semantic transparency is deemed necessary by the service author, or because of security
    1014          or privacy considerations. Certain cache-control directives are therefore provided so that the server can indicate that certain
    1015          resource entities, or portions thereof, are not to be cached regardless of other considerations.
    1016       </p>
    1017       <p id="rfc.section.5.p.3">Note that <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.1"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a> normally prevents a shared cache from saving and returning a response to a previous request if that request included an Authorization
    1018          header.
    1019       </p>
    1020       <p id="rfc.section.5.p.4">A response received with a status code of 200, 203, 206, 300, 301 or 410 <em class="bcp14">MAY</em> be stored by a cache and used in reply to a subsequent request, subject to the expiration mechanism, unless a cache-control
    1021          directive prohibits caching. However, a cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> cache 206 (Partial Content) responses.
    1022       </p>
    1023       <p id="rfc.section.5.p.5">A response received with any other status code (e.g. status codes 302 and 307) <em class="bcp14">MUST NOT</em> be returned in a reply to a subsequent request unless there are cache-control directives or another header(s) that explicitly
    1024          allow it. For example, these include the following: an Expires header (<a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;15.3</a>); a "max-age", "s-maxage", "must-revalidate", "proxy-revalidate", "public" or "private" cache-control directive (<a href="#header.cache-control" id="rfc.xref.header.cache-control.7" title="Cache-Control">Section&nbsp;15.2</a>).
    1025       </p>
    1026       <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses From Caches</a></h1>
    1027       <p id="rfc.section.6.p.1">The purpose of an HTTP cache is to store information received in response to requests for use in responding to future requests.
    1028          In many cases, a cache simply returns the appropriate parts of a response to the requester. However, if the cache holds a
    1029          cache entry based on a previous response, it might have to combine parts of a new response with what is held in the cache
    1030          entry.
    1031       </p>
    1032       <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="end-to-end.and.hop-by-hop.headers" href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></h2>
    1033       <p id="rfc.section.6.1.p.1">For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP headers into two categories: </p>
    1034       <ul>
    1035          <li>End-to-end headers, which are transmitted to the ultimate recipient of a request or response. End-to-end headers in responses <em class="bcp14">MUST</em> be stored as part of a cache entry and <em class="bcp14">MUST</em> be transmitted in any response formed from a cache entry.
    1036          </li>
    1037          <li>Hop-by-hop headers, which are meaningful only for a single transport-level connection, and are not stored by caches or forwarded
    1038             by proxies.
    1039          </li>
    1040       </ul>
    1041       <p id="rfc.section.6.1.p.2">The following HTTP/1.1 headers are hop-by-hop headers: </p>
    1042       <ul>
    1043          <li>Connection</li>
    1044          <li>Keep-Alive</li>
    1045          <li>Proxy-Authenticate</li>
    1046          <li>Proxy-Authorization</li>
    1047          <li>TE</li>
    1048          <li>Trailer</li>
    1049          <li>Transfer-Encoding</li>
    1050          <li>Upgrade</li>
    1051       </ul>
    1052       <p id="rfc.section.6.1.p.3">All other headers defined by HTTP/1.1 are end-to-end headers.</p>
    1053       <p id="rfc.section.6.1.p.4">Other hop-by-hop headers <em class="bcp14">MUST</em> be listed in a Connection header (<a href="p1-messaging.html#header.connection" title="Connection">Section 8.1</a> of <a href="#Part1" id="rfc.xref.Part1.12"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
    1054       </p>
    1055       <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a id="non-modifiable.headers" href="#non-modifiable.headers">Non-modifiable Headers</a></h2>
    1056       <p id="rfc.section.6.2.p.1">Some features of HTTP/1.1, such as Digest Authentication, depend on the value of certain end-to-end headers. A transparent
    1057          proxy <em class="bcp14">SHOULD NOT</em> modify an end-to-end header unless the definition of that header requires or specifically allows that.
    1058       </p>
    1059       <p id="rfc.section.6.2.p.2">A transparent proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a request or response, and it <em class="bcp14">MUST NOT</em> add any of these fields if not already present:
    1060       </p>
    1061       <ul>
    1062          <li>Content-Location</li>
    1063          <li>Content-MD5</li>
    1064          <li>ETag</li>
    1065          <li>Last-Modified</li>
    1066       </ul>
    1067       <p id="rfc.section.6.2.p.3">A transparent proxy <em class="bcp14">MUST NOT</em> modify any of the following fields in a response:
    1068       </p>
    1069       <ul>
    1070          <li>Expires</li>
    1071       </ul>
    1072       <p id="rfc.section.6.2.p.4">but it <em class="bcp14">MAY</em> add any of these fields if not already present. If an Expires header is added, it <em class="bcp14">MUST</em> be given a field-value identical to that of the Date header in that response.
    1073       </p>
    1074       <p id="rfc.section.6.2.p.5">A proxy <em class="bcp14">MUST NOT</em> modify or add any of the following fields in a message that contains the no-transform cache-control directive, or in any request:
    1075       </p>
    1076       <ul>
    1077          <li>Content-Encoding</li>
    1078          <li>Content-Range</li>
    1079          <li>Content-Type</li>
    1080       </ul>
    1081       <p id="rfc.section.6.2.p.6">A non-transparent proxy <em class="bcp14">MAY</em> modify or add these fields to a message that does not include no-transform, but if it does so, it <em class="bcp14">MUST</em> add a Warning 214 (Transformation applied) if one does not already appear in the message (see <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;15.6</a>).
    1082       </p>
    1083       <dl class="empty">
    1084          <dd>Warning: unnecessary modification of end-to-end headers might cause authentication failures if stronger authentication mechanisms
    1085             are introduced in later versions of HTTP. Such authentication mechanisms <em class="bcp14">MAY</em> rely on the values of header fields not listed here.
    1086          </dd>
    1087       </dl>
    1088       <p id="rfc.section.6.2.p.7">The Content-Length field of a request or response is added or deleted according to the rules in <a href="p1-messaging.html#message.length" title="Message Length">Section 4.4</a> of <a href="#Part1" id="rfc.xref.Part1.13"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. A transparent proxy <em class="bcp14">MUST</em> preserve the entity-length (<a href="p3-payload.html#entity.length" title="Entity Length">Section 3.2.2</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>) of the entity-body, although it <em class="bcp14">MAY</em> change the transfer-length (<a href="p1-messaging.html#message.length" title="Message Length">Section 4.4</a> of <a href="#Part1" id="rfc.xref.Part1.14"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
    1089       </p>
    1090       <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a id="combining.headers" href="#combining.headers">Combining Headers</a></h2>
    1091       <p id="rfc.section.6.3.p.1">When a cache makes a validating request to a server, and the server provides a 304 (Not Modified) response or a 206 (Partial
    1092          Content) response, the cache then constructs a response to send to the requesting client.
    1093       </p>
    1094       <p id="rfc.section.6.3.p.2">If the status code is 304 (Not Modified), the cache uses the entity-body stored in the cache entry as the entity-body of this
    1095          outgoing response. If the status code is 206 (Partial Content) and the ETag or Last-Modified headers match exactly, the cache <em class="bcp14">MAY</em> combine the contents stored in the cache entry with the new contents received in the response and use the result as the entity-body
    1096          of this outgoing response, (see <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>).
    1097       </p>
    1098       <p id="rfc.section.6.3.p.3">The end-to-end headers stored in the cache entry are used for the constructed response, except that </p>
    1099       <ul>
    1100          <li>any stored Warning headers with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;15.6</a>) <em class="bcp14">MUST</em> be deleted from the cache entry and the forwarded response.
    1101          </li>
    1102          <li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the cache entry and the forwarded response.
    1103          </li>
    1104          <li>any end-to-end headers provided in the 304 or 206 response <em class="bcp14">MUST</em> replace the corresponding headers from the cache entry.
    1105          </li>
    1106       </ul>
    1107       <p id="rfc.section.6.3.p.4">Unless the cache decides to remove the cache entry, it <em class="bcp14">MUST</em> also replace the end-to-end headers stored with the cache entry with corresponding headers received in the incoming response,
    1108          except for Warning headers as described immediately above. If a header field-name in the incoming response matches more than
    1109          one header in the cache entry, all such old headers <em class="bcp14">MUST</em> be replaced.
    1110       </p>
    1111       <p id="rfc.section.6.3.p.5">In other words, the set of end-to-end headers received in the incoming response overrides all corresponding end-to-end headers
    1112          stored with the cache entry (except for stored Warning headers with warn-code 1xx, which are deleted even if not overridden).
    1113       </p>
    1114       <dl class="empty">
    1115          <dd> <b>Note:</b> this rule allows an origin server to use a 304 (Not Modified) or a 206 (Partial Content) response to update any header associated
    1116             with a previous response for the same entity or sub-ranges thereof, although it might not always be meaningful or correct
    1117             to do so. This rule does not allow an origin server to use a 304 (Not Modified) or a 206 (Partial Content) response to entirely
    1118             delete a header that it had provided with a previous response.
    1119          </dd>
    1120       </dl>
    1121       <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Caching Negotiated Responses</a></h1>
    1122       <p id="rfc.section.7.p.1">Use of server-driven content negotiation (<a href="p3-payload.html#server-driven.negotiation" title="Server-driven Negotiation">Section 4.1</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), as indicated by the presence of a Vary header field in a response, alters the conditions and procedure by which a cache
    1123          can use the response for subsequent requests. See <a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;15.5</a> for use of the Vary header field by servers.
    1124       </p>
    1125       <p id="rfc.section.7.p.2">A server <em class="bcp14">SHOULD</em> use the Vary header field to inform a cache of what request-header fields were used to select among multiple representations
    1126          of a cacheable response subject to server-driven negotiation. The set of header fields named by the Vary field value is known
    1127          as the "selecting" request-headers.
    1128       </p>
    1129       <p id="rfc.section.7.p.3">When the cache receives a subsequent request whose request-target specifies one or more cache entries including a Vary header
    1130          field, the cache <em class="bcp14">MUST NOT</em> use such a cache entry to construct a response to the new request unless all of the selecting request-headers present in the
    1131          new request match the corresponding stored request-headers in the original request.
    1132       </p>
    1133       <p id="rfc.section.7.p.4">The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first
    1134          request can be transformed to the selecting request-headers in the second request by adding or removing linear whitespace
    1135          (LWS) at places where this is allowed by the corresponding BNF, and/or combining multiple message-header fields with the same
    1136          field name following the rules about message headers in <a href="p1-messaging.html#message.headers" title="Message Headers">Section 4.2</a> of <a href="#Part1" id="rfc.xref.Part1.15"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
    1137       </p>
    1138       <p id="rfc.section.7.p.5">A Vary header field-value of "*" always fails to match and subsequent requests on that resource can only be properly interpreted
    1139          by the origin server.
    1140       </p>
    1141       <p id="rfc.section.7.p.6">If the selecting request header fields for the cached entry do not match the selecting request header fields of the new request,
    1142          then the cache <em class="bcp14">MUST NOT</em> use a cached entry to satisfy the request unless it first relays the new request to the origin server in a conditional request
    1143          and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates the entity to
    1144          be used.
    1145       </p>
    1146       <p id="rfc.section.7.p.7">If an entity tag was assigned to a cached representation, the forwarded request <em class="bcp14">SHOULD</em> be conditional and include the entity tags in an If-None-Match header field from all its cache entries for the resource. This
    1147          conveys to the server the set of entities currently held by the cache, so that if any one of these entities matches the requested
    1148          entity, the server can use the ETag header field in its 304 (Not Modified) response to tell the cache which entry is appropriate.
    1149          If the entity-tag of the new response matches that of an existing entry, the new response <em class="bcp14">SHOULD</em> be used to update the header fields of the existing entry, and the result <em class="bcp14">MUST</em> be returned to the client.
    1150       </p>
    1151       <p id="rfc.section.7.p.8">If any of the existing cache entries contains only partial content for the associated entity, its entity-tag <em class="bcp14">SHOULD NOT</em> be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that entry.
    1152       </p>
    1153       <p id="rfc.section.7.p.9">If a cache receives a successful response whose Content-Location field matches that of an existing cache entry for the same
    1154          request-target, whose entity-tag differs from that of the existing entry, and whose Date is more recent than that of the existing
    1155          entry, the existing entry <em class="bcp14">SHOULD NOT</em> be returned in response to future requests and <em class="bcp14">SHOULD</em> be deleted from the cache.
    1156       </p>
    1157       <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="shared.and.non-shared.caches" href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></h1>
    1158       <p id="rfc.section.8.p.1">For reasons of security and privacy, it is necessary to make a distinction between "shared" and "non-shared" caches. A non-shared
    1159          cache is one that is accessible only to a single user. Accessibility in this case <em class="bcp14">SHOULD</em> be enforced by appropriate security mechanisms. All other caches are considered to be "shared." Other sections of this specification
    1160          place certain constraints on the operation of shared caches in order to prevent loss of privacy or failure of access controls.
    1161       </p>
    1162       <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a id="errors.or.incomplete.response.cache.behavior" href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></h1>
    1163       <p id="rfc.section.9.p.1">A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) <em class="bcp14">MAY</em> store the response. However, the cache <em class="bcp14">MUST</em> treat this as a partial response. Partial responses <em class="bcp14">MAY</em> be combined as described in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>; the result might be a full response or might still be partial. A cache <em class="bcp14">MUST NOT</em> return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code.
    1164          A cache <em class="bcp14">MUST NOT</em> return a partial response using a status code of 200 (OK).
    1165       </p>
    1166       <p id="rfc.section.9.p.2">If a cache receives a 5xx response while attempting to revalidate an entry, it <em class="bcp14">MAY</em> either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it <em class="bcp14">MAY</em> return a previously received response unless the cached entry includes the "must-revalidate" cache-control directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.8" title="Cache-Control">Section&nbsp;15.2</a>).
    1167       </p>
    1168       <h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a>&nbsp;<a id="side.effects.of.get.and.head" href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></h1>
    1169       <p id="rfc.section.10.p.1">Unless the origin server explicitly prohibits the caching of their responses, the application of GET and HEAD methods to any
    1170          resources <em class="bcp14">SHOULD NOT</em> have side effects that would lead to erroneous behavior if these responses are taken from a cache. They <em class="bcp14">MAY</em> still have side effects, but a cache is not required to consider such side effects in its caching decisions. Caches are always
    1171          expected to observe an origin server's explicit restrictions on caching.
    1172       </p>
    1173       <p id="rfc.section.10.p.2">We note one exception to this rule: since some applications have traditionally used GET and HEAD requests with URLs containing
    1174          a query part to perform operations with significant side effects, caches <em class="bcp14">MUST NOT</em> treat responses to such URIs as fresh unless the server provides an explicit expiration time. This specifically means that
    1175          responses from HTTP/1.0 servers for such URIs <em class="bcp14">SHOULD NOT</em> be taken from a cache. See <a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> for related information.
    1176       </p>
    1177       <h1 id="rfc.section.11"><a href="#rfc.section.11">11.</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></h1>
    1178       <p id="rfc.section.11.p.1">The effect of certain methods performed on a resource at the origin server might cause one or more existing cache entries
    1179          to become non-transparently invalid. That is, although they might continue to be "fresh," they do not accurately reflect what
    1180          the origin server would return for a new request on that resource.
    1181       </p>
    1182       <p id="rfc.section.11.p.2">There is no way for HTTP to guarantee that all such cache entries are marked invalid. For example, the request that caused
    1183          the change at the origin server might not have gone through the proxy where a cache entry is stored. However, several rules
    1184          help reduce the likelihood of erroneous behavior.
    1185       </p>
    1186       <p id="rfc.section.11.p.3">In this section, the phrase "invalidate an entity" means that the cache will either remove all instances of that entity from
    1187          its storage, or will mark these as "invalid" and in need of a mandatory revalidation before they can be returned in response
    1188          to a subsequent request.
    1189       </p>
    1190       <p id="rfc.section.11.p.4">Some HTTP methods <em class="bcp14">MUST</em> cause a cache to invalidate an entity. This is either the entity referred to by the request-target, or by the Location or
    1191          Content-Location headers (if present). These methods are:
     840</pre> <h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a id="serving.stale.responses" href="#serving.stale.responses">Serving Stale Responses</a></h3>
     841      <p id="rfc.section.2.3.3.p.1">A "stale" response is one that either has explicit expiry information, or is allowed to have heuristic expiry calculated,
     842         but is not fresh according to the calculations in <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>.
     843      </p>
     844      <p id="rfc.section.2.3.3.p.2">Caches <em class="bcp14">MUST NOT</em> return a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
     845         directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
     846         see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>).
     847      </p>
     848      <p id="rfc.section.2.3.3.p.3">Caches <em class="bcp14">SHOULD NOT</em> return stale responses unless they are disconnected (i.e., it cannot contact the origin server or otherwise find a forward
     849         path) or otherwise explicitly allowed (e.g., the max-stale request directive; see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>)..
     850      </p>
     851      <p id="rfc.section.2.3.3.p.4">Stale responses <em class="bcp14">SHOULD</em> have a Warning header with the 110 warn-code (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;3.6</a>). Likewise, the 112 warn-code <em class="bcp14">SHOULD</em> be sent on stale responses if the cache is disconnected.
     852      </p>
     853      <p id="rfc.section.2.3.3.p.5">If a cache receives a first-hand response (either an entire response, or a 304 (Not Modified) response) that it would normally
     854         forward to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">SHOULD</em> forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache <em class="bcp14">SHOULD NOT</em> attempt to validate a response simply because that response became stale in transit.
     855      </p>
     856      <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h2>
     857      <p id="rfc.section.2.4.p.1">Checking with the origin server to see if a stale or otherwise unusable cached response can be reused is called "validating"
     858         or "revalidating." Doing so potentially avoids the overhead of retransmitting the response body when the stored response is
     859         valid.
     860      </p>
     861      <p id="rfc.section.2.4.p.2">HTTP's conditional request mechanism <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a> is used for this purpose. When a stored response includes one or more validators, such as the field values of an ETag or Last-Modified
     862         header field, then a validating request <em class="bcp14">SHOULD</em> be made conditional to those field values.
     863      </p>
     864      <p id="rfc.section.2.4.p.3">A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see <a href="#combining.headers" title="Combining Responses">Section&nbsp;2.7</a>.
     865      </p>
     866      <p id="rfc.section.2.4.p.4">If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the request and replace
     867         the stored response. <span class="comment">[rfc.comment.8: Should there be a requirement here?]</span></p>
     868      <p id="rfc.section.2.4.p.5">If a cache receives a 5xx response while attempting to validate a response, it <em class="bcp14">MAY</em> either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it <em class="bcp14">MAY</em> return a previously stored response (which <em class="bcp14">SHOULD</em> include the 111 warn-code; see <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;3.6</a>) unless the stored response includes the "must-revalidate" cache directive (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;2.3.3</a>).
     869      </p>
     870      <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></h2>
     871      <p id="rfc.section.2.5.p.1">Because unsafe methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) have the potential for changing state on the origin server, intervening caches can use them to keep their contents up-to-date.
     872      </p>
     873      <p id="rfc.section.2.5.p.2">The following HTTP methods <em class="bcp14">MUST</em> cause a cache to invalidate the Request-URI as well as the Location and Content-Location headers (if present):
    1192874      </p>
    1193875      <ul>
     
    1196878         <li>POST</li>
    1197879      </ul>
    1198       <p id="rfc.section.11.p.5">An invalidation based on the URI in a Location or Content-Location header <em class="bcp14">MUST NOT</em> be performed if the host part of that URI differs from the host part in the request-target. This helps prevent denial of service
     880      <p id="rfc.section.2.5.p.3">An invalidation based on the URI in a Location or Content-Location header <em class="bcp14">MUST NOT</em> be performed if the host part of that URI differs from the host part in the Request-URI. This helps prevent denial of service
    1199881         attacks.
    1200882      </p>
    1201       <p id="rfc.section.11.p.6">A cache that passes through requests for methods it does not understand <em class="bcp14">SHOULD</em> invalidate any entities referred to by the request-target.
    1202       </p>
    1203       <h1 id="rfc.section.12"><a href="#rfc.section.12">12.</a>&nbsp;<a id="write-through.mandatory" href="#write-through.mandatory">Write-Through Mandatory</a></h1>
    1204       <p id="rfc.section.12.p.1">All methods that might be expected to cause modifications to the origin server's resources <em class="bcp14">MUST</em> be written through to the origin server. This currently includes all methods except for GET and HEAD. A cache <em class="bcp14">MUST NOT</em> reply to such a request from a client before having transmitted the request to the inbound server, and having received a corresponding
    1205          response from the inbound server. This does not prevent a proxy cache from sending a 100 (Continue) response before the inbound
    1206          server has sent its final reply.
    1207       </p>
    1208       <p id="rfc.section.12.p.2">The alternative (known as "write-back" or "copy-back" caching) is not allowed in HTTP/1.1, due to the difficulty of providing
    1209          consistent updates and the problems arising from server, cache, or network failure prior to write-back.
    1210       </p>
    1211       <h1 id="rfc.section.13"><a href="#rfc.section.13">13.</a>&nbsp;<a id="cache.replacement" href="#cache.replacement">Cache Replacement</a></h1>
    1212       <p id="rfc.section.13.p.1">If a new cacheable (see Sections <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">15.2.2</a>, <a href="#disambiguating.expiration.values" title="Disambiguating Expiration Values">3.5</a>, <a href="#disambiguating.multiple.responses" title="Disambiguating Multiple Responses">3.6</a> and <a href="#errors.or.incomplete.response.cache.behavior" title="Errors or Incomplete Response Cache Behavior">9</a>) response is received from a resource while any existing responses for the same resource are cached, the cache <em class="bcp14">SHOULD</em> use the new response to reply to the current request. It <em class="bcp14">MAY</em> insert it into cache storage and <em class="bcp14">MAY</em>, if it meets all other requirements, use it to respond to any future requests that would previously have caused the old response
    1213          to be returned. If it inserts the new response into cache storage the rules in <a href="#combining.headers" title="Combining Headers">Section&nbsp;6.3</a> apply.
    1214       </p>
    1215       <dl class="empty">
    1216          <dd> <b>Note:</b> a new response that has an older Date header value than existing cached responses is not cacheable.
    1217          </dd>
    1218       </dl>
    1219       <h1 id="rfc.section.14"><a href="#rfc.section.14">14.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
    1220       <p id="rfc.section.14.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, which can be used to redisplay an entity
    1221          retrieved earlier in a session.
    1222       </p>
    1223       <p id="rfc.section.14.p.2">History mechanisms and caches are different. In particular history mechanisms <em class="bcp14">SHOULD NOT</em> try to show a semantically transparent view of the current state of a resource. Rather, a history mechanism is meant to show
    1224          exactly what the user saw at the time when the resource was retrieved.
    1225       </p>
    1226       <p id="rfc.section.14.p.3">By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism <em class="bcp14">SHOULD</em> display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history
    1227          documents.
    1228       </p>
    1229       <p id="rfc.section.14.p.4">This is not to be construed to prohibit the history mechanism from telling the user that a view might be stale. </p>
    1230       <dl class="empty">
    1231          <dd> <b>Note:</b> if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors
    1232             to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider
    1233             it important that users not be presented with error messages or warning messages when they use navigation controls (such as
    1234             BACK) to view previously fetched resources. Even though sometimes such resources ought not be cached, or ought to expire quickly,
    1235             user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs)
    1236             in order not to suffer the effects of improperly functioning history mechanisms.
    1237          </dd>
    1238       </dl>
    1239       <h1 id="rfc.section.15"><a href="#rfc.section.15">15.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
    1240       <p id="rfc.section.15.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
    1241       <p id="rfc.section.15.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
     883      <p id="rfc.section.2.5.p.4"> <span class="comment">[rfc.comment.9: TODO: "host part" needs to be specified better.]</span>
     884      </p>
     885      <p id="rfc.section.2.5.p.5">A cache that passes through requests for methods it does not understand <em class="bcp14">SHOULD</em> invalidate the Request-URI.
     886      </p>
     887      <p id="rfc.section.2.5.p.6">Here, "invalidate" means that the cache will either remove all stored responses related to the Request-URI, or will mark these
     888         as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent request.
     889      </p>
     890      <p id="rfc.section.2.5.p.7">Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the
     891         change at the origin server might not have gone through the cache where a response is stored.
     892      </p>
     893      <p id="rfc.section.2.5.p.8"> <span class="comment">[rfc.comment.10: TODO: specify that only successful (2xx, 3xx?) responses invalidate.]</span>
     894      </p>
     895      <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Caching Negotiated Responses</a></h2>
     896      <p id="rfc.section.2.6.p.1">Use of server-driven content negotiation (<a href="p3-payload.html#server-driven.negotiation" title="Server-driven Negotiation">Section 4.1</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>) alters the conditions under which a cache can use the response for subsequent requests.
     897      </p>
     898      <p id="rfc.section.2.6.p.2">When a cache receives a request that can be satisfied by a stored response that includes a Vary header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;3.5</a>), it <em class="bcp14">MUST NOT</em> use that response unless all of the selecting request-headers in the presented request match the corresponding stored request-headers
     899         from the original request.
     900      </p>
     901      <p id="rfc.section.2.6.p.3">The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first
     902         request can be transformed to the selecting request-headers in the second request by adding or removing linear white space <span class="comment">[rfc.comment.11: [ref]]</span> at places where this is allowed by the corresponding ABNF, and/or combining multiple message-header fields with the same field
     903         name following the rules about message headers in <a href="p1-messaging.html#message.headers" title="Message Headers">Section 4.2</a> of <a href="#Part1" id="rfc.xref.Part1.12"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. <span class="comment">[rfc.comment.12: DISCUSS: header-specific canonicalisation]</span></p>
     904      <p id="rfc.section.2.6.p.4">A Vary header field-value of "*" always fails to match, and subsequent requests to that resource can only be properly interpreted
     905         by the origin server.
     906      </p>
     907      <p id="rfc.section.2.6.p.5">If no stored response matches, the cache <em class="bcp14">MAY</em> forward the presented request to the origin server in a conditional request, and <em class="bcp14">SHOULD</em> include all ETags stored with potentially suitable responses in an If-None-Match request header. If the server responds with
     908         304 (Not Modified) and includes an entity tag or Content-Location that indicates the entity to be used, that cached response <em class="bcp14">MUST</em> be used to satisfy the presented request, and <em class="bcp14">SHOULD</em> be used to update the corresponding stored response; see <a href="#combining.headers" title="Combining Responses">Section&nbsp;2.7</a>.
     909      </p>
     910      <p id="rfc.section.2.6.p.6">If any of the stored responses contains only partial content, its entity-tag <em class="bcp14">SHOULD NOT</em> be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that stored
     911         response.
     912      </p>
     913      <p id="rfc.section.2.6.p.7">If a cache receives a successful response whose Content-Location field matches that of an existing stored response for the
     914         same Request-URI, whose entity-tag differs from that of the existing stored response, and whose Date is more recent than that
     915         of the existing response, the existing response <em class="bcp14">SHOULD NOT</em> be returned in response to future requests and <em class="bcp14">SHOULD</em> be deleted from the cache.<span class="comment">[rfc.comment.13: DISCUSS: Not sure if this is necessary.]</span></p>
     916      <h2 id="rfc.section.2.7"><a href="#rfc.section.2.7">2.7</a>&nbsp;<a id="combining.headers" href="#combining.headers">Combining Responses</a></h2>
     917      <p id="rfc.section.2.7.p.1">When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response, it needs to update the stored response
     918         with the new one, so that the updated response can be sent to the client.
     919      </p>
     920      <p id="rfc.section.2.7.p.2">If the status code is 304 (Not Modified), the cache <em class="bcp14">SHOULD</em> use the stored entity-body as the updated entity-body. If the status code is 206 (Partial Content) and the ETag or Last-Modified
     921         headers match exactly, the cache <em class="bcp14">MAY</em> combine the stored entity-body in the stored response with the updated entity-body received in the response and use the result
     922         as the updated entity-body (see <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>).
     923      </p>
     924      <p id="rfc.section.2.7.p.3">The stored response headers are used for the updated response, except that </p>
     925      <ul>
     926         <li>any stored Warning headers with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;3.6</a>) <em class="bcp14">MUST</em> be deleted from the stored response and the forwarded response.
     927         </li>
     928         <li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the stored response and the forwarded response.
     929         </li>
     930         <li>any headers provided in the 304 or 206 response <em class="bcp14">MUST</em> replace the corresponding headers from the stored response.
     931         </li>
     932      </ul>
     933      <p id="rfc.section.2.7.p.4">A cache <em class="bcp14">MUST</em> also replace any stored headers with corresponding headers received in the incoming response, except for Warning headers as
     934         described immediately above. If a header field-name in the incoming response matches more than one header in the stored response,
     935         all such old headers <em class="bcp14">MUST</em> be replaced. It <em class="bcp14">MAY</em> store the combined entity-body.
     936      </p>
     937      <p id="rfc.section.2.7.p.5"><span class="comment">[rfc.comment.14: ISSUE: discuss how to handle HEAD updates]</span></p>
     938      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
     939      <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
     940      <p id="rfc.section.3.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
    1242941         receives the entity.
    1243942      </p>
    1244943      <div id="rfc.iref.a.2"></div>
    1245944      <div id="rfc.iref.h.2"></div>
    1246       <h2 id="rfc.section.15.1"><a href="#rfc.section.15.1">15.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
    1247       <p id="rfc.section.15.1.p.1">The response-header field "Age" conveys the sender's estimate of the amount of time since the response (or its revalidation)
    1248          was generated at the origin server. A cached response is "fresh" if its age does not exceed its freshness lifetime. Age values
    1249          are calculated as specified in <a href="#age.calculations" title="Age Calculations">Section&nbsp;3.3</a>.
    1250       </p>
    1251       <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span>  <a href="#header.age" class="smpl">Age</a>   = "Age" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.age" class="smpl">Age-v</a>
     945      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
     946      <p id="rfc.section.3.1.p.1">The response-header field "Age" conveys the sender's estimate of the amount of time since the response (or its validation)
     947         was generated at the origin server. Age values are calculated as specified in <a href="#age.calculations" title="Calculating Age">Section&nbsp;2.3.2</a>.
     948      </p>
     949      <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span>  <a href="#header.age" class="smpl">Age</a>   = "Age" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.age" class="smpl">Age-v</a>
    1252950  <a href="#header.age" class="smpl">Age-v</a> = <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
    1253951</pre><div id="rule.delta-seconds">
    1254          <p id="rfc.section.15.1.p.3">  Age values are non-negative decimal integers, representing time in seconds.</p>
     952         <p id="rfc.section.3.1.p.3">  Age field-values are non-negative decimal integers, representing time in seconds.</p>
    1255953      </div>
    1256       <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.3"></span>  <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>  = 1*<a href="#notation" class="smpl">DIGIT</a>
    1257 </pre><p id="rfc.section.15.1.p.5">If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows,
    1258          it <em class="bcp14">MUST</em> transmit an Age header with a value of 2147483648 (2<sup>31</sup>). An HTTP/1.1 server that includes a cache <em class="bcp14">MUST</em> include an Age header field in every response generated from its own cache. Caches <em class="bcp14">SHOULD</em> use an arithmetic type of at least 31 bits of range.
     954      <div id="rfc.figure.u.8"></div> <pre class="inline"><span id="rfc.iref.g.3"></span>  <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>  = 1*<a href="#notation" class="smpl">DIGIT</a>
     955</pre> <p id="rfc.section.3.1.p.5">If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows,
     956         it <em class="bcp14">MUST</em> transmit an Age header with a field-value of 2147483648 (2<sup>31</sup>). Caches <em class="bcp14">SHOULD</em> use an arithmetic type of at least 31 bits of range.
     957      </p>
     958      <p id="rfc.section.3.1.p.6">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
     959         true, since HTTP/1.0 caches may not implement the Age header field.
    1259960      </p>
    1260961      <div id="rfc.iref.c.3"></div>
    1261962      <div id="rfc.iref.h.3"></div>
    1262       <h2 id="rfc.section.15.2"><a href="#rfc.section.15.2">15.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
    1263       <p id="rfc.section.15.2.p.1">The general-header field "Cache-Control" is used to specify directives that <em class="bcp14">MUST</em> be obeyed by all caching mechanisms along the request/response chain. The directives specify behavior intended to prevent
    1264          caches from adversely interfering with the request or response. These directives typically override the default caching algorithms.
    1265          Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive
    1266          is to be given in the response.
    1267       </p>
    1268       <dl class="empty">
    1269          <dd>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;15.4</a>).
    1270          </dd>
    1271       </dl>
    1272       <p id="rfc.section.15.2.p.2">Cache directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
    1273          might be applicable to all recipients along the request/response chain. It is not possible to specify a cache-directive for
    1274          a specific cache.
    1275       </p>
    1276       <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span><span id="rfc.iref.g.7"></span><span id="rfc.iref.g.8"></span><span id="rfc.iref.g.9"></span>  <a href="#header.cache-control" class="smpl">Cache-Control</a>   = "Cache-Control" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.cache-control" class="smpl">Cache-Control-v</a>
     963      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
     964      <p id="rfc.section.3.2.p.1">The general-header field "Cache-Control" is used to specify directives that <em class="bcp14">MUST</em> be obeyed by all caches along the request/response chain. The directives specify behavior intended to prevent caches from
     965         adversely interfering with the request or response. Cache directives are unidirectional in that the presence of a directive
     966         in a request does not imply that the same directive is to be given in the response.
     967      </p>
     968      <dl class="empty">
     969         <dd>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;3.4</a>).
     970         </dd>
     971      </dl>
     972      <p id="rfc.section.3.2.p.2">Cache directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
     973         might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific
     974         cache.
     975      </p>
     976      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#header.cache-control" class="smpl">Cache-Control</a>   = "Cache-Control" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.cache-control" class="smpl">Cache-Control-v</a>
    1277977  <a href="#header.cache-control" class="smpl">Cache-Control-v</a> = 1#<a href="#header.cache-control" class="smpl">cache-directive</a>
    1278978
     
    1280980     / <a href="#header.cache-control" class="smpl">cache-response-directive</a>
    1281981
    1282   <a href="#header.cache-control" class="smpl">cache-request-directive</a> =
    1283        "no-cache"                          ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
    1284      / "no-store"                          ; <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">Section&nbsp;15.2.2</a>
    1285      / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>         ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>, <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">15.2.4</a>
    1286      / "max-stale" [ "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a> ]   ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
    1287      / "min-fresh" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>       ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
    1288      / "no-transform"                      ; <a href="#no-transform.directive" title="No-Transform Directive">Section&nbsp;15.2.5</a>
    1289      / "only-if-cached"                    ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>
    1290      / <a href="#header.cache-control" class="smpl">cache-extension</a>                     ; <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;15.2.6</a>
    1291 
    1292   <a href="#header.cache-control" class="smpl">cache-response-directive</a> =
    1293        "public"                               ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
    1294      / "private" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ] ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
    1295      / "no-cache" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ] ; <a href="#what.is.cacheable" title="What is Cacheable">Section&nbsp;15.2.1</a>
    1296      / "no-store"                             ; <a href="#what.may.be.stored.by.caches" title="What May be Stored by Caches">Section&nbsp;15.2.2</a>
    1297      / "no-transform"                         ; <a href="#no-transform.directive" title="No-Transform Directive">Section&nbsp;15.2.5</a>
    1298      / "must-revalidate"                      ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>
    1299      / "proxy-revalidate"                     ; <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>
    1300      / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>            ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
    1301      / "s-maxage" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>           ; <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>
    1302      / <a href="#header.cache-control" class="smpl">cache-extension</a>                        ; <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;15.2.6</a>
    1303 
    1304982  <a href="#header.cache-control" class="smpl">cache-extension</a> = <a href="#core.rules" class="smpl">token</a> [ "=" ( <a href="#core.rules" class="smpl">token</a> / <a href="#core.rules" class="smpl">quoted-string</a> ) ]
    1305 </pre><p id="rfc.section.15.2.p.4">When a directive appears without any 1#field-name parameter, the directive applies to the entire request or response. When
    1306          such a directive appears with a 1#field-name parameter, it applies only to the named field or fields, and not to the rest
    1307          of the request or response. This mechanism supports extensibility; implementations of future versions of HTTP might apply
    1308          these directives to header fields not defined in HTTP/1.1.
    1309       </p>
    1310       <p id="rfc.section.15.2.p.5">The cache-control directives can be broken down into these general categories: </p>
    1311       <ul>
    1312          <li>Restrictions on what are cacheable; these may only be imposed by the origin server.</li>
    1313          <li>Restrictions on what may be stored by a cache; these may be imposed by either the origin server or the user agent.</li>
    1314          <li>Modifications of the basic expiration mechanism; these may be imposed by either the origin server or the user agent.</li>
    1315          <li>Controls over cache revalidation and reload; these may only be imposed by a user agent.</li>
    1316          <li>Control over transformation of entities.</li>
    1317          <li>Extensions to the caching system.</li>
    1318       </ul>
    1319       <h3 id="rfc.section.15.2.1"><a href="#rfc.section.15.2.1">15.2.1</a>&nbsp;<a id="what.is.cacheable" href="#what.is.cacheable">What is Cacheable</a></h3>
    1320       <p id="rfc.section.15.2.1.p.1">By default, a response is cacheable if the requirements of the request method, request header fields, and the response status
    1321          indicate that it is cacheable. <a href="#response.cacheability" title="Response Cacheability">Section&nbsp;5</a> summarizes these defaults for cacheability. The following Cache-Control response directives allow an origin server to override
    1322          the default cacheability of a response:
    1323       </p>
    1324       <p id="rfc.section.15.2.1.p.2"> <span id="rfc.iref.c.4"></span>  <span id="rfc.iref.p.1"></span> public
    1325       </p>
    1326       <dl class="empty">
    1327          <dd>Indicates that the response <em class="bcp14">MAY</em> be cached by any cache, even if it would normally be non-cacheable or cacheable only within a non-shared cache. (See also
    1328             Authorization, <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.2"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>, for additional details.)
    1329          </dd>
    1330       </dl>
    1331       <p id="rfc.section.15.2.1.p.3"> <span id="rfc.iref.c.5"></span>  <span id="rfc.iref.p.2"></span> private
    1332       </p>
    1333       <dl class="empty">
    1334          <dd>Indicates that all or part of the response message is intended for a single user and <em class="bcp14">MUST NOT</em> be cached by a shared cache. This allows an origin server to state that the specified parts of the response are intended for
    1335             only one user and are not a valid response for requests by other users. A private (non-shared) cache <em class="bcp14">MAY</em> cache the response.
    1336          </dd>
    1337          <dd> <b>Note:</b> This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message
     983</pre><h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="cache-request-directive" href="#cache-request-directive">Request Cache-Control Directives</a></h3>
     984      <div id="rfc.figure.u.10"></div> <pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#header.cache-control" class="smpl">cache-request-directive</a> =
     985       "no-cache"
     986     / "no-store"
     987     / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
     988     / "max-stale" [ "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a> ]
     989     / "min-fresh" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
     990     / "no-transform"
     991     / "only-if-cached"
     992     / <a href="#header.cache-control" class="smpl">cache-extension</a>
     993</pre> <p id="rfc.section.3.2.1.p.2"> <span id="rfc.iref.c.4"></span>  <span id="rfc.iref.n.1"></span> no-cache
     994      </p>
     995      <dl class="empty">
     996         <dd>The no-cache request directive indicates that a stored response <em class="bcp14">MUST NOT</em> be used to satisfy the request without successful validation on the origin server.
     997         </dd>
     998      </dl>
     999      <p id="rfc.section.3.2.1.p.3"> <span id="rfc.iref.c.5"></span>  <span id="rfc.iref.n.2"></span> no-store
     1000      </p>
     1001      <dl class="empty">
     1002         <dd>The no-store request directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either this request or any response to it. This directive applies to both non-shared and shared caches.
     1003            "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
     1004         </dd>
     1005         <dd>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
     1006            might not recognize or obey this directive, and communications networks may be vulnerable to eavesdropping.
     1007         </dd>
     1008      </dl>
     1009      <p id="rfc.section.3.2.1.p.4"> <span id="rfc.iref.c.6"></span>  <span id="rfc.iref.m.1"></span> max-age
     1010      </p>
     1011      <dl class="empty">
     1012         <dd>The max-age request directive indicates that the client is willing to accept a response whose age is no greater than the specified
     1013            time in seconds. Unless max-stale directive is also included, the client is not willing to accept a stale response.
     1014         </dd>
     1015      </dl>
     1016      <p id="rfc.section.3.2.1.p.5"> <span id="rfc.iref.c.7"></span>  <span id="rfc.iref.m.2"></span> max-stale
     1017      </p>
     1018      <dl class="empty">
     1019         <dd>The max-stale request directive indicates that the client is willing to accept a response that has exceeded its expiration
     1020            time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time
     1021            by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept
     1022            a stale response of any age. <span class="comment">[rfc.comment.15: of any staleness? --mnot]</span></dd>
     1023      </dl>
     1024      <p id="rfc.section.3.2.1.p.6"> <span id="rfc.iref.c.8"></span>  <span id="rfc.iref.m.3"></span> min-fresh
     1025      </p>
     1026      <dl class="empty">
     1027         <dd>The min-fresh request directive indicates that the client is willing to accept a response whose freshness lifetime is no less
     1028            than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for
     1029            at least the specified number of seconds.
     1030         </dd>
     1031      </dl>
     1032      <p id="rfc.section.3.2.1.p.7"> <span id="rfc.iref.c.9"></span>  <span id="rfc.iref.n.3"></span> no-transform
     1033      </p>
     1034      <dl class="empty">
     1035         <dd>The no-transform request directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type request headers, nor the request entity-body.
     1036         </dd>
     1037      </dl>
     1038      <p id="rfc.section.3.2.1.p.8"> <span id="rfc.iref.c.10"></span>  <span id="rfc.iref.o.1"></span> only-if-cached
     1039      </p>
     1040      <dl class="empty">
     1041         <dd>The only-if-cached request directive indicates that the client only wishes to return a stored response. If it receives this
     1042            directive, a cache <em class="bcp14">SHOULD</em> either respond using a stored response that is consistent with the other constraints of the request, or respond with a 504
     1043            (Gateway Timeout) status. If a group of caches is being operated as a unified system with good internal connectivity, such
     1044            a request <em class="bcp14">MAY</em> be forwarded within that group of caches.
     1045         </dd>
     1046      </dl>
     1047      <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="cache-response-directive" href="#cache-response-directive">Response Cache-Control Directives</a></h3>
     1048      <div id="rfc.figure.u.11"></div> <pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#header.cache-control" class="smpl">cache-response-directive</a> =
     1049       "public"
     1050     / "private" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ]
     1051     / "no-cache" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ]
     1052     / "no-store"
     1053     / "no-transform"
     1054     / "must-revalidate"
     1055     / "proxy-revalidate"
     1056     / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
     1057     / "s-maxage" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
     1058     / <a href="#header.cache-control" class="smpl">cache-extension</a>
     1059</pre> <p id="rfc.section.3.2.2.p.2"> <span id="rfc.iref.c.11"></span>  <span id="rfc.iref.p.1"></span> public
     1060      </p>
     1061      <dl class="empty">
     1062         <dd>The public response directive indicates that the response <em class="bcp14">MAY</em> be cached, even if it would normally be non-cacheable or cacheable only within a non-shared cache. (See also Authorization, <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.2"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>, for additional details.)
     1063         </dd>
     1064      </dl>
     1065      <p id="rfc.section.3.2.2.p.3"> <span id="rfc.iref.c.12"></span>  <span id="rfc.iref.p.2"></span> private
     1066      </p>
     1067      <dl class="empty">
     1068         <dd>The private response directive indicates that the response message is intended for a single user and <em class="bcp14">MUST NOT</em> be stored by a shared cache. A private (non-shared) cache <em class="bcp14">MAY</em> store the response.
     1069         </dd>
     1070         <dd>If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated
     1071            with the listed response headers. That is, the specified field-names(s) <em class="bcp14">MUST NOT</em> be stored by a shared cache, whereas the remainder of the response message <em class="bcp14">MAY</em> be.
     1072         </dd>
     1073         <dd> <b>Note:</b> This usage of the word private only controls where the response may be stored, and cannot ensure the privacy of the message
    13381074            content.
    13391075         </dd>
    13401076      </dl>
    1341       <p id="rfc.section.15.2.1.p.4"> <span id="rfc.iref.c.6"></span>  <span id="rfc.iref.n.1"></span> no-cache
    1342       </p>
    1343       <dl class="empty">
    1344          <dd>If the no-cache directive does not specify a field-name, then a cache <em class="bcp14">MUST NOT</em> use the response to satisfy a subsequent request without successful revalidation with the origin server. This allows an origin
    1345             server to prevent caching even by caches that have been configured to return stale responses to client requests.
    1346          </dd>
    1347          <dd>If the no-cache directive does specify one or more field-names, then a cache <em class="bcp14">MAY</em> use the response to satisfy a subsequent request, subject to any other restrictions on caching. However, the specified field-name(s) <em class="bcp14">MUST NOT</em> be sent in the response to a subsequent request without successful revalidation with the origin server. This allows an origin
    1348             server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
    1349             <dl class="empty">
    1350                <dd> <b>Note:</b> Most HTTP/1.0 caches will not recognize or obey this directive.
    1351                </dd>
    1352             </dl>
    1353          </dd>
    1354       </dl>
    1355       <h3 id="rfc.section.15.2.2"><a href="#rfc.section.15.2.2">15.2.2</a>&nbsp;<a id="what.may.be.stored.by.caches" href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></h3>
    1356       <p id="rfc.section.15.2.2.p.1"> <span id="rfc.iref.c.7"></span>  <span id="rfc.iref.n.2"></span> no-store
    1357       </p>
    1358       <dl class="empty">
    1359          <dd>The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example,
    1360             on backup tapes). The no-store directive applies to the entire message, and <em class="bcp14">MAY</em> be sent either in a response or in a request. If sent in a request, a cache <em class="bcp14">MUST NOT</em> store any part of either this request or any response to it. If sent in a response, a cache <em class="bcp14">MUST NOT</em> store any part of either this response or the request that elicited it. This directive applies to both non-shared and shared
    1361             caches. "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
    1362          </dd>
    1363          <dd>Even when this directive is associated with a response, users might explicitly store such a response outside of the caching
    1364             system (e.g., with a "Save As" dialog). History buffers <em class="bcp14">MAY</em> store such responses as part of their normal operation.
    1365          </dd>
    1366          <dd>The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about
    1367             accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive might
    1368             improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy.
    1369             In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might
    1370             be vulnerable to eavesdropping.
    1371          </dd>
    1372       </dl>
    1373       <h3 id="rfc.section.15.2.3"><a href="#rfc.section.15.2.3">15.2.3</a>&nbsp;<a id="modifications.of.the.basic.expiration.mechanism" href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></h3>
    1374       <p id="rfc.section.15.2.3.p.1">The expiration time of an entity <em class="bcp14">MAY</em> be specified by the origin server using the Expires header (see <a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;15.3</a>). Alternatively, it <em class="bcp14">MAY</em> be specified using the max-age directive in a response. When the max-age cache-control directive is present in a cached response,
    1375          the response is stale if its current age is greater than the age value given (in seconds) at the time of a new request for
    1376          that resource. The max-age directive on a response implies that the response is cacheable (i.e., "public") unless some other,
    1377          more restrictive cache directive is also present.
    1378       </p>
    1379       <p id="rfc.section.15.2.3.p.2">If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header,
    1380          even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer
    1381          expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be useful if certain HTTP/1.0 caches
    1382          improperly calculate ages or expiration times, perhaps due to desynchronized clocks.
    1383       </p>
    1384       <p id="rfc.section.15.2.3.p.3">Many HTTP/1.0 cache implementations will treat an Expires value that is less than or equal to the response Date value as being
    1385          equivalent to the Cache-Control response directive "no-cache". If an HTTP/1.1 cache receives such a response, and the response
    1386          does not include a Cache-Control header field, it <em class="bcp14">SHOULD</em> consider the response to be non-cacheable in order to retain compatibility with HTTP/1.0 servers.
    1387       </p>
    1388       <dl class="empty">
    1389          <dd> <b>Note:</b> An origin server might wish to use a relatively new HTTP cache control feature, such as the "private" directive, on a network
    1390             including older caches that do not understand that feature. The origin server will need to combine the new feature with an
    1391             Expires field whose value is less than or equal to the Date value. This will prevent older caches from improperly caching
    1392             the response.
    1393          </dd>
    1394       </dl>
    1395       <p id="rfc.section.15.2.3.p.4"> <span id="rfc.iref.c.8"></span>  <span id="rfc.iref.s.3"></span> s-maxage
    1396       </p>
    1397       <dl class="empty">
    1398          <dd>If a response includes an s-maxage directive, then for a shared cache (but not for a private cache), the maximum age specified
    1399             by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage
    1400             directive also implies the semantics of the proxy-revalidate directive (see <a href="#cache.revalidation.and.reload.controls" title="Cache Revalidation and Reload Controls">Section&nbsp;15.2.4</a>), i.e., that the shared cache must not use the entry after it becomes stale to respond to a subsequent request without first
    1401             revalidating it with the origin server. The s-maxage directive is always ignored by a private cache.
    1402          </dd>
    1403       </dl>
    1404       <p id="rfc.section.15.2.3.p.5">Note that most older caches, not compliant with this specification, do not implement any cache-control directives. An origin
    1405          server wishing to use a cache-control directive that restricts, but does not prevent, caching by an HTTP/1.1-compliant cache <em class="bcp14">MAY</em> exploit the requirement that the max-age directive overrides the Expires header, and the fact that pre-HTTP/1.1-compliant
    1406          caches do not observe the max-age directive.
    1407       </p>
    1408       <p id="rfc.section.15.2.3.p.6">Other directives allow a user agent to modify the basic expiration mechanism. These directives <em class="bcp14">MAY</em> be specified on a request:
    1409       </p>
    1410       <p id="rfc.section.15.2.3.p.7"> <span id="rfc.iref.c.9"></span>  <span id="rfc.iref.m.1"></span> max-age
    1411       </p>
    1412       <dl class="empty">
    1413          <dd>Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless
    1414             max-stale directive is also included, the client is not willing to accept a stale response.
    1415          </dd>
    1416       </dl>
    1417       <p id="rfc.section.15.2.3.p.8"> <span id="rfc.iref.c.10"></span>  <span id="rfc.iref.m.2"></span> min-fresh
    1418       </p>
    1419       <dl class="empty">
    1420          <dd>Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the
    1421             specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number
    1422             of seconds.
    1423          </dd>
    1424       </dl>
    1425       <p id="rfc.section.15.2.3.p.9"> <span id="rfc.iref.c.11"></span>  <span id="rfc.iref.m.3"></span> max-stale
    1426       </p>
    1427       <dl class="empty">
    1428          <dd>Indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned
    1429             a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified
    1430             number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age.
    1431          </dd>
    1432       </dl>
    1433       <p id="rfc.section.15.2.3.p.10">If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured
    1434          to override the expiration time of a response, the cache <em class="bcp14">MUST</em> attach a Warning header to the stale response, using Warning 110 (Response is stale).
    1435       </p>
    1436       <p id="rfc.section.15.2.3.p.11">A cache <em class="bcp14">MAY</em> be configured to return stale responses without validation, but only if this does not conflict with any "MUST"-level requirements
    1437          concerning cache validation (e.g., a "must-revalidate" cache-control directive).
    1438       </p>
    1439       <p id="rfc.section.15.2.3.p.12">If both the new request and the cached entry include "max-age" directives, then the lesser of the two values is used for determining
    1440          the freshness of the cached entry for that request.
    1441       </p>
    1442       <h3 id="rfc.section.15.2.4"><a href="#rfc.section.15.2.4">15.2.4</a>&nbsp;<a id="cache.revalidation.and.reload.controls" href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></h3>
    1443       <p id="rfc.section.15.2.4.p.1">Sometimes a user agent might want or need to insist that a cache revalidate its cache entry with the origin server (and not
    1444          just with the next cache along the path to the origin server), or to reload its cache entry from the origin server. End-to-end
    1445          revalidation might be necessary if either the cache or the origin server has overestimated the expiration time of the cached
    1446          response. End-to-end reload may be necessary if the cache entry has become corrupted for some reason.
    1447       </p>
    1448       <p id="rfc.section.15.2.4.p.2">End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we
    1449          call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it
    1450          "specific end-to-end revalidation."
    1451       </p>
    1452       <p id="rfc.section.15.2.4.p.3">The client can specify these three kinds of action using Cache-Control request directives:</p>
    1453       <p id="rfc.section.15.2.4.p.4">End-to-end reload </p>
    1454       <dl class="empty">
    1455          <dd>The request includes a "no-cache" cache-control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache".
    1456             Field names <em class="bcp14">MUST NOT</em> be included with the no-cache directive in a request. The server <em class="bcp14">MUST NOT</em> use a cached copy when responding to such a request.
    1457          </dd>
    1458       </dl>
    1459       <p id="rfc.section.15.2.4.p.5">Specific end-to-end revalidation </p>
    1460       <dl class="empty">
    1461          <dd>The request includes a "max-age=0" cache-control directive, which forces each cache along the path to the origin server to
    1462             revalidate its own entry, if any, with the next cache or server. The initial request includes a cache-validating conditional
    1463             with the client's current validator.
    1464          </dd>
    1465       </dl>
    1466       <p id="rfc.section.15.2.4.p.6">Unspecified end-to-end revalidation </p>
    1467       <dl class="empty">
    1468          <dd>The request includes "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate
    1469             its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional;
    1470             the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional
    1471             with its current validator.
    1472          </dd>
    1473       </dl>
    1474       <p id="rfc.section.15.2.4.p.7"> <span id="rfc.iref.c.12"></span>  <span id="rfc.iref.m.4"></span> max-age
    1475       </p>
    1476       <dl class="empty">
    1477          <dd>When an intermediate cache is forced, by means of a max-age=0 directive, to revalidate its own cache entry, and the client
    1478             has supplied its own validator in the request, the supplied validator might differ from the validator currently stored with
    1479             the cache entry. In this case, the cache <em class="bcp14">MAY</em> use either validator in making its own request without affecting semantic transparency.
    1480          </dd>
    1481          <dd>However, the choice of validator might affect performance. The best approach is for the intermediate cache to use its own
    1482             validator when making its request. If the server replies with 304 (Not Modified), then the cache can return its now validated
    1483             copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate
    1484             cache can compare the returned validator with the one provided in the client's request, using the strong comparison function.
    1485             If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified).
    1486             Otherwise, it returns the new entity with a 200 (OK) response.
    1487          </dd>
    1488          <dd>If a request includes the no-cache directive, it <em class="bcp14">SHOULD NOT</em> include min-fresh, max-stale, or max-age.
    1489          </dd>
    1490       </dl>
    1491       <p id="rfc.section.15.2.4.p.8"> <span id="rfc.iref.c.13"></span>  <span id="rfc.iref.o.1"></span> only-if-cached
    1492       </p>
    1493       <dl class="empty">
    1494          <dd>In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses
    1495             that it currently has stored, and not to reload or revalidate with the origin server. To do this, the client may include the
    1496             only-if-cached directive in a request. If it receives this directive, a cache <em class="bcp14">SHOULD</em> either respond using a cached entry that is consistent with the other constraints of the request, or respond with a 504 (Gateway
    1497             Timeout) status. However, if a group of caches is being operated as a unified system with good internal connectivity, such
    1498             a request <em class="bcp14">MAY</em> be forwarded within that group of caches.
    1499          </dd>
    1500       </dl>
    1501       <p id="rfc.section.15.2.4.p.9"> <span id="rfc.iref.c.14"></span>  <span id="rfc.iref.m.5"></span> must-revalidate
    1502       </p>
    1503       <dl class="empty">
    1504          <dd>Because a cache <em class="bcp14">MAY</em> be configured to ignore a server's specified expiration time, and because a client request <em class="bcp14">MAY</em> include a max-stale directive (which has a similar effect), the protocol also includes a mechanism for the origin server to
    1505             require revalidation of a cache entry on any subsequent use. When the must-revalidate directive is present in a response received
    1506             by a cache, that cache <em class="bcp14">MUST NOT</em> use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server.
    1507             (I.e., the cache <em class="bcp14">MUST</em> do an end-to-end revalidation every time, if, based solely on the origin server's Expires or max-age value, the cached response
    1508             is stale.)
     1077      <p id="rfc.section.3.2.2.p.4"> <span id="rfc.iref.c.13"></span>  <span id="rfc.iref.n.4"></span> no-cache
     1078      </p>
     1079      <dl class="empty">
     1080         <dd>The no-cache response directive indicates that the response <em class="bcp14">MUST NOT</em> be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to
     1081            prevent caching even by caches that have been configured to return stale responses.
     1082         </dd>
     1083         <dd>If the no-cache response directive specifies one or more field-names, this requirement is limited to the field-values assosicated
     1084            with the listed response headers. That is, the specified field-name(s) <em class="bcp14">MUST NOT</em> be sent in the response to a subsequent request without successful validation on the origin server. This allows an origin
     1085            server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
     1086         </dd>
     1087         <dd> <b>Note:</b> Most HTTP/1.0 caches will not recognize or obey this directive.
     1088         </dd>
     1089      </dl>
     1090      <p id="rfc.section.3.2.2.p.5"> <span id="rfc.iref.c.14"></span>  <span id="rfc.iref.n.5"></span> no-store
     1091      </p>
     1092      <dl class="empty">
     1093         <dd>The no-store response directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either the immediate request or response. This directive applies to both non-shared and shared caches. "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
     1094         </dd>
     1095         <dd>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
     1096            might not recognize or obey this directive, and communications networks may be vulnerable to eavesdropping.
     1097         </dd>
     1098      </dl>
     1099      <p id="rfc.section.3.2.2.p.6"> <span id="rfc.iref.c.15"></span>  <span id="rfc.iref.m.4"></span> must-revalidate
     1100      </p>
     1101      <dl class="empty">
     1102         <dd>The must-revalidate response directive indicates that once it has become stale, the response <em class="bcp14">MUST NOT</em> be used to satisfy subsequent requests without successful validation on the origin server.
    15091103         </dd>
    15101104         <dd>The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
    15111105            an HTTP/1.1 cache <em class="bcp14">MUST</em> obey the must-revalidate directive; in particular, if the cache cannot reach the origin server for any reason, it <em class="bcp14">MUST</em> generate a 504 (Gateway Timeout) response.
    15121106         </dd>
    1513          <dd>Servers <em class="bcp14">SHOULD</em> send the must-revalidate directive if and only if failure to revalidate a request on the entity could result in incorrect
    1514             operation, such as a silently unexecuted financial transaction. Recipients <em class="bcp14">MUST NOT</em> take any automated action that violates this directive, and <em class="bcp14">MUST NOT</em> automatically provide an unvalidated copy of the entity if revalidation fails.
    1515          </dd>
    1516          <dd>Although this is not recommended, user agents operating under severe connectivity constraints <em class="bcp14">MAY</em> violate this directive but, if so, <em class="bcp14">MUST</em> explicitly warn the user that an unvalidated response has been provided. The warning <em class="bcp14">MUST</em> be provided on each unvalidated access, and <em class="bcp14">SHOULD</em> require explicit user confirmation.
    1517          </dd>
    1518       </dl>
    1519       <p id="rfc.section.15.2.4.p.10"> <span id="rfc.iref.c.15"></span>  <span id="rfc.iref.p.3"></span> proxy-revalidate
    1520       </p>
    1521       <dl class="empty">
    1522          <dd>The proxy-revalidate directive has the same meaning as the must-revalidate directive, except that it does not apply to non-shared
    1523             user agent caches. It can be used on a response to an authenticated request to permit the user's cache to store and later
    1524             return the response without needing to revalidate it (since it has already been authenticated once by that user), while still
    1525             requiring proxies that service many users to revalidate each time (in order to make sure that each user has been authenticated).
    1526             Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at
    1527             all.
    1528          </dd>
    1529       </dl>
    1530       <h3 id="rfc.section.15.2.5"><a href="#rfc.section.15.2.5">15.2.5</a>&nbsp;<a id="no-transform.directive" href="#no-transform.directive">No-Transform Directive</a></h3>
    1531       <p id="rfc.section.15.2.5.p.1"> <span id="rfc.iref.c.16"></span>  <span id="rfc.iref.n.3"></span> no-transform
    1532       </p>
    1533       <dl class="empty">
    1534          <dd>Implementors of intermediate caches (proxies) have found it useful to convert the media type of certain entity bodies. A non-transparent
    1535             proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on
    1536             a slow link.
    1537          </dd>
    1538          <dd>Serious operational problems occur, however, when these transformations are applied to entity bodies intended for certain
    1539             kinds of applications. For example, applications for medical imaging, scientific data analysis and those using end-to-end
    1540             authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body.
    1541          </dd>
    1542          <dd>Therefore, if a message includes the no-transform directive, an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change those headers that are listed in <a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a> as being subject to the no-transform directive. This implies that the cache or proxy <em class="bcp14">MUST NOT</em> change any aspect of the entity-body that is specified by these headers, including the value of the entity-body itself.
    1543          </dd>
    1544       </dl>
    1545       <h3 id="rfc.section.15.2.6"><a href="#rfc.section.15.2.6">15.2.6</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
    1546       <p id="rfc.section.15.2.6.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
    1547          assigned value. Informational extensions (those which do not require a change in cache behavior) <em class="bcp14">MAY</em> be added without changing the semantics of other directives. Behavioral extensions are designed to work by acting as modifiers
    1548          to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications
    1549          which do not understand the new directive will default to the behavior specified by the standard directive, and those that
    1550          understand the new directive will recognize it as modifying the requirements associated with the standard directive. In this
    1551          way, extensions to the cache-control directives can be made without requiring changes to the base protocol.
    1552       </p>
    1553       <p id="rfc.section.15.2.6.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
     1107         <dd>Servers <em class="bcp14">SHOULD</em> send the must-revalidate directive if and only if failure to validate a request on the entity could result in incorrect operation,
     1108            such as a silently unexecuted financial transaction.
     1109         </dd>
     1110      </dl>
     1111      <p id="rfc.section.3.2.2.p.7"> <span id="rfc.iref.c.16"></span>  <span id="rfc.iref.p.3"></span> proxy-revalidate
     1112      </p>
     1113      <dl class="empty">
     1114         <dd>The proxy-revalidate response directive has the same meaning as the must-revalidate response directive, except that it does
     1115            not apply to non-shared caches.
     1116         </dd>
     1117      </dl>
     1118      <p id="rfc.section.3.2.2.p.8"> <span id="rfc.iref.c.17"></span>  <span id="rfc.iref.m.5"></span> max-age
     1119      </p>
     1120      <dl class="empty">
     1121         <dd>The max-age response directive indicates that response is to be considered stale after its age is greater than the specified
     1122            number of seconds.
     1123         </dd>
     1124      </dl>
     1125      <p id="rfc.section.3.2.2.p.9"> <span id="rfc.iref.c.18"></span>  <span id="rfc.iref.s.2"></span> s-maxage
     1126      </p>
     1127      <dl class="empty">
     1128         <dd>The s-maxage response directive indicates that, in shared caches, the maximum age specified by this directive overrides the
     1129            maximum age specified by either the max-age directive or the Expires header. The s-maxage directive also implies the semantics
     1130            of the proxy-revalidate response directive.
     1131         </dd>
     1132      </dl>
     1133      <p id="rfc.section.3.2.2.p.10"> <span id="rfc.iref.c.19"></span>  <span id="rfc.iref.n.6"></span> no-transform
     1134      </p>
     1135      <dl class="empty">
     1136         <dd>The no-transform response directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type response headers, nor the response entity-body.
     1137         </dd>
     1138      </dl>
     1139      <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
     1140      <p id="rfc.section.3.2.3.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
     1141         value. Informational extensions (those that do not require a change in cache behavior) can be added without changing the semantics
     1142         of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives.
     1143         Both the new directive and the standard directive are supplied, such that applications that do not understand the new directive
     1144         will default to the behavior specified by the standard directive, and those that understand the new directive will recognize
     1145         it as modifying the requirements associated with the standard directive. In this way, extensions to the cache-control directives
     1146         can be made without requiring changes to the base protocol.
     1147      </p>
     1148      <p id="rfc.section.3.2.3.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
    15541149         obeying certain extensions, and ignoring all directives that it does not understand.
    15551150      </p>
    1556       <p id="rfc.section.15.2.6.p.3">For example, consider a hypothetical new response directive called community which acts as a modifier to the private directive.
    1557          We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members
    1558          of the community named within its value may cache the response. An origin server wishing to allow the UCI community to use
    1559          an otherwise private response in their shared cache(s) could do so by including
    1560       </p>
    1561       <div id="rfc.figure.u.14"></div><pre class="text">  Cache-Control: private, community="UCI"
    1562 </pre><p id="rfc.section.15.2.6.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
     1151      <p id="rfc.section.3.2.3.p.3">For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive.
     1152         We define this new directive to mean that, in addition to any non-shared cache, any cache that is shared only by members of
     1153         the community named within its value may cache the response. An origin server wishing to allow the UCI community to use an
     1154         otherwise private response in their shared cache(s) could do so by including
     1155      </p>
     1156      <div id="rfc.figure.u.12"></div> <pre class="text">  Cache-Control: private, community="UCI"
     1157</pre> <p id="rfc.section.3.2.3.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
    15631158         it will also see and understand the private directive and thus default to the safe behavior.
    15641159      </p>
    1565       <p id="rfc.section.15.2.6.p.6">Unrecognized cache-directives <em class="bcp14">MUST</em> be ignored; it is assumed that any cache-directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
     1160      <p id="rfc.section.3.2.3.p.6">Unrecognized cache directives <em class="bcp14">MUST</em> be ignored; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
    15661161         directives (or the response's default cacheability) such that the cache behavior will remain minimally correct even if the
    15671162         cache does not understand the extension(s).
     
    15691164      <div id="rfc.iref.e.2"></div>
    15701165      <div id="rfc.iref.h.4"></div>
    1571       <h2 id="rfc.section.15.3"><a href="#rfc.section.15.3">15.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
    1572       <p id="rfc.section.15.3.p.1">The entity-header field "Expires" gives the date/time after which the response is considered stale. A stale cache entry may
    1573          not normally be returned by a cache (either a proxy cache or a user agent cache) unless it is first validated with the origin
    1574          server (or with an intermediate cache that has a fresh copy of the entity). See <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a> for further discussion of the expiration model.
    1575       </p>
    1576       <p id="rfc.section.15.3.p.2">The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after
     1166      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
     1167      <p id="rfc.section.3.3.p.1">The entity-header field "Expires" gives the date/time after which the response is considered stale. See <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a> for further discussion of the freshness model.
     1168      </p>
     1169      <p id="rfc.section.3.3.p.2">The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after
    15771170         that time.
    15781171      </p>
    1579       <p id="rfc.section.15.3.p.3">The format is an absolute date and time as defined by HTTP-date in <a href="p1-messaging.html#full.date" title="Full Date">Section 3.2.1</a> of <a href="#Part1" id="rfc.xref.Part1.16"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>; it <em class="bcp14">MUST</em> be sent in rfc1123-date format.
    1580       </p>
    1581       <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span>  <a href="#header.expires" class="smpl">Expires</a>   = "Expires" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.expires" class="smpl">Expires-v</a>
     1172      <p id="rfc.section.3.3.p.3">The field-value is an absolute date and time as defined by HTTP-date in <a href="p1-messaging.html#full.date" title="Full Date">Section 3.2.1</a> of <a href="#Part1" id="rfc.xref.Part1.13"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>; it <em class="bcp14">MUST</em> be sent in rfc1123-date format.
     1173      </p>
     1174      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span>  <a href="#header.expires" class="smpl">Expires</a>   = "Expires" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.expires" class="smpl">Expires-v</a>
    15821175  <a href="#header.expires" class="smpl">Expires-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
    1583 </pre><p id="rfc.section.15.3.p.5">An example of its use is</p>
    1584       <div id="rfc.figure.u.16"></div><pre class="text">  Expires: Thu, 01 Dec 1994 16:00:00 GMT
    1585 </pre><p id="rfc.section.15.3.p.7"> </p>
    1586       <dl class="empty">
    1587          <dd> <b>Note:</b> if a response includes a Cache-Control field with the max-age directive (see <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>), that directive overrides the Expires field.
    1588          </dd>
    1589       </dl>
    1590       <p id="rfc.section.15.3.p.8">HTTP/1.1 clients and caches <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
    1591       </p>
    1592       <p id="rfc.section.15.3.p.9">To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header value. (See
    1593          the rules for expiration calculations in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;3.4</a>.)
    1594       </p>
    1595       <p id="rfc.section.15.3.p.10">To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time the response
    1596          is sent. HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
    1597       </p>
    1598       <p id="rfc.section.15.3.p.11">The presence of an Expires header field with a date value of some time in the future on a response that otherwise would by
    1599          default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field
    1600          (<a href="#header.cache-control" id="rfc.xref.header.cache-control.9" title="Cache-Control">Section&nbsp;15.2</a>).
     1176</pre><p id="rfc.section.3.3.p.5">For example</p>
     1177      <div id="rfc.figure.u.14"></div> <pre class="text">  Expires: Thu, 01 Dec 1994 16:00:00 GMT
     1178</pre> <p id="rfc.section.3.3.p.7"> </p>
     1179      <dl class="empty">
     1180         <dd> <b>Note:</b> if a response includes a Cache-Control field with the max-age directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>), that directive overrides the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
     1181         </dd>
     1182      </dl>
     1183      <p id="rfc.section.3.3.p.8">HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
     1184      </p>
     1185      <p id="rfc.section.3.3.p.9">HTTP/1.1 clients and caches <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
    16011186      </p>
    16021187      <div id="rfc.iref.p.4"></div>
    16031188      <div id="rfc.iref.h.5"></div>
    1604       <h2 id="rfc.section.15.4"><a href="#rfc.section.15.4">15.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
    1605       <p id="rfc.section.15.4.p.1">The general-header field "Pragma" is used to include implementation-specific directives that might apply to any recipient
     1189      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
     1190      <p id="rfc.section.3.4.p.1">The general-header field "Pragma" is used to include implementation-specific directives that might apply to any recipient
    16061191         along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however,
    16071192         some systems <em class="bcp14">MAY</em> require that behavior be consistent with the directives.
    16081193      </p>
    1609       <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.12"></span><span id="rfc.iref.g.13"></span><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span>  <a href="#header.pragma" class="smpl">Pragma</a>            = "Pragma" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.pragma" class="smpl">Pragma-v</a>
     1194      <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span><span id="rfc.iref.g.13"></span><span id="rfc.iref.g.14"></span>  <a href="#header.pragma" class="smpl">Pragma</a>            = "Pragma" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.pragma" class="smpl">Pragma-v</a>
    16101195  <a href="#header.pragma" class="smpl">Pragma-v</a>          = 1#<a href="#header.pragma" class="smpl">pragma-directive</a>
    16111196  <a href="#header.pragma" class="smpl">pragma-directive</a>  = "no-cache" / <a href="#header.pragma" class="smpl">extension-pragma</a>
    16121197  <a href="#header.pragma" class="smpl">extension-pragma</a>  = <a href="#core.rules" class="smpl">token</a> [ "=" ( <a href="#core.rules" class="smpl">token</a> / <a href="#core.rules" class="smpl">quoted-string</a> ) ]
    1613 </pre><p id="rfc.section.15.4.p.3">When the no-cache directive is present in a request message, an application <em class="bcp14">SHOULD</em> forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive
    1614          has the same semantics as the no-cache cache-directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.10" title="Cache-Control">Section&nbsp;15.2</a>) and is defined here for backward compatibility with HTTP/1.0. Clients <em class="bcp14">SHOULD</em> include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant.
    1615       </p>
    1616       <p id="rfc.section.15.4.p.4">Pragma directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
    1617          might be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific
    1618          recipient; however, any pragma directive not relevant to a recipient <em class="bcp14">SHOULD</em> be ignored by that recipient.
    1619       </p>
    1620       <p id="rfc.section.15.4.p.5">HTTP/1.1 caches <em class="bcp14">SHOULD</em> treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in
    1621          HTTP.
    1622       </p>
     1198</pre><p id="rfc.section.3.4.p.3">When the no-cache directive is present in a request message, an application <em class="bcp14">SHOULD</em> forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive
     1199         has the same semantics as the no-cache response directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) and is defined here for backward compatibility with HTTP/1.0. Clients <em class="bcp14">SHOULD</em> include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches <em class="bcp14">SHOULD</em> treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
     1200      </p>
     1201      <p id="rfc.section.3.4.p.4"> </p>
    16231202      <dl class="empty">
    16241203         <dd> <b>Note:</b> because the meaning of "Pragma: no-cache" as a response-header field is not actually specified, it does not provide a reliable
     
    16261205         </dd>
    16271206      </dl>
    1628       <div id="rfc.iref.v.2"></div>
     1207      <p id="rfc.section.3.4.p.5">This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</p>
     1208      <div id="rfc.iref.v.3"></div>
    16291209      <div id="rfc.iref.h.6"></div>
    1630       <h2 id="rfc.section.15.5"><a href="#rfc.section.15.5">15.5</a>&nbsp;<a id="header.vary" href="#header.vary">Vary</a></h2>
    1631       <p id="rfc.section.15.5.p.1">The "Vary" response-header field's value indicates the set of request-header fields that fully determines, while the response
    1632          is fresh, whether a cache is permitted to use the response to reply to a subsequent request without revalidation. For uncacheable
    1633          or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation.
    1634          A Vary field value of "*" implies that a cache cannot determine from the request headers of a subsequent request whether this
    1635          response is the appropriate representation. See <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;7</a> for use of the Vary header field by caches.
    1636       </p>
    1637       <div id="rfc.figure.u.18"></div><pre class="inline"><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span>  <a href="#header.vary" class="smpl">Vary</a>   = "Vary" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.vary" class="smpl">Vary-v</a>
     1210      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="header.vary" href="#header.vary">Vary</a></h2>
     1211      <p id="rfc.section.3.5.p.1">The "Vary" response-header field's value indicates the set of request-header fields that determines, while the response is
     1212         fresh, whether a cache is permitted to use the response to reply to a subsequent request without validation; see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.6</a>.
     1213      </p>
     1214      <p id="rfc.section.3.5.p.2">In uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select
     1215         the representation.
     1216      </p>
     1217      <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.15"></span><span id="rfc.iref.g.16"></span>  <a href="#header.vary" class="smpl">Vary</a>   = "Vary" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.vary" class="smpl">Vary-v</a>
    16381218  <a href="#header.vary" class="smpl">Vary-v</a> = "*" / 1#<a href="#abnf.dependencies" class="smpl">field-name</a>
    1639 </pre><p id="rfc.section.15.5.p.3">An HTTP/1.1 server <em class="bcp14">SHOULD</em> include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache
     1219</pre><p id="rfc.section.3.5.p.4">The set of header fields named by the Vary field value is known as the selecting request-headers.</p>
     1220      <p id="rfc.section.3.5.p.5">Servers <em class="bcp14">SHOULD</em> include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache
    16401221         to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that
    16411222         resource. A server <em class="bcp14">MAY</em> include a Vary header field with a non-cacheable response that is subject to server-driven negotiation, since this might provide
    16421223         the user agent with useful information about the dimensions over which the response varies at the time of the response.
    16431224      </p>
    1644       <p id="rfc.section.15.5.p.4">A Vary field value consisting of a list of field-names signals that the representation selected for the response is based
    1645          on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation.
    1646          A cache <em class="bcp14">MAY</em> assume that the same selection will be made for future requests with the same values for the listed field names, for the duration
    1647          of time for which the response is fresh.
    1648       </p>
    1649       <p id="rfc.section.15.5.p.5">The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names
     1225      <p id="rfc.section.3.5.p.6">A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address
     1226         of the client), play a role in the selection of the response representation; therefore, a cache cannot determine whether this
     1227         response is appropriate. The "*" value <em class="bcp14">MUST NOT</em> be generated by a proxy server; it may only be generated by an origin server.
     1228      </p>
     1229      <p id="rfc.section.3.5.p.7">The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names
    16501230         are case-insensitive.
    1651       </p>
    1652       <p id="rfc.section.15.5.p.6">A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address
    1653          of the client), play a role in the selection of the response representation. The "*" value <em class="bcp14">MUST NOT</em> be generated by a proxy server; it may only be generated by an origin server.
    16541231      </p>
    16551232      <div id="rfc.iref.w.1"></div>
    16561233      <div id="rfc.iref.h.7"></div>
    1657       <h2 id="rfc.section.15.6"><a href="#rfc.section.15.6">15.6</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
    1658       <p id="rfc.section.15.6.p.1">The general-header field "Warning" is used to carry additional information about the status or transformation of a message
    1659          which might not be reflected in the message. This information is typically used to warn about a possible lack of semantic
    1660          transparency from caching operations or transformations applied to the entity body of the message.
    1661       </p>
    1662       <p id="rfc.section.15.6.p.2">Warning headers are sent with responses using:</p>
    1663       <div id="rfc.figure.u.19"></div><pre class="inline"><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span><span id="rfc.iref.g.24"></span>  <a href="#header.warning" class="smpl">Warning</a>    = "Warning" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.warning" class="smpl">Warning-v</a>
     1234      <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
     1235      <p id="rfc.section.3.6.p.1">The general-header field "Warning" is used to carry additional information about the status or transformation of a message
     1236         that might not be reflected in the message. This information is typically used to warn about possible incorrectness introduced
     1237         by caching operations or transformations applied to the entity body of the message.
     1238      </p>
     1239      <p id="rfc.section.3.6.p.2">Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status
     1240         code, distinguish these responses from true failures.
     1241      </p>
     1242      <p id="rfc.section.3.6.p.3">Warning headers can in general be applied to any message, however some warn-codes are specific to caches and can only be applied
     1243         to response messages.
     1244      </p>
     1245      <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span>  <a href="#header.warning" class="smpl">Warning</a>    = "Warning" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.warning" class="smpl">Warning-v</a>
    16641246  <a href="#header.warning" class="smpl">Warning-v</a>  = 1#<a href="#header.warning" class="smpl">warning-value</a>
    16651247 
     
    16731255  <a href="#header.warning" class="smpl">warn-text</a>  = <a href="#core.rules" class="smpl">quoted-string</a>
    16741256  <a href="#header.warning" class="smpl">warn-date</a>  = <a href="#notation" class="smpl">DQUOTE</a> <a href="#abnf.dependencies" class="smpl">HTTP-date</a> <a href="#notation" class="smpl">DQUOTE</a>
    1675 </pre><p id="rfc.section.15.6.p.4">A response <em class="bcp14">MAY</em> carry more than one Warning header.
    1676       </p>
    1677       <p id="rfc.section.15.6.p.5">The warn-text <em class="bcp14">SHOULD</em> be in a natural language and character set that is most likely to be intelligible to the human user receiving the response.
    1678          This decision <em class="bcp14">MAY</em> be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the
    1679          Content-Language field in a response, etc. The default language is English and the default character set is ISO-8859-1 (<a href="#ISO-8859-1" id="rfc.xref.ISO-8859-1.1"><cite title="Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1">[ISO-8859-1]</cite></a>).
    1680       </p>
    1681       <p id="rfc.section.15.6.p.6">If a character set other than ISO-8859-1 is used, it <em class="bcp14">MUST</em> be encoded in the warn-text using the method described in <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a>.
    1682       </p>
    1683       <p id="rfc.section.15.6.p.7">Warning headers can in general be applied to any message, however some specific warn-codes are specific to caches and can
    1684          only be applied to response messages. New Warning headers <em class="bcp14">SHOULD</em> be added after any existing Warning headers. A cache <em class="bcp14">MUST NOT</em> delete any Warning header that it received with a message. However, if a cache successfully validates a cache entry, it <em class="bcp14">SHOULD</em> remove any Warning headers previously attached to that entry except as specified for specific Warning codes. It <em class="bcp14">MUST</em> then add any Warning headers received in the validating response. In other words, Warning headers are those that would be
    1685          attached to the most recent relevant response.
    1686       </p>
    1687       <p id="rfc.section.15.6.p.8">When multiple Warning headers are attached to a response, the user agent ought to inform the user of as many of them as possible,
    1688          in the order that they appear in the response. If it is not possible to inform the user of all of the warnings, the user agent <em class="bcp14">SHOULD</em> follow these heuristics:
     1257</pre><p id="rfc.section.3.6.p.5">Multiple warnings can be attached to a response (either by the origin server or by a cache), including multiple warnings with
     1258         the same code number. For example, a server might provide the same warning with texts in both English and Basque.
     1259      </p>
     1260      <p id="rfc.section.3.6.p.6">When this occurs, the user agent <em class="bcp14">SHOULD</em> inform the user of as many of them as possible, in the order that they appear in the response. If it is not possible to inform
     1261         the user of all of the warnings, the user agent <em class="bcp14">SHOULD</em> follow these heuristics:
    16891262      </p>
    16901263      <ul>
     
    16941267         </li>
    16951268      </ul>
    1696       <p id="rfc.section.15.6.p.9">Systems that generate multiple Warning headers <em class="bcp14">SHOULD</em> order them with this user agent behavior in mind.
    1697       </p>
    1698       <p id="rfc.section.15.6.p.10">Requirements for the behavior of caches with respect to Warnings are stated in <a href="#warnings" title="Warnings">Section&nbsp;2.2</a>.
    1699       </p>
    1700       <p id="rfc.section.15.6.p.11">This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its
    1701          meaning.
    1702       </p>
    1703       <p id="rfc.section.15.6.p.12">110 Response is stale </p>
    1704       <dl class="empty">
    1705          <dd> <em class="bcp14">MUST</em> be included whenever the returned response is stale.
    1706          </dd>
    1707       </dl>
    1708       <p id="rfc.section.15.6.p.13">111 Revalidation failed </p>
    1709       <dl class="empty">
    1710          <dd> <em class="bcp14">MUST</em> be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability
    1711             to reach the server.
    1712          </dd>
    1713       </dl>
    1714       <p id="rfc.section.15.6.p.14">112 Disconnected operation </p>
    1715       <dl class="empty">
    1716          <dd> <em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
    1717          </dd>
    1718       </dl>
    1719       <p id="rfc.section.15.6.p.15">113 Heuristic expiration </p>
    1720       <dl class="empty">
    1721          <dd> <em class="bcp14">MUST</em> be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater
     1269      <p id="rfc.section.3.6.p.7">Systems that generate multiple Warning headers <em class="bcp14">SHOULD</em> order them with this user agent behavior in mind. New Warning headers <em class="bcp14">SHOULD</em> be added after any existing Warning headers.
     1270      </p>
     1271      <p id="rfc.section.3.6.p.8">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning is required to be deleted from
     1272         a stored response after validation:
     1273      </p>
     1274      <ul>
     1275         <li>1xx Warnings that describe the freshness or validation status of the response, and so <em class="bcp14">MUST</em> be deleted by caches after validation. They <em class="bcp14">MUST NOT</em> be generated by a cache except when validating a cached entry, and <em class="bcp14">MUST NOT</em> be generated by clients.
     1276         </li>
     1277         <li>2xx Warnings that describe some aspect of the entity body or entity headers that is not rectified by a validation (for example,
     1278            a lossy compression of the entity bodies) and <em class="bcp14">MUST NOT</em> be deleted by caches after validation, unless a full response is returned, in which case they <em class="bcp14">MUST</em> be.
     1279         </li>
     1280      </ul>
     1281      <p id="rfc.section.3.6.p.9">The warn-text <em class="bcp14">SHOULD</em> be in a natural language and character set that is most likely to be intelligible to the human user receiving the response.
     1282         This decision can be based on any available knowledge, such as the location of the cache or user, the Accept-Language field
     1283         in a request, the Content-Language field in a response, etc. The default language is English and the default character set
     1284         is ISO-8859-1 (<a href="#ISO-8859-1" id="rfc.xref.ISO-8859-1.1"><cite title="Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1">[ISO-8859-1]</cite></a>).
     1285      </p>
     1286      <p id="rfc.section.3.6.p.10">If a character set other than ISO-8859-1 is used, it <em class="bcp14">MUST</em> be encoded in the warn-text using the method described in <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a>.
     1287      </p>
     1288      <p id="rfc.section.3.6.p.11">If an implementation sends a message with one or more Warning headers to a receiver whose version is HTTP/1.0 or lower, then
     1289         the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the Date header in the message.
     1290      </p>
     1291      <p id="rfc.section.3.6.p.12">If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from
     1292         the Date value in the response, then that warning-value <em class="bcp14">MUST</em> be deleted from the message before storing, forwarding, or using it. (preventing the consequences of naive caching of Warning
     1293         header fields.) If all of the warning-values are deleted for this reason, the Warning header <em class="bcp14">MUST</em> be deleted as well.
     1294      </p>
     1295      <p id="rfc.section.3.6.p.13">The following warn-codes are defined by this specification, each with a recommended warn-text in English, and a description
     1296         of its meaning.
     1297      </p>
     1298      <p id="rfc.section.3.6.p.14">110 Response is stale </p>
     1299      <dl class="empty">
     1300         <dd><em class="bcp14">SHOULD</em> be included whenever the returned response is stale.
     1301         </dd>
     1302      </dl>
     1303      <p id="rfc.section.3.6.p.15">111 Revalidation failed </p>
     1304      <dl class="empty">
     1305         <dd><em class="bcp14">SHOULD</em> be included if a cache returns a stale response because an attempt to validate the response failed, due to an inability to
     1306            reach the server.
     1307         </dd>
     1308      </dl>
     1309      <p id="rfc.section.3.6.p.16">112 Disconnected operation </p>
     1310      <dl class="empty">
     1311         <dd><em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
     1312         </dd>
     1313      </dl>
     1314      <p id="rfc.section.3.6.p.17">113 Heuristic expiration </p>
     1315      <dl class="empty">
     1316         <dd><em class="bcp14">SHOULD</em> be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater
    17221317            than 24 hours.
    17231318         </dd>
    17241319      </dl>
    1725       <p id="rfc.section.15.6.p.16">199 Miscellaneous warning </p>
    1726       <dl class="empty">
    1727          <dd>The warning text <em class="bcp14">MAY</em> include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action, besides presenting the warning to the user.
    1728          </dd>
    1729       </dl>
    1730       <p id="rfc.section.15.6.p.17">214 Transformation applied </p>
    1731       <dl class="empty">
    1732          <dd> <em class="bcp14">MUST</em> be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the
     1320      <p id="rfc.section.3.6.p.18">199 Miscellaneous warning </p>
     1321      <dl class="empty">
     1322         <dd>The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action, besides presenting the warning to the user.
     1323         </dd>
     1324      </dl>
     1325      <p id="rfc.section.3.6.p.19">214 Transformation applied </p>
     1326      <dl class="empty">
     1327         <dd><em class="bcp14">MUST</em> be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the
    17331328            Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, or the entity-body of the
    17341329            response, unless this Warning code already appears in the response.
    17351330         </dd>
    17361331      </dl>
    1737       <p id="rfc.section.15.6.p.18">299 Miscellaneous persistent warning </p>
    1738       <dl class="empty">
    1739          <dd>The warning text <em class="bcp14">MAY</em> include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action.
    1740          </dd>
    1741       </dl>
    1742       <p id="rfc.section.15.6.p.19">If an implementation sends a message with one or more Warning headers whose version is HTTP/1.0 or lower, then the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the date in the response.
    1743       </p>
    1744       <p id="rfc.section.15.6.p.20">If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from
    1745          the Date value in the response, then that warning-value <em class="bcp14">MUST</em> be deleted from the message before storing, forwarding, or using it. (This prevents bad consequences of naive caching of Warning
    1746          header fields.) If all of the warning-values are deleted for this reason, the Warning header <em class="bcp14">MUST</em> be deleted as well.
    1747       </p>
    1748       <h1 id="rfc.section.16"><a href="#rfc.section.16">16.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
    1749       <h2 id="rfc.section.16.1"><a href="#rfc.section.16.1">16.1</a>&nbsp;<a id="message.header.registration" href="#message.header.registration">Message Header Registration</a></h2>
    1750       <p id="rfc.section.16.1.p.1">The Message Header Registry located at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt; should be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
     1332      <p id="rfc.section.3.6.p.20">299 Miscellaneous persistent warning </p>
     1333      <dl class="empty">
     1334         <dd>The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action.
     1335         </dd>
     1336      </dl>
     1337      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
     1338      <p id="rfc.section.4.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, that can be used to redisplay an entity
     1339         retrieved earlier in a session.
     1340      </p>
     1341      <p id="rfc.section.4.p.2">History mechanisms and caches are different. In particular history mechanisms <em class="bcp14">SHOULD NOT</em> try to show a correct view of the current state of a resource. Rather, a history mechanism is meant to show exactly what the
     1342         user saw at the time when the resource was retrieved.
     1343      </p>
     1344      <p id="rfc.section.4.p.3">By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism <em class="bcp14">SHOULD</em> display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history
     1345         documents.
     1346      </p>
     1347      <p id="rfc.section.4.p.4">This is not to be construed to prohibit the history mechanism from telling the user that a view might be stale. </p>
     1348      <dl class="empty">
     1349         <dd> <b>Note:</b> if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors
     1350            to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider
     1351            it important that users not be presented with error messages or warning messages when they use navigation controls (such as
     1352            BACK) to view previously fetched resources. Even though sometimes such resources ought not be cached, or ought to expire quickly,
     1353            user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs)
     1354            in order not to suffer the effects of improperly functioning history mechanisms.
     1355         </dd>
     1356      </dl>
     1357      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
     1358      <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a>&nbsp;<a id="message.header.registration" href="#message.header.registration">Message Header Registration</a></h2>
     1359      <p id="rfc.section.5.1.p.1">The Message Header Registry located at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt; should be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
    17511360      </p>
    17521361      <div id="rfc.table.1">
     
    17661375                  <td>http</td>
    17671376                  <td>standard</td>
    1768                   <td> <a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;15.1</a>
     1377                  <td> <a href="#header.age" id="rfc.xref.header.age.2" title="Age">Section&nbsp;3.1</a>
    17691378                  </td>
    17701379               </tr>
     
    17731382                  <td>http</td>
    17741383                  <td>standard</td>
    1775                   <td> <a href="#header.cache-control" id="rfc.xref.header.cache-control.11" title="Cache-Control">Section&nbsp;15.2</a>
     1384                  <td> <a href="#header.cache-control" id="rfc.xref.header.cache-control.5" title="Cache-Control">Section&nbsp;3.2</a>
    17761385                  </td>
    17771386               </tr>
     
    17801389                  <td>http</td>
    17811390                  <td>standard</td>
    1782                   <td> <a href="#header.expires" id="rfc.xref.header.expires.3" title="Expires">Section&nbsp;15.3</a>
     1391                  <td> <a href="#header.expires" id="rfc.xref.header.expires.3" title="Expires">Section&nbsp;3.3</a>
    17831392                  </td>
    17841393               </tr>
     
    17871396                  <td>http</td>
    17881397                  <td>standard</td>
    1789                   <td> <a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;15.4</a>
     1398                  <td> <a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;3.4</a>
    17901399                  </td>
    17911400               </tr>
     
    17941403                  <td>http</td>
    17951404                  <td>standard</td>
    1796                   <td> <a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section&nbsp;15.5</a>
     1405                  <td> <a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section&nbsp;3.5</a>
    17971406                  </td>
    17981407               </tr>
     
    18011410                  <td>http</td>
    18021411                  <td>standard</td>
    1803                   <td> <a href="#header.warning" id="rfc.xref.header.warning.6" title="Warning">Section&nbsp;15.6</a>
     1412                  <td> <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;3.6</a>
    18041413                  </td>
    18051414               </tr>
     
    18071416         </table>
    18081417      </div>
    1809       <p id="rfc.section.16.1.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
    1810       <h1 id="rfc.section.17"><a href="#rfc.section.17">17.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
    1811       <p id="rfc.section.17.p.1">Caching proxies provide additional potential vulnerabilities, since the contents of the cache represent an attractive target
    1812          for malicious exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal
    1813          information long after a user believes that the information has been removed from the network. Therefore, cache contents should
    1814          be protected as sensitive information.
    1815       </p>
    1816       <h1 id="rfc.section.18"><a href="#rfc.section.18">18.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
    1817       <p id="rfc.section.18.p.1">Much of the content and presentation of the caching design is due to suggestions and comments from individuals including:
     1418      <p id="rfc.section.5.1.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
     1419      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
     1420      <p id="rfc.section.6.p.1">Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious
     1421         exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal information
     1422         long after a user believes that the information has been removed from the network. Therefore, cache contents should be protected
     1423         as sensitive information.
     1424      </p>
     1425      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
     1426      <p id="rfc.section.7.p.1">Much of the content and presentation of the caching design is due to suggestions and comments from individuals including:
    18181427         Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
    18191428      </p>
    1820       <h1 id="rfc.references"><a id="rfc.section.19" href="#rfc.section.19">19.</a> References
     1429      <h1 id="rfc.references"><a id="rfc.section.8" href="#rfc.section.8">8.</a> References
    18211430      </h1>
    1822       <h2 id="rfc.references.1"><a href="#rfc.section.19.1" id="rfc.section.19.1">19.1</a> Normative References
     1431      <h2 id="rfc.references.1"><a href="#rfc.section.8.1" id="rfc.section.8.1">8.1</a> Normative References
    18231432      </h2>
    18241433      <table summary="Normative References">                   
     
    18731482         </tr>
    18741483      </table>
    1875       <h2 id="rfc.references.2"><a href="#rfc.section.19.2" id="rfc.section.19.2">19.2</a> Informative References
     1484      <h2 id="rfc.references.2"><a href="#rfc.section.8.2" id="rfc.section.8.2">8.2</a> Informative References
    18761485      </h2>
    18771486      <table summary="Informative References">     
     
    19101519      <h1 id="rfc.section.A"><a href="#rfc.section.A">A.</a>&nbsp;<a id="compatibility" href="#compatibility">Compatibility with Previous Versions</a></h1>
    19111520      <h2 id="rfc.section.A.1"><a href="#rfc.section.A.1">A.1</a>&nbsp;<a id="changes.from.rfc.2068" href="#changes.from.rfc.2068">Changes from RFC 2068</a></h2>
    1912       <p id="rfc.section.A.1.p.1">A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add this missing case. (Sections <a href="#response.cacheability" title="Response Cacheability">5</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.12" title="Cache-Control">15.2</a>, <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">15.2.3</a>)
     1521      <p id="rfc.section.A.1.p.1">A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add this missing case. (Sections <a href="#response.cacheability" title="Response Cacheability">2.1</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.6" title="Cache-Control">3.2</a>).
    19131522      </p>
    19141523      <p id="rfc.section.A.1.p.2">Transfer-coding and message lengths all interact in ways that required fixing exactly when chunked encoding is used (to allow
    19151524         for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are
    1916          computed. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a>, see also <a href="#Part1" id="rfc.xref.Part1.17"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="#Part3" id="rfc.xref.Part3.3"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a> and <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>)
    1917       </p>
    1918       <p id="rfc.section.A.1.p.3">Proxies should be able to add Content-Length when appropriate. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a>)
     1525         computed. (see also <a href="#Part1" id="rfc.xref.Part1.14"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a> and <a href="#Part5" id="rfc.xref.Part5.4"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>) <span class="comment">[rfc.comment.16: This used to refer to the text about non-modifiable headers, and will have to be updated later on. --jre]</span>
     1526      </p>
     1527      <p id="rfc.section.A.1.p.3">Proxies should be able to add Content-Length when appropriate. <span class="comment">[rfc.comment.17: This used to refer to the text about non-modifiable headers, and will have to be updated later on. --jre]</span>
    19191528      </p>
    19201529      <p id="rfc.section.A.1.p.4">Range request responses would become very verbose if all meta-data were always returned; by allowing the server to only send
    1921          needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Headers">Section&nbsp;6.3</a>)
    1922       </p>
    1923       <p id="rfc.section.A.1.p.5">The Cache-Control: max-age directive was not properly defined for responses. (<a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">Section&nbsp;15.2.3</a>)
    1924       </p>
    1925       <p id="rfc.section.A.1.p.6">Warnings could be cached incorrectly, or not updated appropriately. (Section <a href="#warnings" title="Warnings">2.2</a>, <a href="#expiration.calculations" title="Expiration Calculations">3.4</a>, <a href="#non-modifiable.headers" title="Non-modifiable Headers">6.2</a>, <a href="#combining.headers" title="Combining Headers">6.3</a>, <a href="#modifications.of.the.basic.expiration.mechanism" title="Modifications of the Basic Expiration Mechanism">15.2.3</a>, and <a href="#header.warning" id="rfc.xref.header.warning.7" title="Warning">15.6</a>) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.
     1530         needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Responses">Section&nbsp;2.7</a>)
     1531      </p>
     1532      <p id="rfc.section.A.1.p.5">The Cache-Control: max-age directive was not properly defined for responses. (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>)
     1533      </p>
     1534      <p id="rfc.section.A.1.p.6">Warnings could be cached incorrectly, or not updated appropriately. (Section <a href="#expiration.model" title="Freshness Model">2.3</a>, <a href="#combining.headers" title="Combining Responses">2.7</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.7" title="Cache-Control">3.2</a>, and <a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">3.6</a>) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.
    19261535      </p>
    19271536      <h2 id="rfc.section.A.2"><a href="#rfc.section.A.2">A.2</a>&nbsp;<a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h2>
    1928       <p id="rfc.section.A.2.p.1">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Invalidation After Updates or Deletions">Section&nbsp;11</a>)
     1537      <p id="rfc.section.A.2.p.1">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;2.5</a>)
    19291538      </p>
    19301539      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
    1931       <div id="rfc.figure.u.20"></div> <pre class="inline"><a href="#header.age" class="smpl">Age</a> = "Age:" OWS Age-v
     1540      <div id="rfc.figure.u.18"></div> <pre class="inline"><a href="#header.age" class="smpl">Age</a> = "Age:" OWS Age-v
    19321541<a href="#header.age" class="smpl">Age-v</a> = delta-seconds
    19331542
     
    19921601
    19931602
    1994 </pre> <div id="rfc.figure.u.21"></div>
     1603</pre> <div id="rfc.figure.u.19"></div>
    19951604      <p>ABNF diagnostics:</p><pre class="inline">; Age defined but not used
    19961605; Cache-Control defined but not used
     
    20661675      </ul>
    20671676      <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a>&nbsp;<a id="changes.since.05" href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></h2>
    2068       <p id="rfc.section.C.7.p.1">Ongoing work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
     1677      <p id="rfc.section.C.7.p.1">This is a total rewrite of this part of the specification.</p>
     1678      <p id="rfc.section.C.7.p.2">In addition: Ongoing work on ABNF conversion (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;):
    20691679      </p>
    20701680      <ul>
     
    20781688            <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
    20791689                  <li class="indline1">age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1">1.2</a></li>
    2080                   <li class="indline1">Age header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.2"><b>15.1</b></a>, <a class="iref" href="#rfc.xref.header.age.1">16.1</a></li>
     1690                  <li class="indline1">Age header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">2.2</a>, <a class="iref" href="#rfc.iref.a.2"><b>3.1</b></a>, <a class="iref" href="#rfc.xref.header.age.2">5.1</a></li>
    20811691               </ul>
    20821692            </li>
     
    20851695                  <li class="indline1">Cache Directives&nbsp;&nbsp;
    20861696                     <ul class="ind">
    2087                         <li class="indline1">max-age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.9"><b>15.2.3</b></a>, <a class="iref" href="#rfc.iref.c.12"><b>15.2.4</b></a></li>
    2088                         <li class="indline1">max-stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.11"><b>15.2.3</b></a></li>
    2089                         <li class="indline1">min-fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.10"><b>15.2.3</b></a></li>
    2090                         <li class="indline1">must-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.14"><b>15.2.4</b></a></li>
    2091                         <li class="indline1">no-cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.6"><b>15.2.1</b></a></li>
    2092                         <li class="indline1">no-store&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>15.2.2</b></a></li>
    2093                         <li class="indline1">no-transform&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.16"><b>15.2.5</b></a></li>
    2094                         <li class="indline1">only-if-cached&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.13"><b>15.2.4</b></a></li>
    2095                         <li class="indline1">private&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5"><b>15.2.1</b></a></li>
    2096                         <li class="indline1">proxy-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.15"><b>15.2.4</b></a></li>
    2097                         <li class="indline1">public&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4"><b>15.2.1</b></a></li>
    2098                         <li class="indline1">s-maxage&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>15.2.3</b></a></li>
    2099                      </ul>
    2100                   </li>
    2101                   <li class="indline1">Cache-Control header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">3.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">3.5</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.8">9</a>, <a class="iref" href="#rfc.iref.c.3"><b>15.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.9">15.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.10">15.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.11">16.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.12">A.1</a></li>
     1697                        <li class="indline1">max-age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.6"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.17"><b>3.2.2</b></a></li>
     1698                        <li class="indline1">max-stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.1</b></a></li>
     1699                        <li class="indline1">min-fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.1</b></a></li>
     1700                        <li class="indline1">must-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.15"><b>3.2.2</b></a></li>
     1701                        <li class="indline1">no-cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.13"><b>3.2.2</b></a></li>
     1702                        <li class="indline1">no-store&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.14"><b>3.2.2</b></a></li>
     1703                        <li class="indline1">no-transform&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.9"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.19"><b>3.2.2</b></a></li>
     1704                        <li class="indline1">only-if-cached&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.10"><b>3.2.1</b></a></li>
     1705                        <li class="indline1">private&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.12"><b>3.2.2</b></a></li>
     1706                        <li class="indline1">proxy-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.16"><b>3.2.2</b></a></li>
     1707                        <li class="indline1">public&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.11"><b>3.2.2</b></a></li>
     1708                        <li class="indline1">s-maxage&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.18"><b>3.2.2</b></a></li>
     1709                     </ul>
     1710                  </li>
     1711                  <li class="indline1">Cache-Control header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">2.2</a>, <a class="iref" href="#rfc.iref.c.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.5">5.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">A.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">A.1</a></li>
    21021712                  <li class="indline1">cacheable&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2">1.2</a></li>
    21031713               </ul>
    21041714            </li>
    21051715            <li class="indline0"><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul class="ind">
    2106                   <li class="indline1">Expires header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">5</a>, <a class="iref" href="#rfc.xref.header.expires.2">15.2.3</a>, <a class="iref" href="#rfc.iref.e.2"><b>15.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.3">16.1</a></li>
     1716                  <li class="indline1">Expires header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.3</a>, <a class="iref" href="#rfc.xref.header.expires.2">2.3.1</a>, <a class="iref" href="#rfc.iref.e.2"><b>3.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.3">5.1</a></li>
    21071717                  <li class="indline1">explicit expiration time&nbsp;&nbsp;<a class="iref" href="#rfc.iref.e.1">1.2</a></li>
    21081718               </ul>
     
    21171727                  <li class="indline1"><tt>Grammar</tt>&nbsp;&nbsp;
    21181728                     <ul class="ind">
    2119                         <li class="indline1"><tt>Age</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1"><b>15.1</b></a></li>
    2120                         <li class="indline1"><tt>Age-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>15.1</b></a></li>
    2121                         <li class="indline1"><tt>Cache-Control</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>15.2</b></a></li>
    2122                         <li class="indline1"><tt>Cache-Control-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>15.2</b></a></li>
    2123                         <li class="indline1"><tt>cache-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>15.2</b></a></li>
    2124                         <li class="indline1"><tt>cache-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>15.2</b></a></li>
    2125                         <li class="indline1"><tt>cache-request-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>15.2</b></a></li>
    2126                         <li class="indline1"><tt>cache-response-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>15.2</b></a></li>
    2127                         <li class="indline1"><tt>delta-seconds</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>15.1</b></a></li>
    2128                         <li class="indline1"><tt>Expires</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>15.3</b></a></li>
    2129                         <li class="indline1"><tt>Expires-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>15.3</b></a></li>
    2130                         <li class="indline1"><tt>extension-pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>15.4</b></a></li>
    2131                         <li class="indline1"><tt>Pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>15.4</b></a></li>
    2132                         <li class="indline1"><tt>pragma-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>15.4</b></a></li>
    2133                         <li class="indline1"><tt>Pragma-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>15.4</b></a></li>
    2134                         <li class="indline1"><tt>Vary</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>15.5</b></a></li>
    2135                         <li class="indline1"><tt>Vary-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>15.5</b></a></li>
    2136                         <li class="indline1"><tt>warn-agent</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.22"><b>15.6</b></a></li>
    2137                         <li class="indline1"><tt>warn-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.21"><b>15.6</b></a></li>
    2138                         <li class="indline1"><tt>warn-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.24"><b>15.6</b></a></li>
    2139                         <li class="indline1"><tt>warn-text</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.23"><b>15.6</b></a></li>
    2140                         <li class="indline1"><tt>Warning</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>15.6</b></a></li>
    2141                         <li class="indline1"><tt>Warning-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>15.6</b></a></li>
    2142                         <li class="indline1"><tt>warning-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.20"><b>15.6</b></a></li>
     1729                        <li class="indline1"><tt>Age</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1"><b>3.1</b></a></li>
     1730                        <li class="indline1"><tt>Age-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>3.1</b></a></li>
     1731                        <li class="indline1"><tt>Cache-Control</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>3.2</b></a></li>
     1732                        <li class="indline1"><tt>Cache-Control-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>3.2</b></a></li>
     1733                        <li class="indline1"><tt>cache-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>3.2</b></a></li>
     1734                        <li class="indline1"><tt>cache-request-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>3.2.1</b></a></li>
     1735                        <li class="indline1"><tt>cache-response-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>3.2.2</b></a></li>
     1736                        <li class="indline1"><tt>delta-seconds</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>3.1</b></a></li>
     1737                        <li class="indline1"><tt>Expires</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>3.3</b></a></li>
     1738                        <li class="indline1"><tt>Expires-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>3.3</b></a></li>
     1739                        <li class="indline1"><tt>extension-pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>3.4</b></a></li>
     1740                        <li class="indline1"><tt>Pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>3.4</b></a></li>
     1741                        <li class="indline1"><tt>pragma-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>3.4</b></a></li>
     1742                        <li class="indline1"><tt>Pragma-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>3.4</b></a></li>
     1743                        <li class="indline1"><tt>Vary</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>3.5</b></a></li>
     1744                        <li class="indline1"><tt>Vary-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>3.5</b></a></li>
     1745                        <li class="indline1"><tt>warn-agent</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.21"><b>3.6</b></a></li>
     1746                        <li class="indline1"><tt>warn-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.20"><b>3.6</b></a></li>
     1747                        <li class="indline1"><tt>warn-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.23"><b>3.6</b></a></li>
     1748                        <li class="indline1"><tt>warn-text</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.22"><b>3.6</b></a></li>
     1749                        <li class="indline1"><tt>Warning</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>3.6</b></a></li>
     1750                        <li class="indline1"><tt>Warning-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>3.6</b></a></li>
     1751                        <li class="indline1"><tt>warning-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>3.6</b></a></li>
    21431752                     </ul>
    21441753                  </li>
     
    21481757                  <li class="indline1">Headers&nbsp;&nbsp;
    21491758                     <ul class="ind">
    2150                         <li class="indline1">Age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.2"><b>15.1</b></a>, <a class="iref" href="#rfc.xref.header.age.1">16.1</a></li>
    2151                         <li class="indline1">Cache-Control&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">3.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.5">3.5</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">5</a>, <a class="iref" href="#rfc.xref.header.cache-control.8">9</a>, <a class="iref" href="#rfc.iref.h.3"><b>15.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.9">15.3</a>, <a class="iref" href="#rfc.xref.header.cache-control.10">15.4</a>, <a class="iref" href="#rfc.xref.header.cache-control.11">16.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.12">A.1</a></li>
    2152                         <li class="indline1">Expires&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">5</a>, <a class="iref" href="#rfc.xref.header.expires.2">15.2.3</a>, <a class="iref" href="#rfc.iref.h.4"><b>15.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.3">16.1</a></li>
    2153                         <li class="indline1">Pragma&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">15.2</a>, <a class="iref" href="#rfc.iref.h.5"><b>15.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.2">16.1</a></li>
    2154                         <li class="indline1">Vary&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">7</a>, <a class="iref" href="#rfc.iref.h.6"><b>15.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">16.1</a></li>
    2155                         <li class="indline1">Warning&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.1</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">6.2</a>, <a class="iref" href="#rfc.xref.header.warning.5">6.3</a>, <a class="iref" href="#rfc.iref.h.7"><b>15.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.6">16.1</a>, <a class="iref" href="#rfc.xref.header.warning.7">A.1</a></li>
     1759                        <li class="indline1">Age&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">2.2</a>, <a class="iref" href="#rfc.iref.h.2"><b>3.1</b></a>, <a class="iref" href="#rfc.xref.header.age.2">5.1</a></li>
     1760                        <li class="indline1">Cache-Control&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">2.2</a>, <a class="iref" href="#rfc.iref.h.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.5">5.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">A.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">A.1</a></li>
     1761                        <li class="indline1">Expires&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.3</a>, <a class="iref" href="#rfc.xref.header.expires.2">2.3.1</a>, <a class="iref" href="#rfc.iref.h.4"><b>3.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.3">5.1</a></li>
     1762                        <li class="indline1">Pragma&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">2.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">3.2</a>, <a class="iref" href="#rfc.iref.h.5"><b>3.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">5.1</a></li>
     1763                        <li class="indline1">Vary&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.6</a>, <a class="iref" href="#rfc.iref.h.6"><b>3.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">5.1</a></li>
     1764                        <li class="indline1">Warning&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.4</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.7</a>, <a class="iref" href="#rfc.iref.h.7"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.4">5.1</a>, <a class="iref" href="#rfc.xref.header.warning.5">A.1</a></li>
    21561765                     </ul>
    21571766                  </li>
     
    21601769            </li>
    21611770            <li class="indline0"><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul class="ind">
    2162                   <li class="indline1"><em>ISO-8859-1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ISO-8859-1.1">15.6</a>, <a class="iref" href="#ISO-8859-1"><b>19.1</b></a></li>
     1771                  <li class="indline1"><em>ISO-8859-1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ISO-8859-1.1">3.6</a>, <a class="iref" href="#ISO-8859-1"><b>8.1</b></a></li>
    21631772               </ul>
    21641773            </li>
     
    21661775                  <li class="indline1">max-age&nbsp;&nbsp;
    21671776                     <ul class="ind">
    2168                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>15.2.3</b></a>, <a class="iref" href="#rfc.iref.m.4"><b>15.2.4</b></a></li>
     1777                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.m.5"><b>3.2.2</b></a></li>
    21691778                     </ul>
    21701779                  </li>
    21711780                  <li class="indline1">max-stale&nbsp;&nbsp;
    21721781                     <ul class="ind">
    2173                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>15.2.3</b></a></li>
     1782                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>3.2.1</b></a></li>
    21741783                     </ul>
    21751784                  </li>
    21761785                  <li class="indline1">min-fresh&nbsp;&nbsp;
    21771786                     <ul class="ind">
    2178                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>15.2.3</b></a></li>
     1787                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>3.2.1</b></a></li>
    21791788                     </ul>
    21801789                  </li>
    21811790                  <li class="indline1">must-revalidate&nbsp;&nbsp;
    21821791                     <ul class="ind">
    2183                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.5"><b>15.2.4</b></a></li>
     1792                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.4"><b>3.2.2</b></a></li>
    21841793                     </ul>
    21851794                  </li>
     
    21891798                  <li class="indline1">no-cache&nbsp;&nbsp;
    21901799                     <ul class="ind">
    2191                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>15.2.1</b></a></li>
     1800                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.4"><b>3.2.2</b></a></li>
    21921801                     </ul>
    21931802                  </li>
    21941803                  <li class="indline1">no-store&nbsp;&nbsp;
    21951804                     <ul class="ind">
    2196                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>15.2.2</b></a></li>
     1805                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.5"><b>3.2.2</b></a></li>
    21971806                     </ul>
    21981807                  </li>
    21991808                  <li class="indline1">no-transform&nbsp;&nbsp;
    22001809                     <ul class="ind">
    2201                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>15.2.5</b></a></li>
     1810                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.6"><b>3.2.2</b></a></li>
    22021811                     </ul>
    22031812                  </li>
     
    22071816                  <li class="indline1">only-if-cached&nbsp;&nbsp;
    22081817                     <ul class="ind">
    2209                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>15.2.4</b></a></li>
     1818                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.1</b></a></li>
    22101819                     </ul>
    22111820                  </li>
     
    22131822            </li>
    22141823            <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
    2215                   <li class="indline1"><em>Part1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">1.4</a>, <a class="iref" href="#rfc.xref.Part1.2">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.3">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.4">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.5">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.8">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.9">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.10">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.11">3.3</a>, <a class="iref" href="#rfc.xref.Part1.12">6.1</a>, <a class="iref" href="#rfc.xref.Part1.13">6.2</a>, <a class="iref" href="#rfc.xref.Part1.14">6.2</a>, <a class="iref" href="#rfc.xref.Part1.15">7</a>, <a class="iref" href="#rfc.xref.Part1.16">15.3</a>, <a class="iref" href="#Part1"><b>19.1</b></a>, <a class="iref" href="#rfc.xref.Part1.17">A.1</a><ul class="ind">
     1824                  <li class="indline1"><em>Part1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">1.4</a>, <a class="iref" href="#rfc.xref.Part1.2">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.3">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.4">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.5">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.8">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.9">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.10">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.11">2.3.2</a>, <a class="iref" href="#rfc.xref.Part1.12">2.6</a>, <a class="iref" href="#rfc.xref.Part1.13">3.3</a>, <a class="iref" href="#Part1"><b>8.1</b></a>, <a class="iref" href="#rfc.xref.Part1.14">A.1</a><ul class="ind">
    22161825                        <li class="indline1"><em>Section 1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">1.4</a></li>
    22171826                        <li class="indline1"><em>Section 1.2.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.2">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.3">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.4">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.5">1.4.1</a></li>
    22181827                        <li class="indline1"><em>Section 2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.8">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.10">1.4.2</a></li>
    2219                         <li class="indline1"><em>Section 3.2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.16">15.3</a></li>
    2220                         <li class="indline1"><em>Section 4.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.15">7</a></li>
    2221                         <li class="indline1"><em>Section 4.4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.13">6.2</a>, <a class="iref" href="#rfc.xref.Part1.14">6.2</a></li>
    2222                         <li class="indline1"><em>Section 8.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.12">6.1</a></li>
    2223                         <li class="indline1"><em>Section 8.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.11">3.3</a></li>
     1828                        <li class="indline1"><em>Section 3.2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.13">3.3</a></li>
     1829                        <li class="indline1"><em>Section 4.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.12">2.6</a></li>
     1830                        <li class="indline1"><em>Section 8.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.11">2.3.2</a></li>
    22241831                        <li class="indline1"><em>Section 8.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.9">1.4.2</a></li>
    22251832                     </ul>
    22261833                  </li>
    2227                   <li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">10</a>, <a class="iref" href="#Part2"><b>19.1</b></a><ul class="ind">
    2228                         <li class="indline1"><em>Section 7.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">10</a></li>
    2229                      </ul>
    2230                   </li>
    2231                   <li class="indline1"><em>Part3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">6.2</a>, <a class="iref" href="#rfc.xref.Part3.2">7</a>, <a class="iref" href="#Part3"><b>19.1</b></a>, <a class="iref" href="#rfc.xref.Part3.3">A.1</a><ul class="ind">
    2232                         <li class="indline1"><em>Section 3.2.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">6.2</a></li>
    2233                         <li class="indline1"><em>Section 4.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.2">7</a></li>
    2234                      </ul>
    2235                   </li>
    2236                   <li class="indline1"><em>Part4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">4</a>, <a class="iref" href="#Part4"><b>19.1</b></a></li>
    2237                   <li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">6.3</a>, <a class="iref" href="#rfc.xref.Part5.2">9</a>, <a class="iref" href="#Part5"><b>19.1</b></a>, <a class="iref" href="#rfc.xref.Part5.3">A.1</a><ul class="ind">
    2238                         <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">6.3</a>, <a class="iref" href="#rfc.xref.Part5.2">9</a></li>
    2239                      </ul>
    2240                   </li>
    2241                   <li class="indline1"><em>Part7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">5</a>, <a class="iref" href="#rfc.xref.Part7.2">15.2.1</a>, <a class="iref" href="#Part7"><b>19.1</b></a><ul class="ind">
    2242                         <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">5</a>, <a class="iref" href="#rfc.xref.Part7.2">15.2.1</a></li>
    2243                      </ul>
    2244                   </li>
    2245                   <li class="indline1">Pragma header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">15.2</a>, <a class="iref" href="#rfc.iref.p.4"><b>15.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.2">16.1</a></li>
     1834                  <li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.2</a>, <a class="iref" href="#rfc.xref.Part2.2">2.5</a>, <a class="iref" href="#Part2"><b>8.1</b></a><ul class="ind">
     1835                        <li class="indline1"><em>Section 7.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.2</a>, <a class="iref" href="#rfc.xref.Part2.2">2.5</a></li>
     1836                     </ul>
     1837                  </li>
     1838                  <li class="indline1"><em>Part3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">2.6</a>, <a class="iref" href="#Part3"><b>8.1</b></a>, <a class="iref" href="#rfc.xref.Part3.2">A.1</a><ul class="ind">
     1839                        <li class="indline1"><em>Section 4.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">2.6</a></li>
     1840                     </ul>
     1841                  </li>
     1842                  <li class="indline1"><em>Part4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3.1.1</a>, <a class="iref" href="#rfc.xref.Part4.2">2.4</a>, <a class="iref" href="#Part4"><b>8.1</b></a><ul class="ind">
     1843                        <li class="indline1"><em>Section 6.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3.1.1</a></li>
     1844                     </ul>
     1845                  </li>
     1846                  <li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">2.1.1</a>, <a class="iref" href="#rfc.xref.Part5.2">2.1.1</a>, <a class="iref" href="#rfc.xref.Part5.3">2.7</a>, <a class="iref" href="#Part5"><b>8.1</b></a>, <a class="iref" href="#rfc.xref.Part5.4">A.1</a><ul class="ind">
     1847                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.2">2.1.1</a>, <a class="iref" href="#rfc.xref.Part5.3">2.7</a></li>
     1848                     </ul>
     1849                  </li>
     1850                  <li class="indline1"><em>Part7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.1</a>, <a class="iref" href="#rfc.xref.Part7.2">3.2.2</a>, <a class="iref" href="#Part7"><b>8.1</b></a><ul class="ind">
     1851                        <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.1</a>, <a class="iref" href="#rfc.xref.Part7.2">3.2.2</a></li>
     1852                     </ul>
     1853                  </li>
     1854                  <li class="indline1">Pragma header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">2.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">3.2</a>, <a class="iref" href="#rfc.iref.p.4"><b>3.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">5.1</a></li>
    22461855                  <li class="indline1">private&nbsp;&nbsp;
    22471856                     <ul class="ind">
    2248                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.2"><b>15.2.1</b></a></li>
     1857                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.2"><b>3.2.2</b></a></li>
    22491858                     </ul>
    22501859                  </li>
    22511860                  <li class="indline1">proxy-revalidate&nbsp;&nbsp;
    22521861                     <ul class="ind">
    2253                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.3"><b>15.2.4</b></a></li>
     1862                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.3"><b>3.2.2</b></a></li>
    22541863                     </ul>
    22551864                  </li>
    22561865                  <li class="indline1">public&nbsp;&nbsp;
    22571866                     <ul class="ind">
    2258                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>15.2.1</b></a></li>
     1867                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>3.2.2</b></a></li>
    22591868                     </ul>
    22601869                  </li>
     
    22621871            </li>
    22631872            <li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind">
    2264                   <li class="indline1"><em>RFC1305</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1305.1">3.3</a>, <a class="iref" href="#RFC1305"><b>19.2</b></a></li>
    2265                   <li class="indline1"><em>RFC2047</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2047.1">15.6</a>, <a class="iref" href="#RFC2047"><b>19.1</b></a></li>
    2266                   <li class="indline1"><em>RFC2119</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2119.1">1.3</a>, <a class="iref" href="#RFC2119"><b>19.1</b></a></li>
    2267                   <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#RFC2616"><b>19.2</b></a>, <a class="iref" href="#rfc.xref.RFC2616.1">C.1</a></li>
    2268                   <li class="indline1"><em>RFC3864</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC3864.1">16.1</a>, <a class="iref" href="#RFC3864"><b>19.2</b></a></li>
    2269                   <li class="indline1"><em>RFC5234</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5234.1">1.4</a>, <a class="iref" href="#RFC5234"><b>19.1</b></a><ul class="ind">
     1873                  <li class="indline1"><em>RFC1305</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1305.1">2.3.2</a>, <a class="iref" href="#RFC1305"><b>8.2</b></a></li>
     1874                  <li class="indline1"><em>RFC2047</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2047.1">3.6</a>, <a class="iref" href="#RFC2047"><b>8.1</b></a></li>
     1875                  <li class="indline1"><em>RFC2119</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2119.1">1.3</a>, <a class="iref" href="#RFC2119"><b>8.1</b></a></li>
     1876                  <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#RFC2616"><b>8.2</b></a>, <a class="iref" href="#rfc.xref.RFC2616.1">C.1</a></li>
     1877                  <li class="indline1"><em>RFC3864</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC3864.1">5.1</a>, <a class="iref" href="#RFC3864"><b>8.2</b></a></li>
     1878                  <li class="indline1"><em>RFC5234</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5234.1">1.4</a>, <a class="iref" href="#RFC5234"><b>8.1</b></a><ul class="ind">
    22701879                        <li class="indline1"><em>Appendix B.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5234.1">1.4</a></li>
    22711880                     </ul>
     
    22761885                  <li class="indline1">s-maxage&nbsp;&nbsp;
    22771886                     <ul class="ind">
    2278                         <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.3"><b>15.2.3</b></a></li>
    2279                      </ul>
    2280                   </li>
    2281                   <li class="indline1">semantically transparent&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1">1.1</a></li>
    2282                   <li class="indline1">stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.2">1.2</a></li>
     1887                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.2"><b>3.2.2</b></a></li>
     1888                     </ul>
     1889                  </li>
     1890                  <li class="indline1">stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1">1.2</a></li>
    22831891               </ul>
    22841892            </li>
    22851893            <li class="indline0"><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul class="ind">
    2286                   <li class="indline1">validator&nbsp;&nbsp;<a class="iref" href="#rfc.iref.v.1">1.2</a></li>
    2287                   <li class="indline1">Vary header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">7</a>, <a class="iref" href="#rfc.iref.v.2"><b>15.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">16.1</a></li>
     1894                  <li class="indline1">validator&nbsp;&nbsp;<a class="iref" href="#rfc.iref.v.1">1.2</a>, <a class="iref" href="#rfc.iref.v.2">1.2</a></li>
     1895                  <li class="indline1">Vary header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.6</a>, <a class="iref" href="#rfc.iref.v.3"><b>3.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">5.1</a></li>
    22881896               </ul>
    22891897            </li>
    22901898            <li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind">
    2291                   <li class="indline1">Warning header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.1</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">6.2</a>, <a class="iref" href="#rfc.xref.header.warning.5">6.3</a>, <a class="iref" href="#rfc.iref.w.1"><b>15.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.6">16.1</a>, <a class="iref" href="#rfc.xref.header.warning.7">A.1</a></li>
     1899                  <li class="indline1">Warning header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.4</a>, <a class="iref" href="#rfc.xref.header.warning.3">2.7</a>, <a class="iref" href="#rfc.iref.w.1"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.4">5.1</a>, <a class="iref" href="#rfc.xref.header.warning.5">A.1</a></li>
    22921900               </ul>
    22931901            </li>
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