Ignore:
Timestamp:
14/06/14 11:20:37 (7 years ago)
Author:
julian.reschke@…
Message:

update to latest version of rfc2629.xslt, regen all HTML

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1 edited

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  • draft-ietf-httpbis/01/p6-cache.html

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    347341      <link rel="Appendix" title="A Compatibility with Previous Versions" href="#rfc.section.A">
    348342      <link rel="Appendix" title="B Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.B">
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    350344      <link rel="schema.dct" href="http://purl.org/dc/terms/">
    351345      <meta name="dct.creator" content="Fielding, R.">
     
    376370            </tr>
    377371            <tr>
    378                <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
     372               <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved)
    379373               </td>
    380374               <td class="right">J. Gettys</td>
     
    447441      </table>
    448442      <p class="title">HTTP/1.1, part 6: Caching<br><span class="filename">draft-ietf-httpbis-p6-cache-01</span></p>
    449       <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
    450       <p>By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she
    451          is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section
    452          6 of BCP 79.
    453       </p>
    454       <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note
    455          that other groups may also distribute working documents as Internet-Drafts.
    456       </p>
    457       <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
    458          documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
    459          in progress”.
    460       </p>
    461       <p>The list of current Internet-Drafts can be accessed at <a href="http://www.ietf.org/ietf/1id-abstracts.txt">http://www.ietf.org/ietf/1id-abstracts.txt</a>.
    462       </p>
    463       <p>The list of Internet-Draft Shadow Directories can be accessed at <a href="http://www.ietf.org/shadow.html">http://www.ietf.org/shadow.html</a>.
    464       </p>
    465       <p>This Internet-Draft will expire on July 15, 2008.</p>
    466       <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
     443      <div id="rfc.status">
     444         <h1><a href="#rfc.status">Status of this Memo</a></h1>
     445         <p>By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she
     446            is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section
     447            6 of BCP 79.
     448         </p>
     449         <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note
     450            that other groups may also distribute working documents as Internet-Drafts.
     451         </p>
     452         <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
     453            documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
     454            in progress”.
     455         </p>
     456         <p>The list of current Internet-Drafts can be accessed at <a href="http://www.ietf.org/ietf/1id-abstracts.txt">http://www.ietf.org/ietf/1id-abstracts.txt</a>.
     457         </p>
     458         <p>The list of Internet-Draft Shadow Directories can be accessed at <a href="http://www.ietf.org/shadow.html">http://www.ietf.org/shadow.html</a>.
     459         </p>
     460         <p>This Internet-Draft will expire on July 15, 2008.</p>
     461      </div>
     462      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    467463      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
    468464         systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 6 of the
     
    470466         6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response
    471467         messages.
    472       </p> 
    473       <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
     468      </p>
     469      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
    474470      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues
    475471         list is at &lt;<a href="http://www.tools.ietf.org/wg/httpbis/trac/report/11">http://www.tools.ietf.org/wg/httpbis/trac/report/11</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://www.tools.ietf.org/wg/httpbis/">http://www.tools.ietf.org/wg/httpbis/</a>&gt;.
    476       </p> 
     472      </p>
    477473      <p>This draft incorporates those issue resolutions that were either collected in the original RFC2616 errata list (&lt;<a href="http://purl.org/NET/http-errata">http://purl.org/NET/http-errata</a>&gt;), or which were agreed upon on the mailing list between October 2006 and November 2007 (as published in "draft-lafon-rfc2616bis-03").
    478       </p> 
     474      </p>
    479475      <hr class="noprint">
    480476      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
    481477      <ul class="toc">
    482          <li>1.&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul>
    483                <li>1.1&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
    484                <li>1.2&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
    485                <li>1.3&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
     478         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul>
     479               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
     480               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
     481               <li><a href="#rfc.section.1.3">1.3</a>&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
    486482            </ul>
    487483         </li>
    488          <li>2.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview</a><ul>
    489                <li>2.1&nbsp;&nbsp;&nbsp;<a href="#cache.correctness">Cache Correctness</a></li>
    490                <li>2.2&nbsp;&nbsp;&nbsp;<a href="#warnings">Warnings</a></li>
    491                <li>2.3&nbsp;&nbsp;&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></li>
    492                <li>2.4&nbsp;&nbsp;&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></li>
    493                <li>2.5&nbsp;&nbsp;&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></li>
    494                <li>2.6&nbsp;&nbsp;&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></li>
     484         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview</a><ul>
     485               <li><a href="#rfc.section.2.1">2.1</a>&nbsp;&nbsp;&nbsp;<a href="#cache.correctness">Cache Correctness</a></li>
     486               <li><a href="#rfc.section.2.2">2.2</a>&nbsp;&nbsp;&nbsp;<a href="#warnings">Warnings</a></li>
     487               <li><a href="#rfc.section.2.3">2.3</a>&nbsp;&nbsp;&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></li>
     488               <li><a href="#rfc.section.2.4">2.4</a>&nbsp;&nbsp;&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></li>
     489               <li><a href="#rfc.section.2.5">2.5</a>&nbsp;&nbsp;&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></li>
     490               <li><a href="#rfc.section.2.6">2.6</a>&nbsp;&nbsp;&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></li>
    495491            </ul>
    496492         </li>
    497          <li>3.&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Expiration Model</a><ul>
    498                <li>3.1&nbsp;&nbsp;&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></li>
    499                <li>3.2&nbsp;&nbsp;&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></li>
    500                <li>3.3&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Age Calculations</a></li>
    501                <li>3.4&nbsp;&nbsp;&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></li>
    502                <li>3.5&nbsp;&nbsp;&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></li>
    503                <li>3.6&nbsp;&nbsp;&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></li>
     493         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Expiration Model</a><ul>
     494               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></li>
     495               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></li>
     496               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Age Calculations</a></li>
     497               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></li>
     498               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></li>
     499               <li><a href="#rfc.section.3.6">3.6</a>&nbsp;&nbsp;&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></li>
    504500            </ul>
    505501         </li>
    506          <li>4.&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul>
    507                <li>4.1&nbsp;&nbsp;&nbsp;<a href="#last-modified.dates">Last-Modified Dates</a></li>
    508                <li>4.2&nbsp;&nbsp;&nbsp;<a href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></li>
    509                <li>4.3&nbsp;&nbsp;&nbsp;<a href="#non-validating.conditionals">Non-validating Conditionals</a></li>
     502         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul>
     503               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#last-modified.dates">Last-Modified Dates</a></li>
     504               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></li>
     505               <li><a href="#rfc.section.4.3">4.3</a>&nbsp;&nbsp;&nbsp;<a href="#non-validating.conditionals">Non-validating Conditionals</a></li>
    510506            </ul>
    511507         </li>
    512          <li>5.&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li>
    513          <li>6.&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a><ul>
    514                <li>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>
    515                <li>6.2&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></li>
    516                <li>6.3&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Headers</a></li>
     508         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a></li>
     509         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a><ul>
     510               <li><a href="#rfc.section.6.1">6.1</a>&nbsp;&nbsp;&nbsp;<a href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></li>
     511               <li><a href="#rfc.section.6.2">6.2</a>&nbsp;&nbsp;&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></li>
     512               <li><a href="#rfc.section.6.3">6.3</a>&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Headers</a></li>
    517513            </ul>
    518514         </li>
    519          <li>7.&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
    520          <li>8.&nbsp;&nbsp;&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></li>
    521          <li>9.&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></li>
    522          <li>10.&nbsp;&nbsp;&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></li>
    523          <li>11.&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></li>
    524          <li>12.&nbsp;&nbsp;&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></li>
    525          <li>13.&nbsp;&nbsp;&nbsp;<a href="#cache.replacement">Cache Replacement</a></li>
    526          <li>14.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
    527          <li>15.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul>
    528                <li>15.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
    529                <li>15.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul>
    530                      <li>15.2.1&nbsp;&nbsp;&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></li>
    531                      <li>15.2.2&nbsp;&nbsp;&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></li>
    532                      <li>15.2.3&nbsp;&nbsp;&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></li>
    533                      <li>15.2.4&nbsp;&nbsp;&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></li>
    534                      <li>15.2.5&nbsp;&nbsp;&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></li>
    535                      <li>15.2.6&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
     515         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
     516         <li><a href="#rfc.section.8">8.</a>&nbsp;&nbsp;&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></li>
     517         <li><a href="#rfc.section.9">9.</a>&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></li>
     518         <li><a href="#rfc.section.10">10.</a>&nbsp;&nbsp;&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></li>
     519         <li><a href="#rfc.section.11">11.</a>&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></li>
     520         <li><a href="#rfc.section.12">12.</a>&nbsp;&nbsp;&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></li>
     521         <li><a href="#rfc.section.13">13.</a>&nbsp;&nbsp;&nbsp;<a href="#cache.replacement">Cache Replacement</a></li>
     522         <li><a href="#rfc.section.14">14.</a>&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
     523         <li><a href="#rfc.section.15">15.</a>&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul>
     524               <li><a href="#rfc.section.15.1">15.1</a>&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
     525               <li><a href="#rfc.section.15.2">15.2</a>&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul>
     526                     <li><a href="#rfc.section.15.2.1">15.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></li>
     527                     <li><a href="#rfc.section.15.2.2">15.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></li>
     528                     <li><a href="#rfc.section.15.2.3">15.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></li>
     529                     <li><a href="#rfc.section.15.2.4">15.2.4</a>&nbsp;&nbsp;&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></li>
     530                     <li><a href="#rfc.section.15.2.5">15.2.5</a>&nbsp;&nbsp;&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></li>
     531                     <li><a href="#rfc.section.15.2.6">15.2.6</a>&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
    536532                  </ul>
    537533               </li>
    538                <li>15.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
    539                <li>15.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
    540                <li>15.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
    541                <li>15.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
     534               <li><a href="#rfc.section.15.3">15.3</a>&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
     535               <li><a href="#rfc.section.15.4">15.4</a>&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
     536               <li><a href="#rfc.section.15.5">15.5</a>&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
     537               <li><a href="#rfc.section.15.6">15.6</a>&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
    542538            </ul>
    543539         </li>
    544          <li>16.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a></li>
    545          <li>17.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
    546          <li>18.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
    547          <li>19.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
    548                <li>19.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
    549                <li>19.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
     540         <li><a href="#rfc.section.16">16.</a>&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a></li>
     541         <li><a href="#rfc.section.17">17.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
     542         <li><a href="#rfc.section.18">18.</a>&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
     543         <li><a href="#rfc.section.19">19.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
     544               <li><a href="#rfc.section.19.1">19.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
     545               <li><a href="#rfc.section.19.2">19.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
    550546            </ul>
    551547         </li>
    552          <li><a href="#rfc.authors">Authors' Addresses</a></li>
    553          <li>A.&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul>
    554                <li>A.1&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></li>
    555                <li>A.2&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
     548         <li><a href="#rfc.section.A">A.</a>&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul>
     549               <li><a href="#rfc.section.A.1">A.1</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></li>
     550               <li><a href="#rfc.section.A.2">A.2</a>&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
    556551            </ul>
    557552         </li>
    558          <li>B.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B">Change Log (to be removed by RFC Editor before publication)</a><ul>
    559                <li>B.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.1">Since RFC2616</a></li>
    560                <li>B.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.2">Since draft-ietf-httpbis-p6-cache-00</a></li>
     553         <li><a href="#rfc.section.B">B.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B">Change Log (to be removed by RFC Editor before publication)</a><ul>
     554               <li><a href="#rfc.section.B.1">B.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.1">Since RFC2616</a></li>
     555               <li><a href="#rfc.section.B.2">B.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.2">Since draft-ietf-httpbis-p6-cache-00</a></li>
    561556            </ul>
    562557         </li>
    563558         <li><a href="#rfc.index">Index</a></li>
     559         <li><a href="#rfc.authors">Authors' Addresses</a></li>
    564560         <li><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
    565561      </ul>
    566       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="caching" href="#caching">Introduction</a></h1>
    567       <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,
    568          and includes a number of elements intended to make caching work as well as possible. Because these elements interact with
    569          each other, it is useful to describe the caching design of HTTP separately. This document defines aspects of HTTP/1.1 related
    570          to caching and reusing response messages.
    571       </p>
    572       <div id="rfc.iref.c.1"></div>
    573       <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a id="intro.purpose" href="#intro.purpose">Purpose</a></h2>
    574       <p id="rfc.section.1.1.p.1">An HTTP <dfn>cache</dfn> is a local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache
    575          stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests.
    576          Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
    577       </p>
    578       <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 eliminate
    579          the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former
    580          reduces the number of network round-trips required for many operations; we use an "expiration" mechanism for this purpose
    581          (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a>). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;4</a>).
    582       </p>
    583       <div id="rfc.iref.s.1"></div>
    584       <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,
    585          except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except
    586          for hop-by-hop headers) that it would have received had its request been handled directly by the origin server.
    587       </p>
    588       <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
    589          transparency is not always necessary and can be effectively traded for the sake of bandwidth scaling, disconnected operation,
    590          and high availability. HTTP/1.1 allows origin servers, caches, and clients to explicitly reduce transparency when necessary.
    591          However, because non-transparent operation may confuse non-expert users and might be incompatible with certain server applications
    592          (such as those for ordering merchandise), the protocol requires that transparency be relaxed
    593       </p>
    594       <ul>
    595          <li>only by an explicit protocol-level request when relaxed by client or origin server</li>
    596          <li>only with an explicit warning to the end user when relaxed by cache or client</li>
    597       </ul>
    598       <p id="rfc.section.1.1.p.5">Therefore, HTTP/1.1 provides these important elements: </p>
    599       <ol>
    600          <li>Protocol features that provide full semantic transparency when this is required by all parties.</li>
    601          <li>Protocol features that allow an origin server or user agent to explicitly request and control non-transparent operation.</li>
    602          <li>Protocol features that allow a cache to attach warnings to responses that do not preserve the requested approximation of semantic
    603             transparency.
    604          </li>
    605       </ol>
    606       <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>
    607       <ul class="empty">
    608          <li> <b>Note:</b> The server, cache, or client implementor might be faced with design decisions not explicitly discussed in this specification.
    609             If a decision might affect semantic transparency, the implementor ought to err on the side of maintaining transparency unless
    610             a careful and complete analysis shows significant benefits in breaking transparency.
    611          </li>
    612       </ul>
    613       <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>
    614       <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>
    615       <p id="rfc.section.1.2.p.2"> <span id="rfc.iref.c.2"></span>  <dfn>cacheable</dfn> 
    616       </p>
    617       <ul class="empty">
    618          <li>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
    619             Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
    620             request.
    621          </li>
    622       </ul>
    623       <p id="rfc.section.1.2.p.3"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
    624       </p>
    625       <ul class="empty">
    626          <li>A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more
    627             proxies. A response is also first-hand if its validity has just been checked directly with the origin server.
    628          </li>
    629       </ul>
    630       <p id="rfc.section.1.2.p.4"> <span id="rfc.iref.e.1"></span>  <dfn>explicit expiration time</dfn> 
    631       </p>
    632       <ul class="empty">
    633          <li>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</li>
    634       </ul>
    635       <p id="rfc.section.1.2.p.5"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
    636       </p>
    637       <ul class="empty">
    638          <li>An expiration time assigned by a cache when no explicit expiration time is available.</li>
    639       </ul>
    640       <p id="rfc.section.1.2.p.6"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
    641       </p>
    642       <ul class="empty">
    643          <li>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</li>
    644       </ul>
    645       <p id="rfc.section.1.2.p.7"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn> 
    646       </p>
    647       <ul class="empty">
    648          <li>The length of time between the generation of a response and its expiration time.</li>
    649       </ul>
    650       <p id="rfc.section.1.2.p.8"> <span id="rfc.iref.f.3"></span>  <dfn>fresh</dfn> 
    651       </p>
    652       <ul class="empty">
    653          <li>A response is fresh if its age has not yet exceeded its freshness lifetime.</li>
    654       </ul>
    655       <p id="rfc.section.1.2.p.9"> <span id="rfc.iref.s.2"></span>  <dfn>stale</dfn> 
    656       </p>
    657       <ul class="empty">
    658          <li>A response is stale if its age has passed its freshness lifetime.</li>
    659       </ul>
    660       <p id="rfc.section.1.2.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>validator</dfn> 
    661       </p>
    662       <ul class="empty">
    663          <li>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
    664             copy of an entity.
    665          </li>
    666       </ul>
    667       <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>
    668       <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"
    669          in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
    670       </p>
    671       <p id="rfc.section.1.3.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
    672       </p>
    673       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Overview</a></h1>
    674       <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>
    675       <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:
    676       </p>
    677       <ol>
    678          <li>It has been checked for equivalence with what the origin server would have returned by revalidating the response with the
    679             origin server (<a href="#validation.model" title="Validation Model">Section&nbsp;4</a>);
    680          </li>
    681          <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
    682             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
    683             not "fresh enough" by the most restrictive freshness requirement of both the client and the origin server, in carefully considered
    684             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;
    685             see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;15.2</a>).
    686          </li>
    687          <li>It is an appropriate 304 (Not Modified), 305 (Use Proxy), or error (4xx or 5xx) response message.</li>
    688       </ol>
    689       <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.
    690       </p>
    691       <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
    692          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.
    693          A user agent that receives a stale response without a Warning <em class="bcp14">MAY</em> display a warning indication to the user.
    694       </p>
    695       <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a id="warnings" href="#warnings">Warnings</a></h2>
    696       <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
    697          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.
    698       </p>
    699       <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
    700          these responses from true failures.
    701       </p>
    702       <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:
    703       </p>
    704       <p id="rfc.section.2.2.p.4"> </p>
    705       <dl>
    706          <dt>1xx</dt>
    707          <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.
    708          </dd>
    709          <dt>2xx</dt>
    710          <dd>Warnings that describe some aspect of the entity body or entity headers that is not rectified by a revalidation (for example,
    711             a lossy compression of the entity bodies) and which <em class="bcp14">MUST NOT</em> be deleted after a successful revalidation.
    712          </dd>
    713       </dl>
    714       <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.
    715       </p>
    716       <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
    717          that are passed to HTTP/1.0 caches carry an extra warning-date field, which prevents a future HTTP/1.1 recipient from believing
    718          an erroneously cached Warning.
    719       </p>
    720       <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.
    721       </p>
    722       <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.
    723          For example, a server might provide the same warning with texts in both English and Basque.
    724       </p>
    725       <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.
    726          This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but
    727          does suggest some heuristics.
    728       </p>
    729       <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>
    730       <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.
    731          In some cases, a server or client might need to provide explicit directives to the HTTP caches. We use the Cache-Control header
    732          for this purpose.
    733       </p>
    734       <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
    735          directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between
    736          header values, the most restrictive interpretation is applied (that is, the one that is most likely to preserve semantic transparency).
    737          However, in some cases, cache-control directives are explicitly specified as weakening the approximation of semantic transparency
    738          (for example, "max-stale" or "public").
    739       </p>
    740       <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>.
    741       </p>
    742       <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>
    743       <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
    744          the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually
    745          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.
    746       </p>
    747       <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
    748          requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent
    749          to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication
    750          need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other indicator.
    751       </p>
    752       <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
    753          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
    754          does not inadvertently consume excess resources or suffer from excessive latency.
    755       </p>
    756       <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>
    757       <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,
    758          but may be necessary for reasons of availability or performance, especially when the cache is poorly connected to the origin
    759          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.
    760       </p>
    761       <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
    762          technical or policy reasons.
    763       </p>
    764       <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>
    765       <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
    766          the primary source of expiration information, in some cases the client might need to control a cache's decision about whether
    767          to return a cached response without validating it. Clients do this using several directives of the Cache-Control header.
    768       </p>
    769       <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)
    770          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
    771          of caches, and so cannot further relax the cache's approximation of semantic transparency.
    772       </p>
    773       <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
    774          the caches, and so might violate the origin server's specified constraints on semantic transparency, but might be necessary
    775          to support disconnected operation, or high availability in the face of poor connectivity.
    776       </p>
    777       <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="expiration.model" href="#expiration.model">Expiration Model</a></h1>
    778       <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>
    779       <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
    780          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.
    781       </p>
    782       <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
    783          not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic
    784          transparency, as long as the server's expiration times are carefully chosen.
    785       </p>
    786       <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
    787          to the requesting client.
    788       </p>
    789       <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.
    790       </p>
    791       <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>).
    792       </p>
    793       <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>
    794       <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
    795          to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource
    796          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.
    797       </p>
    798       <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>
    799       <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,
    800          employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time.
    801          The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results.
    802          Since heuristic expiration times might compromise semantic transparency, they ought to be used cautiously, and we encourage
    803          origin servers to provide explicit expiration times as much as possible.
    804       </p>
    805       <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>
    806       <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
    807          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.
    808       </p>
    809       <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."
    810          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.
    811       </p>
    812       <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
    813          was generated (see <a href="p1-messaging.html#header.date" title="Date">Section 8.3</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.
    814       </p>
    815       <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
    816          Age field value is the cache's estimate of the amount of time since the response was generated or revalidated by the origin
    817          server.
    818       </p>
    819       <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
    820          from the origin server, plus the amount of time it has been in transit along network paths.
    821       </p>
    822       <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>
    823       <p id="rfc.section.3.3.p.7">A response's age can be calculated in two entirely independent ways: </p>
    824       <ol>
    825          <li>now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative,
    826             the result is replaced by zero.
    827          </li>
    828          <li>age_value, if all of the caches along the response path implement HTTP/1.1.</li>
    829       </ol>
    830       <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>
    831       <div id="rfc.figure.u.1"></div><pre class="text">    corrected_received_age = max(now - date_value, age_value)
     562      <div id="caching">
     563         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a href="#caching">Introduction</a></h1>
     564         <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,
     565            and includes a number of elements intended to make caching work as well as possible. Because these elements interact with
     566            each other, it is useful to describe the caching design of HTTP separately. This document defines aspects of HTTP/1.1 related
     567            to caching and reusing response messages.
     568         </p>
     569         <div id="intro.purpose">
     570            <div id="rfc.iref.c.1"></div>
     571            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;<a href="#intro.purpose">Purpose</a></h2>
     572            <p id="rfc.section.1.1.p.1">An HTTP <dfn>cache</dfn> is a local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache
     573               stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests.
     574               Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
     575            </p>
     576            <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 eliminate
     577               the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former
     578               reduces the number of network round-trips required for many operations; we use an "expiration" mechanism for this purpose
     579               (see <a href="#expiration.model" title="Expiration Model">Section&nbsp;3</a>). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;4</a>).
     580            </p>
     581            <div id="rfc.iref.s.1"></div>
     582            <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,
     583               except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except
     584               for hop-by-hop headers) that it would have received had its request been handled directly by the origin server.
     585            </p>
     586            <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
     587               transparency is not always necessary and can be effectively traded for the sake of bandwidth scaling, disconnected operation,
     588               and high availability. HTTP/1.1 allows origin servers, caches, and clients to explicitly reduce transparency when necessary.
     589               However, because non-transparent operation may confuse non-expert users and might be incompatible with certain server applications
     590               (such as those for ordering merchandise), the protocol requires that transparency be relaxed
     591            </p>
     592            <ul>
     593               <li>only by an explicit protocol-level request when relaxed by client or origin server</li>
     594               <li>only with an explicit warning to the end user when relaxed by cache or client</li>
     595            </ul>
     596            <p id="rfc.section.1.1.p.5">Therefore, HTTP/1.1 provides these important elements: </p>
     597            <ol>
     598               <li>Protocol features that provide full semantic transparency when this is required by all parties.</li>
     599               <li>Protocol features that allow an origin server or user agent to explicitly request and control non-transparent operation.</li>
     600               <li>Protocol features that allow a cache to attach warnings to responses that do not preserve the requested approximation of semantic
     601                  transparency.
     602               </li>
     603            </ol>
     604            <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>
     605            <ul class="empty">
     606               <li><b>Note:</b> The server, cache, or client implementor might be faced with design decisions not explicitly discussed in this specification.
     607                  If a decision might affect semantic transparency, the implementor ought to err on the side of maintaining transparency unless
     608                  a careful and complete analysis shows significant benefits in breaking transparency.
     609               </li>
     610            </ul>
     611         </div>
     612         <div id="intro.terminology">
     613            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a href="#intro.terminology">Terminology</a></h2>
     614            <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>
     615            <p id="rfc.section.1.2.p.2"><span id="rfc.iref.c.2"></span> <dfn>cacheable</dfn>
     616            </p>
     617            <ul class="empty">
     618               <li>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
     619                  Even if a resource is cacheable, there may be additional constraints on whether a cache can use the cached copy for a particular
     620                  request.
     621               </li>
     622            </ul>
     623            <p id="rfc.section.1.2.p.3"><span id="rfc.iref.f.1"></span> <dfn>first-hand</dfn>
     624            </p>
     625            <ul class="empty">
     626               <li>A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more
     627                  proxies. A response is also first-hand if its validity has just been checked directly with the origin server.
     628               </li>
     629            </ul>
     630            <p id="rfc.section.1.2.p.4"><span id="rfc.iref.e.1"></span> <dfn>explicit expiration time</dfn>
     631            </p>
     632            <ul class="empty">
     633               <li>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</li>
     634            </ul>
     635            <p id="rfc.section.1.2.p.5"><span id="rfc.iref.h.1"></span> <dfn>heuristic expiration time</dfn>
     636            </p>
     637            <ul class="empty">
     638               <li>An expiration time assigned by a cache when no explicit expiration time is available.</li>
     639            </ul>
     640            <p id="rfc.section.1.2.p.6"><span id="rfc.iref.a.1"></span> <dfn>age</dfn>
     641            </p>
     642            <ul class="empty">
     643               <li>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</li>
     644            </ul>
     645            <p id="rfc.section.1.2.p.7"><span id="rfc.iref.f.2"></span> <dfn>freshness lifetime</dfn>
     646            </p>
     647            <ul class="empty">
     648               <li>The length of time between the generation of a response and its expiration time.</li>
     649            </ul>
     650            <p id="rfc.section.1.2.p.8"><span id="rfc.iref.f.3"></span> <dfn>fresh</dfn>
     651            </p>
     652            <ul class="empty">
     653               <li>A response is fresh if its age has not yet exceeded its freshness lifetime.</li>
     654            </ul>
     655            <p id="rfc.section.1.2.p.9"><span id="rfc.iref.s.2"></span> <dfn>stale</dfn>
     656            </p>
     657            <ul class="empty">
     658               <li>A response is stale if its age has passed its freshness lifetime.</li>
     659            </ul>
     660            <p id="rfc.section.1.2.p.10"><span id="rfc.iref.v.1"></span> <dfn>validator</dfn>
     661            </p>
     662            <ul class="empty">
     663               <li>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
     664                  copy of an entity.
     665               </li>
     666            </ul>
     667         </div>
     668         <div id="intro.requirements">
     669            <h2 id="rfc.section.1.3"><a href="#rfc.section.1.3">1.3</a>&nbsp;<a href="#intro.requirements">Requirements</a></h2>
     670            <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"
     671               in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>.
     672            </p>
     673            <p id="rfc.section.1.3.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant."
     674            </p>
     675         </div>
     676      </div>
     677      <div id="caching.overview">
     678         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a href="#caching.overview">Overview</a></h1>
     679         <div id="cache.correctness">
     680            <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a>&nbsp;<a href="#cache.correctness">Cache Correctness</a></h2>
     681            <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:
     682            </p>
     683            <ol>
     684               <li>It has been checked for equivalence with what the origin server would have returned by revalidating the response with the
     685                  origin server (<a href="#validation.model" title="Validation Model">Section&nbsp;4</a>);
     686               </li>
     687               <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
     688                  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
     689                  not "fresh enough" by the most restrictive freshness requirement of both the client and the origin server, in carefully considered
     690                  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;
     691                  see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;15.2</a>).
     692               </li>
     693               <li>It is an appropriate 304 (Not Modified), 305 (Use Proxy), or error (4xx or 5xx) response message.</li>
     694            </ol>
     695            <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.
     696            </p>
     697            <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
     698               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.
     699               A user agent that receives a stale response without a Warning <em class="bcp14">MAY</em> display a warning indication to the user.
     700            </p>
     701         </div>
     702         <div id="warnings">
     703            <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a>&nbsp;<a href="#warnings">Warnings</a></h2>
     704            <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
     705               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.
     706            </p>
     707            <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
     708               these responses from true failures.
     709            </p>
     710            <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:
     711            </p>
     712            <p id="rfc.section.2.2.p.4"></p>
     713            <dl>
     714               <dt>1xx</dt>
     715               <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.
     716               </dd>
     717               <dt>2xx</dt>
     718               <dd>Warnings that describe some aspect of the entity body or entity headers that is not rectified by a revalidation (for example,
     719                  a lossy compression of the entity bodies) and which <em class="bcp14">MUST NOT</em> be deleted after a successful revalidation.
     720               </dd>
     721            </dl>
     722            <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.
     723            </p>
     724            <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
     725               that are passed to HTTP/1.0 caches carry an extra warning-date field, which prevents a future HTTP/1.1 recipient from believing
     726               an erroneously cached Warning.
     727            </p>
     728            <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.
     729            </p>
     730            <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.
     731               For example, a server might provide the same warning with texts in both English and Basque.
     732            </p>
     733            <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.
     734               This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but
     735               does suggest some heuristics.
     736            </p>
     737         </div>
     738         <div id="cache-control.mechanisms">
     739            <h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a>&nbsp;<a href="#cache-control.mechanisms">Cache-control Mechanisms</a></h2>
     740            <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.
     741               In some cases, a server or client might need to provide explicit directives to the HTTP caches. We use the Cache-Control header
     742               for this purpose.
     743            </p>
     744            <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
     745               directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between
     746               header values, the most restrictive interpretation is applied (that is, the one that is most likely to preserve semantic transparency).
     747               However, in some cases, cache-control directives are explicitly specified as weakening the approximation of semantic transparency
     748               (for example, "max-stale" or "public").
     749            </p>
     750            <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>.
     751            </p>
     752         </div>
     753         <div id="explicit.ua.warnings">
     754            <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a>&nbsp;<a href="#explicit.ua.warnings">Explicit User Agent Warnings</a></h2>
     755            <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
     756               the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually
     757               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.
     758            </p>
     759            <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
     760               requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent
     761               to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication
     762               need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other indicator.
     763            </p>
     764            <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
     765               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
     766               does not inadvertently consume excess resources or suffer from excessive latency.
     767            </p>
     768         </div>
     769         <div id="exceptions.to.the.rules.and.warnings">
     770            <h2 id="rfc.section.2.5"><a href="#rfc.section.2.5">2.5</a>&nbsp;<a href="#exceptions.to.the.rules.and.warnings">Exceptions to the Rules and Warnings</a></h2>
     771            <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,
     772               but may be necessary for reasons of availability or performance, especially when the cache is poorly connected to the origin
     773               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.
     774            </p>
     775            <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
     776               technical or policy reasons.
     777            </p>
     778         </div>
     779         <div id="client-controlled.behavior">
     780            <h2 id="rfc.section.2.6"><a href="#rfc.section.2.6">2.6</a>&nbsp;<a href="#client-controlled.behavior">Client-controlled Behavior</a></h2>
     781            <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
     782               the primary source of expiration information, in some cases the client might need to control a cache's decision about whether
     783               to return a cached response without validating it. Clients do this using several directives of the Cache-Control header.
     784            </p>
     785            <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)
     786               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
     787               of caches, and so cannot further relax the cache's approximation of semantic transparency.
     788            </p>
     789            <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
     790               the caches, and so might violate the origin server's specified constraints on semantic transparency, but might be necessary
     791               to support disconnected operation, or high availability in the face of poor connectivity.
     792            </p>
     793         </div>
     794      </div>
     795      <div id="expiration.model">
     796         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a href="#expiration.model">Expiration Model</a></h1>
     797         <div id="server-specified.expiration">
     798            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a href="#server-specified.expiration">Server-Specified Expiration</a></h2>
     799            <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
     800               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.
     801            </p>
     802            <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
     803               not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic
     804               transparency, as long as the server's expiration times are carefully chosen.
     805            </p>
     806            <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
     807               to the requesting client.
     808            </p>
     809            <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.
     810            </p>
     811            <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>).
     812            </p>
     813            <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>
     814            <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
     815               to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource
     816               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.
     817            </p>
     818         </div>
     819         <div id="heuristic.expiration">
     820            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#heuristic.expiration">Heuristic Expiration</a></h2>
     821            <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,
     822               employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time.
     823               The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results.
     824               Since heuristic expiration times might compromise semantic transparency, they ought to be used cautiously, and we encourage
     825               origin servers to provide explicit expiration times as much as possible.
     826            </p>
     827         </div>
     828         <div id="age.calculations">
     829            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#age.calculations">Age Calculations</a></h2>
     830            <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
     831               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.
     832            </p>
     833            <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."
     834               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.
     835            </p>
     836            <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
     837               was generated (see <a href="p1-messaging.html#header.date" title="Date">Section 8.3</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.
     838            </p>
     839            <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
     840               Age field value is the cache's estimate of the amount of time since the response was generated or revalidated by the origin
     841               server.
     842            </p>
     843            <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
     844               from the origin server, plus the amount of time it has been in transit along network paths.
     845            </p>
     846            <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>
     847            <p id="rfc.section.3.3.p.7">A response's age can be calculated in two entirely independent ways: </p>
     848            <ol>
     849               <li>now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative,
     850                  the result is replaced by zero.
     851               </li>
     852               <li>age_value, if all of the caches along the response path implement HTTP/1.1.</li>
     853            </ol>
     854            <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>
     855            <div id="rfc.figure.u.1"></div><pre class="text">    corrected_received_age = max(now - date_value, age_value)
    832856</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>
    833       <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
    834          and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low
    835          ages.
    836       </p>
    837       <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,
    838          we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age
    839          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
    840          results in conservative behavior no matter how much delay is experienced. So, we compute:
    841       </p>
    842       <div id="rfc.figure.u.2"></div><pre class="text">   corrected_initial_age = corrected_received_age
     857            <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
     858               and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low
     859               ages.
     860            </p>
     861            <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,
     862               we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age
     863               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
     864               results in conservative behavior no matter how much delay is experienced. So, we compute:
     865            </p>
     866            <div id="rfc.figure.u.2"></div><pre class="text">   corrected_initial_age = corrected_received_age
    843867                         + (now - request_time)
    844868</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>
    845       <p id="rfc.section.3.3.p.15">Summary of age calculation algorithm, when a cache receives a response:</p>
    846       <div id="rfc.figure.u.3"></div><pre class="text">   /*
     869            <p id="rfc.section.3.3.p.15">Summary of age calculation algorithm, when a cache receives a response:</p>
     870            <div id="rfc.figure.u.3"></div><pre class="text">   /*
    847871    * age_value
    848872    *      is the value of Age: header received by the cache with
     
    867891   current_age   = corrected_initial_age + resident_time;
    868892</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
    869          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.
    870       </p>
    871       <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
    872          true, since the lack of an Age header field in a response does not imply that the response is first-hand unless all caches
    873          along the request path are compliant with HTTP/1.1 (i.e., older HTTP caches did not implement the Age header field).
    874       </p>
    875       <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>
    876       <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
    877          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
    878          below, the values can be represented in any form appropriate for arithmetic operations.
    879       </p>
    880       <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
    881          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>).
    882       </p>
    883       <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>
    884       <div id="rfc.figure.u.4"></div><pre class="text">   freshness_lifetime = max_age_value
     893               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.
     894            </p>
     895            <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
     896               true, since the lack of an Age header field in a response does not imply that the response is first-hand unless all caches
     897               along the request path are compliant with HTTP/1.1 (i.e., older HTTP caches did not implement the Age header field).
     898            </p>
     899         </div>
     900         <div id="expiration.calculations">
     901            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a href="#expiration.calculations">Expiration Calculations</a></h2>
     902            <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
     903               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
     904               below, the values can be represented in any form appropriate for arithmetic operations.
     905            </p>
     906            <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
     907               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>).
     908            </p>
     909            <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>
     910            <div id="rfc.figure.u.4"></div><pre class="text">   freshness_lifetime = max_age_value
    885911</pre><p id="rfc.section.3.4.p.5">Otherwise, if Expires is present in the response, the calculation is:</p>
    886       <div id="rfc.figure.u.5"></div><pre class="text">   freshness_lifetime = expires_value - date_value
     912            <div id="rfc.figure.u.5"></div><pre class="text">   freshness_lifetime = expires_value - date_value
    887913</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>
    888       <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.
    889       </p>
    890       <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%.
    891       </p>
    892       <p id="rfc.section.3.4.p.10">The calculation to determine if a response has expired is quite simple:</p>
    893       <div id="rfc.figure.u.6"></div><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
    894 </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>
    895       <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
    896          resource that are different.
    897       </p>
    898       <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,
    899          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.
    900       </p>
    901       <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
    902          caches, or because a client has asked for a reload or a revalidation of an apparently fresh cache entry.
    903       </p>
    904       <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>
    905       <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
    906          other responses flow through a different set of caches, a client might receive responses in an order different from that in
    907          which the origin server sent them. We would like the client to use the most recently generated response, even if older responses
    908          are still apparently fresh.
    909       </p>
    910       <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
    911          intentionally carries an earlier expiration time. The Date values are ordered to a granularity of one second.
    912       </p>
    913       <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
    914          than the one for the existing entry, then the client <em class="bcp14">SHOULD</em> repeat the request unconditionally, and include
    915       </p>
    916       <div id="rfc.figure.u.7"></div><pre class="text">    Cache-Control: max-age=0
     914            <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.
     915            </p>
     916            <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%.
     917            </p>
     918            <p id="rfc.section.3.4.p.10">The calculation to determine if a response has expired is quite simple:</p>
     919            <div id="rfc.figure.u.6"></div><pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
     920</pre></div>
     921         <div id="disambiguating.expiration.values">
     922            <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a href="#disambiguating.expiration.values">Disambiguating Expiration Values</a></h2>
     923            <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
     924               resource that are different.
     925            </p>
     926            <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,
     927               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.
     928            </p>
     929            <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
     930               caches, or because a client has asked for a reload or a revalidation of an apparently fresh cache entry.
     931            </p>
     932         </div>
     933         <div id="disambiguating.multiple.responses">
     934            <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a href="#disambiguating.multiple.responses">Disambiguating Multiple Responses</a></h2>
     935            <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
     936               other responses flow through a different set of caches, a client might receive responses in an order different from that in
     937               which the origin server sent them. We would like the client to use the most recently generated response, even if older responses
     938               are still apparently fresh.
     939            </p>
     940            <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
     941               intentionally carries an earlier expiration time. The Date values are ordered to a granularity of one second.
     942            </p>
     943            <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
     944               than the one for the existing entry, then the client <em class="bcp14">SHOULD</em> repeat the request unconditionally, and include
     945            </p>
     946            <div id="rfc.figure.u.7"></div><pre class="text">    Cache-Control: max-age=0
    917947</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>
    918       <div id="rfc.figure.u.8"></div><pre class="text">    Cache-Control: no-cache
     948            <div id="rfc.figure.u.8"></div><pre class="text">    Cache-Control: no-cache
    919949</pre><p id="rfc.section.3.6.p.7">to force any intermediate caches to obtain a new copy from the origin server.</p>
    920       <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
    921          times overlap.
    922       </p>
    923       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h1>
    924       <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
    925          origin server (or possibly an intermediate cache with a fresh response) to see if its cached entry is still usable. We call
    926          this "validating" the cache entry. Since we do not want to have to pay the overhead of retransmitting the full response if
    927          the cached entry is good, and we do not want to pay the overhead of an extra round trip if the cached entry is invalid, the
    928          HTTP/1.1 protocol supports the use of conditional methods.
    929       </p>
    930       <p id="rfc.section.4.p.2">The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server
    931          generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (user
    932          agent or proxy cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated
    933          validator in the request.
    934       </p>
    935       <p id="rfc.section.4.p.3">The server then checks that validator against the current validator for the entity, and, if they match (see <a href="p4-conditional.html#weak.and.strong.validators" title="Weak and Strong Validators">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), it responds with a special status code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns a full response
    936          (including entity-body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round
    937          trip if it does not match.
    938       </p>
    939       <p id="rfc.section.4.p.4">In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries
    940          a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional.
    941       </p>
    942       <p id="rfc.section.4.p.5">The protocol includes both positive and negative senses of cache-validating conditions. That is, it is possible to request
    943          either that a method be performed if and only if a validator matches or if and only if no validators match.
    944       </p>
    945       <ul class="empty">
    946          <li> <b>Note:</b> a response that lacks a validator may still be cached, and served from cache until it expires, unless this is explicitly prohibited
    947             by a cache-control directive. However, a cache cannot do a conditional retrieval if it does not have a validator for the entity,
    948             which means it will not be refreshable after it expires.
    949          </li>
    950       </ul>
    951       <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="last-modified.dates" href="#last-modified.dates">Last-Modified Dates</a></h2>
    952       <p id="rfc.section.4.1.p.1">The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered
    953          to be valid if the entity has not been modified since the Last-Modified value.
    954       </p>
    955       <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="entity.tag.cache.validators" href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></h2>
    956       <p id="rfc.section.4.2.p.1">The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable
    957          validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date
    958          values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification
    959          dates.
    960       </p>
    961       <p id="rfc.section.4.2.p.2">Entity Tags are described in <a href="p4-conditional.html#entity.tags" title="Entity Tags">Section 2</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>. The headers used with entity tags are described in <a href="p4-conditional.html#header.fields" title="Header Field Definitions">Section 6</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
    962       </p>
    963       <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="non-validating.conditionals" href="#non-validating.conditionals">Non-validating Conditionals</a></h2>
    964       <p id="rfc.section.4.3.p.1">The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an
    965          appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality
    966          would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0)
    967          are never used for purposes of validating a cache entry.
    968       </p>
    969       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Response Cacheability</a></h1>
    970       <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
    971          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
    972          such a response was taken from a cache by comparing the Date header to the current time.
    973       </p>
    974       <ul class="empty">
    975          <li> <b>Note:</b> some HTTP/1.0 caches are known to violate this expectation without providing any Warning.
    976          </li>
    977       </ul>
    978       <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
    979          request. This might be because absolute semantic transparency is deemed necessary by the service author, or because of security
    980          or privacy considerations. Certain cache-control directives are therefore provided so that the server can indicate that certain
    981          resource entities, or portions thereof, are not to be cached regardless of other considerations.
    982       </p>
    983       <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
    984          header.
    985       </p>
    986       <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
    987          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.
    988       </p>
    989       <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
    990          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>).
    991       </p>
    992       <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>
    993       <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.
    994          In many cases, a cache simply returns the appropriate parts of a response to the requester. However, if the cache holds a
    995          cache entry based on a previous response, it might have to combine parts of a new response with what is held in the cache
    996          entry.
    997       </p>
    998       <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>
    999       <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>
    1000       <ul>
    1001          <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.
    1002          </li>
    1003          <li>Hop-by-hop headers, which are meaningful only for a single transport-level connection, and are not stored by caches or forwarded
    1004             by proxies.
    1005          </li>
    1006       </ul>
    1007       <p id="rfc.section.6.1.p.2">The following HTTP/1.1 headers are hop-by-hop headers: </p>
    1008       <ul>
    1009          <li>Connection</li>
    1010          <li>Keep-Alive</li>
    1011          <li>Proxy-Authenticate</li>
    1012          <li>Proxy-Authorization</li>
    1013          <li>TE</li>
    1014          <li>Trailer</li>
    1015          <li>Transfer-Encoding</li>
    1016          <li>Upgrade</li>
    1017       </ul>
    1018       <p id="rfc.section.6.1.p.3">All other headers defined by HTTP/1.1 are end-to-end headers.</p>
    1019       <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.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
    1020       </p>
    1021       <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>
    1022       <p id="rfc.section.6.2.p.1">Some features of the HTTP/1.1 protocol, such as Digest Authentication, depend on the value of certain end-to-end headers.
    1023          A transparent proxy <em class="bcp14">SHOULD NOT</em> modify an end-to-end header unless the definition of that header requires or specifically allows that.
    1024       </p>
    1025       <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:
    1026       </p>
    1027       <ul>
    1028          <li>Content-Location</li>
    1029          <li>Content-MD5</li>
    1030          <li>ETag</li>
    1031          <li>Last-Modified</li>
    1032       </ul>
    1033       <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:
    1034       </p>
    1035       <ul>
    1036          <li>Expires</li>
    1037       </ul>
    1038       <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.
    1039       </p>
    1040       <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:
    1041       </p>
    1042       <ul>
    1043          <li>Content-Encoding</li>
    1044          <li>Content-Range</li>
    1045          <li>Content-Type</li>
    1046       </ul>
    1047       <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>).
    1048       </p>
    1049       <ul class="empty">
    1050          <li>Warning: unnecessary modification of end-to-end headers might cause authentication failures if stronger authentication mechanisms
    1051             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.
    1052          </li>
    1053       </ul>
    1054       <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.Part1.3"><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.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
    1055       </p>
    1056       <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>
    1057       <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
    1058          Content) response, the cache then constructs a response to send to the requesting client.
    1059       </p>
    1060       <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
    1061          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
    1062          of this outgoing response, (see <a href="p5-range.html#combining.byte.ranges" title="Combining Byte 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>).
    1063       </p>
    1064       <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>
    1065       <ul>
    1066          <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.
    1067          </li>
    1068          <li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the cache entry and the forwarded response.
    1069          </li>
    1070          <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.
    1071          </li>
    1072       </ul>
    1073       <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,
    1074          except for Warning headers as described immediately above. If a header field-name in the incoming response matches more than
    1075          one header in the cache entry, all such old headers <em class="bcp14">MUST</em> be replaced.
    1076       </p>
    1077       <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
    1078          stored with the cache entry (except for stored Warning headers with warn-code 1xx, which are deleted even if not overridden).
    1079       </p>
    1080       <ul class="empty">
    1081          <li> <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
    1082             with a previous response for the same entity or sub-ranges thereof, although it might not always be meaningful or correct
    1083             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
    1084             delete a header that it had provided with a previous response.
    1085          </li>
    1086       </ul>
    1087       <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>
    1088       <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
    1089          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.
    1090       </p>
    1091       <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
    1092          of a cacheable response subject to server-driven negotiation. The set of header fields named by the Vary field value is known
    1093          as the "selecting" request-headers.
    1094       </p>
    1095       <p id="rfc.section.7.p.3">When the cache receives a subsequent request whose Request-URI specifies one or more cache entries including a Vary header
    1096          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
    1097          new request match the corresponding stored request-headers in the original request.
    1098       </p>
    1099       <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
    1100          request can be transformed to the selecting request-headers in the second request by adding or removing linear white space
    1101          (LWS) at places where this is allowed by the corresponding BNF, and/or combining multiple message-header fields with the same
    1102          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.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
    1103       </p>
    1104       <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
    1105          by the origin server.
    1106       </p>
    1107       <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,
    1108          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
    1109          and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates the entity to
    1110          be used.
    1111       </p>
    1112       <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
    1113          conveys to the server the set of entities currently held by the cache, so that if any one of these entities matches the requested
    1114          entity, the server can use the ETag header field in its 304 (Not Modified) response to tell the cache which entry is appropriate.
    1115          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.
    1116       </p>
    1117       <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.
    1118       </p>
    1119       <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
    1120          Request-URI, whose entity-tag differs from that of the existing entry, and whose Date is more recent than that of the existing
    1121          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.
    1122       </p>
    1123       <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>
    1124       <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
    1125          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
    1126          place certain constraints on the operation of shared caches in order to prevent loss of privacy or failure of access controls.
    1127       </p>
    1128       <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>
    1129       <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 Byte 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.
    1130          A cache <em class="bcp14">MUST NOT</em> return a partial response using a status code of 200 (OK).
    1131       </p>
    1132       <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>).
    1133       </p>
    1134       <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>
    1135       <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
    1136          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
    1137          expected to observe an origin server's explicit restrictions on caching.
    1138       </p>
    1139       <p id="rfc.section.10.p.2">We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those
    1140          containing a "?" in the rel_path 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
    1141          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 8.1.1</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.
    1142       </p>
    1143       <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>
    1144       <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
    1145          to become non-transparently invalid. That is, although they might continue to be "fresh," they do not accurately reflect what
    1146          the origin server would return for a new request on that resource.
    1147       </p>
    1148       <p id="rfc.section.11.p.2">There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request
    1149          that caused the change at the origin server might not have gone through the proxy where a cache entry is stored. However,
    1150          several rules help reduce the likelihood of erroneous behavior.
    1151       </p>
    1152       <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
    1153          its storage, or will mark these as "invalid" and in need of a mandatory revalidation before they can be returned in response
    1154          to a subsequent request.
    1155       </p>
    1156       <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-URI, or by the Location or Content-Location
    1157          headers (if present). These methods are:
    1158       </p>
    1159       <ul>
    1160          <li>PUT</li>
    1161          <li>DELETE</li>
    1162          <li>POST</li>
    1163       </ul>
    1164       <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-URI. This helps prevent denial of service
    1165          attacks.
    1166       </p>
    1167       <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-URI.
    1168       </p>
    1169       <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>
    1170       <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
    1171          response from the inbound server. This does not prevent a proxy cache from sending a 100 (Continue) response before the inbound
    1172          server has sent its final reply.
    1173       </p>
    1174       <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
    1175          consistent updates and the problems arising from server, cache, or network failure prior to write-back.
    1176       </p>
    1177       <h1 id="rfc.section.13"><a href="#rfc.section.13">13.</a>&nbsp;<a id="cache.replacement" href="#cache.replacement">Cache Replacement</a></h1>
    1178       <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
    1179          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.
    1180       </p>
    1181       <ul class="empty">
    1182          <li> <b>Note:</b> a new response that has an older Date header value than existing cached responses is not cacheable.
    1183          </li>
    1184       </ul>
    1185       <h1 id="rfc.section.14"><a href="#rfc.section.14">14.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
    1186       <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
    1187          retrieved earlier in a session.
    1188       </p>
    1189       <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
    1190          exactly what the user saw at the time when the resource was retrieved.
    1191       </p>
    1192       <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
    1193          documents.
    1194       </p>
    1195       <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>
    1196       <ul class="empty">
    1197          <li> <b>Note:</b> if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors
    1198             to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider
    1199             it important that users not be presented with error messages or warning messages when they use navigation controls (such as
    1200             BACK) to view previously fetched resources. Even though sometimes such resources ought not be cached, or ought to expire quickly,
    1201             user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs)
    1202             in order not to suffer the effects of improperly functioning history mechanisms.
    1203          </li>
    1204       </ul>
    1205       <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>
    1206       <p id="rfc.section.15.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
    1207       <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
    1208          receives the entity.
    1209       </p>
    1210       <div id="rfc.iref.a.2"></div>
    1211       <div id="rfc.iref.h.2"></div>
    1212       <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>
    1213       <p id="rfc.section.15.1.p.1">The Age response-header field conveys the sender's estimate of the amount of time since the response (or its revalidation)
    1214          was generated at the origin server. A cached response is "fresh" if its age does not exceed its freshness lifetime. Age values
    1215          are calculated as specified in <a href="#age.calculations" title="Age Calculations">Section&nbsp;3.3</a>.
    1216       </p>
    1217       <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span>  Age = "Age" ":" age-value
     950            <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
     951               times overlap.
     952            </p>
     953         </div>
     954      </div>
     955      <div id="validation.model">
     956         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#validation.model">Validation Model</a></h1>
     957         <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
     958            origin server (or possibly an intermediate cache with a fresh response) to see if its cached entry is still usable. We call
     959            this "validating" the cache entry. Since we do not want to have to pay the overhead of retransmitting the full response if
     960            the cached entry is good, and we do not want to pay the overhead of an extra round trip if the cached entry is invalid, the
     961            HTTP/1.1 protocol supports the use of conditional methods.
     962         </p>
     963         <p id="rfc.section.4.p.2">The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server
     964            generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (user
     965            agent or proxy cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated
     966            validator in the request.
     967         </p>
     968         <p id="rfc.section.4.p.3">The server then checks that validator against the current validator for the entity, and, if they match (see <a href="p4-conditional.html#weak.and.strong.validators" title="Weak and Strong Validators">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), it responds with a special status code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns a full response
     969            (including entity-body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round
     970            trip if it does not match.
     971         </p>
     972         <p id="rfc.section.4.p.4">In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries
     973            a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional.
     974         </p>
     975         <p id="rfc.section.4.p.5">The protocol includes both positive and negative senses of cache-validating conditions. That is, it is possible to request
     976            either that a method be performed if and only if a validator matches or if and only if no validators match.
     977         </p>
     978         <ul class="empty">
     979            <li><b>Note:</b> a response that lacks a validator may still be cached, and served from cache until it expires, unless this is explicitly prohibited
     980               by a cache-control directive. However, a cache cannot do a conditional retrieval if it does not have a validator for the entity,
     981               which means it will not be refreshable after it expires.
     982            </li>
     983         </ul>
     984         <div id="last-modified.dates">
     985            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a href="#last-modified.dates">Last-Modified Dates</a></h2>
     986            <p id="rfc.section.4.1.p.1">The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered
     987               to be valid if the entity has not been modified since the Last-Modified value.
     988            </p>
     989         </div>
     990         <div id="entity.tag.cache.validators">
     991            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a href="#entity.tag.cache.validators">Entity Tag Cache Validators</a></h2>
     992            <p id="rfc.section.4.2.p.1">The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable
     993               validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date
     994               values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification
     995               dates.
     996            </p>
     997            <p id="rfc.section.4.2.p.2">Entity Tags are described in <a href="p4-conditional.html#entity.tags" title="Entity Tags">Section 2</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>. The headers used with entity tags are described in <a href="p4-conditional.html#header.fields" title="Header Field Definitions">Section 6</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
     998            </p>
     999         </div>
     1000         <div id="non-validating.conditionals">
     1001            <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a href="#non-validating.conditionals">Non-validating Conditionals</a></h2>
     1002            <p id="rfc.section.4.3.p.1">The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an
     1003               appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality
     1004               would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0)
     1005               are never used for purposes of validating a cache entry.
     1006            </p>
     1007         </div>
     1008      </div>
     1009      <div id="response.cacheability">
     1010         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a href="#response.cacheability">Response Cacheability</a></h1>
     1011         <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
     1012            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
     1013            such a response was taken from a cache by comparing the Date header to the current time.
     1014         </p>
     1015         <ul class="empty">
     1016            <li><b>Note:</b> some HTTP/1.0 caches are known to violate this expectation without providing any Warning.
     1017            </li>
     1018         </ul>
     1019         <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
     1020            request. This might be because absolute semantic transparency is deemed necessary by the service author, or because of security
     1021            or privacy considerations. Certain cache-control directives are therefore provided so that the server can indicate that certain
     1022            resource entities, or portions thereof, are not to be cached regardless of other considerations.
     1023         </p>
     1024         <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
     1025            header.
     1026         </p>
     1027         <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
     1028            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.
     1029         </p>
     1030         <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
     1031            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>).
     1032         </p>
     1033      </div>
     1034      <div id="constructing.responses.from.caches">
     1035         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses From Caches</a></h1>
     1036         <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.
     1037            In many cases, a cache simply returns the appropriate parts of a response to the requester. However, if the cache holds a
     1038            cache entry based on a previous response, it might have to combine parts of a new response with what is held in the cache
     1039            entry.
     1040         </p>
     1041         <div id="end-to-end.and.hop-by-hop.headers">
     1042            <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a href="#end-to-end.and.hop-by-hop.headers">End-to-end and Hop-by-hop Headers</a></h2>
     1043            <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>
     1044            <ul>
     1045               <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.
     1046               </li>
     1047               <li>Hop-by-hop headers, which are meaningful only for a single transport-level connection, and are not stored by caches or forwarded
     1048                  by proxies.
     1049               </li>
     1050            </ul>
     1051            <p id="rfc.section.6.1.p.2">The following HTTP/1.1 headers are hop-by-hop headers: </p>
     1052            <ul>
     1053               <li>Connection</li>
     1054               <li>Keep-Alive</li>
     1055               <li>Proxy-Authenticate</li>
     1056               <li>Proxy-Authorization</li>
     1057               <li>TE</li>
     1058               <li>Trailer</li>
     1059               <li>Transfer-Encoding</li>
     1060               <li>Upgrade</li>
     1061            </ul>
     1062            <p id="rfc.section.6.1.p.3">All other headers defined by HTTP/1.1 are end-to-end headers.</p>
     1063            <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.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
     1064            </p>
     1065         </div>
     1066         <div id="non-modifiable.headers">
     1067            <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a>&nbsp;<a href="#non-modifiable.headers">Non-modifiable Headers</a></h2>
     1068            <p id="rfc.section.6.2.p.1">Some features of the HTTP/1.1 protocol, such as Digest Authentication, depend on the value of certain end-to-end headers.
     1069               A transparent proxy <em class="bcp14">SHOULD NOT</em> modify an end-to-end header unless the definition of that header requires or specifically allows that.
     1070            </p>
     1071            <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:
     1072            </p>
     1073            <ul>
     1074               <li>Content-Location</li>
     1075               <li>Content-MD5</li>
     1076               <li>ETag</li>
     1077               <li>Last-Modified</li>
     1078            </ul>
     1079            <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:
     1080            </p>
     1081            <ul>
     1082               <li>Expires</li>
     1083            </ul>
     1084            <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.
     1085            </p>
     1086            <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:
     1087            </p>
     1088            <ul>
     1089               <li>Content-Encoding</li>
     1090               <li>Content-Range</li>
     1091               <li>Content-Type</li>
     1092            </ul>
     1093            <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>).
     1094            </p>
     1095            <ul class="empty">
     1096               <li>Warning: unnecessary modification of end-to-end headers might cause authentication failures if stronger authentication mechanisms
     1097                  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.
     1098               </li>
     1099            </ul>
     1100            <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.Part1.3"><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.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
     1101            </p>
     1102         </div>
     1103         <div id="combining.headers">
     1104            <h2 id="rfc.section.6.3"><a href="#rfc.section.6.3">6.3</a>&nbsp;<a href="#combining.headers">Combining Headers</a></h2>
     1105            <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
     1106               Content) response, the cache then constructs a response to send to the requesting client.
     1107            </p>
     1108            <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
     1109               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
     1110               of this outgoing response, (see <a href="p5-range.html#combining.byte.ranges" title="Combining Byte 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>).
     1111            </p>
     1112            <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>
     1113            <ul>
     1114               <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.
     1115               </li>
     1116               <li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the cache entry and the forwarded response.
     1117               </li>
     1118               <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.
     1119               </li>
     1120            </ul>
     1121            <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,
     1122               except for Warning headers as described immediately above. If a header field-name in the incoming response matches more than
     1123               one header in the cache entry, all such old headers <em class="bcp14">MUST</em> be replaced.
     1124            </p>
     1125            <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
     1126               stored with the cache entry (except for stored Warning headers with warn-code 1xx, which are deleted even if not overridden).
     1127            </p>
     1128            <ul class="empty">
     1129               <li><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
     1130                  with a previous response for the same entity or sub-ranges thereof, although it might not always be meaningful or correct
     1131                  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
     1132                  delete a header that it had provided with a previous response.
     1133               </li>
     1134            </ul>
     1135         </div>
     1136      </div>
     1137      <div id="caching.negotiated.responses">
     1138         <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></h1>
     1139         <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
     1140            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.
     1141         </p>
     1142         <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
     1143            of a cacheable response subject to server-driven negotiation. The set of header fields named by the Vary field value is known
     1144            as the "selecting" request-headers.
     1145         </p>
     1146         <p id="rfc.section.7.p.3">When the cache receives a subsequent request whose Request-URI specifies one or more cache entries including a Vary header
     1147            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
     1148            new request match the corresponding stored request-headers in the original request.
     1149         </p>
     1150         <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
     1151            request can be transformed to the selecting request-headers in the second request by adding or removing linear white space
     1152            (LWS) at places where this is allowed by the corresponding BNF, and/or combining multiple message-header fields with the same
     1153            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.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
     1154         </p>
     1155         <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
     1156            by the origin server.
     1157         </p>
     1158         <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,
     1159            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
     1160            and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates the entity to
     1161            be used.
     1162         </p>
     1163         <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
     1164            conveys to the server the set of entities currently held by the cache, so that if any one of these entities matches the requested
     1165            entity, the server can use the ETag header field in its 304 (Not Modified) response to tell the cache which entry is appropriate.
     1166            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.
     1167         </p>
     1168         <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.
     1169         </p>
     1170         <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
     1171            Request-URI, whose entity-tag differs from that of the existing entry, and whose Date is more recent than that of the existing
     1172            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.
     1173         </p>
     1174      </div>
     1175      <div id="shared.and.non-shared.caches">
     1176         <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a href="#shared.and.non-shared.caches">Shared and Non-Shared Caches</a></h1>
     1177         <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
     1178            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
     1179            place certain constraints on the operation of shared caches in order to prevent loss of privacy or failure of access controls.
     1180         </p>
     1181      </div>
     1182      <div id="errors.or.incomplete.response.cache.behavior">
     1183         <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Errors or Incomplete Response Cache Behavior</a></h1>
     1184         <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 Byte 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.
     1185            A cache <em class="bcp14">MUST NOT</em> return a partial response using a status code of 200 (OK).
     1186         </p>
     1187         <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>).
     1188         </p>
     1189      </div>
     1190      <div id="side.effects.of.get.and.head">
     1191         <h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a>&nbsp;<a href="#side.effects.of.get.and.head">Side Effects of GET and HEAD</a></h1>
     1192         <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
     1193            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
     1194            expected to observe an origin server's explicit restrictions on caching.
     1195         </p>
     1196         <p id="rfc.section.10.p.2">We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those
     1197            containing a "?" in the rel_path 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
     1198            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 8.1.1</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.
     1199         </p>
     1200      </div>
     1201      <div id="invalidation.after.updates.or.deletions">
     1202         <h1 id="rfc.section.11"><a href="#rfc.section.11">11.</a>&nbsp;<a href="#invalidation.after.updates.or.deletions">Invalidation After Updates or Deletions</a></h1>
     1203         <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
     1204            to become non-transparently invalid. That is, although they might continue to be "fresh," they do not accurately reflect what
     1205            the origin server would return for a new request on that resource.
     1206         </p>
     1207         <p id="rfc.section.11.p.2">There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request
     1208            that caused the change at the origin server might not have gone through the proxy where a cache entry is stored. However,
     1209            several rules help reduce the likelihood of erroneous behavior.
     1210         </p>
     1211         <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
     1212            its storage, or will mark these as "invalid" and in need of a mandatory revalidation before they can be returned in response
     1213            to a subsequent request.
     1214         </p>
     1215         <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-URI, or by the Location or Content-Location
     1216            headers (if present). These methods are:
     1217         </p>
     1218         <ul>
     1219            <li>PUT</li>
     1220            <li>DELETE</li>
     1221            <li>POST</li>
     1222         </ul>
     1223         <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-URI. This helps prevent denial of service
     1224            attacks.
     1225         </p>
     1226         <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-URI.
     1227         </p>
     1228      </div>
     1229      <div id="write-through.mandatory">
     1230         <h1 id="rfc.section.12"><a href="#rfc.section.12">12.</a>&nbsp;<a href="#write-through.mandatory">Write-Through Mandatory</a></h1>
     1231         <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
     1232            response from the inbound server. This does not prevent a proxy cache from sending a 100 (Continue) response before the inbound
     1233            server has sent its final reply.
     1234         </p>
     1235         <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
     1236            consistent updates and the problems arising from server, cache, or network failure prior to write-back.
     1237         </p>
     1238      </div>
     1239      <div id="cache.replacement">
     1240         <h1 id="rfc.section.13"><a href="#rfc.section.13">13.</a>&nbsp;<a href="#cache.replacement">Cache Replacement</a></h1>
     1241         <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
     1242            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.
     1243         </p>
     1244         <ul class="empty">
     1245            <li><b>Note:</b> a new response that has an older Date header value than existing cached responses is not cacheable.
     1246            </li>
     1247         </ul>
     1248      </div>
     1249      <div id="history.lists">
     1250         <h1 id="rfc.section.14"><a href="#rfc.section.14">14.</a>&nbsp;<a href="#history.lists">History Lists</a></h1>
     1251         <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
     1252            retrieved earlier in a session.
     1253         </p>
     1254         <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
     1255            exactly what the user saw at the time when the resource was retrieved.
     1256         </p>
     1257         <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
     1258            documents.
     1259         </p>
     1260         <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>
     1261         <ul class="empty">
     1262            <li><b>Note:</b> if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors
     1263               to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider
     1264               it important that users not be presented with error messages or warning messages when they use navigation controls (such as
     1265               BACK) to view previously fetched resources. Even though sometimes such resources ought not be cached, or ought to expire quickly,
     1266               user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs)
     1267               in order not to suffer the effects of improperly functioning history mechanisms.
     1268            </li>
     1269         </ul>
     1270      </div>
     1271      <div id="header.fields">
     1272         <h1 id="rfc.section.15"><a href="#rfc.section.15">15.</a>&nbsp;<a href="#header.fields">Header Field Definitions</a></h1>
     1273         <p id="rfc.section.15.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
     1274         <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
     1275            receives the entity.
     1276         </p>
     1277         <div id="header.age">
     1278            <div id="rfc.iref.a.2"></div>
     1279            <div id="rfc.iref.h.2"></div>
     1280            <h2 id="rfc.section.15.1"><a href="#rfc.section.15.1">15.1</a>&nbsp;<a href="#header.age">Age</a></h2>
     1281            <p id="rfc.section.15.1.p.1">The Age response-header field conveys the sender's estimate of the amount of time since the response (or its revalidation)
     1282               was generated at the origin server. A cached response is "fresh" if its age does not exceed its freshness lifetime. Age values
     1283               are calculated as specified in <a href="#age.calculations" title="Age Calculations">Section&nbsp;3.3</a>.
     1284            </p>
     1285            <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span>  Age = "Age" ":" age-value
    12181286  age-value = delta-seconds
    12191287</pre><p id="rfc.section.15.1.p.3">Age values are non-negative decimal integers, representing time in seconds.</p>
    1220       <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.3"></span>  delta-seconds  = 1*DIGIT
     1288            <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.3"></span>  delta-seconds  = 1*DIGIT
    12211289</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,
    1222          it <em class="bcp14">MUST</em> transmit an Age header with a value of 2147483648 (2^31). 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.
    1223       </p>
    1224       <div id="rfc.iref.c.3"></div>
    1225       <div id="rfc.iref.h.3"></div>
    1226       <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>
    1227       <p id="rfc.section.15.2.p.1">The Cache-Control general-header field 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
    1228          caches from adversely interfering with the request or response. These directives typically override the default caching algorithms.
    1229          Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive
    1230          is to be given in the response.
    1231       </p>
    1232       <ul class="empty">
    1233          <li>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>).
    1234          </li>
    1235       </ul>
    1236       <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
    1237          might be applicable to all recipients along the request/response chain. It is not possible to specify a cache-directive for
    1238          a specific cache.
    1239       </p>
    1240       <div id="rfc.figure.u.11"></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>  Cache-Control   = "Cache-Control" ":" 1#cache-directive
     1290               it <em class="bcp14">MUST</em> transmit an Age header with a value of 2147483648 (2^31). 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.
     1291            </p>
     1292         </div>
     1293         <div id="header.cache-control">
     1294            <div id="rfc.iref.c.3"></div>
     1295            <div id="rfc.iref.h.3"></div>
     1296            <h2 id="rfc.section.15.2"><a href="#rfc.section.15.2">15.2</a>&nbsp;<a href="#header.cache-control">Cache-Control</a></h2>
     1297            <p id="rfc.section.15.2.p.1">The Cache-Control general-header field 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
     1298               caches from adversely interfering with the request or response. These directives typically override the default caching algorithms.
     1299               Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive
     1300               is to be given in the response.
     1301            </p>
     1302            <ul class="empty">
     1303               <li>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>).
     1304               </li>
     1305            </ul>
     1306            <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
     1307               might be applicable to all recipients along the request/response chain. It is not possible to specify a cache-directive for
     1308               a specific cache.
     1309            </p>
     1310            <div id="rfc.figure.u.11"></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>  Cache-Control   = "Cache-Control" ":" 1#cache-directive
    12411311
    12421312  cache-directive = cache-request-directive
     
    12671337  cache-extension = token [ "=" ( token | quoted-string ) ]
    12681338</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
    1269          such a directive appears with a 1#field-name parameter, it applies only to the named field or fields, and not to the rest
    1270          of the request or response. This mechanism supports extensibility; implementations of future versions of the HTTP protocol
    1271          might apply these directives to header fields not defined in HTTP/1.1.
    1272       </p>
    1273       <p id="rfc.section.15.2.p.5">The cache-control directives can be broken down into these general categories: </p>
    1274       <ul>
    1275          <li>Restrictions on what are cacheable; these may only be imposed by the origin server.</li>
    1276          <li>Restrictions on what may be stored by a cache; these may be imposed by either the origin server or the user agent.</li>
    1277          <li>Modifications of the basic expiration mechanism; these may be imposed by either the origin server or the user agent.</li>
    1278          <li>Controls over cache revalidation and reload; these may only be imposed by a user agent.</li>
    1279          <li>Control over transformation of entities.</li>
    1280          <li>Extensions to the caching system.</li>
    1281       </ul>
    1282       <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>
    1283       <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
    1284          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
    1285          the default cacheability of a response:
    1286       </p>
    1287       <p id="rfc.section.15.2.1.p.2"> <span id="rfc.iref.c.4"></span>  <span id="rfc.iref.p.1"></span> public
    1288       </p>
    1289       <ul class="empty">
    1290          <li>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
    1291             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.)
    1292          </li>
    1293       </ul>
    1294       <p id="rfc.section.15.2.1.p.3"> <span id="rfc.iref.c.5"></span>  <span id="rfc.iref.p.2"></span> private
    1295       </p>
    1296       <ul class="empty">
    1297          <li>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
    1298             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.
    1299          </li>
    1300          <li> <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
    1301             content.
    1302          </li>
    1303       </ul>
    1304       <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
    1305       </p>
    1306       <ul class="empty">
    1307          <li>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
    1308             server to prevent caching even by caches that have been configured to return stale responses to client requests.
    1309          </li>
    1310          <li>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
    1311             server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
     1339               such a directive appears with a 1#field-name parameter, it applies only to the named field or fields, and not to the rest
     1340               of the request or response. This mechanism supports extensibility; implementations of future versions of the HTTP protocol
     1341               might apply these directives to header fields not defined in HTTP/1.1.
     1342            </p>
     1343            <p id="rfc.section.15.2.p.5">The cache-control directives can be broken down into these general categories: </p>
     1344            <ul>
     1345               <li>Restrictions on what are cacheable; these may only be imposed by the origin server.</li>
     1346               <li>Restrictions on what may be stored by a cache; these may be imposed by either the origin server or the user agent.</li>
     1347               <li>Modifications of the basic expiration mechanism; these may be imposed by either the origin server or the user agent.</li>
     1348               <li>Controls over cache revalidation and reload; these may only be imposed by a user agent.</li>
     1349               <li>Control over transformation of entities.</li>
     1350               <li>Extensions to the caching system.</li>
     1351            </ul>
     1352            <div id="what.is.cacheable">
     1353               <h3 id="rfc.section.15.2.1"><a href="#rfc.section.15.2.1">15.2.1</a>&nbsp;<a href="#what.is.cacheable">What is Cacheable</a></h3>
     1354               <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
     1355                  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
     1356                  the default cacheability of a response:
     1357               </p>
     1358               <p id="rfc.section.15.2.1.p.2"><span id="rfc.iref.c.4"></span> <span id="rfc.iref.p.1"></span> public
     1359               </p>
     1360               <ul class="empty">
     1361                  <li>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
     1362                     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.)
     1363                  </li>
     1364               </ul>
     1365               <p id="rfc.section.15.2.1.p.3"><span id="rfc.iref.c.5"></span> <span id="rfc.iref.p.2"></span> private
     1366               </p>
     1367               <ul class="empty">
     1368                  <li>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
     1369                     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.
     1370                  </li>
     1371                  <li><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
     1372                     content.
     1373                  </li>
     1374               </ul>
     1375               <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
     1376               </p>
     1377               <ul class="empty">
     1378                  <li>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
     1379                     server to prevent caching even by caches that have been configured to return stale responses to client requests.
     1380                  </li>
     1381                  <li>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
     1382                     server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
     1383                     <ul class="empty">
     1384                        <li><b>Note:</b> Most HTTP/1.0 caches will not recognize or obey this directive.
     1385                        </li>
     1386                     </ul>
     1387                  </li>
     1388               </ul>
     1389            </div>
     1390            <div id="what.may.be.stored.by.caches">
     1391               <h3 id="rfc.section.15.2.2"><a href="#rfc.section.15.2.2">15.2.2</a>&nbsp;<a href="#what.may.be.stored.by.caches">What May be Stored by Caches</a></h3>
     1392               <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
     1393               </p>
     1394               <ul class="empty">
     1395                  <li>The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example,
     1396                     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
     1397                     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.
     1398                  </li>
     1399                  <li>Even when this directive is associated with a response, users might explicitly store such a response outside of the caching
     1400                     system (e.g., with a "Save As" dialog). History buffers <em class="bcp14">MAY</em> store such responses as part of their normal operation.
     1401                  </li>
     1402                  <li>The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about
     1403                     accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive might
     1404                     improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy.
     1405                     In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might
     1406                     be vulnerable to eavesdropping.
     1407                  </li>
     1408               </ul>
     1409            </div>
     1410            <div id="modifications.of.the.basic.expiration.mechanism">
     1411               <h3 id="rfc.section.15.2.3"><a href="#rfc.section.15.2.3">15.2.3</a>&nbsp;<a href="#modifications.of.the.basic.expiration.mechanism">Modifications of the Basic Expiration Mechanism</a></h3>
     1412               <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,
     1413                  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
     1414                  that resource. The max-age directive on a response implies that the response is cacheable (i.e., "public") unless some other,
     1415                  more restrictive cache directive is also present.
     1416               </p>
     1417               <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,
     1418                  even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer
     1419                  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
     1420                  improperly calculate ages or expiration times, perhaps due to desynchronized clocks.
     1421               </p>
     1422               <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
     1423                  equivalent to the Cache-Control response directive "no-cache". If an HTTP/1.1 cache receives such a response, and the response
     1424                  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.
     1425               </p>
     1426               <ul class="empty">
     1427                  <li><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
     1428                     including older caches that do not understand that feature. The origin server will need to combine the new feature with an
     1429                     Expires field whose value is less than or equal to the Date value. This will prevent older caches from improperly caching
     1430                     the response.
     1431                  </li>
     1432               </ul>
     1433               <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
     1434               </p>
     1435               <ul class="empty">
     1436                  <li>If a response includes an s-maxage directive, then for a shared cache (but not for a private cache), the maximum age specified
     1437                     by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage
     1438                     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
     1439                     revalidating it with the origin server. The s-maxage directive is always ignored by a private cache.
     1440                  </li>
     1441               </ul>
     1442               <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
     1443                  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
     1444                  caches do not observe the max-age directive.
     1445               </p>
     1446               <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:
     1447               </p>
     1448               <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
     1449               </p>
     1450               <ul class="empty">
     1451                  <li>Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless
     1452                     max-stale directive is also included, the client is not willing to accept a stale response.
     1453                  </li>
     1454               </ul>
     1455               <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
     1456               </p>
     1457               <ul class="empty">
     1458                  <li>Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the
     1459                     specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number
     1460                     of seconds.
     1461                  </li>
     1462               </ul>
     1463               <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
     1464               </p>
     1465               <ul class="empty">
     1466                  <li>Indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned
     1467                     a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified
     1468                     number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age.
     1469                  </li>
     1470               </ul>
     1471               <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
     1472                  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).
     1473               </p>
     1474               <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
     1475                  concerning cache validation (e.g., a "must-revalidate" cache-control directive).
     1476               </p>
     1477               <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
     1478                  the freshness of the cached entry for that request.
     1479               </p>
     1480            </div>
     1481            <div id="cache.revalidation.and.reload.controls">
     1482               <h3 id="rfc.section.15.2.4"><a href="#rfc.section.15.2.4">15.2.4</a>&nbsp;<a href="#cache.revalidation.and.reload.controls">Cache Revalidation and Reload Controls</a></h3>
     1483               <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
     1484                  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
     1485                  revalidation might be necessary if either the cache or the origin server has overestimated the expiration time of the cached
     1486                  response. End-to-end reload may be necessary if the cache entry has become corrupted for some reason.
     1487               </p>
     1488               <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
     1489                  call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it
     1490                  "specific end-to-end revalidation."
     1491               </p>
     1492               <p id="rfc.section.15.2.4.p.3">The client can specify these three kinds of action using Cache-Control request directives:</p>
     1493               <p id="rfc.section.15.2.4.p.4">End-to-end reload </p>
     1494               <ul class="empty">
     1495                  <li>The request includes a "no-cache" cache-control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache".
     1496                     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.
     1497                  </li>
     1498               </ul>
     1499               <p id="rfc.section.15.2.4.p.5">Specific end-to-end revalidation </p>
     1500               <ul class="empty">
     1501                  <li>The request includes a "max-age=0" cache-control directive, which forces each cache along the path to the origin server to
     1502                     revalidate its own entry, if any, with the next cache or server. The initial request includes a cache-validating conditional
     1503                     with the client's current validator.
     1504                  </li>
     1505               </ul>
     1506               <p id="rfc.section.15.2.4.p.6">Unspecified end-to-end revalidation </p>
     1507               <ul class="empty">
     1508                  <li>The request includes "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate
     1509                     its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional;
     1510                     the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional
     1511                     with its current validator.
     1512                  </li>
     1513               </ul>
     1514               <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
     1515               </p>
     1516               <ul class="empty">
     1517                  <li>When an intermediate cache is forced, by means of a max-age=0 directive, to revalidate its own cache entry, and the client
     1518                     has supplied its own validator in the request, the supplied validator might differ from the validator currently stored with
     1519                     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.
     1520                  </li>
     1521                  <li>However, the choice of validator might affect performance. The best approach is for the intermediate cache to use its own
     1522                     validator when making its request. If the server replies with 304 (Not Modified), then the cache can return its now validated
     1523                     copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate
     1524                     cache can compare the returned validator with the one provided in the client's request, using the strong comparison function.
     1525                     If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified).
     1526                     Otherwise, it returns the new entity with a 200 (OK) response.
     1527                  </li>
     1528                  <li>If a request includes the no-cache directive, it <em class="bcp14">SHOULD NOT</em> include min-fresh, max-stale, or max-age.
     1529                  </li>
     1530               </ul>
     1531               <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
     1532               </p>
     1533               <ul class="empty">
     1534                  <li>In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses
     1535                     that it currently has stored, and not to reload or revalidate with the origin server. To do this, the client may include the
     1536                     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
     1537                     Timeout) status. However, if a group of caches is being operated as a unified system with good internal connectivity, such
     1538                     a request <em class="bcp14">MAY</em> be forwarded within that group of caches.
     1539                  </li>
     1540               </ul>
     1541               <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
     1542               </p>
     1543               <ul class="empty">
     1544                  <li>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
     1545                     require revalidation of a cache entry on any subsequent use. When the must-revalidate directive is present in a response received
     1546                     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.
     1547                     (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
     1548                     is stale.)
     1549                  </li>
     1550                  <li>The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
     1551                     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.
     1552                  </li>
     1553                  <li>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
     1554                     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.
     1555                  </li>
     1556                  <li>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.
     1557                  </li>
     1558               </ul>
     1559               <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
     1560               </p>
     1561               <ul class="empty">
     1562                  <li>The proxy-revalidate directive has the same meaning as the must-revalidate directive, except that it does not apply to non-shared
     1563                     user agent caches. It can be used on a response to an authenticated request to permit the user's cache to store and later
     1564                     return the response without needing to revalidate it (since it has already been authenticated once by that user), while still
     1565                     requiring proxies that service many users to revalidate each time (in order to make sure that each user has been authenticated).
     1566                     Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at
     1567                     all.
     1568                  </li>
     1569               </ul>
     1570            </div>
     1571            <div id="no-transform.directive">
     1572               <h3 id="rfc.section.15.2.5"><a href="#rfc.section.15.2.5">15.2.5</a>&nbsp;<a href="#no-transform.directive">No-Transform Directive</a></h3>
     1573               <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
     1574               </p>
     1575               <ul class="empty">
     1576                  <li>Implementors of intermediate caches (proxies) have found it useful to convert the media type of certain entity bodies. A non-transparent
     1577                     proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on
     1578                     a slow link.
     1579                  </li>
     1580                  <li>Serious operational problems occur, however, when these transformations are applied to entity bodies intended for certain
     1581                     kinds of applications. For example, applications for medical imaging, scientific data analysis and those using end-to-end
     1582                     authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body.
     1583                  </li>
     1584                  <li>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.
     1585                  </li>
     1586               </ul>
     1587            </div>
     1588            <div id="cache.control.extensions">
     1589               <h3 id="rfc.section.15.2.6"><a href="#rfc.section.15.2.6">15.2.6</a>&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></h3>
     1590               <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
     1591                  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
     1592                  to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications
     1593                  which do not understand the new directive will default to the behavior specified by the standard directive, and those that
     1594                  understand the new directive will recognize it as modifying the requirements associated with the standard directive. In this
     1595                  way, extensions to the cache-control directives can be made without requiring changes to the base protocol.
     1596               </p>
     1597               <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,
     1598                  obeying certain extensions, and ignoring all directives that it does not understand.
     1599               </p>
     1600               <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.
     1601                  We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members
     1602                  of the community named within its value may cache the response. An origin server wishing to allow the UCI community to use
     1603                  an otherwise private response in their shared cache(s) could do so by including
     1604               </p>
     1605               <div id="rfc.figure.u.12"></div><pre class="text">    Cache-Control: private, community="UCI"
     1606</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
     1607                  it will also see and understand the private directive and thus default to the safe behavior.
     1608               </p>
     1609               <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
     1610                  directives (or the response's default cacheability) such that the cache behavior will remain minimally correct even if the
     1611                  cache does not understand the extension(s).
     1612               </p>
     1613            </div>
     1614         </div>
     1615         <div id="header.expires">
     1616            <div id="rfc.iref.e.2"></div>
     1617            <div id="rfc.iref.h.4"></div>
     1618            <h2 id="rfc.section.15.3"><a href="#rfc.section.15.3">15.3</a>&nbsp;<a href="#header.expires">Expires</a></h2>
     1619            <p id="rfc.section.15.3.p.1">The Expires entity-header field gives the date/time after which the response is considered stale. A stale cache entry may
     1620               not normally be returned by a cache (either a proxy cache or a user agent cache) unless it is first validated with the origin
     1621               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.
     1622            </p>
     1623            <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
     1624               that time.
     1625            </p>
     1626            <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 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.6"><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.
     1627            </p>
     1628            <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span>  Expires = "Expires" ":" HTTP-date
     1629</pre><p id="rfc.section.15.3.p.5">An example of its use is</p>
     1630            <div id="rfc.figure.u.14"></div><pre class="text">   Expires: Thu, 01 Dec 1994 16:00:00 GMT
     1631</pre><p id="rfc.section.15.3.p.7"></p>
    13121632            <ul class="empty">
    1313                <li> <b>Note:</b> Most HTTP/1.0 caches will not recognize or obey this directive.
    1314                </li>
    1315             </ul>
    1316          </li>
    1317       </ul>
    1318       <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>
    1319       <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
    1320       </p>
    1321       <ul class="empty">
    1322          <li>The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example,
    1323             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
    1324             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.
    1325          </li>
    1326          <li>Even when this directive is associated with a response, users might explicitly store such a response outside of the caching
    1327             system (e.g., with a "Save As" dialog). History buffers <em class="bcp14">MAY</em> store such responses as part of their normal operation.
    1328          </li>
    1329          <li>The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about
    1330             accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive might
    1331             improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy.
    1332             In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might
    1333             be vulnerable to eavesdropping.
    1334          </li>
    1335       </ul>
    1336       <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>
    1337       <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,
    1338          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
    1339          that resource. The max-age directive on a response implies that the response is cacheable (i.e., "public") unless some other,
    1340          more restrictive cache directive is also present.
    1341       </p>
    1342       <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,
    1343          even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer
    1344          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
    1345          improperly calculate ages or expiration times, perhaps due to desynchronized clocks.
    1346       </p>
    1347       <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
    1348          equivalent to the Cache-Control response directive "no-cache". If an HTTP/1.1 cache receives such a response, and the response
    1349          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.
    1350       </p>
    1351       <ul class="empty">
    1352          <li> <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
    1353             including older caches that do not understand that feature. The origin server will need to combine the new feature with an
    1354             Expires field whose value is less than or equal to the Date value. This will prevent older caches from improperly caching
    1355             the response.
    1356          </li>
    1357       </ul>
    1358       <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
    1359       </p>
    1360       <ul class="empty">
    1361          <li>If a response includes an s-maxage directive, then for a shared cache (but not for a private cache), the maximum age specified
    1362             by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage
    1363             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
    1364             revalidating it with the origin server. The s-maxage directive is always ignored by a private cache.
    1365          </li>
    1366       </ul>
    1367       <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
    1368          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
    1369          caches do not observe the max-age directive.
    1370       </p>
    1371       <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:
    1372       </p>
    1373       <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
    1374       </p>
    1375       <ul class="empty">
    1376          <li>Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless
    1377             max-stale directive is also included, the client is not willing to accept a stale response.
    1378          </li>
    1379       </ul>
    1380       <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
    1381       </p>
    1382       <ul class="empty">
    1383          <li>Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the
    1384             specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number
    1385             of seconds.
    1386          </li>
    1387       </ul>
    1388       <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
    1389       </p>
    1390       <ul class="empty">
    1391          <li>Indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned
    1392             a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified
    1393             number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age.
    1394          </li>
    1395       </ul>
    1396       <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
    1397          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).
    1398       </p>
    1399       <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
    1400          concerning cache validation (e.g., a "must-revalidate" cache-control directive).
    1401       </p>
    1402       <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
    1403          the freshness of the cached entry for that request.
    1404       </p>
    1405       <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>
    1406       <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
    1407          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
    1408          revalidation might be necessary if either the cache or the origin server has overestimated the expiration time of the cached
    1409          response. End-to-end reload may be necessary if the cache entry has become corrupted for some reason.
    1410       </p>
    1411       <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
    1412          call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it
    1413          "specific end-to-end revalidation."
    1414       </p>
    1415       <p id="rfc.section.15.2.4.p.3">The client can specify these three kinds of action using Cache-Control request directives:</p>
    1416       <p id="rfc.section.15.2.4.p.4">End-to-end reload </p>
    1417       <ul class="empty">
    1418          <li>The request includes a "no-cache" cache-control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache".
    1419             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.
    1420          </li>
    1421       </ul>
    1422       <p id="rfc.section.15.2.4.p.5">Specific end-to-end revalidation </p>
    1423       <ul class="empty">
    1424          <li>The request includes a "max-age=0" cache-control directive, which forces each cache along the path to the origin server to
    1425             revalidate its own entry, if any, with the next cache or server. The initial request includes a cache-validating conditional
    1426             with the client's current validator.
    1427          </li>
    1428       </ul>
    1429       <p id="rfc.section.15.2.4.p.6">Unspecified end-to-end revalidation </p>
    1430       <ul class="empty">
    1431          <li>The request includes "max-age=0" cache-control directive, which forces each cache along the path to the origin server to revalidate
    1432             its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional;
    1433             the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional
    1434             with its current validator.
    1435          </li>
    1436       </ul>
    1437       <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
    1438       </p>
    1439       <ul class="empty">
    1440          <li>When an intermediate cache is forced, by means of a max-age=0 directive, to revalidate its own cache entry, and the client
    1441             has supplied its own validator in the request, the supplied validator might differ from the validator currently stored with
    1442             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.
    1443          </li>
    1444          <li>However, the choice of validator might affect performance. The best approach is for the intermediate cache to use its own
    1445             validator when making its request. If the server replies with 304 (Not Modified), then the cache can return its now validated
    1446             copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate
    1447             cache can compare the returned validator with the one provided in the client's request, using the strong comparison function.
    1448             If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified).
    1449             Otherwise, it returns the new entity with a 200 (OK) response.
    1450          </li>
    1451          <li>If a request includes the no-cache directive, it <em class="bcp14">SHOULD NOT</em> include min-fresh, max-stale, or max-age.
    1452          </li>
    1453       </ul>
    1454       <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
    1455       </p>
    1456       <ul class="empty">
    1457          <li>In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses
    1458             that it currently has stored, and not to reload or revalidate with the origin server. To do this, the client may include the
    1459             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
    1460             Timeout) status. However, if a group of caches is being operated as a unified system with good internal connectivity, such
    1461             a request <em class="bcp14">MAY</em> be forwarded within that group of caches.
    1462          </li>
    1463       </ul>
    1464       <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
    1465       </p>
    1466       <ul class="empty">
    1467          <li>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
    1468             require revalidation of a cache entry on any subsequent use. When the must-revalidate directive is present in a response received
    1469             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.
    1470             (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
    1471             is stale.)
    1472          </li>
    1473          <li>The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
    1474             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.
    1475          </li>
    1476          <li>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
    1477             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.
    1478          </li>
    1479          <li>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.
    1480          </li>
    1481       </ul>
    1482       <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
    1483       </p>
    1484       <ul class="empty">
    1485          <li>The proxy-revalidate directive has the same meaning as the must-revalidate directive, except that it does not apply to non-shared
    1486             user agent caches. It can be used on a response to an authenticated request to permit the user's cache to store and later
    1487             return the response without needing to revalidate it (since it has already been authenticated once by that user), while still
    1488             requiring proxies that service many users to revalidate each time (in order to make sure that each user has been authenticated).
    1489             Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at
    1490             all.
    1491          </li>
    1492       </ul>
    1493       <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>
    1494       <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
    1495       </p>
    1496       <ul class="empty">
    1497          <li>Implementors of intermediate caches (proxies) have found it useful to convert the media type of certain entity bodies. A non-transparent
    1498             proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on
    1499             a slow link.
    1500          </li>
    1501          <li>Serious operational problems occur, however, when these transformations are applied to entity bodies intended for certain
    1502             kinds of applications. For example, applications for medical imaging, scientific data analysis and those using end-to-end
    1503             authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body.
    1504          </li>
    1505          <li>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.
    1506          </li>
    1507       </ul>
    1508       <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>
    1509       <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
    1510          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
    1511          to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications
    1512          which do not understand the new directive will default to the behavior specified by the standard directive, and those that
    1513          understand the new directive will recognize it as modifying the requirements associated with the standard directive. In this
    1514          way, extensions to the cache-control directives can be made without requiring changes to the base protocol.
    1515       </p>
    1516       <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,
    1517          obeying certain extensions, and ignoring all directives that it does not understand.
    1518       </p>
    1519       <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.
    1520          We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members
    1521          of the community named within its value may cache the response. An origin server wishing to allow the UCI community to use
    1522          an otherwise private response in their shared cache(s) could do so by including
    1523       </p>
    1524       <div id="rfc.figure.u.12"></div><pre class="text">    Cache-Control: private, community="UCI"
    1525 </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
    1526          it will also see and understand the private directive and thus default to the safe behavior.
    1527       </p>
    1528       <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
    1529          directives (or the response's default cacheability) such that the cache behavior will remain minimally correct even if the
    1530          cache does not understand the extension(s).
    1531       </p>
    1532       <div id="rfc.iref.e.2"></div>
    1533       <div id="rfc.iref.h.4"></div>
    1534       <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>
    1535       <p id="rfc.section.15.3.p.1">The Expires entity-header field gives the date/time after which the response is considered stale. A stale cache entry may
    1536          not normally be returned by a cache (either a proxy cache or a user agent cache) unless it is first validated with the origin
    1537          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.
    1538       </p>
    1539       <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
    1540          that time.
    1541       </p>
    1542       <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 3.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.6"><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.
    1543       </p>
    1544       <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span>  Expires = "Expires" ":" HTTP-date
    1545 </pre><p id="rfc.section.15.3.p.5">An example of its use is</p>
    1546       <div id="rfc.figure.u.14"></div><pre class="text">   Expires: Thu, 01 Dec 1994 16:00:00 GMT
    1547 </pre><p id="rfc.section.15.3.p.7"> </p>
    1548       <ul class="empty">
    1549          <li> <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.
    1550          </li>
    1551       </ul>
    1552       <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").
    1553       </p>
    1554       <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
    1555          the rules for expiration calculations in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;3.4</a>.)
    1556       </p>
    1557       <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
    1558          is sent. HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
    1559       </p>
    1560       <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
    1561          default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field
    1562          (<a href="#header.cache-control" id="rfc.xref.header.cache-control.9" title="Cache-Control">Section&nbsp;15.2</a>).
    1563       </p>
    1564       <div id="rfc.iref.p.4"></div>
    1565       <div id="rfc.iref.h.5"></div>
    1566       <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>
    1567       <p id="rfc.section.15.4.p.1">The Pragma general-header field is used to include implementation-specific directives that might apply to any recipient along
    1568          the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some
    1569          systems <em class="bcp14">MAY</em> require that behavior be consistent with the directives.
    1570       </p>
    1571       <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span>  Pragma            = "Pragma" ":" 1#pragma-directive
     1633               <li><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.
     1634               </li>
     1635            </ul>
     1636            <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").
     1637            </p>
     1638            <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
     1639               the rules for expiration calculations in <a href="#expiration.calculations" title="Expiration Calculations">Section&nbsp;3.4</a>.)
     1640            </p>
     1641            <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
     1642               is sent. HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
     1643            </p>
     1644            <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
     1645               default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a Cache-Control header field
     1646               (<a href="#header.cache-control" id="rfc.xref.header.cache-control.9" title="Cache-Control">Section&nbsp;15.2</a>).
     1647            </p>
     1648         </div>
     1649         <div id="header.pragma">
     1650            <div id="rfc.iref.p.4"></div>
     1651            <div id="rfc.iref.h.5"></div>
     1652            <h2 id="rfc.section.15.4"><a href="#rfc.section.15.4">15.4</a>&nbsp;<a href="#header.pragma">Pragma</a></h2>
     1653            <p id="rfc.section.15.4.p.1">The Pragma general-header field is used to include implementation-specific directives that might apply to any recipient along
     1654               the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some
     1655               systems <em class="bcp14">MAY</em> require that behavior be consistent with the directives.
     1656            </p>
     1657            <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span>  Pragma            = "Pragma" ":" 1#pragma-directive
    15721658  pragma-directive  = "no-cache" | extension-pragma
    15731659  extension-pragma  = token [ "=" ( token | quoted-string ) ]
    15741660</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
    1575          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.
    1576       </p>
    1577       <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
    1578          might be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific
    1579          recipient; however, any pragma directive not relevant to a recipient <em class="bcp14">SHOULD</em> be ignored by that recipient.
    1580       </p>
    1581       <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
    1582          HTTP.
    1583       </p>
    1584       <ul class="empty">
    1585          <li> <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
    1586             replacement for "Cache-Control: no-cache" in a response.
    1587          </li>
    1588       </ul>
    1589       <div id="rfc.iref.v.2"></div>
    1590       <div id="rfc.iref.h.6"></div>
    1591       <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>
    1592       <p id="rfc.section.15.5.p.1">The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether
    1593          a cache is permitted to use the response to reply to a subsequent request without revalidation. For uncacheable or stale responses,
    1594          the Vary field value advises the user agent about the criteria that were used to select the representation. A Vary field value
    1595          of "*" implies that a cache cannot determine from the request headers of a subsequent request whether this response is the
    1596          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.
    1597       </p>
    1598       <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.13"></span>  Vary  = "Vary" ":" ( "*" | 1#field-name )
     1661               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.
     1662            </p>
     1663            <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
     1664               might be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific
     1665               recipient; however, any pragma directive not relevant to a recipient <em class="bcp14">SHOULD</em> be ignored by that recipient.
     1666            </p>
     1667            <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
     1668               HTTP.
     1669            </p>
     1670            <ul class="empty">
     1671               <li><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
     1672                  replacement for "Cache-Control: no-cache" in a response.
     1673               </li>
     1674            </ul>
     1675         </div>
     1676         <div id="header.vary">
     1677            <div id="rfc.iref.v.2"></div>
     1678            <div id="rfc.iref.h.6"></div>
     1679            <h2 id="rfc.section.15.5"><a href="#rfc.section.15.5">15.5</a>&nbsp;<a href="#header.vary">Vary</a></h2>
     1680            <p id="rfc.section.15.5.p.1">The Vary field value indicates the set of request-header fields that fully determines, while the response is fresh, whether
     1681               a cache is permitted to use the response to reply to a subsequent request without revalidation. For uncacheable or stale responses,
     1682               the Vary field value advises the user agent about the criteria that were used to select the representation. A Vary field value
     1683               of "*" implies that a cache cannot determine from the request headers of a subsequent request whether this response is the
     1684               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.
     1685            </p>
     1686            <div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.13"></span>  Vary  = "Vary" ":" ( "*" | 1#field-name )
    15991687</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
    1600          to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that
    1601          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
    1602          the user agent with useful information about the dimensions over which the response varies at the time of the response.
    1603       </p>
    1604       <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
    1605          on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation.
    1606          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
    1607          of time for which the response is fresh.
    1608       </p>
    1609       <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
    1610          are case-insensitive.
    1611       </p>
    1612       <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
    1613          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.
    1614       </p>
    1615       <div id="rfc.iref.w.1"></div>
    1616       <div id="rfc.iref.h.7"></div>
    1617       <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>
    1618       <p id="rfc.section.15.6.p.1">The Warning general-header field is used to carry additional information about the status or transformation of a message which
    1619          might not be reflected in the message. This information is typically used to warn about a possible lack of semantic transparency
    1620          from caching operations or transformations applied to the entity body of the message.
    1621       </p>
    1622       <p id="rfc.section.15.6.p.2">Warning headers are sent with responses using:</p>
    1623       <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span>  Warning    = "Warning" ":" 1#warning-value
     1688               to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that
     1689               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
     1690               the user agent with useful information about the dimensions over which the response varies at the time of the response.
     1691            </p>
     1692            <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
     1693               on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation.
     1694               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
     1695               of time for which the response is fresh.
     1696            </p>
     1697            <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
     1698               are case-insensitive.
     1699            </p>
     1700            <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
     1701               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.
     1702            </p>
     1703         </div>
     1704         <div id="header.warning">
     1705            <div id="rfc.iref.w.1"></div>
     1706            <div id="rfc.iref.h.7"></div>
     1707            <h2 id="rfc.section.15.6"><a href="#rfc.section.15.6">15.6</a>&nbsp;<a href="#header.warning">Warning</a></h2>
     1708            <p id="rfc.section.15.6.p.1">The Warning general-header field is used to carry additional information about the status or transformation of a message which
     1709               might not be reflected in the message. This information is typically used to warn about a possible lack of semantic transparency
     1710               from caching operations or transformations applied to the entity body of the message.
     1711            </p>
     1712            <p id="rfc.section.15.6.p.2">Warning headers are sent with responses using:</p>
     1713            <div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span><span id="rfc.iref.g.16"></span><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span>  Warning    = "Warning" ":" 1#warning-value
    16241714 
    16251715  warning-value = warn-code SP warn-agent SP warn-text
     
    16331723  warn-date  = DQUOTE HTTP-date DQUOTE
    16341724</pre><p id="rfc.section.15.6.p.4">A response <em class="bcp14">MAY</em> carry more than one Warning header.
    1635       </p>
    1636       <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.
    1637          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
    1638          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>).
    1639       </p>
    1640       <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>.
    1641       </p>
    1642       <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
    1643          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
    1644          attached to the most recent relevant response.
    1645       </p>
    1646       <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,
    1647          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:
    1648       </p>
    1649       <ul>
    1650          <li>Warnings that appear early in the response take priority over those appearing later in the response.</li>
    1651          <li>Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes
    1652             and warn-agents.
    1653          </li>
    1654       </ul>
    1655       <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.
    1656       </p>
    1657       <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>.
    1658       </p>
    1659       <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
    1660          meaning.
    1661       </p>
    1662       <p id="rfc.section.15.6.p.12">110 Response is stale </p>
    1663       <ul class="empty">
    1664          <li> <em class="bcp14">MUST</em> be included whenever the returned response is stale.
    1665          </li>
    1666       </ul>
    1667       <p id="rfc.section.15.6.p.13">111 Revalidation failed </p>
    1668       <ul class="empty">
    1669          <li> <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
    1670             to reach the server.
    1671          </li>
    1672       </ul>
    1673       <p id="rfc.section.15.6.p.14">112 Disconnected operation </p>
    1674       <ul class="empty">
    1675          <li> <em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
    1676          </li>
    1677       </ul>
    1678       <p id="rfc.section.15.6.p.15">113 Heuristic expiration </p>
    1679       <ul class="empty">
    1680          <li> <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
    1681             than 24 hours.
    1682          </li>
    1683       </ul>
    1684       <p id="rfc.section.15.6.p.16">199 Miscellaneous warning </p>
    1685       <ul class="empty">
    1686          <li>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.
    1687          </li>
    1688       </ul>
    1689       <p id="rfc.section.15.6.p.17">214 Transformation applied </p>
    1690       <ul class="empty">
    1691          <li> <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
    1692             Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, or the entity-body of the
    1693             response, unless this Warning code already appears in the response.
    1694          </li>
    1695       </ul>
    1696       <p id="rfc.section.15.6.p.18">299 Miscellaneous persistent warning </p>
    1697       <ul class="empty">
    1698          <li>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.
    1699          </li>
    1700       </ul>
    1701       <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.
    1702       </p>
    1703       <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
    1704          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
    1705          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.
    1706       </p>
    1707       <h1 id="rfc.section.16"><a href="#rfc.section.16">16.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
    1708       <p id="rfc.section.16.p.1">TBD.</p>
    1709       <h1 id="rfc.section.17"><a href="#rfc.section.17">17.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
    1710       <p id="rfc.section.17.p.1">Caching proxies provide additional potential vulnerabilities, since the contents of the cache represent an attractive target
    1711          for malicious exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal
    1712          information long after a user believes that the information has been removed from the network. Therefore, cache contents should
    1713          be protected as sensitive information.
    1714       </p>
    1715       <h1 id="rfc.section.18"><a href="#rfc.section.18">18.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
    1716       <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:
    1717          Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
    1718       </p>
     1725            </p>
     1726            <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.
     1727               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
     1728               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>).
     1729            </p>
     1730            <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>.
     1731            </p>
     1732            <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
     1733               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
     1734               attached to the most recent relevant response.
     1735            </p>
     1736            <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,
     1737               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:
     1738            </p>
     1739            <ul>
     1740               <li>Warnings that appear early in the response take priority over those appearing later in the response.</li>
     1741               <li>Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes
     1742                  and warn-agents.
     1743               </li>
     1744            </ul>
     1745            <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.
     1746            </p>
     1747            <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>.
     1748            </p>
     1749            <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
     1750               meaning.
     1751            </p>
     1752            <p id="rfc.section.15.6.p.12">110 Response is stale </p>
     1753            <ul class="empty">
     1754               <li><em class="bcp14">MUST</em> be included whenever the returned response is stale.
     1755               </li>
     1756            </ul>
     1757            <p id="rfc.section.15.6.p.13">111 Revalidation failed </p>
     1758            <ul class="empty">
     1759               <li><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
     1760                  to reach the server.
     1761               </li>
     1762            </ul>
     1763            <p id="rfc.section.15.6.p.14">112 Disconnected operation </p>
     1764            <ul class="empty">
     1765               <li><em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
     1766               </li>
     1767            </ul>
     1768            <p id="rfc.section.15.6.p.15">113 Heuristic expiration </p>
     1769            <ul class="empty">
     1770               <li><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
     1771                  than 24 hours.
     1772               </li>
     1773            </ul>
     1774            <p id="rfc.section.15.6.p.16">199 Miscellaneous warning </p>
     1775            <ul class="empty">
     1776               <li>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.
     1777               </li>
     1778            </ul>
     1779            <p id="rfc.section.15.6.p.17">214 Transformation applied </p>
     1780            <ul class="empty">
     1781               <li><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
     1782                  Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, or the entity-body of the
     1783                  response, unless this Warning code already appears in the response.
     1784               </li>
     1785            </ul>
     1786            <p id="rfc.section.15.6.p.18">299 Miscellaneous persistent warning </p>
     1787            <ul class="empty">
     1788               <li>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.
     1789               </li>
     1790            </ul>
     1791            <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.
     1792            </p>
     1793            <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
     1794               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
     1795               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.
     1796            </p>
     1797         </div>
     1798      </div>
     1799      <div id="IANA.considerations">
     1800         <h1 id="rfc.section.16"><a href="#rfc.section.16">16.</a>&nbsp;<a href="#IANA.considerations">IANA Considerations</a></h1>
     1801         <p id="rfc.section.16.p.1">TBD.</p>
     1802      </div>
     1803      <div id="security.considerations">
     1804         <h1 id="rfc.section.17"><a href="#rfc.section.17">17.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
     1805         <p id="rfc.section.17.p.1">Caching proxies provide additional potential vulnerabilities, since the contents of the cache represent an attractive target
     1806            for malicious exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal
     1807            information long after a user believes that the information has been removed from the network. Therefore, cache contents should
     1808            be protected as sensitive information.
     1809         </p>
     1810      </div>
     1811      <div id="ack">
     1812         <h1 id="rfc.section.18"><a href="#rfc.section.18">18.</a>&nbsp;<a href="#ack">Acknowledgments</a></h1>
     1813         <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:
     1814            Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
     1815         </p>
     1816      </div>
    17191817      <h1 id="rfc.references"><a id="rfc.section.19" href="#rfc.section.19">19.</a> References
    17201818      </h1>
    17211819      <h2 id="rfc.references.1"><a href="#rfc.section.19.1" id="rfc.section.19.1">19.1</a> Normative References
    17221820      </h2>
    1723       <table>                 
     1821      <table>
    17241822         <tr>
    17251823            <td class="reference"><b id="ISO-8859-1">[ISO-8859-1]</b></td>
     
    17281826         <tr>
    17291827            <td class="reference"><b id="Part1">[Part1]</b></td>
    1730             <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-01">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-01 (work in progress), January&nbsp;2008.
     1828            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-01">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-01 (work in progress), January&nbsp;2008.
    17311829            </td>
    17321830         </tr>
    17331831         <tr>
    17341832            <td class="reference"><b id="Part2">[Part2]</b></td>
    1735             <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-01">HTTP/1.1, part 2: Message Semantics</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-01 (work in progress), January&nbsp;2008.
     1833            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-01">HTTP/1.1, part 2: Message Semantics</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-01 (work in progress), January&nbsp;2008.
    17361834            </td>
    17371835         </tr>
    17381836         <tr>
    17391837            <td class="reference"><b id="Part3">[Part3]</b></td>
    1740             <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-01">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p3-payload-01 (work in progress), January&nbsp;2008.
     1838            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-01">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p3-payload-01 (work in progress), January&nbsp;2008.
    17411839            </td>
    17421840         </tr>
    17431841         <tr>
    17441842            <td class="reference"><b id="Part4">[Part4]</b></td>
    1745             <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-01">HTTP/1.1, part 4: Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-01 (work in progress), January&nbsp;2008.
     1843            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-01">HTTP/1.1, part 4: Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-01 (work in progress), January&nbsp;2008.
    17461844            </td>
    17471845         </tr>
    17481846         <tr>
    17491847            <td class="reference"><b id="Part5">[Part5]</b></td>
    1750             <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-01">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-01 (work in progress), January&nbsp;2008.
     1848            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-01">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-01 (work in progress), January&nbsp;2008.
    17511849            </td>
    17521850         </tr>
    17531851         <tr>
    17541852            <td class="reference"><b id="Part7">[Part7]</b></td>
    1755             <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-01">HTTP/1.1, part 7: Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-01 (work in progress), January&nbsp;2008.
     1853            <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems, Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-01">HTTP/1.1, part 7: Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-01 (work in progress), January&nbsp;2008.
    17561854            </td>
    17571855         </tr>
    17581856         <tr>
    17591857            <td class="reference"><b id="RFC2047">[RFC2047]</b></td>
    1760             <td class="top"><a href="mailto:moore@cs.utk.edu" title="University of Tennessee">Moore, K.</a>, “<a href="http://tools.ietf.org/html/rfc2047">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</a>”, RFC&nbsp;2047, November&nbsp;1996.
     1858            <td class="top"><a href="mailto:moore@cs.utk.edu" title="University of Tennessee">Moore, K.</a>, “<a href="https://tools.ietf.org/html/rfc2047">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</a>”, RFC&nbsp;2047, November&nbsp;1996.
    17611859            </td>
    17621860         </tr>
    17631861         <tr>
    17641862            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
    1765             <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="http://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
     1863            <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP&nbsp;14, RFC&nbsp;2119, March&nbsp;1997.
    17661864            </td>
    17671865         </tr>
     
    17691867      <h2 id="rfc.references.2"><a href="#rfc.section.19.2" id="rfc.section.19.2">19.2</a> Informative References
    17701868      </h2>
    1771       <table>   
     1869      <table>
    17721870         <tr>
    17731871            <td class="reference"><b id="RFC1305">[RFC1305]</b></td>
    1774             <td class="top"><a href="mailto:mills@udel.edu" title="University of Delaware, Electrical Engineering Department">Mills, D.</a>, “<a href="http://tools.ietf.org/html/rfc1305">Network Time Protocol (Version 3) Specification, Implementation</a>”, RFC&nbsp;1305, March&nbsp;1992.
     1872            <td class="top"><a href="mailto:mills@udel.edu" title="University of Delaware, Electrical Engineering Department">Mills, D.</a>, “<a href="https://tools.ietf.org/html/rfc1305">Network Time Protocol (Version 3) Specification, Implementation</a>”, RFC&nbsp;1305, March&nbsp;1992.
    17751873            </td>
    17761874         </tr>
    17771875         <tr>
    17781876            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
    1779             <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
     1877            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
    17801878            </td>
    17811879         </tr>
    17821880      </table>
    1783       <div class="avoidbreak">
    1784          <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
    1785          <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
    1786                (editor)
    1787                <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Day Software</span><span class="adr"><span class="street-address vcardline">23 Corporate Plaza DR, Suite 280</span><span class="vcardline"><span class="locality">Newport Beach</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">92660</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel">Phone: <a href="tel:+1-949-706-5300"><span class="value">+1-949-706-5300</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1-949-706-5305"><span class="value">+1-949-706-5305</span></a></span><span class="vcardline">EMail: <a href="mailto:fielding@gbiv.com"><span class="email">fielding@gbiv.com</span></a></span><span class="vcardline">URI: <a href="http://roy.gbiv.com/" class="url">http://roy.gbiv.com/</a></span></address>
    1788          <address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">One Laptop per Child</span><span class="adr"><span class="street-address vcardline">21 Oak Knoll Road</span><span class="vcardline"><span class="locality">Carlisle</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01741</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:jg@laptop.org"><span class="email">jg@laptop.org</span></a></span><span class="vcardline">URI: <a href="http://www.laptop.org/" class="url">http://www.laptop.org/</a></span></address>
    1789          <address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Hewlett-Packard Company</span><span class="adr"><span class="street-address vcardline">HP Labs, Large Scale Systems Group</span><span class="street-address vcardline">1501 Page Mill Road, MS 1177</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94304</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:JeffMogul@acm.org"><span class="email">JeffMogul@acm.org</span></a></span></address>
    1790          <address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:henrikn@microsoft.com"><span class="email">henrikn@microsoft.com</span></a></span></address>
    1791          <address class="vcard"><span class="vcardline"><span class="fn">Larry Masinter</span><span class="n hidden"><span class="family-name">Masinter</span><span class="given-name">Larry</span></span></span><span class="org vcardline">Adobe Systems, Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:LMM@acm.org"><span class="email">LMM@acm.org</span></a></span><span class="vcardline">URI: <a href="http://larry.masinter.net/" class="url">http://larry.masinter.net/</a></span></address>
    1792          <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span></span><span class="vcardline">EMail: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address>
    1793          <address class="vcard"><span class="vcardline"><span class="fn">Tim Berners-Lee</span><span class="n hidden"><span class="family-name">Berners-Lee</span><span class="given-name">Tim</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">MIT Computer Science and Artificial Intelligence Laboratory</span><span class="street-address vcardline">The Stata Center, Building 32</span><span class="street-address vcardline">32 Vassar Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:timbl@w3.org"><span class="email">timbl@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.w3.org/People/Berners-Lee/" class="url">http://www.w3.org/People/Berners-Lee/</a></span></address>
    1794          <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
    1795                (editor)
    1796                <span class="n hidden"><span class="family-name">Lafon</span><span class="given-name">Yves</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">W3C / ERCIM</span><span class="street-address vcardline">2004, rte des Lucioles</span><span class="vcardline"><span class="locality">Sophia-Antipolis</span>, <span class="region">AM</span>&nbsp;<span class="postal-code">06902</span></span><span class="country-name vcardline">France</span></span><span class="vcardline">EMail: <a href="mailto:ylafon@w3.org"><span class="email">ylafon@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.raubacapeu.net/people/yves/" class="url">http://www.raubacapeu.net/people/yves/</a></span></address>
    1797          <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
    1798                (editor)
    1799                <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span>&nbsp;<span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline tel">Phone: <a href="tel:+492512807760"><span class="value">+49 251 2807760</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+492512807761"><span class="value">+49 251 2807761</span></a></span><span class="vcardline">EMail: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address>
    1800       </div>
    1801       <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a id="compatibility" href="#compatibility">Compatibility with Previous Versions</a></h1>
    1802       <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>
    1803       <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.11" 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>)
    1804       </p>
    1805       <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
    1806          for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are
    1807          computed. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a>, see also <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="#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>)
    1808       </p>
    1809       <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>)
    1810       </p>
    1811       <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
    1812          needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Headers">Section&nbsp;6.3</a>)
    1813       </p>
    1814       <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>)
    1815       </p>
    1816       <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.6" 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.
    1817       </p>
    1818       <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>
    1819       <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>)
    1820       </p>
    1821       <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;Change Log (to be removed by RFC Editor before publication)
    1822       </h1>
    1823       <h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a>&nbsp;Since RFC2616
    1824       </h2>
    1825       <p id="rfc.section.B.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
    1826       </p>
    1827       <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a>&nbsp;Since draft-ietf-httpbis-p6-cache-00
    1828       </h2>
    1829       <p id="rfc.section.B.2.p.1">Closed issues: </p>
    1830       <ul>
    1831          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9</a>&gt;: "Trailer" (&lt;<a href="http://purl.org/NET/http-errata#trailer-hop">http://purl.org/NET/http-errata#trailer-hop</a>&gt;)
    1832          </li>
    1833          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12</a>&gt;: "Invalidation after Update or Delete" (&lt;<a href="http://purl.org/NET/http-errata#invalidupd">http://purl.org/NET/http-errata#invalidupd</a>&gt;)
    1834          </li>
    1835          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35</a>&gt;: "Normative and Informative references"
    1836          </li>
    1837          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48</a>&gt;: "Date reference typo"
    1838          </li>
    1839          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49</a>&gt;: "Connection header text"
    1840          </li>
    1841          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65</a>&gt;: "Informative references"
    1842          </li>
    1843          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66</a>&gt;: "ISO-8859-1 Reference"
    1844          </li>
    1845          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86</a>&gt;: "Normative up-to-date references"
    1846          </li>
    1847          <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87</a>&gt;: "typo in 13.2.2"
    1848          </li>
    1849       </ul>
    1850       <p id="rfc.section.B.2.p.2">Other changes: </p>
    1851       <ul>
    1852          <li>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;)
    1853          </li>
    1854       </ul>
     1881      <div id="compatibility">
     1882         <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a>&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a></h1>
     1883         <div id="changes.from.rfc.2068">
     1884            <h2 id="rfc.section.A.1"><a href="#rfc.section.A.1">A.1</a>&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></h2>
     1885            <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.11" 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>)
     1886            </p>
     1887            <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
     1888               for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are
     1889               computed. (<a href="#non-modifiable.headers" title="Non-modifiable Headers">Section&nbsp;6.2</a>, see also <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="#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>)
     1890            </p>
     1891            <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>)
     1892            </p>
     1893            <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
     1894               needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Headers">Section&nbsp;6.3</a>)
     1895            </p>
     1896            <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>)
     1897            </p>
     1898            <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.6" 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.
     1899            </p>
     1900         </div>
     1901         <div id="changes.from.rfc.2616">
     1902            <h2 id="rfc.section.A.2"><a href="#rfc.section.A.2">A.2</a>&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h2>
     1903            <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>)
     1904            </p>
     1905         </div>
     1906      </div>
     1907      <div>
     1908         <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;Change Log (to be removed by RFC Editor before publication)
     1909         </h1>
     1910         <div>
     1911            <h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a>&nbsp;Since RFC2616
     1912            </h2>
     1913            <p id="rfc.section.B.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>.
     1914            </p>
     1915         </div>
     1916         <div>
     1917            <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a>&nbsp;Since draft-ietf-httpbis-p6-cache-00
     1918            </h2>
     1919            <p id="rfc.section.B.2.p.1">Closed issues: </p>
     1920            <ul>
     1921               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9</a>&gt;: "Trailer" (&lt;<a href="http://purl.org/NET/http-errata#trailer-hop">http://purl.org/NET/http-errata#trailer-hop</a>&gt;)
     1922               </li>
     1923               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12</a>&gt;: "Invalidation after Update or Delete" (&lt;<a href="http://purl.org/NET/http-errata#invalidupd">http://purl.org/NET/http-errata#invalidupd</a>&gt;)
     1924               </li>
     1925               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35</a>&gt;: "Normative and Informative references"
     1926               </li>
     1927               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48</a>&gt;: "Date reference typo"
     1928               </li>
     1929               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49</a>&gt;: "Connection header text"
     1930               </li>
     1931               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65</a>&gt;: "Informative references"
     1932               </li>
     1933               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66</a>&gt;: "ISO-8859-1 Reference"
     1934               </li>
     1935               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86</a>&gt;: "Normative up-to-date references"
     1936               </li>
     1937               <li>&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87</a>&gt;: "typo in 13.2.2"
     1938               </li>
     1939            </ul>
     1940            <p id="rfc.section.B.2.p.2">Other changes: </p>
     1941            <ul>
     1942               <li>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36</a>&gt;)
     1943               </li>
     1944            </ul>
     1945         </div>
     1946      </div>
    18551947      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
    18561948      <p class="noprint"><a href="#rfc.index.A">A</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.F">F</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.I">I</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.N">N</a> <a href="#rfc.index.O">O</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a> <a href="#rfc.index.W">W</a>
     
    20672159         </ul>
    20682160      </div>
    2069       <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
    2070       <p>Copyright © The IETF Trust (2008).</p>
    2071       <p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the
    2072          authors retain all their rights.
    2073       </p>
    2074       <p>This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION
    2075          HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE
    2076          DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
    2077          WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
    2078       </p>
    2079       <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
    2080       <p>The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might
    2081          be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any
    2082          license under such rights might or might not be available; nor does it represent that it has made any independent effort to
    2083          identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and
    2084          BCP 79.
    2085       </p>
    2086       <p>Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result
    2087          of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users
    2088          of this specification can be obtained from the IETF on-line IPR repository at <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.
    2089       </p>
    2090       <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
    2091          rights that may cover technology that may be required to implement this standard. Please address the information to the IETF
    2092          at <a href="mailto:ietf-ipr@ietf.org">ietf-ipr@ietf.org</a>.
    2093       </p>
     2161      <div class="avoidbreak">
     2162         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
     2163         <p><b>Roy T. Fielding</b>
     2164            (editor)
     2165            <br>Day Software<br>23 Corporate Plaza DR, Suite 280<br>Newport Beach, CA&nbsp;92660<br>USA<br>Phone: <a href="tel:+1-949-706-5300">+1-949-706-5300</a><br>Fax: <a href="fax:+1-949-706-5305">+1-949-706-5305</a><br>EMail: <a href="mailto:fielding@gbiv.com">fielding@gbiv.com</a><br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p>
     2166         <p><b>Jim Gettys</b><br>One Laptop per Child<br>21 Oak Knoll Road<br>Carlisle, MA&nbsp;01741<br>USA<br>EMail: <a href="mailto:jg@laptop.org">jg@laptop.org</a><br>URI: <a href="http://www.laptop.org/">http://www.laptop.org/</a></p>
     2167         <p><b>Jeffrey C. Mogul</b><br>Hewlett-Packard Company<br>HP Labs, Large Scale Systems Group<br>1501 Page Mill Road, MS 1177<br>Palo Alto, CA&nbsp;94304<br>USA<br>EMail: <a href="mailto:JeffMogul@acm.org">JeffMogul@acm.org</a></p>
     2168         <p><b>Henrik Frystyk Nielsen</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>USA<br>EMail: <a href="mailto:henrikn@microsoft.com">henrikn@microsoft.com</a></p>
     2169         <p><b>Larry Masinter</b><br>Adobe Systems, Incorporated<br>345 Park Ave<br>San Jose, CA&nbsp;95110<br>USA<br>EMail: <a href="mailto:LMM@acm.org">LMM@acm.org</a><br>URI: <a href="http://larry.masinter.net/">http://larry.masinter.net/</a></p>
     2170         <p><b>Paul J. Leach</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>EMail: <a href="mailto:paulle@microsoft.com">paulle@microsoft.com</a></p>
     2171         <p><b>Tim Berners-Lee</b><br>World Wide Web Consortium<br>MIT Computer Science and Artificial Intelligence Laboratory<br>The Stata Center, Building 32<br>32 Vassar Street<br>Cambridge, MA&nbsp;02139<br>USA<br>EMail: <a href="mailto:timbl@w3.org">timbl@w3.org</a><br>URI: <a href="http://www.w3.org/People/Berners-Lee/">http://www.w3.org/People/Berners-Lee/</a></p>
     2172         <p><b>Yves Lafon</b>
     2173            (editor)
     2174            <br>World Wide Web Consortium<br>W3C / ERCIM<br>2004, rte des Lucioles<br>Sophia-Antipolis, AM&nbsp;06902<br>France<br>EMail: <a href="mailto:ylafon@w3.org">ylafon@w3.org</a><br>URI: <a href="http://www.raubacapeu.net/people/yves/">http://www.raubacapeu.net/people/yves/</a></p>
     2175         <p><b>Julian F. Reschke</b>
     2176            (editor)
     2177            <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW&nbsp;48155<br>Germany<br>Phone: <a href="tel:+492512807760">+49 251 2807760</a><br>Fax: <a href="fax:+492512807761">+49 251 2807761</a><br>EMail: <a href="mailto:julian.reschke@greenbytes.de">julian.reschke@greenbytes.de</a><br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p>
     2178      </div>
     2179      <div id="rfc.copyright">
     2180         <h1><a href="#rfc.copyright">Full Copyright Statement</a></h1>
     2181         <p>Copyright © The IETF Trust (2008).</p>
     2182         <p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the
     2183            authors retain all their rights.
     2184         </p>
     2185         <p>This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION
     2186            HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE
     2187            DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
     2188            WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
     2189         </p>
     2190      </div>
     2191      <div id="rfc.ipr">
     2192         <h1><a href="#rfc.ipr">Intellectual Property</a></h1>
     2193         <p>The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might
     2194            be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any
     2195            license under such rights might or might not be available; nor does it represent that it has made any independent effort to
     2196            identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and
     2197            BCP 79.
     2198         </p>
     2199         <p>Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result
     2200            of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users
     2201            of this specification can be obtained from the IETF on-line IPR repository at <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.
     2202         </p>
     2203         <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
     2204            rights that may cover technology that may be required to implement this standard. Please address the information to the IETF
     2205            at <a href="mailto:ietf-ipr@ietf.org">ietf-ipr@ietf.org</a>.
     2206         </p>
     2207      </div>
    20942208   </body>
    20952209</html>
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