source: draft-ietf-httpbis/latest-roy/p6-cache.html @ 437

Last change on this file since 437 was 437, checked in by julian.reschke@…, 11 years ago

Synchronize with trunk (../latest), with the exception of Part 6. Fix P7 where it pointed into a section that was removed from Part 6.

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/html;charset=utf-8
File size: 147.0 KB
Line 
1<!DOCTYPE html
2  PUBLIC "-//W3C//DTD HTML 4.01//EN">
3<html lang="en">
4   <head profile="http://www.w3.org/2006/03/hcard">
5      <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
6      <title>HTTP/1.1, part 6: Caching</title><style type="text/css" title="Xml2Rfc (sans serif)">
7a {
8  text-decoration: none;
9}
10a.smpl {
11  color: black;
12}
13a:hover {
14  text-decoration: underline;
15}
16a:active {
17  text-decoration: underline;
18}
19address {
20  margin-top: 1em;
21  margin-left: 2em;
22  font-style: normal;
23}
24body {
25  color: black;
26  font-family: verdana, helvetica, arial, sans-serif;
27  font-size: 10pt;
28}
29cite {
30  font-style: normal;
31}
32dd {
33  margin-right: 2em;
34}
35dl {
36  margin-left: 2em;
37}
38
39dl.empty dd {
40  margin-top: .5em;
41}
42dl p {
43  margin-left: 0em;
44}
45dt {
46  margin-top: .5em;
47}
48h1 {
49  font-size: 14pt;
50  line-height: 21pt;
51  page-break-after: avoid;
52}
53h1.np {
54  page-break-before: always;
55}
56h1 a {
57  color: #333333;
58}
59h2 {
60  font-size: 12pt;
61  line-height: 15pt;
62  page-break-after: avoid;
63}
64h2 a {
65  color: black;
66}
67h3 {
68  font-size: 10pt;
69  page-break-after: avoid;
70}
71h3 a {
72  color: black;
73}
74h4 {
75  font-size: 10pt;
76  page-break-after: avoid;
77}
78h4 a {
79  color: black;
80}
81h5 {
82  font-size: 10pt;
83  page-break-after: avoid;
84}
85h5 a {
86  color: black;
87}
88img {
89  margin-left: 3em;
90}
91li {
92  margin-left: 2em;
93  margin-right: 2em;
94}
95ol {
96  margin-left: 2em;
97  margin-right: 2em;
98}
99ol p {
100  margin-left: 0em;
101}
102p {
103  margin-left: 2em;
104  margin-right: 2em;
105}
106pre {
107  margin-left: 3em;
108  background-color: lightyellow;
109  padding: .25em;
110}
111pre.text2 {
112  border-style: dotted;
113  border-width: 1px;
114  background-color: #f0f0f0;
115  width: 69em;
116}
117pre.inline {
118  background-color: white;
119  padding: 0em;
120}
121pre.text {
122  border-style: dotted;
123  border-width: 1px;
124  background-color: #f8f8f8;
125  width: 69em;
126}
127pre.drawing {
128  border-style: solid;
129  border-width: 1px;
130  background-color: #f8f8f8;
131  padding: 2em;
132}
133sup {
134  font-size: 60%;
135}
136table {
137  margin-left: 2em;
138}
139table.tt {
140  vertical-align: top;
141}
142table.full {
143  border-style: outset;
144  border-width: 1px;
145}
146table.headers {
147  border-style: outset;
148  border-width: 1px;
149}
150table.tt td {
151  vertical-align: top;
152}
153table.full td {
154  border-style: inset;
155  border-width: 1px;
156}
157table.tt th {
158  vertical-align: top;
159}
160table.full th {
161  border-style: inset;
162  border-width: 1px;
163}
164table.headers th {
165  border-style: none none inset none;
166  border-width: 1px;
167}
168table.header {
169  width: 95%;
170  font-size: 10pt;
171  color: white;
172}
173td.top {
174  vertical-align: top;
175}
176td.topnowrap {
177  vertical-align: top;
178  white-space: nowrap; 
179}
180td.header {
181  background-color: gray;
182  width: 50%;
183}
184td.header a {
185  color: white;
186}
187td.reference {
188  vertical-align: top;
189  white-space: nowrap;
190  padding-right: 1em;
191}
192thead {
193  display:table-header-group;
194}
195ul.toc {
196  list-style: none;
197  margin-left: 1.5em;
198  margin-right: 0em;
199  padding-left: 0em;
200}
201li.tocline0 {
202  line-height: 150%;
203  font-weight: bold;
204  font-size: 10pt;
205  margin-left: 0em;
206  margin-right: 0em;
207}
208li.tocline1 {
209  line-height: normal;
210  font-weight: normal;
211  font-size: 9pt;
212  margin-left: 0em;
213  margin-right: 0em;
214}
215li.tocline2 {
216  font-size: 0pt;
217}
218ul p {
219  margin-left: 0em;
220}
221ul.ind {
222  list-style: none;
223  margin-left: 1.5em;
224  margin-right: 0em;
225  padding-left: 0em;
226}
227li.indline0 {
228  font-weight: bold;
229  line-height: 200%;
230  margin-left: 0em;
231  margin-right: 0em;
232}
233li.indline1 {
234  font-weight: normal;
235  line-height: 150%;
236  margin-left: 0em;
237  margin-right: 0em;
238}
239.bcp14 {
240  font-style: normal;
241  text-transform: lowercase;
242  font-variant: small-caps;
243}
244.comment {
245  background-color: yellow;
246}
247.center {
248  text-align: center;
249}
250.error {
251  color: red;
252  font-style: italic;
253  font-weight: bold;
254}
255.figure {
256  font-weight: bold;
257  text-align: center;
258  font-size: 9pt;
259}
260.filename {
261  color: #333333;
262  font-weight: bold;
263  font-size: 12pt;
264  line-height: 21pt;
265  text-align: center;
266}
267.fn {
268  font-weight: bold;
269}
270.hidden {
271  display: none;
272}
273.left {
274  text-align: left;
275}
276.right {
277  text-align: right;
278}
279.title {
280  color: #990000;
281  font-size: 18pt;
282  line-height: 18pt;
283  font-weight: bold;
284  text-align: center;
285  margin-top: 36pt;
286}
287.vcardline {
288  display: block;
289}
290.warning {
291  font-size: 14pt;
292  background-color: yellow;
293}
294
295
296@media print {
297  .noprint {
298    display: none;
299  }
300 
301  a {
302    color: black;
303    text-decoration: none;
304  }
305
306  table.header {
307    width: 90%;
308  }
309
310  td.header {
311    width: 50%;
312    color: black;
313    background-color: white;
314    vertical-align: top;
315    font-size: 12pt;
316  }
317
318  ul.toc a::after {
319    content: leader('.') target-counter(attr(href), page);
320  }
321 
322  a.iref {
323    content: target-counter(attr(href), page);
324  }
325 
326  .print2col {
327    column-count: 2;
328    -moz-column-count: 2;
329    column-fill: auto;
330  }
331}
332
333@page {
334  @top-left {
335       content: "INTERNET DRAFT"; 
336  } 
337  @top-right {
338       content: "December 2008"; 
339  } 
340  @top-center {
341       content: "HTTP/1.1, Part 6"; 
342  } 
343  @bottom-left {
344       content: "Fielding, et al."; 
345  } 
346  @bottom-center {
347       content: "Standards Track"; 
348  } 
349  @bottom-right {
350       content: "[Page " counter(page) "]"; 
351  } 
352}
353
354@page:first { 
355    @top-left {
356      content: normal;
357    }
358    @top-right {
359      content: normal;
360    }
361    @top-center {
362      content: normal;
363    }
364}
365</style><link rel="Contents" href="#rfc.toc">
366      <link rel="Author" href="#rfc.authors">
367      <link rel="Copyright" href="#rfc.copyright">
368      <link rel="Index" href="#rfc.index">
369      <link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
370      <link rel="Chapter" title="2 Notational Conventions and Generic Grammar" href="#rfc.section.2">
371      <link rel="Chapter" title="3 Cache Operation" href="#rfc.section.3">
372      <link rel="Chapter" title="4 Header Field Definitions" href="#rfc.section.4">
373      <link rel="Chapter" title="5 History Lists" href="#rfc.section.5">
374      <link rel="Chapter" title="6 IANA Considerations" href="#rfc.section.6">
375      <link rel="Chapter" title="7 Security Considerations" href="#rfc.section.7">
376      <link rel="Chapter" title="8 Acknowledgments" href="#rfc.section.8">
377      <link rel="Chapter" href="#rfc.section.9" title="9 References">
378      <link rel="Appendix" title="A Compatibility with Previous Versions" href="#rfc.section.A">
379      <link rel="Appendix" title="B Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.B">
380      <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.400, 2008-10-10 14:04:14, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
381      <link rel="schema.DC" href="http://purl.org/dc/elements/1.1/">
382      <meta name="DC.Creator" content="Fielding, R.">
383      <meta name="DC.Creator" content="Gettys, J.">
384      <meta name="DC.Creator" content="Mogul, J.">
385      <meta name="DC.Creator" content="Frystyk, H.">
386      <meta name="DC.Creator" content="Masinter, L.">
387      <meta name="DC.Creator" content="Leach, P.">
388      <meta name="DC.Creator" content="Berners-Lee, T.">
389      <meta name="DC.Creator" content="Lafon, Y.">
390      <meta name="DC.Creator" content="Reschke, J. F.">
391      <meta name="DC.Identifier" content="urn:ietf:id:draft-ietf-httpbis-p6-cache-latest">
392      <meta name="DC.Date.Issued" scheme="ISO8601" content="2008-12">
393      <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2616">
394      <meta name="DC.Description.Abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
395   </head>
396   <body>
397      <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
398         <tr>
399            <td class="header left">Network Working Group</td>
400            <td class="header right">R. Fielding, Editor</td>
401         </tr>
402         <tr>
403            <td class="header left">Internet Draft</td>
404            <td class="header right">Day Software</td>
405         </tr>
406         <tr>
407            <td class="header left">
408               &lt;draft-ietf-httpbis-p6-cache-latest&gt;
409               
410            </td>
411            <td class="header right">J. Gettys</td>
412         </tr>
413         <tr>
414            <td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
415            </td>
416            <td class="header right">One Laptop per Child</td>
417         </tr>
418         <tr>
419            <td class="header left">Intended status: Standards Track</td>
420            <td class="header right">J. Mogul</td>
421         </tr>
422         <tr>
423            <td class="header left">Expires: June 2009</td>
424            <td class="header right">HP</td>
425         </tr>
426         <tr>
427            <td class="header left"></td>
428            <td class="header right">H. Frystyk</td>
429         </tr>
430         <tr>
431            <td class="header left"></td>
432            <td class="header right">Microsoft</td>
433         </tr>
434         <tr>
435            <td class="header left"></td>
436            <td class="header right">L. Masinter</td>
437         </tr>
438         <tr>
439            <td class="header left"></td>
440            <td class="header right">Adobe Systems</td>
441         </tr>
442         <tr>
443            <td class="header left"></td>
444            <td class="header right">P. Leach</td>
445         </tr>
446         <tr>
447            <td class="header left"></td>
448            <td class="header right">Microsoft</td>
449         </tr>
450         <tr>
451            <td class="header left"></td>
452            <td class="header right">T. Berners-Lee</td>
453         </tr>
454         <tr>
455            <td class="header left"></td>
456            <td class="header right">W3C/MIT</td>
457         </tr>
458         <tr>
459            <td class="header left"></td>
460            <td class="header right">Y. Lafon, Editor</td>
461         </tr>
462         <tr>
463            <td class="header left"></td>
464            <td class="header right">W3C</td>
465         </tr>
466         <tr>
467            <td class="header left"></td>
468            <td class="header right">J. F. Reschke, Editor</td>
469         </tr>
470         <tr>
471            <td class="header left"></td>
472            <td class="header right">greenbytes</td>
473         </tr>
474         <tr>
475            <td class="header left"></td>
476            <td class="header right">December 11, 2008</td>
477         </tr>
478      </table>
479      <p class="title">HTTP/1.1, part 6: Caching<br><span class="filename">draft-ietf-httpbis-p6-cache-latest</span></p>
480      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
481      <p>By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she
482         is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section
483         6 of BCP 79.
484      </p>
485      <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note
486         that other groups may also distribute working documents as Internet-Drafts.
487      </p>
488      <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
489         documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
490         in progress”.
491      </p>
492      <p>The list of current Internet-Drafts can be accessed at &lt;<a href="http://www.ietf.org/ietf/1id-abstracts.txt">http://www.ietf.org/ietf/1id-abstracts.txt</a>&gt;.
493      </p>
494      <p>The list of Internet-Draft Shadow Directories can be accessed at &lt;<a href="http://www.ietf.org/shadow.html">http://www.ietf.org/shadow.html</a>&gt;.
495      </p>
496      <p>This Internet-Draft will expire in June 2009.</p>
497      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
498      <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information
499         systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and,
500         taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control
501         cache behavior or indicate cacheable response messages.
502      </p> 
503      <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 
504      <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues
505         list is at &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/report/11">http://tools.ietf.org/wg/httpbis/trac/report/11</a>&gt; and related documents (including fancy diffs) can be found at &lt;<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>&gt;.
506      </p> 
507      <p>The changes in this draft are summarized in <a href="#changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">Appendix&nbsp;B.6</a>.
508      </p> 
509      <hr class="noprint">
510      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
511      <ul class="toc">
512         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#caching">Introduction</a><ul class="toc">
513               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#intro.purpose">Purpose</a></li>
514               <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#intro.terminology">Terminology</a></li>
515               <li class="tocline1">1.3&nbsp;&nbsp;&nbsp;<a href="#intro.requirements">Requirements</a></li>
516            </ul>
517         </li>
518         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#notation">Notational Conventions and Generic Grammar</a></li>
519         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Cache Operation</a><ul class="toc">
520               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a><ul class="toc">
521                     <li class="tocline1">3.1.1&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Storing Incomplete Responses</a></li>
522                  </ul>
523               </li>
524               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></li>
525               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness Model</a><ul class="toc">
526                     <li class="tocline1">3.3.1&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a><ul class="toc">
527                           <li class="tocline1">3.3.1.1&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Using Heuristic Freshness</a></li>
528                        </ul>
529                     </li>
530                     <li class="tocline1">3.3.2&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
531                     <li class="tocline1">3.3.3&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
532                  </ul>
533               </li>
534               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a></li>
535               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
536               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
537               <li class="tocline1">3.7&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Responses</a></li>
538            </ul>
539         </li>
540         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
541               <li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
542               <li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
543                     <li class="tocline1">4.2.1&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></li>
544                     <li class="tocline1">4.2.2&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></li>
545                     <li class="tocline1">4.2.3&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
546                  </ul>
547               </li>
548               <li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
549               <li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
550               <li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
551               <li class="tocline1">4.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
552            </ul>
553         </li>
554         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
555         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul class="toc">
556               <li class="tocline1">6.1&nbsp;&nbsp;&nbsp;<a href="#message.header.registration">Message Header Registration</a></li>
557            </ul>
558         </li>
559         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
560         <li class="tocline0">8.&nbsp;&nbsp;&nbsp;<a href="#ack">Acknowledgments</a></li>
561         <li class="tocline0">9.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul class="toc">
562               <li class="tocline1">9.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
563               <li class="tocline1">9.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
564            </ul>
565         </li>
566         <li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li>
567         <li class="tocline0">A.&nbsp;&nbsp;&nbsp;<a href="#compatibility">Compatibility with Previous Versions</a><ul class="toc">
568               <li class="tocline1">A.1&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2068">Changes from RFC 2068</a></li>
569               <li class="tocline1">A.2&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
570            </ul>
571         </li>
572         <li class="tocline0">B.&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul class="toc">
573               <li class="tocline1">B.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.1">Since RFC2616</a></li>
574               <li class="tocline1">B.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.2">Since draft-ietf-httpbis-p6-cache-00</a></li>
575               <li class="tocline1">B.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.B.3">Since draft-ietf-httpbis-p6-cache-01</a></li>
576               <li class="tocline1">B.4&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p6-cache-02</a></li>
577               <li class="tocline1">B.5&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p6-cache-03</a></li>
578               <li class="tocline1">B.6&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p6-cache-04</a></li>
579               <li class="tocline1">B.7&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></li>
580            </ul>
581         </li>
582         <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
583         <li class="tocline0"><a href="#rfc.index">Index</a></li>
584      </ul>
585      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="caching" href="#caching">Introduction</a></h1>
586      <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.
587         This document defines aspects of HTTP/1.1 related to caching and reusing response messages.
588      </p>
589      <div id="rfc.iref.c.1"></div>
590      <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>
591      <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
592         stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests.
593         Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.
594      </p>
595      <p id="rfc.section.1.1.p.2">Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to reuse a prior
596         response message to satisfy a current request. In some cases, a stored response can be reused without the need for a network
597         request, reducing latency and network round-trips; a "freshness" mechanism is used for this purpose (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;3.3</a>). Even when a new request is required, it is often possible to reuse all or parts of the payload of a prior response to satisfy
598         the request, thereby reducing network bandwidth usage; a "validation" mechanism is used for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;3.4</a>).
599      </p>
600      <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>
601      <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>
602      <p id="rfc.section.1.2.p.2"> <span id="rfc.iref.c.2"></span>  <dfn>cacheable</dfn> 
603      </p>
604      <dl class="empty">
605         <dd>A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests.
606            Even when a response is cacheable, there may be additional constraints on whether a cache can use the cached copy to satisfy
607            a particular request.
608         </dd>
609      </dl>
610      <p id="rfc.section.1.2.p.3"> <span id="rfc.iref.e.1"></span>  <dfn>explicit expiration time</dfn> 
611      </p>
612      <dl class="empty">
613         <dd>The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.</dd>
614      </dl>
615      <p id="rfc.section.1.2.p.4"> <span id="rfc.iref.h.1"></span>  <dfn>heuristic expiration time</dfn> 
616      </p>
617      <dl class="empty">
618         <dd>An expiration time assigned by a cache when no explicit expiration time is available.</dd>
619      </dl>
620      <p id="rfc.section.1.2.p.5"> <span id="rfc.iref.a.1"></span>  <dfn>age</dfn> 
621      </p>
622      <dl class="empty">
623         <dd>The age of a response is the time since it was sent by, or successfully validated with, the origin server.</dd>
624      </dl>
625      <p id="rfc.section.1.2.p.6"> <span id="rfc.iref.f.1"></span>  <dfn>first-hand</dfn> 
626      </p>
627      <dl class="empty">
628         <dd>A response is first-hand if the freshness model is not in use; i.e., its age is 0.</dd>
629      </dl>
630      <p id="rfc.section.1.2.p.7"> <span id="rfc.iref.f.2"></span>  <dfn>freshness lifetime</dfn> 
631      </p>
632      <dl class="empty">
633         <dd>The length of time between the generation of a response and its expiration time.</dd>
634      </dl>
635      <p id="rfc.section.1.2.p.8"> <span id="rfc.iref.f.3"></span>  <dfn>fresh</dfn> 
636      </p>
637      <dl class="empty">
638         <dd>A response is fresh if its age has not yet exceeded its freshness lifetime.</dd>
639      </dl>
640      <p id="rfc.section.1.2.p.9"> <span id="rfc.iref.s.1"></span>  <dfn>stale</dfn> 
641      </p>
642      <dl class="empty">
643         <dd>A response is stale if its age has passed its freshness lifetime.</dd>
644      </dl>
645      <p id="rfc.section.1.2.p.10"> <span id="rfc.iref.v.1"></span>  <dfn>validator</dfn> 
646      </p>
647      <dl class="empty">
648         <dd>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a stored response is an
649            equivalent copy of an entity.
650         </dd>
651      </dl>
652      <p id="rfc.section.1.2.p.11"> <span id="rfc.iref.v.2"></span>  <dfn>shared cache</dfn> 
653      </p>
654      <dl class="empty">
655         <dd>A cache that is accessible to more than one user. A non-shared cache is dedicated to a single user.</dd>
656      </dl>
657      <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>
658      <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"
659         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>.
660      </p>
661      <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."
662      </p>
663      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="notation" href="#notation">Notational Conventions and Generic Grammar</a></h1>
664      <p id="rfc.section.2.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation.abnf" title="ABNF Extension: #rule">Section 1.2.1</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> and the core rules defined in <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</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>: <span class="comment">[abnf.dep: ABNF syntax and basic rules will be adopted from RFC 5234, see &lt;<a href="http://ietf.org/wg/httpbis/trac/ticket/36">http://ietf.org/wg/httpbis/trac/ticket/36</a>&gt;.]</span> 
665      </p>
666      <div id="rfc.figure.u.1"></div> <pre class="inline">  <a href="#notation" class="smpl">DIGIT</a>         = &lt;DIGIT, defined in <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
667  <a href="#notation" class="smpl">DQUOTE</a>        = &lt;DQUOTE, defined in <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
668  <a href="#notation" class="smpl">SP</a>            = &lt;SP, defined in <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
669</pre> <div id="rfc.figure.u.2"></div> <pre class="inline">  <a href="#notation" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
670  <a href="#notation" class="smpl">token</a>         = &lt;token, defined in <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="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>&gt;
671</pre> <div id="abnf.dependencies">
672         <p id="rfc.section.2.p.4">          The ABNF rules below are defined in other parts:</p>
673      </div>
674      <div id="rfc.figure.u.3"></div>   <pre class="inline">  <a href="#abnf.dependencies" class="smpl">field-name</a>    = &lt;field-name, defined in <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#message.headers" title="Message Headers">Section 4.2</a>&gt;
675  <a href="#abnf.dependencies" class="smpl">HTTP-date</a>     = &lt;HTTP-date, defined in <a href="#Part1" id="rfc.xref.Part1.9"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#full.date" title="Full Date">Section 3.2.1</a>&gt;
676  <a href="#abnf.dependencies" class="smpl">port</a>          = &lt;port, defined in <a href="#Part1" id="rfc.xref.Part1.10"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.1</a>&gt;
677  <a href="#abnf.dependencies" class="smpl">pseudonym</a>     = &lt;pseudonym, defined in <a href="#Part1" id="rfc.xref.Part1.11"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#header.via" title="Via">Section 8.9</a>&gt; 
678  <a href="#abnf.dependencies" class="smpl">uri-host</a>      = &lt;uri-host, defined in <a href="#Part1" id="rfc.xref.Part1.12"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.1</a>&gt;
679</pre> <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Cache Operation</a></h1>
680      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Response Cacheability</a></h2>
681      <p id="rfc.section.3.1.p.1">A cache <em class="bcp14">MAY</em> store a response to any request, provided that:
682      </p>
683      <ul>
684         <li>the "no-store" cache directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;4.2</a>) does not appear in request or response headers.
685         </li>
686         <li>the cache understands partial responses, if the response is partial or incomplete (see <a href="#errors.or.incomplete.response.cache.behavior" title="Storing Incomplete Responses">Section&nbsp;3.1.1</a>).
687         </li>
688      </ul>
689      <p id="rfc.section.3.1.p.2">Note that in normal operation, most caches will not store a response that has neither a cache validator nor an explicitly
690         expiration time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.
691      </p>
692      <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;<a id="errors.or.incomplete.response.cache.behavior" href="#errors.or.incomplete.response.cache.behavior">Storing Incomplete Responses</a></h3>
693      <p id="rfc.section.3.1.1.p.1">A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) <em class="bcp14">MAY</em> store the response. However, the cache <em class="bcp14">MUST</em> treat this as a partial response <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>. Partial responses <em class="bcp14">MAY</em> be combined as described in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>; the result might be a full response or might still be partial. A cache <em class="bcp14">MUST NOT</em> return a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.
694      </p>
695      <p id="rfc.section.3.1.1.p.2">A cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> store incomplete or partial responses.
696      </p>
697      <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h2>
698      <p id="rfc.section.3.2.p.1">For a given request, a non-shared cache <em class="bcp14">MAY</em> return a stored response, provided that:
699      </p>
700      <ul>
701         <li>the presented request-URI and that of the stored response match (see <span class="comment">[rfc.comment.1: TBD]</span>), and
702         </li>
703         <li>selecting headers nominated by the stored response (if any) match (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;3.6</a>), and
704         </li>
705         <li>the stored response is either fresh (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;3.3</a>) or allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;3.3.3</a>), and
706         </li>
707         <li>the presented request and stored response are free from directives that would prevent it (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;4.2</a> and <a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;4.4</a>).
708         </li>
709      </ul>
710      <p id="rfc.section.3.2.p.2"> <span class="comment">[rfc.comment.2: ISSUE: This doesn't specify whether the request method is part of the cache key.]</span> 
711      </p>
712      <p id="rfc.section.3.2.p.3">A shared cache <em class="bcp14">MAY</em> return a stored response, provided that:
713      </p>
714      <ul>
715         <li>the criteria for non-shared caches above are met (including directives for shared caches; see <a href="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;4.2</a>), and
716         </li>
717         <li>the stored response was not associated with an authenticated request (see <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.1"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>), unless explicitly allowed (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.4" title="Cache-Control">Section&nbsp;4.2</a>).
718         </li>
719      </ul>
720      <p id="rfc.section.3.2.p.4">All responses satisfied from cache <em class="bcp14">MUST</em> include an appropriate Age header field (<a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;4.1</a>).
721      </p>
722      <p id="rfc.section.3.2.p.5">All request methods other than GET and HEAD <em class="bcp14">MUST</em> be written through the cache to the origin server. Note that such requests might invalidate already stored responses; see <a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;3.5</a>.
723      </p>
724      <p id="rfc.section.3.2.p.6">Caches <em class="bcp14">SHOULD</em> use the most recent response (as determined by the Date header) when more than one applicable response is stored. They <em class="bcp14">MAY</em> also send a request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use.
725      </p>
726      <p id="rfc.section.3.2.p.7">In the process of determining whether a stored response is fresh or not, a cache <em class="bcp14">MAY</em> validate that response (see <a href="#validation.model" title="Validation Model">Section&nbsp;3.4</a>).
727      </p>
728      <p id="rfc.section.3.2.p.8"> <span class="comment">[rfc.comment.3: TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1]</span> 
729      </p>
730      <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="expiration.model" href="#expiration.model">Freshness Model</a></h2>
731      <p id="rfc.section.3.3.p.1">HTTP caching works best when caches can entirely avoid making requests to the origin server. When a response is "fresh" in
732         the cache, it can be used to satisfy subsequent requests without contacting the origin server. This is also referred to as
733         "expiration."
734      </p>
735      <p id="rfc.section.3.3.p.2">Expiration applies only to responses taken from a cache and not to first-hand responses. It cannot be used to force a user
736         agent to refresh its display or reload a resource; its semantics apply only to caches. See <a href="#history.lists" title="History Lists">Section&nbsp;5</a> for an explanation of the difference between caches and history mechanisms.
737      </p>
738      <p id="rfc.section.3.3.p.3">The primary mechanism for avoiding requests is for an origin server to provide an explicit expiration time in the future,
739         using either the Expires header <a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;4.3</a> or the max-age response cache directive <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;4.2.2</a>. Generally, origin servers will assign future explicit expiration times to responses in the belief that the entity is not
740         likely to change in a semantically significant way before the expiration time is reached. This normally preserves cache correctness,
741         as long as the server's expiration times are carefully chosen.
742      </p>
743      <p id="rfc.section.3.3.p.4">If an origin server wishes to force a 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, so that caches should validate
744         it before using it for subsequent requests.
745      </p>
746      <p id="rfc.section.3.3.p.5">Since origin servers do not always provide explicit expiration times, HTTP caches may assign heuristic expiration times when
747         they are not specified, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible
748         expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on
749         their results.
750      </p>
751      <p id="rfc.section.3.3.p.6">The calculation to determine if a response has expired is:</p>
752      <div id="rfc.figure.u.4"></div> <pre class="text">   response_is_fresh = (freshness_lifetime &gt; current_age)
753</pre> <p id="rfc.section.3.3.p.8">The freshness_lifetime is defined in <a href="#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section&nbsp;3.3.1</a>; the current_age is defined in <a href="#age.calculations" title="Calculating Age">Section&nbsp;3.3.2</a>.
754      </p>
755      <p id="rfc.section.3.3.p.9">Additionally, clients may need to influence freshness calculation. They can do this using several request cache directives,
756         with the effect of either increasing or loosening constraints on freshness. See <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;4.2.1</a>.
757      </p>
758      <p id="rfc.section.3.3.p.10"> <span class="comment">[rfc.comment.4: ISSUE: there are not requirements directly applying to cache-request-directives and freshness.]</span> 
759      </p>
760      <h3 id="rfc.section.3.3.1"><a href="#rfc.section.3.3.1">3.3.1</a>&nbsp;<a id="calculating.freshness.lifetime" href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h3>
761      <p id="rfc.section.3.3.1.p.1">A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of: </p>
762      <ul>
763         <li>If the cache is shared and the s-maxage response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;4.2.2</a>) is present, use its value, or
764         </li>
765         <li>If the max-age response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;4.2.2</a>) is present, use its value, or
766         </li>
767         <li>If the Expires response header (<a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;4.3</a>) is present, use its value minus the value of the Date response header, or
768         </li>
769         <li>Otherwise, no explicit expiration time is present in the response, but a heuristic may be used; see <a href="#heuristic.freshness" title="Using Heuristic Freshness">Section&nbsp;3.3.1.1</a>.
770         </li>
771      </ul>
772      <p id="rfc.section.3.3.1.p.2">Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server.</p>
773      <h4 id="rfc.section.3.3.1.1"><a href="#rfc.section.3.3.1.1">3.3.1.1</a>&nbsp;<a id="heuristic.freshness" href="#heuristic.freshness">Using Heuristic Freshness</a></h4>
774      <p id="rfc.section.3.3.1.1.p.1">If no explicit expiration time is present in a stored response that has a status code of 200, 203, 206, 300, 301 or 410, a
775         heuristic expiration time <em class="bcp14">MAY</em> be calculated. Heuristics <em class="bcp14">MUST NOT</em> be used for other response status codes.
776      </p>
777      <p id="rfc.section.3.3.1.1.p.2">When a heuristic is used to calculate freshness lifetime, the cache <em class="bcp14">SHOULD</em> attach a Warning header with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning is
778         not already present.
779      </p>
780      <p id="rfc.section.3.3.1.1.p.3">Also, if the response has a Last-Modified header <a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 6.6</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>, the heuristic expiration value <em class="bcp14">SHOULD</em> be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
781      </p>
782      <p id="rfc.section.3.3.1.1.p.4"> <span class="comment">[rfc.comment.5: REVIEW: took away HTTP/1.0 query string heuristic uncacheability.]</span> 
783      </p>
784      <h3 id="rfc.section.3.3.2"><a href="#rfc.section.3.3.2">3.3.2</a>&nbsp;<a id="age.calculations" href="#age.calculations">Calculating Age</a></h3>
785      <p id="rfc.section.3.3.2.p.1">HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The
786         Age field value is the cache's estimate of the amount of time since the response was generated or validated by the origin
787         server. In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the
788         path from the origin server, plus the amount of time it has been in transit along network paths.
789      </p>
790      <p id="rfc.section.3.3.2.p.2">When a response is generated from a stored response, the cache <em class="bcp14">MUST</em> include a single Age header field in the response with a value equal to the stored response's current_age, calculated using
791         the algorithm described in this section.
792      </p>
793      <p id="rfc.section.3.3.2.p.3">The term "age_value" denotes the value of the Age header, in a form appropriate for arithmetic operations.</p>
794      <p id="rfc.section.3.3.2.p.4">HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response
795         was generated (see <a href="p1-messaging.html#header.date" title="Date">Section 8.3</a> of <a href="#Part1" id="rfc.xref.Part1.13"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). The term "date_value" denotes the value of the Date header, in a form appropriate for arithmetic operations.
796      </p>
797      <p id="rfc.section.3.3.2.p.5">The term "now" means "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially
798         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.
799      </p>
800      <p id="rfc.section.3.3.2.p.6">A response's age can be calculated in two entirely independent ways: </p>
801      <ol>
802         <li>now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative,
803            the result is replaced by zero.
804         </li>
805         <li>age_value, if all of the caches along the response path implement HTTP/1.1.</li>
806      </ol>
807      <p id="rfc.section.3.3.2.p.7">These are combined as</p>
808      <div id="rfc.figure.u.5"></div> <pre class="text">    corrected_received_age = max(now - date_value, age_value)
809</pre> <p id="rfc.section.3.3.2.p.9">When an Age value is received, it <em class="bcp14">MUST</em> be interpreted relative to the time the request was initiated, not the time that the response was received.
810      </p>
811      <div id="rfc.figure.u.6"></div> <pre class="text">   corrected_initial_age = corrected_received_age
812                         + (now - request_time)
813</pre> <p id="rfc.section.3.3.2.p.11">where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.</p>
814      <p id="rfc.section.3.3.2.p.12">The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response
815         was last validated by the origin server to the corrected_initial_age.
816      </p>
817      <p id="rfc.section.3.3.2.p.13">In summary:</p>
818      <div id="rfc.figure.u.7"></div> <pre class="text">   /*
819    * age_value
820    *      is the value of Age: header received by the cache with
821    *              this response.
822    * date_value
823    *      is the value of the origin server's Date: header
824    * request_time
825    *      is the (local) time when the cache made the request
826    *              that resulted in this stored response
827    * response_time
828    *      is the (local) time when the cache received the
829    *              response
830    * now
831    *      is the current (local) time
832    */
833
834   apparent_age = max(0, response_time - date_value);
835   corrected_received_age = max(apparent_age, age_value);
836   response_delay = response_time - request_time;
837   corrected_initial_age = corrected_received_age + response_delay;
838   resident_time = now - response_time;
839   current_age   = corrected_initial_age + resident_time;
840</pre> <h3 id="rfc.section.3.3.3"><a href="#rfc.section.3.3.3">3.3.3</a>&nbsp;<a id="serving.stale.responses" href="#serving.stale.responses">Serving Stale Responses</a></h3>
841      <p id="rfc.section.3.3.3.p.1">A "stale" response is one that either has explicit expiry information, or is allowed to have heuristic expiry calculated,
842         but is not fresh according to the calculations in <a href="#expiration.model" title="Freshness Model">Section&nbsp;3.3</a>.
843      </p>
844      <p id="rfc.section.3.3.3.p.2">Caches <em class="bcp14">MUST NOT</em> return a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
845         directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
846         see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;4.2.2</a>).
847      </p>
848      <p id="rfc.section.3.3.3.p.3">Caches <em class="bcp14">MAY</em> return a stale response if disconnected or explicitly allowed (e.g., the max-stale request directive; see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;4.2.1</a>).
849      </p>
850      <p id="rfc.section.3.3.3.p.4">Otherwise, caches <em class="bcp14">SHOULD NOT</em> return stale responses.
851      </p>
852      <p id="rfc.section.3.3.3.p.5">Stale responses <em class="bcp14">SHOULD</em> have a Warning header with the 110 warn-code (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">4.6</a>).
853      </p>
854      <p id="rfc.section.3.3.3.p.6">If a cache receives a first-hand response (either an entire response, or a 304 (Not Modified) response) that it would normally
855         forward to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">SHOULD</em> forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache <em class="bcp14">SHOULD NOT</em> attempt to validate a response simply because that response became stale in transit.
856      </p>
857      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h2>
858      <p id="rfc.section.3.4.p.1">When a cache has a stale response that it would like to use, it <em class="bcp14">SHOULD</em> first check with the origin server (or possibly an intermediate cache with a fresh response) to see if it is still usable.
859         This is called "validating" or "revalidating" the stored response.
860      </p>
861      <p id="rfc.section.3.4.p.2">HTTP's conditional request mechanism, defined in <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>, is used to avoid retransmitting the response payload when the stored response is valid. When a stored response includes
862         one or more "cache validators," such as the field values of an ETag or Last-Modified header field, then a validating GET request <em class="bcp14">SHOULD</em> be made conditional to those field values. The server checks the conditional request's validator against the current state
863         of the requested resource and, if they match, the server responds with a 304 (Not Modified) status code to indicate that the
864         stored response can be refreshed and reused without retransmitting the response payload. If the validator does not match the
865         current state of the requested resource, then the server returns a full response, including payload, so that the request can
866         be satisfied and the stored response supplanted without the need for an additional network round-trip.
867      </p>
868      <p id="rfc.section.3.4.p.3">See <a href="#combining.headers" title="Combining Responses">Section&nbsp;3.7</a> regarding combining cached headers with those in a 304 response.
869      </p>
870      <p id="rfc.section.3.4.p.4">If a cache receives a 5xx response while attempting to validate a response, it <em class="bcp14">MAY</em> either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it <em class="bcp14">MAY</em> return a previously received response unless the stored response includes the "must-revalidate" cache directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;4.2.2</a>).
871      </p>
872      <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></h2>
873      <p id="rfc.section.3.5.p.1">Because unsafe methods <a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a> have the potential for changing state on the origin server, intervening caches have the opportunity to use them to keep their
874         contents up-to-date.
875      </p>
876      <p id="rfc.section.3.5.p.2">The following HTTP methods <em class="bcp14">MUST</em> cause a cache to invalidate the Request-URI as well as the Location and Content-Location headers (if present):
877      </p>
878      <ul>
879         <li>PUT</li>
880         <li>DELETE</li>
881         <li>POST</li>
882      </ul>
883      <p id="rfc.section.3.5.p.3">An invalidation based on the URI in a Location or Content-Location header <em class="bcp14">MUST NOT</em> be performed if the host part of that URI differs from the host part in the Request-URI. This helps prevent denial of service
884         attacks.
885      </p>
886      <p id="rfc.section.3.5.p.4"> <span class="comment">[rfc.comment.6: TODO: "host part" needs to be specified better.]</span> 
887      </p>
888      <p id="rfc.section.3.5.p.5">A cache that passes through requests for methods it does not understand <em class="bcp14">SHOULD</em> invalidate the Request-URI.
889      </p>
890      <p id="rfc.section.3.5.p.6">Here, "invalidate" means that the cache will either remove all stored responses related to the Request-URI, or will mark these
891         as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent request.
892      </p>
893      <p id="rfc.section.3.5.p.7">Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the
894         change at the origin server might not have gone through the cache where a response is stored.
895      </p>
896      <p id="rfc.section.3.5.p.8"> <span class="comment">[rfc.comment.7: TODO: specify that only successful (2xx, 3xx?) responses invalidate.]</span> 
897      </p>
898      <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Caching Negotiated Responses</a></h2>
899      <p id="rfc.section.3.6.p.1">Use of server-driven content negotiation (<a href="p3-payload.html#server-driven.negotiation" title="Server-driven Negotiation">Section 4.1</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), as indicated by the presence of a Vary header field <a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;4.5</a> in a response, alters the conditions and procedure by which a cache can use the response for subsequent requests.
900      </p>
901      <p id="rfc.section.3.6.p.2">When the cache receives a subsequent request which may be satisfied by a stored responses that include a Vary header field,
902         it <em class="bcp14">MUST NOT</em> use it to satisfy the request unless all of the selecting request-headers present in the new request match the corresponding
903         stored request-headers from the original request.
904      </p>
905      <p id="rfc.section.3.6.p.3">The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first
906         request can be transformed to the selecting request-headers in the second request by adding or removing linear white space <span class="comment">[rfc.comment.8: [ref]]</span> at places where this is allowed by the corresponding ABNF, and/or combining multiple message-header fields with the same field
907         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.Part1.14"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>.
908      </p>
909      <p id="rfc.section.3.6.p.4">A Vary header field-value of "*" always fails to match, and subsequent requests on that resource can only be properly interpreted
910         by the origin server.
911      </p>
912      <p id="rfc.section.3.6.p.5">If the selecting request header fields for the stored response do not match the selecting request header fields of the new
913         request, then the cache <em class="bcp14">MUST NOT</em> use the stored response to satisfy the request unless it first relays the new request to the origin server in a conditional
914         request and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates the entity
915         to be used.
916      </p>
917      <p id="rfc.section.3.6.p.6">If one or more applicable stored response has an entity tag, the forwarded request <em class="bcp14">SHOULD</em> be conditional and include all of these entity tags in an If-None-Match header field. This conveys to the server the set of
918         entities currently stored by the cache, so that if any one of these entities matches the requested entity, the server can
919         use the ETag header field in its 304 (Not Modified) response to tell the cache which stored response is appropriate. If the
920         entity-tag of the new response matches that of an existing stored resopnse, the new response <em class="bcp14">SHOULD</em> be used to update its header fields, and the result <em class="bcp14">MUST</em> be returned to the client.
921      </p>
922      <p id="rfc.section.3.6.p.7">If any of the existing stored responses 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 stored
923         response.
924      </p>
925      <p id="rfc.section.3.6.p.8">If a cache receives a successful response whose Content-Location field matches that of an existing stored response for the
926         same Request-URI, whose entity-tag differs from that of the existing stored response, and whose Date is more recent than that
927         of the existing response, the existing response <em class="bcp14">SHOULD NOT</em> be returned in response to future requests and <em class="bcp14">SHOULD</em> be deleted from the cache.
928      </p>
929      <p id="rfc.section.3.6.p.9"> <span class="comment">[rfc.comment.9: TODO: this still needs work.]</span> 
930      </p>
931      <h2 id="rfc.section.3.7"><a href="#rfc.section.3.7">3.7</a>&nbsp;<a id="combining.headers" href="#combining.headers">Combining Responses</a></h2>
932      <p id="rfc.section.3.7.p.1">When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response, it needs to update the stored response
933         with the new one, so that the updated response can be sent to the client.
934      </p>
935      <p id="rfc.section.3.7.p.2">If the status code is 304 (Not Modified), the cache <em class="bcp14">SHOULD</em> use the stored entity-body as the updated entity-body. If the status code is 206 (Partial Content) and the ETag or Last-Modified
936         headers match exactly, the cache <em class="bcp14">MAY</em> combine the stored entity-body in the stored response with the updated entity-body received in the response and use the result
937         as the updated entity-body (see <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>).
938      </p>
939      <p id="rfc.section.3.7.p.3">The stored response headers are used for the updated response, except that </p>
940      <ul>
941         <li>any stored Warning headers with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;4.6</a>) <em class="bcp14">MUST</em> be deleted from the stored response and the forwarded response.
942         </li>
943         <li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the stored response and the forwarded response.
944         </li>
945         <li>any headers provided in the 304 or 206 response <em class="bcp14">MUST</em> replace the corresponding headers from the stored response.
946         </li>
947      </ul>
948      <p id="rfc.section.3.7.p.4">A cache <em class="bcp14">MUST</em> also replace stored headers with corresponding headers received in the incoming response, except for Warning headers as described
949         immediately above. If a header field-name in the incoming response matches more than one header in the stored response, all
950         such old headers <em class="bcp14">MUST</em> be replaced. it <em class="bcp14">MAY</em> store the combined entity-body.
951      </p>
952      <p id="rfc.section.3.7.p.5"><span class="comment">[rfc.comment.10: ISSUE: discuss how to handle HEAD updates]</span></p>
953      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
954      <p id="rfc.section.4.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
955      <p id="rfc.section.4.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
956         receives the entity.
957      </p>
958      <div id="rfc.iref.a.2"></div>
959      <div id="rfc.iref.h.2"></div>
960      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
961      <p id="rfc.section.4.1.p.1">The Age response-header field conveys the sender's estimate of the amount of time since the response (or its validation) was
962         generated at the origin server. Age values are calculated as specified in <a href="#age.calculations" title="Calculating Age">Section&nbsp;3.3.2</a>.
963      </p>
964      <div id="rfc.figure.u.8"></div> <pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span>  <a href="#header.age" class="smpl">Age</a> = "Age" ":" <a href="#header.age" class="smpl">age-value</a>
965  <a href="#header.age" class="smpl">age-value</a> = <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
966</pre> <div id="rule.delta-seconds">
967         <p id="rfc.section.4.1.p.3">  Age field-values are non-negative decimal integers, representing time in seconds.</p>
968      </div>
969      <div id="rfc.figure.u.9"></div> <pre class="inline"><span id="rfc.iref.g.3"></span>  <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>  = 1*<a href="#notation" class="smpl">DIGIT</a>
970</pre> <p id="rfc.section.4.1.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,
971         it <em class="bcp14">MUST</em> transmit an Age header with a field-value of 2147483648 (2<sup>31</sup>). Caches <em class="bcp14">SHOULD</em> use an arithmetic type of at least 31 bits of range.
972      </p>
973      <p id="rfc.section.4.1.p.6">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
974         true, since HTTP/1.0 caches may not implement the Age header field.
975      </p>
976      <div id="rfc.iref.c.3"></div>
977      <div id="rfc.iref.h.3"></div>
978      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
979      <p id="rfc.section.4.2.p.1">The Cache-Control general-header field is used to specify directives that <em class="bcp14">MUST</em> be obeyed by all caches along the request/response chain. The directives specify behavior intended to prevent caches from
980         adversely interfering with the request or response. Cache directives are unidirectional in that the presence of a directive
981         in a request does not imply that the same directive is to be given in the response.
982      </p>
983      <dl class="empty">
984         <dd>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;4.4</a>).
985         </dd>
986      </dl>
987      <p id="rfc.section.4.2.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
988         might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific
989         cache.
990      </p>
991      <div id="rfc.figure.u.10"></div> <pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span>  <a href="#header.cache-control" class="smpl">Cache-Control</a>   = "Cache-Control" ":" 1#<a href="#header.cache-control" class="smpl">cache-directive</a>
992
993  <a href="#header.cache-control" class="smpl">cache-directive</a> = <a href="#cache-request-directive" class="smpl">cache-request-directive</a>
994     / <a href="#cache-response-directive" class="smpl">cache-response-directive</a>
995
996  <a href="#header.cache-control" class="smpl">cache-extension</a> = <a href="#notation" class="smpl">token</a> [ "=" ( <a href="#notation" class="smpl">token</a> / <a href="#notation" class="smpl">quoted-string</a> ) ]
997</pre> <h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a id="cache-request-directive" href="#cache-request-directive">Request Cache-Control Directives</a></h3>
998      <div id="rfc.figure.u.11"></div> <pre class="inline"><span id="rfc.iref.g.7"></span>  <a href="#cache-request-directive" class="smpl">cache-request-directive</a> =
999       "no-cache"
1000     / "no-store"
1001     / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1002     / "max-stale" [ "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a> ]
1003     / "min-fresh" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1004     / "no-transform"
1005     / "only-if-cached"
1006     / <a href="#header.cache-control" class="smpl">cache-extension</a>
1007</pre> <p id="rfc.section.4.2.1.p.2"> <span id="rfc.iref.c.4"></span>  <span id="rfc.iref.n.1"></span> no-cache
1008      </p>
1009      <dl class="empty">
1010         <dd>The no-cache request directive indicates that a stored response <em class="bcp14">MUST NOT</em> be used to satisfy the request without successful validation on the origin server.
1011         </dd>
1012      </dl>
1013      <p id="rfc.section.4.2.1.p.3"> <span id="rfc.iref.c.5"></span>  <span id="rfc.iref.n.2"></span> no-store
1014      </p>
1015      <dl class="empty">
1016         <dd>The no-store request directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either this request or any response to it. This directive applies to both non-shared and shared caches.
1017            "<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.
1018         </dd>
1019         <dd>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1020            might not recognize or obey this directive, and communications networks may be vulnerable to eavesdropping.
1021         </dd>
1022      </dl>
1023      <p id="rfc.section.4.2.1.p.4"> <span id="rfc.iref.c.6"></span>  <span id="rfc.iref.m.1"></span> max-age
1024      </p>
1025      <dl class="empty">
1026         <dd>The max-age request directive indicates that the client is willing to accept a response whose age is no greater than the specified
1027            time in seconds. Unless max-stale directive is also included, the client is not willing to accept a stale response.
1028         </dd>
1029      </dl>
1030      <p id="rfc.section.4.2.1.p.5"> <span id="rfc.iref.c.7"></span>  <span id="rfc.iref.m.2"></span> max-stale
1031      </p>
1032      <dl class="empty">
1033         <dd>The max-stale request directive indicates that the client is willing to accept a response that has exceeded its expiration
1034            time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time
1035            by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept
1036            a stale response of any age.
1037         </dd>
1038      </dl>
1039      <p id="rfc.section.4.2.1.p.6"> <span id="rfc.iref.c.8"></span>  <span id="rfc.iref.m.3"></span> min-fresh
1040      </p>
1041      <dl class="empty">
1042         <dd>The min-fresh request directive indicates that the client is willing to accept a response whose freshness lifetime is no less
1043            than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for
1044            at least the specified number of seconds.
1045         </dd>
1046      </dl>
1047      <p id="rfc.section.4.2.1.p.7"> <span id="rfc.iref.c.9"></span>  <span id="rfc.iref.n.3"></span> no-transform
1048      </p>
1049      <dl class="empty">
1050         <dd>The no-transform request directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type request headers, nor the request entity-body.
1051         </dd>
1052      </dl>
1053      <p id="rfc.section.4.2.1.p.8"> <span id="rfc.iref.c.10"></span>  <span id="rfc.iref.o.1"></span> only-if-cached
1054      </p>
1055      <dl class="empty">
1056         <dd>The only-if-cached request directive indicates that the client only wishes to return a stored response. If it receives this
1057            directive, a cache <em class="bcp14">SHOULD</em> either respond using a stored response that is consistent with the other constraints of the request, or respond with a 504
1058            (Gateway Timeout) status. If a group of caches is being operated as a unified system with good internal connectivity, such
1059            a request <em class="bcp14">MAY</em> be forwarded within that group of caches.
1060         </dd>
1061      </dl>
1062      <h3 id="rfc.section.4.2.2"><a href="#rfc.section.4.2.2">4.2.2</a>&nbsp;<a id="cache-response-directive" href="#cache-response-directive">Response Cache-Control Directives</a></h3>
1063      <div id="rfc.figure.u.12"></div> <pre class="inline"><span id="rfc.iref.g.8"></span>  <a href="#cache-response-directive" class="smpl">cache-response-directive</a> =
1064       "public"
1065     / "private" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ]
1066     / "no-cache" [ "=" <a href="#notation" class="smpl">DQUOTE</a> 1#<a href="#abnf.dependencies" class="smpl">field-name</a> <a href="#notation" class="smpl">DQUOTE</a> ]
1067     / "no-store"
1068     / "no-transform"
1069     / "must-revalidate"
1070     / "proxy-revalidate"
1071     / "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1072     / "s-maxage" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
1073     / <a href="#header.cache-control" class="smpl">cache-extension</a>
1074</pre> <p id="rfc.section.4.2.2.p.2"> <span id="rfc.iref.c.11"></span>  <span id="rfc.iref.p.1"></span> public
1075      </p>
1076      <dl class="empty">
1077         <dd>The public response directive indicates that the response <em class="bcp14">MAY</em> be cached, even if it would normally be non-cacheable or cacheable only within a non-shared cache. (See also Authorization, <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.2"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>, for additional details.)
1078         </dd>
1079      </dl>
1080      <p id="rfc.section.4.2.2.p.3"> <span id="rfc.iref.c.12"></span>  <span id="rfc.iref.p.2"></span> private
1081      </p>
1082      <dl class="empty">
1083         <dd>The private response directive indicates that the response message is intended for a single user and <em class="bcp14">MUST NOT</em> be stored by a shared cache. A private (non-shared) cache <em class="bcp14">MAY</em> store the response.
1084         </dd>
1085         <dd>If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated
1086            with the listed response headers. That is, the specified field-names(s) <em class="bcp14">MUST NOT</em> be stored by a shared cache, whereas the remainder of the response message <em class="bcp14">MAY</em> be.
1087         </dd>
1088         <dd> <b>Note:</b> This usage of the word private only controls where the response may be stored, and cannot ensure the privacy of the message
1089            content.
1090         </dd>
1091      </dl>
1092      <p id="rfc.section.4.2.2.p.4"> <span id="rfc.iref.c.13"></span>  <span id="rfc.iref.n.4"></span> no-cache
1093      </p>
1094      <dl class="empty">
1095         <dd>The no-cache response directive indicates that a response <em class="bcp14">MUST NOT</em> be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to
1096            prevent caching even by caches that have been configured to return stale responses.
1097         </dd>
1098         <dd>If the no-cache response directive specifies one or more field-names, this requirement is limited to the field-values assosicated
1099            with the listed response headers. That is, the specified field-name(s) <em class="bcp14">MUST NOT</em> be sent in the response to a subsequent request without successful validation on the origin server. This allows an origin
1100            server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
1101         </dd>
1102         <dd> <b>Note:</b> Most HTTP/1.0 caches will not recognize or obey this directive.
1103         </dd>
1104      </dl>
1105      <p id="rfc.section.4.2.2.p.5"> <span id="rfc.iref.c.14"></span>  <span id="rfc.iref.n.5"></span> no-store
1106      </p>
1107      <dl class="empty">
1108         <dd>The no-store response directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either the immediate request or response. This directive applies to both non-shared and shared caches. "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
1109         </dd>
1110         <dd>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
1111            might not recognize or obey this directive, and communications networks may be vulnerable to eavesdropping.
1112         </dd>
1113      </dl>
1114      <p id="rfc.section.4.2.2.p.6"> <span id="rfc.iref.c.15"></span>  <span id="rfc.iref.m.4"></span> must-revalidate
1115      </p>
1116      <dl class="empty">
1117         <dd>The must-revalidate response directive indicates that validation is required before the response is used by a cache to satisfy
1118            any request.
1119         </dd>
1120         <dd>When the present, caches <em class="bcp14">MUST NOT</em> use a stored after it becomes stale to respond to a subsequent request without first validating it with the origin server.
1121         </dd>
1122         <dd>The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
1123            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.
1124         </dd>
1125         <dd>Servers <em class="bcp14">SHOULD</em> send the must-revalidate directive if and only if failure to validate a request on the entity could result in incorrect operation,
1126            such as a silently unexecuted financial transaction.
1127         </dd>
1128      </dl>
1129      <p id="rfc.section.4.2.2.p.7"> <span id="rfc.iref.c.16"></span>  <span id="rfc.iref.p.3"></span> proxy-revalidate
1130      </p>
1131      <dl class="empty">
1132         <dd>The proxy-revalidate response directive has the same meaning as the must-revalidate response directive, except that it does
1133            not apply to non-shared caches.
1134         </dd>
1135      </dl>
1136      <p id="rfc.section.4.2.2.p.8"> <span id="rfc.iref.c.17"></span>  <span id="rfc.iref.m.5"></span> max-age
1137      </p>
1138      <dl class="empty">
1139         <dd>The max-age response directive indicates that response is to be considered stale after its age is greater than the specified
1140            number of seconds.
1141         </dd>
1142      </dl>
1143      <p id="rfc.section.4.2.2.p.9"> <span id="rfc.iref.c.18"></span>  <span id="rfc.iref.s.2"></span> s-maxage
1144      </p>
1145      <dl class="empty">
1146         <dd>The s-maxage response directive indicates that, in shared caches, the maximum age specified by this directive overrides the
1147            maximum age specified by either the max-age directive or the Expires header. The s-maxage directive also implies the semantics
1148            of the proxy-revalidate response directive.
1149         </dd>
1150      </dl>
1151      <p id="rfc.section.4.2.2.p.10"> <span id="rfc.iref.c.19"></span>  <span id="rfc.iref.n.6"></span> no-transform
1152      </p>
1153      <dl class="empty">
1154         <dd>The no-transform response directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type response headers, nor the response entity-body.
1155         </dd>
1156      </dl>
1157      <h3 id="rfc.section.4.2.3"><a href="#rfc.section.4.2.3">4.2.3</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
1158      <p id="rfc.section.4.2.3.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
1159         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
1160         to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications
1161         which do not understand the new directive will default to the behavior specified by the standard directive, and those that
1162         understand the new directive will recognize it as modifying the requirements associated with the standard directive. In this
1163         way, extensions to the cache-control directives can be made without requiring changes to the base protocol.
1164      </p>
1165      <p id="rfc.section.4.2.3.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
1166         obeying certain extensions, and ignoring all directives that it does not understand.
1167      </p>
1168      <p id="rfc.section.4.2.3.p.3">For example, consider a hypothetical new response directive called "community" which acts as a modifier to the private directive.
1169         We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members
1170         of the community named within its value may cache the response. An origin server wishing to allow the UCI community to use
1171         an otherwise private response in their shared cache(s) could do so by including
1172      </p>
1173      <div id="rfc.figure.u.13"></div> <pre class="text">    Cache-Control: private, community="UCI"
1174</pre> <p id="rfc.section.4.2.3.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
1175         it will also see and understand the private directive and thus default to the safe behavior.
1176      </p>
1177      <p id="rfc.section.4.2.3.p.6">Unrecognized cache directives <em class="bcp14">MUST</em> be ignored; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1178         directives (or the response's default cacheability) such that the cache behavior will remain minimally correct even if the
1179         cache does not understand the extension(s).
1180      </p>
1181      <div id="rfc.iref.e.2"></div>
1182      <div id="rfc.iref.h.4"></div>
1183      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
1184      <p id="rfc.section.4.3.p.1">The Expires entity-header field gives the date/time after which the response is considered stale. See <a href="#expiration.model" title="Freshness Model">Section&nbsp;3.3</a> for further discussion of the expiration model.
1185      </p>
1186      <p id="rfc.section.4.3.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
1187         that time.
1188      </p>
1189      <p id="rfc.section.4.3.p.3">The field-value is an absolute date and time as defined by HTTP-date in <a href="p1-messaging.html#full.date" title="Full Date">Section 3.2.1</a> of <a href="#Part1" id="rfc.xref.Part1.15"><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.
1190      </p>
1191      <div id="rfc.figure.u.14"></div> <pre class="inline"><span id="rfc.iref.g.9"></span>  <a href="#header.expires" class="smpl">Expires</a> = "Expires" ":" <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
1192</pre> <p id="rfc.section.4.3.p.5">For example</p>
1193      <div id="rfc.figure.u.15"></div> <pre class="text">   Expires: Thu, 01 Dec 1994 16:00:00 GMT
1194</pre> <p id="rfc.section.4.3.p.7"> </p>
1195      <dl class="empty">
1196         <dd> <b>Note:</b> if a response includes a Cache-Control field with the max-age directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;4.2.2</a>), that directive overrides the Expires field.
1197         </dd>
1198      </dl>
1199      <p id="rfc.section.4.3.p.8">HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
1200      </p>
1201      <p id="rfc.section.4.3.p.9">HTTP/1.1 clients and caches <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
1202      </p>
1203      <div id="rfc.iref.p.4"></div>
1204      <div id="rfc.iref.h.5"></div>
1205      <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
1206      <p id="rfc.section.4.4.p.1">The Pragma general-header field is used to include implementation-specific directives that might apply to any recipient along
1207         the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some
1208         systems <em class="bcp14">MAY</em> require that behavior be consistent with the directives.
1209      </p>
1210      <div id="rfc.figure.u.16"></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>  <a href="#header.pragma" class="smpl">Pragma</a>            = "Pragma" ":" 1#<a href="#header.pragma" class="smpl">pragma-directive</a>
1211  <a href="#header.pragma" class="smpl">pragma-directive</a>  = "no-cache" / <a href="#header.pragma" class="smpl">extension-pragma</a>
1212  <a href="#header.pragma" class="smpl">extension-pragma</a>  = <a href="#notation" class="smpl">token</a> [ "=" ( <a href="#notation" class="smpl">token</a> / <a href="#notation" class="smpl">quoted-string</a> ) ]
1213</pre> <p id="rfc.section.4.4.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
1214         has the same semantics as the no-cache response directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;4.2.2</a>) and is defined here for backward compatibility with HTTP/1.0. Clients <em class="bcp14">SHOULD</em> include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches <em class="bcp14">SHOULD</em> treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1215      </p>
1216      <p id="rfc.section.4.4.p.4"> </p>
1217      <dl class="empty">
1218         <dd> <b>Note:</b> because the meaning of "Pragma: no-cache" as a response-header field is not actually specified, it does not provide a reliable
1219            replacement for "Cache-Control: no-cache" in a response.
1220         </dd>
1221      </dl>
1222      <p id="rfc.section.4.4.p.5">This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</p>
1223      <div id="rfc.iref.v.3"></div>
1224      <div id="rfc.iref.h.6"></div>
1225      <h2 id="rfc.section.4.5"><a href="#rfc.section.4.5">4.5</a>&nbsp;<a id="header.vary" href="#header.vary">Vary</a></h2>
1226      <p id="rfc.section.4.5.p.1">The Vary response-header field's value indicates the set of request-header fields that fully determines, while the response
1227         is fresh, whether a cache is permitted to use the response to reply to a subsequent request without validation. For uncacheable
1228         or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation.
1229         A Vary field value of "*" implies that a cache cannot determine from the request headers of a subsequent request whether this
1230         response is the appropriate representation. See <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;3.6</a> for use of the Vary header field by caches.
1231      </p>
1232      <div id="rfc.figure.u.17"></div> <pre class="inline"><span id="rfc.iref.g.13"></span>  <a href="#header.vary" class="smpl">Vary</a>  = "Vary" ":" ( "*" / 1#<a href="#abnf.dependencies" class="smpl">field-name</a> )
1233</pre> <p id="rfc.section.4.5.p.3">The set of header fields named by the Vary field value is known as the "selecting" request-headers.</p>
1234      <p id="rfc.section.4.5.p.4">Servers <em class="bcp14">SHOULD</em> include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache
1235         to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that
1236         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
1237         the user agent with useful information about the dimensions over which the response varies at the time of the response.
1238      </p>
1239      <p id="rfc.section.4.5.p.5">A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address
1240         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.
1241      </p>
1242      <p id="rfc.section.4.5.p.6">The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names
1243         are case-insensitive.
1244      </p>
1245      <div id="rfc.iref.w.1"></div>
1246      <div id="rfc.iref.h.7"></div>
1247      <h2 id="rfc.section.4.6"><a href="#rfc.section.4.6">4.6</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
1248      <p id="rfc.section.4.6.p.1">The Warning general-header field is used to carry additional information about the status or transformation of a message which
1249         might not be reflected in the message. This information is typically used to warn about possible incorrectness introduced
1250         by caching operations or transformations applied to the entity body of the message.
1251      </p>
1252      <p id="rfc.section.4.6.p.2">Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status
1253         code, distinguish these responses from true failures.
1254      </p>
1255      <p id="rfc.section.4.6.p.3">Warning headers can in general be applied to any message, however some warn-codes are specific to caches and can only be applied
1256         to response messages.
1257      </p>
1258      <div id="rfc.figure.u.18"></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>  <a href="#header.warning" class="smpl">Warning</a>    = "Warning" ":" 1#<a href="#header.warning" class="smpl">warning-value</a>
1259 
1260  <a href="#header.warning" class="smpl">warning-value</a> = <a href="#header.warning" class="smpl">warn-code</a> <a href="#notation" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-agent</a> <a href="#notation" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-text</a>
1261                                        [<a href="#notation" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-date</a>]
1262 
1263  <a href="#header.warning" class="smpl">warn-code</a>  = 3<a href="#notation" class="smpl">DIGIT</a>
1264  <a href="#header.warning" class="smpl">warn-agent</a> = ( <a href="#abnf.dependencies" class="smpl">uri-host</a> [ ":" <a href="#abnf.dependencies" class="smpl">port</a> ] ) / <a href="#abnf.dependencies" class="smpl">pseudonym</a>
1265                  ; the name or pseudonym of the server adding
1266                  ; the Warning header, for use in debugging
1267  <a href="#header.warning" class="smpl">warn-text</a>  = <a href="#notation" class="smpl">quoted-string</a>
1268  <a href="#header.warning" class="smpl">warn-date</a>  = <a href="#notation" class="smpl">DQUOTE</a> <a href="#abnf.dependencies" class="smpl">HTTP-date</a> <a href="#notation" class="smpl">DQUOTE</a>
1269</pre> <p id="rfc.section.4.6.p.5">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.
1270         For example, a server might provide the same warning with texts in both English and Basque.
1271      </p>
1272      <p id="rfc.section.4.6.p.6">When this occurs, the user agent <em class="bcp14">SHOULD</em> inform the user of as many of them as possible, in the order that they appear in the response. If it is not possible to inform
1273         the user of all of the warnings, the user agent <em class="bcp14">SHOULD</em> follow these heuristics:
1274      </p>
1275      <ul>
1276         <li>Warnings that appear early in the response take priority over those appearing later in the response.</li>
1277         <li>Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes
1278            and warn-agents.
1279         </li>
1280      </ul>
1281      <p id="rfc.section.4.6.p.7">Systems that generate multiple Warning headers <em class="bcp14">SHOULD</em> order them with this user agent behavior in mind. New Warning headers <em class="bcp14">SHOULD</em> be added after any existing Warning headers.
1282      </p>
1283      <p id="rfc.section.4.6.p.8">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning is required to be deleted from
1284         a stored response after validation:
1285      </p>
1286      <ul>
1287         <li>1xx Warnings that describe the freshness or validation status of the response, and so <em class="bcp14">MUST</em> be deleted by caches after validation. They <em class="bcp14">MUST NOT</em> be generated by a cache except when validating a cached entry, and <em class="bcp14">MUST NOT</em> be generated by clients.
1288         </li>
1289         <li>2xx Warnings that describe some aspect of the entity body or entity headers that is not rectified by a validation (for example,
1290            a lossy compression of the entity bodies) and which <em class="bcp14">MUST NOT</em> be deleted by caches after validation, unless a full response is returned, in which case they <em class="bcp14">MUST</em> be.
1291         </li>
1292      </ul>
1293      <p id="rfc.section.4.6.p.9">The warn-text <em class="bcp14">SHOULD</em> be in a natural language and character set that is most likely to be intelligible to the human user receiving the response.
1294         This decision can be based on any available knowledge, such as the location of the cache or user, the Accept-Language field
1295         in a request, the Content-Language field in a response, etc. The default language is English and the default character set
1296         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>).
1297      </p>
1298      <p id="rfc.section.4.6.p.10">If a character set other than ISO-8859-1 is used, it <em class="bcp14">MUST</em> be encoded in the warn-text using the method described in <a href="#RFC2047" id="rfc.xref.RFC2047.1"><cite title="MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text">[RFC2047]</cite></a>.
1299      </p>
1300      <p id="rfc.section.4.6.p.11">If an implementation sends a message with one or more Warning headers to a receiver whose version is HTTP/1.0 or lower, then
1301         the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the Date header in the message.
1302      </p>
1303      <p id="rfc.section.4.6.p.12">If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from
1304         the Date value in the response, then that warning-value <em class="bcp14">MUST</em> be deleted from the message before storing, forwarding, or using it. (preventing the consequences of naive caching of Warning
1305         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.
1306      </p>
1307      <p id="rfc.section.4.6.p.13">The following warn-codes are defined by this specification, each with a recommended warn-text in English, and a description
1308         of its meaning.
1309      </p>
1310      <p id="rfc.section.4.6.p.14">110 Response is stale </p>
1311      <dl class="empty">
1312         <dd><em class="bcp14">SHOULD</em> be included whenever the returned response is stale.
1313         </dd>
1314      </dl>
1315      <p id="rfc.section.4.6.p.15">111 Revalidation failed </p>
1316      <dl class="empty">
1317         <dd><em class="bcp14">SHOULD</em> be included if a cache returns a stale response because an attempt to validate the response failed, due to an inability to
1318            reach the server.
1319         </dd>
1320      </dl>
1321      <p id="rfc.section.4.6.p.16">112 Disconnected operation </p>
1322      <dl class="empty">
1323         <dd><em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
1324         </dd>
1325      </dl>
1326      <p id="rfc.section.4.6.p.17">113 Heuristic expiration </p>
1327      <dl class="empty">
1328         <dd><em class="bcp14">SHOULD</em> be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater
1329            than 24 hours.
1330         </dd>
1331      </dl>
1332      <p id="rfc.section.4.6.p.18">199 Miscellaneous warning </p>
1333      <dl class="empty">
1334         <dd>The warning text <em class="bcp14">MAY</em> include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action, besides presenting the warning to the user.
1335         </dd>
1336      </dl>
1337      <p id="rfc.section.4.6.p.19">214 Transformation applied </p>
1338      <dl class="empty">
1339         <dd><em class="bcp14">MUST</em> be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the
1340            Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, or the entity-body of the
1341            response, unless this Warning code already appears in the response.
1342         </dd>
1343      </dl>
1344      <p id="rfc.section.4.6.p.20">299 Miscellaneous persistent warning </p>
1345      <dl class="empty">
1346         <dd>The warning text <em class="bcp14">MAY</em> include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action.
1347         </dd>
1348      </dl>
1349      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
1350      <p id="rfc.section.5.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, which can be used to redisplay an entity
1351         retrieved earlier in a session.
1352      </p>
1353      <p id="rfc.section.5.p.2">History mechanisms and caches are different. In particular history mechanisms <em class="bcp14">SHOULD NOT</em> try to show a correct view of the current state of a resource. Rather, a history mechanism is meant to show exactly what the
1354         user saw at the time when the resource was retrieved.
1355      </p>
1356      <p id="rfc.section.5.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
1357         documents.
1358      </p>
1359      <p id="rfc.section.5.p.4">This is not to be construed to prohibit the history mechanism from telling the user that a view might be stale. </p>
1360      <dl class="empty">
1361         <dd> <b>Note:</b> if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors
1362            to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider
1363            it important that users not be presented with error messages or warning messages when they use navigation controls (such as
1364            BACK) to view previously fetched resources. Even though sometimes such resources ought not be cached, or ought to expire quickly,
1365            user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs)
1366            in order not to suffer the effects of improperly functioning history mechanisms.
1367         </dd>
1368      </dl>
1369      <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
1370      <h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a>&nbsp;<a id="message.header.registration" href="#message.header.registration">Message Header Registration</a></h2>
1371      <p id="rfc.section.6.1.p.1">The Message Header Registry located at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt; should be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
1372      </p>
1373      <div id="rfc.table.1">
1374         <div id="iana.header.registration.table"></div>
1375         <table summary="" class="tt full" cellpadding="3" cellspacing="0">
1376            <thead>
1377               <tr>
1378                  <th>Header Field Name</th>
1379                  <th>Protocol</th>
1380                  <th>Status</th>
1381                  <th>Reference</th>
1382               </tr>
1383            </thead>
1384            <tbody>
1385               <tr>
1386                  <td>Age</td>
1387                  <td>http</td>
1388                  <td>standard</td>
1389                  <td> <a href="#header.age" id="rfc.xref.header.age.2" title="Age">Section&nbsp;4.1</a> 
1390                  </td>
1391               </tr>
1392               <tr>
1393                  <td>Cache-Control</td>
1394                  <td>http</td>
1395                  <td>standard</td>
1396                  <td> <a href="#header.cache-control" id="rfc.xref.header.cache-control.5" title="Cache-Control">Section&nbsp;4.2</a> 
1397                  </td>
1398               </tr>
1399               <tr>
1400                  <td>Expires</td>
1401                  <td>http</td>
1402                  <td>standard</td>
1403                  <td> <a href="#header.expires" id="rfc.xref.header.expires.3" title="Expires">Section&nbsp;4.3</a> 
1404                  </td>
1405               </tr>
1406               <tr>
1407                  <td>Pragma</td>
1408                  <td>http</td>
1409                  <td>standard</td>
1410                  <td> <a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;4.4</a> 
1411                  </td>
1412               </tr>
1413               <tr>
1414                  <td>Vary</td>
1415                  <td>http</td>
1416                  <td>standard</td>
1417                  <td> <a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section&nbsp;4.5</a> 
1418                  </td>
1419               </tr>
1420               <tr>
1421                  <td>Warning</td>
1422                  <td>http</td>
1423                  <td>standard</td>
1424                  <td> <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;4.6</a> 
1425                  </td>
1426               </tr>
1427            </tbody>
1428         </table>
1429      </div>
1430      <p id="rfc.section.6.1.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
1431      <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1432      <p id="rfc.section.7.p.1">Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious
1433         exploitation. Because cache contents persist after an HTTP request is complete, an attack on the cache can reveal information
1434         long after a user believes that the information has been removed from the network. Therefore, cache contents should be protected
1435         as sensitive information.
1436      </p>
1437      <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="ack" href="#ack">Acknowledgments</a></h1>
1438      <p id="rfc.section.8.p.1">Much of the content and presentation of the caching design is due to suggestions and comments from individuals including:
1439         Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
1440      </p>
1441      <h1 id="rfc.references"><a id="rfc.section.9" href="#rfc.section.9">9.</a> References
1442      </h1>
1443      <h2 id="rfc.references.1"><a href="#rfc.section.9.1" id="rfc.section.9.1">9.1</a> Normative References
1444      </h2>
1445      <table summary="Normative References">                 
1446         <tr>
1447            <td class="reference"><b id="ISO-8859-1">[ISO-8859-1]</b></td>
1448            <td class="top">International Organization for Standardization, “ Information technology -- 8-bit single-byte coded graphic character sets -- Part
1449               1: Latin alphabet No. 1 ”, ISO/IEC&nbsp;8859-1:1998, 1998.
1450            </td>
1451         </tr>
1452         <tr>
1453            <td class="reference"><b id="Part1">[Part1]</b></td>
1454            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), December&nbsp;2008.
1455            </td>
1456         </tr>
1457         <tr>
1458            <td class="reference"><b id="Part2">[Part2]</b></td>
1459            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">HTTP/1.1, part 2: Message Semantics</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), December&nbsp;2008.
1460            </td>
1461         </tr>
1462         <tr>
1463            <td class="reference"><b id="Part3">[Part3]</b></td>
1464            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-latest">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p3-payload-latest (work in progress), December&nbsp;2008.
1465            </td>
1466         </tr>
1467         <tr>
1468            <td class="reference"><b id="Part4">[Part4]</b></td>
1469            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-latest">HTTP/1.1, part 4: Conditional Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p4-conditional-latest (work in progress), December&nbsp;2008.
1470            </td>
1471         </tr>
1472         <tr>
1473            <td class="reference"><b id="Part5">[Part5]</b></td>
1474            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), December&nbsp;2008.
1475            </td>
1476         </tr>
1477         <tr>
1478            <td class="reference"><b id="Part7">[Part7]</b></td>
1479            <td class="top"><a title="Day Software">Fielding, R., Ed.</a>, <a title="One Laptop per Child">Gettys, J.</a>, <a title="Hewlett-Packard Company">Mogul, J.</a>, <a title="Microsoft Corporation">Frystyk, H.</a>, <a title="Adobe Systems, Incorporated">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, <a title="World Wide Web Consortium">Berners-Lee, T.</a>, <a title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a title="greenbytes GmbH">J. F. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-latest">HTTP/1.1, part 7: Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-latest (work in progress), December&nbsp;2008.
1480            </td>
1481         </tr>
1482         <tr>
1483            <td class="reference"><b id="RFC2047">[RFC2047]</b></td>
1484            <td class="top"><a title="University of Tennessee">Moore, K.</a>, “<a href="http://tools.ietf.org/html/rfc2047">MIME (Multipurpose Internet Mail Extensions)
1485                  Part Three: Message Header Extensions for Non-ASCII Text</a>”, RFC&nbsp;2047, November&nbsp;1996.
1486            </td>
1487         </tr>
1488         <tr>
1489            <td class="reference"><b id="RFC2119">[RFC2119]</b></td>
1490            <td class="top"><a 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.
1491            </td>
1492         </tr>
1493      </table>
1494      <h2 id="rfc.references.2"><a href="#rfc.section.9.2" id="rfc.section.9.2">9.2</a> Informative References
1495      </h2>
1496      <table summary="Informative References">     
1497         <tr>
1498            <td class="reference"><b id="RFC1305">[RFC1305]</b></td>
1499            <td class="top"><a 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.
1500            </td>
1501         </tr>
1502         <tr>
1503            <td class="reference"><b id="RFC2616">[RFC2616]</b></td>
1504            <td class="top"><a title="University of California, Irvine">Fielding, R.</a>, <a title="W3C">Gettys, J.</a>, <a title="Compaq Computer Corporation">Mogul, J.</a>, <a title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a title="Xerox Corporation">Masinter, L.</a>, <a title="Microsoft Corporation">Leach, P.</a>, and <a 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.
1505            </td>
1506         </tr>
1507         <tr>
1508            <td class="reference"><b id="RFC3864">[RFC3864]</b></td>
1509            <td class="top"><a title="Nine by Nine">Klyne, G.</a>, <a title="BEA Systems">Nottingham, M.</a>, and <a title="HP Labs">J. Mogul</a>, “<a href="http://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP&nbsp;90, RFC&nbsp;3864, September&nbsp;2004.
1510            </td>
1511         </tr>
1512      </table>
1513      <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1514      <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
1515            (editor)
1516            <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><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>
1517      <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><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>
1518      <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><span class="email">JeffMogul@acm.org</span></a></span></address>
1519      <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><span class="email">henrikn@microsoft.com</span></a></span></address>
1520      <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><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>
1521      <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><span class="email">paulle@microsoft.com</span></a></span></address>
1522      <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><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>
1523      <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
1524            (editor)
1525            <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><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>
1526      <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
1527            (editor)
1528            <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><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>
1529      <h1 id="rfc.section.A"><a href="#rfc.section.A">A.</a>&nbsp;<a id="compatibility" href="#compatibility">Compatibility with Previous Versions</a></h1>
1530      <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>
1531      <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">3.1</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.6" title="Cache-Control">4.2</a>.
1532      </p>
1533      <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
1534         for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are
1535         computed. (see also <a href="#Part1" id="rfc.xref.Part1.16"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a> and <a href="#Part5" id="rfc.xref.Part5.4"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>)
1536      </p>
1537      <p id="rfc.section.A.1.p.3">Proxies should be able to add Content-Length when appropriate.</p>
1538      <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
1539         needed headers in a 206 response, this problem can be avoided. (<a href="#combining.headers" title="Combining Responses">Section&nbsp;3.7</a>)
1540      </p>
1541      <p id="rfc.section.A.1.p.5">The Cache-Control: max-age directive was not properly defined for responses.</p>
1542      <p id="rfc.section.A.1.p.6">Warnings could be cached incorrectly, or not updated appropriately. <a href="#expiration.model" title="Freshness Model">3.3</a>, <a href="#combining.headers" title="Combining Responses">3.7</a>, <a href="#header.cache-control" id="rfc.xref.header.cache-control.7" title="Cache-Control">4.2</a>, and <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">4.6</a>) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.
1543      </p>
1544      <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>
1545      <p id="rfc.section.A.2.p.1">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;3.5</a>)
1546      </p>
1547      <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
1548      <h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a>&nbsp;Since RFC2616
1549      </h2>
1550      <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>.
1551      </p>
1552      <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a>&nbsp;Since draft-ietf-httpbis-p6-cache-00
1553      </h2>
1554      <p id="rfc.section.B.2.p.1">Closed issues: </p>
1555      <ul>
1556         <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;)
1557         </li>
1558         <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;)
1559         </li>
1560         <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"
1561         </li>
1562         <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"
1563         </li>
1564         <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"
1565         </li>
1566         <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"
1567         </li>
1568         <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"
1569         </li>
1570         <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"
1571         </li>
1572         <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"
1573         </li>
1574      </ul>
1575      <p id="rfc.section.B.2.p.2">Other changes: </p>
1576      <ul>
1577         <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;)
1578         </li>
1579      </ul>
1580      <h2 id="rfc.section.B.3"><a href="#rfc.section.B.3">B.3</a>&nbsp;Since draft-ietf-httpbis-p6-cache-01
1581      </h2>
1582      <p id="rfc.section.B.3.p.1">Closed issues: </p>
1583      <ul>
1584         <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82</a>&gt;: "rel_path not used"
1585         </li>
1586      </ul>
1587      <p id="rfc.section.B.3.p.2">Other changes: </p>
1588      <ul>
1589         <li>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (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;)
1590         </li>
1591         <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li>
1592      </ul>
1593      <h2 id="rfc.section.B.4"><a href="#rfc.section.B.4">B.4</a>&nbsp;<a id="changes.since.02" href="#changes.since.02">Since draft-ietf-httpbis-p6-cache-02</a></h2>
1594      <p id="rfc.section.B.4.p.1">Ongoing work on IANA Message Header Registration (&lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40</a>&gt;):
1595      </p>
1596      <ul>
1597         <li>Reference RFC 3984, and update header registrations for headers defined in this document.</li>
1598      </ul>
1599      <h2 id="rfc.section.B.5"><a href="#rfc.section.B.5">B.5</a>&nbsp;<a id="changes.since.03" href="#changes.since.03">Since draft-ietf-httpbis-p6-cache-03</a></h2>
1600      <p id="rfc.section.B.5.p.1">Closed issues: </p>
1601      <ul>
1602         <li> &lt;<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/106</a>&gt;: "Vary header classification"
1603         </li>
1604      </ul>
1605      <h2 id="rfc.section.B.6"><a href="#rfc.section.B.6">B.6</a>&nbsp;<a id="changes.since.04" href="#changes.since.04">Since draft-ietf-httpbis-p6-cache-04</a></h2>
1606      <h2 id="rfc.section.B.7"><a href="#rfc.section.B.7">B.7</a>&nbsp;<a id="changes.since.05" href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></h2>
1607      <p id="rfc.section.B.7.p.1">This is a total rewrite.</p>
1608      <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
1609      <p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the
1610         authors retain all their rights.
1611      </p>
1612      <p>This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION
1613         HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE
1614         DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
1615         WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
1616      </p>
1617      <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
1618      <p>The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might
1619         be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any
1620         license under such rights might or might not be available; nor does it represent that it has made any independent effort to
1621         identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and
1622         BCP 79.
1623      </p>
1624      <p>Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result
1625         of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users
1626         of this specification can be obtained from the IETF on-line IPR repository at &lt;<a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>&gt;.
1627      </p>
1628      <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
1629         rights that may cover technology that may be required to implement this standard. Please address the information to the IETF
1630         at <a href="mailto:ietf-ipr@ietf.org">ietf-ipr@ietf.org</a>.
1631      </p>
1632      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1633      <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> 
1634      </p>
1635      <div class="print2col">
1636         <ul class="ind">
1637            <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
1638                  <li class="indline1">age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1">1.2</a></li>
1639                  <li class="indline1">Age header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">3.2</a>, <a class="iref" href="#rfc.iref.a.2"><b>4.1</b></a>, <a class="iref" href="#rfc.xref.header.age.2">6.1</a></li>
1640               </ul>
1641            </li>
1642            <li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind">
1643                  <li class="indline1">cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1">1.1</a></li>
1644                  <li class="indline1">Cache Directives&nbsp;&nbsp;
1645                     <ul class="ind">
1646                        <li class="indline1">max-age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.6"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.c.17"><b>4.2.2</b></a></li>
1647                        <li class="indline1">max-stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>4.2.1</b></a></li>
1648                        <li class="indline1">min-fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>4.2.1</b></a></li>
1649                        <li class="indline1">must-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.15"><b>4.2.2</b></a></li>
1650                        <li class="indline1">no-cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.c.13"><b>4.2.2</b></a></li>
1651                        <li class="indline1">no-store&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.c.14"><b>4.2.2</b></a></li>
1652                        <li class="indline1">no-transform&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.9"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.c.19"><b>4.2.2</b></a></li>
1653                        <li class="indline1">only-if-cached&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.10"><b>4.2.1</b></a></li>
1654                        <li class="indline1">private&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.12"><b>4.2.2</b></a></li>
1655                        <li class="indline1">proxy-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.16"><b>4.2.2</b></a></li>
1656                        <li class="indline1">public&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.11"><b>4.2.2</b></a></li>
1657                        <li class="indline1">s-maxage&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.18"><b>4.2.2</b></a></li>
1658                     </ul>
1659                  </li>
1660                  <li class="indline1">Cache-Control header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">3.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">3.2</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">3.2</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">3.2</a>, <a class="iref" href="#rfc.iref.c.3"><b>4.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.5">6.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">A.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">A.1</a></li>
1661                  <li class="indline1">cacheable&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2">1.2</a></li>
1662               </ul>
1663            </li>
1664            <li class="indline0"><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul class="ind">
1665                  <li class="indline1">Expires header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">3.3</a>, <a class="iref" href="#rfc.xref.header.expires.2">3.3.1</a>, <a class="iref" href="#rfc.iref.e.2"><b>4.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.3">6.1</a></li>
1666                  <li class="indline1">explicit expiration time&nbsp;&nbsp;<a class="iref" href="#rfc.iref.e.1">1.2</a></li>
1667               </ul>
1668            </li>
1669            <li class="indline0"><a id="rfc.index.F" href="#rfc.index.F"><b>F</b></a><ul class="ind">
1670                  <li class="indline1">first-hand&nbsp;&nbsp;<a class="iref" href="#rfc.iref.f.1">1.2</a></li>
1671                  <li class="indline1">fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.f.3">1.2</a></li>
1672                  <li class="indline1">freshness lifetime&nbsp;&nbsp;<a class="iref" href="#rfc.iref.f.2">1.2</a></li>
1673               </ul>
1674            </li>
1675            <li class="indline0"><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul class="ind">
1676                  <li class="indline1"><tt>Grammar</tt>&nbsp;&nbsp;
1677                     <ul class="ind">
1678                        <li class="indline1"><tt>Age</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1"><b>4.1</b></a></li>
1679                        <li class="indline1"><tt>age-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>4.1</b></a></li>
1680                        <li class="indline1"><tt>Cache-Control</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>4.2</b></a></li>
1681                        <li class="indline1"><tt>cache-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>4.2</b></a></li>
1682                        <li class="indline1"><tt>cache-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>4.2</b></a></li>
1683                        <li class="indline1"><tt>cache-request-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>4.2.1</b></a></li>
1684                        <li class="indline1"><tt>cache-response-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>4.2.2</b></a></li>
1685                        <li class="indline1"><tt>delta-seconds</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>4.1</b></a></li>
1686                        <li class="indline1"><tt>Expires</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>4.3</b></a></li>
1687                        <li class="indline1"><tt>extension-pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>4.4</b></a></li>
1688                        <li class="indline1"><tt>Pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>4.4</b></a></li>
1689                        <li class="indline1"><tt>pragma-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>4.4</b></a></li>
1690                        <li class="indline1"><tt>Vary</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>4.5</b></a></li>
1691                        <li class="indline1"><tt>warn-agent</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>4.6</b></a></li>
1692                        <li class="indline1"><tt>warn-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>4.6</b></a></li>
1693                        <li class="indline1"><tt>warn-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>4.6</b></a></li>
1694                        <li class="indline1"><tt>warn-text</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>4.6</b></a></li>
1695                        <li class="indline1"><tt>Warning</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>4.6</b></a></li>
1696                        <li class="indline1"><tt>warning-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>4.6</b></a></li>
1697                     </ul>
1698                  </li>
1699               </ul>
1700            </li>
1701            <li class="indline0"><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul class="ind">
1702                  <li class="indline1">Headers&nbsp;&nbsp;
1703                     <ul class="ind">
1704                        <li class="indline1">Age&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">3.2</a>, <a class="iref" href="#rfc.iref.h.2"><b>4.1</b></a>, <a class="iref" href="#rfc.xref.header.age.2">6.1</a></li>
1705                        <li class="indline1">Cache-Control&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">3.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">3.2</a>, <a class="iref" href="#rfc.xref.header.cache-control.3">3.2</a>, <a class="iref" href="#rfc.xref.header.cache-control.4">3.2</a>, <a class="iref" href="#rfc.iref.h.3"><b>4.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.5">6.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.6">A.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.7">A.1</a></li>
1706                        <li class="indline1">Expires&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">3.3</a>, <a class="iref" href="#rfc.xref.header.expires.2">3.3.1</a>, <a class="iref" href="#rfc.iref.h.4"><b>4.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.3">6.1</a></li>
1707                        <li class="indline1">Pragma&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">3.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">4.2</a>, <a class="iref" href="#rfc.iref.h.5"><b>4.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">6.1</a></li>
1708                        <li class="indline1">Vary&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">3.6</a>, <a class="iref" href="#rfc.iref.h.6"><b>4.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">6.1</a></li>
1709                        <li class="indline1">Warning&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">3.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">3.7</a>, <a class="iref" href="#rfc.iref.h.7"><b>4.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.3">6.1</a>, <a class="iref" href="#rfc.xref.header.warning.4">A.1</a></li>
1710                     </ul>
1711                  </li>
1712                  <li class="indline1">heuristic expiration time&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.1">1.2</a></li>
1713               </ul>
1714            </li>
1715            <li class="indline0"><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul class="ind">
1716                  <li class="indline1"><em>ISO-8859-1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ISO-8859-1.1">4.6</a>, <a class="iref" href="#ISO-8859-1"><b>9.1</b></a></li>
1717               </ul>
1718            </li>
1719            <li class="indline0"><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul class="ind">
1720                  <li class="indline1">max-age&nbsp;&nbsp;
1721                     <ul class="ind">
1722                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.m.5"><b>4.2.2</b></a></li>
1723                     </ul>
1724                  </li>
1725                  <li class="indline1">max-stale&nbsp;&nbsp;
1726                     <ul class="ind">
1727                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>4.2.1</b></a></li>
1728                     </ul>
1729                  </li>
1730                  <li class="indline1">min-fresh&nbsp;&nbsp;
1731                     <ul class="ind">
1732                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>4.2.1</b></a></li>
1733                     </ul>
1734                  </li>
1735                  <li class="indline1">must-revalidate&nbsp;&nbsp;
1736                     <ul class="ind">
1737                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.4"><b>4.2.2</b></a></li>
1738                     </ul>
1739                  </li>
1740               </ul>
1741            </li>
1742            <li class="indline0"><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul class="ind">
1743                  <li class="indline1">no-cache&nbsp;&nbsp;
1744                     <ul class="ind">
1745                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.n.4"><b>4.2.2</b></a></li>
1746                     </ul>
1747                  </li>
1748                  <li class="indline1">no-store&nbsp;&nbsp;
1749                     <ul class="ind">
1750                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.n.5"><b>4.2.2</b></a></li>
1751                     </ul>
1752                  </li>
1753                  <li class="indline1">no-transform&nbsp;&nbsp;
1754                     <ul class="ind">
1755                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>4.2.1</b></a>, <a class="iref" href="#rfc.iref.n.6"><b>4.2.2</b></a></li>
1756                     </ul>
1757                  </li>
1758               </ul>
1759            </li>
1760            <li class="indline0"><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul class="ind">
1761                  <li class="indline1">only-if-cached&nbsp;&nbsp;
1762                     <ul class="ind">
1763                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>4.2.1</b></a></li>
1764                     </ul>
1765                  </li>
1766               </ul>
1767            </li>
1768            <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
1769                  <li class="indline1"><em>Part1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">2</a>, <a class="iref" href="#rfc.xref.Part1.2">2</a>, <a class="iref" href="#rfc.xref.Part1.3">2</a>, <a class="iref" href="#rfc.xref.Part1.4">2</a>, <a class="iref" href="#rfc.xref.Part1.5">2</a>, <a class="iref" href="#rfc.xref.Part1.6">2</a>, <a class="iref" href="#rfc.xref.Part1.7">2</a>, <a class="iref" href="#rfc.xref.Part1.8">2</a>, <a class="iref" href="#rfc.xref.Part1.9">2</a>, <a class="iref" href="#rfc.xref.Part1.10">2</a>, <a class="iref" href="#rfc.xref.Part1.11">2</a>, <a class="iref" href="#rfc.xref.Part1.12">2</a>, <a class="iref" href="#rfc.xref.Part1.13">3.3.2</a>, <a class="iref" href="#rfc.xref.Part1.14">3.6</a>, <a class="iref" href="#rfc.xref.Part1.15">4.3</a>, <a class="iref" href="#Part1"><b>9.1</b></a>, <a class="iref" href="#rfc.xref.Part1.16">A.1</a><ul class="ind">
1770                        <li class="indline1"><em>Section 1.2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">2</a></li>
1771                        <li class="indline1"><em>Section 1.2.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.2">2</a>, <a class="iref" href="#rfc.xref.Part1.3">2</a>, <a class="iref" href="#rfc.xref.Part1.4">2</a>, <a class="iref" href="#rfc.xref.Part1.5">2</a>, <a class="iref" href="#rfc.xref.Part1.6">2</a>, <a class="iref" href="#rfc.xref.Part1.7">2</a></li>
1772                        <li class="indline1"><em>Section 2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.10">2</a>, <a class="iref" href="#rfc.xref.Part1.12">2</a></li>
1773                        <li class="indline1"><em>Section 3.2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.9">2</a>, <a class="iref" href="#rfc.xref.Part1.15">4.3</a></li>
1774                        <li class="indline1"><em>Section 4.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.8">2</a>, <a class="iref" href="#rfc.xref.Part1.14">3.6</a></li>
1775                        <li class="indline1"><em>Section 8.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.13">3.3.2</a></li>
1776                        <li class="indline1"><em>Section 8.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.11">2</a></li>
1777                     </ul>
1778                  </li>
1779                  <li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">3.5</a>, <a class="iref" href="#Part2"><b>9.1</b></a><ul class="ind">
1780                        <li class="indline1"><em>Section 7.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">3.5</a></li>
1781                     </ul>
1782                  </li>
1783                  <li class="indline1"><em>Part3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">3.6</a>, <a class="iref" href="#Part3"><b>9.1</b></a>, <a class="iref" href="#rfc.xref.Part3.2">A.1</a><ul class="ind">
1784                        <li class="indline1"><em>Section 4.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part3.1">3.6</a></li>
1785                     </ul>
1786                  </li>
1787                  <li class="indline1"><em>Part4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">3.3.1.1</a>, <a class="iref" href="#rfc.xref.Part4.2">3.4</a>, <a class="iref" href="#Part4"><b>9.1</b></a><ul class="ind">
1788                        <li class="indline1"><em>Section 6.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">3.3.1.1</a></li>
1789                     </ul>
1790                  </li>
1791                  <li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">3.1.1</a>, <a class="iref" href="#rfc.xref.Part5.2">3.1.1</a>, <a class="iref" href="#rfc.xref.Part5.3">3.7</a>, <a class="iref" href="#Part5"><b>9.1</b></a>, <a class="iref" href="#rfc.xref.Part5.4">A.1</a><ul class="ind">
1792                        <li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.2">3.1.1</a>, <a class="iref" href="#rfc.xref.Part5.3">3.7</a></li>
1793                     </ul>
1794                  </li>
1795                  <li class="indline1"><em>Part7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">3.2</a>, <a class="iref" href="#rfc.xref.Part7.2">4.2.2</a>, <a class="iref" href="#Part7"><b>9.1</b></a><ul class="ind">
1796                        <li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">3.2</a>, <a class="iref" href="#rfc.xref.Part7.2">4.2.2</a></li>
1797                     </ul>
1798                  </li>
1799                  <li class="indline1">Pragma header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">3.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">4.2</a>, <a class="iref" href="#rfc.iref.p.4"><b>4.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">6.1</a></li>
1800                  <li class="indline1">private&nbsp;&nbsp;
1801                     <ul class="ind">
1802                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.2"><b>4.2.2</b></a></li>
1803                     </ul>
1804                  </li>
1805                  <li class="indline1">proxy-revalidate&nbsp;&nbsp;
1806                     <ul class="ind">
1807                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.3"><b>4.2.2</b></a></li>
1808                     </ul>
1809                  </li>
1810                  <li class="indline1">public&nbsp;&nbsp;
1811                     <ul class="ind">
1812                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>4.2.2</b></a></li>
1813                     </ul>
1814                  </li>
1815               </ul>
1816            </li>
1817            <li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind">
1818                  <li class="indline1"><em>RFC1305</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1305.1">3.3.2</a>, <a class="iref" href="#RFC1305"><b>9.2</b></a></li>
1819                  <li class="indline1"><em>RFC2047</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2047.1">4.6</a>, <a class="iref" href="#RFC2047"><b>9.1</b></a></li>
1820                  <li class="indline1"><em>RFC2119</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2119.1">1.3</a>, <a class="iref" href="#RFC2119"><b>9.1</b></a></li>
1821                  <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#RFC2616"><b>9.2</b></a>, <a class="iref" href="#rfc.xref.RFC2616.1">B.1</a></li>
1822                  <li class="indline1"><em>RFC3864</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC3864.1">6.1</a>, <a class="iref" href="#RFC3864"><b>9.2</b></a></li>
1823               </ul>
1824            </li>
1825            <li class="indline0"><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul class="ind">
1826                  <li class="indline1">s-maxage&nbsp;&nbsp;
1827                     <ul class="ind">
1828                        <li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.2"><b>4.2.2</b></a></li>
1829                     </ul>
1830                  </li>
1831                  <li class="indline1">stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1">1.2</a></li>
1832               </ul>
1833            </li>
1834            <li class="indline0"><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul class="ind">
1835                  <li class="indline1">validator&nbsp;&nbsp;<a class="iref" href="#rfc.iref.v.1">1.2</a>, <a class="iref" href="#rfc.iref.v.2">1.2</a></li>
1836                  <li class="indline1">Vary header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">3.6</a>, <a class="iref" href="#rfc.iref.v.3"><b>4.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">6.1</a></li>
1837               </ul>
1838            </li>
1839            <li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind">
1840                  <li class="indline1">Warning header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">3.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">3.7</a>, <a class="iref" href="#rfc.iref.w.1"><b>4.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.3">6.1</a>, <a class="iref" href="#rfc.xref.header.warning.4">A.1</a></li>
1841               </ul>
1842            </li>
1843         </ul>
1844      </div>
1845   </body>
1846</html>
Note: See TracBrowser for help on using the repository browser.