source: draft-ietf-httpbis/latest/p6-cache.xml @ 1472

Last change on this file since 1472 was 1472, checked in by julian.reschke@…, 8 years ago

Update to latest version of rfc2629.xslt, add next/prev link relations (does not affect TXT version)

  • Property svn:eol-style set to native
File size: 117.2 KB
Line 
1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "November">
16  <!ENTITY ID-YEAR "2011">
17  <!ENTITY architecture               "<xref target='Part1' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY field-rules                 "<xref target='Part1' x:rel='#field.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY http-date                   "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-date                 "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42]>
43<?rfc toc="yes" ?>
44<?rfc symrefs="yes" ?>
45<?rfc sortrefs="yes" ?>
46<?rfc compact="yes"?>
47<?rfc subcompact="no" ?>
48<?rfc linkmailto="no" ?>
49<?rfc editing="no" ?>
50<?rfc comments="yes"?>
51<?rfc inline="yes"?>
52<?rfc rfcedstyle="yes"?>
53<?rfc-ext allow-markup-in-artwork="yes" ?>
54<?rfc-ext include-references-in-index="yes" ?>
55<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
56  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
57<x:link rel="prev" basename="p5-range"/>
58<x:link rel="next" basename="p7-auth"/>
59<front>
60
61  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
62
63  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
64    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
65    <address>
66      <postal>
67        <street>345 Park Ave</street>
68        <city>San Jose</city>
69        <region>CA</region>
70        <code>95110</code>
71        <country>USA</country>
72      </postal>
73      <email>fielding@gbiv.com</email>
74      <uri>http://roy.gbiv.com/</uri>
75    </address>
76  </author>
77
78  <author initials="J." surname="Gettys" fullname="Jim Gettys">
79    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
80    <address>
81      <postal>
82        <street>21 Oak Knoll Road</street>
83        <city>Carlisle</city>
84        <region>MA</region>
85        <code>01741</code>
86        <country>USA</country>
87      </postal>
88      <email>jg@freedesktop.org</email>
89      <uri>http://gettys.wordpress.com/</uri>
90    </address>
91  </author>
92
93  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
94    <organization abbrev="HP">Hewlett-Packard Company</organization>
95    <address>
96      <postal>
97        <street>HP Labs, Large Scale Systems Group</street>
98        <street>1501 Page Mill Road, MS 1177</street>
99        <city>Palo Alto</city>
100        <region>CA</region>
101        <code>94304</code>
102        <country>USA</country>
103      </postal>
104      <email>JeffMogul@acm.org</email>
105    </address>
106  </author>
107
108  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
109    <organization abbrev="Microsoft">Microsoft Corporation</organization>
110    <address>
111      <postal>
112        <street>1 Microsoft Way</street>
113        <city>Redmond</city>
114        <region>WA</region>
115        <code>98052</code>
116        <country>USA</country>
117      </postal>
118      <email>henrikn@microsoft.com</email>
119    </address>
120  </author>
121
122  <author fullname="Larry Masinter" initials="L." surname="Masinter">
123    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
124    <address>
125      <postal>
126        <street>345 Park Ave</street>
127        <city>San Jose</city>
128        <region>CA</region>
129        <code>95110</code>
130        <country>USA</country>
131      </postal>
132      <email>LMM@acm.org</email>
133      <uri>http://larry.masinter.net/</uri>
134    </address>
135  </author>
136
137  <author fullname="Paul J. Leach" initials="P." surname="Leach">
138    <organization abbrev="Microsoft">Microsoft Corporation</organization>
139    <address>
140      <postal>
141        <street>1 Microsoft Way</street>
142        <city>Redmond</city>
143        <region>WA</region>
144        <code>98052</code>
145      </postal>
146      <email>paulle@microsoft.com</email>
147    </address>
148  </author>
149
150  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
151    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
152    <address>
153      <postal>
154        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
155        <street>The Stata Center, Building 32</street>
156        <street>32 Vassar Street</street>
157        <city>Cambridge</city>
158        <region>MA</region>
159        <code>02139</code>
160        <country>USA</country>
161      </postal>
162      <email>timbl@w3.org</email>
163      <uri>http://www.w3.org/People/Berners-Lee/</uri>
164    </address>
165  </author>
166
167  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
168    <organization abbrev="W3C">World Wide Web Consortium</organization>
169    <address>
170      <postal>
171        <street>W3C / ERCIM</street>
172        <street>2004, rte des Lucioles</street>
173        <city>Sophia-Antipolis</city>
174        <region>AM</region>
175        <code>06902</code>
176        <country>France</country>
177      </postal>
178      <email>ylafon@w3.org</email>
179      <uri>http://www.raubacapeu.net/people/yves/</uri>
180    </address>
181  </author>
182
183  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
184    <organization>Rackspace</organization>
185    <address>
186      <email>mnot@mnot.net</email>
187      <uri>http://www.mnot.net/</uri>
188    </address>
189  </author>
190
191  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
192    <organization abbrev="greenbytes">greenbytes GmbH</organization>
193    <address>
194      <postal>
195        <street>Hafenweg 16</street>
196        <city>Muenster</city><region>NW</region><code>48155</code>
197        <country>Germany</country>
198      </postal>
199      <phone>+49 251 2807760</phone>
200      <facsimile>+49 251 2807761</facsimile>
201      <email>julian.reschke@greenbytes.de</email>
202      <uri>http://greenbytes.de/tech/webdav/</uri>
203    </address>
204  </author>
205
206  <date month="&ID-MONTH;" year="&ID-YEAR;" />
207  <workgroup>HTTPbis Working Group</workgroup>
208
209<abstract>
210<t>
211   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
212   distributed, collaborative, hypertext information systems. HTTP has been in
213   use by the World Wide Web global information initiative since 1990. This
214   document is Part 6 of the seven-part specification that defines the protocol
215   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
216</t>
217<t>
218   Part 6 defines requirements on HTTP caches and the associated header fields
219   that control cache behavior or indicate cacheable response messages.
220</t>
221</abstract>
222
223<note title="Editorial Note (To be removed by RFC Editor)">
224  <t>
225    Discussion of this draft should take place on the HTTPBIS working group
226    mailing list (ietf-http-wg@w3.org), which is archived at
227    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
228  </t>
229  <t>
230    The current issues list is at
231    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
232    documents (including fancy diffs) can be found at
233    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
234  </t>
235  <t>
236    The changes in this draft are summarized in <xref target="changes.since.17"/>.
237  </t>
238</note>
239
240   </front>
241   <middle>
242
243<section anchor="caching" title="Introduction">
244<t>
245   HTTP is typically used for distributed information systems, where
246   performance can be improved by the use of response caches. This document
247   defines aspects of HTTP/1.1 related to caching and reusing response
248   messages.
249</t>
250
251<section anchor="intro.purpose" title="Purpose">
252<iref item="cache" />
253<t>
254   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
255   subsystem that controls its message storage, retrieval, and deletion. A
256   cache stores cacheable responses in order to reduce the response time and
257   network bandwidth consumption on future, equivalent requests. Any client or
258   server &MAY; employ a cache, though a cache cannot be used by a server that
259   is acting as a tunnel.
260</t>
261<t>
262   The goal of caching in HTTP/1.1 is to significantly improve performance
263   by reusing a prior response message to satisfy a current request.
264   A stored response is considered "fresh", as defined in
265   <xref target="expiration.model" />, if the response can be reused without
266   "validation" (checking with the origin server to see if the cached response
267   remains valid for this request).  A fresh cache response can therefore
268   reduce both latency and network transfers each time it is reused.
269   When a cached response is not fresh, it might still be reusable if it can
270   be freshened by validation (<xref target="validation.model" />) or if the
271   origin is unavailable.
272</t>
273</section>
274
275<section anchor="intro.terminology" title="Terminology">
276<t>
277   This specification uses a number of terms to refer to the roles played by
278   participants in, and objects of, HTTP caching.
279</t>
280<t>
281   <iref item="cache" />
282   <x:dfn>cache</x:dfn>
283   <list>
284      <t>A conformant implementation of a HTTP cache. Note that this implies
285        an HTTP/1.1 cache; this specification does not define conformance
286        for HTTP/1.0 caches.</t>
287   </list>
288</t>
289<t anchor="shared.and.non-shared.caches">
290   <iref item="shared cache" />
291   <x:dfn>shared cache</x:dfn>
292   <list>
293      <t>A cache that stores responses to be reused by more than one user;
294         usually (but not always) deployed as part of an intermediary.</t>
295   </list>
296</t>
297<t>
298   <iref item="private cache" />
299   <x:dfn>private cache</x:dfn>
300   <list>
301      <t>A cache that is dedicated to a single user.</t>
302   </list>
303</t>
304<t>
305   <iref item="cacheable" />
306   <x:dfn>cacheable</x:dfn>
307   <list>
308      <t>A response is cacheable if a cache is allowed to store a copy of the
309      response message for use in answering subsequent requests. Even when a
310      response is cacheable, there might be additional constraints on whether
311      a cache can use the stored copy to satisfy a particular request.</t>
312   </list>
313</t>
314<t>
315   <iref item="explicit expiration time" />
316   <x:dfn>explicit expiration time</x:dfn>
317   <list>
318      <t>The time at which the origin server intends that a representation
319      no longer be returned by a cache without further validation.</t>
320   </list>
321</t>
322<t>
323   <iref item="heuristic expiration time" />
324   <x:dfn>heuristic expiration time</x:dfn>
325   <list>
326      <t>An expiration time assigned by a cache when no explicit expiration
327      time is available.</t>
328   </list>
329</t>
330<t>
331   <iref item="age" />
332   <x:dfn>age</x:dfn>
333   <list>
334      <t>The age of a response is the time since it was sent by, or
335      successfully validated with, the origin server.</t>
336   </list>
337</t>
338<t>
339   <iref item="first-hand" />
340   <x:dfn>first-hand</x:dfn>
341   <list>
342      <t>A response is first-hand if the freshness model is not in use; i.e.,
343      its age is 0.</t>
344   </list>
345</t>
346<t>
347   <iref item="freshness lifetime" />
348   <x:dfn>freshness lifetime</x:dfn>
349   <list>
350      <t>The length of time between the generation of a response and its
351      expiration time.</t>
352   </list>
353</t>
354<t>
355   <iref item="fresh" />
356   <x:dfn>fresh</x:dfn>
357   <list>
358      <t>A response is fresh if its age has not yet exceeded its freshness
359      lifetime.</t>
360   </list>
361</t>
362<t>
363   <iref item="stale" />
364   <x:dfn>stale</x:dfn>
365   <list>
366      <t>A response is stale if its age has passed its freshness lifetime
367      (either explicit or heuristic).</t>
368   </list>
369</t>
370<t>
371   <iref item="validator" />
372   <x:dfn>validator</x:dfn>
373   <list>
374      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
375      is used to find out whether a stored response is an equivalent copy of
376      a representation. See &weak-and-strong;.</t>
377   </list>
378</t>
379<t>
380   <iref item="strong validator" />
381   <iref item="validator" subitem="strong" />
382   <x:dfn>strong validator</x:dfn>
383   <list>
384      <t>A validator that is defined by the origin server such that its
385         current value will change if the representation body changes; i.e.,
386         an entity-tag that is not marked as weak (&entity-tags;) or,
387         if no entity-tag is provided, a Last-Modified value that is strong
388         in the sense defined by &lastmod-comparison;.</t>
389   </list>
390</t>
391</section>
392
393<section title="Conformance and Error Handling" anchor="intro.conformance.and.error.handling">
394<t>
395   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
396   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
397   document are to be interpreted as described in <xref target="RFC2119"/>.
398</t>
399<t>
400   This document defines conformance criteria for several roles in HTTP
401   communication, including Senders, Recipients, Clients, Servers, User-Agents,
402   Origin Servers, Intermediaries, Proxies and Gateways. See &architecture;
403   for definitions of these terms.
404</t>
405<t>
406   An implementation is considered conformant if it complies with all of the
407   requirements associated with its role(s). Note that SHOULD-level requirements
408   are relevant here, unless one of the documented exceptions is applicable.
409</t>
410<t>
411   This document also uses ABNF to define valid protocol elements
412   (<xref target="notation"/>). In addition to the prose requirements placed
413   upon them, Senders &MUST-NOT; generate protocol elements that are invalid.
414</t>
415<t>
416   Unless noted otherwise, Recipients &MAY; take steps to recover a usable
417   protocol element from an invalid construct. However, HTTP does not define
418   specific error handling mechanisms, except in cases where it has direct
419   impact on security. This is because different uses of the protocol require
420   different error handling strategies; for example, a Web browser may wish to
421   transparently recover from a response where the Location header field
422   doesn't parse according to the ABNF, whereby in a systems control protocol
423   using HTTP, this type of error recovery could lead to dangerous consequences.
424</t>
425</section>
426
427<section title="Syntax Notation" anchor="notation">
428   <x:anchor-alias value="ALPHA"/>
429   <x:anchor-alias value="CR"/>
430   <x:anchor-alias value="DIGIT"/>
431   <x:anchor-alias value="DQUOTE"/>
432   <x:anchor-alias value="LF"/>
433   <x:anchor-alias value="OCTET"/>
434   <x:anchor-alias value="SP"/>
435   <x:anchor-alias value="VCHAR"/>
436<t>
437   This specification uses the ABNF syntax defined in &notation; (which
438   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
439   <xref target="collected.abnf"/> shows the collected ABNF, with the list
440   rule expanded.
441</t>
442<t>
443   The following core rules are included by reference, as defined in <xref
444   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
445   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
446   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
447   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
448</t>
449
450<section title="Core Rules" anchor="core.rules">
451   <x:anchor-alias value="quoted-string"/>
452   <x:anchor-alias value="token"/>
453   <x:anchor-alias value="OWS"/>
454<t>
455   The core rules below are defined in <xref target="Part1"/>:
456</t>
457<figure><artwork type="abnf2616">
458  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
459  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-rules;&gt;
460  <x:ref>token</x:ref>         = &lt;token, defined in &field-rules;&gt;
461</artwork></figure>
462</section>
463
464<section title="ABNF Rules defined in other Parts of the Specification"
465    anchor="abnf.dependencies">
466   <x:anchor-alias value="field-name"/>
467   <x:anchor-alias value="HTTP-date"/>
468   <x:anchor-alias value="port"/>
469   <x:anchor-alias value="pseudonym"/>
470   <x:anchor-alias value="uri-host"/>
471<t>
472   The ABNF rules below are defined in other parts:
473</t>
474<figure><!--Part1--><artwork type="abnf2616">
475  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
476  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
477  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
478  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
479  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
480</artwork></figure>
481</section>
482</section>
483
484<section title="Delta Seconds" anchor="delta-seconds">
485<t>
486   The delta-seconds rule specifies a non-negative integer, representing time
487   in seconds.
488</t>
489<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
490  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
491</artwork></figure>
492<t>
493   If an implementation receives a delta-seconds value larger than the largest
494   positive integer it can represent, or if any of its subsequent calculations
495   overflows, it &MUST; consider the value to be 2147483648 (2<x:sup>31</x:sup>).
496   Recipients parsing a delta-seconds value &MUST; use an arithmetic type of
497   at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a
498   value greater than 2147483648.
499</t>
500</section>
501
502</section>
503
504<section anchor="caching.overview" title="Cache Operation">
505<iref item="cache entry" />
506<iref item="cache key" />
507<t>
508   Proper cache operation preserves the semantics of HTTP transfers
509   (&semantics;) while eliminating the transfer of information already held
510   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
511   we assume that reusing the cached response is desirable and that such
512   reuse is the default behavior when no requirement or locally-desired
513   configuration prevents it.  Therefore, HTTP cache requirements are focused
514   on preventing a cache from either storing a non-reusable response or
515   reusing a stored response inappropriately.
516</t>
517<t>
518   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
519   HTTP responses corresponding to prior requests that used the same key.
520   The most common form of cache entry is a successful result of a retrieval
521   request: i.e., a 200 (OK) response containing a representation of the
522   resource identified by the request target.  However, it is also possible
523   to cache negative results (e.g., 404 not found), incomplete results
524   (e.g., 206 partial content), and responses to safe methods other than
525   GET if the method's definition allows such caching and defines something
526   suitable for use as a cache key.
527</t>
528<t>
529   The default <x:dfn>cache key</x:dfn> consists of the request method and
530   target URI.  However, since HTTP caches in common use today are typically
531   limited to caching responses to GET, most implementations simply decline
532   other methods and use only the URI as the key.
533</t>
534<t>
535   If a request target is subject to content negotiation, its cache entry
536   might consist of multiple stored responses, each differentiated by a
537   secondary key for the values of the original request's selecting header
538   fields (<xref target="caching.negotiated.responses"/>).
539</t>
540
541<section anchor="response.cacheability" title="Response Cacheability">
542<t>
543   A cache &MUST-NOT; store a response to any request, unless:
544   <list style="symbols">
545      <t>The request method is understood by the cache and defined as being
546      cacheable, and</t>
547      <t>the response status code is understood by the cache, and</t>
548      <t>the "no-store" cache directive (see <xref
549      target="header.cache-control" />) does not appear in request or response
550      header fields, and</t>
551      <t>the "private" cache response directive (see <xref
552      target="cache-response-directive" /> does not appear in the response, if
553      the cache is shared, and</t>
554      <t>the "Authorization" header field (see &header-authorization;) does not
555      appear in the request, if the cache is shared, unless the response
556      explicitly allows it (see <xref target="caching.authenticated.responses"
557      />), and</t>
558      <t>the response either:
559         <list style="symbols">
560            <t>contains an Expires header field (see <xref target="header.expires"
561            />), or</t>
562            <t>contains a max-age response cache directive (see <xref
563            target="cache-response-directive" />), or</t>
564            <t>contains a s-maxage response cache directive and the cache is
565            shared, or</t>
566            <t>contains a Cache Control Extension (see <xref
567            target="cache.control.extensions" />) that allows it to be cached,
568            or</t>
569            <t>has a status code that can be served with heuristic freshness
570            (see <xref target="heuristic.freshness" />).</t>
571         </list>
572      </t>
573   </list>
574</t>
575<t>
576   Note that any of the requirements listed above can be overridden by a
577   cache-control extension; see <xref target="cache.control.extensions" />.
578</t>
579<t>
580   In this context, a cache has "understood" a request method or a response
581   status code if it recognizes it and implements any cache-specific
582   behavior.
583</t>
584<t>
585   Note that, in normal operation, most caches will not store a response that
586   has neither a cache validator nor an explicit expiration time, as such
587   responses are not usually useful to store. However, caches are not
588   prohibited from storing such responses.
589</t>
590<t>
591   A response message is considered complete when all of the octets
592   indicated by the message framing (&messaging;) are received
593   prior to the connection being closed.
594   If the request is GET, the response status is 200 (OK), and the entire
595   response header block has been received, a cache &MAY; store an incomplete
596   response message-body if the cache entry is recorded as incomplete.
597   Likewise, a 206 (Partial Content) response &MAY; be stored as if it were
598   an incomplete 200 (OK) cache entry.  However, a cache &MUST-NOT; store
599   incomplete or partial content responses if it does not support the Range
600   and Content-Range header fields or if it does not understand the
601   range units used in those fields.
602</t>
603<t>
604   A cache &MAY; complete a stored incomplete response by making a subsequent
605   range request (&partial;) and combining the successful response with the
606   stored entry, as defined in <xref target="combining.responses"/>.
607   A cache &MUST-NOT; use an incomplete response to answer requests
608   unless the response has been made complete or the request is partial and
609   specifies a range that is wholly within the incomplete response.
610   A cache &MUST-NOT; send a partial response to a client without explicitly
611   marking it as such using the 206 (Partial Content) status code.
612</t>
613</section>
614
615
616<section anchor="constructing.responses.from.caches" 
617   title="Constructing Responses from Caches">
618<t>
619   For a presented request, a cache &MUST-NOT; return a stored response,
620   unless:
621   <list style="symbols">
622      <t>The presented effective request URI (&effective-request-uri;) and
623      that of the stored response match, and</t>
624      <t>the request method associated with the stored response allows it to
625      be used for the presented request, and</t>
626      <t>selecting header fields nominated by the stored response (if any)
627      match those presented (see <xref target="caching.negotiated.responses"
628      />), and</t>
629      <t>the presented request and stored response are free from directives
630      that would prevent its use (see <xref target="header.cache-control" />
631      and <xref target="header.pragma"/>), and</t>
632      <t>the stored response is either:
633         <list style="symbols">
634            <t>fresh (see <xref target="expiration.model" />), or</t>
635            <t>allowed to be served stale (see <xref
636            target="serving.stale.responses" />), or</t>
637            <t>successfully validated (see <xref target="validation.model"
638            />).</t>
639         </list>
640      </t>
641  </list>
642</t>
643<t>
644   Note that any of the requirements listed above can be overridden by a
645   cache-control extension; see <xref target="cache.control.extensions" />.
646</t>
647<t>
648   When a stored response is used to satisfy a request without validation,
649   a cache &MUST; include a single Age header field (<xref target="header.age"
650   />) in the response with a value equal to the stored response's
651   current_age; see <xref target="age.calculations" />.
652</t>
653<t>
654   A cache &MUST; write through requests with methods that are unsafe
655   (&safe-methods;) to the origin server; i.e., a cache must not generate
656   a reply to such a request before having forwarded the request and having
657   received a corresponding response.
658</t>
659<t>
660   Also, note that unsafe requests might invalidate already stored responses;
661   see <xref target="invalidation.after.updates.or.deletions" />.
662</t>
663<t>
664   When more than one suitable response is stored, a cache &MUST; use the
665   most recent response (as determined by the Date header field). It can also
666   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
667   no-cache" to disambiguate which response to use.
668</t>
669<t>
670   A cache that does not have a clock available &MUST-NOT; use stored responses
671   without revalidating them on every use. A cache, especially a shared
672   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
673   synchronize its clock with a reliable external standard.
674</t>
675
676</section>
677
678<section anchor="expiration.model" title="Freshness Model">
679<t>
680   When a response is "fresh" in the cache, it can be used to satisfy
681   subsequent requests without contacting the origin server, thereby improving
682   efficiency.
683</t>
684<t>
685   The primary mechanism for determining freshness is for an origin server to
686   provide an explicit expiration time in the future, using either the Expires
687   header field (<xref target="header.expires" />) or the max-age response cache
688   directive (<xref target="cache-response-directive" />). Generally, origin
689   servers will assign future explicit expiration times to responses in the
690   belief that the representation is not likely to change in a semantically
691   significant way before the expiration time is reached.
692</t>
693<t>
694   If an origin server wishes to force a cache to validate every request, it
695   can assign an explicit expiration time in the past to indicate that the
696   response is already stale. Compliant caches will normally validate the
697   cached response before reusing it for subsequent requests (see <xref
698   target="serving.stale.responses" />).
699</t>
700<t>
701   Since origin servers do not always provide explicit expiration times,
702   a cache &MAY; assign a heuristic expiration time when an explicit time is not
703   specified, employing algorithms that use other header field values (such as the
704   Last-Modified time) to estimate a plausible expiration time. This
705   specification does not provide specific algorithms, but does impose
706   worst-case constraints on their results.
707</t>
708<figure>
709<preamble>
710  The calculation to determine if a response is fresh is:
711</preamble>
712<artwork type="code">
713   response_is_fresh = (freshness_lifetime &gt; current_age)
714</artwork>
715</figure>
716<t>
717   The freshness_lifetime is defined in <xref
718   target="calculating.freshness.lifetime" />; the current_age is defined in
719   <xref target="age.calculations" />.
720</t>
721<t>
722   Additionally, clients might need to influence freshness calculation. They
723   can do this using several request cache directives, with the effect of
724   either increasing or loosening constraints on freshness. See <xref
725   target="cache-request-directive" />.
726</t>
727<t>
728   Note that freshness applies only to cache operation; it cannot be used to
729   force a user agent to refresh its display or reload a resource. See <xref
730   target="history.lists" /> for an explanation of the difference between
731   caches and history mechanisms.
732</t>
733
734<section anchor="calculating.freshness.lifetime" 
735   title="Calculating Freshness Lifetime">
736<t>
737   A cache can calculate the freshness lifetime (denoted as
738   freshness_lifetime) of a response by using the first match of:
739   <list style="symbols">
740      <t>If the cache is shared and the s-maxage response cache directive
741      (<xref target="cache-response-directive" />) is present, use its value,
742      or</t>
743      <t>If the max-age response cache directive (<xref
744      target="cache-response-directive" />) is present, use its value, or</t>
745      <t>If the Expires response header field (<xref target="header.expires" />) is
746      present, use its value minus the value of the Date response header field,
747      or</t>
748      <t>Otherwise, no explicit expiration time is present in the response. A
749      heuristic freshness lifetime might be applicable; see <xref
750      target="heuristic.freshness" />.</t>
751   </list>
752</t>
753<t>
754   Note that this calculation is not vulnerable to clock skew, since all of
755   the information comes from the origin server.
756</t>
757
758<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
759<t>
760   If no explicit expiration time is present in a stored response that has a
761   status code whose definition allows heuristic freshness to be used
762   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
763   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
764   use heuristics to determine freshness for responses with status codes that do
765   not explicitly allow it.
766</t>
767<t>
768   When a heuristic is used to calculate freshness lifetime, a cache
769   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
770   its current_age is more than 24 hours and such a warning is not already
771   present.
772</t>
773<t>
774   Also, if the response has a Last-Modified header field
775   (&header-last-modified;), caches are encouraged to use a heuristic
776   expiration value that is no more than some fraction of the interval since
777   that time. A typical setting of this fraction might be 10%.
778</t>
779<x:note>
780   <t>
781      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
782      x:sec="13.9"/>) required that caches do not calculate heuristic
783      freshness for URIs with query components (i.e., those containing '?').
784      In practice, this has not been widely implemented. Therefore, servers
785      are encouraged to send explicit directives (e.g., Cache-Control:
786      no-cache) if they wish to preclude caching.
787   </t>
788</x:note>
789</section>
790</section>
791
792<section anchor="age.calculations" title="Calculating Age">
793<t>
794   HTTP/1.1 uses the Age header field to convey the estimated age of the
795   response message when obtained from a cache. The Age field value is the
796   cache's estimate of the amount of time since the response was generated or
797   validated by the origin server. In essence, the Age value is the sum of the
798   time that the response has been resident in each of the caches along the
799   path from the origin server, plus the amount of time it has been in transit
800   along network paths.
801</t>
802<t>
803   The following data is used for the age calculation:
804</t>
805<t>
806   <x:dfn>age_value</x:dfn>
807   <list>
808      <t>
809         The term "age_value" denotes the value of the Age header field (<xref
810         target="header.age"/>), in a form appropriate for arithmetic
811         operation; or 0, if not available.
812      </t>
813   </list>
814</t>
815<t>
816   <x:dfn>date_value</x:dfn>
817   <list>
818      <t>
819         HTTP/1.1 requires origin servers to send a Date header field, if possible,
820         with every response, giving the time at which the response was
821         generated. The term "date_value" denotes the value of the Date
822         header field, in a form appropriate for arithmetic operations. See
823         &header-date; for the definition of the Date header field, and for
824         requirements regarding responses without it.
825      </t>
826   </list>
827</t>
828<t>
829   <x:dfn>now</x:dfn>
830   <list>
831      <t>
832         The term "now" means "the current value of the clock at the host
833         performing the calculation". A cache &SHOULD; use NTP (<xref
834         target="RFC1305"/>) or some similar protocol to synchronize its
835         clocks to a globally accurate time standard.
836      </t>
837   </list>
838</t>
839<t>
840   <x:dfn>request_time</x:dfn>
841   <list>
842      <t>
843         The current value of the clock at the host at the time the request
844         resulting in the stored response was made.
845      </t>
846   </list>
847</t>
848<t>
849   <x:dfn>response_time</x:dfn>
850   <list>
851      <t>
852         The current value of the clock at the host at the time the response
853         was received.
854      </t>
855   </list>
856</t>
857<t>
858   A response's age can be calculated in two entirely independent ways:
859   <list style="numbers">
860      <t>the "apparent_age": response_time minus date_value, if the local
861      clock is reasonably well synchronized to the origin server's clock. If
862      the result is negative, the result is replaced by zero.</t>
863      <t>the "corrected_age_value", if all of the caches along the response
864      path implement HTTP/1.1. A cache &MUST; interpret this value relative
865      to the time the request was initiated, not the time that the response
866      was received.</t>
867   </list>
868</t>
869<figure>
870<artwork type="code">
871  apparent_age = max(0, response_time - date_value);
872
873  response_delay = response_time - request_time;
874  corrected_age_value = age_value + response_delay; 
875</artwork>
876</figure>
877<figure>
878<preamble>These are combined as</preamble>
879<artwork type="code">
880  corrected_initial_age = max(apparent_age, corrected_age_value);
881</artwork></figure>
882<t>
883   The current_age of a stored response can then be calculated by adding the
884   amount of time (in seconds) since the stored response was last validated by
885   the origin server to the corrected_initial_age.
886</t>
887<figure><artwork type="code">
888  resident_time = now - response_time;
889  current_age = corrected_initial_age + resident_time;
890</artwork></figure>
891<t>
892   Additionally, to avoid common problems in date parsing:
893</t>
894<t>
895  <list style="symbols">
896     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
897        which appears to be more than 50 years in the future is in fact
898        in the past (this helps solve the "year 2000" problem).</t>
899
900     <t>Although all date formats are specified to be case-sensitive,
901        recipients &SHOULD; match day, week and timezone names
902        case-insensitively.</t>
903             
904     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
905        Expires date as earlier than the proper value, but &MUST-NOT;
906        internally represent a parsed Expires date as later than the
907        proper value.</t>
908
909     <t>All expiration-related calculations &MUST; be done in GMT. The
910        local time zone &MUST-NOT; influence the calculation or comparison
911        of an age or expiration time.</t>
912
913     <t>If an HTTP header field incorrectly carries a date value with a time
914        zone other than GMT, it &MUST; be converted into GMT using the
915        most conservative possible conversion.</t>
916  </list>
917</t>
918</section>
919
920<section anchor="serving.stale.responses" title="Serving Stale Responses">
921<t>
922   A "stale" response is one that either has explicit expiry information or is
923   allowed to have heuristic expiry calculated, but is not fresh according to
924   the calculations in <xref target="expiration.model" />.
925</t>
926<t>
927   A cache &MUST-NOT; return a stale response if it is prohibited by an
928   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
929   directive, a "must-revalidate" cache-response-directive, or an applicable
930   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
931   target="cache-response-directive"/>).
932</t>
933<t>
934   A cache &MUST-NOT; return stale responses unless it is disconnected
935   (i.e., it cannot contact the origin server or otherwise find a forward
936   path) or doing so is explicitly allowed (e.g., by the max-stale request
937   directive; see <xref target="cache-request-directive" />).
938</t>
939<t>
940   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
941   <xref target="header.warning" />) to stale responses. Likewise, a cache
942   &SHOULD; add the 112 warn-code to stale responses if the cache is
943   disconnected.
944</t>
945<t>
946   If a cache receives a first-hand response (either an entire response, or a
947   304 (Not Modified) response) that it would normally forward to the
948   requesting client, and the received response is no longer fresh, the cache
949   can forward it to the requesting client without adding a new Warning (but
950   without removing any existing Warning header fields). A cache shouldn't
951   attempt to validate a response simply because that response became stale in
952   transit.
953</t>
954</section>
955</section>
956
957<section anchor="validation.model" title="Validation Model">
958<t>
959   When a cache has one or more stored responses for a requested URI, but
960   cannot serve any of them (e.g., because they are not fresh, or one cannot
961   be selected; see <xref target="caching.negotiated.responses"/>), it can use
962   the conditional request mechanism &conditional; in the forwarded request to
963   give the origin server an opportunity to both select a valid stored
964   response to be used, and to update it. This process is known as
965   "validating" or "revalidating" the stored response.
966</t>
967<t>
968   When sending such a conditional request, a cache adds an If-Modified-Since
969   header field whose value is that of the Last-Modified header field from the
970   selected (see <xref target="caching.negotiated.responses"/>) stored
971   response, if available.
972</t>
973<t>
974   Additionally, a cache can add an If-None-Match header field whose value is
975   that of the ETag header field(s) from all responses stored for the
976   requested URI, if present. However, if any of the stored responses contains
977   only partial content, the cache shouldn't include its entity-tag in the
978   If-None-Match header field unless the request is for a range that would be
979   fully satisfied by that stored response.
980</t>
981
982<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
983
984<t>
985   <list style="symbols">
986      <t>
987         A 304 (Not Modified) response status code indicates that the stored
988         response can be updated and reused; see <xref
989         target="freshening.responses"/>.
990      </t>
991      <t>
992         A full response (i.e., one with a response body) indicates that none
993         of the stored responses nominated in the conditional request is
994         suitable. Instead, the cache can use the full response to
995         satisfy the request and &MAY; replace the stored response(s).
996      </t>
997      <t>
998         However, if a cache receives a 5xx response while attempting to
999         validate a response, it can either forward this response to the
1000         requesting client, or act as if the server failed to respond. In the
1001         latter case, it can return a previously stored response (see <xref
1002         target="serving.stale.responses" />).
1003      </t>
1004   </list>
1005</t>
1006
1007<section anchor="freshening.responses" title="Freshening Responses">
1008<t>
1009   When a cache receives a 304 (Not Modified) response and already has one
1010   or more stored 200 (OK) responses for the same cache key, the cache needs
1011   to identify which of the stored responses are updated by this new response
1012   and then update the stored response(s) with the new information provided in
1013   the 304 response.
1014   <list style="symbols">
1015    <t>
1016     If the new response contains a strong validator, then that strong
1017     validator identifies the selected representation.  All of the stored
1018     responses with the same strong validator are selected.
1019     If none of the stored responses contain the same strong validator, then
1020     this new response corresponds to a new selected representation and
1021     &MUST-NOT; update the existing stored responses.
1022    </t>
1023    <t>
1024     If the new response contains a weak validator and that validator
1025     corresponds to one of the cache's stored responses, then the most
1026     recent of those matching stored responses is selected.
1027    </t>
1028    <t>
1029     If the new response does not include any form of validator, there is
1030     only one stored response, and that stored response also lacks a
1031     validator, then that stored response is selected.
1032    </t>
1033   </list>
1034</t>
1035<t>
1036   If a stored response is selected for update, the cache &MUST;:
1037   <list style="symbols">
1038      <t>delete any Warning header fields in the stored response with
1039         warn-code 1xx (see <xref target="header.warning" />);</t>
1040      <t>retain any Warning header fields in the stored response with
1041         warn-code 2xx; and,</t>
1042      <t>use other header fields provided in the 304 response to replace
1043         all instances of the corresponding header fields in the stored
1044         response.</t>
1045   </list>
1046</t>
1047</section>
1048
1049</section>
1050
1051<section anchor="invalidation.after.updates.or.deletions" 
1052   title="Request Methods that Invalidate">
1053<t>
1054   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1055   have the potential for changing state on the origin server, intervening
1056   caches can use them to keep their contents up-to-date.
1057</t>
1058<t>
1059   A cache &MUST; invalidate the effective Request URI
1060   (&effective-request-uri;) as well as the URI(s) in the Location
1061   and Content-Location header fields (if present) when a non-error
1062   response to a request with an unsafe method is received.
1063</t>
1064<t>
1065   However, a cache &MUST-NOT; invalidate a URI from a
1066   Location or Content-Location header field if the host part of that URI
1067   differs from the host part in the effective request URI
1068   (&effective-request-uri;). This helps prevent denial of service attacks.
1069</t>
1070<t>
1071   A cache &MUST; invalidate the effective request URI
1072   (&effective-request-uri;) when it receives a non-error response
1073   to a request with a method whose safety is unknown.
1074</t>
1075<t>
1076   Here, a "non-error response" is one with a 2xx or 3xx status code.
1077   "Invalidate" means that the cache will either remove all stored
1078   responses related to the effective request URI, or will mark these as
1079   "invalid" and in need of a mandatory validation before they can be returned
1080   in response to a subsequent request.
1081</t>
1082<t>
1083   Note that this does not guarantee that all appropriate responses are
1084   invalidated. For example, the request that caused the change at the origin
1085   server might not have gone through the cache where a response is stored.
1086</t>
1087</section>
1088
1089<section anchor="caching.authenticated.responses" 
1090   title="Shared Caching of Authenticated Responses">
1091
1092<t>
1093   A shared cache &MUST-NOT; use a cached response to a request with an
1094   Authorization header field (&header-authorization;) to satisfy any subsequent
1095   request unless a cache directive that allows such responses to be stored is
1096   present in the response.
1097</t>
1098
1099<t>
1100   In this specification, the following Cache-Control response directives
1101   (<xref target="cache-response-directive"/>) have such an effect:
1102   must-revalidate, public, s-maxage.
1103</t>
1104
1105<t>
1106   Note that cached responses that contain the "must-revalidate" and/or
1107   "s-maxage" response directives are not allowed to be served stale (<xref
1108   target="serving.stale.responses"/>) by shared caches. In particular, a
1109   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1110   used to satisfy a subsequent request without revalidating it on the origin
1111   server.
1112</t>
1113</section>
1114
1115<section anchor="caching.negotiated.responses" 
1116   title="Caching Negotiated Responses">
1117<t>
1118   When a cache receives a request that can be satisfied by a stored response
1119   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
1120   use that response unless all of the selecting header fields nominated by
1121   the Vary header field match in both the original request (i.e., that associated
1122   with the stored response), and the presented request.
1123</t>
1124<t>
1125   The selecting header fields from two requests are defined to match if and
1126   only if those in the first request can be transformed to those in the
1127   second request by applying any of the following:
1128   <list style="symbols">
1129      <t>
1130         adding or removing whitespace, where allowed in the header field's syntax
1131      </t>
1132      <t>
1133         combining multiple header fields with the same field name
1134         (see &header-fields;)
1135      </t>
1136      <t>
1137         normalizing both header field values in a way that is known to have
1138         identical semantics, according to the header field's specification (e.g.,
1139         re-ordering field values when order is not significant;
1140         case-normalization, where values are defined to be case-insensitive)
1141      </t>
1142  </list>
1143</t>
1144<t>
1145   If (after any normalization that might take place) a header field is absent
1146   from a request, it can only match another request if it is also absent
1147   there.
1148</t>
1149<t>
1150   A Vary header field-value of "*" always fails to match, and subsequent
1151   requests to that resource can only be properly interpreted by the origin
1152   server.
1153</t>
1154<t>
1155   The stored response with matching selecting header fields is known as the
1156   selected response.
1157</t>
1158<t>
1159   If multiple selected responses are available, the most recent response
1160   (as determined by the Date header field) is used; see <xref 
1161   target="constructing.responses.from.caches"/>.
1162</t>
1163<t>
1164   If no selected response is available, the cache can forward the presented
1165   request to the origin server in a conditional request; see <xref
1166   target="validation.model"/>.
1167</t>
1168</section>
1169
1170<section anchor="combining.responses" title="Combining Partial Content">
1171<t>
1172   A response might transfer only a partial representation if the
1173   connection closed prematurely or if the request used one or more Range
1174   specifiers (&partial;).  After several such transfers, a cache might have
1175   received several ranges of the same representation.  A cache &MAY; combine
1176   these ranges into a single stored response, and reuse that response to
1177   satisfy later requests, if they all share the same strong validator and
1178   the cache complies with the client requirements in &combining-byte-ranges;.
1179</t>
1180<t>
1181   When combining the new response with one or more stored responses, a
1182   cache &MUST;:
1183   <list style="symbols">
1184      <t>delete any Warning header fields in the stored response with
1185         warn-code 1xx (see <xref target="header.warning" />);</t>
1186      <t>retain any Warning header fields in the stored response with
1187         warn-code 2xx; and,</t>
1188      <t>use other header fields provided in the new response, aside
1189         from Content-Range, to replace all instances of the corresponding
1190         header fields in the stored response.</t>
1191   </list>
1192</t>
1193</section>
1194
1195
1196</section>
1197
1198<section anchor="header.field.definitions" title="Header Field Definitions">
1199<t>
1200   This section defines the syntax and semantics of HTTP/1.1 header fields
1201   related to caching.
1202</t>
1203
1204<section anchor="header.age" title="Age">
1205   <iref item="Age header field" primary="true" x:for-anchor="" />
1206   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1207   <x:anchor-alias value="Age"/>
1208   <x:anchor-alias value="age-value"/>
1209<t>
1210   The "Age" header field conveys the sender's estimate of the amount
1211   of time since the response was generated or successfully validated at the
1212   origin server. Age values are calculated as specified in <xref
1213   target="age.calculations" />.
1214</t>
1215<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1216  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1217</artwork></figure>
1218<t>
1219  Age field-values are non-negative integers, representing time in seconds
1220  (see <xref target="delta-seconds"/>).
1221</t>
1222<t>
1223   The presence of an Age header field in a response implies that a response
1224   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1225   might not implement the Age header field.
1226</t>
1227</section>
1228
1229<section anchor="header.cache-control" title="Cache-Control">
1230   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1231   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1232      x:for-anchor="" />
1233   <x:anchor-alias value="Cache-Control"/>
1234   <x:anchor-alias value="cache-directive"/>
1235   <x:anchor-alias value="cache-extension"/>
1236   <x:anchor-alias value="cache-request-directive"/>
1237   <x:anchor-alias value="cache-response-directive"/>
1238<t>
1239   The "Cache-Control" header field is used to specify directives for
1240   caches along the request/response chain. Such cache directives are
1241   unidirectional in that the presence of a directive in a request does not
1242   imply that the same directive is to be given in the response.
1243</t>
1244<t>
1245   A cache &MUST; obey the requirements of the Cache-Control
1246   directives defined in this section. See <xref
1247   target="cache.control.extensions"/> for information about how Cache-Control
1248   directives defined elsewhere are handled.
1249</t>
1250<x:note>
1251   <t>
1252       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1253       might only implement Pragma: no-cache (see <xref target="header.pragma"
1254       />).
1255   </t>
1256</x:note>
1257<t>
1258   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1259   through in forwarded messages, regardless of their
1260   significance to that application, since the directives might be applicable
1261   to all recipients along the request/response chain. It is not possible to
1262   target a directive to a specific cache.
1263</t>
1264<t>Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.</t>
1265<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1266  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1267
1268  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1269     / <x:ref>cache-response-directive</x:ref>
1270
1271  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1272</artwork></figure>
1273
1274<section anchor="cache-request-directive" 
1275   title="Request Cache-Control Directives">
1276   <x:anchor-alias value="cache-request-directive" />
1277
1278<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1279   subitem="cache-request-directive" />
1280  <x:ref>cache-request-directive</x:ref> =
1281       "no-cache"
1282     / "no-store"
1283     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1284     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1285     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1286     / "no-transform"
1287     / "only-if-cached"
1288     / <x:ref>cache-extension</x:ref>
1289</artwork></figure>
1290
1291<t>
1292   <x:dfn>no-cache</x:dfn>
1293   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1294   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1295   <list>
1296      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1297      use a stored response to satisfy the request without successful
1298      validation on the origin server.</t> 
1299   </list>
1300</t>
1301<t>
1302   <x:dfn>no-store</x:dfn>
1303   <iref item="Cache Directives" primary="true" subitem="no-store" />
1304   <iref item="no-store" primary="true" subitem="Cache Directive" />
1305   <list>
1306      <t>The no-store request directive indicates that a cache &MUST-NOT;
1307      store any part of either this request or any response to it. This
1308      directive applies to both private and shared caches. "&MUST-NOT;
1309      store" in this context means that the cache &MUST-NOT; intentionally
1310      store the information in non-volatile storage, and &MUST; make a
1311      best-effort attempt to remove the information from volatile storage as
1312      promptly as possible after forwarding it.</t>
1313      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1314      privacy. In particular, malicious or compromised caches might not
1315      recognize or obey this directive, and communications networks might be
1316      vulnerable to eavesdropping.</t>
1317      <t>Note that if a request containing this directive is satisfied from a
1318      cache, the no-store request directive does not apply to the already
1319      stored response.</t>
1320   </list>
1321</t>
1322<t>
1323   <x:dfn>max-age</x:dfn>
1324   <iref item="Cache Directives" primary="true" subitem="max-age" />
1325   <iref item="max-age" primary="true" subitem="Cache Directive" />
1326   <list>
1327      <t>The max-age request directive indicates that the client is unwilling to
1328      accept a response whose age is greater than the specified number of
1329      seconds. Unless the max-stale request directive is also present, the
1330      client is not willing to accept a stale response.</t>
1331   </list>
1332</t>
1333<t>
1334   <x:dfn>max-stale</x:dfn>
1335   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1336   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1337   <list>
1338      <t>The max-stale request directive indicates that the client is willing
1339      to accept a response that has exceeded its expiration time. If max-stale
1340      is assigned a value, then the client is willing to accept a response
1341      that has exceeded its expiration time by no more than the specified
1342      number of seconds. If no value is assigned to max-stale, then the client
1343      is willing to accept a stale response of any age.</t>
1344   </list>
1345</t>
1346<t>
1347   <x:dfn>min-fresh</x:dfn>
1348   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1349   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1350   <list>
1351      <t>The min-fresh request directive indicates that the client is willing
1352      to accept a response whose freshness lifetime is no less than its
1353      current age plus the specified time in seconds. That is, the client
1354      wants a response that will still be fresh for at least the specified
1355      number of seconds.</t>
1356   </list>
1357</t>
1358<t>
1359   <x:dfn>no-transform</x:dfn>
1360   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1361   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1362   <list>
1363      <t>The no-transform request directive indicates that an intermediary
1364        (whether or not it implements a cache) &MUST-NOT; change the
1365        Content-Encoding, Content-Range or Content-Type request header fields,
1366        nor the request representation.</t>
1367   </list>
1368</t>
1369<t>
1370   <x:dfn>only-if-cached</x:dfn>
1371   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1372   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1373   <list>
1374      <t>The only-if-cached request directive indicates that the client only
1375      wishes to obtain a stored response. If it receives this directive, a
1376      cache &SHOULD; either respond using a stored response that is consistent
1377      with the other constraints of the request, or respond with a 504
1378      (Gateway Timeout) status code. If a group of caches is being operated as
1379      a unified system with good internal connectivity, a member cache &MAY;
1380      forward such a request within that group of caches.</t>
1381   </list>
1382</t>
1383</section>
1384
1385<section anchor="cache-response-directive" 
1386   title="Response Cache-Control Directives">
1387   <x:anchor-alias value="cache-response-directive" />
1388
1389<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1390   subitem="cache-response-directive" />
1391  <x:ref>cache-response-directive</x:ref> =
1392       "public"
1393     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1394     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1395     / "no-store"
1396     / "no-transform"
1397     / "must-revalidate"
1398     / "proxy-revalidate"
1399     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1400     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1401     / <x:ref>cache-extension</x:ref>
1402</artwork></figure>
1403
1404<t>
1405   <x:dfn>public</x:dfn>
1406   <iref item="Cache Directives" primary="true" subitem="public" />
1407   <iref item="public" primary="true" subitem="Cache Directive" />
1408   <list>
1409      <t>The public response directive indicates that a response whose
1410        associated request contains an 'Authentication' header &MAY; be
1411        stored (see <xref target="caching.authenticated.responses" />).</t>
1412  </list>
1413</t>
1414<t>
1415   <x:dfn>private</x:dfn>
1416   <iref item="Cache Directives" primary="true" subitem="private" />
1417   <iref item="private" primary="true" subitem="Cache Directive" />
1418   <list>
1419      <t>The private response directive indicates that the response message is
1420      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1421      private cache &MAY; store the response.</t>
1422      <t>If the private response directive specifies one or more field-names,
1423      this requirement is limited to the field-values associated with the
1424      listed response header fields. That is, a shared cache &MUST-NOT; store
1425      the specified field-names(s), whereas it &MAY; store the remainder of the
1426      response message.</t>
1427      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1428      the response can be stored; it cannot ensure the privacy of the message
1429      content. Also, private response directives with field-names are often
1430      handled by implementations as if an unqualified private directive was
1431      received; i.e., the special handling for the qualified form is not
1432      widely implemented.</t>
1433   </list>
1434</t>
1435<t>
1436   <x:dfn>no-cache</x:dfn>
1437   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1438   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1439   <list>
1440      <t>The no-cache response directive indicates that the response MUST NOT
1441      be used to satisfy a subsequent request without successful validation on
1442      the origin server. This allows an origin server to prevent a cache from
1443      using it to satisfy a request without contacting it, even by caches that
1444      have been configured to return stale responses.</t>
1445      <t>If the no-cache response directive specifies one or more field-names,
1446      this requirement is limited to the field-values associated with the
1447      listed response header fields. That is, a cache &MUST-NOT; send the
1448      specified field-name(s) in the response to a subsequent request without successful
1449      validation on the origin server. This allows an origin server to prevent
1450      the re-use of certain header fields in a response, while still allowing
1451      caching of the rest of the response.</t>
1452      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1453      this directive. Also, no-cache response directives with field-names are
1454      often handled by implementations as if an unqualified no-cache directive
1455      was received; i.e., the special handling for the qualified form is not
1456      widely implemented. </t>
1457   </list>
1458</t>
1459<t>
1460   <x:dfn>no-store</x:dfn>
1461   <iref item="Cache Directives" primary="true" subitem="no-store" />
1462   <iref item="no-store" primary="true" subitem="Cache Directive" />
1463   <list>
1464      <t>The no-store response directive indicates that a cache &MUST-NOT;
1465      store any part of either the immediate request or response. This
1466      directive applies to both private and shared caches. "&MUST-NOT;
1467      store" in this context means that the cache &MUST-NOT; intentionally
1468      store the information in non-volatile storage, and &MUST; make a
1469      best-effort attempt to remove the information from volatile storage as
1470      promptly as possible after forwarding it.</t>
1471      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1472      privacy. In particular, malicious or compromised caches might not
1473      recognize or obey this directive, and communications networks might be
1474      vulnerable to eavesdropping.</t>
1475   </list>
1476</t>
1477<t>
1478   <x:dfn>must-revalidate</x:dfn>
1479   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1480   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1481   <list>
1482      <t>The must-revalidate response directive indicates that once it has
1483      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1484      requests without successful validation on the origin server.</t>
1485      <t>The must-revalidate directive is necessary to support reliable
1486      operation for certain protocol features. In all circumstances a
1487      cache &MUST; obey the must-revalidate directive; in particular,
1488      if a cache cannot reach the origin server for any reason, it &MUST;
1489      generate a 504 (Gateway Timeout) response.</t>
1490      <t>The must-revalidate directive ought to be used by servers if and only
1491      if failure to validate a request on the representation could result in
1492      incorrect operation, such as a silently unexecuted financial
1493      transaction.</t>
1494   </list>
1495</t>
1496<t>
1497   <x:dfn>proxy-revalidate</x:dfn>
1498   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1499   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1500   <list>
1501      <t>The proxy-revalidate response directive has the same meaning as the
1502      must-revalidate response directive, except that it does not apply to
1503      private caches.</t>
1504   </list>
1505</t>
1506<t>
1507   <x:dfn>max-age</x:dfn>
1508   <iref item="Cache Directives" primary="true" subitem="max-age" />
1509   <iref item="max-age" primary="true" subitem="Cache Directive" />
1510   <list>
1511      <t>The max-age response directive indicates that the response is to be
1512      considered stale after its age is greater than the specified number of
1513      seconds.</t>
1514   </list>
1515</t>
1516<t>
1517   <x:dfn>s-maxage</x:dfn>
1518   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1519   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1520   <list>
1521      <t>The s-maxage response directive indicates that, in shared caches, the
1522      maximum age specified by this directive overrides the maximum age
1523      specified by either the max-age directive or the Expires header field. The
1524      s-maxage directive also implies the semantics of the proxy-revalidate
1525      response directive.</t>
1526   </list>
1527</t>
1528<t>
1529   <x:dfn>no-transform</x:dfn>
1530   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1531   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1532   <list>
1533      <t>The no-transform response directive indicates that an intermediary
1534      (regardless of whether it implements a cache) &MUST-NOT; change the
1535      Content-Encoding, Content-Range or Content-Type response header fields,
1536      nor the response representation.</t>
1537   </list>
1538</t>
1539
1540</section>
1541
1542<section anchor="cache.control.extensions" title="Cache Control Extensions">
1543<t>
1544   The Cache-Control header field can be extended through the use of one or
1545   more cache-extension tokens, each with an optional value. Informational
1546   extensions (those that do not require a change in cache behavior) can be
1547   added without changing the semantics of other directives. Behavioral
1548   extensions are designed to work by acting as modifiers to the existing base
1549   of cache directives. Both the new directive and the standard directive are
1550   supplied, such that applications that do not understand the new directive
1551   will default to the behavior specified by the standard directive, and those
1552   that understand the new directive will recognize it as modifying the
1553   requirements associated with the standard directive. In this way,
1554   extensions to the cache-control directives can be made without requiring
1555   changes to the base protocol.
1556</t>
1557<t>
1558   This extension mechanism depends on an HTTP cache obeying all of the
1559   cache-control directives defined for its native HTTP-version, obeying
1560   certain extensions, and ignoring all directives that it does not
1561   understand.
1562</t>
1563<t>
1564   For example, consider a hypothetical new response directive called
1565   "community" that acts as a modifier to the private directive. We define
1566   this new directive to mean that, in addition to any private cache, any
1567   cache that is shared only by members of the community named within its
1568   value may cache the response. An origin server wishing to allow the UCI
1569   community to use an otherwise private response in their shared cache(s)
1570   could do so by including
1571</t>
1572<figure><artwork type="example">
1573  Cache-Control: private, community="UCI"
1574</artwork></figure>
1575<t>
1576   A cache seeing this header field will act correctly even if the cache does
1577   not understand the community cache-extension, since it will also see and
1578   understand the private directive and thus default to the safe behavior.
1579</t>
1580<t>
1581   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1582   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1583   combined with standard directives (or the response's default cacheability)
1584   such that the cache behavior will remain minimally correct even if the
1585   cache does not understand the extension(s).
1586</t>
1587<t>
1588   The HTTP Cache Directive Registry defines the name space for the cache
1589   directives.
1590</t>
1591<t>
1592   A registration &MUST; include the following fields:
1593   <list style="symbols">
1594      <t>Cache Directive Name</t>
1595      <t>Pointer to specification text</t>
1596   </list>
1597</t>
1598<t>
1599   Values to be added to this name space are subject to IETF review (<xref
1600   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1601</t>
1602<t>
1603   The registry itself is maintained at <eref
1604   target="http://www.iana.org/assignments/http-cache-directives"/>.
1605</t>
1606</section>
1607
1608</section>
1609
1610<section anchor="header.expires" title="Expires">
1611   <iref item="Expires header field" primary="true" x:for-anchor="" />
1612   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1613   <x:anchor-alias value="Expires"/>
1614<t>
1615   The "Expires" header field gives the date/time after which the
1616   response is considered stale. See <xref target="expiration.model" /> for
1617   further discussion of the freshness model.
1618</t>
1619<t>
1620   The presence of an Expires field does not imply that the original resource
1621   will change or cease to exist at, before, or after that time.
1622</t>
1623<t>
1624   The field-value is an absolute date and time as defined by HTTP-date in
1625   &http-date;; a sender &MUST; use the rfc1123-date format.
1626</t>
1627<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1628  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1629</artwork></figure>
1630<figure>
1631  <preamble>For example</preamble>
1632<artwork type="example">
1633  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1634</artwork></figure>
1635<t>
1636   A cache &MUST; treat other invalid date formats,
1637   especially including the value "0", as in the past (i.e., "already
1638   expired").
1639</t>
1640<x:note>
1641   <t>
1642       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1643       max-age directive (see <xref target="cache-response-directive" />),
1644       that directive overrides the Expires field. Likewise, the s-maxage
1645       directive overrides Expires in shared caches.
1646   </t>
1647</x:note>
1648<t>
1649   Historically, HTTP required the Expires field-value to be no more than a
1650   year in the future. While longer freshness lifetimes are no longer
1651   prohibited, extremely large values have been demonstrated to cause
1652   problems (e.g., clock overflows due to use of 32-bit integers for
1653   time values), and most caches will evict a response far sooner than
1654   that. Therefore, senders ought not produce them.
1655</t>
1656<t>
1657   An origin server without a clock &MUST-NOT; assign Expires
1658   values to a response unless these values were associated
1659   with the resource by a system or user with a reliable clock. It &MAY;
1660   assign an Expires value that is known, at or before server
1661   configuration time, to be in the past (this allows "pre-expiration"
1662   of responses without storing separate Expires values for each
1663   resource).
1664</t>
1665</section>
1666
1667<section anchor="header.pragma" title="Pragma">
1668   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1669   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1670   <x:anchor-alias value="extension-pragma"/>
1671   <x:anchor-alias value="Pragma"/>
1672   <x:anchor-alias value="pragma-directive"/>
1673<t>
1674   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1675   caches, so that clients can specify a "no-cache" request that they will
1676   understand (as Cache-Control was not defined until HTTP/1.1). When the
1677   Cache-Control header is also present and understood in a request, Pragma is
1678   ignored.
1679</t>
1680<t>
1681   In HTTP/1.0, Pragma was defined as an extensible field for
1682   implementation-specified directives for recipients. This specification
1683   deprecates such extensions to improve interoperability.
1684</t>
1685<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1686  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1687  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1688  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1689</artwork></figure>
1690<t>
1691   When the Cache-Control header is not present in a request, the no-cache
1692   request pragma-directive &MUST; have the same effect on caches as if
1693   "Cache-Control: no-cache" were present (see <xref
1694   target="cache-request-directive" />).
1695</t>
1696<t>
1697   When sending a no-cache request, a client ought to include both the pragma
1698   and cache-control directives, unless Cache-Control: no-cache is
1699   purposefully omitted to target other Cache-Control response directives at
1700   HTTP/1.1 caches. For example:
1701</t>
1702<figure>
1703<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1704GET / HTTP/1.1
1705Host: www.example.com
1706Cache-Control: max-age=30
1707Pragma: no-cache
1708
1709</artwork>
1710</figure>
1711<t>
1712   will constrain HTTP/1.1 caches to serve a response no older than 30
1713   seconds, while precluding implementations that do not understand
1714   Cache-Control from serving a cached response.
1715</t>
1716<x:note>
1717   <t>
1718      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
1719      specified, it does not provide a reliable replacement for
1720      "Cache-Control: no-cache" in them.
1721   </t>
1722</x:note>
1723</section>
1724
1725<section anchor="header.vary" title="Vary">
1726   <iref item="Vary header field" primary="true" x:for-anchor="" />
1727   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1728   <x:anchor-alias value="Vary"/>
1729<t>
1730   The "Vary" header field conveys the set of header fields
1731   that were used to select the representation.
1732</t>
1733<t>
1734   Caches use this information, in part, to determine whether a stored
1735   response can be used to satisfy a given request; see <xref
1736   target="caching.negotiated.responses" />. determines, while the response is
1737   fresh, whether a cache is permitted to use the response to reply to a
1738   subsequent request without validation; see <xref
1739   target="caching.negotiated.responses" />.
1740</t>
1741<t>
1742   In uncacheable or stale responses, the Vary field value advises the user
1743   agent about the criteria that were used to select the representation.
1744</t>
1745<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1746  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1747</artwork></figure>
1748<t>
1749   The set of header fields named by the Vary field value is known as the
1750   selecting header fields.
1751</t>
1752<t>
1753   A server &SHOULD; include a Vary header field with any cacheable response
1754   that is subject to server-driven negotiation. Doing so allows a cache to
1755   properly interpret future requests on that resource and informs the user
1756   agent about the presence of negotiation on that resource. A server &MAY;
1757   include a Vary header field with a non-cacheable response that is subject
1758   to server-driven negotiation, since this might provide the user agent with
1759   useful information about the dimensions over which the response varies at
1760   the time of the response.
1761</t>
1762<t>
1763   A Vary field value of "*" signals that unspecified parameters not limited
1764   to the header fields (e.g., the network address of the client), play a
1765   role in the selection of the response representation; therefore, a cache
1766   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1767   generate the "*" value.
1768</t>
1769<t>
1770   The field-names given are not limited to the set of standard header
1771   fields defined by this specification. Field names are case-insensitive.
1772</t>
1773</section>
1774
1775<section anchor="header.warning" title="Warning">
1776   <iref item="Warning header field" primary="true" x:for-anchor="" />
1777   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1778   <x:anchor-alias value="Warning"/>
1779   <x:anchor-alias value="warning-value"/>
1780   <x:anchor-alias value="warn-agent"/>
1781   <x:anchor-alias value="warn-code"/>
1782   <x:anchor-alias value="warn-date"/>
1783   <x:anchor-alias value="warn-text"/>
1784<t>
1785   The "Warning" header field is used to carry additional information
1786   about the status or transformation of a message that might not be reflected
1787   in the message. This information is typically used to warn about possible
1788   incorrectness introduced by caching operations or transformations applied
1789   to the payload of the message.
1790</t>
1791<t>
1792   Warnings can be used for other purposes, both cache-related and otherwise.
1793   The use of a warning, rather than an error status code, distinguishes these
1794   responses from true failures.
1795</t>
1796<t>
1797   Warning header fields can in general be applied to any message, however some
1798   warn-codes are specific to caches and can only be applied to response
1799   messages.
1800</t>
1801<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
1802  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1803 
1804  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
1805                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1806 
1807  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1808  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1809                  ; the name or pseudonym of the server adding
1810                  ; the Warning header field, for use in debugging
1811  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1812  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1813</artwork></figure>
1814<t>
1815   Multiple warnings can be attached to a response (either by the origin
1816   server or by a cache), including multiple warnings with the same code
1817   number, only differing in warn-text.
1818</t>
1819<t>
1820   When this occurs, the user agent &SHOULD; inform the user of as many of
1821   them as possible, in the order that they appear in the response.
1822</t>
1823<t>
1824   Systems that generate multiple Warning header fields are encouraged to
1825   order them with this user agent behavior in mind. New Warning header fields
1826   are added after any existing Warning headers fields.
1827</t>
1828<t>
1829   Warnings are assigned three digit warn-codes. The first digit indicates
1830   whether the Warning is required to be deleted from a stored response after
1831   validation:
1832   <list style="symbols">
1833      <t>1xx Warnings describe the freshness or validation status of the
1834      response, and so &MUST; be deleted by a cache after validation. They can
1835      only be generated by a cache when validating a cached entry, and
1836      &MUST-NOT; be generated in any other situation.</t>
1837      <t>2xx Warnings describe some aspect of the representation that is not
1838      rectified by a validation (for example, a lossy compression of the
1839      representation) and &MUST-NOT; be deleted by a cache after validation,
1840      unless a full response is returned, in which case they &MUST; be.</t>
1841   </list>
1842</t>
1843<t>
1844   If an implementation sends a message with one or more Warning header fields to a
1845   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1846   in each warning-value a warn-date that matches the Date header field in the
1847   message.
1848</t>
1849<t>
1850   If a system receives a message with a warning-value that includes
1851   a warn-date, and that warn-date is different from the Date value in the
1852   response, then that warning-value &MUST; be deleted from the message before
1853   storing, forwarding, or using it. (preventing the consequences of naive
1854   caching of Warning header fields.) If all of the warning-values are deleted
1855   for this reason, the Warning header field &MUST; be deleted as well.
1856</t>
1857<t>
1858   The following warn-codes are defined by this specification, each with a
1859   recommended warn-text in English, and a description of its meaning.
1860</t>
1861<t><?rfc needLines="4"?>
1862   110 Response is stale
1863   <list>
1864      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
1865   </list>
1866</t>
1867<t><?rfc needLines="4"?>
1868   111 Revalidation failed
1869   <list>
1870      <t>A cache &SHOULD; include this when returning a stale response because an
1871      attempt to validate the response failed, due to an inability to reach
1872      the server.</t>
1873   </list>
1874</t>
1875<t><?rfc needLines="4"?>
1876   112 Disconnected operation
1877   <list>
1878      <t>A cache &SHOULD; include this if it is intentionally disconnected from
1879      the rest of the network for a period of time.</t>
1880   </list>
1881</t>
1882<t><?rfc needLines="4"?>
1883   113 Heuristic expiration
1884   <list>
1885      <t>A cache &SHOULD; include this if it heuristically chose a freshness
1886      lifetime greater than 24 hours and the response's age is greater than 24
1887      hours.</t>
1888   </list>
1889</t>
1890<t><?rfc needLines="4"?>
1891   199 Miscellaneous warning
1892   <list>
1893      <t>The warning text can include arbitrary information to be presented to
1894      a human user, or logged. A system receiving this warning &MUST-NOT; take
1895      any automated action, besides presenting the warning to the user.</t>
1896   </list>
1897</t>
1898<t><?rfc needLines="4"?>
1899   214 Transformation applied
1900   <list>
1901      <t>&MUST; be added by a proxy if it applies any
1902      transformation to the representation, such as changing the
1903      content-coding, media-type, or modifying the representation data, unless
1904      this Warning code already appears in the response.</t>
1905   </list>
1906</t>
1907<t><?rfc needLines="4"?>
1908   299 Miscellaneous persistent warning
1909   <list>
1910      <t>The warning text can include arbitrary information to be presented to
1911      a human user, or logged. A system receiving this warning &MUST-NOT; take
1912      any automated action.</t>
1913   </list>
1914</t>
1915</section>
1916
1917</section>
1918
1919<section anchor="history.lists" title="History Lists">
1920<t>
1921   User agents often have history mechanisms, such as "Back" buttons and
1922   history lists, that can be used to redisplay a representation retrieved
1923   earlier in a session.
1924</t>
1925<t>
1926   The freshness model (<xref target="expiration.model"/>) does not
1927   necessarily apply to history mechanisms. I.e., a history mechanism can
1928   display a previous representation even if it has expired.
1929</t>
1930<t>
1931   This does not prohibit the history mechanism from telling the user that a
1932   view might be stale, or from honoring cache directives (e.g.,
1933   Cache-Control: no-store).
1934</t>
1935</section>
1936
1937
1938<section anchor="IANA.considerations" title="IANA Considerations">
1939
1940<section title="Cache Directive Registry" 
1941   anchor="cache.directive.registration">
1942<t>
1943   The registration procedure for HTTP Cache Directives is defined by <xref
1944   target="cache.control.extensions"/> of this document.
1945</t>
1946<t>
1947   The HTTP Cache Directive Registry shall be created at <eref
1948   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1949   populated with the registrations below:
1950</t>
1951<?BEGININC p6-cache.cache-directives ?>
1952<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1953<texttable xmlns:my="#my" align="left" suppress-title="true"
1954           anchor="iana.cache.directive.registration.table">
1955   <ttcol>Cache Directive</ttcol>
1956   <ttcol>Reference</ttcol>
1957
1958   <c>max-age</c>
1959   <c>
1960      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1961   </c>
1962   <c>max-stale</c>
1963   <c>
1964      <xref target="cache-request-directive"/>
1965   </c>
1966   <c>min-fresh</c>
1967   <c>
1968      <xref target="cache-request-directive"/>
1969   </c>
1970   <c>must-revalidate</c>
1971   <c>
1972      <xref target="cache-response-directive"/>
1973   </c>
1974   <c>no-cache</c>
1975   <c>
1976      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1977   </c>
1978   <c>no-store</c>
1979   <c>
1980      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1981   </c>
1982   <c>no-transform</c>
1983   <c>
1984      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1985   </c>
1986   <c>only-if-cached</c>
1987   <c>
1988      <xref target="cache-request-directive"/>
1989   </c>
1990   <c>private</c>
1991   <c>
1992      <xref target="cache-response-directive"/>
1993   </c>
1994   <c>proxy-revalidate</c>
1995   <c>
1996      <xref target="cache-response-directive"/>
1997   </c>
1998   <c>public</c>
1999   <c>
2000      <xref target="cache-response-directive"/>
2001   </c>
2002   <c>s-maxage</c>
2003   <c>
2004      <xref target="cache-response-directive"/>
2005   </c>
2006   <c>stale-if-error</c>
2007   <c>
2008      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
2009   </c>
2010   <c>stale-while-revalidate</c>
2011   <c>
2012      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
2013   </c>
2014</texttable>
2015<!--(END)-->
2016<?ENDINC p6-cache.cache-directives ?>
2017</section>
2018
2019<section title="Header Field Registration" anchor="header.field.registration">
2020<t>
2021  The Message Header Field Registry located at <eref 
2022  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2023  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
2024</t>
2025<?BEGININC p6-cache.iana-headers ?>
2026<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2027<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2028   <ttcol>Header Field Name</ttcol>
2029   <ttcol>Protocol</ttcol>
2030   <ttcol>Status</ttcol>
2031   <ttcol>Reference</ttcol>
2032
2033   <c>Age</c>
2034   <c>http</c>
2035   <c>standard</c>
2036   <c>
2037      <xref target="header.age"/>
2038   </c>
2039   <c>Cache-Control</c>
2040   <c>http</c>
2041   <c>standard</c>
2042   <c>
2043      <xref target="header.cache-control"/>
2044   </c>
2045   <c>Expires</c>
2046   <c>http</c>
2047   <c>standard</c>
2048   <c>
2049      <xref target="header.expires"/>
2050   </c>
2051   <c>Pragma</c>
2052   <c>http</c>
2053   <c>standard</c>
2054   <c>
2055      <xref target="header.pragma"/>
2056   </c>
2057   <c>Vary</c>
2058   <c>http</c>
2059   <c>standard</c>
2060   <c>
2061      <xref target="header.vary"/>
2062   </c>
2063   <c>Warning</c>
2064   <c>http</c>
2065   <c>standard</c>
2066   <c>
2067      <xref target="header.warning"/>
2068   </c>
2069</texttable>
2070<!--(END)-->
2071<?ENDINC p6-cache.iana-headers ?>
2072<t>
2073   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2074   Force".
2075</t>
2076</section>
2077
2078</section>
2079
2080<section anchor="security.considerations" title="Security Considerations">
2081<t>
2082   Caches expose additional potential vulnerabilities, since the contents of
2083   the cache represent an attractive target for malicious exploitation.
2084   Because cache contents persist after an HTTP request is complete, an attack
2085   on the cache can reveal information long after a user believes that the
2086   information has been removed from the network. Therefore, cache contents
2087   need to be protected as sensitive information.
2088</t>
2089</section>
2090
2091<section title="Acknowledgments" anchor="acks">
2092<t>
2093  See &acks;.
2094</t>
2095</section>
2096
2097</middle>
2098
2099<back>
2100<references title="Normative References">
2101
2102  <reference anchor="Part1">
2103    <front>
2104      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2105      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2106        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2107        <address><email>fielding@gbiv.com</email></address>
2108      </author>
2109      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2110        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2111        <address><email>jg@freedesktop.org</email></address>
2112      </author>
2113      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2114        <organization abbrev="HP">Hewlett-Packard Company</organization>
2115        <address><email>JeffMogul@acm.org</email></address>
2116      </author>
2117      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2118        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2119        <address><email>henrikn@microsoft.com</email></address>
2120      </author>
2121      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2122        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2123        <address><email>LMM@acm.org</email></address>
2124      </author>
2125      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2126        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2127        <address><email>paulle@microsoft.com</email></address>
2128      </author>
2129      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2130        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2131        <address><email>timbl@w3.org</email></address>
2132      </author>
2133      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2134        <organization abbrev="W3C">World Wide Web Consortium</organization>
2135        <address><email>ylafon@w3.org</email></address>
2136      </author>
2137      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2138        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2139        <address><email>julian.reschke@greenbytes.de</email></address>
2140      </author>
2141      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2142    </front>
2143    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2144    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2145  </reference>
2146
2147  <reference anchor="Part2">
2148    <front>
2149      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2150      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2151        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2152        <address><email>fielding@gbiv.com</email></address>
2153      </author>
2154      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2155        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2156        <address><email>jg@freedesktop.org</email></address>
2157      </author>
2158      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2159        <organization abbrev="HP">Hewlett-Packard Company</organization>
2160        <address><email>JeffMogul@acm.org</email></address>
2161      </author>
2162      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2163        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2164        <address><email>henrikn@microsoft.com</email></address>
2165      </author>
2166      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2167        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2168        <address><email>LMM@acm.org</email></address>
2169      </author>
2170      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2171        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2172        <address><email>paulle@microsoft.com</email></address>
2173      </author>
2174      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2175        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2176        <address><email>timbl@w3.org</email></address>
2177      </author>
2178      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2179        <organization abbrev="W3C">World Wide Web Consortium</organization>
2180        <address><email>ylafon@w3.org</email></address>
2181      </author>
2182      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2183        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2184        <address><email>julian.reschke@greenbytes.de</email></address>
2185      </author>
2186      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2187    </front>
2188    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2189    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2190  </reference>
2191
2192  <reference anchor="Part4">
2193    <front>
2194      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2195      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2196        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2197        <address><email>fielding@gbiv.com</email></address>
2198      </author>
2199      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2200        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2201        <address><email>jg@freedesktop.org</email></address>
2202      </author>
2203      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2204        <organization abbrev="HP">Hewlett-Packard Company</organization>
2205        <address><email>JeffMogul@acm.org</email></address>
2206      </author>
2207      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2208        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2209        <address><email>henrikn@microsoft.com</email></address>
2210      </author>
2211      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2212        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2213        <address><email>LMM@acm.org</email></address>
2214      </author>
2215      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2216        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2217        <address><email>paulle@microsoft.com</email></address>
2218      </author>
2219      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2220        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2221        <address><email>timbl@w3.org</email></address>
2222      </author>
2223      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2224        <organization abbrev="W3C">World Wide Web Consortium</organization>
2225        <address><email>ylafon@w3.org</email></address>
2226      </author>
2227      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2228        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2229        <address><email>julian.reschke@greenbytes.de</email></address>
2230      </author>
2231      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2232    </front>
2233    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2234    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2235  </reference>
2236
2237  <reference anchor="Part5">
2238    <front>
2239      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2240      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2241        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2242        <address><email>fielding@gbiv.com</email></address>
2243      </author>
2244      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2245        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2246        <address><email>jg@freedesktop.org</email></address>
2247      </author>
2248      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2249        <organization abbrev="HP">Hewlett-Packard Company</organization>
2250        <address><email>JeffMogul@acm.org</email></address>
2251      </author>
2252      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2253        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2254        <address><email>henrikn@microsoft.com</email></address>
2255      </author>
2256      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2257        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2258        <address><email>LMM@acm.org</email></address>
2259      </author>
2260      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2261        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2262        <address><email>paulle@microsoft.com</email></address>
2263      </author>
2264      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2265        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2266        <address><email>timbl@w3.org</email></address>
2267      </author>
2268      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2269        <organization abbrev="W3C">World Wide Web Consortium</organization>
2270        <address><email>ylafon@w3.org</email></address>
2271      </author>
2272      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2273        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2274        <address><email>julian.reschke@greenbytes.de</email></address>
2275      </author>
2276      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2277    </front>
2278    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2279    <x:source basename="p5-range" href="p5-range.xml" />
2280  </reference>
2281
2282  <reference anchor="Part7">
2283    <front>
2284      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2285      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2286        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2287        <address><email>fielding@gbiv.com</email></address>
2288      </author>
2289      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2290        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2291        <address><email>jg@freedesktop.org</email></address>
2292      </author>
2293      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2294        <organization abbrev="HP">Hewlett-Packard Company</organization>
2295        <address><email>JeffMogul@acm.org</email></address>
2296      </author>
2297      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2298        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2299        <address><email>henrikn@microsoft.com</email></address>
2300      </author>
2301      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2302        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2303        <address><email>LMM@acm.org</email></address>
2304      </author>
2305      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2306        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2307        <address><email>paulle@microsoft.com</email></address>
2308      </author>
2309      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2310        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2311        <address><email>timbl@w3.org</email></address>
2312      </author>
2313      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2314        <organization abbrev="W3C">World Wide Web Consortium</organization>
2315        <address><email>ylafon@w3.org</email></address>
2316      </author>
2317      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2318        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2319        <address><email>julian.reschke@greenbytes.de</email></address>
2320      </author>
2321      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2322    </front>
2323    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2324    <x:source basename="p7-auth" href="p7-auth.xml" />
2325  </reference>
2326
2327  <reference anchor="RFC2119">
2328    <front>
2329      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2330      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2331        <organization>Harvard University</organization>
2332        <address><email>sob@harvard.edu</email></address>
2333      </author>
2334      <date month="March" year="1997" />
2335    </front>
2336    <seriesInfo name="BCP" value="14" />
2337    <seriesInfo name="RFC" value="2119" />
2338  </reference>
2339
2340  <reference anchor="RFC5234">
2341    <front>
2342      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2343      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2344        <organization>Brandenburg InternetWorking</organization>
2345        <address>
2346          <email>dcrocker@bbiw.net</email>
2347        </address> 
2348      </author>
2349      <author initials="P." surname="Overell" fullname="Paul Overell">
2350        <organization>THUS plc.</organization>
2351        <address>
2352          <email>paul.overell@thus.net</email>
2353        </address>
2354      </author>
2355      <date month="January" year="2008"/>
2356    </front>
2357    <seriesInfo name="STD" value="68"/>
2358    <seriesInfo name="RFC" value="5234"/>
2359  </reference>
2360 
2361</references>
2362
2363<references title="Informative References">
2364
2365  <reference anchor="RFC1305">
2366    <front>
2367      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2368      <author fullname="David L. Mills" initials="D." surname="Mills">
2369        <organization>University of Delaware, Electrical Engineering Department</organization>
2370        <address><email>mills@udel.edu</email></address>
2371      </author>
2372      <date month="March" year="1992" />
2373    </front>
2374    <seriesInfo name="RFC" value="1305" />
2375  </reference>
2376
2377  <reference anchor="RFC2616">
2378    <front>
2379      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2380      <author fullname="R. Fielding" initials="R." surname="Fielding">
2381        <organization>University of California, Irvine</organization>
2382        <address><email>fielding@ics.uci.edu</email></address>
2383      </author>
2384      <author fullname="J. Gettys" initials="J." surname="Gettys">
2385        <organization>W3C</organization>
2386        <address><email>jg@w3.org</email></address>
2387      </author>
2388      <author fullname="J. Mogul" initials="J." surname="Mogul">
2389        <organization>Compaq Computer Corporation</organization>
2390        <address><email>mogul@wrl.dec.com</email></address>
2391      </author>
2392      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2393        <organization>MIT Laboratory for Computer Science</organization>
2394        <address><email>frystyk@w3.org</email></address>
2395      </author>
2396      <author fullname="L. Masinter" initials="L." surname="Masinter">
2397        <organization>Xerox Corporation</organization>
2398        <address><email>masinter@parc.xerox.com</email></address>
2399      </author>
2400      <author fullname="P. Leach" initials="P." surname="Leach">
2401        <organization>Microsoft Corporation</organization>
2402        <address><email>paulle@microsoft.com</email></address>
2403      </author>
2404      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2405        <organization>W3C</organization>
2406        <address><email>timbl@w3.org</email></address>
2407      </author>
2408      <date month="June" year="1999" />
2409    </front>
2410    <seriesInfo name="RFC" value="2616" />
2411  </reference>
2412
2413  <reference anchor="RFC3864">
2414    <front>
2415      <title>Registration Procedures for Message Header Fields</title>
2416      <author fullname="G. Klyne" initials="G." surname="Klyne">
2417        <organization>Nine by Nine</organization>
2418        <address><email>GK-IETF@ninebynine.org</email></address>
2419      </author>
2420      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2421        <organization>BEA Systems</organization>
2422        <address><email>mnot@pobox.com</email></address>
2423      </author>
2424      <author fullname="J. Mogul" initials="J." surname="Mogul">
2425        <organization>HP Labs</organization>
2426        <address><email>JeffMogul@acm.org</email></address>
2427      </author>
2428      <date month="September" year="2004" />
2429    </front>
2430    <seriesInfo name="BCP" value="90" />
2431    <seriesInfo name="RFC" value="3864" />
2432  </reference>
2433
2434  <reference anchor='RFC5226'>
2435    <front>
2436      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2437      <author initials='T.' surname='Narten' fullname='T. Narten'>
2438        <organization>IBM</organization>
2439        <address><email>narten@us.ibm.com</email></address>
2440      </author>
2441      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2442        <organization>Google</organization>
2443        <address><email>Harald@Alvestrand.no</email></address>
2444      </author>
2445      <date year='2008' month='May' />
2446    </front>
2447    <seriesInfo name='BCP' value='26' />
2448    <seriesInfo name='RFC' value='5226' />
2449  </reference>
2450
2451  <reference anchor='RFC5861'>
2452    <front>
2453      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2454      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2455        <organization>Yahoo! Inc.</organization>
2456        <address><email>mnot@yahoo-inc.com</email></address>
2457      </author>
2458      <date month="April" year="2010"/>
2459    </front>
2460    <seriesInfo name='RFC' value='5861' />
2461  </reference>
2462
2463</references>
2464
2465<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2466<t>
2467  Make the specified age calculation algorithm less conservative.
2468  (<xref target="age.calculations"/>)
2469</t>
2470<t>
2471  Remove requirement to consider Content-Location in successful responses
2472  in order to determine the appropriate response to use.
2473  (<xref target="validation.model" />)
2474</t>
2475<t>
2476  Clarify denial of service attack avoidance requirement.
2477  (<xref target="invalidation.after.updates.or.deletions" />)
2478</t>
2479<t>
2480  Change ABNF productions for header fields to only define the field value.
2481  (<xref target="header.field.definitions"/>)
2482</t>
2483<t>
2484  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2485  anymore, as these aspects never were implemented.
2486  (<xref target="header.warning" />)
2487</t>
2488</section>
2489
2490<?BEGININC p6-cache.abnf-appendix ?>
2491<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2492<figure>
2493<artwork type="abnf" name="p6-cache.parsed-abnf">
2494<x:ref>Age</x:ref> = delta-seconds
2495
2496<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2497 cache-directive ] )
2498
2499<x:ref>Expires</x:ref> = HTTP-date
2500
2501<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
2502
2503<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2504
2505<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2506 pragma-directive ] )
2507
2508<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2509 ) )
2510
2511<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2512 )
2513
2514<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2515<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2516<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2517 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2518 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2519 cache-extension
2520<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2521 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2522 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2523 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2524 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2525 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2526
2527<x:ref>delta-seconds</x:ref> = 1*DIGIT
2528
2529<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2530
2531<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2532
2533<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2534<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2535<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.8&gt;
2536
2537<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
2538
2539<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.3&gt;
2540
2541<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2542
2543<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2544<x:ref>warn-code</x:ref> = 3DIGIT
2545<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2546<x:ref>warn-text</x:ref> = quoted-string
2547<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2548 ]
2549</artwork>
2550</figure>
2551<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2552; Age defined but not used
2553; Cache-Control defined but not used
2554; Expires defined but not used
2555; Pragma defined but not used
2556; Vary defined but not used
2557; Warning defined but not used
2558</artwork></figure></section>
2559<?ENDINC p6-cache.abnf-appendix ?>
2560
2561<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2562
2563<section title="Since RFC 2616">
2564  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2565</section>
2566
2567<section title="Since draft-ietf-httpbis-p6-cache-00">
2568<t>
2569  Closed issues:
2570  <list style="symbols">
2571    <t>
2572      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2573    <t>
2574      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
2575    <t>
2576      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2577    <t>
2578      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2579    <t>
2580      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2581    <t>
2582      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2583    <t>
2584      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2585    <t>
2586      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2587    <t>
2588      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2589  </list>
2590</t>
2591<t>
2592  Other changes:
2593  <list style="symbols">
2594    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2595        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2596  </list>
2597</t>
2598</section>
2599
2600<section title="Since draft-ietf-httpbis-p6-cache-01">
2601<t>
2602  Closed issues:
2603  <list style="symbols">
2604    <t>
2605      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2606  </list>
2607</t>
2608<t>
2609  Other changes:
2610  <list style="symbols">
2611    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2612      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2613    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2614      specification.</t>
2615  </list>
2616</t>
2617</section>
2618
2619<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2620<t>
2621  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2622  <list style="symbols">
2623    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2624      document.</t>
2625  </list>
2626</t>
2627</section>
2628
2629<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2630<t>
2631  Closed issues:
2632  <list style="symbols">
2633    <t>
2634      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2635  </list>
2636</t>
2637</section>
2638
2639<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2640<t>
2641  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2642  <list style="symbols"> 
2643    <t>
2644      Use "/" instead of "|" for alternatives.
2645    </t>
2646    <t>
2647      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2648      whitespace ("OWS") and required whitespace ("RWS").
2649    </t>
2650    <t>
2651      Rewrite ABNFs to spell out whitespace rules, factor out
2652      header field value format definitions.
2653    </t>
2654  </list>
2655</t>
2656</section>
2657
2658<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2659<t>
2660  This is a total rewrite of this part of the specification.
2661</t>
2662<t>
2663  Affected issues:
2664  <list style="symbols">
2665    <t>
2666      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2667    <t>
2668      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2669    <t>
2670      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2671    <t>
2672      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2673  </list>
2674</t>
2675<t>
2676  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2677  <list style="symbols"> 
2678    <t>
2679      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2680    </t>
2681  </list>
2682</t>
2683</section>
2684
2685<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2686<t>
2687  Closed issues:
2688  <list style="symbols"> 
2689    <t>
2690      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2691      "base for numeric protocol elements"
2692    </t>
2693  </list>
2694</t>
2695<t>
2696  Affected issues:
2697  <list style="symbols">
2698    <t>
2699      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2700      "Vary and non-existant headers"
2701    </t>
2702  </list>
2703</t>
2704</section>
2705
2706<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2707<t>
2708  Closed issues:
2709  <list style="symbols"> 
2710    <t>
2711      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2712      "Definition of 1xx Warn-Codes"
2713    </t>
2714    <t>
2715      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2716      "Content-Location on 304 responses"
2717    </t>
2718    <t>
2719      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2720      "private and no-cache CC directives with headers"
2721    </t>
2722    <t>
2723      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2724      "RFC2047 and warn-text"
2725    </t>
2726  </list>
2727</t>
2728</section>
2729
2730<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2731<t>
2732  Closed issues:
2733  <list style="symbols"> 
2734    <t>
2735      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2736      "serving negotiated responses from cache: header-specific canonicalization"
2737    </t>
2738    <t>
2739      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2740      "Effect of CC directives on history lists"
2741    </t>
2742    <t>
2743      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291" />:
2744      "Cache Extensions can override no-store, etc."
2745    </t>
2746  </list>
2747</t>
2748<t>
2749  Affected issues:
2750  <list style="symbols">
2751    <t>
2752      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2753      Status codes and caching
2754    </t>
2755  </list>
2756</t>
2757<t>
2758  Partly resolved issues:
2759  <list style="symbols"> 
2760    <t>
2761      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2762      "Placement of 13.5.1 and 13.5.2"
2763    </t>
2764  </list>
2765</t>
2766</section>
2767
2768<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2769<t>
2770  Closed issues:
2771  <list style="symbols"> 
2772    <t>
2773      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2774      "Age calculation"
2775    </t>
2776    <t>
2777      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2778      "Clarify differences between / requirements for request and response CC directives"
2779    </t>
2780    <t>
2781      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2782      "Caching authenticated responses"
2783    </t>
2784    <t>
2785      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2786      "IANA registry for cache-control directives"
2787    </t>
2788    <t>
2789      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2790      "Heuristic caching of URLs with query components"
2791    </t>
2792  </list>
2793</t>
2794<t>
2795  Partly resolved issues:
2796  <list style="symbols"> 
2797    <t>
2798      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2799      "Term for the requested resource's URI"
2800    </t>
2801  </list>
2802</t>
2803</section>
2804
2805<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2806<t>
2807  Closed issues:
2808  <list style="symbols"> 
2809    <t>
2810      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2811      "Clarify entity / representation / variant terminology"
2812    </t>
2813    <t>
2814      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2815      "consider removing the 'changes from 2068' sections"
2816    </t>
2817    <t>
2818      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2819      "Allowing heuristic caching for new status codes"
2820    </t>
2821    <t>
2822      Clean up TODOs and prose in "Combining Responses."
2823    </t>
2824  </list>
2825</t>
2826</section>
2827
2828<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2829<t>
2830  Closed issues:
2831  <list style="symbols"> 
2832    <t>
2833      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2834      "Text about clock requirement for caches belongs in p6"
2835    </t>
2836  </list>
2837</t>
2838</section>
2839
2840<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2841<t>
2842  Closed issues:
2843  <list style="symbols"> 
2844    <t>
2845      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2846      "Header Classification"
2847    </t>
2848    <t>
2849      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2850      "Clarify 'public'"
2851    </t>
2852  </list>
2853</t>
2854</section>
2855
2856<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2857<t>
2858  Closed issues:
2859  <list style="symbols">
2860    <t>
2861      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2862      "untangle ABNFs for header fields"
2863    </t>
2864  </list>
2865</t>
2866</section>
2867
2868<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2869<t>
2870  Closed issues:
2871  <list style="symbols">
2872    <t>
2873      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2874      "Mismatch Vary"
2875    </t>
2876    <t>
2877      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2878      "Cache Invalidation only happens upon successful responses"
2879    </t>
2880    <t>
2881      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2882      "Recommend minimum sizes for protocol elements"
2883    </t>
2884    <t>
2885      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2886      "Proxies don't 'understand' methods"
2887    </t>
2888    <t>
2889      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2890      "Cache Extensions can override no-store, etc."
2891    </t>
2892    <t>
2893      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2894      "Pragma"
2895    </t>
2896  </list>
2897</t>
2898</section>
2899
2900<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2901<t>
2902  Closed issues:
2903  <list style="symbols">
2904    <t>
2905      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/290"/>:
2906      "Motivate one-year limit for Expires"
2907    </t>
2908  </list>
2909</t>
2910</section>
2911
2912<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
2913<t>
2914  Closed issues:
2915  <list style="symbols">
2916    <t>
2917      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
2918      "Document HTTP's error-handling philosophy"
2919    </t>
2920    <t>
2921      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/317"/>:
2922      "Cache-Control directive case sensitivity"
2923    </t>
2924  </list>
2925</t>
2926</section>
2927
2928<section title="Since draft-ietf-httpbis-p6-cache-17" anchor="changes.since.17">
2929<t>
2930  No changes yet.
2931</t>
2932</section>
2933
2934</section>
2935  </back>
2936</rfc>
Note: See TracBrowser for help on using the repository browser.