source: draft-ietf-httpbis/latest/p6-cache.xml @ 1483

Last change on this file since 1483 was 1483, checked in by mnot@…, 8 years ago

Clarify effects of request Cache-Control; closes #293.

  • Property svn:eol-style set to native
File size: 117.6 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 "December">
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="proposed" 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 does not contain the no-cache pragma (<xref
630      target="header.pragma"/>), nor the no-cache cache directive (<xref
631      target="cache-request-directive"/>), unless the stored response is
632      successfully validated (<xref target="validation.model"/>), and</t>
633      <t>the stored response does not contain the no-cache cache directive
634      (<xref target="cache-response-directive"/>), unless it is successfully
635      validated (<xref target="validation.model"/>), and</t>
636      <t>the stored response is either:
637         <list style="symbols">
638            <t>fresh (see <xref target="expiration.model" />), or</t>
639            <t>allowed to be served stale (see <xref
640            target="serving.stale.responses" />), or</t>
641            <t>successfully validated (see <xref target="validation.model"
642            />).</t>
643         </list>
644      </t>
645  </list>
646</t>
647<t>
648   Note that any of the requirements listed above can be overridden by a
649   cache-control extension; see <xref target="cache.control.extensions" />.
650</t>
651<t>
652   When a stored response is used to satisfy a request without validation,
653   a cache &MUST; include a single Age header field (<xref target="header.age"
654   />) in the response with a value equal to the stored response's
655   current_age; see <xref target="age.calculations" />.
656</t>
657<t>
658   A cache &MUST; write through requests with methods that are unsafe
659   (&safe-methods;) to the origin server; i.e., a cache must not generate
660   a reply to such a request before having forwarded the request and having
661   received a corresponding response.
662</t>
663<t>
664   Also, note that unsafe requests might invalidate already stored responses;
665   see <xref target="invalidation.after.updates.or.deletions" />.
666</t>
667<t>
668   When more than one suitable response is stored, a cache &MUST; use the
669   most recent response (as determined by the Date header field). It can also
670   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
671   no-cache" to disambiguate which response to use.
672</t>
673<t>
674   A cache that does not have a clock available &MUST-NOT; use stored responses
675   without revalidating them on every use. A cache, especially a shared
676   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
677   synchronize its clock with a reliable external standard.
678</t>
679
680</section>
681
682<section anchor="expiration.model" title="Freshness Model">
683<t>
684   When a response is "fresh" in the cache, it can be used to satisfy
685   subsequent requests without contacting the origin server, thereby improving
686   efficiency.
687</t>
688<t>
689   The primary mechanism for determining freshness is for an origin server to
690   provide an explicit expiration time in the future, using either the Expires
691   header field (<xref target="header.expires" />) or the max-age response cache
692   directive (<xref target="cache-response-directive" />). Generally, origin
693   servers will assign future explicit expiration times to responses in the
694   belief that the representation is not likely to change in a semantically
695   significant way before the expiration time is reached.
696</t>
697<t>
698   If an origin server wishes to force a cache to validate every request, it
699   can assign an explicit expiration time in the past to indicate that the
700   response is already stale. Compliant caches will normally validate the
701   cached response before reusing it for subsequent requests (see <xref
702   target="serving.stale.responses" />).
703</t>
704<t>
705   Since origin servers do not always provide explicit expiration times,
706   a cache &MAY; assign a heuristic expiration time when an explicit time is not
707   specified, employing algorithms that use other header field values (such as the
708   Last-Modified time) to estimate a plausible expiration time. This
709   specification does not provide specific algorithms, but does impose
710   worst-case constraints on their results.
711</t>
712<figure>
713<preamble>
714  The calculation to determine if a response is fresh is:
715</preamble>
716<artwork type="code">
717   response_is_fresh = (freshness_lifetime &gt; current_age)
718</artwork>
719</figure>
720<t>
721   The freshness_lifetime is defined in <xref
722   target="calculating.freshness.lifetime" />; the current_age is defined in
723   <xref target="age.calculations" />.
724</t>
725<t>
726   Additionally, clients can influence freshness calculation -- either
727   constraining it relaxing it -- by using the max-age and min-fresh
728   request cache directives. See <xref target="cache-request-directive" /> 
729   for details.
730</t>
731<t>
732   Note that freshness applies only to cache operation; it cannot be used to
733   force a user agent to refresh its display or reload a resource. See <xref
734   target="history.lists" /> for an explanation of the difference between
735   caches and history mechanisms.
736</t>
737
738<section anchor="calculating.freshness.lifetime" 
739   title="Calculating Freshness Lifetime">
740<t>
741   A cache can calculate the freshness lifetime (denoted as
742   freshness_lifetime) of a response by using the first match of:
743   <list style="symbols">
744      <t>If the cache is shared and the s-maxage response cache directive
745      (<xref target="cache-response-directive" />) is present, use its value,
746      or</t>
747      <t>If the max-age response cache directive (<xref
748      target="cache-response-directive" />) is present, use its value, or</t>
749      <t>If the Expires response header field (<xref target="header.expires" />) is
750      present, use its value minus the value of the Date response header field,
751      or</t>
752      <t>Otherwise, no explicit expiration time is present in the response. A
753      heuristic freshness lifetime might be applicable; see <xref
754      target="heuristic.freshness" />.</t>
755   </list>
756</t>
757<t>
758   Note that this calculation is not vulnerable to clock skew, since all of
759   the information comes from the origin server.
760</t>
761
762<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
763<t>
764   If no explicit expiration time is present in a stored response that has a
765   status code whose definition allows heuristic freshness to be used
766   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
767   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
768   use heuristics to determine freshness for responses with status codes that do
769   not explicitly allow it.
770</t>
771<t>
772   When a heuristic is used to calculate freshness lifetime, a cache
773   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
774   its current_age is more than 24 hours and such a warning is not already
775   present.
776</t>
777<t>
778   Also, if the response has a Last-Modified header field
779   (&header-last-modified;), caches are encouraged to use a heuristic
780   expiration value that is no more than some fraction of the interval since
781   that time. A typical setting of this fraction might be 10%.
782</t>
783<x:note>
784   <t>
785      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
786      x:sec="13.9"/>) required that caches do not calculate heuristic
787      freshness for URIs with query components (i.e., those containing '?').
788      In practice, this has not been widely implemented. Therefore, servers
789      are encouraged to send explicit directives (e.g., Cache-Control:
790      no-cache) if they wish to preclude caching.
791   </t>
792</x:note>
793</section>
794</section>
795
796<section anchor="age.calculations" title="Calculating Age">
797<t>
798   HTTP/1.1 uses the Age header field to convey the estimated age of the
799   response message when obtained from a cache. The Age field value is the
800   cache's estimate of the amount of time since the response was generated or
801   validated by the origin server. In essence, the Age value is the sum of the
802   time that the response has been resident in each of the caches along the
803   path from the origin server, plus the amount of time it has been in transit
804   along network paths.
805</t>
806<t>
807   The following data is used for the age calculation:
808</t>
809<t>
810   <x:dfn>age_value</x:dfn>
811   <list>
812      <t>
813         The term "age_value" denotes the value of the Age header field (<xref
814         target="header.age"/>), in a form appropriate for arithmetic
815         operation; or 0, if not available.
816      </t>
817   </list>
818</t>
819<t>
820   <x:dfn>date_value</x:dfn>
821   <list>
822      <t>
823         HTTP/1.1 requires origin servers to send a Date header field, if possible,
824         with every response, giving the time at which the response was
825         generated. The term "date_value" denotes the value of the Date
826         header field, in a form appropriate for arithmetic operations. See
827         &header-date; for the definition of the Date header field, and for
828         requirements regarding responses without it.
829      </t>
830   </list>
831</t>
832<t>
833   <x:dfn>now</x:dfn>
834   <list>
835      <t>
836         The term "now" means "the current value of the clock at the host
837         performing the calculation". A cache &SHOULD; use NTP (<xref
838         target="RFC1305"/>) or some similar protocol to synchronize its
839         clocks to a globally accurate time standard.
840      </t>
841   </list>
842</t>
843<t>
844   <x:dfn>request_time</x:dfn>
845   <list>
846      <t>
847         The current value of the clock at the host at the time the request
848         resulting in the stored response was made.
849      </t>
850   </list>
851</t>
852<t>
853   <x:dfn>response_time</x:dfn>
854   <list>
855      <t>
856         The current value of the clock at the host at the time the response
857         was received.
858      </t>
859   </list>
860</t>
861<t>
862   A response's age can be calculated in two entirely independent ways:
863   <list style="numbers">
864      <t>the "apparent_age": response_time minus date_value, if the local
865      clock is reasonably well synchronized to the origin server's clock. If
866      the result is negative, the result is replaced by zero.</t>
867      <t>the "corrected_age_value", if all of the caches along the response
868      path implement HTTP/1.1. A cache &MUST; interpret this value relative
869      to the time the request was initiated, not the time that the response
870      was received.</t>
871   </list>
872</t>
873<figure>
874<artwork type="code">
875  apparent_age = max(0, response_time - date_value);
876
877  response_delay = response_time - request_time;
878  corrected_age_value = age_value + response_delay; 
879</artwork>
880</figure>
881<figure>
882<preamble>These are combined as</preamble>
883<artwork type="code">
884  corrected_initial_age = max(apparent_age, corrected_age_value);
885</artwork></figure>
886<t>
887   The current_age of a stored response can then be calculated by adding the
888   amount of time (in seconds) since the stored response was last validated by
889   the origin server to the corrected_initial_age.
890</t>
891<figure><artwork type="code">
892  resident_time = now - response_time;
893  current_age = corrected_initial_age + resident_time;
894</artwork></figure>
895<t>
896   Additionally, to avoid common problems in date parsing:
897</t>
898<t>
899  <list style="symbols">
900     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
901        which appears to be more than 50 years in the future is in fact
902        in the past (this helps solve the "year 2000" problem).</t>
903
904     <t>Although all date formats are specified to be case-sensitive,
905        recipients &SHOULD; match day, week and timezone names
906        case-insensitively.</t>
907             
908     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
909        Expires date as earlier than the proper value, but &MUST-NOT;
910        internally represent a parsed Expires date as later than the
911        proper value.</t>
912
913     <t>All expiration-related calculations &MUST; be done in GMT. The
914        local time zone &MUST-NOT; influence the calculation or comparison
915        of an age or expiration time.</t>
916
917     <t>If an HTTP header field incorrectly carries a date value with a time
918        zone other than GMT, it &MUST; be converted into GMT using the
919        most conservative possible conversion.</t>
920  </list>
921</t>
922</section>
923
924<section anchor="serving.stale.responses" title="Serving Stale Responses">
925<t>
926   A "stale" response is one that either has explicit expiry information or is
927   allowed to have heuristic expiry calculated, but is not fresh according to
928   the calculations in <xref target="expiration.model" />.
929</t>
930<t>
931   A cache &MUST-NOT; return a stale response if it is prohibited by an
932   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
933   directive, a "must-revalidate" cache-response-directive, or an applicable
934   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
935   target="cache-response-directive"/>).
936</t>
937<t>
938   A cache &MUST-NOT; return stale responses unless it is disconnected
939   (i.e., it cannot contact the origin server or otherwise find a forward
940   path) or doing so is explicitly allowed (e.g., by the max-stale request
941   directive; see <xref target="cache-request-directive" />).
942</t>
943<t>
944   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
945   <xref target="header.warning" />) to stale responses. Likewise, a cache
946   &SHOULD; add the 112 warn-code to stale responses if the cache is
947   disconnected.
948</t>
949<t>
950   If a cache receives a first-hand response (either an entire response, or a
951   304 (Not Modified) response) that it would normally forward to the
952   requesting client, and the received response is no longer fresh, the cache
953   can forward it to the requesting client without adding a new Warning (but
954   without removing any existing Warning header fields). A cache shouldn't
955   attempt to validate a response simply because that response became stale in
956   transit.
957</t>
958</section>
959</section>
960
961<section anchor="validation.model" title="Validation Model">
962<t>
963   When a cache has one or more stored responses for a requested URI, but
964   cannot serve any of them (e.g., because they are not fresh, or one cannot
965   be selected; see <xref target="caching.negotiated.responses"/>), it can use
966   the conditional request mechanism &conditional; in the forwarded request to
967   give the origin server an opportunity to both select a valid stored
968   response to be used, and to update it. This process is known as
969   "validating" or "revalidating" the stored response.
970</t>
971<t>
972   When sending such a conditional request, a cache adds an If-Modified-Since
973   header field whose value is that of the Last-Modified header field from the
974   selected (see <xref target="caching.negotiated.responses"/>) stored
975   response, if available.
976</t>
977<t>
978   Additionally, a cache can add an If-None-Match header field whose value is
979   that of the ETag header field(s) from all responses stored for the
980   requested URI, if present. However, if any of the stored responses contains
981   only partial content, the cache shouldn't include its entity-tag in the
982   If-None-Match header field unless the request is for a range that would be
983   fully satisfied by that stored response.
984</t>
985
986<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
987
988<t>
989   <list style="symbols">
990      <t>
991         A 304 (Not Modified) response status code indicates that the stored
992         response can be updated and reused; see <xref
993         target="freshening.responses"/>.
994      </t>
995      <t>
996         A full response (i.e., one with a response body) indicates that none
997         of the stored responses nominated in the conditional request is
998         suitable. Instead, the cache can use the full response to
999         satisfy the request and &MAY; replace the stored response(s).
1000      </t>
1001      <t>
1002         However, if a cache receives a 5xx response while attempting to
1003         validate a response, it can either forward this response to the
1004         requesting client, or act as if the server failed to respond. In the
1005         latter case, it can return a previously stored response (see <xref
1006         target="serving.stale.responses" />).
1007      </t>
1008   </list>
1009</t>
1010
1011<section anchor="freshening.responses" title="Freshening Responses">
1012<t>
1013   When a cache receives a 304 (Not Modified) response and already has one
1014   or more stored 200 (OK) responses for the same cache key, the cache needs
1015   to identify which of the stored responses are updated by this new response
1016   and then update the stored response(s) with the new information provided in
1017   the 304 response.
1018   <list style="symbols">
1019    <t>
1020     If the new response contains a strong validator, then that strong
1021     validator identifies the selected representation.  All of the stored
1022     responses with the same strong validator are selected.
1023     If none of the stored responses contain the same strong validator, then
1024     this new response corresponds to a new selected representation and
1025     &MUST-NOT; update the existing stored responses.
1026    </t>
1027    <t>
1028     If the new response contains a weak validator and that validator
1029     corresponds to one of the cache's stored responses, then the most
1030     recent of those matching stored responses is selected.
1031    </t>
1032    <t>
1033     If the new response does not include any form of validator, there is
1034     only one stored response, and that stored response also lacks a
1035     validator, then that stored response is selected.
1036    </t>
1037   </list>
1038</t>
1039<t>
1040   If a stored response is selected for update, the cache &MUST;:
1041   <list style="symbols">
1042      <t>delete any Warning header fields in the stored response with
1043         warn-code 1xx (see <xref target="header.warning" />);</t>
1044      <t>retain any Warning header fields in the stored response with
1045         warn-code 2xx; and,</t>
1046      <t>use other header fields provided in the 304 response to replace
1047         all instances of the corresponding header fields in the stored
1048         response.</t>
1049   </list>
1050</t>
1051</section>
1052
1053</section>
1054
1055<section anchor="invalidation.after.updates.or.deletions" 
1056   title="Request Methods that Invalidate">
1057<t>
1058   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1059   have the potential for changing state on the origin server, intervening
1060   caches can use them to keep their contents up-to-date.
1061</t>
1062<t>
1063   A cache &MUST; invalidate the effective Request URI
1064   (&effective-request-uri;) as well as the URI(s) in the Location
1065   and Content-Location header fields (if present) when a non-error
1066   response to a request with an unsafe method is received.
1067</t>
1068<t>
1069   However, a cache &MUST-NOT; invalidate a URI from a
1070   Location or Content-Location header field if the host part of that URI
1071   differs from the host part in the effective request URI
1072   (&effective-request-uri;). This helps prevent denial of service attacks.
1073</t>
1074<t>
1075   A cache &MUST; invalidate the effective request URI
1076   (&effective-request-uri;) when it receives a non-error response
1077   to a request with a method whose safety is unknown.
1078</t>
1079<t>
1080   Here, a "non-error response" is one with a 2xx or 3xx status code.
1081   "Invalidate" means that the cache will either remove all stored
1082   responses related to the effective request URI, or will mark these as
1083   "invalid" and in need of a mandatory validation before they can be returned
1084   in response to a subsequent request.
1085</t>
1086<t>
1087   Note that this does not guarantee that all appropriate responses are
1088   invalidated. For example, the request that caused the change at the origin
1089   server might not have gone through the cache where a response is stored.
1090</t>
1091</section>
1092
1093<section anchor="caching.authenticated.responses" 
1094   title="Shared Caching of Authenticated Responses">
1095
1096<t>
1097   A shared cache &MUST-NOT; use a cached response to a request with an
1098   Authorization header field (&header-authorization;) to satisfy any subsequent
1099   request unless a cache directive that allows such responses to be stored is
1100   present in the response.
1101</t>
1102
1103<t>
1104   In this specification, the following Cache-Control response directives
1105   (<xref target="cache-response-directive"/>) have such an effect:
1106   must-revalidate, public, s-maxage.
1107</t>
1108
1109<t>
1110   Note that cached responses that contain the "must-revalidate" and/or
1111   "s-maxage" response directives are not allowed to be served stale (<xref
1112   target="serving.stale.responses"/>) by shared caches. In particular, a
1113   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1114   used to satisfy a subsequent request without revalidating it on the origin
1115   server.
1116</t>
1117</section>
1118
1119<section anchor="caching.negotiated.responses" 
1120   title="Caching Negotiated Responses">
1121<t>
1122   When a cache receives a request that can be satisfied by a stored response
1123   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
1124   use that response unless all of the selecting header fields nominated by
1125   the Vary header field match in both the original request (i.e., that associated
1126   with the stored response), and the presented request.
1127</t>
1128<t>
1129   The selecting header fields from two requests are defined to match if and
1130   only if those in the first request can be transformed to those in the
1131   second request by applying any of the following:
1132   <list style="symbols">
1133      <t>
1134         adding or removing whitespace, where allowed in the header field's syntax
1135      </t>
1136      <t>
1137         combining multiple header fields with the same field name
1138         (see &header-fields;)
1139      </t>
1140      <t>
1141         normalizing both header field values in a way that is known to have
1142         identical semantics, according to the header field's specification (e.g.,
1143         re-ordering field values when order is not significant;
1144         case-normalization, where values are defined to be case-insensitive)
1145      </t>
1146  </list>
1147</t>
1148<t>
1149   If (after any normalization that might take place) a header field is absent
1150   from a request, it can only match another request if it is also absent
1151   there.
1152</t>
1153<t>
1154   A Vary header field-value of "*" always fails to match, and subsequent
1155   requests to that resource can only be properly interpreted by the origin
1156   server.
1157</t>
1158<t>
1159   The stored response with matching selecting header fields is known as the
1160   selected response.
1161</t>
1162<t>
1163   If multiple selected responses are available, the most recent response
1164   (as determined by the Date header field) is used; see <xref 
1165   target="constructing.responses.from.caches"/>.
1166</t>
1167<t>
1168   If no selected response is available, the cache can forward the presented
1169   request to the origin server in a conditional request; see <xref
1170   target="validation.model"/>.
1171</t>
1172</section>
1173
1174<section anchor="combining.responses" title="Combining Partial Content">
1175<t>
1176   A response might transfer only a partial representation if the
1177   connection closed prematurely or if the request used one or more Range
1178   specifiers (&partial;).  After several such transfers, a cache might have
1179   received several ranges of the same representation.  A cache &MAY; combine
1180   these ranges into a single stored response, and reuse that response to
1181   satisfy later requests, if they all share the same strong validator and
1182   the cache complies with the client requirements in &combining-byte-ranges;.
1183</t>
1184<t>
1185   When combining the new response with one or more stored responses, a
1186   cache &MUST;:
1187   <list style="symbols">
1188      <t>delete any Warning header fields in the stored response with
1189         warn-code 1xx (see <xref target="header.warning" />);</t>
1190      <t>retain any Warning header fields in the stored response with
1191         warn-code 2xx; and,</t>
1192      <t>use other header fields provided in the new response, aside
1193         from Content-Range, to replace all instances of the corresponding
1194         header fields in the stored response.</t>
1195   </list>
1196</t>
1197</section>
1198
1199
1200</section>
1201
1202<section anchor="header.field.definitions" title="Header Field Definitions">
1203<t>
1204   This section defines the syntax and semantics of HTTP/1.1 header fields
1205   related to caching.
1206</t>
1207
1208<section anchor="header.age" title="Age">
1209   <iref item="Age header field" primary="true" x:for-anchor="" />
1210   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1211   <x:anchor-alias value="Age"/>
1212   <x:anchor-alias value="age-value"/>
1213<t>
1214   The "Age" header field conveys the sender's estimate of the amount
1215   of time since the response was generated or successfully validated at the
1216   origin server. Age values are calculated as specified in <xref
1217   target="age.calculations" />.
1218</t>
1219<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1220  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1221</artwork></figure>
1222<t>
1223  Age field-values are non-negative integers, representing time in seconds
1224  (see <xref target="delta-seconds"/>).
1225</t>
1226<t>
1227   The presence of an Age header field in a response implies that a response
1228   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1229   might not implement the Age header field.
1230</t>
1231</section>
1232
1233<section anchor="header.cache-control" title="Cache-Control">
1234   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1235   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1236      x:for-anchor="" />
1237   <x:anchor-alias value="Cache-Control"/>
1238   <x:anchor-alias value="cache-directive"/>
1239   <x:anchor-alias value="cache-extension"/>
1240   <x:anchor-alias value="cache-request-directive"/>
1241   <x:anchor-alias value="cache-response-directive"/>
1242<t>
1243   The "Cache-Control" header field is used to specify directives for
1244   caches along the request/response chain. Such cache directives are
1245   unidirectional in that the presence of a directive in a request does not
1246   imply that the same directive is to be given in the response.
1247</t>
1248<t>
1249   A cache &MUST; obey the requirements of the Cache-Control
1250   directives defined in this section. See <xref
1251   target="cache.control.extensions"/> for information about how Cache-Control
1252   directives defined elsewhere are handled.
1253</t>
1254<x:note>
1255   <t>
1256       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1257       might only implement Pragma: no-cache (see <xref target="header.pragma"
1258       />).
1259   </t>
1260</x:note>
1261<t>
1262   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1263   through in forwarded messages, regardless of their
1264   significance to that application, since the directives might be applicable
1265   to all recipients along the request/response chain. It is not possible to
1266   target a directive to a specific cache.
1267</t>
1268<t>Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.</t>
1269<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1270  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1271
1272  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1273     / <x:ref>cache-response-directive</x:ref>
1274
1275  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1276</artwork></figure>
1277
1278<section anchor="cache-request-directive" 
1279   title="Request Cache-Control Directives">
1280   <x:anchor-alias value="cache-request-directive" />
1281
1282<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1283   subitem="cache-request-directive" />
1284  <x:ref>cache-request-directive</x:ref> =
1285       "no-cache"
1286     / "no-store"
1287     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1288     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1289     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1290     / "no-transform"
1291     / "only-if-cached"
1292     / <x:ref>cache-extension</x:ref>
1293</artwork></figure>
1294
1295<t>
1296   <x:dfn>no-cache</x:dfn>
1297   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1298   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1299   <list>
1300      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1301      use a stored response to satisfy the request without successful
1302      validation on the origin server.</t> 
1303   </list>
1304</t>
1305<t>
1306   <x:dfn>no-store</x:dfn>
1307   <iref item="Cache Directives" primary="true" subitem="no-store" />
1308   <iref item="no-store" primary="true" subitem="Cache Directive" />
1309   <list>
1310      <t>The no-store request directive indicates that a cache &MUST-NOT;
1311      store any part of either this request or any response to it. This
1312      directive applies to both private and shared caches. "&MUST-NOT;
1313      store" in this context means that the cache &MUST-NOT; intentionally
1314      store the information in non-volatile storage, and &MUST; make a
1315      best-effort attempt to remove the information from volatile storage as
1316      promptly as possible after forwarding it.</t>
1317      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1318      privacy. In particular, malicious or compromised caches might not
1319      recognize or obey this directive, and communications networks might be
1320      vulnerable to eavesdropping.</t>
1321      <t>Note that if a request containing this directive is satisfied from a
1322      cache, the no-store request directive does not apply to the already
1323      stored response.</t>
1324   </list>
1325</t>
1326<t>
1327   <x:dfn>max-age</x:dfn>
1328   <iref item="Cache Directives" primary="true" subitem="max-age" />
1329   <iref item="max-age" primary="true" subitem="Cache Directive" />
1330   <list>
1331      <t>The max-age request directive indicates that the client is unwilling to
1332      accept a response whose age is greater than the specified number of
1333      seconds. Unless the max-stale request directive is also present, the
1334      client is not willing to accept a stale response.</t>
1335   </list>
1336</t>
1337<t>
1338   <x:dfn>max-stale</x:dfn>
1339   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1340   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1341   <list>
1342      <t>The max-stale request directive indicates that the client is willing
1343      to accept a response that has exceeded its expiration time. If max-stale
1344      is assigned a value, then the client is willing to accept a response
1345      that has exceeded its expiration time by no more than the specified
1346      number of seconds. If no value is assigned to max-stale, then the client
1347      is willing to accept a stale response of any age.</t>
1348   </list>
1349</t>
1350<t>
1351   <x:dfn>min-fresh</x:dfn>
1352   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1353   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1354   <list>
1355      <t>The min-fresh request directive indicates that the client is willing
1356      to accept a response whose freshness lifetime is no less than its
1357      current age plus the specified time in seconds. That is, the client
1358      wants a response that will still be fresh for at least the specified
1359      number of seconds.</t>
1360   </list>
1361</t>
1362<t>
1363   <x:dfn>no-transform</x:dfn>
1364   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1365   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1366   <list>
1367      <t>The no-transform request directive indicates that an intermediary
1368        (whether or not it implements a cache) &MUST-NOT; change the
1369        Content-Encoding, Content-Range or Content-Type request header fields,
1370        nor the request representation.</t>
1371   </list>
1372</t>
1373<t>
1374   <x:dfn>only-if-cached</x:dfn>
1375   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1376   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1377   <list>
1378      <t>The only-if-cached request directive indicates that the client only
1379      wishes to obtain a stored response. If it receives this directive, a
1380      cache &SHOULD; either respond using a stored response that is consistent
1381      with the other constraints of the request, or respond with a 504
1382      (Gateway Timeout) status code. If a group of caches is being operated as
1383      a unified system with good internal connectivity, a member cache &MAY;
1384      forward such a request within that group of caches.</t>
1385   </list>
1386</t>
1387</section>
1388
1389<section anchor="cache-response-directive" 
1390   title="Response Cache-Control Directives">
1391   <x:anchor-alias value="cache-response-directive" />
1392
1393<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1394   subitem="cache-response-directive" />
1395  <x:ref>cache-response-directive</x:ref> =
1396       "public"
1397     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1398     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1399     / "no-store"
1400     / "no-transform"
1401     / "must-revalidate"
1402     / "proxy-revalidate"
1403     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1404     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1405     / <x:ref>cache-extension</x:ref>
1406</artwork></figure>
1407
1408<t>
1409   <x:dfn>public</x:dfn>
1410   <iref item="Cache Directives" primary="true" subitem="public" />
1411   <iref item="public" primary="true" subitem="Cache Directive" />
1412   <list>
1413      <t>The public response directive indicates that a response whose
1414        associated request contains an 'Authentication' header &MAY; be
1415        stored (see <xref target="caching.authenticated.responses" />).</t>
1416  </list>
1417</t>
1418<t>
1419   <x:dfn>private</x:dfn>
1420   <iref item="Cache Directives" primary="true" subitem="private" />
1421   <iref item="private" primary="true" subitem="Cache Directive" />
1422   <list>
1423      <t>The private response directive indicates that the response message is
1424      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1425      private cache &MAY; store the response.</t>
1426      <t>If the private response directive specifies one or more field-names,
1427      this requirement is limited to the field-values associated with the
1428      listed response header fields. That is, a shared cache &MUST-NOT; store
1429      the specified field-names(s), whereas it &MAY; store the remainder of the
1430      response message.</t>
1431      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1432      the response can be stored; it cannot ensure the privacy of the message
1433      content. Also, private response directives with field-names are often
1434      handled by implementations as if an unqualified private directive was
1435      received; i.e., the special handling for the qualified form is not
1436      widely implemented.</t>
1437   </list>
1438</t>
1439<t>
1440   <x:dfn>no-cache</x:dfn>
1441   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1442   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1443   <list>
1444      <t>The no-cache response directive indicates that the response MUST NOT
1445      be used to satisfy a subsequent request without successful validation on
1446      the origin server. This allows an origin server to prevent a cache from
1447      using it to satisfy a request without contacting it, even by caches that
1448      have been configured to return stale responses.</t>
1449      <t>If the no-cache response directive specifies one or more field-names,
1450      this requirement is limited to the field-values associated with the
1451      listed response header fields. That is, a cache &MUST-NOT; send the
1452      specified field-name(s) in the response to a subsequent request without successful
1453      validation on the origin server. This allows an origin server to prevent
1454      the re-use of certain header fields in a response, while still allowing
1455      caching of the rest of the response.</t>
1456      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1457      this directive. Also, no-cache response directives with field-names are
1458      often handled by implementations as if an unqualified no-cache directive
1459      was received; i.e., the special handling for the qualified form is not
1460      widely implemented. </t>
1461   </list>
1462</t>
1463<t>
1464   <x:dfn>no-store</x:dfn>
1465   <iref item="Cache Directives" primary="true" subitem="no-store" />
1466   <iref item="no-store" primary="true" subitem="Cache Directive" />
1467   <list>
1468      <t>The no-store response directive indicates that a cache &MUST-NOT;
1469      store any part of either the immediate request or response. This
1470      directive applies to both private and shared caches. "&MUST-NOT;
1471      store" in this context means that the cache &MUST-NOT; intentionally
1472      store the information in non-volatile storage, and &MUST; make a
1473      best-effort attempt to remove the information from volatile storage as
1474      promptly as possible after forwarding it.</t>
1475      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1476      privacy. In particular, malicious or compromised caches might not
1477      recognize or obey this directive, and communications networks might be
1478      vulnerable to eavesdropping.</t>
1479   </list>
1480</t>
1481<t>
1482   <x:dfn>must-revalidate</x:dfn>
1483   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1484   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1485   <list>
1486      <t>The must-revalidate response directive indicates that once it has
1487      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1488      requests without successful validation on the origin server.</t>
1489      <t>The must-revalidate directive is necessary to support reliable
1490      operation for certain protocol features. In all circumstances a
1491      cache &MUST; obey the must-revalidate directive; in particular,
1492      if a cache cannot reach the origin server for any reason, it &MUST;
1493      generate a 504 (Gateway Timeout) response.</t>
1494      <t>The must-revalidate directive ought to be used by servers if and only
1495      if failure to validate a request on the representation could result in
1496      incorrect operation, such as a silently unexecuted financial
1497      transaction.</t>
1498   </list>
1499</t>
1500<t>
1501   <x:dfn>proxy-revalidate</x:dfn>
1502   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1503   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1504   <list>
1505      <t>The proxy-revalidate response directive has the same meaning as the
1506      must-revalidate response directive, except that it does not apply to
1507      private caches.</t>
1508   </list>
1509</t>
1510<t>
1511   <x:dfn>max-age</x:dfn>
1512   <iref item="Cache Directives" primary="true" subitem="max-age" />
1513   <iref item="max-age" primary="true" subitem="Cache Directive" />
1514   <list>
1515      <t>The max-age response directive indicates that the response is to be
1516      considered stale after its age is greater than the specified number of
1517      seconds.</t>
1518   </list>
1519</t>
1520<t>
1521   <x:dfn>s-maxage</x:dfn>
1522   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1523   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1524   <list>
1525      <t>The s-maxage response directive indicates that, in shared caches, the
1526      maximum age specified by this directive overrides the maximum age
1527      specified by either the max-age directive or the Expires header field. The
1528      s-maxage directive also implies the semantics of the proxy-revalidate
1529      response directive.</t>
1530   </list>
1531</t>
1532<t>
1533   <x:dfn>no-transform</x:dfn>
1534   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1535   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1536   <list>
1537      <t>The no-transform response directive indicates that an intermediary
1538      (regardless of whether it implements a cache) &MUST-NOT; change the
1539      Content-Encoding, Content-Range or Content-Type response header fields,
1540      nor the response representation.</t>
1541   </list>
1542</t>
1543
1544</section>
1545
1546<section anchor="cache.control.extensions" title="Cache Control Extensions">
1547<t>
1548   The Cache-Control header field can be extended through the use of one or
1549   more cache-extension tokens, each with an optional value. Informational
1550   extensions (those that do not require a change in cache behavior) can be
1551   added without changing the semantics of other directives. Behavioral
1552   extensions are designed to work by acting as modifiers to the existing base
1553   of cache directives. Both the new directive and the standard directive are
1554   supplied, such that applications that do not understand the new directive
1555   will default to the behavior specified by the standard directive, and those
1556   that understand the new directive will recognize it as modifying the
1557   requirements associated with the standard directive. In this way,
1558   extensions to the cache-control directives can be made without requiring
1559   changes to the base protocol.
1560</t>
1561<t>
1562   This extension mechanism depends on an HTTP cache obeying all of the
1563   cache-control directives defined for its native HTTP-version, obeying
1564   certain extensions, and ignoring all directives that it does not
1565   understand.
1566</t>
1567<t>
1568   For example, consider a hypothetical new response directive called
1569   "community" that acts as a modifier to the private directive. We define
1570   this new directive to mean that, in addition to any private cache, any
1571   cache that is shared only by members of the community named within its
1572   value may cache the response. An origin server wishing to allow the UCI
1573   community to use an otherwise private response in their shared cache(s)
1574   could do so by including
1575</t>
1576<figure><artwork type="example">
1577  Cache-Control: private, community="UCI"
1578</artwork></figure>
1579<t>
1580   A cache seeing this header field will act correctly even if the cache does
1581   not understand the community cache-extension, since it will also see and
1582   understand the private directive and thus default to the safe behavior.
1583</t>
1584<t>
1585   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1586   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1587   combined with standard directives (or the response's default cacheability)
1588   such that the cache behavior will remain minimally correct even if the
1589   cache does not understand the extension(s).
1590</t>
1591<t>
1592   The HTTP Cache Directive Registry defines the name space for the cache
1593   directives.
1594</t>
1595<t>
1596   A registration &MUST; include the following fields:
1597   <list style="symbols">
1598      <t>Cache Directive Name</t>
1599      <t>Pointer to specification text</t>
1600   </list>
1601</t>
1602<t>
1603   Values to be added to this name space are subject to IETF review (<xref
1604   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1605</t>
1606<t>
1607   The registry itself is maintained at <eref
1608   target="http://www.iana.org/assignments/http-cache-directives"/>.
1609</t>
1610</section>
1611
1612</section>
1613
1614<section anchor="header.expires" title="Expires">
1615   <iref item="Expires header field" primary="true" x:for-anchor="" />
1616   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1617   <x:anchor-alias value="Expires"/>
1618<t>
1619   The "Expires" header field gives the date/time after which the
1620   response is considered stale. See <xref target="expiration.model" /> for
1621   further discussion of the freshness model.
1622</t>
1623<t>
1624   The presence of an Expires field does not imply that the original resource
1625   will change or cease to exist at, before, or after that time.
1626</t>
1627<t>
1628   The field-value is an absolute date and time as defined by HTTP-date in
1629   &http-date;; a sender &MUST; use the rfc1123-date format.
1630</t>
1631<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1632  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1633</artwork></figure>
1634<figure>
1635  <preamble>For example</preamble>
1636<artwork type="example">
1637  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1638</artwork></figure>
1639<t>
1640   A cache &MUST; treat other invalid date formats,
1641   especially including the value "0", as in the past (i.e., "already
1642   expired").
1643</t>
1644<x:note>
1645   <t>
1646       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1647       max-age directive (see <xref target="cache-response-directive" />),
1648       that directive overrides the Expires field. Likewise, the s-maxage
1649       directive overrides Expires in shared caches.
1650   </t>
1651</x:note>
1652<t>
1653   Historically, HTTP required the Expires field-value to be no more than a
1654   year in the future. While longer freshness lifetimes are no longer
1655   prohibited, extremely large values have been demonstrated to cause
1656   problems (e.g., clock overflows due to use of 32-bit integers for
1657   time values), and most caches will evict a response far sooner than
1658   that. Therefore, senders ought not produce them.
1659</t>
1660<t>
1661   An origin server without a clock &MUST-NOT; assign Expires
1662   values to a response unless these values were associated
1663   with the resource by a system or user with a reliable clock. It &MAY;
1664   assign an Expires value that is known, at or before server
1665   configuration time, to be in the past (this allows "pre-expiration"
1666   of responses without storing separate Expires values for each
1667   resource).
1668</t>
1669</section>
1670
1671<section anchor="header.pragma" title="Pragma">
1672   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1673   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1674   <x:anchor-alias value="extension-pragma"/>
1675   <x:anchor-alias value="Pragma"/>
1676   <x:anchor-alias value="pragma-directive"/>
1677<t>
1678   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1679   caches, so that clients can specify a "no-cache" request that they will
1680   understand (as Cache-Control was not defined until HTTP/1.1). When the
1681   Cache-Control header is also present and understood in a request, Pragma is
1682   ignored.
1683</t>
1684<t>
1685   In HTTP/1.0, Pragma was defined as an extensible field for
1686   implementation-specified directives for recipients. This specification
1687   deprecates such extensions to improve interoperability.
1688</t>
1689<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"/>
1690  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1691  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1692  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1693</artwork></figure>
1694<t>
1695   When the Cache-Control header is not present in a request, the no-cache
1696   request pragma-directive &MUST; have the same effect on caches as if
1697   "Cache-Control: no-cache" were present (see <xref
1698   target="cache-request-directive" />).
1699</t>
1700<t>
1701   When sending a no-cache request, a client ought to include both the pragma
1702   and cache-control directives, unless Cache-Control: no-cache is
1703   purposefully omitted to target other Cache-Control response directives at
1704   HTTP/1.1 caches. For example:
1705</t>
1706<figure>
1707<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1708GET / HTTP/1.1
1709Host: www.example.com
1710Cache-Control: max-age=30
1711Pragma: no-cache
1712
1713</artwork>
1714</figure>
1715<t>
1716   will constrain HTTP/1.1 caches to serve a response no older than 30
1717   seconds, while precluding implementations that do not understand
1718   Cache-Control from serving a cached response.
1719</t>
1720<x:note>
1721   <t>
1722      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
1723      specified, it does not provide a reliable replacement for
1724      "Cache-Control: no-cache" in them.
1725   </t>
1726</x:note>
1727</section>
1728
1729<section anchor="header.vary" title="Vary">
1730   <iref item="Vary header field" primary="true" x:for-anchor="" />
1731   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1732   <x:anchor-alias value="Vary"/>
1733<t>
1734   The "Vary" header field conveys the set of header fields
1735   that were used to select the representation.
1736</t>
1737<t>
1738   Caches use this information, in part, to determine whether a stored
1739   response can be used to satisfy a given request; see <xref
1740   target="caching.negotiated.responses" />. determines, while the response is
1741   fresh, whether a cache is permitted to use the response to reply to a
1742   subsequent request without validation; see <xref
1743   target="caching.negotiated.responses" />.
1744</t>
1745<t>
1746   In uncacheable or stale responses, the Vary field value advises the user
1747   agent about the criteria that were used to select the representation.
1748</t>
1749<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1750  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1751</artwork></figure>
1752<t>
1753   The set of header fields named by the Vary field value is known as the
1754   selecting header fields.
1755</t>
1756<t>
1757   A server &SHOULD; include a Vary header field with any cacheable response
1758   that is subject to server-driven negotiation. Doing so allows a cache to
1759   properly interpret future requests on that resource and informs the user
1760   agent about the presence of negotiation on that resource. A server &MAY;
1761   include a Vary header field with a non-cacheable response that is subject
1762   to server-driven negotiation, since this might provide the user agent with
1763   useful information about the dimensions over which the response varies at
1764   the time of the response.
1765</t>
1766<t>
1767   A Vary field value of "*" signals that unspecified parameters not limited
1768   to the header fields (e.g., the network address of the client), play a
1769   role in the selection of the response representation; therefore, a cache
1770   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1771   generate the "*" value.
1772</t>
1773<t>
1774   The field-names given are not limited to the set of standard header
1775   fields defined by this specification. Field names are case-insensitive.
1776</t>
1777</section>
1778
1779<section anchor="header.warning" title="Warning">
1780   <iref item="Warning header field" primary="true" x:for-anchor="" />
1781   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1782   <x:anchor-alias value="Warning"/>
1783   <x:anchor-alias value="warning-value"/>
1784   <x:anchor-alias value="warn-agent"/>
1785   <x:anchor-alias value="warn-code"/>
1786   <x:anchor-alias value="warn-date"/>
1787   <x:anchor-alias value="warn-text"/>
1788<t>
1789   The "Warning" header field is used to carry additional information
1790   about the status or transformation of a message that might not be reflected
1791   in the message. This information is typically used to warn about possible
1792   incorrectness introduced by caching operations or transformations applied
1793   to the payload of the message.
1794</t>
1795<t>
1796   Warnings can be used for other purposes, both cache-related and otherwise.
1797   The use of a warning, rather than an error status code, distinguishes these
1798   responses from true failures.
1799</t>
1800<t>
1801   Warning header fields can in general be applied to any message, however some
1802   warn-codes are specific to caches and can only be applied to response
1803   messages.
1804</t>
1805<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"/>
1806  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1807 
1808  <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>
1809                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1810 
1811  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1812  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1813                  ; the name or pseudonym of the server adding
1814                  ; the Warning header field, for use in debugging
1815  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1816  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1817</artwork></figure>
1818<t>
1819   Multiple warnings can be attached to a response (either by the origin
1820   server or by a cache), including multiple warnings with the same code
1821   number, only differing in warn-text.
1822</t>
1823<t>
1824   When this occurs, the user agent &SHOULD; inform the user of as many of
1825   them as possible, in the order that they appear in the response.
1826</t>
1827<t>
1828   Systems that generate multiple Warning header fields are encouraged to
1829   order them with this user agent behavior in mind. New Warning header fields
1830   are added after any existing Warning headers fields.
1831</t>
1832<t>
1833   Warnings are assigned three digit warn-codes. The first digit indicates
1834   whether the Warning is required to be deleted from a stored response after
1835   validation:
1836   <list style="symbols">
1837      <t>1xx Warnings describe the freshness or validation status of the
1838      response, and so &MUST; be deleted by a cache after validation. They can
1839      only be generated by a cache when validating a cached entry, and
1840      &MUST-NOT; be generated in any other situation.</t>
1841      <t>2xx Warnings describe some aspect of the representation that is not
1842      rectified by a validation (for example, a lossy compression of the
1843      representation) and &MUST-NOT; be deleted by a cache after validation,
1844      unless a full response is returned, in which case they &MUST; be.</t>
1845   </list>
1846</t>
1847<t>
1848   If an implementation sends a message with one or more Warning header fields to a
1849   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1850   in each warning-value a warn-date that matches the Date header field in the
1851   message.
1852</t>
1853<t>
1854   If a system receives a message with a warning-value that includes
1855   a warn-date, and that warn-date is different from the Date value in the
1856   response, then that warning-value &MUST; be deleted from the message before
1857   storing, forwarding, or using it. (preventing the consequences of naive
1858   caching of Warning header fields.) If all of the warning-values are deleted
1859   for this reason, the Warning header field &MUST; be deleted as well.
1860</t>
1861<t>
1862   The following warn-codes are defined by this specification, each with a
1863   recommended warn-text in English, and a description of its meaning.
1864</t>
1865<t><?rfc needLines="4"?>
1866   110 Response is stale
1867   <list>
1868      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
1869   </list>
1870</t>
1871<t><?rfc needLines="4"?>
1872   111 Revalidation failed
1873   <list>
1874      <t>A cache &SHOULD; include this when returning a stale response because an
1875      attempt to validate the response failed, due to an inability to reach
1876      the server.</t>
1877   </list>
1878</t>
1879<t><?rfc needLines="4"?>
1880   112 Disconnected operation
1881   <list>
1882      <t>A cache &SHOULD; include this if it is intentionally disconnected from
1883      the rest of the network for a period of time.</t>
1884   </list>
1885</t>
1886<t><?rfc needLines="4"?>
1887   113 Heuristic expiration
1888   <list>
1889      <t>A cache &SHOULD; include this if it heuristically chose a freshness
1890      lifetime greater than 24 hours and the response's age is greater than 24
1891      hours.</t>
1892   </list>
1893</t>
1894<t><?rfc needLines="4"?>
1895   199 Miscellaneous warning
1896   <list>
1897      <t>The warning text can include arbitrary information to be presented to
1898      a human user, or logged. A system receiving this warning &MUST-NOT; take
1899      any automated action, besides presenting the warning to the user.</t>
1900   </list>
1901</t>
1902<t><?rfc needLines="4"?>
1903   214 Transformation applied
1904   <list>
1905      <t>&MUST; be added by a proxy if it applies any
1906      transformation to the representation, such as changing the
1907      content-coding, media-type, or modifying the representation data, unless
1908      this Warning code already appears in the response.</t>
1909   </list>
1910</t>
1911<t><?rfc needLines="4"?>
1912   299 Miscellaneous persistent warning
1913   <list>
1914      <t>The warning text can include arbitrary information to be presented to
1915      a human user, or logged. A system receiving this warning &MUST-NOT; take
1916      any automated action.</t>
1917   </list>
1918</t>
1919</section>
1920
1921</section>
1922
1923<section anchor="history.lists" title="History Lists">
1924<t>
1925   User agents often have history mechanisms, such as "Back" buttons and
1926   history lists, that can be used to redisplay a representation retrieved
1927   earlier in a session.
1928</t>
1929<t>
1930   The freshness model (<xref target="expiration.model"/>) does not
1931   necessarily apply to history mechanisms. I.e., a history mechanism can
1932   display a previous representation even if it has expired.
1933</t>
1934<t>
1935   This does not prohibit the history mechanism from telling the user that a
1936   view might be stale, or from honoring cache directives (e.g.,
1937   Cache-Control: no-store).
1938</t>
1939</section>
1940
1941
1942<section anchor="IANA.considerations" title="IANA Considerations">
1943
1944<section title="Cache Directive Registry" 
1945   anchor="cache.directive.registration">
1946<t>
1947   The registration procedure for HTTP Cache Directives is defined by <xref
1948   target="cache.control.extensions"/> of this document.
1949</t>
1950<t>
1951   The HTTP Cache Directive Registry shall be created at <eref
1952   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1953   populated with the registrations below:
1954</t>
1955<?BEGININC p6-cache.cache-directives ?>
1956<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1957<texttable xmlns:my="#my" align="left" suppress-title="true"
1958           anchor="iana.cache.directive.registration.table">
1959   <ttcol>Cache Directive</ttcol>
1960   <ttcol>Reference</ttcol>
1961
1962   <c>max-age</c>
1963   <c>
1964      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1965   </c>
1966   <c>max-stale</c>
1967   <c>
1968      <xref target="cache-request-directive"/>
1969   </c>
1970   <c>min-fresh</c>
1971   <c>
1972      <xref target="cache-request-directive"/>
1973   </c>
1974   <c>must-revalidate</c>
1975   <c>
1976      <xref target="cache-response-directive"/>
1977   </c>
1978   <c>no-cache</c>
1979   <c>
1980      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1981   </c>
1982   <c>no-store</c>
1983   <c>
1984      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1985   </c>
1986   <c>no-transform</c>
1987   <c>
1988      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1989   </c>
1990   <c>only-if-cached</c>
1991   <c>
1992      <xref target="cache-request-directive"/>
1993   </c>
1994   <c>private</c>
1995   <c>
1996      <xref target="cache-response-directive"/>
1997   </c>
1998   <c>proxy-revalidate</c>
1999   <c>
2000      <xref target="cache-response-directive"/>
2001   </c>
2002   <c>public</c>
2003   <c>
2004      <xref target="cache-response-directive"/>
2005   </c>
2006   <c>s-maxage</c>
2007   <c>
2008      <xref target="cache-response-directive"/>
2009   </c>
2010   <c>stale-if-error</c>
2011   <c>
2012      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
2013   </c>
2014   <c>stale-while-revalidate</c>
2015   <c>
2016      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
2017   </c>
2018</texttable>
2019<!--(END)-->
2020<?ENDINC p6-cache.cache-directives ?>
2021</section>
2022
2023<section title="Header Field Registration" anchor="header.field.registration">
2024<t>
2025  The Message Header Field Registry located at <eref 
2026  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
2027  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
2028</t>
2029<?BEGININC p6-cache.iana-headers ?>
2030<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2031<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
2032   <ttcol>Header Field Name</ttcol>
2033   <ttcol>Protocol</ttcol>
2034   <ttcol>Status</ttcol>
2035   <ttcol>Reference</ttcol>
2036
2037   <c>Age</c>
2038   <c>http</c>
2039   <c>standard</c>
2040   <c>
2041      <xref target="header.age"/>
2042   </c>
2043   <c>Cache-Control</c>
2044   <c>http</c>
2045   <c>standard</c>
2046   <c>
2047      <xref target="header.cache-control"/>
2048   </c>
2049   <c>Expires</c>
2050   <c>http</c>
2051   <c>standard</c>
2052   <c>
2053      <xref target="header.expires"/>
2054   </c>
2055   <c>Pragma</c>
2056   <c>http</c>
2057   <c>standard</c>
2058   <c>
2059      <xref target="header.pragma"/>
2060   </c>
2061   <c>Vary</c>
2062   <c>http</c>
2063   <c>standard</c>
2064   <c>
2065      <xref target="header.vary"/>
2066   </c>
2067   <c>Warning</c>
2068   <c>http</c>
2069   <c>standard</c>
2070   <c>
2071      <xref target="header.warning"/>
2072   </c>
2073</texttable>
2074<!--(END)-->
2075<?ENDINC p6-cache.iana-headers ?>
2076<t>
2077   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2078   Force".
2079</t>
2080</section>
2081
2082</section>
2083
2084<section anchor="security.considerations" title="Security Considerations">
2085<t>
2086   Caches expose additional potential vulnerabilities, since the contents of
2087   the cache represent an attractive target for malicious exploitation.
2088   Because cache contents persist after an HTTP request is complete, an attack
2089   on the cache can reveal information long after a user believes that the
2090   information has been removed from the network. Therefore, cache contents
2091   need to be protected as sensitive information.
2092</t>
2093</section>
2094
2095<section title="Acknowledgments" anchor="acks">
2096<t>
2097  See &acks;.
2098</t>
2099</section>
2100
2101</middle>
2102
2103<back>
2104<references title="Normative References">
2105
2106  <reference anchor="Part1">
2107    <front>
2108      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2109      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2110        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2111        <address><email>fielding@gbiv.com</email></address>
2112      </author>
2113      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2114        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2115        <address><email>jg@freedesktop.org</email></address>
2116      </author>
2117      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2118        <organization abbrev="HP">Hewlett-Packard Company</organization>
2119        <address><email>JeffMogul@acm.org</email></address>
2120      </author>
2121      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2122        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2123        <address><email>henrikn@microsoft.com</email></address>
2124      </author>
2125      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2126        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2127        <address><email>LMM@acm.org</email></address>
2128      </author>
2129      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2130        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2131        <address><email>paulle@microsoft.com</email></address>
2132      </author>
2133      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2134        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2135        <address><email>timbl@w3.org</email></address>
2136      </author>
2137      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2138        <organization abbrev="W3C">World Wide Web Consortium</organization>
2139        <address><email>ylafon@w3.org</email></address>
2140      </author>
2141      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2142        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2143        <address><email>julian.reschke@greenbytes.de</email></address>
2144      </author>
2145      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2146    </front>
2147    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2148    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2149  </reference>
2150
2151  <reference anchor="Part2">
2152    <front>
2153      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2154      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2155        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2156        <address><email>fielding@gbiv.com</email></address>
2157      </author>
2158      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2159        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2160        <address><email>jg@freedesktop.org</email></address>
2161      </author>
2162      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2163        <organization abbrev="HP">Hewlett-Packard Company</organization>
2164        <address><email>JeffMogul@acm.org</email></address>
2165      </author>
2166      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2167        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2168        <address><email>henrikn@microsoft.com</email></address>
2169      </author>
2170      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2171        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2172        <address><email>LMM@acm.org</email></address>
2173      </author>
2174      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2175        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2176        <address><email>paulle@microsoft.com</email></address>
2177      </author>
2178      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2179        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2180        <address><email>timbl@w3.org</email></address>
2181      </author>
2182      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2183        <organization abbrev="W3C">World Wide Web Consortium</organization>
2184        <address><email>ylafon@w3.org</email></address>
2185      </author>
2186      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2187        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2188        <address><email>julian.reschke@greenbytes.de</email></address>
2189      </author>
2190      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2191    </front>
2192    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2193    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2194  </reference>
2195
2196  <reference anchor="Part4">
2197    <front>
2198      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2199      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2200        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2201        <address><email>fielding@gbiv.com</email></address>
2202      </author>
2203      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2204        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2205        <address><email>jg@freedesktop.org</email></address>
2206      </author>
2207      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2208        <organization abbrev="HP">Hewlett-Packard Company</organization>
2209        <address><email>JeffMogul@acm.org</email></address>
2210      </author>
2211      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2212        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2213        <address><email>henrikn@microsoft.com</email></address>
2214      </author>
2215      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2216        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2217        <address><email>LMM@acm.org</email></address>
2218      </author>
2219      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2220        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2221        <address><email>paulle@microsoft.com</email></address>
2222      </author>
2223      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2224        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2225        <address><email>timbl@w3.org</email></address>
2226      </author>
2227      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2228        <organization abbrev="W3C">World Wide Web Consortium</organization>
2229        <address><email>ylafon@w3.org</email></address>
2230      </author>
2231      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2232        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2233        <address><email>julian.reschke@greenbytes.de</email></address>
2234      </author>
2235      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2236    </front>
2237    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2238    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2239  </reference>
2240
2241  <reference anchor="Part5">
2242    <front>
2243      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2244      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2245        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2246        <address><email>fielding@gbiv.com</email></address>
2247      </author>
2248      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2249        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2250        <address><email>jg@freedesktop.org</email></address>
2251      </author>
2252      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2253        <organization abbrev="HP">Hewlett-Packard Company</organization>
2254        <address><email>JeffMogul@acm.org</email></address>
2255      </author>
2256      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2257        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2258        <address><email>henrikn@microsoft.com</email></address>
2259      </author>
2260      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2261        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2262        <address><email>LMM@acm.org</email></address>
2263      </author>
2264      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2265        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2266        <address><email>paulle@microsoft.com</email></address>
2267      </author>
2268      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2269        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2270        <address><email>timbl@w3.org</email></address>
2271      </author>
2272      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2273        <organization abbrev="W3C">World Wide Web Consortium</organization>
2274        <address><email>ylafon@w3.org</email></address>
2275      </author>
2276      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2277        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2278        <address><email>julian.reschke@greenbytes.de</email></address>
2279      </author>
2280      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2281    </front>
2282    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2283    <x:source basename="p5-range" href="p5-range.xml" />
2284  </reference>
2285
2286  <reference anchor="Part7">
2287    <front>
2288      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2289      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2290        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2291        <address><email>fielding@gbiv.com</email></address>
2292      </author>
2293      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2294        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2295        <address><email>jg@freedesktop.org</email></address>
2296      </author>
2297      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2298        <organization abbrev="HP">Hewlett-Packard Company</organization>
2299        <address><email>JeffMogul@acm.org</email></address>
2300      </author>
2301      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2302        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2303        <address><email>henrikn@microsoft.com</email></address>
2304      </author>
2305      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2306        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2307        <address><email>LMM@acm.org</email></address>
2308      </author>
2309      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2310        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2311        <address><email>paulle@microsoft.com</email></address>
2312      </author>
2313      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2314        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2315        <address><email>timbl@w3.org</email></address>
2316      </author>
2317      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2318        <organization abbrev="W3C">World Wide Web Consortium</organization>
2319        <address><email>ylafon@w3.org</email></address>
2320      </author>
2321      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2322        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2323        <address><email>julian.reschke@greenbytes.de</email></address>
2324      </author>
2325      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2326    </front>
2327    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2328    <x:source basename="p7-auth" href="p7-auth.xml" />
2329  </reference>
2330
2331  <reference anchor="RFC2119">
2332    <front>
2333      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2334      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2335        <organization>Harvard University</organization>
2336        <address><email>sob@harvard.edu</email></address>
2337      </author>
2338      <date month="March" year="1997" />
2339    </front>
2340    <seriesInfo name="BCP" value="14" />
2341    <seriesInfo name="RFC" value="2119" />
2342  </reference>
2343
2344  <reference anchor="RFC5234">
2345    <front>
2346      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2347      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2348        <organization>Brandenburg InternetWorking</organization>
2349        <address>
2350          <email>dcrocker@bbiw.net</email>
2351        </address> 
2352      </author>
2353      <author initials="P." surname="Overell" fullname="Paul Overell">
2354        <organization>THUS plc.</organization>
2355        <address>
2356          <email>paul.overell@thus.net</email>
2357        </address>
2358      </author>
2359      <date month="January" year="2008"/>
2360    </front>
2361    <seriesInfo name="STD" value="68"/>
2362    <seriesInfo name="RFC" value="5234"/>
2363  </reference>
2364 
2365</references>
2366
2367<references title="Informative References">
2368
2369  <reference anchor="RFC1305">
2370    <front>
2371      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2372      <author fullname="David L. Mills" initials="D." surname="Mills">
2373        <organization>University of Delaware, Electrical Engineering Department</organization>
2374        <address><email>mills@udel.edu</email></address>
2375      </author>
2376      <date month="March" year="1992" />
2377    </front>
2378    <seriesInfo name="RFC" value="1305" />
2379  </reference>
2380
2381  <reference anchor="RFC2616">
2382    <front>
2383      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2384      <author fullname="R. Fielding" initials="R." surname="Fielding">
2385        <organization>University of California, Irvine</organization>
2386        <address><email>fielding@ics.uci.edu</email></address>
2387      </author>
2388      <author fullname="J. Gettys" initials="J." surname="Gettys">
2389        <organization>W3C</organization>
2390        <address><email>jg@w3.org</email></address>
2391      </author>
2392      <author fullname="J. Mogul" initials="J." surname="Mogul">
2393        <organization>Compaq Computer Corporation</organization>
2394        <address><email>mogul@wrl.dec.com</email></address>
2395      </author>
2396      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2397        <organization>MIT Laboratory for Computer Science</organization>
2398        <address><email>frystyk@w3.org</email></address>
2399      </author>
2400      <author fullname="L. Masinter" initials="L." surname="Masinter">
2401        <organization>Xerox Corporation</organization>
2402        <address><email>masinter@parc.xerox.com</email></address>
2403      </author>
2404      <author fullname="P. Leach" initials="P." surname="Leach">
2405        <organization>Microsoft Corporation</organization>
2406        <address><email>paulle@microsoft.com</email></address>
2407      </author>
2408      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2409        <organization>W3C</organization>
2410        <address><email>timbl@w3.org</email></address>
2411      </author>
2412      <date month="June" year="1999" />
2413    </front>
2414    <seriesInfo name="RFC" value="2616" />
2415  </reference>
2416
2417  <reference anchor="RFC3864">
2418    <front>
2419      <title>Registration Procedures for Message Header Fields</title>
2420      <author fullname="G. Klyne" initials="G." surname="Klyne">
2421        <organization>Nine by Nine</organization>
2422        <address><email>GK-IETF@ninebynine.org</email></address>
2423      </author>
2424      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2425        <organization>BEA Systems</organization>
2426        <address><email>mnot@pobox.com</email></address>
2427      </author>
2428      <author fullname="J. Mogul" initials="J." surname="Mogul">
2429        <organization>HP Labs</organization>
2430        <address><email>JeffMogul@acm.org</email></address>
2431      </author>
2432      <date month="September" year="2004" />
2433    </front>
2434    <seriesInfo name="BCP" value="90" />
2435    <seriesInfo name="RFC" value="3864" />
2436  </reference>
2437
2438  <reference anchor='RFC5226'>
2439    <front>
2440      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2441      <author initials='T.' surname='Narten' fullname='T. Narten'>
2442        <organization>IBM</organization>
2443        <address><email>narten@us.ibm.com</email></address>
2444      </author>
2445      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2446        <organization>Google</organization>
2447        <address><email>Harald@Alvestrand.no</email></address>
2448      </author>
2449      <date year='2008' month='May' />
2450    </front>
2451    <seriesInfo name='BCP' value='26' />
2452    <seriesInfo name='RFC' value='5226' />
2453  </reference>
2454
2455  <reference anchor='RFC5861'>
2456    <front>
2457      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2458      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2459        <organization>Yahoo! Inc.</organization>
2460        <address><email>mnot@yahoo-inc.com</email></address>
2461      </author>
2462      <date month="April" year="2010"/>
2463    </front>
2464    <seriesInfo name='RFC' value='5861' />
2465  </reference>
2466
2467</references>
2468
2469<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2470<t>
2471  Make the specified age calculation algorithm less conservative.
2472  (<xref target="age.calculations"/>)
2473</t>
2474<t>
2475  Remove requirement to consider Content-Location in successful responses
2476  in order to determine the appropriate response to use.
2477  (<xref target="validation.model" />)
2478</t>
2479<t>
2480  Clarify denial of service attack avoidance requirement.
2481  (<xref target="invalidation.after.updates.or.deletions" />)
2482</t>
2483<t>
2484  Change ABNF productions for header fields to only define the field value.
2485  (<xref target="header.field.definitions"/>)
2486</t>
2487<t>
2488  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2489  anymore, as these aspects never were implemented.
2490  (<xref target="header.warning" />)
2491</t>
2492</section>
2493
2494<?BEGININC p6-cache.abnf-appendix ?>
2495<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2496<figure>
2497<artwork type="abnf" name="p6-cache.parsed-abnf">
2498<x:ref>Age</x:ref> = delta-seconds
2499
2500<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2501 cache-directive ] )
2502
2503<x:ref>Expires</x:ref> = HTTP-date
2504
2505<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 8&gt;
2506
2507<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2508
2509<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2510 pragma-directive ] )
2511
2512<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2513 ) )
2514
2515<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2516 )
2517
2518<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2519<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2520<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2521 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2522 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2523 cache-extension
2524<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2525 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2526 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2527 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2528 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2529 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2530
2531<x:ref>delta-seconds</x:ref> = 1*DIGIT
2532
2533<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2534
2535<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2536
2537<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2538<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2539<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.8&gt;
2540
2541<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
2542
2543<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.3&gt;
2544
2545<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2546
2547<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2548<x:ref>warn-code</x:ref> = 3DIGIT
2549<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2550<x:ref>warn-text</x:ref> = quoted-string
2551<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2552 ]
2553</artwork>
2554</figure>
2555<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2556; Age defined but not used
2557; Cache-Control defined but not used
2558; Expires defined but not used
2559; Pragma defined but not used
2560; Vary defined but not used
2561; Warning defined but not used
2562</artwork></figure></section>
2563<?ENDINC p6-cache.abnf-appendix ?>
2564
2565<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2566
2567<section title="Since RFC 2616">
2568  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2569</section>
2570
2571<section title="Since draft-ietf-httpbis-p6-cache-00">
2572<t>
2573  Closed issues:
2574  <list style="symbols">
2575    <t>
2576      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2577    <t>
2578      <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>
2579    <t>
2580      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2581    <t>
2582      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2583    <t>
2584      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2585    <t>
2586      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2587    <t>
2588      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2589    <t>
2590      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2591    <t>
2592      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2593  </list>
2594</t>
2595<t>
2596  Other changes:
2597  <list style="symbols">
2598    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2599        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2600  </list>
2601</t>
2602</section>
2603
2604<section title="Since draft-ietf-httpbis-p6-cache-01">
2605<t>
2606  Closed issues:
2607  <list style="symbols">
2608    <t>
2609      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2610  </list>
2611</t>
2612<t>
2613  Other changes:
2614  <list style="symbols">
2615    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2616      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2617    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2618      specification.</t>
2619  </list>
2620</t>
2621</section>
2622
2623<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2624<t>
2625  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2626  <list style="symbols">
2627    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2628      document.</t>
2629  </list>
2630</t>
2631</section>
2632
2633<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2634<t>
2635  Closed issues:
2636  <list style="symbols">
2637    <t>
2638      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2639  </list>
2640</t>
2641</section>
2642
2643<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2644<t>
2645  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2646  <list style="symbols"> 
2647    <t>
2648      Use "/" instead of "|" for alternatives.
2649    </t>
2650    <t>
2651      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2652      whitespace ("OWS") and required whitespace ("RWS").
2653    </t>
2654    <t>
2655      Rewrite ABNFs to spell out whitespace rules, factor out
2656      header field value format definitions.
2657    </t>
2658  </list>
2659</t>
2660</section>
2661
2662<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2663<t>
2664  This is a total rewrite of this part of the specification.
2665</t>
2666<t>
2667  Affected issues:
2668  <list style="symbols">
2669    <t>
2670      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2671    <t>
2672      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2673    <t>
2674      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2675    <t>
2676      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2677  </list>
2678</t>
2679<t>
2680  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2681  <list style="symbols"> 
2682    <t>
2683      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2684    </t>
2685  </list>
2686</t>
2687</section>
2688
2689<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2690<t>
2691  Closed issues:
2692  <list style="symbols"> 
2693    <t>
2694      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2695      "base for numeric protocol elements"
2696    </t>
2697  </list>
2698</t>
2699<t>
2700  Affected issues:
2701  <list style="symbols">
2702    <t>
2703      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2704      "Vary and non-existant headers"
2705    </t>
2706  </list>
2707</t>
2708</section>
2709
2710<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2711<t>
2712  Closed issues:
2713  <list style="symbols"> 
2714    <t>
2715      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2716      "Definition of 1xx Warn-Codes"
2717    </t>
2718    <t>
2719      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2720      "Content-Location on 304 responses"
2721    </t>
2722    <t>
2723      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2724      "private and no-cache CC directives with headers"
2725    </t>
2726    <t>
2727      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2728      "RFC2047 and warn-text"
2729    </t>
2730  </list>
2731</t>
2732</section>
2733
2734<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2735<t>
2736  Closed issues:
2737  <list style="symbols"> 
2738    <t>
2739      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2740      "serving negotiated responses from cache: header-specific canonicalization"
2741    </t>
2742    <t>
2743      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2744      "Effect of CC directives on history lists"
2745    </t>
2746    <t>
2747      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291" />:
2748      "Cache Extensions can override no-store, etc."
2749    </t>
2750  </list>
2751</t>
2752<t>
2753  Affected issues:
2754  <list style="symbols">
2755    <t>
2756      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2757      Status codes and caching
2758    </t>
2759  </list>
2760</t>
2761<t>
2762  Partly resolved issues:
2763  <list style="symbols"> 
2764    <t>
2765      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2766      "Placement of 13.5.1 and 13.5.2"
2767    </t>
2768  </list>
2769</t>
2770</section>
2771
2772<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2773<t>
2774  Closed issues:
2775  <list style="symbols"> 
2776    <t>
2777      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2778      "Age calculation"
2779    </t>
2780    <t>
2781      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2782      "Clarify differences between / requirements for request and response CC directives"
2783    </t>
2784    <t>
2785      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2786      "Caching authenticated responses"
2787    </t>
2788    <t>
2789      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2790      "IANA registry for cache-control directives"
2791    </t>
2792    <t>
2793      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2794      "Heuristic caching of URLs with query components"
2795    </t>
2796  </list>
2797</t>
2798<t>
2799  Partly resolved issues:
2800  <list style="symbols"> 
2801    <t>
2802      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2803      "Term for the requested resource's URI"
2804    </t>
2805  </list>
2806</t>
2807</section>
2808
2809<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2810<t>
2811  Closed issues:
2812  <list style="symbols"> 
2813    <t>
2814      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2815      "Clarify entity / representation / variant terminology"
2816    </t>
2817    <t>
2818      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2819      "consider removing the 'changes from 2068' sections"
2820    </t>
2821    <t>
2822      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2823      "Allowing heuristic caching for new status codes"
2824    </t>
2825    <t>
2826      Clean up TODOs and prose in "Combining Responses."
2827    </t>
2828  </list>
2829</t>
2830</section>
2831
2832<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2833<t>
2834  Closed issues:
2835  <list style="symbols"> 
2836    <t>
2837      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2838      "Text about clock requirement for caches belongs in p6"
2839    </t>
2840  </list>
2841</t>
2842</section>
2843
2844<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2845<t>
2846  Closed issues:
2847  <list style="symbols"> 
2848    <t>
2849      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2850      "Header Classification"
2851    </t>
2852    <t>
2853      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2854      "Clarify 'public'"
2855    </t>
2856  </list>
2857</t>
2858</section>
2859
2860<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2861<t>
2862  Closed issues:
2863  <list style="symbols">
2864    <t>
2865      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2866      "untangle ABNFs for header fields"
2867    </t>
2868  </list>
2869</t>
2870</section>
2871
2872<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2873<t>
2874  Closed issues:
2875  <list style="symbols">
2876    <t>
2877      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2878      "Mismatch Vary"
2879    </t>
2880    <t>
2881      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2882      "Cache Invalidation only happens upon successful responses"
2883    </t>
2884    <t>
2885      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2886      "Recommend minimum sizes for protocol elements"
2887    </t>
2888    <t>
2889      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2890      "Proxies don't 'understand' methods"
2891    </t>
2892    <t>
2893      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2894      "Cache Extensions can override no-store, etc."
2895    </t>
2896    <t>
2897      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2898      "Pragma"
2899    </t>
2900  </list>
2901</t>
2902</section>
2903
2904<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2905<t>
2906  Closed issues:
2907  <list style="symbols">
2908    <t>
2909      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/290"/>:
2910      "Motivate one-year limit for Expires"
2911    </t>
2912  </list>
2913</t>
2914</section>
2915
2916<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
2917<t>
2918  Closed issues:
2919  <list style="symbols">
2920    <t>
2921      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/186"/>:
2922      "Document HTTP's error-handling philosophy"
2923    </t>
2924    <t>
2925      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/317"/>:
2926      "Cache-Control directive case sensitivity"
2927    </t>
2928  </list>
2929</t>
2930</section>
2931
2932<section title="Since draft-ietf-httpbis-p6-cache-17" anchor="changes.since.17">
2933<t>
2934  Closed issues:
2935  <list style="symbols">
2936    <t>
2937      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/293"/>:
2938      "Interaction of request and response Cache-Control"
2939    </t>
2940  </list>
2941</t>
2942</section>
2943
2944</section>
2945  </back>
2946</rfc>
Note: See TracBrowser for help on using the repository browser.