source: draft-ietf-httpbis/latest/p6-cache.xml @ 1429

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bump up document dates; update to latest version of rfc2629.xslt, refresh appendices.

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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 "September">
16  <!ENTITY ID-YEAR "2011">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY acks                        "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY field-rules                 "<xref target='Part1' x:rel='#field.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY effective-request-uri       "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY semantics                   "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#header.etag' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY lastmod-comparison          "<xref target='Part4' x:rel='#lastmod.comparison' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY status.2xx                  "<xref target='Part2' x:rel='#status.2xx' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc rfcedstyle="yes"?>
52<?rfc-ext allow-markup-in-artwork="yes" ?>
53<?rfc-ext include-references-in-index="yes" ?>
54<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
55  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
56<front>
57
58  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
59
60  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
61    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
62    <address>
63      <postal>
64        <street>345 Park Ave</street>
65        <city>San Jose</city>
66        <region>CA</region>
67        <code>95110</code>
68        <country>USA</country>
69      </postal>
70      <email>fielding@gbiv.com</email>
71      <uri>http://roy.gbiv.com/</uri>
72    </address>
73  </author>
74
75  <author initials="J." surname="Gettys" fullname="Jim Gettys">
76    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
77    <address>
78      <postal>
79        <street>21 Oak Knoll Road</street>
80        <city>Carlisle</city>
81        <region>MA</region>
82        <code>01741</code>
83        <country>USA</country>
84      </postal>
85      <email>jg@freedesktop.org</email>
86      <uri>http://gettys.wordpress.com/</uri>
87    </address>
88  </author>
89
90  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
91    <organization abbrev="HP">Hewlett-Packard Company</organization>
92    <address>
93      <postal>
94        <street>HP Labs, Large Scale Systems Group</street>
95        <street>1501 Page Mill Road, MS 1177</street>
96        <city>Palo Alto</city>
97        <region>CA</region>
98        <code>94304</code>
99        <country>USA</country>
100      </postal>
101      <email>JeffMogul@acm.org</email>
102    </address>
103  </author>
104
105  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
106    <organization abbrev="Microsoft">Microsoft Corporation</organization>
107    <address>
108      <postal>
109        <street>1 Microsoft Way</street>
110        <city>Redmond</city>
111        <region>WA</region>
112        <code>98052</code>
113        <country>USA</country>
114      </postal>
115      <email>henrikn@microsoft.com</email>
116    </address>
117  </author>
118
119  <author fullname="Larry Masinter" initials="L." surname="Masinter">
120    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
121    <address>
122      <postal>
123        <street>345 Park Ave</street>
124        <city>San Jose</city>
125        <region>CA</region>
126        <code>95110</code>
127        <country>USA</country>
128      </postal>
129      <email>LMM@acm.org</email>
130      <uri>http://larry.masinter.net/</uri>
131    </address>
132  </author>
133
134  <author fullname="Paul J. Leach" initials="P." surname="Leach">
135    <organization abbrev="Microsoft">Microsoft Corporation</organization>
136    <address>
137      <postal>
138        <street>1 Microsoft Way</street>
139        <city>Redmond</city>
140        <region>WA</region>
141        <code>98052</code>
142      </postal>
143      <email>paulle@microsoft.com</email>
144    </address>
145  </author>
146
147  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
148    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
149    <address>
150      <postal>
151        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
152        <street>The Stata Center, Building 32</street>
153        <street>32 Vassar Street</street>
154        <city>Cambridge</city>
155        <region>MA</region>
156        <code>02139</code>
157        <country>USA</country>
158      </postal>
159      <email>timbl@w3.org</email>
160      <uri>http://www.w3.org/People/Berners-Lee/</uri>
161    </address>
162  </author>
163
164  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
165    <organization abbrev="W3C">World Wide Web Consortium</organization>
166    <address>
167      <postal>
168        <street>W3C / ERCIM</street>
169        <street>2004, rte des Lucioles</street>
170        <city>Sophia-Antipolis</city>
171        <region>AM</region>
172        <code>06902</code>
173        <country>France</country>
174      </postal>
175      <email>ylafon@w3.org</email>
176      <uri>http://www.raubacapeu.net/people/yves/</uri>
177    </address>
178  </author>
179
180  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
181    <address>
182      <email>mnot@mnot.net</email>
183      <uri>http://www.mnot.net/</uri>
184    </address>
185  </author>
186
187  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
188    <organization abbrev="greenbytes">greenbytes GmbH</organization>
189    <address>
190      <postal>
191        <street>Hafenweg 16</street>
192        <city>Muenster</city><region>NW</region><code>48155</code>
193        <country>Germany</country>
194      </postal>
195      <phone>+49 251 2807760</phone>
196      <facsimile>+49 251 2807761</facsimile>
197      <email>julian.reschke@greenbytes.de</email>
198      <uri>http://greenbytes.de/tech/webdav/</uri>
199    </address>
200  </author>
201
202  <date month="&ID-MONTH;" year="&ID-YEAR;" />
203  <workgroup>HTTPbis Working Group</workgroup>
204
205<abstract>
206<t>
207   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
208   distributed, collaborative, hypertext information systems. HTTP has been in
209   use by the World Wide Web global information initiative since 1990. This
210   document is Part 6 of the seven-part specification that defines the protocol
211   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616.
212</t>
213<t>
214   Part 6 defines requirements on HTTP caches and the associated header fields
215   that control cache behavior or indicate cacheable response messages.
216</t>
217</abstract>
218
219<note title="Editorial Note (To be removed by RFC Editor)">
220  <t>
221    Discussion of this draft should take place on the HTTPBIS working group
222    mailing list (ietf-http-wg@w3.org), which is archived at
223    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
224  </t>
225  <t>
226    The current issues list is at
227    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
228    documents (including fancy diffs) can be found at
229    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
230  </t>
231  <t>
232    The changes in this draft are summarized in <xref target="changes.since.16"/>.
233  </t>
234</note>
235
236   </front>
237   <middle>
238
239<section anchor="caching" title="Introduction">
240<t>
241   HTTP is typically used for distributed information systems, where
242   performance can be improved by the use of response caches. This document
243   defines aspects of HTTP/1.1 related to caching and reusing response
244   messages.
245</t>
246
247<section anchor="intro.purpose" title="Purpose">
248<iref item="cache" />
249<t>
250   An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the
251   subsystem that controls its message storage, retrieval, and deletion. A
252   cache stores cacheable responses in order to reduce the response time and
253   network bandwidth consumption on future, equivalent requests. Any client or
254   server &MAY; employ a cache, though a cache cannot be used by a server that
255   is acting as a tunnel.
256</t>
257<t>
258   The goal of caching in HTTP/1.1 is to significantly improve performance
259   by reusing a prior response message to satisfy a current request.
260   A stored response is considered "fresh", as defined in
261   <xref target="expiration.model" />, if the response can be reused without
262   "validation" (checking with the origin server to see if the cached response
263   remains valid for this request).  A fresh cache response can therefore
264   reduce both latency and network transfers each time it is reused.
265   When a cached response is not fresh, it might still be reusable if it can
266   be freshened by validation (<xref target="validation.model" />) or if the
267   origin is unavailable.
268</t>
269</section>
270
271<section anchor="intro.terminology" title="Terminology">
272<t>
273   This specification uses a number of terms to refer to the roles played by
274   participants in, and objects of, HTTP caching.
275</t>
276<t>
277   <iref item="cache" />
278   <x:dfn>cache</x:dfn>
279   <list>
280      <t>A conformant implementation of a HTTP cache. Note that this implies
281        an HTTP/1.1 cache; this specification does not define conformance
282        for HTTP/1.0 caches.</t>
283   </list>
284</t>
285<t anchor="shared.and.non-shared.caches">
286   <iref item="shared cache" />
287   <x:dfn>shared cache</x:dfn>
288   <list>
289      <t>A cache that stores responses to be reused by more than one user;
290         usually (but not always) deployed as part of an intermediary.</t>
291   </list>
292</t>
293<t>
294   <iref item="private cache" />
295   <x:dfn>private cache</x:dfn>
296   <list>
297      <t>A cache that is dedicated to a single user.</t>
298   </list>
299</t>
300<t>
301   <iref item="cacheable" />
302   <x:dfn>cacheable</x:dfn>
303   <list>
304      <t>A response is cacheable if a cache is allowed to store a copy of the
305      response message for use in answering subsequent requests. Even when a
306      response is cacheable, there might be additional constraints on whether
307      a cache can use the stored copy to satisfy a particular request.</t>
308   </list>
309</t>
310<t>
311   <iref item="explicit expiration time" />
312   <x:dfn>explicit expiration time</x:dfn>
313   <list>
314      <t>The time at which the origin server intends that a representation
315      no longer be returned by a cache without further validation.</t>
316   </list>
317</t>
318<t>
319   <iref item="heuristic expiration time" />
320   <x:dfn>heuristic expiration time</x:dfn>
321   <list>
322      <t>An expiration time assigned by a cache when no explicit expiration
323      time is available.</t>
324   </list>
325</t>
326<t>
327   <iref item="age" />
328   <x:dfn>age</x:dfn>
329   <list>
330      <t>The age of a response is the time since it was sent by, or
331      successfully validated with, the origin server.</t>
332   </list>
333</t>
334<t>
335   <iref item="first-hand" />
336   <x:dfn>first-hand</x:dfn>
337   <list>
338      <t>A response is first-hand if the freshness model is not in use; i.e.,
339      its age is 0.</t>
340   </list>
341</t>
342<t>
343   <iref item="freshness lifetime" />
344   <x:dfn>freshness lifetime</x:dfn>
345   <list>
346      <t>The length of time between the generation of a response and its
347      expiration time.</t>
348   </list>
349</t>
350<t>
351   <iref item="fresh" />
352   <x:dfn>fresh</x:dfn>
353   <list>
354      <t>A response is fresh if its age has not yet exceeded its freshness
355      lifetime.</t>
356   </list>
357</t>
358<t>
359   <iref item="stale" />
360   <x:dfn>stale</x:dfn>
361   <list>
362      <t>A response is stale if its age has passed its freshness lifetime
363      (either explicit or heuristic).</t>
364   </list>
365</t>
366<t>
367   <iref item="validator" />
368   <x:dfn>validator</x:dfn>
369   <list>
370      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
371      is used to find out whether a stored response is an equivalent copy of
372      a representation. See &weak-and-strong;.</t>
373   </list>
374</t>
375<t>
376   <iref item="strong validator" />
377   <iref item="validator" subitem="strong" />
378   <x:dfn>strong validator</x:dfn>
379   <list>
380      <t>A validator that is defined by the origin server such that its
381         current value will change if the representation body changes; i.e.,
382         an entity-tag that is not marked as weak (&entity-tags;) or,
383         if no entity-tag is provided, a Last-Modified value that is strong
384         in the sense defined by &lastmod-comparison;.</t>
385   </list>
386</t>
387</section>
388
389<section anchor="intro.requirements" title="Requirements">
390<t>
391   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
392   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
393   document are to be interpreted as described in <xref target="RFC2119"/>.
394</t>
395<t>
396   An implementation is not compliant if it fails to satisfy one or more of
397   the "MUST" or "REQUIRED" level requirements for the protocols it
398   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
399   level and all the "SHOULD" level requirements for its protocols is said to
400   be "unconditionally compliant"; one that satisfies all the "MUST" level
401   requirements but not all the "SHOULD" level requirements for its protocols
402   is said to be "conditionally compliant".
403</t>
404</section>
405
406<section title="Syntax Notation" anchor="notation">
407   <x:anchor-alias value="ALPHA"/>
408   <x:anchor-alias value="CR"/>
409   <x:anchor-alias value="DIGIT"/>
410   <x:anchor-alias value="DQUOTE"/>
411   <x:anchor-alias value="LF"/>
412   <x:anchor-alias value="OCTET"/>
413   <x:anchor-alias value="SP"/>
414   <x:anchor-alias value="VCHAR"/>
415<t>
416   This specification uses the ABNF syntax defined in &notation; (which
417   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
418   <xref target="collected.abnf"/> shows the collected ABNF, with the list
419   rule expanded.
420</t>
421<t>
422   The following core rules are included by reference, as defined in <xref
423   target="RFC5234" x:fmt="," x:sec="B.1"/>: ALPHA (letters), CR (carriage
424   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
425   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
426   sequence of data), SP (space), and VCHAR (any visible US-ASCII character).
427</t>
428
429<section title="Core Rules" anchor="core.rules">
430   <x:anchor-alias value="quoted-string"/>
431   <x:anchor-alias value="token"/>
432   <x:anchor-alias value="OWS"/>
433<t>
434   The core rules below are defined in <xref target="Part1"/>:
435</t>
436<figure><artwork type="abnf2616">
437  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
438  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &field-rules;&gt;
439  <x:ref>token</x:ref>         = &lt;token, defined in &field-rules;&gt;
440</artwork></figure>
441</section>
442
443<section title="ABNF Rules defined in other Parts of the Specification"
444    anchor="abnf.dependencies">
445   <x:anchor-alias value="field-name"/>
446   <x:anchor-alias value="HTTP-date"/>
447   <x:anchor-alias value="port"/>
448   <x:anchor-alias value="pseudonym"/>
449   <x:anchor-alias value="uri-host"/>
450<t>
451   The ABNF rules below are defined in other parts:
452</t>
453<figure><!--Part1--><artwork type="abnf2616">
454  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
455  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
456  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
457  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
458  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
459</artwork></figure>
460</section>
461</section>
462
463<section title="Delta Seconds" anchor="delta-seconds">
464<t>
465   The delta-seconds rule specifies a non-negative integer, representing time
466   in seconds.
467</t>
468<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
469  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
470</artwork></figure>
471<t>
472   If an implementation receives a delta-seconds value larger than the largest
473   positive integer it can represent, or if any of its subsequent calculations
474   overflows, it &MUST; consider the value to be 2147483648 (2<x:sup>31</x:sup>).
475   Recipients parsing a delta-seconds value &SHOULD; use an arithmetic type of
476   at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a
477   value greater than 2147483648.
478</t>
479</section>
480
481</section>
482
483<section anchor="caching.overview" title="Cache Operation">
484<iref item="cache entry" />
485<iref item="cache key" />
486<t>
487   Proper cache operation preserves the semantics of HTTP transfers
488   (&semantics;) while eliminating the transfer of information already held
489   in the cache.  Although caching is an entirely &OPTIONAL; feature of HTTP,
490   we assume that reusing the cached response is desirable and that such
491   reuse is the default behavior when no requirement or locally-desired
492   configuration prevents it.  Therefore, HTTP cache requirements are focused
493   on preventing a cache from either storing a non-reusable response or
494   reusing a stored response inappropriately.
495</t>
496<t>
497   Each <x:dfn>cache entry</x:dfn> consists of a cache key and one or more
498   HTTP responses corresponding to prior requests that used the same key.
499   The most common form of cache entry is a successful result of a retrieval
500   request: i.e., a 200 (OK) response containing a representation of the
501   resource identified by the request target.  However, it is also possible
502   to cache negative results (e.g., 404 not found), incomplete results
503   (e.g., 206 partial content), and responses to safe methods other than
504   GET if the method's definition allows such caching and defines something
505   suitable for use as a cache key.
506</t>
507<t>
508   The default <x:dfn>cache key</x:dfn> consists of the request method and
509   target URI.  However, since HTTP caches in common use today are typically
510   limited to caching responses to GET, most implementations simply decline
511   other methods and use only the URI as the key.
512</t>
513<t>
514   If a request target is subject to content negotiation, its cache entry
515   might consist of multiple stored responses, each differentiated by a
516   secondary key for the values of the original request's selecting header
517   fields (<xref target="caching.negotiated.responses"/>).
518</t>
519
520<section anchor="response.cacheability" title="Response Cacheability">
521<t>
522   A cache &MUST-NOT; store a response to any request, unless:
523   <list style="symbols">
524      <t>The request method is understood by the cache and defined as being
525      cacheable, and</t>
526      <t>the response status code is understood by the cache, and</t>
527      <t>the "no-store" cache directive (see <xref
528      target="header.cache-control" />) does not appear in request or response
529      header fields, and</t>
530      <t>the "private" cache response directive (see <xref
531      target="cache-response-directive" /> does not appear in the response, if
532      the cache is shared, and</t>
533      <t>the "Authorization" header field (see &header-authorization;) does not
534      appear in the request, if the cache is shared, unless the response
535      explicitly allows it (see <xref target="caching.authenticated.responses"
536      />), and</t>
537      <t>the response either:
538         <list style="symbols">
539            <t>contains an Expires header field (see <xref target="header.expires"
540            />), or</t>
541            <t>contains a max-age response cache directive (see <xref
542            target="cache-response-directive" />), or</t>
543            <t>contains a s-maxage response cache directive and the cache is
544            shared, or</t>
545            <t>contains a Cache Control Extension (see <xref
546            target="cache.control.extensions" />) that allows it to be cached,
547            or</t>
548            <t>has a status code that can be served with heuristic freshness
549            (see <xref target="heuristic.freshness" />).</t>
550         </list>
551      </t>
552   </list>
553</t>
554<t>
555   Note that any of the requirements listed above can be overridden by a
556   cache-control extension; see <xref target="cache.control.extensions" />.
557</t>
558<t>
559   In this context, a cache has "understood" a request method or a response
560   status code if it recognizes it and implements any cache-specific
561   behavior.
562</t>
563<t>
564   Note that, in normal operation, most caches will not store a response that
565   has neither a cache validator nor an explicit expiration time, as such
566   responses are not usually useful to store. However, caches are not
567   prohibited from storing such responses.
568</t>
569<t>
570   A response message is considered complete when all of the octets
571   indicated by the message framing (&messaging;) are received
572   prior to the connection being closed.
573   If the request is GET, the response status is 200 (OK), and the entire
574   response header block has been received, a cache &MAY; store an incomplete
575   response message-body if the cache entry is recorded as incomplete.
576   Likewise, a 206 (Partial Content) response &MAY; be stored as if it were
577   an incomplete 200 (OK) cache entry.  However, a cache &MUST-NOT; store
578   incomplete or partial content responses if it does not support the Range
579   and Content-Range header fields or if it does not understand the
580   range units used in those fields.
581</t>
582<t>
583   A cache &MAY; complete a stored incomplete response by making a subsequent
584   range request (&partial;) and combining the successful response with the
585   stored entry, as defined in <xref target="combining.responses"/>.
586   A cache &MUST-NOT; use an incomplete response to answer requests
587   unless the response has been made complete or the request is partial and
588   specifies a range that is wholly within the incomplete response.
589   A cache &MUST-NOT; send a partial response to a client without explicitly
590   marking it as such using the 206 (Partial Content) status code.
591</t>
592</section>
593
594
595<section anchor="constructing.responses.from.caches" 
596   title="Constructing Responses from Caches">
597<t>
598   For a presented request, a cache &MUST-NOT; return a stored response,
599   unless:
600   <list style="symbols">
601      <t>The presented effective request URI (&effective-request-uri;) and
602      that of the stored response match, and</t>
603      <t>the request method associated with the stored response allows it to
604      be used for the presented request, and</t>
605      <t>selecting header fields nominated by the stored response (if any)
606      match those presented (see <xref target="caching.negotiated.responses"
607      />), and</t>
608      <t>the presented request and stored response are free from directives
609      that would prevent its use (see <xref target="header.cache-control" />
610      and <xref target="header.pragma"/>), and</t>
611      <t>the stored response is either:
612         <list style="symbols">
613            <t>fresh (see <xref target="expiration.model" />), or</t>
614            <t>allowed to be served stale (see <xref
615            target="serving.stale.responses" />), or</t>
616            <t>successfully validated (see <xref target="validation.model"
617            />).</t>
618         </list>
619      </t>
620  </list>
621</t>
622<t>
623   Note that any of the requirements listed above can be overridden by a
624   cache-control extension; see <xref target="cache.control.extensions" />.
625</t>
626<t>
627   When a stored response is used to satisfy a request without validation,
628   a cache &MUST; include a single Age header field (<xref target="header.age"
629   />) in the response with a value equal to the stored response's
630   current_age; see <xref target="age.calculations" />.
631</t>
632<t>
633   A cache &MUST; write through requests with methods that are unsafe
634   (&safe-methods;) to the origin server; i.e., a cache must not generate
635   a reply to such a request before having forwarded the request and having
636   received a corresponding response.
637</t>
638<t>
639   Also, note that unsafe requests might invalidate already stored responses;
640   see <xref target="invalidation.after.updates.or.deletions" />.
641</t>
642<t>
643   When more than one suitable response is stored, a cache &MUST; use the
644   most recent response (as determined by the Date header field). It can also
645   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
646   no-cache" to disambiguate which response to use.
647</t>
648<t>
649   A cache that does not have a clock available &MUST-NOT; use stored responses
650   without revalidating them on every use. A cache, especially a shared
651   cache, &SHOULD; use a mechanism, such as NTP <xref target="RFC1305"/>, to
652   synchronize its clock with a reliable external standard.
653</t>
654
655</section>
656
657<section anchor="expiration.model" title="Freshness Model">
658<t>
659   When a response is "fresh" in the cache, it can be used to satisfy
660   subsequent requests without contacting the origin server, thereby improving
661   efficiency.
662</t>
663<t>
664   The primary mechanism for determining freshness is for an origin server to
665   provide an explicit expiration time in the future, using either the Expires
666   header field (<xref target="header.expires" />) or the max-age response cache
667   directive (<xref target="cache-response-directive" />). Generally, origin
668   servers will assign future explicit expiration times to responses in the
669   belief that the representation is not likely to change in a semantically
670   significant way before the expiration time is reached.
671</t>
672<t>
673   If an origin server wishes to force a cache to validate every request, it
674   can assign an explicit expiration time in the past to indicate that the
675   response is already stale. Compliant caches will normally validate the
676   cached response before reusing it for subsequent requests (see <xref
677   target="serving.stale.responses" />).
678</t>
679<t>
680   Since origin servers do not always provide explicit expiration times,
681   a cache &MAY; assign a heuristic expiration time when an explicit time is not
682   specified, employing algorithms that use other header field values (such as the
683   Last-Modified time) to estimate a plausible expiration time. This
684   specification does not provide specific algorithms, but does impose
685   worst-case constraints on their results.
686</t>
687<figure>
688<preamble>
689  The calculation to determine if a response is fresh is:
690</preamble>
691<artwork type="code">
692   response_is_fresh = (freshness_lifetime &gt; current_age)
693</artwork>
694</figure>
695<t>
696   The freshness_lifetime is defined in <xref
697   target="calculating.freshness.lifetime" />; the current_age is defined in
698   <xref target="age.calculations" />.
699</t>
700<t>
701   Additionally, clients might need to influence freshness calculation. They
702   can do this using several request cache directives, with the effect of
703   either increasing or loosening constraints on freshness. See <xref
704   target="cache-request-directive" />.
705</t>
706<t>
707   Note that freshness applies only to cache operation; it cannot be used to
708   force a user agent to refresh its display or reload a resource. See <xref
709   target="history.lists" /> for an explanation of the difference between
710   caches and history mechanisms.
711</t>
712
713<section anchor="calculating.freshness.lifetime" 
714   title="Calculating Freshness Lifetime">
715<t>
716   A cache can calculate the freshness lifetime (denoted as
717   freshness_lifetime) of a response by using the first match of:
718   <list style="symbols">
719      <t>If the cache is shared and the s-maxage response cache directive
720      (<xref target="cache-response-directive" />) is present, use its value,
721      or</t>
722      <t>If the max-age response cache directive (<xref
723      target="cache-response-directive" />) is present, use its value, or</t>
724      <t>If the Expires response header field (<xref target="header.expires" />) is
725      present, use its value minus the value of the Date response header field,
726      or</t>
727      <t>Otherwise, no explicit expiration time is present in the response. A
728      heuristic freshness lifetime might be applicable; see <xref
729      target="heuristic.freshness" />.</t>
730   </list>
731</t>
732<t>
733   Note that this calculation is not vulnerable to clock skew, since all of
734   the information comes from the origin server.
735</t>
736
737<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
738<t>
739   If no explicit expiration time is present in a stored response that has a
740   status code whose definition allows heuristic freshness to be used
741   (including the following in &status-codes;: 200, 203, 206, 300, 301 and
742   410), a cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; 
743   use heuristics to determine freshness for responses with status codes that do
744   not explicitly allow it.
745</t>
746<t>
747   When a heuristic is used to calculate freshness lifetime, a cache
748   &SHOULD; attach a Warning header field with a 113 warn-code to the response if
749   its current_age is more than 24 hours and such a warning is not already
750   present.
751</t>
752<t>
753   Also, if the response has a Last-Modified header field (&header-last-modified;),
754   a cache &SHOULD-NOT; use a heuristic expiration value that is more than some
755   fraction of the interval since that time. A typical setting of this fraction
756   might be 10%.
757</t>
758<x:note>
759   <t>
760      <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt=","
761      x:sec="13.9"/>) required that caches do not calculate heuristic
762      freshness for URIs with query components (i.e., those containing '?').
763      In practice, this has not been widely implemented. Therefore, servers
764      are encouraged to send explicit directives (e.g., Cache-Control:
765      no-cache) if they wish to preclude caching.
766   </t>
767</x:note>
768</section>
769</section>
770
771<section anchor="age.calculations" title="Calculating Age">
772<t>
773   HTTP/1.1 uses the Age header field to convey the estimated age of the
774   response message when obtained from a cache. The Age field value is the
775   cache's estimate of the amount of time since the response was generated or
776   validated by the origin server. In essence, the Age value is the sum of the
777   time that the response has been resident in each of the caches along the
778   path from the origin server, plus the amount of time it has been in transit
779   along network paths.
780</t>
781<t>
782   The following data is used for the age calculation:
783</t>
784<t>
785   <x:dfn>age_value</x:dfn>
786   <list>
787      <t>
788         The term "age_value" denotes the value of the Age header field (<xref
789         target="header.age"/>), in a form appropriate for arithmetic
790         operation; or 0, if not available.
791      </t>
792   </list>
793</t>
794<t>
795   <x:dfn>date_value</x:dfn>
796   <list>
797      <t>
798         HTTP/1.1 requires origin servers to send a Date header field, if possible,
799         with every response, giving the time at which the response was
800         generated. The term "date_value" denotes the value of the Date
801         header field, in a form appropriate for arithmetic operations. See
802         &header-date; for the definition of the Date header field, and for
803         requirements regarding responses without it.
804      </t>
805   </list>
806</t>
807<t>
808   <x:dfn>now</x:dfn>
809   <list>
810      <t>
811         The term "now" means "the current value of the clock at the host
812         performing the calculation". A cache &SHOULD; use NTP (<xref
813         target="RFC1305"/>) or some similar protocol to synchronize its
814         clocks to a globally accurate time standard.
815      </t>
816   </list>
817</t>
818<t>
819   <x:dfn>request_time</x:dfn>
820   <list>
821      <t>
822         The current value of the clock at the host at the time the request
823         resulting in the stored response was made.
824      </t>
825   </list>
826</t>
827<t>
828   <x:dfn>response_time</x:dfn>
829   <list>
830      <t>
831         The current value of the clock at the host at the time the response
832         was received.
833      </t>
834   </list>
835</t>
836<t>
837   A response's age can be calculated in two entirely independent ways:
838   <list style="numbers">
839      <t>the "apparent_age": response_time minus date_value, if the local
840      clock is reasonably well synchronized to the origin server's clock. If
841      the result is negative, the result is replaced by zero.</t>
842      <t>the "corrected_age_value", if all of the caches along the response
843      path implement HTTP/1.1. A cache &MUST; interpret this value relative
844      to the time the request was initiated, not the time that the response
845      was received.</t>
846   </list>
847</t>
848<figure>
849<artwork type="code">
850  apparent_age = max(0, response_time - date_value);
851
852  response_delay = response_time - request_time;
853  corrected_age_value = age_value + response_delay; 
854</artwork>
855</figure>
856<figure>
857<preamble>These are combined as</preamble>
858<artwork type="code">
859  corrected_initial_age = max(apparent_age, corrected_age_value);
860</artwork></figure>
861<t>
862   The current_age of a stored response can then be calculated by adding the
863   amount of time (in seconds) since the stored response was last validated by
864   the origin server to the corrected_initial_age.
865</t>
866<figure><artwork type="code">
867  resident_time = now - response_time;
868  current_age = corrected_initial_age + resident_time;
869</artwork></figure>
870<t>
871   Additionally, to avoid common problems in date parsing:
872</t>
873<t>
874  <list style="symbols">
875     <t>HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date
876        which appears to be more than 50 years in the future is in fact
877        in the past (this helps solve the "year 2000" problem).</t>
878
879     <t>Although all date formats are specified to be case-sensitive,
880        recipients &SHOULD; match day, week and timezone names
881        case-insensitively.</t>
882             
883     <t>An HTTP/1.1 implementation &MAY; internally represent a parsed
884        Expires date as earlier than the proper value, but &MUST-NOT;
885        internally represent a parsed Expires date as later than the
886        proper value.</t>
887
888     <t>All expiration-related calculations &MUST; be done in GMT. The
889        local time zone &MUST-NOT; influence the calculation or comparison
890        of an age or expiration time.</t>
891
892     <t>If an HTTP header field incorrectly carries a date value with a time
893        zone other than GMT, it &MUST; be converted into GMT using the
894        most conservative possible conversion.</t>
895  </list>
896</t>
897</section>
898
899<section anchor="serving.stale.responses" title="Serving Stale Responses">
900<t>
901   A "stale" response is one that either has explicit expiry information or is
902   allowed to have heuristic expiry calculated, but is not fresh according to
903   the calculations in <xref target="expiration.model" />.
904</t>
905<t>
906   A cache &MUST-NOT; return a stale response if it is prohibited by an
907   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
908   directive, a "must-revalidate" cache-response-directive, or an applicable
909   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref
910   target="cache-response-directive"/>).
911</t>
912<t>
913   A cache &SHOULD-NOT; return stale responses unless it is disconnected
914   (i.e., it cannot contact the origin server or otherwise find a forward
915   path) or doing so is explicitly allowed (e.g., by the max-stale request
916   directive; see <xref target="cache-request-directive" />).
917</t>
918<t>
919   A cache &SHOULD; append a Warning header field with the 110 warn-code (see
920   <xref target="header.warning" />) to stale responses. Likewise, a cache
921   &SHOULD; add the 112 warn-code to stale responses if the cache is
922   disconnected.
923</t>
924<t>
925   If a cache receives a first-hand response (either an entire response, or a
926   304 (Not Modified) response) that it would normally forward to the
927   requesting client, and the received response is no longer fresh, the cache
928   &SHOULD; forward it to the requesting client without adding a new Warning
929   (but without removing any existing Warning header fields). A cache &SHOULD-NOT;
930   attempt to validate a response simply because that response became stale in
931   transit.
932</t>
933</section>
934</section>
935
936<section anchor="validation.model" title="Validation Model">
937<t>
938   When a cache has one or more stored responses for a requested URI, but
939   cannot serve any of them (e.g., because they are not fresh, or one cannot
940   be selected; see <xref target="caching.negotiated.responses"/>), it can use
941   the conditional request mechanism &conditional; in the forwarded request to
942   give the origin server an opportunity to both select a valid stored
943   response to be used, and to update it. This process is known as
944   "validating" or "revalidating" the stored response.
945</t>
946<t>
947   When sending such a conditional request, a cache &SHOULD; add an
948   If-Modified-Since header field whose value is that of the Last-Modified header
949   field from the selected (see <xref target="caching.negotiated.responses"/>)
950   stored response, if available.
951</t>
952<t>
953   Additionally, a cache &SHOULD; add an If-None-Match header field whose value is
954   that of the ETag header field(s) from all responses stored for the requested URI,
955   if present. However, if any of the stored responses contains only partial
956   content, the cache &SHOULD-NOT; include its entity-tag in the If-None-Match
957   header field unless the request is for a range that would be fully
958   satisfied by that stored response.
959</t>
960
961<t>Cache handling of a response to a conditional request is dependent upon its status code:</t>
962
963<t>
964   <list style="symbols">
965      <t>
966         A 304 (Not Modified) response status code indicates that the stored
967         response can be updated and reused; see <xref
968         target="freshening.responses"/>.
969      </t>
970      <t>
971         A full response (i.e., one with a response body) indicates that none
972         of the stored responses nominated in the conditional request is
973         suitable. Instead, a cache &SHOULD; use the full response to satisfy
974         the request and &MAY; replace the stored response(s).
975      </t>
976      <t>
977         However, if a cache receives a 5xx response while attempting to
978         validate a response, it &MAY; either forward this response to the
979         requesting client, or act as if the server failed to respond. In the
980         latter case, it &MAY; return a previously stored response (see <xref
981         target="serving.stale.responses" />).
982      </t>
983   </list>
984</t>
985
986<section anchor="freshening.responses" title="Freshening Responses">
987<t>
988   When a cache receives a 304 (Not Modified) response and already has one
989   or more stored 200 (OK) responses for the same cache key, the cache needs
990   to identify which of the stored responses are updated by this new response
991   and then update the stored response(s) with the new information provided in
992   the 304 response.
993   <list style="symbols">
994    <t>
995     If the new response contains a strong validator, then that strong
996     validator identifies the selected representation.  All of the stored
997     responses with the same strong validator are selected.
998     If none of the stored responses contain the same strong validator, then
999     this new response corresponds to a new selected representation and
1000     &MUST-NOT; update the existing stored responses.
1001    </t>
1002    <t>
1003     If the new response contains a weak validator and that validator
1004     corresponds to one of the cache's stored responses, then the most
1005     recent of those matching stored responses is selected.
1006    </t>
1007    <t>
1008     If the new response does not include any form of validator, there is
1009     only one stored response, and that stored response also lacks a
1010     validator, then that stored response is selected.
1011    </t>
1012   </list>
1013</t>
1014<t>
1015   If a stored response is selected for update, the cache &MUST;:
1016   <list style="symbols">
1017      <t>delete any Warning header fields in the stored response with
1018         warn-code 1xx (see <xref target="header.warning" />);</t>
1019      <t>retain any Warning header fields in the stored response with
1020         warn-code 2xx; and,</t>
1021      <t>use other header fields provided in the 304 response to replace
1022         all instances of the corresponding header fields in the stored
1023         response.</t>
1024   </list>
1025</t>
1026</section>
1027
1028</section>
1029
1030<section anchor="invalidation.after.updates.or.deletions" 
1031   title="Request Methods that Invalidate">
1032<t>
1033   Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE
1034   have the potential for changing state on the origin server, intervening
1035   caches can use them to keep their contents up-to-date.
1036</t>
1037<t>
1038   A cache &MUST; invalidate the effective Request URI
1039   (&effective-request-uri;) as well as the URI(s) in the Location
1040   and Content-Location header fields (if present) when a non-error
1041   response to a request with an unsafe method is received.
1042</t>
1043<t>
1044   However, a cache &MUST-NOT; invalidate a URI from a
1045   Location or Content-Location header field if the host part of that URI
1046   differs from the host part in the effective request URI
1047   (&effective-request-uri;). This helps prevent denial of service attacks.
1048</t>
1049<t>
1050   A cache &SHOULD; invalidate the effective request URI
1051   (&effective-request-uri;) when it receives a non-error response
1052   to a request with a method whose safety is unknown.
1053</t>
1054<t>
1055   Here, a "non-error response" is one with a 2xx or 3xx status code.
1056   "Invalidate" means that the cache will either remove all stored
1057   responses related to the effective request URI, or will mark these as
1058   "invalid" and in need of a mandatory validation before they can be returned
1059   in response to a subsequent request.
1060</t>
1061<t>
1062   Note that this does not guarantee that all appropriate responses are
1063   invalidated. For example, the request that caused the change at the origin
1064   server might not have gone through the cache where a response is stored.
1065</t>
1066</section>
1067
1068<section anchor="caching.authenticated.responses" 
1069   title="Shared Caching of Authenticated Responses">
1070
1071<t>
1072   A shared cache &MUST-NOT; use a cached response to a request with an
1073   Authorization header field (&header-authorization;) to satisfy any subsequent
1074   request unless a cache directive that allows such responses to be stored is
1075   present in the response.
1076</t>
1077
1078<t>
1079   In this specification, the following Cache-Control response directives
1080   (<xref target="cache-response-directive"/>) have such an effect:
1081   must-revalidate, public, s-maxage.
1082</t>
1083
1084<t>
1085   Note that cached responses that contain the "must-revalidate" and/or
1086   "s-maxage" response directives are not allowed to be served stale (<xref
1087   target="serving.stale.responses"/>) by shared caches. In particular, a
1088   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
1089   used to satisfy a subsequent request without revalidating it on the origin
1090   server.
1091</t>
1092</section>
1093
1094<section anchor="caching.negotiated.responses" 
1095   title="Caching Negotiated Responses">
1096<t>
1097   When a cache receives a request that can be satisfied by a stored response
1098   that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT;
1099   use that response unless all of the selecting header fields nominated by
1100   the Vary header field match in both the original request (i.e., that associated
1101   with the stored response), and the presented request.
1102</t>
1103<t>
1104   The selecting header fields from two requests are defined to match if and
1105   only if those in the first request can be transformed to those in the
1106   second request by applying any of the following:
1107   <list style="symbols">
1108      <t>
1109         adding or removing whitespace, where allowed in the header field's syntax
1110      </t>
1111      <t>
1112         combining multiple header fields with the same field name
1113         (see &header-fields;)
1114      </t>
1115      <t>
1116         normalizing both header field values in a way that is known to have
1117         identical semantics, according to the header field's specification (e.g.,
1118         re-ordering field values when order is not significant;
1119         case-normalization, where values are defined to be case-insensitive)
1120      </t>
1121  </list>
1122</t>
1123<t>
1124   If (after any normalization that might take place) a header field is absent
1125   from a request, it can only match another request if it is also absent
1126   there.
1127</t>
1128<t>
1129   A Vary header field-value of "*" always fails to match, and subsequent
1130   requests to that resource can only be properly interpreted by the origin
1131   server.
1132</t>
1133<t>
1134   The stored response with matching selecting header fields is known as the
1135   selected response.
1136</t>
1137<t>
1138   If multiple selected responses are available, the most recent response
1139   (as determined by the Date header field) is used; see <xref 
1140   target="constructing.responses.from.caches"/>.
1141</t>
1142<t>
1143   If no selected response is available, the cache &MAY; forward the presented
1144   request to the origin server in a conditional request; see <xref
1145   target="validation.model"/>.
1146</t>
1147</section>
1148
1149<section anchor="combining.responses" title="Combining Partial Content">
1150<t>
1151   A response might transfer only a partial representation if the
1152   connection closed prematurely or if the request used one or more Range
1153   specifiers (&partial;).  After several such transfers, a cache might have
1154   received several ranges of the same representation.  A cache &MAY; combine
1155   these ranges into a single stored response, and reuse that response to
1156   satisfy later requests, if they all share the same strong validator and
1157   the cache complies with the client requirements in &combining-byte-ranges;.
1158</t>
1159<t>
1160   When combining the new response with one or more stored responses, a
1161   cache &MUST;:
1162   <list style="symbols">
1163      <t>delete any Warning header fields in the stored response with
1164         warn-code 1xx (see <xref target="header.warning" />);</t>
1165      <t>retain any Warning header fields in the stored response with
1166         warn-code 2xx; and,</t>
1167      <t>use other header fields provided in the new response, aside
1168         from Content-Range, to replace all instances of the corresponding
1169         header fields in the stored response.</t>
1170   </list>
1171</t>
1172</section>
1173
1174
1175</section>
1176
1177<section anchor="header.field.definitions" title="Header Field Definitions">
1178<t>
1179   This section defines the syntax and semantics of HTTP/1.1 header fields
1180   related to caching.
1181</t>
1182
1183<section anchor="header.age" title="Age">
1184   <iref item="Age header field" primary="true" x:for-anchor="" />
1185   <iref item="Header Fields" primary="true" subitem="Age" x:for-anchor="" />
1186   <x:anchor-alias value="Age"/>
1187   <x:anchor-alias value="age-value"/>
1188<t>
1189   The "Age" header field conveys the sender's estimate of the amount
1190   of time since the response was generated or successfully validated at the
1191   origin server. Age values are calculated as specified in <xref
1192   target="age.calculations" />.
1193</t>
1194<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/>
1195  <x:ref>Age</x:ref> = <x:ref>delta-seconds</x:ref>
1196</artwork></figure>
1197<t>
1198  Age field-values are non-negative integers, representing time in seconds
1199  (see <xref target="delta-seconds"/>).
1200</t>
1201<t>
1202   The presence of an Age header field in a response implies that a response
1203   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1204   might not implement the Age header field.
1205</t>
1206</section>
1207
1208<section anchor="header.cache-control" title="Cache-Control">
1209   <iref item="Cache-Control header field" primary="true" x:for-anchor="" />
1210   <iref item="Header Fields" primary="true" subitem="Cache-Control" 
1211      x:for-anchor="" />
1212   <x:anchor-alias value="Cache-Control"/>
1213   <x:anchor-alias value="cache-directive"/>
1214   <x:anchor-alias value="cache-extension"/>
1215   <x:anchor-alias value="cache-request-directive"/>
1216   <x:anchor-alias value="cache-response-directive"/>
1217<t>
1218   The "Cache-Control" header field is used to specify directives for
1219   caches along the request/response chain. Such cache directives are
1220   unidirectional in that the presence of a directive in a request does not
1221   imply that the same directive is to be given in the response.
1222</t>
1223<t>
1224   A cache &MUST; obey the requirements of the Cache-Control
1225   directives defined in this section. See <xref
1226   target="cache.control.extensions"/> for information about how Cache-Control
1227   directives defined elsewhere are handled.
1228</t>
1229<x:note>
1230   <t>
1231       <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1232       might only implement Pragma: no-cache (see <xref target="header.pragma"
1233       />).
1234   </t>
1235</x:note>
1236<t>
1237   A proxy, whether or not it implements a cache, &MUST; pass cache directives
1238   through in forwarded messages, regardless of their
1239   significance to that application, since the directives might be applicable
1240   to all recipients along the request/response chain. It is not possible to
1241   target a directive to a specific cache.
1242</t>
1243<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1244  <x:ref>Cache-Control</x:ref>   = 1#<x:ref>cache-directive</x:ref>
1245
1246  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1247     / <x:ref>cache-response-directive</x:ref>
1248
1249  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1250</artwork></figure>
1251
1252<section anchor="cache-request-directive" 
1253   title="Request Cache-Control Directives">
1254   <x:anchor-alias value="cache-request-directive" />
1255
1256<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1257   subitem="cache-request-directive" />
1258  <x:ref>cache-request-directive</x:ref> =
1259       "no-cache"
1260     / "no-store"
1261     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1262     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1263     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1264     / "no-transform"
1265     / "only-if-cached"
1266     / <x:ref>cache-extension</x:ref>
1267</artwork></figure>
1268
1269<t>
1270   <x:dfn>no-cache</x:dfn>
1271   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1272   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1273   <list>
1274      <t>The no-cache request directive indicates that a cache &MUST-NOT; 
1275      use a stored response to satisfy the request without successful
1276      validation on the origin server.</t> 
1277   </list>
1278</t>
1279<t>
1280   <x:dfn>no-store</x:dfn>
1281   <iref item="Cache Directives" primary="true" subitem="no-store" />
1282   <iref item="no-store" primary="true" subitem="Cache Directive" />
1283   <list>
1284      <t>The no-store request directive indicates that a cache &MUST-NOT;
1285      store any part of either this request or any response to it. This
1286      directive applies to both private and shared caches. "&MUST-NOT;
1287      store" in this context means that the cache &MUST-NOT; intentionally
1288      store the information in non-volatile storage, and &MUST; make a
1289      best-effort attempt to remove the information from volatile storage as
1290      promptly as possible after forwarding it.</t>
1291      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1292      privacy. In particular, malicious or compromised caches might not
1293      recognize or obey this directive, and communications networks might be
1294      vulnerable to eavesdropping.</t>
1295      <t>Note that if a request containing this directive is satisfied from a
1296      cache, the no-store request directive does not apply to the already
1297      stored response.</t>
1298   </list>
1299</t>
1300<t>
1301   <x:dfn>max-age</x:dfn>
1302   <iref item="Cache Directives" primary="true" subitem="max-age" />
1303   <iref item="max-age" primary="true" subitem="Cache Directive" />
1304   <list>
1305      <t>The max-age request directive indicates that the client is unwilling to
1306      accept a response whose age is greater than the specified number of
1307      seconds. Unless the max-stale request directive is also present, the
1308      client is not willing to accept a stale response.</t>
1309   </list>
1310</t>
1311<t>
1312   <x:dfn>max-stale</x:dfn>
1313   <iref item="Cache Directives" primary="true" subitem="max-stale" />
1314   <iref item="max-stale" primary="true" subitem="Cache Directive" />
1315   <list>
1316      <t>The max-stale request directive indicates that the client is willing
1317      to accept a response that has exceeded its expiration time. If max-stale
1318      is assigned a value, then the client is willing to accept a response
1319      that has exceeded its expiration time by no more than the specified
1320      number of seconds. If no value is assigned to max-stale, then the client
1321      is willing to accept a stale response of any age.</t>
1322   </list>
1323</t>
1324<t>
1325   <x:dfn>min-fresh</x:dfn>
1326   <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1327   <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1328   <list>
1329      <t>The min-fresh request directive indicates that the client is willing
1330      to accept a response whose freshness lifetime is no less than its
1331      current age plus the specified time in seconds. That is, the client
1332      wants a response that will still be fresh for at least the specified
1333      number of seconds.</t>
1334   </list>
1335</t>
1336<t>
1337   <x:dfn>no-transform</x:dfn>
1338   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1339   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1340   <list>
1341      <t>The no-transform request directive indicates that an intermediary
1342        (whether or not it implements a cache) &MUST-NOT; change the
1343        Content-Encoding, Content-Range or Content-Type request header fields,
1344        nor the request representation.</t>
1345   </list>
1346</t>
1347<t>
1348   <x:dfn>only-if-cached</x:dfn>
1349   <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1350   <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1351   <list>
1352      <t>The only-if-cached request directive indicates that the client only
1353      wishes to obtain a stored response. If it receives this directive, a
1354      cache &SHOULD; either respond using a stored response that is consistent
1355      with the other constraints of the request, or respond with a 504
1356      (Gateway Timeout) status code. If a group of caches is being operated as
1357      a unified system with good internal connectivity, a member cache &MAY;
1358      forward such a request within that group of caches.</t>
1359   </list>
1360</t>
1361</section>
1362
1363<section anchor="cache-response-directive" 
1364   title="Response Cache-Control Directives">
1365   <x:anchor-alias value="cache-response-directive" />
1366
1367<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" 
1368   subitem="cache-response-directive" />
1369  <x:ref>cache-response-directive</x:ref> =
1370       "public"
1371     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1372     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1373     / "no-store"
1374     / "no-transform"
1375     / "must-revalidate"
1376     / "proxy-revalidate"
1377     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1378     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1379     / <x:ref>cache-extension</x:ref>
1380</artwork></figure>
1381
1382<t>
1383   <x:dfn>public</x:dfn>
1384   <iref item="Cache Directives" primary="true" subitem="public" />
1385   <iref item="public" primary="true" subitem="Cache Directive" />
1386   <list>
1387      <t>The public response directive indicates that a response whose
1388        associated request contains an 'Authentication' header &MAY; be
1389        stored (see <xref target="caching.authenticated.responses" />).</t>
1390  </list>
1391</t>
1392<t>
1393   <x:dfn>private</x:dfn>
1394   <iref item="Cache Directives" primary="true" subitem="private" />
1395   <iref item="private" primary="true" subitem="Cache Directive" />
1396   <list>
1397      <t>The private response directive indicates that the response message is
1398      intended for a single user and &MUST-NOT; be stored by a shared cache. A
1399      private cache &MAY; store the response.</t>
1400      <t>If the private response directive specifies one or more field-names,
1401      this requirement is limited to the field-values associated with the
1402      listed response header fields. That is, a shared cache &MUST-NOT; store
1403      the specified field-names(s), whereas it &MAY; store the remainder of the
1404      response message.</t>
1405      <t> <x:h>Note:</x:h> This usage of the word private only controls where
1406      the response can be stored; it cannot ensure the privacy of the message
1407      content. Also, private response directives with field-names are often
1408      handled by implementations as if an unqualified private directive was
1409      received; i.e., the special handling for the qualified form is not
1410      widely implemented.</t>
1411   </list>
1412</t>
1413<t>
1414   <x:dfn>no-cache</x:dfn>
1415   <iref item="Cache Directives" primary="true" subitem="no-cache" />
1416   <iref item="no-cache" primary="true" subitem="Cache Directive" />
1417   <list>
1418      <t>The no-cache response directive indicates that the response MUST NOT
1419      be used to satisfy a subsequent request without successful validation on
1420      the origin server. This allows an origin server to prevent a cache from
1421      using it to satisfy a request without contacting it, even by caches that
1422      have been configured to return stale responses.</t>
1423      <t>If the no-cache response directive specifies one or more field-names,
1424      this requirement is limited to the field-values associated with the
1425      listed response header fields. That is, a cache &MUST-NOT; send the
1426      specified field-name(s) in the response to a subsequent request without successful
1427      validation on the origin server. This allows an origin server to prevent
1428      the re-use of certain header fields in a response, while still allowing
1429      caching of the rest of the response.</t>
1430      <t> <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey
1431      this directive. Also, no-cache response directives with field-names are
1432      often handled by implementations as if an unqualified no-cache directive
1433      was received; i.e., the special handling for the qualified form is not
1434      widely implemented. </t>
1435   </list>
1436</t>
1437<t>
1438   <x:dfn>no-store</x:dfn>
1439   <iref item="Cache Directives" primary="true" subitem="no-store" />
1440   <iref item="no-store" primary="true" subitem="Cache Directive" />
1441   <list>
1442      <t>The no-store response directive indicates that a cache &MUST-NOT;
1443      store any part of either the immediate request or response. This
1444      directive applies to both private and shared caches. "&MUST-NOT;
1445      store" in this context means that the cache &MUST-NOT; intentionally
1446      store the information in non-volatile storage, and &MUST; make a
1447      best-effort attempt to remove the information from volatile storage as
1448      promptly as possible after forwarding it.</t>
1449      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1450      privacy. In particular, malicious or compromised caches might not
1451      recognize or obey this directive, and communications networks might be
1452      vulnerable to eavesdropping.</t>
1453   </list>
1454</t>
1455<t>
1456   <x:dfn>must-revalidate</x:dfn>
1457   <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1458   <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1459   <list>
1460      <t>The must-revalidate response directive indicates that once it has
1461      become stale, a cache &MUST-NOT; use the response to satisfy subsequent
1462      requests without successful validation on the origin server.</t>
1463      <t>The must-revalidate directive is necessary to support reliable
1464      operation for certain protocol features. In all circumstances a
1465      cache &MUST; obey the must-revalidate directive; in particular,
1466      if a cache cannot reach the origin server for any reason, it &MUST;
1467      generate a 504 (Gateway Timeout) response.</t>
1468      <t>A server &SHOULD; send the must-revalidate directive if and only if
1469      failure to validate a request on the representation could result in
1470      incorrect operation, such as a silently unexecuted financial
1471      transaction.</t>
1472   </list>
1473</t>
1474<t>
1475   <x:dfn>proxy-revalidate</x:dfn>
1476   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1477   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1478   <list>
1479      <t>The proxy-revalidate response directive has the same meaning as the
1480      must-revalidate response directive, except that it does not apply to
1481      private caches.</t>
1482   </list>
1483</t>
1484<t>
1485   <x:dfn>max-age</x:dfn>
1486   <iref item="Cache Directives" primary="true" subitem="max-age" />
1487   <iref item="max-age" primary="true" subitem="Cache Directive" />
1488   <list>
1489      <t>The max-age response directive indicates that the response is to be
1490      considered stale after its age is greater than the specified number of
1491      seconds.</t>
1492   </list>
1493</t>
1494<t>
1495   <x:dfn>s-maxage</x:dfn>
1496   <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1497   <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1498   <list>
1499      <t>The s-maxage response directive indicates that, in shared caches, the
1500      maximum age specified by this directive overrides the maximum age
1501      specified by either the max-age directive or the Expires header field. The
1502      s-maxage directive also implies the semantics of the proxy-revalidate
1503      response directive.</t>
1504   </list>
1505</t>
1506<t>
1507   <x:dfn>no-transform</x:dfn>
1508   <iref item="Cache Directives" primary="true" subitem="no-transform" />
1509   <iref item="no-transform" primary="true" subitem="Cache Directive" />
1510   <list>
1511      <t>The no-transform response directive indicates that an intermediary
1512      (regardless of whether it implements a cache) &MUST-NOT; change the
1513      Content-Encoding, Content-Range or Content-Type response header fields,
1514      nor the response representation.</t>
1515   </list>
1516</t>
1517
1518</section>
1519
1520<section anchor="cache.control.extensions" title="Cache Control Extensions">
1521<t>
1522   The Cache-Control header field can be extended through the use of one or
1523   more cache-extension tokens, each with an optional value. Informational
1524   extensions (those that do not require a change in cache behavior) can be
1525   added without changing the semantics of other directives. Behavioral
1526   extensions are designed to work by acting as modifiers to the existing base
1527   of cache directives. Both the new directive and the standard directive are
1528   supplied, such that applications that do not understand the new directive
1529   will default to the behavior specified by the standard directive, and those
1530   that understand the new directive will recognize it as modifying the
1531   requirements associated with the standard directive. In this way,
1532   extensions to the cache-control directives can be made without requiring
1533   changes to the base protocol.
1534</t>
1535<t>
1536   This extension mechanism depends on an HTTP cache obeying all of the
1537   cache-control directives defined for its native HTTP-version, obeying
1538   certain extensions, and ignoring all directives that it does not
1539   understand.
1540</t>
1541<t>
1542   For example, consider a hypothetical new response directive called
1543   "community" that acts as a modifier to the private directive. We define
1544   this new directive to mean that, in addition to any private cache, any
1545   cache that is shared only by members of the community named within its
1546   value may cache the response. An origin server wishing to allow the UCI
1547   community to use an otherwise private response in their shared cache(s)
1548   could do so by including
1549</t>
1550<figure><artwork type="example">
1551  Cache-Control: private, community="UCI"
1552</artwork></figure>
1553<t>
1554   A cache seeing this header field will act correctly even if the cache does
1555   not understand the community cache-extension, since it will also see and
1556   understand the private directive and thus default to the safe behavior.
1557</t>
1558<t>
1559   A cache &MUST; ignore unrecognized cache directives; it is assumed that any
1560   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1561   combined with standard directives (or the response's default cacheability)
1562   such that the cache behavior will remain minimally correct even if the
1563   cache does not understand the extension(s).
1564</t>
1565<t>
1566   The HTTP Cache Directive Registry defines the name space for the cache
1567   directives.
1568</t>
1569<t>
1570   A registration &MUST; include the following fields:
1571   <list style="symbols">
1572      <t>Cache Directive Name</t>
1573      <t>Pointer to specification text</t>
1574   </list>
1575</t>
1576<t>
1577   Values to be added to this name space are subject to IETF review (<xref
1578   target="RFC5226" x:fmt="," x:sec="4.1"/>).
1579</t>
1580<t>
1581   The registry itself is maintained at <eref
1582   target="http://www.iana.org/assignments/http-cache-directives"/>.
1583</t>
1584</section>
1585
1586</section>
1587
1588<section anchor="header.expires" title="Expires">
1589   <iref item="Expires header field" primary="true" x:for-anchor="" />
1590   <iref item="Header Fields" primary="true" subitem="Expires" x:for-anchor="" />
1591   <x:anchor-alias value="Expires"/>
1592<t>
1593   The "Expires" header field gives the date/time after which the
1594   response is considered stale. See <xref target="expiration.model" /> for
1595   further discussion of the freshness model.
1596</t>
1597<t>
1598   The presence of an Expires field does not imply that the original resource
1599   will change or cease to exist at, before, or after that time.
1600</t>
1601<t>
1602   The field-value is an absolute date and time as defined by HTTP-date in
1603   &full-date;; a sender &MUST; use the rfc1123-date format.
1604</t>
1605<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
1606  <x:ref>Expires</x:ref> = <x:ref>HTTP-date</x:ref>
1607</artwork></figure>
1608<figure>
1609  <preamble>For example</preamble>
1610<artwork type="example">
1611  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1612</artwork></figure>
1613<t>
1614   A cache &MUST; treat other invalid date formats,
1615   especially including the value "0", as in the past (i.e., "already
1616   expired").
1617</t>
1618<x:note>
1619   <t>
1620       <x:h>Note:</x:h> If a response includes a Cache-Control field with the
1621       max-age directive (see <xref target="cache-response-directive" />),
1622       that directive overrides the Expires field. Likewise, the s-maxage
1623       directive overrides Expires in shared caches.
1624   </t>
1625</x:note>
1626<t>
1627   Historically, HTTP required the Expires field-value to be no more than a
1628   year in the future. While longer freshness lifetimes are no longer
1629   prohibited, extremely large values have been demonstrated to cause
1630   problems (e.g., clock overflows due to use of 32-bit integers for
1631   time values), and most caches will evict a response far sooner than
1632   that. Therefore, senders ought not produce them.
1633</t>
1634</section>
1635
1636<section anchor="header.pragma" title="Pragma">
1637   <iref item="Pragma header field" primary="true" x:for-anchor="" />
1638   <iref item="Header Fields" primary="true" subitem="Pragma" x:for-anchor="" />
1639   <x:anchor-alias value="extension-pragma"/>
1640   <x:anchor-alias value="Pragma"/>
1641   <x:anchor-alias value="pragma-directive"/>
1642<t>
1643   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1644   caches, so that clients can specify a "no-cache" request that they will
1645   understand (as Cache-Control was not defined until HTTP/1.1). When the
1646   Cache-Control header is also present and understood in a request, Pragma is
1647   ignored.
1648</t>
1649<t>
1650   In HTTP/1.0, Pragma was defined as an extensible field for
1651   implementation-specified directives for recipients. This specification
1652   deprecates such extensions to improve interoperability.
1653</t>
1654<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"/>
1655  <x:ref>Pragma</x:ref>           = 1#<x:ref>pragma-directive</x:ref>
1656  <x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
1657  <x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1658</artwork></figure>
1659<t>
1660   When the Cache-Control header is not present in a request, the no-cache
1661   request pragma-directive &MUST; have the same effect on caches as if
1662   "Cache-Control: no-cache" were present (see <xref
1663   target="cache-request-directive" />).
1664</t>
1665<t>
1666   When sending a no-cache request, a client &SHOULD; include both pragma and
1667   cache-control directives unless Cache-Control: no-cache is purposefully
1668   omitted to target other Cache-Control response directives at HTTP/1.1
1669   caches. For example:
1670</t>
1671<figure>
1672<artwork type="message/http; msgtype=&#34;response&#34;" x:indent-with="  ">
1673GET / HTTP/1.1
1674Host: www.example.com
1675Cache-Control: max-age=30
1676Pragma: no-cache
1677
1678</artwork>
1679</figure>
1680<t>
1681   will constrain HTTP/1.1 caches to serve a response no older than 30
1682   seconds, while precluding implementations that do not understand
1683   Cache-Control from serving a cached response.
1684</t>
1685<x:note>
1686   <t>
1687      <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" in responses is not
1688      specified, it does not provide a reliable replacement for
1689      "Cache-Control: no-cache" in them.
1690   </t>
1691</x:note>
1692</section>
1693
1694<section anchor="header.vary" title="Vary">
1695   <iref item="Vary header field" primary="true" x:for-anchor="" />
1696   <iref item="Header Fields" primary="true" subitem="Vary" x:for-anchor="" />
1697   <x:anchor-alias value="Vary"/>
1698<t>
1699   The "Vary" header field conveys the set of header fields
1700   that were used to select the representation.
1701</t>
1702<t>
1703   Caches use this information, in part, to determine whether a stored
1704   response can be used to satisfy a given request; see <xref
1705   target="caching.negotiated.responses" />. determines, while the response is
1706   fresh, whether a cache is permitted to use the response to reply to a
1707   subsequent request without validation; see <xref
1708   target="caching.negotiated.responses" />.
1709</t>
1710<t>
1711   In uncacheable or stale responses, the Vary field value advises the user
1712   agent about the criteria that were used to select the representation.
1713</t>
1714<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
1715  <x:ref>Vary</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1716</artwork></figure>
1717<t>
1718   The set of header fields named by the Vary field value is known as the
1719   selecting header fields.
1720</t>
1721<t>
1722   A server &SHOULD; include a Vary header field with any cacheable response
1723   that is subject to server-driven negotiation. Doing so allows a cache to
1724   properly interpret future requests on that resource and informs the user
1725   agent about the presence of negotiation on that resource. A server &MAY;
1726   include a Vary header field with a non-cacheable response that is subject
1727   to server-driven negotiation, since this might provide the user agent with
1728   useful information about the dimensions over which the response varies at
1729   the time of the response.
1730</t>
1731<t>
1732   A Vary field value of "*" signals that unspecified parameters not limited
1733   to the header fields (e.g., the network address of the client), play a
1734   role in the selection of the response representation; therefore, a cache
1735   cannot determine whether this response is appropriate. A proxy &MUST-NOT;
1736   generate the "*" value.
1737</t>
1738<t>
1739   The field-names given are not limited to the set of standard header
1740   fields defined by this specification. Field names are case-insensitive.
1741</t>
1742</section>
1743
1744<section anchor="header.warning" title="Warning">
1745   <iref item="Warning header field" primary="true" x:for-anchor="" />
1746   <iref item="Header Fields" primary="true" subitem="Warning" x:for-anchor="" />
1747   <x:anchor-alias value="Warning"/>
1748   <x:anchor-alias value="warning-value"/>
1749   <x:anchor-alias value="warn-agent"/>
1750   <x:anchor-alias value="warn-code"/>
1751   <x:anchor-alias value="warn-date"/>
1752   <x:anchor-alias value="warn-text"/>
1753<t>
1754   The "Warning" header field is used to carry additional information
1755   about the status or transformation of a message that might not be reflected
1756   in the message. This information is typically used to warn about possible
1757   incorrectness introduced by caching operations or transformations applied
1758   to the payload of the message.
1759</t>
1760<t>
1761   Warnings can be used for other purposes, both cache-related and otherwise.
1762   The use of a warning, rather than an error status code, distinguishes these
1763   responses from true failures.
1764</t>
1765<t>
1766   Warning header fields can in general be applied to any message, however some
1767   warn-codes are specific to caches and can only be applied to response
1768   messages.
1769</t>
1770<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"/>
1771  <x:ref>Warning</x:ref>       = 1#<x:ref>warning-value</x:ref>
1772 
1773  <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>
1774                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1775 
1776  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1777  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1778                  ; the name or pseudonym of the server adding
1779                  ; the Warning header field, for use in debugging
1780  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1781  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1782</artwork></figure>
1783<t>
1784   Multiple warnings can be attached to a response (either by the origin
1785   server or by a cache), including multiple warnings with the same code
1786   number, only differing in warn-text.
1787</t>
1788<t>
1789   When this occurs, the user agent &SHOULD; inform the user of as many of
1790   them as possible, in the order that they appear in the response.
1791</t>
1792<t>
1793   Systems that generate multiple Warning header fields &SHOULD; order them with
1794   this user agent behavior in mind. New Warning header fields &SHOULD; be added
1795   after any existing Warning headers fields.
1796</t>
1797<t>
1798   Warnings are assigned three digit warn-codes. The first digit indicates
1799   whether the Warning is required to be deleted from a stored response after
1800   validation:
1801   <list style="symbols">
1802      <t>1xx Warnings describe the freshness or validation status of the
1803      response, and so &MUST; be deleted by a cache after validation. They can
1804      only be generated by a cache when validating a cached entry, and
1805      &MUST-NOT; be generated in any other situation.</t>
1806      <t>2xx Warnings describe some aspect of the representation that is not
1807      rectified by a validation (for example, a lossy compression of the
1808      representation) and &MUST-NOT; be deleted by a cache after validation,
1809      unless a full response is returned, in which case they &MUST; be.</t>
1810   </list>
1811</t>
1812<t>
1813   If an implementation sends a message with one or more Warning header fields to a
1814   receiver whose version is HTTP/1.0 or lower, then the sender &MUST; include
1815   in each warning-value a warn-date that matches the Date header field in the
1816   message.
1817</t>
1818<t>
1819   If a system receives a message with a warning-value that includes
1820   a warn-date, and that warn-date is different from the Date value in the
1821   response, then that warning-value &MUST; be deleted from the message before
1822   storing, forwarding, or using it. (preventing the consequences of naive
1823   caching of Warning header fields.) If all of the warning-values are deleted
1824   for this reason, the Warning header field &MUST; be deleted as well.
1825</t>
1826<t>
1827   The following warn-codes are defined by this specification, each with a
1828   recommended warn-text in English, and a description of its meaning.
1829</t>
1830<t><?rfc needLines="4"?>
1831   110 Response is stale
1832   <list>
1833      <t>A cache &SHOULD; include this whenever the returned response is stale.</t>
1834   </list>
1835</t>
1836<t><?rfc needLines="4"?>
1837   111 Revalidation failed
1838   <list>
1839      <t>A cache &SHOULD; include this when returning a stale response because an
1840      attempt to validate the response failed, due to an inability to reach
1841      the server.</t>
1842   </list>
1843</t>
1844<t><?rfc needLines="4"?>
1845   112 Disconnected operation
1846   <list>
1847      <t>A cache &SHOULD; b include this if it is intentionally disconnected from
1848      the rest of the network for a period of time.</t>
1849   </list>
1850</t>
1851<t><?rfc needLines="4"?>
1852   113 Heuristic expiration
1853   <list>
1854      <t>A cache &SHOULD; include this if it heuristically chose a freshness
1855      lifetime greater than 24 hours and the response's age is greater than 24
1856      hours.</t>
1857   </list>
1858</t>
1859<t><?rfc needLines="4"?>
1860   199 Miscellaneous warning
1861   <list>
1862      <t>The warning text can include arbitrary information to be presented to
1863      a human user, or logged. A system receiving this warning &MUST-NOT; take
1864      any automated action, besides presenting the warning to the user.</t>
1865   </list>
1866</t>
1867<t><?rfc needLines="4"?>
1868   214 Transformation applied
1869   <list>
1870      <t>&MUST; be added by a proxy if it applies any
1871      transformation to the representation, such as changing the
1872      content-coding, media-type, or modifying the representation data, unless
1873      this Warning code already appears in the response.</t>
1874   </list>
1875</t>
1876<t><?rfc needLines="4"?>
1877   299 Miscellaneous persistent warning
1878   <list>
1879      <t>The warning text can include arbitrary information to be presented to
1880      a human user, or logged. A system receiving this warning &MUST-NOT; take
1881      any automated action.</t>
1882   </list>
1883</t>
1884</section>
1885
1886</section>
1887
1888<section anchor="history.lists" title="History Lists">
1889<t>
1890   User agents often have history mechanisms, such as "Back" buttons and
1891   history lists, that can be used to redisplay a representation retrieved
1892   earlier in a session.
1893</t>
1894<t>
1895   The freshness model (<xref target="expiration.model"/>) does not
1896   necessarily apply to history mechanisms. I.e., a history mechanism can
1897   display a previous representation even if it has expired.
1898</t>
1899<t>
1900   This does not prohibit the history mechanism from telling the user that a
1901   view might be stale, or from honoring cache directives (e.g.,
1902   Cache-Control: no-store).
1903</t>
1904</section>
1905
1906
1907<section anchor="IANA.considerations" title="IANA Considerations">
1908
1909<section title="Cache Directive Registry" 
1910   anchor="cache.directive.registration">
1911<t>
1912   The registration procedure for HTTP Cache Directives is defined by <xref
1913   target="cache.control.extensions"/> of this document.
1914</t>
1915<t>
1916   The HTTP Cache Directive Registry shall be created at <eref
1917   target="http://www.iana.org/assignments/http-cache-directives"/> and be
1918   populated with the registrations below:
1919</t>
1920<?BEGININC p6-cache.cache-directives ?>
1921<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1922<texttable xmlns:my="#my" align="left" suppress-title="true"
1923           anchor="iana.cache.directive.registration.table">
1924   <ttcol>Cache Directive</ttcol>
1925   <ttcol>Reference</ttcol>
1926
1927   <c>max-age</c>
1928   <c>
1929      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1930   </c>
1931   <c>max-stale</c>
1932   <c>
1933      <xref target="cache-request-directive"/>
1934   </c>
1935   <c>min-fresh</c>
1936   <c>
1937      <xref target="cache-request-directive"/>
1938   </c>
1939   <c>must-revalidate</c>
1940   <c>
1941      <xref target="cache-response-directive"/>
1942   </c>
1943   <c>no-cache</c>
1944   <c>
1945      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1946   </c>
1947   <c>no-store</c>
1948   <c>
1949      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1950   </c>
1951   <c>no-transform</c>
1952   <c>
1953      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1954   </c>
1955   <c>only-if-cached</c>
1956   <c>
1957      <xref target="cache-request-directive"/>
1958   </c>
1959   <c>private</c>
1960   <c>
1961      <xref target="cache-response-directive"/>
1962   </c>
1963   <c>proxy-revalidate</c>
1964   <c>
1965      <xref target="cache-response-directive"/>
1966   </c>
1967   <c>public</c>
1968   <c>
1969      <xref target="cache-response-directive"/>
1970   </c>
1971   <c>s-maxage</c>
1972   <c>
1973      <xref target="cache-response-directive"/>
1974   </c>
1975   <c>stale-if-error</c>
1976   <c>
1977      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1978   </c>
1979   <c>stale-while-revalidate</c>
1980   <c>
1981      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1982   </c>
1983</texttable>
1984<!--(END)-->
1985<?ENDINC p6-cache.cache-directives ?>
1986</section>
1987
1988<section title="Header Field Registration" anchor="header.field.registration">
1989<t>
1990  The Message Header Field Registry located at <eref 
1991  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1992  shall be updated with the permanent registrations below (see <xref target="RFC3864" />):
1993</t>
1994<?BEGININC p6-cache.iana-headers ?>
1995<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1996<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1997   <ttcol>Header Field Name</ttcol>
1998   <ttcol>Protocol</ttcol>
1999   <ttcol>Status</ttcol>
2000   <ttcol>Reference</ttcol>
2001
2002   <c>Age</c>
2003   <c>http</c>
2004   <c>standard</c>
2005   <c>
2006      <xref target="header.age"/>
2007   </c>
2008   <c>Cache-Control</c>
2009   <c>http</c>
2010   <c>standard</c>
2011   <c>
2012      <xref target="header.cache-control"/>
2013   </c>
2014   <c>Expires</c>
2015   <c>http</c>
2016   <c>standard</c>
2017   <c>
2018      <xref target="header.expires"/>
2019   </c>
2020   <c>Pragma</c>
2021   <c>http</c>
2022   <c>standard</c>
2023   <c>
2024      <xref target="header.pragma"/>
2025   </c>
2026   <c>Vary</c>
2027   <c>http</c>
2028   <c>standard</c>
2029   <c>
2030      <xref target="header.vary"/>
2031   </c>
2032   <c>Warning</c>
2033   <c>http</c>
2034   <c>standard</c>
2035   <c>
2036      <xref target="header.warning"/>
2037   </c>
2038</texttable>
2039<!--(END)-->
2040<?ENDINC p6-cache.iana-headers ?>
2041<t>
2042   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
2043   Force".
2044</t>
2045</section>
2046
2047</section>
2048
2049<section anchor="security.considerations" title="Security Considerations">
2050<t>
2051   Caches expose additional potential vulnerabilities, since the contents of
2052   the cache represent an attractive target for malicious exploitation.
2053   Because cache contents persist after an HTTP request is complete, an attack
2054   on the cache can reveal information long after a user believes that the
2055   information has been removed from the network. Therefore, cache contents
2056   need to be protected as sensitive information.
2057</t>
2058</section>
2059
2060<section title="Acknowledgments" anchor="acks">
2061<t>
2062  See &acks;.
2063</t>
2064</section>
2065
2066</middle>
2067
2068<back>
2069<references title="Normative References">
2070
2071  <reference anchor="Part1">
2072    <front>
2073      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2074      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2075        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2076        <address><email>fielding@gbiv.com</email></address>
2077      </author>
2078      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2079        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2080        <address><email>jg@freedesktop.org</email></address>
2081      </author>
2082      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2083        <organization abbrev="HP">Hewlett-Packard Company</organization>
2084        <address><email>JeffMogul@acm.org</email></address>
2085      </author>
2086      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2087        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2088        <address><email>henrikn@microsoft.com</email></address>
2089      </author>
2090      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2091        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2092        <address><email>LMM@acm.org</email></address>
2093      </author>
2094      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2095        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2096        <address><email>paulle@microsoft.com</email></address>
2097      </author>
2098      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2099        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2100        <address><email>timbl@w3.org</email></address>
2101      </author>
2102      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2103        <organization abbrev="W3C">World Wide Web Consortium</organization>
2104        <address><email>ylafon@w3.org</email></address>
2105      </author>
2106      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2107        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2108        <address><email>julian.reschke@greenbytes.de</email></address>
2109      </author>
2110      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2111    </front>
2112    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
2113    <x:source basename="p1-messaging" href="p1-messaging.xml" />
2114  </reference>
2115
2116  <reference anchor="Part2">
2117    <front>
2118      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2119      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2120        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2121        <address><email>fielding@gbiv.com</email></address>
2122      </author>
2123      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2124        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2125        <address><email>jg@freedesktop.org</email></address>
2126      </author>
2127      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2128        <organization abbrev="HP">Hewlett-Packard Company</organization>
2129        <address><email>JeffMogul@acm.org</email></address>
2130      </author>
2131      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2132        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2133        <address><email>henrikn@microsoft.com</email></address>
2134      </author>
2135      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2136        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2137        <address><email>LMM@acm.org</email></address>
2138      </author>
2139      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2140        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2141        <address><email>paulle@microsoft.com</email></address>
2142      </author>
2143      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2144        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2145        <address><email>timbl@w3.org</email></address>
2146      </author>
2147      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2148        <organization abbrev="W3C">World Wide Web Consortium</organization>
2149        <address><email>ylafon@w3.org</email></address>
2150      </author>
2151      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2152        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2153        <address><email>julian.reschke@greenbytes.de</email></address>
2154      </author>
2155      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2156    </front>
2157    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
2158    <x:source basename="p2-semantics" href="p2-semantics.xml" />
2159  </reference>
2160
2161  <reference anchor="Part4">
2162    <front>
2163      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2164      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2165        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2166        <address><email>fielding@gbiv.com</email></address>
2167      </author>
2168      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2169        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2170        <address><email>jg@freedesktop.org</email></address>
2171      </author>
2172      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2173        <organization abbrev="HP">Hewlett-Packard Company</organization>
2174        <address><email>JeffMogul@acm.org</email></address>
2175      </author>
2176      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2177        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2178        <address><email>henrikn@microsoft.com</email></address>
2179      </author>
2180      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2181        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2182        <address><email>LMM@acm.org</email></address>
2183      </author>
2184      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2185        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2186        <address><email>paulle@microsoft.com</email></address>
2187      </author>
2188      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2189        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2190        <address><email>timbl@w3.org</email></address>
2191      </author>
2192      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2193        <organization abbrev="W3C">World Wide Web Consortium</organization>
2194        <address><email>ylafon@w3.org</email></address>
2195      </author>
2196      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2197        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2198        <address><email>julian.reschke@greenbytes.de</email></address>
2199      </author>
2200      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2201    </front>
2202    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
2203    <x:source basename="p4-conditional" href="p4-conditional.xml" />
2204  </reference>
2205
2206  <reference anchor="Part5">
2207    <front>
2208      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2209      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2210        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2211        <address><email>fielding@gbiv.com</email></address>
2212      </author>
2213      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2214        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2215        <address><email>jg@freedesktop.org</email></address>
2216      </author>
2217      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2218        <organization abbrev="HP">Hewlett-Packard Company</organization>
2219        <address><email>JeffMogul@acm.org</email></address>
2220      </author>
2221      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2222        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2223        <address><email>henrikn@microsoft.com</email></address>
2224      </author>
2225      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2226        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2227        <address><email>LMM@acm.org</email></address>
2228      </author>
2229      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2230        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2231        <address><email>paulle@microsoft.com</email></address>
2232      </author>
2233      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2234        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2235        <address><email>timbl@w3.org</email></address>
2236      </author>
2237      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2238        <organization abbrev="W3C">World Wide Web Consortium</organization>
2239        <address><email>ylafon@w3.org</email></address>
2240      </author>
2241      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2242        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2243        <address><email>julian.reschke@greenbytes.de</email></address>
2244      </author>
2245      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2246    </front>
2247    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
2248    <x:source basename="p5-range" href="p5-range.xml" />
2249  </reference>
2250
2251  <reference anchor="Part7">
2252    <front>
2253      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2254      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2255        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2256        <address><email>fielding@gbiv.com</email></address>
2257      </author>
2258      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2259        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2260        <address><email>jg@freedesktop.org</email></address>
2261      </author>
2262      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2263        <organization abbrev="HP">Hewlett-Packard Company</organization>
2264        <address><email>JeffMogul@acm.org</email></address>
2265      </author>
2266      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2267        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2268        <address><email>henrikn@microsoft.com</email></address>
2269      </author>
2270      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2271        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2272        <address><email>LMM@acm.org</email></address>
2273      </author>
2274      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2275        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2276        <address><email>paulle@microsoft.com</email></address>
2277      </author>
2278      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2279        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2280        <address><email>timbl@w3.org</email></address>
2281      </author>
2282      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2283        <organization abbrev="W3C">World Wide Web Consortium</organization>
2284        <address><email>ylafon@w3.org</email></address>
2285      </author>
2286      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2287        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2288        <address><email>julian.reschke@greenbytes.de</email></address>
2289      </author>
2290      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2291    </front>
2292    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2293    <x:source basename="p7-auth" href="p7-auth.xml" />
2294  </reference>
2295
2296  <reference anchor="RFC2119">
2297    <front>
2298      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2299      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2300        <organization>Harvard University</organization>
2301        <address><email>sob@harvard.edu</email></address>
2302      </author>
2303      <date month="March" year="1997" />
2304    </front>
2305    <seriesInfo name="BCP" value="14" />
2306    <seriesInfo name="RFC" value="2119" />
2307  </reference>
2308
2309  <reference anchor="RFC5234">
2310    <front>
2311      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2312      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2313        <organization>Brandenburg InternetWorking</organization>
2314        <address>
2315          <email>dcrocker@bbiw.net</email>
2316        </address> 
2317      </author>
2318      <author initials="P." surname="Overell" fullname="Paul Overell">
2319        <organization>THUS plc.</organization>
2320        <address>
2321          <email>paul.overell@thus.net</email>
2322        </address>
2323      </author>
2324      <date month="January" year="2008"/>
2325    </front>
2326    <seriesInfo name="STD" value="68"/>
2327    <seriesInfo name="RFC" value="5234"/>
2328  </reference>
2329 
2330</references>
2331
2332<references title="Informative References">
2333
2334  <reference anchor="RFC1305">
2335    <front>
2336      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2337      <author fullname="David L. Mills" initials="D." surname="Mills">
2338        <organization>University of Delaware, Electrical Engineering Department</organization>
2339        <address><email>mills@udel.edu</email></address>
2340      </author>
2341      <date month="March" year="1992" />
2342    </front>
2343    <seriesInfo name="RFC" value="1305" />
2344  </reference>
2345
2346  <reference anchor="RFC2616">
2347    <front>
2348      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2349      <author fullname="R. Fielding" initials="R." surname="Fielding">
2350        <organization>University of California, Irvine</organization>
2351        <address><email>fielding@ics.uci.edu</email></address>
2352      </author>
2353      <author fullname="J. Gettys" initials="J." surname="Gettys">
2354        <organization>W3C</organization>
2355        <address><email>jg@w3.org</email></address>
2356      </author>
2357      <author fullname="J. Mogul" initials="J." surname="Mogul">
2358        <organization>Compaq Computer Corporation</organization>
2359        <address><email>mogul@wrl.dec.com</email></address>
2360      </author>
2361      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2362        <organization>MIT Laboratory for Computer Science</organization>
2363        <address><email>frystyk@w3.org</email></address>
2364      </author>
2365      <author fullname="L. Masinter" initials="L." surname="Masinter">
2366        <organization>Xerox Corporation</organization>
2367        <address><email>masinter@parc.xerox.com</email></address>
2368      </author>
2369      <author fullname="P. Leach" initials="P." surname="Leach">
2370        <organization>Microsoft Corporation</organization>
2371        <address><email>paulle@microsoft.com</email></address>
2372      </author>
2373      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2374        <organization>W3C</organization>
2375        <address><email>timbl@w3.org</email></address>
2376      </author>
2377      <date month="June" year="1999" />
2378    </front>
2379    <seriesInfo name="RFC" value="2616" />
2380  </reference>
2381
2382  <reference anchor="RFC3864">
2383    <front>
2384      <title>Registration Procedures for Message Header Fields</title>
2385      <author fullname="G. Klyne" initials="G." surname="Klyne">
2386        <organization>Nine by Nine</organization>
2387        <address><email>GK-IETF@ninebynine.org</email></address>
2388      </author>
2389      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2390        <organization>BEA Systems</organization>
2391        <address><email>mnot@pobox.com</email></address>
2392      </author>
2393      <author fullname="J. Mogul" initials="J." surname="Mogul">
2394        <organization>HP Labs</organization>
2395        <address><email>JeffMogul@acm.org</email></address>
2396      </author>
2397      <date month="September" year="2004" />
2398    </front>
2399    <seriesInfo name="BCP" value="90" />
2400    <seriesInfo name="RFC" value="3864" />
2401  </reference>
2402
2403  <reference anchor='RFC5226'>
2404    <front>
2405      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2406      <author initials='T.' surname='Narten' fullname='T. Narten'>
2407        <organization>IBM</organization>
2408        <address><email>narten@us.ibm.com</email></address>
2409      </author>
2410      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2411        <organization>Google</organization>
2412        <address><email>Harald@Alvestrand.no</email></address>
2413      </author>
2414      <date year='2008' month='May' />
2415    </front>
2416    <seriesInfo name='BCP' value='26' />
2417    <seriesInfo name='RFC' value='5226' />
2418  </reference>
2419
2420  <reference anchor='RFC5861'>
2421    <front>
2422      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2423      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2424        <organization>Yahoo! Inc.</organization>
2425        <address><email>mnot@yahoo-inc.com</email></address>
2426      </author>
2427      <date month="April" year="2010"/>
2428    </front>
2429    <seriesInfo name='RFC' value='5861' />
2430  </reference>
2431
2432</references>
2433
2434<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2435<t>
2436  Make the specified age calculation algorithm less conservative.
2437  (<xref target="age.calculations"/>)
2438</t>
2439<t>
2440  Remove requirement to consider Content-Location in successful responses
2441  in order to determine the appropriate response to use.
2442  (<xref target="validation.model" />)
2443</t>
2444<t>
2445  Clarify denial of service attack avoidance requirement.
2446  (<xref target="invalidation.after.updates.or.deletions" />)
2447</t>
2448<t>
2449  Change ABNF productions for header fields to only define the field value.
2450  (<xref target="header.field.definitions"/>)
2451</t>
2452<t>
2453  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2454  anymore, as these aspects never were implemented.
2455  (<xref target="header.warning" />)
2456</t>
2457</section>
2458
2459<?BEGININC p6-cache.abnf-appendix ?>
2460<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2461<figure>
2462<artwork type="abnf" name="p6-cache.parsed-abnf">
2463<x:ref>Age</x:ref> = delta-seconds
2464
2465<x:ref>Cache-Control</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2466 cache-directive ] )
2467
2468<x:ref>Expires</x:ref> = HTTP-date
2469
2470<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2471
2472<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2473
2474<x:ref>Pragma</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2475 pragma-directive ] )
2476
2477<x:ref>Vary</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2478 ) )
2479
2480<x:ref>Warning</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2481 )
2482
2483<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2484<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2485<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2486 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2487 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2488 cache-extension
2489<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2490 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2491 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2492 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2493 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2494 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2495
2496<x:ref>delta-seconds</x:ref> = 1*DIGIT
2497
2498<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2499
2500<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2501
2502<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.7&gt;
2503<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2504<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2505
2506<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 3.2.3&gt;
2507
2508<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 3.2.3&gt;
2509
2510<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.7&gt;
2511
2512<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2513<x:ref>warn-code</x:ref> = 3DIGIT
2514<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2515<x:ref>warn-text</x:ref> = quoted-string
2516<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2517 ]
2518</artwork>
2519</figure>
2520<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2521; Age defined but not used
2522; Cache-Control defined but not used
2523; Expires defined but not used
2524; Pragma defined but not used
2525; Vary defined but not used
2526; Warning defined but not used
2527</artwork></figure></section>
2528<?ENDINC p6-cache.abnf-appendix ?>
2529
2530<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2531
2532<section title="Since RFC 2616">
2533  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2534</section>
2535
2536<section title="Since draft-ietf-httpbis-p6-cache-00">
2537<t>
2538  Closed issues:
2539  <list style="symbols">
2540    <t>
2541      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2542    <t>
2543      <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>
2544    <t>
2545      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2546    <t>
2547      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2548    <t>
2549      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2550    <t>
2551      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2552    <t>
2553      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2554    <t>
2555      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2556    <t>
2557      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2558  </list>
2559</t>
2560<t>
2561  Other changes:
2562  <list style="symbols">
2563    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2564        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2565  </list>
2566</t>
2567</section>
2568
2569<section title="Since draft-ietf-httpbis-p6-cache-01">
2570<t>
2571  Closed issues:
2572  <list style="symbols">
2573    <t>
2574      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2575  </list>
2576</t>
2577<t>
2578  Other changes:
2579  <list style="symbols">
2580    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2581      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2582    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2583      specification.</t>
2584  </list>
2585</t>
2586</section>
2587
2588<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2589<t>
2590  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2591  <list style="symbols">
2592    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2593      document.</t>
2594  </list>
2595</t>
2596</section>
2597
2598<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2599<t>
2600  Closed issues:
2601  <list style="symbols">
2602    <t>
2603      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2604  </list>
2605</t>
2606</section>
2607
2608<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2609<t>
2610  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2611  <list style="symbols"> 
2612    <t>
2613      Use "/" instead of "|" for alternatives.
2614    </t>
2615    <t>
2616      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2617      whitespace ("OWS") and required whitespace ("RWS").
2618    </t>
2619    <t>
2620      Rewrite ABNFs to spell out whitespace rules, factor out
2621      header field value format definitions.
2622    </t>
2623  </list>
2624</t>
2625</section>
2626
2627<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2628<t>
2629  This is a total rewrite of this part of the specification.
2630</t>
2631<t>
2632  Affected issues:
2633  <list style="symbols">
2634    <t>
2635      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2636    <t>
2637      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2638    <t>
2639      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2640    <t>
2641      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2642  </list>
2643</t>
2644<t>
2645  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2646  <list style="symbols"> 
2647    <t>
2648      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2649    </t>
2650  </list>
2651</t>
2652</section>
2653
2654<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2655<t>
2656  Closed issues:
2657  <list style="symbols"> 
2658    <t>
2659      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2660      "base for numeric protocol elements"
2661    </t>
2662  </list>
2663</t>
2664<t>
2665  Affected issues:
2666  <list style="symbols">
2667    <t>
2668      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2669      "Vary and non-existant headers"
2670    </t>
2671  </list>
2672</t>
2673</section>
2674
2675<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2676<t>
2677  Closed issues:
2678  <list style="symbols"> 
2679    <t>
2680      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2681      "Definition of 1xx Warn-Codes"
2682    </t>
2683    <t>
2684      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2685      "Content-Location on 304 responses"
2686    </t>
2687    <t>
2688      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2689      "private and no-cache CC directives with headers"
2690    </t>
2691    <t>
2692      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2693      "RFC2047 and warn-text"
2694    </t>
2695  </list>
2696</t>
2697</section>
2698
2699<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2700<t>
2701  Closed issues:
2702  <list style="symbols"> 
2703    <t>
2704      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2705      "serving negotiated responses from cache: header-specific canonicalization"
2706    </t>
2707    <t>
2708      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2709      "Effect of CC directives on history lists"
2710    </t>
2711    <t>
2712      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291" />:
2713      "Cache Extensions can override no-store, etc."
2714    </t>
2715  </list>
2716</t>
2717<t>
2718  Affected issues:
2719  <list style="symbols">
2720    <t>
2721      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2722      Status codes and caching
2723    </t>
2724  </list>
2725</t>
2726<t>
2727  Partly resolved issues:
2728  <list style="symbols"> 
2729    <t>
2730      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2731      "Placement of 13.5.1 and 13.5.2"
2732    </t>
2733  </list>
2734</t>
2735</section>
2736
2737<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2738<t>
2739  Closed issues:
2740  <list style="symbols"> 
2741    <t>
2742      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2743      "Age calculation"
2744    </t>
2745    <t>
2746      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2747      "Clarify differences between / requirements for request and response CC directives"
2748    </t>
2749    <t>
2750      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2751      "Caching authenticated responses"
2752    </t>
2753    <t>
2754      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2755      "IANA registry for cache-control directives"
2756    </t>
2757    <t>
2758      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2759      "Heuristic caching of URLs with query components"
2760    </t>
2761  </list>
2762</t>
2763<t>
2764  Partly resolved issues:
2765  <list style="symbols"> 
2766    <t>
2767      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2768      "Term for the requested resource's URI"
2769    </t>
2770  </list>
2771</t>
2772</section>
2773
2774<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2775<t>
2776  Closed issues:
2777  <list style="symbols"> 
2778    <t>
2779      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2780      "Clarify entity / representation / variant terminology"
2781    </t>
2782    <t>
2783      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2784      "consider removing the 'changes from 2068' sections"
2785    </t>
2786    <t>
2787      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2788      "Allowing heuristic caching for new status codes"
2789    </t>
2790    <t>
2791      Clean up TODOs and prose in "Combining Responses."
2792    </t>
2793  </list>
2794</t>
2795</section>
2796
2797<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2798<t>
2799  Closed issues:
2800  <list style="symbols"> 
2801    <t>
2802      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2803      "Text about clock requirement for caches belongs in p6"
2804    </t>
2805  </list>
2806</t>
2807</section>
2808
2809<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2810<t>
2811  Closed issues:
2812  <list style="symbols"> 
2813    <t>
2814      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2815      "Header Classification"
2816    </t>
2817    <t>
2818      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2819      "Clarify 'public'"
2820    </t>
2821  </list>
2822</t>
2823</section>
2824
2825<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2826<t>
2827  Closed issues:
2828  <list style="symbols">
2829    <t>
2830      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2831      "untangle ABNFs for header fields"
2832    </t>
2833  </list>
2834</t>
2835</section>
2836
2837<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2838<t>
2839  Closed issues:
2840  <list style="symbols">
2841    <t>
2842      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2843      "Mismatch Vary"
2844    </t>
2845    <t>
2846      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2847      "Cache Invalidation only happens upon successful responses"
2848    </t>
2849    <t>
2850      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2851      "Recommend minimum sizes for protocol elements"
2852    </t>
2853    <t>
2854      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2855      "Proxies don't 'understand' methods"
2856    </t>
2857    <t>
2858      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2859      "Cache Extensions can override no-store, etc."
2860    </t>
2861    <t>
2862      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2863      "Pragma"
2864    </t>
2865  </list>
2866</t>
2867</section>
2868
2869<section title="Since draft-ietf-httpbis-p6-cache-15" anchor="changes.since.15">
2870<t>
2871  Closed issues:
2872  <list style="symbols">
2873    <t>
2874      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/290"/>:
2875      "Motivate one-year limit for Expires"
2876    </t>
2877  </list>
2878</t>
2879</section>
2880
2881<section title="Since draft-ietf-httpbis-p6-cache-16" anchor="changes.since.16">
2882<t>
2883  None yet.
2884</t>
2885</section>
2886
2887</section>
2888  </back>
2889</rfc>
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