source: draft-ietf-httpbis/latest/p6-cache.xml @ 1400

Last change on this file since 1400 was 1400, checked in by mnot@…, 8 years ago

Clarify one-year limit on Expires; addresses #290

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