source: draft-ietf-httpbis/latest/p6-cache.xml @ 914

Last change on this file since 914 was 914, checked in by mnot@…, 9 years ago

Allow new status codes to define whether they allow heuristic freshness (see #223).

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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 "July">
16  <!ENTITY ID-YEAR "2010">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY effective-request-uri      "<xref target='Part1' x:rel='#effective.request.uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY status-codes                "<xref target='Part2' x:rel='#status.codes' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc rfcedstyle="yes"?>
46<?rfc-ext allow-markup-in-artwork="yes" ?>
47<?rfc-ext include-references-in-index="yes" ?>
48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
50<front>
51
52  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
53
54  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
55    <organization abbrev="Day Software">Day Software</organization>
56    <address>
57      <postal>
58        <street>23 Corporate Plaza DR, Suite 280</street>
59        <city>Newport Beach</city>
60        <region>CA</region>
61        <code>92660</code>
62        <country>USA</country>
63      </postal>
64      <phone>+1-949-706-5300</phone>
65      <facsimile>+1-949-706-5305</facsimile>
66      <email>fielding@gbiv.com</email>
67      <uri>http://roy.gbiv.com/</uri>
68    </address>
69  </author>
70
71  <author initials="J." surname="Gettys" fullname="Jim Gettys">
72    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
73    <address>
74      <postal>
75        <street>21 Oak Knoll Road</street>
76        <city>Carlisle</city>
77        <region>MA</region>
78        <code>01741</code>
79        <country>USA</country>
80      </postal>
81      <email>jg@freedesktop.org</email>
82      <uri>http://gettys.wordpress.com/</uri>
83    </address>
84  </author>
85
86  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
87    <organization abbrev="HP">Hewlett-Packard Company</organization>
88    <address>
89      <postal>
90        <street>HP Labs, Large Scale Systems Group</street>
91        <street>1501 Page Mill Road, MS 1177</street>
92        <city>Palo Alto</city>
93        <region>CA</region>
94        <code>94304</code>
95        <country>USA</country>
96      </postal>
97      <email>JeffMogul@acm.org</email>
98    </address>
99  </author>
100
101  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
102    <organization abbrev="Microsoft">Microsoft Corporation</organization>
103    <address>
104      <postal>
105        <street>1 Microsoft Way</street>
106        <city>Redmond</city>
107        <region>WA</region>
108        <code>98052</code>
109        <country>USA</country>
110      </postal>
111      <email>henrikn@microsoft.com</email>
112    </address>
113  </author>
114
115  <author fullname="Larry Masinter" initials="L." surname="Masinter">
116    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
117    <address>
118      <postal>
119        <street>345 Park Ave</street>
120        <city>San Jose</city>
121        <region>CA</region>
122        <code>95110</code>
123        <country>USA</country>
124      </postal>
125      <email>LMM@acm.org</email>
126      <uri>http://larry.masinter.net/</uri>
127    </address>
128  </author>
129
130  <author fullname="Paul J. Leach" initials="P." surname="Leach">
131    <organization abbrev="Microsoft">Microsoft Corporation</organization>
132    <address>
133      <postal>
134        <street>1 Microsoft Way</street>
135        <city>Redmond</city>
136        <region>WA</region>
137        <code>98052</code>
138      </postal>
139      <email>paulle@microsoft.com</email>
140    </address>
141  </author>
142
143  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
144    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
145    <address>
146      <postal>
147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
148        <street>The Stata Center, Building 32</street>
149        <street>32 Vassar Street</street>
150        <city>Cambridge</city>
151        <region>MA</region>
152        <code>02139</code>
153        <country>USA</country>
154      </postal>
155      <email>timbl@w3.org</email>
156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
157    </address>
158  </author>
159
160  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
161    <organization abbrev="W3C">World Wide Web Consortium</organization>
162    <address>
163      <postal>
164        <street>W3C / ERCIM</street>
165        <street>2004, rte des Lucioles</street>
166        <city>Sophia-Antipolis</city>
167        <region>AM</region>
168        <code>06902</code>
169        <country>France</country>
170      </postal>
171      <email>ylafon@w3.org</email>
172      <uri>http://www.raubacapeu.net/people/yves/</uri>
173    </address>
174  </author>
175
176  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
177    <address>
178      <email>mnot@mnot.net</email>
179      <uri>http://www.mnot.net/</uri>
180    </address>
181  </author>
182
183  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
184    <organization abbrev="greenbytes">greenbytes GmbH</organization>
185    <address>
186      <postal>
187        <street>Hafenweg 16</street>
188        <city>Muenster</city><region>NW</region><code>48155</code>
189        <country>Germany</country>
190      </postal>
191      <phone>+49 251 2807760</phone>
192      <facsimile>+49 251 2807761</facsimile>
193      <email>julian.reschke@greenbytes.de</email>
194      <uri>http://greenbytes.de/tech/webdav/</uri>
195    </address>
196  </author>
197
198  <date month="&ID-MONTH;" year="&ID-YEAR;" />
199  <workgroup>HTTPbis Working Group</workgroup>
200
201<abstract>
202<t>
203  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
204  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
205  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
206  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
207  fields that control cache behavior or indicate cacheable response messages.
208</t>
209</abstract>
210
211<note title="Editorial Note (To be removed by RFC Editor)">
212  <t>
213    Discussion of this draft should take place on the HTTPBIS working group
214    mailing list (ietf-http-wg@w3.org). The current issues list is
215    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
216    and related documents (including fancy diffs) can be found at
217    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
218  </t>
219  <t>
220    The changes in this draft are summarized in <xref target="changes.since.10"/>.
221  </t>
222</note>
223
224  </front>
225  <middle>
226
227<section anchor="caching" title="Introduction">
228<t>
229  HTTP is typically used for distributed information systems, where performance can be
230  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
231  caching and reusing response messages.
232</t>
233
234<section anchor="intro.purpose" title="Purpose">
235<iref item="cache" />
236<t>
237  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
238  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
239  in order to reduce the response time and network bandwidth consumption on future,
240  equivalent requests. Any client or server &MAY; employ a cache, though a cache cannot be
241  used by a server that is acting as a tunnel.
242</t>
243<t>
244  Caching would be useless if it did not significantly improve performance. The goal of
245  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
246  some cases, a stored response can be reused without the need for a network request,
247  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
248  (see <xref target="expiration.model" />). Even when a new request is required, it is often
249  possible to reuse all or parts of the payload of a prior response to satisfy the request,
250  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
251  purpose (see <xref target="validation.model" />).
252</t>
253</section>
254
255<section anchor="intro.terminology" title="Terminology">
256<t>
257  This specification uses a number of terms to refer to the roles played by participants
258  in, and objects of, HTTP caching.
259</t>
260<t>
261  <iref item="cacheable" />
262  <x:dfn>cacheable</x:dfn>
263  <list>
264    <t>A response is cacheable if a cache is allowed to store a copy of the response message
265      for use in answering subsequent requests. Even when a response is cacheable, there might
266      be additional constraints on whether a cache can use the cached copy to satisfy a
267      particular request.</t>
268  </list>
269</t>
270<t>
271  <iref item="explicit expiration time" />
272  <x:dfn>explicit expiration time</x:dfn>
273  <list>
274    <t>The time at which the origin server intends that a representation should no longer be
275      returned by a cache without further validation.</t>
276  </list>
277</t>
278<t>
279  <iref item="heuristic expiration time" />
280  <x:dfn>heuristic expiration time</x:dfn>
281  <list>
282    <t>An expiration time assigned by a cache when no explicit expiration time is
283    available.</t>
284  </list>
285</t>
286<t>
287  <iref item="age" />
288  <x:dfn>age</x:dfn>
289  <list>
290    <t>The age of a response is the time since it was sent by, or successfully validated
291      with, the origin server.</t>
292  </list>
293</t>
294<t>
295  <iref item="first-hand" />
296  <x:dfn>first-hand</x:dfn>
297  <list>
298    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
299    0.</t>
300  </list>
301</t>
302<t>
303  <iref item="freshness lifetime" />
304  <x:dfn>freshness lifetime</x:dfn>
305  <list>
306    <t>The length of time between the generation of a response and its expiration time. </t>
307  </list>
308</t>
309<t>
310  <iref item="fresh" />
311  <x:dfn>fresh</x:dfn>
312  <list>
313    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
314  </list>
315</t>
316<t>
317  <iref item="stale" />
318  <x:dfn>stale</x:dfn>
319  <list>
320    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
321  </list>
322</t>
323<t>
324  <iref item="validator" />
325  <x:dfn>validator</x:dfn>
326  <list>
327    <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that is used to find
328      out whether a stored response has an equivalent copy of a representation.</t>
329  </list>
330</t>
331<t anchor="shared.and.non-shared.caches">
332  <iref item="validator" />
333  <x:dfn>shared cache</x:dfn>
334  <list>
335    <t>A cache that is accessible to more than one user. A non-shared cache is
336      dedicated to a single user.</t>
337  </list>
338</t>
339</section>
340
341<section anchor="intro.requirements" title="Requirements">
342<t>
343   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
344   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
345   document are to be interpreted as described in <xref target="RFC2119"/>.
346</t>
347<t>
348   An implementation is not compliant if it fails to satisfy one or more
349   of the "MUST" or "REQUIRED" level requirements for the protocols it
350   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
351   level and all the "SHOULD" level requirements for its protocols is said
352   to be "unconditionally compliant"; one that satisfies all the "MUST"
353   level requirements but not all the "SHOULD" level requirements for its
354   protocols is said to be "conditionally compliant".
355</t>
356</section>
357
358<section title="Syntax Notation" anchor="notation">
359  <x:anchor-alias value="ALPHA"/>
360  <x:anchor-alias value="CR"/>
361  <x:anchor-alias value="DIGIT"/>
362  <x:anchor-alias value="DQUOTE"/>
363  <x:anchor-alias value="LF"/>
364  <x:anchor-alias value="OCTET"/>
365  <x:anchor-alias value="SP"/>
366  <x:anchor-alias value="VCHAR"/>
367  <x:anchor-alias value="WSP"/>
368<t>
369  This specification uses the ABNF syntax defined in &notation; (which
370  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
371  <xref target="collected.abnf"/> shows the collected ABNF, with the list
372  rule expanded.
373</t>
374<t>
375  The following core rules are included by
376  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
377  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
378  DIGIT (decimal 0-9), DQUOTE (double quote),
379  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
380  OCTET (any 8-bit sequence of data), SP (space),
381  VCHAR (any visible USASCII character),
382  and WSP (whitespace).
383</t>
384
385<section title="Core Rules" anchor="core.rules">
386  <x:anchor-alias value="quoted-string"/>
387  <x:anchor-alias value="token"/>
388  <x:anchor-alias value="OWS"/>
389<t>
390  The core rules below are defined in &basic-rules;:
391</t>
392<figure><artwork type="abnf2616">
393  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
394  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
395  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
396</artwork></figure>
397</section>
398
399<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
400  <x:anchor-alias value="field-name"/>
401  <x:anchor-alias value="HTTP-date"/>
402  <x:anchor-alias value="port"/>
403  <x:anchor-alias value="pseudonym"/>
404  <x:anchor-alias value="uri-host"/>
405<t>
406  The ABNF rules below are defined in other parts:
407</t>
408<figure><!--Part1--><artwork type="abnf2616">
409  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
410  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
411  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
412  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
413  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
414</artwork></figure>
415</section>
416
417</section>
418</section>
419
420<section anchor="caching.overview" title="Cache Operation">
421
422<section anchor="response.cacheability" title="Response Cacheability">
423<t>
424  A cache &MUST-NOT; store a response to any request, unless:
425  <list style="symbols">
426    <t>The request method is understood by the cache and defined as being cacheable, and</t>
427    <t>the response status code is understood by the cache, and</t>
428    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
429       appear in request or response headers, and</t>
430    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
431       does not appear in the response, if the cache is shared, and</t>
432    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
433       the cache is shared, unless the response explicitly allows it (see <xref
434       target="caching.authenticated.responses" />), and</t>
435    <t>the response either:
436      <list style="symbols">
437        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
438        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
439        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
440        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
441        <t>has a status code that can be served with heuristic freshness (see <xref
442           target="heuristic.freshness" />).</t>
443      </list>
444    </t>
445  </list>
446</t>
447<t>
448  In this context, a cache has "understood" a request method or a response status
449  code if it recognises it and implements any cache-specific behaviour. In
450  particular, 206 Partial Content responses cannot be cached by an
451  implementation that does not handle partial content
452  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
453</t>
454<t>
455  Note that in normal operation, most caches will not store a response that has neither a
456  cache validator nor an explicit expiration time, as such responses are not usually
457  useful to store. However, caches are not prohibited from storing such responses.
458</t>
459
460<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
461<t>
462  A cache that receives an incomplete response (for example, with fewer bytes of data
463  than specified in a Content-Length header) can store the response, but &MUST; 
464  treat it as a partial response &partial;. Partial responses
465  can be combined as described in &combining-byte-ranges;; the result might be a
466  full response or might still be partial. A cache &MUST-NOT; return a partial
467  response to a client without explicitly marking it as such using the 206 (Partial
468  Content) status code.
469</t>
470<t>
471  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
472  incomplete or partial responses.
473</t>
474</section>
475
476</section>
477
478
479<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
480<t>
481  For a presented request, a cache &MUST-NOT; return a stored response, unless:
482  <list style="symbols">
483    <t>The presented Effective Request URI (&effective-request-uri;) and that of the stored response match, and</t>
484    <t>the request method associated with the stored response allows it to be
485      used for the presented request, and</t>
486    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
487      target="caching.negotiated.responses" />), and</t>
488    <t>the presented request and stored response are free from directives that would prevent
489      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
490      and</t>
491    <t>the stored response is either:
492      <list style="symbols">
493        <t>fresh (see <xref target="expiration.model" />), or</t>
494        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
495        <t>successfully validated (see <xref target="validation.model" />).</t>
496      </list>
497    </t>
498  </list>
499</t>
500<t>
501  When a stored response is used to satisfy a request, caches &MUST; include a
502  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
503  current_age; see <xref target="age.calculations" />.
504  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
505</t>
506<t>
507  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
508  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
509  corresponding response.
510</t>
511<t>
512  Also, note that unsafe requests might invalidate already stored responses; see
513  <xref target="invalidation.after.updates.or.deletions" />.
514</t>
515<t>
516  Caches &MUST; use the most recent response (as determined by the Date header) when
517  more than one suitable response is stored. They can also forward a request with
518  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
519  use.
520</t>
521</section>
522
523<section anchor="expiration.model" title="Freshness Model">
524<t>
525  When a response is "fresh" in the cache, it can be used to satisfy subsequent
526  requests without contacting the origin server, thereby improving efficiency.
527</t>
528<t>
529  The primary mechanism for determining freshness is for an origin server to provide an
530  explicit expiration time in the future, using either the Expires header (<xref
531  target="header.expires" />) or the max-age response cache directive (<xref
532  target="cache-response-directive" />). Generally, origin servers will assign future
533  explicit expiration times to responses in the belief that the representation is not likely to
534  change in a semantically significant way before the expiration time is reached.
535</t>
536<t>
537  If an origin server wishes to force a cache to validate every request, it can
538  assign an explicit expiration time in the past. This means that the response is always
539  stale, so that caches should validate it before using it for subsequent requests.
540  <cref anchor="TODO-response-stale">This wording might cause confusion, because the response might still be served stale.</cref>
541</t>
542<t>
543  Since origin servers do not always provide explicit expiration times, HTTP caches &MAY;
544  assign heuristic expiration times when explicit times are not specified, employing algorithms that
545  use other header values (such as the Last-Modified time) to estimate a plausible
546  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
547  impose worst-case constraints on their results.
548</t>
549<figure>
550<preamble>
551  The calculation to determine if a response is fresh is:
552</preamble>
553<artwork type="code">
554   response_is_fresh = (freshness_lifetime &gt; current_age)
555</artwork>
556</figure>
557<t>
558  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
559  the current_age is defined in <xref target="age.calculations" />.
560</t>
561<t>
562  Additionally, clients might need to influence freshness calculation. They can do this using
563  several request cache directives, with the effect of either increasing or loosening
564  constraints on freshness. See <xref target="cache-request-directive" />.
565</t>
566<t>
567  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
568  freshness.</cref>
569</t>
570<t>
571  Note that freshness applies only to cache operation; it cannot be used to force a user agent
572  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
573  the difference between caches and history mechanisms.
574</t>
575
576<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
577<t>
578  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
579  response by using the first match of:
580  <list style="symbols">
581    <t>If the cache is shared and the s-maxage response cache directive (<xref
582      target="cache-response-directive" />) is present, use its value, or</t>
583    <t>If the max-age response cache directive (<xref target="cache-response-directive"
584      />) is present, use its value, or</t>
585    <t>If the Expires response header (<xref target="header.expires" />) is present, use
586      its value minus the value of the Date response header, or</t>
587    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
588      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
589  </list>
590</t>
591<t>
592  Note that this calculation is not vulnerable to clock skew, since all of the
593  information comes from the origin server.
594</t>
595
596<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
597<t>
598  If no explicit expiration time is present in a stored response that has a status code
599  whose definition allows heuristic freshness to be used (including the following
600  in &status-codes;: 200, 203, 206, 300, 301 and 410), a heuristic expiration time &MAY; be
601  calculated. Heuristics &MUST-NOT; be used for response status codes that do not explicitly
602  allow it.
603</t>
604<t>
605  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
606  attach a Warning header with a 113 warn-code to the response if its current_age is
607  more than 24 hours and such a warning is not already present.
608</t>
609<t>
610  Also, if the response has a Last-Modified header (&header-last-modified;), the
611  heuristic expiration value &SHOULD; be no more than some fraction of the interval
612  since that time. A typical setting of this fraction might be 10%.
613</t>
614<x:note>
615  <t>
616    <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt="," x:sec="13.9"/>)
617    required that caches do not calculate heuristic freshness for URLs with
618    query components (i.e., those containing '?'). In practice, this has not
619    been widely implemented. Therefore, servers are encouraged to send explicit
620    directives (e.g., Cache-Control: no-cache) if they wish to preclude
621    caching.
622  </t>
623</x:note>
624</section>
625</section>
626
627<section anchor="age.calculations" title="Calculating Age">
628<t>
629  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
630  message when obtained from a cache. The Age field value is the cache's estimate of the
631  amount of time since the response was generated or validated by the origin server. In
632  essence, the Age value is the sum of the time that the response has been resident in
633  each of the caches along the path from the origin server, plus the amount of time it has
634  been in transit along network paths.
635</t>
636<t>
637  The following data is used for the age calculation:
638</t>
639<t>
640  <x:dfn>age_value</x:dfn>
641  <list>
642    <t>
643      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
644      in a form appropriate for arithmetic operation; or 0, if not available.
645    </t>
646  </list>
647</t>
648<t>
649  <x:dfn>date_value</x:dfn>
650  <list>
651    <t>
652      HTTP/1.1 requires origin servers to send a Date header, if possible,
653      with every response, giving the time at which the response was generated.
654      The term "date_value" denotes the value of the Date header, in a form
655      appropriate for arithmetic operations. See &header-date; for the definition
656      of the Date header, and for requirements regarding responses without a
657      Date response header.
658    </t>
659  </list>
660</t>
661<t>
662  <x:dfn>now</x:dfn>
663  <list>
664    <t>
665      The term "now" means "the current value of the clock at the host
666      performing the calculation". Hosts that use HTTP, but especially hosts
667      running origin servers and caches, &SHOULD; use NTP
668      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
669      clocks to a globally accurate time standard.
670    </t>
671  </list>
672</t>
673<t>
674  <x:dfn>request_time</x:dfn>
675  <list>
676    <t>
677      The current value of the clock at the host at the time the request
678      resulting in the stored response was made.
679    </t>
680  </list>
681</t>
682<t>
683  <x:dfn>response_time</x:dfn>
684  <list>
685    <t>
686      The current value of the clock at the host at the time the response was
687      received.
688    </t>
689  </list>
690</t>
691<t>
692  A response's age can be calculated in two entirely independent ways:
693  <list style="numbers">
694    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
695      origin server's clock. If the result is negative, the result is replaced by zero.</t>
696    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
697      note this value &MUST; be interpreted relative to the time the
698      request was initiated, not the time that the response was received.</t>
699  </list>
700</t>
701<figure>
702<artwork type="code">
703  apparent_age = max(0, response_time - date_value);
704
705  response_delay = response_time - request_time;
706  corrected_age_value = age_value + response_delay; 
707</artwork>
708</figure>
709<figure>
710<preamble>These are combined as</preamble>
711<artwork type="code">
712  corrected_initial_age = max(apparent_age, corrected_age_value);
713</artwork></figure>
714<t>
715  The current_age of a stored response can then be calculated by adding the amount of
716  time (in seconds) since the stored response was last validated by the origin server to
717  the corrected_initial_age.
718</t>
719<figure><artwork type="code">
720  resident_time = now - response_time;
721  current_age = corrected_initial_age + resident_time;
722</artwork></figure>
723</section>
724
725<section anchor="serving.stale.responses" title="Serving Stale Responses">
726<t>
727  A "stale" response is one that either has explicit expiry information or is allowed to
728  have heuristic expiry calculated, but is not fresh according to the calculations in
729  <xref target="expiration.model" />.
730</t>
731<t>
732  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
733  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
734  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
735  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
736</t>
737<t>
738  Caches &SHOULD-NOT; return stale responses unless they are
739  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
740  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
741</t>
742<t>
743  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
744  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
745  the cache is disconnected.
746</t>
747<t>
748  If a cache receives a first-hand response (either an entire response, or a 304 (Not
749  Modified) response) that it would normally forward to the requesting client, and the
750  received response is no longer fresh, the cache &SHOULD; forward it to the
751  requesting client without adding a new Warning (but without removing any existing
752  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
753  that response became stale in transit.
754</t>
755</section>
756</section>
757
758<section anchor="validation.model" title="Validation Model">
759<t>
760  When a cache has one or more stored responses for a requested URI, but cannot 
761  serve any of them (e.g., because they are not fresh, or one cannot be selected;
762  see <xref target="caching.negotiated.responses"/>),
763  it can use the conditional request mechanism &conditional; in the forwarded
764  request to give the origin server an opportunity to both select a valid stored
765  response to be used, and to update it. This process is known as "validating"
766  or "revalidating" the stored response.
767</t>
768<t>
769  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
770  header whose value is that of the Last-Modified header from the selected
771  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
772</t>
773<t>
774  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
775  is that of the ETag header(s) from all responses stored for the requested URI,
776  if present. However, if any of the stored responses contains only partial
777  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
778  field unless the request is for a range that would be fully satisfied by
779  that stored response.
780</t>
781<t>
782  A 304 (Not Modified) response status code indicates that the stored
783  response can be updated and reused; see <xref target="combining.headers"/>.
784</t>
785<t>
786  A full response (i.e., one with a response body) indicates that none 
787  of the stored responses nominated in the conditional request is
788  suitable. Instead, the full response is used both to satisfy the
789  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
790</t>
791<t>
792  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
793  either forward this response to the requesting client, or act as if the server failed to
794  respond. In the latter case, it &MAY; return a previously stored response (see <xref
795  target="serving.stale.responses" />).
796</t>
797</section>
798
799<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
800<t>
801  Because unsafe methods (&safe-methods;) have the potential for changing state on the
802  origin server, intervening caches can use them to keep their contents
803  up-to-date.
804</t>
805<t>
806  The following HTTP methods &MUST; cause a cache to invalidate the Effective Request URI (&effective-request-uri;) as well
807  as the URI(s) in the Location and Content-Location headers (if present):
808  <list style="symbols">
809    <t>PUT</t>
810    <t>DELETE</t>
811    <t>POST</t>
812  </list>
813</t>
814<t>
815  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
816  be performed if the host part of that URI differs from the host part in the Effective Request URI (&effective-request-uri;).
817  This helps prevent denial of service attacks.
818</t>
819<t>
820  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
821</t>
822<t>
823  A cache that passes through requests for methods it does not understand &SHOULD;
824  invalidate the Effective Request URI (&effective-request-uri;).
825</t>
826<t>
827  Here, "invalidate" means that the cache will either remove all stored responses related
828  to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
829  before they can be returned in response to a subsequent request.
830</t>
831<t>
832  Note that this does not guarantee that all appropriate responses are invalidated. For
833  example, the request that caused the change at the origin server might not have gone
834  through the cache where a response is stored.
835</t>
836<t>
837  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
838</t>
839</section>
840
841<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
842
843<t>Shared caches &MUST-NOT; use a cached response to a request with an Authorization header (&header-authorization;) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.</t>
844
845<t>In this specification, the following Cache-Control response directives (<xref target="cache-response-directive"/>) have such an effect: must-revalidate, public, s-maxage.</t>
846
847<t>Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.</t>
848</section>
849
850<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
851<t>
852  When a cache receives a request that can be satisfied by a stored response
853  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
854  response unless all of the selecting request-headers nominated by the Vary header match
855  in both the original request (i.e., that associated with the stored response),
856  and the presented request.
857</t>
858<t>
859  The selecting request-headers from two requests are defined to match
860  if and only if those in the first request can be transformed to those in the
861  second request by applying any of the following:
862  <list style="symbols">
863    <t>
864      adding or removing whitespace, where allowed in the header's syntax
865    </t>
866    <t>
867      combining multiple message-header fields with the same field name (see
868      &header-fields;)
869    </t>
870    <t>
871      normalizing both header values in a way that is known to have identical
872      semantics, according to the header's specification (e.g., re-ordering field values
873      when order is not significant; case-normalization, where values are defined to be
874      case-insensitive)   
875    </t>
876  </list>
877</t>
878<t>
879  If (after any normalization that might take place) a header field is absent
880  from a request, it can only match another request if it is also absent there.
881</t>
882<t>
883  A Vary header field-value of "*" always fails to match, and subsequent requests to that
884  resource can only be properly interpreted by the origin server.
885</t>
886<t>
887  The stored response with matching selecting request-headers is known as the
888  selected response.
889</t>
890<t>
891  If no selected response is available, the cache &MAY; forward the presented
892  request to the origin server in a conditional request; see <xref target="validation.model"/>.
893</t>
894</section>
895
896<section anchor="combining.headers" title="Combining Responses">
897<t>
898  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
899  (in this section, the "new" response"), it needs to created an updated response by combining
900  the stored response with the new one, so that the updated response can be used to satisfy the request.
901</t>
902<t>
903  If the new response contains an ETag, it identifies the stored 
904  response to use. <cref anchor="TODO-mention-CL">might need language about Content-Location 
905  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
906</t>
907<t>
908  If the status code is 206 (partial content), both the stored and new 
909  responses &MUST; have validators, and those validators &MUST; match using the strong 
910  comparison function (see &weak-and-strong;). Otherwise, the 
911  responses &MUST-NOT; be combined.
912</t>
913<t>
914  The stored response headers are used as those of the updated response, except that
915  <list style="symbols">
916    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
917      &MUST; be deleted from the stored response and the updated response.</t>
918    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
919      response and the updated response.</t>
920    <t>any headers provided in the new response &MUST; replace the corresponding
921      headers from the stored response.</t>
922  </list>
923</t>
924<t>
925  If a header field-name in the new response matches more than one 
926  header in the stored response, all such stored headers &MUST; be replaced.
927</t>
928<t>
929  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
930  stored response in cache. In the case of a 206 response, the combined 
931  representation &MAY; be stored.
932</t>
933</section>
934
935</section>
936
937<section anchor="header.fields" title="Header Field Definitions">
938<t>
939  This section defines the syntax and semantics of HTTP/1.1 header fields
940  related to caching.
941</t>
942<t>
943  For entity-header fields, both sender and recipient refer to either the client or the
944  server, depending on who sends and who receives the message.
945</t>
946
947<section anchor="header.age" title="Age">
948  <iref item="Age header" primary="true" x:for-anchor="" />
949  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
950  <x:anchor-alias value="Age"/>
951  <x:anchor-alias value="Age-v"/>
952  <x:anchor-alias value="age-value"/>
953<t>
954  The "Age" response-header field conveys the sender's estimate of the amount
955  of time since the response was generated or successfully validated at the
956  origin server. Age values are calculated as specified in
957  <xref target="age.calculations" />.
958</t>
959<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
960  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
961  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
962</artwork></figure>
963<t anchor="rule.delta-seconds">
964  <x:anchor-alias value="delta-seconds" />
965  Age field-values are non-negative integers, representing time in seconds.
966</t>
967<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
968  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
969</artwork></figure>
970<t>
971  If a cache receives a value larger than the largest positive integer it can represent, or
972  if any of its age calculations overflows, it &MUST; transmit an Age header with a
973  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
974  of at least 31 bits of range.
975</t>
976<t>
977  The presence of an Age header field in a response implies that a response is not
978  first-hand. However, the converse is not true, since HTTP/1.0 caches might not implement the
979  Age header field.
980</t>
981</section>
982
983<section anchor="header.cache-control" title="Cache-Control">
984  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
985  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
986  <x:anchor-alias value="Cache-Control"/>
987  <x:anchor-alias value="Cache-Control-v"/>
988  <x:anchor-alias value="cache-directive"/>
989  <x:anchor-alias value="cache-extension"/>
990  <x:anchor-alias value="cache-request-directive"/>
991  <x:anchor-alias value="cache-response-directive"/>
992<t>
993  The "Cache-Control" general-header field is used to specify directives for
994  caches along the request/response chain. Such cache directives are
995  unidirectional in that the presence of a directive in a request does not
996  imply that the same directive is to be given in the response.
997</t>
998<t>
999  HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives
1000  defined in this section. See <xref target="cache.control.extensions"/> for
1001  information about how Cache-Control directives defined elsewhere are handled.
1002</t>
1003<x:note>
1004  <t>
1005    <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1006    might only implement Pragma: no-cache (see <xref target="header.pragma" />).
1007  </t>
1008</x:note>
1009<t>
1010  Cache directives &MUST; be passed through by a proxy or gateway application,
1011  regardless of their significance to that application, since the directives might be
1012  applicable to all recipients along the request/response chain. It is not possible to
1013  target a directive to a specific cache.
1014</t>
1015<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1016  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1017  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1018
1019  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1020     / <x:ref>cache-response-directive</x:ref>
1021
1022  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1023</artwork></figure>
1024
1025<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1026  <x:anchor-alias value="cache-request-directive" />
1027
1028<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1029  <x:ref>cache-request-directive</x:ref> =
1030       "no-cache"
1031     / "no-store"
1032     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1033     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1034     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1035     / "no-transform"
1036     / "only-if-cached"
1037     / <x:ref>cache-extension</x:ref>
1038</artwork></figure>
1039
1040<t>
1041  <x:dfn>no-cache</x:dfn>
1042  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1043  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1044  <list>
1045    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1046      used to satisfy the request without successful validation on the origin server.</t>
1047  </list>
1048</t>
1049<t>
1050  <x:dfn>no-store</x:dfn>
1051  <iref item="Cache Directives" primary="true" subitem="no-store" />
1052  <iref item="no-store" primary="true" subitem="Cache Directive" />
1053  <list>
1054    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1055      of either this request or any response to it. This directive applies to both
1056      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1057      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1058      and &MUST; make a best-effort attempt to remove the information from volatile
1059      storage as promptly as possible after forwarding it.</t>
1060    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1061      particular, malicious or compromised caches might not recognize or obey this
1062      directive, and communications networks might be vulnerable to eavesdropping.</t>
1063  </list>
1064</t>
1065<t>
1066  <x:dfn>max-age</x:dfn>
1067  <iref item="Cache Directives" primary="true" subitem="max-age" />
1068  <iref item="max-age" primary="true" subitem="Cache Directive" />
1069  <list>
1070    <t>The max-age request directive indicates that the client is willing to accept a
1071      response whose age is no greater than the specified time in seconds. Unless
1072      the max-stale request directive is also present, the client is not willing to accept a stale
1073      response.</t>
1074  </list>
1075</t>
1076<t>
1077  <x:dfn>max-stale</x:dfn>
1078  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1079  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1080  <list>
1081    <t>The max-stale request directive indicates that the client is willing to accept a
1082      response that has exceeded its expiration time. If max-stale is assigned a value,
1083      then the client is willing to accept a response that has exceeded its expiration
1084      time by no more than the specified number of seconds. If no value is assigned to
1085      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
1086  </list>
1087</t>
1088<t>
1089  <x:dfn>min-fresh</x:dfn>
1090  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1091  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1092  <list>
1093    <t>The min-fresh request directive indicates that the client is willing to accept a
1094      response whose freshness lifetime is no less than its current age plus the specified
1095      time in seconds. That is, the client wants a response that will still be fresh for
1096      at least the specified number of seconds.</t>
1097  </list>
1098</t>
1099<t>
1100  <x:dfn>no-transform</x:dfn>
1101  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1102  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1103  <list>
1104    <t>The no-transform request directive indicates that an intermediate cache or proxy
1105      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1106      headers, nor the request representation.</t>
1107  </list>
1108</t>
1109<t>
1110  <x:dfn>only-if-cached</x:dfn>
1111  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1112  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1113  <list>
1114    <t>The only-if-cached request directive indicates that the client only wishes to
1115      return a stored response. If it receives this directive, a cache &SHOULD; either
1116      respond using a stored response that is consistent with the other constraints of the
1117      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1118      being operated as a unified system with good internal connectivity, such a request
1119      &MAY; be forwarded within that group of caches.</t>
1120  </list>
1121</t>
1122</section>
1123
1124<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1125  <x:anchor-alias value="cache-response-directive" />
1126
1127<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1128  <x:ref>cache-response-directive</x:ref> =
1129       "public"
1130     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1131     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1132     / "no-store"
1133     / "no-transform"
1134     / "must-revalidate"
1135     / "proxy-revalidate"
1136     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1137     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1138     / <x:ref>cache-extension</x:ref>
1139</artwork></figure>
1140
1141<t>
1142  <x:dfn>public</x:dfn>
1143  <iref item="Cache Directives" primary="true" subitem="public" />
1144  <iref item="public" primary="true" subitem="Cache Directive" />
1145  <list>
1146    <t>The public response directive indicates that the response &MAY; be cached, even
1147      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1148      (See also Authorization, &header-authorization;, for additional details.) </t>
1149  </list>
1150</t>
1151<t>
1152  <x:dfn>private</x:dfn>
1153  <iref item="Cache Directives" primary="true" subitem="private" />
1154  <iref item="private" primary="true" subitem="Cache Directive" />
1155  <list>
1156    <t>The private response directive indicates that the response message is intended for
1157      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1158      cache &MAY; store the response.</t>
1159    <t>If the private response directive specifies one or more field-names, this
1160      requirement is limited to the field-values associated with the listed response
1161      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1162      cache, whereas the remainder of the response message &MAY; be.</t>
1163    <t>
1164      <x:h>Note:</x:h> This usage of the word private only controls where the response can
1165      be stored; it cannot ensure the privacy of the message content.
1166      Also, private response directives with field-names are often handled by
1167      implementations as if an unqualified private directive was received; i.e.,
1168      the special handling for the qualified form is not widely implemented.</t>
1169  </list>
1170</t>
1171<t>
1172  <x:dfn>no-cache</x:dfn>
1173  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1174  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1175  <list>
1176    <t>The no-cache response directive indicates that the response MUST NOT be used
1177          to satisfy a subsequent request without successful validation on the origin
1178          server. This allows an origin server to prevent a cache from using it to satisfy
1179          a request without contacting it, even by caches that have been configured to
1180          return stale responses.</t>
1181    <t>If the no-cache response directive specifies one or more field-names, this
1182      requirement is limited to the field-values associated with the listed response
1183      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1184      to a subsequent request without successful validation on the origin server. This
1185      allows an origin server to prevent the re-use of certain header fields in a
1186      response, while still allowing caching of the rest of the response.</t>
1187    <t>
1188      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1189      Also, no-cache response directives with field-names are often handled by
1190      implementations as if an unqualified no-cache directive was received; i.e.,
1191      the special handling for the qualified form is not widely implemented.
1192    </t>
1193  </list>
1194</t>
1195<t>
1196  <x:dfn>no-store</x:dfn>
1197  <iref item="Cache Directives" primary="true" subitem="no-store" />
1198  <iref item="no-store" primary="true" subitem="Cache Directive" />
1199  <list>
1200    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1201      part of either the immediate request or response. This directive applies to both
1202      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1203      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1204      and &MUST; make a best-effort attempt to remove the information from volatile
1205      storage as promptly as possible after forwarding it.</t>
1206    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1207      particular, malicious or compromised caches might not recognize or obey this
1208      directive, and communications networks might be vulnerable to eavesdropping.</t>
1209  </list>
1210</t>
1211<t>
1212  <x:dfn>must-revalidate</x:dfn>
1213  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1214  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1215  <list>
1216    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1217     used to satisfy subsequent requests without successful validation on the origin server.</t>
1218    <t>The must-revalidate directive is necessary to support reliable operation for
1219      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1220      the must-revalidate directive; in particular, if the cache cannot reach the origin
1221      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1222    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1223      validate a request on the representation could result in incorrect operation, such as a
1224      silently unexecuted financial transaction.</t>
1225  </list>
1226</t>
1227<t>
1228  <x:dfn>proxy-revalidate</x:dfn>
1229  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1230  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1231  <list>
1232    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1233      response directive, except that it does not apply to non-shared caches.</t>
1234  </list>
1235</t>
1236<t>
1237  <x:dfn>max-age</x:dfn>
1238  <iref item="Cache Directives" primary="true" subitem="max-age" />
1239  <iref item="max-age" primary="true" subitem="Cache Directive" />
1240  <list>
1241    <t>The max-age response directive indicates that response is to be considered stale
1242      after its age is greater than the specified number of seconds.</t>
1243  </list>
1244</t>
1245<t>
1246  <x:dfn>s-maxage</x:dfn>
1247  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1248  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1249  <list>
1250    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1251      specified by this directive overrides the maximum age specified by either the
1252      max-age directive or the Expires header. The s-maxage directive also implies the
1253      semantics of the proxy-revalidate response directive.</t>
1254  </list>
1255</t>
1256<t>
1257  <x:dfn>no-transform</x:dfn>
1258  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1259  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1260  <list>
1261    <t>The no-transform response directive indicates that an intermediate cache or proxy
1262      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1263      headers, nor the response representation.</t>
1264  </list>
1265</t>
1266
1267</section>
1268
1269<section anchor="cache.control.extensions" title="Cache Control Extensions">
1270<t>
1271  The Cache-Control header field can be extended through the use of one or more
1272  cache-extension tokens, each with an optional value. Informational extensions (those
1273  that do not require a change in cache behavior) can be added without changing the
1274  semantics of other directives. Behavioral extensions are designed to work by acting as
1275  modifiers to the existing base of cache directives. Both the new directive and the
1276  standard directive are supplied, such that applications that do not understand the new
1277  directive will default to the behavior specified by the standard directive, and those
1278  that understand the new directive will recognize it as modifying the requirements
1279  associated with the standard directive. In this way, extensions to the cache-control
1280  directives can be made without requiring changes to the base protocol.
1281</t>
1282<t>
1283  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1284  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1285  all directives that it does not understand.
1286</t>
1287<t>
1288  For example, consider a hypothetical new response directive called "community" that
1289  acts as a modifier to the private directive. We define this new directive to mean that,
1290  in addition to any non-shared cache, any cache that is shared only by members of the
1291  community named within its value may cache the response. An origin server wishing to
1292  allow the UCI community to use an otherwise private response in their shared cache(s)
1293  could do so by including
1294</t>
1295<figure><artwork type="example">
1296  Cache-Control: private, community="UCI"
1297</artwork></figure>
1298<t>
1299  A cache seeing this header field will act correctly even if the cache does not
1300  understand the community cache-extension, since it will also see and understand the
1301  private directive and thus default to the safe behavior.
1302</t>
1303<t>
1304  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1305  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1306  directives (or the response's default cacheability) such that the cache behavior will
1307  remain minimally correct even if the cache does not understand the extension(s).
1308</t>
1309<t>
1310  The HTTP Cache Directive Registry defines the name space for the cache
1311  directives.
1312</t>
1313<t>
1314  Registrations &MUST; include the following fields:
1315  <list style="symbols">
1316    <t>Cache Directive Name</t>
1317    <t>Pointer to specification text</t>
1318  </list>
1319</t>
1320<t>
1321  Values to be added to this name space are subject to IETF review
1322  (<xref target="RFC5226" x:fmt="," x:sec="4.1"/>).
1323</t>
1324<t>
1325  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1326</t>
1327</section>
1328
1329</section>
1330
1331<section anchor="header.expires" title="Expires">
1332  <iref item="Expires header" primary="true" x:for-anchor="" />
1333  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1334  <x:anchor-alias value="Expires"/>
1335  <x:anchor-alias value="Expires-v"/>
1336<t>
1337  The "Expires" entity-header field gives the date/time after which the response is
1338  considered stale. See <xref target="expiration.model" /> for further discussion of the
1339  freshness model.
1340</t>
1341<t>
1342  The presence of an Expires field does not imply that the original resource will change or
1343  cease to exist at, before, or after that time.
1344</t>
1345<t>
1346  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1347  it &MUST; be sent in rfc1123-date format.
1348</t>
1349<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1350  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1351  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1352</artwork></figure>
1353<figure>
1354  <preamble>For example</preamble>
1355<artwork type="example">
1356  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1357</artwork></figure>
1358<x:note>
1359  <t>
1360    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1361    directive (see <xref target="cache-response-directive" />), that directive overrides
1362    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1363  </t>
1364</x:note>
1365<t>
1366  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1367</t>
1368<t>
1369  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1370  including the value "0", as in the past (i.e., "already expired").
1371</t>
1372</section>
1373
1374<section anchor="header.pragma" title="Pragma">
1375  <iref item="Pragma header" primary="true" x:for-anchor="" />
1376  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1377  <x:anchor-alias value="extension-pragma"/>
1378  <x:anchor-alias value="Pragma"/>
1379  <x:anchor-alias value="Pragma-v"/>
1380  <x:anchor-alias value="pragma-directive"/>
1381<t>
1382  The "Pragma" general-header field is used to include implementation-specific directives
1383  that might apply to any recipient along the request/response chain. All pragma directives
1384  specify optional behavior from the viewpoint of the protocol; however, some systems
1385  &MAY; require that behavior be consistent with the directives.
1386</t>
1387<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
1388  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1389  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1390  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1391  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1392</artwork></figure>
1393<t>
1394  When the no-cache directive is present in a request message, an application &SHOULD;
1395  forward the request toward the origin server even if it has a cached copy of what is being
1396  requested. This pragma directive has the same semantics as the no-cache response directive
1397  (see <xref target="cache-response-directive" />) and is defined here for backward
1398  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1399  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1400  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1401</t>
1402<x:note>
1403  <t>
1404    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1405    is not actually specified, it does not provide a reliable replacement for
1406    "Cache-Control: no-cache" in a response.
1407  </t>
1408</x:note>
1409<t>
1410  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1411</t>
1412</section>
1413
1414<section anchor="header.vary" title="Vary">
1415  <iref item="Vary header" primary="true" x:for-anchor="" />
1416  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1417  <x:anchor-alias value="Vary"/>
1418  <x:anchor-alias value="Vary-v"/>
1419<t>
1420  The "Vary" response-header field conveys the set of request-header fields
1421  that were used to select the representation.
1422</t>
1423<t>
1424  Caches use this information, in part, to determine whether a stored response
1425  can be used to satisfy a given request; see
1426  <xref target="caching.negotiated.responses" />.
1427  determines, while the response is fresh, whether a cache is permitted to use the
1428  response to reply to a subsequent request without validation; see <xref 
1429  target="caching.negotiated.responses" />.
1430</t>
1431<t>
1432  In uncacheable or stale responses, the Vary field value advises the user agent about
1433  the criteria that were used to select the representation.
1434</t>
1435<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1436  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1437  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1438</artwork></figure>
1439<t>
1440  The set of header fields named by the Vary field value is known as the selecting
1441  request-headers.
1442</t>
1443<t>
1444  Servers &SHOULD; include a Vary header field with any cacheable response that is
1445  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1446  requests on that resource and informs the user agent about the presence of negotiation on
1447  that resource. A server &MAY; include a Vary header field with a non-cacheable
1448  response that is subject to server-driven negotiation, since this might provide the user
1449  agent with useful information about the dimensions over which the response varies at the
1450  time of the response.
1451</t>
1452<t>
1453  A Vary field value of "*" signals that unspecified parameters not limited to the
1454  request-headers (e.g., the network address of the client), play a role in the selection of
1455  the response representation; therefore, a cache cannot determine whether this response is
1456  appropriate. The "*" value &MUST-NOT; be generated by a proxy server.
1457</t>
1458<t>
1459  The field-names given are not limited to the set of standard request-header fields
1460  defined by this specification. Field names are case-insensitive.
1461</t>
1462</section>
1463
1464<section anchor="header.warning" title="Warning">
1465  <iref item="Warning header" primary="true" x:for-anchor="" />
1466  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1467  <x:anchor-alias value="Warning"/>
1468  <x:anchor-alias value="Warning-v"/>
1469  <x:anchor-alias value="warning-value"/>
1470  <x:anchor-alias value="warn-agent"/>
1471  <x:anchor-alias value="warn-code"/>
1472  <x:anchor-alias value="warn-date"/>
1473  <x:anchor-alias value="warn-text"/>
1474<t>
1475  The "Warning" general-header field is used to carry additional information about the status
1476  or transformation of a message that might not be reflected in the message. This
1477  information is typically used to warn about possible incorrectness introduced by caching
1478  operations or transformations applied to the payload of the message.
1479</t>
1480<t>
1481  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1482  warning, rather than an error status code, distinguishes these responses from true failures.
1483</t>
1484<t>
1485  Warning headers can in general be applied to any message, however some warn-codes are
1486  specific to caches and can only be applied to response messages.
1487</t>
1488<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><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"/>
1489  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1490  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1491 
1492  <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>
1493                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1494 
1495  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1496  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1497                  ; the name or pseudonym of the server adding
1498                  ; the Warning header, for use in debugging
1499  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1500  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1501</artwork></figure>
1502<t>
1503  Multiple warnings can be attached to a response (either by the origin server or by
1504  a cache), including multiple warnings with the same code number, only differing
1505  in warn-text.
1506</t>
1507<t>
1508  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1509  possible, in the order that they appear in the response.
1510</t>
1511<t>
1512  Systems that generate multiple Warning headers &SHOULD; order them with this user
1513  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1514  Warning headers.
1515</t>
1516<t>
1517  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1518  Warning is required to be deleted from a stored response after validation:
1519  <list style="symbols">
1520    <t>1xx Warnings describe the freshness or validation status of the response, and so
1521      &MUST; be deleted by caches after validation. They can only be generated by a cache
1522      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1523    <t>2xx Warnings describe some aspect of the representation that is
1524      not rectified by a validation (for example, a lossy compression of the representation)
1525      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1526      returned, in which case they &MUST; be.</t>
1527  </list>
1528</t>
1529<t>
1530  If an implementation sends a message with one or more Warning headers to a receiver whose
1531  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1532  warn-date that matches the Date header in the message.
1533</t>
1534<t>
1535  If an implementation receives a message with a warning-value that includes a warn-date,
1536  and that warn-date is different from the Date value in the response, then that
1537  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1538  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1539  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1540  well.
1541</t>
1542<t>
1543  The following warn-codes are defined by this specification, each with a recommended
1544  warn-text in English, and a description of its meaning.
1545</t>
1546<t><?rfc needLines="4"?>
1547  110 Response is stale
1548  <list>
1549    <t>&SHOULD; be included whenever the returned response is stale.</t>
1550  </list>
1551</t>
1552<t><?rfc needLines="4"?>
1553  111 Revalidation failed
1554  <list>
1555    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1556      validate the response failed, due to an inability to reach the server.</t>
1557  </list>
1558</t>
1559<t><?rfc needLines="4"?>
1560  112 Disconnected operation
1561  <list>
1562    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1563      the network for a period of time.</t>
1564  </list>
1565</t>
1566<t><?rfc needLines="4"?>
1567  113 Heuristic expiration
1568  <list>
1569    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1570      greater than 24 hours and the response's age is greater than 24 hours.</t>
1571  </list>
1572</t>
1573<t><?rfc needLines="4"?>
1574  199 Miscellaneous warning
1575  <list>
1576    <t>The warning text can include arbitrary information to be presented to a human
1577      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1578      action, besides presenting the warning to the user.</t>
1579  </list>
1580</t>
1581<t><?rfc needLines="4"?>
1582  214 Transformation applied
1583  <list>
1584    <t>&MUST; be added by an intermediate proxy if it applies any
1585      transformation to the representation, such as changing the content-coding,
1586      media-type, or modifying the representation data, unless this Warning code
1587      already appears in the response.</t>
1588  </list>
1589</t>
1590<t><?rfc needLines="4"?>
1591  299 Miscellaneous persistent warning
1592  <list>
1593    <t>The warning text can include arbitrary information to be presented to a human
1594      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1595      action.</t>
1596  </list>
1597</t>
1598</section>
1599
1600</section>
1601
1602<section anchor="history.lists" title="History Lists">
1603<t>
1604  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1605  can be used to redisplay a representation retrieved earlier in a session.
1606</t>
1607<t>
1608  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1609  a history mechanism can display a previous representation even if it has expired.
1610</t>
1611  <t>
1612  This does not prohibit the history mechanism from telling the user that a
1613  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1614  </t>
1615</section>
1616
1617
1618<section anchor="IANA.considerations" title="IANA Considerations">
1619
1620<section title="Cache Directive Registry" anchor="cache.directive.registration">
1621<t>
1622  The registration procedure for HTTP Cache Directives is defined by
1623  <xref target="cache.control.extensions"/> of this document.
1624</t>
1625<t>
1626   The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
1627   and be populated with the registrations below:
1628</t>
1629<?BEGININC p6-cache.cache-directives ?>
1630<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1631<texttable xmlns:my="#my" align="left" suppress-title="true"
1632           anchor="iana.cache.directive.registration.table">
1633   <ttcol>Cache Directive</ttcol>
1634   <ttcol>Reference</ttcol>
1635
1636   <c>max-age</c>
1637   <c>
1638      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1639   </c>
1640   <c>max-stale</c>
1641   <c>
1642      <xref target="cache-request-directive"/>
1643   </c>
1644   <c>min-fresh</c>
1645   <c>
1646      <xref target="cache-request-directive"/>
1647   </c>
1648   <c>must-revalidate</c>
1649   <c>
1650      <xref target="cache-response-directive"/>
1651   </c>
1652   <c>no-cache</c>
1653   <c>
1654      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1655   </c>
1656   <c>no-store</c>
1657   <c>
1658      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1659   </c>
1660   <c>no-transform</c>
1661   <c>
1662      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1663   </c>
1664   <c>only-if-cached</c>
1665   <c>
1666      <xref target="cache-request-directive"/>
1667   </c>
1668   <c>private</c>
1669   <c>
1670      <xref target="cache-response-directive"/>
1671   </c>
1672   <c>proxy-revalidate</c>
1673   <c>
1674      <xref target="cache-response-directive"/>
1675   </c>
1676   <c>public</c>
1677   <c>
1678      <xref target="cache-response-directive"/>
1679   </c>
1680   <c>s-maxage</c>
1681   <c>
1682      <xref target="cache-response-directive"/>
1683   </c>
1684   <c>stale-if-error</c>
1685   <c>
1686      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1687   </c>
1688   <c>stale-while-revalidate</c>
1689   <c>
1690      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1691   </c>
1692</texttable>
1693<!--(END)-->
1694<?ENDINC p6-cache.cache-directives ?>
1695</section>
1696
1697<section anchor="message.header.registration" title="Message Header Registration">
1698<t>
1699  The Message Header Registry located at <eref
1700  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1701  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1702</t>
1703<?BEGININC p6-cache.iana-headers ?>
1704<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1705<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1706   <ttcol>Header Field Name</ttcol>
1707   <ttcol>Protocol</ttcol>
1708   <ttcol>Status</ttcol>
1709   <ttcol>Reference</ttcol>
1710
1711   <c>Age</c>
1712   <c>http</c>
1713   <c>standard</c>
1714   <c>
1715      <xref target="header.age"/>
1716   </c>
1717   <c>Cache-Control</c>
1718   <c>http</c>
1719   <c>standard</c>
1720   <c>
1721      <xref target="header.cache-control"/>
1722   </c>
1723   <c>Expires</c>
1724   <c>http</c>
1725   <c>standard</c>
1726   <c>
1727      <xref target="header.expires"/>
1728   </c>
1729   <c>Pragma</c>
1730   <c>http</c>
1731   <c>standard</c>
1732   <c>
1733      <xref target="header.pragma"/>
1734   </c>
1735   <c>Vary</c>
1736   <c>http</c>
1737   <c>standard</c>
1738   <c>
1739      <xref target="header.vary"/>
1740   </c>
1741   <c>Warning</c>
1742   <c>http</c>
1743   <c>standard</c>
1744   <c>
1745      <xref target="header.warning"/>
1746   </c>
1747</texttable>
1748<!--(END)-->
1749<?ENDINC p6-cache.iana-headers ?>
1750<t>
1751  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1752</t>
1753</section>
1754
1755</section>
1756
1757<section anchor="security.considerations" title="Security Considerations">
1758<t>
1759  Caches expose additional potential vulnerabilities, since the contents of the cache
1760  represent an attractive target for malicious exploitation. Because cache contents persist
1761  after an HTTP request is complete, an attack on the cache can reveal information long after
1762  a user believes that the information has been removed from the network. Therefore, cache
1763  contents should be protected as sensitive information.
1764</t>
1765</section>
1766
1767<section anchor="ack" title="Acknowledgments">
1768<t>
1769  Much of the content and presentation of the caching design is due to suggestions and
1770  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1771  and Larry Masinter.
1772</t>
1773</section>
1774
1775</middle>
1776
1777<back>
1778<references title="Normative References">
1779
1780  <reference anchor="Part1">
1781    <front>
1782      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1783      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1784        <organization abbrev="Day Software">Day Software</organization>
1785        <address><email>fielding@gbiv.com</email></address>
1786      </author>
1787      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1788        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1789        <address><email>jg@freedesktop.org</email></address>
1790      </author>
1791      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1792        <organization abbrev="HP">Hewlett-Packard Company</organization>
1793        <address><email>JeffMogul@acm.org</email></address>
1794      </author>
1795      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1796        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1797        <address><email>henrikn@microsoft.com</email></address>
1798      </author>
1799      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1800        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1801        <address><email>LMM@acm.org</email></address>
1802      </author>
1803      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1804        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1805        <address><email>paulle@microsoft.com</email></address>
1806      </author>
1807      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1808        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1809        <address><email>timbl@w3.org</email></address>
1810      </author>
1811      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1812        <organization abbrev="W3C">World Wide Web Consortium</organization>
1813        <address><email>ylafon@w3.org</email></address>
1814      </author>
1815      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1816        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1817        <address><email>julian.reschke@greenbytes.de</email></address>
1818      </author>
1819      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1820    </front>
1821    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1822    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1823  </reference>
1824
1825  <reference anchor="Part2">
1826    <front>
1827      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1828      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1829        <organization abbrev="Day Software">Day Software</organization>
1830        <address><email>fielding@gbiv.com</email></address>
1831      </author>
1832      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1833        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1834        <address><email>jg@freedesktop.org</email></address>
1835      </author>
1836      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1837        <organization abbrev="HP">Hewlett-Packard Company</organization>
1838        <address><email>JeffMogul@acm.org</email></address>
1839      </author>
1840      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1841        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1842        <address><email>henrikn@microsoft.com</email></address>
1843      </author>
1844      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1845        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1846        <address><email>LMM@acm.org</email></address>
1847      </author>
1848      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1849        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1850        <address><email>paulle@microsoft.com</email></address>
1851      </author>
1852      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1853        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1854        <address><email>timbl@w3.org</email></address>
1855      </author>
1856      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1857        <organization abbrev="W3C">World Wide Web Consortium</organization>
1858        <address><email>ylafon@w3.org</email></address>
1859      </author>
1860      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1861        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1862        <address><email>julian.reschke@greenbytes.de</email></address>
1863      </author>
1864      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1865    </front>
1866    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1867    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1868  </reference>
1869
1870  <reference anchor="Part4">
1871    <front>
1872      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1873      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1874        <organization abbrev="Day Software">Day Software</organization>
1875        <address><email>fielding@gbiv.com</email></address>
1876      </author>
1877      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1878        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1879        <address><email>jg@freedesktop.org</email></address>
1880      </author>
1881      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1882        <organization abbrev="HP">Hewlett-Packard Company</organization>
1883        <address><email>JeffMogul@acm.org</email></address>
1884      </author>
1885      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1886        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1887        <address><email>henrikn@microsoft.com</email></address>
1888      </author>
1889      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1890        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1891        <address><email>LMM@acm.org</email></address>
1892      </author>
1893      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1894        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1895        <address><email>paulle@microsoft.com</email></address>
1896      </author>
1897      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1898        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1899        <address><email>timbl@w3.org</email></address>
1900      </author>
1901      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1902        <organization abbrev="W3C">World Wide Web Consortium</organization>
1903        <address><email>ylafon@w3.org</email></address>
1904      </author>
1905      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1906        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1907        <address><email>julian.reschke@greenbytes.de</email></address>
1908      </author>
1909      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1910    </front>
1911    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1912    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1913  </reference>
1914
1915  <reference anchor="Part5">
1916    <front>
1917      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1918      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1919        <organization abbrev="Day Software">Day Software</organization>
1920        <address><email>fielding@gbiv.com</email></address>
1921      </author>
1922      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1923        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1924        <address><email>jg@freedesktop.org</email></address>
1925      </author>
1926      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1927        <organization abbrev="HP">Hewlett-Packard Company</organization>
1928        <address><email>JeffMogul@acm.org</email></address>
1929      </author>
1930      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1931        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1932        <address><email>henrikn@microsoft.com</email></address>
1933      </author>
1934      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1935        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1936        <address><email>LMM@acm.org</email></address>
1937      </author>
1938      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1939        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1940        <address><email>paulle@microsoft.com</email></address>
1941      </author>
1942      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1943        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1944        <address><email>timbl@w3.org</email></address>
1945      </author>
1946      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1947        <organization abbrev="W3C">World Wide Web Consortium</organization>
1948        <address><email>ylafon@w3.org</email></address>
1949      </author>
1950      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1951        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1952        <address><email>julian.reschke@greenbytes.de</email></address>
1953      </author>
1954      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1955    </front>
1956    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1957    <x:source basename="p5-range" href="p5-range.xml" />
1958  </reference>
1959
1960  <reference anchor="Part7">
1961    <front>
1962      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1963      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1964        <organization abbrev="Day Software">Day Software</organization>
1965        <address><email>fielding@gbiv.com</email></address>
1966      </author>
1967      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1968        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1969        <address><email>jg@freedesktop.org</email></address>
1970      </author>
1971      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1972        <organization abbrev="HP">Hewlett-Packard Company</organization>
1973        <address><email>JeffMogul@acm.org</email></address>
1974      </author>
1975      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1976        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1977        <address><email>henrikn@microsoft.com</email></address>
1978      </author>
1979      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1980        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1981        <address><email>LMM@acm.org</email></address>
1982      </author>
1983      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1984        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1985        <address><email>paulle@microsoft.com</email></address>
1986      </author>
1987      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1988        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1989        <address><email>timbl@w3.org</email></address>
1990      </author>
1991      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1992        <organization abbrev="W3C">World Wide Web Consortium</organization>
1993        <address><email>ylafon@w3.org</email></address>
1994      </author>
1995      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1996        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1997        <address><email>julian.reschke@greenbytes.de</email></address>
1998      </author>
1999      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2000    </front>
2001    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2002    <x:source basename="p7-auth" href="p7-auth.xml" />
2003  </reference>
2004
2005  <reference anchor="RFC2119">
2006    <front>
2007      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2008      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2009        <organization>Harvard University</organization>
2010        <address><email>sob@harvard.edu</email></address>
2011      </author>
2012      <date month="March" year="1997" />
2013    </front>
2014    <seriesInfo name="BCP" value="14" />
2015    <seriesInfo name="RFC" value="2119" />
2016  </reference>
2017
2018  <reference anchor="RFC5234">
2019    <front>
2020      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2021      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2022        <organization>Brandenburg InternetWorking</organization>
2023        <address>
2024          <email>dcrocker@bbiw.net</email>
2025        </address> 
2026      </author>
2027      <author initials="P." surname="Overell" fullname="Paul Overell">
2028        <organization>THUS plc.</organization>
2029        <address>
2030          <email>paul.overell@thus.net</email>
2031        </address>
2032      </author>
2033      <date month="January" year="2008"/>
2034    </front>
2035    <seriesInfo name="STD" value="68"/>
2036    <seriesInfo name="RFC" value="5234"/>
2037  </reference>
2038 
2039</references>
2040
2041<references title="Informative References">
2042
2043  <reference anchor="RFC1305">
2044    <front>
2045      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2046      <author fullname="David L. Mills" initials="D." surname="Mills">
2047        <organization>University of Delaware, Electrical Engineering Department</organization>
2048        <address><email>mills@udel.edu</email></address>
2049      </author>
2050      <date month="March" year="1992" />
2051    </front>
2052    <seriesInfo name="RFC" value="1305" />
2053  </reference>
2054
2055  <reference anchor="RFC2616">
2056    <front>
2057      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2058      <author fullname="R. Fielding" initials="R." surname="Fielding">
2059        <organization>University of California, Irvine</organization>
2060        <address><email>fielding@ics.uci.edu</email></address>
2061      </author>
2062      <author fullname="J. Gettys" initials="J." surname="Gettys">
2063        <organization>W3C</organization>
2064        <address><email>jg@w3.org</email></address>
2065      </author>
2066      <author fullname="J. Mogul" initials="J." surname="Mogul">
2067        <organization>Compaq Computer Corporation</organization>
2068        <address><email>mogul@wrl.dec.com</email></address>
2069      </author>
2070      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2071        <organization>MIT Laboratory for Computer Science</organization>
2072        <address><email>frystyk@w3.org</email></address>
2073      </author>
2074      <author fullname="L. Masinter" initials="L." surname="Masinter">
2075        <organization>Xerox Corporation</organization>
2076        <address><email>masinter@parc.xerox.com</email></address>
2077      </author>
2078      <author fullname="P. Leach" initials="P." surname="Leach">
2079        <organization>Microsoft Corporation</organization>
2080        <address><email>paulle@microsoft.com</email></address>
2081      </author>
2082      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2083        <organization>W3C</organization>
2084        <address><email>timbl@w3.org</email></address>
2085      </author>
2086      <date month="June" year="1999" />
2087    </front>
2088    <seriesInfo name="RFC" value="2616" />
2089  </reference>
2090
2091  <reference anchor="RFC3864">
2092    <front>
2093      <title>Registration Procedures for Message Header Fields</title>
2094      <author fullname="G. Klyne" initials="G." surname="Klyne">
2095        <organization>Nine by Nine</organization>
2096        <address><email>GK-IETF@ninebynine.org</email></address>
2097      </author>
2098      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2099        <organization>BEA Systems</organization>
2100        <address><email>mnot@pobox.com</email></address>
2101      </author>
2102      <author fullname="J. Mogul" initials="J." surname="Mogul">
2103        <organization>HP Labs</organization>
2104        <address><email>JeffMogul@acm.org</email></address>
2105      </author>
2106      <date month="September" year="2004" />
2107    </front>
2108    <seriesInfo name="BCP" value="90" />
2109    <seriesInfo name="RFC" value="3864" />
2110  </reference>
2111
2112  <reference anchor='RFC5226'>
2113    <front>
2114      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2115      <author initials='T.' surname='Narten' fullname='T. Narten'>
2116        <organization>IBM</organization>
2117        <address><email>narten@us.ibm.com</email></address>
2118      </author>
2119      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2120        <organization>Google</organization>
2121        <address><email>Harald@Alvestrand.no</email></address>
2122      </author>
2123      <date year='2008' month='May' />
2124    </front>
2125    <seriesInfo name='BCP' value='26' />
2126    <seriesInfo name='RFC' value='5226' />
2127  </reference>
2128
2129  <reference anchor='RFC5861'>
2130    <front>
2131      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2132      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2133        <organization>Yahoo! Inc.</organization>
2134        <address><email>mnot@yahoo-inc.com</email></address>
2135      </author>
2136      <date month="April" year="2010"/>
2137    </front>
2138    <seriesInfo name='RFC' value='5861' />
2139  </reference>
2140
2141</references>
2142
2143<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2144<t>
2145  Make the specified age calculation algorithm less conservative.
2146  (<xref target="age.calculations"/>)
2147</t>
2148<t>
2149  Remove requirement to consider Content-Location in successful responses
2150  in order to determine the appropriate response to use.
2151  (<xref target="validation.model" />)
2152</t>
2153<t>
2154  Clarify denial of service attack avoidance requirement.
2155  (<xref target="invalidation.after.updates.or.deletions" />)
2156</t>
2157<t>
2158  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2159  anymore, as these aspects never were implemented.
2160  (<xref target="header.warning" />)
2161</t>
2162</section>
2163
2164<?BEGININC p6-cache.abnf-appendix ?>
2165<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2166<figure>
2167<artwork type="abnf" name="p6-cache.parsed-abnf">
2168<x:ref>Age</x:ref> = "Age:" OWS Age-v
2169<x:ref>Age-v</x:ref> = delta-seconds
2170
2171<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2172<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2173 cache-directive ] )
2174
2175<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2176<x:ref>Expires-v</x:ref> = HTTP-date
2177
2178<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2179
2180<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2181
2182<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2183<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2184 pragma-directive ] )
2185
2186<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2187<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2188 ] ) )
2189
2190<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2191<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2192 ] )
2193
2194<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2195<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2196<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2197 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2198 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2199 cache-extension
2200<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2201 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2202 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2203 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2204 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2205 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2206
2207<x:ref>delta-seconds</x:ref> = 1*DIGIT
2208
2209<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2210
2211<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2212
2213<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2214<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2215<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2216
2217<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2218
2219<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2220
2221<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2222
2223<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2224<x:ref>warn-code</x:ref> = 3DIGIT
2225<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2226<x:ref>warn-text</x:ref> = quoted-string
2227<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2228 ]
2229</artwork>
2230</figure>
2231<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2232; Age defined but not used
2233; Cache-Control defined but not used
2234; Expires defined but not used
2235; Pragma defined but not used
2236; Vary defined but not used
2237; Warning defined but not used
2238</artwork></figure></section>
2239<?ENDINC p6-cache.abnf-appendix ?>
2240
2241<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2242
2243<section title="Since RFC2616">
2244  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2245</section>
2246
2247<section title="Since draft-ietf-httpbis-p6-cache-00">
2248<t>
2249  Closed issues:
2250  <list style="symbols">
2251    <t>
2252      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2253    <t>
2254      <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>
2255    <t>
2256      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2257    <t>
2258      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2259    <t>
2260      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2261    <t>
2262      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2263    <t>
2264      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2265    <t>
2266      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2267    <t>
2268      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2269  </list>
2270</t>
2271<t>
2272  Other changes:
2273  <list style="symbols">
2274    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2275        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2276  </list>
2277</t>
2278</section>
2279
2280<section title="Since draft-ietf-httpbis-p6-cache-01">
2281<t>
2282  Closed issues:
2283  <list style="symbols">
2284    <t>
2285      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2286  </list>
2287</t>
2288<t>
2289  Other changes:
2290  <list style="symbols">
2291    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2292      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2293    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2294      specification.</t>
2295  </list>
2296</t>
2297</section>
2298
2299<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2300<t>
2301  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2302  <list style="symbols">
2303    <t>Reference RFC 3984, and update header registrations for headers defined in this
2304      document.</t>
2305  </list>
2306</t>
2307</section>
2308
2309<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2310<t>
2311  Closed issues:
2312  <list style="symbols">
2313    <t>
2314      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2315  </list>
2316</t>
2317</section>
2318
2319<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2320<t>
2321  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2322  <list style="symbols"> 
2323    <t>
2324      Use "/" instead of "|" for alternatives.
2325    </t>
2326    <t>
2327      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2328      whitespace ("OWS") and required whitespace ("RWS").
2329    </t>
2330    <t>
2331      Rewrite ABNFs to spell out whitespace rules, factor out
2332      header value format definitions.
2333    </t>
2334  </list>
2335</t>
2336</section>
2337
2338<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2339<t>
2340  This is a total rewrite of this part of the specification.
2341</t>
2342<t>
2343  Affected issues:
2344  <list style="symbols">
2345    <t>
2346      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2347    <t>
2348      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2349    <t>
2350      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2351    <t>
2352      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2353  </list>
2354</t>
2355<t>
2356  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2357  <list style="symbols"> 
2358    <t>
2359      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2360    </t>
2361  </list>
2362</t>
2363</section>
2364
2365<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2366<t>
2367  Closed issues:
2368  <list style="symbols"> 
2369    <t>
2370      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2371      "base for numeric protocol elements"
2372    </t>
2373  </list>
2374</t>
2375<t>
2376  Affected issues:
2377  <list style="symbols">
2378    <t>
2379      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2380      Vary and non-existant headers
2381    </t>
2382  </list>
2383</t>
2384</section>
2385
2386<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2387<t>
2388  Closed issues:
2389  <list style="symbols"> 
2390    <t>
2391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2392      "Definition of 1xx Warn-Codes"
2393    </t>
2394    <t>
2395      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2396      "Content-Location on 304 responses"
2397    </t>
2398    <t>
2399      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2400      "private and no-cache CC directives with headers"
2401    </t>
2402    <t>
2403      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2404      "RFC2047 and warn-text"
2405    </t>
2406  </list>
2407</t>
2408</section>
2409
2410<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2411<t>
2412  Closed issues:
2413  <list style="symbols"> 
2414    <t>
2415      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2416      "serving negotiated responses from cache: header-specific canonicalization"
2417    </t>
2418    <t>
2419      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2420      "Effect of CC directives on history lists"
2421    </t>
2422  </list>
2423</t>
2424<t>
2425  Affected issues:
2426  <list style="symbols">
2427    <t>
2428      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2429      Status codes and caching
2430    </t>
2431  </list>
2432</t>
2433<t>
2434  Partly resolved issues:
2435  <list style="symbols"> 
2436    <t>
2437      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2438      "Placement of 13.5.1 and 13.5.2"
2439    </t>
2440  </list>
2441</t>
2442</section>
2443
2444<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2445<t>
2446  Closed issues:
2447  <list style="symbols"> 
2448    <t>
2449      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2450      "Age calculation"
2451    </t>
2452    <t>
2453      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2454      "Clarify differences between / requirements for request and response CC directives"
2455    </t>
2456        <t>
2457                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2458                "Caching authenticated responses"
2459        </t>
2460    <t>
2461      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2462      "IANA registry for cache-control directives"
2463    </t>
2464    <t>
2465      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2466      "Heuristic caching of URLs with query components"
2467    </t>
2468  </list>
2469</t>
2470<t>
2471  Partly resolved issues:
2472  <list style="symbols"> 
2473    <t>
2474      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2475      "Term for the requested resource's URI"
2476    </t>
2477  </list>
2478</t>
2479</section>
2480
2481<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2482<t>
2483  Closed issues:
2484  <list style="symbols"> 
2485    <t>
2486      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2487      "consider removing the 'changes from 2068' sections"
2488    </t>
2489  </list>
2490</t>
2491</section>
2492
2493</section>
2494  </back>
2495</rfc>
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