source: draft-ietf-httpbis/latest/p6-cache.xml @ 935

Last change on this file since 935 was 935, checked in by mnot@…, 9 years ago

Disambiguate when Age header is required.

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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 without validation, 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</t>
505<t>
506  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
507  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
508  corresponding response.
509</t>
510<t>
511  Also, note that unsafe requests might invalidate already stored responses; see
512  <xref target="invalidation.after.updates.or.deletions" />.
513</t>
514<t>
515  Caches &MUST; use the most recent response (as determined by the Date header) when
516  more than one suitable response is stored. They can also forward a request with
517  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
518  use.
519</t>
520</section>
521
522<section anchor="expiration.model" title="Freshness Model">
523<t>
524  When a response is "fresh" in the cache, it can be used to satisfy subsequent
525  requests without contacting the origin server, thereby improving efficiency.
526</t>
527<t>
528  The primary mechanism for determining freshness is for an origin server to provide an
529  explicit expiration time in the future, using either the Expires header (<xref
530  target="header.expires" />) or the max-age response cache directive (<xref
531  target="cache-response-directive" />). Generally, origin servers will assign future
532  explicit expiration times to responses in the belief that the representation is not likely to
533  change in a semantically significant way before the expiration time is reached.
534</t>
535<t>
536  If an origin server wishes to force a cache to validate every request, it can
537  assign an explicit expiration time in the past. This means that the response is always
538  stale, so that caches should validate it before using it for subsequent requests.
539  <cref anchor="TODO-response-stale">This wording might cause confusion, because the response might still be served stale.</cref>
540</t>
541<t>
542  Since origin servers do not always provide explicit expiration times, HTTP caches &MAY;
543  assign heuristic expiration times when explicit times are not specified, employing algorithms that
544  use other header values (such as the Last-Modified time) to estimate a plausible
545  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
546  impose worst-case constraints on their results.
547</t>
548<figure>
549<preamble>
550  The calculation to determine if a response is fresh is:
551</preamble>
552<artwork type="code">
553   response_is_fresh = (freshness_lifetime &gt; current_age)
554</artwork>
555</figure>
556<t>
557  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
558  the current_age is defined in <xref target="age.calculations" />.
559</t>
560<t>
561  Additionally, clients might need to influence freshness calculation. They can do this using
562  several request cache directives, with the effect of either increasing or loosening
563  constraints on freshness. See <xref target="cache-request-directive" />.
564</t>
565<t>
566  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
567  freshness.</cref>
568</t>
569<t>
570  Note that freshness applies only to cache operation; it cannot be used to force a user agent
571  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
572  the difference between caches and history mechanisms.
573</t>
574
575<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
576<t>
577  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
578  response by using the first match of:
579  <list style="symbols">
580    <t>If the cache is shared and the s-maxage response cache directive (<xref
581      target="cache-response-directive" />) is present, use its value, or</t>
582    <t>If the max-age response cache directive (<xref target="cache-response-directive"
583      />) is present, use its value, or</t>
584    <t>If the Expires response header (<xref target="header.expires" />) is present, use
585      its value minus the value of the Date response header, or</t>
586    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
587      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
588  </list>
589</t>
590<t>
591  Note that this calculation is not vulnerable to clock skew, since all of the
592  information comes from the origin server.
593</t>
594
595<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
596<t>
597  If no explicit expiration time is present in a stored response that has a status code
598  whose definition allows heuristic freshness to be used (including the following
599  in &status-codes;: 200, 203, 206, 300, 301 and 410), a heuristic expiration time &MAY; be
600  calculated. Heuristics &MUST-NOT; be used for response status codes that do not explicitly
601  allow it.
602</t>
603<t>
604  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
605  attach a Warning header with a 113 warn-code to the response if its current_age is
606  more than 24 hours and such a warning is not already present.
607</t>
608<t>
609  Also, if the response has a Last-Modified header (&header-last-modified;), the
610  heuristic expiration value &SHOULD; be no more than some fraction of the interval
611  since that time. A typical setting of this fraction might be 10%.
612</t>
613<x:note>
614  <t>
615    <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt="," x:sec="13.9"/>)
616    required that caches do not calculate heuristic freshness for URLs with
617    query components (i.e., those containing '?'). In practice, this has not
618    been widely implemented. Therefore, servers are encouraged to send explicit
619    directives (e.g., Cache-Control: no-cache) if they wish to preclude
620    caching.
621  </t>
622</x:note>
623</section>
624</section>
625
626<section anchor="age.calculations" title="Calculating Age">
627<t>
628  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
629  message when obtained from a cache. The Age field value is the cache's estimate of the
630  amount of time since the response was generated or validated by the origin server. In
631  essence, the Age value is the sum of the time that the response has been resident in
632  each of the caches along the path from the origin server, plus the amount of time it has
633  been in transit along network paths.
634</t>
635<t>
636  The following data is used for the age calculation:
637</t>
638<t>
639  <x:dfn>age_value</x:dfn>
640  <list>
641    <t>
642      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
643      in a form appropriate for arithmetic operation; or 0, if not available.
644    </t>
645  </list>
646</t>
647<t>
648  <x:dfn>date_value</x:dfn>
649  <list>
650    <t>
651      HTTP/1.1 requires origin servers to send a Date header, if possible,
652      with every response, giving the time at which the response was generated.
653      The term "date_value" denotes the value of the Date header, in a form
654      appropriate for arithmetic operations. See &header-date; for the definition
655      of the Date header, and for requirements regarding responses without a
656      Date response header.
657    </t>
658  </list>
659</t>
660<t>
661  <x:dfn>now</x:dfn>
662  <list>
663    <t>
664      The term "now" means "the current value of the clock at the host
665      performing the calculation". Hosts that use HTTP, but especially hosts
666      running origin servers and caches, &SHOULD; use NTP
667      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
668      clocks to a globally accurate time standard.
669    </t>
670  </list>
671</t>
672<t>
673  <x:dfn>request_time</x:dfn>
674  <list>
675    <t>
676      The current value of the clock at the host at the time the request
677      resulting in the stored response was made.
678    </t>
679  </list>
680</t>
681<t>
682  <x:dfn>response_time</x:dfn>
683  <list>
684    <t>
685      The current value of the clock at the host at the time the response was
686      received.
687    </t>
688  </list>
689</t>
690<t>
691  A response's age can be calculated in two entirely independent ways:
692  <list style="numbers">
693    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
694      origin server's clock. If the result is negative, the result is replaced by zero.</t>
695    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
696      note this value &MUST; be interpreted relative to the time the
697      request was initiated, not the time that the response was received.</t>
698  </list>
699</t>
700<figure>
701<artwork type="code">
702  apparent_age = max(0, response_time - date_value);
703
704  response_delay = response_time - request_time;
705  corrected_age_value = age_value + response_delay; 
706</artwork>
707</figure>
708<figure>
709<preamble>These are combined as</preamble>
710<artwork type="code">
711  corrected_initial_age = max(apparent_age, corrected_age_value);
712</artwork></figure>
713<t>
714  The current_age of a stored response can then be calculated by adding the amount of
715  time (in seconds) since the stored response was last validated by the origin server to
716  the corrected_initial_age.
717</t>
718<figure><artwork type="code">
719  resident_time = now - response_time;
720  current_age = corrected_initial_age + resident_time;
721</artwork></figure>
722</section>
723
724<section anchor="serving.stale.responses" title="Serving Stale Responses">
725<t>
726  A "stale" response is one that either has explicit expiry information or is allowed to
727  have heuristic expiry calculated, but is not fresh according to the calculations in
728  <xref target="expiration.model" />.
729</t>
730<t>
731  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
732  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
733  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
734  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
735</t>
736<t>
737  Caches &SHOULD-NOT; return stale responses unless they are
738  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
739  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
740</t>
741<t>
742  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
743  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
744  the cache is disconnected.
745</t>
746<t>
747  If a cache receives a first-hand response (either an entire response, or a 304 (Not
748  Modified) response) that it would normally forward to the requesting client, and the
749  received response is no longer fresh, the cache &SHOULD; forward it to the
750  requesting client without adding a new Warning (but without removing any existing
751  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
752  that response became stale in transit.
753</t>
754</section>
755</section>
756
757<section anchor="validation.model" title="Validation Model">
758<t>
759  When a cache has one or more stored responses for a requested URI, but cannot 
760  serve any of them (e.g., because they are not fresh, or one cannot be selected;
761  see <xref target="caching.negotiated.responses"/>),
762  it can use the conditional request mechanism &conditional; in the forwarded
763  request to give the origin server an opportunity to both select a valid stored
764  response to be used, and to update it. This process is known as "validating"
765  or "revalidating" the stored response.
766</t>
767<t>
768  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
769  header whose value is that of the Last-Modified header from the selected
770  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
771</t>
772<t>
773  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
774  is that of the ETag header(s) from all responses stored for the requested URI,
775  if present. However, if any of the stored responses contains only partial
776  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
777  field unless the request is for a range that would be fully satisfied by
778  that stored response.
779</t>
780<t>
781  A 304 (Not Modified) response status code indicates that the stored
782  response can be updated and reused; see <xref target="combining.headers"/>.
783</t>
784<t>
785  A full response (i.e., one with a response body) indicates that none 
786  of the stored responses nominated in the conditional request is
787  suitable. Instead, the full response &SHOULD; be used to satisfy the
788  request and &MAY; replace the stored response.
789</t>
790<t>
791  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
792  either forward this response to the requesting client, or act as if the server failed to
793  respond. In the latter case, it &MAY; return a previously stored response (see <xref
794  target="serving.stale.responses" />).
795</t>
796</section>
797
798<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
799<t>
800  Because unsafe methods (&safe-methods;) have the potential for changing state on the
801  origin server, intervening caches can use them to keep their contents
802  up-to-date.
803</t>
804<t>
805  The following HTTP methods &MUST; cause a cache to invalidate the Effective Request URI (&effective-request-uri;) as well
806  as the URI(s) in the Location and Content-Location headers (if present):
807  <list style="symbols">
808    <t>PUT</t>
809    <t>DELETE</t>
810    <t>POST</t>
811  </list>
812</t>
813<t>
814  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
815  be performed if the host part of that URI differs from the host part in the Effective Request URI (&effective-request-uri;).
816  This helps prevent denial of service attacks.
817</t>
818<t>
819  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
820</t>
821<t>
822  A cache that passes through requests for methods it does not understand &SHOULD;
823  invalidate the Effective Request URI (&effective-request-uri;).
824</t>
825<t>
826  Here, "invalidate" means that the cache will either remove all stored responses related
827  to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
828  before they can be returned in response to a subsequent request.
829</t>
830<t>
831  Note that this does not guarantee that all appropriate responses are invalidated. For
832  example, the request that caused the change at the origin server might not have gone
833  through the cache where a response is stored.
834</t>
835<t>
836  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
837</t>
838</section>
839
840<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
841
842<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>
843
844<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>
845
846<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>
847</section>
848
849<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
850<t>
851  When a cache receives a request that can be satisfied by a stored response
852  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
853  response unless all of the selecting request-headers nominated by the Vary header match
854  in both the original request (i.e., that associated with the stored response),
855  and the presented request.
856</t>
857<t>
858  The selecting request-headers from two requests are defined to match
859  if and only if those in the first request can be transformed to those in the
860  second request by applying any of the following:
861  <list style="symbols">
862    <t>
863      adding or removing whitespace, where allowed in the header's syntax
864    </t>
865    <t>
866      combining multiple message-header fields with the same field name (see
867      &header-fields;)
868    </t>
869    <t>
870      normalizing both header values in a way that is known to have identical
871      semantics, according to the header's specification (e.g., re-ordering field values
872      when order is not significant; case-normalization, where values are defined to be
873      case-insensitive)   
874    </t>
875  </list>
876</t>
877<t>
878  If (after any normalization that might take place) a header field is absent
879  from a request, it can only match another request if it is also absent there.
880</t>
881<t>
882  A Vary header field-value of "*" always fails to match, and subsequent requests to that
883  resource can only be properly interpreted by the origin server.
884</t>
885<t>
886  The stored response with matching selecting request-headers is known as the
887  selected response.
888</t>
889<t>
890  If no selected response is available, the cache &MAY; forward the presented
891  request to the origin server in a conditional request; see <xref target="validation.model"/>.
892</t>
893</section>
894
895<section anchor="combining.headers" title="Combining Responses">
896<t>
897  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
898  (in this section, the "new" response"), it needs to created an updated response by combining
899  the stored response with the new one, so that the updated response can be used to satisfy
900  the request, and potentially update the cached response.
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-select-for-combine">Shouldn't this be the selected response?</cref>
906</t>
907<t>
908  If the new response's 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.</t>
918    <t>any stored Warning headers with warn-code 2xx &MUST; be retained.</t>
919    <t>any other headers provided in the new response &MUST; replace all instances of
920          the corresponding headers from the stored response.</t>
921  </list>
922</t>
923<t>
924  The updated response headers &MUST; be used to replace those of the 
925  stored response in cache (unless the stored response is removed from cache). In the
926  case of a 206 response, the combined representation &MAY; be stored.
927</t>
928</section>
929
930</section>
931
932<section anchor="header.fields" title="Header Field Definitions">
933<t>
934  This section defines the syntax and semantics of HTTP/1.1 header fields
935  related to caching.
936</t>
937<t>
938  For entity-header fields, both sender and recipient refer to either the client or the
939  server, depending on who sends and who receives the message.
940</t>
941
942<section anchor="header.age" title="Age">
943  <iref item="Age header" primary="true" x:for-anchor="" />
944  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
945  <x:anchor-alias value="Age"/>
946  <x:anchor-alias value="Age-v"/>
947  <x:anchor-alias value="age-value"/>
948<t>
949  The "Age" response-header field conveys the sender's estimate of the amount
950  of time since the response was generated or successfully validated at the
951  origin server. Age values are calculated as specified in
952  <xref target="age.calculations" />.
953</t>
954<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
955  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
956  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
957</artwork></figure>
958<t anchor="rule.delta-seconds">
959  <x:anchor-alias value="delta-seconds" />
960  Age field-values are non-negative integers, representing time in seconds.
961</t>
962<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
963  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
964</artwork></figure>
965<t>
966  If a cache receives a value larger than the largest positive integer it can represent, or
967  if any of its age calculations overflows, it &MUST; transmit an Age header with a
968  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
969  of at least 31 bits of range.
970</t>
971<t>
972  The presence of an Age header field in a response implies that a response is not
973  first-hand. However, the converse is not true, since HTTP/1.0 caches might not implement the
974  Age header field.
975</t>
976</section>
977
978<section anchor="header.cache-control" title="Cache-Control">
979  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
980  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
981  <x:anchor-alias value="Cache-Control"/>
982  <x:anchor-alias value="Cache-Control-v"/>
983  <x:anchor-alias value="cache-directive"/>
984  <x:anchor-alias value="cache-extension"/>
985  <x:anchor-alias value="cache-request-directive"/>
986  <x:anchor-alias value="cache-response-directive"/>
987<t>
988  The "Cache-Control" general-header field is used to specify directives for
989  caches along the request/response chain. Such cache directives are
990  unidirectional in that the presence of a directive in a request does not
991  imply that the same directive is to be given in the response.
992</t>
993<t>
994  HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives
995  defined in this section. See <xref target="cache.control.extensions"/> for
996  information about how Cache-Control directives defined elsewhere are handled.
997</t>
998<x:note>
999  <t>
1000    <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1001    might only implement Pragma: no-cache (see <xref target="header.pragma" />).
1002  </t>
1003</x:note>
1004<t>
1005  Cache directives &MUST; be passed through by a proxy or gateway application,
1006  regardless of their significance to that application, since the directives might be
1007  applicable to all recipients along the request/response chain. It is not possible to
1008  target a directive to a specific cache.
1009</t>
1010<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"/>
1011  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1012  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1013
1014  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1015     / <x:ref>cache-response-directive</x:ref>
1016
1017  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1018</artwork></figure>
1019
1020<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1021  <x:anchor-alias value="cache-request-directive" />
1022
1023<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1024  <x:ref>cache-request-directive</x:ref> =
1025       "no-cache"
1026     / "no-store"
1027     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1028     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1029     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1030     / "no-transform"
1031     / "only-if-cached"
1032     / <x:ref>cache-extension</x:ref>
1033</artwork></figure>
1034
1035<t>
1036  <x:dfn>no-cache</x:dfn>
1037  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1038  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1039  <list>
1040    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1041      used to satisfy the request without successful validation on the origin server.</t>
1042  </list>
1043</t>
1044<t>
1045  <x:dfn>no-store</x:dfn>
1046  <iref item="Cache Directives" primary="true" subitem="no-store" />
1047  <iref item="no-store" primary="true" subitem="Cache Directive" />
1048  <list>
1049    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1050      of either this request or any response to it. This directive applies to both
1051      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1052      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1053      and &MUST; make a best-effort attempt to remove the information from volatile
1054      storage as promptly as possible after forwarding it.</t>
1055    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1056      particular, malicious or compromised caches might not recognize or obey this
1057      directive, and communications networks might be vulnerable to eavesdropping.</t>
1058  </list>
1059</t>
1060<t>
1061  <x:dfn>max-age</x:dfn>
1062  <iref item="Cache Directives" primary="true" subitem="max-age" />
1063  <iref item="max-age" primary="true" subitem="Cache Directive" />
1064  <list>
1065    <t>The max-age request directive indicates that the client is willing to accept a
1066      response whose age is no greater than the specified time in seconds. Unless
1067      the max-stale request directive is also present, the client is not willing to accept a stale
1068      response.</t>
1069  </list>
1070</t>
1071<t>
1072  <x:dfn>max-stale</x:dfn>
1073  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1074  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1075  <list>
1076    <t>The max-stale request directive indicates that the client is willing to accept a
1077      response that has exceeded its expiration time. If max-stale is assigned a value,
1078      then the client is willing to accept a response that has exceeded its expiration
1079      time by no more than the specified number of seconds. If no value is assigned to
1080      max-stale, then the client is willing to accept a stale response of any age.</t>
1081  </list>
1082</t>
1083<t>
1084  <x:dfn>min-fresh</x:dfn>
1085  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1086  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1087  <list>
1088    <t>The min-fresh request directive indicates that the client is willing to accept a
1089      response whose freshness lifetime is no less than its current age plus the specified
1090      time in seconds. That is, the client wants a response that will still be fresh for
1091      at least the specified number of seconds.</t>
1092  </list>
1093</t>
1094<t>
1095  <x:dfn>no-transform</x:dfn>
1096  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1097  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1098  <list>
1099    <t>The no-transform request directive indicates that an intermediate cache or proxy
1100      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1101      headers, nor the request representation.</t>
1102  </list>
1103</t>
1104<t>
1105  <x:dfn>only-if-cached</x:dfn>
1106  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1107  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1108  <list>
1109    <t>The only-if-cached request directive indicates that the client only wishes to
1110      return a stored response. If it receives this directive, a cache &SHOULD; either
1111      respond using a stored response that is consistent with the other constraints of the
1112      request, or respond with a 504 (Gateway Timeout) status code. If a group of caches is
1113      being operated as a unified system with good internal connectivity, such a request
1114      &MAY; be forwarded within that group of caches.</t>
1115  </list>
1116</t>
1117</section>
1118
1119<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1120  <x:anchor-alias value="cache-response-directive" />
1121
1122<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1123  <x:ref>cache-response-directive</x:ref> =
1124       "public"
1125     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1126     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1127     / "no-store"
1128     / "no-transform"
1129     / "must-revalidate"
1130     / "proxy-revalidate"
1131     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1132     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1133     / <x:ref>cache-extension</x:ref>
1134</artwork></figure>
1135
1136<t>
1137  <x:dfn>public</x:dfn>
1138  <iref item="Cache Directives" primary="true" subitem="public" />
1139  <iref item="public" primary="true" subitem="Cache Directive" />
1140  <list>
1141    <t>The public response directive indicates that the response &MAY; be cached, even
1142      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1143      (See also Authorization, &header-authorization;, for additional details.) </t>
1144  </list>
1145</t>
1146<t>
1147  <x:dfn>private</x:dfn>
1148  <iref item="Cache Directives" primary="true" subitem="private" />
1149  <iref item="private" primary="true" subitem="Cache Directive" />
1150  <list>
1151    <t>The private response directive indicates that the response message is intended for
1152      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1153      cache &MAY; store the response.</t>
1154    <t>If the private response directive specifies one or more field-names, this
1155      requirement is limited to the field-values associated with the listed response
1156      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1157      cache, whereas the remainder of the response message &MAY; be.</t>
1158    <t>
1159      <x:h>Note:</x:h> This usage of the word private only controls where the response can
1160      be stored; it cannot ensure the privacy of the message content.
1161      Also, private response directives with field-names are often handled by
1162      implementations as if an unqualified private directive was received; i.e.,
1163      the special handling for the qualified form is not widely implemented.</t>
1164  </list>
1165</t>
1166<t>
1167  <x:dfn>no-cache</x:dfn>
1168  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1169  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1170  <list>
1171    <t>The no-cache response directive indicates that the response MUST NOT be used
1172          to satisfy a subsequent request without successful validation on the origin
1173          server. This allows an origin server to prevent a cache from using it to satisfy
1174          a request without contacting it, even by caches that have been configured to
1175          return stale responses.</t>
1176    <t>If the no-cache response directive specifies one or more field-names, this
1177      requirement is limited to the field-values associated with the listed response
1178      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1179      to a subsequent request without successful validation on the origin server. This
1180      allows an origin server to prevent the re-use of certain header fields in a
1181      response, while still allowing caching of the rest of the response.</t>
1182    <t>
1183      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1184      Also, no-cache response directives with field-names are often handled by
1185      implementations as if an unqualified no-cache directive was received; i.e.,
1186      the special handling for the qualified form is not widely implemented.
1187    </t>
1188  </list>
1189</t>
1190<t>
1191  <x:dfn>no-store</x:dfn>
1192  <iref item="Cache Directives" primary="true" subitem="no-store" />
1193  <iref item="no-store" primary="true" subitem="Cache Directive" />
1194  <list>
1195    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1196      part of either the immediate request or response. This directive applies to both
1197      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1198      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1199      and &MUST; make a best-effort attempt to remove the information from volatile
1200      storage as promptly as possible after forwarding it.</t>
1201    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1202      particular, malicious or compromised caches might not recognize or obey this
1203      directive, and communications networks might be vulnerable to eavesdropping.</t>
1204  </list>
1205</t>
1206<t>
1207  <x:dfn>must-revalidate</x:dfn>
1208  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1209  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1210  <list>
1211    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1212     used to satisfy subsequent requests without successful validation on the origin server.</t>
1213    <t>The must-revalidate directive is necessary to support reliable operation for
1214      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1215      the must-revalidate directive; in particular, if the cache cannot reach the origin
1216      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1217    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1218      validate a request on the representation could result in incorrect operation, such as a
1219      silently unexecuted financial transaction.</t>
1220  </list>
1221</t>
1222<t>
1223  <x:dfn>proxy-revalidate</x:dfn>
1224  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1225  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1226  <list>
1227    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1228      response directive, except that it does not apply to non-shared caches.</t>
1229  </list>
1230</t>
1231<t>
1232  <x:dfn>max-age</x:dfn>
1233  <iref item="Cache Directives" primary="true" subitem="max-age" />
1234  <iref item="max-age" primary="true" subitem="Cache Directive" />
1235  <list>
1236    <t>The max-age response directive indicates that response is to be considered stale
1237      after its age is greater than the specified number of seconds.</t>
1238  </list>
1239</t>
1240<t>
1241  <x:dfn>s-maxage</x:dfn>
1242  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1243  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1244  <list>
1245    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1246      specified by this directive overrides the maximum age specified by either the
1247      max-age directive or the Expires header. The s-maxage directive also implies the
1248      semantics of the proxy-revalidate response directive.</t>
1249  </list>
1250</t>
1251<t>
1252  <x:dfn>no-transform</x:dfn>
1253  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1254  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1255  <list>
1256    <t>The no-transform response directive indicates that an intermediate cache or proxy
1257      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1258      headers, nor the response representation.</t>
1259  </list>
1260</t>
1261
1262</section>
1263
1264<section anchor="cache.control.extensions" title="Cache Control Extensions">
1265<t>
1266  The Cache-Control header field can be extended through the use of one or more
1267  cache-extension tokens, each with an optional value. Informational extensions (those
1268  that do not require a change in cache behavior) can be added without changing the
1269  semantics of other directives. Behavioral extensions are designed to work by acting as
1270  modifiers to the existing base of cache directives. Both the new directive and the
1271  standard directive are supplied, such that applications that do not understand the new
1272  directive will default to the behavior specified by the standard directive, and those
1273  that understand the new directive will recognize it as modifying the requirements
1274  associated with the standard directive. In this way, extensions to the cache-control
1275  directives can be made without requiring changes to the base protocol.
1276</t>
1277<t>
1278  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1279  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1280  all directives that it does not understand.
1281</t>
1282<t>
1283  For example, consider a hypothetical new response directive called "community" that
1284  acts as a modifier to the private directive. We define this new directive to mean that,
1285  in addition to any non-shared cache, any cache that is shared only by members of the
1286  community named within its value may cache the response. An origin server wishing to
1287  allow the UCI community to use an otherwise private response in their shared cache(s)
1288  could do so by including
1289</t>
1290<figure><artwork type="example">
1291  Cache-Control: private, community="UCI"
1292</artwork></figure>
1293<t>
1294  A cache seeing this header field will act correctly even if the cache does not
1295  understand the community cache-extension, since it will also see and understand the
1296  private directive and thus default to the safe behavior.
1297</t>
1298<t>
1299  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1300  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1301  directives (or the response's default cacheability) such that the cache behavior will
1302  remain minimally correct even if the cache does not understand the extension(s).
1303</t>
1304<t>
1305  The HTTP Cache Directive Registry defines the name space for the cache
1306  directives.
1307</t>
1308<t>
1309  Registrations &MUST; include the following fields:
1310  <list style="symbols">
1311    <t>Cache Directive Name</t>
1312    <t>Pointer to specification text</t>
1313  </list>
1314</t>
1315<t>
1316  Values to be added to this name space are subject to IETF review
1317  (<xref target="RFC5226" x:fmt="," x:sec="4.1"/>).
1318</t>
1319<t>
1320  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1321</t>
1322</section>
1323
1324</section>
1325
1326<section anchor="header.expires" title="Expires">
1327  <iref item="Expires header" primary="true" x:for-anchor="" />
1328  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1329  <x:anchor-alias value="Expires"/>
1330  <x:anchor-alias value="Expires-v"/>
1331<t>
1332  The "Expires" entity-header field gives the date/time after which the response is
1333  considered stale. See <xref target="expiration.model" /> for further discussion of the
1334  freshness model.
1335</t>
1336<t>
1337  The presence of an Expires field does not imply that the original resource will change or
1338  cease to exist at, before, or after that time.
1339</t>
1340<t>
1341  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1342  it &MUST; be sent in rfc1123-date format.
1343</t>
1344<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1345  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1346  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1347</artwork></figure>
1348<figure>
1349  <preamble>For example</preamble>
1350<artwork type="example">
1351  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1352</artwork></figure>
1353<x:note>
1354  <t>
1355    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1356    directive (see <xref target="cache-response-directive" />), that directive overrides
1357    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1358  </t>
1359</x:note>
1360<t>
1361  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1362</t>
1363<t>
1364  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1365  including the value "0", as in the past (i.e., "already expired").
1366</t>
1367</section>
1368
1369<section anchor="header.pragma" title="Pragma">
1370  <iref item="Pragma header" primary="true" x:for-anchor="" />
1371  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1372  <x:anchor-alias value="extension-pragma"/>
1373  <x:anchor-alias value="Pragma"/>
1374  <x:anchor-alias value="Pragma-v"/>
1375  <x:anchor-alias value="pragma-directive"/>
1376<t>
1377  The "Pragma" general-header field is used to include implementation-specific directives
1378  that might apply to any recipient along the request/response chain. All pragma directives
1379  specify optional behavior from the viewpoint of the protocol; however, some systems
1380  &MAY; require that behavior be consistent with the directives.
1381</t>
1382<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"/>
1383  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1384  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1385  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1386  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1387</artwork></figure>
1388<t>
1389  When the no-cache directive is present in a request message, an application &SHOULD;
1390  forward the request toward the origin server even if it has a cached copy of what is being
1391  requested. This pragma directive has the same semantics as the no-cache response directive
1392  (see <xref target="cache-response-directive" />) and is defined here for backward
1393  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1394  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1395  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1396</t>
1397<x:note>
1398  <t>
1399    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1400    is not actually specified, it does not provide a reliable replacement for
1401    "Cache-Control: no-cache" in a response.
1402  </t>
1403</x:note>
1404<t>
1405  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1406</t>
1407</section>
1408
1409<section anchor="header.vary" title="Vary">
1410  <iref item="Vary header" primary="true" x:for-anchor="" />
1411  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1412  <x:anchor-alias value="Vary"/>
1413  <x:anchor-alias value="Vary-v"/>
1414<t>
1415  The "Vary" response-header field conveys the set of request-header fields
1416  that were used to select the representation.
1417</t>
1418<t>
1419  Caches use this information, in part, to determine whether a stored response
1420  can be used to satisfy a given request; see
1421  <xref target="caching.negotiated.responses" />.
1422  determines, while the response is fresh, whether a cache is permitted to use the
1423  response to reply to a subsequent request without validation; see <xref 
1424  target="caching.negotiated.responses" />.
1425</t>
1426<t>
1427  In uncacheable or stale responses, the Vary field value advises the user agent about
1428  the criteria that were used to select the representation.
1429</t>
1430<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1431  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1432  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1433</artwork></figure>
1434<t>
1435  The set of header fields named by the Vary field value is known as the selecting
1436  request-headers.
1437</t>
1438<t>
1439  Servers &SHOULD; include a Vary header field with any cacheable response that is
1440  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1441  requests on that resource and informs the user agent about the presence of negotiation on
1442  that resource. A server &MAY; include a Vary header field with a non-cacheable
1443  response that is subject to server-driven negotiation, since this might provide the user
1444  agent with useful information about the dimensions over which the response varies at the
1445  time of the response.
1446</t>
1447<t>
1448  A Vary field value of "*" signals that unspecified parameters not limited to the
1449  request-headers (e.g., the network address of the client), play a role in the selection of
1450  the response representation; therefore, a cache cannot determine whether this response is
1451  appropriate. The "*" value &MUST-NOT; be generated by a proxy server.
1452</t>
1453<t>
1454  The field-names given are not limited to the set of standard request-header fields
1455  defined by this specification. Field names are case-insensitive.
1456</t>
1457</section>
1458
1459<section anchor="header.warning" title="Warning">
1460  <iref item="Warning header" primary="true" x:for-anchor="" />
1461  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1462  <x:anchor-alias value="Warning"/>
1463  <x:anchor-alias value="Warning-v"/>
1464  <x:anchor-alias value="warning-value"/>
1465  <x:anchor-alias value="warn-agent"/>
1466  <x:anchor-alias value="warn-code"/>
1467  <x:anchor-alias value="warn-date"/>
1468  <x:anchor-alias value="warn-text"/>
1469<t>
1470  The "Warning" general-header field is used to carry additional information about the status
1471  or transformation of a message that might not be reflected in the message. This
1472  information is typically used to warn about possible incorrectness introduced by caching
1473  operations or transformations applied to the payload of the message.
1474</t>
1475<t>
1476  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1477  warning, rather than an error status code, distinguishes these responses from true failures.
1478</t>
1479<t>
1480  Warning headers can in general be applied to any message, however some warn-codes are
1481  specific to caches and can only be applied to response messages.
1482</t>
1483<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"/>
1484  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1485  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1486 
1487  <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>
1488                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1489 
1490  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1491  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1492                  ; the name or pseudonym of the server adding
1493                  ; the Warning header, for use in debugging
1494  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1495  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1496</artwork></figure>
1497<t>
1498  Multiple warnings can be attached to a response (either by the origin server or by
1499  a cache), including multiple warnings with the same code number, only differing
1500  in warn-text.
1501</t>
1502<t>
1503  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1504  possible, in the order that they appear in the response.
1505</t>
1506<t>
1507  Systems that generate multiple Warning headers &SHOULD; order them with this user
1508  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1509  Warning headers.
1510</t>
1511<t>
1512  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1513  Warning is required to be deleted from a stored response after validation:
1514  <list style="symbols">
1515    <t>1xx Warnings describe the freshness or validation status of the response, and so
1516      &MUST; be deleted by caches after validation. They can only be generated by a cache
1517      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1518    <t>2xx Warnings describe some aspect of the representation that is
1519      not rectified by a validation (for example, a lossy compression of the representation)
1520      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1521      returned, in which case they &MUST; be.</t>
1522  </list>
1523</t>
1524<t>
1525  If an implementation sends a message with one or more Warning headers to a receiver whose
1526  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1527  warn-date that matches the Date header in the message.
1528</t>
1529<t>
1530  If an implementation receives a message with a warning-value that includes a warn-date,
1531  and that warn-date is different from the Date value in the response, then that
1532  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1533  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1534  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1535  well.
1536</t>
1537<t>
1538  The following warn-codes are defined by this specification, each with a recommended
1539  warn-text in English, and a description of its meaning.
1540</t>
1541<t><?rfc needLines="4"?>
1542  110 Response is stale
1543  <list>
1544    <t>&SHOULD; be included whenever the returned response is stale.</t>
1545  </list>
1546</t>
1547<t><?rfc needLines="4"?>
1548  111 Revalidation failed
1549  <list>
1550    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1551      validate the response failed, due to an inability to reach the server.</t>
1552  </list>
1553</t>
1554<t><?rfc needLines="4"?>
1555  112 Disconnected operation
1556  <list>
1557    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1558      the network for a period of time.</t>
1559  </list>
1560</t>
1561<t><?rfc needLines="4"?>
1562  113 Heuristic expiration
1563  <list>
1564    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1565      greater than 24 hours and the response's age is greater than 24 hours.</t>
1566  </list>
1567</t>
1568<t><?rfc needLines="4"?>
1569  199 Miscellaneous warning
1570  <list>
1571    <t>The warning text can include arbitrary information to be presented to a human
1572      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1573      action, besides presenting the warning to the user.</t>
1574  </list>
1575</t>
1576<t><?rfc needLines="4"?>
1577  214 Transformation applied
1578  <list>
1579    <t>&MUST; be added by an intermediate proxy if it applies any
1580      transformation to the representation, such as changing the content-coding,
1581      media-type, or modifying the representation data, unless this Warning code
1582      already appears in the response.</t>
1583  </list>
1584</t>
1585<t><?rfc needLines="4"?>
1586  299 Miscellaneous persistent warning
1587  <list>
1588    <t>The warning text can include arbitrary information to be presented to a human
1589      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1590      action.</t>
1591  </list>
1592</t>
1593</section>
1594
1595</section>
1596
1597<section anchor="history.lists" title="History Lists">
1598<t>
1599  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1600  can be used to redisplay a representation retrieved earlier in a session.
1601</t>
1602<t>
1603  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1604  a history mechanism can display a previous representation even if it has expired.
1605</t>
1606  <t>
1607  This does not prohibit the history mechanism from telling the user that a
1608  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1609  </t>
1610</section>
1611
1612
1613<section anchor="IANA.considerations" title="IANA Considerations">
1614
1615<section title="Cache Directive Registry" anchor="cache.directive.registration">
1616<t>
1617  The registration procedure for HTTP Cache Directives is defined by
1618  <xref target="cache.control.extensions"/> of this document.
1619</t>
1620<t>
1621   The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
1622   and be populated with the registrations below:
1623</t>
1624<?BEGININC p6-cache.cache-directives ?>
1625<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1626<texttable xmlns:my="#my" align="left" suppress-title="true"
1627           anchor="iana.cache.directive.registration.table">
1628   <ttcol>Cache Directive</ttcol>
1629   <ttcol>Reference</ttcol>
1630
1631   <c>max-age</c>
1632   <c>
1633      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1634   </c>
1635   <c>max-stale</c>
1636   <c>
1637      <xref target="cache-request-directive"/>
1638   </c>
1639   <c>min-fresh</c>
1640   <c>
1641      <xref target="cache-request-directive"/>
1642   </c>
1643   <c>must-revalidate</c>
1644   <c>
1645      <xref target="cache-response-directive"/>
1646   </c>
1647   <c>no-cache</c>
1648   <c>
1649      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1650   </c>
1651   <c>no-store</c>
1652   <c>
1653      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1654   </c>
1655   <c>no-transform</c>
1656   <c>
1657      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1658   </c>
1659   <c>only-if-cached</c>
1660   <c>
1661      <xref target="cache-request-directive"/>
1662   </c>
1663   <c>private</c>
1664   <c>
1665      <xref target="cache-response-directive"/>
1666   </c>
1667   <c>proxy-revalidate</c>
1668   <c>
1669      <xref target="cache-response-directive"/>
1670   </c>
1671   <c>public</c>
1672   <c>
1673      <xref target="cache-response-directive"/>
1674   </c>
1675   <c>s-maxage</c>
1676   <c>
1677      <xref target="cache-response-directive"/>
1678   </c>
1679   <c>stale-if-error</c>
1680   <c>
1681      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1682   </c>
1683   <c>stale-while-revalidate</c>
1684   <c>
1685      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1686   </c>
1687</texttable>
1688<!--(END)-->
1689<?ENDINC p6-cache.cache-directives ?>
1690</section>
1691
1692<section title="Header Field Registration" anchor="header.field.registration">
1693<t>
1694  The Message Header Field Registry located at <eref
1695  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1696  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1697</t>
1698<?BEGININC p6-cache.iana-headers ?>
1699<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1700<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1701   <ttcol>Header Field Name</ttcol>
1702   <ttcol>Protocol</ttcol>
1703   <ttcol>Status</ttcol>
1704   <ttcol>Reference</ttcol>
1705
1706   <c>Age</c>
1707   <c>http</c>
1708   <c>standard</c>
1709   <c>
1710      <xref target="header.age"/>
1711   </c>
1712   <c>Cache-Control</c>
1713   <c>http</c>
1714   <c>standard</c>
1715   <c>
1716      <xref target="header.cache-control"/>
1717   </c>
1718   <c>Expires</c>
1719   <c>http</c>
1720   <c>standard</c>
1721   <c>
1722      <xref target="header.expires"/>
1723   </c>
1724   <c>Pragma</c>
1725   <c>http</c>
1726   <c>standard</c>
1727   <c>
1728      <xref target="header.pragma"/>
1729   </c>
1730   <c>Vary</c>
1731   <c>http</c>
1732   <c>standard</c>
1733   <c>
1734      <xref target="header.vary"/>
1735   </c>
1736   <c>Warning</c>
1737   <c>http</c>
1738   <c>standard</c>
1739   <c>
1740      <xref target="header.warning"/>
1741   </c>
1742</texttable>
1743<!--(END)-->
1744<?ENDINC p6-cache.iana-headers ?>
1745<t>
1746  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1747</t>
1748</section>
1749
1750</section>
1751
1752<section anchor="security.considerations" title="Security Considerations">
1753<t>
1754  Caches expose additional potential vulnerabilities, since the contents of the cache
1755  represent an attractive target for malicious exploitation. Because cache contents persist
1756  after an HTTP request is complete, an attack on the cache can reveal information long after
1757  a user believes that the information has been removed from the network. Therefore, cache
1758  contents should be protected as sensitive information.
1759</t>
1760</section>
1761
1762<section anchor="ack" title="Acknowledgments">
1763<t>
1764  Much of the content and presentation of the caching design is due to suggestions and
1765  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1766  and Larry Masinter.
1767</t>
1768</section>
1769
1770</middle>
1771
1772<back>
1773<references title="Normative References">
1774
1775  <reference anchor="Part1">
1776    <front>
1777      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1778      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1779        <organization abbrev="Day Software">Day Software</organization>
1780        <address><email>fielding@gbiv.com</email></address>
1781      </author>
1782      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1783        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1784        <address><email>jg@freedesktop.org</email></address>
1785      </author>
1786      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1787        <organization abbrev="HP">Hewlett-Packard Company</organization>
1788        <address><email>JeffMogul@acm.org</email></address>
1789      </author>
1790      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1791        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1792        <address><email>henrikn@microsoft.com</email></address>
1793      </author>
1794      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1795        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1796        <address><email>LMM@acm.org</email></address>
1797      </author>
1798      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1799        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1800        <address><email>paulle@microsoft.com</email></address>
1801      </author>
1802      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1803        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1804        <address><email>timbl@w3.org</email></address>
1805      </author>
1806      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1807        <organization abbrev="W3C">World Wide Web Consortium</organization>
1808        <address><email>ylafon@w3.org</email></address>
1809      </author>
1810      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1811        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1812        <address><email>julian.reschke@greenbytes.de</email></address>
1813      </author>
1814      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1815    </front>
1816    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1817    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1818  </reference>
1819
1820  <reference anchor="Part2">
1821    <front>
1822      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1823      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1824        <organization abbrev="Day Software">Day Software</organization>
1825        <address><email>fielding@gbiv.com</email></address>
1826      </author>
1827      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1828        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1829        <address><email>jg@freedesktop.org</email></address>
1830      </author>
1831      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1832        <organization abbrev="HP">Hewlett-Packard Company</organization>
1833        <address><email>JeffMogul@acm.org</email></address>
1834      </author>
1835      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1836        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1837        <address><email>henrikn@microsoft.com</email></address>
1838      </author>
1839      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1840        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1841        <address><email>LMM@acm.org</email></address>
1842      </author>
1843      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1844        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1845        <address><email>paulle@microsoft.com</email></address>
1846      </author>
1847      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1848        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1849        <address><email>timbl@w3.org</email></address>
1850      </author>
1851      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1852        <organization abbrev="W3C">World Wide Web Consortium</organization>
1853        <address><email>ylafon@w3.org</email></address>
1854      </author>
1855      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1856        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1857        <address><email>julian.reschke@greenbytes.de</email></address>
1858      </author>
1859      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1860    </front>
1861    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1862    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1863  </reference>
1864
1865  <reference anchor="Part4">
1866    <front>
1867      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1868      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1869        <organization abbrev="Day Software">Day Software</organization>
1870        <address><email>fielding@gbiv.com</email></address>
1871      </author>
1872      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1873        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1874        <address><email>jg@freedesktop.org</email></address>
1875      </author>
1876      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1877        <organization abbrev="HP">Hewlett-Packard Company</organization>
1878        <address><email>JeffMogul@acm.org</email></address>
1879      </author>
1880      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1881        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1882        <address><email>henrikn@microsoft.com</email></address>
1883      </author>
1884      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1885        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1886        <address><email>LMM@acm.org</email></address>
1887      </author>
1888      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1889        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1890        <address><email>paulle@microsoft.com</email></address>
1891      </author>
1892      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1893        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1894        <address><email>timbl@w3.org</email></address>
1895      </author>
1896      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1897        <organization abbrev="W3C">World Wide Web Consortium</organization>
1898        <address><email>ylafon@w3.org</email></address>
1899      </author>
1900      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1901        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1902        <address><email>julian.reschke@greenbytes.de</email></address>
1903      </author>
1904      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1905    </front>
1906    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1907    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1908  </reference>
1909
1910  <reference anchor="Part5">
1911    <front>
1912      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1913      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1914        <organization abbrev="Day Software">Day Software</organization>
1915        <address><email>fielding@gbiv.com</email></address>
1916      </author>
1917      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1918        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1919        <address><email>jg@freedesktop.org</email></address>
1920      </author>
1921      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1922        <organization abbrev="HP">Hewlett-Packard Company</organization>
1923        <address><email>JeffMogul@acm.org</email></address>
1924      </author>
1925      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1926        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1927        <address><email>henrikn@microsoft.com</email></address>
1928      </author>
1929      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1930        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1931        <address><email>LMM@acm.org</email></address>
1932      </author>
1933      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1934        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1935        <address><email>paulle@microsoft.com</email></address>
1936      </author>
1937      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1938        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1939        <address><email>timbl@w3.org</email></address>
1940      </author>
1941      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1942        <organization abbrev="W3C">World Wide Web Consortium</organization>
1943        <address><email>ylafon@w3.org</email></address>
1944      </author>
1945      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1946        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1947        <address><email>julian.reschke@greenbytes.de</email></address>
1948      </author>
1949      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1950    </front>
1951    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1952    <x:source basename="p5-range" href="p5-range.xml" />
1953  </reference>
1954
1955  <reference anchor="Part7">
1956    <front>
1957      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1958      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1959        <organization abbrev="Day Software">Day Software</organization>
1960        <address><email>fielding@gbiv.com</email></address>
1961      </author>
1962      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1963        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1964        <address><email>jg@freedesktop.org</email></address>
1965      </author>
1966      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1967        <organization abbrev="HP">Hewlett-Packard Company</organization>
1968        <address><email>JeffMogul@acm.org</email></address>
1969      </author>
1970      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1971        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1972        <address><email>henrikn@microsoft.com</email></address>
1973      </author>
1974      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1975        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1976        <address><email>LMM@acm.org</email></address>
1977      </author>
1978      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1979        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1980        <address><email>paulle@microsoft.com</email></address>
1981      </author>
1982      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1983        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1984        <address><email>timbl@w3.org</email></address>
1985      </author>
1986      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1987        <organization abbrev="W3C">World Wide Web Consortium</organization>
1988        <address><email>ylafon@w3.org</email></address>
1989      </author>
1990      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1991        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1992        <address><email>julian.reschke@greenbytes.de</email></address>
1993      </author>
1994      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1995    </front>
1996    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1997    <x:source basename="p7-auth" href="p7-auth.xml" />
1998  </reference>
1999
2000  <reference anchor="RFC2119">
2001    <front>
2002      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2003      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2004        <organization>Harvard University</organization>
2005        <address><email>sob@harvard.edu</email></address>
2006      </author>
2007      <date month="March" year="1997" />
2008    </front>
2009    <seriesInfo name="BCP" value="14" />
2010    <seriesInfo name="RFC" value="2119" />
2011  </reference>
2012
2013  <reference anchor="RFC5234">
2014    <front>
2015      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2016      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2017        <organization>Brandenburg InternetWorking</organization>
2018        <address>
2019          <email>dcrocker@bbiw.net</email>
2020        </address> 
2021      </author>
2022      <author initials="P." surname="Overell" fullname="Paul Overell">
2023        <organization>THUS plc.</organization>
2024        <address>
2025          <email>paul.overell@thus.net</email>
2026        </address>
2027      </author>
2028      <date month="January" year="2008"/>
2029    </front>
2030    <seriesInfo name="STD" value="68"/>
2031    <seriesInfo name="RFC" value="5234"/>
2032  </reference>
2033 
2034</references>
2035
2036<references title="Informative References">
2037
2038  <reference anchor="RFC1305">
2039    <front>
2040      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2041      <author fullname="David L. Mills" initials="D." surname="Mills">
2042        <organization>University of Delaware, Electrical Engineering Department</organization>
2043        <address><email>mills@udel.edu</email></address>
2044      </author>
2045      <date month="March" year="1992" />
2046    </front>
2047    <seriesInfo name="RFC" value="1305" />
2048  </reference>
2049
2050  <reference anchor="RFC2616">
2051    <front>
2052      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2053      <author fullname="R. Fielding" initials="R." surname="Fielding">
2054        <organization>University of California, Irvine</organization>
2055        <address><email>fielding@ics.uci.edu</email></address>
2056      </author>
2057      <author fullname="J. Gettys" initials="J." surname="Gettys">
2058        <organization>W3C</organization>
2059        <address><email>jg@w3.org</email></address>
2060      </author>
2061      <author fullname="J. Mogul" initials="J." surname="Mogul">
2062        <organization>Compaq Computer Corporation</organization>
2063        <address><email>mogul@wrl.dec.com</email></address>
2064      </author>
2065      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2066        <organization>MIT Laboratory for Computer Science</organization>
2067        <address><email>frystyk@w3.org</email></address>
2068      </author>
2069      <author fullname="L. Masinter" initials="L." surname="Masinter">
2070        <organization>Xerox Corporation</organization>
2071        <address><email>masinter@parc.xerox.com</email></address>
2072      </author>
2073      <author fullname="P. Leach" initials="P." surname="Leach">
2074        <organization>Microsoft Corporation</organization>
2075        <address><email>paulle@microsoft.com</email></address>
2076      </author>
2077      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2078        <organization>W3C</organization>
2079        <address><email>timbl@w3.org</email></address>
2080      </author>
2081      <date month="June" year="1999" />
2082    </front>
2083    <seriesInfo name="RFC" value="2616" />
2084  </reference>
2085
2086  <reference anchor="RFC3864">
2087    <front>
2088      <title>Registration Procedures for Message Header Fields</title>
2089      <author fullname="G. Klyne" initials="G." surname="Klyne">
2090        <organization>Nine by Nine</organization>
2091        <address><email>GK-IETF@ninebynine.org</email></address>
2092      </author>
2093      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2094        <organization>BEA Systems</organization>
2095        <address><email>mnot@pobox.com</email></address>
2096      </author>
2097      <author fullname="J. Mogul" initials="J." surname="Mogul">
2098        <organization>HP Labs</organization>
2099        <address><email>JeffMogul@acm.org</email></address>
2100      </author>
2101      <date month="September" year="2004" />
2102    </front>
2103    <seriesInfo name="BCP" value="90" />
2104    <seriesInfo name="RFC" value="3864" />
2105  </reference>
2106
2107  <reference anchor='RFC5226'>
2108    <front>
2109      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2110      <author initials='T.' surname='Narten' fullname='T. Narten'>
2111        <organization>IBM</organization>
2112        <address><email>narten@us.ibm.com</email></address>
2113      </author>
2114      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2115        <organization>Google</organization>
2116        <address><email>Harald@Alvestrand.no</email></address>
2117      </author>
2118      <date year='2008' month='May' />
2119    </front>
2120    <seriesInfo name='BCP' value='26' />
2121    <seriesInfo name='RFC' value='5226' />
2122  </reference>
2123
2124  <reference anchor='RFC5861'>
2125    <front>
2126      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2127      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2128        <organization>Yahoo! Inc.</organization>
2129        <address><email>mnot@yahoo-inc.com</email></address>
2130      </author>
2131      <date month="April" year="2010"/>
2132    </front>
2133    <seriesInfo name='RFC' value='5861' />
2134  </reference>
2135
2136</references>
2137
2138<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2139<t>
2140  Make the specified age calculation algorithm less conservative.
2141  (<xref target="age.calculations"/>)
2142</t>
2143<t>
2144  Remove requirement to consider Content-Location in successful responses
2145  in order to determine the appropriate response to use.
2146  (<xref target="validation.model" />)
2147</t>
2148<t>
2149  Clarify denial of service attack avoidance requirement.
2150  (<xref target="invalidation.after.updates.or.deletions" />)
2151</t>
2152<t>
2153  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2154  anymore, as these aspects never were implemented.
2155  (<xref target="header.warning" />)
2156</t>
2157</section>
2158
2159<?BEGININC p6-cache.abnf-appendix ?>
2160<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2161<figure>
2162<artwork type="abnf" name="p6-cache.parsed-abnf">
2163<x:ref>Age</x:ref> = "Age:" OWS Age-v
2164<x:ref>Age-v</x:ref> = delta-seconds
2165
2166<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2167<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2168 cache-directive ] )
2169
2170<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2171<x:ref>Expires-v</x:ref> = HTTP-date
2172
2173<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2174
2175<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2176
2177<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2178<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2179 pragma-directive ] )
2180
2181<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2182<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2183 ] ) )
2184
2185<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2186<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2187 ] )
2188
2189<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2190<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2191<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2192 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2193 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2194 cache-extension
2195<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2196 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2197 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2198 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2199 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2200 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2201
2202<x:ref>delta-seconds</x:ref> = 1*DIGIT
2203
2204<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2205
2206<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2207
2208<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2209<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2210<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2211
2212<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2213
2214<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2215
2216<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2217
2218<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2219<x:ref>warn-code</x:ref> = 3DIGIT
2220<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2221<x:ref>warn-text</x:ref> = quoted-string
2222<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2223 ]
2224</artwork>
2225</figure>
2226<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2227; Age defined but not used
2228; Cache-Control defined but not used
2229; Expires defined but not used
2230; Pragma defined but not used
2231; Vary defined but not used
2232; Warning defined but not used
2233</artwork></figure></section>
2234<?ENDINC p6-cache.abnf-appendix ?>
2235
2236<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2237
2238<section title="Since RFC2616">
2239  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2240</section>
2241
2242<section title="Since draft-ietf-httpbis-p6-cache-00">
2243<t>
2244  Closed issues:
2245  <list style="symbols">
2246    <t>
2247      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2248    <t>
2249      <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>
2250    <t>
2251      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2252    <t>
2253      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2254    <t>
2255      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2256    <t>
2257      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2258    <t>
2259      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2260    <t>
2261      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2262    <t>
2263      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2264  </list>
2265</t>
2266<t>
2267  Other changes:
2268  <list style="symbols">
2269    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2270        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2271  </list>
2272</t>
2273</section>
2274
2275<section title="Since draft-ietf-httpbis-p6-cache-01">
2276<t>
2277  Closed issues:
2278  <list style="symbols">
2279    <t>
2280      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2281  </list>
2282</t>
2283<t>
2284  Other changes:
2285  <list style="symbols">
2286    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2287      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2288    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2289      specification.</t>
2290  </list>
2291</t>
2292</section>
2293
2294<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2295<t>
2296  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2297  <list style="symbols">
2298    <t>Reference RFC 3984, and update header registrations for headers defined in this
2299      document.</t>
2300  </list>
2301</t>
2302</section>
2303
2304<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2305<t>
2306  Closed issues:
2307  <list style="symbols">
2308    <t>
2309      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2310  </list>
2311</t>
2312</section>
2313
2314<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2315<t>
2316  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2317  <list style="symbols"> 
2318    <t>
2319      Use "/" instead of "|" for alternatives.
2320    </t>
2321    <t>
2322      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2323      whitespace ("OWS") and required whitespace ("RWS").
2324    </t>
2325    <t>
2326      Rewrite ABNFs to spell out whitespace rules, factor out
2327      header value format definitions.
2328    </t>
2329  </list>
2330</t>
2331</section>
2332
2333<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2334<t>
2335  This is a total rewrite of this part of the specification.
2336</t>
2337<t>
2338  Affected issues:
2339  <list style="symbols">
2340    <t>
2341      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2342    <t>
2343      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2344    <t>
2345      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2346    <t>
2347      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2348  </list>
2349</t>
2350<t>
2351  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2352  <list style="symbols"> 
2353    <t>
2354      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2355    </t>
2356  </list>
2357</t>
2358</section>
2359
2360<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2361<t>
2362  Closed issues:
2363  <list style="symbols"> 
2364    <t>
2365      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2366      "base for numeric protocol elements"
2367    </t>
2368  </list>
2369</t>
2370<t>
2371  Affected issues:
2372  <list style="symbols">
2373    <t>
2374      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2375      Vary and non-existant headers
2376    </t>
2377  </list>
2378</t>
2379</section>
2380
2381<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2382<t>
2383  Closed issues:
2384  <list style="symbols"> 
2385    <t>
2386      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2387      "Definition of 1xx Warn-Codes"
2388    </t>
2389    <t>
2390      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2391      "Content-Location on 304 responses"
2392    </t>
2393    <t>
2394      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2395      "private and no-cache CC directives with headers"
2396    </t>
2397    <t>
2398      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2399      "RFC2047 and warn-text"
2400    </t>
2401  </list>
2402</t>
2403</section>
2404
2405<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2406<t>
2407  Closed issues:
2408  <list style="symbols"> 
2409    <t>
2410      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2411      "serving negotiated responses from cache: header-specific canonicalization"
2412    </t>
2413    <t>
2414      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2415      "Effect of CC directives on history lists"
2416    </t>
2417  </list>
2418</t>
2419<t>
2420  Affected issues:
2421  <list style="symbols">
2422    <t>
2423      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2424      Status codes and caching
2425    </t>
2426  </list>
2427</t>
2428<t>
2429  Partly resolved issues:
2430  <list style="symbols"> 
2431    <t>
2432      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2433      "Placement of 13.5.1 and 13.5.2"
2434    </t>
2435  </list>
2436</t>
2437</section>
2438
2439<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2440<t>
2441  Closed issues:
2442  <list style="symbols"> 
2443    <t>
2444      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2445      "Age calculation"
2446    </t>
2447    <t>
2448      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2449      "Clarify differences between / requirements for request and response CC directives"
2450    </t>
2451        <t>
2452                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2453                "Caching authenticated responses"
2454        </t>
2455    <t>
2456      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2457      "IANA registry for cache-control directives"
2458    </t>
2459    <t>
2460      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2461      "Heuristic caching of URLs with query components"
2462    </t>
2463  </list>
2464</t>
2465<t>
2466  Partly resolved issues:
2467  <list style="symbols"> 
2468    <t>
2469      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2470      "Term for the requested resource's URI"
2471    </t>
2472  </list>
2473</t>
2474</section>
2475
2476<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2477<t>
2478  Closed issues:
2479  <list style="symbols"> 
2480    <t>
2481      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2482      "consider removing the 'changes from 2068' sections"
2483    </t>
2484    <t>
2485      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2486      "Allowing heuristic caching for new status codes"
2487    </t>
2488        <t>
2489          Clean up TODOs and prose in "Combining Responses."
2490        </t>
2491  </list>
2492</t>
2493</section>
2494
2495</section>
2496  </back>
2497</rfc>
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