source: draft-ietf-httpbis/latest/p6-cache.xml @ 890

Last change on this file since 890 was 890, checked in by mnot@…, 9 years ago

Remove TODO for HEAD updates; now #227.

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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]>
35<?rfc toc="yes" ?>
36<?rfc symrefs="yes" ?>
37<?rfc sortrefs="yes" ?>
38<?rfc compact="yes"?>
39<?rfc subcompact="no" ?>
40<?rfc linkmailto="no" ?>
41<?rfc editing="no" ?>
42<?rfc comments="yes"?>
43<?rfc inline="yes"?>
44<?rfc rfcedstyle="yes"?>
45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
47<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
48  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
49<front>
50
51  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
52
53  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
54    <organization abbrev="Day Software">Day Software</organization>
55    <address>
56      <postal>
57        <street>23 Corporate Plaza DR, Suite 280</street>
58        <city>Newport Beach</city>
59        <region>CA</region>
60        <code>92660</code>
61        <country>USA</country>
62      </postal>
63      <phone>+1-949-706-5300</phone>
64      <facsimile>+1-949-706-5305</facsimile>
65      <email>fielding@gbiv.com</email>
66      <uri>http://roy.gbiv.com/</uri>
67    </address>
68  </author>
69
70  <author initials="J." surname="Gettys" fullname="Jim Gettys">
71    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
72    <address>
73      <postal>
74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
76        <region>MA</region>
77        <code>01741</code>
78        <country>USA</country>
79      </postal>
80      <email>jg@freedesktop.org</email>
81      <uri>http://gettys.wordpress.com/</uri>
82    </address>
83  </author>
84
85  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
86    <organization abbrev="HP">Hewlett-Packard Company</organization>
87    <address>
88      <postal>
89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
91        <city>Palo Alto</city>
92        <region>CA</region>
93        <code>94304</code>
94        <country>USA</country>
95      </postal>
96      <email>JeffMogul@acm.org</email>
97    </address>
98  </author>
99
100  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
109      </postal>
110      <email>henrikn@microsoft.com</email>
111    </address>
112  </author>
113
114  <author fullname="Larry Masinter" initials="L." surname="Masinter">
115    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
116    <address>
117      <postal>
118        <street>345 Park Ave</street>
119        <city>San Jose</city>
120        <region>CA</region>
121        <code>95110</code>
122        <country>USA</country>
123      </postal>
124      <email>LMM@acm.org</email>
125      <uri>http://larry.masinter.net/</uri>
126    </address>
127  </author>
128
129  <author fullname="Paul J. Leach" initials="P." surname="Leach">
130    <organization abbrev="Microsoft">Microsoft Corporation</organization>
131    <address>
132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email>paulle@microsoft.com</email>
139    </address>
140  </author>
141
142  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
143    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144    <address>
145      <postal>
146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
152        <country>USA</country>
153      </postal>
154      <email>timbl@w3.org</email>
155      <uri>http://www.w3.org/People/Berners-Lee/</uri>
156    </address>
157  </author>
158
159  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
160    <organization abbrev="W3C">World Wide Web Consortium</organization>
161    <address>
162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email>ylafon@w3.org</email>
171      <uri>http://www.raubacapeu.net/people/yves/</uri>
172    </address>
173  </author>
174
175  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
176    <address>
177      <email>mnot@mnot.net</email>
178      <uri>http://www.mnot.net/</uri>
179    </address>
180  </author>
181
182  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
183    <organization abbrev="greenbytes">greenbytes GmbH</organization>
184    <address>
185      <postal>
186        <street>Hafenweg 16</street>
187        <city>Muenster</city><region>NW</region><code>48155</code>
188        <country>Germany</country>
189      </postal>
190      <phone>+49 251 2807760</phone>
191      <facsimile>+49 251 2807761</facsimile>
192      <email>julian.reschke@greenbytes.de</email>
193      <uri>http://greenbytes.de/tech/webdav/</uri>
194    </address>
195  </author>
196
197  <date month="&ID-MONTH;" year="&ID-YEAR;" />
198  <workgroup>HTTPbis Working Group</workgroup>
199
200<abstract>
201<t>
202  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
203  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
204  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
205  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
206  fields that control cache behavior or indicate cacheable response messages.
207</t>
208</abstract>
209
210<note title="Editorial Note (To be removed by RFC Editor)">
211  <t>
212    Discussion of this draft should take place on the HTTPBIS working group
213    mailing list (ietf-http-wg@w3.org). The current issues list is
214    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
215    and related documents (including fancy diffs) can be found at
216    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
217  </t>
218  <t>
219    The changes in this draft are summarized in <xref target="changes.since.10"/>.
220  </t>
221</note>
222
223  </front>
224  <middle>
225
226<section anchor="caching" title="Introduction">
227<t>
228  HTTP is typically used for distributed information systems, where performance can be
229  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
230  caching and reusing response messages.
231</t>
232
233<section anchor="intro.purpose" title="Purpose">
234<iref item="cache" />
235<t>
236  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
237  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
238  in order to reduce the response time and network bandwidth consumption on future,
239  equivalent requests. Any client or server may include a cache, though a cache cannot be
240  used by a server that is acting as a tunnel.
241</t>
242<t>
243  Caching would be useless if it did not significantly improve performance. The goal of
244  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
245  some cases, a stored response can be reused without the need for a network request,
246  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
247  (see <xref target="expiration.model" />). Even when a new request is required, it is often
248  possible to reuse all or parts of the payload of a prior response to satisfy the request,
249  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
250  purpose (see <xref target="validation.model" />).
251</t>
252</section>
253
254<section anchor="intro.terminology" title="Terminology">
255<t>
256  This specification uses a number of terms to refer to the roles played by participants
257  in, and objects of, HTTP caching.
258</t>
259<t>
260  <iref item="cacheable" />
261  <x:dfn>cacheable</x:dfn>
262  <list>
263    <t>A response is cacheable if a cache is allowed to store a copy of the response message
264      for use in answering subsequent requests. Even when a response is cacheable, there may
265      be additional constraints on whether a cache can use the cached copy to satisfy a
266      particular request.</t>
267  </list>
268</t>
269<t>
270  <iref item="explicit expiration time" />
271  <x:dfn>explicit expiration time</x:dfn>
272  <list>
273    <t>The time at which the origin server intends that a representation should no longer be
274      returned by a cache without further validation.</t>
275  </list>
276</t>
277<t>
278  <iref item="heuristic expiration time" />
279  <x:dfn>heuristic expiration time</x:dfn>
280  <list>
281    <t>An expiration time assigned by a cache when no explicit expiration time is
282    available.</t>
283  </list>
284</t>
285<t>
286  <iref item="age" />
287  <x:dfn>age</x:dfn>
288  <list>
289    <t>The age of a response is the time since it was sent by, or successfully validated
290      with, the origin server.</t>
291  </list>
292</t>
293<t>
294  <iref item="first-hand" />
295  <x:dfn>first-hand</x:dfn>
296  <list>
297    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
298    0.</t>
299  </list>
300</t>
301<t>
302  <iref item="freshness lifetime" />
303  <x:dfn>freshness lifetime</x:dfn>
304  <list>
305    <t>The length of time between the generation of a response and its expiration time. </t>
306  </list>
307</t>
308<t>
309  <iref item="fresh" />
310  <x:dfn>fresh</x:dfn>
311  <list>
312    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
313  </list>
314</t>
315<t>
316  <iref item="stale" />
317  <x:dfn>stale</x:dfn>
318  <list>
319    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
320  </list>
321</t>
322<t>
323  <iref item="validator" />
324  <x:dfn>validator</x:dfn>
325  <list>
326    <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that is used to find
327      out whether a stored response has an equivalent copy of a representation.</t>
328  </list>
329</t>
330<t anchor="shared.and.non-shared.caches">
331  <iref item="validator" />
332  <x:dfn>shared cache</x:dfn>
333  <list>
334    <t>A cache that is accessible to more than one user. A non-shared cache is
335      dedicated to a single user.</t>
336  </list>
337</t>
338</section>
339
340<section anchor="intro.requirements" title="Requirements">
341<t>
342   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
343   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
344   document are to be interpreted as described in <xref target="RFC2119"/>.
345</t>
346<t>
347   An implementation is not compliant if it fails to satisfy one or more
348   of the "MUST" or "REQUIRED" level requirements for the protocols it
349   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
350   level and all the "SHOULD" level requirements for its protocols is said
351   to be "unconditionally compliant"; one that satisfies all the "MUST"
352   level requirements but not all the "SHOULD" level requirements for its
353   protocols is said to be "conditionally compliant".
354</t>
355</section>
356
357<section title="Syntax Notation" anchor="notation">
358  <x:anchor-alias value="ALPHA"/>
359  <x:anchor-alias value="CR"/>
360  <x:anchor-alias value="DIGIT"/>
361  <x:anchor-alias value="DQUOTE"/>
362  <x:anchor-alias value="LF"/>
363  <x:anchor-alias value="OCTET"/>
364  <x:anchor-alias value="SP"/>
365  <x:anchor-alias value="VCHAR"/>
366  <x:anchor-alias value="WSP"/>
367<t>
368  This specification uses the ABNF syntax defined in &notation; (which
369  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
370  <xref target="collected.abnf"/> shows the collected ABNF, with the list
371  rule expanded.
372</t>
373<t>
374  The following core rules are included by
375  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
376  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
377  DIGIT (decimal 0-9), DQUOTE (double quote),
378  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
379  OCTET (any 8-bit sequence of data), SP (space),
380  VCHAR (any visible USASCII character),
381  and WSP (whitespace).
382</t>
383
384<section title="Core Rules" anchor="core.rules">
385  <x:anchor-alias value="quoted-string"/>
386  <x:anchor-alias value="token"/>
387  <x:anchor-alias value="OWS"/>
388<t>
389  The core rules below are defined in &basic-rules;:
390</t>
391<figure><artwork type="abnf2616">
392  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
393  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
394  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
395</artwork></figure>
396</section>
397
398<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
399  <x:anchor-alias value="field-name"/>
400  <x:anchor-alias value="HTTP-date"/>
401  <x:anchor-alias value="port"/>
402  <x:anchor-alias value="pseudonym"/>
403  <x:anchor-alias value="uri-host"/>
404<t>
405  The ABNF rules below are defined in other parts:
406</t>
407<figure><!--Part1--><artwork type="abnf2616">
408  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
409  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
410  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
411  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
412  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
413</artwork></figure>
414</section>
415
416</section>
417</section>
418
419<section anchor="caching.overview" title="Cache Operation">
420
421<section anchor="response.cacheability" title="Response Cacheability">
422<t>
423  A cache &MUST-NOT; store a response to any request, unless:
424  <list style="symbols">
425    <t>The request method is understood by the cache and defined as being cacheable, and</t>
426    <t>the response status code is understood by the cache, and</t>
427    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
428       appear in request or response headers, and</t>
429    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
430       does not appear in the response, if the cache is shared, and</t>
431    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
432       the cache is shared, unless the response explicitly allows it (see <xref
433       target="caching.authenticated.responses" />), and</t>
434    <t>the response either:
435      <list style="symbols">
436        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
437        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
438        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
439        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
440        <t>has a status code that can be served with heuristic freshness (see <xref
441           target="heuristic.freshness" />).</t>
442      </list>
443    </t>
444  </list>
445</t>
446<t>
447  In this context, a cache has "understood" a request method or a response status
448  code if it recognises it and implements any cache-specific behaviour. In
449  particular, 206 Partial Content responses cannot be cached by an
450  implementation that does not handle partial content
451  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
452</t>
453<t>
454  Note that in normal operation, most caches will not store a response that has neither a
455  cache validator nor an explicit expiration time, as such responses are not usually
456  useful to store. However, caches are not prohibited from storing such responses.
457</t>
458
459<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
460<t>
461  A cache that receives an incomplete response (for example, with fewer bytes of data
462  than specified in a Content-Length header) can store the response, but &MUST; 
463  treat it as a partial response &partial;. Partial responses
464  can be combined as described in &combining-byte-ranges;; the result might be a
465  full response or might still be partial. A cache &MUST-NOT; return a partial
466  response to a client without explicitly marking it as such using the 206 (Partial
467  Content) status code.
468</t>
469<t>
470  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
471  incomplete or partial responses.
472</t>
473</section>
474
475</section>
476
477
478<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
479<t>
480  For a presented request, a cache &MUST-NOT; return a stored response, unless:
481  <list style="symbols">
482    <t>The presented Effective Request URI (&effective-request-uri;) and that of the stored response match, and</t>
483    <t>the request method associated with the stored response allows it to be
484      used for the presented request, and</t>
485    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
486      target="caching.negotiated.responses" />), and</t>
487    <t>the presented request and stored response are free from directives that would prevent
488      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
489      and</t>
490    <t>the stored response is either:
491      <list style="symbols">
492        <t>fresh (see <xref target="expiration.model" />), or</t>
493        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
494        <t>successfully validated (see <xref target="validation.model" />).</t>
495      </list>
496    </t>
497  </list>
498</t>
499<t>
500  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
501</t>
502<t>
503  When a stored response is used to satisfy a request, caches &MUST; include a
504  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
505  current_age; see <xref target="age.calculations" />.
506  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
507</t>
508<t>
509  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
510  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
511  corresponding response.
512</t>
513<t>
514  Also, note that unsafe requests might invalidate already stored responses; see
515  <xref target="invalidation.after.updates.or.deletions" />.
516</t>
517<t>
518  Caches &MUST; use the most recent response (as determined by the Date header) when
519  more than one suitable response is stored. They can also forward a request with
520  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
521  use.
522</t>
523</section>
524
525<section anchor="expiration.model" title="Freshness Model">
526<t>
527  When a response is "fresh" in the cache, it can be used to satisfy subsequent
528  requests without contacting the origin server, thereby improving efficiency.
529</t>
530<t>
531  The primary mechanism for determining freshness is for an origin server to provide an
532  explicit expiration time in the future, using either the Expires header (<xref
533  target="header.expires" />) or the max-age response cache directive (<xref
534  target="cache-response-directive" />). Generally, origin servers will assign future
535  explicit expiration times to responses in the belief that the representation is not likely to
536  change in a semantically significant way before the expiration time is reached.
537</t>
538<t>
539  If an origin server wishes to force a cache to validate every request, it can
540  assign an explicit expiration time in the past. This means that the response is always
541  stale, so that caches should validate it before using it for subsequent requests.
542  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
543</t>
544<t>
545  Since origin servers do not always provide explicit expiration times, HTTP caches may
546  also assign heuristic expiration times when they are not specified, employing algorithms that
547  use other header values (such as the Last-Modified time) to estimate a plausible
548  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
549  impose worst-case constraints on their results.
550</t>
551<figure>
552<preamble>
553  The calculation to determine if a response is fresh is:
554</preamble>
555<artwork type="code">
556   response_is_fresh = (freshness_lifetime &gt; current_age)
557</artwork>
558</figure>
559<t>
560  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
561  the current_age is defined in <xref target="age.calculations" />.
562</t>
563<t>
564  Additionally, clients may need to influence freshness calculation. They can do this using
565  several request cache directives, with the effect of either increasing or loosening
566  constraints on freshness. See <xref target="cache-request-directive" />.
567</t>
568<t>
569  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
570  freshness.</cref>
571</t>
572<t>
573  Note that freshness applies only to cache operation; it cannot be used to force a user agent
574  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
575  the difference between caches and history mechanisms.
576</t>
577
578<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
579<t>
580  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
581  response by using the first match of:
582  <list style="symbols">
583    <t>If the cache is shared and the s-maxage response cache directive (<xref
584      target="cache-response-directive" />) is present, use its value, or</t>
585    <t>If the max-age response cache directive (<xref target="cache-response-directive"
586      />) is present, use its value, or</t>
587    <t>If the Expires response header (<xref target="header.expires" />) is present, use
588      its value minus the value of the Date response header, or</t>
589    <t>Otherwise, no explicit expiration time is present in the response. A heuristic
590      freshness lifetime might be applicable; see <xref target="heuristic.freshness" />.</t>
591  </list>
592</t>
593<t>
594  Note that this calculation is not vulnerable to clock skew, since all of the
595  information comes from the origin server.
596</t>
597
598<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
599<t>
600  If no explicit expiration time is present in a stored response that has a status code
601  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
602  calculated. Heuristics &MUST-NOT; be used for other response status codes.
603</t>
604<t>
605  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
606  attach a Warning header with a 113 warn-code to the response if its current_age is
607  more than 24 hours and such a warning is not already present.
608</t>
609<t>
610  Also, if the response has a Last-Modified header (&header-last-modified;), the
611  heuristic expiration value &SHOULD; be no more than some fraction of the interval
612  since that time. A typical setting of this fraction might be 10%.
613</t>
614<x:note>
615  <t>
616    <x:h>Note:</x:h> RFC 2616 (<xref target="RFC2616" x:fmt="," x:sec="13.9"/>)
617    required that caches do not calculate heuristic freshness for URLs with
618    query components (i.e., those containing '?'). In practice, this has not
619    been widely implemented. Therefore, servers are encouraged to send explicit
620    directives (e.g., Cache-Control: no-cache) if they wish to preclude
621    caching.
622  </t>
623</x:note>
624</section>
625</section>
626
627<section anchor="age.calculations" title="Calculating Age">
628<t>
629  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
630  message when obtained from a cache. The Age field value is the cache's estimate of the
631  amount of time since the response was generated or validated by the origin server. In
632  essence, the Age value is the sum of the time that the response has been resident in
633  each of the caches along the path from the origin server, plus the amount of time it has
634  been in transit along network paths.
635</t>
636<t>
637  The following data is used for the age calculation:
638</t>
639<t>
640  <x:dfn>age_value</x:dfn>
641  <list>
642    <t>
643      The term "age_value" denotes the value of the Age header (<xref target="header.age"/>),
644      in a form appropriate for arithmetic operation; or 0, if not available.
645    </t>
646  </list>
647</t>
648<t>
649  <x:dfn>date_value</x:dfn>
650  <list>
651    <t>
652      HTTP/1.1 requires origin servers to send a Date header, if possible,
653      with every response, giving the time at which the response was generated.
654      The term "date_value" denotes the value of the Date header, in a form
655      appropriate for arithmetic operations. See &header-date; for the definition
656      of the Date header, and for requirements regarding responses without a
657      Date response header.
658    </t>
659  </list>
660</t>
661<t>
662  <x:dfn>now</x:dfn>
663  <list>
664    <t>
665      The term "now" means "the current value of the clock at the host
666      performing the calculation". Hosts that use HTTP, but especially hosts
667      running origin servers and caches, &SHOULD; use NTP
668      (<xref target="RFC1305"/>) or some similar protocol to synchronize their
669      clocks to a globally accurate time standard.
670    </t>
671  </list>
672</t>
673<t>
674  <x:dfn>request_time</x:dfn>
675  <list>
676    <t>
677      The current value of the clock at the host at the time the request
678      resulting in the stored response was made.
679    </t>
680  </list>
681</t>
682<t>
683  <x:dfn>response_time</x:dfn>
684  <list>
685    <t>
686      The current value of the clock at the host at the time the response was
687      received.
688    </t>
689  </list>
690</t>
691<t>
692  A response's age can be calculated in two entirely independent ways:
693  <list style="numbers">
694    <t>the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the
695      origin server's clock. If the result is negative, the result is replaced by zero.</t>
696    <t>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1;
697      note this value &MUST; be interpreted relative to the time the
698      request was initiated, not the time that the response was received.</t>
699  </list>
700</t>
701<figure>
702<artwork type="code">
703  apparent_age = max(0, response_time - date_value);
704
705  response_delay = response_time - request_time;
706  corrected_age_value = age_value + response_delay; 
707</artwork>
708</figure>
709<figure>
710<preamble>These are combined as</preamble>
711<artwork type="code">
712  corrected_initial_age = max(apparent_age, corrected_age_value);
713</artwork></figure>
714<t>
715  The current_age of a stored response can then be calculated by adding the amount of
716  time (in seconds) since the stored response was last validated by the origin server to
717  the corrected_initial_age.
718</t>
719<figure><artwork type="code">
720  resident_time = now - response_time;
721  current_age = corrected_initial_age + resident_time;
722</artwork></figure>
723</section>
724
725<section anchor="serving.stale.responses" title="Serving Stale Responses">
726<t>
727  A "stale" response is one that either has explicit expiry information or is allowed to
728  have heuristic expiry calculated, but is not fresh according to the calculations in
729  <xref target="expiration.model" />.
730</t>
731<t>
732  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
733  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
734  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
735  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
736</t>
737<t>
738  Caches &SHOULD-NOT; return stale responses unless they are
739  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
740  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
741</t>
742<t>
743  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
744  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
745  the cache is disconnected.
746</t>
747<t>
748  If a cache receives a first-hand response (either an entire response, or a 304 (Not
749  Modified) response) that it would normally forward to the requesting client, and the
750  received response is no longer fresh, the cache &SHOULD; forward it to the
751  requesting client without adding a new Warning (but without removing any existing
752  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
753  that response became stale in transit.
754</t>
755</section>
756</section>
757
758<section anchor="validation.model" title="Validation Model">
759<t>
760  When a cache has one or more stored responses for a requested URI, but cannot 
761  serve any of them (e.g., because they are not fresh, or one cannot be selected;
762  see <xref target="caching.negotiated.responses"/>),
763  it can use the conditional request mechanism &conditional; in the forwarded
764  request to give the origin server an opportunity to both select a valid stored
765  response to be used, and to update it. This process is known as "validating"
766  or "revalidating" the stored response.
767</t>
768<t>
769  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
770  header whose value is that of the Last-Modified header from the selected
771  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
772</t>
773<t>
774  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
775  is that of the ETag header(s) from all responses stored for the requested URI,
776  if present. However, if any of the stored responses contains only partial
777  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
778  field unless the request is for a range that would be fully satisfied by
779  that stored response.
780</t>
781<t>
782  A 304 (Not Modified) response status code indicates that the stored
783  response can be updated and reused; see <xref target="combining.headers"/>.
784</t>
785<t>
786  A full response (i.e., one with a response body) indicates that none 
787  of the stored responses nominated in the conditional request is
788  suitable. Instead, the full response is used both to satisfy the
789  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
790</t>
791<t>
792  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
793  either forward this response to the requesting client, or act as if the server failed to
794  respond. In the latter case, it &MAY; return a previously stored response (see <xref
795  target="serving.stale.responses" />).
796</t>
797</section>
798
799<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
800<t>
801  Because unsafe methods (&safe-methods;) have the potential for changing state on the
802  origin server, intervening caches can use them to keep their contents
803  up-to-date.
804</t>
805<t>
806  The following HTTP methods &MUST; cause a cache to invalidate the Effective Request URI (&effective-request-uri;) as well
807  as the URI(s) in the Location and Content-Location headers (if present):
808  <list style="symbols">
809    <t>PUT</t>
810    <t>DELETE</t>
811    <t>POST</t>
812  </list>
813</t>
814<t>
815  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
816  be performed if the host part of that URI differs from the host part in the Effective Request URI (&effective-request-uri;).
817  This helps prevent denial of service attacks.
818</t>
819<t>
820  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
821</t>
822<t>
823  A cache that passes through requests for methods it does not understand &SHOULD;
824  invalidate the Effective Request URI (&effective-request-uri;).
825</t>
826<t>
827  Here, "invalidate" means that the cache will either remove all stored responses related
828  to the Effective Request URI, or will mark these as "invalid" and in need of a mandatory validation
829  before they can be returned in response to a subsequent request.
830</t>
831<t>
832  Note that this does not guarantee that all appropriate responses are invalidated. For
833  example, the request that caused the change at the origin server might not have gone
834  through the cache where a response is stored.
835</t>
836<t>
837  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
838</t>
839</section>
840
841<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
842
843<t>Shared caches &MUST-NOT; use a cached response to a request with an Authorization header (&header-authorization;) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.</t>
844
845<t>In this specification, the following Cache-Control response directives (<xref target="cache-response-directive"/>) have such an effect: must-revalidate, public, s-maxage.</t>
846
847<t>Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.</t>
848</section>
849
850<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
851<t>
852  When a cache receives a request that can be satisfied by a stored response
853  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
854  response unless all of the selecting request-headers nominated by the Vary header match
855  in both the original request (i.e., that associated with the stored response),
856  and the presented request.
857</t>
858<t>
859  The selecting request-headers from two requests are defined to match
860  if and only if those in the first request can be transformed to those in the
861  second request by applying any of the following:
862  <list style="symbols">
863    <t>
864      adding or removing whitespace, where allowed in the header's syntax
865    </t>
866    <t>
867      combining multiple message-header fields with the same field name (see
868      &header-fields;)
869    </t>
870    <t>
871      normalizing both header values in a way that is known to have identical
872      semantics, according to the header's specification (e.g., re-ordering field values
873      when order is not significant; case-normalization, where values are defined to be
874      case-insensitive)   
875    </t>
876  </list>
877</t>
878<t>
879  If (after any normalisation that may take place) a header field is absent
880  from a request, it can only match another request if it is also absent there.
881</t>
882<t>
883  A Vary header field-value of "*" always fails to match, and subsequent requests to that
884  resource can only be properly interpreted by the origin server.
885</t>
886<t>
887  The stored response with matching selecting request-headers is known as the
888  selected response.
889</t>
890<t>
891  If no selected response is available, the cache &MAY; forward the presented
892  request to the origin server in a conditional request; see <xref target="validation.model"/>.
893</t>
894</section>
895
896<section anchor="combining.headers" title="Combining Responses">
897<t>
898  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
899  (in this section, the "new" response"), it needs to created an updated response by combining
900  the stored response with the new one, so that the updated response can be used to satisfy the request.
901</t>
902<t>
903  If the new response contains an ETag, it identifies the stored 
904  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
905  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
906</t>
907<t>
908  If the status code is 206 (partial content), both the stored and new 
909  responses &MUST; have validators, and those validators &MUST; match using the strong 
910  comparison function (see &weak-and-strong;). Otherwise, the 
911  responses &MUST-NOT; be combined.
912</t>
913<t>
914  The stored response headers are used as those of the updated response, except that
915  <list style="symbols">
916    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
917      &MUST; be deleted from the stored response and the updated response.</t>
918    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
919      response and the updated response.</t>
920    <t>any headers provided in the new response &MUST; replace the corresponding
921      headers from the stored response.</t>
922  </list>
923</t>
924<t>
925  If a header field-name in the new response matches more than one 
926  header in the stored response, all such stored headers &MUST; be replaced.
927</t>
928<t>
929  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
930  stored response in cache. In the case of a 206 response, the combined 
931  representation &MAY; be stored.
932</t>
933</section>
934
935</section>
936
937<section anchor="header.fields" title="Header Field Definitions">
938<t>
939  This section defines the syntax and semantics of HTTP/1.1 header fields
940  related to caching.
941</t>
942<t>
943  For entity-header fields, both sender and recipient refer to either the client or the
944  server, depending on who sends and who receives the message.
945</t>
946
947<section anchor="header.age" title="Age">
948  <iref item="Age header" primary="true" x:for-anchor="" />
949  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
950  <x:anchor-alias value="Age"/>
951  <x:anchor-alias value="Age-v"/>
952  <x:anchor-alias value="age-value"/>
953<t>
954  The "Age" response-header field conveys the sender's estimate of the amount
955  of time since the response was generated or successfully validated at the
956  origin server. Age values are calculated as specified in
957  <xref target="age.calculations" />.
958</t>
959<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
960  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
961  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
962</artwork></figure>
963<t anchor="rule.delta-seconds">
964  <x:anchor-alias value="delta-seconds" />
965  Age field-values are non-negative integers, representing time in seconds.
966</t>
967<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
968  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
969</artwork></figure>
970<t>
971  If a cache receives a value larger than the largest positive integer it can represent, or
972  if any of its age calculations overflows, it &MUST; transmit an Age header with a
973  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
974  of at least 31 bits of range.
975</t>
976<t>
977  The presence of an Age header field in a response implies that a response is not
978  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
979  Age header field.
980</t>
981</section>
982
983<section anchor="header.cache-control" title="Cache-Control">
984  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
985  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
986  <x:anchor-alias value="Cache-Control"/>
987  <x:anchor-alias value="Cache-Control-v"/>
988  <x:anchor-alias value="cache-directive"/>
989  <x:anchor-alias value="cache-extension"/>
990  <x:anchor-alias value="cache-request-directive"/>
991  <x:anchor-alias value="cache-response-directive"/>
992<t>
993  The "Cache-Control" general-header field is used to specify directives for
994  caches along the request/response chain. Such cache directives are
995  unidirectional in that the presence of a directive in a request does not
996  imply that the same directive is to be given in the response.
997</t>
998<t>
999  HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives
1000  defined in this section. See <xref target="cache.control.extensions"/> for
1001  information about how Cache-Control directives defined elsewhere are handled.
1002</t>
1003<x:note>
1004  <t>
1005    <x:h>Note:</x:h> HTTP/1.0 caches might not implement Cache-Control and
1006    might only implement Pragma: no-cache (see <xref target="header.pragma" />).
1007  </t>
1008</x:note>
1009<t>
1010  Cache directives &MUST; be passed through by a proxy or gateway application,
1011  regardless of their significance to that application, since the directives might be
1012  applicable to all recipients along the request/response chain. It is not possible to
1013  target a directive to a specific cache.
1014</t>
1015<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
1016  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
1017  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
1018
1019  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
1020     / <x:ref>cache-response-directive</x:ref>
1021
1022  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1023</artwork></figure>
1024
1025<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1026  <x:anchor-alias value="cache-request-directive" />
1027
1028<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
1029  <x:ref>cache-request-directive</x:ref> =
1030       "no-cache"
1031     / "no-store"
1032     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1033     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
1034     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
1035     / "no-transform"
1036     / "only-if-cached"
1037     / <x:ref>cache-extension</x:ref>
1038</artwork></figure>
1039
1040<t>
1041  <x:dfn>no-cache</x:dfn>
1042  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1043  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1044  <list>
1045    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
1046      used to satisfy the request without successful validation on the origin server.</t>
1047  </list>
1048</t>
1049<t>
1050  <x:dfn>no-store</x:dfn>
1051  <iref item="Cache Directives" primary="true" subitem="no-store" />
1052  <iref item="no-store" primary="true" subitem="Cache Directive" />
1053  <list>
1054    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1055      of either this request or any response to it. This directive applies to both
1056      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1057      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1058      and &MUST; make a best-effort attempt to remove the information from volatile
1059      storage as promptly as possible after forwarding it.</t>
1060    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1061      particular, malicious or compromised caches might not recognize or obey this
1062      directive, and communications networks may be vulnerable to eavesdropping.</t>
1063  </list>
1064</t>
1065<t>
1066  <x:dfn>max-age</x:dfn>
1067  <iref item="Cache Directives" primary="true" subitem="max-age" />
1068  <iref item="max-age" primary="true" subitem="Cache Directive" />
1069  <list>
1070    <t>The max-age request directive indicates that the client is willing to accept a
1071      response whose age is no greater than the specified time in seconds. Unless
1072      the max-stale request directive is also present, the client is not willing to accept a stale
1073      response.</t>
1074  </list>
1075</t>
1076<t>
1077  <x:dfn>max-stale</x:dfn>
1078  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1079  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1080  <list>
1081    <t>The max-stale request directive indicates that the client is willing to accept a
1082      response that has exceeded its expiration time. If max-stale is assigned a value,
1083      then the client is willing to accept a response that has exceeded its expiration
1084      time by no more than the specified number of seconds. If no value is assigned to
1085      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
1086  </list>
1087</t>
1088<t>
1089  <x:dfn>min-fresh</x:dfn>
1090  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1091  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1092  <list>
1093    <t>The min-fresh request directive indicates that the client is willing to accept a
1094      response whose freshness lifetime is no less than its current age plus the specified
1095      time in seconds. That is, the client wants a response that will still be fresh for
1096      at least the specified number of seconds.</t>
1097  </list>
1098</t>
1099<t>
1100  <x:dfn>no-transform</x:dfn>
1101  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1102  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1103  <list>
1104    <t>The no-transform request directive indicates that an intermediate cache or proxy
1105      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1106      headers, nor the request representation.</t>
1107  </list>
1108</t>
1109<t>
1110  <x:dfn>only-if-cached</x:dfn>
1111  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1112  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1113  <list>
1114    <t>The only-if-cached request directive indicates that the client only wishes to
1115      return a stored response. If it receives this directive, a cache &SHOULD; either
1116      respond using a stored response that is consistent with the other constraints of the
1117      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1118      being operated as a unified system with good internal connectivity, such a request
1119      &MAY; be forwarded within that group of caches.</t>
1120  </list>
1121</t>
1122</section>
1123
1124<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1125  <x:anchor-alias value="cache-response-directive" />
1126
1127<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1128  <x:ref>cache-response-directive</x:ref> =
1129       "public"
1130     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1131     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1132     / "no-store"
1133     / "no-transform"
1134     / "must-revalidate"
1135     / "proxy-revalidate"
1136     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1137     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1138     / <x:ref>cache-extension</x:ref>
1139</artwork></figure>
1140
1141<t>
1142  <x:dfn>public</x:dfn>
1143  <iref item="Cache Directives" primary="true" subitem="public" />
1144  <iref item="public" primary="true" subitem="Cache Directive" />
1145  <list>
1146    <t>The public response directive indicates that the response &MAY; be cached, even
1147      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1148      (See also Authorization, &header-authorization;, for additional details.) </t>
1149  </list>
1150</t>
1151<t>
1152  <x:dfn>private</x:dfn>
1153  <iref item="Cache Directives" primary="true" subitem="private" />
1154  <iref item="private" primary="true" subitem="Cache Directive" />
1155  <list>
1156    <t>The private response directive indicates that the response message is intended for
1157      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1158      cache &MAY; store the response.</t>
1159    <t>If the private response directive specifies one or more field-names, this
1160      requirement is limited to the field-values associated with the listed response
1161      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1162      cache, whereas the remainder of the response message &MAY; be.</t>
1163    <t>
1164      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1165      be stored, and cannot ensure the privacy of the message content.
1166      Also, private response directives with field-names are often handled by
1167      implementations as if an unqualified private directive was received; i.e.,
1168      the special handling for the qualified form is not widely implemented.</t>
1169  </list>
1170</t>
1171<t>
1172  <x:dfn>no-cache</x:dfn>
1173  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1174  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1175  <list>
1176    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1177      satisfy a subsequent request without successful validation on the origin server.
1178      This allows an origin server to prevent caching even by caches that have been
1179      configured to return stale responses.</t>
1180    <t>If the no-cache response directive specifies one or more field-names, this
1181      requirement is limited to the field-values associated with the listed response
1182      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1183      to a subsequent request without successful validation on the origin server. This
1184      allows an origin server to prevent the re-use of certain header fields in a
1185      response, while still allowing caching of the rest of the response.</t>
1186    <t>
1187      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1188      Also, no-cache response directives with field-names are often handled by
1189      implementations as if an unqualified no-cache directive was received; i.e.,
1190      the special handling for the qualified form is not widely implemented.
1191    </t>
1192  </list>
1193</t>
1194<t>
1195  <x:dfn>no-store</x:dfn>
1196  <iref item="Cache Directives" primary="true" subitem="no-store" />
1197  <iref item="no-store" primary="true" subitem="Cache Directive" />
1198  <list>
1199    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1200      part of either the immediate request or response. This directive applies to both
1201      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1202      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1203      and &MUST; make a best-effort attempt to remove the information from volatile
1204      storage as promptly as possible after forwarding it.</t>
1205    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1206      particular, malicious or compromised caches might not recognize or obey this
1207      directive, and communications networks may be vulnerable to eavesdropping.</t>
1208  </list>
1209</t>
1210<t>
1211  <x:dfn>must-revalidate</x:dfn>
1212  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1213  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1214  <list>
1215    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1216     used to satisfy subsequent requests without successful validation on the origin server.</t>
1217    <t>The must-revalidate directive is necessary to support reliable operation for
1218      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1219      the must-revalidate directive; in particular, if the cache cannot reach the origin
1220      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1221    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1222      validate a request on the representation could result in incorrect operation, such as a
1223      silently unexecuted financial transaction.</t>
1224  </list>
1225</t>
1226<t>
1227  <x:dfn>proxy-revalidate</x:dfn>
1228  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1229  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1230  <list>
1231    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1232      response directive, except that it does not apply to non-shared caches.</t>
1233  </list>
1234</t>
1235<t>
1236  <x:dfn>max-age</x:dfn>
1237  <iref item="Cache Directives" primary="true" subitem="max-age" />
1238  <iref item="max-age" primary="true" subitem="Cache Directive" />
1239  <list>
1240    <t>The max-age response directive indicates that response is to be considered stale
1241      after its age is greater than the specified number of seconds.</t>
1242  </list>
1243</t>
1244<t>
1245  <x:dfn>s-maxage</x:dfn>
1246  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1247  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1248  <list>
1249    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1250      specified by this directive overrides the maximum age specified by either the
1251      max-age directive or the Expires header. The s-maxage directive also implies the
1252      semantics of the proxy-revalidate response directive.</t>
1253  </list>
1254</t>
1255<t>
1256  <x:dfn>no-transform</x:dfn>
1257  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1258  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1259  <list>
1260    <t>The no-transform response directive indicates that an intermediate cache or proxy
1261      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1262      headers, nor the response representation.</t>
1263  </list>
1264</t>
1265
1266</section>
1267
1268<section anchor="cache.control.extensions" title="Cache Control Extensions">
1269<t>
1270  The Cache-Control header field can be extended through the use of one or more
1271  cache-extension tokens, each with an optional value. Informational extensions (those
1272  that do not require a change in cache behavior) can be added without changing the
1273  semantics of other directives. Behavioral extensions are designed to work by acting as
1274  modifiers to the existing base of cache directives. Both the new directive and the
1275  standard directive are supplied, such that applications that do not understand the new
1276  directive will default to the behavior specified by the standard directive, and those
1277  that understand the new directive will recognize it as modifying the requirements
1278  associated with the standard directive. In this way, extensions to the cache-control
1279  directives can be made without requiring changes to the base protocol.
1280</t>
1281<t>
1282  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1283  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1284  all directives that it does not understand.
1285</t>
1286<t>
1287  For example, consider a hypothetical new response directive called "community" that
1288  acts as a modifier to the private directive. We define this new directive to mean that,
1289  in addition to any non-shared cache, any cache that is shared only by members of the
1290  community named within its value may cache the response. An origin server wishing to
1291  allow the UCI community to use an otherwise private response in their shared cache(s)
1292  could do so by including
1293</t>
1294<figure><artwork type="example">
1295  Cache-Control: private, community="UCI"
1296</artwork></figure>
1297<t>
1298  A cache seeing this header field will act correctly even if the cache does not
1299  understand the community cache-extension, since it will also see and understand the
1300  private directive and thus default to the safe behavior.
1301</t>
1302<t>
1303  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1304  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1305  directives (or the response's default cacheability) such that the cache behavior will
1306  remain minimally correct even if the cache does not understand the extension(s).
1307</t>
1308<t>
1309  The HTTP Cache Directive Registry defines the name space for the cache
1310  directives.
1311</t>
1312<t>
1313  Registrations &MUST; include the following fields:
1314  <list style="symbols">
1315    <t>Cache Directive Name</t>
1316    <t>Pointer to specification text</t>
1317  </list>
1318</t>
1319<t>
1320  Values to be added to this name space are subject to IETF review
1321  (<xref target="RFC5226" x:fmt="," x:sec="4.1"/>).
1322</t>
1323<t>
1324  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1325</t>
1326</section>
1327
1328</section>
1329
1330<section anchor="header.expires" title="Expires">
1331  <iref item="Expires header" primary="true" x:for-anchor="" />
1332  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1333  <x:anchor-alias value="Expires"/>
1334  <x:anchor-alias value="Expires-v"/>
1335<t>
1336  The "Expires" entity-header field gives the date/time after which the response is
1337  considered stale. See <xref target="expiration.model" /> for further discussion of the
1338  freshness model.
1339</t>
1340<t>
1341  The presence of an Expires field does not imply that the original resource will change or
1342  cease to exist at, before, or after that time.
1343</t>
1344<t>
1345  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1346  it &MUST; be sent in rfc1123-date format.
1347</t>
1348<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1349  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1350  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1351</artwork></figure>
1352<figure>
1353  <preamble>For example</preamble>
1354<artwork type="example">
1355  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1356</artwork></figure>
1357<x:note>
1358  <t>
1359    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1360    directive (see <xref target="cache-response-directive" />), that directive overrides
1361    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1362  </t>
1363</x:note>
1364<t>
1365  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1366</t>
1367<t>
1368  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1369  including the value "0", as in the past (i.e., "already expired").
1370</t>
1371</section>
1372
1373<section anchor="header.pragma" title="Pragma">
1374  <iref item="Pragma header" primary="true" x:for-anchor="" />
1375  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1376  <x:anchor-alias value="extension-pragma"/>
1377  <x:anchor-alias value="Pragma"/>
1378  <x:anchor-alias value="Pragma-v"/>
1379  <x:anchor-alias value="pragma-directive"/>
1380<t>
1381  The "Pragma" general-header field is used to include implementation-specific directives
1382  that might apply to any recipient along the request/response chain. All pragma directives
1383  specify optional behavior from the viewpoint of the protocol; however, some systems
1384  &MAY; require that behavior be consistent with the directives.
1385</t>
1386<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"/>
1387  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1388  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1389  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1390  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1391</artwork></figure>
1392<t>
1393  When the no-cache directive is present in a request message, an application &SHOULD;
1394  forward the request toward the origin server even if it has a cached copy of what is being
1395  requested. This pragma directive has the same semantics as the no-cache response directive
1396  (see <xref target="cache-response-directive" />) and is defined here for backward
1397  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1398  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1399  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1400</t>
1401<x:note>
1402  <t>
1403    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1404    is not actually specified, it does not provide a reliable replacement for
1405    "Cache-Control: no-cache" in a response.
1406  </t>
1407</x:note>
1408<t>
1409  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1410</t>
1411</section>
1412
1413<section anchor="header.vary" title="Vary">
1414  <iref item="Vary header" primary="true" x:for-anchor="" />
1415  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1416  <x:anchor-alias value="Vary"/>
1417  <x:anchor-alias value="Vary-v"/>
1418<t>
1419  The "Vary" response-header field conveys the set of request-header fields
1420  that were used to select the representation.
1421</t>
1422<t>
1423  Caches use this information, in part, to determine whether a stored response
1424  can be used to satisfy a given request; see
1425  <xref target="caching.negotiated.responses" />.
1426  determines, while the response is fresh, whether a cache is permitted to use the
1427  response to reply to a subsequent request without validation; see <xref 
1428  target="caching.negotiated.responses" />.
1429</t>
1430<t>
1431  In uncacheable or stale responses, the Vary field value advises the user agent about
1432  the criteria that were used to select the representation.
1433</t>
1434<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1435  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1436  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1437</artwork></figure>
1438<t>
1439  The set of header fields named by the Vary field value is known as the selecting
1440  request-headers.
1441</t>
1442<t>
1443  Servers &SHOULD; include a Vary header field with any cacheable response that is
1444  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1445  requests on that resource and informs the user agent about the presence of negotiation on
1446  that resource. A server &MAY; include a Vary header field with a non-cacheable
1447  response that is subject to server-driven negotiation, since this might provide the user
1448  agent with useful information about the dimensions over which the response varies at the
1449  time of the response.
1450</t>
1451<t>
1452  A Vary field value of "*" signals that unspecified parameters not limited to the
1453  request-headers (e.g., the network address of the client), play a role in the selection of
1454  the response representation; therefore, a cache cannot determine whether this response is
1455  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1456  it may only be generated by an origin server.
1457</t>
1458<t>
1459  The field-names given are not limited to the set of standard request-header fields
1460  defined by this specification. Field names are case-insensitive.
1461</t>
1462</section>
1463
1464<section anchor="header.warning" title="Warning">
1465  <iref item="Warning header" primary="true" x:for-anchor="" />
1466  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1467  <x:anchor-alias value="Warning"/>
1468  <x:anchor-alias value="Warning-v"/>
1469  <x:anchor-alias value="warning-value"/>
1470  <x:anchor-alias value="warn-agent"/>
1471  <x:anchor-alias value="warn-code"/>
1472  <x:anchor-alias value="warn-date"/>
1473  <x:anchor-alias value="warn-text"/>
1474<t>
1475  The "Warning" general-header field is used to carry additional information about the status
1476  or transformation of a message that might not be reflected in the message. This
1477  information is typically used to warn about possible incorrectness introduced by caching
1478  operations or transformations applied to the payload of the message.
1479</t>
1480<t>
1481  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1482  warning, rather than an error status code, distinguishes these responses from true failures.
1483</t>
1484<t>
1485  Warning headers can in general be applied to any message, however some warn-codes are
1486  specific to caches and can only be applied to response messages.
1487</t>
1488<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
1489  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1490  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1491 
1492  <x:ref>warning-value</x:ref> = <x:ref>warn-code</x:ref> <x:ref>SP</x:ref> <x:ref>warn-agent</x:ref> <x:ref>SP</x:ref> <x:ref>warn-text</x:ref>
1493                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1494 
1495  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1496  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1497                  ; the name or pseudonym of the server adding
1498                  ; the Warning header, for use in debugging
1499  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1500  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1501</artwork></figure>
1502<t>
1503  Multiple warnings can be attached to a response (either by the origin server or by
1504  a cache), including multiple warnings with the same code number, only differing
1505  in warn-text.
1506</t>
1507<t>
1508  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1509  possible, in the order that they appear in the response.
1510</t>
1511<t>
1512  Systems that generate multiple Warning headers &SHOULD; order them with this user
1513  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1514  Warning headers.
1515</t>
1516<t>
1517  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1518  Warning is required to be deleted from a stored response after validation:
1519  <list style="symbols">
1520    <t>1xx Warnings describe the freshness or validation status of the response, and so
1521      &MUST; be deleted by caches after validation. They can only be generated by a cache
1522      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1523    <t>2xx Warnings describe some aspect of the representation that is
1524      not rectified by a validation (for example, a lossy compression of the representation)
1525      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1526      returned, in which case they &MUST; be.</t>
1527  </list>
1528</t>
1529<t>
1530  If an implementation sends a message with one or more Warning headers to a receiver whose
1531  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1532  warn-date that matches the Date header in the message.
1533</t>
1534<t>
1535  If an implementation receives a message with a warning-value that includes a warn-date,
1536  and that warn-date is different from the Date value in the response, then that
1537  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1538  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1539  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1540  well.
1541</t>
1542<t>
1543  The following warn-codes are defined by this specification, each with a recommended
1544  warn-text in English, and a description of its meaning.
1545</t>
1546<t><?rfc needLines="4"?>
1547  110 Response is stale
1548  <list>
1549    <t>&SHOULD; be included whenever the returned response is stale.</t>
1550  </list>
1551</t>
1552<t><?rfc needLines="4"?>
1553  111 Revalidation failed
1554  <list>
1555    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1556      validate the response failed, due to an inability to reach the server.</t>
1557  </list>
1558</t>
1559<t><?rfc needLines="4"?>
1560  112 Disconnected operation
1561  <list>
1562    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1563      the network for a period of time.</t>
1564  </list>
1565</t>
1566<t><?rfc needLines="4"?>
1567  113 Heuristic expiration
1568  <list>
1569    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1570      greater than 24 hours and the response's age is greater than 24 hours.</t>
1571  </list>
1572</t>
1573<t><?rfc needLines="4"?>
1574  199 Miscellaneous warning
1575  <list>
1576    <t>The warning text can include arbitrary information to be presented to a human
1577      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1578      action, besides presenting the warning to the user.</t>
1579  </list>
1580</t>
1581<t><?rfc needLines="4"?>
1582  214 Transformation applied
1583  <list>
1584    <t>&MUST; be added by an intermediate proxy if it applies any
1585      transformation to the representation, such as changing the content-coding,
1586      media-type, or modifying the representation data, unless this Warning code
1587      already appears in the response.</t>
1588  </list>
1589</t>
1590<t><?rfc needLines="4"?>
1591  299 Miscellaneous persistent warning
1592  <list>
1593    <t>The warning text can include arbitrary information to be presented to a human
1594      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1595      action.</t>
1596  </list>
1597</t>
1598</section>
1599
1600</section>
1601
1602<section anchor="history.lists" title="History Lists">
1603<t>
1604  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1605  can be used to redisplay a representation retrieved earlier in a session.
1606</t>
1607<t>
1608  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1609  a history mechanism can display a previous representation even if it has expired.
1610</t>
1611  <t>
1612  This does not prohibit the history mechanism from telling the user that a
1613  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1614  </t>
1615</section>
1616
1617
1618<section anchor="IANA.considerations" title="IANA Considerations">
1619
1620<section title="Cache Directive Registry" anchor="cache.directive.registration">
1621<t>
1622  The registration procedure for HTTP Cache Directives is defined by
1623  <xref target="cache.control.extensions"/> of this document.
1624</t>
1625<t>
1626   The HTTP Cache Directive Registry should be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/>
1627   and be populated with the registrations below:
1628</t>
1629<?BEGININC p6-cache.cache-directives ?>
1630<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1631<texttable xmlns:my="#my" align="left" suppress-title="true"
1632           anchor="iana.cache.directive.registration.table">
1633   <ttcol>Cache Directive</ttcol>
1634   <ttcol>Reference</ttcol>
1635
1636   <c>max-age</c>
1637   <c>
1638      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1639   </c>
1640   <c>max-stale</c>
1641   <c>
1642      <xref target="cache-request-directive"/>
1643   </c>
1644   <c>min-fresh</c>
1645   <c>
1646      <xref target="cache-request-directive"/>
1647   </c>
1648   <c>must-revalidate</c>
1649   <c>
1650      <xref target="cache-response-directive"/>
1651   </c>
1652   <c>no-cache</c>
1653   <c>
1654      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1655   </c>
1656   <c>no-store</c>
1657   <c>
1658      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1659   </c>
1660   <c>no-transform</c>
1661   <c>
1662      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1663   </c>
1664   <c>only-if-cached</c>
1665   <c>
1666      <xref target="cache-request-directive"/>
1667   </c>
1668   <c>private</c>
1669   <c>
1670      <xref target="cache-response-directive"/>
1671   </c>
1672   <c>proxy-revalidate</c>
1673   <c>
1674      <xref target="cache-response-directive"/>
1675   </c>
1676   <c>public</c>
1677   <c>
1678      <xref target="cache-response-directive"/>
1679   </c>
1680   <c>s-maxage</c>
1681   <c>
1682      <xref target="cache-response-directive"/>
1683   </c>
1684   <c>stale-if-error</c>
1685   <c>
1686      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="4"/>
1687   </c>
1688   <c>stale-while-revalidate</c>
1689   <c>
1690      <xref xmlns:x="http://purl.org/net/xml2rfc/ext" target="RFC5861" x:fmt="," x:sec="3"/>
1691   </c>
1692</texttable>
1693<!--(END)-->
1694<?ENDINC p6-cache.cache-directives ?>
1695</section>
1696
1697<section anchor="message.header.registration" title="Message Header Registration">
1698<t>
1699  The Message Header Registry located at <eref
1700  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1701  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1702</t>
1703<?BEGININC p6-cache.iana-headers ?>
1704<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1705<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1706   <ttcol>Header Field Name</ttcol>
1707   <ttcol>Protocol</ttcol>
1708   <ttcol>Status</ttcol>
1709   <ttcol>Reference</ttcol>
1710
1711   <c>Age</c>
1712   <c>http</c>
1713   <c>standard</c>
1714   <c>
1715      <xref target="header.age"/>
1716   </c>
1717   <c>Cache-Control</c>
1718   <c>http</c>
1719   <c>standard</c>
1720   <c>
1721      <xref target="header.cache-control"/>
1722   </c>
1723   <c>Expires</c>
1724   <c>http</c>
1725   <c>standard</c>
1726   <c>
1727      <xref target="header.expires"/>
1728   </c>
1729   <c>Pragma</c>
1730   <c>http</c>
1731   <c>standard</c>
1732   <c>
1733      <xref target="header.pragma"/>
1734   </c>
1735   <c>Vary</c>
1736   <c>http</c>
1737   <c>standard</c>
1738   <c>
1739      <xref target="header.vary"/>
1740   </c>
1741   <c>Warning</c>
1742   <c>http</c>
1743   <c>standard</c>
1744   <c>
1745      <xref target="header.warning"/>
1746   </c>
1747</texttable>
1748<!--(END)-->
1749<?ENDINC p6-cache.iana-headers ?>
1750<t>
1751  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1752</t>
1753</section>
1754
1755</section>
1756
1757<section anchor="security.considerations" title="Security Considerations">
1758<t>
1759  Caches expose additional potential vulnerabilities, since the contents of the cache
1760  represent an attractive target for malicious exploitation. Because cache contents persist
1761  after an HTTP request is complete, an attack on the cache can reveal information long after
1762  a user believes that the information has been removed from the network. Therefore, cache
1763  contents should be protected as sensitive information.
1764</t>
1765</section>
1766
1767<section anchor="ack" title="Acknowledgments">
1768<t>
1769  Much of the content and presentation of the caching design is due to suggestions and
1770  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1771  and Larry Masinter.
1772</t>
1773</section>
1774
1775</middle>
1776
1777<back>
1778<references title="Normative References">
1779
1780  <reference anchor="Part1">
1781    <front>
1782      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1783      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1784        <organization abbrev="Day Software">Day Software</organization>
1785        <address><email>fielding@gbiv.com</email></address>
1786      </author>
1787      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1788        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1789        <address><email>jg@freedesktop.org</email></address>
1790      </author>
1791      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1792        <organization abbrev="HP">Hewlett-Packard Company</organization>
1793        <address><email>JeffMogul@acm.org</email></address>
1794      </author>
1795      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1796        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1797        <address><email>henrikn@microsoft.com</email></address>
1798      </author>
1799      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1800        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1801        <address><email>LMM@acm.org</email></address>
1802      </author>
1803      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1804        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1805        <address><email>paulle@microsoft.com</email></address>
1806      </author>
1807      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1808        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1809        <address><email>timbl@w3.org</email></address>
1810      </author>
1811      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1812        <organization abbrev="W3C">World Wide Web Consortium</organization>
1813        <address><email>ylafon@w3.org</email></address>
1814      </author>
1815      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1816        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1817        <address><email>julian.reschke@greenbytes.de</email></address>
1818      </author>
1819      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1820    </front>
1821    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1822    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1823  </reference>
1824
1825  <reference anchor="Part2">
1826    <front>
1827      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1828      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1829        <organization abbrev="Day Software">Day Software</organization>
1830        <address><email>fielding@gbiv.com</email></address>
1831      </author>
1832      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1833        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1834        <address><email>jg@freedesktop.org</email></address>
1835      </author>
1836      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1837        <organization abbrev="HP">Hewlett-Packard Company</organization>
1838        <address><email>JeffMogul@acm.org</email></address>
1839      </author>
1840      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1841        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1842        <address><email>henrikn@microsoft.com</email></address>
1843      </author>
1844      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1845        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1846        <address><email>LMM@acm.org</email></address>
1847      </author>
1848      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1849        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1850        <address><email>paulle@microsoft.com</email></address>
1851      </author>
1852      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1853        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1854        <address><email>timbl@w3.org</email></address>
1855      </author>
1856      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1857        <organization abbrev="W3C">World Wide Web Consortium</organization>
1858        <address><email>ylafon@w3.org</email></address>
1859      </author>
1860      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1861        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1862        <address><email>julian.reschke@greenbytes.de</email></address>
1863      </author>
1864      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1865    </front>
1866    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1867    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1868  </reference>
1869
1870  <reference anchor="Part4">
1871    <front>
1872      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1873      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1874        <organization abbrev="Day Software">Day Software</organization>
1875        <address><email>fielding@gbiv.com</email></address>
1876      </author>
1877      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1878        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1879        <address><email>jg@freedesktop.org</email></address>
1880      </author>
1881      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1882        <organization abbrev="HP">Hewlett-Packard Company</organization>
1883        <address><email>JeffMogul@acm.org</email></address>
1884      </author>
1885      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1886        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1887        <address><email>henrikn@microsoft.com</email></address>
1888      </author>
1889      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1890        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1891        <address><email>LMM@acm.org</email></address>
1892      </author>
1893      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1894        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1895        <address><email>paulle@microsoft.com</email></address>
1896      </author>
1897      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1898        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1899        <address><email>timbl@w3.org</email></address>
1900      </author>
1901      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1902        <organization abbrev="W3C">World Wide Web Consortium</organization>
1903        <address><email>ylafon@w3.org</email></address>
1904      </author>
1905      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1906        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1907        <address><email>julian.reschke@greenbytes.de</email></address>
1908      </author>
1909      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1910    </front>
1911    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1912    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1913  </reference>
1914
1915  <reference anchor="Part5">
1916    <front>
1917      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1918      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1919        <organization abbrev="Day Software">Day Software</organization>
1920        <address><email>fielding@gbiv.com</email></address>
1921      </author>
1922      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1923        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1924        <address><email>jg@freedesktop.org</email></address>
1925      </author>
1926      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1927        <organization abbrev="HP">Hewlett-Packard Company</organization>
1928        <address><email>JeffMogul@acm.org</email></address>
1929      </author>
1930      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1931        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1932        <address><email>henrikn@microsoft.com</email></address>
1933      </author>
1934      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1935        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1936        <address><email>LMM@acm.org</email></address>
1937      </author>
1938      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1939        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1940        <address><email>paulle@microsoft.com</email></address>
1941      </author>
1942      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1943        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1944        <address><email>timbl@w3.org</email></address>
1945      </author>
1946      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1947        <organization abbrev="W3C">World Wide Web Consortium</organization>
1948        <address><email>ylafon@w3.org</email></address>
1949      </author>
1950      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1951        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1952        <address><email>julian.reschke@greenbytes.de</email></address>
1953      </author>
1954      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1955    </front>
1956    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1957    <x:source basename="p5-range" href="p5-range.xml" />
1958  </reference>
1959
1960  <reference anchor="Part7">
1961    <front>
1962      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1963      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1964        <organization abbrev="Day Software">Day Software</organization>
1965        <address><email>fielding@gbiv.com</email></address>
1966      </author>
1967      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1968        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1969        <address><email>jg@freedesktop.org</email></address>
1970      </author>
1971      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1972        <organization abbrev="HP">Hewlett-Packard Company</organization>
1973        <address><email>JeffMogul@acm.org</email></address>
1974      </author>
1975      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1976        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1977        <address><email>henrikn@microsoft.com</email></address>
1978      </author>
1979      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1980        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1981        <address><email>LMM@acm.org</email></address>
1982      </author>
1983      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1984        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1985        <address><email>paulle@microsoft.com</email></address>
1986      </author>
1987      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1988        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1989        <address><email>timbl@w3.org</email></address>
1990      </author>
1991      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1992        <organization abbrev="W3C">World Wide Web Consortium</organization>
1993        <address><email>ylafon@w3.org</email></address>
1994      </author>
1995      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1996        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1997        <address><email>julian.reschke@greenbytes.de</email></address>
1998      </author>
1999      <date month="&ID-MONTH;" year="&ID-YEAR;" />
2000    </front>
2001    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
2002    <x:source basename="p7-auth" href="p7-auth.xml" />
2003  </reference>
2004
2005  <reference anchor="RFC2119">
2006    <front>
2007      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2008      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2009        <organization>Harvard University</organization>
2010        <address><email>sob@harvard.edu</email></address>
2011      </author>
2012      <date month="March" year="1997" />
2013    </front>
2014    <seriesInfo name="BCP" value="14" />
2015    <seriesInfo name="RFC" value="2119" />
2016  </reference>
2017
2018  <reference anchor="RFC5234">
2019    <front>
2020      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2021      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2022        <organization>Brandenburg InternetWorking</organization>
2023        <address>
2024          <email>dcrocker@bbiw.net</email>
2025        </address> 
2026      </author>
2027      <author initials="P." surname="Overell" fullname="Paul Overell">
2028        <organization>THUS plc.</organization>
2029        <address>
2030          <email>paul.overell@thus.net</email>
2031        </address>
2032      </author>
2033      <date month="January" year="2008"/>
2034    </front>
2035    <seriesInfo name="STD" value="68"/>
2036    <seriesInfo name="RFC" value="5234"/>
2037  </reference>
2038 
2039</references>
2040
2041<references title="Informative References">
2042
2043  <reference anchor="RFC1305">
2044    <front>
2045      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2046      <author fullname="David L. Mills" initials="D." surname="Mills">
2047        <organization>University of Delaware, Electrical Engineering Department</organization>
2048        <address><email>mills@udel.edu</email></address>
2049      </author>
2050      <date month="March" year="1992" />
2051    </front>
2052    <seriesInfo name="RFC" value="1305" />
2053  </reference>
2054
2055  <reference anchor="RFC2616">
2056    <front>
2057      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2058      <author fullname="R. Fielding" initials="R." surname="Fielding">
2059        <organization>University of California, Irvine</organization>
2060        <address><email>fielding@ics.uci.edu</email></address>
2061      </author>
2062      <author fullname="J. Gettys" initials="J." surname="Gettys">
2063        <organization>W3C</organization>
2064        <address><email>jg@w3.org</email></address>
2065      </author>
2066      <author fullname="J. Mogul" initials="J." surname="Mogul">
2067        <organization>Compaq Computer Corporation</organization>
2068        <address><email>mogul@wrl.dec.com</email></address>
2069      </author>
2070      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2071        <organization>MIT Laboratory for Computer Science</organization>
2072        <address><email>frystyk@w3.org</email></address>
2073      </author>
2074      <author fullname="L. Masinter" initials="L." surname="Masinter">
2075        <organization>Xerox Corporation</organization>
2076        <address><email>masinter@parc.xerox.com</email></address>
2077      </author>
2078      <author fullname="P. Leach" initials="P." surname="Leach">
2079        <organization>Microsoft Corporation</organization>
2080        <address><email>paulle@microsoft.com</email></address>
2081      </author>
2082      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2083        <organization>W3C</organization>
2084        <address><email>timbl@w3.org</email></address>
2085      </author>
2086      <date month="June" year="1999" />
2087    </front>
2088    <seriesInfo name="RFC" value="2616" />
2089  </reference>
2090
2091  <reference anchor="RFC3864">
2092    <front>
2093      <title>Registration Procedures for Message Header Fields</title>
2094      <author fullname="G. Klyne" initials="G." surname="Klyne">
2095        <organization>Nine by Nine</organization>
2096        <address><email>GK-IETF@ninebynine.org</email></address>
2097      </author>
2098      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2099        <organization>BEA Systems</organization>
2100        <address><email>mnot@pobox.com</email></address>
2101      </author>
2102      <author fullname="J. Mogul" initials="J." surname="Mogul">
2103        <organization>HP Labs</organization>
2104        <address><email>JeffMogul@acm.org</email></address>
2105      </author>
2106      <date month="September" year="2004" />
2107    </front>
2108    <seriesInfo name="BCP" value="90" />
2109    <seriesInfo name="RFC" value="3864" />
2110  </reference>
2111
2112  <reference anchor='RFC5226'>
2113    <front>
2114      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2115      <author initials='T.' surname='Narten' fullname='T. Narten'>
2116        <organization>IBM</organization>
2117        <address><email>narten@us.ibm.com</email></address>
2118      </author>
2119      <author initials='H.' surname='Alvestrand' fullname='H. Alvestrand'>
2120        <organization>Google</organization>
2121        <address><email>Harald@Alvestrand.no</email></address>
2122      </author>
2123      <date year='2008' month='May' />
2124    </front>
2125    <seriesInfo name='BCP' value='26' />
2126    <seriesInfo name='RFC' value='5226' />
2127  </reference>
2128
2129  <reference anchor='RFC5861'>
2130    <front>
2131      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2132      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2133        <organization>Yahoo! Inc.</organization>
2134        <address><email>mnot@yahoo-inc.com</email></address>
2135      </author>
2136      <date month="April" year="2010"/>
2137    </front>
2138    <seriesInfo name='RFC' value='5861' />
2139  </reference>
2140
2141</references>
2142
2143<section anchor="compatibility" title="Compatibility with Previous Versions">
2144
2145<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2146<t>
2147  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2148  this missing case.
2149  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2150</t>
2151<t>
2152  Range request responses would become very verbose if all meta-data were always returned;
2153  by allowing the server to only send needed headers in a 206 response, this problem can be
2154  avoided.
2155  (<xref target="combining.headers" />)
2156</t>
2157<t>
2158  The Cache-Control: max-age directive was not properly defined for responses.
2159  (<xref target="cache-response-directive"/>)
2160</t>
2161<t>
2162  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2163    format="counter" target="expiration.model" />, <xref format="counter"
2164    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2165  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2166  header, as PUT or other methods may have need for it in requests.
2167</t>
2168</section>
2169
2170<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2171<t>
2172  Make the specified age calculation algorithm less conservative.
2173  (<xref target="age.calculations"/>)
2174</t>
2175<t>
2176  Remove requirement to consider Content-Location in successful responses
2177  in order to determine the appropriate response to use.
2178  (<xref target="validation.model" />)
2179</t>
2180<t>
2181  Clarify denial of service attack avoidance requirement.
2182  (<xref target="invalidation.after.updates.or.deletions" />)
2183</t>
2184<t>
2185  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2186  anymore, as these aspects never were implemented.
2187  (<xref target="header.warning" />)
2188</t>
2189</section>
2190
2191    </section>
2192
2193<?BEGININC p6-cache.abnf-appendix ?>
2194<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2195<figure>
2196<artwork type="abnf" name="p6-cache.parsed-abnf">
2197<x:ref>Age</x:ref> = "Age:" OWS Age-v
2198<x:ref>Age-v</x:ref> = delta-seconds
2199
2200<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2201<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2202 cache-directive ] )
2203
2204<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2205<x:ref>Expires-v</x:ref> = HTTP-date
2206
2207<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2208
2209<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2210
2211<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2212<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2213 pragma-directive ] )
2214
2215<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2216<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2217 ] ) )
2218
2219<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2220<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2221 ] )
2222
2223<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2224<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2225<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2226 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2227 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2228 cache-extension
2229<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2230 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2231 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2232 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2233 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2234 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2235
2236<x:ref>delta-seconds</x:ref> = 1*DIGIT
2237
2238<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2239
2240<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2241
2242<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2243<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2244<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2245
2246<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2247
2248<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2249
2250<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2251
2252<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2253<x:ref>warn-code</x:ref> = 3DIGIT
2254<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2255<x:ref>warn-text</x:ref> = quoted-string
2256<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2257 ]
2258</artwork>
2259</figure>
2260<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2261; Age defined but not used
2262; Cache-Control defined but not used
2263; Expires defined but not used
2264; Pragma defined but not used
2265; Vary defined but not used
2266; Warning defined but not used
2267</artwork></figure></section>
2268<?ENDINC p6-cache.abnf-appendix ?>
2269
2270<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2271
2272<section title="Since RFC2616">
2273  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2274</section>
2275
2276<section title="Since draft-ietf-httpbis-p6-cache-00">
2277<t>
2278  Closed issues:
2279  <list style="symbols">
2280    <t>
2281      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2282    <t>
2283      <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>
2284    <t>
2285      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2286    <t>
2287      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2288    <t>
2289      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2290    <t>
2291      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2292    <t>
2293      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2294    <t>
2295      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2296    <t>
2297      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2298  </list>
2299</t>
2300<t>
2301  Other changes:
2302  <list style="symbols">
2303    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2304        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2305  </list>
2306</t>
2307</section>
2308
2309<section title="Since draft-ietf-httpbis-p6-cache-01">
2310<t>
2311  Closed issues:
2312  <list style="symbols">
2313    <t>
2314      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2315  </list>
2316</t>
2317<t>
2318  Other changes:
2319  <list style="symbols">
2320    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2321      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2322    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2323      specification.</t>
2324  </list>
2325</t>
2326</section>
2327
2328<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2329<t>
2330  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2331  <list style="symbols">
2332    <t>Reference RFC 3984, and update header registrations for headers defined in this
2333      document.</t>
2334  </list>
2335</t>
2336</section>
2337
2338<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2339<t>
2340  Closed issues:
2341  <list style="symbols">
2342    <t>
2343      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2344  </list>
2345</t>
2346</section>
2347
2348<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2349<t>
2350  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2351  <list style="symbols"> 
2352    <t>
2353      Use "/" instead of "|" for alternatives.
2354    </t>
2355    <t>
2356      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2357      whitespace ("OWS") and required whitespace ("RWS").
2358    </t>
2359    <t>
2360      Rewrite ABNFs to spell out whitespace rules, factor out
2361      header value format definitions.
2362    </t>
2363  </list>
2364</t>
2365</section>
2366
2367<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2368<t>
2369  This is a total rewrite of this part of the specification.
2370</t>
2371<t>
2372  Affected issues:
2373  <list style="symbols">
2374    <t>
2375      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2376    <t>
2377      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2378    <t>
2379      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2380    <t>
2381      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2382  </list>
2383</t>
2384<t>
2385  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2386  <list style="symbols"> 
2387    <t>
2388      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2389    </t>
2390  </list>
2391</t>
2392</section>
2393
2394<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2395<t>
2396  Closed issues:
2397  <list style="symbols"> 
2398    <t>
2399      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2400      "base for numeric protocol elements"
2401    </t>
2402  </list>
2403</t>
2404<t>
2405  Affected issues:
2406  <list style="symbols">
2407    <t>
2408      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2409      Vary and non-existant headers
2410    </t>
2411  </list>
2412</t>
2413</section>
2414
2415<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2416<t>
2417  Closed issues:
2418  <list style="symbols"> 
2419    <t>
2420      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2421      "Definition of 1xx Warn-Codes"
2422    </t>
2423    <t>
2424      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2425      "Content-Location on 304 responses"
2426    </t>
2427    <t>
2428      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2429      "private and no-cache CC directives with headers"
2430    </t>
2431    <t>
2432      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2433      "RFC2047 and warn-text"
2434    </t>
2435  </list>
2436</t>
2437</section>
2438
2439<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2440<t>
2441  Closed issues:
2442  <list style="symbols"> 
2443    <t>
2444      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147" />:
2445      "serving negotiated responses from cache: header-specific canonicalization"
2446    </t>
2447    <t>
2448      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2449      "Effect of CC directives on history lists"
2450    </t>
2451  </list>
2452</t>
2453<t>
2454  Affected issues:
2455  <list style="symbols">
2456    <t>
2457      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2458      Status codes and caching
2459    </t>
2460  </list>
2461</t>
2462<t>
2463  Partly resolved issues:
2464  <list style="symbols"> 
2465    <t>
2466      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2467      "Placement of 13.5.1 and 13.5.2"
2468    </t>
2469  </list>
2470</t>
2471</section>
2472
2473<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2474<t>
2475  Closed issues:
2476  <list style="symbols"> 
2477    <t>
2478      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29" />:
2479      "Age calculation"
2480    </t>
2481    <t>
2482      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168" />:
2483      "Clarify differences between / requirements for request and response CC directives"
2484    </t>
2485        <t>
2486                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174" />:
2487                "Caching authenticated responses"
2488        </t>
2489    <t>
2490      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208" />:
2491      "IANA registry for cache-control directives"
2492    </t>
2493    <t>
2494      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211" />:
2495      "Heuristic caching of URLs with query components"
2496    </t>
2497  </list>
2498</t>
2499<t>
2500  Partly resolved issues:
2501  <list style="symbols"> 
2502    <t>
2503      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2504      "Term for the requested resource's URI"
2505    </t>
2506  </list>
2507</t>
2508</section>
2509
2510<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2511<t>
2512  None yet.
2513</t>
2514</section>
2515
2516</section>
2517  </back>
2518</rfc>
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