source: draft-ietf-httpbis/latest/p6-cache.xml @ 547

Last change on this file since 547 was 547, checked in by julian.reschke@…, 11 years ago

Prepare submission of draft -06 on Monday, March 09.

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='rfc2629.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 "March">
16  <!ENTITY ID-YEAR "2009">
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 messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY conditional                 "<xref target='Part4' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY partial                     "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                   "<xref target='Part1' x:rel='#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 message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc-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 fullname="Jim Gettys" initials="J." surname="Gettys">
71    <organization>One Laptop per Child</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@laptop.org</email>
81      <uri>http://www.laptop.org/</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="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
176    <organization abbrev="greenbytes">greenbytes GmbH</organization>
177    <address>
178      <postal>
179        <street>Hafenweg 16</street>
180        <city>Muenster</city><region>NW</region><code>48155</code>
181        <country>Germany</country>
182      </postal>
183      <phone>+49 251 2807760</phone>   
184      <facsimile>+49 251 2807761</facsimile>   
185      <email>julian.reschke@greenbytes.de</email>       
186      <uri>http://greenbytes.de/tech/webdav/</uri>     
187    </address>
188  </author>
189
190  <date month="&ID-MONTH;" year="&ID-YEAR;" />
191  <workgroup>HTTPbis Working Group</workgroup>
192
193<abstract>
194<t>
195  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
196  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
197  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
198  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
199  fields that control cache behavior or indicate cacheable response messages.
200</t>
201</abstract>
202
203<note title="Editorial Note (To be removed by RFC Editor)">
204<t>
205  Discussion of this draft should take place on the HTTPBIS working group mailing list
206  (ietf-http-wg@w3.org). The current issues list is at <eref
207  target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
208  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
209</t>
210<t>
211  The changes in this draft are summarized in <xref target="changes.since.06" />.
212</t>
213</note>
214
215  </front>
216  <middle>
217
218<section anchor="caching" title="Introduction">
219<t>
220  HTTP is typically used for distributed information systems, where performance can be
221  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
222  caching and reusing response messages.
223</t>
224
225<section anchor="intro.purpose" title="Purpose">
226<iref item="cache" />
227<t>
228  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
229  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
230  in order to reduce the response time and network bandwidth consumption on future,
231  equivalent requests. Any client or server may include a cache, though a cache cannot be
232  used by a server that is acting as a tunnel.
233</t>
234<t>
235  Caching would be useless if it did not significantly improve performance. The goal of
236  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
237  some cases, a stored response can be reused without the need for a network request,
238  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
239  (see <xref target="expiration.model" />). Even when a new request is required, it is often
240  possible to reuse all or parts of the payload of a prior response to satisfy the request,
241  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
242  purpose (see <xref target="validation.model" />).
243</t>
244</section>
245
246<section anchor="intro.terminology" title="Terminology">
247<t
248  >This specification uses a number of terms to refer to the roles played by participants
249  in, and objects of, HTTP caching.
250</t>
251<t>
252  <iref item="cacheable" />
253  <x:dfn>cacheable</x:dfn>
254  <list>
255    <t>A response is cacheable if a cache is allowed to store a copy of the response message
256      for use in answering subsequent requests. Even when a response is cacheable, there may
257      be additional constraints on whether a cache can use the cached copy to satisfy a
258      particular request.</t>
259  </list>
260</t>
261<t>
262  <iref item="explicit expiration time" />
263  <x:dfn>explicit expiration time</x:dfn>
264  <list>
265    <t>The time at which the origin server intends that an entity should no longer be
266      returned by a cache without further validation.</t>
267  </list>
268</t>
269<t>
270  <iref item="heuristic expiration time" />
271  <x:dfn>heuristic expiration time</x:dfn>
272  <list>
273    <t>An expiration time assigned by a cache when no explicit expiration time is
274    available.</t>
275  </list>
276</t>
277<t>
278  <iref item="age" />
279  <x:dfn>age</x:dfn>
280  <list>
281    <t>The age of a response is the time since it was sent by, or successfully validated
282      with, the origin server.</t>
283  </list>
284</t>
285<t>
286  <iref item="first-hand" />
287  <x:dfn>first-hand</x:dfn>
288  <list>
289    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
290    0.</t>
291  </list>
292</t>
293<t>
294  <iref item="freshness lifetime" />
295  <x:dfn>freshness lifetime</x:dfn>
296  <list>
297    <t>The length of time between the generation of a response and its expiration time. </t>
298  </list>
299</t>
300<t>
301  <iref item="fresh" />
302  <x:dfn>fresh</x:dfn>
303  <list>
304    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
305  </list>
306</t>
307<t>
308  <iref item="stale" />
309  <x:dfn>stale</x:dfn>
310  <list>
311    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
312  </list>
313</t>
314<t>
315  <iref item="validator" />
316  <x:dfn>validator</x:dfn>
317  <list>
318    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
319      out whether a stored response is an equivalent copy of an entity.</t>
320  </list>
321</t>
322<t anchor="shared.and.non-shared.caches">
323  <iref item="validator" />
324  <x:dfn>shared cache</x:dfn>
325  <list>
326    <t>A cache that is accessible to more than one user. A non-shared cache is
327      dedicated to a single user.</t>
328  </list>
329</t>
330</section>
331
332<section anchor="intro.requirements" title="Requirements">
333<t>
334  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
335  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
336  described in <xref target="RFC2119" />.
337</t>
338<t>
339  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
340  or &REQUIRED; level requirements for the protocols it implements. An implementation
341  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
342  requirements for its protocols is said to be "unconditionally compliant"; one that
343  satisfies all the &MUST; level requirements but not all the &SHOULD; level
344  requirements for its protocols is said to be "conditionally compliant."
345</t>
346</section>
347
348<section title="Syntax Notation" anchor="notation">
349  <x:anchor-alias value="ALPHA"/>
350  <x:anchor-alias value="CR"/>
351  <x:anchor-alias value="DIGIT"/>
352  <x:anchor-alias value="DQUOTE"/>
353  <x:anchor-alias value="LF"/>
354  <x:anchor-alias value="OCTET"/>
355  <x:anchor-alias value="SP"/>
356  <x:anchor-alias value="VCHAR"/>
357  <x:anchor-alias value="WSP"/>
358<t>
359  This specification uses the ABNF syntax defined in &notation; (which
360  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
361  <xref target="collected.abnf"/> shows the collected ABNF, with the list
362  rule expanded.
363</t>
364<t>
365  The following core rules are included by
366  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
367  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
368  DIGIT (decimal 0-9), DQUOTE (double quote),
369  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
370  OCTET (any 8-bit sequence of data), SP (space),
371  VCHAR (any visible USASCII character),
372  and WSP (whitespace).
373</t>
374
375<section title="Core Rules" anchor="core.rules">
376  <x:anchor-alias value="quoted-string"/>
377  <x:anchor-alias value="token"/>
378  <x:anchor-alias value="OWS"/>
379<t>
380  The core rules below are defined in &basic-rules;:
381</t>
382<figure><artwork type="abnf2616">
383  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
384  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
385  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
386</artwork></figure>
387</section>
388
389<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
390  <x:anchor-alias value="field-name"/>
391  <x:anchor-alias value="HTTP-date"/>
392  <x:anchor-alias value="port"/>
393  <x:anchor-alias value="pseudonym"/>
394  <x:anchor-alias value="uri-host"/>
395<t>
396  The ABNF rules below are defined in other parts:
397</t>
398<figure><!--Part1--><artwork type="abnf2616">
399  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &message-headers;&gt;
400  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
401  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
402  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
403  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
404</artwork></figure>
405</section>
406
407</section>
408</section>
409
410<section anchor="caching.overview" title="Cache Operation">
411
412<section anchor="response.cacheability" title="Response Cacheability">
413<t>
414  A cache &MUST-NOT; store a response to any request, unless:
415  <list style="symbols">
416    <t>The request method is defined as being cacheable, and</t>
417    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
418       appear in request or response headers, and</t>
419    <t>the "private" cache response directive (see <xref target="header.cache-control" />
420       does not appear in the response, if the cache is shared, and</t>
421    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
422       the cache is shared (unless the "public" directive is present; see <xref
423       target="header.cache-control" />), and</t>           
424    <t>the cache understands partial responses, if the response is partial or incomplete
425       (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
426  </list>
427</t>
428<t>
429  Note that in normal operation, most caches will not store a response that has neither a
430  cache validator nor an explicit expiration time, as such responses are not usually
431  useful to store. However, caches are not prohibited from storing such responses.
432</t>
433
434<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
435<t>
436  A cache that receives an incomplete response (for example, with fewer bytes of data
437  than specified in a Content-Length header) can store the response, but &MUST; 
438  treat it as a partial response &partial;. Partial responses
439  can be combined as described in &combining-byte-ranges;; the result might be a
440  full response or might still be partial. A cache &MUST-NOT; return a partial
441  response to a client without explicitly marking it as such using the 206 (Partial
442  Content) status code.
443</t>
444<t>
445  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
446  incomplete or partial responses.
447</t>
448</section>
449
450</section>
451
452
453<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
454<t>
455For a presented request, a cache &MUST-NOT; return a stored response, unless:
456  <list style="symbols">
457    <t>The presented Request-URI and that of the stored response match (see
458      <cref>TBD</cref>), and</t>
459    <t>the request method associated with the stored response allows it to be
460      used for the presented request, and</t>
461    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
462      target="caching.negotiated.responses" />), and</t>
463    <t>the presented request and stored response are free from directives that would prevent
464      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
465      and</t>
466    <t>the stored response is either:
467      <list style="symbols">
468        <t>fresh (see <xref target="expiration.model" />), or</t>
469        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
470        <t>successfully validated (see <xref target="validation.model" />).</t>
471      </list>
472    </t>
473  </list>
474</t>
475<t>
476  <cref>TODO: define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
477</t>
478<t>
479  When a stored response is used to satisfy a request, caches &MUST; include a
480  single Age header field <xref target="header.age" /> in the response with a value equal to the stored response's
481  current_age; see <xref target="age.calculations" />.
482  <cref>DISCUSS: this currently includes successfully validated responses.</cref>
483</t>
484<t>
485  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
486  the origin server; i.e., A cache must not reply to such a request before having forwarded the request and having received a
487  corresponding response.
488</t>
489<t>
490  Also, note that unsafe requests might invalidate already stored responses; see
491  <xref target="invalidation.after.updates.or.deletions" />.
492</t>
493<t>
494  Caches &MUST; use the most recent response (as determined by the Date header) when
495  more than one suitable response is stored. They can also forward a request with
496  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
497  use.
498</t>
499<t>
500  <cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
501</t>
502</section>
503
504<section anchor="expiration.model" title="Freshness Model">
505<t>
506  When a response is "fresh" in the cache, it can be used to satisfy subsequent
507  requests without contacting the origin server, thereby improving efficiency.
508</t>
509<t>
510  The primary mechanism for determining freshness is for an origin server to provide an
511  explicit expiration time in the future, using either the Expires header (<xref
512  target="header.expires" />) or the max-age response cache directive (<xref
513  target="cache-response-directive" />). Generally, origin servers will assign future
514  explicit expiration times to responses in the belief that the entity is not likely to
515  change in a semantically significant way before the expiration time is reached.
516</t>
517<t>
518  If an origin server wishes to force a cache to validate every request, it can
519  assign an explicit expiration time in the past. This means that the response is always
520  stale, so that caches should validate it before using it for subsequent requests.
521  <cref>This wording may cause confusion, because the response may still be served stale.</cref>
522</t>
523<t>
524  Since origin servers do not always provide explicit expiration times, HTTP caches may
525  also assign heuristic expiration times when they are not specified, employing algorithms that
526  use other header values (such as the Last-Modified time) to estimate a plausible
527  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
528  impose worst-case constraints on their results.
529</t>
530<figure>
531<preamble>
532The calculation to determine if a response is fresh is:
533</preamble>
534<artwork type="code">
535   response_is_fresh = (freshness_lifetime &gt; current_age)
536</artwork>
537</figure>
538
539<t>
540  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
541  the current_age is defined in <xref target="age.calculations" />.
542</t>
543<t>
544  Additionally, clients may need to influence freshness calculation. They can do this using
545  several request cache directives, with the effect of either increasing or loosening
546  constraints on freshness. See <xref target="cache-request-directive" />.
547</t>
548<t>
549  <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
550  freshness.</cref>
551</t>
552<t>
553  Note that freshness applies only to cache operation; it cannot be used to force a user agent
554  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
555  the difference between caches and history mechanisms.
556</t>
557
558<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
559<t>
560  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
561  response by using the first match of:
562  <list style="symbols">
563    <t>If the cache is shared and the s-maxage response cache directive (<xref
564      target="cache-response-directive" />) is present, use its value, or</t>
565    <t>If the max-age response cache directive (<xref target="cache-response-directive"
566      />) is present, use its value, or</t>
567    <t>If the Expires response header (<xref target="header.expires" />) is present, use
568      its value minus the value of the Date response header, or</t>
569    <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
570      may be used; see <xref target="heuristic.freshness" />.</t>
571  </list>
572</t>
573<t>
574  Note that this calculation is not vulnerable to clock skew, since all of the
575  information comes from the origin server.
576</t>
577
578<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
579<t>
580  If no explicit expiration time is present in a stored response that has a status code
581  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
582  calculated. Heuristics &MUST-NOT; be used for other response status codes.
583</t>
584<t>
585  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
586  attach a Warning header with a 113 warn-code to the response if its current_age is
587  more than 24 hours and such a warning is not already present.
588</t>
589<t>
590  Also, if the response has a Last-Modified header (&header-last-modified;), the
591  heuristic expiration value &SHOULD; be no more than some fraction of the interval
592  since that time. A typical setting of this fraction might be 10%.
593</t>
594<t>
595  <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
596</t>
597</section>
598</section>
599
600<section anchor="age.calculations" title="Calculating Age">
601<t>
602  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
603  message when obtained from a cache. The Age field value is the cache's estimate of the
604  amount of time since the response was generated or validated by the origin server. In
605  essence, the Age value is the sum of the time that the response has been resident in
606  each of the caches along the path from the origin server, plus the amount of time it has
607  been in transit along network paths.
608</t>
609<t>
610  The term "age_value" denotes the value of the Age header, in a form appropriate for
611  arithmetic operations.
612</t>
613<t>
614  HTTP/1.1 requires origin servers to send a Date header, if possible, with every
615  response, giving the time at which the response was generated (see &header-date;).
616  The term "date_value" denotes the value of the Date header, in a form appropriate for
617  arithmetic operations.
618</t>
619<t>
620  The term "now" means "the current value of the clock at the host performing the
621  calculation." Hosts that use HTTP, but especially hosts running origin servers and
622  caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
623  synchronize their clocks to a globally accurate time standard.
624</t>
625<t>A response's age can be calculated in two entirely independent ways:
626  <list style="numbers">
627    <t>now minus date_value, if the local clock is reasonably well synchronized to the
628      origin server's clock. If the result is negative, the result is replaced by zero.</t>
629    <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
630  </list>
631</t>
632<figure>
633<preamble>These are combined as</preamble>
634<artwork type="code">
635    corrected_received_age = max(now - date_value, age_value)
636</artwork></figure>
637<t>
638  When an Age value is received, it &MUST; be interpreted relative to the time the
639  request was initiated, not the time that the response was received.
640</t>
641<figure><artwork type="code">
642   corrected_initial_age = corrected_received_age
643                         + (now - request_time)
644</artwork></figure>
645<t>
646  where "request_time" is the time (according to the local clock) when the request that
647  elicited this response was sent.
648</t>
649<t>
650  The current_age of a stored response can then be calculated by adding the amount of
651  time (in seconds) since the stored response was last validated by the origin server to
652  the corrected_initial_age.
653</t>
654<t>
655  In summary:
656</t>
657<figure><artwork type="code">
658  age_value     - Age header field-value received with the response
659  date_value    - Date header field-value received with the response
660  request_time  - local time when the cache made the request
661                 resulting in the stored response
662  response_time - local time when the cache received the response
663  now           - current local time
664 
665  apparent_age = max(0, response_time - date_value);
666  corrected_received_age = max(apparent_age, age_value);
667  response_delay = response_time - request_time;
668  corrected_initial_age = corrected_received_age + response_delay;
669  resident_time = now - response_time;
670  current_age   = corrected_initial_age + resident_time;
671</artwork></figure>
672</section>
673
674<section anchor="serving.stale.responses" title="Serving Stale Responses">
675<t>
676  A "stale" response is one that either has explicit expiry information, or is allowed to
677  have heuristic expiry calculated, but is not fresh according to the calculations in
678  <xref target="expiration.model" />.
679</t>
680<t>
681  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
682  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
683  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
684  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
685</t>
686<t>
687  Caches &SHOULD-NOT; return stale responses unless they are
688  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
689  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
690</t>
691<t>
692  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
693  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
694  the cache is disconnected.
695</t>
696<t>
697  If a cache receives a first-hand response (either an entire response, or a 304 (Not
698  Modified) response) that it would normally forward to the requesting client, and the
699  received response is no longer fresh, the cache &SHOULD; forward it to the
700  requesting client without adding a new Warning (but without removing any existing
701  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
702  that response became stale in transit.
703</t>
704</section>
705</section>
706
707<section anchor="validation.model" title="Validation Model">
708<t>
709  Checking with the origin server to see if a stale or otherwise unusable cached response
710  can be reused is called "validating" or "revalidating." Doing so potentially avoids
711  the overhead of retransmitting the response body when the stored response is valid.
712</t>
713<t>
714  HTTP's conditional request mechanism &conditional; is used for this purpose. When a stored
715  response includes one or more validators, such as the field values of an ETag or
716  Last-Modified header field, then a validating request &SHOULD; be made conditional
717  to those field values.
718</t>
719<t>
720  A 304 (Not Modified) response status code indicates that the stored
721  response can be updated and reused; see <xref target="combining.headers"/>.
722</t>
723<t>
724  If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
725  request and replace the stored response. <cref>Should there be a requirement here?</cref>
726</t>
727<t>
728  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
729  either forward this response to the requesting client, or act as if the server failed to
730  respond. In the latter case, it &MAY; return a previously stored response (which &SHOULD; include the
731  111 warn-code; see <xref target="header.warning"/>) unless the
732  stored response includes the "must-revalidate" cache directive (see <xref
733  target="serving.stale.responses" />).
734</t>
735</section>
736
737<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
738<t>
739  Because unsafe methods (&safe-methods;) have the potential for changing state on the
740  origin server, intervening caches can use them to keep their contents
741  up-to-date.
742</t>
743<t>
744  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
745  as the Location and Content-Location headers (if present):
746  <list style="symbols">
747    <t>PUT</t>
748    <t>DELETE</t>
749    <t>POST</t>
750  </list>
751</t>
752<t>
753  An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
754  be performed if the host part of that URI differs from the host part in the Request-URI.
755  This helps prevent denial of service attacks.
756</t>
757<t>
758  <cref>TODO: "host part" needs to be specified better.</cref>
759</t>
760<t>
761  A cache that passes through requests for methods it does not understand &SHOULD;
762  invalidate the Request-URI.
763</t>
764<t>
765  Here, "invalidate" means that the cache will either remove all stored responses related
766  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
767  before they can be returned in response to a subsequent request.
768</t>
769<t>
770  Note that this does not guarantee that all appropriate responses are invalidated. For
771  example, the request that caused the change at the origin server might not have gone
772  through the cache where a response is stored.
773</t>
774<t>
775  <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
776</t>
777</section>
778
779<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
780<t>
781  Use of server-driven content negotiation (&server-driven-negotiation;) alters
782  the conditions under which a cache can use the response for subsequent
783  requests.
784</t>
785<t>
786  When a cache receives a request that can be satisfied by a stored response
787  that includes a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that response unless
788  all of the selecting request-headers in the presented request match the corresponding
789  stored request-headers from the original request.
790</t>
791<t>
792  The selecting request-headers from two requests are defined to match if and only if the
793  selecting request-headers in the first request can be transformed to the selecting
794  request-headers in the second request by adding or removing linear white space
795  <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
796  combining multiple message-header fields with the same field name following the rules
797  about message headers in &message-headers;. <cref>DISCUSS: header-specific canonicalisation</cref>
798</t>
799<t>
800  A Vary header field-value of "*" always fails to match, and subsequent requests to that
801  resource can only be properly interpreted by the origin server.
802</t>
803<t>
804  If no stored response matches, the cache &MAY; forward the presented request to the origin
805  server in a conditional request, and &SHOULD; include all ETags stored with
806  potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
807  includes an entity tag or Content-Location that indicates the entity to be used, that
808  cached response &MUST; be used to satisfy the presented request, and &SHOULD;
809  be used to update the corresponding stored response; see <xref target="combining.headers"/>.
810</t>
811<t>
812  If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT; 
813  be included in the If-None-Match header field unless the request is for a range that would
814  be fully satisfied by that stored response.
815</t>
816<t>
817  If a cache receives a successful response whose Content-Location field matches that of an
818  existing stored response for the same Request-URI, whose entity-tag differs from that of
819  the existing stored response, and whose Date is more recent than that of the existing
820  response, the existing response &SHOULD-NOT; be returned in response to future
821  requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref>
822</t>
823</section>
824
825<section anchor="combining.headers" title="Combining Responses">
826<t>
827  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
828  it needs to update the stored response with the new one, so that the updated response can
829  be sent to the client.
830</t>
831<t>
832  If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
833  the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
834  Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
835  the stored response with the updated entity-body received in the response and use the
836  result as the updated entity-body (see &combining-byte-ranges;).
837</t>
838<t>
839  The stored response headers are used for the updated response, except that
840  <list style="symbols">
841    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
842      &MUST; be deleted from the stored response and the forwarded response.</t>
843    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
844      response and the forwarded response.</t>
845    <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
846      headers from the stored response.</t>
847  </list>
848</t>
849<t>
850  A cache &MUST; also replace any stored headers with corresponding headers received in the
851  incoming response, except for Warning headers as described immediately above. If a header
852  field-name in the incoming response matches more than one header in the stored response,
853  all such old headers &MUST; be replaced. It &MAY; store the combined
854  entity-body.
855</t>
856<t>
857  <cref>ISSUE: discuss how to handle HEAD updates</cref>
858</t>
859</section>
860
861</section>
862
863<section anchor="header.fields" title="Header Field Definitions">
864<t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
865<t>For entity-header fields, both sender and recipient refer to either the client or the
866server, depending on who sends and who receives the entity.</t>
867
868<section anchor="header.age" title="Age">
869  <iref item="Age header" primary="true" x:for-anchor="" />
870  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
871  <x:anchor-alias value="Age"/>
872  <x:anchor-alias value="Age-v"/>
873  <x:anchor-alias value="age-value"/>
874<t>
875  The response-header field "Age" conveys the sender's estimate of the amount of time since
876  the response (or its validation) was generated at the origin server. Age values are
877  calculated as specified in <xref target="age.calculations" />.
878</t>
879<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
880  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
881  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
882</artwork></figure>
883<t anchor="rule.delta-seconds">
884  <x:anchor-alias value="delta-seconds" />
885  Age field-values are non-negative decimal integers, representing time in seconds.
886</t>
887<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
888  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
889</artwork></figure>
890<t>
891  If a cache receives a value larger than the largest positive integer it can represent, or
892  if any of its age calculations overflows, it &MUST; transmit an Age header with a
893  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
894  of at least 31 bits of range.
895</t>
896<t>
897  The presence of an Age header field in a response implies that a response is not
898  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
899  Age header field.
900</t>
901</section>
902
903<section anchor="header.cache-control" title="Cache-Control">
904  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
905  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
906  <x:anchor-alias value="Cache-Control"/>
907  <x:anchor-alias value="Cache-Control-v"/>
908  <x:anchor-alias value="cache-directive"/>
909  <x:anchor-alias value="cache-extension"/>
910  <x:anchor-alias value="cache-request-directive"/>
911  <x:anchor-alias value="cache-response-directive"/>
912<t>
913  The general-header field "Cache-Control" is used to specify directives that &MUST; be
914  obeyed by all caches along the request/response chain. The directives specify behavior
915  intended to prevent caches from adversely interfering with the request or response. Cache
916  directives are unidirectional in that the presence of a directive in a request does not
917  imply that the same directive is to be given in the response.
918</t>
919<x:note>
920  <t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
921  Pragma: no-cache (see <xref target="header.pragma" />).</t>
922</x:note>
923<t>
924  Cache directives &MUST; be passed through by a proxy or gateway application,
925  regardless of their significance to that application, since the directives might be
926  applicable to all recipients along the request/response chain. It is not possible to
927  target a directive to a specific cache.
928</t>
929<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"/>
930  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
931  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
932
933  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
934     / <x:ref>cache-response-directive</x:ref>
935
936  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
937</artwork></figure>
938
939<section anchor="cache-request-directive" title="Request Cache-Control Directives">
940  <x:anchor-alias value="cache-request-directive" />
941
942<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
943  <x:ref>cache-request-directive</x:ref> =
944       "no-cache"
945     / "no-store"
946     / "max-age" "=" <x:ref>delta-seconds</x:ref>
947     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
948     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
949     / "no-transform"
950     / "only-if-cached"
951     / <x:ref>cache-extension</x:ref>
952</artwork></figure>
953
954<t>
955  <iref item="Cache Directives" primary="true" subitem="no-cache" />
956  <iref item="no-cache" primary="true" subitem="Cache Directive" />
957  no-cache
958  <list>
959    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
960      used to satisfy the request without successful validation on the origin server.</t>
961  </list>
962</t>
963<t>
964  <iref item="Cache Directives" primary="true" subitem="no-store" />
965  <iref item="no-store" primary="true" subitem="Cache Directive" />
966  no-store
967  <list>
968    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
969      of either this request or any response to it. This directive applies to both
970      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
971      cache &MUST-NOT; intentionally store the information in non-volatile storage,
972      and &MUST; make a best-effort attempt to remove the information from volatile
973      storage as promptly as possible after forwarding it.</t>
974    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
975      particular, malicious or compromised caches might not recognize or obey this
976      directive, and communications networks may be vulnerable to eavesdropping.</t>
977  </list>
978</t>
979<t>
980  <iref item="Cache Directives" primary="true" subitem="max-age" />
981  <iref item="max-age" primary="true" subitem="Cache Directive" />
982  max-age
983  <list>
984    <t>The max-age request directive indicates that the client is willing to accept a
985      response whose age is no greater than the specified time in seconds. Unless
986      max-stale directive is also included, the client is not willing to accept a stale
987      response.</t>
988  </list>
989</t>
990<t>
991  <iref item="Cache Directives" primary="true" subitem="max-stale" />
992  <iref item="max-stale" primary="true" subitem="Cache Directive" />
993  max-stale
994  <list>
995    <t>The max-stale request directive indicates that the client is willing to accept a
996      response that has exceeded its expiration time. If max-stale is assigned a value,
997      then the client is willing to accept a response that has exceeded its expiration
998      time by no more than the specified number of seconds. If no value is assigned to
999      max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
1000  </list>
1001</t>
1002<t>
1003  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1004  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1005  min-fresh
1006  <list>
1007    <t>The min-fresh request directive indicates that the client is willing to accept a
1008      response whose freshness lifetime is no less than its current age plus the specified
1009      time in seconds. That is, the client wants a response that will still be fresh for
1010      at least the specified number of seconds.</t>
1011  </list>
1012</t>
1013<t>
1014  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1015  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1016  no-transform
1017  <list>
1018    <t>The no-transform request directive indicates that an intermediate cache or proxy
1019      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1020      headers, nor the request entity-body.</t>
1021  </list>
1022</t>
1023<t>
1024  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1025  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1026  only-if-cached
1027  <list>
1028    <t>The only-if-cached request directive indicates that the client only wishes to
1029      return a stored response. If it receives this directive, a cache &SHOULD; either
1030      respond using a stored response that is consistent with the other constraints of the
1031      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1032      being operated as a unified system with good internal connectivity, such a request
1033      &MAY; be forwarded within that group of caches.</t>
1034  </list>
1035</t>
1036</section>
1037
1038<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1039  <x:anchor-alias value="cache-response-directive" />
1040
1041<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1042  <x:ref>cache-response-directive</x:ref> =
1043       "public"
1044     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1045     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1046     / "no-store"
1047     / "no-transform"
1048     / "must-revalidate"
1049     / "proxy-revalidate"
1050     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1051     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1052     / <x:ref>cache-extension</x:ref>
1053</artwork></figure>
1054
1055<t>
1056  <iref item="Cache Directives" primary="true" subitem="public" />
1057  <iref item="public" primary="true" subitem="Cache Directive" />
1058  public
1059  <list>
1060    <t>The public response directive indicates that the response &MAY; be cached, even
1061      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1062      (See also Authorization, &header-authorization;, for additional details.) </t>
1063  </list>
1064</t>
1065<t>
1066  <iref item="Cache Directives" primary="true" subitem="private" />
1067  <iref item="private" primary="true" subitem="Cache Directive" />
1068  private
1069  <list>
1070    <t>The private response directive indicates that the response message is intended for
1071      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1072      cache &MAY; store the response.</t>
1073    <t>If the private response directive specifies one or more field-names, this
1074      requirement is limited to the field-values associated with the listed response
1075      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1076      cache, whereas the remainder of the response message &MAY; be.</t>
1077    <t>
1078      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1079      be stored, and cannot ensure the privacy of the message content.</t>
1080  </list>
1081</t>
1082<t>
1083  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1084  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1085  no-cache
1086  <list>
1087    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1088      satisfy a subsequent request without successful validation on the origin server.
1089      This allows an origin server to prevent caching even by caches that have been
1090      configured to return stale responses.</t>
1091    <t>If the no-cache response directive specifies one or more field-names, this
1092      requirement is limited to the field-values assosicated with the listed response
1093      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1094      to a subsequent request without successful validation on the origin server. This
1095      allows an origin server to prevent the re-use of certain header fields in a
1096      response, while still allowing caching of the rest of the response.</t>
1097    <t>
1098      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1099    </t>
1100  </list>
1101</t>
1102
1103<t>
1104  <iref item="Cache Directives" primary="true" subitem="no-store" />
1105  <iref item="no-store" primary="true" subitem="Cache Directive" />
1106  no-store
1107  <list>
1108    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1109      part of either the immediate request or response. This directive applies to both
1110      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1111      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1112      and &MUST; make a best-effort attempt to remove the information from volatile
1113      storage as promptly as possible after forwarding it.</t>
1114    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1115      particular, malicious or compromised caches might not recognize or obey this
1116      directive, and communications networks may be vulnerable to eavesdropping.</t>
1117  </list>
1118</t>
1119<t>
1120  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1121  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1122  must-revalidate
1123  <list>
1124    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1125     used to satisfy subsequent requests without successful validation on the origin server.</t>
1126    <t>The must-revalidate directive is necessary to support reliable operation for
1127      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1128      the must-revalidate directive; in particular, if the cache cannot reach the origin
1129      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1130    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1131      validate a request on the entity could result in incorrect operation, such as a
1132      silently unexecuted financial transaction.</t>
1133  </list>
1134</t>
1135<t>
1136  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1137  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1138  proxy-revalidate
1139  <list>
1140    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1141      response directive, except that it does not apply to non-shared caches.</t>
1142  </list>
1143</t>
1144<t>
1145  <iref item="Cache Directives" primary="true" subitem="max-age" />
1146  <iref item="max-age" primary="true" subitem="Cache Directive" />
1147  max-age
1148  <list>
1149    <t>The max-age response directive indicates that response is to be considered stale
1150      after its age is greater than the specified number of seconds.</t>
1151  </list>
1152</t>
1153<t>
1154  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1155  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1156  s-maxage
1157  <list>
1158    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1159      specified by this directive overrides the maximum age specified by either the
1160      max-age directive or the Expires header. The s-maxage directive also implies the
1161      semantics of the proxy-revalidate response directive.</t>
1162  </list>
1163</t>
1164<t>
1165  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1166  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1167  no-transform
1168  <list>
1169    <t>The no-transform response directive indicates that an intermediate cache or proxy
1170      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1171      headers, nor the response entity-body.</t>
1172  </list>
1173</t>
1174
1175</section>
1176
1177<section anchor="cache.control.extensions" title="Cache Control Extensions">
1178<t>
1179  The Cache-Control header field can be extended through the use of one or more
1180  cache-extension tokens, each with an optional value. Informational extensions (those
1181  that do not require a change in cache behavior) can be added without changing the
1182  semantics of other directives. Behavioral extensions are designed to work by acting as
1183  modifiers to the existing base of cache directives. Both the new directive and the
1184  standard directive are supplied, such that applications that do not understand the new
1185  directive will default to the behavior specified by the standard directive, and those
1186  that understand the new directive will recognize it as modifying the requirements
1187  associated with the standard directive. In this way, extensions to the cache-control
1188  directives can be made without requiring changes to the base protocol.
1189</t>
1190<t>
1191  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1192  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1193  all directives that it does not understand.
1194</t>
1195<t>
1196  For example, consider a hypothetical new response directive called "community" that
1197  acts as a modifier to the private directive. We define this new directive to mean that,
1198  in addition to any non-shared cache, any cache that is shared only by members of the
1199  community named within its value may cache the response. An origin server wishing to
1200  allow the UCI community to use an otherwise private response in their shared cache(s)
1201  could do so by including
1202</t>
1203<figure><artwork type="example">
1204  Cache-Control: private, community="UCI"
1205</artwork></figure>
1206<t>
1207  A cache seeing this header field will act correctly even if the cache does not
1208  understand the community cache-extension, since it will also see and understand the
1209  private directive and thus default to the safe behavior.
1210</t>
1211<t>
1212  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1213  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1214  directives (or the response's default cacheability) such that the cache behavior will
1215  remain minimally correct even if the cache does not understand the extension(s).
1216</t>
1217</section>
1218
1219</section>
1220
1221<section anchor="header.expires" title="Expires">
1222  <iref item="Expires header" primary="true" x:for-anchor="" />
1223  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1224  <x:anchor-alias value="Expires"/>
1225  <x:anchor-alias value="Expires-v"/>
1226<t>
1227  The entity-header field "Expires" gives the date/time after which the response is
1228  considered stale. See <xref target="expiration.model" /> for further discussion of the
1229  freshness model.
1230</t>
1231<t>
1232  The presence of an Expires field does not imply that the original resource will change or
1233  cease to exist at, before, or after that time.
1234</t>
1235<t>
1236  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1237  it &MUST; be sent in rfc1123-date format.
1238</t>
1239<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1240  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1241  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1242</artwork></figure>
1243<figure>
1244  <preamble>For example</preamble>
1245<artwork type="example">
1246  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1247</artwork></figure>
1248<x:note>
1249    <t>
1250      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1251      directive (see <xref target="cache-response-directive" />), that directive overrides
1252      the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.</t>
1253</x:note>
1254<t>
1255  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1256</t>
1257<t>
1258  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1259  including the value "0", as in the past (i.e., "already expired").
1260</t>
1261</section>
1262
1263<section anchor="header.pragma" title="Pragma">
1264  <iref item="Pragma header" primary="true" x:for-anchor="" />
1265  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1266  <x:anchor-alias value="extension-pragma"/>
1267  <x:anchor-alias value="Pragma"/>
1268  <x:anchor-alias value="Pragma-v"/>
1269  <x:anchor-alias value="pragma-directive"/>
1270<t
1271  >The general-header field "Pragma" is used to include implementation-specific directives
1272  that might apply to any recipient along the request/response chain. All pragma directives
1273  specify optional behavior from the viewpoint of the protocol; however, some systems
1274  &MAY; require that behavior be consistent with the directives.
1275</t>
1276<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"/>
1277  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1278  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1279  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1280  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1281</artwork></figure>
1282<t>
1283  When the no-cache directive is present in a request message, an application &SHOULD;
1284  forward the request toward the origin server even if it has a cached copy of what is being
1285  requested. This pragma directive has the same semantics as the no-cache response directive
1286  (see <xref target="cache-response-directive" />) and is defined here for backward
1287  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1288  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1289  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1290</t>
1291<x:note>
1292  <t>
1293    <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1294    is not actually specified, it does not provide a reliable replacement for
1295    "Cache-Control: no-cache" in a response.</t>
1296</x:note>
1297<t>
1298  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1299</t>
1300</section>
1301
1302<section anchor="header.vary" title="Vary">
1303  <iref item="Vary header" primary="true" x:for-anchor="" />
1304  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1305  <x:anchor-alias value="Vary"/>
1306  <x:anchor-alias value="Vary-v"/>
1307<t>
1308  The "Vary" response-header field's value indicates the set of request-header fields that
1309  determines, while the response is fresh, whether a cache is permitted to use the
1310  response to reply to a subsequent request without validation; see <xref 
1311  target="caching.negotiated.responses" />.
1312</t>
1313<t>
1314  In uncacheable or stale responses, the Vary field value advises the user agent about
1315  the criteria that were used to select the representation.
1316</t>
1317<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1318  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1319  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1320</artwork></figure>
1321<t>
1322  The set of header fields named by the Vary field value is known as the selecting
1323  request-headers.
1324</t>
1325<t>
1326  Servers &SHOULD; include a Vary header field with any cacheable response that is
1327  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1328  requests on that resource and informs the user agent about the presence of negotiation on
1329  that resource. A server &MAY; include a Vary header field with a non-cacheable
1330  response that is subject to server-driven negotiation, since this might provide the user
1331  agent with useful information about the dimensions over which the response varies at the
1332  time of the response.
1333</t>
1334<t>
1335  A Vary field value of "*" signals that unspecified parameters not limited to the
1336  request-headers (e.g., the network address of the client), play a role in the selection of
1337  the response representation; therefore, a cache cannot determine whether this response is
1338  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1339  it may only be generated by an origin server.
1340</t>
1341<t>
1342  The field-names given are not limited to the set of standard request-header fields
1343  defined by this specification. Field names are case-insensitive.
1344</t>
1345</section>
1346
1347<section anchor="header.warning" title="Warning">
1348  <iref item="Warning header" primary="true" x:for-anchor="" />
1349  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1350  <x:anchor-alias value="Warning"/>
1351  <x:anchor-alias value="Warning-v"/>
1352  <x:anchor-alias value="warning-value"/>
1353  <x:anchor-alias value="warn-agent"/>
1354  <x:anchor-alias value="warn-code"/>
1355  <x:anchor-alias value="warn-date"/>
1356  <x:anchor-alias value="warn-text"/>
1357<t>
1358  The general-header field "Warning" is used to carry additional information about the status
1359  or transformation of a message that might not be reflected in the message. This
1360  information is typically used to warn about possible incorrectness introduced by caching
1361  operations or transformations applied to the entity body of the message.
1362</t>
1363<t>
1364  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1365  warning, rather than an error status code, distinguish these responses from true failures.
1366</t>
1367<t>
1368  Warning headers can in general be applied to any message, however some warn-codes are
1369  specific to caches and can only be applied to response messages.
1370</t>
1371<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"/>
1372  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1373  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1374 
1375  <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>
1376                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1377 
1378  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1379  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1380                  ; the name or pseudonym of the server adding
1381                  ; the Warning header, for use in debugging
1382  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1383  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1384</artwork></figure>
1385<t>
1386  Multiple warnings can be attached to a response (either by the origin server or by
1387  a cache), including multiple warnings with the same code number. For example, a server
1388  might provide the same warning with texts in both English and Basque.
1389</t>
1390<t>
1391  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1392  possible, in the order that they appear in the response. If it is not possible to inform
1393  the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
1394  <list style="symbols">
1395    <t>Warnings that appear early in the response take priority over those appearing later
1396      in the response.</t>
1397    <t>Warnings in the user's preferred character set take priority over warnings in other
1398      character sets but with identical warn-codes and warn-agents.</t>
1399  </list>
1400</t>
1401<t>
1402  Systems that generate multiple Warning headers &SHOULD; order them with this user
1403  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1404  Warning headers.
1405</t>
1406<t>
1407  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1408  Warning is required to be deleted from a stored response after validation:
1409  <list style="symbols">
1410    <t>1xx Warnings that describe the freshness or validation status of the response, and so
1411      &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
1412      except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
1413    <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1414      not rectified by a validation (for example, a lossy compression of the entity bodies)
1415      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1416      returned, in which case they &MUST; be.</t>
1417  </list>
1418</t>
1419<t>
1420  The warn-text &SHOULD; be in a natural language and character set that is most likely
1421  to be intelligible to the human user receiving the response. This decision can be based on
1422  any available knowledge, such as the location of the cache or user, the Accept-Language
1423  field in a request, the Content-Language field in a response, etc. The default language is
1424  English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).
1425</t>
1426<t>
1427  If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
1428  warn-text using the method described in <xref target="RFC2047" />.
1429</t>
1430<t>
1431  If an implementation sends a message with one or more Warning headers to a receiver whose
1432  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1433  warn-date that matches the Date header in the message.
1434</t>
1435<t>
1436  If an implementation receives a message with a warning-value that includes a warn-date,
1437  and that warn-date is different from the Date value in the response, then that
1438  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1439  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1440  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1441  well.
1442</t>
1443<t>
1444  The following warn-codes are defined by this specification, each with a recommended
1445  warn-text in English, and a description of its meaning.
1446</t>
1447<t><?rfc needLines="4"?>
1448  110 Response is stale
1449  <list>
1450    <t>&SHOULD; be included whenever the returned response is stale.</t>
1451  </list>
1452</t>
1453<t><?rfc needLines="4"?>
1454  111 Revalidation failed
1455  <list>
1456    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1457      validate the response failed, due to an inability to reach the server.</t>
1458  </list>
1459</t>
1460<t><?rfc needLines="4"?>
1461  112 Disconnected operation
1462  <list>
1463    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1464      the network for a period of time.</t>
1465  </list>
1466</t>
1467<t><?rfc needLines="4"?>
1468  113 Heuristic expiration
1469  <list>
1470    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1471      greater than 24 hours and the response's age is greater than 24 hours.</t>
1472  </list>
1473</t>
1474<t><?rfc needLines="4"?>
1475  199 Miscellaneous warning
1476  <list>
1477    <t>The warning text can include arbitrary information to be presented to a human
1478      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1479      action, besides presenting the warning to the user.</t>
1480  </list>
1481</t>
1482<t><?rfc needLines="4"?>
1483  214 Transformation applied
1484  <list>
1485    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1486      transformation changing the content-coding (as specified in the Content-Encoding
1487      header) or media-type (as specified in the Content-Type header) of the response, or
1488      the entity-body of the response, unless this Warning code already appears in the
1489      response.</t>
1490  </list>
1491</t>
1492<t><?rfc needLines="4"?>
1493  299 Miscellaneous persistent warning
1494  <list>
1495    <t>The warning text can include arbitrary information to be presented to a human
1496      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1497      action.</t>
1498  </list>
1499</t>
1500</section>
1501
1502</section>
1503
1504<section anchor="history.lists" title="History Lists">
1505<t>
1506  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1507  can be used to redisplay an entity retrieved earlier in a session.
1508</t>
1509<t>
1510  History mechanisms and caches are different. In particular history mechanisms
1511  &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1512  history mechanism is meant to show exactly what the user saw at the time when the resource
1513  was retrieved.
1514  </t>
1515<t>
1516  By default, an expiration time does not apply to history mechanisms. If the entity is still
1517  in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1518  unless the user has specifically configured the agent to refresh expired history documents.
1519</t>
1520<t>
1521  This is not to be construed to prohibit the history mechanism from telling the user that a
1522  view might be stale.
1523  <list>
1524    <t>
1525      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1526      stale resources, this will tend to force service authors to avoid using HTTP expiration
1527      controls and cache controls when they would otherwise like to. Service authors may
1528      consider it important that users not be presented with error messages or warning
1529      messages when they use navigation controls (such as BACK) to view previously fetched
1530      resources. Even though sometimes such resources ought not be cached, or ought to expire
1531      quickly, user interface considerations may force service authors to resort to other
1532      means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1533      of improperly functioning history mechanisms.</t>
1534  </list>
1535</t>
1536</section>
1537
1538
1539<section anchor="IANA.considerations" title="IANA Considerations">
1540
1541<section anchor="message.header.registration" title="Message Header Registration">
1542<t>
1543  The Message Header Registry located at <eref
1544  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1545  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1546</t>
1547
1548<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1549<texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1550  <ttcol>Header Field Name</ttcol>
1551  <ttcol>Protocol</ttcol>
1552  <ttcol>Status</ttcol>
1553  <ttcol>Reference</ttcol>
1554
1555  <c>Age</c>
1556  <c>http</c>
1557  <c>standard</c>
1558  <c>
1559    <xref target="header.age" />
1560  </c>
1561
1562  <c>Cache-Control</c>
1563  <c>http</c>
1564  <c>standard</c>
1565  <c>
1566    <xref target="header.cache-control" />
1567  </c>
1568
1569  <c>Expires</c>
1570  <c>http</c>
1571  <c>standard</c>
1572  <c>
1573    <xref target="header.expires" />
1574  </c>
1575
1576  <c>Pragma</c>
1577  <c>http</c>
1578  <c>standard</c>
1579  <c>
1580    <xref target="header.pragma" />
1581  </c>
1582
1583  <c>Vary</c>
1584  <c>http</c>
1585  <c>standard</c>
1586  <c>
1587    <xref target="header.vary" />
1588  </c>
1589
1590  <c>Warning</c>
1591  <c>http</c>
1592  <c>standard</c>
1593  <c>
1594    <xref target="header.warning" />
1595  </c>
1596</texttable>
1597<!--(END)-->
1598<t>
1599  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1600</t>
1601</section>
1602
1603</section>
1604
1605<section anchor="security.considerations" title="Security Considerations">
1606<t>
1607  Caches expose additional potential vulnerabilities, since the contents of the cache
1608  represent an attractive target for malicious exploitation. Because cache contents persist
1609  after an HTTP request is complete, an attack on the cache can reveal information long after
1610  a user believes that the information has been removed from the network. Therefore, cache
1611  contents should be protected as sensitive information.
1612</t>
1613</section>
1614
1615<section anchor="ack" title="Acknowledgments">
1616<t>
1617  Much of the content and presentation of the caching design is due to suggestions and
1618  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1619  and Larry Masinter.
1620</t>
1621</section>
1622
1623</middle>
1624
1625<back>
1626<references title="Normative References">
1627
1628  <reference anchor="ISO-8859-1">
1629    <front>
1630      <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1631        1: Latin alphabet No. 1 </title>
1632      <author>
1633        <organization>International Organization for Standardization</organization>
1634      </author>
1635      <date year="1998" />
1636    </front>
1637    <seriesInfo name="ISO/IEC" value="8859-1:1998" />
1638  </reference>
1639
1640  <reference anchor="Part1">
1641    <front>
1642      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1643      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1644        <organization abbrev="Day Software">Day Software</organization>
1645        <address><email>fielding@gbiv.com</email></address>
1646      </author>
1647      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1648        <organization>One Laptop per Child</organization>
1649        <address><email>jg@laptop.org</email></address>
1650      </author>
1651      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1652        <organization abbrev="HP">Hewlett-Packard Company</organization>
1653        <address><email>JeffMogul@acm.org</email></address>
1654      </author>
1655      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1656        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1657        <address><email>henrikn@microsoft.com</email></address>
1658      </author>
1659      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1660        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1661        <address><email>LMM@acm.org</email></address>
1662      </author>
1663      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1664        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1665        <address><email>paulle@microsoft.com</email></address>
1666      </author>
1667      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1668        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1669        <address><email>timbl@w3.org</email></address>
1670      </author>
1671      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1672        <organization abbrev="W3C">World Wide Web Consortium</organization>
1673        <address><email>ylafon@w3.org</email></address>
1674      </author>
1675      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1676        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1677        <address><email>julian.reschke@greenbytes.de</email></address>
1678      </author>
1679      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1680    </front>
1681    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1682    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1683  </reference>
1684
1685  <reference anchor="Part2">
1686    <front>
1687      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1688      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1689        <organization abbrev="Day Software">Day Software</organization>
1690        <address><email>fielding@gbiv.com</email></address>
1691      </author>
1692      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1693        <organization>One Laptop per Child</organization>
1694        <address><email>jg@laptop.org</email></address>
1695      </author>
1696      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1697        <organization abbrev="HP">Hewlett-Packard Company</organization>
1698        <address><email>JeffMogul@acm.org</email></address>
1699      </author>
1700      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1701        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1702        <address><email>henrikn@microsoft.com</email></address>
1703      </author>
1704      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1705        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1706        <address><email>LMM@acm.org</email></address>
1707      </author>
1708      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1709        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1710        <address><email>paulle@microsoft.com</email></address>
1711      </author>
1712      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1713        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1714        <address><email>timbl@w3.org</email></address>
1715      </author>
1716      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1717        <organization abbrev="W3C">World Wide Web Consortium</organization>
1718        <address><email>ylafon@w3.org</email></address>
1719      </author>
1720      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1721        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1722        <address><email>julian.reschke@greenbytes.de</email></address>
1723      </author>
1724      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1725    </front>
1726    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1727    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1728  </reference>
1729
1730  <reference anchor="Part3">
1731    <front>
1732      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1733      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1734        <organization abbrev="Day Software">Day Software</organization>
1735        <address><email>fielding@gbiv.com</email></address>
1736      </author>
1737      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1738        <organization>One Laptop per Child</organization>
1739        <address><email>jg@laptop.org</email></address>
1740      </author>
1741      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1742        <organization abbrev="HP">Hewlett-Packard Company</organization>
1743        <address><email>JeffMogul@acm.org</email></address>
1744      </author>
1745      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1746        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1747        <address><email>henrikn@microsoft.com</email></address>
1748      </author>
1749      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1750        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1751        <address><email>LMM@acm.org</email></address>
1752      </author>
1753      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1754        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1755        <address><email>paulle@microsoft.com</email></address>
1756      </author>
1757      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1758        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1759        <address><email>timbl@w3.org</email></address>
1760      </author>
1761      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1762        <organization abbrev="W3C">World Wide Web Consortium</organization>
1763        <address><email>ylafon@w3.org</email></address>
1764      </author>
1765      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1766        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1767        <address><email>julian.reschke@greenbytes.de</email></address>
1768      </author>
1769      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1770    </front>
1771    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1772    <x:source basename="p3-payload" href="p3-payload.xml" />
1773  </reference>
1774
1775  <reference anchor="Part4">
1776    <front>
1777      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1778      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1779        <organization abbrev="Day Software">Day Software</organization>
1780        <address><email>fielding@gbiv.com</email></address>
1781      </author>
1782      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1783        <organization>One Laptop per Child</organization>
1784        <address><email>jg@laptop.org</email></address>
1785      </author>
1786      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1787        <organization abbrev="HP">Hewlett-Packard Company</organization>
1788        <address><email>JeffMogul@acm.org</email></address>
1789      </author>
1790      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1791        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1792        <address><email>henrikn@microsoft.com</email></address>
1793      </author>
1794      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1795        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1796        <address><email>LMM@acm.org</email></address>
1797      </author>
1798      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1799        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1800        <address><email>paulle@microsoft.com</email></address>
1801      </author>
1802      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1803        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1804        <address><email>timbl@w3.org</email></address>
1805      </author>
1806      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1807        <organization abbrev="W3C">World Wide Web Consortium</organization>
1808        <address><email>ylafon@w3.org</email></address>
1809      </author>
1810      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1811        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1812        <address><email>julian.reschke@greenbytes.de</email></address>
1813      </author>
1814      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1815    </front>
1816    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1817    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1818  </reference>
1819
1820  <reference anchor="Part5">
1821    <front>
1822      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1823      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1824        <organization abbrev="Day Software">Day Software</organization>
1825        <address><email>fielding@gbiv.com</email></address>
1826      </author>
1827      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1828        <organization>One Laptop per Child</organization>
1829        <address><email>jg@laptop.org</email></address>
1830      </author>
1831      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1832        <organization abbrev="HP">Hewlett-Packard Company</organization>
1833        <address><email>JeffMogul@acm.org</email></address>
1834      </author>
1835      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1836        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1837        <address><email>henrikn@microsoft.com</email></address>
1838      </author>
1839      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1840        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1841        <address><email>LMM@acm.org</email></address>
1842      </author>
1843      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1844        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1845        <address><email>paulle@microsoft.com</email></address>
1846      </author>
1847      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1848        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1849        <address><email>timbl@w3.org</email></address>
1850      </author>
1851      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1852        <organization abbrev="W3C">World Wide Web Consortium</organization>
1853        <address><email>ylafon@w3.org</email></address>
1854      </author>
1855      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1856        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1857        <address><email>julian.reschke@greenbytes.de</email></address>
1858      </author>
1859      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1860    </front>
1861    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1862    <x:source basename="p5-range" href="p5-range.xml" />
1863  </reference>
1864
1865  <reference anchor="Part7">
1866    <front>
1867      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1868      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1869        <organization abbrev="Day Software">Day Software</organization>
1870        <address><email>fielding@gbiv.com</email></address>
1871      </author>
1872      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1873        <organization>One Laptop per Child</organization>
1874        <address><email>jg@laptop.org</email></address>
1875      </author>
1876      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1877        <organization abbrev="HP">Hewlett-Packard Company</organization>
1878        <address><email>JeffMogul@acm.org</email></address>
1879      </author>
1880      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1881        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1882        <address><email>henrikn@microsoft.com</email></address>
1883      </author>
1884      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1885        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1886        <address><email>LMM@acm.org</email></address>
1887      </author>
1888      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1889        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1890        <address><email>paulle@microsoft.com</email></address>
1891      </author>
1892      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1893        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1894        <address><email>timbl@w3.org</email></address>
1895      </author>
1896      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1897        <organization abbrev="W3C">World Wide Web Consortium</organization>
1898        <address><email>ylafon@w3.org</email></address>
1899      </author>
1900      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1901        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1902        <address><email>julian.reschke@greenbytes.de</email></address>
1903      </author>
1904      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1905    </front>
1906    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1907    <x:source basename="p7-auth" href="p7-auth.xml" />
1908  </reference>
1909
1910  <reference anchor="RFC2047">
1911    <front>
1912      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1913        Part Three: Message Header Extensions for Non-ASCII Text</title>
1914      <author fullname="Keith Moore" initials="K." surname="Moore">
1915        <organization>University of Tennessee</organization>
1916        <address><email>moore@cs.utk.edu</email></address>
1917      </author>
1918      <date month="November" year="1996" />
1919    </front>
1920    <seriesInfo name="RFC" value="2047" />
1921  </reference>
1922
1923  <reference anchor="RFC2119">
1924    <front>
1925      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1926      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1927        <organization>Harvard University</organization>
1928        <address><email>sob@harvard.edu</email></address>
1929      </author>
1930      <date month="March" year="1997" />
1931    </front>
1932    <seriesInfo name="BCP" value="14" />
1933    <seriesInfo name="RFC" value="2119" />
1934  </reference>
1935
1936  <reference anchor="RFC5234">
1937    <front>
1938      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1939      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1940        <organization>Brandenburg InternetWorking</organization>
1941        <address>
1942        <postal>
1943        <street>675 Spruce Dr.</street>
1944        <city>Sunnyvale</city>
1945        <region>CA</region>
1946        <code>94086</code>
1947        <country>US</country></postal>
1948        <phone>+1.408.246.8253</phone>
1949        <email>dcrocker@bbiw.net</email></address> 
1950      </author>
1951      <author initials="P." surname="Overell" fullname="Paul Overell">
1952        <organization>THUS plc.</organization>
1953        <address>
1954        <postal>
1955        <street>1/2 Berkeley Square</street>
1956        <street>99 Berkely Street</street>
1957        <city>Glasgow</city>
1958        <code>G3 7HR</code>
1959        <country>UK</country></postal>
1960        <email>paul.overell@thus.net</email></address>
1961      </author>
1962      <date month="January" year="2008"/>
1963    </front>
1964    <seriesInfo name="STD" value="68"/>
1965    <seriesInfo name="RFC" value="5234"/>
1966  </reference>
1967 
1968</references>
1969
1970<references title="Informative References">
1971
1972  <reference anchor="RFC1305">
1973    <front>
1974      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1975      <author fullname="David L. Mills" initials="D." surname="Mills">
1976        <organization>University of Delaware, Electrical Engineering Department</organization>
1977        <address><email>mills@udel.edu</email></address>
1978      </author>
1979      <date month="March" year="1992" />
1980    </front>
1981    <seriesInfo name="RFC" value="1305" />
1982  </reference>
1983
1984  <reference anchor="RFC2616">
1985    <front>
1986      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1987      <author fullname="R. Fielding" initials="R." surname="Fielding">
1988        <organization>University of California, Irvine</organization>
1989        <address><email>fielding@ics.uci.edu</email></address>
1990      </author>
1991      <author fullname="J. Gettys" initials="J." surname="Gettys">
1992        <organization>W3C</organization>
1993        <address><email>jg@w3.org</email></address>
1994      </author>
1995      <author fullname="J. Mogul" initials="J." surname="Mogul">
1996        <organization>Compaq Computer Corporation</organization>
1997        <address><email>mogul@wrl.dec.com</email></address>
1998      </author>
1999      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2000        <organization>MIT Laboratory for Computer Science</organization>
2001        <address><email>frystyk@w3.org</email></address>
2002      </author>
2003      <author fullname="L. Masinter" initials="L." surname="Masinter">
2004        <organization>Xerox Corporation</organization>
2005        <address><email>masinter@parc.xerox.com</email></address>
2006      </author>
2007      <author fullname="P. Leach" initials="P." surname="Leach">
2008        <organization>Microsoft Corporation</organization>
2009        <address><email>paulle@microsoft.com</email></address>
2010      </author>
2011      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2012        <organization>W3C</organization>
2013        <address><email>timbl@w3.org</email></address>
2014      </author>
2015      <date month="June" year="1999" />
2016    </front>
2017    <seriesInfo name="RFC" value="2616" />
2018  </reference>
2019
2020  <reference anchor="RFC3864">
2021    <front>
2022      <title>Registration Procedures for Message Header Fields</title>
2023      <author fullname="G. Klyne" initials="G." surname="Klyne">
2024        <organization>Nine by Nine</organization>
2025        <address><email>GK-IETF@ninebynine.org</email></address>
2026      </author>
2027      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2028        <organization>BEA Systems</organization>
2029        <address><email>mnot@pobox.com</email></address>
2030      </author>
2031      <author fullname="J. Mogul" initials="J." surname="Mogul">
2032        <organization>HP Labs</organization>
2033        <address><email>JeffMogul@acm.org</email></address>
2034      </author>
2035      <date month="September" year="2004" />
2036    </front>
2037    <seriesInfo name="BCP" value="90" />
2038    <seriesInfo name="RFC" value="3864" />
2039  </reference>
2040
2041</references>
2042
2043<section anchor="compatibility" title="Compatibility with Previous Versions">
2044
2045<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2046<t>
2047  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2048  this missing case.
2049  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2050</t>
2051<t>
2052  Transfer-coding and message lengths all interact in ways that required fixing exactly
2053  when chunked encoding is used (to allow for transfer encoding that may not be self
2054  delimiting); it was important to straighten out exactly how message lengths are computed.
2055  (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
2056  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2057</t>
2058<t>
2059  Proxies should be able to add Content-Length when appropriate.
2060  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2061</t>
2062<t
2063  >Range request responses would become very verbose if all meta-data were always returned;
2064  by allowing the server to only send needed headers in a 206 response, this problem can be
2065  avoided.
2066  (<xref target="combining.headers" />)
2067</t>
2068<t>
2069  The Cache-Control: max-age directive was not properly defined for responses.
2070  (<xref target="cache-response-directive"/>)
2071</t>
2072<t>
2073  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2074    format="counter" target="expiration.model" />, <xref format="counter"
2075    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2076  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2077  header, as PUT or other methods may have need for it in requests.
2078</t>
2079</section>
2080
2081<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2082<t>
2083  Clarify denial of service attack avoidance requirement.
2084  (<xref target="invalidation.after.updates.or.deletions" />)
2085</t>
2086</section>
2087
2088    </section>
2089
2090<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2091<figure>
2092<artwork type="abnf" name="p6-cache.parsed-abnf">
2093<x:ref>Age</x:ref> = "Age:" OWS Age-v
2094<x:ref>Age-v</x:ref> = delta-seconds
2095
2096<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2097<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2098 cache-directive ] )
2099
2100<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2101<x:ref>Expires-v</x:ref> = HTTP-date
2102
2103<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
2104
2105<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2106
2107<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2108<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2109 pragma-directive ] )
2110
2111<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2112<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2113 ] ) )
2114
2115<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2116<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2117 ] )
2118
2119<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2120<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2121<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2122 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2123 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2124 cache-extension
2125<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2126 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2127 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2128 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2129 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2130 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2131
2132<x:ref>delta-seconds</x:ref> = 1*DIGIT
2133
2134<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2135
2136<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
2137
2138<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
2139<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2140<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
2141
2142<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2143
2144<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2145
2146<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
2147
2148<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2149<x:ref>warn-code</x:ref> = 3DIGIT
2150<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2151<x:ref>warn-text</x:ref> = quoted-string
2152<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2153 ]
2154
2155
2156</artwork>
2157</figure>
2158<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2159; Age defined but not used
2160; Cache-Control defined but not used
2161; Expires defined but not used
2162; Pragma defined but not used
2163; Vary defined but not used
2164; Warning defined but not used
2165</artwork></figure></section>
2166
2167<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2168
2169<section title="Since RFC2616">
2170  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2171</section>
2172
2173<section title="Since draft-ietf-httpbis-p6-cache-00">
2174<t>
2175  Closed issues:
2176  <list style="symbols">
2177    <t>
2178      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2179    <t>
2180      <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>
2181    <t>
2182      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2183    <t>
2184      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2185    <t>
2186      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2187    <t>
2188      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2189    <t>
2190      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2191    <t>
2192      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2193    <t>
2194      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2195  </list>
2196</t>
2197<t>
2198  Other changes:
2199  <list style="symbols">
2200    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2201        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2202  </list>
2203</t>
2204</section>
2205
2206<section title="Since draft-ietf-httpbis-p6-cache-01">
2207<t>
2208  Closed issues:
2209  <list style="symbols">
2210    <t>
2211      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2212  </list>
2213</t>
2214<t>
2215  Other changes:
2216  <list style="symbols">
2217    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2218      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2219    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2220      specification.</t>
2221  </list>
2222</t>
2223</section>
2224
2225<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2226<t>
2227  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2228  <list style="symbols">
2229    <t>Reference RFC 3984, and update header registrations for headers defined in this
2230      document.</t>
2231  </list>
2232</t>
2233</section>
2234
2235<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2236<t>
2237  Closed issues:
2238  <list style="symbols">
2239    <t>
2240      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2241  </list>
2242</t>
2243</section>
2244
2245<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2246<t>
2247  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2248  <list style="symbols"> 
2249    <t>
2250      Use "/" instead of "|" for alternatives.
2251    </t>
2252    <t>
2253      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2254      whitespace ("OWS") and required whitespace ("RWS").
2255    </t>
2256    <t>
2257      Rewrite ABNFs to spell out whitespace rules, factor out
2258      header value format definitions.
2259    </t>
2260  </list>
2261</t>
2262</section>
2263
2264<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2265<t>
2266  This is a total rewrite of this part of the specification.
2267</t>
2268<t>
2269  Affected issues:
2270  <list style="symbols">
2271    <t>
2272      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2273    <t>
2274      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2275    <t>
2276      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2277    <t>
2278      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2279  </list>
2280</t>
2281<t>
2282  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2283  <list style="symbols"> 
2284    <t>
2285      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2286    </t>
2287  </list>
2288</t>
2289</section>
2290
2291<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2292<t>
2293</t>
2294</section>
2295
2296</section>
2297  </back>
2298</rfc>
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