source: draft-ietf-httpbis/08/p6-cache.xml @ 2462

Last change on this file since 2462 was 1500, checked in by julian.reschke@…, 9 years ago

fix mime types

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  • Property svn:mime-type set to text/xml
File size: 101.1 KB
Line 
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 "08">
15  <!ENTITY ID-MONTH "October">
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='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY 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  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36]>
37<?rfc toc="yes" ?>
38<?rfc symrefs="yes" ?>
39<?rfc sortrefs="yes" ?>
40<?rfc compact="yes"?>
41<?rfc subcompact="no" ?>
42<?rfc linkmailto="no" ?>
43<?rfc editing="no" ?>
44<?rfc comments="yes"?>
45<?rfc inline="yes"?>
46<?rfc-ext allow-markup-in-artwork="yes" ?>
47<?rfc-ext include-references-in-index="yes" ?>
48<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
49  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
50<front>
51
52  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
53
54  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
55    <organization abbrev="Day Software">Day Software</organization>
56    <address>
57      <postal>
58        <street>23 Corporate Plaza DR, Suite 280</street>
59        <city>Newport Beach</city>
60        <region>CA</region>
61        <code>92660</code>
62        <country>USA</country>
63      </postal>
64      <phone>+1-949-706-5300</phone>
65      <facsimile>+1-949-706-5305</facsimile>
66      <email>fielding@gbiv.com</email>
67      <uri>http://roy.gbiv.com/</uri>
68    </address>
69  </author>
70
71  <author fullname="Jim Gettys" initials="J." surname="Gettys">
72    <organization>One Laptop per Child</organization>
73    <address>
74      <postal>
75        <street>21 Oak Knoll Road</street>
76        <city>Carlisle</city>
77        <region>MA</region>
78        <code>01741</code>
79        <country>USA</country>
80      </postal>
81      <email>jg@laptop.org</email>
82      <uri>http://www.laptop.org/</uri>
83    </address>
84  </author>
85
86  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
87    <organization abbrev="HP">Hewlett-Packard Company</organization>
88    <address>
89      <postal>
90        <street>HP Labs, Large Scale Systems Group</street>
91        <street>1501 Page Mill Road, MS 1177</street>
92        <city>Palo Alto</city>
93        <region>CA</region>
94        <code>94304</code>
95        <country>USA</country>
96      </postal>
97      <email>JeffMogul@acm.org</email>
98    </address>
99  </author>
100
101  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
102    <organization abbrev="Microsoft">Microsoft Corporation</organization>
103    <address>
104      <postal>
105        <street>1 Microsoft Way</street>
106        <city>Redmond</city>
107        <region>WA</region>
108        <code>98052</code>
109        <country>USA</country>
110      </postal>
111      <email>henrikn@microsoft.com</email>
112    </address>
113  </author>
114
115  <author fullname="Larry Masinter" initials="L." surname="Masinter">
116    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
117    <address>
118      <postal>
119        <street>345 Park Ave</street>
120        <city>San Jose</city>
121        <region>CA</region>
122        <code>95110</code>
123        <country>USA</country>
124      </postal>
125      <email>LMM@acm.org</email>
126      <uri>http://larry.masinter.net/</uri>
127    </address>
128  </author>
129
130  <author fullname="Paul J. Leach" initials="P." surname="Leach">
131    <organization abbrev="Microsoft">Microsoft Corporation</organization>
132    <address>
133      <postal>
134        <street>1 Microsoft Way</street>
135        <city>Redmond</city>
136        <region>WA</region>
137        <code>98052</code>
138      </postal>
139      <email>paulle@microsoft.com</email>
140    </address>
141  </author>
142
143  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
144    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
145    <address>
146      <postal>
147        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
148        <street>The Stata Center, Building 32</street>
149        <street>32 Vassar Street</street>
150        <city>Cambridge</city>
151        <region>MA</region>
152        <code>02139</code>
153        <country>USA</country>
154      </postal>
155      <email>timbl@w3.org</email>
156      <uri>http://www.w3.org/People/Berners-Lee/</uri>
157    </address>
158  </author>
159
160  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
161    <organization abbrev="W3C">World Wide Web Consortium</organization>
162    <address>
163      <postal>
164        <street>W3C / ERCIM</street>
165        <street>2004, rte des Lucioles</street>
166        <city>Sophia-Antipolis</city>
167        <region>AM</region>
168        <code>06902</code>
169        <country>France</country>
170      </postal>
171      <email>ylafon@w3.org</email>
172      <uri>http://www.raubacapeu.net/people/yves/</uri>
173    </address>
174  </author>
175
176  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
177    <address>
178      <email>mnot@mnot.net</email>
179      <uri>http://www.mnot.net/</uri>
180    </address>
181  </author>
182
183  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
184    <organization abbrev="greenbytes">greenbytes GmbH</organization>
185    <address>
186      <postal>
187        <street>Hafenweg 16</street>
188        <city>Muenster</city><region>NW</region><code>48155</code>
189        <country>Germany</country>
190      </postal>
191      <phone>+49 251 2807760</phone>
192      <facsimile>+49 251 2807761</facsimile>
193      <email>julian.reschke@greenbytes.de</email>
194      <uri>http://greenbytes.de/tech/webdav/</uri>
195    </address>
196  </author>
197
198  <date day="26" month="&ID-MONTH;" year="&ID-YEAR;"/>
199  <workgroup>HTTPbis Working Group</workgroup>
200
201<abstract>
202<t>
203  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
204  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
205  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
206  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
207  fields that control cache behavior or indicate cacheable response messages.
208</t>
209</abstract>
210
211<note title="Editorial Note (To be removed by RFC Editor)">
212<t>
213  Discussion of this draft should take place on the HTTPBIS working group mailing list
214  (ietf-http-wg@w3.org). The current issues list is at <eref
215  target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
216  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
217</t>
218<t>
219  The changes in this draft are summarized in <xref target="changes.since.07" />.
220</t>
221</note>
222
223  </front>
224  <middle>
225
226<section anchor="caching" title="Introduction">
227<t>
228  HTTP is typically used for distributed information systems, where performance can be
229  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
230  caching and reusing response messages.
231</t>
232
233<section anchor="intro.purpose" title="Purpose">
234<iref item="cache" />
235<t>
236  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
237  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
238  in order to reduce the response time and network bandwidth consumption on future,
239  equivalent requests. Any client or server may include a cache, though a cache cannot be
240  used by a server that is acting as a tunnel.
241</t>
242<t>
243  Caching would be useless if it did not significantly improve performance. The goal of
244  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
245  some cases, a stored response can be reused without the need for a network request,
246  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
247  (see <xref target="expiration.model" />). Even when a new request is required, it is often
248  possible to reuse all or parts of the payload of a prior response to satisfy the request,
249  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
250  purpose (see <xref target="validation.model" />).
251</t>
252</section>
253
254<section anchor="intro.terminology" title="Terminology">
255<t>
256  This specification uses a number of terms to refer to the roles played by participants
257  in, and objects of, HTTP caching.
258</t>
259<t>
260  <iref item="cacheable" />
261  <x:dfn>cacheable</x:dfn>
262  <list>
263    <t>A response is cacheable if a cache is allowed to store a copy of the response message
264      for use in answering subsequent requests. Even when a response is cacheable, there may
265      be additional constraints on whether a cache can use the cached copy to satisfy a
266      particular request.</t>
267  </list>
268</t>
269<t>
270  <iref item="explicit expiration time" />
271  <x:dfn>explicit expiration time</x:dfn>
272  <list>
273    <t>The time at which the origin server intends that an entity should no longer be
274      returned by a cache without further validation.</t>
275  </list>
276</t>
277<t>
278  <iref item="heuristic expiration time" />
279  <x:dfn>heuristic expiration time</x:dfn>
280  <list>
281    <t>An expiration time assigned by a cache when no explicit expiration time is
282    available.</t>
283  </list>
284</t>
285<t>
286  <iref item="age" />
287  <x:dfn>age</x:dfn>
288  <list>
289    <t>The age of a response is the time since it was sent by, or successfully validated
290      with, the origin server.</t>
291  </list>
292</t>
293<t>
294  <iref item="first-hand" />
295  <x:dfn>first-hand</x:dfn>
296  <list>
297    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
298    0.</t>
299  </list>
300</t>
301<t>
302  <iref item="freshness lifetime" />
303  <x:dfn>freshness lifetime</x:dfn>
304  <list>
305    <t>The length of time between the generation of a response and its expiration time. </t>
306  </list>
307</t>
308<t>
309  <iref item="fresh" />
310  <x:dfn>fresh</x:dfn>
311  <list>
312    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
313  </list>
314</t>
315<t>
316  <iref item="stale" />
317  <x:dfn>stale</x:dfn>
318  <list>
319    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
320  </list>
321</t>
322<t>
323  <iref item="validator" />
324  <x:dfn>validator</x:dfn>
325  <list>
326    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
327      out whether a stored response is an equivalent copy of an entity.</t>
328  </list>
329</t>
330<t anchor="shared.and.non-shared.caches">
331  <iref item="validator" />
332  <x:dfn>shared cache</x:dfn>
333  <list>
334    <t>A cache that is accessible to more than one user. A non-shared cache is
335      dedicated to a single user.</t>
336  </list>
337</t>
338</section>
339
340<section anchor="intro.requirements" title="Requirements">
341<t>
342  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
343  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
344  described in <xref target="RFC2119" />.
345</t>
346<t>
347  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
348  or &REQUIRED; level requirements for the protocols it implements. An implementation
349  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
350  requirements for its protocols is said to be "unconditionally compliant"; one that
351  satisfies all the &MUST; level requirements but not all the &SHOULD; level
352  requirements for its protocols is said to be "conditionally compliant."
353</t>
354</section>
355
356<section title="Syntax Notation" anchor="notation">
357  <x:anchor-alias value="ALPHA"/>
358  <x:anchor-alias value="CR"/>
359  <x:anchor-alias value="DIGIT"/>
360  <x:anchor-alias value="DQUOTE"/>
361  <x:anchor-alias value="LF"/>
362  <x:anchor-alias value="OCTET"/>
363  <x:anchor-alias value="SP"/>
364  <x:anchor-alias value="VCHAR"/>
365  <x:anchor-alias value="WSP"/>
366<t>
367  This specification uses the ABNF syntax defined in &notation; (which
368  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
369  <xref target="collected.abnf"/> shows the collected ABNF, with the list
370  rule expanded.
371</t>
372<t>
373  The following core rules are included by
374  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
375  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
376  DIGIT (decimal 0-9), DQUOTE (double quote),
377  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
378  OCTET (any 8-bit sequence of data), SP (space),
379  VCHAR (any visible USASCII character),
380  and WSP (whitespace).
381</t>
382
383<section title="Core Rules" anchor="core.rules">
384  <x:anchor-alias value="quoted-string"/>
385  <x:anchor-alias value="token"/>
386  <x:anchor-alias value="OWS"/>
387<t>
388  The core rules below are defined in &basic-rules;:
389</t>
390<figure><artwork type="abnf2616">
391  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
392  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
393  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
394</artwork></figure>
395</section>
396
397<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
398  <x:anchor-alias value="field-name"/>
399  <x:anchor-alias value="HTTP-date"/>
400  <x:anchor-alias value="port"/>
401  <x:anchor-alias value="pseudonym"/>
402  <x:anchor-alias value="uri-host"/>
403<t>
404  The ABNF rules below are defined in other parts:
405</t>
406<figure><!--Part1--><artwork type="abnf2616">
407  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
408  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
409  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
410  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt;
411  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
412</artwork></figure>
413</section>
414
415</section>
416</section>
417
418<section anchor="caching.overview" title="Cache Operation">
419
420<section anchor="response.cacheability" title="Response Cacheability">
421<t>
422  A cache &MUST-NOT; store a response to any request, unless:
423  <list style="symbols">
424    <t>The request method is defined as being cacheable, and</t>
425    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
426       appear in request or response headers, and</t>
427    <t>the "private" cache response directive (see <xref target="header.cache-control" />
428       does not appear in the response, if the cache is shared, and</t>
429    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
430       the cache is shared (unless the "public" directive is present; see <xref
431       target="header.cache-control" />), and</t>           
432    <t>the cache understands partial responses, if the response is partial or incomplete
433       (see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
434  </list>
435</t>
436<t>
437  Note that in normal operation, most caches will not store a response that has neither a
438  cache validator nor an explicit expiration time, as such responses are not usually
439  useful to store. However, caches are not prohibited from storing such responses.
440</t>
441
442<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
443<t>
444  A cache that receives an incomplete response (for example, with fewer bytes of data
445  than specified in a Content-Length header) can store the response, but &MUST;
446  treat it as a partial response &partial;. Partial responses
447  can be combined as described in &combining-byte-ranges;; the result might be a
448  full response or might still be partial. A cache &MUST-NOT; return a partial
449  response to a client without explicitly marking it as such using the 206 (Partial
450  Content) status code.
451</t>
452<t>
453  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
454  incomplete or partial responses.
455</t>
456</section>
457
458</section>
459
460
461<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
462<t>
463  For a presented request, a cache &MUST-NOT; return a stored response, unless:
464  <list style="symbols">
465    <t>The presented Request-URI and that of the stored response match
466      (<cref anchor="TODO-Request-URI">Need to find a new term for this, as Part
467      1 doesn't define Request-URI anymore; the new term request-target does not
468      work for this. (see <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196" />)</cref>), and</t>
469    <t>the request method associated with the stored response allows it to be
470      used for the presented request, and</t>
471    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
472      target="caching.negotiated.responses" />), and</t>
473    <t>the presented request and stored response are free from directives that would prevent
474      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
475      and</t>
476    <t>the stored response is either:
477      <list style="symbols">
478        <t>fresh (see <xref target="expiration.model" />), or</t>
479        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
480        <t>successfully validated (see <xref target="validation.model" />).</t>
481      </list>
482    </t>
483  </list>
484</t>
485<t>
486  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
487</t>
488<t>
489  When a stored response is used to satisfy a request, caches &MUST; include a
490  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
491  current_age; see <xref target="age.calculations" />.
492  <cref>DISCUSS: this currently includes successfully validated responses.</cref>
493</t>
494<t>
495  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
496  the origin server; i.e., A cache must not reply to such a request before having forwarded the request and having received a
497  corresponding response.
498</t>
499<t>
500  Also, note that unsafe requests might invalidate already stored responses; see
501  <xref target="invalidation.after.updates.or.deletions" />.
502</t>
503<t>
504  Caches &MUST; use the most recent response (as determined by the Date header) when
505  more than one suitable response is stored. They can also forward a request with
506  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
507  use.
508</t>
509<t>
510  <cref anchor="TODO-header-properties">end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
511</t>
512</section>
513
514<section anchor="expiration.model" title="Freshness Model">
515<t>
516  When a response is "fresh" in the cache, it can be used to satisfy subsequent
517  requests without contacting the origin server, thereby improving efficiency.
518</t>
519<t>
520  The primary mechanism for determining freshness is for an origin server to provide an
521  explicit expiration time in the future, using either the Expires header (<xref
522  target="header.expires" />) or the max-age response cache directive (<xref
523  target="cache-response-directive" />). Generally, origin servers will assign future
524  explicit expiration times to responses in the belief that the entity is not likely to
525  change in a semantically significant way before the expiration time is reached.
526</t>
527<t>
528  If an origin server wishes to force a cache to validate every request, it can
529  assign an explicit expiration time in the past. This means that the response is always
530  stale, so that caches should validate it before using it for subsequent requests.
531  <cref>This wording may cause confusion, because the response may still be served stale.</cref>
532</t>
533<t>
534  Since origin servers do not always provide explicit expiration times, HTTP caches may
535  also assign heuristic expiration times when they are not specified, employing algorithms that
536  use other header values (such as the Last-Modified time) to estimate a plausible
537  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
538  impose worst-case constraints on their results.
539</t>
540<figure>
541<preamble>
542  The calculation to determine if a response is fresh is:
543</preamble>
544<artwork type="code">
545   response_is_fresh = (freshness_lifetime &gt; current_age)
546</artwork>
547</figure>
548
549<t>
550  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
551  the current_age is defined in <xref target="age.calculations" />.
552</t>
553<t>
554  Additionally, clients may need to influence freshness calculation. They can do this using
555  several request cache directives, with the effect of either increasing or loosening
556  constraints on freshness. See <xref target="cache-request-directive" />.
557</t>
558<t>
559  <cref>ISSUE: there are not requirements directly applying to cache-request-directives and
560  freshness.</cref>
561</t>
562<t>
563  Note that freshness applies only to cache operation; it cannot be used to force a user agent
564  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
565  the difference between caches and history mechanisms.
566</t>
567
568<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
569<t>
570  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
571  response by using the first match of:
572  <list style="symbols">
573    <t>If the cache is shared and the s-maxage response cache directive (<xref
574      target="cache-response-directive" />) is present, use its value, or</t>
575    <t>If the max-age response cache directive (<xref target="cache-response-directive"
576      />) is present, use its value, or</t>
577    <t>If the Expires response header (<xref target="header.expires" />) is present, use
578      its value minus the value of the Date response header, or</t>
579    <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
580      may be used; see <xref target="heuristic.freshness" />.</t>
581  </list>
582</t>
583<t>
584  Note that this calculation is not vulnerable to clock skew, since all of the
585  information comes from the origin server.
586</t>
587
588<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
589<t>
590  If no explicit expiration time is present in a stored response that has a status code
591  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
592  calculated. Heuristics &MUST-NOT; be used for other response status codes.
593</t>
594<t>
595  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
596  attach a Warning header with a 113 warn-code to the response if its current_age is
597  more than 24 hours and such a warning is not already present.
598</t>
599<t>
600  Also, if the response has a Last-Modified header (&header-last-modified;), the
601  heuristic expiration value &SHOULD; be no more than some fraction of the interval
602  since that time. A typical setting of this fraction might be 10%.
603</t>
604<t>
605  <cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
606</t>
607</section>
608</section>
609
610<section anchor="age.calculations" title="Calculating Age">
611<t>
612  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
613  message when obtained from a cache. The Age field value is the cache's estimate of the
614  amount of time since the response was generated or validated by the origin server. In
615  essence, the Age value is the sum of the time that the response has been resident in
616  each of the caches along the path from the origin server, plus the amount of time it has
617  been in transit along network paths.
618</t>
619<t>
620  The term "age_value" denotes the value of the Age header, in a form appropriate for
621  arithmetic operations.
622</t>
623<t>
624  HTTP/1.1 requires origin servers to send a Date header, if possible, with every
625  response, giving the time at which the response was generated (see &header-date;).
626  The term "date_value" denotes the value of the Date header, in a form appropriate for
627  arithmetic operations.
628</t>
629<t>
630  The term "now" means "the current value of the clock at the host performing the
631  calculation." Hosts that use HTTP, but especially hosts running origin servers and
632  caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
633  synchronize their clocks to a globally accurate time standard.
634</t>
635<t>
636  A response's age can be calculated in two entirely independent ways:
637  <list style="numbers">
638    <t>now minus date_value, if the local clock is reasonably well synchronized to the
639      origin server's clock. If the result is negative, the result is replaced by zero.</t>
640    <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
641  </list>
642</t>
643<figure>
644<preamble>These are combined as</preamble>
645<artwork type="code">
646    corrected_received_age = max(now - date_value, age_value)
647</artwork></figure>
648<t>
649  When an Age value is received, it &MUST; be interpreted relative to the time the
650  request was initiated, not the time that the response was received.
651</t>
652<figure><artwork type="code">
653   corrected_initial_age = corrected_received_age
654                         + (now - request_time)
655</artwork></figure>
656<t>
657  where "request_time" is the time (according to the local clock) when the request that
658  elicited this response was sent.
659</t>
660<t>
661  The current_age of a stored response can then be calculated by adding the amount of
662  time (in seconds) since the stored response was last validated by the origin server to
663  the corrected_initial_age.
664</t>
665<t>
666  In summary:
667</t>
668<figure><artwork type="code">
669  age_value     - Age header field-value received with the response
670  date_value    - Date header field-value received with the response
671  request_time  - local time when the cache made the request
672                 resulting in the stored response
673  response_time - local time when the cache received the response
674  now           - current local time
675 
676  apparent_age = max(0, response_time - date_value);
677  corrected_received_age = max(apparent_age, age_value);
678  response_delay = response_time - request_time;
679  corrected_initial_age = corrected_received_age + response_delay;
680  resident_time = now - response_time;
681  current_age   = corrected_initial_age + resident_time;
682</artwork></figure>
683</section>
684
685<section anchor="serving.stale.responses" title="Serving Stale Responses">
686<t>
687  A "stale" response is one that either has explicit expiry information, or is allowed to
688  have heuristic expiry calculated, but is not fresh according to the calculations in
689  <xref target="expiration.model" />.
690</t>
691<t>
692  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
693  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
694  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
695  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
696</t>
697<t>
698  Caches &SHOULD-NOT; return stale responses unless they are
699  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
700  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
701</t>
702<t>
703  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
704  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
705  the cache is disconnected.
706</t>
707<t>
708  If a cache receives a first-hand response (either an entire response, or a 304 (Not
709  Modified) response) that it would normally forward to the requesting client, and the
710  received response is no longer fresh, the cache &SHOULD; forward it to the
711  requesting client without adding a new Warning (but without removing any existing
712  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
713  that response became stale in transit.
714</t>
715</section>
716</section>
717
718<section anchor="validation.model" title="Validation Model">
719<t>
720  When a cache has one or more stored responses for a requested URI, but cannot 
721  serve any of them (e.g., because they are not fresh, or one cannot be selected;
722  see <xref target="caching.negotiated.responses"/>),
723  it can use the conditional request mechanism &conditional; in the forwarded
724  request to give the origin server an opportunity to both select a valid stored
725  response to be used, and to update it. This process is known as "validating"
726  or "revalidating" the stored response.
727</t>
728<t>
729  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
730  header whose value is that of the Last-Modified header from the selected
731  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
732</t>
733<t>
734  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
735  is that of the ETag header(s) from all responses stored for the requested URI,
736  if present. However, if any of the stored responses contains only partial
737  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
738  field unless the request is for a range that would be fully satisfied by
739  that stored response.
740</t>
741<t>
742  A 304 (Not Modified) response status code indicates that the stored
743  response can be updated and reused; see <xref target="combining.headers"/>.
744</t>
745<t>
746  A full response (i.e., one with a response body) indicates that none 
747  of the stored responses nominated in the conditional request is
748  suitable. Instead, the full response is used both to satisfy the
749  request and replace the stored response. <cref>Should there be a requirement here?</cref>
750</t>
751<t>
752  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
753  either forward this response to the requesting client, or act as if the server failed to
754  respond. In the latter case, it &MAY; return a previously stored response (see <xref
755  target="serving.stale.responses" />).
756</t>
757</section>
758
759<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
760<t>
761  Because unsafe methods (&safe-methods;) have the potential for changing state on the
762  origin server, intervening caches can use them to keep their contents
763  up-to-date.
764</t>
765<t>
766  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
767  as the URI(s) in the Location and Content-Location headers (if present):
768  <list style="symbols">
769    <t>PUT</t>
770    <t>DELETE</t>
771    <t>POST</t>
772  </list>
773</t>
774<t>
775  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
776  be performed if the host part of that URI differs from the host part in the Request-URI.
777  This helps prevent denial of service attacks.
778</t>
779<t>
780  <cref>TODO: "host part" needs to be specified better.</cref>
781</t>
782<t>
783  A cache that passes through requests for methods it does not understand &SHOULD;
784  invalidate the Request-URI.
785</t>
786<t>
787  Here, "invalidate" means that the cache will either remove all stored responses related
788  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
789  before they can be returned in response to a subsequent request.
790</t>
791<t>
792  Note that this does not guarantee that all appropriate responses are invalidated. For
793  example, the request that caused the change at the origin server might not have gone
794  through the cache where a response is stored.
795</t>
796<t>
797  <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
798</t>
799</section>
800
801<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
802<t>
803  When a cache receives a request that can be satisfied by a stored response
804  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
805  response unless all of the selecting request-headers nominated by the Vary header match
806  in both the original request (i.e., that associated with the stored response),
807  and the presented request.
808</t>
809<t>
810  The selecting request-headers from two requests are defined to match if and only if the
811  selecting request-headers in the first request can be transformed to the selecting
812  request-headers in the second request by adding or removing linear white space
813  <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
814  combining multiple message-header fields with the same field name following the rules
815  about header fields in &header-fields;.
816</t>
817<t>
818  If a header field is absent from a request, it can only match another request
819  if it is also absent there.
820</t>
821<t>
822  A Vary header field-value of "*" always fails to match, and subsequent requests to that
823  resource can only be properly interpreted by the origin server.
824</t>
825<t>
826  The stored response with matching selecting request-headers is known as the
827  selected response.
828</t>
829<t>
830  If no selected response is available, the cache &MAY; forward the presented
831  request to the origin server in a conditional request; see <xref target="validation.model"/>.
832</t>
833</section>
834
835<section anchor="combining.headers" title="Combining Responses">
836<t>
837  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
838  (in this section, the "new" response"), it needs to created an updated response by combining
839  the stored response with the new one, so that the updated response can be used to satisfy the request.
840</t>
841<t>
842  If the new response contains an ETag, it identifies the stored 
843  response to use. <cref>may need language about Content-Location 
844  here</cref><cref>cover case where INM with multiple etags was sent</cref>
845</t>
846<t>
847  If the status code is 206 (partial content), both the stored and new 
848  responses &MUST; have validators, and those validators &MUST; match using the strong 
849  comparison function (see &weak-and-strong;). Otherwise, the 
850  responses &MUST-NOT; be combined.
851</t>
852<t>
853  The stored response headers are used as those of the updated response, except that
854  <list style="symbols">
855    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
856      &MUST; be deleted from the stored response and the updated response.</t>
857    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
858      response and the updated response.</t>
859    <t>any headers provided in the new response &MUST; replace the corresponding
860      headers from the stored response.</t>
861  </list>
862</t>
863<t>
864  If a header field-name in the new response matches more than one 
865  header in the stored response, all such stored headers &MUST; be replaced.
866</t>
867<t>
868  The updated response can [[<cref>requirement?</cref>]] be used to replace the 
869  stored response in cache. In the case of a 206 response, the combined 
870  entity-body &MAY; be stored.
871</t>
872<t>
873  <cref>ISSUE: discuss how to handle HEAD updates</cref>
874</t>
875</section>
876
877</section>
878
879<section anchor="header.fields" title="Header Field Definitions">
880<t>
881  This section defines the syntax and semantics of HTTP/1.1 header fields
882  related to caching.
883</t>
884<t>
885  For entity-header fields, both sender and recipient refer to either the client or the
886  server, depending on who sends and who receives the entity.
887</t>
888
889<section anchor="header.age" title="Age">
890  <iref item="Age header" primary="true" x:for-anchor="" />
891  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
892  <x:anchor-alias value="Age"/>
893  <x:anchor-alias value="Age-v"/>
894  <x:anchor-alias value="age-value"/>
895<t>
896  The "Age" response-header field conveys the sender's estimate of the amount
897  of time since the response was generated or successfully validated at the
898  origin server. Age values are calculated as specified in
899  <xref target="age.calculations" />.
900</t>
901<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
902  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
903  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
904</artwork></figure>
905<t anchor="rule.delta-seconds">
906  <x:anchor-alias value="delta-seconds" />
907  Age field-values are non-negative integers, representing time in seconds.
908</t>
909<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
910  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
911</artwork></figure>
912<t>
913  If a cache receives a value larger than the largest positive integer it can represent, or
914  if any of its age calculations overflows, it &MUST; transmit an Age header with a
915  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
916  of at least 31 bits of range.
917</t>
918<t>
919  The presence of an Age header field in a response implies that a response is not
920  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
921  Age header field.
922</t>
923</section>
924
925<section anchor="header.cache-control" title="Cache-Control">
926  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
927  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
928  <x:anchor-alias value="Cache-Control"/>
929  <x:anchor-alias value="Cache-Control-v"/>
930  <x:anchor-alias value="cache-directive"/>
931  <x:anchor-alias value="cache-extension"/>
932  <x:anchor-alias value="cache-request-directive"/>
933  <x:anchor-alias value="cache-response-directive"/>
934<t>
935  The "Cache-Control" general-header field is used to specify directives that
936  &MUST; be obeyed by all caches along the request/response chain. Such cache
937  directives are unidirectional in that the presence of a directive in a
938  request does not imply that the same directive is to be given in the
939  response.
940</t>
941<x:note>
942  <t>
943    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
944    Pragma: no-cache (see <xref target="header.pragma" />).
945  </t>
946</x:note>
947<t>
948  Cache directives &MUST; be passed through by a proxy or gateway application,
949  regardless of their significance to that application, since the directives might be
950  applicable to all recipients along the request/response chain. It is not possible to
951  target a directive to a specific cache.
952</t>
953<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"/>
954  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
955  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
956
957  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
958     / <x:ref>cache-response-directive</x:ref>
959
960  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
961</artwork></figure>
962
963<section anchor="cache-request-directive" title="Request Cache-Control Directives">
964  <x:anchor-alias value="cache-request-directive" />
965
966<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
967  <x:ref>cache-request-directive</x:ref> =
968       "no-cache"
969     / "no-store"
970     / "max-age" "=" <x:ref>delta-seconds</x:ref>
971     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
972     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
973     / "no-transform"
974     / "only-if-cached"
975     / <x:ref>cache-extension</x:ref>
976</artwork></figure>
977
978<t>
979  <iref item="Cache Directives" primary="true" subitem="no-cache" />
980  <iref item="no-cache" primary="true" subitem="Cache Directive" />
981  no-cache
982  <list>
983    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
984      used to satisfy the request without successful validation on the origin server.</t>
985  </list>
986</t>
987<t>
988  <iref item="Cache Directives" primary="true" subitem="no-store" />
989  <iref item="no-store" primary="true" subitem="Cache Directive" />
990  no-store
991  <list>
992    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
993      of either this request or any response to it. This directive applies to both
994      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
995      cache &MUST-NOT; intentionally store the information in non-volatile storage,
996      and &MUST; make a best-effort attempt to remove the information from volatile
997      storage as promptly as possible after forwarding it.</t>
998    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
999      particular, malicious or compromised caches might not recognize or obey this
1000      directive, and communications networks may be vulnerable to eavesdropping.</t>
1001  </list>
1002</t>
1003<t>
1004  <iref item="Cache Directives" primary="true" subitem="max-age" />
1005  <iref item="max-age" primary="true" subitem="Cache Directive" />
1006  max-age
1007  <list>
1008    <t>The max-age request directive indicates that the client is willing to accept a
1009      response whose age is no greater than the specified time in seconds. Unless
1010      max-stale directive is also included, the client is not willing to accept a stale
1011      response.</t>
1012  </list>
1013</t>
1014<t>
1015  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1016  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1017  max-stale
1018  <list>
1019    <t>The max-stale request directive indicates that the client is willing to accept a
1020      response that has exceeded its expiration time. If max-stale is assigned a value,
1021      then the client is willing to accept a response that has exceeded its expiration
1022      time by no more than the specified number of seconds. If no value is assigned to
1023      max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
1024  </list>
1025</t>
1026<t>
1027  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1028  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1029  min-fresh
1030  <list>
1031    <t>The min-fresh request directive indicates that the client is willing to accept a
1032      response whose freshness lifetime is no less than its current age plus the specified
1033      time in seconds. That is, the client wants a response that will still be fresh for
1034      at least the specified number of seconds.</t>
1035  </list>
1036</t>
1037<t>
1038  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1039  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1040  no-transform
1041  <list>
1042    <t>The no-transform request directive indicates that an intermediate cache or proxy
1043      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1044      headers, nor the request entity-body.</t>
1045  </list>
1046</t>
1047<t>
1048  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1049  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1050  only-if-cached
1051  <list>
1052    <t>The only-if-cached request directive indicates that the client only wishes to
1053      return a stored response. If it receives this directive, a cache &SHOULD; either
1054      respond using a stored response that is consistent with the other constraints of the
1055      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1056      being operated as a unified system with good internal connectivity, such a request
1057      &MAY; be forwarded within that group of caches.</t>
1058  </list>
1059</t>
1060</section>
1061
1062<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1063  <x:anchor-alias value="cache-response-directive" />
1064
1065<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1066  <x:ref>cache-response-directive</x:ref> =
1067       "public"
1068     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1069     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1070     / "no-store"
1071     / "no-transform"
1072     / "must-revalidate"
1073     / "proxy-revalidate"
1074     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1075     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1076     / <x:ref>cache-extension</x:ref>
1077</artwork></figure>
1078
1079<t>
1080  <iref item="Cache Directives" primary="true" subitem="public" />
1081  <iref item="public" primary="true" subitem="Cache Directive" />
1082  public
1083  <list>
1084    <t>The public response directive indicates that the response &MAY; be cached, even
1085      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1086      (See also Authorization, &header-authorization;, for additional details.) </t>
1087  </list>
1088</t>
1089<t>
1090  <iref item="Cache Directives" primary="true" subitem="private" />
1091  <iref item="private" primary="true" subitem="Cache Directive" />
1092  private
1093  <list>
1094    <t>The private response directive indicates that the response message is intended for
1095      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1096      cache &MAY; store the response.</t>
1097    <t>If the private response directive specifies one or more field-names, this
1098      requirement is limited to the field-values associated with the listed response
1099      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1100      cache, whereas the remainder of the response message &MAY; be.</t>
1101    <t>
1102      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1103      be stored, and cannot ensure the privacy of the message content.
1104      Also, private response directives with field-names are often handled by
1105      implementations as if an unqualified private directive was recieved; i.e.,
1106      the special handling for the qualified form is not widely implemented.
1107</t>
1108  </list>
1109</t>
1110<t>
1111  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1112  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1113  no-cache
1114  <list>
1115    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1116      satisfy a subsequent request without successful validation on the origin server.
1117      This allows an origin server to prevent caching even by caches that have been
1118      configured to return stale responses.</t>
1119    <t>If the no-cache response directive specifies one or more field-names, this
1120      requirement is limited to the field-values associated with the listed response
1121      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1122      to a subsequent request without successful validation on the origin server. This
1123      allows an origin server to prevent the re-use of certain header fields in a
1124      response, while still allowing caching of the rest of the response.</t>
1125    <t>
1126      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1127      Also, no-cache response directives with field-names are often handled by
1128      implementations as if an unqualified no-cache directive was recieved; i.e.,
1129      the special handling for the qualified form is not widely implemented.
1130    </t>
1131  </list>
1132</t>
1133
1134<t>
1135  <iref item="Cache Directives" primary="true" subitem="no-store" />
1136  <iref item="no-store" primary="true" subitem="Cache Directive" />
1137  no-store
1138  <list>
1139    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1140      part of either the immediate request or response. This directive applies to both
1141      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1142      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1143      and &MUST; make a best-effort attempt to remove the information from volatile
1144      storage as promptly as possible after forwarding it.</t>
1145    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1146      particular, malicious or compromised caches might not recognize or obey this
1147      directive, and communications networks may be vulnerable to eavesdropping.</t>
1148  </list>
1149</t>
1150<t>
1151  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1152  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1153  must-revalidate
1154  <list>
1155    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1156     used to satisfy subsequent requests without successful validation on the origin server.</t>
1157    <t>The must-revalidate directive is necessary to support reliable operation for
1158      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1159      the must-revalidate directive; in particular, if the cache cannot reach the origin
1160      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1161    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1162      validate a request on the entity could result in incorrect operation, such as a
1163      silently unexecuted financial transaction.</t>
1164  </list>
1165</t>
1166<t>
1167  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1168  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1169  proxy-revalidate
1170  <list>
1171    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1172      response directive, except that it does not apply to non-shared caches.</t>
1173  </list>
1174</t>
1175<t>
1176  <iref item="Cache Directives" primary="true" subitem="max-age" />
1177  <iref item="max-age" primary="true" subitem="Cache Directive" />
1178  max-age
1179  <list>
1180    <t>The max-age response directive indicates that response is to be considered stale
1181      after its age is greater than the specified number of seconds.</t>
1182  </list>
1183</t>
1184<t>
1185  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1186  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1187  s-maxage
1188  <list>
1189    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1190      specified by this directive overrides the maximum age specified by either the
1191      max-age directive or the Expires header. The s-maxage directive also implies the
1192      semantics of the proxy-revalidate response directive.</t>
1193  </list>
1194</t>
1195<t>
1196  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1197  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1198  no-transform
1199  <list>
1200    <t>The no-transform response directive indicates that an intermediate cache or proxy
1201      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1202      headers, nor the response entity-body.</t>
1203  </list>
1204</t>
1205
1206</section>
1207
1208<section anchor="cache.control.extensions" title="Cache Control Extensions">
1209<t>
1210  The Cache-Control header field can be extended through the use of one or more
1211  cache-extension tokens, each with an optional value. Informational extensions (those
1212  that do not require a change in cache behavior) can be added without changing the
1213  semantics of other directives. Behavioral extensions are designed to work by acting as
1214  modifiers to the existing base of cache directives. Both the new directive and the
1215  standard directive are supplied, such that applications that do not understand the new
1216  directive will default to the behavior specified by the standard directive, and those
1217  that understand the new directive will recognize it as modifying the requirements
1218  associated with the standard directive. In this way, extensions to the cache-control
1219  directives can be made without requiring changes to the base protocol.
1220</t>
1221<t>
1222  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1223  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1224  all directives that it does not understand.
1225</t>
1226<t>
1227  For example, consider a hypothetical new response directive called "community" that
1228  acts as a modifier to the private directive. We define this new directive to mean that,
1229  in addition to any non-shared cache, any cache that is shared only by members of the
1230  community named within its value may cache the response. An origin server wishing to
1231  allow the UCI community to use an otherwise private response in their shared cache(s)
1232  could do so by including
1233</t>
1234<figure><artwork type="example">
1235  Cache-Control: private, community="UCI"
1236</artwork></figure>
1237<t>
1238  A cache seeing this header field will act correctly even if the cache does not
1239  understand the community cache-extension, since it will also see and understand the
1240  private directive and thus default to the safe behavior.
1241</t>
1242<t>
1243  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1244  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1245  directives (or the response's default cacheability) such that the cache behavior will
1246  remain minimally correct even if the cache does not understand the extension(s).
1247</t>
1248</section>
1249
1250</section>
1251
1252<section anchor="header.expires" title="Expires">
1253  <iref item="Expires header" primary="true" x:for-anchor="" />
1254  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1255  <x:anchor-alias value="Expires"/>
1256  <x:anchor-alias value="Expires-v"/>
1257<t>
1258  The "Expires" entity-header field gives the date/time after which the response is
1259  considered stale. See <xref target="expiration.model" /> for further discussion of the
1260  freshness model.
1261</t>
1262<t>
1263  The presence of an Expires field does not imply that the original resource will change or
1264  cease to exist at, before, or after that time.
1265</t>
1266<t>
1267  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1268  it &MUST; be sent in rfc1123-date format.
1269</t>
1270<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1271  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1272  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1273</artwork></figure>
1274<figure>
1275  <preamble>For example</preamble>
1276<artwork type="example">
1277  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1278</artwork></figure>
1279<x:note>
1280  <t>
1281    <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1282    directive (see <xref target="cache-response-directive" />), that directive overrides
1283    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1284  </t>
1285</x:note>
1286<t>
1287  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1288</t>
1289<t>
1290  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1291  including the value "0", as in the past (i.e., "already expired").
1292</t>
1293</section>
1294
1295<section anchor="header.pragma" title="Pragma">
1296  <iref item="Pragma header" primary="true" x:for-anchor="" />
1297  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1298  <x:anchor-alias value="extension-pragma"/>
1299  <x:anchor-alias value="Pragma"/>
1300  <x:anchor-alias value="Pragma-v"/>
1301  <x:anchor-alias value="pragma-directive"/>
1302<t>
1303  The "Pragma" general-header field is used to include implementation-specific directives
1304  that might apply to any recipient along the request/response chain. All pragma directives
1305  specify optional behavior from the viewpoint of the protocol; however, some systems
1306  &MAY; require that behavior be consistent with the directives.
1307</t>
1308<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"/>
1309  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1310  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1311  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1312  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1313</artwork></figure>
1314<t>
1315  When the no-cache directive is present in a request message, an application &SHOULD;
1316  forward the request toward the origin server even if it has a cached copy of what is being
1317  requested. This pragma directive has the same semantics as the no-cache response directive
1318  (see <xref target="cache-response-directive" />) and is defined here for backward
1319  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1320  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1321  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1322</t>
1323<x:note>
1324  <t>
1325    <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1326    is not actually specified, it does not provide a reliable replacement for
1327    "Cache-Control: no-cache" in a response.
1328  </t>
1329</x:note>
1330<t>
1331  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1332</t>
1333</section>
1334
1335<section anchor="header.vary" title="Vary">
1336  <iref item="Vary header" primary="true" x:for-anchor="" />
1337  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1338  <x:anchor-alias value="Vary"/>
1339  <x:anchor-alias value="Vary-v"/>
1340<t>
1341  The "Vary" response-header field conveys the set of request-header fields
1342  that were used to select the representation.
1343</t>
1344<t>
1345  Caches use this information, in part, to determine whether a stored response
1346  can be used to satisdy a given request; see
1347  <xref target="caching.negotiated.responses" />.
1348  determines, while the response is fresh, whether a cache is permitted to use the
1349  response to reply to a subsequent request without validation; see <xref
1350  target="caching.negotiated.responses" />.
1351</t>
1352<t>
1353  In uncacheable or stale responses, the Vary field value advises the user agent about
1354  the criteria that were used to select the representation.
1355</t>
1356<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1357  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1358  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1359</artwork></figure>
1360<t>
1361  The set of header fields named by the Vary field value is known as the selecting
1362  request-headers.
1363</t>
1364<t>
1365  Servers &SHOULD; include a Vary header field with any cacheable response that is
1366  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1367  requests on that resource and informs the user agent about the presence of negotiation on
1368  that resource. A server &MAY; include a Vary header field with a non-cacheable
1369  response that is subject to server-driven negotiation, since this might provide the user
1370  agent with useful information about the dimensions over which the response varies at the
1371  time of the response.
1372</t>
1373<t>
1374  A Vary field value of "*" signals that unspecified parameters not limited to the
1375  request-headers (e.g., the network address of the client), play a role in the selection of
1376  the response representation; therefore, a cache cannot determine whether this response is
1377  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1378  it may only be generated by an origin server.
1379</t>
1380<t>
1381  The field-names given are not limited to the set of standard request-header fields
1382  defined by this specification. Field names are case-insensitive.
1383</t>
1384</section>
1385
1386<section anchor="header.warning" title="Warning">
1387  <iref item="Warning header" primary="true" x:for-anchor="" />
1388  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1389  <x:anchor-alias value="Warning"/>
1390  <x:anchor-alias value="Warning-v"/>
1391  <x:anchor-alias value="warning-value"/>
1392  <x:anchor-alias value="warn-agent"/>
1393  <x:anchor-alias value="warn-code"/>
1394  <x:anchor-alias value="warn-date"/>
1395  <x:anchor-alias value="warn-text"/>
1396<t>
1397  The "Warning" general-header field is used to carry additional information about the status
1398  or transformation of a message that might not be reflected in the message. This
1399  information is typically used to warn about possible incorrectness introduced by caching
1400  operations or transformations applied to the entity body of the message.
1401</t>
1402<t>
1403  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1404  warning, rather than an error status code, distinguish these responses from true failures.
1405</t>
1406<t>
1407  Warning headers can in general be applied to any message, however some warn-codes are
1408  specific to caches and can only be applied to response messages.
1409</t>
1410<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"/>
1411  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1412  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1413 
1414  <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>
1415                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1416 
1417  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1418  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1419                  ; the name or pseudonym of the server adding
1420                  ; the Warning header, for use in debugging
1421  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1422  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1423</artwork></figure>
1424<t>
1425  Multiple warnings can be attached to a response (either by the origin server or by
1426  a cache), including multiple warnings with the same code number, only differing
1427  in warn-text.
1428</t>
1429<t>
1430  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1431  possible, in the order that they appear in the response.
1432</t>
1433<t>
1434  Systems that generate multiple Warning headers &SHOULD; order them with this user
1435  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1436  Warning headers.
1437</t>
1438<t>
1439  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1440  Warning is required to be deleted from a stored response after validation:
1441  <list style="symbols">
1442    <t>1xx Warnings describe the freshness or validation status of the response, and so
1443      &MUST; be deleted by caches after validation. They can only be generated by a cache
1444      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1445    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1446      not rectified by a validation (for example, a lossy compression of the entity bodies)
1447      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1448      returned, in which case they &MUST; be.</t>
1449  </list>
1450</t>
1451<t>
1452  If an implementation sends a message with one or more Warning headers to a receiver whose
1453  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1454  warn-date that matches the Date header in the message.
1455</t>
1456<t>
1457  If an implementation receives a message with a warning-value that includes a warn-date,
1458  and that warn-date is different from the Date value in the response, then that
1459  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1460  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1461  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1462  well.
1463</t>
1464<t>
1465  The following warn-codes are defined by this specification, each with a recommended
1466  warn-text in English, and a description of its meaning.
1467</t>
1468<t><?rfc needLines="4"?>
1469  110 Response is stale
1470  <list>
1471    <t>&SHOULD; be included whenever the returned response is stale.</t>
1472  </list>
1473</t>
1474<t><?rfc needLines="4"?>
1475  111 Revalidation failed
1476  <list>
1477    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1478      validate the response failed, due to an inability to reach the server.</t>
1479  </list>
1480</t>
1481<t><?rfc needLines="4"?>
1482  112 Disconnected operation
1483  <list>
1484    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1485      the network for a period of time.</t>
1486  </list>
1487</t>
1488<t><?rfc needLines="4"?>
1489  113 Heuristic expiration
1490  <list>
1491    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1492      greater than 24 hours and the response's age is greater than 24 hours.</t>
1493  </list>
1494</t>
1495<t><?rfc needLines="4"?>
1496  199 Miscellaneous warning
1497  <list>
1498    <t>The warning text can include arbitrary information to be presented to a human
1499      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1500      action, besides presenting the warning to the user.</t>
1501  </list>
1502</t>
1503<t><?rfc needLines="4"?>
1504  214 Transformation applied
1505  <list>
1506    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1507      transformation changing the content-coding (as specified in the Content-Encoding
1508      header) or media-type (as specified in the Content-Type header) of the response, or
1509      the entity-body of the response, unless this Warning code already appears in the
1510      response.</t>
1511  </list>
1512</t>
1513<t><?rfc needLines="4"?>
1514  299 Miscellaneous persistent warning
1515  <list>
1516    <t>The warning text can include arbitrary information to be presented to a human
1517      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1518      action.</t>
1519  </list>
1520</t>
1521</section>
1522
1523</section>
1524
1525<section anchor="history.lists" title="History Lists">
1526<t>
1527  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1528  can be used to redisplay an entity retrieved earlier in a session.
1529</t>
1530<t>
1531  History mechanisms and caches are different. In particular history mechanisms
1532  &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1533  history mechanism is meant to show exactly what the user saw at the time when the resource
1534  was retrieved.
1535</t>
1536<t>
1537  By default, an expiration time does not apply to history mechanisms. If the entity is still
1538  in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1539  unless the user has specifically configured the agent to refresh expired history documents.
1540</t>
1541<t>
1542  This is not to be construed to prohibit the history mechanism from telling the user that a
1543  view might be stale.
1544</t>
1545<x:note>
1546  <t>
1547    <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1548    stale resources, this will tend to force service authors to avoid using HTTP expiration
1549    controls and cache controls when they would otherwise like to. Service authors may
1550    consider it important that users not be presented with error messages or warning
1551    messages when they use navigation controls (such as BACK) to view previously fetched
1552    resources. Even though sometimes such resources ought not be cached, or ought to expire
1553    quickly, user interface considerations may force service authors to resort to other
1554    means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1555    of improperly functioning history mechanisms.
1556  </t>
1557</x:note>
1558</section>
1559
1560
1561<section anchor="IANA.considerations" title="IANA Considerations">
1562
1563<section anchor="message.header.registration" title="Message Header Registration">
1564<t>
1565  The Message Header Registry located at <eref
1566  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1567  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1568</t>
1569<?BEGININC p6-cache.iana-headers ?>
1570<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1571<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1572   <ttcol>Header Field Name</ttcol>
1573   <ttcol>Protocol</ttcol>
1574   <ttcol>Status</ttcol>
1575   <ttcol>Reference</ttcol>
1576
1577   <c>Age</c>
1578   <c>http</c>
1579   <c>standard</c>
1580   <c>
1581      <xref target="header.age"/>
1582   </c>
1583   <c>Cache-Control</c>
1584   <c>http</c>
1585   <c>standard</c>
1586   <c>
1587      <xref target="header.cache-control"/>
1588   </c>
1589   <c>Expires</c>
1590   <c>http</c>
1591   <c>standard</c>
1592   <c>
1593      <xref target="header.expires"/>
1594   </c>
1595   <c>Pragma</c>
1596   <c>http</c>
1597   <c>standard</c>
1598   <c>
1599      <xref target="header.pragma"/>
1600   </c>
1601   <c>Vary</c>
1602   <c>http</c>
1603   <c>standard</c>
1604   <c>
1605      <xref target="header.vary"/>
1606   </c>
1607   <c>Warning</c>
1608   <c>http</c>
1609   <c>standard</c>
1610   <c>
1611      <xref target="header.warning"/>
1612   </c>
1613</texttable>
1614<!--(END)-->
1615<?ENDINC p6-cache.iana-headers ?>
1616<t>
1617  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1618</t>
1619</section>
1620
1621</section>
1622
1623<section anchor="security.considerations" title="Security Considerations">
1624<t>
1625  Caches expose additional potential vulnerabilities, since the contents of the cache
1626  represent an attractive target for malicious exploitation. Because cache contents persist
1627  after an HTTP request is complete, an attack on the cache can reveal information long after
1628  a user believes that the information has been removed from the network. Therefore, cache
1629  contents should be protected as sensitive information.
1630</t>
1631</section>
1632
1633<section anchor="ack" title="Acknowledgments">
1634<t>
1635  Much of the content and presentation of the caching design is due to suggestions and
1636  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1637  and Larry Masinter.
1638</t>
1639</section>
1640
1641</middle>
1642
1643<back>
1644<references title="Normative References">
1645
1646  <reference anchor="Part1">
1647    <front>
1648      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1649      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1650        <organization abbrev="Day Software">Day Software</organization>
1651        <address><email>fielding@gbiv.com</email></address>
1652      </author>
1653      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1654        <organization>One Laptop per Child</organization>
1655        <address><email>jg@laptop.org</email></address>
1656      </author>
1657      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1658        <organization abbrev="HP">Hewlett-Packard Company</organization>
1659        <address><email>JeffMogul@acm.org</email></address>
1660      </author>
1661      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1662        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1663        <address><email>henrikn@microsoft.com</email></address>
1664      </author>
1665      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1666        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1667        <address><email>LMM@acm.org</email></address>
1668      </author>
1669      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1670        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1671        <address><email>paulle@microsoft.com</email></address>
1672      </author>
1673      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1674        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1675        <address><email>timbl@w3.org</email></address>
1676      </author>
1677      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1678        <organization abbrev="W3C">World Wide Web Consortium</organization>
1679        <address><email>ylafon@w3.org</email></address>
1680      </author>
1681      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1682        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1683        <address><email>julian.reschke@greenbytes.de</email></address>
1684      </author>
1685      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1686    </front>
1687    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1688    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1689  </reference>
1690
1691  <reference anchor="Part2">
1692    <front>
1693      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1694      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1695        <organization abbrev="Day Software">Day Software</organization>
1696        <address><email>fielding@gbiv.com</email></address>
1697      </author>
1698      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1699        <organization>One Laptop per Child</organization>
1700        <address><email>jg@laptop.org</email></address>
1701      </author>
1702      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1703        <organization abbrev="HP">Hewlett-Packard Company</organization>
1704        <address><email>JeffMogul@acm.org</email></address>
1705      </author>
1706      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1707        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1708        <address><email>henrikn@microsoft.com</email></address>
1709      </author>
1710      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1711        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1712        <address><email>LMM@acm.org</email></address>
1713      </author>
1714      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1715        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1716        <address><email>paulle@microsoft.com</email></address>
1717      </author>
1718      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1719        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1720        <address><email>timbl@w3.org</email></address>
1721      </author>
1722      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1723        <organization abbrev="W3C">World Wide Web Consortium</organization>
1724        <address><email>ylafon@w3.org</email></address>
1725      </author>
1726      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1727        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1728        <address><email>julian.reschke@greenbytes.de</email></address>
1729      </author>
1730      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1731    </front>
1732    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1733    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1734  </reference>
1735
1736  <reference anchor="Part3">
1737    <front>
1738      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1739      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1740        <organization abbrev="Day Software">Day Software</organization>
1741        <address><email>fielding@gbiv.com</email></address>
1742      </author>
1743      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1744        <organization>One Laptop per Child</organization>
1745        <address><email>jg@laptop.org</email></address>
1746      </author>
1747      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1748        <organization abbrev="HP">Hewlett-Packard Company</organization>
1749        <address><email>JeffMogul@acm.org</email></address>
1750      </author>
1751      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1752        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1753        <address><email>henrikn@microsoft.com</email></address>
1754      </author>
1755      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1756        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1757        <address><email>LMM@acm.org</email></address>
1758      </author>
1759      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1760        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1761        <address><email>paulle@microsoft.com</email></address>
1762      </author>
1763      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1764        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1765        <address><email>timbl@w3.org</email></address>
1766      </author>
1767      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1768        <organization abbrev="W3C">World Wide Web Consortium</organization>
1769        <address><email>ylafon@w3.org</email></address>
1770      </author>
1771      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1772        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1773        <address><email>julian.reschke@greenbytes.de</email></address>
1774      </author>
1775      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1776    </front>
1777    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1778    <x:source basename="p3-payload" href="p3-payload.xml" />
1779  </reference>
1780
1781  <reference anchor="Part4">
1782    <front>
1783      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1784      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1785        <organization abbrev="Day Software">Day Software</organization>
1786        <address><email>fielding@gbiv.com</email></address>
1787      </author>
1788      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1789        <organization>One Laptop per Child</organization>
1790        <address><email>jg@laptop.org</email></address>
1791      </author>
1792      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1793        <organization abbrev="HP">Hewlett-Packard Company</organization>
1794        <address><email>JeffMogul@acm.org</email></address>
1795      </author>
1796      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1797        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1798        <address><email>henrikn@microsoft.com</email></address>
1799      </author>
1800      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1801        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1802        <address><email>LMM@acm.org</email></address>
1803      </author>
1804      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1805        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1806        <address><email>paulle@microsoft.com</email></address>
1807      </author>
1808      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1809        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1810        <address><email>timbl@w3.org</email></address>
1811      </author>
1812      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1813        <organization abbrev="W3C">World Wide Web Consortium</organization>
1814        <address><email>ylafon@w3.org</email></address>
1815      </author>
1816      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1817        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1818        <address><email>julian.reschke@greenbytes.de</email></address>
1819      </author>
1820      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1821    </front>
1822    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1823    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1824  </reference>
1825
1826  <reference anchor="Part5">
1827    <front>
1828      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1829      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1830        <organization abbrev="Day Software">Day Software</organization>
1831        <address><email>fielding@gbiv.com</email></address>
1832      </author>
1833      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1834        <organization>One Laptop per Child</organization>
1835        <address><email>jg@laptop.org</email></address>
1836      </author>
1837      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1838        <organization abbrev="HP">Hewlett-Packard Company</organization>
1839        <address><email>JeffMogul@acm.org</email></address>
1840      </author>
1841      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1842        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1843        <address><email>henrikn@microsoft.com</email></address>
1844      </author>
1845      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1846        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1847        <address><email>LMM@acm.org</email></address>
1848      </author>
1849      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1850        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1851        <address><email>paulle@microsoft.com</email></address>
1852      </author>
1853      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1854        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1855        <address><email>timbl@w3.org</email></address>
1856      </author>
1857      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1858        <organization abbrev="W3C">World Wide Web Consortium</organization>
1859        <address><email>ylafon@w3.org</email></address>
1860      </author>
1861      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1862        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1863        <address><email>julian.reschke@greenbytes.de</email></address>
1864      </author>
1865      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1866    </front>
1867    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1868    <x:source basename="p5-range" href="p5-range.xml" />
1869  </reference>
1870
1871  <reference anchor="Part7">
1872    <front>
1873      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1874      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1875        <organization abbrev="Day Software">Day Software</organization>
1876        <address><email>fielding@gbiv.com</email></address>
1877      </author>
1878      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1879        <organization>One Laptop per Child</organization>
1880        <address><email>jg@laptop.org</email></address>
1881      </author>
1882      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1883        <organization abbrev="HP">Hewlett-Packard Company</organization>
1884        <address><email>JeffMogul@acm.org</email></address>
1885      </author>
1886      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1887        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1888        <address><email>henrikn@microsoft.com</email></address>
1889      </author>
1890      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1891        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1892        <address><email>LMM@acm.org</email></address>
1893      </author>
1894      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1895        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1896        <address><email>paulle@microsoft.com</email></address>
1897      </author>
1898      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1899        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1900        <address><email>timbl@w3.org</email></address>
1901      </author>
1902      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1903        <organization abbrev="W3C">World Wide Web Consortium</organization>
1904        <address><email>ylafon@w3.org</email></address>
1905      </author>
1906      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1907        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1908        <address><email>julian.reschke@greenbytes.de</email></address>
1909      </author>
1910      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1911    </front>
1912    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1913    <x:source basename="p7-auth" href="p7-auth.xml" />
1914  </reference>
1915
1916  <reference anchor="RFC2119">
1917    <front>
1918      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1919      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1920        <organization>Harvard University</organization>
1921        <address><email>sob@harvard.edu</email></address>
1922      </author>
1923      <date month="March" year="1997" />
1924    </front>
1925    <seriesInfo name="BCP" value="14" />
1926    <seriesInfo name="RFC" value="2119" />
1927  </reference>
1928
1929  <reference anchor="RFC5234">
1930    <front>
1931      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1932      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1933        <organization>Brandenburg InternetWorking</organization>
1934        <address>
1935        <postal>
1936        <street>675 Spruce Dr.</street>
1937        <city>Sunnyvale</city>
1938        <region>CA</region>
1939        <code>94086</code>
1940        <country>US</country></postal>
1941        <phone>+1.408.246.8253</phone>
1942        <email>dcrocker@bbiw.net</email></address> 
1943      </author>
1944      <author initials="P." surname="Overell" fullname="Paul Overell">
1945        <organization>THUS plc.</organization>
1946        <address>
1947        <postal>
1948        <street>1/2 Berkeley Square</street>
1949        <street>99 Berkely Street</street>
1950        <city>Glasgow</city>
1951        <code>G3 7HR</code>
1952        <country>UK</country></postal>
1953        <email>paul.overell@thus.net</email></address>
1954      </author>
1955      <date month="January" year="2008"/>
1956    </front>
1957    <seriesInfo name="STD" value="68"/>
1958    <seriesInfo name="RFC" value="5234"/>
1959  </reference>
1960 
1961</references>
1962
1963<references title="Informative References">
1964
1965  <reference anchor="RFC1305">
1966    <front>
1967      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1968      <author fullname="David L. Mills" initials="D." surname="Mills">
1969        <organization>University of Delaware, Electrical Engineering Department</organization>
1970        <address><email>mills@udel.edu</email></address>
1971      </author>
1972      <date month="March" year="1992" />
1973    </front>
1974    <seriesInfo name="RFC" value="1305" />
1975  </reference>
1976
1977  <reference anchor="RFC2616">
1978    <front>
1979      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1980      <author fullname="R. Fielding" initials="R." surname="Fielding">
1981        <organization>University of California, Irvine</organization>
1982        <address><email>fielding@ics.uci.edu</email></address>
1983      </author>
1984      <author fullname="J. Gettys" initials="J." surname="Gettys">
1985        <organization>W3C</organization>
1986        <address><email>jg@w3.org</email></address>
1987      </author>
1988      <author fullname="J. Mogul" initials="J." surname="Mogul">
1989        <organization>Compaq Computer Corporation</organization>
1990        <address><email>mogul@wrl.dec.com</email></address>
1991      </author>
1992      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1993        <organization>MIT Laboratory for Computer Science</organization>
1994        <address><email>frystyk@w3.org</email></address>
1995      </author>
1996      <author fullname="L. Masinter" initials="L." surname="Masinter">
1997        <organization>Xerox Corporation</organization>
1998        <address><email>masinter@parc.xerox.com</email></address>
1999      </author>
2000      <author fullname="P. Leach" initials="P." surname="Leach">
2001        <organization>Microsoft Corporation</organization>
2002        <address><email>paulle@microsoft.com</email></address>
2003      </author>
2004      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2005        <organization>W3C</organization>
2006        <address><email>timbl@w3.org</email></address>
2007      </author>
2008      <date month="June" year="1999" />
2009    </front>
2010    <seriesInfo name="RFC" value="2616" />
2011  </reference>
2012
2013  <reference anchor="RFC3864">
2014    <front>
2015      <title>Registration Procedures for Message Header Fields</title>
2016      <author fullname="G. Klyne" initials="G." surname="Klyne">
2017        <organization>Nine by Nine</organization>
2018        <address><email>GK-IETF@ninebynine.org</email></address>
2019      </author>
2020      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2021        <organization>BEA Systems</organization>
2022        <address><email>mnot@pobox.com</email></address>
2023      </author>
2024      <author fullname="J. Mogul" initials="J." surname="Mogul">
2025        <organization>HP Labs</organization>
2026        <address><email>JeffMogul@acm.org</email></address>
2027      </author>
2028      <date month="September" year="2004" />
2029    </front>
2030    <seriesInfo name="BCP" value="90" />
2031    <seriesInfo name="RFC" value="3864" />
2032  </reference>
2033
2034</references>
2035
2036<section anchor="compatibility" title="Compatibility with Previous Versions">
2037
2038<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2039<t>
2040  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2041  this missing case.
2042  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2043</t>
2044<t>
2045  Transfer-coding and message lengths all interact in ways that required fixing exactly
2046  when chunked encoding is used (to allow for transfer encoding that may not be self
2047  delimiting); it was important to straighten out exactly how message lengths are computed.
2048  (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
2049  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2050</t>
2051<t>
2052  Proxies should be able to add Content-Length when appropriate.
2053  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2054</t>
2055<t
2056  >Range request responses would become very verbose if all meta-data were always returned;
2057  by allowing the server to only send needed headers in a 206 response, this problem can be
2058  avoided.
2059  (<xref target="combining.headers" />)
2060</t>
2061<t>
2062  The Cache-Control: max-age directive was not properly defined for responses.
2063  (<xref target="cache-response-directive"/>)
2064</t>
2065<t>
2066  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2067    format="counter" target="expiration.model" />, <xref format="counter"
2068    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2069  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2070  header, as PUT or other methods may have need for it in requests.
2071</t>
2072</section>
2073
2074<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2075<t>
2076  Remove requirement to consider Content-Location in successful responses
2077  in order to determine the appropriate response to use.
2078  (<xref target="validation.model" />)
2079</t>
2080<t>
2081  Clarify denial of service attack avoidance requirement.
2082  (<xref target="invalidation.after.updates.or.deletions" />)
2083</t>
2084<t>
2085  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2086  anymore, as these aspects never were implemented.
2087  (<xref target="header.warning" />)
2088</t>
2089</section>
2090
2091    </section>
2092
2093<?BEGININC p6-cache.abnf-appendix ?>
2094<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2095<figure>
2096<artwork type="abnf" name="p6-cache.parsed-abnf">
2097<x:ref>Age</x:ref> = "Age:" OWS Age-v
2098<x:ref>Age-v</x:ref> = delta-seconds
2099
2100<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2101<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2102 cache-directive ] )
2103
2104<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2105<x:ref>Expires-v</x:ref> = HTTP-date
2106
2107<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2108
2109<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2110
2111<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2112<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2113 pragma-directive ] )
2114
2115<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2116<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2117 ] ) )
2118
2119<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2120<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2121 ] )
2122
2123<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2124<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2125<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2126 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2127 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2128 cache-extension
2129<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2130 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2131 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2132 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2133 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2134 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2135
2136<x:ref>delta-seconds</x:ref> = 1*DIGIT
2137
2138<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2139
2140<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2141
2142<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2143<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2144<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2145
2146<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2147
2148<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2149
2150<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2151
2152<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2153<x:ref>warn-code</x:ref> = 3DIGIT
2154<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2155<x:ref>warn-text</x:ref> = quoted-string
2156<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2157 ]
2158</artwork>
2159</figure>
2160<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2161; Age defined but not used
2162; Cache-Control defined but not used
2163; Expires defined but not used
2164; Pragma defined but not used
2165; Vary defined but not used
2166; Warning defined but not used
2167</artwork></figure></section>
2168<?ENDINC p6-cache.abnf-appendix ?>
2169
2170<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2171
2172<section title="Since RFC2616">
2173  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2174</section>
2175
2176<section title="Since draft-ietf-httpbis-p6-cache-00">
2177<t>
2178  Closed issues:
2179  <list style="symbols">
2180    <t>
2181      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2182    <t>
2183      <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>
2184    <t>
2185      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2186    <t>
2187      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2188    <t>
2189      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2190    <t>
2191      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2192    <t>
2193      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2194    <t>
2195      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2196    <t>
2197      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2198  </list>
2199</t>
2200<t>
2201  Other changes:
2202  <list style="symbols">
2203    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2204        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2205  </list>
2206</t>
2207</section>
2208
2209<section title="Since draft-ietf-httpbis-p6-cache-01">
2210<t>
2211  Closed issues:
2212  <list style="symbols">
2213    <t>
2214      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2215  </list>
2216</t>
2217<t>
2218  Other changes:
2219  <list style="symbols">
2220    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2221      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2222    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2223      specification.</t>
2224  </list>
2225</t>
2226</section>
2227
2228<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2229<t>
2230  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2231  <list style="symbols">
2232    <t>Reference RFC 3984, and update header registrations for headers defined in this
2233      document.</t>
2234  </list>
2235</t>
2236</section>
2237
2238<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2239<t>
2240  Closed issues:
2241  <list style="symbols">
2242    <t>
2243      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2244  </list>
2245</t>
2246</section>
2247
2248<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2249<t>
2250  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2251  <list style="symbols">
2252    <t>
2253      Use "/" instead of "|" for alternatives.
2254    </t>
2255    <t>
2256      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2257      whitespace ("OWS") and required whitespace ("RWS").
2258    </t>
2259    <t>
2260      Rewrite ABNFs to spell out whitespace rules, factor out
2261      header value format definitions.
2262    </t>
2263  </list>
2264</t>
2265</section>
2266
2267<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2268<t>
2269  This is a total rewrite of this part of the specification.
2270</t>
2271<t>
2272  Affected issues:
2273  <list style="symbols">
2274    <t>
2275      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2276    <t>
2277      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2278    <t>
2279      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2280    <t>
2281      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2282  </list>
2283</t>
2284<t>
2285  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2286  <list style="symbols">
2287    <t>
2288      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2289    </t>
2290  </list>
2291</t>
2292</section>
2293
2294<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2295<t>
2296  Closed issues:
2297  <list style="symbols">
2298    <t>
2299      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2300      "base for numeric protocol elements"
2301    </t>
2302  </list>
2303</t>
2304<t>
2305  Affected issues:
2306  <list style="symbols">
2307    <t>
2308      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2309      Vary and non-existant headers
2310    </t>
2311  </list>
2312</t>
2313</section>
2314
2315<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2316<t>
2317  Closed issues:
2318  <list style="symbols">
2319    <t>
2320      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2321      "Definition of 1xx Warn-Codes"
2322    </t>
2323    <t>
2324      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2325      "Content-Location on 304 responses"
2326    </t>
2327    <t>
2328      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2329      "private and no-cache CC directives with headers"
2330    </t>
2331    <t>
2332      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2333      "RFC2047 and warn-text"
2334    </t>
2335  </list>
2336</t>
2337</section>
2338
2339</section>
2340  </back>
2341</rfc>
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