source: draft-ietf-httpbis/latest/p6-cache.xml @ 604

Last change on this file since 604 was 604, checked in by julian.reschke@…, 10 years ago

Prepare release of -07 drafts.

  • Property svn:eol-style set to native
File size: 101.8 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 "latest">
15  <!ENTITY ID-MONTH "July">
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 message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35]>
36<?rfc toc="yes" ?>
37<?rfc symrefs="yes" ?>
38<?rfc sortrefs="yes" ?>
39<?rfc compact="yes"?>
40<?rfc subcompact="no" ?>
41<?rfc linkmailto="no" ?>
42<?rfc editing="no" ?>
43<?rfc comments="yes"?>
44<?rfc inline="yes"?>
45<?rfc-ext allow-markup-in-artwork="yes" ?>
46<?rfc-ext include-references-in-index="yes" ?>
47<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
48  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
49<front>
50
51  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
52
53  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
54    <organization abbrev="Day Software">Day Software</organization>
55    <address>
56      <postal>
57        <street>23 Corporate Plaza DR, Suite 280</street>
58        <city>Newport Beach</city>
59        <region>CA</region>
60        <code>92660</code>
61        <country>USA</country>
62      </postal>
63      <phone>+1-949-706-5300</phone>
64      <facsimile>+1-949-706-5305</facsimile>
65      <email>fielding@gbiv.com</email>
66      <uri>http://roy.gbiv.com/</uri>
67    </address>
68  </author>
69
70  <author fullname="Jim Gettys" initials="J." surname="Gettys">
71    <organization>One Laptop per Child</organization>
72    <address>
73      <postal>
74        <street>21 Oak Knoll Road</street>
75        <city>Carlisle</city>
76        <region>MA</region>
77        <code>01741</code>
78        <country>USA</country>
79      </postal>
80      <email>jg@laptop.org</email>
81      <uri>http://www.laptop.org/</uri>
82    </address>
83  </author>
84
85  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
86    <organization abbrev="HP">Hewlett-Packard Company</organization>
87    <address>
88      <postal>
89        <street>HP Labs, Large Scale Systems Group</street>
90        <street>1501 Page Mill Road, MS 1177</street>
91        <city>Palo Alto</city>
92        <region>CA</region>
93        <code>94304</code>
94        <country>USA</country>
95      </postal>
96      <email>JeffMogul@acm.org</email>
97    </address>
98  </author>
99
100  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108        <country>USA</country>
109      </postal>
110      <email>henrikn@microsoft.com</email>
111    </address>
112  </author>
113
114  <author fullname="Larry Masinter" initials="L." surname="Masinter">
115    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
116    <address>
117      <postal>
118        <street>345 Park Ave</street>
119        <city>San Jose</city>
120        <region>CA</region>
121        <code>95110</code>
122        <country>USA</country>
123      </postal>
124      <email>LMM@acm.org</email>
125      <uri>http://larry.masinter.net/</uri>
126    </address>
127  </author>
128
129  <author fullname="Paul J. Leach" initials="P." surname="Leach">
130    <organization abbrev="Microsoft">Microsoft Corporation</organization>
131    <address>
132      <postal>
133        <street>1 Microsoft Way</street>
134        <city>Redmond</city>
135        <region>WA</region>
136        <code>98052</code>
137      </postal>
138      <email>paulle@microsoft.com</email>
139    </address>
140  </author>
141
142  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
143    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
144    <address>
145      <postal>
146        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
147        <street>The Stata Center, Building 32</street>
148        <street>32 Vassar Street</street>
149        <city>Cambridge</city>
150        <region>MA</region>
151        <code>02139</code>
152        <country>USA</country>
153      </postal>
154      <email>timbl@w3.org</email>
155      <uri>http://www.w3.org/People/Berners-Lee/</uri>
156    </address>
157  </author>
158
159  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
160    <organization abbrev="W3C">World Wide Web Consortium</organization>
161    <address>
162      <postal>
163        <street>W3C / ERCIM</street>
164        <street>2004, rte des Lucioles</street>
165        <city>Sophia-Antipolis</city>
166        <region>AM</region>
167        <code>06902</code>
168        <country>France</country>
169      </postal>
170      <email>ylafon@w3.org</email>
171      <uri>http://www.raubacapeu.net/people/yves/</uri>
172    </address>
173  </author>
174
175  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
176    <organization />
177    <address>           
178      <email>mnot@mnot.net</email>     
179      <uri>http://www.mnot.net/</uri>           
180    </address>
181  </author>
182
183  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
184    <organization abbrev="greenbytes">greenbytes GmbH</organization>
185    <address>
186      <postal>
187        <street>Hafenweg 16</street>
188        <city>Muenster</city><region>NW</region><code>48155</code>
189        <country>Germany</country>
190      </postal>
191      <phone>+49 251 2807760</phone>   
192      <facsimile>+49 251 2807761</facsimile>   
193      <email>julian.reschke@greenbytes.de</email>       
194      <uri>http://greenbytes.de/tech/webdav/</uri>     
195    </address>
196  </author>
197
198  <date month="&ID-MONTH;" year="&ID-YEAR;" />
199  <workgroup>HTTPbis Working Group</workgroup>
200
201<abstract>
202<t>
203  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
204  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
205  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
206  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
207  fields that control cache behavior or indicate cacheable response messages.
208</t>
209</abstract>
210
211<note title="Editorial Note (To be removed by RFC Editor)">
212<t>
213  Discussion of this draft should take place on the HTTPBIS working group 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 &message-headers;&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.</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  Checking with the origin server to see if a stale or otherwise unusable cached response
721  can be reused is called "validating" or "revalidating." Doing so potentially avoids
722  the overhead of retransmitting the response body when the stored response is valid.
723</t>
724<t>
725  HTTP's conditional request mechanism &conditional; is used for this purpose. When a stored
726  response includes one or more validators, such as the field values of an ETag or
727  Last-Modified header field, then a validating request &SHOULD; be made conditional
728  to those field values.
729</t>
730<t>
731  A 304 (Not Modified) response status code indicates that the stored
732  response can be updated and reused; see <xref target="combining.headers"/>.
733</t>
734<t>
735  If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
736  request and replace the stored response. <cref>Should there be a requirement here?</cref>
737</t>
738<t>
739  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
740  either forward this response to the requesting client, or act as if the server failed to
741  respond. In the latter case, it &MAY; return a previously stored response (which &SHOULD; include the
742  111 warn-code; see <xref target="header.warning"/>) unless the
743  stored response includes the "must-revalidate" cache directive (see <xref
744  target="serving.stale.responses" />).
745</t>
746</section>
747
748<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
749<t>
750  Because unsafe methods (&safe-methods;) have the potential for changing state on the
751  origin server, intervening caches can use them to keep their contents
752  up-to-date.
753</t>
754<t>
755  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
756  as the URI(s) in the Location and Content-Location headers (if present):
757  <list style="symbols">
758    <t>PUT</t>
759    <t>DELETE</t>
760    <t>POST</t>
761  </list>
762</t>
763<t>
764  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
765  be performed if the host part of that URI differs from the host part in the Request-URI.
766  This helps prevent denial of service attacks.
767</t>
768<t>
769  <cref>TODO: "host part" needs to be specified better.</cref>
770</t>
771<t>
772  A cache that passes through requests for methods it does not understand &SHOULD;
773  invalidate the Request-URI.
774</t>
775<t>
776  Here, "invalidate" means that the cache will either remove all stored responses related
777  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
778  before they can be returned in response to a subsequent request.
779</t>
780<t>
781  Note that this does not guarantee that all appropriate responses are invalidated. For
782  example, the request that caused the change at the origin server might not have gone
783  through the cache where a response is stored.
784</t>
785<t>
786  <cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
787</t>
788</section>
789
790<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
791<t>
792  Use of server-driven content negotiation (&server-driven-negotiation;) alters
793  the conditions under which a cache can use the response for subsequent
794  requests.
795</t>
796<t>
797  When a cache receives a request that can be satisfied by a stored response
798  that includes a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that response unless
799  all of the selecting request-headers in the presented request match the corresponding
800  stored request-headers from the original request.
801</t>
802<t>
803  The selecting request-headers from two requests are defined to match if and only if the
804  selecting request-headers in the first request can be transformed to the selecting
805  request-headers in the second request by adding or removing linear white space
806  <cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
807  combining multiple message-header fields with the same field name following the rules
808  about message headers in &message-headers;. <cref anchor="DISCUSS-header-specific-canonicalization">
809  Should the matching requirement be relaxed so that it would be ok to use a cached response
810  if the selecting request headers match after header-specific canonicalization?
811  (see <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/147">Issue 147</eref>)
812  </cref>
813</t>
814<t>
815  A Vary header field-value of "*" always fails to match, and subsequent requests to that
816  resource can only be properly interpreted by the origin server.
817</t>
818<t>
819  If no stored response matches, the cache &MAY; forward the presented request to the origin
820  server in a conditional request, and &SHOULD; include all ETags stored with
821  potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
822  includes an entity tag or Content-Location that indicates the entity to be used, that
823  cached response &MUST; be used to satisfy the presented request, and &SHOULD;
824  be used to update the corresponding stored response; see <xref target="combining.headers"/>.
825</t>
826<t>
827  If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT; 
828  be included in the If-None-Match header field unless the request is for a range that would
829  be fully satisfied by that stored response.
830</t>
831<t>
832  If a cache receives a successful response whose Content-Location field matches that of an
833  existing stored response for the same Request-URI, whose entity-tag differs from that of
834  the existing stored response, and whose Date is more recent than that of the existing
835  response, the existing response &SHOULD-NOT; be returned in response to future
836  requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref>
837</t>
838</section>
839
840<section anchor="combining.headers" title="Combining Responses">
841<t>
842  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
843  it needs to update the stored response with the new one, so that the updated response can
844  be sent to the client.
845</t>
846<t>
847  If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
848  the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
849  Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
850  the stored response with the updated entity-body received in the response and use the
851  result as the updated entity-body (see &combining-byte-ranges;).
852</t>
853<t>
854  The stored response headers are used for the updated response, except that
855  <list style="symbols">
856    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
857      &MUST; be deleted from the stored response and the forwarded response.</t>
858    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
859      response and the forwarded response.</t>
860    <t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
861      headers from the stored response.</t>
862  </list>
863</t>
864<t>
865  A cache &MUST; also replace any stored headers with corresponding headers received in the
866  incoming response, except for Warning headers as described immediately above. If a header
867  field-name in the incoming response matches more than one header in the stored response,
868  all such old headers &MUST; be replaced. It &MAY; store the combined
869  entity-body.
870</t>
871<t>
872  <cref>ISSUE: discuss how to handle HEAD updates</cref>
873</t>
874</section>
875
876</section>
877
878<section anchor="header.fields" title="Header Field Definitions">
879<t>
880  This section defines the syntax and semantics of HTTP/1.1 header fields
881  related to caching.
882</t>
883<t>
884  For entity-header fields, both sender and recipient refer to either the client or the
885  server, depending on who sends and who receives the entity.
886</t>
887
888<section anchor="header.age" title="Age">
889  <iref item="Age header" primary="true" x:for-anchor="" />
890  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
891  <x:anchor-alias value="Age"/>
892  <x:anchor-alias value="Age-v"/>
893  <x:anchor-alias value="age-value"/>
894<t>
895  The response-header field "Age" conveys the sender's estimate of the amount of time since
896  the response (or its validation) was generated at the origin server. Age values are
897  calculated as specified in <xref target="age.calculations" />.
898</t>
899<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
900  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
901  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
902</artwork></figure>
903<t anchor="rule.delta-seconds">
904  <x:anchor-alias value="delta-seconds" />
905  Age field-values are non-negative integers, representing time in seconds.
906</t>
907<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
908  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
909</artwork></figure>
910<t>
911  If a cache receives a value larger than the largest positive integer it can represent, or
912  if any of its age calculations overflows, it &MUST; transmit an Age header with a
913  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
914  of at least 31 bits of range.
915</t>
916<t>
917  The presence of an Age header field in a response implies that a response is not
918  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
919  Age header field.
920</t>
921</section>
922
923<section anchor="header.cache-control" title="Cache-Control">
924  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
925  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
926  <x:anchor-alias value="Cache-Control"/>
927  <x:anchor-alias value="Cache-Control-v"/>
928  <x:anchor-alias value="cache-directive"/>
929  <x:anchor-alias value="cache-extension"/>
930  <x:anchor-alias value="cache-request-directive"/>
931  <x:anchor-alias value="cache-response-directive"/>
932<t>
933  The general-header field "Cache-Control" is used to specify directives that &MUST; be
934  obeyed by all caches along the request/response chain. The directives specify behavior
935  intended to prevent caches from adversely interfering with the request or response. Cache
936  directives are unidirectional in that the presence of a directive in a request does not
937  imply that the same directive is to be given in the response.
938</t>
939<x:note>
940  <t>
941    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
942    Pragma: no-cache (see <xref target="header.pragma" />).
943  </t>
944</x:note>
945<t>
946  Cache directives &MUST; be passed through by a proxy or gateway application,
947  regardless of their significance to that application, since the directives might be
948  applicable to all recipients along the request/response chain. It is not possible to
949  target a directive to a specific cache.
950</t>
951<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"/>
952  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
953  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
954
955  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
956     / <x:ref>cache-response-directive</x:ref>
957
958  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
959</artwork></figure>
960
961<section anchor="cache-request-directive" title="Request Cache-Control Directives">
962  <x:anchor-alias value="cache-request-directive" />
963
964<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
965  <x:ref>cache-request-directive</x:ref> =
966       "no-cache"
967     / "no-store"
968     / "max-age" "=" <x:ref>delta-seconds</x:ref>
969     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
970     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
971     / "no-transform"
972     / "only-if-cached"
973     / <x:ref>cache-extension</x:ref>
974</artwork></figure>
975
976<t>
977  <iref item="Cache Directives" primary="true" subitem="no-cache" />
978  <iref item="no-cache" primary="true" subitem="Cache Directive" />
979  no-cache
980  <list>
981    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
982      used to satisfy the request without successful validation on the origin server.</t>
983  </list>
984</t>
985<t>
986  <iref item="Cache Directives" primary="true" subitem="no-store" />
987  <iref item="no-store" primary="true" subitem="Cache Directive" />
988  no-store
989  <list>
990    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
991      of either this request or any response to it. This directive applies to both
992      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
993      cache &MUST-NOT; intentionally store the information in non-volatile storage,
994      and &MUST; make a best-effort attempt to remove the information from volatile
995      storage as promptly as possible after forwarding it.</t>
996    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
997      particular, malicious or compromised caches might not recognize or obey this
998      directive, and communications networks may be vulnerable to eavesdropping.</t>
999  </list>
1000</t>
1001<t>
1002  <iref item="Cache Directives" primary="true" subitem="max-age" />
1003  <iref item="max-age" primary="true" subitem="Cache Directive" />
1004  max-age
1005  <list>
1006    <t>The max-age request directive indicates that the client is willing to accept a
1007      response whose age is no greater than the specified time in seconds. Unless
1008      max-stale directive is also included, the client is not willing to accept a stale
1009      response.</t>
1010  </list>
1011</t>
1012<t>
1013  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1014  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1015  max-stale
1016  <list>
1017    <t>The max-stale request directive indicates that the client is willing to accept a
1018      response that has exceeded its expiration time. If max-stale is assigned a value,
1019      then the client is willing to accept a response that has exceeded its expiration
1020      time by no more than the specified number of seconds. If no value is assigned to
1021      max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
1022  </list>
1023</t>
1024<t>
1025  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1026  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1027  min-fresh
1028  <list>
1029    <t>The min-fresh request directive indicates that the client is willing to accept a
1030      response whose freshness lifetime is no less than its current age plus the specified
1031      time in seconds. That is, the client wants a response that will still be fresh for
1032      at least the specified number of seconds.</t>
1033  </list>
1034</t>
1035<t>
1036  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1037  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1038  no-transform
1039  <list>
1040    <t>The no-transform request directive indicates that an intermediate cache or proxy
1041      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1042      headers, nor the request entity-body.</t>
1043  </list>
1044</t>
1045<t>
1046  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1047  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1048  only-if-cached
1049  <list>
1050    <t>The only-if-cached request directive indicates that the client only wishes to
1051      return a stored response. If it receives this directive, a cache &SHOULD; either
1052      respond using a stored response that is consistent with the other constraints of the
1053      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1054      being operated as a unified system with good internal connectivity, such a request
1055      &MAY; be forwarded within that group of caches.</t>
1056  </list>
1057</t>
1058</section>
1059
1060<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1061  <x:anchor-alias value="cache-response-directive" />
1062
1063<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1064  <x:ref>cache-response-directive</x:ref> =
1065       "public"
1066     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1067     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1068     / "no-store"
1069     / "no-transform"
1070     / "must-revalidate"
1071     / "proxy-revalidate"
1072     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1073     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1074     / <x:ref>cache-extension</x:ref>
1075</artwork></figure>
1076
1077<t>
1078  <iref item="Cache Directives" primary="true" subitem="public" />
1079  <iref item="public" primary="true" subitem="Cache Directive" />
1080  public
1081  <list>
1082    <t>The public response directive indicates that the response &MAY; be cached, even
1083      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1084      (See also Authorization, &header-authorization;, for additional details.) </t>
1085  </list>
1086</t>
1087<t>
1088  <iref item="Cache Directives" primary="true" subitem="private" />
1089  <iref item="private" primary="true" subitem="Cache Directive" />
1090  private
1091  <list>
1092    <t>The private response directive indicates that the response message is intended for
1093      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1094      cache &MAY; store the response.</t>
1095    <t>If the private response directive specifies one or more field-names, this
1096      requirement is limited to the field-values associated with the listed response
1097      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1098      cache, whereas the remainder of the response message &MAY; be.</t>
1099    <t>
1100      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1101      be stored, and cannot ensure the privacy of the message content.</t>
1102  </list>
1103</t>
1104<t>
1105  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1106  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1107  no-cache
1108  <list>
1109    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1110      satisfy a subsequent request without successful validation on the origin server.
1111      This allows an origin server to prevent caching even by caches that have been
1112      configured to return stale responses.</t>
1113    <t>If the no-cache response directive specifies one or more field-names, this
1114      requirement is limited to the field-values associated with the listed response
1115      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1116      to a subsequent request without successful validation on the origin server. This
1117      allows an origin server to prevent the re-use of certain header fields in a
1118      response, while still allowing caching of the rest of the response.</t>
1119    <t>
1120      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1121    </t>
1122  </list>
1123</t>
1124
1125<t>
1126  <iref item="Cache Directives" primary="true" subitem="no-store" />
1127  <iref item="no-store" primary="true" subitem="Cache Directive" />
1128  no-store
1129  <list>
1130    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1131      part of either the immediate request or response. This directive applies to both
1132      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1133      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1134      and &MUST; make a best-effort attempt to remove the information from volatile
1135      storage as promptly as possible after forwarding it.</t>
1136    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1137      particular, malicious or compromised caches might not recognize or obey this
1138      directive, and communications networks may be vulnerable to eavesdropping.</t>
1139  </list>
1140</t>
1141<t>
1142  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1143  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1144  must-revalidate
1145  <list>
1146    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1147     used to satisfy subsequent requests without successful validation on the origin server.</t>
1148    <t>The must-revalidate directive is necessary to support reliable operation for
1149      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1150      the must-revalidate directive; in particular, if the cache cannot reach the origin
1151      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1152    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1153      validate a request on the entity could result in incorrect operation, such as a
1154      silently unexecuted financial transaction.</t>
1155  </list>
1156</t>
1157<t>
1158  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1159  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1160  proxy-revalidate
1161  <list>
1162    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1163      response directive, except that it does not apply to non-shared caches.</t>
1164  </list>
1165</t>
1166<t>
1167  <iref item="Cache Directives" primary="true" subitem="max-age" />
1168  <iref item="max-age" primary="true" subitem="Cache Directive" />
1169  max-age
1170  <list>
1171    <t>The max-age response directive indicates that response is to be considered stale
1172      after its age is greater than the specified number of seconds.</t>
1173  </list>
1174</t>
1175<t>
1176  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1177  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1178  s-maxage
1179  <list>
1180    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1181      specified by this directive overrides the maximum age specified by either the
1182      max-age directive or the Expires header. The s-maxage directive also implies the
1183      semantics of the proxy-revalidate response directive.</t>
1184  </list>
1185</t>
1186<t>
1187  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1188  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1189  no-transform
1190  <list>
1191    <t>The no-transform response directive indicates that an intermediate cache or proxy
1192      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1193      headers, nor the response entity-body.</t>
1194  </list>
1195</t>
1196
1197</section>
1198
1199<section anchor="cache.control.extensions" title="Cache Control Extensions">
1200<t>
1201  The Cache-Control header field can be extended through the use of one or more
1202  cache-extension tokens, each with an optional value. Informational extensions (those
1203  that do not require a change in cache behavior) can be added without changing the
1204  semantics of other directives. Behavioral extensions are designed to work by acting as
1205  modifiers to the existing base of cache directives. Both the new directive and the
1206  standard directive are supplied, such that applications that do not understand the new
1207  directive will default to the behavior specified by the standard directive, and those
1208  that understand the new directive will recognize it as modifying the requirements
1209  associated with the standard directive. In this way, extensions to the cache-control
1210  directives can be made without requiring changes to the base protocol.
1211</t>
1212<t>
1213  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1214  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1215  all directives that it does not understand.
1216</t>
1217<t>
1218  For example, consider a hypothetical new response directive called "community" that
1219  acts as a modifier to the private directive. We define this new directive to mean that,
1220  in addition to any non-shared cache, any cache that is shared only by members of the
1221  community named within its value may cache the response. An origin server wishing to
1222  allow the UCI community to use an otherwise private response in their shared cache(s)
1223  could do so by including
1224</t>
1225<figure><artwork type="example">
1226  Cache-Control: private, community="UCI"
1227</artwork></figure>
1228<t>
1229  A cache seeing this header field will act correctly even if the cache does not
1230  understand the community cache-extension, since it will also see and understand the
1231  private directive and thus default to the safe behavior.
1232</t>
1233<t>
1234  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1235  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1236  directives (or the response's default cacheability) such that the cache behavior will
1237  remain minimally correct even if the cache does not understand the extension(s).
1238</t>
1239</section>
1240
1241</section>
1242
1243<section anchor="header.expires" title="Expires">
1244  <iref item="Expires header" primary="true" x:for-anchor="" />
1245  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1246  <x:anchor-alias value="Expires"/>
1247  <x:anchor-alias value="Expires-v"/>
1248<t>
1249  The entity-header field "Expires" gives the date/time after which the response is
1250  considered stale. See <xref target="expiration.model" /> for further discussion of the
1251  freshness model.
1252</t>
1253<t>
1254  The presence of an Expires field does not imply that the original resource will change or
1255  cease to exist at, before, or after that time.
1256</t>
1257<t>
1258  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1259  it &MUST; be sent in rfc1123-date format.
1260</t>
1261<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1262  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1263  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1264</artwork></figure>
1265<figure>
1266  <preamble>For example</preamble>
1267<artwork type="example">
1268  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1269</artwork></figure>
1270<x:note>
1271  <t>
1272    <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
1273    directive (see <xref target="cache-response-directive" />), that directive overrides
1274    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1275  </t>
1276</x:note>
1277<t>
1278  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1279</t>
1280<t>
1281  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1282  including the value "0", as in the past (i.e., "already expired").
1283</t>
1284</section>
1285
1286<section anchor="header.pragma" title="Pragma">
1287  <iref item="Pragma header" primary="true" x:for-anchor="" />
1288  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1289  <x:anchor-alias value="extension-pragma"/>
1290  <x:anchor-alias value="Pragma"/>
1291  <x:anchor-alias value="Pragma-v"/>
1292  <x:anchor-alias value="pragma-directive"/>
1293<t>
1294  The general-header field "Pragma" is used to include implementation-specific directives
1295  that might apply to any recipient along the request/response chain. All pragma directives
1296  specify optional behavior from the viewpoint of the protocol; however, some systems
1297  &MAY; require that behavior be consistent with the directives.
1298</t>
1299<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"/>
1300  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1301  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1302  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1303  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1304</artwork></figure>
1305<t>
1306  When the no-cache directive is present in a request message, an application &SHOULD;
1307  forward the request toward the origin server even if it has a cached copy of what is being
1308  requested. This pragma directive has the same semantics as the no-cache response directive
1309  (see <xref target="cache-response-directive" />) and is defined here for backward
1310  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1311  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1312  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1313</t>
1314<x:note>
1315  <t>
1316    <x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
1317    is not actually specified, it does not provide a reliable replacement for
1318    "Cache-Control: no-cache" in a response.
1319  </t>
1320</x:note>
1321<t>
1322  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1323</t>
1324</section>
1325
1326<section anchor="header.vary" title="Vary">
1327  <iref item="Vary header" primary="true" x:for-anchor="" />
1328  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1329  <x:anchor-alias value="Vary"/>
1330  <x:anchor-alias value="Vary-v"/>
1331<t>
1332  The "Vary" response-header field's value indicates the set of request-header fields that
1333  determines, while the response is fresh, whether a cache is permitted to use the
1334  response to reply to a subsequent request without validation; see <xref 
1335  target="caching.negotiated.responses" />.
1336</t>
1337<t>
1338  In uncacheable or stale responses, the Vary field value advises the user agent about
1339  the criteria that were used to select the representation.
1340</t>
1341<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1342  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1343  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1344</artwork></figure>
1345<t>
1346  The set of header fields named by the Vary field value is known as the selecting
1347  request-headers.
1348</t>
1349<t>
1350  Servers &SHOULD; include a Vary header field with any cacheable response that is
1351  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1352  requests on that resource and informs the user agent about the presence of negotiation on
1353  that resource. A server &MAY; include a Vary header field with a non-cacheable
1354  response that is subject to server-driven negotiation, since this might provide the user
1355  agent with useful information about the dimensions over which the response varies at the
1356  time of the response.
1357</t>
1358<t>
1359  A Vary field value of "*" signals that unspecified parameters not limited to the
1360  request-headers (e.g., the network address of the client), play a role in the selection of
1361  the response representation; therefore, a cache cannot determine whether this response is
1362  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1363  it may only be generated by an origin server.
1364</t>
1365<t>
1366  The field-names given are not limited to the set of standard request-header fields
1367  defined by this specification. Field names are case-insensitive.
1368</t>
1369</section>
1370
1371<section anchor="header.warning" title="Warning">
1372  <iref item="Warning header" primary="true" x:for-anchor="" />
1373  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1374  <x:anchor-alias value="Warning"/>
1375  <x:anchor-alias value="Warning-v"/>
1376  <x:anchor-alias value="warning-value"/>
1377  <x:anchor-alias value="warn-agent"/>
1378  <x:anchor-alias value="warn-code"/>
1379  <x:anchor-alias value="warn-date"/>
1380  <x:anchor-alias value="warn-text"/>
1381<t>
1382  The general-header field "Warning" is used to carry additional information about the status
1383  or transformation of a message that might not be reflected in the message. This
1384  information is typically used to warn about possible incorrectness introduced by caching
1385  operations or transformations applied to the entity body of the message.
1386</t>
1387<t>
1388  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1389  warning, rather than an error status code, distinguish these responses from true failures.
1390</t>
1391<t>
1392  Warning headers can in general be applied to any message, however some warn-codes are
1393  specific to caches and can only be applied to response messages.
1394</t>
1395<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"/>
1396  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1397  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1398 
1399  <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>
1400                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1401 
1402  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1403  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1404                  ; the name or pseudonym of the server adding
1405                  ; the Warning header, for use in debugging
1406  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1407  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1408</artwork></figure>
1409<t>
1410  Multiple warnings can be attached to a response (either by the origin server or by
1411  a cache), including multiple warnings with the same code number. For example, a server
1412  might provide the same warning with texts in both English and Basque.
1413</t>
1414<t>
1415  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1416  possible, in the order that they appear in the response. If it is not possible to inform
1417  the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
1418  <list style="symbols">
1419    <t>Warnings that appear early in the response take priority over those appearing later
1420      in the response.</t>
1421    <t>Warnings in the user's preferred character set take priority over warnings in other
1422      character sets but with identical warn-codes and warn-agents.</t>
1423  </list>
1424</t>
1425<t>
1426  Systems that generate multiple Warning headers &SHOULD; order them with this user
1427  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1428  Warning headers.
1429</t>
1430<t>
1431  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1432  Warning is required to be deleted from a stored response after validation:
1433  <list style="symbols">
1434    <t>1xx Warnings that describe the freshness or validation status of the response, and so
1435      &MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
1436      except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
1437    <t>2xx Warnings that describe some aspect of the entity body or entity headers that is
1438      not rectified by a validation (for example, a lossy compression of the entity bodies)
1439      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1440      returned, in which case they &MUST; be.</t>
1441  </list>
1442</t>
1443<t>
1444  The warn-text &SHOULD; be in a natural language and character set that is most likely
1445  to be intelligible to the human user receiving the response. This decision can be based on
1446  any available knowledge, such as the location of the cache or user, the Accept-Language
1447  field in a request, the Content-Language field in a response, etc. The default language is
1448  English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).
1449</t>
1450<t>
1451  If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
1452  warn-text using the method described in <xref target="RFC2047" />.
1453</t>
1454<t>
1455  If an implementation sends a message with one or more Warning headers to a receiver whose
1456  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1457  warn-date that matches the Date header in the message.
1458</t>
1459<t>
1460  If an implementation receives a message with a warning-value that includes a warn-date,
1461  and that warn-date is different from the Date value in the response, then that
1462  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1463  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1464  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1465  well.
1466</t>
1467<t>
1468  The following warn-codes are defined by this specification, each with a recommended
1469  warn-text in English, and a description of its meaning.
1470</t>
1471<t><?rfc needLines="4"?>
1472  110 Response is stale
1473  <list>
1474    <t>&SHOULD; be included whenever the returned response is stale.</t>
1475  </list>
1476</t>
1477<t><?rfc needLines="4"?>
1478  111 Revalidation failed
1479  <list>
1480    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1481      validate the response failed, due to an inability to reach the server.</t>
1482  </list>
1483</t>
1484<t><?rfc needLines="4"?>
1485  112 Disconnected operation
1486  <list>
1487    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1488      the network for a period of time.</t>
1489  </list>
1490</t>
1491<t><?rfc needLines="4"?>
1492  113 Heuristic expiration
1493  <list>
1494    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1495      greater than 24 hours and the response's age is greater than 24 hours.</t>
1496  </list>
1497</t>
1498<t><?rfc needLines="4"?>
1499  199 Miscellaneous warning
1500  <list>
1501    <t>The warning text can include arbitrary information to be presented to a human
1502      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1503      action, besides presenting the warning to the user.</t>
1504  </list>
1505</t>
1506<t><?rfc needLines="4"?>
1507  214 Transformation applied
1508  <list>
1509    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1510      transformation changing the content-coding (as specified in the Content-Encoding
1511      header) or media-type (as specified in the Content-Type header) of the response, or
1512      the entity-body of the response, unless this Warning code already appears in the
1513      response.</t>
1514  </list>
1515</t>
1516<t><?rfc needLines="4"?>
1517  299 Miscellaneous persistent warning
1518  <list>
1519    <t>The warning text can include arbitrary information to be presented to a human
1520      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1521      action.</t>
1522  </list>
1523</t>
1524</section>
1525
1526</section>
1527
1528<section anchor="history.lists" title="History Lists">
1529<t>
1530  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1531  can be used to redisplay an entity retrieved earlier in a session.
1532</t>
1533<t>
1534  History mechanisms and caches are different. In particular history mechanisms
1535  &SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
1536  history mechanism is meant to show exactly what the user saw at the time when the resource
1537  was retrieved.
1538</t>
1539<t>
1540  By default, an expiration time does not apply to history mechanisms. If the entity is still
1541  in storage, a history mechanism &SHOULD; display it even if the entity has expired,
1542  unless the user has specifically configured the agent to refresh expired history documents.
1543</t>
1544<t>
1545  This is not to be construed to prohibit the history mechanism from telling the user that a
1546  view might be stale.
1547</t>
1548<x:note>
1549  <t>
1550    <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
1551    stale resources, this will tend to force service authors to avoid using HTTP expiration
1552    controls and cache controls when they would otherwise like to. Service authors may
1553    consider it important that users not be presented with error messages or warning
1554    messages when they use navigation controls (such as BACK) to view previously fetched
1555    resources. Even though sometimes such resources ought not be cached, or ought to expire
1556    quickly, user interface considerations may force service authors to resort to other
1557    means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
1558    of improperly functioning history mechanisms.
1559  </t>
1560</x:note>
1561</section>
1562
1563
1564<section anchor="IANA.considerations" title="IANA Considerations">
1565
1566<section anchor="message.header.registration" title="Message Header Registration">
1567<t>
1568  The Message Header Registry located at <eref
1569  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1570  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1571</t>
1572
1573<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1574<texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
1575  <ttcol>Header Field Name</ttcol>
1576  <ttcol>Protocol</ttcol>
1577  <ttcol>Status</ttcol>
1578  <ttcol>Reference</ttcol>
1579
1580  <c>Age</c>
1581  <c>http</c>
1582  <c>standard</c>
1583  <c>
1584    <xref target="header.age" />
1585  </c>
1586
1587  <c>Cache-Control</c>
1588  <c>http</c>
1589  <c>standard</c>
1590  <c>
1591    <xref target="header.cache-control" />
1592  </c>
1593
1594  <c>Expires</c>
1595  <c>http</c>
1596  <c>standard</c>
1597  <c>
1598    <xref target="header.expires" />
1599  </c>
1600
1601  <c>Pragma</c>
1602  <c>http</c>
1603  <c>standard</c>
1604  <c>
1605    <xref target="header.pragma" />
1606  </c>
1607
1608  <c>Vary</c>
1609  <c>http</c>
1610  <c>standard</c>
1611  <c>
1612    <xref target="header.vary" />
1613  </c>
1614
1615  <c>Warning</c>
1616  <c>http</c>
1617  <c>standard</c>
1618  <c>
1619    <xref target="header.warning" />
1620  </c>
1621</texttable>
1622<!--(END)-->
1623<t>
1624  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1625</t>
1626</section>
1627
1628</section>
1629
1630<section anchor="security.considerations" title="Security Considerations">
1631<t>
1632  Caches expose additional potential vulnerabilities, since the contents of the cache
1633  represent an attractive target for malicious exploitation. Because cache contents persist
1634  after an HTTP request is complete, an attack on the cache can reveal information long after
1635  a user believes that the information has been removed from the network. Therefore, cache
1636  contents should be protected as sensitive information.
1637</t>
1638</section>
1639
1640<section anchor="ack" title="Acknowledgments">
1641<t>
1642  Much of the content and presentation of the caching design is due to suggestions and
1643  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1644  and Larry Masinter.
1645</t>
1646</section>
1647
1648</middle>
1649
1650<back>
1651<references title="Normative References">
1652
1653  <reference anchor="ISO-8859-1">
1654    <front>
1655      <title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1656        1: Latin alphabet No. 1 </title>
1657      <author>
1658        <organization>International Organization for Standardization</organization>
1659      </author>
1660      <date year="1998" />
1661    </front>
1662    <seriesInfo name="ISO/IEC" value="8859-1:1998" />
1663  </reference>
1664
1665  <reference anchor="Part1">
1666    <front>
1667      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1668      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1669        <organization abbrev="Day Software">Day Software</organization>
1670        <address><email>fielding@gbiv.com</email></address>
1671      </author>
1672      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1673        <organization>One Laptop per Child</organization>
1674        <address><email>jg@laptop.org</email></address>
1675      </author>
1676      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1677        <organization abbrev="HP">Hewlett-Packard Company</organization>
1678        <address><email>JeffMogul@acm.org</email></address>
1679      </author>
1680      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1681        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1682        <address><email>henrikn@microsoft.com</email></address>
1683      </author>
1684      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1685        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1686        <address><email>LMM@acm.org</email></address>
1687      </author>
1688      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1689        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1690        <address><email>paulle@microsoft.com</email></address>
1691      </author>
1692      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1693        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1694        <address><email>timbl@w3.org</email></address>
1695      </author>
1696      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1697        <organization abbrev="W3C">World Wide Web Consortium</organization>
1698        <address><email>ylafon@w3.org</email></address>
1699      </author>
1700      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1701        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1702        <address><email>julian.reschke@greenbytes.de</email></address>
1703      </author>
1704      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1705    </front>
1706    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1707    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1708  </reference>
1709
1710  <reference anchor="Part2">
1711    <front>
1712      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1713      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1714        <organization abbrev="Day Software">Day Software</organization>
1715        <address><email>fielding@gbiv.com</email></address>
1716      </author>
1717      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1718        <organization>One Laptop per Child</organization>
1719        <address><email>jg@laptop.org</email></address>
1720      </author>
1721      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1722        <organization abbrev="HP">Hewlett-Packard Company</organization>
1723        <address><email>JeffMogul@acm.org</email></address>
1724      </author>
1725      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1726        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1727        <address><email>henrikn@microsoft.com</email></address>
1728      </author>
1729      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1730        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1731        <address><email>LMM@acm.org</email></address>
1732      </author>
1733      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1734        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1735        <address><email>paulle@microsoft.com</email></address>
1736      </author>
1737      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1738        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1739        <address><email>timbl@w3.org</email></address>
1740      </author>
1741      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1742        <organization abbrev="W3C">World Wide Web Consortium</organization>
1743        <address><email>ylafon@w3.org</email></address>
1744      </author>
1745      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1746        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1747        <address><email>julian.reschke@greenbytes.de</email></address>
1748      </author>
1749      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1750    </front>
1751    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1752    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1753  </reference>
1754
1755  <reference anchor="Part3">
1756    <front>
1757      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1758      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1759        <organization abbrev="Day Software">Day Software</organization>
1760        <address><email>fielding@gbiv.com</email></address>
1761      </author>
1762      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1763        <organization>One Laptop per Child</organization>
1764        <address><email>jg@laptop.org</email></address>
1765      </author>
1766      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1767        <organization abbrev="HP">Hewlett-Packard Company</organization>
1768        <address><email>JeffMogul@acm.org</email></address>
1769      </author>
1770      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1771        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1772        <address><email>henrikn@microsoft.com</email></address>
1773      </author>
1774      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1775        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1776        <address><email>LMM@acm.org</email></address>
1777      </author>
1778      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1779        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1780        <address><email>paulle@microsoft.com</email></address>
1781      </author>
1782      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1783        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1784        <address><email>timbl@w3.org</email></address>
1785      </author>
1786      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1787        <organization abbrev="W3C">World Wide Web Consortium</organization>
1788        <address><email>ylafon@w3.org</email></address>
1789      </author>
1790      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1791        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1792        <address><email>julian.reschke@greenbytes.de</email></address>
1793      </author>
1794      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1795    </front>
1796    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
1797    <x:source basename="p3-payload" href="p3-payload.xml" />
1798  </reference>
1799
1800  <reference anchor="Part4">
1801    <front>
1802      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1803      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1804        <organization abbrev="Day Software">Day Software</organization>
1805        <address><email>fielding@gbiv.com</email></address>
1806      </author>
1807      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1808        <organization>One Laptop per Child</organization>
1809        <address><email>jg@laptop.org</email></address>
1810      </author>
1811      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1812        <organization abbrev="HP">Hewlett-Packard Company</organization>
1813        <address><email>JeffMogul@acm.org</email></address>
1814      </author>
1815      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1816        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1817        <address><email>henrikn@microsoft.com</email></address>
1818      </author>
1819      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1820        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1821        <address><email>LMM@acm.org</email></address>
1822      </author>
1823      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1824        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1825        <address><email>paulle@microsoft.com</email></address>
1826      </author>
1827      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1828        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1829        <address><email>timbl@w3.org</email></address>
1830      </author>
1831      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1832        <organization abbrev="W3C">World Wide Web Consortium</organization>
1833        <address><email>ylafon@w3.org</email></address>
1834      </author>
1835      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1836        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1837        <address><email>julian.reschke@greenbytes.de</email></address>
1838      </author>
1839      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1840    </front>
1841    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1842    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1843  </reference>
1844
1845  <reference anchor="Part5">
1846    <front>
1847      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1848      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1849        <organization abbrev="Day Software">Day Software</organization>
1850        <address><email>fielding@gbiv.com</email></address>
1851      </author>
1852      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1853        <organization>One Laptop per Child</organization>
1854        <address><email>jg@laptop.org</email></address>
1855      </author>
1856      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1857        <organization abbrev="HP">Hewlett-Packard Company</organization>
1858        <address><email>JeffMogul@acm.org</email></address>
1859      </author>
1860      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1861        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1862        <address><email>henrikn@microsoft.com</email></address>
1863      </author>
1864      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1865        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1866        <address><email>LMM@acm.org</email></address>
1867      </author>
1868      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1869        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1870        <address><email>paulle@microsoft.com</email></address>
1871      </author>
1872      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1873        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1874        <address><email>timbl@w3.org</email></address>
1875      </author>
1876      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1877        <organization abbrev="W3C">World Wide Web Consortium</organization>
1878        <address><email>ylafon@w3.org</email></address>
1879      </author>
1880      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1881        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1882        <address><email>julian.reschke@greenbytes.de</email></address>
1883      </author>
1884      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1885    </front>
1886    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1887    <x:source basename="p5-range" href="p5-range.xml" />
1888  </reference>
1889
1890  <reference anchor="Part7">
1891    <front>
1892      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1893      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1894        <organization abbrev="Day Software">Day Software</organization>
1895        <address><email>fielding@gbiv.com</email></address>
1896      </author>
1897      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1898        <organization>One Laptop per Child</organization>
1899        <address><email>jg@laptop.org</email></address>
1900      </author>
1901      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1902        <organization abbrev="HP">Hewlett-Packard Company</organization>
1903        <address><email>JeffMogul@acm.org</email></address>
1904      </author>
1905      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1906        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1907        <address><email>henrikn@microsoft.com</email></address>
1908      </author>
1909      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1910        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1911        <address><email>LMM@acm.org</email></address>
1912      </author>
1913      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1914        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1915        <address><email>paulle@microsoft.com</email></address>
1916      </author>
1917      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1918        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1919        <address><email>timbl@w3.org</email></address>
1920      </author>
1921      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1922        <organization abbrev="W3C">World Wide Web Consortium</organization>
1923        <address><email>ylafon@w3.org</email></address>
1924      </author>
1925      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1926        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1927        <address><email>julian.reschke@greenbytes.de</email></address>
1928      </author>
1929      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1930    </front>
1931    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1932    <x:source basename="p7-auth" href="p7-auth.xml" />
1933  </reference>
1934
1935  <reference anchor="RFC2047">
1936    <front>
1937      <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
1938        Part Three: Message Header Extensions for Non-ASCII Text</title>
1939      <author fullname="Keith Moore" initials="K." surname="Moore">
1940        <organization>University of Tennessee</organization>
1941        <address><email>moore@cs.utk.edu</email></address>
1942      </author>
1943      <date month="November" year="1996" />
1944    </front>
1945    <seriesInfo name="RFC" value="2047" />
1946  </reference>
1947
1948  <reference anchor="RFC2119">
1949    <front>
1950      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1951      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1952        <organization>Harvard University</organization>
1953        <address><email>sob@harvard.edu</email></address>
1954      </author>
1955      <date month="March" year="1997" />
1956    </front>
1957    <seriesInfo name="BCP" value="14" />
1958    <seriesInfo name="RFC" value="2119" />
1959  </reference>
1960
1961  <reference anchor="RFC5234">
1962    <front>
1963      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1964      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1965        <organization>Brandenburg InternetWorking</organization>
1966        <address>
1967        <postal>
1968        <street>675 Spruce Dr.</street>
1969        <city>Sunnyvale</city>
1970        <region>CA</region>
1971        <code>94086</code>
1972        <country>US</country></postal>
1973        <phone>+1.408.246.8253</phone>
1974        <email>dcrocker@bbiw.net</email></address> 
1975      </author>
1976      <author initials="P." surname="Overell" fullname="Paul Overell">
1977        <organization>THUS plc.</organization>
1978        <address>
1979        <postal>
1980        <street>1/2 Berkeley Square</street>
1981        <street>99 Berkely Street</street>
1982        <city>Glasgow</city>
1983        <code>G3 7HR</code>
1984        <country>UK</country></postal>
1985        <email>paul.overell@thus.net</email></address>
1986      </author>
1987      <date month="January" year="2008"/>
1988    </front>
1989    <seriesInfo name="STD" value="68"/>
1990    <seriesInfo name="RFC" value="5234"/>
1991  </reference>
1992 
1993</references>
1994
1995<references title="Informative References">
1996
1997  <reference anchor="RFC1305">
1998    <front>
1999      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2000      <author fullname="David L. Mills" initials="D." surname="Mills">
2001        <organization>University of Delaware, Electrical Engineering Department</organization>
2002        <address><email>mills@udel.edu</email></address>
2003      </author>
2004      <date month="March" year="1992" />
2005    </front>
2006    <seriesInfo name="RFC" value="1305" />
2007  </reference>
2008
2009  <reference anchor="RFC2616">
2010    <front>
2011      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2012      <author fullname="R. Fielding" initials="R." surname="Fielding">
2013        <organization>University of California, Irvine</organization>
2014        <address><email>fielding@ics.uci.edu</email></address>
2015      </author>
2016      <author fullname="J. Gettys" initials="J." surname="Gettys">
2017        <organization>W3C</organization>
2018        <address><email>jg@w3.org</email></address>
2019      </author>
2020      <author fullname="J. Mogul" initials="J." surname="Mogul">
2021        <organization>Compaq Computer Corporation</organization>
2022        <address><email>mogul@wrl.dec.com</email></address>
2023      </author>
2024      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2025        <organization>MIT Laboratory for Computer Science</organization>
2026        <address><email>frystyk@w3.org</email></address>
2027      </author>
2028      <author fullname="L. Masinter" initials="L." surname="Masinter">
2029        <organization>Xerox Corporation</organization>
2030        <address><email>masinter@parc.xerox.com</email></address>
2031      </author>
2032      <author fullname="P. Leach" initials="P." surname="Leach">
2033        <organization>Microsoft Corporation</organization>
2034        <address><email>paulle@microsoft.com</email></address>
2035      </author>
2036      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2037        <organization>W3C</organization>
2038        <address><email>timbl@w3.org</email></address>
2039      </author>
2040      <date month="June" year="1999" />
2041    </front>
2042    <seriesInfo name="RFC" value="2616" />
2043  </reference>
2044
2045  <reference anchor="RFC3864">
2046    <front>
2047      <title>Registration Procedures for Message Header Fields</title>
2048      <author fullname="G. Klyne" initials="G." surname="Klyne">
2049        <organization>Nine by Nine</organization>
2050        <address><email>GK-IETF@ninebynine.org</email></address>
2051      </author>
2052      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2053        <organization>BEA Systems</organization>
2054        <address><email>mnot@pobox.com</email></address>
2055      </author>
2056      <author fullname="J. Mogul" initials="J." surname="Mogul">
2057        <organization>HP Labs</organization>
2058        <address><email>JeffMogul@acm.org</email></address>
2059      </author>
2060      <date month="September" year="2004" />
2061    </front>
2062    <seriesInfo name="BCP" value="90" />
2063    <seriesInfo name="RFC" value="3864" />
2064  </reference>
2065
2066</references>
2067
2068<section anchor="compatibility" title="Compatibility with Previous Versions">
2069
2070<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
2071<t>
2072  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
2073  this missing case.
2074  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
2075</t>
2076<t>
2077  Transfer-coding and message lengths all interact in ways that required fixing exactly
2078  when chunked encoding is used (to allow for transfer encoding that may not be self
2079  delimiting); it was important to straighten out exactly how message lengths are computed.
2080  (see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
2081  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2082</t>
2083<t>
2084  Proxies should be able to add Content-Length when appropriate.
2085  <cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
2086</t>
2087<t
2088  >Range request responses would become very verbose if all meta-data were always returned;
2089  by allowing the server to only send needed headers in a 206 response, this problem can be
2090  avoided.
2091  (<xref target="combining.headers" />)
2092</t>
2093<t>
2094  The Cache-Control: max-age directive was not properly defined for responses.
2095  (<xref target="cache-response-directive"/>)
2096</t>
2097<t>
2098  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
2099    format="counter" target="expiration.model" />, <xref format="counter"
2100    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
2101  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
2102  header, as PUT or other methods may have need for it in requests.
2103</t>
2104</section>
2105
2106<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2107<t>
2108  Clarify denial of service attack avoidance requirement.
2109  (<xref target="invalidation.after.updates.or.deletions" />)
2110</t>
2111</section>
2112
2113    </section>
2114
2115<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2116<figure>
2117<artwork type="abnf" name="p6-cache.parsed-abnf">
2118<x:ref>Age</x:ref> = "Age:" OWS Age-v
2119<x:ref>Age-v</x:ref> = delta-seconds
2120
2121<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2122<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2123 cache-directive ] )
2124
2125<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2126<x:ref>Expires-v</x:ref> = HTTP-date
2127
2128<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2&gt;
2129
2130<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2131
2132<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2133<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2134 pragma-directive ] )
2135
2136<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2137<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2138 ] ) )
2139
2140<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2141<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2142 ] )
2143
2144<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2145<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2146<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2147 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2148 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2149 cache-extension
2150<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2151 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2152 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2153 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2154 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2155 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2156
2157<x:ref>delta-seconds</x:ref> = 1*DIGIT
2158
2159<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2160
2161<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
2162
2163<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
2164<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2165<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
2166
2167<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2168
2169<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2170
2171<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
2172
2173<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2174<x:ref>warn-code</x:ref> = 3DIGIT
2175<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2176<x:ref>warn-text</x:ref> = quoted-string
2177<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2178 ]
2179
2180
2181</artwork>
2182</figure>
2183<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2184; Age defined but not used
2185; Cache-Control defined but not used
2186; Expires defined but not used
2187; Pragma defined but not used
2188; Vary defined but not used
2189; Warning defined but not used
2190</artwork></figure></section>
2191
2192<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2193
2194<section title="Since RFC2616">
2195  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2196</section>
2197
2198<section title="Since draft-ietf-httpbis-p6-cache-00">
2199<t>
2200  Closed issues:
2201  <list style="symbols">
2202    <t>
2203      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2204    <t>
2205      <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>
2206    <t>
2207      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2208    <t>
2209      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2210    <t>
2211      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2212    <t>
2213      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2214    <t>
2215      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2216    <t>
2217      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2218    <t>
2219      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2220  </list>
2221</t>
2222<t>
2223  Other changes:
2224  <list style="symbols">
2225    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2226        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2227  </list>
2228</t>
2229</section>
2230
2231<section title="Since draft-ietf-httpbis-p6-cache-01">
2232<t>
2233  Closed issues:
2234  <list style="symbols">
2235    <t>
2236      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2237  </list>
2238</t>
2239<t>
2240  Other changes:
2241  <list style="symbols">
2242    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2243      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2244    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2245      specification.</t>
2246  </list>
2247</t>
2248</section>
2249
2250<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2251<t>
2252  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2253  <list style="symbols">
2254    <t>Reference RFC 3984, and update header registrations for headers defined in this
2255      document.</t>
2256  </list>
2257</t>
2258</section>
2259
2260<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2261<t>
2262  Closed issues:
2263  <list style="symbols">
2264    <t>
2265      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2266  </list>
2267</t>
2268</section>
2269
2270<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2271<t>
2272  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2273  <list style="symbols"> 
2274    <t>
2275      Use "/" instead of "|" for alternatives.
2276    </t>
2277    <t>
2278      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2279      whitespace ("OWS") and required whitespace ("RWS").
2280    </t>
2281    <t>
2282      Rewrite ABNFs to spell out whitespace rules, factor out
2283      header value format definitions.
2284    </t>
2285  </list>
2286</t>
2287</section>
2288
2289<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2290<t>
2291  This is a total rewrite of this part of the specification.
2292</t>
2293<t>
2294  Affected issues:
2295  <list style="symbols">
2296    <t>
2297      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2298    <t>
2299      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2300    <t>
2301      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2302    <t>
2303      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2304  </list>
2305</t>
2306<t>
2307  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2308  <list style="symbols"> 
2309    <t>
2310      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2311    </t>
2312  </list>
2313</t>
2314</section>
2315
2316<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2317<t>
2318  Closed issues:
2319  <list style="symbols"> 
2320    <t>
2321      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2322      "base for numeric protocol elements"
2323    </t>
2324  </list>
2325</t>
2326</section>
2327
2328<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2329<t>
2330  None.
2331</t>
2332</section>
2333
2334</section>
2335  </back>
2336</rfc>
Note: See TracBrowser for help on using the repository browser.