source: draft-ietf-httpbis/latest/p6-cache.xml @ 770

Last change on this file since 770 was 770, checked in by julian.reschke@…, 10 years ago

No need to cite Part 3 anymore.

  • Property svn:eol-style set to native
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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "March">
16  <!ENTITY ID-YEAR "2010">
17  <!ENTITY notation                    "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                 "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY uri                         "<xref target='Part1' x:rel='#uri' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY 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 full-date                   "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY header-via                  "<xref target='Part1' x:rel='#header.via' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY header-last-modified        "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY header-fields               "<xref target='Part1' x:rel='#header.fields' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY weak-and-strong             "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34]>
35<?rfc toc="yes" ?>
36<?rfc symrefs="yes" ?>
37<?rfc sortrefs="yes" ?>
38<?rfc compact="yes"?>
39<?rfc subcompact="no" ?>
40<?rfc linkmailto="no" ?>
41<?rfc editing="no" ?>
42<?rfc comments="yes"?>
43<?rfc inline="yes"?>
44<?rfc-ext allow-markup-in-artwork="yes" ?>
45<?rfc-ext include-references-in-index="yes" ?>
46<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
47  obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
48<front>
49
50  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
51
52  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
53    <organization abbrev="Day Software">Day Software</organization>
54    <address>
55      <postal>
56        <street>23 Corporate Plaza DR, Suite 280</street>
57        <city>Newport Beach</city>
58        <region>CA</region>
59        <code>92660</code>
60        <country>USA</country>
61      </postal>
62      <phone>+1-949-706-5300</phone>
63      <facsimile>+1-949-706-5305</facsimile>
64      <email>fielding@gbiv.com</email>
65      <uri>http://roy.gbiv.com/</uri>
66    </address>
67  </author>
68
69  <author fullname="Jim Gettys" initials="J." surname="Gettys">
70    <organization>One Laptop per Child</organization>
71    <address>
72      <postal>
73        <street>21 Oak Knoll Road</street>
74        <city>Carlisle</city>
75        <region>MA</region>
76        <code>01741</code>
77        <country>USA</country>
78      </postal>
79      <email>jg@laptop.org</email>
80      <uri>http://www.laptop.org/</uri>
81    </address>
82  </author>
83
84  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
85    <organization abbrev="HP">Hewlett-Packard Company</organization>
86    <address>
87      <postal>
88        <street>HP Labs, Large Scale Systems Group</street>
89        <street>1501 Page Mill Road, MS 1177</street>
90        <city>Palo Alto</city>
91        <region>CA</region>
92        <code>94304</code>
93        <country>USA</country>
94      </postal>
95      <email>JeffMogul@acm.org</email>
96    </address>
97  </author>
98
99  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107        <country>USA</country>
108      </postal>
109      <email>henrikn@microsoft.com</email>
110    </address>
111  </author>
112
113  <author fullname="Larry Masinter" initials="L." surname="Masinter">
114    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
115    <address>
116      <postal>
117        <street>345 Park Ave</street>
118        <city>San Jose</city>
119        <region>CA</region>
120        <code>95110</code>
121        <country>USA</country>
122      </postal>
123      <email>LMM@acm.org</email>
124      <uri>http://larry.masinter.net/</uri>
125    </address>
126  </author>
127
128  <author fullname="Paul J. Leach" initials="P." surname="Leach">
129    <organization abbrev="Microsoft">Microsoft Corporation</organization>
130    <address>
131      <postal>
132        <street>1 Microsoft Way</street>
133        <city>Redmond</city>
134        <region>WA</region>
135        <code>98052</code>
136      </postal>
137      <email>paulle@microsoft.com</email>
138    </address>
139  </author>
140
141  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
142    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
143    <address>
144      <postal>
145        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
146        <street>The Stata Center, Building 32</street>
147        <street>32 Vassar Street</street>
148        <city>Cambridge</city>
149        <region>MA</region>
150        <code>02139</code>
151        <country>USA</country>
152      </postal>
153      <email>timbl@w3.org</email>
154      <uri>http://www.w3.org/People/Berners-Lee/</uri>
155    </address>
156  </author>
157
158  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
159    <organization abbrev="W3C">World Wide Web Consortium</organization>
160    <address>
161      <postal>
162        <street>W3C / ERCIM</street>
163        <street>2004, rte des Lucioles</street>
164        <city>Sophia-Antipolis</city>
165        <region>AM</region>
166        <code>06902</code>
167        <country>France</country>
168      </postal>
169      <email>ylafon@w3.org</email>
170      <uri>http://www.raubacapeu.net/people/yves/</uri>
171    </address>
172  </author>
173
174  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
175    <address>
176      <email>mnot@mnot.net</email>
177      <uri>http://www.mnot.net/</uri>
178    </address>
179  </author>
180
181  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
182    <organization abbrev="greenbytes">greenbytes GmbH</organization>
183    <address>
184      <postal>
185        <street>Hafenweg 16</street>
186        <city>Muenster</city><region>NW</region><code>48155</code>
187        <country>Germany</country>
188      </postal>
189      <phone>+49 251 2807760</phone>
190      <facsimile>+49 251 2807761</facsimile>
191      <email>julian.reschke@greenbytes.de</email>
192      <uri>http://greenbytes.de/tech/webdav/</uri>
193    </address>
194  </author>
195
196  <date month="&ID-MONTH;" year="&ID-YEAR;" />
197  <workgroup>HTTPbis Working Group</workgroup>
198
199<abstract>
200<t>
201  The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed,
202  collaborative, hypermedia information systems. This document is Part 6 of the seven-part
203  specification that defines the protocol referred to as "HTTP/1.1" and, taken together,
204  obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header
205  fields that control cache behavior or indicate cacheable response messages.
206</t>
207</abstract>
208
209<note title="Editorial Note (To be removed by RFC Editor)">
210<t>
211  Discussion of this draft should take place on the HTTPBIS working group mailing list
212  (ietf-http-wg@w3.org). The current issues list is at <eref
213  target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
214  (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
215</t>
216<t>
217  The changes in this draft are summarized in <xref target="changes.since.08" />.
218</t>
219</note>
220
221  </front>
222  <middle>
223
224<section anchor="caching" title="Introduction">
225<t>
226  HTTP is typically used for distributed information systems, where performance can be
227  improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
228  caching and reusing response messages.
229</t>
230
231<section anchor="intro.purpose" title="Purpose">
232<iref item="cache" />
233<t>
234  An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
235  controls its message storage, retrieval, and deletion. A cache stores cacheable responses
236  in order to reduce the response time and network bandwidth consumption on future,
237  equivalent requests. Any client or server may include a cache, though a cache cannot be
238  used by a server that is acting as a tunnel.
239</t>
240<t>
241  Caching would be useless if it did not significantly improve performance. The goal of
242  caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In
243  some cases, a stored response can be reused without the need for a network request,
244  reducing latency and network round-trips; a "freshness" mechanism is used for this purpose
245  (see <xref target="expiration.model" />). Even when a new request is required, it is often
246  possible to reuse all or parts of the payload of a prior response to satisfy the request,
247  thereby reducing network bandwidth usage; a "validation" mechanism is used for this
248  purpose (see <xref target="validation.model" />).
249</t>
250</section>
251
252<section anchor="intro.terminology" title="Terminology">
253<t>
254  This specification uses a number of terms to refer to the roles played by participants
255  in, and objects of, HTTP caching.
256</t>
257<t>
258  <iref item="cacheable" />
259  <x:dfn>cacheable</x:dfn>
260  <list>
261    <t>A response is cacheable if a cache is allowed to store a copy of the response message
262      for use in answering subsequent requests. Even when a response is cacheable, there may
263      be additional constraints on whether a cache can use the cached copy to satisfy a
264      particular request.</t>
265  </list>
266</t>
267<t>
268  <iref item="explicit expiration time" />
269  <x:dfn>explicit expiration time</x:dfn>
270  <list>
271    <t>The time at which the origin server intends that an entity should no longer be
272      returned by a cache without further validation.</t>
273  </list>
274</t>
275<t>
276  <iref item="heuristic expiration time" />
277  <x:dfn>heuristic expiration time</x:dfn>
278  <list>
279    <t>An expiration time assigned by a cache when no explicit expiration time is
280    available.</t>
281  </list>
282</t>
283<t>
284  <iref item="age" />
285  <x:dfn>age</x:dfn>
286  <list>
287    <t>The age of a response is the time since it was sent by, or successfully validated
288      with, the origin server.</t>
289  </list>
290</t>
291<t>
292  <iref item="first-hand" />
293  <x:dfn>first-hand</x:dfn>
294  <list>
295    <t>A response is first-hand if the freshness model is not in use; i.e., its age is
296    0.</t>
297  </list>
298</t>
299<t>
300  <iref item="freshness lifetime" />
301  <x:dfn>freshness lifetime</x:dfn>
302  <list>
303    <t>The length of time between the generation of a response and its expiration time. </t>
304  </list>
305</t>
306<t>
307  <iref item="fresh" />
308  <x:dfn>fresh</x:dfn>
309  <list>
310    <t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
311  </list>
312</t>
313<t>
314  <iref item="stale" />
315  <x:dfn>stale</x:dfn>
316  <list>
317    <t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
318  </list>
319</t>
320<t>
321  <iref item="validator" />
322  <x:dfn>validator</x:dfn>
323  <list>
324    <t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
325      out whether a stored response is an equivalent copy of an entity.</t>
326  </list>
327</t>
328<t anchor="shared.and.non-shared.caches">
329  <iref item="validator" />
330  <x:dfn>shared cache</x:dfn>
331  <list>
332    <t>A cache that is accessible to more than one user. A non-shared cache is
333      dedicated to a single user.</t>
334  </list>
335</t>
336</section>
337
338<section anchor="intro.requirements" title="Requirements">
339<t>
340  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
341  NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
342  described in <xref target="RFC2119" />.
343</t>
344<t>
345  An implementation is not compliant if it fails to satisfy one or more of the &MUST;
346  or &REQUIRED; level requirements for the protocols it implements. An implementation
347  that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
348  requirements for its protocols is said to be "unconditionally compliant"; one that
349  satisfies all the &MUST; level requirements but not all the &SHOULD; level
350  requirements for its protocols is said to be "conditionally compliant."
351</t>
352</section>
353
354<section title="Syntax Notation" anchor="notation">
355  <x:anchor-alias value="ALPHA"/>
356  <x:anchor-alias value="CR"/>
357  <x:anchor-alias value="DIGIT"/>
358  <x:anchor-alias value="DQUOTE"/>
359  <x:anchor-alias value="LF"/>
360  <x:anchor-alias value="OCTET"/>
361  <x:anchor-alias value="SP"/>
362  <x:anchor-alias value="VCHAR"/>
363  <x:anchor-alias value="WSP"/>
364<t>
365  This specification uses the ABNF syntax defined in &notation; (which
366  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
367  <xref target="collected.abnf"/> shows the collected ABNF, with the list
368  rule expanded.
369</t>
370<t>
371  The following core rules are included by
372  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
373  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
374  DIGIT (decimal 0-9), DQUOTE (double quote),
375  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
376  OCTET (any 8-bit sequence of data), SP (space),
377  VCHAR (any visible USASCII character),
378  and WSP (whitespace).
379</t>
380
381<section title="Core Rules" anchor="core.rules">
382  <x:anchor-alias value="quoted-string"/>
383  <x:anchor-alias value="token"/>
384  <x:anchor-alias value="OWS"/>
385<t>
386  The core rules below are defined in &basic-rules;:
387</t>
388<figure><artwork type="abnf2616">
389  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
390  <x:ref>token</x:ref>         = &lt;token, defined in &basic-rules;&gt;
391  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
392</artwork></figure>
393</section>
394
395<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
396  <x:anchor-alias value="field-name"/>
397  <x:anchor-alias value="HTTP-date"/>
398  <x:anchor-alias value="port"/>
399  <x:anchor-alias value="pseudonym"/>
400  <x:anchor-alias value="uri-host"/>
401<t>
402  The ABNF rules below are defined in other parts:
403</t>
404<figure><!--Part1--><artwork type="abnf2616">
405  <x:ref>field-name</x:ref>    = &lt;field-name, defined in &header-fields;&gt;
406  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
407  <x:ref>port</x:ref>          = &lt;port, defined in &uri;&gt;
408  <x:ref>pseudonym</x:ref>     = &lt;pseudonym, defined in &header-via;&gt; 
409  <x:ref>uri-host</x:ref>      = &lt;uri-host, defined in &uri;&gt;
410</artwork></figure>
411</section>
412
413</section>
414</section>
415
416<section anchor="caching.overview" title="Cache Operation">
417
418<section anchor="response.cacheability" title="Response Cacheability">
419<t>
420  A cache &MUST-NOT; store a response to any request, unless:
421  <list style="symbols">
422    <t>The request method is understood by the cache and defined as being cacheable, and</t>
423    <t>the response status code is understood by the cache, and</t>
424    <t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
425       appear in request or response headers, and</t>
426    <t>the "private" cache response directive (see <xref target="cache-response-directive" />
427       does not appear in the response, if the cache is shared, and</t>
428    <t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
429       the cache is shared (unless the "public" directive is present; see <xref
430       target="header.cache-control" />), and</t>
431    <t>the response either:
432      <list style="symbols">
433        <t>contains an Expires header (see <xref target="header.expires" />), or</t>
434        <t>contains a max-age response cache directive (see <xref target="cache-response-directive" />), or</t>
435        <t>contains a s-maxage response cache directive and the cache is shared, or</t>
436        <t>contains a Cache Control Extension (see <xref target="cache.control.extensions" />) that allows it to be cached, or</t>
437        <t>has a status code that can be served with heuristic freshness (see <xref
438           target="heuristic.freshness" />).</t>
439      </list>
440    </t>
441  </list>
442</t>
443<t>
444  In this context, a cache has "understood" a request method or a response status
445  code if it recognises it and implements any cache-specific behaviour. In
446  particular, 206 Partial Content responses cannot be cached by an
447  implementation that does not handle partial content
448  (see <xref target="errors.or.incomplete.response.cache.behavior" />).
449</t>
450<t>
451  Note that in normal operation, most caches will not store a response that has neither a
452  cache validator nor an explicit expiration time, as such responses are not usually
453  useful to store. However, caches are not prohibited from storing such responses.
454</t>
455
456<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
457<t>
458  A cache that receives an incomplete response (for example, with fewer bytes of data
459  than specified in a Content-Length header) can store the response, but &MUST; 
460  treat it as a partial response &partial;. Partial responses
461  can be combined as described in &combining-byte-ranges;; the result might be a
462  full response or might still be partial. A cache &MUST-NOT; return a partial
463  response to a client without explicitly marking it as such using the 206 (Partial
464  Content) status code.
465</t>
466<t>
467  A cache that does not support the Range and Content-Range headers &MUST-NOT; store
468  incomplete or partial responses.
469</t>
470</section>
471
472</section>
473
474
475<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
476<t>
477  For a presented request, a cache &MUST-NOT; return a stored response, unless:
478  <list style="symbols">
479    <t>The presented Request-URI and that of the stored response match
480      (<cref anchor="TODO-Request-URI">Need to find a new term for this, as Part
481      1 doesn't define Request-URI anymore; the new term request-target does not
482      work for this. (see <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196" />)</cref>), and</t>
483    <t>the request method associated with the stored response allows it to be
484      used for the presented request, and</t>
485    <t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
486      target="caching.negotiated.responses" />), and</t>
487    <t>the presented request and stored response are free from directives that would prevent
488      its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
489      and</t>
490    <t>the stored response is either:
491      <list style="symbols">
492        <t>fresh (see <xref target="expiration.model" />), or</t>
493        <t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
494        <t>successfully validated (see <xref target="validation.model" />).</t>
495      </list>
496    </t>
497  </list>
498</t>
499<t>
500  <cref anchor="TODO-method-cacheability">define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
501</t>
502<t>
503  When a stored response is used to satisfy a request, caches &MUST; include a
504  single Age header field (<xref target="header.age" />) in the response with a value equal to the stored response's
505  current_age; see <xref target="age.calculations" />.
506  <cref anchor="DISCUSS-includes-validated">this currently includes successfully validated responses.</cref>
507</t>
508<t>
509  Requests with methods that are unsafe (&safe-methods;) &MUST; be written through the cache to
510  the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a
511  corresponding response.
512</t>
513<t>
514  Also, note that unsafe requests might invalidate already stored responses; see
515  <xref target="invalidation.after.updates.or.deletions" />.
516</t>
517<t>
518  Caches &MUST; use the most recent response (as determined by the Date header) when
519  more than one suitable response is stored. They can also forward a request with
520  "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to
521  use.
522</t>
523<t>
524  <cref anchor="TODO-header-properties">end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
525</t>
526</section>
527
528<section anchor="expiration.model" title="Freshness Model">
529<t>
530  When a response is "fresh" in the cache, it can be used to satisfy subsequent
531  requests without contacting the origin server, thereby improving efficiency.
532</t>
533<t>
534  The primary mechanism for determining freshness is for an origin server to provide an
535  explicit expiration time in the future, using either the Expires header (<xref
536  target="header.expires" />) or the max-age response cache directive (<xref
537  target="cache-response-directive" />). Generally, origin servers will assign future
538  explicit expiration times to responses in the belief that the entity is not likely to
539  change in a semantically significant way before the expiration time is reached.
540</t>
541<t>
542  If an origin server wishes to force a cache to validate every request, it can
543  assign an explicit expiration time in the past. This means that the response is always
544  stale, so that caches should validate it before using it for subsequent requests.
545  <cref anchor="TODO-response-stale">This wording may cause confusion, because the response may still be served stale.</cref>
546</t>
547<t>
548  Since origin servers do not always provide explicit expiration times, HTTP caches may
549  also assign heuristic expiration times when they are not specified, employing algorithms that
550  use other header values (such as the Last-Modified time) to estimate a plausible
551  expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does
552  impose worst-case constraints on their results.
553</t>
554<figure>
555<preamble>
556  The calculation to determine if a response is fresh is:
557</preamble>
558<artwork type="code">
559   response_is_fresh = (freshness_lifetime &gt; current_age)
560</artwork>
561</figure>
562
563<t>
564  The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
565  the current_age is defined in <xref target="age.calculations" />.
566</t>
567<t>
568  Additionally, clients may need to influence freshness calculation. They can do this using
569  several request cache directives, with the effect of either increasing or loosening
570  constraints on freshness. See <xref target="cache-request-directive" />.
571</t>
572<t>
573  <cref anchor="ISSUE-no-req-for-directives">there are not requirements directly applying to cache-request-directives and
574  freshness.</cref>
575</t>
576<t>
577  Note that freshness applies only to cache operation; it cannot be used to force a user agent
578  to refresh its display or reload a resource. See <xref target="history.lists" /> for an explanation of
579  the difference between caches and history mechanisms.
580</t>
581
582<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
583<t>
584  A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
585  response by using the first match of:
586  <list style="symbols">
587    <t>If the cache is shared and the s-maxage response cache directive (<xref
588      target="cache-response-directive" />) is present, use its value, or</t>
589    <t>If the max-age response cache directive (<xref target="cache-response-directive"
590      />) is present, use its value, or</t>
591    <t>If the Expires response header (<xref target="header.expires" />) is present, use
592      its value minus the value of the Date response header, or</t>
593    <t>Otherwise, no explicit expiration time is present in the response, but a heuristic
594      may be used; see <xref target="heuristic.freshness" />.</t>
595  </list>
596</t>
597<t>
598  Note that this calculation is not vulnerable to clock skew, since all of the
599  information comes from the origin server.
600</t>
601
602<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
603<t>
604  If no explicit expiration time is present in a stored response that has a status code
605  of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
606  calculated. Heuristics &MUST-NOT; be used for other response status codes.
607</t>
608<t>
609  When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
610  attach a Warning header with a 113 warn-code to the response if its current_age is
611  more than 24 hours and such a warning is not already present.
612</t>
613<t>
614  Also, if the response has a Last-Modified header (&header-last-modified;), the
615  heuristic expiration value &SHOULD; be no more than some fraction of the interval
616  since that time. A typical setting of this fraction might be 10%.
617</t>
618<t>
619  <cref anchor="REVIEW-query-string-heuristics">took away HTTP/1.0 query string heuristic uncacheability.</cref>
620</t>
621</section>
622</section>
623
624<section anchor="age.calculations" title="Calculating Age">
625<t>
626  HTTP/1.1 uses the Age response-header to convey the estimated age of the response
627  message when obtained from a cache. The Age field value is the cache's estimate of the
628  amount of time since the response was generated or validated by the origin server. In
629  essence, the Age value is the sum of the time that the response has been resident in
630  each of the caches along the path from the origin server, plus the amount of time it has
631  been in transit along network paths.
632</t>
633<t>
634  The term "age_value" denotes the value of the Age header, in a form appropriate for
635  arithmetic operations.
636</t>
637<t>
638  HTTP/1.1 requires origin servers to send a Date header, if possible, with every
639  response, giving the time at which the response was generated (see &header-date;).
640  The term "date_value" denotes the value of the Date header, in a form appropriate for
641  arithmetic operations.
642</t>
643<t>
644  The term "now" means "the current value of the clock at the host performing the
645  calculation." Hosts that use HTTP, but especially hosts running origin servers and
646  caches, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
647  synchronize their clocks to a globally accurate time standard.
648</t>
649<t>
650  A response's age can be calculated in two entirely independent ways:
651  <list style="numbers">
652    <t>now minus date_value, if the local clock is reasonably well synchronized to the
653      origin server's clock. If the result is negative, the result is replaced by zero.</t>
654    <t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
655  </list>
656</t>
657<figure>
658<preamble>These are combined as</preamble>
659<artwork type="code">
660    corrected_received_age = max(now - date_value, age_value)
661</artwork></figure>
662<t>
663  When an Age value is received, it &MUST; be interpreted relative to the time the
664  request was initiated, not the time that the response was received.
665</t>
666<figure><artwork type="code">
667   corrected_initial_age = corrected_received_age
668                         + (now - request_time)
669</artwork></figure>
670<t>
671  where "request_time" is the time (according to the local clock) when the request that
672  elicited this response was sent.
673</t>
674<t>
675  The current_age of a stored response can then be calculated by adding the amount of
676  time (in seconds) since the stored response was last validated by the origin server to
677  the corrected_initial_age.
678</t>
679<t>
680  In summary:
681</t>
682<figure><artwork type="code">
683  age_value     - Age header field-value received with the response
684  date_value    - Date header field-value received with the response
685  request_time  - local time when the cache made the request
686                 resulting in the stored response
687  response_time - local time when the cache received the response
688  now           - current local time
689 
690  apparent_age = max(0, response_time - date_value);
691  corrected_received_age = max(apparent_age, age_value);
692  response_delay = response_time - request_time;
693  corrected_initial_age = corrected_received_age + response_delay;
694  resident_time = now - response_time;
695  current_age   = corrected_initial_age + resident_time;
696</artwork></figure>
697</section>
698
699<section anchor="serving.stale.responses" title="Serving Stale Responses">
700<t>
701  A "stale" response is one that either has explicit expiry information, or is allowed to
702  have heuristic expiry calculated, but is not fresh according to the calculations in
703  <xref target="expiration.model" />.
704</t>
705<t>
706  Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
707  in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
708  "must-revalidate" cache-response-directive, or an applicable "s-maxage" or
709  "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
710</t>
711<t>
712  Caches &SHOULD-NOT; return stale responses unless they are
713  disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
714  or otherwise explicitly allowed (e.g., the max-stale request directive; see <xref target="cache-request-directive" />).
715</t>
716<t>
717  Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
718  target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
719  the cache is disconnected.
720</t>
721<t>
722  If a cache receives a first-hand response (either an entire response, or a 304 (Not
723  Modified) response) that it would normally forward to the requesting client, and the
724  received response is no longer fresh, the cache &SHOULD; forward it to the
725  requesting client without adding a new Warning (but without removing any existing
726  Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
727  that response became stale in transit.
728</t>
729</section>
730</section>
731
732<section anchor="validation.model" title="Validation Model">
733<t>
734  When a cache has one or more stored responses for a requested URI, but cannot 
735  serve any of them (e.g., because they are not fresh, or one cannot be selected;
736  see <xref target="caching.negotiated.responses"/>),
737  it can use the conditional request mechanism &conditional; in the forwarded
738  request to give the origin server an opportunity to both select a valid stored
739  response to be used, and to update it. This process is known as "validating"
740  or "revalidating" the stored response.
741</t>
742<t>
743  When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since
744  header whose value is that of the Last-Modified header from the selected
745  (see <xref target="caching.negotiated.responses"/>) stored response, if available.
746</t>
747<t>
748  Additionally, the cache &SHOULD; add an If-None-Match header whose value 
749  is that of the ETag header(s) from all responses stored for the requested URI,
750  if present. However, if any of the stored responses contains only partial
751  content, its entity-tag &SHOULD-NOT; be included in the If-None-Match header
752  field unless the request is for a range that would be fully satisfied by
753  that stored response.
754</t>
755<t>
756  A 304 (Not Modified) response status code indicates that the stored
757  response can be updated and reused; see <xref target="combining.headers"/>.
758</t>
759<t>
760  A full response (i.e., one with a response body) indicates that none 
761  of the stored responses nominated in the conditional request is
762  suitable. Instead, the full response is used both to satisfy the
763  request and replace the stored response. <cref anchor="TODO-req-missing">Should there be a requirement here?</cref>
764</t>
765<t>
766  If a cache receives a 5xx response while attempting to validate a response, it &MAY;
767  either forward this response to the requesting client, or act as if the server failed to
768  respond. In the latter case, it &MAY; return a previously stored response (see <xref
769  target="serving.stale.responses" />).
770</t>
771</section>
772
773<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
774<t>
775  Because unsafe methods (&safe-methods;) have the potential for changing state on the
776  origin server, intervening caches can use them to keep their contents
777  up-to-date.
778</t>
779<t>
780  The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
781  as the URI(s) in the Location and Content-Location headers (if present):
782  <list style="symbols">
783    <t>PUT</t>
784    <t>DELETE</t>
785    <t>POST</t>
786  </list>
787</t>
788<t>
789  An invalidation based on a URI from a Location or Content-Location header &MUST-NOT;
790  be performed if the host part of that URI differs from the host part in the Request-URI.
791  This helps prevent denial of service attacks.
792</t>
793<t>
794  <cref anchor="TODO-def-host-part">"host part" needs to be specified better.</cref>
795</t>
796<t>
797  A cache that passes through requests for methods it does not understand &SHOULD;
798  invalidate the Request-URI.
799</t>
800<t>
801  Here, "invalidate" means that the cache will either remove all stored responses related
802  to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation
803  before they can be returned in response to a subsequent request.
804</t>
805<t>
806  Note that this does not guarantee that all appropriate responses are invalidated. For
807  example, the request that caused the change at the origin server might not have gone
808  through the cache where a response is stored.
809</t>
810<t>
811  <cref anchor="TODO-spec-success-invalidate">specify that only successful (2xx, 3xx?) responses invalidate.</cref>
812</t>
813</section>
814
815<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
816<t>
817  When a cache receives a request that can be satisfied by a stored response
818  that has a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that
819  response unless all of the selecting request-headers nominated by the Vary header match
820  in both the original request (i.e., that associated with the stored response),
821  and the presented request.
822</t>
823<t>
824  The selecting request-headers from two requests are defined to match if and only if the
825  selecting request-headers in the first request can be transformed to the selecting
826  request-headers in the second request by adding or removing linear white space
827  <cref anchor="TODO-missing-ref">[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
828  combining multiple message-header fields with the same field name following the rules
829  about header fields in &header-fields;.
830</t>
831<t>
832  If a header field is absent from a request, it can only match another request
833  if it is also absent there.
834</t>
835<t>
836  A Vary header field-value of "*" always fails to match, and subsequent requests to that
837  resource can only be properly interpreted by the origin server.
838</t>
839<t>
840  The stored response with matching selecting request-headers is known as the
841  selected response.
842</t>
843<t>
844  If no selected response is available, the cache &MAY; forward the presented
845  request to the origin server in a conditional request; see <xref target="validation.model"/>.
846</t>
847</section>
848
849<section anchor="combining.headers" title="Combining Responses">
850<t>
851  When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response
852  (in this section, the "new" response"), it needs to created an updated response by combining
853  the stored response with the new one, so that the updated response can be used to satisfy the request.
854</t>
855<t>
856  If the new response contains an ETag, it identifies the stored 
857  response to use. <cref anchor="TODO-mention-CL">may need language about Content-Location 
858  here</cref><cref anchor="TODO-inm-mult-etags">cover case where INM with multiple etags was sent</cref>
859</t>
860<t>
861  If the status code is 206 (partial content), both the stored and new 
862  responses &MUST; have validators, and those validators &MUST; match using the strong 
863  comparison function (see &weak-and-strong;). Otherwise, the 
864  responses &MUST-NOT; be combined.
865</t>
866<t>
867  The stored response headers are used as those of the updated response, except that
868  <list style="symbols">
869    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
870      &MUST; be deleted from the stored response and the updated response.</t>
871    <t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
872      response and the updated response.</t>
873    <t>any headers provided in the new response &MUST; replace the corresponding
874      headers from the stored response.</t>
875  </list>
876</t>
877<t>
878  If a header field-name in the new response matches more than one 
879  header in the stored response, all such stored headers &MUST; be replaced.
880</t>
881<t>
882  The updated response can <cref anchor="TODO-is-req">requirement?</cref> be used to replace the 
883  stored response in cache. In the case of a 206 response, the combined 
884  entity-body &MAY; be stored.
885</t>
886<t>
887  <cref anchor="ISSUE-how-head">discuss how to handle HEAD updates</cref>
888</t>
889</section>
890
891</section>
892
893<section anchor="header.fields" title="Header Field Definitions">
894<t>
895  This section defines the syntax and semantics of HTTP/1.1 header fields
896  related to caching.
897</t>
898<t>
899  For entity-header fields, both sender and recipient refer to either the client or the
900  server, depending on who sends and who receives the entity.
901</t>
902
903<section anchor="header.age" title="Age">
904  <iref item="Age header" primary="true" x:for-anchor="" />
905  <iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
906  <x:anchor-alias value="Age"/>
907  <x:anchor-alias value="Age-v"/>
908  <x:anchor-alias value="age-value"/>
909<t>
910  The "Age" response-header field conveys the sender's estimate of the amount
911  of time since the response was generated or successfully validated at the
912  origin server. Age values are calculated as specified in
913  <xref target="age.calculations" />.
914</t>
915<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
916  <x:ref>Age</x:ref>   = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
917  <x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
918</artwork></figure>
919<t anchor="rule.delta-seconds">
920  <x:anchor-alias value="delta-seconds" />
921  Age field-values are non-negative integers, representing time in seconds.
922</t>
923<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
924  <x:ref>delta-seconds</x:ref>  = 1*<x:ref>DIGIT</x:ref>
925</artwork></figure>
926<t>
927  If a cache receives a value larger than the largest positive integer it can represent, or
928  if any of its age calculations overflows, it &MUST; transmit an Age header with a
929  field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
930  of at least 31 bits of range.
931</t>
932<t>
933  The presence of an Age header field in a response implies that a response is not
934  first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
935  Age header field.
936</t>
937</section>
938
939<section anchor="header.cache-control" title="Cache-Control">
940  <iref item="Cache-Control header" primary="true" x:for-anchor="" />
941  <iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
942  <x:anchor-alias value="Cache-Control"/>
943  <x:anchor-alias value="Cache-Control-v"/>
944  <x:anchor-alias value="cache-directive"/>
945  <x:anchor-alias value="cache-extension"/>
946  <x:anchor-alias value="cache-request-directive"/>
947  <x:anchor-alias value="cache-response-directive"/>
948<t>
949  The "Cache-Control" general-header field is used to specify directives that
950  &MUST; be obeyed by all caches along the request/response chain. Such cache
951  directives are unidirectional in that the presence of a directive in a
952  request does not imply that the same directive is to be given in the
953  response.
954</t>
955<x:note>
956  <t>
957    Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
958    Pragma: no-cache (see <xref target="header.pragma" />).
959  </t>
960</x:note>
961<t>
962  Cache directives &MUST; be passed through by a proxy or gateway application,
963  regardless of their significance to that application, since the directives might be
964  applicable to all recipients along the request/response chain. It is not possible to
965  target a directive to a specific cache.
966</t>
967<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"/>
968  <x:ref>Cache-Control</x:ref>   = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
969  <x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
970
971  <x:ref>cache-directive</x:ref> = <x:ref>cache-request-directive</x:ref>
972     / <x:ref>cache-response-directive</x:ref>
973
974  <x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
975</artwork></figure>
976
977<section anchor="cache-request-directive" title="Request Cache-Control Directives">
978  <x:anchor-alias value="cache-request-directive" />
979
980<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
981  <x:ref>cache-request-directive</x:ref> =
982       "no-cache"
983     / "no-store"
984     / "max-age" "=" <x:ref>delta-seconds</x:ref>
985     / "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
986     / "min-fresh" "=" <x:ref>delta-seconds</x:ref>
987     / "no-transform"
988     / "only-if-cached"
989     / <x:ref>cache-extension</x:ref>
990</artwork></figure>
991
992<t>
993  <iref item="Cache Directives" primary="true" subitem="no-cache" />
994  <iref item="no-cache" primary="true" subitem="Cache Directive" />
995  no-cache
996  <list>
997    <t>The no-cache request directive indicates that a stored response &MUST-NOT; be
998      used to satisfy the request without successful validation on the origin server.</t>
999  </list>
1000</t>
1001<t>
1002  <iref item="Cache Directives" primary="true" subitem="no-store" />
1003  <iref item="no-store" primary="true" subitem="Cache Directive" />
1004  no-store
1005  <list>
1006    <t>The no-store request directive indicates that a cache &MUST-NOT; store any part
1007      of either this request or any response to it. This directive applies to both
1008      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1009      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1010      and &MUST; make a best-effort attempt to remove the information from volatile
1011      storage as promptly as possible after forwarding it.</t>
1012    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1013      particular, malicious or compromised caches might not recognize or obey this
1014      directive, and communications networks may be vulnerable to eavesdropping.</t>
1015  </list>
1016</t>
1017<t>
1018  <iref item="Cache Directives" primary="true" subitem="max-age" />
1019  <iref item="max-age" primary="true" subitem="Cache Directive" />
1020  max-age
1021  <list>
1022    <t>The max-age request directive indicates that the client is willing to accept a
1023      response whose age is no greater than the specified time in seconds. Unless
1024      max-stale directive is also included, the client is not willing to accept a stale
1025      response.</t>
1026  </list>
1027</t>
1028<t>
1029  <iref item="Cache Directives" primary="true" subitem="max-stale" />
1030  <iref item="max-stale" primary="true" subitem="Cache Directive" />
1031  max-stale
1032  <list>
1033    <t>The max-stale request directive indicates that the client is willing to accept a
1034      response that has exceeded its expiration time. If max-stale is assigned a value,
1035      then the client is willing to accept a response that has exceeded its expiration
1036      time by no more than the specified number of seconds. If no value is assigned to
1037      max-stale, then the client is willing to accept a stale response of any age. <cref anchor="TODO-staleness" source="mnot">of any staleness?</cref></t>
1038  </list>
1039</t>
1040<t>
1041  <iref item="Cache Directives" primary="true" subitem="min-fresh" />
1042  <iref item="min-fresh" primary="true" subitem="Cache Directive" />
1043  min-fresh
1044  <list>
1045    <t>The min-fresh request directive indicates that the client is willing to accept a
1046      response whose freshness lifetime is no less than its current age plus the specified
1047      time in seconds. That is, the client wants a response that will still be fresh for
1048      at least the specified number of seconds.</t>
1049  </list>
1050</t>
1051<t>
1052  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1053  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1054  no-transform
1055  <list>
1056    <t>The no-transform request directive indicates that an intermediate cache or proxy
1057      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
1058      headers, nor the request entity-body.</t>
1059  </list>
1060</t>
1061<t>
1062  <iref item="Cache Directives" primary="true" subitem="only-if-cached" />
1063  <iref item="only-if-cached" primary="true" subitem="Cache Directive" />
1064  only-if-cached
1065  <list>
1066    <t>The only-if-cached request directive indicates that the client only wishes to
1067      return a stored response. If it receives this directive, a cache &SHOULD; either
1068      respond using a stored response that is consistent with the other constraints of the
1069      request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
1070      being operated as a unified system with good internal connectivity, such a request
1071      &MAY; be forwarded within that group of caches.</t>
1072  </list>
1073</t>
1074</section>
1075
1076<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1077  <x:anchor-alias value="cache-response-directive" />
1078
1079<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
1080  <x:ref>cache-response-directive</x:ref> =
1081       "public"
1082     / "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1083     / "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
1084     / "no-store"
1085     / "no-transform"
1086     / "must-revalidate"
1087     / "proxy-revalidate"
1088     / "max-age" "=" <x:ref>delta-seconds</x:ref>
1089     / "s-maxage" "=" <x:ref>delta-seconds</x:ref>
1090     / <x:ref>cache-extension</x:ref>
1091</artwork></figure>
1092
1093<t>
1094  <iref item="Cache Directives" primary="true" subitem="public" />
1095  <iref item="public" primary="true" subitem="Cache Directive" />
1096  public
1097  <list>
1098    <t>The public response directive indicates that the response &MAY; be cached, even
1099      if it would normally be non-cacheable or cacheable only within a non-shared cache.
1100      (See also Authorization, &header-authorization;, for additional details.) </t>
1101  </list>
1102</t>
1103<t>
1104  <iref item="Cache Directives" primary="true" subitem="private" />
1105  <iref item="private" primary="true" subitem="Cache Directive" />
1106  private
1107  <list>
1108    <t>The private response directive indicates that the response message is intended for
1109      a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
1110      cache &MAY; store the response.</t>
1111    <t>If the private response directive specifies one or more field-names, this
1112      requirement is limited to the field-values associated with the listed response
1113      headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
1114      cache, whereas the remainder of the response message &MAY; be.</t>
1115    <t>
1116      <x:h>Note:</x:h> This usage of the word private only controls where the response may
1117      be stored, and cannot ensure the privacy of the message content.
1118      Also, private response directives with field-names are often handled by
1119      implementations as if an unqualified private directive was received; i.e.,
1120      the special handling for the qualified form is not widely implemented.
1121</t>
1122  </list>
1123</t>
1124<t>
1125  <iref item="Cache Directives" primary="true" subitem="no-cache" />
1126  <iref item="no-cache" primary="true" subitem="Cache Directive" />
1127  no-cache
1128  <list>
1129    <t>The no-cache response directive indicates that the response &MUST-NOT; be used to
1130      satisfy a subsequent request without successful validation on the origin server.
1131      This allows an origin server to prevent caching even by caches that have been
1132      configured to return stale responses.</t>
1133    <t>If the no-cache response directive specifies one or more field-names, this
1134      requirement is limited to the field-values associated with the listed response
1135      headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
1136      to a subsequent request without successful validation on the origin server. This
1137      allows an origin server to prevent the re-use of certain header fields in a
1138      response, while still allowing caching of the rest of the response.</t>
1139    <t>
1140      <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
1141      Also, no-cache response directives with field-names are often handled by
1142      implementations as if an unqualified no-cache directive was received; i.e.,
1143      the special handling for the qualified form is not widely implemented.
1144    </t>
1145  </list>
1146</t>
1147
1148<t>
1149  <iref item="Cache Directives" primary="true" subitem="no-store" />
1150  <iref item="no-store" primary="true" subitem="Cache Directive" />
1151  no-store
1152  <list>
1153    <t>The no-store response directive indicates that a cache &MUST-NOT; store any
1154      part of either the immediate request or response. This directive applies to both
1155      non-shared and shared caches. "&MUST-NOT; store" in this context means that the
1156      cache &MUST-NOT; intentionally store the information in non-volatile storage,
1157      and &MUST; make a best-effort attempt to remove the information from volatile
1158      storage as promptly as possible after forwarding it.</t>
1159    <t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
1160      particular, malicious or compromised caches might not recognize or obey this
1161      directive, and communications networks may be vulnerable to eavesdropping.</t>
1162  </list>
1163</t>
1164<t>
1165  <iref item="Cache Directives" primary="true" subitem="must-revalidate" />
1166  <iref item="must-revalidate" primary="true" subitem="Cache Directive" />
1167  must-revalidate
1168  <list>
1169    <t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
1170     used to satisfy subsequent requests without successful validation on the origin server.</t>
1171    <t>The must-revalidate directive is necessary to support reliable operation for
1172      certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
1173      the must-revalidate directive; in particular, if the cache cannot reach the origin
1174      server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
1175    <t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
1176      validate a request on the entity could result in incorrect operation, such as a
1177      silently unexecuted financial transaction.</t>
1178  </list>
1179</t>
1180<t>
1181  <iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
1182  <iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
1183  proxy-revalidate
1184  <list>
1185    <t>The proxy-revalidate response directive has the same meaning as the must-revalidate
1186      response directive, except that it does not apply to non-shared caches.</t>
1187  </list>
1188</t>
1189<t>
1190  <iref item="Cache Directives" primary="true" subitem="max-age" />
1191  <iref item="max-age" primary="true" subitem="Cache Directive" />
1192  max-age
1193  <list>
1194    <t>The max-age response directive indicates that response is to be considered stale
1195      after its age is greater than the specified number of seconds.</t>
1196  </list>
1197</t>
1198<t>
1199  <iref item="Cache Directives" primary="true" subitem="s-maxage" />
1200  <iref item="s-maxage" primary="true" subitem="Cache Directive" />
1201  s-maxage
1202  <list>
1203    <t>The s-maxage response directive indicates that, in shared caches, the maximum age
1204      specified by this directive overrides the maximum age specified by either the
1205      max-age directive or the Expires header. The s-maxage directive also implies the
1206      semantics of the proxy-revalidate response directive.</t>
1207  </list>
1208</t>
1209<t>
1210  <iref item="Cache Directives" primary="true" subitem="no-transform" />
1211  <iref item="no-transform" primary="true" subitem="Cache Directive" />
1212  no-transform
1213  <list>
1214    <t>The no-transform response directive indicates that an intermediate cache or proxy
1215      &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
1216      headers, nor the response entity-body.</t>
1217  </list>
1218</t>
1219
1220</section>
1221
1222<section anchor="cache.control.extensions" title="Cache Control Extensions">
1223<t>
1224  The Cache-Control header field can be extended through the use of one or more
1225  cache-extension tokens, each with an optional value. Informational extensions (those
1226  that do not require a change in cache behavior) can be added without changing the
1227  semantics of other directives. Behavioral extensions are designed to work by acting as
1228  modifiers to the existing base of cache directives. Both the new directive and the
1229  standard directive are supplied, such that applications that do not understand the new
1230  directive will default to the behavior specified by the standard directive, and those
1231  that understand the new directive will recognize it as modifying the requirements
1232  associated with the standard directive. In this way, extensions to the cache-control
1233  directives can be made without requiring changes to the base protocol.
1234</t>
1235<t>
1236  This extension mechanism depends on an HTTP cache obeying all of the cache-control
1237  directives defined for its native HTTP-version, obeying certain extensions, and ignoring
1238  all directives that it does not understand.
1239</t>
1240<t>
1241  For example, consider a hypothetical new response directive called "community" that
1242  acts as a modifier to the private directive. We define this new directive to mean that,
1243  in addition to any non-shared cache, any cache that is shared only by members of the
1244  community named within its value may cache the response. An origin server wishing to
1245  allow the UCI community to use an otherwise private response in their shared cache(s)
1246  could do so by including
1247</t>
1248<figure><artwork type="example">
1249  Cache-Control: private, community="UCI"
1250</artwork></figure>
1251<t>
1252  A cache seeing this header field will act correctly even if the cache does not
1253  understand the community cache-extension, since it will also see and understand the
1254  private directive and thus default to the safe behavior.
1255</t>
1256<t>
1257  Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
1258  directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
1259  directives (or the response's default cacheability) such that the cache behavior will
1260  remain minimally correct even if the cache does not understand the extension(s).
1261</t>
1262</section>
1263
1264</section>
1265
1266<section anchor="header.expires" title="Expires">
1267  <iref item="Expires header" primary="true" x:for-anchor="" />
1268  <iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
1269  <x:anchor-alias value="Expires"/>
1270  <x:anchor-alias value="Expires-v"/>
1271<t>
1272  The "Expires" entity-header field gives the date/time after which the response is
1273  considered stale. See <xref target="expiration.model" /> for further discussion of the
1274  freshness model.
1275</t>
1276<t>
1277  The presence of an Expires field does not imply that the original resource will change or
1278  cease to exist at, before, or after that time.
1279</t>
1280<t>
1281  The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
1282  it &MUST; be sent in rfc1123-date format.
1283</t>
1284<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
1285  <x:ref>Expires</x:ref>   = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
1286  <x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
1287</artwork></figure>
1288<figure>
1289  <preamble>For example</preamble>
1290<artwork type="example">
1291  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1292</artwork></figure>
1293<x:note>
1294  <t>
1295    <x:h>Note:</x:h> If a response includes a Cache-Control field with the max-age
1296    directive (see <xref target="cache-response-directive" />), that directive overrides
1297    the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
1298  </t>
1299</x:note>
1300<t>
1301  HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
1302</t>
1303<t>
1304  HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
1305  including the value "0", as in the past (i.e., "already expired").
1306</t>
1307</section>
1308
1309<section anchor="header.pragma" title="Pragma">
1310  <iref item="Pragma header" primary="true" x:for-anchor="" />
1311  <iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
1312  <x:anchor-alias value="extension-pragma"/>
1313  <x:anchor-alias value="Pragma"/>
1314  <x:anchor-alias value="Pragma-v"/>
1315  <x:anchor-alias value="pragma-directive"/>
1316<t>
1317  The "Pragma" general-header field is used to include implementation-specific directives
1318  that might apply to any recipient along the request/response chain. All pragma directives
1319  specify optional behavior from the viewpoint of the protocol; however, some systems
1320  &MAY; require that behavior be consistent with the directives.
1321</t>
1322<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"/>
1323  <x:ref>Pragma</x:ref>            = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
1324  <x:ref>Pragma-v</x:ref>          = 1#<x:ref>pragma-directive</x:ref>
1325  <x:ref>pragma-directive</x:ref>  = "no-cache" / <x:ref>extension-pragma</x:ref>
1326  <x:ref>extension-pragma</x:ref>  = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
1327</artwork></figure>
1328<t>
1329  When the no-cache directive is present in a request message, an application &SHOULD;
1330  forward the request toward the origin server even if it has a cached copy of what is being
1331  requested. This pragma directive has the same semantics as the no-cache response directive
1332  (see <xref target="cache-response-directive" />) and is defined here for backward
1333  compatibility with HTTP/1.0. Clients &SHOULD; include both header fields when a
1334  no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches
1335  &SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
1336</t>
1337<x:note>
1338  <t>
1339    <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a response-header field
1340    is not actually specified, it does not provide a reliable replacement for
1341    "Cache-Control: no-cache" in a response.
1342  </t>
1343</x:note>
1344<t>
1345  This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
1346</t>
1347</section>
1348
1349<section anchor="header.vary" title="Vary">
1350  <iref item="Vary header" primary="true" x:for-anchor="" />
1351  <iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
1352  <x:anchor-alias value="Vary"/>
1353  <x:anchor-alias value="Vary-v"/>
1354<t>
1355  The "Vary" response-header field conveys the set of request-header fields
1356  that were used to select the representation.
1357</t>
1358<t>
1359  Caches use this information, in part, to determine whether a stored response
1360  can be used to satisfy a given request; see
1361  <xref target="caching.negotiated.responses" />.
1362  determines, while the response is fresh, whether a cache is permitted to use the
1363  response to reply to a subsequent request without validation; see <xref 
1364  target="caching.negotiated.responses" />.
1365</t>
1366<t>
1367  In uncacheable or stale responses, the Vary field value advises the user agent about
1368  the criteria that were used to select the representation.
1369</t>
1370<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
1371  <x:ref>Vary</x:ref>   = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
1372  <x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
1373</artwork></figure>
1374<t>
1375  The set of header fields named by the Vary field value is known as the selecting
1376  request-headers.
1377</t>
1378<t>
1379  Servers &SHOULD; include a Vary header field with any cacheable response that is
1380  subject to server-driven negotiation. Doing so allows a cache to properly interpret future
1381  requests on that resource and informs the user agent about the presence of negotiation on
1382  that resource. A server &MAY; include a Vary header field with a non-cacheable
1383  response that is subject to server-driven negotiation, since this might provide the user
1384  agent with useful information about the dimensions over which the response varies at the
1385  time of the response.
1386</t>
1387<t>
1388  A Vary field value of "*" signals that unspecified parameters not limited to the
1389  request-headers (e.g., the network address of the client), play a role in the selection of
1390  the response representation; therefore, a cache cannot determine whether this response is
1391  appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
1392  it may only be generated by an origin server.
1393</t>
1394<t>
1395  The field-names given are not limited to the set of standard request-header fields
1396  defined by this specification. Field names are case-insensitive.
1397</t>
1398</section>
1399
1400<section anchor="header.warning" title="Warning">
1401  <iref item="Warning header" primary="true" x:for-anchor="" />
1402  <iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
1403  <x:anchor-alias value="Warning"/>
1404  <x:anchor-alias value="Warning-v"/>
1405  <x:anchor-alias value="warning-value"/>
1406  <x:anchor-alias value="warn-agent"/>
1407  <x:anchor-alias value="warn-code"/>
1408  <x:anchor-alias value="warn-date"/>
1409  <x:anchor-alias value="warn-text"/>
1410<t>
1411  The "Warning" general-header field is used to carry additional information about the status
1412  or transformation of a message that might not be reflected in the message. This
1413  information is typically used to warn about possible incorrectness introduced by caching
1414  operations or transformations applied to the entity body of the message.
1415</t>
1416<t>
1417  Warnings can be used for other purposes, both cache-related and otherwise. The use of a
1418  warning, rather than an error status code, distinguishes these responses from true failures.
1419</t>
1420<t>
1421  Warning headers can in general be applied to any message, however some warn-codes are
1422  specific to caches and can only be applied to response messages.
1423</t>
1424<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"/>
1425  <x:ref>Warning</x:ref>    = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
1426  <x:ref>Warning-v</x:ref>  = 1#<x:ref>warning-value</x:ref>
1427 
1428  <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>
1429                                        [<x:ref>SP</x:ref> <x:ref>warn-date</x:ref>]
1430 
1431  <x:ref>warn-code</x:ref>  = 3<x:ref>DIGIT</x:ref>
1432  <x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
1433                  ; the name or pseudonym of the server adding
1434                  ; the Warning header, for use in debugging
1435  <x:ref>warn-text</x:ref>  = <x:ref>quoted-string</x:ref>
1436  <x:ref>warn-date</x:ref>  = <x:ref>DQUOTE</x:ref> <x:ref>HTTP-date</x:ref> <x:ref>DQUOTE</x:ref>
1437</artwork></figure>
1438<t>
1439  Multiple warnings can be attached to a response (either by the origin server or by
1440  a cache), including multiple warnings with the same code number, only differing
1441  in warn-text.
1442</t>
1443<t>
1444  When this occurs, the user agent &SHOULD; inform the user of as many of them as
1445  possible, in the order that they appear in the response.
1446</t>
1447<t>
1448  Systems that generate multiple Warning headers &SHOULD; order them with this user
1449  agent behavior in mind. New Warning headers &SHOULD; be added after any existing
1450  Warning headers.
1451</t>
1452<t>
1453  Warnings are assigned three digit warn-codes. The first digit indicates whether the
1454  Warning is required to be deleted from a stored response after validation:
1455  <list style="symbols">
1456    <t>1xx Warnings describe the freshness or validation status of the response, and so
1457      &MUST; be deleted by caches after validation. They can only be generated by a cache
1458      when validating a cached entry, and &MUST-NOT; be generated in any other situation.</t>
1459    <t>2xx Warnings describe some aspect of the entity body or entity headers that is
1460      not rectified by a validation (for example, a lossy compression of the entity bodies)
1461      and &MUST-NOT; be deleted by caches after validation, unless a full response is
1462      returned, in which case they &MUST; be.</t>
1463  </list>
1464</t>
1465<t>
1466  If an implementation sends a message with one or more Warning headers to a receiver whose
1467  version is HTTP/1.0 or lower, then the sender &MUST; include in each warning-value a
1468  warn-date that matches the Date header in the message.
1469</t>
1470<t>
1471  If an implementation receives a message with a warning-value that includes a warn-date,
1472  and that warn-date is different from the Date value in the response, then that
1473  warning-value &MUST; be deleted from the message before storing, forwarding, or using
1474  it. (preventing the consequences of naive caching of Warning header fields.) If all of the
1475  warning-values are deleted for this reason, the Warning header &MUST; be deleted as
1476  well.
1477</t>
1478<t>
1479  The following warn-codes are defined by this specification, each with a recommended
1480  warn-text in English, and a description of its meaning.
1481</t>
1482<t><?rfc needLines="4"?>
1483  110 Response is stale
1484  <list>
1485    <t>&SHOULD; be included whenever the returned response is stale.</t>
1486  </list>
1487</t>
1488<t><?rfc needLines="4"?>
1489  111 Revalidation failed
1490  <list>
1491    <t>&SHOULD; be included if a cache returns a stale response because an attempt to
1492      validate the response failed, due to an inability to reach the server.</t>
1493  </list>
1494</t>
1495<t><?rfc needLines="4"?>
1496  112 Disconnected operation
1497  <list>
1498    <t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
1499      the network for a period of time.</t>
1500  </list>
1501</t>
1502<t><?rfc needLines="4"?>
1503  113 Heuristic expiration
1504  <list>
1505    <t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
1506      greater than 24 hours and the response's age is greater than 24 hours.</t>
1507  </list>
1508</t>
1509<t><?rfc needLines="4"?>
1510  199 Miscellaneous warning
1511  <list>
1512    <t>The warning text can include arbitrary information to be presented to a human
1513      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1514      action, besides presenting the warning to the user.</t>
1515  </list>
1516</t>
1517<t><?rfc needLines="4"?>
1518  214 Transformation applied
1519  <list>
1520    <t>&MUST; be added by an intermediate cache or proxy if it applies any
1521      transformation changing the content-coding (as specified in the Content-Encoding
1522      header) or media-type (as specified in the Content-Type header) of the response, or
1523      the entity-body of the response, unless this Warning code already appears in the
1524      response.</t>
1525  </list>
1526</t>
1527<t><?rfc needLines="4"?>
1528  299 Miscellaneous persistent warning
1529  <list>
1530    <t>The warning text can include arbitrary information to be presented to a human
1531      user, or logged. A system receiving this warning &MUST-NOT; take any automated
1532      action.</t>
1533  </list>
1534</t>
1535</section>
1536
1537</section>
1538
1539<section anchor="history.lists" title="History Lists">
1540<t>
1541  User agents often have history mechanisms, such as "Back" buttons and history lists, that
1542  can be used to redisplay an entity retrieved earlier in a session.
1543</t>
1544<t>
1545  The freshness model (<xref target="expiration.model"/>) does not necessarily apply to history mechanisms. I.e.,
1546  a history mechanism can display a previous representation even if it has expired.
1547</t>
1548  <t>
1549  This does not prohibit the history mechanism from telling the user that a
1550  view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).
1551  </t>
1552</section>
1553
1554
1555<section anchor="IANA.considerations" title="IANA Considerations">
1556
1557<section anchor="message.header.registration" title="Message Header Registration">
1558<t>
1559  The Message Header Registry located at <eref
1560  target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
1561  should be updated with the permanent registrations below (see <xref target="RFC3864" />):
1562</t>
1563<?BEGININC p6-cache.iana-headers ?>
1564<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1565<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1566   <ttcol>Header Field Name</ttcol>
1567   <ttcol>Protocol</ttcol>
1568   <ttcol>Status</ttcol>
1569   <ttcol>Reference</ttcol>
1570
1571   <c>Age</c>
1572   <c>http</c>
1573   <c>standard</c>
1574   <c>
1575      <xref target="header.age"/>
1576   </c>
1577   <c>Cache-Control</c>
1578   <c>http</c>
1579   <c>standard</c>
1580   <c>
1581      <xref target="header.cache-control"/>
1582   </c>
1583   <c>Expires</c>
1584   <c>http</c>
1585   <c>standard</c>
1586   <c>
1587      <xref target="header.expires"/>
1588   </c>
1589   <c>Pragma</c>
1590   <c>http</c>
1591   <c>standard</c>
1592   <c>
1593      <xref target="header.pragma"/>
1594   </c>
1595   <c>Vary</c>
1596   <c>http</c>
1597   <c>standard</c>
1598   <c>
1599      <xref target="header.vary"/>
1600   </c>
1601   <c>Warning</c>
1602   <c>http</c>
1603   <c>standard</c>
1604   <c>
1605      <xref target="header.warning"/>
1606   </c>
1607</texttable>
1608<!--(END)-->
1609<?ENDINC p6-cache.iana-headers ?>
1610<t>
1611  The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1612</t>
1613</section>
1614
1615</section>
1616
1617<section anchor="security.considerations" title="Security Considerations">
1618<t>
1619  Caches expose additional potential vulnerabilities, since the contents of the cache
1620  represent an attractive target for malicious exploitation. Because cache contents persist
1621  after an HTTP request is complete, an attack on the cache can reveal information long after
1622  a user believes that the information has been removed from the network. Therefore, cache
1623  contents should be protected as sensitive information.
1624</t>
1625</section>
1626
1627<section anchor="ack" title="Acknowledgments">
1628<t>
1629  Much of the content and presentation of the caching design is due to suggestions and
1630  comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
1631  and Larry Masinter.
1632</t>
1633</section>
1634
1635</middle>
1636
1637<back>
1638<references title="Normative References">
1639
1640  <reference anchor="Part1">
1641    <front>
1642      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1643      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1644        <organization abbrev="Day Software">Day Software</organization>
1645        <address><email>fielding@gbiv.com</email></address>
1646      </author>
1647      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1648        <organization>One Laptop per Child</organization>
1649        <address><email>jg@laptop.org</email></address>
1650      </author>
1651      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1652        <organization abbrev="HP">Hewlett-Packard Company</organization>
1653        <address><email>JeffMogul@acm.org</email></address>
1654      </author>
1655      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1656        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1657        <address><email>henrikn@microsoft.com</email></address>
1658      </author>
1659      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1660        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1661        <address><email>LMM@acm.org</email></address>
1662      </author>
1663      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1664        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1665        <address><email>paulle@microsoft.com</email></address>
1666      </author>
1667      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1668        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1669        <address><email>timbl@w3.org</email></address>
1670      </author>
1671      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1672        <organization abbrev="W3C">World Wide Web Consortium</organization>
1673        <address><email>ylafon@w3.org</email></address>
1674      </author>
1675      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1676        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1677        <address><email>julian.reschke@greenbytes.de</email></address>
1678      </author>
1679      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1680    </front>
1681    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
1682    <x:source basename="p1-messaging" href="p1-messaging.xml" />
1683  </reference>
1684
1685  <reference anchor="Part2">
1686    <front>
1687      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1688      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1689        <organization abbrev="Day Software">Day Software</organization>
1690        <address><email>fielding@gbiv.com</email></address>
1691      </author>
1692      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1693        <organization>One Laptop per Child</organization>
1694        <address><email>jg@laptop.org</email></address>
1695      </author>
1696      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1697        <organization abbrev="HP">Hewlett-Packard Company</organization>
1698        <address><email>JeffMogul@acm.org</email></address>
1699      </author>
1700      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1701        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1702        <address><email>henrikn@microsoft.com</email></address>
1703      </author>
1704      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1705        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1706        <address><email>LMM@acm.org</email></address>
1707      </author>
1708      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1709        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1710        <address><email>paulle@microsoft.com</email></address>
1711      </author>
1712      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1713        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1714        <address><email>timbl@w3.org</email></address>
1715      </author>
1716      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1717        <organization abbrev="W3C">World Wide Web Consortium</organization>
1718        <address><email>ylafon@w3.org</email></address>
1719      </author>
1720      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1721        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1722        <address><email>julian.reschke@greenbytes.de</email></address>
1723      </author>
1724      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1725    </front>
1726    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
1727    <x:source basename="p2-semantics" href="p2-semantics.xml" />
1728  </reference>
1729
1730  <reference anchor="Part4">
1731    <front>
1732      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1733      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1734        <organization abbrev="Day Software">Day Software</organization>
1735        <address><email>fielding@gbiv.com</email></address>
1736      </author>
1737      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1738        <organization>One Laptop per Child</organization>
1739        <address><email>jg@laptop.org</email></address>
1740      </author>
1741      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1742        <organization abbrev="HP">Hewlett-Packard Company</organization>
1743        <address><email>JeffMogul@acm.org</email></address>
1744      </author>
1745      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1746        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1747        <address><email>henrikn@microsoft.com</email></address>
1748      </author>
1749      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1750        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1751        <address><email>LMM@acm.org</email></address>
1752      </author>
1753      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1754        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1755        <address><email>paulle@microsoft.com</email></address>
1756      </author>
1757      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1758        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1759        <address><email>timbl@w3.org</email></address>
1760      </author>
1761      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1762        <organization abbrev="W3C">World Wide Web Consortium</organization>
1763        <address><email>ylafon@w3.org</email></address>
1764      </author>
1765      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1766        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1767        <address><email>julian.reschke@greenbytes.de</email></address>
1768      </author>
1769      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1770    </front>
1771    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
1772    <x:source basename="p4-conditional" href="p4-conditional.xml" />
1773  </reference>
1774
1775  <reference anchor="Part5">
1776    <front>
1777      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1778      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1779        <organization abbrev="Day Software">Day Software</organization>
1780        <address><email>fielding@gbiv.com</email></address>
1781      </author>
1782      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1783        <organization>One Laptop per Child</organization>
1784        <address><email>jg@laptop.org</email></address>
1785      </author>
1786      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1787        <organization abbrev="HP">Hewlett-Packard Company</organization>
1788        <address><email>JeffMogul@acm.org</email></address>
1789      </author>
1790      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1791        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1792        <address><email>henrikn@microsoft.com</email></address>
1793      </author>
1794      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1795        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1796        <address><email>LMM@acm.org</email></address>
1797      </author>
1798      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1799        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1800        <address><email>paulle@microsoft.com</email></address>
1801      </author>
1802      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1803        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1804        <address><email>timbl@w3.org</email></address>
1805      </author>
1806      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1807        <organization abbrev="W3C">World Wide Web Consortium</organization>
1808        <address><email>ylafon@w3.org</email></address>
1809      </author>
1810      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1811        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1812        <address><email>julian.reschke@greenbytes.de</email></address>
1813      </author>
1814      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1815    </front>
1816    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
1817    <x:source basename="p5-range" href="p5-range.xml" />
1818  </reference>
1819
1820  <reference anchor="Part7">
1821    <front>
1822      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
1823      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1824        <organization abbrev="Day Software">Day Software</organization>
1825        <address><email>fielding@gbiv.com</email></address>
1826      </author>
1827      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1828        <organization>One Laptop per Child</organization>
1829        <address><email>jg@laptop.org</email></address>
1830      </author>
1831      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1832        <organization abbrev="HP">Hewlett-Packard Company</organization>
1833        <address><email>JeffMogul@acm.org</email></address>
1834      </author>
1835      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1836        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1837        <address><email>henrikn@microsoft.com</email></address>
1838      </author>
1839      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1840        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1841        <address><email>LMM@acm.org</email></address>
1842      </author>
1843      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1844        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1845        <address><email>paulle@microsoft.com</email></address>
1846      </author>
1847      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1848        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1849        <address><email>timbl@w3.org</email></address>
1850      </author>
1851      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1852        <organization abbrev="W3C">World Wide Web Consortium</organization>
1853        <address><email>ylafon@w3.org</email></address>
1854      </author>
1855      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1856        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1857        <address><email>julian.reschke@greenbytes.de</email></address>
1858      </author>
1859      <date month="&ID-MONTH;" year="&ID-YEAR;" />
1860    </front>
1861    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
1862    <x:source basename="p7-auth" href="p7-auth.xml" />
1863  </reference>
1864
1865  <reference anchor="RFC2119">
1866    <front>
1867      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1868      <author fullname="Scott Bradner" initials="S." surname="Bradner">
1869        <organization>Harvard University</organization>
1870        <address><email>sob@harvard.edu</email></address>
1871      </author>
1872      <date month="March" year="1997" />
1873    </front>
1874    <seriesInfo name="BCP" value="14" />
1875    <seriesInfo name="RFC" value="2119" />
1876  </reference>
1877
1878  <reference anchor="RFC5234">
1879    <front>
1880      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1881      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1882        <organization>Brandenburg InternetWorking</organization>
1883        <address>
1884          <email>dcrocker@bbiw.net</email>
1885        </address> 
1886      </author>
1887      <author initials="P." surname="Overell" fullname="Paul Overell">
1888        <organization>THUS plc.</organization>
1889        <address>
1890          <email>paul.overell@thus.net</email>
1891        </address>
1892      </author>
1893      <date month="January" year="2008"/>
1894    </front>
1895    <seriesInfo name="STD" value="68"/>
1896    <seriesInfo name="RFC" value="5234"/>
1897  </reference>
1898 
1899</references>
1900
1901<references title="Informative References">
1902
1903  <reference anchor="RFC1305">
1904    <front>
1905      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
1906      <author fullname="David L. Mills" initials="D." surname="Mills">
1907        <organization>University of Delaware, Electrical Engineering Department</organization>
1908        <address><email>mills@udel.edu</email></address>
1909      </author>
1910      <date month="March" year="1992" />
1911    </front>
1912    <seriesInfo name="RFC" value="1305" />
1913  </reference>
1914
1915  <reference anchor="RFC2616">
1916    <front>
1917      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1918      <author fullname="R. Fielding" initials="R." surname="Fielding">
1919        <organization>University of California, Irvine</organization>
1920        <address><email>fielding@ics.uci.edu</email></address>
1921      </author>
1922      <author fullname="J. Gettys" initials="J." surname="Gettys">
1923        <organization>W3C</organization>
1924        <address><email>jg@w3.org</email></address>
1925      </author>
1926      <author fullname="J. Mogul" initials="J." surname="Mogul">
1927        <organization>Compaq Computer Corporation</organization>
1928        <address><email>mogul@wrl.dec.com</email></address>
1929      </author>
1930      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
1931        <organization>MIT Laboratory for Computer Science</organization>
1932        <address><email>frystyk@w3.org</email></address>
1933      </author>
1934      <author fullname="L. Masinter" initials="L." surname="Masinter">
1935        <organization>Xerox Corporation</organization>
1936        <address><email>masinter@parc.xerox.com</email></address>
1937      </author>
1938      <author fullname="P. Leach" initials="P." surname="Leach">
1939        <organization>Microsoft Corporation</organization>
1940        <address><email>paulle@microsoft.com</email></address>
1941      </author>
1942      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
1943        <organization>W3C</organization>
1944        <address><email>timbl@w3.org</email></address>
1945      </author>
1946      <date month="June" year="1999" />
1947    </front>
1948    <seriesInfo name="RFC" value="2616" />
1949  </reference>
1950
1951  <reference anchor="RFC3864">
1952    <front>
1953      <title>Registration Procedures for Message Header Fields</title>
1954      <author fullname="G. Klyne" initials="G." surname="Klyne">
1955        <organization>Nine by Nine</organization>
1956        <address><email>GK-IETF@ninebynine.org</email></address>
1957      </author>
1958      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
1959        <organization>BEA Systems</organization>
1960        <address><email>mnot@pobox.com</email></address>
1961      </author>
1962      <author fullname="J. Mogul" initials="J." surname="Mogul">
1963        <organization>HP Labs</organization>
1964        <address><email>JeffMogul@acm.org</email></address>
1965      </author>
1966      <date month="September" year="2004" />
1967    </front>
1968    <seriesInfo name="BCP" value="90" />
1969    <seriesInfo name="RFC" value="3864" />
1970  </reference>
1971
1972</references>
1973
1974<section anchor="compatibility" title="Compatibility with Previous Versions">
1975
1976<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
1977<t>
1978  A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
1979  this missing case.
1980  (Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
1981</t>
1982<t>
1983  Range request responses would become very verbose if all meta-data were always returned;
1984  by allowing the server to only send needed headers in a 206 response, this problem can be
1985  avoided.
1986  (<xref target="combining.headers" />)
1987</t>
1988<t>
1989  The Cache-Control: max-age directive was not properly defined for responses.
1990  (<xref target="cache-response-directive"/>)
1991</t>
1992<t>
1993  Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
1994    format="counter" target="expiration.model" />, <xref format="counter"
1995    target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
1996  and <xref format="counter" target="header.warning" />) Warning also needed to be a general
1997  header, as PUT or other methods may have need for it in requests.
1998</t>
1999</section>
2000
2001<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2002<t>
2003  Remove requirement to consider Content-Location in successful responses
2004  in order to determine the appropriate response to use.
2005  (<xref target="validation.model" />)
2006</t>
2007<t>
2008  Clarify denial of service attack avoidance requirement.
2009  (<xref target="invalidation.after.updates.or.deletions" />)
2010</t>
2011<t>
2012  Do not mention RFC 2047 encoding and multiple languages in Warning headers
2013  anymore, as these aspects never were implemented.
2014  (<xref target="header.warning" />)
2015</t>
2016</section>
2017
2018    </section>
2019
2020<?BEGININC p6-cache.abnf-appendix ?>
2021<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
2022<figure>
2023<artwork type="abnf" name="p6-cache.parsed-abnf">
2024<x:ref>Age</x:ref> = "Age:" OWS Age-v
2025<x:ref>Age-v</x:ref> = delta-seconds
2026
2027<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
2028<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
2029 cache-directive ] )
2030
2031<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
2032<x:ref>Expires-v</x:ref> = HTTP-date
2033
2034<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
2035
2036<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
2037
2038<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
2039<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2040 pragma-directive ] )
2041
2042<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
2043<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2044 ] ) )
2045
2046<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
2047<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2048 ] )
2049
2050<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
2051<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
2052<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
2053 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2054 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2055 cache-extension
2056<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
2057 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2058 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2059 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2060 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2061 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2062
2063<x:ref>delta-seconds</x:ref> = 1*DIGIT
2064
2065<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
2066
2067<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 3.2&gt;
2068
2069<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.6&gt;
2070<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
2071<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
2072
2073<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
2074
2075<x:ref>token</x:ref> = &lt;token, defined in [Part1], Section 1.2.2&gt;
2076
2077<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
2078
2079<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
2080<x:ref>warn-code</x:ref> = 3DIGIT
2081<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
2082<x:ref>warn-text</x:ref> = quoted-string
2083<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
2084 ]
2085</artwork>
2086</figure>
2087<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
2088; Age defined but not used
2089; Cache-Control defined but not used
2090; Expires defined but not used
2091; Pragma defined but not used
2092; Vary defined but not used
2093; Warning defined but not used
2094</artwork></figure></section>
2095<?ENDINC p6-cache.abnf-appendix ?>
2096
2097<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2098
2099<section title="Since RFC2616">
2100  <t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
2101</section>
2102
2103<section title="Since draft-ietf-httpbis-p6-cache-00">
2104<t>
2105  Closed issues:
2106  <list style="symbols">
2107    <t>
2108      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
2109    <t>
2110      <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>
2111    <t>
2112      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
2113    <t>
2114      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
2115    <t>
2116      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
2117    <t>
2118      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
2119    <t>
2120      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
2121    <t>
2122      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
2123    <t>
2124      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
2125  </list>
2126</t>
2127<t>
2128  Other changes:
2129  <list style="symbols">
2130    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
2131        target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2132  </list>
2133</t>
2134</section>
2135
2136<section title="Since draft-ietf-httpbis-p6-cache-01">
2137<t>
2138  Closed issues:
2139  <list style="symbols">
2140    <t>
2141      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
2142  </list>
2143</t>
2144<t>
2145  Other changes:
2146  <list style="symbols">
2147    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2148      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
2149    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2150      specification.</t>
2151  </list>
2152</t>
2153</section>
2154
2155<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2156<t>
2157  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
2158  <list style="symbols">
2159    <t>Reference RFC 3984, and update header registrations for headers defined in this
2160      document.</t>
2161  </list>
2162</t>
2163</section>
2164
2165<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2166<t>
2167  Closed issues:
2168  <list style="symbols">
2169    <t>
2170      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
2171  </list>
2172</t>
2173</section>
2174
2175<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2176<t>
2177  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2178  <list style="symbols"> 
2179    <t>
2180      Use "/" instead of "|" for alternatives.
2181    </t>
2182    <t>
2183      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2184      whitespace ("OWS") and required whitespace ("RWS").
2185    </t>
2186    <t>
2187      Rewrite ABNFs to spell out whitespace rules, factor out
2188      header value format definitions.
2189    </t>
2190  </list>
2191</t>
2192</section>
2193
2194<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2195<t>
2196  This is a total rewrite of this part of the specification.
2197</t>
2198<t>
2199  Affected issues:
2200  <list style="symbols">
2201    <t>
2202      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
2203    <t>
2204      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
2205    <t>
2206      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
2207    <t>
2208      <eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
2209  </list>
2210</t>
2211<t>
2212  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2213  <list style="symbols"> 
2214    <t>
2215      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2216    </t>
2217  </list>
2218</t>
2219</section>
2220
2221<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2222<t>
2223  Closed issues:
2224  <list style="symbols"> 
2225    <t>
2226      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2227      "base for numeric protocol elements"
2228    </t>
2229  </list>
2230</t>
2231<t>
2232  Affected issues:
2233  <list style="symbols">
2234    <t>
2235      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2236      Vary and non-existant headers
2237    </t>
2238  </list>
2239</t>
2240</section>
2241
2242<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2243<t>
2244  Closed issues:
2245  <list style="symbols"> 
2246    <t>
2247      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />:
2248      "Definition of 1xx Warn-Codes"
2249    </t>
2250    <t>
2251      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2252      "Content-Location on 304 responses"
2253    </t>
2254    <t>
2255      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169" />:
2256      "private and no-cache CC directives with headers"
2257    </t>
2258    <t>
2259      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2260      "RFC2047 and warn-text"
2261    </t>
2262  </list>
2263</t>
2264</section>
2265
2266<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2267<t>
2268  Closed issues:
2269  <list style="symbols"> 
2270    <t>
2271      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197" />:
2272      "Effect of CC directives on history lists"
2273    </t>
2274  </list>
2275</t>
2276<t>
2277  Affected issues:
2278  <list style="symbols">
2279    <t>
2280      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2281      Status codes and caching
2282    </t>
2283  </list>
2284</t>
2285<t>
2286  Partly resolved issues:
2287  <list style="symbols"> 
2288    <t>
2289      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2290      "Placement of 13.5.1 and 13.5.2"
2291    </t>
2292  </list>
2293</t>
2294</section>
2295
2296</section>
2297  </back>
2298</rfc>
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