source: draft-ietf-httpbis/12/draft-ietf-httpbis-p6-cache-12.xml @ 1515

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fix mime types

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc category="std" docName="draft-ietf-httpbis-p6-cache-12" ipr="pre5378Trust200902" obsoletes="2616">
20<front>
21
22  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
23
24  <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
25    <organization abbrev="Day Software">Day Software</organization>
26    <address>
27      <postal>
28        <street>23 Corporate Plaza DR, Suite 280</street>
29        <city>Newport Beach</city>
30        <region>CA</region>
31        <code>92660</code>
32        <country>USA</country>
33      </postal>
34      <phone>+1-949-706-5300</phone>
35      <facsimile>+1-949-706-5305</facsimile>
36      <email>fielding@gbiv.com</email>
37      <uri>http://roy.gbiv.com/</uri>
38    </address>
39  </author>
40
41  <author initials="J." surname="Gettys" fullname="Jim Gettys">
42    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
43    <address>
44      <postal>
45        <street>21 Oak Knoll Road</street>
46        <city>Carlisle</city>
47        <region>MA</region>
48        <code>01741</code>
49        <country>USA</country>
50      </postal>
51      <email>jg@freedesktop.org</email>
52      <uri>http://gettys.wordpress.com/</uri>
53    </address>
54  </author>
55
56  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
57    <organization abbrev="HP">Hewlett-Packard Company</organization>
58    <address>
59      <postal>
60        <street>HP Labs, Large Scale Systems Group</street>
61        <street>1501 Page Mill Road, MS 1177</street>
62        <city>Palo Alto</city>
63        <region>CA</region>
64        <code>94304</code>
65        <country>USA</country>
66      </postal>
67      <email>JeffMogul@acm.org</email>
68    </address>
69  </author>
70
71  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
72    <organization abbrev="Microsoft">Microsoft Corporation</organization>
73    <address>
74      <postal>
75        <street>1 Microsoft Way</street>
76        <city>Redmond</city>
77        <region>WA</region>
78        <code>98052</code>
79        <country>USA</country>
80      </postal>
81      <email>henrikn@microsoft.com</email>
82    </address>
83  </author>
84
85  <author fullname="Larry Masinter" initials="L." surname="Masinter">
86    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
87    <address>
88      <postal>
89        <street>345 Park Ave</street>
90        <city>San Jose</city>
91        <region>CA</region>
92        <code>95110</code>
93        <country>USA</country>
94      </postal>
95      <email>LMM@acm.org</email>
96      <uri>http://larry.masinter.net/</uri>
97    </address>
98  </author>
99
100  <author fullname="Paul J. Leach" initials="P." surname="Leach">
101    <organization abbrev="Microsoft">Microsoft Corporation</organization>
102    <address>
103      <postal>
104        <street>1 Microsoft Way</street>
105        <city>Redmond</city>
106        <region>WA</region>
107        <code>98052</code>
108      </postal>
109      <email>paulle@microsoft.com</email>
110    </address>
111  </author>
112
113  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
114    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
115    <address>
116      <postal>
117        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
118        <street>The Stata Center, Building 32</street>
119        <street>32 Vassar Street</street>
120        <city>Cambridge</city>
121        <region>MA</region>
122        <code>02139</code>
123        <country>USA</country>
124      </postal>
125      <email>timbl@w3.org</email>
126      <uri>http://www.w3.org/People/Berners-Lee/</uri>
127    </address>
128  </author>
129
130  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
131    <organization abbrev="W3C">World Wide Web Consortium</organization>
132    <address>
133      <postal>
134        <street>W3C / ERCIM</street>
135        <street>2004, rte des Lucioles</street>
136        <city>Sophia-Antipolis</city>
137        <region>AM</region>
138        <code>06902</code>
139        <country>France</country>
140      </postal>
141      <email>ylafon@w3.org</email>
142      <uri>http://www.raubacapeu.net/people/yves/</uri>
143    </address>
144  </author>
145
146  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
147    <address>
148      <email>mnot@mnot.net</email>
149      <uri>http://www.mnot.net/</uri>
150    </address>
151  </author>
152
153  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
154    <organization abbrev="greenbytes">greenbytes GmbH</organization>
155    <address>
156      <postal>
157        <street>Hafenweg 16</street>
158        <city>Muenster</city><region>NW</region><code>48155</code>
159        <country>Germany</country>
160      </postal>
161      <phone>+49 251 2807760</phone>
162      <facsimile>+49 251 2807761</facsimile>
163      <email>julian.reschke@greenbytes.de</email>
164      <uri>http://greenbytes.de/tech/webdav/</uri>
165    </address>
166  </author>
167
168  <date month="October" year="2010" day="25"/>
169  <workgroup>HTTPbis Working Group</workgroup>
170
171<abstract>
172<t>
173   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
174   distributed, collaborative, hypermedia information systems. This document
175   is Part 6 of the seven-part specification that defines the protocol
176   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
177   defines requirements on HTTP caches and the associated header fields that
178   control cache behavior or indicate cacheable response messages.
179</t>
180</abstract>
181
182<note title="Editorial Note (To be removed by RFC Editor)">
183   <t>
184      Discussion of this draft should take place on the HTTPBIS working group
185      mailing list (ietf-http-wg@w3.org). The current issues list is at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
186      documents (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/"/>.
187   </t>
188   <t>
189      The changes in this draft are summarized in <xref target="changes.since.11"/>.
190   </t>
191</note>
192
193   </front>
194   <middle>
195
196<section anchor="caching" title="Introduction">
197<t>
198   HTTP is typically used for distributed information systems, where
199   performance can be improved by the use of response caches. This document
200   defines aspects of HTTP/1.1 related to caching and reusing response
201   messages.
202</t>
203
204<section anchor="intro.purpose" title="Purpose">
205<iref item="cache"/>
206<t>
207   An HTTP cache is a local store of response messages and the
208   subsystem that controls its message storage, retrieval, and deletion. A
209   cache stores cacheable responses in order to reduce the response time and
210   network bandwidth consumption on future, equivalent requests. Any client or
211   server MAY employ a cache, though a cache cannot be used by a server that
212   is acting as a tunnel.
213</t>
214<t>
215   Caching would be useless if it did not significantly improve performance.
216   The goal of caching in HTTP/1.1 is to reuse a prior response message to
217   satisfy a current request. In some cases, a stored response can be reused
218   without the need for a network request, reducing latency and network
219   round-trips; a "freshness" mechanism is used for this purpose (see <xref target="expiration.model"/>). Even when a new request is required, it is
220   often possible to reuse all or parts of the payload of a prior response to
221   satisfy the request, thereby reducing network bandwidth usage; a
222   "validation" mechanism is used for this purpose (see <xref target="validation.model"/>).
223</t>
224</section>
225
226<section anchor="intro.terminology" title="Terminology">
227<t>
228   This specification uses a number of terms to refer to the roles played by
229   participants in, and objects of, HTTP caching.
230</t>
231<t>
232   <iref item="cacheable"/>
233   <?rfc needLines="4"?>cacheable
234   <list>
235      <t>A response is cacheable if a cache is allowed to store a copy of the
236      response message for use in answering subsequent requests. Even when a
237      response is cacheable, there might be additional constraints on whether
238      a cache can use the cached copy to satisfy a particular request.</t>
239   </list>
240</t>
241<t>
242   <iref item="explicit expiration time"/>
243   <?rfc needLines="4"?>explicit expiration time
244   <list>
245      <t>The time at which the origin server intends that a representation
246      no longer be returned by a cache without further validation.</t>
247   </list>
248</t>
249<t>
250   <iref item="heuristic expiration time"/>
251   <?rfc needLines="4"?>heuristic expiration time
252   <list>
253      <t>An expiration time assigned by a cache when no explicit expiration
254      time is available.</t>
255   </list>
256</t>
257<t>
258   <iref item="age"/>
259   <?rfc needLines="4"?>age
260   <list>
261      <t>The age of a response is the time since it was sent by, or
262      successfully validated with, the origin server.</t>
263   </list>
264</t>
265<t>
266   <iref item="first-hand"/>
267   <?rfc needLines="4"?>first-hand
268   <list>
269      <t>A response is first-hand if the freshness model is not in use; i.e.,
270      its age is 0.</t>
271   </list>
272</t>
273<t>
274   <iref item="freshness lifetime"/>
275   <?rfc needLines="4"?>freshness lifetime
276   <list>
277      <t>The length of time between the generation of a response and its
278      expiration time.</t>
279   </list>
280</t>
281<t>
282   <iref item="fresh"/>
283   <?rfc needLines="4"?>fresh
284   <list>
285      <t>A response is fresh if its age has not yet exceeded its freshness
286      lifetime.</t>
287   </list>
288</t>
289<t>
290   <iref item="stale"/>
291   <?rfc needLines="4"?>stale
292   <list>
293      <t>A response is stale if its age has passed its freshness lifetime
294      (either explicit or heuristic).</t>
295   </list>
296</t>
297<t>
298   <iref item="validator"/>
299   <?rfc needLines="4"?>validator
300   <list>
301      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
302      is used to find out whether a stored response has an equivalent copy of
303      a representation.</t>
304   </list>
305</t>
306<t anchor="shared.and.non-shared.caches">
307   <iref item="validator"/>
308   <?rfc needLines="4"?>shared cache
309   <list>
310      <t>A cache that is accessible to more than one user. A non-shared cache
311      is dedicated to a single user.</t>
312   </list>
313</t>
314</section>
315
316<section anchor="intro.requirements" title="Requirements">
317<t>
318   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
319   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
320   document are to be interpreted as described in <xref target="RFC2119"/>.
321</t>
322<t>
323   An implementation is not compliant if it fails to satisfy one or more of
324   the "MUST" or "REQUIRED" level requirements for the protocols it
325   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
326   level and all the "SHOULD" level requirements for its protocols is said to
327   be "unconditionally compliant"; one that satisfies all the "MUST" level
328   requirements but not all the "SHOULD" level requirements for its protocols
329   is said to be "conditionally compliant".
330</t>
331</section>
332
333<section title="Syntax Notation" anchor="notation">
334   
335   
336   
337   
338   
339   
340   
341   
342   
343<t>
344   This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
345   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
346   <xref target="collected.abnf"/> shows the collected ABNF, with the list
347   rule expanded.
348</t>
349<t>
350   The following core rules are included by reference, as defined in <xref target="RFC5234"/>, Appendix B.1: ALPHA (letters), CR (carriage
351   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
352   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
353   sequence of data), SP (space), VCHAR (any visible USASCII character), and
354   WSP (whitespace).
355</t>
356
357<section title="Core Rules" anchor="core.rules">
358   
359   
360   
361<t>
362   The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
363</t>
364<figure><artwork type="abnf2616"><![CDATA[
365  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
366  token         = <token, defined in [Part1], Section 1.2.2>
367  OWS           = <OWS, defined in [Part1], Section 1.2.2>
368]]></artwork></figure>
369</section>
370
371<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
372   
373   
374   
375   
376   
377<t>
378   The ABNF rules below are defined in other parts:
379</t>
380<figure><artwork type="abnf2616"><![CDATA[
381  field-name    = <field-name, defined in [Part1], Section 3.2>
382  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
383  port          = <port, defined in [Part1], Section 2.6>
384  pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
385  uri-host      = <uri-host, defined in [Part1], Section 2.6>
386]]></artwork></figure>
387</section>
388
389</section>
390</section>
391
392<section anchor="caching.overview" title="Cache Operation">
393
394<section anchor="response.cacheability" title="Response Cacheability">
395<t>
396   A cache MUST NOT store a response to any request, unless:
397   <list style="symbols">
398      <t>The request method is understood by the cache and defined as being
399      cacheable, and</t>
400      <t>the response status code is understood by the cache, and</t>
401      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
402      header fields, and</t>
403      <t>the "private" cache response directive (see <xref target="cache-response-directive"/> does not appear in the response, if
404      the cache is shared, and</t>
405      <t>the "Authorization" header field (see Section 4.1 of <xref target="Part7"/>) does not
406      appear in the request, if the cache is shared, unless the response
407      explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
408      <t>the response either:
409         <list style="symbols">
410            <t>contains an Expires header field (see <xref target="header.expires"/>), or</t>
411            <t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
412            <t>contains a s-maxage response cache directive and the cache is
413            shared, or</t>
414            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
415            or</t>
416            <t>has a status code that can be served with heuristic freshness
417            (see <xref target="heuristic.freshness"/>).</t>
418         </list>
419      </t>
420   </list>
421</t>
422<t>
423   In this context, a cache has "understood" a request method or a response
424   status code if it recognises it and implements any cache-specific
425   behaviour. In particular, 206 Partial Content responses cannot be cached by
426   an implementation that does not handle partial content (see <xref target="errors.or.incomplete.response.cache.behavior"/>).
427</t>
428<t>
429   Note that in normal operation, most caches will not store a response that
430   has neither a cache validator nor an explicit expiration time, as such
431   responses are not usually useful to store. However, caches are not
432   prohibited from storing such responses.
433</t>
434
435<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
436<t>
437   A cache that receives an incomplete response (for example, with fewer bytes
438   of data than specified in a Content-Length header field) can store the response,
439   but MUST treat it as a partial response <xref target="Part5"/>. Partial responses can
440   be combined as described in Section 4 of <xref target="Part5"/>; the result might be a
441   full response or might still be partial. A cache MUST NOT return a
442   partial response to a client without explicitly marking it as such using
443   the 206 (Partial Content) status code.
444</t>
445<t>
446   A cache that does not support the Range and Content-Range header fields
447   MUST NOT store incomplete or partial responses.
448</t>
449</section>
450
451</section>
452
453
454<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
455<t>
456   For a presented request, a cache MUST NOT return a stored response,
457   unless:
458   <list style="symbols">
459      <t>The presented effective request URI (Section 4.3 of <xref target="Part1"/>) and
460      that of the stored response match, and</t>
461      <t>the request method associated with the stored response allows it to
462      be used for the presented request, and</t>
463      <t>selecting request-header fields nominated by the stored response (if any)
464      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
465      <t>the presented request and stored response are free from directives
466      that would prevent its use (see <xref target="header.cache-control"/>
467      and <xref target="header.pragma"/>), and</t>
468      <t>the stored response is either:
469         <list style="symbols">
470            <t>fresh (see <xref target="expiration.model"/>), or</t>
471            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
472            <t>successfully validated (see <xref target="validation.model"/>).</t>
473         </list>
474      </t>
475  </list>
476</t>
477<t>
478   When a stored response is used to satisfy a request without validation,
479   caches MUST include a single Age header field (<xref target="header.age"/>) in the response with a value equal to the stored response's
480   current_age; see <xref target="age.calculations"/>.
481</t>
482<t>
483   Requests with methods that are unsafe (Section 7.1.1 of <xref target="Part2"/>) MUST be written
484   through the cache to the origin server; i.e., a cache must not reply to
485   such a request before having forwarded the request and having received a
486   corresponding response.
487</t>
488<t>
489   Also, note that unsafe requests might invalidate already stored responses;
490   see <xref target="invalidation.after.updates.or.deletions"/>.
491</t>
492<t>
493   Caches MUST use the most recent response (as determined by the Date
494   header field) when more than one suitable response is stored. They can also
495   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
496   no-cache" to disambiguate which response to use.
497</t>
498<t>
499   An HTTP implementation without a clock MUST NOT used stored responses
500   without revalidating them on every use. An HTTP cache, especially a shared
501   cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to
502   synchronize its clock with a reliable external standard.
503</t>
504
505</section>
506
507<section anchor="expiration.model" title="Freshness Model">
508<t>
509   When a response is "fresh" in the cache, it can be used to satisfy
510   subsequent requests without contacting the origin server, thereby improving
511   efficiency.
512</t>
513<t>
514   The primary mechanism for determining freshness is for an origin server to
515   provide an explicit expiration time in the future, using either the Expires
516   header field (<xref target="header.expires"/>) or the max-age response cache
517   directive (<xref target="cache-response-directive"/>). Generally, origin
518   servers will assign future explicit expiration times to responses in the
519   belief that the representation is not likely to change in a semantically
520   significant way before the expiration time is reached.
521</t>
522<t>
523   If an origin server wishes to force a cache to validate every request, it
524   can assign an explicit expiration time in the past to indicate that the
525   response is already stale.  Compliant caches will validate the cached response
526   before reusing it for subsequent requests.
527</t>
528<t>
529   Since origin servers do not always provide explicit expiration times, HTTP
530   caches MAY assign heuristic expiration times when explicit times are not
531   specified, employing algorithms that use other heade field values (such as the
532   Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1
533   specification does not provide specific algorithms, but does impose
534   worst-case constraints on their results.
535</t>
536<figure>
537<preamble>
538  The calculation to determine if a response is fresh is:
539</preamble>
540<artwork type="code"><![CDATA[
541   response_is_fresh = (freshness_lifetime > current_age)
542]]></artwork>
543</figure>
544<t>
545   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
546   <xref target="age.calculations"/>.
547</t>
548<t>
549   Additionally, clients might need to influence freshness calculation. They
550   can do this using several request cache directives, with the effect of
551   either increasing or loosening constraints on freshness. See <xref target="cache-request-directive"/>.
552</t>
553<t>
554   <cref anchor="ISSUE-no-req-for-directives">there are not requirements
555   directly applying to cache-request-directives and freshness.</cref>
556</t>
557<t>
558   Note that freshness applies only to cache operation; it cannot be used to
559   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
560   caches and history mechanisms.
561</t>
562
563<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
564<t>
565   A cache can calculate the freshness lifetime (denoted as
566   freshness_lifetime) of a response by using the first match of:
567   <list style="symbols">
568      <t>If the cache is shared and the s-maxage response cache directive
569      (<xref target="cache-response-directive"/>) is present, use its value,
570      or</t>
571      <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
572      <t>If the Expires response header field (<xref target="header.expires"/>) is
573      present, use its value minus the value of the Date response header field,
574      or</t>
575      <t>Otherwise, no explicit expiration time is present in the response. A
576      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
577   </list>
578</t>
579<t>
580   Note that this calculation is not vulnerable to clock skew, since all of
581   the information comes from the origin server.
582</t>
583
584<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
585<t>
586   If no explicit expiration time is present in a stored response that has a
587   status code whose definition allows heuristic freshness to be used
588   (including the following in Section 8 of <xref target="Part2"/>: 200, 203, 206, 300, 301 and
589   410), a heuristic expiration time MAY be calculated. Heuristics
590   MUST NOT be used for response status codes that do not explicitly allow
591   it.
592</t>
593<t>
594   When a heuristic is used to calculate freshness lifetime, the cache
595   SHOULD attach a Warning header field with a 113 warn-code to the response if
596   its current_age is more than 24 hours and such a warning is not already
597   present.
598</t>
599<t>
600   Also, if the response has a Last-Modified header field (Section 6.6 of <xref target="Part4"/>),
601   the heuristic expiration value SHOULD be no more than some fraction of
602   the interval since that time. A typical setting of this fraction might be
603   10%.
604</t>
605<t><list>
606   <t>
607      Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9) required that caches do not calculate heuristic
608      freshness for URLs with query components (i.e., those containing '?').
609      In practice, this has not been widely implemented. Therefore, servers
610      are encouraged to send explicit directives (e.g., Cache-Control:
611      no-cache) if they wish to preclude caching.
612   </t>
613</list></t>
614</section>
615</section>
616
617<section anchor="age.calculations" title="Calculating Age">
618<t>
619   HTTP/1.1 uses the Age response-header field to convey the estimated age of the
620   response message when obtained from a cache. The Age field value is the
621   cache's estimate of the amount of time since the response was generated or
622   validated by the origin server. In essence, the Age value is the sum of the
623   time that the response has been resident in each of the caches along the
624   path from the origin server, plus the amount of time it has been in transit
625   along network paths.
626</t>
627<t>
628   The following data is used for the age calculation:
629</t>
630<t>
631   <?rfc needLines="4"?>age_value
632   <list>
633      <t>
634         The term "age_value" denotes the value of the Age header field (<xref target="header.age"/>), in a form appropriate for arithmetic
635         operation; or 0, if not available.
636      </t>
637   </list>
638</t>
639<t>
640   <?rfc needLines="4"?>date_value
641   <list>
642      <t>
643         HTTP/1.1 requires origin servers to send a Date header field, if possible,
644         with every response, giving the time at which the response was
645         generated. The term "date_value" denotes the value of the Date
646         header field, in a form appropriate for arithmetic operations. See
647         Section 9.3 of <xref target="Part1"/> for the definition of the Date header field, and for
648         requirements regarding responses without it.
649      </t>
650   </list>
651</t>
652<t>
653   <?rfc needLines="4"?>now
654   <list>
655      <t>
656         The term "now" means "the current value of the clock at the host
657         performing the calculation". Hosts that use HTTP, but especially
658         hosts running origin servers and caches, SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize their
659         clocks to a globally accurate time standard.
660      </t>
661   </list>
662</t>
663<t>
664   <?rfc needLines="4"?>request_time
665   <list>
666      <t>
667         The current value of the clock at the host at the time the request
668         resulting in the stored response was made.
669      </t>
670   </list>
671</t>
672<t>
673   <?rfc needLines="4"?>response_time
674   <list>
675      <t>
676         The current value of the clock at the host at the time the response
677         was received.
678      </t>
679   </list>
680</t>
681<t>
682   A response's age can be calculated in two entirely independent ways:
683   <list style="numbers">
684      <t>the "apparent_age": response_time minus date_value, if the local
685      clock is reasonably well synchronized to the origin server's clock. If
686      the result is negative, the result is replaced by zero.</t>
687      <t>the "corrected_age_value", if all of the caches along the response
688      path implement HTTP/1.1; note this value MUST be interpreted relative
689      to the time the request was initiated, not the time that the response
690      was received.</t>
691   </list>
692</t>
693<figure>
694<artwork type="code"><![CDATA[
695  apparent_age = max(0, response_time - date_value);
696
697  response_delay = response_time - request_time;
698  corrected_age_value = age_value + response_delay; 
699]]></artwork>
700</figure>
701<figure>
702<preamble>These are combined as</preamble>
703<artwork type="code"><![CDATA[
704  corrected_initial_age = max(apparent_age, corrected_age_value);
705]]></artwork></figure>
706<t>
707   The current_age of a stored response can then be calculated by adding the
708   amount of time (in seconds) since the stored response was last validated by
709   the origin server to the corrected_initial_age.
710</t>
711<figure><artwork type="code"><![CDATA[
712  resident_time = now - response_time;
713  current_age = corrected_initial_age + resident_time;
714]]></artwork></figure>
715</section>
716
717<section anchor="serving.stale.responses" title="Serving Stale Responses">
718<t>
719   A "stale" response is one that either has explicit expiry information or is
720   allowed to have heuristic expiry calculated, but is not fresh according to
721   the calculations in <xref target="expiration.model"/>.
722</t>
723<t>
724   Caches MUST NOT return a stale response if it is prohibited by an
725   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
726   directive, a "must-revalidate" cache-response-directive, or an applicable
727   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
728</t>
729<t>
730   Caches SHOULD NOT return stale responses unless they are disconnected
731   (i.e., it cannot contact the origin server or otherwise find a forward
732   path) or otherwise explicitly allowed (e.g., the max-stale request
733   directive; see <xref target="cache-request-directive"/>).
734</t>
735<t>
736   Stale responses SHOULD have a Warning header field with the 110 warn-code (see
737   <xref target="header.warning"/>). Likewise, the 112 warn-code SHOULD be
738   sent on stale responses if the cache is disconnected.
739</t>
740<t>
741   If a cache receives a first-hand response (either an entire response, or a
742   304 (Not Modified) response) that it would normally forward to the
743   requesting client, and the received response is no longer fresh, the cache
744   SHOULD forward it to the requesting client without adding a new Warning
745   (but without removing any existing Warning header fields). A cache SHOULD NOT
746   attempt to validate a response simply because that response became stale in
747   transit.
748</t>
749</section>
750</section>
751
752<section anchor="validation.model" title="Validation Model">
753<t>
754   When a cache has one or more stored responses for a requested URI, but
755   cannot serve any of them (e.g., because they are not fresh, or one cannot
756   be selected; see <xref target="caching.negotiated.responses"/>), it can use
757   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
758   give the origin server an opportunity to both select a valid stored
759   response to be used, and to update it. This process is known as
760   "validating" or "revalidating" the stored response.
761</t>
762<t>
763   When sending such a conditional request, the cache SHOULD add an
764   If-Modified-Since header field whose value is that of the Last-Modified header
765   field from the selected (see <xref target="caching.negotiated.responses"/>)
766   stored response, if available.
767</t>
768<t>
769   Additionally, the cache SHOULD add an If-None-Match header field whose value is
770   that of the ETag header field(s) from all responses stored for the requested URI,
771   if present. However, if any of the stored responses contains only partial
772   content, its entity-tag SHOULD NOT be included in the If-None-Match
773   header field unless the request is for a range that would be fully
774   satisfied by that stored response.
775</t>
776<t>
777   A 304 (Not Modified) response status code indicates that the stored
778   response can be updated and reused; see <xref target="combining.responses"/>.
779</t>
780<t>
781   A full response (i.e., one with a response body) indicates that none of the
782   stored responses nominated in the conditional request is suitable. Instead,
783   the full response SHOULD be used to satisfy the request and MAY replace
784   the stored response.
785</t>
786<t>
787   If a cache receives a 5xx response while attempting to validate a response,
788   it MAY either forward this response to the requesting client, or act as
789   if the server failed to respond. In the latter case, it MAY return a
790   previously stored response (see <xref target="serving.stale.responses"/>).
791</t>
792</section>
793
794<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
795<t>
796   Because unsafe methods (Section 7.1.1 of <xref target="Part2"/>) have the potential for changing
797   state on the origin server, intervening caches can use them to keep their
798   contents up-to-date.
799</t>
800<t>
801   The following HTTP methods MUST cause a cache to invalidate the effective
802   Request URI (Section 4.3 of <xref target="Part1"/>) as well as the URI(s) in the Location
803   and Content-Location header fields (if present):
804   <list style="symbols">
805      <t>PUT</t>
806      <t>DELETE</t>
807      <t>POST</t>
808   </list>
809</t>
810<t>
811   An invalidation based on a URI from a Location or Content-Location header field
812   MUST NOT be performed if the host part of that URI differs from the host
813   part in the effective request URI (Section 4.3 of <xref target="Part1"/>). This helps
814   prevent denial of service attacks.
815</t>
816<t>
817   A cache that passes through requests for methods it does not understand
818   SHOULD invalidate the effective request URI (Section 4.3 of <xref target="Part1"/>).
819</t>
820<t>
821   Here, "invalidate" means that the cache will either remove all stored
822   responses related to the effective request URI, or will mark these as
823   "invalid" and in need of a mandatory validation before they can be returned
824   in response to a subsequent request.
825</t>
826<t>
827   Note that this does not guarantee that all appropriate responses are
828   invalidated. For example, the request that caused the change at the origin
829   server might not have gone through the cache where a response is stored.
830</t>
831</section>
832
833<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
834
835<t>
836   Shared caches MUST NOT use a cached response to a request with an
837   Authorization header field (Section 4.1 of <xref target="Part7"/>) to satisfy any subsequent
838   request unless a cache directive that allows such responses to be stored is
839   present in the response.
840</t>
841
842<t>
843   In this specification, the following Cache-Control response directives
844   (<xref target="cache-response-directive"/>) have such an effect:
845   must-revalidate, public, s-maxage.
846</t>
847
848<t>
849   Note that cached responses that contain the "must-revalidate" and/or
850   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
851   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
852   used to satisfy a subsequent request without revalidating it on the origin
853   server.
854</t>
855</section>
856
857<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
858<t>
859   When a cache receives a request that can be satisfied by a stored response
860   that has a Vary header field (<xref target="header.vary"/>), it MUST NOT
861   use that response unless all of the selecting request-header fields nominated by
862   the Vary header field match in both the original request (i.e., that associated
863   with the stored response), and the presented request.
864</t>
865<t>
866   The selecting request-header fields from two requests are defined to match if and
867   only if those in the first request can be transformed to those in the
868   second request by applying any of the following:
869   <list style="symbols">
870      <t>
871         adding or removing whitespace, where allowed in the header field's syntax
872      </t>
873      <t>
874         combining multiple message-header fields with the same field name
875         (see Section 3.2 of <xref target="Part1"/>)
876      </t>
877      <t>
878         normalizing both header field values in a way that is known to have
879         identical semantics, according to the header field's specification (e.g.,
880         re-ordering field values when order is not significant;
881         case-normalization, where values are defined to be case-insensitive)
882      </t>
883  </list>
884</t>
885<t>
886   If (after any normalization that might take place) a header field is absent
887   from a request, it can only match another request if it is also absent
888   there.
889</t>
890<t>
891   A Vary header field-value of "*" always fails to match, and subsequent
892   requests to that resource can only be properly interpreted by the origin
893   server.
894</t>
895<t>
896   The stored response with matching selecting request-header fields is known as the
897   selected response.
898</t>
899<t>
900   If no selected response is available, the cache MAY forward the presented
901   request to the origin server in a conditional request; see <xref target="validation.model"/>.
902</t>
903</section>
904
905<section anchor="combining.responses" title="Combining Responses">
906<t>
907   When a cache receives a 304 (Not Modified) response or a 206 (Partial
908   Content) response (in this section, the "new" response"), it needs to
909   created an updated response by combining the stored response with the new
910   one, so that the updated response can be used to satisfy the request, and
911   potentially update the cached response.
912</t>
913<t>
914   If the new response contains an ETag, it identifies the stored response to
915   use. <cref anchor="TODO-mention-CL">might need language about
916   Content-Location here</cref><cref anchor="TODO-select-for-combine">Shouldn't this be the selected
917   response?</cref>
918</t>
919<t>
920   If the new response's status code is 206 (partial content), both the stored
921   and new responses MUST have validators, and those validators MUST match
922   using the strong comparison function (see Section 4 of <xref target="Part4"/>). Otherwise,
923   the responses MUST NOT be combined.
924</t>
925<t>
926   The stored response header fields are used as those of the updated response,
927   except that
928   <list style="symbols">
929      <t>any stored Warning header fields with warn-code 1xx (see <xref target="header.warning"/>) MUST be deleted.</t>
930      <t>any stored Warning header fields with warn-code 2xx MUST be retained.</t>
931      <t>any other header fields provided in the new response MUST replace all
932      instances of the corresponding header fields from the stored response.</t>
933   </list>
934</t>
935<t>
936   The updated response header fields MUST be used to replace those of the stored
937   response in cache (unless the stored response is removed from cache). In
938   the case of a 206 response, the combined representation MAY be stored.
939</t>
940</section>
941
942</section>
943
944<section anchor="header.fields" title="Header Field Definitions">
945<t>
946   This section defines the syntax and semantics of HTTP/1.1 header fields
947   related to caching.
948</t>
949
950<section anchor="header.age" title="Age">
951   <iref item="Age header" primary="true"/>
952   <iref item="Headers" primary="true" subitem="Age"/>
953   
954   
955   
956<t>
957   The "Age" response-header field conveys the sender's estimate of the amount
958   of time since the response was generated or successfully validated at the
959   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
960</t>
961<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/><artwork type="abnf2616"><![CDATA[
962  Age   = "Age" ":" OWS Age-v
963  Age-v = delta-seconds
964]]></artwork></figure>
965<t anchor="rule.delta-seconds">
966 
967  Age field-values are non-negative integers, representing time in seconds.
968</t>
969<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
970  delta-seconds  = 1*DIGIT
971]]></artwork></figure>
972<t>
973   If a cache receives a value larger than the largest positive integer it can
974   represent, or if any of its age calculations overflows, it MUST transmit
975   an Age header field with a field-value of 2147483648 (2^31). Caches
976   SHOULD use an arithmetic type of at least 31 bits of range.
977</t>
978<t>
979   The presence of an Age header field in a response implies that a response
980   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
981   might not implement the Age header field.
982</t>
983</section>
984
985<section anchor="header.cache-control" title="Cache-Control">
986   <iref item="Cache-Control header" primary="true"/>
987   <iref item="Headers" primary="true" subitem="Cache-Control"/>
988   
989   
990   
991   
992   
993   
994<t>
995   The "Cache-Control" general-header field is used to specify directives for
996   caches along the request/response chain. Such cache directives are
997   unidirectional in that the presence of a directive in a request does not
998   imply that the same directive is to be given in the response.
999</t>
1000<t>
1001   HTTP/1.1 caches MUST obey the requirements of the Cache-Control
1002   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1003   directives defined elsewhere are handled.
1004</t>
1005<t><list>
1006   <t>
1007       Note: HTTP/1.0 caches might not implement Cache-Control and
1008       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1009   </t>
1010</list></t>
1011<t>
1012   Cache directives MUST be passed through by a proxy or gateway
1013   application, regardless of their significance to that application, since
1014   the directives might be applicable to all recipients along the
1015   request/response chain. It is not possible to target a directive to a
1016   specific cache.
1017</t>
1018<figure><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"/><artwork type="abnf2616"><![CDATA[
1019  Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
1020  Cache-Control-v = 1#cache-directive
1021
1022  cache-directive = cache-request-directive
1023     / cache-response-directive
1024
1025  cache-extension = token [ "=" ( token / quoted-string ) ]
1026]]></artwork></figure>
1027
1028<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1029   
1030
1031<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
1032  cache-request-directive =
1033       "no-cache"
1034     / "no-store"
1035     / "max-age" "=" delta-seconds
1036     / "max-stale" [ "=" delta-seconds ]
1037     / "min-fresh" "=" delta-seconds
1038     / "no-transform"
1039     / "only-if-cached"
1040     / cache-extension
1041]]></artwork></figure>
1042
1043<t>
1044   <?rfc needLines="4"?>no-cache
1045   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1046   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1047   <list>
1048      <t>The no-cache request directive indicates that a stored response
1049      MUST NOT be used to satisfy the request without successful validation
1050      on the origin server.</t> 
1051   </list>
1052</t>
1053<t>
1054   <?rfc needLines="4"?>no-store
1055   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1056   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1057   <list>
1058      <t>The no-store request directive indicates that a cache MUST NOT
1059      store any part of either this request or any response to it. This
1060      directive applies to both non-shared and shared caches. "MUST NOT
1061      store" in this context means that the cache MUST NOT intentionally
1062      store the information in non-volatile storage, and MUST make a
1063      best-effort attempt to remove the information from volatile storage as
1064      promptly as possible after forwarding it.</t>
1065      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1066      privacy. In particular, malicious or compromised caches might not
1067      recognize or obey this directive, and communications networks might be
1068      vulnerable to eavesdropping.</t>
1069   </list>
1070</t>
1071<t>
1072   <?rfc needLines="4"?>max-age
1073   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1074   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1075   <list>
1076      <t>The max-age request directive indicates that the client is willing to
1077      accept a response whose age is no greater than the specified time in
1078      seconds. Unless the max-stale request directive is also present, the
1079      client is not willing to accept a stale response.</t>
1080   </list>
1081</t>
1082<t>
1083   <?rfc needLines="4"?>max-stale
1084   <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1085   <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1086   <list>
1087      <t>The max-stale request directive indicates that the client is willing
1088      to accept a response that has exceeded its expiration time. If max-stale
1089      is assigned a value, then the client is willing to accept a response
1090      that has exceeded its expiration time by no more than the specified
1091      number of seconds. If no value is assigned to max-stale, then the client
1092      is willing to accept a stale response of any age.</t>
1093   </list>
1094</t>
1095<t>
1096   <?rfc needLines="4"?>min-fresh
1097   <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1098   <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1099   <list>
1100      <t>The min-fresh request directive indicates that the client is willing
1101      to accept a response whose freshness lifetime is no less than its
1102      current age plus the specified time in seconds. That is, the client
1103      wants a response that will still be fresh for at least the specified
1104      number of seconds.</t>
1105   </list>
1106</t>
1107<t>
1108   <?rfc needLines="4"?>no-transform
1109   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1110   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1111   <list>
1112      <t>The no-transform request directive indicates that an intermediate
1113      cache or proxy MUST NOT change the Content-Encoding, Content-Range or
1114      Content-Type request header fields, nor the request representation.</t>
1115   </list>
1116</t>
1117<t>
1118   <?rfc needLines="4"?>only-if-cached
1119   <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1120   <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1121   <list>
1122      <t>The only-if-cached request directive indicates that the client only
1123      wishes to return a stored response. If it receives this directive, a
1124      cache SHOULD either respond using a stored response that is consistent
1125      with the other constraints of the request, or respond with a 504
1126      (Gateway Timeout) status code. If a group of caches is being operated as
1127      a unified system with good internal connectivity, such a request MAY
1128      be forwarded within that group of caches.</t>
1129   </list>
1130</t>
1131</section>
1132
1133<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1134   
1135
1136<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1137  cache-response-directive =
1138       "public"
1139     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1140     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1141     / "no-store"
1142     / "no-transform"
1143     / "must-revalidate"
1144     / "proxy-revalidate"
1145     / "max-age" "=" delta-seconds
1146     / "s-maxage" "=" delta-seconds
1147     / cache-extension
1148]]></artwork></figure>
1149
1150<t>
1151   <?rfc needLines="4"?>public
1152   <iref item="Cache Directives" primary="true" subitem="public"/>
1153   <iref item="public" primary="true" subitem="Cache Directive"/>
1154   <list>
1155      <t>The public response directive indicates that the response MAY be
1156      cached, even if it would normally be non-cacheable or cacheable only
1157      within a non-shared cache. (See also Authorization,
1158      Section 4.1 of <xref target="Part7"/>, for additional details.) </t>
1159  </list>
1160</t>
1161<t>
1162   <?rfc needLines="4"?>private
1163   <iref item="Cache Directives" primary="true" subitem="private"/>
1164   <iref item="private" primary="true" subitem="Cache Directive"/>
1165   <list>
1166      <t>The private response directive indicates that the response message is
1167      intended for a single user and MUST NOT be stored by a shared cache. A
1168      private (non-shared) cache MAY store the response.</t>
1169      <t>If the private response directive specifies one or more field-names,
1170      this requirement is limited to the field-values associated with the
1171      listed response header fields. That is, the specified field-names(s)
1172      MUST NOT be stored by a shared cache, whereas the remainder of the
1173      response message MAY be.</t>
1174      <t> Note: This usage of the word private only controls where
1175      the response can be stored; it cannot ensure the privacy of the message
1176      content. Also, private response directives with field-names are often
1177      handled by implementations as if an unqualified private directive was
1178      received; i.e., the special handling for the qualified form is not
1179      widely implemented.</t>
1180   </list>
1181</t>
1182<t>
1183   <?rfc needLines="4"?>no-cache
1184   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1185   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1186   <list>
1187      <t>The no-cache response directive indicates that the response MUST NOT
1188      be used to satisfy a subsequent request without successful validation on
1189      the origin server. This allows an origin server to prevent a cache from
1190      using it to satisfy a request without contacting it, even by caches that
1191      have been configured to return stale responses.</t>
1192      <t>If the no-cache response directive specifies one or more field-names,
1193      this requirement is limited to the field-values associated with the
1194      listed response header fields. That is, the specified field-name(s) MUST NOT
1195      be sent in the response to a subsequent request without successful
1196      validation on the origin server. This allows an origin server to prevent
1197      the re-use of certain header fields in a response, while still allowing
1198      caching of the rest of the response.</t>
1199      <t> Note: Most HTTP/1.0 caches will not recognize or obey
1200      this directive. Also, no-cache response directives with field-names are
1201      often handled by implementations as if an unqualified no-cache directive
1202      was received; i.e., the special handling for the qualified form is not
1203      widely implemented. </t>
1204   </list>
1205</t>
1206<t>
1207   <?rfc needLines="4"?>no-store
1208   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1209   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1210   <list>
1211      <t>The no-store response directive indicates that a cache MUST NOT
1212      store any part of either the immediate request or response. This
1213      directive applies to both non-shared and shared caches. "MUST NOT
1214      store" in this context means that the cache MUST NOT intentionally
1215      store the information in non-volatile storage, and MUST make a
1216      best-effort attempt to remove the information from volatile storage as
1217      promptly as possible after forwarding it.</t>
1218      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1219      privacy. In particular, malicious or compromised caches might not
1220      recognize or obey this directive, and communications networks might be
1221      vulnerable to eavesdropping.</t>
1222   </list>
1223</t>
1224<t>
1225   <?rfc needLines="4"?>must-revalidate
1226   <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1227   <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1228   <list>
1229      <t>The must-revalidate response directive indicates that once it has
1230      become stale, the response MUST NOT be used to satisfy subsequent
1231      requests without successful validation on the origin server.</t>
1232      <t>The must-revalidate directive is necessary to support reliable
1233      operation for certain protocol features. In all circumstances an
1234      HTTP/1.1 cache MUST obey the must-revalidate directive; in particular,
1235      if the cache cannot reach the origin server for any reason, it MUST
1236      generate a 504 (Gateway Timeout) response.</t>
1237      <t>Servers SHOULD send the must-revalidate directive if and only if
1238      failure to validate a request on the representation could result in
1239      incorrect operation, such as a silently unexecuted financial
1240      transaction.</t>
1241   </list>
1242</t>
1243<t>
1244   <?rfc needLines="4"?>proxy-revalidate
1245   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1246   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1247   <list>
1248      <t>The proxy-revalidate response directive has the same meaning as the
1249      must-revalidate response directive, except that it does not apply to
1250      non-shared caches.</t>
1251   </list>
1252</t>
1253<t>
1254   <?rfc needLines="4"?>max-age
1255   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1256   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1257   <list>
1258      <t>The max-age response directive indicates that response is to be
1259      considered stale after its age is greater than the specified number of
1260      seconds.</t>
1261   </list>
1262</t>
1263<t>
1264   <?rfc needLines="4"?>s-maxage
1265   <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1266   <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1267   <list>
1268      <t>The s-maxage response directive indicates that, in shared caches, the
1269      maximum age specified by this directive overrides the maximum age
1270      specified by either the max-age directive or the Expires header field. The
1271      s-maxage directive also implies the semantics of the proxy-revalidate
1272      response directive.</t>
1273   </list>
1274</t>
1275<t>
1276   <?rfc needLines="4"?>no-transform
1277   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1278   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1279   <list>
1280      <t>The no-transform response directive indicates that an intermediate
1281      cache or proxy MUST NOT change the Content-Encoding, Content-Range or
1282      Content-Type response header fields, nor the response representation.</t>
1283   </list>
1284</t>
1285
1286</section>
1287
1288<section anchor="cache.control.extensions" title="Cache Control Extensions">
1289<t>
1290   The Cache-Control header field can be extended through the use of one or
1291   more cache-extension tokens, each with an optional value. Informational
1292   extensions (those that do not require a change in cache behavior) can be
1293   added without changing the semantics of other directives. Behavioral
1294   extensions are designed to work by acting as modifiers to the existing base
1295   of cache directives. Both the new directive and the standard directive are
1296   supplied, such that applications that do not understand the new directive
1297   will default to the behavior specified by the standard directive, and those
1298   that understand the new directive will recognize it as modifying the
1299   requirements associated with the standard directive. In this way,
1300   extensions to the cache-control directives can be made without requiring
1301   changes to the base protocol.
1302</t>
1303<t>
1304   This extension mechanism depends on an HTTP cache obeying all of the
1305   cache-control directives defined for its native HTTP-version, obeying
1306   certain extensions, and ignoring all directives that it does not
1307   understand.
1308</t>
1309<t>
1310   For example, consider a hypothetical new response directive called
1311   "community" that acts as a modifier to the private directive. We define
1312   this new directive to mean that, in addition to any non-shared cache, any
1313   cache that is shared only by members of the community named within its
1314   value may cache the response. An origin server wishing to allow the UCI
1315   community to use an otherwise private response in their shared cache(s)
1316   could do so by including
1317</t>
1318<figure><artwork type="example"><![CDATA[
1319  Cache-Control: private, community="UCI"
1320]]></artwork></figure>
1321<t>
1322   A cache seeing this header field will act correctly even if the cache does
1323   not understand the community cache-extension, since it will also see and
1324   understand the private directive and thus default to the safe behavior.
1325</t>
1326<t>
1327   Unrecognized cache directives MUST be ignored; it is assumed that any
1328   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1329   combined with standard directives (or the response's default cacheability)
1330   such that the cache behavior will remain minimally correct even if the
1331   cache does not understand the extension(s).
1332</t>
1333<t>
1334   The HTTP Cache Directive Registry defines the name space for the cache
1335   directives.
1336</t>
1337<t>
1338   Registrations MUST include the following fields:
1339   <list style="symbols">
1340      <t>Cache Directive Name</t>
1341      <t>Pointer to specification text</t>
1342   </list>
1343</t>
1344<t>
1345   Values to be added to this name space are subject to IETF review (<xref target="RFC5226"/>, Section 4.1).
1346</t>
1347<t>
1348   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1349</t>
1350</section>
1351
1352</section>
1353
1354<section anchor="header.expires" title="Expires">
1355   <iref item="Expires header" primary="true"/>
1356   <iref item="Headers" primary="true" subitem="Expires"/>
1357   
1358   
1359<t>
1360   The "Expires" header field gives the date/time after which the
1361   response is considered stale. See <xref target="expiration.model"/> for
1362   further discussion of the freshness model.
1363</t>
1364<t>
1365   The presence of an Expires field does not imply that the original resource
1366   will change or cease to exist at, before, or after that time.
1367</t>
1368<t>
1369   The field-value is an absolute date and time as defined by HTTP-date in
1370   Section 6.1 of <xref target="Part1"/>; it MUST be sent in rfc1123-date format.
1371</t>
1372<figure><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/><artwork type="abnf2616"><![CDATA[
1373  Expires   = "Expires" ":" OWS Expires-v
1374  Expires-v = HTTP-date
1375]]></artwork></figure>
1376<figure>
1377  <preamble>For example</preamble>
1378<artwork type="example"><![CDATA[
1379  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1380]]></artwork></figure>
1381<t><list>
1382   <t>
1383       Note: If a response includes a Cache-Control field with the
1384       max-age directive (see <xref target="cache-response-directive"/>),
1385       that directive overrides the Expires field. Likewise, the s-maxage
1386       directive overrides Expires in shared caches.
1387   </t>
1388</list></t>
1389<t>
1390   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the
1391   future.
1392</t>
1393<t>
1394   HTTP/1.1 clients and caches MUST treat other invalid date formats,
1395   especially including the value "0", as in the past (i.e., "already
1396   expired").
1397</t>
1398</section>
1399
1400<section anchor="header.pragma" title="Pragma">
1401   <iref item="Pragma header" primary="true"/>
1402   <iref item="Headers" primary="true" subitem="Pragma"/>
1403   
1404   
1405   
1406   
1407<t>
1408   The "Pragma" general-header field is used to include
1409   implementation-specific directives that might apply to any recipient along
1410   the request/response chain. All pragma directives specify optional behavior
1411   from the viewpoint of the protocol; however, some systems MAY require
1412   that behavior be consistent with the directives.
1413</t>
1414<figure><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"/><artwork type="abnf2616"><![CDATA[
1415  Pragma            = "Pragma" ":" OWS Pragma-v
1416  Pragma-v          = 1#pragma-directive
1417  pragma-directive  = "no-cache" / extension-pragma
1418  extension-pragma  = token [ "=" ( token / quoted-string ) ]
1419]]></artwork></figure>
1420<t>
1421   When the no-cache directive is present in a request message, an application
1422   SHOULD forward the request toward the origin server even if it has a
1423   cached copy of what is being requested. This pragma directive has the same
1424   semantics as the no-cache response directive (see <xref target="cache-response-directive"/>) and is defined here for backward
1425   compatibility with HTTP/1.0. Clients SHOULD include both header fields
1426   when a no-cache request is sent to a server not known to be HTTP/1.1
1427   compliant. HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the
1428   client had sent "Cache-Control: no-cache".
1429</t>
1430<t><list>
1431   <t>
1432      Note: Because the meaning of "Pragma: no-cache" as a
1433      response-header field is not actually specified, it does not provide a
1434      reliable replacement for "Cache-Control: no-cache" in a response.
1435   </t>
1436</list></t>
1437<t>
1438   This mechanism is deprecated; no new Pragma directives will be defined in
1439   HTTP.
1440</t>
1441</section>
1442
1443<section anchor="header.vary" title="Vary">
1444   <iref item="Vary header" primary="true"/>
1445   <iref item="Headers" primary="true" subitem="Vary"/>
1446   
1447   
1448<t>
1449   The "Vary" response-header field conveys the set of request-header fields
1450   that were used to select the representation.
1451</t>
1452<t>
1453   Caches use this information, in part, to determine whether a stored
1454   response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>. determines, while the response is
1455   fresh, whether a cache is permitted to use the response to reply to a
1456   subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1457</t>
1458<t>
1459   In uncacheable or stale responses, the Vary field value advises the user
1460   agent about the criteria that were used to select the representation.
1461</t>
1462<figure><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/><artwork type="abnf2616"><![CDATA[
1463  Vary   = "Vary" ":" OWS Vary-v
1464  Vary-v = "*" / 1#field-name
1465]]></artwork></figure>
1466<t>
1467   The set of header fields named by the Vary field value is known as the
1468   selecting request-header fields.
1469</t>
1470<t>
1471   Servers SHOULD include a Vary header field with any cacheable response
1472   that is subject to server-driven negotiation. Doing so allows a cache to
1473   properly interpret future requests on that resource and informs the user
1474   agent about the presence of negotiation on that resource. A server MAY
1475   include a Vary header field with a non-cacheable response that is subject
1476   to server-driven negotiation, since this might provide the user agent with
1477   useful information about the dimensions over which the response varies at
1478   the time of the response.
1479</t>
1480<t>
1481   A Vary field value of "*" signals that unspecified parameters not limited
1482   to the request-header fields (e.g., the network address of the client), play a
1483   role in the selection of the response representation; therefore, a cache
1484   cannot determine whether this response is appropriate. The "*" value
1485   MUST NOT be generated by a proxy server.
1486</t>
1487<t>
1488   The field-names given are not limited to the set of standard request-header
1489   fields defined by this specification. Field names are case-insensitive.
1490</t>
1491</section>
1492
1493<section anchor="header.warning" title="Warning">
1494   <iref item="Warning header" primary="true"/>
1495   <iref item="Headers" primary="true" subitem="Warning"/>
1496   
1497   
1498   
1499   
1500   
1501   
1502   
1503<t>
1504   The "Warning" general-header field is used to carry additional information
1505   about the status or transformation of a message that might not be reflected
1506   in the message. This information is typically used to warn about possible
1507   incorrectness introduced by caching operations or transformations applied
1508   to the payload of the message.
1509</t>
1510<t>
1511   Warnings can be used for other purposes, both cache-related and otherwise.
1512   The use of a warning, rather than an error status code, distinguishes these
1513   responses from true failures.
1514</t>
1515<t>
1516   Warning header fields can in general be applied to any message, however some
1517   warn-codes are specific to caches and can only be applied to response
1518   messages.
1519</t>
1520<figure><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"/><artwork type="abnf2616"><![CDATA[
1521  Warning    = "Warning" ":" OWS Warning-v
1522  Warning-v  = 1#warning-value
1523 
1524  warning-value = warn-code SP warn-agent SP warn-text
1525                                        [SP warn-date]
1526 
1527  warn-code  = 3DIGIT
1528  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1529                  ; the name or pseudonym of the server adding
1530                  ; the Warning header field, for use in debugging
1531  warn-text  = quoted-string
1532  warn-date  = DQUOTE HTTP-date DQUOTE
1533]]></artwork></figure>
1534<t>
1535   Multiple warnings can be attached to a response (either by the origin
1536   server or by a cache), including multiple warnings with the same code
1537   number, only differing in warn-text.
1538</t>
1539<t>
1540   When this occurs, the user agent SHOULD inform the user of as many of
1541   them as possible, in the order that they appear in the response.
1542</t>
1543<t>
1544   Systems that generate multiple Warning header fields SHOULD order them with
1545   this user agent behavior in mind. New Warning header fields SHOULD be added
1546   after any existing Warning headers fields.
1547</t>
1548<t>
1549   Warnings are assigned three digit warn-codes. The first digit indicates
1550   whether the Warning is required to be deleted from a stored response after
1551   validation:
1552   <list style="symbols">
1553      <t>1xx Warnings describe the freshness or validation status of the
1554      response, and so MUST be deleted by caches after validation. They can
1555      only be generated by a cache when validating a cached entry, and
1556      MUST NOT be generated in any other situation.</t>
1557      <t>2xx Warnings describe some aspect of the representation that is not
1558      rectified by a validation (for example, a lossy compression of the
1559      representation) and MUST NOT be deleted by caches after validation,
1560      unless a full response is returned, in which case they MUST be.</t>
1561   </list>
1562</t>
1563<t>
1564   If an implementation sends a message with one or more Warning header fields to a
1565   receiver whose version is HTTP/1.0 or lower, then the sender MUST include
1566   in each warning-value a warn-date that matches the Date header field in the
1567   message.
1568</t>
1569<t>
1570   If an implementation receives a message with a warning-value that includes
1571   a warn-date, and that warn-date is different from the Date value in the
1572   response, then that warning-value MUST be deleted from the message before
1573   storing, forwarding, or using it. (preventing the consequences of naive
1574   caching of Warning header fields.) If all of the warning-values are deleted
1575   for this reason, the Warning header field MUST be deleted as well.
1576</t>
1577<t>
1578   The following warn-codes are defined by this specification, each with a
1579   recommended warn-text in English, and a description of its meaning.
1580</t>
1581<t>
1582<?rfc needLines="4"?>
1583   110 Response is stale
1584   <list>
1585      <t>SHOULD be included whenever the returned response is stale.</t>
1586   </list>
1587</t>
1588<t>
1589<?rfc needLines="4"?>
1590   111 Revalidation failed
1591   <list>
1592      <t>SHOULD be included if a cache returns a stale response because an
1593      attempt to validate the response failed, due to an inability to reach
1594      the server.</t>
1595   </list>
1596</t>
1597<t>
1598<?rfc needLines="4"?>
1599   112 Disconnected operation
1600   <list>
1601      <t>SHOULD be included if the cache is intentionally disconnected from
1602      the rest of the network for a period of time.</t>
1603   </list>
1604</t>
1605<t>
1606<?rfc needLines="4"?>
1607   113 Heuristic expiration
1608   <list>
1609      <t>SHOULD be included if the cache heuristically chose a freshness
1610      lifetime greater than 24 hours and the response's age is greater than 24
1611      hours.</t>
1612   </list>
1613</t>
1614<t>
1615<?rfc needLines="4"?>
1616   199 Miscellaneous warning
1617   <list>
1618      <t>The warning text can include arbitrary information to be presented to
1619      a human user, or logged. A system receiving this warning MUST NOT take
1620      any automated action, besides presenting the warning to the user.</t>
1621   </list>
1622</t>
1623<t>
1624<?rfc needLines="4"?>
1625   214 Transformation applied
1626   <list>
1627      <t>MUST be added by an intermediate proxy if it applies any
1628      transformation to the representation, such as changing the
1629      content-coding, media-type, or modifying the representation data, unless
1630      this Warning code already appears in the response.</t>
1631   </list>
1632</t>
1633<t>
1634<?rfc needLines="4"?>
1635   299 Miscellaneous persistent warning
1636   <list>
1637      <t>The warning text can include arbitrary information to be presented to
1638      a human user, or logged. A system receiving this warning MUST NOT take
1639      any automated action.</t>
1640   </list>
1641</t>
1642</section>
1643
1644</section>
1645
1646<section anchor="history.lists" title="History Lists">
1647<t>
1648   User agents often have history mechanisms, such as "Back" buttons and
1649   history lists, that can be used to redisplay a representation retrieved
1650   earlier in a session.
1651</t>
1652<t>
1653   The freshness model (<xref target="expiration.model"/>) does not
1654   necessarily apply to history mechanisms. I.e., a history mechanism can
1655   display a previous representation even if it has expired.
1656</t>
1657<t>
1658   This does not prohibit the history mechanism from telling the user that a
1659   view might be stale, or from honoring cache directives (e.g.,
1660   Cache-Control: no-store).
1661</t>
1662</section>
1663
1664
1665<section anchor="IANA.considerations" title="IANA Considerations">
1666
1667<section title="Cache Directive Registry" anchor="cache.directive.registration">
1668<t>
1669   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1670</t>
1671<t>
1672   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1673   populated with the registrations below:
1674</t>
1675
1676<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1677<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1678   <ttcol>Cache Directive</ttcol>
1679   <ttcol>Reference</ttcol>
1680
1681   <c>max-age</c>
1682   <c>
1683      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1684   </c>
1685   <c>max-stale</c>
1686   <c>
1687      <xref target="cache-request-directive"/>
1688   </c>
1689   <c>min-fresh</c>
1690   <c>
1691      <xref target="cache-request-directive"/>
1692   </c>
1693   <c>must-revalidate</c>
1694   <c>
1695      <xref target="cache-response-directive"/>
1696   </c>
1697   <c>no-cache</c>
1698   <c>
1699      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1700   </c>
1701   <c>no-store</c>
1702   <c>
1703      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1704   </c>
1705   <c>no-transform</c>
1706   <c>
1707      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1708   </c>
1709   <c>only-if-cached</c>
1710   <c>
1711      <xref target="cache-request-directive"/>
1712   </c>
1713   <c>private</c>
1714   <c>
1715      <xref target="cache-response-directive"/>
1716   </c>
1717   <c>proxy-revalidate</c>
1718   <c>
1719      <xref target="cache-response-directive"/>
1720   </c>
1721   <c>public</c>
1722   <c>
1723      <xref target="cache-response-directive"/>
1724   </c>
1725   <c>s-maxage</c>
1726   <c>
1727      <xref target="cache-response-directive"/>
1728   </c>
1729   <c>stale-if-error</c>
1730   <c>
1731      <xref target="RFC5861"/>, Section 4
1732   </c>
1733   <c>stale-while-revalidate</c>
1734   <c>
1735      <xref target="RFC5861"/>, Section 3
1736   </c>
1737</texttable>
1738<!--(END)-->
1739
1740</section>
1741
1742<section title="Header Field Registration" anchor="header.field.registration">
1743<t>
1744  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1745  shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1746</t>
1747
1748<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1749<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1750   <ttcol>Header Field Name</ttcol>
1751   <ttcol>Protocol</ttcol>
1752   <ttcol>Status</ttcol>
1753   <ttcol>Reference</ttcol>
1754
1755   <c>Age</c>
1756   <c>http</c>
1757   <c>standard</c>
1758   <c>
1759      <xref target="header.age"/>
1760   </c>
1761   <c>Cache-Control</c>
1762   <c>http</c>
1763   <c>standard</c>
1764   <c>
1765      <xref target="header.cache-control"/>
1766   </c>
1767   <c>Expires</c>
1768   <c>http</c>
1769   <c>standard</c>
1770   <c>
1771      <xref target="header.expires"/>
1772   </c>
1773   <c>Pragma</c>
1774   <c>http</c>
1775   <c>standard</c>
1776   <c>
1777      <xref target="header.pragma"/>
1778   </c>
1779   <c>Vary</c>
1780   <c>http</c>
1781   <c>standard</c>
1782   <c>
1783      <xref target="header.vary"/>
1784   </c>
1785   <c>Warning</c>
1786   <c>http</c>
1787   <c>standard</c>
1788   <c>
1789      <xref target="header.warning"/>
1790   </c>
1791</texttable>
1792<!--(END)-->
1793
1794<t>
1795   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1796   Force".
1797</t>
1798</section>
1799
1800</section>
1801
1802<section anchor="security.considerations" title="Security Considerations">
1803<t>
1804   Caches expose additional potential vulnerabilities, since the contents of
1805   the cache represent an attractive target for malicious exploitation.
1806   Because cache contents persist after an HTTP request is complete, an attack
1807   on the cache can reveal information long after a user believes that the
1808   information has been removed from the network. Therefore, cache contents
1809   need to be protected as sensitive information.
1810</t>
1811</section>
1812
1813<section anchor="ack" title="Acknowledgments">
1814<t>
1815   Much of the content and presentation of the caching design is due to
1816   suggestions and comments from individuals including: Shel Kaphan, Paul
1817   Leach, Koen Holtman, David Morris, and Larry Masinter.
1818</t>
1819</section>
1820
1821</middle>
1822
1823<back>
1824<references title="Normative References">
1825
1826  <reference anchor="Part1">
1827    <front>
1828      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1829      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1830        <organization abbrev="Day Software">Day Software</organization>
1831        <address><email>fielding@gbiv.com</email></address>
1832      </author>
1833      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1834        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1835        <address><email>jg@freedesktop.org</email></address>
1836      </author>
1837      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1838        <organization abbrev="HP">Hewlett-Packard Company</organization>
1839        <address><email>JeffMogul@acm.org</email></address>
1840      </author>
1841      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1842        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1843        <address><email>henrikn@microsoft.com</email></address>
1844      </author>
1845      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1846        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1847        <address><email>LMM@acm.org</email></address>
1848      </author>
1849      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1850        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1851        <address><email>paulle@microsoft.com</email></address>
1852      </author>
1853      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1854        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1855        <address><email>timbl@w3.org</email></address>
1856      </author>
1857      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1858        <organization abbrev="W3C">World Wide Web Consortium</organization>
1859        <address><email>ylafon@w3.org</email></address>
1860      </author>
1861      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1862        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1863        <address><email>julian.reschke@greenbytes.de</email></address>
1864      </author>
1865      <date month="October" year="2010"/>
1866    </front>
1867    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-12"/>
1868   
1869  </reference>
1870
1871  <reference anchor="Part2">
1872    <front>
1873      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1874      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1875        <organization abbrev="Day Software">Day Software</organization>
1876        <address><email>fielding@gbiv.com</email></address>
1877      </author>
1878      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1879        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1880        <address><email>jg@freedesktop.org</email></address>
1881      </author>
1882      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1883        <organization abbrev="HP">Hewlett-Packard Company</organization>
1884        <address><email>JeffMogul@acm.org</email></address>
1885      </author>
1886      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1887        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1888        <address><email>henrikn@microsoft.com</email></address>
1889      </author>
1890      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1891        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1892        <address><email>LMM@acm.org</email></address>
1893      </author>
1894      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1895        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1896        <address><email>paulle@microsoft.com</email></address>
1897      </author>
1898      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1899        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1900        <address><email>timbl@w3.org</email></address>
1901      </author>
1902      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1903        <organization abbrev="W3C">World Wide Web Consortium</organization>
1904        <address><email>ylafon@w3.org</email></address>
1905      </author>
1906      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1907        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1908        <address><email>julian.reschke@greenbytes.de</email></address>
1909      </author>
1910      <date month="October" year="2010"/>
1911    </front>
1912    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-12"/>
1913   
1914  </reference>
1915
1916  <reference anchor="Part4">
1917    <front>
1918      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1919      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1920        <organization abbrev="Day Software">Day Software</organization>
1921        <address><email>fielding@gbiv.com</email></address>
1922      </author>
1923      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1924        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1925        <address><email>jg@freedesktop.org</email></address>
1926      </author>
1927      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1928        <organization abbrev="HP">Hewlett-Packard Company</organization>
1929        <address><email>JeffMogul@acm.org</email></address>
1930      </author>
1931      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1932        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1933        <address><email>henrikn@microsoft.com</email></address>
1934      </author>
1935      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1936        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1937        <address><email>LMM@acm.org</email></address>
1938      </author>
1939      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1940        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1941        <address><email>paulle@microsoft.com</email></address>
1942      </author>
1943      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1944        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1945        <address><email>timbl@w3.org</email></address>
1946      </author>
1947      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1948        <organization abbrev="W3C">World Wide Web Consortium</organization>
1949        <address><email>ylafon@w3.org</email></address>
1950      </author>
1951      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1952        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1953        <address><email>julian.reschke@greenbytes.de</email></address>
1954      </author>
1955      <date month="October" year="2010"/>
1956    </front>
1957    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-12"/>
1958   
1959  </reference>
1960
1961  <reference anchor="Part5">
1962    <front>
1963      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1964      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1965        <organization abbrev="Day Software">Day Software</organization>
1966        <address><email>fielding@gbiv.com</email></address>
1967      </author>
1968      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1969        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1970        <address><email>jg@freedesktop.org</email></address>
1971      </author>
1972      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1973        <organization abbrev="HP">Hewlett-Packard Company</organization>
1974        <address><email>JeffMogul@acm.org</email></address>
1975      </author>
1976      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1977        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1978        <address><email>henrikn@microsoft.com</email></address>
1979      </author>
1980      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1981        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1982        <address><email>LMM@acm.org</email></address>
1983      </author>
1984      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1985        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1986        <address><email>paulle@microsoft.com</email></address>
1987      </author>
1988      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1989        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1990        <address><email>timbl@w3.org</email></address>
1991      </author>
1992      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1993        <organization abbrev="W3C">World Wide Web Consortium</organization>
1994        <address><email>ylafon@w3.org</email></address>
1995      </author>
1996      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1997        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1998        <address><email>julian.reschke@greenbytes.de</email></address>
1999      </author>
2000      <date month="October" year="2010"/>
2001    </front>
2002    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-12"/>
2003   
2004  </reference>
2005
2006  <reference anchor="Part7">
2007    <front>
2008      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2009      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2010        <organization abbrev="Day Software">Day Software</organization>
2011        <address><email>fielding@gbiv.com</email></address>
2012      </author>
2013      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2014        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2015        <address><email>jg@freedesktop.org</email></address>
2016      </author>
2017      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2018        <organization abbrev="HP">Hewlett-Packard Company</organization>
2019        <address><email>JeffMogul@acm.org</email></address>
2020      </author>
2021      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2022        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2023        <address><email>henrikn@microsoft.com</email></address>
2024      </author>
2025      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2026        <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2027        <address><email>LMM@acm.org</email></address>
2028      </author>
2029      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2030        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2031        <address><email>paulle@microsoft.com</email></address>
2032      </author>
2033      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2034        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2035        <address><email>timbl@w3.org</email></address>
2036      </author>
2037      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2038        <organization abbrev="W3C">World Wide Web Consortium</organization>
2039        <address><email>ylafon@w3.org</email></address>
2040      </author>
2041      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2042        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2043        <address><email>julian.reschke@greenbytes.de</email></address>
2044      </author>
2045      <date month="October" year="2010"/>
2046    </front>
2047    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-12"/>
2048   
2049  </reference>
2050
2051  <reference anchor="RFC2119">
2052    <front>
2053      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2054      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2055        <organization>Harvard University</organization>
2056        <address><email>sob@harvard.edu</email></address>
2057      </author>
2058      <date month="March" year="1997"/>
2059    </front>
2060    <seriesInfo name="BCP" value="14"/>
2061    <seriesInfo name="RFC" value="2119"/>
2062  </reference>
2063
2064  <reference anchor="RFC5234">
2065    <front>
2066      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2067      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2068        <organization>Brandenburg InternetWorking</organization>
2069        <address>
2070          <email>dcrocker@bbiw.net</email>
2071        </address> 
2072      </author>
2073      <author initials="P." surname="Overell" fullname="Paul Overell">
2074        <organization>THUS plc.</organization>
2075        <address>
2076          <email>paul.overell@thus.net</email>
2077        </address>
2078      </author>
2079      <date month="January" year="2008"/>
2080    </front>
2081    <seriesInfo name="STD" value="68"/>
2082    <seriesInfo name="RFC" value="5234"/>
2083  </reference>
2084 
2085</references>
2086
2087<references title="Informative References">
2088
2089  <reference anchor="RFC1305">
2090    <front>
2091      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2092      <author fullname="David L. Mills" initials="D." surname="Mills">
2093        <organization>University of Delaware, Electrical Engineering Department</organization>
2094        <address><email>mills@udel.edu</email></address>
2095      </author>
2096      <date month="March" year="1992"/>
2097    </front>
2098    <seriesInfo name="RFC" value="1305"/>
2099  </reference>
2100
2101  <reference anchor="RFC2616">
2102    <front>
2103      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2104      <author fullname="R. Fielding" initials="R." surname="Fielding">
2105        <organization>University of California, Irvine</organization>
2106        <address><email>fielding@ics.uci.edu</email></address>
2107      </author>
2108      <author fullname="J. Gettys" initials="J." surname="Gettys">
2109        <organization>W3C</organization>
2110        <address><email>jg@w3.org</email></address>
2111      </author>
2112      <author fullname="J. Mogul" initials="J." surname="Mogul">
2113        <organization>Compaq Computer Corporation</organization>
2114        <address><email>mogul@wrl.dec.com</email></address>
2115      </author>
2116      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2117        <organization>MIT Laboratory for Computer Science</organization>
2118        <address><email>frystyk@w3.org</email></address>
2119      </author>
2120      <author fullname="L. Masinter" initials="L." surname="Masinter">
2121        <organization>Xerox Corporation</organization>
2122        <address><email>masinter@parc.xerox.com</email></address>
2123      </author>
2124      <author fullname="P. Leach" initials="P." surname="Leach">
2125        <organization>Microsoft Corporation</organization>
2126        <address><email>paulle@microsoft.com</email></address>
2127      </author>
2128      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2129        <organization>W3C</organization>
2130        <address><email>timbl@w3.org</email></address>
2131      </author>
2132      <date month="June" year="1999"/>
2133    </front>
2134    <seriesInfo name="RFC" value="2616"/>
2135  </reference>
2136
2137  <reference anchor="RFC3864">
2138    <front>
2139      <title>Registration Procedures for Message Header Fields</title>
2140      <author fullname="G. Klyne" initials="G." surname="Klyne">
2141        <organization>Nine by Nine</organization>
2142        <address><email>GK-IETF@ninebynine.org</email></address>
2143      </author>
2144      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2145        <organization>BEA Systems</organization>
2146        <address><email>mnot@pobox.com</email></address>
2147      </author>
2148      <author fullname="J. Mogul" initials="J." surname="Mogul">
2149        <organization>HP Labs</organization>
2150        <address><email>JeffMogul@acm.org</email></address>
2151      </author>
2152      <date month="September" year="2004"/>
2153    </front>
2154    <seriesInfo name="BCP" value="90"/>
2155    <seriesInfo name="RFC" value="3864"/>
2156  </reference>
2157
2158  <reference anchor="RFC5226">
2159    <front>
2160      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2161      <author initials="T." surname="Narten" fullname="T. Narten">
2162        <organization>IBM</organization>
2163        <address><email>narten@us.ibm.com</email></address>
2164      </author>
2165      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2166        <organization>Google</organization>
2167        <address><email>Harald@Alvestrand.no</email></address>
2168      </author>
2169      <date year="2008" month="May"/>
2170    </front>
2171    <seriesInfo name="BCP" value="26"/>
2172    <seriesInfo name="RFC" value="5226"/>
2173  </reference>
2174
2175  <reference anchor="RFC5861">
2176    <front>
2177      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2178      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2179        <organization>Yahoo! Inc.</organization>
2180        <address><email>mnot@yahoo-inc.com</email></address>
2181      </author>
2182      <date month="April" year="2010"/>
2183    </front>
2184    <seriesInfo name="RFC" value="5861"/>
2185  </reference>
2186
2187</references>
2188
2189<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2190<t>
2191  Make the specified age calculation algorithm less conservative.
2192  (<xref target="age.calculations"/>)
2193</t>
2194<t>
2195  Remove requirement to consider Content-Location in successful responses
2196  in order to determine the appropriate response to use.
2197  (<xref target="validation.model"/>)
2198</t>
2199<t>
2200  Clarify denial of service attack avoidance requirement.
2201  (<xref target="invalidation.after.updates.or.deletions"/>)
2202</t>
2203<t>
2204  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2205  anymore, as these aspects never were implemented.
2206  (<xref target="header.warning"/>)
2207</t>
2208</section>
2209
2210
2211<section title="Collected ABNF" anchor="collected.abnf">
2212<figure>
2213<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2214Age = "Age:" OWS Age-v
2215Age-v = delta-seconds
2216
2217Cache-Control = "Cache-Control:" OWS Cache-Control-v
2218Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
2219 cache-directive ] )
2220
2221Expires = "Expires:" OWS Expires-v
2222Expires-v = HTTP-date
2223
2224HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2225
2226OWS = <OWS, defined in [Part1], Section 1.2.2>
2227
2228Pragma = "Pragma:" OWS Pragma-v
2229Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2230 pragma-directive ] )
2231
2232Vary = "Vary:" OWS Vary-v
2233Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2234 ] ) )
2235
2236Warning = "Warning:" OWS Warning-v
2237Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2238 ] )
2239
2240cache-directive = cache-request-directive / cache-response-directive
2241cache-extension = token [ "=" ( token / quoted-string ) ]
2242cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2243 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2244 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2245 cache-extension
2246cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2247 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2248 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2249 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2250 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2251 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2252
2253delta-seconds = 1*DIGIT
2254
2255extension-pragma = token [ "=" ( token / quoted-string ) ]
2256
2257field-name = <field-name, defined in [Part1], Section 3.2>
2258
2259port = <port, defined in [Part1], Section 2.6>
2260pragma-directive = "no-cache" / extension-pragma
2261pseudonym = <pseudonym, defined in [Part1], Section 9.9>
2262
2263quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
2264
2265token = <token, defined in [Part1], Section 1.2.2>
2266
2267uri-host = <uri-host, defined in [Part1], Section 2.6>
2268
2269warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2270warn-code = 3DIGIT
2271warn-date = DQUOTE HTTP-date DQUOTE
2272warn-text = quoted-string
2273warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2274 ]
2275]]></artwork>
2276</figure>
2277<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2278; Age defined but not used
2279; Cache-Control defined but not used
2280; Expires defined but not used
2281; Pragma defined but not used
2282; Vary defined but not used
2283; Warning defined but not used
2284]]></artwork></figure></section>
2285
2286
2287<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2288
2289<section title="Since RFC 2616">
2290  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2291</section>
2292
2293<section title="Since draft-ietf-httpbis-p6-cache-00">
2294<t>
2295  Closed issues:
2296  <list style="symbols">
2297    <t>
2298      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2299    <t>
2300      <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>
2301    <t>
2302      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2303    <t>
2304      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2305    <t>
2306      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2307    <t>
2308      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2309    <t>
2310      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2311    <t>
2312      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2313    <t>
2314      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2315  </list>
2316</t>
2317<t>
2318  Other changes:
2319  <list style="symbols">
2320    <t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2321  </list>
2322</t>
2323</section>
2324
2325<section title="Since draft-ietf-httpbis-p6-cache-01">
2326<t>
2327  Closed issues:
2328  <list style="symbols">
2329    <t>
2330      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2331  </list>
2332</t>
2333<t>
2334  Other changes:
2335  <list style="symbols">
2336    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2337      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2338    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2339      specification.</t>
2340  </list>
2341</t>
2342</section>
2343
2344<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2345<t>
2346  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2347  <list style="symbols">
2348    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2349      document.</t>
2350  </list>
2351</t>
2352</section>
2353
2354<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2355<t>
2356  Closed issues:
2357  <list style="symbols">
2358    <t>
2359      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2360  </list>
2361</t>
2362</section>
2363
2364<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2365<t>
2366  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2367  <list style="symbols"> 
2368    <t>
2369      Use "/" instead of "|" for alternatives.
2370    </t>
2371    <t>
2372      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2373      whitespace ("OWS") and required whitespace ("RWS").
2374    </t>
2375    <t>
2376      Rewrite ABNFs to spell out whitespace rules, factor out
2377      header field value format definitions.
2378    </t>
2379  </list>
2380</t>
2381</section>
2382
2383<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2384<t>
2385  This is a total rewrite of this part of the specification.
2386</t>
2387<t>
2388  Affected issues:
2389  <list style="symbols">
2390    <t>
2391      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2392    <t>
2393      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2394    <t>
2395      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2396    <t>
2397      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2398  </list>
2399</t>
2400<t>
2401  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2402  <list style="symbols"> 
2403    <t>
2404      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2405    </t>
2406  </list>
2407</t>
2408</section>
2409
2410<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2411<t>
2412  Closed issues:
2413  <list style="symbols"> 
2414    <t>
2415      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2416      "base for numeric protocol elements"
2417    </t>
2418  </list>
2419</t>
2420<t>
2421  Affected issues:
2422  <list style="symbols">
2423    <t>
2424      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2425      WVary and non-existant headers"
2426    </t>
2427  </list>
2428</t>
2429</section>
2430
2431<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2432<t>
2433  Closed issues:
2434  <list style="symbols"> 
2435    <t>
2436      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
2437      "Definition of 1xx Warn-Codes"
2438    </t>
2439    <t>
2440      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2441      "Content-Location on 304 responses"
2442    </t>
2443    <t>
2444      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
2445      "private and no-cache CC directives with headers"
2446    </t>
2447    <t>
2448      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2449      "RFC2047 and warn-text"
2450    </t>
2451  </list>
2452</t>
2453</section>
2454
2455<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2456<t>
2457  Closed issues:
2458  <list style="symbols"> 
2459    <t>
2460      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
2461      "serving negotiated responses from cache: header-specific canonicalization"
2462    </t>
2463    <t>
2464      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
2465      "Effect of CC directives on history lists"
2466    </t>
2467  </list>
2468</t>
2469<t>
2470  Affected issues:
2471  <list style="symbols">
2472    <t>
2473      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2474      Status codes and caching
2475    </t>
2476  </list>
2477</t>
2478<t>
2479  Partly resolved issues:
2480  <list style="symbols"> 
2481    <t>
2482      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2483      "Placement of 13.5.1 and 13.5.2"
2484    </t>
2485  </list>
2486</t>
2487</section>
2488
2489<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2490<t>
2491  Closed issues:
2492  <list style="symbols"> 
2493    <t>
2494      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
2495      "Age calculation"
2496    </t>
2497    <t>
2498      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
2499      "Clarify differences between / requirements for request and response CC directives"
2500    </t>
2501        <t>
2502                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
2503                "Caching authenticated responses"
2504        </t>
2505    <t>
2506      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
2507      "IANA registry for cache-control directives"
2508    </t>
2509    <t>
2510      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
2511      "Heuristic caching of URLs with query components"
2512    </t>
2513  </list>
2514</t>
2515<t>
2516  Partly resolved issues:
2517  <list style="symbols"> 
2518    <t>
2519      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2520      "Term for the requested resource's URI"
2521    </t>
2522  </list>
2523</t>
2524</section>
2525
2526<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2527<t>
2528  Closed issues:
2529  <list style="symbols"> 
2530    <t>
2531      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2532      "Clarify entity / representation / variant terminology"
2533    </t>
2534    <t>
2535      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2536      "consider removing the 'changes from 2068' sections"
2537    </t>
2538    <t>
2539      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2540      "Allowing heuristic caching for new status codes"
2541    </t>
2542    <t>
2543      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2544      "Allowing heuristic caching for new status codes"
2545    </t>
2546    <t>
2547      Clean up TODOs and prose in "Combining Responses."
2548    </t>
2549  </list>
2550</t>
2551</section>
2552
2553<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2554<t>
2555  Closed issues:
2556  <list style="symbols"> 
2557    <t>
2558      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2559      "Text about clock requirement for caches belongs in p6"
2560    </t>
2561  </list>
2562</t>
2563</section>
2564
2565</section>
2566  </back>
2567</rfc>
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