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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-14" 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="Adobe">Adobe Systems Incorporated</organization>
26    <address>
27      <postal>
28        <street>345 Park Ave</street>
29        <city>San Jose</city>
30        <region>CA</region>
31        <code>95110</code>
32        <country>USA</country>
33      </postal>
34      <email>fielding@gbiv.com</email>
35      <uri>http://roy.gbiv.com/</uri>
36    </address>
37  </author>
38
39  <author initials="J." surname="Gettys" fullname="Jim Gettys">
40    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
41    <address>
42      <postal>
43        <street>21 Oak Knoll Road</street>
44        <city>Carlisle</city>
45        <region>MA</region>
46        <code>01741</code>
47        <country>USA</country>
48      </postal>
49      <email>jg@freedesktop.org</email>
50      <uri>http://gettys.wordpress.com/</uri>
51    </address>
52  </author>
53
54  <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
55    <organization abbrev="HP">Hewlett-Packard Company</organization>
56    <address>
57      <postal>
58        <street>HP Labs, Large Scale Systems Group</street>
59        <street>1501 Page Mill Road, MS 1177</street>
60        <city>Palo Alto</city>
61        <region>CA</region>
62        <code>94304</code>
63        <country>USA</country>
64      </postal>
65      <email>JeffMogul@acm.org</email>
66    </address>
67  </author>
68
69  <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
70    <organization abbrev="Microsoft">Microsoft Corporation</organization>
71    <address>
72      <postal>
73        <street>1 Microsoft Way</street>
74        <city>Redmond</city>
75        <region>WA</region>
76        <code>98052</code>
77        <country>USA</country>
78      </postal>
79      <email>henrikn@microsoft.com</email>
80    </address>
81  </author>
82
83  <author fullname="Larry Masinter" initials="L." surname="Masinter">
84    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
85    <address>
86      <postal>
87        <street>345 Park Ave</street>
88        <city>San Jose</city>
89        <region>CA</region>
90        <code>95110</code>
91        <country>USA</country>
92      </postal>
93      <email>LMM@acm.org</email>
94      <uri>http://larry.masinter.net/</uri>
95    </address>
96  </author>
97
98  <author fullname="Paul J. Leach" initials="P." surname="Leach">
99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
100    <address>
101      <postal>
102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106      </postal>
107      <email>paulle@microsoft.com</email>
108    </address>
109  </author>
110
111  <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
112    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
113    <address>
114      <postal>
115        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
116        <street>The Stata Center, Building 32</street>
117        <street>32 Vassar Street</street>
118        <city>Cambridge</city>
119        <region>MA</region>
120        <code>02139</code>
121        <country>USA</country>
122      </postal>
123      <email>timbl@w3.org</email>
124      <uri>http://www.w3.org/People/Berners-Lee/</uri>
125    </address>
126  </author>
127
128  <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
129    <organization abbrev="W3C">World Wide Web Consortium</organization>
130    <address>
131      <postal>
132        <street>W3C / ERCIM</street>
133        <street>2004, rte des Lucioles</street>
134        <city>Sophia-Antipolis</city>
135        <region>AM</region>
136        <code>06902</code>
137        <country>France</country>
138      </postal>
139      <email>ylafon@w3.org</email>
140      <uri>http://www.raubacapeu.net/people/yves/</uri>
141    </address>
142  </author>
143
144  <author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
145    <address>
146      <email>mnot@mnot.net</email>
147      <uri>http://www.mnot.net/</uri>
148    </address>
149  </author>
150
151  <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
152    <organization abbrev="greenbytes">greenbytes GmbH</organization>
153    <address>
154      <postal>
155        <street>Hafenweg 16</street>
156        <city>Muenster</city><region>NW</region><code>48155</code>
157        <country>Germany</country>
158      </postal>
159      <phone>+49 251 2807760</phone>
160      <facsimile>+49 251 2807761</facsimile>
161      <email>julian.reschke@greenbytes.de</email>
162      <uri>http://greenbytes.de/tech/webdav/</uri>
163    </address>
164  </author>
165
166  <date month="April" year="2011" day="18"/>
167  <workgroup>HTTPbis Working Group</workgroup>
168
169<abstract>
170<t>
171   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
172   distributed, collaborative, hypermedia information systems. This document
173   is Part 6 of the seven-part specification that defines the protocol
174   referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6
175   defines requirements on HTTP caches and the associated header fields that
176   control cache behavior or indicate cacheable response messages.
177</t>
178</abstract>
179
180<note title="Editorial Note (To be removed by RFC Editor)">
181  <t>
182    Discussion of this draft should take place on the HTTPBIS working group
183    mailing list (ietf-http-wg@w3.org), which is archived at
184    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
185  </t>
186  <t>
187    The current issues list is at
188    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
189    documents (including fancy diffs) can be found at
190    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
191  </t>
192  <t>
193    The changes in this draft are summarized in <xref target="changes.since.13"/>.
194  </t>
195</note>
196
197   </front>
198   <middle>
199
200<section anchor="caching" title="Introduction">
201<t>
202   HTTP is typically used for distributed information systems, where
203   performance can be improved by the use of response caches. This document
204   defines aspects of HTTP/1.1 related to caching and reusing response
205   messages.
206</t>
207
208<section anchor="intro.purpose" title="Purpose">
209<iref item="cache"/>
210<t>
211   An HTTP cache is a local store of response messages and the
212   subsystem that controls its message storage, retrieval, and deletion. A
213   cache stores cacheable responses in order to reduce the response time and
214   network bandwidth consumption on future, equivalent requests. Any client or
215   server MAY employ a cache, though a cache cannot be used by a server that
216   is acting as a tunnel.
217</t>
218<t>
219   Caching would be useless if it did not significantly improve performance.
220   The goal of caching in HTTP/1.1 is to reuse a prior response message to
221   satisfy a current request. In some cases, a stored response can be reused
222   without the need for a network request, reducing latency and network
223   round-trips; a "freshness" mechanism is used for this purpose (see <xref target="expiration.model"/>). Even when a new request is required, it is
224   often possible to reuse all or parts of the payload of a prior response to
225   satisfy the request, thereby reducing network bandwidth usage; a
226   "validation" mechanism is used for this purpose (see <xref target="validation.model"/>).
227</t>
228</section>
229
230<section anchor="intro.terminology" title="Terminology">
231<t>
232   This specification uses a number of terms to refer to the roles played by
233   participants in, and objects of, HTTP caching.
234</t>
235<t>
236   <iref item="cache"/>
237   <?rfc needLines="4"?>cache
238   <list>
239      <t>A conformant implementation of a HTTP cache. Note that this implies
240        an HTTP/1.1 cache; this specification does not define conformance
241        for HTTP/1.0 caches.</t>
242   </list>
243</t>
244<t anchor="shared.and.non-shared.caches">
245   <iref item="shared cache"/>
246   <?rfc needLines="4"?>shared cache
247   <list>
248      <t>A cache that is accessible to more than one user; usually (but
249        not always) deployed as part of an intermediary.</t>
250   </list>
251</t>
252<t>
253   <iref item="private cache"/>
254   <?rfc needLines="4"?>private cache
255   <list>
256      <t>A cache that is dedicated to a single user.</t>
257   </list>
258</t>
259<t>
260   <iref item="cacheable"/>
261   <?rfc needLines="4"?>cacheable
262   <list>
263      <t>A response is cacheable if a cache is allowed to store a copy of the
264      response message for use in answering subsequent requests. Even when a
265      response is cacheable, there might be additional constraints on whether
266      a cache can use the stored copy to satisfy a particular request.</t>
267   </list>
268</t>
269<t>
270   <iref item="explicit expiration time"/>
271   <?rfc needLines="4"?>explicit expiration time
272   <list>
273      <t>The time at which the origin server intends that a representation
274      no longer be returned by a cache without further validation.</t>
275   </list>
276</t>
277<t>
278   <iref item="heuristic expiration time"/>
279   <?rfc needLines="4"?>heuristic expiration time
280   <list>
281      <t>An expiration time assigned by a cache when no explicit expiration
282      time is available.</t>
283   </list>
284</t>
285<t>
286   <iref item="age"/>
287   <?rfc needLines="4"?>age
288   <list>
289      <t>The age of a response is the time since it was sent by, or
290      successfully validated with, the origin server.</t>
291   </list>
292</t>
293<t>
294   <iref item="first-hand"/>
295   <?rfc needLines="4"?>first-hand
296   <list>
297      <t>A response is first-hand if the freshness model is not in use; i.e.,
298      its age is 0.</t>
299   </list>
300</t>
301<t>
302   <iref item="freshness lifetime"/>
303   <?rfc needLines="4"?>freshness lifetime
304   <list>
305      <t>The length of time between the generation of a response and its
306      expiration time.</t>
307   </list>
308</t>
309<t>
310   <iref item="fresh"/>
311   <?rfc needLines="4"?>fresh
312   <list>
313      <t>A response is fresh if its age has not yet exceeded its freshness
314      lifetime.</t>
315   </list>
316</t>
317<t>
318   <iref item="stale"/>
319   <?rfc needLines="4"?>stale
320   <list>
321      <t>A response is stale if its age has passed its freshness lifetime
322      (either explicit or heuristic).</t>
323   </list>
324</t>
325<t>
326   <iref item="validator"/>
327   <?rfc needLines="4"?>validator
328   <list>
329      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
330      is used to find out whether a stored response is an equivalent copy of
331      a representation.</t>
332   </list>
333</t>
334</section>
335
336<section anchor="intro.requirements" title="Requirements">
337<t>
338   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
339   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
340   document are to be interpreted as described in <xref target="RFC2119"/>.
341</t>
342<t>
343   An implementation is not compliant if it fails to satisfy one or more of
344   the "MUST" or "REQUIRED" level requirements for the protocols it
345   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
346   level and all the "SHOULD" level requirements for its protocols is said to
347   be "unconditionally compliant"; one that satisfies all the "MUST" level
348   requirements but not all the "SHOULD" level requirements for its protocols
349   is said to be "conditionally compliant".
350</t>
351</section>
352
353<section title="Syntax Notation" anchor="notation">
354   
355   
356   
357   
358   
359   
360   
361   
362   
363<t>
364   This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
365   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
366   <xref target="collected.abnf"/> shows the collected ABNF, with the list
367   rule expanded.
368</t>
369<t>
370   The following core rules are included by reference, as defined in <xref target="RFC5234"/>, Appendix B.1: ALPHA (letters), CR (carriage
371   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
372   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
373   sequence of data), SP (space), VCHAR (any visible USASCII character), and
374   WSP (whitespace).
375</t>
376
377<section title="Core Rules" anchor="core.rules">
378   
379   
380   
381<t>
382   The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
383</t>
384<figure><artwork type="abnf2616"><![CDATA[
385  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
386  token         = <token, defined in [Part1], Section 1.2.2>
387  OWS           = <OWS, defined in [Part1], Section 1.2.2>
388]]></artwork></figure>
389</section>
390
391<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
392   
393   
394   
395   
396   
397<t>
398   The ABNF rules below are defined in other parts:
399</t>
400<figure><artwork type="abnf2616"><![CDATA[
401  field-name    = <field-name, defined in [Part1], Section 3.2>
402  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
403  port          = <port, defined in [Part1], Section 2.6>
404  pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
405  uri-host      = <uri-host, defined in [Part1], Section 2.6>
406]]></artwork></figure>
407</section>
408
409</section>
410</section>
411
412<section anchor="caching.overview" title="Cache Operation">
413
414<section anchor="response.cacheability" title="Response Cacheability">
415<t>
416   A cache MUST NOT store a response to any request, unless:
417   <list style="symbols">
418      <t>The request method is understood by the cache and defined as being
419      cacheable, and</t>
420      <t>the response status code is understood by the cache, and</t>
421      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
422      header fields, and</t>
423      <t>the "private" cache response directive (see <xref target="cache-response-directive"/> does not appear in the response, if
424      the cache is shared, and</t>
425      <t>the "Authorization" header field (see Section 4.1 of <xref target="Part7"/>) does not
426      appear in the request, if the cache is shared, unless the response
427      explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
428      <t>the response either:
429         <list style="symbols">
430            <t>contains an Expires header field (see <xref target="header.expires"/>), or</t>
431            <t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
432            <t>contains a s-maxage response cache directive and the cache is
433            shared, or</t>
434            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
435            or</t>
436            <t>has a status code that can be served with heuristic freshness
437            (see <xref target="heuristic.freshness"/>).</t>
438         </list>
439      </t>
440   </list>
441</t>
442<t>
443   In this context, a cache has "understood" a request method or a response
444   status code if it recognises it and implements any cache-specific
445   behaviour. In particular, 206 Partial Content responses cannot be cached by
446   an implementation that does not handle partial content (see <xref target="errors.or.incomplete.response.cache.behavior"/>).
447</t>
448<t>
449   Note that in normal operation, most caches will not store a response that
450   has neither a cache validator nor an explicit expiration time, as such
451   responses are not usually useful to store. However, caches are not
452   prohibited from storing such responses.
453</t>
454
455<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
456<t>
457   A cache that receives an incomplete response (for example, with fewer bytes
458   of data than specified in a Content-Length header field) can store the response,
459   but MUST treat it as a partial response <xref target="Part5"/>. Partial responses can
460   be combined as described in Section 4 of <xref target="Part5"/>; the result might be a
461   full response or might still be partial. A cache MUST NOT return a
462   partial response to a client without explicitly marking it as such using
463   the 206 (Partial Content) status code.
464</t>
465<t>
466   A cache that does not support the Range and Content-Range header fields
467   MUST NOT store incomplete or partial responses.
468</t>
469</section>
470
471</section>
472
473
474<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
475<t>
476   For a presented request, a cache MUST NOT return a stored response,
477   unless:
478   <list style="symbols">
479      <t>The presented effective request URI (Section 4.3 of <xref target="Part1"/>) and
480      that of the stored response match, and</t>
481      <t>the request method associated with the stored response allows it to
482      be used for the presented request, and</t>
483      <t>selecting header fields nominated by the stored response (if any)
484      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
485      <t>the presented request and stored response are free from directives
486      that would prevent its use (see <xref target="header.cache-control"/>
487      and <xref target="header.pragma"/>), and</t>
488      <t>the stored response is either:
489         <list style="symbols">
490            <t>fresh (see <xref target="expiration.model"/>), or</t>
491            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
492            <t>successfully validated (see <xref target="validation.model"/>).</t>
493         </list>
494      </t>
495  </list>
496</t>
497<t>
498   When a stored response is used to satisfy a request without validation,
499   a cache MUST include a single Age header field (<xref target="header.age"/>) in the response with a value equal to the stored response's
500   current_age; see <xref target="age.calculations"/>.
501</t>
502<t>
503   A cache MUST write through requests with methods that are unsafe
504   (Section 7.1.1 of <xref target="Part2"/>) to the origin server; i.e., a cache must not generate
505   a reply to such a request before having forwarded the request and having
506   received a corresponding response.
507</t>
508<t>
509   Also, note that unsafe requests might invalidate already stored responses;
510   see <xref target="invalidation.after.updates.or.deletions"/>.
511</t>
512<t>
513   A cache MUST use the most recent response (as determined by the Date
514   header field) when more than one suitable response is stored. It can also
515   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
516   no-cache" to disambiguate which response to use.
517</t>
518<t>
519   A cache that does not have a clock available MUST NOT use stored responses
520   without revalidating them on every use. A cache, especially a shared
521   cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to
522   synchronize its clock with a reliable external standard.
523</t>
524
525</section>
526
527<section anchor="expiration.model" title="Freshness Model">
528<t>
529   When a response is "fresh" in the cache, it can be used to satisfy
530   subsequent requests without contacting the origin server, thereby improving
531   efficiency.
532</t>
533<t>
534   The primary mechanism for determining freshness is for an origin server to
535   provide an explicit expiration time in the future, using either the Expires
536   header field (<xref target="header.expires"/>) or the max-age response cache
537   directive (<xref target="cache-response-directive"/>). Generally, origin
538   servers will assign future explicit expiration times to responses in the
539   belief that the representation is not likely to change in a semantically
540   significant way before the expiration time is reached.
541</t>
542<t>
543   If an origin server wishes to force a cache to validate every request, it
544   can assign an explicit expiration time in the past to indicate that the
545   response is already stale. Compliant caches will normally validate the
546   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
547</t>
548<t>
549   Since origin servers do not always provide explicit expiration times,
550   a cache MAY assign a heuristic expiration time when an explicit time is not
551   specified, employing algorithms that use other header field values (such as the
552   Last-Modified time) to estimate a plausible expiration time. This
553   specification does not provide specific algorithms, but does impose
554   worst-case constraints on their results.
555</t>
556<figure>
557<preamble>
558  The calculation to determine if a response is fresh is:
559</preamble>
560<artwork type="code"><![CDATA[
561   response_is_fresh = (freshness_lifetime > current_age)
562]]></artwork>
563</figure>
564<t>
565   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
566   <xref target="age.calculations"/>.
567</t>
568<t>
569   Additionally, clients might need to influence freshness calculation. They
570   can do this using several request cache directives, with the effect of
571   either increasing or loosening constraints on freshness. See <xref target="cache-request-directive"/>.
572</t>
573<t>
574   <cref anchor="ISSUE-no-req-for-directives">there are not requirements
575   directly applying to cache-request-directives and freshness.</cref>
576</t>
577<t>
578   Note that freshness applies only to cache operation; it cannot be used to
579   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
580   caches and history mechanisms.
581</t>
582
583<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
584<t>
585   A cache can calculate the freshness lifetime (denoted as
586   freshness_lifetime) of a response by using the first match of:
587   <list style="symbols">
588      <t>If the cache is shared and the s-maxage response cache directive
589      (<xref target="cache-response-directive"/>) is present, use its value,
590      or</t>
591      <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
592      <t>If the Expires response header field (<xref target="header.expires"/>) is
593      present, use its value minus the value of the Date response header field,
594      or</t>
595      <t>Otherwise, no explicit expiration time is present in the response. A
596      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
597   </list>
598</t>
599<t>
600   Note that this calculation is not vulnerable to clock skew, since all of
601   the information comes from the origin server.
602</t>
603
604<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
605<t>
606   If no explicit expiration time is present in a stored response that has a
607   status code whose definition allows heuristic freshness to be used
608   (including the following in Section 8 of <xref target="Part2"/>: 200, 203, 206, 300, 301 and
609   410), a cache MAY calculate a heuristic expiration time. A cache MUST NOT
610   use heuristics to determine freshness for responses with status codes that do
611   not explicitly allow it.
612</t>
613<t>
614   When a heuristic is used to calculate freshness lifetime, a cache
615   SHOULD attach a Warning header field with a 113 warn-code to the response if
616   its current_age is more than 24 hours and such a warning is not already
617   present.
618</t>
619<t>
620   Also, if the response has a Last-Modified header field (Section 2.1 of <xref target="Part4"/>),
621   a cache SHOULD NOT use a heuristic expiration value that is more than some
622   fraction of the interval since that time. A typical setting of this fraction
623   might be 10%.
624</t>
625<t><list>
626   <t>
627      Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9) required that caches do not calculate heuristic
628      freshness for URIs with query components (i.e., those containing '?').
629      In practice, this has not been widely implemented. Therefore, servers
630      are encouraged to send explicit directives (e.g., Cache-Control:
631      no-cache) if they wish to preclude caching.
632   </t>
633</list></t>
634</section>
635</section>
636
637<section anchor="age.calculations" title="Calculating Age">
638<t>
639   HTTP/1.1 uses the Age header field to convey the estimated age of the
640   response message when obtained from a cache. The Age field value is the
641   cache's estimate of the amount of time since the response was generated or
642   validated by the origin server. In essence, the Age value is the sum of the
643   time that the response has been resident in each of the caches along the
644   path from the origin server, plus the amount of time it has been in transit
645   along network paths.
646</t>
647<t>
648   The following data is used for the age calculation:
649</t>
650<t>
651   <?rfc needLines="4"?>age_value
652   <list>
653      <t>
654         The term "age_value" denotes the value of the Age header field (<xref target="header.age"/>), in a form appropriate for arithmetic
655         operation; or 0, if not available.
656      </t>
657   </list>
658</t>
659<t>
660   <?rfc needLines="4"?>date_value
661   <list>
662      <t>
663         HTTP/1.1 requires origin servers to send a Date header field, if possible,
664         with every response, giving the time at which the response was
665         generated. The term "date_value" denotes the value of the Date
666         header field, in a form appropriate for arithmetic operations. See
667         Section 9.3 of <xref target="Part1"/> for the definition of the Date header field, and for
668         requirements regarding responses without it.
669      </t>
670   </list>
671</t>
672<t>
673   <?rfc needLines="4"?>now
674   <list>
675      <t>
676         The term "now" means "the current value of the clock at the host
677         performing the calculation". A cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
678         clocks to a globally accurate time standard.
679      </t>
680   </list>
681</t>
682<t>
683   <?rfc needLines="4"?>request_time
684   <list>
685      <t>
686         The current value of the clock at the host at the time the request
687         resulting in the stored response was made.
688      </t>
689   </list>
690</t>
691<t>
692   <?rfc needLines="4"?>response_time
693   <list>
694      <t>
695         The current value of the clock at the host at the time the response
696         was received.
697      </t>
698   </list>
699</t>
700<t>
701   A response's age can be calculated in two entirely independent ways:
702   <list style="numbers">
703      <t>the "apparent_age": response_time minus date_value, if the local
704      clock is reasonably well synchronized to the origin server's clock. If
705      the result is negative, the result is replaced by zero.</t>
706      <t>the "corrected_age_value", if all of the caches along the response
707      path implement HTTP/1.1. A cache MUST interpret this value relative
708      to the time the request was initiated, not the time that the response
709      was received.</t>
710   </list>
711</t>
712<figure>
713<artwork type="code"><![CDATA[
714  apparent_age = max(0, response_time - date_value);
715
716  response_delay = response_time - request_time;
717  corrected_age_value = age_value + response_delay; 
718]]></artwork>
719</figure>
720<figure>
721<preamble>These are combined as</preamble>
722<artwork type="code"><![CDATA[
723  corrected_initial_age = max(apparent_age, corrected_age_value);
724]]></artwork></figure>
725<t>
726   The current_age of a stored response can then be calculated by adding the
727   amount of time (in seconds) since the stored response was last validated by
728   the origin server to the corrected_initial_age.
729</t>
730<figure><artwork type="code"><![CDATA[
731  resident_time = now - response_time;
732  current_age = corrected_initial_age + resident_time;
733]]></artwork></figure>
734</section>
735
736<section anchor="serving.stale.responses" title="Serving Stale Responses">
737<t>
738   A "stale" response is one that either has explicit expiry information or is
739   allowed to have heuristic expiry calculated, but is not fresh according to
740   the calculations in <xref target="expiration.model"/>.
741</t>
742<t>
743   A cache MUST NOT return a stale response if it is prohibited by an
744   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
745   directive, a "must-revalidate" cache-response-directive, or an applicable
746   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
747</t>
748<t>
749   A cache SHOULD NOT return stale responses unless it is disconnected
750   (i.e., it cannot contact the origin server or otherwise find a forward
751   path) or doing so is explicitly allowed (e.g., by the max-stale request
752   directive; see <xref target="cache-request-directive"/>).
753</t>
754<t>
755   A cache SHOULD append a Warning header field with the 110 warn-code (see
756   <xref target="header.warning"/>) to stale responses. Likewise, a cache
757   SHOULD add the 112 warn-code to stale responses if the cache is
758   disconnected.
759</t>
760<t>
761   If a cache receives a first-hand response (either an entire response, or a
762   304 (Not Modified) response) that it would normally forward to the
763   requesting client, and the received response is no longer fresh, the cache
764   SHOULD forward it to the requesting client without adding a new Warning
765   (but without removing any existing Warning header fields). A cache SHOULD NOT
766   attempt to validate a response simply because that response became stale in
767   transit.
768</t>
769</section>
770</section>
771
772<section anchor="validation.model" title="Validation Model">
773<t>
774   When a cache has one or more stored responses for a requested URI, but
775   cannot serve any of them (e.g., because they are not fresh, or one cannot
776   be selected; see <xref target="caching.negotiated.responses"/>), it can use
777   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
778   give the origin server an opportunity to both select a valid stored
779   response to be used, and to update it. This process is known as
780   "validating" or "revalidating" the stored response.
781</t>
782<t>
783   When sending such a conditional request, a cache SHOULD add an
784   If-Modified-Since header field whose value is that of the Last-Modified header
785   field from the selected (see <xref target="caching.negotiated.responses"/>)
786   stored response, if available.
787</t>
788<t>
789   Additionally, a cache SHOULD add an If-None-Match header field whose value is
790   that of the ETag header field(s) from all responses stored for the requested URI,
791   if present. However, if any of the stored responses contains only partial
792   content, the cache SHOULD NOT include its entity-tag in the If-None-Match
793   header field unless the request is for a range that would be fully
794   satisfied by that stored response.
795</t>
796<t>
797   A 304 (Not Modified) response status code indicates that the stored
798   response can be updated and reused; see <xref target="combining.responses"/>.
799</t>
800<t>
801   A full response (i.e., one with a response body) indicates that none of the
802   stored responses nominated in the conditional request is suitable. Instead,
803   a cache SHOULD use the full response to satisfy the request and MAY
804   replace the stored response.
805</t>
806<t>
807   If a cache receives a 5xx response while attempting to validate a response,
808   it MAY either forward this response to the requesting client, or act as
809   if the server failed to respond. In the latter case, it MAY return a
810   previously stored response (see <xref target="serving.stale.responses"/>).
811</t>
812</section>
813
814<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
815<t>
816   Because unsafe request methods (Section 7.1.1 of <xref target="Part2"/>) have the potential for changing
817   state on the origin server, intervening caches can use them to keep their
818   contents up-to-date.
819</t>
820<t>
821   A cache MUST invalidate the effective Request URI
822   (Section 4.3 of <xref target="Part1"/>) as well as the URI(s) in the Location
823   and Content-Location header fields (if present) when the following
824   request methods are received:
825   <list style="symbols">
826      <t>PUT</t>
827      <t>DELETE</t>
828      <t>POST</t>
829   </list>
830</t>
831<t>
832   However, a cache MUST NOT invalidate a URI from a
833   Location or Content-Location header field if the host part of that URI
834   differs from the host part in the effective request URI
835   (Section 4.3 of <xref target="Part1"/>). This helps prevent denial of service attacks.
836</t>
837<t>
838   A cache that passes through requests with methods it does not understand
839   SHOULD invalidate the effective request URI (Section 4.3 of <xref target="Part1"/>).
840</t>
841<t>
842   Here, "invalidate" means that the cache will either remove all stored
843   responses related to the effective request URI, or will mark these as
844   "invalid" and in need of a mandatory validation before they can be returned
845   in response to a subsequent request.
846</t>
847<t>
848   Note that this does not guarantee that all appropriate responses are
849   invalidated. For example, the request that caused the change at the origin
850   server might not have gone through the cache where a response is stored.
851</t>
852</section>
853
854<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
855
856<t>
857   A shared cache MUST NOT use a cached response to a request with an
858   Authorization header field (Section 4.1 of <xref target="Part7"/>) to satisfy any subsequent
859   request unless a cache directive that allows such responses to be stored is
860   present in the response.
861</t>
862
863<t>
864   In this specification, the following Cache-Control response directives
865   (<xref target="cache-response-directive"/>) have such an effect:
866   must-revalidate, public, s-maxage.
867</t>
868
869<t>
870   Note that cached responses that contain the "must-revalidate" and/or
871   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
872   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
873   used to satisfy a subsequent request without revalidating it on the origin
874   server.
875</t>
876</section>
877
878<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
879<t>
880   When a cache receives a request that can be satisfied by a stored response
881   that has a Vary header field (<xref target="header.vary"/>), it MUST NOT
882   use that response unless all of the selecting header fields nominated by
883   the Vary header field match in both the original request (i.e., that associated
884   with the stored response), and the presented request.
885</t>
886<t>
887   The selecting header fields from two requests are defined to match if and
888   only if those in the first request can be transformed to those in the
889   second request by applying any of the following:
890   <list style="symbols">
891      <t>
892         adding or removing whitespace, where allowed in the header field's syntax
893      </t>
894      <t>
895         combining multiple header fields with the same field name
896         (see Section 3.2 of <xref target="Part1"/>)
897      </t>
898      <t>
899         normalizing both header field values in a way that is known to have
900         identical semantics, according to the header field's specification (e.g.,
901         re-ordering field values when order is not significant;
902         case-normalization, where values are defined to be case-insensitive)
903      </t>
904  </list>
905</t>
906<t>
907   If (after any normalization that might take place) a header field is absent
908   from a request, it can only match another request if it is also absent
909   there.
910</t>
911<t>
912   A Vary header field-value of "*" always fails to match, and subsequent
913   requests to that resource can only be properly interpreted by the origin
914   server.
915</t>
916<t>
917   The stored response with matching selecting header fields is known as the
918   selected response.
919</t>
920<t>
921   If no selected response is available, the cache MAY forward the presented
922   request to the origin server in a conditional request; see <xref target="validation.model"/>.
923</t>
924</section>
925
926<section anchor="combining.responses" title="Combining Responses">
927<t>
928   When a cache receives a 304 (Not Modified) response or a 206 (Partial
929   Content) response (in this section, the "new" response"), it needs to
930   create an updated response by combining the stored response with the new
931   one, so that the updated response can be used to satisfy the request, and
932   potentially update the cached response.
933</t>
934<t>
935   If the new response contains an ETag, it identifies the stored response to
936   use. <cref anchor="TODO-mention-CL">might need language about
937   Content-Location here</cref><cref anchor="TODO-select-for-combine">Shouldn't this be the selected
938   response?</cref>
939</t>
940<t>
941   When the new response's status code is 206 (partial content), a cache
942   MUST NOT combine it with the old response if either response does not
943   have a validator, and MUST NOT combine it with the old response when
944   those validators do not match with the strong comparison function
945   (see Section 2.2.2 of <xref target="Part4"/>).
946</t>
947<t>
948   The stored response header fields are used as those of the updated response,
949   except that
950   <list style="symbols">
951      <t>a cache MUST delete any stored Warning header fields with warn-code 1xx (see <xref target="header.warning"/>).</t>
952      <t>a cache MUST retain any stored Warning header fields with warn-code 2xx.</t>
953      <t>a cache MUST use other header fields provided in the new response to replace all
954      instances of the corresponding header fields from the stored response.</t>
955   </list>
956</t>
957<t>
958   A cache MUST use the updated response header fields to replace those of the stored
959   response (unless the stored response is removed). In
960   the case of a 206 response, a cache MAY store the combined representation.
961</t>
962</section>
963
964</section>
965
966<section anchor="header.fields" title="Header Field Definitions">
967<t>
968   This section defines the syntax and semantics of HTTP/1.1 header fields
969   related to caching.
970</t>
971
972<section anchor="header.age" title="Age">
973   <iref item="Age header field" primary="true"/>
974   <iref item="Header Fields" primary="true" subitem="Age"/>
975   
976   
977<t>
978   The "Age" header field conveys the sender's estimate of the amount
979   of time since the response was generated or successfully validated at the
980   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
981</t>
982<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
983  Age = delta-seconds
984]]></artwork></figure>
985<t anchor="rule.delta-seconds">
986 
987  Age field-values are non-negative integers, representing time in seconds.
988</t>
989<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
990  delta-seconds  = 1*DIGIT
991]]></artwork></figure>
992<t>
993   If a cache receives a value larger than the largest positive integer it can
994   represent, or if any of its age calculations overflows, it MUST transmit
995   an Age header field with a field-value of 2147483648 (2^31).
996   Recipients parsing the Age header field-value SHOULD use an arithmetic type of
997   at least 31 bits of range.
998</t>
999<t>
1000   The presence of an Age header field in a response implies that a response
1001   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1002   might not implement the Age header field.
1003</t>
1004</section>
1005
1006<section anchor="header.cache-control" title="Cache-Control">
1007   <iref item="Cache-Control header field" primary="true"/>
1008   <iref item="Header Fields" primary="true" subitem="Cache-Control"/>
1009   
1010   
1011   
1012   
1013   
1014<t>
1015   The "Cache-Control" header field is used to specify directives for
1016   caches along the request/response chain. Such cache directives are
1017   unidirectional in that the presence of a directive in a request does not
1018   imply that the same directive is to be given in the response.
1019</t>
1020<t>
1021   A cache MUST obey the requirements of the Cache-Control
1022   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1023   directives defined elsewhere are handled.
1024</t>
1025<t><list>
1026   <t>
1027       Note: HTTP/1.0 caches might not implement Cache-Control and
1028       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1029   </t>
1030</list></t>
1031<t>
1032   A proxy, whether or not it implements a cache, MUST pass cache directives
1033   through in forwarded messages, regardless of their
1034   significance to that application, since the directives might be applicable
1035   to all recipients along the request/response chain. It is not possible to
1036   target a directive to a specific cache.
1037</t>
1038<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
1039  Cache-Control   = 1#cache-directive
1040
1041  cache-directive = cache-request-directive
1042     / cache-response-directive
1043
1044  cache-extension = token [ "=" ( token / quoted-string ) ]
1045]]></artwork></figure>
1046
1047<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1048   
1049
1050<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
1051  cache-request-directive =
1052       "no-cache"
1053     / "no-store"
1054     / "max-age" "=" delta-seconds
1055     / "max-stale" [ "=" delta-seconds ]
1056     / "min-fresh" "=" delta-seconds
1057     / "no-transform"
1058     / "only-if-cached"
1059     / cache-extension
1060]]></artwork></figure>
1061
1062<t>
1063   <?rfc needLines="4"?>no-cache
1064   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1065   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1066   <list>
1067      <t>The no-cache request directive indicates that a cache MUST NOT
1068      use a stored response to satisfy the request without successful
1069      validation on the origin server.</t> 
1070   </list>
1071</t>
1072<t>
1073   <?rfc needLines="4"?>no-store
1074   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1075   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1076   <list>
1077      <t>The no-store request directive indicates that a cache MUST NOT
1078      store any part of either this request or any response to it. This
1079      directive applies to both private and shared caches. "MUST NOT
1080      store" in this context means that the cache MUST NOT intentionally
1081      store the information in non-volatile storage, and MUST make a
1082      best-effort attempt to remove the information from volatile storage as
1083      promptly as possible after forwarding it.</t>
1084      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1085      privacy. In particular, malicious or compromised caches might not
1086      recognize or obey this directive, and communications networks might be
1087      vulnerable to eavesdropping.</t>
1088      <t>Note that if a request containing this directive is satisfied from a
1089      cache, the no-store request directive does not apply to the already
1090      stored response.</t>
1091   </list>
1092</t>
1093<t>
1094   <?rfc needLines="4"?>max-age
1095   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1096   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1097   <list>
1098      <t>The max-age request directive indicates that the client is willing to
1099      accept a response whose age is no greater than the specified time in
1100      seconds. Unless the max-stale request directive is also present, the
1101      client is not willing to accept a stale response.</t>
1102   </list>
1103</t>
1104<t>
1105   <?rfc needLines="4"?>max-stale
1106   <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1107   <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1108   <list>
1109      <t>The max-stale request directive indicates that the client is willing
1110      to accept a response that has exceeded its expiration time. If max-stale
1111      is assigned a value, then the client is willing to accept a response
1112      that has exceeded its expiration time by no more than the specified
1113      number of seconds. If no value is assigned to max-stale, then the client
1114      is willing to accept a stale response of any age.</t>
1115   </list>
1116</t>
1117<t>
1118   <?rfc needLines="4"?>min-fresh
1119   <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1120   <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1121   <list>
1122      <t>The min-fresh request directive indicates that the client is willing
1123      to accept a response whose freshness lifetime is no less than its
1124      current age plus the specified time in seconds. That is, the client
1125      wants a response that will still be fresh for at least the specified
1126      number of seconds.</t>
1127   </list>
1128</t>
1129<t>
1130   <?rfc needLines="4"?>no-transform
1131   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1132   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1133   <list>
1134      <t>The no-transform request directive indicates that an intermediary
1135        (whether or not it implements a cache) MUST NOT change the
1136        Content-Encoding, Content-Range or Content-Type request header fields,
1137        nor the request representation.</t>
1138   </list>
1139</t>
1140<t>
1141   <?rfc needLines="4"?>only-if-cached
1142   <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1143   <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1144   <list>
1145      <t>The only-if-cached request directive indicates that the client only
1146      wishes to return a stored response. If it receives this directive, a
1147      cache SHOULD either respond using a stored response that is consistent
1148      with the other constraints of the request, or respond with a 504
1149      (Gateway Timeout) status code. If a group of caches is being operated as
1150      a unified system with good internal connectivity, a member cache MAY
1151      forward such a request within that group of caches.</t>
1152   </list>
1153</t>
1154</section>
1155
1156<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1157   
1158
1159<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1160  cache-response-directive =
1161       "public"
1162     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1163     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1164     / "no-store"
1165     / "no-transform"
1166     / "must-revalidate"
1167     / "proxy-revalidate"
1168     / "max-age" "=" delta-seconds
1169     / "s-maxage" "=" delta-seconds
1170     / cache-extension
1171]]></artwork></figure>
1172
1173<t>
1174   <?rfc needLines="4"?>public
1175   <iref item="Cache Directives" primary="true" subitem="public"/>
1176   <iref item="public" primary="true" subitem="Cache Directive"/>
1177   <list>
1178      <t>The public response directive indicates that a response whose
1179        associated request contains an 'Authentication' header MAY be
1180        stored (see <xref target="caching.authenticated.responses"/>).</t>
1181  </list>
1182</t>
1183<t>
1184   <?rfc needLines="4"?>private
1185   <iref item="Cache Directives" primary="true" subitem="private"/>
1186   <iref item="private" primary="true" subitem="Cache Directive"/>
1187   <list>
1188      <t>The private response directive indicates that the response message is
1189      intended for a single user and MUST NOT be stored by a shared cache. A
1190      private cache MAY store the response.</t>
1191      <t>If the private response directive specifies one or more field-names,
1192      this requirement is limited to the field-values associated with the
1193      listed response header fields. That is, a shared cache MUST NOT store
1194      the specified field-names(s), whereas it MAY store the remainder of the
1195      response message.</t>
1196      <t> Note: This usage of the word private only controls where
1197      the response can be stored; it cannot ensure the privacy of the message
1198      content. Also, private response directives with field-names are often
1199      handled by implementations as if an unqualified private directive was
1200      received; i.e., the special handling for the qualified form is not
1201      widely implemented.</t>
1202   </list>
1203</t>
1204<t>
1205   <?rfc needLines="4"?>no-cache
1206   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1207   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1208   <list>
1209      <t>The no-cache response directive indicates that the response MUST NOT
1210      be used to satisfy a subsequent request without successful validation on
1211      the origin server. This allows an origin server to prevent a cache from
1212      using it to satisfy a request without contacting it, even by caches that
1213      have been configured to return stale responses.</t>
1214      <t>If the no-cache response directive specifies one or more field-names,
1215      this requirement is limited to the field-values associated with the
1216      listed response header fields. That is, a cache MUST NOT send the
1217      specified field-name(s) in the response to a subsequent request without successful
1218      validation on the origin server. This allows an origin server to prevent
1219      the re-use of certain header fields in a response, while still allowing
1220      caching of the rest of the response.</t>
1221      <t> Note: Most HTTP/1.0 caches will not recognize or obey
1222      this directive. Also, no-cache response directives with field-names are
1223      often handled by implementations as if an unqualified no-cache directive
1224      was received; i.e., the special handling for the qualified form is not
1225      widely implemented. </t>
1226   </list>
1227</t>
1228<t>
1229   <?rfc needLines="4"?>no-store
1230   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1231   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1232   <list>
1233      <t>The no-store response directive indicates that a cache MUST NOT
1234      store any part of either the immediate request or response. This
1235      directive applies to both private and shared caches. "MUST NOT
1236      store" in this context means that the cache MUST NOT intentionally
1237      store the information in non-volatile storage, and MUST make a
1238      best-effort attempt to remove the information from volatile storage as
1239      promptly as possible after forwarding it.</t>
1240      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1241      privacy. In particular, malicious or compromised caches might not
1242      recognize or obey this directive, and communications networks might be
1243      vulnerable to eavesdropping.</t>
1244   </list>
1245</t>
1246<t>
1247   <?rfc needLines="4"?>must-revalidate
1248   <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1249   <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1250   <list>
1251      <t>The must-revalidate response directive indicates that once it has
1252      become stale, a cache MUST NOT use the response to satisfy subsequent
1253      requests without successful validation on the origin server.</t>
1254      <t>The must-revalidate directive is necessary to support reliable
1255      operation for certain protocol features. In all circumstances a
1256      cache MUST obey the must-revalidate directive; in particular,
1257      if a cache cannot reach the origin server for any reason, it MUST
1258      generate a 504 (Gateway Timeout) response.</t>
1259      <t>A server SHOULD send the must-revalidate directive if and only if
1260      failure to validate a request on the representation could result in
1261      incorrect operation, such as a silently unexecuted financial
1262      transaction.</t>
1263   </list>
1264</t>
1265<t>
1266   <?rfc needLines="4"?>proxy-revalidate
1267   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1268   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1269   <list>
1270      <t>The proxy-revalidate response directive has the same meaning as the
1271      must-revalidate response directive, except that it does not apply to
1272      private caches.</t>
1273   </list>
1274</t>
1275<t>
1276   <?rfc needLines="4"?>max-age
1277   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1278   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1279   <list>
1280      <t>The max-age response directive indicates that the response is to be
1281      considered stale after its age is greater than the specified number of
1282      seconds.</t>
1283   </list>
1284</t>
1285<t>
1286   <?rfc needLines="4"?>s-maxage
1287   <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1288   <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1289   <list>
1290      <t>The s-maxage response directive indicates that, in shared caches, the
1291      maximum age specified by this directive overrides the maximum age
1292      specified by either the max-age directive or the Expires header field. The
1293      s-maxage directive also implies the semantics of the proxy-revalidate
1294      response directive.</t>
1295   </list>
1296</t>
1297<t>
1298   <?rfc needLines="4"?>no-transform
1299   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1300   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1301   <list>
1302      <t>The no-transform response directive indicates that an intermediary
1303      (regardless of whether it implements a cache) MUST NOT change the
1304      Content-Encoding, Content-Range or Content-Type response header fields,
1305      nor the response representation.</t>
1306   </list>
1307</t>
1308
1309</section>
1310
1311<section anchor="cache.control.extensions" title="Cache Control Extensions">
1312<t>
1313   The Cache-Control header field can be extended through the use of one or
1314   more cache-extension tokens, each with an optional value. Informational
1315   extensions (those that do not require a change in cache behavior) can be
1316   added without changing the semantics of other directives. Behavioral
1317   extensions are designed to work by acting as modifiers to the existing base
1318   of cache directives. Both the new directive and the standard directive are
1319   supplied, such that applications that do not understand the new directive
1320   will default to the behavior specified by the standard directive, and those
1321   that understand the new directive will recognize it as modifying the
1322   requirements associated with the standard directive. In this way,
1323   extensions to the cache-control directives can be made without requiring
1324   changes to the base protocol.
1325</t>
1326<t>
1327   This extension mechanism depends on an HTTP cache obeying all of the
1328   cache-control directives defined for its native HTTP-version, obeying
1329   certain extensions, and ignoring all directives that it does not
1330   understand.
1331</t>
1332<t>
1333   For example, consider a hypothetical new response directive called
1334   "community" that acts as a modifier to the private directive. We define
1335   this new directive to mean that, in addition to any private cache, any
1336   cache that is shared only by members of the community named within its
1337   value may cache the response. An origin server wishing to allow the UCI
1338   community to use an otherwise private response in their shared cache(s)
1339   could do so by including
1340</t>
1341<figure><artwork type="example"><![CDATA[
1342  Cache-Control: private, community="UCI"
1343]]></artwork></figure>
1344<t>
1345   A cache seeing this header field will act correctly even if the cache does
1346   not understand the community cache-extension, since it will also see and
1347   understand the private directive and thus default to the safe behavior.
1348</t>
1349<t>
1350   A cache MUST be ignore unrecognized cache directives; it is assumed that any
1351   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1352   combined with standard directives (or the response's default cacheability)
1353   such that the cache behavior will remain minimally correct even if the
1354   cache does not understand the extension(s).
1355</t>
1356<t>
1357   The HTTP Cache Directive Registry defines the name space for the cache
1358   directives.
1359</t>
1360<t>
1361   A registration MUST include the following fields:
1362   <list style="symbols">
1363      <t>Cache Directive Name</t>
1364      <t>Pointer to specification text</t>
1365   </list>
1366</t>
1367<t>
1368   Values to be added to this name space are subject to IETF review (<xref target="RFC5226"/>, Section 4.1).
1369</t>
1370<t>
1371   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1372</t>
1373</section>
1374
1375</section>
1376
1377<section anchor="header.expires" title="Expires">
1378   <iref item="Expires header field" primary="true"/>
1379   <iref item="Header Fields" primary="true" subitem="Expires"/>
1380   
1381<t>
1382   The "Expires" header field gives the date/time after which the
1383   response is considered stale. See <xref target="expiration.model"/> for
1384   further discussion of the freshness model.
1385</t>
1386<t>
1387   The presence of an Expires field does not imply that the original resource
1388   will change or cease to exist at, before, or after that time.
1389</t>
1390<t>
1391   The field-value is an absolute date and time as defined by HTTP-date in
1392   Section 6.1 of <xref target="Part1"/>; a sender MUST use the rfc1123-date format.
1393</t>
1394<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1395  Expires = HTTP-date
1396]]></artwork></figure>
1397<figure>
1398  <preamble>For example</preamble>
1399<artwork type="example"><![CDATA[
1400  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1401]]></artwork></figure>
1402<t><list>
1403   <t>
1404       Note: If a response includes a Cache-Control field with the
1405       max-age directive (see <xref target="cache-response-directive"/>),
1406       that directive overrides the Expires field. Likewise, the s-maxage
1407       directive overrides Expires in shared caches.
1408   </t>
1409</list></t>
1410<t>
1411   A server SHOULD NOT send Expires dates more than one year in the
1412   future.
1413</t>
1414<t>
1415   A cache MUST treat other invalid date formats,
1416   especially including the value "0", as in the past (i.e., "already
1417   expired").
1418</t>
1419</section>
1420
1421<section anchor="header.pragma" title="Pragma">
1422   <iref item="Pragma header field" primary="true"/>
1423   <iref item="Header Fields" primary="true" subitem="Pragma"/>
1424   
1425   
1426   
1427<t>
1428   The "Pragma" header field is used to include
1429   implementation-specific directives that might apply to any recipient along
1430   the request/response chain. All pragma directives specify optional behavior
1431   from the viewpoint of the protocol; however, some systems MAY require
1432   that behavior be consistent with the directives.
1433</t>
1434<figure><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/><artwork type="abnf2616"><![CDATA[
1435  Pragma           = 1#pragma-directive
1436  pragma-directive = "no-cache" / extension-pragma
1437  extension-pragma = token [ "=" ( token / quoted-string ) ]
1438]]></artwork></figure>
1439<t>
1440   When the no-cache directive is present in a request message, a cache
1441   SHOULD forward the request toward the origin server even if it has a
1442   stored copy of what is being requested. This pragma directive has the same
1443   semantics as the no-cache response directive (see <xref target="cache-response-directive"/>) and is defined here for backward
1444   compatibility with HTTP/1.0. A client SHOULD include both header fields
1445   when a no-cache request is sent to a server not known to be HTTP/1.1
1446   compliant. A cache SHOULD treat "Pragma: no-cache" as if the
1447   client had sent "Cache-Control: no-cache".
1448</t>
1449<t><list>
1450   <t>
1451      Note: Because the meaning of "Pragma: no-cache" as a
1452      header field is not actually specified, it does not provide a
1453      reliable replacement for "Cache-Control: no-cache" in a response.
1454   </t>
1455</list></t>
1456<t>
1457   This mechanism is deprecated; no new Pragma directives will be defined in
1458   HTTP.
1459</t>
1460</section>
1461
1462<section anchor="header.vary" title="Vary">
1463   <iref item="Vary header field" primary="true"/>
1464   <iref item="Header Fields" primary="true" subitem="Vary"/>
1465   
1466<t>
1467   The "Vary" header field conveys the set of header fields
1468   that were used to select the representation.
1469</t>
1470<t>
1471   Caches use this information, in part, to determine whether a stored
1472   response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>. determines, while the response is
1473   fresh, whether a cache is permitted to use the response to reply to a
1474   subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1475</t>
1476<t>
1477   In uncacheable or stale responses, the Vary field value advises the user
1478   agent about the criteria that were used to select the representation.
1479</t>
1480<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
1481  Vary = "*" / 1#field-name
1482]]></artwork></figure>
1483<t>
1484   The set of header fields named by the Vary field value is known as the
1485   selecting header fields.
1486</t>
1487<t>
1488   A server SHOULD include a Vary header field with any cacheable response
1489   that is subject to server-driven negotiation. Doing so allows a cache to
1490   properly interpret future requests on that resource and informs the user
1491   agent about the presence of negotiation on that resource. A server MAY
1492   include a Vary header field with a non-cacheable response that is subject
1493   to server-driven negotiation, since this might provide the user agent with
1494   useful information about the dimensions over which the response varies at
1495   the time of the response.
1496</t>
1497<t>
1498   A Vary field value of "*" signals that unspecified parameters not limited
1499   to the header fields (e.g., the network address of the client), play a
1500   role in the selection of the response representation; therefore, a cache
1501   cannot determine whether this response is appropriate. A proxy MUST NOT
1502   generate the "*" value.
1503</t>
1504<t>
1505   The field-names given are not limited to the set of standard header
1506   fields defined by this specification. Field names are case-insensitive.
1507</t>
1508</section>
1509
1510<section anchor="header.warning" title="Warning">
1511   <iref item="Warning header field" primary="true"/>
1512   <iref item="Header Fields" primary="true" subitem="Warning"/>
1513   
1514   
1515   
1516   
1517   
1518   
1519<t>
1520   The "Warning" header field is used to carry additional information
1521   about the status or transformation of a message that might not be reflected
1522   in the message. This information is typically used to warn about possible
1523   incorrectness introduced by caching operations or transformations applied
1524   to the payload of the message.
1525</t>
1526<t>
1527   Warnings can be used for other purposes, both cache-related and otherwise.
1528   The use of a warning, rather than an error status code, distinguishes these
1529   responses from true failures.
1530</t>
1531<t>
1532   Warning header fields can in general be applied to any message, however some
1533   warn-codes are specific to caches and can only be applied to response
1534   messages.
1535</t>
1536<figure><iref primary="true" item="Grammar" subitem="Warning"/><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[
1537  Warning       = 1#warning-value
1538 
1539  warning-value = warn-code SP warn-agent SP warn-text
1540                                        [SP warn-date]
1541 
1542  warn-code  = 3DIGIT
1543  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1544                  ; the name or pseudonym of the server adding
1545                  ; the Warning header field, for use in debugging
1546  warn-text  = quoted-string
1547  warn-date  = DQUOTE HTTP-date DQUOTE
1548]]></artwork></figure>
1549<t>
1550   Multiple warnings can be attached to a response (either by the origin
1551   server or by a cache), including multiple warnings with the same code
1552   number, only differing in warn-text.
1553</t>
1554<t>
1555   When this occurs, the user agent SHOULD inform the user of as many of
1556   them as possible, in the order that they appear in the response.
1557</t>
1558<t>
1559   Systems that generate multiple Warning header fields SHOULD order them with
1560   this user agent behavior in mind. New Warning header fields SHOULD be added
1561   after any existing Warning headers fields.
1562</t>
1563<t>
1564   Warnings are assigned three digit warn-codes. The first digit indicates
1565   whether the Warning is required to be deleted from a stored response after
1566   validation:
1567   <list style="symbols">
1568      <t>1xx Warnings describe the freshness or validation status of the
1569      response, and so MUST be deleted by a cache after validation. They can
1570      only be generated by a cache when validating a cached entry, and
1571      MUST NOT be generated in any other situation.</t>
1572      <t>2xx Warnings describe some aspect of the representation that is not
1573      rectified by a validation (for example, a lossy compression of the
1574      representation) and MUST NOT be deleted by a cache after validation,
1575      unless a full response is returned, in which case they MUST be.</t>
1576   </list>
1577</t>
1578<t>
1579   If an implementation sends a message with one or more Warning header fields to a
1580   receiver whose version is HTTP/1.0 or lower, then the sender MUST include
1581   in each warning-value a warn-date that matches the Date header field in the
1582   message.
1583</t>
1584<t>
1585   If a system receives a message with a warning-value that includes
1586   a warn-date, and that warn-date is different from the Date value in the
1587   response, then that warning-value MUST be deleted from the message before
1588   storing, forwarding, or using it. (preventing the consequences of naive
1589   caching of Warning header fields.) If all of the warning-values are deleted
1590   for this reason, the Warning header field MUST be deleted as well.
1591</t>
1592<t>
1593   The following warn-codes are defined by this specification, each with a
1594   recommended warn-text in English, and a description of its meaning.
1595</t>
1596<t>
1597<?rfc needLines="4"?>
1598   110 Response is stale
1599   <list>
1600      <t>A cache SHOULD include this whenever the returned response is stale.</t>
1601   </list>
1602</t>
1603<t>
1604<?rfc needLines="4"?>
1605   111 Revalidation failed
1606   <list>
1607      <t>A cache SHOULD include this when returning a stale response because an
1608      attempt to validate the response failed, due to an inability to reach
1609      the server.</t>
1610   </list>
1611</t>
1612<t>
1613<?rfc needLines="4"?>
1614   112 Disconnected operation
1615   <list>
1616      <t>A cache SHOULD b include this if it is intentionally disconnected from
1617      the rest of the network for a period of time.</t>
1618   </list>
1619</t>
1620<t>
1621<?rfc needLines="4"?>
1622   113 Heuristic expiration
1623   <list>
1624      <t>A cache SHOULD include this if it heuristically chose a freshness
1625      lifetime greater than 24 hours and the response's age is greater than 24
1626      hours.</t>
1627   </list>
1628</t>
1629<t>
1630<?rfc needLines="4"?>
1631   199 Miscellaneous warning
1632   <list>
1633      <t>The warning text can include arbitrary information to be presented to
1634      a human user, or logged. A system receiving this warning MUST NOT take
1635      any automated action, besides presenting the warning to the user.</t>
1636   </list>
1637</t>
1638<t>
1639<?rfc needLines="4"?>
1640   214 Transformation applied
1641   <list>
1642      <t>MUST be added by a proxy if it applies any
1643      transformation to the representation, such as changing the
1644      content-coding, media-type, or modifying the representation data, unless
1645      this Warning code already appears in the response.</t>
1646   </list>
1647</t>
1648<t>
1649<?rfc needLines="4"?>
1650   299 Miscellaneous persistent warning
1651   <list>
1652      <t>The warning text can include arbitrary information to be presented to
1653      a human user, or logged. A system receiving this warning MUST NOT take
1654      any automated action.</t>
1655   </list>
1656</t>
1657</section>
1658
1659</section>
1660
1661<section anchor="history.lists" title="History Lists">
1662<t>
1663   User agents often have history mechanisms, such as "Back" buttons and
1664   history lists, that can be used to redisplay a representation retrieved
1665   earlier in a session.
1666</t>
1667<t>
1668   The freshness model (<xref target="expiration.model"/>) does not
1669   necessarily apply to history mechanisms. I.e., a history mechanism can
1670   display a previous representation even if it has expired.
1671</t>
1672<t>
1673   This does not prohibit the history mechanism from telling the user that a
1674   view might be stale, or from honoring cache directives (e.g.,
1675   Cache-Control: no-store).
1676</t>
1677</section>
1678
1679
1680<section anchor="IANA.considerations" title="IANA Considerations">
1681
1682<section title="Cache Directive Registry" anchor="cache.directive.registration">
1683<t>
1684   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1685</t>
1686<t>
1687   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1688   populated with the registrations below:
1689</t>
1690
1691<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1692<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1693   <ttcol>Cache Directive</ttcol>
1694   <ttcol>Reference</ttcol>
1695
1696   <c>max-age</c>
1697   <c>
1698      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1699   </c>
1700   <c>max-stale</c>
1701   <c>
1702      <xref target="cache-request-directive"/>
1703   </c>
1704   <c>min-fresh</c>
1705   <c>
1706      <xref target="cache-request-directive"/>
1707   </c>
1708   <c>must-revalidate</c>
1709   <c>
1710      <xref target="cache-response-directive"/>
1711   </c>
1712   <c>no-cache</c>
1713   <c>
1714      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1715   </c>
1716   <c>no-store</c>
1717   <c>
1718      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1719   </c>
1720   <c>no-transform</c>
1721   <c>
1722      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1723   </c>
1724   <c>only-if-cached</c>
1725   <c>
1726      <xref target="cache-request-directive"/>
1727   </c>
1728   <c>private</c>
1729   <c>
1730      <xref target="cache-response-directive"/>
1731   </c>
1732   <c>proxy-revalidate</c>
1733   <c>
1734      <xref target="cache-response-directive"/>
1735   </c>
1736   <c>public</c>
1737   <c>
1738      <xref target="cache-response-directive"/>
1739   </c>
1740   <c>s-maxage</c>
1741   <c>
1742      <xref target="cache-response-directive"/>
1743   </c>
1744   <c>stale-if-error</c>
1745   <c>
1746      <xref target="RFC5861"/>, Section 4
1747   </c>
1748   <c>stale-while-revalidate</c>
1749   <c>
1750      <xref target="RFC5861"/>, Section 3
1751   </c>
1752</texttable>
1753<!--(END)-->
1754
1755</section>
1756
1757<section title="Header Field Registration" anchor="header.field.registration">
1758<t>
1759  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1760  shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1761</t>
1762
1763<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1764<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1765   <ttcol>Header Field Name</ttcol>
1766   <ttcol>Protocol</ttcol>
1767   <ttcol>Status</ttcol>
1768   <ttcol>Reference</ttcol>
1769
1770   <c>Age</c>
1771   <c>http</c>
1772   <c>standard</c>
1773   <c>
1774      <xref target="header.age"/>
1775   </c>
1776   <c>Cache-Control</c>
1777   <c>http</c>
1778   <c>standard</c>
1779   <c>
1780      <xref target="header.cache-control"/>
1781   </c>
1782   <c>Expires</c>
1783   <c>http</c>
1784   <c>standard</c>
1785   <c>
1786      <xref target="header.expires"/>
1787   </c>
1788   <c>Pragma</c>
1789   <c>http</c>
1790   <c>standard</c>
1791   <c>
1792      <xref target="header.pragma"/>
1793   </c>
1794   <c>Vary</c>
1795   <c>http</c>
1796   <c>standard</c>
1797   <c>
1798      <xref target="header.vary"/>
1799   </c>
1800   <c>Warning</c>
1801   <c>http</c>
1802   <c>standard</c>
1803   <c>
1804      <xref target="header.warning"/>
1805   </c>
1806</texttable>
1807<!--(END)-->
1808
1809<t>
1810   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1811   Force".
1812</t>
1813</section>
1814
1815</section>
1816
1817<section anchor="security.considerations" title="Security Considerations">
1818<t>
1819   Caches expose additional potential vulnerabilities, since the contents of
1820   the cache represent an attractive target for malicious exploitation.
1821   Because cache contents persist after an HTTP request is complete, an attack
1822   on the cache can reveal information long after a user believes that the
1823   information has been removed from the network. Therefore, cache contents
1824   need to be protected as sensitive information.
1825</t>
1826</section>
1827
1828<section anchor="ack" title="Acknowledgments">
1829<t>
1830   Much of the content and presentation of the caching design is due to
1831   suggestions and comments from individuals including: Shel Kaphan, Paul
1832   Leach, Koen Holtman, David Morris, and Larry Masinter.
1833</t>
1834</section>
1835
1836</middle>
1837
1838<back>
1839<references title="Normative References">
1840
1841  <reference anchor="Part1">
1842    <front>
1843      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1844      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1845        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1846        <address><email>fielding@gbiv.com</email></address>
1847      </author>
1848      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1849        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1850        <address><email>jg@freedesktop.org</email></address>
1851      </author>
1852      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1853        <organization abbrev="HP">Hewlett-Packard Company</organization>
1854        <address><email>JeffMogul@acm.org</email></address>
1855      </author>
1856      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1857        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1858        <address><email>henrikn@microsoft.com</email></address>
1859      </author>
1860      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1861        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1862        <address><email>LMM@acm.org</email></address>
1863      </author>
1864      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1865        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1866        <address><email>paulle@microsoft.com</email></address>
1867      </author>
1868      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1869        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1870        <address><email>timbl@w3.org</email></address>
1871      </author>
1872      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1873        <organization abbrev="W3C">World Wide Web Consortium</organization>
1874        <address><email>ylafon@w3.org</email></address>
1875      </author>
1876      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1877        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1878        <address><email>julian.reschke@greenbytes.de</email></address>
1879      </author>
1880      <date month="April" year="2011"/>
1881    </front>
1882    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-14"/>
1883   
1884  </reference>
1885
1886  <reference anchor="Part2">
1887    <front>
1888      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1889      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1890        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1891        <address><email>fielding@gbiv.com</email></address>
1892      </author>
1893      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1894        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1895        <address><email>jg@freedesktop.org</email></address>
1896      </author>
1897      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1898        <organization abbrev="HP">Hewlett-Packard Company</organization>
1899        <address><email>JeffMogul@acm.org</email></address>
1900      </author>
1901      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1902        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1903        <address><email>henrikn@microsoft.com</email></address>
1904      </author>
1905      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1906        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1907        <address><email>LMM@acm.org</email></address>
1908      </author>
1909      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1910        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1911        <address><email>paulle@microsoft.com</email></address>
1912      </author>
1913      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1914        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1915        <address><email>timbl@w3.org</email></address>
1916      </author>
1917      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1918        <organization abbrev="W3C">World Wide Web Consortium</organization>
1919        <address><email>ylafon@w3.org</email></address>
1920      </author>
1921      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1922        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1923        <address><email>julian.reschke@greenbytes.de</email></address>
1924      </author>
1925      <date month="April" year="2011"/>
1926    </front>
1927    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-14"/>
1928   
1929  </reference>
1930
1931  <reference anchor="Part4">
1932    <front>
1933      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1934      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1935        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1936        <address><email>fielding@gbiv.com</email></address>
1937      </author>
1938      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1939        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1940        <address><email>jg@freedesktop.org</email></address>
1941      </author>
1942      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1943        <organization abbrev="HP">Hewlett-Packard Company</organization>
1944        <address><email>JeffMogul@acm.org</email></address>
1945      </author>
1946      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1947        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1948        <address><email>henrikn@microsoft.com</email></address>
1949      </author>
1950      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1951        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1952        <address><email>LMM@acm.org</email></address>
1953      </author>
1954      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1955        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1956        <address><email>paulle@microsoft.com</email></address>
1957      </author>
1958      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1959        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1960        <address><email>timbl@w3.org</email></address>
1961      </author>
1962      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1963        <organization abbrev="W3C">World Wide Web Consortium</organization>
1964        <address><email>ylafon@w3.org</email></address>
1965      </author>
1966      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1967        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1968        <address><email>julian.reschke@greenbytes.de</email></address>
1969      </author>
1970      <date month="April" year="2011"/>
1971    </front>
1972    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-14"/>
1973   
1974  </reference>
1975
1976  <reference anchor="Part5">
1977    <front>
1978      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1979      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1980        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1981        <address><email>fielding@gbiv.com</email></address>
1982      </author>
1983      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1984        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1985        <address><email>jg@freedesktop.org</email></address>
1986      </author>
1987      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1988        <organization abbrev="HP">Hewlett-Packard Company</organization>
1989        <address><email>JeffMogul@acm.org</email></address>
1990      </author>
1991      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1992        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1993        <address><email>henrikn@microsoft.com</email></address>
1994      </author>
1995      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1996        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1997        <address><email>LMM@acm.org</email></address>
1998      </author>
1999      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2000        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2001        <address><email>paulle@microsoft.com</email></address>
2002      </author>
2003      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2004        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2005        <address><email>timbl@w3.org</email></address>
2006      </author>
2007      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2008        <organization abbrev="W3C">World Wide Web Consortium</organization>
2009        <address><email>ylafon@w3.org</email></address>
2010      </author>
2011      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2012        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2013        <address><email>julian.reschke@greenbytes.de</email></address>
2014      </author>
2015      <date month="April" year="2011"/>
2016    </front>
2017    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-14"/>
2018   
2019  </reference>
2020
2021  <reference anchor="Part7">
2022    <front>
2023      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2024      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2025        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2026        <address><email>fielding@gbiv.com</email></address>
2027      </author>
2028      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2029        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2030        <address><email>jg@freedesktop.org</email></address>
2031      </author>
2032      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2033        <organization abbrev="HP">Hewlett-Packard Company</organization>
2034        <address><email>JeffMogul@acm.org</email></address>
2035      </author>
2036      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2037        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2038        <address><email>henrikn@microsoft.com</email></address>
2039      </author>
2040      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2041        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2042        <address><email>LMM@acm.org</email></address>
2043      </author>
2044      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2045        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2046        <address><email>paulle@microsoft.com</email></address>
2047      </author>
2048      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2049        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2050        <address><email>timbl@w3.org</email></address>
2051      </author>
2052      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2053        <organization abbrev="W3C">World Wide Web Consortium</organization>
2054        <address><email>ylafon@w3.org</email></address>
2055      </author>
2056      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2057        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2058        <address><email>julian.reschke@greenbytes.de</email></address>
2059      </author>
2060      <date month="April" year="2011"/>
2061    </front>
2062    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-14"/>
2063   
2064  </reference>
2065
2066  <reference anchor="RFC2119">
2067    <front>
2068      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2069      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2070        <organization>Harvard University</organization>
2071        <address><email>sob@harvard.edu</email></address>
2072      </author>
2073      <date month="March" year="1997"/>
2074    </front>
2075    <seriesInfo name="BCP" value="14"/>
2076    <seriesInfo name="RFC" value="2119"/>
2077  </reference>
2078
2079  <reference anchor="RFC5234">
2080    <front>
2081      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2082      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2083        <organization>Brandenburg InternetWorking</organization>
2084        <address>
2085          <email>dcrocker@bbiw.net</email>
2086        </address> 
2087      </author>
2088      <author initials="P." surname="Overell" fullname="Paul Overell">
2089        <organization>THUS plc.</organization>
2090        <address>
2091          <email>paul.overell@thus.net</email>
2092        </address>
2093      </author>
2094      <date month="January" year="2008"/>
2095    </front>
2096    <seriesInfo name="STD" value="68"/>
2097    <seriesInfo name="RFC" value="5234"/>
2098  </reference>
2099 
2100</references>
2101
2102<references title="Informative References">
2103
2104  <reference anchor="RFC1305">
2105    <front>
2106      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2107      <author fullname="David L. Mills" initials="D." surname="Mills">
2108        <organization>University of Delaware, Electrical Engineering Department</organization>
2109        <address><email>mills@udel.edu</email></address>
2110      </author>
2111      <date month="March" year="1992"/>
2112    </front>
2113    <seriesInfo name="RFC" value="1305"/>
2114  </reference>
2115
2116  <reference anchor="RFC2616">
2117    <front>
2118      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2119      <author fullname="R. Fielding" initials="R." surname="Fielding">
2120        <organization>University of California, Irvine</organization>
2121        <address><email>fielding@ics.uci.edu</email></address>
2122      </author>
2123      <author fullname="J. Gettys" initials="J." surname="Gettys">
2124        <organization>W3C</organization>
2125        <address><email>jg@w3.org</email></address>
2126      </author>
2127      <author fullname="J. Mogul" initials="J." surname="Mogul">
2128        <organization>Compaq Computer Corporation</organization>
2129        <address><email>mogul@wrl.dec.com</email></address>
2130      </author>
2131      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2132        <organization>MIT Laboratory for Computer Science</organization>
2133        <address><email>frystyk@w3.org</email></address>
2134      </author>
2135      <author fullname="L. Masinter" initials="L." surname="Masinter">
2136        <organization>Xerox Corporation</organization>
2137        <address><email>masinter@parc.xerox.com</email></address>
2138      </author>
2139      <author fullname="P. Leach" initials="P." surname="Leach">
2140        <organization>Microsoft Corporation</organization>
2141        <address><email>paulle@microsoft.com</email></address>
2142      </author>
2143      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2144        <organization>W3C</organization>
2145        <address><email>timbl@w3.org</email></address>
2146      </author>
2147      <date month="June" year="1999"/>
2148    </front>
2149    <seriesInfo name="RFC" value="2616"/>
2150  </reference>
2151
2152  <reference anchor="RFC3864">
2153    <front>
2154      <title>Registration Procedures for Message Header Fields</title>
2155      <author fullname="G. Klyne" initials="G." surname="Klyne">
2156        <organization>Nine by Nine</organization>
2157        <address><email>GK-IETF@ninebynine.org</email></address>
2158      </author>
2159      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2160        <organization>BEA Systems</organization>
2161        <address><email>mnot@pobox.com</email></address>
2162      </author>
2163      <author fullname="J. Mogul" initials="J." surname="Mogul">
2164        <organization>HP Labs</organization>
2165        <address><email>JeffMogul@acm.org</email></address>
2166      </author>
2167      <date month="September" year="2004"/>
2168    </front>
2169    <seriesInfo name="BCP" value="90"/>
2170    <seriesInfo name="RFC" value="3864"/>
2171  </reference>
2172
2173  <reference anchor="RFC5226">
2174    <front>
2175      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2176      <author initials="T." surname="Narten" fullname="T. Narten">
2177        <organization>IBM</organization>
2178        <address><email>narten@us.ibm.com</email></address>
2179      </author>
2180      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2181        <organization>Google</organization>
2182        <address><email>Harald@Alvestrand.no</email></address>
2183      </author>
2184      <date year="2008" month="May"/>
2185    </front>
2186    <seriesInfo name="BCP" value="26"/>
2187    <seriesInfo name="RFC" value="5226"/>
2188  </reference>
2189
2190  <reference anchor="RFC5861">
2191    <front>
2192      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2193      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2194        <organization>Yahoo! Inc.</organization>
2195        <address><email>mnot@yahoo-inc.com</email></address>
2196      </author>
2197      <date month="April" year="2010"/>
2198    </front>
2199    <seriesInfo name="RFC" value="5861"/>
2200  </reference>
2201
2202</references>
2203
2204<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2205<t>
2206  Make the specified age calculation algorithm less conservative.
2207  (<xref target="age.calculations"/>)
2208</t>
2209<t>
2210  Remove requirement to consider Content-Location in successful responses
2211  in order to determine the appropriate response to use.
2212  (<xref target="validation.model"/>)
2213</t>
2214<t>
2215  Clarify denial of service attack avoidance requirement.
2216  (<xref target="invalidation.after.updates.or.deletions"/>)
2217</t>
2218<t>
2219  Change ABNF productions for header fields to only define the field value.
2220  (<xref target="header.fields"/>)
2221</t>
2222<t>
2223  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2224  anymore, as these aspects never were implemented.
2225  (<xref target="header.warning"/>)
2226</t>
2227</section>
2228
2229
2230<section title="Collected ABNF" anchor="collected.abnf">
2231<figure>
2232<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2233Age = delta-seconds
2234
2235Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2236 cache-directive ] )
2237
2238Expires = HTTP-date
2239
2240HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2241
2242OWS = <OWS, defined in [Part1], Section 1.2.2>
2243
2244Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2245 pragma-directive ] )
2246
2247Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2248 ) )
2249
2250Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2251 )
2252
2253cache-directive = cache-request-directive / cache-response-directive
2254cache-extension = token [ "=" ( token / quoted-string ) ]
2255cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2256 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2257 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2258 cache-extension
2259cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2260 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2261 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2262 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2263 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2264 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2265
2266delta-seconds = 1*DIGIT
2267
2268extension-pragma = token [ "=" ( token / quoted-string ) ]
2269
2270field-name = <field-name, defined in [Part1], Section 3.2>
2271
2272port = <port, defined in [Part1], Section 2.6>
2273pragma-directive = "no-cache" / extension-pragma
2274pseudonym = <pseudonym, defined in [Part1], Section 9.9>
2275
2276quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
2277
2278token = <token, defined in [Part1], Section 1.2.2>
2279
2280uri-host = <uri-host, defined in [Part1], Section 2.6>
2281
2282warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2283warn-code = 3DIGIT
2284warn-date = DQUOTE HTTP-date DQUOTE
2285warn-text = quoted-string
2286warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2287 ]
2288]]></artwork>
2289</figure>
2290<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2291; Age defined but not used
2292; Cache-Control defined but not used
2293; Expires defined but not used
2294; Pragma defined but not used
2295; Vary defined but not used
2296; Warning defined but not used
2297]]></artwork></figure></section>
2298
2299
2300<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2301
2302<section title="Since RFC 2616">
2303  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2304</section>
2305
2306<section title="Since draft-ietf-httpbis-p6-cache-00">
2307<t>
2308  Closed issues:
2309  <list style="symbols">
2310    <t>
2311      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2312    <t>
2313      <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>
2314    <t>
2315      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2316    <t>
2317      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2318    <t>
2319      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2320    <t>
2321      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2322    <t>
2323      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2324    <t>
2325      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2326    <t>
2327      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2328  </list>
2329</t>
2330<t>
2331  Other changes:
2332  <list style="symbols">
2333    <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>
2334  </list>
2335</t>
2336</section>
2337
2338<section title="Since draft-ietf-httpbis-p6-cache-01">
2339<t>
2340  Closed issues:
2341  <list style="symbols">
2342    <t>
2343      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2344  </list>
2345</t>
2346<t>
2347  Other changes:
2348  <list style="symbols">
2349    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2350      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2351    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2352      specification.</t>
2353  </list>
2354</t>
2355</section>
2356
2357<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2358<t>
2359  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2360  <list style="symbols">
2361    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2362      document.</t>
2363  </list>
2364</t>
2365</section>
2366
2367<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2368<t>
2369  Closed issues:
2370  <list style="symbols">
2371    <t>
2372      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2373  </list>
2374</t>
2375</section>
2376
2377<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2378<t>
2379  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2380  <list style="symbols"> 
2381    <t>
2382      Use "/" instead of "|" for alternatives.
2383    </t>
2384    <t>
2385      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2386      whitespace ("OWS") and required whitespace ("RWS").
2387    </t>
2388    <t>
2389      Rewrite ABNFs to spell out whitespace rules, factor out
2390      header field value format definitions.
2391    </t>
2392  </list>
2393</t>
2394</section>
2395
2396<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2397<t>
2398  This is a total rewrite of this part of the specification.
2399</t>
2400<t>
2401  Affected issues:
2402  <list style="symbols">
2403    <t>
2404      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2405    <t>
2406      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2407    <t>
2408      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2409    <t>
2410      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2411  </list>
2412</t>
2413<t>
2414  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2415  <list style="symbols"> 
2416    <t>
2417      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2418    </t>
2419  </list>
2420</t>
2421</section>
2422
2423<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2424<t>
2425  Closed issues:
2426  <list style="symbols"> 
2427    <t>
2428      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2429      "base for numeric protocol elements"
2430    </t>
2431  </list>
2432</t>
2433<t>
2434  Affected issues:
2435  <list style="symbols">
2436    <t>
2437      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2438      "Vary and non-existant headers"
2439    </t>
2440  </list>
2441</t>
2442</section>
2443
2444<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2445<t>
2446  Closed issues:
2447  <list style="symbols"> 
2448    <t>
2449      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
2450      "Definition of 1xx Warn-Codes"
2451    </t>
2452    <t>
2453      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2454      "Content-Location on 304 responses"
2455    </t>
2456    <t>
2457      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
2458      "private and no-cache CC directives with headers"
2459    </t>
2460    <t>
2461      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2462      "RFC2047 and warn-text"
2463    </t>
2464  </list>
2465</t>
2466</section>
2467
2468<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2469<t>
2470  Closed issues:
2471  <list style="symbols"> 
2472    <t>
2473      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
2474      "serving negotiated responses from cache: header-specific canonicalization"
2475    </t>
2476    <t>
2477      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
2478      "Effect of CC directives on history lists"
2479    </t>
2480  </list>
2481</t>
2482<t>
2483  Affected issues:
2484  <list style="symbols">
2485    <t>
2486      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2487      Status codes and caching
2488    </t>
2489  </list>
2490</t>
2491<t>
2492  Partly resolved issues:
2493  <list style="symbols"> 
2494    <t>
2495      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2496      "Placement of 13.5.1 and 13.5.2"
2497    </t>
2498  </list>
2499</t>
2500</section>
2501
2502<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2503<t>
2504  Closed issues:
2505  <list style="symbols"> 
2506    <t>
2507      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
2508      "Age calculation"
2509    </t>
2510    <t>
2511      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
2512      "Clarify differences between / requirements for request and response CC directives"
2513    </t>
2514        <t>
2515                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
2516                "Caching authenticated responses"
2517        </t>
2518    <t>
2519      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
2520      "IANA registry for cache-control directives"
2521    </t>
2522    <t>
2523      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
2524      "Heuristic caching of URLs with query components"
2525    </t>
2526  </list>
2527</t>
2528<t>
2529  Partly resolved issues:
2530  <list style="symbols"> 
2531    <t>
2532      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2533      "Term for the requested resource's URI"
2534    </t>
2535  </list>
2536</t>
2537</section>
2538
2539<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2540<t>
2541  Closed issues:
2542  <list style="symbols"> 
2543    <t>
2544      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2545      "Clarify entity / representation / variant terminology"
2546    </t>
2547    <t>
2548      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2549      "consider removing the 'changes from 2068' sections"
2550    </t>
2551    <t>
2552      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2553      "Allowing heuristic caching for new status codes"
2554    </t>
2555    <t>
2556      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2557      "Allowing heuristic caching for new status codes"
2558    </t>
2559    <t>
2560      Clean up TODOs and prose in "Combining Responses."
2561    </t>
2562  </list>
2563</t>
2564</section>
2565
2566<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2567<t>
2568  Closed issues:
2569  <list style="symbols"> 
2570    <t>
2571      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2572      "Text about clock requirement for caches belongs in p6"
2573    </t>
2574  </list>
2575</t>
2576</section>
2577
2578<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2579<t>
2580  Closed issues:
2581  <list style="symbols"> 
2582    <t>
2583      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2584      "Header Classification"
2585    </t>
2586    <t>
2587      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2588      "Clarify 'public'"
2589    </t>
2590  </list>
2591</t>
2592</section>
2593
2594<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2595<t>
2596  Closed issues:
2597  <list style="symbols">
2598    <t>
2599      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2600      "untangle ABNFs for header fields"
2601    </t>
2602  </list>
2603</t>
2604</section>
2605
2606</section>
2607  </back>
2608</rfc>
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