source: draft-ietf-httpbis/15/draft-ietf-httpbis-p6-cache-15.xml @ 2082

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fix mime types

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc category="std" docName="draft-ietf-httpbis-p6-cache-15" 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="July" year="2011" day="11"/>
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.14"/>.
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 2.7>
404  pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
405  uri-host      = <uri-host, defined in [Part1], Section 2.7>
406]]></artwork></figure>
407</section>
408</section>
409
410<section title="Delta Seconds" anchor="delta-seconds">
411<t>
412   The delta-seconds rule specifies a non-negative integer, representing time
413   in seconds.
414</t>
415<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
416  delta-seconds  = 1*DIGIT
417]]></artwork></figure>
418<t>
419   If an implementation receives a delta-seconds value larger than the largest
420   positive integer it can represent, or if any of its subsequent calculations
421   overflows, it MUST consider the value to be 2147483648 (2^31).
422   Recipients parsing a delta-seconds value SHOULD use an arithmetic type of
423   at least 31 bits of range, and senders MUST NOT send delta-seconds with a
424   value greater than 2147483648.
425</t>
426</section>
427
428</section>
429
430<section anchor="caching.overview" title="Cache Operation">
431
432<section anchor="response.cacheability" title="Response Cacheability">
433<t>
434   A cache MUST NOT store a response to any request, unless:
435   <list style="symbols">
436      <t>The request method is understood by the cache and defined as being
437      cacheable, and</t>
438      <t>the response status code is understood by the cache, and</t>
439      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
440      header fields, and</t>
441      <t>the "private" cache response directive (see <xref target="cache-response-directive"/> does not appear in the response, if
442      the cache is shared, and</t>
443      <t>the "Authorization" header field (see Section 4.1 of <xref target="Part7"/>) does not
444      appear in the request, if the cache is shared, unless the response
445      explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
446      <t>the response either:
447         <list style="symbols">
448            <t>contains an Expires header field (see <xref target="header.expires"/>), or</t>
449            <t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
450            <t>contains a s-maxage response cache directive and the cache is
451            shared, or</t>
452            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
453            or</t>
454            <t>has a status code that can be served with heuristic freshness
455            (see <xref target="heuristic.freshness"/>).</t>
456         </list>
457      </t>
458   </list>
459</t>
460<t>
461   Note that any of the requirements listed above can be overridden by a
462   cache-control extension; see <xref target="cache.control.extensions"/>.
463</t>
464<t>
465   In this context, a cache has "understood" a request method or a response
466   status code if it recognises it and implements any cache-specific
467   behavior. In particular, 206 Partial Content responses cannot be cached by
468   an implementation that does not handle partial content (see <xref target="errors.or.incomplete.response.cache.behavior"/>).
469</t>
470<t>
471   Note that in normal operation, most caches will not store a response that
472   has neither a cache validator nor an explicit expiration time, as such
473   responses are not usually useful to store. However, caches are not
474   prohibited from storing such responses.
475</t>
476
477<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
478<t>
479   A cache that receives an incomplete response (for example, with fewer bytes
480   of data than specified in a Content-Length header field) can store the response,
481   but MUST treat it as a partial response <xref target="Part5"/>. Partial responses can
482   be combined as described in Section 4 of <xref target="Part5"/>; the result might be a
483   full response or might still be partial. A cache MUST NOT return a
484   partial response to a client without explicitly marking it as such using
485   the 206 (Partial Content) status code.
486</t>
487<t>
488   A cache that does not support the Range and Content-Range header fields
489   MUST NOT store incomplete or partial responses.
490</t>
491</section>
492
493</section>
494
495
496<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
497<t>
498   For a presented request, a cache MUST NOT return a stored response,
499   unless:
500   <list style="symbols">
501      <t>The presented effective request URI (Section 4.3 of <xref target="Part1"/>) and
502      that of the stored response match, and</t>
503      <t>the request method associated with the stored response allows it to
504      be used for the presented request, and</t>
505      <t>selecting header fields nominated by the stored response (if any)
506      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
507      <t>the presented request and stored response are free from directives
508      that would prevent its use (see <xref target="header.cache-control"/>
509      and <xref target="header.pragma"/>), and</t>
510      <t>the stored response is either:
511         <list style="symbols">
512            <t>fresh (see <xref target="expiration.model"/>), or</t>
513            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
514            <t>successfully validated (see <xref target="validation.model"/>).</t>
515         </list>
516      </t>
517  </list>
518</t>
519<t>
520   Note that any of the requirements listed above can be overridden by a
521   cache-control extension; see <xref target="cache.control.extensions"/>.
522</t>
523<t>
524   When a stored response is used to satisfy a request without validation,
525   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
526   current_age; see <xref target="age.calculations"/>.
527</t>
528<t>
529   A cache MUST write through requests with methods that are unsafe
530   (Section 7.1.1 of <xref target="Part2"/>) to the origin server; i.e., a cache must not generate
531   a reply to such a request before having forwarded the request and having
532   received a corresponding response.
533</t>
534<t>
535   Also, note that unsafe requests might invalidate already stored responses;
536   see <xref target="invalidation.after.updates.or.deletions"/>.
537</t>
538<t>
539   When more than one suitable response is stored, a cache MUST use the
540   most recent response (as determined by the Date header field). It can also
541   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
542   no-cache" to disambiguate which response to use.
543</t>
544<t>
545   A cache that does not have a clock available MUST NOT use stored responses
546   without revalidating them on every use. A cache, especially a shared
547   cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to
548   synchronize its clock with a reliable external standard.
549</t>
550
551</section>
552
553<section anchor="expiration.model" title="Freshness Model">
554<t>
555   When a response is "fresh" in the cache, it can be used to satisfy
556   subsequent requests without contacting the origin server, thereby improving
557   efficiency.
558</t>
559<t>
560   The primary mechanism for determining freshness is for an origin server to
561   provide an explicit expiration time in the future, using either the Expires
562   header field (<xref target="header.expires"/>) or the max-age response cache
563   directive (<xref target="cache-response-directive"/>). Generally, origin
564   servers will assign future explicit expiration times to responses in the
565   belief that the representation is not likely to change in a semantically
566   significant way before the expiration time is reached.
567</t>
568<t>
569   If an origin server wishes to force a cache to validate every request, it
570   can assign an explicit expiration time in the past to indicate that the
571   response is already stale. Compliant caches will normally validate the
572   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
573</t>
574<t>
575   Since origin servers do not always provide explicit expiration times,
576   a cache MAY assign a heuristic expiration time when an explicit time is not
577   specified, employing algorithms that use other header field values (such as the
578   Last-Modified time) to estimate a plausible expiration time. This
579   specification does not provide specific algorithms, but does impose
580   worst-case constraints on their results.
581</t>
582<figure>
583<preamble>
584  The calculation to determine if a response is fresh is:
585</preamble>
586<artwork type="code"><![CDATA[
587   response_is_fresh = (freshness_lifetime > current_age)
588]]></artwork>
589</figure>
590<t>
591   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
592   <xref target="age.calculations"/>.
593</t>
594<t>
595   Additionally, clients might need to influence freshness calculation. They
596   can do this using several request cache directives, with the effect of
597   either increasing or loosening constraints on freshness. See <xref target="cache-request-directive"/>.
598</t>
599<t>
600   Note that freshness applies only to cache operation; it cannot be used to
601   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
602   caches and history mechanisms.
603</t>
604
605<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
606<t>
607   A cache can calculate the freshness lifetime (denoted as
608   freshness_lifetime) of a response by using the first match of:
609   <list style="symbols">
610      <t>If the cache is shared and the s-maxage response cache directive
611      (<xref target="cache-response-directive"/>) is present, use its value,
612      or</t>
613      <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
614      <t>If the Expires response header field (<xref target="header.expires"/>) is
615      present, use its value minus the value of the Date response header field,
616      or</t>
617      <t>Otherwise, no explicit expiration time is present in the response. A
618      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
619   </list>
620</t>
621<t>
622   Note that this calculation is not vulnerable to clock skew, since all of
623   the information comes from the origin server.
624</t>
625
626<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
627<t>
628   If no explicit expiration time is present in a stored response that has a
629   status code whose definition allows heuristic freshness to be used
630   (including the following in Section 8 of <xref target="Part2"/>: 200, 203, 206, 300, 301 and
631   410), a cache MAY calculate a heuristic expiration time. A cache MUST NOT
632   use heuristics to determine freshness for responses with status codes that do
633   not explicitly allow it.
634</t>
635<t>
636   When a heuristic is used to calculate freshness lifetime, a cache
637   SHOULD attach a Warning header field with a 113 warn-code to the response if
638   its current_age is more than 24 hours and such a warning is not already
639   present.
640</t>
641<t>
642   Also, if the response has a Last-Modified header field (Section 2.1 of <xref target="Part4"/>),
643   a cache SHOULD NOT use a heuristic expiration value that is more than some
644   fraction of the interval since that time. A typical setting of this fraction
645   might be 10%.
646</t>
647<t><list>
648   <t>
649      Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9) required that caches do not calculate heuristic
650      freshness for URIs with query components (i.e., those containing '?').
651      In practice, this has not been widely implemented. Therefore, servers
652      are encouraged to send explicit directives (e.g., Cache-Control:
653      no-cache) if they wish to preclude caching.
654   </t>
655</list></t>
656</section>
657</section>
658
659<section anchor="age.calculations" title="Calculating Age">
660<t>
661   HTTP/1.1 uses the Age header field to convey the estimated age of the
662   response message when obtained from a cache. The Age field value is the
663   cache's estimate of the amount of time since the response was generated or
664   validated by the origin server. In essence, the Age value is the sum of the
665   time that the response has been resident in each of the caches along the
666   path from the origin server, plus the amount of time it has been in transit
667   along network paths.
668</t>
669<t>
670   The following data is used for the age calculation:
671</t>
672<t>
673   <?rfc needLines="4"?>age_value
674   <list>
675      <t>
676         The term "age_value" denotes the value of the Age header field (<xref target="header.age"/>), in a form appropriate for arithmetic
677         operation; or 0, if not available.
678      </t>
679   </list>
680</t>
681<t>
682   <?rfc needLines="4"?>date_value
683   <list>
684      <t>
685         HTTP/1.1 requires origin servers to send a Date header field, if possible,
686         with every response, giving the time at which the response was
687         generated. The term "date_value" denotes the value of the Date
688         header field, in a form appropriate for arithmetic operations. See
689         Section 9.3 of <xref target="Part1"/> for the definition of the Date header field, and for
690         requirements regarding responses without it.
691      </t>
692   </list>
693</t>
694<t>
695   <?rfc needLines="4"?>now
696   <list>
697      <t>
698         The term "now" means "the current value of the clock at the host
699         performing the calculation". A cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
700         clocks to a globally accurate time standard.
701      </t>
702   </list>
703</t>
704<t>
705   <?rfc needLines="4"?>request_time
706   <list>
707      <t>
708         The current value of the clock at the host at the time the request
709         resulting in the stored response was made.
710      </t>
711   </list>
712</t>
713<t>
714   <?rfc needLines="4"?>response_time
715   <list>
716      <t>
717         The current value of the clock at the host at the time the response
718         was received.
719      </t>
720   </list>
721</t>
722<t>
723   A response's age can be calculated in two entirely independent ways:
724   <list style="numbers">
725      <t>the "apparent_age": response_time minus date_value, if the local
726      clock is reasonably well synchronized to the origin server's clock. If
727      the result is negative, the result is replaced by zero.</t>
728      <t>the "corrected_age_value", if all of the caches along the response
729      path implement HTTP/1.1. A cache MUST interpret this value relative
730      to the time the request was initiated, not the time that the response
731      was received.</t>
732   </list>
733</t>
734<figure>
735<artwork type="code"><![CDATA[
736  apparent_age = max(0, response_time - date_value);
737
738  response_delay = response_time - request_time;
739  corrected_age_value = age_value + response_delay; 
740]]></artwork>
741</figure>
742<figure>
743<preamble>These are combined as</preamble>
744<artwork type="code"><![CDATA[
745  corrected_initial_age = max(apparent_age, corrected_age_value);
746]]></artwork></figure>
747<t>
748   The current_age of a stored response can then be calculated by adding the
749   amount of time (in seconds) since the stored response was last validated by
750   the origin server to the corrected_initial_age.
751</t>
752<figure><artwork type="code"><![CDATA[
753  resident_time = now - response_time;
754  current_age = corrected_initial_age + resident_time;
755]]></artwork></figure>
756</section>
757
758<section anchor="serving.stale.responses" title="Serving Stale Responses">
759<t>
760   A "stale" response is one that either has explicit expiry information or is
761   allowed to have heuristic expiry calculated, but is not fresh according to
762   the calculations in <xref target="expiration.model"/>.
763</t>
764<t>
765   A cache MUST NOT return a stale response if it is prohibited by an
766   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
767   directive, a "must-revalidate" cache-response-directive, or an applicable
768   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
769</t>
770<t>
771   A cache SHOULD NOT return stale responses unless it is disconnected
772   (i.e., it cannot contact the origin server or otherwise find a forward
773   path) or doing so is explicitly allowed (e.g., by the max-stale request
774   directive; see <xref target="cache-request-directive"/>).
775</t>
776<t>
777   A cache SHOULD append a Warning header field with the 110 warn-code (see
778   <xref target="header.warning"/>) to stale responses. Likewise, a cache
779   SHOULD add the 112 warn-code to stale responses if the cache is
780   disconnected.
781</t>
782<t>
783   If a cache receives a first-hand response (either an entire response, or a
784   304 (Not Modified) response) that it would normally forward to the
785   requesting client, and the received response is no longer fresh, the cache
786   SHOULD forward it to the requesting client without adding a new Warning
787   (but without removing any existing Warning header fields). A cache SHOULD NOT
788   attempt to validate a response simply because that response became stale in
789   transit.
790</t>
791</section>
792</section>
793
794<section anchor="validation.model" title="Validation Model">
795<t>
796   When a cache has one or more stored responses for a requested URI, but
797   cannot serve any of them (e.g., because they are not fresh, or one cannot
798   be selected; see <xref target="caching.negotiated.responses"/>), it can use
799   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
800   give the origin server an opportunity to both select a valid stored
801   response to be used, and to update it. This process is known as
802   "validating" or "revalidating" the stored response.
803</t>
804<t>
805   When sending such a conditional request, a cache SHOULD add an
806   If-Modified-Since header field whose value is that of the Last-Modified header
807   field from the selected (see <xref target="caching.negotiated.responses"/>)
808   stored response, if available.
809</t>
810<t>
811   Additionally, a cache SHOULD add an If-None-Match header field whose value is
812   that of the ETag header field(s) from all responses stored for the requested URI,
813   if present. However, if any of the stored responses contains only partial
814   content, the cache SHOULD NOT include its entity-tag in the If-None-Match
815   header field unless the request is for a range that would be fully
816   satisfied by that stored response.
817</t>
818<t>
819   A 304 (Not Modified) response status code indicates that the stored
820   response can be updated and reused; see <xref target="combining.responses"/>.
821</t>
822<t>
823   A full response (i.e., one with a response body) indicates that none of the
824   stored responses nominated in the conditional request is suitable. Instead,
825   a cache SHOULD use the full response to satisfy the request and MAY
826   replace the stored response.
827</t>
828<t>
829   If a cache receives a 5xx response while attempting to validate a response,
830   it MAY either forward this response to the requesting client, or act as
831   if the server failed to respond. In the latter case, it MAY return a
832   previously stored response (see <xref target="serving.stale.responses"/>).
833</t>
834</section>
835
836<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
837<t>
838   Because unsafe request methods (Section 7.1.1 of <xref target="Part2"/>) such as PUT, POST or DELETE
839   have the potential for changing state on the origin server, intervening
840   caches can use them to keep their contents up-to-date.
841</t>
842<t>
843   A cache MUST invalidate the effective Request URI
844   (Section 4.3 of <xref target="Part1"/>) as well as the URI(s) in the Location
845   and Content-Location header fields (if present) when a non-error
846   response to a request with an unsafe method is received.
847</t>
848<t>
849   However, a cache MUST NOT invalidate a URI from a
850   Location or Content-Location header field if the host part of that URI
851   differs from the host part in the effective request URI
852   (Section 4.3 of <xref target="Part1"/>). This helps prevent denial of service attacks.
853</t>
854<t>
855   A cache SHOULD invalidate the effective request URI
856   (Section 4.3 of <xref target="Part1"/>) when it receives a non-error response
857   to a request with a method whose safety is unknown.
858</t>
859<t>
860   Here, a "non-error response" is one with a 2xx or 3xx status code.
861   "Invalidate" means that the cache will either remove all stored
862   responses related to the effective request URI, or will mark these as
863   "invalid" and in need of a mandatory validation before they can be returned
864   in response to a subsequent request.
865</t>
866<t>
867   Note that this does not guarantee that all appropriate responses are
868   invalidated. For example, the request that caused the change at the origin
869   server might not have gone through the cache where a response is stored.
870</t>
871</section>
872
873<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
874
875<t>
876   A shared cache MUST NOT use a cached response to a request with an
877   Authorization header field (Section 4.1 of <xref target="Part7"/>) to satisfy any subsequent
878   request unless a cache directive that allows such responses to be stored is
879   present in the response.
880</t>
881
882<t>
883   In this specification, the following Cache-Control response directives
884   (<xref target="cache-response-directive"/>) have such an effect:
885   must-revalidate, public, s-maxage.
886</t>
887
888<t>
889   Note that cached responses that contain the "must-revalidate" and/or
890   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
891   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
892   used to satisfy a subsequent request without revalidating it on the origin
893   server.
894</t>
895</section>
896
897<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
898<t>
899   When a cache receives a request that can be satisfied by a stored response
900   that has a Vary header field (<xref target="header.vary"/>), it MUST NOT
901   use that response unless all of the selecting header fields nominated by
902   the Vary header field match in both the original request (i.e., that associated
903   with the stored response), and the presented request.
904</t>
905<t>
906   The selecting header fields from two requests are defined to match if and
907   only if those in the first request can be transformed to those in the
908   second request by applying any of the following:
909   <list style="symbols">
910      <t>
911         adding or removing whitespace, where allowed in the header field's syntax
912      </t>
913      <t>
914         combining multiple header fields with the same field name
915         (see Section 3.2 of <xref target="Part1"/>)
916      </t>
917      <t>
918         normalizing both header field values in a way that is known to have
919         identical semantics, according to the header field's specification (e.g.,
920         re-ordering field values when order is not significant;
921         case-normalization, where values are defined to be case-insensitive)
922      </t>
923  </list>
924</t>
925<t>
926   If (after any normalization that might take place) a header field is absent
927   from a request, it can only match another request if it is also absent
928   there.
929</t>
930<t>
931   A Vary header field-value of "*" always fails to match, and subsequent
932   requests to that resource can only be properly interpreted by the origin
933   server.
934</t>
935<t>
936   The stored response with matching selecting header fields is known as the
937   selected response.
938</t>
939<t>
940   If multiple selected responses are available, the most recent response
941   (as determined by the Date header field) is used; see <xref target="constructing.responses.from.caches"/>.
942</t>
943<t>
944   If no selected response is available, the cache MAY forward the presented
945   request to the origin server in a conditional request; see <xref target="validation.model"/>.
946</t>
947</section>
948
949<section anchor="combining.responses" title="Combining Responses">
950<t>
951   When a cache receives a 304 (Not Modified) response or a 206 (Partial
952   Content) response (in this section, the "new" response"), it needs to
953   create an updated response by combining the stored response with the new
954   one, so that the updated response can be used to satisfy the request, and
955   potentially update the cached response.
956</t>
957<t>
958   If the new response contains an ETag, it identifies the stored response to
959   use. <cref anchor="TODO-mention-CL">might need language about
960   Content-Location here</cref><cref anchor="TODO-select-for-combine">Shouldn't this be the selected
961   response?</cref>
962</t>
963<t>
964   When the new response's status code is 206 (partial content), a cache
965   MUST NOT combine it with the old response if either response does not
966   have a validator, and MUST NOT combine it with the old response when
967   those validators do not match with the strong comparison function
968   (see Section 2.2.2 of <xref target="Part4"/>).
969</t>
970<t>
971   The stored response header fields are used as those of the updated response,
972   except that
973   <list style="symbols">
974      <t>a cache MUST delete any stored Warning header fields with warn-code 1xx (see <xref target="header.warning"/>).</t>
975      <t>a cache MUST retain any stored Warning header fields with warn-code 2xx.</t>
976      <t>a cache MUST use other header fields provided in the new response to replace all
977      instances of the corresponding header fields from the stored response.</t>
978   </list>
979</t>
980<t>
981   A cache MUST use the updated response header fields to replace those of the stored
982   response (unless the stored response is removed). In
983   the case of a 206 response, a cache MAY store the combined representation.
984</t>
985</section>
986
987</section>
988
989<section anchor="header.fields" title="Header Field Definitions">
990<t>
991   This section defines the syntax and semantics of HTTP/1.1 header fields
992   related to caching.
993</t>
994
995<section anchor="header.age" title="Age">
996   <iref item="Age header field" primary="true"/>
997   <iref item="Header Fields" primary="true" subitem="Age"/>
998   
999   
1000<t>
1001   The "Age" header field conveys the sender's estimate of the amount
1002   of time since the response was generated or successfully validated at the
1003   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
1004</t>
1005<figure><iref primary="true" item="Grammar" subitem="Age"/><artwork type="abnf2616"><![CDATA[
1006  Age = delta-seconds
1007]]></artwork></figure>
1008<t>
1009  Age field-values are non-negative integers, representing time in seconds
1010  (see <xref target="delta-seconds"/>).
1011</t>
1012<t>
1013   The presence of an Age header field in a response implies that a response
1014   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
1015   might not implement the Age header field.
1016</t>
1017</section>
1018
1019<section anchor="header.cache-control" title="Cache-Control">
1020   <iref item="Cache-Control header field" primary="true"/>
1021   <iref item="Header Fields" primary="true" subitem="Cache-Control"/>
1022   
1023   
1024   
1025   
1026   
1027<t>
1028   The "Cache-Control" header field is used to specify directives for
1029   caches along the request/response chain. Such cache directives are
1030   unidirectional in that the presence of a directive in a request does not
1031   imply that the same directive is to be given in the response.
1032</t>
1033<t>
1034   A cache MUST obey the requirements of the Cache-Control
1035   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1036   directives defined elsewhere are handled.
1037</t>
1038<t><list>
1039   <t>
1040       Note: HTTP/1.0 caches might not implement Cache-Control and
1041       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1042   </t>
1043</list></t>
1044<t>
1045   A proxy, whether or not it implements a cache, MUST pass cache directives
1046   through in forwarded messages, regardless of their
1047   significance to that application, since the directives might be applicable
1048   to all recipients along the request/response chain. It is not possible to
1049   target a directive to a specific cache.
1050</t>
1051<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
1052  Cache-Control   = 1#cache-directive
1053
1054  cache-directive = cache-request-directive
1055     / cache-response-directive
1056
1057  cache-extension = token [ "=" ( token / quoted-string ) ]
1058]]></artwork></figure>
1059
1060<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1061   
1062
1063<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
1064  cache-request-directive =
1065       "no-cache"
1066     / "no-store"
1067     / "max-age" "=" delta-seconds
1068     / "max-stale" [ "=" delta-seconds ]
1069     / "min-fresh" "=" delta-seconds
1070     / "no-transform"
1071     / "only-if-cached"
1072     / cache-extension
1073]]></artwork></figure>
1074
1075<t>
1076   <?rfc needLines="4"?>no-cache
1077   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1078   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1079   <list>
1080      <t>The no-cache request directive indicates that a cache MUST NOT
1081      use a stored response to satisfy the request without successful
1082      validation on the origin server.</t> 
1083   </list>
1084</t>
1085<t>
1086   <?rfc needLines="4"?>no-store
1087   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1088   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1089   <list>
1090      <t>The no-store request directive indicates that a cache MUST NOT
1091      store any part of either this request or any response to it. This
1092      directive applies to both private and shared caches. "MUST NOT
1093      store" in this context means that the cache MUST NOT intentionally
1094      store the information in non-volatile storage, and MUST make a
1095      best-effort attempt to remove the information from volatile storage as
1096      promptly as possible after forwarding it.</t>
1097      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1098      privacy. In particular, malicious or compromised caches might not
1099      recognize or obey this directive, and communications networks might be
1100      vulnerable to eavesdropping.</t>
1101      <t>Note that if a request containing this directive is satisfied from a
1102      cache, the no-store request directive does not apply to the already
1103      stored response.</t>
1104   </list>
1105</t>
1106<t>
1107   <?rfc needLines="4"?>max-age
1108   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1109   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1110   <list>
1111      <t>The max-age request directive indicates that the client is unwilling to
1112      accept a response whose age is greater than the specified number of
1113      seconds. Unless the max-stale request directive is also present, the
1114      client is not willing to accept a stale response.</t>
1115   </list>
1116</t>
1117<t>
1118   <?rfc needLines="4"?>max-stale
1119   <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1120   <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1121   <list>
1122      <t>The max-stale request directive indicates that the client is willing
1123      to accept a response that has exceeded its expiration time. If max-stale
1124      is assigned a value, then the client is willing to accept a response
1125      that has exceeded its expiration time by no more than the specified
1126      number of seconds. If no value is assigned to max-stale, then the client
1127      is willing to accept a stale response of any age.</t>
1128   </list>
1129</t>
1130<t>
1131   <?rfc needLines="4"?>min-fresh
1132   <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1133   <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1134   <list>
1135      <t>The min-fresh request directive indicates that the client is willing
1136      to accept a response whose freshness lifetime is no less than its
1137      current age plus the specified time in seconds. That is, the client
1138      wants a response that will still be fresh for at least the specified
1139      number of seconds.</t>
1140   </list>
1141</t>
1142<t>
1143   <?rfc needLines="4"?>no-transform
1144   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1145   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1146   <list>
1147      <t>The no-transform request directive indicates that an intermediary
1148        (whether or not it implements a cache) MUST NOT change the
1149        Content-Encoding, Content-Range or Content-Type request header fields,
1150        nor the request representation.</t>
1151   </list>
1152</t>
1153<t>
1154   <?rfc needLines="4"?>only-if-cached
1155   <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1156   <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1157   <list>
1158      <t>The only-if-cached request directive indicates that the client only
1159      wishes to obtain a stored response. If it receives this directive, a
1160      cache SHOULD either respond using a stored response that is consistent
1161      with the other constraints of the request, or respond with a 504
1162      (Gateway Timeout) status code. If a group of caches is being operated as
1163      a unified system with good internal connectivity, a member cache MAY
1164      forward such a request within that group of caches.</t>
1165   </list>
1166</t>
1167</section>
1168
1169<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1170   
1171
1172<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1173  cache-response-directive =
1174       "public"
1175     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1176     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1177     / "no-store"
1178     / "no-transform"
1179     / "must-revalidate"
1180     / "proxy-revalidate"
1181     / "max-age" "=" delta-seconds
1182     / "s-maxage" "=" delta-seconds
1183     / cache-extension
1184]]></artwork></figure>
1185
1186<t>
1187   <?rfc needLines="4"?>public
1188   <iref item="Cache Directives" primary="true" subitem="public"/>
1189   <iref item="public" primary="true" subitem="Cache Directive"/>
1190   <list>
1191      <t>The public response directive indicates that a response whose
1192        associated request contains an 'Authentication' header MAY be
1193        stored (see <xref target="caching.authenticated.responses"/>).</t>
1194  </list>
1195</t>
1196<t>
1197   <?rfc needLines="4"?>private
1198   <iref item="Cache Directives" primary="true" subitem="private"/>
1199   <iref item="private" primary="true" subitem="Cache Directive"/>
1200   <list>
1201      <t>The private response directive indicates that the response message is
1202      intended for a single user and MUST NOT be stored by a shared cache. A
1203      private cache MAY store the response.</t>
1204      <t>If the private response directive specifies one or more field-names,
1205      this requirement is limited to the field-values associated with the
1206      listed response header fields. That is, a shared cache MUST NOT store
1207      the specified field-names(s), whereas it MAY store the remainder of the
1208      response message.</t>
1209      <t> Note: This usage of the word private only controls where
1210      the response can be stored; it cannot ensure the privacy of the message
1211      content. Also, private response directives with field-names are often
1212      handled by implementations as if an unqualified private directive was
1213      received; i.e., the special handling for the qualified form is not
1214      widely implemented.</t>
1215   </list>
1216</t>
1217<t>
1218   <?rfc needLines="4"?>no-cache
1219   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1220   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1221   <list>
1222      <t>The no-cache response directive indicates that the response MUST NOT
1223      be used to satisfy a subsequent request without successful validation on
1224      the origin server. This allows an origin server to prevent a cache from
1225      using it to satisfy a request without contacting it, even by caches that
1226      have been configured to return stale responses.</t>
1227      <t>If the no-cache response directive specifies one or more field-names,
1228      this requirement is limited to the field-values associated with the
1229      listed response header fields. That is, a cache MUST NOT send the
1230      specified field-name(s) in the response to a subsequent request without successful
1231      validation on the origin server. This allows an origin server to prevent
1232      the re-use of certain header fields in a response, while still allowing
1233      caching of the rest of the response.</t>
1234      <t> Note: Most HTTP/1.0 caches will not recognize or obey
1235      this directive. Also, no-cache response directives with field-names are
1236      often handled by implementations as if an unqualified no-cache directive
1237      was received; i.e., the special handling for the qualified form is not
1238      widely implemented. </t>
1239   </list>
1240</t>
1241<t>
1242   <?rfc needLines="4"?>no-store
1243   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1244   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1245   <list>
1246      <t>The no-store response directive indicates that a cache MUST NOT
1247      store any part of either the immediate request or response. This
1248      directive applies to both private and shared caches. "MUST NOT
1249      store" in this context means that the cache MUST NOT intentionally
1250      store the information in non-volatile storage, and MUST make a
1251      best-effort attempt to remove the information from volatile storage as
1252      promptly as possible after forwarding it.</t>
1253      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1254      privacy. In particular, malicious or compromised caches might not
1255      recognize or obey this directive, and communications networks might be
1256      vulnerable to eavesdropping.</t>
1257   </list>
1258</t>
1259<t>
1260   <?rfc needLines="4"?>must-revalidate
1261   <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1262   <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1263   <list>
1264      <t>The must-revalidate response directive indicates that once it has
1265      become stale, a cache MUST NOT use the response to satisfy subsequent
1266      requests without successful validation on the origin server.</t>
1267      <t>The must-revalidate directive is necessary to support reliable
1268      operation for certain protocol features. In all circumstances a
1269      cache MUST obey the must-revalidate directive; in particular,
1270      if a cache cannot reach the origin server for any reason, it MUST
1271      generate a 504 (Gateway Timeout) response.</t>
1272      <t>A server SHOULD send the must-revalidate directive if and only if
1273      failure to validate a request on the representation could result in
1274      incorrect operation, such as a silently unexecuted financial
1275      transaction.</t>
1276   </list>
1277</t>
1278<t>
1279   <?rfc needLines="4"?>proxy-revalidate
1280   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1281   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1282   <list>
1283      <t>The proxy-revalidate response directive has the same meaning as the
1284      must-revalidate response directive, except that it does not apply to
1285      private caches.</t>
1286   </list>
1287</t>
1288<t>
1289   <?rfc needLines="4"?>max-age
1290   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1291   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1292   <list>
1293      <t>The max-age response directive indicates that the response is to be
1294      considered stale after its age is greater than the specified number of
1295      seconds.</t>
1296   </list>
1297</t>
1298<t>
1299   <?rfc needLines="4"?>s-maxage
1300   <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1301   <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1302   <list>
1303      <t>The s-maxage response directive indicates that, in shared caches, the
1304      maximum age specified by this directive overrides the maximum age
1305      specified by either the max-age directive or the Expires header field. The
1306      s-maxage directive also implies the semantics of the proxy-revalidate
1307      response directive.</t>
1308   </list>
1309</t>
1310<t>
1311   <?rfc needLines="4"?>no-transform
1312   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1313   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1314   <list>
1315      <t>The no-transform response directive indicates that an intermediary
1316      (regardless of whether it implements a cache) MUST NOT change the
1317      Content-Encoding, Content-Range or Content-Type response header fields,
1318      nor the response representation.</t>
1319   </list>
1320</t>
1321
1322</section>
1323
1324<section anchor="cache.control.extensions" title="Cache Control Extensions">
1325<t>
1326   The Cache-Control header field can be extended through the use of one or
1327   more cache-extension tokens, each with an optional value. Informational
1328   extensions (those that do not require a change in cache behavior) can be
1329   added without changing the semantics of other directives. Behavioral
1330   extensions are designed to work by acting as modifiers to the existing base
1331   of cache directives. Both the new directive and the standard directive are
1332   supplied, such that applications that do not understand the new directive
1333   will default to the behavior specified by the standard directive, and those
1334   that understand the new directive will recognize it as modifying the
1335   requirements associated with the standard directive. In this way,
1336   extensions to the cache-control directives can be made without requiring
1337   changes to the base protocol.
1338</t>
1339<t>
1340   This extension mechanism depends on an HTTP cache obeying all of the
1341   cache-control directives defined for its native HTTP-version, obeying
1342   certain extensions, and ignoring all directives that it does not
1343   understand.
1344</t>
1345<t>
1346   For example, consider a hypothetical new response directive called
1347   "community" that acts as a modifier to the private directive. We define
1348   this new directive to mean that, in addition to any private cache, any
1349   cache that is shared only by members of the community named within its
1350   value may cache the response. An origin server wishing to allow the UCI
1351   community to use an otherwise private response in their shared cache(s)
1352   could do so by including
1353</t>
1354<figure><artwork type="example"><![CDATA[
1355  Cache-Control: private, community="UCI"
1356]]></artwork></figure>
1357<t>
1358   A cache seeing this header field will act correctly even if the cache does
1359   not understand the community cache-extension, since it will also see and
1360   understand the private directive and thus default to the safe behavior.
1361</t>
1362<t>
1363   A cache MUST ignore unrecognized cache directives; it is assumed that any
1364   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1365   combined with standard directives (or the response's default cacheability)
1366   such that the cache behavior will remain minimally correct even if the
1367   cache does not understand the extension(s).
1368</t>
1369<t>
1370   The HTTP Cache Directive Registry defines the name space for the cache
1371   directives.
1372</t>
1373<t>
1374   A registration MUST include the following fields:
1375   <list style="symbols">
1376      <t>Cache Directive Name</t>
1377      <t>Pointer to specification text</t>
1378   </list>
1379</t>
1380<t>
1381   Values to be added to this name space are subject to IETF review (<xref target="RFC5226"/>, Section 4.1).
1382</t>
1383<t>
1384   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1385</t>
1386</section>
1387
1388</section>
1389
1390<section anchor="header.expires" title="Expires">
1391   <iref item="Expires header field" primary="true"/>
1392   <iref item="Header Fields" primary="true" subitem="Expires"/>
1393   
1394<t>
1395   The "Expires" header field gives the date/time after which the
1396   response is considered stale. See <xref target="expiration.model"/> for
1397   further discussion of the freshness model.
1398</t>
1399<t>
1400   The presence of an Expires field does not imply that the original resource
1401   will change or cease to exist at, before, or after that time.
1402</t>
1403<t>
1404   The field-value is an absolute date and time as defined by HTTP-date in
1405   Section 6.1 of <xref target="Part1"/>; a sender MUST use the rfc1123-date format.
1406</t>
1407<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
1408  Expires = HTTP-date
1409]]></artwork></figure>
1410<figure>
1411  <preamble>For example</preamble>
1412<artwork type="example"><![CDATA[
1413  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1414]]></artwork></figure>
1415<t><list>
1416   <t>
1417       Note: If a response includes a Cache-Control field with the
1418       max-age directive (see <xref target="cache-response-directive"/>),
1419       that directive overrides the Expires field. Likewise, the s-maxage
1420       directive overrides Expires in shared caches.
1421   </t>
1422</list></t>
1423<t>
1424   A server SHOULD NOT send Expires dates more than one year in the
1425   future.
1426</t>
1427<t>
1428   A cache MUST treat other invalid date formats,
1429   especially including the value "0", as in the past (i.e., "already
1430   expired").
1431</t>
1432</section>
1433
1434<section anchor="header.pragma" title="Pragma">
1435   <iref item="Pragma header field" primary="true"/>
1436   <iref item="Header Fields" primary="true" subitem="Pragma"/>
1437   
1438   
1439   
1440<t>
1441   The "Pragma" header field allows backwards compatibility with HTTP/1.0
1442   caches, so that clients can specify a "no-cache" request that they will
1443   understand (as Cache-Control was not defined until HTTP/1.1). When the
1444   Cache-Control header is also present and understood in a request, Pragma is
1445   ignored.
1446</t>
1447<t>
1448   In HTTP/1.0, Pragma was defined as an extensible field for
1449   implementation-specified directives for recipients. This specification
1450   deprecates such extensions to improve interoperability.
1451</t>
1452<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[
1453  Pragma           = 1#pragma-directive
1454  pragma-directive = "no-cache" / extension-pragma
1455  extension-pragma = token [ "=" ( token / quoted-string ) ]
1456]]></artwork></figure>
1457<t>
1458   When the Cache-Control header is not present in a request, the no-cache
1459   request pragma-directive MUST have the same effect on caches as if
1460   "Cache-Control: no-cache" were present (see <xref target="cache-request-directive"/>).
1461</t>
1462<t>
1463   When sending a no-cache request, a client SHOULD include both pragma and
1464   cache-control directives unless Cache-Control: no-cache is purposefully
1465   omitted to target other Cache-Control response directives at HTTP/1.1
1466   caches. For example:
1467</t>
1468<figure>
1469<artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
1470  GET / HTTP/1.1
1471  Host: www.example.com
1472  Cache-Control: max-age=30
1473  Pragma: no-cache
1474 
1475  ]]></artwork>
1476</figure>
1477<t>
1478   will constrain HTTP/1.1 caches to serve a response no older than 30
1479   seconds, while precluding implementations that do not understand
1480   Cache-Control from serving a cached response.
1481</t>
1482<t><list>
1483   <t>
1484      Note: Because the meaning of "Pragma: no-cache" in responses is not
1485      specified, it does not provide a reliable replacement for
1486      "Cache-Control: no-cache" in them.
1487   </t>
1488</list></t>
1489</section>
1490
1491<section anchor="header.vary" title="Vary">
1492   <iref item="Vary header field" primary="true"/>
1493   <iref item="Header Fields" primary="true" subitem="Vary"/>
1494   
1495<t>
1496   The "Vary" header field conveys the set of header fields
1497   that were used to select the representation.
1498</t>
1499<t>
1500   Caches use this information, in part, to determine whether a stored
1501   response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>. determines, while the response is
1502   fresh, whether a cache is permitted to use the response to reply to a
1503   subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1504</t>
1505<t>
1506   In uncacheable or stale responses, the Vary field value advises the user
1507   agent about the criteria that were used to select the representation.
1508</t>
1509<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
1510  Vary = "*" / 1#field-name
1511]]></artwork></figure>
1512<t>
1513   The set of header fields named by the Vary field value is known as the
1514   selecting header fields.
1515</t>
1516<t>
1517   A server SHOULD include a Vary header field with any cacheable response
1518   that is subject to server-driven negotiation. Doing so allows a cache to
1519   properly interpret future requests on that resource and informs the user
1520   agent about the presence of negotiation on that resource. A server MAY
1521   include a Vary header field with a non-cacheable response that is subject
1522   to server-driven negotiation, since this might provide the user agent with
1523   useful information about the dimensions over which the response varies at
1524   the time of the response.
1525</t>
1526<t>
1527   A Vary field value of "*" signals that unspecified parameters not limited
1528   to the header fields (e.g., the network address of the client), play a
1529   role in the selection of the response representation; therefore, a cache
1530   cannot determine whether this response is appropriate. A proxy MUST NOT
1531   generate the "*" value.
1532</t>
1533<t>
1534   The field-names given are not limited to the set of standard header
1535   fields defined by this specification. Field names are case-insensitive.
1536</t>
1537</section>
1538
1539<section anchor="header.warning" title="Warning">
1540   <iref item="Warning header field" primary="true"/>
1541   <iref item="Header Fields" primary="true" subitem="Warning"/>
1542   
1543   
1544   
1545   
1546   
1547   
1548<t>
1549   The "Warning" header field is used to carry additional information
1550   about the status or transformation of a message that might not be reflected
1551   in the message. This information is typically used to warn about possible
1552   incorrectness introduced by caching operations or transformations applied
1553   to the payload of the message.
1554</t>
1555<t>
1556   Warnings can be used for other purposes, both cache-related and otherwise.
1557   The use of a warning, rather than an error status code, distinguishes these
1558   responses from true failures.
1559</t>
1560<t>
1561   Warning header fields can in general be applied to any message, however some
1562   warn-codes are specific to caches and can only be applied to response
1563   messages.
1564</t>
1565<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[
1566  Warning       = 1#warning-value
1567 
1568  warning-value = warn-code SP warn-agent SP warn-text
1569                                        [SP warn-date]
1570 
1571  warn-code  = 3DIGIT
1572  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1573                  ; the name or pseudonym of the server adding
1574                  ; the Warning header field, for use in debugging
1575  warn-text  = quoted-string
1576  warn-date  = DQUOTE HTTP-date DQUOTE
1577]]></artwork></figure>
1578<t>
1579   Multiple warnings can be attached to a response (either by the origin
1580   server or by a cache), including multiple warnings with the same code
1581   number, only differing in warn-text.
1582</t>
1583<t>
1584   When this occurs, the user agent SHOULD inform the user of as many of
1585   them as possible, in the order that they appear in the response.
1586</t>
1587<t>
1588   Systems that generate multiple Warning header fields SHOULD order them with
1589   this user agent behavior in mind. New Warning header fields SHOULD be added
1590   after any existing Warning headers fields.
1591</t>
1592<t>
1593   Warnings are assigned three digit warn-codes. The first digit indicates
1594   whether the Warning is required to be deleted from a stored response after
1595   validation:
1596   <list style="symbols">
1597      <t>1xx Warnings describe the freshness or validation status of the
1598      response, and so MUST be deleted by a cache after validation. They can
1599      only be generated by a cache when validating a cached entry, and
1600      MUST NOT be generated in any other situation.</t>
1601      <t>2xx Warnings describe some aspect of the representation that is not
1602      rectified by a validation (for example, a lossy compression of the
1603      representation) and MUST NOT be deleted by a cache after validation,
1604      unless a full response is returned, in which case they MUST be.</t>
1605   </list>
1606</t>
1607<t>
1608   If an implementation sends a message with one or more Warning header fields to a
1609   receiver whose version is HTTP/1.0 or lower, then the sender MUST include
1610   in each warning-value a warn-date that matches the Date header field in the
1611   message.
1612</t>
1613<t>
1614   If a system receives a message with a warning-value that includes
1615   a warn-date, and that warn-date is different from the Date value in the
1616   response, then that warning-value MUST be deleted from the message before
1617   storing, forwarding, or using it. (preventing the consequences of naive
1618   caching of Warning header fields.) If all of the warning-values are deleted
1619   for this reason, the Warning header field MUST be deleted as well.
1620</t>
1621<t>
1622   The following warn-codes are defined by this specification, each with a
1623   recommended warn-text in English, and a description of its meaning.
1624</t>
1625<t>
1626<?rfc needLines="4"?>
1627   110 Response is stale
1628   <list>
1629      <t>A cache SHOULD include this whenever the returned response is stale.</t>
1630   </list>
1631</t>
1632<t>
1633<?rfc needLines="4"?>
1634   111 Revalidation failed
1635   <list>
1636      <t>A cache SHOULD include this when returning a stale response because an
1637      attempt to validate the response failed, due to an inability to reach
1638      the server.</t>
1639   </list>
1640</t>
1641<t>
1642<?rfc needLines="4"?>
1643   112 Disconnected operation
1644   <list>
1645      <t>A cache SHOULD b include this if it is intentionally disconnected from
1646      the rest of the network for a period of time.</t>
1647   </list>
1648</t>
1649<t>
1650<?rfc needLines="4"?>
1651   113 Heuristic expiration
1652   <list>
1653      <t>A cache SHOULD include this if it heuristically chose a freshness
1654      lifetime greater than 24 hours and the response's age is greater than 24
1655      hours.</t>
1656   </list>
1657</t>
1658<t>
1659<?rfc needLines="4"?>
1660   199 Miscellaneous warning
1661   <list>
1662      <t>The warning text can include arbitrary information to be presented to
1663      a human user, or logged. A system receiving this warning MUST NOT take
1664      any automated action, besides presenting the warning to the user.</t>
1665   </list>
1666</t>
1667<t>
1668<?rfc needLines="4"?>
1669   214 Transformation applied
1670   <list>
1671      <t>MUST be added by a proxy if it applies any
1672      transformation to the representation, such as changing the
1673      content-coding, media-type, or modifying the representation data, unless
1674      this Warning code already appears in the response.</t>
1675   </list>
1676</t>
1677<t>
1678<?rfc needLines="4"?>
1679   299 Miscellaneous persistent warning
1680   <list>
1681      <t>The warning text can include arbitrary information to be presented to
1682      a human user, or logged. A system receiving this warning MUST NOT take
1683      any automated action.</t>
1684   </list>
1685</t>
1686</section>
1687
1688</section>
1689
1690<section anchor="history.lists" title="History Lists">
1691<t>
1692   User agents often have history mechanisms, such as "Back" buttons and
1693   history lists, that can be used to redisplay a representation retrieved
1694   earlier in a session.
1695</t>
1696<t>
1697   The freshness model (<xref target="expiration.model"/>) does not
1698   necessarily apply to history mechanisms. I.e., a history mechanism can
1699   display a previous representation even if it has expired.
1700</t>
1701<t>
1702   This does not prohibit the history mechanism from telling the user that a
1703   view might be stale, or from honoring cache directives (e.g.,
1704   Cache-Control: no-store).
1705</t>
1706</section>
1707
1708
1709<section anchor="IANA.considerations" title="IANA Considerations">
1710
1711<section title="Cache Directive Registry" anchor="cache.directive.registration">
1712<t>
1713   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1714</t>
1715<t>
1716   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1717   populated with the registrations below:
1718</t>
1719
1720<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1721<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1722   <ttcol>Cache Directive</ttcol>
1723   <ttcol>Reference</ttcol>
1724
1725   <c>max-age</c>
1726   <c>
1727      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1728   </c>
1729   <c>max-stale</c>
1730   <c>
1731      <xref target="cache-request-directive"/>
1732   </c>
1733   <c>min-fresh</c>
1734   <c>
1735      <xref target="cache-request-directive"/>
1736   </c>
1737   <c>must-revalidate</c>
1738   <c>
1739      <xref target="cache-response-directive"/>
1740   </c>
1741   <c>no-cache</c>
1742   <c>
1743      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1744   </c>
1745   <c>no-store</c>
1746   <c>
1747      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1748   </c>
1749   <c>no-transform</c>
1750   <c>
1751      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1752   </c>
1753   <c>only-if-cached</c>
1754   <c>
1755      <xref target="cache-request-directive"/>
1756   </c>
1757   <c>private</c>
1758   <c>
1759      <xref target="cache-response-directive"/>
1760   </c>
1761   <c>proxy-revalidate</c>
1762   <c>
1763      <xref target="cache-response-directive"/>
1764   </c>
1765   <c>public</c>
1766   <c>
1767      <xref target="cache-response-directive"/>
1768   </c>
1769   <c>s-maxage</c>
1770   <c>
1771      <xref target="cache-response-directive"/>
1772   </c>
1773   <c>stale-if-error</c>
1774   <c>
1775      <xref target="RFC5861"/>, Section 4
1776   </c>
1777   <c>stale-while-revalidate</c>
1778   <c>
1779      <xref target="RFC5861"/>, Section 3
1780   </c>
1781</texttable>
1782<!--(END)-->
1783
1784</section>
1785
1786<section title="Header Field Registration" anchor="header.field.registration">
1787<t>
1788  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1789  shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1790</t>
1791
1792<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1793<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1794   <ttcol>Header Field Name</ttcol>
1795   <ttcol>Protocol</ttcol>
1796   <ttcol>Status</ttcol>
1797   <ttcol>Reference</ttcol>
1798
1799   <c>Age</c>
1800   <c>http</c>
1801   <c>standard</c>
1802   <c>
1803      <xref target="header.age"/>
1804   </c>
1805   <c>Cache-Control</c>
1806   <c>http</c>
1807   <c>standard</c>
1808   <c>
1809      <xref target="header.cache-control"/>
1810   </c>
1811   <c>Expires</c>
1812   <c>http</c>
1813   <c>standard</c>
1814   <c>
1815      <xref target="header.expires"/>
1816   </c>
1817   <c>Pragma</c>
1818   <c>http</c>
1819   <c>standard</c>
1820   <c>
1821      <xref target="header.pragma"/>
1822   </c>
1823   <c>Vary</c>
1824   <c>http</c>
1825   <c>standard</c>
1826   <c>
1827      <xref target="header.vary"/>
1828   </c>
1829   <c>Warning</c>
1830   <c>http</c>
1831   <c>standard</c>
1832   <c>
1833      <xref target="header.warning"/>
1834   </c>
1835</texttable>
1836<!--(END)-->
1837
1838<t>
1839   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1840   Force".
1841</t>
1842</section>
1843
1844</section>
1845
1846<section anchor="security.considerations" title="Security Considerations">
1847<t>
1848   Caches expose additional potential vulnerabilities, since the contents of
1849   the cache represent an attractive target for malicious exploitation.
1850   Because cache contents persist after an HTTP request is complete, an attack
1851   on the cache can reveal information long after a user believes that the
1852   information has been removed from the network. Therefore, cache contents
1853   need to be protected as sensitive information.
1854</t>
1855</section>
1856
1857<section anchor="ack" title="Acknowledgments">
1858<t>
1859   Much of the content and presentation of the caching design is due to
1860   suggestions and comments from individuals including: Shel Kaphan, Paul
1861   Leach, Koen Holtman, David Morris, and Larry Masinter.
1862</t>
1863</section>
1864
1865</middle>
1866
1867<back>
1868<references title="Normative References">
1869
1870  <reference anchor="Part1">
1871    <front>
1872      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1873      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1874        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1875        <address><email>fielding@gbiv.com</email></address>
1876      </author>
1877      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1878        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1879        <address><email>jg@freedesktop.org</email></address>
1880      </author>
1881      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1882        <organization abbrev="HP">Hewlett-Packard Company</organization>
1883        <address><email>JeffMogul@acm.org</email></address>
1884      </author>
1885      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1886        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1887        <address><email>henrikn@microsoft.com</email></address>
1888      </author>
1889      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1890        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1891        <address><email>LMM@acm.org</email></address>
1892      </author>
1893      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1894        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1895        <address><email>paulle@microsoft.com</email></address>
1896      </author>
1897      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1898        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1899        <address><email>timbl@w3.org</email></address>
1900      </author>
1901      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1902        <organization abbrev="W3C">World Wide Web Consortium</organization>
1903        <address><email>ylafon@w3.org</email></address>
1904      </author>
1905      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1906        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1907        <address><email>julian.reschke@greenbytes.de</email></address>
1908      </author>
1909      <date month="July" year="2011"/>
1910    </front>
1911    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-15"/>
1912   
1913  </reference>
1914
1915  <reference anchor="Part2">
1916    <front>
1917      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1918      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1919        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1920        <address><email>fielding@gbiv.com</email></address>
1921      </author>
1922      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1923        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1924        <address><email>jg@freedesktop.org</email></address>
1925      </author>
1926      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1927        <organization abbrev="HP">Hewlett-Packard Company</organization>
1928        <address><email>JeffMogul@acm.org</email></address>
1929      </author>
1930      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1931        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1932        <address><email>henrikn@microsoft.com</email></address>
1933      </author>
1934      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1935        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1936        <address><email>LMM@acm.org</email></address>
1937      </author>
1938      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1939        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1940        <address><email>paulle@microsoft.com</email></address>
1941      </author>
1942      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1943        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1944        <address><email>timbl@w3.org</email></address>
1945      </author>
1946      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1947        <organization abbrev="W3C">World Wide Web Consortium</organization>
1948        <address><email>ylafon@w3.org</email></address>
1949      </author>
1950      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1951        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1952        <address><email>julian.reschke@greenbytes.de</email></address>
1953      </author>
1954      <date month="July" year="2011"/>
1955    </front>
1956    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-15"/>
1957   
1958  </reference>
1959
1960  <reference anchor="Part4">
1961    <front>
1962      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1963      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1964        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1965        <address><email>fielding@gbiv.com</email></address>
1966      </author>
1967      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1968        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1969        <address><email>jg@freedesktop.org</email></address>
1970      </author>
1971      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1972        <organization abbrev="HP">Hewlett-Packard Company</organization>
1973        <address><email>JeffMogul@acm.org</email></address>
1974      </author>
1975      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1976        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1977        <address><email>henrikn@microsoft.com</email></address>
1978      </author>
1979      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1980        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1981        <address><email>LMM@acm.org</email></address>
1982      </author>
1983      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1984        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1985        <address><email>paulle@microsoft.com</email></address>
1986      </author>
1987      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1988        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1989        <address><email>timbl@w3.org</email></address>
1990      </author>
1991      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1992        <organization abbrev="W3C">World Wide Web Consortium</organization>
1993        <address><email>ylafon@w3.org</email></address>
1994      </author>
1995      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1996        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1997        <address><email>julian.reschke@greenbytes.de</email></address>
1998      </author>
1999      <date month="July" year="2011"/>
2000    </front>
2001    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-15"/>
2002   
2003  </reference>
2004
2005  <reference anchor="Part5">
2006    <front>
2007      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2008      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2009        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2010        <address><email>fielding@gbiv.com</email></address>
2011      </author>
2012      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2013        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2014        <address><email>jg@freedesktop.org</email></address>
2015      </author>
2016      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2017        <organization abbrev="HP">Hewlett-Packard Company</organization>
2018        <address><email>JeffMogul@acm.org</email></address>
2019      </author>
2020      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2021        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2022        <address><email>henrikn@microsoft.com</email></address>
2023      </author>
2024      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2025        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2026        <address><email>LMM@acm.org</email></address>
2027      </author>
2028      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2029        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2030        <address><email>paulle@microsoft.com</email></address>
2031      </author>
2032      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2033        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2034        <address><email>timbl@w3.org</email></address>
2035      </author>
2036      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2037        <organization abbrev="W3C">World Wide Web Consortium</organization>
2038        <address><email>ylafon@w3.org</email></address>
2039      </author>
2040      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2041        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2042        <address><email>julian.reschke@greenbytes.de</email></address>
2043      </author>
2044      <date month="July" year="2011"/>
2045    </front>
2046    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-15"/>
2047   
2048  </reference>
2049
2050  <reference anchor="Part7">
2051    <front>
2052      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2053      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2054        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2055        <address><email>fielding@gbiv.com</email></address>
2056      </author>
2057      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2058        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2059        <address><email>jg@freedesktop.org</email></address>
2060      </author>
2061      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2062        <organization abbrev="HP">Hewlett-Packard Company</organization>
2063        <address><email>JeffMogul@acm.org</email></address>
2064      </author>
2065      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2066        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2067        <address><email>henrikn@microsoft.com</email></address>
2068      </author>
2069      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2070        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2071        <address><email>LMM@acm.org</email></address>
2072      </author>
2073      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2074        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2075        <address><email>paulle@microsoft.com</email></address>
2076      </author>
2077      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2078        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2079        <address><email>timbl@w3.org</email></address>
2080      </author>
2081      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2082        <organization abbrev="W3C">World Wide Web Consortium</organization>
2083        <address><email>ylafon@w3.org</email></address>
2084      </author>
2085      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2086        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2087        <address><email>julian.reschke@greenbytes.de</email></address>
2088      </author>
2089      <date month="July" year="2011"/>
2090    </front>
2091    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-15"/>
2092   
2093  </reference>
2094
2095  <reference anchor="RFC2119">
2096    <front>
2097      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2098      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2099        <organization>Harvard University</organization>
2100        <address><email>sob@harvard.edu</email></address>
2101      </author>
2102      <date month="March" year="1997"/>
2103    </front>
2104    <seriesInfo name="BCP" value="14"/>
2105    <seriesInfo name="RFC" value="2119"/>
2106  </reference>
2107
2108  <reference anchor="RFC5234">
2109    <front>
2110      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2111      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2112        <organization>Brandenburg InternetWorking</organization>
2113        <address>
2114          <email>dcrocker@bbiw.net</email>
2115        </address> 
2116      </author>
2117      <author initials="P." surname="Overell" fullname="Paul Overell">
2118        <organization>THUS plc.</organization>
2119        <address>
2120          <email>paul.overell@thus.net</email>
2121        </address>
2122      </author>
2123      <date month="January" year="2008"/>
2124    </front>
2125    <seriesInfo name="STD" value="68"/>
2126    <seriesInfo name="RFC" value="5234"/>
2127  </reference>
2128 
2129</references>
2130
2131<references title="Informative References">
2132
2133  <reference anchor="RFC1305">
2134    <front>
2135      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2136      <author fullname="David L. Mills" initials="D." surname="Mills">
2137        <organization>University of Delaware, Electrical Engineering Department</organization>
2138        <address><email>mills@udel.edu</email></address>
2139      </author>
2140      <date month="March" year="1992"/>
2141    </front>
2142    <seriesInfo name="RFC" value="1305"/>
2143  </reference>
2144
2145  <reference anchor="RFC2616">
2146    <front>
2147      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2148      <author fullname="R. Fielding" initials="R." surname="Fielding">
2149        <organization>University of California, Irvine</organization>
2150        <address><email>fielding@ics.uci.edu</email></address>
2151      </author>
2152      <author fullname="J. Gettys" initials="J." surname="Gettys">
2153        <organization>W3C</organization>
2154        <address><email>jg@w3.org</email></address>
2155      </author>
2156      <author fullname="J. Mogul" initials="J." surname="Mogul">
2157        <organization>Compaq Computer Corporation</organization>
2158        <address><email>mogul@wrl.dec.com</email></address>
2159      </author>
2160      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2161        <organization>MIT Laboratory for Computer Science</organization>
2162        <address><email>frystyk@w3.org</email></address>
2163      </author>
2164      <author fullname="L. Masinter" initials="L." surname="Masinter">
2165        <organization>Xerox Corporation</organization>
2166        <address><email>masinter@parc.xerox.com</email></address>
2167      </author>
2168      <author fullname="P. Leach" initials="P." surname="Leach">
2169        <organization>Microsoft Corporation</organization>
2170        <address><email>paulle@microsoft.com</email></address>
2171      </author>
2172      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2173        <organization>W3C</organization>
2174        <address><email>timbl@w3.org</email></address>
2175      </author>
2176      <date month="June" year="1999"/>
2177    </front>
2178    <seriesInfo name="RFC" value="2616"/>
2179  </reference>
2180
2181  <reference anchor="RFC3864">
2182    <front>
2183      <title>Registration Procedures for Message Header Fields</title>
2184      <author fullname="G. Klyne" initials="G." surname="Klyne">
2185        <organization>Nine by Nine</organization>
2186        <address><email>GK-IETF@ninebynine.org</email></address>
2187      </author>
2188      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2189        <organization>BEA Systems</organization>
2190        <address><email>mnot@pobox.com</email></address>
2191      </author>
2192      <author fullname="J. Mogul" initials="J." surname="Mogul">
2193        <organization>HP Labs</organization>
2194        <address><email>JeffMogul@acm.org</email></address>
2195      </author>
2196      <date month="September" year="2004"/>
2197    </front>
2198    <seriesInfo name="BCP" value="90"/>
2199    <seriesInfo name="RFC" value="3864"/>
2200  </reference>
2201
2202  <reference anchor="RFC5226">
2203    <front>
2204      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2205      <author initials="T." surname="Narten" fullname="T. Narten">
2206        <organization>IBM</organization>
2207        <address><email>narten@us.ibm.com</email></address>
2208      </author>
2209      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2210        <organization>Google</organization>
2211        <address><email>Harald@Alvestrand.no</email></address>
2212      </author>
2213      <date year="2008" month="May"/>
2214    </front>
2215    <seriesInfo name="BCP" value="26"/>
2216    <seriesInfo name="RFC" value="5226"/>
2217  </reference>
2218
2219  <reference anchor="RFC5861">
2220    <front>
2221      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2222      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2223        <organization>Yahoo! Inc.</organization>
2224        <address><email>mnot@yahoo-inc.com</email></address>
2225      </author>
2226      <date month="April" year="2010"/>
2227    </front>
2228    <seriesInfo name="RFC" value="5861"/>
2229  </reference>
2230
2231</references>
2232
2233<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2234<t>
2235  Make the specified age calculation algorithm less conservative.
2236  (<xref target="age.calculations"/>)
2237</t>
2238<t>
2239  Remove requirement to consider Content-Location in successful responses
2240  in order to determine the appropriate response to use.
2241  (<xref target="validation.model"/>)
2242</t>
2243<t>
2244  Clarify denial of service attack avoidance requirement.
2245  (<xref target="invalidation.after.updates.or.deletions"/>)
2246</t>
2247<t>
2248  Change ABNF productions for header fields to only define the field value.
2249  (<xref target="header.fields"/>)
2250</t>
2251<t>
2252  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2253  anymore, as these aspects never were implemented.
2254  (<xref target="header.warning"/>)
2255</t>
2256</section>
2257
2258
2259<section title="Collected ABNF" anchor="collected.abnf">
2260<figure>
2261<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2262Age = delta-seconds
2263
2264Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
2265 cache-directive ] )
2266
2267Expires = HTTP-date
2268
2269HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2270
2271OWS = <OWS, defined in [Part1], Section 1.2.2>
2272
2273Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2274 pragma-directive ] )
2275
2276Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
2277 ) )
2278
2279Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
2280 )
2281
2282cache-directive = cache-request-directive / cache-response-directive
2283cache-extension = token [ "=" ( token / quoted-string ) ]
2284cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2285 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2286 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2287 cache-extension
2288cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2289 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2290 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2291 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2292 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2293 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2294
2295delta-seconds = 1*DIGIT
2296
2297extension-pragma = token [ "=" ( token / quoted-string ) ]
2298
2299field-name = <field-name, defined in [Part1], Section 3.2>
2300
2301port = <port, defined in [Part1], Section 2.7>
2302pragma-directive = "no-cache" / extension-pragma
2303pseudonym = <pseudonym, defined in [Part1], Section 9.9>
2304
2305quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
2306
2307token = <token, defined in [Part1], Section 1.2.2>
2308
2309uri-host = <uri-host, defined in [Part1], Section 2.7>
2310
2311warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2312warn-code = 3DIGIT
2313warn-date = DQUOTE HTTP-date DQUOTE
2314warn-text = quoted-string
2315warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2316 ]
2317]]></artwork>
2318</figure>
2319<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2320; Age defined but not used
2321; Cache-Control defined but not used
2322; Expires defined but not used
2323; Pragma defined but not used
2324; Vary defined but not used
2325; Warning defined but not used
2326]]></artwork></figure></section>
2327
2328
2329<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2330
2331<section title="Since RFC 2616">
2332  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2333</section>
2334
2335<section title="Since draft-ietf-httpbis-p6-cache-00">
2336<t>
2337  Closed issues:
2338  <list style="symbols">
2339    <t>
2340      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2341    <t>
2342      <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>
2343    <t>
2344      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2345    <t>
2346      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2347    <t>
2348      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2349    <t>
2350      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2351    <t>
2352      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2353    <t>
2354      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2355    <t>
2356      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2357  </list>
2358</t>
2359<t>
2360  Other changes:
2361  <list style="symbols">
2362    <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>
2363  </list>
2364</t>
2365</section>
2366
2367<section title="Since draft-ietf-httpbis-p6-cache-01">
2368<t>
2369  Closed issues:
2370  <list style="symbols">
2371    <t>
2372      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2373  </list>
2374</t>
2375<t>
2376  Other changes:
2377  <list style="symbols">
2378    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2379      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2380    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2381      specification.</t>
2382  </list>
2383</t>
2384</section>
2385
2386<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2387<t>
2388  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2389  <list style="symbols">
2390    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2391      document.</t>
2392  </list>
2393</t>
2394</section>
2395
2396<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2397<t>
2398  Closed issues:
2399  <list style="symbols">
2400    <t>
2401      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2402  </list>
2403</t>
2404</section>
2405
2406<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2407<t>
2408  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2409  <list style="symbols"> 
2410    <t>
2411      Use "/" instead of "|" for alternatives.
2412    </t>
2413    <t>
2414      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2415      whitespace ("OWS") and required whitespace ("RWS").
2416    </t>
2417    <t>
2418      Rewrite ABNFs to spell out whitespace rules, factor out
2419      header field value format definitions.
2420    </t>
2421  </list>
2422</t>
2423</section>
2424
2425<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2426<t>
2427  This is a total rewrite of this part of the specification.
2428</t>
2429<t>
2430  Affected issues:
2431  <list style="symbols">
2432    <t>
2433      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2434    <t>
2435      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2436    <t>
2437      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2438    <t>
2439      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2440  </list>
2441</t>
2442<t>
2443  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2444  <list style="symbols"> 
2445    <t>
2446      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2447    </t>
2448  </list>
2449</t>
2450</section>
2451
2452<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2453<t>
2454  Closed issues:
2455  <list style="symbols"> 
2456    <t>
2457      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2458      "base for numeric protocol elements"
2459    </t>
2460  </list>
2461</t>
2462<t>
2463  Affected issues:
2464  <list style="symbols">
2465    <t>
2466      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2467      "Vary and non-existant headers"
2468    </t>
2469  </list>
2470</t>
2471</section>
2472
2473<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2474<t>
2475  Closed issues:
2476  <list style="symbols"> 
2477    <t>
2478      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
2479      "Definition of 1xx Warn-Codes"
2480    </t>
2481    <t>
2482      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2483      "Content-Location on 304 responses"
2484    </t>
2485    <t>
2486      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
2487      "private and no-cache CC directives with headers"
2488    </t>
2489    <t>
2490      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2491      "RFC2047 and warn-text"
2492    </t>
2493  </list>
2494</t>
2495</section>
2496
2497<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2498<t>
2499  Closed issues:
2500  <list style="symbols"> 
2501    <t>
2502      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
2503      "serving negotiated responses from cache: header-specific canonicalization"
2504    </t>
2505    <t>
2506      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
2507      "Effect of CC directives on history lists"
2508    </t>
2509    <t>
2510      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/291"/>:
2511      "Cache Extensions can override no-store, etc."
2512    </t>
2513  </list>
2514</t>
2515<t>
2516  Affected issues:
2517  <list style="symbols">
2518    <t>
2519      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2520      Status codes and caching
2521    </t>
2522  </list>
2523</t>
2524<t>
2525  Partly resolved issues:
2526  <list style="symbols"> 
2527    <t>
2528      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2529      "Placement of 13.5.1 and 13.5.2"
2530    </t>
2531  </list>
2532</t>
2533</section>
2534
2535<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2536<t>
2537  Closed issues:
2538  <list style="symbols"> 
2539    <t>
2540      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
2541      "Age calculation"
2542    </t>
2543    <t>
2544      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
2545      "Clarify differences between / requirements for request and response CC directives"
2546    </t>
2547    <t>
2548      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
2549      "Caching authenticated responses"
2550    </t>
2551    <t>
2552      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
2553      "IANA registry for cache-control directives"
2554    </t>
2555    <t>
2556      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
2557      "Heuristic caching of URLs with query components"
2558    </t>
2559  </list>
2560</t>
2561<t>
2562  Partly resolved issues:
2563  <list style="symbols"> 
2564    <t>
2565      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2566      "Term for the requested resource's URI"
2567    </t>
2568  </list>
2569</t>
2570</section>
2571
2572<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2573<t>
2574  Closed issues:
2575  <list style="symbols"> 
2576    <t>
2577      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2578      "Clarify entity / representation / variant terminology"
2579    </t>
2580    <t>
2581      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2582      "consider removing the 'changes from 2068' sections"
2583    </t>
2584    <t>
2585      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2586      "Allowing heuristic caching for new status codes"
2587    </t>
2588    <t>
2589      Clean up TODOs and prose in "Combining Responses."
2590    </t>
2591  </list>
2592</t>
2593</section>
2594
2595<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2596<t>
2597  Closed issues:
2598  <list style="symbols"> 
2599    <t>
2600      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2601      "Text about clock requirement for caches belongs in p6"
2602    </t>
2603  </list>
2604</t>
2605</section>
2606
2607<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2608<t>
2609  Closed issues:
2610  <list style="symbols"> 
2611    <t>
2612      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2613      "Header Classification"
2614    </t>
2615    <t>
2616      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2617      "Clarify 'public'"
2618    </t>
2619  </list>
2620</t>
2621</section>
2622
2623<section title="Since draft-ietf-httpbis-p6-cache-13" anchor="changes.since.13">
2624<t>
2625  Closed issues:
2626  <list style="symbols">
2627    <t>
2628      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
2629      "untangle ABNFs for header fields"
2630    </t>
2631  </list>
2632</t>
2633</section>
2634
2635<section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14">
2636<t>
2637  Closed issues:
2638  <list style="symbols">
2639    <t>
2640      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/38"/>:
2641      "Mismatch Vary"
2642    </t>
2643    <t>
2644      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/235"/>:
2645      "Cache Invalidation only happens upon successful responses"
2646    </t>
2647    <t>
2648      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/282"/>:
2649      "Recommend minimum sizes for protocol elements"
2650    </t>
2651    <t>
2652      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/289"/>:
2653      "Proxies don't 'understand' methods"
2654    </t>
2655    <t>
2656      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>:
2657      "Pragma"
2658    </t>
2659  </list>
2660</t>
2661</section>
2662
2663</section>
2664  </back>
2665</rfc>
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