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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-13" 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="March" year="2011" day="14"/>
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). The current issues list is at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
184      documents (including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/"/>.
185   </t>
186   <t>
187      The changes in this draft are summarized in <xref target="changes.since.12"/>.
188   </t>
189</note>
190
191   </front>
192   <middle>
193
194<section anchor="caching" title="Introduction">
195<t>
196   HTTP is typically used for distributed information systems, where
197   performance can be improved by the use of response caches. This document
198   defines aspects of HTTP/1.1 related to caching and reusing response
199   messages.
200</t>
201
202<section anchor="intro.purpose" title="Purpose">
203<iref item="cache"/>
204<t>
205   An HTTP cache is a local store of response messages and the
206   subsystem that controls its message storage, retrieval, and deletion. A
207   cache stores cacheable responses in order to reduce the response time and
208   network bandwidth consumption on future, equivalent requests. Any client or
209   server MAY employ a cache, though a cache cannot be used by a server that
210   is acting as a tunnel.
211</t>
212<t>
213   Caching would be useless if it did not significantly improve performance.
214   The goal of caching in HTTP/1.1 is to reuse a prior response message to
215   satisfy a current request. In some cases, a stored response can be reused
216   without the need for a network request, reducing latency and network
217   round-trips; a "freshness" mechanism is used for this purpose (see <xref target="expiration.model"/>). Even when a new request is required, it is
218   often possible to reuse all or parts of the payload of a prior response to
219   satisfy the request, thereby reducing network bandwidth usage; a
220   "validation" mechanism is used for this purpose (see <xref target="validation.model"/>).
221</t>
222</section>
223
224<section anchor="intro.terminology" title="Terminology">
225<t>
226   This specification uses a number of terms to refer to the roles played by
227   participants in, and objects of, HTTP caching.
228</t>
229<t>
230   <iref item="cache"/>
231   <?rfc needLines="4"?>cache
232   <list>
233      <t>A conformant implementation of a HTTP cache. Note that this implies
234        an HTTP/1.1 cache; this specification does not define conformance
235        for HTTP/1.0 caches.</t>
236   </list>
237</t>
238<t anchor="shared.and.non-shared.caches">
239   <iref item="shared cache"/>
240   <?rfc needLines="4"?>shared cache
241   <list>
242      <t>A cache that is accessible to more than one user; usually (but
243        not always) deployed as part of an intermediary.</t>
244   </list>
245</t>
246<t>
247   <iref item="private cache"/>
248   <?rfc needLines="4"?>private cache
249   <list>
250      <t>A cache that is dedicated to a single user.</t>
251   </list>
252</t>
253<t>
254   <iref item="cacheable"/>
255   <?rfc needLines="4"?>cacheable
256   <list>
257      <t>A response is cacheable if a cache is allowed to store a copy of the
258      response message for use in answering subsequent requests. Even when a
259      response is cacheable, there might be additional constraints on whether
260      a cache can use the stored copy to satisfy a particular request.</t>
261   </list>
262</t>
263<t>
264   <iref item="explicit expiration time"/>
265   <?rfc needLines="4"?>explicit expiration time
266   <list>
267      <t>The time at which the origin server intends that a representation
268      no longer be returned by a cache without further validation.</t>
269   </list>
270</t>
271<t>
272   <iref item="heuristic expiration time"/>
273   <?rfc needLines="4"?>heuristic expiration time
274   <list>
275      <t>An expiration time assigned by a cache when no explicit expiration
276      time is available.</t>
277   </list>
278</t>
279<t>
280   <iref item="age"/>
281   <?rfc needLines="4"?>age
282   <list>
283      <t>The age of a response is the time since it was sent by, or
284      successfully validated with, the origin server.</t>
285   </list>
286</t>
287<t>
288   <iref item="first-hand"/>
289   <?rfc needLines="4"?>first-hand
290   <list>
291      <t>A response is first-hand if the freshness model is not in use; i.e.,
292      its age is 0.</t>
293   </list>
294</t>
295<t>
296   <iref item="freshness lifetime"/>
297   <?rfc needLines="4"?>freshness lifetime
298   <list>
299      <t>The length of time between the generation of a response and its
300      expiration time.</t>
301   </list>
302</t>
303<t>
304   <iref item="fresh"/>
305   <?rfc needLines="4"?>fresh
306   <list>
307      <t>A response is fresh if its age has not yet exceeded its freshness
308      lifetime.</t>
309   </list>
310</t>
311<t>
312   <iref item="stale"/>
313   <?rfc needLines="4"?>stale
314   <list>
315      <t>A response is stale if its age has passed its freshness lifetime
316      (either explicit or heuristic).</t>
317   </list>
318</t>
319<t>
320   <iref item="validator"/>
321   <?rfc needLines="4"?>validator
322   <list>
323      <t>A protocol element (e.g., an entity-tag or a Last-Modified time) that
324      is used to find out whether a stored response is an equivalent copy of
325      a representation.</t>
326   </list>
327</t>
328</section>
329
330<section anchor="intro.requirements" title="Requirements">
331<t>
332   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
333   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
334   document are to be interpreted as described in <xref target="RFC2119"/>.
335</t>
336<t>
337   An implementation is not compliant if it fails to satisfy one or more of
338   the "MUST" or "REQUIRED" level requirements for the protocols it
339   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
340   level and all the "SHOULD" level requirements for its protocols is said to
341   be "unconditionally compliant"; one that satisfies all the "MUST" level
342   requirements but not all the "SHOULD" level requirements for its protocols
343   is said to be "conditionally compliant".
344</t>
345</section>
346
347<section title="Syntax Notation" anchor="notation">
348   
349   
350   
351   
352   
353   
354   
355   
356   
357<t>
358   This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
359   extends the syntax defined in <xref target="RFC5234"/> with a list rule).
360   <xref target="collected.abnf"/> shows the collected ABNF, with the list
361   rule expanded.
362</t>
363<t>
364   The following core rules are included by reference, as defined in <xref target="RFC5234"/>, Appendix B.1: ALPHA (letters), CR (carriage
365   return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
366   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
367   sequence of data), SP (space), VCHAR (any visible USASCII character), and
368   WSP (whitespace).
369</t>
370
371<section title="Core Rules" anchor="core.rules">
372   
373   
374   
375<t>
376   The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
377</t>
378<figure><artwork type="abnf2616"><![CDATA[
379  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
380  token         = <token, defined in [Part1], Section 1.2.2>
381  OWS           = <OWS, defined in [Part1], Section 1.2.2>
382]]></artwork></figure>
383</section>
384
385<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
386   
387   
388   
389   
390   
391<t>
392   The ABNF rules below are defined in other parts:
393</t>
394<figure><artwork type="abnf2616"><![CDATA[
395  field-name    = <field-name, defined in [Part1], Section 3.2>
396  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
397  port          = <port, defined in [Part1], Section 2.6>
398  pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
399  uri-host      = <uri-host, defined in [Part1], Section 2.6>
400]]></artwork></figure>
401</section>
402
403</section>
404</section>
405
406<section anchor="caching.overview" title="Cache Operation">
407
408<section anchor="response.cacheability" title="Response Cacheability">
409<t>
410   A cache MUST NOT store a response to any request, unless:
411   <list style="symbols">
412      <t>The request method is understood by the cache and defined as being
413      cacheable, and</t>
414      <t>the response status code is understood by the cache, and</t>
415      <t>the "no-store" cache directive (see <xref target="header.cache-control"/>) does not appear in request or response
416      header fields, and</t>
417      <t>the "private" cache response directive (see <xref target="cache-response-directive"/> does not appear in the response, if
418      the cache is shared, and</t>
419      <t>the "Authorization" header field (see Section 4.1 of <xref target="Part7"/>) does not
420      appear in the request, if the cache is shared, unless the response
421      explicitly allows it (see <xref target="caching.authenticated.responses"/>), and</t>
422      <t>the response either:
423         <list style="symbols">
424            <t>contains an Expires header field (see <xref target="header.expires"/>), or</t>
425            <t>contains a max-age response cache directive (see <xref target="cache-response-directive"/>), or</t>
426            <t>contains a s-maxage response cache directive and the cache is
427            shared, or</t>
428            <t>contains a Cache Control Extension (see <xref target="cache.control.extensions"/>) that allows it to be cached,
429            or</t>
430            <t>has a status code that can be served with heuristic freshness
431            (see <xref target="heuristic.freshness"/>).</t>
432         </list>
433      </t>
434   </list>
435</t>
436<t>
437   In this context, a cache has "understood" a request method or a response
438   status code if it recognises it and implements any cache-specific
439   behaviour. In particular, 206 Partial Content responses cannot be cached by
440   an implementation that does not handle partial content (see <xref target="errors.or.incomplete.response.cache.behavior"/>).
441</t>
442<t>
443   Note that in normal operation, most caches will not store a response that
444   has neither a cache validator nor an explicit expiration time, as such
445   responses are not usually useful to store. However, caches are not
446   prohibited from storing such responses.
447</t>
448
449<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
450<t>
451   A cache that receives an incomplete response (for example, with fewer bytes
452   of data than specified in a Content-Length header field) can store the response,
453   but MUST treat it as a partial response <xref target="Part5"/>. Partial responses can
454   be combined as described in Section 4 of <xref target="Part5"/>; the result might be a
455   full response or might still be partial. A cache MUST NOT return a
456   partial response to a client without explicitly marking it as such using
457   the 206 (Partial Content) status code.
458</t>
459<t>
460   A cache that does not support the Range and Content-Range header fields
461   MUST NOT store incomplete or partial responses.
462</t>
463</section>
464
465</section>
466
467
468<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
469<t>
470   For a presented request, a cache MUST NOT return a stored response,
471   unless:
472   <list style="symbols">
473      <t>The presented effective request URI (Section 4.3 of <xref target="Part1"/>) and
474      that of the stored response match, and</t>
475      <t>the request method associated with the stored response allows it to
476      be used for the presented request, and</t>
477      <t>selecting header fields nominated by the stored response (if any)
478      match those presented (see <xref target="caching.negotiated.responses"/>), and</t>
479      <t>the presented request and stored response are free from directives
480      that would prevent its use (see <xref target="header.cache-control"/>
481      and <xref target="header.pragma"/>), and</t>
482      <t>the stored response is either:
483         <list style="symbols">
484            <t>fresh (see <xref target="expiration.model"/>), or</t>
485            <t>allowed to be served stale (see <xref target="serving.stale.responses"/>), or</t>
486            <t>successfully validated (see <xref target="validation.model"/>).</t>
487         </list>
488      </t>
489  </list>
490</t>
491<t>
492   When a stored response is used to satisfy a request without validation,
493   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
494   current_age; see <xref target="age.calculations"/>.
495</t>
496<t>
497   A cache MUST write through requests with methods that are unsafe
498   (Section 7.1.1 of <xref target="Part2"/>) to the origin server; i.e., a cache must not generate
499   a reply to such a request before having forwarded the request and having
500   received a corresponding response.
501</t>
502<t>
503   Also, note that unsafe requests might invalidate already stored responses;
504   see <xref target="invalidation.after.updates.or.deletions"/>.
505</t>
506<t>
507   A cache MUST use the most recent response (as determined by the Date
508   header field) when more than one suitable response is stored. It can also
509   forward a request with "Cache-Control: max-age=0" or "Cache-Control:
510   no-cache" to disambiguate which response to use.
511</t>
512<t>
513   A cache that does not have a clock available MUST NOT used stored responses
514   without revalidating them on every use. A cache, especially a shared
515   cache, SHOULD use a mechanism, such as NTP <xref target="RFC1305"/>, to
516   synchronize its clock with a reliable external standard.
517</t>
518
519</section>
520
521<section anchor="expiration.model" title="Freshness Model">
522<t>
523   When a response is "fresh" in the cache, it can be used to satisfy
524   subsequent requests without contacting the origin server, thereby improving
525   efficiency.
526</t>
527<t>
528   The primary mechanism for determining freshness is for an origin server to
529   provide an explicit expiration time in the future, using either the Expires
530   header field (<xref target="header.expires"/>) or the max-age response cache
531   directive (<xref target="cache-response-directive"/>). Generally, origin
532   servers will assign future explicit expiration times to responses in the
533   belief that the representation is not likely to change in a semantically
534   significant way before the expiration time is reached.
535</t>
536<t>
537   If an origin server wishes to force a cache to validate every request, it
538   can assign an explicit expiration time in the past to indicate that the
539   response is already stale. Compliant caches will normally validate the
540   cached response before reusing it for subsequent requests (see <xref target="serving.stale.responses"/>).
541</t>
542<t>
543   Since origin servers do not always provide explicit expiration times,
544   a cache MAY assign a heuristic expiration time when an explicit time is not
545   specified, employing algorithms that use other header field values (such as the
546   Last-Modified time) to estimate a plausible expiration time. This
547   specification does not provide specific algorithms, but does impose
548   worst-case constraints on their results.
549</t>
550<figure>
551<preamble>
552  The calculation to determine if a response is fresh is:
553</preamble>
554<artwork type="code"><![CDATA[
555   response_is_fresh = (freshness_lifetime > current_age)
556]]></artwork>
557</figure>
558<t>
559   The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime"/>; the current_age is defined in
560   <xref target="age.calculations"/>.
561</t>
562<t>
563   Additionally, clients might need to influence freshness calculation. They
564   can do this using several request cache directives, with the effect of
565   either increasing or loosening constraints on freshness. See <xref target="cache-request-directive"/>.
566</t>
567<t>
568   <cref anchor="ISSUE-no-req-for-directives">there are not requirements
569   directly applying to cache-request-directives and freshness.</cref>
570</t>
571<t>
572   Note that freshness applies only to cache operation; it cannot be used to
573   force a user agent to refresh its display or reload a resource. See <xref target="history.lists"/> for an explanation of the difference between
574   caches and history mechanisms.
575</t>
576
577<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
578<t>
579   A cache can calculate the freshness lifetime (denoted as
580   freshness_lifetime) of a response by using the first match of:
581   <list style="symbols">
582      <t>If the cache is shared and the s-maxage response cache directive
583      (<xref target="cache-response-directive"/>) is present, use its value,
584      or</t>
585      <t>If the max-age response cache directive (<xref target="cache-response-directive"/>) is present, use its value, or</t>
586      <t>If the Expires response header field (<xref target="header.expires"/>) is
587      present, use its value minus the value of the Date response header field,
588      or</t>
589      <t>Otherwise, no explicit expiration time is present in the response. A
590      heuristic freshness lifetime might be applicable; see <xref target="heuristic.freshness"/>.</t>
591   </list>
592</t>
593<t>
594   Note that this calculation is not vulnerable to clock skew, since all of
595   the information comes from the origin server.
596</t>
597
598<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
599<t>
600   If no explicit expiration time is present in a stored response that has a
601   status code whose definition allows heuristic freshness to be used
602   (including the following in Section 8 of <xref target="Part2"/>: 200, 203, 206, 300, 301 and
603   410), a cache MAY calculate a heuristic expiration time. A cache MUST NOT
604   use heuristics to determine freshness for responses with status codes that do
605   not explicitly allow it.
606</t>
607<t>
608   When a heuristic is used to calculate freshness lifetime, a cache
609   SHOULD attach a Warning header field with a 113 warn-code to the response if
610   its current_age is more than 24 hours and such a warning is not already
611   present.
612</t>
613<t>
614   Also, if the response has a Last-Modified header field (Section 6.6 of <xref target="Part4"/>),
615   a cache SHOULD NOT use a heuristic expiration value that is more than some
616   fraction of the interval since that time. A typical setting of this fraction
617   might be 10%.
618</t>
619<t><list>
620   <t>
621      Note: RFC 2616 (<xref target="RFC2616"/>, Section 13.9) required that caches do not calculate heuristic
622      freshness for URIs with query components (i.e., those containing '?').
623      In practice, this has not been widely implemented. Therefore, servers
624      are encouraged to send explicit directives (e.g., Cache-Control:
625      no-cache) if they wish to preclude caching.
626   </t>
627</list></t>
628</section>
629</section>
630
631<section anchor="age.calculations" title="Calculating Age">
632<t>
633   HTTP/1.1 uses the Age header field to convey the estimated age of the
634   response message when obtained from a cache. The Age field value is the
635   cache's estimate of the amount of time since the response was generated or
636   validated by the origin server. In essence, the Age value is the sum of the
637   time that the response has been resident in each of the caches along the
638   path from the origin server, plus the amount of time it has been in transit
639   along network paths.
640</t>
641<t>
642   The following data is used for the age calculation:
643</t>
644<t>
645   <?rfc needLines="4"?>age_value
646   <list>
647      <t>
648         The term "age_value" denotes the value of the Age header field (<xref target="header.age"/>), in a form appropriate for arithmetic
649         operation; or 0, if not available.
650      </t>
651   </list>
652</t>
653<t>
654   <?rfc needLines="4"?>date_value
655   <list>
656      <t>
657         HTTP/1.1 requires origin servers to send a Date header field, if possible,
658         with every response, giving the time at which the response was
659         generated. The term "date_value" denotes the value of the Date
660         header field, in a form appropriate for arithmetic operations. See
661         Section 9.3 of <xref target="Part1"/> for the definition of the Date header field, and for
662         requirements regarding responses without it.
663      </t>
664   </list>
665</t>
666<t>
667   <?rfc needLines="4"?>now
668   <list>
669      <t>
670         The term "now" means "the current value of the clock at the host
671         performing the calculation". A cache SHOULD use NTP (<xref target="RFC1305"/>) or some similar protocol to synchronize its
672         clocks to a globally accurate time standard.
673      </t>
674   </list>
675</t>
676<t>
677   <?rfc needLines="4"?>request_time
678   <list>
679      <t>
680         The current value of the clock at the host at the time the request
681         resulting in the stored response was made.
682      </t>
683   </list>
684</t>
685<t>
686   <?rfc needLines="4"?>response_time
687   <list>
688      <t>
689         The current value of the clock at the host at the time the response
690         was received.
691      </t>
692   </list>
693</t>
694<t>
695   A response's age can be calculated in two entirely independent ways:
696   <list style="numbers">
697      <t>the "apparent_age": response_time minus date_value, if the local
698      clock is reasonably well synchronized to the origin server's clock. If
699      the result is negative, the result is replaced by zero.</t>
700      <t>the "corrected_age_value", if all of the caches along the response
701      path implement HTTP/1.1. A cache MUST interpret this value relative
702      to the time the request was initiated, not the time that the response
703      was received.</t>
704   </list>
705</t>
706<figure>
707<artwork type="code"><![CDATA[
708  apparent_age = max(0, response_time - date_value);
709
710  response_delay = response_time - request_time;
711  corrected_age_value = age_value + response_delay; 
712]]></artwork>
713</figure>
714<figure>
715<preamble>These are combined as</preamble>
716<artwork type="code"><![CDATA[
717  corrected_initial_age = max(apparent_age, corrected_age_value);
718]]></artwork></figure>
719<t>
720   The current_age of a stored response can then be calculated by adding the
721   amount of time (in seconds) since the stored response was last validated by
722   the origin server to the corrected_initial_age.
723</t>
724<figure><artwork type="code"><![CDATA[
725  resident_time = now - response_time;
726  current_age = corrected_initial_age + resident_time;
727]]></artwork></figure>
728</section>
729
730<section anchor="serving.stale.responses" title="Serving Stale Responses">
731<t>
732   A "stale" response is one that either has explicit expiry information or is
733   allowed to have heuristic expiry calculated, but is not fresh according to
734   the calculations in <xref target="expiration.model"/>.
735</t>
736<t>
737   A cache MUST NOT return a stale response if it is prohibited by an
738   explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
739   directive, a "must-revalidate" cache-response-directive, or an applicable
740   "s-maxage" or "proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
741</t>
742<t>
743   A cache SHOULD NOT return stale responses unless it is disconnected
744   (i.e., it cannot contact the origin server or otherwise find a forward
745   path) or otherwise explicitly allowed (e.g., the max-stale request
746   directive; see <xref target="cache-request-directive"/>).
747</t>
748<t>
749   A cache SHOULD append a Warning header field with the 110 warn-code (see
750   <xref target="header.warning"/>) to stale responses. Likewise, a cache
751   SHOULD add the 112 warn-code to stale responses if the cache is
752   disconnected.
753</t>
754<t>
755   If a cache receives a first-hand response (either an entire response, or a
756   304 (Not Modified) response) that it would normally forward to the
757   requesting client, and the received response is no longer fresh, the cache
758   SHOULD forward it to the requesting client without adding a new Warning
759   (but without removing any existing Warning header fields). A cache SHOULD NOT
760   attempt to validate a response simply because that response became stale in
761   transit.
762</t>
763</section>
764</section>
765
766<section anchor="validation.model" title="Validation Model">
767<t>
768   When a cache has one or more stored responses for a requested URI, but
769   cannot serve any of them (e.g., because they are not fresh, or one cannot
770   be selected; see <xref target="caching.negotiated.responses"/>), it can use
771   the conditional request mechanism <xref target="Part4"/> in the forwarded request to
772   give the origin server an opportunity to both select a valid stored
773   response to be used, and to update it. This process is known as
774   "validating" or "revalidating" the stored response.
775</t>
776<t>
777   When sending such a conditional request, a cache SHOULD add an
778   If-Modified-Since header field whose value is that of the Last-Modified header
779   field from the selected (see <xref target="caching.negotiated.responses"/>)
780   stored response, if available.
781</t>
782<t>
783   Additionally, a cache SHOULD add an If-None-Match header field whose value is
784   that of the ETag header field(s) from all responses stored for the requested URI,
785   if present. However, if any of the stored responses contains only partial
786   content, the cache SHOULD NOT include its entity-tag in the If-None-Match
787   header field unless the request is for a range that would be fully
788   satisfied by that stored response.
789</t>
790<t>
791   A 304 (Not Modified) response status code indicates that the stored
792   response can be updated and reused; see <xref target="combining.responses"/>.
793</t>
794<t>
795   A full response (i.e., one with a response body) indicates that none of the
796   stored responses nominated in the conditional request is suitable. Instead,
797   a cache SHOULD use the full response to satisfy the request and MAY
798   replace the stored response.
799</t>
800<t>
801   If a cache receives a 5xx response while attempting to validate a response,
802   it MAY either forward this response to the requesting client, or act as
803   if the server failed to respond. In the latter case, it MAY return a
804   previously stored response (see <xref target="serving.stale.responses"/>).
805</t>
806</section>
807
808<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
809<t>
810   Because unsafe request methods (Section 7.1.1 of <xref target="Part2"/>) have the potential for changing
811   state on the origin server, intervening caches can use them to keep their
812   contents up-to-date.
813</t>
814<t>
815   A cache MUST invalidate the effective Request URI
816   (Section 4.3 of <xref target="Part1"/>) as well as the URI(s) in the Location
817   and Content-Location header fields (if present) when the following
818   request methods are received:
819   <list style="symbols">
820      <t>PUT</t>
821      <t>DELETE</t>
822      <t>POST</t>
823   </list>
824</t>
825<t>
826   However, a cache MUST NOT invalidate a URI from a
827   Location or Content-Location header field if the host part of that URI
828   differs from the host part in the effective request URI
829   (Section 4.3 of <xref target="Part1"/>). This helps prevent denial of service attacks.
830</t>
831<t>
832   A cache that passes through requests with methods it does not understand
833   SHOULD invalidate the effective request URI (Section 4.3 of <xref target="Part1"/>).
834</t>
835<t>
836   Here, "invalidate" means that the cache will either remove all stored
837   responses related to the effective request URI, or will mark these as
838   "invalid" and in need of a mandatory validation before they can be returned
839   in response to a subsequent request.
840</t>
841<t>
842   Note that this does not guarantee that all appropriate responses are
843   invalidated. For example, the request that caused the change at the origin
844   server might not have gone through the cache where a response is stored.
845</t>
846</section>
847
848<section anchor="caching.authenticated.responses" title="Shared Caching of Authenticated Responses">
849
850<t>
851   A shared cache MUST NOT use a cached response to a request with an
852   Authorization header field (Section 4.1 of <xref target="Part7"/>) to satisfy any subsequent
853   request unless a cache directive that allows such responses to be stored is
854   present in the response.
855</t>
856
857<t>
858   In this specification, the following Cache-Control response directives
859   (<xref target="cache-response-directive"/>) have such an effect:
860   must-revalidate, public, s-maxage.
861</t>
862
863<t>
864   Note that cached responses that contain the "must-revalidate" and/or
865   "s-maxage" response directives are not allowed to be served stale (<xref target="serving.stale.responses"/>) by shared caches. In particular, a
866   response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be
867   used to satisfy a subsequent request without revalidating it on the origin
868   server.
869</t>
870</section>
871
872<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
873<t>
874   When a cache receives a request that can be satisfied by a stored response
875   that has a Vary header field (<xref target="header.vary"/>), it MUST NOT
876   use that response unless all of the selecting header fields nominated by
877   the Vary header field match in both the original request (i.e., that associated
878   with the stored response), and the presented request.
879</t>
880<t>
881   The selecting header fields from two requests are defined to match if and
882   only if those in the first request can be transformed to those in the
883   second request by applying any of the following:
884   <list style="symbols">
885      <t>
886         adding or removing whitespace, where allowed in the header field's syntax
887      </t>
888      <t>
889         combining multiple header fields with the same field name
890         (see Section 3.2 of <xref target="Part1"/>)
891      </t>
892      <t>
893         normalizing both header field values in a way that is known to have
894         identical semantics, according to the header field's specification (e.g.,
895         re-ordering field values when order is not significant;
896         case-normalization, where values are defined to be case-insensitive)
897      </t>
898  </list>
899</t>
900<t>
901   If (after any normalization that might take place) a header field is absent
902   from a request, it can only match another request if it is also absent
903   there.
904</t>
905<t>
906   A Vary header field-value of "*" always fails to match, and subsequent
907   requests to that resource can only be properly interpreted by the origin
908   server.
909</t>
910<t>
911   The stored response with matching selecting header fields is known as the
912   selected response.
913</t>
914<t>
915   If no selected response is available, the cache MAY forward the presented
916   request to the origin server in a conditional request; see <xref target="validation.model"/>.
917</t>
918</section>
919
920<section anchor="combining.responses" title="Combining Responses">
921<t>
922   When a cache receives a 304 (Not Modified) response or a 206 (Partial
923   Content) response (in this section, the "new" response"), it needs to
924   create an updated response by combining the stored response with the new
925   one, so that the updated response can be used to satisfy the request, and
926   potentially update the cached response.
927</t>
928<t>
929   If the new response contains an ETag, it identifies the stored response to
930   use. <cref anchor="TODO-mention-CL">might need language about
931   Content-Location here</cref><cref anchor="TODO-select-for-combine">Shouldn't this be the selected
932   response?</cref>
933</t>
934<t>
935   When the new response's status code is 206 (partial content), a cache
936   MUST NOT combine it with the old response if either response does not
937   have a validator, and MUST NOT combine it with the old response when
938   those validators do not match with the strong comparison function
939   (see Section 4 of <xref target="Part4"/>).
940</t>
941<t>
942   The stored response header fields are used as those of the updated response,
943   except that
944   <list style="symbols">
945      <t>a cache MUST delete any stored Warning header fields with warn-code 1xx (see <xref target="header.warning"/>).</t>
946      <t>a cache MUST retain any stored Warning header fields with warn-code 2xx.</t>
947      <t>a cache MUST use other header fields provided in the new response to replace all
948      instances of the corresponding header fields from the stored response.</t>
949   </list>
950</t>
951<t>
952   A cache MUST use the updated response header fields to replace those of the stored
953   response (unless the stored response is removed). In
954   the case of a 206 response, a cache MAY store the combined representation.
955</t>
956</section>
957
958</section>
959
960<section anchor="header.fields" title="Header Field Definitions">
961<t>
962   This section defines the syntax and semantics of HTTP/1.1 header fields
963   related to caching.
964</t>
965
966<section anchor="header.age" title="Age">
967   <iref item="Age header field" primary="true"/>
968   <iref item="Header Fields" primary="true" subitem="Age"/>
969   
970   
971   
972<t>
973   The "Age" header field conveys the sender's estimate of the amount
974   of time since the response was generated or successfully validated at the
975   origin server. Age values are calculated as specified in <xref target="age.calculations"/>.
976</t>
977<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/><artwork type="abnf2616"><![CDATA[
978  Age   = "Age" ":" OWS Age-v
979  Age-v = delta-seconds
980]]></artwork></figure>
981<t anchor="rule.delta-seconds">
982 
983  Age field-values are non-negative integers, representing time in seconds.
984</t>
985<figure><iref item="Grammar" primary="true" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
986  delta-seconds  = 1*DIGIT
987]]></artwork></figure>
988<t>
989   If a cache receives a value larger than the largest positive integer it can
990   represent, or if any of its age calculations overflows, it MUST transmit
991   an Age header field with a field-value of 2147483648 (2^31).
992   Recipients parsing the Age header field-value SHOULD use an arithmetic type of
993   at least 31 bits of range.
994</t>
995<t>
996   The presence of an Age header field in a response implies that a response
997   is not first-hand. However, the converse is not true, since HTTP/1.0 caches
998   might not implement the Age header field.
999</t>
1000</section>
1001
1002<section anchor="header.cache-control" title="Cache-Control">
1003   <iref item="Cache-Control header field" primary="true"/>
1004   <iref item="Header Fields" primary="true" subitem="Cache-Control"/>
1005   
1006   
1007   
1008   
1009   
1010   
1011<t>
1012   The "Cache-Control" header field is used to specify directives for
1013   caches along the request/response chain. Such cache directives are
1014   unidirectional in that the presence of a directive in a request does not
1015   imply that the same directive is to be given in the response.
1016</t>
1017<t>
1018   A cache MUST obey the requirements of the Cache-Control
1019   directives defined in this section. See <xref target="cache.control.extensions"/> for information about how Cache-Control
1020   directives defined elsewhere are handled.
1021</t>
1022<t><list>
1023   <t>
1024       Note: HTTP/1.0 caches might not implement Cache-Control and
1025       might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1026   </t>
1027</list></t>
1028<t>
1029   A proxy, whether or not it implements a cache, MUST pass cache directives
1030   through in forwarded messages, regardless of their
1031   significance to that application, since the directives might be applicable
1032   to all recipients along the request/response chain. It is not possible to
1033   target a directive to a specific cache.
1034</t>
1035<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
1036  Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
1037  Cache-Control-v = 1#cache-directive
1038
1039  cache-directive = cache-request-directive
1040     / cache-response-directive
1041
1042  cache-extension = token [ "=" ( token / quoted-string ) ]
1043]]></artwork></figure>
1044
1045<section anchor="cache-request-directive" title="Request Cache-Control Directives">
1046   
1047
1048<figure><iref item="Grammar" primary="true" subitem="cache-request-directive"/><artwork type="abnf2616"><![CDATA[
1049  cache-request-directive =
1050       "no-cache"
1051     / "no-store"
1052     / "max-age" "=" delta-seconds
1053     / "max-stale" [ "=" delta-seconds ]
1054     / "min-fresh" "=" delta-seconds
1055     / "no-transform"
1056     / "only-if-cached"
1057     / cache-extension
1058]]></artwork></figure>
1059
1060<t>
1061   <?rfc needLines="4"?>no-cache
1062   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1063   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1064   <list>
1065      <t>The no-cache request directive indicates that a cache MUST NOT
1066      use a stored response to satisfy the request without successful
1067      validation on the origin server.</t> 
1068   </list>
1069</t>
1070<t>
1071   <?rfc needLines="4"?>no-store
1072   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1073   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1074   <list>
1075      <t>The no-store request directive indicates that a cache MUST NOT
1076      store any part of either this request or any response to it. This
1077      directive applies to both private and shared caches. "MUST NOT
1078      store" in this context means that the cache MUST NOT intentionally
1079      store the information in non-volatile storage, and MUST make a
1080      best-effort attempt to remove the information from volatile storage as
1081      promptly as possible after forwarding it.</t>
1082      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1083      privacy. In particular, malicious or compromised caches might not
1084      recognize or obey this directive, and communications networks might be
1085      vulnerable to eavesdropping.</t>
1086      <t>Note that if a request containing this directive is satisfied from a
1087      cache, the no-store request directive does not apply to the already
1088      stored response.</t>
1089   </list>
1090</t>
1091<t>
1092   <?rfc needLines="4"?>max-age
1093   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1094   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1095   <list>
1096      <t>The max-age request directive indicates that the client is willing to
1097      accept a response whose age is no greater than the specified time in
1098      seconds. Unless the max-stale request directive is also present, the
1099      client is not willing to accept a stale response.</t>
1100   </list>
1101</t>
1102<t>
1103   <?rfc needLines="4"?>max-stale
1104   <iref item="Cache Directives" primary="true" subitem="max-stale"/>
1105   <iref item="max-stale" primary="true" subitem="Cache Directive"/>
1106   <list>
1107      <t>The max-stale request directive indicates that the client is willing
1108      to accept a response that has exceeded its expiration time. If max-stale
1109      is assigned a value, then the client is willing to accept a response
1110      that has exceeded its expiration time by no more than the specified
1111      number of seconds. If no value is assigned to max-stale, then the client
1112      is willing to accept a stale response of any age.</t>
1113   </list>
1114</t>
1115<t>
1116   <?rfc needLines="4"?>min-fresh
1117   <iref item="Cache Directives" primary="true" subitem="min-fresh"/>
1118   <iref item="min-fresh" primary="true" subitem="Cache Directive"/>
1119   <list>
1120      <t>The min-fresh request directive indicates that the client is willing
1121      to accept a response whose freshness lifetime is no less than its
1122      current age plus the specified time in seconds. That is, the client
1123      wants a response that will still be fresh for at least the specified
1124      number of seconds.</t>
1125   </list>
1126</t>
1127<t>
1128   <?rfc needLines="4"?>no-transform
1129   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1130   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1131   <list>
1132      <t>The no-transform request directive indicates that an intermediary
1133        (whether or not it implements a cache) MUST NOT change the
1134        Content-Encoding, Content-Range or Content-Type request header fields,
1135        nor the request representation.</t>
1136   </list>
1137</t>
1138<t>
1139   <?rfc needLines="4"?>only-if-cached
1140   <iref item="Cache Directives" primary="true" subitem="only-if-cached"/>
1141   <iref item="only-if-cached" primary="true" subitem="Cache Directive"/>
1142   <list>
1143      <t>The only-if-cached request directive indicates that the client only
1144      wishes to return a stored response. If it receives this directive, a
1145      cache SHOULD either respond using a stored response that is consistent
1146      with the other constraints of the request, or respond with a 504
1147      (Gateway Timeout) status code. If a group of caches is being operated as
1148      a unified system with good internal connectivity, a member cache MAY
1149      forward such a request within that group of caches.</t>
1150   </list>
1151</t>
1152</section>
1153
1154<section anchor="cache-response-directive" title="Response Cache-Control Directives">
1155   
1156
1157<figure><iref item="Grammar" primary="true" subitem="cache-response-directive"/><artwork type="abnf2616"><![CDATA[
1158  cache-response-directive =
1159       "public"
1160     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1161     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1162     / "no-store"
1163     / "no-transform"
1164     / "must-revalidate"
1165     / "proxy-revalidate"
1166     / "max-age" "=" delta-seconds
1167     / "s-maxage" "=" delta-seconds
1168     / cache-extension
1169]]></artwork></figure>
1170
1171<t>
1172   <?rfc needLines="4"?>public
1173   <iref item="Cache Directives" primary="true" subitem="public"/>
1174   <iref item="public" primary="true" subitem="Cache Directive"/>
1175   <list>
1176      <t>The public response directive indicates that a response whose
1177        associated request contains an 'Authentication' header MAY be
1178        stored (see <xref target="caching.authenticated.responses"/>).</t>
1179  </list>
1180</t>
1181<t>
1182   <?rfc needLines="4"?>private
1183   <iref item="Cache Directives" primary="true" subitem="private"/>
1184   <iref item="private" primary="true" subitem="Cache Directive"/>
1185   <list>
1186      <t>The private response directive indicates that the response message is
1187      intended for a single user and MUST NOT be stored by a shared cache. A
1188      private cache MAY store the response.</t>
1189      <t>If the private response directive specifies one or more field-names,
1190      this requirement is limited to the field-values associated with the
1191      listed response header fields. That is, a shared cache MUST NOT store
1192      the specified field-names(s), whereas it MAY store the remainder of the
1193      response message.</t>
1194      <t> Note: This usage of the word private only controls where
1195      the response can be stored; it cannot ensure the privacy of the message
1196      content. Also, private response directives with field-names are often
1197      handled by implementations as if an unqualified private directive was
1198      received; i.e., the special handling for the qualified form is not
1199      widely implemented.</t>
1200   </list>
1201</t>
1202<t>
1203   <?rfc needLines="4"?>no-cache
1204   <iref item="Cache Directives" primary="true" subitem="no-cache"/>
1205   <iref item="no-cache" primary="true" subitem="Cache Directive"/>
1206   <list>
1207      <t>The no-cache response directive indicates that the response MUST NOT
1208      be used to satisfy a subsequent request without successful validation on
1209      the origin server. This allows an origin server to prevent a cache from
1210      using it to satisfy a request without contacting it, even by caches that
1211      have been configured to return stale responses.</t>
1212      <t>If the no-cache response directive specifies one or more field-names,
1213      this requirement is limited to the field-values associated with the
1214      listed response header fields. That is, a cache MUST NOT send the
1215      specified field-name(s) in the response to a subsequent request without successful
1216      validation on the origin server. This allows an origin server to prevent
1217      the re-use of certain header fields in a response, while still allowing
1218      caching of the rest of the response.</t>
1219      <t> Note: Most HTTP/1.0 caches will not recognize or obey
1220      this directive. Also, no-cache response directives with field-names are
1221      often handled by implementations as if an unqualified no-cache directive
1222      was received; i.e., the special handling for the qualified form is not
1223      widely implemented. </t>
1224   </list>
1225</t>
1226<t>
1227   <?rfc needLines="4"?>no-store
1228   <iref item="Cache Directives" primary="true" subitem="no-store"/>
1229   <iref item="no-store" primary="true" subitem="Cache Directive"/>
1230   <list>
1231      <t>The no-store response directive indicates that a cache MUST NOT
1232      store any part of either the immediate request or response. This
1233      directive applies to both private and shared caches. "MUST NOT
1234      store" in this context means that the cache MUST NOT intentionally
1235      store the information in non-volatile storage, and MUST make a
1236      best-effort attempt to remove the information from volatile storage as
1237      promptly as possible after forwarding it.</t>
1238      <t>This directive is NOT a reliable or sufficient mechanism for ensuring
1239      privacy. In particular, malicious or compromised caches might not
1240      recognize or obey this directive, and communications networks might be
1241      vulnerable to eavesdropping.</t>
1242   </list>
1243</t>
1244<t>
1245   <?rfc needLines="4"?>must-revalidate
1246   <iref item="Cache Directives" primary="true" subitem="must-revalidate"/>
1247   <iref item="must-revalidate" primary="true" subitem="Cache Directive"/>
1248   <list>
1249      <t>The must-revalidate response directive indicates that once it has
1250      become stale, a cache MUST NOT use the response to satisfy subsequent
1251      requests without successful validation on the origin server.</t>
1252      <t>The must-revalidate directive is necessary to support reliable
1253      operation for certain protocol features. In all circumstances a
1254      cache MUST obey the must-revalidate directive; in particular,
1255      if a cache cannot reach the origin server for any reason, it MUST
1256      generate a 504 (Gateway Timeout) response.</t>
1257      <t>A server SHOULD send the must-revalidate directive if and only if
1258      failure to validate a request on the representation could result in
1259      incorrect operation, such as a silently unexecuted financial
1260      transaction.</t>
1261   </list>
1262</t>
1263<t>
1264   <?rfc needLines="4"?>proxy-revalidate
1265   <iref item="Cache Directives" primary="true" subitem="proxy-revalidate"/>
1266   <iref item="proxy-revalidate" primary="true" subitem="Cache Directive"/>
1267   <list>
1268      <t>The proxy-revalidate response directive has the same meaning as the
1269      must-revalidate response directive, except that it does not apply to
1270      private caches.</t>
1271   </list>
1272</t>
1273<t>
1274   <?rfc needLines="4"?>max-age
1275   <iref item="Cache Directives" primary="true" subitem="max-age"/>
1276   <iref item="max-age" primary="true" subitem="Cache Directive"/>
1277   <list>
1278      <t>The max-age response directive indicates that response is to be
1279      considered stale after its age is greater than the specified number of
1280      seconds.</t>
1281   </list>
1282</t>
1283<t>
1284   <?rfc needLines="4"?>s-maxage
1285   <iref item="Cache Directives" primary="true" subitem="s-maxage"/>
1286   <iref item="s-maxage" primary="true" subitem="Cache Directive"/>
1287   <list>
1288      <t>The s-maxage response directive indicates that, in shared caches, the
1289      maximum age specified by this directive overrides the maximum age
1290      specified by either the max-age directive or the Expires header field. The
1291      s-maxage directive also implies the semantics of the proxy-revalidate
1292      response directive.</t>
1293   </list>
1294</t>
1295<t>
1296   <?rfc needLines="4"?>no-transform
1297   <iref item="Cache Directives" primary="true" subitem="no-transform"/>
1298   <iref item="no-transform" primary="true" subitem="Cache Directive"/>
1299   <list>
1300      <t>The no-transform response directive indicates that an intermediary
1301      (regardless of whether it implements a cache) MUST NOT change the
1302      Content-Encoding, Content-Range or Content-Type response header fields,
1303      nor the response representation.</t>
1304   </list>
1305</t>
1306
1307</section>
1308
1309<section anchor="cache.control.extensions" title="Cache Control Extensions">
1310<t>
1311   The Cache-Control header field can be extended through the use of one or
1312   more cache-extension tokens, each with an optional value. Informational
1313   extensions (those that do not require a change in cache behavior) can be
1314   added without changing the semantics of other directives. Behavioral
1315   extensions are designed to work by acting as modifiers to the existing base
1316   of cache directives. Both the new directive and the standard directive are
1317   supplied, such that applications that do not understand the new directive
1318   will default to the behavior specified by the standard directive, and those
1319   that understand the new directive will recognize it as modifying the
1320   requirements associated with the standard directive. In this way,
1321   extensions to the cache-control directives can be made without requiring
1322   changes to the base protocol.
1323</t>
1324<t>
1325   This extension mechanism depends on an HTTP cache obeying all of the
1326   cache-control directives defined for its native HTTP-version, obeying
1327   certain extensions, and ignoring all directives that it does not
1328   understand.
1329</t>
1330<t>
1331   For example, consider a hypothetical new response directive called
1332   "community" that acts as a modifier to the private directive. We define
1333   this new directive to mean that, in addition to any private cache, any
1334   cache that is shared only by members of the community named within its
1335   value may cache the response. An origin server wishing to allow the UCI
1336   community to use an otherwise private response in their shared cache(s)
1337   could do so by including
1338</t>
1339<figure><artwork type="example"><![CDATA[
1340  Cache-Control: private, community="UCI"
1341]]></artwork></figure>
1342<t>
1343   A cache seeing this header field will act correctly even if the cache does
1344   not understand the community cache-extension, since it will also see and
1345   understand the private directive and thus default to the safe behavior.
1346</t>
1347<t>
1348   A cache MUST be ignore unrecognized cache directives; it is assumed that any
1349   cache directive likely to be unrecognized by an HTTP/1.1 cache will be
1350   combined with standard directives (or the response's default cacheability)
1351   such that the cache behavior will remain minimally correct even if the
1352   cache does not understand the extension(s).
1353</t>
1354<t>
1355   The HTTP Cache Directive Registry defines the name space for the cache
1356   directives.
1357</t>
1358<t>
1359   A registration MUST include the following fields:
1360   <list style="symbols">
1361      <t>Cache Directive Name</t>
1362      <t>Pointer to specification text</t>
1363   </list>
1364</t>
1365<t>
1366   Values to be added to this name space are subject to IETF review (<xref target="RFC5226"/>, Section 4.1).
1367</t>
1368<t>
1369   The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-cache-directives"/>.
1370</t>
1371</section>
1372
1373</section>
1374
1375<section anchor="header.expires" title="Expires">
1376   <iref item="Expires header field" primary="true"/>
1377   <iref item="Header Fields" primary="true" subitem="Expires"/>
1378   
1379   
1380<t>
1381   The "Expires" header field gives the date/time after which the
1382   response is considered stale. See <xref target="expiration.model"/> for
1383   further discussion of the freshness model.
1384</t>
1385<t>
1386   The presence of an Expires field does not imply that the original resource
1387   will change or cease to exist at, before, or after that time.
1388</t>
1389<t>
1390   The field-value is an absolute date and time as defined by HTTP-date in
1391   Section 6.1 of <xref target="Part1"/>; a sender MUST use the rfc1123-date format.
1392</t>
1393<figure><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/><artwork type="abnf2616"><![CDATA[
1394  Expires   = "Expires" ":" OWS Expires-v
1395  Expires-v = HTTP-date
1396]]></artwork></figure>
1397<figure>
1398  <preamble>For example</preamble>
1399<artwork type="example"><![CDATA[
1400  Expires: Thu, 01 Dec 1994 16:00:00 GMT
1401]]></artwork></figure>
1402<t><list>
1403   <t>
1404       Note: If a response includes a Cache-Control field with the
1405       max-age directive (see <xref target="cache-response-directive"/>),
1406       that directive overrides the Expires field. Likewise, the s-maxage
1407       directive overrides Expires in shared caches.
1408   </t>
1409</list></t>
1410<t>
1411   A server SHOULD NOT send Expires dates more than one year in the
1412   future.
1413</t>
1414<t>
1415   A cache MUST treat other invalid date formats,
1416   especially including the value "0", as in the past (i.e., "already
1417   expired").
1418</t>
1419</section>
1420
1421<section anchor="header.pragma" title="Pragma">
1422   <iref item="Pragma header field" primary="true"/>
1423   <iref item="Header Fields" primary="true" subitem="Pragma"/>
1424   
1425   
1426   
1427   
1428<t>
1429   The "Pragma" header field is used to include
1430   implementation-specific directives that might apply to any recipient along
1431   the request/response chain. All pragma directives specify optional behavior
1432   from the viewpoint of the protocol; however, some systems MAY require
1433   that behavior be consistent with the directives.
1434</t>
1435<figure><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/><artwork type="abnf2616"><![CDATA[
1436  Pragma            = "Pragma" ":" OWS Pragma-v
1437  Pragma-v          = 1#pragma-directive
1438  pragma-directive  = "no-cache" / extension-pragma
1439  extension-pragma  = token [ "=" ( token / quoted-string ) ]
1440]]></artwork></figure>
1441<t>
1442   When the no-cache directive is present in a request message, a cache
1443   SHOULD forward the request toward the origin server even if it has a
1444   stored copy of what is being requested. This pragma directive has the same
1445   semantics as the no-cache response directive (see <xref target="cache-response-directive"/>) and is defined here for backward
1446   compatibility with HTTP/1.0. A client SHOULD include both header fields
1447   when a no-cache request is sent to a server not known to be HTTP/1.1
1448   compliant. A cache SHOULD treat "Pragma: no-cache" as if the
1449   client had sent "Cache-Control: no-cache".
1450</t>
1451<t><list>
1452   <t>
1453      Note: Because the meaning of "Pragma: no-cache" as a
1454      header field is not actually specified, it does not provide a
1455      reliable replacement for "Cache-Control: no-cache" in a response.
1456   </t>
1457</list></t>
1458<t>
1459   This mechanism is deprecated; no new Pragma directives will be defined in
1460   HTTP.
1461</t>
1462</section>
1463
1464<section anchor="header.vary" title="Vary">
1465   <iref item="Vary header field" primary="true"/>
1466   <iref item="Header Fields" primary="true" subitem="Vary"/>
1467   
1468   
1469<t>
1470   The "Vary" header field conveys the set of header fields
1471   that were used to select the representation.
1472</t>
1473<t>
1474   Caches use this information, in part, to determine whether a stored
1475   response can be used to satisfy a given request; see <xref target="caching.negotiated.responses"/>. determines, while the response is
1476   fresh, whether a cache is permitted to use the response to reply to a
1477   subsequent request without validation; see <xref target="caching.negotiated.responses"/>.
1478</t>
1479<t>
1480   In uncacheable or stale responses, the Vary field value advises the user
1481   agent about the criteria that were used to select the representation.
1482</t>
1483<figure><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/><artwork type="abnf2616"><![CDATA[
1484  Vary   = "Vary" ":" OWS Vary-v
1485  Vary-v = "*" / 1#field-name
1486]]></artwork></figure>
1487<t>
1488   The set of header fields named by the Vary field value is known as the
1489   selecting header fields.
1490</t>
1491<t>
1492   A server SHOULD include a Vary header field with any cacheable response
1493   that is subject to server-driven negotiation. Doing so allows a cache to
1494   properly interpret future requests on that resource and informs the user
1495   agent about the presence of negotiation on that resource. A server MAY
1496   include a Vary header field with a non-cacheable response that is subject
1497   to server-driven negotiation, since this might provide the user agent with
1498   useful information about the dimensions over which the response varies at
1499   the time of the response.
1500</t>
1501<t>
1502   A Vary field value of "*" signals that unspecified parameters not limited
1503   to the header fields (e.g., the network address of the client), play a
1504   role in the selection of the response representation; therefore, a cache
1505   cannot determine whether this response is appropriate. A proxy MUST NOT
1506   generate the "*" value.
1507</t>
1508<t>
1509   The field-names given are not limited to the set of standard header
1510   fields defined by this specification. Field names are case-insensitive.
1511</t>
1512</section>
1513
1514<section anchor="header.warning" title="Warning">
1515   <iref item="Warning header field" primary="true"/>
1516   <iref item="Header Fields" primary="true" subitem="Warning"/>
1517   
1518   
1519   
1520   
1521   
1522   
1523   
1524<t>
1525   The "Warning" header field is used to carry additional information
1526   about the status or transformation of a message that might not be reflected
1527   in the message. This information is typically used to warn about possible
1528   incorrectness introduced by caching operations or transformations applied
1529   to the payload of the message.
1530</t>
1531<t>
1532   Warnings can be used for other purposes, both cache-related and otherwise.
1533   The use of a warning, rather than an error status code, distinguishes these
1534   responses from true failures.
1535</t>
1536<t>
1537   Warning header fields can in general be applied to any message, however some
1538   warn-codes are specific to caches and can only be applied to response
1539   messages.
1540</t>
1541<figure><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/><artwork type="abnf2616"><![CDATA[
1542  Warning    = "Warning" ":" OWS Warning-v
1543  Warning-v  = 1#warning-value
1544 
1545  warning-value = warn-code SP warn-agent SP warn-text
1546                                        [SP warn-date]
1547 
1548  warn-code  = 3DIGIT
1549  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1550                  ; the name or pseudonym of the server adding
1551                  ; the Warning header field, for use in debugging
1552  warn-text  = quoted-string
1553  warn-date  = DQUOTE HTTP-date DQUOTE
1554]]></artwork></figure>
1555<t>
1556   Multiple warnings can be attached to a response (either by the origin
1557   server or by a cache), including multiple warnings with the same code
1558   number, only differing in warn-text.
1559</t>
1560<t>
1561   When this occurs, the user agent SHOULD inform the user of as many of
1562   them as possible, in the order that they appear in the response.
1563</t>
1564<t>
1565   Systems that generate multiple Warning header fields SHOULD order them with
1566   this user agent behavior in mind. New Warning header fields SHOULD be added
1567   after any existing Warning headers fields.
1568</t>
1569<t>
1570   Warnings are assigned three digit warn-codes. The first digit indicates
1571   whether the Warning is required to be deleted from a stored response after
1572   validation:
1573   <list style="symbols">
1574      <t>1xx Warnings describe the freshness or validation status of the
1575      response, and so MUST be deleted by a cache after validation. They can
1576      only be generated by a cache when validating a cached entry, and
1577      MUST NOT be generated in any other situation.</t>
1578      <t>2xx Warnings describe some aspect of the representation that is not
1579      rectified by a validation (for example, a lossy compression of the
1580      representation) and MUST NOT be deleted by a cache after validation,
1581      unless a full response is returned, in which case they MUST be.</t>
1582   </list>
1583</t>
1584<t>
1585   If an implementation sends a message with one or more Warning header fields to a
1586   receiver whose version is HTTP/1.0 or lower, then the sender MUST include
1587   in each warning-value a warn-date that matches the Date header field in the
1588   message.
1589</t>
1590<t>
1591   If a system receives a message with a warning-value that includes
1592   a warn-date, and that warn-date is different from the Date value in the
1593   response, then that warning-value MUST be deleted from the message before
1594   storing, forwarding, or using it. (preventing the consequences of naive
1595   caching of Warning header fields.) If all of the warning-values are deleted
1596   for this reason, the Warning header field MUST be deleted as well.
1597</t>
1598<t>
1599   The following warn-codes are defined by this specification, each with a
1600   recommended warn-text in English, and a description of its meaning.
1601</t>
1602<t>
1603<?rfc needLines="4"?>
1604   110 Response is stale
1605   <list>
1606      <t>A cache SHOULD include this whenever the returned response is stale.</t>
1607   </list>
1608</t>
1609<t>
1610<?rfc needLines="4"?>
1611   111 Revalidation failed
1612   <list>
1613      <t>A cache SHOULD include this when returning a stale response because an
1614      attempt to validate the response failed, due to an inability to reach
1615      the server.</t>
1616   </list>
1617</t>
1618<t>
1619<?rfc needLines="4"?>
1620   112 Disconnected operation
1621   <list>
1622      <t>A cache SHOULD b include this if it is intentionally disconnected from
1623      the rest of the network for a period of time.</t>
1624   </list>
1625</t>
1626<t>
1627<?rfc needLines="4"?>
1628   113 Heuristic expiration
1629   <list>
1630      <t>A cache SHOULD include this if it heuristically chose a freshness
1631      lifetime greater than 24 hours and the response's age is greater than 24
1632      hours.</t>
1633   </list>
1634</t>
1635<t>
1636<?rfc needLines="4"?>
1637   199 Miscellaneous warning
1638   <list>
1639      <t>The warning text can include arbitrary information to be presented to
1640      a human user, or logged. A system receiving this warning MUST NOT take
1641      any automated action, besides presenting the warning to the user.</t>
1642   </list>
1643</t>
1644<t>
1645<?rfc needLines="4"?>
1646   214 Transformation applied
1647   <list>
1648      <t>MUST be added by a proxy if it applies any
1649      transformation to the representation, such as changing the
1650      content-coding, media-type, or modifying the representation data, unless
1651      this Warning code already appears in the response.</t>
1652   </list>
1653</t>
1654<t>
1655<?rfc needLines="4"?>
1656   299 Miscellaneous persistent warning
1657   <list>
1658      <t>The warning text can include arbitrary information to be presented to
1659      a human user, or logged. A system receiving this warning MUST NOT take
1660      any automated action.</t>
1661   </list>
1662</t>
1663</section>
1664
1665</section>
1666
1667<section anchor="history.lists" title="History Lists">
1668<t>
1669   User agents often have history mechanisms, such as "Back" buttons and
1670   history lists, that can be used to redisplay a representation retrieved
1671   earlier in a session.
1672</t>
1673<t>
1674   The freshness model (<xref target="expiration.model"/>) does not
1675   necessarily apply to history mechanisms. I.e., a history mechanism can
1676   display a previous representation even if it has expired.
1677</t>
1678<t>
1679   This does not prohibit the history mechanism from telling the user that a
1680   view might be stale, or from honoring cache directives (e.g.,
1681   Cache-Control: no-store).
1682</t>
1683</section>
1684
1685
1686<section anchor="IANA.considerations" title="IANA Considerations">
1687
1688<section title="Cache Directive Registry" anchor="cache.directive.registration">
1689<t>
1690   The registration procedure for HTTP Cache Directives is defined by <xref target="cache.control.extensions"/> of this document.
1691</t>
1692<t>
1693   The HTTP Cache Directive Registry shall be created at <eref target="http://www.iana.org/assignments/http-cache-directives"/> and be
1694   populated with the registrations below:
1695</t>
1696
1697<!--AUTOGENERATED FROM extract-cache-directives-defs.xslt, do not edit manually-->
1698<texttable align="left" suppress-title="true" anchor="iana.cache.directive.registration.table">
1699   <ttcol>Cache Directive</ttcol>
1700   <ttcol>Reference</ttcol>
1701
1702   <c>max-age</c>
1703   <c>
1704      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1705   </c>
1706   <c>max-stale</c>
1707   <c>
1708      <xref target="cache-request-directive"/>
1709   </c>
1710   <c>min-fresh</c>
1711   <c>
1712      <xref target="cache-request-directive"/>
1713   </c>
1714   <c>must-revalidate</c>
1715   <c>
1716      <xref target="cache-response-directive"/>
1717   </c>
1718   <c>no-cache</c>
1719   <c>
1720      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1721   </c>
1722   <c>no-store</c>
1723   <c>
1724      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1725   </c>
1726   <c>no-transform</c>
1727   <c>
1728      <xref target="cache-request-directive"/>, <xref target="cache-response-directive"/>
1729   </c>
1730   <c>only-if-cached</c>
1731   <c>
1732      <xref target="cache-request-directive"/>
1733   </c>
1734   <c>private</c>
1735   <c>
1736      <xref target="cache-response-directive"/>
1737   </c>
1738   <c>proxy-revalidate</c>
1739   <c>
1740      <xref target="cache-response-directive"/>
1741   </c>
1742   <c>public</c>
1743   <c>
1744      <xref target="cache-response-directive"/>
1745   </c>
1746   <c>s-maxage</c>
1747   <c>
1748      <xref target="cache-response-directive"/>
1749   </c>
1750   <c>stale-if-error</c>
1751   <c>
1752      <xref target="RFC5861"/>, Section 4
1753   </c>
1754   <c>stale-while-revalidate</c>
1755   <c>
1756      <xref target="RFC5861"/>, Section 3
1757   </c>
1758</texttable>
1759<!--(END)-->
1760
1761</section>
1762
1763<section title="Header Field Registration" anchor="header.field.registration">
1764<t>
1765  The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>
1766  shall be updated with the permanent registrations below (see <xref target="RFC3864"/>):
1767</t>
1768
1769<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1770<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1771   <ttcol>Header Field Name</ttcol>
1772   <ttcol>Protocol</ttcol>
1773   <ttcol>Status</ttcol>
1774   <ttcol>Reference</ttcol>
1775
1776   <c>Age</c>
1777   <c>http</c>
1778   <c>standard</c>
1779   <c>
1780      <xref target="header.age"/>
1781   </c>
1782   <c>Cache-Control</c>
1783   <c>http</c>
1784   <c>standard</c>
1785   <c>
1786      <xref target="header.cache-control"/>
1787   </c>
1788   <c>Expires</c>
1789   <c>http</c>
1790   <c>standard</c>
1791   <c>
1792      <xref target="header.expires"/>
1793   </c>
1794   <c>Pragma</c>
1795   <c>http</c>
1796   <c>standard</c>
1797   <c>
1798      <xref target="header.pragma"/>
1799   </c>
1800   <c>Vary</c>
1801   <c>http</c>
1802   <c>standard</c>
1803   <c>
1804      <xref target="header.vary"/>
1805   </c>
1806   <c>Warning</c>
1807   <c>http</c>
1808   <c>standard</c>
1809   <c>
1810      <xref target="header.warning"/>
1811   </c>
1812</texttable>
1813<!--(END)-->
1814
1815<t>
1816   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task
1817   Force".
1818</t>
1819</section>
1820
1821</section>
1822
1823<section anchor="security.considerations" title="Security Considerations">
1824<t>
1825   Caches expose additional potential vulnerabilities, since the contents of
1826   the cache represent an attractive target for malicious exploitation.
1827   Because cache contents persist after an HTTP request is complete, an attack
1828   on the cache can reveal information long after a user believes that the
1829   information has been removed from the network. Therefore, cache contents
1830   need to be protected as sensitive information.
1831</t>
1832</section>
1833
1834<section anchor="ack" title="Acknowledgments">
1835<t>
1836   Much of the content and presentation of the caching design is due to
1837   suggestions and comments from individuals including: Shel Kaphan, Paul
1838   Leach, Koen Holtman, David Morris, and Larry Masinter.
1839</t>
1840</section>
1841
1842</middle>
1843
1844<back>
1845<references title="Normative References">
1846
1847  <reference anchor="Part1">
1848    <front>
1849      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1850      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1851        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1852        <address><email>fielding@gbiv.com</email></address>
1853      </author>
1854      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1855        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1856        <address><email>jg@freedesktop.org</email></address>
1857      </author>
1858      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1859        <organization abbrev="HP">Hewlett-Packard Company</organization>
1860        <address><email>JeffMogul@acm.org</email></address>
1861      </author>
1862      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1863        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1864        <address><email>henrikn@microsoft.com</email></address>
1865      </author>
1866      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1867        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1868        <address><email>LMM@acm.org</email></address>
1869      </author>
1870      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1871        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1872        <address><email>paulle@microsoft.com</email></address>
1873      </author>
1874      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1875        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1876        <address><email>timbl@w3.org</email></address>
1877      </author>
1878      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1879        <organization abbrev="W3C">World Wide Web Consortium</organization>
1880        <address><email>ylafon@w3.org</email></address>
1881      </author>
1882      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1883        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1884        <address><email>julian.reschke@greenbytes.de</email></address>
1885      </author>
1886      <date month="March" year="2011"/>
1887    </front>
1888    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-13"/>
1889   
1890  </reference>
1891
1892  <reference anchor="Part2">
1893    <front>
1894      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
1895      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1896        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1897        <address><email>fielding@gbiv.com</email></address>
1898      </author>
1899      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1900        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1901        <address><email>jg@freedesktop.org</email></address>
1902      </author>
1903      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1904        <organization abbrev="HP">Hewlett-Packard Company</organization>
1905        <address><email>JeffMogul@acm.org</email></address>
1906      </author>
1907      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1908        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1909        <address><email>henrikn@microsoft.com</email></address>
1910      </author>
1911      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1912        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1913        <address><email>LMM@acm.org</email></address>
1914      </author>
1915      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1916        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1917        <address><email>paulle@microsoft.com</email></address>
1918      </author>
1919      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1920        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1921        <address><email>timbl@w3.org</email></address>
1922      </author>
1923      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1924        <organization abbrev="W3C">World Wide Web Consortium</organization>
1925        <address><email>ylafon@w3.org</email></address>
1926      </author>
1927      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1928        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1929        <address><email>julian.reschke@greenbytes.de</email></address>
1930      </author>
1931      <date month="March" year="2011"/>
1932    </front>
1933    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-13"/>
1934   
1935  </reference>
1936
1937  <reference anchor="Part4">
1938    <front>
1939      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
1940      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1941        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1942        <address><email>fielding@gbiv.com</email></address>
1943      </author>
1944      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1945        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1946        <address><email>jg@freedesktop.org</email></address>
1947      </author>
1948      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1949        <organization abbrev="HP">Hewlett-Packard Company</organization>
1950        <address><email>JeffMogul@acm.org</email></address>
1951      </author>
1952      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1953        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1954        <address><email>henrikn@microsoft.com</email></address>
1955      </author>
1956      <author fullname="Larry Masinter" initials="L." surname="Masinter">
1957        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1958        <address><email>LMM@acm.org</email></address>
1959      </author>
1960      <author fullname="Paul J. Leach" initials="P." surname="Leach">
1961        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1962        <address><email>paulle@microsoft.com</email></address>
1963      </author>
1964      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
1965        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1966        <address><email>timbl@w3.org</email></address>
1967      </author>
1968      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
1969        <organization abbrev="W3C">World Wide Web Consortium</organization>
1970        <address><email>ylafon@w3.org</email></address>
1971      </author>
1972      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
1973        <organization abbrev="greenbytes">greenbytes GmbH</organization>
1974        <address><email>julian.reschke@greenbytes.de</email></address>
1975      </author>
1976      <date month="March" year="2011"/>
1977    </front>
1978    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-13"/>
1979   
1980  </reference>
1981
1982  <reference anchor="Part5">
1983    <front>
1984      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1985      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
1986        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1987        <address><email>fielding@gbiv.com</email></address>
1988      </author>
1989      <author fullname="Jim Gettys" initials="J." surname="Gettys">
1990        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1991        <address><email>jg@freedesktop.org</email></address>
1992      </author>
1993      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
1994        <organization abbrev="HP">Hewlett-Packard Company</organization>
1995        <address><email>JeffMogul@acm.org</email></address>
1996      </author>
1997      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
1998        <organization abbrev="Microsoft">Microsoft Corporation</organization>
1999        <address><email>henrikn@microsoft.com</email></address>
2000      </author>
2001      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2002        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2003        <address><email>LMM@acm.org</email></address>
2004      </author>
2005      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2006        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2007        <address><email>paulle@microsoft.com</email></address>
2008      </author>
2009      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2010        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2011        <address><email>timbl@w3.org</email></address>
2012      </author>
2013      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2014        <organization abbrev="W3C">World Wide Web Consortium</organization>
2015        <address><email>ylafon@w3.org</email></address>
2016      </author>
2017      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2018        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2019        <address><email>julian.reschke@greenbytes.de</email></address>
2020      </author>
2021      <date month="March" year="2011"/>
2022    </front>
2023    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-13"/>
2024   
2025  </reference>
2026
2027  <reference anchor="Part7">
2028    <front>
2029      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2030      <author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
2031        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2032        <address><email>fielding@gbiv.com</email></address>
2033      </author>
2034      <author fullname="Jim Gettys" initials="J." surname="Gettys">
2035        <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
2036        <address><email>jg@freedesktop.org</email></address>
2037      </author>
2038      <author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
2039        <organization abbrev="HP">Hewlett-Packard Company</organization>
2040        <address><email>JeffMogul@acm.org</email></address>
2041      </author>
2042      <author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
2043        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2044        <address><email>henrikn@microsoft.com</email></address>
2045      </author>
2046      <author fullname="Larry Masinter" initials="L." surname="Masinter">
2047        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
2048        <address><email>LMM@acm.org</email></address>
2049      </author>
2050      <author fullname="Paul J. Leach" initials="P." surname="Leach">
2051        <organization abbrev="Microsoft">Microsoft Corporation</organization>
2052        <address><email>paulle@microsoft.com</email></address>
2053      </author>
2054      <author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
2055        <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2056        <address><email>timbl@w3.org</email></address>
2057      </author>
2058      <author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
2059        <organization abbrev="W3C">World Wide Web Consortium</organization>
2060        <address><email>ylafon@w3.org</email></address>
2061      </author>
2062      <author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
2063        <organization abbrev="greenbytes">greenbytes GmbH</organization>
2064        <address><email>julian.reschke@greenbytes.de</email></address>
2065      </author>
2066      <date month="March" year="2011"/>
2067    </front>
2068    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-13"/>
2069   
2070  </reference>
2071
2072  <reference anchor="RFC2119">
2073    <front>
2074      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2075      <author fullname="Scott Bradner" initials="S." surname="Bradner">
2076        <organization>Harvard University</organization>
2077        <address><email>sob@harvard.edu</email></address>
2078      </author>
2079      <date month="March" year="1997"/>
2080    </front>
2081    <seriesInfo name="BCP" value="14"/>
2082    <seriesInfo name="RFC" value="2119"/>
2083  </reference>
2084
2085  <reference anchor="RFC5234">
2086    <front>
2087      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
2088      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
2089        <organization>Brandenburg InternetWorking</organization>
2090        <address>
2091          <email>dcrocker@bbiw.net</email>
2092        </address> 
2093      </author>
2094      <author initials="P." surname="Overell" fullname="Paul Overell">
2095        <organization>THUS plc.</organization>
2096        <address>
2097          <email>paul.overell@thus.net</email>
2098        </address>
2099      </author>
2100      <date month="January" year="2008"/>
2101    </front>
2102    <seriesInfo name="STD" value="68"/>
2103    <seriesInfo name="RFC" value="5234"/>
2104  </reference>
2105 
2106</references>
2107
2108<references title="Informative References">
2109
2110  <reference anchor="RFC1305">
2111    <front>
2112      <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2113      <author fullname="David L. Mills" initials="D." surname="Mills">
2114        <organization>University of Delaware, Electrical Engineering Department</organization>
2115        <address><email>mills@udel.edu</email></address>
2116      </author>
2117      <date month="March" year="1992"/>
2118    </front>
2119    <seriesInfo name="RFC" value="1305"/>
2120  </reference>
2121
2122  <reference anchor="RFC2616">
2123    <front>
2124      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2125      <author fullname="R. Fielding" initials="R." surname="Fielding">
2126        <organization>University of California, Irvine</organization>
2127        <address><email>fielding@ics.uci.edu</email></address>
2128      </author>
2129      <author fullname="J. Gettys" initials="J." surname="Gettys">
2130        <organization>W3C</organization>
2131        <address><email>jg@w3.org</email></address>
2132      </author>
2133      <author fullname="J. Mogul" initials="J." surname="Mogul">
2134        <organization>Compaq Computer Corporation</organization>
2135        <address><email>mogul@wrl.dec.com</email></address>
2136      </author>
2137      <author fullname="H. Frystyk" initials="H." surname="Frystyk">
2138        <organization>MIT Laboratory for Computer Science</organization>
2139        <address><email>frystyk@w3.org</email></address>
2140      </author>
2141      <author fullname="L. Masinter" initials="L." surname="Masinter">
2142        <organization>Xerox Corporation</organization>
2143        <address><email>masinter@parc.xerox.com</email></address>
2144      </author>
2145      <author fullname="P. Leach" initials="P." surname="Leach">
2146        <organization>Microsoft Corporation</organization>
2147        <address><email>paulle@microsoft.com</email></address>
2148      </author>
2149      <author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
2150        <organization>W3C</organization>
2151        <address><email>timbl@w3.org</email></address>
2152      </author>
2153      <date month="June" year="1999"/>
2154    </front>
2155    <seriesInfo name="RFC" value="2616"/>
2156  </reference>
2157
2158  <reference anchor="RFC3864">
2159    <front>
2160      <title>Registration Procedures for Message Header Fields</title>
2161      <author fullname="G. Klyne" initials="G." surname="Klyne">
2162        <organization>Nine by Nine</organization>
2163        <address><email>GK-IETF@ninebynine.org</email></address>
2164      </author>
2165      <author fullname="M. Nottingham" initials="M." surname="Nottingham">
2166        <organization>BEA Systems</organization>
2167        <address><email>mnot@pobox.com</email></address>
2168      </author>
2169      <author fullname="J. Mogul" initials="J." surname="Mogul">
2170        <organization>HP Labs</organization>
2171        <address><email>JeffMogul@acm.org</email></address>
2172      </author>
2173      <date month="September" year="2004"/>
2174    </front>
2175    <seriesInfo name="BCP" value="90"/>
2176    <seriesInfo name="RFC" value="3864"/>
2177  </reference>
2178
2179  <reference anchor="RFC5226">
2180    <front>
2181      <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
2182      <author initials="T." surname="Narten" fullname="T. Narten">
2183        <organization>IBM</organization>
2184        <address><email>narten@us.ibm.com</email></address>
2185      </author>
2186      <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
2187        <organization>Google</organization>
2188        <address><email>Harald@Alvestrand.no</email></address>
2189      </author>
2190      <date year="2008" month="May"/>
2191    </front>
2192    <seriesInfo name="BCP" value="26"/>
2193    <seriesInfo name="RFC" value="5226"/>
2194  </reference>
2195
2196  <reference anchor="RFC5861">
2197    <front>
2198      <title abbrev="HTTP stale controls">HTTP Cache-Control Extensions for Stale Content</title>
2199      <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
2200        <organization>Yahoo! Inc.</organization>
2201        <address><email>mnot@yahoo-inc.com</email></address>
2202      </author>
2203      <date month="April" year="2010"/>
2204    </front>
2205    <seriesInfo name="RFC" value="5861"/>
2206  </reference>
2207
2208</references>
2209
2210<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
2211<t>
2212  Make the specified age calculation algorithm less conservative.
2213  (<xref target="age.calculations"/>)
2214</t>
2215<t>
2216  Remove requirement to consider Content-Location in successful responses
2217  in order to determine the appropriate response to use.
2218  (<xref target="validation.model"/>)
2219</t>
2220<t>
2221  Clarify denial of service attack avoidance requirement.
2222  (<xref target="invalidation.after.updates.or.deletions"/>)
2223</t>
2224<t>
2225  Do not mention RFC 2047 encoding and multiple languages in Warning header fields
2226  anymore, as these aspects never were implemented.
2227  (<xref target="header.warning"/>)
2228</t>
2229</section>
2230
2231
2232<section title="Collected ABNF" anchor="collected.abnf">
2233<figure>
2234<artwork type="abnf" name="p6-cache.parsed-abnf"><![CDATA[
2235Age = "Age:" OWS Age-v
2236Age-v = delta-seconds
2237
2238Cache-Control = "Cache-Control:" OWS Cache-Control-v
2239Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
2240 cache-directive ] )
2241
2242Expires = "Expires:" OWS Expires-v
2243Expires-v = HTTP-date
2244
2245HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2246
2247OWS = <OWS, defined in [Part1], Section 1.2.2>
2248
2249Pragma = "Pragma:" OWS Pragma-v
2250Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
2251 pragma-directive ] )
2252
2253Vary = "Vary:" OWS Vary-v
2254Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
2255 ] ) )
2256
2257Warning = "Warning:" OWS Warning-v
2258Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
2259 ] )
2260
2261cache-directive = cache-request-directive / cache-response-directive
2262cache-extension = token [ "=" ( token / quoted-string ) ]
2263cache-request-directive = "no-cache" / "no-store" / ( "max-age="
2264 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
2265 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
2266 cache-extension
2267cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
2268 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
2269 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
2270 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
2271 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
2272 ) / ( "s-maxage=" delta-seconds ) / cache-extension
2273
2274delta-seconds = 1*DIGIT
2275
2276extension-pragma = token [ "=" ( token / quoted-string ) ]
2277
2278field-name = <field-name, defined in [Part1], Section 3.2>
2279
2280port = <port, defined in [Part1], Section 2.6>
2281pragma-directive = "no-cache" / extension-pragma
2282pseudonym = <pseudonym, defined in [Part1], Section 9.9>
2283
2284quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
2285
2286token = <token, defined in [Part1], Section 1.2.2>
2287
2288uri-host = <uri-host, defined in [Part1], Section 2.6>
2289
2290warn-agent = ( uri-host [ ":" port ] ) / pseudonym
2291warn-code = 3DIGIT
2292warn-date = DQUOTE HTTP-date DQUOTE
2293warn-text = quoted-string
2294warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
2295 ]
2296]]></artwork>
2297</figure>
2298<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
2299; Age defined but not used
2300; Cache-Control defined but not used
2301; Expires defined but not used
2302; Pragma defined but not used
2303; Vary defined but not used
2304; Warning defined but not used
2305]]></artwork></figure></section>
2306
2307
2308<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
2309
2310<section title="Since RFC 2616">
2311  <t>Extracted relevant partitions from <xref target="RFC2616"/>.</t>
2312</section>
2313
2314<section title="Since draft-ietf-httpbis-p6-cache-00">
2315<t>
2316  Closed issues:
2317  <list style="symbols">
2318    <t>
2319      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9"/>: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)</t>
2320    <t>
2321      <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>
2322    <t>
2323      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>: "Normative and Informative references"</t>
2324    <t>
2325      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48"/>: "Date reference typo"</t>
2326    <t>
2327      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49"/>: "Connection header text"</t>
2328    <t>
2329      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65"/>: "Informative references"</t>
2330    <t>
2331      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66"/>: "ISO-8859-1 Reference"</t>
2332    <t>
2333      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86"/>: "Normative up-to-date references"</t>
2334    <t>
2335      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87"/>: "typo in 13.2.2"</t>
2336  </list>
2337</t>
2338<t>
2339  Other changes:
2340  <list style="symbols">
2341    <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>
2342  </list>
2343</t>
2344</section>
2345
2346<section title="Since draft-ietf-httpbis-p6-cache-01">
2347<t>
2348  Closed issues:
2349  <list style="symbols">
2350    <t>
2351      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82"/>: "rel_path not used"</t>
2352  </list>
2353</t>
2354<t>
2355  Other changes:
2356  <list style="symbols">
2357    <t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
2358      on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>)</t>
2359    <t>Add explicit references to BNF syntax and rules imported from other parts of the
2360      specification.</t>
2361  </list>
2362</t>
2363</section>
2364
2365<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
2366<t>
2367  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2368  <list style="symbols">
2369    <t>Reference RFC 3984, and update header field registrations for header fields defined in this
2370      document.</t>
2371  </list>
2372</t>
2373</section>
2374
2375<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
2376<t>
2377  Closed issues:
2378  <list style="symbols">
2379    <t>
2380      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106"/>: "Vary header classification"</t>
2381  </list>
2382</t>
2383</section>
2384
2385<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
2386<t>
2387  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2388  <list style="symbols"> 
2389    <t>
2390      Use "/" instead of "|" for alternatives.
2391    </t>
2392    <t>
2393      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
2394      whitespace ("OWS") and required whitespace ("RWS").
2395    </t>
2396    <t>
2397      Rewrite ABNFs to spell out whitespace rules, factor out
2398      header field value format definitions.
2399    </t>
2400  </list>
2401</t>
2402</section>
2403
2404<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
2405<t>
2406  This is a total rewrite of this part of the specification.
2407</t>
2408<t>
2409  Affected issues:
2410  <list style="symbols">
2411    <t>
2412      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>: "Definition of 1xx Warn-Codes"</t>
2413    <t>
2414      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>: "Placement of 13.5.1 and 13.5.2"</t>
2415    <t>
2416      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/138"/>: "The role of Warning and Semantic Transparency in Caching"</t>
2417    <t>
2418      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/139"/>: "Methods and Caching"</t>
2419  </list>
2420</t>
2421<t>
2422  In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
2423  <list style="symbols"> 
2424    <t>
2425      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
2426    </t>
2427  </list>
2428</t>
2429</section>
2430
2431<section anchor="changes.since.06" title="Since draft-ietf-httpbis-p6-cache-06">
2432<t>
2433  Closed issues:
2434  <list style="symbols"> 
2435    <t>
2436      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/161"/>:
2437      "base for numeric protocol elements"
2438    </t>
2439  </list>
2440</t>
2441<t>
2442  Affected issues:
2443  <list style="symbols">
2444    <t>
2445      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/37"/>:
2446      "Vary and non-existant headers"
2447    </t>
2448  </list>
2449</t>
2450</section>
2451
2452<section anchor="changes.since.07" title="Since draft-ietf-httpbis-p6-cache-07">
2453<t>
2454  Closed issues:
2455  <list style="symbols"> 
2456    <t>
2457      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54"/>:
2458      "Definition of 1xx Warn-Codes"
2459    </t>
2460    <t>
2461      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/167"/>:
2462      "Content-Location on 304 responses"
2463    </t>
2464    <t>
2465      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/169"/>:
2466      "private and no-cache CC directives with headers"
2467    </t>
2468    <t>
2469      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/187"/>:
2470      "RFC2047 and warn-text"
2471    </t>
2472  </list>
2473</t>
2474</section>
2475
2476<section anchor="changes.since.08" title="Since draft-ietf-httpbis-p6-cache-08">
2477<t>
2478  Closed issues:
2479  <list style="symbols"> 
2480    <t>
2481      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/147"/>:
2482      "serving negotiated responses from cache: header-specific canonicalization"
2483    </t>
2484    <t>
2485      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/197"/>:
2486      "Effect of CC directives on history lists"
2487    </t>
2488  </list>
2489</t>
2490<t>
2491  Affected issues:
2492  <list style="symbols">
2493    <t>
2494      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/199"/>:
2495      Status codes and caching
2496    </t>
2497  </list>
2498</t>
2499<t>
2500  Partly resolved issues:
2501  <list style="symbols"> 
2502    <t>
2503      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/60"/>:
2504      "Placement of 13.5.1 and 13.5.2"
2505    </t>
2506  </list>
2507</t>
2508</section>
2509
2510<section title="Since draft-ietf-httpbis-p6-cache-09" anchor="changes.since.09">
2511<t>
2512  Closed issues:
2513  <list style="symbols"> 
2514    <t>
2515      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/29"/>:
2516      "Age calculation"
2517    </t>
2518    <t>
2519      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/168"/>:
2520      "Clarify differences between / requirements for request and response CC directives"
2521    </t>
2522        <t>
2523                <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/174"/>:
2524                "Caching authenticated responses"
2525        </t>
2526    <t>
2527      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/208"/>:
2528      "IANA registry for cache-control directives"
2529    </t>
2530    <t>
2531      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/211"/>:
2532      "Heuristic caching of URLs with query components"
2533    </t>
2534  </list>
2535</t>
2536<t>
2537  Partly resolved issues:
2538  <list style="symbols"> 
2539    <t>
2540      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/196"/>:
2541      "Term for the requested resource's URI"
2542    </t>
2543  </list>
2544</t>
2545</section>
2546
2547<section title="Since draft-ietf-httpbis-p6-cache-10" anchor="changes.since.10">
2548<t>
2549  Closed issues:
2550  <list style="symbols"> 
2551    <t>
2552      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
2553      "Clarify entity / representation / variant terminology"
2554    </t>
2555    <t>
2556      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
2557      "consider removing the 'changes from 2068' sections"
2558    </t>
2559    <t>
2560      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2561      "Allowing heuristic caching for new status codes"
2562    </t>
2563    <t>
2564      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/223"/>:
2565      "Allowing heuristic caching for new status codes"
2566    </t>
2567    <t>
2568      Clean up TODOs and prose in "Combining Responses."
2569    </t>
2570  </list>
2571</t>
2572</section>
2573
2574<section title="Since draft-ietf-httpbis-p6-cache-11" anchor="changes.since.11">
2575<t>
2576  Closed issues:
2577  <list style="symbols"> 
2578    <t>
2579      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/204"/>:
2580      "Text about clock requirement for caches belongs in p6"
2581    </t>
2582  </list>
2583</t>
2584</section>
2585
2586<section title="Since draft-ietf-httpbis-p6-cache-12" anchor="changes.since.12">
2587<t>
2588  Closed issues:
2589  <list style="symbols"> 
2590    <t>
2591      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
2592      "Header Classification"
2593    </t>
2594    <t>
2595      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/268"/>:
2596      "Clarify 'public'"
2597    </t>
2598  </list>
2599</t>
2600</section>
2601
2602</section>
2603  </back>
2604</rfc>
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