source: draft-ietf-httpbis/01/draft-ietf-httpbis-p6-cache-01.xml @ 377

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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<!DOCTYPE rfc
15  PUBLIC "" "rfc2629.dtd">
16<rfc obsoletes="2616" category="std" ipr="full3978" docName="draft-ietf-httpbis-p6-cache-01">
17<front>
18
19  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
20
21  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
22    <organization abbrev="Day Software">Day Software</organization>
23    <address>
24      <postal>
25        <street>23 Corporate Plaza DR, Suite 280</street>
26        <city>Newport Beach</city>
27        <region>CA</region>
28        <code>92660</code>
29        <country>USA</country>
30      </postal>
31      <phone>+1-949-706-5300</phone>
32      <facsimile>+1-949-706-5305</facsimile>
33      <email>fielding@gbiv.com</email>
34      <uri>http://roy.gbiv.com/</uri>
35    </address>
36  </author>
37
38  <author initials="J." surname="Gettys" fullname="Jim Gettys">
39    <organization>One Laptop per Child</organization>
40    <address>
41      <postal>
42        <street>21 Oak Knoll Road</street>
43        <city>Carlisle</city>
44        <region>MA</region>
45        <code>01741</code>
46        <country>USA</country>
47      </postal>
48      <email>jg@laptop.org</email>
49      <uri>http://www.laptop.org/</uri>
50    </address>
51  </author>
52 
53  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
54    <organization abbrev="HP">Hewlett-Packard Company</organization>
55    <address>
56      <postal>
57        <street>HP Labs, Large Scale Systems Group</street>
58        <street>1501 Page Mill Road, MS 1177</street>
59        <city>Palo Alto</city>
60        <region>CA</region>
61        <code>94304</code>
62        <country>USA</country>
63      </postal>
64      <email>JeffMogul@acm.org</email>
65    </address>
66  </author>
67
68  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
69    <organization abbrev="Microsoft">Microsoft Corporation</organization>
70    <address>
71      <postal>
72        <street>1 Microsoft Way</street>
73        <city>Redmond</city>
74        <region>WA</region>
75        <code>98052</code>
76        <country>USA</country>
77      </postal>
78      <email>henrikn@microsoft.com</email>
79    </address>
80  </author>
81
82  <author initials="L." surname="Masinter" fullname="Larry Masinter">
83    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
84    <address>
85      <postal>
86        <street>345 Park Ave</street>
87        <city>San Jose</city>
88        <region>CA</region>
89        <code>95110</code>
90        <country>USA</country>
91      </postal>
92      <email>LMM@acm.org</email>
93      <uri>http://larry.masinter.net/</uri>
94    </address>
95  </author>
96 
97  <author initials="P." surname="Leach" fullname="Paul J. Leach">
98    <organization abbrev="Microsoft">Microsoft Corporation</organization>
99    <address>
100      <postal>
101        <street>1 Microsoft Way</street>
102        <city>Redmond</city>
103        <region>WA</region>
104        <code>98052</code>
105      </postal>
106      <email>paulle@microsoft.com</email>
107    </address>
108  </author>
109   
110  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
111    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
112    <address>
113      <postal>
114        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
115        <street>The Stata Center, Building 32</street>
116        <street>32 Vassar Street</street>
117        <city>Cambridge</city>
118        <region>MA</region>
119        <code>02139</code>
120        <country>USA</country>
121      </postal>
122      <email>timbl@w3.org</email>
123      <uri>http://www.w3.org/People/Berners-Lee/</uri>
124    </address>
125  </author>
126
127  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
128    <organization abbrev="W3C">World Wide Web Consortium</organization>
129    <address>
130      <postal>
131        <street>W3C / ERCIM</street>
132        <street>2004, rte des Lucioles</street>
133        <city>Sophia-Antipolis</city>
134        <region>AM</region>
135        <code>06902</code>
136        <country>France</country>
137      </postal>
138      <email>ylafon@w3.org</email>
139      <uri>http://www.raubacapeu.net/people/yves/</uri>
140    </address>
141  </author>
142
143  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
144    <organization abbrev="greenbytes">greenbytes GmbH</organization>
145    <address>
146      <postal>
147        <street>Hafenweg 16</street>
148        <city>Muenster</city><region>NW</region><code>48155</code>
149        <country>Germany</country>
150      </postal>
151      <phone>+49 251 2807760</phone>   
152      <facsimile>+49 251 2807761</facsimile>   
153      <email>julian.reschke@greenbytes.de</email>       
154      <uri>http://greenbytes.de/tech/webdav/</uri>     
155    </address>
156  </author>
157
158  <date month="January" year="2008" day="12"/>
159
160<abstract>
161<t>
162   The Hypertext Transfer Protocol (HTTP) is an application-level
163   protocol for distributed, collaborative, hypermedia information
164   systems. HTTP has been in use by the World Wide Web global information
165   initiative since 1990. This document is Part 6 of the seven-part specification
166   that defines the protocol referred to as "HTTP/1.1" and, taken together,
167   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
168   and the associated header fields that control cache behavior or indicate
169   cacheable response messages.
170</t>
171</abstract>
172
173<note title="Editorial Note (To be removed by RFC Editor)">
174  <t>
175    Discussion of this draft should take place on the HTTPBIS working group
176    mailing list (ietf-http-wg@w3.org). The current issues list is
177    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
178    and related documents (including fancy diffs) can be found at
179    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
180  </t>
181  <t>
182    This draft incorporates those issue resolutions that were either
183    collected in the original RFC2616 errata list (<eref target="http://purl.org/NET/http-errata"/>),
184    or which were agreed upon on the mailing list between October 2006 and
185    November 2007 (as published in "draft-lafon-rfc2616bis-03").
186  </t>
187</note>
188</front>
189<middle>
190<section title="Introduction" anchor="caching">
191<t>
192   HTTP is typically used for distributed information systems, where
193   performance can be improved by the use of response caches, and includes
194   a number of elements intended to make caching work as well as possible.
195   Because these elements interact with each other, it is useful to describe
196   the caching design of HTTP separately.  This document defines aspects of
197   HTTP/1.1 related to caching and reusing response messages.
198</t>
199
200<section title="Purpose" anchor="intro.purpose">
201<iref item="cache"/>
202<t>
203   An HTTP cache is a local store of response messages
204   and the subsystem that controls its message storage, retrieval, and
205   deletion. A cache stores cacheable responses in order to reduce the
206   response time and network bandwidth consumption on future, equivalent
207   requests. Any client or server may include a cache, though a cache
208   cannot be used by a server that is acting as a tunnel.
209</t>
210<t>
211   Caching would be useless if it did not significantly improve
212   performance. The goal of caching in HTTP/1.1 is to eliminate the need
213   to send requests in many cases, and to eliminate the need to send
214   full responses in many other cases. The former reduces the number of
215   network round-trips required for many operations; we use an
216   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
217   latter reduces network bandwidth requirements; we use a "validation"
218   mechanism for this purpose (see <xref target="validation.model"/>).
219</t>
220<iref item="semantically transparent"/>
221<t>
222   A cache behaves in a "semantically transparent" manner, with
223   respect to a particular response, when its use affects neither the
224   requesting client nor the origin server, except to improve
225   performance. When a cache is semantically transparent, the client
226   receives exactly the same response (except for hop-by-hop headers)
227   that it would have received had its request been handled directly
228   by the origin server.
229</t>
230<t>
231   In an ideal world, all interactions with an HTTP cache would be
232   semantically transparent.  However, for some resources, semantic
233   transparency is not always necessary and can be effectively traded
234   for the sake of bandwidth scaling, disconnected operation, and
235   high availability.  HTTP/1.1 allows origin servers, caches,
236   and clients to explicitly reduce transparency when necessary.
237   However, because non-transparent operation may confuse non-expert
238   users and might be incompatible with certain server applications
239   (such as those for ordering merchandise), the protocol requires that
240   transparency be relaxed
241  <list style="symbols">
242     <t>only by an explicit protocol-level request when relaxed by
243        client or origin server</t>
244
245     <t>only with an explicit warning to the end user when relaxed by
246        cache or client</t>
247  </list>
248</t>
249<t>
250   Therefore, HTTP/1.1 provides these important elements:
251  <list style="numbers">
252      <t>Protocol features that provide full semantic transparency when
253         this is required by all parties.</t>
254
255      <t>Protocol features that allow an origin server or user agent to
256         explicitly request and control non-transparent operation.</t>
257
258      <t>Protocol features that allow a cache to attach warnings to
259         responses that do not preserve the requested approximation of
260         semantic transparency.</t>
261  </list>
262</t>
263<t>
264   A basic principle is that it must be possible for the clients to
265   detect any potential relaxation of semantic transparency.
266  <list><t>
267      Note: The server, cache, or client implementor might be faced with
268      design decisions not explicitly discussed in this specification.
269      If a decision might affect semantic transparency, the implementor
270      ought to err on the side of maintaining transparency unless a
271      careful and complete analysis shows significant benefits in
272      breaking transparency.
273    </t></list>
274</t>
275</section>
276
277<section title="Terminology" anchor="intro.terminology">
278<t>
279   This specification uses a number of terms to refer to the roles
280   played by participants in, and objects of, HTTP caching.
281</t>
282<t>
283  <iref item="cacheable"/>
284  cacheable
285  <list>
286    <t>
287      A response is cacheable if a cache is allowed to store a copy of
288      the response message for use in answering subsequent requests.
289      Even if a resource is cacheable, there may
290      be additional constraints on whether a cache can use the cached
291      copy for a particular request.
292    </t>
293  </list>
294</t>
295<t>
296  <iref item="first-hand"/>
297  first-hand
298  <list>
299    <t>
300      A response is first-hand if it comes directly and without
301      unnecessary delay from the origin server, perhaps via one or more
302      proxies. A response is also first-hand if its validity has just
303      been checked directly with the origin server.
304    </t>
305  </list>
306</t>
307<t>
308  <iref item="explicit expiration time"/>
309  explicit expiration time
310  <list>
311    <t>
312      The time at which the origin server intends that an entity should
313      no longer be returned by a cache without further validation.
314    </t>
315  </list>
316</t>
317<t>
318  <iref item="heuristic expiration time"/>
319  heuristic expiration time
320  <list>
321    <t>
322      An expiration time assigned by a cache when no explicit expiration
323      time is available.
324    </t>
325  </list>
326</t>
327<t>
328  <iref item="age"/>
329  age
330  <list>
331    <t>
332      The age of a response is the time since it was sent by, or
333      successfully validated with, the origin server.
334    </t>
335  </list>
336</t>
337<t>
338  <iref item="freshness lifetime"/>
339  freshness lifetime
340  <list>
341    <t>
342      The length of time between the generation of a response and its
343      expiration time.
344    </t>
345  </list>
346</t>
347<t>
348  <iref item="fresh"/>
349  fresh
350  <list>
351    <t>
352      A response is fresh if its age has not yet exceeded its freshness
353      lifetime.
354    </t>
355  </list>
356</t>
357<t>
358  <iref item="stale"/>
359  stale
360  <list>
361    <t>
362      A response is stale if its age has passed its freshness lifetime.
363    </t>
364  </list>
365</t>
366<t>
367  <iref item="validator"/>
368  validator
369  <list>
370    <t>
371      A protocol element (e.g., an entity tag or a Last-Modified time)
372      that is used to find out whether a cache entry is an equivalent
373      copy of an entity.
374    </t>
375  </list>
376</t>
377</section>
378
379<section title="Requirements" anchor="intro.requirements">
380<t>
381   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
382   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
383   document are to be interpreted as described in <xref target="RFC2119"/>.
384</t>
385<t>
386   An implementation is not compliant if it fails to satisfy one or more
387   of the MUST or REQUIRED level requirements for the protocols it
388   implements. An implementation that satisfies all the MUST or REQUIRED
389   level and all the SHOULD level requirements for its protocols is said
390   to be "unconditionally compliant"; one that satisfies all the MUST
391   level requirements but not all the SHOULD level requirements for its
392   protocols is said to be "conditionally compliant."
393</t>
394</section>
395</section>
396
397<section title="Overview" anchor="caching.overview">
398<section title="Cache Correctness" anchor="cache.correctness">
399<t>
400   A correct cache MUST respond to a request with the most up-to-date
401   response held by the cache that is appropriate to the request (see
402   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
403   <xref target="disambiguating.multiple.responses" format="counter"/>,
404   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
405   conditions:
406  <list style="numbers">
407      <t>It has been checked for equivalence with what the origin server
408         would have returned by revalidating the response with the
409         origin server (<xref target="validation.model"/>);</t>
410
411      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
412         this means it meets the least restrictive freshness requirement
413         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
414         the origin server so specifies, it is the freshness requirement
415         of the origin server alone.
416
417         If a stored response is not "fresh enough" by the most
418         restrictive freshness requirement of both the client and the
419         origin server, in carefully considered circumstances the cache
420         MAY still return the response with the appropriate Warning
421         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
422         and <xref target="header.warning" format="counter"/>), unless such a response
423         is prohibited (e.g., by a "no-store" cache-directive, or by a
424         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
425
426      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
427         or error (4xx or 5xx) response message.</t>
428  </list>
429</t>
430<t>
431   If the cache can not communicate with the origin server, then a
432   correct cache SHOULD respond as above if the response can be
433   correctly served from the cache; if not it MUST return an error or
434   warning indicating that there was a communication failure.
435</t>
436<t>
437   If a cache receives a response (either an entire response, or a 304
438   (Not Modified) response) that it would normally forward to the
439   requesting client, and the received response is no longer fresh, the
440   cache SHOULD forward it to the requesting client without adding a new
441   Warning (but without removing any existing Warning headers). A cache
442   SHOULD NOT  attempt to revalidate a response simply because that
443   response became stale in transit; this might lead to an infinite
444   loop. A user agent that receives a stale response without a Warning
445   MAY display a warning indication to the user.
446</t>
447</section>
448
449<section title="Warnings" anchor="warnings">
450<t>
451   Whenever a cache returns a response that is neither first-hand nor
452   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
453   MUST attach a warning to that effect, using a Warning general-header.
454   The Warning header and the currently defined warnings are described
455   in <xref target="header.warning"/>. The warning allows clients to take appropriate
456   action.
457</t>
458<t>
459   Warnings MAY be used for other purposes, both cache-related and
460   otherwise. The use of a warning, rather than an error status code,
461   distinguish these responses from true failures.
462</t>
463<t>
464   Warnings are assigned three digit warn-codes. The first digit
465   indicates whether the Warning MUST or MUST NOT be deleted from a
466   stored cache entry after a successful revalidation:
467</t>
468<t>
469  <list style="hanging">
470    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
471     the response, and so MUST be deleted after a successful
472     revalidation. 1xx warn-codes MAY be generated by a cache only when
473     validating a cached entry. It MUST NOT be generated by clients.</t>
474
475    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
476     headers that is not rectified by a revalidation (for example, a
477     lossy compression of the entity bodies) and which MUST NOT be
478     deleted after a successful revalidation.</t>
479    </list>
480</t>
481<t>
482   See <xref target="header.warning"/> for the definitions of the codes themselves.
483</t>
484<t>
485   HTTP/1.0 caches will cache all Warnings in responses, without
486   deleting the ones in the first category. Warnings in responses that
487   are passed to HTTP/1.0 caches carry an extra warning-date field,
488   which prevents a future HTTP/1.1 recipient from believing an
489   erroneously cached Warning.
490</t>
491<t>
492   Warnings also carry a warning text. The text MAY be in any
493   appropriate natural language (perhaps based on the client's Accept
494   headers), and include an OPTIONAL indication of what character set is
495   used.
496</t>
497<t>
498   Multiple warnings MAY be attached to a response (either by the origin
499   server or by a cache), including multiple warnings with the same code
500   number. For example, a server might provide the same warning with
501   texts in both English and Basque.
502</t>
503<t>
504   When multiple warnings are attached to a response, it might not be
505   practical or reasonable to display all of them to the user. This
506   version of HTTP does not specify strict priority rules for deciding
507   which warnings to display and in what order, but does suggest some
508   heuristics.
509</t>
510</section>
511
512<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
513<t>
514   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
515   times and validators) are implicit directives to caches. In some
516   cases, a server or client might need to provide explicit directives
517   to the HTTP caches. We use the Cache-Control header for this purpose.
518</t>
519<t>
520   The Cache-Control header allows a client or server to transmit a
521   variety of directives in either requests or responses. These
522   directives typically override the default caching algorithms. As a
523   general rule, if there is any apparent conflict between header
524   values, the most restrictive interpretation is applied (that is, the
525   one that is most likely to preserve semantic transparency). However,
526   in some cases, cache-control directives are explicitly specified as
527   weakening the approximation of semantic transparency (for example,
528   "max-stale" or "public").
529</t>
530<t>
531   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
532</t>
533</section>
534
535<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
536<t>
537   Many user agents make it possible for users to override the basic
538   caching mechanisms. For example, the user agent might allow the user
539   to specify that cached entities (even explicitly stale ones) are
540   never validated. Or the user agent might habitually add "Cache-Control:
541   max-stale=3600" to every request. The user agent SHOULD NOT
542   default to either non-transparent behavior, or behavior that results
543   in abnormally ineffective caching, but MAY be explicitly configured
544   to do so by an explicit action of the user.
545</t>
546<t>
547   If the user has overridden the basic caching mechanisms, the user
548   agent SHOULD explicitly indicate to the user whenever this results in
549   the display of information that might not meet the server's
550   transparency requirements (in particular, if the displayed entity is
551   known to be stale). Since the protocol normally allows the user agent
552   to determine if responses are stale or not, this indication need only
553   be displayed when this actually happens. The indication need not be a
554   dialog box; it could be an icon (for example, a picture of a rotting
555   fish) or some other indicator.
556</t>
557<t>
558   If the user has overridden the caching mechanisms in a way that would
559   abnormally reduce the effectiveness of caches, the user agent SHOULD
560   continually indicate this state to the user (for example, by a
561   display of a picture of currency in flames) so that the user does not
562   inadvertently consume excess resources or suffer from excessive
563   latency.
564</t>
565</section>
566
567<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
568<t>
569   In some cases, the operator of a cache MAY choose to configure it to
570   return stale responses even when not requested by clients. This
571   decision ought not be made lightly, but may be necessary for reasons
572   of availability or performance, especially when the cache is poorly
573   connected to the origin server. Whenever a cache returns a stale
574   response, it MUST mark it as such (using a Warning header) enabling
575   the client software to alert the user that there might be a potential
576   problem.
577</t>
578<t>
579   It also allows the user agent to take steps to obtain a first-hand or
580   fresh response. For this reason, a cache SHOULD NOT  return a stale
581   response if the client explicitly requests a first-hand or fresh one,
582   unless it is impossible to comply for technical or policy reasons.
583</t>
584</section>
585
586<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
587<t>
588   While the origin server (and to a lesser extent, intermediate caches,
589   by their contribution to the age of a response) are the primary
590   source of expiration information, in some cases the client might need
591   to control a cache's decision about whether to return a cached
592   response without validating it. Clients do this using several
593   directives of the Cache-Control header.
594</t>
595<t>
596   A client's request MAY specify the maximum age it is willing to
597   accept of an unvalidated response; specifying a value of zero forces
598   the cache(s) to revalidate all responses. A client MAY also specify
599   the minimum time remaining before a response expires. Both of these
600   options increase constraints on the behavior of caches, and so cannot
601   further relax the cache's approximation of semantic transparency.
602</t>
603<t>
604   A client MAY also specify that it will accept stale responses, up to
605   some maximum amount of staleness. This loosens the constraints on the
606   caches, and so might violate the origin server's specified
607   constraints on semantic transparency, but might be necessary to
608   support disconnected operation, or high availability in the face of
609   poor connectivity.
610</t>
611</section>
612</section>
613
614<section title="Expiration Model" anchor="expiration.model">
615
616<section title="Server-Specified Expiration" anchor="server-specified.expiration">
617<t>
618   HTTP caching works best when caches can entirely avoid making
619   requests to the origin server. The primary mechanism for avoiding
620   requests is for an origin server to provide an explicit expiration
621   time in the future, indicating that a response MAY be used to satisfy
622   subsequent requests. In other words, a cache can return a fresh
623   response without first contacting the server.
624</t>
625<t>
626   Our expectation is that servers will assign future explicit
627   expiration times to responses in the belief that the entity is not
628   likely to change, in a semantically significant way, before the
629   expiration time is reached. This normally preserves semantic
630   transparency, as long as the server's expiration times are carefully
631   chosen.
632</t>
633<t>
634   The expiration mechanism applies only to responses taken from a cache
635   and not to first-hand responses forwarded immediately to the
636   requesting client.
637</t>
638<t>
639   If an origin server wishes to force a semantically transparent cache
640   to validate every request, it MAY assign an explicit expiration time
641   in the past. This means that the response is always stale, and so the
642   cache SHOULD validate it before using it for subsequent requests. See
643   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
644</t>
645<t>
646   If an origin server wishes to force any HTTP/1.1 cache, no matter how
647   it is configured, to validate every request, it SHOULD use the "must-revalidate"
648   cache-control directive (see <xref target="header.cache-control"/>).
649</t>
650<t>
651   Servers specify explicit expiration times using either the Expires
652   header, or the max-age directive of the Cache-Control header.
653</t>
654<t>
655   An expiration time cannot be used to force a user agent to refresh
656   its display or reload a resource; its semantics apply only to caching
657   mechanisms, and such mechanisms need only check a resource's
658   expiration status when a new request for that resource is initiated.
659   See <xref target="history.lists"/> for an explanation of the difference between caches
660   and history mechanisms.
661</t>
662</section>
663
664<section title="Heuristic Expiration" anchor="heuristic.expiration">
665<t>
666   Since origin servers do not always provide explicit expiration times,
667   HTTP caches typically assign heuristic expiration times, employing
668   algorithms that use other header values (such as the Last-Modified
669   time) to estimate a plausible expiration time. The HTTP/1.1
670   specification does not provide specific algorithms, but does impose
671   worst-case constraints on their results. Since heuristic expiration
672   times might compromise semantic transparency, they ought to be used
673   cautiously, and we encourage origin servers to provide explicit
674   expiration times as much as possible.
675</t>
676</section>
677
678<section title="Age Calculations" anchor="age.calculations">
679<t>
680   In order to know if a cached entry is fresh, a cache needs to know if
681   its age exceeds its freshness lifetime. We discuss how to calculate
682   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
683   the age of a response or cache entry.
684</t>
685<t>
686   In this discussion, we use the term "now" to mean "the current value
687   of the clock at the host performing the calculation." Hosts that use
688   HTTP, but especially hosts running origin servers and caches, SHOULD
689   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
690   a globally accurate time standard.
691</t>
692<t>
693   HTTP/1.1 requires origin servers to send a Date header, if possible,
694   with every response, giving the time at which the response was
695   generated (see Section 8.3 of <xref target="Part1"/>). We use the term "date_value" to denote
696   the value of the Date header, in a form appropriate for arithmetic
697   operations.
698</t>
699<t>
700   HTTP/1.1 uses the Age response-header to convey the estimated age of
701   the response message when obtained from a cache. The Age field value
702   is the cache's estimate of the amount of time since the response was
703   generated or revalidated by the origin server.
704</t>
705<t>
706   In essence, the Age value is the sum of the time that the response
707   has been resident in each of the caches along the path from the
708   origin server, plus the amount of time it has been in transit along
709   network paths.
710</t>
711<t>
712   We use the term "age_value" to denote the value of the Age header, in
713   a form appropriate for arithmetic operations.
714</t>
715<t>
716   A response's age can be calculated in two entirely independent ways:
717  <list style="numbers">
718      <t>now minus date_value, if the local clock is reasonably well
719         synchronized to the origin server's clock. If the result is
720         negative, the result is replaced by zero.</t>
721
722      <t>age_value, if all of the caches along the response path
723         implement HTTP/1.1.</t>
724  </list>
725</t>
726<t>
727   Given that we have two independent ways to compute the age of a
728   response when it is received, we can combine these as
729</t>
730<figure><artwork type="code"><![CDATA[
731    corrected_received_age = max(now - date_value, age_value)
732]]></artwork></figure>
733<t>
734   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
735   paths, one gets a reliable (conservative) result.
736</t>
737<t>
738   Because of network-imposed delays, some significant interval might
739   pass between the time that a server generates a response and the time
740   it is received at the next outbound cache or client. If uncorrected,
741   this delay could result in improperly low ages.
742</t>
743<t>
744   Because the request that resulted in the returned Age value must have
745   been initiated prior to that Age value's generation, we can correct
746   for delays imposed by the network by recording the time at which the
747   request was initiated. Then, when an Age value is received, it MUST
748   be interpreted relative to the time the request was initiated, not
749   the time that the response was received. This algorithm results in
750   conservative behavior no matter how much delay is experienced. So, we
751   compute:
752</t>
753<figure><artwork type="code"><![CDATA[
754   corrected_initial_age = corrected_received_age
755                         + (now - request_time)
756]]></artwork></figure>
757<t>
758   where "request_time" is the time (according to the local clock) when
759   the request that elicited this response was sent.
760</t>
761<t>
762   Summary of age calculation algorithm, when a cache receives a
763   response:
764</t>
765<figure><artwork type="code"><![CDATA[
766   /*
767    * age_value
768    *      is the value of Age: header received by the cache with
769    *              this response.
770    * date_value
771    *      is the value of the origin server's Date: header
772    * request_time
773    *      is the (local) time when the cache made the request
774    *              that resulted in this cached response
775    * response_time
776    *      is the (local) time when the cache received the
777    *              response
778    * now
779    *      is the current (local) time
780    */
781
782   apparent_age = max(0, response_time - date_value);
783   corrected_received_age = max(apparent_age, age_value);
784   response_delay = response_time - request_time;
785   corrected_initial_age = corrected_received_age + response_delay;
786   resident_time = now - response_time;
787   current_age   = corrected_initial_age + resident_time;
788]]></artwork></figure>
789<t>
790   The current_age of a cache entry is calculated by adding the amount
791   of time (in seconds) since the cache entry was last validated by the
792   origin server to the corrected_initial_age. When a response is
793   generated from a cache entry, the cache MUST include a single Age
794   header field in the response with a value equal to the cache entry's
795   current_age.
796</t>
797<t>
798   The presence of an Age header field in a response implies that a
799   response is not first-hand. However, the converse is not true, since
800   the lack of an Age header field in a response does not imply that the
801   response is first-hand unless all caches along the request path are
802   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
803   the Age header field).
804</t>
805</section>
806
807<section title="Expiration Calculations" anchor="expiration.calculations">
808<t>
809   In order to decide whether a response is fresh or stale, we need to
810   compare its freshness lifetime to its age. The age is calculated as
811   described in <xref target="age.calculations"/>; this section describes how to calculate
812   the freshness lifetime, and to determine if a response has expired.
813   In the discussion below, the values can be represented in any form
814   appropriate for arithmetic operations.
815</t>
816<t>
817   We use the term "expires_value" to denote the value of the Expires
818   header. We use the term "max_age_value" to denote an appropriate
819   value of the number of seconds carried by the "max-age" directive of
820   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
821</t>
822<t>
823   The max-age directive takes priority over Expires, so if max-age is
824   present in a response, the calculation is simply:
825</t>
826<figure><artwork type="code"><![CDATA[
827   freshness_lifetime = max_age_value
828]]></artwork></figure>
829<t>
830   Otherwise, if Expires is present in the response, the calculation is:
831</t>
832<figure><artwork type="code"><![CDATA[
833   freshness_lifetime = expires_value - date_value
834]]></artwork></figure>
835<t>
836   Note that neither of these calculations is vulnerable to clock skew,
837   since all of the information comes from the origin server.
838</t>
839<t>
840   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
841   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
842   does not include other restrictions on caching, the cache MAY compute
843   a freshness lifetime using a heuristic. The cache MUST attach Warning
844   113 to any response whose age is more than 24 hours if such warning
845   has not already been added.
846</t>
847<t>
848   Also, if the response does have a Last-Modified time, the heuristic
849   expiration value SHOULD be no more than some fraction of the interval
850   since that time. A typical setting of this fraction might be 10%.
851</t>
852<t>
853   The calculation to determine if a response has expired is quite
854   simple:
855</t>
856<figure><artwork type="code"><![CDATA[
857   response_is_fresh = (freshness_lifetime > current_age)
858]]></artwork></figure>
859</section>
860
861<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
862<t>
863   Because expiration values are assigned optimistically, it is possible
864   for two caches to contain fresh values for the same resource that are
865   different.
866</t>
867<t>
868   If a client performing a retrieval receives a non-first-hand response
869   for a request that was already fresh in its own cache, and the Date
870   header in its existing cache entry is newer than the Date on the new
871   response, then the client MAY ignore the response. If so, it MAY
872   retry the request with a "Cache-Control: max-age=0" directive (see
873   <xref target="header.cache-control"/>), to force a check with the origin server.
874</t>
875<t>
876   If a cache has two fresh responses for the same representation with
877   different validators, it MUST use the one with the more recent Date
878   header. This situation might arise because the cache is pooling
879   responses from other caches, or because a client has asked for a
880   reload or a revalidation of an apparently fresh cache entry.
881</t>
882</section>
883
884<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
885<t>
886   Because a client might be receiving responses via multiple paths, so
887   that some responses flow through one set of caches and other
888   responses flow through a different set of caches, a client might
889   receive responses in an order different from that in which the origin
890   server sent them. We would like the client to use the most recently
891   generated response, even if older responses are still apparently
892   fresh.
893</t>
894<t>
895   Neither the entity tag nor the expiration value can impose an
896   ordering on responses, since it is possible that a later response
897   intentionally carries an earlier expiration time. The Date values are
898   ordered to a granularity of one second.
899</t>
900<t>
901   When a client tries to revalidate a cache entry, and the response it
902   receives contains a Date header that appears to be older than the one
903   for the existing entry, then the client SHOULD repeat the request
904   unconditionally, and include
905</t>
906<figure><artwork type="example"><![CDATA[
907    Cache-Control: max-age=0
908]]></artwork></figure>
909<t>
910   to force any intermediate caches to validate their copies directly
911   with the origin server, or
912</t>
913<figure><artwork type="example"><![CDATA[
914    Cache-Control: no-cache
915]]></artwork></figure>
916<t>
917   to force any intermediate caches to obtain a new copy from the origin
918   server.
919</t>
920<t>
921   If the Date values are equal, then the client MAY use either response
922   (or MAY, if it is being extremely prudent, request a new response).
923   Servers MUST NOT depend on clients being able to choose
924   deterministically between responses generated during the same second,
925   if their expiration times overlap.
926</t>
927</section>
928</section>
929
930<section title="Validation Model" anchor="validation.model">
931<t>
932   When a cache has a stale entry that it would like to use as a
933   response to a client's request, it first has to check with the origin
934   server (or possibly an intermediate cache with a fresh response) to
935   see if its cached entry is still usable. We call this "validating"
936   the cache entry. Since we do not want to have to pay the overhead of
937   retransmitting the full response if the cached entry is good, and we
938   do not want to pay the overhead of an extra round trip if the cached
939   entry is invalid, the HTTP/1.1 protocol supports the use of
940   conditional methods.
941</t>
942<t>
943   The key protocol features for supporting conditional methods are
944   those concerned with "cache validators." When an origin server
945   generates a full response, it attaches some sort of validator to it,
946   which is kept with the cache entry. When a client (user agent or
947   proxy cache) makes a conditional request for a resource for which it
948   has a cache entry, it includes the associated validator in the
949   request.
950</t>
951<t>
952   The server then checks that validator against the current validator
953   for the entity, and, if they match (see Section 4 of <xref target="Part4"/>), it responds
954   with a special status code (usually, 304 (Not Modified)) and no
955   entity-body. Otherwise, it returns a full response (including
956   entity-body). Thus, we avoid transmitting the full response if the
957   validator matches, and we avoid an extra round trip if it does not
958   match.
959</t>
960<t>
961   In HTTP/1.1, a conditional request looks exactly the same as a normal
962   request for the same resource, except that it carries a special
963   header (which includes the validator) that implicitly turns the
964   method (usually, GET) into a conditional.
965</t>
966<t>
967   The protocol includes both positive and negative senses of cache-validating
968   conditions. That is, it is possible to request either that
969   a method be performed if and only if a validator matches or if and
970   only if no validators match.
971  <list><t>
972      Note: a response that lacks a validator may still be cached, and
973      served from cache until it expires, unless this is explicitly
974      prohibited by a cache-control directive. However, a cache cannot
975      do a conditional retrieval if it does not have a validator for the
976      entity, which means it will not be refreshable after it expires.
977  </t></list>
978</t>
979
980<section title="Last-Modified Dates" anchor="last-modified.dates">
981<t>
982   The Last-Modified entity-header field value is often used as a cache
983   validator. In simple terms, a cache entry is considered to be valid
984   if the entity has not been modified since the Last-Modified value.
985</t>
986</section>
987
988<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
989<t>
990   The ETag response-header field value, an entity tag, provides for an
991   "opaque" cache validator. This might allow more reliable validation
992   in situations where it is inconvenient to store modification dates,
993   where the one-second resolution of HTTP date values is not
994   sufficient, or where the origin server wishes to avoid certain
995   paradoxes that might arise from the use of modification dates.
996</t>
997<t>
998   Entity Tags are described in Section 2 of <xref target="Part4"/>. The headers used with entity
999   tags are described in Section 6 of <xref target="Part4"/>.
1000</t>
1001</section>
1002
1003<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1004<t>
1005   The principle behind entity tags is that only the service author
1006   knows the semantics of a resource well enough to select an
1007   appropriate cache validation mechanism, and the specification of any
1008   validator comparison function more complex than byte-equality would
1009   open up a can of worms. Thus, comparisons of any other headers
1010   (except Last-Modified, for compatibility with HTTP/1.0) are never
1011   used for purposes of validating a cache entry.
1012</t>
1013</section>
1014</section>
1015
1016<section title="Response Cacheability" anchor="response.cacheability">
1017<t>
1018   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1019   directive, a caching system MAY always store a successful response
1020   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, MAY return it without validation
1021   if it is fresh, and MAY return it after successful validation. If
1022   there is neither a cache validator nor an explicit expiration time
1023   associated with a response, we do not expect it to be cached, but
1024   certain caches MAY violate this expectation (for example, when little
1025   or no network connectivity is available). A client can usually detect
1026   that such a response was taken from a cache by comparing the Date
1027   header to the current time.
1028  <list><t>
1029      Note: some HTTP/1.0 caches are known to violate this expectation
1030      without providing any Warning.
1031  </t></list>
1032</t>
1033<t>
1034   However, in some cases it might be inappropriate for a cache to
1035   retain an entity, or to return it in response to a subsequent
1036   request. This might be because absolute semantic transparency is
1037   deemed necessary by the service author, or because of security or
1038   privacy considerations. Certain cache-control directives are
1039   therefore provided so that the server can indicate that certain
1040   resource entities, or portions thereof, are not to be cached
1041   regardless of other considerations.
1042</t>
1043<t>
1044   Note that Section 3.1 of <xref target="Part7"/> normally prevents a shared cache from saving
1045   and returning a response to a previous request if that request
1046   included an Authorization header.
1047</t>
1048<t>
1049   A response received with a status code of 200, 203, 206, 300, 301 or
1050   410 MAY be stored by a cache and used in reply to a subsequent
1051   request, subject to the expiration mechanism, unless a cache-control
1052   directive prohibits caching. However, a cache that does not support
1053   the Range and Content-Range headers MUST NOT cache 206 (Partial
1054   Content) responses.
1055</t>
1056<t>
1057   A response received with any other status code (e.g. status codes 302
1058   and 307) MUST NOT be returned in a reply to a subsequent request
1059   unless there are cache-control directives or another header(s) that
1060   explicitly allow it. For example, these include the following: an
1061   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1062   "proxy-revalidate", "public" or "private" cache-control
1063   directive (<xref target="header.cache-control"/>).
1064</t>
1065</section>
1066
1067<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1068<t>
1069   The purpose of an HTTP cache is to store information received in
1070   response to requests for use in responding to future requests. In
1071   many cases, a cache simply returns the appropriate parts of a
1072   response to the requester. However, if the cache holds a cache entry
1073   based on a previous response, it might have to combine parts of a new
1074   response with what is held in the cache entry.
1075</t>
1076
1077<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1078<t>
1079   For the purpose of defining the behavior of caches and non-caching
1080   proxies, we divide HTTP headers into two categories:
1081  <list style="symbols">
1082      <t>End-to-end headers, which are  transmitted to the ultimate
1083        recipient of a request or response. End-to-end headers in
1084        responses MUST be stored as part of a cache entry and MUST be
1085        transmitted in any response formed from a cache entry.</t>
1086
1087      <t>Hop-by-hop headers, which are meaningful only for a single
1088        transport-level connection, and are not stored by caches or
1089        forwarded by proxies.</t>
1090  </list>
1091</t>
1092<t>
1093   The following HTTP/1.1 headers are hop-by-hop headers:
1094  <list style="symbols">
1095      <t>Connection</t>
1096      <t>Keep-Alive</t>
1097      <t>Proxy-Authenticate</t>
1098      <t>Proxy-Authorization</t>
1099      <t>TE</t>
1100      <t>Trailer</t>
1101      <t>Transfer-Encoding</t>
1102      <t>Upgrade</t>
1103  </list>
1104</t>
1105<t>
1106   All other headers defined by HTTP/1.1 are end-to-end headers.
1107</t>
1108<t>
1109   Other hop-by-hop headers MUST be listed in a Connection header
1110   (Section 8.1 of <xref target="Part1"/>).
1111</t>
1112</section>
1113
1114<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1115<t>
1116   Some features of the HTTP/1.1 protocol, such as Digest
1117   Authentication, depend on the value of certain end-to-end headers. A
1118   transparent proxy SHOULD NOT  modify an end-to-end header unless the
1119   definition of that header requires or specifically allows that.
1120</t>
1121<t>
1122   A transparent proxy MUST NOT modify any of the following fields in a
1123   request or response, and it MUST NOT add any of these fields if not
1124   already present:
1125  <list style="symbols">
1126      <t>Content-Location</t>
1127      <t>Content-MD5</t>
1128      <t>ETag</t>
1129      <t>Last-Modified</t>
1130  </list>
1131</t>
1132<t>
1133   A transparent proxy MUST NOT modify any of the following fields in a
1134   response:
1135  <list style="symbols">
1136    <t>Expires</t>
1137  </list>
1138</t>
1139<t>
1140   but it MAY add any of these fields if not already present. If an
1141   Expires header is added, it MUST be given a field-value identical to
1142   that of the Date header in that response.
1143</t>
1144<t>
1145   A  proxy MUST NOT modify or add any of the following fields in a
1146   message that contains the no-transform cache-control directive, or in
1147   any request:
1148  <list style="symbols">
1149    <t>Content-Encoding</t>
1150    <t>Content-Range</t>
1151    <t>Content-Type</t>
1152  </list>
1153</t>
1154<t>
1155   A non-transparent proxy MAY modify or add these fields to a message
1156   that does not include no-transform, but if it does so, it MUST add a
1157   Warning 214 (Transformation applied) if one does not already appear
1158   in the message (see <xref target="header.warning"/>).
1159  <list><t>
1160      Warning: unnecessary modification of end-to-end headers might
1161      cause authentication failures if stronger authentication
1162      mechanisms are introduced in later versions of HTTP. Such
1163      authentication mechanisms MAY rely on the values of header fields
1164      not listed here.
1165    </t></list>
1166</t>
1167<t>
1168   The Content-Length field of a request or response is added or deleted
1169   according to the rules in Section 4.4 of <xref target="Part1"/>. A transparent proxy MUST
1170   preserve the entity-length (Section 3.2.2 of <xref target="Part3"/>) of the entity-body,
1171   although it MAY change the transfer-length (Section 4.4 of <xref target="Part1"/>).
1172</t>
1173</section>
1174
1175<section title="Combining Headers" anchor="combining.headers">
1176<t>
1177   When a cache makes a validating request to a server, and the server
1178   provides a 304 (Not Modified) response or a 206 (Partial Content)
1179   response, the cache then constructs a response to send to the
1180   requesting client.
1181</t>
1182<t>
1183   If the status code is 304 (Not Modified), the cache uses the entity-body
1184   stored in the cache entry as the entity-body of this outgoing
1185   response. If the status code is 206 (Partial Content) and the ETag or
1186   Last-Modified headers match exactly, the cache MAY combine the
1187   contents stored in the cache entry with the new contents received in
1188   the response and use the result as the entity-body of this outgoing
1189   response, (see Section 4 of <xref target="Part5"/>).
1190</t>
1191<t>
1192   The end-to-end headers stored in the cache entry are used for the
1193   constructed response, except that
1194  <list style="symbols">
1195    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1196      MUST be deleted from the cache entry and the forwarded response.</t>
1197    <t>any stored Warning headers with warn-code 2xx MUST be retained
1198        in the cache entry and the forwarded response.</t>
1199    <t>any end-to-end headers provided in the 304 or 206 response MUST
1200        replace the corresponding headers from the cache entry.</t>
1201  </list>
1202</t>
1203<t>
1204   Unless the cache decides to remove the cache entry, it MUST also
1205   replace the end-to-end headers stored with the cache entry with
1206   corresponding headers received in the incoming response, except for
1207   Warning headers as described immediately above. If a header field-name
1208   in the incoming response matches more than one header in the
1209   cache entry, all such old headers MUST be replaced.
1210</t>
1211<t>
1212   In other words, the set of end-to-end headers received in the
1213   incoming response overrides all corresponding end-to-end headers
1214   stored with the cache entry (except for stored Warning headers with
1215   warn-code 1xx, which are deleted even if not overridden).
1216  <list><t>
1217      Note: this rule allows an origin server to use a 304 (Not
1218      Modified) or a 206 (Partial Content) response to update any header
1219      associated with a previous response for the same entity or sub-ranges
1220      thereof, although it might not always be meaningful or
1221      correct to do so. This rule does not allow an origin server to use
1222      a 304 (Not Modified) or a 206 (Partial Content) response to
1223      entirely delete a header that it had provided with a previous
1224      response.
1225  </t></list>
1226</t>
1227</section>
1228
1229</section>
1230
1231<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1232<t>
1233   Use of server-driven content negotiation (Section 4.1 of <xref target="Part3"/>), as indicated
1234   by the presence of a Vary header field in a response, alters the
1235   conditions and procedure by which a cache can use the response for
1236   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1237   field by servers.
1238</t>
1239<t>
1240   A server SHOULD use the Vary header field to inform a cache of what
1241   request-header fields were used to select among multiple
1242   representations of a cacheable response subject to server-driven
1243   negotiation. The set of header fields named by the Vary field value
1244   is known as the "selecting" request-headers.
1245</t>
1246<t>
1247   When the cache receives a subsequent request whose Request-URI
1248   specifies one or more cache entries including a Vary header field,
1249   the cache MUST NOT use such a cache entry to construct a response to
1250   the new request unless all of the selecting request-headers present
1251   in the new request match the corresponding stored request-headers in
1252   the original request.
1253</t>
1254<t>
1255   The selecting request-headers from two requests are defined to match
1256   if and only if the selecting request-headers in the first request can
1257   be transformed to the selecting request-headers in the second request
1258   by adding or removing linear white space (LWS) at places where this
1259   is allowed by the corresponding BNF, and/or combining multiple
1260   message-header fields with the same field name following the rules
1261   about message headers in Section 4.2 of <xref target="Part1"/>.
1262</t>
1263<t>
1264   A Vary header field-value of "*" always fails to match and subsequent
1265   requests on that resource can only be properly interpreted by the
1266   origin server.
1267</t>
1268<t>
1269   If the selecting request header fields for the cached entry do not
1270   match the selecting request header fields of the new request, then
1271   the cache MUST NOT use a cached entry to satisfy the request unless
1272   it first relays the new request to the origin server in a conditional
1273   request and the server responds with 304 (Not Modified), including an
1274   entity tag or Content-Location that indicates the entity to be used.
1275</t>
1276<t>
1277   If an entity tag was assigned to a cached representation, the
1278   forwarded request SHOULD be conditional and include the entity tags
1279   in an If-None-Match header field from all its cache entries for the
1280   resource. This conveys to the server the set of entities currently
1281   held by the cache, so that if any one of these entities matches the
1282   requested entity, the server can use the ETag header field in its 304
1283   (Not Modified) response to tell the cache which entry is appropriate.
1284   If the entity-tag of the new response matches that of an existing
1285   entry, the new response SHOULD be used to update the header fields of
1286   the existing entry, and the result MUST be returned to the client.
1287</t>
1288<t>
1289   If any of the existing cache entries contains only partial content
1290   for the associated entity, its entity-tag SHOULD NOT  be included in
1291   the If-None-Match header field unless the request is for a range that
1292   would be fully satisfied by that entry.
1293</t>
1294<t>
1295   If a cache receives a successful response whose Content-Location
1296   field matches that of an existing cache entry for the same Request-URI,
1297   whose entity-tag differs from that of the existing entry, and
1298   whose Date is more recent than that of the existing entry, the
1299   existing entry SHOULD NOT  be returned in response to future requests
1300   and SHOULD be deleted from the cache.
1301</t>
1302</section>
1303
1304<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1305<t>
1306   For reasons of security and privacy, it is necessary to make a
1307   distinction between "shared" and "non-shared" caches. A non-shared
1308   cache is one that is accessible only to a single user. Accessibility
1309   in this case SHOULD be enforced by appropriate security mechanisms.
1310   All other caches are considered to be "shared." Other sections of
1311   this specification place certain constraints on the operation of
1312   shared caches in order to prevent loss of privacy or failure of
1313   access controls.
1314</t>
1315</section>
1316
1317<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1318<t>
1319   A cache that receives an incomplete response (for example, with fewer
1320   bytes of data than specified in a Content-Length header) MAY store
1321   the response. However, the cache MUST treat this as a partial
1322   response. Partial responses MAY be combined as described in Section 4 of <xref target="Part5"/>;
1323   the result might be a full response or might still be
1324   partial. A cache MUST NOT return a partial response to a client
1325   without explicitly marking it as such, using the 206 (Partial
1326   Content) status code. A cache MUST NOT return a partial response
1327   using a status code of 200 (OK).
1328</t>
1329<t>
1330   If a cache receives a 5xx response while attempting to revalidate an
1331   entry, it MAY either forward this response to the requesting client,
1332   or act as if the server failed to respond. In the latter case, it MAY
1333   return a previously received response unless the cached entry
1334   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1335</t>
1336</section>
1337
1338<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1339<t>
1340   Unless the origin server explicitly prohibits the caching of their
1341   responses, the application of GET and HEAD methods to any resources
1342   SHOULD NOT  have side effects that would lead to erroneous behavior if
1343   these responses are taken from a cache. They MAY still have side
1344   effects, but a cache is not required to consider such side effects in
1345   its caching decisions. Caches are always expected to observe an
1346   origin server's explicit restrictions on caching.
1347</t>
1348<t>
1349   We note one exception to this rule: since some applications have
1350   traditionally used GETs and HEADs with query URLs (those containing a
1351   "?" in the rel_path part) to perform operations with significant side
1352   effects, caches MUST NOT treat responses to such URIs as fresh unless
1353   the server provides an explicit expiration time. This specifically
1354   means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
1355   be taken from a cache. See Section 8.1.1 of <xref target="Part2"/> for related information.
1356</t>
1357</section>
1358
1359<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1360<t>
1361   The effect of certain methods performed on a resource at the origin
1362   server might cause one or more existing cache entries to become non-transparently
1363   invalid. That is, although they might continue to be
1364   "fresh," they do not accurately reflect what the origin server would
1365   return for a new request on that resource.
1366</t>
1367<t>
1368   There is no way for the HTTP protocol to guarantee that all such
1369   cache entries are marked invalid. For example, the request that
1370   caused the change at the origin server might not have gone through
1371   the proxy where a cache entry is stored. However, several rules help
1372   reduce the likelihood of erroneous behavior.
1373</t>
1374<t>
1375   In this section, the phrase "invalidate an entity" means that the
1376   cache will either remove all instances of that entity from its
1377   storage, or will mark these as "invalid" and in need of a mandatory
1378   revalidation before they can be returned in response to a subsequent
1379   request.
1380</t>
1381<t>
1382   Some HTTP methods MUST cause a cache to invalidate an entity. This is
1383   either the entity referred to by the Request-URI, or by the Location
1384   or Content-Location headers (if present). These methods are:
1385  <list style="symbols">
1386      <t>PUT</t>
1387      <t>DELETE</t>
1388      <t>POST</t>
1389  </list>
1390</t> 
1391<t>
1392   An invalidation based
1393   on the URI in a Location or Content-Location header MUST NOT be
1394   performed if the host part of that URI differs from the host part
1395   in the Request-URI. This helps prevent denial of service attacks.
1396</t>
1397<t>
1398   A cache that passes through requests for methods it does not
1399   understand SHOULD invalidate any entities referred to by the
1400   Request-URI.
1401</t>
1402</section>
1403
1404<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1405<t>
1406   All methods that might be expected to cause modifications to the
1407   origin server's resources MUST be written through to the origin
1408   server. This currently includes all methods except for GET and HEAD.
1409   A cache MUST NOT reply to such a request from a client before having
1410   transmitted the request to the inbound server, and having received a
1411   corresponding response from the inbound server. This does not prevent
1412   a proxy cache from sending a 100 (Continue) response before the
1413   inbound server has sent its final reply.
1414</t>
1415<t>
1416   The alternative (known as "write-back" or "copy-back" caching) is not
1417   allowed in HTTP/1.1, due to the difficulty of providing consistent
1418   updates and the problems arising from server, cache, or network
1419   failure prior to write-back.
1420</t>
1421</section>
1422
1423<section title="Cache Replacement" anchor="cache.replacement">
1424<t>
1425   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1426   <xref target="disambiguating.expiration.values" format="counter"/>,
1427   <xref target="disambiguating.multiple.responses" format="counter"/>
1428   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1429   response is received from a resource while any existing responses for
1430   the same resource are cached, the cache SHOULD use the new response
1431   to reply to the current request. It MAY insert it into cache storage
1432   and MAY, if it meets all other requirements, use it to respond to any
1433   future requests that would previously have caused the old response to
1434   be returned. If it inserts the new response into cache storage  the
1435   rules in <xref target="combining.headers"/> apply.
1436  <list><t>
1437      Note: a new response that has an older Date header value than
1438      existing cached responses is not cacheable.
1439  </t></list>
1440</t>
1441</section>
1442
1443<section title="History Lists" anchor="history.lists">
1444<t>
1445   User agents often have history mechanisms, such as "Back" buttons and
1446   history lists, which can be used to redisplay an entity retrieved
1447   earlier in a session.
1448</t>
1449<t>
1450   History mechanisms and caches are different. In particular history
1451   mechanisms SHOULD NOT  try to show a semantically transparent view of
1452   the current state of a resource. Rather, a history mechanism is meant
1453   to show exactly what the user saw at the time when the resource was
1454   retrieved.
1455</t>
1456<t>
1457   By default, an expiration time does not apply to history mechanisms.
1458   If the entity is still in storage, a history mechanism SHOULD display
1459   it even if the entity has expired, unless the user has specifically
1460   configured the agent to refresh expired history documents.
1461</t>
1462<t>
1463   This is not to be construed to prohibit the history mechanism from
1464   telling the user that a view might be stale.
1465  <list><t>
1466      Note: if history list mechanisms unnecessarily prevent users from
1467      viewing stale resources, this will tend to force service authors
1468      to avoid using HTTP expiration controls and cache controls when
1469      they would otherwise like to. Service authors may consider it
1470      important that users not be presented with error messages or
1471      warning messages when they use navigation controls (such as BACK)
1472      to view previously fetched resources. Even though sometimes such
1473      resources ought not be cached, or ought to expire quickly, user
1474      interface considerations may force service authors to resort to
1475      other means of preventing caching (e.g. "once-only" URLs) in order
1476      not to suffer the effects of improperly functioning history
1477      mechanisms.
1478  </t></list>
1479</t>
1480</section>
1481
1482<section title="Header Field Definitions" anchor="header.fields">
1483<t>
1484   This section defines the syntax and semantics of HTTP/1.1 header fields
1485   related to caching.
1486</t>
1487<t>
1488   For entity-header fields, both sender and recipient refer to either the
1489   client or the server, depending on who sends and who receives the entity.
1490</t>
1491
1492<section title="Age" anchor="header.age">
1493  <iref primary="true" item="Age header"/>
1494  <iref primary="true" item="Headers" subitem="Age"/>
1495<t>
1496      The Age response-header field conveys the sender's estimate of the
1497      amount of time since the response (or its revalidation) was
1498      generated at the origin server. A cached response is "fresh" if
1499      its age does not exceed its freshness lifetime. Age values are
1500      calculated as specified in <xref target="age.calculations"/>.
1501</t>
1502<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/><artwork type="abnf2616"><![CDATA[
1503  Age = "Age" ":" age-value
1504  age-value = delta-seconds
1505]]></artwork></figure>
1506<t>
1507      Age values are non-negative decimal integers, representing time in
1508      seconds.
1509</t>
1510<figure><iref primary="true" item="Grammar" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
1511  delta-seconds  = 1*DIGIT
1512]]></artwork></figure>
1513<t>
1514      If a cache receives a value larger than the largest positive
1515      integer it can represent, or if any of its age calculations
1516      overflows, it MUST transmit an Age header with a value of
1517      2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
1518      include an Age header field in every response generated from its
1519      own cache. Caches SHOULD use an arithmetic type of at least 31
1520      bits of range.
1521</t>
1522</section>
1523
1524<section title="Cache-Control" anchor="header.cache-control">
1525  <iref primary="true" item="Cache-Control header"/>
1526  <iref primary="true" item="Headers" subitem="Cache-Control"/>
1527<t>
1528   The Cache-Control general-header field is used to specify directives
1529   that MUST be obeyed by all caching mechanisms along the
1530   request/response chain. The directives specify behavior intended to
1531   prevent caches from adversely interfering with the request or
1532   response. These directives typically override the default caching
1533   algorithms. Cache directives are unidirectional in that the presence
1534   of a directive in a request does not imply that the same directive is
1535   to be given in the response.
1536  <list><t>
1537      Note that HTTP/1.0 caches might not implement Cache-Control and
1538      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1539  </t></list>
1540</t>
1541<t>
1542   Cache directives MUST be passed through by a proxy or gateway
1543   application, regardless of their significance to that application,
1544   since the directives might be applicable to all recipients along the
1545   request/response chain. It is not possible to specify a cache-directive
1546   for a specific cache.
1547</t>
1548<figure><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="cache-directive"/><iref primary="true" item="Grammar" subitem="cache-request-directive"/><iref primary="true" item="Grammar" subitem="cache-response-directive"/><iref primary="true" item="Grammar" subitem="cache-extension"/><artwork type="abnf2616"><![CDATA[
1549  Cache-Control   = "Cache-Control" ":" 1#cache-directive
1550
1551  cache-directive = cache-request-directive
1552       | cache-response-directive
1553
1554  cache-request-directive =
1555         "no-cache"                          ; Section 15.2.1
1556       | "no-store"                          ; Section 15.2.2
1557       | "max-age" "=" delta-seconds         ; Section 15.2.3, 15.2.4
1558       | "max-stale" [ "=" delta-seconds ]   ; Section 15.2.3
1559       | "min-fresh" "=" delta-seconds       ; Section 15.2.3
1560       | "no-transform"                      ; Section 15.2.5
1561       | "only-if-cached"                    ; Section 15.2.4
1562       | cache-extension                     ; Section 15.2.6
1563
1564  cache-response-directive =
1565         "public"                               ; Section 15.2.1
1566       | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 15.2.1
1567       | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]; Section 15.2.1
1568       | "no-store"                             ; Section 15.2.2
1569       | "no-transform"                         ; Section 15.2.5
1570       | "must-revalidate"                      ; Section 15.2.4
1571       | "proxy-revalidate"                     ; Section 15.2.4
1572       | "max-age" "=" delta-seconds            ; Section 15.2.3
1573       | "s-maxage" "=" delta-seconds           ; Section 15.2.3
1574       | cache-extension                        ; Section 15.2.6
1575
1576  cache-extension = token [ "=" ( token | quoted-string ) ]
1577]]></artwork></figure>
1578<t>
1579   When a directive appears without any 1#field-name parameter, the
1580   directive applies to the entire request or response. When such a
1581   directive appears with a 1#field-name parameter, it applies only to
1582   the named field or fields, and not to the rest of the request or
1583   response. This mechanism supports extensibility; implementations of
1584   future versions of the HTTP protocol might apply these directives to
1585   header fields not defined in HTTP/1.1.
1586</t>
1587<t>
1588   The cache-control directives can be broken down into these general
1589   categories:
1590  <list style="symbols">
1591     <t>Restrictions on what are cacheable; these may only be imposed by
1592        the origin server.</t>
1593
1594     <t>Restrictions on what may be stored by a cache; these may be
1595        imposed by either the origin server or the user agent.</t>
1596
1597     <t>Modifications of the basic expiration mechanism; these may be
1598        imposed by either the origin server or the user agent.</t>
1599
1600     <t>Controls over cache revalidation and reload; these may only be
1601        imposed by a user agent.</t>
1602
1603     <t>Control over transformation of entities.</t>
1604
1605     <t>Extensions to the caching system.</t>
1606  </list>
1607</t>
1608
1609<section title="What is Cacheable" anchor="what.is.cacheable">
1610<t>
1611   By default, a response is cacheable if the requirements of the
1612   request method, request header fields, and the response status
1613   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1614   for cacheability. The following Cache-Control response directives
1615   allow an origin server to override the default cacheability of a
1616   response:
1617</t>
1618<t>
1619  <iref item="Cache Directives" subitem="public" primary="true"/>
1620  <iref item="public" subitem="Cache Directive" primary="true"/>
1621   public
1622  <list><t>
1623      Indicates that the response MAY be cached by any cache, even if it
1624      would normally be non-cacheable or cacheable only within a non-shared
1625      cache. (See also Authorization, Section 3.1 of <xref target="Part7"/>, for
1626      additional details.)
1627  </t></list>
1628</t>
1629<t>
1630  <iref item="Cache Directives" subitem="private" primary="true"/>
1631  <iref item="private" subitem="Cache Directive" primary="true"/>
1632   private
1633  <list><t>
1634      Indicates that all or part of the response message is intended for
1635      a single user and MUST NOT be cached by a shared cache. This
1636      allows an origin server to state that the specified parts of the
1637      response are intended for only one user and are not a valid
1638      response for requests by other users. A private (non-shared) cache
1639      MAY cache the response.
1640    </t><t>
1641       Note: This usage of the word private only controls where the
1642       response may be cached, and cannot ensure the privacy of the
1643       message content.
1644  </t></list>
1645</t>
1646<t>
1647  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1648  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1649   no-cache
1650  <list><t>
1651       If the no-cache directive does not specify a field-name, then a
1652      cache MUST NOT use the response to satisfy a subsequent request
1653      without successful revalidation with the origin server. This
1654      allows an origin server to prevent caching even by caches that
1655      have been configured to return stale responses to client requests.
1656    </t><t>
1657      If the no-cache directive does specify one or more field-names,
1658      then a cache MAY use the response to satisfy a subsequent request,
1659      subject to any other restrictions on caching. However, the
1660      specified field-name(s) MUST NOT be sent in the response to a
1661      subsequent request without successful revalidation with the origin
1662      server. This allows an origin server to prevent the re-use of
1663      certain header fields in a response, while still allowing caching
1664      of the rest of the response.
1665    <list><t>
1666       Note: Most HTTP/1.0 caches will not recognize or obey this
1667       directive.
1668    </t></list>
1669  </t></list>
1670</t>
1671</section>
1672
1673<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1674<t>
1675  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1676  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1677   no-store
1678  <list><t>   
1679      The purpose of the no-store directive is to prevent the
1680      inadvertent release or retention of sensitive information (for
1681      example, on backup tapes). The no-store directive applies to the
1682      entire message, and MAY be sent either in a response or in a
1683      request. If sent in a request, a cache MUST NOT store any part of
1684      either this request or any response to it. If sent in a response,
1685      a cache MUST NOT store any part of either this response or the
1686      request that elicited it. This directive applies to both non-shared
1687      and shared caches. "MUST NOT store" in this context means
1688      that the cache MUST NOT intentionally store the information in
1689      non-volatile storage, and MUST make a best-effort attempt to
1690      remove the information from volatile storage as promptly as
1691      possible after forwarding it.
1692  </t><t>
1693      Even when this directive is associated with a response, users
1694      might explicitly store such a response outside of the caching
1695      system (e.g., with a "Save As" dialog). History buffers MAY store
1696      such responses as part of their normal operation.
1697  </t><t>
1698      The purpose of this directive is to meet the stated requirements
1699      of certain users and service authors who are concerned about
1700      accidental releases of information via unanticipated accesses to
1701      cache data structures. While the use of this directive might
1702      improve privacy in some cases, we caution that it is NOT in any
1703      way a reliable or sufficient mechanism for ensuring privacy. In
1704      particular, malicious or compromised caches might not recognize or
1705      obey this directive, and communications networks might be
1706      vulnerable to eavesdropping.
1707  </t></list>
1708</t>
1709</section>
1710
1711<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1712<t>
1713   The expiration time of an entity MAY be specified by the origin
1714   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1715   it MAY be specified using the max-age directive in a response. When
1716   the max-age cache-control directive is present in a cached response,
1717   the response is stale if its current age is greater than the age
1718   value given (in seconds) at the time of a new request for that
1719   resource. The max-age directive on a response implies that the
1720   response is cacheable (i.e., "public") unless some other, more
1721   restrictive cache directive is also present.
1722</t>
1723<t>
1724   If a response includes both an Expires header and a max-age
1725   directive, the max-age directive overrides the Expires header, even
1726   if the Expires header is more restrictive. This rule allows an origin
1727   server to provide, for a given response, a longer expiration time to
1728   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1729   useful if certain HTTP/1.0 caches improperly calculate ages or
1730   expiration times, perhaps due to desynchronized clocks.
1731</t>
1732<t>
1733   Many HTTP/1.0 cache implementations will treat an Expires value that
1734   is less than or equal to the response Date value as being equivalent
1735   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1736   cache receives such a response, and the response does not include a
1737   Cache-Control header field, it SHOULD consider the response to be
1738   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1739  <list><t>
1740       Note: An origin server might wish to use a relatively new HTTP
1741       cache control feature, such as the "private" directive, on a
1742       network including older caches that do not understand that
1743       feature. The origin server will need to combine the new feature
1744       with an Expires field whose value is less than or equal to the
1745       Date value. This will prevent older caches from improperly
1746       caching the response.
1747  </t></list>
1748</t>
1749<t>
1750  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1751  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1752   s-maxage
1753  <list><t>
1754       If a response includes an s-maxage directive, then for a shared
1755       cache (but not for a private cache), the maximum age specified by
1756       this directive overrides the maximum age specified by either the
1757       max-age directive or the Expires header. The s-maxage directive
1758       also implies the semantics of the proxy-revalidate directive (see
1759       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1760       entry after it becomes stale to respond to a subsequent request
1761       without first revalidating it with the origin server. The s-maxage
1762       directive is always ignored by a private cache.
1763  </t></list>
1764</t>
1765<t>
1766   Note that most older caches, not compliant with this specification,
1767   do not implement any cache-control directives. An origin server
1768   wishing to use a cache-control directive that restricts, but does not
1769   prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
1770   requirement that the max-age directive overrides the Expires header,
1771   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1772   max-age directive.
1773</t>
1774<t>
1775   Other directives allow a user agent to modify the basic expiration
1776   mechanism. These directives MAY be specified on a request:
1777</t>
1778<t>
1779  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1780  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1781   max-age
1782  <list><t>
1783      Indicates that the client is willing to accept a response whose
1784      age is no greater than the specified time in seconds. Unless max-stale
1785      directive is also included, the client is not willing to
1786      accept a stale response.
1787  </t></list>
1788</t>
1789<t>
1790  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1791  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1792   min-fresh
1793  <list><t>
1794      Indicates that the client is willing to accept a response whose
1795      freshness lifetime is no less than its current age plus the
1796      specified time in seconds. That is, the client wants a response
1797      that will still be fresh for at least the specified number of
1798      seconds.
1799  </t></list>
1800</t>
1801<t>
1802  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1803  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1804   max-stale
1805  <list><t>
1806      Indicates that the client is willing to accept a response that has
1807      exceeded its expiration time. If max-stale is assigned a value,
1808      then the client is willing to accept a response that has exceeded
1809      its expiration time by no more than the specified number of
1810      seconds. If no value is assigned to max-stale, then the client is
1811      willing to accept a stale response of any age.
1812  </t></list>
1813</t>
1814<t>
1815   If a cache returns a stale response, either because of a max-stale
1816   directive on a request, or because the cache is configured to
1817   override the expiration time of a response, the cache MUST attach a
1818   Warning header to the stale response, using Warning 110 (Response is
1819   stale).
1820</t>
1821<t>
1822   A cache MAY be configured to return stale responses without
1823   validation, but only if this does not conflict with any "MUST"-level
1824   requirements concerning cache validation (e.g., a "must-revalidate"
1825   cache-control directive).
1826</t>
1827<t>
1828   If both the new request and the cached entry include "max-age"
1829   directives, then the lesser of the two values is used for determining
1830   the freshness of the cached entry for that request.
1831</t>
1832</section>
1833
1834<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1835<t>
1836   Sometimes a user agent might want or need to insist that a cache
1837   revalidate its cache entry with the origin server (and not just with
1838   the next cache along the path to the origin server), or to reload its
1839   cache entry from the origin server. End-to-end revalidation might be
1840   necessary if either the cache or the origin server has overestimated
1841   the expiration time of the cached response. End-to-end reload may be
1842   necessary if the cache entry has become corrupted for some reason.
1843</t>
1844<t>
1845   End-to-end revalidation may be requested either when the client does
1846   not have its own local cached copy, in which case we call it
1847   "unspecified end-to-end revalidation", or when the client does have a
1848   local cached copy, in which case we call it "specific end-to-end
1849   revalidation."
1850</t>
1851<t>
1852   The client can specify these three kinds of action using Cache-Control
1853   request directives:
1854</t>
1855<t>
1856   End-to-end reload
1857  <list><t>
1858      The request includes a "no-cache" cache-control directive or, for
1859      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1860      names MUST NOT be included with the no-cache directive in a
1861      request. The server MUST NOT use a cached copy when responding to
1862      such a request.
1863  </t></list>
1864</t>
1865<t>
1866   Specific end-to-end revalidation
1867  <list><t>
1868      The request includes a "max-age=0" cache-control directive, which
1869      forces each cache along the path to the origin server to
1870      revalidate its own entry, if any, with the next cache or server.
1871      The initial request includes a cache-validating conditional with
1872      the client's current validator.
1873  </t></list>
1874</t>
1875<t>
1876   Unspecified end-to-end revalidation
1877  <list><t>
1878      The request includes "max-age=0" cache-control directive, which
1879      forces each cache along the path to the origin server to
1880      revalidate its own entry, if any, with the next cache or server.
1881      The initial request does not include a cache-validating
1882      conditional; the first cache along the path (if any) that holds a
1883      cache entry for this resource includes a cache-validating
1884      conditional with its current validator.
1885  </t></list>
1886</t>
1887<t>
1888  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1889  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1890   max-age
1891  <list><t>
1892      When an intermediate cache is forced, by means of a max-age=0
1893      directive, to revalidate its own cache entry, and the client has
1894      supplied its own validator in the request, the supplied validator
1895      might differ from the validator currently stored with the cache
1896      entry. In this case, the cache MAY use either validator in making
1897      its own request without affecting semantic transparency.
1898  </t><t>
1899      However, the choice of validator might affect performance. The
1900      best approach is for the intermediate cache to use its own
1901      validator when making its request. If the server replies with 304
1902      (Not Modified), then the cache can return its now validated copy
1903      to the client with a 200 (OK) response. If the server replies with
1904      a new entity and cache validator, however, the intermediate cache
1905      can compare the returned validator with the one provided in the
1906      client's request, using the strong comparison function. If the
1907      client's validator is equal to the origin server's, then the
1908      intermediate cache simply returns 304 (Not Modified). Otherwise,
1909      it returns the new entity with a 200 (OK) response.
1910  </t><t>
1911      If a request includes the no-cache directive, it SHOULD NOT
1912      include min-fresh, max-stale, or max-age.
1913  </t></list>
1914</t>
1915<t>
1916  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1917  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1918   only-if-cached
1919  <list><t>
1920      In some cases, such as times of extremely poor network
1921      connectivity, a client may want a cache to return only those
1922      responses that it currently has stored, and not to reload or
1923      revalidate with the origin server. To do this, the client may
1924      include the only-if-cached directive in a request. If it receives
1925      this directive, a cache SHOULD either respond using a cached entry
1926      that is consistent with the other constraints of the request, or
1927      respond with a 504 (Gateway Timeout) status. However, if a group
1928      of caches is being operated as a unified system with good internal
1929      connectivity, such a request MAY be forwarded within that group of
1930      caches.
1931  </t></list>
1932</t>
1933<t>
1934  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1935  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1936   must-revalidate
1937  <list><t>
1938      Because a cache MAY be configured to ignore a server's specified
1939      expiration time, and because a client request MAY include a max-stale
1940      directive (which has a similar effect), the protocol also
1941      includes a mechanism for the origin server to require revalidation
1942      of a cache entry on any subsequent use. When the must-revalidate
1943      directive is present in a response received by a cache, that cache
1944      MUST NOT use the entry after it becomes stale to respond to a
1945      subsequent request without first revalidating it with the origin
1946      server. (I.e., the cache MUST do an end-to-end revalidation every
1947      time, if, based solely on the origin server's Expires or max-age
1948      value, the cached response is stale.)
1949  </t><t>
1950      The must-revalidate directive is necessary to support reliable
1951      operation for certain protocol features. In all circumstances an
1952      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1953      particular, if the cache cannot reach the origin server for any
1954      reason, it MUST generate a 504 (Gateway Timeout) response.
1955  </t><t>
1956      Servers SHOULD send the must-revalidate directive if and only if
1957      failure to revalidate a request on the entity could result in
1958      incorrect operation, such as a silently unexecuted financial
1959      transaction. Recipients MUST NOT take any automated action that
1960      violates this directive, and MUST NOT automatically provide an
1961      unvalidated copy of the entity if revalidation fails.
1962  </t><t>
1963      Although this is not recommended, user agents operating under
1964      severe connectivity constraints MAY violate this directive but, if
1965      so, MUST explicitly warn the user that an unvalidated response has
1966      been provided. The warning MUST be provided on each unvalidated
1967      access, and SHOULD require explicit user confirmation.
1968  </t></list>
1969</t>
1970<t>
1971  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1972  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1973   proxy-revalidate
1974  <list><t>
1975      The proxy-revalidate directive has the same meaning as the must-revalidate
1976      directive, except that it does not apply to non-shared
1977      user agent caches. It can be used on a response to an
1978      authenticated request to permit the user's cache to store and
1979      later return the response without needing to revalidate it (since
1980      it has already been authenticated once by that user), while still
1981      requiring proxies that service many users to revalidate each time
1982      (in order to make sure that each user has been authenticated).
1983      Note that such authenticated responses also need the public cache
1984      control directive in order to allow them to be cached at all.
1985  </t></list>
1986</t>
1987</section>
1988
1989<section title="No-Transform Directive" anchor="no-transform.directive">
1990<t>
1991  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
1992  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
1993   no-transform
1994  <list><t>
1995      Implementors of intermediate caches (proxies) have found it useful
1996      to convert the media type of certain entity bodies. A non-transparent
1997      proxy might, for example, convert between image
1998      formats in order to save cache space or to reduce the amount of
1999      traffic on a slow link.
2000  </t><t>
2001      Serious operational problems occur, however, when these
2002      transformations are applied to entity bodies intended for certain
2003      kinds of applications. For example, applications for medical
2004      imaging, scientific data analysis and those using end-to-end
2005      authentication, all depend on receiving an entity body that is bit
2006      for bit identical to the original entity-body.
2007  </t><t>
2008      Therefore, if a message includes the no-transform directive, an
2009      intermediate cache or proxy MUST NOT change those headers that are
2010      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2011      directive. This implies that the cache or proxy MUST NOT change
2012      any aspect of the entity-body that is specified by these headers,
2013      including the value of the entity-body itself.
2014  </t></list>
2015</t>
2016</section>
2017
2018<section title="Cache Control Extensions" anchor="cache.control.extensions">
2019<t>
2020   The Cache-Control header field can be extended through the use of one
2021   or more cache-extension tokens, each with an optional assigned value.
2022   Informational extensions (those which do not require a change in
2023   cache behavior) MAY be added without changing the semantics of other
2024   directives. Behavioral extensions are designed to work by acting as
2025   modifiers to the existing base of cache directives. Both the new
2026   directive and the standard directive are supplied, such that
2027   applications which do not understand the new directive will default
2028   to the behavior specified by the standard directive, and those that
2029   understand the new directive will recognize it as modifying the
2030   requirements associated with the standard directive. In this way,
2031   extensions to the cache-control directives can be made without
2032   requiring changes to the base protocol.
2033</t>
2034<t>
2035   This extension mechanism depends on an HTTP cache obeying all of the
2036   cache-control directives defined for its native HTTP-version, obeying
2037   certain extensions, and ignoring all directives that it does not
2038   understand.
2039</t>
2040<t>
2041   For example, consider a hypothetical new response directive called
2042   community which acts as a modifier to the private directive. We
2043   define this new directive to mean that, in addition to any non-shared
2044   cache, any cache which is shared only by members of the community
2045   named within its value may cache the response. An origin server
2046   wishing to allow the UCI community to use an otherwise private
2047   response in their shared cache(s) could do so by including
2048</t>
2049<figure><artwork type="example"><![CDATA[
2050    Cache-Control: private, community="UCI"
2051]]></artwork></figure>
2052<t>
2053   A cache seeing this header field will act correctly even if the cache
2054   does not understand the community cache-extension, since it will also
2055   see and understand the private directive and thus default to the safe
2056   behavior.
2057</t>
2058<t>
2059   Unrecognized cache-directives MUST be ignored; it is assumed that any
2060   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2061   be combined with standard directives (or the response's default
2062   cacheability) such that the cache behavior will remain minimally
2063   correct even if the cache does not understand the extension(s).
2064</t>
2065</section>
2066</section>
2067
2068<section title="Expires" anchor="header.expires">
2069  <iref primary="true" item="Expires header"/>
2070  <iref primary="true" item="Headers" subitem="Expires"/>
2071<t>
2072   The Expires entity-header field gives the date/time after which the
2073   response is considered stale. A stale cache entry may not normally be
2074   returned by a cache (either a proxy cache or a user agent cache)
2075   unless it is first validated with the origin server (or with an
2076   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2077   for further discussion of the expiration model.
2078</t>
2079<t>
2080   The presence of an Expires field does not imply that the original
2081   resource will change or cease to exist at, before, or after that
2082   time.
2083</t>
2084<t>
2085   The format is an absolute date and time as defined by HTTP-date in
2086   Section 3.3.1 of <xref target="Part1"/>; it MUST be sent in rfc1123-date format.
2087</t>
2088<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
2089  Expires = "Expires" ":" HTTP-date
2090]]></artwork></figure>
2091<t>
2092   An example of its use is
2093</t>
2094<figure><artwork type="example"><![CDATA[
2095   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2096]]></artwork></figure>
2097<t>
2098  <list><t>
2099      Note: if a response includes a Cache-Control field with the max-age
2100      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2101      Expires field.
2102  </t></list>
2103</t>
2104<t>
2105   HTTP/1.1 clients and caches MUST treat other invalid date formats,
2106   especially including the value "0", as in the past (i.e., "already
2107   expired").
2108</t>
2109<t>
2110   To mark a response as "already expired," an origin server sends an
2111   Expires date that is equal to the Date header value. (See the rules
2112   for expiration calculations in <xref target="expiration.calculations"/>.)
2113</t>
2114<t>
2115   To mark a response as "never expires," an origin server sends an
2116   Expires date approximately one year from the time the response is
2117   sent. HTTP/1.1 servers SHOULD NOT  send Expires dates more than one
2118   year in the future.
2119</t>
2120<t>
2121   The presence of an Expires header field with a date value of some
2122   time in the future on a response that otherwise would by default be
2123   non-cacheable indicates that the response is cacheable, unless
2124   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2125</t>
2126</section>
2127
2128<section title="Pragma" anchor="header.pragma">
2129  <iref primary="true" item="Pragma header"/>
2130  <iref primary="true" item="Headers" subitem="Pragma"/>
2131<t>
2132   The Pragma general-header field is used to include implementation-specific
2133   directives that might apply to any recipient along the
2134   request/response chain. All pragma directives specify optional
2135   behavior from the viewpoint of the protocol; however, some systems
2136   MAY require that behavior be consistent with the directives.
2137</t>
2138<figure><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/><artwork type="abnf2616"><![CDATA[
2139  Pragma            = "Pragma" ":" 1#pragma-directive
2140  pragma-directive  = "no-cache" | extension-pragma
2141  extension-pragma  = token [ "=" ( token | quoted-string ) ]
2142]]></artwork></figure>
2143<t>
2144   When the no-cache directive is present in a request message, an
2145   application SHOULD forward the request toward the origin server even
2146   if it has a cached copy of what is being requested. This pragma
2147   directive has the same semantics as the no-cache cache-directive (see
2148   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2149   HTTP/1.0. Clients SHOULD include both header fields when a no-cache
2150   request is sent to a server not known to be HTTP/1.1 compliant.
2151</t>
2152<t>
2153   Pragma directives MUST be passed through by a proxy or gateway
2154   application, regardless of their significance to that application,
2155   since the directives might be applicable to all recipients along the
2156   request/response chain. It is not possible to specify a pragma for a
2157   specific recipient; however, any pragma directive not relevant to a
2158   recipient SHOULD be ignored by that recipient.
2159</t>
2160<t>
2161   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
2162   sent "Cache-Control: no-cache". No new Pragma directives will be
2163   defined in HTTP.
2164  <list><t>
2165      Note: because the meaning of "Pragma: no-cache" as a
2166      response-header field is not actually specified, it does not provide a
2167      reliable replacement for "Cache-Control: no-cache" in a response.
2168  </t></list>
2169</t>
2170</section>
2171
2172<section title="Vary" anchor="header.vary">
2173  <iref primary="true" item="Vary header"/>
2174  <iref primary="true" item="Headers" subitem="Vary"/>
2175<t>
2176   The Vary field value indicates the set of request-header fields that
2177   fully determines, while the response is fresh, whether a cache is
2178   permitted to use the response to reply to a subsequent request
2179   without revalidation. For uncacheable or stale responses, the Vary
2180   field value advises the user agent about the criteria that were used
2181   to select the representation. A Vary field value of "*" implies that
2182   a cache cannot determine from the request headers of a subsequent
2183   request whether this response is the appropriate representation. See
2184   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2185</t>
2186<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
2187  Vary  = "Vary" ":" ( "*" | 1#field-name )
2188]]></artwork></figure>
2189<t>
2190   An HTTP/1.1 server SHOULD include a Vary header field with any
2191   cacheable response that is subject to server-driven negotiation.
2192   Doing so allows a cache to properly interpret future requests on that
2193   resource and informs the user agent about the presence of negotiation
2194   on that resource. A server MAY include a Vary header field with a
2195   non-cacheable response that is subject to server-driven negotiation,
2196   since this might provide the user agent with useful information about
2197   the dimensions over which the response varies at the time of the
2198   response.
2199</t>
2200<t>
2201   A Vary field value consisting of a list of field-names signals that
2202   the representation selected for the response is based on a selection
2203   algorithm which considers ONLY the listed request-header field values
2204   in selecting the most appropriate representation. A cache MAY assume
2205   that the same selection will be made for future requests with the
2206   same values for the listed field names, for the duration of time for
2207   which the response is fresh.
2208</t>
2209<t>
2210   The field-names given are not limited to the set of standard
2211   request-header fields defined by this specification. Field names are
2212   case-insensitive.
2213</t>
2214<t>
2215   A Vary field value of "*" signals that unspecified parameters not
2216   limited to the request-headers (e.g., the network address of the
2217   client), play a role in the selection of the response representation.
2218   The "*" value MUST NOT be generated by a proxy server; it may only be
2219   generated by an origin server.
2220</t>
2221</section>
2222
2223<section title="Warning" anchor="header.warning">
2224  <iref primary="true" item="Warning header"/>
2225  <iref primary="true" item="Headers" subitem="Warning"/>
2226<t>
2227   The Warning general-header field is used to carry additional
2228   information about the status or transformation of a message which
2229   might not be reflected in the message. This information is typically
2230   used to warn about a possible lack of semantic transparency from
2231   caching operations or transformations applied to the entity body of
2232   the message.
2233</t>
2234<t>
2235   Warning headers are sent with responses using:
2236</t>
2237<figure><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/><artwork type="abnf2616"><![CDATA[
2238  Warning    = "Warning" ":" 1#warning-value
2239 
2240  warning-value = warn-code SP warn-agent SP warn-text
2241                                        [SP warn-date]
2242 
2243  warn-code  = 3DIGIT
2244  warn-agent = ( host [ ":" port ] ) | pseudonym
2245                  ; the name or pseudonym of the server adding
2246                  ; the Warning header, for use in debugging
2247  warn-text  = quoted-string
2248  warn-date  = DQUOTE HTTP-date DQUOTE
2249]]></artwork></figure>
2250<t>
2251   A response MAY carry more than one Warning header.
2252</t>
2253<t>
2254   The warn-text SHOULD be in a natural language and character set that
2255   is most likely to be intelligible to the human user receiving the
2256   response. This decision MAY be based on any available knowledge, such
2257   as the location of the cache or user, the Accept-Language field in a
2258   request, the Content-Language field in a response, etc. The default
2259   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2260</t>
2261<t>
2262   If a character set other than ISO-8859-1 is used, it MUST be encoded
2263   in the warn-text using the method described in <xref target="RFC2047"/>.
2264</t>
2265<t>
2266   Warning headers can in general be applied to any message, however
2267   some specific warn-codes are specific to caches and can only be
2268   applied to response messages. New Warning headers SHOULD be added
2269   after any existing Warning headers. A cache MUST NOT delete any
2270   Warning header that it received with a message. However, if a cache
2271   successfully validates a cache entry, it SHOULD remove any Warning
2272   headers previously attached to that entry except as specified for
2273   specific Warning codes. It MUST then add any Warning headers received
2274   in the validating response. In other words, Warning headers are those
2275   that would be attached to the most recent relevant response.
2276</t>
2277<t>
2278   When multiple Warning headers are attached to a response, the user
2279   agent ought to inform the user of as many of them as possible, in the
2280   order that they appear in the response. If it is not possible to
2281   inform the user of all of the warnings, the user agent SHOULD follow
2282   these heuristics:
2283  <list style="symbols">
2284    <t>Warnings that appear early in the response take priority over
2285        those appearing later in the response.</t>
2286
2287    <t>Warnings in the user's preferred character set take priority
2288        over warnings in other character sets but with identical warn-codes
2289        and warn-agents.</t>
2290  </list>
2291</t>
2292<t>
2293   Systems that generate multiple Warning headers SHOULD order them with
2294   this user agent behavior in mind.
2295</t>
2296<t>
2297   Requirements for the behavior of caches with respect to Warnings are
2298   stated in <xref target="warnings"/>.
2299</t>
2300<t>
2301   This is a list of the currently-defined warn-codes, each with a
2302   recommended warn-text in English, and a description of its meaning.
2303</t>
2304<t>
2305   110 Response is stale
2306  <list><t>
2307     MUST be included whenever the returned response is stale.
2308  </t></list>
2309</t>
2310<t>
2311   111 Revalidation failed
2312  <list><t>
2313     MUST be included if a cache returns a stale response because an
2314     attempt to revalidate the response failed, due to an inability to
2315     reach the server.
2316  </t></list>
2317</t>
2318<t>
2319   112 Disconnected operation
2320  <list><t>
2321     SHOULD be included if the cache is intentionally disconnected from
2322     the rest of the network for a period of time.
2323  </t></list>
2324</t>
2325<t>
2326   113 Heuristic expiration
2327  <list><t>
2328     MUST be included if the cache heuristically chose a freshness
2329     lifetime greater than 24 hours and the response's age is greater
2330     than 24 hours.
2331  </t></list>
2332</t>
2333<t>
2334   199 Miscellaneous warning
2335  <list><t>
2336     The warning text MAY include arbitrary information to be presented
2337     to a human user, or logged. A system receiving this warning MUST NOT
2338     take any automated action, besides presenting the warning to
2339     the user.
2340  </t></list>
2341</t>
2342<t>
2343   214 Transformation applied
2344  <list><t>
2345     MUST be added by an intermediate cache or proxy if it applies any
2346     transformation changing the content-coding (as specified in the
2347     Content-Encoding header) or media-type (as specified in the
2348     Content-Type header) of the response, or the entity-body of the
2349     response, unless this Warning code already appears in the response.
2350  </t></list>
2351</t>
2352<t>
2353   299 Miscellaneous persistent warning
2354  <list><t>
2355     The warning text MAY include arbitrary information to be presented
2356     to a human user, or logged. A system receiving this warning MUST NOT
2357     take any automated action.
2358  </t></list>
2359</t>
2360<t>
2361   If an implementation sends a message with one or more Warning headers
2362   whose version is HTTP/1.0 or lower, then the sender MUST include in
2363   each warning-value a warn-date that matches the date in the response.
2364</t>
2365<t>
2366   If an implementation receives a message with a warning-value that
2367   includes a warn-date, and that warn-date is different from the Date
2368   value in the response, then that warning-value MUST be deleted from
2369   the message before storing, forwarding, or using it. (This prevents
2370   bad consequences of naive caching of Warning header fields.) If all
2371   of the warning-values are deleted for this reason, the Warning header
2372   MUST be deleted as well.
2373</t>
2374</section>
2375
2376</section>
2377
2378<section title="IANA Considerations" anchor="IANA.considerations">
2379<t>
2380   TBD.
2381</t>
2382</section>
2383
2384<section title="Security Considerations" anchor="security.considerations">
2385<t>
2386   Caching proxies provide additional potential vulnerabilities, since
2387   the contents of the cache represent an attractive target for
2388   malicious exploitation. Because cache contents persist after an HTTP
2389   request is complete, an attack on the cache can reveal information
2390   long after a user believes that the information has been removed from
2391   the network. Therefore, cache contents should be protected as
2392   sensitive information.
2393</t>
2394</section>
2395
2396<section title="Acknowledgments" anchor="ack">
2397<t>
2398   Much of the content and presentation of the caching design is due to
2399   suggestions and comments from individuals including: Shel Kaphan,
2400   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2401</t>
2402</section>
2403</middle>
2404<back>
2405
2406<references title="Normative References">
2407
2408<reference anchor="ISO-8859-1">
2409  <front>
2410    <title>
2411     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2412    </title>
2413    <author>
2414      <organization>International Organization for Standardization</organization>
2415    </author>
2416    <date year="1998"/>
2417  </front>
2418  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2419</reference>
2420
2421<reference anchor="Part1">
2422  <front>
2423    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2424    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2425      <organization abbrev="Day Software">Day Software</organization>
2426      <address><email>fielding@gbiv.com</email></address>
2427    </author>
2428    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2429      <organization>One Laptop per Child</organization>
2430      <address><email>jg@laptop.org</email></address>
2431    </author>
2432    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2433      <organization abbrev="HP">Hewlett-Packard Company</organization>
2434      <address><email>JeffMogul@acm.org</email></address>
2435    </author>
2436    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2437      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2438      <address><email>henrikn@microsoft.com</email></address>
2439    </author>
2440    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2441      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2442      <address><email>LMM@acm.org</email></address>
2443    </author>
2444    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2445      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2446      <address><email>paulle@microsoft.com</email></address>
2447    </author>
2448    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2449      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2450      <address><email>timbl@w3.org</email></address>
2451    </author>
2452    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2453      <organization abbrev="W3C">World Wide Web Consortium</organization>
2454      <address><email>ylafon@w3.org</email></address>
2455    </author>
2456    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2457      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2458      <address><email>julian.reschke@greenbytes.de</email></address>
2459    </author>
2460    <date month="January" year="2008"/>
2461  </front>
2462  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-01"/>
2463 
2464</reference>
2465
2466<reference anchor="Part2">
2467  <front>
2468    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2469    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2470      <organization abbrev="Day Software">Day Software</organization>
2471      <address><email>fielding@gbiv.com</email></address>
2472    </author>
2473    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2474      <organization>One Laptop per Child</organization>
2475      <address><email>jg@laptop.org</email></address>
2476    </author>
2477    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2478      <organization abbrev="HP">Hewlett-Packard Company</organization>
2479      <address><email>JeffMogul@acm.org</email></address>
2480    </author>
2481    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2482      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2483      <address><email>henrikn@microsoft.com</email></address>
2484    </author>
2485    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2486      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2487      <address><email>LMM@acm.org</email></address>
2488    </author>
2489    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2490      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2491      <address><email>paulle@microsoft.com</email></address>
2492    </author>
2493    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2494      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2495      <address><email>timbl@w3.org</email></address>
2496    </author>
2497    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2498      <organization abbrev="W3C">World Wide Web Consortium</organization>
2499      <address><email>ylafon@w3.org</email></address>
2500    </author>
2501    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2502      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2503      <address><email>julian.reschke@greenbytes.de</email></address>
2504    </author>
2505    <date month="January" year="2008"/>
2506  </front>
2507  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-01"/>
2508 
2509</reference>
2510
2511<reference anchor="Part3">
2512  <front>
2513    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2514    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2515      <organization abbrev="Day Software">Day Software</organization>
2516      <address><email>fielding@gbiv.com</email></address>
2517    </author>
2518    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2519      <organization>One Laptop per Child</organization>
2520      <address><email>jg@laptop.org</email></address>
2521    </author>
2522    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2523      <organization abbrev="HP">Hewlett-Packard Company</organization>
2524      <address><email>JeffMogul@acm.org</email></address>
2525    </author>
2526    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2527      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2528      <address><email>henrikn@microsoft.com</email></address>
2529    </author>
2530    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2531      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2532      <address><email>LMM@acm.org</email></address>
2533    </author>
2534    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2535      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2536      <address><email>paulle@microsoft.com</email></address>
2537    </author>
2538    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2539      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2540      <address><email>timbl@w3.org</email></address>
2541    </author>
2542    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2543      <organization abbrev="W3C">World Wide Web Consortium</organization>
2544      <address><email>ylafon@w3.org</email></address>
2545    </author>
2546    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2547      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2548      <address><email>julian.reschke@greenbytes.de</email></address>
2549    </author>
2550    <date month="January" year="2008"/>
2551  </front>
2552  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-01"/>
2553 
2554</reference>
2555
2556<reference anchor="Part4">
2557  <front>
2558    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2559    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2560      <organization abbrev="Day Software">Day Software</organization>
2561      <address><email>fielding@gbiv.com</email></address>
2562    </author>
2563    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2564      <organization>One Laptop per Child</organization>
2565      <address><email>jg@laptop.org</email></address>
2566    </author>
2567    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2568      <organization abbrev="HP">Hewlett-Packard Company</organization>
2569      <address><email>JeffMogul@acm.org</email></address>
2570    </author>
2571    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2572      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2573      <address><email>henrikn@microsoft.com</email></address>
2574    </author>
2575    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2576      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2577      <address><email>LMM@acm.org</email></address>
2578    </author>
2579    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2580      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2581      <address><email>paulle@microsoft.com</email></address>
2582    </author>
2583    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2584      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2585      <address><email>timbl@w3.org</email></address>
2586    </author>
2587    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2588      <organization abbrev="W3C">World Wide Web Consortium</organization>
2589      <address><email>ylafon@w3.org</email></address>
2590    </author>
2591    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2592      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2593      <address><email>julian.reschke@greenbytes.de</email></address>
2594    </author>
2595    <date month="January" year="2008"/>
2596  </front>
2597  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-01"/>
2598 
2599</reference>
2600
2601<reference anchor="Part5">
2602  <front>
2603    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2604    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2605      <organization abbrev="Day Software">Day Software</organization>
2606      <address><email>fielding@gbiv.com</email></address>
2607    </author>
2608    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2609      <organization>One Laptop per Child</organization>
2610      <address><email>jg@laptop.org</email></address>
2611    </author>
2612    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2613      <organization abbrev="HP">Hewlett-Packard Company</organization>
2614      <address><email>JeffMogul@acm.org</email></address>
2615    </author>
2616    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2617      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2618      <address><email>henrikn@microsoft.com</email></address>
2619    </author>
2620    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2621      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2622      <address><email>LMM@acm.org</email></address>
2623    </author>
2624    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2625      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2626      <address><email>paulle@microsoft.com</email></address>
2627    </author>
2628    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2629      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2630      <address><email>timbl@w3.org</email></address>
2631    </author>
2632    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2633      <organization abbrev="W3C">World Wide Web Consortium</organization>
2634      <address><email>ylafon@w3.org</email></address>
2635    </author>
2636    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2637      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2638      <address><email>julian.reschke@greenbytes.de</email></address>
2639    </author>
2640    <date month="January" year="2008"/>
2641  </front>
2642  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-01"/>
2643 
2644</reference>
2645
2646<reference anchor="Part7">
2647  <front>
2648    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2649    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2650      <organization abbrev="Day Software">Day Software</organization>
2651      <address><email>fielding@gbiv.com</email></address>
2652    </author>
2653    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2654      <organization>One Laptop per Child</organization>
2655      <address><email>jg@laptop.org</email></address>
2656    </author>
2657    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2658      <organization abbrev="HP">Hewlett-Packard Company</organization>
2659      <address><email>JeffMogul@acm.org</email></address>
2660    </author>
2661    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2662      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2663      <address><email>henrikn@microsoft.com</email></address>
2664    </author>
2665    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2666      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2667      <address><email>LMM@acm.org</email></address>
2668    </author>
2669    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2670      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2671      <address><email>paulle@microsoft.com</email></address>
2672    </author>
2673    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2674      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2675      <address><email>timbl@w3.org</email></address>
2676    </author>
2677    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2678      <organization abbrev="W3C">World Wide Web Consortium</organization>
2679      <address><email>ylafon@w3.org</email></address>
2680    </author>
2681    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2682      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2683      <address><email>julian.reschke@greenbytes.de</email></address>
2684    </author>
2685    <date month="January" year="2008"/>
2686  </front>
2687  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-01"/>
2688 
2689</reference>
2690
2691<reference anchor="RFC2047">
2692  <front>
2693    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2694    <author initials="K." surname="Moore" fullname="Keith Moore">
2695      <organization>University of Tennessee</organization>
2696      <address><email>moore@cs.utk.edu</email></address>
2697    </author>
2698    <date month="November" year="1996"/>
2699  </front>
2700  <seriesInfo name="RFC" value="2047"/>
2701</reference>
2702
2703<reference anchor="RFC2119">
2704  <front>
2705    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2706    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2707      <organization>Harvard University</organization>
2708      <address><email>sob@harvard.edu</email></address>
2709    </author>
2710    <date month="March" year="1997"/>
2711  </front>
2712  <seriesInfo name="BCP" value="14"/>
2713  <seriesInfo name="RFC" value="2119"/>
2714</reference>
2715
2716</references>
2717
2718<references title="Informative References">
2719
2720<reference anchor="RFC1305">
2721  <front>
2722    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2723    <author initials="D." surname="Mills" fullname="David L. Mills">
2724      <organization>University of Delaware, Electrical Engineering Department</organization>
2725      <address><email>mills@udel.edu</email></address>
2726    </author>
2727    <date month="March" year="1992"/>
2728  </front>
2729  <seriesInfo name="RFC" value="1305"/>
2730</reference>
2731
2732<reference anchor="RFC2616">
2733  <front>
2734    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2735    <author initials="R." surname="Fielding" fullname="R. Fielding">
2736      <organization>University of California, Irvine</organization>
2737      <address><email>fielding@ics.uci.edu</email></address>
2738    </author>
2739    <author initials="J." surname="Gettys" fullname="J. Gettys">
2740      <organization>W3C</organization>
2741      <address><email>jg@w3.org</email></address>
2742    </author>
2743    <author initials="J." surname="Mogul" fullname="J. Mogul">
2744      <organization>Compaq Computer Corporation</organization>
2745      <address><email>mogul@wrl.dec.com</email></address>
2746    </author>
2747    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2748      <organization>MIT Laboratory for Computer Science</organization>
2749      <address><email>frystyk@w3.org</email></address>
2750    </author>
2751    <author initials="L." surname="Masinter" fullname="L. Masinter">
2752      <organization>Xerox Corporation</organization>
2753      <address><email>masinter@parc.xerox.com</email></address>
2754    </author>
2755    <author initials="P." surname="Leach" fullname="P. Leach">
2756      <organization>Microsoft Corporation</organization>
2757      <address><email>paulle@microsoft.com</email></address>
2758    </author>
2759    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2760      <organization>W3C</organization>
2761      <address><email>timbl@w3.org</email></address>
2762    </author>
2763    <date month="June" year="1999"/>
2764  </front>
2765  <seriesInfo name="RFC" value="2616"/>
2766</reference>
2767
2768</references>
2769
2770<section title="Compatibility with Previous Versions" anchor="compatibility">
2771
2772<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2773<t>
2774   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2775   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2776   <xref target="header.cache-control" format="counter"/>,
2777   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2778</t>
2779<t>
2780   Transfer-coding and message lengths all interact in ways that
2781   required fixing exactly when chunked encoding is used (to allow for
2782   transfer encoding that may not be self delimiting); it was important
2783   to straighten out exactly how message lengths are computed.
2784   (<xref target="non-modifiable.headers"/>,
2785   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2786</t>
2787<t>
2788   Proxies should be able to add Content-Length when appropriate.
2789   (<xref target="non-modifiable.headers"/>)
2790</t>
2791<t>
2792   Range request responses would become very verbose if all meta-data
2793   were always returned; by allowing the server to only send needed
2794   headers in a 206 response, this problem can be avoided.
2795   (<xref target="combining.headers"/>)
2796</t>
2797<t>
2798   The Cache-Control: max-age directive was not properly defined for
2799   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2800</t>
2801<t>
2802   Warnings could be cached incorrectly, or not updated appropriately.
2803   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2804   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2805   and <xref target="header.warning" format="counter"/>) Warning
2806   also needed to be a general header, as PUT or other methods may have
2807   need for it in requests.
2808</t>
2809</section>
2810
2811<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2812<t>
2813  Clarify denial of service attack avoidance requirement.
2814  (<xref target="invalidation.after.updates.or.deletions"/>)
2815</t>
2816</section>
2817
2818</section>
2819
2820<section title="Change Log (to be removed by RFC Editor before publication)">
2821
2822<section title="Since RFC2616">
2823<t>
2824  Extracted relevant partitions from <xref target="RFC2616"/>.
2825</t>
2826</section>
2827
2828<section title="Since draft-ietf-httpbis-p6-cache-00">
2829<t>
2830  Closed issues:
2831  <list style="symbols"> 
2832    <t>
2833      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2834      "Trailer"
2835      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2836    </t>
2837    <t>
2838      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2839      "Invalidation after Update or Delete"
2840      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2841    </t>
2842    <t>
2843      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2844      "Normative and Informative references"
2845    </t>
2846    <t>
2847      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2848      "Date reference typo"
2849    </t>
2850    <t>
2851      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2852      "Connection header text"
2853    </t>
2854    <t>
2855      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2856      "Informative references"
2857    </t>
2858    <t>
2859      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2860      "ISO-8859-1 Reference"
2861    </t>
2862    <t>
2863      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2864      "Normative up-to-date references"
2865    </t>
2866    <t>
2867      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2868      "typo in 13.2.2"
2869    </t>
2870  </list>
2871</t>
2872<t>
2873  Other changes:
2874  <list style="symbols"> 
2875    <t>
2876      Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
2877    </t>
2878  </list>
2879</t>
2880</section>
2881
2882</section>
2883
2884</back>
2885</rfc>
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