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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<!DOCTYPE rfc
17  PUBLIC "" "rfc2629.dtd">
18<rfc obsoletes="2616" category="std" ipr="full3978" docName="draft-ietf-httpbis-p6-cache-03">
19<front>
20
21  <title abbrev="HTTP/1.1, Part 6">HTTP/1.1, part 6: Caching</title>
22
23  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
24    <organization abbrev="Day Software">Day Software</organization>
25    <address>
26      <postal>
27        <street>23 Corporate Plaza DR, Suite 280</street>
28        <city>Newport Beach</city>
29        <region>CA</region>
30        <code>92660</code>
31        <country>USA</country>
32      </postal>
33      <phone>+1-949-706-5300</phone>
34      <facsimile>+1-949-706-5305</facsimile>
35      <email>fielding@gbiv.com</email>
36      <uri>http://roy.gbiv.com/</uri>
37    </address>
38  </author>
39
40  <author initials="J." surname="Gettys" fullname="Jim Gettys">
41    <organization>One Laptop per Child</organization>
42    <address>
43      <postal>
44        <street>21 Oak Knoll Road</street>
45        <city>Carlisle</city>
46        <region>MA</region>
47        <code>01741</code>
48        <country>USA</country>
49      </postal>
50      <email>jg@laptop.org</email>
51      <uri>http://www.laptop.org/</uri>
52    </address>
53  </author>
54 
55  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
56    <organization abbrev="HP">Hewlett-Packard Company</organization>
57    <address>
58      <postal>
59        <street>HP Labs, Large Scale Systems Group</street>
60        <street>1501 Page Mill Road, MS 1177</street>
61        <city>Palo Alto</city>
62        <region>CA</region>
63        <code>94304</code>
64        <country>USA</country>
65      </postal>
66      <email>JeffMogul@acm.org</email>
67    </address>
68  </author>
69
70  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
71    <organization abbrev="Microsoft">Microsoft Corporation</organization>
72    <address>
73      <postal>
74        <street>1 Microsoft Way</street>
75        <city>Redmond</city>
76        <region>WA</region>
77        <code>98052</code>
78        <country>USA</country>
79      </postal>
80      <email>henrikn@microsoft.com</email>
81    </address>
82  </author>
83
84  <author initials="L." surname="Masinter" fullname="Larry Masinter">
85    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
86    <address>
87      <postal>
88        <street>345 Park Ave</street>
89        <city>San Jose</city>
90        <region>CA</region>
91        <code>95110</code>
92        <country>USA</country>
93      </postal>
94      <email>LMM@acm.org</email>
95      <uri>http://larry.masinter.net/</uri>
96    </address>
97  </author>
98 
99  <author initials="P." surname="Leach" fullname="Paul J. Leach">
100    <organization abbrev="Microsoft">Microsoft Corporation</organization>
101    <address>
102      <postal>
103        <street>1 Microsoft Way</street>
104        <city>Redmond</city>
105        <region>WA</region>
106        <code>98052</code>
107      </postal>
108      <email>paulle@microsoft.com</email>
109    </address>
110  </author>
111   
112  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
113    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
114    <address>
115      <postal>
116        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
117        <street>The Stata Center, Building 32</street>
118        <street>32 Vassar Street</street>
119        <city>Cambridge</city>
120        <region>MA</region>
121        <code>02139</code>
122        <country>USA</country>
123      </postal>
124      <email>timbl@w3.org</email>
125      <uri>http://www.w3.org/People/Berners-Lee/</uri>
126    </address>
127  </author>
128
129  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
130    <organization abbrev="W3C">World Wide Web Consortium</organization>
131    <address>
132      <postal>
133        <street>W3C / ERCIM</street>
134        <street>2004, rte des Lucioles</street>
135        <city>Sophia-Antipolis</city>
136        <region>AM</region>
137        <code>06902</code>
138        <country>France</country>
139      </postal>
140      <email>ylafon@w3.org</email>
141      <uri>http://www.raubacapeu.net/people/yves/</uri>
142    </address>
143  </author>
144
145  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
146    <organization abbrev="greenbytes">greenbytes GmbH</organization>
147    <address>
148      <postal>
149        <street>Hafenweg 16</street>
150        <city>Muenster</city><region>NW</region><code>48155</code>
151        <country>Germany</country>
152      </postal>
153      <phone>+49 251 2807760</phone>   
154      <facsimile>+49 251 2807761</facsimile>   
155      <email>julian.reschke@greenbytes.de</email>       
156      <uri>http://greenbytes.de/tech/webdav/</uri>     
157    </address>
158  </author>
159
160  <date month="June" year="2008" day="17"/>
161
162<abstract>
163<t>
164   The Hypertext Transfer Protocol (HTTP) is an application-level
165   protocol for distributed, collaborative, hypermedia information
166   systems. HTTP has been in use by the World Wide Web global information
167   initiative since 1990. This document is Part 6 of the seven-part specification
168   that defines the protocol referred to as "HTTP/1.1" and, taken together,
169   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
170   and the associated header fields that control cache behavior or indicate
171   cacheable response messages.
172</t>
173</abstract>
174
175<note title="Editorial Note (To be removed by RFC Editor)">
176  <t>
177    Discussion of this draft should take place on the HTTPBIS working group
178    mailing list (ietf-http-wg@w3.org). The current issues list is
179    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
180    and related documents (including fancy diffs) can be found at
181    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
182  </t>
183  <t>
184    The changes in this draft are summarized in <xref target="changes.since.02"/>.
185  </t>
186</note>
187</front>
188<middle>
189<section title="Introduction" anchor="caching">
190<t>
191   HTTP is typically used for distributed information systems, where
192   performance can be improved by the use of response caches, and includes
193   a number of elements intended to make caching work as well as possible.
194   Because these elements interact with each other, it is useful to describe
195   the caching design of HTTP separately.  This document defines aspects of
196   HTTP/1.1 related to caching and reusing response messages.
197</t>
198
199<section title="Purpose" anchor="intro.purpose">
200<iref item="cache"/>
201<t>
202   An HTTP cache is a local store of response messages
203   and the subsystem that controls its message storage, retrieval, and
204   deletion. A cache stores cacheable responses in order to reduce the
205   response time and network bandwidth consumption on future, equivalent
206   requests. Any client or server may include a cache, though a cache
207   cannot be used by a server that is acting as a tunnel.
208</t>
209<t>
210   Caching would be useless if it did not significantly improve
211   performance. The goal of caching in HTTP/1.1 is to reuse a prior response
212   message to satisfy a current request.  In some cases, the existing response
213   can be reused without the need for a network request, reducing latency and
214   network round-trips; we use an "expiration" mechanism for this purpose
215   (see <xref target="expiration.model"/>).  Even when a new request is required,
216   it is often possible to reuse all or parts of the payload of a prior response
217   to satisfy the request, thereby reducing network bandwidth usage; we use a
218   "validation" 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 status and payload
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 when a response 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
398<section title="Notational Conventions and Generic Grammar" anchor="notation">
399 
400 
401 
402 
403 
404<t>
405  This specification uses the ABNF syntax defined in Section 2.1 of <xref target="Part1"/> and
406  the core rules defined in Section 2.2 of <xref target="Part1"/>:
407  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
408  &lt;http://tools.ietf.org/wg/httpbis/trac/ticket/36&gt;.</cref>
409</t>
410<figure><artwork type="abnf2616"><![CDATA[
411  DIGIT         = <DIGIT, defined in [Part1], Section 2.2>
412  DQUOTE        = <DQUOTE, defined in [Part1], Section 2.2>
413  SP            = <SP, defined in [Part1], Section 2.2>
414]]></artwork></figure>
415<figure><artwork type="abnf2616"><![CDATA[
416  quoted-string = <quoted-string, defined in [Part1], Section 2.2>
417  token         = <token, defined in [Part1], Section 2.2>
418]]></artwork></figure>
419<t anchor="abnf.dependencies">
420 
421 
422 
423 
424 
425  The ABNF rules below are defined in other parts:
426</t>
427<figure><!--Part1--><artwork type="abnf2616"><![CDATA[
428  field-name    = <field-name, defined in [Part1], Section 4.2>
429  HTTP-date     = <HTTP-date, defined in [Part1], Section 3.3.1>
430  port          = <port, defined in [Part1], Section 3.2.1>
431  pseudonym     = <pseudonym, defined in [Part1], Section 8.9>
432  uri-host      = <uri-host, defined in [Part1], Section 3.2.1>
433]]></artwork></figure>
434</section>
435
436<section title="Overview" anchor="caching.overview">
437<section title="Cache Correctness" anchor="cache.correctness">
438<t>
439   A correct cache MUST respond to a request with the most up-to-date
440   response held by the cache that is appropriate to the request (see
441   Sections <xref target="disambiguating.expiration.values" format="counter"/>,
442   <xref target="disambiguating.multiple.responses" format="counter"/>,
443   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
444   conditions:
445  <list style="numbers">
446      <t>It has been checked for equivalence with what the origin server
447         would have returned by revalidating the response with the
448         origin server (<xref target="validation.model"/>);</t>
449
450      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
451         this means it meets the least restrictive freshness requirement
452         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
453         the origin server so specifies, it is the freshness requirement
454         of the origin server alone.
455
456         If a stored response is not "fresh enough" by the most
457         restrictive freshness requirement of both the client and the
458         origin server, in carefully considered circumstances the cache
459         MAY still return the response with the appropriate Warning
460         header (see Sections <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
461         and <xref target="header.warning" format="counter"/>), unless such a response
462         is prohibited (e.g., by a "no-store" cache-directive, or by a
463         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
464
465      <t>It is an appropriate 304 (Not Modified), 305 (Use Proxy),
466         or error (4xx or 5xx) response message.</t>
467  </list>
468</t>
469<t>
470   If the cache can not communicate with the origin server, then a
471   correct cache SHOULD respond as above if the response can be
472   correctly served from the cache; if not it MUST return an error or
473   warning indicating that there was a communication failure.
474</t>
475<t>
476   If a cache receives a response (either an entire response, or a 304
477   (Not Modified) response) that it would normally forward to the
478   requesting client, and the received response is no longer fresh, the
479   cache SHOULD forward it to the requesting client without adding a new
480   Warning (but without removing any existing Warning headers). A cache
481   SHOULD NOT  attempt to revalidate a response simply because that
482   response became stale in transit; this might lead to an infinite
483   loop. A user agent that receives a stale response without a Warning
484   MAY display a warning indication to the user.
485</t>
486</section>
487
488<section title="Warnings" anchor="warnings">
489<t>
490   Whenever a cache returns a response that is neither first-hand nor
491   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
492   MUST attach a warning to that effect, using a Warning general-header.
493   The Warning header and the currently defined warnings are described
494   in <xref target="header.warning"/>. The warning allows clients to take appropriate
495   action.
496</t>
497<t>
498   Warnings MAY be used for other purposes, both cache-related and
499   otherwise. The use of a warning, rather than an error status code,
500   distinguish these responses from true failures.
501</t>
502<t>
503   Warnings are assigned three digit warn-codes. The first digit
504   indicates whether the Warning MUST or MUST NOT be deleted from a
505   stored cache entry after a successful revalidation:
506</t>
507<t>
508  <list style="hanging">
509    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
510     the response, and so MUST be deleted after a successful
511     revalidation. 1xx warn-codes MAY be generated by a cache only when
512     validating a cached entry. It MUST NOT be generated by clients.</t>
513
514    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
515     headers that is not rectified by a revalidation (for example, a
516     lossy compression of the entity bodies) and which MUST NOT be
517     deleted after a successful revalidation.</t>
518    </list>
519</t>
520<t>
521   See <xref target="header.warning"/> for the definitions of the codes themselves.
522</t>
523<t>
524   HTTP/1.0 caches will cache all Warnings in responses, without
525   deleting the ones in the first category. Warnings in responses that
526   are passed to HTTP/1.0 caches carry an extra warning-date field,
527   which prevents a future HTTP/1.1 recipient from believing an
528   erroneously cached Warning.
529</t>
530<t>
531   Warnings also carry a warning text. The text MAY be in any
532   appropriate natural language (perhaps based on the client's Accept
533   headers), and include an OPTIONAL indication of what character set is
534   used.
535</t>
536<t>
537   Multiple warnings MAY be attached to a response (either by the origin
538   server or by a cache), including multiple warnings with the same code
539   number. For example, a server might provide the same warning with
540   texts in both English and Basque.
541</t>
542<t>
543   When multiple warnings are attached to a response, it might not be
544   practical or reasonable to display all of them to the user. This
545   version of HTTP does not specify strict priority rules for deciding
546   which warnings to display and in what order, but does suggest some
547   heuristics.
548</t>
549</section>
550
551<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
552<t>
553   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
554   times and validators) are implicit directives to caches. In some
555   cases, a server or client might need to provide explicit directives
556   to the HTTP caches. We use the Cache-Control header for this purpose.
557</t>
558<t>
559   The Cache-Control header allows a client or server to transmit a
560   variety of directives in either requests or responses. These
561   directives typically override the default caching algorithms. As a
562   general rule, if there is any apparent conflict between header
563   values, the most restrictive interpretation is applied (that is, the
564   one that is most likely to preserve semantic transparency). However,
565   in some cases, cache-control directives are explicitly specified as
566   weakening the approximation of semantic transparency (for example,
567   "max-stale" or "public").
568</t>
569<t>
570   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
571</t>
572</section>
573
574<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
575<t>
576   Many user agents make it possible for users to override the basic
577   caching mechanisms. For example, the user agent might allow the user
578   to specify that cached entities (even explicitly stale ones) are
579   never validated. Or the user agent might habitually add "Cache-Control:
580   max-stale=3600" to every request. The user agent SHOULD NOT
581   default to either non-transparent behavior, or behavior that results
582   in abnormally ineffective caching, but MAY be explicitly configured
583   to do so by an explicit action of the user.
584</t>
585<t>
586   If the user has overridden the basic caching mechanisms, the user
587   agent SHOULD explicitly indicate to the user whenever this results in
588   the display of information that might not meet the server's
589   transparency requirements (in particular, if the displayed entity is
590   known to be stale). Since the protocol normally allows the user agent
591   to determine if responses are stale or not, this indication need only
592   be displayed when this actually happens. The indication need not be a
593   dialog box; it could be an icon (for example, a picture of a rotting
594   fish) or some other indicator.
595</t>
596<t>
597   If the user has overridden the caching mechanisms in a way that would
598   abnormally reduce the effectiveness of caches, the user agent SHOULD
599   continually indicate this state to the user (for example, by a
600   display of a picture of currency in flames) so that the user does not
601   inadvertently consume excess resources or suffer from excessive
602   latency.
603</t>
604</section>
605
606<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
607<t>
608   In some cases, the operator of a cache MAY choose to configure it to
609   return stale responses even when not requested by clients. This
610   decision ought not be made lightly, but may be necessary for reasons
611   of availability or performance, especially when the cache is poorly
612   connected to the origin server. Whenever a cache returns a stale
613   response, it MUST mark it as such (using a Warning header) enabling
614   the client software to alert the user that there might be a potential
615   problem.
616</t>
617<t>
618   It also allows the user agent to take steps to obtain a first-hand or
619   fresh response. For this reason, a cache SHOULD NOT  return a stale
620   response if the client explicitly requests a first-hand or fresh one,
621   unless it is impossible to comply for technical or policy reasons.
622</t>
623</section>
624
625<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
626<t>
627   While the origin server (and to a lesser extent, intermediate caches,
628   by their contribution to the age of a response) are the primary
629   source of expiration information, in some cases the client might need
630   to control a cache's decision about whether to return a cached
631   response without validating it. Clients do this using several
632   directives of the Cache-Control header.
633</t>
634<t>
635   A client's request MAY specify the maximum age it is willing to
636   accept of an unvalidated response; specifying a value of zero forces
637   the cache(s) to revalidate all responses. A client MAY also specify
638   the minimum time remaining before a response expires. Both of these
639   options increase constraints on the behavior of caches, and so cannot
640   further relax the cache's approximation of semantic transparency.
641</t>
642<t>
643   A client MAY also specify that it will accept stale responses, up to
644   some maximum amount of staleness. This loosens the constraints on the
645   caches, and so might violate the origin server's specified
646   constraints on semantic transparency, but might be necessary to
647   support disconnected operation, or high availability in the face of
648   poor connectivity.
649</t>
650</section>
651</section>
652
653<section title="Expiration Model" anchor="expiration.model">
654
655<section title="Server-Specified Expiration" anchor="server-specified.expiration">
656<t>
657   HTTP caching works best when caches can entirely avoid making
658   requests to the origin server. The primary mechanism for avoiding
659   requests is for an origin server to provide an explicit expiration
660   time in the future, indicating that a response MAY be used to satisfy
661   subsequent requests. In other words, a cache can return a fresh
662   response without first contacting the server.
663</t>
664<t>
665   Our expectation is that servers will assign future explicit
666   expiration times to responses in the belief that the entity is not
667   likely to change, in a semantically significant way, before the
668   expiration time is reached. This normally preserves semantic
669   transparency, as long as the server's expiration times are carefully
670   chosen.
671</t>
672<t>
673   The expiration mechanism applies only to responses taken from a cache
674   and not to first-hand responses forwarded immediately to the
675   requesting client.
676</t>
677<t>
678   If an origin server wishes to force a semantically transparent cache
679   to validate every request, it MAY assign an explicit expiration time
680   in the past. This means that the response is always stale, and so the
681   cache SHOULD validate it before using it for subsequent requests. See
682   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
683</t>
684<t>
685   If an origin server wishes to force any HTTP/1.1 cache, no matter how
686   it is configured, to validate every request, it SHOULD use the "must-revalidate"
687   cache-control directive (see <xref target="header.cache-control"/>).
688</t>
689<t>
690   Servers specify explicit expiration times using either the Expires
691   header, or the max-age directive of the Cache-Control header.
692</t>
693<t>
694   An expiration time cannot be used to force a user agent to refresh
695   its display or reload a resource; its semantics apply only to caching
696   mechanisms, and such mechanisms need only check a resource's
697   expiration status when a new request for that resource is initiated.
698   See <xref target="history.lists"/> for an explanation of the difference between caches
699   and history mechanisms.
700</t>
701</section>
702
703<section title="Heuristic Expiration" anchor="heuristic.expiration">
704<t>
705   Since origin servers do not always provide explicit expiration times,
706   HTTP caches typically assign heuristic expiration times, employing
707   algorithms that use other header values (such as the Last-Modified
708   time) to estimate a plausible expiration time. The HTTP/1.1
709   specification does not provide specific algorithms, but does impose
710   worst-case constraints on their results. Since heuristic expiration
711   times might compromise semantic transparency, they ought to be used
712   cautiously, and we encourage origin servers to provide explicit
713   expiration times as much as possible.
714</t>
715</section>
716
717<section title="Age Calculations" anchor="age.calculations">
718<t>
719   In order to know if a cached entry is fresh, a cache needs to know if
720   its age exceeds its freshness lifetime. We discuss how to calculate
721   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
722   the age of a response or cache entry.
723</t>
724<t>
725   In this discussion, we use the term "now" to mean "the current value
726   of the clock at the host performing the calculation." Hosts that use
727   HTTP, but especially hosts running origin servers and caches, SHOULD
728   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
729   a globally accurate time standard.
730</t>
731<t>
732   HTTP/1.1 requires origin servers to send a Date header, if possible,
733   with every response, giving the time at which the response was
734   generated (see Section 8.3 of <xref target="Part1"/>). We use the term "date_value" to denote
735   the value of the Date header, in a form appropriate for arithmetic
736   operations.
737</t>
738<t>
739   HTTP/1.1 uses the Age response-header to convey the estimated age of
740   the response message when obtained from a cache. The Age field value
741   is the cache's estimate of the amount of time since the response was
742   generated or revalidated by the origin server.
743</t>
744<t>
745   In essence, the Age value is the sum of the time that the response
746   has been resident in each of the caches along the path from the
747   origin server, plus the amount of time it has been in transit along
748   network paths.
749</t>
750<t>
751   We use the term "age_value" to denote the value of the Age header, in
752   a form appropriate for arithmetic operations.
753</t>
754<t>
755   A response's age can be calculated in two entirely independent ways:
756  <list style="numbers">
757      <t>now minus date_value, if the local clock is reasonably well
758         synchronized to the origin server's clock. If the result is
759         negative, the result is replaced by zero.</t>
760
761      <t>age_value, if all of the caches along the response path
762         implement HTTP/1.1.</t>
763  </list>
764</t>
765<t>
766   Given that we have two independent ways to compute the age of a
767   response when it is received, we can combine these as
768</t>
769<figure><artwork type="code"><![CDATA[
770    corrected_received_age = max(now - date_value, age_value)
771]]></artwork></figure>
772<t>
773   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
774   paths, one gets a reliable (conservative) result.
775</t>
776<t>
777   Because of network-imposed delays, some significant interval might
778   pass between the time that a server generates a response and the time
779   it is received at the next outbound cache or client. If uncorrected,
780   this delay could result in improperly low ages.
781</t>
782<t>
783   Because the request that resulted in the returned Age value must have
784   been initiated prior to that Age value's generation, we can correct
785   for delays imposed by the network by recording the time at which the
786   request was initiated. Then, when an Age value is received, it MUST
787   be interpreted relative to the time the request was initiated, not
788   the time that the response was received. This algorithm results in
789   conservative behavior no matter how much delay is experienced. So, we
790   compute:
791</t>
792<figure><artwork type="code"><![CDATA[
793   corrected_initial_age = corrected_received_age
794                         + (now - request_time)
795]]></artwork></figure>
796<t>
797   where "request_time" is the time (according to the local clock) when
798   the request that elicited this response was sent.
799</t>
800<t>
801   Summary of age calculation algorithm, when a cache receives a
802   response:
803</t>
804<figure><artwork type="code"><![CDATA[
805   /*
806    * age_value
807    *      is the value of Age: header received by the cache with
808    *              this response.
809    * date_value
810    *      is the value of the origin server's Date: header
811    * request_time
812    *      is the (local) time when the cache made the request
813    *              that resulted in this cached response
814    * response_time
815    *      is the (local) time when the cache received the
816    *              response
817    * now
818    *      is the current (local) time
819    */
820
821   apparent_age = max(0, response_time - date_value);
822   corrected_received_age = max(apparent_age, age_value);
823   response_delay = response_time - request_time;
824   corrected_initial_age = corrected_received_age + response_delay;
825   resident_time = now - response_time;
826   current_age   = corrected_initial_age + resident_time;
827]]></artwork></figure>
828<t>
829   The current_age of a cache entry is calculated by adding the amount
830   of time (in seconds) since the cache entry was last validated by the
831   origin server to the corrected_initial_age. When a response is
832   generated from a cache entry, the cache MUST include a single Age
833   header field in the response with a value equal to the cache entry's
834   current_age.
835</t>
836<t>
837   The presence of an Age header field in a response implies that a
838   response is not first-hand. However, the converse is not true, since
839   the lack of an Age header field in a response does not imply that the
840   response is first-hand unless all caches along the request path are
841   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
842   the Age header field).
843</t>
844</section>
845
846<section title="Expiration Calculations" anchor="expiration.calculations">
847<t>
848   In order to decide whether a response is fresh or stale, we need to
849   compare its freshness lifetime to its age. The age is calculated as
850   described in <xref target="age.calculations"/>; this section describes how to calculate
851   the freshness lifetime, and to determine if a response has expired.
852   In the discussion below, the values can be represented in any form
853   appropriate for arithmetic operations.
854</t>
855<t>
856   We use the term "expires_value" to denote the value of the Expires
857   header. We use the term "max_age_value" to denote an appropriate
858   value of the number of seconds carried by the "max-age" directive of
859   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
860</t>
861<t>
862   The max-age directive takes priority over Expires, so if max-age is
863   present in a response, the calculation is simply:
864</t>
865<figure><artwork type="code"><![CDATA[
866   freshness_lifetime = max_age_value
867]]></artwork></figure>
868<t>
869   Otherwise, if Expires is present in the response, the calculation is:
870</t>
871<figure><artwork type="code"><![CDATA[
872   freshness_lifetime = expires_value - date_value
873]]></artwork></figure>
874<t>
875   Note that neither of these calculations is vulnerable to clock skew,
876   since all of the information comes from the origin server.
877</t>
878<t>
879   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
880   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
881   does not include other restrictions on caching, the cache MAY compute
882   a freshness lifetime using a heuristic. The cache MUST attach Warning
883   113 to any response whose age is more than 24 hours if such warning
884   has not already been added.
885</t>
886<t>
887   Also, if the response does have a Last-Modified time, the heuristic
888   expiration value SHOULD be no more than some fraction of the interval
889   since that time. A typical setting of this fraction might be 10%.
890</t>
891<t>
892   The calculation to determine if a response has expired is quite
893   simple:
894</t>
895<figure><artwork type="code"><![CDATA[
896   response_is_fresh = (freshness_lifetime > current_age)
897]]></artwork></figure>
898</section>
899
900<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
901<t>
902   Because expiration values are assigned optimistically, it is possible
903   for two caches to contain fresh values for the same resource that are
904   different.
905</t>
906<t>
907   If a client performing a retrieval receives a non-first-hand response
908   for a request that was already fresh in its own cache, and the Date
909   header in its existing cache entry is newer than the Date on the new
910   response, then the client MAY ignore the response. If so, it MAY
911   retry the request with a "Cache-Control: max-age=0" directive (see
912   <xref target="header.cache-control"/>), to force a check with the origin server.
913</t>
914<t>
915   If a cache has two fresh responses for the same representation with
916   different validators, it MUST use the one with the more recent Date
917   header. This situation might arise because the cache is pooling
918   responses from other caches, or because a client has asked for a
919   reload or a revalidation of an apparently fresh cache entry.
920</t>
921</section>
922
923<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
924<t>
925   Because a client might be receiving responses via multiple paths, so
926   that some responses flow through one set of caches and other
927   responses flow through a different set of caches, a client might
928   receive responses in an order different from that in which the origin
929   server sent them. We would like the client to use the most recently
930   generated response, even if older responses are still apparently
931   fresh.
932</t>
933<t>
934   Neither the entity tag nor the expiration value can impose an
935   ordering on responses, since it is possible that a later response
936   intentionally carries an earlier expiration time. The Date values are
937   ordered to a granularity of one second.
938</t>
939<t>
940   When a client tries to revalidate a cache entry, and the response it
941   receives contains a Date header that appears to be older than the one
942   for the existing entry, then the client SHOULD repeat the request
943   unconditionally, and include
944</t>
945<figure><artwork type="example"><![CDATA[
946    Cache-Control: max-age=0
947]]></artwork></figure>
948<t>
949   to force any intermediate caches to validate their copies directly
950   with the origin server, or
951</t>
952<figure><artwork type="example"><![CDATA[
953    Cache-Control: no-cache
954]]></artwork></figure>
955<t>
956   to force any intermediate caches to obtain a new copy from the origin
957   server.
958</t>
959<t>
960   If the Date values are equal, then the client MAY use either response
961   (or MAY, if it is being extremely prudent, request a new response).
962   Servers MUST NOT depend on clients being able to choose
963   deterministically between responses generated during the same second,
964   if their expiration times overlap.
965</t>
966</section>
967</section>
968
969<section title="Validation Model" anchor="validation.model">
970<t>
971   When a cache has a stale entry that it would like to use as a
972   response to a client's request, it first has to check with the origin
973   server (or possibly an intermediate cache with a fresh response) to
974   see if its cached entry is still usable. We call this "validating"
975   the cache entry.
976</t>
977<t>
978   HTTP's conditional request mechanism, defined in <xref target="Part4"/>, is
979   used to avoid retransmitting the response payload when the cached entry
980   is valid.  When a cached response includes one or more "cache validators,"
981   such as the field values of an ETag or Last-Modified header field, then
982   a validating GET request SHOULD be made conditional to those field values.
983   The server checks the conditional request's validator against the current
984   state of the requested resource and, if they match, the server responds
985   with a 304 (Not Modified) status code to indicate that the cached response
986   can be refreshed and reused without retransmitting the response payload.
987   If the validator does not match the current state of the requested
988   resource, then the server returns a full response, including payload,
989   so that the request can be satisfied and the cache entry supplanted
990   without the need for an additional network round-trip.
991</t>
992</section>
993
994<section title="Response Cacheability" anchor="response.cacheability">
995<t>
996   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
997   directive, a caching system MAY always store a successful response
998   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, MAY return it without validation
999   if it is fresh, and MAY return it after successful validation. If
1000   there is neither a cache validator nor an explicit expiration time
1001   associated with a response, we do not expect it to be cached, but
1002   certain caches MAY violate this expectation (for example, when little
1003   or no network connectivity is available). A client can usually detect
1004   that such a response was taken from a cache by comparing the Date
1005   header to the current time.
1006  <list><t>
1007      Note: some HTTP/1.0 caches are known to violate this expectation
1008      without providing any Warning.
1009  </t></list>
1010</t>
1011<t>
1012   However, in some cases it might be inappropriate for a cache to
1013   retain an entity, or to return it in response to a subsequent
1014   request. This might be because absolute semantic transparency is
1015   deemed necessary by the service author, or because of security or
1016   privacy considerations. Certain cache-control directives are
1017   therefore provided so that the server can indicate that certain
1018   resource entities, or portions thereof, are not to be cached
1019   regardless of other considerations.
1020</t>
1021<t>
1022   Note that Section 4.1 of <xref target="Part7"/> normally prevents a shared cache from saving
1023   and returning a response to a previous request if that request
1024   included an Authorization header.
1025</t>
1026<t>
1027   A response received with a status code of 200, 203, 206, 300, 301 or
1028   410 MAY be stored by a cache and used in reply to a subsequent
1029   request, subject to the expiration mechanism, unless a cache-control
1030   directive prohibits caching. However, a cache that does not support
1031   the Range and Content-Range headers MUST NOT cache 206 (Partial
1032   Content) responses.
1033</t>
1034<t>
1035   A response received with any other status code (e.g. status codes 302
1036   and 307) MUST NOT be returned in a reply to a subsequent request
1037   unless there are cache-control directives or another header(s) that
1038   explicitly allow it. For example, these include the following: an
1039   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1040   "proxy-revalidate", "public" or "private" cache-control
1041   directive (<xref target="header.cache-control"/>).
1042</t>
1043</section>
1044
1045<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1046<t>
1047   The purpose of an HTTP cache is to store information received in
1048   response to requests for use in responding to future requests. In
1049   many cases, a cache simply returns the appropriate parts of a
1050   response to the requester. However, if the cache holds a cache entry
1051   based on a previous response, it might have to combine parts of a new
1052   response with what is held in the cache entry.
1053</t>
1054
1055<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1056<t>
1057   For the purpose of defining the behavior of caches and non-caching
1058   proxies, we divide HTTP headers into two categories:
1059  <list style="symbols">
1060      <t>End-to-end headers, which are  transmitted to the ultimate
1061        recipient of a request or response. End-to-end headers in
1062        responses MUST be stored as part of a cache entry and MUST be
1063        transmitted in any response formed from a cache entry.</t>
1064
1065      <t>Hop-by-hop headers, which are meaningful only for a single
1066        transport-level connection, and are not stored by caches or
1067        forwarded by proxies.</t>
1068  </list>
1069</t>
1070<t>
1071   The following HTTP/1.1 headers are hop-by-hop headers:
1072  <list style="symbols">
1073      <t>Connection</t>
1074      <t>Keep-Alive</t>
1075      <t>Proxy-Authenticate</t>
1076      <t>Proxy-Authorization</t>
1077      <t>TE</t>
1078      <t>Trailer</t>
1079      <t>Transfer-Encoding</t>
1080      <t>Upgrade</t>
1081  </list>
1082</t>
1083<t>
1084   All other headers defined by HTTP/1.1 are end-to-end headers.
1085</t>
1086<t>
1087   Other hop-by-hop headers MUST be listed in a Connection header
1088   (Section 8.1 of <xref target="Part1"/>).
1089</t>
1090</section>
1091
1092<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1093<t>
1094   Some features of HTTP/1.1, such as Digest
1095   Authentication, depend on the value of certain end-to-end headers. A
1096   transparent proxy SHOULD NOT  modify an end-to-end header unless the
1097   definition of that header requires or specifically allows that.
1098</t>
1099<t>
1100   A transparent proxy MUST NOT modify any of the following fields in a
1101   request or response, and it MUST NOT add any of these fields if not
1102   already present:
1103  <list style="symbols">
1104      <t>Content-Location</t>
1105      <t>Content-MD5</t>
1106      <t>ETag</t>
1107      <t>Last-Modified</t>
1108  </list>
1109</t>
1110<t>
1111   A transparent proxy MUST NOT modify any of the following fields in a
1112   response:
1113  <list style="symbols">
1114    <t>Expires</t>
1115  </list>
1116</t>
1117<t>
1118   but it MAY add any of these fields if not already present. If an
1119   Expires header is added, it MUST be given a field-value identical to
1120   that of the Date header in that response.
1121</t>
1122<t>
1123   A  proxy MUST NOT modify or add any of the following fields in a
1124   message that contains the no-transform cache-control directive, or in
1125   any request:
1126  <list style="symbols">
1127    <t>Content-Encoding</t>
1128    <t>Content-Range</t>
1129    <t>Content-Type</t>
1130  </list>
1131</t>
1132<t>
1133   A non-transparent proxy MAY modify or add these fields to a message
1134   that does not include no-transform, but if it does so, it MUST add a
1135   Warning 214 (Transformation applied) if one does not already appear
1136   in the message (see <xref target="header.warning"/>).
1137  <list><t>
1138      Warning: unnecessary modification of end-to-end headers might
1139      cause authentication failures if stronger authentication
1140      mechanisms are introduced in later versions of HTTP. Such
1141      authentication mechanisms MAY rely on the values of header fields
1142      not listed here.
1143    </t></list>
1144</t>
1145<t>
1146   The Content-Length field of a request or response is added or deleted
1147   according to the rules in Section 4.4 of <xref target="Part1"/>. A transparent proxy MUST
1148   preserve the entity-length (Section 4.2.2 of <xref target="Part3"/>) of the entity-body,
1149   although it MAY change the transfer-length (Section 4.4 of <xref target="Part1"/>).
1150</t>
1151</section>
1152
1153<section title="Combining Headers" anchor="combining.headers">
1154<t>
1155   When a cache makes a validating request to a server, and the server
1156   provides a 304 (Not Modified) response or a 206 (Partial Content)
1157   response, the cache then constructs a response to send to the
1158   requesting client.
1159</t>
1160<t>
1161   If the status code is 304 (Not Modified), the cache uses the entity-body
1162   stored in the cache entry as the entity-body of this outgoing
1163   response. If the status code is 206 (Partial Content) and the ETag or
1164   Last-Modified headers match exactly, the cache MAY combine the
1165   contents stored in the cache entry with the new contents received in
1166   the response and use the result as the entity-body of this outgoing
1167   response, (see Section 5 of <xref target="Part5"/>).
1168</t>
1169<t>
1170   The end-to-end headers stored in the cache entry are used for the
1171   constructed response, except that
1172  <list style="symbols">
1173    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1174      MUST be deleted from the cache entry and the forwarded response.</t>
1175    <t>any stored Warning headers with warn-code 2xx MUST be retained
1176        in the cache entry and the forwarded response.</t>
1177    <t>any end-to-end headers provided in the 304 or 206 response MUST
1178        replace the corresponding headers from the cache entry.</t>
1179  </list>
1180</t>
1181<t>
1182   Unless the cache decides to remove the cache entry, it MUST also
1183   replace the end-to-end headers stored with the cache entry with
1184   corresponding headers received in the incoming response, except for
1185   Warning headers as described immediately above. If a header field-name
1186   in the incoming response matches more than one header in the
1187   cache entry, all such old headers MUST be replaced.
1188</t>
1189<t>
1190   In other words, the set of end-to-end headers received in the
1191   incoming response overrides all corresponding end-to-end headers
1192   stored with the cache entry (except for stored Warning headers with
1193   warn-code 1xx, which are deleted even if not overridden).
1194  <list><t>
1195      Note: this rule allows an origin server to use a 304 (Not
1196      Modified) or a 206 (Partial Content) response to update any header
1197      associated with a previous response for the same entity or sub-ranges
1198      thereof, although it might not always be meaningful or
1199      correct to do so. This rule does not allow an origin server to use
1200      a 304 (Not Modified) or a 206 (Partial Content) response to
1201      entirely delete a header that it had provided with a previous
1202      response.
1203  </t></list>
1204</t>
1205</section>
1206
1207</section>
1208
1209<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1210<t>
1211   Use of server-driven content negotiation (Section 5.1 of <xref target="Part3"/>), as indicated
1212   by the presence of a Vary header field in a response, alters the
1213   conditions and procedure by which a cache can use the response for
1214   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1215   field by servers.
1216</t>
1217<t>
1218   A server SHOULD use the Vary header field to inform a cache of what
1219   request-header fields were used to select among multiple
1220   representations of a cacheable response subject to server-driven
1221   negotiation. The set of header fields named by the Vary field value
1222   is known as the "selecting" request-headers.
1223</t>
1224<t>
1225   When the cache receives a subsequent request whose Request-URI
1226   specifies one or more cache entries including a Vary header field,
1227   the cache MUST NOT use such a cache entry to construct a response to
1228   the new request unless all of the selecting request-headers present
1229   in the new request match the corresponding stored request-headers in
1230   the original request.
1231</t>
1232<t>
1233   The selecting request-headers from two requests are defined to match
1234   if and only if the selecting request-headers in the first request can
1235   be transformed to the selecting request-headers in the second request
1236   by adding or removing linear white space (LWS) at places where this
1237   is allowed by the corresponding BNF, and/or combining multiple
1238   message-header fields with the same field name following the rules
1239   about message headers in Section 4.2 of <xref target="Part1"/>.
1240</t>
1241<t>
1242   A Vary header field-value of "*" always fails to match and subsequent
1243   requests on that resource can only be properly interpreted by the
1244   origin server.
1245</t>
1246<t>
1247   If the selecting request header fields for the cached entry do not
1248   match the selecting request header fields of the new request, then
1249   the cache MUST NOT use a cached entry to satisfy the request unless
1250   it first relays the new request to the origin server in a conditional
1251   request and the server responds with 304 (Not Modified), including an
1252   entity tag or Content-Location that indicates the entity to be used.
1253</t>
1254<t>
1255   If an entity tag was assigned to a cached representation, the
1256   forwarded request SHOULD be conditional and include the entity tags
1257   in an If-None-Match header field from all its cache entries for the
1258   resource. This conveys to the server the set of entities currently
1259   held by the cache, so that if any one of these entities matches the
1260   requested entity, the server can use the ETag header field in its 304
1261   (Not Modified) response to tell the cache which entry is appropriate.
1262   If the entity-tag of the new response matches that of an existing
1263   entry, the new response SHOULD be used to update the header fields of
1264   the existing entry, and the result MUST be returned to the client.
1265</t>
1266<t>
1267   If any of the existing cache entries contains only partial content
1268   for the associated entity, its entity-tag SHOULD NOT  be included in
1269   the If-None-Match header field unless the request is for a range that
1270   would be fully satisfied by that entry.
1271</t>
1272<t>
1273   If a cache receives a successful response whose Content-Location
1274   field matches that of an existing cache entry for the same Request-URI,
1275   whose entity-tag differs from that of the existing entry, and
1276   whose Date is more recent than that of the existing entry, the
1277   existing entry SHOULD NOT  be returned in response to future requests
1278   and SHOULD be deleted from the cache.
1279</t>
1280</section>
1281
1282<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1283<t>
1284   For reasons of security and privacy, it is necessary to make a
1285   distinction between "shared" and "non-shared" caches. A non-shared
1286   cache is one that is accessible only to a single user. Accessibility
1287   in this case SHOULD be enforced by appropriate security mechanisms.
1288   All other caches are considered to be "shared." Other sections of
1289   this specification place certain constraints on the operation of
1290   shared caches in order to prevent loss of privacy or failure of
1291   access controls.
1292</t>
1293</section>
1294
1295<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1296<t>
1297   A cache that receives an incomplete response (for example, with fewer
1298   bytes of data than specified in a Content-Length header) MAY store
1299   the response. However, the cache MUST treat this as a partial
1300   response. Partial responses MAY be combined as described in Section 5 of <xref target="Part5"/>;
1301   the result might be a full response or might still be
1302   partial. A cache MUST NOT return a partial response to a client
1303   without explicitly marking it as such, using the 206 (Partial
1304   Content) status code. A cache MUST NOT return a partial response
1305   using a status code of 200 (OK).
1306</t>
1307<t>
1308   If a cache receives a 5xx response while attempting to revalidate an
1309   entry, it MAY either forward this response to the requesting client,
1310   or act as if the server failed to respond. In the latter case, it MAY
1311   return a previously received response unless the cached entry
1312   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1313</t>
1314</section>
1315
1316<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1317<t>
1318   Unless the origin server explicitly prohibits the caching of their
1319   responses, the application of GET and HEAD methods to any resources
1320   SHOULD NOT  have side effects that would lead to erroneous behavior if
1321   these responses are taken from a cache. They MAY still have side
1322   effects, but a cache is not required to consider such side effects in
1323   its caching decisions. Caches are always expected to observe an
1324   origin server's explicit restrictions on caching.
1325</t>
1326<t>
1327   We note one exception to this rule: since some applications have
1328   traditionally used GET and HEAD requests with URLs containing a query part
1329   to perform operations with significant side
1330   effects, caches MUST NOT treat responses to such URIs as fresh unless
1331   the server provides an explicit expiration time. This specifically
1332   means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
1333   be taken from a cache. See Section 8.1.1 of <xref target="Part2"/> for related information.
1334</t>
1335</section>
1336
1337<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1338<t>
1339   The effect of certain methods performed on a resource at the origin
1340   server might cause one or more existing cache entries to become non-transparently
1341   invalid. That is, although they might continue to be
1342   "fresh," they do not accurately reflect what the origin server would
1343   return for a new request on that resource.
1344</t>
1345<t>
1346   There is no way for HTTP to guarantee that all such
1347   cache entries are marked invalid. For example, the request that
1348   caused the change at the origin server might not have gone through
1349   the proxy where a cache entry is stored. However, several rules help
1350   reduce the likelihood of erroneous behavior.
1351</t>
1352<t>
1353   In this section, the phrase "invalidate an entity" means that the
1354   cache will either remove all instances of that entity from its
1355   storage, or will mark these as "invalid" and in need of a mandatory
1356   revalidation before they can be returned in response to a subsequent
1357   request.
1358</t>
1359<t>
1360   Some HTTP methods MUST cause a cache to invalidate an entity. This is
1361   either the entity referred to by the Request-URI, or by the Location
1362   or Content-Location headers (if present). These methods are:
1363  <list style="symbols">
1364      <t>PUT</t>
1365      <t>DELETE</t>
1366      <t>POST</t>
1367  </list>
1368</t> 
1369<t>
1370   An invalidation based
1371   on the URI in a Location or Content-Location header MUST NOT be
1372   performed if the host part of that URI differs from the host part
1373   in the Request-URI. This helps prevent denial of service attacks.
1374</t>
1375<t>
1376   A cache that passes through requests for methods it does not
1377   understand SHOULD invalidate any entities referred to by the
1378   Request-URI.
1379</t>
1380</section>
1381
1382<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1383<t>
1384   All methods that might be expected to cause modifications to the
1385   origin server's resources MUST be written through to the origin
1386   server. This currently includes all methods except for GET and HEAD.
1387   A cache MUST NOT reply to such a request from a client before having
1388   transmitted the request to the inbound server, and having received a
1389   corresponding response from the inbound server. This does not prevent
1390   a proxy cache from sending a 100 (Continue) response before the
1391   inbound server has sent its final reply.
1392</t>
1393<t>
1394   The alternative (known as "write-back" or "copy-back" caching) is not
1395   allowed in HTTP/1.1, due to the difficulty of providing consistent
1396   updates and the problems arising from server, cache, or network
1397   failure prior to write-back.
1398</t>
1399</section>
1400
1401<section title="Cache Replacement" anchor="cache.replacement">
1402<t>
1403   If a new cacheable (see Sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1404   <xref target="disambiguating.expiration.values" format="counter"/>,
1405   <xref target="disambiguating.multiple.responses" format="counter"/>
1406   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1407   response is received from a resource while any existing responses for
1408   the same resource are cached, the cache SHOULD use the new response
1409   to reply to the current request. It MAY insert it into cache storage
1410   and MAY, if it meets all other requirements, use it to respond to any
1411   future requests that would previously have caused the old response to
1412   be returned. If it inserts the new response into cache storage  the
1413   rules in <xref target="combining.headers"/> apply.
1414  <list><t>
1415      Note: a new response that has an older Date header value than
1416      existing cached responses is not cacheable.
1417  </t></list>
1418</t>
1419</section>
1420
1421<section title="History Lists" anchor="history.lists">
1422<t>
1423   User agents often have history mechanisms, such as "Back" buttons and
1424   history lists, which can be used to redisplay an entity retrieved
1425   earlier in a session.
1426</t>
1427<t>
1428   History mechanisms and caches are different. In particular history
1429   mechanisms SHOULD NOT  try to show a semantically transparent view of
1430   the current state of a resource. Rather, a history mechanism is meant
1431   to show exactly what the user saw at the time when the resource was
1432   retrieved.
1433</t>
1434<t>
1435   By default, an expiration time does not apply to history mechanisms.
1436   If the entity is still in storage, a history mechanism SHOULD display
1437   it even if the entity has expired, unless the user has specifically
1438   configured the agent to refresh expired history documents.
1439</t>
1440<t>
1441   This is not to be construed to prohibit the history mechanism from
1442   telling the user that a view might be stale.
1443  <list><t>
1444      Note: if history list mechanisms unnecessarily prevent users from
1445      viewing stale resources, this will tend to force service authors
1446      to avoid using HTTP expiration controls and cache controls when
1447      they would otherwise like to. Service authors may consider it
1448      important that users not be presented with error messages or
1449      warning messages when they use navigation controls (such as BACK)
1450      to view previously fetched resources. Even though sometimes such
1451      resources ought not be cached, or ought to expire quickly, user
1452      interface considerations may force service authors to resort to
1453      other means of preventing caching (e.g. "once-only" URLs) in order
1454      not to suffer the effects of improperly functioning history
1455      mechanisms.
1456  </t></list>
1457</t>
1458</section>
1459
1460<section title="Header Field Definitions" anchor="header.fields">
1461<t>
1462   This section defines the syntax and semantics of HTTP/1.1 header fields
1463   related to caching.
1464</t>
1465<t>
1466   For entity-header fields, both sender and recipient refer to either the
1467   client or the server, depending on who sends and who receives the entity.
1468</t>
1469
1470<section title="Age" anchor="header.age">
1471  <iref primary="true" item="Age header"/>
1472  <iref primary="true" item="Headers" subitem="Age"/>
1473 
1474 
1475<t>
1476      The Age response-header field conveys the sender's estimate of the
1477      amount of time since the response (or its revalidation) was
1478      generated at the origin server. A cached response is "fresh" if
1479      its age does not exceed its freshness lifetime. Age values are
1480      calculated as specified in <xref target="age.calculations"/>.
1481</t>
1482<figure><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/><artwork type="abnf2616"><![CDATA[
1483  Age = "Age" ":" age-value
1484  age-value = delta-seconds
1485]]></artwork></figure>
1486<t anchor="rule.delta-seconds">
1487 
1488      Age values are non-negative decimal integers, representing time in
1489      seconds.
1490</t>
1491<figure><iref primary="true" item="Grammar" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
1492  delta-seconds  = 1*DIGIT
1493]]></artwork></figure>
1494<t>
1495      If a cache receives a value larger than the largest positive
1496      integer it can represent, or if any of its age calculations
1497      overflows, it MUST transmit an Age header with a value of
1498      2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
1499      include an Age header field in every response generated from its
1500      own cache. Caches SHOULD use an arithmetic type of at least 31
1501      bits of range.
1502</t>
1503</section>
1504
1505<section title="Cache-Control" anchor="header.cache-control">
1506  <iref primary="true" item="Cache-Control header"/>
1507  <iref primary="true" item="Headers" subitem="Cache-Control"/>
1508 
1509 
1510 
1511 
1512 
1513<t>
1514   The Cache-Control general-header field is used to specify directives
1515   that MUST be obeyed by all caching mechanisms along the
1516   request/response chain. The directives specify behavior intended to
1517   prevent caches from adversely interfering with the request or
1518   response. These directives typically override the default caching
1519   algorithms. Cache directives are unidirectional in that the presence
1520   of a directive in a request does not imply that the same directive is
1521   to be given in the response.
1522  <list><t>
1523      Note that HTTP/1.0 caches might not implement Cache-Control and
1524      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1525  </t></list>
1526</t>
1527<t>
1528   Cache directives MUST be passed through by a proxy or gateway
1529   application, regardless of their significance to that application,
1530   since the directives might be applicable to all recipients along the
1531   request/response chain. It is not possible to specify a cache-directive
1532   for a specific cache.
1533</t>
1534<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[
1535  Cache-Control   = "Cache-Control" ":" 1#cache-directive
1536
1537  cache-directive = cache-request-directive
1538     | cache-response-directive
1539
1540  cache-request-directive =
1541       "no-cache"                          ; Section 16.2.1
1542     | "no-store"                          ; Section 16.2.2
1543     | "max-age" "=" delta-seconds         ; Section 16.2.3, 16.2.4
1544     | "max-stale" [ "=" delta-seconds ]   ; Section 16.2.3
1545     | "min-fresh" "=" delta-seconds       ; Section 16.2.3
1546     | "no-transform"                      ; Section 16.2.5
1547     | "only-if-cached"                    ; Section 16.2.4
1548     | cache-extension                     ; Section 16.2.6
1549
1550  cache-response-directive =
1551       "public"                               ; Section 16.2.1
1552     | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
1553     | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
1554     | "no-store"                             ; Section 16.2.2
1555     | "no-transform"                         ; Section 16.2.5
1556     | "must-revalidate"                      ; Section 16.2.4
1557     | "proxy-revalidate"                     ; Section 16.2.4
1558     | "max-age" "=" delta-seconds            ; Section 16.2.3
1559     | "s-maxage" "=" delta-seconds           ; Section 16.2.3
1560     | cache-extension                        ; Section 16.2.6
1561
1562  cache-extension = token [ "=" ( token | quoted-string ) ]
1563]]></artwork></figure>
1564<t>
1565   When a directive appears without any 1#field-name parameter, the
1566   directive applies to the entire request or response. When such a
1567   directive appears with a 1#field-name parameter, it applies only to
1568   the named field or fields, and not to the rest of the request or
1569   response. This mechanism supports extensibility; implementations of
1570   future versions of HTTP might apply these directives to
1571   header fields not defined in HTTP/1.1.
1572</t>
1573<t>
1574   The cache-control directives can be broken down into these general
1575   categories:
1576  <list style="symbols">
1577     <t>Restrictions on what are cacheable; these may only be imposed by
1578        the origin server.</t>
1579
1580     <t>Restrictions on what may be stored by a cache; these may be
1581        imposed by either the origin server or the user agent.</t>
1582
1583     <t>Modifications of the basic expiration mechanism; these may be
1584        imposed by either the origin server or the user agent.</t>
1585
1586     <t>Controls over cache revalidation and reload; these may only be
1587        imposed by a user agent.</t>
1588
1589     <t>Control over transformation of entities.</t>
1590
1591     <t>Extensions to the caching system.</t>
1592  </list>
1593</t>
1594
1595<section title="What is Cacheable" anchor="what.is.cacheable">
1596<t>
1597   By default, a response is cacheable if the requirements of the
1598   request method, request header fields, and the response status
1599   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1600   for cacheability. The following Cache-Control response directives
1601   allow an origin server to override the default cacheability of a
1602   response:
1603</t>
1604<t>
1605  <iref item="Cache Directives" subitem="public" primary="true"/>
1606  <iref item="public" subitem="Cache Directive" primary="true"/>
1607   public
1608  <list><t>
1609      Indicates that the response MAY be cached by any cache, even if it
1610      would normally be non-cacheable or cacheable only within a non-shared
1611      cache. (See also Authorization, Section 4.1 of <xref target="Part7"/>, for
1612      additional details.)
1613  </t></list>
1614</t>
1615<t>
1616  <iref item="Cache Directives" subitem="private" primary="true"/>
1617  <iref item="private" subitem="Cache Directive" primary="true"/>
1618   private
1619  <list><t>
1620      Indicates that all or part of the response message is intended for
1621      a single user and MUST NOT be cached by a shared cache. This
1622      allows an origin server to state that the specified parts of the
1623      response are intended for only one user and are not a valid
1624      response for requests by other users. A private (non-shared) cache
1625      MAY cache the response.
1626    </t><t>
1627       Note: This usage of the word private only controls where the
1628       response may be cached, and cannot ensure the privacy of the
1629       message content.
1630  </t></list>
1631</t>
1632<t>
1633  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1634  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1635   no-cache
1636  <list><t>
1637       If the no-cache directive does not specify a field-name, then a
1638      cache MUST NOT use the response to satisfy a subsequent request
1639      without successful revalidation with the origin server. This
1640      allows an origin server to prevent caching even by caches that
1641      have been configured to return stale responses to client requests.
1642    </t><t>
1643      If the no-cache directive does specify one or more field-names,
1644      then a cache MAY use the response to satisfy a subsequent request,
1645      subject to any other restrictions on caching. However, the
1646      specified field-name(s) MUST NOT be sent in the response to a
1647      subsequent request without successful revalidation with the origin
1648      server. This allows an origin server to prevent the re-use of
1649      certain header fields in a response, while still allowing caching
1650      of the rest of the response.
1651    <list><t>
1652       Note: Most HTTP/1.0 caches will not recognize or obey this
1653       directive.
1654    </t></list>
1655  </t></list>
1656</t>
1657</section>
1658
1659<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1660<t>
1661  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1662  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1663   no-store
1664  <list><t>   
1665      The purpose of the no-store directive is to prevent the
1666      inadvertent release or retention of sensitive information (for
1667      example, on backup tapes). The no-store directive applies to the
1668      entire message, and MAY be sent either in a response or in a
1669      request. If sent in a request, a cache MUST NOT store any part of
1670      either this request or any response to it. If sent in a response,
1671      a cache MUST NOT store any part of either this response or the
1672      request that elicited it. This directive applies to both non-shared
1673      and shared caches. "MUST NOT store" in this context means
1674      that the cache MUST NOT intentionally store the information in
1675      non-volatile storage, and MUST make a best-effort attempt to
1676      remove the information from volatile storage as promptly as
1677      possible after forwarding it.
1678  </t><t>
1679      Even when this directive is associated with a response, users
1680      might explicitly store such a response outside of the caching
1681      system (e.g., with a "Save As" dialog). History buffers MAY store
1682      such responses as part of their normal operation.
1683  </t><t>
1684      The purpose of this directive is to meet the stated requirements
1685      of certain users and service authors who are concerned about
1686      accidental releases of information via unanticipated accesses to
1687      cache data structures. While the use of this directive might
1688      improve privacy in some cases, we caution that it is NOT in any
1689      way a reliable or sufficient mechanism for ensuring privacy. In
1690      particular, malicious or compromised caches might not recognize or
1691      obey this directive, and communications networks might be
1692      vulnerable to eavesdropping.
1693  </t></list>
1694</t>
1695</section>
1696
1697<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1698<t>
1699   The expiration time of an entity MAY be specified by the origin
1700   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1701   it MAY be specified using the max-age directive in a response. When
1702   the max-age cache-control directive is present in a cached response,
1703   the response is stale if its current age is greater than the age
1704   value given (in seconds) at the time of a new request for that
1705   resource. The max-age directive on a response implies that the
1706   response is cacheable (i.e., "public") unless some other, more
1707   restrictive cache directive is also present.
1708</t>
1709<t>
1710   If a response includes both an Expires header and a max-age
1711   directive, the max-age directive overrides the Expires header, even
1712   if the Expires header is more restrictive. This rule allows an origin
1713   server to provide, for a given response, a longer expiration time to
1714   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1715   useful if certain HTTP/1.0 caches improperly calculate ages or
1716   expiration times, perhaps due to desynchronized clocks.
1717</t>
1718<t>
1719   Many HTTP/1.0 cache implementations will treat an Expires value that
1720   is less than or equal to the response Date value as being equivalent
1721   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1722   cache receives such a response, and the response does not include a
1723   Cache-Control header field, it SHOULD consider the response to be
1724   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1725  <list><t>
1726       Note: An origin server might wish to use a relatively new HTTP
1727       cache control feature, such as the "private" directive, on a
1728       network including older caches that do not understand that
1729       feature. The origin server will need to combine the new feature
1730       with an Expires field whose value is less than or equal to the
1731       Date value. This will prevent older caches from improperly
1732       caching the response.
1733  </t></list>
1734</t>
1735<t>
1736  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1737  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1738   s-maxage
1739  <list><t>
1740       If a response includes an s-maxage directive, then for a shared
1741       cache (but not for a private cache), the maximum age specified by
1742       this directive overrides the maximum age specified by either the
1743       max-age directive or the Expires header. The s-maxage directive
1744       also implies the semantics of the proxy-revalidate directive (see
1745       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1746       entry after it becomes stale to respond to a subsequent request
1747       without first revalidating it with the origin server. The s-maxage
1748       directive is always ignored by a private cache.
1749  </t></list>
1750</t>
1751<t>
1752   Note that most older caches, not compliant with this specification,
1753   do not implement any cache-control directives. An origin server
1754   wishing to use a cache-control directive that restricts, but does not
1755   prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
1756   requirement that the max-age directive overrides the Expires header,
1757   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1758   max-age directive.
1759</t>
1760<t>
1761   Other directives allow a user agent to modify the basic expiration
1762   mechanism. These directives MAY be specified on a request:
1763</t>
1764<t>
1765  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1766  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1767   max-age
1768  <list><t>
1769      Indicates that the client is willing to accept a response whose
1770      age is no greater than the specified time in seconds. Unless max-stale
1771      directive is also included, the client is not willing to
1772      accept a stale response.
1773  </t></list>
1774</t>
1775<t>
1776  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1777  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1778   min-fresh
1779  <list><t>
1780      Indicates that the client is willing to accept a response whose
1781      freshness lifetime is no less than its current age plus the
1782      specified time in seconds. That is, the client wants a response
1783      that will still be fresh for at least the specified number of
1784      seconds.
1785  </t></list>
1786</t>
1787<t>
1788  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1789  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1790   max-stale
1791  <list><t>
1792      Indicates that the client is willing to accept a response that has
1793      exceeded its expiration time. If max-stale is assigned a value,
1794      then the client is willing to accept a response that has exceeded
1795      its expiration time by no more than the specified number of
1796      seconds. If no value is assigned to max-stale, then the client is
1797      willing to accept a stale response of any age.
1798  </t></list>
1799</t>
1800<t>
1801   If a cache returns a stale response, either because of a max-stale
1802   directive on a request, or because the cache is configured to
1803   override the expiration time of a response, the cache MUST attach a
1804   Warning header to the stale response, using Warning 110 (Response is
1805   stale).
1806</t>
1807<t>
1808   A cache MAY be configured to return stale responses without
1809   validation, but only if this does not conflict with any "MUST"-level
1810   requirements concerning cache validation (e.g., a "must-revalidate"
1811   cache-control directive).
1812</t>
1813<t>
1814   If both the new request and the cached entry include "max-age"
1815   directives, then the lesser of the two values is used for determining
1816   the freshness of the cached entry for that request.
1817</t>
1818</section>
1819
1820<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1821<t>
1822   Sometimes a user agent might want or need to insist that a cache
1823   revalidate its cache entry with the origin server (and not just with
1824   the next cache along the path to the origin server), or to reload its
1825   cache entry from the origin server. End-to-end revalidation might be
1826   necessary if either the cache or the origin server has overestimated
1827   the expiration time of the cached response. End-to-end reload may be
1828   necessary if the cache entry has become corrupted for some reason.
1829</t>
1830<t>
1831   End-to-end revalidation may be requested either when the client does
1832   not have its own local cached copy, in which case we call it
1833   "unspecified end-to-end revalidation", or when the client does have a
1834   local cached copy, in which case we call it "specific end-to-end
1835   revalidation."
1836</t>
1837<t>
1838   The client can specify these three kinds of action using Cache-Control
1839   request directives:
1840</t>
1841<t>
1842   End-to-end reload
1843  <list><t>
1844      The request includes a "no-cache" cache-control directive or, for
1845      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1846      names MUST NOT be included with the no-cache directive in a
1847      request. The server MUST NOT use a cached copy when responding to
1848      such a request.
1849  </t></list>
1850</t>
1851<t>
1852   Specific end-to-end revalidation
1853  <list><t>
1854      The request includes a "max-age=0" cache-control directive, which
1855      forces each cache along the path to the origin server to
1856      revalidate its own entry, if any, with the next cache or server.
1857      The initial request includes a cache-validating conditional with
1858      the client's current validator.
1859  </t></list>
1860</t>
1861<t>
1862   Unspecified end-to-end revalidation
1863  <list><t>
1864      The request includes "max-age=0" cache-control directive, which
1865      forces each cache along the path to the origin server to
1866      revalidate its own entry, if any, with the next cache or server.
1867      The initial request does not include a cache-validating
1868      conditional; the first cache along the path (if any) that holds a
1869      cache entry for this resource includes a cache-validating
1870      conditional with its current validator.
1871  </t></list>
1872</t>
1873<t>
1874  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1875  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1876   max-age
1877  <list><t>
1878      When an intermediate cache is forced, by means of a max-age=0
1879      directive, to revalidate its own cache entry, and the client has
1880      supplied its own validator in the request, the supplied validator
1881      might differ from the validator currently stored with the cache
1882      entry. In this case, the cache MAY use either validator in making
1883      its own request without affecting semantic transparency.
1884  </t><t>
1885      However, the choice of validator might affect performance. The
1886      best approach is for the intermediate cache to use its own
1887      validator when making its request. If the server replies with 304
1888      (Not Modified), then the cache can return its now validated copy
1889      to the client with a 200 (OK) response. If the server replies with
1890      a new entity and cache validator, however, the intermediate cache
1891      can compare the returned validator with the one provided in the
1892      client's request, using the strong comparison function. If the
1893      client's validator is equal to the origin server's, then the
1894      intermediate cache simply returns 304 (Not Modified). Otherwise,
1895      it returns the new entity with a 200 (OK) response.
1896  </t><t>
1897      If a request includes the no-cache directive, it SHOULD NOT
1898      include min-fresh, max-stale, or max-age.
1899  </t></list>
1900</t>
1901<t>
1902  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1903  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1904   only-if-cached
1905  <list><t>
1906      In some cases, such as times of extremely poor network
1907      connectivity, a client may want a cache to return only those
1908      responses that it currently has stored, and not to reload or
1909      revalidate with the origin server. To do this, the client may
1910      include the only-if-cached directive in a request. If it receives
1911      this directive, a cache SHOULD either respond using a cached entry
1912      that is consistent with the other constraints of the request, or
1913      respond with a 504 (Gateway Timeout) status. However, if a group
1914      of caches is being operated as a unified system with good internal
1915      connectivity, such a request MAY be forwarded within that group of
1916      caches.
1917  </t></list>
1918</t>
1919<t>
1920  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1921  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1922   must-revalidate
1923  <list><t>
1924      Because a cache MAY be configured to ignore a server's specified
1925      expiration time, and because a client request MAY include a max-stale
1926      directive (which has a similar effect), the protocol also
1927      includes a mechanism for the origin server to require revalidation
1928      of a cache entry on any subsequent use. When the must-revalidate
1929      directive is present in a response received by a cache, that cache
1930      MUST NOT use the entry after it becomes stale to respond to a
1931      subsequent request without first revalidating it with the origin
1932      server. (I.e., the cache MUST do an end-to-end revalidation every
1933      time, if, based solely on the origin server's Expires or max-age
1934      value, the cached response is stale.)
1935  </t><t>
1936      The must-revalidate directive is necessary to support reliable
1937      operation for certain protocol features. In all circumstances an
1938      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1939      particular, if the cache cannot reach the origin server for any
1940      reason, it MUST generate a 504 (Gateway Timeout) response.
1941  </t><t>
1942      Servers SHOULD send the must-revalidate directive if and only if
1943      failure to revalidate a request on the entity could result in
1944      incorrect operation, such as a silently unexecuted financial
1945      transaction. Recipients MUST NOT take any automated action that
1946      violates this directive, and MUST NOT automatically provide an
1947      unvalidated copy of the entity if revalidation fails.
1948  </t><t>
1949      Although this is not recommended, user agents operating under
1950      severe connectivity constraints MAY violate this directive but, if
1951      so, MUST explicitly warn the user that an unvalidated response has
1952      been provided. The warning MUST be provided on each unvalidated
1953      access, and SHOULD require explicit user confirmation.
1954  </t></list>
1955</t>
1956<t>
1957  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1958  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1959   proxy-revalidate
1960  <list><t>
1961      The proxy-revalidate directive has the same meaning as the must-revalidate
1962      directive, except that it does not apply to non-shared
1963      user agent caches. It can be used on a response to an
1964      authenticated request to permit the user's cache to store and
1965      later return the response without needing to revalidate it (since
1966      it has already been authenticated once by that user), while still
1967      requiring proxies that service many users to revalidate each time
1968      (in order to make sure that each user has been authenticated).
1969      Note that such authenticated responses also need the public cache
1970      control directive in order to allow them to be cached at all.
1971  </t></list>
1972</t>
1973</section>
1974
1975<section title="No-Transform Directive" anchor="no-transform.directive">
1976<t>
1977  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
1978  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
1979   no-transform
1980  <list><t>
1981      Implementors of intermediate caches (proxies) have found it useful
1982      to convert the media type of certain entity bodies. A non-transparent
1983      proxy might, for example, convert between image
1984      formats in order to save cache space or to reduce the amount of
1985      traffic on a slow link.
1986  </t><t>
1987      Serious operational problems occur, however, when these
1988      transformations are applied to entity bodies intended for certain
1989      kinds of applications. For example, applications for medical
1990      imaging, scientific data analysis and those using end-to-end
1991      authentication, all depend on receiving an entity body that is bit
1992      for bit identical to the original entity-body.
1993  </t><t>
1994      Therefore, if a message includes the no-transform directive, an
1995      intermediate cache or proxy MUST NOT change those headers that are
1996      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
1997      directive. This implies that the cache or proxy MUST NOT change
1998      any aspect of the entity-body that is specified by these headers,
1999      including the value of the entity-body itself.
2000  </t></list>
2001</t>
2002</section>
2003
2004<section title="Cache Control Extensions" anchor="cache.control.extensions">
2005<t>
2006   The Cache-Control header field can be extended through the use of one
2007   or more cache-extension tokens, each with an optional assigned value.
2008   Informational extensions (those which do not require a change in
2009   cache behavior) MAY be added without changing the semantics of other
2010   directives. Behavioral extensions are designed to work by acting as
2011   modifiers to the existing base of cache directives. Both the new
2012   directive and the standard directive are supplied, such that
2013   applications which do not understand the new directive will default
2014   to the behavior specified by the standard directive, and those that
2015   understand the new directive will recognize it as modifying the
2016   requirements associated with the standard directive. In this way,
2017   extensions to the cache-control directives can be made without
2018   requiring changes to the base protocol.
2019</t>
2020<t>
2021   This extension mechanism depends on an HTTP cache obeying all of the
2022   cache-control directives defined for its native HTTP-version, obeying
2023   certain extensions, and ignoring all directives that it does not
2024   understand.
2025</t>
2026<t>
2027   For example, consider a hypothetical new response directive called
2028   community which acts as a modifier to the private directive. We
2029   define this new directive to mean that, in addition to any non-shared
2030   cache, any cache which is shared only by members of the community
2031   named within its value may cache the response. An origin server
2032   wishing to allow the UCI community to use an otherwise private
2033   response in their shared cache(s) could do so by including
2034</t>
2035<figure><artwork type="example"><![CDATA[
2036    Cache-Control: private, community="UCI"
2037]]></artwork></figure>
2038<t>
2039   A cache seeing this header field will act correctly even if the cache
2040   does not understand the community cache-extension, since it will also
2041   see and understand the private directive and thus default to the safe
2042   behavior.
2043</t>
2044<t>
2045   Unrecognized cache-directives MUST be ignored; it is assumed that any
2046   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2047   be combined with standard directives (or the response's default
2048   cacheability) such that the cache behavior will remain minimally
2049   correct even if the cache does not understand the extension(s).
2050</t>
2051</section>
2052</section>
2053
2054<section title="Expires" anchor="header.expires">
2055  <iref primary="true" item="Expires header"/>
2056  <iref primary="true" item="Headers" subitem="Expires"/>
2057 
2058<t>
2059   The Expires entity-header field gives the date/time after which the
2060   response is considered stale. A stale cache entry may not normally be
2061   returned by a cache (either a proxy cache or a user agent cache)
2062   unless it is first validated with the origin server (or with an
2063   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2064   for further discussion of the expiration model.
2065</t>
2066<t>
2067   The presence of an Expires field does not imply that the original
2068   resource will change or cease to exist at, before, or after that
2069   time.
2070</t>
2071<t>
2072   The format is an absolute date and time as defined by HTTP-date in
2073   Section 3.3.1 of <xref target="Part1"/>; it MUST be sent in rfc1123-date format.
2074</t>
2075<figure><iref primary="true" item="Grammar" subitem="Expires"/><artwork type="abnf2616"><![CDATA[
2076  Expires = "Expires" ":" HTTP-date
2077]]></artwork></figure>
2078<t>
2079   An example of its use is
2080</t>
2081<figure><artwork type="example"><![CDATA[
2082   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2083]]></artwork></figure>
2084<t>
2085  <list><t>
2086      Note: if a response includes a Cache-Control field with the max-age
2087      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2088      Expires field.
2089  </t></list>
2090</t>
2091<t>
2092   HTTP/1.1 clients and caches MUST treat other invalid date formats,
2093   especially including the value "0", as in the past (i.e., "already
2094   expired").
2095</t>
2096<t>
2097   To mark a response as "already expired," an origin server sends an
2098   Expires date that is equal to the Date header value. (See the rules
2099   for expiration calculations in <xref target="expiration.calculations"/>.)
2100</t>
2101<t>
2102   To mark a response as "never expires," an origin server sends an
2103   Expires date approximately one year from the time the response is
2104   sent. HTTP/1.1 servers SHOULD NOT  send Expires dates more than one
2105   year in the future.
2106</t>
2107<t>
2108   The presence of an Expires header field with a date value of some
2109   time in the future on a response that otherwise would by default be
2110   non-cacheable indicates that the response is cacheable, unless
2111   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2112</t>
2113</section>
2114
2115<section title="Pragma" anchor="header.pragma">
2116  <iref primary="true" item="Pragma header"/>
2117  <iref primary="true" item="Headers" subitem="Pragma"/>
2118 
2119 
2120 
2121<t>
2122   The Pragma general-header field is used to include implementation-specific
2123   directives that might apply to any recipient along the
2124   request/response chain. All pragma directives specify optional
2125   behavior from the viewpoint of the protocol; however, some systems
2126   MAY require that behavior be consistent with the directives.
2127</t>
2128<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[
2129  Pragma            = "Pragma" ":" 1#pragma-directive
2130  pragma-directive  = "no-cache" | extension-pragma
2131  extension-pragma  = token [ "=" ( token | quoted-string ) ]
2132]]></artwork></figure>
2133<t>
2134   When the no-cache directive is present in a request message, an
2135   application SHOULD forward the request toward the origin server even
2136   if it has a cached copy of what is being requested. This pragma
2137   directive has the same semantics as the no-cache cache-directive (see
2138   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2139   HTTP/1.0. Clients SHOULD include both header fields when a no-cache
2140   request is sent to a server not known to be HTTP/1.1 compliant.
2141</t>
2142<t>
2143   Pragma directives MUST be passed through by a proxy or gateway
2144   application, regardless of their significance to that application,
2145   since the directives might be applicable to all recipients along the
2146   request/response chain. It is not possible to specify a pragma for a
2147   specific recipient; however, any pragma directive not relevant to a
2148   recipient SHOULD be ignored by that recipient.
2149</t>
2150<t>
2151   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
2152   sent "Cache-Control: no-cache". No new Pragma directives will be
2153   defined in HTTP.
2154  <list><t>
2155      Note: because the meaning of "Pragma: no-cache" as a
2156      response-header field is not actually specified, it does not provide a
2157      reliable replacement for "Cache-Control: no-cache" in a response.
2158  </t></list>
2159</t>
2160</section>
2161
2162<section title="Vary" anchor="header.vary">
2163  <iref primary="true" item="Vary header"/>
2164  <iref primary="true" item="Headers" subitem="Vary"/>
2165 
2166<t>
2167   The Vary field value indicates the set of request-header fields that
2168   fully determines, while the response is fresh, whether a cache is
2169   permitted to use the response to reply to a subsequent request
2170   without revalidation. For uncacheable or stale responses, the Vary
2171   field value advises the user agent about the criteria that were used
2172   to select the representation. A Vary field value of "*" implies that
2173   a cache cannot determine from the request headers of a subsequent
2174   request whether this response is the appropriate representation. See
2175   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2176</t>
2177<figure><iref primary="true" item="Grammar" subitem="Vary"/><artwork type="abnf2616"><![CDATA[
2178  Vary  = "Vary" ":" ( "*" | 1#field-name )
2179]]></artwork></figure>
2180<t>
2181   An HTTP/1.1 server SHOULD include a Vary header field with any
2182   cacheable response that is subject to server-driven negotiation.
2183   Doing so allows a cache to properly interpret future requests on that
2184   resource and informs the user agent about the presence of negotiation
2185   on that resource. A server MAY include a Vary header field with a
2186   non-cacheable response that is subject to server-driven negotiation,
2187   since this might provide the user agent with useful information about
2188   the dimensions over which the response varies at the time of the
2189   response.
2190</t>
2191<t>
2192   A Vary field value consisting of a list of field-names signals that
2193   the representation selected for the response is based on a selection
2194   algorithm which considers ONLY the listed request-header field values
2195   in selecting the most appropriate representation. A cache MAY assume
2196   that the same selection will be made for future requests with the
2197   same values for the listed field names, for the duration of time for
2198   which the response is fresh.
2199</t>
2200<t>
2201   The field-names given are not limited to the set of standard
2202   request-header fields defined by this specification. Field names are
2203   case-insensitive.
2204</t>
2205<t>
2206   A Vary field value of "*" signals that unspecified parameters not
2207   limited to the request-headers (e.g., the network address of the
2208   client), play a role in the selection of the response representation.
2209   The "*" value MUST NOT be generated by a proxy server; it may only be
2210   generated by an origin server.
2211</t>
2212</section>
2213
2214<section title="Warning" anchor="header.warning">
2215  <iref primary="true" item="Warning header"/>
2216  <iref primary="true" item="Headers" subitem="Warning"/>
2217 
2218 
2219 
2220 
2221 
2222 
2223<t>
2224   The Warning general-header field is used to carry additional
2225   information about the status or transformation of a message which
2226   might not be reflected in the message. This information is typically
2227   used to warn about a possible lack of semantic transparency from
2228   caching operations or transformations applied to the entity body of
2229   the message.
2230</t>
2231<t>
2232   Warning headers are sent with responses using:
2233</t>
2234<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[
2235  Warning    = "Warning" ":" 1#warning-value
2236 
2237  warning-value = warn-code SP warn-agent SP warn-text
2238                                        [SP warn-date]
2239 
2240  warn-code  = 3DIGIT
2241  warn-agent = ( uri-host [ ":" port ] ) | pseudonym
2242                  ; the name or pseudonym of the server adding
2243                  ; the Warning header, for use in debugging
2244  warn-text  = quoted-string
2245  warn-date  = DQUOTE HTTP-date DQUOTE
2246]]></artwork></figure>
2247<t>
2248   A response MAY carry more than one Warning header.
2249</t>
2250<t>
2251   The warn-text SHOULD be in a natural language and character set that
2252   is most likely to be intelligible to the human user receiving the
2253   response. This decision MAY be based on any available knowledge, such
2254   as the location of the cache or user, the Accept-Language field in a
2255   request, the Content-Language field in a response, etc. The default
2256   language is English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1"/>).
2257</t>
2258<t>
2259   If a character set other than ISO-8859-1 is used, it MUST be encoded
2260   in the warn-text using the method described in <xref target="RFC2047"/>.
2261</t>
2262<t>
2263   Warning headers can in general be applied to any message, however
2264   some specific warn-codes are specific to caches and can only be
2265   applied to response messages. New Warning headers SHOULD be added
2266   after any existing Warning headers. A cache MUST NOT delete any
2267   Warning header that it received with a message. However, if a cache
2268   successfully validates a cache entry, it SHOULD remove any Warning
2269   headers previously attached to that entry except as specified for
2270   specific Warning codes. It MUST then add any Warning headers received
2271   in the validating response. In other words, Warning headers are those
2272   that would be attached to the most recent relevant response.
2273</t>
2274<t>
2275   When multiple Warning headers are attached to a response, the user
2276   agent ought to inform the user of as many of them as possible, in the
2277   order that they appear in the response. If it is not possible to
2278   inform the user of all of the warnings, the user agent SHOULD follow
2279   these heuristics:
2280  <list style="symbols">
2281    <t>Warnings that appear early in the response take priority over
2282        those appearing later in the response.</t>
2283
2284    <t>Warnings in the user's preferred character set take priority
2285        over warnings in other character sets but with identical warn-codes
2286        and warn-agents.</t>
2287  </list>
2288</t>
2289<t>
2290   Systems that generate multiple Warning headers SHOULD order them with
2291   this user agent behavior in mind.
2292</t>
2293<t>
2294   Requirements for the behavior of caches with respect to Warnings are
2295   stated in <xref target="warnings"/>.
2296</t>
2297<t>
2298   This is a list of the currently-defined warn-codes, each with a
2299   recommended warn-text in English, and a description of its meaning.
2300</t>
2301<t>
2302   110 Response is stale
2303  <list><t>
2304     MUST be included whenever the returned response is stale.
2305  </t></list>
2306</t>
2307<t>
2308   111 Revalidation failed
2309  <list><t>
2310     MUST be included if a cache returns a stale response because an
2311     attempt to revalidate the response failed, due to an inability to
2312     reach the server.
2313  </t></list>
2314</t>
2315<t>
2316   112 Disconnected operation
2317  <list><t>
2318     SHOULD be included if the cache is intentionally disconnected from
2319     the rest of the network for a period of time.
2320  </t></list>
2321</t>
2322<t>
2323   113 Heuristic expiration
2324  <list><t>
2325     MUST be included if the cache heuristically chose a freshness
2326     lifetime greater than 24 hours and the response's age is greater
2327     than 24 hours.
2328  </t></list>
2329</t>
2330<t>
2331   199 Miscellaneous warning
2332  <list><t>
2333     The warning text MAY include arbitrary information to be presented
2334     to a human user, or logged. A system receiving this warning MUST NOT
2335     take any automated action, besides presenting the warning to
2336     the user.
2337  </t></list>
2338</t>
2339<t>
2340   214 Transformation applied
2341  <list><t>
2342     MUST be added by an intermediate cache or proxy if it applies any
2343     transformation changing the content-coding (as specified in the
2344     Content-Encoding header) or media-type (as specified in the
2345     Content-Type header) of the response, or the entity-body of the
2346     response, unless this Warning code already appears in the response.
2347  </t></list>
2348</t>
2349<t>
2350   299 Miscellaneous persistent warning
2351  <list><t>
2352     The warning text MAY include arbitrary information to be presented
2353     to a human user, or logged. A system receiving this warning MUST NOT
2354     take any automated action.
2355  </t></list>
2356</t>
2357<t>
2358   If an implementation sends a message with one or more Warning headers
2359   whose version is HTTP/1.0 or lower, then the sender MUST include in
2360   each warning-value a warn-date that matches the date in the response.
2361</t>
2362<t>
2363   If an implementation receives a message with a warning-value that
2364   includes a warn-date, and that warn-date is different from the Date
2365   value in the response, then that warning-value MUST be deleted from
2366   the message before storing, forwarding, or using it. (This prevents
2367   bad consequences of naive caching of Warning header fields.) If all
2368   of the warning-values are deleted for this reason, the Warning header
2369   MUST be deleted as well.
2370</t>
2371</section>
2372
2373</section>
2374
2375<section title="IANA Considerations" anchor="IANA.considerations">
2376<section title="Message Header Registration" anchor="message.header.registration">
2377<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
2378<!--(START)-->
2379<t>
2380    The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
2381    with the permanent registrations below (see <xref target="RFC3864"/>):
2382  </t>
2383<texttable>
2384   <ttcol>Header Field Name</ttcol>
2385   <ttcol>Protocol</ttcol>
2386   <ttcol>Status</ttcol>
2387   <ttcol>Reference</ttcol>
2388
2389   <c>Age</c>
2390   <c>http</c>
2391   <c>standard</c>
2392   <c>
2393      <xref target="header.age"/>
2394   </c>
2395
2396   <c>Cache-Control</c>
2397   <c>http</c>
2398   <c>standard</c>
2399   <c>
2400      <xref target="header.cache-control"/>
2401   </c>
2402
2403   <c>Expires</c>
2404   <c>http</c>
2405   <c>standard</c>
2406   <c>
2407      <xref target="header.expires"/>
2408   </c>
2409
2410   <c>Pragma</c>
2411   <c>http</c>
2412   <c>standard</c>
2413   <c>
2414      <xref target="header.pragma"/>
2415   </c>
2416
2417   <c>Vary</c>
2418   <c>http</c>
2419   <c>standard</c>
2420   <c>
2421      <xref target="header.vary"/>
2422   </c>
2423
2424   <c>Warning</c>
2425   <c>http</c>
2426   <c>standard</c>
2427   <c>
2428      <xref target="header.warning"/>
2429   </c>
2430</texttable>
2431<t>
2432    The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
2433  </t>
2434<!--(END)-->
2435</section>
2436</section>
2437
2438<section title="Security Considerations" anchor="security.considerations">
2439<t>
2440   Caching proxies provide additional potential vulnerabilities, since
2441   the contents of the cache represent an attractive target for
2442   malicious exploitation. Because cache contents persist after an HTTP
2443   request is complete, an attack on the cache can reveal information
2444   long after a user believes that the information has been removed from
2445   the network. Therefore, cache contents should be protected as
2446   sensitive information.
2447</t>
2448</section>
2449
2450<section title="Acknowledgments" anchor="ack">
2451<t>
2452   Much of the content and presentation of the caching design is due to
2453   suggestions and comments from individuals including: Shel Kaphan,
2454   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2455</t>
2456</section>
2457</middle>
2458<back>
2459
2460<references title="Normative References">
2461
2462<reference anchor="ISO-8859-1">
2463  <front>
2464    <title>
2465     Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
2466    </title>
2467    <author>
2468      <organization>International Organization for Standardization</organization>
2469    </author>
2470    <date year="1998"/>
2471  </front>
2472  <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
2473</reference>
2474
2475<reference anchor="Part1">
2476  <front>
2477    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2478    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2479      <organization abbrev="Day Software">Day Software</organization>
2480      <address><email>fielding@gbiv.com</email></address>
2481    </author>
2482    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2483      <organization>One Laptop per Child</organization>
2484      <address><email>jg@laptop.org</email></address>
2485    </author>
2486    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2487      <organization abbrev="HP">Hewlett-Packard Company</organization>
2488      <address><email>JeffMogul@acm.org</email></address>
2489    </author>
2490    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2491      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2492      <address><email>henrikn@microsoft.com</email></address>
2493    </author>
2494    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2495      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2496      <address><email>LMM@acm.org</email></address>
2497    </author>
2498    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2499      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2500      <address><email>paulle@microsoft.com</email></address>
2501    </author>
2502    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2503      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2504      <address><email>timbl@w3.org</email></address>
2505    </author>
2506    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2507      <organization abbrev="W3C">World Wide Web Consortium</organization>
2508      <address><email>ylafon@w3.org</email></address>
2509    </author>
2510    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2511      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2512      <address><email>julian.reschke@greenbytes.de</email></address>
2513    </author>
2514    <date month="June" year="2008"/>
2515  </front>
2516  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-03"/>
2517 
2518</reference>
2519
2520<reference anchor="Part2">
2521  <front>
2522    <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2523    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2524      <organization abbrev="Day Software">Day Software</organization>
2525      <address><email>fielding@gbiv.com</email></address>
2526    </author>
2527    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2528      <organization>One Laptop per Child</organization>
2529      <address><email>jg@laptop.org</email></address>
2530    </author>
2531    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2532      <organization abbrev="HP">Hewlett-Packard Company</organization>
2533      <address><email>JeffMogul@acm.org</email></address>
2534    </author>
2535    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2536      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2537      <address><email>henrikn@microsoft.com</email></address>
2538    </author>
2539    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2540      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2541      <address><email>LMM@acm.org</email></address>
2542    </author>
2543    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2544      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2545      <address><email>paulle@microsoft.com</email></address>
2546    </author>
2547    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2548      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2549      <address><email>timbl@w3.org</email></address>
2550    </author>
2551    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2552      <organization abbrev="W3C">World Wide Web Consortium</organization>
2553      <address><email>ylafon@w3.org</email></address>
2554    </author>
2555    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2556      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2557      <address><email>julian.reschke@greenbytes.de</email></address>
2558    </author>
2559    <date month="June" year="2008"/>
2560  </front>
2561  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-03"/>
2562 
2563</reference>
2564
2565<reference anchor="Part3">
2566  <front>
2567    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2568    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2569      <organization abbrev="Day Software">Day Software</organization>
2570      <address><email>fielding@gbiv.com</email></address>
2571    </author>
2572    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2573      <organization>One Laptop per Child</organization>
2574      <address><email>jg@laptop.org</email></address>
2575    </author>
2576    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2577      <organization abbrev="HP">Hewlett-Packard Company</organization>
2578      <address><email>JeffMogul@acm.org</email></address>
2579    </author>
2580    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2581      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2582      <address><email>henrikn@microsoft.com</email></address>
2583    </author>
2584    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2585      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2586      <address><email>LMM@acm.org</email></address>
2587    </author>
2588    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2589      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2590      <address><email>paulle@microsoft.com</email></address>
2591    </author>
2592    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2593      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2594      <address><email>timbl@w3.org</email></address>
2595    </author>
2596    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2597      <organization abbrev="W3C">World Wide Web Consortium</organization>
2598      <address><email>ylafon@w3.org</email></address>
2599    </author>
2600    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2601      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2602      <address><email>julian.reschke@greenbytes.de</email></address>
2603    </author>
2604    <date month="June" year="2008"/>
2605  </front>
2606  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-03"/>
2607 
2608</reference>
2609
2610<reference anchor="Part4">
2611  <front>
2612    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2613    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2614      <organization abbrev="Day Software">Day Software</organization>
2615      <address><email>fielding@gbiv.com</email></address>
2616    </author>
2617    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2618      <organization>One Laptop per Child</organization>
2619      <address><email>jg@laptop.org</email></address>
2620    </author>
2621    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2622      <organization abbrev="HP">Hewlett-Packard Company</organization>
2623      <address><email>JeffMogul@acm.org</email></address>
2624    </author>
2625    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2626      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2627      <address><email>henrikn@microsoft.com</email></address>
2628    </author>
2629    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2630      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2631      <address><email>LMM@acm.org</email></address>
2632    </author>
2633    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2634      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2635      <address><email>paulle@microsoft.com</email></address>
2636    </author>
2637    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2638      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2639      <address><email>timbl@w3.org</email></address>
2640    </author>
2641    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2642      <organization abbrev="W3C">World Wide Web Consortium</organization>
2643      <address><email>ylafon@w3.org</email></address>
2644    </author>
2645    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2646      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2647      <address><email>julian.reschke@greenbytes.de</email></address>
2648    </author>
2649    <date month="June" year="2008"/>
2650  </front>
2651  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-03"/>
2652 
2653</reference>
2654
2655<reference anchor="Part5">
2656  <front>
2657    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2658    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2659      <organization abbrev="Day Software">Day Software</organization>
2660      <address><email>fielding@gbiv.com</email></address>
2661    </author>
2662    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2663      <organization>One Laptop per Child</organization>
2664      <address><email>jg@laptop.org</email></address>
2665    </author>
2666    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2667      <organization abbrev="HP">Hewlett-Packard Company</organization>
2668      <address><email>JeffMogul@acm.org</email></address>
2669    </author>
2670    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2671      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2672      <address><email>henrikn@microsoft.com</email></address>
2673    </author>
2674    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2675      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2676      <address><email>LMM@acm.org</email></address>
2677    </author>
2678    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2679      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2680      <address><email>paulle@microsoft.com</email></address>
2681    </author>
2682    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2683      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2684      <address><email>timbl@w3.org</email></address>
2685    </author>
2686    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2687      <organization abbrev="W3C">World Wide Web Consortium</organization>
2688      <address><email>ylafon@w3.org</email></address>
2689    </author>
2690    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2691      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2692      <address><email>julian.reschke@greenbytes.de</email></address>
2693    </author>
2694    <date month="June" year="2008"/>
2695  </front>
2696  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-03"/>
2697 
2698</reference>
2699
2700<reference anchor="Part7">
2701  <front>
2702    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2703    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2704      <organization abbrev="Day Software">Day Software</organization>
2705      <address><email>fielding@gbiv.com</email></address>
2706    </author>
2707    <author initials="J." surname="Gettys" fullname="Jim Gettys">
2708      <organization>One Laptop per Child</organization>
2709      <address><email>jg@laptop.org</email></address>
2710    </author>
2711    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2712      <organization abbrev="HP">Hewlett-Packard Company</organization>
2713      <address><email>JeffMogul@acm.org</email></address>
2714    </author>
2715    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2716      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2717      <address><email>henrikn@microsoft.com</email></address>
2718    </author>
2719    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2720      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2721      <address><email>LMM@acm.org</email></address>
2722    </author>
2723    <author initials="P." surname="Leach" fullname="Paul J. Leach">
2724      <organization abbrev="Microsoft">Microsoft Corporation</organization>
2725      <address><email>paulle@microsoft.com</email></address>
2726    </author>
2727    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2728      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2729      <address><email>timbl@w3.org</email></address>
2730    </author>
2731    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
2732      <organization abbrev="W3C">World Wide Web Consortium</organization>
2733      <address><email>ylafon@w3.org</email></address>
2734    </author>
2735    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
2736      <organization abbrev="greenbytes">greenbytes GmbH</organization>
2737      <address><email>julian.reschke@greenbytes.de</email></address>
2738    </author>
2739    <date month="June" year="2008"/>
2740  </front>
2741  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-03"/>
2742 
2743</reference>
2744
2745<reference anchor="RFC2047">
2746  <front>
2747    <title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2748    <author initials="K." surname="Moore" fullname="Keith Moore">
2749      <organization>University of Tennessee</organization>
2750      <address><email>moore@cs.utk.edu</email></address>
2751    </author>
2752    <date month="November" year="1996"/>
2753  </front>
2754  <seriesInfo name="RFC" value="2047"/>
2755</reference>
2756
2757<reference anchor="RFC2119">
2758  <front>
2759    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
2760    <author initials="S." surname="Bradner" fullname="Scott Bradner">
2761      <organization>Harvard University</organization>
2762      <address><email>sob@harvard.edu</email></address>
2763    </author>
2764    <date month="March" year="1997"/>
2765  </front>
2766  <seriesInfo name="BCP" value="14"/>
2767  <seriesInfo name="RFC" value="2119"/>
2768</reference>
2769
2770</references>
2771
2772<references title="Informative References">
2773
2774<reference anchor="RFC1305">
2775  <front>
2776    <title>Network Time Protocol (Version 3) Specification, Implementation</title>
2777    <author initials="D." surname="Mills" fullname="David L. Mills">
2778      <organization>University of Delaware, Electrical Engineering Department</organization>
2779      <address><email>mills@udel.edu</email></address>
2780    </author>
2781    <date month="March" year="1992"/>
2782  </front>
2783  <seriesInfo name="RFC" value="1305"/>
2784</reference>
2785
2786<reference anchor="RFC2616">
2787  <front>
2788    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2789    <author initials="R." surname="Fielding" fullname="R. Fielding">
2790      <organization>University of California, Irvine</organization>
2791      <address><email>fielding@ics.uci.edu</email></address>
2792    </author>
2793    <author initials="J." surname="Gettys" fullname="J. Gettys">
2794      <organization>W3C</organization>
2795      <address><email>jg@w3.org</email></address>
2796    </author>
2797    <author initials="J." surname="Mogul" fullname="J. Mogul">
2798      <organization>Compaq Computer Corporation</organization>
2799      <address><email>mogul@wrl.dec.com</email></address>
2800    </author>
2801    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2802      <organization>MIT Laboratory for Computer Science</organization>
2803      <address><email>frystyk@w3.org</email></address>
2804    </author>
2805    <author initials="L." surname="Masinter" fullname="L. Masinter">
2806      <organization>Xerox Corporation</organization>
2807      <address><email>masinter@parc.xerox.com</email></address>
2808    </author>
2809    <author initials="P." surname="Leach" fullname="P. Leach">
2810      <organization>Microsoft Corporation</organization>
2811      <address><email>paulle@microsoft.com</email></address>
2812    </author>
2813    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2814      <organization>W3C</organization>
2815      <address><email>timbl@w3.org</email></address>
2816    </author>
2817    <date month="June" year="1999"/>
2818  </front>
2819  <seriesInfo name="RFC" value="2616"/>
2820</reference>
2821
2822<reference anchor="RFC3864">
2823  <front>
2824    <title>Registration Procedures for Message Header Fields</title>
2825    <author initials="G." surname="Klyne" fullname="G. Klyne">
2826      <organization>Nine by Nine</organization>
2827      <address><email>GK-IETF@ninebynine.org</email></address>
2828    </author>
2829    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
2830      <organization>BEA Systems</organization>
2831      <address><email>mnot@pobox.com</email></address>
2832    </author>
2833    <author initials="J." surname="Mogul" fullname="J. Mogul">
2834      <organization>HP Labs</organization>
2835      <address><email>JeffMogul@acm.org</email></address>
2836    </author>
2837    <date year="2004" month="September"/>
2838  </front>
2839  <seriesInfo name="BCP" value="90"/>
2840  <seriesInfo name="RFC" value="3864"/>
2841</reference>
2842
2843</references>
2844
2845<section title="Compatibility with Previous Versions" anchor="compatibility">
2846
2847<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2848<t>
2849   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2850   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2851   <xref target="header.cache-control" format="counter"/>,
2852   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2853</t>
2854<t>
2855   Transfer-coding and message lengths all interact in ways that
2856   required fixing exactly when chunked encoding is used (to allow for
2857   transfer encoding that may not be self delimiting); it was important
2858   to straighten out exactly how message lengths are computed.
2859   (<xref target="non-modifiable.headers"/>,
2860   see also <xref target="Part1"/>, <xref target="Part3"/> and <xref target="Part5"/>)
2861</t>
2862<t>
2863   Proxies should be able to add Content-Length when appropriate.
2864   (<xref target="non-modifiable.headers"/>)
2865</t>
2866<t>
2867   Range request responses would become very verbose if all meta-data
2868   were always returned; by allowing the server to only send needed
2869   headers in a 206 response, this problem can be avoided.
2870   (<xref target="combining.headers"/>)
2871</t>
2872<t>
2873   The Cache-Control: max-age directive was not properly defined for
2874   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2875</t>
2876<t>
2877   Warnings could be cached incorrectly, or not updated appropriately.
2878   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2879   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2880   and <xref target="header.warning" format="counter"/>) Warning
2881   also needed to be a general header, as PUT or other methods may have
2882   need for it in requests.
2883</t>
2884</section>
2885
2886<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
2887<t>
2888  Clarify denial of service attack avoidance requirement.
2889  (<xref target="invalidation.after.updates.or.deletions"/>)
2890</t>
2891</section>
2892
2893</section>
2894
2895<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
2896
2897<section title="Since RFC2616">
2898<t>
2899  Extracted relevant partitions from <xref target="RFC2616"/>.
2900</t>
2901</section>
2902
2903<section title="Since draft-ietf-httpbis-p6-cache-00">
2904<t>
2905  Closed issues:
2906  <list style="symbols"> 
2907    <t>
2908      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9"/>:
2909      "Trailer"
2910      (<eref target="http://purl.org/NET/http-errata#trailer-hop"/>)
2911    </t>
2912    <t>
2913      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12"/>:
2914      "Invalidation after Update or Delete"
2915      (<eref target="http://purl.org/NET/http-errata#invalidupd"/>)
2916    </t>
2917    <t>
2918      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
2919      "Normative and Informative references"
2920    </t>
2921    <t>
2922      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48"/>:
2923      "Date reference typo"
2924    </t>
2925    <t>
2926      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49"/>:
2927      "Connection header text"
2928    </t>
2929    <t>
2930      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
2931      "Informative references"
2932    </t>
2933    <t>
2934      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66"/>:
2935      "ISO-8859-1 Reference"
2936    </t>
2937    <t>
2938      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86"/>:
2939      "Normative up-to-date references"
2940    </t>
2941    <t>
2942      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87"/>:
2943      "typo in 13.2.2"
2944    </t>
2945  </list>
2946</t>
2947<t>
2948  Other changes:
2949  <list style="symbols"> 
2950    <t>
2951      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"/>)
2952    </t>
2953  </list>
2954</t>
2955</section>
2956
2957<section title="Since draft-ietf-httpbis-p6-cache-01">
2958<t>
2959  Closed issues:
2960  <list style="symbols"> 
2961    <t>
2962      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82"/>:
2963      "rel_path not used"
2964    </t>
2965  </list>
2966</t>
2967<t>
2968  Other changes:
2969  <list style="symbols"> 
2970    <t>
2971       Get rid of duplicate BNF rule names ("host" -&gt; "uri-host")
2972       (work in progress on <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>)
2973    </t>
2974    <t>
2975      Add explicit references to BNF syntax and rules imported from other parts of the specification.
2976    </t>
2977  </list>
2978</t>
2979</section>
2980
2981<section title="Since draft-ietf-httpbis-p6-cache-02" anchor="changes.since.02">
2982<t>
2983  Ongoing work on IANA Message Header Registration (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
2984  <list style="symbols"> 
2985    <t>
2986      Reference RFC 3984, and update header registrations for headers defined
2987      in this document.
2988    </t>
2989  </list>
2990</t>
2991</section>
2992
2993</section>
2994
2995</back>
2996</rfc>
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