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1<?xml version="1.0" encoding="utf-8"?>
2<!DOCTYPE rfc [
3  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
4  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
5  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
6  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
7  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
8  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
9  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
10  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
11  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
12  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
13  <!ENTITY ID-VERSION "latest">
14  <!ENTITY ID-MONTH "December">
15  <!ENTITY ID-YEAR "2007">
16  <!ENTITY messaging                   "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
17  <!ENTITY combining-byte-ranges       "<xref target='Part5' x:rel='#combining.byte.ranges' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY entity-length               "<xref target='Part3' x:rel='#entity.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY entity-tags                 "<xref target='Part4' x:rel='#entity.tags' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY full-date                   "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY header-authorization        "<xref target='Part7' x:rel='#header.authorization' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-connection           "<xref target='Part1' x:rel='#header.connection' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-date                 "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY weak-and-strong-validators  "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY message-headers             "<xref target='Part1' x:rel='#message.headers' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY message-length              "<xref target='Part1' x:rel='#message.length' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY safe-methods                "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY server-driven-negotiation   "<xref target='Part3' x:rel='#server-driven.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29]>
30<?rfc toc="yes" ?>
31<?rfc symrefs="yes" ?>
32<?rfc sortrefs="yes" ?>
33<?rfc compact="yes"?>
34<?rfc subcompact="no" ?>
35<?rfc linkmailto="no" ?>
36<?rfc editing="no" ?>
37<?rfc-ext allow-markup-in-artwork="yes" ?>
38<?rfc-ext include-references-in-index="yes" ?>
39<rfc obsoletes="2068, 2616" category="std"
40     ipr="full3978" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;"
41     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
42<front>
43
44  <title abbrev="HTTP/1.1, part 6">HTTP/1.1, part 6: Caching</title>
45
46  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
47    <organization abbrev="Day Software">Day Software</organization>
48    <address>
49      <postal>
50        <street>23 Corporate Plaza DR, Suite 280</street>
51        <city>Newport Beach</city>
52        <region>CA</region>
53        <code>92660</code>
54        <country>USA</country>
55      </postal>
56      <phone>+1-949-706-5300</phone>
57      <facsimile>+1-949-706-5305</facsimile>
58      <email>fielding@gbiv.com</email>
59      <uri>http://roy.gbiv.com/</uri>
60    </address>
61  </author>
62
63  <author initials="J." surname="Gettys" fullname="Jim Gettys">
64    <organization>One Laptop per Child</organization>
65    <address>
66      <postal>
67        <street>21 Oak Knoll Road</street>
68        <city>Carlisle</city>
69        <region>MA</region>
70        <code>01741</code>
71        <country>USA</country>
72      </postal>
73      <email>jg@laptop.org</email>
74      <uri>http://www.laptop.org/</uri>
75    </address>
76  </author>
77 
78  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
79    <organization abbrev="HP">Hewlett-Packard Company</organization>
80    <address>
81      <postal>
82        <street>HP Labs, Large Scale Systems Group</street>
83        <street>1501 Page Mill Road, MS 1177</street>
84        <city>Palo Alto</city>
85        <region>CA</region>
86        <code>94304</code>
87        <country>USA</country>
88      </postal>
89      <email>JeffMogul@acm.org</email>
90    </address>
91  </author>
92
93  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
94    <organization abbrev="Microsoft">Microsoft Corporation</organization>
95    <address>
96      <postal>
97        <street>1 Microsoft Way</street>
98        <city>Redmond</city>
99        <region>WA</region>
100        <code>98052</code>
101        <country>USA</country>
102      </postal>
103      <email>henrikn@microsoft.com</email>
104    </address>
105  </author>
106
107  <author initials="L." surname="Masinter" fullname="Larry Masinter">
108    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
109    <address>
110      <postal>
111        <street>345 Park Ave</street>
112        <city>San Jose</city>
113        <region>CA</region>
114        <code>95110</code>
115        <country>USA</country>
116      </postal>
117      <email>LMM@acm.org</email>
118      <uri>http://larry.masinter.net/</uri>
119    </address>
120  </author>
121 
122  <author initials="P." surname="Leach" fullname="Paul J. Leach">
123    <organization abbrev="Microsoft">Microsoft Corporation</organization>
124    <address>
125      <postal>
126        <street>1 Microsoft Way</street>
127        <city>Redmond</city>
128        <region>WA</region>
129        <code>98052</code>
130      </postal>
131      <email>paulle@microsoft.com</email>
132    </address>
133  </author>
134   
135  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
136    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
137    <address>
138      <postal>
139        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
140        <street>The Stata Center, Building 32</street>
141        <street>32 Vassar Street</street>
142        <city>Cambridge</city>
143        <region>MA</region>
144        <code>02139</code>
145        <country>USA</country>
146      </postal>
147      <email>timbl@w3.org</email>
148      <uri>http://www.w3.org/People/Berners-Lee/</uri>
149    </address>
150  </author>
151
152  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
153
154<abstract>
155<t>
156   The Hypertext Transfer Protocol (HTTP) is an application-level
157   protocol for distributed, collaborative, hypermedia information
158   systems. HTTP has been in use by the World Wide Web global information
159   initiative since 1990. This document is Part 6 of the seven-part specification
160   that defines the protocol referred to as "HTTP/1.1" and, taken together,
161   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
162   and the associated header fields that control cache behavior or indicate
163   cacheable response messages.
164</t>
165</abstract>
166
167<note title="Editorial Note (To be removed by RFC Editor)">
168  <t>
169    This version of the HTTP specification contains only minimal editorial
170    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
171    and authors' addresses).  All other changes are due to partitioning the
172    original into seven mostly independent parts.  The intent is for readers
173    of future drafts to able to use draft 00 as the basis for comparison
174    when the WG makes later changes to the specification text.  This draft
175    will shortly be followed by draft 01 (containing the first round of changes
176    that have already been agreed to on the mailing list). There is no point in
177    reviewing this draft other than to verify that the partitioning has been
178    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
179    will be the editors after draft 00 is submitted.
180  </t>
181  <t>
182    Discussion of this draft should take place on the HTTPBIS working group
183    mailing list (ietf-http-wg@w3.org). The current issues list is
184    at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
185    and related documents (including fancy diffs) can be found at
186    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
187  </t>
188</note>
189</front>
190<middle>
191<section title="Introduction" anchor="introduction">
192<t>
193   This document will define aspects of HTTP related to caching response
194   messages.  Right now it only includes the extracted relevant sections
195   of <xref target="RFC2616">RFC 2616</xref> without edit.
196</t>
197
198<section title="Terminology" anchor="intro.terminology">
199<t>
200   This specification uses a number of terms to refer to the roles
201   played by participants in, and objects of, the HTTP communication.
202</t>
203<t>
204  <iref item="cache"/>
205  <x:dfn>cache</x:dfn>
206  <list>
207    <t>
208      A program's local store of response messages and the subsystem
209      that controls its message storage, retrieval, and deletion. A
210      cache stores cacheable responses in order to reduce the response
211      time and network bandwidth consumption on future, equivalent
212      requests. Any client or server may include a cache, though a cache
213      cannot be used by a server that is acting as a tunnel.
214    </t>
215  </list>
216</t>
217<t>
218  <iref item="cacheable"/>
219  <x:dfn>cacheable</x:dfn>
220  <list>
221    <t>
222      A response is cacheable if a cache is allowed to store a copy of
223      the response message for use in answering subsequent requests. The
224      rules for determining the cacheability of HTTP responses are
225      defined in <xref target="caching"/>. Even if a resource is cacheable, there may
226      be additional constraints on whether a cache can use the cached
227      copy for a particular request.
228    </t>
229  </list>
230</t>
231<t>
232  <iref item="first-hand"/>
233  <x:dfn>first-hand</x:dfn>
234  <list>
235    <t>
236      A response is first-hand if it comes directly and without
237      unnecessary delay from the origin server, perhaps via one or more
238      proxies. A response is also first-hand if its validity has just
239      been checked directly with the origin server.
240    </t>
241  </list>
242</t>
243<t>
244  <iref item="explicit expiration time"/>
245  <x:dfn>explicit expiration time</x:dfn>
246  <list>
247    <t>
248      The time at which the origin server intends that an entity should
249      no longer be returned by a cache without further validation.
250    </t>
251  </list>
252</t>
253<t>
254  <iref item="heuristic expiration time"/>
255  <x:dfn>heuristic expiration time</x:dfn>
256  <list>
257    <t>
258      An expiration time assigned by a cache when no explicit expiration
259      time is available.
260    </t>
261  </list>
262</t>
263<t>
264  <iref item="age"/>
265  <x:dfn>age</x:dfn>
266  <list>
267    <t>
268      The age of a response is the time since it was sent by, or
269      successfully validated with, the origin server.
270    </t>
271  </list>
272</t>
273<t>
274  <iref item="freshness lifetime"/>
275  <x:dfn>freshness lifetime</x:dfn>
276  <list>
277    <t>
278      The length of time between the generation of a response and its
279      expiration time.
280    </t>
281  </list>
282</t>
283<t>
284  <iref item="fresh"/>
285  <x:dfn>fresh</x:dfn>
286  <list>
287    <t>
288      A response is fresh if its age has not yet exceeded its freshness
289      lifetime.
290    </t>
291  </list>
292</t>
293<t>
294  <iref item="stale"/>
295  <x:dfn>stale</x:dfn>
296  <list>
297    <t>
298      A response is stale if its age has passed its freshness lifetime.
299    </t>
300  </list>
301</t>
302<t>
303  <iref item="semantically transparent"/>
304  <x:dfn>semantically transparent</x:dfn>
305  <list>
306    <t>
307      A cache behaves in a "semantically transparent" manner, with
308      respect to a particular response, when its use affects neither the
309      requesting client nor the origin server, except to improve
310      performance. When a cache is semantically transparent, the client
311      receives exactly the same response (except for hop-by-hop headers)
312      that it would have received had its request been handled directly
313      by the origin server.
314    </t>
315  </list>
316</t>
317<t>
318  <iref item="validator"/>
319  <x:dfn>validator</x:dfn>
320  <list>
321    <t>
322      A protocol element (e.g., an entity tag or a Last-Modified time)
323      that is used to find out whether a cache entry is an equivalent
324      copy of an entity.
325    </t>
326  </list>
327</t>
328</section>
329
330<section title="Delta Seconds" anchor="delta.seconds">
331<t>
332   Some HTTP header fields allow a time value to be specified as an
333   integer number of seconds, represented in decimal, after the time
334   that the message was received.
335</t>
336<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
337    delta-seconds  = 1*DIGIT
338</artwork></figure>
339</section>
340</section>
341
342<section title="Caching in HTTP" anchor="caching">
343<section title="Overview" anchor="caching.overview">
344<t>
345   HTTP is typically used for distributed information systems, where
346   performance can be improved by the use of response caches. The
347   HTTP/1.1 protocol includes a number of elements intended to make
348   caching work as well as possible. Because these elements are
349   inextricable from other aspects of the protocol, and because they
350   interact with each other, it is useful to describe the basic caching
351   design of HTTP separately from the detailed descriptions of methods,
352   headers, response codes, etc.
353</t>
354<t>
355   Caching would be useless if it did not significantly improve
356   performance. The goal of caching in HTTP/1.1 is to eliminate the need
357   to send requests in many cases, and to eliminate the need to send
358   full responses in many other cases. The former reduces the number of
359   network round-trips required for many operations; we use an
360   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
361   latter reduces network bandwidth requirements; we use a "validation"
362   mechanism for this purpose (see <xref target="validation.model"/>).
363</t>
364<t>
365   Requirements for performance, availability, and disconnected
366   operation require us to be able to relax the goal of semantic
367   transparency. The HTTP/1.1 protocol allows origin servers, caches,
368   and clients to explicitly reduce transparency when necessary.
369   However, because non-transparent operation may confuse non-expert
370   users, and might be incompatible with certain server applications
371   (such as those for ordering merchandise), the protocol requires that
372   transparency be relaxed
373  <list style="symbols">
374     <t>only by an explicit protocol-level request when relaxed by
375        client or origin server</t>
376
377     <t>only with an explicit warning to the end user when relaxed by
378        cache or client</t>
379  </list>
380</t>
381<t>
382   Therefore, the HTTP/1.1 protocol provides these important elements:
383  <list style="numbers">
384      <t>Protocol features that provide full semantic transparency when
385         this is required by all parties.</t>
386
387      <t>Protocol features that allow an origin server or user agent to
388         explicitly request and control non-transparent operation.</t>
389
390      <t>Protocol features that allow a cache to attach warnings to
391         responses that do not preserve the requested approximation of
392         semantic transparency.</t>
393  </list>
394</t>
395<t>
396   A basic principle is that it must be possible for the clients to
397   detect any potential relaxation of semantic transparency.
398  <list><t>
399      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
400      design decisions not explicitly discussed in this specification.
401      If a decision might affect semantic transparency, the implementor
402      ought to err on the side of maintaining transparency unless a
403      careful and complete analysis shows significant benefits in
404      breaking transparency.
405    </t></list>
406</t>
407
408<section title="Cache Correctness" anchor="cache.correctness">
409<t>
410   A correct cache &MUST; respond to a request with the most up-to-date
411   response held by the cache that is appropriate to the request (see
412   sections <xref target="disambiguating.expiration.values" format="counter"/>,
413   <xref target="disambiguating.multiple.responses" format="counter"/>,
414   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
415   conditions:
416  <list style="numbers">
417      <t>It has been checked for equivalence with what the origin server
418         would have returned by revalidating the response with the
419         origin server (<xref target="validation.model"/>);</t>
420
421      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
422         this means it meets the least restrictive freshness requirement
423         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
424         the origin server so specifies, it is the freshness requirement
425         of the origin server alone.
426
427         If a stored response is not "fresh enough" by the most
428         restrictive freshness requirement of both the client and the
429         origin server, in carefully considered circumstances the cache
430         &MAY; still return the response with the appropriate Warning
431         header (see section <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
432         and <xref target="header.warning" format="counter"/>), unless such a response
433         is prohibited (e.g., by a "no-store" cache-directive, or by a
434         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
435
436      <t>It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
437         or error (4xx or 5xx) response message.</t>
438  </list>
439</t>
440<t>
441   If the cache can not communicate with the origin server, then a
442   correct cache &SHOULD; respond as above if the response can be
443   correctly served from the cache; if not it &MUST; return an error or
444   warning indicating that there was a communication failure.
445</t>
446<t>
447   If a cache receives a response (either an entire response, or a 304
448   (Not Modified) response) that it would normally forward to the
449   requesting client, and the received response is no longer fresh, the
450   cache &SHOULD; forward it to the requesting client without adding a new
451   Warning (but without removing any existing Warning headers). A cache
452   &SHOULD-NOT;  attempt to revalidate a response simply because that
453   response became stale in transit; this might lead to an infinite
454   loop. A user agent that receives a stale response without a Warning
455   &MAY; display a warning indication to the user.
456</t>
457</section>
458
459<section title="Warnings" anchor="warnings">
460<t>
461   Whenever a cache returns a response that is neither first-hand nor
462   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
463   &MUST; attach a warning to that effect, using a Warning general-header.
464   The Warning header and the currently defined warnings are described
465   in <xref target="header.warning"/>. The warning allows clients to take appropriate
466   action.
467</t>
468<t>
469   Warnings &MAY; be used for other purposes, both cache-related and
470   otherwise. The use of a warning, rather than an error status code,
471   distinguish these responses from true failures.
472</t>
473<t>
474   Warnings are assigned three digit warn-codes. The first digit
475   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
476   stored cache entry after a successful revalidation:
477</t>
478<t>
479  <list style="hanging">
480    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
481     the response, and so &MUST; be deleted after a successful
482     revalidation. 1XX warn-codes &MAY; be generated by a cache only when
483     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
484
485    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
486     headers that is not rectified by a revalidation (for example, a
487     lossy compression of the entity bodies) and which &MUST-NOT; be
488     deleted after a successful revalidation.</t>
489    </list>
490</t>
491<t>
492   See <xref target="header.warning"/> for the definitions of the codes themselves.
493</t>
494<t>
495   HTTP/1.0 caches will cache all Warnings in responses, without
496   deleting the ones in the first category. Warnings in responses that
497   are passed to HTTP/1.0 caches carry an extra warning-date field,
498   which prevents a future HTTP/1.1 recipient from believing an
499   erroneously cached Warning.
500</t>
501<t>
502   Warnings also carry a warning text. The text &MAY; be in any
503   appropriate natural language (perhaps based on the client's Accept
504   headers), and include an &OPTIONAL; indication of what character set is
505   used.
506</t>
507<t>
508   Multiple warnings &MAY; be attached to a response (either by the origin
509   server or by a cache), including multiple warnings with the same code
510   number. For example, a server might provide the same warning with
511   texts in both English and Basque.
512</t>
513<t>
514   When multiple warnings are attached to a response, it might not be
515   practical or reasonable to display all of them to the user. This
516   version of HTTP does not specify strict priority rules for deciding
517   which warnings to display and in what order, but does suggest some
518   heuristics.
519</t>
520</section>
521
522<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
523<t>
524   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
525   times and validators) are implicit directives to caches. In some
526   cases, a server or client might need to provide explicit directives
527   to the HTTP caches. We use the Cache-Control header for this purpose.
528</t>
529<t>
530   The Cache-Control header allows a client or server to transmit a
531   variety of directives in either requests or responses. These
532   directives typically override the default caching algorithms. As a
533   general rule, if there is any apparent conflict between header
534   values, the most restrictive interpretation is applied (that is, the
535   one that is most likely to preserve semantic transparency). However,
536   in some cases, cache-control directives are explicitly specified as
537   weakening the approximation of semantic transparency (for example,
538   "max-stale" or "public").
539</t>
540<t>
541   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
542</t>
543</section>
544
545<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
546<t>
547   Many user agents make it possible for users to override the basic
548   caching mechanisms. For example, the user agent might allow the user
549   to specify that cached entities (even explicitly stale ones) are
550   never validated. Or the user agent might habitually add "Cache-Control:
551   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
552   default to either non-transparent behavior, or behavior that results
553   in abnormally ineffective caching, but &MAY; be explicitly configured
554   to do so by an explicit action of the user.
555</t>
556<t>
557   If the user has overridden the basic caching mechanisms, the user
558   agent &SHOULD; explicitly indicate to the user whenever this results in
559   the display of information that might not meet the server's
560   transparency requirements (in particular, if the displayed entity is
561   known to be stale). Since the protocol normally allows the user agent
562   to determine if responses are stale or not, this indication need only
563   be displayed when this actually happens. The indication need not be a
564   dialog box; it could be an icon (for example, a picture of a rotting
565   fish) or some other indicator.
566</t>
567<t>
568   If the user has overridden the caching mechanisms in a way that would
569   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
570   continually indicate this state to the user (for example, by a
571   display of a picture of currency in flames) so that the user does not
572   inadvertently consume excess resources or suffer from excessive
573   latency.
574</t>
575</section>
576
577<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
578<t>
579   In some cases, the operator of a cache &MAY; choose to configure it to
580   return stale responses even when not requested by clients. This
581   decision ought not be made lightly, but may be necessary for reasons
582   of availability or performance, especially when the cache is poorly
583   connected to the origin server. Whenever a cache returns a stale
584   response, it &MUST; mark it as such (using a Warning header) enabling
585   the client software to alert the user that there might be a potential
586   problem.
587</t>
588<t>
589   It also allows the user agent to take steps to obtain a first-hand or
590   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
591   response if the client explicitly requests a first-hand or fresh one,
592   unless it is impossible to comply for technical or policy reasons.
593</t>
594</section>
595
596<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
597<t>
598   While the origin server (and to a lesser extent, intermediate caches,
599   by their contribution to the age of a response) are the primary
600   source of expiration information, in some cases the client might need
601   to control a cache's decision about whether to return a cached
602   response without validating it. Clients do this using several
603   directives of the Cache-Control header.
604</t>
605<t>
606   A client's request &MAY; specify the maximum age it is willing to
607   accept of an unvalidated response; specifying a value of zero forces
608   the cache(s) to revalidate all responses. A client &MAY; also specify
609   the minimum time remaining before a response expires. Both of these
610   options increase constraints on the behavior of caches, and so cannot
611   further relax the cache's approximation of semantic transparency.
612</t>
613<t>
614   A client &MAY; also specify that it will accept stale responses, up to
615   some maximum amount of staleness. This loosens the constraints on the
616   caches, and so might violate the origin server's specified
617   constraints on semantic transparency, but might be necessary to
618   support disconnected operation, or high availability in the face of
619   poor connectivity.
620</t>
621</section>
622</section>
623
624<section title="Expiration Model" anchor="expiration.model">
625
626<section title="Server-Specified Expiration" anchor="server-specified.expiration">
627<t>
628   HTTP caching works best when caches can entirely avoid making
629   requests to the origin server. The primary mechanism for avoiding
630   requests is for an origin server to provide an explicit expiration
631   time in the future, indicating that a response &MAY; be used to satisfy
632   subsequent requests. In other words, a cache can return a fresh
633   response without first contacting the server.
634</t>
635<t>
636   Our expectation is that servers will assign future explicit
637   expiration times to responses in the belief that the entity is not
638   likely to change, in a semantically significant way, before the
639   expiration time is reached. This normally preserves semantic
640   transparency, as long as the server's expiration times are carefully
641   chosen.
642</t>
643<t>
644   The expiration mechanism applies only to responses taken from a cache
645   and not to first-hand responses forwarded immediately to the
646   requesting client.
647</t>
648<t>
649   If an origin server wishes to force a semantically transparent cache
650   to validate every request, it &MAY; assign an explicit expiration time
651   in the past. This means that the response is always stale, and so the
652   cache &SHOULD; validate it before using it for subsequent requests. See
653   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
654</t>
655<t>
656   If an origin server wishes to force any HTTP/1.1 cache, no matter how
657   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
658   cache-control directive (see <xref target="header.cache-control"/>).
659</t>
660<t>
661   Servers specify explicit expiration times using either the Expires
662   header, or the max-age directive of the Cache-Control header.
663</t>
664<t>
665   An expiration time cannot be used to force a user agent to refresh
666   its display or reload a resource; its semantics apply only to caching
667   mechanisms, and such mechanisms need only check a resource's
668   expiration status when a new request for that resource is initiated.
669   See <xref target="history.lists"/> for an explanation of the difference between caches
670   and history mechanisms.
671</t>
672</section>
673
674<section title="Heuristic Expiration" anchor="heuristic.expiration">
675<t>
676   Since origin servers do not always provide explicit expiration times,
677   HTTP caches typically assign heuristic expiration times, employing
678   algorithms that use other header values (such as the Last-Modified
679   time) to estimate a plausible expiration time. The HTTP/1.1
680   specification does not provide specific algorithms, but does impose
681   worst-case constraints on their results. Since heuristic expiration
682   times might compromise semantic transparency, they ought to used
683   cautiously, and we encourage origin servers to provide explicit
684   expiration times as much as possible.
685</t>
686</section>
687
688<section title="Age Calculations" anchor="age.calculations">
689<t>
690   In order to know if a cached entry is fresh, a cache needs to know if
691   its age exceeds its freshness lifetime. We discuss how to calculate
692   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
693   the age of a response or cache entry.
694</t>
695<t>
696   In this discussion, we use the term "now" to mean "the current value
697   of the clock at the host performing the calculation." Hosts that use
698   HTTP, but especially hosts running origin servers and caches, &SHOULD;
699   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
700   a globally accurate time standard.
701</t>
702<t>
703   HTTP/1.1 requires origin servers to send a Date header, if possible,
704   with every response, giving the time at which the response was
705   generated (see &header-date;). We use the term "date_value" to denote
706   the value of the Date header, in a form appropriate for arithmetic
707   operations.
708</t>
709<t>
710   HTTP/1.1 uses the Age response-header to convey the estimated age of
711   the response message when obtained from a cache. The Age field value
712   is the cache's estimate of the amount of time since the response was
713   generated or revalidated by the origin server.
714</t>
715<t>
716   In essence, the Age value is the sum of the time that the response
717   has been resident in each of the caches along the path from the
718   origin server, plus the amount of time it has been in transit along
719   network paths.
720</t>
721<t>
722   We use the term "age_value" to denote the value of the Age header, in
723   a form appropriate for arithmetic operations.
724</t>
725<t>
726   A response's age can be calculated in two entirely independent ways:
727  <list style="numbers">
728      <t>now minus date_value, if the local clock is reasonably well
729         synchronized to the origin server's clock. If the result is
730         negative, the result is replaced by zero.</t>
731
732      <t>age_value, if all of the caches along the response path
733         implement HTTP/1.1.</t>
734  </list>
735</t>
736<t>
737   Given that we have two independent ways to compute the age of a
738   response when it is received, we can combine these as
739</t>
740<figure><artwork type="code">
741    corrected_received_age = max(now - date_value, age_value)
742</artwork></figure>
743<t>
744   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
745   paths, one gets a reliable (conservative) result.
746</t>
747<t>
748   Because of network-imposed delays, some significant interval might
749   pass between the time that a server generates a response and the time
750   it is received at the next outbound cache or client. If uncorrected,
751   this delay could result in improperly low ages.
752</t>
753<t>
754   Because the request that resulted in the returned Age value must have
755   been initiated prior to that Age value's generation, we can correct
756   for delays imposed by the network by recording the time at which the
757   request was initiated. Then, when an Age value is received, it &MUST;
758   be interpreted relative to the time the request was initiated, not
759   the time that the response was received. This algorithm results in
760   conservative behavior no matter how much delay is experienced. So, we
761   compute:
762</t>
763<figure><artwork type="code">
764   corrected_initial_age = corrected_received_age
765                         + (now - request_time)
766</artwork></figure>
767<t>
768   where "request_time" is the time (according to the local clock) when
769   the request that elicited this response was sent.
770</t>
771<t>
772   Summary of age calculation algorithm, when a cache receives a
773   response:
774</t>
775<figure><artwork type="code">
776   /*
777    * age_value
778    *      is the value of Age: header received by the cache with
779    *              this response.
780    * date_value
781    *      is the value of the origin server's Date: header
782    * request_time
783    *      is the (local) time when the cache made the request
784    *              that resulted in this cached response
785    * response_time
786    *      is the (local) time when the cache received the
787    *              response
788    * now
789    *      is the current (local) time
790    */
791
792   apparent_age = max(0, response_time - date_value);
793   corrected_received_age = max(apparent_age, age_value);
794   response_delay = response_time - request_time;
795   corrected_initial_age = corrected_received_age + response_delay;
796   resident_time = now - response_time;
797   current_age   = corrected_initial_age + resident_time;
798</artwork></figure>
799<t>
800   The current_age of a cache entry is calculated by adding the amount
801   of time (in seconds) since the cache entry was last validated by the
802   origin server to the corrected_initial_age. When a response is
803   generated from a cache entry, the cache &MUST; include a single Age
804   header field in the response with a value equal to the cache entry's
805   current_age.
806</t>
807<t>
808   The presence of an Age header field in a response implies that a
809   response is not first-hand. However, the converse is not true, since
810   the lack of an Age header field in a response does not imply that the
811   response is first-hand unless all caches along the request path are
812   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
813   the Age header field).
814</t>
815</section>
816
817<section title="Expiration Calculations" anchor="expiration.calculations">
818<t>
819   In order to decide whether a response is fresh or stale, we need to
820   compare its freshness lifetime to its age. The age is calculated as
821   described in <xref target="age.calculations"/>; this section describes how to calculate
822   the freshness lifetime, and to determine if a response has expired.
823   In the discussion below, the values can be represented in any form
824   appropriate for arithmetic operations.
825</t>
826<t>
827   We use the term "expires_value" to denote the value of the Expires
828   header. We use the term "max_age_value" to denote an appropriate
829   value of the number of seconds carried by the "max-age" directive of
830   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
831</t>
832<t>
833   The max-age directive takes priority over Expires, so if max-age is
834   present in a response, the calculation is simply:
835</t>
836<figure><artwork type="code">
837   freshness_lifetime = max_age_value
838</artwork></figure>
839<t>
840   Otherwise, if Expires is present in the response, the calculation is:
841</t>
842<figure><artwork type="code">
843   freshness_lifetime = expires_value - date_value
844</artwork></figure>
845<t>
846   Note that neither of these calculations is vulnerable to clock skew,
847   since all of the information comes from the origin server.
848</t>
849<t>
850   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
851   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
852   does not include other restrictions on caching, the cache &MAY; compute
853   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
854   113 to any response whose age is more than 24 hours if such warning
855   has not already been added.
856</t>
857<t>
858   Also, if the response does have a Last-Modified time, the heuristic
859   expiration value &SHOULD; be no more than some fraction of the interval
860   since that time. A typical setting of this fraction might be 10%.
861</t>
862<t>
863   The calculation to determine if a response has expired is quite
864   simple:
865</t>
866<figure><artwork type="code">
867   response_is_fresh = (freshness_lifetime &gt; current_age)
868</artwork></figure>
869</section>
870
871<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
872<t>
873   Because expiration values are assigned optimistically, it is possible
874   for two caches to contain fresh values for the same resource that are
875   different.
876</t>
877<t>
878   If a client performing a retrieval receives a non-first-hand response
879   for a request that was already fresh in its own cache, and the Date
880   header in its existing cache entry is newer than the Date on the new
881   response, then the client &MAY; ignore the response. If so, it &MAY;
882   retry the request with a "Cache-Control: max-age=0" directive (see
883   <xref target="header.cache-control"/>), to force a check with the origin server.
884</t>
885<t>
886   If a cache has two fresh responses for the same representation with
887   different validators, it &MUST; use the one with the more recent Date
888   header. This situation might arise because the cache is pooling
889   responses from other caches, or because a client has asked for a
890   reload or a revalidation of an apparently fresh cache entry.
891</t>
892</section>
893
894<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
895<t>
896   Because a client might be receiving responses via multiple paths, so
897   that some responses flow through one set of caches and other
898   responses flow through a different set of caches, a client might
899   receive responses in an order different from that in which the origin
900   server sent them. We would like the client to use the most recently
901   generated response, even if older responses are still apparently
902   fresh.
903</t>
904<t>
905   Neither the entity tag nor the expiration value can impose an
906   ordering on responses, since it is possible that a later response
907   intentionally carries an earlier expiration time. The Date values are
908   ordered to a granularity of one second.
909</t>
910<t>
911   When a client tries to revalidate a cache entry, and the response it
912   receives contains a Date header that appears to be older than the one
913   for the existing entry, then the client &SHOULD; repeat the request
914   unconditionally, and include
915</t>
916<figure><artwork type="example">
917    Cache-Control: max-age=0
918</artwork></figure>
919<t>
920   to force any intermediate caches to validate their copies directly
921   with the origin server, or
922</t>
923<figure><artwork type="example">
924    Cache-Control: no-cache
925</artwork></figure>
926<t>
927   to force any intermediate caches to obtain a new copy from the origin
928   server.
929</t>
930<t>
931   If the Date values are equal, then the client &MAY; use either response
932   (or &MAY;, if it is being extremely prudent, request a new response).
933   Servers &MUST-NOT; depend on clients being able to choose
934   deterministically between responses generated during the same second,
935   if their expiration times overlap.
936</t>
937</section>
938</section>
939
940<section title="Validation Model" anchor="validation.model">
941<t>
942   When a cache has a stale entry that it would like to use as a
943   response to a client's request, it first has to check with the origin
944   server (or possibly an intermediate cache with a fresh response) to
945   see if its cached entry is still usable. We call this "validating"
946   the cache entry. Since we do not want to have to pay the overhead of
947   retransmitting the full response if the cached entry is good, and we
948   do not want to pay the overhead of an extra round trip if the cached
949   entry is invalid, the HTTP/1.1 protocol supports the use of
950   conditional methods.
951</t>
952<t>
953   The key protocol features for supporting conditional methods are
954   those concerned with "cache validators." When an origin server
955   generates a full response, it attaches some sort of validator to it,
956   which is kept with the cache entry. When a client (user agent or
957   proxy cache) makes a conditional request for a resource for which it
958   has a cache entry, it includes the associated validator in the
959   request.
960</t>
961<t>
962   The server then checks that validator against the current validator
963   for the entity, and, if they match (see &weak-and-strong-validators;), it responds
964   with a special status code (usually, 304 (Not Modified)) and no
965   entity-body. Otherwise, it returns a full response (including
966   entity-body). Thus, we avoid transmitting the full response if the
967   validator matches, and we avoid an extra round trip if it does not
968   match.
969</t>
970<t>
971   In HTTP/1.1, a conditional request looks exactly the same as a normal
972   request for the same resource, except that it carries a special
973   header (which includes the validator) that implicitly turns the
974   method (usually, GET) into a conditional.
975</t>
976<t>
977   The protocol includes both positive and negative senses of cache-validating
978   conditions. That is, it is possible to request either that
979   a method be performed if and only if a validator matches or if and
980   only if no validators match.
981  <list><t>
982      <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
983      served from cache until it expires, unless this is explicitly
984      prohibited by a cache-control directive. However, a cache cannot
985      do a conditional retrieval if it does not have a validator for the
986      entity, which means it will not be refreshable after it expires.
987  </t></list>
988</t>
989
990<section title="Last-Modified Dates" anchor="last-modified.dates">
991<t>
992   The Last-Modified entity-header field value is often used as a cache
993   validator. In simple terms, a cache entry is considered to be valid
994   if the entity has not been modified since the Last-Modified value.
995</t>
996</section>
997
998<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
999<t>
1000   The ETag response-header field value, an entity tag, provides for an
1001   "opaque" cache validator. This might allow more reliable validation
1002   in situations where it is inconvenient to store modification dates,
1003   where the one-second resolution of HTTP date values is not
1004   sufficient, or where the origin server wishes to avoid certain
1005   paradoxes that might arise from the use of modification dates.
1006</t>
1007<t>
1008   Entity Tags are described in &entity-tags;.
1009</t>
1010</section>
1011
1012<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1013<t>
1014   The principle behind entity tags is that only the service author
1015   knows the semantics of a resource well enough to select an
1016   appropriate cache validation mechanism, and the specification of any
1017   validator comparison function more complex than byte-equality would
1018   open up a can of worms. Thus, comparisons of any other headers
1019   (except Last-Modified, for compatibility with HTTP/1.0) are never
1020   used for purposes of validating a cache entry.
1021</t>
1022</section>
1023</section>
1024
1025<section title="Response Cacheability" anchor="response.cacheability">
1026<t>
1027   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1028   directive, a caching system &MAY; always store a successful response
1029   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1030   if it is fresh, and &MAY; return it after successful validation. If
1031   there is neither a cache validator nor an explicit expiration time
1032   associated with a response, we do not expect it to be cached, but
1033   certain caches &MAY; violate this expectation (for example, when little
1034   or no network connectivity is available). A client can usually detect
1035   that such a response was taken from a cache by comparing the Date
1036   header to the current time.
1037  <list><t>
1038      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1039      without providing any Warning.
1040  </t></list>
1041</t>
1042<t>
1043   However, in some cases it might be inappropriate for a cache to
1044   retain an entity, or to return it in response to a subsequent
1045   request. This might be because absolute semantic transparency is
1046   deemed necessary by the service author, or because of security or
1047   privacy considerations. Certain cache-control directives are
1048   therefore provided so that the server can indicate that certain
1049   resource entities, or portions thereof, are not to be cached
1050   regardless of other considerations.
1051</t>
1052<t>
1053   Note that &header-authorization; normally prevents a shared cache from saving
1054   and returning a response to a previous request if that request
1055   included an Authorization header.
1056</t>
1057<t>
1058   A response received with a status code of 200, 203, 206, 300, 301 or
1059   410 &MAY; be stored by a cache and used in reply to a subsequent
1060   request, subject to the expiration mechanism, unless a cache-control
1061   directive prohibits caching. However, a cache that does not support
1062   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1063   Content) responses.
1064</t>
1065<t>
1066   A response received with any other status code (e.g. status codes 302
1067   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1068   unless there are cache-control directives or another header(s) that
1069   explicitly allow it. For example, these include the following: an
1070   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1071   "proxy-revalidate", "public" or "private" cache-control
1072   directive (<xref target="header.cache-control"/>).
1073</t>
1074</section>
1075
1076<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1077<t>
1078   The purpose of an HTTP cache is to store information received in
1079   response to requests for use in responding to future requests. In
1080   many cases, a cache simply returns the appropriate parts of a
1081   response to the requester. However, if the cache holds a cache entry
1082   based on a previous response, it might have to combine parts of a new
1083   response with what is held in the cache entry.
1084</t>
1085
1086<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1087<t>
1088   For the purpose of defining the behavior of caches and non-caching
1089   proxies, we divide HTTP headers into two categories:
1090  <list style="symbols">
1091      <t>End-to-end headers, which are  transmitted to the ultimate
1092        recipient of a request or response. End-to-end headers in
1093        responses &MUST; be stored as part of a cache entry and &MUST; be
1094        transmitted in any response formed from a cache entry.</t>
1095
1096      <t>Hop-by-hop headers, which are meaningful only for a single
1097        transport-level connection, and are not stored by caches or
1098        forwarded by proxies.</t>
1099  </list>
1100</t>
1101<t>
1102   The following HTTP/1.1 headers are hop-by-hop headers:
1103  <list style="symbols">
1104      <t>Connection</t>
1105      <t>Keep-Alive</t>
1106      <t>Proxy-Authenticate</t>
1107      <t>Proxy-Authorization</t>
1108      <t>TE</t>
1109      <t>Trailers</t>
1110      <t>Transfer-Encoding</t>
1111      <t>Upgrade</t>
1112  </list>
1113</t>
1114<t>
1115   All other headers defined by HTTP/1.1 are end-to-end headers.
1116</t>
1117<t>
1118   Other hop-by-hop headers &MUST; be listed in a Connection header,
1119   (&header-connection;) to be introduced into HTTP/1.1 (or later).
1120</t>
1121</section>
1122
1123<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1124<t>
1125   Some features of the HTTP/1.1 protocol, such as Digest
1126   Authentication, depend on the value of certain end-to-end headers. A
1127   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1128   definition of that header requires or specifically allows that.
1129</t>
1130<t>
1131   A transparent proxy &MUST-NOT; modify any of the following fields in a
1132   request or response, and it &MUST-NOT; add any of these fields if not
1133   already present:
1134  <list style="symbols">
1135      <t>Content-Location</t>
1136      <t>Content-MD5</t>
1137      <t>ETag</t>
1138      <t>Last-Modified</t>
1139  </list>
1140</t>
1141<t>
1142   A transparent proxy &MUST-NOT; modify any of the following fields in a
1143   response:
1144  <list style="symbols">
1145    <t>Expires</t>
1146  </list>
1147</t>
1148<t>
1149   but it &MAY; add any of these fields if not already present. If an
1150   Expires header is added, it &MUST; be given a field-value identical to
1151   that of the Date header in that response.
1152</t>
1153<t>
1154   A  proxy &MUST-NOT; modify or add any of the following fields in a
1155   message that contains the no-transform cache-control directive, or in
1156   any request:
1157  <list style="symbols">
1158    <t>Content-Encoding</t>
1159    <t>Content-Range</t>
1160    <t>Content-Type</t>
1161  </list>
1162</t>
1163<t>
1164   A non-transparent proxy &MAY; modify or add these fields to a message
1165   that does not include no-transform, but if it does so, it &MUST; add a
1166   Warning 214 (Transformation applied) if one does not already appear
1167   in the message (see <xref target="header.warning"/>).
1168  <list><t>
1169      Warning: unnecessary modification of end-to-end headers might
1170      cause authentication failures if stronger authentication
1171      mechanisms are introduced in later versions of HTTP. Such
1172      authentication mechanisms &MAY; rely on the values of header fields
1173      not listed here.
1174    </t></list>
1175</t>
1176<t>
1177   The Content-Length field of a request or response is added or deleted
1178   according to the rules in &message-length;. A transparent proxy &MUST;
1179   preserve the entity-length (&entity-length;) of the entity-body,
1180   although it &MAY; change the transfer-length (&message-length;).
1181</t>
1182</section>
1183
1184<section title="Combining Headers" anchor="combining.headers">
1185<t>
1186   When a cache makes a validating request to a server, and the server
1187   provides a 304 (Not Modified) response or a 206 (Partial Content)
1188   response, the cache then constructs a response to send to the
1189   requesting client.
1190</t>
1191<t>
1192   If the status code is 304 (Not Modified), the cache uses the entity-body
1193   stored in the cache entry as the entity-body of this outgoing
1194   response. If the status code is 206 (Partial Content) and the ETag or
1195   Last-Modified headers match exactly, the cache &MAY; combine the
1196   contents stored in the cache entry with the new contents received in
1197   the response and use the result as the entity-body of this outgoing
1198   response, (see &combining-byte-ranges;).
1199</t>
1200<t>
1201   The end-to-end headers stored in the cache entry are used for the
1202   constructed response, except that
1203  <list style="symbols">
1204    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1205      &MUST; be deleted from the cache entry and the forwarded response.</t>
1206    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1207        in the cache entry and the forwarded response.</t>
1208    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1209        replace the corresponding headers from the cache entry.</t>
1210  </list>
1211</t>
1212<t>
1213   Unless the cache decides to remove the cache entry, it &MUST; also
1214   replace the end-to-end headers stored with the cache entry with
1215   corresponding headers received in the incoming response, except for
1216   Warning headers as described immediately above. If a header field-name
1217   in the incoming response matches more than one header in the
1218   cache entry, all such old headers &MUST; be replaced.
1219</t>
1220<t>
1221   In other words, the set of end-to-end headers received in the
1222   incoming response overrides all corresponding end-to-end headers
1223   stored with the cache entry (except for stored Warning headers with
1224   warn-code 1xx, which are deleted even if not overridden).
1225  <list><t>
1226      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1227      Modified) or a 206 (Partial Content) response to update any header
1228      associated with a previous response for the same entity or sub-ranges
1229      thereof, although it might not always be meaningful or
1230      correct to do so. This rule does not allow an origin server to use
1231      a 304 (Not Modified) or a 206 (Partial Content) response to
1232      entirely delete a header that it had provided with a previous
1233      response.
1234  </t></list>
1235</t>
1236</section>
1237
1238</section>
1239
1240<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1241<t>
1242   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1243   by the presence of a Vary header field in a response, alters the
1244   conditions and procedure by which a cache can use the response for
1245   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1246   field by servers.
1247</t>
1248<t>
1249   A server &SHOULD; use the Vary header field to inform a cache of what
1250   request-header fields were used to select among multiple
1251   representations of a cacheable response subject to server-driven
1252   negotiation. The set of header fields named by the Vary field value
1253   is known as the "selecting" request-headers.
1254</t>
1255<t>
1256   When the cache receives a subsequent request whose Request-URI
1257   specifies one or more cache entries including a Vary header field,
1258   the cache &MUST-NOT; use such a cache entry to construct a response to
1259   the new request unless all of the selecting request-headers present
1260   in the new request match the corresponding stored request-headers in
1261   the original request.
1262</t>
1263<t>
1264   The selecting request-headers from two requests are defined to match
1265   if and only if the selecting request-headers in the first request can
1266   be transformed to the selecting request-headers in the second request
1267   by adding or removing linear white space (LWS) at places where this
1268   is allowed by the corresponding BNF, and/or combining multiple
1269   message-header fields with the same field name following the rules
1270   about message headers in &message-headers;.
1271</t>
1272<t>
1273   A Vary header field-value of "*" always fails to match and subsequent
1274   requests on that resource can only be properly interpreted by the
1275   origin server.
1276</t>
1277<t>
1278   If the selecting request header fields for the cached entry do not
1279   match the selecting request header fields of the new request, then
1280   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1281   it first relays the new request to the origin server in a conditional
1282   request and the server responds with 304 (Not Modified), including an
1283   entity tag or Content-Location that indicates the entity to be used.
1284</t>
1285<t>
1286   If an entity tag was assigned to a cached representation, the
1287   forwarded request &SHOULD; be conditional and include the entity tags
1288   in an If-None-Match header field from all its cache entries for the
1289   resource. This conveys to the server the set of entities currently
1290   held by the cache, so that if any one of these entities matches the
1291   requested entity, the server can use the ETag header field in its 304
1292   (Not Modified) response to tell the cache which entry is appropriate.
1293   If the entity-tag of the new response matches that of an existing
1294   entry, the new response &SHOULD; be used to update the header fields of
1295   the existing entry, and the result &MUST; be returned to the client.
1296</t>
1297<t>
1298   If any of the existing cache entries contains only partial content
1299   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1300   the If-None-Match header field unless the request is for a range that
1301   would be fully satisfied by that entry.
1302</t>
1303<t>
1304   If a cache receives a successful response whose Content-Location
1305   field matches that of an existing cache entry for the same Request-URI,
1306   whose entity-tag differs from that of the existing entry, and
1307   whose Date is more recent than that of the existing entry, the
1308   existing entry &SHOULD-NOT;  be returned in response to future requests
1309   and &SHOULD; be deleted from the cache.
1310</t>
1311</section>
1312
1313<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1314<t>
1315   For reasons of security and privacy, it is necessary to make a
1316   distinction between "shared" and "non-shared" caches. A non-shared
1317   cache is one that is accessible only to a single user. Accessibility
1318   in this case &SHOULD; be enforced by appropriate security mechanisms.
1319   All other caches are considered to be "shared." Other sections of
1320   this specification place certain constraints on the operation of
1321   shared caches in order to prevent loss of privacy or failure of
1322   access controls.
1323</t>
1324</section>
1325
1326<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1327<t>
1328   A cache that receives an incomplete response (for example, with fewer
1329   bytes of data than specified in a Content-Length header) &MAY; store
1330   the response. However, the cache &MUST; treat this as a partial
1331   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1332   the result might be a full response or might still be
1333   partial. A cache &MUST-NOT; return a partial response to a client
1334   without explicitly marking it as such, using the 206 (Partial
1335   Content) status code. A cache &MUST-NOT; return a partial response
1336   using a status code of 200 (OK).
1337</t>
1338<t>
1339   If a cache receives a 5xx response while attempting to revalidate an
1340   entry, it &MAY; either forward this response to the requesting client,
1341   or act as if the server failed to respond. In the latter case, it &MAY;
1342   return a previously received response unless the cached entry
1343   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1344</t>
1345</section>
1346
1347<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1348<t>
1349   Unless the origin server explicitly prohibits the caching of their
1350   responses, the application of GET and HEAD methods to any resources
1351   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1352   these responses are taken from a cache. They &MAY; still have side
1353   effects, but a cache is not required to consider such side effects in
1354   its caching decisions. Caches are always expected to observe an
1355   origin server's explicit restrictions on caching.
1356</t>
1357<t>
1358   We note one exception to this rule: since some applications have
1359   traditionally used GETs and HEADs with query URLs (those containing a
1360   "?" in the rel_path part) to perform operations with significant side
1361   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1362   the server provides an explicit expiration time. This specifically
1363   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1364   be taken from a cache. See &safe-methods; for related information.
1365</t>
1366</section>
1367
1368<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1369<t>
1370   The effect of certain methods performed on a resource at the origin
1371   server might cause one or more existing cache entries to become non-transparently
1372   invalid. That is, although they might continue to be
1373   "fresh," they do not accurately reflect what the origin server would
1374   return for a new request on that resource.
1375</t>
1376<t>
1377   There is no way for the HTTP protocol to guarantee that all such
1378   cache entries are marked invalid. For example, the request that
1379   caused the change at the origin server might not have gone through
1380   the proxy where a cache entry is stored. However, several rules help
1381   reduce the likelihood of erroneous behavior.
1382</t>
1383<t>
1384   In this section, the phrase "invalidate an entity" means that the
1385   cache will either remove all instances of that entity from its
1386   storage, or will mark these as "invalid" and in need of a mandatory
1387   revalidation before they can be returned in response to a subsequent
1388   request.
1389</t>
1390<t>
1391   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1392   either the entity referred to by the Request-URI, or by the Location
1393   or Content-Location headers (if present). These methods are:
1394  <list style="symbols">
1395      <t>PUT</t>
1396      <t>DELETE</t>
1397      <t>POST</t>
1398  </list>
1399</t> 
1400<t>
1401   In order to prevent denial of service attacks, an invalidation based
1402   on the URI in a Location or Content-Location header &MUST; only be
1403   performed if the host part is the same as in the Request-URI.
1404</t>
1405<t>
1406   A cache that passes through requests for methods it does not
1407   understand &SHOULD; invalidate any entities referred to by the
1408   Request-URI.
1409</t>
1410</section>
1411
1412<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1413<t>
1414   All methods that might be expected to cause modifications to the
1415   origin server's resources &MUST; be written through to the origin
1416   server. This currently includes all methods except for GET and HEAD.
1417   A cache &MUST-NOT; reply to such a request from a client before having
1418   transmitted the request to the inbound server, and having received a
1419   corresponding response from the inbound server. This does not prevent
1420   a proxy cache from sending a 100 (Continue) response before the
1421   inbound server has sent its final reply.
1422</t>
1423<t>
1424   The alternative (known as "write-back" or "copy-back" caching) is not
1425   allowed in HTTP/1.1, due to the difficulty of providing consistent
1426   updates and the problems arising from server, cache, or network
1427   failure prior to write-back.
1428</t>
1429</section>
1430
1431<section title="Cache Replacement" anchor="cache.replacement">
1432<t>
1433   If a new cacheable (see sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1434   <xref target="disambiguating.expiration.values" format="counter"/>,
1435   <xref target="disambiguating.multiple.responses" format="counter"/>
1436   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1437   response is received from a resource while any existing responses for
1438   the same resource are cached, the cache &SHOULD; use the new response
1439   to reply to the current request. It &MAY; insert it into cache storage
1440   and &MAY;, if it meets all other requirements, use it to respond to any
1441   future requests that would previously have caused the old response to
1442   be returned. If it inserts the new response into cache storage  the
1443   rules in <xref target="combining.headers"/> apply.
1444  <list><t>
1445      <x:h>Note:</x:h> a new response that has an older Date header value than
1446      existing cached responses is not cacheable.
1447  </t></list>
1448</t>
1449</section>
1450
1451<section title="History Lists" anchor="history.lists">
1452<t>
1453   User agents often have history mechanisms, such as "Back" buttons and
1454   history lists, which can be used to redisplay an entity retrieved
1455   earlier in a session.
1456</t>
1457<t>
1458   History mechanisms and caches are different. In particular history
1459   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1460   the current state of a resource. Rather, a history mechanism is meant
1461   to show exactly what the user saw at the time when the resource was
1462   retrieved.
1463</t>
1464<t>
1465   By default, an expiration time does not apply to history mechanisms.
1466   If the entity is still in storage, a history mechanism &SHOULD; display
1467   it even if the entity has expired, unless the user has specifically
1468   configured the agent to refresh expired history documents.
1469</t>
1470<t>
1471   This is not to be construed to prohibit the history mechanism from
1472   telling the user that a view might be stale.
1473  <list><t>
1474      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1475      viewing stale resources, this will tend to force service authors
1476      to avoid using HTTP expiration controls and cache controls when
1477      they would otherwise like to. Service authors may consider it
1478      important that users not be presented with error messages or
1479      warning messages when they use navigation controls (such as BACK)
1480      to view previously fetched resources. Even though sometimes such
1481      resources ought not to cached, or ought to expire quickly, user
1482      interface considerations may force service authors to resort to
1483      other means of preventing caching (e.g. "once-only" URLs) in order
1484      not to suffer the effects of improperly functioning history
1485      mechanisms.
1486  </t></list>
1487</t>
1488</section>
1489</section>
1490
1491<section title="Header Field Definitions" anchor="header.fields">
1492<t>
1493   This section defines the syntax and semantics of all standard
1494   HTTP/1.1 header fields. For entity-header fields, both sender and
1495   recipient refer to either the client or the server, depending on who
1496   sends and who receives the entity.
1497</t>
1498
1499<section title="Age" anchor="header.age">
1500  <iref primary="true" item="Age header" x:for-anchor=""/>
1501  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1502<t>
1503      The Age response-header field conveys the sender's estimate of the
1504      amount of time since the response (or its revalidation) was
1505      generated at the origin server. A cached response is "fresh" if
1506      its age does not exceed its freshness lifetime. Age values are
1507      calculated as specified in <xref target="age.calculations"/>.
1508</t>
1509<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1510        Age = "Age" ":" age-value
1511        age-value = delta-seconds
1512</artwork></figure>
1513<t>
1514      Age values are non-negative decimal integers, representing time in
1515      seconds.
1516</t>
1517<t>
1518      If a cache receives a value larger than the largest positive
1519      integer it can represent, or if any of its age calculations
1520      overflows, it &MUST; transmit an Age header with a value of
1521      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1522      include an Age header field in every response generated from its
1523      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1524      bits of range.
1525</t>
1526</section>
1527
1528<section title="Cache-Control" anchor="header.cache-control">
1529  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1530  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1531<t>
1532   The Cache-Control general-header field is used to specify directives
1533   that &MUST; be obeyed by all caching mechanisms along the
1534   request/response chain. The directives specify behavior intended to
1535   prevent caches from adversely interfering with the request or
1536   response. These directives typically override the default caching
1537   algorithms. Cache directives are unidirectional in that the presence
1538   of a directive in a request does not imply that the same directive is
1539   to be given in the response.
1540  <list><t>
1541      Note that HTTP/1.0 caches might not implement Cache-Control and
1542      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1543  </t></list>
1544</t>
1545<t>
1546   Cache directives &MUST; be passed through by a proxy or gateway
1547   application, regardless of their significance to that application,
1548   since the directives might be applicable to all recipients along the
1549   request/response chain. It is not possible to specify a cache-directive
1550   for a specific cache.
1551</t>
1552<figure><artwork type="abnf2616"><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"/>
1553   Cache-Control   = "Cache-Control" ":" 1#cache-directive
1554
1555   cache-directive = cache-request-directive
1556        | cache-response-directive
1557
1558   cache-request-directive =
1559          "no-cache"                          ; <xref target="what.is.cacheable"/>
1560        | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1561        | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1562        | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1563        | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1564        | "no-transform"                      ; <xref target="no-transform.directive"/>
1565        | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1566        | cache-extension                     ; <xref target="cache.control.extensions"/>
1567
1568    cache-response-directive =
1569          "public"                               ; <xref target="what.is.cacheable"/>
1570        | "private" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ] ; <xref target="what.is.cacheable"/>
1571        | "no-cache" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ]; <xref target="what.is.cacheable"/>
1572        | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1573        | "no-transform"                         ; <xref target="no-transform.directive"/>
1574        | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1575        | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1576        | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1577        | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1578        | cache-extension                        ; <xref target="cache.control.extensions"/>
1579
1580   cache-extension = token [ "=" ( token | quoted-string ) ]
1581</artwork></figure>
1582<t>
1583   When a directive appears without any 1#field-name parameter, the
1584   directive applies to the entire request or response. When such a
1585   directive appears with a 1#field-name parameter, it applies only to
1586   the named field or fields, and not to the rest of the request or
1587   response. This mechanism supports extensibility; implementations of
1588   future versions of the HTTP protocol might apply these directives to
1589   header fields not defined in HTTP/1.1.
1590</t>
1591<t>
1592   The cache-control directives can be broken down into these general
1593   categories:
1594  <list style="symbols">
1595     <t>Restrictions on what are cacheable; these may only be imposed by
1596        the origin server.</t>
1597
1598     <t>Restrictions on what may be stored by a cache; these may be
1599        imposed by either the origin server or the user agent.</t>
1600
1601     <t>Modifications of the basic expiration mechanism; these may be
1602        imposed by either the origin server or the user agent.</t>
1603
1604     <t>Controls over cache revalidation and reload; these may only be
1605        imposed by a user agent.</t>
1606
1607     <t>Control over transformation of entities.</t>
1608
1609     <t>Extensions to the caching system.</t>
1610  </list>
1611</t>
1612
1613<section title="What is Cacheable" anchor="what.is.cacheable">
1614<t>
1615   By default, a response is cacheable if the requirements of the
1616   request method, request header fields, and the response status
1617   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1618   for cacheability. The following Cache-Control response directives
1619   allow an origin server to override the default cacheability of a
1620   response:
1621</t>
1622<t>
1623  <iref item="Cache Directives" subitem="public" primary="true"/>
1624  <iref item="public" subitem="Cache Directive" primary="true"/>
1625   public
1626  <list><t>
1627      Indicates that the response &MAY; be cached by any cache, even if it
1628      would normally be non-cacheable or cacheable only within a non-shared
1629      cache. (See also Authorization, &header-authorization;, for
1630      additional details.)
1631  </t></list>
1632</t>
1633<t>
1634  <iref item="Cache Directives" subitem="private" primary="true"/>
1635  <iref item="private" subitem="Cache Directive" primary="true"/>
1636   private
1637  <list><t>
1638      Indicates that all or part of the response message is intended for
1639      a single user and &MUST-NOT; be cached by a shared cache. This
1640      allows an origin server to state that the specified parts of the
1641      response are intended for only one user and are not a valid
1642      response for requests by other users. A private (non-shared) cache
1643      &MAY; cache the response.
1644    </t><t>
1645       <x:h>Note:</x:h> This usage of the word private only controls where the
1646       response may be cached, and cannot ensure the privacy of the
1647       message content.
1648  </t></list>
1649</t>
1650<t>
1651  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1652  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1653   no-cache
1654  <list><t>
1655       If the no-cache directive does not specify a field-name, then a
1656      cache &MUST-NOT; use the response to satisfy a subsequent request
1657      without successful revalidation with the origin server. This
1658      allows an origin server to prevent caching even by caches that
1659      have been configured to return stale responses to client requests.
1660    </t><t>
1661      If the no-cache directive does specify one or more field-names,
1662      then a cache &MAY; use the response to satisfy a subsequent request,
1663      subject to any other restrictions on caching. However, the
1664      specified field-name(s) &MUST-NOT; be sent in the response to a
1665      subsequent request without successful revalidation with the origin
1666      server. This allows an origin server to prevent the re-use of
1667      certain header fields in a response, while still allowing caching
1668      of the rest of the response.
1669    <list><t>
1670       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1671       directive.
1672    </t></list>
1673  </t></list>
1674</t>
1675</section>
1676
1677<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1678<t>
1679  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1680  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1681   no-store
1682  <list><t>   
1683      The purpose of the no-store directive is to prevent the
1684      inadvertent release or retention of sensitive information (for
1685      example, on backup tapes). The no-store directive applies to the
1686      entire message, and &MAY; be sent either in a response or in a
1687      request. If sent in a request, a cache &MUST-NOT; store any part of
1688      either this request or any response to it. If sent in a response,
1689      a cache &MUST-NOT; store any part of either this response or the
1690      request that elicited it. This directive applies to both non-shared
1691      and shared caches. "&MUST-NOT; store" in this context means
1692      that the cache &MUST-NOT; intentionally store the information in
1693      non-volatile storage, and &MUST; make a best-effort attempt to
1694      remove the information from volatile storage as promptly as
1695      possible after forwarding it.
1696  </t><t>
1697      Even when this directive is associated with a response, users
1698      might explicitly store such a response outside of the caching
1699      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1700      such responses as part of their normal operation.
1701  </t><t>
1702      The purpose of this directive is to meet the stated requirements
1703      of certain users and service authors who are concerned about
1704      accidental releases of information via unanticipated accesses to
1705      cache data structures. While the use of this directive might
1706      improve privacy in some cases, we caution that it is NOT in any
1707      way a reliable or sufficient mechanism for ensuring privacy. In
1708      particular, malicious or compromised caches might not recognize or
1709      obey this directive, and communications networks might be
1710      vulnerable to eavesdropping.
1711  </t></list>
1712</t>
1713</section>
1714
1715<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1716<t>
1717   The expiration time of an entity &MAY; be specified by the origin
1718   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1719   it &MAY; be specified using the max-age directive in a response. When
1720   the max-age cache-control directive is present in a cached response,
1721   the response is stale if its current age is greater than the age
1722   value given (in seconds) at the time of a new request for that
1723   resource. The max-age directive on a response implies that the
1724   response is cacheable (i.e., "public") unless some other, more
1725   restrictive cache directive is also present.
1726</t>
1727<t>
1728   If a response includes both an Expires header and a max-age
1729   directive, the max-age directive overrides the Expires header, even
1730   if the Expires header is more restrictive. This rule allows an origin
1731   server to provide, for a given response, a longer expiration time to
1732   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1733   useful if certain HTTP/1.0 caches improperly calculate ages or
1734   expiration times, perhaps due to desynchronized clocks.
1735</t>
1736<t>
1737   Many HTTP/1.0 cache implementations will treat an Expires value that
1738   is less than or equal to the response Date value as being equivalent
1739   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1740   cache receives such a response, and the response does not include a
1741   Cache-Control header field, it &SHOULD; consider the response to be
1742   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1743  <list><t>
1744       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1745       cache control feature, such as the "private" directive, on a
1746       network including older caches that do not understand that
1747       feature. The origin server will need to combine the new feature
1748       with an Expires field whose value is less than or equal to the
1749       Date value. This will prevent older caches from improperly
1750       caching the response.
1751  </t></list>
1752</t>
1753<t>
1754  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1755  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1756   s-maxage
1757  <list><t>
1758       If a response includes an s-maxage directive, then for a shared
1759       cache (but not for a private cache), the maximum age specified by
1760       this directive overrides the maximum age specified by either the
1761       max-age directive or the Expires header. The s-maxage directive
1762       also implies the semantics of the proxy-revalidate directive (see
1763       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1764       entry after it becomes stale to respond to a subsequent request
1765       without first revalidating it with the origin server. The s-maxage
1766       directive is always ignored by a private cache.
1767  </t></list>
1768</t>
1769<t>
1770   Note that most older caches, not compliant with this specification,
1771   do not implement any cache-control directives. An origin server
1772   wishing to use a cache-control directive that restricts, but does not
1773   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1774   requirement that the max-age directive overrides the Expires header,
1775   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1776   max-age directive.
1777</t>
1778<t>
1779   Other directives allow a user agent to modify the basic expiration
1780   mechanism. These directives &MAY; be specified on a request:
1781</t>
1782<t>
1783  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1784  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1785   max-age
1786  <list><t>
1787      Indicates that the client is willing to accept a response whose
1788      age is no greater than the specified time in seconds. Unless max-stale
1789      directive is also included, the client is not willing to
1790      accept a stale response.
1791  </t></list>
1792</t>
1793<t>
1794  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1795  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1796   min-fresh
1797  <list><t>
1798      Indicates that the client is willing to accept a response whose
1799      freshness lifetime is no less than its current age plus the
1800      specified time in seconds. That is, the client wants a response
1801      that will still be fresh for at least the specified number of
1802      seconds.
1803  </t></list>
1804</t>
1805<t>
1806  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1807  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1808   max-stale
1809  <list><t>
1810      Indicates that the client is willing to accept a response that has
1811      exceeded its expiration time. If max-stale is assigned a value,
1812      then the client is willing to accept a response that has exceeded
1813      its expiration time by no more than the specified number of
1814      seconds. If no value is assigned to max-stale, then the client is
1815      willing to accept a stale response of any age.
1816  </t></list>
1817</t>
1818<t>
1819   If a cache returns a stale response, either because of a max-stale
1820   directive on a request, or because the cache is configured to
1821   override the expiration time of a response, the cache &MUST; attach a
1822   Warning header to the stale response, using Warning 110 (Response is
1823   stale).
1824</t>
1825<t>
1826   A cache &MAY; be configured to return stale responses without
1827   validation, but only if this does not conflict with any "MUST"-level
1828   requirements concerning cache validation (e.g., a "must-revalidate"
1829   cache-control directive).
1830</t>
1831<t>
1832   If both the new request and the cached entry include "max-age"
1833   directives, then the lesser of the two values is used for determining
1834   the freshness of the cached entry for that request.
1835</t>
1836</section>
1837
1838<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1839<t>
1840   Sometimes a user agent might want or need to insist that a cache
1841   revalidate its cache entry with the origin server (and not just with
1842   the next cache along the path to the origin server), or to reload its
1843   cache entry from the origin server. End-to-end revalidation might be
1844   necessary if either the cache or the origin server has overestimated
1845   the expiration time of the cached response. End-to-end reload may be
1846   necessary if the cache entry has become corrupted for some reason.
1847</t>
1848<t>
1849   End-to-end revalidation may be requested either when the client does
1850   not have its own local cached copy, in which case we call it
1851   "unspecified end-to-end revalidation", or when the client does have a
1852   local cached copy, in which case we call it "specific end-to-end
1853   revalidation."
1854</t>
1855<t>
1856   The client can specify these three kinds of action using Cache-Control
1857   request directives:
1858</t>
1859<t>
1860   End-to-end reload
1861  <list><t>
1862      The request includes a "no-cache" cache-control directive or, for
1863      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1864      names &MUST-NOT; be included with the no-cache directive in a
1865      request. The server &MUST-NOT; use a cached copy when responding to
1866      such a request.
1867  </t></list>
1868</t>
1869<t>
1870   Specific end-to-end revalidation
1871  <list><t>
1872      The request includes a "max-age=0" cache-control directive, which
1873      forces each cache along the path to the origin server to
1874      revalidate its own entry, if any, with the next cache or server.
1875      The initial request includes a cache-validating conditional with
1876      the client's current validator.
1877  </t></list>
1878</t>
1879<t>
1880   Unspecified end-to-end revalidation
1881  <list><t>
1882      The request includes "max-age=0" cache-control directive, which
1883      forces each cache along the path to the origin server to
1884      revalidate its own entry, if any, with the next cache or server.
1885      The initial request does not include a cache-validating
1886      conditional; the first cache along the path (if any) that holds a
1887      cache entry for this resource includes a cache-validating
1888      conditional with its current validator.
1889  </t></list>
1890</t>
1891<t>
1892  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1893  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1894   max-age
1895  <list><t>
1896      When an intermediate cache is forced, by means of a max-age=0
1897      directive, to revalidate its own cache entry, and the client has
1898      supplied its own validator in the request, the supplied validator
1899      might differ from the validator currently stored with the cache
1900      entry. In this case, the cache &MAY; use either validator in making
1901      its own request without affecting semantic transparency.
1902  </t><t>
1903      However, the choice of validator might affect performance. The
1904      best approach is for the intermediate cache to use its own
1905      validator when making its request. If the server replies with 304
1906      (Not Modified), then the cache can return its now validated copy
1907      to the client with a 200 (OK) response. If the server replies with
1908      a new entity and cache validator, however, the intermediate cache
1909      can compare the returned validator with the one provided in the
1910      client's request, using the strong comparison function. If the
1911      client's validator is equal to the origin server's, then the
1912      intermediate cache simply returns 304 (Not Modified). Otherwise,
1913      it returns the new entity with a 200 (OK) response.
1914  </t><t>
1915      If a request includes the no-cache directive, it &SHOULD-NOT; 
1916      include min-fresh, max-stale, or max-age.
1917  </t></list>
1918</t>
1919<t>
1920  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1921  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1922   only-if-cached
1923  <list><t>
1924      In some cases, such as times of extremely poor network
1925      connectivity, a client may want a cache to return only those
1926      responses that it currently has stored, and not to reload or
1927      revalidate with the origin server. To do this, the client may
1928      include the only-if-cached directive in a request. If it receives
1929      this directive, a cache &SHOULD; either respond using a cached entry
1930      that is consistent with the other constraints of the request, or
1931      respond with a 504 (Gateway Timeout) status. However, if a group
1932      of caches is being operated as a unified system with good internal
1933      connectivity, such a request &MAY; be forwarded within that group of
1934      caches.
1935  </t></list>
1936</t>
1937<t>
1938  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1939  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1940   must-revalidate
1941  <list><t>
1942      Because a cache &MAY; be configured to ignore a server's specified
1943      expiration time, and because a client request &MAY; include a max-stale
1944      directive (which has a similar effect), the protocol also
1945      includes a mechanism for the origin server to require revalidation
1946      of a cache entry on any subsequent use. When the must-revalidate
1947      directive is present in a response received by a cache, that cache
1948      &MUST-NOT; use the entry after it becomes stale to respond to a
1949      subsequent request without first revalidating it with the origin
1950      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1951      time, if, based solely on the origin server's Expires or max-age
1952      value, the cached response is stale.)
1953  </t><t>
1954      The must-revalidate directive is necessary to support reliable
1955      operation for certain protocol features. In all circumstances an
1956      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1957      particular, if the cache cannot reach the origin server for any
1958      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1959  </t><t>
1960      Servers &SHOULD; send the must-revalidate directive if and only if
1961      failure to revalidate a request on the entity could result in
1962      incorrect operation, such as a silently unexecuted financial
1963      transaction. Recipients &MUST-NOT; take any automated action that
1964      violates this directive, and &MUST-NOT; automatically provide an
1965      unvalidated copy of the entity if revalidation fails.
1966  </t><t>
1967      Although this is not recommended, user agents operating under
1968      severe connectivity constraints &MAY; violate this directive but, if
1969      so, &MUST; explicitly warn the user that an unvalidated response has
1970      been provided. The warning &MUST; be provided on each unvalidated
1971      access, and &SHOULD; require explicit user confirmation.
1972  </t></list>
1973</t>
1974<t>
1975  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1976  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1977   proxy-revalidate
1978  <list><t>
1979      The proxy-revalidate directive has the same meaning as the must-revalidate
1980      directive, except that it does not apply to non-shared
1981      user agent caches. It can be used on a response to an
1982      authenticated request to permit the user's cache to store and
1983      later return the response without needing to revalidate it (since
1984      it has already been authenticated once by that user), while still
1985      requiring proxies that service many users to revalidate each time
1986      (in order to make sure that each user has been authenticated).
1987      Note that such authenticated responses also need the public cache
1988      control directive in order to allow them to be cached at all.
1989  </t></list>
1990</t>
1991</section>
1992
1993<section title="No-Transform Directive" anchor="no-transform.directive">
1994<t>
1995  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
1996  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
1997   no-transform
1998  <list><t>
1999      Implementors of intermediate caches (proxies) have found it useful
2000      to convert the media type of certain entity bodies. A non-transparent
2001      proxy might, for example, convert between image
2002      formats in order to save cache space or to reduce the amount of
2003      traffic on a slow link.
2004  </t><t>
2005      Serious operational problems occur, however, when these
2006      transformations are applied to entity bodies intended for certain
2007      kinds of applications. For example, applications for medical
2008      imaging, scientific data analysis and those using end-to-end
2009      authentication, all depend on receiving an entity body that is bit
2010      for bit identical to the original entity-body.
2011  </t><t>
2012      Therefore, if a message includes the no-transform directive, an
2013      intermediate cache or proxy &MUST-NOT; change those headers that are
2014      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2015      directive. This implies that the cache or proxy &MUST-NOT; change
2016      any aspect of the entity-body that is specified by these headers,
2017      including the value of the entity-body itself.
2018  </t></list>
2019</t>
2020</section>
2021
2022<section title="Cache Control Extensions" anchor="cache.control.extensions">
2023<t>
2024   The Cache-Control header field can be extended through the use of one
2025   or more cache-extension tokens, each with an optional assigned value.
2026   Informational extensions (those which do not require a change in
2027   cache behavior) &MAY; be added without changing the semantics of other
2028   directives. Behavioral extensions are designed to work by acting as
2029   modifiers to the existing base of cache directives. Both the new
2030   directive and the standard directive are supplied, such that
2031   applications which do not understand the new directive will default
2032   to the behavior specified by the standard directive, and those that
2033   understand the new directive will recognize it as modifying the
2034   requirements associated with the standard directive. In this way,
2035   extensions to the cache-control directives can be made without
2036   requiring changes to the base protocol.
2037</t>
2038<t>
2039   This extension mechanism depends on an HTTP cache obeying all of the
2040   cache-control directives defined for its native HTTP-version, obeying
2041   certain extensions, and ignoring all directives that it does not
2042   understand.
2043</t>
2044<t>
2045   For example, consider a hypothetical new response directive called
2046   community which acts as a modifier to the private directive. We
2047   define this new directive to mean that, in addition to any non-shared
2048   cache, any cache which is shared only by members of the community
2049   named within its value may cache the response. An origin server
2050   wishing to allow the UCI community to use an otherwise private
2051   response in their shared cache(s) could do so by including
2052</t>
2053<figure><artwork type="example">
2054    Cache-Control: private, community="UCI"
2055</artwork></figure>
2056<t>
2057   A cache seeing this header field will act correctly even if the cache
2058   does not understand the community cache-extension, since it will also
2059   see and understand the private directive and thus default to the safe
2060   behavior.
2061</t>
2062<t>
2063   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2064   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2065   be combined with standard directives (or the response's default
2066   cacheability) such that the cache behavior will remain minimally
2067   correct even if the cache does not understand the extension(s).
2068</t>
2069</section>
2070</section>
2071
2072<section title="Expires" anchor="header.expires">
2073  <iref primary="true" item="Expires header" x:for-anchor=""/>
2074  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2075<t>
2076   The Expires entity-header field gives the date/time after which the
2077   response is considered stale. A stale cache entry may not normally be
2078   returned by a cache (either a proxy cache or a user agent cache)
2079   unless it is first validated with the origin server (or with an
2080   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2081   for further discussion of the expiration model.
2082</t>
2083<t>
2084   The presence of an Expires field does not imply that the original
2085   resource will change or cease to exist at, before, or after that
2086   time.
2087</t>
2088<t>
2089   The format is an absolute date and time as defined by HTTP-date in
2090   &full-date;; it &MUST; be in RFC 1123 date format:
2091</t>
2092<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2093   Expires = "Expires" ":" HTTP-date
2094</artwork></figure>
2095<t>
2096   An example of its use is
2097</t>
2098<figure><artwork type="example">
2099   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2100</artwork></figure>
2101<t>
2102  <list><t>
2103      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2104      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2105      Expires field.
2106  </t></list>
2107</t>
2108<t>
2109   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2110   especially including the value "0", as in the past (i.e., "already
2111   expired").
2112</t>
2113<t>
2114   To mark a response as "already expired," an origin server sends an
2115   Expires date that is equal to the Date header value. (See the rules
2116   for expiration calculations in <xref target="expiration.calculations"/>.)
2117</t>
2118<t>
2119   To mark a response as "never expires," an origin server sends an
2120   Expires date approximately one year from the time the response is
2121   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2122   year in the future.
2123</t>
2124<t>
2125   The presence of an Expires header field with a date value of some
2126   time in the future on a response that otherwise would by default be
2127   non-cacheable indicates that the response is cacheable, unless
2128   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2129</t>
2130</section>
2131
2132<section title="Pragma" anchor="header.pragma">
2133  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2134  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2135<t>
2136   The Pragma general-header field is used to include implementation-specific
2137   directives that might apply to any recipient along the
2138   request/response chain. All pragma directives specify optional
2139   behavior from the viewpoint of the protocol; however, some systems
2140   &MAY; require that behavior be consistent with the directives.
2141</t>
2142<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
2143    Pragma            = "Pragma" ":" 1#pragma-directive
2144    pragma-directive  = "no-cache" | extension-pragma
2145    extension-pragma  = token [ "=" ( token | quoted-string ) ]
2146</artwork></figure>
2147<t>
2148   When the no-cache directive is present in a request message, an
2149   application &SHOULD; forward the request toward the origin server even
2150   if it has a cached copy of what is being requested. This pragma
2151   directive has the same semantics as the no-cache cache-directive (see
2152   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2153   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2154   request is sent to a server not known to be HTTP/1.1 compliant.
2155</t>
2156<t>
2157   Pragma directives &MUST; be passed through by a proxy or gateway
2158   application, regardless of their significance to that application,
2159   since the directives might be applicable to all recipients along the
2160   request/response chain. It is not possible to specify a pragma for a
2161   specific recipient; however, any pragma directive not relevant to a
2162   recipient &SHOULD; be ignored by that recipient.
2163</t>
2164<t>
2165   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2166   sent "Cache-Control: no-cache". No new Pragma directives will be
2167   defined in HTTP.
2168  <list><t>
2169      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache as a response
2170      header field is not actually specified, it does not provide a
2171      reliable replacement for "Cache-Control: no-cache" in a response
2172  </t></list>
2173</t>
2174</section>
2175
2176<section title="Vary" anchor="header.vary">
2177  <iref primary="true" item="Vary header" x:for-anchor=""/>
2178  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2179<t>
2180   The Vary field value indicates the set of request-header fields that
2181   fully determines, while the response is fresh, whether a cache is
2182   permitted to use the response to reply to a subsequent request
2183   without revalidation. For uncacheable or stale responses, the Vary
2184   field value advises the user agent about the criteria that were used
2185   to select the representation. A Vary field value of "*" implies that
2186   a cache cannot determine from the request headers of a subsequent
2187   request whether this response is the appropriate representation. See
2188   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2189</t>
2190<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2191    Vary  = "Vary" ":" ( "*" | 1#field-name )
2192</artwork></figure>
2193<t>
2194   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2195   cacheable response that is subject to server-driven negotiation.
2196   Doing so allows a cache to properly interpret future requests on that
2197   resource and informs the user agent about the presence of negotiation
2198   on that resource. A server &MAY; include a Vary header field with a
2199   non-cacheable response that is subject to server-driven negotiation,
2200   since this might provide the user agent with useful information about
2201   the dimensions over which the response varies at the time of the
2202   response.
2203</t>
2204<t>
2205   A Vary field value consisting of a list of field-names signals that
2206   the representation selected for the response is based on a selection
2207   algorithm which considers ONLY the listed request-header field values
2208   in selecting the most appropriate representation. A cache &MAY; assume
2209   that the same selection will be made for future requests with the
2210   same values for the listed field names, for the duration of time for
2211   which the response is fresh.
2212</t>
2213<t>
2214   The field-names given are not limited to the set of standard
2215   request-header fields defined by this specification. Field names are
2216   case-insensitive.
2217</t>
2218<t>
2219   A Vary field value of "*" signals that unspecified parameters not
2220   limited to the request-headers (e.g., the network address of the
2221   client), play a role in the selection of the response representation.
2222   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2223   generated by an origin server.
2224</t>
2225</section>
2226
2227<section title="Warning" anchor="header.warning">
2228  <iref primary="true" item="Warning header" x:for-anchor=""/>
2229  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2230<t>
2231   The Warning general-header field is used to carry additional
2232   information about the status or transformation of a message which
2233   might not be reflected in the message. This information is typically
2234   used to warn about a possible lack of semantic transparency from
2235   caching operations or transformations applied to the entity body of
2236   the message.
2237</t>
2238<t>
2239   Warning headers are sent with responses using:
2240</t>
2241<figure><artwork type="abnf2616"><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"/>
2242    Warning    = "Warning" ":" 1#warning-value
2243
2244    warning-value = warn-code SP warn-agent SP warn-text
2245                                          [SP warn-date]
2246
2247    warn-code  = 3DIGIT
2248    warn-agent = ( host [ ":" port ] ) | pseudonym
2249                    ; the name or pseudonym of the server adding
2250                    ; the Warning header, for use in debugging
2251    warn-text  = quoted-string
2252    warn-date  = &lt;"&gt; HTTP-date &lt;"&gt;
2253</artwork></figure>
2254<t>
2255   A response &MAY; carry more than one Warning header.
2256</t>
2257<t>
2258   The warn-text &SHOULD; be in a natural language and character set that
2259   is most likely to be intelligible to the human user receiving the
2260   response. This decision &MAY; be based on any available knowledge, such
2261   as the location of the cache or user, the Accept-Language field in a
2262   request, the Content-Language field in a response, etc. The default
2263   language is English and the default character set is ISO-8859-1.
2264</t>
2265<t>
2266   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2267   in the warn-text using the method described in RFC 2047 <xref target="RFC2047"/>.
2268</t>
2269<t>
2270   Warning headers can in general be applied to any message, however
2271   some specific warn-codes are specific to caches and can only be
2272   applied to response messages. New Warning headers &SHOULD; be added
2273   after any existing Warning headers. A cache &MUST-NOT; delete any
2274   Warning header that it received with a message. However, if a cache
2275   successfully validates a cache entry, it &SHOULD; remove any Warning
2276   headers previously attached to that entry except as specified for
2277   specific Warning codes. It &MUST; then add any Warning headers received
2278   in the validating response. In other words, Warning headers are those
2279   that would be attached to the most recent relevant response.
2280</t>
2281<t>
2282   When multiple Warning headers are attached to a response, the user
2283   agent ought to inform the user of as many of them as possible, in the
2284   order that they appear in the response. If it is not possible to
2285   inform the user of all of the warnings, the user agent &SHOULD; follow
2286   these heuristics:
2287  <list style="symbols">
2288    <t>Warnings that appear early in the response take priority over
2289        those appearing later in the response.</t>
2290
2291    <t>Warnings in the user's preferred character set take priority
2292        over warnings in other character sets but with identical warn-codes
2293        and warn-agents.</t>
2294  </list>
2295</t>
2296<t>
2297   Systems that generate multiple Warning headers &SHOULD; order them with
2298   this user agent behavior in mind.
2299</t>
2300<t>
2301   Requirements for the behavior of caches with respect to Warnings are
2302   stated in <xref target="warnings"/>.
2303</t>
2304<t>
2305   This is a list of the currently-defined warn-codes, each with a
2306   recommended warn-text in English, and a description of its meaning.
2307</t>
2308<t>
2309   110 Response is stale
2310  <list><t>
2311     &MUST; be included whenever the returned response is stale.
2312  </t></list>
2313</t>
2314<t>
2315   111 Revalidation failed
2316  <list><t>
2317     &MUST; be included if a cache returns a stale response because an
2318     attempt to revalidate the response failed, due to an inability to
2319     reach the server.
2320  </t></list>
2321</t>
2322<t>
2323   112 Disconnected operation
2324  <list><t>
2325     &SHOULD; be included if the cache is intentionally disconnected from
2326     the rest of the network for a period of time.
2327  </t></list>
2328</t>
2329<t>
2330   113 Heuristic expiration
2331  <list><t>
2332     &MUST; be included if the cache heuristically chose a freshness
2333     lifetime greater than 24 hours and the response's age is greater
2334     than 24 hours.
2335  </t></list>
2336</t>
2337<t>
2338   199 Miscellaneous warning
2339  <list><t>
2340     The warning text &MAY; include arbitrary information to be presented
2341     to a human user, or logged. A system receiving this warning &MUST-NOT;
2342     take any automated action, besides presenting the warning to
2343     the user.
2344  </t></list>
2345</t>
2346<t>
2347   214 Transformation applied
2348  <list><t>
2349     &MUST; be added by an intermediate cache or proxy if it applies any
2350     transformation changing the content-coding (as specified in the
2351     Content-Encoding header) or media-type (as specified in the
2352     Content-Type header) of the response, or the entity-body of the
2353     response, unless this Warning code already appears in the response.
2354  </t></list>
2355</t>
2356<t>
2357   299 Miscellaneous persistent warning
2358  <list><t>
2359     The warning text &MAY; include arbitrary information to be presented
2360     to a human user, or logged. A system receiving this warning &MUST-NOT;
2361     take any automated action.
2362  </t></list>
2363</t>
2364<t>
2365   If an implementation sends a message with one or more Warning headers
2366   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2367   each warning-value a warn-date that matches the date in the response.
2368</t>
2369<t>
2370   If an implementation receives a message with a warning-value that
2371   includes a warn-date, and that warn-date is different from the Date
2372   value in the response, then that warning-value &MUST; be deleted from
2373   the message before storing, forwarding, or using it. (This prevents
2374   bad consequences of naive caching of Warning header fields.) If all
2375   of the warning-values are deleted for this reason, the Warning header
2376   &MUST; be deleted as well.
2377</t>
2378</section>
2379
2380</section>
2381
2382<section title="IANA Considerations" anchor="IANA.considerations">
2383<t>
2384   TBD.
2385</t>
2386</section>
2387
2388<section title="Security Considerations" anchor="security.considerations">
2389<t>
2390   Caching proxies provide additional potential vulnerabilities, since
2391   the contents of the cache represent an attractive target for
2392   malicious exploitation. Because cache contents persist after an HTTP
2393   request is complete, an attack on the cache can reveal information
2394   long after a user believes that the information has been removed from
2395   the network. Therefore, cache contents should be protected as
2396   sensitive information.
2397</t>
2398</section>
2399
2400<section title="Acknowledgments" anchor="ack">
2401<t>
2402   Much of the content and presentation of the caching design is due to
2403   suggestions and comments from individuals including: Shel Kaphan,
2404   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2405</t>
2406<t>
2407   Based on an XML translation of RFC 2616 by Julian Reschke.
2408</t>
2409</section>
2410</middle>
2411<back>
2412<references>
2413
2414<reference anchor="Part1">
2415   <front>
2416      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2417      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2418         <organization abbrev="Day Software">Day Software</organization>
2419         <address><email>fielding@gbiv.com</email></address>
2420      </author>
2421      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2422         <organization>One Laptop per Child</organization>
2423         <address><email>jg@laptop.org</email></address>
2424      </author>
2425      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2426         <organization abbrev="HP">Hewlett-Packard Company</organization>
2427         <address><email>JeffMogul@acm.org</email></address>
2428      </author>
2429      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2430         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2431         <address><email>henrikn@microsoft.com</email></address>
2432      </author>
2433      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2434         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2435         <address><email>LMM@acm.org</email></address>
2436      </author>
2437      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2438         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2439         <address><email>paulle@microsoft.com</email></address>
2440      </author>
2441      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2442         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2443         <address><email>timbl@w3.org</email></address>
2444      </author>
2445      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2446   </front>
2447   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2448   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2449</reference>
2450
2451<reference anchor="Part2">
2452   <front>
2453      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2454      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2455         <organization abbrev="Day Software">Day Software</organization>
2456         <address><email>fielding@gbiv.com</email></address>
2457      </author>
2458      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2459         <organization>One Laptop per Child</organization>
2460         <address><email>jg@laptop.org</email></address>
2461      </author>
2462      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2463         <organization abbrev="HP">Hewlett-Packard Company</organization>
2464         <address><email>JeffMogul@acm.org</email></address>
2465      </author>
2466      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2467         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2468         <address><email>henrikn@microsoft.com</email></address>
2469      </author>
2470      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2471         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2472         <address><email>LMM@acm.org</email></address>
2473      </author>
2474      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2475         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2476         <address><email>paulle@microsoft.com</email></address>
2477      </author>
2478      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2479         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2480         <address><email>timbl@w3.org</email></address>
2481      </author>
2482      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2483   </front>
2484   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2485   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2486</reference>
2487
2488<reference anchor="Part3">
2489   <front>
2490      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2491      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2492         <organization abbrev="Day Software">Day Software</organization>
2493         <address><email>fielding@gbiv.com</email></address>
2494      </author>
2495      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2496         <organization>One Laptop per Child</organization>
2497         <address><email>jg@laptop.org</email></address>
2498      </author>
2499      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2500         <organization abbrev="HP">Hewlett-Packard Company</organization>
2501         <address><email>JeffMogul@acm.org</email></address>
2502      </author>
2503      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2504         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2505         <address><email>henrikn@microsoft.com</email></address>
2506      </author>
2507      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2508         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2509         <address><email>LMM@acm.org</email></address>
2510      </author>
2511      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2512         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2513         <address><email>paulle@microsoft.com</email></address>
2514      </author>
2515      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2516         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2517         <address><email>timbl@w3.org</email></address>
2518      </author>
2519      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2520   </front>
2521   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2522   <x:source href="p3-payload.xml" basename="p3-payload"/>
2523</reference>
2524
2525<reference anchor="Part4">
2526   <front>
2527      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2528      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2529         <organization abbrev="Day Software">Day Software</organization>
2530         <address><email>fielding@gbiv.com</email></address>
2531      </author>
2532      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2533         <organization>One Laptop per Child</organization>
2534         <address><email>jg@laptop.org</email></address>
2535      </author>
2536      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2537         <organization abbrev="HP">Hewlett-Packard Company</organization>
2538         <address><email>JeffMogul@acm.org</email></address>
2539      </author>
2540      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2541         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2542         <address><email>henrikn@microsoft.com</email></address>
2543      </author>
2544      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2545         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2546         <address><email>LMM@acm.org</email></address>
2547      </author>
2548      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2549         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2550         <address><email>paulle@microsoft.com</email></address>
2551      </author>
2552      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2553         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2554         <address><email>timbl@w3.org</email></address>
2555      </author>
2556      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2557   </front>
2558   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2559   <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2560</reference>
2561
2562<reference anchor="Part5">
2563   <front>
2564      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2565      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2566         <organization abbrev="Day Software">Day Software</organization>
2567         <address><email>fielding@gbiv.com</email></address>
2568      </author>
2569      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2570         <organization>One Laptop per Child</organization>
2571         <address><email>jg@laptop.org</email></address>
2572      </author>
2573      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2574         <organization abbrev="HP">Hewlett-Packard Company</organization>
2575         <address><email>JeffMogul@acm.org</email></address>
2576      </author>
2577      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2578         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2579         <address><email>henrikn@microsoft.com</email></address>
2580      </author>
2581      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2582         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2583         <address><email>LMM@acm.org</email></address>
2584      </author>
2585      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2586         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2587         <address><email>paulle@microsoft.com</email></address>
2588      </author>
2589      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2590         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2591         <address><email>timbl@w3.org</email></address>
2592      </author>
2593      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2594   </front>
2595   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2596   <x:source href="p5-range.xml" basename="p5-range"/>
2597</reference>
2598
2599<reference anchor="Part7">
2600   <front>
2601      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2602      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2603         <organization abbrev="Day Software">Day Software</organization>
2604         <address><email>fielding@gbiv.com</email></address>
2605      </author>
2606      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2607         <organization>One Laptop per Child</organization>
2608         <address><email>jg@laptop.org</email></address>
2609      </author>
2610      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2611         <organization abbrev="HP">Hewlett-Packard Company</organization>
2612         <address><email>JeffMogul@acm.org</email></address>
2613      </author>
2614      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2615         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2616         <address><email>henrikn@microsoft.com</email></address>
2617      </author>
2618      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2619         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2620         <address><email>LMM@acm.org</email></address>
2621      </author>
2622      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2623         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2624         <address><email>paulle@microsoft.com</email></address>
2625      </author>
2626      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2627         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2628         <address><email>timbl@w3.org</email></address>
2629      </author>
2630      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2631   </front>
2632   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2633   <x:source href="p7-auth.xml" basename="p7-auth"/>
2634</reference>
2635
2636<reference anchor="RFC2616">
2637   <front>
2638      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2639      <author initials="R." surname="Fielding" fullname="R. Fielding">
2640         <organization>University of California, Irvine</organization>
2641         <address><email>fielding@ics.uci.edu</email></address>
2642      </author>
2643      <author initials="J." surname="Gettys" fullname="J. Gettys">
2644         <organization>W3C</organization>
2645         <address><email>jg@w3.org</email></address>
2646      </author>
2647      <author initials="J." surname="Mogul" fullname="J. Mogul">
2648         <organization>Compaq Computer Corporation</organization>
2649         <address><email>mogul@wrl.dec.com</email></address>
2650      </author>
2651      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2652         <organization>MIT Laboratory for Computer Science</organization>
2653         <address><email>frystyk@w3.org</email></address>
2654      </author>
2655      <author initials="L." surname="Masinter" fullname="L. Masinter">
2656         <organization>Xerox Corporation</organization>
2657         <address><email>masinter@parc.xerox.com</email></address>
2658      </author>
2659      <author initials="P." surname="Leach" fullname="P. Leach">
2660         <organization>Microsoft Corporation</organization>
2661         <address><email>paulle@microsoft.com</email></address>
2662      </author>
2663      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2664         <organization>W3C</organization>
2665         <address><email>timbl@w3.org</email></address>
2666      </author>
2667      <date month="June" year="1999"/>
2668   </front>
2669   <seriesInfo name="RFC" value="2616"/>
2670</reference>
2671
2672<reference anchor="RFC2047">
2673<front>
2674<title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2675<author initials="K." surname="Moore" fullname="Keith Moore">
2676<organization>University of Tennessee</organization>
2677<address>
2678<postal>
2679<street>107 Ayres Hall</street>
2680<street>Knoxville TN 37996-1301</street></postal>
2681<email>moore@cs.utk.edu</email></address></author>
2682<date month="November" year="1996"/>
2683<area>Applications</area>
2684<keyword>Amercian Standard Code for Information Interchange</keyword>
2685<keyword>mail</keyword>
2686<keyword>multipurpose internet mail extensions</keyword>
2687</front>
2688<seriesInfo name="RFC" value="2047"/>
2689</reference>
2690
2691<reference anchor="RFC1305">
2692<front>
2693<title>Network Time Protocol (Version 3) Specification, Implementation</title>
2694<author initials="D." surname="Mills" fullname="David L. Mills">
2695<organization>University of Delaware, Electrical Engineering Department</organization>
2696<address>
2697<postal>
2698<street/>
2699<city>Newark</city>
2700<region>DE</region>
2701<code>19716</code>
2702<country>US</country></postal>
2703<phone>+1 302 451 8247</phone>
2704<email>mills@udel.edu</email></address></author>
2705<date month="March" year="1992"/>
2706<abstract>
2707<t>This document describes the Network Time Protocol (NTP), specifies its normal structure and summarizes information useful for its implementation. NTP provides the mechanisms to synchronize time and coordinate time distribution in a large, diverse internet operating at rates from mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time servers operating in a self-organizing, hierarchical-master-slave configuration synchronizes local clocks within the subnet and to national time standards via wire or radio. The servers can also redistribute reference time via local routing algorithms and time daemons.</t></abstract></front>
2708<seriesInfo name="RFC" value="1305"/>
2709</reference>
2710
2711</references>
2712
2713<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2714<t>
2715   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2716   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2717   <xref target="header.cache-control" format="counter"/>,
2718   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2719</t>
2720<t>
2721   The Cache-Control: max-age directive was not properly defined for
2722   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2723</t>
2724<t>
2725   Warnings could be cached incorrectly, or not updated appropriately.
2726   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2727   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2728   and <xref target="header.warning" format="counter"/>) Warning
2729   also needed to be a general header, as PUT or other methods may have
2730   need for it in requests.
2731</t>
2732</section>
2733</back>
2734</rfc>
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