source: draft-ietf-httpbis/latest/p6-cache.xml @ 93

Last change on this file since 93 was 93, checked in by ylafon@…, 12 years ago

Resolve #87: Typo in Heuristic Expiration

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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  <author initials="Y." surname="Lafon" fullname="Yves Lafon">
153    <organization abbrev="W3C">World Wide Web Consortium</organization>
154    <address>
155      <postal>
156        <street>W3C / ERCIM</street>
157        <street>2004, rte des Lucioles</street>
158        <city>Sophia-Antipolis</city>
159        <region>AM</region>
160        <code>06902</code>
161        <country>France</country>
162      </postal>
163      <email>ylafon@w3.org</email>
164      <uri>http://www.raubacapeu.net/people/yves/</uri>
165    </address>
166  </author>
167
168  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
169
170<abstract>
171<t>
172   The Hypertext Transfer Protocol (HTTP) is an application-level
173   protocol for distributed, collaborative, hypermedia information
174   systems. HTTP has been in use by the World Wide Web global information
175   initiative since 1990. This document is Part 6 of the seven-part specification
176   that defines the protocol referred to as "HTTP/1.1" and, taken together,
177   obsoletes RFC 2616.  Part 6 defines requirements on HTTP caches
178   and the associated header fields that control cache behavior or indicate
179   cacheable response messages.
180</t>
181</abstract>
182
183<note title="Editorial Note (To be removed by RFC Editor)">
184  <t>
185    This version of the HTTP specification contains only minimal editorial
186    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
187    and authors' addresses).  All other changes are due to partitioning the
188    original into seven mostly independent parts.  The intent is for readers
189    of future drafts to able to use draft 00 as the basis for comparison
190    when the WG makes later changes to the specification text.  This draft
191    will shortly be followed by draft 01 (containing the first round of changes
192    that have already been agreed to on the mailing list). There is no point in
193    reviewing this draft other than to verify that the partitioning has been
194    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
195    will be the editors after draft 00 is submitted.
196  </t>
197  <t>
198    Discussion of this draft should take place on the HTTPBIS working group
199    mailing list (ietf-http-wg@w3.org). The current issues list is
200    at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
201    and related documents (including fancy diffs) can be found at
202    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
203  </t>
204</note>
205</front>
206<middle>
207<section title="Introduction" anchor="introduction">
208<t>
209   This document will define aspects of HTTP related to caching response
210   messages.  Right now it only includes the extracted relevant sections
211   of <xref target="RFC2616">RFC 2616</xref> without edit.
212</t>
213
214<section title="Terminology" anchor="intro.terminology">
215<t>
216   This specification uses a number of terms to refer to the roles
217   played by participants in, and objects of, the HTTP communication.
218</t>
219<t>
220  <iref item="cache"/>
221  <x:dfn>cache</x:dfn>
222  <list>
223    <t>
224      A program's local store of response messages and the subsystem
225      that controls its message storage, retrieval, and deletion. A
226      cache stores cacheable responses in order to reduce the response
227      time and network bandwidth consumption on future, equivalent
228      requests. Any client or server may include a cache, though a cache
229      cannot be used by a server that is acting as a tunnel.
230    </t>
231  </list>
232</t>
233<t>
234  <iref item="cacheable"/>
235  <x:dfn>cacheable</x:dfn>
236  <list>
237    <t>
238      A response is cacheable if a cache is allowed to store a copy of
239      the response message for use in answering subsequent requests. The
240      rules for determining the cacheability of HTTP responses are
241      defined in <xref target="caching"/>. Even if a resource is cacheable, there may
242      be additional constraints on whether a cache can use the cached
243      copy for a particular request.
244    </t>
245  </list>
246</t>
247<t>
248  <iref item="first-hand"/>
249  <x:dfn>first-hand</x:dfn>
250  <list>
251    <t>
252      A response is first-hand if it comes directly and without
253      unnecessary delay from the origin server, perhaps via one or more
254      proxies. A response is also first-hand if its validity has just
255      been checked directly with the origin server.
256    </t>
257  </list>
258</t>
259<t>
260  <iref item="explicit expiration time"/>
261  <x:dfn>explicit expiration time</x:dfn>
262  <list>
263    <t>
264      The time at which the origin server intends that an entity should
265      no longer be returned by a cache without further validation.
266    </t>
267  </list>
268</t>
269<t>
270  <iref item="heuristic expiration time"/>
271  <x:dfn>heuristic expiration time</x:dfn>
272  <list>
273    <t>
274      An expiration time assigned by a cache when no explicit expiration
275      time is available.
276    </t>
277  </list>
278</t>
279<t>
280  <iref item="age"/>
281  <x:dfn>age</x:dfn>
282  <list>
283    <t>
284      The age of a response is the time since it was sent by, or
285      successfully validated with, the origin server.
286    </t>
287  </list>
288</t>
289<t>
290  <iref item="freshness lifetime"/>
291  <x:dfn>freshness lifetime</x:dfn>
292  <list>
293    <t>
294      The length of time between the generation of a response and its
295      expiration time.
296    </t>
297  </list>
298</t>
299<t>
300  <iref item="fresh"/>
301  <x:dfn>fresh</x:dfn>
302  <list>
303    <t>
304      A response is fresh if its age has not yet exceeded its freshness
305      lifetime.
306    </t>
307  </list>
308</t>
309<t>
310  <iref item="stale"/>
311  <x:dfn>stale</x:dfn>
312  <list>
313    <t>
314      A response is stale if its age has passed its freshness lifetime.
315    </t>
316  </list>
317</t>
318<t>
319  <iref item="semantically transparent"/>
320  <x:dfn>semantically transparent</x:dfn>
321  <list>
322    <t>
323      A cache behaves in a "semantically transparent" manner, with
324      respect to a particular response, when its use affects neither the
325      requesting client nor the origin server, except to improve
326      performance. When a cache is semantically transparent, the client
327      receives exactly the same response (except for hop-by-hop headers)
328      that it would have received had its request been handled directly
329      by the origin server.
330    </t>
331  </list>
332</t>
333<t>
334  <iref item="validator"/>
335  <x:dfn>validator</x:dfn>
336  <list>
337    <t>
338      A protocol element (e.g., an entity tag or a Last-Modified time)
339      that is used to find out whether a cache entry is an equivalent
340      copy of an entity.
341    </t>
342  </list>
343</t>
344</section>
345
346<section title="Delta Seconds" anchor="delta.seconds">
347<t>
348   Some HTTP header fields allow a time value to be specified as an
349   integer number of seconds, represented in decimal, after the time
350   that the message was received.
351</t>
352<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="delta-seconds"/>
353    delta-seconds  = 1*DIGIT
354</artwork></figure>
355</section>
356</section>
357
358<section title="Caching in HTTP" anchor="caching">
359<section title="Overview" anchor="caching.overview">
360<t>
361   HTTP is typically used for distributed information systems, where
362   performance can be improved by the use of response caches. The
363   HTTP/1.1 protocol includes a number of elements intended to make
364   caching work as well as possible. Because these elements are
365   inextricable from other aspects of the protocol, and because they
366   interact with each other, it is useful to describe the basic caching
367   design of HTTP separately from the detailed descriptions of methods,
368   headers, response codes, etc.
369</t>
370<t>
371   Caching would be useless if it did not significantly improve
372   performance. The goal of caching in HTTP/1.1 is to eliminate the need
373   to send requests in many cases, and to eliminate the need to send
374   full responses in many other cases. The former reduces the number of
375   network round-trips required for many operations; we use an
376   "expiration" mechanism for this purpose (see <xref target="expiration.model"/>). The
377   latter reduces network bandwidth requirements; we use a "validation"
378   mechanism for this purpose (see <xref target="validation.model"/>).
379</t>
380<t>
381   Requirements for performance, availability, and disconnected
382   operation require us to be able to relax the goal of semantic
383   transparency. The HTTP/1.1 protocol allows origin servers, caches,
384   and clients to explicitly reduce transparency when necessary.
385   However, because non-transparent operation may confuse non-expert
386   users, and might be incompatible with certain server applications
387   (such as those for ordering merchandise), the protocol requires that
388   transparency be relaxed
389  <list style="symbols">
390     <t>only by an explicit protocol-level request when relaxed by
391        client or origin server</t>
392
393     <t>only with an explicit warning to the end user when relaxed by
394        cache or client</t>
395  </list>
396</t>
397<t>
398   Therefore, the HTTP/1.1 protocol provides these important elements:
399  <list style="numbers">
400      <t>Protocol features that provide full semantic transparency when
401         this is required by all parties.</t>
402
403      <t>Protocol features that allow an origin server or user agent to
404         explicitly request and control non-transparent operation.</t>
405
406      <t>Protocol features that allow a cache to attach warnings to
407         responses that do not preserve the requested approximation of
408         semantic transparency.</t>
409  </list>
410</t>
411<t>
412   A basic principle is that it must be possible for the clients to
413   detect any potential relaxation of semantic transparency.
414  <list><t>
415      <x:h>Note:</x:h> The server, cache, or client implementor might be faced with
416      design decisions not explicitly discussed in this specification.
417      If a decision might affect semantic transparency, the implementor
418      ought to err on the side of maintaining transparency unless a
419      careful and complete analysis shows significant benefits in
420      breaking transparency.
421    </t></list>
422</t>
423
424<section title="Cache Correctness" anchor="cache.correctness">
425<t>
426   A correct cache &MUST; respond to a request with the most up-to-date
427   response held by the cache that is appropriate to the request (see
428   sections <xref target="disambiguating.expiration.values" format="counter"/>,
429   <xref target="disambiguating.multiple.responses" format="counter"/>,
430   and <xref target="cache.replacement" format="counter"/>) which meets one of the following
431   conditions:
432  <list style="numbers">
433      <t>It has been checked for equivalence with what the origin server
434         would have returned by revalidating the response with the
435         origin server (<xref target="validation.model"/>);</t>
436
437      <t>It is "fresh enough" (see <xref target="expiration.model"/>). In the default case,
438         this means it meets the least restrictive freshness requirement
439         of the client, origin server, and cache (see <xref target="header.cache-control"/>); if
440         the origin server so specifies, it is the freshness requirement
441         of the origin server alone.
442
443         If a stored response is not "fresh enough" by the most
444         restrictive freshness requirement of both the client and the
445         origin server, in carefully considered circumstances the cache
446         &MAY; still return the response with the appropriate Warning
447         header (see section <xref target="exceptions.to.the.rules.and.warnings" format="counter"/>
448         and <xref target="header.warning" format="counter"/>), unless such a response
449         is prohibited (e.g., by a "no-store" cache-directive, or by a
450         "no-cache" cache-request-directive; see <xref target="header.cache-control"/>).</t>
451
452      <t>It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
453         or error (4xx or 5xx) response message.</t>
454  </list>
455</t>
456<t>
457   If the cache can not communicate with the origin server, then a
458   correct cache &SHOULD; respond as above if the response can be
459   correctly served from the cache; if not it &MUST; return an error or
460   warning indicating that there was a communication failure.
461</t>
462<t>
463   If a cache receives a response (either an entire response, or a 304
464   (Not Modified) response) that it would normally forward to the
465   requesting client, and the received response is no longer fresh, the
466   cache &SHOULD; forward it to the requesting client without adding a new
467   Warning (but without removing any existing Warning headers). A cache
468   &SHOULD-NOT;  attempt to revalidate a response simply because that
469   response became stale in transit; this might lead to an infinite
470   loop. A user agent that receives a stale response without a Warning
471   &MAY; display a warning indication to the user.
472</t>
473</section>
474
475<section title="Warnings" anchor="warnings">
476<t>
477   Whenever a cache returns a response that is neither first-hand nor
478   "fresh enough" (in the sense of condition 2 in <xref target="cache.correctness"/>), it
479   &MUST; attach a warning to that effect, using a Warning general-header.
480   The Warning header and the currently defined warnings are described
481   in <xref target="header.warning"/>. The warning allows clients to take appropriate
482   action.
483</t>
484<t>
485   Warnings &MAY; be used for other purposes, both cache-related and
486   otherwise. The use of a warning, rather than an error status code,
487   distinguish these responses from true failures.
488</t>
489<t>
490   Warnings are assigned three digit warn-codes. The first digit
491   indicates whether the Warning &MUST; or &MUST-NOT; be deleted from a
492   stored cache entry after a successful revalidation:
493</t>
494<t>
495  <list style="hanging">
496    <t hangText="1xx">Warnings that describe the freshness or revalidation status of
497     the response, and so &MUST; be deleted after a successful
498     revalidation. 1XX warn-codes &MAY; be generated by a cache only when
499     validating a cached entry. It &MUST-NOT; be generated by clients.</t>
500
501    <t hangText="2xx">Warnings that describe some aspect of the entity body or entity
502     headers that is not rectified by a revalidation (for example, a
503     lossy compression of the entity bodies) and which &MUST-NOT; be
504     deleted after a successful revalidation.</t>
505    </list>
506</t>
507<t>
508   See <xref target="header.warning"/> for the definitions of the codes themselves.
509</t>
510<t>
511   HTTP/1.0 caches will cache all Warnings in responses, without
512   deleting the ones in the first category. Warnings in responses that
513   are passed to HTTP/1.0 caches carry an extra warning-date field,
514   which prevents a future HTTP/1.1 recipient from believing an
515   erroneously cached Warning.
516</t>
517<t>
518   Warnings also carry a warning text. The text &MAY; be in any
519   appropriate natural language (perhaps based on the client's Accept
520   headers), and include an &OPTIONAL; indication of what character set is
521   used.
522</t>
523<t>
524   Multiple warnings &MAY; be attached to a response (either by the origin
525   server or by a cache), including multiple warnings with the same code
526   number. For example, a server might provide the same warning with
527   texts in both English and Basque.
528</t>
529<t>
530   When multiple warnings are attached to a response, it might not be
531   practical or reasonable to display all of them to the user. This
532   version of HTTP does not specify strict priority rules for deciding
533   which warnings to display and in what order, but does suggest some
534   heuristics.
535</t>
536</section>
537
538<section title="Cache-control Mechanisms" anchor="cache-control.mechanisms">
539<t>
540   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
541   times and validators) are implicit directives to caches. In some
542   cases, a server or client might need to provide explicit directives
543   to the HTTP caches. We use the Cache-Control header for this purpose.
544</t>
545<t>
546   The Cache-Control header allows a client or server to transmit a
547   variety of directives in either requests or responses. These
548   directives typically override the default caching algorithms. As a
549   general rule, if there is any apparent conflict between header
550   values, the most restrictive interpretation is applied (that is, the
551   one that is most likely to preserve semantic transparency). However,
552   in some cases, cache-control directives are explicitly specified as
553   weakening the approximation of semantic transparency (for example,
554   "max-stale" or "public").
555</t>
556<t>
557   The cache-control directives are described in detail in <xref target="header.cache-control"/>.
558</t>
559</section>
560
561<section title="Explicit User Agent Warnings" anchor="explicit.ua.warnings">
562<t>
563   Many user agents make it possible for users to override the basic
564   caching mechanisms. For example, the user agent might allow the user
565   to specify that cached entities (even explicitly stale ones) are
566   never validated. Or the user agent might habitually add "Cache-Control:
567   max-stale=3600" to every request. The user agent &SHOULD-NOT; 
568   default to either non-transparent behavior, or behavior that results
569   in abnormally ineffective caching, but &MAY; be explicitly configured
570   to do so by an explicit action of the user.
571</t>
572<t>
573   If the user has overridden the basic caching mechanisms, the user
574   agent &SHOULD; explicitly indicate to the user whenever this results in
575   the display of information that might not meet the server's
576   transparency requirements (in particular, if the displayed entity is
577   known to be stale). Since the protocol normally allows the user agent
578   to determine if responses are stale or not, this indication need only
579   be displayed when this actually happens. The indication need not be a
580   dialog box; it could be an icon (for example, a picture of a rotting
581   fish) or some other indicator.
582</t>
583<t>
584   If the user has overridden the caching mechanisms in a way that would
585   abnormally reduce the effectiveness of caches, the user agent &SHOULD;
586   continually indicate this state to the user (for example, by a
587   display of a picture of currency in flames) so that the user does not
588   inadvertently consume excess resources or suffer from excessive
589   latency.
590</t>
591</section>
592
593<section title="Exceptions to the Rules and Warnings" anchor="exceptions.to.the.rules.and.warnings">
594<t>
595   In some cases, the operator of a cache &MAY; choose to configure it to
596   return stale responses even when not requested by clients. This
597   decision ought not be made lightly, but may be necessary for reasons
598   of availability or performance, especially when the cache is poorly
599   connected to the origin server. Whenever a cache returns a stale
600   response, it &MUST; mark it as such (using a Warning header) enabling
601   the client software to alert the user that there might be a potential
602   problem.
603</t>
604<t>
605   It also allows the user agent to take steps to obtain a first-hand or
606   fresh response. For this reason, a cache &SHOULD-NOT;  return a stale
607   response if the client explicitly requests a first-hand or fresh one,
608   unless it is impossible to comply for technical or policy reasons.
609</t>
610</section>
611
612<section title="Client-controlled Behavior" anchor="client-controlled.behavior">
613<t>
614   While the origin server (and to a lesser extent, intermediate caches,
615   by their contribution to the age of a response) are the primary
616   source of expiration information, in some cases the client might need
617   to control a cache's decision about whether to return a cached
618   response without validating it. Clients do this using several
619   directives of the Cache-Control header.
620</t>
621<t>
622   A client's request &MAY; specify the maximum age it is willing to
623   accept of an unvalidated response; specifying a value of zero forces
624   the cache(s) to revalidate all responses. A client &MAY; also specify
625   the minimum time remaining before a response expires. Both of these
626   options increase constraints on the behavior of caches, and so cannot
627   further relax the cache's approximation of semantic transparency.
628</t>
629<t>
630   A client &MAY; also specify that it will accept stale responses, up to
631   some maximum amount of staleness. This loosens the constraints on the
632   caches, and so might violate the origin server's specified
633   constraints on semantic transparency, but might be necessary to
634   support disconnected operation, or high availability in the face of
635   poor connectivity.
636</t>
637</section>
638</section>
639
640<section title="Expiration Model" anchor="expiration.model">
641
642<section title="Server-Specified Expiration" anchor="server-specified.expiration">
643<t>
644   HTTP caching works best when caches can entirely avoid making
645   requests to the origin server. The primary mechanism for avoiding
646   requests is for an origin server to provide an explicit expiration
647   time in the future, indicating that a response &MAY; be used to satisfy
648   subsequent requests. In other words, a cache can return a fresh
649   response without first contacting the server.
650</t>
651<t>
652   Our expectation is that servers will assign future explicit
653   expiration times to responses in the belief that the entity is not
654   likely to change, in a semantically significant way, before the
655   expiration time is reached. This normally preserves semantic
656   transparency, as long as the server's expiration times are carefully
657   chosen.
658</t>
659<t>
660   The expiration mechanism applies only to responses taken from a cache
661   and not to first-hand responses forwarded immediately to the
662   requesting client.
663</t>
664<t>
665   If an origin server wishes to force a semantically transparent cache
666   to validate every request, it &MAY; assign an explicit expiration time
667   in the past. This means that the response is always stale, and so the
668   cache &SHOULD; validate it before using it for subsequent requests. See
669   <xref target="cache.revalidation.and.reload.controls"/> for a more restrictive way to force revalidation.
670</t>
671<t>
672   If an origin server wishes to force any HTTP/1.1 cache, no matter how
673   it is configured, to validate every request, it &SHOULD; use the "must-revalidate"
674   cache-control directive (see <xref target="header.cache-control"/>).
675</t>
676<t>
677   Servers specify explicit expiration times using either the Expires
678   header, or the max-age directive of the Cache-Control header.
679</t>
680<t>
681   An expiration time cannot be used to force a user agent to refresh
682   its display or reload a resource; its semantics apply only to caching
683   mechanisms, and such mechanisms need only check a resource's
684   expiration status when a new request for that resource is initiated.
685   See <xref target="history.lists"/> for an explanation of the difference between caches
686   and history mechanisms.
687</t>
688</section>
689
690<section title="Heuristic Expiration" anchor="heuristic.expiration">
691<t>
692   Since origin servers do not always provide explicit expiration times,
693   HTTP caches typically assign heuristic expiration times, employing
694   algorithms that use other header values (such as the Last-Modified
695   time) to estimate a plausible expiration time. The HTTP/1.1
696   specification does not provide specific algorithms, but does impose
697   worst-case constraints on their results. Since heuristic expiration
698   times might compromise semantic transparency, they ought to be used
699   cautiously, and we encourage origin servers to provide explicit
700   expiration times as much as possible.
701</t>
702</section>
703
704<section title="Age Calculations" anchor="age.calculations">
705<t>
706   In order to know if a cached entry is fresh, a cache needs to know if
707   its age exceeds its freshness lifetime. We discuss how to calculate
708   the latter in <xref target="expiration.calculations"/>; this section describes how to calculate
709   the age of a response or cache entry.
710</t>
711<t>
712   In this discussion, we use the term "now" to mean "the current value
713   of the clock at the host performing the calculation." Hosts that use
714   HTTP, but especially hosts running origin servers and caches, &SHOULD;
715   use NTP <xref target="RFC1305"/> or some similar protocol to synchronize their clocks to
716   a globally accurate time standard.
717</t>
718<t>
719   HTTP/1.1 requires origin servers to send a Date header, if possible,
720   with every response, giving the time at which the response was
721   generated (see &header-date;). We use the term "date_value" to denote
722   the value of the Date header, in a form appropriate for arithmetic
723   operations.
724</t>
725<t>
726   HTTP/1.1 uses the Age response-header to convey the estimated age of
727   the response message when obtained from a cache. The Age field value
728   is the cache's estimate of the amount of time since the response was
729   generated or revalidated by the origin server.
730</t>
731<t>
732   In essence, the Age value is the sum of the time that the response
733   has been resident in each of the caches along the path from the
734   origin server, plus the amount of time it has been in transit along
735   network paths.
736</t>
737<t>
738   We use the term "age_value" to denote the value of the Age header, in
739   a form appropriate for arithmetic operations.
740</t>
741<t>
742   A response's age can be calculated in two entirely independent ways:
743  <list style="numbers">
744      <t>now minus date_value, if the local clock is reasonably well
745         synchronized to the origin server's clock. If the result is
746         negative, the result is replaced by zero.</t>
747
748      <t>age_value, if all of the caches along the response path
749         implement HTTP/1.1.</t>
750  </list>
751</t>
752<t>
753   Given that we have two independent ways to compute the age of a
754   response when it is received, we can combine these as
755</t>
756<figure><artwork type="code">
757    corrected_received_age = max(now - date_value, age_value)
758</artwork></figure>
759<t>
760   and as long as we have either nearly synchronized clocks or all-HTTP/1.1
761   paths, one gets a reliable (conservative) result.
762</t>
763<t>
764   Because of network-imposed delays, some significant interval might
765   pass between the time that a server generates a response and the time
766   it is received at the next outbound cache or client. If uncorrected,
767   this delay could result in improperly low ages.
768</t>
769<t>
770   Because the request that resulted in the returned Age value must have
771   been initiated prior to that Age value's generation, we can correct
772   for delays imposed by the network by recording the time at which the
773   request was initiated. Then, when an Age value is received, it &MUST;
774   be interpreted relative to the time the request was initiated, not
775   the time that the response was received. This algorithm results in
776   conservative behavior no matter how much delay is experienced. So, we
777   compute:
778</t>
779<figure><artwork type="code">
780   corrected_initial_age = corrected_received_age
781                         + (now - request_time)
782</artwork></figure>
783<t>
784   where "request_time" is the time (according to the local clock) when
785   the request that elicited this response was sent.
786</t>
787<t>
788   Summary of age calculation algorithm, when a cache receives a
789   response:
790</t>
791<figure><artwork type="code">
792   /*
793    * age_value
794    *      is the value of Age: header received by the cache with
795    *              this response.
796    * date_value
797    *      is the value of the origin server's Date: header
798    * request_time
799    *      is the (local) time when the cache made the request
800    *              that resulted in this cached response
801    * response_time
802    *      is the (local) time when the cache received the
803    *              response
804    * now
805    *      is the current (local) time
806    */
807
808   apparent_age = max(0, response_time - date_value);
809   corrected_received_age = max(apparent_age, age_value);
810   response_delay = response_time - request_time;
811   corrected_initial_age = corrected_received_age + response_delay;
812   resident_time = now - response_time;
813   current_age   = corrected_initial_age + resident_time;
814</artwork></figure>
815<t>
816   The current_age of a cache entry is calculated by adding the amount
817   of time (in seconds) since the cache entry was last validated by the
818   origin server to the corrected_initial_age. When a response is
819   generated from a cache entry, the cache &MUST; include a single Age
820   header field in the response with a value equal to the cache entry's
821   current_age.
822</t>
823<t>
824   The presence of an Age header field in a response implies that a
825   response is not first-hand. However, the converse is not true, since
826   the lack of an Age header field in a response does not imply that the
827   response is first-hand unless all caches along the request path are
828   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
829   the Age header field).
830</t>
831</section>
832
833<section title="Expiration Calculations" anchor="expiration.calculations">
834<t>
835   In order to decide whether a response is fresh or stale, we need to
836   compare its freshness lifetime to its age. The age is calculated as
837   described in <xref target="age.calculations"/>; this section describes how to calculate
838   the freshness lifetime, and to determine if a response has expired.
839   In the discussion below, the values can be represented in any form
840   appropriate for arithmetic operations.
841</t>
842<t>
843   We use the term "expires_value" to denote the value of the Expires
844   header. We use the term "max_age_value" to denote an appropriate
845   value of the number of seconds carried by the "max-age" directive of
846   the Cache-Control header in a response (see <xref target="modifications.of.the.basic.expiration.mechanism"/>).
847</t>
848<t>
849   The max-age directive takes priority over Expires, so if max-age is
850   present in a response, the calculation is simply:
851</t>
852<figure><artwork type="code">
853   freshness_lifetime = max_age_value
854</artwork></figure>
855<t>
856   Otherwise, if Expires is present in the response, the calculation is:
857</t>
858<figure><artwork type="code">
859   freshness_lifetime = expires_value - date_value
860</artwork></figure>
861<t>
862   Note that neither of these calculations is vulnerable to clock skew,
863   since all of the information comes from the origin server.
864</t>
865<t>
866   If none of Expires, Cache-Control: max-age, or Cache-Control: s-maxage
867   (see <xref target="modifications.of.the.basic.expiration.mechanism"/>) appears in the response, and the response
868   does not include other restrictions on caching, the cache &MAY; compute
869   a freshness lifetime using a heuristic. The cache &MUST; attach Warning
870   113 to any response whose age is more than 24 hours if such warning
871   has not already been added.
872</t>
873<t>
874   Also, if the response does have a Last-Modified time, the heuristic
875   expiration value &SHOULD; be no more than some fraction of the interval
876   since that time. A typical setting of this fraction might be 10%.
877</t>
878<t>
879   The calculation to determine if a response has expired is quite
880   simple:
881</t>
882<figure><artwork type="code">
883   response_is_fresh = (freshness_lifetime &gt; current_age)
884</artwork></figure>
885</section>
886
887<section title="Disambiguating Expiration Values" anchor="disambiguating.expiration.values">
888<t>
889   Because expiration values are assigned optimistically, it is possible
890   for two caches to contain fresh values for the same resource that are
891   different.
892</t>
893<t>
894   If a client performing a retrieval receives a non-first-hand response
895   for a request that was already fresh in its own cache, and the Date
896   header in its existing cache entry is newer than the Date on the new
897   response, then the client &MAY; ignore the response. If so, it &MAY;
898   retry the request with a "Cache-Control: max-age=0" directive (see
899   <xref target="header.cache-control"/>), to force a check with the origin server.
900</t>
901<t>
902   If a cache has two fresh responses for the same representation with
903   different validators, it &MUST; use the one with the more recent Date
904   header. This situation might arise because the cache is pooling
905   responses from other caches, or because a client has asked for a
906   reload or a revalidation of an apparently fresh cache entry.
907</t>
908</section>
909
910<section title="Disambiguating Multiple Responses" anchor="disambiguating.multiple.responses">
911<t>
912   Because a client might be receiving responses via multiple paths, so
913   that some responses flow through one set of caches and other
914   responses flow through a different set of caches, a client might
915   receive responses in an order different from that in which the origin
916   server sent them. We would like the client to use the most recently
917   generated response, even if older responses are still apparently
918   fresh.
919</t>
920<t>
921   Neither the entity tag nor the expiration value can impose an
922   ordering on responses, since it is possible that a later response
923   intentionally carries an earlier expiration time. The Date values are
924   ordered to a granularity of one second.
925</t>
926<t>
927   When a client tries to revalidate a cache entry, and the response it
928   receives contains a Date header that appears to be older than the one
929   for the existing entry, then the client &SHOULD; repeat the request
930   unconditionally, and include
931</t>
932<figure><artwork type="example">
933    Cache-Control: max-age=0
934</artwork></figure>
935<t>
936   to force any intermediate caches to validate their copies directly
937   with the origin server, or
938</t>
939<figure><artwork type="example">
940    Cache-Control: no-cache
941</artwork></figure>
942<t>
943   to force any intermediate caches to obtain a new copy from the origin
944   server.
945</t>
946<t>
947   If the Date values are equal, then the client &MAY; use either response
948   (or &MAY;, if it is being extremely prudent, request a new response).
949   Servers &MUST-NOT; depend on clients being able to choose
950   deterministically between responses generated during the same second,
951   if their expiration times overlap.
952</t>
953</section>
954</section>
955
956<section title="Validation Model" anchor="validation.model">
957<t>
958   When a cache has a stale entry that it would like to use as a
959   response to a client's request, it first has to check with the origin
960   server (or possibly an intermediate cache with a fresh response) to
961   see if its cached entry is still usable. We call this "validating"
962   the cache entry. Since we do not want to have to pay the overhead of
963   retransmitting the full response if the cached entry is good, and we
964   do not want to pay the overhead of an extra round trip if the cached
965   entry is invalid, the HTTP/1.1 protocol supports the use of
966   conditional methods.
967</t>
968<t>
969   The key protocol features for supporting conditional methods are
970   those concerned with "cache validators." When an origin server
971   generates a full response, it attaches some sort of validator to it,
972   which is kept with the cache entry. When a client (user agent or
973   proxy cache) makes a conditional request for a resource for which it
974   has a cache entry, it includes the associated validator in the
975   request.
976</t>
977<t>
978   The server then checks that validator against the current validator
979   for the entity, and, if they match (see &weak-and-strong-validators;), it responds
980   with a special status code (usually, 304 (Not Modified)) and no
981   entity-body. Otherwise, it returns a full response (including
982   entity-body). Thus, we avoid transmitting the full response if the
983   validator matches, and we avoid an extra round trip if it does not
984   match.
985</t>
986<t>
987   In HTTP/1.1, a conditional request looks exactly the same as a normal
988   request for the same resource, except that it carries a special
989   header (which includes the validator) that implicitly turns the
990   method (usually, GET) into a conditional.
991</t>
992<t>
993   The protocol includes both positive and negative senses of cache-validating
994   conditions. That is, it is possible to request either that
995   a method be performed if and only if a validator matches or if and
996   only if no validators match.
997  <list><t>
998      <x:h>Note:</x:h> a response that lacks a validator may still be cached, and
999      served from cache until it expires, unless this is explicitly
1000      prohibited by a cache-control directive. However, a cache cannot
1001      do a conditional retrieval if it does not have a validator for the
1002      entity, which means it will not be refreshable after it expires.
1003  </t></list>
1004</t>
1005
1006<section title="Last-Modified Dates" anchor="last-modified.dates">
1007<t>
1008   The Last-Modified entity-header field value is often used as a cache
1009   validator. In simple terms, a cache entry is considered to be valid
1010   if the entity has not been modified since the Last-Modified value.
1011</t>
1012</section>
1013
1014<section title="Entity Tag Cache Validators" anchor="entity.tag.cache.validators">
1015<t>
1016   The ETag response-header field value, an entity tag, provides for an
1017   "opaque" cache validator. This might allow more reliable validation
1018   in situations where it is inconvenient to store modification dates,
1019   where the one-second resolution of HTTP date values is not
1020   sufficient, or where the origin server wishes to avoid certain
1021   paradoxes that might arise from the use of modification dates.
1022</t>
1023<t>
1024   Entity Tags are described in &entity-tags;.
1025</t>
1026</section>
1027
1028<section title="Non-validating Conditionals" anchor="non-validating.conditionals">
1029<t>
1030   The principle behind entity tags is that only the service author
1031   knows the semantics of a resource well enough to select an
1032   appropriate cache validation mechanism, and the specification of any
1033   validator comparison function more complex than byte-equality would
1034   open up a can of worms. Thus, comparisons of any other headers
1035   (except Last-Modified, for compatibility with HTTP/1.0) are never
1036   used for purposes of validating a cache entry.
1037</t>
1038</section>
1039</section>
1040
1041<section title="Response Cacheability" anchor="response.cacheability">
1042<t>
1043   Unless specifically constrained by a cache-control (<xref target="header.cache-control"/>)
1044   directive, a caching system &MAY; always store a successful response
1045   (see <xref target="errors.or.incomplete.response.cache.behavior"/>) as a cache entry, &MAY; return it without validation
1046   if it is fresh, and &MAY; return it after successful validation. If
1047   there is neither a cache validator nor an explicit expiration time
1048   associated with a response, we do not expect it to be cached, but
1049   certain caches &MAY; violate this expectation (for example, when little
1050   or no network connectivity is available). A client can usually detect
1051   that such a response was taken from a cache by comparing the Date
1052   header to the current time.
1053  <list><t>
1054      <x:h>Note:</x:h> some HTTP/1.0 caches are known to violate this expectation
1055      without providing any Warning.
1056  </t></list>
1057</t>
1058<t>
1059   However, in some cases it might be inappropriate for a cache to
1060   retain an entity, or to return it in response to a subsequent
1061   request. This might be because absolute semantic transparency is
1062   deemed necessary by the service author, or because of security or
1063   privacy considerations. Certain cache-control directives are
1064   therefore provided so that the server can indicate that certain
1065   resource entities, or portions thereof, are not to be cached
1066   regardless of other considerations.
1067</t>
1068<t>
1069   Note that &header-authorization; normally prevents a shared cache from saving
1070   and returning a response to a previous request if that request
1071   included an Authorization header.
1072</t>
1073<t>
1074   A response received with a status code of 200, 203, 206, 300, 301 or
1075   410 &MAY; be stored by a cache and used in reply to a subsequent
1076   request, subject to the expiration mechanism, unless a cache-control
1077   directive prohibits caching. However, a cache that does not support
1078   the Range and Content-Range headers &MUST-NOT; cache 206 (Partial
1079   Content) responses.
1080</t>
1081<t>
1082   A response received with any other status code (e.g. status codes 302
1083   and 307) &MUST-NOT; be returned in a reply to a subsequent request
1084   unless there are cache-control directives or another header(s) that
1085   explicitly allow it. For example, these include the following: an
1086   Expires header (<xref target="header.expires"/>); a "max-age", "s-maxage",  "must-revalidate",
1087   "proxy-revalidate", "public" or "private" cache-control
1088   directive (<xref target="header.cache-control"/>).
1089</t>
1090</section>
1091
1092<section title="Constructing Responses From Caches" anchor="constructing.responses.from.caches">
1093<t>
1094   The purpose of an HTTP cache is to store information received in
1095   response to requests for use in responding to future requests. In
1096   many cases, a cache simply returns the appropriate parts of a
1097   response to the requester. However, if the cache holds a cache entry
1098   based on a previous response, it might have to combine parts of a new
1099   response with what is held in the cache entry.
1100</t>
1101
1102<section title="End-to-end and Hop-by-hop Headers" anchor="end-to-end.and.hop-by-hop.headers">
1103<t>
1104   For the purpose of defining the behavior of caches and non-caching
1105   proxies, we divide HTTP headers into two categories:
1106  <list style="symbols">
1107      <t>End-to-end headers, which are  transmitted to the ultimate
1108        recipient of a request or response. End-to-end headers in
1109        responses &MUST; be stored as part of a cache entry and &MUST; be
1110        transmitted in any response formed from a cache entry.</t>
1111
1112      <t>Hop-by-hop headers, which are meaningful only for a single
1113        transport-level connection, and are not stored by caches or
1114        forwarded by proxies.</t>
1115  </list>
1116</t>
1117<t>
1118   The following HTTP/1.1 headers are hop-by-hop headers:
1119  <list style="symbols">
1120      <t>Connection</t>
1121      <t>Keep-Alive</t>
1122      <t>Proxy-Authenticate</t>
1123      <t>Proxy-Authorization</t>
1124      <t>TE</t>
1125      <t>Trailer</t>
1126      <t>Transfer-Encoding</t>
1127      <t>Upgrade</t>
1128  </list>
1129</t>
1130<t>
1131   All other headers defined by HTTP/1.1 are end-to-end headers.
1132</t>
1133<t>
1134   Other hop-by-hop headers &MUST; be listed in a Connection header
1135   (&header-connection;).
1136</t>
1137</section>
1138
1139<section title="Non-modifiable Headers" anchor="non-modifiable.headers">
1140<t>
1141   Some features of the HTTP/1.1 protocol, such as Digest
1142   Authentication, depend on the value of certain end-to-end headers. A
1143   transparent proxy &SHOULD-NOT;  modify an end-to-end header unless the
1144   definition of that header requires or specifically allows that.
1145</t>
1146<t>
1147   A transparent proxy &MUST-NOT; modify any of the following fields in a
1148   request or response, and it &MUST-NOT; add any of these fields if not
1149   already present:
1150  <list style="symbols">
1151      <t>Content-Location</t>
1152      <t>Content-MD5</t>
1153      <t>ETag</t>
1154      <t>Last-Modified</t>
1155  </list>
1156</t>
1157<t>
1158   A transparent proxy &MUST-NOT; modify any of the following fields in a
1159   response:
1160  <list style="symbols">
1161    <t>Expires</t>
1162  </list>
1163</t>
1164<t>
1165   but it &MAY; add any of these fields if not already present. If an
1166   Expires header is added, it &MUST; be given a field-value identical to
1167   that of the Date header in that response.
1168</t>
1169<t>
1170   A  proxy &MUST-NOT; modify or add any of the following fields in a
1171   message that contains the no-transform cache-control directive, or in
1172   any request:
1173  <list style="symbols">
1174    <t>Content-Encoding</t>
1175    <t>Content-Range</t>
1176    <t>Content-Type</t>
1177  </list>
1178</t>
1179<t>
1180   A non-transparent proxy &MAY; modify or add these fields to a message
1181   that does not include no-transform, but if it does so, it &MUST; add a
1182   Warning 214 (Transformation applied) if one does not already appear
1183   in the message (see <xref target="header.warning"/>).
1184  <list><t>
1185      Warning: unnecessary modification of end-to-end headers might
1186      cause authentication failures if stronger authentication
1187      mechanisms are introduced in later versions of HTTP. Such
1188      authentication mechanisms &MAY; rely on the values of header fields
1189      not listed here.
1190    </t></list>
1191</t>
1192<t>
1193   The Content-Length field of a request or response is added or deleted
1194   according to the rules in &message-length;. A transparent proxy &MUST;
1195   preserve the entity-length (&entity-length;) of the entity-body,
1196   although it &MAY; change the transfer-length (&message-length;).
1197</t>
1198</section>
1199
1200<section title="Combining Headers" anchor="combining.headers">
1201<t>
1202   When a cache makes a validating request to a server, and the server
1203   provides a 304 (Not Modified) response or a 206 (Partial Content)
1204   response, the cache then constructs a response to send to the
1205   requesting client.
1206</t>
1207<t>
1208   If the status code is 304 (Not Modified), the cache uses the entity-body
1209   stored in the cache entry as the entity-body of this outgoing
1210   response. If the status code is 206 (Partial Content) and the ETag or
1211   Last-Modified headers match exactly, the cache &MAY; combine the
1212   contents stored in the cache entry with the new contents received in
1213   the response and use the result as the entity-body of this outgoing
1214   response, (see &combining-byte-ranges;).
1215</t>
1216<t>
1217   The end-to-end headers stored in the cache entry are used for the
1218   constructed response, except that
1219  <list style="symbols">
1220    <t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning"/>)
1221      &MUST; be deleted from the cache entry and the forwarded response.</t>
1222    <t>any stored Warning headers with warn-code 2xx &MUST; be retained
1223        in the cache entry and the forwarded response.</t>
1224    <t>any end-to-end headers provided in the 304 or 206 response &MUST;
1225        replace the corresponding headers from the cache entry.</t>
1226  </list>
1227</t>
1228<t>
1229   Unless the cache decides to remove the cache entry, it &MUST; also
1230   replace the end-to-end headers stored with the cache entry with
1231   corresponding headers received in the incoming response, except for
1232   Warning headers as described immediately above. If a header field-name
1233   in the incoming response matches more than one header in the
1234   cache entry, all such old headers &MUST; be replaced.
1235</t>
1236<t>
1237   In other words, the set of end-to-end headers received in the
1238   incoming response overrides all corresponding end-to-end headers
1239   stored with the cache entry (except for stored Warning headers with
1240   warn-code 1xx, which are deleted even if not overridden).
1241  <list><t>
1242      <x:h>Note:</x:h> this rule allows an origin server to use a 304 (Not
1243      Modified) or a 206 (Partial Content) response to update any header
1244      associated with a previous response for the same entity or sub-ranges
1245      thereof, although it might not always be meaningful or
1246      correct to do so. This rule does not allow an origin server to use
1247      a 304 (Not Modified) or a 206 (Partial Content) response to
1248      entirely delete a header that it had provided with a previous
1249      response.
1250  </t></list>
1251</t>
1252</section>
1253
1254</section>
1255
1256<section title="Caching Negotiated Responses" anchor="caching.negotiated.responses">
1257<t>
1258   Use of server-driven content negotiation (&server-driven-negotiation;), as indicated
1259   by the presence of a Vary header field in a response, alters the
1260   conditions and procedure by which a cache can use the response for
1261   subsequent requests. See <xref target="header.vary"/> for use of the Vary header
1262   field by servers.
1263</t>
1264<t>
1265   A server &SHOULD; use the Vary header field to inform a cache of what
1266   request-header fields were used to select among multiple
1267   representations of a cacheable response subject to server-driven
1268   negotiation. The set of header fields named by the Vary field value
1269   is known as the "selecting" request-headers.
1270</t>
1271<t>
1272   When the cache receives a subsequent request whose Request-URI
1273   specifies one or more cache entries including a Vary header field,
1274   the cache &MUST-NOT; use such a cache entry to construct a response to
1275   the new request unless all of the selecting request-headers present
1276   in the new request match the corresponding stored request-headers in
1277   the original request.
1278</t>
1279<t>
1280   The selecting request-headers from two requests are defined to match
1281   if and only if the selecting request-headers in the first request can
1282   be transformed to the selecting request-headers in the second request
1283   by adding or removing linear white space (LWS) at places where this
1284   is allowed by the corresponding BNF, and/or combining multiple
1285   message-header fields with the same field name following the rules
1286   about message headers in &message-headers;.
1287</t>
1288<t>
1289   A Vary header field-value of "*" always fails to match and subsequent
1290   requests on that resource can only be properly interpreted by the
1291   origin server.
1292</t>
1293<t>
1294   If the selecting request header fields for the cached entry do not
1295   match the selecting request header fields of the new request, then
1296   the cache &MUST-NOT; use a cached entry to satisfy the request unless
1297   it first relays the new request to the origin server in a conditional
1298   request and the server responds with 304 (Not Modified), including an
1299   entity tag or Content-Location that indicates the entity to be used.
1300</t>
1301<t>
1302   If an entity tag was assigned to a cached representation, the
1303   forwarded request &SHOULD; be conditional and include the entity tags
1304   in an If-None-Match header field from all its cache entries for the
1305   resource. This conveys to the server the set of entities currently
1306   held by the cache, so that if any one of these entities matches the
1307   requested entity, the server can use the ETag header field in its 304
1308   (Not Modified) response to tell the cache which entry is appropriate.
1309   If the entity-tag of the new response matches that of an existing
1310   entry, the new response &SHOULD; be used to update the header fields of
1311   the existing entry, and the result &MUST; be returned to the client.
1312</t>
1313<t>
1314   If any of the existing cache entries contains only partial content
1315   for the associated entity, its entity-tag &SHOULD-NOT;  be included in
1316   the If-None-Match header field unless the request is for a range that
1317   would be fully satisfied by that entry.
1318</t>
1319<t>
1320   If a cache receives a successful response whose Content-Location
1321   field matches that of an existing cache entry for the same Request-URI,
1322   whose entity-tag differs from that of the existing entry, and
1323   whose Date is more recent than that of the existing entry, the
1324   existing entry &SHOULD-NOT;  be returned in response to future requests
1325   and &SHOULD; be deleted from the cache.
1326</t>
1327</section>
1328
1329<section title="Shared and Non-Shared Caches" anchor="shared.and.non-shared.caches">
1330<t>
1331   For reasons of security and privacy, it is necessary to make a
1332   distinction between "shared" and "non-shared" caches. A non-shared
1333   cache is one that is accessible only to a single user. Accessibility
1334   in this case &SHOULD; be enforced by appropriate security mechanisms.
1335   All other caches are considered to be "shared." Other sections of
1336   this specification place certain constraints on the operation of
1337   shared caches in order to prevent loss of privacy or failure of
1338   access controls.
1339</t>
1340</section>
1341
1342<section title="Errors or Incomplete Response Cache Behavior" anchor="errors.or.incomplete.response.cache.behavior">
1343<t>
1344   A cache that receives an incomplete response (for example, with fewer
1345   bytes of data than specified in a Content-Length header) &MAY; store
1346   the response. However, the cache &MUST; treat this as a partial
1347   response. Partial responses &MAY; be combined as described in &combining-byte-ranges;;
1348   the result might be a full response or might still be
1349   partial. A cache &MUST-NOT; return a partial response to a client
1350   without explicitly marking it as such, using the 206 (Partial
1351   Content) status code. A cache &MUST-NOT; return a partial response
1352   using a status code of 200 (OK).
1353</t>
1354<t>
1355   If a cache receives a 5xx response while attempting to revalidate an
1356   entry, it &MAY; either forward this response to the requesting client,
1357   or act as if the server failed to respond. In the latter case, it &MAY;
1358   return a previously received response unless the cached entry
1359   includes the "must-revalidate" cache-control directive (see <xref target="header.cache-control"/>).
1360</t>
1361</section>
1362
1363<section title="Side Effects of GET and HEAD" anchor="side.effects.of.get.and.head">
1364<t>
1365   Unless the origin server explicitly prohibits the caching of their
1366   responses, the application of GET and HEAD methods to any resources
1367   &SHOULD-NOT;  have side effects that would lead to erroneous behavior if
1368   these responses are taken from a cache. They &MAY; still have side
1369   effects, but a cache is not required to consider such side effects in
1370   its caching decisions. Caches are always expected to observe an
1371   origin server's explicit restrictions on caching.
1372</t>
1373<t>
1374   We note one exception to this rule: since some applications have
1375   traditionally used GETs and HEADs with query URLs (those containing a
1376   "?" in the rel_path part) to perform operations with significant side
1377   effects, caches &MUST-NOT; treat responses to such URIs as fresh unless
1378   the server provides an explicit expiration time. This specifically
1379   means that responses from HTTP/1.0 servers for such URIs &SHOULD-NOT; 
1380   be taken from a cache. See &safe-methods; for related information.
1381</t>
1382</section>
1383
1384<section title="Invalidation After Updates or Deletions" anchor="invalidation.after.updates.or.deletions">
1385<t>
1386   The effect of certain methods performed on a resource at the origin
1387   server might cause one or more existing cache entries to become non-transparently
1388   invalid. That is, although they might continue to be
1389   "fresh," they do not accurately reflect what the origin server would
1390   return for a new request on that resource.
1391</t>
1392<t>
1393   There is no way for the HTTP protocol to guarantee that all such
1394   cache entries are marked invalid. For example, the request that
1395   caused the change at the origin server might not have gone through
1396   the proxy where a cache entry is stored. However, several rules help
1397   reduce the likelihood of erroneous behavior.
1398</t>
1399<t>
1400   In this section, the phrase "invalidate an entity" means that the
1401   cache will either remove all instances of that entity from its
1402   storage, or will mark these as "invalid" and in need of a mandatory
1403   revalidation before they can be returned in response to a subsequent
1404   request.
1405</t>
1406<t>
1407   Some HTTP methods &MUST; cause a cache to invalidate an entity. This is
1408   either the entity referred to by the Request-URI, or by the Location
1409   or Content-Location headers (if present). These methods are:
1410  <list style="symbols">
1411      <t>PUT</t>
1412      <t>DELETE</t>
1413      <t>POST</t>
1414  </list>
1415</t> 
1416<t>
1417   An invalidation based
1418   on the URI in a Location or Content-Location header &MUST-NOT; be
1419   performed if the host part of that URI differs from the host part
1420   in the Request-URI. This helps prevent denial of service attacks.
1421</t>
1422<t>
1423   A cache that passes through requests for methods it does not
1424   understand &SHOULD; invalidate any entities referred to by the
1425   Request-URI.
1426</t>
1427</section>
1428
1429<section title="Write-Through Mandatory" anchor="write-through.mandatory">
1430<t>
1431   All methods that might be expected to cause modifications to the
1432   origin server's resources &MUST; be written through to the origin
1433   server. This currently includes all methods except for GET and HEAD.
1434   A cache &MUST-NOT; reply to such a request from a client before having
1435   transmitted the request to the inbound server, and having received a
1436   corresponding response from the inbound server. This does not prevent
1437   a proxy cache from sending a 100 (Continue) response before the
1438   inbound server has sent its final reply.
1439</t>
1440<t>
1441   The alternative (known as "write-back" or "copy-back" caching) is not
1442   allowed in HTTP/1.1, due to the difficulty of providing consistent
1443   updates and the problems arising from server, cache, or network
1444   failure prior to write-back.
1445</t>
1446</section>
1447
1448<section title="Cache Replacement" anchor="cache.replacement">
1449<t>
1450   If a new cacheable (see sections <xref target="what.may.be.stored.by.caches" format="counter"/>,
1451   <xref target="disambiguating.expiration.values" format="counter"/>,
1452   <xref target="disambiguating.multiple.responses" format="counter"/>
1453   and <xref target="errors.or.incomplete.response.cache.behavior" format="counter"/>)
1454   response is received from a resource while any existing responses for
1455   the same resource are cached, the cache &SHOULD; use the new response
1456   to reply to the current request. It &MAY; insert it into cache storage
1457   and &MAY;, if it meets all other requirements, use it to respond to any
1458   future requests that would previously have caused the old response to
1459   be returned. If it inserts the new response into cache storage  the
1460   rules in <xref target="combining.headers"/> apply.
1461  <list><t>
1462      <x:h>Note:</x:h> a new response that has an older Date header value than
1463      existing cached responses is not cacheable.
1464  </t></list>
1465</t>
1466</section>
1467
1468<section title="History Lists" anchor="history.lists">
1469<t>
1470   User agents often have history mechanisms, such as "Back" buttons and
1471   history lists, which can be used to redisplay an entity retrieved
1472   earlier in a session.
1473</t>
1474<t>
1475   History mechanisms and caches are different. In particular history
1476   mechanisms &SHOULD-NOT;  try to show a semantically transparent view of
1477   the current state of a resource. Rather, a history mechanism is meant
1478   to show exactly what the user saw at the time when the resource was
1479   retrieved.
1480</t>
1481<t>
1482   By default, an expiration time does not apply to history mechanisms.
1483   If the entity is still in storage, a history mechanism &SHOULD; display
1484   it even if the entity has expired, unless the user has specifically
1485   configured the agent to refresh expired history documents.
1486</t>
1487<t>
1488   This is not to be construed to prohibit the history mechanism from
1489   telling the user that a view might be stale.
1490  <list><t>
1491      <x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from
1492      viewing stale resources, this will tend to force service authors
1493      to avoid using HTTP expiration controls and cache controls when
1494      they would otherwise like to. Service authors may consider it
1495      important that users not be presented with error messages or
1496      warning messages when they use navigation controls (such as BACK)
1497      to view previously fetched resources. Even though sometimes such
1498      resources ought not to cached, or ought to expire quickly, user
1499      interface considerations may force service authors to resort to
1500      other means of preventing caching (e.g. "once-only" URLs) in order
1501      not to suffer the effects of improperly functioning history
1502      mechanisms.
1503  </t></list>
1504</t>
1505</section>
1506</section>
1507
1508<section title="Header Field Definitions" anchor="header.fields">
1509<t>
1510   This section defines the syntax and semantics of all standard
1511   HTTP/1.1 header fields. For entity-header fields, both sender and
1512   recipient refer to either the client or the server, depending on who
1513   sends and who receives the entity.
1514</t>
1515
1516<section title="Age" anchor="header.age">
1517  <iref primary="true" item="Age header" x:for-anchor=""/>
1518  <iref primary="true" item="Headers" subitem="Age" x:for-anchor=""/>
1519<t>
1520      The Age response-header field conveys the sender's estimate of the
1521      amount of time since the response (or its revalidation) was
1522      generated at the origin server. A cached response is "fresh" if
1523      its age does not exceed its freshness lifetime. Age values are
1524      calculated as specified in <xref target="age.calculations"/>.
1525</t>
1526<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="age-value"/>
1527        Age = "Age" ":" age-value
1528        age-value = delta-seconds
1529</artwork></figure>
1530<t>
1531      Age values are non-negative decimal integers, representing time in
1532      seconds.
1533</t>
1534<t>
1535      If a cache receives a value larger than the largest positive
1536      integer it can represent, or if any of its age calculations
1537      overflows, it &MUST; transmit an Age header with a value of
1538      2147483648 (2^31). An HTTP/1.1 server that includes a cache &MUST;
1539      include an Age header field in every response generated from its
1540      own cache. Caches &SHOULD; use an arithmetic type of at least 31
1541      bits of range.
1542</t>
1543</section>
1544
1545<section title="Cache-Control" anchor="header.cache-control">
1546  <iref primary="true" item="Cache-Control header" x:for-anchor=""/>
1547  <iref primary="true" item="Headers" subitem="Cache-Control" x:for-anchor=""/>
1548<t>
1549   The Cache-Control general-header field is used to specify directives
1550   that &MUST; be obeyed by all caching mechanisms along the
1551   request/response chain. The directives specify behavior intended to
1552   prevent caches from adversely interfering with the request or
1553   response. These directives typically override the default caching
1554   algorithms. Cache directives are unidirectional in that the presence
1555   of a directive in a request does not imply that the same directive is
1556   to be given in the response.
1557  <list><t>
1558      Note that HTTP/1.0 caches might not implement Cache-Control and
1559      might only implement Pragma: no-cache (see <xref target="header.pragma"/>).
1560  </t></list>
1561</t>
1562<t>
1563   Cache directives &MUST; be passed through by a proxy or gateway
1564   application, regardless of their significance to that application,
1565   since the directives might be applicable to all recipients along the
1566   request/response chain. It is not possible to specify a cache-directive
1567   for a specific cache.
1568</t>
1569<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"/>
1570   Cache-Control   = "Cache-Control" ":" 1#cache-directive
1571
1572   cache-directive = cache-request-directive
1573        | cache-response-directive
1574
1575   cache-request-directive =
1576          "no-cache"                          ; <xref target="what.is.cacheable"/>
1577        | "no-store"                          ; <xref target="what.may.be.stored.by.caches"/>
1578        | "max-age" "=" delta-seconds         ; <xref target="modifications.of.the.basic.expiration.mechanism"/>, <xref format="counter" target="cache.revalidation.and.reload.controls"/>
1579        | "max-stale" [ "=" delta-seconds ]   ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1580        | "min-fresh" "=" delta-seconds       ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1581        | "no-transform"                      ; <xref target="no-transform.directive"/>
1582        | "only-if-cached"                    ; <xref target="cache.revalidation.and.reload.controls"/>
1583        | cache-extension                     ; <xref target="cache.control.extensions"/>
1584
1585    cache-response-directive =
1586          "public"                               ; <xref target="what.is.cacheable"/>
1587        | "private" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ] ; <xref target="what.is.cacheable"/>
1588        | "no-cache" [ "=" &lt;"&gt; 1#field-name &lt;"&gt; ]; <xref target="what.is.cacheable"/>
1589        | "no-store"                             ; <xref target="what.may.be.stored.by.caches"/>
1590        | "no-transform"                         ; <xref target="no-transform.directive"/>
1591        | "must-revalidate"                      ; <xref target="cache.revalidation.and.reload.controls"/>
1592        | "proxy-revalidate"                     ; <xref target="cache.revalidation.and.reload.controls"/>
1593        | "max-age" "=" delta-seconds            ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1594        | "s-maxage" "=" delta-seconds           ; <xref target="modifications.of.the.basic.expiration.mechanism"/>
1595        | cache-extension                        ; <xref target="cache.control.extensions"/>
1596
1597   cache-extension = token [ "=" ( token | quoted-string ) ]
1598</artwork></figure>
1599<t>
1600   When a directive appears without any 1#field-name parameter, the
1601   directive applies to the entire request or response. When such a
1602   directive appears with a 1#field-name parameter, it applies only to
1603   the named field or fields, and not to the rest of the request or
1604   response. This mechanism supports extensibility; implementations of
1605   future versions of the HTTP protocol might apply these directives to
1606   header fields not defined in HTTP/1.1.
1607</t>
1608<t>
1609   The cache-control directives can be broken down into these general
1610   categories:
1611  <list style="symbols">
1612     <t>Restrictions on what are cacheable; these may only be imposed by
1613        the origin server.</t>
1614
1615     <t>Restrictions on what may be stored by a cache; these may be
1616        imposed by either the origin server or the user agent.</t>
1617
1618     <t>Modifications of the basic expiration mechanism; these may be
1619        imposed by either the origin server or the user agent.</t>
1620
1621     <t>Controls over cache revalidation and reload; these may only be
1622        imposed by a user agent.</t>
1623
1624     <t>Control over transformation of entities.</t>
1625
1626     <t>Extensions to the caching system.</t>
1627  </list>
1628</t>
1629
1630<section title="What is Cacheable" anchor="what.is.cacheable">
1631<t>
1632   By default, a response is cacheable if the requirements of the
1633   request method, request header fields, and the response status
1634   indicate that it is cacheable. <xref target="response.cacheability"/> summarizes these defaults
1635   for cacheability. The following Cache-Control response directives
1636   allow an origin server to override the default cacheability of a
1637   response:
1638</t>
1639<t>
1640  <iref item="Cache Directives" subitem="public" primary="true"/>
1641  <iref item="public" subitem="Cache Directive" primary="true"/>
1642   public
1643  <list><t>
1644      Indicates that the response &MAY; be cached by any cache, even if it
1645      would normally be non-cacheable or cacheable only within a non-shared
1646      cache. (See also Authorization, &header-authorization;, for
1647      additional details.)
1648  </t></list>
1649</t>
1650<t>
1651  <iref item="Cache Directives" subitem="private" primary="true"/>
1652  <iref item="private" subitem="Cache Directive" primary="true"/>
1653   private
1654  <list><t>
1655      Indicates that all or part of the response message is intended for
1656      a single user and &MUST-NOT; be cached by a shared cache. This
1657      allows an origin server to state that the specified parts of the
1658      response are intended for only one user and are not a valid
1659      response for requests by other users. A private (non-shared) cache
1660      &MAY; cache the response.
1661    </t><t>
1662       <x:h>Note:</x:h> This usage of the word private only controls where the
1663       response may be cached, and cannot ensure the privacy of the
1664       message content.
1665  </t></list>
1666</t>
1667<t>
1668  <iref item="Cache Directives" subitem="no-cache" primary="true"/>
1669  <iref item="no-cache" subitem="Cache Directive" primary="true"/>
1670   no-cache
1671  <list><t>
1672       If the no-cache directive does not specify a field-name, then a
1673      cache &MUST-NOT; use the response to satisfy a subsequent request
1674      without successful revalidation with the origin server. This
1675      allows an origin server to prevent caching even by caches that
1676      have been configured to return stale responses to client requests.
1677    </t><t>
1678      If the no-cache directive does specify one or more field-names,
1679      then a cache &MAY; use the response to satisfy a subsequent request,
1680      subject to any other restrictions on caching. However, the
1681      specified field-name(s) &MUST-NOT; be sent in the response to a
1682      subsequent request without successful revalidation with the origin
1683      server. This allows an origin server to prevent the re-use of
1684      certain header fields in a response, while still allowing caching
1685      of the rest of the response.
1686    <list><t>
1687       <x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this
1688       directive.
1689    </t></list>
1690  </t></list>
1691</t>
1692</section>
1693
1694<section title="What May be Stored by Caches" anchor="what.may.be.stored.by.caches">
1695<t>
1696  <iref item="Cache Directives" subitem="no-store" primary="true"/>
1697  <iref item="no-store" subitem="Cache Directive" primary="true"/>
1698   no-store
1699  <list><t>   
1700      The purpose of the no-store directive is to prevent the
1701      inadvertent release or retention of sensitive information (for
1702      example, on backup tapes). The no-store directive applies to the
1703      entire message, and &MAY; be sent either in a response or in a
1704      request. If sent in a request, a cache &MUST-NOT; store any part of
1705      either this request or any response to it. If sent in a response,
1706      a cache &MUST-NOT; store any part of either this response or the
1707      request that elicited it. This directive applies to both non-shared
1708      and shared caches. "&MUST-NOT; store" in this context means
1709      that the cache &MUST-NOT; intentionally store the information in
1710      non-volatile storage, and &MUST; make a best-effort attempt to
1711      remove the information from volatile storage as promptly as
1712      possible after forwarding it.
1713  </t><t>
1714      Even when this directive is associated with a response, users
1715      might explicitly store such a response outside of the caching
1716      system (e.g., with a "Save As" dialog). History buffers &MAY; store
1717      such responses as part of their normal operation.
1718  </t><t>
1719      The purpose of this directive is to meet the stated requirements
1720      of certain users and service authors who are concerned about
1721      accidental releases of information via unanticipated accesses to
1722      cache data structures. While the use of this directive might
1723      improve privacy in some cases, we caution that it is NOT in any
1724      way a reliable or sufficient mechanism for ensuring privacy. In
1725      particular, malicious or compromised caches might not recognize or
1726      obey this directive, and communications networks might be
1727      vulnerable to eavesdropping.
1728  </t></list>
1729</t>
1730</section>
1731
1732<section title="Modifications of the Basic Expiration Mechanism" anchor="modifications.of.the.basic.expiration.mechanism">
1733<t>
1734   The expiration time of an entity &MAY; be specified by the origin
1735   server using the Expires header (see <xref target="header.expires"/>). Alternatively,
1736   it &MAY; be specified using the max-age directive in a response. When
1737   the max-age cache-control directive is present in a cached response,
1738   the response is stale if its current age is greater than the age
1739   value given (in seconds) at the time of a new request for that
1740   resource. The max-age directive on a response implies that the
1741   response is cacheable (i.e., "public") unless some other, more
1742   restrictive cache directive is also present.
1743</t>
1744<t>
1745   If a response includes both an Expires header and a max-age
1746   directive, the max-age directive overrides the Expires header, even
1747   if the Expires header is more restrictive. This rule allows an origin
1748   server to provide, for a given response, a longer expiration time to
1749   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
1750   useful if certain HTTP/1.0 caches improperly calculate ages or
1751   expiration times, perhaps due to desynchronized clocks.
1752</t>
1753<t>
1754   Many HTTP/1.0 cache implementations will treat an Expires value that
1755   is less than or equal to the response Date value as being equivalent
1756   to the Cache-Control response directive "no-cache". If an HTTP/1.1
1757   cache receives such a response, and the response does not include a
1758   Cache-Control header field, it &SHOULD; consider the response to be
1759   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1760  <list><t>
1761       <x:h>Note:</x:h> An origin server might wish to use a relatively new HTTP
1762       cache control feature, such as the "private" directive, on a
1763       network including older caches that do not understand that
1764       feature. The origin server will need to combine the new feature
1765       with an Expires field whose value is less than or equal to the
1766       Date value. This will prevent older caches from improperly
1767       caching the response.
1768  </t></list>
1769</t>
1770<t>
1771  <iref item="Cache Directives" subitem="s-maxage" primary="true"/>
1772  <iref item="s-maxage" subitem="Cache Directive" primary="true"/>
1773   s-maxage
1774  <list><t>
1775       If a response includes an s-maxage directive, then for a shared
1776       cache (but not for a private cache), the maximum age specified by
1777       this directive overrides the maximum age specified by either the
1778       max-age directive or the Expires header. The s-maxage directive
1779       also implies the semantics of the proxy-revalidate directive (see
1780       <xref target="cache.revalidation.and.reload.controls"/>), i.e., that the shared cache must not use the
1781       entry after it becomes stale to respond to a subsequent request
1782       without first revalidating it with the origin server. The s-maxage
1783       directive is always ignored by a private cache.
1784  </t></list>
1785</t>
1786<t>
1787   Note that most older caches, not compliant with this specification,
1788   do not implement any cache-control directives. An origin server
1789   wishing to use a cache-control directive that restricts, but does not
1790   prevent, caching by an HTTP/1.1-compliant cache &MAY; exploit the
1791   requirement that the max-age directive overrides the Expires header,
1792   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1793   max-age directive.
1794</t>
1795<t>
1796   Other directives allow a user agent to modify the basic expiration
1797   mechanism. These directives &MAY; be specified on a request:
1798</t>
1799<t>
1800  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1801  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1802   max-age
1803  <list><t>
1804      Indicates that the client is willing to accept a response whose
1805      age is no greater than the specified time in seconds. Unless max-stale
1806      directive is also included, the client is not willing to
1807      accept a stale response.
1808  </t></list>
1809</t>
1810<t>
1811  <iref item="Cache Directives" subitem="min-fresh" primary="true"/>
1812  <iref item="min-fresh" subitem="Cache Directive" primary="true"/>
1813   min-fresh
1814  <list><t>
1815      Indicates that the client is willing to accept a response whose
1816      freshness lifetime is no less than its current age plus the
1817      specified time in seconds. That is, the client wants a response
1818      that will still be fresh for at least the specified number of
1819      seconds.
1820  </t></list>
1821</t>
1822<t>
1823  <iref item="Cache Directives" subitem="max-stale" primary="true"/>
1824  <iref item="max-stale" subitem="Cache Directive" primary="true"/>
1825   max-stale
1826  <list><t>
1827      Indicates that the client is willing to accept a response that has
1828      exceeded its expiration time. If max-stale is assigned a value,
1829      then the client is willing to accept a response that has exceeded
1830      its expiration time by no more than the specified number of
1831      seconds. If no value is assigned to max-stale, then the client is
1832      willing to accept a stale response of any age.
1833  </t></list>
1834</t>
1835<t>
1836   If a cache returns a stale response, either because of a max-stale
1837   directive on a request, or because the cache is configured to
1838   override the expiration time of a response, the cache &MUST; attach a
1839   Warning header to the stale response, using Warning 110 (Response is
1840   stale).
1841</t>
1842<t>
1843   A cache &MAY; be configured to return stale responses without
1844   validation, but only if this does not conflict with any "MUST"-level
1845   requirements concerning cache validation (e.g., a "must-revalidate"
1846   cache-control directive).
1847</t>
1848<t>
1849   If both the new request and the cached entry include "max-age"
1850   directives, then the lesser of the two values is used for determining
1851   the freshness of the cached entry for that request.
1852</t>
1853</section>
1854
1855<section title="Cache Revalidation and Reload Controls" anchor="cache.revalidation.and.reload.controls">
1856<t>
1857   Sometimes a user agent might want or need to insist that a cache
1858   revalidate its cache entry with the origin server (and not just with
1859   the next cache along the path to the origin server), or to reload its
1860   cache entry from the origin server. End-to-end revalidation might be
1861   necessary if either the cache or the origin server has overestimated
1862   the expiration time of the cached response. End-to-end reload may be
1863   necessary if the cache entry has become corrupted for some reason.
1864</t>
1865<t>
1866   End-to-end revalidation may be requested either when the client does
1867   not have its own local cached copy, in which case we call it
1868   "unspecified end-to-end revalidation", or when the client does have a
1869   local cached copy, in which case we call it "specific end-to-end
1870   revalidation."
1871</t>
1872<t>
1873   The client can specify these three kinds of action using Cache-Control
1874   request directives:
1875</t>
1876<t>
1877   End-to-end reload
1878  <list><t>
1879      The request includes a "no-cache" cache-control directive or, for
1880      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
1881      names &MUST-NOT; be included with the no-cache directive in a
1882      request. The server &MUST-NOT; use a cached copy when responding to
1883      such a request.
1884  </t></list>
1885</t>
1886<t>
1887   Specific end-to-end revalidation
1888  <list><t>
1889      The request includes a "max-age=0" cache-control directive, which
1890      forces each cache along the path to the origin server to
1891      revalidate its own entry, if any, with the next cache or server.
1892      The initial request includes a cache-validating conditional with
1893      the client's current validator.
1894  </t></list>
1895</t>
1896<t>
1897   Unspecified end-to-end revalidation
1898  <list><t>
1899      The request includes "max-age=0" cache-control directive, which
1900      forces each cache along the path to the origin server to
1901      revalidate its own entry, if any, with the next cache or server.
1902      The initial request does not include a cache-validating
1903      conditional; the first cache along the path (if any) that holds a
1904      cache entry for this resource includes a cache-validating
1905      conditional with its current validator.
1906  </t></list>
1907</t>
1908<t>
1909  <iref item="Cache Directives" subitem="max-age" primary="true"/>
1910  <iref item="max-age" subitem="Cache Directive" primary="true"/>
1911   max-age
1912  <list><t>
1913      When an intermediate cache is forced, by means of a max-age=0
1914      directive, to revalidate its own cache entry, and the client has
1915      supplied its own validator in the request, the supplied validator
1916      might differ from the validator currently stored with the cache
1917      entry. In this case, the cache &MAY; use either validator in making
1918      its own request without affecting semantic transparency.
1919  </t><t>
1920      However, the choice of validator might affect performance. The
1921      best approach is for the intermediate cache to use its own
1922      validator when making its request. If the server replies with 304
1923      (Not Modified), then the cache can return its now validated copy
1924      to the client with a 200 (OK) response. If the server replies with
1925      a new entity and cache validator, however, the intermediate cache
1926      can compare the returned validator with the one provided in the
1927      client's request, using the strong comparison function. If the
1928      client's validator is equal to the origin server's, then the
1929      intermediate cache simply returns 304 (Not Modified). Otherwise,
1930      it returns the new entity with a 200 (OK) response.
1931  </t><t>
1932      If a request includes the no-cache directive, it &SHOULD-NOT; 
1933      include min-fresh, max-stale, or max-age.
1934  </t></list>
1935</t>
1936<t>
1937  <iref item="Cache Directives" subitem="only-if-cached" primary="true"/>
1938  <iref item="only-if-cached" subitem="Cache Directive" primary="true"/>
1939   only-if-cached
1940  <list><t>
1941      In some cases, such as times of extremely poor network
1942      connectivity, a client may want a cache to return only those
1943      responses that it currently has stored, and not to reload or
1944      revalidate with the origin server. To do this, the client may
1945      include the only-if-cached directive in a request. If it receives
1946      this directive, a cache &SHOULD; either respond using a cached entry
1947      that is consistent with the other constraints of the request, or
1948      respond with a 504 (Gateway Timeout) status. However, if a group
1949      of caches is being operated as a unified system with good internal
1950      connectivity, such a request &MAY; be forwarded within that group of
1951      caches.
1952  </t></list>
1953</t>
1954<t>
1955  <iref item="Cache Directives" subitem="must-revalidate" primary="true"/>
1956  <iref item="must-revalidate" subitem="Cache Directive" primary="true"/>
1957   must-revalidate
1958  <list><t>
1959      Because a cache &MAY; be configured to ignore a server's specified
1960      expiration time, and because a client request &MAY; include a max-stale
1961      directive (which has a similar effect), the protocol also
1962      includes a mechanism for the origin server to require revalidation
1963      of a cache entry on any subsequent use. When the must-revalidate
1964      directive is present in a response received by a cache, that cache
1965      &MUST-NOT; use the entry after it becomes stale to respond to a
1966      subsequent request without first revalidating it with the origin
1967      server. (I.e., the cache &MUST; do an end-to-end revalidation every
1968      time, if, based solely on the origin server's Expires or max-age
1969      value, the cached response is stale.)
1970  </t><t>
1971      The must-revalidate directive is necessary to support reliable
1972      operation for certain protocol features. In all circumstances an
1973      HTTP/1.1 cache &MUST; obey the must-revalidate directive; in
1974      particular, if the cache cannot reach the origin server for any
1975      reason, it &MUST; generate a 504 (Gateway Timeout) response.
1976  </t><t>
1977      Servers &SHOULD; send the must-revalidate directive if and only if
1978      failure to revalidate a request on the entity could result in
1979      incorrect operation, such as a silently unexecuted financial
1980      transaction. Recipients &MUST-NOT; take any automated action that
1981      violates this directive, and &MUST-NOT; automatically provide an
1982      unvalidated copy of the entity if revalidation fails.
1983  </t><t>
1984      Although this is not recommended, user agents operating under
1985      severe connectivity constraints &MAY; violate this directive but, if
1986      so, &MUST; explicitly warn the user that an unvalidated response has
1987      been provided. The warning &MUST; be provided on each unvalidated
1988      access, and &SHOULD; require explicit user confirmation.
1989  </t></list>
1990</t>
1991<t>
1992  <iref item="Cache Directives" subitem="proxy-revalidate" primary="true"/>
1993  <iref item="proxy-revalidate" subitem="Cache Directive" primary="true"/>
1994   proxy-revalidate
1995  <list><t>
1996      The proxy-revalidate directive has the same meaning as the must-revalidate
1997      directive, except that it does not apply to non-shared
1998      user agent caches. It can be used on a response to an
1999      authenticated request to permit the user's cache to store and
2000      later return the response without needing to revalidate it (since
2001      it has already been authenticated once by that user), while still
2002      requiring proxies that service many users to revalidate each time
2003      (in order to make sure that each user has been authenticated).
2004      Note that such authenticated responses also need the public cache
2005      control directive in order to allow them to be cached at all.
2006  </t></list>
2007</t>
2008</section>
2009
2010<section title="No-Transform Directive" anchor="no-transform.directive">
2011<t>
2012  <iref item="Cache Directives" subitem="no-transform" primary="true"/>
2013  <iref item="no-transform" subitem="Cache Directive" primary="true"/>
2014   no-transform
2015  <list><t>
2016      Implementors of intermediate caches (proxies) have found it useful
2017      to convert the media type of certain entity bodies. A non-transparent
2018      proxy might, for example, convert between image
2019      formats in order to save cache space or to reduce the amount of
2020      traffic on a slow link.
2021  </t><t>
2022      Serious operational problems occur, however, when these
2023      transformations are applied to entity bodies intended for certain
2024      kinds of applications. For example, applications for medical
2025      imaging, scientific data analysis and those using end-to-end
2026      authentication, all depend on receiving an entity body that is bit
2027      for bit identical to the original entity-body.
2028  </t><t>
2029      Therefore, if a message includes the no-transform directive, an
2030      intermediate cache or proxy &MUST-NOT; change those headers that are
2031      listed in <xref target="non-modifiable.headers"/> as being subject to the no-transform
2032      directive. This implies that the cache or proxy &MUST-NOT; change
2033      any aspect of the entity-body that is specified by these headers,
2034      including the value of the entity-body itself.
2035  </t></list>
2036</t>
2037</section>
2038
2039<section title="Cache Control Extensions" anchor="cache.control.extensions">
2040<t>
2041   The Cache-Control header field can be extended through the use of one
2042   or more cache-extension tokens, each with an optional assigned value.
2043   Informational extensions (those which do not require a change in
2044   cache behavior) &MAY; be added without changing the semantics of other
2045   directives. Behavioral extensions are designed to work by acting as
2046   modifiers to the existing base of cache directives. Both the new
2047   directive and the standard directive are supplied, such that
2048   applications which do not understand the new directive will default
2049   to the behavior specified by the standard directive, and those that
2050   understand the new directive will recognize it as modifying the
2051   requirements associated with the standard directive. In this way,
2052   extensions to the cache-control directives can be made without
2053   requiring changes to the base protocol.
2054</t>
2055<t>
2056   This extension mechanism depends on an HTTP cache obeying all of the
2057   cache-control directives defined for its native HTTP-version, obeying
2058   certain extensions, and ignoring all directives that it does not
2059   understand.
2060</t>
2061<t>
2062   For example, consider a hypothetical new response directive called
2063   community which acts as a modifier to the private directive. We
2064   define this new directive to mean that, in addition to any non-shared
2065   cache, any cache which is shared only by members of the community
2066   named within its value may cache the response. An origin server
2067   wishing to allow the UCI community to use an otherwise private
2068   response in their shared cache(s) could do so by including
2069</t>
2070<figure><artwork type="example">
2071    Cache-Control: private, community="UCI"
2072</artwork></figure>
2073<t>
2074   A cache seeing this header field will act correctly even if the cache
2075   does not understand the community cache-extension, since it will also
2076   see and understand the private directive and thus default to the safe
2077   behavior.
2078</t>
2079<t>
2080   Unrecognized cache-directives &MUST; be ignored; it is assumed that any
2081   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2082   be combined with standard directives (or the response's default
2083   cacheability) such that the cache behavior will remain minimally
2084   correct even if the cache does not understand the extension(s).
2085</t>
2086</section>
2087</section>
2088
2089<section title="Expires" anchor="header.expires">
2090  <iref primary="true" item="Expires header" x:for-anchor=""/>
2091  <iref primary="true" item="Headers" subitem="Expires" x:for-anchor=""/>
2092<t>
2093   The Expires entity-header field gives the date/time after which the
2094   response is considered stale. A stale cache entry may not normally be
2095   returned by a cache (either a proxy cache or a user agent cache)
2096   unless it is first validated with the origin server (or with an
2097   intermediate cache that has a fresh copy of the entity). See <xref target="expiration.model"/>
2098   for further discussion of the expiration model.
2099</t>
2100<t>
2101   The presence of an Expires field does not imply that the original
2102   resource will change or cease to exist at, before, or after that
2103   time.
2104</t>
2105<t>
2106   The format is an absolute date and time as defined by HTTP-date in
2107   &full-date;; it &MUST; be in RFC 1123 date format:
2108</t>
2109<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/>
2110   Expires = "Expires" ":" HTTP-date
2111</artwork></figure>
2112<t>
2113   An example of its use is
2114</t>
2115<figure><artwork type="example">
2116   Expires: Thu, 01 Dec 1994 16:00:00 GMT
2117</artwork></figure>
2118<t>
2119  <list><t>
2120      <x:h>Note:</x:h> if a response includes a Cache-Control field with the max-age
2121      directive (see <xref target="modifications.of.the.basic.expiration.mechanism"/>), that directive overrides the
2122      Expires field.
2123  </t></list>
2124</t>
2125<t>
2126   HTTP/1.1 clients and caches &MUST; treat other invalid date formats,
2127   especially including the value "0", as in the past (i.e., "already
2128   expired").
2129</t>
2130<t>
2131   To mark a response as "already expired," an origin server sends an
2132   Expires date that is equal to the Date header value. (See the rules
2133   for expiration calculations in <xref target="expiration.calculations"/>.)
2134</t>
2135<t>
2136   To mark a response as "never expires," an origin server sends an
2137   Expires date approximately one year from the time the response is
2138   sent. HTTP/1.1 servers &SHOULD-NOT;  send Expires dates more than one
2139   year in the future.
2140</t>
2141<t>
2142   The presence of an Expires header field with a date value of some
2143   time in the future on a response that otherwise would by default be
2144   non-cacheable indicates that the response is cacheable, unless
2145   indicated otherwise by a Cache-Control header field (<xref target="header.cache-control"/>).
2146</t>
2147</section>
2148
2149<section title="Pragma" anchor="header.pragma">
2150  <iref primary="true" item="Pragma header" x:for-anchor=""/>
2151  <iref primary="true" item="Headers" subitem="Pragma" x:for-anchor=""/>
2152<t>
2153   The Pragma general-header field is used to include implementation-specific
2154   directives that might apply to any recipient along the
2155   request/response chain. All pragma directives specify optional
2156   behavior from the viewpoint of the protocol; however, some systems
2157   &MAY; require that behavior be consistent with the directives.
2158</t>
2159<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"/>
2160    Pragma            = "Pragma" ":" 1#pragma-directive
2161    pragma-directive  = "no-cache" | extension-pragma
2162    extension-pragma  = token [ "=" ( token | quoted-string ) ]
2163</artwork></figure>
2164<t>
2165   When the no-cache directive is present in a request message, an
2166   application &SHOULD; forward the request toward the origin server even
2167   if it has a cached copy of what is being requested. This pragma
2168   directive has the same semantics as the no-cache cache-directive (see
2169   <xref target="header.cache-control"/>) and is defined here for backward compatibility with
2170   HTTP/1.0. Clients &SHOULD; include both header fields when a no-cache
2171   request is sent to a server not known to be HTTP/1.1 compliant.
2172</t>
2173<t>
2174   Pragma directives &MUST; be passed through by a proxy or gateway
2175   application, regardless of their significance to that application,
2176   since the directives might be applicable to all recipients along the
2177   request/response chain. It is not possible to specify a pragma for a
2178   specific recipient; however, any pragma directive not relevant to a
2179   recipient &SHOULD; be ignored by that recipient.
2180</t>
2181<t>
2182   HTTP/1.1 caches &SHOULD; treat "Pragma: no-cache" as if the client had
2183   sent "Cache-Control: no-cache". No new Pragma directives will be
2184   defined in HTTP.
2185  <list><t>
2186      <x:h>Note:</x:h> because the meaning of "Pragma: no-cache as a response
2187      header field is not actually specified, it does not provide a
2188      reliable replacement for "Cache-Control: no-cache" in a response
2189  </t></list>
2190</t>
2191</section>
2192
2193<section title="Vary" anchor="header.vary">
2194  <iref primary="true" item="Vary header" x:for-anchor=""/>
2195  <iref primary="true" item="Headers" subitem="Vary" x:for-anchor=""/>
2196<t>
2197   The Vary field value indicates the set of request-header fields that
2198   fully determines, while the response is fresh, whether a cache is
2199   permitted to use the response to reply to a subsequent request
2200   without revalidation. For uncacheable or stale responses, the Vary
2201   field value advises the user agent about the criteria that were used
2202   to select the representation. A Vary field value of "*" implies that
2203   a cache cannot determine from the request headers of a subsequent
2204   request whether this response is the appropriate representation. See
2205   <xref target="caching.negotiated.responses"/> for use of the Vary header field by caches.
2206</t>
2207<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/>
2208    Vary  = "Vary" ":" ( "*" | 1#field-name )
2209</artwork></figure>
2210<t>
2211   An HTTP/1.1 server &SHOULD; include a Vary header field with any
2212   cacheable response that is subject to server-driven negotiation.
2213   Doing so allows a cache to properly interpret future requests on that
2214   resource and informs the user agent about the presence of negotiation
2215   on that resource. A server &MAY; include a Vary header field with a
2216   non-cacheable response that is subject to server-driven negotiation,
2217   since this might provide the user agent with useful information about
2218   the dimensions over which the response varies at the time of the
2219   response.
2220</t>
2221<t>
2222   A Vary field value consisting of a list of field-names signals that
2223   the representation selected for the response is based on a selection
2224   algorithm which considers ONLY the listed request-header field values
2225   in selecting the most appropriate representation. A cache &MAY; assume
2226   that the same selection will be made for future requests with the
2227   same values for the listed field names, for the duration of time for
2228   which the response is fresh.
2229</t>
2230<t>
2231   The field-names given are not limited to the set of standard
2232   request-header fields defined by this specification. Field names are
2233   case-insensitive.
2234</t>
2235<t>
2236   A Vary field value of "*" signals that unspecified parameters not
2237   limited to the request-headers (e.g., the network address of the
2238   client), play a role in the selection of the response representation.
2239   The "*" value &MUST-NOT; be generated by a proxy server; it may only be
2240   generated by an origin server.
2241</t>
2242</section>
2243
2244<section title="Warning" anchor="header.warning">
2245  <iref primary="true" item="Warning header" x:for-anchor=""/>
2246  <iref primary="true" item="Headers" subitem="Warning" x:for-anchor=""/>
2247<t>
2248   The Warning general-header field is used to carry additional
2249   information about the status or transformation of a message which
2250   might not be reflected in the message. This information is typically
2251   used to warn about a possible lack of semantic transparency from
2252   caching operations or transformations applied to the entity body of
2253   the message.
2254</t>
2255<t>
2256   Warning headers are sent with responses using:
2257</t>
2258<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"/>
2259    Warning    = "Warning" ":" 1#warning-value
2260
2261    warning-value = warn-code SP warn-agent SP warn-text
2262                                          [SP warn-date]
2263
2264    warn-code  = 3DIGIT
2265    warn-agent = ( host [ ":" port ] ) | pseudonym
2266                    ; the name or pseudonym of the server adding
2267                    ; the Warning header, for use in debugging
2268    warn-text  = quoted-string
2269    warn-date  = &lt;"&gt; HTTP-date &lt;"&gt;
2270</artwork></figure>
2271<t>
2272   A response &MAY; carry more than one Warning header.
2273</t>
2274<t>
2275   The warn-text &SHOULD; be in a natural language and character set that
2276   is most likely to be intelligible to the human user receiving the
2277   response. This decision &MAY; be based on any available knowledge, such
2278   as the location of the cache or user, the Accept-Language field in a
2279   request, the Content-Language field in a response, etc. The default
2280   language is English and the default character set is ISO-8859-1.
2281</t>
2282<t>
2283   If a character set other than ISO-8859-1 is used, it &MUST; be encoded
2284   in the warn-text using the method described in RFC 2047 <xref target="RFC2047"/>.
2285</t>
2286<t>
2287   Warning headers can in general be applied to any message, however
2288   some specific warn-codes are specific to caches and can only be
2289   applied to response messages. New Warning headers &SHOULD; be added
2290   after any existing Warning headers. A cache &MUST-NOT; delete any
2291   Warning header that it received with a message. However, if a cache
2292   successfully validates a cache entry, it &SHOULD; remove any Warning
2293   headers previously attached to that entry except as specified for
2294   specific Warning codes. It &MUST; then add any Warning headers received
2295   in the validating response. In other words, Warning headers are those
2296   that would be attached to the most recent relevant response.
2297</t>
2298<t>
2299   When multiple Warning headers are attached to a response, the user
2300   agent ought to inform the user of as many of them as possible, in the
2301   order that they appear in the response. If it is not possible to
2302   inform the user of all of the warnings, the user agent &SHOULD; follow
2303   these heuristics:
2304  <list style="symbols">
2305    <t>Warnings that appear early in the response take priority over
2306        those appearing later in the response.</t>
2307
2308    <t>Warnings in the user's preferred character set take priority
2309        over warnings in other character sets but with identical warn-codes
2310        and warn-agents.</t>
2311  </list>
2312</t>
2313<t>
2314   Systems that generate multiple Warning headers &SHOULD; order them with
2315   this user agent behavior in mind.
2316</t>
2317<t>
2318   Requirements for the behavior of caches with respect to Warnings are
2319   stated in <xref target="warnings"/>.
2320</t>
2321<t>
2322   This is a list of the currently-defined warn-codes, each with a
2323   recommended warn-text in English, and a description of its meaning.
2324</t>
2325<t>
2326   110 Response is stale
2327  <list><t>
2328     &MUST; be included whenever the returned response is stale.
2329  </t></list>
2330</t>
2331<t>
2332   111 Revalidation failed
2333  <list><t>
2334     &MUST; be included if a cache returns a stale response because an
2335     attempt to revalidate the response failed, due to an inability to
2336     reach the server.
2337  </t></list>
2338</t>
2339<t>
2340   112 Disconnected operation
2341  <list><t>
2342     &SHOULD; be included if the cache is intentionally disconnected from
2343     the rest of the network for a period of time.
2344  </t></list>
2345</t>
2346<t>
2347   113 Heuristic expiration
2348  <list><t>
2349     &MUST; be included if the cache heuristically chose a freshness
2350     lifetime greater than 24 hours and the response's age is greater
2351     than 24 hours.
2352  </t></list>
2353</t>
2354<t>
2355   199 Miscellaneous warning
2356  <list><t>
2357     The warning text &MAY; include arbitrary information to be presented
2358     to a human user, or logged. A system receiving this warning &MUST-NOT;
2359     take any automated action, besides presenting the warning to
2360     the user.
2361  </t></list>
2362</t>
2363<t>
2364   214 Transformation applied
2365  <list><t>
2366     &MUST; be added by an intermediate cache or proxy if it applies any
2367     transformation changing the content-coding (as specified in the
2368     Content-Encoding header) or media-type (as specified in the
2369     Content-Type header) of the response, or the entity-body of the
2370     response, unless this Warning code already appears in the response.
2371  </t></list>
2372</t>
2373<t>
2374   299 Miscellaneous persistent warning
2375  <list><t>
2376     The warning text &MAY; include arbitrary information to be presented
2377     to a human user, or logged. A system receiving this warning &MUST-NOT;
2378     take any automated action.
2379  </t></list>
2380</t>
2381<t>
2382   If an implementation sends a message with one or more Warning headers
2383   whose version is HTTP/1.0 or lower, then the sender &MUST; include in
2384   each warning-value a warn-date that matches the date in the response.
2385</t>
2386<t>
2387   If an implementation receives a message with a warning-value that
2388   includes a warn-date, and that warn-date is different from the Date
2389   value in the response, then that warning-value &MUST; be deleted from
2390   the message before storing, forwarding, or using it. (This prevents
2391   bad consequences of naive caching of Warning header fields.) If all
2392   of the warning-values are deleted for this reason, the Warning header
2393   &MUST; be deleted as well.
2394</t>
2395</section>
2396
2397</section>
2398
2399<section title="IANA Considerations" anchor="IANA.considerations">
2400<t>
2401   TBD.
2402</t>
2403</section>
2404
2405<section title="Security Considerations" anchor="security.considerations">
2406<t>
2407   Caching proxies provide additional potential vulnerabilities, since
2408   the contents of the cache represent an attractive target for
2409   malicious exploitation. Because cache contents persist after an HTTP
2410   request is complete, an attack on the cache can reveal information
2411   long after a user believes that the information has been removed from
2412   the network. Therefore, cache contents should be protected as
2413   sensitive information.
2414</t>
2415</section>
2416
2417<section title="Acknowledgments" anchor="ack">
2418<t>
2419   Much of the content and presentation of the caching design is due to
2420   suggestions and comments from individuals including: Shel Kaphan,
2421   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2422</t>
2423<t>
2424   Based on an XML translation of RFC 2616 by Julian Reschke.
2425</t>
2426</section>
2427</middle>
2428<back>
2429<references>
2430
2431<reference anchor="Part1">
2432   <front>
2433      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
2434      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2435         <organization abbrev="Day Software">Day Software</organization>
2436         <address><email>fielding@gbiv.com</email></address>
2437      </author>
2438      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2439         <organization>One Laptop per Child</organization>
2440         <address><email>jg@laptop.org</email></address>
2441      </author>
2442      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2443         <organization abbrev="HP">Hewlett-Packard Company</organization>
2444         <address><email>JeffMogul@acm.org</email></address>
2445      </author>
2446      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2447         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2448         <address><email>henrikn@microsoft.com</email></address>
2449      </author>
2450      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2451         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2452         <address><email>LMM@acm.org</email></address>
2453      </author>
2454      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2455         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2456         <address><email>paulle@microsoft.com</email></address>
2457      </author>
2458      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2459         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2460         <address><email>timbl@w3.org</email></address>
2461      </author>
2462      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2463   </front>
2464   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
2465   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
2466</reference>
2467
2468<reference anchor="Part2">
2469   <front>
2470      <title abbrev="HTTP/1.1">HTTP/1.1, part 2: Message Semantics</title>
2471      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2472         <organization abbrev="Day Software">Day Software</organization>
2473         <address><email>fielding@gbiv.com</email></address>
2474      </author>
2475      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2476         <organization>One Laptop per Child</organization>
2477         <address><email>jg@laptop.org</email></address>
2478      </author>
2479      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2480         <organization abbrev="HP">Hewlett-Packard Company</organization>
2481         <address><email>JeffMogul@acm.org</email></address>
2482      </author>
2483      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2484         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2485         <address><email>henrikn@microsoft.com</email></address>
2486      </author>
2487      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2488         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2489         <address><email>LMM@acm.org</email></address>
2490      </author>
2491      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2492         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2493         <address><email>paulle@microsoft.com</email></address>
2494      </author>
2495      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2496         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2497         <address><email>timbl@w3.org</email></address>
2498      </author>
2499      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2500   </front>
2501   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
2502   <x:source href="p2-semantics.xml" basename="p2-semantics"/>
2503</reference>
2504
2505<reference anchor="Part3">
2506   <front>
2507      <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
2508      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2509         <organization abbrev="Day Software">Day Software</organization>
2510         <address><email>fielding@gbiv.com</email></address>
2511      </author>
2512      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2513         <organization>One Laptop per Child</organization>
2514         <address><email>jg@laptop.org</email></address>
2515      </author>
2516      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2517         <organization abbrev="HP">Hewlett-Packard Company</organization>
2518         <address><email>JeffMogul@acm.org</email></address>
2519      </author>
2520      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2521         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2522         <address><email>henrikn@microsoft.com</email></address>
2523      </author>
2524      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2525         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2526         <address><email>LMM@acm.org</email></address>
2527      </author>
2528      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2529         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2530         <address><email>paulle@microsoft.com</email></address>
2531      </author>
2532      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2533         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2534         <address><email>timbl@w3.org</email></address>
2535      </author>
2536      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2537   </front>
2538   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
2539   <x:source href="p3-payload.xml" basename="p3-payload"/>
2540</reference>
2541
2542<reference anchor="Part4">
2543   <front>
2544      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
2545      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2546         <organization abbrev="Day Software">Day Software</organization>
2547         <address><email>fielding@gbiv.com</email></address>
2548      </author>
2549      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2550         <organization>One Laptop per Child</organization>
2551         <address><email>jg@laptop.org</email></address>
2552      </author>
2553      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2554         <organization abbrev="HP">Hewlett-Packard Company</organization>
2555         <address><email>JeffMogul@acm.org</email></address>
2556      </author>
2557      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2558         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2559         <address><email>henrikn@microsoft.com</email></address>
2560      </author>
2561      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2562         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2563         <address><email>LMM@acm.org</email></address>
2564      </author>
2565      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2566         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2567         <address><email>paulle@microsoft.com</email></address>
2568      </author>
2569      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2570         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2571         <address><email>timbl@w3.org</email></address>
2572      </author>
2573      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2574   </front>
2575   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
2576   <x:source href="p4-conditional.xml" basename="p4-conditional"/>
2577</reference>
2578
2579<reference anchor="Part5">
2580   <front>
2581      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
2582      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2583         <organization abbrev="Day Software">Day Software</organization>
2584         <address><email>fielding@gbiv.com</email></address>
2585      </author>
2586      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2587         <organization>One Laptop per Child</organization>
2588         <address><email>jg@laptop.org</email></address>
2589      </author>
2590      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2591         <organization abbrev="HP">Hewlett-Packard Company</organization>
2592         <address><email>JeffMogul@acm.org</email></address>
2593      </author>
2594      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2595         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2596         <address><email>henrikn@microsoft.com</email></address>
2597      </author>
2598      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2599         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2600         <address><email>LMM@acm.org</email></address>
2601      </author>
2602      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2603         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2604         <address><email>paulle@microsoft.com</email></address>
2605      </author>
2606      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2607         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2608         <address><email>timbl@w3.org</email></address>
2609      </author>
2610      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2611   </front>
2612   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
2613   <x:source href="p5-range.xml" basename="p5-range"/>
2614</reference>
2615
2616<reference anchor="Part7">
2617   <front>
2618      <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
2619      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
2620         <organization abbrev="Day Software">Day Software</organization>
2621         <address><email>fielding@gbiv.com</email></address>
2622      </author>
2623      <author initials="J." surname="Gettys" fullname="Jim Gettys">
2624         <organization>One Laptop per Child</organization>
2625         <address><email>jg@laptop.org</email></address>
2626      </author>
2627      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
2628         <organization abbrev="HP">Hewlett-Packard Company</organization>
2629         <address><email>JeffMogul@acm.org</email></address>
2630      </author>
2631      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
2632         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2633         <address><email>henrikn@microsoft.com</email></address>
2634      </author>
2635      <author initials="L." surname="Masinter" fullname="Larry Masinter">
2636         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
2637         <address><email>LMM@acm.org</email></address>
2638      </author>
2639      <author initials="P." surname="Leach" fullname="Paul J. Leach">
2640         <organization abbrev="Microsoft">Microsoft Corporation</organization>
2641         <address><email>paulle@microsoft.com</email></address>
2642      </author>
2643      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2644         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
2645         <address><email>timbl@w3.org</email></address>
2646      </author>
2647      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
2648   </front>
2649   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;"/>
2650   <x:source href="p7-auth.xml" basename="p7-auth"/>
2651</reference>
2652
2653<reference anchor="RFC2616">
2654   <front>
2655      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
2656      <author initials="R." surname="Fielding" fullname="R. Fielding">
2657         <organization>University of California, Irvine</organization>
2658         <address><email>fielding@ics.uci.edu</email></address>
2659      </author>
2660      <author initials="J." surname="Gettys" fullname="J. Gettys">
2661         <organization>W3C</organization>
2662         <address><email>jg@w3.org</email></address>
2663      </author>
2664      <author initials="J." surname="Mogul" fullname="J. Mogul">
2665         <organization>Compaq Computer Corporation</organization>
2666         <address><email>mogul@wrl.dec.com</email></address>
2667      </author>
2668      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
2669         <organization>MIT Laboratory for Computer Science</organization>
2670         <address><email>frystyk@w3.org</email></address>
2671      </author>
2672      <author initials="L." surname="Masinter" fullname="L. Masinter">
2673         <organization>Xerox Corporation</organization>
2674         <address><email>masinter@parc.xerox.com</email></address>
2675      </author>
2676      <author initials="P." surname="Leach" fullname="P. Leach">
2677         <organization>Microsoft Corporation</organization>
2678         <address><email>paulle@microsoft.com</email></address>
2679      </author>
2680      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
2681         <organization>W3C</organization>
2682         <address><email>timbl@w3.org</email></address>
2683      </author>
2684      <date month="June" year="1999"/>
2685   </front>
2686   <seriesInfo name="RFC" value="2616"/>
2687</reference>
2688
2689<reference anchor="RFC2047">
2690<front>
2691<title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text</title>
2692<author initials="K." surname="Moore" fullname="Keith Moore">
2693<organization>University of Tennessee</organization>
2694<address>
2695<postal>
2696<street>107 Ayres Hall</street>
2697<street>Knoxville TN 37996-1301</street></postal>
2698<email>moore@cs.utk.edu</email></address></author>
2699<date month="November" year="1996"/>
2700<area>Applications</area>
2701<keyword>Amercian Standard Code for Information Interchange</keyword>
2702<keyword>mail</keyword>
2703<keyword>multipurpose internet mail extensions</keyword>
2704</front>
2705<seriesInfo name="RFC" value="2047"/>
2706</reference>
2707
2708<reference anchor="RFC1305">
2709<front>
2710<title>Network Time Protocol (Version 3) Specification, Implementation</title>
2711<author initials="D." surname="Mills" fullname="David L. Mills">
2712<organization>University of Delaware, Electrical Engineering Department</organization>
2713<address>
2714<postal>
2715<street/>
2716<city>Newark</city>
2717<region>DE</region>
2718<code>19716</code>
2719<country>US</country></postal>
2720<phone>+1 302 451 8247</phone>
2721<email>mills@udel.edu</email></address></author>
2722<date month="March" year="1992"/>
2723<abstract>
2724<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>
2725<seriesInfo name="RFC" value="1305"/>
2726</reference>
2727
2728</references>
2729
2730<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
2731<t>
2732   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2733   was introduced to add this missing case. (Sections <xref target="response.cacheability" format="counter"/>,
2734   <xref target="header.cache-control" format="counter"/>,
2735   <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>)
2736</t>
2737<t>
2738   The Cache-Control: max-age directive was not properly defined for
2739   responses. (<xref target="modifications.of.the.basic.expiration.mechanism"/>)
2740</t>
2741<t>
2742   Warnings could be cached incorrectly, or not updated appropriately.
2743   (Section <xref target="warnings" format="counter"/>, <xref target="expiration.calculations" format="counter"/>, <xref target="non-modifiable.headers" format="counter"/>,
2744   <xref target="combining.headers" format="counter"/>, <xref target="modifications.of.the.basic.expiration.mechanism" format="counter"/>,
2745   and <xref target="header.warning" format="counter"/>) Warning
2746   also needed to be a general header, as PUT or other methods may have
2747   need for it in requests.
2748</t>
2749</section>
2750</back>
2751</rfc>
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