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4Network Working Group                                   R. Fielding, Ed.
5Internet-Draft                                              Day Software
6Obsoletes: 2068, 2616                                          J. Gettys
7(if approved)                                       One Laptop per Child
8Intended status: Standards Track                                J. Mogul
9Expires: June 22, 2008                                                HP
10                                                              H. Frystyk
11                                                               Microsoft
12                                                             L. Masinter
13                                                           Adobe Systems
14                                                                P. Leach
15                                                               Microsoft
16                                                          T. Berners-Lee
17                                                                 W3C/MIT
18                                                       December 20, 2007
19
20
21                       HTTP/1.1, part 6: Caching
22                     draft-ietf-httpbis-p6-cache-00
23
24Status of this Memo
25
26   By submitting this Internet-Draft, each author represents that any
27   applicable patent or other IPR claims of which he or she is aware
28   have been or will be disclosed, and any of which he or she becomes
29   aware will be disclosed, in accordance with Section 6 of BCP 79.
30
31   Internet-Drafts are working documents of the Internet Engineering
32   Task Force (IETF), its areas, and its working groups.  Note that
33   other groups may also distribute working documents as Internet-
34   Drafts.
35
36   Internet-Drafts are draft documents valid for a maximum of six months
37   and may be updated, replaced, or obsoleted by other documents at any
38   time.  It is inappropriate to use Internet-Drafts as reference
39   material or to cite them other than as "work in progress."
40
41   The list of current Internet-Drafts can be accessed at
42   http://www.ietf.org/ietf/1id-abstracts.txt.
43
44   The list of Internet-Draft Shadow Directories can be accessed at
45   http://www.ietf.org/shadow.html.
46
47   This Internet-Draft will expire on June 22, 2008.
48
49Copyright Notice
50
51   Copyright (C) The IETF Trust (2007).
52
53
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57Internet-Draft              HTTP/1.1, part 6               December 2007
58
59
60Abstract
61
62   The Hypertext Transfer Protocol (HTTP) is an application-level
63   protocol for distributed, collaborative, hypermedia information
64   systems.  HTTP has been in use by the World Wide Web global
65   information initiative since 1990.  This document is Part 6 of the
66   seven-part specification that defines the protocol referred to as
67   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 6 defines
68   requirements on HTTP caches and the associated header fields that
69   control cache behavior or indicate cacheable response messages.
70
71Editorial Note (To be removed by RFC Editor)
72
73   This version of the HTTP specification contains only minimal
74   editorial changes from [RFC2616] (abstract, introductory paragraph,
75   and authors' addresses).  All other changes are due to partitioning
76   the original into seven mostly independent parts.  The intent is for
77   readers of future drafts to able to use draft 00 as the basis for
78   comparison when the WG makes later changes to the specification text.
79   This draft will shortly be followed by draft 01 (containing the first
80   round of changes that have already been agreed to on the mailing
81   list).  There is no point in reviewing this draft other than to
82   verify that the partitioning has been done correctly.  Roy T.
83   Fielding, Yves Lafon, and Julian Reschke will be the editors after
84   draft 00 is submitted.
85
86   Discussion of this draft should take place on the HTTPBIS working
87   group mailing list (ietf-http-wg@w3.org).  The current issues list is
88   at <http://www3.tools.ietf.org/wg/httpbis/trac/report/11> and related
89   documents (including fancy diffs) can be found at
90   <http://www3.tools.ietf.org/wg/httpbis/>.
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113Internet-Draft              HTTP/1.1, part 6               December 2007
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115
116Table of Contents
117
118   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
119     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
120     1.2.  Delta Seconds  . . . . . . . . . . . . . . . . . . . . . .  6
121   2.  Caching in HTTP  . . . . . . . . . . . . . . . . . . . . . . .  6
122     2.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
123       2.1.1.  Cache Correctness  . . . . . . . . . . . . . . . . . .  8
124       2.1.2.  Warnings . . . . . . . . . . . . . . . . . . . . . . .  9
125       2.1.3.  Cache-control Mechanisms . . . . . . . . . . . . . . . 10
126       2.1.4.  Explicit User Agent Warnings . . . . . . . . . . . . . 10
127       2.1.5.  Exceptions to the Rules and Warnings . . . . . . . . . 11
128       2.1.6.  Client-controlled Behavior . . . . . . . . . . . . . . 11
129     2.2.  Expiration Model . . . . . . . . . . . . . . . . . . . . . 11
130       2.2.1.  Server-Specified Expiration  . . . . . . . . . . . . . 11
131       2.2.2.  Heuristic Expiration . . . . . . . . . . . . . . . . . 12
132       2.2.3.  Age Calculations . . . . . . . . . . . . . . . . . . . 13
133       2.2.4.  Expiration Calculations  . . . . . . . . . . . . . . . 15
134       2.2.5.  Disambiguating Expiration Values . . . . . . . . . . . 16
135       2.2.6.  Disambiguating Multiple Responses  . . . . . . . . . . 16
136     2.3.  Validation Model . . . . . . . . . . . . . . . . . . . . . 17
137       2.3.1.  Last-Modified Dates  . . . . . . . . . . . . . . . . . 18
138       2.3.2.  Entity Tag Cache Validators  . . . . . . . . . . . . . 18
139       2.3.3.  Non-validating Conditionals  . . . . . . . . . . . . . 18
140     2.4.  Response Cacheability  . . . . . . . . . . . . . . . . . . 18
141     2.5.  Constructing Responses From Caches . . . . . . . . . . . . 19
142       2.5.1.  End-to-end and Hop-by-hop Headers  . . . . . . . . . . 19
143       2.5.2.  Non-modifiable Headers . . . . . . . . . . . . . . . . 20
144       2.5.3.  Combining Headers  . . . . . . . . . . . . . . . . . . 21
145     2.6.  Caching Negotiated Responses . . . . . . . . . . . . . . . 22
146     2.7.  Shared and Non-Shared Caches . . . . . . . . . . . . . . . 24
147     2.8.  Errors or Incomplete Response Cache Behavior . . . . . . . 24
148     2.9.  Side Effects of GET and HEAD . . . . . . . . . . . . . . . 24
149     2.10. Invalidation After Updates or Deletions  . . . . . . . . . 25
150     2.11. Write-Through Mandatory  . . . . . . . . . . . . . . . . . 25
151     2.12. Cache Replacement  . . . . . . . . . . . . . . . . . . . . 26
152     2.13. History Lists  . . . . . . . . . . . . . . . . . . . . . . 26
153   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 27
154     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
155     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 27
156       3.2.1.  What is Cacheable  . . . . . . . . . . . . . . . . . . 29
157       3.2.2.  What May be Stored by Caches . . . . . . . . . . . . . 30
158       3.2.3.  Modifications of the Basic Expiration Mechanism  . . . 31
159       3.2.4.  Cache Revalidation and Reload Controls . . . . . . . . 33
160       3.2.5.  No-Transform Directive . . . . . . . . . . . . . . . . 35
161       3.2.6.  Cache Control Extensions . . . . . . . . . . . . . . . 36
162     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 37
163     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 38
164
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172     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
173     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 39
174   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
175   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 42
176   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 42
177   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
178   Appendix A.  Changes from RFC 2068 . . . . . . . . . . . . . . . . 43
179   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
180   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46
181   Intellectual Property and Copyright Statements . . . . . . . . . . 49
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227
2281.  Introduction
229
230   This document will define aspects of HTTP related to caching response
231   messages.  Right now it only includes the extracted relevant sections
232   of RFC 2616 [RFC2616] without edit.
233
2341.1.  Terminology
235
236   This specification uses a number of terms to refer to the roles
237   played by participants in, and objects of, the HTTP communication.
238
239   cache
240
241      A program's local store of response messages and the subsystem
242      that controls its message storage, retrieval, and deletion.  A
243      cache stores cacheable responses in order to reduce the response
244      time and network bandwidth consumption on future, equivalent
245      requests.  Any client or server may include a cache, though a
246      cache cannot be used by a server that is acting as a tunnel.
247
248   cacheable
249
250      A response is cacheable if a cache is allowed to store a copy of
251      the response message for use in answering subsequent requests.
252      The rules for determining the cacheability of HTTP responses are
253      defined in Section 2.  Even if a resource is cacheable, there may
254      be additional constraints on whether a cache can use the cached
255      copy for a particular request.
256
257   first-hand
258
259      A response is first-hand if it comes directly and without
260      unnecessary delay from the origin server, perhaps via one or more
261      proxies.  A response is also first-hand if its validity has just
262      been checked directly with the origin server.
263
264   explicit expiration time
265
266      The time at which the origin server intends that an entity should
267      no longer be returned by a cache without further validation.
268
269   heuristic expiration time
270
271      An expiration time assigned by a cache when no explicit expiration
272      time is available.
273
274   age
275
276
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283
284      The age of a response is the time since it was sent by, or
285      successfully validated with, the origin server.
286
287   freshness lifetime
288
289      The length of time between the generation of a response and its
290      expiration time.
291
292   fresh
293
294      A response is fresh if its age has not yet exceeded its freshness
295      lifetime.
296
297   stale
298
299      A response is stale if its age has passed its freshness lifetime.
300
301   semantically transparent
302
303      A cache behaves in a "semantically transparent" manner, with
304      respect to a particular response, when its use affects neither the
305      requesting client nor the origin server, except to improve
306      performance.  When a cache is semantically transparent, the client
307      receives exactly the same response (except for hop-by-hop headers)
308      that it would have received had its request been handled directly
309      by the origin server.
310
311   validator
312
313      A protocol element (e.g., an entity tag or a Last-Modified time)
314      that is used to find out whether a cache entry is an equivalent
315      copy of an entity.
316
3171.2.  Delta Seconds
318
319   Some HTTP header fields allow a time value to be specified as an
320   integer number of seconds, represented in decimal, after the time
321   that the message was received.
322
323       delta-seconds  = 1*DIGIT
324
325
3262.  Caching in HTTP
327
3282.1.  Overview
329
330   HTTP is typically used for distributed information systems, where
331   performance can be improved by the use of response caches.  The
332
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339
340   HTTP/1.1 protocol includes a number of elements intended to make
341   caching work as well as possible.  Because these elements are
342   inextricable from other aspects of the protocol, and because they
343   interact with each other, it is useful to describe the basic caching
344   design of HTTP separately from the detailed descriptions of methods,
345   headers, response codes, etc.
346
347   Caching would be useless if it did not significantly improve
348   performance.  The goal of caching in HTTP/1.1 is to eliminate the
349   need to send requests in many cases, and to eliminate the need to
350   send full responses in many other cases.  The former reduces the
351   number of network round-trips required for many operations; we use an
352   "expiration" mechanism for this purpose (see Section 2.2).  The
353   latter reduces network bandwidth requirements; we use a "validation"
354   mechanism for this purpose (see Section 2.3).
355
356   Requirements for performance, availability, and disconnected
357   operation require us to be able to relax the goal of semantic
358   transparency.  The HTTP/1.1 protocol allows origin servers, caches,
359   and clients to explicitly reduce transparency when necessary.
360   However, because non-transparent operation may confuse non-expert
361   users, and might be incompatible with certain server applications
362   (such as those for ordering merchandise), the protocol requires that
363   transparency be relaxed
364
365   o  only by an explicit protocol-level request when relaxed by client
366      or origin server
367
368   o  only with an explicit warning to the end user when relaxed by
369      cache or client
370
371   Therefore, the HTTP/1.1 protocol provides these important elements:
372
373   1.  Protocol features that provide full semantic transparency when
374       this is required by all parties.
375
376   2.  Protocol features that allow an origin server or user agent to
377       explicitly request and control non-transparent operation.
378
379   3.  Protocol features that allow a cache to attach warnings to
380       responses that do not preserve the requested approximation of
381       semantic transparency.
382
383   A basic principle is that it must be possible for the clients to
384   detect any potential relaxation of semantic transparency.
385
386      Note: The server, cache, or client implementor might be faced with
387      design decisions not explicitly discussed in this specification.
388
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396      If a decision might affect semantic transparency, the implementor
397      ought to err on the side of maintaining transparency unless a
398      careful and complete analysis shows significant benefits in
399      breaking transparency.
400
4012.1.1.  Cache Correctness
402
403   A correct cache MUST respond to a request with the most up-to-date
404   response held by the cache that is appropriate to the request (see
405   sections 2.2.5, 2.2.6, and 2.12) which meets one of the following
406   conditions:
407
408   1.  It has been checked for equivalence with what the origin server
409       would have returned by revalidating the response with the origin
410       server (Section 2.3);
411
412   2.  It is "fresh enough" (see Section 2.2).  In the default case,
413       this means it meets the least restrictive freshness requirement
414       of the client, origin server, and cache (see Section 3.2); if the
415       origin server so specifies, it is the freshness requirement of
416       the origin server alone.  If a stored response is not "fresh
417       enough" by the most restrictive freshness requirement of both the
418       client and the origin server, in carefully considered
419       circumstances the cache MAY still return the response with the
420       appropriate Warning header (see section 2.1.5 and 3.6), unless
421       such a response is prohibited (e.g., by a "no-store" cache-
422       directive, or by a "no-cache" cache-request-directive; see
423       Section 3.2).
424
425   3.  It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or
426       error (4xx or 5xx) response message.
427
428   If the cache can not communicate with the origin server, then a
429   correct cache SHOULD respond as above if the response can be
430   correctly served from the cache; if not it MUST return an error or
431   warning indicating that there was a communication failure.
432
433   If a cache receives a response (either an entire response, or a 304
434   (Not Modified) response) that it would normally forward to the
435   requesting client, and the received response is no longer fresh, the
436   cache SHOULD forward it to the requesting client without adding a new
437   Warning (but without removing any existing Warning headers).  A cache
438   SHOULD NOT attempt to revalidate a response simply because that
439   response became stale in transit; this might lead to an infinite
440   loop.  A user agent that receives a stale response without a Warning
441   MAY display a warning indication to the user.
442
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4522.1.2.  Warnings
453
454   Whenever a cache returns a response that is neither first-hand nor
455   "fresh enough" (in the sense of condition 2 in Section 2.1.1), it
456   MUST attach a warning to that effect, using a Warning general-header.
457   The Warning header and the currently defined warnings are described
458   in Section 3.6.  The warning allows clients to take appropriate
459   action.
460
461   Warnings MAY be used for other purposes, both cache-related and
462   otherwise.  The use of a warning, rather than an error status code,
463   distinguish these responses from true failures.
464
465   Warnings are assigned three digit warn-codes.  The first digit
466   indicates whether the Warning MUST or MUST NOT be deleted from a
467   stored cache entry after a successful revalidation:
468
469   1xx  Warnings that describe the freshness or revalidation status of
470      the response, and so MUST be deleted after a successful
471      revalidation. 1XX warn-codes MAY be generated by a cache only when
472      validating a cached entry.  It MUST NOT be generated by clients.
473
474   2xx  Warnings that describe some aspect of the entity body or entity
475      headers that is not rectified by a revalidation (for example, a
476      lossy compression of the entity bodies) and which MUST NOT be
477      deleted after a successful revalidation.
478
479   See Section 3.6 for the definitions of the codes themselves.
480
481   HTTP/1.0 caches will cache all Warnings in responses, without
482   deleting the ones in the first category.  Warnings in responses that
483   are passed to HTTP/1.0 caches carry an extra warning-date field,
484   which prevents a future HTTP/1.1 recipient from believing an
485   erroneously cached Warning.
486
487   Warnings also carry a warning text.  The text MAY be in any
488   appropriate natural language (perhaps based on the client's Accept
489   headers), and include an OPTIONAL indication of what character set is
490   used.
491
492   Multiple warnings MAY be attached to a response (either by the origin
493   server or by a cache), including multiple warnings with the same code
494   number.  For example, a server might provide the same warning with
495   texts in both English and Basque.
496
497   When multiple warnings are attached to a response, it might not be
498   practical or reasonable to display all of them to the user.  This
499   version of HTTP does not specify strict priority rules for deciding
500
501
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507
508   which warnings to display and in what order, but does suggest some
509   heuristics.
510
5112.1.3.  Cache-control Mechanisms
512
513   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
514   times and validators) are implicit directives to caches.  In some
515   cases, a server or client might need to provide explicit directives
516   to the HTTP caches.  We use the Cache-Control header for this
517   purpose.
518
519   The Cache-Control header allows a client or server to transmit a
520   variety of directives in either requests or responses.  These
521   directives typically override the default caching algorithms.  As a
522   general rule, if there is any apparent conflict between header
523   values, the most restrictive interpretation is applied (that is, the
524   one that is most likely to preserve semantic transparency).  However,
525   in some cases, cache-control directives are explicitly specified as
526   weakening the approximation of semantic transparency (for example,
527   "max-stale" or "public").
528
529   The cache-control directives are described in detail in Section 3.2.
530
5312.1.4.  Explicit User Agent Warnings
532
533   Many user agents make it possible for users to override the basic
534   caching mechanisms.  For example, the user agent might allow the user
535   to specify that cached entities (even explicitly stale ones) are
536   never validated.  Or the user agent might habitually add "Cache-
537   Control: max-stale=3600" to every request.  The user agent SHOULD NOT
538   default to either non-transparent behavior, or behavior that results
539   in abnormally ineffective caching, but MAY be explicitly configured
540   to do so by an explicit action of the user.
541
542   If the user has overridden the basic caching mechanisms, the user
543   agent SHOULD explicitly indicate to the user whenever this results in
544   the display of information that might not meet the server's
545   transparency requirements (in particular, if the displayed entity is
546   known to be stale).  Since the protocol normally allows the user
547   agent to determine if responses are stale or not, this indication
548   need only be displayed when this actually happens.  The indication
549   need not be a dialog box; it could be an icon (for example, a picture
550   of a rotting fish) or some other indicator.
551
552   If the user has overridden the caching mechanisms in a way that would
553   abnormally reduce the effectiveness of caches, the user agent SHOULD
554   continually indicate this state to the user (for example, by a
555   display of a picture of currency in flames) so that the user does not
556
557
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563
564   inadvertently consume excess resources or suffer from excessive
565   latency.
566
5672.1.5.  Exceptions to the Rules and Warnings
568
569   In some cases, the operator of a cache MAY choose to configure it to
570   return stale responses even when not requested by clients.  This
571   decision ought not be made lightly, but may be necessary for reasons
572   of availability or performance, especially when the cache is poorly
573   connected to the origin server.  Whenever a cache returns a stale
574   response, it MUST mark it as such (using a Warning header) enabling
575   the client software to alert the user that there might be a potential
576   problem.
577
578   It also allows the user agent to take steps to obtain a first-hand or
579   fresh response.  For this reason, a cache SHOULD NOT return a stale
580   response if the client explicitly requests a first-hand or fresh one,
581   unless it is impossible to comply for technical or policy reasons.
582
5832.1.6.  Client-controlled Behavior
584
585   While the origin server (and to a lesser extent, intermediate caches,
586   by their contribution to the age of a response) are the primary
587   source of expiration information, in some cases the client might need
588   to control a cache's decision about whether to return a cached
589   response without validating it.  Clients do this using several
590   directives of the Cache-Control header.
591
592   A client's request MAY specify the maximum age it is willing to
593   accept of an unvalidated response; specifying a value of zero forces
594   the cache(s) to revalidate all responses.  A client MAY also specify
595   the minimum time remaining before a response expires.  Both of these
596   options increase constraints on the behavior of caches, and so cannot
597   further relax the cache's approximation of semantic transparency.
598
599   A client MAY also specify that it will accept stale responses, up to
600   some maximum amount of staleness.  This loosens the constraints on
601   the caches, and so might violate the origin server's specified
602   constraints on semantic transparency, but might be necessary to
603   support disconnected operation, or high availability in the face of
604   poor connectivity.
605
6062.2.  Expiration Model
607
6082.2.1.  Server-Specified Expiration
609
610   HTTP caching works best when caches can entirely avoid making
611   requests to the origin server.  The primary mechanism for avoiding
612
613
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619
620   requests is for an origin server to provide an explicit expiration
621   time in the future, indicating that a response MAY be used to satisfy
622   subsequent requests.  In other words, a cache can return a fresh
623   response without first contacting the server.
624
625   Our expectation is that servers will assign future explicit
626   expiration times to responses in the belief that the entity is not
627   likely to change, in a semantically significant way, before the
628   expiration time is reached.  This normally preserves semantic
629   transparency, as long as the server's expiration times are carefully
630   chosen.
631
632   The expiration mechanism applies only to responses taken from a cache
633   and not to first-hand responses forwarded immediately to the
634   requesting client.
635
636   If an origin server wishes to force a semantically transparent cache
637   to validate every request, it MAY assign an explicit expiration time
638   in the past.  This means that the response is always stale, and so
639   the cache SHOULD validate it before using it for subsequent requests.
640   See Section 3.2.4 for a more restrictive way to force revalidation.
641
642   If an origin server wishes to force any HTTP/1.1 cache, no matter how
643   it is configured, to validate every request, it SHOULD use the "must-
644   revalidate" cache-control directive (see Section 3.2).
645
646   Servers specify explicit expiration times using either the Expires
647   header, or the max-age directive of the Cache-Control header.
648
649   An expiration time cannot be used to force a user agent to refresh
650   its display or reload a resource; its semantics apply only to caching
651   mechanisms, and such mechanisms need only check a resource's
652   expiration status when a new request for that resource is initiated.
653   See Section 2.13 for an explanation of the difference between caches
654   and history mechanisms.
655
6562.2.2.  Heuristic Expiration
657
658   Since origin servers do not always provide explicit expiration times,
659   HTTP caches typically assign heuristic expiration times, employing
660   algorithms that use other header values (such as the Last-Modified
661   time) to estimate a plausible expiration time.  The HTTP/1.1
662   specification does not provide specific algorithms, but does impose
663   worst-case constraints on their results.  Since heuristic expiration
664   times might compromise semantic transparency, they ought to used
665   cautiously, and we encourage origin servers to provide explicit
666   expiration times as much as possible.
667
668
669
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675
6762.2.3.  Age Calculations
677
678   In order to know if a cached entry is fresh, a cache needs to know if
679   its age exceeds its freshness lifetime.  We discuss how to calculate
680   the latter in Section 2.2.4; this section describes how to calculate
681   the age of a response or cache entry.
682
683   In this discussion, we use the term "now" to mean "the current value
684   of the clock at the host performing the calculation."  Hosts that use
685   HTTP, but especially hosts running origin servers and caches, SHOULD
686   use NTP [RFC1305] or some similar protocol to synchronize their
687   clocks to a globally accurate time standard.
688
689   HTTP/1.1 requires origin servers to send a Date header, if possible,
690   with every response, giving the time at which the response was
691   generated (see Section 8.3 of [Part1]).  We use the term "date_value"
692   to denote the value of the Date header, in a form appropriate for
693   arithmetic operations.
694
695   HTTP/1.1 uses the Age response-header to convey the estimated age of
696   the response message when obtained from a cache.  The Age field value
697   is the cache's estimate of the amount of time since the response was
698   generated or revalidated by the origin server.
699
700   In essence, the Age value is the sum of the time that the response
701   has been resident in each of the caches along the path from the
702   origin server, plus the amount of time it has been in transit along
703   network paths.
704
705   We use the term "age_value" to denote the value of the Age header, in
706   a form appropriate for arithmetic operations.
707
708   A response's age can be calculated in two entirely independent ways:
709
710   1.  now minus date_value, if the local clock is reasonably well
711       synchronized to the origin server's clock.  If the result is
712       negative, the result is replaced by zero.
713
714   2.  age_value, if all of the caches along the response path implement
715       HTTP/1.1.
716
717   Given that we have two independent ways to compute the age of a
718   response when it is received, we can combine these as
719
720       corrected_received_age = max(now - date_value, age_value)
721
722   and as long as we have either nearly synchronized clocks or all-
723   HTTP/1.1 paths, one gets a reliable (conservative) result.
724
725
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731
732   Because of network-imposed delays, some significant interval might
733   pass between the time that a server generates a response and the time
734   it is received at the next outbound cache or client.  If uncorrected,
735   this delay could result in improperly low ages.
736
737   Because the request that resulted in the returned Age value must have
738   been initiated prior to that Age value's generation, we can correct
739   for delays imposed by the network by recording the time at which the
740   request was initiated.  Then, when an Age value is received, it MUST
741   be interpreted relative to the time the request was initiated, not
742   the time that the response was received.  This algorithm results in
743   conservative behavior no matter how much delay is experienced.  So,
744   we compute:
745
746      corrected_initial_age = corrected_received_age
747                            + (now - request_time)
748
749   where "request_time" is the time (according to the local clock) when
750   the request that elicited this response was sent.
751
752   Summary of age calculation algorithm, when a cache receives a
753   response:
754
755      /*
756       * age_value
757       *      is the value of Age: header received by the cache with
758       *              this response.
759       * date_value
760       *      is the value of the origin server's Date: header
761       * request_time
762       *      is the (local) time when the cache made the request
763       *              that resulted in this cached response
764       * response_time
765       *      is the (local) time when the cache received the
766       *              response
767       * now
768       *      is the current (local) time
769       */
770
771      apparent_age = max(0, response_time - date_value);
772      corrected_received_age = max(apparent_age, age_value);
773      response_delay = response_time - request_time;
774      corrected_initial_age = corrected_received_age + response_delay;
775      resident_time = now - response_time;
776      current_age   = corrected_initial_age + resident_time;
777
778   The current_age of a cache entry is calculated by adding the amount
779   of time (in seconds) since the cache entry was last validated by the
780
781
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787
788   origin server to the corrected_initial_age.  When a response is
789   generated from a cache entry, the cache MUST include a single Age
790   header field in the response with a value equal to the cache entry's
791   current_age.
792
793   The presence of an Age header field in a response implies that a
794   response is not first-hand.  However, the converse is not true, since
795   the lack of an Age header field in a response does not imply that the
796   response is first-hand unless all caches along the request path are
797   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
798   the Age header field).
799
8002.2.4.  Expiration Calculations
801
802   In order to decide whether a response is fresh or stale, we need to
803   compare its freshness lifetime to its age.  The age is calculated as
804   described in Section 2.2.3; this section describes how to calculate
805   the freshness lifetime, and to determine if a response has expired.
806   In the discussion below, the values can be represented in any form
807   appropriate for arithmetic operations.
808
809   We use the term "expires_value" to denote the value of the Expires
810   header.  We use the term "max_age_value" to denote an appropriate
811   value of the number of seconds carried by the "max-age" directive of
812   the Cache-Control header in a response (see Section 3.2.3).
813
814   The max-age directive takes priority over Expires, so if max-age is
815   present in a response, the calculation is simply:
816
817      freshness_lifetime = max_age_value
818
819   Otherwise, if Expires is present in the response, the calculation is:
820
821      freshness_lifetime = expires_value - date_value
822
823   Note that neither of these calculations is vulnerable to clock skew,
824   since all of the information comes from the origin server.
825
826   If none of Expires, Cache-Control: max-age, or Cache-Control:
827   s-maxage (see Section 3.2.3) appears in the response, and the
828   response does not include other restrictions on caching, the cache
829   MAY compute a freshness lifetime using a heuristic.  The cache MUST
830   attach Warning 113 to any response whose age is more than 24 hours if
831   such warning has not already been added.
832
833   Also, if the response does have a Last-Modified time, the heuristic
834   expiration value SHOULD be no more than some fraction of the interval
835   since that time.  A typical setting of this fraction might be 10%.
836
837
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843
844   The calculation to determine if a response has expired is quite
845   simple:
846
847      response_is_fresh = (freshness_lifetime > current_age)
848
8492.2.5.  Disambiguating Expiration Values
850
851   Because expiration values are assigned optimistically, it is possible
852   for two caches to contain fresh values for the same resource that are
853   different.
854
855   If a client performing a retrieval receives a non-first-hand response
856   for a request that was already fresh in its own cache, and the Date
857   header in its existing cache entry is newer than the Date on the new
858   response, then the client MAY ignore the response.  If so, it MAY
859   retry the request with a "Cache-Control: max-age=0" directive (see
860   Section 3.2), to force a check with the origin server.
861
862   If a cache has two fresh responses for the same representation with
863   different validators, it MUST use the one with the more recent Date
864   header.  This situation might arise because the cache is pooling
865   responses from other caches, or because a client has asked for a
866   reload or a revalidation of an apparently fresh cache entry.
867
8682.2.6.  Disambiguating Multiple Responses
869
870   Because a client might be receiving responses via multiple paths, so
871   that some responses flow through one set of caches and other
872   responses flow through a different set of caches, a client might
873   receive responses in an order different from that in which the origin
874   server sent them.  We would like the client to use the most recently
875   generated response, even if older responses are still apparently
876   fresh.
877
878   Neither the entity tag nor the expiration value can impose an
879   ordering on responses, since it is possible that a later response
880   intentionally carries an earlier expiration time.  The Date values
881   are ordered to a granularity of one second.
882
883   When a client tries to revalidate a cache entry, and the response it
884   receives contains a Date header that appears to be older than the one
885   for the existing entry, then the client SHOULD repeat the request
886   unconditionally, and include
887
888       Cache-Control: max-age=0
889
890   to force any intermediate caches to validate their copies directly
891   with the origin server, or
892
893
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899
900       Cache-Control: no-cache
901
902   to force any intermediate caches to obtain a new copy from the origin
903   server.
904
905   If the Date values are equal, then the client MAY use either response
906   (or MAY, if it is being extremely prudent, request a new response).
907   Servers MUST NOT depend on clients being able to choose
908   deterministically between responses generated during the same second,
909   if their expiration times overlap.
910
9112.3.  Validation Model
912
913   When a cache has a stale entry that it would like to use as a
914   response to a client's request, it first has to check with the origin
915   server (or possibly an intermediate cache with a fresh response) to
916   see if its cached entry is still usable.  We call this "validating"
917   the cache entry.  Since we do not want to have to pay the overhead of
918   retransmitting the full response if the cached entry is good, and we
919   do not want to pay the overhead of an extra round trip if the cached
920   entry is invalid, the HTTP/1.1 protocol supports the use of
921   conditional methods.
922
923   The key protocol features for supporting conditional methods are
924   those concerned with "cache validators."  When an origin server
925   generates a full response, it attaches some sort of validator to it,
926   which is kept with the cache entry.  When a client (user agent or
927   proxy cache) makes a conditional request for a resource for which it
928   has a cache entry, it includes the associated validator in the
929   request.
930
931   The server then checks that validator against the current validator
932   for the entity, and, if they match (see Section 4 of [Part4]), it
933   responds with a special status code (usually, 304 (Not Modified)) and
934   no entity-body.  Otherwise, it returns a full response (including
935   entity-body).  Thus, we avoid transmitting the full response if the
936   validator matches, and we avoid an extra round trip if it does not
937   match.
938
939   In HTTP/1.1, a conditional request looks exactly the same as a normal
940   request for the same resource, except that it carries a special
941   header (which includes the validator) that implicitly turns the
942   method (usually, GET) into a conditional.
943
944   The protocol includes both positive and negative senses of cache-
945   validating conditions.  That is, it is possible to request either
946   that a method be performed if and only if a validator matches or if
947   and only if no validators match.
948
949
950
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955
956      Note: a response that lacks a validator may still be cached, and
957      served from cache until it expires, unless this is explicitly
958      prohibited by a cache-control directive.  However, a cache cannot
959      do a conditional retrieval if it does not have a validator for the
960      entity, which means it will not be refreshable after it expires.
961
9622.3.1.  Last-Modified Dates
963
964   The Last-Modified entity-header field value is often used as a cache
965   validator.  In simple terms, a cache entry is considered to be valid
966   if the entity has not been modified since the Last-Modified value.
967
9682.3.2.  Entity Tag Cache Validators
969
970   The ETag response-header field value, an entity tag, provides for an
971   "opaque" cache validator.  This might allow more reliable validation
972   in situations where it is inconvenient to store modification dates,
973   where the one-second resolution of HTTP date values is not
974   sufficient, or where the origin server wishes to avoid certain
975   paradoxes that might arise from the use of modification dates.
976
977   Entity Tags are described in Section 2 of [Part4].
978
9792.3.3.  Non-validating Conditionals
980
981   The principle behind entity tags is that only the service author
982   knows the semantics of a resource well enough to select an
983   appropriate cache validation mechanism, and the specification of any
984   validator comparison function more complex than byte-equality would
985   open up a can of worms.  Thus, comparisons of any other headers
986   (except Last-Modified, for compatibility with HTTP/1.0) are never
987   used for purposes of validating a cache entry.
988
9892.4.  Response Cacheability
990
991   Unless specifically constrained by a cache-control (Section 3.2)
992   directive, a caching system MAY always store a successful response
993   (see Section 2.8) as a cache entry, MAY return it without validation
994   if it is fresh, and MAY return it after successful validation.  If
995   there is neither a cache validator nor an explicit expiration time
996   associated with a response, we do not expect it to be cached, but
997   certain caches MAY violate this expectation (for example, when little
998   or no network connectivity is available).  A client can usually
999   detect that such a response was taken from a cache by comparing the
1000   Date header to the current time.
1001
1002      Note: some HTTP/1.0 caches are known to violate this expectation
1003      without providing any Warning.
1004
1005
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1011
1012   However, in some cases it might be inappropriate for a cache to
1013   retain an entity, or to return it in response to a subsequent
1014   request.  This might be because absolute semantic transparency is
1015   deemed necessary by the service author, or because of security or
1016   privacy considerations.  Certain cache-control directives are
1017   therefore provided so that the server can indicate that certain
1018   resource entities, or portions thereof, are not to be cached
1019   regardless of other considerations.
1020
1021   Note that Section 3.1 of [Part7] normally prevents a shared cache
1022   from saving and returning a response to a previous request if that
1023   request included an Authorization header.
1024
1025   A response received with a status code of 200, 203, 206, 300, 301 or
1026   410 MAY be stored by a cache and used in reply to a subsequent
1027   request, subject to the expiration mechanism, unless a cache-control
1028   directive prohibits caching.  However, a cache that does not support
1029   the Range and Content-Range headers MUST NOT cache 206 (Partial
1030   Content) responses.
1031
1032   A response received with any other status code (e.g. status codes 302
1033   and 307) MUST NOT be returned in a reply to a subsequent request
1034   unless there are cache-control directives or another header(s) that
1035   explicitly allow it.  For example, these include the following: an
1036   Expires header (Section 3.3); a "max-age", "s-maxage", "must-
1037   revalidate", "proxy-revalidate", "public" or "private" cache-control
1038   directive (Section 3.2).
1039
10402.5.  Constructing Responses From Caches
1041
1042   The purpose of an HTTP cache is to store information received in
1043   response to requests for use in responding to future requests.  In
1044   many cases, a cache simply returns the appropriate parts of a
1045   response to the requester.  However, if the cache holds a cache entry
1046   based on a previous response, it might have to combine parts of a new
1047   response with what is held in the cache entry.
1048
10492.5.1.  End-to-end and Hop-by-hop Headers
1050
1051   For the purpose of defining the behavior of caches and non-caching
1052   proxies, we divide HTTP headers into two categories:
1053
1054   o  End-to-end headers, which are transmitted to the ultimate
1055      recipient of a request or response.  End-to-end headers in
1056      responses MUST be stored as part of a cache entry and MUST be
1057      transmitted in any response formed from a cache entry.
1058
1059
1060
1061
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1067
1068   o  Hop-by-hop headers, which are meaningful only for a single
1069      transport-level connection, and are not stored by caches or
1070      forwarded by proxies.
1071
1072   The following HTTP/1.1 headers are hop-by-hop headers:
1073
1074   o  Connection
1075
1076   o  Keep-Alive
1077
1078   o  Proxy-Authenticate
1079
1080   o  Proxy-Authorization
1081
1082   o  TE
1083
1084   o  Trailers
1085
1086   o  Transfer-Encoding
1087
1088   o  Upgrade
1089
1090   All other headers defined by HTTP/1.1 are end-to-end headers.
1091
1092   Other hop-by-hop headers MUST be listed in a Connection header,
1093   (Section 8.1 of [Part1]) to be introduced into HTTP/1.1 (or later).
1094
10952.5.2.  Non-modifiable Headers
1096
1097   Some features of the HTTP/1.1 protocol, such as Digest
1098   Authentication, depend on the value of certain end-to-end headers.  A
1099   transparent proxy SHOULD NOT modify an end-to-end header unless the
1100   definition of that header requires or specifically allows that.
1101
1102   A transparent proxy MUST NOT modify any of the following fields in a
1103   request or response, and it MUST NOT add any of these fields if not
1104   already present:
1105
1106   o  Content-Location
1107
1108   o  Content-MD5
1109
1110   o  ETag
1111
1112   o  Last-Modified
1113
1114   A transparent proxy MUST NOT modify any of the following fields in a
1115   response:
1116
1117
1118
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1122
1123
1124   o  Expires
1125
1126   but it MAY add any of these fields if not already present.  If an
1127   Expires header is added, it MUST be given a field-value identical to
1128   that of the Date header in that response.
1129
1130   A proxy MUST NOT modify or add any of the following fields in a
1131   message that contains the no-transform cache-control directive, or in
1132   any request:
1133
1134   o  Content-Encoding
1135
1136   o  Content-Range
1137
1138   o  Content-Type
1139
1140   A non-transparent proxy MAY modify or add these fields to a message
1141   that does not include no-transform, but if it does so, it MUST add a
1142   Warning 214 (Transformation applied) if one does not already appear
1143   in the message (see Section 3.6).
1144
1145      Warning: unnecessary modification of end-to-end headers might
1146      cause authentication failures if stronger authentication
1147      mechanisms are introduced in later versions of HTTP.  Such
1148      authentication mechanisms MAY rely on the values of header fields
1149      not listed here.
1150
1151   The Content-Length field of a request or response is added or deleted
1152   according to the rules in Section 4.4 of [Part1].  A transparent
1153   proxy MUST preserve the entity-length (Section 3.2.2 of [Part3]) of
1154   the entity-body, although it MAY change the transfer-length (Section
1155   4.4 of [Part1]).
1156
11572.5.3.  Combining Headers
1158
1159   When a cache makes a validating request to a server, and the server
1160   provides a 304 (Not Modified) response or a 206 (Partial Content)
1161   response, the cache then constructs a response to send to the
1162   requesting client.
1163
1164   If the status code is 304 (Not Modified), the cache uses the entity-
1165   body stored in the cache entry as the entity-body of this outgoing
1166   response.  If the status code is 206 (Partial Content) and the ETag
1167   or Last-Modified headers match exactly, the cache MAY combine the
1168   contents stored in the cache entry with the new contents received in
1169   the response and use the result as the entity-body of this outgoing
1170   response, (see Section 4 of [Part5]).
1171
1172
1173
1174
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1178
1179
1180   The end-to-end headers stored in the cache entry are used for the
1181   constructed response, except that
1182
1183   o  any stored Warning headers with warn-code 1xx (see Section 3.6)
1184      MUST be deleted from the cache entry and the forwarded response.
1185
1186   o  any stored Warning headers with warn-code 2xx MUST be retained in
1187      the cache entry and the forwarded response.
1188
1189   o  any end-to-end headers provided in the 304 or 206 response MUST
1190      replace the corresponding headers from the cache entry.
1191
1192   Unless the cache decides to remove the cache entry, it MUST also
1193   replace the end-to-end headers stored with the cache entry with
1194   corresponding headers received in the incoming response, except for
1195   Warning headers as described immediately above.  If a header field-
1196   name in the incoming response matches more than one header in the
1197   cache entry, all such old headers MUST be replaced.
1198
1199   In other words, the set of end-to-end headers received in the
1200   incoming response overrides all corresponding end-to-end headers
1201   stored with the cache entry (except for stored Warning headers with
1202   warn-code 1xx, which are deleted even if not overridden).
1203
1204      Note: this rule allows an origin server to use a 304 (Not
1205      Modified) or a 206 (Partial Content) response to update any header
1206      associated with a previous response for the same entity or sub-
1207      ranges thereof, although it might not always be meaningful or
1208      correct to do so.  This rule does not allow an origin server to
1209      use a 304 (Not Modified) or a 206 (Partial Content) response to
1210      entirely delete a header that it had provided with a previous
1211      response.
1212
12132.6.  Caching Negotiated Responses
1214
1215   Use of server-driven content negotiation (Section 4.1 of [Part3]), as
1216   indicated by the presence of a Vary header field in a response,
1217   alters the conditions and procedure by which a cache can use the
1218   response for subsequent requests.  See Section 3.5 for use of the
1219   Vary header field by servers.
1220
1221   A server SHOULD use the Vary header field to inform a cache of what
1222   request-header fields were used to select among multiple
1223   representations of a cacheable response subject to server-driven
1224   negotiation.  The set of header fields named by the Vary field value
1225   is known as the "selecting" request-headers.
1226
1227   When the cache receives a subsequent request whose Request-URI
1228
1229
1230
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1235
1236   specifies one or more cache entries including a Vary header field,
1237   the cache MUST NOT use such a cache entry to construct a response to
1238   the new request unless all of the selecting request-headers present
1239   in the new request match the corresponding stored request-headers in
1240   the original request.
1241
1242   The selecting request-headers from two requests are defined to match
1243   if and only if the selecting request-headers in the first request can
1244   be transformed to the selecting request-headers in the second request
1245   by adding or removing linear white space (LWS) at places where this
1246   is allowed by the corresponding BNF, and/or combining multiple
1247   message-header fields with the same field name following the rules
1248   about message headers in Section 4.2 of [Part1].
1249
1250   A Vary header field-value of "*" always fails to match and subsequent
1251   requests on that resource can only be properly interpreted by the
1252   origin server.
1253
1254   If the selecting request header fields for the cached entry do not
1255   match the selecting request header fields of the new request, then
1256   the cache MUST NOT use a cached entry to satisfy the request unless
1257   it first relays the new request to the origin server in a conditional
1258   request and the server responds with 304 (Not Modified), including an
1259   entity tag or Content-Location that indicates the entity to be used.
1260
1261   If an entity tag was assigned to a cached representation, the
1262   forwarded request SHOULD be conditional and include the entity tags
1263   in an If-None-Match header field from all its cache entries for the
1264   resource.  This conveys to the server the set of entities currently
1265   held by the cache, so that if any one of these entities matches the
1266   requested entity, the server can use the ETag header field in its 304
1267   (Not Modified) response to tell the cache which entry is appropriate.
1268   If the entity-tag of the new response matches that of an existing
1269   entry, the new response SHOULD be used to update the header fields of
1270   the existing entry, and the result MUST be returned to the client.
1271
1272   If any of the existing cache entries contains only partial content
1273   for the associated entity, its entity-tag SHOULD NOT be included in
1274   the If-None-Match header field unless the request is for a range that
1275   would be fully satisfied by that entry.
1276
1277   If a cache receives a successful response whose Content-Location
1278   field matches that of an existing cache entry for the same Request-
1279   URI, whose entity-tag differs from that of the existing entry, and
1280   whose Date is more recent than that of the existing entry, the
1281   existing entry SHOULD NOT be returned in response to future requests
1282   and SHOULD be deleted from the cache.
1283
1284
1285
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1291
12922.7.  Shared and Non-Shared Caches
1293
1294   For reasons of security and privacy, it is necessary to make a
1295   distinction between "shared" and "non-shared" caches.  A non-shared
1296   cache is one that is accessible only to a single user.  Accessibility
1297   in this case SHOULD be enforced by appropriate security mechanisms.
1298   All other caches are considered to be "shared."  Other sections of
1299   this specification place certain constraints on the operation of
1300   shared caches in order to prevent loss of privacy or failure of
1301   access controls.
1302
13032.8.  Errors or Incomplete Response Cache Behavior
1304
1305   A cache that receives an incomplete response (for example, with fewer
1306   bytes of data than specified in a Content-Length header) MAY store
1307   the response.  However, the cache MUST treat this as a partial
1308   response.  Partial responses MAY be combined as described in Section
1309   4 of [Part5]; the result might be a full response or might still be
1310   partial.  A cache MUST NOT return a partial response to a client
1311   without explicitly marking it as such, using the 206 (Partial
1312   Content) status code.  A cache MUST NOT return a partial response
1313   using a status code of 200 (OK).
1314
1315   If a cache receives a 5xx response while attempting to revalidate an
1316   entry, it MAY either forward this response to the requesting client,
1317   or act as if the server failed to respond.  In the latter case, it
1318   MAY return a previously received response unless the cached entry
1319   includes the "must-revalidate" cache-control directive (see
1320   Section 3.2).
1321
13222.9.  Side Effects of GET and HEAD
1323
1324   Unless the origin server explicitly prohibits the caching of their
1325   responses, the application of GET and HEAD methods to any resources
1326   SHOULD NOT have side effects that would lead to erroneous behavior if
1327   these responses are taken from a cache.  They MAY still have side
1328   effects, but a cache is not required to consider such side effects in
1329   its caching decisions.  Caches are always expected to observe an
1330   origin server's explicit restrictions on caching.
1331
1332   We note one exception to this rule: since some applications have
1333   traditionally used GETs and HEADs with query URLs (those containing a
1334   "?" in the rel_path part) to perform operations with significant side
1335   effects, caches MUST NOT treat responses to such URIs as fresh unless
1336   the server provides an explicit expiration time.  This specifically
1337   means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
1338   be taken from a cache.  See Section 8.1.1 of [Part2] for related
1339   information.
1340
1341
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1347
13482.10.  Invalidation After Updates or Deletions
1349
1350   The effect of certain methods performed on a resource at the origin
1351   server might cause one or more existing cache entries to become non-
1352   transparently invalid.  That is, although they might continue to be
1353   "fresh," they do not accurately reflect what the origin server would
1354   return for a new request on that resource.
1355
1356   There is no way for the HTTP protocol to guarantee that all such
1357   cache entries are marked invalid.  For example, the request that
1358   caused the change at the origin server might not have gone through
1359   the proxy where a cache entry is stored.  However, several rules help
1360   reduce the likelihood of erroneous behavior.
1361
1362   In this section, the phrase "invalidate an entity" means that the
1363   cache will either remove all instances of that entity from its
1364   storage, or will mark these as "invalid" and in need of a mandatory
1365   revalidation before they can be returned in response to a subsequent
1366   request.
1367
1368   Some HTTP methods MUST cause a cache to invalidate an entity.  This
1369   is either the entity referred to by the Request-URI, or by the
1370   Location or Content-Location headers (if present).  These methods
1371   are:
1372
1373   o  PUT
1374
1375   o  DELETE
1376
1377   o  POST
1378
1379   In order to prevent denial of service attacks, an invalidation based
1380   on the URI in a Location or Content-Location header MUST only be
1381   performed if the host part is the same as in the Request-URI.
1382
1383   A cache that passes through requests for methods it does not
1384   understand SHOULD invalidate any entities referred to by the Request-
1385   URI.
1386
13872.11.  Write-Through Mandatory
1388
1389   All methods that might be expected to cause modifications to the
1390   origin server's resources MUST be written through to the origin
1391   server.  This currently includes all methods except for GET and HEAD.
1392   A cache MUST NOT reply to such a request from a client before having
1393   transmitted the request to the inbound server, and having received a
1394   corresponding response from the inbound server.  This does not
1395   prevent a proxy cache from sending a 100 (Continue) response before
1396
1397
1398
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1403
1404   the inbound server has sent its final reply.
1405
1406   The alternative (known as "write-back" or "copy-back" caching) is not
1407   allowed in HTTP/1.1, due to the difficulty of providing consistent
1408   updates and the problems arising from server, cache, or network
1409   failure prior to write-back.
1410
14112.12.  Cache Replacement
1412
1413   If a new cacheable (see sections 3.2.2, 2.2.5, 2.2.6 and 2.8)
1414   response is received from a resource while any existing responses for
1415   the same resource are cached, the cache SHOULD use the new response
1416   to reply to the current request.  It MAY insert it into cache storage
1417   and MAY, if it meets all other requirements, use it to respond to any
1418   future requests that would previously have caused the old response to
1419   be returned.  If it inserts the new response into cache storage the
1420   rules in Section 2.5.3 apply.
1421
1422      Note: a new response that has an older Date header value than
1423      existing cached responses is not cacheable.
1424
14252.13.  History Lists
1426
1427   User agents often have history mechanisms, such as "Back" buttons and
1428   history lists, which can be used to redisplay an entity retrieved
1429   earlier in a session.
1430
1431   History mechanisms and caches are different.  In particular history
1432   mechanisms SHOULD NOT try to show a semantically transparent view of
1433   the current state of a resource.  Rather, a history mechanism is
1434   meant to show exactly what the user saw at the time when the resource
1435   was retrieved.
1436
1437   By default, an expiration time does not apply to history mechanisms.
1438   If the entity is still in storage, a history mechanism SHOULD display
1439   it even if the entity has expired, unless the user has specifically
1440   configured the agent to refresh expired history documents.
1441
1442   This is not to be construed to prohibit the history mechanism from
1443   telling the user that a view might be stale.
1444
1445      Note: if history list mechanisms unnecessarily prevent users from
1446      viewing stale resources, this will tend to force service authors
1447      to avoid using HTTP expiration controls and cache controls when
1448      they would otherwise like to.  Service authors may consider it
1449      important that users not be presented with error messages or
1450      warning messages when they use navigation controls (such as BACK)
1451      to view previously fetched resources.  Even though sometimes such
1452
1453
1454
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1458
1459
1460      resources ought not to cached, or ought to expire quickly, user
1461      interface considerations may force service authors to resort to
1462      other means of preventing caching (e.g. "once-only" URLs) in order
1463      not to suffer the effects of improperly functioning history
1464      mechanisms.
1465
1466
14673.  Header Field Definitions
1468
1469   This section defines the syntax and semantics of all standard
1470   HTTP/1.1 header fields.  For entity-header fields, both sender and
1471   recipient refer to either the client or the server, depending on who
1472   sends and who receives the entity.
1473
14743.1.  Age
1475
1476   The Age response-header field conveys the sender's estimate of the
1477   amount of time since the response (or its revalidation) was generated
1478   at the origin server.  A cached response is "fresh" if its age does
1479   not exceed its freshness lifetime.  Age values are calculated as
1480   specified in Section 2.2.3.
1481
1482           Age = "Age" ":" age-value
1483           age-value = delta-seconds
1484
1485   Age values are non-negative decimal integers, representing time in
1486   seconds.
1487
1488   If a cache receives a value larger than the largest positive integer
1489   it can represent, or if any of its age calculations overflows, it
1490   MUST transmit an Age header with a value of 2147483648 (2^31).  An
1491   HTTP/1.1 server that includes a cache MUST include an Age header
1492   field in every response generated from its own cache.  Caches SHOULD
1493   use an arithmetic type of at least 31 bits of range.
1494
14953.2.  Cache-Control
1496
1497   The Cache-Control general-header field is used to specify directives
1498   that MUST be obeyed by all caching mechanisms along the request/
1499   response chain.  The directives specify behavior intended to prevent
1500   caches from adversely interfering with the request or response.
1501   These directives typically override the default caching algorithms.
1502   Cache directives are unidirectional in that the presence of a
1503   directive in a request does not imply that the same directive is to
1504   be given in the response.
1505
1506      Note that HTTP/1.0 caches might not implement Cache-Control and
1507      might only implement Pragma: no-cache (see Section 3.4).
1508
1509
1510
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1514
1515
1516   Cache directives MUST be passed through by a proxy or gateway
1517   application, regardless of their significance to that application,
1518   since the directives might be applicable to all recipients along the
1519   request/response chain.  It is not possible to specify a cache-
1520   directive for a specific cache.
1521
1522      Cache-Control   = "Cache-Control" ":" 1#cache-directive
1523
1524      cache-directive = cache-request-directive
1525           | cache-response-directive
1526
1527      cache-request-directive =
1528             "no-cache"                          ; Section 3.2.1
1529           | "no-store"                          ; Section 3.2.2
1530           | "max-age" "=" delta-seconds         ; Section 3.2.3, 3.2.4
1531           | "max-stale" [ "=" delta-seconds ]   ; Section 3.2.3
1532           | "min-fresh" "=" delta-seconds       ; Section 3.2.3
1533           | "no-transform"                      ; Section 3.2.5
1534           | "only-if-cached"                    ; Section 3.2.4
1535           | cache-extension                     ; Section 3.2.6
1536
1537       cache-response-directive =
1538             "public"                               ; Section 3.2.1
1539           | "private" [ "=" <"> 1#field-name <"> ] ; Section 3.2.1
1540           | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 3.2.1
1541           | "no-store"                             ; Section 3.2.2
1542           | "no-transform"                         ; Section 3.2.5
1543           | "must-revalidate"                      ; Section 3.2.4
1544           | "proxy-revalidate"                     ; Section 3.2.4
1545           | "max-age" "=" delta-seconds            ; Section 3.2.3
1546           | "s-maxage" "=" delta-seconds           ; Section 3.2.3
1547           | cache-extension                        ; Section 3.2.6
1548
1549      cache-extension = token [ "=" ( token | quoted-string ) ]
1550
1551   When a directive appears without any 1#field-name parameter, the
1552   directive applies to the entire request or response.  When such a
1553   directive appears with a 1#field-name parameter, it applies only to
1554   the named field or fields, and not to the rest of the request or
1555   response.  This mechanism supports extensibility; implementations of
1556   future versions of the HTTP protocol might apply these directives to
1557   header fields not defined in HTTP/1.1.
1558
1559   The cache-control directives can be broken down into these general
1560   categories:
1561
1562   o  Restrictions on what are cacheable; these may only be imposed by
1563      the origin server.
1564
1565
1566
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1570
1571
1572   o  Restrictions on what may be stored by a cache; these may be
1573      imposed by either the origin server or the user agent.
1574
1575   o  Modifications of the basic expiration mechanism; these may be
1576      imposed by either the origin server or the user agent.
1577
1578   o  Controls over cache revalidation and reload; these may only be
1579      imposed by a user agent.
1580
1581   o  Control over transformation of entities.
1582
1583   o  Extensions to the caching system.
1584
15853.2.1.  What is Cacheable
1586
1587   By default, a response is cacheable if the requirements of the
1588   request method, request header fields, and the response status
1589   indicate that it is cacheable.  Section 2.4 summarizes these defaults
1590   for cacheability.  The following Cache-Control response directives
1591   allow an origin server to override the default cacheability of a
1592   response:
1593
1594   public
1595
1596      Indicates that the response MAY be cached by any cache, even if it
1597      would normally be non-cacheable or cacheable only within a non-
1598      shared cache.  (See also Authorization, Section 3.1 of [Part7],
1599      for additional details.)
1600
1601   private
1602
1603      Indicates that all or part of the response message is intended for
1604      a single user and MUST NOT be cached by a shared cache.  This
1605      allows an origin server to state that the specified parts of the
1606      response are intended for only one user and are not a valid
1607      response for requests by other users.  A private (non-shared)
1608      cache MAY cache the response.
1609
1610      Note: This usage of the word private only controls where the
1611      response may be cached, and cannot ensure the privacy of the
1612      message content.
1613
1614   no-cache
1615
1616      If the no-cache directive does not specify a field-name, then a
1617      cache MUST NOT use the response to satisfy a subsequent request
1618      without successful revalidation with the origin server.  This
1619      allows an origin server to prevent caching even by caches that
1620
1621
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1627
1628      have been configured to return stale responses to client requests.
1629
1630      If the no-cache directive does specify one or more field-names,
1631      then a cache MAY use the response to satisfy a subsequent request,
1632      subject to any other restrictions on caching.  However, the
1633      specified field-name(s) MUST NOT be sent in the response to a
1634      subsequent request without successful revalidation with the origin
1635      server.  This allows an origin server to prevent the re-use of
1636      certain header fields in a response, while still allowing caching
1637      of the rest of the response.
1638
1639         Note: Most HTTP/1.0 caches will not recognize or obey this
1640         directive.
1641
16423.2.2.  What May be Stored by Caches
1643
1644   no-store
1645
1646      The purpose of the no-store directive is to prevent the
1647      inadvertent release or retention of sensitive information (for
1648      example, on backup tapes).  The no-store directive applies to the
1649      entire message, and MAY be sent either in a response or in a
1650      request.  If sent in a request, a cache MUST NOT store any part of
1651      either this request or any response to it.  If sent in a response,
1652      a cache MUST NOT store any part of either this response or the
1653      request that elicited it.  This directive applies to both non-
1654      shared and shared caches.  "MUST NOT store" in this context means
1655      that the cache MUST NOT intentionally store the information in
1656      non-volatile storage, and MUST make a best-effort attempt to
1657      remove the information from volatile storage as promptly as
1658      possible after forwarding it.
1659
1660      Even when this directive is associated with a response, users
1661      might explicitly store such a response outside of the caching
1662      system (e.g., with a "Save As" dialog).  History buffers MAY store
1663      such responses as part of their normal operation.
1664
1665      The purpose of this directive is to meet the stated requirements
1666      of certain users and service authors who are concerned about
1667      accidental releases of information via unanticipated accesses to
1668      cache data structures.  While the use of this directive might
1669      improve privacy in some cases, we caution that it is NOT in any
1670      way a reliable or sufficient mechanism for ensuring privacy.  In
1671      particular, malicious or compromised caches might not recognize or
1672      obey this directive, and communications networks might be
1673      vulnerable to eavesdropping.
1674
1675
1676
1677
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1683
16843.2.3.  Modifications of the Basic Expiration Mechanism
1685
1686   The expiration time of an entity MAY be specified by the origin
1687   server using the Expires header (see Section 3.3).  Alternatively, it
1688   MAY be specified using the max-age directive in a response.  When the
1689   max-age cache-control directive is present in a cached response, the
1690   response is stale if its current age is greater than the age value
1691   given (in seconds) at the time of a new request for that resource.
1692   The max-age directive on a response implies that the response is
1693   cacheable (i.e., "public") unless some other, more restrictive cache
1694   directive is also present.
1695
1696   If a response includes both an Expires header and a max-age
1697   directive, the max-age directive overrides the Expires header, even
1698   if the Expires header is more restrictive.  This rule allows an
1699   origin server to provide, for a given response, a longer expiration
1700   time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache.  This
1701   might be useful if certain HTTP/1.0 caches improperly calculate ages
1702   or expiration times, perhaps due to desynchronized clocks.
1703
1704   Many HTTP/1.0 cache implementations will treat an Expires value that
1705   is less than or equal to the response Date value as being equivalent
1706   to the Cache-Control response directive "no-cache".  If an HTTP/1.1
1707   cache receives such a response, and the response does not include a
1708   Cache-Control header field, it SHOULD consider the response to be
1709   non-cacheable in order to retain compatibility with HTTP/1.0 servers.
1710
1711      Note: An origin server might wish to use a relatively new HTTP
1712      cache control feature, such as the "private" directive, on a
1713      network including older caches that do not understand that
1714      feature.  The origin server will need to combine the new feature
1715      with an Expires field whose value is less than or equal to the
1716      Date value.  This will prevent older caches from improperly
1717      caching the response.
1718
1719   s-maxage
1720
1721      If a response includes an s-maxage directive, then for a shared
1722      cache (but not for a private cache), the maximum age specified by
1723      this directive overrides the maximum age specified by either the
1724      max-age directive or the Expires header.  The s-maxage directive
1725      also implies the semantics of the proxy-revalidate directive (see
1726      Section 3.2.4), i.e., that the shared cache must not use the entry
1727      after it becomes stale to respond to a subsequent request without
1728      first revalidating it with the origin server.  The s-maxage
1729      directive is always ignored by a private cache.
1730
1731   Note that most older caches, not compliant with this specification,
1732
1733
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1739
1740   do not implement any cache-control directives.  An origin server
1741   wishing to use a cache-control directive that restricts, but does not
1742   prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
1743   requirement that the max-age directive overrides the Expires header,
1744   and the fact that pre-HTTP/1.1-compliant caches do not observe the
1745   max-age directive.
1746
1747   Other directives allow a user agent to modify the basic expiration
1748   mechanism.  These directives MAY be specified on a request:
1749
1750   max-age
1751
1752      Indicates that the client is willing to accept a response whose
1753      age is no greater than the specified time in seconds.  Unless max-
1754      stale directive is also included, the client is not willing to
1755      accept a stale response.
1756
1757   min-fresh
1758
1759      Indicates that the client is willing to accept a response whose
1760      freshness lifetime is no less than its current age plus the
1761      specified time in seconds.  That is, the client wants a response
1762      that will still be fresh for at least the specified number of
1763      seconds.
1764
1765   max-stale
1766
1767      Indicates that the client is willing to accept a response that has
1768      exceeded its expiration time.  If max-stale is assigned a value,
1769      then the client is willing to accept a response that has exceeded
1770      its expiration time by no more than the specified number of
1771      seconds.  If no value is assigned to max-stale, then the client is
1772      willing to accept a stale response of any age.
1773
1774   If a cache returns a stale response, either because of a max-stale
1775   directive on a request, or because the cache is configured to
1776   override the expiration time of a response, the cache MUST attach a
1777   Warning header to the stale response, using Warning 110 (Response is
1778   stale).
1779
1780   A cache MAY be configured to return stale responses without
1781   validation, but only if this does not conflict with any "MUST"-level
1782   requirements concerning cache validation (e.g., a "must-revalidate"
1783   cache-control directive).
1784
1785   If both the new request and the cached entry include "max-age"
1786   directives, then the lesser of the two values is used for determining
1787   the freshness of the cached entry for that request.
1788
1789
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1795
17963.2.4.  Cache Revalidation and Reload Controls
1797
1798   Sometimes a user agent might want or need to insist that a cache
1799   revalidate its cache entry with the origin server (and not just with
1800   the next cache along the path to the origin server), or to reload its
1801   cache entry from the origin server.  End-to-end revalidation might be
1802   necessary if either the cache or the origin server has overestimated
1803   the expiration time of the cached response.  End-to-end reload may be
1804   necessary if the cache entry has become corrupted for some reason.
1805
1806   End-to-end revalidation may be requested either when the client does
1807   not have its own local cached copy, in which case we call it
1808   "unspecified end-to-end revalidation", or when the client does have a
1809   local cached copy, in which case we call it "specific end-to-end
1810   revalidation."
1811
1812   The client can specify these three kinds of action using Cache-
1813   Control request directives:
1814
1815   End-to-end reload
1816
1817      The request includes a "no-cache" cache-control directive or, for
1818      compatibility with HTTP/1.0 clients, "Pragma: no-cache".  Field
1819      names MUST NOT be included with the no-cache directive in a
1820      request.  The server MUST NOT use a cached copy when responding to
1821      such a request.
1822
1823   Specific end-to-end revalidation
1824
1825      The request includes a "max-age=0" cache-control directive, which
1826      forces each cache along the path to the origin server to
1827      revalidate its own entry, if any, with the next cache or server.
1828      The initial request includes a cache-validating conditional with
1829      the client's current validator.
1830
1831   Unspecified end-to-end revalidation
1832
1833      The request includes "max-age=0" cache-control directive, which
1834      forces each cache along the path to the origin server to
1835      revalidate its own entry, if any, with the next cache or server.
1836      The initial request does not include a cache-validating
1837      conditional; the first cache along the path (if any) that holds a
1838      cache entry for this resource includes a cache-validating
1839      conditional with its current validator.
1840
1841   max-age
1842
1843
1844
1845
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1851
1852      When an intermediate cache is forced, by means of a max-age=0
1853      directive, to revalidate its own cache entry, and the client has
1854      supplied its own validator in the request, the supplied validator
1855      might differ from the validator currently stored with the cache
1856      entry.  In this case, the cache MAY use either validator in making
1857      its own request without affecting semantic transparency.
1858
1859      However, the choice of validator might affect performance.  The
1860      best approach is for the intermediate cache to use its own
1861      validator when making its request.  If the server replies with 304
1862      (Not Modified), then the cache can return its now validated copy
1863      to the client with a 200 (OK) response.  If the server replies
1864      with a new entity and cache validator, however, the intermediate
1865      cache can compare the returned validator with the one provided in
1866      the client's request, using the strong comparison function.  If
1867      the client's validator is equal to the origin server's, then the
1868      intermediate cache simply returns 304 (Not Modified).  Otherwise,
1869      it returns the new entity with a 200 (OK) response.
1870
1871      If a request includes the no-cache directive, it SHOULD NOT
1872      include min-fresh, max-stale, or max-age.
1873
1874   only-if-cached
1875
1876      In some cases, such as times of extremely poor network
1877      connectivity, a client may want a cache to return only those
1878      responses that it currently has stored, and not to reload or
1879      revalidate with the origin server.  To do this, the client may
1880      include the only-if-cached directive in a request.  If it receives
1881      this directive, a cache SHOULD either respond using a cached entry
1882      that is consistent with the other constraints of the request, or
1883      respond with a 504 (Gateway Timeout) status.  However, if a group
1884      of caches is being operated as a unified system with good internal
1885      connectivity, such a request MAY be forwarded within that group of
1886      caches.
1887
1888   must-revalidate
1889
1890      Because a cache MAY be configured to ignore a server's specified
1891      expiration time, and because a client request MAY include a max-
1892      stale directive (which has a similar effect), the protocol also
1893      includes a mechanism for the origin server to require revalidation
1894      of a cache entry on any subsequent use.  When the must-revalidate
1895      directive is present in a response received by a cache, that cache
1896      MUST NOT use the entry after it becomes stale to respond to a
1897      subsequent request without first revalidating it with the origin
1898      server.  (I.e., the cache MUST do an end-to-end revalidation every
1899      time, if, based solely on the origin server's Expires or max-age
1900
1901
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1907
1908      value, the cached response is stale.)
1909
1910      The must-revalidate directive is necessary to support reliable
1911      operation for certain protocol features.  In all circumstances an
1912      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1913      particular, if the cache cannot reach the origin server for any
1914      reason, it MUST generate a 504 (Gateway Timeout) response.
1915
1916      Servers SHOULD send the must-revalidate directive if and only if
1917      failure to revalidate a request on the entity could result in
1918      incorrect operation, such as a silently unexecuted financial
1919      transaction.  Recipients MUST NOT take any automated action that
1920      violates this directive, and MUST NOT automatically provide an
1921      unvalidated copy of the entity if revalidation fails.
1922
1923      Although this is not recommended, user agents operating under
1924      severe connectivity constraints MAY violate this directive but, if
1925      so, MUST explicitly warn the user that an unvalidated response has
1926      been provided.  The warning MUST be provided on each unvalidated
1927      access, and SHOULD require explicit user confirmation.
1928
1929   proxy-revalidate
1930
1931      The proxy-revalidate directive has the same meaning as the must-
1932      revalidate directive, except that it does not apply to non-shared
1933      user agent caches.  It can be used on a response to an
1934      authenticated request to permit the user's cache to store and
1935      later return the response without needing to revalidate it (since
1936      it has already been authenticated once by that user), while still
1937      requiring proxies that service many users to revalidate each time
1938      (in order to make sure that each user has been authenticated).
1939      Note that such authenticated responses also need the public cache
1940      control directive in order to allow them to be cached at all.
1941
19423.2.5.  No-Transform Directive
1943
1944   no-transform
1945
1946      Implementors of intermediate caches (proxies) have found it useful
1947      to convert the media type of certain entity bodies.  A non-
1948      transparent proxy might, for example, convert between image
1949      formats in order to save cache space or to reduce the amount of
1950      traffic on a slow link.
1951
1952      Serious operational problems occur, however, when these
1953      transformations are applied to entity bodies intended for certain
1954      kinds of applications.  For example, applications for medical
1955      imaging, scientific data analysis and those using end-to-end
1956
1957
1958
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1963
1964      authentication, all depend on receiving an entity body that is bit
1965      for bit identical to the original entity-body.
1966
1967      Therefore, if a message includes the no-transform directive, an
1968      intermediate cache or proxy MUST NOT change those headers that are
1969      listed in Section 2.5.2 as being subject to the no-transform
1970      directive.  This implies that the cache or proxy MUST NOT change
1971      any aspect of the entity-body that is specified by these headers,
1972      including the value of the entity-body itself.
1973
19743.2.6.  Cache Control Extensions
1975
1976   The Cache-Control header field can be extended through the use of one
1977   or more cache-extension tokens, each with an optional assigned value.
1978   Informational extensions (those which do not require a change in
1979   cache behavior) MAY be added without changing the semantics of other
1980   directives.  Behavioral extensions are designed to work by acting as
1981   modifiers to the existing base of cache directives.  Both the new
1982   directive and the standard directive are supplied, such that
1983   applications which do not understand the new directive will default
1984   to the behavior specified by the standard directive, and those that
1985   understand the new directive will recognize it as modifying the
1986   requirements associated with the standard directive.  In this way,
1987   extensions to the cache-control directives can be made without
1988   requiring changes to the base protocol.
1989
1990   This extension mechanism depends on an HTTP cache obeying all of the
1991   cache-control directives defined for its native HTTP-version, obeying
1992   certain extensions, and ignoring all directives that it does not
1993   understand.
1994
1995   For example, consider a hypothetical new response directive called
1996   community which acts as a modifier to the private directive.  We
1997   define this new directive to mean that, in addition to any non-shared
1998   cache, any cache which is shared only by members of the community
1999   named within its value may cache the response.  An origin server
2000   wishing to allow the UCI community to use an otherwise private
2001   response in their shared cache(s) could do so by including
2002
2003       Cache-Control: private, community="UCI"
2004
2005   A cache seeing this header field will act correctly even if the cache
2006   does not understand the community cache-extension, since it will also
2007   see and understand the private directive and thus default to the safe
2008   behavior.
2009
2010   Unrecognized cache-directives MUST be ignored; it is assumed that any
2011   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
2012
2013
2014
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2018
2019
2020   be combined with standard directives (or the response's default
2021   cacheability) such that the cache behavior will remain minimally
2022   correct even if the cache does not understand the extension(s).
2023
20243.3.  Expires
2025
2026   The Expires entity-header field gives the date/time after which the
2027   response is considered stale.  A stale cache entry may not normally
2028   be returned by a cache (either a proxy cache or a user agent cache)
2029   unless it is first validated with the origin server (or with an
2030   intermediate cache that has a fresh copy of the entity).  See
2031   Section 2.2 for further discussion of the expiration model.
2032
2033   The presence of an Expires field does not imply that the original
2034   resource will change or cease to exist at, before, or after that
2035   time.
2036
2037   The format is an absolute date and time as defined by HTTP-date in
2038   Section 3.3.1 of [Part1]; it MUST be in RFC 1123 date format:
2039
2040      Expires = "Expires" ":" HTTP-date
2041
2042   An example of its use is
2043
2044      Expires: Thu, 01 Dec 1994 16:00:00 GMT
2045
2046      Note: if a response includes a Cache-Control field with the max-
2047      age directive (see Section 3.2.3), that directive overrides the
2048      Expires field.
2049
2050   HTTP/1.1 clients and caches MUST treat other invalid date formats,
2051   especially including the value "0", as in the past (i.e., "already
2052   expired").
2053
2054   To mark a response as "already expired," an origin server sends an
2055   Expires date that is equal to the Date header value.  (See the rules
2056   for expiration calculations in Section 2.2.4.)
2057
2058   To mark a response as "never expires," an origin server sends an
2059   Expires date approximately one year from the time the response is
2060   sent.  HTTP/1.1 servers SHOULD NOT send Expires dates more than one
2061   year in the future.
2062
2063   The presence of an Expires header field with a date value of some
2064   time in the future on a response that otherwise would by default be
2065   non-cacheable indicates that the response is cacheable, unless
2066   indicated otherwise by a Cache-Control header field (Section 3.2).
2067
2068
2069
2070
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2075
20763.4.  Pragma
2077
2078   The Pragma general-header field is used to include implementation-
2079   specific directives that might apply to any recipient along the
2080   request/response chain.  All pragma directives specify optional
2081   behavior from the viewpoint of the protocol; however, some systems
2082   MAY require that behavior be consistent with the directives.
2083
2084       Pragma            = "Pragma" ":" 1#pragma-directive
2085       pragma-directive  = "no-cache" | extension-pragma
2086       extension-pragma  = token [ "=" ( token | quoted-string ) ]
2087
2088   When the no-cache directive is present in a request message, an
2089   application SHOULD forward the request toward the origin server even
2090   if it has a cached copy of what is being requested.  This pragma
2091   directive has the same semantics as the no-cache cache-directive (see
2092   Section 3.2) and is defined here for backward compatibility with
2093   HTTP/1.0.  Clients SHOULD include both header fields when a no-cache
2094   request is sent to a server not known to be HTTP/1.1 compliant.
2095
2096   Pragma directives MUST be passed through by a proxy or gateway
2097   application, regardless of their significance to that application,
2098   since the directives might be applicable to all recipients along the
2099   request/response chain.  It is not possible to specify a pragma for a
2100   specific recipient; however, any pragma directive not relevant to a
2101   recipient SHOULD be ignored by that recipient.
2102
2103   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
2104   sent "Cache-Control: no-cache".  No new Pragma directives will be
2105   defined in HTTP.
2106
2107      Note: because the meaning of "Pragma: no-cache as a response
2108      header field is not actually specified, it does not provide a
2109      reliable replacement for "Cache-Control: no-cache" in a response
2110
21113.5.  Vary
2112
2113   The Vary field value indicates the set of request-header fields that
2114   fully determines, while the response is fresh, whether a cache is
2115   permitted to use the response to reply to a subsequent request
2116   without revalidation.  For uncacheable or stale responses, the Vary
2117   field value advises the user agent about the criteria that were used
2118   to select the representation.  A Vary field value of "*" implies that
2119   a cache cannot determine from the request headers of a subsequent
2120   request whether this response is the appropriate representation.  See
2121   Section 2.6 for use of the Vary header field by caches.
2122
2123       Vary  = "Vary" ":" ( "*" | 1#field-name )
2124
2125
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2129Internet-Draft              HTTP/1.1, part 6               December 2007
2130
2131
2132   An HTTP/1.1 server SHOULD include a Vary header field with any
2133   cacheable response that is subject to server-driven negotiation.
2134   Doing so allows a cache to properly interpret future requests on that
2135   resource and informs the user agent about the presence of negotiation
2136   on that resource.  A server MAY include a Vary header field with a
2137   non-cacheable response that is subject to server-driven negotiation,
2138   since this might provide the user agent with useful information about
2139   the dimensions over which the response varies at the time of the
2140   response.
2141
2142   A Vary field value consisting of a list of field-names signals that
2143   the representation selected for the response is based on a selection
2144   algorithm which considers ONLY the listed request-header field values
2145   in selecting the most appropriate representation.  A cache MAY assume
2146   that the same selection will be made for future requests with the
2147   same values for the listed field names, for the duration of time for
2148   which the response is fresh.
2149
2150   The field-names given are not limited to the set of standard request-
2151   header fields defined by this specification.  Field names are case-
2152   insensitive.
2153
2154   A Vary field value of "*" signals that unspecified parameters not
2155   limited to the request-headers (e.g., the network address of the
2156   client), play a role in the selection of the response representation.
2157   The "*" value MUST NOT be generated by a proxy server; it may only be
2158   generated by an origin server.
2159
21603.6.  Warning
2161
2162   The Warning general-header field is used to carry additional
2163   information about the status or transformation of a message which
2164   might not be reflected in the message.  This information is typically
2165   used to warn about a possible lack of semantic transparency from
2166   caching operations or transformations applied to the entity body of
2167   the message.
2168
2169   Warning headers are sent with responses using:
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183Fielding, et al.          Expires June 22, 2008                [Page 39]
2184
2185Internet-Draft              HTTP/1.1, part 6               December 2007
2186
2187
2188       Warning    = "Warning" ":" 1#warning-value
2189
2190       warning-value = warn-code SP warn-agent SP warn-text
2191                                             [SP warn-date]
2192
2193       warn-code  = 3DIGIT
2194       warn-agent = ( host [ ":" port ] ) | pseudonym
2195                       ; the name or pseudonym of the server adding
2196                       ; the Warning header, for use in debugging
2197       warn-text  = quoted-string
2198       warn-date  = <"> HTTP-date <">
2199
2200   A response MAY carry more than one Warning header.
2201
2202   The warn-text SHOULD be in a natural language and character set that
2203   is most likely to be intelligible to the human user receiving the
2204   response.  This decision MAY be based on any available knowledge,
2205   such as the location of the cache or user, the Accept-Language field
2206   in a request, the Content-Language field in a response, etc.  The
2207   default language is English and the default character set is ISO-
2208   8859-1.
2209
2210   If a character set other than ISO-8859-1 is used, it MUST be encoded
2211   in the warn-text using the method described in RFC 2047 [RFC2047].
2212
2213   Warning headers can in general be applied to any message, however
2214   some specific warn-codes are specific to caches and can only be
2215   applied to response messages.  New Warning headers SHOULD be added
2216   after any existing Warning headers.  A cache MUST NOT delete any
2217   Warning header that it received with a message.  However, if a cache
2218   successfully validates a cache entry, it SHOULD remove any Warning
2219   headers previously attached to that entry except as specified for
2220   specific Warning codes.  It MUST then add any Warning headers
2221   received in the validating response.  In other words, Warning headers
2222   are those that would be attached to the most recent relevant
2223   response.
2224
2225   When multiple Warning headers are attached to a response, the user
2226   agent ought to inform the user of as many of them as possible, in the
2227   order that they appear in the response.  If it is not possible to
2228   inform the user of all of the warnings, the user agent SHOULD follow
2229   these heuristics:
2230
2231   o  Warnings that appear early in the response take priority over
2232      those appearing later in the response.
2233
2234   o  Warnings in the user's preferred character set take priority over
2235      warnings in other character sets but with identical warn-codes and
2236
2237
2238
2239Fielding, et al.          Expires June 22, 2008                [Page 40]
2240
2241Internet-Draft              HTTP/1.1, part 6               December 2007
2242
2243
2244      warn-agents.
2245
2246   Systems that generate multiple Warning headers SHOULD order them with
2247   this user agent behavior in mind.
2248
2249   Requirements for the behavior of caches with respect to Warnings are
2250   stated in Section 2.1.2.
2251
2252   This is a list of the currently-defined warn-codes, each with a
2253   recommended warn-text in English, and a description of its meaning.
2254
2255   110 Response is stale
2256
2257      MUST be included whenever the returned response is stale.
2258
2259   111 Revalidation failed
2260
2261      MUST be included if a cache returns a stale response because an
2262      attempt to revalidate the response failed, due to an inability to
2263      reach the server.
2264
2265   112 Disconnected operation
2266
2267      SHOULD be included if the cache is intentionally disconnected from
2268      the rest of the network for a period of time.
2269
2270   113 Heuristic expiration
2271
2272      MUST be included if the cache heuristically chose a freshness
2273      lifetime greater than 24 hours and the response's age is greater
2274      than 24 hours.
2275
2276   199 Miscellaneous warning
2277
2278      The warning text MAY include arbitrary information to be presented
2279      to a human user, or logged.  A system receiving this warning MUST
2280      NOT take any automated action, besides presenting the warning to
2281      the user.
2282
2283   214 Transformation applied
2284
2285      MUST be added by an intermediate cache or proxy if it applies any
2286      transformation changing the content-coding (as specified in the
2287      Content-Encoding header) or media-type (as specified in the
2288      Content-Type header) of the response, or the entity-body of the
2289      response, unless this Warning code already appears in the
2290      response.
2291
2292
2293
2294
2295Fielding, et al.          Expires June 22, 2008                [Page 41]
2296
2297Internet-Draft              HTTP/1.1, part 6               December 2007
2298
2299
2300   299 Miscellaneous persistent warning
2301
2302      The warning text MAY include arbitrary information to be presented
2303      to a human user, or logged.  A system receiving this warning MUST
2304      NOT take any automated action.
2305
2306   If an implementation sends a message with one or more Warning headers
2307   whose version is HTTP/1.0 or lower, then the sender MUST include in
2308   each warning-value a warn-date that matches the date in the response.
2309
2310   If an implementation receives a message with a warning-value that
2311   includes a warn-date, and that warn-date is different from the Date
2312   value in the response, then that warning-value MUST be deleted from
2313   the message before storing, forwarding, or using it.  (This prevents
2314   bad consequences of naive caching of Warning header fields.)  If all
2315   of the warning-values are deleted for this reason, the Warning header
2316   MUST be deleted as well.
2317
2318
23194.  IANA Considerations
2320
2321   TBD.
2322
2323
23245.  Security Considerations
2325
2326   Caching proxies provide additional potential vulnerabilities, since
2327   the contents of the cache represent an attractive target for
2328   malicious exploitation.  Because cache contents persist after an HTTP
2329   request is complete, an attack on the cache can reveal information
2330   long after a user believes that the information has been removed from
2331   the network.  Therefore, cache contents should be protected as
2332   sensitive information.
2333
2334
23356.  Acknowledgments
2336
2337   Much of the content and presentation of the caching design is due to
2338   suggestions and comments from individuals including: Shel Kaphan,
2339   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2340
2341   Based on an XML translation of RFC 2616 by Julian Reschke.
2342
2343
23447.  References
2345
2346   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2347              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
2348
2349
2350
2351Fielding, et al.          Expires June 22, 2008                [Page 42]
2352
2353Internet-Draft              HTTP/1.1, part 6               December 2007
2354
2355
2356              part 1: URIs, Connections, and Message Parsing",
2357              draft-ietf-httpbis-p1-messaging-00 (work in progress),
2358              December 2007.
2359
2360   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2361              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
2362              part 2: Message Semantics",
2363              draft-ietf-httpbis-p2-semantics-00 (work in progress),
2364              December 2007.
2365
2366   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2367              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
2368              part 3: Message Payload and Content Negotiation",
2369              draft-ietf-httpbis-p3-payload-00 (work in progress),
2370              December 2007.
2371
2372   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2373              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
2374              part 4: Conditional Requests",
2375              draft-ietf-httpbis-p4-conditional-00 (work in progress),
2376              December 2007.
2377
2378   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2379              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
2380              part 5: Range Requests and Partial Responses",
2381              draft-ietf-httpbis-p5-range-00 (work in progress),
2382              December 2007.
2383
2384   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2385              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
2386              part 7: Authentication", draft-ietf-httpbis-p7-auth-00
2387              (work in progress), December 2007.
2388
2389   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
2390              Specification, Implementation", RFC 1305, March 1992.
2391
2392   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
2393              Part Three: Message Header Extensions for Non-ASCII Text",
2394              RFC 2047, November 1996.
2395
2396   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
2397              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
2398              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
2399
2400
2401Appendix A.  Changes from RFC 2068
2402
2403   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2404
2405
2406
2407Fielding, et al.          Expires June 22, 2008                [Page 43]
2408
2409Internet-Draft              HTTP/1.1, part 6               December 2007
2410
2411
2412   was introduced to add this missing case.  (Sections 2.4, 3.2, 3.2.3)
2413
2414   The Cache-Control: max-age directive was not properly defined for
2415   responses.  (Section 3.2.3)
2416
2417   Warnings could be cached incorrectly, or not updated appropriately.
2418   (Section 2.1.2, 2.2.4, 2.5.2, 2.5.3, 3.2.3, and 3.6) Warning also
2419   needed to be a general header, as PUT or other methods may have need
2420   for it in requests.
2421
2422
2423Index
2424
2425   A
2426      age  5
2427      Age header  27
2428
2429   C
2430      cache  5
2431      Cache Directives
2432         max-age  32
2433         max-age  33
2434         max-stale  32
2435         min-fresh  32
2436         must-revalidate  34
2437         no-cache  29
2438         no-store  30
2439         no-transform  35
2440         only-if-cached  34
2441         private  29
2442         proxy-revalidate  35
2443         public  29
2444         s-maxage  31
2445      Cache-Control header  27
2446      cacheable  5
2447
2448   E
2449      Expires header  37
2450      explicit expiration time  5
2451
2452   F
2453      first-hand  5
2454      fresh  6
2455      freshness lifetime  6
2456
2457   G
2458      Grammar
2459         Age  27
2460
2461
2462
2463Fielding, et al.          Expires June 22, 2008                [Page 44]
2464
2465Internet-Draft              HTTP/1.1, part 6               December 2007
2466
2467
2468         age-value  27
2469         Cache-Control  28
2470         cache-directive  28
2471         cache-extension  28
2472         cache-request-directive  28
2473         cache-response-directive  28
2474         delta-seconds  6
2475         Expires  37
2476         extension-pragma  38
2477         Pragma  38
2478         pragma-directive  38
2479         Vary  38
2480         warn-agent  40
2481         warn-code  40
2482         warn-date  40
2483         warn-text  40
2484         Warning  40
2485         warning-value  40
2486
2487   H
2488      Headers
2489         Age  27
2490         Cache-Control  27
2491         Expires  37
2492         Pragma  38
2493         Vary  38
2494         Warning  39
2495      heuristic expiration time  5
2496
2497   M
2498      max-age
2499         Cache Directive  32
2500         Cache Directive  33
2501      max-stale
2502         Cache Directive  32
2503      min-fresh
2504         Cache Directive  32
2505      must-revalidate
2506         Cache Directive  34
2507
2508   N
2509      no-cache
2510         Cache Directive  29
2511      no-store
2512         Cache Directive  30
2513      no-transform
2514         Cache Directive  35
2515
2516
2517
2518
2519Fielding, et al.          Expires June 22, 2008                [Page 45]
2520
2521Internet-Draft              HTTP/1.1, part 6               December 2007
2522
2523
2524   O
2525      only-if-cached
2526         Cache Directive  34
2527
2528   P
2529      Pragma header  38
2530      private
2531         Cache Directive  29
2532      proxy-revalidate
2533         Cache Directive  35
2534      public
2535         Cache Directive  29
2536
2537   S
2538      s-maxage
2539         Cache Directive  31
2540      semantically transparent  6
2541      stale  6
2542
2543   V
2544      validator  6
2545      Vary header  38
2546
2547   W
2548      Warning header  39
2549
2550
2551Authors' Addresses
2552
2553   Roy T. Fielding (editor)
2554   Day Software
2555   23 Corporate Plaza DR, Suite 280
2556   Newport Beach, CA  92660
2557   USA
2558
2559   Phone: +1-949-706-5300
2560   Fax:   +1-949-706-5305
2561   Email: fielding@gbiv.com
2562   URI:   http://roy.gbiv.com/
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575Fielding, et al.          Expires June 22, 2008                [Page 46]
2576
2577Internet-Draft              HTTP/1.1, part 6               December 2007
2578
2579
2580   Jim Gettys
2581   One Laptop per Child
2582   21 Oak Knoll Road
2583   Carlisle, MA  01741
2584   USA
2585
2586   Email: jg@laptop.org
2587   URI:   http://www.laptop.org/
2588
2589
2590   Jeffrey C. Mogul
2591   Hewlett-Packard Company
2592   HP Labs, Large Scale Systems Group
2593   1501 Page Mill Road, MS 1177
2594   Palo Alto, CA  94304
2595   USA
2596
2597   Email: JeffMogul@acm.org
2598
2599
2600   Henrik Frystyk Nielsen
2601   Microsoft Corporation
2602   1 Microsoft Way
2603   Redmond, WA  98052
2604   USA
2605
2606   Email: henrikn@microsoft.com
2607
2608
2609   Larry Masinter
2610   Adobe Systems, Incorporated
2611   345 Park Ave
2612   San Jose, CA  95110
2613   USA
2614
2615   Email: LMM@acm.org
2616   URI:   http://larry.masinter.net/
2617
2618
2619   Paul J. Leach
2620   Microsoft Corporation
2621   1 Microsoft Way
2622   Redmond, WA  98052
2623
2624   Email: paulle@microsoft.com
2625
2626
2627
2628
2629
2630
2631Fielding, et al.          Expires June 22, 2008                [Page 47]
2632
2633Internet-Draft              HTTP/1.1, part 6               December 2007
2634
2635
2636   Tim Berners-Lee
2637   World Wide Web Consortium
2638   MIT Computer Science and Artificial Intelligence Laboratory
2639   The Stata Center, Building 32
2640   32 Vassar Street
2641   Cambridge, MA  02139
2642   USA
2643
2644   Email: timbl@w3.org
2645   URI:   http://www.w3.org/People/Berners-Lee/
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687Fielding, et al.          Expires June 22, 2008                [Page 48]
2688
2689Internet-Draft              HTTP/1.1, part 6               December 2007
2690
2691
2692Full Copyright Statement
2693
2694   Copyright (C) The IETF Trust (2007).
2695
2696   This document is subject to the rights, licenses and restrictions
2697   contained in BCP 78, and except as set forth therein, the authors
2698   retain all their rights.
2699
2700   This document and the information contained herein are provided on an
2701   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
2702   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
2703   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
2704   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
2705   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
2706   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
2707
2708
2709Intellectual Property
2710
2711   The IETF takes no position regarding the validity or scope of any
2712   Intellectual Property Rights or other rights that might be claimed to
2713   pertain to the implementation or use of the technology described in
2714   this document or the extent to which any license under such rights
2715   might or might not be available; nor does it represent that it has
2716   made any independent effort to identify any such rights.  Information
2717   on the procedures with respect to rights in RFC documents can be
2718   found in BCP 78 and BCP 79.
2719
2720   Copies of IPR disclosures made to the IETF Secretariat and any
2721   assurances of licenses to be made available, or the result of an
2722   attempt made to obtain a general license or permission for the use of
2723   such proprietary rights by implementers or users of this
2724   specification can be obtained from the IETF on-line IPR repository at
2725   http://www.ietf.org/ipr.
2726
2727   The IETF invites any interested party to bring to its attention any
2728   copyrights, patents or patent applications, or other proprietary
2729   rights that may cover technology that may be required to implement
2730   this standard.  Please address the information to the IETF at
2731   ietf-ipr@ietf.org.
2732
2733
2734Acknowledgment
2735
2736   Funding for the RFC Editor function is provided by the IETF
2737   Administrative Support Activity (IASA).
2738
2739
2740
2741
2742
2743Fielding, et al.          Expires June 22, 2008                [Page 49]
2744
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