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4Network Working Group                                   R. Fielding, Ed.
5Internet-Draft                                              Day Software
6Obsoletes: 2616 (if approved)                                  J. Gettys
7Intended status: Standards Track                    One Laptop per Child
8Expires: December 19, 2008                                      J. Mogul
9                                                                      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                                                           Y. Lafon, Ed.
19                                                                     W3C
20                                                         J. Reschke, Ed.
21                                                              greenbytes
22                                                           June 17, 2008
23
24
25                       HTTP/1.1, part 6: Caching
26                     draft-ietf-httpbis-p6-cache-03
27
28Status of this Memo
29
30   By submitting this Internet-Draft, each author represents that any
31   applicable patent or other IPR claims of which he or she is aware
32   have been or will be disclosed, and any of which he or she becomes
33   aware will be disclosed, in accordance with Section 6 of BCP 79.
34
35   Internet-Drafts are working documents of the Internet Engineering
36   Task Force (IETF), its areas, and its working groups.  Note that
37   other groups may also distribute working documents as Internet-
38   Drafts.
39
40   Internet-Drafts are draft documents valid for a maximum of six months
41   and may be updated, replaced, or obsoleted by other documents at any
42   time.  It is inappropriate to use Internet-Drafts as reference
43   material or to cite them other than as "work in progress."
44
45   The list of current Internet-Drafts can be accessed at
46   http://www.ietf.org/ietf/1id-abstracts.txt.
47
48   The list of Internet-Draft Shadow Directories can be accessed at
49   http://www.ietf.org/shadow.html.
50
51   This Internet-Draft will expire on December 19, 2008.
52
53
54
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57Internet-Draft              HTTP/1.1, Part 6                   June 2008
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   Discussion of this draft should take place on the HTTPBIS working
74   group mailing list (ietf-http-wg@w3.org).  The current issues list is
75   at <http://www.tools.ietf.org/wg/httpbis/trac/report/11> and related
76   documents (including fancy diffs) can be found at
77   <http://www.tools.ietf.org/wg/httpbis/>.
78
79   The changes in this draft are summarized in Appendix B.4.
80
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113Internet-Draft              HTTP/1.1, Part 6                   June 2008
114
115
116Table of Contents
117
118   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
119     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
120     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
121     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
122   2.  Notational Conventions and Generic Grammar . . . . . . . . . .  8
123   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
124     3.1.  Cache Correctness  . . . . . . . . . . . . . . . . . . . .  8
125     3.2.  Warnings . . . . . . . . . . . . . . . . . . . . . . . . .  9
126     3.3.  Cache-control Mechanisms . . . . . . . . . . . . . . . . . 10
127     3.4.  Explicit User Agent Warnings . . . . . . . . . . . . . . . 10
128     3.5.  Exceptions to the Rules and Warnings . . . . . . . . . . . 11
129     3.6.  Client-controlled Behavior . . . . . . . . . . . . . . . . 11
130   4.  Expiration Model . . . . . . . . . . . . . . . . . . . . . . . 12
131     4.1.  Server-Specified Expiration  . . . . . . . . . . . . . . . 12
132     4.2.  Heuristic Expiration . . . . . . . . . . . . . . . . . . . 13
133     4.3.  Age Calculations . . . . . . . . . . . . . . . . . . . . . 13
134     4.4.  Expiration Calculations  . . . . . . . . . . . . . . . . . 15
135     4.5.  Disambiguating Expiration Values . . . . . . . . . . . . . 16
136     4.6.  Disambiguating Multiple Responses  . . . . . . . . . . . . 17
137   5.  Validation Model . . . . . . . . . . . . . . . . . . . . . . . 17
138   6.  Response Cacheability  . . . . . . . . . . . . . . . . . . . . 18
139   7.  Constructing Responses From Caches . . . . . . . . . . . . . . 19
140     7.1.  End-to-end and Hop-by-hop Headers  . . . . . . . . . . . . 19
141     7.2.  Non-modifiable Headers . . . . . . . . . . . . . . . . . . 20
142     7.3.  Combining Headers  . . . . . . . . . . . . . . . . . . . . 21
143   8.  Caching Negotiated Responses . . . . . . . . . . . . . . . . . 22
144   9.  Shared and Non-Shared Caches . . . . . . . . . . . . . . . . . 23
145   10. Errors or Incomplete Response Cache Behavior . . . . . . . . . 24
146   11. Side Effects of GET and HEAD . . . . . . . . . . . . . . . . . 24
147   12. Invalidation After Updates or Deletions  . . . . . . . . . . . 24
148   13. Write-Through Mandatory  . . . . . . . . . . . . . . . . . . . 25
149   14. Cache Replacement  . . . . . . . . . . . . . . . . . . . . . . 26
150   15. History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 26
151   16. Header Field Definitions . . . . . . . . . . . . . . . . . . . 27
152     16.1. Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
153     16.2. Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 27
154       16.2.1.  What is Cacheable . . . . . . . . . . . . . . . . . . 29
155       16.2.2.  What May be Stored by Caches  . . . . . . . . . . . . 30
156       16.2.3.  Modifications of the Basic Expiration Mechanism . . . 31
157       16.2.4.  Cache Revalidation and Reload Controls  . . . . . . . 33
158       16.2.5.  No-Transform Directive  . . . . . . . . . . . . . . . 35
159       16.2.6.  Cache Control Extensions  . . . . . . . . . . . . . . 36
160     16.3. Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 37
161     16.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 38
162     16.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
163     16.6. Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 39
164
165
166
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169Internet-Draft              HTTP/1.1, Part 6                   June 2008
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171
172   17. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
173     17.1. Message Header Registration  . . . . . . . . . . . . . . . 42
174   18. Security Considerations  . . . . . . . . . . . . . . . . . . . 42
175   19. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 43
176   20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
177     20.1. Normative References . . . . . . . . . . . . . . . . . . . 43
178     20.2. Informative References . . . . . . . . . . . . . . . . . . 44
179   Appendix A.  Compatibility with Previous Versions  . . . . . . . . 44
180     A.1.  Changes from RFC 2068  . . . . . . . . . . . . . . . . . . 44
181     A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 45
182   Appendix B.  Change Log (to be removed by RFC Editor before
183                publication)  . . . . . . . . . . . . . . . . . . . . 45
184     B.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 45
185     B.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 45
186     B.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 46
187     B.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 46
188   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
189   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
190   Intellectual Property and Copyright Statements . . . . . . . . . . 52
191
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227
2281.  Introduction
229
230   HTTP is typically used for distributed information systems, where
231   performance can be improved by the use of response caches, and
232   includes a number of elements intended to make caching work as well
233   as possible.  Because these elements interact with each other, it is
234   useful to describe the caching design of HTTP separately.  This
235   document defines aspects of HTTP/1.1 related to caching and reusing
236   response messages.
237
2381.1.  Purpose
239
240   An HTTP cache is a local store of response messages and the subsystem
241   that controls its message storage, retrieval, and deletion.  A cache
242   stores cacheable responses in order to reduce the response time and
243   network bandwidth consumption on future, equivalent requests.  Any
244   client or server may include a cache, though a cache cannot be used
245   by a server that is acting as a tunnel.
246
247   Caching would be useless if it did not significantly improve
248   performance.  The goal of caching in HTTP/1.1 is to reuse a prior
249   response message to satisfy a current request.  In some cases, the
250   existing response can be reused without the need for a network
251   request, reducing latency and network round-trips; we use an
252   "expiration" mechanism for this purpose (see Section 4).  Even when a
253   new request is required, it is often possible to reuse all or parts
254   of the payload of a prior response to satisfy the request, thereby
255   reducing network bandwidth usage; we use a "validation" mechanism for
256   this purpose (see Section 5).
257
258   A cache behaves in a "semantically transparent" manner, with respect
259   to a particular response, when its use affects neither the requesting
260   client nor the origin server, except to improve performance.  When a
261   cache is semantically transparent, the client receives exactly the
262   same response status and payload that it would have received had its
263   request been handled directly by the origin server.
264
265   In an ideal world, all interactions with an HTTP cache would be
266   semantically transparent.  However, for some resources, semantic
267   transparency is not always necessary and can be effectively traded
268   for the sake of bandwidth scaling, disconnected operation, and high
269   availability.  HTTP/1.1 allows origin servers, caches, and clients to
270   explicitly reduce transparency when necessary.  However, because non-
271   transparent operation may confuse non-expert users and might be
272   incompatible with certain server applications (such as those for
273   ordering merchandise), the protocol requires that transparency be
274   relaxed
275
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283
284   o  only by an explicit protocol-level request when relaxed by client
285      or origin server
286
287   o  only with an explicit warning to the end user when relaxed by
288      cache or client
289
290   Therefore, HTTP/1.1 provides these important elements:
291
292   1.  Protocol features that provide full semantic transparency when
293       this is required by all parties.
294
295   2.  Protocol features that allow an origin server or user agent to
296       explicitly request and control non-transparent operation.
297
298   3.  Protocol features that allow a cache to attach warnings to
299       responses that do not preserve the requested approximation of
300       semantic transparency.
301
302   A basic principle is that it must be possible for the clients to
303   detect any potential relaxation of semantic transparency.
304
305      Note: The server, cache, or client implementor might be faced with
306      design decisions not explicitly discussed in this specification.
307      If a decision might affect semantic transparency, the implementor
308      ought to err on the side of maintaining transparency unless a
309      careful and complete analysis shows significant benefits in
310      breaking transparency.
311
3121.2.  Terminology
313
314   This specification uses a number of terms to refer to the roles
315   played by participants in, and objects of, HTTP caching.
316
317   cacheable
318
319      A response is cacheable if a cache is allowed to store a copy of
320      the response message for use in answering subsequent requests.
321      Even when a response is cacheable, there may be additional
322      constraints on whether a cache can use the cached copy for a
323      particular request.
324
325   first-hand
326
327      A response is first-hand if it comes directly and without
328      unnecessary delay from the origin server, perhaps via one or more
329      proxies.  A response is also first-hand if its validity has just
330      been checked directly with the origin server.
331
332
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339
340   explicit expiration time
341
342      The time at which the origin server intends that an entity should
343      no longer be returned by a cache without further validation.
344
345   heuristic expiration time
346
347      An expiration time assigned by a cache when no explicit expiration
348      time is available.
349
350   age
351
352      The age of a response is the time since it was sent by, or
353      successfully validated with, the origin server.
354
355   freshness lifetime
356
357      The length of time between the generation of a response and its
358      expiration time.
359
360   fresh
361
362      A response is fresh if its age has not yet exceeded its freshness
363      lifetime.
364
365   stale
366
367      A response is stale if its age has passed its freshness lifetime.
368
369   validator
370
371      A protocol element (e.g., an entity tag or a Last-Modified time)
372      that is used to find out whether a cache entry is an equivalent
373      copy of an entity.
374
3751.3.  Requirements
376
377   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
378   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
379   document are to be interpreted as described in [RFC2119].
380
381   An implementation is not compliant if it fails to satisfy one or more
382   of the MUST or REQUIRED level requirements for the protocols it
383   implements.  An implementation that satisfies all the MUST or
384   REQUIRED level and all the SHOULD level requirements for its
385   protocols is said to be "unconditionally compliant"; one that
386   satisfies all the MUST level requirements but not all the SHOULD
387   level requirements for its protocols is said to be "conditionally
388
389
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395
396   compliant."
397
398
3992.  Notational Conventions and Generic Grammar
400
401   This specification uses the ABNF syntax defined in Section 2.1 of
402   [Part1] and the core rules defined in Section 2.2 of [Part1]:
403   [[abnf.dep: ABNF syntax and basic rules will be adopted from RFC
404   5234, see <http://tools.ietf.org/wg/httpbis/trac/ticket/36>.]]
405
406     DIGIT         = <DIGIT, defined in [Part1], Section 2.2>
407     DQUOTE        = <DQUOTE, defined in [Part1], Section 2.2>
408     SP            = <SP, defined in [Part1], Section 2.2>
409
410
411     quoted-string = <quoted-string, defined in [Part1], Section 2.2>
412     token         = <token, defined in [Part1], Section 2.2>
413
414   The ABNF rules below are defined in other parts:
415
416     field-name    = <field-name, defined in [Part1], Section 4.2>
417     HTTP-date     = <HTTP-date, defined in [Part1], Section 3.3.1>
418     port          = <port, defined in [Part1], Section 3.2.1>
419     pseudonym     = <pseudonym, defined in [Part1], Section 8.9>
420     uri-host      = <uri-host, defined in [Part1], Section 3.2.1>
421
422
4233.  Overview
424
4253.1.  Cache Correctness
426
427   A correct cache MUST respond to a request with the most up-to-date
428   response held by the cache that is appropriate to the request (see
429   Sections 4.5, 4.6, and 14) which meets one of the following
430   conditions:
431
432   1.  It has been checked for equivalence with what the origin server
433       would have returned by revalidating the response with the origin
434       server (Section 5);
435
436   2.  It is "fresh enough" (see Section 4).  In the default case, this
437       means it meets the least restrictive freshness requirement of the
438       client, origin server, and cache (see Section 16.2); if the
439       origin server so specifies, it is the freshness requirement of
440       the origin server alone.  If a stored response is not "fresh
441       enough" by the most restrictive freshness requirement of both the
442       client and the origin server, in carefully considered
443       circumstances the cache MAY still return the response with the
444
445
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451
452       appropriate Warning header (see Sections 3.5 and 16.6), unless
453       such a response is prohibited (e.g., by a "no-store" cache-
454       directive, or by a "no-cache" cache-request-directive; see
455       Section 16.2).
456
457   3.  It is an appropriate 304 (Not Modified), 305 (Use Proxy), or
458       error (4xx or 5xx) response message.
459
460   If the cache can not communicate with the origin server, then a
461   correct cache SHOULD respond as above if the response can be
462   correctly served from the cache; if not it MUST return an error or
463   warning indicating that there was a communication failure.
464
465   If a cache receives a response (either an entire response, or a 304
466   (Not Modified) response) that it would normally forward to the
467   requesting client, and the received response is no longer fresh, the
468   cache SHOULD forward it to the requesting client without adding a new
469   Warning (but without removing any existing Warning headers).  A cache
470   SHOULD NOT attempt to revalidate a response simply because that
471   response became stale in transit; this might lead to an infinite
472   loop.  A user agent that receives a stale response without a Warning
473   MAY display a warning indication to the user.
474
4753.2.  Warnings
476
477   Whenever a cache returns a response that is neither first-hand nor
478   "fresh enough" (in the sense of condition 2 in Section 3.1), it MUST
479   attach a warning to that effect, using a Warning general-header.  The
480   Warning header and the currently defined warnings are described in
481   Section 16.6.  The warning allows clients to take appropriate action.
482
483   Warnings MAY be used for other purposes, both cache-related and
484   otherwise.  The use of a warning, rather than an error status code,
485   distinguish these responses from true failures.
486
487   Warnings are assigned three digit warn-codes.  The first digit
488   indicates whether the Warning MUST or MUST NOT be deleted from a
489   stored cache entry after a successful revalidation:
490
491   1xx  Warnings that describe the freshness or revalidation status of
492      the response, and so MUST be deleted after a successful
493      revalidation. 1xx warn-codes MAY be generated by a cache only when
494      validating a cached entry.  It MUST NOT be generated by clients.
495
496   2xx  Warnings that describe some aspect of the entity body or entity
497      headers that is not rectified by a revalidation (for example, a
498      lossy compression of the entity bodies) and which MUST NOT be
499      deleted after a successful revalidation.
500
501
502
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507
508   See Section 16.6 for the definitions of the codes themselves.
509
510   HTTP/1.0 caches will cache all Warnings in responses, without
511   deleting the ones in the first category.  Warnings in responses that
512   are passed to HTTP/1.0 caches carry an extra warning-date field,
513   which prevents a future HTTP/1.1 recipient from believing an
514   erroneously cached Warning.
515
516   Warnings also carry a warning text.  The text MAY be in any
517   appropriate natural language (perhaps based on the client's Accept
518   headers), and include an OPTIONAL indication of what character set is
519   used.
520
521   Multiple warnings MAY be attached to a response (either by the origin
522   server or by a cache), including multiple warnings with the same code
523   number.  For example, a server might provide the same warning with
524   texts in both English and Basque.
525
526   When multiple warnings are attached to a response, it might not be
527   practical or reasonable to display all of them to the user.  This
528   version of HTTP does not specify strict priority rules for deciding
529   which warnings to display and in what order, but does suggest some
530   heuristics.
531
5323.3.  Cache-control Mechanisms
533
534   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
535   times and validators) are implicit directives to caches.  In some
536   cases, a server or client might need to provide explicit directives
537   to the HTTP caches.  We use the Cache-Control header for this
538   purpose.
539
540   The Cache-Control header allows a client or server to transmit a
541   variety of directives in either requests or responses.  These
542   directives typically override the default caching algorithms.  As a
543   general rule, if there is any apparent conflict between header
544   values, the most restrictive interpretation is applied (that is, the
545   one that is most likely to preserve semantic transparency).  However,
546   in some cases, cache-control directives are explicitly specified as
547   weakening the approximation of semantic transparency (for example,
548   "max-stale" or "public").
549
550   The cache-control directives are described in detail in Section 16.2.
551
5523.4.  Explicit User Agent Warnings
553
554   Many user agents make it possible for users to override the basic
555   caching mechanisms.  For example, the user agent might allow the user
556
557
558
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563
564   to specify that cached entities (even explicitly stale ones) are
565   never validated.  Or the user agent might habitually add "Cache-
566   Control: max-stale=3600" to every request.  The user agent SHOULD NOT
567   default to either non-transparent behavior, or behavior that results
568   in abnormally ineffective caching, but MAY be explicitly configured
569   to do so by an explicit action of the user.
570
571   If the user has overridden the basic caching mechanisms, the user
572   agent SHOULD explicitly indicate to the user whenever this results in
573   the display of information that might not meet the server's
574   transparency requirements (in particular, if the displayed entity is
575   known to be stale).  Since the protocol normally allows the user
576   agent to determine if responses are stale or not, this indication
577   need only be displayed when this actually happens.  The indication
578   need not be a dialog box; it could be an icon (for example, a picture
579   of a rotting fish) or some other indicator.
580
581   If the user has overridden the caching mechanisms in a way that would
582   abnormally reduce the effectiveness of caches, the user agent SHOULD
583   continually indicate this state to the user (for example, by a
584   display of a picture of currency in flames) so that the user does not
585   inadvertently consume excess resources or suffer from excessive
586   latency.
587
5883.5.  Exceptions to the Rules and Warnings
589
590   In some cases, the operator of a cache MAY choose to configure it to
591   return stale responses even when not requested by clients.  This
592   decision ought not be made lightly, but may be necessary for reasons
593   of availability or performance, especially when the cache is poorly
594   connected to the origin server.  Whenever a cache returns a stale
595   response, it MUST mark it as such (using a Warning header) enabling
596   the client software to alert the user that there might be a potential
597   problem.
598
599   It also allows the user agent to take steps to obtain a first-hand or
600   fresh response.  For this reason, a cache SHOULD NOT return a stale
601   response if the client explicitly requests a first-hand or fresh one,
602   unless it is impossible to comply for technical or policy reasons.
603
6043.6.  Client-controlled Behavior
605
606   While the origin server (and to a lesser extent, intermediate caches,
607   by their contribution to the age of a response) are the primary
608   source of expiration information, in some cases the client might need
609   to control a cache's decision about whether to return a cached
610   response without validating it.  Clients do this using several
611   directives of the Cache-Control header.
612
613
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619
620   A client's request MAY specify the maximum age it is willing to
621   accept of an unvalidated response; specifying a value of zero forces
622   the cache(s) to revalidate all responses.  A client MAY also specify
623   the minimum time remaining before a response expires.  Both of these
624   options increase constraints on the behavior of caches, and so cannot
625   further relax the cache's approximation of semantic transparency.
626
627   A client MAY also specify that it will accept stale responses, up to
628   some maximum amount of staleness.  This loosens the constraints on
629   the caches, and so might violate the origin server's specified
630   constraints on semantic transparency, but might be necessary to
631   support disconnected operation, or high availability in the face of
632   poor connectivity.
633
634
6354.  Expiration Model
636
6374.1.  Server-Specified Expiration
638
639   HTTP caching works best when caches can entirely avoid making
640   requests to the origin server.  The primary mechanism for avoiding
641   requests is for an origin server to provide an explicit expiration
642   time in the future, indicating that a response MAY be used to satisfy
643   subsequent requests.  In other words, a cache can return a fresh
644   response without first contacting the server.
645
646   Our expectation is that servers will assign future explicit
647   expiration times to responses in the belief that the entity is not
648   likely to change, in a semantically significant way, before the
649   expiration time is reached.  This normally preserves semantic
650   transparency, as long as the server's expiration times are carefully
651   chosen.
652
653   The expiration mechanism applies only to responses taken from a cache
654   and not to first-hand responses forwarded immediately to the
655   requesting client.
656
657   If an origin server wishes to force a semantically transparent cache
658   to validate every request, it MAY assign an explicit expiration time
659   in the past.  This means that the response is always stale, and so
660   the cache SHOULD validate it before using it for subsequent requests.
661   See Section 16.2.4 for a more restrictive way to force revalidation.
662
663   If an origin server wishes to force any HTTP/1.1 cache, no matter how
664   it is configured, to validate every request, it SHOULD use the "must-
665   revalidate" cache-control directive (see Section 16.2).
666
667   Servers specify explicit expiration times using either the Expires
668
669
670
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675
676   header, or the max-age directive of the Cache-Control header.
677
678   An expiration time cannot be used to force a user agent to refresh
679   its display or reload a resource; its semantics apply only to caching
680   mechanisms, and such mechanisms need only check a resource's
681   expiration status when a new request for that resource is initiated.
682   See Section 15 for an explanation of the difference between caches
683   and history mechanisms.
684
6854.2.  Heuristic Expiration
686
687   Since origin servers do not always provide explicit expiration times,
688   HTTP caches typically assign heuristic expiration times, employing
689   algorithms that use other header values (such as the Last-Modified
690   time) to estimate a plausible expiration time.  The HTTP/1.1
691   specification does not provide specific algorithms, but does impose
692   worst-case constraints on their results.  Since heuristic expiration
693   times might compromise semantic transparency, they ought to be used
694   cautiously, and we encourage origin servers to provide explicit
695   expiration times as much as possible.
696
6974.3.  Age Calculations
698
699   In order to know if a cached entry is fresh, a cache needs to know if
700   its age exceeds its freshness lifetime.  We discuss how to calculate
701   the latter in Section 4.4; this section describes how to calculate
702   the age of a response or cache entry.
703
704   In this discussion, we use the term "now" to mean "the current value
705   of the clock at the host performing the calculation."  Hosts that use
706   HTTP, but especially hosts running origin servers and caches, SHOULD
707   use NTP [RFC1305] or some similar protocol to synchronize their
708   clocks to a globally accurate time standard.
709
710   HTTP/1.1 requires origin servers to send a Date header, if possible,
711   with every response, giving the time at which the response was
712   generated (see Section 8.3 of [Part1]).  We use the term "date_value"
713   to denote the value of the Date header, in a form appropriate for
714   arithmetic operations.
715
716   HTTP/1.1 uses the Age response-header to convey the estimated age of
717   the response message when obtained from a cache.  The Age field value
718   is the cache's estimate of the amount of time since the response was
719   generated or revalidated by the origin server.
720
721   In essence, the Age value is the sum of the time that the response
722   has been resident in each of the caches along the path from the
723   origin server, plus the amount of time it has been in transit along
724
725
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731
732   network paths.
733
734   We use the term "age_value" to denote the value of the Age header, in
735   a form appropriate for arithmetic operations.
736
737   A response's age can be calculated in two entirely independent ways:
738
739   1.  now minus date_value, if the local clock is reasonably well
740       synchronized to the origin server's clock.  If the result is
741       negative, the result is replaced by zero.
742
743   2.  age_value, if all of the caches along the response path implement
744       HTTP/1.1.
745
746   Given that we have two independent ways to compute the age of a
747   response when it is received, we can combine these as
748
749       corrected_received_age = max(now - date_value, age_value)
750
751   and as long as we have either nearly synchronized clocks or all-
752   HTTP/1.1 paths, one gets a reliable (conservative) result.
753
754   Because of network-imposed delays, some significant interval might
755   pass between the time that a server generates a response and the time
756   it is received at the next outbound cache or client.  If uncorrected,
757   this delay could result in improperly low ages.
758
759   Because the request that resulted in the returned Age value must have
760   been initiated prior to that Age value's generation, we can correct
761   for delays imposed by the network by recording the time at which the
762   request was initiated.  Then, when an Age value is received, it MUST
763   be interpreted relative to the time the request was initiated, not
764   the time that the response was received.  This algorithm results in
765   conservative behavior no matter how much delay is experienced.  So,
766   we compute:
767
768      corrected_initial_age = corrected_received_age
769                            + (now - request_time)
770
771   where "request_time" is the time (according to the local clock) when
772   the request that elicited this response was sent.
773
774   Summary of age calculation algorithm, when a cache receives a
775   response:
776
777
778
779
780
781
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787
788      /*
789       * age_value
790       *      is the value of Age: header received by the cache with
791       *              this response.
792       * date_value
793       *      is the value of the origin server's Date: header
794       * request_time
795       *      is the (local) time when the cache made the request
796       *              that resulted in this cached response
797       * response_time
798       *      is the (local) time when the cache received the
799       *              response
800       * now
801       *      is the current (local) time
802       */
803
804      apparent_age = max(0, response_time - date_value);
805      corrected_received_age = max(apparent_age, age_value);
806      response_delay = response_time - request_time;
807      corrected_initial_age = corrected_received_age + response_delay;
808      resident_time = now - response_time;
809      current_age   = corrected_initial_age + resident_time;
810
811   The current_age of a cache entry is calculated by adding the amount
812   of time (in seconds) since the cache entry was last validated by the
813   origin server to the corrected_initial_age.  When a response is
814   generated from a cache entry, the cache MUST include a single Age
815   header field in the response with a value equal to the cache entry's
816   current_age.
817
818   The presence of an Age header field in a response implies that a
819   response is not first-hand.  However, the converse is not true, since
820   the lack of an Age header field in a response does not imply that the
821   response is first-hand unless all caches along the request path are
822   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
823   the Age header field).
824
8254.4.  Expiration Calculations
826
827   In order to decide whether a response is fresh or stale, we need to
828   compare its freshness lifetime to its age.  The age is calculated as
829   described in Section 4.3; this section describes how to calculate the
830   freshness lifetime, and to determine if a response has expired.  In
831   the discussion below, the values can be represented in any form
832   appropriate for arithmetic operations.
833
834   We use the term "expires_value" to denote the value of the Expires
835   header.  We use the term "max_age_value" to denote an appropriate
836
837
838
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843
844   value of the number of seconds carried by the "max-age" directive of
845   the Cache-Control header in a response (see Section 16.2.3).
846
847   The max-age directive takes priority over Expires, so if max-age is
848   present in a response, the calculation is simply:
849
850      freshness_lifetime = max_age_value
851
852   Otherwise, if Expires is present in the response, the calculation is:
853
854      freshness_lifetime = expires_value - date_value
855
856   Note that neither of these calculations is vulnerable to clock skew,
857   since all of the information comes from the origin server.
858
859   If none of Expires, Cache-Control: max-age, or Cache-Control:
860   s-maxage (see Section 16.2.3) appears in the response, and the
861   response does not include other restrictions on caching, the cache
862   MAY compute a freshness lifetime using a heuristic.  The cache MUST
863   attach Warning 113 to any response whose age is more than 24 hours if
864   such warning has not already been added.
865
866   Also, if the response does have a Last-Modified time, the heuristic
867   expiration value SHOULD be no more than some fraction of the interval
868   since that time.  A typical setting of this fraction might be 10%.
869
870   The calculation to determine if a response has expired is quite
871   simple:
872
873      response_is_fresh = (freshness_lifetime > current_age)
874
8754.5.  Disambiguating Expiration Values
876
877   Because expiration values are assigned optimistically, it is possible
878   for two caches to contain fresh values for the same resource that are
879   different.
880
881   If a client performing a retrieval receives a non-first-hand response
882   for a request that was already fresh in its own cache, and the Date
883   header in its existing cache entry is newer than the Date on the new
884   response, then the client MAY ignore the response.  If so, it MAY
885   retry the request with a "Cache-Control: max-age=0" directive (see
886   Section 16.2), to force a check with the origin server.
887
888   If a cache has two fresh responses for the same representation with
889   different validators, it MUST use the one with the more recent Date
890   header.  This situation might arise because the cache is pooling
891   responses from other caches, or because a client has asked for a
892
893
894
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899
900   reload or a revalidation of an apparently fresh cache entry.
901
9024.6.  Disambiguating Multiple Responses
903
904   Because a client might be receiving responses via multiple paths, so
905   that some responses flow through one set of caches and other
906   responses flow through a different set of caches, a client might
907   receive responses in an order different from that in which the origin
908   server sent them.  We would like the client to use the most recently
909   generated response, even if older responses are still apparently
910   fresh.
911
912   Neither the entity tag nor the expiration value can impose an
913   ordering on responses, since it is possible that a later response
914   intentionally carries an earlier expiration time.  The Date values
915   are ordered to a granularity of one second.
916
917   When a client tries to revalidate a cache entry, and the response it
918   receives contains a Date header that appears to be older than the one
919   for the existing entry, then the client SHOULD repeat the request
920   unconditionally, and include
921
922       Cache-Control: max-age=0
923
924   to force any intermediate caches to validate their copies directly
925   with the origin server, or
926
927       Cache-Control: no-cache
928
929   to force any intermediate caches to obtain a new copy from the origin
930   server.
931
932   If the Date values are equal, then the client MAY use either response
933   (or MAY, if it is being extremely prudent, request a new response).
934   Servers MUST NOT depend on clients being able to choose
935   deterministically between responses generated during the same second,
936   if their expiration times overlap.
937
938
9395.  Validation Model
940
941   When a cache has a stale entry that it would like to use as a
942   response to a client's request, it first has to check with the origin
943   server (or possibly an intermediate cache with a fresh response) to
944   see if its cached entry is still usable.  We call this "validating"
945   the cache entry.
946
947   HTTP's conditional request mechanism, defined in [Part4], is used to
948
949
950
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955
956   avoid retransmitting the response payload when the cached entry is
957   valid.  When a cached response includes one or more "cache
958   validators," such as the field values of an ETag or Last-Modified
959   header field, then a validating GET request SHOULD be made
960   conditional to those field values.  The server checks the conditional
961   request's validator against the current state of the requested
962   resource and, if they match, the server responds with a 304 (Not
963   Modified) status code to indicate that the cached response can be
964   refreshed and reused without retransmitting the response payload.  If
965   the validator does not match the current state of the requested
966   resource, then the server returns a full response, including payload,
967   so that the request can be satisfied and the cache entry supplanted
968   without the need for an additional network round-trip.
969
970
9716.  Response Cacheability
972
973   Unless specifically constrained by a cache-control (Section 16.2)
974   directive, a caching system MAY always store a successful response
975   (see Section 10) as a cache entry, MAY return it without validation
976   if it is fresh, and MAY return it after successful validation.  If
977   there is neither a cache validator nor an explicit expiration time
978   associated with a response, we do not expect it to be cached, but
979   certain caches MAY violate this expectation (for example, when little
980   or no network connectivity is available).  A client can usually
981   detect that such a response was taken from a cache by comparing the
982   Date header to the current time.
983
984      Note: some HTTP/1.0 caches are known to violate this expectation
985      without providing any Warning.
986
987   However, in some cases it might be inappropriate for a cache to
988   retain an entity, or to return it in response to a subsequent
989   request.  This might be because absolute semantic transparency is
990   deemed necessary by the service author, or because of security or
991   privacy considerations.  Certain cache-control directives are
992   therefore provided so that the server can indicate that certain
993   resource entities, or portions thereof, are not to be cached
994   regardless of other considerations.
995
996   Note that Section 4.1 of [Part7] normally prevents a shared cache
997   from saving and returning a response to a previous request if that
998   request included an Authorization header.
999
1000   A response received with a status code of 200, 203, 206, 300, 301 or
1001   410 MAY be stored by a cache and used in reply to a subsequent
1002   request, subject to the expiration mechanism, unless a cache-control
1003   directive prohibits caching.  However, a cache that does not support
1004
1005
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1011
1012   the Range and Content-Range headers MUST NOT cache 206 (Partial
1013   Content) responses.
1014
1015   A response received with any other status code (e.g. status codes 302
1016   and 307) MUST NOT be returned in a reply to a subsequent request
1017   unless there are cache-control directives or another header(s) that
1018   explicitly allow it.  For example, these include the following: an
1019   Expires header (Section 16.3); a "max-age", "s-maxage", "must-
1020   revalidate", "proxy-revalidate", "public" or "private" cache-control
1021   directive (Section 16.2).
1022
1023
10247.  Constructing Responses From Caches
1025
1026   The purpose of an HTTP cache is to store information received in
1027   response to requests for use in responding to future requests.  In
1028   many cases, a cache simply returns the appropriate parts of a
1029   response to the requester.  However, if the cache holds a cache entry
1030   based on a previous response, it might have to combine parts of a new
1031   response with what is held in the cache entry.
1032
10337.1.  End-to-end and Hop-by-hop Headers
1034
1035   For the purpose of defining the behavior of caches and non-caching
1036   proxies, we divide HTTP headers into two categories:
1037
1038   o  End-to-end headers, which are transmitted to the ultimate
1039      recipient of a request or response.  End-to-end headers in
1040      responses MUST be stored as part of a cache entry and MUST be
1041      transmitted in any response formed from a cache entry.
1042
1043   o  Hop-by-hop headers, which are meaningful only for a single
1044      transport-level connection, and are not stored by caches or
1045      forwarded by proxies.
1046
1047   The following HTTP/1.1 headers are hop-by-hop headers:
1048
1049   o  Connection
1050
1051   o  Keep-Alive
1052
1053   o  Proxy-Authenticate
1054
1055   o  Proxy-Authorization
1056
1057   o  TE
1058
1059
1060
1061
1062
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1066
1067
1068   o  Trailer
1069
1070   o  Transfer-Encoding
1071
1072   o  Upgrade
1073
1074   All other headers defined by HTTP/1.1 are end-to-end headers.
1075
1076   Other hop-by-hop headers MUST be listed in a Connection header
1077   (Section 8.1 of [Part1]).
1078
10797.2.  Non-modifiable Headers
1080
1081   Some features of HTTP/1.1, such as Digest Authentication, depend on
1082   the value of certain end-to-end headers.  A transparent proxy SHOULD
1083   NOT modify an end-to-end header unless the definition of that header
1084   requires or specifically allows that.
1085
1086   A transparent proxy MUST NOT modify any of the following fields in a
1087   request or response, and it MUST NOT add any of these fields if not
1088   already present:
1089
1090   o  Content-Location
1091
1092   o  Content-MD5
1093
1094   o  ETag
1095
1096   o  Last-Modified
1097
1098   A transparent proxy MUST NOT modify any of the following fields in a
1099   response:
1100
1101   o  Expires
1102
1103   but it MAY add any of these fields if not already present.  If an
1104   Expires header is added, it MUST be given a field-value identical to
1105   that of the Date header in that response.
1106
1107   A proxy MUST NOT modify or add any of the following fields in a
1108   message that contains the no-transform cache-control directive, or in
1109   any request:
1110
1111   o  Content-Encoding
1112
1113   o  Content-Range
1114
1115
1116
1117
1118
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1123
1124   o  Content-Type
1125
1126   A non-transparent proxy MAY modify or add these fields to a message
1127   that does not include no-transform, but if it does so, it MUST add a
1128   Warning 214 (Transformation applied) if one does not already appear
1129   in the message (see Section 16.6).
1130
1131      Warning: unnecessary modification of end-to-end headers might
1132      cause authentication failures if stronger authentication
1133      mechanisms are introduced in later versions of HTTP.  Such
1134      authentication mechanisms MAY rely on the values of header fields
1135      not listed here.
1136
1137   The Content-Length field of a request or response is added or deleted
1138   according to the rules in Section 4.4 of [Part1].  A transparent
1139   proxy MUST preserve the entity-length (Section 4.2.2 of [Part3]) of
1140   the entity-body, although it MAY change the transfer-length (Section
1141   4.4 of [Part1]).
1142
11437.3.  Combining Headers
1144
1145   When a cache makes a validating request to a server, and the server
1146   provides a 304 (Not Modified) response or a 206 (Partial Content)
1147   response, the cache then constructs a response to send to the
1148   requesting client.
1149
1150   If the status code is 304 (Not Modified), the cache uses the entity-
1151   body stored in the cache entry as the entity-body of this outgoing
1152   response.  If the status code is 206 (Partial Content) and the ETag
1153   or Last-Modified headers match exactly, the cache MAY combine the
1154   contents stored in the cache entry with the new contents received in
1155   the response and use the result as the entity-body of this outgoing
1156   response, (see Section 5 of [Part5]).
1157
1158   The end-to-end headers stored in the cache entry are used for the
1159   constructed response, except that
1160
1161   o  any stored Warning headers with warn-code 1xx (see Section 16.6)
1162      MUST be deleted from the cache entry and the forwarded response.
1163
1164   o  any stored Warning headers with warn-code 2xx MUST be retained in
1165      the cache entry and the forwarded response.
1166
1167   o  any end-to-end headers provided in the 304 or 206 response MUST
1168      replace the corresponding headers from the cache entry.
1169
1170   Unless the cache decides to remove the cache entry, it MUST also
1171   replace the end-to-end headers stored with the cache entry with
1172
1173
1174
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1179
1180   corresponding headers received in the incoming response, except for
1181   Warning headers as described immediately above.  If a header field-
1182   name in the incoming response matches more than one header in the
1183   cache entry, all such old headers MUST be replaced.
1184
1185   In other words, the set of end-to-end headers received in the
1186   incoming response overrides all corresponding end-to-end headers
1187   stored with the cache entry (except for stored Warning headers with
1188   warn-code 1xx, which are deleted even if not overridden).
1189
1190      Note: this rule allows an origin server to use a 304 (Not
1191      Modified) or a 206 (Partial Content) response to update any header
1192      associated with a previous response for the same entity or sub-
1193      ranges thereof, although it might not always be meaningful or
1194      correct to do so.  This rule does not allow an origin server to
1195      use a 304 (Not Modified) or a 206 (Partial Content) response to
1196      entirely delete a header that it had provided with a previous
1197      response.
1198
1199
12008.  Caching Negotiated Responses
1201
1202   Use of server-driven content negotiation (Section 5.1 of [Part3]), as
1203   indicated by the presence of a Vary header field in a response,
1204   alters the conditions and procedure by which a cache can use the
1205   response for subsequent requests.  See Section 16.5 for use of the
1206   Vary header field by servers.
1207
1208   A server SHOULD use the Vary header field to inform a cache of what
1209   request-header fields were used to select among multiple
1210   representations of a cacheable response subject to server-driven
1211   negotiation.  The set of header fields named by the Vary field value
1212   is known as the "selecting" request-headers.
1213
1214   When the cache receives a subsequent request whose Request-URI
1215   specifies one or more cache entries including a Vary header field,
1216   the cache MUST NOT use such a cache entry to construct a response to
1217   the new request unless all of the selecting request-headers present
1218   in the new request match the corresponding stored request-headers in
1219   the original request.
1220
1221   The selecting request-headers from two requests are defined to match
1222   if and only if the selecting request-headers in the first request can
1223   be transformed to the selecting request-headers in the second request
1224   by adding or removing linear white space (LWS) at places where this
1225   is allowed by the corresponding BNF, and/or combining multiple
1226   message-header fields with the same field name following the rules
1227   about message headers in Section 4.2 of [Part1].
1228
1229
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1235
1236   A Vary header field-value of "*" always fails to match and subsequent
1237   requests on that resource can only be properly interpreted by the
1238   origin server.
1239
1240   If the selecting request header fields for the cached entry do not
1241   match the selecting request header fields of the new request, then
1242   the cache MUST NOT use a cached entry to satisfy the request unless
1243   it first relays the new request to the origin server in a conditional
1244   request and the server responds with 304 (Not Modified), including an
1245   entity tag or Content-Location that indicates the entity to be used.
1246
1247   If an entity tag was assigned to a cached representation, the
1248   forwarded request SHOULD be conditional and include the entity tags
1249   in an If-None-Match header field from all its cache entries for the
1250   resource.  This conveys to the server the set of entities currently
1251   held by the cache, so that if any one of these entities matches the
1252   requested entity, the server can use the ETag header field in its 304
1253   (Not Modified) response to tell the cache which entry is appropriate.
1254   If the entity-tag of the new response matches that of an existing
1255   entry, the new response SHOULD be used to update the header fields of
1256   the existing entry, and the result MUST be returned to the client.
1257
1258   If any of the existing cache entries contains only partial content
1259   for the associated entity, its entity-tag SHOULD NOT be included in
1260   the If-None-Match header field unless the request is for a range that
1261   would be fully satisfied by that entry.
1262
1263   If a cache receives a successful response whose Content-Location
1264   field matches that of an existing cache entry for the same Request-
1265   URI, whose entity-tag differs from that of the existing entry, and
1266   whose Date is more recent than that of the existing entry, the
1267   existing entry SHOULD NOT be returned in response to future requests
1268   and SHOULD be deleted from the cache.
1269
1270
12719.  Shared and Non-Shared Caches
1272
1273   For reasons of security and privacy, it is necessary to make a
1274   distinction between "shared" and "non-shared" caches.  A non-shared
1275   cache is one that is accessible only to a single user.  Accessibility
1276   in this case SHOULD be enforced by appropriate security mechanisms.
1277   All other caches are considered to be "shared."  Other sections of
1278   this specification place certain constraints on the operation of
1279   shared caches in order to prevent loss of privacy or failure of
1280   access controls.
1281
1282
1283
1284
1285
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1291
129210.  Errors or Incomplete Response Cache Behavior
1293
1294   A cache that receives an incomplete response (for example, with fewer
1295   bytes of data than specified in a Content-Length header) MAY store
1296   the response.  However, the cache MUST treat this as a partial
1297   response.  Partial responses MAY be combined as described in Section
1298   5 of [Part5]; the result might be a full response or might still be
1299   partial.  A cache MUST NOT return a partial response to a client
1300   without explicitly marking it as such, using the 206 (Partial
1301   Content) status code.  A cache MUST NOT return a partial response
1302   using a status code of 200 (OK).
1303
1304   If a cache receives a 5xx response while attempting to revalidate an
1305   entry, it MAY either forward this response to the requesting client,
1306   or act as if the server failed to respond.  In the latter case, it
1307   MAY return a previously received response unless the cached entry
1308   includes the "must-revalidate" cache-control directive (see
1309   Section 16.2).
1310
1311
131211.  Side Effects of GET and HEAD
1313
1314   Unless the origin server explicitly prohibits the caching of their
1315   responses, the application of GET and HEAD methods to any resources
1316   SHOULD NOT have side effects that would lead to erroneous behavior if
1317   these responses are taken from a cache.  They MAY still have side
1318   effects, but a cache is not required to consider such side effects in
1319   its caching decisions.  Caches are always expected to observe an
1320   origin server's explicit restrictions on caching.
1321
1322   We note one exception to this rule: since some applications have
1323   traditionally used GET and HEAD requests with URLs containing a query
1324   part to perform operations with significant side effects, caches MUST
1325   NOT treat responses to such URIs as fresh unless the server provides
1326   an explicit expiration time.  This specifically means that responses
1327   from HTTP/1.0 servers for such URIs SHOULD NOT be taken from a cache.
1328   See Section 8.1.1 of [Part2] for related information.
1329
1330
133112.  Invalidation After Updates or Deletions
1332
1333   The effect of certain methods performed on a resource at the origin
1334   server might cause one or more existing cache entries to become non-
1335   transparently invalid.  That is, although they might continue to be
1336   "fresh," they do not accurately reflect what the origin server would
1337   return for a new request on that resource.
1338
1339   There is no way for HTTP to guarantee that all such cache entries are
1340
1341
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1347
1348   marked invalid.  For example, the request that caused the change at
1349   the origin server might not have gone through the proxy where a cache
1350   entry is stored.  However, several rules help reduce the likelihood
1351   of erroneous behavior.
1352
1353   In this section, the phrase "invalidate an entity" means that the
1354   cache will either remove all instances of that entity from its
1355   storage, or will mark these as "invalid" and in need of a mandatory
1356   revalidation before they can be returned in response to a subsequent
1357   request.
1358
1359   Some HTTP methods MUST cause a cache to invalidate an entity.  This
1360   is either the entity referred to by the Request-URI, or by the
1361   Location or Content-Location headers (if present).  These methods
1362   are:
1363
1364   o  PUT
1365
1366   o  DELETE
1367
1368   o  POST
1369
1370   An invalidation based on the URI in a Location or Content-Location
1371   header MUST NOT be performed if the host part of that URI differs
1372   from the host part in the Request-URI.  This helps prevent denial of
1373   service attacks.
1374
1375   A cache that passes through requests for methods it does not
1376   understand SHOULD invalidate any entities referred to by the Request-
1377   URI.
1378
1379
138013.  Write-Through Mandatory
1381
1382   All methods that might be expected to cause modifications to the
1383   origin server's resources MUST be written through to the origin
1384   server.  This currently includes all methods except for GET and HEAD.
1385   A cache MUST NOT reply to such a request from a client before having
1386   transmitted the request to the inbound server, and having received a
1387   corresponding response from the inbound server.  This does not
1388   prevent a proxy cache from sending a 100 (Continue) response before
1389   the inbound server has sent its final reply.
1390
1391   The alternative (known as "write-back" or "copy-back" caching) is not
1392   allowed in HTTP/1.1, due to the difficulty of providing consistent
1393   updates and the problems arising from server, cache, or network
1394   failure prior to write-back.
1395
1396
1397
1398
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1403
140414.  Cache Replacement
1405
1406   If a new cacheable (see Sections 16.2.2, 4.5, 4.6 and 10) response is
1407   received from a resource while any existing responses for the same
1408   resource are cached, the cache SHOULD use the new response to reply
1409   to the current request.  It MAY insert it into cache storage and MAY,
1410   if it meets all other requirements, use it to respond to any future
1411   requests that would previously have caused the old response to be
1412   returned.  If it inserts the new response into cache storage the
1413   rules in Section 7.3 apply.
1414
1415      Note: a new response that has an older Date header value than
1416      existing cached responses is not cacheable.
1417
1418
141915.  History Lists
1420
1421   User agents often have history mechanisms, such as "Back" buttons and
1422   history lists, which can be used to redisplay an entity retrieved
1423   earlier in a session.
1424
1425   History mechanisms and caches are different.  In particular history
1426   mechanisms SHOULD NOT try to show a semantically transparent view of
1427   the current state of a resource.  Rather, a history mechanism is
1428   meant to show exactly what the user saw at the time when the resource
1429   was retrieved.
1430
1431   By default, an expiration time does not apply to history mechanisms.
1432   If the entity is still in storage, a history mechanism SHOULD display
1433   it even if the entity has expired, unless the user has specifically
1434   configured the agent to refresh expired history documents.
1435
1436   This is not to be construed to prohibit the history mechanism from
1437   telling the user that a view might be stale.
1438
1439      Note: if history list mechanisms unnecessarily prevent users from
1440      viewing stale resources, this will tend to force service authors
1441      to avoid using HTTP expiration controls and cache controls when
1442      they would otherwise like to.  Service authors may consider it
1443      important that users not be presented with error messages or
1444      warning messages when they use navigation controls (such as BACK)
1445      to view previously fetched resources.  Even though sometimes such
1446      resources ought not be cached, or ought to expire quickly, user
1447      interface considerations may force service authors to resort to
1448      other means of preventing caching (e.g. "once-only" URLs) in order
1449      not to suffer the effects of improperly functioning history
1450      mechanisms.
1451
1452
1453
1454
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1459
146016.  Header Field Definitions
1461
1462   This section defines the syntax and semantics of HTTP/1.1 header
1463   fields related to caching.
1464
1465   For entity-header fields, both sender and recipient refer to either
1466   the client or the server, depending on who sends and who receives the
1467   entity.
1468
146916.1.  Age
1470
1471   The Age response-header field conveys the sender's estimate of the
1472   amount of time since the response (or its revalidation) was generated
1473   at the origin server.  A cached response is "fresh" if its age does
1474   not exceed its freshness lifetime.  Age values are calculated as
1475   specified in Section 4.3.
1476
1477     Age = "Age" ":" age-value
1478     age-value = delta-seconds
1479
1480   Age values are non-negative decimal integers, representing time in
1481   seconds.
1482
1483     delta-seconds  = 1*DIGIT
1484
1485   If a cache receives a value larger than the largest positive integer
1486   it can represent, or if any of its age calculations overflows, it
1487   MUST transmit an Age header with a value of 2147483648 (2^31).  An
1488   HTTP/1.1 server that includes a cache MUST include an Age header
1489   field in every response generated from its own cache.  Caches SHOULD
1490   use an arithmetic type of at least 31 bits of range.
1491
149216.2.  Cache-Control
1493
1494   The Cache-Control general-header field is used to specify directives
1495   that MUST be obeyed by all caching mechanisms along the request/
1496   response chain.  The directives specify behavior intended to prevent
1497   caches from adversely interfering with the request or response.
1498   These directives typically override the default caching algorithms.
1499   Cache directives are unidirectional in that the presence of a
1500   directive in a request does not imply that the same directive is to
1501   be given in the response.
1502
1503      Note that HTTP/1.0 caches might not implement Cache-Control and
1504      might only implement Pragma: no-cache (see Section 16.4).
1505
1506   Cache directives MUST be passed through by a proxy or gateway
1507   application, regardless of their significance to that application,
1508
1509
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1515
1516   since the directives might be applicable to all recipients along the
1517   request/response chain.  It is not possible to specify a cache-
1518   directive for a specific cache.
1519
1520     Cache-Control   = "Cache-Control" ":" 1#cache-directive
1521
1522     cache-directive = cache-request-directive
1523        | cache-response-directive
1524
1525     cache-request-directive =
1526          "no-cache"                          ; Section 16.2.1
1527        | "no-store"                          ; Section 16.2.2
1528        | "max-age" "=" delta-seconds         ; Section 16.2.3, 16.2.4
1529        | "max-stale" [ "=" delta-seconds ]   ; Section 16.2.3
1530        | "min-fresh" "=" delta-seconds       ; Section 16.2.3
1531        | "no-transform"                      ; Section 16.2.5
1532        | "only-if-cached"                    ; Section 16.2.4
1533        | cache-extension                     ; Section 16.2.6
1534
1535     cache-response-directive =
1536          "public"                               ; Section 16.2.1
1537        | "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
1538        | "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
1539        | "no-store"                             ; Section 16.2.2
1540        | "no-transform"                         ; Section 16.2.5
1541        | "must-revalidate"                      ; Section 16.2.4
1542        | "proxy-revalidate"                     ; Section 16.2.4
1543        | "max-age" "=" delta-seconds            ; Section 16.2.3
1544        | "s-maxage" "=" delta-seconds           ; Section 16.2.3
1545        | cache-extension                        ; Section 16.2.6
1546
1547     cache-extension = token [ "=" ( token | quoted-string ) ]
1548
1549   When a directive appears without any 1#field-name parameter, the
1550   directive applies to the entire request or response.  When such a
1551   directive appears with a 1#field-name parameter, it applies only to
1552   the named field or fields, and not to the rest of the request or
1553   response.  This mechanism supports extensibility; implementations of
1554   future versions of HTTP might apply these directives to header fields
1555   not defined in HTTP/1.1.
1556
1557   The cache-control directives can be broken down into these general
1558   categories:
1559
1560   o  Restrictions on what are cacheable; these may only be imposed by
1561      the origin server.
1562
1563
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
158516.2.1.  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 6 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 4.1 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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1626
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
164216.2.2.  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
1678
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1682
1683
168416.2.3.  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 16.3).  Alternatively,
1688   it MAY be specified using the max-age directive in a response.  When
1689   the max-age cache-control directive is present in a cached response,
1690   the response is stale if its current age is greater than the age
1691   value given (in seconds) at the time of a new request for that
1692   resource.  The max-age directive on a response implies that the
1693   response is cacheable (i.e., "public") unless some other, more
1694   restrictive cache 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 16.2.4), i.e., that the shared cache must not use the
1727      entry after it becomes stale to respond to a subsequent request
1728      without first revalidating it with the origin server.  The
1729      s-maxage 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
179616.2.4.  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
1846
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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
194216.2.5.  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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1962
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 7.2 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
197416.2.6.  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
202416.3.  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 4 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 sent in rfc1123-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 16.2.3), 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 4.4.)
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 16.2).
2067
2068
2069
2070
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2075
207616.4.  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 16.2) 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
211116.5.  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 8 for use of the Vary header field by caches.
2122
2123     Vary  = "Vary" ":" ( "*" | 1#field-name )
2124
2125
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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
216016.6.  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 December 19, 2008              [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                   June 2008
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 = ( uri-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  = DQUOTE HTTP-date DQUOTE
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 ([ISO-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 [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 December 19, 2008              [Page 40]
2240
2241Internet-Draft              HTTP/1.1, Part 6                   June 2008
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 3.2.
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 December 19, 2008              [Page 41]
2296
2297Internet-Draft              HTTP/1.1, Part 6                   June 2008
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
231917.  IANA Considerations
2320
232117.1.  Message Header Registration
2322
2323   The Message Header Registry located at <http://www.iana.org/
2324   assignments/message-headers/message-header-index.html> should be
2325   updated with the permanent registrations below (see [RFC3864]):
2326
2327        +-------------------+----------+----------+--------------+
2328        | Header Field Name | Protocol | Status   | Reference    |
2329        +-------------------+----------+----------+--------------+
2330        | Age               | http     | standard | Section 16.1 |
2331        | Cache-Control     | http     | standard | Section 16.2 |
2332        | Expires           | http     | standard | Section 16.3 |
2333        | Pragma            | http     | standard | Section 16.4 |
2334        | Vary              | http     | standard | Section 16.5 |
2335        | Warning           | http     | standard | Section 16.6 |
2336        +-------------------+----------+----------+--------------+
2337
2338   The change controller is: "IETF (iesg@ietf.org) - Internet
2339   Engineering Task Force".
2340
2341
234218.  Security Considerations
2343
2344   Caching proxies provide additional potential vulnerabilities, since
2345   the contents of the cache represent an attractive target for
2346   malicious exploitation.  Because cache contents persist after an HTTP
2347   request is complete, an attack on the cache can reveal information
2348
2349
2350
2351Fielding, et al.        Expires December 19, 2008              [Page 42]
2352
2353Internet-Draft              HTTP/1.1, Part 6                   June 2008
2354
2355
2356   long after a user believes that the information has been removed from
2357   the network.  Therefore, cache contents should be protected as
2358   sensitive information.
2359
2360
236119.  Acknowledgments
2362
2363   Much of the content and presentation of the caching design is due to
2364   suggestions and comments from individuals including: Shel Kaphan,
2365   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
2366
2367
236820.  References
2369
237020.1.  Normative References
2371
2372   [ISO-8859-1]
2373              International Organization for Standardization,
2374              "Information technology -- 8-bit single-byte coded graphic
2375              character sets -- Part 1: Latin alphabet No. 1", ISO/
2376              IEC 8859-1:1998, 1998.
2377
2378   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2379              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
2380              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
2381              and Message Parsing", draft-ietf-httpbis-p1-messaging-03
2382              (work in progress), June 2008.
2383
2384   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2385              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
2386              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
2387              Semantics", draft-ietf-httpbis-p2-semantics-03 (work in
2388              progress), June 2008.
2389
2390   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2391              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
2392              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
2393              and Content Negotiation", draft-ietf-httpbis-p3-payload-03
2394              (work in progress), June 2008.
2395
2396   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2397              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
2398              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
2399              Requests", draft-ietf-httpbis-p4-conditional-03 (work in
2400              progress), June 2008.
2401
2402   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2403              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
2404
2405
2406
2407Fielding, et al.        Expires December 19, 2008              [Page 43]
2408
2409Internet-Draft              HTTP/1.1, Part 6                   June 2008
2410
2411
2412              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
2413              Partial Responses", draft-ietf-httpbis-p5-range-03 (work
2414              in progress), June 2008.
2415
2416   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
2417              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
2418              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
2419              draft-ietf-httpbis-p7-auth-03 (work in progress),
2420              June 2008.
2421
2422   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
2423              Part Three: Message Header Extensions for Non-ASCII Text",
2424              RFC 2047, November 1996.
2425
2426   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
2427              Requirement Levels", BCP 14, RFC 2119, March 1997.
2428
242920.2.  Informative References
2430
2431   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
2432              Specification, Implementation", RFC 1305, March 1992.
2433
2434   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
2435              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
2436              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
2437
2438   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
2439              Procedures for Message Header Fields", BCP 90, RFC 3864,
2440              September 2004.
2441
2442
2443Appendix A.  Compatibility with Previous Versions
2444
2445A.1.  Changes from RFC 2068
2446
2447   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
2448   was introduced to add this missing case.  (Sections 6, 16.2, 16.2.3)
2449
2450   Transfer-coding and message lengths all interact in ways that
2451   required fixing exactly when chunked encoding is used (to allow for
2452   transfer encoding that may not be self delimiting); it was important
2453   to straighten out exactly how message lengths are computed.
2454   (Section 7.2, see also [Part1], [Part3] and [Part5])
2455
2456   Proxies should be able to add Content-Length when appropriate.
2457   (Section 7.2)
2458
2459   Range request responses would become very verbose if all meta-data
2460
2461
2462
2463Fielding, et al.        Expires December 19, 2008              [Page 44]
2464
2465Internet-Draft              HTTP/1.1, Part 6                   June 2008
2466
2467
2468   were always returned; by allowing the server to only send needed
2469   headers in a 206 response, this problem can be avoided.
2470   (Section 7.3)
2471
2472   The Cache-Control: max-age directive was not properly defined for
2473   responses.  (Section 16.2.3)
2474
2475   Warnings could be cached incorrectly, or not updated appropriately.
2476   (Section 3.2, 4.4, 7.2, 7.3, 16.2.3, and 16.6) Warning also needed to
2477   be a general header, as PUT or other methods may have need for it in
2478   requests.
2479
2480A.2.  Changes from RFC 2616
2481
2482   Clarify denial of service attack avoidance requirement.  (Section 12)
2483
2484
2485Appendix B.  Change Log (to be removed by RFC Editor before publication)
2486
2487B.1.  Since RFC2616
2488
2489   Extracted relevant partitions from [RFC2616].
2490
2491B.2.  Since draft-ietf-httpbis-p6-cache-00
2492
2493   Closed issues:
2494
2495   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
2496      (<http://purl.org/NET/http-errata#trailer-hop>)
2497
2498   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/12>:
2499      "Invalidation after Update or Delete"
2500      (<http://purl.org/NET/http-errata#invalidupd>)
2501
2502   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative
2503      and Informative references"
2504
2505   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date
2506      reference typo"
2507
2508   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/49>:
2509      "Connection header text"
2510
2511   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65>:
2512      "Informative references"
2513
2514   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/66>:
2515      "ISO-8859-1 Reference"
2516
2517
2518
2519Fielding, et al.        Expires December 19, 2008              [Page 45]
2520
2521Internet-Draft              HTTP/1.1, Part 6                   June 2008
2522
2523
2524   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative
2525      up-to-date references"
2526
2527   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
2528      13.2.2"
2529
2530   Other changes:
2531
2532   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
2533      on <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36>)
2534
2535B.3.  Since draft-ietf-httpbis-p6-cache-01
2536
2537   Closed issues:
2538
2539   o  <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path
2540      not used"
2541
2542   Other changes:
2543
2544   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
2545      in progress on
2546      <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36>)
2547
2548   o  Add explicit references to BNF syntax and rules imported from
2549      other parts of the specification.
2550
2551B.4.  Since draft-ietf-httpbis-p6-cache-02
2552
2553   Ongoing work on IANA Message Header Registration
2554   (<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40>):
2555
2556   o  Reference RFC 3984, and update header registrations for headers
2557      defined in this document.
2558
2559
2560Index
2561
2562   A
2563      age  7
2564      Age header  27
2565
2566   C
2567      cache  5
2568      Cache Directives
2569         max-age  32-33
2570         max-stale  32
2571         min-fresh  32
2572
2573
2574
2575Fielding, et al.        Expires December 19, 2008              [Page 46]
2576
2577Internet-Draft              HTTP/1.1, Part 6                   June 2008
2578
2579
2580         must-revalidate  34
2581         no-cache  29
2582         no-store  30
2583         no-transform  35
2584         only-if-cached  34
2585         private  29
2586         proxy-revalidate  35
2587         public  29
2588         s-maxage  31
2589      Cache-Control header  27
2590      cacheable  6
2591
2592   E
2593      Expires header  37
2594      explicit expiration time  7
2595
2596   F
2597      first-hand  6
2598      fresh  7
2599      freshness lifetime  7
2600
2601   G
2602      Grammar
2603         Age  27
2604         age-value  27
2605         Cache-Control  28
2606         cache-directive  28
2607         cache-extension  28
2608         cache-request-directive  28
2609         cache-response-directive  28
2610         delta-seconds  27
2611         Expires  37
2612         extension-pragma  38
2613         Pragma  38
2614         pragma-directive  38
2615         Vary  38
2616         warn-agent  40
2617         warn-code  40
2618         warn-date  40
2619         warn-text  40
2620         Warning  40
2621         warning-value  40
2622
2623   H
2624      Headers
2625         Age  27
2626         Cache-Control  27
2627         Expires  37
2628
2629
2630
2631Fielding, et al.        Expires December 19, 2008              [Page 47]
2632
2633Internet-Draft              HTTP/1.1, Part 6                   June 2008
2634
2635
2636         Pragma  38
2637         Vary  38
2638         Warning  39
2639      heuristic expiration time  7
2640
2641   M
2642      max-age
2643         Cache Directive  32-33
2644      max-stale
2645         Cache Directive  32
2646      min-fresh
2647         Cache Directive  32
2648      must-revalidate
2649         Cache Directive  34
2650
2651   N
2652      no-cache
2653         Cache Directive  29
2654      no-store
2655         Cache Directive  30
2656      no-transform
2657         Cache Directive  35
2658
2659   O
2660      only-if-cached
2661         Cache Directive  34
2662
2663   P
2664      Pragma header  38
2665      private
2666         Cache Directive  29
2667      proxy-revalidate
2668         Cache Directive  35
2669      public
2670         Cache Directive  29
2671
2672   S
2673      s-maxage
2674         Cache Directive  31
2675      semantically transparent  5
2676      stale  7
2677
2678   V
2679      validator  7
2680      Vary header  38
2681
2682   W
2683      Warning header  39
2684
2685
2686
2687Fielding, et al.        Expires December 19, 2008              [Page 48]
2688
2689Internet-Draft              HTTP/1.1, Part 6                   June 2008
2690
2691
2692Authors' Addresses
2693
2694   Roy T. Fielding (editor)
2695   Day Software
2696   23 Corporate Plaza DR, Suite 280
2697   Newport Beach, CA  92660
2698   USA
2699
2700   Phone: +1-949-706-5300
2701   Fax:   +1-949-706-5305
2702   Email: fielding@gbiv.com
2703   URI:   http://roy.gbiv.com/
2704
2705
2706   Jim Gettys
2707   One Laptop per Child
2708   21 Oak Knoll Road
2709   Carlisle, MA  01741
2710   USA
2711
2712   Email: jg@laptop.org
2713   URI:   http://www.laptop.org/
2714
2715
2716   Jeffrey C. Mogul
2717   Hewlett-Packard Company
2718   HP Labs, Large Scale Systems Group
2719   1501 Page Mill Road, MS 1177
2720   Palo Alto, CA  94304
2721   USA
2722
2723   Email: JeffMogul@acm.org
2724
2725
2726   Henrik Frystyk Nielsen
2727   Microsoft Corporation
2728   1 Microsoft Way
2729   Redmond, WA  98052
2730   USA
2731
2732   Email: henrikn@microsoft.com
2733
2734
2735
2736
2737
2738
2739
2740
2741
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2743Fielding, et al.        Expires December 19, 2008              [Page 49]
2744
2745Internet-Draft              HTTP/1.1, Part 6                   June 2008
2746
2747
2748   Larry Masinter
2749   Adobe Systems, Incorporated
2750   345 Park Ave
2751   San Jose, CA  95110
2752   USA
2753
2754   Email: LMM@acm.org
2755   URI:   http://larry.masinter.net/
2756
2757
2758   Paul J. Leach
2759   Microsoft Corporation
2760   1 Microsoft Way
2761   Redmond, WA  98052
2762
2763   Email: paulle@microsoft.com
2764
2765
2766   Tim Berners-Lee
2767   World Wide Web Consortium
2768   MIT Computer Science and Artificial Intelligence Laboratory
2769   The Stata Center, Building 32
2770   32 Vassar Street
2771   Cambridge, MA  02139
2772   USA
2773
2774   Email: timbl@w3.org
2775   URI:   http://www.w3.org/People/Berners-Lee/
2776
2777
2778   Yves Lafon (editor)
2779   World Wide Web Consortium
2780   W3C / ERCIM
2781   2004, rte des Lucioles
2782   Sophia-Antipolis, AM  06902
2783   France
2784
2785   Email: ylafon@w3.org
2786   URI:   http://www.raubacapeu.net/people/yves/
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799Fielding, et al.        Expires December 19, 2008              [Page 50]
2800
2801Internet-Draft              HTTP/1.1, Part 6                   June 2008
2802
2803
2804   Julian F. Reschke (editor)
2805   greenbytes GmbH
2806   Hafenweg 16
2807   Muenster, NW  48155
2808   Germany
2809
2810   Phone: +49 251 2807760
2811   Fax:   +49 251 2807761
2812   Email: julian.reschke@greenbytes.de
2813   URI:   http://greenbytes.de/tech/webdav/
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
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2837
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2849
2850
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2852
2853
2854
2855Fielding, et al.        Expires December 19, 2008              [Page 51]
2856
2857Internet-Draft              HTTP/1.1, Part 6                   June 2008
2858
2859
2860Full Copyright Statement
2861
2862   Copyright (C) The IETF Trust (2008).
2863
2864   This document is subject to the rights, licenses and restrictions
2865   contained in BCP 78, and except as set forth therein, the authors
2866   retain all their rights.
2867
2868   This document and the information contained herein are provided on an
2869   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
2870   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
2871   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
2872   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
2873   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
2874   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
2875
2876
2877Intellectual Property
2878
2879   The IETF takes no position regarding the validity or scope of any
2880   Intellectual Property Rights or other rights that might be claimed to
2881   pertain to the implementation or use of the technology described in
2882   this document or the extent to which any license under such rights
2883   might or might not be available; nor does it represent that it has
2884   made any independent effort to identify any such rights.  Information
2885   on the procedures with respect to rights in RFC documents can be
2886   found in BCP 78 and BCP 79.
2887
2888   Copies of IPR disclosures made to the IETF Secretariat and any
2889   assurances of licenses to be made available, or the result of an
2890   attempt made to obtain a general license or permission for the use of
2891   such proprietary rights by implementers or users of this
2892   specification can be obtained from the IETF on-line IPR repository at
2893   http://www.ietf.org/ipr.
2894
2895   The IETF invites any interested party to bring to its attention any
2896   copyrights, patents or patent applications, or other proprietary
2897   rights that may cover technology that may be required to implement
2898   this standard.  Please address the information to the IETF at
2899   ietf-ipr@ietf.org.
2900
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2911Fielding, et al.        Expires December 19, 2008              [Page 52]
2912
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