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4HTTPbis Working Group                                   R. Fielding, Ed.
5Internet-Draft                                              Day Software
6Obsoletes: 2616 (if approved)                                  J. Gettys
7Intended status: Standards Track                    One Laptop per Child
8Expires: September 10, 2009                                     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                                                           March 9, 2009
23
24
25                       HTTP/1.1, part 6: Caching
26                     draft-ietf-httpbis-p6-cache-06
27
28Status of this Memo
29
30   This Internet-Draft is submitted to IETF in full conformance with the
31   provisions of BCP 78 and BCP 79.  This document may contain material
32   from IETF Documents or IETF Contributions published or made publicly
33   available before November 10, 2008.  The person(s) controlling the
34   copyright in some of this material may not have granted the IETF
35   Trust the right to allow modifications of such material outside the
36   IETF Standards Process.  Without obtaining an adequate license from
37   the person(s) controlling the copyright in such materials, this
38   document may not be modified outside the IETF Standards Process, and
39   derivative works of it may not be created outside the IETF Standards
40   Process, except to format it for publication as an RFC or to
41   translate it into languages other than English.
42
43   Internet-Drafts are working documents of the Internet Engineering
44   Task Force (IETF), its areas, and its working groups.  Note that
45   other groups may also distribute working documents as Internet-
46   Drafts.
47
48   Internet-Drafts are draft documents valid for a maximum of six months
49   and may be updated, replaced, or obsoleted by other documents at any
50   time.  It is inappropriate to use Internet-Drafts as reference
51   material or to cite them other than as "work in progress."
52
53
54
55Fielding, et al.       Expires September 10, 2009               [Page 1]
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57Internet-Draft              HTTP/1.1, Part 6                  March 2009
58
59
60   The list of current Internet-Drafts can be accessed at
61   http://www.ietf.org/ietf/1id-abstracts.txt.
62
63   The list of Internet-Draft Shadow Directories can be accessed at
64   http://www.ietf.org/shadow.html.
65
66   This Internet-Draft will expire on September 10, 2009.
67
68Copyright Notice
69
70   Copyright (c) 2009 IETF Trust and the persons identified as the
71   document authors.  All rights reserved.
72
73   This document is subject to BCP 78 and the IETF Trust's Legal
74   Provisions Relating to IETF Documents in effect on the date of
75   publication of this document (http://trustee.ietf.org/license-info).
76   Please review these documents carefully, as they describe your rights
77   and restrictions with respect to this document.
78
79Abstract
80
81   The Hypertext Transfer Protocol (HTTP) is an application-level
82   protocol for distributed, collaborative, hypermedia information
83   systems.  This document is Part 6 of the seven-part specification
84   that defines the protocol referred to as "HTTP/1.1" and, taken
85   together, obsoletes RFC 2616.  Part 6 defines requirements on HTTP
86   caches and the associated header fields that control cache behavior
87   or indicate cacheable response messages.
88
89Editorial Note (To be removed by RFC Editor)
90
91   Discussion of this draft should take place on the HTTPBIS working
92   group mailing list (ietf-http-wg@w3.org).  The current issues list is
93   at <http://tools.ietf.org/wg/httpbis/trac/report/11> and related
94   documents (including fancy diffs) can be found at
95   <http://tools.ietf.org/wg/httpbis/>.
96
97   The changes in this draft are summarized in Appendix C.7.
98
99
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113Internet-Draft              HTTP/1.1, Part 6                  March 2009
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115
116Table of Contents
117
118   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
119     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
120     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
121     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
122     1.4.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
123       1.4.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7
124       1.4.2.  ABNF Rules defined in other Parts of the
125               Specification  . . . . . . . . . . . . . . . . . . . .  7
126   2.  Cache Operation  . . . . . . . . . . . . . . . . . . . . . . .  7
127     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  7
128       2.1.1.  Storing Partial and Incomplete Responses . . . . . . .  8
129     2.2.  Constructing Responses from Caches . . . . . . . . . . . .  8
130     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . .  9
131       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 10
132       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 11
133       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 13
134     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 13
135     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 14
136     2.6.  Caching Negotiated Responses . . . . . . . . . . . . . . . 15
137     2.7.  Combining Responses  . . . . . . . . . . . . . . . . . . . 16
138   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 16
139     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
140     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 17
141       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 18
142       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 20
143       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 22
144     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 23
145     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 23
146     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
147     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 25
148   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 28
149   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
150     5.1.  Message Header Registration  . . . . . . . . . . . . . . . 28
151   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
152   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
153   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
154     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 29
155     8.2.  Informative References . . . . . . . . . . . . . . . . . . 30
156   Appendix A.  Compatibility with Previous Versions  . . . . . . . . 31
157     A.1.  Changes from RFC 2068  . . . . . . . . . . . . . . . . . . 31
158     A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 31
159   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 31
160   Appendix C.  Change Log (to be removed by RFC Editor before
161                publication)  . . . . . . . . . . . . . . . . . . . . 33
162     C.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 33
163     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33
164
165
166
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169Internet-Draft              HTTP/1.1, Part 6                  March 2009
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171
172     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
173     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
174     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
175     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35
176     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
177   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
178   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
179
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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.  This
232   document defines aspects of HTTP/1.1 related to caching and reusing
233   response messages.
234
2351.1.  Purpose
236
237   An HTTP cache is a local store of response messages and the subsystem
238   that controls its message storage, retrieval, and deletion.  A cache
239   stores cacheable responses in order to reduce the response time and
240   network bandwidth consumption on future, equivalent requests.  Any
241   client or server may include a cache, though a cache cannot be used
242   by a server that is acting as a tunnel.
243
244   Caching would be useless if it did not significantly improve
245   performance.  The goal of caching in HTTP/1.1 is to reuse a prior
246   response message to satisfy a current request.  In some cases, a
247   stored response can be reused without the need for a network request,
248   reducing latency and network round-trips; a "freshness" mechanism is
249   used for this purpose (see Section 2.3).  Even when a new request is
250   required, it is often possible to reuse all or parts of the payload
251   of a prior response to satisfy the request, thereby reducing network
252   bandwidth usage; a "validation" mechanism is used for this purpose
253   (see Section 2.4).
254
2551.2.  Terminology
256
257   This specification uses a number of terms to refer to the roles
258   played by participants in, and objects of, HTTP caching.
259
260   cacheable
261
262      A response is cacheable if a cache is allowed to store a copy of
263      the response message for use in answering subsequent requests.
264      Even when a response is cacheable, there may be additional
265      constraints on whether a cache can use the cached copy to satisfy
266      a particular request.
267
268   explicit expiration time
269
270      The time at which the origin server intends that an entity should
271      no longer be returned by a cache without further validation.
272
273
274
275
276
277
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283
284   heuristic expiration time
285
286      An expiration time assigned by a cache when no explicit expiration
287      time is available.
288
289   age
290
291      The age of a response is the time since it was sent by, or
292      successfully validated with, the origin server.
293
294   first-hand
295
296      A response is first-hand if the freshness model is not in use;
297      i.e., its age is 0.
298
299   freshness lifetime
300
301      The length of time between the generation of a response and its
302      expiration time.
303
304   fresh
305
306      A response is fresh if its age has not yet exceeded its freshness
307      lifetime.
308
309   stale
310
311      A response is stale if its age has passed its freshness lifetime
312      (either explicit or heuristic).
313
314   validator
315
316      A protocol element (e.g., an entity tag or a Last-Modified time)
317      that is used to find out whether a stored response is an
318      equivalent copy of an entity.
319
320   shared cache
321
322      A cache that is accessible to more than one user.  A non-shared
323      cache is dedicated to a single user.
324
3251.3.  Requirements
326
327   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
328   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
329   document are to be interpreted as described in [RFC2119].
330
331   An implementation is not compliant if it fails to satisfy one or more
332
333
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339
340   of the MUST or REQUIRED level requirements for the protocols it
341   implements.  An implementation that satisfies all the MUST or
342   REQUIRED level and all the SHOULD level requirements for its
343   protocols is said to be "unconditionally compliant"; one that
344   satisfies all the MUST level requirements but not all the SHOULD
345   level requirements for its protocols is said to be "conditionally
346   compliant."
347
3481.4.  Syntax Notation
349
350   This specification uses the ABNF syntax defined in Section 1.2 of
351   [Part1] (which extends the syntax defined in [RFC5234] with a list
352   rule).  Appendix B shows the collected ABNF, with the list rule
353   expanded.
354
355   The following core rules are included by reference, as defined in
356   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
357   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
358   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
359   sequence of data), SP (space), VCHAR (any visible USASCII character),
360   and WSP (whitespace).
361
3621.4.1.  Core Rules
363
364   The core rules below are defined in Section 1.2.2 of [Part1]:
365
366     quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
367     token         = <token, defined in [Part1], Section 1.2.2>
368     OWS           = <OWS, defined in [Part1], Section 1.2.2>
369
3701.4.2.  ABNF Rules defined in other Parts of the Specification
371
372   The ABNF rules below are defined in other parts:
373
374     field-name    = <field-name, defined in [Part1], Section 4.2>
375     HTTP-date     = <HTTP-date, defined in [Part1], Section 3.2.1>
376     port          = <port, defined in [Part1], Section 2.1>
377     pseudonym     = <pseudonym, defined in [Part1], Section 8.9>
378     uri-host      = <uri-host, defined in [Part1], Section 2.1>
379
380
3812.  Cache Operation
382
3832.1.  Response Cacheability
384
385   A cache MUST NOT store a response to any request, unless:
386
387
388
389
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395
396   o  The request method is defined as being cacheable, and
397
398   o  the "no-store" cache directive (see Section 3.2) does not appear
399      in request or response headers, and
400
401   o  the "private" cache response directive (see Section 3.2 does not
402      appear in the response, if the cache is shared, and
403
404   o  the "Authorization" header (see Section 3.1 of [Part7]) does not
405      appear in the request, if the cache is shared (unless the "public"
406      directive is present; see Section 3.2), and
407
408   o  the cache understands partial responses, if the response is
409      partial or incomplete (see Section 2.1.1).
410
411   Note that in normal operation, most caches will not store a response
412   that has neither a cache validator nor an explicit expiration time,
413   as such responses are not usually useful to store.  However, caches
414   are not prohibited from storing such responses.
415
4162.1.1.  Storing Partial and Incomplete Responses
417
418   A cache that receives an incomplete response (for example, with fewer
419   bytes of data than specified in a Content-Length header) can store
420   the response, but MUST treat it as a partial response [Part5].
421   Partial responses can be combined as described in Section 4 of
422   [Part5]; the result might be a full response or might still be
423   partial.  A cache MUST NOT return a partial response to a client
424   without explicitly marking it as such using the 206 (Partial Content)
425   status code.
426
427   A cache that does not support the Range and Content-Range headers
428   MUST NOT store incomplete or partial responses.
429
4302.2.  Constructing Responses from Caches
431
432   For a presented request, a cache MUST NOT return a stored response,
433   unless:
434
435   o  The presented Request-URI and that of the stored response match
436      (see [[anchor1: TBD]]), and
437
438   o  the request method associated with the stored response allows it
439      to be used for the presented request, and
440
441   o  selecting request-headers nominated by the stored response (if
442      any) match those presented (see Section 2.6), and
443
444
445
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451
452   o  the presented request and stored response are free from directives
453      that would prevent its use (see Section 3.2 and Section 3.4), and
454
455   o  the stored response is either:
456
457      *  fresh (see Section 2.3), or
458
459      *  allowed to be served stale (see Section 2.3.3), or
460
461      *  successfully validated (see Section 2.4).
462
463   [[anchor2: TODO: define method cacheability for GET, HEAD and POST in
464   p2-semantics.]]
465
466   When a stored response is used to satisfy a request, caches MUST
467   include a single Age header field Section 3.1 in the response with a
468   value equal to the stored response's current_age; see Section 2.3.2.
469   [[anchor3: DISCUSS: this currently includes successfully validated
470   responses.]]
471
472   Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
473   be written through the cache to the origin server; i.e., A cache must
474   not reply to such a request before having forwarded the request and
475   having received a corresponding response.
476
477   Also, note that unsafe requests might invalidate already stored
478   responses; see Section 2.5.
479
480   Caches MUST use the most recent response (as determined by the Date
481   header) when more than one suitable response is stored.  They can
482   also forward a request with "Cache-Control: max-age=0" or "Cache-
483   Control: no-cache" to disambiguate which response to use.
484
485   [[anchor4: TODO: end-to-end and hop-by-hop headers, non-modifiable
486   headers removed; re-spec in p1]]
487
4882.3.  Freshness Model
489
490   When a response is "fresh" in the cache, it can be used to satisfy
491   subsequent requests without contacting the origin server, thereby
492   improving efficiency.
493
494   The primary mechanism for determining freshness is for an origin
495   server to provide an explicit expiration time in the future, using
496   either the Expires header (Section 3.3) or the max-age response cache
497   directive (Section 3.2.2).  Generally, origin servers will assign
498   future explicit expiration times to responses in the belief that the
499   entity is not likely to change in a semantically significant way
500
501
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507
508   before the expiration time is reached.
509
510   If an origin server wishes to force a cache to validate every
511   request, it can assign an explicit expiration time in the past.  This
512   means that the response is always stale, so that caches should
513   validate it before using it for subsequent requests. [[anchor5: This
514   wording may cause confusion, because the response may still be served
515   stale.]]
516
517   Since origin servers do not always provide explicit expiration times,
518   HTTP caches may also assign heuristic expiration times when they are
519   not specified, employing algorithms that use other header values
520   (such as the Last-Modified time) to estimate a plausible expiration
521   time.  The HTTP/1.1 specification does not provide specific
522   algorithms, but does impose worst-case constraints on their results.
523
524   The calculation to determine if a response is fresh is:
525
526      response_is_fresh = (freshness_lifetime > current_age)
527
528   The freshness_lifetime is defined in Section 2.3.1; the current_age
529   is defined in Section 2.3.2.
530
531   Additionally, clients may need to influence freshness calculation.
532   They can do this using several request cache directives, with the
533   effect of either increasing or loosening constraints on freshness.
534   See Section 3.2.1.
535
536   [[anchor6: ISSUE: there are not requirements directly applying to
537   cache-request-directives and freshness.]]
538
539   Note that freshness applies only to cache operation; it cannot be
540   used to force a user agent to refresh its display or reload a
541   resource.  See Section 4 for an explanation of the difference between
542   caches and history mechanisms.
543
5442.3.1.  Calculating Freshness Lifetime
545
546   A cache can calculate the freshness lifetime (denoted as
547   freshness_lifetime) of a response by using the first match of:
548
549   o  If the cache is shared and the s-maxage response cache directive
550      (Section 3.2.2) is present, use its value, or
551
552   o  If the max-age response cache directive (Section 3.2.2) is
553      present, use its value, or
554
555
556
557
558
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563
564   o  If the Expires response header (Section 3.3) is present, use its
565      value minus the value of the Date response header, or
566
567   o  Otherwise, no explicit expiration time is present in the response,
568      but a heuristic may be used; see Section 2.3.1.1.
569
570   Note that this calculation is not vulnerable to clock skew, since all
571   of the information comes from the origin server.
572
5732.3.1.1.  Calculating Heuristic Freshness
574
575   If no explicit expiration time is present in a stored response that
576   has a status code of 200, 203, 206, 300, 301 or 410, a heuristic
577   expiration time can be calculated.  Heuristics MUST NOT be used for
578   other response status codes.
579
580   When a heuristic is used to calculate freshness lifetime, the cache
581   SHOULD attach a Warning header with a 113 warn-code to the response
582   if its current_age is more than 24 hours and such a warning is not
583   already present.
584
585   Also, if the response has a Last-Modified header (Section 6.6 of
586   [Part4]), the heuristic expiration value SHOULD be no more than some
587   fraction of the interval since that time.  A typical setting of this
588   fraction might be 10%.
589
590   [[anchor7: REVIEW: took away HTTP/1.0 query string heuristic
591   uncacheability.]]
592
5932.3.2.  Calculating Age
594
595   HTTP/1.1 uses the Age response-header to convey the estimated age of
596   the response message when obtained from a cache.  The Age field value
597   is the cache's estimate of the amount of time since the response was
598   generated or validated by the origin server.  In essence, the Age
599   value is the sum of the time that the response has been resident in
600   each of the caches along the path from the origin server, plus the
601   amount of time it has been in transit along network paths.
602
603   The term "age_value" denotes the value of the Age header, in a form
604   appropriate for arithmetic operations.
605
606   HTTP/1.1 requires origin servers to send a Date header, if possible,
607   with every response, giving the time at which the response was
608   generated (see Section 8.3 of [Part1]).  The term "date_value"
609   denotes the value of the Date header, in a form appropriate for
610   arithmetic operations.
611
612
613
614
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619
620   The term "now" means "the current value of the clock at the host
621   performing the calculation."  Hosts that use HTTP, but especially
622   hosts running origin servers and caches, SHOULD use NTP [RFC1305] or
623   some similar protocol to synchronize their clocks to a globally
624   accurate time standard.
625
626   A response's age can be calculated in two entirely independent ways:
627
628   1.  now minus date_value, if the local clock is reasonably well
629       synchronized to the origin server's clock.  If the result is
630       negative, the result is replaced by zero.
631
632   2.  age_value, if all of the caches along the response path implement
633       HTTP/1.1.
634
635   These are combined as
636
637       corrected_received_age = max(now - date_value, age_value)
638
639   When an Age value is received, it MUST be interpreted relative to the
640   time the request was initiated, not the time that the response was
641   received.
642
643      corrected_initial_age = corrected_received_age
644                            + (now - request_time)
645
646   where "request_time" is the time (according to the local clock) when
647   the request that elicited this response was sent.
648
649   The current_age of a stored response can then be calculated by adding
650   the amount of time (in seconds) since the stored response was last
651   validated by the origin server to the corrected_initial_age.
652
653   In summary:
654
655     age_value     - Age header field-value received with the response
656     date_value    - Date header field-value received with the response
657     request_time  - local time when the cache made the request
658                    resulting in the stored response
659     response_time - local time when the cache received the response
660     now           - current local time
661
662     apparent_age = max(0, response_time - date_value);
663     corrected_received_age = max(apparent_age, age_value);
664     response_delay = response_time - request_time;
665     corrected_initial_age = corrected_received_age + response_delay;
666     resident_time = now - response_time;
667     current_age   = corrected_initial_age + resident_time;
668
669
670
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675
6762.3.3.  Serving Stale Responses
677
678   A "stale" response is one that either has explicit expiry
679   information, or is allowed to have heuristic expiry calculated, but
680   is not fresh according to the calculations in Section 2.3.
681
682   Caches MUST NOT return a stale response if it is prohibited by an
683   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
684   cache directive, a "must-revalidate" cache-response-directive, or an
685   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
686   see Section 3.2.2).
687
688   Caches SHOULD NOT return stale responses unless they are disconnected
689   (i.e., it cannot contact the origin server or otherwise find a
690   forward path) or otherwise explicitly allowed (e.g., the max-stale
691   request directive; see Section 3.2.1).
692
693   Stale responses SHOULD have a Warning header with the 110 warn-code
694   (see Section 3.6).  Likewise, the 112 warn-code SHOULD be sent on
695   stale responses if the cache is disconnected.
696
697   If a cache receives a first-hand response (either an entire response,
698   or a 304 (Not Modified) response) that it would normally forward to
699   the requesting client, and the received response is no longer fresh,
700   the cache SHOULD forward it to the requesting client without adding a
701   new Warning (but without removing any existing Warning headers).  A
702   cache SHOULD NOT attempt to validate a response simply because that
703   response became stale in transit.
704
7052.4.  Validation Model
706
707   Checking with the origin server to see if a stale or otherwise
708   unusable cached response can be reused is called "validating" or
709   "revalidating."  Doing so potentially avoids the overhead of
710   retransmitting the response body when the stored response is valid.
711
712   HTTP's conditional request mechanism [Part4] is used for this
713   purpose.  When a stored response includes one or more validators,
714   such as the field values of an ETag or Last-Modified header field,
715   then a validating request SHOULD be made conditional to those field
716   values.
717
718   A 304 (Not Modified) response status code indicates that the stored
719   response can be updated and reused; see Section 2.7.
720
721   If instead the cache receives a full response (i.e., one with a
722   response body), it is used to satisfy the request and replace the
723   stored response. [[anchor8: Should there be a requirement here?]]
724
725
726
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731
732   If a cache receives a 5xx response while attempting to validate a
733   response, it MAY either forward this response to the requesting
734   client, or act as if the server failed to respond.  In the latter
735   case, it MAY return a previously stored response (which SHOULD
736   include the 111 warn-code; see Section 3.6) unless the stored
737   response includes the "must-revalidate" cache directive (see
738   Section 2.3.3).
739
7402.5.  Request Methods that Invalidate
741
742   Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
743   for changing state on the origin server, intervening caches can use
744   them to keep their contents up-to-date.
745
746   The following HTTP methods MUST cause a cache to invalidate the
747   Request-URI as well as the Location and Content-Location headers (if
748   present):
749
750   o  PUT
751
752   o  DELETE
753
754   o  POST
755
756   An invalidation based on the URI in a Location or Content-Location
757   header MUST NOT be performed if the host part of that URI differs
758   from the host part in the Request-URI.  This helps prevent denial of
759   service attacks.
760
761   [[anchor9: TODO: "host part" needs to be specified better.]]
762
763   A cache that passes through requests for methods it does not
764   understand SHOULD invalidate the Request-URI.
765
766   Here, "invalidate" means that the cache will either remove all stored
767   responses related to the Request-URI, or will mark these as "invalid"
768   and in need of a mandatory validation before they can be returned in
769   response to a subsequent request.
770
771   Note that this does not guarantee that all appropriate responses are
772   invalidated.  For example, the request that caused the change at the
773   origin server might not have gone through the cache where a response
774   is stored.
775
776   [[anchor10: TODO: specify that only successful (2xx, 3xx?) responses
777   invalidate.]]
778
779
780
781
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787
7882.6.  Caching Negotiated Responses
789
790   Use of server-driven content negotiation (Section 4.1 of [Part3])
791   alters the conditions under which a cache can use the response for
792   subsequent requests.
793
794   When a cache receives a request that can be satisfied by a stored
795   response that includes a Vary header field (Section 3.5), it MUST NOT
796   use that response unless all of the selecting request-headers in the
797   presented request match the corresponding stored request-headers from
798   the original request.
799
800   The selecting request-headers from two requests are defined to match
801   if and only if the selecting request-headers in the first request can
802   be transformed to the selecting request-headers in the second request
803   by adding or removing linear white space [[anchor11: [ref]]] at
804   places where this is allowed by the corresponding ABNF, and/or
805   combining multiple message-header fields with the same field name
806   following the rules about message headers in Section 4.2 of [Part1].
807   [[anchor12: DISCUSS: header-specific canonicalisation]]
808
809   A Vary header field-value of "*" always fails to match, and
810   subsequent requests to that resource can only be properly interpreted
811   by the origin server.
812
813   If no stored response matches, the cache MAY forward the presented
814   request to the origin server in a conditional request, and SHOULD
815   include all ETags stored with potentially suitable responses in an
816   If-None-Match request header.  If the server responds with 304 (Not
817   Modified) and includes an entity tag or Content-Location that
818   indicates the entity to be used, that cached response MUST be used to
819   satisfy the presented request, and SHOULD be used to update the
820   corresponding stored response; see Section 2.7.
821
822   If any of the stored responses contains only partial content, its
823   entity-tag SHOULD NOT be included in the If-None-Match header field
824   unless the request is for a range that would be fully satisfied by
825   that stored response.
826
827   If a cache receives a successful response whose Content-Location
828   field matches that of an existing stored response for the same
829   Request-URI, whose entity-tag differs from that of the existing
830   stored response, and whose Date is more recent than that of the
831   existing response, the existing response SHOULD NOT be returned in
832   response to future requests and SHOULD be deleted from the
833   cache.[[anchor13: DISCUSS: Not sure if this is necessary.]]
834
835
836
837
838
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843
8442.7.  Combining Responses
845
846   When a cache receives a 304 (Not Modified) response or a 206 (Partial
847   Content) response, it needs to update the stored response with the
848   new one, so that the updated response can be sent to the client.
849
850   If the status code is 304 (Not Modified), the cache SHOULD use the
851   stored entity-body as the updated entity-body.  If the status code is
852   206 (Partial Content) and the ETag or Last-Modified headers match
853   exactly, the cache MAY combine the stored entity-body in the stored
854   response with the updated entity-body received in the response and
855   use the result as the updated entity-body (see Section 4 of [Part5]).
856
857   The stored response headers are used for the updated response, except
858   that
859
860   o  any stored Warning headers with warn-code 1xx (see Section 3.6)
861      MUST be deleted from the stored response and the forwarded
862      response.
863
864   o  any stored Warning headers with warn-code 2xx MUST be retained in
865      the stored response and the forwarded response.
866
867   o  any headers provided in the 304 or 206 response MUST replace the
868      corresponding headers from the stored response.
869
870   A cache MUST also replace any stored headers with corresponding
871   headers received in the incoming response, except for Warning headers
872   as described immediately above.  If a header field-name in the
873   incoming response matches more than one header in the stored
874   response, all such old headers MUST be replaced.  It MAY store the
875   combined entity-body.
876
877   [[anchor14: ISSUE: discuss how to handle HEAD updates]]
878
879
8803.  Header Field Definitions
881
882   This section defines the syntax and semantics of HTTP/1.1 header
883   fields related to caching.
884
885   For entity-header fields, both sender and recipient refer to either
886   the client or the server, depending on who sends and who receives the
887   entity.
888
889
890
891
892
893
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899
9003.1.  Age
901
902   The response-header field "Age" conveys the sender's estimate of the
903   amount of time since the response (or its validation) was generated
904   at the origin server.  Age values are calculated as specified in
905   Section 2.3.2.
906
907     Age   = "Age" ":" OWS Age-v
908     Age-v = delta-seconds
909
910   Age field-values are non-negative decimal integers, representing time
911   in seconds.
912
913     delta-seconds  = 1*DIGIT
914
915   If a cache receives a value larger than the largest positive integer
916   it can represent, or if any of its age calculations overflows, it
917   MUST transmit an Age header with a field-value of 2147483648 (2^31).
918   Caches SHOULD use an arithmetic type of at least 31 bits of range.
919
920   The presence of an Age header field in a response implies that a
921   response is not first-hand.  However, the converse is not true, since
922   HTTP/1.0 caches may not implement the Age header field.
923
9243.2.  Cache-Control
925
926   The general-header field "Cache-Control" is used to specify
927   directives that MUST be obeyed by all caches along the request/
928   response chain.  The directives specify behavior intended to prevent
929   caches from adversely interfering with the request or response.
930   Cache directives are unidirectional in that the presence of a
931   directive in a request does not imply that the same directive is to
932   be given in the response.
933
934      Note that HTTP/1.0 caches might not implement Cache-Control and
935      might only implement Pragma: no-cache (see Section 3.4).
936
937   Cache directives MUST be passed through by a proxy or gateway
938   application, regardless of their significance to that application,
939   since the directives might be applicable to all recipients along the
940   request/response chain.  It is not possible to target a directive to
941   a specific cache.
942
943
944
945
946
947
948
949
950
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955
956     Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
957     Cache-Control-v = 1#cache-directive
958
959     cache-directive = cache-request-directive
960        / cache-response-directive
961
962     cache-extension = token [ "=" ( token / quoted-string ) ]
963
9643.2.1.  Request Cache-Control Directives
965
966     cache-request-directive =
967          "no-cache"
968        / "no-store"
969        / "max-age" "=" delta-seconds
970        / "max-stale" [ "=" delta-seconds ]
971        / "min-fresh" "=" delta-seconds
972        / "no-transform"
973        / "only-if-cached"
974        / cache-extension
975
976   no-cache
977
978      The no-cache request directive indicates that a stored response
979      MUST NOT be used to satisfy the request without successful
980      validation on the origin server.
981
982   no-store
983
984      The no-store request directive indicates that a cache MUST NOT
985      store any part of either this request or any response to it.  This
986      directive applies to both non-shared and shared caches.  "MUST NOT
987      store" in this context means that the cache MUST NOT intentionally
988      store the information in non-volatile storage, and MUST make a
989      best-effort attempt to remove the information from volatile
990      storage as promptly as possible after forwarding it.
991
992      This directive is NOT a reliable or sufficient mechanism for
993      ensuring privacy.  In particular, malicious or compromised caches
994      might not recognize or obey this directive, and communications
995      networks may be vulnerable to eavesdropping.
996
997   max-age
998
999      The max-age request directive indicates that the client is willing
1000      to accept a response whose age is no greater than the specified
1001      time in seconds.  Unless max-stale directive is also included, the
1002      client is not willing to accept a stale response.
1003
1004
1005
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1011
1012   max-stale
1013
1014      The max-stale request directive indicates that the client is
1015      willing to accept a response that has exceeded its expiration
1016      time.  If max-stale is assigned a value, then the client is
1017      willing to accept a response that has exceeded its expiration time
1018      by no more than the specified number of seconds.  If no value is
1019      assigned to max-stale, then the client is willing to accept a
1020      stale response of any age. [[anchor15: of any staleness? --mnot]]
1021
1022   min-fresh
1023
1024      The min-fresh request directive indicates that the client is
1025      willing to accept a response whose freshness lifetime is no less
1026      than its current age plus the specified time in seconds.  That is,
1027      the client wants a response that will still be fresh for at least
1028      the specified number of seconds.
1029
1030   no-transform
1031
1032      The no-transform request directive indicates that an intermediate
1033      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1034      or Content-Type request headers, nor the request entity-body.
1035
1036   only-if-cached
1037
1038      The only-if-cached request directive indicates that the client
1039      only wishes to return a stored response.  If it receives this
1040      directive, a cache SHOULD either respond using a stored response
1041      that is consistent with the other constraints of the request, or
1042      respond with a 504 (Gateway Timeout) status.  If a group of caches
1043      is being operated as a unified system with good internal
1044      connectivity, such a request MAY be forwarded within that group of
1045      caches.
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
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1067
10683.2.2.  Response Cache-Control Directives
1069
1070     cache-response-directive =
1071          "public"
1072        / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1073        / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1074        / "no-store"
1075        / "no-transform"
1076        / "must-revalidate"
1077        / "proxy-revalidate"
1078        / "max-age" "=" delta-seconds
1079        / "s-maxage" "=" delta-seconds
1080        / cache-extension
1081
1082   public
1083
1084      The public response directive indicates that the response MAY be
1085      cached, even if it would normally be non-cacheable or cacheable
1086      only within a non-shared cache.  (See also Authorization, Section
1087      3.1 of [Part7], for additional details.)
1088
1089   private
1090
1091      The private response directive indicates that the response message
1092      is intended for a single user and MUST NOT be stored by a shared
1093      cache.  A private (non-shared) cache MAY store the response.
1094
1095      If the private response directive specifies one or more field-
1096      names, this requirement is limited to the field-values associated
1097      with the listed response headers.  That is, the specified field-
1098      names(s) MUST NOT be stored by a shared cache, whereas the
1099      remainder of the response message MAY be.
1100
1101      Note: This usage of the word private only controls where the
1102      response may be stored, and cannot ensure the privacy of the
1103      message content.
1104
1105   no-cache
1106
1107      The no-cache response directive indicates that the response MUST
1108      NOT be used to satisfy a subsequent request without successful
1109      validation on the origin server.  This allows an origin server to
1110      prevent caching even by caches that have been configured to return
1111      stale responses.
1112
1113      If the no-cache response directive specifies one or more field-
1114      names, this requirement is limited to the field-values assosicated
1115      with the listed response headers.  That is, the specified field-
1116
1117
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1122
1123
1124      name(s) MUST NOT be sent in the response to a subsequent request
1125      without successful validation on the origin server.  This allows
1126      an origin server to prevent the re-use of certain header fields in
1127      a response, while still allowing caching of the rest of the
1128      response.
1129
1130      Note: Most HTTP/1.0 caches will not recognize or obey this
1131      directive.
1132
1133   no-store
1134
1135      The no-store response directive indicates that a cache MUST NOT
1136      store any part of either the immediate request or response.  This
1137      directive applies to both non-shared and shared caches.  "MUST NOT
1138      store" in this context means that the cache MUST NOT intentionally
1139      store the information in non-volatile storage, and MUST make a
1140      best-effort attempt to remove the information from volatile
1141      storage as promptly as possible after forwarding it.
1142
1143      This directive is NOT a reliable or sufficient mechanism for
1144      ensuring privacy.  In particular, malicious or compromised caches
1145      might not recognize or obey this directive, and communications
1146      networks may be vulnerable to eavesdropping.
1147
1148   must-revalidate
1149
1150      The must-revalidate response directive indicates that once it has
1151      become stale, the response MUST NOT be used to satisfy subsequent
1152      requests without successful validation on the origin server.
1153
1154      The must-revalidate directive is necessary to support reliable
1155      operation for certain protocol features.  In all circumstances an
1156      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1157      particular, if the cache cannot reach the origin server for any
1158      reason, it MUST generate a 504 (Gateway Timeout) response.
1159
1160      Servers SHOULD send the must-revalidate directive if and only if
1161      failure to validate a request on the entity could result in
1162      incorrect operation, such as a silently unexecuted financial
1163      transaction.
1164
1165   proxy-revalidate
1166
1167      The proxy-revalidate response directive has the same meaning as
1168      the must-revalidate response directive, except that it does not
1169      apply to non-shared caches.
1170
1171   max-age
1172
1173
1174
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1178
1179
1180      The max-age response directive indicates that response is to be
1181      considered stale after its age is greater than the specified
1182      number of seconds.
1183
1184   s-maxage
1185
1186      The s-maxage response directive indicates that, in shared caches,
1187      the maximum age specified by this directive overrides the maximum
1188      age specified by either the max-age directive or the Expires
1189      header.  The s-maxage directive also implies the semantics of the
1190      proxy-revalidate response directive.
1191
1192   no-transform
1193
1194      The no-transform response directive indicates that an intermediate
1195      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1196      or Content-Type response headers, nor the response entity-body.
1197
11983.2.3.  Cache Control Extensions
1199
1200   The Cache-Control header field can be extended through the use of one
1201   or more cache-extension tokens, each with an optional value.
1202   Informational extensions (those that do not require a change in cache
1203   behavior) can be added without changing the semantics of other
1204   directives.  Behavioral extensions are designed to work by acting as
1205   modifiers to the existing base of cache directives.  Both the new
1206   directive and the standard directive are supplied, such that
1207   applications that do not understand the new directive will default to
1208   the behavior specified by the standard directive, and those that
1209   understand the new directive will recognize it as modifying the
1210   requirements associated with the standard directive.  In this way,
1211   extensions to the cache-control directives can be made without
1212   requiring changes to the base protocol.
1213
1214   This extension mechanism depends on an HTTP cache obeying all of the
1215   cache-control directives defined for its native HTTP-version, obeying
1216   certain extensions, and ignoring all directives that it does not
1217   understand.
1218
1219   For example, consider a hypothetical new response directive called
1220   "community" that acts as a modifier to the private directive.  We
1221   define this new directive to mean that, in addition to any non-shared
1222   cache, any cache that is shared only by members of the community
1223   named within its value may cache the response.  An origin server
1224   wishing to allow the UCI community to use an otherwise private
1225   response in their shared cache(s) could do so by including
1226
1227     Cache-Control: private, community="UCI"
1228
1229
1230
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1235
1236   A cache seeing this header field will act correctly even if the cache
1237   does not understand the community cache-extension, since it will also
1238   see and understand the private directive and thus default to the safe
1239   behavior.
1240
1241   Unrecognized cache directives MUST be ignored; it is assumed that any
1242   cache directive likely to be unrecognized by an HTTP/1.1 cache will
1243   be combined with standard directives (or the response's default
1244   cacheability) such that the cache behavior will remain minimally
1245   correct even if the cache does not understand the extension(s).
1246
12473.3.  Expires
1248
1249   The entity-header field "Expires" gives the date/time after which the
1250   response is considered stale.  See Section 2.3 for further discussion
1251   of the freshness model.
1252
1253   The presence of an Expires field does not imply that the original
1254   resource will change or cease to exist at, before, or after that
1255   time.
1256
1257   The field-value is an absolute date and time as defined by HTTP-date
1258   in Section 3.2.1 of [Part1]; it MUST be sent in rfc1123-date format.
1259
1260     Expires   = "Expires" ":" OWS Expires-v
1261     Expires-v = HTTP-date
1262
1263   For example
1264
1265     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1266
1267      Note: if a response includes a Cache-Control field with the max-
1268      age directive (see Section 3.2.2), that directive overrides the
1269      Expires field.  Likewise, the s-maxage directive overrides Expires
1270      in shared caches.
1271
1272   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
1273   the future.
1274
1275   HTTP/1.1 clients and caches MUST treat other invalid date formats,
1276   especially including the value "0", as in the past (i.e., "already
1277   expired").
1278
12793.4.  Pragma
1280
1281   The general-header field "Pragma" is used to include implementation-
1282   specific directives that might apply to any recipient along the
1283   request/response chain.  All pragma directives specify optional
1284
1285
1286
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1290
1291
1292   behavior from the viewpoint of the protocol; however, some systems
1293   MAY require that behavior be consistent with the directives.
1294
1295     Pragma            = "Pragma" ":" OWS Pragma-v
1296     Pragma-v          = 1#pragma-directive
1297     pragma-directive  = "no-cache" / extension-pragma
1298     extension-pragma  = token [ "=" ( token / quoted-string ) ]
1299
1300   When the no-cache directive is present in a request message, an
1301   application SHOULD forward the request toward the origin server even
1302   if it has a cached copy of what is being requested.  This pragma
1303   directive has the same semantics as the no-cache response directive
1304   (see Section 3.2.2) and is defined here for backward compatibility
1305   with HTTP/1.0.  Clients SHOULD include both header fields when a no-
1306   cache request is sent to a server not known to be HTTP/1.1 compliant.
1307   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
1308   sent "Cache-Control: no-cache".
1309
1310      Note: because the meaning of "Pragma: no-cache" as a response-
1311      header field is not actually specified, it does not provide a
1312      reliable replacement for "Cache-Control: no-cache" in a response.
1313
1314   This mechanism is deprecated; no new Pragma directives will be
1315   defined in HTTP.
1316
13173.5.  Vary
1318
1319   The "Vary" response-header field's value indicates the set of
1320   request-header fields that determines, while the response is fresh,
1321   whether a cache is permitted to use the response to reply to a
1322   subsequent request without validation; see Section 2.6.
1323
1324   In uncacheable or stale responses, the Vary field value advises the
1325   user agent about the criteria that were used to select the
1326   representation.
1327
1328     Vary   = "Vary" ":" OWS Vary-v
1329     Vary-v = "*" / 1#field-name
1330
1331   The set of header fields named by the Vary field value is known as
1332   the selecting request-headers.
1333
1334   Servers SHOULD include a Vary header field with any cacheable
1335   response that is subject to server-driven negotiation.  Doing so
1336   allows a cache to properly interpret future requests on that resource
1337   and informs the user agent about the presence of negotiation on that
1338   resource.  A server MAY include a Vary header field with a non-
1339   cacheable response that is subject to server-driven negotiation,
1340
1341
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1346
1347
1348   since this might provide the user agent with useful information about
1349   the dimensions over which the response varies at the time of the
1350   response.
1351
1352   A Vary field value of "*" signals that unspecified parameters not
1353   limited to the request-headers (e.g., the network address of the
1354   client), play a role in the selection of the response representation;
1355   therefore, a cache cannot determine whether this response is
1356   appropriate.  The "*" value MUST NOT be generated by a proxy server;
1357   it may only be generated by an origin server.
1358
1359   The field-names given are not limited to the set of standard request-
1360   header fields defined by this specification.  Field names are case-
1361   insensitive.
1362
13633.6.  Warning
1364
1365   The general-header field "Warning" is used to carry additional
1366   information about the status or transformation of a message that
1367   might not be reflected in the message.  This information is typically
1368   used to warn about possible incorrectness introduced by caching
1369   operations or transformations applied to the entity body of the
1370   message.
1371
1372   Warnings can be used for other purposes, both cache-related and
1373   otherwise.  The use of a warning, rather than an error status code,
1374   distinguish these responses from true failures.
1375
1376   Warning headers can in general be applied to any message, however
1377   some warn-codes are specific to caches and can only be applied to
1378   response messages.
1379
1380     Warning    = "Warning" ":" OWS Warning-v
1381     Warning-v  = 1#warning-value
1382
1383     warning-value = warn-code SP warn-agent SP warn-text
1384                                           [SP warn-date]
1385
1386     warn-code  = 3DIGIT
1387     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1388                     ; the name or pseudonym of the server adding
1389                     ; the Warning header, for use in debugging
1390     warn-text  = quoted-string
1391     warn-date  = DQUOTE HTTP-date DQUOTE
1392
1393   Multiple warnings can be attached to a response (either by the origin
1394   server or by a cache), including multiple warnings with the same code
1395   number.  For example, a server might provide the same warning with
1396
1397
1398
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1402
1403
1404   texts in both English and Basque.
1405
1406   When this occurs, the user agent SHOULD inform the user of as many of
1407   them as possible, in the order that they appear in the response.  If
1408   it is not possible to inform the user of all of the warnings, the
1409   user agent SHOULD follow these heuristics:
1410
1411   o  Warnings that appear early in the response take priority over
1412      those appearing later in the response.
1413
1414   o  Warnings in the user's preferred character set take priority over
1415      warnings in other character sets but with identical warn-codes and
1416      warn-agents.
1417
1418   Systems that generate multiple Warning headers SHOULD order them with
1419   this user agent behavior in mind.  New Warning headers SHOULD be
1420   added after any existing Warning headers.
1421
1422   Warnings are assigned three digit warn-codes.  The first digit
1423   indicates whether the Warning is required to be deleted from a stored
1424   response after validation:
1425
1426   o  1xx Warnings that describe the freshness or validation status of
1427      the response, and so MUST be deleted by caches after validation.
1428      They MUST NOT be generated by a cache except when validating a
1429      cached entry, and MUST NOT be generated by clients.
1430
1431   o  2xx Warnings that describe some aspect of the entity body or
1432      entity headers that is not rectified by a validation (for example,
1433      a lossy compression of the entity bodies) and MUST NOT be deleted
1434      by caches after validation, unless a full response is returned, in
1435      which case they MUST be.
1436
1437   The warn-text SHOULD be in a natural language and character set that
1438   is most likely to be intelligible to the human user receiving the
1439   response.  This decision can be based on any available knowledge,
1440   such as the location of the cache or user, the Accept-Language field
1441   in a request, the Content-Language field in a response, etc.  The
1442   default language is English and the default character set is ISO-
1443   8859-1 ([ISO-8859-1]).
1444
1445   If a character set other than ISO-8859-1 is used, it MUST be encoded
1446   in the warn-text using the method described in [RFC2047].
1447
1448   If an implementation sends a message with one or more Warning headers
1449   to a receiver whose version is HTTP/1.0 or lower, then the sender
1450   MUST include in each warning-value a warn-date that matches the Date
1451   header in the message.
1452
1453
1454
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1458
1459
1460   If an implementation receives a message with a warning-value that
1461   includes a warn-date, and that warn-date is different from the Date
1462   value in the response, then that warning-value MUST be deleted from
1463   the message before storing, forwarding, or using it. (preventing the
1464   consequences of naive caching of Warning header fields.)  If all of
1465   the warning-values are deleted for this reason, the Warning header
1466   MUST be deleted as well.
1467
1468   The following warn-codes are defined by this specification, each with
1469   a recommended warn-text in English, and a description of its meaning.
1470
1471   110 Response is stale
1472
1473      SHOULD be included whenever the returned response is stale.
1474
1475   111 Revalidation failed
1476
1477      SHOULD be included if a cache returns a stale response because an
1478      attempt to validate the response failed, due to an inability to
1479      reach the server.
1480
1481   112 Disconnected operation
1482
1483      SHOULD be included if the cache is intentionally disconnected from
1484      the rest of the network for a period of time.
1485
1486   113 Heuristic expiration
1487
1488      SHOULD be included if the cache heuristically chose a freshness
1489      lifetime greater than 24 hours and the response's age is greater
1490      than 24 hours.
1491
1492   199 Miscellaneous warning
1493
1494      The warning text can include arbitrary information to be presented
1495      to a human user, or logged.  A system receiving this warning MUST
1496      NOT take any automated action, besides presenting the warning to
1497      the user.
1498
1499   214 Transformation applied
1500
1501      MUST be added by an intermediate cache or proxy if it applies any
1502      transformation changing the content-coding (as specified in the
1503      Content-Encoding header) or media-type (as specified in the
1504      Content-Type header) of the response, or the entity-body of the
1505      response, unless this Warning code already appears in the
1506      response.
1507
1508
1509
1510
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1514
1515
1516   299 Miscellaneous persistent warning
1517
1518      The warning text can include arbitrary information to be presented
1519      to a human user, or logged.  A system receiving this warning MUST
1520      NOT take any automated action.
1521
1522
15234.  History Lists
1524
1525   User agents often have history mechanisms, such as "Back" buttons and
1526   history lists, that can be used to redisplay an entity retrieved
1527   earlier in a session.
1528
1529   History mechanisms and caches are different.  In particular history
1530   mechanisms SHOULD NOT try to show a correct view of the current state
1531   of a resource.  Rather, a history mechanism is meant to show exactly
1532   what the user saw at the time when the resource was retrieved.
1533
1534   By default, an expiration time does not apply to history mechanisms.
1535   If the entity is still in storage, a history mechanism SHOULD display
1536   it even if the entity has expired, unless the user has specifically
1537   configured the agent to refresh expired history documents.
1538
1539   This is not to be construed to prohibit the history mechanism from
1540   telling the user that a view might be stale.
1541
1542      Note: if history list mechanisms unnecessarily prevent users from
1543      viewing stale resources, this will tend to force service authors
1544      to avoid using HTTP expiration controls and cache controls when
1545      they would otherwise like to.  Service authors may consider it
1546      important that users not be presented with error messages or
1547      warning messages when they use navigation controls (such as BACK)
1548      to view previously fetched resources.  Even though sometimes such
1549      resources ought not be cached, or ought to expire quickly, user
1550      interface considerations may force service authors to resort to
1551      other means of preventing caching (e.g. "once-only" URLs) in order
1552      not to suffer the effects of improperly functioning history
1553      mechanisms.
1554
1555
15565.  IANA Considerations
1557
15585.1.  Message Header Registration
1559
1560   The Message Header Registry located at <http://www.iana.org/
1561   assignments/message-headers/message-header-index.html> should be
1562   updated with the permanent registrations below (see [RFC3864]):
1563
1564
1565
1566
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1570
1571
1572   +-------------------+----------+----------+-------------+
1573   | Header Field Name | Protocol | Status   | Reference   |
1574   +-------------------+----------+----------+-------------+
1575   | Age               | http     | standard | Section 3.1 |
1576   | Cache-Control     | http     | standard | Section 3.2 |
1577   | Expires           | http     | standard | Section 3.3 |
1578   | Pragma            | http     | standard | Section 3.4 |
1579   | Vary              | http     | standard | Section 3.5 |
1580   | Warning           | http     | standard | Section 3.6 |
1581   +-------------------+----------+----------+-------------+
1582
1583   The change controller is: "IETF (iesg@ietf.org) - Internet
1584   Engineering Task Force".
1585
1586
15876.  Security Considerations
1588
1589   Caches expose additional potential vulnerabilities, since the
1590   contents of the cache represent an attractive target for malicious
1591   exploitation.  Because cache contents persist after an HTTP request
1592   is complete, an attack on the cache can reveal information long after
1593   a user believes that the information has been removed from the
1594   network.  Therefore, cache contents should be protected as sensitive
1595   information.
1596
1597
15987.  Acknowledgments
1599
1600   Much of the content and presentation of the caching design is due to
1601   suggestions and comments from individuals including: Shel Kaphan,
1602   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
1603
1604
16058.  References
1606
16078.1.  Normative References
1608
1609   [ISO-8859-1]
1610              International Organization for Standardization,
1611              "Information technology -- 8-bit single-byte coded graphic
1612              character sets -- Part 1: Latin alphabet No. 1", ISO/
1613              IEC 8859-1:1998, 1998.
1614
1615   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1616              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1617              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
1618              and Message Parsing", draft-ietf-httpbis-p1-messaging-06
1619              (work in progress), March 2009.
1620
1621
1622
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1626
1627
1628   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1629              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1630              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
1631              Semantics", draft-ietf-httpbis-p2-semantics-06 (work in
1632              progress), March 2009.
1633
1634   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1635              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1636              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
1637              and Content Negotiation", draft-ietf-httpbis-p3-payload-06
1638              (work in progress), March 2009.
1639
1640   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1641              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1642              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
1643              Requests", draft-ietf-httpbis-p4-conditional-06 (work in
1644              progress), March 2009.
1645
1646   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1647              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1648              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1649              Partial Responses", draft-ietf-httpbis-p5-range-06 (work
1650              in progress), March 2009.
1651
1652   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1653              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1654              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
1655              draft-ietf-httpbis-p7-auth-06 (work in progress),
1656              March 2009.
1657
1658   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
1659              Part Three: Message Header Extensions for Non-ASCII Text",
1660              RFC 2047, November 1996.
1661
1662   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1663              Requirement Levels", BCP 14, RFC 2119, March 1997.
1664
1665   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1666              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1667
16688.2.  Informative References
1669
1670   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
1671              Specification, Implementation", RFC 1305, March 1992.
1672
1673   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1674              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1675              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1676
1677
1678
1679Fielding, et al.       Expires September 10, 2009              [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                  March 2009
1682
1683
1684   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1685              Procedures for Message Header Fields", BCP 90, RFC 3864,
1686              September 2004.
1687
1688
1689Appendix A.  Compatibility with Previous Versions
1690
1691A.1.  Changes from RFC 2068
1692
1693   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
1694   was introduced to add this missing case.  (Sections 2.1, 3.2).
1695
1696   Transfer-coding and message lengths all interact in ways that
1697   required fixing exactly when chunked encoding is used (to allow for
1698   transfer encoding that may not be self delimiting); it was important
1699   to straighten out exactly how message lengths are computed. (see also
1700   [Part1], [Part3] and [Part5]) [[anchor18: This used to refer to the
1701   text about non-modifiable headers, and will have to be updated later
1702   on. --jre]]
1703
1704   Proxies should be able to add Content-Length when appropriate.
1705   [[anchor19: This used to refer to the text about non-modifiable
1706   headers, and will have to be updated later on. --jre]]
1707
1708   Range request responses would become very verbose if all meta-data
1709   were always returned; by allowing the server to only send needed
1710   headers in a 206 response, this problem can be avoided.
1711   (Section 2.7)
1712
1713   The Cache-Control: max-age directive was not properly defined for
1714   responses.  (Section 3.2.2)
1715
1716   Warnings could be cached incorrectly, or not updated appropriately.
1717   (Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general
1718   header, as PUT or other methods may have need for it in requests.
1719
1720A.2.  Changes from RFC 2616
1721
1722   Clarify denial of service attack avoidance requirement.
1723   (Section 2.5)
1724
1725
1726Appendix B.  Collected ABNF
1727
1728   Age = "Age:" OWS Age-v
1729   Age-v = delta-seconds
1730
1731   Cache-Control = "Cache-Control:" OWS Cache-Control-v
1732
1733
1734
1735Fielding, et al.       Expires September 10, 2009              [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                  March 2009
1738
1739
1740   Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
1741    cache-directive ] )
1742
1743   Expires = "Expires:" OWS Expires-v
1744   Expires-v = HTTP-date
1745
1746   HTTP-date = <HTTP-date, defined in [Part1], Section 3.2.1>
1747
1748   OWS = <OWS, defined in [Part1], Section 1.2.2>
1749
1750   Pragma = "Pragma:" OWS Pragma-v
1751   Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1752    pragma-directive ] )
1753
1754   Vary = "Vary:" OWS Vary-v
1755   Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1756    ] ) )
1757
1758   Warning = "Warning:" OWS Warning-v
1759   Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1760    ] )
1761
1762   cache-directive = cache-request-directive / cache-response-directive
1763   cache-extension = token [ "=" ( token / quoted-string ) ]
1764   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
1765    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1766    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1767    cache-extension
1768   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
1769    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1770    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1771    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1772    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1773    ) / ( "s-maxage=" delta-seconds ) / cache-extension
1774
1775   delta-seconds = 1*DIGIT
1776
1777   extension-pragma = token [ "=" ( token / quoted-string ) ]
1778
1779   field-name = <field-name, defined in [Part1], Section 4.2>
1780
1781   port = <port, defined in [Part1], Section 2.1>
1782   pragma-directive = "no-cache" / extension-pragma
1783   pseudonym = <pseudonym, defined in [Part1], Section 8.9>
1784
1785   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1786
1787   token = <token, defined in [Part1], Section 1.2.2>
1788
1789
1790
1791Fielding, et al.       Expires September 10, 2009              [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                  March 2009
1794
1795
1796   uri-host = <uri-host, defined in [Part1], Section 2.1>
1797
1798   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1799   warn-code = 3DIGIT
1800   warn-date = DQUOTE HTTP-date DQUOTE
1801   warn-text = quoted-string
1802   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
1803    ]
1804
1805
1806
1807   ABNF diagnostics:
1808
1809   ; Age defined but not used
1810   ; Cache-Control defined but not used
1811   ; Expires defined but not used
1812   ; Pragma defined but not used
1813   ; Vary defined but not used
1814   ; Warning defined but not used
1815
1816
1817Appendix C.  Change Log (to be removed by RFC Editor before publication)
1818
1819C.1.  Since RFC2616
1820
1821   Extracted relevant partitions from [RFC2616].
1822
1823C.2.  Since draft-ietf-httpbis-p6-cache-00
1824
1825   Closed issues:
1826
1827   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
1828      (<http://purl.org/NET/http-errata#trailer-hop>)
1829
1830   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
1831      after Update or Delete"
1832      (<http://purl.org/NET/http-errata#invalidupd>)
1833
1834   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1835      Informative references"
1836
1837   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
1838      typo"
1839
1840   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
1841      header text"
1842
1843
1844
1845
1846
1847Fielding, et al.       Expires September 10, 2009              [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                  March 2009
1850
1851
1852   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
1853      references"
1854
1855   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
1856      Reference"
1857
1858   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
1859      to-date references"
1860
1861   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
1862      13.2.2"
1863
1864   Other changes:
1865
1866   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
1867      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1868
1869C.3.  Since draft-ietf-httpbis-p6-cache-01
1870
1871   Closed issues:
1872
1873   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
1874      used"
1875
1876   Other changes:
1877
1878   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
1879      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1880
1881   o  Add explicit references to BNF syntax and rules imported from
1882      other parts of the specification.
1883
1884C.4.  Since draft-ietf-httpbis-p6-cache-02
1885
1886   Ongoing work on IANA Message Header Registration
1887   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1888
1889   o  Reference RFC 3984, and update header registrations for headers
1890      defined in this document.
1891
1892C.5.  Since draft-ietf-httpbis-p6-cache-03
1893
1894   Closed issues:
1895
1896   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
1897      classification"
1898
1899
1900
1901
1902
1903Fielding, et al.       Expires September 10, 2009              [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                  March 2009
1906
1907
1908C.6.  Since draft-ietf-httpbis-p6-cache-04
1909
1910   Ongoing work on ABNF conversion
1911   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1912
1913   o  Use "/" instead of "|" for alternatives.
1914
1915   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1916      whitespace ("OWS") and required whitespace ("RWS").
1917
1918   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1919      value format definitions.
1920
1921C.7.  Since draft-ietf-httpbis-p6-cache-05
1922
1923   This is a total rewrite of this part of the specification.
1924
1925   Affected issues:
1926
1927   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1928      1xx Warn-Codes"
1929
1930   o  <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement
1931      of 13.5.1 and 13.5.2"
1932
1933   o  <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role
1934      of Warning and Semantic Transparency in Caching"
1935
1936   o  <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods
1937      and Caching"
1938
1939   In addition: Final work on ABNF conversion
1940   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1941
1942   o  Add appendix containing collected and expanded ABNF, reorganize
1943      ABNF introduction.
1944
1945
1946Index
1947
1948   A
1949      age  6
1950      Age header  17
1951
1952   C
1953      cache  5
1954      Cache Directives
1955         max-age  18, 21
1956
1957
1958
1959Fielding, et al.       Expires September 10, 2009              [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                  March 2009
1962
1963
1964         max-stale  19
1965         min-fresh  19
1966         must-revalidate  21
1967         no-cache  18, 20
1968         no-store  18, 21
1969         no-transform  19, 22
1970         only-if-cached  19
1971         private  20
1972         proxy-revalidate  21
1973         public  20
1974         s-maxage  22
1975      Cache-Control header  17
1976      cacheable  5
1977
1978   E
1979      Expires header  23
1980      explicit expiration time  5
1981
1982   F
1983      first-hand  6
1984      fresh  6
1985      freshness lifetime  6
1986
1987   G
1988      Grammar
1989         Age  17
1990         Age-v  17
1991         Cache-Control  18
1992         Cache-Control-v  18
1993         cache-extension  18
1994         cache-request-directive  18
1995         cache-response-directive  20
1996         delta-seconds  17
1997         Expires  23
1998         Expires-v  23
1999         extension-pragma  24
2000         Pragma  24
2001         pragma-directive  24
2002         Pragma-v  24
2003         Vary  24
2004         Vary-v  24
2005         warn-agent  25
2006         warn-code  25
2007         warn-date  25
2008         warn-text  25
2009         Warning  25
2010         Warning-v  25
2011         warning-value  25
2012
2013
2014
2015Fielding, et al.       Expires September 10, 2009              [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                  March 2009
2018
2019
2020   H
2021      Headers
2022         Age  17
2023         Cache-Control  17
2024         Expires  23
2025         Pragma  23
2026         Vary  24
2027         Warning  25
2028      heuristic expiration time  5
2029
2030   M
2031      max-age
2032         Cache Directive  18, 21
2033      max-stale
2034         Cache Directive  19
2035      min-fresh
2036         Cache Directive  19
2037      must-revalidate
2038         Cache Directive  21
2039
2040   N
2041      no-cache
2042         Cache Directive  18, 20
2043      no-store
2044         Cache Directive  18, 21
2045      no-transform
2046         Cache Directive  19, 22
2047
2048   O
2049      only-if-cached
2050         Cache Directive  19
2051
2052   P
2053      Pragma header  23
2054      private
2055         Cache Directive  20
2056      proxy-revalidate
2057         Cache Directive  21
2058      public
2059         Cache Directive  20
2060
2061   S
2062      s-maxage
2063         Cache Directive  22
2064      stale  6
2065
2066   V
2067      validator  6
2068
2069
2070
2071Fielding, et al.       Expires September 10, 2009              [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                  March 2009
2074
2075
2076      Vary header  24
2077
2078   W
2079      Warning header  25
2080
2081
2082Authors' Addresses
2083
2084   Roy T. Fielding (editor)
2085   Day Software
2086   23 Corporate Plaza DR, Suite 280
2087   Newport Beach, CA  92660
2088   USA
2089
2090   Phone: +1-949-706-5300
2091   Fax:   +1-949-706-5305
2092   Email: fielding@gbiv.com
2093   URI:   http://roy.gbiv.com/
2094
2095
2096   Jim Gettys
2097   One Laptop per Child
2098   21 Oak Knoll Road
2099   Carlisle, MA  01741
2100   USA
2101
2102   Email: jg@laptop.org
2103   URI:   http://www.laptop.org/
2104
2105
2106   Jeffrey C. Mogul
2107   Hewlett-Packard Company
2108   HP Labs, Large Scale Systems Group
2109   1501 Page Mill Road, MS 1177
2110   Palo Alto, CA  94304
2111   USA
2112
2113   Email: JeffMogul@acm.org
2114
2115
2116   Henrik Frystyk Nielsen
2117   Microsoft Corporation
2118   1 Microsoft Way
2119   Redmond, WA  98052
2120   USA
2121
2122   Email: henrikn@microsoft.com
2123
2124
2125
2126
2127Fielding, et al.       Expires September 10, 2009              [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                  March 2009
2130
2131
2132   Larry Masinter
2133   Adobe Systems, Incorporated
2134   345 Park Ave
2135   San Jose, CA  95110
2136   USA
2137
2138   Email: LMM@acm.org
2139   URI:   http://larry.masinter.net/
2140
2141
2142   Paul J. Leach
2143   Microsoft Corporation
2144   1 Microsoft Way
2145   Redmond, WA  98052
2146
2147   Email: paulle@microsoft.com
2148
2149
2150   Tim Berners-Lee
2151   World Wide Web Consortium
2152   MIT Computer Science and Artificial Intelligence Laboratory
2153   The Stata Center, Building 32
2154   32 Vassar Street
2155   Cambridge, MA  02139
2156   USA
2157
2158   Email: timbl@w3.org
2159   URI:   http://www.w3.org/People/Berners-Lee/
2160
2161
2162   Yves Lafon (editor)
2163   World Wide Web Consortium
2164   W3C / ERCIM
2165   2004, rte des Lucioles
2166   Sophia-Antipolis, AM  06902
2167   France
2168
2169   Email: ylafon@w3.org
2170   URI:   http://www.raubacapeu.net/people/yves/
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183Fielding, et al.       Expires September 10, 2009              [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                  March 2009
2186
2187
2188   Julian F. Reschke (editor)
2189   greenbytes GmbH
2190   Hafenweg 16
2191   Muenster, NW  48155
2192   Germany
2193
2194   Phone: +49 251 2807760
2195   Fax:   +49 251 2807761
2196   Email: julian.reschke@greenbytes.de
2197   URI:   http://greenbytes.de/tech/webdav/
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239Fielding, et al.       Expires September 10, 2009              [Page 40]
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