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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: April 29, 2010                                         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                                                      M. Nottingham, Ed.
21
22                                                         J. Reschke, Ed.
23                                                              greenbytes
24                                                        October 26, 2009
25
26
27                       HTTP/1.1, part 6: Caching
28                     draft-ietf-httpbis-p6-cache-08
29
30Status of this Memo
31
32   This Internet-Draft is submitted to IETF in full conformance with the
33   provisions of BCP 78 and BCP 79.  This document may contain material
34   from IETF Documents or IETF Contributions published or made publicly
35   available before November 10, 2008.  The person(s) controlling the
36   copyright in some of this material may not have granted the IETF
37   Trust the right to allow modifications of such material outside the
38   IETF Standards Process.  Without obtaining an adequate license from
39   the person(s) controlling the copyright in such materials, this
40   document may not be modified outside the IETF Standards Process, and
41   derivative works of it may not be created outside the IETF Standards
42   Process, except to format it for publication as an RFC or to
43   translate it into languages other than English.
44
45   Internet-Drafts are working documents of the Internet Engineering
46   Task Force (IETF), its areas, and its working groups.  Note that
47   other groups may also distribute working documents as Internet-
48   Drafts.
49
50   Internet-Drafts are draft documents valid for a maximum of six months
51   and may be updated, replaced, or obsoleted by other documents at any
52
53
54
55Fielding, et al.         Expires April 29, 2010                 [Page 1]
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57Internet-Draft              HTTP/1.1, Part 6                October 2009
58
59
60   time.  It is inappropriate to use Internet-Drafts as reference
61   material or to cite them other than as "work in progress."
62
63   The list of current Internet-Drafts can be accessed at
64   http://www.ietf.org/ietf/1id-abstracts.txt.
65
66   The list of Internet-Draft Shadow Directories can be accessed at
67   http://www.ietf.org/shadow.html.
68
69   This Internet-Draft will expire on April 29, 2010.
70
71Copyright Notice
72
73   Copyright (c) 2009 IETF Trust and the persons identified as the
74   document authors.  All rights reserved.
75
76   This document is subject to BCP 78 and the IETF Trust's Legal
77   Provisions Relating to IETF Documents in effect on the date of
78   publication of this document (http://trustee.ietf.org/license-info).
79   Please review these documents carefully, as they describe your rights
80   and restrictions with respect to this document.
81
82Abstract
83
84   The Hypertext Transfer Protocol (HTTP) is an application-level
85   protocol for distributed, collaborative, hypermedia information
86   systems.  This document is Part 6 of the seven-part specification
87   that defines the protocol referred to as "HTTP/1.1" and, taken
88   together, obsoletes RFC 2616.  Part 6 defines requirements on HTTP
89   caches and the associated header fields that control cache behavior
90   or indicate cacheable response messages.
91
92Editorial Note (To be removed by RFC Editor)
93
94   Discussion of this draft should take place on the HTTPBIS working
95   group mailing list (ietf-http-wg@w3.org).  The current issues list is
96   at <http://tools.ietf.org/wg/httpbis/trac/report/11> and related
97   documents (including fancy diffs) can be found at
98   <http://tools.ietf.org/wg/httpbis/>.
99
100   The changes in this draft are summarized in Appendix C.9.
101
102
103
104
105
106
107
108
109
110
111Fielding, et al.         Expires April 29, 2010                 [Page 2]
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113Internet-Draft              HTTP/1.1, Part 6                October 2009
114
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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  . . . . . . . . 30
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                October 2009
170
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     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35
178     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36
179   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
180   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
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200
201
202
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221
222
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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
278
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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
334
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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 3.2>
375     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
376     port          = <port, defined in [Part1], Section 2.6>
377     pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
378     uri-host      = <uri-host, defined in [Part1], Section 2.6>
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
390
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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      ([[TODO-Request-URI: Need to find a new term for this, as Part 1
437      doesn't define Request-URI anymore; the new term request-target
438      does not work for this. (see
439      <http://tools.ietf.org/wg/httpbis/trac/ticket/196>)]]), and
440
441   o  the request method associated with the stored response allows it
442      to be used for the presented request, and
443
444
445
446
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451
452   o  selecting request-headers nominated by the stored response (if
453      any) match those presented (see Section 2.6), and
454
455   o  the presented request and stored response are free from directives
456      that would prevent its use (see Section 3.2 and Section 3.4), and
457
458   o  the stored response is either:
459
460      *  fresh (see Section 2.3), or
461
462      *  allowed to be served stale (see Section 2.3.3), or
463
464      *  successfully validated (see Section 2.4).
465
466   [[TODO-method-cacheability: define method cacheability for GET, HEAD
467   and POST in p2-semantics.]]
468
469   When a stored response is used to satisfy a request, caches MUST
470   include a single Age header field (Section 3.1) in the response with
471   a value equal to the stored response's current_age; see
472   Section 2.3.2. [[anchor1: DISCUSS: this currently includes
473   successfully validated responses.]]
474
475   Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
476   be written through the cache to the origin server; i.e., A cache must
477   not reply to such a request before having forwarded the request and
478   having received a corresponding response.
479
480   Also, note that unsafe requests might invalidate already stored
481   responses; see Section 2.5.
482
483   Caches MUST use the most recent response (as determined by the Date
484   header) when more than one suitable response is stored.  They can
485   also forward a request with "Cache-Control: max-age=0" or "Cache-
486   Control: no-cache" to disambiguate which response to use.
487
488   [[TODO-header-properties: end-to-end and hop-by-hop headers, non-
489   modifiable headers removed; re-spec in p1]]
490
4912.3.  Freshness Model
492
493   When a response is "fresh" in the cache, it can be used to satisfy
494   subsequent requests without contacting the origin server, thereby
495   improving efficiency.
496
497   The primary mechanism for determining freshness is for an origin
498   server to provide an explicit expiration time in the future, using
499   either the Expires header (Section 3.3) or the max-age response cache
500
501
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507
508   directive (Section 3.2.2).  Generally, origin servers will assign
509   future explicit expiration times to responses in the belief that the
510   entity is not likely to change in a semantically significant way
511   before the expiration time is reached.
512
513   If an origin server wishes to force a cache to validate every
514   request, it can assign an explicit expiration time in the past.  This
515   means that the response is always stale, so that caches should
516   validate it before using it for subsequent requests. [[anchor2: This
517   wording may cause confusion, because the response may still be served
518   stale.]]
519
520   Since origin servers do not always provide explicit expiration times,
521   HTTP caches may also assign heuristic expiration times when they are
522   not specified, employing algorithms that use other header values
523   (such as the Last-Modified time) to estimate a plausible expiration
524   time.  The HTTP/1.1 specification does not provide specific
525   algorithms, but does impose worst-case constraints on their results.
526
527   The calculation to determine if a response is fresh is:
528
529      response_is_fresh = (freshness_lifetime > current_age)
530
531   The freshness_lifetime is defined in Section 2.3.1; the current_age
532   is defined in Section 2.3.2.
533
534   Additionally, clients may need to influence freshness calculation.
535   They can do this using several request cache directives, with the
536   effect of either increasing or loosening constraints on freshness.
537   See Section 3.2.1.
538
539   [[anchor3: ISSUE: there are not requirements directly applying to
540   cache-request-directives and freshness.]]
541
542   Note that freshness applies only to cache operation; it cannot be
543   used to force a user agent to refresh its display or reload a
544   resource.  See Section 4 for an explanation of the difference between
545   caches and history mechanisms.
546
5472.3.1.  Calculating Freshness Lifetime
548
549   A cache can calculate the freshness lifetime (denoted as
550   freshness_lifetime) of a response by using the first match of:
551
552   o  If the cache is shared and the s-maxage response cache directive
553      (Section 3.2.2) is present, use its value, or
554
555
556
557
558
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563
564   o  If the max-age response cache directive (Section 3.2.2) is
565      present, use its value, or
566
567   o  If the Expires response header (Section 3.3) is present, use its
568      value minus the value of the Date response header, or
569
570   o  Otherwise, no explicit expiration time is present in the response,
571      but a heuristic may be used; see Section 2.3.1.1.
572
573   Note that this calculation is not vulnerable to clock skew, since all
574   of the information comes from the origin server.
575
5762.3.1.1.  Calculating Heuristic Freshness
577
578   If no explicit expiration time is present in a stored response that
579   has a status code of 200, 203, 206, 300, 301 or 410, a heuristic
580   expiration time can be calculated.  Heuristics MUST NOT be used for
581   other response status codes.
582
583   When a heuristic is used to calculate freshness lifetime, the cache
584   SHOULD attach a Warning header with a 113 warn-code to the response
585   if its current_age is more than 24 hours and such a warning is not
586   already present.
587
588   Also, if the response has a Last-Modified header (Section 6.6 of
589   [Part4]), the heuristic expiration value SHOULD be no more than some
590   fraction of the interval since that time.  A typical setting of this
591   fraction might be 10%.
592
593   [[anchor4: REVIEW: took away HTTP/1.0 query string heuristic
594   uncacheability.]]
595
5962.3.2.  Calculating Age
597
598   HTTP/1.1 uses the Age response-header to convey the estimated age of
599   the response message when obtained from a cache.  The Age field value
600   is the cache's estimate of the amount of time since the response was
601   generated or validated by the origin server.  In essence, the Age
602   value is the sum of the time that the response has been resident in
603   each of the caches along the path from the origin server, plus the
604   amount of time it has been in transit along network paths.
605
606   The term "age_value" denotes the value of the Age header, in a form
607   appropriate for arithmetic operations.
608
609   HTTP/1.1 requires origin servers to send a Date header, if possible,
610   with every response, giving the time at which the response was
611   generated (see Section 9.3 of [Part1]).  The term "date_value"
612
613
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619
620   denotes the value of the Date header, in a form appropriate for
621   arithmetic operations.
622
623   The term "now" means "the current value of the clock at the host
624   performing the calculation."  Hosts that use HTTP, but especially
625   hosts running origin servers and caches, SHOULD use NTP [RFC1305] or
626   some similar protocol to synchronize their clocks to a globally
627   accurate time standard.
628
629   A response's age can be calculated in two entirely independent ways:
630
631   1.  now minus date_value, if the local clock is reasonably well
632       synchronized to the origin server's clock.  If the result is
633       negative, the result is replaced by zero.
634
635   2.  age_value, if all of the caches along the response path implement
636       HTTP/1.1.
637
638   These are combined as
639
640       corrected_received_age = max(now - date_value, age_value)
641
642   When an Age value is received, it MUST be interpreted relative to the
643   time the request was initiated, not the time that the response was
644   received.
645
646      corrected_initial_age = corrected_received_age
647                            + (now - request_time)
648
649   where "request_time" is the time (according to the local clock) when
650   the request that elicited this response was sent.
651
652   The current_age of a stored response can then be calculated by adding
653   the amount of time (in seconds) since the stored response was last
654   validated by the origin server to the corrected_initial_age.
655
656   In summary:
657
658
659
660
661
662
663
664
665
666
667
668
669
670
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675
676     age_value     - Age header field-value received with the response
677     date_value    - Date header field-value received with the response
678     request_time  - local time when the cache made the request
679                    resulting in the stored response
680     response_time - local time when the cache received the response
681     now           - current local time
682
683     apparent_age = max(0, response_time - date_value);
684     corrected_received_age = max(apparent_age, age_value);
685     response_delay = response_time - request_time;
686     corrected_initial_age = corrected_received_age + response_delay;
687     resident_time = now - response_time;
688     current_age   = corrected_initial_age + resident_time;
689
6902.3.3.  Serving Stale Responses
691
692   A "stale" response is one that either has explicit expiry
693   information, or is allowed to have heuristic expiry calculated, but
694   is not fresh according to the calculations in Section 2.3.
695
696   Caches MUST NOT return a stale response if it is prohibited by an
697   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
698   cache directive, a "must-revalidate" cache-response-directive, or an
699   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
700   see Section 3.2.2).
701
702   Caches SHOULD NOT return stale responses unless they are disconnected
703   (i.e., it cannot contact the origin server or otherwise find a
704   forward path) or otherwise explicitly allowed (e.g., the max-stale
705   request directive; see Section 3.2.1).
706
707   Stale responses SHOULD have a Warning header with the 110 warn-code
708   (see Section 3.6).  Likewise, the 112 warn-code SHOULD be sent on
709   stale responses if the cache is disconnected.
710
711   If a cache receives a first-hand response (either an entire response,
712   or a 304 (Not Modified) response) that it would normally forward to
713   the requesting client, and the received response is no longer fresh,
714   the cache SHOULD forward it to the requesting client without adding a
715   new Warning (but without removing any existing Warning headers).  A
716   cache SHOULD NOT attempt to validate a response simply because that
717   response became stale in transit.
718
7192.4.  Validation Model
720
721   When a cache has one or more stored responses for a requested URI,
722   but cannot serve any of them (e.g., because they are not fresh, or
723   one cannot be selected; see Section 2.6), it can use the conditional
724
725
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731
732   request mechanism [Part4] in the forwarded request to give the origin
733   server an opportunity to both select a valid stored response to be
734   used, and to update it.  This process is known as "validating" or
735   "revalidating" the stored response.
736
737   When sending such a conditional request, the cache SHOULD add an If-
738   Modified-Since header whose value is that of the Last-Modified header
739   from the selected (see Section 2.6) stored response, if available.
740
741   Additionally, the cache SHOULD add an If-None-Match header whose
742   value is that of the ETag header(s) from all responses stored for the
743   requested URI, if present.  However, if any of the stored responses
744   contains only partial content, its entity-tag SHOULD NOT be included
745   in the If-None-Match header field unless the request is for a range
746   that would be fully satisfied by that stored response.
747
748   A 304 (Not Modified) response status code indicates that the stored
749   response can be updated and reused; see Section 2.7.
750
751   A full response (i.e., one with a response body) indicates that none
752   of the stored responses nominated in the conditional request is
753   suitable.  Instead, the full response is used both to satisfy the
754   request and replace the stored response. [[anchor5: Should there be a
755   requirement here?]]
756
757   If a cache receives a 5xx response while attempting to validate a
758   response, it MAY either forward this response to the requesting
759   client, or act as if the server failed to respond.  In the latter
760   case, it MAY return a previously stored response (see Section 2.3.3).
761
7622.5.  Request Methods that Invalidate
763
764   Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
765   for changing state on the origin server, intervening caches can use
766   them to keep their contents up-to-date.
767
768   The following HTTP methods MUST cause a cache to invalidate the
769   Request-URI as well as the URI(s) in the Location and Content-
770   Location headers (if present):
771
772   o  PUT
773
774   o  DELETE
775
776   o  POST
777
778   An invalidation based on a URI from a Location or Content-Location
779   header MUST NOT be performed if the host part of that URI differs
780
781
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787
788   from the host part in the Request-URI.  This helps prevent denial of
789   service attacks.
790
791   [[anchor6: TODO: "host part" needs to be specified better.]]
792
793   A cache that passes through requests for methods it does not
794   understand SHOULD invalidate the Request-URI.
795
796   Here, "invalidate" means that the cache will either remove all stored
797   responses related to the Request-URI, or will mark these as "invalid"
798   and in need of a mandatory validation before they can be returned in
799   response to a subsequent request.
800
801   Note that this does not guarantee that all appropriate responses are
802   invalidated.  For example, the request that caused the change at the
803   origin server might not have gone through the cache where a response
804   is stored.
805
806   [[anchor7: TODO: specify that only successful (2xx, 3xx?) responses
807   invalidate.]]
808
8092.6.  Caching Negotiated Responses
810
811   When a cache receives a request that can be satisfied by a stored
812   response that has a Vary header field (Section 3.5), it MUST NOT use
813   that response unless all of the selecting request-headers nominated
814   by the Vary header match in both the original request (i.e., that
815   associated with the stored response), and the presented request.
816
817   The selecting request-headers from two requests are defined to match
818   if and only if the selecting request-headers in the first request can
819   be transformed to the selecting request-headers in the second request
820   by adding or removing linear white space [[anchor8: [ref]]] at places
821   where this is allowed by the corresponding ABNF, and/or combining
822   multiple message-header fields with the same field name following the
823   rules about header fields in Section 3.2 of [Part1].
824
825   If a header field is absent from a request, it can only match another
826   request if it is also absent there.
827
828   A Vary header field-value of "*" always fails to match, and
829   subsequent requests to that resource can only be properly interpreted
830   by the origin server.
831
832   The stored response with matching selecting request-headers is known
833   as the selected response.
834
835   If no selected response is available, the cache MAY forward the
836
837
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843
844   presented request to the origin server in a conditional request; see
845   Section 2.4.
846
8472.7.  Combining Responses
848
849   When a cache receives a 304 (Not Modified) response or a 206 (Partial
850   Content) response (in this section, the "new" response"), it needs to
851   created an updated response by combining the stored response with the
852   new one, so that the updated response can be used to satisfy the
853   request.
854
855   If the new response contains an ETag, it identifies the stored
856   response to use. [[anchor9: may need language about Content-Location
857   here]][[anchor10: cover case where INM with multiple etags was sent]]
858
859   If the status code is 206 (partial content), both the stored and new
860   responses MUST have validators, and those validators MUST match using
861   the strong comparison function (see Section 4 of [Part4]).
862   Otherwise, the responses MUST NOT be combined.
863
864   The stored response headers are used as those of the updated
865   response, except that
866
867   o  any stored Warning headers with warn-code 1xx (see Section 3.6)
868      MUST be deleted from the stored response and the updated response.
869
870   o  any stored Warning headers with warn-code 2xx MUST be retained in
871      the stored response and the updated response.
872
873   o  any headers provided in the new response MUST replace the
874      corresponding headers from the stored response.
875
876   If a header field-name in the new response matches more than one
877   header in the stored response, all such stored headers MUST be
878   replaced.
879
880   The updated response can [[[[anchor11: requirement?]]]] be used to
881   replace the stored response in cache.  In the case of a 206 response,
882   the combined entity-body MAY be stored.
883
884   [[anchor12: ISSUE: discuss how to handle HEAD updates]]
885
886
8873.  Header Field Definitions
888
889   This section defines the syntax and semantics of HTTP/1.1 header
890   fields related to caching.
891
892
893
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899
900   For entity-header fields, both sender and recipient refer to either
901   the client or the server, depending on who sends and who receives the
902   entity.
903
9043.1.  Age
905
906   The "Age" response-header field conveys the sender's estimate of the
907   amount of time since the response was generated or successfully
908   validated at the origin server.  Age values are calculated as
909   specified in Section 2.3.2.
910
911     Age   = "Age" ":" OWS Age-v
912     Age-v = delta-seconds
913
914   Age field-values are non-negative integers, representing time in
915   seconds.
916
917     delta-seconds  = 1*DIGIT
918
919   If a cache receives a value larger than the largest positive integer
920   it can represent, or if any of its age calculations overflows, it
921   MUST transmit an Age header with a field-value of 2147483648 (2^31).
922   Caches SHOULD use an arithmetic type of at least 31 bits of range.
923
924   The presence of an Age header field in a response implies that a
925   response is not first-hand.  However, the converse is not true, since
926   HTTP/1.0 caches may not implement the Age header field.
927
9283.2.  Cache-Control
929
930   The "Cache-Control" general-header field is used to specify
931   directives that MUST be obeyed by all caches along the request/
932   response chain.  Such cache directives are unidirectional in that the
933   presence of a directive in a request does not imply that the same
934   directive is to be given in the response.
935
936      Note that HTTP/1.0 caches might not implement Cache-Control and
937      might only implement Pragma: no-cache (see Section 3.4).
938
939   Cache directives MUST be passed through by a proxy or gateway
940   application, regardless of their significance to that application,
941   since the directives might be applicable to all recipients along the
942   request/response chain.  It is not possible to target a directive to
943   a specific cache.
944
945
946
947
948
949
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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. [[anchor13: 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.  Also, private response directives with field-
1104      names are often handled by implementations as if an unqualified
1105      private directive was recieved; i.e., the special handling for the
1106      qualified form is not widely implemented.
1107
1108   no-cache
1109
1110      The no-cache response directive indicates that the response MUST
1111      NOT be used to satisfy a subsequent request without successful
1112      validation on the origin server.  This allows an origin server to
1113      prevent caching even by caches that have been configured to return
1114      stale responses.
1115
1116
1117
1118
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1122
1123
1124      If the no-cache response directive specifies one or more field-
1125      names, this requirement is limited to the field-values associated
1126      with the listed response headers.  That is, the specified field-
1127      name(s) MUST NOT be sent in the response to a subsequent request
1128      without successful validation on the origin server.  This allows
1129      an origin server to prevent the re-use of certain header fields in
1130      a response, while still allowing caching of the rest of the
1131      response.
1132
1133      Note: Most HTTP/1.0 caches will not recognize or obey this
1134      directive.  Also, no-cache response directives with field-names
1135      are often handled by implementations as if an unqualified no-cache
1136      directive was recieved; i.e., the special handling for the
1137      qualified form is not widely implemented.
1138
1139   no-store
1140
1141      The no-store response directive indicates that a cache MUST NOT
1142      store any part of either the immediate request or response.  This
1143      directive applies to both non-shared and shared caches.  "MUST NOT
1144      store" in this context means that the cache MUST NOT intentionally
1145      store the information in non-volatile storage, and MUST make a
1146      best-effort attempt to remove the information from volatile
1147      storage as promptly as possible after forwarding it.
1148
1149      This directive is NOT a reliable or sufficient mechanism for
1150      ensuring privacy.  In particular, malicious or compromised caches
1151      might not recognize or obey this directive, and communications
1152      networks may be vulnerable to eavesdropping.
1153
1154   must-revalidate
1155
1156      The must-revalidate response directive indicates that once it has
1157      become stale, the response MUST NOT be used to satisfy subsequent
1158      requests without successful validation on the origin server.
1159
1160      The must-revalidate directive is necessary to support reliable
1161      operation for certain protocol features.  In all circumstances an
1162      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1163      particular, if the cache cannot reach the origin server for any
1164      reason, it MUST generate a 504 (Gateway Timeout) response.
1165
1166      Servers SHOULD send the must-revalidate directive if and only if
1167      failure to validate a request on the entity could result in
1168      incorrect operation, such as a silently unexecuted financial
1169      transaction.
1170
1171   proxy-revalidate
1172
1173
1174
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1178
1179
1180      The proxy-revalidate response directive has the same meaning as
1181      the must-revalidate response directive, except that it does not
1182      apply to non-shared caches.
1183
1184   max-age
1185
1186      The max-age response directive indicates that response is to be
1187      considered stale after its age is greater than the specified
1188      number of seconds.
1189
1190   s-maxage
1191
1192      The s-maxage response directive indicates that, in shared caches,
1193      the maximum age specified by this directive overrides the maximum
1194      age specified by either the max-age directive or the Expires
1195      header.  The s-maxage directive also implies the semantics of the
1196      proxy-revalidate response directive.
1197
1198   no-transform
1199
1200      The no-transform response directive indicates that an intermediate
1201      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1202      or Content-Type response headers, nor the response entity-body.
1203
12043.2.3.  Cache Control Extensions
1205
1206   The Cache-Control header field can be extended through the use of one
1207   or more cache-extension tokens, each with an optional value.
1208   Informational extensions (those that do not require a change in cache
1209   behavior) can be added without changing the semantics of other
1210   directives.  Behavioral extensions are designed to work by acting as
1211   modifiers to the existing base of cache directives.  Both the new
1212   directive and the standard directive are supplied, such that
1213   applications that do not understand the new directive will default to
1214   the behavior specified by the standard directive, and those that
1215   understand the new directive will recognize it as modifying the
1216   requirements associated with the standard directive.  In this way,
1217   extensions to the cache-control directives can be made without
1218   requiring changes to the base protocol.
1219
1220   This extension mechanism depends on an HTTP cache obeying all of the
1221   cache-control directives defined for its native HTTP-version, obeying
1222   certain extensions, and ignoring all directives that it does not
1223   understand.
1224
1225   For example, consider a hypothetical new response directive called
1226   "community" that acts as a modifier to the private directive.  We
1227   define this new directive to mean that, in addition to any non-shared
1228
1229
1230
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1234
1235
1236   cache, any cache that is shared only by members of the community
1237   named within its value may cache the response.  An origin server
1238   wishing to allow the UCI community to use an otherwise private
1239   response in their shared cache(s) could do so by including
1240
1241     Cache-Control: private, community="UCI"
1242
1243   A cache seeing this header field will act correctly even if the cache
1244   does not understand the community cache-extension, since it will also
1245   see and understand the private directive and thus default to the safe
1246   behavior.
1247
1248   Unrecognized cache directives MUST be ignored; it is assumed that any
1249   cache directive likely to be unrecognized by an HTTP/1.1 cache will
1250   be combined with standard directives (or the response's default
1251   cacheability) such that the cache behavior will remain minimally
1252   correct even if the cache does not understand the extension(s).
1253
12543.3.  Expires
1255
1256   The "Expires" entity-header field gives the date/time after which the
1257   response is considered stale.  See Section 2.3 for further discussion
1258   of the freshness model.
1259
1260   The presence of an Expires field does not imply that the original
1261   resource will change or cease to exist at, before, or after that
1262   time.
1263
1264   The field-value is an absolute date and time as defined by HTTP-date
1265   in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format.
1266
1267     Expires   = "Expires" ":" OWS Expires-v
1268     Expires-v = HTTP-date
1269
1270   For example
1271
1272     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1273
1274      Note: if a response includes a Cache-Control field with the max-
1275      age directive (see Section 3.2.2), that directive overrides the
1276      Expires field.  Likewise, the s-maxage directive overrides Expires
1277      in shared caches.
1278
1279   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
1280   the future.
1281
1282   HTTP/1.1 clients and caches MUST treat other invalid date formats,
1283   especially including the value "0", as in the past (i.e., "already
1284
1285
1286
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1290
1291
1292   expired").
1293
12943.4.  Pragma
1295
1296   The "Pragma" general-header field is used to include implementation-
1297   specific directives that might apply to any recipient along the
1298   request/response chain.  All pragma directives specify optional
1299   behavior from the viewpoint of the protocol; however, some systems
1300   MAY require that behavior be consistent with the directives.
1301
1302     Pragma            = "Pragma" ":" OWS Pragma-v
1303     Pragma-v          = 1#pragma-directive
1304     pragma-directive  = "no-cache" / extension-pragma
1305     extension-pragma  = token [ "=" ( token / quoted-string ) ]
1306
1307   When the no-cache directive is present in a request message, an
1308   application SHOULD forward the request toward the origin server even
1309   if it has a cached copy of what is being requested.  This pragma
1310   directive has the same semantics as the no-cache response directive
1311   (see Section 3.2.2) and is defined here for backward compatibility
1312   with HTTP/1.0.  Clients SHOULD include both header fields when a no-
1313   cache request is sent to a server not known to be HTTP/1.1 compliant.
1314   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
1315   sent "Cache-Control: no-cache".
1316
1317      Note: because the meaning of "Pragma: no-cache" as a response-
1318      header field is not actually specified, it does not provide a
1319      reliable replacement for "Cache-Control: no-cache" in a response.
1320
1321   This mechanism is deprecated; no new Pragma directives will be
1322   defined in HTTP.
1323
13243.5.  Vary
1325
1326   The "Vary" response-header field conveys the set of request-header
1327   fields that were used to select the representation.
1328
1329   Caches use this information, in part, to determine whether a stored
1330   response can be used to satisdy a given request; see Section 2.6.
1331   determines, while the response is fresh, whether a cache is permitted
1332   to use the response to reply to a subsequent request without
1333   validation; see Section 2.6.
1334
1335   In uncacheable or stale responses, the Vary field value advises the
1336   user agent about the criteria that were used to select the
1337   representation.
1338
1339     Vary   = "Vary" ":" OWS Vary-v
1340
1341
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1346
1347
1348     Vary-v = "*" / 1#field-name
1349
1350   The set of header fields named by the Vary field value is known as
1351   the selecting request-headers.
1352
1353   Servers SHOULD include a Vary header field with any cacheable
1354   response that is subject to server-driven negotiation.  Doing so
1355   allows a cache to properly interpret future requests on that resource
1356   and informs the user agent about the presence of negotiation on that
1357   resource.  A server MAY include a Vary header field with a non-
1358   cacheable response that is subject to server-driven negotiation,
1359   since this might provide the user agent with useful information about
1360   the dimensions over which the response varies at the time of the
1361   response.
1362
1363   A Vary field value of "*" signals that unspecified parameters not
1364   limited to the request-headers (e.g., the network address of the
1365   client), play a role in the selection of the response representation;
1366   therefore, a cache cannot determine whether this response is
1367   appropriate.  The "*" value MUST NOT be generated by a proxy server;
1368   it may only be generated by an origin server.
1369
1370   The field-names given are not limited to the set of standard request-
1371   header fields defined by this specification.  Field names are case-
1372   insensitive.
1373
13743.6.  Warning
1375
1376   The "Warning" general-header field is used to carry additional
1377   information about the status or transformation of a message that
1378   might not be reflected in the message.  This information is typically
1379   used to warn about possible incorrectness introduced by caching
1380   operations or transformations applied to the entity body of the
1381   message.
1382
1383   Warnings can be used for other purposes, both cache-related and
1384   otherwise.  The use of a warning, rather than an error status code,
1385   distinguish these responses from true failures.
1386
1387   Warning headers can in general be applied to any message, however
1388   some warn-codes are specific to caches and can only be applied to
1389   response messages.
1390
1391
1392
1393
1394
1395
1396
1397
1398
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1402
1403
1404     Warning    = "Warning" ":" OWS Warning-v
1405     Warning-v  = 1#warning-value
1406
1407     warning-value = warn-code SP warn-agent SP warn-text
1408                                           [SP warn-date]
1409
1410     warn-code  = 3DIGIT
1411     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1412                     ; the name or pseudonym of the server adding
1413                     ; the Warning header, for use in debugging
1414     warn-text  = quoted-string
1415     warn-date  = DQUOTE HTTP-date DQUOTE
1416
1417   Multiple warnings can be attached to a response (either by the origin
1418   server or by a cache), including multiple warnings with the same code
1419   number, only differing in warn-text.
1420
1421   When this occurs, the user agent SHOULD inform the user of as many of
1422   them as possible, in the order that they appear in the response.
1423
1424   Systems that generate multiple Warning headers SHOULD order them with
1425   this user agent behavior in mind.  New Warning headers SHOULD be
1426   added after any existing Warning headers.
1427
1428   Warnings are assigned three digit warn-codes.  The first digit
1429   indicates whether the Warning is required to be deleted from a stored
1430   response after validation:
1431
1432   o  1xx Warnings describe the freshness or validation status of the
1433      response, and so MUST be deleted by caches after validation.  They
1434      can only be generated by a cache when validating a cached entry,
1435      and MUST NOT be generated in any other situation.
1436
1437   o  2xx Warnings describe some aspect of the entity body or entity
1438      headers that is not rectified by a validation (for example, a
1439      lossy compression of the entity bodies) and MUST NOT be deleted by
1440      caches after validation, unless a full response is returned, in
1441      which case they MUST be.
1442
1443   If an implementation sends a message with one or more Warning headers
1444   to a receiver whose version is HTTP/1.0 or lower, then the sender
1445   MUST include in each warning-value a warn-date that matches the Date
1446   header in the message.
1447
1448   If an implementation receives a message with a warning-value that
1449   includes a warn-date, and that warn-date is different from the Date
1450   value in the response, then that warning-value MUST be deleted from
1451   the message before storing, forwarding, or using it. (preventing the
1452
1453
1454
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1458
1459
1460   consequences of naive caching of Warning header fields.)  If all of
1461   the warning-values are deleted for this reason, the Warning header
1462   MUST be deleted as well.
1463
1464   The following warn-codes are defined by this specification, each with
1465   a recommended warn-text in English, and a description of its meaning.
1466
1467   110 Response is stale
1468
1469      SHOULD be included whenever the returned response is stale.
1470
1471   111 Revalidation failed
1472
1473      SHOULD be included if a cache returns a stale response because an
1474      attempt to validate the response failed, due to an inability to
1475      reach the server.
1476
1477   112 Disconnected operation
1478
1479      SHOULD be included if the cache is intentionally disconnected from
1480      the rest of the network for a period of time.
1481
1482   113 Heuristic expiration
1483
1484      SHOULD be included if the cache heuristically chose a freshness
1485      lifetime greater than 24 hours and the response's age is greater
1486      than 24 hours.
1487
1488   199 Miscellaneous warning
1489
1490      The warning text can include arbitrary information to be presented
1491      to a human user, or logged.  A system receiving this warning MUST
1492      NOT take any automated action, besides presenting the warning to
1493      the user.
1494
1495   214 Transformation applied
1496
1497      MUST be added by an intermediate cache or proxy if it applies any
1498      transformation changing the content-coding (as specified in the
1499      Content-Encoding header) or media-type (as specified in the
1500      Content-Type header) of the response, or the entity-body of the
1501      response, unless this Warning code already appears in the
1502      response.
1503
1504   299 Miscellaneous persistent warning
1505
1506      The warning text can include arbitrary information to be presented
1507      to a human user, or logged.  A system receiving this warning MUST
1508
1509
1510
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1514
1515
1516      NOT take any automated action.
1517
1518
15194.  History Lists
1520
1521   User agents often have history mechanisms, such as "Back" buttons and
1522   history lists, that can be used to redisplay an entity retrieved
1523   earlier in a session.
1524
1525   History mechanisms and caches are different.  In particular history
1526   mechanisms SHOULD NOT try to show a correct view of the current state
1527   of a resource.  Rather, a history mechanism is meant to show exactly
1528   what the user saw at the time when the resource was retrieved.
1529
1530   By default, an expiration time does not apply to history mechanisms.
1531   If the entity is still in storage, a history mechanism SHOULD display
1532   it even if the entity has expired, unless the user has specifically
1533   configured the agent to refresh expired history documents.
1534
1535   This is not to be construed to prohibit the history mechanism from
1536   telling the user that a view might be stale.
1537
1538      Note: if history list mechanisms unnecessarily prevent users from
1539      viewing stale resources, this will tend to force service authors
1540      to avoid using HTTP expiration controls and cache controls when
1541      they would otherwise like to.  Service authors may consider it
1542      important that users not be presented with error messages or
1543      warning messages when they use navigation controls (such as BACK)
1544      to view previously fetched resources.  Even though sometimes such
1545      resources ought not be cached, or ought to expire quickly, user
1546      interface considerations may force service authors to resort to
1547      other means of preventing caching (e.g. "once-only" URLs) in order
1548      not to suffer the effects of improperly functioning history
1549      mechanisms.
1550
1551
15525.  IANA Considerations
1553
15545.1.  Message Header Registration
1555
1556   The Message Header Registry located at <http://www.iana.org/
1557   assignments/message-headers/message-header-index.html> should be
1558   updated with the permanent registrations below (see [RFC3864]):
1559
1560
1561
1562
1563
1564
1565
1566
1567Fielding, et al.         Expires April 29, 2010                [Page 28]
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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   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1610              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1611              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
1612              and Message Parsing", draft-ietf-httpbis-p1-messaging-08
1613              (work in progress), October 2009.
1614
1615   [Part2]    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 2: Message
1618              Semantics", draft-ietf-httpbis-p2-semantics-08 (work in
1619              progress), October 2009.
1620
1621
1622
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1626
1627
1628   [Part3]    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 3: Message Payload
1631              and Content Negotiation", draft-ietf-httpbis-p3-payload-08
1632              (work in progress), October 2009.
1633
1634   [Part4]    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 4: Conditional
1637              Requests", draft-ietf-httpbis-p4-conditional-08 (work in
1638              progress), October 2009.
1639
1640   [Part5]    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 5: Range Requests and
1643              Partial Responses", draft-ietf-httpbis-p5-range-08 (work
1644              in progress), October 2009.
1645
1646   [Part7]    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 7: Authentication",
1649              draft-ietf-httpbis-p7-auth-08 (work in progress),
1650              October 2009.
1651
1652   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1653              Requirement Levels", BCP 14, RFC 2119, March 1997.
1654
1655   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1656              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1657
16588.2.  Informative References
1659
1660   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
1661              Specification, Implementation", RFC 1305, March 1992.
1662
1663   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1664              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1665              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1666
1667   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1668              Procedures for Message Header Fields", BCP 90, RFC 3864,
1669              September 2004.
1670
1671
1672Appendix A.  Compatibility with Previous Versions
1673
1674
1675
1676
1677
1678
1679Fielding, et al.         Expires April 29, 2010                [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                October 2009
1682
1683
1684A.1.  Changes from RFC 2068
1685
1686   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
1687   was introduced to add this missing case.  (Sections 2.1, 3.2).
1688
1689   Transfer-coding and message lengths all interact in ways that
1690   required fixing exactly when chunked encoding is used (to allow for
1691   transfer encoding that may not be self delimiting); it was important
1692   to straighten out exactly how message lengths are computed. (see also
1693   [Part1], [Part3] and [Part5]) [[anchor16: This used to refer to the
1694   text about non-modifiable headers, and will have to be updated later
1695   on. --jre]]
1696
1697   Proxies should be able to add Content-Length when appropriate.
1698   [[anchor17: This used to refer to the text about non-modifiable
1699   headers, and will have to be updated later on. --jre]]
1700
1701   Range request responses would become very verbose if all meta-data
1702   were always returned; by allowing the server to only send needed
1703   headers in a 206 response, this problem can be avoided.
1704   (Section 2.7)
1705
1706   The Cache-Control: max-age directive was not properly defined for
1707   responses.  (Section 3.2.2)
1708
1709   Warnings could be cached incorrectly, or not updated appropriately.
1710   (Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general
1711   header, as PUT or other methods may have need for it in requests.
1712
1713A.2.  Changes from RFC 2616
1714
1715   Remove requirement to consider Content-Location in successful
1716   responses in order to determine the appropriate response to use.
1717   (Section 2.4)
1718
1719   Clarify denial of service attack avoidance requirement.
1720   (Section 2.5)
1721
1722   Do not mention RFC 2047 encoding and multiple languages in Warning
1723   headers anymore, as these aspects never were implemented.
1724   (Section 3.6)
1725
1726
1727Appendix B.  Collected ABNF
1728
1729   Age = "Age:" OWS Age-v
1730   Age-v = delta-seconds
1731
1732
1733
1734
1735Fielding, et al.         Expires April 29, 2010                [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                October 2009
1738
1739
1740   Cache-Control = "Cache-Control:" OWS Cache-Control-v
1741   Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
1742    cache-directive ] )
1743
1744   Expires = "Expires:" OWS Expires-v
1745   Expires-v = HTTP-date
1746
1747   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1748
1749   OWS = <OWS, defined in [Part1], Section 1.2.2>
1750
1751   Pragma = "Pragma:" OWS Pragma-v
1752   Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1753    pragma-directive ] )
1754
1755   Vary = "Vary:" OWS Vary-v
1756   Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1757    ] ) )
1758
1759   Warning = "Warning:" OWS Warning-v
1760   Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1761    ] )
1762
1763   cache-directive = cache-request-directive / cache-response-directive
1764   cache-extension = token [ "=" ( token / quoted-string ) ]
1765   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
1766    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1767    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1768    cache-extension
1769   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
1770    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1771    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1772    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1773    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1774    ) / ( "s-maxage=" delta-seconds ) / cache-extension
1775
1776   delta-seconds = 1*DIGIT
1777
1778   extension-pragma = token [ "=" ( token / quoted-string ) ]
1779
1780   field-name = <field-name, defined in [Part1], Section 3.2>
1781
1782   port = <port, defined in [Part1], Section 2.6>
1783   pragma-directive = "no-cache" / extension-pragma
1784   pseudonym = <pseudonym, defined in [Part1], Section 9.9>
1785
1786   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1787
1788
1789
1790
1791Fielding, et al.         Expires April 29, 2010                [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                October 2009
1794
1795
1796   token = <token, defined in [Part1], Section 1.2.2>
1797
1798   uri-host = <uri-host, defined in [Part1], Section 2.6>
1799
1800   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1801   warn-code = 3DIGIT
1802   warn-date = DQUOTE HTTP-date DQUOTE
1803   warn-text = quoted-string
1804   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
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 April 29, 2010                [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                October 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 April 29, 2010                [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                October 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
1945C.8.  Since draft-ietf-httpbis-p6-cache-06
1946
1947   Closed issues:
1948
1949   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
1950      numeric protocol elements"
1951
1952   Affected issues:
1953
1954   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: Vary and non-
1955      existant headers
1956
1957
1958
1959Fielding, et al.         Expires April 29, 2010                [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                October 2009
1962
1963
1964C.9.  Since draft-ietf-httpbis-p6-cache-07
1965
1966   Closed issues:
1967
1968   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1969      1xx Warn-Codes"
1970
1971   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
1972      Location on 304 responses"
1973
1974   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
1975      no-cache CC directives with headers"
1976
1977   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
1978      warn-text"
1979
1980
1981Index
1982
1983   A
1984      age  6
1985      Age header  17
1986
1987   C
1988      cache  5
1989      Cache Directives
1990         max-age  18, 22
1991         max-stale  19
1992         min-fresh  19
1993         must-revalidate  21
1994         no-cache  18, 20
1995         no-store  18, 21
1996         no-transform  19, 22
1997         only-if-cached  19
1998         private  20
1999         proxy-revalidate  21
2000         public  20
2001         s-maxage  22
2002      Cache-Control header  17
2003      cacheable  5
2004
2005   E
2006      Expires header  23
2007      explicit expiration time  5
2008
2009   F
2010      first-hand  6
2011      fresh  6
2012
2013
2014
2015Fielding, et al.         Expires April 29, 2010                [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                October 2009
2018
2019
2020      freshness lifetime  6
2021
2022   G
2023      Grammar
2024         Age  17
2025         Age-v  17
2026         Cache-Control  18
2027         Cache-Control-v  18
2028         cache-extension  18
2029         cache-request-directive  18
2030         cache-response-directive  20
2031         delta-seconds  17
2032         Expires  23
2033         Expires-v  23
2034         extension-pragma  24
2035         Pragma  24
2036         pragma-directive  24
2037         Pragma-v  24
2038         Vary  24
2039         Vary-v  24
2040         warn-agent  26
2041         warn-code  26
2042         warn-date  26
2043         warn-text  26
2044         Warning  26
2045         Warning-v  26
2046         warning-value  26
2047
2048   H
2049      Headers
2050         Age  17
2051         Cache-Control  17
2052         Expires  23
2053         Pragma  24
2054         Vary  24
2055         Warning  25
2056      heuristic expiration time  5
2057
2058   M
2059      max-age
2060         Cache Directive  18, 22
2061      max-stale
2062         Cache Directive  19
2063      min-fresh
2064         Cache Directive  19
2065      must-revalidate
2066         Cache Directive  21
2067
2068
2069
2070
2071Fielding, et al.         Expires April 29, 2010                [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                October 2009
2074
2075
2076   N
2077      no-cache
2078         Cache Directive  18, 20
2079      no-store
2080         Cache Directive  18, 21
2081      no-transform
2082         Cache Directive  19, 22
2083
2084   O
2085      only-if-cached
2086         Cache Directive  19
2087
2088   P
2089      Pragma header  24
2090      private
2091         Cache Directive  20
2092      proxy-revalidate
2093         Cache Directive  21
2094      public
2095         Cache Directive  20
2096
2097   S
2098      s-maxage
2099         Cache Directive  22
2100      stale  6
2101
2102   V
2103      validator  6
2104      Vary header  24
2105
2106   W
2107      Warning header  25
2108
2109
2110Authors' Addresses
2111
2112   Roy T. Fielding (editor)
2113   Day Software
2114   23 Corporate Plaza DR, Suite 280
2115   Newport Beach, CA  92660
2116   USA
2117
2118   Phone: +1-949-706-5300
2119   Fax:   +1-949-706-5305
2120   Email: fielding@gbiv.com
2121   URI:   http://roy.gbiv.com/
2122
2123
2124
2125
2126
2127Fielding, et al.         Expires April 29, 2010                [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                October 2009
2130
2131
2132   Jim Gettys
2133   One Laptop per Child
2134   21 Oak Knoll Road
2135   Carlisle, MA  01741
2136   USA
2137
2138   Email: jg@laptop.org
2139   URI:   http://www.laptop.org/
2140
2141
2142   Jeffrey C. Mogul
2143   Hewlett-Packard Company
2144   HP Labs, Large Scale Systems Group
2145   1501 Page Mill Road, MS 1177
2146   Palo Alto, CA  94304
2147   USA
2148
2149   Email: JeffMogul@acm.org
2150
2151
2152   Henrik Frystyk Nielsen
2153   Microsoft Corporation
2154   1 Microsoft Way
2155   Redmond, WA  98052
2156   USA
2157
2158   Email: henrikn@microsoft.com
2159
2160
2161   Larry Masinter
2162   Adobe Systems, Incorporated
2163   345 Park Ave
2164   San Jose, CA  95110
2165   USA
2166
2167   Email: LMM@acm.org
2168   URI:   http://larry.masinter.net/
2169
2170
2171   Paul J. Leach
2172   Microsoft Corporation
2173   1 Microsoft Way
2174   Redmond, WA  98052
2175
2176   Email: paulle@microsoft.com
2177
2178
2179
2180
2181
2182
2183Fielding, et al.         Expires April 29, 2010                [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                October 2009
2186
2187
2188   Tim Berners-Lee
2189   World Wide Web Consortium
2190   MIT Computer Science and Artificial Intelligence Laboratory
2191   The Stata Center, Building 32
2192   32 Vassar Street
2193   Cambridge, MA  02139
2194   USA
2195
2196   Email: timbl@w3.org
2197   URI:   http://www.w3.org/People/Berners-Lee/
2198
2199
2200   Yves Lafon (editor)
2201   World Wide Web Consortium
2202   W3C / ERCIM
2203   2004, rte des Lucioles
2204   Sophia-Antipolis, AM  06902
2205   France
2206
2207   Email: ylafon@w3.org
2208   URI:   http://www.raubacapeu.net/people/yves/
2209
2210
2211   Mark Nottingham (editor)
2212
2213   Email: mnot@mnot.net
2214   URI:   http://www.mnot.net/
2215
2216
2217   Julian F. Reschke (editor)
2218   greenbytes GmbH
2219   Hafenweg 16
2220   Muenster, NW  48155
2221   Germany
2222
2223   Phone: +49 251 2807760
2224   Fax:   +49 251 2807761
2225   Email: julian.reschke@greenbytes.de
2226   URI:   http://greenbytes.de/tech/webdav/
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239Fielding, et al.         Expires April 29, 2010                [Page 40]
2240
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