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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 9, 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                                                           March 8, 2010
25
26
27                       HTTP/1.1, part 6: Caching
28                     draft-ietf-httpbis-p6-cache-09
29
30Abstract
31
32   The Hypertext Transfer Protocol (HTTP) is an application-level
33   protocol for distributed, collaborative, hypermedia information
34   systems.  This document is Part 6 of the seven-part specification
35   that defines the protocol referred to as "HTTP/1.1" and, taken
36   together, obsoletes RFC 2616.  Part 6 defines requirements on HTTP
37   caches and the associated header fields that control cache behavior
38   or indicate cacheable response messages.
39
40Editorial Note (To be removed by RFC Editor)
41
42   Discussion of this draft should take place on the HTTPBIS working
43   group mailing list (ietf-http-wg@w3.org).  The current issues list is
44   at <http://tools.ietf.org/wg/httpbis/trac/report/11> and related
45   documents (including fancy diffs) can be found at
46   <http://tools.ietf.org/wg/httpbis/>.
47
48   The changes in this draft are summarized in Appendix C.10.
49
50Status of this Memo
51
52
53
54
55Fielding, et al.        Expires September 9, 2010               [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 6                  March 2010
58
59
60   This Internet-Draft is submitted to IETF in full conformance with the
61   provisions of BCP 78 and BCP 79.
62
63   Internet-Drafts are working documents of the Internet Engineering
64   Task Force (IETF), its areas, and its working groups.  Note that
65   other groups may also distribute working documents as Internet-
66   Drafts.
67
68   Internet-Drafts are draft documents valid for a maximum of six months
69   and may be updated, replaced, or obsoleted by other documents at any
70   time.  It is inappropriate to use Internet-Drafts as reference
71   material or to cite them other than as "work in progress."
72
73   The list of current Internet-Drafts can be accessed at
74   http://www.ietf.org/ietf/1id-abstracts.txt.
75
76   The list of Internet-Draft Shadow Directories can be accessed at
77   http://www.ietf.org/shadow.html.
78
79   This Internet-Draft will expire on September 9, 2010.
80
81Copyright Notice
82
83   Copyright (c) 2010 IETF Trust and the persons identified as the
84   document authors.  All rights reserved.
85
86   This document is subject to BCP 78 and the IETF Trust's Legal
87   Provisions Relating to IETF Documents
88   (http://trustee.ietf.org/license-info) in effect on the date of
89   publication of this document.  Please review these documents
90   carefully, as they describe your rights and restrictions with respect
91   to this document.  Code Components extracted from this document must
92   include Simplified BSD License text as described in Section 4.e of
93   the Trust Legal Provisions and are provided without warranty as
94   described in the BSD License.
95
96   This document may contain material from IETF Documents or IETF
97   Contributions published or made publicly available before November
98   10, 2008.  The person(s) controlling the copyright in some of this
99   material may not have granted the IETF Trust the right to allow
100   modifications of such material outside the IETF Standards Process.
101   Without obtaining an adequate license from the person(s) controlling
102   the copyright in such materials, this document may not be modified
103   outside the IETF Standards Process, and derivative works of it may
104   not be created outside the IETF Standards Process, except to format
105   it for publication as an RFC or to translate it into languages other
106   than English.
107
108
109
110
111Fielding, et al.        Expires September 9, 2010               [Page 2]
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113Internet-Draft              HTTP/1.1, Part 6                  March 2010
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 . . . . . . . . . . . .  9
130     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 10
131       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 11
132       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 12
133       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 13
134     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 14
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 . . . . . . . . . . . . . . . . . . . 17
139     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
140     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 18
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  . . . . . . . . . . . . . . . . . . . 28
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  . . . . . . . . . . . . . . . . . . 30
158     A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 30
159   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 31
160   Appendix C.  Change Log (to be removed by RFC Editor before
161                publication)  . . . . . . . . . . . . . . . . . . . . 32
162     C.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 32
163     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 32
164
165
166
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169Internet-Draft              HTTP/1.1, Part 6                  March 2010
170
171
172     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 33
173     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 33
174     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
175     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 34
176     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 34
177     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35
178     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35
179     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 35
180   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
181   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
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 understood by the cache and defined as being
397      cacheable, and
398
399   o  the response status code is understood by the cache, and
400
401   o  the "no-store" cache directive (see Section 3.2) does not appear
402      in request or response headers, and
403
404   o  the "private" cache response directive (see Section 3.2.2 does not
405      appear in the response, if the cache is shared, and
406
407   o  the "Authorization" header (see Section 3.1 of [Part7]) does not
408      appear in the request, if the cache is shared (unless the "public"
409      directive is present; see Section 3.2), and
410
411   o  the response either:
412
413      *  contains an Expires header (see Section 3.3), or
414
415      *  contains a max-age response cache directive (see
416         Section 3.2.2), or
417
418      *  contains a s-maxage response cache directive and the cache is
419         shared, or
420
421      *  contains a Cache Control Extension (see Section 3.2.3) that
422         allows it to be cached, or
423
424      *  has a status code that can be served with heuristic freshness
425         (see Section 2.3.1.1).
426
427   In this context, a cache has "understood" a request method or a
428   response status code if it recognises it and implements any cache-
429   specific behaviour.  In particular, 206 Partial Content responses
430   cannot be cached by an implementation that does not handle partial
431   content (see Section 2.1.1).
432
433   Note that in normal operation, most caches will not store a response
434   that has neither a cache validator nor an explicit expiration time,
435   as such responses are not usually useful to store.  However, caches
436   are not prohibited from storing such responses.
437
4382.1.1.  Storing Partial and Incomplete Responses
439
440   A cache that receives an incomplete response (for example, with fewer
441   bytes of data than specified in a Content-Length header) can store
442   the response, but MUST treat it as a partial response [Part5].
443   Partial responses can be combined as described in Section 4 of
444
445
446
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451
452   [Part5]; the result might be a full response or might still be
453   partial.  A cache MUST NOT return a partial response to a client
454   without explicitly marking it as such using the 206 (Partial Content)
455   status code.
456
457   A cache that does not support the Range and Content-Range headers
458   MUST NOT store incomplete or partial responses.
459
4602.2.  Constructing Responses from Caches
461
462   For a presented request, a cache MUST NOT return a stored response,
463   unless:
464
465   o  The presented Request-URI and that of the stored response match
466      ([[TODO-Request-URI: Need to find a new term for this, as Part 1
467      doesn't define Request-URI anymore; the new term request-target
468      does not work for this. (see
469      <http://tools.ietf.org/wg/httpbis/trac/ticket/196>)]]), and
470
471   o  the request method associated with the stored response allows it
472      to be used for the presented request, and
473
474   o  selecting request-headers nominated by the stored response (if
475      any) match those presented (see Section 2.6), and
476
477   o  the presented request and stored response are free from directives
478      that would prevent its use (see Section 3.2 and Section 3.4), and
479
480   o  the stored response is either:
481
482      *  fresh (see Section 2.3), or
483
484      *  allowed to be served stale (see Section 2.3.3), or
485
486      *  successfully validated (see Section 2.4).
487
488   [[TODO-method-cacheability: define method cacheability for GET, HEAD
489   and POST in p2-semantics.]]
490
491   When a stored response is used to satisfy a request, caches MUST
492   include a single Age header field (Section 3.1) in the response with
493   a value equal to the stored response's current_age; see
494   Section 2.3.2.  [[DISCUSS-includes-validated: this currently includes
495   successfully validated responses.]]
496
497   Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
498   be written through the cache to the origin server; i.e., a cache must
499   not reply to such a request before having forwarded the request and
500
501
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507
508   having received a corresponding response.
509
510   Also, note that unsafe requests might invalidate already stored
511   responses; see Section 2.5.
512
513   Caches MUST use the most recent response (as determined by the Date
514   header) when more than one suitable response is stored.  They can
515   also forward a request with "Cache-Control: max-age=0" or "Cache-
516   Control: no-cache" to disambiguate which response to use.
517
518   [[TODO-header-properties: end-to-end and hop-by-hop headers, non-
519   modifiable headers removed; re-spec in p1]]
520
5212.3.  Freshness Model
522
523   When a response is "fresh" in the cache, it can be used to satisfy
524   subsequent requests without contacting the origin server, thereby
525   improving efficiency.
526
527   The primary mechanism for determining freshness is for an origin
528   server to provide an explicit expiration time in the future, using
529   either the Expires header (Section 3.3) or the max-age response cache
530   directive (Section 3.2.2).  Generally, origin servers will assign
531   future explicit expiration times to responses in the belief that the
532   entity is not likely to change in a semantically significant way
533   before the expiration time is reached.
534
535   If an origin server wishes to force a cache to validate every
536   request, it can assign an explicit expiration time in the past.  This
537   means that the response is always stale, so that caches should
538   validate it before using it for subsequent requests.  [[TODO-
539   response-stale: This wording may cause confusion, because the
540   response may still be served stale.]]
541
542   Since origin servers do not always provide explicit expiration times,
543   HTTP caches may also assign heuristic expiration times when they are
544   not specified, employing algorithms that use other header values
545   (such as the Last-Modified time) to estimate a plausible expiration
546   time.  The HTTP/1.1 specification does not provide specific
547   algorithms, but does impose worst-case constraints on their results.
548
549   The calculation to determine if a response is fresh is:
550
551      response_is_fresh = (freshness_lifetime > current_age)
552
553   The freshness_lifetime is defined in Section 2.3.1; the current_age
554   is defined in Section 2.3.2.
555
556
557
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563
564   Additionally, clients may need to influence freshness calculation.
565   They can do this using several request cache directives, with the
566   effect of either increasing or loosening constraints on freshness.
567   See Section 3.2.1.
568
569   [[ISSUE-no-req-for-directives: there are not requirements directly
570   applying to cache-request-directives and freshness.]]
571
572   Note that freshness applies only to cache operation; it cannot be
573   used to force a user agent to refresh its display or reload a
574   resource.  See Section 4 for an explanation of the difference between
575   caches and history mechanisms.
576
5772.3.1.  Calculating Freshness Lifetime
578
579   A cache can calculate the freshness lifetime (denoted as
580   freshness_lifetime) of a response by using the first match of:
581
582   o  If the cache is shared and the s-maxage response cache directive
583      (Section 3.2.2) is present, use its value, or
584
585   o  If the max-age response cache directive (Section 3.2.2) is
586      present, use its value, or
587
588   o  If the Expires response header (Section 3.3) is present, use its
589      value minus the value of the Date response header, or
590
591   o  Otherwise, no explicit expiration time is present in the response,
592      but a heuristic may be used; see Section 2.3.1.1.
593
594   Note that this calculation is not vulnerable to clock skew, since all
595   of the information comes from the origin server.
596
5972.3.1.1.  Calculating Heuristic Freshness
598
599   If no explicit expiration time is present in a stored response that
600   has a status code of 200, 203, 206, 300, 301 or 410, a heuristic
601   expiration time can be calculated.  Heuristics MUST NOT be used for
602   other response status codes.
603
604   When a heuristic is used to calculate freshness lifetime, the cache
605   SHOULD attach a Warning header with a 113 warn-code to the response
606   if its current_age is more than 24 hours and such a warning is not
607   already present.
608
609   Also, if the response has a Last-Modified header (Section 6.6 of
610   [Part4]), the heuristic expiration value SHOULD be no more than some
611   fraction of the interval since that time.  A typical setting of this
612
613
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619
620   fraction might be 10%.
621
622   [[REVIEW-query-string-heuristics: took away HTTP/1.0 query string
623   heuristic uncacheability.]]
624
6252.3.2.  Calculating Age
626
627   HTTP/1.1 uses the Age response-header to convey the estimated age of
628   the response message when obtained from a cache.  The Age field value
629   is the cache's estimate of the amount of time since the response was
630   generated or validated by the origin server.  In essence, the Age
631   value is the sum of the time that the response has been resident in
632   each of the caches along the path from the origin server, plus the
633   amount of time it has been in transit along network paths.
634
635   The term "age_value" denotes the value of the Age header, in a form
636   appropriate for arithmetic operations.
637
638   HTTP/1.1 requires origin servers to send a Date header, if possible,
639   with every response, giving the time at which the response was
640   generated (see Section 9.3 of [Part1]).  The term "date_value"
641   denotes the value of the Date header, in a form appropriate for
642   arithmetic operations.
643
644   The term "now" means "the current value of the clock at the host
645   performing the calculation."  Hosts that use HTTP, but especially
646   hosts running origin servers and caches, SHOULD use NTP [RFC1305] or
647   some similar protocol to synchronize their clocks to a globally
648   accurate time standard.
649
650   A response's age can be calculated in two entirely independent ways:
651
652   1.  now minus date_value, if the local clock is reasonably well
653       synchronized to the origin server's clock.  If the result is
654       negative, the result is replaced by zero.
655
656   2.  age_value, if all of the caches along the response path implement
657       HTTP/1.1.
658
659   These are combined as
660
661       corrected_received_age = max(now - date_value, age_value)
662
663   When an Age value is received, it MUST be interpreted relative to the
664   time the request was initiated, not the time that the response was
665   received.
666
667      corrected_initial_age = corrected_received_age
668
669
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675
676                            + (now - request_time)
677
678   where "request_time" is the time (according to the local clock) when
679   the request that elicited this response was sent.
680
681   The current_age of a stored response can then be calculated by adding
682   the amount of time (in seconds) since the stored response was last
683   validated by the origin server to the corrected_initial_age.
684
685   In summary:
686
687     age_value     - Age header field-value received with the response
688     date_value    - Date header field-value received with the response
689     request_time  - local time when the cache made the request
690                    resulting in the stored response
691     response_time - local time when the cache received the response
692     now           - current local time
693
694     apparent_age = max(0, response_time - date_value);
695     corrected_received_age = max(apparent_age, age_value);
696     response_delay = response_time - request_time;
697     corrected_initial_age = corrected_received_age + response_delay;
698     resident_time = now - response_time;
699     current_age   = corrected_initial_age + resident_time;
700
7012.3.3.  Serving Stale Responses
702
703   A "stale" response is one that either has explicit expiry
704   information, or is allowed to have heuristic expiry calculated, but
705   is not fresh according to the calculations in Section 2.3.
706
707   Caches MUST NOT return a stale response if it is prohibited by an
708   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
709   cache directive, a "must-revalidate" cache-response-directive, or an
710   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
711   see Section 3.2.2).
712
713   Caches SHOULD NOT return stale responses unless they are disconnected
714   (i.e., it cannot contact the origin server or otherwise find a
715   forward path) or otherwise explicitly allowed (e.g., the max-stale
716   request directive; see Section 3.2.1).
717
718   Stale responses SHOULD have a Warning header with the 110 warn-code
719   (see Section 3.6).  Likewise, the 112 warn-code SHOULD be sent on
720   stale responses if the cache is disconnected.
721
722   If a cache receives a first-hand response (either an entire response,
723   or a 304 (Not Modified) response) that it would normally forward to
724
725
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731
732   the requesting client, and the received response is no longer fresh,
733   the cache SHOULD forward it to the requesting client without adding a
734   new Warning (but without removing any existing Warning headers).  A
735   cache SHOULD NOT attempt to validate a response simply because that
736   response became stale in transit.
737
7382.4.  Validation Model
739
740   When a cache has one or more stored responses for a requested URI,
741   but cannot serve any of them (e.g., because they are not fresh, or
742   one cannot be selected; see Section 2.6), it can use the conditional
743   request mechanism [Part4] in the forwarded request to give the origin
744   server an opportunity to both select a valid stored response to be
745   used, and to update it.  This process is known as "validating" or
746   "revalidating" the stored response.
747
748   When sending such a conditional request, the cache SHOULD add an If-
749   Modified-Since header whose value is that of the Last-Modified header
750   from the selected (see Section 2.6) stored response, if available.
751
752   Additionally, the cache SHOULD add an If-None-Match header whose
753   value is that of the ETag header(s) from all responses stored for the
754   requested URI, if present.  However, if any of the stored responses
755   contains only partial content, its entity-tag SHOULD NOT be included
756   in the If-None-Match header field unless the request is for a range
757   that would be fully satisfied by that stored response.
758
759   A 304 (Not Modified) response status code indicates that the stored
760   response can be updated and reused; see Section 2.7.
761
762   A full response (i.e., one with a response body) indicates that none
763   of the stored responses nominated in the conditional request is
764   suitable.  Instead, the full response is used both to satisfy the
765   request and replace the stored response.  [[TODO-req-missing: Should
766   there be a requirement here?]]
767
768   If a cache receives a 5xx response while attempting to validate a
769   response, it MAY either forward this response to the requesting
770   client, or act as if the server failed to respond.  In the latter
771   case, it MAY return a previously stored response (see Section 2.3.3).
772
7732.5.  Request Methods that Invalidate
774
775   Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
776   for changing state on the origin server, intervening caches can use
777   them to keep their contents up-to-date.
778
779   The following HTTP methods MUST cause a cache to invalidate the
780
781
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787
788   Request-URI as well as the URI(s) in the Location and Content-
789   Location headers (if present):
790
791   o  PUT
792
793   o  DELETE
794
795   o  POST
796
797   An invalidation based on a URI from a Location or Content-Location
798   header MUST NOT be performed if the host part of that URI differs
799   from the host part in the Request-URI.  This helps prevent denial of
800   service attacks.
801
802   [[TODO-def-host-part: "host part" needs to be specified better.]]
803
804   A cache that passes through requests for methods it does not
805   understand SHOULD invalidate the Request-URI.
806
807   Here, "invalidate" means that the cache will either remove all stored
808   responses related to the Request-URI, or will mark these as "invalid"
809   and in need of a mandatory validation before they can be returned in
810   response to a subsequent request.
811
812   Note that this does not guarantee that all appropriate responses are
813   invalidated.  For example, the request that caused the change at the
814   origin server might not have gone through the cache where a response
815   is stored.
816
817   [[TODO-spec-success-invalidate: specify that only successful (2xx,
818   3xx?) responses invalidate.]]
819
8202.6.  Caching Negotiated Responses
821
822   When a cache receives a request that can be satisfied by a stored
823   response that has a Vary header field (Section 3.5), it MUST NOT use
824   that response unless all of the selecting request-headers nominated
825   by the Vary header match in both the original request (i.e., that
826   associated with the stored response), and the presented request.
827
828   The selecting request-headers from two requests are defined to match
829   if and only if those in the first request can be transformed to those
830   in the second request by applying any of the following:
831
832   o  adding or removing whitespace, where allowed in the header's
833      syntax
834
835
836
837
838
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843
844   o  combining multiple message-header fields with the same field name
845      (see Section 3.2 of [Part1])
846
847   o  normalizing both header values in a way that is known to have
848      identical semantics, according to the header's specification
849      (e.g., re-ordering field values when order is not significant;
850      case-normalization, where values are defined to be case-
851      insensitive)
852
853   If (after any normalisation that may take place) a header field is
854   absent from a request, it can only match another request if it is
855   also absent there.
856
857   A Vary header field-value of "*" always fails to match, and
858   subsequent requests to that resource can only be properly interpreted
859   by the origin server.
860
861   The stored response with matching selecting request-headers is known
862   as the selected response.
863
864   If no selected response is available, the cache MAY forward the
865   presented request to the origin server in a conditional request; see
866   Section 2.4.
867
8682.7.  Combining Responses
869
870   When a cache receives a 304 (Not Modified) response or a 206 (Partial
871   Content) response (in this section, the "new" response"), it needs to
872   created an updated response by combining the stored response with the
873   new one, so that the updated response can be used to satisfy the
874   request.
875
876   If the new response contains an ETag, it identifies the stored
877   response to use.  [[TODO-mention-CL: may need language about Content-
878   Location here]][[TODO-inm-mult-etags: cover case where INM with
879   multiple etags was sent]]
880
881   If the status code is 206 (partial content), both the stored and new
882   responses MUST have validators, and those validators MUST match using
883   the strong comparison function (see Section 4 of [Part4]).
884   Otherwise, the responses MUST NOT be combined.
885
886   The stored response headers are used as those of the updated
887   response, except that
888
889   o  any stored Warning headers with warn-code 1xx (see Section 3.6)
890      MUST be deleted from the stored response and the updated response.
891
892
893
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899
900   o  any stored Warning headers with warn-code 2xx MUST be retained in
901      the stored response and the updated response.
902
903   o  any headers provided in the new response MUST replace the
904      corresponding headers from the stored response.
905
906   If a header field-name in the new response matches more than one
907   header in the stored response, all such stored headers MUST be
908   replaced.
909
910   The updated response can [[TODO-is-req: requirement?]] be used to
911   replace the stored response in cache.  In the case of a 206 response,
912   the combined entity-body MAY be stored.
913
914   [[ISSUE-how-head: discuss how to handle HEAD updates]]
915
916
9173.  Header Field Definitions
918
919   This section defines the syntax and semantics of HTTP/1.1 header
920   fields related to caching.
921
922   For entity-header fields, both sender and recipient refer to either
923   the client or the server, depending on who sends and who receives the
924   entity.
925
9263.1.  Age
927
928   The "Age" response-header field conveys the sender's estimate of the
929   amount of time since the response was generated or successfully
930   validated at the origin server.  Age values are calculated as
931   specified in Section 2.3.2.
932
933     Age   = "Age" ":" OWS Age-v
934     Age-v = delta-seconds
935
936   Age field-values are non-negative integers, representing time in
937   seconds.
938
939     delta-seconds  = 1*DIGIT
940
941   If a cache receives a value larger than the largest positive integer
942   it can represent, or if any of its age calculations overflows, it
943   MUST transmit an Age header with a field-value of 2147483648 (2^31).
944   Caches SHOULD use an arithmetic type of at least 31 bits of range.
945
946   The presence of an Age header field in a response implies that a
947   response is not first-hand.  However, the converse is not true, since
948
949
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955
956   HTTP/1.0 caches may not implement the Age header field.
957
9583.2.  Cache-Control
959
960   The "Cache-Control" general-header field is used to specify
961   directives that MUST be obeyed by all caches along the request/
962   response chain.  Such cache directives are unidirectional in that the
963   presence of a directive in a request does not imply that the same
964   directive is to be given in the response.
965
966      Note that HTTP/1.0 caches might not implement Cache-Control and
967      might only implement Pragma: no-cache (see Section 3.4).
968
969   Cache directives MUST be passed through by a proxy or gateway
970   application, regardless of their significance to that application,
971   since the directives might be applicable to all recipients along the
972   request/response chain.  It is not possible to target a directive to
973   a specific cache.
974
975     Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
976     Cache-Control-v = 1#cache-directive
977
978     cache-directive = cache-request-directive
979        / cache-response-directive
980
981     cache-extension = token [ "=" ( token / quoted-string ) ]
982
9833.2.1.  Request Cache-Control Directives
984
985     cache-request-directive =
986          "no-cache"
987        / "no-store"
988        / "max-age" "=" delta-seconds
989        / "max-stale" [ "=" delta-seconds ]
990        / "min-fresh" "=" delta-seconds
991        / "no-transform"
992        / "only-if-cached"
993        / cache-extension
994
995   no-cache
996
997      The no-cache request directive indicates that a stored response
998      MUST NOT be used to satisfy the request without successful
999      validation on the origin server.
1000
1001   no-store
1002
1003
1004
1005
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1011
1012      The no-store request directive indicates that a cache MUST NOT
1013      store any part of either this request or any response to it.  This
1014      directive applies to both non-shared and shared caches.  "MUST NOT
1015      store" in this context means that the cache MUST NOT intentionally
1016      store the information in non-volatile storage, and MUST make a
1017      best-effort attempt to remove the information from volatile
1018      storage as promptly as possible after forwarding it.
1019
1020      This directive is NOT a reliable or sufficient mechanism for
1021      ensuring privacy.  In particular, malicious or compromised caches
1022      might not recognize or obey this directive, and communications
1023      networks may be vulnerable to eavesdropping.
1024
1025   max-age
1026
1027      The max-age request directive indicates that the client is willing
1028      to accept a response whose age is no greater than the specified
1029      time in seconds.  Unless max-stale directive is also included, the
1030      client is not willing to accept a stale response.
1031
1032   max-stale
1033
1034      The max-stale request directive indicates that the client is
1035      willing to accept a response that has exceeded its expiration
1036      time.  If max-stale is assigned a value, then the client is
1037      willing to accept a response that has exceeded its expiration time
1038      by no more than the specified number of seconds.  If no value is
1039      assigned to max-stale, then the client is willing to accept a
1040      stale response of any age.  [[TODO-staleness: of any staleness?
1041      --mnot]]
1042
1043   min-fresh
1044
1045      The min-fresh request directive indicates that the client is
1046      willing to accept a response whose freshness lifetime is no less
1047      than its current age plus the specified time in seconds.  That is,
1048      the client wants a response that will still be fresh for at least
1049      the specified number of seconds.
1050
1051   no-transform
1052
1053      The no-transform request directive indicates that an intermediate
1054      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1055      or Content-Type request headers, nor the request entity-body.
1056
1057   only-if-cached
1058
1059
1060
1061
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1067
1068      The only-if-cached request directive indicates that the client
1069      only wishes to return a stored response.  If it receives this
1070      directive, a cache SHOULD either respond using a stored response
1071      that is consistent with the other constraints of the request, or
1072      respond with a 504 (Gateway Timeout) status.  If a group of caches
1073      is being operated as a unified system with good internal
1074      connectivity, such a request MAY be forwarded within that group of
1075      caches.
1076
10773.2.2.  Response Cache-Control Directives
1078
1079     cache-response-directive =
1080          "public"
1081        / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1082        / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1083        / "no-store"
1084        / "no-transform"
1085        / "must-revalidate"
1086        / "proxy-revalidate"
1087        / "max-age" "=" delta-seconds
1088        / "s-maxage" "=" delta-seconds
1089        / cache-extension
1090
1091   public
1092
1093      The public response directive indicates that the response MAY be
1094      cached, even if it would normally be non-cacheable or cacheable
1095      only within a non-shared cache.  (See also Authorization, Section
1096      3.1 of [Part7], for additional details.)
1097
1098   private
1099
1100      The private response directive indicates that the response message
1101      is intended for a single user and MUST NOT be stored by a shared
1102      cache.  A private (non-shared) cache MAY store the response.
1103
1104      If the private response directive specifies one or more field-
1105      names, this requirement is limited to the field-values associated
1106      with the listed response headers.  That is, the specified field-
1107      names(s) MUST NOT be stored by a shared cache, whereas the
1108      remainder of the response message MAY be.
1109
1110      Note: This usage of the word private only controls where the
1111      response may be stored, and cannot ensure the privacy of the
1112      message content.  Also, private response directives with field-
1113      names are often handled by implementations as if an unqualified
1114      private directive was received; i.e., the special handling for the
1115      qualified form is not widely implemented.
1116
1117
1118
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1122
1123
1124   no-cache
1125
1126      The no-cache response directive indicates that the response MUST
1127      NOT be used to satisfy a subsequent request without successful
1128      validation on the origin server.  This allows an origin server to
1129      prevent caching even by caches that have been configured to return
1130      stale responses.
1131
1132      If the no-cache response directive specifies one or more field-
1133      names, this requirement is limited to the field-values associated
1134      with the listed response headers.  That is, the specified field-
1135      name(s) MUST NOT be sent in the response to a subsequent request
1136      without successful validation on the origin server.  This allows
1137      an origin server to prevent the re-use of certain header fields in
1138      a response, while still allowing caching of the rest of the
1139      response.
1140
1141      Note: Most HTTP/1.0 caches will not recognize or obey this
1142      directive.  Also, no-cache response directives with field-names
1143      are often handled by implementations as if an unqualified no-cache
1144      directive was received; i.e., the special handling for the
1145      qualified form is not widely implemented.
1146
1147   no-store
1148
1149      The no-store response directive indicates that a cache MUST NOT
1150      store any part of either the immediate request or response.  This
1151      directive applies to both non-shared and shared caches.  "MUST NOT
1152      store" in this context means that the cache MUST NOT intentionally
1153      store the information in non-volatile storage, and MUST make a
1154      best-effort attempt to remove the information from volatile
1155      storage as promptly as possible after forwarding it.
1156
1157      This directive is NOT a reliable or sufficient mechanism for
1158      ensuring privacy.  In particular, malicious or compromised caches
1159      might not recognize or obey this directive, and communications
1160      networks may be vulnerable to eavesdropping.
1161
1162   must-revalidate
1163
1164      The must-revalidate response directive indicates that once it has
1165      become stale, the response MUST NOT be used to satisfy subsequent
1166      requests without successful validation on the origin server.
1167
1168      The must-revalidate directive is necessary to support reliable
1169      operation for certain protocol features.  In all circumstances an
1170      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1171      particular, if the cache cannot reach the origin server for any
1172
1173
1174
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1178
1179
1180      reason, it MUST generate a 504 (Gateway Timeout) response.
1181
1182      Servers SHOULD send the must-revalidate directive if and only if
1183      failure to validate a request on the entity could result in
1184      incorrect operation, such as a silently unexecuted financial
1185      transaction.
1186
1187   proxy-revalidate
1188
1189      The proxy-revalidate response directive has the same meaning as
1190      the must-revalidate response directive, except that it does not
1191      apply to non-shared caches.
1192
1193   max-age
1194
1195      The max-age response directive indicates that response is to be
1196      considered stale after its age is greater than the specified
1197      number of seconds.
1198
1199   s-maxage
1200
1201      The s-maxage response directive indicates that, in shared caches,
1202      the maximum age specified by this directive overrides the maximum
1203      age specified by either the max-age directive or the Expires
1204      header.  The s-maxage directive also implies the semantics of the
1205      proxy-revalidate response directive.
1206
1207   no-transform
1208
1209      The no-transform response directive indicates that an intermediate
1210      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1211      or Content-Type response headers, nor the response entity-body.
1212
12133.2.3.  Cache Control Extensions
1214
1215   The Cache-Control header field can be extended through the use of one
1216   or more cache-extension tokens, each with an optional value.
1217   Informational extensions (those that do not require a change in cache
1218   behavior) can be added without changing the semantics of other
1219   directives.  Behavioral extensions are designed to work by acting as
1220   modifiers to the existing base of cache directives.  Both the new
1221   directive and the standard directive are supplied, such that
1222   applications that do not understand the new directive will default to
1223   the behavior specified by the standard directive, and those that
1224   understand the new directive will recognize it as modifying the
1225   requirements associated with the standard directive.  In this way,
1226   extensions to the cache-control directives can be made without
1227   requiring changes to the base protocol.
1228
1229
1230
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1234
1235
1236   This extension mechanism depends on an HTTP cache obeying all of the
1237   cache-control directives defined for its native HTTP-version, obeying
1238   certain extensions, and ignoring all directives that it does not
1239   understand.
1240
1241   For example, consider a hypothetical new response directive called
1242   "community" that acts as a modifier to the private directive.  We
1243   define this new directive to mean that, in addition to any non-shared
1244   cache, any cache that is shared only by members of the community
1245   named within its value may cache the response.  An origin server
1246   wishing to allow the UCI community to use an otherwise private
1247   response in their shared cache(s) could do so by including
1248
1249     Cache-Control: private, community="UCI"
1250
1251   A cache seeing this header field will act correctly even if the cache
1252   does not understand the community cache-extension, since it will also
1253   see and understand the private directive and thus default to the safe
1254   behavior.
1255
1256   Unrecognized cache directives MUST be ignored; it is assumed that any
1257   cache directive likely to be unrecognized by an HTTP/1.1 cache will
1258   be combined with standard directives (or the response's default
1259   cacheability) such that the cache behavior will remain minimally
1260   correct even if the cache does not understand the extension(s).
1261
12623.3.  Expires
1263
1264   The "Expires" entity-header field gives the date/time after which the
1265   response is considered stale.  See Section 2.3 for further discussion
1266   of the freshness model.
1267
1268   The presence of an Expires field does not imply that the original
1269   resource will change or cease to exist at, before, or after that
1270   time.
1271
1272   The field-value is an absolute date and time as defined by HTTP-date
1273   in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format.
1274
1275     Expires   = "Expires" ":" OWS Expires-v
1276     Expires-v = HTTP-date
1277
1278   For example
1279
1280     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1281
1282
1283
1284
1285
1286
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1290
1291
1292      Note: If a response includes a Cache-Control field with the max-
1293      age directive (see Section 3.2.2), that directive overrides the
1294      Expires field.  Likewise, the s-maxage directive overrides Expires
1295      in shared caches.
1296
1297   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
1298   the future.
1299
1300   HTTP/1.1 clients and caches MUST treat other invalid date formats,
1301   especially including the value "0", as in the past (i.e., "already
1302   expired").
1303
13043.4.  Pragma
1305
1306   The "Pragma" general-header field is used to include implementation-
1307   specific directives that might apply to any recipient along the
1308   request/response chain.  All pragma directives specify optional
1309   behavior from the viewpoint of the protocol; however, some systems
1310   MAY require that behavior be consistent with the directives.
1311
1312     Pragma            = "Pragma" ":" OWS Pragma-v
1313     Pragma-v          = 1#pragma-directive
1314     pragma-directive  = "no-cache" / extension-pragma
1315     extension-pragma  = token [ "=" ( token / quoted-string ) ]
1316
1317   When the no-cache directive is present in a request message, an
1318   application SHOULD forward the request toward the origin server even
1319   if it has a cached copy of what is being requested.  This pragma
1320   directive has the same semantics as the no-cache response directive
1321   (see Section 3.2.2) and is defined here for backward compatibility
1322   with HTTP/1.0.  Clients SHOULD include both header fields when a no-
1323   cache request is sent to a server not known to be HTTP/1.1 compliant.
1324   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
1325   sent "Cache-Control: no-cache".
1326
1327      Note: Because the meaning of "Pragma: no-cache" as a response-
1328      header field is not actually specified, it does not provide a
1329      reliable replacement for "Cache-Control: no-cache" in a response.
1330
1331   This mechanism is deprecated; no new Pragma directives will be
1332   defined in HTTP.
1333
13343.5.  Vary
1335
1336   The "Vary" response-header field conveys the set of request-header
1337   fields that were used to select the representation.
1338
1339   Caches use this information, in part, to determine whether a stored
1340
1341
1342
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1346
1347
1348   response can be used to satisfy a given request; see Section 2.6.
1349   determines, while the response is fresh, whether a cache is permitted
1350   to use the response to reply to a subsequent request without
1351   validation; see Section 2.6.
1352
1353   In uncacheable or stale responses, the Vary field value advises the
1354   user agent about the criteria that were used to select the
1355   representation.
1356
1357     Vary   = "Vary" ":" OWS Vary-v
1358     Vary-v = "*" / 1#field-name
1359
1360   The set of header fields named by the Vary field value is known as
1361   the selecting request-headers.
1362
1363   Servers SHOULD include a Vary header field with any cacheable
1364   response that is subject to server-driven negotiation.  Doing so
1365   allows a cache to properly interpret future requests on that resource
1366   and informs the user agent about the presence of negotiation on that
1367   resource.  A server MAY include a Vary header field with a non-
1368   cacheable response that is subject to server-driven negotiation,
1369   since this might provide the user agent with useful information about
1370   the dimensions over which the response varies at the time of the
1371   response.
1372
1373   A Vary field value of "*" signals that unspecified parameters not
1374   limited to the request-headers (e.g., the network address of the
1375   client), play a role in the selection of the response representation;
1376   therefore, a cache cannot determine whether this response is
1377   appropriate.  The "*" value MUST NOT be generated by a proxy server;
1378   it may only be generated by an origin server.
1379
1380   The field-names given are not limited to the set of standard request-
1381   header fields defined by this specification.  Field names are case-
1382   insensitive.
1383
13843.6.  Warning
1385
1386   The "Warning" general-header field is used to carry additional
1387   information about the status or transformation of a message that
1388   might not be reflected in the message.  This information is typically
1389   used to warn about possible incorrectness introduced by caching
1390   operations or transformations applied to the entity body of the
1391   message.
1392
1393   Warnings can be used for other purposes, both cache-related and
1394   otherwise.  The use of a warning, rather than an error status code,
1395   distinguishes these responses from true failures.
1396
1397
1398
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1402
1403
1404   Warning headers can in general be applied to any message, however
1405   some warn-codes are specific to caches and can only be applied to
1406   response messages.
1407
1408     Warning    = "Warning" ":" OWS Warning-v
1409     Warning-v  = 1#warning-value
1410
1411     warning-value = warn-code SP warn-agent SP warn-text
1412                                           [SP warn-date]
1413
1414     warn-code  = 3DIGIT
1415     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1416                     ; the name or pseudonym of the server adding
1417                     ; the Warning header, for use in debugging
1418     warn-text  = quoted-string
1419     warn-date  = DQUOTE HTTP-date DQUOTE
1420
1421   Multiple warnings can be attached to a response (either by the origin
1422   server or by a cache), including multiple warnings with the same code
1423   number, only differing in warn-text.
1424
1425   When this occurs, the user agent SHOULD inform the user of as many of
1426   them as possible, in the order that they appear in the response.
1427
1428   Systems that generate multiple Warning headers SHOULD order them with
1429   this user agent behavior in mind.  New Warning headers SHOULD be
1430   added after any existing Warning headers.
1431
1432   Warnings are assigned three digit warn-codes.  The first digit
1433   indicates whether the Warning is required to be deleted from a stored
1434   response after validation:
1435
1436   o  1xx Warnings describe the freshness or validation status of the
1437      response, and so MUST be deleted by caches after validation.  They
1438      can only be generated by a cache when validating a cached entry,
1439      and MUST NOT be generated in any other situation.
1440
1441   o  2xx Warnings describe some aspect of the entity body or entity
1442      headers that is not rectified by a validation (for example, a
1443      lossy compression of the entity bodies) and MUST NOT be deleted by
1444      caches after validation, unless a full response is returned, in
1445      which case they MUST be.
1446
1447   If an implementation sends a message with one or more Warning headers
1448   to a receiver whose version is HTTP/1.0 or lower, then the sender
1449   MUST include in each warning-value a warn-date that matches the Date
1450   header in the message.
1451
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   The freshness model (Section 2.3) does not necessarily apply to
1530   history mechanisms.  I.e., a history mechanism can display a previous
1531   representation even if it has expired.
1532
1533   This does not prohibit the history mechanism from telling the user
1534   that a view might be stale, or from honoring cache directives (e.g.,
1535   Cache-Control: no-store).
1536
1537
15385.  IANA Considerations
1539
15405.1.  Message Header Registration
1541
1542   The Message Header Registry located at <http://www.iana.org/
1543   assignments/message-headers/message-header-index.html> should be
1544   updated with the permanent registrations below (see [RFC3864]):
1545
1546   +-------------------+----------+----------+-------------+
1547   | Header Field Name | Protocol | Status   | Reference   |
1548   +-------------------+----------+----------+-------------+
1549   | Age               | http     | standard | Section 3.1 |
1550   | Cache-Control     | http     | standard | Section 3.2 |
1551   | Expires           | http     | standard | Section 3.3 |
1552   | Pragma            | http     | standard | Section 3.4 |
1553   | Vary              | http     | standard | Section 3.5 |
1554   | Warning           | http     | standard | Section 3.6 |
1555   +-------------------+----------+----------+-------------+
1556
1557   The change controller is: "IETF (iesg@ietf.org) - Internet
1558   Engineering Task Force".
1559
1560
15616.  Security Considerations
1562
1563   Caches expose additional potential vulnerabilities, since the
1564
1565
1566
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1570
1571
1572   contents of the cache represent an attractive target for malicious
1573   exploitation.  Because cache contents persist after an HTTP request
1574   is complete, an attack on the cache can reveal information long after
1575   a user believes that the information has been removed from the
1576   network.  Therefore, cache contents should be protected as sensitive
1577   information.
1578
1579
15807.  Acknowledgments
1581
1582   Much of the content and presentation of the caching design is due to
1583   suggestions and comments from individuals including: Shel Kaphan,
1584   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
1585
1586
15878.  References
1588
15898.1.  Normative References
1590
1591   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1592              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1593              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
1594              and Message Parsing", draft-ietf-httpbis-p1-messaging-09
1595              (work in progress), March 2010.
1596
1597   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1598              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1599              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
1600              Semantics", draft-ietf-httpbis-p2-semantics-09 (work in
1601              progress), March 2010.
1602
1603   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1604              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1605              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
1606              Requests", draft-ietf-httpbis-p4-conditional-09 (work in
1607              progress), March 2010.
1608
1609   [Part5]    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 5: Range Requests and
1612              Partial Responses", draft-ietf-httpbis-p5-range-09 (work
1613              in progress), March 2010.
1614
1615   [Part7]    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 7: Authentication",
1618              draft-ietf-httpbis-p7-auth-09 (work in progress),
1619              March 2010.
1620
1621
1622
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1626
1627
1628   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1629              Requirement Levels", BCP 14, RFC 2119, March 1997.
1630
1631   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1632              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1633
16348.2.  Informative References
1635
1636   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
1637              Specification, Implementation", RFC 1305, March 1992.
1638
1639   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1640              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1641              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1642
1643   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1644              Procedures for Message Header Fields", BCP 90, RFC 3864,
1645              September 2004.
1646
1647
1648Appendix A.  Compatibility with Previous Versions
1649
1650A.1.  Changes from RFC 2068
1651
1652   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
1653   was introduced to add this missing case.  (Sections 2.1, 3.2).
1654
1655   Range request responses would become very verbose if all meta-data
1656   were always returned; by allowing the server to only send needed
1657   headers in a 206 response, this problem can be avoided.
1658   (Section 2.7)
1659
1660   The Cache-Control: max-age directive was not properly defined for
1661   responses.  (Section 3.2.2)
1662
1663   Warnings could be cached incorrectly, or not updated appropriately.
1664   (Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general
1665   header, as PUT or other methods may have need for it in requests.
1666
1667A.2.  Changes from RFC 2616
1668
1669   Remove requirement to consider Content-Location in successful
1670   responses in order to determine the appropriate response to use.
1671   (Section 2.4)
1672
1673   Clarify denial of service attack avoidance requirement.
1674   (Section 2.5)
1675
1676
1677
1678
1679Fielding, et al.        Expires September 9, 2010              [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                  March 2010
1682
1683
1684   Do not mention RFC 2047 encoding and multiple languages in Warning
1685   headers anymore, as these aspects never were implemented.
1686   (Section 3.6)
1687
1688
1689Appendix B.  Collected ABNF
1690
1691   Age = "Age:" OWS Age-v
1692   Age-v = delta-seconds
1693
1694   Cache-Control = "Cache-Control:" OWS Cache-Control-v
1695   Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
1696    cache-directive ] )
1697
1698   Expires = "Expires:" OWS Expires-v
1699   Expires-v = HTTP-date
1700
1701   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1702
1703   OWS = <OWS, defined in [Part1], Section 1.2.2>
1704
1705   Pragma = "Pragma:" OWS Pragma-v
1706   Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1707    pragma-directive ] )
1708
1709   Vary = "Vary:" OWS Vary-v
1710   Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1711    ] ) )
1712
1713   Warning = "Warning:" OWS Warning-v
1714   Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1715    ] )
1716
1717   cache-directive = cache-request-directive / cache-response-directive
1718   cache-extension = token [ "=" ( token / quoted-string ) ]
1719   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
1720    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1721    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1722    cache-extension
1723   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
1724    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1725    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1726    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1727    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1728    ) / ( "s-maxage=" delta-seconds ) / cache-extension
1729
1730   delta-seconds = 1*DIGIT
1731
1732
1733
1734
1735Fielding, et al.        Expires September 9, 2010              [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                  March 2010
1738
1739
1740   extension-pragma = token [ "=" ( token / quoted-string ) ]
1741
1742   field-name = <field-name, defined in [Part1], Section 3.2>
1743
1744   port = <port, defined in [Part1], Section 2.6>
1745   pragma-directive = "no-cache" / extension-pragma
1746   pseudonym = <pseudonym, defined in [Part1], Section 9.9>
1747
1748   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1749
1750   token = <token, defined in [Part1], Section 1.2.2>
1751
1752   uri-host = <uri-host, defined in [Part1], Section 2.6>
1753
1754   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1755   warn-code = 3DIGIT
1756   warn-date = DQUOTE HTTP-date DQUOTE
1757   warn-text = quoted-string
1758   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
1759    ]
1760
1761   ABNF diagnostics:
1762
1763   ; Age defined but not used
1764   ; Cache-Control defined but not used
1765   ; Expires defined but not used
1766   ; Pragma defined but not used
1767   ; Vary defined but not used
1768   ; Warning defined but not used
1769
1770
1771Appendix C.  Change Log (to be removed by RFC Editor before publication)
1772
1773C.1.  Since RFC2616
1774
1775   Extracted relevant partitions from [RFC2616].
1776
1777C.2.  Since draft-ietf-httpbis-p6-cache-00
1778
1779   Closed issues:
1780
1781   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
1782      (<http://purl.org/NET/http-errata#trailer-hop>)
1783
1784   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
1785      after Update or Delete"
1786      (<http://purl.org/NET/http-errata#invalidupd>)
1787
1788
1789
1790
1791Fielding, et al.        Expires September 9, 2010              [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                  March 2010
1794
1795
1796   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1797      Informative references"
1798
1799   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
1800      typo"
1801
1802   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
1803      header text"
1804
1805   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
1806      references"
1807
1808   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
1809      Reference"
1810
1811   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
1812      to-date references"
1813
1814   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
1815      13.2.2"
1816
1817   Other changes:
1818
1819   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
1820      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1821
1822C.3.  Since draft-ietf-httpbis-p6-cache-01
1823
1824   Closed issues:
1825
1826   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
1827      used"
1828
1829   Other changes:
1830
1831   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
1832      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1833
1834   o  Add explicit references to BNF syntax and rules imported from
1835      other parts of the specification.
1836
1837C.4.  Since draft-ietf-httpbis-p6-cache-02
1838
1839   Ongoing work on IANA Message Header Registration
1840   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1841
1842   o  Reference RFC 3984, and update header registrations for headers
1843      defined in this document.
1844
1845
1846
1847Fielding, et al.        Expires September 9, 2010              [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                  March 2010
1850
1851
1852C.5.  Since draft-ietf-httpbis-p6-cache-03
1853
1854   Closed issues:
1855
1856   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
1857      classification"
1858
1859C.6.  Since draft-ietf-httpbis-p6-cache-04
1860
1861   Ongoing work on ABNF conversion
1862   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1863
1864   o  Use "/" instead of "|" for alternatives.
1865
1866   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1867      whitespace ("OWS") and required whitespace ("RWS").
1868
1869   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1870      value format definitions.
1871
1872C.7.  Since draft-ietf-httpbis-p6-cache-05
1873
1874   This is a total rewrite of this part of the specification.
1875
1876   Affected issues:
1877
1878   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1879      1xx Warn-Codes"
1880
1881   o  <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement
1882      of 13.5.1 and 13.5.2"
1883
1884   o  <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role
1885      of Warning and Semantic Transparency in Caching"
1886
1887   o  <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods
1888      and Caching"
1889
1890   In addition: Final work on ABNF conversion
1891   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1892
1893   o  Add appendix containing collected and expanded ABNF, reorganize
1894      ABNF introduction.
1895
1896
1897
1898
1899
1900
1901
1902
1903Fielding, et al.        Expires September 9, 2010              [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                  March 2010
1906
1907
1908C.8.  Since draft-ietf-httpbis-p6-cache-06
1909
1910   Closed issues:
1911
1912   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
1913      numeric protocol elements"
1914
1915   Affected issues:
1916
1917   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: Vary and non-
1918      existant headers
1919
1920C.9.  Since draft-ietf-httpbis-p6-cache-07
1921
1922   Closed issues:
1923
1924   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1925      1xx Warn-Codes"
1926
1927   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
1928      Location on 304 responses"
1929
1930   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
1931      no-cache CC directives with headers"
1932
1933   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
1934      warn-text"
1935
1936C.10.  Since draft-ietf-httpbis-p6-cache-08
1937
1938   Closed issues:
1939
1940   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
1941      negotiated responses from cache: header-specific canonicalization"
1942
1943   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
1944      directives on history lists"
1945
1946   Affected issues:
1947
1948   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
1949      and caching
1950
1951   Partly resolved issues:
1952
1953   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
1954      13.5.1 and 13.5.2"
1955
1956
1957
1958
1959Fielding, et al.        Expires September 9, 2010              [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                  March 2010
1962
1963
1964Index
1965
1966   A
1967      age  6
1968      Age header  17
1969
1970   C
1971      cache  5
1972      Cache Directives
1973         max-age  19, 22
1974         max-stale  19
1975         min-fresh  19
1976         must-revalidate  21
1977         no-cache  18, 21
1978         no-store  18, 21
1979         no-transform  19, 22
1980         only-if-cached  19
1981         private  20
1982         proxy-revalidate  22
1983         public  20
1984         s-maxage  22
1985      Cache-Control header  18
1986      cacheable  5
1987
1988   E
1989      Expires header  23
1990      explicit expiration time  5
1991
1992   F
1993      first-hand  6
1994      fresh  6
1995      freshness lifetime  6
1996
1997   G
1998      Grammar
1999         Age  17
2000         Age-v  17
2001         Cache-Control  18
2002         Cache-Control-v  18
2003         cache-extension  18
2004         cache-request-directive  18
2005         cache-response-directive  20
2006         delta-seconds  17
2007         Expires  23
2008         Expires-v  23
2009         extension-pragma  24
2010         Pragma  24
2011         pragma-directive  24
2012
2013
2014
2015Fielding, et al.        Expires September 9, 2010              [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                  March 2010
2018
2019
2020         Pragma-v  24
2021         Vary  25
2022         Vary-v  25
2023         warn-agent  26
2024         warn-code  26
2025         warn-date  26
2026         warn-text  26
2027         Warning  26
2028         Warning-v  26
2029         warning-value  26
2030
2031   H
2032      Headers
2033         Age  17
2034         Cache-Control  18
2035         Expires  23
2036         Pragma  24
2037         Vary  24
2038         Warning  25
2039      heuristic expiration time  5
2040
2041   M
2042      max-age
2043         Cache Directive  19, 22
2044      max-stale
2045         Cache Directive  19
2046      min-fresh
2047         Cache Directive  19
2048      must-revalidate
2049         Cache Directive  21
2050
2051   N
2052      no-cache
2053         Cache Directive  18, 21
2054      no-store
2055         Cache Directive  18, 21
2056      no-transform
2057         Cache Directive  19, 22
2058
2059   O
2060      only-if-cached
2061         Cache Directive  19
2062
2063   P
2064      Pragma header  24
2065      private
2066         Cache Directive  20
2067      proxy-revalidate
2068
2069
2070
2071Fielding, et al.        Expires September 9, 2010              [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                  March 2010
2074
2075
2076         Cache Directive  22
2077      public
2078         Cache Directive  20
2079
2080   S
2081      s-maxage
2082         Cache Directive  22
2083      stale  6
2084
2085   V
2086      validator  6
2087      Vary header  24
2088
2089   W
2090      Warning header  25
2091
2092
2093Authors' Addresses
2094
2095   Roy T. Fielding (editor)
2096   Day Software
2097   23 Corporate Plaza DR, Suite 280
2098   Newport Beach, CA  92660
2099   USA
2100
2101   Phone: +1-949-706-5300
2102   Fax:   +1-949-706-5305
2103   Email: fielding@gbiv.com
2104   URI:   http://roy.gbiv.com/
2105
2106
2107   Jim Gettys
2108   One Laptop per Child
2109   21 Oak Knoll Road
2110   Carlisle, MA  01741
2111   USA
2112
2113   Email: jg@laptop.org
2114   URI:   http://www.laptop.org/
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127Fielding, et al.        Expires September 9, 2010              [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                  March 2010
2130
2131
2132   Jeffrey C. Mogul
2133   Hewlett-Packard Company
2134   HP Labs, Large Scale Systems Group
2135   1501 Page Mill Road, MS 1177
2136   Palo Alto, CA  94304
2137   USA
2138
2139   Email: JeffMogul@acm.org
2140
2141
2142   Henrik Frystyk Nielsen
2143   Microsoft Corporation
2144   1 Microsoft Way
2145   Redmond, WA  98052
2146   USA
2147
2148   Email: henrikn@microsoft.com
2149
2150
2151   Larry Masinter
2152   Adobe Systems, Incorporated
2153   345 Park Ave
2154   San Jose, CA  95110
2155   USA
2156
2157   Email: LMM@acm.org
2158   URI:   http://larry.masinter.net/
2159
2160
2161   Paul J. Leach
2162   Microsoft Corporation
2163   1 Microsoft Way
2164   Redmond, WA  98052
2165
2166   Email: paulle@microsoft.com
2167
2168
2169   Tim Berners-Lee
2170   World Wide Web Consortium
2171   MIT Computer Science and Artificial Intelligence Laboratory
2172   The Stata Center, Building 32
2173   32 Vassar Street
2174   Cambridge, MA  02139
2175   USA
2176
2177   Email: timbl@w3.org
2178   URI:   http://www.w3.org/People/Berners-Lee/
2179
2180
2181
2182
2183Fielding, et al.        Expires September 9, 2010              [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                  March 2010
2186
2187
2188   Yves Lafon (editor)
2189   World Wide Web Consortium
2190   W3C / ERCIM
2191   2004, rte des Lucioles
2192   Sophia-Antipolis, AM  06902
2193   France
2194
2195   Email: ylafon@w3.org
2196   URI:   http://www.raubacapeu.net/people/yves/
2197
2198
2199   Mark Nottingham (editor)
2200
2201   Email: mnot@mnot.net
2202   URI:   http://www.mnot.net/
2203
2204
2205   Julian F. Reschke (editor)
2206   greenbytes GmbH
2207   Hafenweg 16
2208   Muenster, NW  48155
2209   Germany
2210
2211   Phone: +49 251 2807760
2212   Fax:   +49 251 2807761
2213   Email: julian.reschke@greenbytes.de
2214   URI:   http://greenbytes.de/tech/webdav/
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 9, 2010              [Page 40]
2240
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