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4HTTPbis Working Group                                   R. Fielding, Ed.
5Internet-Draft                                              Day Software
6Obsoletes: 2616 (if approved)                                  J. Gettys
7Intended status: Standards Track                          Alcatel-Lucent
8Expires: April 28, 2011                                         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 25, 2010
25
26
27                       HTTP/1.1, part 6: Caching
28                     draft-ietf-httpbis-p6-cache-12
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/3> 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.13.
49
50Status of This Memo
51
52
53
54
55Fielding, et al.         Expires April 28, 2011                 [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 6                October 2010
58
59
60   This Internet-Draft is submitted 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).  Note that other groups may also distribute
65   working documents as Internet-Drafts.  The list of current Internet-
66   Drafts is at http://datatracker.ietf.org/drafts/current/.
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   This Internet-Draft will expire on April 28, 2011.
74
75Copyright Notice
76
77   Copyright (c) 2010 IETF Trust and the persons identified as the
78   document authors.  All rights reserved.
79
80   This document is subject to BCP 78 and the IETF Trust's Legal
81   Provisions Relating to IETF Documents
82   (http://trustee.ietf.org/license-info) in effect on the date of
83   publication of this document.  Please review these documents
84   carefully, as they describe your rights and restrictions with respect
85   to this document.  Code Components extracted from this document must
86   include Simplified BSD License text as described in Section 4.e of
87   the Trust Legal Provisions and are provided without warranty as
88   described in the Simplified BSD License.
89
90   This document may contain material from IETF Documents or IETF
91   Contributions published or made publicly available before November
92   10, 2008.  The person(s) controlling the copyright in some of this
93   material may not have granted the IETF Trust the right to allow
94   modifications of such material outside the IETF Standards Process.
95   Without obtaining an adequate license from the person(s) controlling
96   the copyright in such materials, this document may not be modified
97   outside the IETF Standards Process, and derivative works of it may
98   not be created outside the IETF Standards Process, except to format
99   it for publication as an RFC or to translate it into languages other
100   than English.
101
102Table of Contents
103
104   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
105     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
106     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
107     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
108
109
110
111Fielding, et al.         Expires April 28, 2011                 [Page 2]
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113Internet-Draft              HTTP/1.1, Part 6                October 2010
114
115
116     1.4.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
117       1.4.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7
118       1.4.2.  ABNF Rules defined in other Parts of the
119               Specification  . . . . . . . . . . . . . . . . . . . .  7
120   2.  Cache Operation  . . . . . . . . . . . . . . . . . . . . . . .  7
121     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  7
122       2.1.1.  Storing Partial and Incomplete Responses . . . . . . .  8
123     2.2.  Constructing Responses from Caches . . . . . . . . . . . .  9
124     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 10
125       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 11
126       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 12
127       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 13
128     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 14
129     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 15
130     2.6.  Shared Caching of Authenticated Responses  . . . . . . . . 15
131     2.7.  Caching Negotiated Responses . . . . . . . . . . . . . . . 16
132     2.8.  Combining Responses  . . . . . . . . . . . . . . . . . . . 17
133   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 17
134     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
135     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 18
136       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 19
137       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 20
138       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 23
139     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 24
140     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 25
141     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
142     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 26
143   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 29
144   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
145     5.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 29
146     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 30
147   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
148   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
149   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
150     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 30
151     8.2.  Informative References . . . . . . . . . . . . . . . . . . 31
152   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 32
153   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 32
154   Appendix C.  Change Log (to be removed by RFC Editor before
155                publication)  . . . . . . . . . . . . . . . . . . . . 33
156     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 34
157     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 34
158     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
159     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 35
160     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 35
161     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35
162     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
163     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 36
164
165
166
167Fielding, et al.         Expires April 28, 2011                 [Page 3]
168
169Internet-Draft              HTTP/1.1, Part 6                October 2010
170
171
172     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36
173     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 36
174     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 37
175     C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 37
176     C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 38
177   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
178
179
180
181
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
223Fielding, et al.         Expires April 28, 2011                 [Page 4]
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225Internet-Draft              HTTP/1.1, Part 6                October 2010
226
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 employ a cache, though a cache cannot be used by
242   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 might 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 a representation
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 has an
318      equivalent copy of a representation.
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
3802.  Cache Operation
381
3822.1.  Response Cacheability
383
384   A cache MUST NOT store a response to any request, unless:
385
386   o  The request method is understood by the cache and defined as being
387      cacheable, and
388
389
390
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395
396   o  the response status code is understood by the cache, and
397
398   o  the "no-store" cache directive (see Section 3.2) does not appear
399      in request or response header fields, and
400
401   o  the "private" cache response directive (see Section 3.2.2 does not
402      appear in the response, if the cache is shared, and
403
404   o  the "Authorization" header field (see Section 4.1 of [Part7]) does
405      not appear in the request, if the cache is shared, unless the
406      response explicitly allows it (see Section 2.6), and
407
408   o  the response either:
409
410      *  contains an Expires header field (see Section 3.3), or
411
412      *  contains a max-age response cache directive (see
413         Section 3.2.2), or
414
415      *  contains a s-maxage response cache directive and the cache is
416         shared, or
417
418      *  contains a Cache Control Extension (see Section 3.2.3) that
419         allows it to be cached, or
420
421      *  has a status code that can be served with heuristic freshness
422         (see Section 2.3.1.1).
423
424   In this context, a cache has "understood" a request method or a
425   response status code if it recognises it and implements any cache-
426   specific behaviour.  In particular, 206 Partial Content responses
427   cannot be cached by an implementation that does not handle partial
428   content (see Section 2.1.1).
429
430   Note that in normal operation, most caches will not store a response
431   that has neither a cache validator nor an explicit expiration time,
432   as such responses are not usually useful to store.  However, caches
433   are not prohibited from storing such responses.
434
4352.1.1.  Storing Partial and Incomplete Responses
436
437   A cache that receives an incomplete response (for example, with fewer
438   bytes of data than specified in a Content-Length header field) can
439   store the response, but MUST treat it as a partial response [Part5].
440   Partial responses can be combined as described in Section 4 of
441   [Part5]; the result might be a full response or might still be
442   partial.  A cache MUST NOT return a partial response to a client
443   without explicitly marking it as such using the 206 (Partial Content)
444
445
446
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451
452   status code.
453
454   A cache that does not support the Range and Content-Range header
455   fields MUST NOT store incomplete or partial responses.
456
4572.2.  Constructing Responses from Caches
458
459   For a presented request, a cache MUST NOT return a stored response,
460   unless:
461
462   o  The presented effective request URI (Section 4.3 of [Part1]) and
463      that of the stored response match, and
464
465   o  the request method associated with the stored response allows it
466      to be used for the presented request, and
467
468   o  selecting request-header fields nominated by the stored response
469      (if any) match those presented (see Section 2.7), and
470
471   o  the presented request and stored response are free from directives
472      that would prevent its use (see Section 3.2 and Section 3.4), and
473
474   o  the stored response is either:
475
476      *  fresh (see Section 2.3), or
477
478      *  allowed to be served stale (see Section 2.3.3), or
479
480      *  successfully validated (see Section 2.4).
481
482   When a stored response is used to satisfy a request without
483   validation, caches MUST include a single Age header field
484   (Section 3.1) in the response with a value equal to the stored
485   response's current_age; see Section 2.3.2.
486
487   Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
488   be written through the cache to the origin server; i.e., a cache must
489   not reply to such a request before having forwarded the request and
490   having received a corresponding response.
491
492   Also, note that unsafe requests might invalidate already stored
493   responses; see Section 2.5.
494
495   Caches MUST use the most recent response (as determined by the Date
496   header field) when more than one suitable response is stored.  They
497   can also forward a request with "Cache-Control: max-age=0" or "Cache-
498   Control: no-cache" to disambiguate which response to use.
499
500
501
502
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507
508   An HTTP implementation without a clock MUST NOT used stored responses
509   without revalidating them on every use.  An HTTP cache, especially a
510   shared cache, SHOULD use a mechanism, such as NTP [RFC1305], to
511   synchronize its clock with a reliable external standard.
512
5132.3.  Freshness Model
514
515   When a response is "fresh" in the cache, it can be used to satisfy
516   subsequent requests without contacting the origin server, thereby
517   improving efficiency.
518
519   The primary mechanism for determining freshness is for an origin
520   server to provide an explicit expiration time in the future, using
521   either the Expires header field (Section 3.3) or the max-age response
522   cache directive (Section 3.2.2).  Generally, origin servers will
523   assign future explicit expiration times to responses in the belief
524   that the representation is not likely to change in a semantically
525   significant way before the expiration time is reached.
526
527   If an origin server wishes to force a cache to validate every
528   request, it can assign an explicit expiration time in the past to
529   indicate that the response is already stale.  Compliant caches will
530   validate the cached response before reusing it for subsequent
531   requests.
532
533   Since origin servers do not always provide explicit expiration times,
534   HTTP caches MAY assign heuristic expiration times when explicit times
535   are not specified, employing algorithms that use other heade field
536   values (such as the Last-Modified time) to estimate a plausible
537   expiration time.  The HTTP/1.1 specification does not provide
538   specific algorithms, but does impose worst-case constraints on their
539   results.
540
541   The calculation to determine if a response is fresh is:
542
543      response_is_fresh = (freshness_lifetime > current_age)
544
545   The freshness_lifetime is defined in Section 2.3.1; the current_age
546   is defined in Section 2.3.2.
547
548   Additionally, clients might need to influence freshness calculation.
549   They can do this using several request cache directives, with the
550   effect of either increasing or loosening constraints on freshness.
551   See Section 3.2.1.
552
553   [[ISSUE-no-req-for-directives: there are not requirements directly
554   applying to cache-request-directives and freshness.]]
555
556
557
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563
564   Note that freshness applies only to cache operation; it cannot be
565   used to force a user agent to refresh its display or reload a
566   resource.  See Section 4 for an explanation of the difference between
567   caches and history mechanisms.
568
5692.3.1.  Calculating Freshness Lifetime
570
571   A cache can calculate the freshness lifetime (denoted as
572   freshness_lifetime) of a response by using the first match of:
573
574   o  If the cache is shared and the s-maxage response cache directive
575      (Section 3.2.2) is present, use its value, or
576
577   o  If the max-age response cache directive (Section 3.2.2) is
578      present, use its value, or
579
580   o  If the Expires response header field (Section 3.3) is present, use
581      its value minus the value of the Date response header field, or
582
583   o  Otherwise, no explicit expiration time is present in the response.
584      A heuristic freshness lifetime might be applicable; see
585      Section 2.3.1.1.
586
587   Note that this calculation is not vulnerable to clock skew, since all
588   of the information comes from the origin server.
589
5902.3.1.1.  Calculating Heuristic Freshness
591
592   If no explicit expiration time is present in a stored response that
593   has a status code whose definition allows heuristic freshness to be
594   used (including the following in Section 8 of [Part2]: 200, 203, 206,
595   300, 301 and 410), a heuristic expiration time MAY be calculated.
596   Heuristics MUST NOT be used for response status codes that do not
597   explicitly allow it.
598
599   When a heuristic is used to calculate freshness lifetime, the cache
600   SHOULD attach a Warning header field with a 113 warn-code to the
601   response if its current_age is more than 24 hours and such a warning
602   is not already present.
603
604   Also, if the response has a Last-Modified header field (Section 6.6
605   of [Part4]), the heuristic expiration value SHOULD be no more than
606   some fraction of the interval since that time.  A typical setting of
607   this fraction might be 10%.
608
609      Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
610      not calculate heuristic freshness for URLs with query components
611      (i.e., those containing '?').  In practice, this has not been
612
613
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619
620      widely implemented.  Therefore, servers are encouraged to send
621      explicit directives (e.g., Cache-Control: no-cache) if they wish
622      to preclude caching.
623
6242.3.2.  Calculating Age
625
626   HTTP/1.1 uses the Age response-header field to convey the estimated
627   age of the response message when obtained from a cache.  The Age
628   field value is the cache's estimate of the amount of time since the
629   response was generated or validated by the origin server.  In
630   essence, the Age value is the sum of the time that the response has
631   been resident in each of the caches along the path from the origin
632   server, plus the amount of time it has been in transit along network
633   paths.
634
635   The following data is used for the age calculation:
636
637   age_value
638
639      The term "age_value" denotes the value of the Age header field
640      (Section 3.1), in a form appropriate for arithmetic operation; or
641      0, if not available.
642
643   date_value
644
645      HTTP/1.1 requires origin servers to send a Date header field, if
646      possible, with every response, giving the time at which the
647      response was generated.  The term "date_value" denotes the value
648      of the Date header field, in a form appropriate for arithmetic
649      operations.  See Section 9.3 of [Part1] for the definition of the
650      Date header field, and for requirements regarding responses
651      without it.
652
653   now
654
655      The term "now" means "the current value of the clock at the host
656      performing the calculation".  Hosts that use HTTP, but especially
657      hosts running origin servers and caches, SHOULD use NTP
658      ([RFC1305]) or some similar protocol to synchronize their clocks
659      to a globally accurate time standard.
660
661   request_time
662
663      The current value of the clock at the host at the time the request
664      resulting in the stored response was made.
665
666
667
668
669
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675
676   response_time
677
678      The current value of the clock at the host at the time the
679      response was received.
680
681   A response's age can be calculated in two entirely independent ways:
682
683   1.  the "apparent_age": response_time minus date_value, if the local
684       clock is reasonably well synchronized to the origin server's
685       clock.  If the result is negative, the result is replaced by
686       zero.
687
688   2.  the "corrected_age_value", if all of the caches along the
689       response path implement HTTP/1.1; note this value MUST be
690       interpreted relative to the time the request was initiated, not
691       the time that the response was received.
692
693
694     apparent_age = max(0, response_time - date_value);
695
696     response_delay = response_time - request_time;
697     corrected_age_value = age_value + response_delay;
698
699   These are combined as
700
701     corrected_initial_age = max(apparent_age, corrected_age_value);
702
703   The current_age of a stored response can then be calculated by adding
704   the amount of time (in seconds) since the stored response was last
705   validated by the origin server to the corrected_initial_age.
706
707     resident_time = now - response_time;
708     current_age = corrected_initial_age + resident_time;
709
7102.3.3.  Serving Stale Responses
711
712   A "stale" response is one that either has explicit expiry information
713   or is allowed to have heuristic expiry calculated, but is not fresh
714   according to the calculations in Section 2.3.
715
716   Caches MUST NOT return a stale response if it is prohibited by an
717   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
718   cache directive, a "must-revalidate" cache-response-directive, or an
719   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
720   see Section 3.2.2).
721
722   Caches SHOULD NOT return stale responses unless they are disconnected
723   (i.e., it cannot contact the origin server or otherwise find a
724
725
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731
732   forward path) or otherwise explicitly allowed (e.g., the max-stale
733   request directive; see Section 3.2.1).
734
735   Stale responses SHOULD have a Warning header field with the 110 warn-
736   code (see Section 3.6).  Likewise, the 112 warn-code SHOULD be sent
737   on stale responses if the cache is disconnected.
738
739   If a cache receives a first-hand response (either an entire response,
740   or a 304 (Not Modified) response) that it would normally forward to
741   the requesting client, and the received response is no longer fresh,
742   the cache SHOULD forward it to the requesting client without adding a
743   new Warning (but without removing any existing Warning header
744   fields).  A cache SHOULD NOT attempt to validate a response simply
745   because that response became stale in transit.
746
7472.4.  Validation Model
748
749   When a cache has one or more stored responses for a requested URI,
750   but cannot serve any of them (e.g., because they are not fresh, or
751   one cannot be selected; see Section 2.7), it can use the conditional
752   request mechanism [Part4] in the forwarded request to give the origin
753   server an opportunity to both select a valid stored response to be
754   used, and to update it.  This process is known as "validating" or
755   "revalidating" the stored response.
756
757   When sending such a conditional request, the cache SHOULD add an If-
758   Modified-Since header field whose value is that of the Last-Modified
759   header field from the selected (see Section 2.7) stored response, if
760   available.
761
762   Additionally, the cache SHOULD add an If-None-Match header field
763   whose value is that of the ETag header field(s) from all responses
764   stored for the requested URI, if present.  However, if any of the
765   stored responses contains only partial content, its entity-tag SHOULD
766   NOT be included in the If-None-Match header field unless the request
767   is for a range that would be fully satisfied by that stored response.
768
769   A 304 (Not Modified) response status code indicates that the stored
770   response can be updated and reused; see Section 2.8.
771
772   A full response (i.e., one with a response body) indicates that none
773   of the stored responses nominated in the conditional request is
774   suitable.  Instead, the full response SHOULD be used to satisfy the
775   request and MAY replace the stored response.
776
777   If a cache receives a 5xx response while attempting to validate a
778   response, it MAY either forward this response to the requesting
779   client, or act as if the server failed to respond.  In the latter
780
781
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787
788   case, it MAY return a previously stored response (see Section 2.3.3).
789
7902.5.  Request Methods that Invalidate
791
792   Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
793   for changing state on the origin server, intervening caches can use
794   them to keep their contents up-to-date.
795
796   The following HTTP methods MUST cause a cache to invalidate the
797   effective Request URI (Section 4.3 of [Part1]) as well as the URI(s)
798   in the Location and Content-Location header fields (if present):
799
800   o  PUT
801
802   o  DELETE
803
804   o  POST
805
806   An invalidation based on a URI from a Location or Content-Location
807   header field MUST NOT be performed if the host part of that URI
808   differs from the host part in the effective request URI (Section 4.3
809   of [Part1]).  This helps prevent denial of service attacks.
810
811   A cache that passes through requests for methods it does not
812   understand SHOULD invalidate the effective request URI (Section 4.3
813   of [Part1]).
814
815   Here, "invalidate" means that the cache will either remove all stored
816   responses related to the effective request URI, or will mark these as
817   "invalid" and in need of a mandatory validation before they can be
818   returned in response to a subsequent request.
819
820   Note that this does not guarantee that all appropriate responses are
821   invalidated.  For example, the request that caused the change at the
822   origin server might not have gone through the cache where a response
823   is stored.
824
8252.6.  Shared Caching of Authenticated Responses
826
827   Shared caches MUST NOT use a cached response to a request with an
828   Authorization header field (Section 4.1 of [Part7]) to satisfy any
829   subsequent request unless a cache directive that allows such
830   responses to be stored is present in the response.
831
832   In this specification, the following Cache-Control response
833   directives (Section 3.2.2) have such an effect: must-revalidate,
834   public, s-maxage.
835
836
837
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843
844   Note that cached responses that contain the "must-revalidate" and/or
845   "s-maxage" response directives are not allowed to be served stale
846   (Section 2.3.3) by shared caches.  In particular, a response with
847   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
848   satisfy a subsequent request without revalidating it on the origin
849   server.
850
8512.7.  Caching Negotiated Responses
852
853   When a cache receives a request that can be satisfied by a stored
854   response that has a Vary header field (Section 3.5), it MUST NOT use
855   that response unless all of the selecting request-header fields
856   nominated by the Vary header field match in both the original request
857   (i.e., that associated with the stored response), and the presented
858   request.
859
860   The selecting request-header fields from two requests are defined to
861   match if and only if those in the first request can be transformed to
862   those in the second request by applying any of the following:
863
864   o  adding or removing whitespace, where allowed in the header field's
865      syntax
866
867   o  combining multiple message-header fields with the same field name
868      (see Section 3.2 of [Part1])
869
870   o  normalizing both header field values in a way that is known to
871      have identical semantics, according to the header field's
872      specification (e.g., re-ordering field values when order is not
873      significant; case-normalization, where values are defined to be
874      case-insensitive)
875
876   If (after any normalization that might take place) a header field is
877   absent from a request, it can only match another request if it is
878   also absent there.
879
880   A Vary header field-value of "*" always fails to match, and
881   subsequent requests to that resource can only be properly interpreted
882   by the origin server.
883
884   The stored response with matching selecting request-header fields is
885   known as the selected response.
886
887   If no selected response is available, the cache MAY forward the
888   presented request to the origin server in a conditional request; see
889   Section 2.4.
890
891
892
893
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899
9002.8.  Combining Responses
901
902   When a cache receives a 304 (Not Modified) response or a 206 (Partial
903   Content) response (in this section, the "new" response"), it needs to
904   created an updated response by combining the stored response with the
905   new one, so that the updated response can be used to satisfy the
906   request, and potentially update the cached response.
907
908   If the new response contains an ETag, it identifies the stored
909   response to use.  [[TODO-mention-CL: might need language about
910   Content-Location here]][[TODO-select-for-combine: Shouldn't this be
911   the selected response?]]
912
913   If the new response's status code is 206 (partial content), both the
914   stored and new responses MUST have validators, and those validators
915   MUST match using the strong comparison function (see Section 4 of
916   [Part4]).  Otherwise, the responses MUST NOT be combined.
917
918   The stored response header fields are used as those of the updated
919   response, except that
920
921   o  any stored Warning header fields with warn-code 1xx (see
922      Section 3.6) MUST be deleted.
923
924   o  any stored Warning header fields with warn-code 2xx MUST be
925      retained.
926
927   o  any other header fields provided in the new response MUST replace
928      all instances of the corresponding header fields from the stored
929      response.
930
931   The updated response header fields MUST be used to replace those of
932   the stored response in cache (unless the stored response is removed
933   from cache).  In the case of a 206 response, the combined
934   representation MAY be stored.
935
9363.  Header Field Definitions
937
938   This section defines the syntax and semantics of HTTP/1.1 header
939   fields related to caching.
940
9413.1.  Age
942
943   The "Age" response-header field conveys the sender's estimate of the
944   amount of time since the response was generated or successfully
945   validated at the origin server.  Age values are calculated as
946   specified in Section 2.3.2.
947
948
949
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955
956     Age   = "Age" ":" OWS Age-v
957     Age-v = delta-seconds
958
959   Age field-values are non-negative integers, representing time in
960   seconds.
961
962     delta-seconds  = 1*DIGIT
963
964   If a cache receives a value larger than the largest positive integer
965   it can represent, or if any of its age calculations overflows, it
966   MUST transmit an Age header field with a field-value of 2147483648
967   (2^31).  Caches SHOULD use an arithmetic type of at least 31 bits of
968   range.
969
970   The presence of an Age header field in a response implies that a
971   response is not first-hand.  However, the converse is not true, since
972   HTTP/1.0 caches might not implement the Age header field.
973
9743.2.  Cache-Control
975
976   The "Cache-Control" general-header field is used to specify
977   directives for caches along the request/response chain.  Such cache
978   directives are unidirectional in that the presence of a directive in
979   a request does not imply that the same directive is to be given in
980   the response.
981
982   HTTP/1.1 caches MUST obey the requirements of the Cache-Control
983   directives defined in this section.  See Section 3.2.3 for
984   information about how Cache-Control directives defined elsewhere are
985   handled.
986
987      Note: HTTP/1.0 caches might not implement Cache-Control and might
988      only implement Pragma: no-cache (see Section 3.4).
989
990   Cache directives MUST be passed through by a proxy or gateway
991   application, regardless of their significance to that application,
992   since the directives might be applicable to all recipients along the
993   request/response chain.  It is not possible to target a directive to
994   a specific cache.
995
996     Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
997     Cache-Control-v = 1#cache-directive
998
999     cache-directive = cache-request-directive
1000        / cache-response-directive
1001
1002     cache-extension = token [ "=" ( token / quoted-string ) ]
1003
1004
1005
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1011
10123.2.1.  Request Cache-Control Directives
1013
1014     cache-request-directive =
1015          "no-cache"
1016        / "no-store"
1017        / "max-age" "=" delta-seconds
1018        / "max-stale" [ "=" delta-seconds ]
1019        / "min-fresh" "=" delta-seconds
1020        / "no-transform"
1021        / "only-if-cached"
1022        / cache-extension
1023
1024   no-cache
1025
1026      The no-cache request directive indicates that a stored response
1027      MUST NOT be used to satisfy the request without successful
1028      validation on the origin server.
1029
1030   no-store
1031
1032      The no-store request directive indicates that a cache MUST NOT
1033      store any part of either this request or any response to it.  This
1034      directive applies to both non-shared and shared caches.  "MUST NOT
1035      store" in this context means that the cache MUST NOT intentionally
1036      store the information in non-volatile storage, and MUST make a
1037      best-effort attempt to remove the information from volatile
1038      storage as promptly as possible after forwarding it.
1039
1040      This directive is NOT a reliable or sufficient mechanism for
1041      ensuring privacy.  In particular, malicious or compromised caches
1042      might not recognize or obey this directive, and communications
1043      networks might be vulnerable to eavesdropping.
1044
1045   max-age
1046
1047      The max-age request directive indicates that the client is willing
1048      to accept a response whose age is no greater than the specified
1049      time in seconds.  Unless the max-stale request directive is also
1050      present, the client is not willing to accept a stale response.
1051
1052   max-stale
1053
1054      The max-stale request directive indicates that the client is
1055      willing to accept a response that has exceeded its expiration
1056      time.  If max-stale is assigned a value, then the client is
1057      willing to accept a response that has exceeded its expiration time
1058      by no more than the specified number of seconds.  If no value is
1059      assigned to max-stale, then the client is willing to accept a
1060
1061
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1067
1068      stale response of any age.
1069
1070   min-fresh
1071
1072      The min-fresh request directive indicates that the client is
1073      willing to accept a response whose freshness lifetime is no less
1074      than its current age plus the specified time in seconds.  That is,
1075      the client wants a response that will still be fresh for at least
1076      the specified number of seconds.
1077
1078   no-transform
1079
1080      The no-transform request directive indicates that an intermediate
1081      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1082      or Content-Type request header fields, nor the request
1083      representation.
1084
1085   only-if-cached
1086
1087      The only-if-cached request directive indicates that the client
1088      only wishes to return a stored response.  If it receives this
1089      directive, a cache SHOULD either respond using a stored response
1090      that is consistent with the other constraints of the request, or
1091      respond with a 504 (Gateway Timeout) status code.  If a group of
1092      caches is being operated as a unified system with good internal
1093      connectivity, such a request MAY be forwarded within that group of
1094      caches.
1095
10963.2.2.  Response Cache-Control Directives
1097
1098     cache-response-directive =
1099          "public"
1100        / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1101        / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1102        / "no-store"
1103        / "no-transform"
1104        / "must-revalidate"
1105        / "proxy-revalidate"
1106        / "max-age" "=" delta-seconds
1107        / "s-maxage" "=" delta-seconds
1108        / cache-extension
1109
1110   public
1111
1112      The public response directive indicates that the response MAY be
1113      cached, even if it would normally be non-cacheable or cacheable
1114      only within a non-shared cache.  (See also Authorization, Section
1115      4.1 of [Part7], for additional details.)
1116
1117
1118
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1122
1123
1124   private
1125
1126      The private response directive indicates that the response message
1127      is intended for a single user and MUST NOT be stored by a shared
1128      cache.  A private (non-shared) cache MAY store the response.
1129
1130      If the private response directive specifies one or more field-
1131      names, this requirement is limited to the field-values associated
1132      with the listed response header fields.  That is, the specified
1133      field-names(s) MUST NOT be stored by a shared cache, whereas the
1134      remainder of the response message MAY be.
1135
1136      Note: This usage of the word private only controls where the
1137      response can be stored; it cannot ensure the privacy of the
1138      message content.  Also, private response directives with field-
1139      names are often handled by implementations as if an unqualified
1140      private directive was received; i.e., the special handling for the
1141      qualified form is not widely implemented.
1142
1143   no-cache
1144
1145      The no-cache response directive indicates that the response MUST
1146      NOT be used to satisfy a subsequent request without successful
1147      validation on the origin server.  This allows an origin server to
1148      prevent a cache from using it to satisfy a request without
1149      contacting it, even by caches that have been configured to return
1150      stale responses.
1151
1152      If the no-cache response directive specifies one or more field-
1153      names, this requirement is limited to the field-values associated
1154      with the listed response header fields.  That is, the specified
1155      field-name(s) MUST NOT be sent in the response to a subsequent
1156      request without successful validation on the origin server.  This
1157      allows an origin server to prevent the re-use of certain header
1158      fields in a response, while still allowing caching of the rest of
1159      the response.
1160
1161      Note: Most HTTP/1.0 caches will not recognize or obey this
1162      directive.  Also, no-cache response directives with field-names
1163      are often handled by implementations as if an unqualified no-cache
1164      directive was received; i.e., the special handling for the
1165      qualified form is not widely implemented.
1166
1167   no-store
1168
1169      The no-store response directive indicates that a cache MUST NOT
1170      store any part of either the immediate request or response.  This
1171      directive applies to both non-shared and shared caches.  "MUST NOT
1172
1173
1174
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1178
1179
1180      store" in this context means that the cache MUST NOT intentionally
1181      store the information in non-volatile storage, and MUST make a
1182      best-effort attempt to remove the information from volatile
1183      storage as promptly as possible after forwarding it.
1184
1185      This directive is NOT a reliable or sufficient mechanism for
1186      ensuring privacy.  In particular, malicious or compromised caches
1187      might not recognize or obey this directive, and communications
1188      networks might be vulnerable to eavesdropping.
1189
1190   must-revalidate
1191
1192      The must-revalidate response directive indicates that once it has
1193      become stale, the response MUST NOT be used to satisfy subsequent
1194      requests without successful validation on the origin server.
1195
1196      The must-revalidate directive is necessary to support reliable
1197      operation for certain protocol features.  In all circumstances an
1198      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1199      particular, if the cache cannot reach the origin server for any
1200      reason, it MUST generate a 504 (Gateway Timeout) response.
1201
1202      Servers SHOULD send the must-revalidate directive if and only if
1203      failure to validate a request on the representation could result
1204      in incorrect operation, such as a silently unexecuted financial
1205      transaction.
1206
1207   proxy-revalidate
1208
1209      The proxy-revalidate response directive has the same meaning as
1210      the must-revalidate response directive, except that it does not
1211      apply to non-shared caches.
1212
1213   max-age
1214
1215      The max-age response directive indicates that response is to be
1216      considered stale after its age is greater than the specified
1217      number of seconds.
1218
1219   s-maxage
1220
1221      The s-maxage response directive indicates that, in shared caches,
1222      the maximum age specified by this directive overrides the maximum
1223      age specified by either the max-age directive or the Expires
1224      header field.  The s-maxage directive also implies the semantics
1225      of the proxy-revalidate response directive.
1226
1227
1228
1229
1230
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1234
1235
1236   no-transform
1237
1238      The no-transform response directive indicates that an intermediate
1239      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1240      or Content-Type response header fields, nor the response
1241      representation.
1242
12433.2.3.  Cache Control Extensions
1244
1245   The Cache-Control header field can be extended through the use of one
1246   or more cache-extension tokens, each with an optional value.
1247   Informational extensions (those that do not require a change in cache
1248   behavior) can be added without changing the semantics of other
1249   directives.  Behavioral extensions are designed to work by acting as
1250   modifiers to the existing base of cache directives.  Both the new
1251   directive and the standard directive are supplied, such that
1252   applications that do not understand the new directive will default to
1253   the behavior specified by the standard directive, and those that
1254   understand the new directive will recognize it as modifying the
1255   requirements associated with the standard directive.  In this way,
1256   extensions to the cache-control directives can be made without
1257   requiring changes to the base protocol.
1258
1259   This extension mechanism depends on an HTTP cache obeying all of the
1260   cache-control directives defined for its native HTTP-version, obeying
1261   certain extensions, and ignoring all directives that it does not
1262   understand.
1263
1264   For example, consider a hypothetical new response directive called
1265   "community" that acts as a modifier to the private directive.  We
1266   define this new directive to mean that, in addition to any non-shared
1267   cache, any cache that is shared only by members of the community
1268   named within its value may cache the response.  An origin server
1269   wishing to allow the UCI community to use an otherwise private
1270   response in their shared cache(s) could do so by including
1271
1272     Cache-Control: private, community="UCI"
1273
1274   A cache seeing this header field will act correctly even if the cache
1275   does not understand the community cache-extension, since it will also
1276   see and understand the private directive and thus default to the safe
1277   behavior.
1278
1279   Unrecognized cache directives MUST be ignored; it is assumed that any
1280   cache directive likely to be unrecognized by an HTTP/1.1 cache will
1281   be combined with standard directives (or the response's default
1282   cacheability) such that the cache behavior will remain minimally
1283   correct even if the cache does not understand the extension(s).
1284
1285
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1290
1291
1292   The HTTP Cache Directive Registry defines the name space for the
1293   cache directives.
1294
1295   Registrations MUST include the following fields:
1296
1297   o  Cache Directive Name
1298
1299   o  Pointer to specification text
1300
1301   Values to be added to this name space are subject to IETF review
1302   ([RFC5226], Section 4.1).
1303
1304   The registry itself is maintained at
1305   <http://www.iana.org/assignments/http-cache-directives>.
1306
13073.3.  Expires
1308
1309   The "Expires" header field gives the date/time after which the
1310   response is considered stale.  See Section 2.3 for further discussion
1311   of the freshness model.
1312
1313   The presence of an Expires field does not imply that the original
1314   resource will change or cease to exist at, before, or after that
1315   time.
1316
1317   The field-value is an absolute date and time as defined by HTTP-date
1318   in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format.
1319
1320     Expires   = "Expires" ":" OWS Expires-v
1321     Expires-v = HTTP-date
1322
1323   For example
1324
1325     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1326
1327      Note: If a response includes a Cache-Control field with the max-
1328      age directive (see Section 3.2.2), that directive overrides the
1329      Expires field.  Likewise, the s-maxage directive overrides Expires
1330      in shared caches.
1331
1332   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
1333   the future.
1334
1335   HTTP/1.1 clients and caches MUST treat other invalid date formats,
1336   especially including the value "0", as in the past (i.e., "already
1337   expired").
1338
1339
1340
1341
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1346
1347
13483.4.  Pragma
1349
1350   The "Pragma" general-header field is used to include implementation-
1351   specific directives that might apply to any recipient along the
1352   request/response chain.  All pragma directives specify optional
1353   behavior from the viewpoint of the protocol; however, some systems
1354   MAY require that behavior be consistent with the directives.
1355
1356     Pragma            = "Pragma" ":" OWS Pragma-v
1357     Pragma-v          = 1#pragma-directive
1358     pragma-directive  = "no-cache" / extension-pragma
1359     extension-pragma  = token [ "=" ( token / quoted-string ) ]
1360
1361   When the no-cache directive is present in a request message, an
1362   application SHOULD forward the request toward the origin server even
1363   if it has a cached copy of what is being requested.  This pragma
1364   directive has the same semantics as the no-cache response directive
1365   (see Section 3.2.2) and is defined here for backward compatibility
1366   with HTTP/1.0.  Clients SHOULD include both header fields when a no-
1367   cache request is sent to a server not known to be HTTP/1.1 compliant.
1368   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
1369   sent "Cache-Control: no-cache".
1370
1371      Note: Because the meaning of "Pragma: no-cache" as a response-
1372      header field is not actually specified, it does not provide a
1373      reliable replacement for "Cache-Control: no-cache" in a response.
1374
1375   This mechanism is deprecated; no new Pragma directives will be
1376   defined in HTTP.
1377
13783.5.  Vary
1379
1380   The "Vary" response-header field conveys the set of request-header
1381   fields that were used to select the representation.
1382
1383   Caches use this information, in part, to determine whether a stored
1384   response can be used to satisfy a given request; see Section 2.7.
1385   determines, while the response is fresh, whether a cache is permitted
1386   to use the response to reply to a subsequent request without
1387   validation; see Section 2.7.
1388
1389   In uncacheable or stale responses, the Vary field value advises the
1390   user agent about the criteria that were used to select the
1391   representation.
1392
1393     Vary   = "Vary" ":" OWS Vary-v
1394     Vary-v = "*" / 1#field-name
1395
1396
1397
1398
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1402
1403
1404   The set of header fields named by the Vary field value is known as
1405   the selecting request-header fields.
1406
1407   Servers SHOULD include a Vary header field with any cacheable
1408   response that is subject to server-driven negotiation.  Doing so
1409   allows a cache to properly interpret future requests on that resource
1410   and informs the user agent about the presence of negotiation on that
1411   resource.  A server MAY include a Vary header field with a non-
1412   cacheable response that is subject to server-driven negotiation,
1413   since this might provide the user agent with useful information about
1414   the dimensions over which the response varies at the time of the
1415   response.
1416
1417   A Vary field value of "*" signals that unspecified parameters not
1418   limited to the request-header fields (e.g., the network address of
1419   the client), play a role in the selection of the response
1420   representation; therefore, a cache cannot determine whether this
1421   response is appropriate.  The "*" value MUST NOT be generated by a
1422   proxy server.
1423
1424   The field-names given are not limited to the set of standard request-
1425   header fields defined by this specification.  Field names are case-
1426   insensitive.
1427
14283.6.  Warning
1429
1430   The "Warning" general-header field is used to carry additional
1431   information about the status or transformation of a message that
1432   might not be reflected in the message.  This information is typically
1433   used to warn about possible incorrectness introduced by caching
1434   operations or transformations applied to the payload of the message.
1435
1436   Warnings can be used for other purposes, both cache-related and
1437   otherwise.  The use of a warning, rather than an error status code,
1438   distinguishes these responses from true failures.
1439
1440   Warning header fields can in general be applied to any message,
1441   however some warn-codes are specific to caches and can only be
1442   applied to response messages.
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
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1458
1459
1460     Warning    = "Warning" ":" OWS Warning-v
1461     Warning-v  = 1#warning-value
1462
1463     warning-value = warn-code SP warn-agent SP warn-text
1464                                           [SP warn-date]
1465
1466     warn-code  = 3DIGIT
1467     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1468                     ; the name or pseudonym of the server adding
1469                     ; the Warning header field, for use in debugging
1470     warn-text  = quoted-string
1471     warn-date  = DQUOTE HTTP-date DQUOTE
1472
1473   Multiple warnings can be attached to a response (either by the origin
1474   server or by a cache), including multiple warnings with the same code
1475   number, only differing in warn-text.
1476
1477   When this occurs, the user agent SHOULD inform the user of as many of
1478   them as possible, in the order that they appear in the response.
1479
1480   Systems that generate multiple Warning header fields SHOULD order
1481   them with this user agent behavior in mind.  New Warning header
1482   fields SHOULD be added after any existing Warning headers fields.
1483
1484   Warnings are assigned three digit warn-codes.  The first digit
1485   indicates whether the Warning is required to be deleted from a stored
1486   response after validation:
1487
1488   o  1xx Warnings describe the freshness or validation status of the
1489      response, and so MUST be deleted by caches after validation.  They
1490      can only be generated by a cache when validating a cached entry,
1491      and MUST NOT be generated in any other situation.
1492
1493   o  2xx Warnings describe some aspect of the representation that is
1494      not rectified by a validation (for example, a lossy compression of
1495      the representation) and MUST NOT be deleted by caches after
1496      validation, unless a full response is returned, in which case they
1497      MUST be.
1498
1499   If an implementation sends a message with one or more Warning header
1500   fields to a receiver whose version is HTTP/1.0 or lower, then the
1501   sender MUST include in each warning-value a warn-date that matches
1502   the Date header field in the message.
1503
1504   If an implementation receives a message with a warning-value that
1505   includes a warn-date, and that warn-date is different from the Date
1506   value in the response, then that warning-value MUST be deleted from
1507   the message before storing, forwarding, or using it. (preventing the
1508
1509
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1514
1515
1516   consequences of naive caching of Warning header fields.)  If all of
1517   the warning-values are deleted for this reason, the Warning header
1518   field MUST be deleted as well.
1519
1520   The following warn-codes are defined by this specification, each with
1521   a recommended warn-text in English, and a description of its meaning.
1522
1523   110 Response is stale
1524
1525      SHOULD be included whenever the returned response is stale.
1526
1527   111 Revalidation failed
1528
1529      SHOULD be included if a cache returns a stale response because an
1530      attempt to validate the response failed, due to an inability to
1531      reach the server.
1532
1533   112 Disconnected operation
1534
1535      SHOULD be included if the cache is intentionally disconnected from
1536      the rest of the network for a period of time.
1537
1538   113 Heuristic expiration
1539
1540      SHOULD be included if the cache heuristically chose a freshness
1541      lifetime greater than 24 hours and the response's age is greater
1542      than 24 hours.
1543
1544   199 Miscellaneous warning
1545
1546      The warning text can include arbitrary information to be presented
1547      to a human user, or logged.  A system receiving this warning MUST
1548      NOT take any automated action, besides presenting the warning to
1549      the user.
1550
1551   214 Transformation applied
1552
1553      MUST be added by an intermediate proxy if it applies any
1554      transformation to the representation, such as changing the
1555      content-coding, media-type, or modifying the representation data,
1556      unless this Warning code already appears in the response.
1557
1558   299 Miscellaneous persistent warning
1559
1560      The warning text can include arbitrary information to be presented
1561      to a human user, or logged.  A system receiving this warning MUST
1562      NOT take any automated action.
1563
1564
1565
1566
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1570
1571
15724.  History Lists
1573
1574   User agents often have history mechanisms, such as "Back" buttons and
1575   history lists, that can be used to redisplay a representation
1576   retrieved earlier in a session.
1577
1578   The freshness model (Section 2.3) does not necessarily apply to
1579   history mechanisms.  I.e., a history mechanism can display a previous
1580   representation even if it has expired.
1581
1582   This does not prohibit the history mechanism from telling the user
1583   that a view might be stale, or from honoring cache directives (e.g.,
1584   Cache-Control: no-store).
1585
15865.  IANA Considerations
1587
15885.1.  Cache Directive Registry
1589
1590   The registration procedure for HTTP Cache Directives is defined by
1591   Section 3.2.3 of this document.
1592
1593   The HTTP Cache Directive Registry shall be created at
1594   <http://www.iana.org/assignments/http-cache-directives> and be
1595   populated with the registrations below:
1596
1597   +------------------------+------------------------------+
1598   | Cache Directive        | Reference                    |
1599   +------------------------+------------------------------+
1600   | max-age                | Section 3.2.1, Section 3.2.2 |
1601   | max-stale              | Section 3.2.1                |
1602   | min-fresh              | Section 3.2.1                |
1603   | must-revalidate        | Section 3.2.2                |
1604   | no-cache               | Section 3.2.1, Section 3.2.2 |
1605   | no-store               | Section 3.2.1, Section 3.2.2 |
1606   | no-transform           | Section 3.2.1, Section 3.2.2 |
1607   | only-if-cached         | Section 3.2.1                |
1608   | private                | Section 3.2.2                |
1609   | proxy-revalidate       | Section 3.2.2                |
1610   | public                 | Section 3.2.2                |
1611   | s-maxage               | Section 3.2.2                |
1612   | stale-if-error         | [RFC5861], Section 4         |
1613   | stale-while-revalidate | [RFC5861], Section 3         |
1614   +------------------------+------------------------------+
1615
1616
1617
1618
1619
1620
1621
1622
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1626
1627
16285.2.  Header Field Registration
1629
1630   The Message Header Field Registry located at <http://www.iana.org/
1631   assignments/message-headers/message-header-index.html> shall be
1632   updated with the permanent registrations below (see [RFC3864]):
1633
1634   +-------------------+----------+----------+-------------+
1635   | Header Field Name | Protocol | Status   | Reference   |
1636   +-------------------+----------+----------+-------------+
1637   | Age               | http     | standard | Section 3.1 |
1638   | Cache-Control     | http     | standard | Section 3.2 |
1639   | Expires           | http     | standard | Section 3.3 |
1640   | Pragma            | http     | standard | Section 3.4 |
1641   | Vary              | http     | standard | Section 3.5 |
1642   | Warning           | http     | standard | Section 3.6 |
1643   +-------------------+----------+----------+-------------+
1644
1645   The change controller is: "IETF (iesg@ietf.org) - Internet
1646   Engineering Task Force".
1647
16486.  Security Considerations
1649
1650   Caches expose additional potential vulnerabilities, since the
1651   contents of the cache represent an attractive target for malicious
1652   exploitation.  Because cache contents persist after an HTTP request
1653   is complete, an attack on the cache can reveal information long after
1654   a user believes that the information has been removed from the
1655   network.  Therefore, cache contents need to be protected as sensitive
1656   information.
1657
16587.  Acknowledgments
1659
1660   Much of the content and presentation of the caching design is due to
1661   suggestions and comments from individuals including: Shel Kaphan,
1662   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
1663
16648.  References
1665
16668.1.  Normative References
1667
1668   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1669              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1670              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
1671              and Message Parsing", draft-ietf-httpbis-p1-messaging-12
1672              (work in progress), October 2010.
1673
1674   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1675              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1676
1677
1678
1679Fielding, et al.         Expires April 28, 2011                [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                October 2010
1682
1683
1684              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
1685              Semantics", draft-ietf-httpbis-p2-semantics-12 (work in
1686              progress), October 2010.
1687
1688   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1689              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1690              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
1691              Requests", draft-ietf-httpbis-p4-conditional-12 (work in
1692              progress), October 2010.
1693
1694   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1695              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1696              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1697              Partial Responses", draft-ietf-httpbis-p5-range-12 (work
1698              in progress), October 2010.
1699
1700   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1701              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1702              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
1703              draft-ietf-httpbis-p7-auth-12 (work in progress),
1704              October 2010.
1705
1706   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1707              Requirement Levels", BCP 14, RFC 2119, March 1997.
1708
1709   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1710              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1711
17128.2.  Informative References
1713
1714   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
1715              Specification, Implementation", RFC 1305, March 1992.
1716
1717   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1718              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1719              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1720
1721   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1722              Procedures for Message Header Fields", BCP 90, RFC 3864,
1723              September 2004.
1724
1725   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
1726              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
1727              May 2008.
1728
1729   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
1730              Content", RFC 5861, April 2010.
1731
1732
1733
1734
1735Fielding, et al.         Expires April 28, 2011                [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                October 2010
1738
1739
1740Appendix A.  Changes from RFC 2616
1741
1742   Make the specified age calculation algorithm less conservative.
1743   (Section 2.3.2)
1744
1745   Remove requirement to consider Content-Location in successful
1746   responses in order to determine the appropriate response to use.
1747   (Section 2.4)
1748
1749   Clarify denial of service attack avoidance requirement.
1750   (Section 2.5)
1751
1752   Do not mention RFC 2047 encoding and multiple languages in Warning
1753   header fields anymore, as these aspects never were implemented.
1754   (Section 3.6)
1755
1756Appendix B.  Collected ABNF
1757
1758   Age = "Age:" OWS Age-v
1759   Age-v = delta-seconds
1760
1761   Cache-Control = "Cache-Control:" OWS Cache-Control-v
1762   Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
1763    cache-directive ] )
1764
1765   Expires = "Expires:" OWS Expires-v
1766   Expires-v = HTTP-date
1767
1768   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1769
1770   OWS = <OWS, defined in [Part1], Section 1.2.2>
1771
1772   Pragma = "Pragma:" OWS Pragma-v
1773   Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1774    pragma-directive ] )
1775
1776   Vary = "Vary:" OWS Vary-v
1777   Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1778    ] ) )
1779
1780   Warning = "Warning:" OWS Warning-v
1781   Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1782    ] )
1783
1784   cache-directive = cache-request-directive / cache-response-directive
1785   cache-extension = token [ "=" ( token / quoted-string ) ]
1786
1787
1788
1789
1790
1791Fielding, et al.         Expires April 28, 2011                [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                October 2010
1794
1795
1796   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
1797    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1798    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1799    cache-extension
1800   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
1801    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1802    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1803    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1804    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1805    ) / ( "s-maxage=" delta-seconds ) / cache-extension
1806
1807   delta-seconds = 1*DIGIT
1808
1809   extension-pragma = token [ "=" ( token / quoted-string ) ]
1810
1811   field-name = <field-name, defined in [Part1], Section 3.2>
1812
1813   port = <port, defined in [Part1], Section 2.6>
1814   pragma-directive = "no-cache" / extension-pragma
1815   pseudonym = <pseudonym, defined in [Part1], Section 9.9>
1816
1817   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1818
1819   token = <token, defined in [Part1], Section 1.2.2>
1820
1821   uri-host = <uri-host, defined in [Part1], Section 2.6>
1822
1823   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1824   warn-code = 3DIGIT
1825   warn-date = DQUOTE HTTP-date DQUOTE
1826   warn-text = quoted-string
1827   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
1828    ]
1829
1830   ABNF diagnostics:
1831
1832   ; Age defined but not used
1833   ; Cache-Control defined but not used
1834   ; Expires defined but not used
1835   ; Pragma defined but not used
1836   ; Vary defined but not used
1837   ; Warning defined but not used
1838
1839Appendix C.  Change Log (to be removed by RFC Editor before publication)
1840
1841
1842
1843
1844
1845
1846
1847Fielding, et al.         Expires April 28, 2011                [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                October 2010
1850
1851
1852C.1.  Since RFC 2616
1853
1854   Extracted relevant partitions from [RFC2616].
1855
1856C.2.  Since draft-ietf-httpbis-p6-cache-00
1857
1858   Closed issues:
1859
1860   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
1861      (<http://purl.org/NET/http-errata#trailer-hop>)
1862
1863   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
1864      after Update or Delete"
1865      (<http://purl.org/NET/http-errata#invalidupd>)
1866
1867   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1868      Informative references"
1869
1870   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
1871      typo"
1872
1873   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
1874      header text"
1875
1876   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
1877      references"
1878
1879   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
1880      Reference"
1881
1882   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
1883      to-date references"
1884
1885   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
1886      13.2.2"
1887
1888   Other changes:
1889
1890   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
1891      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1892
1893C.3.  Since draft-ietf-httpbis-p6-cache-01
1894
1895   Closed issues:
1896
1897   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
1898      used"
1899
1900
1901
1902
1903Fielding, et al.         Expires April 28, 2011                [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                October 2010
1906
1907
1908   Other changes:
1909
1910   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
1911      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1912
1913   o  Add explicit references to BNF syntax and rules imported from
1914      other parts of the specification.
1915
1916C.4.  Since draft-ietf-httpbis-p6-cache-02
1917
1918   Ongoing work on IANA Message Header Field Registration
1919   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1920
1921   o  Reference RFC 3984, and update header field registrations for
1922      header fields defined in this document.
1923
1924C.5.  Since draft-ietf-httpbis-p6-cache-03
1925
1926   Closed issues:
1927
1928   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
1929      classification"
1930
1931C.6.  Since draft-ietf-httpbis-p6-cache-04
1932
1933   Ongoing work on ABNF conversion
1934   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1935
1936   o  Use "/" instead of "|" for alternatives.
1937
1938   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1939      whitespace ("OWS") and required whitespace ("RWS").
1940
1941   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1942      field value format definitions.
1943
1944C.7.  Since draft-ietf-httpbis-p6-cache-05
1945
1946   This is a total rewrite of this part of the specification.
1947
1948   Affected issues:
1949
1950   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1951      1xx Warn-Codes"
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 April 28, 2011                [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                October 2010
1962
1963
1964   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role of
1965      Warning and Semantic Transparency in Caching"
1966
1967   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
1968      Caching"
1969
1970   In addition: Final work on ABNF conversion
1971   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1972
1973   o  Add appendix containing collected and expanded ABNF, reorganize
1974      ABNF introduction.
1975
1976C.8.  Since draft-ietf-httpbis-p6-cache-06
1977
1978   Closed issues:
1979
1980   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
1981      numeric protocol elements"
1982
1983   Affected issues:
1984
1985   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: WVary and non-
1986      existant headers"
1987
1988C.9.  Since draft-ietf-httpbis-p6-cache-07
1989
1990   Closed issues:
1991
1992   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1993      1xx Warn-Codes"
1994
1995   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
1996      Location on 304 responses"
1997
1998   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
1999      no-cache CC directives with headers"
2000
2001   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
2002      warn-text"
2003
2004C.10.  Since draft-ietf-httpbis-p6-cache-08
2005
2006   Closed issues:
2007
2008   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
2009      negotiated responses from cache: header-specific canonicalization"
2010
2011
2012
2013
2014
2015Fielding, et al.         Expires April 28, 2011                [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                October 2010
2018
2019
2020   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
2021      directives on history lists"
2022
2023   Affected issues:
2024
2025   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
2026      and caching
2027
2028   Partly resolved issues:
2029
2030   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
2031      13.5.1 and 13.5.2"
2032
2033C.11.  Since draft-ietf-httpbis-p6-cache-09
2034
2035   Closed issues:
2036
2037   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/29>: "Age
2038      calculation"
2039
2040   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/168>: "Clarify
2041      differences between / requirements for request and response CC
2042      directives"
2043
2044   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/174>: "Caching
2045      authenticated responses"
2046
2047   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/208>: "IANA registry
2048      for cache-control directives"
2049
2050   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
2051      caching of URLs with query components"
2052
2053   Partly resolved issues:
2054
2055   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
2056      requested resource's URI"
2057
2058C.12.  Since draft-ietf-httpbis-p6-cache-10
2059
2060   Closed issues:
2061
2062   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
2063      entity / representation / variant terminology"
2064
2065   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
2066      removing the 'changes from 2068' sections"
2067
2068
2069
2070
2071Fielding, et al.         Expires April 28, 2011                [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                October 2010
2074
2075
2076   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2077      heuristic caching for new status codes"
2078
2079   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2080      heuristic caching for new status codes"
2081
2082   o  Clean up TODOs and prose in "Combining Responses."
2083
2084C.13.  Since draft-ietf-httpbis-p6-cache-11
2085
2086   Closed issues:
2087
2088   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/204>: "Text about
2089      clock requirement for caches belongs in p6"
2090
2091Index
2092
2093   A
2094      age  6
2095      Age header  17
2096
2097   C
2098      cache  5
2099      Cache Directives
2100         max-age  19, 22
2101         max-stale  19
2102         min-fresh  20
2103         must-revalidate  22
2104         no-cache  19, 21
2105         no-store  19, 21
2106         no-transform  20, 23
2107         only-if-cached  20
2108         private  21
2109         proxy-revalidate  22
2110         public  20
2111         s-maxage  22
2112      Cache-Control header  18
2113      cacheable  5
2114
2115   E
2116      Expires header  24
2117      explicit expiration time  5
2118
2119   F
2120      first-hand  6
2121      fresh  6
2122      freshness lifetime  6
2123
2124
2125
2126
2127Fielding, et al.         Expires April 28, 2011                [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                October 2010
2130
2131
2132   G
2133      Grammar
2134         Age  18
2135         Age-v  18
2136         Cache-Control  18
2137         Cache-Control-v  18
2138         cache-extension  18
2139         cache-request-directive  19
2140         cache-response-directive  20
2141         delta-seconds  18
2142         Expires  24
2143         Expires-v  24
2144         extension-pragma  25
2145         Pragma  25
2146         pragma-directive  25
2147         Pragma-v  25
2148         Vary  25
2149         Vary-v  25
2150         warn-agent  27
2151         warn-code  27
2152         warn-date  27
2153         warn-text  27
2154         Warning  27
2155         Warning-v  27
2156         warning-value  27
2157
2158   H
2159      Headers
2160         Age  17
2161         Cache-Control  18
2162         Expires  24
2163         Pragma  25
2164         Vary  25
2165         Warning  26
2166      heuristic expiration time  5
2167
2168   M
2169      max-age
2170         Cache Directive  19, 22
2171      max-stale
2172         Cache Directive  19
2173      min-fresh
2174         Cache Directive  20
2175      must-revalidate
2176         Cache Directive  22
2177
2178   N
2179      no-cache
2180
2181
2182
2183Fielding, et al.         Expires April 28, 2011                [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                October 2010
2186
2187
2188         Cache Directive  19, 21
2189      no-store
2190         Cache Directive  19, 21
2191      no-transform
2192         Cache Directive  20, 23
2193
2194   O
2195      only-if-cached
2196         Cache Directive  20
2197
2198   P
2199      Pragma header  25
2200      private
2201         Cache Directive  21
2202      proxy-revalidate
2203         Cache Directive  22
2204      public
2205         Cache Directive  20
2206
2207   S
2208      s-maxage
2209         Cache Directive  22
2210      stale  6
2211
2212   V
2213      validator  6
2214      Vary header  25
2215
2216   W
2217      Warning header  26
2218
2219Authors' Addresses
2220
2221   Roy T. Fielding (editor)
2222   Day Software
2223   23 Corporate Plaza DR, Suite 280
2224   Newport Beach, CA  92660
2225   USA
2226
2227   Phone: +1-949-706-5300
2228   Fax:   +1-949-706-5305
2229   EMail: fielding@gbiv.com
2230   URI:   http://roy.gbiv.com/
2231
2232
2233
2234
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2237
2238
2239Fielding, et al.         Expires April 28, 2011                [Page 40]
2240
2241Internet-Draft              HTTP/1.1, Part 6                October 2010
2242
2243
2244   Jim Gettys
2245   Alcatel-Lucent Bell Labs
2246   21 Oak Knoll Road
2247   Carlisle, MA  01741
2248   USA
2249
2250   EMail: jg@freedesktop.org
2251   URI:   http://gettys.wordpress.com/
2252
2253
2254   Jeffrey C. Mogul
2255   Hewlett-Packard Company
2256   HP Labs, Large Scale Systems Group
2257   1501 Page Mill Road, MS 1177
2258   Palo Alto, CA  94304
2259   USA
2260
2261   EMail: JeffMogul@acm.org
2262
2263
2264   Henrik Frystyk Nielsen
2265   Microsoft Corporation
2266   1 Microsoft Way
2267   Redmond, WA  98052
2268   USA
2269
2270   EMail: henrikn@microsoft.com
2271
2272
2273   Larry Masinter
2274   Adobe Systems, Incorporated
2275   345 Park Ave
2276   San Jose, CA  95110
2277   USA
2278
2279   EMail: LMM@acm.org
2280   URI:   http://larry.masinter.net/
2281
2282
2283   Paul J. Leach
2284   Microsoft Corporation
2285   1 Microsoft Way
2286   Redmond, WA  98052
2287
2288   EMail: paulle@microsoft.com
2289
2290
2291
2292
2293
2294
2295Fielding, et al.         Expires April 28, 2011                [Page 41]
2296
2297Internet-Draft              HTTP/1.1, Part 6                October 2010
2298
2299
2300   Tim Berners-Lee
2301   World Wide Web Consortium
2302   MIT Computer Science and Artificial Intelligence Laboratory
2303   The Stata Center, Building 32
2304   32 Vassar Street
2305   Cambridge, MA  02139
2306   USA
2307
2308   EMail: timbl@w3.org
2309   URI:   http://www.w3.org/People/Berners-Lee/
2310
2311
2312   Yves Lafon (editor)
2313   World Wide Web Consortium
2314   W3C / ERCIM
2315   2004, rte des Lucioles
2316   Sophia-Antipolis, AM  06902
2317   France
2318
2319   EMail: ylafon@w3.org
2320   URI:   http://www.raubacapeu.net/people/yves/
2321
2322
2323   Mark Nottingham (editor)
2324
2325   EMail: mnot@mnot.net
2326   URI:   http://www.mnot.net/
2327
2328
2329   Julian F. Reschke (editor)
2330   greenbytes GmbH
2331   Hafenweg 16
2332   Muenster, NW  48155
2333   Germany
2334
2335   Phone: +49 251 2807760
2336   Fax:   +49 251 2807761
2337   EMail: julian.reschke@greenbytes.de
2338   URI:   http://greenbytes.de/tech/webdav/
2339
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2351Fielding, et al.         Expires April 28, 2011                [Page 42]
2352
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