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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: February 5, 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                                                          August 4, 2010
25
26
27                       HTTP/1.1, part 6: Caching
28                     draft-ietf-httpbis-p6-cache-11
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.12.
49
50Status of This Memo
51
52
53
54
55Fielding, et al.        Expires February 5, 2011                [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 6                 August 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 February 5, 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 February 5, 2011                [Page 2]
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113Internet-Draft              HTTP/1.1, Part 6                 August 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  . . . . . . . . . . . . . 14
130     2.6.  Shared Caching of Authenticated Responses  . . . . . . . . 15
131     2.7.  Caching Negotiated Responses . . . . . . . . . . . . . . . 16
132     2.8.  Combining Responses  . . . . . . . . . . . . . . . . . . . 16
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 . . . . . . . . . . . 18
137       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 20
138       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 22
139     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 24
140     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 24
141     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
142     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 26
143   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 28
144   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
145     5.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 28
146     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 29
147   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
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 . . . . . . . . . . . . . . . . 31
153   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 31
154   Appendix C.  Change Log (to be removed by RFC Editor before
155                publication)  . . . . . . . . . . . . . . . . . . . . 33
156     C.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 33
157     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33
158     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
159     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
160     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
161     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 34
162     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
163     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35
164
165
166
167Fielding, et al.        Expires February 5, 2011                [Page 3]
168
169Internet-Draft              HTTP/1.1, Part 6                 August 2010
170
171
172     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35
173     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 36
174     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 36
175     C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 37
176   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
177
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 February 5, 2011                [Page 4]
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225Internet-Draft              HTTP/1.1, Part 6                 August 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 headers, 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 (see Section 3.1 of [Part7]) does not
405      appear in the request, if the cache is shared, unless the response
406      explicitly allows it (see Section 2.6), and
407
408   o  the response either:
409
410      *  contains an Expires header (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) can store
439   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 headers
455   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-headers nominated by the stored response (if
469      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) when more than one suitable response is stored.  They can
497   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
5082.3.  Freshness Model
509
510   When a response is "fresh" in the cache, it can be used to satisfy
511   subsequent requests without contacting the origin server, thereby
512   improving efficiency.
513
514   The primary mechanism for determining freshness is for an origin
515   server to provide an explicit expiration time in the future, using
516   either the Expires header (Section 3.3) or the max-age response cache
517   directive (Section 3.2.2).  Generally, origin servers will assign
518   future explicit expiration times to responses in the belief that the
519   representation is not likely to change in a semantically significant
520   way before the expiration time is reached.
521
522   If an origin server wishes to force a cache to validate every
523   request, it can assign an explicit expiration time in the past to
524   indicate that the response is already stale.  Compliant caches will
525   validate the cached response before reusing it for subsequent
526   requests.
527
528   Since origin servers do not always provide explicit expiration times,
529   HTTP caches MAY assign heuristic expiration times when explicit times
530   are not specified, employing algorithms that use other header values
531   (such as the Last-Modified time) to estimate a plausible expiration
532   time.  The HTTP/1.1 specification does not provide specific
533   algorithms, but does impose worst-case constraints on their results.
534
535   The calculation to determine if a response is fresh is:
536
537      response_is_fresh = (freshness_lifetime > current_age)
538
539   The freshness_lifetime is defined in Section 2.3.1; the current_age
540   is defined in Section 2.3.2.
541
542   Additionally, clients might need to influence freshness calculation.
543   They can do this using several request cache directives, with the
544   effect of either increasing or loosening constraints on freshness.
545   See Section 3.2.1.
546
547   [[ISSUE-no-req-for-directives: there are not requirements directly
548   applying to cache-request-directives and freshness.]]
549
550   Note that freshness applies only to cache operation; it cannot be
551   used to force a user agent to refresh its display or reload a
552   resource.  See Section 4 for an explanation of the difference between
553   caches and history mechanisms.
554
555
556
557
558
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563
5642.3.1.  Calculating Freshness Lifetime
565
566   A cache can calculate the freshness lifetime (denoted as
567   freshness_lifetime) of a response by using the first match of:
568
569   o  If the cache is shared and the s-maxage response cache directive
570      (Section 3.2.2) is present, use its value, or
571
572   o  If the max-age response cache directive (Section 3.2.2) is
573      present, use its value, or
574
575   o  If the Expires response header (Section 3.3) is present, use its
576      value minus the value of the Date response header, or
577
578   o  Otherwise, no explicit expiration time is present in the response.
579      A heuristic freshness lifetime might be applicable; see
580      Section 2.3.1.1.
581
582   Note that this calculation is not vulnerable to clock skew, since all
583   of the information comes from the origin server.
584
5852.3.1.1.  Calculating Heuristic Freshness
586
587   If no explicit expiration time is present in a stored response that
588   has a status code whose definition allows heuristic freshness to be
589   used (including the following in Section 8 of [Part2]: 200, 203, 206,
590   300, 301 and 410), a heuristic expiration time MAY be calculated.
591   Heuristics MUST NOT be used for response status codes that do not
592   explicitly allow it.
593
594   When a heuristic is used to calculate freshness lifetime, the cache
595   SHOULD attach a Warning header with a 113 warn-code to the response
596   if its current_age is more than 24 hours and such a warning is not
597   already present.
598
599   Also, if the response has a Last-Modified header (Section 6.6 of
600   [Part4]), the heuristic expiration value SHOULD be no more than some
601   fraction of the interval since that time.  A typical setting of this
602   fraction might be 10%.
603
604      Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
605      not calculate heuristic freshness for URLs with query components
606      (i.e., those containing '?').  In practice, this has not been
607      widely implemented.  Therefore, servers are encouraged to send
608      explicit directives (e.g., Cache-Control: no-cache) if they wish
609      to preclude caching.
610
611
612
613
614
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619
6202.3.2.  Calculating Age
621
622   HTTP/1.1 uses the Age response-header to convey the estimated age of
623   the response message when obtained from a cache.  The Age field value
624   is the cache's estimate of the amount of time since the response was
625   generated or validated by the origin server.  In essence, the Age
626   value is the sum of the time that the response has been resident in
627   each of the caches along the path from the origin server, plus the
628   amount of time it has been in transit along network paths.
629
630   The following data is used for the age calculation:
631
632   age_value
633
634      The term "age_value" denotes the value of the Age header
635      (Section 3.1), in a form appropriate for arithmetic operation; or
636      0, if not available.
637
638   date_value
639
640      HTTP/1.1 requires origin servers to send a Date header, if
641      possible, with every response, giving the time at which the
642      response was generated.  The term "date_value" denotes the value
643      of the Date header, in a form appropriate for arithmetic
644      operations.  See Section 9.3 of [Part1] for the definition of the
645      Date header, and for requirements regarding responses without a
646      Date response header.
647
648   now
649
650      The term "now" means "the current value of the clock at the host
651      performing the calculation".  Hosts that use HTTP, but especially
652      hosts running origin servers and caches, SHOULD use NTP
653      ([RFC1305]) or some similar protocol to synchronize their clocks
654      to a globally accurate time standard.
655
656   request_time
657
658      The current value of the clock at the host at the time the request
659      resulting in the stored response was made.
660
661   response_time
662
663      The current value of the clock at the host at the time the
664      response was received.
665
666   A response's age can be calculated in two entirely independent ways:
667
668
669
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675
676   1.  the "apparent_age": response_time minus date_value, if the local
677       clock is reasonably well synchronized to the origin server's
678       clock.  If the result is negative, the result is replaced by
679       zero.
680
681   2.  the "corrected_age_value", if all of the caches along the
682       response path implement HTTP/1.1; note this value MUST be
683       interpreted relative to the time the request was initiated, not
684       the time that the response was received.
685
686
687     apparent_age = max(0, response_time - date_value);
688
689     response_delay = response_time - request_time;
690     corrected_age_value = age_value + response_delay;
691
692   These are combined as
693
694     corrected_initial_age = max(apparent_age, corrected_age_value);
695
696   The current_age of a stored response can then be calculated by adding
697   the amount of time (in seconds) since the stored response was last
698   validated by the origin server to the corrected_initial_age.
699
700     resident_time = now - response_time;
701     current_age = corrected_initial_age + resident_time;
702
7032.3.3.  Serving Stale Responses
704
705   A "stale" response is one that either has explicit expiry information
706   or is allowed to have heuristic expiry calculated, but is not fresh
707   according to the calculations in Section 2.3.
708
709   Caches MUST NOT return a stale response if it is prohibited by an
710   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
711   cache directive, a "must-revalidate" cache-response-directive, or an
712   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
713   see Section 3.2.2).
714
715   Caches SHOULD NOT return stale responses unless they are disconnected
716   (i.e., it cannot contact the origin server or otherwise find a
717   forward path) or otherwise explicitly allowed (e.g., the max-stale
718   request directive; see Section 3.2.1).
719
720   Stale responses SHOULD have a Warning header with the 110 warn-code
721   (see Section 3.6).  Likewise, the 112 warn-code SHOULD be sent on
722   stale responses if the cache is disconnected.
723
724
725
726
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731
732   If a cache receives a first-hand response (either an entire response,
733   or a 304 (Not Modified) response) that it would normally forward to
734   the requesting client, and the received response is no longer fresh,
735   the cache SHOULD forward it to the requesting client without adding a
736   new Warning (but without removing any existing Warning headers).  A
737   cache SHOULD NOT attempt to validate a response simply because that
738   response became stale in transit.
739
7402.4.  Validation Model
741
742   When a cache has one or more stored responses for a requested URI,
743   but cannot serve any of them (e.g., because they are not fresh, or
744   one cannot be selected; see Section 2.7), it can use the conditional
745   request mechanism [Part4] in the forwarded request to give the origin
746   server an opportunity to both select a valid stored response to be
747   used, and to update it.  This process is known as "validating" or
748   "revalidating" the stored response.
749
750   When sending such a conditional request, the cache SHOULD add an If-
751   Modified-Since header whose value is that of the Last-Modified header
752   from the selected (see Section 2.7) stored response, if available.
753
754   Additionally, the cache SHOULD add an If-None-Match header whose
755   value is that of the ETag header(s) from all responses stored for the
756   requested URI, if present.  However, if any of the stored responses
757   contains only partial content, its entity-tag SHOULD NOT be included
758   in the If-None-Match header field unless the request is for a range
759   that would be fully satisfied by that stored response.
760
761   A 304 (Not Modified) response status code indicates that the stored
762   response can be updated and reused; see Section 2.8.
763
764   A full response (i.e., one with a response body) indicates that none
765   of the stored responses nominated in the conditional request is
766   suitable.  Instead, the full response SHOULD be used to satisfy the
767   request and MAY replace the stored response.
768
769   If a cache receives a 5xx response while attempting to validate a
770   response, it MAY either forward this response to the requesting
771   client, or act as if the server failed to respond.  In the latter
772   case, it MAY return a previously stored response (see Section 2.3.3).
773
7742.5.  Request Methods that Invalidate
775
776   Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
777   for changing state on the origin server, intervening caches can use
778   them to keep their contents up-to-date.
779
780
781
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787
788   The following HTTP methods MUST cause a cache to invalidate the
789   effective Request URI (Section 4.3 of [Part1]) as well as the URI(s)
790   in the Location and Content-Location headers (if present):
791
792   o  PUT
793
794   o  DELETE
795
796   o  POST
797
798   An invalidation based on a URI from a Location or Content-Location
799   header MUST NOT be performed if the host part of that URI differs
800   from the host part in the effective request URI (Section 4.3 of
801   [Part1]).  This helps prevent denial of service attacks.
802
803   A cache that passes through requests for methods it does not
804   understand SHOULD invalidate the effective request URI (Section 4.3
805   of [Part1]).
806
807   Here, "invalidate" means that the cache will either remove all stored
808   responses related to the effective request URI, or will mark these as
809   "invalid" and in need of a mandatory validation before they can be
810   returned in 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
8172.6.  Shared Caching of Authenticated Responses
818
819   Shared caches MUST NOT use a cached response to a request with an
820   Authorization header (Section 3.1 of [Part7]) to satisfy any
821   subsequent request unless a cache directive that allows such
822   responses to be stored is present in the response.
823
824   In this specification, the following Cache-Control response
825   directives (Section 3.2.2) have such an effect: must-revalidate,
826   public, s-maxage.
827
828   Note that cached responses that contain the "must-revalidate" and/or
829   "s-maxage" response directives are not allowed to be served stale
830   (Section 2.3.3) by shared caches.  In particular, a response with
831   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
832   satisfy a subsequent request without revalidating it on the origin
833   server.
834
835
836
837
838
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843
8442.7.  Caching Negotiated Responses
845
846   When a cache receives a request that can be satisfied by a stored
847   response that has a Vary header field (Section 3.5), it MUST NOT use
848   that response unless all of the selecting request-headers nominated
849   by the Vary header match in both the original request (i.e., that
850   associated with the stored response), and the presented request.
851
852   The selecting request-headers from two requests are defined to match
853   if and only if those in the first request can be transformed to those
854   in the second request by applying any of the following:
855
856   o  adding or removing whitespace, where allowed in the header's
857      syntax
858
859   o  combining multiple message-header fields with the same field name
860      (see Section 3.2 of [Part1])
861
862   o  normalizing both header values in a way that is known to have
863      identical semantics, according to the header's specification
864      (e.g., re-ordering field values when order is not significant;
865      case-normalization, where values are defined to be case-
866      insensitive)
867
868   If (after any normalization that might take place) a header field is
869   absent from a request, it can only match another request if it is
870   also absent there.
871
872   A Vary header field-value of "*" always fails to match, and
873   subsequent requests to that resource can only be properly interpreted
874   by the origin server.
875
876   The stored response with matching selecting request-headers is known
877   as the selected response.
878
879   If no selected response is available, the cache MAY forward the
880   presented request to the origin server in a conditional request; see
881   Section 2.4.
882
8832.8.  Combining Responses
884
885   When a cache receives a 304 (Not Modified) response or a 206 (Partial
886   Content) response (in this section, the "new" response"), it needs to
887   created an updated response by combining the stored response with the
888   new one, so that the updated response can be used to satisfy the
889   request, and potentially update the cached response.
890
891   If the new response contains an ETag, it identifies the stored
892
893
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899
900   response to use.  [[TODO-mention-CL: might need language about
901   Content-Location here]][[TODO-select-for-combine: Shouldn't this be
902   the selected response?]]
903
904   If the new response's status code is 206 (partial content), both the
905   stored and new responses MUST have validators, and those validators
906   MUST match using the strong comparison function (see Section 4 of
907   [Part4]).  Otherwise, the responses MUST NOT be combined.
908
909   The stored response headers are used as those of the updated
910   response, except that
911
912   o  any stored Warning headers with warn-code 1xx (see Section 3.6)
913      MUST be deleted.
914
915   o  any stored Warning headers with warn-code 2xx MUST be retained.
916
917   o  any other headers provided in the new response MUST replace all
918      instances of the corresponding headers from the stored response.
919
920   The updated response headers MUST be used to replace those of the
921   stored response in cache (unless the stored response is removed from
922   cache).  In the case of a 206 response, the combined representation
923   MAY be stored.
924
9253.  Header Field Definitions
926
927   This section defines the syntax and semantics of HTTP/1.1 header
928   fields related to caching.
929
9303.1.  Age
931
932   The "Age" response-header field conveys the sender's estimate of the
933   amount of time since the response was generated or successfully
934   validated at the origin server.  Age values are calculated as
935   specified in Section 2.3.2.
936
937     Age   = "Age" ":" OWS Age-v
938     Age-v = delta-seconds
939
940   Age field-values are non-negative integers, representing time in
941   seconds.
942
943     delta-seconds  = 1*DIGIT
944
945   If a cache receives a value larger than the largest positive integer
946   it can represent, or if any of its age calculations overflows, it
947   MUST transmit an Age header with a field-value of 2147483648 (2^31).
948
949
950
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955
956   Caches SHOULD use an arithmetic type of at least 31 bits of range.
957
958   The presence of an Age header field in a response implies that a
959   response is not first-hand.  However, the converse is not true, since
960   HTTP/1.0 caches might not implement the Age header field.
961
9623.2.  Cache-Control
963
964   The "Cache-Control" general-header field is used to specify
965   directives for caches along the request/response chain.  Such cache
966   directives are unidirectional in that the presence of a directive in
967   a request does not imply that the same directive is to be given in
968   the response.
969
970   HTTP/1.1 caches MUST obey the requirements of the Cache-Control
971   directives defined in this section.  See Section 3.2.3 for
972   information about how Cache-Control directives defined elsewhere are
973   handled.
974
975      Note: HTTP/1.0 caches might not implement Cache-Control and might
976      only implement Pragma: no-cache (see Section 3.4).
977
978   Cache directives MUST be passed through by a proxy or gateway
979   application, regardless of their significance to that application,
980   since the directives might be applicable to all recipients along the
981   request/response chain.  It is not possible to target a directive to
982   a specific cache.
983
984     Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
985     Cache-Control-v = 1#cache-directive
986
987     cache-directive = cache-request-directive
988        / cache-response-directive
989
990     cache-extension = token [ "=" ( token / quoted-string ) ]
991
9923.2.1.  Request Cache-Control Directives
993
994     cache-request-directive =
995          "no-cache"
996        / "no-store"
997        / "max-age" "=" delta-seconds
998        / "max-stale" [ "=" delta-seconds ]
999        / "min-fresh" "=" delta-seconds
1000        / "no-transform"
1001        / "only-if-cached"
1002        / cache-extension
1003
1004
1005
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1011
1012   no-cache
1013
1014      The no-cache request directive indicates that a stored response
1015      MUST NOT be used to satisfy the request without successful
1016      validation on the origin server.
1017
1018   no-store
1019
1020      The no-store request directive indicates that a cache MUST NOT
1021      store any part of either this request or any response to it.  This
1022      directive applies to both non-shared and shared caches.  "MUST NOT
1023      store" in this context means that the cache MUST NOT intentionally
1024      store the information in non-volatile storage, and MUST make a
1025      best-effort attempt to remove the information from volatile
1026      storage as promptly as possible after forwarding it.
1027
1028      This directive is NOT a reliable or sufficient mechanism for
1029      ensuring privacy.  In particular, malicious or compromised caches
1030      might not recognize or obey this directive, and communications
1031      networks might be vulnerable to eavesdropping.
1032
1033   max-age
1034
1035      The max-age request directive indicates that the client is willing
1036      to accept a response whose age is no greater than the specified
1037      time in seconds.  Unless the max-stale request directive is also
1038      present, the client is not willing to accept a stale response.
1039
1040   max-stale
1041
1042      The max-stale request directive indicates that the client is
1043      willing to accept a response that has exceeded its expiration
1044      time.  If max-stale is assigned a value, then the client is
1045      willing to accept a response that has exceeded its expiration time
1046      by no more than the specified number of seconds.  If no value is
1047      assigned to max-stale, then the client is willing to accept a
1048      stale response of any age.
1049
1050   min-fresh
1051
1052      The min-fresh request directive indicates that the client is
1053      willing to accept a response whose freshness lifetime is no less
1054      than its current age plus the specified time in seconds.  That is,
1055      the client wants a response that will still be fresh for at least
1056      the specified number of seconds.
1057
1058
1059
1060
1061
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1067
1068   no-transform
1069
1070      The no-transform request directive indicates that an intermediate
1071      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1072      or Content-Type request headers, nor the request representation.
1073
1074   only-if-cached
1075
1076      The only-if-cached request directive indicates that the client
1077      only wishes to return a stored response.  If it receives this
1078      directive, a cache SHOULD either respond using a stored response
1079      that is consistent with the other constraints of the request, or
1080      respond with a 504 (Gateway Timeout) status code.  If a group of
1081      caches is being operated as a unified system with good internal
1082      connectivity, such a request MAY be forwarded within that group of
1083      caches.
1084
10853.2.2.  Response Cache-Control Directives
1086
1087     cache-response-directive =
1088          "public"
1089        / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1090        / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1091        / "no-store"
1092        / "no-transform"
1093        / "must-revalidate"
1094        / "proxy-revalidate"
1095        / "max-age" "=" delta-seconds
1096        / "s-maxage" "=" delta-seconds
1097        / cache-extension
1098
1099   public
1100
1101      The public response directive indicates that the response MAY be
1102      cached, even if it would normally be non-cacheable or cacheable
1103      only within a non-shared cache.  (See also Authorization, Section
1104      3.1 of [Part7], for additional details.)
1105
1106   private
1107
1108      The private response directive indicates that the response message
1109      is intended for a single user and MUST NOT be stored by a shared
1110      cache.  A private (non-shared) cache MAY store the response.
1111
1112      If the private response directive specifies one or more field-
1113      names, this requirement is limited to the field-values associated
1114      with the listed response headers.  That is, the specified field-
1115      names(s) MUST NOT be stored by a shared cache, whereas the
1116
1117
1118
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1122
1123
1124      remainder of the response message MAY be.
1125
1126      Note: This usage of the word private only controls where the
1127      response can be stored; it cannot ensure the privacy of the
1128      message content.  Also, private response directives with field-
1129      names are often handled by implementations as if an unqualified
1130      private directive was received; i.e., the special handling for the
1131      qualified form is not widely implemented.
1132
1133   no-cache
1134
1135      The no-cache response directive indicates that the response MUST
1136      NOT be used to satisfy a subsequent request without successful
1137      validation on the origin server.  This allows an origin server to
1138      prevent a cache from using it to satisfy a request without
1139      contacting it, even by caches that have been configured to return
1140      stale responses.
1141
1142      If the no-cache response directive specifies one or more field-
1143      names, this requirement is limited to the field-values associated
1144      with the listed response headers.  That is, the specified field-
1145      name(s) MUST NOT be sent in the response to a subsequent request
1146      without successful validation on the origin server.  This allows
1147      an origin server to prevent the re-use of certain header fields in
1148      a response, while still allowing caching of the rest of the
1149      response.
1150
1151      Note: Most HTTP/1.0 caches will not recognize or obey this
1152      directive.  Also, no-cache response directives with field-names
1153      are often handled by implementations as if an unqualified no-cache
1154      directive was received; i.e., the special handling for the
1155      qualified form is not widely implemented.
1156
1157   no-store
1158
1159      The no-store response directive indicates that a cache MUST NOT
1160      store any part of either the immediate request or response.  This
1161      directive applies to both non-shared and shared caches.  "MUST NOT
1162      store" in this context means that the cache MUST NOT intentionally
1163      store the information in non-volatile storage, and MUST make a
1164      best-effort attempt to remove the information from volatile
1165      storage as promptly as possible after forwarding it.
1166
1167      This directive is NOT a reliable or sufficient mechanism for
1168      ensuring privacy.  In particular, malicious or compromised caches
1169      might not recognize or obey this directive, and communications
1170      networks might be vulnerable to eavesdropping.
1171
1172
1173
1174
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1179
1180   must-revalidate
1181
1182      The must-revalidate response directive indicates that once it has
1183      become stale, the response MUST NOT be used to satisfy subsequent
1184      requests without successful validation on the origin server.
1185
1186      The must-revalidate directive is necessary to support reliable
1187      operation for certain protocol features.  In all circumstances an
1188      HTTP/1.1 cache MUST obey the must-revalidate directive; in
1189      particular, if the cache cannot reach the origin server for any
1190      reason, it MUST generate a 504 (Gateway Timeout) response.
1191
1192      Servers SHOULD send the must-revalidate directive if and only if
1193      failure to validate a request on the representation could result
1194      in incorrect operation, such as a silently unexecuted financial
1195      transaction.
1196
1197   proxy-revalidate
1198
1199      The proxy-revalidate response directive has the same meaning as
1200      the must-revalidate response directive, except that it does not
1201      apply to non-shared caches.
1202
1203   max-age
1204
1205      The max-age response directive indicates that response is to be
1206      considered stale after its age is greater than the specified
1207      number of seconds.
1208
1209   s-maxage
1210
1211      The s-maxage response directive indicates that, in shared caches,
1212      the maximum age specified by this directive overrides the maximum
1213      age specified by either the max-age directive or the Expires
1214      header.  The s-maxage directive also implies the semantics of the
1215      proxy-revalidate response directive.
1216
1217   no-transform
1218
1219      The no-transform response directive indicates that an intermediate
1220      cache or proxy MUST NOT change the Content-Encoding, Content-Range
1221      or Content-Type response headers, nor the response representation.
1222
12233.2.3.  Cache Control Extensions
1224
1225   The Cache-Control header field can be extended through the use of one
1226   or more cache-extension tokens, each with an optional value.
1227   Informational extensions (those that do not require a change in cache
1228
1229
1230
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1234
1235
1236   behavior) can be added without changing the semantics of other
1237   directives.  Behavioral extensions are designed to work by acting as
1238   modifiers to the existing base of cache directives.  Both the new
1239   directive and the standard directive are supplied, such that
1240   applications that do not understand the new directive will default to
1241   the behavior specified by the standard directive, and those that
1242   understand the new directive will recognize it as modifying the
1243   requirements associated with the standard directive.  In this way,
1244   extensions to the cache-control directives can be made without
1245   requiring changes to the base protocol.
1246
1247   This extension mechanism depends on an HTTP cache obeying all of the
1248   cache-control directives defined for its native HTTP-version, obeying
1249   certain extensions, and ignoring all directives that it does not
1250   understand.
1251
1252   For example, consider a hypothetical new response directive called
1253   "community" that acts as a modifier to the private directive.  We
1254   define this new directive to mean that, in addition to any non-shared
1255   cache, any cache that is shared only by members of the community
1256   named within its value may cache the response.  An origin server
1257   wishing to allow the UCI community to use an otherwise private
1258   response in their shared cache(s) could do so by including
1259
1260     Cache-Control: private, community="UCI"
1261
1262   A cache seeing this header field will act correctly even if the cache
1263   does not understand the community cache-extension, since it will also
1264   see and understand the private directive and thus default to the safe
1265   behavior.
1266
1267   Unrecognized cache directives MUST be ignored; it is assumed that any
1268   cache directive likely to be unrecognized by an HTTP/1.1 cache will
1269   be combined with standard directives (or the response's default
1270   cacheability) such that the cache behavior will remain minimally
1271   correct even if the cache does not understand the extension(s).
1272
1273   The HTTP Cache Directive Registry defines the name space for the
1274   cache directives.
1275
1276   Registrations MUST include the following fields:
1277
1278   o  Cache Directive Name
1279
1280   o  Pointer to specification text
1281
1282   Values to be added to this name space are subject to IETF review
1283   ([RFC5226], Section 4.1).
1284
1285
1286
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1290
1291
1292   The registry itself is maintained at
1293   <http://www.iana.org/assignments/http-cache-directives>.
1294
12953.3.  Expires
1296
1297   The "Expires" header field gives the date/time after which the
1298   response is considered stale.  See Section 2.3 for further discussion
1299   of the freshness model.
1300
1301   The presence of an Expires field does not imply that the original
1302   resource will change or cease to exist at, before, or after that
1303   time.
1304
1305   The field-value is an absolute date and time as defined by HTTP-date
1306   in Section 6.1 of [Part1]; it MUST be sent in rfc1123-date format.
1307
1308     Expires   = "Expires" ":" OWS Expires-v
1309     Expires-v = HTTP-date
1310
1311   For example
1312
1313     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1314
1315      Note: If a response includes a Cache-Control field with the max-
1316      age directive (see Section 3.2.2), that directive overrides the
1317      Expires field.  Likewise, the s-maxage directive overrides Expires
1318      in shared caches.
1319
1320   HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
1321   the future.
1322
1323   HTTP/1.1 clients and caches MUST treat other invalid date formats,
1324   especially including the value "0", as in the past (i.e., "already
1325   expired").
1326
13273.4.  Pragma
1328
1329   The "Pragma" general-header field is used to include implementation-
1330   specific directives that might apply to any recipient along the
1331   request/response chain.  All pragma directives specify optional
1332   behavior from the viewpoint of the protocol; however, some systems
1333   MAY require that behavior be consistent with the directives.
1334
1335     Pragma            = "Pragma" ":" OWS Pragma-v
1336     Pragma-v          = 1#pragma-directive
1337     pragma-directive  = "no-cache" / extension-pragma
1338     extension-pragma  = token [ "=" ( token / quoted-string ) ]
1339
1340
1341
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1347
1348   When the no-cache directive is present in a request message, an
1349   application SHOULD forward the request toward the origin server even
1350   if it has a cached copy of what is being requested.  This pragma
1351   directive has the same semantics as the no-cache response directive
1352   (see Section 3.2.2) and is defined here for backward compatibility
1353   with HTTP/1.0.  Clients SHOULD include both header fields when a no-
1354   cache request is sent to a server not known to be HTTP/1.1 compliant.
1355   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
1356   sent "Cache-Control: no-cache".
1357
1358      Note: Because the meaning of "Pragma: no-cache" as a response-
1359      header field is not actually specified, it does not provide a
1360      reliable replacement for "Cache-Control: no-cache" in a response.
1361
1362   This mechanism is deprecated; no new Pragma directives will be
1363   defined in HTTP.
1364
13653.5.  Vary
1366
1367   The "Vary" response-header field conveys the set of request-header
1368   fields that were used to select the representation.
1369
1370   Caches use this information, in part, to determine whether a stored
1371   response can be used to satisfy a given request; see Section 2.7.
1372   determines, while the response is fresh, whether a cache is permitted
1373   to use the response to reply to a subsequent request without
1374   validation; see Section 2.7.
1375
1376   In uncacheable or stale responses, the Vary field value advises the
1377   user agent about the criteria that were used to select the
1378   representation.
1379
1380     Vary   = "Vary" ":" OWS Vary-v
1381     Vary-v = "*" / 1#field-name
1382
1383   The set of header fields named by the Vary field value is known as
1384   the selecting request-headers.
1385
1386   Servers SHOULD include a Vary header field with any cacheable
1387   response that is subject to server-driven negotiation.  Doing so
1388   allows a cache to properly interpret future requests on that resource
1389   and informs the user agent about the presence of negotiation on that
1390   resource.  A server MAY include a Vary header field with a non-
1391   cacheable response that is subject to server-driven negotiation,
1392   since this might provide the user agent with useful information about
1393   the dimensions over which the response varies at the time of the
1394   response.
1395
1396
1397
1398
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1402
1403
1404   A Vary field value of "*" signals that unspecified parameters not
1405   limited to the request-headers (e.g., the network address of the
1406   client), play a role in the selection of the response representation;
1407   therefore, a cache cannot determine whether this response is
1408   appropriate.  The "*" value MUST NOT be generated by a proxy server.
1409
1410   The field-names given are not limited to the set of standard request-
1411   header fields defined by this specification.  Field names are case-
1412   insensitive.
1413
14143.6.  Warning
1415
1416   The "Warning" general-header field is used to carry additional
1417   information about the status or transformation of a message that
1418   might not be reflected in the message.  This information is typically
1419   used to warn about possible incorrectness introduced by caching
1420   operations or transformations applied to the payload of the message.
1421
1422   Warnings can be used for other purposes, both cache-related and
1423   otherwise.  The use of a warning, rather than an error status code,
1424   distinguishes these responses from true failures.
1425
1426   Warning headers can in general be applied to any message, however
1427   some warn-codes are specific to caches and can only be applied to
1428   response messages.
1429
1430     Warning    = "Warning" ":" OWS Warning-v
1431     Warning-v  = 1#warning-value
1432
1433     warning-value = warn-code SP warn-agent SP warn-text
1434                                           [SP warn-date]
1435
1436     warn-code  = 3DIGIT
1437     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1438                     ; the name or pseudonym of the server adding
1439                     ; the Warning header, for use in debugging
1440     warn-text  = quoted-string
1441     warn-date  = DQUOTE HTTP-date DQUOTE
1442
1443   Multiple warnings can be attached to a response (either by the origin
1444   server or by a cache), including multiple warnings with the same code
1445   number, only differing in warn-text.
1446
1447   When this occurs, the user agent SHOULD inform the user of as many of
1448   them as possible, in the order that they appear in the response.
1449
1450   Systems that generate multiple Warning headers SHOULD order them with
1451   this user agent behavior in mind.  New Warning headers SHOULD be
1452
1453
1454
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1458
1459
1460   added after any existing Warning headers.
1461
1462   Warnings are assigned three digit warn-codes.  The first digit
1463   indicates whether the Warning is required to be deleted from a stored
1464   response after validation:
1465
1466   o  1xx Warnings describe the freshness or validation status of the
1467      response, and so MUST be deleted by caches after validation.  They
1468      can only be generated by a cache when validating a cached entry,
1469      and MUST NOT be generated in any other situation.
1470
1471   o  2xx Warnings describe some aspect of the representation that is
1472      not rectified by a validation (for example, a lossy compression of
1473      the representation) and MUST NOT be deleted by caches after
1474      validation, unless a full response is returned, in which case they
1475      MUST be.
1476
1477   If an implementation sends a message with one or more Warning headers
1478   to a receiver whose version is HTTP/1.0 or lower, then the sender
1479   MUST include in each warning-value a warn-date that matches the Date
1480   header in the message.
1481
1482   If an implementation receives a message with a warning-value that
1483   includes a warn-date, and that warn-date is different from the Date
1484   value in the response, then that warning-value MUST be deleted from
1485   the message before storing, forwarding, or using it. (preventing the
1486   consequences of naive caching of Warning header fields.)  If all of
1487   the warning-values are deleted for this reason, the Warning header
1488   MUST be deleted as well.
1489
1490   The following warn-codes are defined by this specification, each with
1491   a recommended warn-text in English, and a description of its meaning.
1492
1493   110 Response is stale
1494
1495      SHOULD be included whenever the returned response is stale.
1496
1497   111 Revalidation failed
1498
1499      SHOULD be included if a cache returns a stale response because an
1500      attempt to validate the response failed, due to an inability to
1501      reach the server.
1502
1503   112 Disconnected operation
1504
1505      SHOULD be included if the cache is intentionally disconnected from
1506      the rest of the network for a period of time.
1507
1508
1509
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1514
1515
1516   113 Heuristic expiration
1517
1518      SHOULD be included if the cache heuristically chose a freshness
1519      lifetime greater than 24 hours and the response's age is greater
1520      than 24 hours.
1521
1522   199 Miscellaneous warning
1523
1524      The warning text can include arbitrary information to be presented
1525      to a human user, or logged.  A system receiving this warning MUST
1526      NOT take any automated action, besides presenting the warning to
1527      the user.
1528
1529   214 Transformation applied
1530
1531      MUST be added by an intermediate proxy if it applies any
1532      transformation to the representation, such as changing the
1533      content-coding, media-type, or modifying the representation data,
1534      unless this Warning code already appears in the response.
1535
1536   299 Miscellaneous persistent warning
1537
1538      The warning text can include arbitrary information to be presented
1539      to a human user, or logged.  A system receiving this warning MUST
1540      NOT take any automated action.
1541
15424.  History Lists
1543
1544   User agents often have history mechanisms, such as "Back" buttons and
1545   history lists, that can be used to redisplay a representation
1546   retrieved earlier in a session.
1547
1548   The freshness model (Section 2.3) does not necessarily apply to
1549   history mechanisms.  I.e., a history mechanism can display a previous
1550   representation even if it has expired.
1551
1552   This does not prohibit the history mechanism from telling the user
1553   that a view might be stale, or from honoring cache directives (e.g.,
1554   Cache-Control: no-store).
1555
15565.  IANA Considerations
1557
15585.1.  Cache Directive Registry
1559
1560   The registration procedure for HTTP Cache Directives is defined by
1561   Section 3.2.3 of this document.
1562
1563   The HTTP Cache Directive Registry shall be created at
1564
1565
1566
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1570
1571
1572   <http://www.iana.org/assignments/http-cache-directives> and be
1573   populated with the registrations below:
1574
1575   +------------------------+------------------------------+
1576   | Cache Directive        | Reference                    |
1577   +------------------------+------------------------------+
1578   | max-age                | Section 3.2.1, Section 3.2.2 |
1579   | max-stale              | Section 3.2.1                |
1580   | min-fresh              | Section 3.2.1                |
1581   | must-revalidate        | Section 3.2.2                |
1582   | no-cache               | Section 3.2.1, Section 3.2.2 |
1583   | no-store               | Section 3.2.1, Section 3.2.2 |
1584   | no-transform           | Section 3.2.1, Section 3.2.2 |
1585   | only-if-cached         | Section 3.2.1                |
1586   | private                | Section 3.2.2                |
1587   | proxy-revalidate       | Section 3.2.2                |
1588   | public                 | Section 3.2.2                |
1589   | s-maxage               | Section 3.2.2                |
1590   | stale-if-error         | [RFC5861], Section 4         |
1591   | stale-while-revalidate | [RFC5861], Section 3         |
1592   +------------------------+------------------------------+
1593
15945.2.  Header Field Registration
1595
1596   The Message Header Field Registry located at <http://www.iana.org/
1597   assignments/message-headers/message-header-index.html> shall be
1598   updated with the permanent registrations below (see [RFC3864]):
1599
1600   +-------------------+----------+----------+-------------+
1601   | Header Field Name | Protocol | Status   | Reference   |
1602   +-------------------+----------+----------+-------------+
1603   | Age               | http     | standard | Section 3.1 |
1604   | Cache-Control     | http     | standard | Section 3.2 |
1605   | Expires           | http     | standard | Section 3.3 |
1606   | Pragma            | http     | standard | Section 3.4 |
1607   | Vary              | http     | standard | Section 3.5 |
1608   | Warning           | http     | standard | Section 3.6 |
1609   +-------------------+----------+----------+-------------+
1610
1611   The change controller is: "IETF (iesg@ietf.org) - Internet
1612   Engineering Task Force".
1613
16146.  Security Considerations
1615
1616   Caches expose additional potential vulnerabilities, since the
1617   contents of the cache represent an attractive target for malicious
1618   exploitation.  Because cache contents persist after an HTTP request
1619   is complete, an attack on the cache can reveal information long after
1620
1621
1622
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1626
1627
1628   a user believes that the information has been removed from the
1629   network.  Therefore, cache contents need to be protected as sensitive
1630   information.
1631
16327.  Acknowledgments
1633
1634   Much of the content and presentation of the caching design is due to
1635   suggestions and comments from individuals including: Shel Kaphan,
1636   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
1637
16388.  References
1639
16408.1.  Normative References
1641
1642   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1643              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1644              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
1645              and Message Parsing", draft-ietf-httpbis-p1-messaging-11
1646              (work in progress), August 2010.
1647
1648   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1649              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1650              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
1651              Semantics", draft-ietf-httpbis-p2-semantics-11 (work in
1652              progress), August 2010.
1653
1654   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1655              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1656              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
1657              Requests", draft-ietf-httpbis-p4-conditional-11 (work in
1658              progress), August 2010.
1659
1660   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1661              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1662              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1663              Partial Responses", draft-ietf-httpbis-p5-range-11 (work
1664              in progress), August 2010.
1665
1666   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1667              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1668              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
1669              draft-ietf-httpbis-p7-auth-11 (work in progress),
1670              August 2010.
1671
1672   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1673              Requirement Levels", BCP 14, RFC 2119, March 1997.
1674
1675   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1676
1677
1678
1679Fielding, et al.        Expires February 5, 2011               [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                 August 2010
1682
1683
1684              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1685
16868.2.  Informative References
1687
1688   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
1689              Specification, Implementation", RFC 1305, March 1992.
1690
1691   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1692              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1693              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1694
1695   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1696              Procedures for Message Header Fields", BCP 90, RFC 3864,
1697              September 2004.
1698
1699   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
1700              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
1701              May 2008.
1702
1703   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
1704              Content", RFC 5861, April 2010.
1705
1706Appendix A.  Changes from RFC 2616
1707
1708   Make the specified age calculation algorithm less conservative.
1709   (Section 2.3.2)
1710
1711   Remove requirement to consider Content-Location in successful
1712   responses in order to determine the appropriate response to use.
1713   (Section 2.4)
1714
1715   Clarify denial of service attack avoidance requirement.
1716   (Section 2.5)
1717
1718   Do not mention RFC 2047 encoding and multiple languages in Warning
1719   headers anymore, as these aspects never were implemented.
1720   (Section 3.6)
1721
1722Appendix B.  Collected ABNF
1723
1724   Age = "Age:" OWS Age-v
1725   Age-v = delta-seconds
1726
1727   Cache-Control = "Cache-Control:" OWS Cache-Control-v
1728   Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
1729    cache-directive ] )
1730
1731   Expires = "Expires:" OWS Expires-v
1732
1733
1734
1735Fielding, et al.        Expires February 5, 2011               [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                 August 2010
1738
1739
1740   Expires-v = HTTP-date
1741
1742   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1743
1744   OWS = <OWS, defined in [Part1], Section 1.2.2>
1745
1746   Pragma = "Pragma:" OWS Pragma-v
1747   Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1748    pragma-directive ] )
1749
1750   Vary = "Vary:" OWS Vary-v
1751   Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1752    ] ) )
1753
1754   Warning = "Warning:" OWS Warning-v
1755   Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1756    ] )
1757
1758   cache-directive = cache-request-directive / cache-response-directive
1759   cache-extension = token [ "=" ( token / quoted-string ) ]
1760   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
1761    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1762    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1763    cache-extension
1764   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
1765    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1766    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1767    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1768    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1769    ) / ( "s-maxage=" delta-seconds ) / cache-extension
1770
1771   delta-seconds = 1*DIGIT
1772
1773   extension-pragma = token [ "=" ( token / quoted-string ) ]
1774
1775   field-name = <field-name, defined in [Part1], Section 3.2>
1776
1777   port = <port, defined in [Part1], Section 2.6>
1778   pragma-directive = "no-cache" / extension-pragma
1779   pseudonym = <pseudonym, defined in [Part1], Section 9.9>
1780
1781   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1782
1783   token = <token, defined in [Part1], Section 1.2.2>
1784
1785   uri-host = <uri-host, defined in [Part1], Section 2.6>
1786
1787   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1788
1789
1790
1791Fielding, et al.        Expires February 5, 2011               [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                 August 2010
1794
1795
1796   warn-code = 3DIGIT
1797   warn-date = DQUOTE HTTP-date DQUOTE
1798   warn-text = quoted-string
1799   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
1800    ]
1801
1802   ABNF diagnostics:
1803
1804   ; Age defined but not used
1805   ; Cache-Control defined but not used
1806   ; Expires defined but not used
1807   ; Pragma defined but not used
1808   ; Vary defined but not used
1809   ; Warning defined but not used
1810
1811Appendix C.  Change Log (to be removed by RFC Editor before publication)
1812
1813C.1.  Since RFC2616
1814
1815   Extracted relevant partitions from [RFC2616].
1816
1817C.2.  Since draft-ietf-httpbis-p6-cache-00
1818
1819   Closed issues:
1820
1821   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
1822      (<http://purl.org/NET/http-errata#trailer-hop>)
1823
1824   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
1825      after Update or Delete"
1826      (<http://purl.org/NET/http-errata#invalidupd>)
1827
1828   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1829      Informative references"
1830
1831   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
1832      typo"
1833
1834   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
1835      header text"
1836
1837   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
1838      references"
1839
1840   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
1841      Reference"
1842
1843
1844
1845
1846
1847Fielding, et al.        Expires February 5, 2011               [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                 August 2010
1850
1851
1852   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
1853      to-date references"
1854
1855   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
1856      13.2.2"
1857
1858   Other changes:
1859
1860   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
1861      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1862
1863C.3.  Since draft-ietf-httpbis-p6-cache-01
1864
1865   Closed issues:
1866
1867   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
1868      used"
1869
1870   Other changes:
1871
1872   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
1873      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1874
1875   o  Add explicit references to BNF syntax and rules imported from
1876      other parts of the specification.
1877
1878C.4.  Since draft-ietf-httpbis-p6-cache-02
1879
1880   Ongoing work on IANA Message Header Registration
1881   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1882
1883   o  Reference RFC 3984, and update header registrations for headers
1884      defined in this document.
1885
1886C.5.  Since draft-ietf-httpbis-p6-cache-03
1887
1888   Closed issues:
1889
1890   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
1891      classification"
1892
1893C.6.  Since draft-ietf-httpbis-p6-cache-04
1894
1895   Ongoing work on ABNF conversion
1896   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1897
1898   o  Use "/" instead of "|" for alternatives.
1899
1900
1901
1902
1903Fielding, et al.        Expires February 5, 2011               [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                 August 2010
1906
1907
1908   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1909      whitespace ("OWS") and required whitespace ("RWS").
1910
1911   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1912      value format definitions.
1913
1914C.7.  Since draft-ietf-httpbis-p6-cache-05
1915
1916   This is a total rewrite of this part of the specification.
1917
1918   Affected issues:
1919
1920   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1921      1xx Warn-Codes"
1922
1923   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
1924      13.5.1 and 13.5.2"
1925
1926   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role of
1927      Warning and Semantic Transparency in Caching"
1928
1929   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
1930      Caching"
1931
1932   In addition: Final work on ABNF conversion
1933   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1934
1935   o  Add appendix containing collected and expanded ABNF, reorganize
1936      ABNF introduction.
1937
1938C.8.  Since draft-ietf-httpbis-p6-cache-06
1939
1940   Closed issues:
1941
1942   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
1943      numeric protocol elements"
1944
1945   Affected issues:
1946
1947   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: Vary and non-
1948      existant headers
1949
1950C.9.  Since draft-ietf-httpbis-p6-cache-07
1951
1952   Closed issues:
1953
1954   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1955      1xx Warn-Codes"
1956
1957
1958
1959Fielding, et al.        Expires February 5, 2011               [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                 August 2010
1962
1963
1964   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
1965      Location on 304 responses"
1966
1967   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
1968      no-cache CC directives with headers"
1969
1970   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
1971      warn-text"
1972
1973C.10.  Since draft-ietf-httpbis-p6-cache-08
1974
1975   Closed issues:
1976
1977   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
1978      negotiated responses from cache: header-specific canonicalization"
1979
1980   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
1981      directives on history lists"
1982
1983   Affected issues:
1984
1985   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
1986      and caching
1987
1988   Partly resolved issues:
1989
1990   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
1991      13.5.1 and 13.5.2"
1992
1993C.11.  Since draft-ietf-httpbis-p6-cache-09
1994
1995   Closed issues:
1996
1997   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/29>: "Age
1998      calculation"
1999
2000   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/168>: "Clarify
2001      differences between / requirements for request and response CC
2002      directives"
2003
2004   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/174>: "Caching
2005      authenticated responses"
2006
2007   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/208>: "IANA registry
2008      for cache-control directives"
2009
2010   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
2011      caching of URLs with query components"
2012
2013
2014
2015Fielding, et al.        Expires February 5, 2011               [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                 August 2010
2018
2019
2020   Partly resolved issues:
2021
2022   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
2023      requested resource's URI"
2024
2025C.12.  Since draft-ietf-httpbis-p6-cache-10
2026
2027   Closed issues:
2028
2029   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
2030      entity / representation / variant terminology"
2031
2032   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
2033      removing the 'changes from 2068' sections"
2034
2035   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2036      heuristic caching for new status codes"
2037
2038   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2039      heuristic caching for new status codes"
2040
2041   o  Clean up TODOs and prose in "Combining Responses."
2042
2043Index
2044
2045   A
2046      age  6
2047      Age header  17
2048
2049   C
2050      cache  5
2051      Cache Directives
2052         max-age  19, 22
2053         max-stale  19
2054         min-fresh  19
2055         must-revalidate  22
2056         no-cache  19, 21
2057         no-store  19, 21
2058         no-transform  20, 22
2059         only-if-cached  20
2060         private  20
2061         proxy-revalidate  22
2062         public  20
2063         s-maxage  22
2064      Cache-Control header  18
2065      cacheable  5
2066
2067   E
2068
2069
2070
2071Fielding, et al.        Expires February 5, 2011               [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                 August 2010
2074
2075
2076      Expires header  24
2077      explicit expiration time  5
2078
2079   F
2080      first-hand  6
2081      fresh  6
2082      freshness lifetime  6
2083
2084   G
2085      Grammar
2086         Age  17
2087         Age-v  17
2088         Cache-Control  18
2089         Cache-Control-v  18
2090         cache-extension  18
2091         cache-request-directive  18
2092         cache-response-directive  20
2093         delta-seconds  17
2094         Expires  24
2095         Expires-v  24
2096         extension-pragma  24
2097         Pragma  24
2098         pragma-directive  24
2099         Pragma-v  24
2100         Vary  25
2101         Vary-v  25
2102         warn-agent  26
2103         warn-code  26
2104         warn-date  26
2105         warn-text  26
2106         Warning  26
2107         Warning-v  26
2108         warning-value  26
2109
2110   H
2111      Headers
2112         Age  17
2113         Cache-Control  18
2114         Expires  24
2115         Pragma  24
2116         Vary  25
2117         Warning  26
2118      heuristic expiration time  5
2119
2120   M
2121      max-age
2122         Cache Directive  19, 22
2123      max-stale
2124
2125
2126
2127Fielding, et al.        Expires February 5, 2011               [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                 August 2010
2130
2131
2132         Cache Directive  19
2133      min-fresh
2134         Cache Directive  19
2135      must-revalidate
2136         Cache Directive  22
2137
2138   N
2139      no-cache
2140         Cache Directive  19, 21
2141      no-store
2142         Cache Directive  19, 21
2143      no-transform
2144         Cache Directive  20, 22
2145
2146   O
2147      only-if-cached
2148         Cache Directive  20
2149
2150   P
2151      Pragma header  24
2152      private
2153         Cache Directive  20
2154      proxy-revalidate
2155         Cache Directive  22
2156      public
2157         Cache Directive  20
2158
2159   S
2160      s-maxage
2161         Cache Directive  22
2162      stale  6
2163
2164   V
2165      validator  6
2166      Vary header  25
2167
2168   W
2169      Warning header  26
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183Fielding, et al.        Expires February 5, 2011               [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                 August 2010
2186
2187
2188Authors' Addresses
2189
2190   Roy T. Fielding (editor)
2191   Day Software
2192   23 Corporate Plaza DR, Suite 280
2193   Newport Beach, CA  92660
2194   USA
2195
2196   Phone: +1-949-706-5300
2197   Fax:   +1-949-706-5305
2198   EMail: fielding@gbiv.com
2199   URI:   http://roy.gbiv.com/
2200
2201
2202   Jim Gettys
2203   Alcatel-Lucent Bell Labs
2204   21 Oak Knoll Road
2205   Carlisle, MA  01741
2206   USA
2207
2208   EMail: jg@freedesktop.org
2209   URI:   http://gettys.wordpress.com/
2210
2211
2212   Jeffrey C. Mogul
2213   Hewlett-Packard Company
2214   HP Labs, Large Scale Systems Group
2215   1501 Page Mill Road, MS 1177
2216   Palo Alto, CA  94304
2217   USA
2218
2219   EMail: JeffMogul@acm.org
2220
2221
2222   Henrik Frystyk Nielsen
2223   Microsoft Corporation
2224   1 Microsoft Way
2225   Redmond, WA  98052
2226   USA
2227
2228   EMail: henrikn@microsoft.com
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2241Internet-Draft              HTTP/1.1, Part 6                 August 2010
2242
2243
2244   Larry Masinter
2245   Adobe Systems, Incorporated
2246   345 Park Ave
2247   San Jose, CA  95110
2248   USA
2249
2250   EMail: LMM@acm.org
2251   URI:   http://larry.masinter.net/
2252
2253
2254   Paul J. Leach
2255   Microsoft Corporation
2256   1 Microsoft Way
2257   Redmond, WA  98052
2258
2259   EMail: paulle@microsoft.com
2260
2261
2262   Tim Berners-Lee
2263   World Wide Web Consortium
2264   MIT Computer Science and Artificial Intelligence Laboratory
2265   The Stata Center, Building 32
2266   32 Vassar Street
2267   Cambridge, MA  02139
2268   USA
2269
2270   EMail: timbl@w3.org
2271   URI:   http://www.w3.org/People/Berners-Lee/
2272
2273
2274   Yves Lafon (editor)
2275   World Wide Web Consortium
2276   W3C / ERCIM
2277   2004, rte des Lucioles
2278   Sophia-Antipolis, AM  06902
2279   France
2280
2281   EMail: ylafon@w3.org
2282   URI:   http://www.raubacapeu.net/people/yves/
2283
2284
2285   Mark Nottingham (editor)
2286
2287   EMail: mnot@mnot.net
2288   URI:   http://www.mnot.net/
2289
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2297Internet-Draft              HTTP/1.1, Part 6                 August 2010
2298
2299
2300   Julian F. Reschke (editor)
2301   greenbytes GmbH
2302   Hafenweg 16
2303   Muenster, NW  48155
2304   Germany
2305
2306   Phone: +49 251 2807760
2307   Fax:   +49 251 2807761
2308   EMail: julian.reschke@greenbytes.de
2309   URI:   http://greenbytes.de/tech/webdav/
2310
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2352
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