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4HTTPbis Working Group                                   R. Fielding, Ed.
5Internet-Draft                                                     Adobe
6Obsoletes: 2616 (if approved)                                  J. Gettys
7Intended status: Standards Track                          Alcatel-Lucent
8Expires: September 15, 2011                                     J. Mogul
9                                                                      HP
10                                                              H. Frystyk
11                                                               Microsoft
12                                                             L. Masinter
13                                                                   Adobe
14                                                                P. Leach
15                                                               Microsoft
16                                                          T. Berners-Lee
17                                                                 W3C/MIT
18                                                           Y. Lafon, Ed.
19                                                                     W3C
20                                                      M. Nottingham, Ed.
21
22                                                         J. Reschke, Ed.
23                                                              greenbytes
24                                                          March 14, 2011
25
26
27                       HTTP/1.1, part 6: Caching
28                     draft-ietf-httpbis-p6-cache-13
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.14.
49
50Status of This Memo
51
52
53
54
55Fielding, et al.       Expires September 15, 2011               [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 6                  March 2011
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 September 15, 2011.
74
75Copyright Notice
76
77   Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . . . . . .  7
108
109
110
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113Internet-Draft              HTTP/1.1, Part 6                  March 2011
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  . . . . . . . . . . . . . . . . . . . . . . .  8
121     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  8
122       2.1.1.  Storing Partial and Incomplete Responses . . . . . . .  9
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  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
135     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 18
136       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 19
137       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 21
138       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 23
139     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 24
140     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 25
141     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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  . . . . . . . . . . . . . . . . . . . . . . . 31
149   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
150     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 31
151     8.2.  Informative References . . . . . . . . . . . . . . . . . . 32
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)  . . . . . . . . . . . . . . . . . . . . 34
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 . . . . . . . . . . . 35
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 . . . . . . . . . . . 36
163     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 36
164
165
166
167Fielding, et al.       Expires September 15, 2011               [Page 3]
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169Internet-Draft              HTTP/1.1, Part 6                  March 2011
170
171
172     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36
173     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 37
174     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 37
175     C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 38
176     C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 38
177     C.14. Since draft-ietf-httpbis-p6-cache-12 . . . . . . . . . . . 38
178   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
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
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227
2281.  Introduction
229
230   HTTP is typically used for distributed information systems, where
231   performance can be improved by the use of response caches.  This
232   document defines aspects of HTTP/1.1 related to caching and reusing
233   response messages.
234
2351.1.  Purpose
236
237   An HTTP cache is a local store of response messages and the subsystem
238   that controls its message storage, retrieval, and deletion.  A cache
239   stores cacheable responses in order to reduce the response time and
240   network bandwidth consumption on future, equivalent requests.  Any
241   client or server MAY 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   cache
261
262      A conformant implementation of a HTTP cache.  Note that this
263      implies an HTTP/1.1 cache; this specification does not define
264      conformance for HTTP/1.0 caches.
265
266   shared cache
267
268      A cache that is accessible to more than one user; usually (but not
269      always) deployed as part of an intermediary.
270
271   private cache
272
273      A cache that is dedicated to a single user.
274
275
276
277
278
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283
284   cacheable
285
286      A response is cacheable if a cache is allowed to store a copy of
287      the response message for use in answering subsequent requests.
288      Even when a response is cacheable, there might be additional
289      constraints on whether a cache can use the stored copy to satisfy
290      a particular request.
291
292   explicit expiration time
293
294      The time at which the origin server intends that a representation
295      no longer be returned by a cache without further validation.
296
297   heuristic expiration time
298
299      An expiration time assigned by a cache when no explicit expiration
300      time is available.
301
302   age
303
304      The age of a response is the time since it was sent by, or
305      successfully validated with, the origin server.
306
307   first-hand
308
309      A response is first-hand if the freshness model is not in use;
310      i.e., its age is 0.
311
312   freshness lifetime
313
314      The length of time between the generation of a response and its
315      expiration time.
316
317   fresh
318
319      A response is fresh if its age has not yet exceeded its freshness
320      lifetime.
321
322   stale
323
324      A response is stale if its age has passed its freshness lifetime
325      (either explicit or heuristic).
326
327   validator
328
329      A protocol element (e.g., an entity-tag or a Last-Modified time)
330      that is used to find out whether a stored response is an
331      equivalent copy of a representation.
332
333
334
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339
3401.3.  Requirements
341
342   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
343   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
344   document are to be interpreted as described in [RFC2119].
345
346   An implementation is not compliant if it fails to satisfy one or more
347   of the "MUST" or "REQUIRED" level requirements for the protocols it
348   implements.  An implementation that satisfies all the "MUST" or
349   "REQUIRED" level and all the "SHOULD" level requirements for its
350   protocols is said to be "unconditionally compliant"; one that
351   satisfies all the "MUST" level requirements but not all the "SHOULD"
352   level requirements for its protocols is said to be "conditionally
353   compliant".
354
3551.4.  Syntax Notation
356
357   This specification uses the ABNF syntax defined in Section 1.2 of
358   [Part1] (which extends the syntax defined in [RFC5234] with a list
359   rule).  Appendix B shows the collected ABNF, with the list rule
360   expanded.
361
362   The following core rules are included by reference, as defined in
363   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
364   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
365   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
366   sequence of data), SP (space), VCHAR (any visible USASCII character),
367   and WSP (whitespace).
368
3691.4.1.  Core Rules
370
371   The core rules below are defined in Section 1.2.2 of [Part1]:
372
373     quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
374     token         = <token, defined in [Part1], Section 1.2.2>
375     OWS           = <OWS, defined in [Part1], Section 1.2.2>
376
3771.4.2.  ABNF Rules defined in other Parts of the Specification
378
379   The ABNF rules below are defined in other parts:
380
381     field-name    = <field-name, defined in [Part1], Section 3.2>
382     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
383     port          = <port, defined in [Part1], Section 2.6>
384     pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
385     uri-host      = <uri-host, defined in [Part1], Section 2.6>
386
387
388
389
390
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395
3962.  Cache Operation
397
3982.1.  Response Cacheability
399
400   A cache MUST NOT store a response to any request, unless:
401
402   o  The request method is understood by the cache and defined as being
403      cacheable, and
404
405   o  the response status code is understood by the cache, and
406
407   o  the "no-store" cache directive (see Section 3.2) does not appear
408      in request or response header fields, and
409
410   o  the "private" cache response directive (see Section 3.2.2 does not
411      appear in the response, if the cache is shared, and
412
413   o  the "Authorization" header field (see Section 4.1 of [Part7]) does
414      not appear in the request, if the cache is shared, unless the
415      response explicitly allows it (see Section 2.6), and
416
417   o  the response either:
418
419      *  contains an Expires header field (see Section 3.3), or
420
421      *  contains a max-age response cache directive (see
422         Section 3.2.2), or
423
424      *  contains a s-maxage response cache directive and the cache is
425         shared, or
426
427      *  contains a Cache Control Extension (see Section 3.2.3) that
428         allows it to be cached, or
429
430      *  has a status code that can be served with heuristic freshness
431         (see Section 2.3.1.1).
432
433   In this context, a cache has "understood" a request method or a
434   response status code if it recognises it and implements any cache-
435   specific behaviour.  In particular, 206 Partial Content responses
436   cannot be cached by an implementation that does not handle partial
437   content (see Section 2.1.1).
438
439   Note that in normal operation, most caches will not store a response
440   that has neither a cache validator nor an explicit expiration time,
441   as such responses are not usually useful to store.  However, caches
442   are not prohibited from storing such responses.
443
444
445
446
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451
4522.1.1.  Storing Partial and Incomplete Responses
453
454   A cache that receives an incomplete response (for example, with fewer
455   bytes of data than specified in a Content-Length header field) can
456   store the response, but MUST treat it as a partial response [Part5].
457   Partial responses can be combined as described in Section 4 of
458   [Part5]; the result might be a full response or might still be
459   partial.  A cache MUST NOT return a partial response to a client
460   without explicitly marking it as such using the 206 (Partial Content)
461   status code.
462
463   A cache that does not support the Range and Content-Range header
464   fields MUST NOT store incomplete or partial responses.
465
4662.2.  Constructing Responses from Caches
467
468   For a presented request, a cache MUST NOT return a stored response,
469   unless:
470
471   o  The presented effective request URI (Section 4.3 of [Part1]) and
472      that of the stored response match, and
473
474   o  the request method associated with the stored response allows it
475      to be used for the presented request, and
476
477   o  selecting header fields nominated by the stored response (if any)
478      match those presented (see Section 2.7), and
479
480   o  the presented request and stored response are free from directives
481      that would prevent its use (see Section 3.2 and Section 3.4), and
482
483   o  the stored response is either:
484
485      *  fresh (see Section 2.3), or
486
487      *  allowed to be served stale (see Section 2.3.3), or
488
489      *  successfully validated (see Section 2.4).
490
491   When a stored response is used to satisfy a request without
492   validation, a cache MUST include a single Age header field
493   (Section 3.1) in the response with a value equal to the stored
494   response's current_age; see Section 2.3.2.
495
496   A cache MUST write through requests with methods that are unsafe
497   (Section 7.1.1 of [Part2]) to the origin server; i.e., a cache must
498   not generate a reply to such a request before having forwarded the
499   request and having received a corresponding response.
500
501
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506
507
508   Also, note that unsafe requests might invalidate already stored
509   responses; see Section 2.5.
510
511   A cache MUST use the most recent response (as determined by the Date
512   header field) when more than one suitable response is stored.  It can
513   also forward a request with "Cache-Control: max-age=0" or "Cache-
514   Control: no-cache" to disambiguate which response to use.
515
516   A cache that does not have a clock available MUST NOT used stored
517   responses without revalidating them on every use.  A cache,
518   especially a shared cache, SHOULD use a mechanism, such as NTP
519   [RFC1305], to synchronize its clock with a reliable external
520   standard.
521
5222.3.  Freshness Model
523
524   When a response is "fresh" in the cache, it can be used to satisfy
525   subsequent requests without contacting the origin server, thereby
526   improving efficiency.
527
528   The primary mechanism for determining freshness is for an origin
529   server to provide an explicit expiration time in the future, using
530   either the Expires header field (Section 3.3) or the max-age response
531   cache directive (Section 3.2.2).  Generally, origin servers will
532   assign future explicit expiration times to responses in the belief
533   that the representation is not likely to change in a semantically
534   significant way before the expiration time is reached.
535
536   If an origin server wishes to force a cache to validate every
537   request, it can assign an explicit expiration time in the past to
538   indicate that the response is already stale.  Compliant caches will
539   normally validate the cached response before reusing it for
540   subsequent requests (see Section 2.3.3).
541
542   Since origin servers do not always provide explicit expiration times,
543   a cache MAY assign a heuristic expiration time when an explicit time
544   is not specified, employing algorithms that use other header field
545   values (such as the Last-Modified time) to estimate a plausible
546   expiration time.  This specification does not provide specific
547   algorithms, but does impose worst-case constraints on their results.
548
549   The calculation to determine if a response is fresh is:
550
551      response_is_fresh = (freshness_lifetime > current_age)
552
553   The freshness_lifetime is defined in Section 2.3.1; the current_age
554   is defined in Section 2.3.2.
555
556
557
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563
564   Additionally, clients might need to influence freshness calculation.
565   They can do this using several request cache directives, with the
566   effect of either increasing or loosening constraints on freshness.
567   See Section 3.2.1.
568
569   [[ISSUE-no-req-for-directives: there are not requirements directly
570   applying to cache-request-directives and freshness.]]
571
572   Note that freshness applies only to cache operation; it cannot be
573   used to force a user agent to refresh its display or reload a
574   resource.  See Section 4 for an explanation of the difference between
575   caches and history mechanisms.
576
5772.3.1.  Calculating Freshness Lifetime
578
579   A cache can calculate the freshness lifetime (denoted as
580   freshness_lifetime) of a response by using the first match of:
581
582   o  If the cache is shared and the s-maxage response cache directive
583      (Section 3.2.2) is present, use its value, or
584
585   o  If the max-age response cache directive (Section 3.2.2) is
586      present, use its value, or
587
588   o  If the Expires response header field (Section 3.3) is present, use
589      its value minus the value of the Date response header field, or
590
591   o  Otherwise, no explicit expiration time is present in the response.
592      A heuristic freshness lifetime might be applicable; see
593      Section 2.3.1.1.
594
595   Note that this calculation is not vulnerable to clock skew, since all
596   of the information comes from the origin server.
597
5982.3.1.1.  Calculating Heuristic Freshness
599
600   If no explicit expiration time is present in a stored response that
601   has a status code whose definition allows heuristic freshness to be
602   used (including the following in Section 8 of [Part2]: 200, 203, 206,
603   300, 301 and 410), a cache MAY calculate a heuristic expiration time.
604   A cache MUST NOT use heuristics to determine freshness for responses
605   with status codes that do not explicitly allow it.
606
607   When a heuristic is used to calculate freshness lifetime, a cache
608   SHOULD attach a Warning header field with a 113 warn-code to the
609   response if its current_age is more than 24 hours and such a warning
610   is not already present.
611
612
613
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619
620   Also, if the response has a Last-Modified header field (Section 6.6
621   of [Part4]), a cache SHOULD NOT use a heuristic expiration value that
622   is more than some fraction of the interval since that time.  A
623   typical setting of this fraction might be 10%.
624
625      Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
626      not calculate heuristic freshness for URIs with query components
627      (i.e., those containing '?').  In practice, this has not been
628      widely implemented.  Therefore, servers are encouraged to send
629      explicit directives (e.g., Cache-Control: no-cache) if they wish
630      to preclude caching.
631
6322.3.2.  Calculating Age
633
634   HTTP/1.1 uses the Age header field to convey the estimated age of the
635   response message when obtained from a cache.  The Age field value is
636   the cache's estimate of the amount of time since the response was
637   generated or validated by the origin server.  In essence, the Age
638   value is the sum of the time that the response has been resident in
639   each of the caches along the path from the origin server, plus the
640   amount of time it has been in transit along network paths.
641
642   The following data is used for the age calculation:
643
644   age_value
645
646      The term "age_value" denotes the value of the Age header field
647      (Section 3.1), in a form appropriate for arithmetic operation; or
648      0, if not available.
649
650   date_value
651
652      HTTP/1.1 requires origin servers to send a Date header field, if
653      possible, with every response, giving the time at which the
654      response was generated.  The term "date_value" denotes the value
655      of the Date header field, in a form appropriate for arithmetic
656      operations.  See Section 9.3 of [Part1] for the definition of the
657      Date header field, and for requirements regarding responses
658      without it.
659
660   now
661
662      The term "now" means "the current value of the clock at the host
663      performing the calculation".  A cache SHOULD use NTP ([RFC1305])
664      or some similar protocol to synchronize its clocks to a globally
665      accurate time standard.
666
667
668
669
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675
676   request_time
677
678      The current value of the clock at the host at the time the request
679      resulting in the stored response was made.
680
681   response_time
682
683      The current value of the clock at the host at the time the
684      response was received.
685
686   A response's age can be calculated in two entirely independent ways:
687
688   1.  the "apparent_age": response_time minus date_value, if the local
689       clock is reasonably well synchronized to the origin server's
690       clock.  If the result is negative, the result is replaced by
691       zero.
692
693   2.  the "corrected_age_value", if all of the caches along the
694       response path implement HTTP/1.1.  A cache MUST interpret this
695       value relative to the time the request was initiated, not the
696       time that the response was received.
697
698
699     apparent_age = max(0, response_time - date_value);
700
701     response_delay = response_time - request_time;
702     corrected_age_value = age_value + response_delay;
703
704   These are combined as
705
706     corrected_initial_age = max(apparent_age, corrected_age_value);
707
708   The current_age of a stored response can then be calculated by adding
709   the amount of time (in seconds) since the stored response was last
710   validated by the origin server to the corrected_initial_age.
711
712     resident_time = now - response_time;
713     current_age = corrected_initial_age + resident_time;
714
7152.3.3.  Serving Stale Responses
716
717   A "stale" response is one that either has explicit expiry information
718   or is allowed to have heuristic expiry calculated, but is not fresh
719   according to the calculations in Section 2.3.
720
721   A cache MUST NOT return a stale response if it is prohibited by an
722   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
723   cache directive, a "must-revalidate" cache-response-directive, or an
724
725
726
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731
732   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
733   see Section 3.2.2).
734
735   A cache SHOULD NOT return stale responses unless it is disconnected
736   (i.e., it cannot contact the origin server or otherwise find a
737   forward path) or otherwise explicitly allowed (e.g., the max-stale
738   request directive; see Section 3.2.1).
739
740   A cache SHOULD append a Warning header field with the 110 warn-code
741   (see Section 3.6) to stale responses.  Likewise, a cache SHOULD add
742   the 112 warn-code to stale responses if the cache is disconnected.
743
744   If a cache receives a first-hand response (either an entire response,
745   or a 304 (Not Modified) response) that it would normally forward to
746   the requesting client, and the received response is no longer fresh,
747   the cache SHOULD forward it to the requesting client without adding a
748   new Warning (but without removing any existing Warning header
749   fields).  A cache SHOULD NOT attempt to validate a response simply
750   because that response became stale in transit.
751
7522.4.  Validation Model
753
754   When a cache has one or more stored responses for a requested URI,
755   but cannot serve any of them (e.g., because they are not fresh, or
756   one cannot be selected; see Section 2.7), it can use the conditional
757   request mechanism [Part4] in the forwarded request to give the origin
758   server an opportunity to both select a valid stored response to be
759   used, and to update it.  This process is known as "validating" or
760   "revalidating" the stored response.
761
762   When sending such a conditional request, a cache SHOULD add an If-
763   Modified-Since header field whose value is that of the Last-Modified
764   header field from the selected (see Section 2.7) stored response, if
765   available.
766
767   Additionally, a cache SHOULD add an If-None-Match header field whose
768   value is that of the ETag header field(s) from all responses stored
769   for the requested URI, if present.  However, if any of the stored
770   responses contains only partial content, the cache SHOULD NOT include
771   its entity-tag in the If-None-Match header field unless the request
772   is for a range that would be fully satisfied by that stored response.
773
774   A 304 (Not Modified) response status code indicates that the stored
775   response can be updated and reused; see Section 2.8.
776
777   A full response (i.e., one with a response body) indicates that none
778   of the stored responses nominated in the conditional request is
779   suitable.  Instead, a cache SHOULD use the full response to satisfy
780
781
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787
788   the request and MAY replace the stored response.
789
790   If a cache receives a 5xx response while attempting to validate a
791   response, it MAY either forward this response to the requesting
792   client, or act as if the server failed to respond.  In the latter
793   case, it MAY return a previously stored response (see Section 2.3.3).
794
7952.5.  Request Methods that Invalidate
796
797   Because unsafe request methods (Section 7.1.1 of [Part2]) have the
798   potential for changing state on the origin server, intervening caches
799   can use them to keep their contents up-to-date.
800
801   A cache MUST invalidate the effective Request URI (Section 4.3 of
802   [Part1]) as well as the URI(s) in the Location and Content-Location
803   header fields (if present) when the following request methods are
804   received:
805
806   o  PUT
807
808   o  DELETE
809
810   o  POST
811
812   However, a cache MUST NOT invalidate a URI from a Location or
813   Content-Location header field if the host part of that URI differs
814   from the host part in the effective request URI (Section 4.3 of
815   [Part1]).  This helps prevent denial of service attacks.
816
817   A cache that passes through requests with methods it does not
818   understand SHOULD invalidate the effective request URI (Section 4.3
819   of [Part1]).
820
821   Here, "invalidate" means that the cache will either remove all stored
822   responses related to the effective request URI, or will mark these as
823   "invalid" and in need of a mandatory validation before they can be
824   returned in response to a subsequent request.
825
826   Note that this does not guarantee that all appropriate responses are
827   invalidated.  For example, the request that caused the change at the
828   origin server might not have gone through the cache where a response
829   is stored.
830
8312.6.  Shared Caching of Authenticated Responses
832
833   A shared cache MUST NOT use a cached response to a request with an
834   Authorization header field (Section 4.1 of [Part7]) to satisfy any
835   subsequent request unless a cache directive that allows such
836
837
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843
844   responses to be stored is present in the response.
845
846   In this specification, the following Cache-Control response
847   directives (Section 3.2.2) have such an effect: must-revalidate,
848   public, s-maxage.
849
850   Note that cached responses that contain the "must-revalidate" and/or
851   "s-maxage" response directives are not allowed to be served stale
852   (Section 2.3.3) by shared caches.  In particular, a response with
853   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
854   satisfy a subsequent request without revalidating it on the origin
855   server.
856
8572.7.  Caching Negotiated Responses
858
859   When a cache receives a request that can be satisfied by a stored
860   response that has a Vary header field (Section 3.5), it MUST NOT use
861   that response unless all of the selecting header fields nominated by
862   the Vary header field match in both the original request (i.e., that
863   associated with the stored response), and the presented request.
864
865   The selecting header fields from two requests are defined to match if
866   and only if those in the first request can be transformed to those in
867   the second request by applying any of the following:
868
869   o  adding or removing whitespace, where allowed in the header field's
870      syntax
871
872   o  combining multiple header fields with the same field name (see
873      Section 3.2 of [Part1])
874
875   o  normalizing both header field values in a way that is known to
876      have identical semantics, according to the header field's
877      specification (e.g., re-ordering field values when order is not
878      significant; case-normalization, where values are defined to be
879      case-insensitive)
880
881   If (after any normalization that might take place) a header field is
882   absent from a request, it can only match another request if it is
883   also absent there.
884
885   A Vary header field-value of "*" always fails to match, and
886   subsequent requests to that resource can only be properly interpreted
887   by the origin server.
888
889   The stored response with matching selecting header fields is known as
890   the selected response.
891
892
893
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899
900   If no selected response is available, the cache MAY forward the
901   presented request to the origin server in a conditional request; see
902   Section 2.4.
903
9042.8.  Combining Responses
905
906   When a cache receives a 304 (Not Modified) response or a 206 (Partial
907   Content) response (in this section, the "new" response"), it needs to
908   create an updated response by combining the stored response with the
909   new one, so that the updated response can be used to satisfy the
910   request, and potentially update the cached response.
911
912   If the new response contains an ETag, it identifies the stored
913   response to use.  [[TODO-mention-CL: might need language about
914   Content-Location here]][[TODO-select-for-combine: Shouldn't this be
915   the selected response?]]
916
917   When the new response's status code is 206 (partial content), a cache
918   MUST NOT combine it with the old response if either response does not
919   have a validator, and MUST NOT combine it with the old response when
920   those validators do not match with the strong comparison function
921   (see Section 4 of [Part4]).
922
923   The stored response header fields are used as those of the updated
924   response, except that
925
926   o  a cache MUST delete any stored Warning header fields with warn-
927      code 1xx (see Section 3.6).
928
929   o  a cache MUST retain any stored Warning header fields with warn-
930      code 2xx.
931
932   o  a cache MUST use other header fields provided in the new response
933      to replace all instances of the corresponding header fields from
934      the stored response.
935
936   A cache MUST use the updated response header fields to replace those
937   of the stored response (unless the stored response is removed).  In
938   the case of a 206 response, a cache MAY store the combined
939   representation.
940
9413.  Header Field Definitions
942
943   This section defines the syntax and semantics of HTTP/1.1 header
944   fields related to caching.
945
946
947
948
949
950
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955
9563.1.  Age
957
958   The "Age" header field conveys the sender's estimate of the amount of
959   time since the response was generated or successfully validated at
960   the origin server.  Age values are calculated as specified in
961   Section 2.3.2.
962
963     Age   = "Age" ":" OWS Age-v
964     Age-v = delta-seconds
965
966   Age field-values are non-negative integers, representing time in
967   seconds.
968
969     delta-seconds  = 1*DIGIT
970
971   If a cache receives a value larger than the largest positive integer
972   it can represent, or if any of its age calculations overflows, it
973   MUST transmit an Age header field with a field-value of 2147483648
974   (2^31).  Recipients parsing the Age header field-value SHOULD use an
975   arithmetic type of at least 31 bits of range.
976
977   The presence of an Age header field in a response implies that a
978   response is not first-hand.  However, the converse is not true, since
979   HTTP/1.0 caches might not implement the Age header field.
980
9813.2.  Cache-Control
982
983   The "Cache-Control" header field is used to specify directives for
984   caches along the request/response chain.  Such cache directives are
985   unidirectional in that the presence of a directive in a request does
986   not imply that the same directive is to be given in the response.
987
988   A cache MUST obey the requirements of the Cache-Control directives
989   defined in this section.  See Section 3.2.3 for information about how
990   Cache-Control directives defined elsewhere are handled.
991
992      Note: HTTP/1.0 caches might not implement Cache-Control and might
993      only implement Pragma: no-cache (see Section 3.4).
994
995   A proxy, whether or not it implements a cache, MUST pass cache
996   directives through in forwarded messages, regardless of their
997   significance to that application, since the directives might be
998   applicable to all recipients along the request/response chain.  It is
999   not possible to target a directive to a specific cache.
1000
1001
1002
1003
1004
1005
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1011
1012     Cache-Control   = "Cache-Control" ":" OWS Cache-Control-v
1013     Cache-Control-v = 1#cache-directive
1014
1015     cache-directive = cache-request-directive
1016        / cache-response-directive
1017
1018     cache-extension = token [ "=" ( token / quoted-string ) ]
1019
10203.2.1.  Request Cache-Control Directives
1021
1022     cache-request-directive =
1023          "no-cache"
1024        / "no-store"
1025        / "max-age" "=" delta-seconds
1026        / "max-stale" [ "=" delta-seconds ]
1027        / "min-fresh" "=" delta-seconds
1028        / "no-transform"
1029        / "only-if-cached"
1030        / cache-extension
1031
1032   no-cache
1033
1034      The no-cache request directive indicates that a cache MUST NOT use
1035      a stored response to satisfy the request without successful
1036      validation on the origin server.
1037
1038   no-store
1039
1040      The no-store request directive indicates that a cache MUST NOT
1041      store any part of either this request or any response to it.  This
1042      directive applies to both private and shared caches.  "MUST NOT
1043      store" in this context means that the cache MUST NOT intentionally
1044      store the information in non-volatile storage, and MUST make a
1045      best-effort attempt to remove the information from volatile
1046      storage as promptly as possible after forwarding it.
1047
1048      This directive is NOT a reliable or sufficient mechanism for
1049      ensuring privacy.  In particular, malicious or compromised caches
1050      might not recognize or obey this directive, and communications
1051      networks might be vulnerable to eavesdropping.
1052
1053      Note that if a request containing this directive is satisfied from
1054      a cache, the no-store request directive does not apply to the
1055      already stored response.
1056
1057
1058
1059
1060
1061
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1067
1068   max-age
1069
1070      The max-age request directive indicates that the client is willing
1071      to accept a response whose age is no greater than the specified
1072      time in seconds.  Unless the max-stale request directive is also
1073      present, the client is not willing to accept a stale response.
1074
1075   max-stale
1076
1077      The max-stale request directive indicates that the client is
1078      willing to accept a response that has exceeded its expiration
1079      time.  If max-stale is assigned a value, then the client is
1080      willing to accept a response that has exceeded its expiration time
1081      by no more than the specified number of seconds.  If no value is
1082      assigned to max-stale, then the client is willing to accept a
1083      stale response of any age.
1084
1085   min-fresh
1086
1087      The min-fresh request directive indicates that the client is
1088      willing to accept a response whose freshness lifetime is no less
1089      than its current age plus the specified time in seconds.  That is,
1090      the client wants a response that will still be fresh for at least
1091      the specified number of seconds.
1092
1093   no-transform
1094
1095      The no-transform request directive indicates that an intermediary
1096      (whether or not it implements a cache) MUST NOT change the
1097      Content-Encoding, Content-Range or Content-Type request header
1098      fields, nor the request representation.
1099
1100   only-if-cached
1101
1102      The only-if-cached request directive indicates that the client
1103      only wishes to return a stored response.  If it receives this
1104      directive, a cache SHOULD either respond using a stored response
1105      that is consistent with the other constraints of the request, or
1106      respond with a 504 (Gateway Timeout) status code.  If a group of
1107      caches is being operated as a unified system with good internal
1108      connectivity, a member cache MAY forward such a request within
1109      that group of caches.
1110
1111
1112
1113
1114
1115
1116
1117
1118
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1123
11243.2.2.  Response Cache-Control Directives
1125
1126     cache-response-directive =
1127          "public"
1128        / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
1129        / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
1130        / "no-store"
1131        / "no-transform"
1132        / "must-revalidate"
1133        / "proxy-revalidate"
1134        / "max-age" "=" delta-seconds
1135        / "s-maxage" "=" delta-seconds
1136        / cache-extension
1137
1138   public
1139
1140      The public response directive indicates that a response whose
1141      associated request contains an 'Authentication' header MAY be
1142      stored (see Section 2.6).
1143
1144   private
1145
1146      The private response directive indicates that the response message
1147      is intended for a single user and MUST NOT be stored by a shared
1148      cache.  A private cache MAY store the response.
1149
1150      If the private response directive specifies one or more field-
1151      names, this requirement is limited to the field-values associated
1152      with the listed response header fields.  That is, a shared cache
1153      MUST NOT store the specified field-names(s), whereas it MAY store
1154      the remainder of the response message.
1155
1156      Note: This usage of the word private only controls where the
1157      response can be stored; it cannot ensure the privacy of the
1158      message content.  Also, private response directives with field-
1159      names are often handled by implementations as if an unqualified
1160      private directive was received; i.e., the special handling for the
1161      qualified form is not widely implemented.
1162
1163   no-cache
1164
1165      The no-cache response directive indicates that the response MUST
1166      NOT be used to satisfy a subsequent request without successful
1167      validation on the origin server.  This allows an origin server to
1168      prevent a cache from using it to satisfy a request without
1169      contacting it, even by caches that have been configured to return
1170      stale responses.
1171
1172
1173
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1179
1180      If the no-cache response directive specifies one or more field-
1181      names, this requirement is limited to the field-values associated
1182      with the listed response header fields.  That is, a cache MUST NOT
1183      send the specified field-name(s) in the response to a subsequent
1184      request without successful validation on the origin server.  This
1185      allows an origin server to prevent the re-use of certain header
1186      fields in a response, while still allowing caching of the rest of
1187      the response.
1188
1189      Note: Most HTTP/1.0 caches will not recognize or obey this
1190      directive.  Also, no-cache response directives with field-names
1191      are often handled by implementations as if an unqualified no-cache
1192      directive was received; i.e., the special handling for the
1193      qualified form is not widely implemented.
1194
1195   no-store
1196
1197      The no-store response directive indicates that a cache MUST NOT
1198      store any part of either the immediate request or response.  This
1199      directive applies to both private and shared caches.  "MUST NOT
1200      store" in this context means that the cache MUST NOT intentionally
1201      store the information in non-volatile storage, and MUST make a
1202      best-effort attempt to remove the information from volatile
1203      storage as promptly as possible after forwarding it.
1204
1205      This directive is NOT a reliable or sufficient mechanism for
1206      ensuring privacy.  In particular, malicious or compromised caches
1207      might not recognize or obey this directive, and communications
1208      networks might be vulnerable to eavesdropping.
1209
1210   must-revalidate
1211
1212      The must-revalidate response directive indicates that once it has
1213      become stale, a cache MUST NOT use the response to satisfy
1214      subsequent requests without successful validation on the origin
1215      server.
1216
1217      The must-revalidate directive is necessary to support reliable
1218      operation for certain protocol features.  In all circumstances a
1219      cache MUST obey the must-revalidate directive; in particular, if a
1220      cache cannot reach the origin server for any reason, it MUST
1221      generate a 504 (Gateway Timeout) response.
1222
1223      A server SHOULD send the must-revalidate directive if and only if
1224      failure to validate a request on the representation could result
1225      in incorrect operation, such as a silently unexecuted financial
1226      transaction.
1227
1228
1229
1230
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1235
1236   proxy-revalidate
1237
1238      The proxy-revalidate response directive has the same meaning as
1239      the must-revalidate response directive, except that it does not
1240      apply to private caches.
1241
1242   max-age
1243
1244      The max-age response directive indicates that response is to be
1245      considered stale after its age is greater than the specified
1246      number of seconds.
1247
1248   s-maxage
1249
1250      The s-maxage response directive indicates that, in shared caches,
1251      the maximum age specified by this directive overrides the maximum
1252      age specified by either the max-age directive or the Expires
1253      header field.  The s-maxage directive also implies the semantics
1254      of the proxy-revalidate response directive.
1255
1256   no-transform
1257
1258      The no-transform response directive indicates that an intermediary
1259      (regardless of whether it implements a cache) MUST NOT change the
1260      Content-Encoding, Content-Range or Content-Type response header
1261      fields, nor the response representation.
1262
12633.2.3.  Cache Control Extensions
1264
1265   The Cache-Control header field can be extended through the use of one
1266   or more cache-extension tokens, each with an optional value.
1267   Informational extensions (those that do not require a change in cache
1268   behavior) can be added without changing the semantics of other
1269   directives.  Behavioral extensions are designed to work by acting as
1270   modifiers to the existing base of cache directives.  Both the new
1271   directive and the standard directive are supplied, such that
1272   applications that do not understand the new directive will default to
1273   the behavior specified by the standard directive, and those that
1274   understand the new directive will recognize it as modifying the
1275   requirements associated with the standard directive.  In this way,
1276   extensions to the cache-control directives can be made without
1277   requiring changes to the base protocol.
1278
1279   This extension mechanism depends on an HTTP cache obeying all of the
1280   cache-control directives defined for its native HTTP-version, obeying
1281   certain extensions, and ignoring all directives that it does not
1282   understand.
1283
1284
1285
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1291
1292   For example, consider a hypothetical new response directive called
1293   "community" that acts as a modifier to the private directive.  We
1294   define this new directive to mean that, in addition to any private
1295   cache, any cache that is shared only by members of the community
1296   named within its value may cache the response.  An origin server
1297   wishing to allow the UCI community to use an otherwise private
1298   response in their shared cache(s) could do so by including
1299
1300     Cache-Control: private, community="UCI"
1301
1302   A cache seeing this header field will act correctly even if the cache
1303   does not understand the community cache-extension, since it will also
1304   see and understand the private directive and thus default to the safe
1305   behavior.
1306
1307   A cache MUST be ignore unrecognized cache directives; it is assumed
1308   that any cache directive likely to be unrecognized by an HTTP/1.1
1309   cache will be combined with standard directives (or the response's
1310   default cacheability) such that the cache behavior will remain
1311   minimally correct even if the cache does not understand the
1312   extension(s).
1313
1314   The HTTP Cache Directive Registry defines the name space for the
1315   cache directives.
1316
1317   A registration MUST include the following fields:
1318
1319   o  Cache Directive Name
1320
1321   o  Pointer to specification text
1322
1323   Values to be added to this name space are subject to IETF review
1324   ([RFC5226], Section 4.1).
1325
1326   The registry itself is maintained at
1327   <http://www.iana.org/assignments/http-cache-directives>.
1328
13293.3.  Expires
1330
1331   The "Expires" header field gives the date/time after which the
1332   response is considered stale.  See Section 2.3 for further discussion
1333   of the freshness model.
1334
1335   The presence of an Expires field does not imply that the original
1336   resource will change or cease to exist at, before, or after that
1337   time.
1338
1339   The field-value is an absolute date and time as defined by HTTP-date
1340
1341
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1347
1348   in Section 6.1 of [Part1]; a sender MUST use the rfc1123-date format.
1349
1350     Expires   = "Expires" ":" OWS Expires-v
1351     Expires-v = HTTP-date
1352
1353   For example
1354
1355     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1356
1357      Note: If a response includes a Cache-Control field with the max-
1358      age directive (see Section 3.2.2), that directive overrides the
1359      Expires field.  Likewise, the s-maxage directive overrides Expires
1360      in shared caches.
1361
1362   A server SHOULD NOT send Expires dates more than one year in the
1363   future.
1364
1365   A cache MUST treat other invalid date formats, especially including
1366   the value "0", as in the past (i.e., "already expired").
1367
13683.4.  Pragma
1369
1370   The "Pragma" header field is used to include implementation-specific
1371   directives that might apply to any recipient along the request/
1372   response chain.  All pragma directives specify optional behavior from
1373   the viewpoint of the protocol; however, some systems MAY require that
1374   behavior be consistent with the directives.
1375
1376     Pragma            = "Pragma" ":" OWS Pragma-v
1377     Pragma-v          = 1#pragma-directive
1378     pragma-directive  = "no-cache" / extension-pragma
1379     extension-pragma  = token [ "=" ( token / quoted-string ) ]
1380
1381   When the no-cache directive is present in a request message, a cache
1382   SHOULD forward the request toward the origin server even if it has a
1383   stored copy of what is being requested.  This pragma directive has
1384   the same semantics as the no-cache response directive (see
1385   Section 3.2.2) and is defined here for backward compatibility with
1386   HTTP/1.0.  A client SHOULD include both header fields when a no-cache
1387   request is sent to a server not known to be HTTP/1.1 compliant.  A
1388   cache SHOULD treat "Pragma: no-cache" as if the client had sent
1389   "Cache-Control: no-cache".
1390
1391      Note: Because the meaning of "Pragma: no-cache" as a header field
1392      is not actually specified, it does not provide a reliable
1393      replacement for "Cache-Control: no-cache" in a response.
1394
1395   This mechanism is deprecated; no new Pragma directives will be
1396
1397
1398
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1402
1403
1404   defined in HTTP.
1405
14063.5.  Vary
1407
1408   The "Vary" header field conveys the set of header fields that were
1409   used to select the representation.
1410
1411   Caches use this information, in part, to determine whether a stored
1412   response can be used to satisfy a given request; see Section 2.7.
1413   determines, while the response is fresh, whether a cache is permitted
1414   to use the response to reply to a subsequent request without
1415   validation; see Section 2.7.
1416
1417   In uncacheable or stale responses, the Vary field value advises the
1418   user agent about the criteria that were used to select the
1419   representation.
1420
1421     Vary   = "Vary" ":" OWS Vary-v
1422     Vary-v = "*" / 1#field-name
1423
1424   The set of header fields named by the Vary field value is known as
1425   the selecting header fields.
1426
1427   A server SHOULD include a Vary header field with any cacheable
1428   response that is subject to server-driven negotiation.  Doing so
1429   allows a cache to properly interpret future requests on that resource
1430   and informs the user agent about the presence of negotiation on that
1431   resource.  A server MAY include a Vary header field with a non-
1432   cacheable response that is subject to server-driven negotiation,
1433   since this might provide the user agent with useful information about
1434   the dimensions over which the response varies at the time of the
1435   response.
1436
1437   A Vary field value of "*" signals that unspecified parameters not
1438   limited to the header fields (e.g., the network address of the
1439   client), play a role in the selection of the response representation;
1440   therefore, a cache cannot determine whether this response is
1441   appropriate.  A proxy MUST NOT generate the "*" value.
1442
1443   The field-names given are not limited to the set of standard header
1444   fields defined by this specification.  Field names are case-
1445   insensitive.
1446
14473.6.  Warning
1448
1449   The "Warning" header field is used to carry additional information
1450   about the status or transformation of a message that might not be
1451   reflected in the message.  This information is typically used to warn
1452
1453
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1459
1460   about possible incorrectness introduced by caching operations or
1461   transformations applied to the payload of the message.
1462
1463   Warnings can be used for other purposes, both cache-related and
1464   otherwise.  The use of a warning, rather than an error status code,
1465   distinguishes these responses from true failures.
1466
1467   Warning header fields can in general be applied to any message,
1468   however some warn-codes are specific to caches and can only be
1469   applied to response messages.
1470
1471     Warning    = "Warning" ":" OWS Warning-v
1472     Warning-v  = 1#warning-value
1473
1474     warning-value = warn-code SP warn-agent SP warn-text
1475                                           [SP warn-date]
1476
1477     warn-code  = 3DIGIT
1478     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1479                     ; the name or pseudonym of the server adding
1480                     ; the Warning header field, for use in debugging
1481     warn-text  = quoted-string
1482     warn-date  = DQUOTE HTTP-date DQUOTE
1483
1484   Multiple warnings can be attached to a response (either by the origin
1485   server or by a cache), including multiple warnings with the same code
1486   number, only differing in warn-text.
1487
1488   When this occurs, the user agent SHOULD inform the user of as many of
1489   them as possible, in the order that they appear in the response.
1490
1491   Systems that generate multiple Warning header fields SHOULD order
1492   them with this user agent behavior in mind.  New Warning header
1493   fields SHOULD be added after any existing Warning headers fields.
1494
1495   Warnings are assigned three digit warn-codes.  The first digit
1496   indicates whether the Warning is required to be deleted from a stored
1497   response after validation:
1498
1499   o  1xx Warnings describe the freshness or validation status of the
1500      response, and so MUST be deleted by a cache after validation.
1501      They can only be generated by a cache when validating a cached
1502      entry, and MUST NOT be generated in any other situation.
1503
1504   o  2xx Warnings describe some aspect of the representation that is
1505      not rectified by a validation (for example, a lossy compression of
1506      the representation) and MUST NOT be deleted by a cache after
1507      validation, unless a full response is returned, in which case they
1508
1509
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1514
1515
1516      MUST be.
1517
1518   If an implementation sends a message with one or more Warning header
1519   fields to a receiver whose version is HTTP/1.0 or lower, then the
1520   sender MUST include in each warning-value a warn-date that matches
1521   the Date header field in the message.
1522
1523   If a system receives a message with a warning-value that includes a
1524   warn-date, and that warn-date is different from the Date value in the
1525   response, then that warning-value MUST be deleted from the message
1526   before storing, forwarding, or using it. (preventing the consequences
1527   of naive caching of Warning header fields.)  If all of the warning-
1528   values are deleted for this reason, the Warning header field MUST be
1529   deleted as well.
1530
1531   The following warn-codes are defined by this specification, each with
1532   a recommended warn-text in English, and a description of its meaning.
1533
1534   110 Response is stale
1535
1536      A cache SHOULD include this whenever the returned response is
1537      stale.
1538
1539   111 Revalidation failed
1540
1541      A cache SHOULD include this when returning a stale response
1542      because an attempt to validate the response failed, due to an
1543      inability to reach the server.
1544
1545   112 Disconnected operation
1546
1547      A cache SHOULD b include this if it is intentionally disconnected
1548      from the rest of the network for a period of time.
1549
1550   113 Heuristic expiration
1551
1552      A cache SHOULD include this if it heuristically chose a freshness
1553      lifetime greater than 24 hours and the response's age is greater
1554      than 24 hours.
1555
1556   199 Miscellaneous warning
1557
1558      The warning text can include arbitrary information to be presented
1559      to a human user, or logged.  A system receiving this warning MUST
1560      NOT take any automated action, besides presenting the warning to
1561      the user.
1562
1563
1564
1565
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1571
1572   214 Transformation applied
1573
1574      MUST be added by a proxy if it applies any transformation to the
1575      representation, such as changing the content-coding, media-type,
1576      or modifying the representation data, unless this Warning code
1577      already appears in the response.
1578
1579   299 Miscellaneous persistent warning
1580
1581      The warning text can include arbitrary information to be presented
1582      to a human user, or logged.  A system receiving this warning MUST
1583      NOT take any automated action.
1584
15854.  History Lists
1586
1587   User agents often have history mechanisms, such as "Back" buttons and
1588   history lists, that can be used to redisplay a representation
1589   retrieved earlier in a session.
1590
1591   The freshness model (Section 2.3) does not necessarily apply to
1592   history mechanisms.  I.e., a history mechanism can display a previous
1593   representation even if it has expired.
1594
1595   This does not prohibit the history mechanism from telling the user
1596   that a view might be stale, or from honoring cache directives (e.g.,
1597   Cache-Control: no-store).
1598
15995.  IANA Considerations
1600
16015.1.  Cache Directive Registry
1602
1603   The registration procedure for HTTP Cache Directives is defined by
1604   Section 3.2.3 of this document.
1605
1606   The HTTP Cache Directive Registry shall be created at
1607   <http://www.iana.org/assignments/http-cache-directives> and be
1608   populated with the registrations below:
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
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1626
1627
1628   +------------------------+------------------------------+
1629   | Cache Directive        | Reference                    |
1630   +------------------------+------------------------------+
1631   | max-age                | Section 3.2.1, Section 3.2.2 |
1632   | max-stale              | Section 3.2.1                |
1633   | min-fresh              | Section 3.2.1                |
1634   | must-revalidate        | Section 3.2.2                |
1635   | no-cache               | Section 3.2.1, Section 3.2.2 |
1636   | no-store               | Section 3.2.1, Section 3.2.2 |
1637   | no-transform           | Section 3.2.1, Section 3.2.2 |
1638   | only-if-cached         | Section 3.2.1                |
1639   | private                | Section 3.2.2                |
1640   | proxy-revalidate       | Section 3.2.2                |
1641   | public                 | Section 3.2.2                |
1642   | s-maxage               | Section 3.2.2                |
1643   | stale-if-error         | [RFC5861], Section 4         |
1644   | stale-while-revalidate | [RFC5861], Section 3         |
1645   +------------------------+------------------------------+
1646
16475.2.  Header Field Registration
1648
1649   The Message Header Field Registry located at <http://www.iana.org/
1650   assignments/message-headers/message-header-index.html> shall be
1651   updated with the permanent registrations below (see [RFC3864]):
1652
1653   +-------------------+----------+----------+-------------+
1654   | Header Field Name | Protocol | Status   | Reference   |
1655   +-------------------+----------+----------+-------------+
1656   | Age               | http     | standard | Section 3.1 |
1657   | Cache-Control     | http     | standard | Section 3.2 |
1658   | Expires           | http     | standard | Section 3.3 |
1659   | Pragma            | http     | standard | Section 3.4 |
1660   | Vary              | http     | standard | Section 3.5 |
1661   | Warning           | http     | standard | Section 3.6 |
1662   +-------------------+----------+----------+-------------+
1663
1664   The change controller is: "IETF (iesg@ietf.org) - Internet
1665   Engineering Task Force".
1666
16676.  Security Considerations
1668
1669   Caches expose additional potential vulnerabilities, since the
1670   contents of the cache represent an attractive target for malicious
1671   exploitation.  Because cache contents persist after an HTTP request
1672   is complete, an attack on the cache can reveal information long after
1673   a user believes that the information has been removed from the
1674   network.  Therefore, cache contents need to be protected as sensitive
1675   information.
1676
1677
1678
1679Fielding, et al.       Expires September 15, 2011              [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                  March 2011
1682
1683
16847.  Acknowledgments
1685
1686   Much of the content and presentation of the caching design is due to
1687   suggestions and comments from individuals including: Shel Kaphan,
1688   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
1689
16908.  References
1691
16928.1.  Normative References
1693
1694   [Part1]    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 1: URIs, Connections,
1697              and Message Parsing", draft-ietf-httpbis-p1-messaging-13
1698              (work in progress), March 2011.
1699
1700   [Part2]    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 2: Message
1703              Semantics", draft-ietf-httpbis-p2-semantics-13 (work in
1704              progress), March 2011.
1705
1706   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1707              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1708              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
1709              Requests", draft-ietf-httpbis-p4-conditional-13 (work in
1710              progress), March 2011.
1711
1712   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1713              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1714              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1715              Partial Responses", draft-ietf-httpbis-p5-range-13 (work
1716              in progress), March 2011.
1717
1718   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1719              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1720              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
1721              draft-ietf-httpbis-p7-auth-13 (work in progress),
1722              March 2011.
1723
1724   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1725              Requirement Levels", BCP 14, RFC 2119, March 1997.
1726
1727   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1728              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1729
1730
1731
1732
1733
1734
1735Fielding, et al.       Expires September 15, 2011              [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                  March 2011
1738
1739
17408.2.  Informative References
1741
1742   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
1743              Specification, Implementation", RFC 1305, March 1992.
1744
1745   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1746              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1747              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1748
1749   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1750              Procedures for Message Header Fields", BCP 90, RFC 3864,
1751              September 2004.
1752
1753   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
1754              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
1755              May 2008.
1756
1757   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
1758              Content", RFC 5861, April 2010.
1759
1760Appendix A.  Changes from RFC 2616
1761
1762   Make the specified age calculation algorithm less conservative.
1763   (Section 2.3.2)
1764
1765   Remove requirement to consider Content-Location in successful
1766   responses in order to determine the appropriate response to use.
1767   (Section 2.4)
1768
1769   Clarify denial of service attack avoidance requirement.
1770   (Section 2.5)
1771
1772   Do not mention RFC 2047 encoding and multiple languages in Warning
1773   header fields anymore, as these aspects never were implemented.
1774   (Section 3.6)
1775
1776Appendix B.  Collected ABNF
1777
1778   Age = "Age:" OWS Age-v
1779   Age-v = delta-seconds
1780
1781   Cache-Control = "Cache-Control:" OWS Cache-Control-v
1782   Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
1783    cache-directive ] )
1784
1785   Expires = "Expires:" OWS Expires-v
1786   Expires-v = HTTP-date
1787
1788
1789
1790
1791Fielding, et al.       Expires September 15, 2011              [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                  March 2011
1794
1795
1796   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1797
1798   OWS = <OWS, defined in [Part1], Section 1.2.2>
1799
1800   Pragma = "Pragma:" OWS Pragma-v
1801   Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1802    pragma-directive ] )
1803
1804   Vary = "Vary:" OWS Vary-v
1805   Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
1806    ] ) )
1807
1808   Warning = "Warning:" OWS Warning-v
1809   Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
1810    ] )
1811
1812   cache-directive = cache-request-directive / cache-response-directive
1813   cache-extension = token [ "=" ( token / quoted-string ) ]
1814   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
1815    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1816    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1817    cache-extension
1818   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
1819    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1820    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1821    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1822    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1823    ) / ( "s-maxage=" delta-seconds ) / cache-extension
1824
1825   delta-seconds = 1*DIGIT
1826
1827   extension-pragma = token [ "=" ( token / quoted-string ) ]
1828
1829   field-name = <field-name, defined in [Part1], Section 3.2>
1830
1831   port = <port, defined in [Part1], Section 2.6>
1832   pragma-directive = "no-cache" / extension-pragma
1833   pseudonym = <pseudonym, defined in [Part1], Section 9.9>
1834
1835   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1836
1837   token = <token, defined in [Part1], Section 1.2.2>
1838
1839   uri-host = <uri-host, defined in [Part1], Section 2.6>
1840
1841   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1842   warn-code = 3DIGIT
1843   warn-date = DQUOTE HTTP-date DQUOTE
1844
1845
1846
1847Fielding, et al.       Expires September 15, 2011              [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                  March 2011
1850
1851
1852   warn-text = quoted-string
1853   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
1854    ]
1855
1856   ABNF diagnostics:
1857
1858   ; Age defined but not used
1859   ; Cache-Control defined but not used
1860   ; Expires defined but not used
1861   ; Pragma defined but not used
1862   ; Vary defined but not used
1863   ; Warning defined but not used
1864
1865Appendix C.  Change Log (to be removed by RFC Editor before publication)
1866
1867C.1.  Since RFC 2616
1868
1869   Extracted relevant partitions from [RFC2616].
1870
1871C.2.  Since draft-ietf-httpbis-p6-cache-00
1872
1873   Closed issues:
1874
1875   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
1876      (<http://purl.org/NET/http-errata#trailer-hop>)
1877
1878   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
1879      after Update or Delete"
1880      (<http://purl.org/NET/http-errata#invalidupd>)
1881
1882   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1883      Informative references"
1884
1885   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
1886      typo"
1887
1888   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
1889      header text"
1890
1891   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
1892      references"
1893
1894   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
1895      Reference"
1896
1897   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
1898      to-date references"
1899
1900
1901
1902
1903Fielding, et al.       Expires September 15, 2011              [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                  March 2011
1906
1907
1908   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
1909      13.2.2"
1910
1911   Other changes:
1912
1913   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
1914      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1915
1916C.3.  Since draft-ietf-httpbis-p6-cache-01
1917
1918   Closed issues:
1919
1920   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
1921      used"
1922
1923   Other changes:
1924
1925   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
1926      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1927
1928   o  Add explicit references to BNF syntax and rules imported from
1929      other parts of the specification.
1930
1931C.4.  Since draft-ietf-httpbis-p6-cache-02
1932
1933   Ongoing work on IANA Message Header Field Registration
1934   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1935
1936   o  Reference RFC 3984, and update header field registrations for
1937      header fields defined in this document.
1938
1939C.5.  Since draft-ietf-httpbis-p6-cache-03
1940
1941   Closed issues:
1942
1943   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
1944      classification"
1945
1946C.6.  Since draft-ietf-httpbis-p6-cache-04
1947
1948   Ongoing work on ABNF conversion
1949   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1950
1951   o  Use "/" instead of "|" for alternatives.
1952
1953   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1954      whitespace ("OWS") and required whitespace ("RWS").
1955
1956
1957
1958
1959Fielding, et al.       Expires September 15, 2011              [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                  March 2011
1962
1963
1964   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1965      field value format definitions.
1966
1967C.7.  Since draft-ietf-httpbis-p6-cache-05
1968
1969   This is a total rewrite of this part of the specification.
1970
1971   Affected issues:
1972
1973   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1974      1xx Warn-Codes"
1975
1976   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
1977      13.5.1 and 13.5.2"
1978
1979   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role of
1980      Warning and Semantic Transparency in Caching"
1981
1982   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
1983      Caching"
1984
1985   In addition: Final work on ABNF conversion
1986   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1987
1988   o  Add appendix containing collected and expanded ABNF, reorganize
1989      ABNF introduction.
1990
1991C.8.  Since draft-ietf-httpbis-p6-cache-06
1992
1993   Closed issues:
1994
1995   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
1996      numeric protocol elements"
1997
1998   Affected issues:
1999
2000   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: "Vary and non-
2001      existant headers"
2002
2003C.9.  Since draft-ietf-httpbis-p6-cache-07
2004
2005   Closed issues:
2006
2007   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
2008      1xx Warn-Codes"
2009
2010   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
2011      Location on 304 responses"
2012
2013
2014
2015Fielding, et al.       Expires September 15, 2011              [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                  March 2011
2018
2019
2020   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
2021      no-cache CC directives with headers"
2022
2023   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
2024      warn-text"
2025
2026C.10.  Since draft-ietf-httpbis-p6-cache-08
2027
2028   Closed issues:
2029
2030   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
2031      negotiated responses from cache: header-specific canonicalization"
2032
2033   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
2034      directives on history lists"
2035
2036   Affected issues:
2037
2038   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
2039      and caching
2040
2041   Partly resolved issues:
2042
2043   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
2044      13.5.1 and 13.5.2"
2045
2046C.11.  Since draft-ietf-httpbis-p6-cache-09
2047
2048   Closed issues:
2049
2050   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/29>: "Age
2051      calculation"
2052
2053   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/168>: "Clarify
2054      differences between / requirements for request and response CC
2055      directives"
2056
2057   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/174>: "Caching
2058      authenticated responses"
2059
2060   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/208>: "IANA registry
2061      for cache-control directives"
2062
2063   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
2064      caching of URLs with query components"
2065
2066   Partly resolved issues:
2067
2068
2069
2070
2071Fielding, et al.       Expires September 15, 2011              [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                  March 2011
2074
2075
2076   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
2077      requested resource's URI"
2078
2079C.12.  Since draft-ietf-httpbis-p6-cache-10
2080
2081   Closed issues:
2082
2083   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
2084      entity / representation / variant terminology"
2085
2086   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
2087      removing the 'changes from 2068' sections"
2088
2089   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2090      heuristic caching for new status codes"
2091
2092   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2093      heuristic caching for new status codes"
2094
2095   o  Clean up TODOs and prose in "Combining Responses."
2096
2097C.13.  Since draft-ietf-httpbis-p6-cache-11
2098
2099   Closed issues:
2100
2101   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/204>: "Text about
2102      clock requirement for caches belongs in p6"
2103
2104C.14.  Since draft-ietf-httpbis-p6-cache-12
2105
2106   Closed issues:
2107
2108   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
2109      Classification"
2110
2111   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/268>: "Clarify
2112      'public'"
2113
2114Index
2115
2116   A
2117      age  6
2118      Age header field  18
2119
2120   C
2121      cache  5
2122      Cache Directives
2123         max-age  20, 23
2124
2125
2126
2127Fielding, et al.       Expires September 15, 2011              [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                  March 2011
2130
2131
2132         max-stale  20
2133         min-fresh  20
2134         must-revalidate  22
2135         no-cache  19, 21
2136         no-store  19, 22
2137         no-transform  20, 23
2138         only-if-cached  20
2139         private  21
2140         proxy-revalidate  23
2141         public  21
2142         s-maxage  23
2143      Cache-Control header field  18
2144      cacheable  5
2145
2146   E
2147      Expires header field  24
2148      explicit expiration time  6
2149
2150   F
2151      first-hand  6
2152      fresh  6
2153      freshness lifetime  6
2154
2155   G
2156      Grammar
2157         Age  18
2158         Age-v  18
2159         Cache-Control  19
2160         Cache-Control-v  19
2161         cache-extension  19
2162         cache-request-directive  19
2163         cache-response-directive  21
2164         delta-seconds  18
2165         Expires  25
2166         Expires-v  25
2167         extension-pragma  25
2168         Pragma  25
2169         pragma-directive  25
2170         Pragma-v  25
2171         Vary  26
2172         Vary-v  26
2173         warn-agent  27
2174         warn-code  27
2175         warn-date  27
2176         warn-text  27
2177         Warning  27
2178         Warning-v  27
2179         warning-value  27
2180
2181
2182
2183Fielding, et al.       Expires September 15, 2011              [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                  March 2011
2186
2187
2188   H
2189      Header Fields
2190         Age  18
2191         Cache-Control  18
2192         Expires  24
2193         Pragma  25
2194         Vary  26
2195         Warning  26
2196      heuristic expiration time  6
2197
2198   M
2199      max-age
2200         Cache Directive  20, 23
2201      max-stale
2202         Cache Directive  20
2203      min-fresh
2204         Cache Directive  20
2205      must-revalidate
2206         Cache Directive  22
2207
2208   N
2209      no-cache
2210         Cache Directive  19, 21
2211      no-store
2212         Cache Directive  19, 22
2213      no-transform
2214         Cache Directive  20, 23
2215
2216   O
2217      only-if-cached
2218         Cache Directive  20
2219
2220   P
2221      Pragma header field  25
2222      private
2223         Cache Directive  21
2224      private cache  5
2225      proxy-revalidate
2226         Cache Directive  23
2227      public
2228         Cache Directive  21
2229
2230   S
2231      s-maxage
2232         Cache Directive  23
2233      shared cache  5
2234      stale  6
2235
2236
2237
2238
2239Fielding, et al.       Expires September 15, 2011              [Page 40]
2240
2241Internet-Draft              HTTP/1.1, Part 6                  March 2011
2242
2243
2244   V
2245      validator  6
2246      Vary header field  26
2247
2248   W
2249      Warning header field  26
2250
2251Authors' Addresses
2252
2253   Roy T. Fielding (editor)
2254   Adobe Systems Incorporated
2255   345 Park Ave
2256   San Jose, CA  95110
2257   USA
2258
2259   EMail: fielding@gbiv.com
2260   URI:   http://roy.gbiv.com/
2261
2262
2263   Jim Gettys
2264   Alcatel-Lucent Bell Labs
2265   21 Oak Knoll Road
2266   Carlisle, MA  01741
2267   USA
2268
2269   EMail: jg@freedesktop.org
2270   URI:   http://gettys.wordpress.com/
2271
2272
2273   Jeffrey C. Mogul
2274   Hewlett-Packard Company
2275   HP Labs, Large Scale Systems Group
2276   1501 Page Mill Road, MS 1177
2277   Palo Alto, CA  94304
2278   USA
2279
2280   EMail: JeffMogul@acm.org
2281
2282
2283   Henrik Frystyk Nielsen
2284   Microsoft Corporation
2285   1 Microsoft Way
2286   Redmond, WA  98052
2287   USA
2288
2289   EMail: henrikn@microsoft.com
2290
2291
2292
2293
2294
2295Fielding, et al.       Expires September 15, 2011              [Page 41]
2296
2297Internet-Draft              HTTP/1.1, Part 6                  March 2011
2298
2299
2300   Larry Masinter
2301   Adobe Systems Incorporated
2302   345 Park Ave
2303   San Jose, CA  95110
2304   USA
2305
2306   EMail: LMM@acm.org
2307   URI:   http://larry.masinter.net/
2308
2309
2310   Paul J. Leach
2311   Microsoft Corporation
2312   1 Microsoft Way
2313   Redmond, WA  98052
2314
2315   EMail: paulle@microsoft.com
2316
2317
2318   Tim Berners-Lee
2319   World Wide Web Consortium
2320   MIT Computer Science and Artificial Intelligence Laboratory
2321   The Stata Center, Building 32
2322   32 Vassar Street
2323   Cambridge, MA  02139
2324   USA
2325
2326   EMail: timbl@w3.org
2327   URI:   http://www.w3.org/People/Berners-Lee/
2328
2329
2330   Yves Lafon (editor)
2331   World Wide Web Consortium
2332   W3C / ERCIM
2333   2004, rte des Lucioles
2334   Sophia-Antipolis, AM  06902
2335   France
2336
2337   EMail: ylafon@w3.org
2338   URI:   http://www.raubacapeu.net/people/yves/
2339
2340
2341   Mark Nottingham (editor)
2342
2343   EMail: mnot@mnot.net
2344   URI:   http://www.mnot.net/
2345
2346
2347
2348
2349
2350
2351Fielding, et al.       Expires September 15, 2011              [Page 42]
2352
2353Internet-Draft              HTTP/1.1, Part 6                  March 2011
2354
2355
2356   Julian F. Reschke (editor)
2357   greenbytes GmbH
2358   Hafenweg 16
2359   Muenster, NW  48155
2360   Germany
2361
2362   Phone: +49 251 2807760
2363   Fax:   +49 251 2807761
2364   EMail: julian.reschke@greenbytes.de
2365   URI:   http://greenbytes.de/tech/webdav/
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407Fielding, et al.       Expires September 15, 2011              [Page 43]
2408
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