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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: October 20, 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                                                          April 18, 2011
25
26
27                       HTTP/1.1, part 6: Caching
28                     draft-ietf-httpbis-p6-cache-14
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), which is archived at
44   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
45
46   The current issues list is at
47   <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
48   documents (including fancy diffs) can be found at
49   <http://tools.ietf.org/wg/httpbis/>.
50
51   The changes in this draft are summarized in Appendix C.15.
52
53
54
55Fielding, et al.        Expires October 20, 2011                [Page 1]
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57Internet-Draft              HTTP/1.1, Part 6                  April 2011
58
59
60Status of This Memo
61
62   This Internet-Draft is submitted in full conformance with the
63   provisions of BCP 78 and BCP 79.
64
65   Internet-Drafts are working documents of the Internet Engineering
66   Task Force (IETF).  Note that other groups may also distribute
67   working documents as Internet-Drafts.  The list of current Internet-
68   Drafts is at http://datatracker.ietf.org/drafts/current/.
69
70   Internet-Drafts are draft documents valid for a maximum of six months
71   and may be updated, replaced, or obsoleted by other documents at any
72   time.  It is inappropriate to use Internet-Drafts as reference
73   material or to cite them other than as "work in progress."
74
75   This Internet-Draft will expire on October 20, 2011.
76
77Copyright Notice
78
79   Copyright (c) 2011 IETF Trust and the persons identified as the
80   document authors.  All rights reserved.
81
82   This document is subject to BCP 78 and the IETF Trust's Legal
83   Provisions Relating to IETF Documents
84   (http://trustee.ietf.org/license-info) in effect on the date of
85   publication of this document.  Please review these documents
86   carefully, as they describe your rights and restrictions with respect
87   to this document.  Code Components extracted from this document must
88   include Simplified BSD License text as described in Section 4.e of
89   the Trust Legal Provisions and are provided without warranty as
90   described in the Simplified BSD License.
91
92   This document may contain material from IETF Documents or IETF
93   Contributions published or made publicly available before November
94   10, 2008.  The person(s) controlling the copyright in some of this
95   material may not have granted the IETF Trust the right to allow
96   modifications of such material outside the IETF Standards Process.
97   Without obtaining an adequate license from the person(s) controlling
98   the copyright in such materials, this document may not be modified
99   outside the IETF Standards Process, and derivative works of it may
100   not be created outside the IETF Standards Process, except to format
101   it for publication as an RFC or to translate it into languages other
102   than English.
103
104Table of Contents
105
106   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
107     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
108
109
110
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114
115
116     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
117     1.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
118     1.4.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
119       1.4.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7
120       1.4.2.  ABNF Rules defined in other Parts of the
121               Specification  . . . . . . . . . . . . . . . . . . . .  7
122   2.  Cache Operation  . . . . . . . . . . . . . . . . . . . . . . .  8
123     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  8
124       2.1.1.  Storing Partial and Incomplete Responses . . . . . . .  9
125     2.2.  Constructing Responses from Caches . . . . . . . . . . . .  9
126     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 10
127       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 11
128       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 12
129       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 13
130     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 14
131     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 15
132     2.6.  Shared Caching of Authenticated Responses  . . . . . . . . 15
133     2.7.  Caching Negotiated Responses . . . . . . . . . . . . . . . 16
134     2.8.  Combining Responses  . . . . . . . . . . . . . . . . . . . 17
135   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 17
136     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
137     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 18
138       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 19
139       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 21
140       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 23
141     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 24
142     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 25
143     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
144     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 26
145   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 29
146   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
147     5.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 29
148     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 30
149   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
150   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 31
151   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
152     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 31
153     8.2.  Informative References . . . . . . . . . . . . . . . . . . 32
154   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 32
155   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 32
156   Appendix C.  Change Log (to be removed by RFC Editor before
157                publication)  . . . . . . . . . . . . . . . . . . . . 34
158     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 34
159     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 34
160     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 35
161     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 35
162     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 35
163     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35
164
165
166
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169Internet-Draft              HTTP/1.1, Part 6                  April 2011
170
171
172     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 36
173     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 36
174     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36
175     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 37
176     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 37
177     C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 38
178     C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 38
179     C.14. Since draft-ietf-httpbis-p6-cache-12 . . . . . . . . . . . 38
180     C.15. Since draft-ietf-httpbis-p6-cache-13 . . . . . . . . . . . 38
181   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
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227
2281.  Introduction
229
230   HTTP is typically used for distributed information systems, where
231   performance can be improved by the use of response caches.  This
232   document defines aspects of HTTP/1.1 related to caching and reusing
233   response messages.
234
2351.1.  Purpose
236
237   An HTTP cache is a local store of response messages and the subsystem
238   that controls its message storage, retrieval, and deletion.  A cache
239   stores cacheable responses in order to reduce the response time and
240   network bandwidth consumption on future, equivalent requests.  Any
241   client or server MAY 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
502
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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 use 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
558
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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
614
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619
620   Also, if the response has a Last-Modified header field (Section 2.1
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
670
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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 doing so is explicitly allowed (e.g., by the max-
738   stale 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 2.2.2 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 = delta-seconds
964
965   Age field-values are non-negative integers, representing time in
966   seconds.
967
968     delta-seconds  = 1*DIGIT
969
970   If a cache receives a value larger than the largest positive integer
971   it can represent, or if any of its age calculations overflows, it
972   MUST transmit an Age header field with a field-value of 2147483648
973   (2^31).  Recipients parsing the Age header field-value SHOULD use an
974   arithmetic type of at least 31 bits of range.
975
976   The presence of an Age header field in a response implies that a
977   response is not first-hand.  However, the converse is not true, since
978   HTTP/1.0 caches might not implement the Age header field.
979
9803.2.  Cache-Control
981
982   The "Cache-Control" header field is used to specify directives for
983   caches along the request/response chain.  Such cache directives are
984   unidirectional in that the presence of a directive in a request does
985   not imply that the same directive is to be given in the response.
986
987   A cache MUST obey the requirements of the Cache-Control directives
988   defined in this section.  See Section 3.2.3 for information about how
989   Cache-Control directives defined elsewhere are handled.
990
991      Note: HTTP/1.0 caches might not implement Cache-Control and might
992      only implement Pragma: no-cache (see Section 3.4).
993
994   A proxy, whether or not it implements a cache, MUST pass cache
995   directives through in forwarded messages, regardless of their
996   significance to that application, since the directives might be
997   applicable to all recipients along the request/response chain.  It is
998   not possible to target a directive to a specific cache.
999
1000
1001
1002
1003
1004
1005
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1011
1012     Cache-Control   = 1#cache-directive
1013
1014     cache-directive = cache-request-directive
1015        / cache-response-directive
1016
1017     cache-extension = token [ "=" ( token / quoted-string ) ]
1018
10193.2.1.  Request Cache-Control Directives
1020
1021     cache-request-directive =
1022          "no-cache"
1023        / "no-store"
1024        / "max-age" "=" delta-seconds
1025        / "max-stale" [ "=" delta-seconds ]
1026        / "min-fresh" "=" delta-seconds
1027        / "no-transform"
1028        / "only-if-cached"
1029        / cache-extension
1030
1031   no-cache
1032
1033      The no-cache request directive indicates that a cache MUST NOT use
1034      a stored response to satisfy the request without successful
1035      validation on the origin server.
1036
1037   no-store
1038
1039      The no-store request directive indicates that a cache MUST NOT
1040      store any part of either this request or any response to it.  This
1041      directive applies to both private and shared caches.  "MUST NOT
1042      store" in this context means that the cache MUST NOT intentionally
1043      store the information in non-volatile storage, and MUST make a
1044      best-effort attempt to remove the information from volatile
1045      storage as promptly as possible after forwarding it.
1046
1047      This directive is NOT a reliable or sufficient mechanism for
1048      ensuring privacy.  In particular, malicious or compromised caches
1049      might not recognize or obey this directive, and communications
1050      networks might be vulnerable to eavesdropping.
1051
1052      Note that if a request containing this directive is satisfied from
1053      a cache, the no-store request directive does not apply to the
1054      already stored response.
1055
1056   max-age
1057
1058      The max-age request directive indicates that the client is willing
1059      to accept a response whose age is no greater than the specified
1060
1061
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1067
1068      time in seconds.  Unless the max-stale request directive is also
1069      present, the client is not willing to accept a stale response.
1070
1071   max-stale
1072
1073      The max-stale request directive indicates that the client is
1074      willing to accept a response that has exceeded its expiration
1075      time.  If max-stale is assigned a value, then the client is
1076      willing to accept a response that has exceeded its expiration time
1077      by no more than the specified number of seconds.  If no value is
1078      assigned to max-stale, then the client is willing to accept a
1079      stale response of any age.
1080
1081   min-fresh
1082
1083      The min-fresh request directive indicates that the client is
1084      willing to accept a response whose freshness lifetime is no less
1085      than its current age plus the specified time in seconds.  That is,
1086      the client wants a response that will still be fresh for at least
1087      the specified number of seconds.
1088
1089   no-transform
1090
1091      The no-transform request directive indicates that an intermediary
1092      (whether or not it implements a cache) MUST NOT change the
1093      Content-Encoding, Content-Range or Content-Type request header
1094      fields, nor the request representation.
1095
1096   only-if-cached
1097
1098      The only-if-cached request directive indicates that the client
1099      only wishes to return a stored response.  If it receives this
1100      directive, a cache SHOULD either respond using a stored response
1101      that is consistent with the other constraints of the request, or
1102      respond with a 504 (Gateway Timeout) status code.  If a group of
1103      caches is being operated as a unified system with good internal
1104      connectivity, a member cache MAY forward such a request within
1105      that group of caches.
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
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1122
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
1174
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1178
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 the response is to
1245      be 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
1286
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1290
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 = HTTP-date
1351
1352   For example
1353
1354     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1355
1356      Note: If a response includes a Cache-Control field with the max-
1357      age directive (see Section 3.2.2), that directive overrides the
1358      Expires field.  Likewise, the s-maxage directive overrides Expires
1359      in shared caches.
1360
1361   A server SHOULD NOT send Expires dates more than one year in the
1362   future.
1363
1364   A cache MUST treat other invalid date formats, especially including
1365   the value "0", as in the past (i.e., "already expired").
1366
13673.4.  Pragma
1368
1369   The "Pragma" header field is used to include implementation-specific
1370   directives that might apply to any recipient along the request/
1371   response chain.  All pragma directives specify optional behavior from
1372   the viewpoint of the protocol; however, some systems MAY require that
1373   behavior be consistent with the directives.
1374
1375     Pragma           = 1#pragma-directive
1376     pragma-directive = "no-cache" / extension-pragma
1377     extension-pragma = token [ "=" ( token / quoted-string ) ]
1378
1379   When the no-cache directive is present in a request message, a cache
1380   SHOULD forward the request toward the origin server even if it has a
1381   stored copy of what is being requested.  This pragma directive has
1382   the same semantics as the no-cache response directive (see
1383   Section 3.2.2) and is defined here for backward compatibility with
1384   HTTP/1.0.  A client SHOULD include both header fields when a no-cache
1385   request is sent to a server not known to be HTTP/1.1 compliant.  A
1386   cache SHOULD treat "Pragma: no-cache" as if the client had sent
1387   "Cache-Control: no-cache".
1388
1389      Note: Because the meaning of "Pragma: no-cache" as a header field
1390      is not actually specified, it does not provide a reliable
1391      replacement for "Cache-Control: no-cache" in a response.
1392
1393   This mechanism is deprecated; no new Pragma directives will be
1394   defined in HTTP.
1395
1396
1397
1398
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1402
1403
14043.5.  Vary
1405
1406   The "Vary" header field conveys the set of header fields that were
1407   used to select the representation.
1408
1409   Caches use this information, in part, to determine whether a stored
1410   response can be used to satisfy a given request; see Section 2.7.
1411   determines, while the response is fresh, whether a cache is permitted
1412   to use the response to reply to a subsequent request without
1413   validation; see Section 2.7.
1414
1415   In uncacheable or stale responses, the Vary field value advises the
1416   user agent about the criteria that were used to select the
1417   representation.
1418
1419     Vary = "*" / 1#field-name
1420
1421   The set of header fields named by the Vary field value is known as
1422   the selecting header fields.
1423
1424   A server SHOULD include a Vary header field with any cacheable
1425   response that is subject to server-driven negotiation.  Doing so
1426   allows a cache to properly interpret future requests on that resource
1427   and informs the user agent about the presence of negotiation on that
1428   resource.  A server MAY include a Vary header field with a non-
1429   cacheable response that is subject to server-driven negotiation,
1430   since this might provide the user agent with useful information about
1431   the dimensions over which the response varies at the time of the
1432   response.
1433
1434   A Vary field value of "*" signals that unspecified parameters not
1435   limited to the header fields (e.g., the network address of the
1436   client), play a role in the selection of the response representation;
1437   therefore, a cache cannot determine whether this response is
1438   appropriate.  A proxy MUST NOT generate the "*" value.
1439
1440   The field-names given are not limited to the set of standard header
1441   fields defined by this specification.  Field names are case-
1442   insensitive.
1443
14443.6.  Warning
1445
1446   The "Warning" header field is used to carry additional information
1447   about the status or transformation of a message that might not be
1448   reflected in the message.  This information is typically used to warn
1449   about possible incorrectness introduced by caching operations or
1450   transformations applied to the payload of the message.
1451
1452
1453
1454
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1459
1460   Warnings can be used for other purposes, both cache-related and
1461   otherwise.  The use of a warning, rather than an error status code,
1462   distinguishes these responses from true failures.
1463
1464   Warning header fields can in general be applied to any message,
1465   however some warn-codes are specific to caches and can only be
1466   applied to response messages.
1467
1468     Warning       = 1#warning-value
1469
1470     warning-value = warn-code SP warn-agent SP warn-text
1471                                           [SP warn-date]
1472
1473     warn-code  = 3DIGIT
1474     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1475                     ; the name or pseudonym of the server adding
1476                     ; the Warning header field, for use in debugging
1477     warn-text  = quoted-string
1478     warn-date  = DQUOTE HTTP-date DQUOTE
1479
1480   Multiple warnings can be attached to a response (either by the origin
1481   server or by a cache), including multiple warnings with the same code
1482   number, only differing in warn-text.
1483
1484   When this occurs, the user agent SHOULD inform the user of as many of
1485   them as possible, in the order that they appear in the response.
1486
1487   Systems that generate multiple Warning header fields SHOULD order
1488   them with this user agent behavior in mind.  New Warning header
1489   fields SHOULD be added after any existing Warning headers fields.
1490
1491   Warnings are assigned three digit warn-codes.  The first digit
1492   indicates whether the Warning is required to be deleted from a stored
1493   response after validation:
1494
1495   o  1xx Warnings describe the freshness or validation status of the
1496      response, and so MUST be deleted by a cache after validation.
1497      They can only be generated by a cache when validating a cached
1498      entry, and MUST NOT be generated in any other situation.
1499
1500   o  2xx Warnings describe some aspect of the representation that is
1501      not rectified by a validation (for example, a lossy compression of
1502      the representation) and MUST NOT be deleted by a cache after
1503      validation, unless a full response is returned, in which case they
1504      MUST be.
1505
1506   If an implementation sends a message with one or more Warning header
1507   fields to a receiver whose version is HTTP/1.0 or lower, then the
1508
1509
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1514
1515
1516   sender MUST include in each warning-value a warn-date that matches
1517   the Date header field in the message.
1518
1519   If a system receives a message with a warning-value that includes a
1520   warn-date, and that warn-date is different from the Date value in the
1521   response, then that warning-value MUST be deleted from the message
1522   before storing, forwarding, or using it. (preventing the consequences
1523   of naive caching of Warning header fields.)  If all of the warning-
1524   values are deleted for this reason, the Warning header field MUST be
1525   deleted as well.
1526
1527   The following warn-codes are defined by this specification, each with
1528   a recommended warn-text in English, and a description of its meaning.
1529
1530   110 Response is stale
1531
1532      A cache SHOULD include this whenever the returned response is
1533      stale.
1534
1535   111 Revalidation failed
1536
1537      A cache SHOULD include this when returning a stale response
1538      because an attempt to validate the response failed, due to an
1539      inability to reach the server.
1540
1541   112 Disconnected operation
1542
1543      A cache SHOULD b include this if it is intentionally disconnected
1544      from the rest of the network for a period of time.
1545
1546   113 Heuristic expiration
1547
1548      A cache SHOULD include this if it heuristically chose a freshness
1549      lifetime greater than 24 hours and the response's age is greater
1550      than 24 hours.
1551
1552   199 Miscellaneous warning
1553
1554      The warning text can include arbitrary information to be presented
1555      to a human user, or logged.  A system receiving this warning MUST
1556      NOT take any automated action, besides presenting the warning to
1557      the user.
1558
1559   214 Transformation applied
1560
1561      MUST be added by a proxy if it applies any transformation to the
1562      representation, such as changing the content-coding, media-type,
1563      or modifying the representation data, unless this Warning code
1564
1565
1566
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1570
1571
1572      already appears in the response.
1573
1574   299 Miscellaneous persistent warning
1575
1576      The warning text can include arbitrary information to be presented
1577      to a human user, or logged.  A system receiving this warning MUST
1578      NOT take any automated action.
1579
15804.  History Lists
1581
1582   User agents often have history mechanisms, such as "Back" buttons and
1583   history lists, that can be used to redisplay a representation
1584   retrieved earlier in a session.
1585
1586   The freshness model (Section 2.3) does not necessarily apply to
1587   history mechanisms.  I.e., a history mechanism can display a previous
1588   representation even if it has expired.
1589
1590   This does not prohibit the history mechanism from telling the user
1591   that a view might be stale, or from honoring cache directives (e.g.,
1592   Cache-Control: no-store).
1593
15945.  IANA Considerations
1595
15965.1.  Cache Directive Registry
1597
1598   The registration procedure for HTTP Cache Directives is defined by
1599   Section 3.2.3 of this document.
1600
1601   The HTTP Cache Directive Registry shall be created at
1602   <http://www.iana.org/assignments/http-cache-directives> and be
1603   populated with the registrations below:
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
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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 October 20, 2011               [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                  April 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-14
1698              (work in progress), April 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-14 (work in
1704              progress), April 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-14 (work in
1710              progress), April 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-14 (work
1716              in progress), April 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-14 (work in progress),
1722              April 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 October 20, 2011               [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                  April 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   Change ABNF productions for header fields to only define the field
1773   value.  (Section 3)
1774
1775   Do not mention RFC 2047 encoding and multiple languages in Warning
1776   header fields anymore, as these aspects never were implemented.
1777   (Section 3.6)
1778
1779Appendix B.  Collected ABNF
1780
1781   Age = delta-seconds
1782
1783   Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
1784    cache-directive ] )
1785
1786   Expires = HTTP-date
1787
1788
1789
1790
1791Fielding, et al.        Expires October 20, 2011               [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                  April 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 = *( "," OWS ) pragma-directive *( OWS "," [ OWS
1801    pragma-directive ] )
1802
1803   Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
1804    ) )
1805
1806   Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
1807    )
1808
1809   cache-directive = cache-request-directive / cache-response-directive
1810   cache-extension = token [ "=" ( token / quoted-string ) ]
1811   cache-request-directive = "no-cache" / "no-store" / ( "max-age="
1812    delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
1813    "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
1814    cache-extension
1815   cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
1816    OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
1817    "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
1818    field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
1819    "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
1820    ) / ( "s-maxage=" delta-seconds ) / cache-extension
1821
1822   delta-seconds = 1*DIGIT
1823
1824   extension-pragma = token [ "=" ( token / quoted-string ) ]
1825
1826   field-name = <field-name, defined in [Part1], Section 3.2>
1827
1828   port = <port, defined in [Part1], Section 2.6>
1829   pragma-directive = "no-cache" / extension-pragma
1830   pseudonym = <pseudonym, defined in [Part1], Section 9.9>
1831
1832   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1833
1834   token = <token, defined in [Part1], Section 1.2.2>
1835
1836   uri-host = <uri-host, defined in [Part1], Section 2.6>
1837
1838   warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1839   warn-code = 3DIGIT
1840   warn-date = DQUOTE HTTP-date DQUOTE
1841   warn-text = quoted-string
1842   warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
1843    ]
1844
1845
1846
1847Fielding, et al.        Expires October 20, 2011               [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                  April 2011
1850
1851
1852   ABNF diagnostics:
1853
1854   ; Age defined but not used
1855   ; Cache-Control defined but not used
1856   ; Expires defined but not used
1857   ; Pragma defined but not used
1858   ; Vary defined but not used
1859   ; Warning defined but not used
1860
1861Appendix C.  Change Log (to be removed by RFC Editor before publication)
1862
1863C.1.  Since RFC 2616
1864
1865   Extracted relevant partitions from [RFC2616].
1866
1867C.2.  Since draft-ietf-httpbis-p6-cache-00
1868
1869   Closed issues:
1870
1871   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
1872      (<http://purl.org/NET/http-errata#trailer-hop>)
1873
1874   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
1875      after Update or Delete"
1876      (<http://purl.org/NET/http-errata#invalidupd>)
1877
1878   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1879      Informative references"
1880
1881   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
1882      typo"
1883
1884   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/49>: "Connection
1885      header text"
1886
1887   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
1888      references"
1889
1890   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
1891      Reference"
1892
1893   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
1894      to-date references"
1895
1896   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
1897      13.2.2"
1898
1899   Other changes:
1900
1901
1902
1903Fielding, et al.        Expires October 20, 2011               [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                  April 2011
1906
1907
1908   o  Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
1909      on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1910
1911C.3.  Since draft-ietf-httpbis-p6-cache-01
1912
1913   Closed issues:
1914
1915   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
1916      used"
1917
1918   Other changes:
1919
1920   o  Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
1921      in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
1922
1923   o  Add explicit references to BNF syntax and rules imported from
1924      other parts of the specification.
1925
1926C.4.  Since draft-ietf-httpbis-p6-cache-02
1927
1928   Ongoing work on IANA Message Header Field Registration
1929   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1930
1931   o  Reference RFC 3984, and update header field registrations for
1932      header fields defined in this document.
1933
1934C.5.  Since draft-ietf-httpbis-p6-cache-03
1935
1936   Closed issues:
1937
1938   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
1939      classification"
1940
1941C.6.  Since draft-ietf-httpbis-p6-cache-04
1942
1943   Ongoing work on ABNF conversion
1944   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1945
1946   o  Use "/" instead of "|" for alternatives.
1947
1948   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1949      whitespace ("OWS") and required whitespace ("RWS").
1950
1951   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1952      field value format definitions.
1953
1954
1955
1956
1957
1958
1959Fielding, et al.        Expires October 20, 2011               [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                  April 2011
1962
1963
1964C.7.  Since draft-ietf-httpbis-p6-cache-05
1965
1966   This is a total rewrite of this part of the specification.
1967
1968   Affected issues:
1969
1970   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
1971      1xx Warn-Codes"
1972
1973   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
1974      13.5.1 and 13.5.2"
1975
1976   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role of
1977      Warning and Semantic Transparency in Caching"
1978
1979   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
1980      Caching"
1981
1982   In addition: Final work on ABNF conversion
1983   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1984
1985   o  Add appendix containing collected and expanded ABNF, reorganize
1986      ABNF introduction.
1987
1988C.8.  Since draft-ietf-httpbis-p6-cache-06
1989
1990   Closed issues:
1991
1992   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
1993      numeric protocol elements"
1994
1995   Affected issues:
1996
1997   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/37>: "Vary and non-
1998      existant headers"
1999
2000C.9.  Since draft-ietf-httpbis-p6-cache-07
2001
2002   Closed issues:
2003
2004   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
2005      1xx Warn-Codes"
2006
2007   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
2008      Location on 304 responses"
2009
2010   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/169>: "private and
2011      no-cache CC directives with headers"
2012
2013
2014
2015Fielding, et al.        Expires October 20, 2011               [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                  April 2011
2018
2019
2020   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/187>: "RFC2047 and
2021      warn-text"
2022
2023C.10.  Since draft-ietf-httpbis-p6-cache-08
2024
2025   Closed issues:
2026
2027   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/147>: "serving
2028      negotiated responses from cache: header-specific canonicalization"
2029
2030   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/197>: "Effect of CC
2031      directives on history lists"
2032
2033   Affected issues:
2034
2035   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: Status codes
2036      and caching
2037
2038   Partly resolved issues:
2039
2040   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
2041      13.5.1 and 13.5.2"
2042
2043C.11.  Since draft-ietf-httpbis-p6-cache-09
2044
2045   Closed issues:
2046
2047   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/29>: "Age
2048      calculation"
2049
2050   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/168>: "Clarify
2051      differences between / requirements for request and response CC
2052      directives"
2053
2054   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/174>: "Caching
2055      authenticated responses"
2056
2057   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/208>: "IANA registry
2058      for cache-control directives"
2059
2060   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/211>: "Heuristic
2061      caching of URLs with query components"
2062
2063   Partly resolved issues:
2064
2065   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
2066      requested resource's URI"
2067
2068
2069
2070
2071Fielding, et al.        Expires October 20, 2011               [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                  April 2011
2074
2075
2076C.12.  Since draft-ietf-httpbis-p6-cache-10
2077
2078   Closed issues:
2079
2080   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
2081      entity / representation / variant terminology"
2082
2083   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
2084      removing the 'changes from 2068' sections"
2085
2086   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2087      heuristic caching for new status codes"
2088
2089   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
2090      heuristic caching for new status codes"
2091
2092   o  Clean up TODOs and prose in "Combining Responses."
2093
2094C.13.  Since draft-ietf-httpbis-p6-cache-11
2095
2096   Closed issues:
2097
2098   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/204>: "Text about
2099      clock requirement for caches belongs in p6"
2100
2101C.14.  Since draft-ietf-httpbis-p6-cache-12
2102
2103   Closed issues:
2104
2105   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
2106      Classification"
2107
2108   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/268>: "Clarify
2109      'public'"
2110
2111C.15.  Since draft-ietf-httpbis-p6-cache-13
2112
2113   Closed issues:
2114
2115   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
2116      ABNFs for header fields"
2117
2118Index
2119
2120   A
2121      age  6
2122      Age header field  18
2123
2124
2125
2126
2127Fielding, et al.        Expires October 20, 2011               [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                  April 2011
2130
2131
2132   C
2133      cache  5
2134      Cache Directives
2135         max-age  19, 23
2136         max-stale  20
2137         min-fresh  20
2138         must-revalidate  22
2139         no-cache  19, 21
2140         no-store  19, 22
2141         no-transform  20, 23
2142         only-if-cached  20
2143         private  21
2144         proxy-revalidate  23
2145         public  21
2146         s-maxage  23
2147      Cache-Control header field  18
2148      cacheable  5
2149
2150   E
2151      Expires header field  24
2152      explicit expiration time  6
2153
2154   F
2155      first-hand  6
2156      fresh  6
2157      freshness lifetime  6
2158
2159   G
2160      Grammar
2161         Age  18
2162         Cache-Control  19
2163         cache-extension  19
2164         cache-request-directive  19
2165         cache-response-directive  21
2166         delta-seconds  18
2167         Expires  25
2168         extension-pragma  25
2169         Pragma  25
2170         pragma-directive  25
2171         Vary  26
2172         warn-agent  27
2173         warn-code  27
2174         warn-date  27
2175         warn-text  27
2176         Warning  27
2177         warning-value  27
2178
2179   H
2180
2181
2182
2183Fielding, et al.        Expires October 20, 2011               [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                  April 2011
2186
2187
2188      Header Fields
2189         Age  18
2190         Cache-Control  18
2191         Expires  24
2192         Pragma  25
2193         Vary  26
2194         Warning  26
2195      heuristic expiration time  6
2196
2197   M
2198      max-age
2199         Cache Directive  19, 23
2200      max-stale
2201         Cache Directive  20
2202      min-fresh
2203         Cache Directive  20
2204      must-revalidate
2205         Cache Directive  22
2206
2207   N
2208      no-cache
2209         Cache Directive  19, 21
2210      no-store
2211         Cache Directive  19, 22
2212      no-transform
2213         Cache Directive  20, 23
2214
2215   O
2216      only-if-cached
2217         Cache Directive  20
2218
2219   P
2220      Pragma header field  25
2221      private
2222         Cache Directive  21
2223      private cache  5
2224      proxy-revalidate
2225         Cache Directive  23
2226      public
2227         Cache Directive  21
2228
2229   S
2230      s-maxage
2231         Cache Directive  23
2232      shared cache  5
2233      stale  6
2234
2235   V
2236
2237
2238
2239Fielding, et al.        Expires October 20, 2011               [Page 40]
2240
2241Internet-Draft              HTTP/1.1, Part 6                  April 2011
2242
2243
2244      validator  6
2245      Vary header field  26
2246
2247   W
2248      Warning header field  26
2249
2250Authors' Addresses
2251
2252   Roy T. Fielding (editor)
2253   Adobe Systems Incorporated
2254   345 Park Ave
2255   San Jose, CA  95110
2256   USA
2257
2258   EMail: fielding@gbiv.com
2259   URI:   http://roy.gbiv.com/
2260
2261
2262   Jim Gettys
2263   Alcatel-Lucent Bell Labs
2264   21 Oak Knoll Road
2265   Carlisle, MA  01741
2266   USA
2267
2268   EMail: jg@freedesktop.org
2269   URI:   http://gettys.wordpress.com/
2270
2271
2272   Jeffrey C. Mogul
2273   Hewlett-Packard Company
2274   HP Labs, Large Scale Systems Group
2275   1501 Page Mill Road, MS 1177
2276   Palo Alto, CA  94304
2277   USA
2278
2279   EMail: JeffMogul@acm.org
2280
2281
2282   Henrik Frystyk Nielsen
2283   Microsoft Corporation
2284   1 Microsoft Way
2285   Redmond, WA  98052
2286   USA
2287
2288   EMail: henrikn@microsoft.com
2289
2290
2291
2292
2293
2294
2295Fielding, et al.        Expires October 20, 2011               [Page 41]
2296
2297Internet-Draft              HTTP/1.1, Part 6                  April 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 October 20, 2011               [Page 42]
2352
2353Internet-Draft              HTTP/1.1, Part 6                  April 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 October 20, 2011               [Page 43]
2408
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