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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: January 12, 2012                                       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                                                           July 11, 2011
25
26
27                       HTTP/1.1, part 6: Caching
28                     draft-ietf-httpbis-p6-cache-15
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.16.
52
53
54
55Fielding, et al.        Expires January 12, 2012                [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 6                   July 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 January 12, 2012.
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
111Fielding, et al.        Expires January 12, 2012                [Page 2]
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113Internet-Draft              HTTP/1.1, Part 6                   July 2011
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     1.5.  Delta Seconds  . . . . . . . . . . . . . . . . . . . . . .  8
123   2.  Cache Operation  . . . . . . . . . . . . . . . . . . . . . . .  8
124     2.1.  Response Cacheability  . . . . . . . . . . . . . . . . . .  8
125       2.1.1.  Storing Partial and Incomplete Responses . . . . . . .  9
126     2.2.  Constructing Responses from Caches . . . . . . . . . . . .  9
127     2.3.  Freshness Model  . . . . . . . . . . . . . . . . . . . . . 10
128       2.3.1.  Calculating Freshness Lifetime . . . . . . . . . . . . 11
129       2.3.2.  Calculating Age  . . . . . . . . . . . . . . . . . . . 12
130       2.3.3.  Serving Stale Responses  . . . . . . . . . . . . . . . 14
131     2.4.  Validation Model . . . . . . . . . . . . . . . . . . . . . 14
132     2.5.  Request Methods that Invalidate  . . . . . . . . . . . . . 15
133     2.6.  Shared Caching of Authenticated Responses  . . . . . . . . 16
134     2.7.  Caching Negotiated Responses . . . . . . . . . . . . . . . 16
135     2.8.  Combining Responses  . . . . . . . . . . . . . . . . . . . 17
136   3.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 18
137     3.1.  Age  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
138     3.2.  Cache-Control  . . . . . . . . . . . . . . . . . . . . . . 18
139       3.2.1.  Request Cache-Control Directives . . . . . . . . . . . 19
140       3.2.2.  Response Cache-Control Directives  . . . . . . . . . . 21
141       3.2.3.  Cache Control Extensions . . . . . . . . . . . . . . . 23
142     3.3.  Expires  . . . . . . . . . . . . . . . . . . . . . . . . . 24
143     3.4.  Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 25
144     3.5.  Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
145     3.6.  Warning  . . . . . . . . . . . . . . . . . . . . . . . . . 27
146   4.  History Lists  . . . . . . . . . . . . . . . . . . . . . . . . 29
147   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
148     5.1.  Cache Directive Registry . . . . . . . . . . . . . . . . . 29
149     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 30
150   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
151   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 31
152   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
153     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 31
154     8.2.  Informative References . . . . . . . . . . . . . . . . . . 32
155   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 32
156   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 32
157   Appendix C.  Change Log (to be removed by RFC Editor before
158                publication)  . . . . . . . . . . . . . . . . . . . . 34
159     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 34
160     C.2.  Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 34
161     C.3.  Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 35
162     C.4.  Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 35
163     C.5.  Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 35
164
165
166
167Fielding, et al.        Expires January 12, 2012                [Page 3]
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169Internet-Draft              HTTP/1.1, Part 6                   July 2011
170
171
172     C.6.  Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35
173     C.7.  Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 36
174     C.8.  Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 36
175     C.9.  Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36
176     C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 37
177     C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 37
178     C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 38
179     C.13. Since draft-ietf-httpbis-p6-cache-11 . . . . . . . . . . . 38
180     C.14. Since draft-ietf-httpbis-p6-cache-12 . . . . . . . . . . . 38
181     C.15. Since draft-ietf-httpbis-p6-cache-13 . . . . . . . . . . . 38
182     C.16. Since draft-ietf-httpbis-p6-cache-14 . . . . . . . . . . . 38
183   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
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 2.7>
384     pseudonym     = <pseudonym, defined in [Part1], Section 9.9>
385     uri-host      = <uri-host, defined in [Part1], Section 2.7>
386
387
388
389
390
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395
3961.5.  Delta Seconds
397
398   The delta-seconds rule specifies a non-negative integer, representing
399   time in seconds.
400
401     delta-seconds  = 1*DIGIT
402
403   If an implementation receives a delta-seconds value larger than the
404   largest positive integer it can represent, or if any of its
405   subsequent calculations overflows, it MUST consider the value to be
406   2147483648 (2^31).  Recipients parsing a delta-seconds value SHOULD
407   use an arithmetic type of at least 31 bits of range, and senders MUST
408   NOT send delta-seconds with a value greater than 2147483648.
409
4102.  Cache Operation
411
4122.1.  Response Cacheability
413
414   A cache MUST NOT store a response to any request, unless:
415
416   o  The request method is understood by the cache and defined as being
417      cacheable, and
418
419   o  the response status code is understood by the cache, and
420
421   o  the "no-store" cache directive (see Section 3.2) does not appear
422      in request or response header fields, and
423
424   o  the "private" cache response directive (see Section 3.2.2 does not
425      appear in the response, if the cache is shared, and
426
427   o  the "Authorization" header field (see Section 4.1 of [Part7]) does
428      not appear in the request, if the cache is shared, unless the
429      response explicitly allows it (see Section 2.6), and
430
431   o  the response either:
432
433      *  contains an Expires header field (see Section 3.3), or
434
435      *  contains a max-age response cache directive (see
436         Section 3.2.2), or
437
438      *  contains a s-maxage response cache directive and the cache is
439         shared, or
440
441      *  contains a Cache Control Extension (see Section 3.2.3) that
442         allows it to be cached, or
443
444
445
446
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451
452      *  has a status code that can be served with heuristic freshness
453         (see Section 2.3.1.1).
454
455   Note that any of the requirements listed above can be overridden by a
456   cache-control extension; see Section 3.2.3.
457
458   In this context, a cache has "understood" a request method or a
459   response status code if it recognises it and implements any cache-
460   specific behavior.  In particular, 206 Partial Content responses
461   cannot be cached by an implementation that does not handle partial
462   content (see Section 2.1.1).
463
464   Note that in normal operation, most caches will not store a response
465   that has neither a cache validator nor an explicit expiration time,
466   as such responses are not usually useful to store.  However, caches
467   are not prohibited from storing such responses.
468
4692.1.1.  Storing Partial and Incomplete Responses
470
471   A cache that receives an incomplete response (for example, with fewer
472   bytes of data than specified in a Content-Length header field) can
473   store the response, but MUST treat it as a partial response [Part5].
474   Partial responses can be combined as described in Section 4 of
475   [Part5]; the result might be a full response or might still be
476   partial.  A cache MUST NOT return a partial response to a client
477   without explicitly marking it as such using the 206 (Partial Content)
478   status code.
479
480   A cache that does not support the Range and Content-Range header
481   fields MUST NOT store incomplete or partial responses.
482
4832.2.  Constructing Responses from Caches
484
485   For a presented request, a cache MUST NOT return a stored response,
486   unless:
487
488   o  The presented effective request URI (Section 4.3 of [Part1]) and
489      that of the stored response match, and
490
491   o  the request method associated with the stored response allows it
492      to be used for the presented request, and
493
494   o  selecting header fields nominated by the stored response (if any)
495      match those presented (see Section 2.7), and
496
497   o  the presented request and stored response are free from directives
498      that would prevent its use (see Section 3.2 and Section 3.4), and
499
500
501
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507
508   o  the stored response is either:
509
510      *  fresh (see Section 2.3), or
511
512      *  allowed to be served stale (see Section 2.3.3), or
513
514      *  successfully validated (see Section 2.4).
515
516   Note that any of the requirements listed above can be overridden by a
517   cache-control extension; see Section 3.2.3.
518
519   When a stored response is used to satisfy a request without
520   validation, a cache MUST include a single Age header field
521   (Section 3.1) in the response with a value equal to the stored
522   response's current_age; see Section 2.3.2.
523
524   A cache MUST write through requests with methods that are unsafe
525   (Section 7.1.1 of [Part2]) to the origin server; i.e., a cache must
526   not generate a reply to such a request before having forwarded the
527   request and having received a corresponding response.
528
529   Also, note that unsafe requests might invalidate already stored
530   responses; see Section 2.5.
531
532   When more than one suitable response is stored, a cache MUST use the
533   most recent response (as determined by the Date header field).  It
534   can also forward a request with "Cache-Control: max-age=0" or "Cache-
535   Control: no-cache" to disambiguate which response to use.
536
537   A cache that does not have a clock available MUST NOT use stored
538   responses without revalidating them on every use.  A cache,
539   especially a shared cache, SHOULD use a mechanism, such as NTP
540   [RFC1305], to synchronize its clock with a reliable external
541   standard.
542
5432.3.  Freshness Model
544
545   When a response is "fresh" in the cache, it can be used to satisfy
546   subsequent requests without contacting the origin server, thereby
547   improving efficiency.
548
549   The primary mechanism for determining freshness is for an origin
550   server to provide an explicit expiration time in the future, using
551   either the Expires header field (Section 3.3) or the max-age response
552   cache directive (Section 3.2.2).  Generally, origin servers will
553   assign future explicit expiration times to responses in the belief
554   that the representation is not likely to change in a semantically
555   significant way before the expiration time is reached.
556
557
558
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563
564   If an origin server wishes to force a cache to validate every
565   request, it can assign an explicit expiration time in the past to
566   indicate that the response is already stale.  Compliant caches will
567   normally validate the cached response before reusing it for
568   subsequent requests (see Section 2.3.3).
569
570   Since origin servers do not always provide explicit expiration times,
571   a cache MAY assign a heuristic expiration time when an explicit time
572   is not specified, employing algorithms that use other header field
573   values (such as the Last-Modified time) to estimate a plausible
574   expiration time.  This specification does not provide specific
575   algorithms, but does impose worst-case constraints on their results.
576
577   The calculation to determine if a response is fresh is:
578
579      response_is_fresh = (freshness_lifetime > current_age)
580
581   The freshness_lifetime is defined in Section 2.3.1; the current_age
582   is defined in Section 2.3.2.
583
584   Additionally, clients might need to influence freshness calculation.
585   They can do this using several request cache directives, with the
586   effect of either increasing or loosening constraints on freshness.
587   See Section 3.2.1.
588
589   Note that freshness applies only to cache operation; it cannot be
590   used to force a user agent to refresh its display or reload a
591   resource.  See Section 4 for an explanation of the difference between
592   caches and history mechanisms.
593
5942.3.1.  Calculating Freshness Lifetime
595
596   A cache can calculate the freshness lifetime (denoted as
597   freshness_lifetime) of a response by using the first match of:
598
599   o  If the cache is shared and the s-maxage response cache directive
600      (Section 3.2.2) is present, use its value, or
601
602   o  If the max-age response cache directive (Section 3.2.2) is
603      present, use its value, or
604
605   o  If the Expires response header field (Section 3.3) is present, use
606      its value minus the value of the Date response header field, or
607
608   o  Otherwise, no explicit expiration time is present in the response.
609      A heuristic freshness lifetime might be applicable; see
610      Section 2.3.1.1.
611
612
613
614
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619
620   Note that this calculation is not vulnerable to clock skew, since all
621   of the information comes from the origin server.
622
6232.3.1.1.  Calculating Heuristic Freshness
624
625   If no explicit expiration time is present in a stored response that
626   has a status code whose definition allows heuristic freshness to be
627   used (including the following in Section 8 of [Part2]: 200, 203, 206,
628   300, 301 and 410), a cache MAY calculate a heuristic expiration time.
629   A cache MUST NOT use heuristics to determine freshness for responses
630   with status codes that do not explicitly allow it.
631
632   When a heuristic is used to calculate freshness lifetime, a cache
633   SHOULD attach a Warning header field with a 113 warn-code to the
634   response if its current_age is more than 24 hours and such a warning
635   is not already present.
636
637   Also, if the response has a Last-Modified header field (Section 2.1
638   of [Part4]), a cache SHOULD NOT use a heuristic expiration value that
639   is more than some fraction of the interval since that time.  A
640   typical setting of this fraction might be 10%.
641
642      Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
643      not calculate heuristic freshness for URIs with query components
644      (i.e., those containing '?').  In practice, this has not been
645      widely implemented.  Therefore, servers are encouraged to send
646      explicit directives (e.g., Cache-Control: no-cache) if they wish
647      to preclude caching.
648
6492.3.2.  Calculating Age
650
651   HTTP/1.1 uses the Age header field to convey the estimated age of the
652   response message when obtained from a cache.  The Age field value is
653   the cache's estimate of the amount of time since the response was
654   generated or validated by the origin server.  In essence, the Age
655   value is the sum of the time that the response has been resident in
656   each of the caches along the path from the origin server, plus the
657   amount of time it has been in transit along network paths.
658
659   The following data is used for the age calculation:
660
661   age_value
662
663      The term "age_value" denotes the value of the Age header field
664      (Section 3.1), in a form appropriate for arithmetic operation; or
665      0, if not available.
666
667
668
669
670
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675
676   date_value
677
678      HTTP/1.1 requires origin servers to send a Date header field, if
679      possible, with every response, giving the time at which the
680      response was generated.  The term "date_value" denotes the value
681      of the Date header field, in a form appropriate for arithmetic
682      operations.  See Section 9.3 of [Part1] for the definition of the
683      Date header field, and for requirements regarding responses
684      without it.
685
686   now
687
688      The term "now" means "the current value of the clock at the host
689      performing the calculation".  A cache SHOULD use NTP ([RFC1305])
690      or some similar protocol to synchronize its clocks to a globally
691      accurate time standard.
692
693   request_time
694
695      The current value of the clock at the host at the time the request
696      resulting in the stored response was made.
697
698   response_time
699
700      The current value of the clock at the host at the time the
701      response was received.
702
703   A response's age can be calculated in two entirely independent ways:
704
705   1.  the "apparent_age": response_time minus date_value, if the local
706       clock is reasonably well synchronized to the origin server's
707       clock.  If the result is negative, the result is replaced by
708       zero.
709
710   2.  the "corrected_age_value", if all of the caches along the
711       response path implement HTTP/1.1.  A cache MUST interpret this
712       value relative to the time the request was initiated, not the
713       time that the response was received.
714
715
716     apparent_age = max(0, response_time - date_value);
717
718     response_delay = response_time - request_time;
719     corrected_age_value = age_value + response_delay;
720
721
722
723
724
725
726
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731
732   These are combined as
733
734     corrected_initial_age = max(apparent_age, corrected_age_value);
735
736   The current_age of a stored response can then be calculated by adding
737   the amount of time (in seconds) since the stored response was last
738   validated by the origin server to the corrected_initial_age.
739
740     resident_time = now - response_time;
741     current_age = corrected_initial_age + resident_time;
742
7432.3.3.  Serving Stale Responses
744
745   A "stale" response is one that either has explicit expiry information
746   or is allowed to have heuristic expiry calculated, but is not fresh
747   according to the calculations in Section 2.3.
748
749   A cache MUST NOT return a stale response if it is prohibited by an
750   explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
751   cache directive, a "must-revalidate" cache-response-directive, or an
752   applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
753   see Section 3.2.2).
754
755   A cache SHOULD NOT return stale responses unless it is disconnected
756   (i.e., it cannot contact the origin server or otherwise find a
757   forward path) or doing so is explicitly allowed (e.g., by the max-
758   stale request directive; see Section 3.2.1).
759
760   A cache SHOULD append a Warning header field with the 110 warn-code
761   (see Section 3.6) to stale responses.  Likewise, a cache SHOULD add
762   the 112 warn-code to stale responses if the cache is disconnected.
763
764   If a cache receives a first-hand response (either an entire response,
765   or a 304 (Not Modified) response) that it would normally forward to
766   the requesting client, and the received response is no longer fresh,
767   the cache SHOULD forward it to the requesting client without adding a
768   new Warning (but without removing any existing Warning header
769   fields).  A cache SHOULD NOT attempt to validate a response simply
770   because that response became stale in transit.
771
7722.4.  Validation Model
773
774   When a cache has one or more stored responses for a requested URI,
775   but cannot serve any of them (e.g., because they are not fresh, or
776   one cannot be selected; see Section 2.7), it can use the conditional
777   request mechanism [Part4] in the forwarded request to give the origin
778   server an opportunity to both select a valid stored response to be
779   used, and to update it.  This process is known as "validating" or
780
781
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787
788   "revalidating" the stored response.
789
790   When sending such a conditional request, a cache SHOULD add an If-
791   Modified-Since header field whose value is that of the Last-Modified
792   header field from the selected (see Section 2.7) stored response, if
793   available.
794
795   Additionally, a cache SHOULD add an If-None-Match header field whose
796   value is that of the ETag header field(s) from all responses stored
797   for the requested URI, if present.  However, if any of the stored
798   responses contains only partial content, the cache SHOULD NOT include
799   its entity-tag in the If-None-Match header field unless the request
800   is for a range that would be fully satisfied by that stored response.
801
802   A 304 (Not Modified) response status code indicates that the stored
803   response can be updated and reused; see Section 2.8.
804
805   A full response (i.e., one with a response body) indicates that none
806   of the stored responses nominated in the conditional request is
807   suitable.  Instead, a cache SHOULD use the full response to satisfy
808   the request and MAY replace the stored response.
809
810   If a cache receives a 5xx response while attempting to validate a
811   response, it MAY either forward this response to the requesting
812   client, or act as if the server failed to respond.  In the latter
813   case, it MAY return a previously stored response (see Section 2.3.3).
814
8152.5.  Request Methods that Invalidate
816
817   Because unsafe request methods (Section 7.1.1 of [Part2]) such as
818   PUT, POST or DELETE have the potential for changing state on the
819   origin server, intervening caches can use them to keep their contents
820   up-to-date.
821
822   A cache MUST invalidate the effective Request URI (Section 4.3 of
823   [Part1]) as well as the URI(s) in the Location and Content-Location
824   header fields (if present) when a non-error response to a request
825   with an unsafe method is received.
826
827   However, a cache MUST NOT invalidate a URI from a Location or
828   Content-Location header field if the host part of that URI differs
829   from the host part in the effective request URI (Section 4.3 of
830   [Part1]).  This helps prevent denial of service attacks.
831
832   A cache SHOULD invalidate the effective request URI (Section 4.3 of
833   [Part1]) when it receives a non-error response to a request with a
834   method whose safety is unknown.
835
836
837
838
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843
844   Here, a "non-error response" is one with a 2xx or 3xx status code.
845   "Invalidate" means that the cache will either remove all stored
846   responses related to the effective request URI, or will mark these as
847   "invalid" and in need of a mandatory validation before they can be
848   returned in response to a subsequent request.
849
850   Note that this does not guarantee that all appropriate responses are
851   invalidated.  For example, the request that caused the change at the
852   origin server might not have gone through the cache where a response
853   is stored.
854
8552.6.  Shared Caching of Authenticated Responses
856
857   A shared cache MUST NOT use a cached response to a request with an
858   Authorization header field (Section 4.1 of [Part7]) to satisfy any
859   subsequent request unless a cache directive that allows such
860   responses to be stored is present in the response.
861
862   In this specification, the following Cache-Control response
863   directives (Section 3.2.2) have such an effect: must-revalidate,
864   public, s-maxage.
865
866   Note that cached responses that contain the "must-revalidate" and/or
867   "s-maxage" response directives are not allowed to be served stale
868   (Section 2.3.3) by shared caches.  In particular, a response with
869   either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
870   satisfy a subsequent request without revalidating it on the origin
871   server.
872
8732.7.  Caching Negotiated Responses
874
875   When a cache receives a request that can be satisfied by a stored
876   response that has a Vary header field (Section 3.5), it MUST NOT use
877   that response unless all of the selecting header fields nominated by
878   the Vary header field match in both the original request (i.e., that
879   associated with the stored response), and the presented request.
880
881   The selecting header fields from two requests are defined to match if
882   and only if those in the first request can be transformed to those in
883   the second request by applying any of the following:
884
885   o  adding or removing whitespace, where allowed in the header field's
886      syntax
887
888   o  combining multiple header fields with the same field name (see
889      Section 3.2 of [Part1])
890
891
892
893
894
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899
900   o  normalizing both header field values in a way that is known to
901      have identical semantics, according to the header field's
902      specification (e.g., re-ordering field values when order is not
903      significant; case-normalization, where values are defined to be
904      case-insensitive)
905
906   If (after any normalization that might take place) a header field is
907   absent from a request, it can only match another request if it is
908   also absent there.
909
910   A Vary header field-value of "*" always fails to match, and
911   subsequent requests to that resource can only be properly interpreted
912   by the origin server.
913
914   The stored response with matching selecting header fields is known as
915   the selected response.
916
917   If multiple selected responses are available, the most recent
918   response (as determined by the Date header field) is used; see
919   Section 2.2.
920
921   If no selected response is available, the cache MAY forward the
922   presented request to the origin server in a conditional request; see
923   Section 2.4.
924
9252.8.  Combining Responses
926
927   When a cache receives a 304 (Not Modified) response or a 206 (Partial
928   Content) response (in this section, the "new" response"), it needs to
929   create an updated response by combining the stored response with the
930   new one, so that the updated response can be used to satisfy the
931   request, and potentially update the cached response.
932
933   If the new response contains an ETag, it identifies the stored
934   response to use.  [[TODO-mention-CL: might need language about
935   Content-Location here]][[TODO-select-for-combine: Shouldn't this be
936   the selected response?]]
937
938   When the new response's status code is 206 (partial content), a cache
939   MUST NOT combine it with the old response if either response does not
940   have a validator, and MUST NOT combine it with the old response when
941   those validators do not match with the strong comparison function
942   (see Section 2.2.2 of [Part4]).
943
944   The stored response header fields are used as those of the updated
945   response, except that
946
947
948
949
950
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955
956   o  a cache MUST delete any stored Warning header fields with warn-
957      code 1xx (see Section 3.6).
958
959   o  a cache MUST retain any stored Warning header fields with warn-
960      code 2xx.
961
962   o  a cache MUST use other header fields provided in the new response
963      to replace all instances of the corresponding header fields from
964      the stored response.
965
966   A cache MUST use the updated response header fields to replace those
967   of the stored response (unless the stored response is removed).  In
968   the case of a 206 response, a cache MAY store the combined
969   representation.
970
9713.  Header Field Definitions
972
973   This section defines the syntax and semantics of HTTP/1.1 header
974   fields related to caching.
975
9763.1.  Age
977
978   The "Age" header field conveys the sender's estimate of the amount of
979   time since the response was generated or successfully validated at
980   the origin server.  Age values are calculated as specified in
981   Section 2.3.2.
982
983     Age = delta-seconds
984
985   Age field-values are non-negative integers, representing time in
986   seconds (see Section 1.5).
987
988   The presence of an Age header field in a response implies that a
989   response is not first-hand.  However, the converse is not true, since
990   HTTP/1.0 caches might not implement the Age header field.
991
9923.2.  Cache-Control
993
994   The "Cache-Control" header field is used to specify directives for
995   caches along the request/response chain.  Such cache directives are
996   unidirectional in that the presence of a directive in a request does
997   not imply that the same directive is to be given in the response.
998
999   A cache MUST obey the requirements of the Cache-Control directives
1000   defined in this section.  See Section 3.2.3 for information about how
1001   Cache-Control directives defined elsewhere are handled.
1002
1003
1004
1005
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1011
1012      Note: HTTP/1.0 caches might not implement Cache-Control and might
1013      only implement Pragma: no-cache (see Section 3.4).
1014
1015   A proxy, whether or not it implements a cache, MUST pass cache
1016   directives through in forwarded messages, regardless of their
1017   significance to that application, since the directives might be
1018   applicable to all recipients along the request/response chain.  It is
1019   not possible to target a directive to a specific cache.
1020
1021     Cache-Control   = 1#cache-directive
1022
1023     cache-directive = cache-request-directive
1024        / cache-response-directive
1025
1026     cache-extension = token [ "=" ( token / quoted-string ) ]
1027
10283.2.1.  Request Cache-Control Directives
1029
1030     cache-request-directive =
1031          "no-cache"
1032        / "no-store"
1033        / "max-age" "=" delta-seconds
1034        / "max-stale" [ "=" delta-seconds ]
1035        / "min-fresh" "=" delta-seconds
1036        / "no-transform"
1037        / "only-if-cached"
1038        / cache-extension
1039
1040   no-cache
1041
1042      The no-cache request directive indicates that a cache MUST NOT use
1043      a stored response to satisfy the request without successful
1044      validation on the origin server.
1045
1046   no-store
1047
1048      The no-store request directive indicates that a cache MUST NOT
1049      store any part of either this request or any response to it.  This
1050      directive applies to both private and shared caches.  "MUST NOT
1051      store" in this context means that the cache MUST NOT intentionally
1052      store the information in non-volatile storage, and MUST make a
1053      best-effort attempt to remove the information from volatile
1054      storage as promptly as possible after forwarding it.
1055
1056      This directive is NOT a reliable or sufficient mechanism for
1057      ensuring privacy.  In particular, malicious or compromised caches
1058      might not recognize or obey this directive, and communications
1059      networks might be vulnerable to eavesdropping.
1060
1061
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1067
1068      Note that if a request containing this directive is satisfied from
1069      a cache, the no-store request directive does not apply to the
1070      already stored response.
1071
1072   max-age
1073
1074      The max-age request directive indicates that the client is
1075      unwilling to accept a response whose age is greater than the
1076      specified number of seconds.  Unless the max-stale request
1077      directive is also present, the client is not willing to accept a
1078      stale response.
1079
1080   max-stale
1081
1082      The max-stale request directive indicates that the client is
1083      willing to accept a response that has exceeded its expiration
1084      time.  If max-stale is assigned a value, then the client is
1085      willing to accept a response that has exceeded its expiration time
1086      by no more than the specified number of seconds.  If no value is
1087      assigned to max-stale, then the client is willing to accept a
1088      stale response of any age.
1089
1090   min-fresh
1091
1092      The min-fresh request directive indicates that the client is
1093      willing to accept a response whose freshness lifetime is no less
1094      than its current age plus the specified time in seconds.  That is,
1095      the client wants a response that will still be fresh for at least
1096      the specified number of seconds.
1097
1098   no-transform
1099
1100      The no-transform request directive indicates that an intermediary
1101      (whether or not it implements a cache) MUST NOT change the
1102      Content-Encoding, Content-Range or Content-Type request header
1103      fields, nor the request representation.
1104
1105   only-if-cached
1106
1107      The only-if-cached request directive indicates that the client
1108      only wishes to obtain a stored response.  If it receives this
1109      directive, a cache SHOULD either respond using a stored response
1110      that is consistent with the other constraints of the request, or
1111      respond with a 504 (Gateway Timeout) status code.  If a group of
1112      caches is being operated as a unified system with good internal
1113      connectivity, a member cache MAY forward such a request within
1114      that group of caches.
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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1234
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
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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 ignore unrecognized cache directives; it is assumed that
1308   any cache directive likely to be unrecognized by an HTTP/1.1 cache
1309   will be combined with standard directives (or the response's default
1310   cacheability) such that the cache behavior will remain minimally
1311   correct even if the cache does not understand the extension(s).
1312
1313   The HTTP Cache Directive Registry defines the name space for the
1314   cache directives.
1315
1316   A registration MUST include the following fields:
1317
1318   o  Cache Directive Name
1319
1320   o  Pointer to specification text
1321
1322   Values to be added to this name space are subject to IETF review
1323   ([RFC5226], Section 4.1).
1324
1325   The registry itself is maintained at
1326   <http://www.iana.org/assignments/http-cache-directives>.
1327
13283.3.  Expires
1329
1330   The "Expires" header field gives the date/time after which the
1331   response is considered stale.  See Section 2.3 for further discussion
1332   of the freshness model.
1333
1334   The presence of an Expires field does not imply that the original
1335   resource will change or cease to exist at, before, or after that
1336   time.
1337
1338   The field-value is an absolute date and time as defined by HTTP-date
1339   in Section 6.1 of [Part1]; a sender MUST use the rfc1123-date format.
1340
1341
1342
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1347
1348     Expires = HTTP-date
1349
1350   For example
1351
1352     Expires: Thu, 01 Dec 1994 16:00:00 GMT
1353
1354      Note: If a response includes a Cache-Control field with the max-
1355      age directive (see Section 3.2.2), that directive overrides the
1356      Expires field.  Likewise, the s-maxage directive overrides Expires
1357      in shared caches.
1358
1359   A server SHOULD NOT send Expires dates more than one year in the
1360   future.
1361
1362   A cache MUST treat other invalid date formats, especially including
1363   the value "0", as in the past (i.e., "already expired").
1364
13653.4.  Pragma
1366
1367   The "Pragma" header field allows backwards compatibility with
1368   HTTP/1.0 caches, so that clients can specify a "no-cache" request
1369   that they will understand (as Cache-Control was not defined until
1370   HTTP/1.1).  When the Cache-Control header is also present and
1371   understood in a request, Pragma is ignored.
1372
1373   In HTTP/1.0, Pragma was defined as an extensible field for
1374   implementation-specified directives for recipients.  This
1375   specification deprecates such extensions to improve interoperability.
1376
1377     Pragma           = 1#pragma-directive
1378     pragma-directive = "no-cache" / extension-pragma
1379     extension-pragma = token [ "=" ( token / quoted-string ) ]
1380
1381   When the Cache-Control header is not present in a request, the no-
1382   cache request pragma-directive MUST have the same effect on caches as
1383   if "Cache-Control: no-cache" were present (see Section 3.2.1).
1384
1385   When sending a no-cache request, a client SHOULD include both pragma
1386   and cache-control directives unless Cache-Control: no-cache is
1387   purposefully omitted to target other Cache-Control response
1388   directives at HTTP/1.1 caches.  For example:
1389
1390     GET / HTTP/1.1
1391     Host: www.example.com
1392     Cache-Control: max-age=30
1393     Pragma: no-cache
1394
1395
1396
1397
1398
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1402
1403
1404   will constrain HTTP/1.1 caches to serve a response no older than 30
1405   seconds, while precluding implementations that do not understand
1406   Cache-Control from serving a cached response.
1407
1408      Note: Because the meaning of "Pragma: no-cache" in responses is
1409      not specified, it does not provide a reliable replacement for
1410      "Cache-Control: no-cache" in them.
1411
14123.5.  Vary
1413
1414   The "Vary" header field conveys the set of header fields that were
1415   used to select the representation.
1416
1417   Caches use this information, in part, to determine whether a stored
1418   response can be used to satisfy a given request; see Section 2.7.
1419   determines, while the response is fresh, whether a cache is permitted
1420   to use the response to reply to a subsequent request without
1421   validation; see Section 2.7.
1422
1423   In uncacheable or stale responses, the Vary field value advises the
1424   user agent about the criteria that were used to select the
1425   representation.
1426
1427     Vary = "*" / 1#field-name
1428
1429   The set of header fields named by the Vary field value is known as
1430   the selecting header fields.
1431
1432   A server SHOULD include a Vary header field with any cacheable
1433   response that is subject to server-driven negotiation.  Doing so
1434   allows a cache to properly interpret future requests on that resource
1435   and informs the user agent about the presence of negotiation on that
1436   resource.  A server MAY include a Vary header field with a non-
1437   cacheable response that is subject to server-driven negotiation,
1438   since this might provide the user agent with useful information about
1439   the dimensions over which the response varies at the time of the
1440   response.
1441
1442   A Vary field value of "*" signals that unspecified parameters not
1443   limited to the header fields (e.g., the network address of the
1444   client), play a role in the selection of the response representation;
1445   therefore, a cache cannot determine whether this response is
1446   appropriate.  A proxy MUST NOT generate the "*" value.
1447
1448   The field-names given are not limited to the set of standard header
1449   fields defined by this specification.  Field names are case-
1450   insensitive.
1451
1452
1453
1454
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1459
14603.6.  Warning
1461
1462   The "Warning" header field is used to carry additional information
1463   about the status or transformation of a message that might not be
1464   reflected in the message.  This information is typically used to warn
1465   about possible incorrectness introduced by caching operations or
1466   transformations applied to the payload of the message.
1467
1468   Warnings can be used for other purposes, both cache-related and
1469   otherwise.  The use of a warning, rather than an error status code,
1470   distinguishes these responses from true failures.
1471
1472   Warning header fields can in general be applied to any message,
1473   however some warn-codes are specific to caches and can only be
1474   applied to response messages.
1475
1476     Warning       = 1#warning-value
1477
1478     warning-value = warn-code SP warn-agent SP warn-text
1479                                           [SP warn-date]
1480
1481     warn-code  = 3DIGIT
1482     warn-agent = ( uri-host [ ":" port ] ) / pseudonym
1483                     ; the name or pseudonym of the server adding
1484                     ; the Warning header field, for use in debugging
1485     warn-text  = quoted-string
1486     warn-date  = DQUOTE HTTP-date DQUOTE
1487
1488   Multiple warnings can be attached to a response (either by the origin
1489   server or by a cache), including multiple warnings with the same code
1490   number, only differing in warn-text.
1491
1492   When this occurs, the user agent SHOULD inform the user of as many of
1493   them as possible, in the order that they appear in the response.
1494
1495   Systems that generate multiple Warning header fields SHOULD order
1496   them with this user agent behavior in mind.  New Warning header
1497   fields SHOULD be added after any existing Warning headers fields.
1498
1499   Warnings are assigned three digit warn-codes.  The first digit
1500   indicates whether the Warning is required to be deleted from a stored
1501   response after validation:
1502
1503   o  1xx Warnings describe the freshness or validation status of the
1504      response, and so MUST be deleted by a cache after validation.
1505      They can only be generated by a cache when validating a cached
1506      entry, and MUST NOT be generated in any other situation.
1507
1508
1509
1510
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1514
1515
1516   o  2xx Warnings describe some aspect of the representation that is
1517      not rectified by a validation (for example, a lossy compression of
1518      the representation) and MUST NOT be deleted by a cache after
1519      validation, unless a full response is returned, in which case they
1520      MUST be.
1521
1522   If an implementation sends a message with one or more Warning header
1523   fields to a receiver whose version is HTTP/1.0 or lower, then the
1524   sender MUST include in each warning-value a warn-date that matches
1525   the Date header field in the message.
1526
1527   If a system receives a message with a warning-value that includes a
1528   warn-date, and that warn-date is different from the Date value in the
1529   response, then that warning-value MUST be deleted from the message
1530   before storing, forwarding, or using it. (preventing the consequences
1531   of naive caching of Warning header fields.)  If all of the warning-
1532   values are deleted for this reason, the Warning header field MUST be
1533   deleted as well.
1534
1535   The following warn-codes are defined by this specification, each with
1536   a recommended warn-text in English, and a description of its meaning.
1537
1538   110 Response is stale
1539
1540      A cache SHOULD include this whenever the returned response is
1541      stale.
1542
1543   111 Revalidation failed
1544
1545      A cache SHOULD include this when returning a stale response
1546      because an attempt to validate the response failed, due to an
1547      inability to reach the server.
1548
1549   112 Disconnected operation
1550
1551      A cache SHOULD b include this if it is intentionally disconnected
1552      from the rest of the network for a period of time.
1553
1554   113 Heuristic expiration
1555
1556      A cache SHOULD include this if it heuristically chose a freshness
1557      lifetime greater than 24 hours and the response's age is greater
1558      than 24 hours.
1559
1560   199 Miscellaneous warning
1561
1562      The warning text can include arbitrary information to be presented
1563      to a human user, or logged.  A system receiving this warning MUST
1564
1565
1566
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1570
1571
1572      NOT take any automated action, besides presenting the warning to
1573      the user.
1574
1575   214 Transformation applied
1576
1577      MUST be added by a proxy if it applies any transformation to the
1578      representation, such as changing the content-coding, media-type,
1579      or modifying the representation data, unless this Warning code
1580      already appears in the response.
1581
1582   299 Miscellaneous persistent warning
1583
1584      The warning text can include arbitrary information to be presented
1585      to a human user, or logged.  A system receiving this warning MUST
1586      NOT take any automated action.
1587
15884.  History Lists
1589
1590   User agents often have history mechanisms, such as "Back" buttons and
1591   history lists, that can be used to redisplay a representation
1592   retrieved earlier in a session.
1593
1594   The freshness model (Section 2.3) does not necessarily apply to
1595   history mechanisms.  I.e., a history mechanism can display a previous
1596   representation even if it has expired.
1597
1598   This does not prohibit the history mechanism from telling the user
1599   that a view might be stale, or from honoring cache directives (e.g.,
1600   Cache-Control: no-store).
1601
16025.  IANA Considerations
1603
16045.1.  Cache Directive Registry
1605
1606   The registration procedure for HTTP Cache Directives is defined by
1607   Section 3.2.3 of this document.
1608
1609   The HTTP Cache Directive Registry shall be created at
1610   <http://www.iana.org/assignments/http-cache-directives> and be
1611   populated with the registrations below:
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 January 12, 2012               [Page 30]
1680
1681Internet-Draft              HTTP/1.1, Part 6                   July 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-15
1698              (work in progress), July 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-15 (work in
1704              progress), July 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-15 (work in
1710              progress), July 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-15 (work
1716              in progress), July 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-15 (work in progress),
1722              July 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 January 12, 2012               [Page 31]
1736
1737Internet-Draft              HTTP/1.1, Part 6                   July 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 January 12, 2012               [Page 32]
1792
1793Internet-Draft              HTTP/1.1, Part 6                   July 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 2.7>
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 2.7>
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 January 12, 2012               [Page 33]
1848
1849Internet-Draft              HTTP/1.1, Part 6                   July 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 January 12, 2012               [Page 34]
1904
1905Internet-Draft              HTTP/1.1, Part 6                   July 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 January 12, 2012               [Page 35]
1960
1961Internet-Draft              HTTP/1.1, Part 6                   July 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 January 12, 2012               [Page 36]
2016
2017Internet-Draft              HTTP/1.1, Part 6                   July 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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/291>: "Cache
2034      Extensions can override no-store, etc."
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 January 12, 2012               [Page 37]
2072
2073Internet-Draft              HTTP/1.1, Part 6                   July 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  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
2118C.16.  Since draft-ietf-httpbis-p6-cache-14
2119
2120   Closed issues:
2121
2122   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/38>: "Mismatch Vary"
2123
2124
2125
2126
2127Fielding, et al.        Expires January 12, 2012               [Page 38]
2128
2129Internet-Draft              HTTP/1.1, Part 6                   July 2011
2130
2131
2132   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/235>: "Cache
2133      Invalidation only happens upon successful responses"
2134
2135   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/282>: "Recommend
2136      minimum sizes for protocol elements"
2137
2138   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/289>: "Proxies don't
2139      'understand' methods"
2140
2141   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/292>: "Pragma"
2142
2143Index
2144
2145   A
2146      age  6
2147      Age header field  18
2148
2149   C
2150      cache  5
2151      Cache Directives
2152         max-age  20, 23
2153         max-stale  20
2154         min-fresh  20
2155         must-revalidate  22
2156         no-cache  19, 21
2157         no-store  19, 22
2158         no-transform  20, 23
2159         only-if-cached  20
2160         private  21
2161         proxy-revalidate  23
2162         public  21
2163         s-maxage  23
2164      Cache-Control header field  18
2165      cacheable  5
2166
2167   E
2168      Expires header field  24
2169      explicit expiration time  6
2170
2171   F
2172      first-hand  6
2173      fresh  6
2174      freshness lifetime  6
2175
2176   G
2177      Grammar
2178         Age  18
2179         Cache-Control  19
2180
2181
2182
2183Fielding, et al.        Expires January 12, 2012               [Page 39]
2184
2185Internet-Draft              HTTP/1.1, Part 6                   July 2011
2186
2187
2188         cache-extension  19
2189         cache-request-directive  19
2190         cache-response-directive  21
2191         delta-seconds  8
2192         Expires  25
2193         extension-pragma  25
2194         Pragma  25
2195         pragma-directive  25
2196         Vary  26
2197         warn-agent  27
2198         warn-code  27
2199         warn-date  27
2200         warn-text  27
2201         Warning  27
2202         warning-value  27
2203
2204   H
2205      Header Fields
2206         Age  18
2207         Cache-Control  18
2208         Expires  24
2209         Pragma  25
2210         Vary  26
2211         Warning  27
2212      heuristic expiration time  6
2213
2214   M
2215      max-age
2216         Cache Directive  20, 23
2217      max-stale
2218         Cache Directive  20
2219      min-fresh
2220         Cache Directive  20
2221      must-revalidate
2222         Cache Directive  22
2223
2224   N
2225      no-cache
2226         Cache Directive  19, 21
2227      no-store
2228         Cache Directive  19, 22
2229      no-transform
2230         Cache Directive  20, 23
2231
2232   O
2233      only-if-cached
2234         Cache Directive  20
2235
2236
2237
2238
2239Fielding, et al.        Expires January 12, 2012               [Page 40]
2240
2241Internet-Draft              HTTP/1.1, Part 6                   July 2011
2242
2243
2244   P
2245      Pragma header field  25
2246      private
2247         Cache Directive  21
2248      private cache  5
2249      proxy-revalidate
2250         Cache Directive  23
2251      public
2252         Cache Directive  21
2253
2254   S
2255      s-maxage
2256         Cache Directive  23
2257      shared cache  5
2258      stale  6
2259
2260   V
2261      validator  6
2262      Vary header field  26
2263
2264   W
2265      Warning header field  27
2266
2267Authors' Addresses
2268
2269   Roy T. Fielding (editor)
2270   Adobe Systems Incorporated
2271   345 Park Ave
2272   San Jose, CA  95110
2273   USA
2274
2275   EMail: fielding@gbiv.com
2276   URI:   http://roy.gbiv.com/
2277
2278
2279   Jim Gettys
2280   Alcatel-Lucent Bell Labs
2281   21 Oak Knoll Road
2282   Carlisle, MA  01741
2283   USA
2284
2285   EMail: jg@freedesktop.org
2286   URI:   http://gettys.wordpress.com/
2287
2288
2289
2290
2291
2292
2293
2294
2295Fielding, et al.        Expires January 12, 2012               [Page 41]
2296
2297Internet-Draft              HTTP/1.1, Part 6                   July 2011
2298
2299
2300   Jeffrey C. Mogul
2301   Hewlett-Packard Company
2302   HP Labs, Large Scale Systems Group
2303   1501 Page Mill Road, MS 1177
2304   Palo Alto, CA  94304
2305   USA
2306
2307   EMail: JeffMogul@acm.org
2308
2309
2310   Henrik Frystyk Nielsen
2311   Microsoft Corporation
2312   1 Microsoft Way
2313   Redmond, WA  98052
2314   USA
2315
2316   EMail: henrikn@microsoft.com
2317
2318
2319   Larry Masinter
2320   Adobe Systems Incorporated
2321   345 Park Ave
2322   San Jose, CA  95110
2323   USA
2324
2325   EMail: LMM@acm.org
2326   URI:   http://larry.masinter.net/
2327
2328
2329   Paul J. Leach
2330   Microsoft Corporation
2331   1 Microsoft Way
2332   Redmond, WA  98052
2333
2334   EMail: paulle@microsoft.com
2335
2336
2337   Tim Berners-Lee
2338   World Wide Web Consortium
2339   MIT Computer Science and Artificial Intelligence Laboratory
2340   The Stata Center, Building 32
2341   32 Vassar Street
2342   Cambridge, MA  02139
2343   USA
2344
2345   EMail: timbl@w3.org
2346   URI:   http://www.w3.org/People/Berners-Lee/
2347
2348
2349
2350
2351Fielding, et al.        Expires January 12, 2012               [Page 42]
2352
2353Internet-Draft              HTTP/1.1, Part 6                   July 2011
2354
2355
2356   Yves Lafon (editor)
2357   World Wide Web Consortium
2358   W3C / ERCIM
2359   2004, rte des Lucioles
2360   Sophia-Antipolis, AM  06902
2361   France
2362
2363   EMail: ylafon@w3.org
2364   URI:   http://www.raubacapeu.net/people/yves/
2365
2366
2367   Mark Nottingham (editor)
2368
2369   EMail: mnot@mnot.net
2370   URI:   http://www.mnot.net/
2371
2372
2373   Julian F. Reschke (editor)
2374   greenbytes GmbH
2375   Hafenweg 16
2376   Muenster, NW  48155
2377   Germany
2378
2379   Phone: +49 251 2807760
2380   Fax:   +49 251 2807761
2381   EMail: julian.reschke@greenbytes.de
2382   URI:   http://greenbytes.de/tech/webdav/
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 January 12, 2012               [Page 43]
2408
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