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4HTTPbis Working Group                                   R. Fielding, Ed.
5Internet-Draft                                                     Adobe
6Obsoletes: 2616 (if approved)                                  J. Gettys
7Intended status: Standards Track                          Alcatel-Lucent
8Expires: September 15, 2011                                     J. Mogul
9                                                                      HP
10                                                              H. Frystyk
11                                                               Microsoft
12                                                             L. Masinter
13                                                                   Adobe
14                                                                P. Leach
15                                                               Microsoft
16                                                          T. Berners-Lee
17                                                                 W3C/MIT
18                                                           Y. Lafon, Ed.
19                                                                     W3C
20                                                         J. Reschke, Ed.
21                                                              greenbytes
22                                                          March 14, 2011
23
24
25                 HTTP/1.1, part 4: Conditional Requests
26                  draft-ietf-httpbis-p4-conditional-13
27
28Abstract
29
30   The Hypertext Transfer Protocol (HTTP) is an application-level
31   protocol for distributed, collaborative, hypermedia information
32   systems.  HTTP has been in use by the World Wide Web global
33   information initiative since 1990.  This document is Part 4 of the
34   seven-part specification that defines the protocol referred to as
35   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 4 defines
36   request header fields for indicating conditional requests and the
37   rules for constructing responses to those requests.
38
39Editorial Note (To be removed by RFC Editor)
40
41   Discussion of this draft should take place on the HTTPBIS working
42   group mailing list (ietf-http-wg@w3.org).  The current issues list is
43   at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
44   documents (including fancy diffs) can be found at
45   <http://tools.ietf.org/wg/httpbis/>.
46
47   The changes in this draft are summarized in Appendix C.14.
48
49Status of This Memo
50
51   This Internet-Draft is submitted in full conformance with the
52
53
54
55Fielding, et al.       Expires September 15, 2011               [Page 1]
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57Internet-Draft              HTTP/1.1, Part 4                  March 2011
58
59
60   provisions of BCP 78 and BCP 79.
61
62   Internet-Drafts are working documents of the Internet Engineering
63   Task Force (IETF).  Note that other groups may also distribute
64   working documents as Internet-Drafts.  The list of current Internet-
65   Drafts is at http://datatracker.ietf.org/drafts/current/.
66
67   Internet-Drafts are draft documents valid for a maximum of six months
68   and may be updated, replaced, or obsoleted by other documents at any
69   time.  It is inappropriate to use Internet-Drafts as reference
70   material or to cite them other than as "work in progress."
71
72   This Internet-Draft will expire on September 15, 2011.
73
74Copyright Notice
75
76   Copyright (c) 2011 IETF Trust and the persons identified as the
77   document authors.  All rights reserved.
78
79   This document is subject to BCP 78 and the IETF Trust's Legal
80   Provisions Relating to IETF Documents
81   (http://trustee.ietf.org/license-info) in effect on the date of
82   publication of this document.  Please review these documents
83   carefully, as they describe your rights and restrictions with respect
84   to this document.  Code Components extracted from this document must
85   include Simplified BSD License text as described in Section 4.e of
86   the Trust Legal Provisions and are provided without warranty as
87   described in the Simplified BSD License.
88
89   This document may contain material from IETF Documents or IETF
90   Contributions published or made publicly available before November
91   10, 2008.  The person(s) controlling the copyright in some of this
92   material may not have granted the IETF Trust the right to allow
93   modifications of such material outside the IETF Standards Process.
94   Without obtaining an adequate license from the person(s) controlling
95   the copyright in such materials, this document may not be modified
96   outside the IETF Standards Process, and derivative works of it may
97   not be created outside the IETF Standards Process, except to format
98   it for publication as an RFC or to translate it into languages other
99   than English.
100
101
102
103
104
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107
108
109
110
111Fielding, et al.       Expires September 15, 2011               [Page 2]
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113Internet-Draft              HTTP/1.1, Part 4                  March 2011
114
115
116Table of Contents
117
118   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
119     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
120     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  5
121       1.2.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  6
122       1.2.2.  ABNF Rules defined in other Parts of the
123               Specification  . . . . . . . . . . . . . . . . . . . .  6
124   2.  Entity-Tags  . . . . . . . . . . . . . . . . . . . . . . . . .  6
125     2.1.  Example: Entity-tags varying on Content-Negotiated
126           Resources  . . . . . . . . . . . . . . . . . . . . . . . .  7
127   3.  Status Code Definitions  . . . . . . . . . . . . . . . . . . .  8
128     3.1.  304 Not Modified . . . . . . . . . . . . . . . . . . . . .  8
129     3.2.  412 Precondition Failed  . . . . . . . . . . . . . . . . .  8
130   4.  Weak and Strong Validators . . . . . . . . . . . . . . . . . .  9
131   5.  Rules for When to Use Entity-tags and Last-Modified Dates  . . 11
132   6.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 13
133     6.1.  ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
134     6.2.  If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
135     6.3.  If-Modified-Since  . . . . . . . . . . . . . . . . . . . . 15
136     6.4.  If-None-Match  . . . . . . . . . . . . . . . . . . . . . . 17
137     6.5.  If-Unmodified-Since  . . . . . . . . . . . . . . . . . . . 18
138     6.6.  Last-Modified  . . . . . . . . . . . . . . . . . . . . . . 19
139   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
140     7.1.  Status Code Registration . . . . . . . . . . . . . . . . . 19
141     7.2.  Header Field Registration  . . . . . . . . . . . . . . . . 20
142   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
143   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
144   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
145     10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
146     10.2. Informative References . . . . . . . . . . . . . . . . . . 21
147   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 21
148   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 22
149   Appendix C.  Change Log (to be removed by RFC Editor before
150                publication)  . . . . . . . . . . . . . . . . . . . . 22
151     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 22
152     C.2.  Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23
153     C.3.  Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23
154     C.4.  Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23
155     C.5.  Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23
156     C.6.  Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24
157     C.7.  Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24
158     C.8.  Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24
159     C.9.  Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24
160     C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24
161     C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 24
162     C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25
163     C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25
164
165
166
167Fielding, et al.       Expires September 15, 2011               [Page 3]
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169Internet-Draft              HTTP/1.1, Part 4                  March 2011
170
171
172     C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25
173   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
174
175
176
177
178
179
180
181
182
183
184
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222
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225Internet-Draft              HTTP/1.1, Part 4                  March 2011
226
227
2281.  Introduction
229
230   This document defines HTTP/1.1 response metadata for indicating
231   potential changes to payload content, including modification time
232   stamps and opaque entity-tags, and the HTTP conditional request
233   mechanisms that allow preconditions to be placed on a request method.
234   Conditional GET requests allow for efficient cache updates.  Other
235   conditional request methods are used to protect against overwriting
236   or misunderstanding the state of a resource that has been changed
237   unbeknownst to the requesting client.
238
239   This document is currently disorganized in order to minimize the
240   changes between drafts and enable reviewers to see the smaller errata
241   changes.  A future draft will reorganize the sections to better
242   reflect the content.  In particular, the sections on resource
243   metadata will be discussed first and then followed by each
244   conditional request header field, concluding with a definition of
245   precedence and the expectation of ordering strong validator checks
246   before weak validator checks.  It is likely that more content from
247   [Part6] will migrate to this part, where appropriate.  The current
248   mess reflects how widely dispersed these topics and associated
249   requirements had become in [RFC2616].
250
2511.1.  Requirements
252
253   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
254   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
255   document are to be interpreted as described in [RFC2119].
256
257   An implementation is not compliant if it fails to satisfy one or more
258   of the "MUST" or "REQUIRED" level requirements for the protocols it
259   implements.  An implementation that satisfies all the "MUST" or
260   "REQUIRED" level and all the "SHOULD" level requirements for its
261   protocols is said to be "unconditionally compliant"; one that
262   satisfies all the "MUST" level requirements but not all the "SHOULD"
263   level requirements for its protocols is said to be "conditionally
264   compliant".
265
2661.2.  Syntax Notation
267
268   This specification uses the ABNF syntax defined in Section 1.2 of
269   [Part1] (which extends the syntax defined in [RFC5234] with a list
270   rule).  Appendix B shows the collected ABNF, with the list rule
271   expanded.
272
273   The following core rules are included by reference, as defined in
274   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
275   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
276
277
278
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283
284   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
285   sequence of data), SP (space), VCHAR (any visible USASCII character),
286   and WSP (whitespace).
287
2881.2.1.  Core Rules
289
290   The core rules below are defined in Section 1.2.2 of [Part1]:
291
292     quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
293     OWS           = <OWS, defined in [Part1], Section 1.2.2>
294
2951.2.2.  ABNF Rules defined in other Parts of the Specification
296
297   The ABNF rules below are defined in other parts:
298
299     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
300
3012.  Entity-Tags
302
303   Entity-tags are used for comparing two or more representations of the
304   same resource.  HTTP/1.1 uses entity-tags in the ETag (Section 6.1),
305   If-Match (Section 6.2), If-None-Match (Section 6.4), and If-Range
306   (Section 5.3 of [Part5]) header fields.  The definition of how they
307   are used and compared as cache validators is in Section 4.  An
308   entity-tag consists of an opaque quoted string, possibly prefixed by
309   a weakness indicator.
310
311     entity-tag = [ weak ] opaque-tag
312     weak       = %x57.2F ; "W/", case-sensitive
313     opaque-tag = quoted-string
314
315   A "strong entity-tag" MAY be shared by two representations of a
316   resource only if they are equivalent by octet equality.
317
318   A "weak entity-tag", indicated by the "W/" prefix, MAY be shared by
319   two representations of a resource only if the representations are
320   equivalent and could be substituted for each other with no
321   significant change in semantics.  A weak entity-tag can only be used
322   for weak comparison.
323
324   An entity-tag MUST be unique across all versions of all
325   representations associated with a particular resource.  A given
326   entity-tag value MAY be used for representations obtained by requests
327   on different URIs.  The use of the same entity-tag value in
328   conjunction with representations obtained by requests on different
329   URIs does not imply the equivalence of those representations.
330
331
332
333
334
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339
3402.1.  Example: Entity-tags varying on Content-Negotiated Resources
341
342   Consider a resource that is subject to content negotiation (Section 5
343   of [Part3]), and where the representations returned upon a GET
344   request vary based on the Accept-Encoding request header field
345   (Section 6.3 of [Part3]):
346
347   >> Request:
348
349     GET /index HTTP/1.1
350     Host: www.example.com
351     Accept-Encoding: gzip
352
353
354   In this case, the response might or might not use the gzip content
355   coding.  If it does not, the response might look like:
356
357   >> Response:
358
359     HTTP/1.1 200 OK
360     Date: Thu, 26 Mar 2010 00:05:00 GMT
361     ETag: "123-a"
362     Content-Length: 70
363     Vary: Accept-Encoding
364     Content-Type: text/plain
365
366     Hello World!
367     Hello World!
368     Hello World!
369     Hello World!
370     Hello World!
371
372   An alternative representation that does use gzip content coding would
373   be:
374
375   >> Response:
376
377     HTTP/1.1 200 OK
378     Date: Thu, 26 Mar 2010 00:05:00 GMT
379     ETag: "123-b"
380     Content-Length: 43
381     Vary: Accept-Encoding
382     Content-Type: text/plain
383     Content-Encoding: gzip
384
385     ...binary data...
386
387
388
389
390
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395
396      Note: Content codings are a property of the representation, so
397      therefore an entity-tag of an encoded representation must be
398      distinct from an unencoded representation to prevent conflicts
399      during cache updates and range requests.  In contrast, transfer
400      codings (Section 6.2 of [Part1]) apply only during message
401      transfer and do not require distinct entity-tags.
402
4033.  Status Code Definitions
404
4053.1.  304 Not Modified
406
407   If the client has performed a conditional GET request and access is
408   allowed, but the document has not been modified, the server SHOULD
409   respond with this status code.  The 304 response MUST NOT contain a
410   message-body, and thus is always terminated by the first empty line
411   after the header fields.
412
413   A 304 response MUST include a Date header field (Section 9.3 of
414   [Part1]) unless its omission is required by Section 9.3.1 of [Part1].
415   If a 200 response to the same request would have included any of the
416   header fields Cache-Control, Content-Location, ETag, Expires, Last-
417   Modified, or Vary, then those same header fields MUST be sent in a
418   304 response.
419
420   Since the goal of a 304 response is to minimize information transfer
421   when the recipient already has one or more cached representations,
422   the response SHOULD NOT include representation metadata other than
423   the above listed fields unless said metadata exists for the purpose
424   of guiding cache updates (e.g., future HTTP extensions).
425
426   If a 304 response includes an entity-tag that indicates a
427   representation not currently cached, then the recipient MUST NOT use
428   the 304 to update its own cache.  If that conditional request
429   originated with an outbound client, such as a user agent with its own
430   cache sending a conditional GET to a shared proxy, then the 304
431   response MAY be forwarded to the outbound client.  Otherwise,
432   disregard the response and repeat the request without the
433   conditional.
434
435   If a cache uses a received 304 response to update a cache entry, the
436   cache MUST update the entry to reflect any new field values given in
437   the response.
438
4393.2.  412 Precondition Failed
440
441   The precondition given in one or more of the header fields evaluated
442   to false when it was tested on the server.  This response code allows
443   the client to place preconditions on the current resource metadata
444
445
446
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451
452   (header field data) and thus prevent the requested method from being
453   applied to a resource other than the one intended.
454
4554.  Weak and Strong Validators
456
457   Since both origin servers and caches will compare two validators to
458   decide if they represent the same or different representations, one
459   normally would expect that if the representation (including both
460   representation header fields and representation body) changes in any
461   way, then the associated validator would change as well.  If this is
462   true, then we call this validator a "strong validator".
463
464   However, there might be cases when a server prefers to change the
465   validator only on semantically significant changes, and not when
466   insignificant aspects of the representation change.  A validator that
467   does not always change when the representation changes is a "weak
468   validator".
469
470   An entity-tag is normally a strong validator, but the protocol
471   provides a mechanism to tag an entity-tag as "weak".  One can think
472   of a strong validator as one that changes whenever the sequence of
473   bits in a representation changes, while a weak value changes whenever
474   the meaning of a representation changes.  Alternatively, one can
475   think of a strong validator as part of an identifier for a specific
476   representation, whereas a weak validator is part of an identifier for
477   a set of semantically equivalent representations.
478
479      Note: One example of a strong validator is an integer that is
480      incremented in stable storage every time a representation is
481      changed.
482
483      A representation's modification time, if defined with only one-
484      second resolution, could be a weak validator, since it is possible
485      that the representation might be modified twice during a single
486      second.
487
488      Support for weak validators is optional.  However, weak validators
489      allow for more efficient caching of equivalent objects; for
490      example, a hit counter on a site is probably good enough if it is
491      updated every few days or weeks, and any value during that period
492      is likely "good enough" to be equivalent.
493
494   A "use" of a validator is either when a client generates a request
495   and includes the validator in a validating header field, or when a
496   server compares two validators.
497
498   Strong validators are usable in any context.  Weak validators are
499   only usable in contexts that do not depend on exact equality of a
500
501
502
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507
508   representation.  For example, either kind is usable for a normal
509   conditional GET.  However, only a strong validator is usable for a
510   sub-range retrieval, since otherwise the client might end up with an
511   internally inconsistent representation.
512
513   Clients MUST NOT use weak validators in range requests ([Part5]).
514
515   The only function that HTTP/1.1 defines on validators is comparison.
516   There are two validator comparison functions, depending on whether
517   the comparison context allows the use of weak validators or not:
518
519   o  The strong comparison function: in order to be considered equal,
520      both opaque-tags MUST be identical character-by-character, and
521      both MUST NOT be weak.
522
523   o  The weak comparison function: in order to be considered equal,
524      both opaque-tags MUST be identical character-by-character, but
525      either or both of them MAY be tagged as "weak" without affecting
526      the result.
527
528   The example below shows the results for a set of entity-tag pairs,
529   and both the weak and strong comparison function results:
530
531   +--------+--------+-------------------+-----------------+
532   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
533   +--------+--------+-------------------+-----------------+
534   | W/"1"  | W/"1"  | no match          | match           |
535   | W/"1"  | W/"2"  | no match          | no match        |
536   | W/"1"  | "1"    | no match          | match           |
537   | "1"    | "1"    | match             | match           |
538   +--------+--------+-------------------+-----------------+
539
540   An entity-tag is strong unless it is explicitly tagged as weak.
541   Section 2 gives the syntax for entity-tags.
542
543   A Last-Modified time, when used as a validator in a request, is
544   implicitly weak unless it is possible to deduce that it is strong,
545   using the following rules:
546
547   o  The validator is being compared by an origin server to the actual
548      current validator for the representation and,
549
550   o  That origin server reliably knows that the associated
551      representation did not change twice during the second covered by
552      the presented validator.
553
554   or
555
556
557
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563
564   o  The validator is about to be used by a client in an If-Modified-
565      Since or If-Unmodified-Since header field, because the client has
566      a cache entry for the associated representation, and
567
568   o  That cache entry includes a Date value, which gives the time when
569      the origin server sent the original response, and
570
571   o  The presented Last-Modified time is at least 60 seconds before the
572      Date value.
573
574   or
575
576   o  The validator is being compared by an intermediate cache to the
577      validator stored in its cache entry for the representation, and
578
579   o  That cache entry includes a Date value, which gives the time when
580      the origin server sent the original response, and
581
582   o  The presented Last-Modified time is at least 60 seconds before the
583      Date value.
584
585   This method relies on the fact that if two different responses were
586   sent by the origin server during the same second, but both had the
587   same Last-Modified time, then at least one of those responses would
588   have a Date value equal to its Last-Modified time.  The arbitrary 60-
589   second limit guards against the possibility that the Date and Last-
590   Modified values are generated from different clocks, or at somewhat
591   different times during the preparation of the response.  An
592   implementation MAY use a value larger than 60 seconds, if it is
593   believed that 60 seconds is too short.
594
595   If a client wishes to perform a sub-range retrieval on a value for
596   which it has only a Last-Modified time and no opaque validator, it
597   MAY do this only if the Last-Modified time is strong in the sense
598   described here.
599
600   A cache or origin server receiving a conditional range request
601   ([Part5]) MUST use the strong comparison function to evaluate the
602   condition.
603
604   These rules allow HTTP/1.1 caches and clients to safely perform sub-
605   range retrievals on values that have been obtained from HTTP/1.0
606   servers.
607
6085.  Rules for When to Use Entity-tags and Last-Modified Dates
609
610   We adopt a set of rules and recommendations for origin servers,
611   clients, and caches regarding when various validator types ought to
612
613
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619
620   be used, and for what purposes.
621
622   HTTP/1.1 origin servers:
623
624   o  SHOULD send an entity-tag validator unless it is not feasible to
625      generate one.
626
627   o  MAY send a weak entity-tag instead of a strong entity-tag, if
628      performance considerations support the use of weak entity-tags, or
629      if it is unfeasible to send a strong entity-tag.
630
631   o  SHOULD send a Last-Modified value if it is feasible to send one,
632      unless the risk of a breakdown in semantic transparency that could
633      result from using this date in an If-Modified-Since header field
634      would lead to serious problems.
635
636   In other words, the preferred behavior for an HTTP/1.1 origin server
637   is to send both a strong entity-tag and a Last-Modified value.
638
639   In order to be legitimate, a strong entity-tag MUST change whenever
640   the associated representation changes in any way.  A weak entity-tag
641   SHOULD change whenever the associated representation changes in a
642   semantically significant way.
643
644      Note: In order to provide semantically transparent caching, an
645      origin server must avoid reusing a specific strong entity-tag
646      value for two different representations, or reusing a specific
647      weak entity-tag value for two semantically different
648      representations.  Cache entries might persist for arbitrarily long
649      periods, regardless of expiration times, so it might be
650      inappropriate to expect that a cache will never again attempt to
651      validate an entry using a validator that it obtained at some point
652      in the past.
653
654   HTTP/1.1 clients:
655
656   o  MUST use that entity-tag in any cache-conditional request (using
657      If-Match or If-None-Match) if an entity-tag has been provided by
658      the origin server.
659
660   o  SHOULD use the Last-Modified value in non-subrange cache-
661      conditional requests (using If-Modified-Since) if only a Last-
662      Modified value has been provided by the origin server.
663
664   o  MAY use the Last-Modified value in subrange cache-conditional
665      requests (using If-Unmodified-Since) if only a Last-Modified value
666      has been provided by an HTTP/1.0 origin server.  The user agent
667      SHOULD provide a way to disable this, in case of difficulty.
668
669
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675
676   o  SHOULD use both validators in cache-conditional requests if both
677      an entity-tag and a Last-Modified value have been provided by the
678      origin server.  This allows both HTTP/1.0 and HTTP/1.1 caches to
679      respond appropriately.
680
681   An HTTP/1.1 origin server, upon receiving a conditional request that
682   includes both a Last-Modified date (e.g., in an If-Modified-Since or
683   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
684   in an If-Match, If-None-Match, or If-Range header field) as cache
685   validators, MUST NOT return a response status code of 304 (Not
686   Modified) unless doing so is consistent with all of the conditional
687   header fields in the request.
688
689   An HTTP/1.1 caching proxy, upon receiving a conditional request that
690   includes both a Last-Modified date and one or more entity-tags as
691   cache validators, MUST NOT return a locally cached response to the
692   client unless that cached response is consistent with all of the
693   conditional header fields in the request.
694
695      Note: The general principle behind these rules is that HTTP/1.1
696      servers and clients ought to transmit as much non-redundant
697      information as is available in their responses and requests.
698      HTTP/1.1 systems receiving this information will make the most
699      conservative assumptions about the validators they receive.
700
701      HTTP/1.0 clients and caches might ignore entity-tags.  Generally,
702      last-modified values received or used by these systems will
703      support transparent and efficient caching, and so HTTP/1.1 origin
704      servers should provide Last-Modified values.  In those rare cases
705      where the use of a Last-Modified value as a validator by an
706      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
707      origin servers should not provide one.
708
7096.  Header Field Definitions
710
711   This section defines the syntax and semantics of HTTP/1.1 header
712   fields related to conditional requests.
713
7146.1.  ETag
715
716   The "ETag" header field provides the current value of the entity-tag
717   (see Section 2) for one representation of the target resource.  An
718   entity-tag is intended for use as a resource-local identifier for
719   differentiating between representations of the same resource that
720   vary over time or via content negotiation (see Section 4).
721
722     ETag   = "ETag" ":" OWS ETag-v
723     ETag-v = entity-tag
724
725
726
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731
732   Examples:
733
734     ETag: "xyzzy"
735     ETag: W/"xyzzy"
736     ETag: ""
737
738   An entity-tag provides an "opaque" cache validator that allows for
739   more reliable validation than modification dates in situations where
740   it is inconvenient to store modification dates, where the one-second
741   resolution of HTTP date values is not sufficient, or where the origin
742   server wishes to avoid certain paradoxes that might arise from the
743   use of modification dates.
744
745   The principle behind entity-tags is that only the service author
746   knows the semantics of a resource well enough to select an
747   appropriate cache validation mechanism, and the specification of any
748   validator comparison function more complex than byte-equality would
749   open up a can of worms.  Thus, comparisons of any other header fields
750   (except Last-Modified, for compatibility with HTTP/1.0) are never
751   used for purposes of validating a cache entry.
752
7536.2.  If-Match
754
755   The "If-Match" header field is used to make a request method
756   conditional.  A client that has one or more representations
757   previously obtained from the resource can verify that one of those
758   representations is current by including a list of their associated
759   entity-tags in the If-Match header field.
760
761   This allows efficient updates of cached information with a minimum
762   amount of transaction overhead.  It is also used when updating
763   resources, to prevent inadvertent modification of the wrong version
764   of a resource.  As a special case, the value "*" matches any current
765   representation of the resource.
766
767     If-Match   = "If-Match" ":" OWS If-Match-v
768     If-Match-v = "*" / 1#entity-tag
769
770   If any of the entity-tags match the entity-tag of the representation
771   that would have been returned in the response to a similar GET
772   request (without the If-Match header field) on that resource, or if
773   "*" is given and any current representation exists for that resource,
774   then the server MAY perform the requested method as if the If-Match
775   header field did not exist.
776
777   If none of the entity-tags match, or if "*" is given and no current
778   representation exists, the server MUST NOT perform the requested
779   method, and MUST return a 412 (Precondition Failed) response.  This
780
781
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787
788   behavior is most useful when the client wants to prevent an updating
789   request method, such as PUT, from modifying a resource that has
790   changed since the client last retrieved it.
791
792   If the request would, without the If-Match header field, result in
793   anything other than a 2xx or 412 status code, then the If-Match
794   header field MUST be ignored.
795
796   The meaning of "If-Match: *" is that the request method SHOULD be
797   performed if the representation selected by the origin server (or by
798   a cache, possibly using the Vary mechanism, see Section 3.5 of
799   [Part6]) exists, and MUST NOT be performed if the representation does
800   not exist.
801
802   A request intended to update a resource (e.g., a PUT) MAY include an
803   If-Match header field to signal that the request method MUST NOT be
804   applied if the representation corresponding to the If-Match value (a
805   single entity-tag) is no longer a representation of that resource.
806   This allows the user to indicate that they do not wish the request to
807   be successful if the resource has been changed without their
808   knowledge.  Examples:
809
810     If-Match: "xyzzy"
811     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
812     If-Match: *
813
814   The result of a request having both an If-Match header field and
815   either an If-None-Match or an If-Modified-Since header fields is
816   undefined by this specification.
817
8186.3.  If-Modified-Since
819
820   The "If-Modified-Since" header field is used to make a request method
821   conditional by date: if the representation that would have been
822   transferred in a 200 response to a GET request has not been modified
823   since the time specified in this field, then do not perform the
824   method; instead, respond as detailed below.
825
826     If-Modified-Since   = "If-Modified-Since" ":" OWS
827                           If-Modified-Since-v
828     If-Modified-Since-v = HTTP-date
829
830   An example of the field is:
831
832     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
833
834   A GET method with an If-Modified-Since header field and no Range
835   header field requests that the representation be transferred only if
836
837
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843
844   it has been modified since the date given by the If-Modified-Since
845   header field.  The algorithm for determining this includes the
846   following cases:
847
848   1.  If the request would normally result in anything other than a 200
849       (OK) status code, or if the passed If-Modified-Since date is
850       invalid, the response is exactly the same as for a normal GET.  A
851       date which is later than the server's current time is invalid.
852
853   2.  If the representation has been modified since the If-Modified-
854       Since date, the response is exactly the same as for a normal GET.
855
856   3.  If the representation has not been modified since a valid If-
857       Modified-Since date, the server SHOULD return a 304 (Not
858       Modified) response.
859
860   The purpose of this feature is to allow efficient updates of cached
861   information with a minimum amount of transaction overhead.
862
863      Note: The Range header field modifies the meaning of If-Modified-
864      Since; see Section 5.4 of [Part5] for full details.
865
866      Note: If-Modified-Since times are interpreted by the server, whose
867      clock might not be synchronized with the client.
868
869      Note: When handling an If-Modified-Since header field, some
870      servers will use an exact date comparison function, rather than a
871      less-than function, for deciding whether to send a 304 (Not
872      Modified) response.  To get best results when sending an If-
873      Modified-Since header field for cache validation, clients are
874      advised to use the exact date string received in a previous Last-
875      Modified header field whenever possible.
876
877      Note: If a client uses an arbitrary date in the If-Modified-Since
878      header field instead of a date taken from the Last-Modified header
879      field for the same request, the client needs to be aware that this
880      date is interpreted in the server's understanding of time.
881      Unsynchronized clocks and rounding problems, due to the different
882      encodings of time between the client and server, are concerns.
883      This includes the possibility of race conditions if the document
884      has changed between the time it was first requested and the If-
885      Modified-Since date of a subsequent request, and the possibility
886      of clock-skew-related problems if the If-Modified-Since date is
887      derived from the client's clock without correction to the server's
888      clock.  Corrections for different time bases between client and
889      server are at best approximate due to network latency.
890
891   The result of a request having both an If-Modified-Since header field
892
893
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899
900   and either an If-Match or an If-Unmodified-Since header fields is
901   undefined by this specification.
902
9036.4.  If-None-Match
904
905   The "If-None-Match" header field is used to make a request method
906   conditional.  A client that has one or more representations
907   previously obtained from the resource can verify that none of those
908   representations is current by including a list of their associated
909   entity-tags in the If-None-Match header field.
910
911   This allows efficient updates of cached information with a minimum
912   amount of transaction overhead.  It is also used to prevent a request
913   method (e.g., PUT) from inadvertently modifying an existing resource
914   when the client believes that the resource does not exist.
915
916   As a special case, the value "*" matches any current representation
917   of the resource.
918
919     If-None-Match   = "If-None-Match" ":" OWS If-None-Match-v
920     If-None-Match-v = "*" / 1#entity-tag
921
922   If any of the entity-tags match the entity-tag of the representation
923   that would have been returned in the response to a similar GET
924   request (without the If-None-Match header field) on that resource, or
925   if "*" is given and any current representation exists for that
926   resource, then the server MUST NOT perform the requested method,
927   unless required to do so because the resource's modification date
928   fails to match that supplied in an If-Modified-Since header field in
929   the request.  Instead, if the request method was GET or HEAD, the
930   server SHOULD respond with a 304 (Not Modified) response, including
931   the cache-related header fields (particularly ETag) of one of the
932   representations that matched.  For all other request methods, the
933   server MUST respond with a 412 (Precondition Failed) status code.
934
935   If none of the entity-tags match, then the server MAY perform the
936   requested method as if the If-None-Match header field did not exist,
937   but MUST also ignore any If-Modified-Since header field(s) in the
938   request.  That is, if no entity-tags match, then the server MUST NOT
939   return a 304 (Not Modified) response.
940
941   If the request would, without the If-None-Match header field, result
942   in anything other than a 2xx or 304 status code, then the If-None-
943   Match header field MUST be ignored.  (See Section 5 for a discussion
944   of server behavior when both If-Modified-Since and If-None-Match
945   appear in the same request.)
946
947   The meaning of "If-None-Match: *" is that the request method MUST NOT
948
949
950
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955
956   be performed if the representation selected by the origin server (or
957   by a cache, possibly using the Vary mechanism, see Section 3.5 of
958   [Part6]) exists, and SHOULD be performed if the representation does
959   not exist.  This feature is intended to be useful in preventing races
960   between PUT operations.
961
962   Examples:
963
964     If-None-Match: "xyzzy"
965     If-None-Match: W/"xyzzy"
966     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
967     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
968     If-None-Match: *
969
970   The result of a request having both an If-None-Match header field and
971   either an If-Match or an If-Unmodified-Since header fields is
972   undefined by this specification.
973
9746.5.  If-Unmodified-Since
975
976   The "If-Unmodified-Since" header field is used to make a request
977   method conditional.  If the representation that would have been
978   transferred in a 200 response to a GET request on the same resource
979   has not been modified since the time specified in this field, the
980   server SHOULD perform the requested operation as if the If-
981   Unmodified-Since header field were not present.
982
983   If the representation has been modified since the specified time, the
984   server MUST NOT perform the requested operation, and MUST return a
985   412 (Precondition Failed).
986
987     If-Unmodified-Since   = "If-Unmodified-Since" ":" OWS
988                             If-Unmodified-Since-v
989     If-Unmodified-Since-v = HTTP-date
990
991   An example of the field is:
992
993     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
994
995   If the request normally (i.e., without the If-Unmodified-Since header
996   field) would result in anything other than a 2xx or 412 status code,
997   the If-Unmodified-Since header field SHOULD be ignored.
998
999   If the specified date is invalid, the header field is ignored.
1000
1001   The result of a request having both an If-Unmodified-Since header
1002   field and either an If-None-Match or an If-Modified-Since header
1003   fields is undefined by this specification.
1004
1005
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1011
10126.6.  Last-Modified
1013
1014   The "Last-Modified" header field indicates the date and time at which
1015   the origin server believes the representation was last modified.
1016
1017     Last-Modified   = "Last-Modified" ":" OWS Last-Modified-v
1018     Last-Modified-v = HTTP-date
1019
1020   An example of its use is
1021
1022     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1023
1024   A representation is typically the sum of many parts behind the
1025   resource interface.  The last-modified time would usually be the most
1026   recent time that any of those parts were changed.  How that value is
1027   determined for any given resource is an implementation detail beyond
1028   the scope of this specification.  What matters to HTTP is how
1029   recipients of the Last-Modified header field can use its value to
1030   make conditional requests and test the validity of locally cached
1031   responses.
1032
1033   An origin server MUST NOT send a Last-Modified date which is later
1034   than the server's time of message origination.  In such cases, where
1035   the resource's last modification would indicate some time in the
1036   future, the server MUST replace that date with the message
1037   origination date.
1038
1039   An origin server SHOULD obtain the Last-Modified value of the
1040   representation as close as possible to the time that it generates the
1041   Date value of its response.  This allows a recipient to make an
1042   accurate assessment of the representation's modification time,
1043   especially if the representation changes near the time that the
1044   response is generated.
1045
1046   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
1047
1048   The Last-Modified header field value is often used as a cache
1049   validator.  In simple terms, a cache entry is considered to be valid
1050   if the representation has not been modified since the Last-Modified
1051   value.
1052
10537.  IANA Considerations
1054
10557.1.  Status Code Registration
1056
1057   The HTTP Status Code Registry located at
1058   <http://www.iana.org/assignments/http-status-codes> shall be updated
1059   with the registrations below:
1060
1061
1062
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1067
1068   +-------+---------------------+-------------+
1069   | Value | Description         | Reference   |
1070   +-------+---------------------+-------------+
1071   | 304   | Not Modified        | Section 3.1 |
1072   | 412   | Precondition Failed | Section 3.2 |
1073   +-------+---------------------+-------------+
1074
10757.2.  Header Field Registration
1076
1077   The Message Header Field Registry located at <http://www.iana.org/
1078   assignments/message-headers/message-header-index.html> shall be
1079   updated with the permanent registrations below (see [RFC3864]):
1080
1081   +---------------------+----------+----------+-------------+
1082   | Header Field Name   | Protocol | Status   | Reference   |
1083   +---------------------+----------+----------+-------------+
1084   | ETag                | http     | standard | Section 6.1 |
1085   | If-Match            | http     | standard | Section 6.2 |
1086   | If-Modified-Since   | http     | standard | Section 6.3 |
1087   | If-None-Match       | http     | standard | Section 6.4 |
1088   | If-Unmodified-Since | http     | standard | Section 6.5 |
1089   | Last-Modified       | http     | standard | Section 6.6 |
1090   +---------------------+----------+----------+-------------+
1091
1092   The change controller is: "IETF (iesg@ietf.org) - Internet
1093   Engineering Task Force".
1094
10958.  Security Considerations
1096
1097   No additional security considerations have been identified beyond
1098   those applicable to HTTP in general [Part1].
1099
11009.  Acknowledgments
1101
110210.  References
1103
110410.1.  Normative References
1105
1106   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1107              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1108              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
1109              and Message Parsing", draft-ietf-httpbis-p1-messaging-13
1110              (work in progress), March 2011.
1111
1112   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1113              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1114              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
1115              and Content Negotiation", draft-ietf-httpbis-p3-payload-13
1116
1117
1118
1119Fielding, et al.       Expires September 15, 2011              [Page 20]
1120
1121Internet-Draft              HTTP/1.1, Part 4                  March 2011
1122
1123
1124              (work in progress), March 2011.
1125
1126   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1127              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1128              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1129              Partial Responses", draft-ietf-httpbis-p5-range-13 (work
1130              in progress), March 2011.
1131
1132   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1133              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1134              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
1135              6: Caching", draft-ietf-httpbis-p6-cache-13 (work in
1136              progress), March 2011.
1137
1138   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1139              Requirement Levels", BCP 14, RFC 2119, March 1997.
1140
1141   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1142              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1143
114410.2.  Informative References
1145
1146   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1147              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1148              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1149
1150   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1151              Procedures for Message Header Fields", BCP 90, RFC 3864,
1152              September 2004.
1153
1154Appendix A.  Changes from RFC 2616
1155
1156   Allow weak entity-tags in all requests except range requests
1157   (Sections 4 and 6.4).
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175Fielding, et al.       Expires September 15, 2011              [Page 21]
1176
1177Internet-Draft              HTTP/1.1, Part 4                  March 2011
1178
1179
1180Appendix B.  Collected ABNF
1181
1182   ETag = "ETag:" OWS ETag-v
1183   ETag-v = entity-tag
1184
1185   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1186
1187   If-Match = "If-Match:" OWS If-Match-v
1188   If-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1189    entity-tag ] ) )
1190   If-Modified-Since = "If-Modified-Since:" OWS If-Modified-Since-v
1191   If-Modified-Since-v = HTTP-date
1192   If-None-Match = "If-None-Match:" OWS If-None-Match-v
1193   If-None-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1194    entity-tag ] ) )
1195   If-Unmodified-Since = "If-Unmodified-Since:" OWS
1196    If-Unmodified-Since-v
1197   If-Unmodified-Since-v = HTTP-date
1198
1199   Last-Modified = "Last-Modified:" OWS Last-Modified-v
1200   Last-Modified-v = HTTP-date
1201
1202   OWS = <OWS, defined in [Part1], Section 1.2.2>
1203
1204   entity-tag = [ weak ] opaque-tag
1205
1206   opaque-tag = quoted-string
1207
1208   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1209
1210   weak = %x57.2F ; W/
1211
1212   ABNF diagnostics:
1213
1214   ; ETag defined but not used
1215   ; If-Match defined but not used
1216   ; If-Modified-Since defined but not used
1217   ; If-None-Match defined but not used
1218   ; If-Unmodified-Since defined but not used
1219   ; Last-Modified defined but not used
1220
1221Appendix C.  Change Log (to be removed by RFC Editor before publication)
1222
1223C.1.  Since RFC 2616
1224
1225   Extracted relevant partitions from [RFC2616].
1226
1227
1228
1229
1230
1231Fielding, et al.       Expires September 15, 2011              [Page 22]
1232
1233Internet-Draft              HTTP/1.1, Part 4                  March 2011
1234
1235
1236C.2.  Since draft-ietf-httpbis-p4-conditional-00
1237
1238   Closed issues:
1239
1240   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1241      Informative references"
1242
1243   Other changes:
1244
1245   o  Move definitions of 304 and 412 condition codes from Part2.
1246
1247C.3.  Since draft-ietf-httpbis-p4-conditional-01
1248
1249   Ongoing work on ABNF conversion
1250   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1251
1252   o  Add explicit references to BNF syntax and rules imported from
1253      other parts of the specification.
1254
1255C.4.  Since draft-ietf-httpbis-p4-conditional-02
1256
1257   Closed issues:
1258
1259   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1260      non-GET requests"
1261
1262   Ongoing work on IANA Message Header Field Registration
1263   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1264
1265   o  Reference RFC 3984, and update header field registrations for
1266      header fields defined in this document.
1267
1268C.5.  Since draft-ietf-httpbis-p4-conditional-03
1269
1270   Closed issues:
1271
1272   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for
1273      ETag matching"
1274
1275   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity
1276      value' undefined"
1277
1278   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068
1279      Date header reference"
1280
1281
1282
1283
1284
1285
1286
1287Fielding, et al.       Expires September 15, 2011              [Page 23]
1288
1289Internet-Draft              HTTP/1.1, Part 4                  March 2011
1290
1291
1292C.6.  Since draft-ietf-httpbis-p4-conditional-04
1293
1294   Ongoing work on ABNF conversion
1295   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1296
1297   o  Use "/" instead of "|" for alternatives.
1298
1299   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1300      whitespace ("OWS") and required whitespace ("RWS").
1301
1302   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1303      field value format definitions.
1304
1305C.7.  Since draft-ietf-httpbis-p4-conditional-05
1306
1307   Final work on ABNF conversion
1308   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1309
1310   o  Add appendix containing collected and expanded ABNF, reorganize
1311      ABNF introduction.
1312
1313C.8.  Since draft-ietf-httpbis-p4-conditional-06
1314
1315   Closed issues:
1316
1317   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-
1318      sensitivity of etag weakness indicator"
1319
1320C.9.  Since draft-ietf-httpbis-p4-conditional-07
1321
1322   Closed issues:
1323
1324   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1325      non-GET requests" (If-Match still was defined to require strong
1326      matching)
1327
1328   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
1329      registrations for optional status codes"
1330
1331C.10.  Since draft-ietf-httpbis-p4-conditional-08
1332
1333   No significant changes.
1334
1335C.11.  Since draft-ietf-httpbis-p4-conditional-09
1336
1337   No significant changes.
1338
1339
1340
1341
1342
1343Fielding, et al.       Expires September 15, 2011              [Page 24]
1344
1345Internet-Draft              HTTP/1.1, Part 4                  March 2011
1346
1347
1348C.12.  Since draft-ietf-httpbis-p4-conditional-10
1349
1350   Closed issues:
1351
1352   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
1353      'Requested Variant'"
1354
1355   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
1356      entity / representation / variant terminology"
1357
1358   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
1359      removing the 'changes from 2068' sections"
1360
1361C.13.  Since draft-ietf-httpbis-p4-conditional-11
1362
1363   None.
1364
1365C.14.  Since draft-ietf-httpbis-p4-conditional-12
1366
1367   Closed issues:
1368
1369   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
1370      Classification"
1371
1372Index
1373
1374   3
1375      304 Not Modified (status code)  8
1376
1377   4
1378      412 Precondition Failed (status code)  8
1379
1380   E
1381      ETag header field  13
1382
1383   G
1384      Grammar
1385         entity-tag  6
1386         ETag  13
1387         ETag-v  13
1388         If-Match  14
1389         If-Match-v  14
1390         If-Modified-Since  15
1391         If-Modified-Since-v  15
1392         If-None-Match  17
1393         If-None-Match-v  17
1394         If-Unmodified-Since  18
1395         If-Unmodified-Since-v  18
1396
1397
1398
1399Fielding, et al.       Expires September 15, 2011              [Page 25]
1400
1401Internet-Draft              HTTP/1.1, Part 4                  March 2011
1402
1403
1404         Last-Modified  19
1405         Last-Modified-v  19
1406         opaque-tag  6
1407         weak  6
1408
1409   H
1410      Header Fields
1411         ETag  13
1412         If-Match  14
1413         If-Modified-Since  15
1414         If-None-Match  17
1415         If-Unmodified-Since  18
1416         Last-Modified  19
1417
1418   I
1419      If-Match header field  14
1420      If-Modified-Since header field  15
1421      If-None-Match header field  17
1422      If-Unmodified-Since header field  18
1423
1424   L
1425      Last-Modified header field  19
1426
1427   S
1428      Status Codes
1429         304 Not Modified  8
1430         412 Precondition Failed  8
1431
1432Authors' Addresses
1433
1434   Roy T. Fielding (editor)
1435   Adobe Systems Incorporated
1436   345 Park Ave
1437   San Jose, CA  95110
1438   USA
1439
1440   EMail: fielding@gbiv.com
1441   URI:   http://roy.gbiv.com/
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455Fielding, et al.       Expires September 15, 2011              [Page 26]
1456
1457Internet-Draft              HTTP/1.1, Part 4                  March 2011
1458
1459
1460   Jim Gettys
1461   Alcatel-Lucent Bell Labs
1462   21 Oak Knoll Road
1463   Carlisle, MA  01741
1464   USA
1465
1466   EMail: jg@freedesktop.org
1467   URI:   http://gettys.wordpress.com/
1468
1469
1470   Jeffrey C. Mogul
1471   Hewlett-Packard Company
1472   HP Labs, Large Scale Systems Group
1473   1501 Page Mill Road, MS 1177
1474   Palo Alto, CA  94304
1475   USA
1476
1477   EMail: JeffMogul@acm.org
1478
1479
1480   Henrik Frystyk Nielsen
1481   Microsoft Corporation
1482   1 Microsoft Way
1483   Redmond, WA  98052
1484   USA
1485
1486   EMail: henrikn@microsoft.com
1487
1488
1489   Larry Masinter
1490   Adobe Systems Incorporated
1491   345 Park Ave
1492   San Jose, CA  95110
1493   USA
1494
1495   EMail: LMM@acm.org
1496   URI:   http://larry.masinter.net/
1497
1498
1499   Paul J. Leach
1500   Microsoft Corporation
1501   1 Microsoft Way
1502   Redmond, WA  98052
1503
1504   EMail: paulle@microsoft.com
1505
1506
1507
1508
1509
1510
1511Fielding, et al.       Expires September 15, 2011              [Page 27]
1512
1513Internet-Draft              HTTP/1.1, Part 4                  March 2011
1514
1515
1516   Tim Berners-Lee
1517   World Wide Web Consortium
1518   MIT Computer Science and Artificial Intelligence Laboratory
1519   The Stata Center, Building 32
1520   32 Vassar Street
1521   Cambridge, MA  02139
1522   USA
1523
1524   EMail: timbl@w3.org
1525   URI:   http://www.w3.org/People/Berners-Lee/
1526
1527
1528   Yves Lafon (editor)
1529   World Wide Web Consortium
1530   W3C / ERCIM
1531   2004, rte des Lucioles
1532   Sophia-Antipolis, AM  06902
1533   France
1534
1535   EMail: ylafon@w3.org
1536   URI:   http://www.raubacapeu.net/people/yves/
1537
1538
1539   Julian F. Reschke (editor)
1540   greenbytes GmbH
1541   Hafenweg 16
1542   Muenster, NW  48155
1543   Germany
1544
1545   Phone: +49 251 2807760
1546   Fax:   +49 251 2807761
1547   EMail: julian.reschke@greenbytes.de
1548   URI:   http://greenbytes.de/tech/webdav/
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567Fielding, et al.       Expires September 15, 2011              [Page 28]
1568
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