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4HTTPbis Working Group                                   R. Fielding, Ed.
5Internet-Draft                                              Day Software
6Obsoletes: 2616 (if approved)                                  J. Gettys
7Intended status: Standards Track                          Alcatel-Lucent
8Expires: April 28, 2011                                         J. Mogul
9                                                                      HP
10                                                              H. Frystyk
11                                                               Microsoft
12                                                             L. Masinter
13                                                           Adobe Systems
14                                                                P. Leach
15                                                               Microsoft
16                                                          T. Berners-Lee
17                                                                 W3C/MIT
18                                                           Y. Lafon, Ed.
19                                                                     W3C
20                                                         J. Reschke, Ed.
21                                                              greenbytes
22                                                        October 25, 2010
23
24
25                 HTTP/1.1, part 4: Conditional Requests
26                  draft-ietf-httpbis-p4-conditional-12
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.13.
48
49Status of This Memo
50
51   This Internet-Draft is submitted in full conformance with the
52
53
54
55Fielding, et al.         Expires April 28, 2011                 [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 4                October 2010
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 April 28, 2011.
73
74Copyright Notice
75
76   Copyright (c) 2010 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
105
106
107
108
109
110
111Fielding, et al.         Expires April 28, 2011                 [Page 2]
112
113Internet-Draft              HTTP/1.1, Part 4                October 2010
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 . . . . . . . . . . . . . . . . . 20
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 April 28, 2011                 [Page 3]
168
169Internet-Draft              HTTP/1.1, Part 4                October 2010
170
171
172   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
173
174
175
176
177
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179
180
181
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183
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221
222
223Fielding, et al.         Expires April 28, 2011                 [Page 4]
224
225Internet-Draft              HTTP/1.1, Part 4                October 2010
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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281Internet-Draft              HTTP/1.1, Part 4                October 2010
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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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393Internet-Draft              HTTP/1.1, Part 4                October 2010
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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 request-header fields
442   evaluated to false when it was tested on the server.  This response
443   code allows the client to place preconditions on the current resource
444
445
446
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451
452   metadata (header field data) and thus prevent the requested method
453   from being 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
558
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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 legal, a strong entity-tag MUST change whenever the
640   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
670
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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 will 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" response-header field provides the current value of the
717   entity-tag (see Section 2) for one representation of the target
718   resource.  An entity-tag is intended for use as a resource-local
719   identifier for differentiating between representations of the same
720   resource that vary over time or via content negotiation (see
721   Section 4).
722
723     ETag   = "ETag" ":" OWS ETag-v
724
725
726
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731
732     ETag-v = entity-tag
733
734   Examples:
735
736     ETag: "xyzzy"
737     ETag: W/"xyzzy"
738     ETag: ""
739
740   An entity-tag provides an "opaque" cache validator that allows for
741   more reliable validation than modification dates in situations where
742   it is inconvenient to store modification dates, where the one-second
743   resolution of HTTP date values is not sufficient, or where the origin
744   server wishes to avoid certain paradoxes that might arise from the
745   use of modification dates.
746
747   The principle behind entity-tags is that only the service author
748   knows the semantics of a resource well enough to select an
749   appropriate cache validation mechanism, and the specification of any
750   validator comparison function more complex than byte-equality would
751   open up a can of worms.  Thus, comparisons of any other header fields
752   (except Last-Modified, for compatibility with HTTP/1.0) are never
753   used for purposes of validating a cache entry.
754
7556.2.  If-Match
756
757   The "If-Match" request-header field is used to make a request method
758   conditional.  A client that has one or more representations
759   previously obtained from the resource can verify that one of those
760   representations is current by including a list of their associated
761   entity-tags in the If-Match header field.
762
763   This allows efficient updates of cached information with a minimum
764   amount of transaction overhead.  It is also used when updating
765   resources, to prevent inadvertent modification of the wrong version
766   of a resource.  As a special case, the value "*" matches any current
767   representation of the resource.
768
769     If-Match   = "If-Match" ":" OWS If-Match-v
770     If-Match-v = "*" / 1#entity-tag
771
772   If any of the entity-tags match the entity-tag of the representation
773   that would have been returned in the response to a similar GET
774   request (without the If-Match header field) on that resource, or if
775   "*" is given and any current representation exists for that resource,
776   then the server MAY perform the requested method as if the If-Match
777   header field did not exist.
778
779   If none of the entity-tags match, or if "*" is given and no current
780
781
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787
788   representation exists, the server MUST NOT perform the requested
789   method, and MUST return a 412 (Precondition Failed) response.  This
790   behavior is most useful when the client wants to prevent an updating
791   method, such as PUT, from modifying a resource that has changed since
792   the client last retrieved it.
793
794   If the request would, without the If-Match header field, result in
795   anything other than a 2xx or 412 status code, then the If-Match
796   header field MUST be ignored.
797
798   The meaning of "If-Match: *" is that the method SHOULD be performed
799   if the representation selected by the origin server (or by a cache,
800   possibly using the Vary mechanism, see Section 3.5 of [Part6])
801   exists, and MUST NOT be performed if the representation does not
802   exist.
803
804   A request intended to update a resource (e.g., a PUT) MAY include an
805   If-Match header field to signal that the request method MUST NOT be
806   applied if the representation corresponding to the If-Match value (a
807   single entity-tag) is no longer a representation of that resource.
808   This allows the user to indicate that they do not wish the request to
809   be successful if the resource has been changed without their
810   knowledge.  Examples:
811
812     If-Match: "xyzzy"
813     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
814     If-Match: *
815
816   The result of a request having both an If-Match header field and
817   either an If-None-Match or an If-Modified-Since header fields is
818   undefined by this specification.
819
8206.3.  If-Modified-Since
821
822   The "If-Modified-Since" request-header field is used to make a
823   request method conditional by date: if the representation that would
824   have been transferred in a 200 response to a GET request has not been
825   modified since the time specified in this field, then do not perform
826   the method; instead, respond as detailed below.
827
828     If-Modified-Since   = "If-Modified-Since" ":" OWS
829                           If-Modified-Since-v
830     If-Modified-Since-v = HTTP-date
831
832   An example of the field is:
833
834     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
835
836
837
838
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843
844   A GET method with an If-Modified-Since header field and no Range
845   header field requests that the representation be transferred only if
846   it has been modified since the date given by the If-Modified-Since
847   header field.  The algorithm for determining this includes the
848   following cases:
849
850   1.  If the request would normally result in anything other than a 200
851       (OK) status code, or if the passed If-Modified-Since date is
852       invalid, the response is exactly the same as for a normal GET.  A
853       date which is later than the server's current time is invalid.
854
855   2.  If the representation has been modified since the If-Modified-
856       Since date, the response is exactly the same as for a normal GET.
857
858   3.  If the representation has not been modified since a valid If-
859       Modified-Since date, the server SHOULD return a 304 (Not
860       Modified) response.
861
862   The purpose of this feature is to allow efficient updates of cached
863   information with a minimum amount of transaction overhead.
864
865      Note: The Range request-header field modifies the meaning of If-
866      Modified-Since; see Section 5.4 of [Part5] for full details.
867
868      Note: If-Modified-Since times are interpreted by the server, whose
869      clock might not be synchronized with the client.
870
871      Note: When handling an If-Modified-Since header field, some
872      servers will use an exact date comparison function, rather than a
873      less-than function, for deciding whether to send a 304 (Not
874      Modified) response.  To get best results when sending an If-
875      Modified-Since header field for cache validation, clients are
876      advised to use the exact date string received in a previous Last-
877      Modified header field whenever possible.
878
879      Note: If a client uses an arbitrary date in the If-Modified-Since
880      header field instead of a date taken from the Last-Modified header
881      field for the same request, the client needs to be aware that this
882      date is interpreted in the server's understanding of time.
883      Unsynchronized clocks and rounding problems, due to the different
884      encodings of time between the client and server, are concerns.
885      This includes the possibility of race conditions if the document
886      has changed between the time it was first requested and the If-
887      Modified-Since date of a subsequent request, and the possibility
888      of clock-skew-related problems if the If-Modified-Since date is
889      derived from the client's clock without correction to the server's
890      clock.  Corrections for different time bases between client and
891      server are at best approximate due to network latency.
892
893
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899
900   The result of a request having both an If-Modified-Since header field
901   and either an If-Match or an If-Unmodified-Since header fields is
902   undefined by this specification.
903
9046.4.  If-None-Match
905
906   The "If-None-Match" request-header field is used to make a request
907   method conditional.  A client that has one or more representations
908   previously obtained from the resource can verify that none of those
909   representations is current by including a list of their associated
910   entity-tags in the If-None-Match header field.
911
912   This allows efficient updates of cached information with a minimum
913   amount of transaction overhead.  It is also used to prevent a method
914   (e.g., PUT) from inadvertently modifying an existing resource when
915   the client believes that the resource does not exist.
916
917   As a special case, the value "*" matches any current representation
918   of the resource.
919
920     If-None-Match   = "If-None-Match" ":" OWS If-None-Match-v
921     If-None-Match-v = "*" / 1#entity-tag
922
923   If any of the entity-tags match the entity-tag of the representation
924   that would have been returned in the response to a similar GET
925   request (without the If-None-Match header field) on that resource, or
926   if "*" is given and any current representation exists for that
927   resource, then the server MUST NOT perform the requested method,
928   unless required to do so because the resource's modification date
929   fails to match that supplied in an If-Modified-Since header field in
930   the request.  Instead, if the request method was GET or HEAD, the
931   server SHOULD respond with a 304 (Not Modified) response, including
932   the cache-related header fields (particularly ETag) of one of the
933   representations that matched.  For all other request methods, the
934   server MUST respond with a 412 (Precondition Failed) status code.
935
936   If none of the entity-tags match, then the server MAY perform the
937   requested method as if the If-None-Match header field did not exist,
938   but MUST also ignore any If-Modified-Since header field(s) in the
939   request.  That is, if no entity-tags match, then the server MUST NOT
940   return a 304 (Not Modified) response.
941
942   If the request would, without the If-None-Match header field, result
943   in anything other than a 2xx or 304 status code, then the If-None-
944   Match header field MUST be ignored.  (See Section 5 for a discussion
945   of server behavior when both If-Modified-Since and If-None-Match
946   appear in the same request.)
947
948
949
950
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955
956   The meaning of "If-None-Match: *" is that the method MUST NOT be
957   performed if the representation selected by the origin server (or by
958   a cache, possibly using the Vary mechanism, see Section 3.5 of
959   [Part6]) exists, and SHOULD be performed if the representation does
960   not exist.  This feature is intended to be useful in preventing races
961   between PUT operations.
962
963   Examples:
964
965     If-None-Match: "xyzzy"
966     If-None-Match: W/"xyzzy"
967     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
968     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
969     If-None-Match: *
970
971   The result of a request having both an If-None-Match header field and
972   either an If-Match or an If-Unmodified-Since header fields is
973   undefined by this specification.
974
9756.5.  If-Unmodified-Since
976
977   The "If-Unmodified-Since" request-header field is used to make a
978   request method conditional.  If the representation that would have
979   been transferred in a 200 response to a GET request on the same
980   resource has not been modified since the time specified in this
981   field, the server SHOULD perform the requested operation as if the
982   If-Unmodified-Since header field were not present.
983
984   If the representation has been modified since the specified time, the
985   server MUST NOT perform the requested operation, and MUST return a
986   412 (Precondition Failed).
987
988     If-Unmodified-Since   = "If-Unmodified-Since" ":" OWS
989                             If-Unmodified-Since-v
990     If-Unmodified-Since-v = HTTP-date
991
992   An example of the field is:
993
994     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
995
996   If the request normally (i.e., without the If-Unmodified-Since header
997   field) would result in anything other than a 2xx or 412 status code,
998   the If-Unmodified-Since header field SHOULD be ignored.
999
1000   If the specified date is invalid, the header field is ignored.
1001
1002   The result of a request having both an If-Unmodified-Since header
1003   field and either an If-None-Match or an If-Modified-Since header
1004
1005
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1011
1012   fields is undefined by this specification.
1013
10146.6.  Last-Modified
1015
1016   The "Last-Modified" header field indicates the date and time at which
1017   the origin server believes the representation was last modified.
1018
1019     Last-Modified   = "Last-Modified" ":" OWS Last-Modified-v
1020     Last-Modified-v = HTTP-date
1021
1022   An example of its use is
1023
1024     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1025
1026   The exact meaning of this header field depends on the implementation
1027   of the origin server and the nature of the original resource.  For
1028   files, it might be just the file system last-modified time.  For
1029   representations with dynamically included parts, it might be the most
1030   recent of the set of last-modify times for its component parts.  For
1031   database gateways, it might be the last-update time stamp of the
1032   record.  For virtual objects, it might be the last time the internal
1033   state changed.
1034
1035   An origin server MUST NOT send a Last-Modified date which is later
1036   than the server's time of message origination.  In such cases, where
1037   the resource's last modification would indicate some time in the
1038   future, the server MUST replace that date with the message
1039   origination date.
1040
1041   An origin server SHOULD obtain the Last-Modified value of the
1042   representation as close as possible to the time that it generates the
1043   Date value of its response.  This allows a recipient to make an
1044   accurate assessment of the representation's modification time,
1045   especially if the representation changes near the time that the
1046   response is generated.
1047
1048   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
1049
1050   The Last-Modified header field value is often used as a cache
1051   validator.  In simple terms, a cache entry is considered to be valid
1052   if the representation has not been modified since the Last-Modified
1053   value.
1054
10557.  IANA Considerations
1056
1057
1058
1059
1060
1061
1062
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1067
10687.1.  Status Code Registration
1069
1070   The HTTP Status Code Registry located at
1071   <http://www.iana.org/assignments/http-status-codes> shall be updated
1072   with the registrations below:
1073
1074   +-------+---------------------+-------------+
1075   | Value | Description         | Reference   |
1076   +-------+---------------------+-------------+
1077   | 304   | Not Modified        | Section 3.1 |
1078   | 412   | Precondition Failed | Section 3.2 |
1079   +-------+---------------------+-------------+
1080
10817.2.  Header Field Registration
1082
1083   The Message Header Field Registry located at <http://www.iana.org/
1084   assignments/message-headers/message-header-index.html> shall be
1085   updated with the permanent registrations below (see [RFC3864]):
1086
1087   +---------------------+----------+----------+-------------+
1088   | Header Field Name   | Protocol | Status   | Reference   |
1089   +---------------------+----------+----------+-------------+
1090   | ETag                | http     | standard | Section 6.1 |
1091   | If-Match            | http     | standard | Section 6.2 |
1092   | If-Modified-Since   | http     | standard | Section 6.3 |
1093   | If-None-Match       | http     | standard | Section 6.4 |
1094   | If-Unmodified-Since | http     | standard | Section 6.5 |
1095   | Last-Modified       | http     | standard | Section 6.6 |
1096   +---------------------+----------+----------+-------------+
1097
1098   The change controller is: "IETF (iesg@ietf.org) - Internet
1099   Engineering Task Force".
1100
11018.  Security Considerations
1102
1103   No additional security considerations have been identified beyond
1104   those applicable to HTTP in general [Part1].
1105
11069.  Acknowledgments
1107
110810.  References
1109
111010.1.  Normative References
1111
1112   [Part1]    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 1: URIs, Connections,
1115              and Message Parsing", draft-ietf-httpbis-p1-messaging-12
1116
1117
1118
1119Fielding, et al.         Expires April 28, 2011                [Page 20]
1120
1121Internet-Draft              HTTP/1.1, Part 4                October 2010
1122
1123
1124              (work in progress), October 2010.
1125
1126   [Part3]    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 3: Message Payload
1129              and Content Negotiation", draft-ietf-httpbis-p3-payload-12
1130              (work in progress), October 2010.
1131
1132   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1133              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1134              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1135              Partial Responses", draft-ietf-httpbis-p5-range-12 (work
1136              in progress), October 2010.
1137
1138   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1139              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1140              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
1141              6: Caching", draft-ietf-httpbis-p6-cache-12 (work in
1142              progress), October 2010.
1143
1144   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1145              Requirement Levels", BCP 14, RFC 2119, March 1997.
1146
1147   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1148              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1149
115010.2.  Informative References
1151
1152   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1153              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1154              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1155
1156   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1157              Procedures for Message Header Fields", BCP 90, RFC 3864,
1158              September 2004.
1159
1160Appendix A.  Changes from RFC 2616
1161
1162   Allow weak entity-tags in all requests except range requests
1163   (Sections 4 and 6.4).
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175Fielding, et al.         Expires April 28, 2011                [Page 21]
1176
1177Internet-Draft              HTTP/1.1, Part 4                October 2010
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 April 28, 2011                [Page 22]
1232
1233Internet-Draft              HTTP/1.1, Part 4                October 2010
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 April 28, 2011                [Page 23]
1288
1289Internet-Draft              HTTP/1.1, Part 4                October 2010
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 April 28, 2011                [Page 24]
1344
1345Internet-Draft              HTTP/1.1, Part 4                October 2010
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
1365Index
1366
1367   3
1368      304 Not Modified (status code)  8
1369
1370   4
1371      412 Precondition Failed (status code)  8
1372
1373   E
1374      ETag header  13
1375
1376   G
1377      Grammar
1378         entity-tag  6
1379         ETag  13
1380         ETag-v  13
1381         If-Match  14
1382         If-Match-v  14
1383         If-Modified-Since  15
1384         If-Modified-Since-v  15
1385         If-None-Match  17
1386         If-None-Match-v  17
1387         If-Unmodified-Since  18
1388         If-Unmodified-Since-v  18
1389         Last-Modified  19
1390         Last-Modified-v  19
1391         opaque-tag  6
1392         weak  6
1393
1394   H
1395      Headers
1396
1397
1398
1399Fielding, et al.         Expires April 28, 2011                [Page 25]
1400
1401Internet-Draft              HTTP/1.1, Part 4                October 2010
1402
1403
1404         ETag  13
1405         If-Match  14
1406         If-Modified-Since  15
1407         If-None-Match  17
1408         If-Unmodified-Since  18
1409         Last-Modified  19
1410
1411   I
1412      If-Match header  14
1413      If-Modified-Since header  15
1414      If-None-Match header  17
1415      If-Unmodified-Since header  18
1416
1417   L
1418      Last-Modified header  19
1419
1420   S
1421      Status Codes
1422         304 Not Modified  8
1423         412 Precondition Failed  8
1424
1425Authors' Addresses
1426
1427   Roy T. Fielding (editor)
1428   Day Software
1429   23 Corporate Plaza DR, Suite 280
1430   Newport Beach, CA  92660
1431   USA
1432
1433   Phone: +1-949-706-5300
1434   Fax:   +1-949-706-5305
1435   EMail: fielding@gbiv.com
1436   URI:   http://roy.gbiv.com/
1437
1438
1439   Jim Gettys
1440   Alcatel-Lucent Bell Labs
1441   21 Oak Knoll Road
1442   Carlisle, MA  01741
1443   USA
1444
1445   EMail: jg@freedesktop.org
1446   URI:   http://gettys.wordpress.com/
1447
1448
1449
1450
1451
1452
1453
1454
1455Fielding, et al.         Expires April 28, 2011                [Page 26]
1456
1457Internet-Draft              HTTP/1.1, Part 4                October 2010
1458
1459
1460   Jeffrey C. Mogul
1461   Hewlett-Packard Company
1462   HP Labs, Large Scale Systems Group
1463   1501 Page Mill Road, MS 1177
1464   Palo Alto, CA  94304
1465   USA
1466
1467   EMail: JeffMogul@acm.org
1468
1469
1470   Henrik Frystyk Nielsen
1471   Microsoft Corporation
1472   1 Microsoft Way
1473   Redmond, WA  98052
1474   USA
1475
1476   EMail: henrikn@microsoft.com
1477
1478
1479   Larry Masinter
1480   Adobe Systems, Incorporated
1481   345 Park Ave
1482   San Jose, CA  95110
1483   USA
1484
1485   EMail: LMM@acm.org
1486   URI:   http://larry.masinter.net/
1487
1488
1489   Paul J. Leach
1490   Microsoft Corporation
1491   1 Microsoft Way
1492   Redmond, WA  98052
1493
1494   EMail: paulle@microsoft.com
1495
1496
1497   Tim Berners-Lee
1498   World Wide Web Consortium
1499   MIT Computer Science and Artificial Intelligence Laboratory
1500   The Stata Center, Building 32
1501   32 Vassar Street
1502   Cambridge, MA  02139
1503   USA
1504
1505   EMail: timbl@w3.org
1506   URI:   http://www.w3.org/People/Berners-Lee/
1507
1508
1509
1510
1511Fielding, et al.         Expires April 28, 2011                [Page 27]
1512
1513Internet-Draft              HTTP/1.1, Part 4                October 2010
1514
1515
1516   Yves Lafon (editor)
1517   World Wide Web Consortium
1518   W3C / ERCIM
1519   2004, rte des Lucioles
1520   Sophia-Antipolis, AM  06902
1521   France
1522
1523   EMail: ylafon@w3.org
1524   URI:   http://www.raubacapeu.net/people/yves/
1525
1526
1527   Julian F. Reschke (editor)
1528   greenbytes GmbH
1529   Hafenweg 16
1530   Muenster, NW  48155
1531   Germany
1532
1533   Phone: +49 251 2807760
1534   Fax:   +49 251 2807761
1535   EMail: julian.reschke@greenbytes.de
1536   URI:   http://greenbytes.de/tech/webdav/
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567Fielding, et al.         Expires April 28, 2011                [Page 28]
1568
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