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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: February 25, 2012                                      J. Mogul
9                                                                      HP
10                                                              H. Frystyk
11                                                               Microsoft
12                                                             L. Masinter
13                                                                   Adobe
14                                                                P. Leach
15                                                               Microsoft
16                                                          T. Berners-Lee
17                                                                 W3C/MIT
18                                                           Y. Lafon, Ed.
19                                                                     W3C
20                                                         J. Reschke, Ed.
21                                                              greenbytes
22                                                         August 24, 2011
23
24
25                 HTTP/1.1, part 4: Conditional Requests
26                  draft-ietf-httpbis-p4-conditional-16
27
28Abstract
29
30   The Hypertext Transfer Protocol (HTTP) is an application-level
31   protocol for distributed, collaborative, hypertext 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.
36
37   Part 4 defines request header fields for indicating conditional
38   requests and the rules for constructing responses to those requests.
39
40Editorial Note (To be removed by RFC Editor)
41
42   Discussion of this draft should take place on the HTTPBIS working
43   group mailing list (ietf-http-wg@w3.org), which is archived at
44   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
45
46   The current issues list is at
47   <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
48   documents (including fancy diffs) can be found at
49   <http://tools.ietf.org/wg/httpbis/>.
50
51   The changes in this draft are summarized in Appendix C.17.
52
53
54
55Fielding, et al.        Expires February 25, 2012               [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 4                 August 2011
58
59
60Status of This Memo
61
62   This Internet-Draft is submitted in full conformance with the
63   provisions of BCP 78 and BCP 79.
64
65   Internet-Drafts are working documents of the Internet Engineering
66   Task Force (IETF).  Note that other groups may also distribute
67   working documents as Internet-Drafts.  The list of current Internet-
68   Drafts is at http://datatracker.ietf.org/drafts/current/.
69
70   Internet-Drafts are draft documents valid for a maximum of six months
71   and may be updated, replaced, or obsoleted by other documents at any
72   time.  It is inappropriate to use Internet-Drafts as reference
73   material or to cite them other than as "work in progress."
74
75   This Internet-Draft will expire on February 25, 2012.
76
77Copyright Notice
78
79   Copyright (c) 2011 IETF Trust and the persons identified as the
80   document authors.  All rights reserved.
81
82   This document is subject to BCP 78 and the IETF Trust's Legal
83   Provisions Relating to IETF Documents
84   (http://trustee.ietf.org/license-info) in effect on the date of
85   publication of this document.  Please review these documents
86   carefully, as they describe your rights and restrictions with respect
87   to this document.  Code Components extracted from this document must
88   include Simplified BSD License text as described in Section 4.e of
89   the Trust Legal Provisions and are provided without warranty as
90   described in the Simplified BSD License.
91
92   This document may contain material from IETF Documents or IETF
93   Contributions published or made publicly available before November
94   10, 2008.  The person(s) controlling the copyright in some of this
95   material may not have granted the IETF Trust the right to allow
96   modifications of such material outside the IETF Standards Process.
97   Without obtaining an adequate license from the person(s) controlling
98   the copyright in such materials, this document may not be modified
99   outside the IETF Standards Process, and derivative works of it may
100   not be created outside the IETF Standards Process, except to format
101   it for publication as an RFC or to translate it into languages other
102   than English.
103
104Table of Contents
105
106   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
107     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
108
109
110
111Fielding, et al.        Expires February 25, 2012               [Page 2]
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113Internet-Draft              HTTP/1.1, Part 4                 August 2011
114
115
116     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  6
117   2.  Validators . . . . . . . . . . . . . . . . . . . . . . . . . .  6
118     2.1.  Weak versus Strong . . . . . . . . . . . . . . . . . . . .  6
119     2.2.  Last-Modified  . . . . . . . . . . . . . . . . . . . . . .  8
120       2.2.1.  Generation . . . . . . . . . . . . . . . . . . . . . .  8
121       2.2.2.  Comparison . . . . . . . . . . . . . . . . . . . . . .  9
122     2.3.  ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
123       2.3.1.  Generation . . . . . . . . . . . . . . . . . . . . . . 11
124       2.3.2.  Comparison . . . . . . . . . . . . . . . . . . . . . . 11
125       2.3.3.  Example: Entity-tags varying on Content-Negotiated
126               Resources  . . . . . . . . . . . . . . . . . . . . . . 12
127     2.4.  Rules for When to Use Entity-tags and Last-Modified
128           Dates  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
129   3.  Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
130     3.1.  If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
131     3.2.  If-None-Match  . . . . . . . . . . . . . . . . . . . . . . 15
132     3.3.  If-Modified-Since  . . . . . . . . . . . . . . . . . . . . 17
133     3.4.  If-Unmodified-Since  . . . . . . . . . . . . . . . . . . . 18
134     3.5.  If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 19
135   4.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 19
136     4.1.  304 Not Modified . . . . . . . . . . . . . . . . . . . . . 19
137     4.2.  412 Precondition Failed  . . . . . . . . . . . . . . . . . 20
138   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
139     5.1.  Status Code Registration . . . . . . . . . . . . . . . . . 20
140     5.2.  Header Field Registration  . . . . . . . . . . . . . . . . 20
141   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
142   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
143   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
144     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
145     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
146   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
147   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 22
148   Appendix C.  Change Log (to be removed by RFC Editor before
149                publication)  . . . . . . . . . . . . . . . . . . . . 23
150     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 23
151     C.2.  Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23
152     C.3.  Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23
153     C.4.  Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23
154     C.5.  Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23
155     C.6.  Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24
156     C.7.  Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24
157     C.8.  Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24
158     C.9.  Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24
159     C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24
160     C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 25
161     C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25
162     C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25
163     C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25
164
165
166
167Fielding, et al.        Expires February 25, 2012               [Page 3]
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169Internet-Draft              HTTP/1.1, Part 4                 August 2011
170
171
172     C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25
173     C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 26
174     C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 26
175   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
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201
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215
216
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219
220
221
222
223Fielding, et al.        Expires February 25, 2012               [Page 4]
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226
227
2281.  Introduction
229
230   This document defines the HTTP/1.1 conditional request mechanisms,
231   including both metadata for indicating/observing changes in resource
232   representations and request header fields that specify preconditions
233   on that metadata be checked before performing the request method.
234   Conditional GET requests are the most efficient mechanism for HTTP
235   cache updates [Part6].  Conditionals can also be applied to state-
236   changing methods, such as PUT and DELETE, to prevent the "lost
237   update" problem: one client accidentally overwriting the work of
238   another client that has been acting in parallel.
239
240   Conditional request preconditions are based on the state of the
241   target resource as a whole (its current value set) or the state as
242   observed in a previously obtained representation (one value in that
243   set).  A resource might have multiple current representations, each
244   with its own observable state.  The conditional request mechanisms
245   assume that the mapping of requests to corresponding representations
246   will be consistent over time if the server intends to take advantage
247   of conditionals.  Regardless, if the mapping is inconsistent and the
248   server is unable to select the appropriate representation, then no
249   harm will result when the precondition evaluates to false.
250
251   We use the term "selected representation" to refer to the current
252   representation of the target resource that would have been selected
253   in a successful response if the same request had used the method GET
254   and had excluded all of the conditional request header fields.  The
255   conditional request preconditions are evaluated by comparing the
256   values provided in the request header fields to the current metadata
257   for the selected representation.
258
2591.1.  Requirements
260
261   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
262   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
263   document are to be interpreted as described in [RFC2119].
264
265   An implementation is not compliant if it fails to satisfy one or more
266   of the "MUST" or "REQUIRED" level requirements for the protocols it
267   implements.  An implementation that satisfies all the "MUST" or
268   "REQUIRED" level and all the "SHOULD" level requirements for its
269   protocols is said to be "unconditionally compliant"; one that
270   satisfies all the "MUST" level requirements but not all the "SHOULD"
271   level requirements for its protocols is said to be "conditionally
272   compliant".
273
274
275
276
277
278
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283
2841.2.  Syntax Notation
285
286   This specification uses the ABNF syntax defined in Section 1.2 of
287   [Part1] (which extends the syntax defined in [RFC5234] with a list
288   rule).  Appendix B shows the collected ABNF, with the list rule
289   expanded.
290
291   The following core rules are included by reference, as defined in
292   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
293   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
294   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
295   sequence of data), SP (space), VCHAR (any visible USASCII character),
296   and WSP (whitespace).
297
298   The ABNF rules below are defined in [Part1]:
299
300     OWS           = <OWS, defined in [Part1], Section 1.2.2>
301     quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
302     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
303
3042.  Validators
305
306   This specification defines two forms of metadata that are commonly
307   used to observe resource state and test for preconditions:
308   modification dates and opaque entity tags.  Additional metadata that
309   reflects resource state has been defined by various extensions of
310   HTTP, such as WebDAV [RFC4918], that are beyond the scope of this
311   specification.  A resource metadata value is referred to as a
312   "validator" when it is used within a precondition.
313
3142.1.  Weak versus Strong
315
316   Validators come in two flavors: strong or weak.  Weak validators are
317   easy to generate but are far less useful for comparisons.  Strong
318   validators are ideal for comparisons but can be very difficult (and
319   occasionally impossible) to generate efficiently.  Rather than impose
320   that all forms of resource adhere to the same strength of validator,
321   HTTP exposes the type of validator in use and imposes restrictions on
322   when weak validators can be used as preconditions.
323
324   A "strong validator" is a representation metadata value that MUST be
325   changed to a new, previously unused or guaranteed unique, value
326   whenever a change occurs to the representation data such that a
327   change would be observable in the payload body of a 200 response to
328   GET.  A strong validator MAY be changed for other reasons, such as
329   when a semantically significant part of the representation metadata
330   is changed (e.g., Content-Type), but it is in the best interests of
331   the origin server to only change the value when it is necessary to
332
333
334
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339
340   invalidate the stored responses held by remote caches and authoring
341   tools.  A strong validator MUST be unique across all representations
342   of a given resource, such that no two representations of that
343   resource share the same validator unless their payload body would be
344   identical.
345
346   Cache entries might persist for arbitrarily long periods, regardless
347   of expiration times.  Thus, a cache might attempt to validate an
348   entry using a validator that it obtained in the distant past.  A
349   strong validator MUST be unique across all versions of all
350   representations associated with a particular resource over time.
351   However, there is no implication of uniqueness across representations
352   of different resources (i.e., the same strong validator might be in
353   use for representations of multiple resources at the same time and
354   does not imply that those representations are equivalent).
355
356   There are a variety of strong validators used in practice.  The best
357   are based on strict revision control, wherein each change to a
358   representation always results in a unique node name and revision
359   identifier being assigned before the representation is made
360   accessible to GET.  A cryptographic hash function applied to the
361   representation data is also sufficient if the data is available prior
362   to the response header fields being sent and the digest does not need
363   to be recalculated every time a validation request is received.
364   However, if a resource has distinct representations that differ only
365   in their metadata, such as might occur with content negotiation over
366   media types that happen to share the same data format, then a server
367   SHOULD incorporate additional information in the validator to
368   distinguish those representations and avoid confusing cache behavior.
369
370   In contrast, a "weak validator" is a representation metadata value
371   that might not be changed for every change to the representation
372   data.  This weakness might be due to limitations in how the value is
373   calculated, such as clock resolution or an inability to ensure
374   uniqueness for all possible representations of the resource, or due
375   to a desire by the resource owner to group representations by some
376   self-determined set of equivalency rather than unique sequences of
377   data.  A weak entity-tag SHOULD change whenever the origin server
378   considers prior representations to be unacceptable as a substitute
379   for the current representation.  In other words, a weak entity-tag
380   SHOULD change whenever the origin server wants caches to invalidate
381   old responses.
382
383   For example, the representation of a weather report that changes in
384   content every second, based on dynamic measurements, might be grouped
385   into sets of equivalent representations (from the origin server's
386   perspective) with the same weak validator in order to allow cached
387   representations to be valid for a reasonable period of time (perhaps
388
389
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395
396   adjusted dynamically based on server load or weather quality).
397   Likewise, a representation's modification time, if defined with only
398   one-second resolution, might be a weak validator if it is possible
399   for the representation to be modified twice during a single second
400   and retrieved between those modifications.
401
402   A "use" of a validator occurs when either a client generates a
403   request and includes the validator in a precondition or when a server
404   compares two validators.  Weak validators are only usable in contexts
405   that do not depend on exact equality of a representation's payload
406   body.  Strong validators are usable and preferred for all conditional
407   requests, including cache validation, partial content ranges, and
408   "lost update" avoidance.
409
4102.2.  Last-Modified
411
412   The "Last-Modified" header field indicates the date and time at which
413   the origin server believes the selected representation was last
414   modified.
415
416     Last-Modified = HTTP-date
417
418   An example of its use is
419
420     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
421
4222.2.1.  Generation
423
424   Origin servers SHOULD send Last-Modified for any selected
425   representation for which a last modification date can be reasonably
426   and consistently determined, since its use in conditional requests
427   and evaluating cache freshness ([Part6]) results in a substantial
428   reduction of HTTP traffic on the Internet and can be a significant
429   factor in improving service scalability and reliability.
430
431   A representation is typically the sum of many parts behind the
432   resource interface.  The last-modified time would usually be the most
433   recent time that any of those parts were changed.  How that value is
434   determined for any given resource is an implementation detail beyond
435   the scope of this specification.  What matters to HTTP is how
436   recipients of the Last-Modified header field can use its value to
437   make conditional requests and test the validity of locally cached
438   responses.
439
440   An origin server SHOULD obtain the Last-Modified value of the
441   representation as close as possible to the time that it generates the
442   Date field-value for its response.  This allows a recipient to make
443   an accurate assessment of the representation's modification time,
444
445
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451
452   especially if the representation changes near the time that the
453   response is generated.
454
455   An origin server with a clock MUST NOT send a Last-Modified date that
456   is later than the server's time of message origination (Date).  If
457   the last modification time is derived from implementation-specific
458   metadata that evaluates to some time in the future, according to the
459   origin server's clock, then the origin server MUST replace that value
460   with the message origination date.  This prevents a future
461   modification date from having an adverse impact on cache validation.
462
4632.2.2.  Comparison
464
465   A Last-Modified time, when used as a validator in a request, is
466   implicitly weak unless it is possible to deduce that it is strong,
467   using the following rules:
468
469   o  The validator is being compared by an origin server to the actual
470      current validator for the representation and,
471
472   o  That origin server reliably knows that the associated
473      representation did not change twice during the second covered by
474      the presented validator.
475
476   or
477
478   o  The validator is about to be used by a client in an If-Modified-
479      Since, If-Unmodified-Since header field, because the client has a
480      cache entry, or If-Range for the associated representation, and
481
482   o  That cache entry includes a Date value, which gives the time when
483      the origin server sent the original response, and
484
485   o  The presented Last-Modified time is at least 60 seconds before the
486      Date value.
487
488   or
489
490   o  The validator is being compared by an intermediate cache to the
491      validator stored in its cache entry for the representation, and
492
493   o  That cache entry includes a Date value, which gives the time when
494      the origin server sent the original response, and
495
496   o  The presented Last-Modified time is at least 60 seconds before the
497      Date value.
498
499   This method relies on the fact that if two different responses were
500
501
502
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507
508   sent by the origin server during the same second, but both had the
509   same Last-Modified time, then at least one of those responses would
510   have a Date value equal to its Last-Modified time.  The arbitrary 60-
511   second limit guards against the possibility that the Date and Last-
512   Modified values are generated from different clocks, or at somewhat
513   different times during the preparation of the response.  An
514   implementation MAY use a value larger than 60 seconds, if it is
515   believed that 60 seconds is too short.
516
5172.3.  ETag
518
519   The ETag header field provides the current entity-tag for the
520   selected representation.  An entity-tag is an opaque validator for
521   differentiating between multiple representations of the same
522   resource, regardless of whether those multiple representations are
523   due to resource state changes over time, content negotiation
524   resulting in multiple representations being valid at the same time,
525   or both.  An entity-tag consists of an opaque quoted string, possibly
526   prefixed by a weakness indicator.
527
528     ETag       = entity-tag
529
530     entity-tag = [ weak ] opaque-tag
531     weak       = %x57.2F ; "W/", case-sensitive
532     opaque-tag = quoted-string
533
534   An entity-tag can be more reliable for validation than a modification
535   date in situations where it is inconvenient to store modification
536   dates, where the one-second resolution of HTTP date values is not
537   sufficient, or where modification dates are not consistently
538   maintained.
539
540   Examples:
541
542     ETag: "xyzzy"
543     ETag: W/"xyzzy"
544     ETag: ""
545
546   An entity-tag can be either a weak or strong validator, with strong
547   being the default.  If an origin server provides an entity-tag for a
548   representation and the generation of that entity-tag does not satisfy
549   the requirements for a strong validator (Section 2.1), then that
550   entity-tag MUST be marked as weak by prefixing its opaque value with
551   "W/" (case-sensitive).
552
553
554
555
556
557
558
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563
5642.3.1.  Generation
565
566   The principle behind entity-tags is that only the service author
567   knows the implementation of a resource well enough to select the most
568   accurate and efficient validation mechanism for that resource, and
569   that any such mechanism can be mapped to a simple sequence of octets
570   for easy comparison.  Since the value is opaque, there is no need for
571   the client to be aware of how each entity-tag is constructed.
572
573   For example, a resource that has implementation-specific versioning
574   applied to all changes might use an internal revision number, perhaps
575   combined with a variance identifier for content negotiation, to
576   accurately differentiate between representations.  Other
577   implementations might use a stored hash of representation content, a
578   combination of various filesystem attributes, or a modification
579   timestamp that has sub-second resolution.
580
581   Origin servers SHOULD send ETag for any selected representation for
582   which detection of changes can be reasonably and consistently
583   determined, since the entity-tag's use in conditional requests and
584   evaluating cache freshness ([Part6]) can result in a substantial
585   reduction of HTTP network traffic and can be a significant factor in
586   improving service scalability and reliability.
587
5882.3.2.  Comparison
589
590   There are two entity-tag comparison functions, depending on whether
591   the comparison context allows the use of weak validators or not:
592
593   o  The strong comparison function: in order to be considered equal,
594      both opaque-tags MUST be identical character-by-character, and
595      both MUST NOT be weak.
596
597   o  The weak comparison function: in order to be considered equal,
598      both opaque-tags MUST be identical character-by-character, but
599      either or both of them MAY be tagged as "weak" without affecting
600      the result.
601
602   The example below shows the results for a set of entity-tag pairs,
603   and both the weak and strong comparison function results:
604
605   +--------+--------+-------------------+-----------------+
606   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
607   +--------+--------+-------------------+-----------------+
608   | W/"1"  | W/"1"  | no match          | match           |
609   | W/"1"  | W/"2"  | no match          | no match        |
610   | W/"1"  | "1"    | no match          | match           |
611
612
613
614
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619
620   | "1"    | "1"    | match             | match           |
621   +--------+--------+-------------------+-----------------+
622
6232.3.3.  Example: Entity-tags varying on Content-Negotiated Resources
624
625   Consider a resource that is subject to content negotiation (Section 5
626   of [Part3]), and where the representations returned upon a GET
627   request vary based on the Accept-Encoding request header field
628   (Section 6.3 of [Part3]):
629
630   >> Request:
631
632     GET /index HTTP/1.1
633     Host: www.example.com
634     Accept-Encoding: gzip
635
636
637   In this case, the response might or might not use the gzip content
638   coding.  If it does not, the response might look like:
639
640   >> Response:
641
642     HTTP/1.1 200 OK
643     Date: Thu, 26 Mar 2010 00:05:00 GMT
644     ETag: "123-a"
645     Content-Length: 70
646     Vary: Accept-Encoding
647     Content-Type: text/plain
648
649     Hello World!
650     Hello World!
651     Hello World!
652     Hello World!
653     Hello World!
654
655   An alternative representation that does use gzip content coding would
656   be:
657
658
659
660
661
662
663
664
665
666
667
668
669
670
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675
676   >> Response:
677
678     HTTP/1.1 200 OK
679     Date: Thu, 26 Mar 2010 00:05:00 GMT
680     ETag: "123-b"
681     Content-Length: 43
682     Vary: Accept-Encoding
683     Content-Type: text/plain
684     Content-Encoding: gzip
685
686     ...binary data...
687
688      Note: Content codings are a property of the representation, so
689      therefore an entity-tag of an encoded representation must be
690      distinct from an unencoded representation to prevent conflicts
691      during cache updates and range requests.  In contrast, transfer
692      codings (Section 6.2 of [Part1]) apply only during message
693      transfer and do not require distinct entity-tags.
694
6952.4.  Rules for When to Use Entity-tags and Last-Modified Dates
696
697   We adopt a set of rules and recommendations for origin servers,
698   clients, and caches regarding when various validator types ought to
699   be used, and for what purposes.
700
701   HTTP/1.1 origin servers:
702
703   o  SHOULD send an entity-tag validator unless it is not feasible to
704      generate one.
705
706   o  MAY send a weak entity-tag instead of a strong entity-tag, if
707      performance considerations support the use of weak entity-tags, or
708      if it is unfeasible to send a strong entity-tag.
709
710   o  SHOULD send a Last-Modified value if it is feasible to send one.
711
712   In other words, the preferred behavior for an HTTP/1.1 origin server
713   is to send both a strong entity-tag and a Last-Modified value.
714
715   HTTP/1.1 clients:
716
717   o  MUST use that entity-tag in any cache-conditional request (using
718      If-Match or If-None-Match) if an entity-tag has been provided by
719      the origin server.
720
721   o  SHOULD use the Last-Modified value in non-subrange cache-
722      conditional requests (using If-Modified-Since) if only a Last-
723      Modified value has been provided by the origin server.
724
725
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731
732   o  MAY use the Last-Modified value in subrange cache-conditional
733      requests (using If-Unmodified-Since) if only a Last-Modified value
734      has been provided by an HTTP/1.0 origin server.  The user agent
735      SHOULD provide a way to disable this, in case of difficulty.
736
737   o  SHOULD use both validators in cache-conditional requests if both
738      an entity-tag and a Last-Modified value have been provided by the
739      origin server.  This allows both HTTP/1.0 and HTTP/1.1 caches to
740      respond appropriately.
741
742   An HTTP/1.1 origin server, upon receiving a conditional request that
743   includes both a Last-Modified date (e.g., in an If-Modified-Since or
744   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
745   in an If-Match, If-None-Match, or If-Range header field) as cache
746   validators, MUST NOT return a response status code of 304 (Not
747   Modified) unless doing so is consistent with all of the conditional
748   header fields in the request.
749
750   An HTTP/1.1 caching proxy, upon receiving a conditional request that
751   includes both a Last-Modified date and one or more entity-tags as
752   cache validators, MUST NOT return a locally cached response to the
753   client unless that cached response is consistent with all of the
754   conditional header fields in the request.
755
756      Note: The general principle behind these rules is that HTTP/1.1
757      servers and clients ought to transmit as much non-redundant
758      information as is available in their responses and requests.
759      HTTP/1.1 systems receiving this information will make the most
760      conservative assumptions about the validators they receive.
761
762      HTTP/1.0 clients and caches might ignore entity-tags.  Generally,
763      last-modified values received or used by these systems will
764      support transparent and efficient caching, and so HTTP/1.1 origin
765      servers should provide Last-Modified values.  In those rare cases
766      where the use of a Last-Modified value as a validator by an
767      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
768      origin servers should not provide one.
769
7703.  Precondition Header Fields
771
772   This section defines the syntax and semantics of HTTP/1.1 header
773   fields for applying preconditions on requests.
774
7753.1.  If-Match
776
777   The "If-Match" header field MAY be used to make a request method
778   conditional on the current existence or value of an entity-tag for
779   one or more representations of the target resource.  If-Match is
780
781
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787
788   generally useful for resource update requests, such as PUT requests,
789   as a means for protecting against accidental overwrites when multiple
790   clients are acting in parallel on the same resource (i.e., the "lost
791   update" problem).  An If-Match field-value of "*" places the
792   precondition on the existence of any current representation for the
793   target resource.
794
795     If-Match = "*" / 1#entity-tag
796
797   If any of the entity-tags listed in the If-Match field value match
798   (as per Section 2.3.2) the entity-tag of the selected representation
799   for the target resource, or if "*" is given and any current
800   representation exists for the target resource, then the server MAY
801   perform the request method as if the If-Match header field was not
802   present.
803
804   If none of the entity-tags match, or if "*" is given and no current
805   representation exists, the server MUST NOT perform the requested
806   method.  Instead, the server MUST respond with the 412 (Precondition
807   Failed) status code.
808
809   If the request would, without the If-Match header field, result in
810   anything other than a 2xx or 412 status code, then the If-Match
811   header field MUST be ignored.
812
813   Examples:
814
815     If-Match: "xyzzy"
816     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
817     If-Match: *
818
819   The result of a request having both an If-Match header field and
820   either an If-None-Match or an If-Modified-Since header fields is
821   undefined by this specification.
822
8233.2.  If-None-Match
824
825   The "If-None-Match" header field MAY be used to make a request method
826   conditional on not matching any of the current entity-tag values for
827   representations of the target resource.  If-None-Match is primarily
828   used in conditional GET requests to enable efficient updates of
829   cached information with a minimum amount of transaction overhead.  A
830   client that has one or more representations previously obtained from
831   the target resource can send If-None-Match with a list of the
832   associated entity-tags in the hope of receiving a 304 response if at
833   least one of those representations matches the selected
834   representation.
835
836
837
838
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843
844   If-None-Match MAY also be used with a value of "*" to prevent an
845   unsafe request method (e.g., PUT) from inadvertently modifying an
846   existing representation of the target resource when the client
847   believes that the resource does not have a current representation.
848   This is a variation on the "lost update" problem that might arise if
849   more than one client attempts to create an initial representation for
850   the target resource.
851
852     If-None-Match = "*" / 1#entity-tag
853
854   If any of the entity-tags listed in the If-None-Match field-value
855   match (as per Section 2.3.2) the entity-tag of the selected
856   representation, or if "*" is given and any current representation
857   exists for that resource, then the server MUST NOT perform the
858   requested method.  Instead, if the request method was GET or HEAD,
859   the server SHOULD respond with a 304 (Not Modified) status code,
860   including the cache-related header fields (particularly ETag) of the
861   selected representation that has a matching entity-tag.  For all
862   other request methods, the server MUST respond with a 412
863   (Precondition Failed) status code.
864
865   If none of the entity-tags match, then the server MAY perform the
866   requested method as if the If-None-Match header field did not exist,
867   but MUST also ignore any If-Modified-Since header field(s) in the
868   request.  That is, if no entity-tags match, then the server MUST NOT
869   return a 304 (Not Modified) response.
870
871   If the request would, without the If-None-Match header field, result
872   in anything other than a 2xx or 304 status code, then the If-None-
873   Match header field MUST be ignored.  (See Section 2.4 for a
874   discussion of server behavior when both If-Modified-Since and If-
875   None-Match appear in the same request.)
876
877   Examples:
878
879     If-None-Match: "xyzzy"
880     If-None-Match: W/"xyzzy"
881     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
882     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
883     If-None-Match: *
884
885   The result of a request having both an If-None-Match header field and
886   either an If-Match or an If-Unmodified-Since header fields is
887   undefined by this specification.
888
889
890
891
892
893
894
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899
9003.3.  If-Modified-Since
901
902   The "If-Modified-Since" header field MAY be used to make a request
903   method conditional by modification date: if the selected
904   representation has not been modified since the time specified in this
905   field, then do not perform the request method; instead, respond as
906   detailed below.
907
908     If-Modified-Since = HTTP-date
909
910   An example of the field is:
911
912     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
913
914   A GET method with an If-Modified-Since header field and no Range
915   header field requests that the selected representation be transferred
916   only if it has been modified since the date given by the If-Modified-
917   Since header field.  The algorithm for determining this includes the
918   following cases:
919
920   1.  If the request would normally result in anything other than a 200
921       (OK) status code, or if the passed If-Modified-Since date is
922       invalid, the response is exactly the same as for a normal GET.  A
923       date which is later than the server's current time is invalid.
924
925   2.  If the selected representation has been modified since the If-
926       Modified-Since date, the response is exactly the same as for a
927       normal GET.
928
929   3.  If the selected representation has not been modified since a
930       valid If-Modified-Since date, the server SHOULD return a 304 (Not
931       Modified) response.
932
933   The purpose of this feature is to allow efficient updates of cached
934   information with a minimum amount of transaction overhead.
935
936      Note: The Range header field modifies the meaning of If-Modified-
937      Since; see Section 5.4 of [Part5] for full details.
938
939      Note: If-Modified-Since times are interpreted by the server, whose
940      clock might not be synchronized with the client.
941
942      Note: When handling an If-Modified-Since header field, some
943      servers will use an exact date comparison function, rather than a
944      less-than function, for deciding whether to send a 304 (Not
945      Modified) response.  To get best results when sending an If-
946      Modified-Since header field for cache validation, clients are
947      advised to use the exact date string received in a previous Last-
948
949
950
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955
956      Modified header field whenever possible.
957
958      Note: If a client uses an arbitrary date in the If-Modified-Since
959      header field instead of a date taken from the Last-Modified header
960      field for the same request, the client needs to be aware that this
961      date is interpreted in the server's understanding of time.
962      Unsynchronized clocks and rounding problems, due to the different
963      encodings of time between the client and server, are concerns.
964      This includes the possibility of race conditions if the document
965      has changed between the time it was first requested and the If-
966      Modified-Since date of a subsequent request, and the possibility
967      of clock-skew-related problems if the If-Modified-Since date is
968      derived from the client's clock without correction to the server's
969      clock.  Corrections for different time bases between client and
970      server are at best approximate due to network latency.
971
972   The result of a request having both an If-Modified-Since header field
973   and either an If-Match or an If-Unmodified-Since header fields is
974   undefined by this specification.
975
9763.4.  If-Unmodified-Since
977
978   The "If-Unmodified-Since" header field MAY be used to make a request
979   method conditional by modification date: if the selected
980   representation has been modified since the time specified in this
981   field, then the server MUST NOT perform the requested operation and
982   MUST instead respond with the 412 (Precondition Failed) status code.
983   If the selected representation has not been modified since the time
984   specified in this field, the server SHOULD perform the request method
985   as if the If-Unmodified-Since header field were not present.
986
987     If-Unmodified-Since = HTTP-date
988
989   An example of the field is:
990
991     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
992
993   If the request normally (i.e., without the If-Unmodified-Since header
994   field) would result in anything other than a 2xx or 412 status code,
995   the If-Unmodified-Since header field SHOULD be ignored.
996
997   If the specified date is invalid, the header field MUST be ignored.
998
999   The result of a request having both an If-Unmodified-Since header
1000   field and either an If-None-Match or an If-Modified-Since header
1001   fields is undefined by this specification.
1002
1003
1004
1005
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1011
10123.5.  If-Range
1013
1014   The If-Range header field provides a special conditional request
1015   mechanism that is similar to If-Match and If-Unmodified-Since but
1016   specific to HTTP range requests.  If-Range is defined in Section 5.3
1017   of [Part5].
1018
10194.  Status Code Definitions
1020
10214.1.  304 Not Modified
1022
1023   The 304 status code indicates that a conditional GET request has been
1024   received and would have resulted in a 200 (OK) response if it were
1025   not for the fact that the condition has evaluated to false.  In other
1026   words, there is no need for the server to transfer a representation
1027   of the target resource because the client's request indicates that it
1028   already has a valid representation, as indicated by the 304 response
1029   header fields, and is therefore redirecting the client to make use of
1030   that stored representation as if it were the payload of a 200
1031   response.  The 304 response MUST NOT contain a message-body, and thus
1032   is always terminated by the first empty line after the header fields.
1033
1034   A 304 response MUST include a Date header field (Section 9.3 of
1035   [Part1]) unless its omission is required by Section 9.3.1 of [Part1].
1036   If a 200 response to the same request would have included any of the
1037   header fields Cache-Control, Content-Location, ETag, Expires, Last-
1038   Modified, or Vary, then those same header fields MUST be sent in a
1039   304 response.
1040
1041   Since the goal of a 304 response is to minimize information transfer
1042   when the recipient already has one or more cached representations,
1043   the response SHOULD NOT include representation metadata other than
1044   the above listed fields unless said metadata exists for the purpose
1045   of guiding cache updates (e.g., future HTTP extensions).
1046
1047   If the recipient of a 304 response does not have a cached
1048   representation corresponding to the entity-tag indicated by the 304
1049   response, then the recipient MUST NOT use the 304 to update its own
1050   cache.  If this conditional request originated with an outbound
1051   client, such as a user agent with its own cache sending a conditional
1052   GET to a shared proxy, then the 304 response MAY be forwarded to the
1053   outbound client.  Otherwise, the recipient MUST disregard the 304
1054   response and repeat the request without any preconditions.
1055
1056   If a cache uses a received 304 response to update a cache entry, the
1057   cache MUST update the entry to reflect any new field values given in
1058   the response.
1059
1060
1061
1062
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1067
10684.2.  412 Precondition Failed
1069
1070   The 412 status code indicates that one or more preconditions given in
1071   the request header fields evaluated to false when tested on the
1072   server.  This response code allows the client to place preconditions
1073   on the current resource state (its current representations and
1074   metadata) and thus prevent the request method from being applied if
1075   the target resource is in an unexpected state.
1076
10775.  IANA Considerations
1078
10795.1.  Status Code Registration
1080
1081   The HTTP Status Code Registry located at
1082   <http://www.iana.org/assignments/http-status-codes> shall be updated
1083   with the registrations below:
1084
1085   +-------+---------------------+-------------+
1086   | Value | Description         | Reference   |
1087   +-------+---------------------+-------------+
1088   | 304   | Not Modified        | Section 4.1 |
1089   | 412   | Precondition Failed | Section 4.2 |
1090   +-------+---------------------+-------------+
1091
10925.2.  Header Field Registration
1093
1094   The Message Header Field Registry located at <http://www.iana.org/
1095   assignments/message-headers/message-header-index.html> shall be
1096   updated with the permanent registrations below (see [RFC3864]):
1097
1098   +---------------------+----------+----------+-------------+
1099   | Header Field Name   | Protocol | Status   | Reference   |
1100   +---------------------+----------+----------+-------------+
1101   | ETag                | http     | standard | Section 2.3 |
1102   | If-Match            | http     | standard | Section 3.1 |
1103   | If-Modified-Since   | http     | standard | Section 3.3 |
1104   | If-None-Match       | http     | standard | Section 3.2 |
1105   | If-Unmodified-Since | http     | standard | Section 3.4 |
1106   | Last-Modified       | http     | standard | Section 2.2 |
1107   +---------------------+----------+----------+-------------+
1108
1109   The change controller is: "IETF (iesg@ietf.org) - Internet
1110   Engineering Task Force".
1111
11126.  Security Considerations
1113
1114   No additional security considerations have been identified beyond
1115   those applicable to HTTP in general [Part1].
1116
1117
1118
1119Fielding, et al.        Expires February 25, 2012              [Page 20]
1120
1121Internet-Draft              HTTP/1.1, Part 4                 August 2011
1122
1123
11247.  Acknowledgments
1125
1126   See Section 12 of [Part1].
1127
11288.  References
1129
11308.1.  Normative References
1131
1132   [Part1]    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 1: URIs, Connections,
1135              and Message Parsing", draft-ietf-httpbis-p1-messaging-16
1136              (work in progress), August 2011.
1137
1138   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1139              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1140              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
1141              and Content Negotiation", draft-ietf-httpbis-p3-payload-16
1142              (work in progress), August 2011.
1143
1144   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1145              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1146              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1147              Partial Responses", draft-ietf-httpbis-p5-range-16 (work
1148              in progress), August 2011.
1149
1150   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1151              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1152              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
1153              6: Caching", draft-ietf-httpbis-p6-cache-16 (work in
1154              progress), August 2011.
1155
1156   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1157              Requirement Levels", BCP 14, RFC 2119, March 1997.
1158
1159   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1160              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1161
11628.2.  Informative References
1163
1164   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1165              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1166              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1167
1168   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1169              Procedures for Message Header Fields", BCP 90, RFC 3864,
1170              September 2004.
1171
1172
1173
1174
1175Fielding, et al.        Expires February 25, 2012              [Page 21]
1176
1177Internet-Draft              HTTP/1.1, Part 4                 August 2011
1178
1179
1180   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
1181              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
1182
1183Appendix A.  Changes from RFC 2616
1184
1185   Allow weak entity-tags in all requests except range requests
1186   (Sections 2.1 and 3.2).
1187
1188   Change ABNF productions for header fields to only define the field
1189   value.  (Section 3)
1190
1191Appendix B.  Collected ABNF
1192
1193   ETag = entity-tag
1194
1195   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1196
1197   If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1198    entity-tag ] ) )
1199   If-Modified-Since = HTTP-date
1200   If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1201    entity-tag ] ) )
1202   If-Unmodified-Since = HTTP-date
1203
1204   Last-Modified = HTTP-date
1205
1206   OWS = <OWS, defined in [Part1], Section 1.2.2>
1207
1208   entity-tag = [ weak ] opaque-tag
1209
1210   opaque-tag = quoted-string
1211
1212   quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
1213
1214   weak = %x57.2F ; W/
1215
1216   ABNF diagnostics:
1217
1218   ; ETag defined but not used
1219   ; If-Match defined but not used
1220   ; If-Modified-Since defined but not used
1221   ; If-None-Match defined but not used
1222   ; If-Unmodified-Since defined but not used
1223   ; Last-Modified defined but not used
1224
1225
1226
1227
1228
1229
1230
1231Fielding, et al.        Expires February 25, 2012              [Page 22]
1232
1233Internet-Draft              HTTP/1.1, Part 4                 August 2011
1234
1235
1236Appendix C.  Change Log (to be removed by RFC Editor before publication)
1237
1238C.1.  Since RFC 2616
1239
1240   Extracted relevant partitions from [RFC2616].
1241
1242C.2.  Since draft-ietf-httpbis-p4-conditional-00
1243
1244   Closed issues:
1245
1246   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1247      Informative references"
1248
1249   Other changes:
1250
1251   o  Move definitions of 304 and 412 condition codes from Part2.
1252
1253C.3.  Since draft-ietf-httpbis-p4-conditional-01
1254
1255   Ongoing work on ABNF conversion
1256   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1257
1258   o  Add explicit references to BNF syntax and rules imported from
1259      other parts of the specification.
1260
1261C.4.  Since draft-ietf-httpbis-p4-conditional-02
1262
1263   Closed issues:
1264
1265   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1266      non-GET requests"
1267
1268   Ongoing work on IANA Message Header Field Registration
1269   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1270
1271   o  Reference RFC 3984, and update header field registrations for
1272      header fields defined in this document.
1273
1274C.5.  Since draft-ietf-httpbis-p4-conditional-03
1275
1276   Closed issues:
1277
1278   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for
1279      ETag matching"
1280
1281   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity
1282      value' undefined"
1283
1284
1285
1286
1287Fielding, et al.        Expires February 25, 2012              [Page 23]
1288
1289Internet-Draft              HTTP/1.1, Part 4                 August 2011
1290
1291
1292   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068
1293      Date header reference"
1294
1295C.6.  Since draft-ietf-httpbis-p4-conditional-04
1296
1297   Ongoing work on ABNF conversion
1298   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1299
1300   o  Use "/" instead of "|" for alternatives.
1301
1302   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1303      whitespace ("OWS") and required whitespace ("RWS").
1304
1305   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1306      field value format definitions.
1307
1308C.7.  Since draft-ietf-httpbis-p4-conditional-05
1309
1310   Final work on ABNF conversion
1311   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1312
1313   o  Add appendix containing collected and expanded ABNF, reorganize
1314      ABNF introduction.
1315
1316C.8.  Since draft-ietf-httpbis-p4-conditional-06
1317
1318   Closed issues:
1319
1320   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-
1321      sensitivity of etag weakness indicator"
1322
1323C.9.  Since draft-ietf-httpbis-p4-conditional-07
1324
1325   Closed issues:
1326
1327   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1328      non-GET requests" (If-Match still was defined to require strong
1329      matching)
1330
1331   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
1332      registrations for optional status codes"
1333
1334C.10.  Since draft-ietf-httpbis-p4-conditional-08
1335
1336   No significant changes.
1337
1338
1339
1340
1341
1342
1343Fielding, et al.        Expires February 25, 2012              [Page 24]
1344
1345Internet-Draft              HTTP/1.1, Part 4                 August 2011
1346
1347
1348C.11.  Since draft-ietf-httpbis-p4-conditional-09
1349
1350   No significant changes.
1351
1352C.12.  Since draft-ietf-httpbis-p4-conditional-10
1353
1354   Closed issues:
1355
1356   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
1357      'Requested Variant'"
1358
1359   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
1360      entity / representation / variant terminology"
1361
1362   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
1363      removing the 'changes from 2068' sections"
1364
1365C.13.  Since draft-ietf-httpbis-p4-conditional-11
1366
1367   None.
1368
1369C.14.  Since draft-ietf-httpbis-p4-conditional-12
1370
1371   Closed issues:
1372
1373   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
1374      Classification"
1375
1376C.15.  Since draft-ietf-httpbis-p4-conditional-13
1377
1378   Closed issues:
1379
1380   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/89>: "If-* and
1381      entities"
1382
1383   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/101>: "Definition of
1384      validator weakness"
1385
1386   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
1387      ABNFs for header fields"
1388
1389   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/269>: "ETags and
1390      Quotes"
1391
1392
1393
1394
1395
1396
1397
1398
1399Fielding, et al.        Expires February 25, 2012              [Page 25]
1400
1401Internet-Draft              HTTP/1.1, Part 4                 August 2011
1402
1403
1404C.16.  Since draft-ietf-httpbis-p4-conditional-14
1405
1406   None.
1407
1408C.17.  Since draft-ietf-httpbis-p4-conditional-15
1409
1410   Closed issues:
1411
1412   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/304>: "If-Range
1413      should be listed when dicussing contexts where L-M can be
1414      considered strong"
1415
1416Index
1417
1418   3
1419      304 Not Modified (status code)  19
1420
1421   4
1422      412 Precondition Failed (status code)  20
1423
1424   E
1425      ETag header field  10
1426
1427   G
1428      Grammar
1429         entity-tag  10
1430         ETag  10
1431         If-Match  15
1432         If-Modified-Since  17
1433         If-None-Match  16
1434         If-Unmodified-Since  18
1435         Last-Modified  8
1436         opaque-tag  10
1437         weak  10
1438
1439   H
1440      Header Fields
1441         ETag  10
1442         If-Match  14
1443         If-Modified-Since  17
1444         If-None-Match  15
1445         If-Unmodified-Since  18
1446         Last-Modified  8
1447
1448   I
1449      If-Match header field  14
1450      If-Modified-Since header field  17
1451      If-None-Match header field  15
1452
1453
1454
1455Fielding, et al.        Expires February 25, 2012              [Page 26]
1456
1457Internet-Draft              HTTP/1.1, Part 4                 August 2011
1458
1459
1460      If-Unmodified-Since header field  18
1461
1462   L
1463      Last-Modified header field  8
1464
1465   M
1466      metadata  6
1467
1468   S
1469      selected representation  5
1470      Status Codes
1471         304 Not Modified  19
1472         412 Precondition Failed  20
1473
1474   V
1475      validator  6
1476         strong  6
1477         weak  6
1478
1479Authors' Addresses
1480
1481   Roy T. Fielding (editor)
1482   Adobe Systems Incorporated
1483   345 Park Ave
1484   San Jose, CA  95110
1485   USA
1486
1487   EMail: fielding@gbiv.com
1488   URI:   http://roy.gbiv.com/
1489
1490
1491   Jim Gettys
1492   Alcatel-Lucent Bell Labs
1493   21 Oak Knoll Road
1494   Carlisle, MA  01741
1495   USA
1496
1497   EMail: jg@freedesktop.org
1498   URI:   http://gettys.wordpress.com/
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511Fielding, et al.        Expires February 25, 2012              [Page 27]
1512
1513Internet-Draft              HTTP/1.1, Part 4                 August 2011
1514
1515
1516   Jeffrey C. Mogul
1517   Hewlett-Packard Company
1518   HP Labs, Large Scale Systems Group
1519   1501 Page Mill Road, MS 1177
1520   Palo Alto, CA  94304
1521   USA
1522
1523   EMail: JeffMogul@acm.org
1524
1525
1526   Henrik Frystyk Nielsen
1527   Microsoft Corporation
1528   1 Microsoft Way
1529   Redmond, WA  98052
1530   USA
1531
1532   EMail: henrikn@microsoft.com
1533
1534
1535   Larry Masinter
1536   Adobe Systems Incorporated
1537   345 Park Ave
1538   San Jose, CA  95110
1539   USA
1540
1541   EMail: LMM@acm.org
1542   URI:   http://larry.masinter.net/
1543
1544
1545   Paul J. Leach
1546   Microsoft Corporation
1547   1 Microsoft Way
1548   Redmond, WA  98052
1549
1550   EMail: paulle@microsoft.com
1551
1552
1553   Tim Berners-Lee
1554   World Wide Web Consortium
1555   MIT Computer Science and Artificial Intelligence Laboratory
1556   The Stata Center, Building 32
1557   32 Vassar Street
1558   Cambridge, MA  02139
1559   USA
1560
1561   EMail: timbl@w3.org
1562   URI:   http://www.w3.org/People/Berners-Lee/
1563
1564
1565
1566
1567Fielding, et al.        Expires February 25, 2012              [Page 28]
1568
1569Internet-Draft              HTTP/1.1, Part 4                 August 2011
1570
1571
1572   Yves Lafon (editor)
1573   World Wide Web Consortium
1574   W3C / ERCIM
1575   2004, rte des Lucioles
1576   Sophia-Antipolis, AM  06902
1577   France
1578
1579   EMail: ylafon@w3.org
1580   URI:   http://www.raubacapeu.net/people/yves/
1581
1582
1583   Julian F. Reschke (editor)
1584   greenbytes GmbH
1585   Hafenweg 16
1586   Muenster, NW  48155
1587   Germany
1588
1589   Phone: +49 251 2807760
1590   Fax:   +49 251 2807761
1591   EMail: julian.reschke@greenbytes.de
1592   URI:   http://greenbytes.de/tech/webdav/
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623Fielding, et al.        Expires February 25, 2012              [Page 29]
1624
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