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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: May 3, 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                                                        October 31, 2011
23
24
25                 HTTP/1.1, part 4: Conditional Requests
26                  draft-ietf-httpbis-p4-conditional-17
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.18.
52
53
54
55Fielding, et al.           Expires May 3, 2012                  [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 4                October 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 May 3, 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.  Conformance and Error Handling . . . . . . . . . . . . . .  5
108
109
110
111Fielding, et al.           Expires May 3, 2012                  [Page 2]
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113Internet-Draft              HTTP/1.1, Part 4                October 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 . . . . . . . . . . . . . . . . . . . . . . . . . 15
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 . . . . . . . . 24
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 . . . . . . . . 25
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 May 3, 2012                  [Page 3]
168
169Internet-Draft              HTTP/1.1, Part 4                October 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     C.18. Since draft-ietf-httpbis-p4-conditional-16 . . . . . . . . 26
176   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
177
178
179
180
181
182
183
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185
186
187
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194
195
196
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221
222
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225Internet-Draft              HTTP/1.1, Part 4                October 2011
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.  Conformance and Error Handling
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   This document defines conformance criteria for several roles in HTTP
266   communication, including Senders, Recipients, Clients, Servers, User-
267   Agents, Origin Servers, Intermediaries, Proxies and Gateways.  See
268   Section 2 of [Part1] for definitions of these terms.
269
270   An implementation is considered conformant if it complies with all of
271   the requirements associated with its role(s).  Note that SHOULD-level
272   requirements are relevant here, unless one of the documented
273   exceptions is applicable.
274
275   This document also uses ABNF to define valid protocol elements
276
277
278
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283
284   (Section 1.2).  In addition to the prose requirements placed upon
285   them, Senders MUST NOT generate protocol elements that are invalid.
286
287   Unless noted otherwise, Recipients MAY take steps to recover a usable
288   protocol element from an invalid construct.  However, HTTP does not
289   define specific error handling mechanisms, except in cases where it
290   has direct impact on security.  This is because different uses of the
291   protocol require different error handling strategies; for example, a
292   Web browser may wish to transparently recover from a response where
293   the Location header field doesn't parse according to the ABNF,
294   whereby in a systems control protocol using HTTP, this type of error
295   recovery could lead to dangerous consequences.
296
2971.2.  Syntax Notation
298
299   This specification uses the ABNF syntax defined in Section 1.2 of
300   [Part1] (which extends the syntax defined in [RFC5234] with a list
301   rule).  Appendix B shows the collected ABNF, with the list rule
302   expanded.
303
304   The following core rules are included by reference, as defined in
305   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
306   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
307   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
308   sequence of data), SP (space), and VCHAR (any visible US-ASCII
309   character).
310
311   The ABNF rules below are defined in [Part1] and [Part2]:
312
313     OWS           = <OWS, defined in [Part1], Section 1.2.2>
314     quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
315     HTTP-date     = <HTTP-date, defined in [Part2], Section 8>
316
3172.  Validators
318
319   This specification defines two forms of metadata that are commonly
320   used to observe resource state and test for preconditions:
321   modification dates and opaque entity tags.  Additional metadata that
322   reflects resource state has been defined by various extensions of
323   HTTP, such as WebDAV [RFC4918], that are beyond the scope of this
324   specification.  A resource metadata value is referred to as a
325   "validator" when it is used within a precondition.
326
3272.1.  Weak versus Strong
328
329   Validators come in two flavors: strong or weak.  Weak validators are
330   easy to generate but are far less useful for comparisons.  Strong
331   validators are ideal for comparisons but can be very difficult (and
332
333
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339
340   occasionally impossible) to generate efficiently.  Rather than impose
341   that all forms of resource adhere to the same strength of validator,
342   HTTP exposes the type of validator in use and imposes restrictions on
343   when weak validators can be used as preconditions.
344
345   A "strong validator" is a representation metadata value that MUST be
346   changed to a new, previously unused or guaranteed unique, value
347   whenever a change occurs to the representation data such that a
348   change would be observable in the payload body of a 200 response to
349   GET.  A strong validator MAY be changed for other reasons, such as
350   when a semantically significant part of the representation metadata
351   is changed (e.g., Content-Type), but it is in the best interests of
352   the origin server to only change the value when it is necessary to
353   invalidate the stored responses held by remote caches and authoring
354   tools.  A strong validator MUST be unique across all representations
355   of a given resource, such that no two representations of that
356   resource share the same validator unless their payload body would be
357   identical.
358
359   Cache entries might persist for arbitrarily long periods, regardless
360   of expiration times.  Thus, a cache might attempt to validate an
361   entry using a validator that it obtained in the distant past.  A
362   strong validator MUST be unique across all versions of all
363   representations associated with a particular resource over time.
364   However, there is no implication of uniqueness across representations
365   of different resources (i.e., the same strong validator might be in
366   use for representations of multiple resources at the same time and
367   does not imply that those representations are equivalent).
368
369   There are a variety of strong validators used in practice.  The best
370   are based on strict revision control, wherein each change to a
371   representation always results in a unique node name and revision
372   identifier being assigned before the representation is made
373   accessible to GET.  A cryptographic hash function applied to the
374   representation data is also sufficient if the data is available prior
375   to the response header fields being sent and the digest does not need
376   to be recalculated every time a validation request is received.
377   However, if a resource has distinct representations that differ only
378   in their metadata, such as might occur with content negotiation over
379   media types that happen to share the same data format, then a server
380   SHOULD incorporate additional information in the validator to
381   distinguish those representations and avoid confusing cache behavior.
382
383   In contrast, a "weak validator" is a representation metadata value
384   that might not be changed for every change to the representation
385   data.  This weakness might be due to limitations in how the value is
386   calculated, such as clock resolution or an inability to ensure
387   uniqueness for all possible representations of the resource, or due
388
389
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395
396   to a desire by the resource owner to group representations by some
397   self-determined set of equivalency rather than unique sequences of
398   data.  A weak entity-tag SHOULD change whenever the origin server
399   considers prior representations to be unacceptable as a substitute
400   for the current representation.  In other words, a weak entity-tag
401   SHOULD change whenever the origin server wants caches to invalidate
402   old responses.
403
404   For example, the representation of a weather report that changes in
405   content every second, based on dynamic measurements, might be grouped
406   into sets of equivalent representations (from the origin server's
407   perspective) with the same weak validator in order to allow cached
408   representations to be valid for a reasonable period of time (perhaps
409   adjusted dynamically based on server load or weather quality).
410   Likewise, a representation's modification time, if defined with only
411   one-second resolution, might be a weak validator if it is possible
412   for the representation to be modified twice during a single second
413   and retrieved between those modifications.
414
415   A "use" of a validator occurs when either a client generates a
416   request and includes the validator in a precondition or when a server
417   compares two validators.  Weak validators are only usable in contexts
418   that do not depend on exact equality of a representation's payload
419   body.  Strong validators are usable and preferred for all conditional
420   requests, including cache validation, partial content ranges, and
421   "lost update" avoidance.
422
4232.2.  Last-Modified
424
425   The "Last-Modified" header field indicates the date and time at which
426   the origin server believes the selected representation was last
427   modified.
428
429     Last-Modified = HTTP-date
430
431   An example of its use is
432
433     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
434
4352.2.1.  Generation
436
437   Origin servers SHOULD send Last-Modified for any selected
438   representation for which a last modification date can be reasonably
439   and consistently determined, since its use in conditional requests
440   and evaluating cache freshness ([Part6]) results in a substantial
441   reduction of HTTP traffic on the Internet and can be a significant
442   factor in improving service scalability and reliability.
443
444
445
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451
452   A representation is typically the sum of many parts behind the
453   resource interface.  The last-modified time would usually be the most
454   recent time that any of those parts were changed.  How that value is
455   determined for any given resource is an implementation detail beyond
456   the scope of this specification.  What matters to HTTP is how
457   recipients of the Last-Modified header field can use its value to
458   make conditional requests and test the validity of locally cached
459   responses.
460
461   An origin server SHOULD obtain the Last-Modified value of the
462   representation as close as possible to the time that it generates the
463   Date field-value for its response.  This allows a recipient to make
464   an accurate assessment of the representation's modification time,
465   especially if the representation changes near the time that the
466   response is generated.
467
468   An origin server with a clock MUST NOT send a Last-Modified date that
469   is later than the server's time of message origination (Date).  If
470   the last modification time is derived from implementation-specific
471   metadata that evaluates to some time in the future, according to the
472   origin server's clock, then the origin server MUST replace that value
473   with the message origination date.  This prevents a future
474   modification date from having an adverse impact on cache validation.
475
476   An origin server without a clock MUST NOT assign Last-Modified values
477   to a response unless these values were associated with the resource
478   by some other system or user with a reliable clock.
479
4802.2.2.  Comparison
481
482   A Last-Modified time, when used as a validator in a request, is
483   implicitly weak unless it is possible to deduce that it is strong,
484   using the following rules:
485
486   o  The validator is being compared by an origin server to the actual
487      current validator for the representation and,
488
489   o  That origin server reliably knows that the associated
490      representation did not change twice during the second covered by
491      the presented validator.
492
493   or
494
495   o  The validator is about to be used by a client in an If-Modified-
496      Since, If-Unmodified-Since header field, because the client has a
497      cache entry, or If-Range for the associated representation, and
498
499
500
501
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507
508   o  That cache entry includes a Date value, which gives the time when
509      the origin server sent the original response, and
510
511   o  The presented Last-Modified time is at least 60 seconds before the
512      Date value.
513
514   or
515
516   o  The validator is being compared by an intermediate cache to the
517      validator stored in its cache entry for the representation, and
518
519   o  That cache entry includes a Date value, which gives the time when
520      the origin server sent the original response, and
521
522   o  The presented Last-Modified time is at least 60 seconds before the
523      Date value.
524
525   This method relies on the fact that if two different responses were
526   sent by the origin server during the same second, but both had the
527   same Last-Modified time, then at least one of those responses would
528   have a Date value equal to its Last-Modified time.  The arbitrary 60-
529   second limit guards against the possibility that the Date and Last-
530   Modified values are generated from different clocks, or at somewhat
531   different times during the preparation of the response.  An
532   implementation MAY use a value larger than 60 seconds, if it is
533   believed that 60 seconds is too short.
534
5352.3.  ETag
536
537   The ETag header field provides the current entity-tag for the
538   selected representation.  An entity-tag is an opaque validator for
539   differentiating between multiple representations of the same
540   resource, regardless of whether those multiple representations are
541   due to resource state changes over time, content negotiation
542   resulting in multiple representations being valid at the same time,
543   or both.  An entity-tag consists of an opaque quoted string, possibly
544   prefixed by a weakness indicator.
545
546     ETag       = entity-tag
547
548     entity-tag = [ weak ] opaque-tag
549     weak       = %x57.2F ; "W/", case-sensitive
550     opaque-tag = quoted-string
551
552   An entity-tag can be more reliable for validation than a modification
553   date in situations where it is inconvenient to store modification
554   dates, where the one-second resolution of HTTP date values is not
555   sufficient, or where modification dates are not consistently
556
557
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563
564   maintained.
565
566   Examples:
567
568     ETag: "xyzzy"
569     ETag: W/"xyzzy"
570     ETag: ""
571
572   An entity-tag can be either a weak or strong validator, with strong
573   being the default.  If an origin server provides an entity-tag for a
574   representation and the generation of that entity-tag does not satisfy
575   the requirements for a strong validator (Section 2.1), then that
576   entity-tag MUST be marked as weak by prefixing its opaque value with
577   "W/" (case-sensitive).
578
5792.3.1.  Generation
580
581   The principle behind entity-tags is that only the service author
582   knows the implementation of a resource well enough to select the most
583   accurate and efficient validation mechanism for that resource, and
584   that any such mechanism can be mapped to a simple sequence of octets
585   for easy comparison.  Since the value is opaque, there is no need for
586   the client to be aware of how each entity-tag is constructed.
587
588   For example, a resource that has implementation-specific versioning
589   applied to all changes might use an internal revision number, perhaps
590   combined with a variance identifier for content negotiation, to
591   accurately differentiate between representations.  Other
592   implementations might use a stored hash of representation content, a
593   combination of various filesystem attributes, or a modification
594   timestamp that has sub-second resolution.
595
596   Origin servers SHOULD send ETag for any selected representation for
597   which detection of changes can be reasonably and consistently
598   determined, since the entity-tag's use in conditional requests and
599   evaluating cache freshness ([Part6]) can result in a substantial
600   reduction of HTTP network traffic and can be a significant factor in
601   improving service scalability and reliability.
602
6032.3.2.  Comparison
604
605   There are two entity-tag comparison functions, depending on whether
606   the comparison context allows the use of weak validators or not:
607
608   o  The strong comparison function: in order to be considered equal,
609      both opaque-tags MUST be identical character-by-character, and
610      both MUST NOT be weak.
611
612
613
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619
620   o  The weak comparison function: in order to be considered equal,
621      both opaque-tags MUST be identical character-by-character, but
622      either or both of them MAY be tagged as "weak" without affecting
623      the result.
624
625   The example below shows the results for a set of entity-tag pairs,
626   and both the weak and strong comparison function results:
627
628   +--------+--------+-------------------+-----------------+
629   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
630   +--------+--------+-------------------+-----------------+
631   | W/"1"  | W/"1"  | no match          | match           |
632   | W/"1"  | W/"2"  | no match          | no match        |
633   | W/"1"  | "1"    | no match          | match           |
634   | "1"    | "1"    | match             | match           |
635   +--------+--------+-------------------+-----------------+
636
6372.3.3.  Example: Entity-tags varying on Content-Negotiated Resources
638
639   Consider a resource that is subject to content negotiation (Section 5
640   of [Part3]), and where the representations returned upon a GET
641   request vary based on the Accept-Encoding request header field
642   (Section 6.3 of [Part3]):
643
644   >> Request:
645
646     GET /index HTTP/1.1
647     Host: www.example.com
648     Accept-Encoding: gzip
649
650
651   In this case, the response might or might not use the gzip content
652   coding.  If it does not, the response might look like:
653
654   >> Response:
655
656     HTTP/1.1 200 OK
657     Date: Thu, 26 Mar 2010 00:05:00 GMT
658     ETag: "123-a"
659     Content-Length: 70
660     Vary: Accept-Encoding
661     Content-Type: text/plain
662
663     Hello World!
664     Hello World!
665     Hello World!
666     Hello World!
667     Hello World!
668
669
670
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675
676   An alternative representation that does use gzip content coding would
677   be:
678
679   >> Response:
680
681     HTTP/1.1 200 OK
682     Date: Thu, 26 Mar 2010 00:05:00 GMT
683     ETag: "123-b"
684     Content-Length: 43
685     Vary: Accept-Encoding
686     Content-Type: text/plain
687     Content-Encoding: gzip
688
689     ...binary data...
690
691      Note: Content codings are a property of the representation, so
692      therefore an entity-tag of an encoded representation must be
693      distinct from an unencoded representation to prevent conflicts
694      during cache updates and range requests.  In contrast, transfer
695      codings (Section 5.1 of [Part1]) apply only during message
696      transfer and do not require distinct entity-tags.
697
6982.4.  Rules for When to Use Entity-tags and Last-Modified Dates
699
700   We adopt a set of rules and recommendations for origin servers,
701   clients, and caches regarding when various validator types ought to
702   be used, and for what purposes.
703
704   HTTP/1.1 origin servers:
705
706   o  SHOULD send an entity-tag validator unless it is not feasible to
707      generate one.
708
709   o  MAY send a weak entity-tag instead of a strong entity-tag, if
710      performance considerations support the use of weak entity-tags, or
711      if it is unfeasible to send a strong entity-tag.
712
713   o  SHOULD send a Last-Modified value if it is feasible to send one.
714
715   In other words, the preferred behavior for an HTTP/1.1 origin server
716   is to send both a strong entity-tag and a Last-Modified value.
717
718   HTTP/1.1 clients:
719
720   o  MUST use that entity-tag in any cache-conditional request (using
721      If-Match or If-None-Match) if an entity-tag has been provided by
722      the origin server.
723
724
725
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731
732   o  SHOULD use the Last-Modified value in non-subrange cache-
733      conditional requests (using If-Modified-Since) if only a Last-
734      Modified value has been provided by the origin server.
735
736   o  MAY use the Last-Modified value in subrange cache-conditional
737      requests (using If-Unmodified-Since) if only a Last-Modified value
738      has been provided by an HTTP/1.0 origin server.  The user agent
739      SHOULD provide a way to disable this, in case of difficulty.
740
741   o  SHOULD use both validators in cache-conditional requests if both
742      an entity-tag and a Last-Modified value have been provided by the
743      origin server.  This allows both HTTP/1.0 and HTTP/1.1 caches to
744      respond appropriately.
745
746   An HTTP/1.1 origin server, upon receiving a conditional request that
747   includes both a Last-Modified date (e.g., in an If-Modified-Since or
748   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
749   in an If-Match, If-None-Match, or If-Range header field) as cache
750   validators, MUST NOT return a response status code of 304 (Not
751   Modified) unless doing so is consistent with all of the conditional
752   header fields in the request.
753
754   An HTTP/1.1 caching proxy, upon receiving a conditional request that
755   includes both a Last-Modified date and one or more entity-tags as
756   cache validators, MUST NOT return a locally cached response to the
757   client unless that cached response is consistent with all of the
758   conditional header fields in the request.
759
760      Note: The general principle behind these rules is that HTTP/1.1
761      servers and clients ought to transmit as much non-redundant
762      information as is available in their responses and requests.
763      HTTP/1.1 systems receiving this information will make the most
764      conservative assumptions about the validators they receive.
765
766      HTTP/1.0 clients and caches might ignore entity-tags.  Generally,
767      last-modified values received or used by these systems will
768      support transparent and efficient caching, and so HTTP/1.1 origin
769      servers should provide Last-Modified values.  In those rare cases
770      where the use of a Last-Modified value as a validator by an
771      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
772      origin servers should not provide one.
773
7743.  Precondition Header Fields
775
776   This section defines the syntax and semantics of HTTP/1.1 header
777   fields for applying preconditions on requests.
778
779
780
781
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787
7883.1.  If-Match
789
790   The "If-Match" header field MAY be used to make a request method
791   conditional on the current existence or value of an entity-tag for
792   one or more representations of the target resource.  If-Match is
793   generally useful for resource update requests, such as PUT requests,
794   as a means for protecting against accidental overwrites when multiple
795   clients are acting in parallel on the same resource (i.e., the "lost
796   update" problem).  An If-Match field-value of "*" places the
797   precondition on the existence of any current representation for the
798   target resource.
799
800     If-Match = "*" / 1#entity-tag
801
802   If any of the entity-tags listed in the If-Match field value match
803   (as per Section 2.3.2) the entity-tag of the selected representation
804   for the target resource, or if "*" is given and any current
805   representation exists for the target resource, then the server MAY
806   perform the request method as if the If-Match header field was not
807   present.
808
809   If none of the entity-tags match, or if "*" is given and no current
810   representation exists, the server MUST NOT perform the requested
811   method.  Instead, the server MUST respond with the 412 (Precondition
812   Failed) status code.
813
814   If the request would, without the If-Match header field, result in
815   anything other than a 2xx or 412 status code, then the If-Match
816   header field MUST be ignored.
817
818   Examples:
819
820     If-Match: "xyzzy"
821     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
822     If-Match: *
823
824   The result of a request having both an If-Match header field and
825   either an If-None-Match or an If-Modified-Since header fields is
826   undefined by this specification.
827
8283.2.  If-None-Match
829
830   The "If-None-Match" header field MAY be used to make a request method
831   conditional on not matching any of the current entity-tag values for
832   representations of the target resource.  If-None-Match is primarily
833   used in conditional GET requests to enable efficient updates of
834   cached information with a minimum amount of transaction overhead.  A
835   client that has one or more representations previously obtained from
836
837
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843
844   the target resource can send If-None-Match with a list of the
845   associated entity-tags in the hope of receiving a 304 response if at
846   least one of those representations matches the selected
847   representation.
848
849   If-None-Match MAY also be used with a value of "*" to prevent an
850   unsafe request method (e.g., PUT) from inadvertently modifying an
851   existing representation of the target resource when the client
852   believes that the resource does not have a current representation.
853   This is a variation on the "lost update" problem that might arise if
854   more than one client attempts to create an initial representation for
855   the target resource.
856
857     If-None-Match = "*" / 1#entity-tag
858
859   If any of the entity-tags listed in the If-None-Match field-value
860   match (as per Section 2.3.2) the entity-tag of the selected
861   representation, or if "*" is given and any current representation
862   exists for that resource, then the server MUST NOT perform the
863   requested method.  Instead, if the request method was GET or HEAD,
864   the server SHOULD respond with a 304 (Not Modified) status code,
865   including the cache-related header fields (particularly ETag) of the
866   selected representation that has a matching entity-tag.  For all
867   other request methods, the server MUST respond with a 412
868   (Precondition Failed) status code.
869
870   If none of the entity-tags match, then the server MAY perform the
871   requested method as if the If-None-Match header field did not exist,
872   but MUST also ignore any If-Modified-Since header field(s) in the
873   request.  That is, if no entity-tags match, then the server MUST NOT
874   return a 304 (Not Modified) response.
875
876   If the request would, without the If-None-Match header field, result
877   in anything other than a 2xx or 304 status code, then the If-None-
878   Match header field MUST be ignored.  (See Section 2.4 for a
879   discussion of server behavior when both If-Modified-Since and If-
880   None-Match appear in the same request.)
881
882   Examples:
883
884     If-None-Match: "xyzzy"
885     If-None-Match: W/"xyzzy"
886     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
887     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
888     If-None-Match: *
889
890   The result of a request having both an If-None-Match header field and
891   either an If-Match or an If-Unmodified-Since header fields is
892
893
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899
900   undefined by this specification.
901
9023.3.  If-Modified-Since
903
904   The "If-Modified-Since" header field MAY be used to make a request
905   method conditional by modification date: if the selected
906   representation has not been modified since the time specified in this
907   field, then do not perform the request method; instead, respond as
908   detailed below.
909
910     If-Modified-Since = HTTP-date
911
912   An example of the field is:
913
914     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
915
916   A GET method with an If-Modified-Since header field and no Range
917   header field requests that the selected representation be transferred
918   only if it has been modified since the date given by the If-Modified-
919   Since header field.  The algorithm for determining this includes the
920   following cases:
921
922   1.  If the request would normally result in anything other than a 200
923       (OK) status code, or if the passed If-Modified-Since date is
924       invalid, the response is exactly the same as for a normal GET.  A
925       date which is later than the server's current time is invalid.
926
927   2.  If the selected representation has been modified since the If-
928       Modified-Since date, the response is exactly the same as for a
929       normal GET.
930
931   3.  If the selected representation has not been modified since a
932       valid If-Modified-Since date, the server SHOULD return a 304 (Not
933       Modified) response.
934
935   The purpose of this feature is to allow efficient updates of cached
936   information with a minimum amount of transaction overhead.
937
938      Note: The Range header field modifies the meaning of If-Modified-
939      Since; see Section 5.4 of [Part5] for full details.
940
941      Note: If-Modified-Since times are interpreted by the server, whose
942      clock might not be synchronized with the client.
943
944      Note: When handling an If-Modified-Since header field, some
945      servers will use an exact date comparison function, rather than a
946      less-than function, for deciding whether to send a 304 (Not
947      Modified) response.  To get best results when sending an If-
948
949
950
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955
956      Modified-Since header field for cache validation, clients are
957      advised to use the exact date string received in a previous Last-
958      Modified header field whenever possible.
959
960      Note: If a client uses an arbitrary date in the If-Modified-Since
961      header field instead of a date taken from the Last-Modified header
962      field for the same request, the client needs to be aware that this
963      date is interpreted in the server's understanding of time.
964      Unsynchronized clocks and rounding problems, due to the different
965      encodings of time between the client and server, are concerns.
966      This includes the possibility of race conditions if the document
967      has changed between the time it was first requested and the If-
968      Modified-Since date of a subsequent request, and the possibility
969      of clock-skew-related problems if the If-Modified-Since date is
970      derived from the client's clock without correction to the server's
971      clock.  Corrections for different time bases between client and
972      server are at best approximate due to network latency.
973
974   The result of a request having both an If-Modified-Since header field
975   and either an If-Match or an If-Unmodified-Since header fields is
976   undefined by this specification.
977
9783.4.  If-Unmodified-Since
979
980   The "If-Unmodified-Since" header field MAY be used to make a request
981   method conditional by modification date: if the selected
982   representation has been modified since the time specified in this
983   field, then the server MUST NOT perform the requested operation and
984   MUST instead respond with the 412 (Precondition Failed) status code.
985   If the selected representation has not been modified since the time
986   specified in this field, the server SHOULD perform the request method
987   as if the If-Unmodified-Since header field were not present.
988
989     If-Unmodified-Since = HTTP-date
990
991   An example of the field is:
992
993     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
994
995   If the request normally (i.e., without the If-Unmodified-Since header
996   field) would result in anything other than a 2xx or 412 status code,
997   the If-Unmodified-Since header field SHOULD be ignored.
998
999   If the specified date is invalid, the header field MUST be ignored.
1000
1001   The result of a request having both an If-Unmodified-Since header
1002   field and either an If-None-Match or an If-Modified-Since header
1003   fields is undefined by this specification.
1004
1005
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1011
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 9.2 of
1035   [Part2]) unless the origin server does not have a clock that can
1036   provide a reasonable approximation of the current time.  If a 200
1037   response to the same request would have included any of the header
1038   fields Cache-Control, Content-Location, ETag, Expires, Last-Modified,
1039   or Vary, then those same header fields MUST be sent in a 304
1040   response.
1041
1042   Since the goal of a 304 response is to minimize information transfer
1043   when the recipient already has one or more cached representations,
1044   the response SHOULD NOT include representation metadata other than
1045   the above listed fields unless said metadata exists for the purpose
1046   of guiding cache updates (e.g., future HTTP extensions).
1047
1048   If the recipient of a 304 response does not have a cached
1049   representation corresponding to the entity-tag indicated by the 304
1050   response, then the recipient MUST NOT use the 304 to update its own
1051   cache.  If this conditional request originated with an outbound
1052   client, such as a user agent with its own cache sending a conditional
1053   GET to a shared proxy, then the 304 response MAY be forwarded to the
1054   outbound client.  Otherwise, the recipient MUST disregard the 304
1055   response and repeat the request without any preconditions.
1056
1057   If a cache uses a received 304 response to update a cache entry, the
1058   cache MUST update the entry to reflect any new field values given in
1059   the response.
1060
1061
1062
1063Fielding, et al.           Expires May 3, 2012                 [Page 19]
1064
1065Internet-Draft              HTTP/1.1, Part 4                October 2011
1066
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 May 3, 2012                 [Page 20]
1120
1121Internet-Draft              HTTP/1.1, Part 4                October 2011
1122
1123
11247.  Acknowledgments
1125
1126   See Section 11 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-17
1136              (work in progress), October 2011.
1137
1138   [Part2]    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 2: Message
1141              Semantics", draft-ietf-httpbis-p2-semantics-17 (work in
1142              progress), October 2011.
1143
1144   [Part3]    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 3: Message Payload
1147              and Content Negotiation", draft-ietf-httpbis-p3-payload-17
1148              (work in progress), October 2011.
1149
1150   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1151              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1152              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1153              Partial Responses", draft-ietf-httpbis-p5-range-17 (work
1154              in progress), October 2011.
1155
1156   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1157              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1158              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
1159              6: Caching", draft-ietf-httpbis-p6-cache-17 (work in
1160              progress), October 2011.
1161
1162   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1163              Requirement Levels", BCP 14, RFC 2119, March 1997.
1164
1165   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1166              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1167
11688.2.  Informative References
1169
1170   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1171              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1172
1173
1174
1175Fielding, et al.           Expires May 3, 2012                 [Page 21]
1176
1177Internet-Draft              HTTP/1.1, Part 4                October 2011
1178
1179
1180              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1181
1182   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1183              Procedures for Message Header Fields", BCP 90, RFC 3864,
1184              September 2004.
1185
1186   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
1187              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
1188
1189Appendix A.  Changes from RFC 2616
1190
1191   Allow weak entity-tags in all requests except range requests
1192   (Sections 2.1 and 3.2).
1193
1194   Change ABNF productions for header fields to only define the field
1195   value.  (Section 3)
1196
1197Appendix B.  Collected ABNF
1198
1199   ETag = entity-tag
1200
1201   HTTP-date = <HTTP-date, defined in [Part2], Section 8>
1202
1203   If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1204    entity-tag ] ) )
1205   If-Modified-Since = HTTP-date
1206   If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1207    entity-tag ] ) )
1208   If-Unmodified-Since = HTTP-date
1209
1210   Last-Modified = HTTP-date
1211
1212   OWS = <OWS, defined in [Part1], Section 1.2.2>
1213
1214   entity-tag = [ weak ] opaque-tag
1215
1216   opaque-tag = quoted-string
1217
1218   quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
1219
1220   weak = %x57.2F ; W/
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231Fielding, et al.           Expires May 3, 2012                 [Page 22]
1232
1233Internet-Draft              HTTP/1.1, Part 4                October 2011
1234
1235
1236   ABNF diagnostics:
1237
1238   ; ETag defined but not used
1239   ; If-Match defined but not used
1240   ; If-Modified-Since defined but not used
1241   ; If-None-Match defined but not used
1242   ; If-Unmodified-Since defined but not used
1243   ; Last-Modified defined but not used
1244
1245Appendix C.  Change Log (to be removed by RFC Editor before publication)
1246
1247C.1.  Since RFC 2616
1248
1249   Extracted relevant partitions from [RFC2616].
1250
1251C.2.  Since draft-ietf-httpbis-p4-conditional-00
1252
1253   Closed issues:
1254
1255   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1256      Informative references"
1257
1258   Other changes:
1259
1260   o  Move definitions of 304 and 412 condition codes from Part2.
1261
1262C.3.  Since draft-ietf-httpbis-p4-conditional-01
1263
1264   Ongoing work on ABNF conversion
1265   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1266
1267   o  Add explicit references to BNF syntax and rules imported from
1268      other parts of the specification.
1269
1270C.4.  Since draft-ietf-httpbis-p4-conditional-02
1271
1272   Closed issues:
1273
1274   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1275      non-GET requests"
1276
1277   Ongoing work on IANA Message Header Field Registration
1278   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1279
1280   o  Reference RFC 3984, and update header field registrations for
1281      header fields defined in this document.
1282
1283
1284
1285
1286
1287Fielding, et al.           Expires May 3, 2012                 [Page 23]
1288
1289Internet-Draft              HTTP/1.1, Part 4                October 2011
1290
1291
1292C.5.  Since draft-ietf-httpbis-p4-conditional-03
1293
1294   Closed issues:
1295
1296   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for
1297      ETag matching"
1298
1299   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity
1300      value' undefined"
1301
1302   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068
1303      Date header reference"
1304
1305C.6.  Since draft-ietf-httpbis-p4-conditional-04
1306
1307   Ongoing work on ABNF conversion
1308   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1309
1310   o  Use "/" instead of "|" for alternatives.
1311
1312   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1313      whitespace ("OWS") and required whitespace ("RWS").
1314
1315   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1316      field value format definitions.
1317
1318C.7.  Since draft-ietf-httpbis-p4-conditional-05
1319
1320   Final work on ABNF conversion
1321   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1322
1323   o  Add appendix containing collected and expanded ABNF, reorganize
1324      ABNF introduction.
1325
1326C.8.  Since draft-ietf-httpbis-p4-conditional-06
1327
1328   Closed issues:
1329
1330   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-
1331      sensitivity of etag weakness indicator"
1332
1333C.9.  Since draft-ietf-httpbis-p4-conditional-07
1334
1335   Closed issues:
1336
1337   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1338      non-GET requests" (If-Match still was defined to require strong
1339      matching)
1340
1341
1342
1343Fielding, et al.           Expires May 3, 2012                 [Page 24]
1344
1345Internet-Draft              HTTP/1.1, Part 4                October 2011
1346
1347
1348   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
1349      registrations for optional status codes"
1350
1351C.10.  Since draft-ietf-httpbis-p4-conditional-08
1352
1353   No significant changes.
1354
1355C.11.  Since draft-ietf-httpbis-p4-conditional-09
1356
1357   No significant changes.
1358
1359C.12.  Since draft-ietf-httpbis-p4-conditional-10
1360
1361   Closed issues:
1362
1363   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
1364      'Requested Variant'"
1365
1366   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
1367      entity / representation / variant terminology"
1368
1369   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
1370      removing the 'changes from 2068' sections"
1371
1372C.13.  Since draft-ietf-httpbis-p4-conditional-11
1373
1374   None.
1375
1376C.14.  Since draft-ietf-httpbis-p4-conditional-12
1377
1378   Closed issues:
1379
1380   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
1381      Classification"
1382
1383C.15.  Since draft-ietf-httpbis-p4-conditional-13
1384
1385   Closed issues:
1386
1387   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/89>: "If-* and
1388      entities"
1389
1390   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/101>: "Definition of
1391      validator weakness"
1392
1393   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
1394      ABNFs for header fields"
1395
1396
1397
1398
1399Fielding, et al.           Expires May 3, 2012                 [Page 25]
1400
1401Internet-Draft              HTTP/1.1, Part 4                October 2011
1402
1403
1404   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/269>: "ETags and
1405      Quotes"
1406
1407C.16.  Since draft-ietf-httpbis-p4-conditional-14
1408
1409   None.
1410
1411C.17.  Since draft-ietf-httpbis-p4-conditional-15
1412
1413   Closed issues:
1414
1415   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/304>: "If-Range
1416      should be listed when dicussing contexts where L-M can be
1417      considered strong"
1418
1419C.18.  Since draft-ietf-httpbis-p4-conditional-16
1420
1421   Closed issues:
1422
1423   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document
1424      HTTP's error-handling philosophy"
1425
1426Index
1427
1428   3
1429      304 Not Modified (status code)  19
1430
1431   4
1432      412 Precondition Failed (status code)  20
1433
1434   E
1435      ETag header field  10
1436
1437   G
1438      Grammar
1439         entity-tag  10
1440         ETag  10
1441         If-Match  15
1442         If-Modified-Since  17
1443         If-None-Match  16
1444         If-Unmodified-Since  18
1445         Last-Modified  8
1446         opaque-tag  10
1447         weak  10
1448
1449   H
1450      Header Fields
1451         ETag  10
1452
1453
1454
1455Fielding, et al.           Expires May 3, 2012                 [Page 26]
1456
1457Internet-Draft              HTTP/1.1, Part 4                October 2011
1458
1459
1460         If-Match  15
1461         If-Modified-Since  17
1462         If-None-Match  15
1463         If-Unmodified-Since  18
1464         Last-Modified  8
1465
1466   I
1467      If-Match header field  15
1468      If-Modified-Since header field  17
1469      If-None-Match header field  15
1470      If-Unmodified-Since header field  18
1471
1472   L
1473      Last-Modified header field  8
1474
1475   M
1476      metadata  6
1477
1478   S
1479      selected representation  5
1480      Status Codes
1481         304 Not Modified  19
1482         412 Precondition Failed  20
1483
1484   V
1485      validator  6
1486         strong  6
1487         weak  6
1488
1489Authors' Addresses
1490
1491   Roy T. Fielding (editor)
1492   Adobe Systems Incorporated
1493   345 Park Ave
1494   San Jose, CA  95110
1495   USA
1496
1497   EMail: fielding@gbiv.com
1498   URI:   http://roy.gbiv.com/
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511Fielding, et al.           Expires May 3, 2012                 [Page 27]
1512
1513Internet-Draft              HTTP/1.1, Part 4                October 2011
1514
1515
1516   Jim Gettys
1517   Alcatel-Lucent Bell Labs
1518   21 Oak Knoll Road
1519   Carlisle, MA  01741
1520   USA
1521
1522   EMail: jg@freedesktop.org
1523   URI:   http://gettys.wordpress.com/
1524
1525
1526   Jeffrey C. Mogul
1527   Hewlett-Packard Company
1528   HP Labs, Large Scale Systems Group
1529   1501 Page Mill Road, MS 1177
1530   Palo Alto, CA  94304
1531   USA
1532
1533   EMail: JeffMogul@acm.org
1534
1535
1536   Henrik Frystyk Nielsen
1537   Microsoft Corporation
1538   1 Microsoft Way
1539   Redmond, WA  98052
1540   USA
1541
1542   EMail: henrikn@microsoft.com
1543
1544
1545   Larry Masinter
1546   Adobe Systems Incorporated
1547   345 Park Ave
1548   San Jose, CA  95110
1549   USA
1550
1551   EMail: LMM@acm.org
1552   URI:   http://larry.masinter.net/
1553
1554
1555   Paul J. Leach
1556   Microsoft Corporation
1557   1 Microsoft Way
1558   Redmond, WA  98052
1559
1560   EMail: paulle@microsoft.com
1561
1562
1563
1564
1565
1566
1567Fielding, et al.           Expires May 3, 2012                 [Page 28]
1568
1569Internet-Draft              HTTP/1.1, Part 4                October 2011
1570
1571
1572   Tim Berners-Lee
1573   World Wide Web Consortium
1574   MIT Computer Science and Artificial Intelligence Laboratory
1575   The Stata Center, Building 32
1576   32 Vassar Street
1577   Cambridge, MA  02139
1578   USA
1579
1580   EMail: timbl@w3.org
1581   URI:   http://www.w3.org/People/Berners-Lee/
1582
1583
1584   Yves Lafon (editor)
1585   World Wide Web Consortium
1586   W3C / ERCIM
1587   2004, rte des Lucioles
1588   Sophia-Antipolis, AM  06902
1589   France
1590
1591   EMail: ylafon@w3.org
1592   URI:   http://www.raubacapeu.net/people/yves/
1593
1594
1595   Julian F. Reschke (editor)
1596   greenbytes GmbH
1597   Hafenweg 16
1598   Muenster, NW  48155
1599   Germany
1600
1601   Phone: +49 251 2807760
1602   Fax:   +49 251 2807761
1603   EMail: julian.reschke@greenbytes.de
1604   URI:   http://greenbytes.de/tech/webdav/
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623Fielding, et al.           Expires May 3, 2012                 [Page 29]
1624
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