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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: July 7, 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                                                         January 4, 2012
23
24
25                 HTTP/1.1, part 4: Conditional Requests
26                  draft-ietf-httpbis-p4-conditional-18
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.19.
52
53
54
55Fielding, et al.          Expires July 7, 2012                  [Page 1]
56
57Internet-Draft              HTTP/1.1, Part 4                January 2012
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 July 7, 2012.
76
77Copyright Notice
78
79   Copyright (c) 2012 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 July 7, 2012                  [Page 2]
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113Internet-Draft              HTTP/1.1, Part 4                January 2012
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 . . . . . . . . . . . . . . . . . . . . . . 12
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 . . . . . . . . . . . . . . . . . . 15
130     3.1.  If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 15
131     3.2.  If-None-Match  . . . . . . . . . . . . . . . . . . . . . . 16
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  . . . . . . . . . . . . . . . . . . . 21
142   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
143   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
144     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
145     8.2.  Informative References . . . . . . . . . . . . . . . . . . 22
146   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
147   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 23
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 . . . . . . . . 24
153     C.4.  Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 24
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 . . . . . . . . 25
157     C.8.  Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 25
158     C.9.  Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 25
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 . . . . . . . . 26
163     C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 26
164
165
166
167Fielding, et al.          Expires July 7, 2012                  [Page 3]
168
169Internet-Draft              HTTP/1.1, Part 4                January 2012
170
171
172     C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 26
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     C.19. Since draft-ietf-httpbis-p4-conditional-17 . . . . . . . . 27
177   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
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216
217
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220
221
222
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225Internet-Draft              HTTP/1.1, Part 4                January 2012
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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     obs-text      = <obs-text, 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
502
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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 = DQUOTE *etagc DQUOTE
551     etagc      = %x21 / %x23-7E / obs-text
552                ; VCHAR except double quotes, plus obs-text
553
554
555
556
557
558
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563
564      Note: Previously, opaque-tag was defined to be a quoted-string
565      ([RFC2616], Section 3.11), thus some recipients might perform
566      backslash unescaping.  Servers therefore ought to avoid backslash
567      characters in entity tags.
568
569   An entity-tag can be more reliable for validation than a modification
570   date in situations where it is inconvenient to store modification
571   dates, where the one-second resolution of HTTP date values is not
572   sufficient, or where modification dates are not consistently
573   maintained.
574
575   Examples:
576
577     ETag: "xyzzy"
578     ETag: W/"xyzzy"
579     ETag: ""
580
581   An entity-tag can be either a weak or strong validator, with strong
582   being the default.  If an origin server provides an entity-tag for a
583   representation and the generation of that entity-tag does not satisfy
584   the requirements for a strong validator (Section 2.1), then that
585   entity-tag MUST be marked as weak by prefixing its opaque value with
586   "W/" (case-sensitive).
587
5882.3.1.  Generation
589
590   The principle behind entity-tags is that only the service author
591   knows the implementation of a resource well enough to select the most
592   accurate and efficient validation mechanism for that resource, and
593   that any such mechanism can be mapped to a simple sequence of octets
594   for easy comparison.  Since the value is opaque, there is no need for
595   the client to be aware of how each entity-tag is constructed.
596
597   For example, a resource that has implementation-specific versioning
598   applied to all changes might use an internal revision number, perhaps
599   combined with a variance identifier for content negotiation, to
600   accurately differentiate between representations.  Other
601   implementations might use a stored hash of representation content, a
602   combination of various filesystem attributes, or a modification
603   timestamp that has sub-second resolution.
604
605   Origin servers SHOULD send ETag for any selected representation for
606   which detection of changes can be reasonably and consistently
607   determined, since the entity-tag's use in conditional requests and
608   evaluating cache freshness ([Part6]) can result in a substantial
609   reduction of HTTP network traffic and can be a significant factor in
610   improving service scalability and reliability.
611
612
613
614
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619
6202.3.2.  Comparison
621
622   There are two entity-tag comparison functions, depending on whether
623   the comparison context allows the use of weak validators or not:
624
625   o  The strong comparison function: in order to be considered equal,
626      both opaque-tags MUST be identical character-by-character, and
627      both MUST NOT be weak.
628
629   o  The weak comparison function: in order to be considered equal,
630      both opaque-tags MUST be identical character-by-character, but
631      either or both of them MAY be tagged as "weak" without affecting
632      the result.
633
634   The example below shows the results for a set of entity-tag pairs,
635   and both the weak and strong comparison function results:
636
637   +--------+--------+-------------------+-----------------+
638   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
639   +--------+--------+-------------------+-----------------+
640   | W/"1"  | W/"1"  | no match          | match           |
641   | W/"1"  | W/"2"  | no match          | no match        |
642   | W/"1"  | "1"    | no match          | match           |
643   | "1"    | "1"    | match             | match           |
644   +--------+--------+-------------------+-----------------+
645
6462.3.3.  Example: Entity-tags varying on Content-Negotiated Resources
647
648   Consider a resource that is subject to content negotiation (Section 5
649   of [Part3]), and where the representations returned upon a GET
650   request vary based on the Accept-Encoding request header field
651   (Section 6.3 of [Part3]):
652
653   >> Request:
654
655     GET /index HTTP/1.1
656     Host: www.example.com
657     Accept-Encoding: gzip
658
659
660   In this case, the response might or might not use the gzip content
661   coding.  If it does not, the response might look like:
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-a"
681     Content-Length: 70
682     Vary: Accept-Encoding
683     Content-Type: text/plain
684
685     Hello World!
686     Hello World!
687     Hello World!
688     Hello World!
689     Hello World!
690
691   An alternative representation that does use gzip content coding would
692   be:
693
694   >> Response:
695
696     HTTP/1.1 200 OK
697     Date: Thu, 26 Mar 2010 00:05:00 GMT
698     ETag: "123-b"
699     Content-Length: 43
700     Vary: Accept-Encoding
701     Content-Type: text/plain
702     Content-Encoding: gzip
703
704     ...binary data...
705
706      Note: Content codings are a property of the representation, so
707      therefore an entity-tag of an encoded representation must be
708      distinct from an unencoded representation to prevent conflicts
709      during cache updates and range requests.  In contrast, transfer
710      codings (Section 5.1 of [Part1]) apply only during message
711      transfer and do not require distinct entity-tags.
712
7132.4.  Rules for When to Use Entity-tags and Last-Modified Dates
714
715   We adopt a set of rules and recommendations for origin servers,
716   clients, and caches regarding when various validator types ought to
717   be used, and for what purposes.
718
719   HTTP/1.1 origin servers:
720
721   o  SHOULD send an entity-tag validator unless it is not feasible to
722      generate one.
723
724
725
726
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731
732   o  MAY send a weak entity-tag instead of a strong entity-tag, if
733      performance considerations support the use of weak entity-tags, or
734      if it is unfeasible to send a strong entity-tag.
735
736   o  SHOULD send a Last-Modified value if it is feasible to send one.
737
738   In other words, the preferred behavior for an HTTP/1.1 origin server
739   is to send both a strong entity-tag and a Last-Modified value.
740
741   HTTP/1.1 clients:
742
743   o  MUST use that entity-tag in any cache-conditional request (using
744      If-Match or If-None-Match) if an entity-tag has been provided by
745      the origin server.
746
747   o  SHOULD use the Last-Modified value in non-subrange cache-
748      conditional requests (using If-Modified-Since) if only a Last-
749      Modified value has been provided by the origin server.
750
751   o  MAY use the Last-Modified value in subrange cache-conditional
752      requests (using If-Unmodified-Since) if only a Last-Modified value
753      has been provided by an HTTP/1.0 origin server.  The user agent
754      SHOULD provide a way to disable this, in case of difficulty.
755
756   o  SHOULD use both validators in cache-conditional requests if both
757      an entity-tag and a Last-Modified value have been provided by the
758      origin server.  This allows both HTTP/1.0 and HTTP/1.1 caches to
759      respond appropriately.
760
761   An HTTP/1.1 origin server, upon receiving a conditional request that
762   includes both a Last-Modified date (e.g., in an If-Modified-Since or
763   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
764   in an If-Match, If-None-Match, or If-Range header field) as cache
765   validators, MUST NOT return a response status code of 304 (Not
766   Modified) unless doing so is consistent with all of the conditional
767   header fields in the request.
768
769   An HTTP/1.1 caching proxy, upon receiving a conditional request that
770   includes both a Last-Modified date and one or more entity-tags as
771   cache validators, MUST NOT return a locally cached response to the
772   client unless that cached response is consistent with all of the
773   conditional header fields in the request.
774
775      Note: The general principle behind these rules is that HTTP/1.1
776      servers and clients ought to transmit as much non-redundant
777      information as is available in their responses and requests.
778      HTTP/1.1 systems receiving this information will make the most
779      conservative assumptions about the validators they receive.
780
781
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787
788      HTTP/1.0 clients and caches might ignore entity-tags.  Generally,
789      last-modified values received or used by these systems will
790      support transparent and efficient caching, and so HTTP/1.1 origin
791      servers should provide Last-Modified values.  In those rare cases
792      where the use of a Last-Modified value as a validator by an
793      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
794      origin servers should not provide one.
795
7963.  Precondition Header Fields
797
798   This section defines the syntax and semantics of HTTP/1.1 header
799   fields for applying preconditions on requests.
800
8013.1.  If-Match
802
803   The "If-Match" header field MAY be used to make a request method
804   conditional on the current existence or value of an entity-tag for
805   one or more representations of the target resource.  If-Match is
806   generally useful for resource update requests, such as PUT requests,
807   as a means for protecting against accidental overwrites when multiple
808   clients are acting in parallel on the same resource (i.e., the "lost
809   update" problem).  An If-Match field-value of "*" places the
810   precondition on the existence of any current representation for the
811   target resource.
812
813     If-Match = "*" / 1#entity-tag
814
815   If any of the entity-tags listed in the If-Match field value match
816   (as per Section 2.3.2) the entity-tag of the selected representation
817   for the target resource, or if "*" is given and any current
818   representation exists for the target resource, then the server MAY
819   perform the request method as if the If-Match header field was not
820   present.
821
822   If none of the entity-tags match, or if "*" is given and no current
823   representation exists, the server MUST NOT perform the requested
824   method.  Instead, the server MUST respond with the 412 (Precondition
825   Failed) status code.
826
827   If the request would, without the If-Match header field, result in
828   anything other than a 2xx or 412 status code, then the If-Match
829   header field MUST be ignored.
830
831   Examples:
832
833     If-Match: "xyzzy"
834     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
835     If-Match: *
836
837
838
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843
844   The result of a request having both an If-Match header field and
845   either an If-None-Match or an If-Modified-Since header fields is
846   undefined by this specification.
847
8483.2.  If-None-Match
849
850   The "If-None-Match" header field MAY be used to make a request method
851   conditional on not matching any of the current entity-tag values for
852   representations of the target resource.  If-None-Match is primarily
853   used in conditional GET requests to enable efficient updates of
854   cached information with a minimum amount of transaction overhead.  A
855   client that has one or more representations previously obtained from
856   the target resource can send If-None-Match with a list of the
857   associated entity-tags in the hope of receiving a 304 response if at
858   least one of those representations matches the selected
859   representation.
860
861   If-None-Match MAY also be used with a value of "*" to prevent an
862   unsafe request method (e.g., PUT) from inadvertently modifying an
863   existing representation of the target resource when the client
864   believes that the resource does not have a current representation.
865   This is a variation on the "lost update" problem that might arise if
866   more than one client attempts to create an initial representation for
867   the target resource.
868
869     If-None-Match = "*" / 1#entity-tag
870
871   If any of the entity-tags listed in the If-None-Match field-value
872   match (as per Section 2.3.2) the entity-tag of the selected
873   representation, or if "*" is given and any current representation
874   exists for that resource, then the server MUST NOT perform the
875   requested method.  Instead, if the request method was GET or HEAD,
876   the server SHOULD respond with a 304 (Not Modified) status code,
877   including the cache-related header fields (particularly ETag) of the
878   selected representation that has a matching entity-tag.  For all
879   other request methods, the server MUST respond with a 412
880   (Precondition Failed) status code.
881
882   If none of the entity-tags match, then the server MAY perform the
883   requested method as if the If-None-Match header field did not exist,
884   but MUST also ignore any If-Modified-Since header field(s) in the
885   request.  That is, if no entity-tags match, then the server MUST NOT
886   return a 304 (Not Modified) response.
887
888   If the request would, without the If-None-Match header field, result
889   in anything other than a 2xx or 304 status code, then the If-None-
890   Match header field MUST be ignored.  (See Section 2.4 for a
891   discussion of server behavior when both If-Modified-Since and If-
892
893
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899
900   None-Match appear in the same request.)
901
902   Examples:
903
904     If-None-Match: "xyzzy"
905     If-None-Match: W/"xyzzy"
906     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
907     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
908     If-None-Match: *
909
910   The result of a request having both an If-None-Match header field and
911   either an If-Match or an If-Unmodified-Since header fields is
912   undefined by this specification.
913
9143.3.  If-Modified-Since
915
916   The "If-Modified-Since" header field MAY be used to make a request
917   method conditional by modification date: if the selected
918   representation has not been modified since the time specified in this
919   field, then do not perform the request method; instead, respond as
920   detailed below.
921
922     If-Modified-Since = HTTP-date
923
924   An example of the field is:
925
926     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
927
928   A GET method with an If-Modified-Since header field and no Range
929   header field requests that the selected representation be transferred
930   only if it has been modified since the date given by the If-Modified-
931   Since header field.  The algorithm for determining this includes the
932   following cases:
933
934   1.  If the request would normally result in anything other than a 200
935       (OK) status code, or if the passed If-Modified-Since date is
936       invalid, the response is exactly the same as for a normal GET.  A
937       date which is later than the server's current time is invalid.
938
939   2.  If the selected representation has been modified since the If-
940       Modified-Since date, the response is exactly the same as for a
941       normal GET.
942
943   3.  If the selected representation has not been modified since a
944       valid If-Modified-Since date, the server SHOULD return a 304 (Not
945       Modified) response.
946
947   The purpose of this feature is to allow efficient updates of cached
948
949
950
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955
956   information with a minimum amount of transaction overhead.
957
958      Note: The Range header field modifies the meaning of If-Modified-
959      Since; see Section 5.4 of [Part5] for full details.
960
961      Note: If-Modified-Since times are interpreted by the server, whose
962      clock might not be synchronized with the client.
963
964      Note: When handling an If-Modified-Since header field, some
965      servers will use an exact date comparison function, rather than a
966      less-than function, for deciding whether to send a 304 (Not
967      Modified) response.  To get best results when sending an If-
968      Modified-Since header field for cache validation, clients are
969      advised to use the exact date string received in a previous Last-
970      Modified header field whenever possible.
971
972      Note: If a client uses an arbitrary date in the If-Modified-Since
973      header field instead of a date taken from the Last-Modified header
974      field for the same request, the client needs to be aware that this
975      date is interpreted in the server's understanding of time.
976      Unsynchronized clocks and rounding problems, due to the different
977      encodings of time between the client and server, are concerns.
978      This includes the possibility of race conditions if the document
979      has changed between the time it was first requested and the If-
980      Modified-Since date of a subsequent request, and the possibility
981      of clock-skew-related problems if the If-Modified-Since date is
982      derived from the client's clock without correction to the server's
983      clock.  Corrections for different time bases between client and
984      server are at best approximate due to network latency.
985
986   The result of a request having both an If-Modified-Since header field
987   and either an If-Match or an If-Unmodified-Since header fields is
988   undefined by this specification.
989
9903.4.  If-Unmodified-Since
991
992   The "If-Unmodified-Since" header field MAY be used to make a request
993   method conditional by modification date: if the selected
994   representation has been modified since the time specified in this
995   field, then the server MUST NOT perform the requested operation and
996   MUST instead respond with the 412 (Precondition Failed) status code.
997   If the selected representation has not been modified since the time
998   specified in this field, the server SHOULD perform the request method
999   as if the If-Unmodified-Since header field were not present.
1000
1001     If-Unmodified-Since = HTTP-date
1002
1003   An example of the field is:
1004
1005
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1011
1012     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1013
1014   If the request normally (i.e., without the If-Unmodified-Since header
1015   field) would result in anything other than a 2xx or 412 status code,
1016   the If-Unmodified-Since header field SHOULD be ignored.
1017
1018   If the specified date is invalid, the header field MUST be ignored.
1019
1020   The result of a request having both an If-Unmodified-Since header
1021   field and either an If-None-Match or an If-Modified-Since header
1022   fields is undefined by this specification.
1023
10243.5.  If-Range
1025
1026   The If-Range header field provides a special conditional request
1027   mechanism that is similar to If-Match and If-Unmodified-Since but
1028   specific to HTTP range requests.  If-Range is defined in Section 5.3
1029   of [Part5].
1030
10314.  Status Code Definitions
1032
10334.1.  304 Not Modified
1034
1035   The 304 status code indicates that a conditional GET request has been
1036   received and would have resulted in a 200 (OK) response if it were
1037   not for the fact that the condition has evaluated to false.  In other
1038   words, there is no need for the server to transfer a representation
1039   of the target resource because the client's request indicates that it
1040   already has a valid representation, as indicated by the 304 response
1041   header fields, and is therefore redirecting the client to make use of
1042   that stored representation as if it were the payload of a 200
1043   response.  The 304 response MUST NOT contain a message-body, and thus
1044   is always terminated by the first empty line after the header fields.
1045
1046   A 304 response MUST include a Date header field (Section 9.2 of
1047   [Part2]) unless the origin server does not have a clock that can
1048   provide a reasonable approximation of the current time.  If a 200
1049   response to the same request would have included any of the header
1050   fields Cache-Control, Content-Location, ETag, Expires, Last-Modified,
1051   or Vary, then those same header fields MUST be sent in a 304
1052   response.
1053
1054   Since the goal of a 304 response is to minimize information transfer
1055   when the recipient already has one or more cached representations,
1056   the response SHOULD NOT include representation metadata other than
1057   the above listed fields unless said metadata exists for the purpose
1058   of guiding cache updates (e.g., future HTTP extensions).
1059
1060
1061
1062
1063Fielding, et al.          Expires July 7, 2012                 [Page 19]
1064
1065Internet-Draft              HTTP/1.1, Part 4                January 2012
1066
1067
1068   If the recipient of a 304 response does not have a cached
1069   representation corresponding to the entity-tag indicated by the 304
1070   response, then the recipient MUST NOT use the 304 to update its own
1071   cache.  If this conditional request originated with an outbound
1072   client, such as a user agent with its own cache sending a conditional
1073   GET to a shared proxy, then the 304 response MAY be forwarded to the
1074   outbound client.  Otherwise, the recipient MUST disregard the 304
1075   response and repeat the request without any preconditions.
1076
1077   If a cache uses a received 304 response to update a cache entry, the
1078   cache MUST update the entry to reflect any new field values given in
1079   the response.
1080
10814.2.  412 Precondition Failed
1082
1083   The 412 status code indicates that one or more preconditions given in
1084   the request header fields evaluated to false when tested on the
1085   server.  This response code allows the client to place preconditions
1086   on the current resource state (its current representations and
1087   metadata) and thus prevent the request method from being applied if
1088   the target resource is in an unexpected state.
1089
10905.  IANA Considerations
1091
10925.1.  Status Code Registration
1093
1094   The HTTP Status Code Registry located at
1095   <http://www.iana.org/assignments/http-status-codes> shall be updated
1096   with the registrations below:
1097
1098   +-------+---------------------+-------------+
1099   | Value | Description         | Reference   |
1100   +-------+---------------------+-------------+
1101   | 304   | Not Modified        | Section 4.1 |
1102   | 412   | Precondition Failed | Section 4.2 |
1103   +-------+---------------------+-------------+
1104
11055.2.  Header Field Registration
1106
1107   The Message Header Field Registry located at <http://www.iana.org/
1108   assignments/message-headers/message-header-index.html> shall be
1109   updated with the permanent registrations below (see [RFC3864]):
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119Fielding, et al.          Expires July 7, 2012                 [Page 20]
1120
1121Internet-Draft              HTTP/1.1, Part 4                January 2012
1122
1123
1124   +---------------------+----------+----------+-------------+
1125   | Header Field Name   | Protocol | Status   | Reference   |
1126   +---------------------+----------+----------+-------------+
1127   | ETag                | http     | standard | Section 2.3 |
1128   | If-Match            | http     | standard | Section 3.1 |
1129   | If-Modified-Since   | http     | standard | Section 3.3 |
1130   | If-None-Match       | http     | standard | Section 3.2 |
1131   | If-Unmodified-Since | http     | standard | Section 3.4 |
1132   | Last-Modified       | http     | standard | Section 2.2 |
1133   +---------------------+----------+----------+-------------+
1134
1135   The change controller is: "IETF (iesg@ietf.org) - Internet
1136   Engineering Task Force".
1137
11386.  Security Considerations
1139
1140   No additional security considerations have been identified beyond
1141   those applicable to HTTP in general [Part1].
1142
11437.  Acknowledgments
1144
1145   See Section 11 of [Part1].
1146
11478.  References
1148
11498.1.  Normative References
1150
1151   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1152              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1153              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
1154              and Message Parsing", draft-ietf-httpbis-p1-messaging-18
1155              (work in progress), January 2012.
1156
1157   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1158              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1159              and J. Reschke, Ed., "HTTP/1.1, part 2: Message
1160              Semantics", draft-ietf-httpbis-p2-semantics-18 (work in
1161              progress), January 2012.
1162
1163   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1164              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1165              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
1166              and Content Negotiation", draft-ietf-httpbis-p3-payload-18
1167              (work in progress), January 2012.
1168
1169   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1170              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1171              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
1172
1173
1174
1175Fielding, et al.          Expires July 7, 2012                 [Page 21]
1176
1177Internet-Draft              HTTP/1.1, Part 4                January 2012
1178
1179
1180              Partial Responses", draft-ietf-httpbis-p5-range-18 (work
1181              in progress), January 2012.
1182
1183   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
1184              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
1185              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
1186              6: Caching", draft-ietf-httpbis-p6-cache-18 (work in
1187              progress), January 2012.
1188
1189   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1190              Requirement Levels", BCP 14, RFC 2119, March 1997.
1191
1192   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1193              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1194
11958.2.  Informative References
1196
1197   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1198              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1199              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1200
1201   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1202              Procedures for Message Header Fields", BCP 90, RFC 3864,
1203              September 2004.
1204
1205   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
1206              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
1207
1208Appendix A.  Changes from RFC 2616
1209
1210   Allow weak entity-tags in all requests except range requests
1211   (Sections 2.1 and 3.2).
1212
1213   Change ETag header field ABNF not to use quoted-string, thus avoiding
1214   escaping issues.  (Section 2.3)
1215
1216   Change ABNF productions for header fields to only define the field
1217   value.  (Section 3)
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231Fielding, et al.          Expires July 7, 2012                 [Page 22]
1232
1233Internet-Draft              HTTP/1.1, Part 4                January 2012
1234
1235
1236Appendix B.  Collected ABNF
1237
1238   ETag = entity-tag
1239
1240   HTTP-date = <HTTP-date, defined in [Part2], Section 8>
1241
1242   If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1243    entity-tag ] ) )
1244   If-Modified-Since = HTTP-date
1245   If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1246    entity-tag ] ) )
1247   If-Unmodified-Since = HTTP-date
1248
1249   Last-Modified = HTTP-date
1250
1251   OWS = <OWS, defined in [Part1], Section 1.2.2>
1252
1253   entity-tag = [ weak ] opaque-tag
1254   etagc = "!" / %x23-7E ; '#'-'~'
1255    / obs-text
1256
1257   obs-text = <obs-text, defined in [Part1], Section 3.2.3>
1258   opaque-tag = DQUOTE *etagc DQUOTE
1259
1260   weak = %x57.2F ; W/
1261
1262   ABNF diagnostics:
1263
1264   ; ETag defined but not used
1265   ; If-Match defined but not used
1266   ; If-Modified-Since defined but not used
1267   ; If-None-Match defined but not used
1268   ; If-Unmodified-Since defined but not used
1269   ; Last-Modified defined but not used
1270
1271Appendix C.  Change Log (to be removed by RFC Editor before publication)
1272
1273C.1.  Since RFC 2616
1274
1275   Extracted relevant partitions from [RFC2616].
1276
1277C.2.  Since draft-ietf-httpbis-p4-conditional-00
1278
1279   Closed issues:
1280
1281   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
1282      Informative references"
1283
1284
1285
1286
1287Fielding, et al.          Expires July 7, 2012                 [Page 23]
1288
1289Internet-Draft              HTTP/1.1, Part 4                January 2012
1290
1291
1292   Other changes:
1293
1294   o  Move definitions of 304 and 412 condition codes from Part2.
1295
1296C.3.  Since draft-ietf-httpbis-p4-conditional-01
1297
1298   Ongoing work on ABNF conversion
1299   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1300
1301   o  Add explicit references to BNF syntax and rules imported from
1302      other parts of the specification.
1303
1304C.4.  Since draft-ietf-httpbis-p4-conditional-02
1305
1306   Closed issues:
1307
1308   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1309      non-GET requests"
1310
1311   Ongoing work on IANA Message Header Field Registration
1312   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
1313
1314   o  Reference RFC 3984, and update header field registrations for
1315      header fields defined in this document.
1316
1317C.5.  Since draft-ietf-httpbis-p4-conditional-03
1318
1319   Closed issues:
1320
1321   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for
1322      ETag matching"
1323
1324   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity
1325      value' undefined"
1326
1327   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068
1328      Date header reference"
1329
1330C.6.  Since draft-ietf-httpbis-p4-conditional-04
1331
1332   Ongoing work on ABNF conversion
1333   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1334
1335   o  Use "/" instead of "|" for alternatives.
1336
1337   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1338      whitespace ("OWS") and required whitespace ("RWS").
1339
1340
1341
1342
1343Fielding, et al.          Expires July 7, 2012                 [Page 24]
1344
1345Internet-Draft              HTTP/1.1, Part 4                January 2012
1346
1347
1348   o  Rewrite ABNFs to spell out whitespace rules, factor out header
1349      field value format definitions.
1350
1351C.7.  Since draft-ietf-httpbis-p4-conditional-05
1352
1353   Final work on ABNF conversion
1354   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
1355
1356   o  Add appendix containing collected and expanded ABNF, reorganize
1357      ABNF introduction.
1358
1359C.8.  Since draft-ietf-httpbis-p4-conditional-06
1360
1361   Closed issues:
1362
1363   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-
1364      sensitivity of etag weakness indicator"
1365
1366C.9.  Since draft-ietf-httpbis-p4-conditional-07
1367
1368   Closed issues:
1369
1370   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
1371      non-GET requests" (If-Match still was defined to require strong
1372      matching)
1373
1374   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
1375      registrations for optional status codes"
1376
1377C.10.  Since draft-ietf-httpbis-p4-conditional-08
1378
1379   No significant changes.
1380
1381C.11.  Since draft-ietf-httpbis-p4-conditional-09
1382
1383   No significant changes.
1384
1385C.12.  Since draft-ietf-httpbis-p4-conditional-10
1386
1387   Closed issues:
1388
1389   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
1390      'Requested Variant'"
1391
1392   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
1393      entity / representation / variant terminology"
1394
1395
1396
1397
1398
1399Fielding, et al.          Expires July 7, 2012                 [Page 25]
1400
1401Internet-Draft              HTTP/1.1, Part 4                January 2012
1402
1403
1404   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
1405      removing the 'changes from 2068' sections"
1406
1407C.13.  Since draft-ietf-httpbis-p4-conditional-11
1408
1409   None.
1410
1411C.14.  Since draft-ietf-httpbis-p4-conditional-12
1412
1413   Closed issues:
1414
1415   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
1416      Classification"
1417
1418C.15.  Since draft-ietf-httpbis-p4-conditional-13
1419
1420   Closed issues:
1421
1422   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/89>: "If-* and
1423      entities"
1424
1425   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/101>: "Definition of
1426      validator weakness"
1427
1428   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
1429      ABNFs for header fields"
1430
1431   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/269>: "ETags and
1432      Quotes"
1433
1434C.16.  Since draft-ietf-httpbis-p4-conditional-14
1435
1436   None.
1437
1438C.17.  Since draft-ietf-httpbis-p4-conditional-15
1439
1440   Closed issues:
1441
1442   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/304>: "If-Range
1443      should be listed when dicussing contexts where L-M can be
1444      considered strong"
1445
1446C.18.  Since draft-ietf-httpbis-p4-conditional-16
1447
1448   Closed issues:
1449
1450   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document
1451      HTTP's error-handling philosophy"
1452
1453
1454
1455Fielding, et al.          Expires July 7, 2012                 [Page 26]
1456
1457Internet-Draft              HTTP/1.1, Part 4                January 2012
1458
1459
1460C.19.  Since draft-ietf-httpbis-p4-conditional-17
1461
1462   Closed issues:
1463
1464   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/306>: "does etag
1465      value really use quoted-string"
1466
1467Index
1468
1469   3
1470      304 Not Modified (status code)  19
1471
1472   4
1473      412 Precondition Failed (status code)  20
1474
1475   E
1476      ETag header field  10
1477
1478   G
1479      Grammar
1480         entity-tag  10
1481         ETag  10
1482         etagc  10
1483         If-Match  15
1484         If-Modified-Since  17
1485         If-None-Match  16
1486         If-Unmodified-Since  18
1487         Last-Modified  8
1488         opaque-tag  10
1489         weak  10
1490
1491   H
1492      Header Fields
1493         ETag  10
1494         If-Match  15
1495         If-Modified-Since  17
1496         If-None-Match  16
1497         If-Unmodified-Since  18
1498         Last-Modified  8
1499
1500   I
1501      If-Match header field  15
1502      If-Modified-Since header field  17
1503      If-None-Match header field  16
1504      If-Unmodified-Since header field  18
1505
1506   L
1507      Last-Modified header field  8
1508
1509
1510
1511Fielding, et al.          Expires July 7, 2012                 [Page 27]
1512
1513Internet-Draft              HTTP/1.1, Part 4                January 2012
1514
1515
1516   M
1517      metadata  6
1518
1519   S
1520      selected representation  5
1521      Status Codes
1522         304 Not Modified  19
1523         412 Precondition Failed  20
1524
1525   V
1526      validator  6
1527         strong  6
1528         weak  6
1529
1530Authors' Addresses
1531
1532   Roy T. Fielding (editor)
1533   Adobe Systems Incorporated
1534   345 Park Ave
1535   San Jose, CA  95110
1536   USA
1537
1538   EMail: fielding@gbiv.com
1539   URI:   http://roy.gbiv.com/
1540
1541
1542   Jim Gettys
1543   Alcatel-Lucent Bell Labs
1544   21 Oak Knoll Road
1545   Carlisle, MA  01741
1546   USA
1547
1548   EMail: jg@freedesktop.org
1549   URI:   http://gettys.wordpress.com/
1550
1551
1552   Jeffrey C. Mogul
1553   Hewlett-Packard Company
1554   HP Labs, Large Scale Systems Group
1555   1501 Page Mill Road, MS 1177
1556   Palo Alto, CA  94304
1557   USA
1558
1559   EMail: JeffMogul@acm.org
1560
1561
1562
1563
1564
1565
1566
1567Fielding, et al.          Expires July 7, 2012                 [Page 28]
1568
1569Internet-Draft              HTTP/1.1, Part 4                January 2012
1570
1571
1572   Henrik Frystyk Nielsen
1573   Microsoft Corporation
1574   1 Microsoft Way
1575   Redmond, WA  98052
1576   USA
1577
1578   EMail: henrikn@microsoft.com
1579
1580
1581   Larry Masinter
1582   Adobe Systems Incorporated
1583   345 Park Ave
1584   San Jose, CA  95110
1585   USA
1586
1587   EMail: LMM@acm.org
1588   URI:   http://larry.masinter.net/
1589
1590
1591   Paul J. Leach
1592   Microsoft Corporation
1593   1 Microsoft Way
1594   Redmond, WA  98052
1595
1596   EMail: paulle@microsoft.com
1597
1598
1599   Tim Berners-Lee
1600   World Wide Web Consortium
1601   MIT Computer Science and Artificial Intelligence Laboratory
1602   The Stata Center, Building 32
1603   32 Vassar Street
1604   Cambridge, MA  02139
1605   USA
1606
1607   EMail: timbl@w3.org
1608   URI:   http://www.w3.org/People/Berners-Lee/
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623Fielding, et al.          Expires July 7, 2012                 [Page 29]
1624
1625Internet-Draft              HTTP/1.1, Part 4                January 2012
1626
1627
1628   Yves Lafon (editor)
1629   World Wide Web Consortium
1630   W3C / ERCIM
1631   2004, rte des Lucioles
1632   Sophia-Antipolis, AM  06902
1633   France
1634
1635   EMail: ylafon@w3.org
1636   URI:   http://www.raubacapeu.net/people/yves/
1637
1638
1639   Julian F. Reschke (editor)
1640   greenbytes GmbH
1641   Hafenweg 16
1642   Muenster, NW  48155
1643   Germany
1644
1645   Phone: +49 251 2807760
1646   Fax:   +49 251 2807761
1647   EMail: julian.reschke@greenbytes.de
1648   URI:   http://greenbytes.de/tech/webdav/
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
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