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4HTTPbis Working Group                                   R. Fielding, Ed.
5Internet-Draft                                                     Adobe
6Obsoletes: 2616 (if approved)                              Y. Lafon, Ed.
7Intended status: Standards Track                                     W3C
8Expires: January 17, 2013                                J. Reschke, Ed.
9                                                              greenbytes
10                                                           July 16, 2012
11
12
13                 HTTP/1.1, part 4: Conditional Requests
14                  draft-ietf-httpbis-p4-conditional-20
15
16Abstract
17
18   The Hypertext Transfer Protocol (HTTP) is an application-level
19   protocol for distributed, collaborative, hypertext information
20   systems.  This document defines HTTP/1.1 conditional requests,
21   including metadata header fields for indicating state changes,
22   request header fields for making preconditions on such state, and
23   rules for constructing the responses to a conditional request when
24   one or more preconditions evaluate to false.
25
26Editorial Note (To be removed by RFC Editor)
27
28   Discussion of this draft takes place on the HTTPBIS working group
29   mailing list (ietf-http-wg@w3.org), which is archived at
30   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
31
32   The current issues list is at
33   <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
34   documents (including fancy diffs) can be found at
35   <http://tools.ietf.org/wg/httpbis/>.
36
37   The changes in this draft are summarized in Appendix D.1.
38
39Status of This Memo
40
41   This Internet-Draft is submitted in full conformance with the
42   provisions of BCP 78 and BCP 79.
43
44   Internet-Drafts are working documents of the Internet Engineering
45   Task Force (IETF).  Note that other groups may also distribute
46   working documents as Internet-Drafts.  The list of current Internet-
47   Drafts is at http://datatracker.ietf.org/drafts/current/.
48
49   Internet-Drafts are draft documents valid for a maximum of six months
50   and may be updated, replaced, or obsoleted by other documents at any
51   time.  It is inappropriate to use Internet-Drafts as reference
52
53
54
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57Internet-Draft              HTTP/1.1, Part 4                   July 2012
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59
60   material or to cite them other than as "work in progress."
61
62   This Internet-Draft will expire on January 17, 2013.
63
64Copyright Notice
65
66   Copyright (c) 2012 IETF Trust and the persons identified as the
67   document authors.  All rights reserved.
68
69   This document is subject to BCP 78 and the IETF Trust's Legal
70   Provisions Relating to IETF Documents
71   (http://trustee.ietf.org/license-info) in effect on the date of
72   publication of this document.  Please review these documents
73   carefully, as they describe your rights and restrictions with respect
74   to this document.  Code Components extracted from this document must
75   include Simplified BSD License text as described in Section 4.e of
76   the Trust Legal Provisions and are provided without warranty as
77   described in the Simplified BSD License.
78
79   This document may contain material from IETF Documents or IETF
80   Contributions published or made publicly available before November
81   10, 2008.  The person(s) controlling the copyright in some of this
82   material may not have granted the IETF Trust the right to allow
83   modifications of such material outside the IETF Standards Process.
84   Without obtaining an adequate license from the person(s) controlling
85   the copyright in such materials, this document may not be modified
86   outside the IETF Standards Process, and derivative works of it may
87   not be created outside the IETF Standards Process, except to format
88   it for publication as an RFC or to translate it into languages other
89   than English.
90
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113Internet-Draft              HTTP/1.1, Part 4                   July 2012
114
115
116Table of Contents
117
118   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
119     1.1.  Conformance and Error Handling . . . . . . . . . . . . . .  4
120     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  5
121   2.  Validators . . . . . . . . . . . . . . . . . . . . . . . . . .  5
122     2.1.  Weak versus Strong . . . . . . . . . . . . . . . . . . . .  6
123     2.2.  Last-Modified  . . . . . . . . . . . . . . . . . . . . . .  7
124       2.2.1.  Generation . . . . . . . . . . . . . . . . . . . . . .  8
125       2.2.2.  Comparison . . . . . . . . . . . . . . . . . . . . . .  8
126     2.3.  ETag . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
127       2.3.1.  Generation . . . . . . . . . . . . . . . . . . . . . . 10
128       2.3.2.  Comparison . . . . . . . . . . . . . . . . . . . . . . 11
129       2.3.3.  Example: Entity-tags varying on Content-Negotiated
130               Resources  . . . . . . . . . . . . . . . . . . . . . . 11
131     2.4.  Rules for When to Use Entity-tags and Last-Modified
132           Dates  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
133   3.  Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
134     3.1.  If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
135     3.2.  If-None-Match  . . . . . . . . . . . . . . . . . . . . . . 15
136     3.3.  If-Modified-Since  . . . . . . . . . . . . . . . . . . . . 16
137     3.4.  If-Unmodified-Since  . . . . . . . . . . . . . . . . . . . 17
138     3.5.  If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18
139   4.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 18
140     4.1.  304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18
141     4.2.  412 Precondition Failed  . . . . . . . . . . . . . . . . . 19
142   5.  Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . 19
143   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
144     6.1.  Status Code Registration . . . . . . . . . . . . . . . . . 20
145     6.2.  Header Field Registration  . . . . . . . . . . . . . . . . 21
146   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
147   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
148   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
149     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
150     9.2.  Informative References . . . . . . . . . . . . . . . . . . 22
151   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
152   Appendix B.  Imported ABNF . . . . . . . . . . . . . . . . . . . . 23
153   Appendix C.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 23
154   Appendix D.  Change Log (to be removed by RFC Editor before
155                publication)  . . . . . . . . . . . . . . . . . . . . 24
156     D.1.  Since draft-ietf-httpbis-p4-conditional-19 . . . . . . . . 24
157   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
158
159
160
161
162
163
164
165
166
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169Internet-Draft              HTTP/1.1, Part 4                   July 2012
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171
1721.  Introduction
173
174   Conditional requests are HTTP requests [Part2] that include one or
175   more header fields indicating a precondition to be tested before
176   applying the method semantics to the target resource.  Each
177   precondition is based on metadata that is expected to change if the
178   selected representation of the target resource is changed.  This
179   document defines the HTTP/1.1 conditional request mechanisms in terms
180   of the architecture, syntax notation, and conformance criteria
181   defined in [Part1].
182
183   Conditional GET requests are the most efficient mechanism for HTTP
184   cache updates [Part6].  Conditionals can also be applied to state-
185   changing methods, such as PUT and DELETE, to prevent the "lost
186   update" problem: one client accidentally overwriting the work of
187   another client that has been acting in parallel.
188
189   Conditional request preconditions are based on the state of the
190   target resource as a whole (its current value set) or the state as
191   observed in a previously obtained representation (one value in that
192   set).  A resource might have multiple current representations, each
193   with its own observable state.  The conditional request mechanisms
194   assume that the mapping of requests to corresponding representations
195   will be consistent over time if the server intends to take advantage
196   of conditionals.  Regardless, if the mapping is inconsistent and the
197   server is unable to select the appropriate representation, then no
198   harm will result when the precondition evaluates to false.
199
200   We use the term "selected representation" to refer to the current
201   representation of the target resource that would have been selected
202   in a successful response if the same request had used the method GET
203   and had excluded all of the conditional request header fields.  The
204   conditional request preconditions are evaluated by comparing the
205   values provided in the request header fields to the current metadata
206   for the selected representation.
207
2081.1.  Conformance and Error Handling
209
210   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
211   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
212   document are to be interpreted as described in [RFC2119].
213
214   This specification targets conformance criteria according to the role
215   of a participant in HTTP communication.  Hence, HTTP requirements are
216   placed on senders, recipients, clients, servers, user agents,
217   intermediaries, origin servers, proxies, gateways, or caches,
218   depending on what behavior is being constrained by the requirement.
219   See Section 2 of [Part1] for definitions of these terms.
220
221
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227
228   The verb "generate" is used instead of "send" where a requirement
229   differentiates between creating a protocol element and merely
230   forwarding a received element downstream.
231
232   An implementation is considered conformant if it complies with all of
233   the requirements associated with the roles it partakes in HTTP.  Note
234   that SHOULD-level requirements are relevant here, unless one of the
235   documented exceptions is applicable.
236
237   This document also uses ABNF to define valid protocol elements
238   (Section 1.2).  In addition to the prose requirements placed upon
239   them, senders MUST NOT generate protocol elements that do not match
240   the grammar defined by the ABNF rules for those protocol elements
241   that are applicable to the sender's role.  If a received protocol
242   element is processed, the recipient MUST be able to parse any value
243   that would match the ABNF rules for that protocol element, excluding
244   only those rules not applicable to the recipient's role.
245
246   Unless noted otherwise, a recipient MAY attempt to recover a usable
247   protocol element from an invalid construct.  HTTP does not define
248   specific error handling mechanisms except when they have a direct
249   impact on security, since different applications of the protocol
250   require different error handling strategies.  For example, a Web
251   browser might wish to transparently recover from a response where the
252   Location header field doesn't parse according to the ABNF, whereas a
253   systems control client might consider any form of error recovery to
254   be dangerous.
255
2561.2.  Syntax Notation
257
258   This specification uses the Augmented Backus-Naur Form (ABNF)
259   notation of [RFC5234] with the list rule extension defined in Section
260   1.2 of [Part1].  Appendix B describes rules imported from other
261   documents.  Appendix C shows the collected ABNF with the list rule
262   expanded.
263
2642.  Validators
265
266   This specification defines two forms of metadata that are commonly
267   used to observe resource state and test for preconditions:
268   modification dates (Section 2.2) and opaque entity tags
269   (Section 2.3).  Additional metadata that reflects resource state has
270   been defined by various extensions of HTTP, such as WebDAV [RFC4918],
271   that are beyond the scope of this specification.  A resource metadata
272   value is referred to as a "validator" when it is used within a
273   precondition.
274
275
276
277
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283
2842.1.  Weak versus Strong
285
286   Validators come in two flavors: strong or weak.  Weak validators are
287   easy to generate but are far less useful for comparisons.  Strong
288   validators are ideal for comparisons but can be very difficult (and
289   occasionally impossible) to generate efficiently.  Rather than impose
290   that all forms of resource adhere to the same strength of validator,
291   HTTP exposes the type of validator in use and imposes restrictions on
292   when weak validators can be used as preconditions.
293
294   A "strong validator" is a representation metadata value that MUST be
295   changed to a new, previously unused or guaranteed unique, value
296   whenever a change occurs to the representation data such that a
297   change would be observable in the payload body of a 200 (OK) response
298   to GET.
299
300   A strong validator MAY be changed for other reasons, such as when a
301   semantically significant part of the representation metadata is
302   changed (e.g., Content-Type), but it is in the best interests of the
303   origin server to only change the value when it is necessary to
304   invalidate the stored responses held by remote caches and authoring
305   tools.  A strong validator MUST be unique across all representations
306   of a given resource, such that no two representations of that
307   resource share the same validator unless their payload body would be
308   identical.
309
310   Cache entries might persist for arbitrarily long periods, regardless
311   of expiration times.  Thus, a cache might attempt to validate an
312   entry using a validator that it obtained in the distant past.  A
313   strong validator MUST be unique across all versions of all
314   representations associated with a particular resource over time.
315   However, there is no implication of uniqueness across representations
316   of different resources (i.e., the same strong validator might be in
317   use for representations of multiple resources at the same time and
318   does not imply that those representations are equivalent).
319
320   There are a variety of strong validators used in practice.  The best
321   are based on strict revision control, wherein each change to a
322   representation always results in a unique node name and revision
323   identifier being assigned before the representation is made
324   accessible to GET.  A collision-resistant hash function applied to
325   the representation data is also sufficient if the data is available
326   prior to the response header fields being sent and the digest does
327   not need to be recalculated every time a validation request is
328   received.  However, if a resource has distinct representations that
329   differ only in their metadata, such as might occur with content
330   negotiation over media types that happen to share the same data
331   format, then the origin server SHOULD incorporate additional
332
333
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339
340   information in the validator to distinguish those representations and
341   avoid confusing cache behavior.
342
343   In contrast, a "weak validator" is a representation metadata value
344   that might not be changed for every change to the representation
345   data.  This weakness might be due to limitations in how the value is
346   calculated, such as clock resolution or an inability to ensure
347   uniqueness for all possible representations of the resource, or due
348   to a desire by the resource owner to group representations by some
349   self-determined set of equivalency rather than unique sequences of
350   data.  An origin server SHOULD change a weak entity-tag whenever it
351   considers prior representations to be unacceptable as a substitute
352   for the current representation.  In other words, a weak entity-tag
353   ought to change whenever the origin server wants caches to invalidate
354   old responses.
355
356   For example, the representation of a weather report that changes in
357   content every second, based on dynamic measurements, might be grouped
358   into sets of equivalent representations (from the origin server's
359   perspective) with the same weak validator in order to allow cached
360   representations to be valid for a reasonable period of time (perhaps
361   adjusted dynamically based on server load or weather quality).
362   Likewise, a representation's modification time, if defined with only
363   one-second resolution, might be a weak validator if it is possible
364   for the representation to be modified twice during a single second
365   and retrieved between those modifications.
366
367   A "use" of a validator occurs when either a client generates a
368   request and includes the validator in a precondition or when a server
369   compares two validators.  Weak validators are only usable in contexts
370   that do not depend on exact equality of a representation's payload
371   body.  Strong validators are usable and preferred for all conditional
372   requests, including cache validation, partial content ranges, and
373   "lost update" avoidance.
374
3752.2.  Last-Modified
376
377   The "Last-Modified" header field indicates the date and time at which
378   the origin server believes the selected representation was last
379   modified.
380
381     Last-Modified = HTTP-date
382
383   An example of its use is
384
385     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
386
387
388
389
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395
3962.2.1.  Generation
397
398   Origin servers SHOULD send Last-Modified for any selected
399   representation for which a last modification date can be reasonably
400   and consistently determined, since its use in conditional requests
401   and evaluating cache freshness ([Part6]) results in a substantial
402   reduction of HTTP traffic on the Internet and can be a significant
403   factor in improving service scalability and reliability.
404
405   A representation is typically the sum of many parts behind the
406   resource interface.  The last-modified time would usually be the most
407   recent time that any of those parts were changed.  How that value is
408   determined for any given resource is an implementation detail beyond
409   the scope of this specification.  What matters to HTTP is how
410   recipients of the Last-Modified header field can use its value to
411   make conditional requests and test the validity of locally cached
412   responses.
413
414   An origin server SHOULD obtain the Last-Modified value of the
415   representation as close as possible to the time that it generates the
416   Date field value for its response.  This allows a recipient to make
417   an accurate assessment of the representation's modification time,
418   especially if the representation changes near the time that the
419   response is generated.
420
421   An origin server with a clock MUST NOT send a Last-Modified date that
422   is later than the server's time of message origination (Date).  If
423   the last modification time is derived from implementation-specific
424   metadata that evaluates to some time in the future, according to the
425   origin server's clock, then the origin server MUST replace that value
426   with the message origination date.  This prevents a future
427   modification date from having an adverse impact on cache validation.
428
429   An origin server without a clock MUST NOT assign Last-Modified values
430   to a response unless these values were associated with the resource
431   by some other system or user with a reliable clock.
432
4332.2.2.  Comparison
434
435   A Last-Modified time, when used as a validator in a request, is
436   implicitly weak unless it is possible to deduce that it is strong,
437   using the following rules:
438
439   o  The validator is being compared by an origin server to the actual
440      current validator for the representation and,
441
442   o  That origin server reliably knows that the associated
443      representation did not change twice during the second covered by
444
445
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451
452      the presented validator.
453
454   or
455
456   o  The validator is about to be used by a client in an If-Modified-
457      Since, If-Unmodified-Since header field, because the client has a
458      cache entry, or If-Range for the associated representation, and
459
460   o  That cache entry includes a Date value, which gives the time when
461      the origin server sent the original response, and
462
463   o  The presented Last-Modified time is at least 60 seconds before the
464      Date value.
465
466   or
467
468   o  The validator is being compared by an intermediate cache to the
469      validator stored in its cache entry for the representation, and
470
471   o  That cache entry includes a Date value, which gives the time when
472      the origin server sent the original response, and
473
474   o  The presented Last-Modified time is at least 60 seconds before the
475      Date value.
476
477   This method relies on the fact that if two different responses were
478   sent by the origin server during the same second, but both had the
479   same Last-Modified time, then at least one of those responses would
480   have a Date value equal to its Last-Modified time.  The arbitrary 60-
481   second limit guards against the possibility that the Date and Last-
482   Modified values are generated from different clocks, or at somewhat
483   different times during the preparation of the response.  An
484   implementation MAY use a value larger than 60 seconds, if it is
485   believed that 60 seconds is too short.
486
4872.3.  ETag
488
489   The "ETag" header field provides the current entity-tag for the
490   selected representation.  An entity-tag is an opaque validator for
491   differentiating between multiple representations of the same
492   resource, regardless of whether those multiple representations are
493   due to resource state changes over time, content negotiation
494   resulting in multiple representations being valid at the same time,
495   or both.  An entity-tag consists of an opaque quoted string, possibly
496   prefixed by a weakness indicator.
497
498
499
500
501
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507
508     ETag       = entity-tag
509
510     entity-tag = [ weak ] opaque-tag
511     weak       = %x57.2F ; "W/", case-sensitive
512     opaque-tag = DQUOTE *etagc DQUOTE
513     etagc      = %x21 / %x23-7E / obs-text
514                ; VCHAR except double quotes, plus obs-text
515
516      Note: Previously, opaque-tag was defined to be a quoted-string
517      ([RFC2616], Section 3.11), thus some recipients might perform
518      backslash unescaping.  Servers therefore ought to avoid backslash
519      characters in entity tags.
520
521   An entity-tag can be more reliable for validation than a modification
522   date in situations where it is inconvenient to store modification
523   dates, where the one-second resolution of HTTP date values is not
524   sufficient, or where modification dates are not consistently
525   maintained.
526
527   Examples:
528
529     ETag: "xyzzy"
530     ETag: W/"xyzzy"
531     ETag: ""
532
533   An entity-tag can be either a weak or strong validator, with strong
534   being the default.  If an origin server provides an entity-tag for a
535   representation and the generation of that entity-tag does not satisfy
536   the requirements for a strong validator (Section 2.1), then that
537   entity-tag MUST be marked as weak by prefixing its opaque value with
538   "W/" (case-sensitive).
539
5402.3.1.  Generation
541
542   The principle behind entity-tags is that only the service author
543   knows the implementation of a resource well enough to select the most
544   accurate and efficient validation mechanism for that resource, and
545   that any such mechanism can be mapped to a simple sequence of octets
546   for easy comparison.  Since the value is opaque, there is no need for
547   the client to be aware of how each entity-tag is constructed.
548
549   For example, a resource that has implementation-specific versioning
550   applied to all changes might use an internal revision number, perhaps
551   combined with a variance identifier for content negotiation, to
552   accurately differentiate between representations.  Other
553   implementations might use a collision-resistant hash of
554   representation content, a combination of various filesystem
555   attributes, or a modification timestamp that has sub-second
556
557
558
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563
564   resolution.
565
566   Origin servers SHOULD send ETag for any selected representation for
567   which detection of changes can be reasonably and consistently
568   determined, since the entity-tag's use in conditional requests and
569   evaluating cache freshness ([Part6]) can result in a substantial
570   reduction of HTTP network traffic and can be a significant factor in
571   improving service scalability and reliability.
572
5732.3.2.  Comparison
574
575   There are two entity-tag comparison functions, depending on whether
576   the comparison context allows the use of weak validators or not:
577
578   o  The strong comparison function: in order to be considered equal,
579      both opaque-tags MUST be identical character-by-character, and
580      both MUST NOT be weak.
581
582   o  The weak comparison function: in order to be considered equal,
583      both opaque-tags MUST be identical character-by-character, but
584      either or both of them MAY be tagged as "weak" without affecting
585      the result.
586
587   The example below shows the results for a set of entity-tag pairs,
588   and both the weak and strong comparison function results:
589
590   +--------+--------+-------------------+-----------------+
591   | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
592   +--------+--------+-------------------+-----------------+
593   | W/"1"  | W/"1"  | no match          | match           |
594   | W/"1"  | W/"2"  | no match          | no match        |
595   | W/"1"  | "1"    | no match          | match           |
596   | "1"    | "1"    | match             | match           |
597   +--------+--------+-------------------+-----------------+
598
5992.3.3.  Example: Entity-tags varying on Content-Negotiated Resources
600
601   Consider a resource that is subject to content negotiation (Section 8
602   of [Part2]), and where the representations returned upon a GET
603   request vary based on the Accept-Encoding request header field
604   (Section 9.3 of [Part2]):
605
606   >> Request:
607
608     GET /index HTTP/1.1
609     Host: www.example.com
610     Accept-Encoding: gzip
611
612
613
614
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619
620   In this case, the response might or might not use the gzip content
621   coding.  If it does not, the response might look like:
622
623   >> Response:
624
625     HTTP/1.1 200 OK
626     Date: Thu, 26 Mar 2010 00:05:00 GMT
627     ETag: "123-a"
628     Content-Length: 70
629     Vary: Accept-Encoding
630     Content-Type: text/plain
631
632     Hello World!
633     Hello World!
634     Hello World!
635     Hello World!
636     Hello World!
637
638   An alternative representation that does use gzip content coding would
639   be:
640
641   >> Response:
642
643     HTTP/1.1 200 OK
644     Date: Thu, 26 Mar 2010 00:05:00 GMT
645     ETag: "123-b"
646     Content-Length: 43
647     Vary: Accept-Encoding
648     Content-Type: text/plain
649     Content-Encoding: gzip
650
651     ...binary data...
652
653      Note: Content codings are a property of the representation, so
654      therefore an entity-tag of an encoded representation has to be
655      distinct from an unencoded representation to prevent conflicts
656      during cache updates and range requests.  In contrast, transfer
657      codings (Section 4 of [Part1]) apply only during message transfer
658      and do not require distinct entity-tags.
659
6602.4.  Rules for When to Use Entity-tags and Last-Modified Dates
661
662   We adopt a set of rules and recommendations for origin servers,
663   clients, and caches regarding when various validator types ought to
664   be used, and for what purposes.
665
666   HTTP/1.1 origin servers:
667
668
669
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675
676   o  SHOULD send an entity-tag validator unless it is not feasible to
677      generate one.
678
679   o  MAY send a weak entity-tag instead of a strong entity-tag, if
680      performance considerations support the use of weak entity-tags, or
681      if it is unfeasible to send a strong entity-tag.
682
683   o  SHOULD send a Last-Modified value if it is feasible to send one.
684
685   In other words, the preferred behavior for an HTTP/1.1 origin server
686   is to send both a strong entity-tag and a Last-Modified value.
687
688   HTTP/1.1 clients:
689
690   o  MUST use that entity-tag in any cache-conditional request (using
691      If-Match or If-None-Match) if an entity-tag has been provided by
692      the origin server.
693
694   o  SHOULD use the Last-Modified value in non-subrange cache-
695      conditional requests (using If-Modified-Since) if only a Last-
696      Modified value has been provided by the origin server.
697
698   o  MAY use the Last-Modified value in subrange cache-conditional
699      requests (using If-Unmodified-Since) if only a Last-Modified value
700      has been provided by an HTTP/1.0 origin server.  The user agent
701      SHOULD provide a way to disable this, in case of difficulty.
702
703   o  SHOULD use both validators in cache-conditional requests if both
704      an entity-tag and a Last-Modified value have been provided by the
705      origin server.  This allows both HTTP/1.0 and HTTP/1.1 caches to
706      respond appropriately.
707
708   An HTTP/1.1 origin server, upon receiving a conditional request that
709   includes both a Last-Modified date (e.g., in an If-Modified-Since or
710   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
711   in an If-Match, If-None-Match, or If-Range header field) as cache
712   validators, MUST NOT return a response status code of 304 (Not
713   Modified) unless doing so is consistent with all of the conditional
714   header fields in the request.
715
716   An HTTP/1.1 caching proxy, upon receiving a conditional request that
717   includes both a Last-Modified date and one or more entity-tags as
718   cache validators, MUST NOT return a locally cached response to the
719   client unless that cached response is consistent with all of the
720   conditional header fields in the request.
721
722      Note: The general principle behind these rules is that HTTP/1.1
723      servers and clients ought to transmit as much non-redundant
724
725
726
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731
732      information as is available in their responses and requests.
733      HTTP/1.1 systems receiving this information will make the most
734      conservative assumptions about the validators they receive.
735
736      HTTP/1.0 clients and caches might ignore entity-tags.  Generally,
737      last-modified values received or used by these systems will
738      support transparent and efficient caching, and so HTTP/1.1 origin
739      servers still ought to provide Last-Modified values.
740
7413.  Precondition Header Fields
742
743   This section defines the syntax and semantics of HTTP/1.1 header
744   fields for applying preconditions on requests.  Section 5 defines the
745   order of evaluation when more than one precondition is present in a
746   request.
747
7483.1.  If-Match
749
750   The "If-Match" header field can be used to make a request method
751   conditional on the current existence or value of an entity-tag for
752   one or more representations of the target resource.
753
754   If-Match is generally useful for resource update requests, such as
755   PUT requests, as a means for protecting against accidental overwrites
756   when multiple clients are acting in parallel on the same resource
757   (i.e., the "lost update" problem).  An If-Match field-value of "*"
758   places the precondition on the existence of any current
759   representation for the target resource.
760
761     If-Match = "*" / 1#entity-tag
762
763   The If-Match condition is met if and only if any of the entity-tags
764   listed in the If-Match field value match the entity-tag of the
765   selected representation for the target resource (as per
766   Section 2.3.2), or if "*" is given and any current representation
767   exists for the target resource.
768
769   If the condition is met, the server MAY perform the request method as
770   if the If-Match header field was not present.
771
772   Origin servers MUST NOT perform the requested method if the condition
773   is not met; instead they MUST respond with the 412 (Precondition
774   Failed) status code.
775
776   Proxy servers using a cached response as the selected representation
777   MUST NOT perform the requested method if the condition is not met;
778   instead, they MUST forward the request towards the origin server.
779
780
781
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787
788   If the request would, without the If-Match header field, result in
789   anything other than a 2xx (Successful) or 412 (Precondition Failed)
790   status code, then the If-Match header field MUST be ignored.
791
792   Examples:
793
794     If-Match: "xyzzy"
795     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
796     If-Match: *
797
7983.2.  If-None-Match
799
800   The "If-None-Match" header field can be used to make a request method
801   conditional on not matching any of the current entity-tag values for
802   representations of the target resource.
803
804   If-None-Match is primarily used in conditional GET requests to enable
805   efficient updates of cached information with a minimum amount of
806   transaction overhead.  A client that has one or more representations
807   previously obtained from the target resource can send If-None-Match
808   with a list of the associated entity-tags in the hope of receiving a
809   304 (Not Modified) response if at least one of those representations
810   matches the selected representation.
811
812   If-None-Match can also be used with a value of "*" to prevent an
813   unsafe request method (e.g., PUT) from inadvertently modifying an
814   existing representation of the target resource when the client
815   believes that the resource does not have a current representation.
816   This is a variation on the "lost update" problem that might arise if
817   more than one client attempts to create an initial representation for
818   the target resource.
819
820     If-None-Match = "*" / 1#entity-tag
821
822   The If-None-Match condition is met if and only if none of the entity-
823   tags listed in the If-None-Match field value match the entity-tag of
824   the selected representation for the target resource (as per
825   Section 2.3.2), or if "*" is given and no current representation
826   exists for that resource.
827
828   If the condition is not met, the server MUST NOT perform the
829   requested method.  Instead, if the request method was GET or HEAD,
830   the server SHOULD respond with a 304 (Not Modified) status code,
831   including the cache-related header fields (particularly ETag) of the
832   selected representation that has a matching entity-tag.  For all
833   other request methods, the server MUST respond with a 412
834   (Precondition Failed) status code.
835
836
837
838
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843
844   If the condition is met, the server MAY perform the requested method
845   as if the If-None-Match header field did not exist, but MUST also
846   ignore any If-Modified-Since header field(s) in the request.  That
847   is, if no entity-tags match, then the server MUST NOT return a 304
848   (Not Modified) response.
849
850   If the request would, without the If-None-Match header field, result
851   in anything other than a 2xx (Successful) or 304 (Not Modified)
852   status code, then the If-None-Match header field MUST be ignored.
853   (See Section 2.4 for a discussion of server behavior when both If-
854   Modified-Since and If-None-Match appear in the same request.)
855
856   Examples:
857
858     If-None-Match: "xyzzy"
859     If-None-Match: W/"xyzzy"
860     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
861     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
862     If-None-Match: *
863
8643.3.  If-Modified-Since
865
866   The "If-Modified-Since" header field can be used with GET or HEAD to
867   make the method conditional by modification date: if the selected
868   representation has not been modified since the time specified in this
869   field, then do not perform the request method; instead, respond as
870   detailed below.
871
872     If-Modified-Since = HTTP-date
873
874   An example of the field is:
875
876     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
877
878   A GET method with an If-Modified-Since header field and no Range
879   header field requests that the selected representation be transferred
880   only if it has been modified since the date given by the If-Modified-
881   Since header field.  The algorithm for determining this includes the
882   following cases:
883
884   1.  If the request would normally result in anything other than a 200
885       (OK) status code, or if the passed If-Modified-Since date is
886       invalid, the response is exactly the same as for a normal GET.  A
887       date which is later than the server's current time is invalid.
888
889   2.  If the selected representation has been modified since the If-
890       Modified-Since date, the response is exactly the same as for a
891       normal GET.
892
893
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899
900   3.  If the selected representation has not been modified since a
901       valid If-Modified-Since date, the server SHOULD return a 304 (Not
902       Modified) response.
903
904   The purpose of this feature is to allow efficient updates of cached
905   information with a minimum amount of transaction overhead.
906
907      Note: The Range header field modifies the meaning of If-Modified-
908      Since; see Section 5.4 of [Part5] for full details.
909
910      Note: If-Modified-Since times are interpreted by the server, whose
911      clock might not be synchronized with the client.
912
913      Note: When handling an If-Modified-Since header field, some
914      servers will use an exact date comparison function, rather than a
915      less-than function, for deciding whether to send a 304 (Not
916      Modified) response.  To get best results when sending an If-
917      Modified-Since header field for cache validation, clients are
918      advised to use the exact date string received in a previous Last-
919      Modified header field whenever possible.
920
921      Note: If a client uses an arbitrary date in the If-Modified-Since
922      header field instead of a date taken from the Last-Modified header
923      field for the same request, the client needs to be aware that this
924      date is interpreted in the server's understanding of time.
925      Unsynchronized clocks and rounding problems, due to the different
926      encodings of time between the client and server, are concerns.
927      This includes the possibility of race conditions if the document
928      has changed between the time it was first requested and the If-
929      Modified-Since date of a subsequent request, and the possibility
930      of clock-skew-related problems if the If-Modified-Since date is
931      derived from the client's clock without correction to the server's
932      clock.  Corrections for different time bases between client and
933      server are at best approximate due to network latency.
934
9353.4.  If-Unmodified-Since
936
937   The "If-Unmodified-Since" header field can be used to make a request
938   method conditional by modification date: if the selected
939   representation has been modified since the time specified in this
940   field, then the server MUST NOT perform the requested operation and
941   MUST instead respond with the 412 (Precondition Failed) status code.
942   If the selected representation has not been modified since the time
943   specified in this field, the server SHOULD perform the request method
944   as if the If-Unmodified-Since header field were not present.
945
946     If-Unmodified-Since = HTTP-date
947
948
949
950
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955
956   An example of the field is:
957
958     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
959
960   If a request normally (i.e., in absence of the If-Unmodified-Since
961   header field) would result in anything other than a 2xx (Successful)
962   or 412 (Precondition Failed) status code, the If-Unmodified-Since
963   header field SHOULD be ignored.
964
965   If the specified date is invalid, the header field MUST be ignored.
966
9673.5.  If-Range
968
969   The "If-Range" header field provides a special conditional request
970   mechanism that is similar to If-Match and If-Unmodified-Since but
971   specific to HTTP range requests.  If-Range is defined in Section 5.3
972   of [Part5].
973
9744.  Status Code Definitions
975
9764.1.  304 Not Modified
977
978   The 304 status code indicates that a conditional GET request has been
979   received and would have resulted in a 200 (OK) response if it were
980   not for the fact that the condition has evaluated to false.  In other
981   words, there is no need for the server to transfer a representation
982   of the target resource because the client's request indicates that it
983   already has a valid representation, as indicated by the 304 response
984   header fields, and is therefore redirecting the client to make use of
985   that stored representation as if it were the payload of a 200
986   response.  The 304 response MUST NOT contain a message-body, and thus
987   is always terminated by the first empty line after the header fields.
988
989   A 304 response MUST include a Date header field (Section 9.10 of
990   [Part2]) unless the origin server does not have a clock that can
991   provide a reasonable approximation of the current time.  If a 200
992   (OK) response to the same request would have included any of the
993   header fields Cache-Control, Content-Location, ETag, Expires, or
994   Vary, then those same header fields MUST be sent in a 304 response.
995
996   Since the goal of a 304 response is to minimize information transfer
997   when the recipient already has one or more cached representations,
998   the response SHOULD NOT include representation metadata other than
999   the above listed fields unless said metadata exists for the purpose
1000   of guiding cache updates (e.g., future HTTP extensions).
1001
1002   If the recipient of a 304 response does not have a cached
1003   representation corresponding to the entity-tag indicated by the 304
1004
1005
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1011
1012   response, then the recipient MUST NOT use the 304 to update its own
1013   cache.  If this conditional request originated with an outbound
1014   client, such as a user agent with its own cache sending a conditional
1015   GET to a shared proxy, then the 304 response MAY be forwarded to that
1016   client.  Otherwise, the recipient MUST disregard the 304 response and
1017   repeat the request without any preconditions.
1018
1019   If a cache uses a received 304 response to update a cache entry, the
1020   cache MUST update the entry to reflect any new field values given in
1021   the response.
1022
10234.2.  412 Precondition Failed
1024
1025   The 412 status code indicates that one or more preconditions given in
1026   the request header fields evaluated to false when tested on the
1027   server.  This response code allows the client to place preconditions
1028   on the current resource state (its current representations and
1029   metadata) and thus prevent the request method from being applied if
1030   the target resource is in an unexpected state.
1031
10325.  Precedence
1033
1034   When more than one conditional request header field is present in a
1035   request, the order in which the fields are evaluated becomes
1036   important.  In practice, the fields defined in this document are
1037   consistently implemented in a single, logical order, due to the fact
1038   that entity tags are presumed to be more accurate than date
1039   validators.  For example, the only reason to send both If-Modified-
1040   Since and If-None-Match in the same GET request is to support
1041   intermediary caches that might not have implemented If-None-Match, so
1042   it makes sense to ignore the If-Modified-Since when entity tags are
1043   understood and available for the selected representation.
1044
1045   The general rule of conditional precedence is that exact match
1046   conditions are evaluated before cache-validating conditions and,
1047   within that order, last-modified conditions are only evaluated if the
1048   corresponding entity tag condition is not present (or not applicable
1049   because the selected representation does not have an entity tag).
1050
1051   Specifically, the fields defined by this specification are evaluated
1052   as follows:
1053
1054   1.  When If-Match is present, evaluate it:
1055
1056       *  if true, continue to step 3
1057
1058       *  if false, respond 412 (Precondition Failed)
1059
1060
1061
1062
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1067
1068   2.  When If-Match is not present and If-Unmodified-Since is present,
1069       evaluate it:
1070
1071       *  if true, continue to step 3
1072
1073       *  if false, respond 412 (Precondition Failed)
1074
1075   3.  When the method is GET and both Range and If-Range are present,
1076       evaluate it:
1077
1078       *  if the validator matches, respond 206 (Partial Content)
1079
1080       *  if the validator does not match, respond 200 (OK)
1081
1082   4.  When If-None-Match is present, evaluate it:
1083
1084       *  if true, all conditions are met
1085
1086       *  if false for GET/HEAD, respond 304 (Not Modified)
1087
1088       *  if false for other methods, respond 412 (Precondition Failed)
1089
1090   5.  When the method is GET or HEAD, If-None-Match is not present, and
1091       If-Modified-Since is present, evaluate it:
1092
1093       *  if true, all conditions are met
1094
1095       *  if false, respond 304 (Not Modified)
1096
1097   Any extension to HTTP/1.1 that defines additional conditional request
1098   header fields ought to define its own expectations regarding the
1099   order for evaluating such fields in relation to those defined in this
1100   document and other conditionals that might be found in practice.
1101
11026.  IANA Considerations
1103
11046.1.  Status Code Registration
1105
1106   The HTTP Status Code Registry located at
1107   <http://www.iana.org/assignments/http-status-codes> shall be updated
1108   with the registrations below:
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119Fielding, et al.        Expires January 17, 2013               [Page 20]
1120
1121Internet-Draft              HTTP/1.1, Part 4                   July 2012
1122
1123
1124   +-------+---------------------+-------------+
1125   | Value | Description         | Reference   |
1126   +-------+---------------------+-------------+
1127   | 304   | Not Modified        | Section 4.1 |
1128   | 412   | Precondition Failed | Section 4.2 |
1129   +-------+---------------------+-------------+
1130
11316.2.  Header Field Registration
1132
1133   The Message Header Field Registry located at <http://www.iana.org/
1134   assignments/message-headers/message-header-index.html> shall be
1135   updated with the permanent registrations below (see [RFC3864]):
1136
1137   +---------------------+----------+----------+-------------+
1138   | Header Field Name   | Protocol | Status   | Reference   |
1139   +---------------------+----------+----------+-------------+
1140   | ETag                | http     | standard | Section 2.3 |
1141   | If-Match            | http     | standard | Section 3.1 |
1142   | If-Modified-Since   | http     | standard | Section 3.3 |
1143   | If-None-Match       | http     | standard | Section 3.2 |
1144   | If-Unmodified-Since | http     | standard | Section 3.4 |
1145   | Last-Modified       | http     | standard | Section 2.2 |
1146   +---------------------+----------+----------+-------------+
1147
1148   The change controller is: "IETF (iesg@ietf.org) - Internet
1149   Engineering Task Force".
1150
11517.  Security Considerations
1152
1153   No additional security considerations have been identified beyond
1154   those applicable to HTTP in general [Part1].
1155
1156   The validators defined by this specification are not intended to
1157   ensure the validity of a representation, guard against malicious
1158   changes, or detect man-in-the-middle attacks.  At best, they enable
1159   more efficient cache updates and optimistic concurrent writes when
1160   all participants are behaving nicely.  At worst, the conditions will
1161   fail and the client will receive a response that is no more harmful
1162   than an HTTP exchange without conditional requests.
1163
11648.  Acknowledgments
1165
1166   See Section 9 of [Part1].
1167
11689.  References
1169
1170
1171
1172
1173
1174
1175Fielding, et al.        Expires January 17, 2013               [Page 21]
1176
1177Internet-Draft              HTTP/1.1, Part 4                   July 2012
1178
1179
11809.1.  Normative References
1181
1182   [Part1]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
1183              "HTTP/1.1, part 1: Message Routing and Syntax"",
1184              draft-ietf-httpbis-p1-messaging-20 (work in progress),
1185              July 2012.
1186
1187   [Part2]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
1188              "HTTP/1.1, part 2: Semantics and Payloads",
1189              draft-ietf-httpbis-p2-semantics-20 (work in progress),
1190              July 2012.
1191
1192   [Part5]    Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
1193              "HTTP/1.1, part 5: Range Requests",
1194              draft-ietf-httpbis-p5-range-20 (work in progress),
1195              July 2012.
1196
1197   [Part6]    Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed.,
1198              and J. Reschke, Ed., "HTTP/1.1, part 6: Caching",
1199              draft-ietf-httpbis-p6-cache-20 (work in progress),
1200              July 2012.
1201
1202   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
1203              Requirement Levels", BCP 14, RFC 2119, March 1997.
1204
1205   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1206              Specifications: ABNF", STD 68, RFC 5234, January 2008.
1207
12089.2.  Informative References
1209
1210   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1211              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1212              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1213
1214   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
1215              Procedures for Message Header Fields", BCP 90, RFC 3864,
1216              September 2004.
1217
1218   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
1219              Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
1220
1221Appendix A.  Changes from RFC 2616
1222
1223   Allow weak entity-tags in all requests except range requests
1224   (Sections 2.1 and 3.2).
1225
1226   Change ETag header field ABNF not to use quoted-string, thus avoiding
1227   escaping issues.  (Section 2.3)
1228
1229
1230
1231Fielding, et al.        Expires January 17, 2013               [Page 22]
1232
1233Internet-Draft              HTTP/1.1, Part 4                   July 2012
1234
1235
1236   Change ABNF productions for header fields to only define the field
1237   value.  (Section 3)
1238
1239Appendix B.  Imported ABNF
1240
1241   The following core rules are included by reference, as defined in
1242   Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
1243   CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
1244   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
1245   8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
1246   character).
1247
1248   The rules below are defined in [Part1]:
1249
1250     OWS           = <OWS, defined in [Part1], Section 3.2.1>
1251     obs-text      = <obs-text, defined in [Part1], Section 3.2.4>
1252
1253   The rules below are defined in other parts:
1254
1255     HTTP-date     = <HTTP-date, defined in [Part2], Section 5.1>
1256
1257Appendix C.  Collected ABNF
1258
1259   ETag = entity-tag
1260
1261   HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>
1262
1263   If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1264    entity-tag ] ) )
1265   If-Modified-Since = HTTP-date
1266   If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1267    entity-tag ] ) )
1268   If-Unmodified-Since = HTTP-date
1269
1270   Last-Modified = HTTP-date
1271
1272   OWS = <OWS, defined in [Part1], Section 3.2.1>
1273
1274   entity-tag = [ weak ] opaque-tag
1275   etagc = "!" / %x23-7E ; '#'-'~'
1276    / obs-text
1277
1278   obs-text = <obs-text, defined in [Part1], Section 3.2.4>
1279   opaque-tag = DQUOTE *etagc DQUOTE
1280
1281   weak = %x57.2F ; W/
1282
1283
1284
1285
1286
1287Fielding, et al.        Expires January 17, 2013               [Page 23]
1288
1289Internet-Draft              HTTP/1.1, Part 4                   July 2012
1290
1291
1292Appendix D.  Change Log (to be removed by RFC Editor before publication)
1293
1294   Changes up to the first Working Group Last Call draft are summarized
1295   in <http://tools.ietf.org/html/
1296   draft-ietf-httpbis-p4-conditional-19#appendix-C>.
1297
1298D.1.  Since draft-ietf-httpbis-p4-conditional-19
1299
1300   Closed issues:
1301
1302   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/241>: "Need to
1303      clarify eval order/interaction of conditional headers"
1304
1305   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/354>: "ETags and
1306      Conditional Requests"
1307
1308   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
1309      requirements for recipients"
1310
1311   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/363>: "Rare cases"
1312
1313   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/365>: "Conditional
1314      Request Security Considerations"
1315
1316   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/371>: "If-Modified-
1317      Since lacks definition for method != GET"
1318
1319   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/372>: "refactor
1320      conditional header field descriptions"
1321
1322Index
1323
1324   3
1325      304 Not Modified (status code)  18
1326
1327   4
1328      412 Precondition Failed (status code)  19
1329
1330   E
1331      ETag header field  9
1332
1333   G
1334      Grammar
1335         entity-tag  10
1336         ETag  10
1337         etagc  10
1338         If-Match  14
1339         If-Modified-Since  16
1340
1341
1342
1343Fielding, et al.        Expires January 17, 2013               [Page 24]
1344
1345Internet-Draft              HTTP/1.1, Part 4                   July 2012
1346
1347
1348         If-None-Match  15
1349         If-Unmodified-Since  17
1350         Last-Modified  7
1351         opaque-tag  10
1352         weak  10
1353
1354   H
1355      Header Fields
1356         ETag  9
1357         If-Match  14
1358         If-Modified-Since  16
1359         If-None-Match  15
1360         If-Unmodified-Since  17
1361         Last-Modified  7
1362
1363   I
1364      If-Match header field  14
1365      If-Modified-Since header field  16
1366      If-None-Match header field  15
1367      If-Unmodified-Since header field  17
1368
1369   L
1370      Last-Modified header field  7
1371
1372   M
1373      metadata  5
1374
1375   S
1376      selected representation  4
1377      Status Codes
1378         304 Not Modified  18
1379         412 Precondition Failed  19
1380
1381   V
1382      validator  5
1383         strong  6
1384         weak  6
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399Fielding, et al.        Expires January 17, 2013               [Page 25]
1400
1401Internet-Draft              HTTP/1.1, Part 4                   July 2012
1402
1403
1404Authors' Addresses
1405
1406   Roy T. Fielding (editor)
1407   Adobe Systems Incorporated
1408   345 Park Ave
1409   San Jose, CA  95110
1410   USA
1411
1412   EMail: fielding@gbiv.com
1413   URI:   http://roy.gbiv.com/
1414
1415
1416   Yves Lafon (editor)
1417   World Wide Web Consortium
1418   W3C / ERCIM
1419   2004, rte des Lucioles
1420   Sophia-Antipolis, AM  06902
1421   France
1422
1423   EMail: ylafon@w3.org
1424   URI:   http://www.raubacapeu.net/people/yves/
1425
1426
1427   Julian F. Reschke (editor)
1428   greenbytes GmbH
1429   Hafenweg 16
1430   Muenster, NW  48155
1431   Germany
1432
1433   EMail: julian.reschke@greenbytes.de
1434   URI:   http://greenbytes.de/tech/webdav/
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455Fielding, et al.        Expires January 17, 2013               [Page 26]
1456
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