source: draft-ietf-httpbis/15/draft-ietf-httpbis-p4-conditional-15.xml @ 2082

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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
7<?rfc toc="yes" ?>
8<?rfc symrefs="yes" ?>
9<?rfc sortrefs="yes" ?>
10<?rfc compact="yes"?>
11<?rfc subcompact="no" ?>
12<?rfc linkmailto="no" ?>
13<?rfc editing="no" ?>
14<?rfc comments="yes"?>
15<?rfc inline="yes"?>
16<?rfc rfcedstyle="yes"?>
17<!DOCTYPE rfc
18  PUBLIC "" "rfc2629.dtd">
19<rfc obsoletes="2616" category="std" ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-15">
20<front>
21
22  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
23
24  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
25    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
26    <address>
27      <postal>
28        <street>345 Park Ave</street>
29        <city>San Jose</city>
30        <region>CA</region>
31        <code>95110</code>
32        <country>USA</country>
33      </postal>
34      <email>fielding@gbiv.com</email>
35      <uri>http://roy.gbiv.com/</uri>
36    </address>
37  </author>
38
39  <author initials="J." surname="Gettys" fullname="Jim Gettys">
40    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
41    <address>
42      <postal>
43        <street>21 Oak Knoll Road</street>
44        <city>Carlisle</city>
45        <region>MA</region>
46        <code>01741</code>
47        <country>USA</country>
48      </postal>
49      <email>jg@freedesktop.org</email>
50      <uri>http://gettys.wordpress.com/</uri>
51    </address>
52  </author>
53 
54  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
55    <organization abbrev="HP">Hewlett-Packard Company</organization>
56    <address>
57      <postal>
58        <street>HP Labs, Large Scale Systems Group</street>
59        <street>1501 Page Mill Road, MS 1177</street>
60        <city>Palo Alto</city>
61        <region>CA</region>
62        <code>94304</code>
63        <country>USA</country>
64      </postal>
65      <email>JeffMogul@acm.org</email>
66    </address>
67  </author>
68
69  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
70    <organization abbrev="Microsoft">Microsoft Corporation</organization>
71    <address>
72      <postal>
73        <street>1 Microsoft Way</street>
74        <city>Redmond</city>
75        <region>WA</region>
76        <code>98052</code>
77        <country>USA</country>
78      </postal>
79      <email>henrikn@microsoft.com</email>
80    </address>
81  </author>
82
83  <author initials="L." surname="Masinter" fullname="Larry Masinter">
84    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
85    <address>
86      <postal>
87        <street>345 Park Ave</street>
88        <city>San Jose</city>
89        <region>CA</region>
90        <code>95110</code>
91        <country>USA</country>
92      </postal>
93      <email>LMM@acm.org</email>
94      <uri>http://larry.masinter.net/</uri>
95    </address>
96  </author>
97 
98  <author initials="P." surname="Leach" fullname="Paul J. Leach">
99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
100    <address>
101      <postal>
102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106      </postal>
107      <email>paulle@microsoft.com</email>
108    </address>
109  </author>
110   
111  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
112    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
113    <address>
114      <postal>
115        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
116        <street>The Stata Center, Building 32</street>
117        <street>32 Vassar Street</street>
118        <city>Cambridge</city>
119        <region>MA</region>
120        <code>02139</code>
121        <country>USA</country>
122      </postal>
123      <email>timbl@w3.org</email>
124      <uri>http://www.w3.org/People/Berners-Lee/</uri>
125    </address>
126  </author>
127
128  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
129    <organization abbrev="W3C">World Wide Web Consortium</organization>
130    <address>
131      <postal>
132        <street>W3C / ERCIM</street>
133        <street>2004, rte des Lucioles</street>
134        <city>Sophia-Antipolis</city>
135        <region>AM</region>
136        <code>06902</code>
137        <country>France</country>
138      </postal>
139      <email>ylafon@w3.org</email>
140      <uri>http://www.raubacapeu.net/people/yves/</uri>
141    </address>
142  </author>
143
144  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
145    <organization abbrev="greenbytes">greenbytes GmbH</organization>
146    <address>
147      <postal>
148        <street>Hafenweg 16</street>
149        <city>Muenster</city><region>NW</region><code>48155</code>
150        <country>Germany</country>
151      </postal>
152      <phone>+49 251 2807760</phone>
153      <facsimile>+49 251 2807761</facsimile>
154      <email>julian.reschke@greenbytes.de</email>
155      <uri>http://greenbytes.de/tech/webdav/</uri>
156    </address>
157  </author>
158
159  <date month="July" year="2011" day="11"/>
160  <workgroup>HTTPbis Working Group</workgroup>
161
162<abstract>
163<t>
164   The Hypertext Transfer Protocol (HTTP) is an application-level
165   protocol for distributed, collaborative, hypermedia information
166   systems. HTTP has been in use by the World Wide Web global information
167   initiative since 1990. This document is Part 4 of the seven-part specification
168   that defines the protocol referred to as "HTTP/1.1" and, taken together,
169   obsoletes RFC 2616.  Part 4 defines request header fields for
170   indicating conditional requests and the rules for constructing responses
171   to those requests.
172</t>
173</abstract>
174
175<note title="Editorial Note (To be removed by RFC Editor)">
176  <t>
177    Discussion of this draft should take place on the HTTPBIS working group
178    mailing list (ietf-http-wg@w3.org), which is archived at
179    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
180  </t>
181  <t>
182    The current issues list is at
183    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
184    documents (including fancy diffs) can be found at
185    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
186  </t>
187  <t>
188    The changes in this draft are summarized in <xref target="changes.since.14"/>.
189  </t>
190</note>
191</front>
192<middle>
193<section title="Introduction" anchor="introduction">
194<t>
195   This document defines the HTTP/1.1 conditional request mechanisms,
196   including both response metadata that can be used to indicate or
197   observe changes to resource state and request header fields that
198   specify preconditions to be checked before performing the action
199   given by the request method.  Conditional GET requests are the most
200   efficient mechanism for HTTP cache updates <xref target="Part6"/>.  Conditionals
201   can also be
202   applied to state-changing methods, such as PUT and DELETE, to prevent
203   the "lost update" problem: one client accidentally overwriting
204   the work of another client that has been acting in parallel.
205</t>
206<t>
207   Conditional request preconditions are based on the state of the target
208   resource as a whole (its current value set) or the state as observed
209   in a previously obtained representation (one value in that set).
210   A resource might have multiple current representations, each with its
211   own observable state.  The conditional request mechanisms assume that
212   the mapping of requests to corresponding representations will be
213   consistent over time if the server intends to take advantage of
214   conditionals.  Regardless, if the mapping is inconsistent and
215   the server is unable to select the appropriate representation, then
216   no harm will result when the precondition evaluates to false.
217</t>
218<t><iref primary="true" item="selected representation"/>
219   We use the term "selected representation" to refer to
220   the current representation of the target resource that would have been
221   selected in a successful response if the same request had used the method
222   GET and had excluded all of the conditional request header fields.
223   The conditional request preconditions are evaluated by comparing the
224   values provided in the request header fields to the current metadata
225   for the selected representation.
226</t>
227
228<section title="Requirements" anchor="intro.requirements">
229<t>
230   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
231   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
232   document are to be interpreted as described in <xref target="RFC2119"/>.
233</t>
234<t>
235   An implementation is not compliant if it fails to satisfy one or more
236   of the "MUST" or "REQUIRED" level requirements for the protocols it
237   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
238   level and all the "SHOULD" level requirements for its protocols is said
239   to be "unconditionally compliant"; one that satisfies all the "MUST"
240   level requirements but not all the "SHOULD" level requirements for its
241   protocols is said to be "conditionally compliant".
242</t>
243</section>
244
245<section title="Syntax Notation" anchor="notation">
246 
247 
248 
249 
250 
251 
252 
253 
254 
255 
256 
257<t>
258  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
259  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
260  <xref target="collected.abnf"/> shows the collected ABNF, with the list
261  rule expanded.
262</t>
263<t>
264  The following core rules are included by
265  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
266  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
267  DIGIT (decimal 0-9), DQUOTE (double quote),
268  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
269  OCTET (any 8-bit sequence of data), SP (space),
270  VCHAR (any visible USASCII character),
271  and WSP (whitespace).
272</t>
273<t>
274  The ABNF rules below are defined in other parts:
275</t>
276<figure><artwork type="abnf2616"><![CDATA[
277  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
278  OWS           = <OWS, defined in [Part1], Section 1.2.2>
279  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
280]]></artwork></figure>
281</section>
282</section>
283
284<section title="Resource State Metadata (Validators)" anchor="resource.metadata">
285   <iref primary="true" item="metadata"/>
286   <iref primary="true" item="validator"/>
287<t>
288   This specification defines two forms of metadata that are commonly used
289   to observe resource state and test for preconditions: modification dates
290   and opaque entity tags.  Additional metadata that reflects resource state
291   has been defined by various extensions of HTTP, such as WebDAV
292   <xref target="RFC4918"/>, that are beyond the scope of this specification.
293   A resource metadata value is referred to as a "validator"
294   when it is used within a precondition.
295</t>
296
297<section title="Last-Modified" anchor="header.last-modified">
298  <iref primary="true" item="Last-Modified header field"/>
299  <iref primary="true" item="Header Fields" subitem="Last-Modified"/>
300 
301<t>
302   The "Last-Modified" header field indicates the date and time at
303   which the origin server believes the selected representation was
304   last modified.
305</t>
306<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><artwork type="abnf2616"><![CDATA[
307  Last-Modified = HTTP-date
308]]></artwork></figure>
309<t>
310   An example of its use is
311</t>
312<figure><artwork type="example"><![CDATA[
313  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
314]]></artwork></figure>
315
316<section title="Generation" anchor="lastmod.generation">
317<t>
318   Origin servers SHOULD send Last-Modified for any selected
319   representation for which a last modification date can be reasonably
320   and consistently determined, since its use in conditional requests
321   and evaluating cache freshness (<xref target="Part6"/>) results in a substantial
322   reduction of HTTP traffic on the Internet and can be a significant
323   factor in improving service scalability and reliability.
324</t>
325<t>
326   A representation is typically the sum of many parts behind the
327   resource interface.  The last-modified time would usually be
328   the most recent time that any of those parts were changed.
329   How that value is determined for any given resource is an
330   implementation detail beyond the scope of this specification.
331   What matters to HTTP is how recipients of the Last-Modified
332   header field can use its value to make conditional requests
333   and test the validity of locally cached responses.
334</t>
335<t>
336   An origin server SHOULD obtain the Last-Modified value of the
337   representation as close as possible to the time that it generates
338   the Date field-value for its response. This allows a recipient to
339   make an accurate assessment of the representation's modification time,
340   especially if the representation changes near the time that the
341   response is generated.
342</t>
343<t>
344   An origin server with a clock MUST NOT send a Last-Modified date
345   that is later than the server's time of message origination (Date).
346   If the last modification time is derived from implementation-specific
347   metadata that evaluates to some time in the future, according to the
348   origin server's clock, then the origin server MUST replace that
349   value with the message origination date. This prevents a future
350   modification date from having an adverse impact on cache validation.
351</t>
352</section>
353
354<section title="Comparison" anchor="lastmod.comparison">
355<t>
356   A Last-Modified time, when used as a validator in a request, is
357   implicitly weak unless it is possible to deduce that it is strong,
358   using the following rules:
359  <list style="symbols">
360     <t>The validator is being compared by an origin server to the
361        actual current validator for the representation and,</t>
362     <t>That origin server reliably knows that the associated representation did
363        not change twice during the second covered by the presented
364        validator.</t>
365  </list>
366</t>
367<t>
368   or
369  <list style="symbols">
370     <t>The validator is about to be used by a client in an If-Modified-Since
371        or If-Unmodified-Since header field, because the client
372        has a cache entry for the associated representation, and</t>
373     <t>That cache entry includes a Date value, which gives the time
374        when the origin server sent the original response, and</t>
375     <t>The presented Last-Modified time is at least 60 seconds before
376        the Date value.</t>
377  </list>
378</t>
379<t>
380   or
381  <list style="symbols">
382     <t>The validator is being compared by an intermediate cache to the
383        validator stored in its cache entry for the representation, and</t>
384     <t>That cache entry includes a Date value, which gives the time
385        when the origin server sent the original response, and</t>
386     <t>The presented Last-Modified time is at least 60 seconds before
387        the Date value.</t>
388  </list>
389</t>
390<t>
391   This method relies on the fact that if two different responses were
392   sent by the origin server during the same second, but both had the
393   same Last-Modified time, then at least one of those responses would
394   have a Date value equal to its Last-Modified time. The arbitrary 60-second
395   limit guards against the possibility that the Date and Last-Modified
396   values are generated from different clocks, or at somewhat
397   different times during the preparation of the response. An
398   implementation MAY use a value larger than 60 seconds, if it is
399   believed that 60 seconds is too short.
400</t>
401</section>
402</section>
403
404<section title="ETag" anchor="header.etag">
405  <iref primary="true" item="ETag header field"/>
406  <iref primary="true" item="Header Fields" subitem="ETag"/>
407 
408 
409 
410 
411 
412<t>
413   The ETag header field provides the current entity-tag for the
414   selected representation.
415   An entity-tag is an opaque validator for differentiating between
416   multiple representations of the same resource, regardless of whether
417   those multiple representations are due to resource state changes over
418   time, content negotiation resulting in multiple representations being
419   valid at the same time, or both. An entity-tag consists of an opaque
420   quoted string, possibly prefixed by a weakness indicator.
421</t>
422<figure><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/><artwork type="abnf2616"><![CDATA[
423  ETag       = entity-tag
424
425  entity-tag = [ weak ] opaque-tag
426  weak       = %x57.2F ; "W/", case-sensitive
427  opaque-tag = quoted-string
428]]></artwork></figure>
429<t>
430   An entity-tag can be more reliable for validation than a modification
431   date in situations where it is inconvenient to store modification
432   dates, where the one-second resolution of HTTP date values is not
433   sufficient, or where modification dates are not consistently maintained.
434</t>
435<figure><preamble>
436  Examples:
437</preamble>
438<artwork type="example"><![CDATA[
439  ETag: "xyzzy"
440  ETag: W/"xyzzy"
441  ETag: ""
442]]></artwork></figure>
443
444<section title="Generation" anchor="entity.tag.generation">
445<t>
446   The principle behind entity-tags is that only the service author
447   knows the implementation of a resource well enough to select the
448   most accurate and efficient validation mechanism for that resource,
449   and that any such mechanism can be mapped to a simple sequence of
450   octets for easy comparison.  Since the value is opaque, there is no
451   need for the client to be aware of how each entity-tag is constructed.
452</t>
453<t>
454   For example, a resource that has implementation-specific versioning
455   applied to all changes might use an internal revision number, perhaps
456   combined with a variance identifier for content negotiation, to
457   accurately differentiate between representations.
458   Other implementations might use a stored hash of representation content,
459   a combination of various filesystem attributes, or a modification
460   timestamp that has sub-second resolution.
461</t>
462<t>
463   Origin servers SHOULD send ETag for any selected representation
464   for which detection of changes can be reasonably and consistently
465   determined, since the entity-tag's use in conditional requests and
466   evaluating cache freshness (<xref target="Part6"/>) can result in a substantial
467   reduction of HTTP network traffic and can be a significant factor in
468   improving service scalability and reliability.
469</t>
470</section>
471
472<section title="Weak versus Strong" anchor="weak.and.strong.validators">
473<t>
474   Since both origin servers and caches will compare two validators to
475   decide if they indicate the same or different representations, one
476   normally would expect that if the representation (including both
477   representation header fields and representation body) changes in any
478   way, then the associated validator would change as well. If this is
479   true, then we call that validator a "strong validator".  One example
480   of a strong validator is an integer that is incremented in stable
481   storage every time a representation is changed.
482</t>
483<t>
484   However, there might be cases when a server prefers to change the
485   validator only when it desires cached representations to be invalidated.
486   For example, the representation of a weather report that changes in
487   content every second, based on dynamic measurements, might be grouped
488   into sets of equivalent representations (from the origin server's
489   perspective) in order to allow cached representations to be valid
490   for a reasonable period of time (perhaps adjusted dynamically based
491   on server load or weather quality).
492   A validator that does not always change when the representation
493   changes is a "weak validator".
494</t>
495<t>
496   One can think of a strong validator as part of an identifier for a
497   specific representation, whereas a weak validator is part of an
498   identifier for a set of equivalent representations (where this notion
499   of equivalence is entirely governed by the origin server and beyond
500   the scope of this specification).
501</t>
502<t>
503   An entity-tag is normally a strong validator, but the protocol
504   provides a mechanism to tag an entity-tag as "weak".
505  <list><t>
506      A representation's modification time, if defined with only one-second
507      resolution, could be a weak validator, since it is possible that
508      the representation might be modified twice during a single second.
509    </t><t>
510      Support for weak validators is optional. However, weak validators
511      allow for more efficient caching of equivalent objects; for
512      example, a hit counter on a site is probably good enough if it is
513      updated every few days or weeks, and any value during that period
514      is likely "good enough" to be equivalent.
515    </t></list>
516</t>
517<t>
518   A strong entity-tag MUST change whenever the associated representation
519   changes in any way. A weak entity-tag SHOULD change whenever the origin
520   server considers prior representations to be unacceptable as a substitute
521   for the current representation. In other words, a weak entity tag SHOULD
522   change whenever the origin server wants caches to invalidate old responses.
523</t>
524<t>
525   A "strong entity-tag" MAY be shared by two representations of a resource
526   only if they are equivalent by octet equality.
527</t>
528<t>
529   A "weak entity-tag", indicated by the "W/" prefix, MAY be shared by
530   two representations of a resource. A weak entity-tag can only be used
531   for weak comparison.
532</t>
533<t>
534   Cache entries might persist for arbitrarily long periods, regardless
535   of expiration times.  Thus, a cache might attempt to validate an
536   entry using a validator that it obtained in the distant past.
537   A strong entity-tag MUST be unique across all versions of all
538   representations associated with a particular resource over time.
539   However, there is no implication of uniqueness across entity-tags
540   of different resources (i.e., the same entity-tag value might be
541   in use for representations of multiple resources at the same time
542   and does not imply that those representations are equivalent).
543</t>
544</section>
545
546<section title="Comparison" anchor="entity.tag.comparison">
547 
548<t>
549   There are two entity-tag comparison functions, depending
550   on whether the comparison context allows the use of weak validators
551   or not:
552  <list style="symbols">
553     <t>The strong comparison function: in order to be considered equal,
554        both opaque-tags MUST be identical character-by-character, and both
555        MUST NOT be weak.</t>
556     <t>The weak comparison function: in order to be considered equal, both
557        opaque-tags MUST be identical character-by-character, but
558        either or both of them MAY be tagged as "weak" without affecting
559        the result.</t>
560  </list>
561</t>
562<t>
563   A "use" of a validator is either when a client generates a request
564   and includes the validator in a precondition, or when a server
565   compares two validators.
566</t>
567<t>
568   Strong validators are usable in any context. Weak validators are only
569   usable in contexts that do not depend on exact equality of a representation.
570   For example, either kind is usable for a normal conditional GET.
571</t>
572<t>
573   The example below shows the results for a set of entity-tag pairs,
574   and both the weak and strong comparison function results:
575</t>
576<texttable align="left">
577  <ttcol>ETag 1</ttcol>
578  <ttcol>ETag 2</ttcol>
579  <ttcol>Strong Comparison</ttcol>
580  <ttcol>Weak Comparison</ttcol>
581
582  <c>W/"1"</c>
583  <c>W/"1"</c>
584  <c>no match</c>
585  <c>match</c>
586 
587  <c>W/"1"</c>
588  <c>W/"2"</c>
589  <c>no match</c>
590  <c>no match</c>
591
592  <c>W/"1"</c>
593  <c>"1"</c>
594  <c>no match</c>
595  <c>match</c>
596
597  <c>"1"</c>
598  <c>"1"</c>
599  <c>match</c>
600  <c>match</c>
601</texttable>
602<t>
603   An entity-tag is strong unless it is explicitly tagged as weak.
604</t>
605</section>
606
607<section title="Rules for When to Use Entity-tags and Last-Modified Dates" anchor="rules.for.when.to.use.entity.tags.and.last-modified.dates">
608<t>
609   We adopt a set of rules and recommendations for origin servers,
610   clients, and caches regarding when various validator types ought to
611   be used, and for what purposes.
612</t>
613<t>
614   HTTP/1.1 origin servers:
615  <list style="symbols">
616     <t>SHOULD send an entity-tag validator unless it is not feasible to
617        generate one.</t>
618
619     <t>MAY send a weak entity-tag instead of a strong entity-tag, if
620        performance considerations support the use of weak entity-tags,
621        or if it is unfeasible to send a strong entity-tag.</t>
622
623     <t>SHOULD send a Last-Modified value if it is feasible to send one.</t>
624  </list>
625</t>
626<t>
627   In other words, the preferred behavior for an HTTP/1.1 origin server
628   is to send both a strong entity-tag and a Last-Modified value.
629</t>
630<t>
631   HTTP/1.1 clients:
632  <list style="symbols">
633     <t>MUST use that entity-tag in any cache-conditional request (using
634        If-Match or If-None-Match) if an entity-tag has been provided by the
635        origin server.</t>
636
637     <t>SHOULD use the Last-Modified value in non-subrange cache-conditional
638        requests (using If-Modified-Since) if only a Last-Modified value has
639        been provided by the origin server. </t>
640
641     <t>MAY use the Last-Modified value in subrange cache-conditional
642        requests (using If-Unmodified-Since) if only a Last-Modified value has
643        been provided by an HTTP/1.0 origin server. The user agent SHOULD
644        provide a way to disable this, in case of difficulty.</t>
645
646     <t>SHOULD use both validators in cache-conditional requests if both an
647        entity-tag and a Last-Modified value have been provided by the origin
648        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
649        appropriately.</t>
650  </list>
651</t>
652<t>
653   An HTTP/1.1 origin server, upon receiving a conditional request that
654   includes both a Last-Modified date (e.g., in an If-Modified-Since or
655   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
656   in an If-Match, If-None-Match, or If-Range header field) as cache
657   validators, MUST NOT return a response status code of 304 (Not Modified)
658   unless doing so is consistent with all of the conditional header
659   fields in the request.
660</t>
661<t>
662   An HTTP/1.1 caching proxy, upon receiving a conditional request that
663   includes both a Last-Modified date and one or more entity-tags as
664   cache validators, MUST NOT return a locally cached response to the
665   client unless that cached response is consistent with all of the
666   conditional header fields in the request.
667  <list><t>
668      Note: The general principle behind these rules is that HTTP/1.1
669      servers and clients ought to transmit as much non-redundant
670      information as is available in their responses and requests.
671      HTTP/1.1 systems receiving this information will make the most
672      conservative assumptions about the validators they receive.
673  </t><t>
674      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
675      last-modified values received or used by these systems will
676      support transparent and efficient caching, and so HTTP/1.1 origin
677      servers should provide Last-Modified values. In those rare cases
678      where the use of a Last-Modified value as a validator by an
679      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
680      origin servers should not provide one.
681  </t></list>
682</t>
683</section>
684
685<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
686<t>
687   Consider a resource that is subject to content negotiation (Section 5 of <xref target="Part3"/>),
688   and where the representations returned upon a GET request vary based on
689   the Accept-Encoding request header field (Section 6.3 of <xref target="Part3"/>):
690</t>
691<figure><preamble>&gt;&gt; Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
692  GET /index HTTP/1.1
693  Host: www.example.com
694  Accept-Encoding: gzip
695 
696  ]]></artwork></figure>
697<t>
698   In this case, the response might or might not use the gzip content coding.
699   If it does not, the response might look like:
700</t>
701<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
702  HTTP/1.1 200 OK
703  Date: Thu, 26 Mar 2010 00:05:00 GMT
704  ETag: "123-a"
705  Content-Length: 70
706  Vary: Accept-Encoding
707  Content-Type: text/plain
708 
709  Hello World!
710  Hello World!
711  Hello World!
712  Hello World!
713  Hello World!
714  ]]></artwork></figure>
715<t>
716   An alternative representation that does use gzip content coding would be:
717</t>
718<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
719  HTTP/1.1 200 OK
720  Date: Thu, 26 Mar 2010 00:05:00 GMT
721  ETag: "123-b"
722  Content-Length: 43
723  Vary: Accept-Encoding
724  Content-Type: text/plain
725  Content-Encoding: gzip
726 
727  ...binary data...]]></artwork></figure>
728<t><list>
729  <t>
730    Note: Content codings are a property of the representation,
731    so therefore an entity-tag of an encoded representation must be distinct
732    from an unencoded representation to prevent conflicts during cache updates
733    and range requests.  In contrast, transfer codings (Section 6.2 of <xref target="Part1"/>)
734    apply only during message transfer and do not require distinct entity-tags.
735  </t>
736</list></t>
737</section>
738</section>
739</section>
740
741<section title="Precondition Header Fields" anchor="header.fields">
742<t>
743   This section defines the syntax and semantics of HTTP/1.1 header fields
744   for applying preconditions on requests.
745</t>
746
747<section title="If-Match" anchor="header.if-match">
748  <iref primary="true" item="If-Match header field"/>
749  <iref primary="true" item="Header Fields" subitem="If-Match"/>
750 
751<t>
752   The "If-Match" header field MAY be used to make a request method
753   conditional on the current existence or value of an entity-tag for
754   one or more representations of the target resource.  If-Match is
755   generally useful for resource update requests, such as PUT requests,
756   as a means for protecting against accidental overwrites when multiple
757   clients are acting in parallel on the same resource (i.e., the
758   "lost update" problem).  An If-Match field-value of "*" places the
759   precondition on the existence of any current representation for the
760   target resource.
761</t>
762<figure><iref primary="true" item="Grammar" subitem="If-Match"/><artwork type="abnf2616"><![CDATA[
763  If-Match = "*" / 1#entity-tag
764]]></artwork></figure>
765<t>
766   If any of the entity-tags listed in the If-Match field value match
767   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
768   selected representation for the target resource,
769   or if "*" is given and any current representation exists for the
770   target resource, then the server MAY perform the request method
771   as if the If-Match header field was not present.
772</t>
773<t>
774   If none of the entity-tags match, or if "*" is given and no current
775   representation exists, the server MUST NOT perform the requested method.
776   Instead, the server MUST respond with the 412 (Precondition Failed)
777   status code.
778</t>
779<t>
780   If the request would, without the If-Match header field, result in
781   anything other than a 2xx or 412 status code, then the If-Match header field
782   MUST be ignored.
783</t>
784<t>
785   Examples:
786</t>
787<figure><artwork type="example"><![CDATA[
788  If-Match: "xyzzy"
789  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
790  If-Match: *
791]]></artwork></figure>
792<t>
793   The result of a request having both an If-Match header field and
794   either an If-None-Match or an If-Modified-Since header fields is
795   undefined by this specification.
796</t>
797</section>
798
799<section title="If-None-Match" anchor="header.if-none-match">
800  <iref primary="true" item="If-None-Match header field"/>
801  <iref primary="true" item="Header Fields" subitem="If-None-Match"/>
802 
803<t>
804   The "If-None-Match" header field MAY be used to make a request method
805   conditional on not matching any of the current entity-tag values for
806   representations of the target resource.  If-None-Match is primarily
807   used in conditional GET requests to enable efficient updates of cached
808   information with a minimum amount of transaction overhead.  A client
809   that has one or more representations previously obtained from the
810   target resource can send If-None-Match with a list of the associated
811   entity-tags in the hope of receiving a 304 response if at least one
812   of those representations matches the selected representation.
813</t>
814<t>
815   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
816   request method (e.g., PUT) from inadvertently modifying an existing
817   representation of the target resource when the client believes that
818   the resource does not have a current representation.  This is a variation
819   on the "lost update" problem that might arise if more than one client
820   attempts to create an initial representation for the target resource.
821</t>
822<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><artwork type="abnf2616"><![CDATA[
823  If-None-Match = "*" / 1#entity-tag
824]]></artwork></figure>
825<t>
826   If any of the entity-tags listed in the If-None-Match field-value match
827   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
828   selected representation, or if "*" is
829   given and any current representation exists for that resource, then the
830   server MUST NOT perform the requested method.
831   Instead, if the request method was GET or HEAD, the server SHOULD
832   respond with a 304 (Not Modified) status code, including the cache-related
833   header fields (particularly ETag) of the selected representation that has
834   a matching entity-tag.  For all other request methods, the server MUST
835   respond with a 412 (Precondition Failed) status code.
836</t>
837<t>
838   If none of the entity-tags match, then the server MAY perform the
839   requested method as if the If-None-Match header field did not exist,
840   but MUST also ignore any If-Modified-Since header field(s) in the
841   request. That is, if no entity-tags match, then the server MUST NOT
842   return a 304 (Not Modified) response.
843</t>
844<t>
845   If the request would, without the If-None-Match header field, result
846   in anything other than a 2xx or 304 status code, then the If-None-Match
847   header field MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
848   a discussion of server behavior when both If-Modified-Since and
849   If-None-Match appear in the same request.)
850</t>
851<t>
852   Examples:
853</t>
854<figure><artwork type="example"><![CDATA[
855  If-None-Match: "xyzzy"
856  If-None-Match: W/"xyzzy"
857  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
858  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
859  If-None-Match: *
860]]></artwork></figure>
861<t>
862   The result of a request having both an If-None-Match header field and
863   either an If-Match or an If-Unmodified-Since header fields is
864   undefined by this specification.
865</t>
866</section>
867
868<section title="If-Modified-Since" anchor="header.if-modified-since">
869  <iref primary="true" item="If-Modified-Since header field"/>
870  <iref primary="true" item="Header Fields" subitem="If-Modified-Since"/>
871 
872<t>
873   The "If-Modified-Since" header field MAY be used to make a request
874   method conditional by modification date: if the selected representation
875   has not been modified since the time specified in this field, then
876   do not perform the request method; instead, respond as detailed below.
877</t>
878<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><artwork type="abnf2616"><![CDATA[
879  If-Modified-Since = HTTP-date
880]]></artwork></figure>
881<t>
882   An example of the field is:
883</t>
884<figure><artwork type="example"><![CDATA[
885  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
886]]></artwork></figure>
887<t>
888   A GET method with an If-Modified-Since header field and no Range header
889   field requests that the selected representation be transferred only if
890   it has been modified since the date given by the If-Modified-Since
891   header field.
892   The algorithm for determining this includes the following cases:
893  <list style="numbers">
894      <t>If the request would normally result in anything other than a
895         200 (OK) status code, or if the passed If-Modified-Since date is
896         invalid, the response is exactly the same as for a normal GET.
897         A date which is later than the server's current time is
898         invalid.</t>
899
900      <t>If the selected representation has been modified since the
901         If-Modified-Since date, the response is exactly the same as for
902         a normal GET.</t>
903
904      <t>If the selected representation has not been modified since a valid
905         If-Modified-Since date, the server SHOULD return a
906         304 (Not Modified) response.</t>
907  </list>
908</t>
909<t>
910   The purpose of this feature is to allow efficient updates of cached
911   information with a minimum amount of transaction overhead.
912  <list><t>
913      Note: The Range header field modifies the meaning of If-Modified-Since;
914      see Section 5.4 of <xref target="Part5"/> for full details.
915    </t><t>
916      Note: If-Modified-Since times are interpreted by the server, whose
917      clock might not be synchronized with the client.
918    </t><t>
919      Note: When handling an If-Modified-Since header field, some
920      servers will use an exact date comparison function, rather than a
921      less-than function, for deciding whether to send a 304 (Not
922      Modified) response. To get best results when sending an If-Modified-Since
923      header field for cache validation, clients are
924      advised to use the exact date string received in a previous Last-Modified
925      header field whenever possible.
926    </t><t>
927      Note: If a client uses an arbitrary date in the If-Modified-Since
928      header field instead of a date taken from the Last-Modified header field for
929      the same request, the client needs to be aware that this
930      date is interpreted in the server's understanding of time.
931      Unsynchronized clocks and rounding problems, due to the different
932      encodings of time between the client and server, are concerns.
933      This includes the possibility of race conditions if the
934      document has changed between the time it was first requested and
935      the If-Modified-Since date of a subsequent request, and the
936      possibility of clock-skew-related problems if the If-Modified-Since
937      date is derived from the client's clock without correction
938      to the server's clock. Corrections for different time bases
939      between client and server are at best approximate due to network
940      latency.
941    </t>
942  </list>
943</t>
944<t>
945   The result of a request having both an If-Modified-Since header field
946   and either an If-Match or an If-Unmodified-Since header fields is
947   undefined by this specification.
948</t>
949</section>
950
951<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
952  <iref primary="true" item="If-Unmodified-Since header field"/>
953  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since"/>
954 
955<t>
956   The "If-Unmodified-Since" header field MAY be used to make a request
957   method conditional by modification date: if the selected representation
958   has been modified since the time specified in this field, then the
959   server MUST NOT perform the requested operation and MUST instead
960   respond with the 412 (Precondition Failed) status code.
961   If the selected representation has not been modified since the time
962   specified in this field, the server SHOULD perform the request
963   method as if the If-Unmodified-Since header field were not present.
964</t>
965<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><artwork type="abnf2616"><![CDATA[
966  If-Unmodified-Since = HTTP-date
967]]></artwork></figure>
968<t>
969   An example of the field is:
970</t>
971<figure><artwork type="example"><![CDATA[
972  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
973]]></artwork></figure>
974<t>
975   If the request normally (i.e., without the If-Unmodified-Since
976   header field) would result in anything other than a 2xx or 412 status code,
977   the If-Unmodified-Since header field SHOULD be ignored.
978</t>
979<t>
980   If the specified date is invalid, the header field MUST be ignored.
981</t>
982<t>
983   The result of a request having both an If-Unmodified-Since header
984   field and either an If-None-Match or an If-Modified-Since header
985   fields is undefined by this specification.
986</t>
987</section>
988
989<section title="If-Range" anchor="header.if-range">
990<t>
991   The If-Range header field provides a special conditional request
992   mechanism that is similar to If-Match and If-Unmodified-Since but
993   specific to HTTP range requests. If-Range is defined in Section 5.3 of <xref target="Part5"/>.
994</t>
995</section>
996
997</section>
998
999<section title="Status Code Definitions" anchor="status.code.definitions">
1000<section title="304 Not Modified" anchor="status.304">
1001  <iref primary="true" item="304 Not Modified (status code)"/>
1002  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
1003<t>
1004   The 304 status code indicates that a conditional GET request has been
1005   received and would have resulted in a 200 (OK) response if it were not
1006   for the fact that the condition has evaluated to false.  In other words,
1007   there is no need for the server to transfer a representation of the
1008   target resource because the client's request indicates that it already
1009   has a valid representation, as indicated by the 304 response header
1010   fields, and is therefore redirecting the client to make use of that
1011   stored representation as if it were the payload of a 200 response.
1012   The 304 response MUST NOT contain a message-body, and thus is always
1013   terminated by the first empty line after the header fields.
1014</t>
1015<t>
1016   A 304 response MUST include a Date header field (Section 9.3 of <xref target="Part1"/>)
1017   unless its omission is required by Section 9.3.1 of <xref target="Part1"/>.  If a 200 response
1018   to the same request would have included any of the header fields
1019   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1020   Vary, then those same header fields MUST be sent in a 304 response.
1021</t>
1022<t>
1023   Since the goal of a 304 response is to minimize information transfer
1024   when the recipient already has one or more cached representations,
1025   the response SHOULD NOT include representation metadata other
1026   than the above listed fields unless said metadata exists for the
1027   purpose of guiding cache updates (e.g., future HTTP extensions).
1028</t>
1029<t>
1030   If the recipient of a 304 response does not have a cached representation
1031   corresponding to the entity-tag indicated by the 304 response, then the
1032   recipient MUST NOT use the 304 to update its own cache.  If this
1033   conditional request originated with an outbound client, such as a
1034   user agent with its own cache sending a conditional GET to a shared
1035   proxy, then the 304 response MAY be forwarded to the outbound client.
1036   Otherwise, the recipient MUST disregard the 304 response and repeat
1037   the request without any preconditions.
1038</t>
1039<t>
1040   If a cache uses a received 304 response to update a cache entry, the
1041   cache MUST update the entry to reflect any new field values given in
1042   the response.
1043</t>
1044</section>
1045
1046<section title="412 Precondition Failed" anchor="status.412">
1047  <iref primary="true" item="412 Precondition Failed (status code)"/>
1048  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
1049<t>
1050   The 412 status code indicates that one or more preconditions given in
1051   the request header fields evaluated to false when tested on the server.
1052   This response code allows the client to place preconditions on the
1053   current resource state (its current representations and metadata)
1054   and thus prevent the request method from being applied if the target
1055   resource is in an unexpected state.
1056</t>
1057</section>
1058</section>
1059
1060<section title="IANA Considerations" anchor="IANA.considerations">
1061
1062<section title="Status Code Registration" anchor="status.code.registration">
1063<t>
1064   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1065   shall be updated with the registrations below:
1066</t>
1067
1068<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1069<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1070   <ttcol>Value</ttcol>
1071   <ttcol>Description</ttcol>
1072   <ttcol>Reference</ttcol>
1073   <c>304</c>
1074   <c>Not Modified</c>
1075   <c>
1076      <xref target="status.304"/>
1077   </c>
1078   <c>412</c>
1079   <c>Precondition Failed</c>
1080   <c>
1081      <xref target="status.412"/>
1082   </c>
1083</texttable>
1084<!--(END)-->
1085
1086</section>
1087
1088<section title="Header Field Registration" anchor="header.field.registration">
1089<t>
1090   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1091   with the permanent registrations below (see <xref target="RFC3864"/>):
1092</t>
1093
1094<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1095<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1096   <ttcol>Header Field Name</ttcol>
1097   <ttcol>Protocol</ttcol>
1098   <ttcol>Status</ttcol>
1099   <ttcol>Reference</ttcol>
1100
1101   <c>ETag</c>
1102   <c>http</c>
1103   <c>standard</c>
1104   <c>
1105      <xref target="header.etag"/>
1106   </c>
1107   <c>If-Match</c>
1108   <c>http</c>
1109   <c>standard</c>
1110   <c>
1111      <xref target="header.if-match"/>
1112   </c>
1113   <c>If-Modified-Since</c>
1114   <c>http</c>
1115   <c>standard</c>
1116   <c>
1117      <xref target="header.if-modified-since"/>
1118   </c>
1119   <c>If-None-Match</c>
1120   <c>http</c>
1121   <c>standard</c>
1122   <c>
1123      <xref target="header.if-none-match"/>
1124   </c>
1125   <c>If-Unmodified-Since</c>
1126   <c>http</c>
1127   <c>standard</c>
1128   <c>
1129      <xref target="header.if-unmodified-since"/>
1130   </c>
1131   <c>Last-Modified</c>
1132   <c>http</c>
1133   <c>standard</c>
1134   <c>
1135      <xref target="header.last-modified"/>
1136   </c>
1137</texttable>
1138<!--(END)-->
1139
1140<t>
1141   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1142</t>
1143</section>
1144</section>
1145
1146<section title="Security Considerations" anchor="security.considerations">
1147<t>
1148   No additional security considerations have been identified beyond
1149   those applicable to HTTP in general <xref target="Part1"/>.
1150</t>
1151</section>
1152
1153<section title="Acknowledgments" anchor="ack">
1154</section>
1155</middle>
1156<back>
1157
1158<references title="Normative References">
1159
1160<reference anchor="Part1">
1161  <front>
1162    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1163    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1164      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1165      <address><email>fielding@gbiv.com</email></address>
1166    </author>
1167    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1168      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1169      <address><email>jg@freedesktop.org</email></address>
1170    </author>
1171    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1172      <organization abbrev="HP">Hewlett-Packard Company</organization>
1173      <address><email>JeffMogul@acm.org</email></address>
1174    </author>
1175    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1176      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1177      <address><email>henrikn@microsoft.com</email></address>
1178    </author>
1179    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1180      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1181      <address><email>LMM@acm.org</email></address>
1182    </author>
1183    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1184      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1185      <address><email>paulle@microsoft.com</email></address>
1186    </author>
1187    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1188      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1189      <address><email>timbl@w3.org</email></address>
1190    </author>
1191    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1192      <organization abbrev="W3C">World Wide Web Consortium</organization>
1193      <address><email>ylafon@w3.org</email></address>
1194    </author>
1195    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1196      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1197      <address><email>julian.reschke@greenbytes.de</email></address>
1198    </author>
1199    <date month="July" year="2011"/>
1200  </front>
1201  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-15"/>
1202 
1203</reference>
1204
1205<reference anchor="Part3">
1206  <front>
1207    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1208    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1209      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1210      <address><email>fielding@gbiv.com</email></address>
1211    </author>
1212    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1213      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1214      <address><email>jg@freedesktop.org</email></address>
1215    </author>
1216    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1217      <organization abbrev="HP">Hewlett-Packard Company</organization>
1218      <address><email>JeffMogul@acm.org</email></address>
1219    </author>
1220    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1221      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1222      <address><email>henrikn@microsoft.com</email></address>
1223    </author>
1224    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1225      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1226      <address><email>LMM@acm.org</email></address>
1227    </author>
1228    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1229      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1230      <address><email>paulle@microsoft.com</email></address>
1231    </author>
1232    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1233      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1234      <address><email>timbl@w3.org</email></address>
1235    </author>
1236    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1237      <organization abbrev="W3C">World Wide Web Consortium</organization>
1238      <address><email>ylafon@w3.org</email></address>
1239    </author>
1240    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1241      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1242      <address><email>julian.reschke@greenbytes.de</email></address>
1243    </author>
1244    <date month="July" year="2011"/>
1245  </front>
1246  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-15"/>
1247 
1248</reference>
1249
1250<reference anchor="Part5">
1251  <front>
1252    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1253    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1254      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1255      <address><email>fielding@gbiv.com</email></address>
1256    </author>
1257    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1258      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1259      <address><email>jg@freedesktop.org</email></address>
1260    </author>
1261    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1262      <organization abbrev="HP">Hewlett-Packard Company</organization>
1263      <address><email>JeffMogul@acm.org</email></address>
1264    </author>
1265    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1266      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1267      <address><email>henrikn@microsoft.com</email></address>
1268    </author>
1269    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1270      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1271      <address><email>LMM@acm.org</email></address>
1272    </author>
1273    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1274      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1275      <address><email>paulle@microsoft.com</email></address>
1276    </author>
1277    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1278      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1279      <address><email>timbl@w3.org</email></address>
1280    </author>
1281    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1282      <organization abbrev="W3C">World Wide Web Consortium</organization>
1283      <address><email>ylafon@w3.org</email></address>
1284    </author>
1285    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1286      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1287      <address><email>julian.reschke@greenbytes.de</email></address>
1288    </author>
1289    <date month="July" year="2011"/>
1290  </front>
1291  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-15"/>
1292 
1293</reference>
1294
1295<reference anchor="Part6">
1296  <front>
1297    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1298    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1299      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1300      <address><email>fielding@gbiv.com</email></address>
1301    </author>
1302    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1303      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1304      <address><email>jg@freedesktop.org</email></address>
1305    </author>
1306    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1307      <organization abbrev="HP">Hewlett-Packard Company</organization>
1308      <address><email>JeffMogul@acm.org</email></address>
1309    </author>
1310    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1311      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1312      <address><email>henrikn@microsoft.com</email></address>
1313    </author>
1314    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1315      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1316      <address><email>LMM@acm.org</email></address>
1317    </author>
1318    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1319      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1320      <address><email>paulle@microsoft.com</email></address>
1321    </author>
1322    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1323      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1324      <address><email>timbl@w3.org</email></address>
1325    </author>
1326    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1327      <organization abbrev="W3C">World Wide Web Consortium</organization>
1328      <address><email>ylafon@w3.org</email></address>
1329    </author>
1330    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1331      <address><email>mnot@mnot.net</email></address>
1332    </author>
1333    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1334      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1335      <address><email>julian.reschke@greenbytes.de</email></address>
1336    </author>
1337    <date month="July" year="2011"/>
1338  </front>
1339  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-15"/>
1340 
1341</reference>
1342
1343<reference anchor="RFC2119">
1344  <front>
1345    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1346    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1347      <organization>Harvard University</organization>
1348      <address><email>sob@harvard.edu</email></address>
1349    </author>
1350    <date month="March" year="1997"/>
1351  </front>
1352  <seriesInfo name="BCP" value="14"/>
1353  <seriesInfo name="RFC" value="2119"/>
1354</reference>
1355
1356<reference anchor="RFC5234">
1357  <front>
1358    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1359    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1360      <organization>Brandenburg InternetWorking</organization>
1361      <address>
1362        <email>dcrocker@bbiw.net</email>
1363      </address> 
1364    </author>
1365    <author initials="P." surname="Overell" fullname="Paul Overell">
1366      <organization>THUS plc.</organization>
1367      <address>
1368        <email>paul.overell@thus.net</email>
1369      </address>
1370    </author>
1371    <date month="January" year="2008"/>
1372  </front>
1373  <seriesInfo name="STD" value="68"/>
1374  <seriesInfo name="RFC" value="5234"/>
1375</reference>
1376
1377</references>
1378
1379<references title="Informative References">
1380
1381<reference anchor="RFC2616">
1382  <front>
1383    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1384    <author initials="R." surname="Fielding" fullname="R. Fielding">
1385      <organization>University of California, Irvine</organization>
1386      <address><email>fielding@ics.uci.edu</email></address>
1387    </author>
1388    <author initials="J." surname="Gettys" fullname="J. Gettys">
1389      <organization>W3C</organization>
1390      <address><email>jg@w3.org</email></address>
1391    </author>
1392    <author initials="J." surname="Mogul" fullname="J. Mogul">
1393      <organization>Compaq Computer Corporation</organization>
1394      <address><email>mogul@wrl.dec.com</email></address>
1395    </author>
1396    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1397      <organization>MIT Laboratory for Computer Science</organization>
1398      <address><email>frystyk@w3.org</email></address>
1399    </author>
1400    <author initials="L." surname="Masinter" fullname="L. Masinter">
1401      <organization>Xerox Corporation</organization>
1402      <address><email>masinter@parc.xerox.com</email></address>
1403    </author>
1404    <author initials="P." surname="Leach" fullname="P. Leach">
1405      <organization>Microsoft Corporation</organization>
1406      <address><email>paulle@microsoft.com</email></address>
1407    </author>
1408    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1409      <organization>W3C</organization>
1410      <address><email>timbl@w3.org</email></address>
1411    </author>
1412    <date month="June" year="1999"/>
1413  </front>
1414  <seriesInfo name="RFC" value="2616"/>
1415</reference>
1416
1417<reference anchor="RFC3864">
1418  <front>
1419    <title>Registration Procedures for Message Header Fields</title>
1420    <author initials="G." surname="Klyne" fullname="G. Klyne">
1421      <organization>Nine by Nine</organization>
1422      <address><email>GK-IETF@ninebynine.org</email></address>
1423    </author>
1424    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1425      <organization>BEA Systems</organization>
1426      <address><email>mnot@pobox.com</email></address>
1427    </author>
1428    <author initials="J." surname="Mogul" fullname="J. Mogul">
1429      <organization>HP Labs</organization>
1430      <address><email>JeffMogul@acm.org</email></address>
1431    </author>
1432    <date year="2004" month="September"/>
1433  </front>
1434  <seriesInfo name="BCP" value="90"/>
1435  <seriesInfo name="RFC" value="3864"/>
1436</reference>
1437
1438<reference anchor="RFC4918">
1439  <front>
1440    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1441    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor">
1442      <organization abbrev="CommerceNet">CommerceNet</organization>
1443      <address><email>ldusseault@commerce.net</email></address>
1444    </author>
1445    <date month="June" year="2007"/>
1446  </front>
1447  <seriesInfo name="RFC" value="4918"/>
1448</reference>
1449</references>
1450
1451<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1452<t>
1453  Allow weak entity-tags in all requests except range requests (Sections
1454  <xref target="weak.and.strong.validators" format="counter"/> and
1455  <xref target="header.if-none-match" format="counter"/>).
1456</t>
1457<t>
1458  Change ABNF productions for header fields to only define the field value.
1459  (<xref target="header.fields"/>)
1460</t>
1461</section>
1462
1463
1464<section title="Collected ABNF" anchor="collected.abnf">
1465<figure>
1466<artwork type="abnf" name="p4-conditional.parsed-abnf"><![CDATA[
1467ETag = entity-tag
1468
1469HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1470
1471If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1472 entity-tag ] ) )
1473If-Modified-Since = HTTP-date
1474If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1475 entity-tag ] ) )
1476If-Unmodified-Since = HTTP-date
1477
1478Last-Modified = HTTP-date
1479
1480OWS = <OWS, defined in [Part1], Section 1.2.2>
1481
1482entity-tag = [ weak ] opaque-tag
1483
1484opaque-tag = quoted-string
1485
1486quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1487
1488weak = %x57.2F ; W/
1489]]></artwork>
1490</figure>
1491<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
1492; ETag defined but not used
1493; If-Match defined but not used
1494; If-Modified-Since defined but not used
1495; If-None-Match defined but not used
1496; If-Unmodified-Since defined but not used
1497; Last-Modified defined but not used
1498]]></artwork></figure></section>
1499
1500
1501<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1502
1503<section title="Since RFC 2616">
1504<t>
1505  Extracted relevant partitions from <xref target="RFC2616"/>.
1506</t>
1507</section>
1508
1509<section title="Since draft-ietf-httpbis-p4-conditional-00">
1510<t>
1511  Closed issues:
1512  <list style="symbols"> 
1513    <t>
1514      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1515      "Normative and Informative references"
1516    </t>
1517  </list>
1518</t>
1519<t>
1520  Other changes:
1521  <list style="symbols"> 
1522    <t>
1523      Move definitions of 304 and 412 condition codes from Part2.
1524    </t>
1525  </list>
1526</t>
1527</section>
1528
1529<section title="Since draft-ietf-httpbis-p4-conditional-01">
1530<t>
1531  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1532  <list style="symbols"> 
1533    <t>
1534      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1535    </t>
1536  </list>
1537</t>
1538</section>
1539
1540<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1541<t>
1542  Closed issues:
1543  <list style="symbols"> 
1544    <t>
1545      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1546      "Weak ETags on non-GET requests"
1547    </t>
1548  </list>
1549</t>
1550<t>
1551  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1552  <list style="symbols"> 
1553    <t>
1554      Reference RFC 3984, and update header field registrations for header fields defined
1555      in this document.
1556    </t>
1557  </list>
1558</t>
1559</section>
1560
1561<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1562<t>
1563  Closed issues:
1564  <list style="symbols"> 
1565    <t>
1566      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1567      "Examples for ETag matching"
1568    </t>
1569    <t>
1570      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1571      "'entity value' undefined"
1572    </t>
1573    <t>
1574      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1575      "bogus 2068 Date header reference"
1576    </t>
1577  </list>
1578</t>
1579</section>
1580
1581<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1582<t>
1583  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1584  <list style="symbols"> 
1585    <t>
1586      Use "/" instead of "|" for alternatives.
1587    </t>
1588    <t>
1589      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1590      whitespace ("OWS") and required whitespace ("RWS").
1591    </t>
1592    <t>
1593      Rewrite ABNFs to spell out whitespace rules, factor out
1594      header field value format definitions.
1595    </t>
1596  </list>
1597</t>
1598</section>
1599
1600<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1601<t>
1602  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1603  <list style="symbols"> 
1604    <t>
1605      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1606    </t>
1607  </list>
1608</t>
1609</section>
1610
1611<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1612<t>
1613  Closed issues:
1614  <list style="symbols"> 
1615    <t>
1616      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1617      "case-sensitivity of etag weakness indicator"
1618    </t>
1619  </list>
1620</t>
1621</section>
1622
1623<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1624<t>
1625  Closed issues:
1626  <list style="symbols"> 
1627    <t>
1628      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1629      "Weak ETags on non-GET requests" (If-Match still was defined to require
1630      strong matching)
1631    </t>
1632    <t>
1633      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1634      "move IANA registrations for optional status codes"
1635    </t>
1636  </list>
1637</t>
1638</section>
1639
1640<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1641<t>
1642  No significant changes.
1643</t>
1644</section>
1645
1646<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1647<t>
1648  No significant changes.
1649</t>
1650</section>
1651
1652<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1653<t>
1654  Closed issues:
1655  <list style="symbols"> 
1656    <t>
1657      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1658      "Clarify 'Requested Variant'"
1659    </t>
1660    <t>
1661      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1662      "Clarify entity / representation / variant terminology"
1663    </t>
1664    <t>
1665      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1666      "consider removing the 'changes from 2068' sections"
1667    </t>
1668  </list>
1669</t>
1670</section>
1671
1672<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1673<t>
1674  None.
1675</t>
1676</section>
1677
1678<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1679<t>
1680  Closed issues:
1681  <list style="symbols"> 
1682    <t>
1683      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1684      "Header Classification"
1685    </t>
1686  </list>
1687</t>
1688</section>
1689
1690<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1691<t>
1692  Closed issues:
1693  <list style="symbols"> 
1694    <t>
1695      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1696      "If-* and entities"
1697    </t>
1698    <t>
1699      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1700      "Definition of validator weakness"
1701    </t>
1702    <t>
1703      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1704      "untangle ABNFs for header fields"
1705    </t>
1706    <t>
1707      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1708      "ETags and Quotes"
1709    </t>
1710  </list>
1711</t>
1712</section>
1713
1714<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1715<t>
1716  None.
1717</t>
1718</section>
1719
1720</section>
1721
1722</back>
1723</rfc>
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