source: draft-ietf-httpbis/13/draft-ietf-httpbis-p4-conditional-13.xml @ 1629

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fix mime types

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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-13">
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="March" year="2011" day="14"/>
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). The current issues list is
179    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
180    and related documents (including fancy diffs) can be found at
181    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
182  </t>
183  <t>
184    The changes in this draft are summarized in <xref target="changes.since.12"/>.
185  </t>
186</note>
187</front>
188<middle>
189<section title="Introduction" anchor="introduction">
190<t>
191   This document defines HTTP/1.1 response metadata for indicating potential
192   changes to payload content, including modification time stamps and opaque
193   entity-tags, and the HTTP conditional request mechanisms that allow
194   preconditions to be placed on a request method.  Conditional GET requests
195   allow for efficient cache updates.  Other conditional request methods are
196   used to protect against overwriting or misunderstanding the state of a
197   resource that has been changed unbeknownst to the requesting client.
198</t>
199<t>
200   This document is currently disorganized in order to minimize the changes
201   between drafts and enable reviewers to see the smaller errata changes.
202   A future draft will reorganize the sections to better reflect the content.
203   In particular, the sections on resource metadata will be discussed first
204   and then followed by each conditional request header field, concluding with a
205   definition of precedence and the expectation of ordering strong validator
206   checks before weak validator checks.  It is likely that more content from
207   <xref target="Part6"/> will migrate to this part, where appropriate.
208   The current mess reflects how widely dispersed these topics and associated
209   requirements had become in <xref target="RFC2616"/>.
210</t>
211
212<section title="Requirements" anchor="intro.requirements">
213<t>
214   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
215   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
216   document are to be interpreted as described in <xref target="RFC2119"/>.
217</t>
218<t>
219   An implementation is not compliant if it fails to satisfy one or more
220   of the "MUST" or "REQUIRED" level requirements for the protocols it
221   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
222   level and all the "SHOULD" level requirements for its protocols is said
223   to be "unconditionally compliant"; one that satisfies all the "MUST"
224   level requirements but not all the "SHOULD" level requirements for its
225   protocols is said to be "conditionally compliant".
226</t>
227</section>
228
229<section title="Syntax Notation" anchor="notation">
230 
231 
232 
233 
234 
235 
236 
237<t>
238  This specification uses the ABNF syntax defined in Section 1.2 of <xref target="Part1"/> (which
239  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
240  <xref target="collected.abnf"/> shows the collected ABNF, with the list
241  rule expanded.
242</t>
243<t>
244  The following core rules are included by
245  reference, as defined in <xref target="RFC5234"/>, Appendix B.1:
246  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
247  DIGIT (decimal 0-9), DQUOTE (double quote),
248  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
249  OCTET (any 8-bit sequence of data), SP (space),
250  VCHAR (any visible USASCII character),
251  and WSP (whitespace).
252</t>
253
254<section title="Core Rules" anchor="core.rules">
255 
256 
257<t>
258  The core rules below are defined in Section 1.2.2 of <xref target="Part1"/>:
259</t>
260<figure><artwork type="abnf2616"><![CDATA[
261  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
262  OWS           = <OWS, defined in [Part1], Section 1.2.2>
263]]></artwork></figure>
264</section>
265
266<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
267 
268<t>
269  The ABNF rules below are defined in other parts:
270</t>
271<figure><artwork type="abnf2616"><![CDATA[
272  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
273]]></artwork></figure>
274</section>
275
276</section>
277
278</section>
279
280<section title="Entity-Tags" anchor="entity.tags">
281 
282 
283 
284<t>
285   Entity-tags are used for comparing two or more representations of the same
286   resource. HTTP/1.1 uses entity-tags in the ETag (<xref target="header.etag"/>),
287   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
288   If-Range (Section 5.3 of <xref target="Part5"/>) header fields. The definition of how they
289   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
290   entity-tag consists of an opaque quoted string, possibly prefixed by
291   a weakness indicator.
292</t>
293<figure><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[
294  entity-tag = [ weak ] opaque-tag
295  weak       = %x57.2F ; "W/", case-sensitive
296  opaque-tag = quoted-string
297]]></artwork></figure>
298<t>
299   A "strong entity-tag" MAY be shared by two representations of a resource
300   only if they are equivalent by octet equality.
301</t>
302<t>
303   A "weak entity-tag", indicated by the "W/" prefix, MAY be shared by
304   two representations of a resource only if the representations are equivalent and
305   could be substituted for each other with no significant change in
306   semantics. A weak entity-tag can only be used for weak comparison.
307</t>
308<t>
309   An entity-tag MUST be unique across all versions of all representations
310   associated with a particular resource. A given entity-tag value MAY
311   be used for representations obtained by requests on different URIs. The use
312   of the same entity-tag value in conjunction with representations obtained by
313   requests on different URIs does not imply the equivalence of those
314   representations.
315</t>
316
317<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
318<t>
319   Consider a resource that is subject to content negotiation (Section 5 of <xref target="Part3"/>),
320   and where the representations returned upon a GET request vary based on
321   the Accept-Encoding request header field (Section 6.3 of <xref target="Part3"/>):
322</t>
323<figure><preamble>&gt;&gt; Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"><![CDATA[
324  GET /index HTTP/1.1
325  Host: www.example.com
326  Accept-Encoding: gzip
327 
328  ]]></artwork></figure>
329<t>
330   In this case, the response might or might not use the gzip content coding.
331   If it does not, the response might look like:
332</t>
333<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
334  HTTP/1.1 200 OK
335  Date: Thu, 26 Mar 2010 00:05:00 GMT
336  ETag: "123-a"
337  Content-Length: 70
338  Vary: Accept-Encoding
339  Content-Type: text/plain
340 
341  Hello World!
342  Hello World!
343  Hello World!
344  Hello World!
345  Hello World!
346  ]]></artwork></figure>
347<t>
348   An alternative representation that does use gzip content coding would be:
349</t>
350<figure><preamble>&gt;&gt; Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"><![CDATA[
351  HTTP/1.1 200 OK
352  Date: Thu, 26 Mar 2010 00:05:00 GMT
353  ETag: "123-b"
354  Content-Length: 43
355  Vary: Accept-Encoding
356  Content-Type: text/plain
357  Content-Encoding: gzip
358 
359  ...binary data...]]></artwork></figure>
360<t><list>
361  <t>
362    Note: Content codings are a property of the representation,
363    so therefore an entity-tag of an encoded representation must be distinct
364    from an unencoded representation to prevent conflicts during cache updates
365    and range requests.  In contrast, transfer codings (Section 6.2 of <xref target="Part1"/>)
366    apply only during message transfer and do not require distinct entity-tags.
367  </t>
368</list></t>
369</section>
370</section>
371
372<section title="Status Code Definitions" anchor="status.code.definitions">
373<section title="304 Not Modified" anchor="status.304">
374  <iref primary="true" item="304 Not Modified (status code)"/>
375  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
376<t>
377   If the client has performed a conditional GET request and access is
378   allowed, but the document has not been modified, the server SHOULD
379   respond with this status code. The 304 response MUST NOT contain a
380   message-body, and thus is always terminated by the first empty line
381   after the header fields.
382</t>
383<t>
384   A 304 response MUST include a Date header field (Section 9.3 of <xref target="Part1"/>)
385   unless its omission is required by Section 9.3.1 of <xref target="Part1"/>.  If a 200 response
386   to the same request would have included any of the header fields
387   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
388   Vary, then those same header fields MUST be sent in a 304 response.
389</t>
390<t>
391   Since the goal of a 304 response is to minimize information transfer
392   when the recipient already has one or more cached representations,
393   the response SHOULD NOT include representation metadata other
394   than the above listed fields unless said metadata exists for the
395   purpose of guiding cache updates (e.g., future HTTP extensions).
396</t>
397<t>
398   If a 304 response includes an entity-tag that indicates a
399   representation not currently cached, then the recipient MUST NOT
400   use the 304 to update its own cache.  If that conditional request originated
401   with an outbound client, such as a user agent with its own cache sending a
402   conditional GET to a shared proxy, then the 304 response MAY be
403   forwarded to the outbound client.  Otherwise, disregard the response
404   and repeat the request without the conditional.
405</t>
406<t>
407   If a cache uses a received 304 response to update a cache entry, the
408   cache MUST update the entry to reflect any new field values given in
409   the response.
410</t>
411</section>
412
413<section title="412 Precondition Failed" anchor="status.412">
414  <iref primary="true" item="412 Precondition Failed (status code)"/>
415  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
416<t>
417   The precondition given in one or more of the header fields
418   evaluated to false when it was tested on the server. This response
419   code allows the client to place preconditions on the current resource
420   metadata (header field data) and thus prevent the requested
421   method from being applied to a resource other than the one intended.
422</t>
423</section>
424</section>
425
426<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
427<t>
428   Since both origin servers and caches will compare two validators to
429   decide if they represent the same or different representations, one normally
430   would expect that if the representation (including both representation
431   header fields and representation body) changes in any way, then the
432   associated validator would change as well. If this is true, then we
433   call this validator a "strong validator".
434</t>
435<t>
436   However, there might be cases when a server prefers to change the
437   validator only on semantically significant changes, and not when
438   insignificant aspects of the representation change. A validator that does not
439   always change when the representation changes is a "weak validator".
440</t>
441<t>
442   An entity-tag is normally a strong validator, but the protocol
443   provides a mechanism to tag an entity-tag as "weak". One can think of
444   a strong validator as one that changes whenever the sequence of bits
445   in a representation changes, while a weak value changes whenever the
446   meaning of a representation changes. Alternatively, one can think of
447   a strong validator as part of an identifier for a specific representation,
448   whereas a weak validator is part of an identifier for a set of semantically
449   equivalent representations.
450  <list><t>
451      Note: One example of a strong validator is an integer that is
452      incremented in stable storage every time a representation is changed.
453    </t><t>
454      A representation's modification time, if defined with only one-second
455      resolution, could be a weak validator, since it is possible that
456      the representation might be modified twice during a single second.
457    </t><t>
458      Support for weak validators is optional. However, weak validators
459      allow for more efficient caching of equivalent objects; for
460      example, a hit counter on a site is probably good enough if it is
461      updated every few days or weeks, and any value during that period
462      is likely "good enough" to be equivalent.
463    </t></list>
464</t>
465<t>
466   A "use" of a validator is either when a client generates a request
467   and includes the validator in a validating header field, or when a
468   server compares two validators.
469</t>
470<t>
471   Strong validators are usable in any context. Weak validators are only
472   usable in contexts that do not depend on exact equality of a representation.
473   For example, either kind is usable for a normal conditional GET.
474   However, only a strong validator is usable for a sub-range
475   retrieval, since otherwise the client might end up with an internally
476   inconsistent representation.
477</t>
478<t>
479   Clients MUST NOT use weak validators in range requests (<xref target="Part5"/>).
480</t>
481<t>
482   The only function that HTTP/1.1 defines on validators is
483   comparison. There are two validator comparison functions, depending
484   on whether the comparison context allows the use of weak validators
485   or not:
486  <list style="symbols">
487     <t>The strong comparison function: in order to be considered equal,
488        both opaque-tags MUST be identical character-by-character, and both
489        MUST NOT be weak.</t>
490     <t>The weak comparison function: in order to be considered equal, both
491        opaque-tags MUST be identical character-by-character, but
492        either or both of them MAY be tagged as "weak" without affecting
493        the result.</t>
494  </list>
495</t>
496<t>
497   The example below shows the results for a set of entity-tag pairs,
498   and both the weak and strong comparison function results:
499</t>
500<texttable align="left">
501  <ttcol>ETag 1</ttcol>
502  <ttcol>ETag 2</ttcol>
503  <ttcol>Strong Comparison</ttcol>
504  <ttcol>Weak Comparison</ttcol>
505
506  <c>W/"1"</c>
507  <c>W/"1"</c>
508  <c>no match</c>
509  <c>match</c>
510 
511  <c>W/"1"</c>
512  <c>W/"2"</c>
513  <c>no match</c>
514  <c>no match</c>
515
516  <c>W/"1"</c>
517  <c>"1"</c>
518  <c>no match</c>
519  <c>match</c>
520
521  <c>"1"</c>
522  <c>"1"</c>
523  <c>match</c>
524  <c>match</c>
525</texttable>
526<t>
527   An entity-tag is strong unless it is explicitly tagged as weak.
528   <xref target="entity.tags"/> gives the syntax for entity-tags.
529</t>
530<t>
531   A Last-Modified time, when used as a validator in a request, is
532   implicitly weak unless it is possible to deduce that it is strong,
533   using the following rules:
534  <list style="symbols">
535     <t>The validator is being compared by an origin server to the
536        actual current validator for the representation and,</t>
537     <t>That origin server reliably knows that the associated representation did
538        not change twice during the second covered by the presented
539        validator.</t>
540  </list>
541</t>
542<t>
543   or
544  <list style="symbols">
545     <t>The validator is about to be used by a client in an If-Modified-Since
546        or If-Unmodified-Since header field, because the client
547        has a cache entry for the associated representation, and</t>
548     <t>That cache entry includes a Date value, which gives the time
549        when the origin server sent the original response, and</t>
550     <t>The presented Last-Modified time is at least 60 seconds before
551        the Date value.</t>
552  </list>
553</t>
554<t>
555   or
556  <list style="symbols">
557     <t>The validator is being compared by an intermediate cache to the
558        validator stored in its cache entry for the representation, and</t>
559     <t>That cache entry includes a Date value, which gives the time
560        when the origin server sent the original response, and</t>
561     <t>The presented Last-Modified time is at least 60 seconds before
562        the Date value.</t>
563  </list>
564</t>
565<t>
566   This method relies on the fact that if two different responses were
567   sent by the origin server during the same second, but both had the
568   same Last-Modified time, then at least one of those responses would
569   have a Date value equal to its Last-Modified time. The arbitrary 60-second
570   limit guards against the possibility that the Date and Last-Modified
571   values are generated from different clocks, or at somewhat
572   different times during the preparation of the response. An
573   implementation MAY use a value larger than 60 seconds, if it is
574   believed that 60 seconds is too short.
575</t>
576<t>
577   If a client wishes to perform a sub-range retrieval on a value for
578   which it has only a Last-Modified time and no opaque validator, it
579   MAY do this only if the Last-Modified time is strong in the sense
580   described here.
581</t>
582<t>
583   A cache or origin server receiving a conditional range request
584   (<xref target="Part5"/>) MUST use the strong comparison function to
585   evaluate the condition.
586</t>
587<t>
588   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
589   retrievals on values that have been obtained from HTTP/1.0
590   servers.
591</t>
592</section>
593
594<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">
595<t>
596   We adopt a set of rules and recommendations for origin servers,
597   clients, and caches regarding when various validator types ought to
598   be used, and for what purposes.
599</t>
600<t>
601   HTTP/1.1 origin servers:
602  <list style="symbols">
603     <t>SHOULD send an entity-tag validator unless it is not feasible to
604        generate one.</t>
605
606     <t>MAY send a weak entity-tag instead of a strong entity-tag, if
607        performance considerations support the use of weak entity-tags,
608        or if it is unfeasible to send a strong entity-tag.</t>
609
610     <t>SHOULD send a Last-Modified value if it is feasible to send one,
611        unless the risk of a breakdown in semantic transparency that
612        could result from using this date in an If-Modified-Since header
613        field would lead to serious problems.</t>
614  </list>
615</t>
616<t>
617   In other words, the preferred behavior for an HTTP/1.1 origin server
618   is to send both a strong entity-tag and a Last-Modified value.
619</t>
620<t>
621   In order to be legitimate, a strong entity-tag MUST change whenever the
622   associated representation changes in any way. A weak entity-tag SHOULD
623   change whenever the associated representation changes in a semantically
624   significant way.
625</t>
626<t><list>
627  <t>
628    Note: In order to provide semantically transparent caching, an
629    origin server must avoid reusing a specific strong entity-tag
630    value for two different representations, or reusing a specific weak
631    entity-tag value for two semantically different representations. Cache
632    entries might persist for arbitrarily long periods, regardless of
633    expiration times, so it might be inappropriate to expect that a
634    cache will never again attempt to validate an entry using a
635    validator that it obtained at some point in the past.
636  </t>
637</list></t>
638<t>
639   HTTP/1.1 clients:
640  <list style="symbols">
641     <t>MUST use that entity-tag in any cache-conditional request (using
642        If-Match or If-None-Match) if an entity-tag has been provided by the
643        origin server.</t>
644
645     <t>SHOULD use the Last-Modified value in non-subrange cache-conditional
646        requests (using If-Modified-Since) if only a Last-Modified value has
647        been provided by the origin server. </t>
648
649     <t>MAY use the Last-Modified value in subrange cache-conditional
650        requests (using If-Unmodified-Since) if only a Last-Modified value has
651        been provided by an HTTP/1.0 origin server. The user agent SHOULD
652        provide a way to disable this, in case of difficulty.</t>
653
654     <t>SHOULD use both validators in cache-conditional requests if both an
655        entity-tag and a Last-Modified value have been provided by the origin
656        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
657        appropriately.</t>
658  </list>
659</t>
660<t>
661   An HTTP/1.1 origin server, upon receiving a conditional request that
662   includes both a Last-Modified date (e.g., in an If-Modified-Since or
663   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
664   in an If-Match, If-None-Match, or If-Range header field) as cache
665   validators, MUST NOT return a response status code of 304 (Not Modified)
666   unless doing so is consistent with all of the conditional header
667   fields in the request.
668</t>
669<t>
670   An HTTP/1.1 caching proxy, upon receiving a conditional request that
671   includes both a Last-Modified date and one or more entity-tags as
672   cache validators, MUST NOT return a locally cached response to the
673   client unless that cached response is consistent with all of the
674   conditional header fields in the request.
675  <list><t>
676      Note: The general principle behind these rules is that HTTP/1.1
677      servers and clients ought to transmit as much non-redundant
678      information as is available in their responses and requests.
679      HTTP/1.1 systems receiving this information will make the most
680      conservative assumptions about the validators they receive.
681  </t><t>
682      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
683      last-modified values received or used by these systems will
684      support transparent and efficient caching, and so HTTP/1.1 origin
685      servers should provide Last-Modified values. In those rare cases
686      where the use of a Last-Modified value as a validator by an
687      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
688      origin servers should not provide one.
689  </t></list>
690</t>
691</section>
692
693<section title="Header Field Definitions" anchor="header.fields">
694<t>
695   This section defines the syntax and semantics of HTTP/1.1 header fields
696   related to conditional requests.
697</t>
698
699<section title="ETag" anchor="header.etag">
700  <iref primary="true" item="ETag header field"/>
701  <iref primary="true" item="Header Fields" subitem="ETag"/>
702 
703 
704<t>
705   The "ETag" header field provides the current value of the
706   entity-tag (see <xref target="entity.tags"/>) for one representation of
707   the target resource.  An entity-tag
708   is intended for use as a resource-local identifier for differentiating
709   between representations of the same resource that vary over time or via
710   content negotiation (see <xref target="weak.and.strong.validators"/>).
711</t>
712<figure><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/><artwork type="abnf2616"><![CDATA[
713  ETag   = "ETag" ":" OWS ETag-v
714  ETag-v = entity-tag
715]]></artwork></figure>
716<figure><preamble>
717  Examples:
718</preamble>
719<artwork type="example"><![CDATA[
720  ETag: "xyzzy"
721  ETag: W/"xyzzy"
722  ETag: ""
723]]></artwork></figure>
724<t>
725   An entity-tag provides an "opaque" cache validator that allows for
726   more reliable validation than modification dates in situations where
727   it is inconvenient to store modification dates,
728   where the one-second resolution of HTTP date values is not
729   sufficient, or where the origin server wishes to avoid certain
730   paradoxes that might arise from the use of modification dates.
731</t>
732<t>
733   The principle behind entity-tags is that only the service author
734   knows the semantics of a resource well enough to select an
735   appropriate cache validation mechanism, and the specification of any
736   validator comparison function more complex than byte-equality would
737   open up a can of worms. Thus, comparisons of any other header fields
738   (except Last-Modified, for compatibility with HTTP/1.0) are never
739   used for purposes of validating a cache entry.
740</t>
741</section>
742
743<section title="If-Match" anchor="header.if-match">
744  <iref primary="true" item="If-Match header field"/>
745  <iref primary="true" item="Header Fields" subitem="If-Match"/>
746 
747 
748<t>
749   The "If-Match" header field is used to make a request method
750   conditional. A client that has one or more representations previously
751   obtained from the resource can verify that one of those representations is
752   current by including a list of their associated entity-tags in the
753   If-Match header field.
754</t>
755<t>
756   This allows efficient updates of cached information with a minimum amount of
757   transaction overhead. It is also used when updating resources, to prevent
758   inadvertent modification of the wrong version of a resource. As a special
759   case, the value "*" matches any current representation of the resource.
760</t>
761<figure><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/><artwork type="abnf2616"><![CDATA[
762  If-Match   = "If-Match" ":" OWS If-Match-v
763  If-Match-v = "*" / 1#entity-tag
764]]></artwork></figure>
765<t>
766   If any of the entity-tags match the entity-tag of the representation that
767   would have been returned in the response to a similar GET request
768   (without the If-Match header field) on that resource, or if "*" is given
769   and any current representation exists for that resource, then the server MAY
770   perform the requested method as if the If-Match header field did not
771   exist.
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, and
776   MUST return a 412 (Precondition Failed) response. This behavior is
777   most useful when the client wants to prevent an updating request method,
778   such as PUT, from modifying a resource that has changed since the client
779   last retrieved it.
780</t>
781<t>
782   If the request would, without the If-Match header field, result in
783   anything other than a 2xx or 412 status code, then the If-Match header field
784   MUST be ignored.
785</t>
786<t>
787   The meaning of "If-Match: *" is that the request method SHOULD be performed
788   if the representation selected by the origin server (or by a cache,
789   possibly using the Vary mechanism, see Section 3.5 of <xref target="Part6"/>) exists, and
790   MUST NOT be performed if the representation does not exist.
791</t>
792<t>
793   A request intended to update a resource (e.g., a PUT) MAY include an
794   If-Match header field to signal that the request method MUST NOT be
795   applied if the representation corresponding to the If-Match value (a single
796   entity-tag) is no longer a representation of that resource. This
797   allows the user to indicate that they do not wish the request to be
798   successful if the resource has been changed without their knowledge.
799   Examples:
800</t>
801<figure><artwork type="example"><![CDATA[
802  If-Match: "xyzzy"
803  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
804  If-Match: *
805]]></artwork></figure>
806<t>
807   The result of a request having both an If-Match header field and
808   either an If-None-Match or an If-Modified-Since header fields is
809   undefined by this specification.
810</t>
811</section>
812
813<section title="If-Modified-Since" anchor="header.if-modified-since">
814  <iref primary="true" item="If-Modified-Since header field"/>
815  <iref primary="true" item="Header Fields" subitem="If-Modified-Since"/>
816 
817 
818<t>
819   The "If-Modified-Since" header field is used to make a request
820   method conditional by date: if the representation that would have been
821   transferred in a 200 response to a GET request has not been modified since
822   the time specified in this field, then do not perform the method;
823   instead, respond as detailed below.
824</t>
825<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/><artwork type="abnf2616"><![CDATA[
826  If-Modified-Since   = "If-Modified-Since" ":" OWS
827                        If-Modified-Since-v
828  If-Modified-Since-v = HTTP-date
829]]></artwork></figure>
830<t>
831   An example of the field is:
832</t>
833<figure><artwork type="example"><![CDATA[
834  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
835]]></artwork></figure>
836<t>
837   A GET method with an If-Modified-Since header field and no Range header
838   field requests that the representation be transferred only if it has
839   been modified since the date given by the If-Modified-Since header field.
840   The algorithm for determining this includes the following cases:
841  <list style="numbers">
842      <t>If the request would normally result in anything other than a
843         200 (OK) status code, or if the passed If-Modified-Since date is
844         invalid, the response is exactly the same as for a normal GET.
845         A date which is later than the server's current time is
846         invalid.</t>
847
848      <t>If the representation has been modified since the If-Modified-Since
849         date, the response is exactly the same as for a normal GET.</t>
850
851      <t>If the representation has not been modified since a valid
852         If-Modified-Since date, the server SHOULD return a
853         304 (Not Modified) response.</t>
854  </list>
855</t>
856<t>
857   The purpose of this feature is to allow efficient updates of cached
858   information with a minimum amount of transaction overhead.
859  <list><t>
860      Note: The Range header field modifies the meaning of If-Modified-Since;
861      see Section 5.4 of <xref target="Part5"/> for full details.
862    </t><t>
863      Note: If-Modified-Since times are interpreted by the server, whose
864      clock might not be synchronized with the client.
865    </t><t>
866      Note: When handling an If-Modified-Since header field, some
867      servers will use an exact date comparison function, rather than a
868      less-than function, for deciding whether to send a 304 (Not
869      Modified) response. To get best results when sending an If-Modified-Since
870      header field for cache validation, clients are
871      advised to use the exact date string received in a previous Last-Modified
872      header field whenever possible.
873    </t><t>
874      Note: If a client uses an arbitrary date in the If-Modified-Since
875      header field instead of a date taken from the Last-Modified header field for
876      the same request, the client needs to be aware that this
877      date is interpreted in the server's understanding of time.
878      Unsynchronized clocks and rounding problems, due to the different
879      encodings of time between the client and server, are concerns.
880      This includes the possibility of race conditions if the
881      document has changed between the time it was first requested and
882      the If-Modified-Since date of a subsequent request, and the
883      possibility of clock-skew-related problems if the If-Modified-Since
884      date is derived from the client's clock without correction
885      to the server's clock. Corrections for different time bases
886      between client and server are at best approximate due to network
887      latency.
888    </t>
889  </list>
890</t>
891<t>
892   The result of a request having both an If-Modified-Since header field
893   and either an If-Match or an If-Unmodified-Since header fields is
894   undefined by this specification.
895</t>
896</section>
897
898<section title="If-None-Match" anchor="header.if-none-match">
899  <iref primary="true" item="If-None-Match header field"/>
900  <iref primary="true" item="Header Fields" subitem="If-None-Match"/>
901 
902 
903<t>
904   The "If-None-Match" header field is used to make a request method
905   conditional. A client that has one or more representations previously
906   obtained from the resource can verify that none of those representations is
907   current by including a list of their associated entity-tags in the
908   If-None-Match header field.
909</t>
910<t>
911   This allows efficient updates of cached information with a minimum amount of
912   transaction overhead. It is also used to prevent a request method (e.g., PUT)
913   from inadvertently modifying an existing resource when the client
914   believes that the resource does not exist.
915</t>
916<t>
917   As a special case, the value "*" matches any current representation of the
918   resource.
919</t>
920<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/><artwork type="abnf2616"><![CDATA[
921  If-None-Match   = "If-None-Match" ":" OWS If-None-Match-v
922  If-None-Match-v = "*" / 1#entity-tag
923]]></artwork></figure>
924<t>
925   If any of the entity-tags match the entity-tag of the representation that
926   would have been returned in the response to a similar GET request
927   (without the If-None-Match header field) on that resource, or if "*" is
928   given and any current representation exists for that resource, then the
929   server MUST NOT perform the requested method, unless required to do
930   so because the resource's modification date fails to match that
931   supplied in an If-Modified-Since header field in the request.
932   Instead, if the request method was GET or HEAD, the server SHOULD
933   respond with a 304 (Not Modified) response, including the cache-related
934   header fields (particularly ETag) of one of the representations that
935   matched. For all other request methods, the server MUST respond with
936   a 412 (Precondition Failed) status code.
937</t>
938<t>
939   If none of the entity-tags match, then the server MAY perform the
940   requested method as if the If-None-Match header field did not exist,
941   but MUST also ignore any If-Modified-Since header field(s) in the
942   request. That is, if no entity-tags match, then the server MUST NOT
943   return a 304 (Not Modified) response.
944</t>
945<t>
946   If the request would, without the If-None-Match header field, result
947   in anything other than a 2xx or 304 status code, then the If-None-Match
948   header field MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
949   server behavior when both If-Modified-Since and If-None-Match appear
950   in the same request.)
951</t>
952<t>
953   The meaning of "If-None-Match: *" is that the request method MUST NOT be
954   performed if the representation selected by the origin server (or by
955   a cache, possibly using the Vary mechanism, see Section 3.5 of <xref target="Part6"/>)
956   exists, and SHOULD be performed if the representation does not exist.
957   This feature is intended to be useful in preventing races between PUT
958   operations.
959</t>
960<t>
961   Examples:
962</t>
963<figure><artwork type="example"><![CDATA[
964  If-None-Match: "xyzzy"
965  If-None-Match: W/"xyzzy"
966  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
967  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
968  If-None-Match: *
969]]></artwork></figure>
970<t>
971   The result of a request having both an If-None-Match header field and
972   either an If-Match or an If-Unmodified-Since header fields is
973   undefined by this specification.
974</t>
975</section>
976
977<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
978  <iref primary="true" item="If-Unmodified-Since header field"/>
979  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since"/>
980 
981 
982<t>
983   The "If-Unmodified-Since" header field is used to make a request
984   method conditional.  If the representation that would have been transferred
985   in a 200 response to a GET request on the same resource has not been modified
986   since the time specified in this field, the server SHOULD perform the
987   requested operation as if the If-Unmodified-Since header field were not
988   present.
989</t>
990<t>
991   If the representation has been modified since the specified time,
992   the server MUST NOT perform the requested operation, and MUST return
993   a 412 (Precondition Failed).
994</t>
995<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/><artwork type="abnf2616"><![CDATA[
996  If-Unmodified-Since   = "If-Unmodified-Since" ":" OWS
997                          If-Unmodified-Since-v
998  If-Unmodified-Since-v = HTTP-date
999]]></artwork></figure>
1000<t>
1001   An example of the field is:
1002</t>
1003<figure><artwork type="example"><![CDATA[
1004  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1005]]></artwork></figure>
1006<t>
1007   If the request normally (i.e., without the If-Unmodified-Since
1008   header field) would result in anything other than a 2xx or 412 status code,
1009   the If-Unmodified-Since header field SHOULD be ignored.
1010</t>
1011<t>
1012   If the specified date is invalid, the header field is ignored.
1013</t>
1014<t>
1015   The result of a request having both an If-Unmodified-Since header
1016   field and either an If-None-Match or an If-Modified-Since header
1017   fields is undefined by this specification.
1018</t>
1019</section>
1020
1021<section title="Last-Modified" anchor="header.last-modified">
1022  <iref primary="true" item="Last-Modified header field"/>
1023  <iref primary="true" item="Header Fields" subitem="Last-Modified"/>
1024 
1025 
1026<t>
1027   The "Last-Modified" header field indicates the date and time at
1028   which the origin server believes the representation was last modified.
1029</t>
1030<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/><artwork type="abnf2616"><![CDATA[
1031  Last-Modified   = "Last-Modified" ":" OWS Last-Modified-v
1032  Last-Modified-v = HTTP-date
1033]]></artwork></figure>
1034<t>
1035   An example of its use is
1036</t>
1037<figure><artwork type="example"><![CDATA[
1038  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1039]]></artwork></figure>
1040<t>
1041   A representation is typically the sum of many parts behind the
1042   resource interface.  The last-modified time would usually be
1043   the most recent time that any of those parts were changed.
1044   How that value is determined for any given resource is an
1045   implementation detail beyond the scope of this specification.
1046   What matters to HTTP is how recipients of the Last-Modified
1047   header field can use its value to make conditional requests
1048   and test the validity of locally cached responses.
1049</t>
1050<t>
1051   An origin server MUST NOT send a Last-Modified date which is later
1052   than the server's time of message origination. In such cases, where
1053   the resource's last modification would indicate some time in the
1054   future, the server MUST replace that date with the message
1055   origination date.
1056</t>
1057<t>
1058   An origin server SHOULD obtain the Last-Modified value of the representation
1059   as close as possible to the time that it generates the Date value of
1060   its response. This allows a recipient to make an accurate assessment
1061   of the representation's modification time, especially if the representation changes
1062   near the time that the response is generated.
1063</t>
1064<t>
1065   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
1066</t>
1067<t>
1068   The Last-Modified header field value is often used as a cache
1069   validator. In simple terms, a cache entry is considered to be valid
1070   if the representation has not been modified since the Last-Modified value.
1071</t>
1072</section>
1073
1074</section>
1075
1076<section title="IANA Considerations" anchor="IANA.considerations">
1077
1078<section title="Status Code Registration" anchor="status.code.registration">
1079<t>
1080   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1081   shall be updated with the registrations below:
1082</t>
1083
1084<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1085<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1086   <ttcol>Value</ttcol>
1087   <ttcol>Description</ttcol>
1088   <ttcol>Reference</ttcol>
1089   <c>304</c>
1090   <c>Not Modified</c>
1091   <c>
1092      <xref target="status.304"/>
1093   </c>
1094   <c>412</c>
1095   <c>Precondition Failed</c>
1096   <c>
1097      <xref target="status.412"/>
1098   </c>
1099</texttable>
1100<!--(END)-->
1101
1102</section>
1103
1104<section title="Header Field Registration" anchor="header.field.registration">
1105<t>
1106   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1107   with the permanent registrations below (see <xref target="RFC3864"/>):
1108</t>
1109
1110<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1111<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1112   <ttcol>Header Field Name</ttcol>
1113   <ttcol>Protocol</ttcol>
1114   <ttcol>Status</ttcol>
1115   <ttcol>Reference</ttcol>
1116
1117   <c>ETag</c>
1118   <c>http</c>
1119   <c>standard</c>
1120   <c>
1121      <xref target="header.etag"/>
1122   </c>
1123   <c>If-Match</c>
1124   <c>http</c>
1125   <c>standard</c>
1126   <c>
1127      <xref target="header.if-match"/>
1128   </c>
1129   <c>If-Modified-Since</c>
1130   <c>http</c>
1131   <c>standard</c>
1132   <c>
1133      <xref target="header.if-modified-since"/>
1134   </c>
1135   <c>If-None-Match</c>
1136   <c>http</c>
1137   <c>standard</c>
1138   <c>
1139      <xref target="header.if-none-match"/>
1140   </c>
1141   <c>If-Unmodified-Since</c>
1142   <c>http</c>
1143   <c>standard</c>
1144   <c>
1145      <xref target="header.if-unmodified-since"/>
1146   </c>
1147   <c>Last-Modified</c>
1148   <c>http</c>
1149   <c>standard</c>
1150   <c>
1151      <xref target="header.last-modified"/>
1152   </c>
1153</texttable>
1154<!--(END)-->
1155
1156<t>
1157   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1158</t>
1159</section>
1160</section>
1161
1162<section title="Security Considerations" anchor="security.considerations">
1163<t>
1164   No additional security considerations have been identified beyond
1165   those applicable to HTTP in general <xref target="Part1"/>.
1166</t>
1167</section>
1168
1169<section title="Acknowledgments" anchor="ack">
1170</section>
1171</middle>
1172<back>
1173
1174<references title="Normative References">
1175
1176<reference anchor="Part1">
1177  <front>
1178    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1179    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1180      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1181      <address><email>fielding@gbiv.com</email></address>
1182    </author>
1183    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1184      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1185      <address><email>jg@freedesktop.org</email></address>
1186    </author>
1187    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1188      <organization abbrev="HP">Hewlett-Packard Company</organization>
1189      <address><email>JeffMogul@acm.org</email></address>
1190    </author>
1191    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1192      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1193      <address><email>henrikn@microsoft.com</email></address>
1194    </author>
1195    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1196      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1197      <address><email>LMM@acm.org</email></address>
1198    </author>
1199    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1200      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1201      <address><email>paulle@microsoft.com</email></address>
1202    </author>
1203    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1204      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1205      <address><email>timbl@w3.org</email></address>
1206    </author>
1207    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1208      <organization abbrev="W3C">World Wide Web Consortium</organization>
1209      <address><email>ylafon@w3.org</email></address>
1210    </author>
1211    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1212      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1213      <address><email>julian.reschke@greenbytes.de</email></address>
1214    </author>
1215    <date month="March" year="2011"/>
1216  </front>
1217  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-13"/>
1218 
1219</reference>
1220
1221<reference anchor="Part3">
1222  <front>
1223    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1224    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1225      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1226      <address><email>fielding@gbiv.com</email></address>
1227    </author>
1228    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1229      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1230      <address><email>jg@freedesktop.org</email></address>
1231    </author>
1232    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1233      <organization abbrev="HP">Hewlett-Packard Company</organization>
1234      <address><email>JeffMogul@acm.org</email></address>
1235    </author>
1236    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1237      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1238      <address><email>henrikn@microsoft.com</email></address>
1239    </author>
1240    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1241      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1242      <address><email>LMM@acm.org</email></address>
1243    </author>
1244    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1245      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1246      <address><email>paulle@microsoft.com</email></address>
1247    </author>
1248    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1249      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1250      <address><email>timbl@w3.org</email></address>
1251    </author>
1252    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1253      <organization abbrev="W3C">World Wide Web Consortium</organization>
1254      <address><email>ylafon@w3.org</email></address>
1255    </author>
1256    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1257      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1258      <address><email>julian.reschke@greenbytes.de</email></address>
1259    </author>
1260    <date month="March" year="2011"/>
1261  </front>
1262  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-13"/>
1263 
1264</reference>
1265
1266<reference anchor="Part5">
1267  <front>
1268    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1269    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1270      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1271      <address><email>fielding@gbiv.com</email></address>
1272    </author>
1273    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1274      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1275      <address><email>jg@freedesktop.org</email></address>
1276    </author>
1277    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1278      <organization abbrev="HP">Hewlett-Packard Company</organization>
1279      <address><email>JeffMogul@acm.org</email></address>
1280    </author>
1281    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1282      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1283      <address><email>henrikn@microsoft.com</email></address>
1284    </author>
1285    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1286      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1287      <address><email>LMM@acm.org</email></address>
1288    </author>
1289    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1290      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1291      <address><email>paulle@microsoft.com</email></address>
1292    </author>
1293    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1294      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1295      <address><email>timbl@w3.org</email></address>
1296    </author>
1297    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1298      <organization abbrev="W3C">World Wide Web Consortium</organization>
1299      <address><email>ylafon@w3.org</email></address>
1300    </author>
1301    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1302      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1303      <address><email>julian.reschke@greenbytes.de</email></address>
1304    </author>
1305    <date month="March" year="2011"/>
1306  </front>
1307  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-13"/>
1308 
1309</reference>
1310
1311<reference anchor="Part6">
1312  <front>
1313    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1314    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1315      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1316      <address><email>fielding@gbiv.com</email></address>
1317    </author>
1318    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1319      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1320      <address><email>jg@freedesktop.org</email></address>
1321    </author>
1322    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1323      <organization abbrev="HP">Hewlett-Packard Company</organization>
1324      <address><email>JeffMogul@acm.org</email></address>
1325    </author>
1326    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1327      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1328      <address><email>henrikn@microsoft.com</email></address>
1329    </author>
1330    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1331      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1332      <address><email>LMM@acm.org</email></address>
1333    </author>
1334    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1335      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1336      <address><email>paulle@microsoft.com</email></address>
1337    </author>
1338    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1339      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1340      <address><email>timbl@w3.org</email></address>
1341    </author>
1342    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1343      <organization abbrev="W3C">World Wide Web Consortium</organization>
1344      <address><email>ylafon@w3.org</email></address>
1345    </author>
1346    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1347      <address><email>mnot@mnot.net</email></address>
1348    </author>
1349    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1350      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1351      <address><email>julian.reschke@greenbytes.de</email></address>
1352    </author>
1353    <date month="March" year="2011"/>
1354  </front>
1355  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-13"/>
1356 
1357</reference>
1358
1359<reference anchor="RFC2119">
1360  <front>
1361    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1362    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1363      <organization>Harvard University</organization>
1364      <address><email>sob@harvard.edu</email></address>
1365    </author>
1366    <date month="March" year="1997"/>
1367  </front>
1368  <seriesInfo name="BCP" value="14"/>
1369  <seriesInfo name="RFC" value="2119"/>
1370</reference>
1371
1372<reference anchor="RFC5234">
1373  <front>
1374    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1375    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1376      <organization>Brandenburg InternetWorking</organization>
1377      <address>
1378        <email>dcrocker@bbiw.net</email>
1379      </address> 
1380    </author>
1381    <author initials="P." surname="Overell" fullname="Paul Overell">
1382      <organization>THUS plc.</organization>
1383      <address>
1384        <email>paul.overell@thus.net</email>
1385      </address>
1386    </author>
1387    <date month="January" year="2008"/>
1388  </front>
1389  <seriesInfo name="STD" value="68"/>
1390  <seriesInfo name="RFC" value="5234"/>
1391</reference>
1392
1393</references>
1394
1395<references title="Informative References">
1396
1397<reference anchor="RFC2616">
1398  <front>
1399    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1400    <author initials="R." surname="Fielding" fullname="R. Fielding">
1401      <organization>University of California, Irvine</organization>
1402      <address><email>fielding@ics.uci.edu</email></address>
1403    </author>
1404    <author initials="J." surname="Gettys" fullname="J. Gettys">
1405      <organization>W3C</organization>
1406      <address><email>jg@w3.org</email></address>
1407    </author>
1408    <author initials="J." surname="Mogul" fullname="J. Mogul">
1409      <organization>Compaq Computer Corporation</organization>
1410      <address><email>mogul@wrl.dec.com</email></address>
1411    </author>
1412    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1413      <organization>MIT Laboratory for Computer Science</organization>
1414      <address><email>frystyk@w3.org</email></address>
1415    </author>
1416    <author initials="L." surname="Masinter" fullname="L. Masinter">
1417      <organization>Xerox Corporation</organization>
1418      <address><email>masinter@parc.xerox.com</email></address>
1419    </author>
1420    <author initials="P." surname="Leach" fullname="P. Leach">
1421      <organization>Microsoft Corporation</organization>
1422      <address><email>paulle@microsoft.com</email></address>
1423    </author>
1424    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1425      <organization>W3C</organization>
1426      <address><email>timbl@w3.org</email></address>
1427    </author>
1428    <date month="June" year="1999"/>
1429  </front>
1430  <seriesInfo name="RFC" value="2616"/>
1431</reference>
1432
1433<reference anchor="RFC3864">
1434  <front>
1435    <title>Registration Procedures for Message Header Fields</title>
1436    <author initials="G." surname="Klyne" fullname="G. Klyne">
1437      <organization>Nine by Nine</organization>
1438      <address><email>GK-IETF@ninebynine.org</email></address>
1439    </author>
1440    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
1441      <organization>BEA Systems</organization>
1442      <address><email>mnot@pobox.com</email></address>
1443    </author>
1444    <author initials="J." surname="Mogul" fullname="J. Mogul">
1445      <organization>HP Labs</organization>
1446      <address><email>JeffMogul@acm.org</email></address>
1447    </author>
1448    <date year="2004" month="September"/>
1449  </front>
1450  <seriesInfo name="BCP" value="90"/>
1451  <seriesInfo name="RFC" value="3864"/>
1452</reference>
1453
1454</references>
1455
1456<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1457<t>
1458  Allow weak entity-tags in all requests except range requests (Sections
1459  <xref target="weak.and.strong.validators" format="counter"/> and
1460  <xref target="header.if-none-match" format="counter"/>).
1461</t>
1462</section>
1463
1464
1465<section title="Collected ABNF" anchor="collected.abnf">
1466<figure>
1467<artwork type="abnf" name="p4-conditional.parsed-abnf"><![CDATA[
1468ETag = "ETag:" OWS ETag-v
1469ETag-v = entity-tag
1470
1471HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
1472
1473If-Match = "If-Match:" OWS If-Match-v
1474If-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1475 entity-tag ] ) )
1476If-Modified-Since = "If-Modified-Since:" OWS If-Modified-Since-v
1477If-Modified-Since-v = HTTP-date
1478If-None-Match = "If-None-Match:" OWS If-None-Match-v
1479If-None-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1480 entity-tag ] ) )
1481If-Unmodified-Since = "If-Unmodified-Since:" OWS
1482 If-Unmodified-Since-v
1483If-Unmodified-Since-v = HTTP-date
1484
1485Last-Modified = "Last-Modified:" OWS Last-Modified-v
1486Last-Modified-v = HTTP-date
1487
1488OWS = <OWS, defined in [Part1], Section 1.2.2>
1489
1490entity-tag = [ weak ] opaque-tag
1491
1492opaque-tag = quoted-string
1493
1494quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
1495
1496weak = %x57.2F ; W/
1497]]></artwork>
1498</figure>
1499<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline"><![CDATA[
1500; ETag defined but not used
1501; If-Match defined but not used
1502; If-Modified-Since defined but not used
1503; If-None-Match defined but not used
1504; If-Unmodified-Since defined but not used
1505; Last-Modified defined but not used
1506]]></artwork></figure></section>
1507
1508
1509<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1510
1511<section title="Since RFC 2616">
1512<t>
1513  Extracted relevant partitions from <xref target="RFC2616"/>.
1514</t>
1515</section>
1516
1517<section title="Since draft-ietf-httpbis-p4-conditional-00">
1518<t>
1519  Closed issues:
1520  <list style="symbols">
1521    <t>
1522      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1523      "Normative and Informative references"
1524    </t>
1525  </list>
1526</t>
1527<t>
1528  Other changes:
1529  <list style="symbols">
1530    <t>
1531      Move definitions of 304 and 412 condition codes from Part2.
1532    </t>
1533  </list>
1534</t>
1535</section>
1536
1537<section title="Since draft-ietf-httpbis-p4-conditional-01">
1538<t>
1539  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1540  <list style="symbols">
1541    <t>
1542      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1543    </t>
1544  </list>
1545</t>
1546</section>
1547
1548<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1549<t>
1550  Closed issues:
1551  <list style="symbols">
1552    <t>
1553      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1554      "Weak ETags on non-GET requests"
1555    </t>
1556  </list>
1557</t>
1558<t>
1559  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1560  <list style="symbols">
1561    <t>
1562      Reference RFC 3984, and update header field registrations for header fields defined
1563      in this document.
1564    </t>
1565  </list>
1566</t>
1567</section>
1568
1569<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1570<t>
1571  Closed issues:
1572  <list style="symbols">
1573    <t>
1574      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1575      "Examples for ETag matching"
1576    </t>
1577    <t>
1578      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1579      "'entity value' undefined"
1580    </t>
1581    <t>
1582      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1583      "bogus 2068 Date header reference"
1584    </t>
1585  </list>
1586</t>
1587</section>
1588
1589<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1590<t>
1591  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1592  <list style="symbols">
1593    <t>
1594      Use "/" instead of "|" for alternatives.
1595    </t>
1596    <t>
1597      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1598      whitespace ("OWS") and required whitespace ("RWS").
1599    </t>
1600    <t>
1601      Rewrite ABNFs to spell out whitespace rules, factor out
1602      header field value format definitions.
1603    </t>
1604  </list>
1605</t>
1606</section>
1607
1608<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1609<t>
1610  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1611  <list style="symbols">
1612    <t>
1613      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1614    </t>
1615  </list>
1616</t>
1617</section>
1618
1619<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1620<t>
1621  Closed issues:
1622  <list style="symbols">
1623    <t>
1624      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1625      "case-sensitivity of etag weakness indicator"
1626    </t>
1627  </list>
1628</t>
1629</section>
1630
1631<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1632<t>
1633  Closed issues:
1634  <list style="symbols">
1635    <t>
1636      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1637      "Weak ETags on non-GET requests" (If-Match still was defined to require
1638      strong matching)
1639    </t>
1640    <t>
1641      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1642      "move IANA registrations for optional status codes"
1643    </t>
1644  </list>
1645</t>
1646</section>
1647
1648<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1649<t>
1650  No significant changes.
1651</t>
1652</section>
1653
1654<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1655<t>
1656  No significant changes.
1657</t>
1658</section>
1659
1660<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1661<t>
1662  Closed issues:
1663  <list style="symbols">
1664    <t>
1665      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1666      "Clarify 'Requested Variant'"
1667    </t>
1668    <t>
1669      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1670      "Clarify entity / representation / variant terminology"
1671    </t>
1672    <t>
1673      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1674      "consider removing the 'changes from 2068' sections"
1675    </t>
1676  </list>
1677</t>
1678</section>
1679
1680<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1681<t>
1682  None.
1683</t>
1684</section>
1685
1686<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1687<t>
1688  Closed issues:
1689  <list style="symbols">
1690    <t>
1691      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1692      "Header Classification"
1693    </t>
1694  </list>
1695</t>
1696</section>
1697
1698</section>
1699
1700</back>
1701</rfc>
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