source: draft-ietf-httpbis/latest/p4-conditional.xml @ 366

Last change on this file since 366 was 366, checked in by julian.reschke@…, 11 years ago

Rewrite header ABNFs to spell out whitespace rules, factor out value format definitions. (related to #36)

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "November">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY full-date                  "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27]>
28<?rfc toc="yes" ?>
29<?rfc symrefs="yes" ?>
30<?rfc sortrefs="yes" ?>
31<?rfc compact="yes"?>
32<?rfc subcompact="no" ?>
33<?rfc linkmailto="no" ?>
34<?rfc editing="no" ?>
35<?rfc comments="yes"?>
36<?rfc inline="yes"?>
37<?rfc-ext allow-markup-in-artwork="yes" ?>
38<?rfc-ext include-references-in-index="yes" ?>
39<rfc obsoletes="2616" category="std" x:maturity-level="draft"
40     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
41     xmlns:x='http://purl.org/net/xml2rfc/ext'>
42<front>
43
44  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
45
46  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
47    <organization abbrev="Day Software">Day Software</organization>
48    <address>
49      <postal>
50        <street>23 Corporate Plaza DR, Suite 280</street>
51        <city>Newport Beach</city>
52        <region>CA</region>
53        <code>92660</code>
54        <country>USA</country>
55      </postal>
56      <phone>+1-949-706-5300</phone>
57      <facsimile>+1-949-706-5305</facsimile>
58      <email>fielding@gbiv.com</email>
59      <uri>http://roy.gbiv.com/</uri>
60    </address>
61  </author>
62
63  <author initials="J." surname="Gettys" fullname="Jim Gettys">
64    <organization>One Laptop per Child</organization>
65    <address>
66      <postal>
67        <street>21 Oak Knoll Road</street>
68        <city>Carlisle</city>
69        <region>MA</region>
70        <code>01741</code>
71        <country>USA</country>
72      </postal>
73      <email>jg@laptop.org</email>
74      <uri>http://www.laptop.org/</uri>
75    </address>
76  </author>
77 
78  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
79    <organization abbrev="HP">Hewlett-Packard Company</organization>
80    <address>
81      <postal>
82        <street>HP Labs, Large Scale Systems Group</street>
83        <street>1501 Page Mill Road, MS 1177</street>
84        <city>Palo Alto</city>
85        <region>CA</region>
86        <code>94304</code>
87        <country>USA</country>
88      </postal>
89      <email>JeffMogul@acm.org</email>
90    </address>
91  </author>
92
93  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
94    <organization abbrev="Microsoft">Microsoft Corporation</organization>
95    <address>
96      <postal>
97        <street>1 Microsoft Way</street>
98        <city>Redmond</city>
99        <region>WA</region>
100        <code>98052</code>
101        <country>USA</country>
102      </postal>
103      <email>henrikn@microsoft.com</email>
104    </address>
105  </author>
106
107  <author initials="L." surname="Masinter" fullname="Larry Masinter">
108    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
109    <address>
110      <postal>
111        <street>345 Park Ave</street>
112        <city>San Jose</city>
113        <region>CA</region>
114        <code>95110</code>
115        <country>USA</country>
116      </postal>
117      <email>LMM@acm.org</email>
118      <uri>http://larry.masinter.net/</uri>
119    </address>
120  </author>
121 
122  <author initials="P." surname="Leach" fullname="Paul J. Leach">
123    <organization abbrev="Microsoft">Microsoft Corporation</organization>
124    <address>
125      <postal>
126        <street>1 Microsoft Way</street>
127        <city>Redmond</city>
128        <region>WA</region>
129        <code>98052</code>
130      </postal>
131      <email>paulle@microsoft.com</email>
132    </address>
133  </author>
134   
135  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
136    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
137    <address>
138      <postal>
139        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
140        <street>The Stata Center, Building 32</street>
141        <street>32 Vassar Street</street>
142        <city>Cambridge</city>
143        <region>MA</region>
144        <code>02139</code>
145        <country>USA</country>
146      </postal>
147      <email>timbl@w3.org</email>
148      <uri>http://www.w3.org/People/Berners-Lee/</uri>
149    </address>
150  </author>
151
152  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
153    <organization abbrev="W3C">World Wide Web Consortium</organization>
154    <address>
155      <postal>
156        <street>W3C / ERCIM</street>
157        <street>2004, rte des Lucioles</street>
158        <city>Sophia-Antipolis</city>
159        <region>AM</region>
160        <code>06902</code>
161        <country>France</country>
162      </postal>
163      <email>ylafon@w3.org</email>
164      <uri>http://www.raubacapeu.net/people/yves/</uri>
165    </address>
166  </author>
167
168  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
169    <organization abbrev="greenbytes">greenbytes GmbH</organization>
170    <address>
171      <postal>
172        <street>Hafenweg 16</street>
173        <city>Muenster</city><region>NW</region><code>48155</code>
174        <country>Germany</country>
175      </postal>
176      <phone>+49 251 2807760</phone>   
177      <facsimile>+49 251 2807761</facsimile>   
178      <email>julian.reschke@greenbytes.de</email>       
179      <uri>http://greenbytes.de/tech/webdav/</uri>     
180    </address>
181  </author>
182
183  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
184
185<abstract>
186<t>
187   The Hypertext Transfer Protocol (HTTP) is an application-level
188   protocol for distributed, collaborative, hypermedia information
189   systems. HTTP has been in use by the World Wide Web global information
190   initiative since 1990. This document is Part 4 of the seven-part specification
191   that defines the protocol referred to as "HTTP/1.1" and, taken together,
192   obsoletes RFC 2616.  Part 4 defines request header fields for
193   indicating conditional requests and the rules for constructing responses
194   to those requests.
195</t>
196</abstract>
197
198<note title="Editorial Note (To be removed by RFC Editor)">
199  <t>
200    Discussion of this draft should take place on the HTTPBIS working group
201    mailing list (ietf-http-wg@w3.org). The current issues list is
202    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
203    and related documents (including fancy diffs) can be found at
204    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
205  </t>
206  <t>
207    The changes in this draft are summarized in <xref target="changes.since.04"/>.
208  </t>
209</note>
210</front>
211<middle>
212<section title="Introduction" anchor="introduction">
213<t>
214   This document defines HTTP/1.1 response metadata for indicating potential
215   changes to payload content, including modification time stamps and opaque
216   entity-tags, and the HTTP conditional request mechanisms that allow
217   preconditions to be placed on a request method.  Conditional GET requests
218   allow for efficient cache updates.  Other conditional request methods are
219   used to protect against overwriting or misunderstanding the state of a
220   resource that has been changed unbeknownst to the requesting client.
221</t>
222<t>
223   This document is currently disorganized in order to minimize the changes
224   between drafts and enable reviewers to see the smaller errata changes.
225   The next draft will reorganize the sections to better reflect the content.
226   In particular, the sections on resource metadata will be discussed first
227   and then followed by each conditional request-header, concluding with a
228   definition of precedence and the expectation of ordering strong validator
229   checks before weak validator checks.  It is likely that more content from
230   &caching; will migrate to this part, where appropriate.
231   The current mess reflects how widely dispersed these topics and associated
232   requirements had become in <xref target="RFC2616"/>.
233</t>
234
235<section title="Requirements" anchor="intro.requirements">
236<t>
237   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
238   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
239   document are to be interpreted as described in <xref target="RFC2119"/>.
240</t>
241<t>
242   An implementation is not compliant if it fails to satisfy one or more
243   of the &MUST; or &REQUIRED; level requirements for the protocols it
244   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
245   level and all the &SHOULD; level requirements for its protocols is said
246   to be "unconditionally compliant"; one that satisfies all the &MUST;
247   level requirements but not all the &SHOULD; level requirements for its
248   protocols is said to be "conditionally compliant."
249</t>
250</section>
251</section>
252
253<section title="Notational Conventions and Generic Grammar" anchor="notation">
254  <x:anchor-alias value="quoted-string"/>
255  <x:anchor-alias value="BWS"/>
256  <x:anchor-alias value="OWS"/>
257  <x:anchor-alias value="RWS"/>
258<t>
259  This specification uses the ABNF syntax defined in &notation-abnf; and
260  the core rules defined in &basic-rules;:
261  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
262  <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
263</t>
264<figure><artwork type="abnf2616">
265  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
266  <x:ref>BWS</x:ref>           = &lt;BWS, defined in &basic-rules;&gt;
267  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
268  <x:ref>RWS</x:ref>           = &lt;RWS, defined in &basic-rules;&gt;
269</artwork></figure>
270<t anchor="abnf.dependencies">
271  <x:anchor-alias value="HTTP-date"/>
272  The ABNF rules below are defined in other parts:
273</t>
274<figure><!--Part1--><artwork type="abnf2616">
275  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
276</artwork></figure>
277</section>
278
279<section title="Entity Tags" anchor="entity.tags">
280  <x:anchor-alias value="entity-tag"/>
281  <x:anchor-alias value="opaque-tag"/>
282  <x:anchor-alias value="weak"/>
283<t>
284   Entity tags are used for comparing two or more entities from the same
285   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
286   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
287   If-Range (&header-if-range;) header fields. The definition of how they
288   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
289   entity tag consists of an opaque quoted string, possibly prefixed by
290   a weakness indicator.
291</t>
292<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/>
293  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
294  <x:ref>weak</x:ref>       = "W/"
295  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
296</artwork></figure>
297<t>
298   A "strong entity tag" &MAY; be shared by two entities of a resource
299   only if they are equivalent by octet equality.
300</t>
301<t>
302   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
303   two entities of a resource only if the entities are equivalent and
304   could be substituted for each other with no significant change in
305   semantics. A weak entity tag can only be used for weak comparison.
306</t>
307<t>
308   An entity tag &MUST; be unique across all versions of all entities
309   associated with a particular resource. A given entity tag value &MAY;
310   be used for entities obtained by requests on different URIs. The use
311   of the same entity tag value in conjunction with entities obtained by
312   requests on different URIs does not imply the equivalence of those
313   entities.
314</t>
315</section>
316
317<section title="Status Code Definitions">
318<section title="304 Not Modified" anchor="status.304">
319  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
320  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
321<t>
322   If the client has performed a conditional GET request and access is
323   allowed, but the document has not been modified, the server &SHOULD;
324   respond with this status code. The 304 response &MUST-NOT; contain a
325   message-body, and thus is always terminated by the first empty line
326   after the header fields.
327</t>
328<t>
329   The response &MUST; include the following header fields:
330  <list style="symbols">
331    <x:lt>
332      <t>Date, unless its omission is required by &clockless;.</t>
333      <t>
334         If a clockless origin server obeys these rules, and proxies and
335         clients add their own Date to any response received without one (as
336         already specified by &header-date;, caches will operate
337         correctly.</t>
338    </x:lt>
339    <x:lt>
340      <t>ETag and/or Content-Location, if the header would have been sent
341         in a 200 response to the same request.</t>
342    </x:lt>
343    <x:lt>
344      <t>Expires, Cache-Control, and/or Vary, if the field-value might
345         differ from that sent in any previous response for the same
346         variant.</t>
347    </x:lt>
348  </list>
349</t>
350<t>
351   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
352   the response &SHOULD-NOT;  include other entity-headers.
353   Otherwise (i.e., the conditional GET used a weak validator), the
354   response &MUST-NOT; include other entity-headers; this prevents
355   inconsistencies between cached entity-bodies and updated headers.
356</t>
357<t>
358   If a 304 response indicates an entity not currently cached, then the
359   cache &MUST; disregard the response and repeat the request without the
360   conditional.
361</t>
362<t>
363   If a cache uses a received 304 response to update a cache entry, the
364   cache &MUST; update the entry to reflect any new field values given in
365   the response.
366</t>
367</section>
368
369<section title="412 Precondition Failed" anchor="status.412">
370  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
371  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
372<t>
373   The precondition given in one or more of the request-header fields
374   evaluated to false when it was tested on the server. This response
375   code allows the client to place preconditions on the current resource
376   metainformation (header field data) and thus prevent the requested
377   method from being applied to a resource other than the one intended.
378</t>
379</section>
380</section>
381
382<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
383<t>
384   Since both origin servers and caches will compare two validators to
385   decide if they represent the same or different entities, one normally
386   would expect that if the entity (the entity-body or any entity-headers)
387   changes in any way, then the associated validator would
388   change as well. If this is true, then we call this validator a
389   "strong validator."
390</t>
391<t>
392   However, there might be cases when a server prefers to change the
393   validator only on semantically significant changes, and not when
394   insignificant aspects of the entity change. A validator that does not
395   always change when the resource changes is a "weak validator."
396</t>
397<t>
398   Entity tags are normally "strong validators," but the protocol
399   provides a mechanism to tag an entity tag as "weak." One can think of
400   a strong validator as one that changes whenever the bits of an entity
401   changes, while a weak value changes whenever the meaning of an entity
402   changes. Alternatively, one can think of a strong validator as part
403   of an identifier for a specific entity, while a weak validator is
404   part of an identifier for a set of semantically equivalent entities.
405  <list><t>
406      <x:h>Note:</x:h> One example of a strong validator is an integer that is
407      incremented in stable storage every time an entity is changed.
408    </t><t>
409      An entity's modification time, if represented with one-second
410      resolution, could be a weak validator, since it is possible that
411      the resource might be modified twice during a single second.
412    </t><t>
413      Support for weak validators is optional. However, weak validators
414      allow for more efficient caching of equivalent objects; for
415      example, a hit counter on a site is probably good enough if it is
416      updated every few days or weeks, and any value during that period
417      is likely "good enough" to be equivalent.
418    </t></list>
419</t>
420<t>
421   A "use" of a validator is either when a client generates a request
422   and includes the validator in a validating header field, or when a
423   server compares two validators.
424</t>
425<t>
426   Strong validators are usable in any context. Weak validators are only
427   usable in contexts that do not depend on exact equality of an entity.
428   For example, either kind is usable for a conditional GET of a full
429   entity. However, only a strong validator is usable for a sub-range
430   retrieval, since otherwise the client might end up with an internally
431   inconsistent entity.
432</t>
433<t>
434   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
435</t>
436<t>
437   The only function that HTTP/1.1 defines on validators is
438   comparison. There are two validator comparison functions, depending
439   on whether the comparison context allows the use of weak validators
440   or not:
441  <list style="symbols">
442     <t>The strong comparison function: in order to be considered equal,
443        both opaque-tags &MUST; be identical character-by-character, and both
444        &MUST-NOT; be weak.</t>
445     <t>The weak comparison function: in order to be considered equal, both
446        opaque-tags &MUST; be identical character-by-character.</t>
447  </list>
448</t>
449<t>
450   The example below shows the results for a set of entity tag pairs,
451   and both the weak and strong comparison function results:
452</t>
453<texttable align="left">
454  <ttcol>ETag 1</ttcol>
455  <ttcol>ETag 2</ttcol>
456  <ttcol>Strong Comparison</ttcol>
457  <ttcol>Weak Comparison</ttcol>
458
459  <c>W/"1"</c>
460  <c>W/"1"</c>
461  <c>no match</c>
462  <c>match</c>
463 
464  <c>W/"1"</c>
465  <c>W/"2"</c>
466  <c>no match</c>
467  <c>no match</c>
468
469  <c>W/"1"</c>
470  <c>"1"</c>
471  <c>no match</c>
472  <c>match</c>
473
474  <c>"1"</c>
475  <c>"1"</c>
476  <c>match</c>
477  <c>match</c>
478</texttable>
479<t>
480   An entity tag is strong unless it is explicitly tagged as weak.
481   <xref target="entity.tags"/> gives the syntax for entity tags.
482</t>
483<t>
484   A Last-Modified time, when used as a validator in a request, is
485   implicitly weak unless it is possible to deduce that it is strong,
486   using the following rules:
487  <list style="symbols">
488     <t>The validator is being compared by an origin server to the
489        actual current validator for the entity and,</t>
490     <t>That origin server reliably knows that the associated entity did
491        not change twice during the second covered by the presented
492        validator.</t>
493  </list>
494</t>
495<t>
496   or
497  <list style="symbols">
498     <t>The validator is about to be used by a client in an If-Modified-Since
499        or If-Unmodified-Since header, because the client
500        has a cache entry for the associated entity, and</t>
501     <t>That cache entry includes a Date value, which gives the time
502        when the origin server sent the original response, and</t>
503     <t>The presented Last-Modified time is at least 60 seconds before
504        the Date value.</t>
505  </list>
506</t>
507<t>
508   or
509  <list style="symbols">
510     <t>The validator is being compared by an intermediate cache to the
511        validator stored in its cache entry for the entity, and</t>
512     <t>That cache entry includes a Date value, which gives the time
513        when the origin server sent the original response, and</t>
514     <t>The presented Last-Modified time is at least 60 seconds before
515        the Date value.</t>
516  </list>
517</t>
518<t>
519   This method relies on the fact that if two different responses were
520   sent by the origin server during the same second, but both had the
521   same Last-Modified time, then at least one of those responses would
522   have a Date value equal to its Last-Modified time. The arbitrary 60-second
523   limit guards against the possibility that the Date and Last-Modified
524   values are generated from different clocks, or at somewhat
525   different times during the preparation of the response. An
526   implementation &MAY; use a value larger than 60 seconds, if it is
527   believed that 60 seconds is too short.
528</t>
529<t>
530   If a client wishes to perform a sub-range retrieval on a value for
531   which it has only a Last-Modified time and no opaque validator, it
532   &MAY; do this only if the Last-Modified time is strong in the sense
533   described here.
534</t>
535<t>
536   A cache or origin server receiving a conditional range request
537   (<xref target="Part5"/>) &MUST; use the strong comparison function to
538   evaluate the condition.
539</t>
540<t>
541   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
542   retrievals on values that have been obtained from HTTP/1.0
543   servers.
544</t>
545</section>
546
547<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">
548<t>
549   We adopt a set of rules and recommendations for origin servers,
550   clients, and caches regarding when various validator types ought to
551   be used, and for what purposes.
552</t>
553<t>
554   HTTP/1.1 origin servers:
555  <list style="symbols">
556     <t>&SHOULD; send an entity tag validator unless it is not feasible to
557        generate one.</t>
558
559     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
560        performance considerations support the use of weak entity tags,
561        or if it is unfeasible to send a strong entity tag.</t>
562
563     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
564        unless the risk of a breakdown in semantic transparency that
565        could result from using this date in an If-Modified-Since header
566        would lead to serious problems.</t>
567  </list>
568</t>
569<t>
570   In other words, the preferred behavior for an HTTP/1.1 origin server
571   is to send both a strong entity tag and a Last-Modified value.
572</t>
573<t>
574   In order to be legal, a strong entity tag &MUST; change whenever the
575   associated entity changes in any way. A weak entity tag &SHOULD;
576   change whenever the associated entity changes in a semantically
577   significant way.
578  <list><t>
579      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
580      origin server must avoid reusing a specific strong entity tag
581      value for two different entities, or reusing a specific weak
582      entity tag value for two semantically different entities. Cache
583      entries might persist for arbitrarily long periods, regardless of
584      expiration times, so it might be inappropriate to expect that a
585      cache will never again attempt to validate an entry using a
586      validator that it obtained at some point in the past.
587  </t></list>
588</t>
589<t>
590   HTTP/1.1 clients:
591  <list style="symbols">
592     <t>If an entity tag has been provided by the origin server, &MUST;
593        use that entity tag in any cache-conditional request (using If-Match
594        or If-None-Match).</t>
595
596     <t>If only a Last-Modified value has been provided by the origin
597        server, &SHOULD; use that value in non-subrange cache-conditional
598        requests (using If-Modified-Since).</t>
599
600     <t>If only a Last-Modified value has been provided by an HTTP/1.0
601        origin server, &MAY; use that value in subrange cache-conditional
602        requests (using If-Unmodified-Since:). The user agent &SHOULD;
603        provide a way to disable this, in case of difficulty.</t>
604
605     <t>If both an entity tag and a Last-Modified value have been
606        provided by the origin server, &SHOULD; use both validators in
607        cache-conditional requests. This allows both HTTP/1.0 and
608        HTTP/1.1 caches to respond appropriately.</t>
609  </list>
610</t>
611<t>
612   An HTTP/1.1 origin server, upon receiving a conditional request that
613   includes both a Last-Modified date (e.g., in an If-Modified-Since or
614   If-Unmodified-Since header field) and one or more entity tags (e.g.,
615   in an If-Match, If-None-Match, or If-Range header field) as cache
616   validators, &MUST-NOT; return a response status of 304 (Not Modified)
617   unless doing so is consistent with all of the conditional header
618   fields in the request.
619</t>
620<t>
621   An HTTP/1.1 caching proxy, upon receiving a conditional request that
622   includes both a Last-Modified date and one or more entity tags as
623   cache validators, &MUST-NOT; return a locally cached response to the
624   client unless that cached response is consistent with all of the
625   conditional header fields in the request.
626  <list><t>
627      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
628      servers and clients should transmit as much non-redundant
629      information as is available in their responses and requests.
630      HTTP/1.1 systems receiving this information will make the most
631      conservative assumptions about the validators they receive.
632  </t><t>
633      HTTP/1.0 clients and caches will ignore entity tags. Generally,
634      last-modified values received or used by these systems will
635      support transparent and efficient caching, and so HTTP/1.1 origin
636      servers should provide Last-Modified values. In those rare cases
637      where the use of a Last-Modified value as a validator by an
638      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
639      origin servers should not provide one.
640  </t></list>
641</t>
642</section>
643
644<section title="Header Field Definitions" anchor="header.fields">
645<t>
646   This section defines the syntax and semantics of HTTP/1.1 header fields
647   related to conditional requests.
648</t>
649<t>
650   For entity-header fields, both sender and recipient refer to either the
651   client or the server, depending on who sends and who receives the entity.
652</t>
653
654<section title="ETag" anchor="header.etag">
655  <iref primary="true" item="ETag header" x:for-anchor=""/>
656  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
657  <x:anchor-alias value="ETag"/>
658  <x:anchor-alias value="ETag-v"/>
659<t>
660   The response-header field "ETag" provides the current value of the
661   entity tag (see <xref target="entity.tags"/>) for the requested variant.
662   The headers used with entity
663   tags are described in Sections <xref target="header.if-match" format="counter"/>
664   and <xref target="header.if-none-match" format="counter"/> of this document,
665   and in &header-if-range;. The entity tag
666   &MAY; be used for comparison with other entities from the same resource
667   (see <xref target="weak.and.strong.validators"/>).
668</t>
669<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
670  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
671  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
672</artwork></figure>
673<figure><preamble>
674  Examples:
675</preamble>
676<artwork type="example">
677  ETag: "xyzzy"
678  ETag: W/"xyzzy"
679  ETag: ""
680</artwork></figure>
681<t>
682   The ETag response-header field value, an entity tag, provides for an
683   "opaque" cache validator. This might allow more reliable validation
684   in situations where it is inconvenient to store modification dates,
685   where the one-second resolution of HTTP date values is not
686   sufficient, or where the origin server wishes to avoid certain
687   paradoxes that might arise from the use of modification dates.
688</t>
689<t>
690   The principle behind entity tags is that only the service author
691   knows the semantics of a resource well enough to select an
692   appropriate cache validation mechanism, and the specification of any
693   validator comparison function more complex than byte-equality would
694   open up a can of worms. Thus, comparisons of any other headers
695   (except Last-Modified, for compatibility with HTTP/1.0) are never
696   used for purposes of validating a cache entry.
697</t>
698</section>
699
700<section title="If-Match" anchor="header.if-match">
701  <iref primary="true" item="If-Match header" x:for-anchor=""/>
702  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
703  <x:anchor-alias value="If-Match"/>
704  <x:anchor-alias value="If-Match-v"/>
705<t>
706   The request-header field "If-Match" is used with a method to make it
707   conditional. A client that has one or more entities previously
708   obtained from the resource can verify that one of those entities is
709   current by including a list of their associated entity tags in the
710   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
711   purpose of this feature is to allow efficient updates of cached
712   information with a minimum amount of transaction overhead. It is also
713   used, on updating requests, to prevent inadvertent modification of
714   the wrong version of a resource. As a special case, the value "*"
715   matches any current entity of the resource.
716</t>
717<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
718  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
719  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
720</artwork></figure>
721<t>
722   If any of the entity tags match the entity tag of the entity that
723   would have been returned in the response to a similar GET request
724   (without the If-Match header) on that resource, or if "*" is given
725   and any current entity exists for that resource, then the server &MAY;
726   perform the requested method as if the If-Match header field did not
727   exist.
728</t>
729<t>
730   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
731   to compare the entity tags in If-Match.
732</t>
733<t>
734   If none of the entity tags match, or if "*" is given and no current
735   entity exists, the server &MUST-NOT; perform the requested method, and
736   &MUST; return a 412 (Precondition Failed) response. This behavior is
737   most useful when the client wants to prevent an updating method, such
738   as PUT, from modifying a resource that has changed since the client
739   last retrieved it.
740</t>
741<t>
742   If the request would, without the If-Match header field, result in
743   anything other than a 2xx or 412 status, then the If-Match header
744   &MUST; be ignored.
745</t>
746<t>
747   The meaning of "If-Match: *" is that the method &SHOULD; be performed
748   if the representation selected by the origin server (or by a cache,
749   possibly using the Vary mechanism, see &header-vary;) exists, and
750   &MUST-NOT; be performed if the representation does not exist.
751</t>
752<t>
753   A request intended to update a resource (e.g., a PUT) &MAY; include an
754   If-Match header field to signal that the request method &MUST-NOT; be
755   applied if the entity corresponding to the If-Match value (a single
756   entity tag) is no longer a representation of that resource. This
757   allows the user to indicate that they do not wish the request to be
758   successful if the resource has been changed without their knowledge.
759   Examples:
760</t>
761<figure><artwork type="example">
762  If-Match: "xyzzy"
763  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
764  If-Match: *
765</artwork></figure>
766<t>
767   The result of a request having both an If-Match header field and
768   either an If-None-Match or an If-Modified-Since header fields is
769   undefined by this specification.
770</t>
771</section>
772
773<section title="If-Modified-Since" anchor="header.if-modified-since">
774  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
775  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
776  <x:anchor-alias value="If-Modified-Since"/>
777  <x:anchor-alias value="If-Modified-Since-v"/>
778<t>
779   The request-header field "If-Modified-Since" is used with a method to
780   make it conditional: if the requested variant has not been modified
781   since the time specified in this field, an entity will not be
782   returned from the server; instead, a 304 (Not Modified) response will
783   be returned without any message-body.
784</t>
785<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
786  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref> <x:ref>If-Modified-Since-v</x:ref>
787  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
788</artwork></figure>
789<t>
790   An example of the field is:
791</t>
792<figure><artwork type="example">
793  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
794</artwork></figure>
795<t>
796   A GET method with an If-Modified-Since header and no Range header
797   requests that the identified entity be transferred only if it has
798   been modified since the date given by the If-Modified-Since header.
799   The algorithm for determining this includes the following cases:
800  <list style="numbers">
801      <t>If the request would normally result in anything other than a
802         200 (OK) status, or if the passed If-Modified-Since date is
803         invalid, the response is exactly the same as for a normal GET.
804         A date which is later than the server's current time is
805         invalid.</t>
806
807      <t>If the variant has been modified since the If-Modified-Since
808         date, the response is exactly the same as for a normal GET.</t>
809
810      <t>If the variant has not been modified since a valid If-Modified-Since
811         date, the server &SHOULD; return a 304 (Not
812         Modified) response.</t>
813  </list>
814</t>
815<t>
816   The purpose of this feature is to allow efficient updates of cached
817   information with a minimum amount of transaction overhead.
818  <list><t>
819      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
820      see &header-range; for full details.
821    </t><t>
822      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
823      clock might not be synchronized with the client.
824    </t><t>
825      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
826      servers will use an exact date comparison function, rather than a
827      less-than function, for deciding whether to send a 304 (Not
828      Modified) response. To get best results when sending an If-Modified-Since
829      header field for cache validation, clients are
830      advised to use the exact date string received in a previous Last-Modified
831      header field whenever possible.
832    </t><t>
833      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
834      header instead of a date taken from the Last-Modified header for
835      the same request, the client should be aware of the fact that this
836      date is interpreted in the server's understanding of time. The
837      client should consider unsynchronized clocks and rounding problems
838      due to the different encodings of time between the client and
839      server. This includes the possibility of race conditions if the
840      document has changed between the time it was first requested and
841      the If-Modified-Since date of a subsequent request, and the
842      possibility of clock-skew-related problems if the If-Modified-Since
843      date is derived from the client's clock without correction
844      to the server's clock. Corrections for different time bases
845      between client and server are at best approximate due to network
846      latency.
847    </t>
848  </list>
849</t>
850<t>
851   The result of a request having both an If-Modified-Since header field
852   and either an If-Match or an If-Unmodified-Since header fields is
853   undefined by this specification.
854</t>
855</section>
856
857<section title="If-None-Match" anchor="header.if-none-match">
858  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
859  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
860  <x:anchor-alias value="If-None-Match"/>
861  <x:anchor-alias value="If-None-Match-v"/>
862<t>
863   The request-header field "If-None-Match" is used with a method to make
864   it conditional. A client that has one or more entities previously
865   obtained from the resource can verify that none of those entities is
866   current by including a list of their associated entity tags in the
867   If-None-Match header field. The purpose of this feature is to allow
868   efficient updates of cached information with a minimum amount of
869   transaction overhead. It is also used to prevent a method (e.g. PUT)
870   from inadvertently modifying an existing resource when the client
871   believes that the resource does not exist.
872</t>
873<t>
874   As a special case, the value "*" matches any current entity of the
875   resource.
876</t>
877<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
878  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
879  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
880</artwork></figure>
881<t>
882   If any of the entity tags match the entity tag of the entity that
883   would have been returned in the response to a similar GET request
884   (without the If-None-Match header) on that resource, or if "*" is
885   given and any current entity exists for that resource, then the
886   server &MUST-NOT; perform the requested method, unless required to do
887   so because the resource's modification date fails to match that
888   supplied in an If-Modified-Since header field in the request.
889   Instead, if the request method was GET or HEAD, the server &SHOULD;
890   respond with a 304 (Not Modified) response, including the cache-related
891   header fields (particularly ETag) of one of the entities that
892   matched. For all other request methods, the server &MUST; respond with
893   a status of 412 (Precondition Failed).
894</t>
895<t>
896   See <xref target="weak.and.strong.validators"/> for rules on how to
897   determine if two entity tags match.
898</t>
899<t>
900   If none of the entity tags match, then the server &MAY; perform the
901   requested method as if the If-None-Match header field did not exist,
902   but &MUST; also ignore any If-Modified-Since header field(s) in the
903   request. That is, if no entity tags match, then the server &MUST-NOT;
904   return a 304 (Not Modified) response.
905</t>
906<t>
907   If the request would, without the If-None-Match header field, result
908   in anything other than a 2xx or 304 status, then the If-None-Match
909   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
910   server behavior when both If-Modified-Since and If-None-Match appear
911   in the same request.)
912</t>
913<t>
914   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
915   performed if the representation selected by the origin server (or by
916   a cache, possibly using the Vary mechanism, see &header-vary;)
917   exists, and &SHOULD; be performed if the representation does not exist.
918   This feature is intended to be useful in preventing races between PUT
919   operations.
920</t>
921<t>
922   Examples:
923</t>
924<figure><artwork type="example">
925  If-None-Match: "xyzzy"
926  If-None-Match: W/"xyzzy"
927  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
928  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
929  If-None-Match: *
930</artwork></figure>
931<t>
932   The result of a request having both an If-None-Match header field and
933   either an If-Match or an If-Unmodified-Since header fields is
934   undefined by this specification.
935</t>
936</section>
937
938<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
939  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
940  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
941  <x:anchor-alias value="If-Unmodified-Since"/>
942  <x:anchor-alias value="If-Unmodified-Since-v"/>
943<t>
944   The request-header field "If-Unmodified-Since" is used with a method to
945   make it conditional. If the requested resource has not been modified
946   since the time specified in this field, the server &SHOULD; perform the
947   requested operation as if the If-Unmodified-Since header were not
948   present.
949</t>
950<t>
951   If the requested variant has been modified since the specified time,
952   the server &MUST-NOT; perform the requested operation, and &MUST; return
953   a 412 (Precondition Failed).
954</t>
955<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
956  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref> <x:ref>If-Unmodified-Since-v</x:ref>
957  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
958</artwork></figure>
959<t>
960   An example of the field is:
961</t>
962<figure><artwork type="example">
963  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
964</artwork></figure>
965<t>
966   If the request normally (i.e., without the If-Unmodified-Since
967   header) would result in anything other than a 2xx or 412 status, the
968   If-Unmodified-Since header &SHOULD; be ignored.
969</t>
970<t>
971   If the specified date is invalid, the header is ignored.
972</t>
973<t>
974   The result of a request having both an If-Unmodified-Since header
975   field and either an If-None-Match or an If-Modified-Since header
976   fields is undefined by this specification.
977</t>
978</section>
979
980<section title="Last-Modified" anchor="header.last-modified">
981  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
982  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
983  <x:anchor-alias value="Last-Modified"/>
984  <x:anchor-alias value="Last-Modified-v"/>
985<t>
986   The entity-header field "Last-Modified" indicates the date and time at
987   which the origin server believes the variant was last modified.
988</t>
989<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
990  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
991  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
992</artwork></figure>
993<t>
994   An example of its use is
995</t>
996<figure><artwork type="example">
997  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
998</artwork></figure>
999<t>
1000   The exact meaning of this header field depends on the implementation
1001   of the origin server and the nature of the original resource. For
1002   files, it may be just the file system last-modified time. For
1003   entities with dynamically included parts, it may be the most recent
1004   of the set of last-modify times for its component parts. For database
1005   gateways, it may be the last-update time stamp of the record. For
1006   virtual objects, it may be the last time the internal state changed.
1007</t>
1008<t>
1009   An origin server &MUST-NOT; send a Last-Modified date which is later
1010   than the server's time of message origination. In such cases, where
1011   the resource's last modification would indicate some time in the
1012   future, the server &MUST; replace that date with the message
1013   origination date.
1014</t>
1015<t>
1016   An origin server &SHOULD; obtain the Last-Modified value of the entity
1017   as close as possible to the time that it generates the Date value of
1018   its response. This allows a recipient to make an accurate assessment
1019   of the entity's modification time, especially if the entity changes
1020   near the time that the response is generated.
1021</t>
1022<t>
1023   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1024</t>
1025<t>
1026   The Last-Modified entity-header field value is often used as a cache
1027   validator. In simple terms, a cache entry is considered to be valid
1028   if the entity has not been modified since the Last-Modified value.
1029</t>
1030</section>
1031
1032</section>
1033
1034<section title="IANA Considerations" anchor="IANA.considerations">
1035<section title="Message Header Registration" anchor="message.header.registration">
1036<t>
1037   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1038   with the permanent registrations below (see <xref target="RFC3864"/>):
1039</t>
1040<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1041<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1042   <ttcol>Header Field Name</ttcol>
1043   <ttcol>Protocol</ttcol>
1044   <ttcol>Status</ttcol>
1045   <ttcol>Reference</ttcol>
1046
1047   <c>ETag</c>
1048   <c>http</c>
1049   <c>standard</c>
1050   <c>
1051      <xref target="header.etag"/>
1052   </c>
1053   <c>If-Match</c>
1054   <c>http</c>
1055   <c>standard</c>
1056   <c>
1057      <xref target="header.if-match"/>
1058   </c>
1059   <c>If-Modified-Since</c>
1060   <c>http</c>
1061   <c>standard</c>
1062   <c>
1063      <xref target="header.if-modified-since"/>
1064   </c>
1065   <c>If-None-Match</c>
1066   <c>http</c>
1067   <c>standard</c>
1068   <c>
1069      <xref target="header.if-none-match"/>
1070   </c>
1071   <c>If-Unmodified-Since</c>
1072   <c>http</c>
1073   <c>standard</c>
1074   <c>
1075      <xref target="header.if-unmodified-since"/>
1076   </c>
1077   <c>Last-Modified</c>
1078   <c>http</c>
1079   <c>standard</c>
1080   <c>
1081      <xref target="header.last-modified"/>
1082   </c>
1083</texttable>
1084<!--(END)-->
1085<t>
1086   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1087</t>
1088</section>
1089</section>
1090
1091<section title="Security Considerations" anchor="security.considerations">
1092<t>
1093   No additional security considerations have been identified beyond
1094   those applicable to HTTP in general &messaging;.
1095</t>
1096</section>
1097
1098<section title="Acknowledgments" anchor="ack">
1099</section>
1100</middle>
1101<back>
1102
1103<references title="Normative References">
1104
1105<reference anchor="Part1">
1106  <front>
1107    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1108    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1109      <organization abbrev="Day Software">Day Software</organization>
1110      <address><email>fielding@gbiv.com</email></address>
1111    </author>
1112    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1113      <organization>One Laptop per Child</organization>
1114      <address><email>jg@laptop.org</email></address>
1115    </author>
1116    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1117      <organization abbrev="HP">Hewlett-Packard Company</organization>
1118      <address><email>JeffMogul@acm.org</email></address>
1119    </author>
1120    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1121      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1122      <address><email>henrikn@microsoft.com</email></address>
1123    </author>
1124    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1125      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1126      <address><email>LMM@acm.org</email></address>
1127    </author>
1128    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1129      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1130      <address><email>paulle@microsoft.com</email></address>
1131    </author>
1132    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1133      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1134      <address><email>timbl@w3.org</email></address>
1135    </author>
1136    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1137      <organization abbrev="W3C">World Wide Web Consortium</organization>
1138      <address><email>ylafon@w3.org</email></address>
1139    </author>
1140    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1141      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1142      <address><email>julian.reschke@greenbytes.de</email></address>
1143    </author>
1144    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1145  </front>
1146  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1147  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1148</reference>
1149
1150<reference anchor="Part5">
1151  <front>
1152    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1153    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1154      <organization abbrev="Day Software">Day Software</organization>
1155      <address><email>fielding@gbiv.com</email></address>
1156    </author>
1157    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1158      <organization>One Laptop per Child</organization>
1159      <address><email>jg@laptop.org</email></address>
1160    </author>
1161    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1162      <organization abbrev="HP">Hewlett-Packard Company</organization>
1163      <address><email>JeffMogul@acm.org</email></address>
1164    </author>
1165    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1166      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1167      <address><email>henrikn@microsoft.com</email></address>
1168    </author>
1169    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1170      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1171      <address><email>LMM@acm.org</email></address>
1172    </author>
1173    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1174      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1175      <address><email>paulle@microsoft.com</email></address>
1176    </author>
1177    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1178      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1179      <address><email>timbl@w3.org</email></address>
1180    </author>
1181    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1182      <organization abbrev="W3C">World Wide Web Consortium</organization>
1183      <address><email>ylafon@w3.org</email></address>
1184    </author>
1185    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1186      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1187      <address><email>julian.reschke@greenbytes.de</email></address>
1188    </author>
1189    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1190  </front>
1191  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1192  <x:source href="p5-range.xml" basename="p5-range"/>
1193</reference>
1194
1195<reference anchor="Part6">
1196  <front>
1197    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1198    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1199      <organization abbrev="Day Software">Day Software</organization>
1200      <address><email>fielding@gbiv.com</email></address>
1201    </author>
1202    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1203      <organization>One Laptop per Child</organization>
1204      <address><email>jg@laptop.org</email></address>
1205    </author>
1206    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1207      <organization abbrev="HP">Hewlett-Packard Company</organization>
1208      <address><email>JeffMogul@acm.org</email></address>
1209    </author>
1210    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1211      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1212      <address><email>henrikn@microsoft.com</email></address>
1213    </author>
1214    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1215      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1216      <address><email>LMM@acm.org</email></address>
1217    </author>
1218    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1219      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1220      <address><email>paulle@microsoft.com</email></address>
1221    </author>
1222    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1223      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1224      <address><email>timbl@w3.org</email></address>
1225    </author>
1226    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1227      <organization abbrev="W3C">World Wide Web Consortium</organization>
1228      <address><email>ylafon@w3.org</email></address>
1229    </author>
1230    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1231      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1232      <address><email>julian.reschke@greenbytes.de</email></address>
1233    </author>
1234    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1235  </front>
1236  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1237  <x:source href="p6-cache.xml" basename="p6-cache"/>
1238</reference>
1239
1240<reference anchor="RFC2119">
1241  <front>
1242    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1243    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1244      <organization>Harvard University</organization>
1245      <address><email>sob@harvard.edu</email></address>
1246    </author>
1247    <date month="March" year="1997"/>
1248  </front>
1249  <seriesInfo name="BCP" value="14"/>
1250  <seriesInfo name="RFC" value="2119"/>
1251</reference>
1252
1253</references>
1254
1255<references title="Informative References">
1256
1257<reference anchor="RFC2616">
1258  <front>
1259    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1260    <author initials="R." surname="Fielding" fullname="R. Fielding">
1261      <organization>University of California, Irvine</organization>
1262      <address><email>fielding@ics.uci.edu</email></address>
1263    </author>
1264    <author initials="J." surname="Gettys" fullname="J. Gettys">
1265      <organization>W3C</organization>
1266      <address><email>jg@w3.org</email></address>
1267    </author>
1268    <author initials="J." surname="Mogul" fullname="J. Mogul">
1269      <organization>Compaq Computer Corporation</organization>
1270      <address><email>mogul@wrl.dec.com</email></address>
1271    </author>
1272    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1273      <organization>MIT Laboratory for Computer Science</organization>
1274      <address><email>frystyk@w3.org</email></address>
1275    </author>
1276    <author initials="L." surname="Masinter" fullname="L. Masinter">
1277      <organization>Xerox Corporation</organization>
1278      <address><email>masinter@parc.xerox.com</email></address>
1279    </author>
1280    <author initials="P." surname="Leach" fullname="P. Leach">
1281      <organization>Microsoft Corporation</organization>
1282      <address><email>paulle@microsoft.com</email></address>
1283    </author>
1284    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1285      <organization>W3C</organization>
1286      <address><email>timbl@w3.org</email></address>
1287    </author>
1288    <date month="June" year="1999"/>
1289  </front>
1290  <seriesInfo name="RFC" value="2616"/>
1291</reference>
1292
1293<reference anchor='RFC3864'>
1294  <front>
1295    <title>Registration Procedures for Message Header Fields</title>
1296    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1297      <organization>Nine by Nine</organization>
1298      <address><email>GK-IETF@ninebynine.org</email></address>
1299    </author>
1300    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1301      <organization>BEA Systems</organization>
1302      <address><email>mnot@pobox.com</email></address>
1303    </author>
1304    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1305      <organization>HP Labs</organization>
1306      <address><email>JeffMogul@acm.org</email></address>
1307    </author>
1308    <date year='2004' month='September' />
1309  </front>
1310  <seriesInfo name='BCP' value='90' />
1311  <seriesInfo name='RFC' value='3864' />
1312</reference>
1313
1314</references>
1315
1316<section title="Compatibility with Previous Versions" anchor="compatibility">
1317
1318<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1319<t>
1320  Allow weak entity tags in all requests except range requests (Sections
1321  <xref target="weak.and.strong.validators" format="counter"/> and
1322  <xref target="header.if-none-match" format="counter"/>).
1323</t>
1324</section>
1325
1326</section>
1327
1328<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1329
1330<section title="Since RFC2616">
1331<t>
1332  Extracted relevant partitions from <xref target="RFC2616"/>.
1333</t>
1334</section>
1335
1336<section title="Since draft-ietf-httpbis-p4-conditional-00">
1337<t>
1338  Closed issues:
1339  <list style="symbols"> 
1340    <t>
1341      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1342      "Normative and Informative references"
1343    </t>
1344  </list>
1345</t>
1346<t>
1347  Other changes:
1348  <list style="symbols"> 
1349    <t>
1350      Move definitions of 304 and 412 condition codes from Part2.
1351    </t>
1352  </list>
1353</t>
1354</section>
1355
1356<section title="Since draft-ietf-httpbis-p4-conditional-01">
1357<t>
1358  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1359  <list style="symbols"> 
1360    <t>
1361      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1362    </t>
1363  </list>
1364</t>
1365</section>
1366
1367<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1368<t>
1369  Closed issues:
1370  <list style="symbols"> 
1371    <t>
1372      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1373      "Weak ETags on non-GET requests"
1374    </t>
1375  </list>
1376</t>
1377<t>
1378  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1379  <list style="symbols"> 
1380    <t>
1381      Reference RFC 3984, and update header registrations for headers defined
1382      in this document.
1383    </t>
1384  </list>
1385</t>
1386</section>
1387
1388<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1389<t>
1390  Closed issues:
1391  <list style="symbols"> 
1392    <t>
1393      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1394      "Examples for ETag matching"
1395    </t>
1396    <t>
1397      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1398      "'entity value' undefined"
1399    </t>
1400    <t>
1401      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1402      "bogus 2068 Date header reference"
1403    </t>
1404  </list>
1405</t>
1406</section>
1407
1408<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1409<t>
1410  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1411  <list style="symbols"> 
1412    <t>
1413      Use "/" instead of "|" for alternatives.
1414    </t>
1415    <t>
1416      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1417      whitespace ("OWS") and required whitespace ("RWS").
1418    </t>
1419    <t>
1420      Rewrite ABNFs to spell out whitespace rules, factor out
1421      header value format definitions.
1422    </t>
1423  </list>
1424</t>
1425</section>
1426
1427</section>
1428
1429</back>
1430</rfc>
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