source: draft-ietf-httpbis/latest/p4-conditional.xml @ 424

Last change on this file since 424 was 424, checked in by julian.reschke@…, 11 years ago

reorganize ABNF introductions to match Part1 (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                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY full-date                  "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28]>
29<?rfc toc="yes" ?>
30<?rfc symrefs="yes" ?>
31<?rfc sortrefs="yes" ?>
32<?rfc compact="yes"?>
33<?rfc subcompact="no" ?>
34<?rfc linkmailto="no" ?>
35<?rfc editing="no" ?>
36<?rfc comments="yes"?>
37<?rfc inline="yes"?>
38<?rfc-ext allow-markup-in-artwork="yes" ?>
39<?rfc-ext include-references-in-index="yes" ?>
40<rfc obsoletes="2616" category="std" x:maturity-level="draft"
41     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
42     xmlns:x='http://purl.org/net/xml2rfc/ext'>
43<front>
44
45  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
46
47  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
48    <organization abbrev="Day Software">Day Software</organization>
49    <address>
50      <postal>
51        <street>23 Corporate Plaza DR, Suite 280</street>
52        <city>Newport Beach</city>
53        <region>CA</region>
54        <code>92660</code>
55        <country>USA</country>
56      </postal>
57      <phone>+1-949-706-5300</phone>
58      <facsimile>+1-949-706-5305</facsimile>
59      <email>fielding@gbiv.com</email>
60      <uri>http://roy.gbiv.com/</uri>
61    </address>
62  </author>
63
64  <author initials="J." surname="Gettys" fullname="Jim Gettys">
65    <organization>One Laptop per Child</organization>
66    <address>
67      <postal>
68        <street>21 Oak Knoll Road</street>
69        <city>Carlisle</city>
70        <region>MA</region>
71        <code>01741</code>
72        <country>USA</country>
73      </postal>
74      <email>jg@laptop.org</email>
75      <uri>http://www.laptop.org/</uri>
76    </address>
77  </author>
78 
79  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
80    <organization abbrev="HP">Hewlett-Packard Company</organization>
81    <address>
82      <postal>
83        <street>HP Labs, Large Scale Systems Group</street>
84        <street>1501 Page Mill Road, MS 1177</street>
85        <city>Palo Alto</city>
86        <region>CA</region>
87        <code>94304</code>
88        <country>USA</country>
89      </postal>
90      <email>JeffMogul@acm.org</email>
91    </address>
92  </author>
93
94  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
95    <organization abbrev="Microsoft">Microsoft Corporation</organization>
96    <address>
97      <postal>
98        <street>1 Microsoft Way</street>
99        <city>Redmond</city>
100        <region>WA</region>
101        <code>98052</code>
102        <country>USA</country>
103      </postal>
104      <email>henrikn@microsoft.com</email>
105    </address>
106  </author>
107
108  <author initials="L." surname="Masinter" fullname="Larry Masinter">
109    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
110    <address>
111      <postal>
112        <street>345 Park Ave</street>
113        <city>San Jose</city>
114        <region>CA</region>
115        <code>95110</code>
116        <country>USA</country>
117      </postal>
118      <email>LMM@acm.org</email>
119      <uri>http://larry.masinter.net/</uri>
120    </address>
121  </author>
122 
123  <author initials="P." surname="Leach" fullname="Paul J. Leach">
124    <organization abbrev="Microsoft">Microsoft Corporation</organization>
125    <address>
126      <postal>
127        <street>1 Microsoft Way</street>
128        <city>Redmond</city>
129        <region>WA</region>
130        <code>98052</code>
131      </postal>
132      <email>paulle@microsoft.com</email>
133    </address>
134  </author>
135   
136  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
137    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
138    <address>
139      <postal>
140        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
141        <street>The Stata Center, Building 32</street>
142        <street>32 Vassar Street</street>
143        <city>Cambridge</city>
144        <region>MA</region>
145        <code>02139</code>
146        <country>USA</country>
147      </postal>
148      <email>timbl@w3.org</email>
149      <uri>http://www.w3.org/People/Berners-Lee/</uri>
150    </address>
151  </author>
152
153  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
154    <organization abbrev="W3C">World Wide Web Consortium</organization>
155    <address>
156      <postal>
157        <street>W3C / ERCIM</street>
158        <street>2004, rte des Lucioles</street>
159        <city>Sophia-Antipolis</city>
160        <region>AM</region>
161        <code>06902</code>
162        <country>France</country>
163      </postal>
164      <email>ylafon@w3.org</email>
165      <uri>http://www.raubacapeu.net/people/yves/</uri>
166    </address>
167  </author>
168
169  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
170    <organization abbrev="greenbytes">greenbytes GmbH</organization>
171    <address>
172      <postal>
173        <street>Hafenweg 16</street>
174        <city>Muenster</city><region>NW</region><code>48155</code>
175        <country>Germany</country>
176      </postal>
177      <phone>+49 251 2807760</phone>   
178      <facsimile>+49 251 2807761</facsimile>   
179      <email>julian.reschke@greenbytes.de</email>       
180      <uri>http://greenbytes.de/tech/webdav/</uri>     
181    </address>
182  </author>
183
184  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
185
186<abstract>
187<t>
188   The Hypertext Transfer Protocol (HTTP) is an application-level
189   protocol for distributed, collaborative, hypermedia information
190   systems. HTTP has been in use by the World Wide Web global information
191   initiative since 1990. This document is Part 4 of the seven-part specification
192   that defines the protocol referred to as "HTTP/1.1" and, taken together,
193   obsoletes RFC 2616.  Part 4 defines request header fields for
194   indicating conditional requests and the rules for constructing responses
195   to those requests.
196</t>
197</abstract>
198
199<note title="Editorial Note (To be removed by RFC Editor)">
200  <t>
201    Discussion of this draft should take place on the HTTPBIS working group
202    mailing list (ietf-http-wg@w3.org). The current issues list is
203    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/11"/>
204    and related documents (including fancy diffs) can be found at
205    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
206  </t>
207  <t>
208    The changes in this draft are summarized in <xref target="changes.since.05"/>.
209  </t>
210</note>
211</front>
212<middle>
213<section title="Introduction" anchor="introduction">
214<t>
215   This document defines HTTP/1.1 response metadata for indicating potential
216   changes to payload content, including modification time stamps and opaque
217   entity-tags, and the HTTP conditional request mechanisms that allow
218   preconditions to be placed on a request method.  Conditional GET requests
219   allow for efficient cache updates.  Other conditional request methods are
220   used to protect against overwriting or misunderstanding the state of a
221   resource that has been changed unbeknownst to the requesting client.
222</t>
223<t>
224   This document is currently disorganized in order to minimize the changes
225   between drafts and enable reviewers to see the smaller errata changes.
226   The next draft will reorganize the sections to better reflect the content.
227   In particular, the sections on resource metadata will be discussed first
228   and then followed by each conditional request-header, concluding with a
229   definition of precedence and the expectation of ordering strong validator
230   checks before weak validator checks.  It is likely that more content from
231   &caching; will migrate to this part, where appropriate.
232   The current mess reflects how widely dispersed these topics and associated
233   requirements had become in <xref target="RFC2616"/>.
234</t>
235
236<section title="Requirements" anchor="intro.requirements">
237<t>
238   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
239   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
240   document are to be interpreted as described in <xref target="RFC2119"/>.
241</t>
242<t>
243   An implementation is not compliant if it fails to satisfy one or more
244   of the &MUST; or &REQUIRED; level requirements for the protocols it
245   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
246   level and all the &SHOULD; level requirements for its protocols is said
247   to be "unconditionally compliant"; one that satisfies all the &MUST;
248   level requirements but not all the &SHOULD; level requirements for its
249   protocols is said to be "conditionally compliant."
250</t>
251</section>
252
253<section title="Syntax Notation" anchor="notation">
254<t>
255  This specification uses the ABNF syntax defined in &notation;.
256</t>
257
258<section title="Core Rules" anchor="core.rules">
259  <x:anchor-alias value="quoted-string"/>
260  <x:anchor-alias value="OWS"/>
261<t>
262  The core rules below are defined in &basic-rules;:
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>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
267</artwork></figure>
268</section>
269
270<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
271  <x:anchor-alias value="HTTP-date"/>
272<t>
273  The ABNF rules below are defined in other parts:
274</t>
275<figure><!--Part1--><artwork type="abnf2616">
276  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
277</artwork></figure>
278</section>
279
280</section>
281
282</section>
283
284<section title="Entity Tags" anchor="entity.tags">
285  <x:anchor-alias value="entity-tag"/>
286  <x:anchor-alias value="opaque-tag"/>
287  <x:anchor-alias value="weak"/>
288<t>
289   Entity tags are used for comparing two or more entities from the same
290   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
291   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
292   If-Range (&header-if-range;) header fields. The definition of how they
293   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
294   entity tag consists of an opaque quoted string, possibly prefixed by
295   a weakness indicator.
296</t>
297<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"/>
298  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
299  <x:ref>weak</x:ref>       = "W/"
300  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
301</artwork></figure>
302<t>
303   A "strong entity tag" &MAY; be shared by two entities of a resource
304   only if they are equivalent by octet equality.
305</t>
306<t>
307   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
308   two entities of a resource only if the entities are equivalent and
309   could be substituted for each other with no significant change in
310   semantics. A weak entity tag can only be used for weak comparison.
311</t>
312<t>
313   An entity tag &MUST; be unique across all versions of all entities
314   associated with a particular resource. A given entity tag value &MAY;
315   be used for entities obtained by requests on different URIs. The use
316   of the same entity tag value in conjunction with entities obtained by
317   requests on different URIs does not imply the equivalence of those
318   entities.
319</t>
320</section>
321
322<section title="Status Code Definitions">
323<section title="304 Not Modified" anchor="status.304">
324  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
325  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
326<t>
327   If the client has performed a conditional GET request and access is
328   allowed, but the document has not been modified, the server &SHOULD;
329   respond with this status code. The 304 response &MUST-NOT; contain a
330   message-body, and thus is always terminated by the first empty line
331   after the header fields.
332</t>
333<t>
334   The response &MUST; include the following header fields:
335  <list style="symbols">
336    <x:lt>
337      <t>Date, unless its omission is required by &clockless;.</t>
338      <t>
339         If a clockless origin server obeys these rules, and proxies and
340         clients add their own Date to any response received without one (as
341         already specified by &header-date;, caches will operate
342         correctly.</t>
343    </x:lt>
344    <x:lt>
345      <t>ETag and/or Content-Location, if the header would have been sent
346         in a 200 response to the same request.</t>
347    </x:lt>
348    <x:lt>
349      <t>Expires, Cache-Control, and/or Vary, if the field-value might
350         differ from that sent in any previous response for the same
351         variant.</t>
352    </x:lt>
353  </list>
354</t>
355<t>
356   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
357   the response &SHOULD-NOT;  include other entity-headers.
358   Otherwise (i.e., the conditional GET used a weak validator), the
359   response &MUST-NOT; include other entity-headers; this prevents
360   inconsistencies between cached entity-bodies and updated headers.
361</t>
362<t>
363   If a 304 response indicates an entity not currently cached, then the
364   cache &MUST; disregard the response and repeat the request without the
365   conditional.
366</t>
367<t>
368   If a cache uses a received 304 response to update a cache entry, the
369   cache &MUST; update the entry to reflect any new field values given in
370   the response.
371</t>
372</section>
373
374<section title="412 Precondition Failed" anchor="status.412">
375  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
376  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
377<t>
378   The precondition given in one or more of the request-header fields
379   evaluated to false when it was tested on the server. This response
380   code allows the client to place preconditions on the current resource
381   metainformation (header field data) and thus prevent the requested
382   method from being applied to a resource other than the one intended.
383</t>
384</section>
385</section>
386
387<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
388<t>
389   Since both origin servers and caches will compare two validators to
390   decide if they represent the same or different entities, one normally
391   would expect that if the entity (the entity-body or any entity-headers)
392   changes in any way, then the associated validator would
393   change as well. If this is true, then we call this validator a
394   "strong validator."
395</t>
396<t>
397   However, there might be cases when a server prefers to change the
398   validator only on semantically significant changes, and not when
399   insignificant aspects of the entity change. A validator that does not
400   always change when the resource changes is a "weak validator."
401</t>
402<t>
403   Entity tags are normally "strong validators," but the protocol
404   provides a mechanism to tag an entity tag as "weak." One can think of
405   a strong validator as one that changes whenever the bits of an entity
406   changes, while a weak value changes whenever the meaning of an entity
407   changes. Alternatively, one can think of a strong validator as part
408   of an identifier for a specific entity, while a weak validator is
409   part of an identifier for a set of semantically equivalent entities.
410  <list><t>
411      <x:h>Note:</x:h> One example of a strong validator is an integer that is
412      incremented in stable storage every time an entity is changed.
413    </t><t>
414      An entity's modification time, if represented with one-second
415      resolution, could be a weak validator, since it is possible that
416      the resource might be modified twice during a single second.
417    </t><t>
418      Support for weak validators is optional. However, weak validators
419      allow for more efficient caching of equivalent objects; for
420      example, a hit counter on a site is probably good enough if it is
421      updated every few days or weeks, and any value during that period
422      is likely "good enough" to be equivalent.
423    </t></list>
424</t>
425<t>
426   A "use" of a validator is either when a client generates a request
427   and includes the validator in a validating header field, or when a
428   server compares two validators.
429</t>
430<t>
431   Strong validators are usable in any context. Weak validators are only
432   usable in contexts that do not depend on exact equality of an entity.
433   For example, either kind is usable for a conditional GET of a full
434   entity. However, only a strong validator is usable for a sub-range
435   retrieval, since otherwise the client might end up with an internally
436   inconsistent entity.
437</t>
438<t>
439   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
440</t>
441<t>
442   The only function that HTTP/1.1 defines on validators is
443   comparison. There are two validator comparison functions, depending
444   on whether the comparison context allows the use of weak validators
445   or not:
446  <list style="symbols">
447     <t>The strong comparison function: in order to be considered equal,
448        both opaque-tags &MUST; be identical character-by-character, and both
449        &MUST-NOT; be weak.</t>
450     <t>The weak comparison function: in order to be considered equal, both
451        opaque-tags &MUST; be identical character-by-character.</t>
452  </list>
453</t>
454<t>
455   The example below shows the results for a set of entity tag pairs,
456   and both the weak and strong comparison function results:
457</t>
458<texttable align="left">
459  <ttcol>ETag 1</ttcol>
460  <ttcol>ETag 2</ttcol>
461  <ttcol>Strong Comparison</ttcol>
462  <ttcol>Weak Comparison</ttcol>
463
464  <c>W/"1"</c>
465  <c>W/"1"</c>
466  <c>no match</c>
467  <c>match</c>
468 
469  <c>W/"1"</c>
470  <c>W/"2"</c>
471  <c>no match</c>
472  <c>no match</c>
473
474  <c>W/"1"</c>
475  <c>"1"</c>
476  <c>no match</c>
477  <c>match</c>
478
479  <c>"1"</c>
480  <c>"1"</c>
481  <c>match</c>
482  <c>match</c>
483</texttable>
484<t>
485   An entity tag is strong unless it is explicitly tagged as weak.
486   <xref target="entity.tags"/> gives the syntax for entity tags.
487</t>
488<t>
489   A Last-Modified time, when used as a validator in a request, is
490   implicitly weak unless it is possible to deduce that it is strong,
491   using the following rules:
492  <list style="symbols">
493     <t>The validator is being compared by an origin server to the
494        actual current validator for the entity and,</t>
495     <t>That origin server reliably knows that the associated entity did
496        not change twice during the second covered by the presented
497        validator.</t>
498  </list>
499</t>
500<t>
501   or
502  <list style="symbols">
503     <t>The validator is about to be used by a client in an If-Modified-Since
504        or If-Unmodified-Since header, because the client
505        has a cache entry for the associated entity, and</t>
506     <t>That cache entry includes a Date value, which gives the time
507        when the origin server sent the original response, and</t>
508     <t>The presented Last-Modified time is at least 60 seconds before
509        the Date value.</t>
510  </list>
511</t>
512<t>
513   or
514  <list style="symbols">
515     <t>The validator is being compared by an intermediate cache to the
516        validator stored in its cache entry for the entity, and</t>
517     <t>That cache entry includes a Date value, which gives the time
518        when the origin server sent the original response, and</t>
519     <t>The presented Last-Modified time is at least 60 seconds before
520        the Date value.</t>
521  </list>
522</t>
523<t>
524   This method relies on the fact that if two different responses were
525   sent by the origin server during the same second, but both had the
526   same Last-Modified time, then at least one of those responses would
527   have a Date value equal to its Last-Modified time. The arbitrary 60-second
528   limit guards against the possibility that the Date and Last-Modified
529   values are generated from different clocks, or at somewhat
530   different times during the preparation of the response. An
531   implementation &MAY; use a value larger than 60 seconds, if it is
532   believed that 60 seconds is too short.
533</t>
534<t>
535   If a client wishes to perform a sub-range retrieval on a value for
536   which it has only a Last-Modified time and no opaque validator, it
537   &MAY; do this only if the Last-Modified time is strong in the sense
538   described here.
539</t>
540<t>
541   A cache or origin server receiving a conditional range request
542   (<xref target="Part5"/>) &MUST; use the strong comparison function to
543   evaluate the condition.
544</t>
545<t>
546   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
547   retrievals on values that have been obtained from HTTP/1.0
548   servers.
549</t>
550</section>
551
552<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">
553<t>
554   We adopt a set of rules and recommendations for origin servers,
555   clients, and caches regarding when various validator types ought to
556   be used, and for what purposes.
557</t>
558<t>
559   HTTP/1.1 origin servers:
560  <list style="symbols">
561     <t>&SHOULD; send an entity tag validator unless it is not feasible to
562        generate one.</t>
563
564     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
565        performance considerations support the use of weak entity tags,
566        or if it is unfeasible to send a strong entity tag.</t>
567
568     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
569        unless the risk of a breakdown in semantic transparency that
570        could result from using this date in an If-Modified-Since header
571        would lead to serious problems.</t>
572  </list>
573</t>
574<t>
575   In other words, the preferred behavior for an HTTP/1.1 origin server
576   is to send both a strong entity tag and a Last-Modified value.
577</t>
578<t>
579   In order to be legal, a strong entity tag &MUST; change whenever the
580   associated entity changes in any way. A weak entity tag &SHOULD;
581   change whenever the associated entity changes in a semantically
582   significant way.
583  <list><t>
584      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
585      origin server must avoid reusing a specific strong entity tag
586      value for two different entities, or reusing a specific weak
587      entity tag value for two semantically different entities. Cache
588      entries might persist for arbitrarily long periods, regardless of
589      expiration times, so it might be inappropriate to expect that a
590      cache will never again attempt to validate an entry using a
591      validator that it obtained at some point in the past.
592  </t></list>
593</t>
594<t>
595   HTTP/1.1 clients:
596  <list style="symbols">
597     <t>If an entity tag has been provided by the origin server, &MUST;
598        use that entity tag in any cache-conditional request (using If-Match
599        or If-None-Match).</t>
600
601     <t>If only a Last-Modified value has been provided by the origin
602        server, &SHOULD; use that value in non-subrange cache-conditional
603        requests (using If-Modified-Since).</t>
604
605     <t>If only a Last-Modified value has been provided by an HTTP/1.0
606        origin server, &MAY; use that value in subrange cache-conditional
607        requests (using If-Unmodified-Since:). The user agent &SHOULD;
608        provide a way to disable this, in case of difficulty.</t>
609
610     <t>If both an entity tag and a Last-Modified value have been
611        provided by the origin server, &SHOULD; use both validators in
612        cache-conditional requests. This allows both HTTP/1.0 and
613        HTTP/1.1 caches to respond appropriately.</t>
614  </list>
615</t>
616<t>
617   An HTTP/1.1 origin server, upon receiving a conditional request that
618   includes both a Last-Modified date (e.g., in an If-Modified-Since or
619   If-Unmodified-Since header field) and one or more entity tags (e.g.,
620   in an If-Match, If-None-Match, or If-Range header field) as cache
621   validators, &MUST-NOT; return a response status of 304 (Not Modified)
622   unless doing so is consistent with all of the conditional header
623   fields in the request.
624</t>
625<t>
626   An HTTP/1.1 caching proxy, upon receiving a conditional request that
627   includes both a Last-Modified date and one or more entity tags as
628   cache validators, &MUST-NOT; return a locally cached response to the
629   client unless that cached response is consistent with all of the
630   conditional header fields in the request.
631  <list><t>
632      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
633      servers and clients should transmit as much non-redundant
634      information as is available in their responses and requests.
635      HTTP/1.1 systems receiving this information will make the most
636      conservative assumptions about the validators they receive.
637  </t><t>
638      HTTP/1.0 clients and caches will ignore entity tags. Generally,
639      last-modified values received or used by these systems will
640      support transparent and efficient caching, and so HTTP/1.1 origin
641      servers should provide Last-Modified values. In those rare cases
642      where the use of a Last-Modified value as a validator by an
643      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
644      origin servers should not provide one.
645  </t></list>
646</t>
647</section>
648
649<section title="Header Field Definitions" anchor="header.fields">
650<t>
651   This section defines the syntax and semantics of HTTP/1.1 header fields
652   related to conditional requests.
653</t>
654<t>
655   For entity-header fields, both sender and recipient refer to either the
656   client or the server, depending on who sends and who receives the entity.
657</t>
658
659<section title="ETag" anchor="header.etag">
660  <iref primary="true" item="ETag header" x:for-anchor=""/>
661  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
662  <x:anchor-alias value="ETag"/>
663  <x:anchor-alias value="ETag-v"/>
664<t>
665   The response-header field "ETag" provides the current value of the
666   entity tag (see <xref target="entity.tags"/>) for the requested variant.
667   The headers used with entity
668   tags are described in Sections <xref target="header.if-match" format="counter"/>
669   and <xref target="header.if-none-match" format="counter"/> of this document,
670   and in &header-if-range;. The entity tag
671   &MAY; be used for comparison with other entities from the same resource
672   (see <xref target="weak.and.strong.validators"/>).
673</t>
674<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
675  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
676  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
677</artwork></figure>
678<figure><preamble>
679  Examples:
680</preamble>
681<artwork type="example">
682  ETag: "xyzzy"
683  ETag: W/"xyzzy"
684  ETag: ""
685</artwork></figure>
686<t>
687   The ETag response-header field value, an entity tag, provides for an
688   "opaque" cache validator. This might allow more reliable validation
689   in situations where it is inconvenient to store modification dates,
690   where the one-second resolution of HTTP date values is not
691   sufficient, or where the origin server wishes to avoid certain
692   paradoxes that might arise from the use of modification dates.
693</t>
694<t>
695   The principle behind entity tags is that only the service author
696   knows the semantics of a resource well enough to select an
697   appropriate cache validation mechanism, and the specification of any
698   validator comparison function more complex than byte-equality would
699   open up a can of worms. Thus, comparisons of any other headers
700   (except Last-Modified, for compatibility with HTTP/1.0) are never
701   used for purposes of validating a cache entry.
702</t>
703</section>
704
705<section title="If-Match" anchor="header.if-match">
706  <iref primary="true" item="If-Match header" x:for-anchor=""/>
707  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
708  <x:anchor-alias value="If-Match"/>
709  <x:anchor-alias value="If-Match-v"/>
710<t>
711   The request-header field "If-Match" is used with a method to make it
712   conditional. A client that has one or more entities previously
713   obtained from the resource can verify that one of those entities is
714   current by including a list of their associated entity tags in the
715   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
716   purpose of this feature is to allow efficient updates of cached
717   information with a minimum amount of transaction overhead. It is also
718   used, on updating requests, to prevent inadvertent modification of
719   the wrong version of a resource. As a special case, the value "*"
720   matches any current entity of the resource.
721</t>
722<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
723  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
724  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
725</artwork></figure>
726<t>
727   If any of the entity tags match the entity tag of the entity that
728   would have been returned in the response to a similar GET request
729   (without the If-Match header) on that resource, or if "*" is given
730   and any current entity exists for that resource, then the server &MAY;
731   perform the requested method as if the If-Match header field did not
732   exist.
733</t>
734<t>
735   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
736   to compare the entity tags in If-Match.
737</t>
738<t>
739   If none of the entity tags match, or if "*" is given and no current
740   entity exists, the server &MUST-NOT; perform the requested method, and
741   &MUST; return a 412 (Precondition Failed) response. This behavior is
742   most useful when the client wants to prevent an updating method, such
743   as PUT, from modifying a resource that has changed since the client
744   last retrieved it.
745</t>
746<t>
747   If the request would, without the If-Match header field, result in
748   anything other than a 2xx or 412 status, then the If-Match header
749   &MUST; be ignored.
750</t>
751<t>
752   The meaning of "If-Match: *" is that the method &SHOULD; be performed
753   if the representation selected by the origin server (or by a cache,
754   possibly using the Vary mechanism, see &header-vary;) exists, and
755   &MUST-NOT; be performed if the representation does not exist.
756</t>
757<t>
758   A request intended to update a resource (e.g., a PUT) &MAY; include an
759   If-Match header field to signal that the request method &MUST-NOT; be
760   applied if the entity corresponding to the If-Match value (a single
761   entity tag) is no longer a representation of that resource. This
762   allows the user to indicate that they do not wish the request to be
763   successful if the resource has been changed without their knowledge.
764   Examples:
765</t>
766<figure><artwork type="example">
767  If-Match: "xyzzy"
768  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
769  If-Match: *
770</artwork></figure>
771<t>
772   The result of a request having both an If-Match header field and
773   either an If-None-Match or an If-Modified-Since header fields is
774   undefined by this specification.
775</t>
776</section>
777
778<section title="If-Modified-Since" anchor="header.if-modified-since">
779  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
780  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
781  <x:anchor-alias value="If-Modified-Since"/>
782  <x:anchor-alias value="If-Modified-Since-v"/>
783<t>
784   The request-header field "If-Modified-Since" is used with a method to
785   make it conditional: if the requested variant has not been modified
786   since the time specified in this field, an entity will not be
787   returned from the server; instead, a 304 (Not Modified) response will
788   be returned without any message-body.
789</t>
790<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
791  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
792                        <x:ref>If-Modified-Since-v</x:ref>
793  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
794</artwork></figure>
795<t>
796   An example of the field is:
797</t>
798<figure><artwork type="example">
799  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
800</artwork></figure>
801<t>
802   A GET method with an If-Modified-Since header and no Range header
803   requests that the identified entity be transferred only if it has
804   been modified since the date given by the If-Modified-Since header.
805   The algorithm for determining this includes the following cases:
806  <list style="numbers">
807      <t>If the request would normally result in anything other than a
808         200 (OK) status, or if the passed If-Modified-Since date is
809         invalid, the response is exactly the same as for a normal GET.
810         A date which is later than the server's current time is
811         invalid.</t>
812
813      <t>If the variant has been modified since the If-Modified-Since
814         date, the response is exactly the same as for a normal GET.</t>
815
816      <t>If the variant has not been modified since a valid If-Modified-Since
817         date, the server &SHOULD; return a 304 (Not
818         Modified) response.</t>
819  </list>
820</t>
821<t>
822   The purpose of this feature is to allow efficient updates of cached
823   information with a minimum amount of transaction overhead.
824  <list><t>
825      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
826      see &header-range; for full details.
827    </t><t>
828      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
829      clock might not be synchronized with the client.
830    </t><t>
831      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
832      servers will use an exact date comparison function, rather than a
833      less-than function, for deciding whether to send a 304 (Not
834      Modified) response. To get best results when sending an If-Modified-Since
835      header field for cache validation, clients are
836      advised to use the exact date string received in a previous Last-Modified
837      header field whenever possible.
838    </t><t>
839      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
840      header instead of a date taken from the Last-Modified header for
841      the same request, the client should be aware of the fact that this
842      date is interpreted in the server's understanding of time. The
843      client should consider unsynchronized clocks and rounding problems
844      due to the different encodings of time between the client and
845      server. This includes the possibility of race conditions if the
846      document has changed between the time it was first requested and
847      the If-Modified-Since date of a subsequent request, and the
848      possibility of clock-skew-related problems if the If-Modified-Since
849      date is derived from the client's clock without correction
850      to the server's clock. Corrections for different time bases
851      between client and server are at best approximate due to network
852      latency.
853    </t>
854  </list>
855</t>
856<t>
857   The result of a request having both an If-Modified-Since header field
858   and either an If-Match or an If-Unmodified-Since header fields is
859   undefined by this specification.
860</t>
861</section>
862
863<section title="If-None-Match" anchor="header.if-none-match">
864  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
865  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
866  <x:anchor-alias value="If-None-Match"/>
867  <x:anchor-alias value="If-None-Match-v"/>
868<t>
869   The request-header field "If-None-Match" is used with a method to make
870   it conditional. A client that has one or more entities previously
871   obtained from the resource can verify that none of those entities is
872   current by including a list of their associated entity tags in the
873   If-None-Match header field. The purpose of this feature is to allow
874   efficient updates of cached information with a minimum amount of
875   transaction overhead. It is also used to prevent a method (e.g. PUT)
876   from inadvertently modifying an existing resource when the client
877   believes that the resource does not exist.
878</t>
879<t>
880   As a special case, the value "*" matches any current entity of the
881   resource.
882</t>
883<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
884  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
885  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
886</artwork></figure>
887<t>
888   If any of the entity tags match the entity tag of the entity that
889   would have been returned in the response to a similar GET request
890   (without the If-None-Match header) on that resource, or if "*" is
891   given and any current entity exists for that resource, then the
892   server &MUST-NOT; perform the requested method, unless required to do
893   so because the resource's modification date fails to match that
894   supplied in an If-Modified-Since header field in the request.
895   Instead, if the request method was GET or HEAD, the server &SHOULD;
896   respond with a 304 (Not Modified) response, including the cache-related
897   header fields (particularly ETag) of one of the entities that
898   matched. For all other request methods, the server &MUST; respond with
899   a status of 412 (Precondition Failed).
900</t>
901<t>
902   See <xref target="weak.and.strong.validators"/> for rules on how to
903   determine if two entity tags match.
904</t>
905<t>
906   If none of the entity tags match, then the server &MAY; perform the
907   requested method as if the If-None-Match header field did not exist,
908   but &MUST; also ignore any If-Modified-Since header field(s) in the
909   request. That is, if no entity tags match, then the server &MUST-NOT;
910   return a 304 (Not Modified) response.
911</t>
912<t>
913   If the request would, without the If-None-Match header field, result
914   in anything other than a 2xx or 304 status, then the If-None-Match
915   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
916   server behavior when both If-Modified-Since and If-None-Match appear
917   in the same request.)
918</t>
919<t>
920   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
921   performed if the representation selected by the origin server (or by
922   a cache, possibly using the Vary mechanism, see &header-vary;)
923   exists, and &SHOULD; be performed if the representation does not exist.
924   This feature is intended to be useful in preventing races between PUT
925   operations.
926</t>
927<t>
928   Examples:
929</t>
930<figure><artwork type="example">
931  If-None-Match: "xyzzy"
932  If-None-Match: W/"xyzzy"
933  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
934  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
935  If-None-Match: *
936</artwork></figure>
937<t>
938   The result of a request having both an If-None-Match header field and
939   either an If-Match or an If-Unmodified-Since header fields is
940   undefined by this specification.
941</t>
942</section>
943
944<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
945  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
946  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
947  <x:anchor-alias value="If-Unmodified-Since"/>
948  <x:anchor-alias value="If-Unmodified-Since-v"/>
949<t>
950   The request-header field "If-Unmodified-Since" is used with a method to
951   make it conditional. If the requested resource has not been modified
952   since the time specified in this field, the server &SHOULD; perform the
953   requested operation as if the If-Unmodified-Since header were not
954   present.
955</t>
956<t>
957   If the requested variant has been modified since the specified time,
958   the server &MUST-NOT; perform the requested operation, and &MUST; return
959   a 412 (Precondition Failed).
960</t>
961<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
962  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
963                          <x:ref>If-Unmodified-Since-v</x:ref>
964  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
965</artwork></figure>
966<t>
967   An example of the field is:
968</t>
969<figure><artwork type="example">
970  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
971</artwork></figure>
972<t>
973   If the request normally (i.e., without the If-Unmodified-Since
974   header) would result in anything other than a 2xx or 412 status, the
975   If-Unmodified-Since header &SHOULD; be ignored.
976</t>
977<t>
978   If the specified date is invalid, the header is ignored.
979</t>
980<t>
981   The result of a request having both an If-Unmodified-Since header
982   field and either an If-None-Match or an If-Modified-Since header
983   fields is undefined by this specification.
984</t>
985</section>
986
987<section title="Last-Modified" anchor="header.last-modified">
988  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
989  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
990  <x:anchor-alias value="Last-Modified"/>
991  <x:anchor-alias value="Last-Modified-v"/>
992<t>
993   The entity-header field "Last-Modified" indicates the date and time at
994   which the origin server believes the variant was last modified.
995</t>
996<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
997  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
998  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
999</artwork></figure>
1000<t>
1001   An example of its use is
1002</t>
1003<figure><artwork type="example">
1004  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1005</artwork></figure>
1006<t>
1007   The exact meaning of this header field depends on the implementation
1008   of the origin server and the nature of the original resource. For
1009   files, it may be just the file system last-modified time. For
1010   entities with dynamically included parts, it may be the most recent
1011   of the set of last-modify times for its component parts. For database
1012   gateways, it may be the last-update time stamp of the record. For
1013   virtual objects, it may be the last time the internal state changed.
1014</t>
1015<t>
1016   An origin server &MUST-NOT; send a Last-Modified date which is later
1017   than the server's time of message origination. In such cases, where
1018   the resource's last modification would indicate some time in the
1019   future, the server &MUST; replace that date with the message
1020   origination date.
1021</t>
1022<t>
1023   An origin server &SHOULD; obtain the Last-Modified value of the entity
1024   as close as possible to the time that it generates the Date value of
1025   its response. This allows a recipient to make an accurate assessment
1026   of the entity's modification time, especially if the entity changes
1027   near the time that the response is generated.
1028</t>
1029<t>
1030   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1031</t>
1032<t>
1033   The Last-Modified entity-header field value is often used as a cache
1034   validator. In simple terms, a cache entry is considered to be valid
1035   if the entity has not been modified since the Last-Modified value.
1036</t>
1037</section>
1038
1039</section>
1040
1041<section title="IANA Considerations" anchor="IANA.considerations">
1042<section title="Message Header Registration" anchor="message.header.registration">
1043<t>
1044   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1045   with the permanent registrations below (see <xref target="RFC3864"/>):
1046</t>
1047<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1048<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1049   <ttcol>Header Field Name</ttcol>
1050   <ttcol>Protocol</ttcol>
1051   <ttcol>Status</ttcol>
1052   <ttcol>Reference</ttcol>
1053
1054   <c>ETag</c>
1055   <c>http</c>
1056   <c>standard</c>
1057   <c>
1058      <xref target="header.etag"/>
1059   </c>
1060   <c>If-Match</c>
1061   <c>http</c>
1062   <c>standard</c>
1063   <c>
1064      <xref target="header.if-match"/>
1065   </c>
1066   <c>If-Modified-Since</c>
1067   <c>http</c>
1068   <c>standard</c>
1069   <c>
1070      <xref target="header.if-modified-since"/>
1071   </c>
1072   <c>If-None-Match</c>
1073   <c>http</c>
1074   <c>standard</c>
1075   <c>
1076      <xref target="header.if-none-match"/>
1077   </c>
1078   <c>If-Unmodified-Since</c>
1079   <c>http</c>
1080   <c>standard</c>
1081   <c>
1082      <xref target="header.if-unmodified-since"/>
1083   </c>
1084   <c>Last-Modified</c>
1085   <c>http</c>
1086   <c>standard</c>
1087   <c>
1088      <xref target="header.last-modified"/>
1089   </c>
1090</texttable>
1091<!--(END)-->
1092<t>
1093   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1094</t>
1095</section>
1096</section>
1097
1098<section title="Security Considerations" anchor="security.considerations">
1099<t>
1100   No additional security considerations have been identified beyond
1101   those applicable to HTTP in general &messaging;.
1102</t>
1103</section>
1104
1105<section title="Acknowledgments" anchor="ack">
1106</section>
1107</middle>
1108<back>
1109
1110<references title="Normative References">
1111
1112<reference anchor="Part1">
1113  <front>
1114    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1115    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1116      <organization abbrev="Day Software">Day Software</organization>
1117      <address><email>fielding@gbiv.com</email></address>
1118    </author>
1119    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1120      <organization>One Laptop per Child</organization>
1121      <address><email>jg@laptop.org</email></address>
1122    </author>
1123    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1124      <organization abbrev="HP">Hewlett-Packard Company</organization>
1125      <address><email>JeffMogul@acm.org</email></address>
1126    </author>
1127    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1128      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1129      <address><email>henrikn@microsoft.com</email></address>
1130    </author>
1131    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1132      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1133      <address><email>LMM@acm.org</email></address>
1134    </author>
1135    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1136      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1137      <address><email>paulle@microsoft.com</email></address>
1138    </author>
1139    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1140      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1141      <address><email>timbl@w3.org</email></address>
1142    </author>
1143    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1144      <organization abbrev="W3C">World Wide Web Consortium</organization>
1145      <address><email>ylafon@w3.org</email></address>
1146    </author>
1147    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1148      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1149      <address><email>julian.reschke@greenbytes.de</email></address>
1150    </author>
1151    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1152  </front>
1153  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1154  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1155</reference>
1156
1157<reference anchor="Part5">
1158  <front>
1159    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1160    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1161      <organization abbrev="Day Software">Day Software</organization>
1162      <address><email>fielding@gbiv.com</email></address>
1163    </author>
1164    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1165      <organization>One Laptop per Child</organization>
1166      <address><email>jg@laptop.org</email></address>
1167    </author>
1168    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1169      <organization abbrev="HP">Hewlett-Packard Company</organization>
1170      <address><email>JeffMogul@acm.org</email></address>
1171    </author>
1172    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1173      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1174      <address><email>henrikn@microsoft.com</email></address>
1175    </author>
1176    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1177      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1178      <address><email>LMM@acm.org</email></address>
1179    </author>
1180    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1181      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1182      <address><email>paulle@microsoft.com</email></address>
1183    </author>
1184    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1185      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1186      <address><email>timbl@w3.org</email></address>
1187    </author>
1188    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1189      <organization abbrev="W3C">World Wide Web Consortium</organization>
1190      <address><email>ylafon@w3.org</email></address>
1191    </author>
1192    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1193      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1194      <address><email>julian.reschke@greenbytes.de</email></address>
1195    </author>
1196    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1197  </front>
1198  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1199  <x:source href="p5-range.xml" basename="p5-range"/>
1200</reference>
1201
1202<reference anchor="Part6">
1203  <front>
1204    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1205    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1206      <organization abbrev="Day Software">Day Software</organization>
1207      <address><email>fielding@gbiv.com</email></address>
1208    </author>
1209    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1210      <organization>One Laptop per Child</organization>
1211      <address><email>jg@laptop.org</email></address>
1212    </author>
1213    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1214      <organization abbrev="HP">Hewlett-Packard Company</organization>
1215      <address><email>JeffMogul@acm.org</email></address>
1216    </author>
1217    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1218      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1219      <address><email>henrikn@microsoft.com</email></address>
1220    </author>
1221    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1222      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1223      <address><email>LMM@acm.org</email></address>
1224    </author>
1225    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1226      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1227      <address><email>paulle@microsoft.com</email></address>
1228    </author>
1229    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1230      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1231      <address><email>timbl@w3.org</email></address>
1232    </author>
1233    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1234      <organization abbrev="W3C">World Wide Web Consortium</organization>
1235      <address><email>ylafon@w3.org</email></address>
1236    </author>
1237    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1238      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1239      <address><email>julian.reschke@greenbytes.de</email></address>
1240    </author>
1241    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1242  </front>
1243  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1244  <x:source href="p6-cache.xml" basename="p6-cache"/>
1245</reference>
1246
1247<reference anchor="RFC2119">
1248  <front>
1249    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1250    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1251      <organization>Harvard University</organization>
1252      <address><email>sob@harvard.edu</email></address>
1253    </author>
1254    <date month="March" year="1997"/>
1255  </front>
1256  <seriesInfo name="BCP" value="14"/>
1257  <seriesInfo name="RFC" value="2119"/>
1258</reference>
1259
1260</references>
1261
1262<references title="Informative References">
1263
1264<reference anchor="RFC2616">
1265  <front>
1266    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1267    <author initials="R." surname="Fielding" fullname="R. Fielding">
1268      <organization>University of California, Irvine</organization>
1269      <address><email>fielding@ics.uci.edu</email></address>
1270    </author>
1271    <author initials="J." surname="Gettys" fullname="J. Gettys">
1272      <organization>W3C</organization>
1273      <address><email>jg@w3.org</email></address>
1274    </author>
1275    <author initials="J." surname="Mogul" fullname="J. Mogul">
1276      <organization>Compaq Computer Corporation</organization>
1277      <address><email>mogul@wrl.dec.com</email></address>
1278    </author>
1279    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1280      <organization>MIT Laboratory for Computer Science</organization>
1281      <address><email>frystyk@w3.org</email></address>
1282    </author>
1283    <author initials="L." surname="Masinter" fullname="L. Masinter">
1284      <organization>Xerox Corporation</organization>
1285      <address><email>masinter@parc.xerox.com</email></address>
1286    </author>
1287    <author initials="P." surname="Leach" fullname="P. Leach">
1288      <organization>Microsoft Corporation</organization>
1289      <address><email>paulle@microsoft.com</email></address>
1290    </author>
1291    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1292      <organization>W3C</organization>
1293      <address><email>timbl@w3.org</email></address>
1294    </author>
1295    <date month="June" year="1999"/>
1296  </front>
1297  <seriesInfo name="RFC" value="2616"/>
1298</reference>
1299
1300<reference anchor='RFC3864'>
1301  <front>
1302    <title>Registration Procedures for Message Header Fields</title>
1303    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1304      <organization>Nine by Nine</organization>
1305      <address><email>GK-IETF@ninebynine.org</email></address>
1306    </author>
1307    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1308      <organization>BEA Systems</organization>
1309      <address><email>mnot@pobox.com</email></address>
1310    </author>
1311    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1312      <organization>HP Labs</organization>
1313      <address><email>JeffMogul@acm.org</email></address>
1314    </author>
1315    <date year='2004' month='September' />
1316  </front>
1317  <seriesInfo name='BCP' value='90' />
1318  <seriesInfo name='RFC' value='3864' />
1319</reference>
1320
1321</references>
1322
1323<section title="Compatibility with Previous Versions" anchor="compatibility">
1324
1325<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1326<t>
1327  Allow weak entity tags in all requests except range requests (Sections
1328  <xref target="weak.and.strong.validators" format="counter"/> and
1329  <xref target="header.if-none-match" format="counter"/>).
1330</t>
1331</section>
1332
1333</section>
1334
1335<section title="Collected ABNF" anchor="collected.abnf">
1336   <figure>
1337      <artwork type="abnf" name="p4-conditional.parsed-abnf">ETag = "ETag:" OWS ETag-v&#xD;
1338ETag-v = entity-tag&#xD;
1339HTTP-date = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;&#xD;
1340If-Match = "If-Match:" OWS If-Match-v&#xD;
1341If-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS&#xD;
1342 entity-tag ] ) )&#xD;
1343If-Modified-Since = "If-Modified-Since:" OWS If-Modified-Since-v&#xD;
1344If-Modified-Since-v = HTTP-date&#xD;
1345If-None-Match = "If-None-Match:" OWS If-None-Match-v&#xD;
1346If-None-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS&#xD;
1347 entity-tag ] ) )&#xD;
1348If-Unmodified-Since = "If-Unmodified-Since:" OWS&#xD;
1349 If-Unmodified-Since-v&#xD;
1350If-Unmodified-Since-v = HTTP-date&#xD;
1351Last-Modified = "Last-Modified:" OWS Last-Modified-v&#xD;
1352Last-Modified-v = HTTP-date&#xD;
1353OWS = &lt;OWS, defined in [Part1], Section 1.2.2&gt;&#xD;
1354entity-tag = [ weak ] opaque-tag&#xD;
1355opaque-tag = quoted-string&#xD;
1356quoted-string = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;&#xD;
1357weak = "W/"&#xD;
1358; ETag defined but not used&#xD;
1359; If-Match defined but not used&#xD;
1360; If-Modified-Since defined but not used&#xD;
1361; If-None-Match defined but not used&#xD;
1362; If-Unmodified-Since defined but not used&#xD;
1363; Last-Modified defined but not used&#xD;
1364</artwork>
1365   </figure>
1366</section>
1367
1368<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1369
1370<section title="Since RFC2616">
1371<t>
1372  Extracted relevant partitions from <xref target="RFC2616"/>.
1373</t>
1374</section>
1375
1376<section title="Since draft-ietf-httpbis-p4-conditional-00">
1377<t>
1378  Closed issues:
1379  <list style="symbols"> 
1380    <t>
1381      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1382      "Normative and Informative references"
1383    </t>
1384  </list>
1385</t>
1386<t>
1387  Other changes:
1388  <list style="symbols"> 
1389    <t>
1390      Move definitions of 304 and 412 condition codes from Part2.
1391    </t>
1392  </list>
1393</t>
1394</section>
1395
1396<section title="Since draft-ietf-httpbis-p4-conditional-01">
1397<t>
1398  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1399  <list style="symbols"> 
1400    <t>
1401      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1402    </t>
1403  </list>
1404</t>
1405</section>
1406
1407<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1408<t>
1409  Closed issues:
1410  <list style="symbols"> 
1411    <t>
1412      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1413      "Weak ETags on non-GET requests"
1414    </t>
1415  </list>
1416</t>
1417<t>
1418  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1419  <list style="symbols"> 
1420    <t>
1421      Reference RFC 3984, and update header registrations for headers defined
1422      in this document.
1423    </t>
1424  </list>
1425</t>
1426</section>
1427
1428<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1429<t>
1430  Closed issues:
1431  <list style="symbols"> 
1432    <t>
1433      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1434      "Examples for ETag matching"
1435    </t>
1436    <t>
1437      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1438      "'entity value' undefined"
1439    </t>
1440    <t>
1441      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1442      "bogus 2068 Date header reference"
1443    </t>
1444  </list>
1445</t>
1446</section>
1447
1448<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1449<t>
1450  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1451  <list style="symbols"> 
1452    <t>
1453      Use "/" instead of "|" for alternatives.
1454    </t>
1455    <t>
1456      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1457      whitespace ("OWS") and required whitespace ("RWS").
1458    </t>
1459    <t>
1460      Rewrite ABNFs to spell out whitespace rules, factor out
1461      header value format definitions.
1462    </t>
1463  </list>
1464</t>
1465</section>
1466
1467<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1468<t>
1469  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1470  <list style="symbols"> 
1471    <t>
1472      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1473    </t>
1474  </list>
1475</t>
1476</section>
1477
1478</section>
1479
1480</back>
1481</rfc>
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