source: draft-ietf-httpbis/latest/p4-conditional.xml @ 425

Last change on this file since 425 was 425, checked in by julian.reschke@…, 11 years ago

reference RFC5234 core rules directly, fix line end bug in appendix generation (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  <x:anchor-alias value="ALPHA"/>
255  <x:anchor-alias value="CR"/>
256  <x:anchor-alias value="DIGIT"/>
257  <x:anchor-alias value="LF"/>
258  <x:anchor-alias value="OCTET"/>
259  <x:anchor-alias value="VCHAR"/>
260  <x:anchor-alias value="WSP"/>
261<t>
262  This specification uses the ABNF syntax defined in &notation;.
263  The following core rules are included by
264  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
265  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
266  DIGIT (decimal 0-9), DQUOTE (double quote),
267  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
268  OCTET (any 8-bit sequence of data), SP (space),
269  VCHAR (any visible USASCII character),
270  and WSP (whitespace).
271</t>
272
273<section title="Core Rules" anchor="core.rules">
274  <x:anchor-alias value="quoted-string"/>
275  <x:anchor-alias value="OWS"/>
276<t>
277  The core rules below are defined in &basic-rules;:
278</t>
279<figure><artwork type="abnf2616">
280  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
281  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
282</artwork></figure>
283</section>
284
285<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
286  <x:anchor-alias value="HTTP-date"/>
287<t>
288  The ABNF rules below are defined in other parts:
289</t>
290<figure><!--Part1--><artwork type="abnf2616">
291  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
292</artwork></figure>
293</section>
294
295</section>
296
297</section>
298
299<section title="Entity Tags" anchor="entity.tags">
300  <x:anchor-alias value="entity-tag"/>
301  <x:anchor-alias value="opaque-tag"/>
302  <x:anchor-alias value="weak"/>
303<t>
304   Entity tags are used for comparing two or more entities from the same
305   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
306   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
307   If-Range (&header-if-range;) header fields. The definition of how they
308   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
309   entity tag consists of an opaque quoted string, possibly prefixed by
310   a weakness indicator.
311</t>
312<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"/>
313  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
314  <x:ref>weak</x:ref>       = "W/"
315  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
316</artwork></figure>
317<t>
318   A "strong entity tag" &MAY; be shared by two entities of a resource
319   only if they are equivalent by octet equality.
320</t>
321<t>
322   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
323   two entities of a resource only if the entities are equivalent and
324   could be substituted for each other with no significant change in
325   semantics. A weak entity tag can only be used for weak comparison.
326</t>
327<t>
328   An entity tag &MUST; be unique across all versions of all entities
329   associated with a particular resource. A given entity tag value &MAY;
330   be used for entities obtained by requests on different URIs. The use
331   of the same entity tag value in conjunction with entities obtained by
332   requests on different URIs does not imply the equivalence of those
333   entities.
334</t>
335</section>
336
337<section title="Status Code Definitions">
338<section title="304 Not Modified" anchor="status.304">
339  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
340  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
341<t>
342   If the client has performed a conditional GET request and access is
343   allowed, but the document has not been modified, the server &SHOULD;
344   respond with this status code. The 304 response &MUST-NOT; contain a
345   message-body, and thus is always terminated by the first empty line
346   after the header fields.
347</t>
348<t>
349   The response &MUST; include the following header fields:
350  <list style="symbols">
351    <x:lt>
352      <t>Date, unless its omission is required by &clockless;.</t>
353      <t>
354         If a clockless origin server obeys these rules, and proxies and
355         clients add their own Date to any response received without one (as
356         already specified by &header-date;, caches will operate
357         correctly.</t>
358    </x:lt>
359    <x:lt>
360      <t>ETag and/or Content-Location, if the header would have been sent
361         in a 200 response to the same request.</t>
362    </x:lt>
363    <x:lt>
364      <t>Expires, Cache-Control, and/or Vary, if the field-value might
365         differ from that sent in any previous response for the same
366         variant.</t>
367    </x:lt>
368  </list>
369</t>
370<t>
371   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
372   the response &SHOULD-NOT;  include other entity-headers.
373   Otherwise (i.e., the conditional GET used a weak validator), the
374   response &MUST-NOT; include other entity-headers; this prevents
375   inconsistencies between cached entity-bodies and updated headers.
376</t>
377<t>
378   If a 304 response indicates an entity not currently cached, then the
379   cache &MUST; disregard the response and repeat the request without the
380   conditional.
381</t>
382<t>
383   If a cache uses a received 304 response to update a cache entry, the
384   cache &MUST; update the entry to reflect any new field values given in
385   the response.
386</t>
387</section>
388
389<section title="412 Precondition Failed" anchor="status.412">
390  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
391  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
392<t>
393   The precondition given in one or more of the request-header fields
394   evaluated to false when it was tested on the server. This response
395   code allows the client to place preconditions on the current resource
396   metainformation (header field data) and thus prevent the requested
397   method from being applied to a resource other than the one intended.
398</t>
399</section>
400</section>
401
402<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
403<t>
404   Since both origin servers and caches will compare two validators to
405   decide if they represent the same or different entities, one normally
406   would expect that if the entity (the entity-body or any entity-headers)
407   changes in any way, then the associated validator would
408   change as well. If this is true, then we call this validator a
409   "strong validator."
410</t>
411<t>
412   However, there might be cases when a server prefers to change the
413   validator only on semantically significant changes, and not when
414   insignificant aspects of the entity change. A validator that does not
415   always change when the resource changes is a "weak validator."
416</t>
417<t>
418   Entity tags are normally "strong validators," but the protocol
419   provides a mechanism to tag an entity tag as "weak." One can think of
420   a strong validator as one that changes whenever the bits of an entity
421   changes, while a weak value changes whenever the meaning of an entity
422   changes. Alternatively, one can think of a strong validator as part
423   of an identifier for a specific entity, while a weak validator is
424   part of an identifier for a set of semantically equivalent entities.
425  <list><t>
426      <x:h>Note:</x:h> One example of a strong validator is an integer that is
427      incremented in stable storage every time an entity is changed.
428    </t><t>
429      An entity's modification time, if represented with one-second
430      resolution, could be a weak validator, since it is possible that
431      the resource might be modified twice during a single second.
432    </t><t>
433      Support for weak validators is optional. However, weak validators
434      allow for more efficient caching of equivalent objects; for
435      example, a hit counter on a site is probably good enough if it is
436      updated every few days or weeks, and any value during that period
437      is likely "good enough" to be equivalent.
438    </t></list>
439</t>
440<t>
441   A "use" of a validator is either when a client generates a request
442   and includes the validator in a validating header field, or when a
443   server compares two validators.
444</t>
445<t>
446   Strong validators are usable in any context. Weak validators are only
447   usable in contexts that do not depend on exact equality of an entity.
448   For example, either kind is usable for a conditional GET of a full
449   entity. However, only a strong validator is usable for a sub-range
450   retrieval, since otherwise the client might end up with an internally
451   inconsistent entity.
452</t>
453<t>
454   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
455</t>
456<t>
457   The only function that HTTP/1.1 defines on validators is
458   comparison. There are two validator comparison functions, depending
459   on whether the comparison context allows the use of weak validators
460   or not:
461  <list style="symbols">
462     <t>The strong comparison function: in order to be considered equal,
463        both opaque-tags &MUST; be identical character-by-character, and both
464        &MUST-NOT; be weak.</t>
465     <t>The weak comparison function: in order to be considered equal, both
466        opaque-tags &MUST; be identical character-by-character.</t>
467  </list>
468</t>
469<t>
470   The example below shows the results for a set of entity tag pairs,
471   and both the weak and strong comparison function results:
472</t>
473<texttable align="left">
474  <ttcol>ETag 1</ttcol>
475  <ttcol>ETag 2</ttcol>
476  <ttcol>Strong Comparison</ttcol>
477  <ttcol>Weak Comparison</ttcol>
478
479  <c>W/"1"</c>
480  <c>W/"1"</c>
481  <c>no match</c>
482  <c>match</c>
483 
484  <c>W/"1"</c>
485  <c>W/"2"</c>
486  <c>no match</c>
487  <c>no match</c>
488
489  <c>W/"1"</c>
490  <c>"1"</c>
491  <c>no match</c>
492  <c>match</c>
493
494  <c>"1"</c>
495  <c>"1"</c>
496  <c>match</c>
497  <c>match</c>
498</texttable>
499<t>
500   An entity tag is strong unless it is explicitly tagged as weak.
501   <xref target="entity.tags"/> gives the syntax for entity tags.
502</t>
503<t>
504   A Last-Modified time, when used as a validator in a request, is
505   implicitly weak unless it is possible to deduce that it is strong,
506   using the following rules:
507  <list style="symbols">
508     <t>The validator is being compared by an origin server to the
509        actual current validator for the entity and,</t>
510     <t>That origin server reliably knows that the associated entity did
511        not change twice during the second covered by the presented
512        validator.</t>
513  </list>
514</t>
515<t>
516   or
517  <list style="symbols">
518     <t>The validator is about to be used by a client in an If-Modified-Since
519        or If-Unmodified-Since header, because the client
520        has a cache entry for the associated entity, and</t>
521     <t>That cache entry includes a Date value, which gives the time
522        when the origin server sent the original response, and</t>
523     <t>The presented Last-Modified time is at least 60 seconds before
524        the Date value.</t>
525  </list>
526</t>
527<t>
528   or
529  <list style="symbols">
530     <t>The validator is being compared by an intermediate cache to the
531        validator stored in its cache entry for the entity, and</t>
532     <t>That cache entry includes a Date value, which gives the time
533        when the origin server sent the original response, and</t>
534     <t>The presented Last-Modified time is at least 60 seconds before
535        the Date value.</t>
536  </list>
537</t>
538<t>
539   This method relies on the fact that if two different responses were
540   sent by the origin server during the same second, but both had the
541   same Last-Modified time, then at least one of those responses would
542   have a Date value equal to its Last-Modified time. The arbitrary 60-second
543   limit guards against the possibility that the Date and Last-Modified
544   values are generated from different clocks, or at somewhat
545   different times during the preparation of the response. An
546   implementation &MAY; use a value larger than 60 seconds, if it is
547   believed that 60 seconds is too short.
548</t>
549<t>
550   If a client wishes to perform a sub-range retrieval on a value for
551   which it has only a Last-Modified time and no opaque validator, it
552   &MAY; do this only if the Last-Modified time is strong in the sense
553   described here.
554</t>
555<t>
556   A cache or origin server receiving a conditional range request
557   (<xref target="Part5"/>) &MUST; use the strong comparison function to
558   evaluate the condition.
559</t>
560<t>
561   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
562   retrievals on values that have been obtained from HTTP/1.0
563   servers.
564</t>
565</section>
566
567<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">
568<t>
569   We adopt a set of rules and recommendations for origin servers,
570   clients, and caches regarding when various validator types ought to
571   be used, and for what purposes.
572</t>
573<t>
574   HTTP/1.1 origin servers:
575  <list style="symbols">
576     <t>&SHOULD; send an entity tag validator unless it is not feasible to
577        generate one.</t>
578
579     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
580        performance considerations support the use of weak entity tags,
581        or if it is unfeasible to send a strong entity tag.</t>
582
583     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
584        unless the risk of a breakdown in semantic transparency that
585        could result from using this date in an If-Modified-Since header
586        would lead to serious problems.</t>
587  </list>
588</t>
589<t>
590   In other words, the preferred behavior for an HTTP/1.1 origin server
591   is to send both a strong entity tag and a Last-Modified value.
592</t>
593<t>
594   In order to be legal, a strong entity tag &MUST; change whenever the
595   associated entity changes in any way. A weak entity tag &SHOULD;
596   change whenever the associated entity changes in a semantically
597   significant way.
598  <list><t>
599      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
600      origin server must avoid reusing a specific strong entity tag
601      value for two different entities, or reusing a specific weak
602      entity tag value for two semantically different entities. Cache
603      entries might persist for arbitrarily long periods, regardless of
604      expiration times, so it might be inappropriate to expect that a
605      cache will never again attempt to validate an entry using a
606      validator that it obtained at some point in the past.
607  </t></list>
608</t>
609<t>
610   HTTP/1.1 clients:
611  <list style="symbols">
612     <t>If an entity tag has been provided by the origin server, &MUST;
613        use that entity tag in any cache-conditional request (using If-Match
614        or If-None-Match).</t>
615
616     <t>If only a Last-Modified value has been provided by the origin
617        server, &SHOULD; use that value in non-subrange cache-conditional
618        requests (using If-Modified-Since).</t>
619
620     <t>If only a Last-Modified value has been provided by an HTTP/1.0
621        origin server, &MAY; use that value in subrange cache-conditional
622        requests (using If-Unmodified-Since:). The user agent &SHOULD;
623        provide a way to disable this, in case of difficulty.</t>
624
625     <t>If both an entity tag and a Last-Modified value have been
626        provided by the origin server, &SHOULD; use both validators in
627        cache-conditional requests. This allows both HTTP/1.0 and
628        HTTP/1.1 caches to respond appropriately.</t>
629  </list>
630</t>
631<t>
632   An HTTP/1.1 origin server, upon receiving a conditional request that
633   includes both a Last-Modified date (e.g., in an If-Modified-Since or
634   If-Unmodified-Since header field) and one or more entity tags (e.g.,
635   in an If-Match, If-None-Match, or If-Range header field) as cache
636   validators, &MUST-NOT; return a response status of 304 (Not Modified)
637   unless doing so is consistent with all of the conditional header
638   fields in the request.
639</t>
640<t>
641   An HTTP/1.1 caching proxy, upon receiving a conditional request that
642   includes both a Last-Modified date and one or more entity tags as
643   cache validators, &MUST-NOT; return a locally cached response to the
644   client unless that cached response is consistent with all of the
645   conditional header fields in the request.
646  <list><t>
647      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
648      servers and clients should transmit as much non-redundant
649      information as is available in their responses and requests.
650      HTTP/1.1 systems receiving this information will make the most
651      conservative assumptions about the validators they receive.
652  </t><t>
653      HTTP/1.0 clients and caches will ignore entity tags. Generally,
654      last-modified values received or used by these systems will
655      support transparent and efficient caching, and so HTTP/1.1 origin
656      servers should provide Last-Modified values. In those rare cases
657      where the use of a Last-Modified value as a validator by an
658      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
659      origin servers should not provide one.
660  </t></list>
661</t>
662</section>
663
664<section title="Header Field Definitions" anchor="header.fields">
665<t>
666   This section defines the syntax and semantics of HTTP/1.1 header fields
667   related to conditional requests.
668</t>
669<t>
670   For entity-header fields, both sender and recipient refer to either the
671   client or the server, depending on who sends and who receives the entity.
672</t>
673
674<section title="ETag" anchor="header.etag">
675  <iref primary="true" item="ETag header" x:for-anchor=""/>
676  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
677  <x:anchor-alias value="ETag"/>
678  <x:anchor-alias value="ETag-v"/>
679<t>
680   The response-header field "ETag" provides the current value of the
681   entity tag (see <xref target="entity.tags"/>) for the requested variant.
682   The headers used with entity
683   tags are described in Sections <xref target="header.if-match" format="counter"/>
684   and <xref target="header.if-none-match" format="counter"/> of this document,
685   and in &header-if-range;. The entity tag
686   &MAY; be used for comparison with other entities from the same resource
687   (see <xref target="weak.and.strong.validators"/>).
688</t>
689<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
690  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
691  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
692</artwork></figure>
693<figure><preamble>
694  Examples:
695</preamble>
696<artwork type="example">
697  ETag: "xyzzy"
698  ETag: W/"xyzzy"
699  ETag: ""
700</artwork></figure>
701<t>
702   The ETag response-header field value, an entity tag, provides for an
703   "opaque" cache validator. This might allow more reliable validation
704   in situations where it is inconvenient to store modification dates,
705   where the one-second resolution of HTTP date values is not
706   sufficient, or where the origin server wishes to avoid certain
707   paradoxes that might arise from the use of modification dates.
708</t>
709<t>
710   The principle behind entity tags is that only the service author
711   knows the semantics of a resource well enough to select an
712   appropriate cache validation mechanism, and the specification of any
713   validator comparison function more complex than byte-equality would
714   open up a can of worms. Thus, comparisons of any other headers
715   (except Last-Modified, for compatibility with HTTP/1.0) are never
716   used for purposes of validating a cache entry.
717</t>
718</section>
719
720<section title="If-Match" anchor="header.if-match">
721  <iref primary="true" item="If-Match header" x:for-anchor=""/>
722  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
723  <x:anchor-alias value="If-Match"/>
724  <x:anchor-alias value="If-Match-v"/>
725<t>
726   The request-header field "If-Match" is used with a method to make it
727   conditional. A client that has one or more entities previously
728   obtained from the resource can verify that one of those entities is
729   current by including a list of their associated entity tags in the
730   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
731   purpose of this feature is to allow efficient updates of cached
732   information with a minimum amount of transaction overhead. It is also
733   used, on updating requests, to prevent inadvertent modification of
734   the wrong version of a resource. As a special case, the value "*"
735   matches any current entity of the resource.
736</t>
737<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
738  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
739  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
740</artwork></figure>
741<t>
742   If any of the entity tags match the entity tag of the entity that
743   would have been returned in the response to a similar GET request
744   (without the If-Match header) on that resource, or if "*" is given
745   and any current entity exists for that resource, then the server &MAY;
746   perform the requested method as if the If-Match header field did not
747   exist.
748</t>
749<t>
750   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
751   to compare the entity tags in If-Match.
752</t>
753<t>
754   If none of the entity tags match, or if "*" is given and no current
755   entity exists, the server &MUST-NOT; perform the requested method, and
756   &MUST; return a 412 (Precondition Failed) response. This behavior is
757   most useful when the client wants to prevent an updating method, such
758   as PUT, from modifying a resource that has changed since the client
759   last retrieved it.
760</t>
761<t>
762   If the request would, without the If-Match header field, result in
763   anything other than a 2xx or 412 status, then the If-Match header
764   &MUST; be ignored.
765</t>
766<t>
767   The meaning of "If-Match: *" is that the method &SHOULD; be performed
768   if the representation selected by the origin server (or by a cache,
769   possibly using the Vary mechanism, see &header-vary;) exists, and
770   &MUST-NOT; be performed if the representation does not exist.
771</t>
772<t>
773   A request intended to update a resource (e.g., a PUT) &MAY; include an
774   If-Match header field to signal that the request method &MUST-NOT; be
775   applied if the entity corresponding to the If-Match value (a single
776   entity tag) is no longer a representation of that resource. This
777   allows the user to indicate that they do not wish the request to be
778   successful if the resource has been changed without their knowledge.
779   Examples:
780</t>
781<figure><artwork type="example">
782  If-Match: "xyzzy"
783  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
784  If-Match: *
785</artwork></figure>
786<t>
787   The result of a request having both an If-Match header field and
788   either an If-None-Match or an If-Modified-Since header fields is
789   undefined by this specification.
790</t>
791</section>
792
793<section title="If-Modified-Since" anchor="header.if-modified-since">
794  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
795  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
796  <x:anchor-alias value="If-Modified-Since"/>
797  <x:anchor-alias value="If-Modified-Since-v"/>
798<t>
799   The request-header field "If-Modified-Since" is used with a method to
800   make it conditional: if the requested variant has not been modified
801   since the time specified in this field, an entity will not be
802   returned from the server; instead, a 304 (Not Modified) response will
803   be returned without any message-body.
804</t>
805<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
806  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
807                        <x:ref>If-Modified-Since-v</x:ref>
808  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
809</artwork></figure>
810<t>
811   An example of the field is:
812</t>
813<figure><artwork type="example">
814  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
815</artwork></figure>
816<t>
817   A GET method with an If-Modified-Since header and no Range header
818   requests that the identified entity be transferred only if it has
819   been modified since the date given by the If-Modified-Since header.
820   The algorithm for determining this includes the following cases:
821  <list style="numbers">
822      <t>If the request would normally result in anything other than a
823         200 (OK) status, or if the passed If-Modified-Since date is
824         invalid, the response is exactly the same as for a normal GET.
825         A date which is later than the server's current time is
826         invalid.</t>
827
828      <t>If the variant has been modified since the If-Modified-Since
829         date, the response is exactly the same as for a normal GET.</t>
830
831      <t>If the variant has not been modified since a valid If-Modified-Since
832         date, the server &SHOULD; return a 304 (Not
833         Modified) response.</t>
834  </list>
835</t>
836<t>
837   The purpose of this feature is to allow efficient updates of cached
838   information with a minimum amount of transaction overhead.
839  <list><t>
840      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
841      see &header-range; for full details.
842    </t><t>
843      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
844      clock might not be synchronized with the client.
845    </t><t>
846      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
847      servers will use an exact date comparison function, rather than a
848      less-than function, for deciding whether to send a 304 (Not
849      Modified) response. To get best results when sending an If-Modified-Since
850      header field for cache validation, clients are
851      advised to use the exact date string received in a previous Last-Modified
852      header field whenever possible.
853    </t><t>
854      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
855      header instead of a date taken from the Last-Modified header for
856      the same request, the client should be aware of the fact that this
857      date is interpreted in the server's understanding of time. The
858      client should consider unsynchronized clocks and rounding problems
859      due to the different encodings of time between the client and
860      server. This includes the possibility of race conditions if the
861      document has changed between the time it was first requested and
862      the If-Modified-Since date of a subsequent request, and the
863      possibility of clock-skew-related problems if the If-Modified-Since
864      date is derived from the client's clock without correction
865      to the server's clock. Corrections for different time bases
866      between client and server are at best approximate due to network
867      latency.
868    </t>
869  </list>
870</t>
871<t>
872   The result of a request having both an If-Modified-Since header field
873   and either an If-Match or an If-Unmodified-Since header fields is
874   undefined by this specification.
875</t>
876</section>
877
878<section title="If-None-Match" anchor="header.if-none-match">
879  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
880  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
881  <x:anchor-alias value="If-None-Match"/>
882  <x:anchor-alias value="If-None-Match-v"/>
883<t>
884   The request-header field "If-None-Match" is used with a method to make
885   it conditional. A client that has one or more entities previously
886   obtained from the resource can verify that none of those entities is
887   current by including a list of their associated entity tags in the
888   If-None-Match header field. The purpose of this feature is to allow
889   efficient updates of cached information with a minimum amount of
890   transaction overhead. It is also used to prevent a method (e.g. PUT)
891   from inadvertently modifying an existing resource when the client
892   believes that the resource does not exist.
893</t>
894<t>
895   As a special case, the value "*" matches any current entity of the
896   resource.
897</t>
898<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
899  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
900  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
901</artwork></figure>
902<t>
903   If any of the entity tags match the entity tag of the entity that
904   would have been returned in the response to a similar GET request
905   (without the If-None-Match header) on that resource, or if "*" is
906   given and any current entity exists for that resource, then the
907   server &MUST-NOT; perform the requested method, unless required to do
908   so because the resource's modification date fails to match that
909   supplied in an If-Modified-Since header field in the request.
910   Instead, if the request method was GET or HEAD, the server &SHOULD;
911   respond with a 304 (Not Modified) response, including the cache-related
912   header fields (particularly ETag) of one of the entities that
913   matched. For all other request methods, the server &MUST; respond with
914   a status of 412 (Precondition Failed).
915</t>
916<t>
917   See <xref target="weak.and.strong.validators"/> for rules on how to
918   determine if two entity tags match.
919</t>
920<t>
921   If none of the entity tags match, then the server &MAY; perform the
922   requested method as if the If-None-Match header field did not exist,
923   but &MUST; also ignore any If-Modified-Since header field(s) in the
924   request. That is, if no entity tags match, then the server &MUST-NOT;
925   return a 304 (Not Modified) response.
926</t>
927<t>
928   If the request would, without the If-None-Match header field, result
929   in anything other than a 2xx or 304 status, then the If-None-Match
930   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
931   server behavior when both If-Modified-Since and If-None-Match appear
932   in the same request.)
933</t>
934<t>
935   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
936   performed if the representation selected by the origin server (or by
937   a cache, possibly using the Vary mechanism, see &header-vary;)
938   exists, and &SHOULD; be performed if the representation does not exist.
939   This feature is intended to be useful in preventing races between PUT
940   operations.
941</t>
942<t>
943   Examples:
944</t>
945<figure><artwork type="example">
946  If-None-Match: "xyzzy"
947  If-None-Match: W/"xyzzy"
948  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
949  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
950  If-None-Match: *
951</artwork></figure>
952<t>
953   The result of a request having both an If-None-Match header field and
954   either an If-Match or an If-Unmodified-Since header fields is
955   undefined by this specification.
956</t>
957</section>
958
959<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
960  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
961  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
962  <x:anchor-alias value="If-Unmodified-Since"/>
963  <x:anchor-alias value="If-Unmodified-Since-v"/>
964<t>
965   The request-header field "If-Unmodified-Since" is used with a method to
966   make it conditional. If the requested resource has not been modified
967   since the time specified in this field, the server &SHOULD; perform the
968   requested operation as if the If-Unmodified-Since header were not
969   present.
970</t>
971<t>
972   If the requested variant has been modified since the specified time,
973   the server &MUST-NOT; perform the requested operation, and &MUST; return
974   a 412 (Precondition Failed).
975</t>
976<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
977  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
978                          <x:ref>If-Unmodified-Since-v</x:ref>
979  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
980</artwork></figure>
981<t>
982   An example of the field is:
983</t>
984<figure><artwork type="example">
985  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
986</artwork></figure>
987<t>
988   If the request normally (i.e., without the If-Unmodified-Since
989   header) would result in anything other than a 2xx or 412 status, the
990   If-Unmodified-Since header &SHOULD; be ignored.
991</t>
992<t>
993   If the specified date is invalid, the header is ignored.
994</t>
995<t>
996   The result of a request having both an If-Unmodified-Since header
997   field and either an If-None-Match or an If-Modified-Since header
998   fields is undefined by this specification.
999</t>
1000</section>
1001
1002<section title="Last-Modified" anchor="header.last-modified">
1003  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
1004  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
1005  <x:anchor-alias value="Last-Modified"/>
1006  <x:anchor-alias value="Last-Modified-v"/>
1007<t>
1008   The entity-header field "Last-Modified" indicates the date and time at
1009   which the origin server believes the variant was last modified.
1010</t>
1011<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
1012  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
1013  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
1014</artwork></figure>
1015<t>
1016   An example of its use is
1017</t>
1018<figure><artwork type="example">
1019  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1020</artwork></figure>
1021<t>
1022   The exact meaning of this header field depends on the implementation
1023   of the origin server and the nature of the original resource. For
1024   files, it may be just the file system last-modified time. For
1025   entities with dynamically included parts, it may be the most recent
1026   of the set of last-modify times for its component parts. For database
1027   gateways, it may be the last-update time stamp of the record. For
1028   virtual objects, it may be the last time the internal state changed.
1029</t>
1030<t>
1031   An origin server &MUST-NOT; send a Last-Modified date which is later
1032   than the server's time of message origination. In such cases, where
1033   the resource's last modification would indicate some time in the
1034   future, the server &MUST; replace that date with the message
1035   origination date.
1036</t>
1037<t>
1038   An origin server &SHOULD; obtain the Last-Modified value of the entity
1039   as close as possible to the time that it generates the Date value of
1040   its response. This allows a recipient to make an accurate assessment
1041   of the entity's modification time, especially if the entity changes
1042   near the time that the response is generated.
1043</t>
1044<t>
1045   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1046</t>
1047<t>
1048   The Last-Modified entity-header field value is often used as a cache
1049   validator. In simple terms, a cache entry is considered to be valid
1050   if the entity has not been modified since the Last-Modified value.
1051</t>
1052</section>
1053
1054</section>
1055
1056<section title="IANA Considerations" anchor="IANA.considerations">
1057<section title="Message Header Registration" anchor="message.header.registration">
1058<t>
1059   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1060   with the permanent registrations below (see <xref target="RFC3864"/>):
1061</t>
1062<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1063<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1064   <ttcol>Header Field Name</ttcol>
1065   <ttcol>Protocol</ttcol>
1066   <ttcol>Status</ttcol>
1067   <ttcol>Reference</ttcol>
1068
1069   <c>ETag</c>
1070   <c>http</c>
1071   <c>standard</c>
1072   <c>
1073      <xref target="header.etag"/>
1074   </c>
1075   <c>If-Match</c>
1076   <c>http</c>
1077   <c>standard</c>
1078   <c>
1079      <xref target="header.if-match"/>
1080   </c>
1081   <c>If-Modified-Since</c>
1082   <c>http</c>
1083   <c>standard</c>
1084   <c>
1085      <xref target="header.if-modified-since"/>
1086   </c>
1087   <c>If-None-Match</c>
1088   <c>http</c>
1089   <c>standard</c>
1090   <c>
1091      <xref target="header.if-none-match"/>
1092   </c>
1093   <c>If-Unmodified-Since</c>
1094   <c>http</c>
1095   <c>standard</c>
1096   <c>
1097      <xref target="header.if-unmodified-since"/>
1098   </c>
1099   <c>Last-Modified</c>
1100   <c>http</c>
1101   <c>standard</c>
1102   <c>
1103      <xref target="header.last-modified"/>
1104   </c>
1105</texttable>
1106<!--(END)-->
1107<t>
1108   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1109</t>
1110</section>
1111</section>
1112
1113<section title="Security Considerations" anchor="security.considerations">
1114<t>
1115   No additional security considerations have been identified beyond
1116   those applicable to HTTP in general &messaging;.
1117</t>
1118</section>
1119
1120<section title="Acknowledgments" anchor="ack">
1121</section>
1122</middle>
1123<back>
1124
1125<references title="Normative References">
1126
1127<reference anchor="Part1">
1128  <front>
1129    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1130    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1131      <organization abbrev="Day Software">Day Software</organization>
1132      <address><email>fielding@gbiv.com</email></address>
1133    </author>
1134    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1135      <organization>One Laptop per Child</organization>
1136      <address><email>jg@laptop.org</email></address>
1137    </author>
1138    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1139      <organization abbrev="HP">Hewlett-Packard Company</organization>
1140      <address><email>JeffMogul@acm.org</email></address>
1141    </author>
1142    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1143      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1144      <address><email>henrikn@microsoft.com</email></address>
1145    </author>
1146    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1147      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1148      <address><email>LMM@acm.org</email></address>
1149    </author>
1150    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1151      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1152      <address><email>paulle@microsoft.com</email></address>
1153    </author>
1154    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1155      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1156      <address><email>timbl@w3.org</email></address>
1157    </author>
1158    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1159      <organization abbrev="W3C">World Wide Web Consortium</organization>
1160      <address><email>ylafon@w3.org</email></address>
1161    </author>
1162    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1163      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1164      <address><email>julian.reschke@greenbytes.de</email></address>
1165    </author>
1166    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1167  </front>
1168  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1169  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1170</reference>
1171
1172<reference anchor="Part5">
1173  <front>
1174    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1175    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1176      <organization abbrev="Day Software">Day Software</organization>
1177      <address><email>fielding@gbiv.com</email></address>
1178    </author>
1179    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1180      <organization>One Laptop per Child</organization>
1181      <address><email>jg@laptop.org</email></address>
1182    </author>
1183    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1184      <organization abbrev="HP">Hewlett-Packard Company</organization>
1185      <address><email>JeffMogul@acm.org</email></address>
1186    </author>
1187    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1188      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1189      <address><email>henrikn@microsoft.com</email></address>
1190    </author>
1191    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1192      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1193      <address><email>LMM@acm.org</email></address>
1194    </author>
1195    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1196      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1197      <address><email>paulle@microsoft.com</email></address>
1198    </author>
1199    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1200      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1201      <address><email>timbl@w3.org</email></address>
1202    </author>
1203    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1204      <organization abbrev="W3C">World Wide Web Consortium</organization>
1205      <address><email>ylafon@w3.org</email></address>
1206    </author>
1207    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1208      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1209      <address><email>julian.reschke@greenbytes.de</email></address>
1210    </author>
1211    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1212  </front>
1213  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1214  <x:source href="p5-range.xml" basename="p5-range"/>
1215</reference>
1216
1217<reference anchor="Part6">
1218  <front>
1219    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1220    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1221      <organization abbrev="Day Software">Day Software</organization>
1222      <address><email>fielding@gbiv.com</email></address>
1223    </author>
1224    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1225      <organization>One Laptop per Child</organization>
1226      <address><email>jg@laptop.org</email></address>
1227    </author>
1228    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1229      <organization abbrev="HP">Hewlett-Packard Company</organization>
1230      <address><email>JeffMogul@acm.org</email></address>
1231    </author>
1232    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1233      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1234      <address><email>henrikn@microsoft.com</email></address>
1235    </author>
1236    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1237      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1238      <address><email>LMM@acm.org</email></address>
1239    </author>
1240    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1241      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1242      <address><email>paulle@microsoft.com</email></address>
1243    </author>
1244    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1245      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1246      <address><email>timbl@w3.org</email></address>
1247    </author>
1248    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1249      <organization abbrev="W3C">World Wide Web Consortium</organization>
1250      <address><email>ylafon@w3.org</email></address>
1251    </author>
1252    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1253      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1254      <address><email>julian.reschke@greenbytes.de</email></address>
1255    </author>
1256    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1257  </front>
1258  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1259  <x:source href="p6-cache.xml" basename="p6-cache"/>
1260</reference>
1261
1262<reference anchor="RFC2119">
1263  <front>
1264    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1265    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1266      <organization>Harvard University</organization>
1267      <address><email>sob@harvard.edu</email></address>
1268    </author>
1269    <date month="March" year="1997"/>
1270  </front>
1271  <seriesInfo name="BCP" value="14"/>
1272  <seriesInfo name="RFC" value="2119"/>
1273</reference>
1274
1275<reference anchor="RFC5234">
1276  <front>
1277    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1278    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1279      <organization>Brandenburg InternetWorking</organization>
1280      <address>
1281      <postal>
1282      <street>675 Spruce Dr.</street>
1283      <city>Sunnyvale</city>
1284      <region>CA</region>
1285      <code>94086</code>
1286      <country>US</country></postal>
1287      <phone>+1.408.246.8253</phone>
1288      <email>dcrocker@bbiw.net</email></address> 
1289    </author>
1290    <author initials="P." surname="Overell" fullname="Paul Overell">
1291      <organization>THUS plc.</organization>
1292      <address>
1293      <postal>
1294      <street>1/2 Berkeley Square</street>
1295      <street>99 Berkely Street</street>
1296      <city>Glasgow</city>
1297      <code>G3 7HR</code>
1298      <country>UK</country></postal>
1299      <email>paul.overell@thus.net</email></address>
1300    </author>
1301    <date month="January" year="2008"/>
1302  </front>
1303  <seriesInfo name="STD" value="68"/>
1304  <seriesInfo name="RFC" value="5234"/>
1305</reference>
1306
1307</references>
1308
1309<references title="Informative References">
1310
1311<reference anchor="RFC2616">
1312  <front>
1313    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1314    <author initials="R." surname="Fielding" fullname="R. Fielding">
1315      <organization>University of California, Irvine</organization>
1316      <address><email>fielding@ics.uci.edu</email></address>
1317    </author>
1318    <author initials="J." surname="Gettys" fullname="J. Gettys">
1319      <organization>W3C</organization>
1320      <address><email>jg@w3.org</email></address>
1321    </author>
1322    <author initials="J." surname="Mogul" fullname="J. Mogul">
1323      <organization>Compaq Computer Corporation</organization>
1324      <address><email>mogul@wrl.dec.com</email></address>
1325    </author>
1326    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1327      <organization>MIT Laboratory for Computer Science</organization>
1328      <address><email>frystyk@w3.org</email></address>
1329    </author>
1330    <author initials="L." surname="Masinter" fullname="L. Masinter">
1331      <organization>Xerox Corporation</organization>
1332      <address><email>masinter@parc.xerox.com</email></address>
1333    </author>
1334    <author initials="P." surname="Leach" fullname="P. Leach">
1335      <organization>Microsoft Corporation</organization>
1336      <address><email>paulle@microsoft.com</email></address>
1337    </author>
1338    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1339      <organization>W3C</organization>
1340      <address><email>timbl@w3.org</email></address>
1341    </author>
1342    <date month="June" year="1999"/>
1343  </front>
1344  <seriesInfo name="RFC" value="2616"/>
1345</reference>
1346
1347<reference anchor='RFC3864'>
1348  <front>
1349    <title>Registration Procedures for Message Header Fields</title>
1350    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1351      <organization>Nine by Nine</organization>
1352      <address><email>GK-IETF@ninebynine.org</email></address>
1353    </author>
1354    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1355      <organization>BEA Systems</organization>
1356      <address><email>mnot@pobox.com</email></address>
1357    </author>
1358    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1359      <organization>HP Labs</organization>
1360      <address><email>JeffMogul@acm.org</email></address>
1361    </author>
1362    <date year='2004' month='September' />
1363  </front>
1364  <seriesInfo name='BCP' value='90' />
1365  <seriesInfo name='RFC' value='3864' />
1366</reference>
1367
1368</references>
1369
1370<section title="Compatibility with Previous Versions" anchor="compatibility">
1371
1372<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1373<t>
1374  Allow weak entity tags in all requests except range requests (Sections
1375  <xref target="weak.and.strong.validators" format="counter"/> and
1376  <xref target="header.if-none-match" format="counter"/>).
1377</t>
1378</section>
1379
1380</section>
1381
1382<section title="Collected ABNF" anchor="collected.abnf">
1383   <figure>
1384      <artwork type="abnf" name="p4-conditional.parsed-abnf">ETag = "ETag:" OWS ETag-v&#xD;
1385ETag-v = entity-tag&#xD;
1386HTTP-date = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;&#xD;
1387If-Match = "If-Match:" OWS If-Match-v&#xD;
1388If-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS&#xD;
1389 entity-tag ] ) )&#xD;
1390If-Modified-Since = "If-Modified-Since:" OWS If-Modified-Since-v&#xD;
1391If-Modified-Since-v = HTTP-date&#xD;
1392If-None-Match = "If-None-Match:" OWS If-None-Match-v&#xD;
1393If-None-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS&#xD;
1394 entity-tag ] ) )&#xD;
1395If-Unmodified-Since = "If-Unmodified-Since:" OWS&#xD;
1396 If-Unmodified-Since-v&#xD;
1397If-Unmodified-Since-v = HTTP-date&#xD;
1398Last-Modified = "Last-Modified:" OWS Last-Modified-v&#xD;
1399Last-Modified-v = HTTP-date&#xD;
1400OWS = &lt;OWS, defined in [Part1], Section 1.2.2&gt;&#xD;
1401entity-tag = [ weak ] opaque-tag&#xD;
1402opaque-tag = quoted-string&#xD;
1403quoted-string = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;&#xD;
1404weak = "W/"&#xD;
1405; ETag defined but not used&#xD;
1406; If-Match defined but not used&#xD;
1407; If-Modified-Since defined but not used&#xD;
1408; If-None-Match defined but not used&#xD;
1409; If-Unmodified-Since defined but not used&#xD;
1410; Last-Modified defined but not used&#xD;
1411</artwork>
1412   </figure>
1413</section>
1414
1415<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1416
1417<section title="Since RFC2616">
1418<t>
1419  Extracted relevant partitions from <xref target="RFC2616"/>.
1420</t>
1421</section>
1422
1423<section title="Since draft-ietf-httpbis-p4-conditional-00">
1424<t>
1425  Closed issues:
1426  <list style="symbols"> 
1427    <t>
1428      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1429      "Normative and Informative references"
1430    </t>
1431  </list>
1432</t>
1433<t>
1434  Other changes:
1435  <list style="symbols"> 
1436    <t>
1437      Move definitions of 304 and 412 condition codes from Part2.
1438    </t>
1439  </list>
1440</t>
1441</section>
1442
1443<section title="Since draft-ietf-httpbis-p4-conditional-01">
1444<t>
1445  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1446  <list style="symbols"> 
1447    <t>
1448      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1449    </t>
1450  </list>
1451</t>
1452</section>
1453
1454<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1455<t>
1456  Closed issues:
1457  <list style="symbols"> 
1458    <t>
1459      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1460      "Weak ETags on non-GET requests"
1461    </t>
1462  </list>
1463</t>
1464<t>
1465  Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1466  <list style="symbols"> 
1467    <t>
1468      Reference RFC 3984, and update header registrations for headers defined
1469      in this document.
1470    </t>
1471  </list>
1472</t>
1473</section>
1474
1475<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1476<t>
1477  Closed issues:
1478  <list style="symbols"> 
1479    <t>
1480      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1481      "Examples for ETag matching"
1482    </t>
1483    <t>
1484      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1485      "'entity value' undefined"
1486    </t>
1487    <t>
1488      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1489      "bogus 2068 Date header reference"
1490    </t>
1491  </list>
1492</t>
1493</section>
1494
1495<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1496<t>
1497  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1498  <list style="symbols"> 
1499    <t>
1500      Use "/" instead of "|" for alternatives.
1501    </t>
1502    <t>
1503      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1504      whitespace ("OWS") and required whitespace ("RWS").
1505    </t>
1506    <t>
1507      Rewrite ABNFs to spell out whitespace rules, factor out
1508      header value format definitions.
1509    </t>
1510  </list>
1511</t>
1512</section>
1513
1514<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1515<t>
1516  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1517  <list style="symbols"> 
1518    <t>
1519      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1520    </t>
1521  </list>
1522</t>
1523</section>
1524
1525</section>
1526
1527</back>
1528</rfc>
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