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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 "August">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY full-date                  "<xref target='Part1' x:rel='#full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26]>
27<?rfc toc="yes" ?>
28<?rfc symrefs="yes" ?>
29<?rfc sortrefs="yes" ?>
30<?rfc compact="yes"?>
31<?rfc subcompact="no" ?>
32<?rfc linkmailto="no" ?>
33<?rfc editing="no" ?>
34<?rfc comments="yes"?>
35<?rfc inline="yes"?>
36<?rfc-ext allow-markup-in-artwork="yes" ?>
37<?rfc-ext include-references-in-index="yes" ?>
38<rfc obsoletes="2616" category="std"
39     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
40     xmlns:x='http://purl.org/net/xml2rfc/ext'>
41<front>
42
43  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
44
45  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
46    <organization abbrev="Day Software">Day Software</organization>
47    <address>
48      <postal>
49        <street>23 Corporate Plaza DR, Suite 280</street>
50        <city>Newport Beach</city>
51        <region>CA</region>
52        <code>92660</code>
53        <country>USA</country>
54      </postal>
55      <phone>+1-949-706-5300</phone>
56      <facsimile>+1-949-706-5305</facsimile>
57      <email>fielding@gbiv.com</email>
58      <uri>http://roy.gbiv.com/</uri>
59    </address>
60  </author>
61
62  <author initials="J." surname="Gettys" fullname="Jim Gettys">
63    <organization>One Laptop per Child</organization>
64    <address>
65      <postal>
66        <street>21 Oak Knoll Road</street>
67        <city>Carlisle</city>
68        <region>MA</region>
69        <code>01741</code>
70        <country>USA</country>
71      </postal>
72      <email>jg@laptop.org</email>
73      <uri>http://www.laptop.org/</uri>
74    </address>
75  </author>
76 
77  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
78    <organization abbrev="HP">Hewlett-Packard Company</organization>
79    <address>
80      <postal>
81        <street>HP Labs, Large Scale Systems Group</street>
82        <street>1501 Page Mill Road, MS 1177</street>
83        <city>Palo Alto</city>
84        <region>CA</region>
85        <code>94304</code>
86        <country>USA</country>
87      </postal>
88      <email>JeffMogul@acm.org</email>
89    </address>
90  </author>
91
92  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
93    <organization abbrev="Microsoft">Microsoft Corporation</organization>
94    <address>
95      <postal>
96        <street>1 Microsoft Way</street>
97        <city>Redmond</city>
98        <region>WA</region>
99        <code>98052</code>
100        <country>USA</country>
101      </postal>
102      <email>henrikn@microsoft.com</email>
103    </address>
104  </author>
105
106  <author initials="L." surname="Masinter" fullname="Larry Masinter">
107    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
108    <address>
109      <postal>
110        <street>345 Park Ave</street>
111        <city>San Jose</city>
112        <region>CA</region>
113        <code>95110</code>
114        <country>USA</country>
115      </postal>
116      <email>LMM@acm.org</email>
117      <uri>http://larry.masinter.net/</uri>
118    </address>
119  </author>
120 
121  <author initials="P." surname="Leach" fullname="Paul J. Leach">
122    <organization abbrev="Microsoft">Microsoft Corporation</organization>
123    <address>
124      <postal>
125        <street>1 Microsoft Way</street>
126        <city>Redmond</city>
127        <region>WA</region>
128        <code>98052</code>
129      </postal>
130      <email>paulle@microsoft.com</email>
131    </address>
132  </author>
133   
134  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
135    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
136    <address>
137      <postal>
138        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
139        <street>The Stata Center, Building 32</street>
140        <street>32 Vassar Street</street>
141        <city>Cambridge</city>
142        <region>MA</region>
143        <code>02139</code>
144        <country>USA</country>
145      </postal>
146      <email>timbl@w3.org</email>
147      <uri>http://www.w3.org/People/Berners-Lee/</uri>
148    </address>
149  </author>
150
151  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
152    <organization abbrev="W3C">World Wide Web Consortium</organization>
153    <address>
154      <postal>
155        <street>W3C / ERCIM</street>
156        <street>2004, rte des Lucioles</street>
157        <city>Sophia-Antipolis</city>
158        <region>AM</region>
159        <code>06902</code>
160        <country>France</country>
161      </postal>
162      <email>ylafon@w3.org</email>
163      <uri>http://www.raubacapeu.net/people/yves/</uri>
164    </address>
165  </author>
166
167  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
168    <organization abbrev="greenbytes">greenbytes GmbH</organization>
169    <address>
170      <postal>
171        <street>Hafenweg 16</street>
172        <city>Muenster</city><region>NW</region><code>48155</code>
173        <country>Germany</country>
174      </postal>
175      <phone>+49 251 2807760</phone>   
176      <facsimile>+49 251 2807761</facsimile>   
177      <email>julian.reschke@greenbytes.de</email>       
178      <uri>http://greenbytes.de/tech/webdav/</uri>     
179    </address>
180  </author>
181
182  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
183
184<abstract>
185<t>
186   The Hypertext Transfer Protocol (HTTP) is an application-level
187   protocol for distributed, collaborative, hypermedia information
188   systems. HTTP has been in use by the World Wide Web global information
189   initiative since 1990. This document is Part 4 of the seven-part specification
190   that defines the protocol referred to as "HTTP/1.1" and, taken together,
191   obsoletes RFC 2616.  Part 4 defines request header fields for
192   indicating conditional requests and the rules for constructing responses
193   to those requests.
194</t>
195</abstract>
196
197<note title="Editorial Note (To be removed by RFC Editor)">
198  <t>
199    Discussion of this draft should take place on the HTTPBIS working group
200    mailing list (ietf-http-wg@w3.org). The current issues list is
201    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
202    and related documents (including fancy diffs) can be found at
203    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
204  </t>
205  <t>
206    The changes in this draft are summarized in <xref target="changes.since.02"/>.
207  </t>
208</note>
209</front>
210<middle>
211<section title="Introduction" anchor="introduction">
212<t>
213   This document defines HTTP/1.1 response metadata for indicating potential
214   changes to payload content, including modification time stamps and opaque
215   entity-tags, and the HTTP conditional request mechanisms that allow
216   preconditions to be placed on a request method.  Conditional GET requests
217   allow for efficient cache updates.  Other conditional request methods are
218   used to protect against overwriting or misunderstanding the state of a
219   resource that has been changed unbeknownst to the requesting client.
220</t>
221<t>
222   This document is currently disorganized in order to minimize the changes
223   between drafts and enable reviewers to see the smaller errata changes.
224   The next draft will reorganize the sections to better reflect the content.
225   In particular, the sections on resource metadata will be discussed first
226   and then followed by each conditional request-header, concluding with a
227   definition of precedence and the expectation of ordering strong validator
228   checks before weak validator checks.  It is likely that more content from
229   &caching; will migrate to this part, where appropriate.
230   The current mess reflects how widely dispersed these topics and associated
231   requirements had become in <xref target="RFC2616"/>.
232</t>
233
234<section title="Requirements" anchor="intro.requirements">
235<t>
236   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
237   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
238   document are to be interpreted as described in <xref target="RFC2119"/>.
239</t>
240<t>
241   An implementation is not compliant if it fails to satisfy one or more
242   of the &MUST; or &REQUIRED; level requirements for the protocols it
243   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
244   level and all the &SHOULD; level requirements for its protocols is said
245   to be "unconditionally compliant"; one that satisfies all the &MUST;
246   level requirements but not all the &SHOULD; level requirements for its
247   protocols is said to be "conditionally compliant."
248</t>
249</section>
250</section>
251
252<section title="Notational Conventions and Generic Grammar" anchor="notation">
253  <x:anchor-alias value="quoted-string"/>
254<t>
255  This specification uses the ABNF syntax defined in &notation-abnf; and
256  the core rules defined in &basic-rules;:
257  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
258  <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>.</cref>
259</t>
260<figure><artwork type="abnf2616">
261  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
262</artwork></figure>
263<t anchor="abnf.dependencies">
264  <x:anchor-alias value="HTTP-date"/>
265  The ABNF rules below are defined in other parts:
266</t>
267<figure><!--Part1--><artwork type="abnf2616">
268  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
269</artwork></figure>
270</section>
271
272<section title="Entity Tags" anchor="entity.tags">
273  <x:anchor-alias value="entity-tag"/>
274  <x:anchor-alias value="opaque-tag"/>
275  <x:anchor-alias value="weak"/>
276<t>
277   Entity tags are used for comparing two or more entities from the same
278   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
279   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
280   If-Range (&header-if-range;) header fields. The definition of how they
281   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
282   entity tag consists of an opaque quoted string, possibly prefixed by
283   a weakness indicator.
284</t>
285<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"/>
286  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
287  <x:ref>weak</x:ref>       = "W/"
288  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
289</artwork></figure>
290<t>
291   A "strong entity tag" &MAY; be shared by two entities of a resource
292   only if they are equivalent by octet equality.
293</t>
294<t>
295   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
296   two entities of a resource only if the entities are equivalent and
297   could be substituted for each other with no significant change in
298   semantics. A weak entity tag can only be used for weak comparison.
299</t>
300<t>
301   An entity tag &MUST; be unique across all versions of all entities
302   associated with a particular resource. A given entity tag value &MAY;
303   be used for entities obtained by requests on different URIs. The use
304   of the same entity tag value in conjunction with entities obtained by
305   requests on different URIs does not imply the equivalence of those
306   entities.
307</t>
308</section>
309
310<section title="Status Code Definitions">
311<section title="304 Not Modified" anchor="status.304">
312  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
313  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
314<t>
315   If the client has performed a conditional GET request and access is
316   allowed, but the document has not been modified, the server &SHOULD;
317   respond with this status code. The 304 response &MUST-NOT; contain a
318   message-body, and thus is always terminated by the first empty line
319   after the header fields.
320</t>
321<t>
322   The response &MUST; include the following header fields:
323  <list style="symbols">
324    <t>Date, unless its omission is required by &clockless;</t>
325  </list>
326</t>
327<t>
328   If a clockless origin server obeys these rules, and proxies and
329   clients add their own Date to any response received without one (as
330   already specified by <xref target="RFC2068" x:sec="14.19" x:fmt=","/>), caches will operate
331   correctly.
332  <list style="symbols">
333    <t>ETag and/or Content-Location, if the header would have been sent
334        in a 200 response to the same request</t>
335    <t>Expires, Cache-Control, and/or Vary, if the field-value might
336        differ from that sent in any previous response for the same
337        variant</t>
338  </list>
339</t>
340<t>
341   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
342   the response &SHOULD-NOT;  include other entity-headers.
343   Otherwise (i.e., the conditional GET used a weak validator), the
344   response &MUST-NOT; include other entity-headers; this prevents
345   inconsistencies between cached entity-bodies and updated headers.
346</t>
347<t>
348   If a 304 response indicates an entity not currently cached, then the
349   cache &MUST; disregard the response and repeat the request without the
350   conditional.
351</t>
352<t>
353   If a cache uses a received 304 response to update a cache entry, the
354   cache &MUST; update the entry to reflect any new field values given in
355   the response.
356</t>
357</section>
358
359<section title="412 Precondition Failed" anchor="status.412">
360  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
361  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
362<t>
363   The precondition given in one or more of the request-header fields
364   evaluated to false when it was tested on the server. This response
365   code allows the client to place preconditions on the current resource
366   metainformation (header field data) and thus prevent the requested
367   method from being applied to a resource other than the one intended.
368</t>
369</section>
370</section>
371
372<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
373<t>
374   Since both origin servers and caches will compare two validators to
375   decide if they represent the same or different entities, one normally
376   would expect that if the entity (the entity-body or any entity-headers)
377   changes in any way, then the associated validator would
378   change as well. If this is true, then we call this validator a
379   "strong validator."
380</t>
381<t>
382   However, there might be cases when a server prefers to change the
383   validator only on semantically significant changes, and not when
384   insignificant aspects of the entity change. A validator that does not
385   always change when the resource changes is a "weak validator."
386</t>
387<t>
388   Entity tags are normally "strong validators," but the protocol
389   provides a mechanism to tag an entity tag as "weak." One can think of
390   a strong validator as one that changes whenever the bits of an entity
391   changes, while a weak value changes whenever the meaning of an entity
392   changes. Alternatively, one can think of a strong validator as part
393   of an identifier for a specific entity, while a weak validator is
394   part of an identifier for a set of semantically equivalent entities.
395  <list><t>
396      <x:h>Note:</x:h> One example of a strong validator is an integer that is
397      incremented in stable storage every time an entity is changed.
398    </t><t>
399      An entity's modification time, if represented with one-second
400      resolution, could be a weak validator, since it is possible that
401      the resource might be modified twice during a single second.
402    </t><t>
403      Support for weak validators is optional. However, weak validators
404      allow for more efficient caching of equivalent objects; for
405      example, a hit counter on a site is probably good enough if it is
406      updated every few days or weeks, and any value during that period
407      is likely "good enough" to be equivalent.
408    </t></list>
409</t>
410<t>
411   A "use" of a validator is either when a client generates a request
412   and includes the validator in a validating header field, or when a
413   server compares two validators.
414</t>
415<t>
416   Strong validators are usable in any context. Weak validators are only
417   usable in contexts that do not depend on exact equality of an entity.
418   For example, either kind is usable for a conditional GET of a full
419   entity. However, only a strong validator is usable for a sub-range
420   retrieval, since otherwise the client might end up with an internally
421   inconsistent entity.
422</t>
423<t>
424   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
425</t>
426<t>
427   The only function that HTTP/1.1 defines on validators is
428   comparison. There are two validator comparison functions, depending
429   on whether the comparison context allows the use of weak validators
430   or not:
431  <list style="symbols">
432     <t>The strong comparison function: in order to be considered equal,
433        both opaque-tags &MUST; be identical character-by-character, and both
434        &MUST-NOT; be weak.</t>
435     <t>The weak comparison function: in order to be considered equal, both
436        opaque-tags &MUST; be identical character-by-character.</t>
437  </list>
438</t>
439<t>
440   The example below shows the results for a set of entity tag pairs,
441   and both the weak and strong comparison function results:
442</t>
443<texttable align="left">
444  <ttcol>ETag 1</ttcol>
445  <ttcol>ETag 2</ttcol>
446  <ttcol>Strong Comparison</ttcol>
447  <ttcol>Weak Comparison</ttcol>
448
449  <c>W/"1"</c>
450  <c>W/"1"</c>
451  <c>no match</c>
452  <c>match</c>
453 
454  <c>W/"1"</c>
455  <c>W/"2"</c>
456  <c>no match</c>
457  <c>no match</c>
458
459  <c>W/"1"</c>
460  <c>"1"</c>
461  <c>no match</c>
462  <c>match</c>
463
464  <c>"1"</c>
465  <c>"1"</c>
466  <c>match</c>
467  <c>match</c>
468</texttable>
469<t>
470   An entity tag is strong unless it is explicitly tagged as weak.
471   <xref target="entity.tags"/> gives the syntax for entity tags.
472</t>
473<t>
474   A Last-Modified time, when used as a validator in a request, is
475   implicitly weak unless it is possible to deduce that it is strong,
476   using the following rules:
477  <list style="symbols">
478     <t>The validator is being compared by an origin server to the
479        actual current validator for the entity and,</t>
480     <t>That origin server reliably knows that the associated entity did
481        not change twice during the second covered by the presented
482        validator.</t>
483  </list>
484</t>
485<t>
486   or
487  <list style="symbols">
488     <t>The validator is about to be used by a client in an If-Modified-Since
489        or If-Unmodified-Since header, because the client
490        has a cache entry for the associated entity, and</t>
491     <t>That cache entry includes a Date value, which gives the time
492        when the origin server sent the original response, and</t>
493     <t>The presented Last-Modified time is at least 60 seconds before
494        the Date value.</t>
495  </list>
496</t>
497<t>
498   or
499  <list style="symbols">
500     <t>The validator is being compared by an intermediate cache to the
501        validator stored in its cache entry for the entity, and</t>
502     <t>That cache entry includes a Date value, which gives the time
503        when the origin server sent the original response, and</t>
504     <t>The presented Last-Modified time is at least 60 seconds before
505        the Date value.</t>
506  </list>
507</t>
508<t>
509   This method relies on the fact that if two different responses were
510   sent by the origin server during the same second, but both had the
511   same Last-Modified time, then at least one of those responses would
512   have a Date value equal to its Last-Modified time. The arbitrary 60-second
513   limit guards against the possibility that the Date and Last-Modified
514   values are generated from different clocks, or at somewhat
515   different times during the preparation of the response. An
516   implementation &MAY; use a value larger than 60 seconds, if it is
517   believed that 60 seconds is too short.
518</t>
519<t>
520   If a client wishes to perform a sub-range retrieval on a value for
521   which it has only a Last-Modified time and no opaque validator, it
522   &MAY; do this only if the Last-Modified time is strong in the sense
523   described here.
524</t>
525<t>
526   A cache or origin server receiving a conditional range request
527   (<xref target="Part5"/>) &MUST; use the strong comparison function to
528   evaluate the condition.
529</t>
530<t>
531   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
532   retrievals on values that have been obtained from HTTP/1.0
533   servers.
534</t>
535</section>
536
537<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">
538<t>
539   We adopt a set of rules and recommendations for origin servers,
540   clients, and caches regarding when various validator types ought to
541   be used, and for what purposes.
542</t>
543<t>
544   HTTP/1.1 origin servers:
545  <list style="symbols">
546     <t>&SHOULD; send an entity tag validator unless it is not feasible to
547        generate one.</t>
548
549     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
550        performance considerations support the use of weak entity tags,
551        or if it is unfeasible to send a strong entity tag.</t>
552
553     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
554        unless the risk of a breakdown in semantic transparency that
555        could result from using this date in an If-Modified-Since header
556        would lead to serious problems.</t>
557  </list>
558</t>
559<t>
560   In other words, the preferred behavior for an HTTP/1.1 origin server
561   is to send both a strong entity tag and a Last-Modified value.
562</t>
563<t>
564   In order to be legal, a strong entity tag &MUST; change whenever the
565   associated entity value changes in any way. A weak entity tag &SHOULD;
566   change whenever the associated entity changes in a semantically
567   significant way.
568  <list><t>
569      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
570      origin server must avoid reusing a specific strong entity tag
571      value for two different entities, or reusing a specific weak
572      entity tag value for two semantically different entities. Cache
573      entries might persist for arbitrarily long periods, regardless of
574      expiration times, so it might be inappropriate to expect that a
575      cache will never again attempt to validate an entry using a
576      validator that it obtained at some point in the past.
577  </t></list>
578</t>
579<t>
580   HTTP/1.1 clients:
581  <list style="symbols">
582     <t>If an entity tag has been provided by the origin server, &MUST;
583        use that entity tag in any cache-conditional request (using If-Match
584        or If-None-Match).</t>
585
586     <t>If only a Last-Modified value has been provided by the origin
587        server, &SHOULD; use that value in non-subrange cache-conditional
588        requests (using If-Modified-Since).</t>
589
590     <t>If only a Last-Modified value has been provided by an HTTP/1.0
591        origin server, &MAY; use that value in subrange cache-conditional
592        requests (using If-Unmodified-Since:). The user agent &SHOULD;
593        provide a way to disable this, in case of difficulty.</t>
594
595     <t>If both an entity tag and a Last-Modified value have been
596        provided by the origin server, &SHOULD; use both validators in
597        cache-conditional requests. This allows both HTTP/1.0 and
598        HTTP/1.1 caches to respond appropriately.</t>
599  </list>
600</t>
601<t>
602   An HTTP/1.1 origin server, upon receiving a conditional request that
603   includes both a Last-Modified date (e.g., in an If-Modified-Since or
604   If-Unmodified-Since header field) and one or more entity tags (e.g.,
605   in an If-Match, If-None-Match, or If-Range header field) as cache
606   validators, &MUST-NOT; return a response status of 304 (Not Modified)
607   unless doing so is consistent with all of the conditional header
608   fields in the request.
609</t>
610<t>
611   An HTTP/1.1 caching proxy, upon receiving a conditional request that
612   includes both a Last-Modified date and one or more entity tags as
613   cache validators, &MUST-NOT; return a locally cached response to the
614   client unless that cached response is consistent with all of the
615   conditional header fields in the request.
616  <list><t>
617      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
618      servers and clients should transmit as much non-redundant
619      information as is available in their responses and requests.
620      HTTP/1.1 systems receiving this information will make the most
621      conservative assumptions about the validators they receive.
622  </t><t>
623      HTTP/1.0 clients and caches will ignore entity tags. Generally,
624      last-modified values received or used by these systems will
625      support transparent and efficient caching, and so HTTP/1.1 origin
626      servers should provide Last-Modified values. In those rare cases
627      where the use of a Last-Modified value as a validator by an
628      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
629      origin servers should not provide one.
630  </t></list>
631</t>
632</section>
633
634<section title="Header Field Definitions" anchor="header.fields">
635<t>
636   This section defines the syntax and semantics of HTTP/1.1 header fields
637   related to conditional requests.
638</t>
639<t>
640   For entity-header fields, both sender and recipient refer to either the
641   client or the server, depending on who sends and who receives the entity.
642</t>
643
644<section title="ETag" anchor="header.etag">
645  <iref primary="true" item="ETag header" x:for-anchor=""/>
646  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
647  <x:anchor-alias value="ETag"/>
648<t>
649   The ETag response-header field provides the current value of the
650   entity tag for the requested variant. The headers used with entity
651   tags are described in Sections <xref target="header.if-match" format="counter"/>
652   and <xref target="header.if-none-match" format="counter"/> of this document,
653   and in &header-if-range;. The entity tag
654   &MAY; be used for comparison with other entities from the same resource
655   (see <xref target="weak.and.strong.validators"/>).
656</t>
657<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
658  <x:ref>ETag</x:ref> = "ETag" ":" <x:ref>entity-tag</x:ref>
659</artwork></figure>
660<figure><preamble>
661   Examples:
662</preamble>
663<artwork type="example">
664   ETag: "xyzzy"
665   ETag: W/"xyzzy"
666   ETag: ""
667</artwork></figure>
668<t>
669   The ETag response-header field value, an entity tag, provides for an
670   "opaque" cache validator. This might allow more reliable validation
671   in situations where it is inconvenient to store modification dates,
672   where the one-second resolution of HTTP date values is not
673   sufficient, or where the origin server wishes to avoid certain
674   paradoxes that might arise from the use of modification dates.
675</t>
676<t>
677   The principle behind entity tags is that only the service author
678   knows the semantics of a resource well enough to select an
679   appropriate cache validation mechanism, and the specification of any
680   validator comparison function more complex than byte-equality would
681   open up a can of worms. Thus, comparisons of any other headers
682   (except Last-Modified, for compatibility with HTTP/1.0) are never
683   used for purposes of validating a cache entry.
684</t>
685</section>
686
687<section title="If-Match" anchor="header.if-match">
688  <iref primary="true" item="If-Match header" x:for-anchor=""/>
689  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
690  <x:anchor-alias value="If-Match"/>
691<t>
692   The If-Match request-header field is used with a method to make it
693   conditional. A client that has one or more entities previously
694   obtained from the resource can verify that one of those entities is
695   current by including a list of their associated entity tags in the
696   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
697   purpose of this feature is to allow efficient updates of cached
698   information with a minimum amount of transaction overhead. It is also
699   used, on updating requests, to prevent inadvertent modification of
700   the wrong version of a resource. As a special case, the value "*"
701   matches any current entity of the resource.
702</t>
703<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
704  <x:ref>If-Match</x:ref> = "If-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
705</artwork></figure>
706<t>
707   If any of the entity tags match the entity tag of the entity that
708   would have been returned in the response to a similar GET request
709   (without the If-Match header) on that resource, or if "*" is given
710   and any current entity exists for that resource, then the server &MAY;
711   perform the requested method as if the If-Match header field did not
712   exist.
713</t>
714<t>
715   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
716   to compare the entity tags in If-Match.
717</t>
718<t>
719   If none of the entity tags match, or if "*" is given and no current
720   entity exists, the server &MUST-NOT; perform the requested method, and
721   &MUST; return a 412 (Precondition Failed) response. This behavior is
722   most useful when the client wants to prevent an updating method, such
723   as PUT, from modifying a resource that has changed since the client
724   last retrieved it.
725</t>
726<t>
727   If the request would, without the If-Match header field, result in
728   anything other than a 2xx or 412 status, then the If-Match header
729   &MUST; be ignored.
730</t>
731<t>
732   The meaning of "If-Match: *" is that the method &SHOULD; be performed
733   if the representation selected by the origin server (or by a cache,
734   possibly using the Vary mechanism, see &header-vary;) exists, and
735   &MUST-NOT; be performed if the representation does not exist.
736</t>
737<t>
738   A request intended to update a resource (e.g., a PUT) &MAY; include an
739   If-Match header field to signal that the request method &MUST-NOT; be
740   applied if the entity corresponding to the If-Match value (a single
741   entity tag) is no longer a representation of that resource. This
742   allows the user to indicate that they do not wish the request to be
743   successful if the resource has been changed without their knowledge.
744   Examples:
745</t>
746<figure><artwork type="example">
747    If-Match: "xyzzy"
748    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
749    If-Match: *
750</artwork></figure>
751<t>
752   The result of a request having both an If-Match header field and
753   either an If-None-Match or an If-Modified-Since header fields is
754   undefined by this specification.
755</t>
756</section>
757
758<section title="If-Modified-Since" anchor="header.if-modified-since">
759  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
760  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
761  <x:anchor-alias value="If-Modified-Since"/>
762<t>
763   The If-Modified-Since request-header field is used with a method to
764   make it conditional: if the requested variant has not been modified
765   since the time specified in this field, an entity will not be
766   returned from the server; instead, a 304 (Not Modified) response will
767   be returned without any message-body.
768</t>
769<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
770  <x:ref>If-Modified-Since</x:ref> = "If-Modified-Since" ":" <x:ref>HTTP-date</x:ref>
771</artwork></figure>
772<t>
773   An example of the field is:
774</t>
775<figure><artwork type="example">
776    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
777</artwork></figure>
778<t>
779   A GET method with an If-Modified-Since header and no Range header
780   requests that the identified entity be transferred only if it has
781   been modified since the date given by the If-Modified-Since header.
782   The algorithm for determining this includes the following cases:
783  <list style="numbers">
784      <t>If the request would normally result in anything other than a
785         200 (OK) status, or if the passed If-Modified-Since date is
786         invalid, the response is exactly the same as for a normal GET.
787         A date which is later than the server's current time is
788         invalid.</t>
789
790      <t>If the variant has been modified since the If-Modified-Since
791         date, the response is exactly the same as for a normal GET.</t>
792
793      <t>If the variant has not been modified since a valid If-Modified-Since
794         date, the server &SHOULD; return a 304 (Not
795         Modified) response.</t>
796  </list>
797</t>
798<t>
799   The purpose of this feature is to allow efficient updates of cached
800   information with a minimum amount of transaction overhead.
801  <list><t>
802      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
803      see &header-range; for full details.
804    </t><t>
805      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
806      clock might not be synchronized with the client.
807    </t><t>
808      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
809      servers will use an exact date comparison function, rather than a
810      less-than function, for deciding whether to send a 304 (Not
811      Modified) response. To get best results when sending an If-Modified-Since
812      header field for cache validation, clients are
813      advised to use the exact date string received in a previous Last-Modified
814      header field whenever possible.
815    </t><t>
816      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
817      header instead of a date taken from the Last-Modified header for
818      the same request, the client should be aware of the fact that this
819      date is interpreted in the server's understanding of time. The
820      client should consider unsynchronized clocks and rounding problems
821      due to the different encodings of time between the client and
822      server. This includes the possibility of race conditions if the
823      document has changed between the time it was first requested and
824      the If-Modified-Since date of a subsequent request, and the
825      possibility of clock-skew-related problems if the If-Modified-Since
826      date is derived from the client's clock without correction
827      to the server's clock. Corrections for different time bases
828      between client and server are at best approximate due to network
829      latency.
830    </t>
831  </list>
832</t>
833<t>
834   The result of a request having both an If-Modified-Since header field
835   and either an If-Match or an If-Unmodified-Since header fields is
836   undefined by this specification.
837</t>
838</section>
839
840<section title="If-None-Match" anchor="header.if-none-match">
841  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
842  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
843  <x:anchor-alias value="If-None-Match"/>
844<t>
845   The If-None-Match request-header field is used with a method to make
846   it conditional. A client that has one or more entities previously
847   obtained from the resource can verify that none of those entities is
848   current by including a list of their associated entity tags in the
849   If-None-Match header field. The purpose of this feature is to allow
850   efficient updates of cached information with a minimum amount of
851   transaction overhead. It is also used to prevent a method (e.g. PUT)
852   from inadvertently modifying an existing resource when the client
853   believes that the resource does not exist.
854</t>
855<t>
856   As a special case, the value "*" matches any current entity of the
857   resource.
858</t>
859<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
860  <x:ref>If-None-Match</x:ref> = "If-None-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
861</artwork></figure>
862<t>
863   If any of the entity tags match the entity tag of the entity that
864   would have been returned in the response to a similar GET request
865   (without the If-None-Match header) on that resource, or if "*" is
866   given and any current entity exists for that resource, then the
867   server &MUST-NOT; perform the requested method, unless required to do
868   so because the resource's modification date fails to match that
869   supplied in an If-Modified-Since header field in the request.
870   Instead, if the request method was GET or HEAD, the server &SHOULD;
871   respond with a 304 (Not Modified) response, including the cache-related
872   header fields (particularly ETag) of one of the entities that
873   matched. For all other request methods, the server &MUST; respond with
874   a status of 412 (Precondition Failed).
875</t>
876<t>
877   See <xref target="weak.and.strong.validators"/> for rules on how to
878   determine if two entity tags match.
879</t>
880<t>
881   If none of the entity tags match, then the server &MAY; perform the
882   requested method as if the If-None-Match header field did not exist,
883   but &MUST; also ignore any If-Modified-Since header field(s) in the
884   request. That is, if no entity tags match, then the server &MUST-NOT;
885   return a 304 (Not Modified) response.
886</t>
887<t>
888   If the request would, without the If-None-Match header field, result
889   in anything other than a 2xx or 304 status, then the If-None-Match
890   header &MUST; be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
891   server behavior when both If-Modified-Since and If-None-Match appear
892   in the same request.)
893</t>
894<t>
895   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
896   performed if the representation selected by the origin server (or by
897   a cache, possibly using the Vary mechanism, see &header-vary;)
898   exists, and &SHOULD; be performed if the representation does not exist.
899   This feature is intended to be useful in preventing races between PUT
900   operations.
901</t>
902<t>
903   Examples:
904</t>
905<figure><artwork type="example">
906    If-None-Match: "xyzzy"
907    If-None-Match: W/"xyzzy"
908    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
909    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
910    If-None-Match: *
911</artwork></figure>
912<t>
913   The result of a request having both an If-None-Match header field and
914   either an If-Match or an If-Unmodified-Since header fields is
915   undefined by this specification.
916</t>
917</section>
918
919<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
920  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
921  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
922  <x:anchor-alias value="If-Unmodified-Since"/>
923<t>
924   The If-Unmodified-Since request-header field is used with a method to
925   make it conditional. If the requested resource has not been modified
926   since the time specified in this field, the server &SHOULD; perform the
927   requested operation as if the If-Unmodified-Since header were not
928   present.
929</t>
930<t>
931   If the requested variant has been modified since the specified time,
932   the server &MUST-NOT; perform the requested operation, and &MUST; return
933   a 412 (Precondition Failed).
934</t>
935<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
936  <x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since" ":" <x:ref>HTTP-date</x:ref>
937</artwork></figure>
938<t>
939   An example of the field is:
940</t>
941<figure><artwork type="example">
942    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
943</artwork></figure>
944<t>
945   If the request normally (i.e., without the If-Unmodified-Since
946   header) would result in anything other than a 2xx or 412 status, the
947   If-Unmodified-Since header &SHOULD; be ignored.
948</t>
949<t>
950   If the specified date is invalid, the header is ignored.
951</t>
952<t>
953   The result of a request having both an If-Unmodified-Since header
954   field and either an If-None-Match or an If-Modified-Since header
955   fields is undefined by this specification.
956</t>
957</section>
958
959<section title="Last-Modified" anchor="header.last-modified">
960  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
961  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
962  <x:anchor-alias value="Last-Modified"/>
963<t>
964   The Last-Modified entity-header field indicates the date and time at
965   which the origin server believes the variant was last modified.
966</t>
967<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
968  <x:ref>Last-Modified</x:ref>  = "Last-Modified" ":" <x:ref>HTTP-date</x:ref>
969</artwork></figure>
970<t>
971   An example of its use is
972</t>
973<figure><artwork type="example">
974    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
975</artwork></figure>
976<t>
977   The exact meaning of this header field depends on the implementation
978   of the origin server and the nature of the original resource. For
979   files, it may be just the file system last-modified time. For
980   entities with dynamically included parts, it may be the most recent
981   of the set of last-modify times for its component parts. For database
982   gateways, it may be the last-update time stamp of the record. For
983   virtual objects, it may be the last time the internal state changed.
984</t>
985<t>
986   An origin server &MUST-NOT; send a Last-Modified date which is later
987   than the server's time of message origination. In such cases, where
988   the resource's last modification would indicate some time in the
989   future, the server &MUST; replace that date with the message
990   origination date.
991</t>
992<t>
993   An origin server &SHOULD; obtain the Last-Modified value of the entity
994   as close as possible to the time that it generates the Date value of
995   its response. This allows a recipient to make an accurate assessment
996   of the entity's modification time, especially if the entity changes
997   near the time that the response is generated.
998</t>
999<t>
1000   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1001</t>
1002<t>
1003   The Last-Modified entity-header field value is often used as a cache
1004   validator. In simple terms, a cache entry is considered to be valid
1005   if the entity has not been modified since the Last-Modified value.
1006</t>
1007</section>
1008
1009</section>
1010
1011<section title="IANA Considerations" anchor="IANA.considerations">
1012<section title="Message Header Registration" anchor="message.header.registration">
1013<t>
1014   The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
1015   with the permanent registrations below (see <xref target="RFC3864"/>):
1016</t>
1017<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1018<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1019   <ttcol>Header Field Name</ttcol>
1020   <ttcol>Protocol</ttcol>
1021   <ttcol>Status</ttcol>
1022   <ttcol>Reference</ttcol>
1023
1024   <c>ETag</c>
1025   <c>http</c>
1026   <c>standard</c>
1027   <c>
1028      <xref target="header.etag"/>
1029   </c>
1030   <c>If-Match</c>
1031   <c>http</c>
1032   <c>standard</c>
1033   <c>
1034      <xref target="header.if-match"/>
1035   </c>
1036   <c>If-Modified-Since</c>
1037   <c>http</c>
1038   <c>standard</c>
1039   <c>
1040      <xref target="header.if-modified-since"/>
1041   </c>
1042   <c>If-None-Match</c>
1043   <c>http</c>
1044   <c>standard</c>
1045   <c>
1046      <xref target="header.if-none-match"/>
1047   </c>
1048   <c>If-Unmodified-Since</c>
1049   <c>http</c>
1050   <c>standard</c>
1051   <c>
1052      <xref target="header.if-unmodified-since"/>
1053   </c>
1054   <c>Last-Modified</c>
1055   <c>http</c>
1056   <c>standard</c>
1057   <c>
1058      <xref target="header.last-modified"/>
1059   </c>
1060</texttable>
1061<!--(END)-->
1062<t>
1063   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1064</t>
1065</section>
1066</section>
1067
1068<section title="Security Considerations" anchor="security.considerations">
1069<t>
1070   No additional security considerations have been identified beyond
1071   those applicable to HTTP in general &messaging;.
1072</t>
1073</section>
1074
1075<section title="Acknowledgments" anchor="ack">
1076</section>
1077</middle>
1078<back>
1079
1080<references title="Normative References">
1081
1082<reference anchor="Part1">
1083  <front>
1084    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1085    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1086      <organization abbrev="Day Software">Day Software</organization>
1087      <address><email>fielding@gbiv.com</email></address>
1088    </author>
1089    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1090      <organization>One Laptop per Child</organization>
1091      <address><email>jg@laptop.org</email></address>
1092    </author>
1093    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1094      <organization abbrev="HP">Hewlett-Packard Company</organization>
1095      <address><email>JeffMogul@acm.org</email></address>
1096    </author>
1097    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1098      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1099      <address><email>henrikn@microsoft.com</email></address>
1100    </author>
1101    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1102      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1103      <address><email>LMM@acm.org</email></address>
1104    </author>
1105    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1106      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1107      <address><email>paulle@microsoft.com</email></address>
1108    </author>
1109    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1110      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1111      <address><email>timbl@w3.org</email></address>
1112    </author>
1113    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1114      <organization abbrev="W3C">World Wide Web Consortium</organization>
1115      <address><email>ylafon@w3.org</email></address>
1116    </author>
1117    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1118      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1119      <address><email>julian.reschke@greenbytes.de</email></address>
1120    </author>
1121    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1122  </front>
1123  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1124  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1125</reference>
1126
1127<reference anchor="Part5">
1128  <front>
1129    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</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-p5-range-&ID-VERSION;"/>
1169  <x:source href="p5-range.xml" basename="p5-range"/>
1170</reference>
1171
1172<reference anchor="Part6">
1173  <front>
1174    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</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-p6-cache-&ID-VERSION;"/>
1214  <x:source href="p6-cache.xml" basename="p6-cache"/>
1215</reference>
1216
1217<reference anchor="RFC2119">
1218  <front>
1219    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1220    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1221      <organization>Harvard University</organization>
1222      <address><email>sob@harvard.edu</email></address>
1223    </author>
1224    <date month="March" year="1997"/>
1225  </front>
1226  <seriesInfo name="BCP" value="14"/>
1227  <seriesInfo name="RFC" value="2119"/>
1228</reference>
1229
1230</references>
1231
1232<references title="Informative References">
1233
1234<reference anchor="RFC2068">
1235  <front>
1236    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1237    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1238      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1239      <address><email>fielding@ics.uci.edu</email></address>
1240    </author>
1241    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1242      <organization>MIT Laboratory for Computer Science</organization>
1243      <address><email>jg@w3.org</email></address>
1244    </author>
1245    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1246      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1247      <address><email>mogul@wrl.dec.com</email></address>
1248    </author>
1249    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1250      <organization>MIT Laboratory for Computer Science</organization>
1251      <address><email>frystyk@w3.org</email></address>
1252    </author>
1253    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1254      <organization>MIT Laboratory for Computer Science</organization>
1255      <address><email>timbl@w3.org</email></address>
1256    </author>
1257    <date month="January" year="1997"/>
1258  </front>
1259  <seriesInfo name="RFC" value="2068"/>
1260</reference>
1261
1262<reference anchor="RFC2616">
1263  <front>
1264    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1265    <author initials="R." surname="Fielding" fullname="R. Fielding">
1266      <organization>University of California, Irvine</organization>
1267      <address><email>fielding@ics.uci.edu</email></address>
1268    </author>
1269    <author initials="J." surname="Gettys" fullname="J. Gettys">
1270      <organization>W3C</organization>
1271      <address><email>jg@w3.org</email></address>
1272    </author>
1273    <author initials="J." surname="Mogul" fullname="J. Mogul">
1274      <organization>Compaq Computer Corporation</organization>
1275      <address><email>mogul@wrl.dec.com</email></address>
1276    </author>
1277    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1278      <organization>MIT Laboratory for Computer Science</organization>
1279      <address><email>frystyk@w3.org</email></address>
1280    </author>
1281    <author initials="L." surname="Masinter" fullname="L. Masinter">
1282      <organization>Xerox Corporation</organization>
1283      <address><email>masinter@parc.xerox.com</email></address>
1284    </author>
1285    <author initials="P." surname="Leach" fullname="P. Leach">
1286      <organization>Microsoft Corporation</organization>
1287      <address><email>paulle@microsoft.com</email></address>
1288    </author>
1289    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1290      <organization>W3C</organization>
1291      <address><email>timbl@w3.org</email></address>
1292    </author>
1293    <date month="June" year="1999"/>
1294  </front>
1295  <seriesInfo name="RFC" value="2616"/>
1296</reference>
1297
1298<reference anchor='RFC3864'>
1299  <front>
1300    <title>Registration Procedures for Message Header Fields</title>
1301    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1302      <organization>Nine by Nine</organization>
1303      <address><email>GK-IETF@ninebynine.org</email></address>
1304    </author>
1305    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1306      <organization>BEA Systems</organization>
1307      <address><email>mnot@pobox.com</email></address>
1308    </author>
1309    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1310      <organization>HP Labs</organization>
1311      <address><email>JeffMogul@acm.org</email></address>
1312    </author>
1313    <date year='2004' month='September' />
1314  </front>
1315  <seriesInfo name='BCP' value='90' />
1316  <seriesInfo name='RFC' value='3864' />
1317</reference>
1318
1319</references>
1320
1321<section title="Compatibility with Previous Versions" anchor="compatibility">
1322
1323<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1324<t>
1325  Allow weak entity tags in all requests except range requests (Sections
1326  <xref target="weak.and.strong.validators" format="counter"/> and
1327  <xref target="header.if-none-match" format="counter"/>).
1328</t>
1329</section>
1330
1331</section>
1332
1333<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1334
1335<section title="Since RFC2616">
1336<t>
1337  Extracted relevant partitions from <xref target="RFC2616"/>.
1338</t>
1339</section>
1340
1341<section title="Since draft-ietf-httpbis-p4-conditional-00">
1342<t>
1343  Closed issues:
1344  <list style="symbols"> 
1345    <t>
1346      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1347      "Normative and Informative references"
1348    </t>
1349  </list>
1350</t>
1351<t>
1352  Other changes:
1353  <list style="symbols"> 
1354    <t>
1355      Move definitions of 304 and 412 condition codes from Part2.
1356    </t>
1357  </list>
1358</t>
1359</section>
1360
1361<section title="Since draft-ietf-httpbis-p4-conditional-01">
1362<t>
1363  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1364  <list style="symbols"> 
1365    <t>
1366      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1367    </t>
1368  </list>
1369</t>
1370</section>
1371
1372<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1373<t>
1374  Closed issues:
1375  <list style="symbols"> 
1376    <t>
1377      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1378      "Weak ETags on non-GET requests"
1379    </t>
1380  </list>
1381</t>
1382<t>
1383  Ongoing work on IANA Message Header Registration (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1384  <list style="symbols"> 
1385    <t>
1386      Reference RFC 3984, and update header registrations for headers defined
1387      in this document.
1388    </t>
1389  </list>
1390</t>
1391</section>
1392
1393<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1394<t>
1395  Closed issues:
1396  <list style="symbols"> 
1397    <t>
1398      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1399      "Examples for ETag matching"
1400    </t>
1401  </list>
1402</t>
1403</section>
1404
1405</section>
1406
1407</back>
1408</rfc>
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