source: draft-ietf-httpbis/latest/p4-conditional.xml @ 138

Last change on this file since 138 was 137, checked in by julian.reschke@…, 15 years ago

Use consistent status reason phrases.

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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=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "January">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY messaging                  "<xref target='Part1' xmlns:x=''/>">
18  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x=''/>">
19  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x=''/>">
20  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x=''/>">
21  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x=''/>">
23<?rfc toc="yes" ?>
24<?rfc symrefs="yes" ?>
25<?rfc sortrefs="yes" ?>
26<?rfc compact="yes"?>
27<?rfc subcompact="no" ?>
28<?rfc linkmailto="no" ?>
29<?rfc editing="no" ?>
30<?rfc-ext allow-markup-in-artwork="yes" ?>
31<?rfc-ext include-references-in-index="yes" ?>
32<rfc obsoletes="2068, 2616" category="std"
33     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
34     xmlns:x='' xmlns:ed="">
37  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
39  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
40    <organization abbrev="Day Software">Day Software</organization>
41    <address>
42      <postal>
43        <street>23 Corporate Plaza DR, Suite 280</street>
44        <city>Newport Beach</city>
45        <region>CA</region>
46        <code>92660</code>
47        <country>USA</country>
48      </postal>
49      <phone>+1-949-706-5300</phone>
50      <facsimile>+1-949-706-5305</facsimile>
51      <email></email>
52      <uri></uri>
53    </address>
54  </author>
56  <author initials="J." surname="Gettys" fullname="Jim Gettys">
57    <organization>One Laptop per Child</organization>
58    <address>
59      <postal>
60        <street>21 Oak Knoll Road</street>
61        <city>Carlisle</city>
62        <region>MA</region>
63        <code>01741</code>
64        <country>USA</country>
65      </postal>
66      <email></email>
67      <uri></uri>
68    </address>
69  </author>
71  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
72    <organization abbrev="HP">Hewlett-Packard Company</organization>
73    <address>
74      <postal>
75        <street>HP Labs, Large Scale Systems Group</street>
76        <street>1501 Page Mill Road, MS 1177</street>
77        <city>Palo Alto</city>
78        <region>CA</region>
79        <code>94304</code>
80        <country>USA</country>
81      </postal>
82      <email></email>
83    </address>
84  </author>
86  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
87    <organization abbrev="Microsoft">Microsoft Corporation</organization>
88    <address>
89      <postal>
90        <street>1 Microsoft Way</street>
91        <city>Redmond</city>
92        <region>WA</region>
93        <code>98052</code>
94        <country>USA</country>
95      </postal>
96      <email></email>
97    </address>
98  </author>
100  <author initials="L." surname="Masinter" fullname="Larry Masinter">
101    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
102    <address>
103      <postal>
104        <street>345 Park Ave</street>
105        <city>San Jose</city>
106        <region>CA</region>
107        <code>95110</code>
108        <country>USA</country>
109      </postal>
110      <email></email>
111      <uri></uri>
112    </address>
113  </author>
115  <author initials="P." surname="Leach" fullname="Paul J. Leach">
116    <organization abbrev="Microsoft">Microsoft Corporation</organization>
117    <address>
118      <postal>
119        <street>1 Microsoft Way</street>
120        <city>Redmond</city>
121        <region>WA</region>
122        <code>98052</code>
123      </postal>
124      <email></email>
125    </address>
126  </author>
128  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
129    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
130    <address>
131      <postal>
132        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
133        <street>The Stata Center, Building 32</street>
134        <street>32 Vassar Street</street>
135        <city>Cambridge</city>
136        <region>MA</region>
137        <code>02139</code>
138        <country>USA</country>
139      </postal>
140      <email></email>
141      <uri></uri>
142    </address>
143  </author>
145  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
146    <organization abbrev="W3C">World Wide Web Consortium</organization>
147    <address>
148      <postal>
149        <street>W3C / ERCIM</street>
150        <street>2004, rte des Lucioles</street>
151        <city>Sophia-Antipolis</city>
152        <region>AM</region>
153        <code>06902</code>
154        <country>France</country>
155      </postal>
156      <email></email>
157      <uri></uri>
158    </address>
159  </author>
161  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
162    <organization abbrev="greenbytes">greenbytes GmbH</organization>
163    <address>
164      <postal>
165        <street>Hafenweg 16</street>
166        <city>Muenster</city><region>NW</region><code>48155</code>
167        <country>Germany</country>
168      </postal>
169      <phone>+49 251 2807760</phone>   
170      <facsimile>+49 251 2807761</facsimile>   
171      <email></email>       
172      <uri></uri>     
173    </address>
174  </author>
176  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
180   The Hypertext Transfer Protocol (HTTP) is an application-level
181   protocol for distributed, collaborative, hypermedia information
182   systems. HTTP has been in use by the World Wide Web global information
183   initiative since 1990. This document is Part 4 of the seven-part specification
184   that defines the protocol referred to as "HTTP/1.1" and, taken together,
185   obsoletes RFC 2616.  Part 4 defines request header fields for
186   indicating conditional requests and the rules for constructing responses
187   to those requests.
191<note title="Editorial Note (To be removed by RFC Editor)">
192  <t>
193    Discussion of this draft should take place on the HTTPBIS working group
194    mailing list ( The current issues list is
195    at <eref target=""/>
196    and related documents (including fancy diffs) can be found at
197    <eref target=""/>.
198  </t>
202<section title="Introduction" anchor="introduction">
204   This document will define aspects of HTTP related to conditional
205   request messages based on time stamps and entity-tags.  Right now it
206   only includes the extracted relevant sections of <xref target="RFC2616">RFC 2616</xref>
207   without edit.
210<section title="Requirements" anchor="intro.requirements">
212   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
213   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
214   document are to be interpreted as described in <xref target="RFC2119"/>.
217   An implementation is not compliant if it fails to satisfy one or more
218   of the &MUST; or &REQUIRED; level requirements for the protocols it
219   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
220   level and all the &SHOULD; level requirements for its protocols is said
221   to be "unconditionally compliant"; one that satisfies all the &MUST;
222   level requirements but not all the &SHOULD; level requirements for its
223   protocols is said to be "conditionally compliant."
228<section title="Entity Tags" anchor="entity.tags">
230   Entity tags are used for comparing two or more entities from the same
231   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
232   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
233   If-Range (&header-if-range;) header fields. The definition of how they
234   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
235   entity tag consists of an opaque quoted string, possibly prefixed by
236   a weakness indicator.
238<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"/>
239  entity-tag = [ weak ] opaque-tag
240  weak       = "W/"
241  opaque-tag = quoted-string
244   A "strong entity tag" &MAY; be shared by two entities of a resource
245   only if they are equivalent by octet equality.
248   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
249   two entities of a resource only if the entities are equivalent and
250   could be substituted for each other with no significant change in
251   semantics. A weak entity tag can only be used for weak comparison.
254   An entity tag &MUST; be unique across all versions of all entities
255   associated with a particular resource. A given entity tag value &MAY;
256   be used for entities obtained by requests on different URIs. The use
257   of the same entity tag value in conjunction with entities obtained by
258   requests on different URIs does not imply the equivalence of those
259   entities.
263<section title="Status Code Definitions">
264<section title="304 Not Modified" anchor="status.304">
265  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
266  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
268   If the client has performed a conditional GET request and access is
269   allowed, but the document has not been modified, the server &SHOULD;
270   respond with this status code. The 304 response &MUST-NOT; contain a
271   message-body, and thus is always terminated by the first empty line
272   after the header fields.
275   The response &MUST; include the following header fields:
276  <list style="symbols">
277    <t>Date, unless its omission is required by &clockless;</t>
278  </list>
281   If a clockless origin server obeys these rules, and proxies and
282   clients add their own Date to any response received without one (as
283   already specified by <xref target="RFC2068" x:sec="14.19" x:fmt=","/>), caches will operate
284   correctly.
285  <list style="symbols">
286    <t>ETag and/or Content-Location, if the header would have been sent
287        in a 200 response to the same request</t>
288    <t>Expires, Cache-Control, and/or Vary, if the field-value might
289        differ from that sent in any previous response for the same
290        variant</t>
291  </list>
294   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
295   the response &SHOULD-NOT;  include other entity-headers.
296   Otherwise (i.e., the conditional GET used a weak validator), the
297   response &MUST-NOT; include other entity-headers; this prevents
298   inconsistencies between cached entity-bodies and updated headers.
301   If a 304 response indicates an entity not currently cached, then the
302   cache &MUST; disregard the response and repeat the request without the
303   conditional.
306   If a cache uses a received 304 response to update a cache entry, the
307   cache &MUST; update the entry to reflect any new field values given in
308   the response.
312<section title="412 Precondition Failed" anchor="status.412">
313  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
314  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
316   The precondition given in one or more of the request-header fields
317   evaluated to false when it was tested on the server. This response
318   code allows the client to place preconditions on the current resource
319   metainformation (header field data) and thus prevent the requested
320   method from being applied to a resource other than the one intended.
325<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
327   Since both origin servers and caches will compare two validators to
328   decide if they represent the same or different entities, one normally
329   would expect that if the entity (the entity-body or any entity-headers)
330   changes in any way, then the associated validator would
331   change as well. If this is true, then we call this validator a
332   "strong validator."
335   However, there might be cases when a server prefers to change the
336   validator only on semantically significant changes, and not when
337   insignificant aspects of the entity change. A validator that does not
338   always change when the resource changes is a "weak validator."
341   Entity tags are normally "strong validators," but the protocol
342   provides a mechanism to tag an entity tag as "weak." One can think of
343   a strong validator as one that changes whenever the bits of an entity
344   changes, while a weak value changes whenever the meaning of an entity
345   changes. Alternatively, one can think of a strong validator as part
346   of an identifier for a specific entity, while a weak validator is
347   part of an identifier for a set of semantically equivalent entities.
348  <list><t>
349      <x:h>Note:</x:h> One example of a strong validator is an integer that is
350      incremented in stable storage every time an entity is changed.
351    </t><t>
352      An entity's modification time, if represented with one-second
353      resolution, could be a weak validator, since it is possible that
354      the resource might be modified twice during a single second.
355    </t><t>
356      Support for weak validators is optional. However, weak validators
357      allow for more efficient caching of equivalent objects; for
358      example, a hit counter on a site is probably good enough if it is
359      updated every few days or weeks, and any value during that period
360      is likely "good enough" to be equivalent.
361    </t></list>
364   A "use" of a validator is either when a client generates a request
365   and includes the validator in a validating header field, or when a
366   server compares two validators.
369   Strong validators are usable in any context. Weak validators are only
370   usable in contexts that do not depend on exact equality of an entity.
371   For example, either kind is usable for a conditional GET of a full
372   entity. However, only a strong validator is usable for a sub-range
373   retrieval, since otherwise the client might end up with an internally
374   inconsistent entity.
377   Clients &MAY; issue simple (non-subrange) GET requests with either weak
378   validators or strong validators. Clients &MUST-NOT; use weak validators
379   in other forms of request.
382   The only function that the HTTP/1.1 protocol defines on validators is
383   comparison. There are two validator comparison functions, depending
384   on whether the comparison context allows the use of weak validators
385   or not:
386  <list style="symbols">
387     <t>The strong comparison function: in order to be considered equal,
388        both validators &MUST; be identical in every way, and both &MUST-NOT;
389        be weak.</t>
390     <t>The weak comparison function: in order to be considered equal,
391        both validators &MUST; be identical in every way, but either or
392        both of them &MAY; be tagged as "weak" without affecting the
393        result.</t>
394  </list>
397   An entity tag is strong unless it is explicitly tagged as weak.
398   <xref target="entity.tags"/> gives the syntax for entity tags.
401   A Last-Modified time, when used as a validator in a request, is
402   implicitly weak unless it is possible to deduce that it is strong,
403   using the following rules:
404  <list style="symbols">
405     <t>The validator is being compared by an origin server to the
406        actual current validator for the entity and,</t>
407     <t>That origin server reliably knows that the associated entity did
408        not change twice during the second covered by the presented
409        validator.</t>
410  </list>
413   or
414  <list style="symbols">
415     <t>The validator is about to be used by a client in an If-Modified-Since
416        or If-Unmodified-Since header, because the client
417        has a cache entry for the associated entity, and</t>
418     <t>That cache entry includes a Date value, which gives the time
419        when the origin server sent the original response, and</t>
420     <t>The presented Last-Modified time is at least 60 seconds before
421        the Date value.</t>
422  </list>
425   or
426  <list style="symbols">
427     <t>The validator is being compared by an intermediate cache to the
428        validator stored in its cache entry for the entity, and</t>
429     <t>That cache entry includes a Date value, which gives the time
430        when the origin server sent the original response, and</t>
431     <t>The presented Last-Modified time is at least 60 seconds before
432        the Date value.</t>
433  </list>
436   This method relies on the fact that if two different responses were
437   sent by the origin server during the same second, but both had the
438   same Last-Modified time, then at least one of those responses would
439   have a Date value equal to its Last-Modified time. The arbitrary 60-second
440   limit guards against the possibility that the Date and Last-Modified
441   values are generated from different clocks, or at somewhat
442   different times during the preparation of the response. An
443   implementation &MAY; use a value larger than 60 seconds, if it is
444   believed that 60 seconds is too short.
447   If a client wishes to perform a sub-range retrieval on a value for
448   which it has only a Last-Modified time and no opaque validator, it
449   &MAY; do this only if the Last-Modified time is strong in the sense
450   described here.
453   A cache or origin server receiving a conditional request, other than
454   a full-body GET request, &MUST; use the strong comparison function to
455   evaluate the condition.
458   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
459   retrievals on values that have been obtained from HTTP/1.0
460   servers.
464<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="">
466   We adopt a set of rules and recommendations for origin servers,
467   clients, and caches regarding when various validator types ought to
468   be used, and for what purposes.
471   HTTP/1.1 origin servers:
472  <list style="symbols">
473     <t>&SHOULD; send an entity tag validator unless it is not feasible to
474        generate one.</t>
476     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
477        performance considerations support the use of weak entity tags,
478        or if it is unfeasible to send a strong entity tag.</t>
480     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
481        unless the risk of a breakdown in semantic transparency that
482        could result from using this date in an If-Modified-Since header
483        would lead to serious problems.</t>
484  </list>
487   In other words, the preferred behavior for an HTTP/1.1 origin server
488   is to send both a strong entity tag and a Last-Modified value.
491   In order to be legal, a strong entity tag &MUST; change whenever the
492   associated entity value changes in any way. A weak entity tag &SHOULD;
493   change whenever the associated entity changes in a semantically
494   significant way.
495  <list><t>
496      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
497      origin server must avoid reusing a specific strong entity tag
498      value for two different entities, or reusing a specific weak
499      entity tag value for two semantically different entities. Cache
500      entries might persist for arbitrarily long periods, regardless of
501      expiration times, so it might be inappropriate to expect that a
502      cache will never again attempt to validate an entry using a
503      validator that it obtained at some point in the past.
504  </t></list>
507   HTTP/1.1 clients:
508  <list style="symbols">
509     <t>If an entity tag has been provided by the origin server, &MUST;
510        use that entity tag in any cache-conditional request (using If-Match
511        or If-None-Match).</t>
513     <t>If only a Last-Modified value has been provided by the origin
514        server, &SHOULD; use that value in non-subrange cache-conditional
515        requests (using If-Modified-Since).</t>
517     <t>If only a Last-Modified value has been provided by an HTTP/1.0
518        origin server, &MAY; use that value in subrange cache-conditional
519        requests (using If-Unmodified-Since:). The user agent &SHOULD;
520        provide a way to disable this, in case of difficulty.</t>
522     <t>If both an entity tag and a Last-Modified value have been
523        provided by the origin server, &SHOULD; use both validators in
524        cache-conditional requests. This allows both HTTP/1.0 and
525        HTTP/1.1 caches to respond appropriately.</t>
526  </list>
529   An HTTP/1.1 origin server, upon receiving a conditional request that
530   includes both a Last-Modified date (e.g., in an If-Modified-Since or
531   If-Unmodified-Since header field) and one or more entity tags (e.g.,
532   in an If-Match, If-None-Match, or If-Range header field) as cache
533   validators, &MUST-NOT; return a response status of 304 (Not Modified)
534   unless doing so is consistent with all of the conditional header
535   fields in the request.
538   An HTTP/1.1 caching proxy, upon receiving a conditional request that
539   includes both a Last-Modified date and one or more entity tags as
540   cache validators, &MUST-NOT; return a locally cached response to the
541   client unless that cached response is consistent with all of the
542   conditional header fields in the request.
543  <list><t>
544      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
545      servers and clients should transmit as much non-redundant
546      information as is available in their responses and requests.
547      HTTP/1.1 systems receiving this information will make the most
548      conservative assumptions about the validators they receive.
549  </t><t>
550      HTTP/1.0 clients and caches will ignore entity tags. Generally,
551      last-modified values received or used by these systems will
552      support transparent and efficient caching, and so HTTP/1.1 origin
553      servers should provide Last-Modified values. In those rare cases
554      where the use of a Last-Modified value as a validator by an
555      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
556      origin servers should not provide one.
557  </t></list>
561<section title="Header Field Definitions" anchor="header.fields">
563   This section defines the syntax and semantics of HTTP/1.1 header fields
564   related to conditional requests.
567   For entity-header fields, both sender and recipient refer to either the
568   client or the server, depending on who sends and who receives the entity.
571<section title="ETag" anchor="header.etag">
572  <iref primary="true" item="ETag header" x:for-anchor=""/>
573  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
575   The ETag response-header field provides the current value of the
576   entity tag for the requested variant. The headers used with entity
577   tags are described in Sections <xref target="header.if-match" format="counter"/>
578   and <xref target="header.if-none-match" format="counter"/> of this document,
579   and in &header-if-range;. The entity tag
580   &MAY; be used for comparison with other entities from the same resource
581   (see <xref target="weak.and.strong.validators"/>).
583<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
584  ETag = "ETag" ":" entity-tag
587   Examples:
589<artwork type="example">
590   ETag: "xyzzy"
591   ETag: W/"xyzzy"
592   ETag: ""
596<section title="If-Match" anchor="header.if-match">
597  <iref primary="true" item="If-Match header" x:for-anchor=""/>
598  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
600   The If-Match request-header field is used with a method to make it
601   conditional. A client that has one or more entities previously
602   obtained from the resource can verify that one of those entities is
603   current by including a list of their associated entity tags in the
604   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
605   purpose of this feature is to allow efficient updates of cached
606   information with a minimum amount of transaction overhead. It is also
607   used, on updating requests, to prevent inadvertent modification of
608   the wrong version of a resource. As a special case, the value "*"
609   matches any current entity of the resource.
611<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
612  If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
615   If any of the entity tags match the entity tag of the entity that
616   would have been returned in the response to a similar GET request
617   (without the If-Match header) on that resource, or if "*" is given
618   and any current entity exists for that resource, then the server &MAY;
619   perform the requested method as if the If-Match header field did not
620   exist.
623   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
624   to compare the entity tags in If-Match.
627   If none of the entity tags match, or if "*" is given and no current
628   entity exists, the server &MUST-NOT; perform the requested method, and
629   &MUST; return a 412 (Precondition Failed) response. This behavior is
630   most useful when the client wants to prevent an updating method, such
631   as PUT, from modifying a resource that has changed since the client
632   last retrieved it.
635   If the request would, without the If-Match header field, result in
636   anything other than a 2xx or 412 status, then the If-Match header
637   &MUST; be ignored.
640   The meaning of "If-Match: *" is that the method &SHOULD; be performed
641   if the representation selected by the origin server (or by a cache,
642   possibly using the Vary mechanism, see &header-vary;) exists, and
643   &MUST-NOT; be performed if the representation does not exist.
646   A request intended to update a resource (e.g., a PUT) &MAY; include an
647   If-Match header field to signal that the request method &MUST-NOT; be
648   applied if the entity corresponding to the If-Match value (a single
649   entity tag) is no longer a representation of that resource. This
650   allows the user to indicate that they do not wish the request to be
651   successful if the resource has been changed without their knowledge.
652   Examples:
654<figure><artwork type="example">
655    If-Match: "xyzzy"
656    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
657    If-Match: *
660   The result of a request having both an If-Match header field and
661   either an If-None-Match or an If-Modified-Since header fields is
662   undefined by this specification.
666<section title="If-Modified-Since" anchor="header.if-modified-since">
667  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
668  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
670   The If-Modified-Since request-header field is used with a method to
671   make it conditional: if the requested variant has not been modified
672   since the time specified in this field, an entity will not be
673   returned from the server; instead, a 304 (Not Modified) response will
674   be returned without any message-body.
676<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
677  If-Modified-Since = "If-Modified-Since" ":" HTTP-date
680   An example of the field is:
682<figure><artwork type="example">
683    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
686   A GET method with an If-Modified-Since header and no Range header
687   requests that the identified entity be transferred only if it has
688   been modified since the date given by the If-Modified-Since header.
689   The algorithm for determining this includes the following cases:
690  <list style="numbers">
691      <t>If the request would normally result in anything other than a
692         200 (OK) status, or if the passed If-Modified-Since date is
693         invalid, the response is exactly the same as for a normal GET.
694         A date which is later than the server's current time is
695         invalid.</t>
697      <t>If the variant has been modified since the If-Modified-Since
698         date, the response is exactly the same as for a normal GET.</t>
700      <t>If the variant has not been modified since a valid If-Modified-Since
701         date, the server &SHOULD; return a 304 (Not
702         Modified) response.</t>
703  </list>
706   The purpose of this feature is to allow efficient updates of cached
707   information with a minimum amount of transaction overhead.
708  <list><t>
709      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
710      see &header-range; for full details.
711    </t><t>
712      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
713      clock might not be synchronized with the client.
714    </t><t>
715      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
716      servers will use an exact date comparison function, rather than a
717      less-than function, for deciding whether to send a 304 (Not
718      Modified) response. To get best results when sending an If-Modified-Since
719      header field for cache validation, clients are
720      advised to use the exact date string received in a previous Last-Modified
721      header field whenever possible.
722    </t><t>
723      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
724      header instead of a date taken from the Last-Modified header for
725      the same request, the client should be aware of the fact that this
726      date is interpreted in the server's understanding of time. The
727      client should consider unsynchronized clocks and rounding problems
728      due to the different encodings of time between the client and
729      server. This includes the possibility of race conditions if the
730      document has changed between the time it was first requested and
731      the If-Modified-Since date of a subsequent request, and the
732      possibility of clock-skew-related problems if the If-Modified-Since
733      date is derived from the client's clock without correction
734      to the server's clock. Corrections for different time bases
735      between client and server are at best approximate due to network
736      latency.
737    </t>
738  </list>
741   The result of a request having both an If-Modified-Since header field
742   and either an If-Match or an If-Unmodified-Since header fields is
743   undefined by this specification.
747<section title="If-None-Match" anchor="header.if-none-match">
748  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
749  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
751   The If-None-Match request-header field is used with a method to make
752   it conditional. A client that has one or more entities previously
753   obtained from the resource can verify that none of those entities is
754   current by including a list of their associated entity tags in the
755   If-None-Match header field. The purpose of this feature is to allow
756   efficient updates of cached information with a minimum amount of
757   transaction overhead. It is also used to prevent a method (e.g. PUT)
758   from inadvertently modifying an existing resource when the client
759   believes that the resource does not exist.
762   As a special case, the value "*" matches any current entity of the
763   resource.
765<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
766  If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
769   If any of the entity tags match the entity tag of the entity that
770   would have been returned in the response to a similar GET request
771   (without the If-None-Match header) on that resource, or if "*" is
772   given and any current entity exists for that resource, then the
773   server &MUST-NOT; perform the requested method, unless required to do
774   so because the resource's modification date fails to match that
775   supplied in an If-Modified-Since header field in the request.
776   Instead, if the request method was GET or HEAD, the server &SHOULD;
777   respond with a 304 (Not Modified) response, including the cache-related
778   header fields (particularly ETag) of one of the entities that
779   matched. For all other request methods, the server &MUST; respond with
780   a status of 412 (Precondition Failed).
783   See <xref target="weak.and.strong.validators"/> for rules on how to determine if two entities tags
784   match. The weak comparison function can only be used with GET or HEAD
785   requests.
788   If none of the entity tags match, then the server &MAY; perform the
789   requested method as if the If-None-Match header field did not exist,
790   but &MUST; also ignore any If-Modified-Since header field(s) in the
791   request. That is, if no entity tags match, then the server &MUST-NOT;
792   return a 304 (Not Modified) response.
795   If the request would, without the If-None-Match header field, result
796   in anything other than a 2xx or 304 status, then the If-None-Match
797   header &MUST; be ignored. (See <xref target=""/> for a discussion of
798   server behavior when both If-Modified-Since and If-None-Match appear
799   in the same request.)
802   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
803   performed if the representation selected by the origin server (or by
804   a cache, possibly using the Vary mechanism, see &header-vary;)
805   exists, and &SHOULD; be performed if the representation does not exist.
806   This feature is intended to be useful in preventing races between PUT
807   operations.
810   Examples:
812<figure><artwork type="example">
813    If-None-Match: "xyzzy"
814    If-None-Match: W/"xyzzy"
815    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
816    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
817    If-None-Match: *
820   The result of a request having both an If-None-Match header field and
821   either an If-Match or an If-Unmodified-Since header fields is
822   undefined by this specification.
826<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
827  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
828  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
830   The If-Unmodified-Since request-header field is used with a method to
831   make it conditional. If the requested resource has not been modified
832   since the time specified in this field, the server &SHOULD; perform the
833   requested operation as if the If-Unmodified-Since header were not
834   present.
837   If the requested variant has been modified since the specified time,
838   the server &MUST-NOT; perform the requested operation, and &MUST; return
839   a 412 (Precondition Failed).
841<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
842  If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
845   An example of the field is:
847<figure><artwork type="example">
848    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
851   If the request normally (i.e., without the If-Unmodified-Since
852   header) would result in anything other than a 2xx or 412 status, the
853   If-Unmodified-Since header &SHOULD; be ignored.
856   If the specified date is invalid, the header is ignored.
859   The result of a request having both an If-Unmodified-Since header
860   field and either an If-None-Match or an If-Modified-Since header
861   fields is undefined by this specification.
865<section title="Last-Modified" anchor="header.last-modified">
866  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
867  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
869   The Last-Modified entity-header field indicates the date and time at
870   which the origin server believes the variant was last modified.
872<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
873  Last-Modified  = "Last-Modified" ":" HTTP-date
876   An example of its use is
878<figure><artwork type="example">
879    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
882   The exact meaning of this header field depends on the implementation
883   of the origin server and the nature of the original resource. For
884   files, it may be just the file system last-modified time. For
885   entities with dynamically included parts, it may be the most recent
886   of the set of last-modify times for its component parts. For database
887   gateways, it may be the last-update time stamp of the record. For
888   virtual objects, it may be the last time the internal state changed.
891   An origin server &MUST-NOT; send a Last-Modified date which is later
892   than the server's time of message origination. In such cases, where
893   the resource's last modification would indicate some time in the
894   future, the server &MUST; replace that date with the message
895   origination date.
898   An origin server &SHOULD; obtain the Last-Modified value of the entity
899   as close as possible to the time that it generates the Date value of
900   its response. This allows a recipient to make an accurate assessment
901   of the entity's modification time, especially if the entity changes
902   near the time that the response is generated.
905   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
911<section title="IANA Considerations" anchor="IANA.considerations">
913   TBD.
917<section title="Security Considerations" anchor="security.considerations">
919   No additional security considerations have been identified beyond
920   those applicable to HTTP in general &messaging;.
924<section title="Acknowledgments" anchor="ack">
929<references title="Normative References">
931<reference anchor="Part1">
932  <front>
933    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
934    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
935      <organization abbrev="Day Software">Day Software</organization>
936      <address><email></email></address>
937    </author>
938    <author initials="J." surname="Gettys" fullname="Jim Gettys">
939      <organization>One Laptop per Child</organization>
940      <address><email></email></address>
941    </author>
942    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
943      <organization abbrev="HP">Hewlett-Packard Company</organization>
944      <address><email></email></address>
945    </author>
946    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
947      <organization abbrev="Microsoft">Microsoft Corporation</organization>
948      <address><email></email></address>
949    </author>
950    <author initials="L." surname="Masinter" fullname="Larry Masinter">
951      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
952      <address><email></email></address>
953    </author>
954    <author initials="P." surname="Leach" fullname="Paul J. Leach">
955      <organization abbrev="Microsoft">Microsoft Corporation</organization>
956      <address><email></email></address>
957    </author>
958    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
959      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
960      <address><email></email></address>
961    </author>
962    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
963      <organization abbrev="W3C">World Wide Web Consortium</organization>
964      <address><email></email></address>
965    </author>
966    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
967      <organization abbrev="greenbytes">greenbytes GmbH</organization>
968      <address><email></email></address>
969    </author>
970    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
971  </front>
972  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
973  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
976<reference anchor="Part5">
977  <front>
978    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
979    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
980      <organization abbrev="Day Software">Day Software</organization>
981      <address><email></email></address>
982    </author>
983    <author initials="J." surname="Gettys" fullname="Jim Gettys">
984      <organization>One Laptop per Child</organization>
985      <address><email></email></address>
986    </author>
987    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
988      <organization abbrev="HP">Hewlett-Packard Company</organization>
989      <address><email></email></address>
990    </author>
991    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
992      <organization abbrev="Microsoft">Microsoft Corporation</organization>
993      <address><email></email></address>
994    </author>
995    <author initials="L." surname="Masinter" fullname="Larry Masinter">
996      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
997      <address><email></email></address>
998    </author>
999    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1000      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1001      <address><email></email></address>
1002    </author>
1003    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1004      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1005      <address><email></email></address>
1006    </author>
1007    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1008      <organization abbrev="W3C">World Wide Web Consortium</organization>
1009      <address><email></email></address>
1010    </author>
1011    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1012      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1013      <address><email></email></address>
1014    </author>
1015    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1016  </front>
1017  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1018  <x:source href="p5-range.xml" basename="p5-range"/>
1021<reference anchor="Part6">
1022  <front>
1023    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1024    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1025      <organization abbrev="Day Software">Day Software</organization>
1026      <address><email></email></address>
1027    </author>
1028    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1029      <organization>One Laptop per Child</organization>
1030      <address><email></email></address>
1031    </author>
1032    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1033      <organization abbrev="HP">Hewlett-Packard Company</organization>
1034      <address><email></email></address>
1035    </author>
1036    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1037      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1038      <address><email></email></address>
1039    </author>
1040    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1041      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1042      <address><email></email></address>
1043    </author>
1044    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1045      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1046      <address><email></email></address>
1047    </author>
1048    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1049      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1050      <address><email></email></address>
1051    </author>
1052    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1053      <organization abbrev="W3C">World Wide Web Consortium</organization>
1054      <address><email></email></address>
1055    </author>
1056    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1057      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1058      <address><email></email></address>
1059    </author>
1060    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1061  </front>
1062  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1063  <x:source href="p6-cache.xml" basename="p6-cache"/>
1066<reference anchor="RFC2119">
1067  <front>
1068    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1069    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1070      <organization>Harvard University</organization>
1071      <address><email></email></address>
1072    </author>
1073    <date month="March" year="1997"/>
1074  </front>
1075  <seriesInfo name="BCP" value="14"/>
1076  <seriesInfo name="RFC" value="2119"/>
1081<references title="Informative References">
1083<reference anchor="RFC2068">
1084  <front>
1085    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1086    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1087      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1088      <address><email></email></address>
1089    </author>
1090    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1091      <organization>MIT Laboratory for Computer Science</organization>
1092      <address><email></email></address>
1093    </author>
1094    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1095      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1096      <address><email></email></address>
1097    </author>
1098    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1099      <organization>MIT Laboratory for Computer Science</organization>
1100      <address><email></email></address>
1101    </author>
1102    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1103      <organization>MIT Laboratory for Computer Science</organization>
1104      <address><email></email></address>
1105    </author>
1106    <date month="January" year="1997"/>
1107  </front>
1108  <seriesInfo name="RFC" value="2068"/>
1111<reference anchor="RFC2616">
1112  <front>
1113    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1114    <author initials="R." surname="Fielding" fullname="R. Fielding">
1115      <organization>University of California, Irvine</organization>
1116      <address><email></email></address>
1117    </author>
1118    <author initials="J." surname="Gettys" fullname="J. Gettys">
1119      <organization>W3C</organization>
1120      <address><email></email></address>
1121    </author>
1122    <author initials="J." surname="Mogul" fullname="J. Mogul">
1123      <organization>Compaq Computer Corporation</organization>
1124      <address><email></email></address>
1125    </author>
1126    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1127      <organization>MIT Laboratory for Computer Science</organization>
1128      <address><email></email></address>
1129    </author>
1130    <author initials="L." surname="Masinter" fullname="L. Masinter">
1131      <organization>Xerox Corporation</organization>
1132      <address><email></email></address>
1133    </author>
1134    <author initials="P." surname="Leach" fullname="P. Leach">
1135      <organization>Microsoft Corporation</organization>
1136      <address><email></email></address>
1137    </author>
1138    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1139      <organization>W3C</organization>
1140      <address><email></email></address>
1141    </author>
1142    <date month="June" year="1999"/>
1143  </front>
1144  <seriesInfo name="RFC" value="2616"/>
1149<section title="Compatibility with Previous Versions" anchor="compatibility">
1151<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1156<section title="Change Log (to be removed by RFC Editor before publication)">
1158<section title="Since RFC2616">
1160  Extracted relevant partitions from <xref target="RFC2616"/>.
1164<section title="Since draft-ietf-httpbis-p4-conditional-00">
1166  Other changes:
1167  <list style="symbols">
1168    <t>
1169      Move definitions of 304 and 412 condition codes from Part2.
1170    </t>
1171    <t>
1172      Categorize references as "Normative" or "Informative".
1173    </t>
1174  </list>
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