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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 "December">
16  <!ENTITY ID-YEAR "2007">
17  <!ENTITY messaging                  "<xref target='Part1' xmlns:x=''/>">
18  <!ENTITY caching                    "<xref target='Part6' xmlns:x=''/>">
19  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x=''/>">
20  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x=''/>">
21  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x=''/>">
22  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x=''/>">
24<?rfc toc="yes" ?>
25<?rfc symrefs="yes" ?>
26<?rfc sortrefs="yes" ?>
27<?rfc compact="yes"?>
28<?rfc subcompact="no" ?>
29<?rfc linkmailto="no" ?>
30<?rfc editing="no" ?>
31<?rfc-ext allow-markup-in-artwork="yes" ?>
32<?rfc-ext include-references-in-index="yes" ?>
33<rfc obsoletes="2068, 2616" category="std"
34     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
35     xmlns:x='' xmlns:ed="">
38  <title abbrev="HTTP/1.1, part 4">HTTP/1.1, part 4: Conditional Requests</title>
40  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
41    <organization abbrev="Day Software">Day Software</organization>
42    <address>
43      <postal>
44        <street>23 Corporate Plaza DR, Suite 280</street>
45        <city>Newport Beach</city>
46        <region>CA</region>
47        <code>92660</code>
48        <country>USA</country>
49      </postal>
50      <phone>+1-949-706-5300</phone>
51      <facsimile>+1-949-706-5305</facsimile>
52      <email></email>
53      <uri></uri>
54    </address>
55  </author>
57  <author initials="J." surname="Gettys" fullname="Jim Gettys">
58    <organization>One Laptop per Child</organization>
59    <address>
60      <postal>
61        <street>21 Oak Knoll Road</street>
62        <city>Carlisle</city>
63        <region>MA</region>
64        <code>01741</code>
65        <country>USA</country>
66      </postal>
67      <email></email>
68      <uri></uri>
69    </address>
70  </author>
72  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
73    <organization abbrev="HP">Hewlett-Packard Company</organization>
74    <address>
75      <postal>
76        <street>HP Labs, Large Scale Systems Group</street>
77        <street>1501 Page Mill Road, MS 1177</street>
78        <city>Palo Alto</city>
79        <region>CA</region>
80        <code>94304</code>
81        <country>USA</country>
82      </postal>
83      <email></email>
84    </address>
85  </author>
87  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
88    <organization abbrev="Microsoft">Microsoft Corporation</organization>
89    <address>
90      <postal>
91        <street>1 Microsoft Way</street>
92        <city>Redmond</city>
93        <region>WA</region>
94        <code>98052</code>
95        <country>USA</country>
96      </postal>
97      <email></email>
98    </address>
99  </author>
101  <author initials="L." surname="Masinter" fullname="Larry Masinter">
102    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
103    <address>
104      <postal>
105        <street>345 Park Ave</street>
106        <city>San Jose</city>
107        <region>CA</region>
108        <code>95110</code>
109        <country>USA</country>
110      </postal>
111      <email></email>
112      <uri></uri>
113    </address>
114  </author>
116  <author initials="P." surname="Leach" fullname="Paul J. Leach">
117    <organization abbrev="Microsoft">Microsoft Corporation</organization>
118    <address>
119      <postal>
120        <street>1 Microsoft Way</street>
121        <city>Redmond</city>
122        <region>WA</region>
123        <code>98052</code>
124      </postal>
125      <email></email>
126    </address>
127  </author>
129  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
130    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
131    <address>
132      <postal>
133        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
134        <street>The Stata Center, Building 32</street>
135        <street>32 Vassar Street</street>
136        <city>Cambridge</city>
137        <region>MA</region>
138        <code>02139</code>
139        <country>USA</country>
140      </postal>
141      <email></email>
142      <uri></uri>
143    </address>
144  </author>
146  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
147    <organization abbrev="W3C">World Wide Web Consortium</organization>
148    <address>
149      <postal>
150        <street>W3C / ERCIM</street>
151        <street>2004, rte des Lucioles</street>
152        <city>Sophia-Antipolis</city>
153        <region>AM</region>
154        <code>06902</code>
155        <country>France</country>
156      </postal>
157      <email></email>
158      <uri></uri>
159    </address>
160  </author>
162  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
163    <organization abbrev="greenbytes">greenbytes GmbH</organization>
164    <address>
165      <postal>
166        <street>Hafenweg 16</street>
167        <city>Muenster</city><region>NW</region><code>48155</code>
168        <country>Germany</country>
169      </postal>
170      <phone>+49 251 2807760</phone>   
171      <facsimile>+49 251 2807761</facsimile>   
172      <email></email>       
173      <uri></uri>     
174    </address>
175  </author>
177  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
181   The Hypertext Transfer Protocol (HTTP) is an application-level
182   protocol for distributed, collaborative, hypermedia information
183   systems. HTTP has been in use by the World Wide Web global information
184   initiative since 1990. This document is Part 4 of the seven-part specification
185   that defines the protocol referred to as "HTTP/1.1" and, taken together,
186   obsoletes RFC 2616.  Part 4 defines request header fields for
187   indicating conditional requests and the rules for constructing responses
188   to those requests.
192<note title="Editorial Note (To be removed by RFC Editor)">
193  <t>
194    This version of the HTTP specification contains only minimal editorial
195    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
196    and authors' addresses).  All other changes are due to partitioning the
197    original into seven mostly independent parts.  The intent is for readers
198    of future drafts to able to use draft 00 as the basis for comparison
199    when the WG makes later changes to the specification text.  This draft
200    will shortly be followed by draft 01 (containing the first round of changes
201    that have already been agreed to on the mailing list). There is no point in
202    reviewing this draft other than to verify that the partitioning has been
203    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
204    will be the editors after draft 00 is submitted.
205  </t>
206  <t>
207    Discussion of this draft should take place on the HTTPBIS working group
208    mailing list ( The current issues list is
209    at <eref target=""/>
210    and related documents (including fancy diffs) can be found at
211    <eref target=""/>.
212  </t>
216<section title="Introduction" anchor="introduction">
218   This document will define aspects of HTTP related to conditional
219   request messages based on time stamps and entity-tags.  Right now it
220   only includes the extracted relevant sections of <xref target="RFC2616">RFC 2616</xref>
221   without edit.
224<section title="Requirements" anchor="intro.requirements">
226   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
227   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
228   document are to be interpreted as described in <xref target="RFC2119"/>.
231   An implementation is not compliant if it fails to satisfy one or more
232   of the &MUST; or &REQUIRED; level requirements for the protocols it
233   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
234   level and all the &SHOULD; level requirements for its protocols is said
235   to be "unconditionally compliant"; one that satisfies all the &MUST;
236   level requirements but not all the &SHOULD; level requirements for its
237   protocols is said to be "conditionally compliant."
242<section title="Entity Tags" anchor="entity.tags">
244   Entity tags are used for comparing two or more entities from the same
245   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
246   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
247   If-Range (&header-if-range;) header fields. The definition of how they
248   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
249   entity tag consists of an opaque quoted string, possibly prefixed by
250   a weakness indicator.
252<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"/>
253   entity-tag = [ weak ] opaque-tag
254   weak       = "W/"
255   opaque-tag = quoted-string
258   A "strong entity tag" &MAY; be shared by two entities of a resource
259   only if they are equivalent by octet equality.
262   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
263   two entities of a resource only if the entities are equivalent and
264   could be substituted for each other with no significant change in
265   semantics. A weak entity tag can only be used for weak comparison.
268   An entity tag &MUST; be unique across all versions of all entities
269   associated with a particular resource. A given entity tag value &MAY;
270   be used for entities obtained by requests on different URIs. The use
271   of the same entity tag value in conjunction with entities obtained by
272   requests on different URIs does not imply the equivalence of those
273   entities.
277<section title="Status Code Definitions">
278<section title="304 Not Modified" anchor="status.304">
279  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
280  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
282   If the client has performed a conditional GET request and access is
283   allowed, but the document has not been modified, the server &SHOULD;
284   respond with this status code. The 304 response &MUST-NOT; contain a
285   message-body, and thus is always terminated by the first empty line
286   after the header fields.
289   The response &MUST; include the following header fields:
290  <list style="symbols">
291    <t>Date, unless its omission is required by &clockless;</t>
292  </list>
295   If a clockless origin server obeys these rules, and proxies and
296   clients add their own Date to any response received without one (as
297   already specified by <xref target="RFC2068" x:sec="14.19" x:fmt=","/>), caches will operate
298   correctly.
299  <list style="symbols">
300    <t>ETag and/or Content-Location, if the header would have been sent
301        in a 200 response to the same request</t>
302    <t>Expires, Cache-Control, and/or Vary, if the field-value might
303        differ from that sent in any previous response for the same
304        variant</t>
305  </list>
308   If the conditional GET used a strong cache validator (see &caching;),
309   the response &SHOULD-NOT;  include other entity-headers.
310   Otherwise (i.e., the conditional GET used a weak validator), the
311   response &MUST-NOT; include other entity-headers; this prevents
312   inconsistencies between cached entity-bodies and updated headers.
315   If a 304 response indicates an entity not currently cached, then the
316   cache &MUST; disregard the response and repeat the request without the
317   conditional.
320   If a cache uses a received 304 response to update a cache entry, the
321   cache &MUST; update the entry to reflect any new field values given in
322   the response.
326<section title="412 Precondition Failed" anchor="status.412">
327  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
328  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
330   The precondition given in one or more of the request-header fields
331   evaluated to false when it was tested on the server. This response
332   code allows the client to place preconditions on the current resource
333   metainformation (header field data) and thus prevent the requested
334   method from being applied to a resource other than the one intended.
339<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
341   Since both origin servers and caches will compare two validators to
342   decide if they represent the same or different entities, one normally
343   would expect that if the entity (the entity-body or any entity-headers)
344   changes in any way, then the associated validator would
345   change as well. If this is true, then we call this validator a
346   "strong validator."
349   However, there might be cases when a server prefers to change the
350   validator only on semantically significant changes, and not when
351   insignificant aspects of the entity change. A validator that does not
352   always change when the resource changes is a "weak validator."
355   Entity tags are normally "strong validators," but the protocol
356   provides a mechanism to tag an entity tag as "weak." One can think of
357   a strong validator as one that changes whenever the bits of an entity
358   changes, while a weak value changes whenever the meaning of an entity
359   changes. Alternatively, one can think of a strong validator as part
360   of an identifier for a specific entity, while a weak validator is
361   part of an identifier for a set of semantically equivalent entities.
362  <list><t>
363      <x:h>Note:</x:h> One example of a strong validator is an integer that is
364      incremented in stable storage every time an entity is changed.
365    </t><t>
366      An entity's modification time, if represented with one-second
367      resolution, could be a weak validator, since it is possible that
368      the resource might be modified twice during a single second.
369    </t><t>
370      Support for weak validators is optional. However, weak validators
371      allow for more efficient caching of equivalent objects; for
372      example, a hit counter on a site is probably good enough if it is
373      updated every few days or weeks, and any value during that period
374      is likely "good enough" to be equivalent.
375    </t></list>
378   A "use" of a validator is either when a client generates a request
379   and includes the validator in a validating header field, or when a
380   server compares two validators.
383   Strong validators are usable in any context. Weak validators are only
384   usable in contexts that do not depend on exact equality of an entity.
385   For example, either kind is usable for a conditional GET of a full
386   entity. However, only a strong validator is usable for a sub-range
387   retrieval, since otherwise the client might end up with an internally
388   inconsistent entity.
391   Clients &MAY; issue simple (non-subrange) GET requests with either weak
392   validators or strong validators. Clients &MUST-NOT; use weak validators
393   in other forms of request.
396   The only function that the HTTP/1.1 protocol defines on validators is
397   comparison. There are two validator comparison functions, depending
398   on whether the comparison context allows the use of weak validators
399   or not:
400  <list style="symbols">
401     <t>The strong comparison function: in order to be considered equal,
402        both validators &MUST; be identical in every way, and both &MUST-NOT;
403        be weak.</t>
404     <t>The weak comparison function: in order to be considered equal,
405        both validators &MUST; be identical in every way, but either or
406        both of them &MAY; be tagged as "weak" without affecting the
407        result.</t>
408  </list>
411   An entity tag is strong unless it is explicitly tagged as weak.
412   <xref target="entity.tags"/> gives the syntax for entity tags.
415   A Last-Modified time, when used as a validator in a request, is
416   implicitly weak unless it is possible to deduce that it is strong,
417   using the following rules:
418  <list style="symbols">
419     <t>The validator is being compared by an origin server to the
420        actual current validator for the entity and,</t>
421     <t>That origin server reliably knows that the associated entity did
422        not change twice during the second covered by the presented
423        validator.</t>
424  </list>
427   or
428  <list style="symbols">
429     <t>The validator is about to be used by a client in an If-Modified-Since
430        or If-Unmodified-Since header, because the client
431        has a cache entry for the associated entity, and</t>
432     <t>That cache entry includes a Date value, which gives the time
433        when the origin server sent the original response, and</t>
434     <t>The presented Last-Modified time is at least 60 seconds before
435        the Date value.</t>
436  </list>
439   or
440  <list style="symbols">
441     <t>The validator is being compared by an intermediate cache to the
442        validator stored in its cache entry for the entity, and</t>
443     <t>That cache entry includes a Date value, which gives the time
444        when the origin server sent the original response, and</t>
445     <t>The presented Last-Modified time is at least 60 seconds before
446        the Date value.</t>
447  </list>
450   This method relies on the fact that if two different responses were
451   sent by the origin server during the same second, but both had the
452   same Last-Modified time, then at least one of those responses would
453   have a Date value equal to its Last-Modified time. The arbitrary 60-second
454   limit guards against the possibility that the Date and Last-Modified
455   values are generated from different clocks, or at somewhat
456   different times during the preparation of the response. An
457   implementation &MAY; use a value larger than 60 seconds, if it is
458   believed that 60 seconds is too short.
461   If a client wishes to perform a sub-range retrieval on a value for
462   which it has only a Last-Modified time and no opaque validator, it
463   &MAY; do this only if the Last-Modified time is strong in the sense
464   described here.
467   A cache or origin server receiving a conditional request, other than
468   a full-body GET request, &MUST; use the strong comparison function to
469   evaluate the condition.
472   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
473   retrievals on values that have been obtained from HTTP/1.0
474   servers.
478<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="">
480   We adopt a set of rules and recommendations for origin servers,
481   clients, and caches regarding when various validator types ought to
482   be used, and for what purposes.
485   HTTP/1.1 origin servers:
486  <list style="symbols">
487     <t>&SHOULD; send an entity tag validator unless it is not feasible to
488        generate one.</t>
490     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
491        performance considerations support the use of weak entity tags,
492        or if it is unfeasible to send a strong entity tag.</t>
494     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
495        unless the risk of a breakdown in semantic transparency that
496        could result from using this date in an If-Modified-Since header
497        would lead to serious problems.</t>
498  </list>
501   In other words, the preferred behavior for an HTTP/1.1 origin server
502   is to send both a strong entity tag and a Last-Modified value.
505   In order to be legal, a strong entity tag &MUST; change whenever the
506   associated entity value changes in any way. A weak entity tag &SHOULD;
507   change whenever the associated entity changes in a semantically
508   significant way.
509  <list><t>
510      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
511      origin server must avoid reusing a specific strong entity tag
512      value for two different entities, or reusing a specific weak
513      entity tag value for two semantically different entities. Cache
514      entries might persist for arbitrarily long periods, regardless of
515      expiration times, so it might be inappropriate to expect that a
516      cache will never again attempt to validate an entry using a
517      validator that it obtained at some point in the past.
518  </t></list>
521   HTTP/1.1 clients:
522  <list style="symbols">
523     <t>If an entity tag has been provided by the origin server, &MUST;
524        use that entity tag in any cache-conditional request (using If-Match
525        or If-None-Match).</t>
527     <t>If only a Last-Modified value has been provided by the origin
528        server, &SHOULD; use that value in non-subrange cache-conditional
529        requests (using If-Modified-Since).</t>
531     <t>If only a Last-Modified value has been provided by an HTTP/1.0
532        origin server, &MAY; use that value in subrange cache-conditional
533        requests (using If-Unmodified-Since:). The user agent &SHOULD;
534        provide a way to disable this, in case of difficulty.</t>
536     <t>If both an entity tag and a Last-Modified value have been
537        provided by the origin server, &SHOULD; use both validators in
538        cache-conditional requests. This allows both HTTP/1.0 and
539        HTTP/1.1 caches to respond appropriately.</t>
540  </list>
543   An HTTP/1.1 origin server, upon receiving a conditional request that
544   includes both a Last-Modified date (e.g., in an If-Modified-Since or
545   If-Unmodified-Since header field) and one or more entity tags (e.g.,
546   in an If-Match, If-None-Match, or If-Range header field) as cache
547   validators, &MUST-NOT; return a response status of 304 (Not Modified)
548   unless doing so is consistent with all of the conditional header
549   fields in the request.
552   An HTTP/1.1 caching proxy, upon receiving a conditional request that
553   includes both a Last-Modified date and one or more entity tags as
554   cache validators, &MUST-NOT; return a locally cached response to the
555   client unless that cached response is consistent with all of the
556   conditional header fields in the request.
557  <list><t>
558      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
559      servers and clients should transmit as much non-redundant
560      information as is available in their responses and requests.
561      HTTP/1.1 systems receiving this information will make the most
562      conservative assumptions about the validators they receive.
563  </t><t>
564      HTTP/1.0 clients and caches will ignore entity tags. Generally,
565      last-modified values received or used by these systems will
566      support transparent and efficient caching, and so HTTP/1.1 origin
567      servers should provide Last-Modified values. In those rare cases
568      where the use of a Last-Modified value as a validator by an
569      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
570      origin servers should not provide one.
571  </t></list>
575<section title="Header Field Definitions" anchor="header.fields">
577   This section defines the syntax and semantics of all standard
578   HTTP/1.1 header fields. For entity-header fields, both sender and
579   recipient refer to either the client or the server, depending on who
580   sends and who receives the entity.
583<section title="ETag" anchor="header.etag">
584  <iref primary="true" item="ETag header" x:for-anchor=""/>
585  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
587   The ETag response-header field provides the current value of the
588   entity tag for the requested variant. The headers used with entity
589   tags are described in Sections <xref target="header.if-match" format="counter"/>, <xref target="header.if-none-match" format="counter"/> and &header-if-range;. The entity tag
590   &MAY; be used for comparison with other entities from the same resource
591   (see <xref target="weak.and.strong.validators"/>).
593<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
594    ETag = "ETag" ":" entity-tag
597   Examples:
599<artwork type="example">
600   ETag: "xyzzy"
601   ETag: W/"xyzzy"
602   ETag: ""
606<section title="If-Match" anchor="header.if-match">
607  <iref primary="true" item="If-Match header" x:for-anchor=""/>
608  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
610   The If-Match request-header field is used with a method to make it
611   conditional. A client that has one or more entities previously
612   obtained from the resource can verify that one of those entities is
613   current by including a list of their associated entity tags in the
614   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
615   purpose of this feature is to allow efficient updates of cached
616   information with a minimum amount of transaction overhead. It is also
617   used, on updating requests, to prevent inadvertent modification of
618   the wrong version of a resource. As a special case, the value "*"
619   matches any current entity of the resource.
621<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
622    If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
625   If any of the entity tags match the entity tag of the entity that
626   would have been returned in the response to a similar GET request
627   (without the If-Match header) on that resource, or if "*" is given
628   and any current entity exists for that resource, then the server &MAY;
629   perform the requested method as if the If-Match header field did not
630   exist.
633   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
634   to compare the entity tags in If-Match.
637   If none of the entity tags match, or if "*" is given and no current
638   entity exists, the server &MUST-NOT; perform the requested method, and
639   &MUST; return a 412 (Precondition Failed) response. This behavior is
640   most useful when the client wants to prevent an updating method, such
641   as PUT, from modifying a resource that has changed since the client
642   last retrieved it.
645   If the request would, without the If-Match header field, result in
646   anything other than a 2xx or 412 status, then the If-Match header
647   &MUST; be ignored.
650   The meaning of "If-Match: *" is that the method &SHOULD; be performed
651   if the representation selected by the origin server (or by a cache,
652   possibly using the Vary mechanism, see &header-vary;) exists, and
653   &MUST-NOT; be performed if the representation does not exist.
656   A request intended to update a resource (e.g., a PUT) &MAY; include an
657   If-Match header field to signal that the request method &MUST-NOT; be
658   applied if the entity corresponding to the If-Match value (a single
659   entity tag) is no longer a representation of that resource. This
660   allows the user to indicate that they do not wish the request to be
661   successful if the resource has been changed without their knowledge.
662   Examples:
664<figure><artwork type="example">
665    If-Match: "xyzzy"
666    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
667    If-Match: *
670   The result of a request having both an If-Match header field and
671   either an If-None-Match or an If-Modified-Since header fields is
672   undefined by this specification.
676<section title="If-Modified-Since" anchor="header.if-modified-since">
677  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
678  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
680   The If-Modified-Since request-header field is used with a method to
681   make it conditional: if the requested variant has not been modified
682   since the time specified in this field, an entity will not be
683   returned from the server; instead, a 304 (not modified) response will
684   be returned without any message-body.
686<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
687    If-Modified-Since = "If-Modified-Since" ":" HTTP-date
690   An example of the field is:
692<figure><artwork type="example">
693    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
696   A GET method with an If-Modified-Since header and no Range header
697   requests that the identified entity be transferred only if it has
698   been modified since the date given by the If-Modified-Since header.
699   The algorithm for determining this includes the following cases:
700  <list style="numbers">
701      <t>If the request would normally result in anything other than a
702         200 (OK) status, or if the passed If-Modified-Since date is
703         invalid, the response is exactly the same as for a normal GET.
704         A date which is later than the server's current time is
705         invalid.</t>
707      <t>If the variant has been modified since the If-Modified-Since
708         date, the response is exactly the same as for a normal GET.</t>
710      <t>If the variant has not been modified since a valid If-Modified-Since
711         date, the server &SHOULD; return a 304 (Not
712         Modified) response.</t>
713  </list>
716   The purpose of this feature is to allow efficient updates of cached
717   information with a minimum amount of transaction overhead.
718  <list><t>
719      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
720      see &header-range; for full details.
721    </t><t>
722      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
723      clock might not be synchronized with the client.
724    </t><t>
725      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
726      servers will use an exact date comparison function, rather than a
727      less-than function, for deciding whether to send a 304 (Not
728      Modified) response. To get best results when sending an If-Modified-Since
729      header field for cache validation, clients are
730      advised to use the exact date string received in a previous Last-Modified
731      header field whenever possible.
732    </t><t>
733      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
734      header instead of a date taken from the Last-Modified header for
735      the same request, the client should be aware of the fact that this
736      date is interpreted in the server's understanding of time. The
737      client should consider unsynchronized clocks and rounding problems
738      due to the different encodings of time between the client and
739      server. This includes the possibility of race conditions if the
740      document has changed between the time it was first requested and
741      the If-Modified-Since date of a subsequent request, and the
742      possibility of clock-skew-related problems if the If-Modified-Since
743      date is derived from the client's clock without correction
744      to the server's clock. Corrections for different time bases
745      between client and server are at best approximate due to network
746      latency.
747    </t>
748  </list>
751   The result of a request having both an If-Modified-Since header field
752   and either an If-Match or an If-Unmodified-Since header fields is
753   undefined by this specification.
757<section title="If-None-Match" anchor="header.if-none-match">
758  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
759  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
761   The If-None-Match request-header field is used with a method to make
762   it conditional. A client that has one or more entities previously
763   obtained from the resource can verify that none of those entities is
764   current by including a list of their associated entity tags in the
765   If-None-Match header field. The purpose of this feature is to allow
766   efficient updates of cached information with a minimum amount of
767   transaction overhead. It is also used to prevent a method (e.g. PUT)
768   from inadvertently modifying an existing resource when the client
769   believes that the resource does not exist.
772   As a special case, the value "*" matches any current entity of the
773   resource.
775<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
776    If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
779   If any of the entity tags match the entity tag of the entity that
780   would have been returned in the response to a similar GET request
781   (without the If-None-Match header) on that resource, or if "*" is
782   given and any current entity exists for that resource, then the
783   server &MUST-NOT; perform the requested method, unless required to do
784   so because the resource's modification date fails to match that
785   supplied in an If-Modified-Since header field in the request.
786   Instead, if the request method was GET or HEAD, the server &SHOULD;
787   respond with a 304 (Not Modified) response, including the cache-related
788   header fields (particularly ETag) of one of the entities that
789   matched. For all other request methods, the server &MUST; respond with
790   a status of 412 (Precondition Failed).
793   See <xref target="weak.and.strong.validators"/> for rules on how to determine if two entities tags
794   match. The weak comparison function can only be used with GET or HEAD
795   requests.
798   If none of the entity tags match, then the server &MAY; perform the
799   requested method as if the If-None-Match header field did not exist,
800   but &MUST; also ignore any If-Modified-Since header field(s) in the
801   request. That is, if no entity tags match, then the server &MUST-NOT;
802   return a 304 (Not Modified) response.
805   If the request would, without the If-None-Match header field, result
806   in anything other than a 2xx or 304 status, then the If-None-Match
807   header &MUST; be ignored. (See <xref target=""/> for a discussion of
808   server behavior when both If-Modified-Since and If-None-Match appear
809   in the same request.)
812   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
813   performed if the representation selected by the origin server (or by
814   a cache, possibly using the Vary mechanism, see &header-vary;)
815   exists, and &SHOULD; be performed if the representation does not exist.
816   This feature is intended to be useful in preventing races between PUT
817   operations.
820   Examples:
822<figure><artwork type="example">
823    If-None-Match: "xyzzy"
824    If-None-Match: W/"xyzzy"
825    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
826    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
827    If-None-Match: *
830   The result of a request having both an If-None-Match header field and
831   either an If-Match or an If-Unmodified-Since header fields is
832   undefined by this specification.
836<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
837  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
838  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
840   The If-Unmodified-Since request-header field is used with a method to
841   make it conditional. If the requested resource has not been modified
842   since the time specified in this field, the server &SHOULD; perform the
843   requested operation as if the If-Unmodified-Since header were not
844   present.
847   If the requested variant has been modified since the specified time,
848   the server &MUST-NOT; perform the requested operation, and &MUST; return
849   a 412 (Precondition Failed).
851<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
852   If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
855   An example of the field is:
857<figure><artwork type="example">
858    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
861   If the request normally (i.e., without the If-Unmodified-Since
862   header) would result in anything other than a 2xx or 412 status, the
863   If-Unmodified-Since header &SHOULD; be ignored.
866   If the specified date is invalid, the header is ignored.
869   The result of a request having both an If-Unmodified-Since header
870   field and either an If-None-Match or an If-Modified-Since header
871   fields is undefined by this specification.
875<section title="Last-Modified" anchor="header.last-modified">
876  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
877  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
879   The Last-Modified entity-header field indicates the date and time at
880   which the origin server believes the variant was last modified.
882<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
883    Last-Modified  = "Last-Modified" ":" HTTP-date
886   An example of its use is
888<figure><artwork type="example">
889    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
892   The exact meaning of this header field depends on the implementation
893   of the origin server and the nature of the original resource. For
894   files, it may be just the file system last-modified time. For
895   entities with dynamically included parts, it may be the most recent
896   of the set of last-modify times for its component parts. For database
897   gateways, it may be the last-update time stamp of the record. For
898   virtual objects, it may be the last time the internal state changed.
901   An origin server &MUST-NOT; send a Last-Modified date which is later
902   than the server's time of message origination. In such cases, where
903   the resource's last modification would indicate some time in the
904   future, the server &MUST; replace that date with the message
905   origination date.
908   An origin server &SHOULD; obtain the Last-Modified value of the entity
909   as close as possible to the time that it generates the Date value of
910   its response. This allows a recipient to make an accurate assessment
911   of the entity's modification time, especially if the entity changes
912   near the time that the response is generated.
915   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
921<section title="IANA Considerations" anchor="IANA.considerations">
923   TBD.
927<section title="Security Considerations" anchor="security.considerations">
929   No additional security considerations have been identified beyond
930   those applicable to HTTP in general &messaging;.
934<section title="Acknowledgments" anchor="ack">
940<reference anchor="Part1">
941   <front>
942      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
943      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
944         <organization abbrev="Day Software">Day Software</organization>
945         <address><email></email></address>
946      </author>
947      <author initials="J." surname="Gettys" fullname="Jim Gettys">
948         <organization>One Laptop per Child</organization>
949         <address><email></email></address>
950      </author>
951      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
952         <organization abbrev="HP">Hewlett-Packard Company</organization>
953         <address><email></email></address>
954      </author>
955      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
956         <organization abbrev="Microsoft">Microsoft Corporation</organization>
957         <address><email></email></address>
958      </author>
959      <author initials="L." surname="Masinter" fullname="Larry Masinter">
960         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
961         <address><email></email></address>
962      </author>
963      <author initials="P." surname="Leach" fullname="Paul J. Leach">
964         <organization abbrev="Microsoft">Microsoft Corporation</organization>
965         <address><email></email></address>
966      </author>
967      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
968         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
969         <address><email></email></address>
970      </author>
971      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
972         <organization abbrev="W3C">World Wide Web Consortium</organization>
973         <address><email></email></address>
974      </author>
975      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
976         <organization abbrev="greenbytes">greenbytes GmbH</organization>
977         <address><email></email></address>
978      </author>
979      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
980   </front>
981   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
982   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
985<reference anchor="Part5">
986   <front>
987      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
988      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
989         <organization abbrev="Day Software">Day Software</organization>
990         <address><email></email></address>
991      </author>
992      <author initials="J." surname="Gettys" fullname="Jim Gettys">
993         <organization>One Laptop per Child</organization>
994         <address><email></email></address>
995      </author>
996      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
997         <organization abbrev="HP">Hewlett-Packard Company</organization>
998         <address><email></email></address>
999      </author>
1000      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1001         <organization abbrev="Microsoft">Microsoft Corporation</organization>
1002         <address><email></email></address>
1003      </author>
1004      <author initials="L." surname="Masinter" fullname="Larry Masinter">
1005         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1006         <address><email></email></address>
1007      </author>
1008      <author initials="P." surname="Leach" fullname="Paul J. Leach">
1009         <organization abbrev="Microsoft">Microsoft Corporation</organization>
1010         <address><email></email></address>
1011      </author>
1012      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1013         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1014         <address><email></email></address>
1015      </author>
1016      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1017         <organization abbrev="W3C">World Wide Web Consortium</organization>
1018         <address><email></email></address>
1019      </author>
1020      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1021         <organization abbrev="greenbytes">greenbytes GmbH</organization>
1022         <address><email></email></address>
1023      </author>
1024      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1025   </front>
1026   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1027   <x:source href="p5-range.xml" basename="p5-range"/>
1030<reference anchor="Part6">
1031   <front>
1032      <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1033      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1034         <organization abbrev="Day Software">Day Software</organization>
1035         <address><email></email></address>
1036      </author>
1037      <author initials="J." surname="Gettys" fullname="Jim Gettys">
1038         <organization>One Laptop per Child</organization>
1039         <address><email></email></address>
1040      </author>
1041      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1042         <organization abbrev="HP">Hewlett-Packard Company</organization>
1043         <address><email></email></address>
1044      </author>
1045      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1046         <organization abbrev="Microsoft">Microsoft Corporation</organization>
1047         <address><email></email></address>
1048      </author>
1049      <author initials="L." surname="Masinter" fullname="Larry Masinter">
1050         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1051         <address><email></email></address>
1052      </author>
1053      <author initials="P." surname="Leach" fullname="Paul J. Leach">
1054         <organization abbrev="Microsoft">Microsoft Corporation</organization>
1055         <address><email></email></address>
1056      </author>
1057      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1058         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1059         <address><email></email></address>
1060      </author>
1061      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1062         <organization abbrev="W3C">World Wide Web Consortium</organization>
1063         <address><email></email></address>
1064      </author>
1065      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1066         <organization abbrev="greenbytes">greenbytes GmbH</organization>
1067         <address><email></email></address>
1068      </author>
1069      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1070   </front>
1071   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1072   <x:source href="p6-cache.xml" basename="p6-cache"/>
1075<reference anchor="RFC2616">
1076   <front>
1077      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1078      <author initials="R." surname="Fielding" fullname="R. Fielding">
1079         <organization>University of California, Irvine</organization>
1080         <address><email></email></address>
1081      </author>
1082      <author initials="J." surname="Gettys" fullname="J. Gettys">
1083         <organization>W3C</organization>
1084         <address><email></email></address>
1085      </author>
1086      <author initials="J." surname="Mogul" fullname="J. Mogul">
1087         <organization>Compaq Computer Corporation</organization>
1088         <address><email></email></address>
1089      </author>
1090      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1091         <organization>MIT Laboratory for Computer Science</organization>
1092         <address><email></email></address>
1093      </author>
1094      <author initials="L." surname="Masinter" fullname="L. Masinter">
1095         <organization>Xerox Corporation</organization>
1096         <address><email></email></address>
1097      </author>
1098      <author initials="P." surname="Leach" fullname="P. Leach">
1099         <organization>Microsoft Corporation</organization>
1100         <address><email></email></address>
1101      </author>
1102      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1103         <organization>W3C</organization>
1104         <address><email></email></address>
1105      </author>
1106      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1107         <organization abbrev="W3C">World Wide Web Consortium</organization>
1108         <address><email></email></address>
1109      </author>
1110      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1111         <organization abbrev="greenbytes">greenbytes GmbH</organization>
1112         <address><email></email></address>
1113      </author>
1114      <date month="June" year="1999"/>
1115   </front>
1116   <seriesInfo name="RFC" value="2616"/>
1119<reference anchor="RFC2119">
1120  <front>
1121    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1122    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1123      <organization>Harvard University</organization>
1124      <address><email></email></address>
1125    </author>
1126    <date month="March" year="1997"/>
1127  </front>
1128  <seriesInfo name="BCP" value="14"/>
1129  <seriesInfo name="RFC" value="2119"/>
1132<reference anchor="RFC2068">
1133  <front>
1134    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1135    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1136      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1137      <address><email></email></address>
1138    </author>
1139    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1140      <organization>MIT Laboratory for Computer Science</organization>
1141      <address><email></email></address>
1142    </author>
1143    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1144      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1145      <address><email></email></address>
1146    </author>
1147    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1148      <organization>MIT Laboratory for Computer Science</organization>
1149      <address><email></email></address>
1150    </author>
1151    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1152      <organization>MIT Laboratory for Computer Science</organization>
1153      <address><email></email></address>
1154    </author>
1155    <date month="January" year="1997"/>
1156  </front>
1157  <seriesInfo name="RFC" value="2068"/>
1162<section title="Compatibility with Previous Versions" anchor="compatibility">
1164<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
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