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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 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 all standard
564   HTTP/1.1 header fields. For entity-header fields, both sender and
565   recipient refer to either the client or the server, depending on who
566   sends and who receives the entity.
569<section title="ETag" anchor="header.etag">
570  <iref primary="true" item="ETag header" x:for-anchor=""/>
571  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
573   The ETag response-header field provides the current value of the
574   entity tag for the requested variant. The headers used with entity
575   tags are described in Sections <xref target="header.if-match" format="counter"/>
576   and <xref target="header.if-none-match" format="counter"/> of this document,
577   and in &header-if-range;. The entity tag
578   &MAY; be used for comparison with other entities from the same resource
579   (see <xref target="weak.and.strong.validators"/>).
581<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
582    ETag = "ETag" ":" entity-tag
585   Examples:
587<artwork type="example">
588   ETag: "xyzzy"
589   ETag: W/"xyzzy"
590   ETag: ""
594<section title="If-Match" anchor="header.if-match">
595  <iref primary="true" item="If-Match header" x:for-anchor=""/>
596  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
598   The If-Match request-header field is used with a method to make it
599   conditional. A client that has one or more entities previously
600   obtained from the resource can verify that one of those entities is
601   current by including a list of their associated entity tags in the
602   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
603   purpose of this feature is to allow efficient updates of cached
604   information with a minimum amount of transaction overhead. It is also
605   used, on updating requests, to prevent inadvertent modification of
606   the wrong version of a resource. As a special case, the value "*"
607   matches any current entity of the resource.
609<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
610    If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
613   If any of the entity tags match the entity tag of the entity that
614   would have been returned in the response to a similar GET request
615   (without the If-Match header) on that resource, or if "*" is given
616   and any current entity exists for that resource, then the server &MAY;
617   perform the requested method as if the If-Match header field did not
618   exist.
621   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
622   to compare the entity tags in If-Match.
625   If none of the entity tags match, or if "*" is given and no current
626   entity exists, the server &MUST-NOT; perform the requested method, and
627   &MUST; return a 412 (Precondition Failed) response. This behavior is
628   most useful when the client wants to prevent an updating method, such
629   as PUT, from modifying a resource that has changed since the client
630   last retrieved it.
633   If the request would, without the If-Match header field, result in
634   anything other than a 2xx or 412 status, then the If-Match header
635   &MUST; be ignored.
638   The meaning of "If-Match: *" is that the method &SHOULD; be performed
639   if the representation selected by the origin server (or by a cache,
640   possibly using the Vary mechanism, see &header-vary;) exists, and
641   &MUST-NOT; be performed if the representation does not exist.
644   A request intended to update a resource (e.g., a PUT) &MAY; include an
645   If-Match header field to signal that the request method &MUST-NOT; be
646   applied if the entity corresponding to the If-Match value (a single
647   entity tag) is no longer a representation of that resource. This
648   allows the user to indicate that they do not wish the request to be
649   successful if the resource has been changed without their knowledge.
650   Examples:
652<figure><artwork type="example">
653    If-Match: "xyzzy"
654    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
655    If-Match: *
658   The result of a request having both an If-Match header field and
659   either an If-None-Match or an If-Modified-Since header fields is
660   undefined by this specification.
664<section title="If-Modified-Since" anchor="header.if-modified-since">
665  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
666  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
668   The If-Modified-Since request-header field is used with a method to
669   make it conditional: if the requested variant has not been modified
670   since the time specified in this field, an entity will not be
671   returned from the server; instead, a 304 (not modified) response will
672   be returned without any message-body.
674<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
675    If-Modified-Since = "If-Modified-Since" ":" HTTP-date
678   An example of the field is:
680<figure><artwork type="example">
681    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
684   A GET method with an If-Modified-Since header and no Range header
685   requests that the identified entity be transferred only if it has
686   been modified since the date given by the If-Modified-Since header.
687   The algorithm for determining this includes the following cases:
688  <list style="numbers">
689      <t>If the request would normally result in anything other than a
690         200 (OK) status, or if the passed If-Modified-Since date is
691         invalid, the response is exactly the same as for a normal GET.
692         A date which is later than the server's current time is
693         invalid.</t>
695      <t>If the variant has been modified since the If-Modified-Since
696         date, the response is exactly the same as for a normal GET.</t>
698      <t>If the variant has not been modified since a valid If-Modified-Since
699         date, the server &SHOULD; return a 304 (Not
700         Modified) response.</t>
701  </list>
704   The purpose of this feature is to allow efficient updates of cached
705   information with a minimum amount of transaction overhead.
706  <list><t>
707      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
708      see &header-range; for full details.
709    </t><t>
710      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
711      clock might not be synchronized with the client.
712    </t><t>
713      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
714      servers will use an exact date comparison function, rather than a
715      less-than function, for deciding whether to send a 304 (Not
716      Modified) response. To get best results when sending an If-Modified-Since
717      header field for cache validation, clients are
718      advised to use the exact date string received in a previous Last-Modified
719      header field whenever possible.
720    </t><t>
721      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
722      header instead of a date taken from the Last-Modified header for
723      the same request, the client should be aware of the fact that this
724      date is interpreted in the server's understanding of time. The
725      client should consider unsynchronized clocks and rounding problems
726      due to the different encodings of time between the client and
727      server. This includes the possibility of race conditions if the
728      document has changed between the time it was first requested and
729      the If-Modified-Since date of a subsequent request, and the
730      possibility of clock-skew-related problems if the If-Modified-Since
731      date is derived from the client's clock without correction
732      to the server's clock. Corrections for different time bases
733      between client and server are at best approximate due to network
734      latency.
735    </t>
736  </list>
739   The result of a request having both an If-Modified-Since header field
740   and either an If-Match or an If-Unmodified-Since header fields is
741   undefined by this specification.
745<section title="If-None-Match" anchor="header.if-none-match">
746  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
747  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
749   The If-None-Match request-header field is used with a method to make
750   it conditional. A client that has one or more entities previously
751   obtained from the resource can verify that none of those entities is
752   current by including a list of their associated entity tags in the
753   If-None-Match header field. The purpose of this feature is to allow
754   efficient updates of cached information with a minimum amount of
755   transaction overhead. It is also used to prevent a method (e.g. PUT)
756   from inadvertently modifying an existing resource when the client
757   believes that the resource does not exist.
760   As a special case, the value "*" matches any current entity of the
761   resource.
763<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
764    If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
767   If any of the entity tags match the entity tag of the entity that
768   would have been returned in the response to a similar GET request
769   (without the If-None-Match header) on that resource, or if "*" is
770   given and any current entity exists for that resource, then the
771   server &MUST-NOT; perform the requested method, unless required to do
772   so because the resource's modification date fails to match that
773   supplied in an If-Modified-Since header field in the request.
774   Instead, if the request method was GET or HEAD, the server &SHOULD;
775   respond with a 304 (Not Modified) response, including the cache-related
776   header fields (particularly ETag) of one of the entities that
777   matched. For all other request methods, the server &MUST; respond with
778   a status of 412 (Precondition Failed).
781   See <xref target="weak.and.strong.validators"/> for rules on how to determine if two entities tags
782   match. The weak comparison function can only be used with GET or HEAD
783   requests.
786   If none of the entity tags match, then the server &MAY; perform the
787   requested method as if the If-None-Match header field did not exist,
788   but &MUST; also ignore any If-Modified-Since header field(s) in the
789   request. That is, if no entity tags match, then the server &MUST-NOT;
790   return a 304 (Not Modified) response.
793   If the request would, without the If-None-Match header field, result
794   in anything other than a 2xx or 304 status, then the If-None-Match
795   header &MUST; be ignored. (See <xref target=""/> for a discussion of
796   server behavior when both If-Modified-Since and If-None-Match appear
797   in the same request.)
800   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
801   performed if the representation selected by the origin server (or by
802   a cache, possibly using the Vary mechanism, see &header-vary;)
803   exists, and &SHOULD; be performed if the representation does not exist.
804   This feature is intended to be useful in preventing races between PUT
805   operations.
808   Examples:
810<figure><artwork type="example">
811    If-None-Match: "xyzzy"
812    If-None-Match: W/"xyzzy"
813    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
814    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
815    If-None-Match: *
818   The result of a request having both an If-None-Match header field and
819   either an If-Match or an If-Unmodified-Since header fields is
820   undefined by this specification.
824<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
825  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
826  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
828   The If-Unmodified-Since request-header field is used with a method to
829   make it conditional. If the requested resource has not been modified
830   since the time specified in this field, the server &SHOULD; perform the
831   requested operation as if the If-Unmodified-Since header were not
832   present.
835   If the requested variant has been modified since the specified time,
836   the server &MUST-NOT; perform the requested operation, and &MUST; return
837   a 412 (Precondition Failed).
839<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
840   If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
843   An example of the field is:
845<figure><artwork type="example">
846    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
849   If the request normally (i.e., without the If-Unmodified-Since
850   header) would result in anything other than a 2xx or 412 status, the
851   If-Unmodified-Since header &SHOULD; be ignored.
854   If the specified date is invalid, the header is ignored.
857   The result of a request having both an If-Unmodified-Since header
858   field and either an If-None-Match or an If-Modified-Since header
859   fields is undefined by this specification.
863<section title="Last-Modified" anchor="header.last-modified">
864  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
865  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
867   The Last-Modified entity-header field indicates the date and time at
868   which the origin server believes the variant was last modified.
870<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
871    Last-Modified  = "Last-Modified" ":" HTTP-date
874   An example of its use is
876<figure><artwork type="example">
877    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
880   The exact meaning of this header field depends on the implementation
881   of the origin server and the nature of the original resource. For
882   files, it may be just the file system last-modified time. For
883   entities with dynamically included parts, it may be the most recent
884   of the set of last-modify times for its component parts. For database
885   gateways, it may be the last-update time stamp of the record. For
886   virtual objects, it may be the last time the internal state changed.
889   An origin server &MUST-NOT; send a Last-Modified date which is later
890   than the server's time of message origination. In such cases, where
891   the resource's last modification would indicate some time in the
892   future, the server &MUST; replace that date with the message
893   origination date.
896   An origin server &SHOULD; obtain the Last-Modified value of the entity
897   as close as possible to the time that it generates the Date value of
898   its response. This allows a recipient to make an accurate assessment
899   of the entity's modification time, especially if the entity changes
900   near the time that the response is generated.
903   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
909<section title="IANA Considerations" anchor="IANA.considerations">
911   TBD.
915<section title="Security Considerations" anchor="security.considerations">
917   No additional security considerations have been identified beyond
918   those applicable to HTTP in general &messaging;.
922<section title="Acknowledgments" anchor="ack">
928<reference anchor="Part1">
929   <front>
930      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
931      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
932         <organization abbrev="Day Software">Day Software</organization>
933         <address><email></email></address>
934      </author>
935      <author initials="J." surname="Gettys" fullname="Jim Gettys">
936         <organization>One Laptop per Child</organization>
937         <address><email></email></address>
938      </author>
939      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
940         <organization abbrev="HP">Hewlett-Packard Company</organization>
941         <address><email></email></address>
942      </author>
943      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
944         <organization abbrev="Microsoft">Microsoft Corporation</organization>
945         <address><email></email></address>
946      </author>
947      <author initials="L." surname="Masinter" fullname="Larry Masinter">
948         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
949         <address><email></email></address>
950      </author>
951      <author initials="P." surname="Leach" fullname="Paul J. Leach">
952         <organization abbrev="Microsoft">Microsoft Corporation</organization>
953         <address><email></email></address>
954      </author>
955      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
956         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
957         <address><email></email></address>
958      </author>
959      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
960         <organization abbrev="W3C">World Wide Web Consortium</organization>
961         <address><email></email></address>
962      </author>
963      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
964         <organization abbrev="greenbytes">greenbytes GmbH</organization>
965         <address><email></email></address>
966      </author>
967      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
968   </front>
969   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
970   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
973<reference anchor="Part5">
974   <front>
975      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
976      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
977         <organization abbrev="Day Software">Day Software</organization>
978         <address><email></email></address>
979      </author>
980      <author initials="J." surname="Gettys" fullname="Jim Gettys">
981         <organization>One Laptop per Child</organization>
982         <address><email></email></address>
983      </author>
984      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
985         <organization abbrev="HP">Hewlett-Packard Company</organization>
986         <address><email></email></address>
987      </author>
988      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
989         <organization abbrev="Microsoft">Microsoft Corporation</organization>
990         <address><email></email></address>
991      </author>
992      <author initials="L." surname="Masinter" fullname="Larry Masinter">
993         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
994         <address><email></email></address>
995      </author>
996      <author initials="P." surname="Leach" fullname="Paul J. Leach">
997         <organization abbrev="Microsoft">Microsoft Corporation</organization>
998         <address><email></email></address>
999      </author>
1000      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1001         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1002         <address><email></email></address>
1003      </author>
1004      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1005         <organization abbrev="W3C">World Wide Web Consortium</organization>
1006         <address><email></email></address>
1007      </author>
1008      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1009         <organization abbrev="greenbytes">greenbytes GmbH</organization>
1010         <address><email></email></address>
1011      </author>
1012      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1013   </front>
1014   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1015   <x:source href="p5-range.xml" basename="p5-range"/>
1018<reference anchor="Part6">
1019   <front>
1020      <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1021      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1022         <organization abbrev="Day Software">Day Software</organization>
1023         <address><email></email></address>
1024      </author>
1025      <author initials="J." surname="Gettys" fullname="Jim Gettys">
1026         <organization>One Laptop per Child</organization>
1027         <address><email></email></address>
1028      </author>
1029      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1030         <organization abbrev="HP">Hewlett-Packard Company</organization>
1031         <address><email></email></address>
1032      </author>
1033      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1034         <organization abbrev="Microsoft">Microsoft Corporation</organization>
1035         <address><email></email></address>
1036      </author>
1037      <author initials="L." surname="Masinter" fullname="Larry Masinter">
1038         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1039         <address><email></email></address>
1040      </author>
1041      <author initials="P." surname="Leach" fullname="Paul J. Leach">
1042         <organization abbrev="Microsoft">Microsoft Corporation</organization>
1043         <address><email></email></address>
1044      </author>
1045      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1046         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1047         <address><email></email></address>
1048      </author>
1049      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1050         <organization abbrev="W3C">World Wide Web Consortium</organization>
1051         <address><email></email></address>
1052      </author>
1053      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1054         <organization abbrev="greenbytes">greenbytes GmbH</organization>
1055         <address><email></email></address>
1056      </author>
1057      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1058   </front>
1059   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1060   <x:source href="p6-cache.xml" basename="p6-cache"/>
1063<reference anchor="RFC2616">
1064   <front>
1065      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1066      <author initials="R." surname="Fielding" fullname="R. Fielding">
1067         <organization>University of California, Irvine</organization>
1068         <address><email></email></address>
1069      </author>
1070      <author initials="J." surname="Gettys" fullname="J. Gettys">
1071         <organization>W3C</organization>
1072         <address><email></email></address>
1073      </author>
1074      <author initials="J." surname="Mogul" fullname="J. Mogul">
1075         <organization>Compaq Computer Corporation</organization>
1076         <address><email></email></address>
1077      </author>
1078      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1079         <organization>MIT Laboratory for Computer Science</organization>
1080         <address><email></email></address>
1081      </author>
1082      <author initials="L." surname="Masinter" fullname="L. Masinter">
1083         <organization>Xerox Corporation</organization>
1084         <address><email></email></address>
1085      </author>
1086      <author initials="P." surname="Leach" fullname="P. Leach">
1087         <organization>Microsoft Corporation</organization>
1088         <address><email></email></address>
1089      </author>
1090      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1091         <organization>W3C</organization>
1092         <address><email></email></address>
1093      </author>
1094      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1095         <organization abbrev="W3C">World Wide Web Consortium</organization>
1096         <address><email></email></address>
1097      </author>
1098      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1099         <organization abbrev="greenbytes">greenbytes GmbH</organization>
1100         <address><email></email></address>
1101      </author>
1102      <date month="June" year="1999"/>
1103   </front>
1104   <seriesInfo name="RFC" value="2616"/>
1107<reference anchor="RFC2119">
1108  <front>
1109    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1110    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1111      <organization>Harvard University</organization>
1112      <address><email></email></address>
1113    </author>
1114    <date month="March" year="1997"/>
1115  </front>
1116  <seriesInfo name="BCP" value="14"/>
1117  <seriesInfo name="RFC" value="2119"/>
1120<reference anchor="RFC2068">
1121  <front>
1122    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1123    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1124      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1125      <address><email></email></address>
1126    </author>
1127    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1128      <organization>MIT Laboratory for Computer Science</organization>
1129      <address><email></email></address>
1130    </author>
1131    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1132      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1133      <address><email></email></address>
1134    </author>
1135    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1136      <organization>MIT Laboratory for Computer Science</organization>
1137      <address><email></email></address>
1138    </author>
1139    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1140      <organization>MIT Laboratory for Computer Science</organization>
1141      <address><email></email></address>
1142    </author>
1143    <date month="January" year="1997"/>
1144  </front>
1145  <seriesInfo name="RFC" value="2068"/>
1150<section title="Compatibility with Previous Versions" anchor="compatibility">
1152<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1157<section title="Change Log (to be removed by RFC Editor before publication)">
1159<section title="Since RFC2616">
1161  Extracted relevant partitions from <xref target="RFC2616"/>.
1165<section title="Since draft-ietf-httpbis-p4-conditional-00">
1167  Other changes:
1168  <list style="symbols">
1169    <t>
1170      Move definitions of 304 and 412 condition codes from Part2.
1171    </t>
1172  </list>
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