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1<?xml version="1.0" encoding="UTF-8"?>
2<!--
3    This XML document is the output of clean-for-DTD.xslt; a tool that strips
4    extensions to RFC2629(bis) from documents for processing with xml2rfc.
5-->
6<?rfc toc="yes" ?>
7<?rfc symrefs="yes" ?>
8<?rfc sortrefs="yes" ?>
9<?rfc compact="yes"?>
10<?rfc subcompact="no" ?>
11<?rfc linkmailto="no" ?>
12<?rfc editing="no" ?>
13<!DOCTYPE rfc
14  PUBLIC "" "rfc2629.dtd">
15<rfc obsoletes="2068, 2616" category="std" ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-00">
16<front>
17
18  <title abbrev="HTTP/1.1, part 4">HTTP/1.1, part 4: Conditional Requests</title>
19
20  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
21    <organization abbrev="Day Software">Day Software</organization>
22    <address>
23      <postal>
24        <street>23 Corporate Plaza DR, Suite 280</street>
25        <city>Newport Beach</city>
26        <region>CA</region>
27        <code>92660</code>
28        <country>USA</country>
29      </postal>
30      <phone>+1-949-706-5300</phone>
31      <facsimile>+1-949-706-5305</facsimile>
32      <email>fielding@gbiv.com</email>
33      <uri>http://roy.gbiv.com/</uri>
34    </address>
35  </author>
36
37  <author initials="J." surname="Gettys" fullname="Jim Gettys">
38    <organization>One Laptop per Child</organization>
39    <address>
40      <postal>
41        <street>21 Oak Knoll Road</street>
42        <city>Carlisle</city>
43        <region>MA</region>
44        <code>01741</code>
45        <country>USA</country>
46      </postal>
47      <email>jg@laptop.org</email>
48      <uri>http://www.laptop.org/</uri>
49    </address>
50  </author>
51 
52  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
53    <organization abbrev="HP">Hewlett-Packard Company</organization>
54    <address>
55      <postal>
56        <street>HP Labs, Large Scale Systems Group</street>
57        <street>1501 Page Mill Road, MS 1177</street>
58        <city>Palo Alto</city>
59        <region>CA</region>
60        <code>94304</code>
61        <country>USA</country>
62      </postal>
63      <email>JeffMogul@acm.org</email>
64    </address>
65  </author>
66
67  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
68    <organization abbrev="Microsoft">Microsoft Corporation</organization>
69    <address>
70      <postal>
71        <street>1 Microsoft Way</street>
72        <city>Redmond</city>
73        <region>WA</region>
74        <code>98052</code>
75        <country>USA</country>
76      </postal>
77      <email>henrikn@microsoft.com</email>
78    </address>
79  </author>
80
81  <author initials="L." surname="Masinter" fullname="Larry Masinter">
82    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
83    <address>
84      <postal>
85        <street>345 Park Ave</street>
86        <city>San Jose</city>
87        <region>CA</region>
88        <code>95110</code>
89        <country>USA</country>
90      </postal>
91      <email>LMM@acm.org</email>
92      <uri>http://larry.masinter.net/</uri>
93    </address>
94  </author>
95 
96  <author initials="P." surname="Leach" fullname="Paul J. Leach">
97    <organization abbrev="Microsoft">Microsoft Corporation</organization>
98    <address>
99      <postal>
100        <street>1 Microsoft Way</street>
101        <city>Redmond</city>
102        <region>WA</region>
103        <code>98052</code>
104      </postal>
105      <email>paulle@microsoft.com</email>
106    </address>
107  </author>
108   
109  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
110    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
111    <address>
112      <postal>
113        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
114        <street>The Stata Center, Building 32</street>
115        <street>32 Vassar Street</street>
116        <city>Cambridge</city>
117        <region>MA</region>
118        <code>02139</code>
119        <country>USA</country>
120      </postal>
121      <email>timbl@w3.org</email>
122      <uri>http://www.w3.org/People/Berners-Lee/</uri>
123    </address>
124  </author>
125
126  <date month="December" year="2007"/>
127
128<abstract>
129<t>
130   The Hypertext Transfer Protocol (HTTP) is an application-level
131   protocol for distributed, collaborative, hypermedia information
132   systems. HTTP has been in use by the World Wide Web global information
133   initiative since 1990. This document is Part 4 of the seven-part specification
134   that defines the protocol referred to as "HTTP/1.1" and, taken together,
135   obsoletes RFC 2616.  Part 4 defines request header fields for
136   indicating conditional requests and the rules for constructing responses
137   to those requests.
138</t>
139</abstract>
140
141<note title="Editorial Note (To be removed by RFC Editor)">
142  <t>
143    This version of the HTTP specification contains only minimal editorial
144    changes from <xref target="RFC2616"/> (abstract, introductory paragraph,
145    and authors' addresses).  All other changes are due to partitioning the
146    original into seven mostly independent parts.  The intent is for readers
147    of future drafts to able to use draft 00 as the basis for comparison
148    when the WG makes later changes to the specification text.  This draft
149    will shortly be followed by draft 01 (containing the first round of changes
150    that have already been agreed to on the mailing list). There is no point in
151    reviewing this draft other than to verify that the partitioning has been
152    done correctly.  Roy T. Fielding, Yves Lafon, and Julian Reschke
153    will be the editors after draft 00 is submitted.
154  </t>
155  <t>
156    Discussion of this draft should take place on the HTTPBIS working group
157    mailing list (ietf-http-wg@w3.org). The current issues list is
158    at <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
159    and related documents (including fancy diffs) can be found at
160    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
161  </t>
162</note>
163</front>
164<middle>
165<section title="Introduction" anchor="introduction">
166<t>
167   This document will define aspects of HTTP related to conditional
168   request messages based on time stamps and entity-tags.  Right now it
169   only includes the extracted relevant sections of <xref target="RFC2616">RFC 2616</xref>
170   without edit.
171</t>
172</section>
173
174<section title="Entity Tags" anchor="entity.tags">
175<t>
176   Entity tags are used for comparing two or more entities from the same
177   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
178   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
179   If-Range (Section 5.3 of <xref target="Part5"/>) header fields. The definition of how they
180   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
181   entity tag consists of an opaque quoted string, possibly prefixed by
182   a weakness indicator.
183</t>
184<figure><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/><artwork type="abnf2616"><![CDATA[
185   entity-tag = [ weak ] opaque-tag
186   weak       = "W/"
187   opaque-tag = quoted-string
188]]></artwork></figure>
189<t>
190   A "strong entity tag" MAY be shared by two entities of a resource
191   only if they are equivalent by octet equality.
192</t>
193<t>
194   A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
195   two entities of a resource only if the entities are equivalent and
196   could be substituted for each other with no significant change in
197   semantics. A weak entity tag can only be used for weak comparison.
198</t>
199<t>
200   An entity tag MUST be unique across all versions of all entities
201   associated with a particular resource. A given entity tag value MAY
202   be used for entities obtained by requests on different URIs. The use
203   of the same entity tag value in conjunction with entities obtained by
204   requests on different URIs does not imply the equivalence of those
205   entities.
206</t>
207</section>
208
209<section title="Status Code Definitions">
210<section title="304 Not Modified" anchor="status.304">
211  <iref primary="true" item="304 Not Modified (status code)"/>
212  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
213<t>
214   If the client has performed a conditional GET request and access is
215   allowed, but the document has not been modified, the server SHOULD
216   respond with this status code. The 304 response MUST NOT contain a
217   message-body, and thus is always terminated by the first empty line
218   after the header fields.
219</t>
220<t>
221   The response MUST include the following header fields:
222  <list style="symbols">
223    <t>Date, unless its omission is required by Section 8.3.1 of <xref target="Part1"/></t>
224  </list>
225</t>
226<t>
227   If a clockless origin server obeys these rules, and proxies and
228   clients add their own Date to any response received without one (as
229   already specified by <xref target="RFC2068"/>, section 14.19), caches will operate
230   correctly.
231  <list style="symbols">
232    <t>ETag and/or Content-Location, if the header would have been sent
233        in a 200 response to the same request</t>
234    <t>Expires, Cache-Control, and/or Vary, if the field-value might
235        differ from that sent in any previous response for the same
236        variant</t>
237  </list>
238</t>
239<t>
240   If the conditional GET used a strong cache validator (see <xref target="Part6"/>),
241   the response SHOULD NOT  include other entity-headers.
242   Otherwise (i.e., the conditional GET used a weak validator), the
243   response MUST NOT include other entity-headers; this prevents
244   inconsistencies between cached entity-bodies and updated headers.
245</t>
246<t>
247   If a 304 response indicates an entity not currently cached, then the
248   cache MUST disregard the response and repeat the request without the
249   conditional.
250</t>
251<t>
252   If a cache uses a received 304 response to update a cache entry, the
253   cache MUST update the entry to reflect any new field values given in
254   the response.
255</t>
256</section>
257
258<section title="412 Precondition Failed" anchor="status.412">
259  <iref primary="true" item="412 Precondition Failed (status code)"/>
260  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
261<t>
262   The precondition given in one or more of the request-header fields
263   evaluated to false when it was tested on the server. This response
264   code allows the client to place preconditions on the current resource
265   metainformation (header field data) and thus prevent the requested
266   method from being applied to a resource other than the one intended.
267</t>
268</section>
269</section>
270
271<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
272<t>
273   Since both origin servers and caches will compare two validators to
274   decide if they represent the same or different entities, one normally
275   would expect that if the entity (the entity-body or any entity-headers)
276   changes in any way, then the associated validator would
277   change as well. If this is true, then we call this validator a
278   "strong validator."
279</t>
280<t>
281   However, there might be cases when a server prefers to change the
282   validator only on semantically significant changes, and not when
283   insignificant aspects of the entity change. A validator that does not
284   always change when the resource changes is a "weak validator."
285</t>
286<t>
287   Entity tags are normally "strong validators," but the protocol
288   provides a mechanism to tag an entity tag as "weak." One can think of
289   a strong validator as one that changes whenever the bits of an entity
290   changes, while a weak value changes whenever the meaning of an entity
291   changes. Alternatively, one can think of a strong validator as part
292   of an identifier for a specific entity, while a weak validator is
293   part of an identifier for a set of semantically equivalent entities.
294  <list><t>
295      Note: One example of a strong validator is an integer that is
296      incremented in stable storage every time an entity is changed.
297    </t><t>
298      An entity's modification time, if represented with one-second
299      resolution, could be a weak validator, since it is possible that
300      the resource might be modified twice during a single second.
301    </t><t>
302      Support for weak validators is optional. However, weak validators
303      allow for more efficient caching of equivalent objects; for
304      example, a hit counter on a site is probably good enough if it is
305      updated every few days or weeks, and any value during that period
306      is likely "good enough" to be equivalent.
307    </t></list>
308</t>
309<t>
310   A "use" of a validator is either when a client generates a request
311   and includes the validator in a validating header field, or when a
312   server compares two validators.
313</t>
314<t>
315   Strong validators are usable in any context. Weak validators are only
316   usable in contexts that do not depend on exact equality of an entity.
317   For example, either kind is usable for a conditional GET of a full
318   entity. However, only a strong validator is usable for a sub-range
319   retrieval, since otherwise the client might end up with an internally
320   inconsistent entity.
321</t>
322<t>
323   Clients MAY issue simple (non-subrange) GET requests with either weak
324   validators or strong validators. Clients MUST NOT use weak validators
325   in other forms of request.
326</t>
327<t>
328   The only function that the HTTP/1.1 protocol defines on validators is
329   comparison. There are two validator comparison functions, depending
330   on whether the comparison context allows the use of weak validators
331   or not:
332  <list style="symbols">
333     <t>The strong comparison function: in order to be considered equal,
334        both validators MUST be identical in every way, and both MUST NOT
335        be weak.</t>
336     <t>The weak comparison function: in order to be considered equal,
337        both validators MUST be identical in every way, but either or
338        both of them MAY be tagged as "weak" without affecting the
339        result.</t>
340  </list>
341</t>
342<t>
343   An entity tag is strong unless it is explicitly tagged as weak.
344   <xref target="entity.tags"/> gives the syntax for entity tags.
345</t>
346<t>
347   A Last-Modified time, when used as a validator in a request, is
348   implicitly weak unless it is possible to deduce that it is strong,
349   using the following rules:
350  <list style="symbols">
351     <t>The validator is being compared by an origin server to the
352        actual current validator for the entity and,</t>
353     <t>That origin server reliably knows that the associated entity did
354        not change twice during the second covered by the presented
355        validator.</t>
356  </list>
357</t>
358<t>
359   or
360  <list style="symbols">
361     <t>The validator is about to be used by a client in an If-Modified-Since
362        or If-Unmodified-Since header, because the client
363        has a cache entry for the associated entity, and</t>
364     <t>That cache entry includes a Date value, which gives the time
365        when the origin server sent the original response, and</t>
366     <t>The presented Last-Modified time is at least 60 seconds before
367        the Date value.</t>
368  </list>
369</t>
370<t>
371   or
372  <list style="symbols">
373     <t>The validator is being compared by an intermediate cache to the
374        validator stored in its cache entry for the entity, and</t>
375     <t>That cache entry includes a Date value, which gives the time
376        when the origin server sent the original response, and</t>
377     <t>The presented Last-Modified time is at least 60 seconds before
378        the Date value.</t>
379  </list>
380</t>
381<t>
382   This method relies on the fact that if two different responses were
383   sent by the origin server during the same second, but both had the
384   same Last-Modified time, then at least one of those responses would
385   have a Date value equal to its Last-Modified time. The arbitrary 60-second
386   limit guards against the possibility that the Date and Last-Modified
387   values are generated from different clocks, or at somewhat
388   different times during the preparation of the response. An
389   implementation MAY use a value larger than 60 seconds, if it is
390   believed that 60 seconds is too short.
391</t>
392<t>
393   If a client wishes to perform a sub-range retrieval on a value for
394   which it has only a Last-Modified time and no opaque validator, it
395   MAY do this only if the Last-Modified time is strong in the sense
396   described here.
397</t>
398<t>
399   A cache or origin server receiving a conditional request, other than
400   a full-body GET request, MUST use the strong comparison function to
401   evaluate the condition.
402</t>
403<t>
404   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
405   retrievals on values that have been obtained from HTTP/1.0
406   servers.
407</t>
408</section>
409
410<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="rules.for.when.to.use.entity.tags.and.last-modified.dates">
411<t>
412   We adopt a set of rules and recommendations for origin servers,
413   clients, and caches regarding when various validator types ought to
414   be used, and for what purposes.
415</t>
416<t>
417   HTTP/1.1 origin servers:
418  <list style="symbols">
419     <t>SHOULD send an entity tag validator unless it is not feasible to
420        generate one.</t>
421
422     <t>MAY send a weak entity tag instead of a strong entity tag, if
423        performance considerations support the use of weak entity tags,
424        or if it is unfeasible to send a strong entity tag.</t>
425
426     <t>SHOULD send a Last-Modified value if it is feasible to send one,
427        unless the risk of a breakdown in semantic transparency that
428        could result from using this date in an If-Modified-Since header
429        would lead to serious problems.</t>
430  </list>
431</t>
432<t>
433   In other words, the preferred behavior for an HTTP/1.1 origin server
434   is to send both a strong entity tag and a Last-Modified value.
435</t>
436<t>
437   In order to be legal, a strong entity tag MUST change whenever the
438   associated entity value changes in any way. A weak entity tag SHOULD
439   change whenever the associated entity changes in a semantically
440   significant way.
441  <list><t>
442      Note: in order to provide semantically transparent caching, an
443      origin server must avoid reusing a specific strong entity tag
444      value for two different entities, or reusing a specific weak
445      entity tag value for two semantically different entities. Cache
446      entries might persist for arbitrarily long periods, regardless of
447      expiration times, so it might be inappropriate to expect that a
448      cache will never again attempt to validate an entry using a
449      validator that it obtained at some point in the past.
450  </t></list>
451</t>
452<t>
453   HTTP/1.1 clients:
454  <list style="symbols">
455     <t>If an entity tag has been provided by the origin server, MUST
456        use that entity tag in any cache-conditional request (using If-Match
457        or If-None-Match).</t>
458
459     <t>If only a Last-Modified value has been provided by the origin
460        server, SHOULD use that value in non-subrange cache-conditional
461        requests (using If-Modified-Since).</t>
462
463     <t>If only a Last-Modified value has been provided by an HTTP/1.0
464        origin server, MAY use that value in subrange cache-conditional
465        requests (using If-Unmodified-Since:). The user agent SHOULD
466        provide a way to disable this, in case of difficulty.</t>
467
468     <t>If both an entity tag and a Last-Modified value have been
469        provided by the origin server, SHOULD use both validators in
470        cache-conditional requests. This allows both HTTP/1.0 and
471        HTTP/1.1 caches to respond appropriately.</t>
472  </list>
473</t>
474<t>
475   An HTTP/1.1 origin server, upon receiving a conditional request that
476   includes both a Last-Modified date (e.g., in an If-Modified-Since or
477   If-Unmodified-Since header field) and one or more entity tags (e.g.,
478   in an If-Match, If-None-Match, or If-Range header field) as cache
479   validators, MUST NOT return a response status of 304 (Not Modified)
480   unless doing so is consistent with all of the conditional header
481   fields in the request.
482</t>
483<t>
484   An HTTP/1.1 caching proxy, upon receiving a conditional request that
485   includes both a Last-Modified date and one or more entity tags as
486   cache validators, MUST NOT return a locally cached response to the
487   client unless that cached response is consistent with all of the
488   conditional header fields in the request.
489  <list><t>
490      Note: The general principle behind these rules is that HTTP/1.1
491      servers and clients should transmit as much non-redundant
492      information as is available in their responses and requests.
493      HTTP/1.1 systems receiving this information will make the most
494      conservative assumptions about the validators they receive.
495  </t><t>
496      HTTP/1.0 clients and caches will ignore entity tags. Generally,
497      last-modified values received or used by these systems will
498      support transparent and efficient caching, and so HTTP/1.1 origin
499      servers should provide Last-Modified values. In those rare cases
500      where the use of a Last-Modified value as a validator by an
501      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
502      origin servers should not provide one.
503  </t></list>
504</t>
505</section>
506
507<section title="Header Field Definitions" anchor="header.fields">
508<t>
509   This section defines the syntax and semantics of all standard
510   HTTP/1.1 header fields. For entity-header fields, both sender and
511   recipient refer to either the client or the server, depending on who
512   sends and who receives the entity.
513</t>
514
515<section title="ETag" anchor="header.etag">
516  <iref primary="true" item="ETag header"/>
517  <iref primary="true" item="Headers" subitem="ETag"/>
518<t>
519   The ETag response-header field provides the current value of the
520   entity tag for the requested variant. The headers used with entity
521   tags are described in sections <xref target="header.if-match" format="counter"/>, <xref target="header.if-none-match" format="counter"/> and Section 5.3 of <xref target="Part5"/>. The entity tag
522   MAY be used for comparison with other entities from the same resource
523   (see <xref target="weak.and.strong.validators"/>).
524</t>
525<figure><iref primary="true" item="Grammar" subitem="ETag"/><artwork type="abnf2616"><![CDATA[
526    ETag = "ETag" ":" entity-tag
527]]></artwork></figure>
528<figure><preamble>
529   Examples:
530</preamble>
531<artwork type="example"><![CDATA[
532   ETag: "xyzzy"
533   ETag: W/"xyzzy"
534   ETag: ""
535]]></artwork></figure>
536</section>
537
538<section title="If-Match" anchor="header.if-match">
539  <iref primary="true" item="If-Match header"/>
540  <iref primary="true" item="Headers" subitem="If-Match"/>
541<t>
542   The If-Match request-header field is used with a method to make it
543   conditional. A client that has one or more entities previously
544   obtained from the resource can verify that one of those entities is
545   current by including a list of their associated entity tags in the
546   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
547   purpose of this feature is to allow efficient updates of cached
548   information with a minimum amount of transaction overhead. It is also
549   used, on updating requests, to prevent inadvertent modification of
550   the wrong version of a resource. As a special case, the value "*"
551   matches any current entity of the resource.
552</t>
553<figure><iref primary="true" item="Grammar" subitem="If-Match"/><artwork type="abnf2616"><![CDATA[
554    If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
555]]></artwork></figure>
556<t>
557   If any of the entity tags match the entity tag of the entity that
558   would have been returned in the response to a similar GET request
559   (without the If-Match header) on that resource, or if "*" is given
560   and any current entity exists for that resource, then the server MAY
561   perform the requested method as if the If-Match header field did not
562   exist.
563</t>
564<t>
565   A server MUST use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
566   to compare the entity tags in If-Match.
567</t>
568<t>
569   If none of the entity tags match, or if "*" is given and no current
570   entity exists, the server MUST NOT perform the requested method, and
571   MUST return a 412 (Precondition Failed) response. This behavior is
572   most useful when the client wants to prevent an updating method, such
573   as PUT, from modifying a resource that has changed since the client
574   last retrieved it.
575</t>
576<t>
577   If the request would, without the If-Match header field, result in
578   anything other than a 2xx or 412 status, then the If-Match header
579   MUST be ignored.
580</t>
581<t>
582   The meaning of "If-Match: *" is that the method SHOULD be performed
583   if the representation selected by the origin server (or by a cache,
584   possibly using the Vary mechanism, see Section 3.5 of <xref target="Part6"/>) exists, and
585   MUST NOT be performed if the representation does not exist.
586</t>
587<t>
588   A request intended to update a resource (e.g., a PUT) MAY include an
589   If-Match header field to signal that the request method MUST NOT be
590   applied if the entity corresponding to the If-Match value (a single
591   entity tag) is no longer a representation of that resource. This
592   allows the user to indicate that they do not wish the request to be
593   successful if the resource has been changed without their knowledge.
594   Examples:
595</t>
596<figure><artwork type="example"><![CDATA[
597    If-Match: "xyzzy"
598    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
599    If-Match: *
600]]></artwork></figure>
601<t>
602   The result of a request having both an If-Match header field and
603   either an If-None-Match or an If-Modified-Since header fields is
604   undefined by this specification.
605</t>
606</section>
607
608<section title="If-Modified-Since" anchor="header.if-modified-since">
609  <iref primary="true" item="If-Modified-Since header"/>
610  <iref primary="true" item="Headers" subitem="If-Modified-Since"/>
611<t>
612   The If-Modified-Since request-header field is used with a method to
613   make it conditional: if the requested variant has not been modified
614   since the time specified in this field, an entity will not be
615   returned from the server; instead, a 304 (not modified) response will
616   be returned without any message-body.
617</t>
618<figure><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><artwork type="abnf2616"><![CDATA[
619    If-Modified-Since = "If-Modified-Since" ":" HTTP-date
620]]></artwork></figure>
621<t>
622   An example of the field is:
623</t>
624<figure><artwork type="example"><![CDATA[
625    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
626]]></artwork></figure>
627<t>
628   A GET method with an If-Modified-Since header and no Range header
629   requests that the identified entity be transferred only if it has
630   been modified since the date given by the If-Modified-Since header.
631   The algorithm for determining this includes the following cases:
632  <list style="numbers">
633      <t>If the request would normally result in anything other than a
634         200 (OK) status, or if the passed If-Modified-Since date is
635         invalid, the response is exactly the same as for a normal GET.
636         A date which is later than the server's current time is
637         invalid.</t>
638
639      <t>If the variant has been modified since the If-Modified-Since
640         date, the response is exactly the same as for a normal GET.</t>
641
642      <t>If the variant has not been modified since a valid If-Modified-Since
643         date, the server SHOULD return a 304 (Not
644         Modified) response.</t>
645  </list>
646</t>
647<t>
648   The purpose of this feature is to allow efficient updates of cached
649   information with a minimum amount of transaction overhead.
650  <list><t>
651      Note: The Range request-header field modifies the meaning of If-Modified-Since;
652      see Section 5.4 of <xref target="Part5"/> for full details.
653    </t><t>
654      Note: If-Modified-Since times are interpreted by the server, whose
655      clock might not be synchronized with the client.
656    </t><t>
657      Note: When handling an If-Modified-Since header field, some
658      servers will use an exact date comparison function, rather than a
659      less-than function, for deciding whether to send a 304 (Not
660      Modified) response. To get best results when sending an If-Modified-Since
661      header field for cache validation, clients are
662      advised to use the exact date string received in a previous Last-Modified
663      header field whenever possible.
664    </t><t>
665      Note: If a client uses an arbitrary date in the If-Modified-Since
666      header instead of a date taken from the Last-Modified header for
667      the same request, the client should be aware of the fact that this
668      date is interpreted in the server's understanding of time. The
669      client should consider unsynchronized clocks and rounding problems
670      due to the different encodings of time between the client and
671      server. This includes the possibility of race conditions if the
672      document has changed between the time it was first requested and
673      the If-Modified-Since date of a subsequent request, and the
674      possibility of clock-skew-related problems if the If-Modified-Since
675      date is derived from the client's clock without correction
676      to the server's clock. Corrections for different time bases
677      between client and server are at best approximate due to network
678      latency.
679    </t>
680  </list>
681</t>
682<t>
683   The result of a request having both an If-Modified-Since header field
684   and either an If-Match or an If-Unmodified-Since header fields is
685   undefined by this specification.
686</t>
687</section>
688
689<section title="If-None-Match" anchor="header.if-none-match">
690  <iref primary="true" item="If-None-Match header"/>
691  <iref primary="true" item="Headers" subitem="If-None-Match"/>
692<t>
693   The If-None-Match request-header field is used with a method to make
694   it conditional. A client that has one or more entities previously
695   obtained from the resource can verify that none of those entities is
696   current by including a list of their associated entity tags in the
697   If-None-Match header field. The purpose of this feature is to allow
698   efficient updates of cached information with a minimum amount of
699   transaction overhead. It is also used to prevent a method (e.g. PUT)
700   from inadvertently modifying an existing resource when the client
701   believes that the resource does not exist.
702</t>
703<t>
704   As a special case, the value "*" matches any current entity of the
705   resource.
706</t>
707<figure><iref primary="true" item="Grammar" subitem="If-None-Match"/><artwork type="abnf2616"><![CDATA[
708    If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
709]]></artwork></figure>
710<t>
711   If any of the entity tags match the entity tag of the entity that
712   would have been returned in the response to a similar GET request
713   (without the If-None-Match header) on that resource, or if "*" is
714   given and any current entity exists for that resource, then the
715   server MUST NOT perform the requested method, unless required to do
716   so because the resource's modification date fails to match that
717   supplied in an If-Modified-Since header field in the request.
718   Instead, if the request method was GET or HEAD, the server SHOULD
719   respond with a 304 (Not Modified) response, including the cache-related
720   header fields (particularly ETag) of one of the entities that
721   matched. For all other request methods, the server MUST respond with
722   a status of 412 (Precondition Failed).
723</t>
724<t>
725   See <xref target="weak.and.strong.validators"/> for rules on how to determine if two entities tags
726   match. The weak comparison function can only be used with GET or HEAD
727   requests.
728</t>
729<t>
730   If none of the entity tags match, then the server MAY perform the
731   requested method as if the If-None-Match header field did not exist,
732   but MUST also ignore any If-Modified-Since header field(s) in the
733   request. That is, if no entity tags match, then the server MUST NOT
734   return a 304 (Not Modified) response.
735</t>
736<t>
737   If the request would, without the If-None-Match header field, result
738   in anything other than a 2xx or 304 status, then the If-None-Match
739   header MUST be ignored. (See <xref target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for a discussion of
740   server behavior when both If-Modified-Since and If-None-Match appear
741   in the same request.)
742</t>
743<t>
744   The meaning of "If-None-Match: *" is that the method MUST NOT be
745   performed if the representation selected by the origin server (or by
746   a cache, possibly using the Vary mechanism, see Section 3.5 of <xref target="Part6"/>)
747   exists, and SHOULD be performed if the representation does not exist.
748   This feature is intended to be useful in preventing races between PUT
749   operations.
750</t>
751<t>
752   Examples:
753</t>
754<figure><artwork type="example"><![CDATA[
755    If-None-Match: "xyzzy"
756    If-None-Match: W/"xyzzy"
757    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
758    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
759    If-None-Match: *
760]]></artwork></figure>
761<t>
762   The result of a request having both an If-None-Match header field and
763   either an If-Match or an If-Unmodified-Since header fields is
764   undefined by this specification.
765</t>
766</section>
767
768<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
769  <iref primary="true" item="If-Unmodified-Since header"/>
770  <iref primary="true" item="Headers" subitem="If-Unmodified-Since"/>
771<t>
772   The If-Unmodified-Since request-header field is used with a method to
773   make it conditional. If the requested resource has not been modified
774   since the time specified in this field, the server SHOULD perform the
775   requested operation as if the If-Unmodified-Since header were not
776   present.
777</t>
778<t>
779   If the requested variant has been modified since the specified time,
780   the server MUST NOT perform the requested operation, and MUST return
781   a 412 (Precondition Failed).
782</t>
783<figure><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><artwork type="abnf2616"><![CDATA[
784   If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
785]]></artwork></figure>
786<t>
787   An example of the field is:
788</t>
789<figure><artwork type="example"><![CDATA[
790    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
791]]></artwork></figure>
792<t>
793   If the request normally (i.e., without the If-Unmodified-Since
794   header) would result in anything other than a 2xx or 412 status, the
795   If-Unmodified-Since header SHOULD be ignored.
796</t>
797<t>
798   If the specified date is invalid, the header is ignored.
799</t>
800<t>
801   The result of a request having both an If-Unmodified-Since header
802   field and either an If-None-Match or an If-Modified-Since header
803   fields is undefined by this specification.
804</t>
805</section>
806
807<section title="Last-Modified" anchor="header.last-modified">
808  <iref primary="true" item="Last-Modified header"/>
809  <iref primary="true" item="Headers" subitem="Last-Modified"/>
810<t>
811   The Last-Modified entity-header field indicates the date and time at
812   which the origin server believes the variant was last modified.
813</t>
814<figure><iref primary="true" item="Grammar" subitem="Last-Modified"/><artwork type="abnf2616"><![CDATA[
815    Last-Modified  = "Last-Modified" ":" HTTP-date
816]]></artwork></figure>
817<t>
818   An example of its use is
819</t>
820<figure><artwork type="example"><![CDATA[
821    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
822]]></artwork></figure>
823<t>
824   The exact meaning of this header field depends on the implementation
825   of the origin server and the nature of the original resource. For
826   files, it may be just the file system last-modified time. For
827   entities with dynamically included parts, it may be the most recent
828   of the set of last-modify times for its component parts. For database
829   gateways, it may be the last-update time stamp of the record. For
830   virtual objects, it may be the last time the internal state changed.
831</t>
832<t>
833   An origin server MUST NOT send a Last-Modified date which is later
834   than the server's time of message origination. In such cases, where
835   the resource's last modification would indicate some time in the
836   future, the server MUST replace that date with the message
837   origination date.
838</t>
839<t>
840   An origin server SHOULD obtain the Last-Modified value of the entity
841   as close as possible to the time that it generates the Date value of
842   its response. This allows a recipient to make an accurate assessment
843   of the entity's modification time, especially if the entity changes
844   near the time that the response is generated.
845</t>
846<t>
847   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
848</t>
849</section>
850
851</section>
852
853<section title="IANA Considerations" anchor="IANA.considerations">
854<t>
855   TBD.
856</t>
857</section>
858
859<section title="Security Considerations" anchor="security.considerations">
860<t>
861   No additional security considerations have been identified beyond
862   those applicable to HTTP in general <xref target="Part1"/>.
863</t>
864</section>
865
866<section title="Acknowledgments" anchor="ack">
867<t>
868   Based on an XML translation of RFC 2616 by Julian Reschke.
869</t>
870</section>
871</middle>
872<back>
873<references>
874   
875<reference anchor="Part1">
876   <front>
877      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
878      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
879         <organization abbrev="Day Software">Day Software</organization>
880         <address><email>fielding@gbiv.com</email></address>
881      </author>
882      <author initials="J." surname="Gettys" fullname="Jim Gettys">
883         <organization>One Laptop per Child</organization>
884         <address><email>jg@laptop.org</email></address>
885      </author>
886      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
887         <organization abbrev="HP">Hewlett-Packard Company</organization>
888         <address><email>JeffMogul@acm.org</email></address>
889      </author>
890      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
891         <organization abbrev="Microsoft">Microsoft Corporation</organization>
892         <address><email>henrikn@microsoft.com</email></address>
893      </author>
894      <author initials="L." surname="Masinter" fullname="Larry Masinter">
895         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
896         <address><email>LMM@acm.org</email></address>
897      </author>
898      <author initials="P." surname="Leach" fullname="Paul J. Leach">
899         <organization abbrev="Microsoft">Microsoft Corporation</organization>
900         <address><email>paulle@microsoft.com</email></address>
901      </author>
902      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
903         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
904         <address><email>timbl@w3.org</email></address>
905      </author>
906      <date month="December" year="2007"/>
907   </front>
908   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-00"/>
909   
910</reference>
911
912<reference anchor="Part5">
913   <front>
914      <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
915      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
916         <organization abbrev="Day Software">Day Software</organization>
917         <address><email>fielding@gbiv.com</email></address>
918      </author>
919      <author initials="J." surname="Gettys" fullname="Jim Gettys">
920         <organization>One Laptop per Child</organization>
921         <address><email>jg@laptop.org</email></address>
922      </author>
923      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
924         <organization abbrev="HP">Hewlett-Packard Company</organization>
925         <address><email>JeffMogul@acm.org</email></address>
926      </author>
927      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
928         <organization abbrev="Microsoft">Microsoft Corporation</organization>
929         <address><email>henrikn@microsoft.com</email></address>
930      </author>
931      <author initials="L." surname="Masinter" fullname="Larry Masinter">
932         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
933         <address><email>LMM@acm.org</email></address>
934      </author>
935      <author initials="P." surname="Leach" fullname="Paul J. Leach">
936         <organization abbrev="Microsoft">Microsoft Corporation</organization>
937         <address><email>paulle@microsoft.com</email></address>
938      </author>
939      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
940         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
941         <address><email>timbl@w3.org</email></address>
942      </author>
943      <date month="December" year="2007"/>
944   </front>
945   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-00"/>
946   
947</reference>
948
949<reference anchor="Part6">
950   <front>
951      <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
952      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
953         <organization abbrev="Day Software">Day Software</organization>
954         <address><email>fielding@gbiv.com</email></address>
955      </author>
956      <author initials="J." surname="Gettys" fullname="Jim Gettys">
957         <organization>One Laptop per Child</organization>
958         <address><email>jg@laptop.org</email></address>
959      </author>
960      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
961         <organization abbrev="HP">Hewlett-Packard Company</organization>
962         <address><email>JeffMogul@acm.org</email></address>
963      </author>
964      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
965         <organization abbrev="Microsoft">Microsoft Corporation</organization>
966         <address><email>henrikn@microsoft.com</email></address>
967      </author>
968      <author initials="L." surname="Masinter" fullname="Larry Masinter">
969         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
970         <address><email>LMM@acm.org</email></address>
971      </author>
972      <author initials="P." surname="Leach" fullname="Paul J. Leach">
973         <organization abbrev="Microsoft">Microsoft Corporation</organization>
974         <address><email>paulle@microsoft.com</email></address>
975      </author>
976      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
977         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
978         <address><email>timbl@w3.org</email></address>
979      </author>
980      <date month="December" year="2007"/>
981   </front>
982   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-00"/>
983   
984</reference>
985
986<reference anchor="RFC2616">
987   <front>
988      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
989      <author initials="R." surname="Fielding" fullname="R. Fielding">
990         <organization>University of California, Irvine</organization>
991         <address><email>fielding@ics.uci.edu</email></address>
992      </author>
993      <author initials="J." surname="Gettys" fullname="J. Gettys">
994         <organization>W3C</organization>
995         <address><email>jg@w3.org</email></address>
996      </author>
997      <author initials="J." surname="Mogul" fullname="J. Mogul">
998         <organization>Compaq Computer Corporation</organization>
999         <address><email>mogul@wrl.dec.com</email></address>
1000      </author>
1001      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1002         <organization>MIT Laboratory for Computer Science</organization>
1003         <address><email>frystyk@w3.org</email></address>
1004      </author>
1005      <author initials="L." surname="Masinter" fullname="L. Masinter">
1006         <organization>Xerox Corporation</organization>
1007         <address><email>masinter@parc.xerox.com</email></address>
1008      </author>
1009      <author initials="P." surname="Leach" fullname="P. Leach">
1010         <organization>Microsoft Corporation</organization>
1011         <address><email>paulle@microsoft.com</email></address>
1012      </author>
1013      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1014         <organization>W3C</organization>
1015         <address><email>timbl@w3.org</email></address>
1016      </author>
1017      <date month="June" year="1999"/>
1018   </front>
1019   <seriesInfo name="RFC" value="2616"/>
1020</reference>
1021
1022<reference anchor="RFC2068">
1023<front>
1024<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1025<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1026<organization>University of California, Irvine, Department of Information and Computer Science</organization>
1027<address>
1028<postal>
1029<street/>
1030<city>Irvine</city>
1031<region>CA</region>
1032<code>92717-3425</code>
1033<country>US</country></postal>
1034<facsimile>+1 714 824 4056</facsimile>
1035<email>fielding@ics.uci.edu</email></address></author>
1036<author initials="J." surname="Gettys" fullname="Jim Gettys">
1037<organization>MIT Laboratory for Computer Science</organization>
1038<address>
1039<postal>
1040<street>545 Technology Square</street>
1041<city>Cambridge</city>
1042<region>MA</region>
1043<code>02139</code>
1044<country>US</country></postal>
1045<facsimile>+1 617 258 8682</facsimile>
1046<email>jg@w3.org</email></address></author>
1047<author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1048<organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1049<address>
1050<postal>
1051<street>250 University Avenue</street>
1052<city>Palo Alto</city>
1053<region>CA</region>
1054<code>94301</code>
1055<country>US</country></postal>
1056<email>mogul@wrl.dec.com</email></address></author>
1057<author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1058<organization>MIT Laboratory for Computer Science</organization>
1059<address>
1060<postal>
1061<street>545 Technology Square</street>
1062<city>Cambridge</city>
1063<region>MA</region>
1064<code>02139</code>
1065<country>US</country></postal>
1066<facsimile>+1 617 258 8682</facsimile>
1067<email>frystyk@w3.org</email></address></author>
1068<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1069<organization>MIT Laboratory for Computer Science</organization>
1070<address>
1071<postal>
1072<street>545 Technology Square</street>
1073<city>Cambridge</city>
1074<region>MA</region>
1075<code>02139</code>
1076<country>US</country></postal>
1077<facsimile>+1 617 258 8682</facsimile>
1078<email>timbl@w3.org</email></address></author>
1079<date month="January" year="1997"/>
1080<abstract>
1081<t>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, object-oriented protocol which can be used for many tasks, such as name servers and distributed object management systems, through extension of its request methods. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.</t>
1082<t>HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1".</t></abstract></front>
1083<seriesInfo name="RFC" value="2068"/>
1084</reference>
1085
1086</references>
1087</back>
1088</rfc>
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