source: draft-ietf-httpbis/latest/p4-conditional.xml @ 543

Last change on this file since 543 was 543, checked in by julian.reschke@…, 11 years ago

Resolve #36: In P2..P7, mention list rule defined in P1; also point to Appendix containing collected ABNF with list rule being expanded (closes #36)

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File size: 62.8 KB
[29]1<?xml version="1.0" encoding="utf-8"?>
[101]2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
[8]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>">
[29]14  <!ENTITY ID-VERSION "latest">
[460]15  <!ENTITY ID-MONTH "March">
[439]16  <!ENTITY ID-YEAR "2009">
[424]17  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x=''/>">
[205]18  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x=''/>">
19  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x=''/>">
[306]20  <!ENTITY header-date                "<xref target='Part1' x:rel='' xmlns:x=''/>">
[31]21  <!ENTITY messaging                  "<xref target='Part1' xmlns:x=''/>">
[163]22  <!ENTITY caching                    "<xref target='Part6' xmlns:x=''/>">
[31]23  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x=''/>">
24  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x=''/>">
25  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x=''/>">
[45]26  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x=''/>">
[205]27  <!ENTITY full-date                  "<xref target='Part1' x:rel='' xmlns:x=''/>">
29<?rfc toc="yes" ?>
[29]30<?rfc symrefs="yes" ?>
31<?rfc sortrefs="yes" ?>
[8]32<?rfc compact="yes"?>
33<?rfc subcompact="no" ?>
34<?rfc linkmailto="no" ?>
35<?rfc editing="no" ?>
[203]36<?rfc comments="yes"?>
37<?rfc inline="yes"?>
[8]38<?rfc-ext allow-markup-in-artwork="yes" ?>
39<?rfc-ext include-references-in-index="yes" ?>
[308]40<rfc obsoletes="2616" category="std" x:maturity-level="draft"
[446]41     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
[153]42     xmlns:x=''>
[120]45  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
[29]47  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
48    <organization abbrev="Day Software">Day Software</organization>
[8]49    <address>
50      <postal>
[29]51        <street>23 Corporate Plaza DR, Suite 280</street>
52        <city>Newport Beach</city>
[8]53        <region>CA</region>
[29]54        <code>92660</code>
55        <country>USA</country>
[8]56      </postal>
[29]57      <phone>+1-949-706-5300</phone>
58      <facsimile>+1-949-706-5305</facsimile>
59      <email></email>
60      <uri></uri>
[8]61    </address>
62  </author>
[29]64  <author initials="J." surname="Gettys" fullname="Jim Gettys">
65    <organization>One Laptop per Child</organization>
[8]66    <address>
67      <postal>
[29]68        <street>21 Oak Knoll Road</street>
69        <city>Carlisle</city>
[8]70        <region>MA</region>
[29]71        <code>01741</code>
72        <country>USA</country>
[8]73      </postal>
[29]74      <email></email>
75      <uri></uri>
[8]76    </address>
77  </author>
79  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
[29]80    <organization abbrev="HP">Hewlett-Packard Company</organization>
[8]81    <address>
82      <postal>
[29]83        <street>HP Labs, Large Scale Systems Group</street>
84        <street>1501 Page Mill Road, MS 1177</street>
[8]85        <city>Palo Alto</city>
86        <region>CA</region>
[29]87        <code>94304</code>
88        <country>USA</country>
[8]89      </postal>
[29]90      <email></email>
[8]91    </address>
92  </author>
94  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
[29]95    <organization abbrev="Microsoft">Microsoft Corporation</organization>
[8]96    <address>
97      <postal>
[29]98        <street>1 Microsoft Way</street>
99        <city>Redmond</city>
100        <region>WA</region>
101        <code>98052</code>
102        <country>USA</country>
[8]103      </postal>
[29]104      <email></email>
[8]105    </address>
106  </author>
108  <author initials="L." surname="Masinter" fullname="Larry Masinter">
[29]109    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
[8]110    <address>
111      <postal>
[29]112        <street>345 Park Ave</street>
113        <city>San Jose</city>
[8]114        <region>CA</region>
[29]115        <code>95110</code>
116        <country>USA</country>
[8]117      </postal>
[29]118      <email></email>
119      <uri></uri>
[8]120    </address>
121  </author>
123  <author initials="P." surname="Leach" fullname="Paul J. Leach">
124    <organization abbrev="Microsoft">Microsoft Corporation</organization>
125    <address>
126      <postal>
127        <street>1 Microsoft Way</street>
128        <city>Redmond</city>
129        <region>WA</region>
130        <code>98052</code>
131      </postal>
132      <email></email>
133    </address>
134  </author>
136  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
137    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
138    <address>
139      <postal>
[34]140        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
141        <street>The Stata Center, Building 32</street>
142        <street>32 Vassar Street</street>
[8]143        <city>Cambridge</city>
144        <region>MA</region>
145        <code>02139</code>
[29]146        <country>USA</country>
[8]147      </postal>
148      <email></email>
[34]149      <uri></uri>
[8]150    </address>
151  </author>
[95]153  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
[94]154    <organization abbrev="W3C">World Wide Web Consortium</organization>
155    <address>
156      <postal>
157        <street>W3C / ERCIM</street>
158        <street>2004, rte des Lucioles</street>
159        <city>Sophia-Antipolis</city>
160        <region>AM</region>
161        <code>06902</code>
162        <country>France</country>
163      </postal>
164      <email></email>
165      <uri></uri>
166    </address>
167  </author>
169  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
170    <organization abbrev="greenbytes">greenbytes GmbH</organization>
171    <address>
172      <postal>
173        <street>Hafenweg 16</street>
174        <city>Muenster</city><region>NW</region><code>48155</code>
175        <country>Germany</country>
176      </postal>
177      <phone>+49 251 2807760</phone>   
178      <facsimile>+49 251 2807761</facsimile>   
179      <email></email>       
180      <uri></uri>     
181    </address>
182  </author>
[31]184  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
[440]185  <workgroup>HTTPbis Working Group</workgroup>
189   The Hypertext Transfer Protocol (HTTP) is an application-level
190   protocol for distributed, collaborative, hypermedia information
[29]191   systems. HTTP has been in use by the World Wide Web global information
[35]192   initiative since 1990. This document is Part 4 of the seven-part specification
[29]193   that defines the protocol referred to as "HTTP/1.1" and, taken together,
[42]194   obsoletes RFC 2616.  Part 4 defines request header fields for
[29]195   indicating conditional requests and the rules for constructing responses
196   to those requests.
200<note title="Editorial Note (To be removed by RFC Editor)">
201  <t>
202    Discussion of this draft should take place on the HTTPBIS working group
203    mailing list ( The current issues list is
[324]204    at <eref target=""/>
[36]205    and related documents (including fancy diffs) can be found at
[324]206    <eref target=""/>.
[36]207  </t>
[153]208  <t>
[382]209    The changes in this draft are summarized in <xref target="changes.since.05"/>.
[153]210  </t>
214<section title="Introduction" anchor="introduction">
[163]216   This document defines HTTP/1.1 response metadata for indicating potential
217   changes to payload content, including modification time stamps and opaque
218   entity-tags, and the HTTP conditional request mechanisms that allow
219   preconditions to be placed on a request method.  Conditional GET requests
220   allow for efficient cache updates.  Other conditional request methods are
221   used to protect against overwriting or misunderstanding the state of a
222   resource that has been changed unbeknownst to the requesting client.
225   This document is currently disorganized in order to minimize the changes
226   between drafts and enable reviewers to see the smaller errata changes.
227   The next draft will reorganize the sections to better reflect the content.
228   In particular, the sections on resource metadata will be discussed first
229   and then followed by each conditional request-header, concluding with a
230   definition of precedence and the expectation of ordering strong validator
231   checks before weak validator checks.  It is likely that more content from
232   &caching; will migrate to this part, where appropriate.
233   The current mess reflects how widely dispersed these topics and associated
234   requirements had become in <xref target="RFC2616"/>.
237<section title="Requirements" anchor="intro.requirements">
239   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
240   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
241   document are to be interpreted as described in <xref target="RFC2119"/>.
244   An implementation is not compliant if it fails to satisfy one or more
245   of the &MUST; or &REQUIRED; level requirements for the protocols it
246   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
247   level and all the &SHOULD; level requirements for its protocols is said
248   to be "unconditionally compliant"; one that satisfies all the &MUST;
249   level requirements but not all the &SHOULD; level requirements for its
250   protocols is said to be "conditionally compliant."
[424]254<section title="Syntax Notation" anchor="notation">
[425]255  <x:anchor-alias value="ALPHA"/>
256  <x:anchor-alias value="CR"/>
257  <x:anchor-alias value="DIGIT"/>
258  <x:anchor-alias value="LF"/>
259  <x:anchor-alias value="OCTET"/>
260  <x:anchor-alias value="VCHAR"/>
261  <x:anchor-alias value="WSP"/>
[543]263  This specification uses the ABNF syntax defined in &notation; (which
264  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
265  <xref target="collected.abnf"/> shows the collected ABNF, with the list
266  rule expanded.
[425]269  The following core rules are included by
270  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
271  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
272  DIGIT (decimal 0-9), DQUOTE (double quote),
273  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
274  OCTET (any 8-bit sequence of data), SP (space),
275  VCHAR (any visible USASCII character),
276  and WSP (whitespace).
279<section title="Core Rules" anchor="core.rules">
[229]280  <x:anchor-alias value="quoted-string"/>
[362]281  <x:anchor-alias value="OWS"/>
[424]283  The core rules below are defined in &basic-rules;:
285<figure><artwork type="abnf2616">
[229]286  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
[362]287  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
291<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
[229]292  <x:anchor-alias value="HTTP-date"/>
[205]294  The ABNF rules below are defined in other parts:
[207]296<figure><!--Part1--><artwork type="abnf2616">
[229]297  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
[8]305<section title="Entity Tags" anchor="entity.tags">
[229]306  <x:anchor-alias value="entity-tag"/>
307  <x:anchor-alias value="opaque-tag"/>
308  <x:anchor-alias value="weak"/>
310   Entity tags are used for comparing two or more entities from the same
311   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
312   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
[29]313   If-Range (&header-if-range;) header fields. The definition of how they
[8]314   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
315   entity tag consists of an opaque quoted string, possibly prefixed by
316   a weakness indicator.
318<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"/>
[229]319  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
320  <x:ref>weak</x:ref>       = "W/"
321  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
324   A "strong entity tag" &MAY; be shared by two entities of a resource
325   only if they are equivalent by octet equality.
328   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
329   two entities of a resource only if the entities are equivalent and
330   could be substituted for each other with no significant change in
331   semantics. A weak entity tag can only be used for weak comparison.
334   An entity tag &MUST; be unique across all versions of all entities
335   associated with a particular resource. A given entity tag value &MAY;
336   be used for entities obtained by requests on different URIs. The use
337   of the same entity tag value in conjunction with entities obtained by
338   requests on different URIs does not imply the equivalence of those
339   entities.
[45]343<section title="Status Code Definitions">
344<section title="304 Not Modified" anchor="status.304">
345  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
346  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
348   If the client has performed a conditional GET request and access is
349   allowed, but the document has not been modified, the server &SHOULD;
350   respond with this status code. The 304 response &MUST-NOT; contain a
351   message-body, and thus is always terminated by the first empty line
352   after the header fields.
355   The response &MUST; include the following header fields:
356  <list style="symbols">
[305]357    <x:lt>
[306]358      <t>Date, unless its omission is required by &clockless;.</t>
359      <t>
360         If a clockless origin server obeys these rules, and proxies and
[305]361         clients add their own Date to any response received without one (as
[306]362         already specified by &header-date;, caches will operate
[305]363         correctly.</t>
364    </x:lt>
365    <x:lt>
366      <t>ETag and/or Content-Location, if the header would have been sent
[306]367         in a 200 response to the same request.</t>
[305]368    </x:lt>
369    <x:lt>
370      <t>Expires, Cache-Control, and/or Vary, if the field-value might
[306]371         differ from that sent in any previous response for the same
372         variant.</t>
[305]373    </x:lt>
[45]374  </list>
[115]377   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
[45]378   the response &SHOULD-NOT;  include other entity-headers.
379   Otherwise (i.e., the conditional GET used a weak validator), the
380   response &MUST-NOT; include other entity-headers; this prevents
381   inconsistencies between cached entity-bodies and updated headers.
384   If a 304 response indicates an entity not currently cached, then the
385   cache &MUST; disregard the response and repeat the request without the
386   conditional.
389   If a cache uses a received 304 response to update a cache entry, the
390   cache &MUST; update the entry to reflect any new field values given in
391   the response.
395<section title="412 Precondition Failed" anchor="status.412">
396  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
397  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
399   The precondition given in one or more of the request-header fields
400   evaluated to false when it was tested on the server. This response
401   code allows the client to place preconditions on the current resource
402   metainformation (header field data) and thus prevent the requested
403   method from being applied to a resource other than the one intended.
[8]408<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
410   Since both origin servers and caches will compare two validators to
411   decide if they represent the same or different entities, one normally
412   would expect that if the entity (the entity-body or any entity-headers)
413   changes in any way, then the associated validator would
414   change as well. If this is true, then we call this validator a
415   "strong validator."
418   However, there might be cases when a server prefers to change the
419   validator only on semantically significant changes, and not when
420   insignificant aspects of the entity change. A validator that does not
421   always change when the resource changes is a "weak validator."
424   Entity tags are normally "strong validators," but the protocol
425   provides a mechanism to tag an entity tag as "weak." One can think of
426   a strong validator as one that changes whenever the bits of an entity
427   changes, while a weak value changes whenever the meaning of an entity
428   changes. Alternatively, one can think of a strong validator as part
429   of an identifier for a specific entity, while a weak validator is
430   part of an identifier for a set of semantically equivalent entities.
431  <list><t>
432      <x:h>Note:</x:h> One example of a strong validator is an integer that is
433      incremented in stable storage every time an entity is changed.
434    </t><t>
435      An entity's modification time, if represented with one-second
436      resolution, could be a weak validator, since it is possible that
437      the resource might be modified twice during a single second.
438    </t><t>
439      Support for weak validators is optional. However, weak validators
440      allow for more efficient caching of equivalent objects; for
441      example, a hit counter on a site is probably good enough if it is
442      updated every few days or weeks, and any value during that period
443      is likely "good enough" to be equivalent.
444    </t></list>
447   A "use" of a validator is either when a client generates a request
448   and includes the validator in a validating header field, or when a
449   server compares two validators.
452   Strong validators are usable in any context. Weak validators are only
453   usable in contexts that do not depend on exact equality of an entity.
454   For example, either kind is usable for a conditional GET of a full
455   entity. However, only a strong validator is usable for a sub-range
456   retrieval, since otherwise the client might end up with an internally
457   inconsistent entity.
[245]460   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
[172]463   The only function that HTTP/1.1 defines on validators is
[8]464   comparison. There are two validator comparison functions, depending
465   on whether the comparison context allows the use of weak validators
466   or not:
467  <list style="symbols">
468     <t>The strong comparison function: in order to be considered equal,
[298]469        both opaque-tags &MUST; be identical character-by-character, and both
470        &MUST-NOT; be weak.</t>
471     <t>The weak comparison function: in order to be considered equal, both
472        opaque-tags &MUST; be identical character-by-character.</t>
[8]473  </list>
[298]476   The example below shows the results for a set of entity tag pairs,
477   and both the weak and strong comparison function results:
479<texttable align="left">
480  <ttcol>ETag 1</ttcol>
481  <ttcol>ETag 2</ttcol>
482  <ttcol>Strong Comparison</ttcol>
483  <ttcol>Weak Comparison</ttcol>
485  <c>W/"1"</c>
486  <c>W/"1"</c>
487  <c>no match</c>
488  <c>match</c>
490  <c>W/"1"</c>
491  <c>W/"2"</c>
492  <c>no match</c>
493  <c>no match</c>
495  <c>W/"1"</c>
496  <c>"1"</c>
497  <c>no match</c>
498  <c>match</c>
500  <c>"1"</c>
501  <c>"1"</c>
502  <c>match</c>
503  <c>match</c>
[8]506   An entity tag is strong unless it is explicitly tagged as weak.
507   <xref target="entity.tags"/> gives the syntax for entity tags.
510   A Last-Modified time, when used as a validator in a request, is
511   implicitly weak unless it is possible to deduce that it is strong,
512   using the following rules:
513  <list style="symbols">
514     <t>The validator is being compared by an origin server to the
515        actual current validator for the entity and,</t>
516     <t>That origin server reliably knows that the associated entity did
517        not change twice during the second covered by the presented
518        validator.</t>
519  </list>
522   or
523  <list style="symbols">
524     <t>The validator is about to be used by a client in an If-Modified-Since
525        or If-Unmodified-Since header, because the client
526        has a cache entry for the associated entity, and</t>
527     <t>That cache entry includes a Date value, which gives the time
528        when the origin server sent the original response, and</t>
529     <t>The presented Last-Modified time is at least 60 seconds before
530        the Date value.</t>
531  </list>
534   or
535  <list style="symbols">
536     <t>The validator is being compared by an intermediate cache to the
537        validator stored in its cache entry for the entity, and</t>
538     <t>That cache entry includes a Date value, which gives the time
539        when the origin server sent the original response, and</t>
540     <t>The presented Last-Modified time is at least 60 seconds before
541        the Date value.</t>
542  </list>
545   This method relies on the fact that if two different responses were
546   sent by the origin server during the same second, but both had the
547   same Last-Modified time, then at least one of those responses would
548   have a Date value equal to its Last-Modified time. The arbitrary 60-second
549   limit guards against the possibility that the Date and Last-Modified
550   values are generated from different clocks, or at somewhat
551   different times during the preparation of the response. An
552   implementation &MAY; use a value larger than 60 seconds, if it is
553   believed that 60 seconds is too short.
556   If a client wishes to perform a sub-range retrieval on a value for
557   which it has only a Last-Modified time and no opaque validator, it
558   &MAY; do this only if the Last-Modified time is strong in the sense
559   described here.
[245]562   A cache or origin server receiving a conditional range request
563   (<xref target="Part5"/>) &MUST; use the strong comparison function to
[8]564   evaluate the condition.
567   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
568   retrievals on values that have been obtained from HTTP/1.0
569   servers.
573<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="">
575   We adopt a set of rules and recommendations for origin servers,
576   clients, and caches regarding when various validator types ought to
577   be used, and for what purposes.
580   HTTP/1.1 origin servers:
581  <list style="symbols">
582     <t>&SHOULD; send an entity tag validator unless it is not feasible to
583        generate one.</t>
585     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
586        performance considerations support the use of weak entity tags,
587        or if it is unfeasible to send a strong entity tag.</t>
589     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
590        unless the risk of a breakdown in semantic transparency that
591        could result from using this date in an If-Modified-Since header
592        would lead to serious problems.</t>
593  </list>
596   In other words, the preferred behavior for an HTTP/1.1 origin server
597   is to send both a strong entity tag and a Last-Modified value.
600   In order to be legal, a strong entity tag &MUST; change whenever the
[302]601   associated entity changes in any way. A weak entity tag &SHOULD;
[8]602   change whenever the associated entity changes in a semantically
603   significant way.
604  <list><t>
605      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
606      origin server must avoid reusing a specific strong entity tag
607      value for two different entities, or reusing a specific weak
608      entity tag value for two semantically different entities. Cache
609      entries might persist for arbitrarily long periods, regardless of
610      expiration times, so it might be inappropriate to expect that a
611      cache will never again attempt to validate an entry using a
612      validator that it obtained at some point in the past.
613  </t></list>
616   HTTP/1.1 clients:
617  <list style="symbols">
618     <t>If an entity tag has been provided by the origin server, &MUST;
619        use that entity tag in any cache-conditional request (using If-Match
620        or If-None-Match).</t>
622     <t>If only a Last-Modified value has been provided by the origin
623        server, &SHOULD; use that value in non-subrange cache-conditional
624        requests (using If-Modified-Since).</t>
626     <t>If only a Last-Modified value has been provided by an HTTP/1.0
627        origin server, &MAY; use that value in subrange cache-conditional
628        requests (using If-Unmodified-Since:). The user agent &SHOULD;
629        provide a way to disable this, in case of difficulty.</t>
631     <t>If both an entity tag and a Last-Modified value have been
632        provided by the origin server, &SHOULD; use both validators in
633        cache-conditional requests. This allows both HTTP/1.0 and
634        HTTP/1.1 caches to respond appropriately.</t>
635  </list>
638   An HTTP/1.1 origin server, upon receiving a conditional request that
639   includes both a Last-Modified date (e.g., in an If-Modified-Since or
640   If-Unmodified-Since header field) and one or more entity tags (e.g.,
641   in an If-Match, If-None-Match, or If-Range header field) as cache
642   validators, &MUST-NOT; return a response status of 304 (Not Modified)
643   unless doing so is consistent with all of the conditional header
644   fields in the request.
647   An HTTP/1.1 caching proxy, upon receiving a conditional request that
648   includes both a Last-Modified date and one or more entity tags as
649   cache validators, &MUST-NOT; return a locally cached response to the
650   client unless that cached response is consistent with all of the
651   conditional header fields in the request.
652  <list><t>
653      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
654      servers and clients should transmit as much non-redundant
655      information as is available in their responses and requests.
656      HTTP/1.1 systems receiving this information will make the most
657      conservative assumptions about the validators they receive.
658  </t><t>
659      HTTP/1.0 clients and caches will ignore entity tags. Generally,
660      last-modified values received or used by these systems will
661      support transparent and efficient caching, and so HTTP/1.1 origin
662      servers should provide Last-Modified values. In those rare cases
663      where the use of a Last-Modified value as a validator by an
664      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
665      origin servers should not provide one.
666  </t></list>
670<section title="Header Field Definitions" anchor="header.fields">
[117]672   This section defines the syntax and semantics of HTTP/1.1 header fields
673   related to conditional requests.
676   For entity-header fields, both sender and recipient refer to either the
677   client or the server, depending on who sends and who receives the entity.
680<section title="ETag" anchor="header.etag">
681  <iref primary="true" item="ETag header" x:for-anchor=""/>
682  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
[229]683  <x:anchor-alias value="ETag"/>
[362]684  <x:anchor-alias value="ETag-v"/>
[362]686   The response-header field "ETag" provides the current value of the
[321]687   entity tag (see <xref target="entity.tags"/>) for the requested variant.
688   The headers used with entity
[115]689   tags are described in Sections <xref target="header.if-match" format="counter"/>
690   and <xref target="header.if-none-match" format="counter"/> of this document,
691   and in &header-if-range;. The entity tag
[8]692   &MAY; be used for comparison with other entities from the same resource
693   (see <xref target="weak.and.strong.validators"/>).
[362]695<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
[366]696  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
[362]697  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
[362]700  Examples:
702<artwork type="example">
[362]703  ETag: "xyzzy"
704  ETag: W/"xyzzy"
705  ETag: ""
708   The ETag response-header field value, an entity tag, provides for an
709   "opaque" cache validator. This might allow more reliable validation
710   in situations where it is inconvenient to store modification dates,
711   where the one-second resolution of HTTP date values is not
712   sufficient, or where the origin server wishes to avoid certain
713   paradoxes that might arise from the use of modification dates.
716   The principle behind entity tags is that only the service author
717   knows the semantics of a resource well enough to select an
718   appropriate cache validation mechanism, and the specification of any
719   validator comparison function more complex than byte-equality would
720   open up a can of worms. Thus, comparisons of any other headers
721   (except Last-Modified, for compatibility with HTTP/1.0) are never
722   used for purposes of validating a cache entry.
726<section title="If-Match" anchor="header.if-match">
727  <iref primary="true" item="If-Match header" x:for-anchor=""/>
728  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
[229]729  <x:anchor-alias value="If-Match"/>
[362]730  <x:anchor-alias value="If-Match-v"/>
[362]732   The request-header field "If-Match" is used with a method to make it
[8]733   conditional. A client that has one or more entities previously
734   obtained from the resource can verify that one of those entities is
735   current by including a list of their associated entity tags in the
736   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
737   purpose of this feature is to allow efficient updates of cached
738   information with a minimum amount of transaction overhead. It is also
739   used, on updating requests, to prevent inadvertent modification of
740   the wrong version of a resource. As a special case, the value "*"
741   matches any current entity of the resource.
[362]743<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
[366]744  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
[362]745  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
748   If any of the entity tags match the entity tag of the entity that
749   would have been returned in the response to a similar GET request
750   (without the If-Match header) on that resource, or if "*" is given
751   and any current entity exists for that resource, then the server &MAY;
752   perform the requested method as if the If-Match header field did not
753   exist.
756   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
757   to compare the entity tags in If-Match.
760   If none of the entity tags match, or if "*" is given and no current
761   entity exists, the server &MUST-NOT; perform the requested method, and
762   &MUST; return a 412 (Precondition Failed) response. This behavior is
763   most useful when the client wants to prevent an updating method, such
764   as PUT, from modifying a resource that has changed since the client
765   last retrieved it.
768   If the request would, without the If-Match header field, result in
769   anything other than a 2xx or 412 status, then the If-Match header
770   &MUST; be ignored.
773   The meaning of "If-Match: *" is that the method &SHOULD; be performed
774   if the representation selected by the origin server (or by a cache,
[29]775   possibly using the Vary mechanism, see &header-vary;) exists, and
[8]776   &MUST-NOT; be performed if the representation does not exist.
779   A request intended to update a resource (e.g., a PUT) &MAY; include an
780   If-Match header field to signal that the request method &MUST-NOT; be
781   applied if the entity corresponding to the If-Match value (a single
782   entity tag) is no longer a representation of that resource. This
783   allows the user to indicate that they do not wish the request to be
784   successful if the resource has been changed without their knowledge.
785   Examples:
787<figure><artwork type="example">
[362]788  If-Match: "xyzzy"
789  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
790  If-Match: *
793   The result of a request having both an If-Match header field and
794   either an If-None-Match or an If-Modified-Since header fields is
795   undefined by this specification.
799<section title="If-Modified-Since" anchor="header.if-modified-since">
800  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
801  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
[229]802  <x:anchor-alias value="If-Modified-Since"/>
[362]803  <x:anchor-alias value="If-Modified-Since-v"/>
[362]805   The request-header field "If-Modified-Since" is used with a method to
[8]806   make it conditional: if the requested variant has not been modified
807   since the time specified in this field, an entity will not be
[137]808   returned from the server; instead, a 304 (Not Modified) response will
[8]809   be returned without any message-body.
[362]811<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
[376]812  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
813                        <x:ref>If-Modified-Since-v</x:ref>
[362]814  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
817   An example of the field is:
819<figure><artwork type="example">
[362]820  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
823   A GET method with an If-Modified-Since header and no Range header
824   requests that the identified entity be transferred only if it has
825   been modified since the date given by the If-Modified-Since header.
826   The algorithm for determining this includes the following cases:
827  <list style="numbers">
828      <t>If the request would normally result in anything other than a
829         200 (OK) status, or if the passed If-Modified-Since date is
830         invalid, the response is exactly the same as for a normal GET.
831         A date which is later than the server's current time is
832         invalid.</t>
834      <t>If the variant has been modified since the If-Modified-Since
835         date, the response is exactly the same as for a normal GET.</t>
837      <t>If the variant has not been modified since a valid If-Modified-Since
838         date, the server &SHOULD; return a 304 (Not
839         Modified) response.</t>
840  </list>
843   The purpose of this feature is to allow efficient updates of cached
844   information with a minimum amount of transaction overhead.
845  <list><t>
846      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
[29]847      see &header-range; for full details.
[8]848    </t><t>
849      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
850      clock might not be synchronized with the client.
851    </t><t>
852      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
853      servers will use an exact date comparison function, rather than a
854      less-than function, for deciding whether to send a 304 (Not
855      Modified) response. To get best results when sending an If-Modified-Since
856      header field for cache validation, clients are
857      advised to use the exact date string received in a previous Last-Modified
858      header field whenever possible.
859    </t><t>
860      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
861      header instead of a date taken from the Last-Modified header for
862      the same request, the client should be aware of the fact that this
863      date is interpreted in the server's understanding of time. The
864      client should consider unsynchronized clocks and rounding problems
865      due to the different encodings of time between the client and
866      server. This includes the possibility of race conditions if the
867      document has changed between the time it was first requested and
868      the If-Modified-Since date of a subsequent request, and the
869      possibility of clock-skew-related problems if the If-Modified-Since
870      date is derived from the client's clock without correction
871      to the server's clock. Corrections for different time bases
872      between client and server are at best approximate due to network
873      latency.
874    </t>
875  </list>
878   The result of a request having both an If-Modified-Since header field
879   and either an If-Match or an If-Unmodified-Since header fields is
880   undefined by this specification.
884<section title="If-None-Match" anchor="header.if-none-match">
885  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
886  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
[229]887  <x:anchor-alias value="If-None-Match"/>
[362]888  <x:anchor-alias value="If-None-Match-v"/>
[362]890   The request-header field "If-None-Match" is used with a method to make
[8]891   it conditional. A client that has one or more entities previously
892   obtained from the resource can verify that none of those entities is
893   current by including a list of their associated entity tags in the
894   If-None-Match header field. The purpose of this feature is to allow
895   efficient updates of cached information with a minimum amount of
896   transaction overhead. It is also used to prevent a method (e.g. PUT)
897   from inadvertently modifying an existing resource when the client
898   believes that the resource does not exist.
901   As a special case, the value "*" matches any current entity of the
902   resource.
[362]904<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
[366]905  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
[362]906  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
909   If any of the entity tags match the entity tag of the entity that
910   would have been returned in the response to a similar GET request
911   (without the If-None-Match header) on that resource, or if "*" is
912   given and any current entity exists for that resource, then the
913   server &MUST-NOT; perform the requested method, unless required to do
914   so because the resource's modification date fails to match that
915   supplied in an If-Modified-Since header field in the request.
916   Instead, if the request method was GET or HEAD, the server &SHOULD;
917   respond with a 304 (Not Modified) response, including the cache-related
918   header fields (particularly ETag) of one of the entities that
919   matched. For all other request methods, the server &MUST; respond with
920   a status of 412 (Precondition Failed).
[245]923   See <xref target="weak.and.strong.validators"/> for rules on how to
924   determine if two entity tags match.
927   If none of the entity tags match, then the server &MAY; perform the
928   requested method as if the If-None-Match header field did not exist,
929   but &MUST; also ignore any If-Modified-Since header field(s) in the
930   request. That is, if no entity tags match, then the server &MUST-NOT;
931   return a 304 (Not Modified) response.
934   If the request would, without the If-None-Match header field, result
935   in anything other than a 2xx or 304 status, then the If-None-Match
936   header &MUST; be ignored. (See <xref target=""/> for a discussion of
937   server behavior when both If-Modified-Since and If-None-Match appear
938   in the same request.)
941   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
942   performed if the representation selected by the origin server (or by
[29]943   a cache, possibly using the Vary mechanism, see &header-vary;)
[8]944   exists, and &SHOULD; be performed if the representation does not exist.
945   This feature is intended to be useful in preventing races between PUT
946   operations.
949   Examples:
951<figure><artwork type="example">
[362]952  If-None-Match: "xyzzy"
953  If-None-Match: W/"xyzzy"
954  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
955  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
956  If-None-Match: *
959   The result of a request having both an If-None-Match header field and
960   either an If-Match or an If-Unmodified-Since header fields is
961   undefined by this specification.
965<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
966  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
967  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
[229]968  <x:anchor-alias value="If-Unmodified-Since"/>
[362]969  <x:anchor-alias value="If-Unmodified-Since-v"/>
[362]971   The request-header field "If-Unmodified-Since" is used with a method to
[8]972   make it conditional. If the requested resource has not been modified
973   since the time specified in this field, the server &SHOULD; perform the
974   requested operation as if the If-Unmodified-Since header were not
975   present.
978   If the requested variant has been modified since the specified time,
979   the server &MUST-NOT; perform the requested operation, and &MUST; return
980   a 412 (Precondition Failed).
[362]982<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
[376]983  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
984                          <x:ref>If-Unmodified-Since-v</x:ref>
[362]985  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
988   An example of the field is:
990<figure><artwork type="example">
[362]991  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
994   If the request normally (i.e., without the If-Unmodified-Since
995   header) would result in anything other than a 2xx or 412 status, the
996   If-Unmodified-Since header &SHOULD; be ignored.
999   If the specified date is invalid, the header is ignored.
1002   The result of a request having both an If-Unmodified-Since header
1003   field and either an If-None-Match or an If-Modified-Since header
1004   fields is undefined by this specification.
1008<section title="Last-Modified" anchor="header.last-modified">
1009  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
1010  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
[229]1011  <x:anchor-alias value="Last-Modified"/>
[362]1012  <x:anchor-alias value="Last-Modified-v"/>
[362]1014   The entity-header field "Last-Modified" indicates the date and time at
[8]1015   which the origin server believes the variant was last modified.
[362]1017<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
[366]1018  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
[362]1019  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
1022   An example of its use is
1024<figure><artwork type="example">
[362]1025  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1028   The exact meaning of this header field depends on the implementation
1029   of the origin server and the nature of the original resource. For
1030   files, it may be just the file system last-modified time. For
1031   entities with dynamically included parts, it may be the most recent
1032   of the set of last-modify times for its component parts. For database
1033   gateways, it may be the last-update time stamp of the record. For
1034   virtual objects, it may be the last time the internal state changed.
1037   An origin server &MUST-NOT; send a Last-Modified date which is later
1038   than the server's time of message origination. In such cases, where
1039   the resource's last modification would indicate some time in the
1040   future, the server &MUST; replace that date with the message
1041   origination date.
1044   An origin server &SHOULD; obtain the Last-Modified value of the entity
1045   as close as possible to the time that it generates the Date value of
1046   its response. This allows a recipient to make an accurate assessment
1047   of the entity's modification time, especially if the entity changes
1048   near the time that the response is generated.
1051   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1054   The Last-Modified entity-header field value is often used as a cache
1055   validator. In simple terms, a cache entry is considered to be valid
1056   if the entity has not been modified since the Last-Modified value.
[29]1062<section title="IANA Considerations" anchor="IANA.considerations">
[253]1063<section title="Message Header Registration" anchor="message.header.registration">
1065   The Message Header Registry located at <eref target=""/> should be updated
1066   with the permanent registrations below (see <xref target="RFC3864"/>):
[253]1068<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
[290]1069<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
[253]1070   <ttcol>Header Field Name</ttcol>
1071   <ttcol>Protocol</ttcol>
1072   <ttcol>Status</ttcol>
1073   <ttcol>Reference</ttcol>
1075   <c>ETag</c>
1076   <c>http</c>
1077   <c>standard</c>
1078   <c>
1079      <xref target="header.etag"/>
1080   </c>
1081   <c>If-Match</c>
1082   <c>http</c>
1083   <c>standard</c>
1084   <c>
1085      <xref target="header.if-match"/>
1086   </c>
1087   <c>If-Modified-Since</c>
1088   <c>http</c>
1089   <c>standard</c>
1090   <c>
1091      <xref target="header.if-modified-since"/>
1092   </c>
1093   <c>If-None-Match</c>
1094   <c>http</c>
1095   <c>standard</c>
1096   <c>
1097      <xref target="header.if-none-match"/>
1098   </c>
1099   <c>If-Unmodified-Since</c>
1100   <c>http</c>
1101   <c>standard</c>
1102   <c>
1103      <xref target="header.if-unmodified-since"/>
1104   </c>
1105   <c>Last-Modified</c>
1106   <c>http</c>
1107   <c>standard</c>
1108   <c>
1109      <xref target="header.last-modified"/>
1110   </c>
[290]1114   The change controller is: "IETF ( - Internet Engineering Task Force".
1119<section title="Security Considerations" anchor="security.considerations">
[29]1121   No additional security considerations have been identified beyond
1122   those applicable to HTTP in general &messaging;.
1126<section title="Acknowledgments" anchor="ack">
1131<references title="Normative References">
[31]1133<reference anchor="Part1">
[119]1134  <front>
1135    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1136    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1137      <organization abbrev="Day Software">Day Software</organization>
1138      <address><email></email></address>
1139    </author>
1140    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1141      <organization>One Laptop per Child</organization>
1142      <address><email></email></address>
1143    </author>
1144    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1145      <organization abbrev="HP">Hewlett-Packard Company</organization>
1146      <address><email></email></address>
1147    </author>
1148    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1149      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1150      <address><email></email></address>
1151    </author>
1152    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1153      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1154      <address><email></email></address>
1155    </author>
1156    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1157      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1158      <address><email></email></address>
1159    </author>
1160    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1161      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1162      <address><email></email></address>
1163    </author>
1164    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1165      <organization abbrev="W3C">World Wide Web Consortium</organization>
1166      <address><email></email></address>
1167    </author>
1168    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1169      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1170      <address><email></email></address>
1171    </author>
1172    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1173  </front>
1174  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1175  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1178<reference anchor="Part5">
[119]1179  <front>
1180    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1181    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1182      <organization abbrev="Day Software">Day Software</organization>
1183      <address><email></email></address>
1184    </author>
1185    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1186      <organization>One Laptop per Child</organization>
1187      <address><email></email></address>
1188    </author>
1189    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1190      <organization abbrev="HP">Hewlett-Packard Company</organization>
1191      <address><email></email></address>
1192    </author>
1193    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1194      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1195      <address><email></email></address>
1196    </author>
1197    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1198      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1199      <address><email></email></address>
1200    </author>
1201    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1202      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1203      <address><email></email></address>
1204    </author>
1205    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1206      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1207      <address><email></email></address>
1208    </author>
1209    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1210      <organization abbrev="W3C">World Wide Web Consortium</organization>
1211      <address><email></email></address>
1212    </author>
1213    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1214      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1215      <address><email></email></address>
1216    </author>
1217    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1218  </front>
1219  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1220  <x:source href="p5-range.xml" basename="p5-range"/>
1223<reference anchor="Part6">
[119]1224  <front>
1225    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1226    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1227      <organization abbrev="Day Software">Day Software</organization>
1228      <address><email></email></address>
1229    </author>
1230    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1231      <organization>One Laptop per Child</organization>
1232      <address><email></email></address>
1233    </author>
1234    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1235      <organization abbrev="HP">Hewlett-Packard Company</organization>
1236      <address><email></email></address>
1237    </author>
1238    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1239      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1240      <address><email></email></address>
1241    </author>
1242    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1243      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1244      <address><email></email></address>
1245    </author>
1246    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1247      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1248      <address><email></email></address>
1249    </author>
1250    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1251      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1252      <address><email></email></address>
1253    </author>
1254    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1255      <organization abbrev="W3C">World Wide Web Consortium</organization>
1256      <address><email></email></address>
1257    </author>
1258    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1259      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1260      <address><email></email></address>
1261    </author>
1262    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1263  </front>
1264  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1265  <x:source href="p6-cache.xml" basename="p6-cache"/>
[96]1268<reference anchor="RFC2119">
1269  <front>
1270    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1271    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1272      <organization>Harvard University</organization>
1273      <address><email></email></address>
1274    </author>
1275    <date month="March" year="1997"/>
1276  </front>
1277  <seriesInfo name="BCP" value="14"/>
1278  <seriesInfo name="RFC" value="2119"/>
[425]1281<reference anchor="RFC5234">
1282  <front>
1283    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1284    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1285      <organization>Brandenburg InternetWorking</organization>
1286      <address>
1287      <postal>
1288      <street>675 Spruce Dr.</street>
1289      <city>Sunnyvale</city>
1290      <region>CA</region>
1291      <code>94086</code>
1292      <country>US</country></postal>
1293      <phone>+1.408.246.8253</phone>
1294      <email></email></address> 
1295    </author>
1296    <author initials="P." surname="Overell" fullname="Paul Overell">
1297      <organization>THUS plc.</organization>
1298      <address>
1299      <postal>
1300      <street>1/2 Berkeley Square</street>
1301      <street>99 Berkely Street</street>
1302      <city>Glasgow</city>
1303      <code>G3 7HR</code>
1304      <country>UK</country></postal>
1305      <email></email></address>
1306    </author>
1307    <date month="January" year="2008"/>
1308  </front>
1309  <seriesInfo name="STD" value="68"/>
1310  <seriesInfo name="RFC" value="5234"/>
1315<references title="Informative References">
1317<reference anchor="RFC2616">
1318  <front>
1319    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1320    <author initials="R." surname="Fielding" fullname="R. Fielding">
1321      <organization>University of California, Irvine</organization>
1322      <address><email></email></address>
1323    </author>
1324    <author initials="J." surname="Gettys" fullname="J. Gettys">
1325      <organization>W3C</organization>
1326      <address><email></email></address>
1327    </author>
1328    <author initials="J." surname="Mogul" fullname="J. Mogul">
1329      <organization>Compaq Computer Corporation</organization>
1330      <address><email></email></address>
1331    </author>
1332    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1333      <organization>MIT Laboratory for Computer Science</organization>
1334      <address><email></email></address>
1335    </author>
1336    <author initials="L." surname="Masinter" fullname="L. Masinter">
1337      <organization>Xerox Corporation</organization>
1338      <address><email></email></address>
1339    </author>
1340    <author initials="P." surname="Leach" fullname="P. Leach">
1341      <organization>Microsoft Corporation</organization>
1342      <address><email></email></address>
1343    </author>
1344    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1345      <organization>W3C</organization>
1346      <address><email></email></address>
1347    </author>
1348    <date month="June" year="1999"/>
1349  </front>
1350  <seriesInfo name="RFC" value="2616"/>
[253]1353<reference anchor='RFC3864'>
1354  <front>
1355    <title>Registration Procedures for Message Header Fields</title>
1356    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1357      <organization>Nine by Nine</organization>
1358      <address><email></email></address>
1359    </author>
1360    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1361      <organization>BEA Systems</organization>
1362      <address><email></email></address>
1363    </author>
1364    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1365      <organization>HP Labs</organization>
1366      <address><email></email></address>
1367    </author>
1368    <date year='2004' month='September' />
1369  </front>
1370  <seriesInfo name='BCP' value='90' />
1371  <seriesInfo name='RFC' value='3864' />
1376<section title="Compatibility with Previous Versions" anchor="compatibility">
1378<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1380  Allow weak entity tags in all requests except range requests (Sections
1381  <xref target="weak.and.strong.validators" format="counter"/> and
1382  <xref target="header.if-none-match" format="counter"/>).
[427]1388<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
1390<artwork type="abnf" name="p4-conditional.parsed-abnf">
1391<x:ref>ETag</x:ref> = "ETag:" OWS ETag-v
1392<x:ref>ETag-v</x:ref> = entity-tag
1394<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
1396<x:ref>If-Match</x:ref> = "If-Match:" OWS If-Match-v
1397<x:ref>If-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1398 entity-tag ] ) )
1399<x:ref>If-Modified-Since</x:ref> = "If-Modified-Since:" OWS If-Modified-Since-v
1400<x:ref>If-Modified-Since-v</x:ref> = HTTP-date
1401<x:ref>If-None-Match</x:ref> = "If-None-Match:" OWS If-None-Match-v
1402<x:ref>If-None-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1403 entity-tag ] ) )
1404<x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since:" OWS
1405 If-Unmodified-Since-v
1406<x:ref>If-Unmodified-Since-v</x:ref> = HTTP-date
1408<x:ref>Last-Modified</x:ref> = "Last-Modified:" OWS Last-Modified-v
1409<x:ref>Last-Modified-v</x:ref> = HTTP-date
1411<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1413<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1415<x:ref>opaque-tag</x:ref> = quoted-string
1417<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1419<x:ref>weak</x:ref> = "W/"
[532]1424<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1425; ETag defined but not used
[427]1426; If-Match defined but not used
1427; If-Modified-Since defined but not used
1428; If-None-Match defined but not used
1429; If-Unmodified-Since defined but not used
1430; Last-Modified defined but not used
[252]1433<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1435<section title="Since RFC2616">
1437  Extracted relevant partitions from <xref target="RFC2616"/>.
1441<section title="Since draft-ietf-httpbis-p4-conditional-00">
[152]1443  Closed issues:
1444  <list style="symbols">
1445    <t>
[324]1446      <eref target=""/>:
[152]1447      "Normative and Informative references"
1448    </t>
1449  </list>
[116]1452  Other changes:
1453  <list style="symbols">
1454    <t>
1455      Move definitions of 304 and 412 condition codes from Part2.
1456    </t>
1457  </list>
[170]1461<section title="Since draft-ietf-httpbis-p4-conditional-01">
[324]1463  Ongoing work on ABNF conversion (<eref target=""/>):
[205]1464  <list style="symbols">
1465    <t>
1466      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1467    </t>
1468  </list>
[252]1472<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
[245]1474  Closed issues:
1475  <list style="symbols">
1476    <t>
[324]1477      <eref target=""/>:
[245]1478      "Weak ETags on non-GET requests"
1479    </t>
1480  </list>
[324]1483  Ongoing work on IANA Message Header Registration (<eref target=""/>):
[253]1484  <list style="symbols">
1485    <t>
1486      Reference RFC 3984, and update header registrations for headers defined
1487      in this document.
1488    </t>
1489  </list>
[267]1493<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
[298]1495  Closed issues:
1496  <list style="symbols">
1497    <t>
[324]1498      <eref target=""/>:
[298]1499      "Examples for ETag matching"
1500    </t>
[302]1501    <t>
[324]1502      <eref target=""/>:
[302]1503      "'entity value' undefined"
1504    </t>
[306]1505    <t>
[324]1506      <eref target=""/>:
[306]1507      "bogus 2068 Date header reference"
1508    </t>
[298]1509  </list>
[323]1513<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
[334]1515  Ongoing work on ABNF conversion (<eref target=""/>):
1516  <list style="symbols">
1517    <t>
1518      Use "/" instead of "|" for alternatives.
1519    </t>
[362]1520    <t>
1521      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1522      whitespace ("OWS") and required whitespace ("RWS").
1523    </t>
1524    <t>
1525      Rewrite ABNFs to spell out whitespace rules, factor out
1526      header value format definitions.
1527    </t>
[334]1528  </list>
[382]1532<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
[543]1534  Final work on ABNF conversion (<eref target=""/>):
[421]1535  <list style="symbols">
1536    <t>
[424]1537      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
[421]1538    </t>
1539  </list>
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