source: draft-ietf-httpbis/10/p4-conditional.xml @ 2656

Last change on this file since 2656 was 1500, checked in by julian.reschke@…, 9 years ago

fix mime types

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 68.8 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>">
[841]14  <!ENTITY ID-VERSION "10">
[832]15  <!ENTITY ID-MONTH "July">
[741]16  <!ENTITY ID-YEAR "2010">
[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=''/>">
[800]23  <!ENTITY header-accept-encoding     "<xref target='Part3' x:rel='#header.accept-encoding' xmlns:x=''/>">
[31]24  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x=''/>">
25  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x=''/>">
26  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x=''/>">
[45]27  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x=''/>">
[580]28  <!ENTITY full-date                  "<xref target='Part1' x:rel='' xmlns:x=''/>">
[800]29  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x=''/>">
30  <!ENTITY content-negotiation        "<xref target='Part3' x:rel='#content.negotiation' xmlns:x=''/>">
32<?rfc toc="yes" ?>
[29]33<?rfc symrefs="yes" ?>
34<?rfc sortrefs="yes" ?>
[8]35<?rfc compact="yes"?>
36<?rfc subcompact="no" ?>
37<?rfc linkmailto="no" ?>
38<?rfc editing="no" ?>
[203]39<?rfc comments="yes"?>
40<?rfc inline="yes"?>
[799]41<?rfc rfcedstyle="yes"?>
[8]42<?rfc-ext allow-markup-in-artwork="yes" ?>
43<?rfc-ext include-references-in-index="yes" ?>
[308]44<rfc obsoletes="2616" category="std" x:maturity-level="draft"
[446]45     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
[153]46     xmlns:x=''>
[120]49  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
[29]51  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
52    <organization abbrev="Day Software">Day Software</organization>
[8]53    <address>
54      <postal>
[29]55        <street>23 Corporate Plaza DR, Suite 280</street>
56        <city>Newport Beach</city>
[8]57        <region>CA</region>
[29]58        <code>92660</code>
59        <country>USA</country>
[8]60      </postal>
[29]61      <phone>+1-949-706-5300</phone>
62      <facsimile>+1-949-706-5305</facsimile>
63      <email></email>
64      <uri></uri>
[8]65    </address>
66  </author>
[29]68  <author initials="J." surname="Gettys" fullname="Jim Gettys">
[845]69    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
[8]70    <address>
71      <postal>
[29]72        <street>21 Oak Knoll Road</street>
73        <city>Carlisle</city>
[8]74        <region>MA</region>
[29]75        <code>01741</code>
76        <country>USA</country>
[8]77      </postal>
[845]78      <email></email>
79      <uri></uri>
[8]80    </address>
81  </author>
83  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
[29]84    <organization abbrev="HP">Hewlett-Packard Company</organization>
[8]85    <address>
86      <postal>
[29]87        <street>HP Labs, Large Scale Systems Group</street>
88        <street>1501 Page Mill Road, MS 1177</street>
[8]89        <city>Palo Alto</city>
90        <region>CA</region>
[29]91        <code>94304</code>
92        <country>USA</country>
[8]93      </postal>
[29]94      <email></email>
[8]95    </address>
96  </author>
98  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
[29]99    <organization abbrev="Microsoft">Microsoft Corporation</organization>
[8]100    <address>
101      <postal>
[29]102        <street>1 Microsoft Way</street>
103        <city>Redmond</city>
104        <region>WA</region>
105        <code>98052</code>
106        <country>USA</country>
[8]107      </postal>
[29]108      <email></email>
[8]109    </address>
110  </author>
112  <author initials="L." surname="Masinter" fullname="Larry Masinter">
[29]113    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
[8]114    <address>
115      <postal>
[29]116        <street>345 Park Ave</street>
117        <city>San Jose</city>
[8]118        <region>CA</region>
[29]119        <code>95110</code>
120        <country>USA</country>
[8]121      </postal>
[29]122      <email></email>
123      <uri></uri>
[8]124    </address>
125  </author>
127  <author initials="P." surname="Leach" fullname="Paul J. Leach">
128    <organization abbrev="Microsoft">Microsoft Corporation</organization>
129    <address>
130      <postal>
131        <street>1 Microsoft Way</street>
132        <city>Redmond</city>
133        <region>WA</region>
134        <code>98052</code>
135      </postal>
136      <email></email>
137    </address>
138  </author>
140  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
141    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
142    <address>
143      <postal>
[34]144        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
145        <street>The Stata Center, Building 32</street>
146        <street>32 Vassar Street</street>
[8]147        <city>Cambridge</city>
148        <region>MA</region>
149        <code>02139</code>
[29]150        <country>USA</country>
[8]151      </postal>
152      <email></email>
[34]153      <uri></uri>
[8]154    </address>
155  </author>
[95]157  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
[94]158    <organization abbrev="W3C">World Wide Web Consortium</organization>
159    <address>
160      <postal>
161        <street>W3C / ERCIM</street>
162        <street>2004, rte des Lucioles</street>
163        <city>Sophia-Antipolis</city>
164        <region>AM</region>
165        <code>06902</code>
166        <country>France</country>
167      </postal>
168      <email></email>
169      <uri></uri>
170    </address>
171  </author>
173  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
174    <organization abbrev="greenbytes">greenbytes GmbH</organization>
175    <address>
176      <postal>
177        <street>Hafenweg 16</street>
178        <city>Muenster</city><region>NW</region><code>48155</code>
179        <country>Germany</country>
180      </postal>
[609]181      <phone>+49 251 2807760</phone>
182      <facsimile>+49 251 2807761</facsimile>
183      <email></email>
184      <uri></uri>
[95]185    </address>
186  </author>
[841]188  <date month="&ID-MONTH;" year="&ID-YEAR;" day="12"/>
[440]189  <workgroup>HTTPbis Working Group</workgroup>
193   The Hypertext Transfer Protocol (HTTP) is an application-level
194   protocol for distributed, collaborative, hypermedia information
[29]195   systems. HTTP has been in use by the World Wide Web global information
[35]196   initiative since 1990. This document is Part 4 of the seven-part specification
[29]197   that defines the protocol referred to as "HTTP/1.1" and, taken together,
[42]198   obsoletes RFC 2616.  Part 4 defines request header fields for
[29]199   indicating conditional requests and the rules for constructing responses
200   to those requests.
204<note title="Editorial Note (To be removed by RFC Editor)">
205  <t>
206    Discussion of this draft should take place on the HTTPBIS working group
207    mailing list ( The current issues list is
[848]208    at <eref target=""/>
[36]209    and related documents (including fancy diffs) can be found at
[324]210    <eref target=""/>.
[36]211  </t>
[153]212  <t>
[773]213    The changes in this draft are summarized in <xref target="changes.since.09"/>.
[153]214  </t>
218<section title="Introduction" anchor="introduction">
[163]220   This document defines HTTP/1.1 response metadata for indicating potential
221   changes to payload content, including modification time stamps and opaque
222   entity-tags, and the HTTP conditional request mechanisms that allow
223   preconditions to be placed on a request method.  Conditional GET requests
224   allow for efficient cache updates.  Other conditional request methods are
225   used to protect against overwriting or misunderstanding the state of a
226   resource that has been changed unbeknownst to the requesting client.
229   This document is currently disorganized in order to minimize the changes
230   between drafts and enable reviewers to see the smaller errata changes.
231   The next draft will reorganize the sections to better reflect the content.
232   In particular, the sections on resource metadata will be discussed first
233   and then followed by each conditional request-header, concluding with a
234   definition of precedence and the expectation of ordering strong validator
235   checks before weak validator checks.  It is likely that more content from
236   &caching; will migrate to this part, where appropriate.
237   The current mess reflects how widely dispersed these topics and associated
238   requirements had become in <xref target="RFC2616"/>.
241<section title="Requirements" anchor="intro.requirements">
243   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
244   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
245   document are to be interpreted as described in <xref target="RFC2119"/>.
248   An implementation is not compliant if it fails to satisfy one or more
[847]249   of the "MUST" or "REQUIRED" level requirements for the protocols it
250   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
251   level and all the "SHOULD" level requirements for its protocols is said
252   to be "unconditionally compliant"; one that satisfies all the "MUST"
253   level requirements but not all the "SHOULD" level requirements for its
254   protocols is said to be "conditionally compliant".
[424]258<section title="Syntax Notation" anchor="notation">
[425]259  <x:anchor-alias value="ALPHA"/>
260  <x:anchor-alias value="CR"/>
261  <x:anchor-alias value="DIGIT"/>
262  <x:anchor-alias value="LF"/>
263  <x:anchor-alias value="OCTET"/>
264  <x:anchor-alias value="VCHAR"/>
265  <x:anchor-alias value="WSP"/>
[543]267  This specification uses the ABNF syntax defined in &notation; (which
268  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
269  <xref target="collected.abnf"/> shows the collected ABNF, with the list
270  rule expanded.
[425]273  The following core rules are included by
274  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
275  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
276  DIGIT (decimal 0-9), DQUOTE (double quote),
277  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
278  OCTET (any 8-bit sequence of data), SP (space),
279  VCHAR (any visible USASCII character),
280  and WSP (whitespace).
283<section title="Core Rules" anchor="core.rules">
[229]284  <x:anchor-alias value="quoted-string"/>
[362]285  <x:anchor-alias value="OWS"/>
[424]287  The core rules below are defined in &basic-rules;:
289<figure><artwork type="abnf2616">
[229]290  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
[362]291  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
295<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
[229]296  <x:anchor-alias value="HTTP-date"/>
[205]298  The ABNF rules below are defined in other parts:
[207]300<figure><!--Part1--><artwork type="abnf2616">
[229]301  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
[8]309<section title="Entity Tags" anchor="entity.tags">
[229]310  <x:anchor-alias value="entity-tag"/>
311  <x:anchor-alias value="opaque-tag"/>
312  <x:anchor-alias value="weak"/>
314   Entity tags are used for comparing two or more entities from the same
315   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
316   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
[29]317   If-Range (&header-if-range;) header fields. The definition of how they
[8]318   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
319   entity tag consists of an opaque quoted string, possibly prefixed by
320   a weakness indicator.
322<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]323  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
[570]324  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
[229]325  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
328   A "strong entity tag" &MAY; be shared by two entities of a resource
329   only if they are equivalent by octet equality.
332   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
333   two entities of a resource only if the entities are equivalent and
334   could be substituted for each other with no significant change in
335   semantics. A weak entity tag can only be used for weak comparison.
338   An entity tag &MUST; be unique across all versions of all entities
339   associated with a particular resource. A given entity tag value &MAY;
340   be used for entities obtained by requests on different URIs. The use
341   of the same entity tag value in conjunction with entities obtained by
342   requests on different URIs does not imply the equivalence of those
343   entities.
346<section title="Example: Entity Tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
348   Consider a resource that is subject to content negotiation (&content-negotiation;),
349   and where the representations returned upon a GET request vary based on
350   the Accept-Encoding request header field (&header-accept-encoding;):
352<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
353GET /index HTTP/1.1
355Accept-Encoding: gzip
359   In this case, the response may use the gzip Content Coding or not.
360   If it does, it might look like that:
362<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
363HTTP/1.1 200 OK
364Date: Thu, 26 Mar 2010 00:05:00 GMT
365ETag: "123-a"
366Content-Length: <x:length-of target="exbody"/>
367Vary: Accept-Encoding
368Content-Type: text/plain
370<x:span anchor="exbody">Hello World!
371Hello World!
372Hello World!
373Hello World!
374Hello World!
377   A variant that does use gzip Content Coding would be:
379<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
380HTTP/1.1 200 OK
381Date: Thu, 26 Mar 2010 00:05:00 GMT
382ETag: "123-b"
383Content-Length: 43
384Vary: Accept-Encoding
385Content-Type: text/plain
386Content-Encoding: gzip
388<spanx>...binary data...</spanx></artwork></figure>
390  <t>
391    <x:h>Note:</x:h> Content Codings are a property of the response
392    entity, thus affect the Entity Tag. An alternative are Transfer
393    Codings (&transfer-codings;) which apply only to the transfer of
394    the message, and thus do not require assigning distinct entity tags.
395  </t>
[838]400<section title="Status Code Definitions" anchor="status.code.definitions">
[45]401<section title="304 Not Modified" anchor="status.304">
402  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
403  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
405   If the client has performed a conditional GET request and access is
406   allowed, but the document has not been modified, the server &SHOULD;
407   respond with this status code. The 304 response &MUST-NOT; contain a
408   message-body, and thus is always terminated by the first empty line
409   after the header fields.
412   The response &MUST; include the following header fields:
413  <list style="symbols">
[305]414    <x:lt>
[306]415      <t>Date, unless its omission is required by &clockless;.</t>
416      <t>
417         If a clockless origin server obeys these rules, and proxies and
[305]418         clients add their own Date to any response received without one (as
[306]419         already specified by &header-date;, caches will operate
[305]420         correctly.</t>
421    </x:lt>
422    <x:lt>
423      <t>ETag and/or Content-Location, if the header would have been sent
[306]424         in a 200 response to the same request.</t>
[305]425    </x:lt>
426    <x:lt>
427      <t>Expires, Cache-Control, and/or Vary, if the field-value might
[306]428         differ from that sent in any previous response for the same
429         variant.</t>
[305]430    </x:lt>
[45]431  </list>
[115]434   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
[45]435   the response &SHOULD-NOT;  include other entity-headers.
436   Otherwise (i.e., the conditional GET used a weak validator), the
437   response &MUST-NOT; include other entity-headers; this prevents
438   inconsistencies between cached entity-bodies and updated headers.
441   If a 304 response indicates an entity not currently cached, then the
442   cache &MUST; disregard the response and repeat the request without the
443   conditional.
446   If a cache uses a received 304 response to update a cache entry, the
447   cache &MUST; update the entry to reflect any new field values given in
448   the response.
452<section title="412 Precondition Failed" anchor="status.412">
453  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
454  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
456   The precondition given in one or more of the request-header fields
457   evaluated to false when it was tested on the server. This response
458   code allows the client to place preconditions on the current resource
459   metainformation (header field data) and thus prevent the requested
460   method from being applied to a resource other than the one intended.
[8]465<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
467   Since both origin servers and caches will compare two validators to
468   decide if they represent the same or different entities, one normally
469   would expect that if the entity (the entity-body or any entity-headers)
470   changes in any way, then the associated validator would
471   change as well. If this is true, then we call this validator a
472   "strong validator."
475   However, there might be cases when a server prefers to change the
476   validator only on semantically significant changes, and not when
477   insignificant aspects of the entity change. A validator that does not
478   always change when the resource changes is a "weak validator."
481   Entity tags are normally "strong validators," but the protocol
482   provides a mechanism to tag an entity tag as "weak." One can think of
483   a strong validator as one that changes whenever the bits of an entity
484   changes, while a weak value changes whenever the meaning of an entity
485   changes. Alternatively, one can think of a strong validator as part
486   of an identifier for a specific entity, while a weak validator is
487   part of an identifier for a set of semantically equivalent entities.
488  <list><t>
489      <x:h>Note:</x:h> One example of a strong validator is an integer that is
490      incremented in stable storage every time an entity is changed.
491    </t><t>
492      An entity's modification time, if represented with one-second
493      resolution, could be a weak validator, since it is possible that
494      the resource might be modified twice during a single second.
495    </t><t>
496      Support for weak validators is optional. However, weak validators
497      allow for more efficient caching of equivalent objects; for
498      example, a hit counter on a site is probably good enough if it is
499      updated every few days or weeks, and any value during that period
500      is likely "good enough" to be equivalent.
501    </t></list>
504   A "use" of a validator is either when a client generates a request
505   and includes the validator in a validating header field, or when a
506   server compares two validators.
509   Strong validators are usable in any context. Weak validators are only
510   usable in contexts that do not depend on exact equality of an entity.
511   For example, either kind is usable for a conditional GET of a full
512   entity. However, only a strong validator is usable for a sub-range
513   retrieval, since otherwise the client might end up with an internally
514   inconsistent entity.
[245]517   Clients &MUST-NOT; use weak validators in range requests (<xref target="Part5"/>).
[172]520   The only function that HTTP/1.1 defines on validators is
[8]521   comparison. There are two validator comparison functions, depending
522   on whether the comparison context allows the use of weak validators
523   or not:
524  <list style="symbols">
525     <t>The strong comparison function: in order to be considered equal,
[298]526        both opaque-tags &MUST; be identical character-by-character, and both
527        &MUST-NOT; be weak.</t>
528     <t>The weak comparison function: in order to be considered equal, both
[610]529        opaque-tags &MUST; be identical character-by-character, but
530        either or both of them &MAY; be tagged as "weak" without affecting
531        the result.</t>
[8]532  </list>
[298]535   The example below shows the results for a set of entity tag pairs,
536   and both the weak and strong comparison function results:
538<texttable align="left">
539  <ttcol>ETag 1</ttcol>
540  <ttcol>ETag 2</ttcol>
541  <ttcol>Strong Comparison</ttcol>
542  <ttcol>Weak Comparison</ttcol>
544  <c>W/"1"</c>
545  <c>W/"1"</c>
546  <c>no match</c>
547  <c>match</c>
549  <c>W/"1"</c>
550  <c>W/"2"</c>
551  <c>no match</c>
552  <c>no match</c>
554  <c>W/"1"</c>
555  <c>"1"</c>
556  <c>no match</c>
557  <c>match</c>
559  <c>"1"</c>
560  <c>"1"</c>
561  <c>match</c>
562  <c>match</c>
[8]565   An entity tag is strong unless it is explicitly tagged as weak.
566   <xref target="entity.tags"/> gives the syntax for entity tags.
569   A Last-Modified time, when used as a validator in a request, is
570   implicitly weak unless it is possible to deduce that it is strong,
571   using the following rules:
572  <list style="symbols">
573     <t>The validator is being compared by an origin server to the
574        actual current validator for the entity and,</t>
575     <t>That origin server reliably knows that the associated entity did
576        not change twice during the second covered by the presented
577        validator.</t>
578  </list>
581   or
582  <list style="symbols">
583     <t>The validator is about to be used by a client in an If-Modified-Since
584        or If-Unmodified-Since header, because the client
585        has a cache entry for the associated entity, and</t>
586     <t>That cache entry includes a Date value, which gives the time
587        when the origin server sent the original response, and</t>
588     <t>The presented Last-Modified time is at least 60 seconds before
589        the Date value.</t>
590  </list>
593   or
594  <list style="symbols">
595     <t>The validator is being compared by an intermediate cache to the
596        validator stored in its cache entry for the entity, and</t>
597     <t>That cache entry includes a Date value, which gives the time
598        when the origin server sent the original response, and</t>
599     <t>The presented Last-Modified time is at least 60 seconds before
600        the Date value.</t>
601  </list>
604   This method relies on the fact that if two different responses were
605   sent by the origin server during the same second, but both had the
606   same Last-Modified time, then at least one of those responses would
607   have a Date value equal to its Last-Modified time. The arbitrary 60-second
608   limit guards against the possibility that the Date and Last-Modified
609   values are generated from different clocks, or at somewhat
610   different times during the preparation of the response. An
611   implementation &MAY; use a value larger than 60 seconds, if it is
612   believed that 60 seconds is too short.
615   If a client wishes to perform a sub-range retrieval on a value for
616   which it has only a Last-Modified time and no opaque validator, it
617   &MAY; do this only if the Last-Modified time is strong in the sense
618   described here.
[245]621   A cache or origin server receiving a conditional range request
622   (<xref target="Part5"/>) &MUST; use the strong comparison function to
[8]623   evaluate the condition.
626   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
627   retrievals on values that have been obtained from HTTP/1.0
628   servers.
632<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="">
634   We adopt a set of rules and recommendations for origin servers,
635   clients, and caches regarding when various validator types ought to
636   be used, and for what purposes.
639   HTTP/1.1 origin servers:
640  <list style="symbols">
641     <t>&SHOULD; send an entity tag validator unless it is not feasible to
642        generate one.</t>
644     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
645        performance considerations support the use of weak entity tags,
646        or if it is unfeasible to send a strong entity tag.</t>
648     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
649        unless the risk of a breakdown in semantic transparency that
650        could result from using this date in an If-Modified-Since header
651        would lead to serious problems.</t>
652  </list>
655   In other words, the preferred behavior for an HTTP/1.1 origin server
656   is to send both a strong entity tag and a Last-Modified value.
659   In order to be legal, a strong entity tag &MUST; change whenever the
[302]660   associated entity changes in any way. A weak entity tag &SHOULD;
[8]661   change whenever the associated entity changes in a semantically
662   significant way.
665  <t>
[756]666    <x:h>Note:</x:h> In order to provide semantically transparent caching, an
[563]667    origin server must avoid reusing a specific strong entity tag
668    value for two different entities, or reusing a specific weak
669    entity tag value for two semantically different entities. Cache
670    entries might persist for arbitrarily long periods, regardless of
671    expiration times, so it might be inappropriate to expect that a
672    cache will never again attempt to validate an entry using a
673    validator that it obtained at some point in the past.
674  </t>
677   HTTP/1.1 clients:
678  <list style="symbols">
[755]679     <t>&MUST; use that entity tag in any cache-conditional request (using
680        If-Match or If-None-Match) if an entity tag has been provided by the
681        origin server.</t>
[755]683     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
684        requests (using If-Modified-Since) if only a Last-Modified value has
685        been provided by the origin server. </t>
[755]687     <t>&MAY; use the Last-Modified value in subrange cache-conditional
688        requests (using If-Unmodified-Since) if only a Last-Modified value has
689        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
[8]690        provide a way to disable this, in case of difficulty.</t>
[755]692     <t>&SHOULD; use both validators in cache-conditional requests if both an
693        entity tag and a Last-Modified value have been provided by the origin
694        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
695        appropriately.</t>
[8]696  </list>
699   An HTTP/1.1 origin server, upon receiving a conditional request that
700   includes both a Last-Modified date (e.g., in an If-Modified-Since or
701   If-Unmodified-Since header field) and one or more entity tags (e.g.,
702   in an If-Match, If-None-Match, or If-Range header field) as cache
703   validators, &MUST-NOT; return a response status of 304 (Not Modified)
704   unless doing so is consistent with all of the conditional header
705   fields in the request.
708   An HTTP/1.1 caching proxy, upon receiving a conditional request that
709   includes both a Last-Modified date and one or more entity tags as
710   cache validators, &MUST-NOT; return a locally cached response to the
711   client unless that cached response is consistent with all of the
712   conditional header fields in the request.
713  <list><t>
714      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
715      servers and clients should transmit as much non-redundant
716      information as is available in their responses and requests.
717      HTTP/1.1 systems receiving this information will make the most
718      conservative assumptions about the validators they receive.
719  </t><t>
720      HTTP/1.0 clients and caches will ignore entity tags. Generally,
721      last-modified values received or used by these systems will
722      support transparent and efficient caching, and so HTTP/1.1 origin
723      servers should provide Last-Modified values. In those rare cases
724      where the use of a Last-Modified value as a validator by an
725      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
726      origin servers should not provide one.
727  </t></list>
731<section title="Header Field Definitions" anchor="header.fields">
[117]733   This section defines the syntax and semantics of HTTP/1.1 header fields
734   related to conditional requests.
737   For entity-header fields, both sender and recipient refer to either the
738   client or the server, depending on who sends and who receives the entity.
741<section title="ETag" anchor="header.etag">
742  <iref primary="true" item="ETag header" x:for-anchor=""/>
743  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
[229]744  <x:anchor-alias value="ETag"/>
[362]745  <x:anchor-alias value="ETag-v"/>
[697]747   The "ETag" response-header field provides the current value of the
[698]748   entity tag (see <xref target="entity.tags"/>) for the requested variant,
749   which may be used for comparison with other entities from the same resource
[8]750   (see <xref target="weak.and.strong.validators"/>).
[362]752<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="ETag-v"/>
[366]753  <x:ref>ETag</x:ref>   = "ETag" ":" <x:ref>OWS</x:ref> <x:ref>ETag-v</x:ref>
[362]754  <x:ref>ETag-v</x:ref> = <x:ref>entity-tag</x:ref>
[362]757  Examples:
759<artwork type="example">
[362]760  ETag: "xyzzy"
761  ETag: W/"xyzzy"
762  ETag: ""
765   The ETag response-header field value, an entity tag, provides for an
766   "opaque" cache validator. This might allow more reliable validation
767   in situations where it is inconvenient to store modification dates,
768   where the one-second resolution of HTTP date values is not
769   sufficient, or where the origin server wishes to avoid certain
770   paradoxes that might arise from the use of modification dates.
773   The principle behind entity tags is that only the service author
774   knows the semantics of a resource well enough to select an
775   appropriate cache validation mechanism, and the specification of any
776   validator comparison function more complex than byte-equality would
777   open up a can of worms. Thus, comparisons of any other headers
778   (except Last-Modified, for compatibility with HTTP/1.0) are never
779   used for purposes of validating a cache entry.
783<section title="If-Match" anchor="header.if-match">
784  <iref primary="true" item="If-Match header" x:for-anchor=""/>
785  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
[229]786  <x:anchor-alias value="If-Match"/>
[362]787  <x:anchor-alias value="If-Match-v"/>
[698]789   The "If-Match" request-header field is used to make a request method
[8]790   conditional. A client that has one or more entities previously
791   obtained from the resource can verify that one of those entities is
792   current by including a list of their associated entity tags in the
[698]793   If-Match header field.
796   This allows efficient updates of cached information with a minimum amount of
797   transaction overhead. It is also used when updating resources, to prevent
798   inadvertent modification of the wrong version of a resource. As a special
799   case, the value "*" matches any current entity of the resource.
[362]801<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/><iref primary="true" item="Grammar" subitem="If-Match-v"/>
[366]802  <x:ref>If-Match</x:ref>   = "If-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-Match-v</x:ref>
[362]803  <x:ref>If-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
806   If any of the entity tags match the entity tag of the entity that
807   would have been returned in the response to a similar GET request
808   (without the If-Match header) on that resource, or if "*" is given
809   and any current entity exists for that resource, then the server &MAY;
810   perform the requested method as if the If-Match header field did not
811   exist.
814   If none of the entity tags match, or if "*" is given and no current
815   entity exists, the server &MUST-NOT; perform the requested method, and
816   &MUST; return a 412 (Precondition Failed) response. This behavior is
817   most useful when the client wants to prevent an updating method, such
818   as PUT, from modifying a resource that has changed since the client
819   last retrieved it.
822   If the request would, without the If-Match header field, result in
823   anything other than a 2xx or 412 status, then the If-Match header
824   &MUST; be ignored.
827   The meaning of "If-Match: *" is that the method &SHOULD; be performed
828   if the representation selected by the origin server (or by a cache,
[29]829   possibly using the Vary mechanism, see &header-vary;) exists, and
[8]830   &MUST-NOT; be performed if the representation does not exist.
833   A request intended to update a resource (e.g., a PUT) &MAY; include an
834   If-Match header field to signal that the request method &MUST-NOT; be
835   applied if the entity corresponding to the If-Match value (a single
836   entity tag) is no longer a representation of that resource. This
837   allows the user to indicate that they do not wish the request to be
838   successful if the resource has been changed without their knowledge.
839   Examples:
841<figure><artwork type="example">
[362]842  If-Match: "xyzzy"
843  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
844  If-Match: *
847   The result of a request having both an If-Match header field and
848   either an If-None-Match or an If-Modified-Since header fields is
849   undefined by this specification.
853<section title="If-Modified-Since" anchor="header.if-modified-since">
854  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
855  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
[229]856  <x:anchor-alias value="If-Modified-Since"/>
[362]857  <x:anchor-alias value="If-Modified-Since-v"/>
[698]859   The "If-Modified-Since" request-header field is used to make a request
860   method conditional: if the requested variant has not been modified since the
861   time specified in this field, the server will not return an entity; instead,
862   a 304 (Not Modified) response will be returned.
[362]864<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/><iref primary="true" item="Grammar" subitem="If-Modified-Since-v"/>
[376]865  <x:ref>If-Modified-Since</x:ref>   = "If-Modified-Since" ":" <x:ref>OWS</x:ref>
866                        <x:ref>If-Modified-Since-v</x:ref>
[362]867  <x:ref>If-Modified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
870   An example of the field is:
872<figure><artwork type="example">
[362]873  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
876   A GET method with an If-Modified-Since header and no Range header
877   requests that the identified entity be transferred only if it has
878   been modified since the date given by the If-Modified-Since header.
879   The algorithm for determining this includes the following cases:
880  <list style="numbers">
881      <t>If the request would normally result in anything other than a
882         200 (OK) status, or if the passed If-Modified-Since date is
883         invalid, the response is exactly the same as for a normal GET.
884         A date which is later than the server's current time is
885         invalid.</t>
887      <t>If the variant has been modified since the If-Modified-Since
888         date, the response is exactly the same as for a normal GET.</t>
890      <t>If the variant has not been modified since a valid If-Modified-Since
891         date, the server &SHOULD; return a 304 (Not
892         Modified) response.</t>
893  </list>
896   The purpose of this feature is to allow efficient updates of cached
897   information with a minimum amount of transaction overhead.
898  <list><t>
899      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
[29]900      see &header-range; for full details.
[8]901    </t><t>
902      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
903      clock might not be synchronized with the client.
904    </t><t>
905      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
906      servers will use an exact date comparison function, rather than a
907      less-than function, for deciding whether to send a 304 (Not
908      Modified) response. To get best results when sending an If-Modified-Since
909      header field for cache validation, clients are
910      advised to use the exact date string received in a previous Last-Modified
911      header field whenever possible.
912    </t><t>
913      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
914      header instead of a date taken from the Last-Modified header for
915      the same request, the client should be aware of the fact that this
916      date is interpreted in the server's understanding of time. The
917      client should consider unsynchronized clocks and rounding problems
918      due to the different encodings of time between the client and
919      server. This includes the possibility of race conditions if the
920      document has changed between the time it was first requested and
921      the If-Modified-Since date of a subsequent request, and the
922      possibility of clock-skew-related problems if the If-Modified-Since
923      date is derived from the client's clock without correction
924      to the server's clock. Corrections for different time bases
925      between client and server are at best approximate due to network
926      latency.
927    </t>
928  </list>
931   The result of a request having both an If-Modified-Since header field
932   and either an If-Match or an If-Unmodified-Since header fields is
933   undefined by this specification.
937<section title="If-None-Match" anchor="header.if-none-match">
938  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
939  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
[229]940  <x:anchor-alias value="If-None-Match"/>
[362]941  <x:anchor-alias value="If-None-Match-v"/>
[698]943   The "If-None-Match" request-header field is used to make a request method
944   conditional. A client that has one or more entities previously
[8]945   obtained from the resource can verify that none of those entities is
946   current by including a list of their associated entity tags in the
[698]947   If-None-Match header field.
950   This allows efficient updates of cached information with a minimum amount of
[763]951   transaction overhead. It is also used to prevent a method (e.g., PUT)
[8]952   from inadvertently modifying an existing resource when the client
953   believes that the resource does not exist.
956   As a special case, the value "*" matches any current entity of the
957   resource.
[362]959<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/><iref primary="true" item="Grammar" subitem="If-None-Match-v"/>
[366]960  <x:ref>If-None-Match</x:ref>   = "If-None-Match" ":" <x:ref>OWS</x:ref> <x:ref>If-None-Match-v</x:ref>
[362]961  <x:ref>If-None-Match-v</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
964   If any of the entity tags match the entity tag of the entity that
965   would have been returned in the response to a similar GET request
966   (without the If-None-Match header) on that resource, or if "*" is
967   given and any current entity exists for that resource, then the
968   server &MUST-NOT; perform the requested method, unless required to do
969   so because the resource's modification date fails to match that
970   supplied in an If-Modified-Since header field in the request.
971   Instead, if the request method was GET or HEAD, the server &SHOULD;
972   respond with a 304 (Not Modified) response, including the cache-related
973   header fields (particularly ETag) of one of the entities that
974   matched. For all other request methods, the server &MUST; respond with
975   a status of 412 (Precondition Failed).
978   If none of the entity tags match, then the server &MAY; perform the
979   requested method as if the If-None-Match header field did not exist,
980   but &MUST; also ignore any If-Modified-Since header field(s) in the
981   request. That is, if no entity tags match, then the server &MUST-NOT;
982   return a 304 (Not Modified) response.
985   If the request would, without the If-None-Match header field, result
986   in anything other than a 2xx or 304 status, then the If-None-Match
987   header &MUST; be ignored. (See <xref target=""/> for a discussion of
988   server behavior when both If-Modified-Since and If-None-Match appear
989   in the same request.)
992   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
993   performed if the representation selected by the origin server (or by
[29]994   a cache, possibly using the Vary mechanism, see &header-vary;)
[8]995   exists, and &SHOULD; be performed if the representation does not exist.
996   This feature is intended to be useful in preventing races between PUT
997   operations.
1000   Examples:
1002<figure><artwork type="example">
[362]1003  If-None-Match: "xyzzy"
1004  If-None-Match: W/"xyzzy"
1005  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
1006  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
1007  If-None-Match: *
1010   The result of a request having both an If-None-Match header field and
1011   either an If-Match or an If-Unmodified-Since header fields is
1012   undefined by this specification.
1016<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
1017  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
1018  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
[229]1019  <x:anchor-alias value="If-Unmodified-Since"/>
[362]1020  <x:anchor-alias value="If-Unmodified-Since-v"/>
[698]1022   The "If-Unmodified-Since" request-header field is used to make a request
1023   method conditional. If the requested resource has not been modified
[8]1024   since the time specified in this field, the server &SHOULD; perform the
1025   requested operation as if the If-Unmodified-Since header were not
1026   present.
1029   If the requested variant has been modified since the specified time,
1030   the server &MUST-NOT; perform the requested operation, and &MUST; return
1031   a 412 (Precondition Failed).
[362]1033<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/><iref primary="true" item="Grammar" subitem="If-Unmodified-Since-v"/>
[376]1034  <x:ref>If-Unmodified-Since</x:ref>   = "If-Unmodified-Since" ":" <x:ref>OWS</x:ref>
1035                          <x:ref>If-Unmodified-Since-v</x:ref>
[362]1036  <x:ref>If-Unmodified-Since-v</x:ref> = <x:ref>HTTP-date</x:ref>
1039   An example of the field is:
1041<figure><artwork type="example">
[362]1042  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1045   If the request normally (i.e., without the If-Unmodified-Since
1046   header) would result in anything other than a 2xx or 412 status, the
1047   If-Unmodified-Since header &SHOULD; be ignored.
1050   If the specified date is invalid, the header is ignored.
1053   The result of a request having both an If-Unmodified-Since header
1054   field and either an If-None-Match or an If-Modified-Since header
1055   fields is undefined by this specification.
1059<section title="Last-Modified" anchor="header.last-modified">
1060  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
1061  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
[229]1062  <x:anchor-alias value="Last-Modified"/>
[362]1063  <x:anchor-alias value="Last-Modified-v"/>
[697]1065   The "Last-Modified" entity-header field indicates the date and time at
[8]1066   which the origin server believes the variant was last modified.
[362]1068<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/><iref primary="true" item="Grammar" subitem="Last-Modified-v"/>
[366]1069  <x:ref>Last-Modified</x:ref>   = "Last-Modified" ":" <x:ref>OWS</x:ref> <x:ref>Last-Modified-v</x:ref>
[362]1070  <x:ref>Last-Modified-v</x:ref> = <x:ref>HTTP-date</x:ref>
1073   An example of its use is
1075<figure><artwork type="example">
[362]1076  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1079   The exact meaning of this header field depends on the implementation
1080   of the origin server and the nature of the original resource. For
1081   files, it may be just the file system last-modified time. For
1082   entities with dynamically included parts, it may be the most recent
1083   of the set of last-modify times for its component parts. For database
1084   gateways, it may be the last-update time stamp of the record. For
1085   virtual objects, it may be the last time the internal state changed.
1088   An origin server &MUST-NOT; send a Last-Modified date which is later
1089   than the server's time of message origination. In such cases, where
1090   the resource's last modification would indicate some time in the
1091   future, the server &MUST; replace that date with the message
1092   origination date.
1095   An origin server &SHOULD; obtain the Last-Modified value of the entity
1096   as close as possible to the time that it generates the Date value of
1097   its response. This allows a recipient to make an accurate assessment
1098   of the entity's modification time, especially if the entity changes
1099   near the time that the response is generated.
1102   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1105   The Last-Modified entity-header field value is often used as a cache
1106   validator. In simple terms, a cache entry is considered to be valid
1107   if the entity has not been modified since the Last-Modified value.
[29]1113<section title="IANA Considerations" anchor="IANA.considerations">
1115<section title="Status Code Registration" anchor="status.code.registration">
1117   The HTTP Status Code Registry located at <eref target=""/>
1118   should be updated with the registrations below:
1120<?BEGININC p4-conditional.iana-status-codes ?>
1121<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1122<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1123   <ttcol>Value</ttcol>
1124   <ttcol>Description</ttcol>
1125   <ttcol>Reference</ttcol>
1126   <c>304</c>
1127   <c>Not Modified</c>
1128   <c>
1129      <xref target="status.304"/>
1130   </c>
1131   <c>412</c>
1132   <c>Precondition Failed</c>
1133   <c>
1134      <xref target="status.412"/>
1135   </c>
1138<?ENDINC p4-conditional.iana-status-codes ?>
[253]1141<section title="Message Header Registration" anchor="message.header.registration">
1143   The Message Header Registry located at <eref target=""/> should be updated
1144   with the permanent registrations below (see <xref target="RFC3864"/>):
[680]1146<?BEGININC p4-conditional.iana-headers ?>
[253]1147<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
[290]1148<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
[253]1149   <ttcol>Header Field Name</ttcol>
1150   <ttcol>Protocol</ttcol>
1151   <ttcol>Status</ttcol>
1152   <ttcol>Reference</ttcol>
1154   <c>ETag</c>
1155   <c>http</c>
1156   <c>standard</c>
1157   <c>
1158      <xref target="header.etag"/>
1159   </c>
1160   <c>If-Match</c>
1161   <c>http</c>
1162   <c>standard</c>
1163   <c>
1164      <xref target="header.if-match"/>
1165   </c>
1166   <c>If-Modified-Since</c>
1167   <c>http</c>
1168   <c>standard</c>
1169   <c>
1170      <xref target="header.if-modified-since"/>
1171   </c>
1172   <c>If-None-Match</c>
1173   <c>http</c>
1174   <c>standard</c>
1175   <c>
1176      <xref target="header.if-none-match"/>
1177   </c>
1178   <c>If-Unmodified-Since</c>
1179   <c>http</c>
1180   <c>standard</c>
1181   <c>
1182      <xref target="header.if-unmodified-since"/>
1183   </c>
1184   <c>Last-Modified</c>
1185   <c>http</c>
1186   <c>standard</c>
1187   <c>
1188      <xref target="header.last-modified"/>
1189   </c>
[680]1192<?ENDINC p4-conditional.iana-headers ?>
[290]1194   The change controller is: "IETF ( - Internet Engineering Task Force".
1199<section title="Security Considerations" anchor="security.considerations">
[29]1201   No additional security considerations have been identified beyond
1202   those applicable to HTTP in general &messaging;.
1206<section title="Acknowledgments" anchor="ack">
1211<references title="Normative References">
[31]1213<reference anchor="Part1">
[119]1214  <front>
1215    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1216    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1217      <organization abbrev="Day Software">Day Software</organization>
1218      <address><email></email></address>
1219    </author>
1220    <author initials="J." surname="Gettys" fullname="Jim Gettys">
[845]1221      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1222      <address><email></email></address>
[119]1223    </author>
1224    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1225      <organization abbrev="HP">Hewlett-Packard Company</organization>
1226      <address><email></email></address>
1227    </author>
1228    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1229      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1230      <address><email></email></address>
1231    </author>
1232    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1233      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1234      <address><email></email></address>
1235    </author>
1236    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1237      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1238      <address><email></email></address>
1239    </author>
1240    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1241      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1242      <address><email></email></address>
1243    </author>
1244    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1245      <organization abbrev="W3C">World Wide Web Consortium</organization>
1246      <address><email></email></address>
1247    </author>
1248    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1249      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1250      <address><email></email></address>
1251    </author>
1252    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1253  </front>
1254  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1255  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
[800]1258<reference anchor="Part3">
1259  <front>
1260    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1261    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1262      <organization abbrev="Day Software">Day Software</organization>
1263      <address><email></email></address>
1264    </author>
1265    <author initials="J." surname="Gettys" fullname="Jim Gettys">
[845]1266      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1267      <address><email></email></address>
[800]1268    </author>
1269    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1270      <organization abbrev="HP">Hewlett-Packard Company</organization>
1271      <address><email></email></address>
1272    </author>
1273    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1274      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1275      <address><email></email></address>
1276    </author>
1277    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1278      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1279      <address><email></email></address>
1280    </author>
1281    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1282      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1283      <address><email></email></address>
1284    </author>
1285    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1286      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1287      <address><email></email></address>
1288    </author>
1289    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1290      <organization abbrev="W3C">World Wide Web Consortium</organization>
1291      <address><email></email></address>
1292    </author>
1293    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1294      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1295      <address><email></email></address>
1296    </author>
1297    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1298  </front>
1299  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1300  <x:source href="p3-payload.xml" basename="p3-payload"/>
[31]1303<reference anchor="Part5">
[119]1304  <front>
1305    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1306    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1307      <organization abbrev="Day Software">Day Software</organization>
1308      <address><email></email></address>
1309    </author>
1310    <author initials="J." surname="Gettys" fullname="Jim Gettys">
[845]1311      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1312      <address><email></email></address>
[119]1313    </author>
1314    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1315      <organization abbrev="HP">Hewlett-Packard Company</organization>
1316      <address><email></email></address>
1317    </author>
1318    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1319      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1320      <address><email></email></address>
1321    </author>
1322    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1323      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1324      <address><email></email></address>
1325    </author>
1326    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1327      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1328      <address><email></email></address>
1329    </author>
1330    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1331      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1332      <address><email></email></address>
1333    </author>
1334    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1335      <organization abbrev="W3C">World Wide Web Consortium</organization>
1336      <address><email></email></address>
1337    </author>
1338    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1339      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1340      <address><email></email></address>
1341    </author>
1342    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1343  </front>
1344  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1345  <x:source href="p5-range.xml" basename="p5-range"/>
1348<reference anchor="Part6">
[119]1349  <front>
1350    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1351    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1352      <organization abbrev="Day Software">Day Software</organization>
1353      <address><email></email></address>
1354    </author>
1355    <author initials="J." surname="Gettys" fullname="Jim Gettys">
[845]1356      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1357      <address><email></email></address>
[119]1358    </author>
1359    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1360      <organization abbrev="HP">Hewlett-Packard Company</organization>
1361      <address><email></email></address>
1362    </author>
1363    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1364      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1365      <address><email></email></address>
1366    </author>
1367    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1368      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1369      <address><email></email></address>
1370    </author>
1371    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1372      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1373      <address><email></email></address>
1374    </author>
1375    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1376      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1377      <address><email></email></address>
1378    </author>
1379    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1380      <organization abbrev="W3C">World Wide Web Consortium</organization>
1381      <address><email></email></address>
1382    </author>
[601]1383    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1384      <address><email></email></address>
1385    </author>
[119]1386    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1387      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1388      <address><email></email></address>
1389    </author>
1390    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1391  </front>
1392  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1393  <x:source href="p6-cache.xml" basename="p6-cache"/>
[96]1396<reference anchor="RFC2119">
1397  <front>
1398    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1399    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1400      <organization>Harvard University</organization>
1401      <address><email></email></address>
1402    </author>
1403    <date month="March" year="1997"/>
1404  </front>
1405  <seriesInfo name="BCP" value="14"/>
1406  <seriesInfo name="RFC" value="2119"/>
[425]1409<reference anchor="RFC5234">
1410  <front>
1411    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1412    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1413      <organization>Brandenburg InternetWorking</organization>
1414      <address>
[728]1415        <email></email>
1416      </address> 
[425]1417    </author>
1418    <author initials="P." surname="Overell" fullname="Paul Overell">
1419      <organization>THUS plc.</organization>
1420      <address>
[728]1421        <email></email>
1422      </address>
[425]1423    </author>
1424    <date month="January" year="2008"/>
1425  </front>
1426  <seriesInfo name="STD" value="68"/>
1427  <seriesInfo name="RFC" value="5234"/>
1432<references title="Informative References">
1434<reference anchor="RFC2616">
1435  <front>
1436    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1437    <author initials="R." surname="Fielding" fullname="R. Fielding">
1438      <organization>University of California, Irvine</organization>
1439      <address><email></email></address>
1440    </author>
1441    <author initials="J." surname="Gettys" fullname="J. Gettys">
1442      <organization>W3C</organization>
1443      <address><email></email></address>
1444    </author>
1445    <author initials="J." surname="Mogul" fullname="J. Mogul">
1446      <organization>Compaq Computer Corporation</organization>
1447      <address><email></email></address>
1448    </author>
1449    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1450      <organization>MIT Laboratory for Computer Science</organization>
1451      <address><email></email></address>
1452    </author>
1453    <author initials="L." surname="Masinter" fullname="L. Masinter">
1454      <organization>Xerox Corporation</organization>
1455      <address><email></email></address>
1456    </author>
1457    <author initials="P." surname="Leach" fullname="P. Leach">
1458      <organization>Microsoft Corporation</organization>
1459      <address><email></email></address>
1460    </author>
1461    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1462      <organization>W3C</organization>
1463      <address><email></email></address>
1464    </author>
1465    <date month="June" year="1999"/>
1466  </front>
1467  <seriesInfo name="RFC" value="2616"/>
[253]1470<reference anchor='RFC3864'>
1471  <front>
1472    <title>Registration Procedures for Message Header Fields</title>
1473    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1474      <organization>Nine by Nine</organization>
1475      <address><email></email></address>
1476    </author>
1477    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1478      <organization>BEA Systems</organization>
1479      <address><email></email></address>
1480    </author>
1481    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1482      <organization>HP Labs</organization>
1483      <address><email></email></address>
1484    </author>
1485    <date year='2004' month='September' />
1486  </front>
1487  <seriesInfo name='BCP' value='90' />
1488  <seriesInfo name='RFC' value='3864' />
1493<section title="Compatibility with Previous Versions" anchor="compatibility">
1495<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1497  Allow weak entity tags in all requests except range requests (Sections
1498  <xref target="weak.and.strong.validators" format="counter"/> and
1499  <xref target="header.if-none-match" format="counter"/>).
[680]1505<?BEGININC p4-conditional.abnf-appendix ?>
[427]1506<section xmlns:x="" title="Collected ABNF" anchor="collected.abnf">
1508<artwork type="abnf" name="p4-conditional.parsed-abnf">
1509<x:ref>ETag</x:ref> = "ETag:" OWS ETag-v
1510<x:ref>ETag-v</x:ref> = entity-tag
[678]1512<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1514<x:ref>If-Match</x:ref> = "If-Match:" OWS If-Match-v
1515<x:ref>If-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1516 entity-tag ] ) )
1517<x:ref>If-Modified-Since</x:ref> = "If-Modified-Since:" OWS If-Modified-Since-v
1518<x:ref>If-Modified-Since-v</x:ref> = HTTP-date
1519<x:ref>If-None-Match</x:ref> = "If-None-Match:" OWS If-None-Match-v
1520<x:ref>If-None-Match-v</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1521 entity-tag ] ) )
1522<x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since:" OWS
1523 If-Unmodified-Since-v
1524<x:ref>If-Unmodified-Since-v</x:ref> = HTTP-date
1526<x:ref>Last-Modified</x:ref> = "Last-Modified:" OWS Last-Modified-v
1527<x:ref>Last-Modified-v</x:ref> = HTTP-date
1529<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1531<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1533<x:ref>opaque-tag</x:ref> = quoted-string
1535<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
[581]1537<x:ref>weak</x:ref> = %x57.2F ; W/
[532]1540<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1541; ETag defined but not used
[427]1542; If-Match defined but not used
1543; If-Modified-Since defined but not used
1544; If-None-Match defined but not used
1545; If-Unmodified-Since defined but not used
1546; Last-Modified defined but not used
[680]1548<?ENDINC p4-conditional.abnf-appendix ?>
[252]1550<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1552<section title="Since RFC2616">
1554  Extracted relevant partitions from <xref target="RFC2616"/>.
1558<section title="Since draft-ietf-httpbis-p4-conditional-00">
[152]1560  Closed issues:
1561  <list style="symbols">
1562    <t>
[324]1563      <eref target=""/>:
[152]1564      "Normative and Informative references"
1565    </t>
1566  </list>
[116]1569  Other changes:
1570  <list style="symbols">
1571    <t>
1572      Move definitions of 304 and 412 condition codes from Part2.
1573    </t>
1574  </list>
[170]1578<section title="Since draft-ietf-httpbis-p4-conditional-01">
[324]1580  Ongoing work on ABNF conversion (<eref target=""/>):
[205]1581  <list style="symbols">
1582    <t>
1583      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1584    </t>
1585  </list>
[252]1589<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
[245]1591  Closed issues:
1592  <list style="symbols">
1593    <t>
[324]1594      <eref target=""/>:
[245]1595      "Weak ETags on non-GET requests"
1596    </t>
1597  </list>
[324]1600  Ongoing work on IANA Message Header Registration (<eref target=""/>):
[253]1601  <list style="symbols">
1602    <t>
1603      Reference RFC 3984, and update header registrations for headers defined
1604      in this document.
1605    </t>
1606  </list>
[267]1610<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
[298]1612  Closed issues:
1613  <list style="symbols">
1614    <t>
[324]1615      <eref target=""/>:
[298]1616      "Examples for ETag matching"
1617    </t>
[302]1618    <t>
[324]1619      <eref target=""/>:
[302]1620      "'entity value' undefined"
1621    </t>
[306]1622    <t>
[324]1623      <eref target=""/>:
[306]1624      "bogus 2068 Date header reference"
1625    </t>
[298]1626  </list>
[323]1630<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
[334]1632  Ongoing work on ABNF conversion (<eref target=""/>):
1633  <list style="symbols">
1634    <t>
1635      Use "/" instead of "|" for alternatives.
1636    </t>
[362]1637    <t>
1638      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1639      whitespace ("OWS") and required whitespace ("RWS").
1640    </t>
1641    <t>
1642      Rewrite ABNFs to spell out whitespace rules, factor out
1643      header value format definitions.
1644    </t>
[334]1645  </list>
[382]1649<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
[543]1651  Final work on ABNF conversion (<eref target=""/>):
[421]1652  <list style="symbols">
1653    <t>
[424]1654      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
[421]1655    </t>
1656  </list>
[547]1660<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
[570]1662  Closed issues:
1663  <list style="symbols">
1664    <t>
1665      <eref target=""/>:
1666      "case-sensitivity of etag weakness indicator"
1667    </t>
1668  </list>
[604]1672<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
[656]1674  Closed issues:
1675  <list style="symbols">
1676    <t>
1677      <eref target=""/>:
1678      "Weak ETags on non-GET requests" (If-Match still was defined to require
1679      strong matching)
1680    </t>
[700]1681    <t>
1682      <eref target=""/>:
1683      "move IANA registrations for optional status codes"
1684    </t>
[656]1685  </list>
[720]1689<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
[765]1691  No significant changes.
[773]1695<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
[841]1697  No significant changes.
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