source: draft-ietf-httpbis/latest/p4-conditional.xml @ 234

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1<?xml version="1.0" encoding="utf-8"?>
2<?xml-stylesheet type='text/xsl' href='../myxml2rfc.xslt'?>
3<!DOCTYPE rfc [
4  <!ENTITY MAY "<bcp14 xmlns=''>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns=''>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns=''>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns=''>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns=''>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns=''>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns=''>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns=''>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns=''>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns=''>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "April">
16  <!ENTITY ID-YEAR "2008">
17  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x=''/>">
18  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x=''/>">
19  <!ENTITY messaging                  "<xref target='Part1' xmlns:x=''/>">
20  <!ENTITY caching                    "<xref target='Part6' xmlns:x=''/>">
21  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x=''/>">
22  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x=''/>">
23  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x=''/>">
24  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x=''/>">
25  <!ENTITY full-date                  "<xref target='Part1' x:rel='' xmlns:x=''/>">
27<?rfc toc="yes" ?>
28<?rfc symrefs="yes" ?>
29<?rfc sortrefs="yes" ?>
30<?rfc compact="yes"?>
31<?rfc subcompact="no" ?>
32<?rfc linkmailto="no" ?>
33<?rfc editing="no" ?>
34<?rfc comments="yes"?>
35<?rfc inline="yes"?>
36<?rfc-ext allow-markup-in-artwork="yes" ?>
37<?rfc-ext include-references-in-index="yes" ?>
38<rfc obsoletes="2616" category="std"
39     ipr="full3978" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
40     xmlns:x=''>
43  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
45  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
46    <organization abbrev="Day Software">Day Software</organization>
47    <address>
48      <postal>
49        <street>23 Corporate Plaza DR, Suite 280</street>
50        <city>Newport Beach</city>
51        <region>CA</region>
52        <code>92660</code>
53        <country>USA</country>
54      </postal>
55      <phone>+1-949-706-5300</phone>
56      <facsimile>+1-949-706-5305</facsimile>
57      <email></email>
58      <uri></uri>
59    </address>
60  </author>
62  <author initials="J." surname="Gettys" fullname="Jim Gettys">
63    <organization>One Laptop per Child</organization>
64    <address>
65      <postal>
66        <street>21 Oak Knoll Road</street>
67        <city>Carlisle</city>
68        <region>MA</region>
69        <code>01741</code>
70        <country>USA</country>
71      </postal>
72      <email></email>
73      <uri></uri>
74    </address>
75  </author>
77  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
78    <organization abbrev="HP">Hewlett-Packard Company</organization>
79    <address>
80      <postal>
81        <street>HP Labs, Large Scale Systems Group</street>
82        <street>1501 Page Mill Road, MS 1177</street>
83        <city>Palo Alto</city>
84        <region>CA</region>
85        <code>94304</code>
86        <country>USA</country>
87      </postal>
88      <email></email>
89    </address>
90  </author>
92  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
93    <organization abbrev="Microsoft">Microsoft Corporation</organization>
94    <address>
95      <postal>
96        <street>1 Microsoft Way</street>
97        <city>Redmond</city>
98        <region>WA</region>
99        <code>98052</code>
100        <country>USA</country>
101      </postal>
102      <email></email>
103    </address>
104  </author>
106  <author initials="L." surname="Masinter" fullname="Larry Masinter">
107    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
108    <address>
109      <postal>
110        <street>345 Park Ave</street>
111        <city>San Jose</city>
112        <region>CA</region>
113        <code>95110</code>
114        <country>USA</country>
115      </postal>
116      <email></email>
117      <uri></uri>
118    </address>
119  </author>
121  <author initials="P." surname="Leach" fullname="Paul J. Leach">
122    <organization abbrev="Microsoft">Microsoft Corporation</organization>
123    <address>
124      <postal>
125        <street>1 Microsoft Way</street>
126        <city>Redmond</city>
127        <region>WA</region>
128        <code>98052</code>
129      </postal>
130      <email></email>
131    </address>
132  </author>
134  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
135    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
136    <address>
137      <postal>
138        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
139        <street>The Stata Center, Building 32</street>
140        <street>32 Vassar Street</street>
141        <city>Cambridge</city>
142        <region>MA</region>
143        <code>02139</code>
144        <country>USA</country>
145      </postal>
146      <email></email>
147      <uri></uri>
148    </address>
149  </author>
151  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
152    <organization abbrev="W3C">World Wide Web Consortium</organization>
153    <address>
154      <postal>
155        <street>W3C / ERCIM</street>
156        <street>2004, rte des Lucioles</street>
157        <city>Sophia-Antipolis</city>
158        <region>AM</region>
159        <code>06902</code>
160        <country>France</country>
161      </postal>
162      <email></email>
163      <uri></uri>
164    </address>
165  </author>
167  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
168    <organization abbrev="greenbytes">greenbytes GmbH</organization>
169    <address>
170      <postal>
171        <street>Hafenweg 16</street>
172        <city>Muenster</city><region>NW</region><code>48155</code>
173        <country>Germany</country>
174      </postal>
175      <phone>+49 251 2807760</phone>   
176      <facsimile>+49 251 2807761</facsimile>   
177      <email></email>       
178      <uri></uri>     
179    </address>
180  </author>
182  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
186   The Hypertext Transfer Protocol (HTTP) is an application-level
187   protocol for distributed, collaborative, hypermedia information
188   systems. HTTP has been in use by the World Wide Web global information
189   initiative since 1990. This document is Part 4 of the seven-part specification
190   that defines the protocol referred to as "HTTP/1.1" and, taken together,
191   obsoletes RFC 2616.  Part 4 defines request header fields for
192   indicating conditional requests and the rules for constructing responses
193   to those requests.
197<note title="Editorial Note (To be removed by RFC Editor)">
198  <t>
199    Discussion of this draft should take place on the HTTPBIS working group
200    mailing list ( The current issues list is
201    at <eref target=""/>
202    and related documents (including fancy diffs) can be found at
203    <eref target=""/>.
204  </t>
205  <t>
206    This draft incorporates those issue resolutions that were either
207    collected in the original RFC2616 errata list (<eref target=""/>),
208    or which were agreed upon on the mailing list between October 2006 and
209    November 2007 (as published in "draft-lafon-rfc2616bis-03").
210  </t>
214<section title="Introduction" anchor="introduction">
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."
255<section title="Notational Conventions and Generic Grammar" anchor="notation">
256  <x:anchor-alias value="quoted-string"/>
258  This specification uses the ABNF syntax defined in &notation-abnf; and
259  the core rules defined in &basic-rules;:
260  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
261  <eref target=""/>.</cref>
263<figure><artwork type="abnf2616">
264  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
266<t anchor="abnf.dependencies">
267  <x:anchor-alias value="HTTP-date"/>
268  The ABNF rules below are defined in other parts:
270<figure><!--Part1--><artwork type="abnf2616">
271  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
275<section title="Entity Tags" anchor="entity.tags">
276  <x:anchor-alias value="entity-tag"/>
277  <x:anchor-alias value="opaque-tag"/>
278  <x:anchor-alias value="weak"/>
280   Entity tags are used for comparing two or more entities from the same
281   requested resource. HTTP/1.1 uses entity tags in the ETag (<xref target="header.etag"/>),
282   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
283   If-Range (&header-if-range;) header fields. The definition of how they
284   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
285   entity tag consists of an opaque quoted string, possibly prefixed by
286   a weakness indicator.
288<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"/>
289  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
290  <x:ref>weak</x:ref>       = "W/"
291  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
294   A "strong entity tag" &MAY; be shared by two entities of a resource
295   only if they are equivalent by octet equality.
298   A "weak entity tag," indicated by the "W/" prefix, &MAY; be shared by
299   two entities of a resource only if the entities are equivalent and
300   could be substituted for each other with no significant change in
301   semantics. A weak entity tag can only be used for weak comparison.
304   An entity tag &MUST; be unique across all versions of all entities
305   associated with a particular resource. A given entity tag value &MAY;
306   be used for entities obtained by requests on different URIs. The use
307   of the same entity tag value in conjunction with entities obtained by
308   requests on different URIs does not imply the equivalence of those
309   entities.
313<section title="Status Code Definitions">
314<section title="304 Not Modified" anchor="status.304">
315  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
316  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
318   If the client has performed a conditional GET request and access is
319   allowed, but the document has not been modified, the server &SHOULD;
320   respond with this status code. The 304 response &MUST-NOT; contain a
321   message-body, and thus is always terminated by the first empty line
322   after the header fields.
325   The response &MUST; include the following header fields:
326  <list style="symbols">
327    <t>Date, unless its omission is required by &clockless;</t>
328  </list>
331   If a clockless origin server obeys these rules, and proxies and
332   clients add their own Date to any response received without one (as
333   already specified by <xref target="RFC2068" x:sec="14.19" x:fmt=","/>), caches will operate
334   correctly.
335  <list style="symbols">
336    <t>ETag and/or Content-Location, if the header would have been sent
337        in a 200 response to the same request</t>
338    <t>Expires, Cache-Control, and/or Vary, if the field-value might
339        differ from that sent in any previous response for the same
340        variant</t>
341  </list>
344   If the conditional GET used a strong cache validator (see <xref target="weak.and.strong.validators"/>),
345   the response &SHOULD-NOT;  include other entity-headers.
346   Otherwise (i.e., the conditional GET used a weak validator), the
347   response &MUST-NOT; include other entity-headers; this prevents
348   inconsistencies between cached entity-bodies and updated headers.
351   If a 304 response indicates an entity not currently cached, then the
352   cache &MUST; disregard the response and repeat the request without the
353   conditional.
356   If a cache uses a received 304 response to update a cache entry, the
357   cache &MUST; update the entry to reflect any new field values given in
358   the response.
362<section title="412 Precondition Failed" anchor="status.412">
363  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
364  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
366   The precondition given in one or more of the request-header fields
367   evaluated to false when it was tested on the server. This response
368   code allows the client to place preconditions on the current resource
369   metainformation (header field data) and thus prevent the requested
370   method from being applied to a resource other than the one intended.
375<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
377   Since both origin servers and caches will compare two validators to
378   decide if they represent the same or different entities, one normally
379   would expect that if the entity (the entity-body or any entity-headers)
380   changes in any way, then the associated validator would
381   change as well. If this is true, then we call this validator a
382   "strong validator."
385   However, there might be cases when a server prefers to change the
386   validator only on semantically significant changes, and not when
387   insignificant aspects of the entity change. A validator that does not
388   always change when the resource changes is a "weak validator."
391   Entity tags are normally "strong validators," but the protocol
392   provides a mechanism to tag an entity tag as "weak." One can think of
393   a strong validator as one that changes whenever the bits of an entity
394   changes, while a weak value changes whenever the meaning of an entity
395   changes. Alternatively, one can think of a strong validator as part
396   of an identifier for a specific entity, while a weak validator is
397   part of an identifier for a set of semantically equivalent entities.
398  <list><t>
399      <x:h>Note:</x:h> One example of a strong validator is an integer that is
400      incremented in stable storage every time an entity is changed.
401    </t><t>
402      An entity's modification time, if represented with one-second
403      resolution, could be a weak validator, since it is possible that
404      the resource might be modified twice during a single second.
405    </t><t>
406      Support for weak validators is optional. However, weak validators
407      allow for more efficient caching of equivalent objects; for
408      example, a hit counter on a site is probably good enough if it is
409      updated every few days or weeks, and any value during that period
410      is likely "good enough" to be equivalent.
411    </t></list>
414   A "use" of a validator is either when a client generates a request
415   and includes the validator in a validating header field, or when a
416   server compares two validators.
419   Strong validators are usable in any context. Weak validators are only
420   usable in contexts that do not depend on exact equality of an entity.
421   For example, either kind is usable for a conditional GET of a full
422   entity. However, only a strong validator is usable for a sub-range
423   retrieval, since otherwise the client might end up with an internally
424   inconsistent entity.
427   Clients &MAY; issue simple (non-subrange) GET requests with either weak
428   validators or strong validators. Clients &MUST-NOT; use weak validators
429   in other forms of request.
432   The only function that HTTP/1.1 defines on validators is
433   comparison. There are two validator comparison functions, depending
434   on whether the comparison context allows the use of weak validators
435   or not:
436  <list style="symbols">
437     <t>The strong comparison function: in order to be considered equal,
438        both validators &MUST; be identical in every way, and both &MUST-NOT;
439        be weak.</t>
440     <t>The weak comparison function: in order to be considered equal,
441        both validators &MUST; be identical in every way, but either or
442        both of them &MAY; be tagged as "weak" without affecting the
443        result.</t>
444  </list>
447   An entity tag is strong unless it is explicitly tagged as weak.
448   <xref target="entity.tags"/> gives the syntax for entity tags.
451   A Last-Modified time, when used as a validator in a request, is
452   implicitly weak unless it is possible to deduce that it is strong,
453   using the following rules:
454  <list style="symbols">
455     <t>The validator is being compared by an origin server to the
456        actual current validator for the entity and,</t>
457     <t>That origin server reliably knows that the associated entity did
458        not change twice during the second covered by the presented
459        validator.</t>
460  </list>
463   or
464  <list style="symbols">
465     <t>The validator is about to be used by a client in an If-Modified-Since
466        or If-Unmodified-Since header, because the client
467        has a cache entry for the associated entity, and</t>
468     <t>That cache entry includes a Date value, which gives the time
469        when the origin server sent the original response, and</t>
470     <t>The presented Last-Modified time is at least 60 seconds before
471        the Date value.</t>
472  </list>
475   or
476  <list style="symbols">
477     <t>The validator is being compared by an intermediate cache to the
478        validator stored in its cache entry for the entity, and</t>
479     <t>That cache entry includes a Date value, which gives the time
480        when the origin server sent the original response, and</t>
481     <t>The presented Last-Modified time is at least 60 seconds before
482        the Date value.</t>
483  </list>
486   This method relies on the fact that if two different responses were
487   sent by the origin server during the same second, but both had the
488   same Last-Modified time, then at least one of those responses would
489   have a Date value equal to its Last-Modified time. The arbitrary 60-second
490   limit guards against the possibility that the Date and Last-Modified
491   values are generated from different clocks, or at somewhat
492   different times during the preparation of the response. An
493   implementation &MAY; use a value larger than 60 seconds, if it is
494   believed that 60 seconds is too short.
497   If a client wishes to perform a sub-range retrieval on a value for
498   which it has only a Last-Modified time and no opaque validator, it
499   &MAY; do this only if the Last-Modified time is strong in the sense
500   described here.
503   A cache or origin server receiving a conditional request, other than
504   a full-body GET request, &MUST; use the strong comparison function to
505   evaluate the condition.
508   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
509   retrievals on values that have been obtained from HTTP/1.0
510   servers.
514<section title="Rules for When to Use Entity Tags and Last-Modified Dates" anchor="">
516   We adopt a set of rules and recommendations for origin servers,
517   clients, and caches regarding when various validator types ought to
518   be used, and for what purposes.
521   HTTP/1.1 origin servers:
522  <list style="symbols">
523     <t>&SHOULD; send an entity tag validator unless it is not feasible to
524        generate one.</t>
526     <t>&MAY; send a weak entity tag instead of a strong entity tag, if
527        performance considerations support the use of weak entity tags,
528        or if it is unfeasible to send a strong entity tag.</t>
530     <t>&SHOULD; send a Last-Modified value if it is feasible to send one,
531        unless the risk of a breakdown in semantic transparency that
532        could result from using this date in an If-Modified-Since header
533        would lead to serious problems.</t>
534  </list>
537   In other words, the preferred behavior for an HTTP/1.1 origin server
538   is to send both a strong entity tag and a Last-Modified value.
541   In order to be legal, a strong entity tag &MUST; change whenever the
542   associated entity value changes in any way. A weak entity tag &SHOULD;
543   change whenever the associated entity changes in a semantically
544   significant way.
545  <list><t>
546      <x:h>Note:</x:h> in order to provide semantically transparent caching, an
547      origin server must avoid reusing a specific strong entity tag
548      value for two different entities, or reusing a specific weak
549      entity tag value for two semantically different entities. Cache
550      entries might persist for arbitrarily long periods, regardless of
551      expiration times, so it might be inappropriate to expect that a
552      cache will never again attempt to validate an entry using a
553      validator that it obtained at some point in the past.
554  </t></list>
557   HTTP/1.1 clients:
558  <list style="symbols">
559     <t>If an entity tag has been provided by the origin server, &MUST;
560        use that entity tag in any cache-conditional request (using If-Match
561        or If-None-Match).</t>
563     <t>If only a Last-Modified value has been provided by the origin
564        server, &SHOULD; use that value in non-subrange cache-conditional
565        requests (using If-Modified-Since).</t>
567     <t>If only a Last-Modified value has been provided by an HTTP/1.0
568        origin server, &MAY; use that value in subrange cache-conditional
569        requests (using If-Unmodified-Since:). The user agent &SHOULD;
570        provide a way to disable this, in case of difficulty.</t>
572     <t>If both an entity tag and a Last-Modified value have been
573        provided by the origin server, &SHOULD; use both validators in
574        cache-conditional requests. This allows both HTTP/1.0 and
575        HTTP/1.1 caches to respond appropriately.</t>
576  </list>
579   An HTTP/1.1 origin server, upon receiving a conditional request that
580   includes both a Last-Modified date (e.g., in an If-Modified-Since or
581   If-Unmodified-Since header field) and one or more entity tags (e.g.,
582   in an If-Match, If-None-Match, or If-Range header field) as cache
583   validators, &MUST-NOT; return a response status of 304 (Not Modified)
584   unless doing so is consistent with all of the conditional header
585   fields in the request.
588   An HTTP/1.1 caching proxy, upon receiving a conditional request that
589   includes both a Last-Modified date and one or more entity tags as
590   cache validators, &MUST-NOT; return a locally cached response to the
591   client unless that cached response is consistent with all of the
592   conditional header fields in the request.
593  <list><t>
594      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
595      servers and clients should transmit as much non-redundant
596      information as is available in their responses and requests.
597      HTTP/1.1 systems receiving this information will make the most
598      conservative assumptions about the validators they receive.
599  </t><t>
600      HTTP/1.0 clients and caches will ignore entity tags. Generally,
601      last-modified values received or used by these systems will
602      support transparent and efficient caching, and so HTTP/1.1 origin
603      servers should provide Last-Modified values. In those rare cases
604      where the use of a Last-Modified value as a validator by an
605      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
606      origin servers should not provide one.
607  </t></list>
611<section title="Header Field Definitions" anchor="header.fields">
613   This section defines the syntax and semantics of HTTP/1.1 header fields
614   related to conditional requests.
617   For entity-header fields, both sender and recipient refer to either the
618   client or the server, depending on who sends and who receives the entity.
621<section title="ETag" anchor="header.etag">
622  <iref primary="true" item="ETag header" x:for-anchor=""/>
623  <iref primary="true" item="Headers" subitem="ETag" x:for-anchor=""/>
624  <x:anchor-alias value="ETag"/>
626   The ETag response-header field provides the current value of the
627   entity tag for the requested variant. The headers used with entity
628   tags are described in Sections <xref target="header.if-match" format="counter"/>
629   and <xref target="header.if-none-match" format="counter"/> of this document,
630   and in &header-if-range;. The entity tag
631   &MAY; be used for comparison with other entities from the same resource
632   (see <xref target="weak.and.strong.validators"/>).
634<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
635  <x:ref>ETag</x:ref> = "ETag" ":" <x:ref>entity-tag</x:ref>
638   Examples:
640<artwork type="example">
641   ETag: "xyzzy"
642   ETag: W/"xyzzy"
643   ETag: ""
646   The ETag response-header field value, an entity tag, provides for an
647   "opaque" cache validator. This might allow more reliable validation
648   in situations where it is inconvenient to store modification dates,
649   where the one-second resolution of HTTP date values is not
650   sufficient, or where the origin server wishes to avoid certain
651   paradoxes that might arise from the use of modification dates.
654   The principle behind entity tags is that only the service author
655   knows the semantics of a resource well enough to select an
656   appropriate cache validation mechanism, and the specification of any
657   validator comparison function more complex than byte-equality would
658   open up a can of worms. Thus, comparisons of any other headers
659   (except Last-Modified, for compatibility with HTTP/1.0) are never
660   used for purposes of validating a cache entry.
664<section title="If-Match" anchor="header.if-match">
665  <iref primary="true" item="If-Match header" x:for-anchor=""/>
666  <iref primary="true" item="Headers" subitem="If-Match" x:for-anchor=""/>
667  <x:anchor-alias value="If-Match"/>
669   The If-Match request-header field is used with a method to make it
670   conditional. A client that has one or more entities previously
671   obtained from the resource can verify that one of those entities is
672   current by including a list of their associated entity tags in the
673   If-Match header field. Entity tags are defined in <xref target="entity.tags"/>. The
674   purpose of this feature is to allow efficient updates of cached
675   information with a minimum amount of transaction overhead. It is also
676   used, on updating requests, to prevent inadvertent modification of
677   the wrong version of a resource. As a special case, the value "*"
678   matches any current entity of the resource.
680<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
681  <x:ref>If-Match</x:ref> = "If-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
684   If any of the entity tags match the entity tag of the entity that
685   would have been returned in the response to a similar GET request
686   (without the If-Match header) on that resource, or if "*" is given
687   and any current entity exists for that resource, then the server &MAY;
688   perform the requested method as if the If-Match header field did not
689   exist.
692   A server &MUST; use the strong comparison function (see <xref target="weak.and.strong.validators"/>)
693   to compare the entity tags in If-Match.
696   If none of the entity tags match, or if "*" is given and no current
697   entity exists, the server &MUST-NOT; perform the requested method, and
698   &MUST; return a 412 (Precondition Failed) response. This behavior is
699   most useful when the client wants to prevent an updating method, such
700   as PUT, from modifying a resource that has changed since the client
701   last retrieved it.
704   If the request would, without the If-Match header field, result in
705   anything other than a 2xx or 412 status, then the If-Match header
706   &MUST; be ignored.
709   The meaning of "If-Match: *" is that the method &SHOULD; be performed
710   if the representation selected by the origin server (or by a cache,
711   possibly using the Vary mechanism, see &header-vary;) exists, and
712   &MUST-NOT; be performed if the representation does not exist.
715   A request intended to update a resource (e.g., a PUT) &MAY; include an
716   If-Match header field to signal that the request method &MUST-NOT; be
717   applied if the entity corresponding to the If-Match value (a single
718   entity tag) is no longer a representation of that resource. This
719   allows the user to indicate that they do not wish the request to be
720   successful if the resource has been changed without their knowledge.
721   Examples:
723<figure><artwork type="example">
724    If-Match: "xyzzy"
725    If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
726    If-Match: *
729   The result of a request having both an If-Match header field and
730   either an If-None-Match or an If-Modified-Since header fields is
731   undefined by this specification.
735<section title="If-Modified-Since" anchor="header.if-modified-since">
736  <iref primary="true" item="If-Modified-Since header" x:for-anchor=""/>
737  <iref primary="true" item="Headers" subitem="If-Modified-Since" x:for-anchor=""/>
738  <x:anchor-alias value="If-Modified-Since"/>
740   The If-Modified-Since request-header field is used with a method to
741   make it conditional: if the requested variant has not been modified
742   since the time specified in this field, an entity will not be
743   returned from the server; instead, a 304 (Not Modified) response will
744   be returned without any message-body.
746<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
747  <x:ref>If-Modified-Since</x:ref> = "If-Modified-Since" ":" <x:ref>HTTP-date</x:ref>
750   An example of the field is:
752<figure><artwork type="example">
753    If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
756   A GET method with an If-Modified-Since header and no Range header
757   requests that the identified entity be transferred only if it has
758   been modified since the date given by the If-Modified-Since header.
759   The algorithm for determining this includes the following cases:
760  <list style="numbers">
761      <t>If the request would normally result in anything other than a
762         200 (OK) status, or if the passed If-Modified-Since date is
763         invalid, the response is exactly the same as for a normal GET.
764         A date which is later than the server's current time is
765         invalid.</t>
767      <t>If the variant has been modified since the If-Modified-Since
768         date, the response is exactly the same as for a normal GET.</t>
770      <t>If the variant has not been modified since a valid If-Modified-Since
771         date, the server &SHOULD; return a 304 (Not
772         Modified) response.</t>
773  </list>
776   The purpose of this feature is to allow efficient updates of cached
777   information with a minimum amount of transaction overhead.
778  <list><t>
779      <x:h>Note:</x:h> The Range request-header field modifies the meaning of If-Modified-Since;
780      see &header-range; for full details.
781    </t><t>
782      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
783      clock might not be synchronized with the client.
784    </t><t>
785      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
786      servers will use an exact date comparison function, rather than a
787      less-than function, for deciding whether to send a 304 (Not
788      Modified) response. To get best results when sending an If-Modified-Since
789      header field for cache validation, clients are
790      advised to use the exact date string received in a previous Last-Modified
791      header field whenever possible.
792    </t><t>
793      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
794      header instead of a date taken from the Last-Modified header for
795      the same request, the client should be aware of the fact that this
796      date is interpreted in the server's understanding of time. The
797      client should consider unsynchronized clocks and rounding problems
798      due to the different encodings of time between the client and
799      server. This includes the possibility of race conditions if the
800      document has changed between the time it was first requested and
801      the If-Modified-Since date of a subsequent request, and the
802      possibility of clock-skew-related problems if the If-Modified-Since
803      date is derived from the client's clock without correction
804      to the server's clock. Corrections for different time bases
805      between client and server are at best approximate due to network
806      latency.
807    </t>
808  </list>
811   The result of a request having both an If-Modified-Since header field
812   and either an If-Match or an If-Unmodified-Since header fields is
813   undefined by this specification.
817<section title="If-None-Match" anchor="header.if-none-match">
818  <iref primary="true" item="If-None-Match header" x:for-anchor=""/>
819  <iref primary="true" item="Headers" subitem="If-None-Match" x:for-anchor=""/>
820  <x:anchor-alias value="If-None-Match"/>
822   The If-None-Match request-header field is used with a method to make
823   it conditional. A client that has one or more entities previously
824   obtained from the resource can verify that none of those entities is
825   current by including a list of their associated entity tags in the
826   If-None-Match header field. The purpose of this feature is to allow
827   efficient updates of cached information with a minimum amount of
828   transaction overhead. It is also used to prevent a method (e.g. PUT)
829   from inadvertently modifying an existing resource when the client
830   believes that the resource does not exist.
833   As a special case, the value "*" matches any current entity of the
834   resource.
836<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
837  <x:ref>If-None-Match</x:ref> = "If-None-Match" ":" ( "*" | 1#<x:ref>entity-tag</x:ref> )
840   If any of the entity tags match the entity tag of the entity that
841   would have been returned in the response to a similar GET request
842   (without the If-None-Match header) on that resource, or if "*" is
843   given and any current entity exists for that resource, then the
844   server &MUST-NOT; perform the requested method, unless required to do
845   so because the resource's modification date fails to match that
846   supplied in an If-Modified-Since header field in the request.
847   Instead, if the request method was GET or HEAD, the server &SHOULD;
848   respond with a 304 (Not Modified) response, including the cache-related
849   header fields (particularly ETag) of one of the entities that
850   matched. For all other request methods, the server &MUST; respond with
851   a status of 412 (Precondition Failed).
854   See <xref target="weak.and.strong.validators"/> for rules on how to determine if two entities tags
855   match. The weak comparison function can only be used with GET or HEAD
856   requests.
859   If none of the entity tags match, then the server &MAY; perform the
860   requested method as if the If-None-Match header field did not exist,
861   but &MUST; also ignore any If-Modified-Since header field(s) in the
862   request. That is, if no entity tags match, then the server &MUST-NOT;
863   return a 304 (Not Modified) response.
866   If the request would, without the If-None-Match header field, result
867   in anything other than a 2xx or 304 status, then the If-None-Match
868   header &MUST; be ignored. (See <xref target=""/> for a discussion of
869   server behavior when both If-Modified-Since and If-None-Match appear
870   in the same request.)
873   The meaning of "If-None-Match: *" is that the method &MUST-NOT; be
874   performed if the representation selected by the origin server (or by
875   a cache, possibly using the Vary mechanism, see &header-vary;)
876   exists, and &SHOULD; be performed if the representation does not exist.
877   This feature is intended to be useful in preventing races between PUT
878   operations.
881   Examples:
883<figure><artwork type="example">
884    If-None-Match: "xyzzy"
885    If-None-Match: W/"xyzzy"
886    If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
887    If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
888    If-None-Match: *
891   The result of a request having both an If-None-Match header field and
892   either an If-Match or an If-Unmodified-Since header fields is
893   undefined by this specification.
897<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
898  <iref primary="true" item="If-Unmodified-Since header" x:for-anchor=""/>
899  <iref primary="true" item="Headers" subitem="If-Unmodified-Since" x:for-anchor=""/>
900  <x:anchor-alias value="If-Unmodified-Since"/>
902   The If-Unmodified-Since request-header field is used with a method to
903   make it conditional. If the requested resource has not been modified
904   since the time specified in this field, the server &SHOULD; perform the
905   requested operation as if the If-Unmodified-Since header were not
906   present.
909   If the requested variant has been modified since the specified time,
910   the server &MUST-NOT; perform the requested operation, and &MUST; return
911   a 412 (Precondition Failed).
913<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
914  <x:ref>If-Unmodified-Since</x:ref> = "If-Unmodified-Since" ":" <x:ref>HTTP-date</x:ref>
917   An example of the field is:
919<figure><artwork type="example">
920    If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
923   If the request normally (i.e., without the If-Unmodified-Since
924   header) would result in anything other than a 2xx or 412 status, the
925   If-Unmodified-Since header &SHOULD; be ignored.
928   If the specified date is invalid, the header is ignored.
931   The result of a request having both an If-Unmodified-Since header
932   field and either an If-None-Match or an If-Modified-Since header
933   fields is undefined by this specification.
937<section title="Last-Modified" anchor="header.last-modified">
938  <iref primary="true" item="Last-Modified header" x:for-anchor=""/>
939  <iref primary="true" item="Headers" subitem="Last-Modified" x:for-anchor=""/>
940  <x:anchor-alias value="Last-Modified"/>
942   The Last-Modified entity-header field indicates the date and time at
943   which the origin server believes the variant was last modified.
945<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
946  <x:ref>Last-Modified</x:ref>  = "Last-Modified" ":" <x:ref>HTTP-date</x:ref>
949   An example of its use is
951<figure><artwork type="example">
952    Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
955   The exact meaning of this header field depends on the implementation
956   of the origin server and the nature of the original resource. For
957   files, it may be just the file system last-modified time. For
958   entities with dynamically included parts, it may be the most recent
959   of the set of last-modify times for its component parts. For database
960   gateways, it may be the last-update time stamp of the record. For
961   virtual objects, it may be the last time the internal state changed.
964   An origin server &MUST-NOT; send a Last-Modified date which is later
965   than the server's time of message origination. In such cases, where
966   the resource's last modification would indicate some time in the
967   future, the server &MUST; replace that date with the message
968   origination date.
971   An origin server &SHOULD; obtain the Last-Modified value of the entity
972   as close as possible to the time that it generates the Date value of
973   its response. This allows a recipient to make an accurate assessment
974   of the entity's modification time, especially if the entity changes
975   near the time that the response is generated.
978   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
981   The Last-Modified entity-header field value is often used as a cache
982   validator. In simple terms, a cache entry is considered to be valid
983   if the entity has not been modified since the Last-Modified value.
989<section title="IANA Considerations" anchor="IANA.considerations">
991   <cref>TBD.</cref>
995<section title="Security Considerations" anchor="security.considerations">
997   No additional security considerations have been identified beyond
998   those applicable to HTTP in general &messaging;.
1002<section title="Acknowledgments" anchor="ack">
1007<references title="Normative References">
1009<reference anchor="Part1">
1010  <front>
1011    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1012    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1013      <organization abbrev="Day Software">Day Software</organization>
1014      <address><email></email></address>
1015    </author>
1016    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1017      <organization>One Laptop per Child</organization>
1018      <address><email></email></address>
1019    </author>
1020    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1021      <organization abbrev="HP">Hewlett-Packard Company</organization>
1022      <address><email></email></address>
1023    </author>
1024    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1025      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1026      <address><email></email></address>
1027    </author>
1028    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1029      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1030      <address><email></email></address>
1031    </author>
1032    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1033      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1034      <address><email></email></address>
1035    </author>
1036    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1037      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1038      <address><email></email></address>
1039    </author>
1040    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1041      <organization abbrev="W3C">World Wide Web Consortium</organization>
1042      <address><email></email></address>
1043    </author>
1044    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1045      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1046      <address><email></email></address>
1047    </author>
1048    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1049  </front>
1050  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1051  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1054<reference anchor="Part5">
1055  <front>
1056    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1057    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1058      <organization abbrev="Day Software">Day Software</organization>
1059      <address><email></email></address>
1060    </author>
1061    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1062      <organization>One Laptop per Child</organization>
1063      <address><email></email></address>
1064    </author>
1065    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1066      <organization abbrev="HP">Hewlett-Packard Company</organization>
1067      <address><email></email></address>
1068    </author>
1069    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1070      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1071      <address><email></email></address>
1072    </author>
1073    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1074      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1075      <address><email></email></address>
1076    </author>
1077    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1078      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1079      <address><email></email></address>
1080    </author>
1081    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1082      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1083      <address><email></email></address>
1084    </author>
1085    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1086      <organization abbrev="W3C">World Wide Web Consortium</organization>
1087      <address><email></email></address>
1088    </author>
1089    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1090      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1091      <address><email></email></address>
1092    </author>
1093    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1094  </front>
1095  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1096  <x:source href="p5-range.xml" basename="p5-range"/>
1099<reference anchor="Part6">
1100  <front>
1101    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1102    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1103      <organization abbrev="Day Software">Day Software</organization>
1104      <address><email></email></address>
1105    </author>
1106    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1107      <organization>One Laptop per Child</organization>
1108      <address><email></email></address>
1109    </author>
1110    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1111      <organization abbrev="HP">Hewlett-Packard Company</organization>
1112      <address><email></email></address>
1113    </author>
1114    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1115      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1116      <address><email></email></address>
1117    </author>
1118    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1119      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
1120      <address><email></email></address>
1121    </author>
1122    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1123      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1124      <address><email></email></address>
1125    </author>
1126    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1127      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1128      <address><email></email></address>
1129    </author>
1130    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1131      <organization abbrev="W3C">World Wide Web Consortium</organization>
1132      <address><email></email></address>
1133    </author>
1134    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1135      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1136      <address><email></email></address>
1137    </author>
1138    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1139  </front>
1140  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1141  <x:source href="p6-cache.xml" basename="p6-cache"/>
1144<reference anchor="RFC2119">
1145  <front>
1146    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1147    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1148      <organization>Harvard University</organization>
1149      <address><email></email></address>
1150    </author>
1151    <date month="March" year="1997"/>
1152  </front>
1153  <seriesInfo name="BCP" value="14"/>
1154  <seriesInfo name="RFC" value="2119"/>
1159<references title="Informative References">
1161<reference anchor="RFC2068">
1162  <front>
1163    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1164    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
1165      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
1166      <address><email></email></address>
1167    </author>
1168    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1169      <organization>MIT Laboratory for Computer Science</organization>
1170      <address><email></email></address>
1171    </author>
1172    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1173      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
1174      <address><email></email></address>
1175    </author>
1176    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1177      <organization>MIT Laboratory for Computer Science</organization>
1178      <address><email></email></address>
1179    </author>
1180    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1181      <organization>MIT Laboratory for Computer Science</organization>
1182      <address><email></email></address>
1183    </author>
1184    <date month="January" year="1997"/>
1185  </front>
1186  <seriesInfo name="RFC" value="2068"/>
1189<reference anchor="RFC2616">
1190  <front>
1191    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1192    <author initials="R." surname="Fielding" fullname="R. Fielding">
1193      <organization>University of California, Irvine</organization>
1194      <address><email></email></address>
1195    </author>
1196    <author initials="J." surname="Gettys" fullname="J. Gettys">
1197      <organization>W3C</organization>
1198      <address><email></email></address>
1199    </author>
1200    <author initials="J." surname="Mogul" fullname="J. Mogul">
1201      <organization>Compaq Computer Corporation</organization>
1202      <address><email></email></address>
1203    </author>
1204    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1205      <organization>MIT Laboratory for Computer Science</organization>
1206      <address><email></email></address>
1207    </author>
1208    <author initials="L." surname="Masinter" fullname="L. Masinter">
1209      <organization>Xerox Corporation</organization>
1210      <address><email></email></address>
1211    </author>
1212    <author initials="P." surname="Leach" fullname="P. Leach">
1213      <organization>Microsoft Corporation</organization>
1214      <address><email></email></address>
1215    </author>
1216    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1217      <organization>W3C</organization>
1218      <address><email></email></address>
1219    </author>
1220    <date month="June" year="1999"/>
1221  </front>
1222  <seriesInfo name="RFC" value="2616"/>
1227<section title="Compatibility with Previous Versions" anchor="compatibility">
1229<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1234<section title="Change Log (to be removed by RFC Editor before publication)">
1236<section title="Since RFC2616">
1238  Extracted relevant partitions from <xref target="RFC2616"/>.
1242<section title="Since draft-ietf-httpbis-p4-conditional-00">
1244  Closed issues:
1245  <list style="symbols">
1246    <t>
1247      <eref target=""/>:
1248      "Normative and Informative references"
1249    </t>
1250  </list>
1253  Other changes:
1254  <list style="symbols">
1255    <t>
1256      Move definitions of 304 and 412 condition codes from Part2.
1257    </t>
1258  </list>
1262<section title="Since draft-ietf-httpbis-p4-conditional-01">
1264  Ongoing work on ABNF conversion (<eref target=""/>):
1265  <list style="symbols">
1266    <t>
1267      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1268    </t>
1269  </list>
1273<section title="Since draft-ietf-httpbis-p4-conditional-02">
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