source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1254

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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='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
5  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
6  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
7  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
8  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
9  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
10  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
11  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
12  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
13  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
14  <!ENTITY ID-VERSION "latest">
15  <!ENTITY ID-MONTH "April">
16  <!ENTITY ID-YEAR "2011">
17  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
18  <!ENTITY notation-abnf              "<xref target='Part1' x:rel='#notation.abnf' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY basic-rules                "<xref target='Part1' x:rel='#basic.rules' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY header-date                "<xref target='Part1' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY header-accept-encoding     "<xref target='Part3' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-vary                "<xref target='Part6' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY clockless                  "<xref target='Part1' x:rel='#clockless.origin.server.operation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY full-date                  "<xref target='Part1' x:rel='#date.time.formats.full.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY content-negotiation        "<xref target='Part3' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31]>
32<?rfc toc="yes" ?>
33<?rfc symrefs="yes" ?>
34<?rfc sortrefs="yes" ?>
35<?rfc compact="yes"?>
36<?rfc subcompact="no" ?>
37<?rfc linkmailto="no" ?>
38<?rfc editing="no" ?>
39<?rfc comments="yes"?>
40<?rfc inline="yes"?>
41<?rfc rfcedstyle="yes"?>
42<?rfc-ext allow-markup-in-artwork="yes" ?>
43<?rfc-ext include-references-in-index="yes" ?>
44<rfc obsoletes="2616" category="std" x:maturity-level="draft"
45     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
46     xmlns:x='http://purl.org/net/xml2rfc/ext'>
47<front>
48
49  <title abbrev="HTTP/1.1, Part 4">HTTP/1.1, part 4: Conditional Requests</title>
50
51  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
52    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
53    <address>
54      <postal>
55        <street>345 Park Ave</street>
56        <city>San Jose</city>
57        <region>CA</region>
58        <code>95110</code>
59        <country>USA</country>
60      </postal>
61      <email>fielding@gbiv.com</email>
62      <uri>http://roy.gbiv.com/</uri>
63    </address>
64  </author>
65
66  <author initials="J." surname="Gettys" fullname="Jim Gettys">
67    <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
68    <address>
69      <postal>
70        <street>21 Oak Knoll Road</street>
71        <city>Carlisle</city>
72        <region>MA</region>
73        <code>01741</code>
74        <country>USA</country>
75      </postal>
76      <email>jg@freedesktop.org</email>
77      <uri>http://gettys.wordpress.com/</uri>
78    </address>
79  </author>
80 
81  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
82    <organization abbrev="HP">Hewlett-Packard Company</organization>
83    <address>
84      <postal>
85        <street>HP Labs, Large Scale Systems Group</street>
86        <street>1501 Page Mill Road, MS 1177</street>
87        <city>Palo Alto</city>
88        <region>CA</region>
89        <code>94304</code>
90        <country>USA</country>
91      </postal>
92      <email>JeffMogul@acm.org</email>
93    </address>
94  </author>
95
96  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
97    <organization abbrev="Microsoft">Microsoft Corporation</organization>
98    <address>
99      <postal>
100        <street>1 Microsoft Way</street>
101        <city>Redmond</city>
102        <region>WA</region>
103        <code>98052</code>
104        <country>USA</country>
105      </postal>
106      <email>henrikn@microsoft.com</email>
107    </address>
108  </author>
109
110  <author initials="L." surname="Masinter" fullname="Larry Masinter">
111    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
112    <address>
113      <postal>
114        <street>345 Park Ave</street>
115        <city>San Jose</city>
116        <region>CA</region>
117        <code>95110</code>
118        <country>USA</country>
119      </postal>
120      <email>LMM@acm.org</email>
121      <uri>http://larry.masinter.net/</uri>
122    </address>
123  </author>
124 
125  <author initials="P." surname="Leach" fullname="Paul J. Leach">
126    <organization abbrev="Microsoft">Microsoft Corporation</organization>
127    <address>
128      <postal>
129        <street>1 Microsoft Way</street>
130        <city>Redmond</city>
131        <region>WA</region>
132        <code>98052</code>
133      </postal>
134      <email>paulle@microsoft.com</email>
135    </address>
136  </author>
137   
138  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
139    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
140    <address>
141      <postal>
142        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
143        <street>The Stata Center, Building 32</street>
144        <street>32 Vassar Street</street>
145        <city>Cambridge</city>
146        <region>MA</region>
147        <code>02139</code>
148        <country>USA</country>
149      </postal>
150      <email>timbl@w3.org</email>
151      <uri>http://www.w3.org/People/Berners-Lee/</uri>
152    </address>
153  </author>
154
155  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
156    <organization abbrev="W3C">World Wide Web Consortium</organization>
157    <address>
158      <postal>
159        <street>W3C / ERCIM</street>
160        <street>2004, rte des Lucioles</street>
161        <city>Sophia-Antipolis</city>
162        <region>AM</region>
163        <code>06902</code>
164        <country>France</country>
165      </postal>
166      <email>ylafon@w3.org</email>
167      <uri>http://www.raubacapeu.net/people/yves/</uri>
168    </address>
169  </author>
170
171  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
172    <organization abbrev="greenbytes">greenbytes GmbH</organization>
173    <address>
174      <postal>
175        <street>Hafenweg 16</street>
176        <city>Muenster</city><region>NW</region><code>48155</code>
177        <country>Germany</country>
178      </postal>
179      <phone>+49 251 2807760</phone>
180      <facsimile>+49 251 2807761</facsimile>
181      <email>julian.reschke@greenbytes.de</email>
182      <uri>http://greenbytes.de/tech/webdav/</uri>
183    </address>
184  </author>
185
186  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
187  <workgroup>HTTPbis Working Group</workgroup>
188
189<abstract>
190<t>
191   The Hypertext Transfer Protocol (HTTP) is an application-level
192   protocol for distributed, collaborative, hypermedia information
193   systems. HTTP has been in use by the World Wide Web global information
194   initiative since 1990. This document is Part 4 of the seven-part specification
195   that defines the protocol referred to as "HTTP/1.1" and, taken together,
196   obsoletes RFC 2616.  Part 4 defines request header fields for
197   indicating conditional requests and the rules for constructing responses
198   to those requests.
199</t>
200</abstract>
201
202<note title="Editorial Note (To be removed by RFC Editor)">
203  <t>
204    Discussion of this draft should take place on the HTTPBIS working group
205    mailing list (ietf-http-wg@w3.org). The current issues list is
206    at <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/>
207    and related documents (including fancy diffs) can be found at
208    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
209  </t>
210  <t>
211    The changes in this draft are summarized in <xref target="changes.since.13"/>.
212  </t>
213</note>
214</front>
215<middle>
216<section title="Introduction" anchor="introduction">
217<t>
218   This document defines the HTTP/1.1 conditional request mechanisms,
219   including both response metadata that can be used to indicate or
220   observe changes to resource state and request header fields that
221   specify preconditions to be checked before performing the action
222   given by the request method.  Conditional GET requests are the most
223   efficient mechanism for HTTP cache updates &caching;.  Conditionals can be
224   applied to state-changing methods, such as PUT and DELETE, to prevent
225   the "lost update" problem: one client accidentally overwriting
226   the work of another client that has been acting in parallel.
227</t>
228<t>
229   Conditional request preconditions are based on the state of the target
230   resource as a whole (its current value set) or the state as observed
231   in a previously obtained representation (one value in that set).
232   A resource might have multiple current representations, each with its
233   own observable state.  The conditional request mechanisms assume that
234   the mapping of requests to corresponding representations will be
235   consistent over time if the server intends to take advantage of
236   conditionals.  Regardless, if the mapping is inconsistent and
237   the server is unable to select the appropriate representation, then
238   no harm will result when the precondition evaluates to false.
239</t>
240<t><iref primary="true" item="selected representation"/>
241   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
242   the current representation of the target resource that would have been
243   selected in a successful response if the same request had used the method
244   GET and had excluded all of the conditional request header fields.
245   The conditional request preconditions are evaluated by comparing the
246   values provided in the request header fields to the current metadata
247   for the selected representation.
248</t>
249
250<section title="Requirements" anchor="intro.requirements">
251<t>
252   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
253   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
254   document are to be interpreted as described in <xref target="RFC2119"/>.
255</t>
256<t>
257   An implementation is not compliant if it fails to satisfy one or more
258   of the "MUST" or "REQUIRED" level requirements for the protocols it
259   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
260   level and all the "SHOULD" level requirements for its protocols is said
261   to be "unconditionally compliant"; one that satisfies all the "MUST"
262   level requirements but not all the "SHOULD" level requirements for its
263   protocols is said to be "conditionally compliant".
264</t>
265</section>
266
267<section title="Syntax Notation" anchor="notation">
268  <x:anchor-alias value="ALPHA"/>
269  <x:anchor-alias value="CR"/>
270  <x:anchor-alias value="DIGIT"/>
271  <x:anchor-alias value="LF"/>
272  <x:anchor-alias value="OCTET"/>
273  <x:anchor-alias value="VCHAR"/>
274  <x:anchor-alias value="WSP"/>
275  <x:anchor-alias value="core.rules"/>
276  <x:anchor-alias value="quoted-string"/>
277  <x:anchor-alias value="OWS"/>
278  <x:anchor-alias value="HTTP-date"/>
279<t>
280  This specification uses the ABNF syntax defined in &notation; (which
281  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
282  <xref target="collected.abnf"/> shows the collected ABNF, with the list
283  rule expanded.
284</t>
285<t>
286  The following core rules are included by
287  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
288  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
289  DIGIT (decimal 0-9), DQUOTE (double quote),
290  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
291  OCTET (any 8-bit sequence of data), SP (space),
292  VCHAR (any visible USASCII character),
293  and WSP (whitespace).
294</t>
295<t>
296  The ABNF rules below are defined in other parts:
297</t>
298<figure><artwork type="abnf2616">
299  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
300  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
301  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
302</artwork></figure>
303</section>
304</section>
305
306<section title="Resource State Metadata (Validators)" anchor="resource.metadata">
307   <iref primary="true" item="metadata"/>
308   <iref primary="true" item="validator"/>
309<t>
310   This specification defines two forms of metadata that are commonly used
311   to observe resource state and test for preconditions: modification dates
312   and opaque entity tags.  Additional metadata that reflects resource state
313   has been defined by various extensions of HTTP, such as WebDAV
314   <xref target="RFC4918"/>, that are beyond the scope of this specification.
315   Such metadata is referred to as a "<x:dfn>validator</x:dfn>" when it is
316   used within a precondition.
317</t>
318
319<section title="Last-Modified" anchor="header.last-modified">
320  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
321  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
322  <x:anchor-alias value="Last-Modified"/>
323<t>
324   The "Last-Modified" header field indicates the date and time at
325   which the origin server believes the representation was last modified.
326</t>
327<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
328  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
329</artwork></figure>
330<t>
331   An example of its use is
332</t>
333<figure><artwork type="example">
334  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
335</artwork></figure>
336<t>
337   A representation is typically the sum of many parts behind the
338   resource interface.  The last-modified time would usually be
339   the most recent time that any of those parts were changed.
340   How that value is determined for any given resource is an
341   implementation detail beyond the scope of this specification.
342   What matters to HTTP is how recipients of the Last-Modified
343   header field can use its value to make conditional requests
344   and test the validity of locally cached responses.
345</t>
346<t>
347   An origin server &MUST-NOT; send a Last-Modified date which is later
348   than the server's time of message origination. In such cases, where
349   the resource's last modification would indicate some time in the
350   future, the server &MUST; replace that date with the message
351   origination date.
352</t>
353<t>
354   An origin server &SHOULD; obtain the Last-Modified value of the representation
355   as close as possible to the time that it generates the Date value of
356   its response. This allows a recipient to make an accurate assessment
357   of the representation's modification time, especially if the representation changes
358   near the time that the response is generated.
359</t>
360<t>
361   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
362</t>
363<t>
364   The Last-Modified header field value is often used as a cache
365   validator. In simple terms, a cache entry is considered to be valid
366   if the representation has not been modified since the Last-Modified value.
367</t>
368</section>
369
370<section title="Entity Tags" anchor="entity.tags">
371  <x:anchor-alias value="entity-tag"/>
372  <x:anchor-alias value="opaque-tag"/>
373  <x:anchor-alias value="weak"/>
374<t>
375   Entity-tags are used for comparing two or more representations of the same
376   resource. HTTP/1.1 uses entity-tags in the ETag (<xref target="header.etag"/>),
377   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
378   If-Range (&header-if-range;) header fields. The definition of how they
379   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
380   entity-tag consists of an opaque quoted string, possibly prefixed by
381   a weakness indicator.
382</t>
383<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"/>
384  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
385  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
386  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
387</artwork></figure>
388<t>
389   A "strong entity-tag" &MAY; be shared by two representations of a resource
390   only if they are equivalent by octet equality.
391</t>
392<t>
393   A "weak entity-tag", indicated by the "W/" prefix, &MAY; be shared by
394   two representations of a resource. A weak entity-tag can only be used
395   for weak comparison.
396</t>
397<t>
398   Cache entries might persist for arbitrarily long periods, regardless
399   of expiration times, so it is inappropriate to expect that a cache will
400   never again attempt to validate an entry using a validator that it
401   obtained at some point in the past.
402   A strong entity-tag &MUST; be unique across all versions of all
403   representations associated with a particular resource over time.
404   However, there is no implication of uniqueness across entity-tags
405   of different resources (i.e., the same entity-tag value might be
406   in use for representations of multiple resources at the same time
407   and does not imply that those representations are equivalent).
408</t>
409
410<section title="ETag" anchor="header.etag">
411  <iref primary="true" item="ETag header field" x:for-anchor=""/>
412  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
413  <x:anchor-alias value="ETag"/>
414<t>
415   The "ETag" header field provides the current value of the
416   entity-tag (see <xref target="entity.tags"/>) for one representation of
417   the target resource.  An entity-tag
418   is intended for use as a resource-local identifier for differentiating
419   between representations of the same resource that vary over time or via
420   content negotiation (see <xref target="weak.and.strong.validators"/>).
421</t>
422<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
423  <x:ref>ETag</x:ref> = <x:ref>entity-tag</x:ref>
424</artwork></figure>
425<figure><preamble>
426  Examples:
427</preamble>
428<artwork type="example">
429  ETag: "xyzzy"
430  ETag: W/"xyzzy"
431  ETag: ""
432</artwork></figure>
433<t>
434   An entity-tag provides an "opaque" cache validator that allows for
435   more reliable validation than modification dates in situations where
436   it is inconvenient to store modification dates,
437   where the one-second resolution of HTTP date values is not
438   sufficient, or where the origin server wishes to avoid certain
439   paradoxes that might arise from the use of modification dates.
440</t>
441<t>
442   The principle behind entity-tags is that only the service author
443   knows the semantics of a resource well enough to select an
444   appropriate cache validation mechanism, and the specification of any
445   validator comparison function more complex than byte-equality would
446   open up a can of worms. Thus, comparisons of any other header fields
447   (except Last-Modified, for compatibility with HTTP/1.0) are never
448   used for purposes of validating a cache entry.
449</t>
450</section>
451
452<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
453<t>
454   Consider a resource that is subject to content negotiation (&content-negotiation;),
455   and where the representations returned upon a GET request vary based on
456   the Accept-Encoding request header field (&header-accept-encoding;):
457</t>
458<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
459GET /index HTTP/1.1
460Host: www.example.com
461Accept-Encoding: gzip
462
463</artwork></figure>
464<t>
465   In this case, the response might or might not use the gzip content coding.
466   If it does not, the response might look like:
467</t>
468<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
469HTTP/1.1 200 OK
470Date: Thu, 26 Mar 2010 00:05:00 GMT
471ETag: "123-a"
472Content-Length: <x:length-of target="exbody"/>
473Vary: Accept-Encoding
474Content-Type: text/plain
475
476<x:span anchor="exbody">Hello World!
477Hello World!
478Hello World!
479Hello World!
480Hello World!
481</x:span></artwork></figure>
482<t>
483   An alternative representation that does use gzip content coding would be:
484</t>
485<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
486HTTP/1.1 200 OK
487Date: Thu, 26 Mar 2010 00:05:00 GMT
488ETag: "123-b"
489Content-Length: 43
490Vary: Accept-Encoding
491Content-Type: text/plain
492Content-Encoding: gzip
493
494<spanx>...binary data...</spanx></artwork></figure>
495<x:note>
496  <t>
497    <x:h>Note:</x:h> Content codings are a property of the representation,
498    so therefore an entity-tag of an encoded representation must be distinct
499    from an unencoded representation to prevent conflicts during cache updates
500    and range requests.  In contrast, transfer codings (&transfer-codings;)
501    apply only during message transfer and do not require distinct entity-tags.
502  </t>
503</x:note>
504</section>
505</section>
506
507<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
508<t>
509   Since both origin servers and caches will compare two validators to
510   decide if they represent the same or different representations, one
511   normally would expect that if the representation (including both
512   representation header fields and representation body) changes in any
513   way, then the associated validator would change as well. If this is
514   true, then we call this validator a "strong validator".  One example
515   of a strong validator is an integer that is incremented in stable
516   storage every time a representation is changed.
517</t>
518<t>
519   However, there might be cases when a server prefers to change the
520   validator only when it desires cached representations to be invalidated.
521   For example, the representation of a weather report that changes in
522   content every second, based on dynamic measurements, might be grouped
523   into sets of equivalent representations (from the origin server's
524   perspective) in order to allow cached representations to be valid
525   for a reasonable period of time (perhaps adjusted dynamically based
526   on server load or weather quality).
527   A validator that does not always change when the representation
528   changes is a "weak validator".
529</t>
530<t>
531   An entity-tag is normally a strong validator, but the protocol
532   provides a mechanism to tag an entity-tag as "weak". One can think
533   of a strong validator as part of an identifier for a specific
534   representation, whereas a weak validator is part of an identifier
535   for a set of equivalent representations (where this notion of
536   equivalence is entirely governed by the origin server and beyond
537   the scope of this specification).
538  <list><t>
539      A representation's modification time, if defined with only one-second
540      resolution, could be a weak validator, since it is possible that
541      the representation might be modified twice during a single second.
542    </t><t>
543      Support for weak validators is optional. However, weak validators
544      allow for more efficient caching of equivalent objects; for
545      example, a hit counter on a site is probably good enough if it is
546      updated every few days or weeks, and any value during that period
547      is likely "good enough" to be equivalent.
548    </t></list>
549</t>
550<t>
551   A strong entity-tag &MUST; change whenever the associated representation
552   changes in any way. A weak entity-tag &SHOULD; change whenever the origin
553   server considers prior representations to be unacceptable as a substitute
554   for the current representation. In other words, a weak entity tag &SHOULD;
555   change whenever the origin server wants caches to invalidate old responses.
556</t>
557<t>
558   A "use" of a validator is either when a client generates a request
559   and includes the validator in a validating header field, or when a
560   server compares two validators.
561</t>
562<t>
563   Strong validators are usable in any context. Weak validators are only
564   usable in contexts that do not depend on exact equality of a representation.
565   For example, either kind is usable for a normal conditional GET.
566   However, only a strong validator is usable for range retrieval
567   (<xref target="Part5"/>), since otherwise the client might end up
568   with an internally inconsistent representation.
569   Clients &MUST-NOT; use weak validators in range requests.
570</t>
571<t>
572   The only function that HTTP/1.1 defines on validators is
573   comparison. There are two validator comparison functions, depending
574   on whether the comparison context allows the use of weak validators
575   or not:
576  <list style="symbols">
577     <t>The strong comparison function: in order to be considered equal,
578        both opaque-tags &MUST; be identical character-by-character, and both
579        &MUST-NOT; be weak.</t>
580     <t>The weak comparison function: in order to be considered equal, both
581        opaque-tags &MUST; be identical character-by-character, but
582        either or both of them &MAY; be tagged as "weak" without affecting
583        the result.</t>
584  </list>
585</t>
586<t>
587   The example below shows the results for a set of entity-tag pairs,
588   and both the weak and strong comparison function results:
589</t>
590<texttable align="left">
591  <ttcol>ETag 1</ttcol>
592  <ttcol>ETag 2</ttcol>
593  <ttcol>Strong Comparison</ttcol>
594  <ttcol>Weak Comparison</ttcol>
595
596  <c>W/"1"</c>
597  <c>W/"1"</c>
598  <c>no match</c>
599  <c>match</c>
600 
601  <c>W/"1"</c>
602  <c>W/"2"</c>
603  <c>no match</c>
604  <c>no match</c>
605
606  <c>W/"1"</c>
607  <c>"1"</c>
608  <c>no match</c>
609  <c>match</c>
610
611  <c>"1"</c>
612  <c>"1"</c>
613  <c>match</c>
614  <c>match</c>
615</texttable>
616<t>
617   An entity-tag is strong unless it is explicitly tagged as weak.
618   <xref target="entity.tags"/> gives the syntax for entity-tags.
619</t>
620<t>
621   A Last-Modified time, when used as a validator in a request, is
622   implicitly weak unless it is possible to deduce that it is strong,
623   using the following rules:
624  <list style="symbols">
625     <t>The validator is being compared by an origin server to the
626        actual current validator for the representation and,</t>
627     <t>That origin server reliably knows that the associated representation did
628        not change twice during the second covered by the presented
629        validator.</t>
630  </list>
631</t>
632<t>
633   or
634  <list style="symbols">
635     <t>The validator is about to be used by a client in an If-Modified-Since
636        or If-Unmodified-Since header field, because the client
637        has a cache entry for the associated representation, and</t>
638     <t>That cache entry includes a Date value, which gives the time
639        when the origin server sent the original response, and</t>
640     <t>The presented Last-Modified time is at least 60 seconds before
641        the Date value.</t>
642  </list>
643</t>
644<t>
645   or
646  <list style="symbols">
647     <t>The validator is being compared by an intermediate cache to the
648        validator stored in its cache entry for the representation, and</t>
649     <t>That cache entry includes a Date value, which gives the time
650        when the origin server sent the original response, and</t>
651     <t>The presented Last-Modified time is at least 60 seconds before
652        the Date value.</t>
653  </list>
654</t>
655<t>
656   This method relies on the fact that if two different responses were
657   sent by the origin server during the same second, but both had the
658   same Last-Modified time, then at least one of those responses would
659   have a Date value equal to its Last-Modified time. The arbitrary 60-second
660   limit guards against the possibility that the Date and Last-Modified
661   values are generated from different clocks, or at somewhat
662   different times during the preparation of the response. An
663   implementation &MAY; use a value larger than 60 seconds, if it is
664   believed that 60 seconds is too short.
665</t>
666<t>
667   If a client wishes to perform a sub-range retrieval on a value for
668   which it has only a Last-Modified time and no opaque validator, it
669   &MAY; do this only if the Last-Modified time is strong in the sense
670   described here.
671</t>
672<t>
673   A cache or origin server receiving a conditional range request
674   (<xref target="Part5"/>) &MUST; use the strong comparison function to
675   evaluate the condition.
676</t>
677<t>
678   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
679   retrievals on values that have been obtained from HTTP/1.0
680   servers.
681</t>
682</section>
683
684<section title="Rules for When to Use Entity-tags and Last-Modified Dates" anchor="rules.for.when.to.use.entity.tags.and.last-modified.dates">
685<t>
686   We adopt a set of rules and recommendations for origin servers,
687   clients, and caches regarding when various validator types ought to
688   be used, and for what purposes.
689</t>
690<t>
691   HTTP/1.1 origin servers:
692  <list style="symbols">
693     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
694        generate one.</t>
695
696     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
697        performance considerations support the use of weak entity-tags,
698        or if it is unfeasible to send a strong entity-tag.</t>
699
700     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
701  </list>
702</t>
703<t>
704   In other words, the preferred behavior for an HTTP/1.1 origin server
705   is to send both a strong entity-tag and a Last-Modified value.
706</t>
707<t>
708   HTTP/1.1 clients:
709  <list style="symbols">
710     <t>&MUST; use that entity-tag in any cache-conditional request (using
711        If-Match or If-None-Match) if an entity-tag has been provided by the
712        origin server.</t>
713
714     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
715        requests (using If-Modified-Since) if only a Last-Modified value has
716        been provided by the origin server. </t>
717
718     <t>&MAY; use the Last-Modified value in subrange cache-conditional
719        requests (using If-Unmodified-Since) if only a Last-Modified value has
720        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
721        provide a way to disable this, in case of difficulty.</t>
722
723     <t>&SHOULD; use both validators in cache-conditional requests if both an
724        entity-tag and a Last-Modified value have been provided by the origin
725        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
726        appropriately.</t>
727  </list>
728</t>
729<t>
730   An HTTP/1.1 origin server, upon receiving a conditional request that
731   includes both a Last-Modified date (e.g., in an If-Modified-Since or
732   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
733   in an If-Match, If-None-Match, or If-Range header field) as cache
734   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
735   unless doing so is consistent with all of the conditional header
736   fields in the request.
737</t>
738<t>
739   An HTTP/1.1 caching proxy, upon receiving a conditional request that
740   includes both a Last-Modified date and one or more entity-tags as
741   cache validators, &MUST-NOT; return a locally cached response to the
742   client unless that cached response is consistent with all of the
743   conditional header fields in the request.
744  <list><t>
745      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
746      servers and clients ought to transmit as much non-redundant
747      information as is available in their responses and requests.
748      HTTP/1.1 systems receiving this information will make the most
749      conservative assumptions about the validators they receive.
750  </t><t>
751      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
752      last-modified values received or used by these systems will
753      support transparent and efficient caching, and so HTTP/1.1 origin
754      servers should provide Last-Modified values. In those rare cases
755      where the use of a Last-Modified value as a validator by an
756      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
757      origin servers should not provide one.
758  </t></list>
759</t>
760</section>
761
762</section>
763
764<section title="Precondition Header Fields" anchor="header.fields">
765<t>
766   This section defines the syntax and semantics of HTTP/1.1 header fields
767   for applying preconditions on requests.
768</t>
769
770<section title="If-Match" anchor="header.if-match">
771  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
772  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
773  <x:anchor-alias value="If-Match"/>
774<t>
775   The "If-Match" header field &MAY; be used to make a request method
776   conditional on the current existence or value of an entity-tag for
777   one or more representations of the target resource.  If-Match is
778   generally useful for resource update requests, such as PUT requests,
779   as a means for protecting against accidental overwrites when multiple
780   clients are acting in parallel on the same resource (i.e., the
781   "lost update" problem).  An If-Match field-value of "*" places the
782   precondition on the existence of any current representation for the
783   target resource.
784</t>
785<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
786  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
787</artwork></figure>
788<t>
789   If any of the entity-tags listed in the If-Match field value match
790   the entity-tag of the selected representation for the target resource,
791   or if "*" is given and any current representation exists for the
792   target resource, then the server &MAY; perform the request method
793   as if the If-Match header field was not present.
794</t>
795<t>
796   If none of the entity-tags match, or if "*" is given and no current
797   representation exists, the server &MUST-NOT; perform the requested method.
798   Instead, the server &MUST; respond with the 412 (Precondition Failed)
799   status code.
800</t>
801<t>
802   If the request would, without the If-Match header field, result in
803   anything other than a 2xx or 412 status code, then the If-Match header field
804   &MUST; be ignored.
805</t>
806<t>
807   Examples:
808</t>
809<figure><artwork type="example">
810  If-Match: "xyzzy"
811  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
812  If-Match: *
813</artwork></figure>
814<t>
815   The result of a request having both an If-Match header field and
816   either an If-None-Match or an If-Modified-Since header fields is
817   undefined by this specification.
818</t>
819</section>
820
821<section title="If-None-Match" anchor="header.if-none-match">
822  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
823  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
824  <x:anchor-alias value="If-None-Match"/>
825<t>
826   The "If-None-Match" header field &MAY; be used to make a request method
827   conditional on not matching any of the current entity-tag values for
828   representations of the target resource.  If-None-Match is primarily
829   used in conditional GET requests to enable efficient updates of cached
830   information with a minimum amount of transaction overhead.  A client
831   that has one or more representations previously obtained from the
832   target resource can send If-None-Match with a list of the associated
833   entity-tags in the hope of receiving a 304 response if at least one
834   of those representations matches the selected representation.
835</t>
836<t>
837   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
838   request method (e.g., PUT) from inadvertently modifying an existing
839   representation of the target resource when the client believes that
840   the resource does not have a current representation.  This is a variation
841   on the "lost update" problem that might arise if more than one client
842   attempts to create an initial representation for the target resource.
843</t>
844<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
845  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
846</artwork></figure>
847<t>
848   If any of the entity-tags listed in the If-None-Match field-value match
849   the entity-tag of the selected representation, or if "*" is
850   given and any current representation exists for that resource, then the
851   server &MUST-NOT; perform the requested method.
852   Instead, if the request method was GET or HEAD, the server &SHOULD;
853   respond with a 304 (Not Modified) status code, including the cache-related
854   header fields (particularly ETag) of the selected representation that has
855   a matching entity-tag.  For all other request methods, the server &MUST;
856   respond with a 412 (Precondition Failed) status code.
857</t>
858<t>
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.
864</t>
865<t>
866   If the request would, without the If-None-Match header field, result
867   in anything other than a 2xx or 304 status code, then the If-None-Match
868   header field &MUST; be ignored. (See <xref
869   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
870   a discussion of server behavior when both If-Modified-Since and
871   If-None-Match appear in the same request.)
872</t>
873<t>
874   Examples:
875</t>
876<figure><artwork type="example">
877  If-None-Match: "xyzzy"
878  If-None-Match: W/"xyzzy"
879  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
880  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
881  If-None-Match: *
882</artwork></figure>
883<t>
884   The result of a request having both an If-None-Match header field and
885   either an If-Match or an If-Unmodified-Since header fields is
886   undefined by this specification.
887</t>
888</section>
889
890<section title="If-Modified-Since" anchor="header.if-modified-since">
891  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
892  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
893  <x:anchor-alias value="If-Modified-Since"/>
894<t>
895   The "If-Modified-Since" header field &MAY; be used to make a request
896   method conditional by modification date: if the selected representation
897   has not been modified since the time specified in this field, then
898   do not perform the request method; instead, respond as detailed below.
899</t>
900<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
901  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
902</artwork></figure>
903<t>
904   An example of the field is:
905</t>
906<figure><artwork type="example">
907  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
908</artwork></figure>
909<t>
910   A GET method with an If-Modified-Since header field and no Range header
911   field requests that the selected representation be transferred only if
912   it has been modified since the date given by the If-Modified-Since
913   header field.
914   The algorithm for determining this includes the following cases:
915  <list style="numbers">
916      <t>If the request would normally result in anything other than a
917         200 (OK) status code, or if the passed If-Modified-Since date is
918         invalid, the response is exactly the same as for a normal GET.
919         A date which is later than the server's current time is
920         invalid.</t>
921
922      <t>If the selected representation has been modified since the
923         If-Modified-Since date, the response is exactly the same as for
924         a normal GET.</t>
925
926      <t>If the selected representation has not been modified since a valid
927         If-Modified-Since date, the server &SHOULD; return a
928         304 (Not Modified) response.</t>
929  </list>
930</t>
931<t>
932   The purpose of this feature is to allow efficient updates of cached
933   information with a minimum amount of transaction overhead.
934  <list><t>
935      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
936      see &header-range; for full details.
937    </t><t>
938      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
939      clock might not be synchronized with the client.
940    </t><t>
941      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
942      servers will use an exact date comparison function, rather than a
943      less-than function, for deciding whether to send a 304 (Not
944      Modified) response. To get best results when sending an If-Modified-Since
945      header field for cache validation, clients are
946      advised to use the exact date string received in a previous Last-Modified
947      header field whenever possible.
948    </t><t>
949      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
950      header field instead of a date taken from the Last-Modified header field for
951      the same request, the client needs to be aware that this
952      date is interpreted in the server's understanding of time.
953      Unsynchronized clocks and rounding problems, due to the different
954      encodings of time between the client and server, are concerns.
955      This includes the possibility of race conditions if the
956      document has changed between the time it was first requested and
957      the If-Modified-Since date of a subsequent request, and the
958      possibility of clock-skew-related problems if the If-Modified-Since
959      date is derived from the client's clock without correction
960      to the server's clock. Corrections for different time bases
961      between client and server are at best approximate due to network
962      latency.
963    </t>
964  </list>
965</t>
966<t>
967   The result of a request having both an If-Modified-Since header field
968   and either an If-Match or an If-Unmodified-Since header fields is
969   undefined by this specification.
970</t>
971</section>
972
973<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
974  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
975  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
976  <x:anchor-alias value="If-Unmodified-Since"/>
977<t>
978   The "If-Unmodified-Since" header field &MAY; be used to make a request
979   method conditional by modification date: if the selected representation
980   has been modified since the time specified in this field, then the
981   server &MUST-NOT; perform the requested operation and &MUST; instead
982   respond with the 412 (Precondition Failed) status code.
983   If the selected representation has not been modified since the time
984   specified in this field, the server &SHOULD; perform the request
985   method as if the If-Unmodified-Since header field were not present.
986</t>
987<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
988  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
989</artwork></figure>
990<t>
991   An example of the field is:
992</t>
993<figure><artwork type="example">
994  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
995</artwork></figure>
996<t>
997   If the request normally (i.e., without the If-Unmodified-Since
998   header field) would result in anything other than a 2xx or 412 status code,
999   the If-Unmodified-Since header field &SHOULD; be ignored.
1000</t>
1001<t>
1002   If the specified date is invalid, the header field &MUST; be ignored.
1003</t>
1004<t>
1005   The result of a request having both an If-Unmodified-Since header
1006   field and either an If-None-Match or an If-Modified-Since header
1007   fields is undefined by this specification.
1008</t>
1009</section>
1010
1011</section>
1012
1013<section title="Status Code Definitions" anchor="status.code.definitions">
1014<section title="304 Not Modified" anchor="status.304">
1015  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1016  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1017<t>
1018   The 304 status code indicates that a conditional GET request has been
1019   received and would have resulted in a 200 (OK) response if it were not
1020   for the fact that the condition has evaluated to false.  In other words,
1021   there is no need for the server to transfer a representation of the
1022   target resource because the client's request indicates that it already
1023   has a valid representation, as indicated by the 304 response header
1024   fields, and is therefore redirecting the client to make use of that
1025   stored representation as if it were the payload of a 200 response.
1026   The 304 response &MUST-NOT; contain a message-body, and thus is always
1027   terminated by the first empty line after the header fields.
1028</t>
1029<t>
1030   A 304 response &MUST; include a Date header field (&header-date;)
1031   unless its omission is required by &clockless;.  If a 200 response
1032   to the same request would have included any of the header fields
1033   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1034   Vary, then those same header fields &MUST; be sent in a 304 response.
1035</t>
1036<t>
1037   Since the goal of a 304 response is to minimize information transfer
1038   when the recipient already has one or more cached representations,
1039   the response &SHOULD-NOT; include representation metadata other
1040   than the above listed fields unless said metadata exists for the
1041   purpose of guiding cache updates (e.g., future HTTP extensions).
1042</t>
1043<t>
1044   If the recipient of a 304 response does not have a cached representation
1045   corresponding to the entity-tag indicated by the 304 response, then the
1046   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1047   conditional request originated with an outbound client, such as a
1048   user agent with its own cache sending a conditional GET to a shared
1049   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1050   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1051   the request without any preconditions.
1052</t>
1053<t>
1054   If a cache uses a received 304 response to update a cache entry, the
1055   cache &MUST; update the entry to reflect any new field values given in
1056   the response.
1057</t>
1058</section>
1059
1060<section title="412 Precondition Failed" anchor="status.412">
1061  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1062  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1063<t>
1064   The 412 status code indicates that one or more preconditions given in
1065   the request header fields evaluated to false when tested on the server.
1066   This response code allows the client to place preconditions on the
1067   current resource state (its current representations and metadata)
1068   and thus prevent the request method from being applied if the target
1069   resource is in an unexpected state.
1070</t>
1071</section>
1072</section>
1073
1074<section title="IANA Considerations" anchor="IANA.considerations">
1075
1076<section title="Status Code Registration" anchor="status.code.registration">
1077<t>
1078   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1079   shall be updated with the registrations below:
1080</t>
1081<?BEGININC p4-conditional.iana-status-codes ?>
1082<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1083<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1084   <ttcol>Value</ttcol>
1085   <ttcol>Description</ttcol>
1086   <ttcol>Reference</ttcol>
1087   <c>304</c>
1088   <c>Not Modified</c>
1089   <c>
1090      <xref target="status.304"/>
1091   </c>
1092   <c>412</c>
1093   <c>Precondition Failed</c>
1094   <c>
1095      <xref target="status.412"/>
1096   </c>
1097</texttable>
1098<!--(END)-->
1099<?ENDINC p4-conditional.iana-status-codes ?>
1100</section>
1101
1102<section title="Header Field Registration" anchor="header.field.registration">
1103<t>
1104   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1105   with the permanent registrations below (see <xref target="RFC3864"/>):
1106</t>
1107<?BEGININC p4-conditional.iana-headers ?>
1108<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1109<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1110   <ttcol>Header Field Name</ttcol>
1111   <ttcol>Protocol</ttcol>
1112   <ttcol>Status</ttcol>
1113   <ttcol>Reference</ttcol>
1114
1115   <c>ETag</c>
1116   <c>http</c>
1117   <c>standard</c>
1118   <c>
1119      <xref target="header.etag"/>
1120   </c>
1121   <c>If-Match</c>
1122   <c>http</c>
1123   <c>standard</c>
1124   <c>
1125      <xref target="header.if-match"/>
1126   </c>
1127   <c>If-Modified-Since</c>
1128   <c>http</c>
1129   <c>standard</c>
1130   <c>
1131      <xref target="header.if-modified-since"/>
1132   </c>
1133   <c>If-None-Match</c>
1134   <c>http</c>
1135   <c>standard</c>
1136   <c>
1137      <xref target="header.if-none-match"/>
1138   </c>
1139   <c>If-Unmodified-Since</c>
1140   <c>http</c>
1141   <c>standard</c>
1142   <c>
1143      <xref target="header.if-unmodified-since"/>
1144   </c>
1145   <c>Last-Modified</c>
1146   <c>http</c>
1147   <c>standard</c>
1148   <c>
1149      <xref target="header.last-modified"/>
1150   </c>
1151</texttable>
1152<!--(END)-->
1153<?ENDINC p4-conditional.iana-headers ?>
1154<t>
1155   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1156</t>
1157</section>
1158</section>
1159
1160<section title="Security Considerations" anchor="security.considerations">
1161<t>
1162   No additional security considerations have been identified beyond
1163   those applicable to HTTP in general &messaging;.
1164</t>
1165</section>
1166
1167<section title="Acknowledgments" anchor="ack">
1168</section>
1169</middle>
1170<back>
1171
1172<references title="Normative References">
1173
1174<reference anchor="Part1">
1175  <front>
1176    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1177    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1178      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1179      <address><email>fielding@gbiv.com</email></address>
1180    </author>
1181    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1182      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1183      <address><email>jg@freedesktop.org</email></address>
1184    </author>
1185    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1186      <organization abbrev="HP">Hewlett-Packard Company</organization>
1187      <address><email>JeffMogul@acm.org</email></address>
1188    </author>
1189    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1190      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1191      <address><email>henrikn@microsoft.com</email></address>
1192    </author>
1193    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1194      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1195      <address><email>LMM@acm.org</email></address>
1196    </author>
1197    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1198      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1199      <address><email>paulle@microsoft.com</email></address>
1200    </author>
1201    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1202      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1203      <address><email>timbl@w3.org</email></address>
1204    </author>
1205    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1206      <organization abbrev="W3C">World Wide Web Consortium</organization>
1207      <address><email>ylafon@w3.org</email></address>
1208    </author>
1209    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1210      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1211      <address><email>julian.reschke@greenbytes.de</email></address>
1212    </author>
1213    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1214  </front>
1215  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1216  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1217</reference>
1218
1219<reference anchor="Part3">
1220  <front>
1221    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1222    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1223      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1224      <address><email>fielding@gbiv.com</email></address>
1225    </author>
1226    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1227      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1228      <address><email>jg@freedesktop.org</email></address>
1229    </author>
1230    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1231      <organization abbrev="HP">Hewlett-Packard Company</organization>
1232      <address><email>JeffMogul@acm.org</email></address>
1233    </author>
1234    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1235      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1236      <address><email>henrikn@microsoft.com</email></address>
1237    </author>
1238    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1239      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1240      <address><email>LMM@acm.org</email></address>
1241    </author>
1242    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1243      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1244      <address><email>paulle@microsoft.com</email></address>
1245    </author>
1246    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1247      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1248      <address><email>timbl@w3.org</email></address>
1249    </author>
1250    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1251      <organization abbrev="W3C">World Wide Web Consortium</organization>
1252      <address><email>ylafon@w3.org</email></address>
1253    </author>
1254    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1255      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1256      <address><email>julian.reschke@greenbytes.de</email></address>
1257    </author>
1258    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1259  </front>
1260  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1261  <x:source href="p3-payload.xml" basename="p3-payload"/>
1262</reference>
1263
1264<reference anchor="Part5">
1265  <front>
1266    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1267    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1268      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1269      <address><email>fielding@gbiv.com</email></address>
1270    </author>
1271    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1272      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1273      <address><email>jg@freedesktop.org</email></address>
1274    </author>
1275    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1276      <organization abbrev="HP">Hewlett-Packard Company</organization>
1277      <address><email>JeffMogul@acm.org</email></address>
1278    </author>
1279    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1280      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1281      <address><email>henrikn@microsoft.com</email></address>
1282    </author>
1283    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1284      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1285      <address><email>LMM@acm.org</email></address>
1286    </author>
1287    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1288      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1289      <address><email>paulle@microsoft.com</email></address>
1290    </author>
1291    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1292      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1293      <address><email>timbl@w3.org</email></address>
1294    </author>
1295    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1296      <organization abbrev="W3C">World Wide Web Consortium</organization>
1297      <address><email>ylafon@w3.org</email></address>
1298    </author>
1299    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1300      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1301      <address><email>julian.reschke@greenbytes.de</email></address>
1302    </author>
1303    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1304  </front>
1305  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1306  <x:source href="p5-range.xml" basename="p5-range"/>
1307</reference>
1308
1309<reference anchor="Part6">
1310  <front>
1311    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1312    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1313      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1314      <address><email>fielding@gbiv.com</email></address>
1315    </author>
1316    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1317      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1318      <address><email>jg@freedesktop.org</email></address>
1319    </author>
1320    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1321      <organization abbrev="HP">Hewlett-Packard Company</organization>
1322      <address><email>JeffMogul@acm.org</email></address>
1323    </author>
1324    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1325      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1326      <address><email>henrikn@microsoft.com</email></address>
1327    </author>
1328    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1329      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1330      <address><email>LMM@acm.org</email></address>
1331    </author>
1332    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1333      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1334      <address><email>paulle@microsoft.com</email></address>
1335    </author>
1336    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1337      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1338      <address><email>timbl@w3.org</email></address>
1339    </author>
1340    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1341      <organization abbrev="W3C">World Wide Web Consortium</organization>
1342      <address><email>ylafon@w3.org</email></address>
1343    </author>
1344    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1345      <address><email>mnot@mnot.net</email></address>
1346    </author>
1347    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1348      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1349      <address><email>julian.reschke@greenbytes.de</email></address>
1350    </author>
1351    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1352  </front>
1353  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1354  <x:source href="p6-cache.xml" basename="p6-cache"/>
1355</reference>
1356
1357<reference anchor="RFC2119">
1358  <front>
1359    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1360    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1361      <organization>Harvard University</organization>
1362      <address><email>sob@harvard.edu</email></address>
1363    </author>
1364    <date month="March" year="1997"/>
1365  </front>
1366  <seriesInfo name="BCP" value="14"/>
1367  <seriesInfo name="RFC" value="2119"/>
1368</reference>
1369
1370<reference anchor="RFC5234">
1371  <front>
1372    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1373    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1374      <organization>Brandenburg InternetWorking</organization>
1375      <address>
1376        <email>dcrocker@bbiw.net</email>
1377      </address> 
1378    </author>
1379    <author initials="P." surname="Overell" fullname="Paul Overell">
1380      <organization>THUS plc.</organization>
1381      <address>
1382        <email>paul.overell@thus.net</email>
1383      </address>
1384    </author>
1385    <date month="January" year="2008"/>
1386  </front>
1387  <seriesInfo name="STD" value="68"/>
1388  <seriesInfo name="RFC" value="5234"/>
1389</reference>
1390
1391</references>
1392
1393<references title="Informative References">
1394
1395<reference anchor="RFC2616">
1396  <front>
1397    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1398    <author initials="R." surname="Fielding" fullname="R. Fielding">
1399      <organization>University of California, Irvine</organization>
1400      <address><email>fielding@ics.uci.edu</email></address>
1401    </author>
1402    <author initials="J." surname="Gettys" fullname="J. Gettys">
1403      <organization>W3C</organization>
1404      <address><email>jg@w3.org</email></address>
1405    </author>
1406    <author initials="J." surname="Mogul" fullname="J. Mogul">
1407      <organization>Compaq Computer Corporation</organization>
1408      <address><email>mogul@wrl.dec.com</email></address>
1409    </author>
1410    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1411      <organization>MIT Laboratory for Computer Science</organization>
1412      <address><email>frystyk@w3.org</email></address>
1413    </author>
1414    <author initials="L." surname="Masinter" fullname="L. Masinter">
1415      <organization>Xerox Corporation</organization>
1416      <address><email>masinter@parc.xerox.com</email></address>
1417    </author>
1418    <author initials="P." surname="Leach" fullname="P. Leach">
1419      <organization>Microsoft Corporation</organization>
1420      <address><email>paulle@microsoft.com</email></address>
1421    </author>
1422    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1423      <organization>W3C</organization>
1424      <address><email>timbl@w3.org</email></address>
1425    </author>
1426    <date month="June" year="1999"/>
1427  </front>
1428  <seriesInfo name="RFC" value="2616"/>
1429</reference>
1430
1431<reference anchor='RFC3864'>
1432  <front>
1433    <title>Registration Procedures for Message Header Fields</title>
1434    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1435      <organization>Nine by Nine</organization>
1436      <address><email>GK-IETF@ninebynine.org</email></address>
1437    </author>
1438    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1439      <organization>BEA Systems</organization>
1440      <address><email>mnot@pobox.com</email></address>
1441    </author>
1442    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1443      <organization>HP Labs</organization>
1444      <address><email>JeffMogul@acm.org</email></address>
1445    </author>
1446    <date year='2004' month='September' />
1447  </front>
1448  <seriesInfo name='BCP' value='90' />
1449  <seriesInfo name='RFC' value='3864' />
1450</reference>
1451
1452<reference anchor='RFC4918'>
1453  <front>
1454    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1455    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1456      <organization abbrev="CommerceNet">CommerceNet</organization>
1457      <address><email>ldusseault@commerce.net</email></address>
1458    </author>
1459    <date month="June" year="2007" />
1460  </front>
1461  <seriesInfo name='RFC' value='4918' />
1462</reference>
1463</references>
1464
1465<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1466<t>
1467  Allow weak entity-tags in all requests except range requests (Sections
1468  <xref target="weak.and.strong.validators" format="counter"/> and
1469  <xref target="header.if-none-match" format="counter"/>).
1470</t>
1471<t>
1472  Change ABNF productions for header fields to only define the field value.
1473  (<xref target="header.fields"/>)
1474</t>
1475</section>
1476
1477<?BEGININC p4-conditional.abnf-appendix ?>
1478<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1479<figure>
1480<artwork type="abnf" name="p4-conditional.parsed-abnf">
1481<x:ref>ETag</x:ref> = entity-tag
1482
1483<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1484
1485<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1486 entity-tag ] ) )
1487<x:ref>If-Modified-Since</x:ref> = HTTP-date
1488<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1489 entity-tag ] ) )
1490<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1491
1492<x:ref>Last-Modified</x:ref> = HTTP-date
1493
1494<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1495
1496<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1497
1498<x:ref>opaque-tag</x:ref> = quoted-string
1499
1500<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1501
1502<x:ref>weak</x:ref> = %x57.2F ; W/
1503</artwork>
1504</figure>
1505<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1506; ETag defined but not used
1507; If-Match defined but not used
1508; If-Modified-Since defined but not used
1509; If-None-Match defined but not used
1510; If-Unmodified-Since defined but not used
1511; Last-Modified defined but not used
1512</artwork></figure></section>
1513<?ENDINC p4-conditional.abnf-appendix ?>
1514
1515<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1516
1517<section title="Since RFC 2616">
1518<t>
1519  Extracted relevant partitions from <xref target="RFC2616"/>.
1520</t>
1521</section>
1522
1523<section title="Since draft-ietf-httpbis-p4-conditional-00">
1524<t>
1525  Closed issues:
1526  <list style="symbols"> 
1527    <t>
1528      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1529      "Normative and Informative references"
1530    </t>
1531  </list>
1532</t>
1533<t>
1534  Other changes:
1535  <list style="symbols"> 
1536    <t>
1537      Move definitions of 304 and 412 condition codes from Part2.
1538    </t>
1539  </list>
1540</t>
1541</section>
1542
1543<section title="Since draft-ietf-httpbis-p4-conditional-01">
1544<t>
1545  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1546  <list style="symbols"> 
1547    <t>
1548      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1549    </t>
1550  </list>
1551</t>
1552</section>
1553
1554<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1555<t>
1556  Closed issues:
1557  <list style="symbols"> 
1558    <t>
1559      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1560      "Weak ETags on non-GET requests"
1561    </t>
1562  </list>
1563</t>
1564<t>
1565  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1566  <list style="symbols"> 
1567    <t>
1568      Reference RFC 3984, and update header field registrations for header fields defined
1569      in this document.
1570    </t>
1571  </list>
1572</t>
1573</section>
1574
1575<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1576<t>
1577  Closed issues:
1578  <list style="symbols"> 
1579    <t>
1580      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1581      "Examples for ETag matching"
1582    </t>
1583    <t>
1584      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1585      "'entity value' undefined"
1586    </t>
1587    <t>
1588      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1589      "bogus 2068 Date header reference"
1590    </t>
1591  </list>
1592</t>
1593</section>
1594
1595<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1596<t>
1597  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1598  <list style="symbols"> 
1599    <t>
1600      Use "/" instead of "|" for alternatives.
1601    </t>
1602    <t>
1603      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1604      whitespace ("OWS") and required whitespace ("RWS").
1605    </t>
1606    <t>
1607      Rewrite ABNFs to spell out whitespace rules, factor out
1608      header field value format definitions.
1609    </t>
1610  </list>
1611</t>
1612</section>
1613
1614<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1615<t>
1616  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1617  <list style="symbols"> 
1618    <t>
1619      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1620    </t>
1621  </list>
1622</t>
1623</section>
1624
1625<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1626<t>
1627  Closed issues:
1628  <list style="symbols"> 
1629    <t>
1630      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1631      "case-sensitivity of etag weakness indicator"
1632    </t>
1633  </list>
1634</t>
1635</section>
1636
1637<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1638<t>
1639  Closed issues:
1640  <list style="symbols"> 
1641    <t>
1642      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1643      "Weak ETags on non-GET requests" (If-Match still was defined to require
1644      strong matching)
1645    </t>
1646    <t>
1647      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1648      "move IANA registrations for optional status codes"
1649    </t>
1650  </list>
1651</t>
1652</section>
1653
1654<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1655<t>
1656  No significant changes.
1657</t>
1658</section>
1659
1660<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1661<t>
1662  No significant changes.
1663</t>
1664</section>
1665
1666<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1667<t>
1668  Closed issues:
1669  <list style="symbols"> 
1670    <t>
1671      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1672      "Clarify 'Requested Variant'"
1673    </t>
1674    <t>
1675      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1676      "Clarify entity / representation / variant terminology"
1677    </t>
1678    <t>
1679      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1680      "consider removing the 'changes from 2068' sections"
1681    </t>
1682  </list>
1683</t>
1684</section>
1685
1686<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1687<t>
1688  None.
1689</t>
1690</section>
1691
1692<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1693<t>
1694  Closed issues:
1695  <list style="symbols"> 
1696    <t>
1697      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1698      "Header Classification"
1699    </t>
1700  </list>
1701</t>
1702</section>
1703
1704<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1705<t>
1706  Closed issues:
1707  <list style="symbols"> 
1708    <t>
1709      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1710      "If-* and entities"
1711    </t>
1712    <t>
1713      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1714      "Definition of validator weakness"
1715    </t>
1716    <t>
1717      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1718      "untangle ABNFs for header fields"
1719    </t>
1720  </list>
1721</t>
1722</section>
1723
1724</section>
1725
1726</back>
1727</rfc>
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