source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1243

Last change on this file since 1243 was 1243, checked in by julian.reschke@…, 9 years ago

note that #101 was resolved by [1123] (see #101)

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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 "March">
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 HTTP/1.1 response metadata for indicating potential
219   changes to payload content, including modification time stamps and opaque
220   entity-tags, and the HTTP conditional request mechanisms that allow
221   preconditions to be placed on a request method.  Conditional GET requests
222   allow for efficient cache updates.  Other conditional request methods are
223   used to protect against overwriting or misunderstanding the state of a
224   resource that has been changed unbeknownst to the requesting client.
225</t>
226<t><iref primary="true" item="selected representation"/>
227   We use the term "<x:dfn>selected representation</x:dfn>" to refer to the current
228   representation of the target resource that would have been selected
229   and sent as payload in a 200 (OK) response if the same request had used
230   the method GET and had excluded all of the conditional request header
231   fields.  The conditions found within conditional requests are evaluated
232   on the basis of either the state of the target resource as a whole or
233   the state of the target resource's selected representation.
234</t>
235<t>
236   This document is currently disorganized in order to minimize the changes
237   between drafts and enable reviewers to see the smaller errata changes.
238   A future draft will reorganize the sections to better reflect the content.
239   In particular, the sections on resource metadata will be discussed first
240   and then followed by each conditional request header field, concluding with a
241   definition of precedence and the expectation of ordering strong validator
242   checks before weak validator checks.  It is likely that more content from
243   &caching; will migrate to this part, where appropriate.
244   The current mess reflects how widely dispersed these topics and associated
245   requirements had become in <xref target="RFC2616"/>.
246</t>
247
248<section title="Requirements" anchor="intro.requirements">
249<t>
250   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
251   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
252   document are to be interpreted as described in <xref target="RFC2119"/>.
253</t>
254<t>
255   An implementation is not compliant if it fails to satisfy one or more
256   of the "MUST" or "REQUIRED" level requirements for the protocols it
257   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
258   level and all the "SHOULD" level requirements for its protocols is said
259   to be "unconditionally compliant"; one that satisfies all the "MUST"
260   level requirements but not all the "SHOULD" level requirements for its
261   protocols is said to be "conditionally compliant".
262</t>
263</section>
264
265<section title="Syntax Notation" anchor="notation">
266  <x:anchor-alias value="ALPHA"/>
267  <x:anchor-alias value="CR"/>
268  <x:anchor-alias value="DIGIT"/>
269  <x:anchor-alias value="LF"/>
270  <x:anchor-alias value="OCTET"/>
271  <x:anchor-alias value="VCHAR"/>
272  <x:anchor-alias value="WSP"/>
273<t>
274  This specification uses the ABNF syntax defined in &notation; (which
275  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
276  <xref target="collected.abnf"/> shows the collected ABNF, with the list
277  rule expanded.
278</t>
279<t>
280  The following core rules are included by
281  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
282  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
283  DIGIT (decimal 0-9), DQUOTE (double quote),
284  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
285  OCTET (any 8-bit sequence of data), SP (space),
286  VCHAR (any visible USASCII character),
287  and WSP (whitespace).
288</t>
289
290<section title="Core Rules" anchor="core.rules">
291  <x:anchor-alias value="quoted-string"/>
292  <x:anchor-alias value="OWS"/>
293<t>
294  The core rules below are defined in &basic-rules;:
295</t>
296<figure><artwork type="abnf2616">
297  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
298  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
299</artwork></figure>
300</section>
301
302<section title="ABNF Rules defined in other Parts of the Specification" anchor="abnf.dependencies">
303  <x:anchor-alias value="HTTP-date"/>
304<t>
305  The ABNF rules below are defined in other parts:
306</t>
307<figure><!--Part1--><artwork type="abnf2616">
308  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
309</artwork></figure>
310</section>
311
312</section>
313
314</section>
315
316<section title="Entity-Tags" anchor="entity.tags">
317  <x:anchor-alias value="entity-tag"/>
318  <x:anchor-alias value="opaque-tag"/>
319  <x:anchor-alias value="weak"/>
320<t>
321   Entity-tags are used for comparing two or more representations of the same
322   resource. HTTP/1.1 uses entity-tags in the ETag (<xref target="header.etag"/>),
323   If-Match (<xref target="header.if-match"/>), If-None-Match (<xref target="header.if-none-match"/>), and
324   If-Range (&header-if-range;) header fields. The definition of how they
325   are used and compared as cache validators is in <xref target="weak.and.strong.validators"/>. An
326   entity-tag consists of an opaque quoted string, possibly prefixed by
327   a weakness indicator.
328</t>
329<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"/>
330  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
331  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
332  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
333</artwork></figure>
334<t>
335   A "strong entity-tag" &MAY; be shared by two representations of a resource
336   only if they are equivalent by octet equality.
337</t>
338<t>
339   A "weak entity-tag", indicated by the "W/" prefix, &MAY; be shared by
340   two representations of a resource. A weak entity-tag can only be used
341   for weak comparison.
342</t>
343<t>
344   Cache entries might persist for arbitrarily long periods, regardless
345   of expiration times, so it is inappropriate to expect that a cache will
346   never again attempt to validate an entry using a validator that it
347   obtained at some point in the past.
348   A strong entity-tag &MUST; be unique across all versions of all
349   representations associated with a particular resource over time.
350   However, there is no implication of uniqueness across entity-tags
351   of different resources (i.e., the same entity-tag value might be
352   in use for representations of multiple resources at the same time
353   and does not imply that those representations are equivalent).
354</t>
355
356<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
357<t>
358   Consider a resource that is subject to content negotiation (&content-negotiation;),
359   and where the representations returned upon a GET request vary based on
360   the Accept-Encoding request header field (&header-accept-encoding;):
361</t>
362<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
363GET /index HTTP/1.1
364Host: www.example.com
365Accept-Encoding: gzip
366
367</artwork></figure>
368<t>
369   In this case, the response might or might not use the gzip content coding.
370   If it does not, the response might look like:
371</t>
372<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
373HTTP/1.1 200 OK
374Date: Thu, 26 Mar 2010 00:05:00 GMT
375ETag: "123-a"
376Content-Length: <x:length-of target="exbody"/>
377Vary: Accept-Encoding
378Content-Type: text/plain
379
380<x:span anchor="exbody">Hello World!
381Hello World!
382Hello World!
383Hello World!
384Hello World!
385</x:span></artwork></figure>
386<t>
387   An alternative representation that does use gzip content coding would be:
388</t>
389<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
390HTTP/1.1 200 OK
391Date: Thu, 26 Mar 2010 00:05:00 GMT
392ETag: "123-b"
393Content-Length: 43
394Vary: Accept-Encoding
395Content-Type: text/plain
396Content-Encoding: gzip
397
398<spanx>...binary data...</spanx></artwork></figure>
399<x:note>
400  <t>
401    <x:h>Note:</x:h> Content codings are a property of the representation,
402    so therefore an entity-tag of an encoded representation must be distinct
403    from an unencoded representation to prevent conflicts during cache updates
404    and range requests.  In contrast, transfer codings (&transfer-codings;)
405    apply only during message transfer and do not require distinct entity-tags.
406  </t>
407</x:note>
408</section>
409</section>
410
411<section title="Status Code Definitions" anchor="status.code.definitions">
412<section title="304 Not Modified" anchor="status.304">
413  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
414  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
415<t>
416   The 304 status code indicates that a conditional GET request has been
417   received and would have resulted in a 200 (OK) response if it were not
418   for the fact that the condition has evaluated to false.  In other words,
419   there is no need for the server to transfer a representation of the
420   target resource because the client's request indicates that it already
421   has a valid representation, as indicated by the 304 response header
422   fields, and is therefore redirecting the client to make use of that
423   stored representation as if it were the payload of a 200 response.
424   The 304 response &MUST-NOT; contain a message-body, and thus is always
425   terminated by the first empty line after the header fields.
426</t>
427<t>
428   A 304 response &MUST; include a Date header field (&header-date;)
429   unless its omission is required by &clockless;.  If a 200 response
430   to the same request would have included any of the header fields
431   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
432   Vary, then those same header fields &MUST; be sent in a 304 response.
433</t>
434<t>
435   Since the goal of a 304 response is to minimize information transfer
436   when the recipient already has one or more cached representations,
437   the response &SHOULD-NOT; include representation metadata other
438   than the above listed fields unless said metadata exists for the
439   purpose of guiding cache updates (e.g., future HTTP extensions).
440</t>
441<t>
442   If the recipient of a 304 response does not have a cached representation
443   corresponding to the entity-tag indicated by the 304 response, then the
444   recipient &MUST-NOT; use the 304 to update its own cache.  If this
445   conditional request originated with an outbound client, such as a
446   user agent with its own cache sending a conditional GET to a shared
447   proxy, then the 304 response &MAY; be forwarded to the outbound client.
448   Otherwise, the recipient &MUST; disregard the 304 response and repeat
449   the request without any preconditions.
450</t>
451<t>
452   If a cache uses a received 304 response to update a cache entry, the
453   cache &MUST; update the entry to reflect any new field values given in
454   the response.
455</t>
456</section>
457
458<section title="412 Precondition Failed" anchor="status.412">
459  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
460  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
461<t>
462   The 412 status code indicates that one or more preconditions given in
463   the request header fields evaluated to false when tested on the server.
464   This response code allows the client to place preconditions on the
465   current resource state (its current representations and metadata)
466   and thus prevent the request method from being applied if the target
467   resource is in an unexpected state.
468</t>
469</section>
470</section>
471
472<section title="Weak and Strong Validators" anchor="weak.and.strong.validators">
473<t>
474   Since both origin servers and caches will compare two validators to
475   decide if they represent the same or different representations, one
476   normally would expect that if the representation (including both
477   representation header fields and representation body) changes in any
478   way, then the associated validator would change as well. If this is
479   true, then we call this validator a "strong validator".  One example
480   of a strong validator is an integer that is incremented in stable
481   storage every time a representation is changed.
482</t>
483<t>
484   However, there might be cases when a server prefers to change the
485   validator only when it desires cached representations to be invalidated.
486   For example, the representation of a weather report that changes in
487   content every second, based on dynamic measurements, might be grouped
488   into sets of equivalent representations (from the origin server's
489   perspective) in order to allow cached representations to be valid
490   for a reasonable period of time (perhaps adjusted dynamically based
491   on server load or weather quality).
492   A validator that does not always change when the representation
493   changes is a "weak validator".
494</t>
495<t>
496   An entity-tag is normally a strong validator, but the protocol
497   provides a mechanism to tag an entity-tag as "weak". One can think
498   of a strong validator as part of an identifier for a specific
499   representation, whereas a weak validator is part of an identifier
500   for a set of equivalent representations (where this notion of
501   equivalence is entirely governed by the origin server and beyond
502   the scope of this specification).
503  <list><t>
504      A representation's modification time, if defined with only one-second
505      resolution, could be a weak validator, since it is possible that
506      the representation might be modified twice during a single second.
507    </t><t>
508      Support for weak validators is optional. However, weak validators
509      allow for more efficient caching of equivalent objects; for
510      example, a hit counter on a site is probably good enough if it is
511      updated every few days or weeks, and any value during that period
512      is likely "good enough" to be equivalent.
513    </t></list>
514</t>
515<t>
516   A strong entity-tag &MUST; change whenever the associated representation
517   changes in any way. A weak entity-tag &SHOULD; change whenever the origin
518   server considers prior representations to be unacceptable as a substitute
519   for the current representation. In other words, a weak entity tag &SHOULD;
520   change whenever the origin server wants caches to invalidate old responses.
521</t>
522<t>
523   A "use" of a validator is either when a client generates a request
524   and includes the validator in a validating header field, or when a
525   server compares two validators.
526</t>
527<t>
528   Strong validators are usable in any context. Weak validators are only
529   usable in contexts that do not depend on exact equality of a representation.
530   For example, either kind is usable for a normal conditional GET.
531   However, only a strong validator is usable for range retrieval
532   (<xref target="Part5"/>), since otherwise the client might end up
533   with an internally inconsistent representation.
534   Clients &MUST-NOT; use weak validators in range requests.
535</t>
536<t>
537   The only function that HTTP/1.1 defines on validators is
538   comparison. There are two validator comparison functions, depending
539   on whether the comparison context allows the use of weak validators
540   or not:
541  <list style="symbols">
542     <t>The strong comparison function: in order to be considered equal,
543        both opaque-tags &MUST; be identical character-by-character, and both
544        &MUST-NOT; be weak.</t>
545     <t>The weak comparison function: in order to be considered equal, both
546        opaque-tags &MUST; be identical character-by-character, but
547        either or both of them &MAY; be tagged as "weak" without affecting
548        the result.</t>
549  </list>
550</t>
551<t>
552   The example below shows the results for a set of entity-tag pairs,
553   and both the weak and strong comparison function results:
554</t>
555<texttable align="left">
556  <ttcol>ETag 1</ttcol>
557  <ttcol>ETag 2</ttcol>
558  <ttcol>Strong Comparison</ttcol>
559  <ttcol>Weak Comparison</ttcol>
560
561  <c>W/"1"</c>
562  <c>W/"1"</c>
563  <c>no match</c>
564  <c>match</c>
565 
566  <c>W/"1"</c>
567  <c>W/"2"</c>
568  <c>no match</c>
569  <c>no match</c>
570
571  <c>W/"1"</c>
572  <c>"1"</c>
573  <c>no match</c>
574  <c>match</c>
575
576  <c>"1"</c>
577  <c>"1"</c>
578  <c>match</c>
579  <c>match</c>
580</texttable>
581<t>
582   An entity-tag is strong unless it is explicitly tagged as weak.
583   <xref target="entity.tags"/> gives the syntax for entity-tags.
584</t>
585<t>
586   A Last-Modified time, when used as a validator in a request, is
587   implicitly weak unless it is possible to deduce that it is strong,
588   using the following rules:
589  <list style="symbols">
590     <t>The validator is being compared by an origin server to the
591        actual current validator for the representation and,</t>
592     <t>That origin server reliably knows that the associated representation did
593        not change twice during the second covered by the presented
594        validator.</t>
595  </list>
596</t>
597<t>
598   or
599  <list style="symbols">
600     <t>The validator is about to be used by a client in an If-Modified-Since
601        or If-Unmodified-Since header field, because the client
602        has a cache entry for the associated representation, and</t>
603     <t>That cache entry includes a Date value, which gives the time
604        when the origin server sent the original response, and</t>
605     <t>The presented Last-Modified time is at least 60 seconds before
606        the Date value.</t>
607  </list>
608</t>
609<t>
610   or
611  <list style="symbols">
612     <t>The validator is being compared by an intermediate cache to the
613        validator stored in its cache entry for the representation, and</t>
614     <t>That cache entry includes a Date value, which gives the time
615        when the origin server sent the original response, and</t>
616     <t>The presented Last-Modified time is at least 60 seconds before
617        the Date value.</t>
618  </list>
619</t>
620<t>
621   This method relies on the fact that if two different responses were
622   sent by the origin server during the same second, but both had the
623   same Last-Modified time, then at least one of those responses would
624   have a Date value equal to its Last-Modified time. The arbitrary 60-second
625   limit guards against the possibility that the Date and Last-Modified
626   values are generated from different clocks, or at somewhat
627   different times during the preparation of the response. An
628   implementation &MAY; use a value larger than 60 seconds, if it is
629   believed that 60 seconds is too short.
630</t>
631<t>
632   If a client wishes to perform a sub-range retrieval on a value for
633   which it has only a Last-Modified time and no opaque validator, it
634   &MAY; do this only if the Last-Modified time is strong in the sense
635   described here.
636</t>
637<t>
638   A cache or origin server receiving a conditional range request
639   (<xref target="Part5"/>) &MUST; use the strong comparison function to
640   evaluate the condition.
641</t>
642<t>
643   These rules allow HTTP/1.1 caches and clients to safely perform sub-range
644   retrievals on values that have been obtained from HTTP/1.0
645   servers.
646</t>
647</section>
648
649<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">
650<t>
651   We adopt a set of rules and recommendations for origin servers,
652   clients, and caches regarding when various validator types ought to
653   be used, and for what purposes.
654</t>
655<t>
656   HTTP/1.1 origin servers:
657  <list style="symbols">
658     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
659        generate one.</t>
660
661     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
662        performance considerations support the use of weak entity-tags,
663        or if it is unfeasible to send a strong entity-tag.</t>
664
665     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
666  </list>
667</t>
668<t>
669   In other words, the preferred behavior for an HTTP/1.1 origin server
670   is to send both a strong entity-tag and a Last-Modified value.
671</t>
672<t>
673   HTTP/1.1 clients:
674  <list style="symbols">
675     <t>&MUST; use that entity-tag in any cache-conditional request (using
676        If-Match or If-None-Match) if an entity-tag has been provided by the
677        origin server.</t>
678
679     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
680        requests (using If-Modified-Since) if only a Last-Modified value has
681        been provided by the origin server. </t>
682
683     <t>&MAY; use the Last-Modified value in subrange cache-conditional
684        requests (using If-Unmodified-Since) if only a Last-Modified value has
685        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
686        provide a way to disable this, in case of difficulty.</t>
687
688     <t>&SHOULD; use both validators in cache-conditional requests if both an
689        entity-tag and a Last-Modified value have been provided by the origin
690        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
691        appropriately.</t>
692  </list>
693</t>
694<t>
695   An HTTP/1.1 origin server, upon receiving a conditional request that
696   includes both a Last-Modified date (e.g., in an If-Modified-Since or
697   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
698   in an If-Match, If-None-Match, or If-Range header field) as cache
699   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
700   unless doing so is consistent with all of the conditional header
701   fields in the request.
702</t>
703<t>
704   An HTTP/1.1 caching proxy, upon receiving a conditional request that
705   includes both a Last-Modified date and one or more entity-tags as
706   cache validators, &MUST-NOT; return a locally cached response to the
707   client unless that cached response is consistent with all of the
708   conditional header fields in the request.
709  <list><t>
710      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
711      servers and clients ought to transmit as much non-redundant
712      information as is available in their responses and requests.
713      HTTP/1.1 systems receiving this information will make the most
714      conservative assumptions about the validators they receive.
715  </t><t>
716      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
717      last-modified values received or used by these systems will
718      support transparent and efficient caching, and so HTTP/1.1 origin
719      servers should provide Last-Modified values. In those rare cases
720      where the use of a Last-Modified value as a validator by an
721      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
722      origin servers should not provide one.
723  </t></list>
724</t>
725</section>
726
727<section title="Header Field Definitions" anchor="header.fields">
728<t>
729   This section defines the syntax and semantics of HTTP/1.1 header fields
730   related to conditional requests.
731</t>
732
733<section title="ETag" anchor="header.etag">
734  <iref primary="true" item="ETag header field" x:for-anchor=""/>
735  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
736  <x:anchor-alias value="ETag"/>
737<t>
738   The "ETag" header field provides the current value of the
739   entity-tag (see <xref target="entity.tags"/>) for one representation of
740   the target resource.  An entity-tag
741   is intended for use as a resource-local identifier for differentiating
742   between representations of the same resource that vary over time or via
743   content negotiation (see <xref target="weak.and.strong.validators"/>).
744</t>
745<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/>
746  <x:ref>ETag</x:ref> = <x:ref>entity-tag</x:ref>
747</artwork></figure>
748<figure><preamble>
749  Examples:
750</preamble>
751<artwork type="example">
752  ETag: "xyzzy"
753  ETag: W/"xyzzy"
754  ETag: ""
755</artwork></figure>
756<t>
757   An entity-tag provides an "opaque" cache validator that allows for
758   more reliable validation than modification dates in situations where
759   it is inconvenient to store modification dates,
760   where the one-second resolution of HTTP date values is not
761   sufficient, or where the origin server wishes to avoid certain
762   paradoxes that might arise from the use of modification dates.
763</t>
764<t>
765   The principle behind entity-tags is that only the service author
766   knows the semantics of a resource well enough to select an
767   appropriate cache validation mechanism, and the specification of any
768   validator comparison function more complex than byte-equality would
769   open up a can of worms. Thus, comparisons of any other header fields
770   (except Last-Modified, for compatibility with HTTP/1.0) are never
771   used for purposes of validating a cache entry.
772</t>
773</section>
774
775<section title="If-Match" anchor="header.if-match">
776  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
777  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
778  <x:anchor-alias value="If-Match"/>
779<t>
780   The "If-Match" header field &MAY; be used to make a request method
781   conditional on the current existence or value of an entity-tag for
782   one or more representations of the target resource.  If-Match is
783   generally useful for resource update requests, such as PUT requests,
784   as a means for protecting against accidental overwrites when multiple
785   clients are acting in parallel on the same resource (i.e., the
786   "lost update" problem).  An If-Match field-value of "*" places the
787   precondition on the existence of any current representation for the
788   target resource.
789</t>
790<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
791  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
792</artwork></figure>
793<t>
794   If any of the entity-tags listed in the If-Match field value match
795   the entity-tag of the selected representation for the target resource,
796   or if "*" is given and any current representation exists for the
797   target resource, then the server &MAY; perform the request method
798   as if the If-Match header field was not present.
799</t>
800<t>
801   If none of the entity-tags match, or if "*" is given and no current
802   representation exists, the server &MUST-NOT; perform the requested method.
803   Instead, the server &MUST; respond with the 412 (Precondition Failed)
804   status code.
805</t>
806<t>
807   If the request would, without the If-Match header field, result in
808   anything other than a 2xx or 412 status code, then the If-Match header field
809   &MUST; be ignored.
810</t>
811<t>
812   Examples:
813</t>
814<figure><artwork type="example">
815  If-Match: "xyzzy"
816  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
817  If-Match: *
818</artwork></figure>
819<t>
820   The result of a request having both an If-Match header field and
821   either an If-None-Match or an If-Modified-Since header fields is
822   undefined by this specification.
823</t>
824</section>
825
826<section title="If-Modified-Since" anchor="header.if-modified-since">
827  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
828  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
829  <x:anchor-alias value="If-Modified-Since"/>
830<t>
831   The "If-Modified-Since" header field &MAY; be used to make a request
832   method conditional by modification date: if the selected representation
833   has not been modified since the time specified in this field, then
834   do not perform the request method; instead, respond as detailed below.
835</t>
836<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
837  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
838</artwork></figure>
839<t>
840   An example of the field is:
841</t>
842<figure><artwork type="example">
843  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
844</artwork></figure>
845<t>
846   A GET method with an If-Modified-Since header field and no Range header
847   field requests that the selected representation be transferred only if
848   it has been modified since the date given by the If-Modified-Since
849   header field.
850   The algorithm for determining this includes the following cases:
851  <list style="numbers">
852      <t>If the request would normally result in anything other than a
853         200 (OK) status code, or if the passed If-Modified-Since date is
854         invalid, the response is exactly the same as for a normal GET.
855         A date which is later than the server's current time is
856         invalid.</t>
857
858      <t>If the selected representation has been modified since the
859         If-Modified-Since date, the response is exactly the same as for
860         a normal GET.</t>
861
862      <t>If the selected representation has not been modified since a valid
863         If-Modified-Since date, the server &SHOULD; return a
864         304 (Not Modified) response.</t>
865  </list>
866</t>
867<t>
868   The purpose of this feature is to allow efficient updates of cached
869   information with a minimum amount of transaction overhead.
870  <list><t>
871      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
872      see &header-range; for full details.
873    </t><t>
874      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
875      clock might not be synchronized with the client.
876    </t><t>
877      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
878      servers will use an exact date comparison function, rather than a
879      less-than function, for deciding whether to send a 304 (Not
880      Modified) response. To get best results when sending an If-Modified-Since
881      header field for cache validation, clients are
882      advised to use the exact date string received in a previous Last-Modified
883      header field whenever possible.
884    </t><t>
885      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
886      header field instead of a date taken from the Last-Modified header field for
887      the same request, the client needs to be aware that this
888      date is interpreted in the server's understanding of time.
889      Unsynchronized clocks and rounding problems, due to the different
890      encodings of time between the client and server, are concerns.
891      This includes the possibility of race conditions if the
892      document has changed between the time it was first requested and
893      the If-Modified-Since date of a subsequent request, and the
894      possibility of clock-skew-related problems if the If-Modified-Since
895      date is derived from the client's clock without correction
896      to the server's clock. Corrections for different time bases
897      between client and server are at best approximate due to network
898      latency.
899    </t>
900  </list>
901</t>
902<t>
903   The result of a request having both an If-Modified-Since header field
904   and either an If-Match or an If-Unmodified-Since header fields is
905   undefined by this specification.
906</t>
907</section>
908
909<section title="If-None-Match" anchor="header.if-none-match">
910  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
911  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
912  <x:anchor-alias value="If-None-Match"/>
913<t>
914   The "If-None-Match" header field &MAY; be used to make a request method
915   conditional on not matching any of the current entity-tag values for
916   representations of the target resource.  If-None-Match is primarily
917   used in conditional GET requests to enable efficient updates of cached
918   information with a minimum amount of transaction overhead.  A client
919   that has one or more representations previously obtained from the
920   target resource can send If-None-Match with a list of the associated
921   entity-tags in the hope of receiving a 304 response if at least one
922   of those representations matches the selected representation.
923</t>
924<t>
925   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
926   request method (e.g., PUT) from inadvertently modifying an existing
927   representation of the target resource when the client believes that
928   the resource does not have a current representation.  This is a variation
929   on the "lost update" problem that might arise if more than one client
930   attempts to create an initial representation for the target resource.
931</t>
932<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
933  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
934</artwork></figure>
935<t>
936   If any of the entity-tags listed in the If-None-Match field-value match
937   the entity-tag of the selected representation, or if "*" is
938   given and any current representation exists for that resource, then the
939   server &MUST-NOT; perform the requested method.
940   Instead, if the request method was GET or HEAD, the server &SHOULD;
941   respond with a 304 (Not Modified) status code, including the cache-related
942   header fields (particularly ETag) of the selected representation that has
943   a matching entity-tag.  For all other request methods, the server &MUST;
944   respond with a 412 (Precondition Failed) status code.
945</t>
946<t>
947   If none of the entity-tags match, then the server &MAY; perform the
948   requested method as if the If-None-Match header field did not exist,
949   but &MUST; also ignore any If-Modified-Since header field(s) in the
950   request. That is, if no entity-tags match, then the server &MUST-NOT;
951   return a 304 (Not Modified) response.
952</t>
953<t>
954   If the request would, without the If-None-Match header field, result
955   in anything other than a 2xx or 304 status code, then the If-None-Match
956   header field &MUST; be ignored. (See <xref
957   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
958   a discussion of server behavior when both If-Modified-Since and
959   If-None-Match appear in the same request.)
960</t>
961<t>
962   Examples:
963</t>
964<figure><artwork type="example">
965  If-None-Match: "xyzzy"
966  If-None-Match: W/"xyzzy"
967  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
968  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
969  If-None-Match: *
970</artwork></figure>
971<t>
972   The result of a request having both an If-None-Match header field and
973   either an If-Match or an If-Unmodified-Since header fields is
974   undefined by this specification.
975</t>
976</section>
977
978<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
979  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
980  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
981  <x:anchor-alias value="If-Unmodified-Since"/>
982<t>
983   The "If-Unmodified-Since" header field &MAY; be used to make a request
984   method conditional by modification date: if the selected representation
985   has been modified since the time specified in this field, then the
986   server &MUST-NOT; perform the requested operation and &MUST; instead
987   respond with the 412 (Precondition Failed) status code.
988   If the selected representation has not been modified since the time
989   specified in this field, the server &SHOULD; perform the request
990   method as if the If-Unmodified-Since header field were not present.
991</t>
992<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
993  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
994</artwork></figure>
995<t>
996   An example of the field is:
997</t>
998<figure><artwork type="example">
999  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1000</artwork></figure>
1001<t>
1002   If the request normally (i.e., without the If-Unmodified-Since
1003   header field) would result in anything other than a 2xx or 412 status code,
1004   the If-Unmodified-Since header field &SHOULD; be ignored.
1005</t>
1006<t>
1007   If the specified date is invalid, the header field &MUST; be ignored.
1008</t>
1009<t>
1010   The result of a request having both an If-Unmodified-Since header
1011   field and either an If-None-Match or an If-Modified-Since header
1012   fields is undefined by this specification.
1013</t>
1014</section>
1015
1016<section title="Last-Modified" anchor="header.last-modified">
1017  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
1018  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
1019  <x:anchor-alias value="Last-Modified"/>
1020<t>
1021   The "Last-Modified" header field indicates the date and time at
1022   which the origin server believes the representation was last modified.
1023</t>
1024<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
1025  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
1026</artwork></figure>
1027<t>
1028   An example of its use is
1029</t>
1030<figure><artwork type="example">
1031  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
1032</artwork></figure>
1033<t>
1034   A representation is typically the sum of many parts behind the
1035   resource interface.  The last-modified time would usually be
1036   the most recent time that any of those parts were changed.
1037   How that value is determined for any given resource is an
1038   implementation detail beyond the scope of this specification.
1039   What matters to HTTP is how recipients of the Last-Modified
1040   header field can use its value to make conditional requests
1041   and test the validity of locally cached responses.
1042</t>
1043<t>
1044   An origin server &MUST-NOT; send a Last-Modified date which is later
1045   than the server's time of message origination. In such cases, where
1046   the resource's last modification would indicate some time in the
1047   future, the server &MUST; replace that date with the message
1048   origination date.
1049</t>
1050<t>
1051   An origin server &SHOULD; obtain the Last-Modified value of the representation
1052   as close as possible to the time that it generates the Date value of
1053   its response. This allows a recipient to make an accurate assessment
1054   of the representation's modification time, especially if the representation changes
1055   near the time that the response is generated.
1056</t>
1057<t>
1058   HTTP/1.1 servers &SHOULD; send Last-Modified whenever feasible.
1059</t>
1060<t>
1061   The Last-Modified header field value is often used as a cache
1062   validator. In simple terms, a cache entry is considered to be valid
1063   if the representation has not been modified since the Last-Modified value.
1064</t>
1065</section>
1066
1067</section>
1068
1069<section title="IANA Considerations" anchor="IANA.considerations">
1070
1071<section title="Status Code Registration" anchor="status.code.registration">
1072<t>
1073   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1074   shall be updated with the registrations below:
1075</t>
1076<?BEGININC p4-conditional.iana-status-codes ?>
1077<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1078<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1079   <ttcol>Value</ttcol>
1080   <ttcol>Description</ttcol>
1081   <ttcol>Reference</ttcol>
1082   <c>304</c>
1083   <c>Not Modified</c>
1084   <c>
1085      <xref target="status.304"/>
1086   </c>
1087   <c>412</c>
1088   <c>Precondition Failed</c>
1089   <c>
1090      <xref target="status.412"/>
1091   </c>
1092</texttable>
1093<!--(END)-->
1094<?ENDINC p4-conditional.iana-status-codes ?>
1095</section>
1096
1097<section title="Header Field Registration" anchor="header.field.registration">
1098<t>
1099   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1100   with the permanent registrations below (see <xref target="RFC3864"/>):
1101</t>
1102<?BEGININC p4-conditional.iana-headers ?>
1103<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1104<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1105   <ttcol>Header Field Name</ttcol>
1106   <ttcol>Protocol</ttcol>
1107   <ttcol>Status</ttcol>
1108   <ttcol>Reference</ttcol>
1109
1110   <c>ETag</c>
1111   <c>http</c>
1112   <c>standard</c>
1113   <c>
1114      <xref target="header.etag"/>
1115   </c>
1116   <c>If-Match</c>
1117   <c>http</c>
1118   <c>standard</c>
1119   <c>
1120      <xref target="header.if-match"/>
1121   </c>
1122   <c>If-Modified-Since</c>
1123   <c>http</c>
1124   <c>standard</c>
1125   <c>
1126      <xref target="header.if-modified-since"/>
1127   </c>
1128   <c>If-None-Match</c>
1129   <c>http</c>
1130   <c>standard</c>
1131   <c>
1132      <xref target="header.if-none-match"/>
1133   </c>
1134   <c>If-Unmodified-Since</c>
1135   <c>http</c>
1136   <c>standard</c>
1137   <c>
1138      <xref target="header.if-unmodified-since"/>
1139   </c>
1140   <c>Last-Modified</c>
1141   <c>http</c>
1142   <c>standard</c>
1143   <c>
1144      <xref target="header.last-modified"/>
1145   </c>
1146</texttable>
1147<!--(END)-->
1148<?ENDINC p4-conditional.iana-headers ?>
1149<t>
1150   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1151</t>
1152</section>
1153</section>
1154
1155<section title="Security Considerations" anchor="security.considerations">
1156<t>
1157   No additional security considerations have been identified beyond
1158   those applicable to HTTP in general &messaging;.
1159</t>
1160</section>
1161
1162<section title="Acknowledgments" anchor="ack">
1163</section>
1164</middle>
1165<back>
1166
1167<references title="Normative References">
1168
1169<reference anchor="Part1">
1170  <front>
1171    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1172    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1173      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1174      <address><email>fielding@gbiv.com</email></address>
1175    </author>
1176    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1177      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1178      <address><email>jg@freedesktop.org</email></address>
1179    </author>
1180    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1181      <organization abbrev="HP">Hewlett-Packard Company</organization>
1182      <address><email>JeffMogul@acm.org</email></address>
1183    </author>
1184    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1185      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1186      <address><email>henrikn@microsoft.com</email></address>
1187    </author>
1188    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1189      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1190      <address><email>LMM@acm.org</email></address>
1191    </author>
1192    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1193      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1194      <address><email>paulle@microsoft.com</email></address>
1195    </author>
1196    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1197      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1198      <address><email>timbl@w3.org</email></address>
1199    </author>
1200    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1201      <organization abbrev="W3C">World Wide Web Consortium</organization>
1202      <address><email>ylafon@w3.org</email></address>
1203    </author>
1204    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1205      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1206      <address><email>julian.reschke@greenbytes.de</email></address>
1207    </author>
1208    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1209  </front>
1210  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1211  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1212</reference>
1213
1214<reference anchor="Part3">
1215  <front>
1216    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1217    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1218      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1219      <address><email>fielding@gbiv.com</email></address>
1220    </author>
1221    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1222      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1223      <address><email>jg@freedesktop.org</email></address>
1224    </author>
1225    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1226      <organization abbrev="HP">Hewlett-Packard Company</organization>
1227      <address><email>JeffMogul@acm.org</email></address>
1228    </author>
1229    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1230      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1231      <address><email>henrikn@microsoft.com</email></address>
1232    </author>
1233    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1234      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1235      <address><email>LMM@acm.org</email></address>
1236    </author>
1237    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1238      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1239      <address><email>paulle@microsoft.com</email></address>
1240    </author>
1241    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1242      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1243      <address><email>timbl@w3.org</email></address>
1244    </author>
1245    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1246      <organization abbrev="W3C">World Wide Web Consortium</organization>
1247      <address><email>ylafon@w3.org</email></address>
1248    </author>
1249    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1250      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1251      <address><email>julian.reschke@greenbytes.de</email></address>
1252    </author>
1253    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1254  </front>
1255  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1256  <x:source href="p3-payload.xml" basename="p3-payload"/>
1257</reference>
1258
1259<reference anchor="Part5">
1260  <front>
1261    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1262    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1263      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1264      <address><email>fielding@gbiv.com</email></address>
1265    </author>
1266    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1267      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1268      <address><email>jg@freedesktop.org</email></address>
1269    </author>
1270    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1271      <organization abbrev="HP">Hewlett-Packard Company</organization>
1272      <address><email>JeffMogul@acm.org</email></address>
1273    </author>
1274    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1275      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1276      <address><email>henrikn@microsoft.com</email></address>
1277    </author>
1278    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1279      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1280      <address><email>LMM@acm.org</email></address>
1281    </author>
1282    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1283      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1284      <address><email>paulle@microsoft.com</email></address>
1285    </author>
1286    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1287      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1288      <address><email>timbl@w3.org</email></address>
1289    </author>
1290    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1291      <organization abbrev="W3C">World Wide Web Consortium</organization>
1292      <address><email>ylafon@w3.org</email></address>
1293    </author>
1294    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1295      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1296      <address><email>julian.reschke@greenbytes.de</email></address>
1297    </author>
1298    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1299  </front>
1300  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1301  <x:source href="p5-range.xml" basename="p5-range"/>
1302</reference>
1303
1304<reference anchor="Part6">
1305  <front>
1306    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1307    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1308      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1309      <address><email>fielding@gbiv.com</email></address>
1310    </author>
1311    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1312      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1313      <address><email>jg@freedesktop.org</email></address>
1314    </author>
1315    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1316      <organization abbrev="HP">Hewlett-Packard Company</organization>
1317      <address><email>JeffMogul@acm.org</email></address>
1318    </author>
1319    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1320      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1321      <address><email>henrikn@microsoft.com</email></address>
1322    </author>
1323    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1324      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1325      <address><email>LMM@acm.org</email></address>
1326    </author>
1327    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1328      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1329      <address><email>paulle@microsoft.com</email></address>
1330    </author>
1331    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1332      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1333      <address><email>timbl@w3.org</email></address>
1334    </author>
1335    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1336      <organization abbrev="W3C">World Wide Web Consortium</organization>
1337      <address><email>ylafon@w3.org</email></address>
1338    </author>
1339    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1340      <address><email>mnot@mnot.net</email></address>
1341    </author>
1342    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1343      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1344      <address><email>julian.reschke@greenbytes.de</email></address>
1345    </author>
1346    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1347  </front>
1348  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1349  <x:source href="p6-cache.xml" basename="p6-cache"/>
1350</reference>
1351
1352<reference anchor="RFC2119">
1353  <front>
1354    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1355    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1356      <organization>Harvard University</organization>
1357      <address><email>sob@harvard.edu</email></address>
1358    </author>
1359    <date month="March" year="1997"/>
1360  </front>
1361  <seriesInfo name="BCP" value="14"/>
1362  <seriesInfo name="RFC" value="2119"/>
1363</reference>
1364
1365<reference anchor="RFC5234">
1366  <front>
1367    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1368    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1369      <organization>Brandenburg InternetWorking</organization>
1370      <address>
1371        <email>dcrocker@bbiw.net</email>
1372      </address> 
1373    </author>
1374    <author initials="P." surname="Overell" fullname="Paul Overell">
1375      <organization>THUS plc.</organization>
1376      <address>
1377        <email>paul.overell@thus.net</email>
1378      </address>
1379    </author>
1380    <date month="January" year="2008"/>
1381  </front>
1382  <seriesInfo name="STD" value="68"/>
1383  <seriesInfo name="RFC" value="5234"/>
1384</reference>
1385
1386</references>
1387
1388<references title="Informative References">
1389
1390<reference anchor="RFC2616">
1391  <front>
1392    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1393    <author initials="R." surname="Fielding" fullname="R. Fielding">
1394      <organization>University of California, Irvine</organization>
1395      <address><email>fielding@ics.uci.edu</email></address>
1396    </author>
1397    <author initials="J." surname="Gettys" fullname="J. Gettys">
1398      <organization>W3C</organization>
1399      <address><email>jg@w3.org</email></address>
1400    </author>
1401    <author initials="J." surname="Mogul" fullname="J. Mogul">
1402      <organization>Compaq Computer Corporation</organization>
1403      <address><email>mogul@wrl.dec.com</email></address>
1404    </author>
1405    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1406      <organization>MIT Laboratory for Computer Science</organization>
1407      <address><email>frystyk@w3.org</email></address>
1408    </author>
1409    <author initials="L." surname="Masinter" fullname="L. Masinter">
1410      <organization>Xerox Corporation</organization>
1411      <address><email>masinter@parc.xerox.com</email></address>
1412    </author>
1413    <author initials="P." surname="Leach" fullname="P. Leach">
1414      <organization>Microsoft Corporation</organization>
1415      <address><email>paulle@microsoft.com</email></address>
1416    </author>
1417    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1418      <organization>W3C</organization>
1419      <address><email>timbl@w3.org</email></address>
1420    </author>
1421    <date month="June" year="1999"/>
1422  </front>
1423  <seriesInfo name="RFC" value="2616"/>
1424</reference>
1425
1426<reference anchor='RFC3864'>
1427  <front>
1428    <title>Registration Procedures for Message Header Fields</title>
1429    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1430      <organization>Nine by Nine</organization>
1431      <address><email>GK-IETF@ninebynine.org</email></address>
1432    </author>
1433    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1434      <organization>BEA Systems</organization>
1435      <address><email>mnot@pobox.com</email></address>
1436    </author>
1437    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1438      <organization>HP Labs</organization>
1439      <address><email>JeffMogul@acm.org</email></address>
1440    </author>
1441    <date year='2004' month='September' />
1442  </front>
1443  <seriesInfo name='BCP' value='90' />
1444  <seriesInfo name='RFC' value='3864' />
1445</reference>
1446
1447</references>
1448
1449<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1450<t>
1451  Allow weak entity-tags in all requests except range requests (Sections
1452  <xref target="weak.and.strong.validators" format="counter"/> and
1453  <xref target="header.if-none-match" format="counter"/>).
1454</t>
1455<t>
1456  Change ABNF productions for header fields to only define the field value.
1457  (<xref target="header.fields"/>)
1458</t>
1459</section>
1460
1461<?BEGININC p4-conditional.abnf-appendix ?>
1462<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1463<figure>
1464<artwork type="abnf" name="p4-conditional.parsed-abnf">
1465<x:ref>ETag</x:ref> = entity-tag
1466
1467<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1468
1469<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1470 entity-tag ] ) )
1471<x:ref>If-Modified-Since</x:ref> = HTTP-date
1472<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1473 entity-tag ] ) )
1474<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1475
1476<x:ref>Last-Modified</x:ref> = HTTP-date
1477
1478<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1479
1480<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1481
1482<x:ref>opaque-tag</x:ref> = quoted-string
1483
1484<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1485
1486<x:ref>weak</x:ref> = %x57.2F ; W/
1487</artwork>
1488</figure>
1489<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1490; ETag defined but not used
1491; If-Match defined but not used
1492; If-Modified-Since defined but not used
1493; If-None-Match defined but not used
1494; If-Unmodified-Since defined but not used
1495; Last-Modified defined but not used
1496</artwork></figure></section>
1497<?ENDINC p4-conditional.abnf-appendix ?>
1498
1499<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1500
1501<section title="Since RFC 2616">
1502<t>
1503  Extracted relevant partitions from <xref target="RFC2616"/>.
1504</t>
1505</section>
1506
1507<section title="Since draft-ietf-httpbis-p4-conditional-00">
1508<t>
1509  Closed issues:
1510  <list style="symbols"> 
1511    <t>
1512      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1513      "Normative and Informative references"
1514    </t>
1515  </list>
1516</t>
1517<t>
1518  Other changes:
1519  <list style="symbols"> 
1520    <t>
1521      Move definitions of 304 and 412 condition codes from Part2.
1522    </t>
1523  </list>
1524</t>
1525</section>
1526
1527<section title="Since draft-ietf-httpbis-p4-conditional-01">
1528<t>
1529  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1530  <list style="symbols"> 
1531    <t>
1532      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1533    </t>
1534  </list>
1535</t>
1536</section>
1537
1538<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1539<t>
1540  Closed issues:
1541  <list style="symbols"> 
1542    <t>
1543      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1544      "Weak ETags on non-GET requests"
1545    </t>
1546  </list>
1547</t>
1548<t>
1549  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1550  <list style="symbols"> 
1551    <t>
1552      Reference RFC 3984, and update header field registrations for header fields defined
1553      in this document.
1554    </t>
1555  </list>
1556</t>
1557</section>
1558
1559<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1560<t>
1561  Closed issues:
1562  <list style="symbols"> 
1563    <t>
1564      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1565      "Examples for ETag matching"
1566    </t>
1567    <t>
1568      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1569      "'entity value' undefined"
1570    </t>
1571    <t>
1572      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1573      "bogus 2068 Date header reference"
1574    </t>
1575  </list>
1576</t>
1577</section>
1578
1579<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1580<t>
1581  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1582  <list style="symbols"> 
1583    <t>
1584      Use "/" instead of "|" for alternatives.
1585    </t>
1586    <t>
1587      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1588      whitespace ("OWS") and required whitespace ("RWS").
1589    </t>
1590    <t>
1591      Rewrite ABNFs to spell out whitespace rules, factor out
1592      header field value format definitions.
1593    </t>
1594  </list>
1595</t>
1596</section>
1597
1598<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1599<t>
1600  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1601  <list style="symbols"> 
1602    <t>
1603      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1604    </t>
1605  </list>
1606</t>
1607</section>
1608
1609<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1610<t>
1611  Closed issues:
1612  <list style="symbols"> 
1613    <t>
1614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1615      "case-sensitivity of etag weakness indicator"
1616    </t>
1617  </list>
1618</t>
1619</section>
1620
1621<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1622<t>
1623  Closed issues:
1624  <list style="symbols"> 
1625    <t>
1626      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1627      "Weak ETags on non-GET requests" (If-Match still was defined to require
1628      strong matching)
1629    </t>
1630    <t>
1631      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1632      "move IANA registrations for optional status codes"
1633    </t>
1634  </list>
1635</t>
1636</section>
1637
1638<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1639<t>
1640  No significant changes.
1641</t>
1642</section>
1643
1644<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1645<t>
1646  No significant changes.
1647</t>
1648</section>
1649
1650<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1651<t>
1652  Closed issues:
1653  <list style="symbols"> 
1654    <t>
1655      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1656      "Clarify 'Requested Variant'"
1657    </t>
1658    <t>
1659      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1660      "Clarify entity / representation / variant terminology"
1661    </t>
1662    <t>
1663      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1664      "consider removing the 'changes from 2068' sections"
1665    </t>
1666  </list>
1667</t>
1668</section>
1669
1670<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1671<t>
1672  None.
1673</t>
1674</section>
1675
1676<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1677<t>
1678  Closed issues:
1679  <list style="symbols"> 
1680    <t>
1681      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1682      "Header Classification"
1683    </t>
1684  </list>
1685</t>
1686</section>
1687
1688<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1689<t>
1690  Closed issues:
1691  <list style="symbols"> 
1692    <t>
1693      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1694      "If-* and entities"
1695    </t>
1696    <t>
1697      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1698      "Definition of validator weakness"
1699    </t>
1700    <t>
1701      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1702      "untangle ABNFs for header fields"
1703    </t>
1704  </list>
1705</t>
1706</section>
1707
1708</section>
1709
1710</back>
1711</rfc>
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