source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1260

Last change on this file since 1260 was 1260, checked in by fielding@…, 9 years ago

editorial: rearrange p4 a bit more to split discussion of last-modified
from that of entity-tags, move entity-tags definition within the ETag
header field, and introduce sections for requirements on generation
and comparison of each (separately). This roughly halves the
cognitive complexity of the existing descriptions and allows us
to get rid of a lot of duplication (with more to come).

Moved some stray requirements on range requests to p5.

  • Property svn:eol-style set to native
File size: 69.8 KB
Line 
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
224   can also be
225   applied to state-changing methods, such as PUT and DELETE, to prevent
226   the "lost update" problem: one client accidentally overwriting
227   the work of another client that has been acting in parallel.
228</t>
229<t>
230   Conditional request preconditions are based on the state of the target
231   resource as a whole (its current value set) or the state as observed
232   in a previously obtained representation (one value in that set).
233   A resource might have multiple current representations, each with its
234   own observable state.  The conditional request mechanisms assume that
235   the mapping of requests to corresponding representations will be
236   consistent over time if the server intends to take advantage of
237   conditionals.  Regardless, if the mapping is inconsistent and
238   the server is unable to select the appropriate representation, then
239   no harm will result when the precondition evaluates to false.
240</t>
241<t><iref primary="true" item="selected representation"/>
242   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
243   the current representation of the target resource that would have been
244   selected in a successful response if the same request had used the method
245   GET and had excluded all of the conditional request header fields.
246   The conditional request preconditions are evaluated by comparing the
247   values provided in the request header fields to the current metadata
248   for the selected representation.
249</t>
250
251<section title="Requirements" anchor="intro.requirements">
252<t>
253   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
254   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
255   document are to be interpreted as described in <xref target="RFC2119"/>.
256</t>
257<t>
258   An implementation is not compliant if it fails to satisfy one or more
259   of the "MUST" or "REQUIRED" level requirements for the protocols it
260   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
261   level and all the "SHOULD" level requirements for its protocols is said
262   to be "unconditionally compliant"; one that satisfies all the "MUST"
263   level requirements but not all the "SHOULD" level requirements for its
264   protocols is said to be "conditionally compliant".
265</t>
266</section>
267
268<section title="Syntax Notation" anchor="notation">
269  <x:anchor-alias value="ALPHA"/>
270  <x:anchor-alias value="CR"/>
271  <x:anchor-alias value="DIGIT"/>
272  <x:anchor-alias value="LF"/>
273  <x:anchor-alias value="OCTET"/>
274  <x:anchor-alias value="VCHAR"/>
275  <x:anchor-alias value="WSP"/>
276  <x:anchor-alias value="core.rules"/>
277  <x:anchor-alias value="quoted-string"/>
278  <x:anchor-alias value="OWS"/>
279  <x:anchor-alias value="HTTP-date"/>
280<t>
281  This specification uses the ABNF syntax defined in &notation; (which
282  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
283  <xref target="collected.abnf"/> shows the collected ABNF, with the list
284  rule expanded.
285</t>
286<t>
287  The following core rules are included by
288  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
289  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
290  DIGIT (decimal 0-9), DQUOTE (double quote),
291  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
292  OCTET (any 8-bit sequence of data), SP (space),
293  VCHAR (any visible USASCII character),
294  and WSP (whitespace).
295</t>
296<t>
297  The ABNF rules below are defined in other parts:
298</t>
299<figure><artwork type="abnf2616">
300  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
301  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
302  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
303</artwork></figure>
304</section>
305</section>
306
307<section title="Resource State Metadata (Validators)" anchor="resource.metadata">
308   <iref primary="true" item="metadata"/>
309   <iref primary="true" item="validator"/>
310<t>
311   This specification defines two forms of metadata that are commonly used
312   to observe resource state and test for preconditions: modification dates
313   and opaque entity tags.  Additional metadata that reflects resource state
314   has been defined by various extensions of HTTP, such as WebDAV
315   <xref target="RFC4918"/>, that are beyond the scope of this specification.
316   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
317   when it is used within a precondition.
318</t>
319
320<section title="Last-Modified" anchor="header.last-modified">
321  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
322  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
323  <x:anchor-alias value="Last-Modified"/>
324<t>
325   The "Last-Modified" header field indicates the date and time at
326   which the origin server believes the selected representation was
327   last modified.
328</t>
329<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
330  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
331</artwork></figure>
332<t>
333   An example of its use is
334</t>
335<figure><artwork type="example">
336  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
337</artwork></figure>
338
339<section title="Generation" anchor="lastmod.generation">
340<t>
341   Origin servers &SHOULD; send Last-Modified for any selected
342   representation for which a last modification date can be reasonably
343   and consistently determined, since its use in conditional requests
344   and evaluating cache freshness (&caching;) results in a substantial
345   reduction of HTTP traffic on the Internet and can be a significant
346   factor in improving service scalability and reliability.
347</t>
348<t>
349   A representation is typically the sum of many parts behind the
350   resource interface.  The last-modified time would usually be
351   the most recent time that any of those parts were changed.
352   How that value is determined for any given resource is an
353   implementation detail beyond the scope of this specification.
354   What matters to HTTP is how recipients of the Last-Modified
355   header field can use its value to make conditional requests
356   and test the validity of locally cached responses.
357</t>
358<t>
359   An origin server &SHOULD; obtain the Last-Modified value of the
360   representation as close as possible to the time that it generates
361   the Date field-value for its response. This allows a recipient to
362   make an accurate assessment of the representation's modification time,
363   especially if the representation changes near the time that the
364   response is generated.
365</t>
366<t>
367   An origin server with a clock &MUST-NOT; send a Last-Modified date
368   that is later than the server's time of message origination (Date).
369   If the last modification time is derived from implementation-specific
370   metadata that evaluates to some time in the future, according to the
371   origin server's clock, then the origin server &MUST; replace that
372   value with the message origination date. This prevents a future
373   modification date from having an adverse impact on cache validation.
374</t>
375</section>
376
377<section title="Comparison" anchor="lastmod.comparison">
378<t>
379   A Last-Modified time, when used as a validator in a request, is
380   implicitly weak unless it is possible to deduce that it is strong,
381   using the following rules:
382  <list style="symbols">
383     <t>The validator is being compared by an origin server to the
384        actual current validator for the representation and,</t>
385     <t>That origin server reliably knows that the associated representation did
386        not change twice during the second covered by the presented
387        validator.</t>
388  </list>
389</t>
390<t>
391   or
392  <list style="symbols">
393     <t>The validator is about to be used by a client in an If-Modified-Since
394        or If-Unmodified-Since header field, because the client
395        has a cache entry for the associated representation, and</t>
396     <t>That cache entry includes a Date value, which gives the time
397        when the origin server sent the original response, and</t>
398     <t>The presented Last-Modified time is at least 60 seconds before
399        the Date value.</t>
400  </list>
401</t>
402<t>
403   or
404  <list style="symbols">
405     <t>The validator is being compared by an intermediate cache to the
406        validator stored in its cache entry for the representation, and</t>
407     <t>That cache entry includes a Date value, which gives the time
408        when the origin server sent the original response, and</t>
409     <t>The presented Last-Modified time is at least 60 seconds before
410        the Date value.</t>
411  </list>
412</t>
413<t>
414   This method relies on the fact that if two different responses were
415   sent by the origin server during the same second, but both had the
416   same Last-Modified time, then at least one of those responses would
417   have a Date value equal to its Last-Modified time. The arbitrary 60-second
418   limit guards against the possibility that the Date and Last-Modified
419   values are generated from different clocks, or at somewhat
420   different times during the preparation of the response. An
421   implementation &MAY; use a value larger than 60 seconds, if it is
422   believed that 60 seconds is too short.
423</t>
424</section>
425</section>
426
427<section title="ETag" anchor="header.etag">
428  <iref primary="true" item="ETag header field" x:for-anchor=""/>
429  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
430  <x:anchor-alias value="ETag"/>
431  <x:anchor-alias value="entity-tag"/>
432  <x:anchor-alias value="entity.tags"/>
433  <x:anchor-alias value="opaque-tag"/>
434  <x:anchor-alias value="weak"/>
435<t>
436   The ETag header field provides the current entity-tag for the
437   selected representation.
438   An entity-tag is an opaque validator for differentiating between
439   multiple representations of the same resource, regardless of whether
440   those multiple representations are due to resource state changes over
441   time, content negotiation resulting in multiple representations being
442   valid at the same time, or both. An entity-tag consists of an opaque
443   quoted string, possibly prefixed by a weakness indicator.
444</t>
445<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ETag"/><iref primary="true" item="Grammar" subitem="entity-tag"/><iref primary="true" item="Grammar" subitem="weak"/><iref primary="true" item="Grammar" subitem="opaque-tag"/>
446  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
447
448  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
449  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
450  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
451</artwork></figure>
452<t>
453   An entity-tag can be more reliable for validation than a modification
454   date in situations where it is inconvenient to store modification
455   dates, where the one-second resolution of HTTP date values is not
456   sufficient, or where modification dates are not consistently maintained.
457</t>
458<figure><preamble>
459  Examples:
460</preamble>
461<artwork type="example">
462  ETag: "xyzzy"
463  ETag: W/"xyzzy"
464  ETag: ""
465</artwork></figure>
466
467<section title="Generation" anchor="entity.tag.generation">
468<t>
469   The principle behind entity-tags is that only the service author
470   knows the implementation of a resource well enough to select the
471   most accurate and efficient validation mechanism for that resource,
472   and that any such mechanism can be mapped to a simple sequence of
473   octets for easy comparison.  Since the value is opaque, there is no
474   need for the client to be aware of how each entity-tag is constructed.
475</t>
476<t>
477   For example, a resource that has implementation-specific versioning
478   applied to all changes might use an internal revision number, perhaps
479   combined with a variance identifier for content negotiation, to
480   accurately differentiate between representations.
481   Other implementations might use a stored hash of representation content,
482   a combination of various filesystem attributes, or a modification
483   timestamp that has sub-second resolution.
484</t>
485<t>
486   Origin servers &SHOULD; send ETag for any selected representation
487   for which detection of changes can be reasonably and consistently
488   determined, since the entity-tag's use in conditional requests and
489   evaluating cache freshness (&caching;) can result in a substantial
490   reduction of HTTP network traffic and can be a significant factor in
491   improving service scalability and reliability.
492</t>
493</section>
494
495<section title="Weak versus Strong" anchor="weak.and.strong.validators">
496<t>
497   Since both origin servers and caches will compare two validators to
498   decide if they indicate the same or different representations, one
499   normally would expect that if the representation (including both
500   representation header fields and representation body) changes in any
501   way, then the associated validator would change as well. If this is
502   true, then we call that validator a "strong validator".  One example
503   of a strong validator is an integer that is incremented in stable
504   storage every time a representation is changed.
505</t>
506<t>
507   However, there might be cases when a server prefers to change the
508   validator only when it desires cached representations to be invalidated.
509   For example, the representation of a weather report that changes in
510   content every second, based on dynamic measurements, might be grouped
511   into sets of equivalent representations (from the origin server's
512   perspective) in order to allow cached representations to be valid
513   for a reasonable period of time (perhaps adjusted dynamically based
514   on server load or weather quality).
515   A validator that does not always change when the representation
516   changes is a "weak validator".
517</t>
518<t>
519   One can think of a strong validator as part of an identifier for a
520   specific representation, whereas a weak validator is part of an
521   identifier for a set of equivalent representations (where this notion
522   of equivalence is entirely governed by the origin server and beyond
523   the scope of this specification).
524</t>
525<t>
526   An entity-tag is normally a strong validator, but the protocol
527   provides a mechanism to tag an entity-tag as "weak".
528  <list><t>
529      A representation's modification time, if defined with only one-second
530      resolution, could be a weak validator, since it is possible that
531      the representation might be modified twice during a single second.
532    </t><t>
533      Support for weak validators is optional. However, weak validators
534      allow for more efficient caching of equivalent objects; for
535      example, a hit counter on a site is probably good enough if it is
536      updated every few days or weeks, and any value during that period
537      is likely "good enough" to be equivalent.
538    </t></list>
539</t>
540<t>
541   A strong entity-tag &MUST; change whenever the associated representation
542   changes in any way. A weak entity-tag &SHOULD; change whenever the origin
543   server considers prior representations to be unacceptable as a substitute
544   for the current representation. In other words, a weak entity tag &SHOULD;
545   change whenever the origin server wants caches to invalidate old responses.
546</t>
547<t>
548   A "strong entity-tag" &MAY; be shared by two representations of a resource
549   only if they are equivalent by octet equality.
550</t>
551<t>
552   A "weak entity-tag", indicated by the "W/" prefix, &MAY; be shared by
553   two representations of a resource. A weak entity-tag can only be used
554   for weak comparison.
555</t>
556<t>
557   Cache entries might persist for arbitrarily long periods, regardless
558   of expiration times.  Thus, a cache might attempt to validate an
559   entry using a validator that it obtained in the distant past.
560   A strong entity-tag &MUST; be unique across all versions of all
561   representations associated with a particular resource over time.
562   However, there is no implication of uniqueness across entity-tags
563   of different resources (i.e., the same entity-tag value might be
564   in use for representations of multiple resources at the same time
565   and does not imply that those representations are equivalent).
566</t>
567</section>
568
569<section title="Comparison" anchor="entity.tag.comparison">
570  <x:anchor-alias value="validator.comparison"/>
571<t>
572   There are two entity-tag comparison functions, depending
573   on whether the comparison context allows the use of weak validators
574   or not:
575  <list style="symbols">
576     <t>The strong comparison function: in order to be considered equal,
577        both opaque-tags &MUST; be identical character-by-character, and both
578        &MUST-NOT; be weak.</t>
579     <t>The weak comparison function: in order to be considered equal, both
580        opaque-tags &MUST; be identical character-by-character, but
581        either or both of them &MAY; be tagged as "weak" without affecting
582        the result.</t>
583  </list>
584</t>
585<t>
586   A "use" of a validator is either when a client generates a request
587   and includes the validator in a precondition, or when a server
588   compares two validators.
589</t>
590<t>
591   Strong validators are usable in any context. Weak validators are only
592   usable in contexts that do not depend on exact equality of a representation.
593   For example, either kind is usable for a normal conditional GET.
594</t>
595<t>
596   The example below shows the results for a set of entity-tag pairs,
597   and both the weak and strong comparison function results:
598</t>
599<texttable align="left">
600  <ttcol>ETag 1</ttcol>
601  <ttcol>ETag 2</ttcol>
602  <ttcol>Strong Comparison</ttcol>
603  <ttcol>Weak Comparison</ttcol>
604
605  <c>W/"1"</c>
606  <c>W/"1"</c>
607  <c>no match</c>
608  <c>match</c>
609 
610  <c>W/"1"</c>
611  <c>W/"2"</c>
612  <c>no match</c>
613  <c>no match</c>
614
615  <c>W/"1"</c>
616  <c>"1"</c>
617  <c>no match</c>
618  <c>match</c>
619
620  <c>"1"</c>
621  <c>"1"</c>
622  <c>match</c>
623  <c>match</c>
624</texttable>
625<t>
626   An entity-tag is strong unless it is explicitly tagged as weak.
627</t>
628</section>
629
630<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">
631<t>
632   We adopt a set of rules and recommendations for origin servers,
633   clients, and caches regarding when various validator types ought to
634   be used, and for what purposes.
635</t>
636<t>
637   HTTP/1.1 origin servers:
638  <list style="symbols">
639     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
640        generate one.</t>
641
642     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
643        performance considerations support the use of weak entity-tags,
644        or if it is unfeasible to send a strong entity-tag.</t>
645
646     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
647  </list>
648</t>
649<t>
650   In other words, the preferred behavior for an HTTP/1.1 origin server
651   is to send both a strong entity-tag and a Last-Modified value.
652</t>
653<t>
654   HTTP/1.1 clients:
655  <list style="symbols">
656     <t>&MUST; use that entity-tag in any cache-conditional request (using
657        If-Match or If-None-Match) if an entity-tag has been provided by the
658        origin server.</t>
659
660     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
661        requests (using If-Modified-Since) if only a Last-Modified value has
662        been provided by the origin server. </t>
663
664     <t>&MAY; use the Last-Modified value in subrange cache-conditional
665        requests (using If-Unmodified-Since) if only a Last-Modified value has
666        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
667        provide a way to disable this, in case of difficulty.</t>
668
669     <t>&SHOULD; use both validators in cache-conditional requests if both an
670        entity-tag and a Last-Modified value have been provided by the origin
671        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
672        appropriately.</t>
673  </list>
674</t>
675<t>
676   An HTTP/1.1 origin server, upon receiving a conditional request that
677   includes both a Last-Modified date (e.g., in an If-Modified-Since or
678   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
679   in an If-Match, If-None-Match, or If-Range header field) as cache
680   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
681   unless doing so is consistent with all of the conditional header
682   fields in the request.
683</t>
684<t>
685   An HTTP/1.1 caching proxy, upon receiving a conditional request that
686   includes both a Last-Modified date and one or more entity-tags as
687   cache validators, &MUST-NOT; return a locally cached response to the
688   client unless that cached response is consistent with all of the
689   conditional header fields in the request.
690  <list><t>
691      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
692      servers and clients ought to transmit as much non-redundant
693      information as is available in their responses and requests.
694      HTTP/1.1 systems receiving this information will make the most
695      conservative assumptions about the validators they receive.
696  </t><t>
697      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
698      last-modified values received or used by these systems will
699      support transparent and efficient caching, and so HTTP/1.1 origin
700      servers should provide Last-Modified values. In those rare cases
701      where the use of a Last-Modified value as a validator by an
702      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
703      origin servers should not provide one.
704  </t></list>
705</t>
706</section>
707
708<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
709<t>
710   Consider a resource that is subject to content negotiation (&content-negotiation;),
711   and where the representations returned upon a GET request vary based on
712   the Accept-Encoding request header field (&header-accept-encoding;):
713</t>
714<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
715GET /index HTTP/1.1
716Host: www.example.com
717Accept-Encoding: gzip
718
719</artwork></figure>
720<t>
721   In this case, the response might or might not use the gzip content coding.
722   If it does not, the response might look like:
723</t>
724<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
725HTTP/1.1 200 OK
726Date: Thu, 26 Mar 2010 00:05:00 GMT
727ETag: "123-a"
728Content-Length: <x:length-of target="exbody"/>
729Vary: Accept-Encoding
730Content-Type: text/plain
731
732<x:span anchor="exbody">Hello World!
733Hello World!
734Hello World!
735Hello World!
736Hello World!
737</x:span></artwork></figure>
738<t>
739   An alternative representation that does use gzip content coding would be:
740</t>
741<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
742HTTP/1.1 200 OK
743Date: Thu, 26 Mar 2010 00:05:00 GMT
744ETag: "123-b"
745Content-Length: 43
746Vary: Accept-Encoding
747Content-Type: text/plain
748Content-Encoding: gzip
749
750<spanx>...binary data...</spanx></artwork></figure>
751<x:note>
752  <t>
753    <x:h>Note:</x:h> Content codings are a property of the representation,
754    so therefore an entity-tag of an encoded representation must be distinct
755    from an unencoded representation to prevent conflicts during cache updates
756    and range requests.  In contrast, transfer codings (&transfer-codings;)
757    apply only during message transfer and do not require distinct entity-tags.
758  </t>
759</x:note>
760</section>
761</section>
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 title="If-Range" anchor="header.if-range">
1012<t>
1013   The If-Range header field provides a special conditional request
1014   mechanism that is similar to If-Match and If-Unmodified-Since but
1015   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1016</t>
1017</section>
1018
1019</section>
1020
1021<section title="Status Code Definitions" anchor="status.code.definitions">
1022<section title="304 Not Modified" anchor="status.304">
1023  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1024  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1025<t>
1026   The 304 status code indicates that a conditional GET request has been
1027   received and would have resulted in a 200 (OK) response if it were not
1028   for the fact that the condition has evaluated to false.  In other words,
1029   there is no need for the server to transfer a representation of the
1030   target resource because the client's request indicates that it already
1031   has a valid representation, as indicated by the 304 response header
1032   fields, and is therefore redirecting the client to make use of that
1033   stored representation as if it were the payload of a 200 response.
1034   The 304 response &MUST-NOT; contain a message-body, and thus is always
1035   terminated by the first empty line after the header fields.
1036</t>
1037<t>
1038   A 304 response &MUST; include a Date header field (&header-date;)
1039   unless its omission is required by &clockless;.  If a 200 response
1040   to the same request would have included any of the header fields
1041   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1042   Vary, then those same header fields &MUST; be sent in a 304 response.
1043</t>
1044<t>
1045   Since the goal of a 304 response is to minimize information transfer
1046   when the recipient already has one or more cached representations,
1047   the response &SHOULD-NOT; include representation metadata other
1048   than the above listed fields unless said metadata exists for the
1049   purpose of guiding cache updates (e.g., future HTTP extensions).
1050</t>
1051<t>
1052   If the recipient of a 304 response does not have a cached representation
1053   corresponding to the entity-tag indicated by the 304 response, then the
1054   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1055   conditional request originated with an outbound client, such as a
1056   user agent with its own cache sending a conditional GET to a shared
1057   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1058   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1059   the request without any preconditions.
1060</t>
1061<t>
1062   If a cache uses a received 304 response to update a cache entry, the
1063   cache &MUST; update the entry to reflect any new field values given in
1064   the response.
1065</t>
1066</section>
1067
1068<section title="412 Precondition Failed" anchor="status.412">
1069  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1070  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1071<t>
1072   The 412 status code indicates that one or more preconditions given in
1073   the request header fields evaluated to false when tested on the server.
1074   This response code allows the client to place preconditions on the
1075   current resource state (its current representations and metadata)
1076   and thus prevent the request method from being applied if the target
1077   resource is in an unexpected state.
1078</t>
1079</section>
1080</section>
1081
1082<section title="IANA Considerations" anchor="IANA.considerations">
1083
1084<section title="Status Code Registration" anchor="status.code.registration">
1085<t>
1086   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1087   shall be updated with the registrations below:
1088</t>
1089<?BEGININC p4-conditional.iana-status-codes ?>
1090<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1091<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1092   <ttcol>Value</ttcol>
1093   <ttcol>Description</ttcol>
1094   <ttcol>Reference</ttcol>
1095   <c>304</c>
1096   <c>Not Modified</c>
1097   <c>
1098      <xref target="status.304"/>
1099   </c>
1100   <c>412</c>
1101   <c>Precondition Failed</c>
1102   <c>
1103      <xref target="status.412"/>
1104   </c>
1105</texttable>
1106<!--(END)-->
1107<?ENDINC p4-conditional.iana-status-codes ?>
1108</section>
1109
1110<section title="Header Field Registration" anchor="header.field.registration">
1111<t>
1112   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1113   with the permanent registrations below (see <xref target="RFC3864"/>):
1114</t>
1115<?BEGININC p4-conditional.iana-headers ?>
1116<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1117<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1118   <ttcol>Header Field Name</ttcol>
1119   <ttcol>Protocol</ttcol>
1120   <ttcol>Status</ttcol>
1121   <ttcol>Reference</ttcol>
1122
1123   <c>ETag</c>
1124   <c>http</c>
1125   <c>standard</c>
1126   <c>
1127      <xref target="header.etag"/>
1128   </c>
1129   <c>If-Match</c>
1130   <c>http</c>
1131   <c>standard</c>
1132   <c>
1133      <xref target="header.if-match"/>
1134   </c>
1135   <c>If-Modified-Since</c>
1136   <c>http</c>
1137   <c>standard</c>
1138   <c>
1139      <xref target="header.if-modified-since"/>
1140   </c>
1141   <c>If-None-Match</c>
1142   <c>http</c>
1143   <c>standard</c>
1144   <c>
1145      <xref target="header.if-none-match"/>
1146   </c>
1147   <c>If-Unmodified-Since</c>
1148   <c>http</c>
1149   <c>standard</c>
1150   <c>
1151      <xref target="header.if-unmodified-since"/>
1152   </c>
1153   <c>Last-Modified</c>
1154   <c>http</c>
1155   <c>standard</c>
1156   <c>
1157      <xref target="header.last-modified"/>
1158   </c>
1159</texttable>
1160<!--(END)-->
1161<?ENDINC p4-conditional.iana-headers ?>
1162<t>
1163   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1164</t>
1165</section>
1166</section>
1167
1168<section title="Security Considerations" anchor="security.considerations">
1169<t>
1170   No additional security considerations have been identified beyond
1171   those applicable to HTTP in general &messaging;.
1172</t>
1173</section>
1174
1175<section title="Acknowledgments" anchor="ack">
1176</section>
1177</middle>
1178<back>
1179
1180<references title="Normative References">
1181
1182<reference anchor="Part1">
1183  <front>
1184    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1185    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1186      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1187      <address><email>fielding@gbiv.com</email></address>
1188    </author>
1189    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1190      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1191      <address><email>jg@freedesktop.org</email></address>
1192    </author>
1193    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1194      <organization abbrev="HP">Hewlett-Packard Company</organization>
1195      <address><email>JeffMogul@acm.org</email></address>
1196    </author>
1197    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1198      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1199      <address><email>henrikn@microsoft.com</email></address>
1200    </author>
1201    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1202      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1203      <address><email>LMM@acm.org</email></address>
1204    </author>
1205    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1206      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1207      <address><email>paulle@microsoft.com</email></address>
1208    </author>
1209    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1210      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1211      <address><email>timbl@w3.org</email></address>
1212    </author>
1213    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1214      <organization abbrev="W3C">World Wide Web Consortium</organization>
1215      <address><email>ylafon@w3.org</email></address>
1216    </author>
1217    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1218      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1219      <address><email>julian.reschke@greenbytes.de</email></address>
1220    </author>
1221    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1222  </front>
1223  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1224  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1225</reference>
1226
1227<reference anchor="Part3">
1228  <front>
1229    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1230    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1231      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1232      <address><email>fielding@gbiv.com</email></address>
1233    </author>
1234    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1235      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1236      <address><email>jg@freedesktop.org</email></address>
1237    </author>
1238    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1239      <organization abbrev="HP">Hewlett-Packard Company</organization>
1240      <address><email>JeffMogul@acm.org</email></address>
1241    </author>
1242    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1243      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1244      <address><email>henrikn@microsoft.com</email></address>
1245    </author>
1246    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1247      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1248      <address><email>LMM@acm.org</email></address>
1249    </author>
1250    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1251      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1252      <address><email>paulle@microsoft.com</email></address>
1253    </author>
1254    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1255      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1256      <address><email>timbl@w3.org</email></address>
1257    </author>
1258    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1259      <organization abbrev="W3C">World Wide Web Consortium</organization>
1260      <address><email>ylafon@w3.org</email></address>
1261    </author>
1262    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1263      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1264      <address><email>julian.reschke@greenbytes.de</email></address>
1265    </author>
1266    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1267  </front>
1268  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1269  <x:source href="p3-payload.xml" basename="p3-payload"/>
1270</reference>
1271
1272<reference anchor="Part5">
1273  <front>
1274    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1275    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1276      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1277      <address><email>fielding@gbiv.com</email></address>
1278    </author>
1279    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1280      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1281      <address><email>jg@freedesktop.org</email></address>
1282    </author>
1283    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1284      <organization abbrev="HP">Hewlett-Packard Company</organization>
1285      <address><email>JeffMogul@acm.org</email></address>
1286    </author>
1287    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1288      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1289      <address><email>henrikn@microsoft.com</email></address>
1290    </author>
1291    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1292      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1293      <address><email>LMM@acm.org</email></address>
1294    </author>
1295    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1296      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1297      <address><email>paulle@microsoft.com</email></address>
1298    </author>
1299    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1300      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1301      <address><email>timbl@w3.org</email></address>
1302    </author>
1303    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1304      <organization abbrev="W3C">World Wide Web Consortium</organization>
1305      <address><email>ylafon@w3.org</email></address>
1306    </author>
1307    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1308      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1309      <address><email>julian.reschke@greenbytes.de</email></address>
1310    </author>
1311    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1312  </front>
1313  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1314  <x:source href="p5-range.xml" basename="p5-range"/>
1315</reference>
1316
1317<reference anchor="Part6">
1318  <front>
1319    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1320    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1321      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1322      <address><email>fielding@gbiv.com</email></address>
1323    </author>
1324    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1325      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1326      <address><email>jg@freedesktop.org</email></address>
1327    </author>
1328    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1329      <organization abbrev="HP">Hewlett-Packard Company</organization>
1330      <address><email>JeffMogul@acm.org</email></address>
1331    </author>
1332    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1333      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1334      <address><email>henrikn@microsoft.com</email></address>
1335    </author>
1336    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1337      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1338      <address><email>LMM@acm.org</email></address>
1339    </author>
1340    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1341      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1342      <address><email>paulle@microsoft.com</email></address>
1343    </author>
1344    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1345      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1346      <address><email>timbl@w3.org</email></address>
1347    </author>
1348    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1349      <organization abbrev="W3C">World Wide Web Consortium</organization>
1350      <address><email>ylafon@w3.org</email></address>
1351    </author>
1352    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1353      <address><email>mnot@mnot.net</email></address>
1354    </author>
1355    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1356      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1357      <address><email>julian.reschke@greenbytes.de</email></address>
1358    </author>
1359    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1360  </front>
1361  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1362  <x:source href="p6-cache.xml" basename="p6-cache"/>
1363</reference>
1364
1365<reference anchor="RFC2119">
1366  <front>
1367    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1368    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1369      <organization>Harvard University</organization>
1370      <address><email>sob@harvard.edu</email></address>
1371    </author>
1372    <date month="March" year="1997"/>
1373  </front>
1374  <seriesInfo name="BCP" value="14"/>
1375  <seriesInfo name="RFC" value="2119"/>
1376</reference>
1377
1378<reference anchor="RFC5234">
1379  <front>
1380    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1381    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1382      <organization>Brandenburg InternetWorking</organization>
1383      <address>
1384        <email>dcrocker@bbiw.net</email>
1385      </address> 
1386    </author>
1387    <author initials="P." surname="Overell" fullname="Paul Overell">
1388      <organization>THUS plc.</organization>
1389      <address>
1390        <email>paul.overell@thus.net</email>
1391      </address>
1392    </author>
1393    <date month="January" year="2008"/>
1394  </front>
1395  <seriesInfo name="STD" value="68"/>
1396  <seriesInfo name="RFC" value="5234"/>
1397</reference>
1398
1399</references>
1400
1401<references title="Informative References">
1402
1403<reference anchor="RFC2616">
1404  <front>
1405    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1406    <author initials="R." surname="Fielding" fullname="R. Fielding">
1407      <organization>University of California, Irvine</organization>
1408      <address><email>fielding@ics.uci.edu</email></address>
1409    </author>
1410    <author initials="J." surname="Gettys" fullname="J. Gettys">
1411      <organization>W3C</organization>
1412      <address><email>jg@w3.org</email></address>
1413    </author>
1414    <author initials="J." surname="Mogul" fullname="J. Mogul">
1415      <organization>Compaq Computer Corporation</organization>
1416      <address><email>mogul@wrl.dec.com</email></address>
1417    </author>
1418    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1419      <organization>MIT Laboratory for Computer Science</organization>
1420      <address><email>frystyk@w3.org</email></address>
1421    </author>
1422    <author initials="L." surname="Masinter" fullname="L. Masinter">
1423      <organization>Xerox Corporation</organization>
1424      <address><email>masinter@parc.xerox.com</email></address>
1425    </author>
1426    <author initials="P." surname="Leach" fullname="P. Leach">
1427      <organization>Microsoft Corporation</organization>
1428      <address><email>paulle@microsoft.com</email></address>
1429    </author>
1430    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1431      <organization>W3C</organization>
1432      <address><email>timbl@w3.org</email></address>
1433    </author>
1434    <date month="June" year="1999"/>
1435  </front>
1436  <seriesInfo name="RFC" value="2616"/>
1437</reference>
1438
1439<reference anchor='RFC3864'>
1440  <front>
1441    <title>Registration Procedures for Message Header Fields</title>
1442    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1443      <organization>Nine by Nine</organization>
1444      <address><email>GK-IETF@ninebynine.org</email></address>
1445    </author>
1446    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1447      <organization>BEA Systems</organization>
1448      <address><email>mnot@pobox.com</email></address>
1449    </author>
1450    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1451      <organization>HP Labs</organization>
1452      <address><email>JeffMogul@acm.org</email></address>
1453    </author>
1454    <date year='2004' month='September' />
1455  </front>
1456  <seriesInfo name='BCP' value='90' />
1457  <seriesInfo name='RFC' value='3864' />
1458</reference>
1459
1460<reference anchor='RFC4918'>
1461  <front>
1462    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1463    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1464      <organization abbrev="CommerceNet">CommerceNet</organization>
1465      <address><email>ldusseault@commerce.net</email></address>
1466    </author>
1467    <date month="June" year="2007" />
1468  </front>
1469  <seriesInfo name='RFC' value='4918' />
1470</reference>
1471</references>
1472
1473<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1474<t>
1475  Allow weak entity-tags in all requests except range requests (Sections
1476  <xref target="weak.and.strong.validators" format="counter"/> and
1477  <xref target="header.if-none-match" format="counter"/>).
1478</t>
1479<t>
1480  Change ABNF productions for header fields to only define the field value.
1481  (<xref target="header.fields"/>)
1482</t>
1483</section>
1484
1485<?BEGININC p4-conditional.abnf-appendix ?>
1486<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1487<figure>
1488<artwork type="abnf" name="p4-conditional.parsed-abnf">
1489<x:ref>ETag</x:ref> = entity-tag
1490
1491<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1492
1493<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1494 entity-tag ] ) )
1495<x:ref>If-Modified-Since</x:ref> = HTTP-date
1496<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1497 entity-tag ] ) )
1498<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1499
1500<x:ref>Last-Modified</x:ref> = HTTP-date
1501
1502<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1503
1504<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1505
1506<x:ref>opaque-tag</x:ref> = quoted-string
1507
1508<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1509
1510<x:ref>weak</x:ref> = %x57.2F ; W/
1511</artwork>
1512</figure>
1513<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1514; ETag defined but not used
1515; If-Match defined but not used
1516; If-Modified-Since defined but not used
1517; If-None-Match defined but not used
1518; If-Unmodified-Since defined but not used
1519; Last-Modified defined but not used
1520</artwork></figure></section>
1521<?ENDINC p4-conditional.abnf-appendix ?>
1522
1523<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1524
1525<section title="Since RFC 2616">
1526<t>
1527  Extracted relevant partitions from <xref target="RFC2616"/>.
1528</t>
1529</section>
1530
1531<section title="Since draft-ietf-httpbis-p4-conditional-00">
1532<t>
1533  Closed issues:
1534  <list style="symbols"> 
1535    <t>
1536      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1537      "Normative and Informative references"
1538    </t>
1539  </list>
1540</t>
1541<t>
1542  Other changes:
1543  <list style="symbols"> 
1544    <t>
1545      Move definitions of 304 and 412 condition codes from Part2.
1546    </t>
1547  </list>
1548</t>
1549</section>
1550
1551<section title="Since draft-ietf-httpbis-p4-conditional-01">
1552<t>
1553  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1554  <list style="symbols"> 
1555    <t>
1556      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1557    </t>
1558  </list>
1559</t>
1560</section>
1561
1562<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1563<t>
1564  Closed issues:
1565  <list style="symbols"> 
1566    <t>
1567      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1568      "Weak ETags on non-GET requests"
1569    </t>
1570  </list>
1571</t>
1572<t>
1573  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1574  <list style="symbols"> 
1575    <t>
1576      Reference RFC 3984, and update header field registrations for header fields defined
1577      in this document.
1578    </t>
1579  </list>
1580</t>
1581</section>
1582
1583<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1584<t>
1585  Closed issues:
1586  <list style="symbols"> 
1587    <t>
1588      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1589      "Examples for ETag matching"
1590    </t>
1591    <t>
1592      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1593      "'entity value' undefined"
1594    </t>
1595    <t>
1596      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1597      "bogus 2068 Date header reference"
1598    </t>
1599  </list>
1600</t>
1601</section>
1602
1603<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1604<t>
1605  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1606  <list style="symbols"> 
1607    <t>
1608      Use "/" instead of "|" for alternatives.
1609    </t>
1610    <t>
1611      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1612      whitespace ("OWS") and required whitespace ("RWS").
1613    </t>
1614    <t>
1615      Rewrite ABNFs to spell out whitespace rules, factor out
1616      header field value format definitions.
1617    </t>
1618  </list>
1619</t>
1620</section>
1621
1622<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1623<t>
1624  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1625  <list style="symbols"> 
1626    <t>
1627      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1628    </t>
1629  </list>
1630</t>
1631</section>
1632
1633<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1634<t>
1635  Closed issues:
1636  <list style="symbols"> 
1637    <t>
1638      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1639      "case-sensitivity of etag weakness indicator"
1640    </t>
1641  </list>
1642</t>
1643</section>
1644
1645<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1646<t>
1647  Closed issues:
1648  <list style="symbols"> 
1649    <t>
1650      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1651      "Weak ETags on non-GET requests" (If-Match still was defined to require
1652      strong matching)
1653    </t>
1654    <t>
1655      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1656      "move IANA registrations for optional status codes"
1657    </t>
1658  </list>
1659</t>
1660</section>
1661
1662<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1663<t>
1664  No significant changes.
1665</t>
1666</section>
1667
1668<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1669<t>
1670  No significant changes.
1671</t>
1672</section>
1673
1674<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1675<t>
1676  Closed issues:
1677  <list style="symbols"> 
1678    <t>
1679      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1680      "Clarify 'Requested Variant'"
1681    </t>
1682    <t>
1683      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1684      "Clarify entity / representation / variant terminology"
1685    </t>
1686    <t>
1687      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1688      "consider removing the 'changes from 2068' sections"
1689    </t>
1690  </list>
1691</t>
1692</section>
1693
1694<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1695<t>
1696  None.
1697</t>
1698</section>
1699
1700<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1701<t>
1702  Closed issues:
1703  <list style="symbols"> 
1704    <t>
1705      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1706      "Header Classification"
1707    </t>
1708  </list>
1709</t>
1710</section>
1711
1712<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1713<t>
1714  Closed issues:
1715  <list style="symbols"> 
1716    <t>
1717      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1718      "If-* and entities"
1719    </t>
1720    <t>
1721      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1722      "Definition of validator weakness"
1723    </t>
1724    <t>
1725      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1726      "untangle ABNFs for header fields"
1727    </t>
1728  </list>
1729</t>
1730</section>
1731
1732</section>
1733
1734</back>
1735</rfc>
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