source: draft-ietf-httpbis/latest/p4-conditional.xml @ 1303

Last change on this file since 1303 was 1303, checked in by julian.reschke@…, 9 years ago

update rfc2629.xslt, bump up document dates

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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 "June">
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), which is archived at
206    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
207  </t>
208  <t>
209    The current issues list is at
210    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
211    documents (including fancy diffs) can be found at
212    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
213  </t>
214  <t>
215    The changes in this draft are summarized in <xref target="changes.since.14"/>.
216  </t>
217</note>
218</front>
219<middle>
220<section title="Introduction" anchor="introduction">
221<t>
222   This document defines the HTTP/1.1 conditional request mechanisms,
223   including both response metadata that can be used to indicate or
224   observe changes to resource state and request header fields that
225   specify preconditions to be checked before performing the action
226   given by the request method.  Conditional GET requests are the most
227   efficient mechanism for HTTP cache updates &caching;.  Conditionals
228   can also be
229   applied to state-changing methods, such as PUT and DELETE, to prevent
230   the "lost update" problem: one client accidentally overwriting
231   the work of another client that has been acting in parallel.
232</t>
233<t>
234   Conditional request preconditions are based on the state of the target
235   resource as a whole (its current value set) or the state as observed
236   in a previously obtained representation (one value in that set).
237   A resource might have multiple current representations, each with its
238   own observable state.  The conditional request mechanisms assume that
239   the mapping of requests to corresponding representations will be
240   consistent over time if the server intends to take advantage of
241   conditionals.  Regardless, if the mapping is inconsistent and
242   the server is unable to select the appropriate representation, then
243   no harm will result when the precondition evaluates to false.
244</t>
245<t><iref primary="true" item="selected representation"/>
246   We use the term "<x:dfn>selected representation</x:dfn>" to refer to
247   the current representation of the target resource that would have been
248   selected in a successful response if the same request had used the method
249   GET and had excluded all of the conditional request header fields.
250   The conditional request preconditions are evaluated by comparing the
251   values provided in the request header fields to the current metadata
252   for the selected representation.
253</t>
254
255<section title="Requirements" anchor="intro.requirements">
256<t>
257   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
258   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
259   document are to be interpreted as described in <xref target="RFC2119"/>.
260</t>
261<t>
262   An implementation is not compliant if it fails to satisfy one or more
263   of the "MUST" or "REQUIRED" level requirements for the protocols it
264   implements. An implementation that satisfies all the "MUST" or "REQUIRED"
265   level and all the "SHOULD" level requirements for its protocols is said
266   to be "unconditionally compliant"; one that satisfies all the "MUST"
267   level requirements but not all the "SHOULD" level requirements for its
268   protocols is said to be "conditionally compliant".
269</t>
270</section>
271
272<section title="Syntax Notation" anchor="notation">
273  <x:anchor-alias value="ALPHA"/>
274  <x:anchor-alias value="CR"/>
275  <x:anchor-alias value="DIGIT"/>
276  <x:anchor-alias value="LF"/>
277  <x:anchor-alias value="OCTET"/>
278  <x:anchor-alias value="VCHAR"/>
279  <x:anchor-alias value="WSP"/>
280  <x:anchor-alias value="core.rules"/>
281  <x:anchor-alias value="quoted-string"/>
282  <x:anchor-alias value="OWS"/>
283  <x:anchor-alias value="HTTP-date"/>
284<t>
285  This specification uses the ABNF syntax defined in &notation; (which
286  extends the syntax defined in <xref target="RFC5234"/> with a list rule).
287  <xref target="collected.abnf"/> shows the collected ABNF, with the list
288  rule expanded.
289</t>
290<t>
291  The following core rules are included by
292  reference, as defined in <xref target="RFC5234" x:fmt="," x:sec="B.1"/>:
293  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
294  DIGIT (decimal 0-9), DQUOTE (double quote),
295  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
296  OCTET (any 8-bit sequence of data), SP (space),
297  VCHAR (any visible USASCII character),
298  and WSP (whitespace).
299</t>
300<t>
301  The ABNF rules below are defined in other parts:
302</t>
303<figure><artwork type="abnf2616">
304  <x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in &basic-rules;&gt;
305  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &basic-rules;&gt;
306  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &full-date;&gt;
307</artwork></figure>
308</section>
309</section>
310
311<section title="Resource State Metadata (Validators)" anchor="resource.metadata">
312   <iref primary="true" item="metadata"/>
313   <iref primary="true" item="validator"/>
314<t>
315   This specification defines two forms of metadata that are commonly used
316   to observe resource state and test for preconditions: modification dates
317   and opaque entity tags.  Additional metadata that reflects resource state
318   has been defined by various extensions of HTTP, such as WebDAV
319   <xref target="RFC4918"/>, that are beyond the scope of this specification.
320   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
321   when it is used within a precondition.
322</t>
323
324<section title="Last-Modified" anchor="header.last-modified">
325  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
326  <iref primary="true" item="Header Fields" subitem="Last-Modified" x:for-anchor=""/>
327  <x:anchor-alias value="Last-Modified"/>
328<t>
329   The "Last-Modified" header field indicates the date and time at
330   which the origin server believes the selected representation was
331   last modified.
332</t>
333<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
334  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
335</artwork></figure>
336<t>
337   An example of its use is
338</t>
339<figure><artwork type="example">
340  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
341</artwork></figure>
342
343<section title="Generation" anchor="lastmod.generation">
344<t>
345   Origin servers &SHOULD; send Last-Modified for any selected
346   representation for which a last modification date can be reasonably
347   and consistently determined, since its use in conditional requests
348   and evaluating cache freshness (&caching;) results in a substantial
349   reduction of HTTP traffic on the Internet and can be a significant
350   factor in improving service scalability and reliability.
351</t>
352<t>
353   A representation is typically the sum of many parts behind the
354   resource interface.  The last-modified time would usually be
355   the most recent time that any of those parts were changed.
356   How that value is determined for any given resource is an
357   implementation detail beyond the scope of this specification.
358   What matters to HTTP is how recipients of the Last-Modified
359   header field can use its value to make conditional requests
360   and test the validity of locally cached responses.
361</t>
362<t>
363   An origin server &SHOULD; obtain the Last-Modified value of the
364   representation as close as possible to the time that it generates
365   the Date field-value for its response. This allows a recipient to
366   make an accurate assessment of the representation's modification time,
367   especially if the representation changes near the time that the
368   response is generated.
369</t>
370<t>
371   An origin server with a clock &MUST-NOT; send a Last-Modified date
372   that is later than the server's time of message origination (Date).
373   If the last modification time is derived from implementation-specific
374   metadata that evaluates to some time in the future, according to the
375   origin server's clock, then the origin server &MUST; replace that
376   value with the message origination date. This prevents a future
377   modification date from having an adverse impact on cache validation.
378</t>
379</section>
380
381<section title="Comparison" anchor="lastmod.comparison">
382<t>
383   A Last-Modified time, when used as a validator in a request, is
384   implicitly weak unless it is possible to deduce that it is strong,
385   using the following rules:
386  <list style="symbols">
387     <t>The validator is being compared by an origin server to the
388        actual current validator for the representation and,</t>
389     <t>That origin server reliably knows that the associated representation did
390        not change twice during the second covered by the presented
391        validator.</t>
392  </list>
393</t>
394<t>
395   or
396  <list style="symbols">
397     <t>The validator is about to be used by a client in an If-Modified-Since
398        or If-Unmodified-Since header field, because the client
399        has a cache entry for the associated representation, and</t>
400     <t>That cache entry includes a Date value, which gives the time
401        when the origin server sent the original response, and</t>
402     <t>The presented Last-Modified time is at least 60 seconds before
403        the Date value.</t>
404  </list>
405</t>
406<t>
407   or
408  <list style="symbols">
409     <t>The validator is being compared by an intermediate cache to the
410        validator stored in its cache entry for the representation, and</t>
411     <t>That cache entry includes a Date value, which gives the time
412        when the origin server sent the original response, and</t>
413     <t>The presented Last-Modified time is at least 60 seconds before
414        the Date value.</t>
415  </list>
416</t>
417<t>
418   This method relies on the fact that if two different responses were
419   sent by the origin server during the same second, but both had the
420   same Last-Modified time, then at least one of those responses would
421   have a Date value equal to its Last-Modified time. The arbitrary 60-second
422   limit guards against the possibility that the Date and Last-Modified
423   values are generated from different clocks, or at somewhat
424   different times during the preparation of the response. An
425   implementation &MAY; use a value larger than 60 seconds, if it is
426   believed that 60 seconds is too short.
427</t>
428</section>
429</section>
430
431<section title="ETag" anchor="header.etag">
432  <iref primary="true" item="ETag header field" x:for-anchor=""/>
433  <iref primary="true" item="Header Fields" subitem="ETag" x:for-anchor=""/>
434  <x:anchor-alias value="ETag"/>
435  <x:anchor-alias value="entity-tag"/>
436  <x:anchor-alias value="entity.tags"/>
437  <x:anchor-alias value="opaque-tag"/>
438  <x:anchor-alias value="weak"/>
439<t>
440   The ETag header field provides the current entity-tag for the
441   selected representation.
442   An entity-tag is an opaque validator for differentiating between
443   multiple representations of the same resource, regardless of whether
444   those multiple representations are due to resource state changes over
445   time, content negotiation resulting in multiple representations being
446   valid at the same time, or both. An entity-tag consists of an opaque
447   quoted string, possibly prefixed by a weakness indicator.
448</t>
449<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"/>
450  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
451
452  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
453  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
454  <x:ref>opaque-tag</x:ref> = <x:ref>quoted-string</x:ref>
455</artwork></figure>
456<t>
457   An entity-tag can be more reliable for validation than a modification
458   date in situations where it is inconvenient to store modification
459   dates, where the one-second resolution of HTTP date values is not
460   sufficient, or where modification dates are not consistently maintained.
461</t>
462<figure><preamble>
463  Examples:
464</preamble>
465<artwork type="example">
466  ETag: "xyzzy"
467  ETag: W/"xyzzy"
468  ETag: ""
469</artwork></figure>
470
471<section title="Generation" anchor="entity.tag.generation">
472<t>
473   The principle behind entity-tags is that only the service author
474   knows the implementation of a resource well enough to select the
475   most accurate and efficient validation mechanism for that resource,
476   and that any such mechanism can be mapped to a simple sequence of
477   octets for easy comparison.  Since the value is opaque, there is no
478   need for the client to be aware of how each entity-tag is constructed.
479</t>
480<t>
481   For example, a resource that has implementation-specific versioning
482   applied to all changes might use an internal revision number, perhaps
483   combined with a variance identifier for content negotiation, to
484   accurately differentiate between representations.
485   Other implementations might use a stored hash of representation content,
486   a combination of various filesystem attributes, or a modification
487   timestamp that has sub-second resolution.
488</t>
489<t>
490   Origin servers &SHOULD; send ETag for any selected representation
491   for which detection of changes can be reasonably and consistently
492   determined, since the entity-tag's use in conditional requests and
493   evaluating cache freshness (&caching;) can result in a substantial
494   reduction of HTTP network traffic and can be a significant factor in
495   improving service scalability and reliability.
496</t>
497</section>
498
499<section title="Weak versus Strong" anchor="weak.and.strong.validators">
500<t>
501   Since both origin servers and caches will compare two validators to
502   decide if they indicate the same or different representations, one
503   normally would expect that if the representation (including both
504   representation header fields and representation body) changes in any
505   way, then the associated validator would change as well. If this is
506   true, then we call that validator a "strong validator".  One example
507   of a strong validator is an integer that is incremented in stable
508   storage every time a representation is changed.
509</t>
510<t>
511   However, there might be cases when a server prefers to change the
512   validator only when it desires cached representations to be invalidated.
513   For example, the representation of a weather report that changes in
514   content every second, based on dynamic measurements, might be grouped
515   into sets of equivalent representations (from the origin server's
516   perspective) in order to allow cached representations to be valid
517   for a reasonable period of time (perhaps adjusted dynamically based
518   on server load or weather quality).
519   A validator that does not always change when the representation
520   changes is a "weak validator".
521</t>
522<t>
523   One can think of a strong validator as part of an identifier for a
524   specific representation, whereas a weak validator is part of an
525   identifier for a set of equivalent representations (where this notion
526   of equivalence is entirely governed by the origin server and beyond
527   the scope of this specification).
528</t>
529<t>
530   An entity-tag is normally a strong validator, but the protocol
531   provides a mechanism to tag an entity-tag as "weak".
532  <list><t>
533      A representation's modification time, if defined with only one-second
534      resolution, could be a weak validator, since it is possible that
535      the representation might be modified twice during a single second.
536    </t><t>
537      Support for weak validators is optional. However, weak validators
538      allow for more efficient caching of equivalent objects; for
539      example, a hit counter on a site is probably good enough if it is
540      updated every few days or weeks, and any value during that period
541      is likely "good enough" to be equivalent.
542    </t></list>
543</t>
544<t>
545   A strong entity-tag &MUST; change whenever the associated representation
546   changes in any way. A weak entity-tag &SHOULD; change whenever the origin
547   server considers prior representations to be unacceptable as a substitute
548   for the current representation. In other words, a weak entity tag &SHOULD;
549   change whenever the origin server wants caches to invalidate old responses.
550</t>
551<t>
552   A "strong entity-tag" &MAY; be shared by two representations of a resource
553   only if they are equivalent by octet equality.
554</t>
555<t>
556   A "weak entity-tag", indicated by the "W/" prefix, &MAY; be shared by
557   two representations of a resource. A weak entity-tag can only be used
558   for weak comparison.
559</t>
560<t>
561   Cache entries might persist for arbitrarily long periods, regardless
562   of expiration times.  Thus, a cache might attempt to validate an
563   entry using a validator that it obtained in the distant past.
564   A strong entity-tag &MUST; be unique across all versions of all
565   representations associated with a particular resource over time.
566   However, there is no implication of uniqueness across entity-tags
567   of different resources (i.e., the same entity-tag value might be
568   in use for representations of multiple resources at the same time
569   and does not imply that those representations are equivalent).
570</t>
571</section>
572
573<section title="Comparison" anchor="entity.tag.comparison">
574  <x:anchor-alias value="validator.comparison"/>
575<t>
576   There are two entity-tag comparison functions, depending
577   on whether the comparison context allows the use of weak validators
578   or not:
579  <list style="symbols">
580     <t>The strong comparison function: in order to be considered equal,
581        both opaque-tags &MUST; be identical character-by-character, and both
582        &MUST-NOT; be weak.</t>
583     <t>The weak comparison function: in order to be considered equal, both
584        opaque-tags &MUST; be identical character-by-character, but
585        either or both of them &MAY; be tagged as "weak" without affecting
586        the result.</t>
587  </list>
588</t>
589<t>
590   A "use" of a validator is either when a client generates a request
591   and includes the validator in a precondition, or when a server
592   compares two validators.
593</t>
594<t>
595   Strong validators are usable in any context. Weak validators are only
596   usable in contexts that do not depend on exact equality of a representation.
597   For example, either kind is usable for a normal conditional GET.
598</t>
599<t>
600   The example below shows the results for a set of entity-tag pairs,
601   and both the weak and strong comparison function results:
602</t>
603<texttable align="left">
604  <ttcol>ETag 1</ttcol>
605  <ttcol>ETag 2</ttcol>
606  <ttcol>Strong Comparison</ttcol>
607  <ttcol>Weak Comparison</ttcol>
608
609  <c>W/"1"</c>
610  <c>W/"1"</c>
611  <c>no match</c>
612  <c>match</c>
613 
614  <c>W/"1"</c>
615  <c>W/"2"</c>
616  <c>no match</c>
617  <c>no match</c>
618
619  <c>W/"1"</c>
620  <c>"1"</c>
621  <c>no match</c>
622  <c>match</c>
623
624  <c>"1"</c>
625  <c>"1"</c>
626  <c>match</c>
627  <c>match</c>
628</texttable>
629<t>
630   An entity-tag is strong unless it is explicitly tagged as weak.
631</t>
632</section>
633
634<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">
635<t>
636   We adopt a set of rules and recommendations for origin servers,
637   clients, and caches regarding when various validator types ought to
638   be used, and for what purposes.
639</t>
640<t>
641   HTTP/1.1 origin servers:
642  <list style="symbols">
643     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
644        generate one.</t>
645
646     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
647        performance considerations support the use of weak entity-tags,
648        or if it is unfeasible to send a strong entity-tag.</t>
649
650     <t>&SHOULD; send a Last-Modified value if it is feasible to send one.</t>
651  </list>
652</t>
653<t>
654   In other words, the preferred behavior for an HTTP/1.1 origin server
655   is to send both a strong entity-tag and a Last-Modified value.
656</t>
657<t>
658   HTTP/1.1 clients:
659  <list style="symbols">
660     <t>&MUST; use that entity-tag in any cache-conditional request (using
661        If-Match or If-None-Match) if an entity-tag has been provided by the
662        origin server.</t>
663
664     <t>&SHOULD; use the Last-Modified value in non-subrange cache-conditional
665        requests (using If-Modified-Since) if only a Last-Modified value has
666        been provided by the origin server. </t>
667
668     <t>&MAY; use the Last-Modified value in subrange cache-conditional
669        requests (using If-Unmodified-Since) if only a Last-Modified value has
670        been provided by an HTTP/1.0 origin server. The user agent &SHOULD;
671        provide a way to disable this, in case of difficulty.</t>
672
673     <t>&SHOULD; use both validators in cache-conditional requests if both an
674        entity-tag and a Last-Modified value have been provided by the origin
675        server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond
676        appropriately.</t>
677  </list>
678</t>
679<t>
680   An HTTP/1.1 origin server, upon receiving a conditional request that
681   includes both a Last-Modified date (e.g., in an If-Modified-Since or
682   If-Unmodified-Since header field) and one or more entity-tags (e.g.,
683   in an If-Match, If-None-Match, or If-Range header field) as cache
684   validators, &MUST-NOT; return a response status code of 304 (Not Modified)
685   unless doing so is consistent with all of the conditional header
686   fields in the request.
687</t>
688<t>
689   An HTTP/1.1 caching proxy, upon receiving a conditional request that
690   includes both a Last-Modified date and one or more entity-tags as
691   cache validators, &MUST-NOT; return a locally cached response to the
692   client unless that cached response is consistent with all of the
693   conditional header fields in the request.
694  <list><t>
695      <x:h>Note:</x:h> The general principle behind these rules is that HTTP/1.1
696      servers and clients ought to transmit as much non-redundant
697      information as is available in their responses and requests.
698      HTTP/1.1 systems receiving this information will make the most
699      conservative assumptions about the validators they receive.
700  </t><t>
701      HTTP/1.0 clients and caches might ignore entity-tags. Generally,
702      last-modified values received or used by these systems will
703      support transparent and efficient caching, and so HTTP/1.1 origin
704      servers should provide Last-Modified values. In those rare cases
705      where the use of a Last-Modified value as a validator by an
706      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
707      origin servers should not provide one.
708  </t></list>
709</t>
710</section>
711
712<section title="Example: Entity-tags varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
713<t>
714   Consider a resource that is subject to content negotiation (&content-negotiation;),
715   and where the representations returned upon a GET request vary based on
716   the Accept-Encoding request header field (&header-accept-encoding;):
717</t>
718<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
719GET /index HTTP/1.1
720Host: www.example.com
721Accept-Encoding: gzip
722
723</artwork></figure>
724<t>
725   In this case, the response might or might not use the gzip content coding.
726   If it does not, the response might look like:
727</t>
728<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
729HTTP/1.1 200 OK
730Date: Thu, 26 Mar 2010 00:05:00 GMT
731ETag: "123-a"
732Content-Length: <x:length-of target="exbody"/>
733Vary: Accept-Encoding
734Content-Type: text/plain
735
736<x:span anchor="exbody">Hello World!
737Hello World!
738Hello World!
739Hello World!
740Hello World!
741</x:span></artwork></figure>
742<t>
743   An alternative representation that does use gzip content coding would be:
744</t>
745<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
746HTTP/1.1 200 OK
747Date: Thu, 26 Mar 2010 00:05:00 GMT
748ETag: "123-b"
749Content-Length: 43
750Vary: Accept-Encoding
751Content-Type: text/plain
752Content-Encoding: gzip
753
754<spanx>...binary data...</spanx></artwork></figure>
755<x:note>
756  <t>
757    <x:h>Note:</x:h> Content codings are a property of the representation,
758    so therefore an entity-tag of an encoded representation must be distinct
759    from an unencoded representation to prevent conflicts during cache updates
760    and range requests.  In contrast, transfer codings (&transfer-codings;)
761    apply only during message transfer and do not require distinct entity-tags.
762  </t>
763</x:note>
764</section>
765</section>
766</section>
767
768<section title="Precondition Header Fields" anchor="header.fields">
769<t>
770   This section defines the syntax and semantics of HTTP/1.1 header fields
771   for applying preconditions on requests.
772</t>
773
774<section title="If-Match" anchor="header.if-match">
775  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
776  <iref primary="true" item="Header Fields" subitem="If-Match" x:for-anchor=""/>
777  <x:anchor-alias value="If-Match"/>
778<t>
779   The "If-Match" header field &MAY; be used to make a request method
780   conditional on the current existence or value of an entity-tag for
781   one or more representations of the target resource.  If-Match is
782   generally useful for resource update requests, such as PUT requests,
783   as a means for protecting against accidental overwrites when multiple
784   clients are acting in parallel on the same resource (i.e., the
785   "lost update" problem).  An If-Match field-value of "*" places the
786   precondition on the existence of any current representation for the
787   target resource.
788</t>
789<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
790  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
791</artwork></figure>
792<t>
793   If any of the entity-tags listed in the If-Match field value match
794   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
795   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-None-Match" anchor="header.if-none-match">
827  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
828  <iref primary="true" item="Header Fields" subitem="If-None-Match" x:for-anchor=""/>
829  <x:anchor-alias value="If-None-Match"/>
830<t>
831   The "If-None-Match" header field &MAY; be used to make a request method
832   conditional on not matching any of the current entity-tag values for
833   representations of the target resource.  If-None-Match is primarily
834   used in conditional GET requests to enable efficient updates of cached
835   information with a minimum amount of transaction overhead.  A client
836   that has one or more representations previously obtained from the
837   target resource can send If-None-Match with a list of the associated
838   entity-tags in the hope of receiving a 304 response if at least one
839   of those representations matches the selected representation.
840</t>
841<t>
842   If-None-Match MAY also be used with a value of "*" to prevent an unsafe
843   request method (e.g., PUT) from inadvertently modifying an existing
844   representation of the target resource when the client believes that
845   the resource does not have a current representation.  This is a variation
846   on the "lost update" problem that might arise if more than one client
847   attempts to create an initial representation for the target resource.
848</t>
849<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
850  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
851</artwork></figure>
852<t>
853   If any of the entity-tags listed in the If-None-Match field-value match
854   (as per <xref target="entity.tag.comparison"/>) the entity-tag of the
855   selected representation, or if "*" is
856   given and any current representation exists for that resource, then the
857   server &MUST-NOT; perform the requested method.
858   Instead, if the request method was GET or HEAD, the server &SHOULD;
859   respond with a 304 (Not Modified) status code, including the cache-related
860   header fields (particularly ETag) of the selected representation that has
861   a matching entity-tag.  For all other request methods, the server &MUST;
862   respond with a 412 (Precondition Failed) status code.
863</t>
864<t>
865   If none of the entity-tags match, then the server &MAY; perform the
866   requested method as if the If-None-Match header field did not exist,
867   but &MUST; also ignore any If-Modified-Since header field(s) in the
868   request. That is, if no entity-tags match, then the server &MUST-NOT;
869   return a 304 (Not Modified) response.
870</t>
871<t>
872   If the request would, without the If-None-Match header field, result
873   in anything other than a 2xx or 304 status code, then the If-None-Match
874   header field &MUST; be ignored. (See <xref
875   target="rules.for.when.to.use.entity.tags.and.last-modified.dates"/> for
876   a discussion of server behavior when both If-Modified-Since and
877   If-None-Match appear in the same request.)
878</t>
879<t>
880   Examples:
881</t>
882<figure><artwork type="example">
883  If-None-Match: "xyzzy"
884  If-None-Match: W/"xyzzy"
885  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
886  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
887  If-None-Match: *
888</artwork></figure>
889<t>
890   The result of a request having both an If-None-Match header field and
891   either an If-Match or an If-Unmodified-Since header fields is
892   undefined by this specification.
893</t>
894</section>
895
896<section title="If-Modified-Since" anchor="header.if-modified-since">
897  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
898  <iref primary="true" item="Header Fields" subitem="If-Modified-Since" x:for-anchor=""/>
899  <x:anchor-alias value="If-Modified-Since"/>
900<t>
901   The "If-Modified-Since" header field &MAY; be used to make a request
902   method conditional by modification date: if the selected representation
903   has not been modified since the time specified in this field, then
904   do not perform the request method; instead, respond as detailed below.
905</t>
906<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
907  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
908</artwork></figure>
909<t>
910   An example of the field is:
911</t>
912<figure><artwork type="example">
913  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
914</artwork></figure>
915<t>
916   A GET method with an If-Modified-Since header field and no Range header
917   field requests that the selected representation be transferred only if
918   it has been modified since the date given by the If-Modified-Since
919   header field.
920   The algorithm for determining this includes the following cases:
921  <list style="numbers">
922      <t>If the request would normally result in anything other than a
923         200 (OK) status code, or if the passed If-Modified-Since date is
924         invalid, the response is exactly the same as for a normal GET.
925         A date which is later than the server's current time is
926         invalid.</t>
927
928      <t>If the selected representation has been modified since the
929         If-Modified-Since date, the response is exactly the same as for
930         a normal GET.</t>
931
932      <t>If the selected representation has not been modified since a valid
933         If-Modified-Since date, the server &SHOULD; return a
934         304 (Not Modified) response.</t>
935  </list>
936</t>
937<t>
938   The purpose of this feature is to allow efficient updates of cached
939   information with a minimum amount of transaction overhead.
940  <list><t>
941      <x:h>Note:</x:h> The Range header field modifies the meaning of If-Modified-Since;
942      see &header-range; for full details.
943    </t><t>
944      <x:h>Note:</x:h> If-Modified-Since times are interpreted by the server, whose
945      clock might not be synchronized with the client.
946    </t><t>
947      <x:h>Note:</x:h> When handling an If-Modified-Since header field, some
948      servers will use an exact date comparison function, rather than a
949      less-than function, for deciding whether to send a 304 (Not
950      Modified) response. To get best results when sending an If-Modified-Since
951      header field for cache validation, clients are
952      advised to use the exact date string received in a previous Last-Modified
953      header field whenever possible.
954    </t><t>
955      <x:h>Note:</x:h> If a client uses an arbitrary date in the If-Modified-Since
956      header field instead of a date taken from the Last-Modified header field for
957      the same request, the client needs to be aware that this
958      date is interpreted in the server's understanding of time.
959      Unsynchronized clocks and rounding problems, due to the different
960      encodings of time between the client and server, are concerns.
961      This includes the possibility of race conditions if the
962      document has changed between the time it was first requested and
963      the If-Modified-Since date of a subsequent request, and the
964      possibility of clock-skew-related problems if the If-Modified-Since
965      date is derived from the client's clock without correction
966      to the server's clock. Corrections for different time bases
967      between client and server are at best approximate due to network
968      latency.
969    </t>
970  </list>
971</t>
972<t>
973   The result of a request having both an If-Modified-Since header field
974   and either an If-Match or an If-Unmodified-Since header fields is
975   undefined by this specification.
976</t>
977</section>
978
979<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
980  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
981  <iref primary="true" item="Header Fields" subitem="If-Unmodified-Since" x:for-anchor=""/>
982  <x:anchor-alias value="If-Unmodified-Since"/>
983<t>
984   The "If-Unmodified-Since" header field &MAY; be used to make a request
985   method conditional by modification date: if the selected representation
986   has been modified since the time specified in this field, then the
987   server &MUST-NOT; perform the requested operation and &MUST; instead
988   respond with the 412 (Precondition Failed) status code.
989   If the selected representation has not been modified since the time
990   specified in this field, the server &SHOULD; perform the request
991   method as if the If-Unmodified-Since header field were not present.
992</t>
993<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
994  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
995</artwork></figure>
996<t>
997   An example of the field is:
998</t>
999<figure><artwork type="example">
1000  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
1001</artwork></figure>
1002<t>
1003   If the request normally (i.e., without the If-Unmodified-Since
1004   header field) would result in anything other than a 2xx or 412 status code,
1005   the If-Unmodified-Since header field &SHOULD; be ignored.
1006</t>
1007<t>
1008   If the specified date is invalid, the header field &MUST; be ignored.
1009</t>
1010<t>
1011   The result of a request having both an If-Unmodified-Since header
1012   field and either an If-None-Match or an If-Modified-Since header
1013   fields is undefined by this specification.
1014</t>
1015</section>
1016
1017<section title="If-Range" anchor="header.if-range">
1018<t>
1019   The If-Range header field provides a special conditional request
1020   mechanism that is similar to If-Match and If-Unmodified-Since but
1021   specific to HTTP range requests. If-Range is defined in &header-if-range;.
1022</t>
1023</section>
1024
1025</section>
1026
1027<section title="Status Code Definitions" anchor="status.code.definitions">
1028<section title="304 Not Modified" anchor="status.304">
1029  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
1030  <iref primary="true" item="Status Codes" subitem="304 Not Modified" x:for-anchor=""/>
1031<t>
1032   The 304 status code indicates that a conditional GET request has been
1033   received and would have resulted in a 200 (OK) response if it were not
1034   for the fact that the condition has evaluated to false.  In other words,
1035   there is no need for the server to transfer a representation of the
1036   target resource because the client's request indicates that it already
1037   has a valid representation, as indicated by the 304 response header
1038   fields, and is therefore redirecting the client to make use of that
1039   stored representation as if it were the payload of a 200 response.
1040   The 304 response &MUST-NOT; contain a message-body, and thus is always
1041   terminated by the first empty line after the header fields.
1042</t>
1043<t>
1044   A 304 response &MUST; include a Date header field (&header-date;)
1045   unless its omission is required by &clockless;.  If a 200 response
1046   to the same request would have included any of the header fields
1047   Cache-Control, Content-Location, ETag, Expires, Last-Modified, or
1048   Vary, then those same header fields &MUST; be sent in a 304 response.
1049</t>
1050<t>
1051   Since the goal of a 304 response is to minimize information transfer
1052   when the recipient already has one or more cached representations,
1053   the response &SHOULD-NOT; include representation metadata other
1054   than the above listed fields unless said metadata exists for the
1055   purpose of guiding cache updates (e.g., future HTTP extensions).
1056</t>
1057<t>
1058   If the recipient of a 304 response does not have a cached representation
1059   corresponding to the entity-tag indicated by the 304 response, then the
1060   recipient &MUST-NOT; use the 304 to update its own cache.  If this
1061   conditional request originated with an outbound client, such as a
1062   user agent with its own cache sending a conditional GET to a shared
1063   proxy, then the 304 response &MAY; be forwarded to the outbound client.
1064   Otherwise, the recipient &MUST; disregard the 304 response and repeat
1065   the request without any preconditions.
1066</t>
1067<t>
1068   If a cache uses a received 304 response to update a cache entry, the
1069   cache &MUST; update the entry to reflect any new field values given in
1070   the response.
1071</t>
1072</section>
1073
1074<section title="412 Precondition Failed" anchor="status.412">
1075  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1076  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed" x:for-anchor=""/>
1077<t>
1078   The 412 status code indicates that one or more preconditions given in
1079   the request header fields evaluated to false when tested on the server.
1080   This response code allows the client to place preconditions on the
1081   current resource state (its current representations and metadata)
1082   and thus prevent the request method from being applied if the target
1083   resource is in an unexpected state.
1084</t>
1085</section>
1086</section>
1087
1088<section title="IANA Considerations" anchor="IANA.considerations">
1089
1090<section title="Status Code Registration" anchor="status.code.registration">
1091<t>
1092   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1093   shall be updated with the registrations below:
1094</t>
1095<?BEGININC p4-conditional.iana-status-codes ?>
1096<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1097<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1098   <ttcol>Value</ttcol>
1099   <ttcol>Description</ttcol>
1100   <ttcol>Reference</ttcol>
1101   <c>304</c>
1102   <c>Not Modified</c>
1103   <c>
1104      <xref target="status.304"/>
1105   </c>
1106   <c>412</c>
1107   <c>Precondition Failed</c>
1108   <c>
1109      <xref target="status.412"/>
1110   </c>
1111</texttable>
1112<!--(END)-->
1113<?ENDINC p4-conditional.iana-status-codes ?>
1114</section>
1115
1116<section title="Header Field Registration" anchor="header.field.registration">
1117<t>
1118   The Message Header Field Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> shall be updated
1119   with the permanent registrations below (see <xref target="RFC3864"/>):
1120</t>
1121<?BEGININC p4-conditional.iana-headers ?>
1122<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1123<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1124   <ttcol>Header Field Name</ttcol>
1125   <ttcol>Protocol</ttcol>
1126   <ttcol>Status</ttcol>
1127   <ttcol>Reference</ttcol>
1128
1129   <c>ETag</c>
1130   <c>http</c>
1131   <c>standard</c>
1132   <c>
1133      <xref target="header.etag"/>
1134   </c>
1135   <c>If-Match</c>
1136   <c>http</c>
1137   <c>standard</c>
1138   <c>
1139      <xref target="header.if-match"/>
1140   </c>
1141   <c>If-Modified-Since</c>
1142   <c>http</c>
1143   <c>standard</c>
1144   <c>
1145      <xref target="header.if-modified-since"/>
1146   </c>
1147   <c>If-None-Match</c>
1148   <c>http</c>
1149   <c>standard</c>
1150   <c>
1151      <xref target="header.if-none-match"/>
1152   </c>
1153   <c>If-Unmodified-Since</c>
1154   <c>http</c>
1155   <c>standard</c>
1156   <c>
1157      <xref target="header.if-unmodified-since"/>
1158   </c>
1159   <c>Last-Modified</c>
1160   <c>http</c>
1161   <c>standard</c>
1162   <c>
1163      <xref target="header.last-modified"/>
1164   </c>
1165</texttable>
1166<!--(END)-->
1167<?ENDINC p4-conditional.iana-headers ?>
1168<t>
1169   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1170</t>
1171</section>
1172</section>
1173
1174<section title="Security Considerations" anchor="security.considerations">
1175<t>
1176   No additional security considerations have been identified beyond
1177   those applicable to HTTP in general &messaging;.
1178</t>
1179</section>
1180
1181<section title="Acknowledgments" anchor="ack">
1182</section>
1183</middle>
1184<back>
1185
1186<references title="Normative References">
1187
1188<reference anchor="Part1">
1189  <front>
1190    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
1191    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1192      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1193      <address><email>fielding@gbiv.com</email></address>
1194    </author>
1195    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1196      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1197      <address><email>jg@freedesktop.org</email></address>
1198    </author>
1199    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1200      <organization abbrev="HP">Hewlett-Packard Company</organization>
1201      <address><email>JeffMogul@acm.org</email></address>
1202    </author>
1203    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1204      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1205      <address><email>henrikn@microsoft.com</email></address>
1206    </author>
1207    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1208      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1209      <address><email>LMM@acm.org</email></address>
1210    </author>
1211    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1212      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1213      <address><email>paulle@microsoft.com</email></address>
1214    </author>
1215    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1216      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1217      <address><email>timbl@w3.org</email></address>
1218    </author>
1219    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1220      <organization abbrev="W3C">World Wide Web Consortium</organization>
1221      <address><email>ylafon@w3.org</email></address>
1222    </author>
1223    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1224      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1225      <address><email>julian.reschke@greenbytes.de</email></address>
1226    </author>
1227    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1228  </front>
1229  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1230  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1231</reference>
1232
1233<reference anchor="Part3">
1234  <front>
1235    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
1236    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1237      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1238      <address><email>fielding@gbiv.com</email></address>
1239    </author>
1240    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1241      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1242      <address><email>jg@freedesktop.org</email></address>
1243    </author>
1244    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1245      <organization abbrev="HP">Hewlett-Packard Company</organization>
1246      <address><email>JeffMogul@acm.org</email></address>
1247    </author>
1248    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1249      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1250      <address><email>henrikn@microsoft.com</email></address>
1251    </author>
1252    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1253      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1254      <address><email>LMM@acm.org</email></address>
1255    </author>
1256    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1257      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1258      <address><email>paulle@microsoft.com</email></address>
1259    </author>
1260    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1261      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1262      <address><email>timbl@w3.org</email></address>
1263    </author>
1264    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1265      <organization abbrev="W3C">World Wide Web Consortium</organization>
1266      <address><email>ylafon@w3.org</email></address>
1267    </author>
1268    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1269      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1270      <address><email>julian.reschke@greenbytes.de</email></address>
1271    </author>
1272    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1273  </front>
1274  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;"/>
1275  <x:source href="p3-payload.xml" basename="p3-payload"/>
1276</reference>
1277
1278<reference anchor="Part5">
1279  <front>
1280    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
1281    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1282      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1283      <address><email>fielding@gbiv.com</email></address>
1284    </author>
1285    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1286      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1287      <address><email>jg@freedesktop.org</email></address>
1288    </author>
1289    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1290      <organization abbrev="HP">Hewlett-Packard Company</organization>
1291      <address><email>JeffMogul@acm.org</email></address>
1292    </author>
1293    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1294      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1295      <address><email>henrikn@microsoft.com</email></address>
1296    </author>
1297    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1298      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1299      <address><email>LMM@acm.org</email></address>
1300    </author>
1301    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1302      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1303      <address><email>paulle@microsoft.com</email></address>
1304    </author>
1305    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1306      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1307      <address><email>timbl@w3.org</email></address>
1308    </author>
1309    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1310      <organization abbrev="W3C">World Wide Web Consortium</organization>
1311      <address><email>ylafon@w3.org</email></address>
1312    </author>
1313    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1314      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1315      <address><email>julian.reschke@greenbytes.de</email></address>
1316    </author>
1317    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1318  </front>
1319  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1320  <x:source href="p5-range.xml" basename="p5-range"/>
1321</reference>
1322
1323<reference anchor="Part6">
1324  <front>
1325    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
1326    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1327      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1328      <address><email>fielding@gbiv.com</email></address>
1329    </author>
1330    <author initials="J." surname="Gettys" fullname="Jim Gettys">
1331      <organization abbrev="Alcatel-Lucent">Alcatel-Lucent Bell Labs</organization>
1332      <address><email>jg@freedesktop.org</email></address>
1333    </author>
1334    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
1335      <organization abbrev="HP">Hewlett-Packard Company</organization>
1336      <address><email>JeffMogul@acm.org</email></address>
1337    </author>
1338    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
1339      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1340      <address><email>henrikn@microsoft.com</email></address>
1341    </author>
1342    <author initials="L." surname="Masinter" fullname="Larry Masinter">
1343      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1344      <address><email>LMM@acm.org</email></address>
1345    </author>
1346    <author initials="P." surname="Leach" fullname="Paul J. Leach">
1347      <organization abbrev="Microsoft">Microsoft Corporation</organization>
1348      <address><email>paulle@microsoft.com</email></address>
1349    </author>
1350    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1351      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
1352      <address><email>timbl@w3.org</email></address>
1353    </author>
1354    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1355      <organization abbrev="W3C">World Wide Web Consortium</organization>
1356      <address><email>ylafon@w3.org</email></address>
1357    </author>
1358    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1359      <address><email>mnot@mnot.net</email></address>
1360    </author>
1361    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1362      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1363      <address><email>julian.reschke@greenbytes.de</email></address>
1364    </author>
1365    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1366  </front>
1367  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1368  <x:source href="p6-cache.xml" basename="p6-cache"/>
1369</reference>
1370
1371<reference anchor="RFC2119">
1372  <front>
1373    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1374    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1375      <organization>Harvard University</organization>
1376      <address><email>sob@harvard.edu</email></address>
1377    </author>
1378    <date month="March" year="1997"/>
1379  </front>
1380  <seriesInfo name="BCP" value="14"/>
1381  <seriesInfo name="RFC" value="2119"/>
1382</reference>
1383
1384<reference anchor="RFC5234">
1385  <front>
1386    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1387    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1388      <organization>Brandenburg InternetWorking</organization>
1389      <address>
1390        <email>dcrocker@bbiw.net</email>
1391      </address> 
1392    </author>
1393    <author initials="P." surname="Overell" fullname="Paul Overell">
1394      <organization>THUS plc.</organization>
1395      <address>
1396        <email>paul.overell@thus.net</email>
1397      </address>
1398    </author>
1399    <date month="January" year="2008"/>
1400  </front>
1401  <seriesInfo name="STD" value="68"/>
1402  <seriesInfo name="RFC" value="5234"/>
1403</reference>
1404
1405</references>
1406
1407<references title="Informative References">
1408
1409<reference anchor="RFC2616">
1410  <front>
1411    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1412    <author initials="R." surname="Fielding" fullname="R. Fielding">
1413      <organization>University of California, Irvine</organization>
1414      <address><email>fielding@ics.uci.edu</email></address>
1415    </author>
1416    <author initials="J." surname="Gettys" fullname="J. Gettys">
1417      <organization>W3C</organization>
1418      <address><email>jg@w3.org</email></address>
1419    </author>
1420    <author initials="J." surname="Mogul" fullname="J. Mogul">
1421      <organization>Compaq Computer Corporation</organization>
1422      <address><email>mogul@wrl.dec.com</email></address>
1423    </author>
1424    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1425      <organization>MIT Laboratory for Computer Science</organization>
1426      <address><email>frystyk@w3.org</email></address>
1427    </author>
1428    <author initials="L." surname="Masinter" fullname="L. Masinter">
1429      <organization>Xerox Corporation</organization>
1430      <address><email>masinter@parc.xerox.com</email></address>
1431    </author>
1432    <author initials="P." surname="Leach" fullname="P. Leach">
1433      <organization>Microsoft Corporation</organization>
1434      <address><email>paulle@microsoft.com</email></address>
1435    </author>
1436    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1437      <organization>W3C</organization>
1438      <address><email>timbl@w3.org</email></address>
1439    </author>
1440    <date month="June" year="1999"/>
1441  </front>
1442  <seriesInfo name="RFC" value="2616"/>
1443</reference>
1444
1445<reference anchor='RFC3864'>
1446  <front>
1447    <title>Registration Procedures for Message Header Fields</title>
1448    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1449      <organization>Nine by Nine</organization>
1450      <address><email>GK-IETF@ninebynine.org</email></address>
1451    </author>
1452    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1453      <organization>BEA Systems</organization>
1454      <address><email>mnot@pobox.com</email></address>
1455    </author>
1456    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1457      <organization>HP Labs</organization>
1458      <address><email>JeffMogul@acm.org</email></address>
1459    </author>
1460    <date year='2004' month='September' />
1461  </front>
1462  <seriesInfo name='BCP' value='90' />
1463  <seriesInfo name='RFC' value='3864' />
1464</reference>
1465
1466<reference anchor='RFC4918'>
1467  <front>
1468    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1469    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1470      <organization abbrev="CommerceNet">CommerceNet</organization>
1471      <address><email>ldusseault@commerce.net</email></address>
1472    </author>
1473    <date month="June" year="2007" />
1474  </front>
1475  <seriesInfo name='RFC' value='4918' />
1476</reference>
1477</references>
1478
1479<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1480<t>
1481  Allow weak entity-tags in all requests except range requests (Sections
1482  <xref target="weak.and.strong.validators" format="counter"/> and
1483  <xref target="header.if-none-match" format="counter"/>).
1484</t>
1485<t>
1486  Change ABNF productions for header fields to only define the field value.
1487  (<xref target="header.fields"/>)
1488</t>
1489</section>
1490
1491<?BEGININC p4-conditional.abnf-appendix ?>
1492<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1493<figure>
1494<artwork type="abnf" name="p4-conditional.parsed-abnf">
1495<x:ref>ETag</x:ref> = entity-tag
1496
1497<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 6.1&gt;
1498
1499<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1500 entity-tag ] ) )
1501<x:ref>If-Modified-Since</x:ref> = HTTP-date
1502<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1503 entity-tag ] ) )
1504<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1505
1506<x:ref>Last-Modified</x:ref> = HTTP-date
1507
1508<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 1.2.2&gt;
1509
1510<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1511
1512<x:ref>opaque-tag</x:ref> = quoted-string
1513
1514<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
1515
1516<x:ref>weak</x:ref> = %x57.2F ; W/
1517</artwork>
1518</figure>
1519<figure><preamble>ABNF diagnostics:</preamble><artwork type="inline">
1520; ETag defined but not used
1521; If-Match defined but not used
1522; If-Modified-Since defined but not used
1523; If-None-Match defined but not used
1524; If-Unmodified-Since defined but not used
1525; Last-Modified defined but not used
1526</artwork></figure></section>
1527<?ENDINC p4-conditional.abnf-appendix ?>
1528
1529<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1530
1531<section title="Since RFC 2616">
1532<t>
1533  Extracted relevant partitions from <xref target="RFC2616"/>.
1534</t>
1535</section>
1536
1537<section title="Since draft-ietf-httpbis-p4-conditional-00">
1538<t>
1539  Closed issues:
1540  <list style="symbols"> 
1541    <t>
1542      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
1543      "Normative and Informative references"
1544    </t>
1545  </list>
1546</t>
1547<t>
1548  Other changes:
1549  <list style="symbols"> 
1550    <t>
1551      Move definitions of 304 and 412 condition codes from Part2.
1552    </t>
1553  </list>
1554</t>
1555</section>
1556
1557<section title="Since draft-ietf-httpbis-p4-conditional-01">
1558<t>
1559  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1560  <list style="symbols"> 
1561    <t>
1562      Add explicit references to BNF syntax and rules imported from other parts of the specification.
1563    </t>
1564  </list>
1565</t>
1566</section>
1567
1568<section title="Since draft-ietf-httpbis-p4-conditional-02" anchor="changes.since.02">
1569<t>
1570  Closed issues:
1571  <list style="symbols"> 
1572    <t>
1573      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1574      "Weak ETags on non-GET requests"
1575    </t>
1576  </list>
1577</t>
1578<t>
1579  Ongoing work on IANA Message Header Field Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
1580  <list style="symbols"> 
1581    <t>
1582      Reference RFC 3984, and update header field registrations for header fields defined
1583      in this document.
1584    </t>
1585  </list>
1586</t>
1587</section>
1588
1589<section title="Since draft-ietf-httpbis-p4-conditional-03" anchor="changes.since.03">
1590<t>
1591  Closed issues:
1592  <list style="symbols"> 
1593    <t>
1594      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/71"/>:
1595      "Examples for ETag matching"
1596    </t>
1597    <t>
1598      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/124"/>:
1599      "'entity value' undefined"
1600    </t>
1601    <t>
1602      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/126"/>:
1603      "bogus 2068 Date header reference"
1604    </t>
1605  </list>
1606</t>
1607</section>
1608
1609<section title="Since draft-ietf-httpbis-p4-conditional-04" anchor="changes.since.04">
1610<t>
1611  Ongoing work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1612  <list style="symbols"> 
1613    <t>
1614      Use "/" instead of "|" for alternatives.
1615    </t>
1616    <t>
1617      Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1618      whitespace ("OWS") and required whitespace ("RWS").
1619    </t>
1620    <t>
1621      Rewrite ABNFs to spell out whitespace rules, factor out
1622      header field value format definitions.
1623    </t>
1624  </list>
1625</t>
1626</section>
1627
1628<section title="Since draft-ietf-httpbis-p4-conditional-05" anchor="changes.since.05">
1629<t>
1630  Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
1631  <list style="symbols"> 
1632    <t>
1633      Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.
1634    </t>
1635  </list>
1636</t>
1637</section>
1638
1639<section title="Since draft-ietf-httpbis-p4-conditional-06" anchor="changes.since.06">
1640<t>
1641  Closed issues:
1642  <list style="symbols"> 
1643    <t>
1644      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/153"/>:
1645      "case-sensitivity of etag weakness indicator"
1646    </t>
1647  </list>
1648</t>
1649</section>
1650
1651<section title="Since draft-ietf-httpbis-p4-conditional-07" anchor="changes.since.07">
1652<t>
1653  Closed issues:
1654  <list style="symbols"> 
1655    <t>
1656      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/116"/>:
1657      "Weak ETags on non-GET requests" (If-Match still was defined to require
1658      strong matching)
1659    </t>
1660    <t>
1661      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/198"/>:
1662      "move IANA registrations for optional status codes"
1663    </t>
1664  </list>
1665</t>
1666</section>
1667
1668<section title="Since draft-ietf-httpbis-p4-conditional-08" anchor="changes.since.08">
1669<t>
1670  No significant changes.
1671</t>
1672</section>
1673
1674<section title="Since draft-ietf-httpbis-p4-conditional-09" anchor="changes.since.09">
1675<t>
1676  No significant changes.
1677</t>
1678</section>
1679
1680<section title="Since draft-ietf-httpbis-p4-conditional-10" anchor="changes.since.10">
1681<t>
1682  Closed issues:
1683  <list style="symbols"> 
1684    <t>
1685      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/69"/>:
1686      "Clarify 'Requested Variant'"
1687    </t>
1688    <t>
1689      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/109"/>:
1690      "Clarify entity / representation / variant terminology"
1691    </t>
1692    <t>
1693      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/220"/>:
1694      "consider removing the 'changes from 2068' sections"
1695    </t>
1696  </list>
1697</t>
1698</section>
1699
1700<section title="Since draft-ietf-httpbis-p4-conditional-11" anchor="changes.since.11">
1701<t>
1702  None.
1703</t>
1704</section>
1705
1706<section title="Since draft-ietf-httpbis-p4-conditional-12" anchor="changes.since.12">
1707<t>
1708  Closed issues:
1709  <list style="symbols"> 
1710    <t>
1711      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/224"/>:
1712      "Header Classification"
1713    </t>
1714  </list>
1715</t>
1716</section>
1717
1718<section title="Since draft-ietf-httpbis-p4-conditional-13" anchor="changes.since.13">
1719<t>
1720  Closed issues:
1721  <list style="symbols"> 
1722    <t>
1723      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/89"/>:
1724      "If-* and entities"
1725    </t>
1726    <t>
1727      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/101"/>:
1728      "Definition of validator weakness"
1729    </t>
1730    <t>
1731      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/276"/>:
1732      "untangle ABNFs for header fields"
1733    </t>
1734    <t>
1735      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/269"/>:
1736      "ETags and Quotes"
1737    </t>
1738  </list>
1739</t>
1740</section>
1741
1742<section title="Since draft-ietf-httpbis-p4-conditional-14" anchor="changes.since.14">
1743<t>
1744  None yet.
1745</t>
1746</section>
1747
1748</section>
1749
1750</back>
1751</rfc>
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