source: draft-ietf-httpbis/latest/p4-conditional.xml @ 2388

Last change on this file since 2388 was 2388, checked in by julian.reschke@…, 7 years ago

typo fixed (see #495)

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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 "September">
16  <!ENTITY ID-YEAR "2013">
17  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
18  <!ENTITY architecture               "<xref target='Part1' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY conformance                "<xref target='Part1' x:rel='#conformance' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY abnf-extension             "<xref target='Part1' x:rel='#abnf.extension' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY acks                       "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY whitespace                 "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY field-components           "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-date                "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY safe-methods               "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation             "<xref target='Part2' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY semantics                  "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY cache-key                  "<xref target='Part6' x:rel='#constructing.responses.from.caches' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY cache-validation-received  "<xref target='Part6' x:rel='#validation.received' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY freshening-responses       "<xref target='Part6' x:rel='#freshening.responses' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-accept-encoding     "<xref target='Part2' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-vary                "<xref target='Part2' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY http-date                  "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY content-negotiation        "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc rfcedstyle="yes"?>
52<?rfc-ext allow-markup-in-artwork="yes" ?>
53<?rfc-ext include-references-in-index="yes" ?>
54<rfc obsoletes="2616" category="std" x:maturity-level="proposed"
55     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
56     xmlns:x='http://purl.org/net/xml2rfc/ext'>
57<x:link rel="prev" basename="p2-semantics"/>
58<x:link rel="next" basename="p5-range"/>
59<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
60<front>
61
62  <title abbrev="HTTP/1.1 Conditional Requests">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
63
64  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
65    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
66    <address>
67      <postal>
68        <street>345 Park Ave</street>
69        <city>San Jose</city>
70        <region>CA</region>
71        <code>95110</code>
72        <country>USA</country>
73      </postal>
74      <email>fielding@gbiv.com</email>
75      <uri>http://roy.gbiv.com/</uri>
76    </address>
77  </author>
78
79  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
80    <organization abbrev="greenbytes">greenbytes GmbH</organization>
81    <address>
82      <postal>
83        <street>Hafenweg 16</street>
84        <city>Muenster</city><region>NW</region><code>48155</code>
85        <country>Germany</country>
86      </postal>
87      <email>julian.reschke@greenbytes.de</email>
88      <uri>http://greenbytes.de/tech/webdav/</uri>
89    </address>
90  </author>
91
92  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
93  <workgroup>HTTPbis Working Group</workgroup>
94
95<abstract>
96<t>
97   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
98   distributed, collaborative, hypertext information systems. This document
99   defines HTTP/1.1 conditional requests, including metadata header fields
100   for indicating state changes, request header fields for making
101   preconditions on such state, and rules for constructing the responses to a
102   conditional request when one or more preconditions evaluate to false.
103</t>
104</abstract>
105
106<note title="Editorial Note (To be removed by RFC Editor)">
107  <t>
108    Discussion of this draft takes place on the HTTPBIS working group
109    mailing list (ietf-http-wg@w3.org), which is archived at
110    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
111  </t>
112  <t>
113    The current issues list is at
114    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
115    documents (including fancy diffs) can be found at
116    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
117  </t>
118  <t>
119    The changes in this draft are summarized in <xref target="changes.since.23"/>.
120  </t>
121</note>
122</front>
123
124<middle>
125<section title="Introduction" anchor="introduction">
126<t>
127   Conditional requests are HTTP requests <xref target="Part2"/> that include
128   one or more header fields indicating a precondition to be tested before
129   applying the method semantics to the target resource.
130   This document defines the HTTP/1.1 conditional request mechanisms in terms
131   of the architecture, syntax notation, and conformance criteria defined in
132   <xref target="Part1"/>.
133</t>
134<t>
135   Conditional GET requests are the most efficient mechanism for HTTP
136   cache updates &caching;.  Conditionals can also be
137   applied to state-changing methods, such as PUT and DELETE, to prevent
138   the "lost update" problem: one client accidentally overwriting
139   the work of another client that has been acting in parallel.
140</t>
141<t><iref primary="true" item="selected representation"/>
142   Conditional request preconditions are based on the state of the target
143   resource as a whole (its current value set) or the state as observed
144   in a previously obtained representation (one value in that set).
145   A resource might have multiple current representations, each with its
146   own observable state.  The conditional request mechanisms assume that
147   the mapping of requests to a "selected representation" (&representation;)
148   will be consistent over time if the server intends to take advantage of
149   conditionals. Regardless, if the mapping is inconsistent and the server is
150   unable to select the appropriate representation, then no harm will result
151   when the precondition evaluates to false.
152</t>
153<t>
154   The conditional request preconditions defined by this specification
155   (<xref target="preconditions"/>) are evaluated when applicable to the
156   recipient (<xref target="evaluation"/>) according to their order of
157   precedence (<xref target="precedence"/>).
158</t>
159
160<section title="Conformance and Error Handling" anchor="conformance">
161<t>
162   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
163   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
164   document are to be interpreted as described in <xref target="RFC2119"/>.
165</t>
166<t>
167   Conformance criteria and considerations regarding error handling
168   are defined in &conformance;.
169</t>
170</section>
171
172<section title="Syntax Notation" anchor="notation">
173<t>
174   This specification uses the Augmented Backus-Naur Form (ABNF) notation
175   of <xref target="RFC5234"/> with the list rule extension defined in
176   &abnf-extension;. <xref target="imported.abnf"/> describes rules imported from
177   other documents. <xref target="collected.abnf"/> shows the collected ABNF
178   with the list rule expanded.
179</t>
180</section>
181</section>
182
183<section title="Validators" anchor="validators">
184   <iref primary="true" item="metadata"/>
185   <iref primary="true" item="validator"/>
186<t>
187   This specification defines two forms of metadata that are commonly used
188   to observe resource state and test for preconditions: modification dates
189   (<xref target="header.last-modified"/>) and opaque entity tags
190   (<xref target="header.etag"/>).  Additional metadata that reflects resource state
191   has been defined by various extensions of HTTP, such as WebDAV
192   <xref target="RFC4918"/>, that are beyond the scope of this specification.
193   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
194   when it is used within a precondition.
195</t>
196
197<section title="Weak versus Strong" anchor="weak.and.strong.validators">
198   <iref primary="true" item="validator" subitem="weak"/>
199   <iref primary="true" item="validator" subitem="strong"/>
200<t>
201   Validators come in two flavors: strong or weak.  Weak validators are easy
202   to generate but are far less useful for comparisons.  Strong validators
203   are ideal for comparisons but can be very difficult (and occasionally
204   impossible) to generate efficiently.  Rather than impose that all forms
205   of resource adhere to the same strength of validator, HTTP exposes the
206   type of validator in use and imposes restrictions on when weak validators
207   can be used as preconditions.
208</t>
209<t>
210   A "strong validator" is representation metadata that changes value whenever
211   a change occurs to the representation data that would be observable in the
212   payload body of a <x:ref>200 (OK)</x:ref> response to GET.
213</t>
214<t>  
215   A strong validator might change for other reasons, such as when a
216   semantically significant part of the representation metadata is changed
217   (e.g., <x:ref>Content-Type</x:ref>), but it is in the best interests of the
218   origin server to only change the value when it is necessary to invalidate
219   the stored responses held by remote caches and authoring tools. A strong
220   validator is unique across all representations of a given resource, such
221   that no two representations of that resource can share the same validator
222   unless their representation data is identical.
223</t>
224<t>
225   Cache entries might persist for arbitrarily long periods, regardless
226   of expiration times.  Thus, a cache might attempt to validate an
227   entry using a validator that it obtained in the distant past.
228   A strong validator is unique across all versions of all
229   representations associated with a particular resource over time.
230   However, there is no implication of uniqueness across representations
231   of different resources (i.e., the same strong validator might be
232   in use for representations of multiple resources at the same time
233   and does not imply that those representations are equivalent).
234</t>
235<t>
236   There are a variety of strong validators used in practice.  The best are
237   based on strict revision control, wherein each change to a representation
238   always results in a unique node name and revision identifier being assigned
239   before the representation is made accessible to GET.  A collision-resistant hash
240   function applied to the representation data is also sufficient if the data
241   is available prior to the response header fields being sent and the digest
242   does not need to be recalculated every time a validation request is
243   received.  However, if a resource has distinct representations that differ
244   only in their metadata, such as might occur with content negotiation over
245   media types that happen to share the same data format, then the origin
246   server &SHOULD; incorporate additional information in the validator to
247   distinguish those representations.
248</t>
249<t>
250   In contrast, a "weak validator" is representation metadata that
251   might not change for every change to the representation data.  This
252   weakness might be due to limitations in how the value is calculated, such
253   as clock resolution or an inability to ensure uniqueness for all possible
254   representations of the resource, or due to a desire by the resource owner
255   to group representations by some self-determined set of equivalency
256   rather than unique sequences of data.  An origin server &SHOULD; change a
257   weak entity-tag whenever it considers prior representations to be
258   unacceptable as a substitute for the current representation. In other words,
259   a weak entity-tag ought to change whenever the origin server wants caches to
260   invalidate old responses.
261</t>
262<t>
263   For example, the representation of a weather report that changes in
264   content every second, based on dynamic measurements, might be grouped
265   into sets of equivalent representations (from the origin server's
266   perspective) with the same weak validator in order to allow cached
267   representations to be valid for a reasonable period of time (perhaps
268   adjusted dynamically based on server load or weather quality).
269   Likewise, a representation's modification time, if defined with only
270   one-second resolution, might be a weak validator if it is possible
271   for the representation to be modified twice during a single second and
272   retrieved between those modifications.
273</t>
274<t>
275   Likewise, a validator is weak if it is shared by two or more
276   representations of a given resource at the same time, unless those
277   representations have identical representation data. For example, if the
278   origin server sends the same validator for a representation with a gzip
279   content coding applied as it does for a representation with no content
280   coding, then that validator is weak. However, two simultaneous
281   representations might share the same strong validator if they differ only
282   in the representation metadata, such as when two different media types are
283   available for the same representation data.
284</t>
285<t>
286   A "use" of a validator occurs when either a client generates a request
287   and includes the validator in a precondition or when a server
288   compares two validators.
289   Weak validators are only usable in contexts that do not depend on exact
290   equality of the representation data.
291   Strong validators are usable and preferred for all conditional requests,
292   including cache validation, partial content ranges, and "lost update"
293   avoidance.
294</t>
295</section>
296
297<section title="Last-Modified" anchor="header.last-modified">
298  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
299  <x:anchor-alias value="Last-Modified"/>
300<t>
301   The "Last-Modified" header field in a response provides a timestamp
302   indicating the date and time at which the origin server believes the
303   selected representation was last modified, as determined at the conclusion
304   of handling the request.
305</t>
306<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
307  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
308</artwork></figure>
309<t>
310   An example of its use is
311</t>
312<figure><artwork type="example">
313  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
314</artwork></figure>
315
316<section title="Generation" anchor="lastmod.generation">
317<t>
318   Origin servers &SHOULD; send Last-Modified for any selected
319   representation for which a last modification date can be reasonably
320   and consistently determined, since its use in conditional requests
321   and evaluating cache freshness (&caching;) results in a substantial
322   reduction of HTTP traffic on the Internet and can be a significant
323   factor in improving service scalability and reliability.
324</t>
325<t>
326   A representation is typically the sum of many parts behind the
327   resource interface.  The last-modified time would usually be
328   the most recent time that any of those parts were changed.
329   How that value is determined for any given resource is an
330   implementation detail beyond the scope of this specification.
331   What matters to HTTP is how recipients of the Last-Modified
332   header field can use its value to make conditional requests
333   and test the validity of locally cached responses.
334</t>
335<t>
336   An origin server &SHOULD; obtain the Last-Modified value of the
337   representation as close as possible to the time that it generates the
338   <x:ref>Date</x:ref> field value for its response. This allows a recipient to
339   make an accurate assessment of the representation's modification time,
340   especially if the representation changes near the time that the
341   response is generated.
342</t>
343<t>
344   An origin server with a clock &MUST-NOT; send a Last-Modified date
345   that is later than the server's time of message origination (<x:ref>Date</x:ref>).
346   If the last modification time is derived from implementation-specific
347   metadata that evaluates to some time in the future, according to the
348   origin server's clock, then the origin server &MUST; replace that
349   value with the message origination date. This prevents a future
350   modification date from having an adverse impact on cache validation.
351</t>
352<t>
353   An origin server without a clock &MUST-NOT; assign Last-Modified
354   values to a response unless these values were associated
355   with the resource by some other system or user with a reliable clock.
356</t>
357</section>
358
359<section title="Comparison" anchor="lastmod.comparison">
360<t>
361   A Last-Modified time, when used as a validator in a request, is
362   implicitly weak unless it is possible to deduce that it is strong,
363   using the following rules:
364  <list style="symbols">
365     <t>The validator is being compared by an origin server to the
366        actual current validator for the representation and,</t>
367     <t>That origin server reliably knows that the associated representation did
368        not change twice during the second covered by the presented
369        validator.</t>
370  </list>
371</t>
372<t>
373   or
374  <list style="symbols">
375     <t>The validator is about to be used by a client in an <x:ref>If-Modified-Since</x:ref>,
376        <x:ref>If-Unmodified-Since</x:ref> header field, because the client has
377        a cache entry, or <x:ref>If-Range</x:ref> for the associated
378        representation, and</t>
379     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
380        time when the origin server sent the original response, and</t>
381     <t>The presented Last-Modified time is at least 60 seconds before
382        the Date value.</t>
383  </list>
384</t>
385<t>
386   or
387  <list style="symbols">
388     <t>The validator is being compared by an intermediate cache to the
389        validator stored in its cache entry for the representation, and</t>
390     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
391        time when the origin server sent the original response, and</t>
392     <t>The presented Last-Modified time is at least 60 seconds before
393        the Date value.</t>
394  </list>
395</t>
396<t>
397   This method relies on the fact that if two different responses were
398   sent by the origin server during the same second, but both had the
399   same Last-Modified time, then at least one of those responses would
400   have a <x:ref>Date</x:ref> value equal to its Last-Modified time. The
401   arbitrary 60-second limit guards against the possibility that the Date and
402   Last-Modified values are generated from different clocks, or at somewhat
403   different times during the preparation of the response. An
404   implementation &MAY; use a value larger than 60 seconds, if it is
405   believed that 60 seconds is too short.
406</t>
407</section>
408</section>
409
410<section title="ETag" anchor="header.etag">
411  <iref primary="true" item="ETag header field" x:for-anchor=""/>
412  <x:anchor-alias value="ETag"/>
413  <x:anchor-alias value="entity-tag"/>
414  <x:anchor-alias value="opaque-tag"/>
415  <x:anchor-alias value="weak"/>
416  <x:anchor-alias value="etagc"/>
417<t>
418   The "ETag" header field in a response provides the current entity-tag for
419   the selected representation, as determined at the conclusion of handling
420   the request.
421   An entity-tag is an opaque validator for differentiating between
422   multiple representations of the same resource, regardless of whether
423   those multiple representations are due to resource state changes over
424   time, content negotiation resulting in multiple representations being
425   valid at the same time, or both. An entity-tag consists of an opaque
426   quoted string, possibly prefixed by a weakness indicator.
427</t>
428<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"/><iref primary="true" item="Grammar" subitem="etagc"/>
429  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
430
431  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
432  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
433  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
434  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
435             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
436</artwork></figure>
437<x:note>
438  <t>
439    &Note; Previously, opaque-tag was defined to be a quoted-string
440    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
441    might perform backslash unescaping. Servers therefore ought to avoid
442    backslash characters in entity tags.
443  </t>
444</x:note>
445<t>
446   An entity-tag can be more reliable for validation than a modification
447   date in situations where it is inconvenient to store modification
448   dates, where the one-second resolution of HTTP date values is not
449   sufficient, or where modification dates are not consistently maintained.
450</t>
451<figure><preamble>
452  Examples:
453</preamble>
454<artwork type="example">
455  ETag: "xyzzy"
456  ETag: W/"xyzzy"
457  ETag: ""
458</artwork></figure>
459<t>
460   An entity-tag can be either a weak or strong validator, with
461   strong being the default.  If an origin server provides an entity-tag
462   for a representation and the generation of that entity-tag does not satisfy
463   all of the characteristics of a strong validator
464   (<xref target="weak.and.strong.validators"/>), then the origin server
465   &MUST; mark the entity-tag as weak by prefixing its opaque value
466   with "W/" (case-sensitive).
467</t>
468
469<section title="Generation" anchor="entity.tag.generation">
470<t>
471   The principle behind entity-tags is that only the service author
472   knows the implementation of a resource well enough to select the
473   most accurate and efficient validation mechanism for that resource,
474   and that any such mechanism can be mapped to a simple sequence of
475   octets for easy comparison.  Since the value is opaque, there is no
476   need for the client to be aware of how each entity-tag is constructed.
477</t>
478<t>
479   For example, a resource that has implementation-specific versioning
480   applied to all changes might use an internal revision number, perhaps
481   combined with a variance identifier for content negotiation, to
482   accurately differentiate between representations.
483   Other implementations might use a collision-resistant hash of
484   representation content,
485   a combination of various filesystem attributes, or a modification
486   timestamp that has sub-second resolution.
487</t>
488<t>
489   Origin servers &SHOULD; send ETag for any selected representation
490   for which detection of changes can be reasonably and consistently
491   determined, since the entity-tag's use in conditional requests and
492   evaluating cache freshness (&caching;) can result in a substantial
493   reduction of HTTP network traffic and can be a significant factor in
494   improving service scalability and reliability.
495</t>
496</section>
497
498<section title="Comparison" anchor="entity.tag.comparison">
499  <x:anchor-alias value="validator.comparison"/>
500  <x:anchor-alias value="strong comparison"/>
501  <x:anchor-alias value="weak comparison"/>
502<t>
503   There are two entity-tag comparison functions, depending
504   on whether the comparison context allows the use of weak validators
505   or not:
506  <list style="symbols">
507     <t><x:dfn>Strong comparison</x:dfn>: two entity-tags are equivalent if both
508        are not weak and their opaque-tags match character-by-character.</t>
509     <t><x:dfn>Weak comparison</x:dfn>: two entity-tags are equivalent if their opaque-tags
510        match character-by-character, regardless of either or both
511        being tagged as "weak".</t>
512  </list>
513</t>
514<t>
515   The example below shows the results for a set of entity-tag pairs,
516   and both the weak and strong comparison function results:
517</t>
518<texttable align="left">
519  <ttcol>ETag 1</ttcol>
520  <ttcol>ETag 2</ttcol>
521  <ttcol>Strong Comparison</ttcol>
522  <ttcol>Weak Comparison</ttcol>
523
524  <c>W/"1"</c>
525  <c>W/"1"</c>
526  <c>no match</c>
527  <c>match</c>
528 
529  <c>W/"1"</c>
530  <c>W/"2"</c>
531  <c>no match</c>
532  <c>no match</c>
533
534  <c>W/"1"</c>
535  <c>"1"</c>
536  <c>no match</c>
537  <c>match</c>
538
539  <c>"1"</c>
540  <c>"1"</c>
541  <c>match</c>
542  <c>match</c>
543</texttable>
544</section>
545
546<section title="Example: Entity-tags Varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
547<t>
548   Consider a resource that is subject to content negotiation
549   (&content-negotiation;), and where the representations sent in response to
550   a GET request vary based on the <x:ref>Accept-Encoding</x:ref> request
551   header field (&header-accept-encoding;):
552</t>
553<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
554GET /index HTTP/1.1
555Host: www.example.com
556Accept-Encoding: gzip
557
558</artwork></figure>
559<t>
560   In this case, the response might or might not use the gzip content coding.
561   If it does not, the response might look like:
562</t>
563<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
564HTTP/1.1 200 OK
565Date: Fri, 26 Mar 2010 00:05:00 GMT
566ETag: "123-a"
567Content-Length: <x:length-of target="exbody"/>
568Vary: Accept-Encoding
569Content-Type: text/plain
570
571<x:span anchor="exbody">Hello World!
572Hello World!
573Hello World!
574Hello World!
575Hello World!
576</x:span></artwork></figure>
577<t>
578   An alternative representation that does use gzip content coding would be:
579</t>
580<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
581HTTP/1.1 200 OK
582Date: Fri, 26 Mar 2010 00:05:00 GMT
583ETag: "123-b"
584Content-Length: 43
585Vary: Accept-Encoding
586Content-Type: text/plain
587Content-Encoding: gzip
588
589<spanx>...binary data...</spanx></artwork></figure>
590<x:note>
591  <t>
592    &Note; Content codings are a property of the representation,
593    so therefore an entity-tag of an encoded representation has to be distinct
594    from an unencoded representation to prevent conflicts during cache updates
595    and range requests.  In contrast, transfer codings (&transfer-codings;)
596    apply only during message transfer and do not require distinct entity-tags.
597  </t>
598</x:note>
599</section>
600</section>
601
602<section title="When to Use Entity-tags and Last-Modified Dates" anchor="when.to.use.entity.tags.and.last-modified.dates">
603<t>
604   We adopt a set of rules and recommendations for origin servers,
605   clients, and caches regarding when various validator types ought to
606   be used, and for what purposes.
607</t>
608<t>
609   In <x:ref>200 (OK)</x:ref> responses to GET or HEAD, an origin server:
610  <list style="symbols">
611     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
612        generate one.</t>
613
614     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
615        performance considerations support the use of weak entity-tags,
616        or if it is unfeasible to send a strong entity-tag.</t>
617
618     <t>&SHOULD; send a <x:ref>Last-Modified</x:ref> value if it is feasible to
619        send one.</t>
620  </list>
621</t>
622<t>
623   In other words, the preferred behavior for an origin server
624   is to send both a strong entity-tag and a <x:ref>Last-Modified</x:ref>
625   value in successful responses to a retrieval request.
626</t>
627<t>
628   A client:
629  <list style="symbols">
630     <t>&MUST; use that entity-tag in any cache-conditional request (using
631        <x:ref>If-Match</x:ref> or <x:ref>If-None-Match</x:ref>) if an
632        entity-tag has been provided by the origin server.</t>
633
634     <t>&SHOULD; use the <x:ref>Last-Modified</x:ref> value in non-subrange
635        cache-conditional requests (using <x:ref>If-Modified-Since</x:ref>)
636        if only a Last-Modified value has been provided by the origin server.</t>
637
638     <t>&MAY; use the <x:ref>Last-Modified</x:ref> value in subrange
639        cache-conditional requests (using <x:ref>If-Unmodified-Since</x:ref>)
640        if only a Last-Modified value has been provided by an HTTP/1.0 origin
641        server. The user agent &SHOULD; provide a way to disable this, in case
642        of difficulty.</t>
643
644     <t>&SHOULD; use both validators in cache-conditional requests if both an
645        entity-tag and a <x:ref>Last-Modified</x:ref> value have been provided
646        by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
647        respond appropriately.</t>
648  </list>
649</t>
650</section>
651</section>
652
653<section title="Precondition Header Fields" anchor="preconditions">
654<t>
655   This section defines the syntax and semantics of HTTP/1.1 header fields
656   for applying preconditions on requests.
657   <xref target="evaluation"/> defines when the preconditions are applied.
658   <xref target="precedence"/> defines the order of evaluation when more than
659   one precondition is present.
660</t>
661<t>
662   Requirements on a cache when handling a received conditional request are
663   defined in &cache-validation-received;.
664</t>
665
666<section title="If-Match" anchor="header.if-match">
667  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
668  <x:anchor-alias value="If-Match"/>
669<t>
670   The "If-Match" header field makes the request method conditional on the
671   recipient cache or origin server either having at least one current
672   representation of the target resource, when the field-value is "*", or
673   having a current representation of the target resource that has an
674   entity-tag matching a member of the list of entity-tags provided in the
675   field-value.
676</t>
677<t>
678   A recipient &MUST; use the strong comparison function when comparing
679   entity-tags for If-Match (<xref target="entity.tag.comparison"/>).
680</t>
681<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
682  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
683</artwork></figure>
684<t>
685   Examples:
686</t>
687<figure><artwork type="example">
688  If-Match: "xyzzy"
689  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
690  If-Match: *
691</artwork></figure>
692<t>
693   If-Match is most often used with state-changing methods (e.g., POST, PUT,
694   DELETE) to prevent accidental overwrites when multiple user agents might be
695   acting in parallel on the same resource (i.e., to prevent the "lost update"
696   problem). It can also be used with safe methods to abort a request if the
697   <x:ref>selected representation</x:ref> does not match one already stored
698   (or partially stored) from a prior request.
699</t>
700<t>
701   An origin server that receives an If-Match header field &MUST; evaluate the
702   condition prior to performing the method (<xref target="evaluation"/>).
703   If the field-value is "*", the condition is false if the origin server
704   does not have a current representation for the target resource.
705   If the field-value is a list of entity-tags, the condition is false if
706   none of the listed tags match the entity-tag of the selected representation.
707</t>
708<t>
709   An origin server &MUST-NOT; perform the requested method if a received
710   If-Match condition evaluates to false; instead the origin server &MUST;
711   respond with either:
712   a) the <x:ref>412 (Precondition Failed)</x:ref> status code; or,
713   b) one of the <x:ref>2xx (Successful)</x:ref> status codes if the origin
714   server has verified that a state change is being requested and the final
715   state is already reflected in the current state of the target resource
716   (i.e., the change requested by the user agent has already succeeded, but
717   the user agent might not be aware of it, perhaps because the prior response
718   was lost or a compatible change was made by some other user agent).
719   In the latter case, the origin server &MUST-NOT; send a validator header
720   field in the response unless it can verify that the request is a duplicate
721   of an immediately prior change made by the same user agent.
722</t>
723</section>
724
725<section title="If-None-Match" anchor="header.if-none-match">
726  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
727  <x:anchor-alias value="If-None-Match"/>
728<t>
729   The "If-None-Match" header field makes the request method conditional on
730   the recipient cache or origin server either not having a current
731   representation of the target resource, when the field-value is "*", or
732   not having a current representation of the target resource that has an
733   entity-tag matching a member of the list of entity-tags provided in the
734   field-value.
735</t>
736<t>
737   A recipient &MUST; use the weak comparison function when comparing
738   entity-tags for If-None-Match (<xref target="entity.tag.comparison"/>).
739</t>
740<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
741  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
742</artwork></figure>
743<t>
744   Examples:
745</t>
746<figure><artwork type="example">
747  If-None-Match: "xyzzy"
748  If-None-Match: W/"xyzzy"
749  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
750  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
751  If-None-Match: *
752</artwork></figure>
753<t>
754   If-None-Match is primarily used in conditional GET requests to enable
755   efficient updates of cached information with a minimum amount of
756   transaction overhead. When a client desires to update one or more stored
757   responses that have entity-tags, the client &SHOULD; generate an
758   If-None-Match header field containing a list of those entity-tags when
759   making a GET request; this allows recipient servers to send a
760   <x:ref>304 (Not Modified)</x:ref> response to indicate when one of those
761   stored responses matches the selected representation.
762</t>
763<t>
764   If-None-Match can also be used with a value of "*" to prevent an unsafe
765   request method (e.g., PUT) from inadvertently modifying an existing
766   representation of the target resource when the client believes that
767   the resource does not have a current representation (&safe-methods;).
768   This is a variation on the "lost update" problem that might arise if more
769   than one client attempts to create an initial representation for the target
770   resource.
771</t>
772<t>
773   An origin server that receives an If-None-Match header field &MUST;
774   evaluate the condition prior to performing the method
775   (<xref target="evaluation"/>).
776   If the field-value is "*", the condition is false if the origin server
777   has a current representation for the target resource.
778   If the field-value is a list of entity-tags, the condition is false if
779   one of the listed tags match the entity-tag of the selected representation.
780</t>
781<t>
782   An origin server &MUST-NOT; perform the requested method if the condition
783   evaluates to false; instead, the origin server &MUST; respond with either
784   a) the <x:ref>304 (Not Modified)</x:ref> status code if the request method
785   is GET or HEAD; or,
786   b) the <x:ref>412 (Precondition Failed)</x:ref> status code for all other
787   request methods.
788</t>
789</section>
790
791<section title="If-Modified-Since" anchor="header.if-modified-since">
792  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
793  <x:anchor-alias value="If-Modified-Since"/>
794<t>
795   The "If-Modified-Since" header field makes the GET or HEAD request method
796   conditional on the selected representation's modification date being more
797   recent than the date provided in the field-value: if the selected representation
798   has not been modified since the time specified in this field, then respond
799   with a <x:ref>304 (Not Modified)</x:ref> status code and only send
800   metadata that might be useful for identifying or updating a cached
801   representation. Hence, transfer of the selected representation's data is
802   avoided if that data has not changed.
803</t>
804<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
805  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
806</artwork></figure>
807<t>
808   An example of the field is:
809</t>
810<figure><artwork type="example">
811  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
812</artwork></figure>
813<t>
814   A recipient &MUST; ignore If-Modified-Since if the request contains an
815   <x:ref>If-None-Match</x:ref> header field; the condition in
816   <x:ref>If-None-Match</x:ref> is considered to be a more accurate
817   replacement for the condition in If-Modified-Since and the two are only
818   combined for the sake of interoperating with older intermediaries that
819   might not implement <x:ref>If-None-Match</x:ref>.
820</t>
821<t>
822   A recipient &MUST; ignore the If-Modified-Since header field if the
823   received field-value is not a valid HTTP-date, or if the request method
824   is neither GET nor HEAD.
825</t>
826<t>
827   A recipient &MUST; interpret an If-Modified-Since field-value's timestamp
828   in terms of the origin server's clock.  A recipient cache that evaluates
829   this precondition &SHOULD; make its comparisons with
830   respect to a stored <x:ref>Last-Modified</x:ref> or <x:ref>Date</x:ref>
831   header field, when available in a prior response from that origin server,
832   rather than comparing it to a timestamp generated by its own clock.
833</t>
834<t>
835   If-Modified-Since is typically used for two distinct purposes:
836   1) to allow efficient updates of a cached representation that does not
837   have an entity-tag; and,
838   2) to limit the scope of a web traversal to resources that have recently
839   changed.
840</t>
841<t>
842   When used for cache updates, a cache will typically use the value of the
843   cached message's <x:ref>Last-Modified</x:ref> field to generate the field
844   value of If-Modified-Since. This behavior is most interoperable for cases
845   where clocks are poorly synchronized or when the server has chosen to only
846   honor exact timestamp matches (due to a problem with Last-Modified dates
847   that appear to go "back in time" when the origin server's clock is
848   corrected or a representation is restored from an archived backup).
849   However, caches occasionally generate the field value based on other data,
850   such as the <x:ref>Date</x:ref> header field of the cached message or the
851   local clock time that the message was received, particularly when the
852   cached message does not contain a <x:ref>Last-Modified</x:ref> field.
853</t>
854<t>
855   When used for limiting the scope of retrieval to a recent time window, a
856   user agent will generate an If-Modified-Since field value based on either
857   its own local clock or a <x:ref>Date</x:ref> header field received from the
858   server in a prior response. Origin servers that choose an exact timestamp
859   match based on the selected representation's <x:ref>Last-Modified</x:ref>
860   field will not be able to help the user agent limit its data transfers to
861   only those changed during the specified window.
862</t>
863</section>
864
865<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
866  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
867  <x:anchor-alias value="If-Unmodified-Since"/>
868<t>
869   The "If-Unmodified-Since" header field makes the request method conditional
870   on the selected representation's last modification date being earlier than or
871   equal to the date provided in the field-value. This field accomplishes the
872   same purpose as <x:ref>If-Match</x:ref> for cases where the user agent does
873   not have an entity-tag for the representation.
874</t>
875<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
876  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
877</artwork></figure>
878<t>
879   An example of the field is:
880</t>
881<figure><artwork type="example">
882  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
883</artwork></figure>
884<t>
885   A recipient &MUST; ignore If-Unmodified-Since if the request contains an
886   <x:ref>If-Match</x:ref> header field; the condition in
887   <x:ref>If-Match</x:ref> is considered to be a more accurate replacement for
888   the condition in If-Unmodified-Since and the two are only combined for the
889   sake of interoperating with older intermediaries that might not implement
890   <x:ref>If-Match</x:ref>.
891</t>
892<t>
893   A recipient &MUST; ignore the If-Unmodified-Since header field if the
894   received field-value is not a valid HTTP-date.
895</t>
896<t>
897   A recipient &MUST; interpret an If-Unmodified-Since field-value's timestamp
898   in terms of the origin server's clock.
899</t>
900<t>
901   If-Unmodified-Since is most often used with state-changing methods
902   (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple
903   user agents might be acting in parallel on a resource that does
904   not supply entity-tags with its representations (i.e., to prevent the
905   "lost update" problem). It can also be used with safe methods to abort a
906   request if the <x:ref>selected representation</x:ref> does not match one
907   already stored (or partially stored) from a prior request.
908</t>
909<t>
910   An origin server that receives an If-Unmodified-Since header field &MUST;
911   evaluate the condition prior to performing the method
912   (<xref target="evaluation"/>). The condition is false if the selected
913   representation's last modification date is more recent than the date
914   provided in the field-value.
915</t>
916<t>
917   An origin server &MUST-NOT; perform the requested method if a received
918   If-Unmodified-Since condition evaluates to false; instead the
919   origin server &MUST; respond with either:
920   a) the <x:ref>412 (Precondition Failed)</x:ref> status code; or,
921   b) one of the <x:ref>2xx (Successful)</x:ref> status codes if the origin
922   server has verified that a state change is being requested and the final
923   state is already reflected in the current state of the target resource
924   (i.e., the change requested by the user agent has already succeeded, but
925   the user agent might not be aware of that because the prior response message
926   was lost or a compatible change was made by some other user agent).
927   In the latter case, the origin server &MUST-NOT; send a validator header
928   field in the response unless it can verify that the request is a duplicate
929   of an immediately prior change made by the same user agent.
930</t>
931</section>
932
933<section title="If-Range" anchor="header.if-range">
934<t>
935   The "If-Range" header field provides a special conditional request
936   mechanism that is similar to the <x:ref>If-Match</x:ref> and
937   <x:ref>If-Unmodified-Since</x:ref> header fields but instructs the recipient to
938   ignore the <x:ref>Range</x:ref> header field if the validator doesn't
939   match, resulting in transfer of the new selected representation instead of
940   a 412 response. If-Range is defined in &header-if-range;.
941</t>
942</section>
943</section>
944
945<section title="Status Code Definitions" anchor="status.code.definitions">
946<section title="304 Not Modified" anchor="status.304">
947  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
948  <x:anchor-alias value="304"/>
949  <x:anchor-alias value="304 (Not Modified)"/>
950<t>
951   The <x:dfn>304 (Not Modified)</x:dfn> status code indicates that a
952   conditional GET or HEAD request has been
953   received and would have resulted in a <x:ref>200 (OK)</x:ref> response
954   if it were not for the fact that the condition has evaluated to false.
955   In other words, there is no need for the server to transfer a
956   representation of the target resource because the request indicates that
957   the client, which made the request conditional, already has a valid
958   representation; the server is therefore redirecting the client to make
959   use of that stored representation as if it were the payload of a
960   <x:ref>200 (OK)</x:ref> response.
961</t>
962<t>
963   The server generating a 304 response &MUST; generate any of the following
964   header fields that would have been sent in a <x:ref>200 (OK)</x:ref>
965   response to the same request:
966   <x:ref>Cache-Control</x:ref>,
967   <x:ref>Content-Location</x:ref>,
968   <x:ref>ETag</x:ref>,
969   <x:ref>Expires</x:ref>, and
970   <x:ref>Vary</x:ref>.
971</t>
972<t>
973   Since the goal of a 304 response is to minimize information transfer
974   when the recipient already has one or more cached representations,
975   a sender &SHOULD-NOT; generate representation metadata other
976   than the above listed fields unless said metadata exists for the
977   purpose of guiding cache updates (e.g., <x:ref>Last-Modified</x:ref> might
978   be useful if the response does not have an <x:ref>ETag</x:ref> field).
979</t>
980<t>
981   Requirements on a cache that receives a 304 response are defined in
982   &freshening-responses;. If the conditional request originated with an
983   outbound client, such as a user agent with its own cache sending a
984   conditional GET to a shared proxy, then the proxy &SHOULD; forward the
985   304 response to that client.
986</t>
987<t>
988   A 304 response cannot contain a message-body; it is always
989   terminated by the first empty line after the header fields.
990</t>
991</section>
992
993<section title="412 Precondition Failed" anchor="status.412">
994  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
995  <x:anchor-alias value="412 (Precondition Failed)"/>
996<t>
997   The <x:dfn>412 (Precondition Failed)</x:dfn> status code indicates that one
998   or more conditions given in the request header fields evaluated to false
999   when tested on the server. This response code allows the client to place
1000   preconditions on the current resource state (its current representations
1001   and metadata) and thus prevent the request method from being applied if the
1002   target resource is in an unexpected state.
1003</t>
1004</section>
1005</section>
1006
1007<section title="Evaluation" anchor="evaluation">
1008<t>
1009   Except when excluded below, a server &MUST; evaluate received request
1010   preconditions after it has successfully performed its normal request checks
1011   and just before it would perform the action associated with the request
1012   method.
1013   A server &MUST; ignore all received preconditions if its response to the
1014   same request without those conditions would have been a status code other
1015   than a <x:ref>2xx</x:ref> or <x:ref>412 (Precondition Failed)</x:ref>.
1016   In other words, redirects and failures take precedence over the evaluation
1017   of preconditions in conditional requests.
1018</t>
1019<t>
1020   A server that is not the origin server for the target resource and cannot
1021   act as a cache for requests on the target resource &MUST-NOT; evaluate the
1022   conditional request header fields defined by this specification, and
1023   &MUST; forward them if the request is forwarded, since the generating
1024   client intends that they be evaluated by a server that can provide a
1025   current representation.
1026   Likewise, a server &MUST; ignore the conditional request header fields
1027   defined by this specification when received with a request method that does
1028   not involve the selection or modification of a
1029   <x:ref>selected representation</x:ref>, such as CONNECT, OPTIONS, or TRACE.
1030</t>
1031<t>
1032   Conditional request header fields that are defined by extensions to HTTP
1033   might place conditions on all recipients, on the state of the target
1034   resource in general, or on a group of resources. For instance, the "If"
1035   header field in WebDAV can make a request conditional on various aspects
1036   of multiple resources, such as locks, if the recipient understands and
1037   implements that field (<xref target="RFC4918" x:fmt="," x:sec="10.4"/>).
1038</t>
1039<t>
1040   Although conditional request header fields are defined as being usable with
1041   the HEAD method (to keep HEAD's semantics consistent with those of GET),
1042   there is no point in sending a conditional HEAD because a successful
1043   response is around the same size as a <x:ref>304 (Not Modified)</x:ref>
1044   response and more useful than a <x:ref>412 (Precondition Failed)</x:ref>
1045   response.
1046</t>
1047</section>
1048
1049<section title="Precedence" anchor="precedence">
1050<t>
1051   When more than one conditional request header field is present in a request,
1052   the order in which the fields are evaluated becomes important. In practice,
1053   the fields defined in this document are consistently implemented in a
1054   single, logical order, since "lost update" preconditions have more strict
1055   requirements than cache validation, a validated cache is more efficient
1056   than a partial response, and entity tags are presumed to be more accurate
1057   than date validators.
1058</t>
1059<t>
1060   A recipient cache or origin server &MUST; evaluate the request
1061   preconditions defined by this specification in the following order:
1062   <list style="numbers">
1063     <t anchor="precedence1">When <x:ref>If-Match</x:ref> is present, evaluate it:
1064       <list style="symbols">
1065         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
1066         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref> unless
1067            it can be determined that the state-changing request has already
1068            succeeded (see <xref target="header.if-match"/>)</t>
1069       </list>
1070     </t>
1071     <t anchor="precedence2">When <x:ref>If-Match</x:ref> is not present and
1072        <x:ref>If-Unmodified-Since</x:ref> is present, evaluate it:
1073       <list style="symbols">
1074         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
1075         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref> unless
1076            it can be determined that the state-changing request has already
1077            succeeded (see <xref target="header.if-unmodified-since"/>)</t>
1078       </list>
1079     </t>
1080     <t anchor="precedence3">When <x:ref>If-None-Match</x:ref> is present, evaluate it:
1081       <list style="symbols">
1082         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
1083         <t>if false for GET/HEAD, respond <x:ref>304 (Not Modified)</x:ref></t>
1084         <t>if false for other methods, respond <x:ref>412 (Precondition Failed)</x:ref></t>
1085       </list>
1086     </t>
1087     <t anchor="precedence4">When the method is GET or HEAD,
1088        <x:ref>If-None-Match</x:ref> is not present, and
1089        <x:ref>If-Modified-Since</x:ref> is present, evaluate it:
1090       <list style="symbols">
1091         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
1092         <t>if false, respond <x:ref>304 (Not Modified)</x:ref></t>
1093       </list>
1094     </t>
1095     <t anchor="precedence5">When the method is GET and both <x:ref>Range</x:ref> and
1096        <x:ref>If-Range</x:ref> are present, evaluate If-Range:
1097       <list style="symbols">
1098         <t>if the validator matches and the Range specification is
1099            applicable to the selected representation, respond
1100            <x:ref>206 (Partial Content)</x:ref> <xref target="Part5"/></t>
1101       </list>
1102     </t>
1103     <t anchor="precedencelast">Otherwise,
1104       <list style="symbols">
1105         <t>all conditions are met, so perform the requested action and
1106            respond according to its success or failure.</t>
1107       </list>
1108     </t>
1109   </list>
1110</t>
1111<t>
1112   Any extension to HTTP/1.1 that defines additional conditional request
1113   header fields ought to define its own expectations regarding the order
1114   for evaluating such fields in relation to those defined in this document
1115   and other conditionals that might be found in practice.
1116</t>
1117</section>
1118
1119<section title="IANA Considerations" anchor="IANA.considerations">
1120
1121<section title="Status Code Registration" anchor="status.code.registration">
1122<t>
1123   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1124   shall be updated with the registrations below:
1125</t>
1126<?BEGININC p4-conditional.iana-status-codes ?>
1127<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1128<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1129   <ttcol>Value</ttcol>
1130   <ttcol>Description</ttcol>
1131   <ttcol>Reference</ttcol>
1132   <c>304</c>
1133   <c>Not Modified</c>
1134   <c>
1135      <xref target="status.304"/>
1136   </c>
1137   <c>412</c>
1138   <c>Precondition Failed</c>
1139   <c>
1140      <xref target="status.412"/>
1141   </c>
1142</texttable>
1143<!--(END)-->
1144<?ENDINC p4-conditional.iana-status-codes ?>
1145</section>
1146
1147<section title="Header Field Registration" anchor="header.field.registration">
1148<t>
1149   HTTP header fields are registered within the Message Header Field Registry
1150   maintained at
1151   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
1152</t>
1153<t>
1154   This document defines the following HTTP header fields, so their
1155   associated registry entries shall be updated according to the permanent
1156   registrations below (see <xref target="BCP90"/>):
1157</t>
1158<?BEGININC p4-conditional.iana-headers ?>
1159<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1160<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1161   <ttcol>Header Field Name</ttcol>
1162   <ttcol>Protocol</ttcol>
1163   <ttcol>Status</ttcol>
1164   <ttcol>Reference</ttcol>
1165
1166   <c>ETag</c>
1167   <c>http</c>
1168   <c>standard</c>
1169   <c>
1170      <xref target="header.etag"/>
1171   </c>
1172   <c>If-Match</c>
1173   <c>http</c>
1174   <c>standard</c>
1175   <c>
1176      <xref target="header.if-match"/>
1177   </c>
1178   <c>If-Modified-Since</c>
1179   <c>http</c>
1180   <c>standard</c>
1181   <c>
1182      <xref target="header.if-modified-since"/>
1183   </c>
1184   <c>If-None-Match</c>
1185   <c>http</c>
1186   <c>standard</c>
1187   <c>
1188      <xref target="header.if-none-match"/>
1189   </c>
1190   <c>If-Unmodified-Since</c>
1191   <c>http</c>
1192   <c>standard</c>
1193   <c>
1194      <xref target="header.if-unmodified-since"/>
1195   </c>
1196   <c>Last-Modified</c>
1197   <c>http</c>
1198   <c>standard</c>
1199   <c>
1200      <xref target="header.last-modified"/>
1201   </c>
1202</texttable>
1203<!--(END)-->
1204<?ENDINC p4-conditional.iana-headers ?>
1205<t>
1206   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1207</t>
1208</section>
1209</section>
1210
1211<section title="Security Considerations" anchor="security.considerations">
1212<t>
1213   This section is meant to inform developers, information providers, and
1214   users of known security concerns specific to the HTTP/1.1 conditional
1215   request mechanisms. More general security considerations are addressed
1216   in HTTP messaging &messaging; and semantics &semantics;.
1217</t>
1218<t>
1219   The validators defined by this specification are not intended to ensure
1220   the validity of a representation, guard against malicious changes, or
1221   detect man-in-the-middle attacks. At best, they enable more efficient cache
1222   updates and optimistic concurrent writes when all participants are behaving
1223   nicely. At worst, the conditions will fail and the client will receive a
1224   response that is no more harmful than an HTTP exchange without conditional
1225   requests.
1226</t>
1227<t>
1228   An entity-tag can be abused in ways that create privacy risks. For example,
1229   a site might deliberately construct a semantically invalid entity-tag that
1230   is unique to the user or user agent, send it in a cacheable response with a
1231   long freshness time, and then read that entity-tag in later conditional
1232   requests as a means of re-identifying that user or user agent. Such an
1233   identifying tag would become a persistent identifier for as long as the
1234   user agent retained the original cache entry. User agents that cache
1235   representations ought to ensure that the cache is cleared or replaced
1236   whenever the user performs privacy-maintaining actions, such as clearing
1237   stored cookies or changing to a private browsing mode.
1238</t>
1239</section>
1240
1241<section title="Acknowledgments" anchor="acks">
1242<t>
1243  See &acks;.
1244</t>
1245</section>
1246</middle>
1247<back>
1248
1249<references title="Normative References">
1250
1251<reference anchor="Part1">
1252  <front>
1253    <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
1254    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1255      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1256      <address><email>fielding@gbiv.com</email></address>
1257    </author>
1258    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1259      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1260      <address><email>julian.reschke@greenbytes.de</email></address>
1261    </author>
1262    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1263  </front>
1264  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1265  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1266</reference>
1267
1268<reference anchor="Part2">
1269  <front>
1270    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
1271    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1272      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1273      <address><email>fielding@gbiv.com</email></address>
1274    </author>
1275    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1276      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1277      <address><email>julian.reschke@greenbytes.de</email></address>
1278    </author>
1279    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1280  </front>
1281  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1282  <x:source href="p2-semantics.xml" basename="p2-semantics">
1283    <x:defines>2xx</x:defines>
1284    <x:defines>2xx (Successful)</x:defines>
1285    <x:defines>200 (OK)</x:defines>
1286    <x:defines>204 (No Content)</x:defines>
1287    <x:defines>Accept-Encoding</x:defines>
1288    <x:defines>Content-Location</x:defines>
1289    <x:defines>Content-Type</x:defines>
1290    <x:defines>Date</x:defines>
1291    <x:defines>Location</x:defines>
1292    <x:defines>Vary</x:defines>
1293    <x:defines>selected representation</x:defines>
1294  </x:source>
1295</reference>
1296
1297<reference anchor="Part5">
1298  <front>
1299    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
1300    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1301      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1302      <address><email>fielding@gbiv.com</email></address>
1303    </author>
1304    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1305      <organization abbrev="W3C">World Wide Web Consortium</organization>
1306      <address><email>ylafon@w3.org</email></address>
1307    </author>
1308    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1309      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1310      <address><email>julian.reschke@greenbytes.de</email></address>
1311    </author>
1312    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1313  </front>
1314  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1315  <x:source href="p5-range.xml" basename="p5-range">
1316    <x:defines>If-Range</x:defines>
1317    <x:defines>Range</x:defines>
1318    <x:defines>206 (Partial Content)</x:defines>
1319  </x:source>
1320</reference>
1321
1322<reference anchor="Part6">
1323  <front>
1324    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
1325    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1326      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1327      <address><email>fielding@gbiv.com</email></address>
1328    </author>
1329    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1330      <organization>Akamai</organization>
1331      <address><email>mnot@mnot.net</email></address>
1332    </author>
1333    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1334      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1335      <address><email>julian.reschke@greenbytes.de</email></address>
1336    </author>
1337    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1338  </front>
1339  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1340  <x:source href="p6-cache.xml" basename="p6-cache">
1341    <x:defines>Cache-Control</x:defines>
1342    <x:defines>Expires</x:defines>
1343  </x:source>
1344</reference>
1345
1346<reference anchor="RFC2119">
1347  <front>
1348    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1349    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1350      <organization>Harvard University</organization>
1351      <address><email>sob@harvard.edu</email></address>
1352    </author>
1353    <date month="March" year="1997"/>
1354  </front>
1355  <seriesInfo name="BCP" value="14"/>
1356  <seriesInfo name="RFC" value="2119"/>
1357</reference>
1358
1359<reference anchor="RFC5234">
1360  <front>
1361    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1362    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1363      <organization>Brandenburg InternetWorking</organization>
1364      <address>
1365        <email>dcrocker@bbiw.net</email>
1366      </address> 
1367    </author>
1368    <author initials="P." surname="Overell" fullname="Paul Overell">
1369      <organization>THUS plc.</organization>
1370      <address>
1371        <email>paul.overell@thus.net</email>
1372      </address>
1373    </author>
1374    <date month="January" year="2008"/>
1375  </front>
1376  <seriesInfo name="STD" value="68"/>
1377  <seriesInfo name="RFC" value="5234"/>
1378</reference>
1379
1380</references>
1381
1382<references title="Informative References">
1383
1384<reference anchor="RFC2616">
1385  <front>
1386    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1387    <author initials="R." surname="Fielding" fullname="R. Fielding">
1388      <organization>University of California, Irvine</organization>
1389      <address><email>fielding@ics.uci.edu</email></address>
1390    </author>
1391    <author initials="J." surname="Gettys" fullname="J. Gettys">
1392      <organization>W3C</organization>
1393      <address><email>jg@w3.org</email></address>
1394    </author>
1395    <author initials="J." surname="Mogul" fullname="J. Mogul">
1396      <organization>Compaq Computer Corporation</organization>
1397      <address><email>mogul@wrl.dec.com</email></address>
1398    </author>
1399    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1400      <organization>MIT Laboratory for Computer Science</organization>
1401      <address><email>frystyk@w3.org</email></address>
1402    </author>
1403    <author initials="L." surname="Masinter" fullname="L. Masinter">
1404      <organization>Xerox Corporation</organization>
1405      <address><email>masinter@parc.xerox.com</email></address>
1406    </author>
1407    <author initials="P." surname="Leach" fullname="P. Leach">
1408      <organization>Microsoft Corporation</organization>
1409      <address><email>paulle@microsoft.com</email></address>
1410    </author>
1411    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1412      <organization>W3C</organization>
1413      <address><email>timbl@w3.org</email></address>
1414    </author>
1415    <date month="June" year="1999"/>
1416  </front>
1417  <seriesInfo name="RFC" value="2616"/>
1418</reference>
1419
1420<reference anchor='BCP90'>
1421  <front>
1422    <title>Registration Procedures for Message Header Fields</title>
1423    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1424      <organization>Nine by Nine</organization>
1425      <address><email>GK-IETF@ninebynine.org</email></address>
1426    </author>
1427    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1428      <organization>BEA Systems</organization>
1429      <address><email>mnot@pobox.com</email></address>
1430    </author>
1431    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1432      <organization>HP Labs</organization>
1433      <address><email>JeffMogul@acm.org</email></address>
1434    </author>
1435    <date year='2004' month='September' />
1436  </front>
1437  <seriesInfo name='BCP' value='90' />
1438  <seriesInfo name='RFC' value='3864' />
1439</reference>
1440
1441<reference anchor='RFC4918'>
1442  <front>
1443    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1444    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1445      <organization abbrev="CommerceNet">CommerceNet</organization>
1446      <address><email>ldusseault@commerce.net</email></address>
1447    </author>
1448    <date month="June" year="2007" />
1449  </front>
1450  <seriesInfo name='RFC' value='4918' />
1451</reference>
1452</references>
1453
1454<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1455<t>
1456  The definition of validator weakness has been expanded and clarified.
1457  (<xref target="weak.and.strong.validators" />)
1458</t>
1459<t>
1460  Weak entity-tags are now allowed in all requests except range requests.
1461  (Sections <xref target="weak.and.strong.validators" format="counter"/> and
1462  <xref target="header.if-none-match" format="counter"/>)
1463</t>
1464<t>
1465  The <x:ref>ETag</x:ref> header field ABNF has been changed to not use
1466  quoted-string, thus avoiding escaping issues.
1467  (<xref target="header.etag" />)
1468</t>
1469<t>
1470  ETag is defined to provide an entity tag for the selected representation,
1471  thereby clarifying what it applies to in various situations (such as a
1472  PUT response).
1473  (<xref target="header.etag" />)
1474</t>
1475<t>
1476  The precedence for evaluation of conditional requests has been defined.
1477  (<xref target="precedence" />)
1478</t>
1479</section>
1480
1481<section title="Imported ABNF" anchor="imported.abnf">
1482  <x:anchor-alias value="ALPHA"/>
1483  <x:anchor-alias value="CR"/>
1484  <x:anchor-alias value="DIGIT"/>
1485  <x:anchor-alias value="DQUOTE"/>
1486  <x:anchor-alias value="LF"/>
1487  <x:anchor-alias value="OCTET"/>
1488  <x:anchor-alias value="VCHAR"/>
1489  <x:anchor-alias value="core.rules"/>
1490  <x:anchor-alias value="obs-text"/>
1491  <x:anchor-alias value="OWS"/>
1492  <x:anchor-alias value="HTTP-date"/>
1493<t>
1494  The following core rules are included by
1495  reference, as defined in <xref target="RFC5234" x:fmt="of" x:sec="B.1"/>:
1496  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
1497  DIGIT (decimal 0-9), DQUOTE (double quote),
1498  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
1499  OCTET (any 8-bit sequence of data), SP (space), and
1500  VCHAR (any visible US-ASCII character).
1501</t>
1502<t>
1503  The rules below are defined in <xref target="Part1"/>:
1504</t>
1505<figure><artwork type="abnf2616">
1506  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
1507  <x:ref>obs-text</x:ref>      = &lt;obs-text, defined in &field-components;&gt;
1508</artwork></figure>
1509<t>
1510  The rules below are defined in other parts:
1511</t>
1512<figure><artwork type="abnf2616">
1513  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
1514</artwork></figure>
1515</section>
1516
1517<?BEGININC p4-conditional.abnf-appendix ?>
1518<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1519<t>
1520  In the collected ABNF below, list rules are expanded as per <xref target="Part1" x:rel="#notation"/>.
1521</t><figure>
1522<artwork type="abnf" name="p4-conditional.parsed-abnf">
1523<x:ref>ETag</x:ref> = entity-tag
1524
1525<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
1526
1527<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1528 entity-tag ] ) )
1529<x:ref>If-Modified-Since</x:ref> = HTTP-date
1530<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1531 entity-tag ] ) )
1532<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1533
1534<x:ref>Last-Modified</x:ref> = HTTP-date
1535
1536<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
1537
1538<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1539<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1540 / obs-text
1541
1542<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.6&gt;
1543<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1544
1545<x:ref>weak</x:ref> = %x57.2F ; W/
1546</artwork>
1547</figure>
1548</section>
1549<?ENDINC p4-conditional.abnf-appendix ?>
1550
1551<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1552<t>
1553  Changes up to the first Working Group Last Call draft are summarized
1554  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C"/>.
1555</t>
1556
1557<section title="Since draft-ietf-httpbis-p4-conditional-19" anchor="changes.since.19">
1558<t>
1559  Closed issues:
1560  <list style="symbols">
1561    <t>
1562      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/241"/>:
1563      "Need to clarify eval order/interaction of conditional headers"
1564    </t>
1565    <t>
1566      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/345"/>:
1567      "Required headers on 304 and 206"
1568    </t>
1569    <t>
1570      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/350"/>:
1571      "Optionality of Conditional Request Support"
1572    </t>
1573    <t>
1574      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/354"/>:
1575      "ETags and Conditional Requests"
1576    </t>
1577    <t>
1578      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
1579      "ABNF requirements for recipients"
1580    </t>
1581    <t>
1582      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/363"/>:
1583      "Rare cases"
1584    </t>
1585    <t>
1586      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/365"/>:
1587      "Conditional Request Security Considerations"
1588    </t>
1589    <t>
1590      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/371"/>:
1591      "If-Modified-Since lacks definition for method != GET"
1592    </t>
1593    <t>
1594      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/372"/>:
1595      "refactor conditional header field descriptions"
1596    </t>
1597  </list>
1598</t>
1599</section>
1600
1601<section title="Since draft-ietf-httpbis-p4-conditional-20" anchor="changes.since.20">
1602<t>
1603  <list style="symbols">
1604    <t>
1605      Conformance criteria and considerations regarding error handling are
1606      now defined in Part 1.
1607    </t>
1608  </list>
1609</t>
1610</section>
1611
1612<section title="Since draft-ietf-httpbis-p4-conditional-21" anchor="changes.since.21">
1613<t>
1614  Closed issues:
1615  <list style="symbols">
1616    <t>
1617      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/96"/>:
1618      "Conditional GET text"
1619    </t>
1620    <t>
1621      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/350"/>:
1622      "Optionality of Conditional Request Support"
1623    </t>
1624    <t>
1625      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/384"/>:
1626      "unclear prose in definition of 304"
1627    </t>
1628    <t>
1629      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/401"/>:
1630      "ETags and Conneg"
1631    </t>
1632    <t>
1633      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/402"/>:
1634      "Comparison function for If-Match and If-None-Match"
1635    </t>
1636    <t>
1637      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
1638      "304 without validator"
1639    </t>
1640    <t>
1641      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/427"/>:
1642      "If-Match and 428"
1643    </t>
1644  </list>
1645</t>
1646</section>
1647
1648<section title="Since draft-ietf-httpbis-p4-conditional-22" anchor="changes.since.22">
1649<t>
1650  Closed issues:
1651  <list style="symbols">
1652    <t>
1653      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/436"/>:
1654      "explain list expansion in ABNF appendices"
1655    </t>
1656    <t>
1657      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/437"/>:
1658      "incorrect example dates"
1659    </t>
1660  </list>
1661</t>
1662<t>
1663  Partly resolved issues:
1664  <list style="symbols">
1665    <t>
1666      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/461"/>:
1667      "Editorial suggestions"
1668    </t>
1669  </list>
1670</t>
1671</section>
1672
1673<section title="Since draft-ietf-httpbis-p4-conditional-23" anchor="changes.since.23">
1674<t>
1675  Closed issues:
1676  <list style="symbols">
1677    <t>
1678      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/455"/>:
1679      "PUT + If-Match over-constrained?"
1680    </t>
1681    <t>
1682      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/479"/>:
1683      "MUSTs and other feedback"
1684    </t>
1685  </list>
1686</t>
1687<t>
1688  Partly resolved issues:
1689  <list style="symbols">
1690    <t>
1691      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/495"/>:
1692      "p4 editorial nits"
1693    </t>
1694  </list>
1695</t>
1696</section>
1697</section>
1698
1699</back>
1700</rfc>
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