source: draft-ietf-httpbis/24/p4-conditional.xml @ 2656

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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 "24">
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;" day="25"/>
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 needs to 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   Strong validators are usable for all conditional requests, including cache
287   validation, partial content ranges, and "lost update" avoidance.
288   Weak validators are only usable when the client does not require exact
289   equality with previously obtained representation data, such as when
290   validating a cache entry or limiting a web traversal to recent changes.
291</t>
292</section>
293
294<section title="Last-Modified" anchor="header.last-modified">
295  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
296  <x:anchor-alias value="Last-Modified"/>
297<t>
298   The "Last-Modified" header field in a response provides a timestamp
299   indicating the date and time at which the origin server believes the
300   selected representation was last modified, as determined at the conclusion
301   of handling the request.
302</t>
303<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
304  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
305</artwork></figure>
306<t>
307   An example of its use is
308</t>
309<figure><artwork type="example">
310  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
311</artwork></figure>
312
313<section title="Generation" anchor="lastmod.generation">
314<t>
315   An origin server &SHOULD; send Last-Modified for any selected
316   representation for which a last modification date can be reasonably
317   and consistently determined, since its use in conditional requests
318   and evaluating cache freshness (&caching;) results in a substantial
319   reduction of HTTP traffic on the Internet and can be a significant
320   factor in improving service scalability and reliability.
321</t>
322<t>
323   A representation is typically the sum of many parts behind the
324   resource interface.  The last-modified time would usually be
325   the most recent time that any of those parts were changed.
326   How that value is determined for any given resource is an
327   implementation detail beyond the scope of this specification.
328   What matters to HTTP is how recipients of the Last-Modified
329   header field can use its value to make conditional requests
330   and test the validity of locally cached responses.
331</t>
332<t>
333   An origin server &SHOULD; obtain the Last-Modified value of the
334   representation as close as possible to the time that it generates the
335   <x:ref>Date</x:ref> field value for its response. This allows a recipient to
336   make an accurate assessment of the representation's modification time,
337   especially if the representation changes near the time that the
338   response is generated.
339</t>
340<t>
341   An origin server with a clock &MUST-NOT; send a Last-Modified date
342   that is later than the server's time of message origination (<x:ref>Date</x:ref>).
343   If the last modification time is derived from implementation-specific
344   metadata that evaluates to some time in the future, according to the
345   origin server's clock, then the origin server &MUST; replace that
346   value with the message origination date. This prevents a future
347   modification date from having an adverse impact on cache validation.
348</t>
349<t>
350   An origin server without a clock &MUST-NOT; assign Last-Modified
351   values to a response unless these values were associated
352   with the resource by some other system or user with a reliable clock.
353</t>
354</section>
355
356<section title="Comparison" anchor="lastmod.comparison">
357<t>
358   A Last-Modified time, when used as a validator in a request, is
359   implicitly weak unless it is possible to deduce that it is strong,
360   using the following rules:
361  <list style="symbols">
362     <t>The validator is being compared by an origin server to the
363        actual current validator for the representation and,</t>
364     <t>That origin server reliably knows that the associated representation did
365        not change twice during the second covered by the presented
366        validator.</t>
367  </list>
368</t>
369<t>
370   or
371  <list style="symbols">
372     <t>The validator is about to be used by a client in an <x:ref>If-Modified-Since</x:ref>,
373        <x:ref>If-Unmodified-Since</x:ref> header field, because the client has
374        a cache entry, or <x:ref>If-Range</x:ref> for the associated
375        representation, and</t>
376     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
377        time when the origin server sent the original response, and</t>
378     <t>The presented Last-Modified time is at least 60 seconds before
379        the Date value.</t>
380  </list>
381</t>
382<t>
383   or
384  <list style="symbols">
385     <t>The validator is being compared by an intermediate cache to the
386        validator stored in its cache entry for the representation, and</t>
387     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
388        time when the origin server sent the original response, and</t>
389     <t>The presented Last-Modified time is at least 60 seconds before
390        the Date value.</t>
391  </list>
392</t>
393<t>
394   This method relies on the fact that if two different responses were
395   sent by the origin server during the same second, but both had the
396   same Last-Modified time, then at least one of those responses would
397   have a <x:ref>Date</x:ref> value equal to its Last-Modified time. The
398   arbitrary 60-second limit guards against the possibility that the Date and
399   Last-Modified values are generated from different clocks, or at somewhat
400   different times during the preparation of the response. An
401   implementation &MAY; use a value larger than 60 seconds, if it is
402   believed that 60 seconds is too short.
403</t>
404</section>
405</section>
406
407<section title="ETag" anchor="header.etag">
408  <iref primary="true" item="ETag header field" x:for-anchor=""/>
409  <x:anchor-alias value="ETag"/>
410  <x:anchor-alias value="entity-tag"/>
411  <x:anchor-alias value="opaque-tag"/>
412  <x:anchor-alias value="weak"/>
413  <x:anchor-alias value="etagc"/>
414<t>
415   The "ETag" header field in a response provides the current entity-tag for
416   the selected representation, as determined at the conclusion of handling
417   the request.
418   An entity-tag is an opaque validator for differentiating between
419   multiple representations of the same resource, regardless of whether
420   those multiple representations are due to resource state changes over
421   time, content negotiation resulting in multiple representations being
422   valid at the same time, or both. An entity-tag consists of an opaque
423   quoted string, possibly prefixed by a weakness indicator.
424</t>
425<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"/>
426  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
427
428  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
429  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
430  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
431  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
432             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
433</artwork></figure>
434<x:note>
435  <t>
436    &Note; Previously, opaque-tag was defined to be a quoted-string
437    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
438    might perform backslash unescaping. Servers therefore ought to avoid
439    backslash characters in entity tags.
440  </t>
441</x:note>
442<t>
443   An entity-tag can be more reliable for validation than a modification
444   date in situations where it is inconvenient to store modification
445   dates, where the one-second resolution of HTTP date values is not
446   sufficient, or where modification dates are not consistently maintained.
447</t>
448<figure><preamble>
449  Examples:
450</preamble>
451<artwork type="example">
452  ETag: "xyzzy"
453  ETag: W/"xyzzy"
454  ETag: ""
455</artwork></figure>
456<t>
457   An entity-tag can be either a weak or strong validator, with
458   strong being the default.  If an origin server provides an entity-tag
459   for a representation and the generation of that entity-tag does not satisfy
460   all of the characteristics of a strong validator
461   (<xref target="weak.and.strong.validators"/>), then the origin server
462   &MUST; mark the entity-tag as weak by prefixing its opaque value
463   with "W/" (case-sensitive).
464</t>
465
466<section title="Generation" anchor="entity.tag.generation">
467<t>
468   The principle behind entity-tags is that only the service author
469   knows the implementation of a resource well enough to select the
470   most accurate and efficient validation mechanism for that resource,
471   and that any such mechanism can be mapped to a simple sequence of
472   octets for easy comparison.  Since the value is opaque, there is no
473   need for the client to be aware of how each entity-tag is constructed.
474</t>
475<t>
476   For example, a resource that has implementation-specific versioning
477   applied to all changes might use an internal revision number, perhaps
478   combined with a variance identifier for content negotiation, to
479   accurately differentiate between representations.
480   Other implementations might use a collision-resistant hash of
481   representation content,
482   a combination of various filesystem attributes, or a modification
483   timestamp that has sub-second resolution.
484</t>
485<t>
486   An origin server &SHOULD; send ETag for any selected representation
487   for which detection of changes can be reasonably and consistently
488   determined, since the entity-tag's use in conditional requests and
489   evaluating cache freshness (&caching;) can result in a substantial
490   reduction of HTTP network traffic and can be a significant factor in
491   improving service scalability and reliability.
492</t>
493</section>
494
495<section title="Comparison" anchor="entity.tag.comparison">
496  <x:anchor-alias value="validator.comparison"/>
497  <x:anchor-alias value="strong comparison"/>
498  <x:anchor-alias value="weak comparison"/>
499<t>
500   There are two entity-tag comparison functions, depending
501   on whether the comparison context allows the use of weak validators
502   or not:
503  <list style="symbols">
504     <t><x:dfn>Strong comparison</x:dfn>: two entity-tags are equivalent if both
505        are not weak and their opaque-tags match character-by-character.</t>
506     <t><x:dfn>Weak comparison</x:dfn>: two entity-tags are equivalent if their opaque-tags
507        match character-by-character, regardless of either or both
508        being tagged as "weak".</t>
509  </list>
510</t>
511<t>
512   The example below shows the results for a set of entity-tag pairs,
513   and both the weak and strong comparison function results:
514</t>
515<texttable align="left">
516  <ttcol>ETag 1</ttcol>
517  <ttcol>ETag 2</ttcol>
518  <ttcol>Strong Comparison</ttcol>
519  <ttcol>Weak Comparison</ttcol>
520
521  <c>W/"1"</c>
522  <c>W/"1"</c>
523  <c>no match</c>
524  <c>match</c>
525 
526  <c>W/"1"</c>
527  <c>W/"2"</c>
528  <c>no match</c>
529  <c>no match</c>
530
531  <c>W/"1"</c>
532  <c>"1"</c>
533  <c>no match</c>
534  <c>match</c>
535
536  <c>"1"</c>
537  <c>"1"</c>
538  <c>match</c>
539  <c>match</c>
540</texttable>
541</section>
542
543<section title="Example: Entity-tags Varying on Content-Negotiated Resources" anchor="example.entity.tag.vs.conneg">
544<t>
545   Consider a resource that is subject to content negotiation
546   (&content-negotiation;), and where the representations sent in response to
547   a GET request vary based on the <x:ref>Accept-Encoding</x:ref> request
548   header field (&header-accept-encoding;):
549</t>
550<figure><preamble>>> Request:</preamble><artwork type="message/http; msgtype=&#34;request&#34;"  x:indent-with="  ">
551GET /index HTTP/1.1
552Host: www.example.com
553Accept-Encoding: gzip
554
555</artwork></figure>
556<t>
557   In this case, the response might or might not use the gzip content coding.
558   If it does not, the response might look like:
559</t>
560<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
561HTTP/1.1 200 OK
562Date: Fri, 26 Mar 2010 00:05:00 GMT
563ETag: "123-a"
564Content-Length: <x:length-of target="exbody"/>
565Vary: Accept-Encoding
566Content-Type: text/plain
567
568<x:span anchor="exbody">Hello World!
569Hello World!
570Hello World!
571Hello World!
572Hello World!
573</x:span></artwork></figure>
574<t>
575   An alternative representation that does use gzip content coding would be:
576</t>
577<figure><preamble>>> Response:</preamble><artwork type="message/http; msgtype=&#34;response&#34;"  x:indent-with="  ">
578HTTP/1.1 200 OK
579Date: Fri, 26 Mar 2010 00:05:00 GMT
580ETag: "123-b"
581Content-Length: 43
582Vary: Accept-Encoding
583Content-Type: text/plain
584Content-Encoding: gzip
585
586<spanx>...binary data...</spanx></artwork></figure>
587<x:note>
588  <t>
589    &Note; Content codings are a property of the representation,
590    so therefore an entity-tag of an encoded representation has to be distinct
591    from an unencoded representation to prevent conflicts during cache updates
592    and range requests.  In contrast, transfer codings (&transfer-codings;)
593    apply only during message transfer and do not require distinct entity-tags.
594  </t>
595</x:note>
596</section>
597</section>
598
599<section title="When to Use Entity-tags and Last-Modified Dates" anchor="when.to.use.entity.tags.and.last-modified.dates">
600<t>
601   We adopt a set of rules and recommendations for origin servers,
602   clients, and caches regarding when various validator types ought to
603   be used, and for what purposes.
604</t>
605<t>
606   In <x:ref>200 (OK)</x:ref> responses to GET or HEAD, an origin server:
607  <list style="symbols">
608     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
609        generate one.</t>
610
611     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
612        performance considerations support the use of weak entity-tags,
613        or if it is unfeasible to send a strong entity-tag.</t>
614
615     <t>&SHOULD; send a <x:ref>Last-Modified</x:ref> value if it is feasible to
616        send one.</t>
617  </list>
618</t>
619<t>
620   In other words, the preferred behavior for an origin server
621   is to send both a strong entity-tag and a <x:ref>Last-Modified</x:ref>
622   value in successful responses to a retrieval request.
623</t>
624<t>
625   A client:
626  <list style="symbols">
627     <t>&MUST; send that entity-tag in any cache validation request (using
628        <x:ref>If-Match</x:ref> or <x:ref>If-None-Match</x:ref>) if an
629        entity-tag has been provided by the origin server.</t>
630
631     <t>&SHOULD; send the <x:ref>Last-Modified</x:ref> value in non-subrange
632        cache validation requests (using <x:ref>If-Modified-Since</x:ref>)
633        if only a Last-Modified value has been provided by the origin server.</t>
634
635     <t>&MAY; send the <x:ref>Last-Modified</x:ref> value in subrange
636        cache validation requests (using <x:ref>If-Unmodified-Since</x:ref>)
637        if only a Last-Modified value has been provided by an HTTP/1.0 origin
638        server. The user agent &SHOULD; provide a way to disable this, in case
639        of difficulty.</t>
640
641     <t>&SHOULD; send both validators in cache validation requests if both an
642        entity-tag and a <x:ref>Last-Modified</x:ref> value have been provided
643        by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
644        respond appropriately.</t>
645  </list>
646</t>
647</section>
648</section>
649
650<section title="Precondition Header Fields" anchor="preconditions">
651<t>
652   This section defines the syntax and semantics of HTTP/1.1 header fields
653   for applying preconditions on requests.
654   <xref target="evaluation"/> defines when the preconditions are applied.
655   <xref target="precedence"/> defines the order of evaluation when more than
656   one precondition is present.
657</t>
658
659<section title="If-Match" anchor="header.if-match">
660  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
661  <x:anchor-alias value="If-Match"/>
662<t>
663   The "If-Match" header field makes the request method conditional on the
664   recipient origin server either having at least one current
665   representation of the target resource, when the field-value is "*", or
666   having a current representation of the target resource that has an
667   entity-tag matching a member of the list of entity-tags provided in the
668   field-value.
669</t>
670<t>
671   An origin server &MUST; use the strong comparison function when comparing
672   entity-tags for If-Match (<xref target="entity.tag.comparison"/>), since
673   the client intends this precondition to prevent the method from being
674   applied if there have been any changes to the representation data.
675</t>
676<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
677  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
678</artwork></figure>
679<t>
680   Examples:
681</t>
682<figure><artwork type="example">
683  If-Match: "xyzzy"
684  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
685  If-Match: *
686</artwork></figure>
687<t>
688   If-Match is most often used with state-changing methods (e.g., POST, PUT,
689   DELETE) to prevent accidental overwrites when multiple user agents might be
690   acting in parallel on the same resource (i.e., to prevent the "lost update"
691   problem). It can also be used with safe methods to abort a request if the
692   <x:ref>selected representation</x:ref> does not match one already stored
693   (or partially stored) from a prior request.
694</t>
695<t>
696   An origin server that receives an If-Match header field &MUST; evaluate the
697   condition prior to performing the method (<xref target="evaluation"/>).
698   If the field-value is "*", the condition is false if the origin server
699   does not have a current representation for the target resource.
700   If the field-value is a list of entity-tags, the condition is false if
701   none of the listed tags match the entity-tag of the selected representation.
702</t>
703<t>
704   An origin server &MUST-NOT; perform the requested method if a received
705   If-Match condition evaluates to false; instead the origin server &MUST;
706   respond with either:
707   a) the <x:ref>412 (Precondition Failed)</x:ref> status code; or,
708   b) one of the <x:ref>2xx (Successful)</x:ref> status codes if the origin
709   server has verified that a state change is being requested and the final
710   state is already reflected in the current state of the target resource
711   (i.e., the change requested by the user agent has already succeeded, but
712   the user agent might not be aware of it, perhaps because the prior response
713   was lost or a compatible change was made by some other user agent).
714   In the latter case, the origin server &MUST-NOT; send a validator header
715   field in the response unless it can verify that the request is a duplicate
716   of an immediately prior change made by the same user agent.
717</t>
718<t>
719   The If-Match header field can be ignored by caches and intermediaries
720   because it is not applicable to a stored response.
721</t>
722</section>
723
724<section title="If-None-Match" anchor="header.if-none-match">
725  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
726  <x:anchor-alias value="If-None-Match"/>
727<t>
728   The "If-None-Match" header field makes the request method conditional on
729   a recipient cache or origin server either not having any current
730   representation of the target resource, when the field-value is "*", or
731   having a selected representation with an entity-tag that does not match any
732   of those listed in the field-value.
733</t>
734<t>
735   A recipient &MUST; use the weak comparison function when comparing
736   entity-tags for If-None-Match (<xref target="entity.tag.comparison"/>),
737   since weak entity-tags can be used for cache validation even if there have
738   been changes to the representation data.
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<t>
790   Requirements on cache handling of a received If-None-Match header field
791   are defined in &cache-validation-received;.
792</t>
793</section>
794
795<section title="If-Modified-Since" anchor="header.if-modified-since">
796  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
797  <x:anchor-alias value="If-Modified-Since"/>
798<t>
799   The "If-Modified-Since" header field makes a GET or HEAD request method
800   conditional on the selected representation's modification date being more
801   recent than the date provided in the field-value. Transfer of the selected
802   representation's data is 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.
829</t>
830<t>
831   If-Modified-Since is typically used for two distinct purposes:
832   1) to allow efficient updates of a cached representation that does not
833   have an entity-tag; and,
834   2) to limit the scope of a web traversal to resources that have recently
835   changed.
836</t>
837<t>
838   When used for cache updates, a cache will typically use the value of the
839   cached message's <x:ref>Last-Modified</x:ref> field to generate the field
840   value of If-Modified-Since. This behavior is most interoperable for cases
841   where clocks are poorly synchronized or when the server has chosen to only
842   honor exact timestamp matches (due to a problem with Last-Modified dates
843   that appear to go "back in time" when the origin server's clock is
844   corrected or a representation is restored from an archived backup).
845   However, caches occasionally generate the field value based on other data,
846   such as the <x:ref>Date</x:ref> header field of the cached message or the
847   local clock time that the message was received, particularly when the
848   cached message does not contain a <x:ref>Last-Modified</x:ref> field.
849</t>
850<t>
851   When used for limiting the scope of retrieval to a recent time window, a
852   user agent will generate an If-Modified-Since field value based on either
853   its own local clock or a <x:ref>Date</x:ref> header field received from the
854   server in a prior response. Origin servers that choose an exact timestamp
855   match based on the selected representation's <x:ref>Last-Modified</x:ref>
856   field will not be able to help the user agent limit its data transfers to
857   only those changed during the specified window.
858</t>
859<t>
860   An origin server that receives an If-Modified-Since header field &SHOULD;
861   evaluate the condition prior to performing the method
862   (<xref target="evaluation"/>).
863   The origin server &SHOULD-NOT; perform the requested method if the selected
864   representation's last modification date is earlier than or equal to the
865   date provided in the field-value; instead, the origin server &SHOULD;
866   generate a <x:ref>304 (Not Modified)</x:ref> response, including only those
867   metadata that are useful for identifying or updating a previously cached
868   response.
869</t>
870<t>
871   Requirements on cache handling of a received If-Modified-Since header field
872   are defined in &cache-validation-received;.
873</t>
874</section>
875
876<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
877  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
878  <x:anchor-alias value="If-Unmodified-Since"/>
879<t>
880   The "If-Unmodified-Since" header field makes the request method conditional
881   on the selected representation's last modification date being earlier than or
882   equal to the date provided in the field-value. This field accomplishes the
883   same purpose as <x:ref>If-Match</x:ref> for cases where the user agent does
884   not have an entity-tag for the representation.
885</t>
886<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
887  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
888</artwork></figure>
889<t>
890   An example of the field is:
891</t>
892<figure><artwork type="example">
893  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
894</artwork></figure>
895<t>
896   A recipient &MUST; ignore If-Unmodified-Since if the request contains an
897   <x:ref>If-Match</x:ref> header field; the condition in
898   <x:ref>If-Match</x:ref> is considered to be a more accurate replacement for
899   the condition in If-Unmodified-Since and the two are only combined for the
900   sake of interoperating with older intermediaries that might not implement
901   <x:ref>If-Match</x:ref>.
902</t>
903<t>
904   A recipient &MUST; ignore the If-Unmodified-Since header field if the
905   received field-value is not a valid HTTP-date.
906</t>
907<t>
908   A recipient &MUST; interpret an If-Unmodified-Since field-value's timestamp
909   in terms of the origin server's clock.
910</t>
911<t>
912   If-Unmodified-Since is most often used with state-changing methods
913   (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple
914   user agents might be acting in parallel on a resource that does
915   not supply entity-tags with its representations (i.e., to prevent the
916   "lost update" problem). It can also be used with safe methods to abort a
917   request if the <x:ref>selected representation</x:ref> does not match one
918   already stored (or partially stored) from a prior request.
919</t>
920<t>
921   An origin server that receives an If-Unmodified-Since header field &MUST;
922   evaluate the condition prior to performing the method
923   (<xref target="evaluation"/>).
924   The origin server &MUST-NOT; perform the requested method if the selected
925   representation's last modification date is more recent than the date
926   provided in the field-value; instead the
927   origin server &MUST; respond with either:
928   a) the <x:ref>412 (Precondition Failed)</x:ref> status code; or,
929   b) one of the <x:ref>2xx (Successful)</x:ref> status codes if the origin
930   server has verified that a state change is being requested and the final
931   state is already reflected in the current state of the target resource
932   (i.e., the change requested by the user agent has already succeeded, but
933   the user agent might not be aware of that because the prior response message
934   was lost or a compatible change was made by some other user agent).
935   In the latter case, the origin server &MUST-NOT; send a validator header
936   field in the response unless it can verify that the request is a duplicate
937   of an immediately prior change made by the same user agent.
938</t>
939<t>
940   The If-Unmodified-Since header field can be ignored by caches and
941   intermediaries because it is not applicable to a stored response.
942</t>
943</section>
944
945<section title="If-Range" anchor="header.if-range">
946<t>
947   The "If-Range" header field provides a special conditional request
948   mechanism that is similar to the <x:ref>If-Match</x:ref> and
949   <x:ref>If-Unmodified-Since</x:ref> header fields but instructs the
950   recipient to ignore the <x:ref>Range</x:ref> header field if the validator
951   doesn't match, resulting in transfer of the new selected representation
952   instead of a 412 response. If-Range is defined in &header-if-range;.
953</t>
954</section>
955</section>
956
957<section title="Status Code Definitions" anchor="status.code.definitions">
958<section title="304 Not Modified" anchor="status.304">
959  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
960  <x:anchor-alias value="304"/>
961  <x:anchor-alias value="304 (Not Modified)"/>
962<t>
963   The <x:dfn>304 (Not Modified)</x:dfn> status code indicates that a
964   conditional GET or HEAD request has been
965   received and would have resulted in a <x:ref>200 (OK)</x:ref> response
966   if it were not for the fact that the condition has evaluated to false.
967   In other words, there is no need for the server to transfer a
968   representation of the target resource because the request indicates that
969   the client, which made the request conditional, already has a valid
970   representation; the server is therefore redirecting the client to make
971   use of that stored representation as if it were the payload of a
972   <x:ref>200 (OK)</x:ref> response.
973</t>
974<t>
975   The server generating a 304 response &MUST; generate any of the following
976   header fields that would have been sent in a <x:ref>200 (OK)</x:ref>
977   response to the same request:
978   <x:ref>Cache-Control</x:ref>,
979   <x:ref>Content-Location</x:ref>,
980   <x:ref>Date</x:ref>,
981   <x:ref>ETag</x:ref>,
982   <x:ref>Expires</x:ref>, and
983   <x:ref>Vary</x:ref>.
984</t>
985<t>
986   Since the goal of a 304 response is to minimize information transfer
987   when the recipient already has one or more cached representations,
988   a sender &SHOULD-NOT; generate representation metadata other
989   than the above listed fields unless said metadata exists for the
990   purpose of guiding cache updates (e.g., <x:ref>Last-Modified</x:ref> might
991   be useful if the response does not have an <x:ref>ETag</x:ref> field).
992</t>
993<t>
994   Requirements on a cache that receives a 304 response are defined in
995   &freshening-responses;. If the conditional request originated with an
996   outbound client, such as a user agent with its own cache sending a
997   conditional GET to a shared proxy, then the proxy &SHOULD; forward the
998   304 response to that client.
999</t>
1000<t>
1001   A 304 response cannot contain a message-body; it is always
1002   terminated by the first empty line after the header fields.
1003</t>
1004</section>
1005
1006<section title="412 Precondition Failed" anchor="status.412">
1007  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1008  <x:anchor-alias value="412 (Precondition Failed)"/>
1009<t>
1010   The <x:dfn>412 (Precondition Failed)</x:dfn> status code indicates that one
1011   or more conditions given in the request header fields evaluated to false
1012   when tested on the server. This response code allows the client to place
1013   preconditions on the current resource state (its current representations
1014   and metadata) and thus prevent the request method from being applied if the
1015   target resource is in an unexpected state.
1016</t>
1017</section>
1018</section>
1019
1020<section title="Evaluation" anchor="evaluation">
1021<t>
1022   Except when excluded below, a recipient cache or origin server &MUST;
1023   evaluate received request preconditions after it has successfully performed
1024   its normal request checks and just before it would perform the action
1025   associated with the request method.
1026   A server &MUST; ignore all received preconditions if its response to the
1027   same request without those conditions would have been a status code other
1028   than a <x:ref>2xx</x:ref> or <x:ref>412 (Precondition Failed)</x:ref>.
1029   In other words, redirects and failures take precedence over the evaluation
1030   of preconditions in conditional requests.
1031</t>
1032<t>
1033   A server that is not the origin server for the target resource and cannot
1034   act as a cache for requests on the target resource &MUST-NOT; evaluate the
1035   conditional request header fields defined by this specification, and
1036   &MUST; forward them if the request is forwarded, since the generating
1037   client intends that they be evaluated by a server that can provide a
1038   current representation.
1039   Likewise, a server &MUST; ignore the conditional request header fields
1040   defined by this specification when received with a request method that does
1041   not involve the selection or modification of a
1042   <x:ref>selected representation</x:ref>, such as CONNECT, OPTIONS, or TRACE.
1043</t>
1044<t>
1045   Conditional request header fields that are defined by extensions to HTTP
1046   might place conditions on all recipients, on the state of the target
1047   resource in general, or on a group of resources. For instance, the "If"
1048   header field in WebDAV can make a request conditional on various aspects
1049   of multiple resources, such as locks, if the recipient understands and
1050   implements that field (<xref target="RFC4918" x:fmt="," x:sec="10.4"/>).
1051</t>
1052<t>
1053   Although conditional request header fields are defined as being usable with
1054   the HEAD method (to keep HEAD's semantics consistent with those of GET),
1055   there is no point in sending a conditional HEAD because a successful
1056   response is around the same size as a <x:ref>304 (Not Modified)</x:ref>
1057   response and more useful than a <x:ref>412 (Precondition Failed)</x:ref>
1058   response.
1059</t>
1060</section>
1061
1062<section title="Precedence" anchor="precedence">
1063<t>
1064   When more than one conditional request header field is present in a request,
1065   the order in which the fields are evaluated becomes important. In practice,
1066   the fields defined in this document are consistently implemented in a
1067   single, logical order, since "lost update" preconditions have more strict
1068   requirements than cache validation, a validated cache is more efficient
1069   than a partial response, and entity tags are presumed to be more accurate
1070   than date validators.
1071</t>
1072<t>
1073   A recipient cache or origin server &MUST; evaluate the request
1074   preconditions defined by this specification in the following order:
1075   <list style="numbers">
1076     <t anchor="precedence1">When recipient is the origin server and
1077       <x:ref>If-Match</x:ref> is present,
1078       evaluate the <x:ref>If-Match</x:ref> precondition:
1079       <list style="symbols">
1080         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
1081         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref> unless
1082            it can be determined that the state-changing request has already
1083            succeeded (see <xref target="header.if-match"/>)</t>
1084       </list>
1085     </t>
1086     <t anchor="precedence2">When recipient is the origin server,
1087       <x:ref>If-Match</x:ref> is not present, and
1088       <x:ref>If-Unmodified-Since</x:ref> is present,
1089       evaluate the <x:ref>If-Unmodified-Since</x:ref> precondition:
1090       <list style="symbols">
1091         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
1092         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref> unless
1093            it can be determined that the state-changing request has already
1094            succeeded (see <xref target="header.if-unmodified-since"/>)</t>
1095       </list>
1096     </t>
1097     <t anchor="precedence3">When <x:ref>If-None-Match</x:ref> is present,
1098       evaluate the <x:ref>If-None-Match</x:ref> precondition:
1099       <list style="symbols">
1100         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
1101         <t>if false for GET/HEAD, respond <x:ref>304 (Not Modified)</x:ref></t>
1102         <t>if false for other methods, respond <x:ref>412 (Precondition Failed)</x:ref></t>
1103       </list>
1104     </t>
1105     <t anchor="precedence4">When the method is GET or HEAD,
1106       <x:ref>If-None-Match</x:ref> is not present, and
1107       <x:ref>If-Modified-Since</x:ref> is present,
1108       evaluate the <x:ref>If-Modified-Since</x:ref> precondition:
1109       <list style="symbols">
1110         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
1111         <t>if false, respond <x:ref>304 (Not Modified)</x:ref></t>
1112       </list>
1113     </t>
1114     <t anchor="precedence5">When the method is GET and both
1115       <x:ref>Range</x:ref> and <x:ref>If-Range</x:ref> are present,
1116       evaluate the <x:ref>If-Range</x:ref> precondition:
1117       <list style="symbols">
1118         <t>if the validator matches and the Range specification is
1119            applicable to the selected representation, respond
1120            <x:ref>206 (Partial Content)</x:ref> <xref target="Part5"/></t>
1121       </list>
1122     </t>
1123     <t anchor="precedencelast">Otherwise,
1124       <list style="symbols">
1125         <t>all conditions are met, so perform the requested action and
1126            respond according to its success or failure.</t>
1127       </list>
1128     </t>
1129   </list>
1130</t>
1131<t>
1132   Any extension to HTTP/1.1 that defines additional conditional request
1133   header fields ought to define its own expectations regarding the order
1134   for evaluating such fields in relation to those defined in this document
1135   and other conditionals that might be found in practice.
1136</t>
1137</section>
1138
1139<section title="IANA Considerations" anchor="IANA.considerations">
1140
1141<section title="Status Code Registration" anchor="status.code.registration">
1142<t>
1143   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1144   shall be updated with the registrations below:
1145</t>
1146<?BEGININC p4-conditional.iana-status-codes ?>
1147<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1148<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1149   <ttcol>Value</ttcol>
1150   <ttcol>Description</ttcol>
1151   <ttcol>Reference</ttcol>
1152   <c>304</c>
1153   <c>Not Modified</c>
1154   <c>
1155      <xref target="status.304"/>
1156   </c>
1157   <c>412</c>
1158   <c>Precondition Failed</c>
1159   <c>
1160      <xref target="status.412"/>
1161   </c>
1162</texttable>
1163<!--(END)-->
1164<?ENDINC p4-conditional.iana-status-codes ?>
1165</section>
1166
1167<section title="Header Field Registration" anchor="header.field.registration">
1168<t>
1169   HTTP header fields are registered within the Message Header Field Registry
1170   maintained at
1171   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
1172</t>
1173<t>
1174   This document defines the following HTTP header fields, so their
1175   associated registry entries shall be updated according to the permanent
1176   registrations below (see <xref target="BCP90"/>):
1177</t>
1178<?BEGININC p4-conditional.iana-headers ?>
1179<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1180<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1181   <ttcol>Header Field Name</ttcol>
1182   <ttcol>Protocol</ttcol>
1183   <ttcol>Status</ttcol>
1184   <ttcol>Reference</ttcol>
1185
1186   <c>ETag</c>
1187   <c>http</c>
1188   <c>standard</c>
1189   <c>
1190      <xref target="header.etag"/>
1191   </c>
1192   <c>If-Match</c>
1193   <c>http</c>
1194   <c>standard</c>
1195   <c>
1196      <xref target="header.if-match"/>
1197   </c>
1198   <c>If-Modified-Since</c>
1199   <c>http</c>
1200   <c>standard</c>
1201   <c>
1202      <xref target="header.if-modified-since"/>
1203   </c>
1204   <c>If-None-Match</c>
1205   <c>http</c>
1206   <c>standard</c>
1207   <c>
1208      <xref target="header.if-none-match"/>
1209   </c>
1210   <c>If-Unmodified-Since</c>
1211   <c>http</c>
1212   <c>standard</c>
1213   <c>
1214      <xref target="header.if-unmodified-since"/>
1215   </c>
1216   <c>Last-Modified</c>
1217   <c>http</c>
1218   <c>standard</c>
1219   <c>
1220      <xref target="header.last-modified"/>
1221   </c>
1222</texttable>
1223<!--(END)-->
1224<?ENDINC p4-conditional.iana-headers ?>
1225<t>
1226   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1227</t>
1228</section>
1229</section>
1230
1231<section title="Security Considerations" anchor="security.considerations">
1232<t>
1233   This section is meant to inform developers, information providers, and
1234   users of known security concerns specific to the HTTP/1.1 conditional
1235   request mechanisms. More general security considerations are addressed
1236   in HTTP messaging &messaging; and semantics &semantics;.
1237</t>
1238<t>
1239   The validators defined by this specification are not intended to ensure
1240   the validity of a representation, guard against malicious changes, or
1241   detect man-in-the-middle attacks. At best, they enable more efficient cache
1242   updates and optimistic concurrent writes when all participants are behaving
1243   nicely. At worst, the conditions will fail and the client will receive a
1244   response that is no more harmful than an HTTP exchange without conditional
1245   requests.
1246</t>
1247<t>
1248   An entity-tag can be abused in ways that create privacy risks. For example,
1249   a site might deliberately construct a semantically invalid entity-tag that
1250   is unique to the user or user agent, send it in a cacheable response with a
1251   long freshness time, and then read that entity-tag in later conditional
1252   requests as a means of re-identifying that user or user agent. Such an
1253   identifying tag would become a persistent identifier for as long as the
1254   user agent retained the original cache entry. User agents that cache
1255   representations ought to ensure that the cache is cleared or replaced
1256   whenever the user performs privacy-maintaining actions, such as clearing
1257   stored cookies or changing to a private browsing mode.
1258</t>
1259</section>
1260
1261<section title="Acknowledgments" anchor="acks">
1262<t>
1263  See &acks;.
1264</t>
1265</section>
1266</middle>
1267<back>
1268
1269<references title="Normative References">
1270
1271<reference anchor="Part1">
1272  <front>
1273    <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
1274    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1275      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1276      <address><email>fielding@gbiv.com</email></address>
1277    </author>
1278    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1279      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1280      <address><email>julian.reschke@greenbytes.de</email></address>
1281    </author>
1282    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1283  </front>
1284  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1285  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1286</reference>
1287
1288<reference anchor="Part2">
1289  <front>
1290    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
1291    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1292      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1293      <address><email>fielding@gbiv.com</email></address>
1294    </author>
1295    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1296      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1297      <address><email>julian.reschke@greenbytes.de</email></address>
1298    </author>
1299    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1300  </front>
1301  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1302  <x:source href="p2-semantics.xml" basename="p2-semantics">
1303    <x:defines>2xx</x:defines>
1304    <x:defines>2xx (Successful)</x:defines>
1305    <x:defines>200 (OK)</x:defines>
1306    <x:defines>204 (No Content)</x:defines>
1307    <x:defines>Accept-Encoding</x:defines>
1308    <x:defines>Content-Location</x:defines>
1309    <x:defines>Content-Type</x:defines>
1310    <x:defines>Date</x:defines>
1311    <x:defines>Location</x:defines>
1312    <x:defines>Vary</x:defines>
1313    <x:defines>selected representation</x:defines>
1314  </x:source>
1315</reference>
1316
1317<reference anchor="Part5">
1318  <front>
1319    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
1320    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1321      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1322      <address><email>fielding@gbiv.com</email></address>
1323    </author>
1324    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1325      <organization abbrev="W3C">World Wide Web Consortium</organization>
1326      <address><email>ylafon@w3.org</email></address>
1327    </author>
1328    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1329      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1330      <address><email>julian.reschke@greenbytes.de</email></address>
1331    </author>
1332    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1333  </front>
1334  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1335  <x:source href="p5-range.xml" basename="p5-range">
1336    <x:defines>If-Range</x:defines>
1337    <x:defines>Range</x:defines>
1338    <x:defines>206 (Partial Content)</x:defines>
1339  </x:source>
1340</reference>
1341
1342<reference anchor="Part6">
1343  <front>
1344    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
1345    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1346      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1347      <address><email>fielding@gbiv.com</email></address>
1348    </author>
1349    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1350      <organization>Akamai</organization>
1351      <address><email>mnot@mnot.net</email></address>
1352    </author>
1353    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1354      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1355      <address><email>julian.reschke@greenbytes.de</email></address>
1356    </author>
1357    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1358  </front>
1359  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1360  <x:source href="p6-cache.xml" basename="p6-cache">
1361    <x:defines>Cache-Control</x:defines>
1362    <x:defines>Expires</x:defines>
1363  </x:source>
1364</reference>
1365
1366<reference anchor="RFC2119">
1367  <front>
1368    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1369    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1370      <organization>Harvard University</organization>
1371      <address><email>sob@harvard.edu</email></address>
1372    </author>
1373    <date month="March" year="1997"/>
1374  </front>
1375  <seriesInfo name="BCP" value="14"/>
1376  <seriesInfo name="RFC" value="2119"/>
1377</reference>
1378
1379<reference anchor="RFC5234">
1380  <front>
1381    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1382    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1383      <organization>Brandenburg InternetWorking</organization>
1384      <address>
1385        <email>dcrocker@bbiw.net</email>
1386      </address> 
1387    </author>
1388    <author initials="P." surname="Overell" fullname="Paul Overell">
1389      <organization>THUS plc.</organization>
1390      <address>
1391        <email>paul.overell@thus.net</email>
1392      </address>
1393    </author>
1394    <date month="January" year="2008"/>
1395  </front>
1396  <seriesInfo name="STD" value="68"/>
1397  <seriesInfo name="RFC" value="5234"/>
1398</reference>
1399
1400</references>
1401
1402<references title="Informative References">
1403
1404<reference anchor="RFC2616">
1405  <front>
1406    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1407    <author initials="R." surname="Fielding" fullname="R. Fielding">
1408      <organization>University of California, Irvine</organization>
1409      <address><email>fielding@ics.uci.edu</email></address>
1410    </author>
1411    <author initials="J." surname="Gettys" fullname="J. Gettys">
1412      <organization>W3C</organization>
1413      <address><email>jg@w3.org</email></address>
1414    </author>
1415    <author initials="J." surname="Mogul" fullname="J. Mogul">
1416      <organization>Compaq Computer Corporation</organization>
1417      <address><email>mogul@wrl.dec.com</email></address>
1418    </author>
1419    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1420      <organization>MIT Laboratory for Computer Science</organization>
1421      <address><email>frystyk@w3.org</email></address>
1422    </author>
1423    <author initials="L." surname="Masinter" fullname="L. Masinter">
1424      <organization>Xerox Corporation</organization>
1425      <address><email>masinter@parc.xerox.com</email></address>
1426    </author>
1427    <author initials="P." surname="Leach" fullname="P. Leach">
1428      <organization>Microsoft Corporation</organization>
1429      <address><email>paulle@microsoft.com</email></address>
1430    </author>
1431    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1432      <organization>W3C</organization>
1433      <address><email>timbl@w3.org</email></address>
1434    </author>
1435    <date month="June" year="1999"/>
1436  </front>
1437  <seriesInfo name="RFC" value="2616"/>
1438</reference>
1439
1440<reference anchor='BCP90'>
1441  <front>
1442    <title>Registration Procedures for Message Header Fields</title>
1443    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1444      <organization>Nine by Nine</organization>
1445      <address><email>GK-IETF@ninebynine.org</email></address>
1446    </author>
1447    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1448      <organization>BEA Systems</organization>
1449      <address><email>mnot@pobox.com</email></address>
1450    </author>
1451    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1452      <organization>HP Labs</organization>
1453      <address><email>JeffMogul@acm.org</email></address>
1454    </author>
1455    <date year='2004' month='September' />
1456  </front>
1457  <seriesInfo name='BCP' value='90' />
1458  <seriesInfo name='RFC' value='3864' />
1459</reference>
1460
1461<reference anchor='RFC4918'>
1462  <front>
1463    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1464    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1465      <organization abbrev="CommerceNet">CommerceNet</organization>
1466      <address><email>ldusseault@commerce.net</email></address>
1467    </author>
1468    <date month="June" year="2007" />
1469  </front>
1470  <seriesInfo name='RFC' value='4918' />
1471</reference>
1472</references>
1473
1474<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1475<t>
1476  The definition of validator weakness has been expanded and clarified.
1477  (<xref target="weak.and.strong.validators" />)
1478</t>
1479<t>
1480  Weak entity-tags are now allowed in all requests except range requests.
1481  (Sections <xref target="weak.and.strong.validators" format="counter"/> and
1482  <xref target="header.if-none-match" format="counter"/>)
1483</t>
1484<t>
1485  The <x:ref>ETag</x:ref> header field ABNF has been changed to not use
1486  quoted-string, thus avoiding escaping issues.
1487  (<xref target="header.etag" />)
1488</t>
1489<t>
1490  ETag is defined to provide an entity tag for the selected representation,
1491  thereby clarifying what it applies to in various situations (such as a
1492  PUT response).
1493  (<xref target="header.etag" />)
1494</t>
1495<t>
1496  The precedence for evaluation of conditional requests has been defined.
1497  (<xref target="precedence" />)
1498</t>
1499</section>
1500
1501<section title="Imported ABNF" anchor="imported.abnf">
1502  <x:anchor-alias value="ALPHA"/>
1503  <x:anchor-alias value="CR"/>
1504  <x:anchor-alias value="DIGIT"/>
1505  <x:anchor-alias value="DQUOTE"/>
1506  <x:anchor-alias value="LF"/>
1507  <x:anchor-alias value="OCTET"/>
1508  <x:anchor-alias value="VCHAR"/>
1509  <x:anchor-alias value="core.rules"/>
1510  <x:anchor-alias value="obs-text"/>
1511  <x:anchor-alias value="OWS"/>
1512  <x:anchor-alias value="HTTP-date"/>
1513<t>
1514  The following core rules are included by
1515  reference, as defined in <xref target="RFC5234" x:fmt="of" x:sec="B.1"/>:
1516  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
1517  DIGIT (decimal 0-9), DQUOTE (double quote),
1518  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
1519  OCTET (any 8-bit sequence of data), SP (space), and
1520  VCHAR (any visible US-ASCII character).
1521</t>
1522<t>
1523  The rules below are defined in <xref target="Part1"/>:
1524</t>
1525<figure><artwork type="abnf2616">
1526  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
1527  <x:ref>obs-text</x:ref>      = &lt;obs-text, defined in &field-components;&gt;
1528</artwork></figure>
1529<t>
1530  The rules below are defined in other parts:
1531</t>
1532<figure><artwork type="abnf2616">
1533  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
1534</artwork></figure>
1535</section>
1536
1537<?BEGININC p4-conditional.abnf-appendix ?>
1538<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1539<t>
1540  In the collected ABNF below, list rules are expanded as per <xref target="Part1" x:rel="#notation"/>.
1541</t><figure>
1542<artwork type="abnf" name="p4-conditional.parsed-abnf">
1543<x:ref>ETag</x:ref> = entity-tag
1544
1545<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
1546
1547<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1548 entity-tag ] ) )
1549<x:ref>If-Modified-Since</x:ref> = HTTP-date
1550<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1551 entity-tag ] ) )
1552<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1553
1554<x:ref>Last-Modified</x:ref> = HTTP-date
1555
1556<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
1557
1558<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1559<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1560 / obs-text
1561
1562<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.6&gt;
1563<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1564
1565<x:ref>weak</x:ref> = %x57.2F ; W/
1566</artwork>
1567</figure>
1568</section>
1569<?ENDINC p4-conditional.abnf-appendix ?>
1570
1571<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1572<t>
1573  Changes up to the first Working Group Last Call draft are summarized
1574  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C"/>.
1575</t>
1576
1577<section title="Since draft-ietf-httpbis-p4-conditional-19" anchor="changes.since.19">
1578<t>
1579  Closed issues:
1580  <list style="symbols">
1581    <t>
1582      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/241"/>:
1583      "Need to clarify eval order/interaction of conditional headers"
1584    </t>
1585    <t>
1586      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/345"/>:
1587      "Required headers on 304 and 206"
1588    </t>
1589    <t>
1590      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/350"/>:
1591      "Optionality of Conditional Request Support"
1592    </t>
1593    <t>
1594      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/354"/>:
1595      "ETags and Conditional Requests"
1596    </t>
1597    <t>
1598      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/361"/>:
1599      "ABNF requirements for recipients"
1600    </t>
1601    <t>
1602      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/363"/>:
1603      "Rare cases"
1604    </t>
1605    <t>
1606      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/365"/>:
1607      "Conditional Request Security Considerations"
1608    </t>
1609    <t>
1610      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/371"/>:
1611      "If-Modified-Since lacks definition for method != GET"
1612    </t>
1613    <t>
1614      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/372"/>:
1615      "refactor conditional header field descriptions"
1616    </t>
1617  </list>
1618</t>
1619</section>
1620
1621<section title="Since draft-ietf-httpbis-p4-conditional-20" anchor="changes.since.20">
1622<t>
1623  <list style="symbols">
1624    <t>
1625      Conformance criteria and considerations regarding error handling are
1626      now defined in Part 1.
1627    </t>
1628  </list>
1629</t>
1630</section>
1631
1632<section title="Since draft-ietf-httpbis-p4-conditional-21" anchor="changes.since.21">
1633<t>
1634  Closed issues:
1635  <list style="symbols">
1636    <t>
1637      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/96"/>:
1638      "Conditional GET text"
1639    </t>
1640    <t>
1641      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/350"/>:
1642      "Optionality of Conditional Request Support"
1643    </t>
1644    <t>
1645      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/384"/>:
1646      "unclear prose in definition of 304"
1647    </t>
1648    <t>
1649      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/401"/>:
1650      "ETags and Conneg"
1651    </t>
1652    <t>
1653      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/402"/>:
1654      "Comparison function for If-Match and If-None-Match"
1655    </t>
1656    <t>
1657      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/406"/>:
1658      "304 without validator"
1659    </t>
1660    <t>
1661      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/427"/>:
1662      "If-Match and 428"
1663    </t>
1664  </list>
1665</t>
1666</section>
1667
1668<section title="Since draft-ietf-httpbis-p4-conditional-22" anchor="changes.since.22">
1669<t>
1670  Closed issues:
1671  <list style="symbols">
1672    <t>
1673      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/436"/>:
1674      "explain list expansion in ABNF appendices"
1675    </t>
1676    <t>
1677      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/437"/>:
1678      "incorrect example dates"
1679    </t>
1680  </list>
1681</t>
1682<t>
1683  Partly resolved issues:
1684  <list style="symbols">
1685    <t>
1686      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/461"/>:
1687      "Editorial suggestions"
1688    </t>
1689  </list>
1690</t>
1691</section>
1692
1693<section title="Since draft-ietf-httpbis-p4-conditional-23" anchor="changes.since.23">
1694<t>
1695  Closed issues:
1696  <list style="symbols">
1697    <t>
1698      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/455"/>:
1699      "PUT + If-Match over-constrained?"
1700    </t>
1701    <t>
1702      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/479"/>:
1703      "MUSTs and other feedback"
1704    </t>
1705    <t>
1706      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/495"/>:
1707      "p4 editorial nits"
1708    </t>
1709  </list>
1710</t>
1711</section>
1712</section>
1713
1714</back>
1715</rfc>
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