source: draft-ietf-httpbis/latest/p4-conditional.xml @ 2398

Last change on this file since 2398 was 2398, checked in by fielding@…, 8 years ago

rephrase misused SHOULDs; addresses #472

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