source: draft-ietf-httpbis/latest/p4-conditional.xml @ 2629

Last change on this file since 2629 was 2629, checked in by julian.reschke@…, 6 years ago

insert RFC numbers (#553)

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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 "May">
16  <!ENTITY ID-YEAR "2014">
17  <!ENTITY mdash "&#8212;">
18  <!ENTITY Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
19  <!ENTITY architecture               "<xref target='RFC7230' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY conformance                "<xref target='RFC7230' x:rel='#conformance' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY notation                   "<xref target='RFC7230' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY abnf-extension             "<xref target='RFC7230' x:rel='#abnf.extension' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY acks                       "<xref target='RFC7230' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY whitespace                 "<xref target='RFC7230' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY field-components           "<xref target='RFC7230' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY header-date                "<xref target='RFC7231' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY safe-methods               "<xref target='RFC7231' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY representation             "<xref target='RFC7231' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY messaging                  "<xref target='RFC7230' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY semantics                  "<xref target='RFC7231' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY caching                    "<xref target='RFC7234' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY cache-key                  "<xref target='RFC7234' x:rel='#constructing.responses.from.caches' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY cache-validation-received  "<xref target='RFC7234' x:rel='#validation.received' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY freshening-responses       "<xref target='RFC7234' x:rel='#freshening.responses' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-accept-encoding     "<xref target='RFC7231' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-if-range            "<xref target='RFC7233' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-range               "<xref target='RFC7233' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY header-vary                "<xref target='RFC7231' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY http-date                  "<xref target='RFC7231' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY transfer-codings           "<xref target='RFC7230' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41  <!ENTITY content-negotiation        "<xref target='RFC7231' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
42]>
43<?rfc toc="yes" ?>
44<?rfc symrefs="yes" ?>
45<?rfc sortrefs="yes" ?>
46<?rfc compact="yes"?>
47<?rfc subcompact="no" ?>
48<?rfc linkmailto="no" ?>
49<?rfc editing="no" ?>
50<?rfc comments="yes"?>
51<?rfc inline="yes"?>
52<?rfc rfcedstyle="yes"?>
53<?rfc-ext allow-markup-in-artwork="yes" ?>
54<?rfc-ext include-references-in-index="yes" ?>
55<rfc obsoletes="2616" category="std" x:maturity-level="proposed"
56     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
57     xmlns:x='http://purl.org/net/xml2rfc/ext'>
58<x:link rel="prev" basename="p2-semantics"/>
59<x:link rel="next" basename="p5-range"/>
60<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
61<front>
62
63  <title abbrev="HTTP/1.1 Conditional Requests">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
64
65  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
66    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
67    <address>
68      <postal>
69        <street>345 Park Ave</street>
70        <city>San Jose</city>
71        <region>CA</region>
72        <code>95110</code>
73        <country>USA</country>
74      </postal>
75      <email>fielding@gbiv.com</email>
76      <uri>http://roy.gbiv.com/</uri>
77    </address>
78  </author>
79
80  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
81    <organization abbrev="greenbytes">greenbytes GmbH</organization>
82    <address>
83      <postal>
84        <street>Hafenweg 16</street>
85        <city>Muenster</city><region>NW</region><code>48155</code>
86        <country>Germany</country>
87      </postal>
88      <email>julian.reschke@greenbytes.de</email>
89      <uri>http://greenbytes.de/tech/webdav/</uri>
90    </address>
91  </author>
92
93  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
94
95  <area>Applications</area>
96  <workgroup>HTTPbis</workgroup>
97
98<abstract>
99<t>
100   The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for
101   distributed, collaborative, hypertext information systems. This document
102   defines HTTP/1.1 conditional requests, including metadata header fields
103   for indicating state changes, request header fields for making
104   preconditions on such state, and rules for constructing the responses to a
105   conditional request when one or more preconditions evaluate to false.
106</t>
107</abstract>
108
109<note title="Editorial Note (To be removed by RFC Editor)">
110  <t>
111    Discussion of this draft takes place on the HTTPBIS working group
112    mailing list (ietf-http-wg@w3.org), which is archived at
113    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
114  </t>
115  <t>
116    The current issues list is at
117    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
118    documents (including fancy diffs) can be found at
119    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
120  </t>
121  <t>
122    The changes in this draft are summarized in <xref target="changes.since.26"/>.
123  </t>
124</note>
125</front>
126
127<middle>
128<section title="Introduction" anchor="introduction">
129<t>
130   Conditional requests are HTTP requests <xref target="RFC7231"/> that include
131   one or more header fields indicating a precondition to be tested before
132   applying the method semantics to the target resource.
133   This document defines the HTTP/1.1 conditional request mechanisms in terms
134   of the architecture, syntax notation, and conformance criteria defined in
135   <xref target="RFC7230"/>.
136</t>
137<t>
138   Conditional GET requests are the most efficient mechanism for HTTP
139   cache updates &caching;.  Conditionals can also be
140   applied to state-changing methods, such as PUT and DELETE, to prevent
141   the "lost update" problem: one client accidentally overwriting
142   the work of another client that has been acting in parallel.
143</t>
144<t><iref primary="true" item="selected representation"/>
145   Conditional request preconditions are based on the state of the target
146   resource as a whole (its current value set) or the state as observed
147   in a previously obtained representation (one value in that set).
148   A resource might have multiple current representations, each with its
149   own observable state.  The conditional request mechanisms assume that
150   the mapping of requests to a "selected representation" (&representation;)
151   will be consistent over time if the server intends to take advantage of
152   conditionals. Regardless, if the mapping is inconsistent and the server is
153   unable to select the appropriate representation, then no harm will result
154   when the precondition evaluates to false.
155</t>
156<t>
157   The conditional request preconditions defined by this specification
158   (<xref target="preconditions"/>) are evaluated when applicable to the
159   recipient (<xref target="evaluation"/>) according to their order of
160   precedence (<xref target="precedence"/>).
161</t>
162
163<section title="Conformance and Error Handling" anchor="conformance">
164<t>
165   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
166   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
167   document are to be interpreted as described in <xref target="RFC2119"/>.
168</t>
169<t>
170   Conformance criteria and considerations regarding error handling
171   are defined in &conformance;.
172</t>
173</section>
174
175<section title="Syntax Notation" anchor="notation">
176<t>
177   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
178   <xref target="RFC5234"/> with a list extension, defined in
179   &abnf-extension;, that allows for compact definition of
180   comma-separated lists using a '#' operator (similar to how the '*' operator
181   indicates repetition).
182   <xref target="imported.abnf"/> describes rules imported from
183   other documents.
184   <xref target="collected.abnf"/> shows the collected grammar with all list
185   operators expanded to standard ABNF notation.
186</t>
187</section>
188</section>
189
190<section title="Validators" anchor="validators">
191   <iref primary="true" item="metadata"/>
192   <iref primary="true" item="validator"/>
193<t>
194   This specification defines two forms of metadata that are commonly used
195   to observe resource state and test for preconditions: modification dates
196   (<xref target="header.last-modified"/>) and opaque entity tags
197   (<xref target="header.etag"/>).  Additional metadata that reflects resource state
198   has been defined by various extensions of HTTP, such as WebDAV
199   <xref target="RFC4918"/>, that are beyond the scope of this specification.
200   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
201   when it is used within a precondition.
202</t>
203
204<section title="Weak versus Strong" anchor="weak.and.strong.validators">
205   <iref primary="true" item="validator" subitem="weak"/>
206   <iref primary="true" item="validator" subitem="strong"/>
207<t>
208   Validators come in two flavors: strong or weak.  Weak validators are easy
209   to generate but are far less useful for comparisons.  Strong validators
210   are ideal for comparisons but can be very difficult (and occasionally
211   impossible) to generate efficiently.  Rather than impose that all forms
212   of resource adhere to the same strength of validator, HTTP exposes the
213   type of validator in use and imposes restrictions on when weak validators
214   can be used as preconditions.
215</t>
216<t>
217   A "strong validator" is representation metadata that changes value whenever
218   a change occurs to the representation data that would be observable in the
219   payload body of a <x:ref>200 (OK)</x:ref> response to GET.
220</t>
221<t>  
222   A strong validator might change for reasons other than a change to the
223   representation data, such as when a
224   semantically significant part of the representation metadata is changed
225   (e.g., <x:ref>Content-Type</x:ref>), but it is in the best interests of the
226   origin server to only change the value when it is necessary to invalidate
227   the stored responses held by remote caches and authoring tools.
228</t>
229<t>
230   Cache entries might persist for arbitrarily long periods, regardless
231   of expiration times.  Thus, a cache might attempt to validate an
232   entry using a validator that it obtained in the distant past.
233   A strong validator is unique across all versions of all
234   representations associated with a particular resource over time.
235   However, there is no implication of uniqueness across representations
236   of different resources (i.e., the same strong validator might be
237   in use for representations of multiple resources at the same time
238   and does not imply that those representations are equivalent).
239</t>
240<t>
241   There are a variety of strong validators used in practice.  The best are
242   based on strict revision control, wherein each change to a representation
243   always results in a unique node name and revision identifier being assigned
244   before the representation is made accessible to GET.  A collision-resistant hash
245   function applied to the representation data is also sufficient if the data
246   is available prior to the response header fields being sent and the digest
247   does not need to be recalculated every time a validation request is
248   received.  However, if a resource has distinct representations that differ
249   only in their metadata, such as might occur with content negotiation over
250   media types that happen to share the same data format, then the origin
251   server needs to incorporate additional information in the validator to
252   distinguish those representations.
253</t>
254<t>
255   In contrast, a "weak validator" is representation metadata that
256   might not change for every change to the representation data.  This
257   weakness might be due to limitations in how the value is calculated, such
258   as clock resolution or an inability to ensure uniqueness for all possible
259   representations of the resource, or due to a desire by the resource owner
260   to group representations by some self-determined set of equivalency
261   rather than unique sequences of data.  An origin server &SHOULD; change a
262   weak entity-tag whenever it considers prior representations to be
263   unacceptable as a substitute for the current representation. In other words,
264   a weak entity-tag ought to change whenever the origin server wants caches to
265   invalidate old responses.
266</t>
267<t>
268   For example, the representation of a weather report that changes in
269   content every second, based on dynamic measurements, might be grouped
270   into sets of equivalent representations (from the origin server's
271   perspective) with the same weak validator in order to allow cached
272   representations to be valid for a reasonable period of time (perhaps
273   adjusted dynamically based on server load or weather quality).
274   Likewise, a representation's modification time, if defined with only
275   one-second resolution, might be a weak validator if it is possible
276   for the representation to be modified twice during a single second and
277   retrieved between those modifications.
278</t>
279<t>
280   Likewise, a validator is weak if it is shared by two or more
281   representations of a given resource at the same time, unless those
282   representations have identical representation data. For example, if the
283   origin server sends the same validator for a representation with a gzip
284   content coding applied as it does for a representation with no content
285   coding, then that validator is weak. However, two simultaneous
286   representations might share the same strong validator if they differ only
287   in the representation metadata, such as when two different media types are
288   available for the same representation data.
289</t>
290<t>
291   Strong validators are usable for all conditional requests, including cache
292   validation, partial content ranges, and "lost update" avoidance.
293   Weak validators are only usable when the client does not require exact
294   equality with previously obtained representation data, such as when
295   validating a cache entry or limiting a web traversal to recent changes.
296</t>
297</section>
298
299<section title="Last-Modified" anchor="header.last-modified">
300  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
301  <x:anchor-alias value="Last-Modified"/>
302<t>
303   The "Last-Modified" header field in a response provides a timestamp
304   indicating the date and time at which the origin server believes the
305   selected representation was last modified, as determined at the conclusion
306   of handling the request.
307</t>
308<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
309  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
310</artwork></figure>
311<t>
312   An example of its use is
313</t>
314<figure><artwork type="example">
315  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
316</artwork></figure>
317
318<section title="Generation" anchor="lastmod.generation">
319<t>
320   An origin server &SHOULD; send Last-Modified for any selected
321   representation for which a last modification date can be reasonably
322   and consistently determined, since its use in conditional requests
323   and evaluating cache freshness (&caching;) results in a substantial
324   reduction of HTTP traffic on the Internet and can be a significant
325   factor in improving service scalability and reliability.
326</t>
327<t>
328   A representation is typically the sum of many parts behind the
329   resource interface.  The last-modified time would usually be
330   the most recent time that any of those parts were changed.
331   How that value is determined for any given resource is an
332   implementation detail beyond the scope of this specification.
333   What matters to HTTP is how recipients of the Last-Modified
334   header field can use its value to make conditional requests
335   and test the validity of locally cached responses.
336</t>
337<t>
338   An origin server &SHOULD; obtain the Last-Modified value of the
339   representation as close as possible to the time that it generates the
340   <x:ref>Date</x:ref> field value for its response. This allows a recipient to
341   make an accurate assessment of the representation's modification time,
342   especially if the representation changes near the time that the
343   response is generated.
344</t>
345<t>
346   An origin server with a clock &MUST-NOT; send a Last-Modified date
347   that is later than the server's time of message origination (<x:ref>Date</x:ref>).
348   If the last modification time is derived from implementation-specific
349   metadata that evaluates to some time in the future, according to the
350   origin server's clock, then the origin server &MUST; replace that
351   value with the message origination date. This prevents a future
352   modification date from having an adverse impact on cache validation.
353</t>
354<t>
355   An origin server without a clock &MUST-NOT; assign Last-Modified
356   values to a response unless these values were associated
357   with the resource by some other system or user with a reliable clock.
358</t>
359</section>
360
361<section title="Comparison" anchor="lastmod.comparison">
362<t>
363   A Last-Modified time, when used as a validator in a request, is
364   implicitly weak unless it is possible to deduce that it is strong,
365   using the following rules:
366  <list style="symbols">
367     <t>The validator is being compared by an origin server to the
368        actual current validator for the representation and,</t>
369     <t>That origin server reliably knows that the associated representation did
370        not change twice during the second covered by the presented
371        validator.</t>
372  </list>
373</t>
374<t>
375   or
376  <list style="symbols">
377     <t>The validator is about to be used by a client in an <x:ref>If-Modified-Since</x:ref>,
378        <x:ref>If-Unmodified-Since</x:ref> header field, because the client has
379        a cache entry, or <x:ref>If-Range</x:ref> for the associated
380        representation, and</t>
381     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
382        time when the origin server sent the original response, and</t>
383     <t>The presented Last-Modified time is at least 60 seconds before
384        the Date value.</t>
385  </list>
386</t>
387<t>
388   or
389  <list style="symbols">
390     <t>The validator is being compared by an intermediate cache to the
391        validator stored in its cache entry for the representation, and</t>
392     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
393        time when the origin server sent the original response, and</t>
394     <t>The presented Last-Modified time is at least 60 seconds before
395        the Date value.</t>
396  </list>
397</t>
398<t>
399   This method relies on the fact that if two different responses were
400   sent by the origin server during the same second, but both had the
401   same Last-Modified time, then at least one of those responses would
402   have a <x:ref>Date</x:ref> value equal to its Last-Modified time. The
403   arbitrary 60-second limit guards against the possibility that the Date and
404   Last-Modified values are generated from different clocks, or at somewhat
405   different times during the preparation of the response. An
406   implementation &MAY; use a value larger than 60 seconds, if it is
407   believed that 60 seconds is too short.
408</t>
409</section>
410</section>
411
412<section title="ETag" anchor="header.etag">
413  <iref primary="true" item="ETag header field" x:for-anchor=""/>
414  <x:anchor-alias value="ETag"/>
415  <x:anchor-alias value="entity-tag"/>
416  <x:anchor-alias value="opaque-tag"/>
417  <x:anchor-alias value="weak"/>
418  <x:anchor-alias value="etagc"/>
419<t>
420   The "ETag" header field in a response provides the current entity-tag for
421   the selected representation, as determined at the conclusion of handling
422   the request.
423   An entity-tag is an opaque validator for differentiating between
424   multiple representations of the same resource, regardless of whether
425   those multiple representations are due to resource state changes over
426   time, content negotiation resulting in multiple representations being
427   valid at the same time, or both. An entity-tag consists of an opaque
428   quoted string, possibly prefixed by a weakness indicator.
429</t>
430<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"/>
431  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
432
433  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
434  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
435  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
436  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
437             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
438</artwork></figure>
439<x:note>
440  <t>
441    &Note; Previously, opaque-tag was defined to be a quoted-string
442    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
443    might perform backslash unescaping. Servers therefore ought to avoid
444    backslash characters in entity tags.
445  </t>
446</x:note>
447<t>
448   An entity-tag can be more reliable for validation than a modification
449   date in situations where it is inconvenient to store modification
450   dates, where the one-second resolution of HTTP date values is not
451   sufficient, or where modification dates are not consistently maintained.
452</t>
453<figure><preamble>
454  Examples:
455</preamble>
456<artwork type="example">
457  ETag: "xyzzy"
458  ETag: W/"xyzzy"
459  ETag: ""
460</artwork></figure>
461<t>
462   An entity-tag can be either a weak or strong validator, with
463   strong being the default.  If an origin server provides an entity-tag
464   for a representation and the generation of that entity-tag does not satisfy
465   all of the characteristics of a strong validator
466   (<xref target="weak.and.strong.validators"/>), then the origin server
467   &MUST; mark the entity-tag as weak by prefixing its opaque value
468   with "W/" (case-sensitive).
469</t>
470
471<section title="Generation" anchor="entity.tag.generation">
472<t>
473   The principle behind entity-tags is that only the service author
474   knows the implementation of a resource well enough to select the
475   most accurate and efficient validation mechanism for that resource,
476   and that any such mechanism can be mapped to a simple sequence of
477   octets for easy comparison.  Since the value is opaque, there is no
478   need for the client to be aware of how each entity-tag is constructed.
479</t>
480<t>
481   For example, a resource that has implementation-specific versioning
482   applied to all changes might use an internal revision number, perhaps
483   combined with a variance identifier for content negotiation, to
484   accurately differentiate between representations.
485   Other implementations might use a collision-resistant hash of
486   representation content, a combination of various file attributes, or
487   a modification timestamp that has sub-second resolution.
488</t>
489<t>
490   An origin server &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 data,
594    so a strong entity-tag for a content-encoded representation has to be
595    distinct from the entity tag of an unencoded representation to prevent
596    potential conflicts during cache updates and range requests. In contrast,
597    transfer codings (&transfer-codings;) apply only during message transfer
598    and do not result in distinct entity-tags.
599  </t>
600</x:note>
601</section>
602</section>
603
604<section title="When to Use Entity-tags and Last-Modified Dates" anchor="when.to.use.entity.tags.and.last-modified.dates">
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="RFC7233"/></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 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="RFC7230">
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="RFC7231">
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="RFC7233">
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="RFC7234">
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="RFC7230"/>:
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="RFC7230" 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 [RFC7231], 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 [RFC7230], 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 [RFC7230], 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 IETF Last Call draft are summarized
1574  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-24#appendix-D"/>.
1575</t>
1576
1577<section title="Since draft-ietf-httpbis-p4-conditional-24" anchor="changes.since.24">
1578<t>
1579  Closed issues:
1580  <list style="symbols">
1581    <t>
1582      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/518"/>:
1583      "APPSDIR review of draft-ietf-httpbis-p4-conditional-24"
1584    </t>
1585  </list>
1586</t>
1587</section>
1588
1589<section title="Since draft-ietf-httpbis-p4-conditional-25" anchor="changes.since.25">
1590<t>
1591  Closed issues:
1592  <list style="symbols">
1593    <t>
1594      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/538"/>:
1595      "add 'stateless' to Abstract"
1596    </t>
1597    <t>
1598      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/542"/>:
1599      "improve introduction of list rule"
1600    </t>
1601    <t>
1602      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/549"/>:
1603      "augment security considerations with pointers to current research"
1604    </t>
1605  </list>
1606</t>
1607</section>
1608
1609<section title="Since draft-ietf-httpbis-p4-conditional-26" anchor="changes.since.26">
1610<t>
1611  None yet.
1612</t>
1613</section>
1614</section>
1615
1616</back>
1617</rfc>
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