source: draft-ietf-httpbis/latest/p4-conditional.xml @ 2554

Last change on this file since 2554 was 2554, checked in by julian.reschke@…, 6 years ago

add the "stateless" term to abstracts (see #538)

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