source: draft-ietf-httpbis/latest/p4-conditional.xml @ 2281

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