source: draft-ietf-httpbis/latest/p4-conditional.xml @ 2534

Last change on this file since 2534 was 2531, checked in by fielding@…, 7 years ago

slightly rephrase [2525]; see #542

  • Property svn:eol-style set to native
  • Property svn:mime-type set to text/xml
File size: 69.6 KB
Line 
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 Note "<x:h xmlns:x='http://purl.org/net/xml2rfc/ext'>Note:</x:h>">
18  <!ENTITY architecture               "<xref target='Part1' x:rel='#architecture' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
19  <!ENTITY conformance                "<xref target='Part1' x:rel='#conformance' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
20  <!ENTITY notation                   "<xref target='Part1' x:rel='#notation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
21  <!ENTITY abnf-extension             "<xref target='Part1' x:rel='#abnf.extension' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
22  <!ENTITY acks                       "<xref target='Part1' x:rel='#acks' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
23  <!ENTITY whitespace                 "<xref target='Part1' x:rel='#whitespace' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
24  <!ENTITY field-components           "<xref target='Part1' x:rel='#field.components' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
25  <!ENTITY header-date                "<xref target='Part2' x:rel='#header.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
26  <!ENTITY safe-methods               "<xref target='Part2' x:rel='#safe.methods' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
27  <!ENTITY representation             "<xref target='Part2' x:rel='#representations' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
28  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
29  <!ENTITY semantics                  "<xref target='Part2' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
30  <!ENTITY caching                    "<xref target='Part6' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
31  <!ENTITY cache-key                  "<xref target='Part6' x:rel='#constructing.responses.from.caches' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
32  <!ENTITY cache-validation-received  "<xref target='Part6' x:rel='#validation.received' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
33  <!ENTITY freshening-responses       "<xref target='Part6' x:rel='#freshening.responses' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
34  <!ENTITY header-accept-encoding     "<xref target='Part2' x:rel='#header.accept-encoding' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
35  <!ENTITY header-if-range            "<xref target='Part5' x:rel='#header.if-range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
36  <!ENTITY header-range               "<xref target='Part5' x:rel='#header.range' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
37  <!ENTITY header-vary                "<xref target='Part2' x:rel='#header.vary' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
38  <!ENTITY http-date                  "<xref target='Part2' x:rel='#http.date' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
39  <!ENTITY transfer-codings           "<xref target='Part1' x:rel='#transfer.codings' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
40  <!ENTITY content-negotiation        "<xref target='Part2' x:rel='#content.negotiation' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
41]>
42<?rfc toc="yes" ?>
43<?rfc symrefs="yes" ?>
44<?rfc sortrefs="yes" ?>
45<?rfc compact="yes"?>
46<?rfc subcompact="no" ?>
47<?rfc linkmailto="no" ?>
48<?rfc editing="no" ?>
49<?rfc comments="yes"?>
50<?rfc inline="yes"?>
51<?rfc rfcedstyle="yes"?>
52<?rfc-ext allow-markup-in-artwork="yes" ?>
53<?rfc-ext include-references-in-index="yes" ?>
54<rfc obsoletes="2616" category="std" x:maturity-level="proposed"
55     ipr="pre5378Trust200902" docName="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"
56     xmlns:x='http://purl.org/net/xml2rfc/ext'>
57<x:link rel="prev" basename="p2-semantics"/>
58<x:link rel="next" basename="p5-range"/>
59<x:feedback template="mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&amp;body=&lt;{ref}&gt;:"/>
60<front>
61
62  <title abbrev="HTTP/1.1 Conditional Requests">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title>
63
64  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
65    <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
66    <address>
67      <postal>
68        <street>345 Park Ave</street>
69        <city>San Jose</city>
70        <region>CA</region>
71        <code>95110</code>
72        <country>USA</country>
73      </postal>
74      <email>fielding@gbiv.com</email>
75      <uri>http://roy.gbiv.com/</uri>
76    </address>
77  </author>
78
79  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
80    <organization abbrev="greenbytes">greenbytes GmbH</organization>
81    <address>
82      <postal>
83        <street>Hafenweg 16</street>
84        <city>Muenster</city><region>NW</region><code>48155</code>
85        <country>Germany</country>
86      </postal>
87      <email>julian.reschke@greenbytes.de</email>
88      <uri>http://greenbytes.de/tech/webdav/</uri>
89    </address>
90  </author>
91
92  <date month="&ID-MONTH;" year="&ID-YEAR;"/>
93  <workgroup>HTTPbis Working Group</workgroup>
94
95<abstract>
96<t>
97   The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
98   distributed, collaborative, hypertext information systems. This document
99   defines HTTP/1.1 conditional requests, including metadata header fields
100   for indicating state changes, request header fields for making
101   preconditions on such state, and rules for constructing the responses to a
102   conditional request when one or more preconditions evaluate to false.
103</t>
104</abstract>
105
106<note title="Editorial Note (To be removed by RFC Editor)">
107  <t>
108    Discussion of this draft takes place on the HTTPBIS working group
109    mailing list (ietf-http-wg@w3.org), which is archived at
110    <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
111  </t>
112  <t>
113    The current issues list is at
114    <eref target="http://tools.ietf.org/wg/httpbis/trac/report/3"/> and related
115    documents (including fancy diffs) can be found at
116    <eref target="http://tools.ietf.org/wg/httpbis/"/>.
117  </t>
118  <t>
119    The changes in this draft are summarized in <xref target="changes.since.25"/>.
120  </t>
121</note>
122</front>
123
124<middle>
125<section title="Introduction" anchor="introduction">
126<t>
127   Conditional requests are HTTP requests <xref target="Part2"/> that include
128   one or more header fields indicating a precondition to be tested before
129   applying the method semantics to the target resource.
130   This document defines the HTTP/1.1 conditional request mechanisms in terms
131   of the architecture, syntax notation, and conformance criteria defined in
132   <xref target="Part1"/>.
133</t>
134<t>
135   Conditional GET requests are the most efficient mechanism for HTTP
136   cache updates &caching;.  Conditionals can also be
137   applied to state-changing methods, such as PUT and DELETE, to prevent
138   the "lost update" problem: one client accidentally overwriting
139   the work of another client that has been acting in parallel.
140</t>
141<t><iref primary="true" item="selected representation"/>
142   Conditional request preconditions are based on the state of the target
143   resource as a whole (its current value set) or the state as observed
144   in a previously obtained representation (one value in that set).
145   A resource might have multiple current representations, each with its
146   own observable state.  The conditional request mechanisms assume that
147   the mapping of requests to a "selected representation" (&representation;)
148   will be consistent over time if the server intends to take advantage of
149   conditionals. Regardless, if the mapping is inconsistent and the server is
150   unable to select the appropriate representation, then no harm will result
151   when the precondition evaluates to false.
152</t>
153<t>
154   The conditional request preconditions defined by this specification
155   (<xref target="preconditions"/>) are evaluated when applicable to the
156   recipient (<xref target="evaluation"/>) according to their order of
157   precedence (<xref target="precedence"/>).
158</t>
159
160<section title="Conformance and Error Handling" anchor="conformance">
161<t>
162   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
163   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
164   document are to be interpreted as described in <xref target="RFC2119"/>.
165</t>
166<t>
167   Conformance criteria and considerations regarding error handling
168   are defined in &conformance;.
169</t>
170</section>
171
172<section title="Syntax Notation" anchor="notation">
173<t>
174   This specification uses the Augmented Backus-Naur Form (ABNF) notation of
175   <xref target="RFC5234"/> with a list extension, defined in
176   &abnf-extension;, that allows for compact definition of
177   comma-separated lists using a '#' operator (similar to how the '*' operator
178   indicates repetition).
179   <xref target="imported.abnf"/> describes rules imported from
180   other documents.
181   <xref target="collected.abnf"/> shows the collected grammar with all list
182   operators expanded to standard ABNF notation.
183</t>
184</section>
185</section>
186
187<section title="Validators" anchor="validators">
188   <iref primary="true" item="metadata"/>
189   <iref primary="true" item="validator"/>
190<t>
191   This specification defines two forms of metadata that are commonly used
192   to observe resource state and test for preconditions: modification dates
193   (<xref target="header.last-modified"/>) and opaque entity tags
194   (<xref target="header.etag"/>).  Additional metadata that reflects resource state
195   has been defined by various extensions of HTTP, such as WebDAV
196   <xref target="RFC4918"/>, that are beyond the scope of this specification.
197   A resource metadata value is referred to as a "<x:dfn>validator</x:dfn>"
198   when it is used within a precondition.
199</t>
200
201<section title="Weak versus Strong" anchor="weak.and.strong.validators">
202   <iref primary="true" item="validator" subitem="weak"/>
203   <iref primary="true" item="validator" subitem="strong"/>
204<t>
205   Validators come in two flavors: strong or weak.  Weak validators are easy
206   to generate but are far less useful for comparisons.  Strong validators
207   are ideal for comparisons but can be very difficult (and occasionally
208   impossible) to generate efficiently.  Rather than impose that all forms
209   of resource adhere to the same strength of validator, HTTP exposes the
210   type of validator in use and imposes restrictions on when weak validators
211   can be used as preconditions.
212</t>
213<t>
214   A "strong validator" is representation metadata that changes value whenever
215   a change occurs to the representation data that would be observable in the
216   payload body of a <x:ref>200 (OK)</x:ref> response to GET.
217</t>
218<t>  
219   A strong validator might change for reasons other than a change to the
220   representation data, such as when a
221   semantically significant part of the representation metadata is changed
222   (e.g., <x:ref>Content-Type</x:ref>), but it is in the best interests of the
223   origin server to only change the value when it is necessary to invalidate
224   the stored responses held by remote caches and authoring tools.
225</t>
226<t>
227   Cache entries might persist for arbitrarily long periods, regardless
228   of expiration times.  Thus, a cache might attempt to validate an
229   entry using a validator that it obtained in the distant past.
230   A strong validator is unique across all versions of all
231   representations associated with a particular resource over time.
232   However, there is no implication of uniqueness across representations
233   of different resources (i.e., the same strong validator might be
234   in use for representations of multiple resources at the same time
235   and does not imply that those representations are equivalent).
236</t>
237<t>
238   There are a variety of strong validators used in practice.  The best are
239   based on strict revision control, wherein each change to a representation
240   always results in a unique node name and revision identifier being assigned
241   before the representation is made accessible to GET.  A collision-resistant hash
242   function applied to the representation data is also sufficient if the data
243   is available prior to the response header fields being sent and the digest
244   does not need to be recalculated every time a validation request is
245   received.  However, if a resource has distinct representations that differ
246   only in their metadata, such as might occur with content negotiation over
247   media types that happen to share the same data format, then the origin
248   server needs to incorporate additional information in the validator to
249   distinguish those representations.
250</t>
251<t>
252   In contrast, a "weak validator" is representation metadata that
253   might not change for every change to the representation data.  This
254   weakness might be due to limitations in how the value is calculated, such
255   as clock resolution or an inability to ensure uniqueness for all possible
256   representations of the resource, or due to a desire by the resource owner
257   to group representations by some self-determined set of equivalency
258   rather than unique sequences of data.  An origin server &SHOULD; change a
259   weak entity-tag whenever it considers prior representations to be
260   unacceptable as a substitute for the current representation. In other words,
261   a weak entity-tag ought to change whenever the origin server wants caches to
262   invalidate old responses.
263</t>
264<t>
265   For example, the representation of a weather report that changes in
266   content every second, based on dynamic measurements, might be grouped
267   into sets of equivalent representations (from the origin server's
268   perspective) with the same weak validator in order to allow cached
269   representations to be valid for a reasonable period of time (perhaps
270   adjusted dynamically based on server load or weather quality).
271   Likewise, a representation's modification time, if defined with only
272   one-second resolution, might be a weak validator if it is possible
273   for the representation to be modified twice during a single second and
274   retrieved between those modifications.
275</t>
276<t>
277   Likewise, a validator is weak if it is shared by two or more
278   representations of a given resource at the same time, unless those
279   representations have identical representation data. For example, if the
280   origin server sends the same validator for a representation with a gzip
281   content coding applied as it does for a representation with no content
282   coding, then that validator is weak. However, two simultaneous
283   representations might share the same strong validator if they differ only
284   in the representation metadata, such as when two different media types are
285   available for the same representation data.
286</t>
287<t>
288   Strong validators are usable for all conditional requests, including cache
289   validation, partial content ranges, and "lost update" avoidance.
290   Weak validators are only usable when the client does not require exact
291   equality with previously obtained representation data, such as when
292   validating a cache entry or limiting a web traversal to recent changes.
293</t>
294</section>
295
296<section title="Last-Modified" anchor="header.last-modified">
297  <iref primary="true" item="Last-Modified header field" x:for-anchor=""/>
298  <x:anchor-alias value="Last-Modified"/>
299<t>
300   The "Last-Modified" header field in a response provides a timestamp
301   indicating the date and time at which the origin server believes the
302   selected representation was last modified, as determined at the conclusion
303   of handling the request.
304</t>
305<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Last-Modified"/>
306  <x:ref>Last-Modified</x:ref> = <x:ref>HTTP-date</x:ref>
307</artwork></figure>
308<t>
309   An example of its use is
310</t>
311<figure><artwork type="example">
312  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
313</artwork></figure>
314
315<section title="Generation" anchor="lastmod.generation">
316<t>
317   An origin server &SHOULD; send Last-Modified for any selected
318   representation for which a last modification date can be reasonably
319   and consistently determined, since its use in conditional requests
320   and evaluating cache freshness (&caching;) results in a substantial
321   reduction of HTTP traffic on the Internet and can be a significant
322   factor in improving service scalability and reliability.
323</t>
324<t>
325   A representation is typically the sum of many parts behind the
326   resource interface.  The last-modified time would usually be
327   the most recent time that any of those parts were changed.
328   How that value is determined for any given resource is an
329   implementation detail beyond the scope of this specification.
330   What matters to HTTP is how recipients of the Last-Modified
331   header field can use its value to make conditional requests
332   and test the validity of locally cached responses.
333</t>
334<t>
335   An origin server &SHOULD; obtain the Last-Modified value of the
336   representation as close as possible to the time that it generates the
337   <x:ref>Date</x:ref> field value for its response. This allows a recipient to
338   make an accurate assessment of the representation's modification time,
339   especially if the representation changes near the time that the
340   response is generated.
341</t>
342<t>
343   An origin server with a clock &MUST-NOT; send a Last-Modified date
344   that is later than the server's time of message origination (<x:ref>Date</x:ref>).
345   If the last modification time is derived from implementation-specific
346   metadata that evaluates to some time in the future, according to the
347   origin server's clock, then the origin server &MUST; replace that
348   value with the message origination date. This prevents a future
349   modification date from having an adverse impact on cache validation.
350</t>
351<t>
352   An origin server without a clock &MUST-NOT; assign Last-Modified
353   values to a response unless these values were associated
354   with the resource by some other system or user with a reliable clock.
355</t>
356</section>
357
358<section title="Comparison" anchor="lastmod.comparison">
359<t>
360   A Last-Modified time, when used as a validator in a request, is
361   implicitly weak unless it is possible to deduce that it is strong,
362   using the following rules:
363  <list style="symbols">
364     <t>The validator is being compared by an origin server to the
365        actual current validator for the representation and,</t>
366     <t>That origin server reliably knows that the associated representation did
367        not change twice during the second covered by the presented
368        validator.</t>
369  </list>
370</t>
371<t>
372   or
373  <list style="symbols">
374     <t>The validator is about to be used by a client in an <x:ref>If-Modified-Since</x:ref>,
375        <x:ref>If-Unmodified-Since</x:ref> header field, because the client has
376        a cache entry, or <x:ref>If-Range</x:ref> for the associated
377        representation, and</t>
378     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
379        time when the origin server sent the original response, and</t>
380     <t>The presented Last-Modified time is at least 60 seconds before
381        the Date value.</t>
382  </list>
383</t>
384<t>
385   or
386  <list style="symbols">
387     <t>The validator is being compared by an intermediate cache to the
388        validator stored in its cache entry for the representation, and</t>
389     <t>That cache entry includes a <x:ref>Date</x:ref> value, which gives the
390        time when the origin server sent the original response, and</t>
391     <t>The presented Last-Modified time is at least 60 seconds before
392        the Date value.</t>
393  </list>
394</t>
395<t>
396   This method relies on the fact that if two different responses were
397   sent by the origin server during the same second, but both had the
398   same Last-Modified time, then at least one of those responses would
399   have a <x:ref>Date</x:ref> value equal to its Last-Modified time. The
400   arbitrary 60-second limit guards against the possibility that the Date and
401   Last-Modified values are generated from different clocks, or at somewhat
402   different times during the preparation of the response. An
403   implementation &MAY; use a value larger than 60 seconds, if it is
404   believed that 60 seconds is too short.
405</t>
406</section>
407</section>
408
409<section title="ETag" anchor="header.etag">
410  <iref primary="true" item="ETag header field" x:for-anchor=""/>
411  <x:anchor-alias value="ETag"/>
412  <x:anchor-alias value="entity-tag"/>
413  <x:anchor-alias value="opaque-tag"/>
414  <x:anchor-alias value="weak"/>
415  <x:anchor-alias value="etagc"/>
416<t>
417   The "ETag" header field in a response provides the current entity-tag for
418   the selected representation, as determined at the conclusion of handling
419   the request.
420   An entity-tag is an opaque validator for differentiating between
421   multiple representations of the same resource, regardless of whether
422   those multiple representations are due to resource state changes over
423   time, content negotiation resulting in multiple representations being
424   valid at the same time, or both. An entity-tag consists of an opaque
425   quoted string, possibly prefixed by a weakness indicator.
426</t>
427<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"/>
428  <x:ref>ETag</x:ref>       = <x:ref>entity-tag</x:ref>
429
430  <x:ref>entity-tag</x:ref> = [ <x:ref>weak</x:ref> ] <x:ref>opaque-tag</x:ref>
431  <x:ref>weak</x:ref>       = <x:abnf-char-sequence>"W/"</x:abnf-char-sequence> ; "W/", case-sensitive
432  <x:ref>opaque-tag</x:ref> = <x:ref>DQUOTE</x:ref> *<x:ref>etagc</x:ref> <x:ref>DQUOTE</x:ref>
433  <x:ref>etagc</x:ref>      = %x21 / %x23-7E / <x:ref>obs-text</x:ref>
434             ; <x:ref>VCHAR</x:ref> except double quotes, plus obs-text
435</artwork></figure>
436<x:note>
437  <t>
438    &Note; Previously, opaque-tag was defined to be a quoted-string
439    (<xref target="RFC2616" x:fmt="," x:sec="3.11"/>), thus some recipients
440    might perform backslash unescaping. Servers therefore ought to avoid
441    backslash characters in entity tags.
442  </t>
443</x:note>
444<t>
445   An entity-tag can be more reliable for validation than a modification
446   date in situations where it is inconvenient to store modification
447   dates, where the one-second resolution of HTTP date values is not
448   sufficient, or where modification dates are not consistently maintained.
449</t>
450<figure><preamble>
451  Examples:
452</preamble>
453<artwork type="example">
454  ETag: "xyzzy"
455  ETag: W/"xyzzy"
456  ETag: ""
457</artwork></figure>
458<t>
459   An entity-tag can be either a weak or strong validator, with
460   strong being the default.  If an origin server provides an entity-tag
461   for a representation and the generation of that entity-tag does not satisfy
462   all of the characteristics of a strong validator
463   (<xref target="weak.and.strong.validators"/>), then the origin server
464   &MUST; mark the entity-tag as weak by prefixing its opaque value
465   with "W/" (case-sensitive).
466</t>
467
468<section title="Generation" anchor="entity.tag.generation">
469<t>
470   The principle behind entity-tags is that only the service author
471   knows the implementation of a resource well enough to select the
472   most accurate and efficient validation mechanism for that resource,
473   and that any such mechanism can be mapped to a simple sequence of
474   octets for easy comparison.  Since the value is opaque, there is no
475   need for the client to be aware of how each entity-tag is constructed.
476</t>
477<t>
478   For example, a resource that has implementation-specific versioning
479   applied to all changes might use an internal revision number, perhaps
480   combined with a variance identifier for content negotiation, to
481   accurately differentiate between representations.
482   Other implementations might use a collision-resistant hash of
483   representation content, a combination of various file attributes, or
484   a modification timestamp that has sub-second resolution.
485</t>
486<t>
487   An origin server &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 data,
591    so a strong entity-tag for a content-encoded representation has to be
592    distinct from the entity tag of an unencoded representation to prevent
593    potential conflicts during cache updates and range requests. In contrast,
594    transfer codings (&transfer-codings;) apply only during message transfer
595    and do not result in distinct entity-tags.
596  </t>
597</x:note>
598</section>
599</section>
600
601<section title="When to Use Entity-tags and Last-Modified Dates" anchor="when.to.use.entity.tags.and.last-modified.dates">
602<t>
603   We adopt a set of rules and recommendations for origin servers,
604   clients, and caches regarding when various validator types ought to
605   be used, and for what purposes.
606</t>
607<t>
608   In <x:ref>200 (OK)</x:ref> responses to GET or HEAD, an origin server:
609  <list style="symbols">
610     <t>&SHOULD; send an entity-tag validator unless it is not feasible to
611        generate one.</t>
612
613     <t>&MAY; send a weak entity-tag instead of a strong entity-tag, if
614        performance considerations support the use of weak entity-tags,
615        or if it is unfeasible to send a strong entity-tag.</t>
616
617     <t>&SHOULD; send a <x:ref>Last-Modified</x:ref> value if it is feasible to
618        send one.</t>
619  </list>
620</t>
621<t>
622   In other words, the preferred behavior for an origin server
623   is to send both a strong entity-tag and a <x:ref>Last-Modified</x:ref>
624   value in successful responses to a retrieval request.
625</t>
626<t>
627   A client:
628  <list style="symbols">
629     <t>&MUST; send that entity-tag in any cache validation request (using
630        <x:ref>If-Match</x:ref> or <x:ref>If-None-Match</x:ref>) if an
631        entity-tag has been provided by the origin server.</t>
632
633     <t>&SHOULD; send the <x:ref>Last-Modified</x:ref> value in non-subrange
634        cache validation requests (using <x:ref>If-Modified-Since</x:ref>)
635        if only a Last-Modified value has been provided by the origin server.</t>
636
637     <t>&MAY; send the <x:ref>Last-Modified</x:ref> value in subrange
638        cache validation requests (using <x:ref>If-Unmodified-Since</x:ref>)
639        if only a Last-Modified value has been provided by an HTTP/1.0 origin
640        server. The user agent &SHOULD; provide a way to disable this, in case
641        of difficulty.</t>
642
643     <t>&SHOULD; send both validators in cache validation requests if both an
644        entity-tag and a <x:ref>Last-Modified</x:ref> value have been provided
645        by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
646        respond appropriately.</t>
647  </list>
648</t>
649</section>
650</section>
651
652<section title="Precondition Header Fields" anchor="preconditions">
653<t>
654   This section defines the syntax and semantics of HTTP/1.1 header fields
655   for applying preconditions on requests.
656   <xref target="evaluation"/> defines when the preconditions are applied.
657   <xref target="precedence"/> defines the order of evaluation when more than
658   one precondition is present.
659</t>
660
661<section title="If-Match" anchor="header.if-match">
662  <iref primary="true" item="If-Match header field" x:for-anchor=""/>
663  <x:anchor-alias value="If-Match"/>
664<t>
665   The "If-Match" header field makes the request method conditional on the
666   recipient origin server either having at least one current
667   representation of the target resource, when the field-value is "*", or
668   having a current representation of the target resource that has an
669   entity-tag matching a member of the list of entity-tags provided in the
670   field-value.
671</t>
672<t>
673   An origin server &MUST; use the strong comparison function when comparing
674   entity-tags for If-Match (<xref target="entity.tag.comparison"/>), since
675   the client intends this precondition to prevent the method from being
676   applied if there have been any changes to the representation data.
677</t>
678<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Match"/>
679  <x:ref>If-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
680</artwork></figure>
681<t>
682   Examples:
683</t>
684<figure><artwork type="example">
685  If-Match: "xyzzy"
686  If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
687  If-Match: *
688</artwork></figure>
689<t>
690   If-Match is most often used with state-changing methods (e.g., POST, PUT,
691   DELETE) to prevent accidental overwrites when multiple user agents might be
692   acting in parallel on the same resource (i.e., to prevent the "lost update"
693   problem). It can also be used with safe methods to abort a request if the
694   <x:ref>selected representation</x:ref> does not match one already stored
695   (or partially stored) from a prior request.
696</t>
697<t>
698   An origin server that receives an If-Match header field &MUST; evaluate the
699   condition prior to performing the method (<xref target="evaluation"/>).
700   If the field-value is "*", the condition is false if the origin server
701   does not have a current representation for the target resource.
702   If the field-value is a list of entity-tags, the condition is false if
703   none of the listed tags match the entity-tag of the selected representation.
704</t>
705<t>
706   An origin server &MUST-NOT; perform the requested method if a received
707   If-Match condition evaluates to false; instead the origin server &MUST;
708   respond with either:
709   a) the <x:ref>412 (Precondition Failed)</x:ref> status code; or,
710   b) one of the <x:ref>2xx (Successful)</x:ref> status codes if the origin
711   server has verified that a state change is being requested and the final
712   state is already reflected in the current state of the target resource
713   (i.e., the change requested by the user agent has already succeeded, but
714   the user agent might not be aware of it, perhaps because the prior response
715   was lost or a compatible change was made by some other user agent).
716   In the latter case, the origin server &MUST-NOT; send a validator header
717   field in the response unless it can verify that the request is a duplicate
718   of an immediately prior change made by the same user agent.
719</t>
720<t>
721   The If-Match header field can be ignored by caches and intermediaries
722   because it is not applicable to a stored response.
723</t>
724</section>
725
726<section title="If-None-Match" anchor="header.if-none-match">
727  <iref primary="true" item="If-None-Match header field" x:for-anchor=""/>
728  <x:anchor-alias value="If-None-Match"/>
729<t>
730   The "If-None-Match" header field makes the request method conditional on
731   a recipient cache or origin server either not having any current
732   representation of the target resource, when the field-value is "*", or
733   having a selected representation with an entity-tag that does not match any
734   of those listed in the field-value.
735</t>
736<t>
737   A recipient &MUST; use the weak comparison function when comparing
738   entity-tags for If-None-Match (<xref target="entity.tag.comparison"/>),
739   since weak entity-tags can be used for cache validation even if there have
740   been changes to the representation data.
741</t>
742<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-None-Match"/>
743  <x:ref>If-None-Match</x:ref> = "*" / 1#<x:ref>entity-tag</x:ref>
744</artwork></figure>
745<t>
746   Examples:
747</t>
748<figure><artwork type="example">
749  If-None-Match: "xyzzy"
750  If-None-Match: W/"xyzzy"
751  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
752  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
753  If-None-Match: *
754</artwork></figure>
755<t>
756   If-None-Match is primarily used in conditional GET requests to enable
757   efficient updates of cached information with a minimum amount of
758   transaction overhead. When a client desires to update one or more stored
759   responses that have entity-tags, the client &SHOULD; generate an
760   If-None-Match header field containing a list of those entity-tags when
761   making a GET request; this allows recipient servers to send a
762   <x:ref>304 (Not Modified)</x:ref> response to indicate when one of those
763   stored responses matches the selected representation.
764</t>
765<t>
766   If-None-Match can also be used with a value of "*" to prevent an unsafe
767   request method (e.g., PUT) from inadvertently modifying an existing
768   representation of the target resource when the client believes that
769   the resource does not have a current representation (&safe-methods;).
770   This is a variation on the "lost update" problem that might arise if more
771   than one client attempts to create an initial representation for the target
772   resource.
773</t>
774<t>
775   An origin server that receives an If-None-Match header field &MUST;
776   evaluate the condition prior to performing the method
777   (<xref target="evaluation"/>).
778   If the field-value is "*", the condition is false if the origin server
779   has a current representation for the target resource.
780   If the field-value is a list of entity-tags, the condition is false if
781   one of the listed tags match the entity-tag of the selected representation.
782</t>
783<t>
784   An origin server &MUST-NOT; perform the requested method if the condition
785   evaluates to false; instead, the origin server &MUST; respond with either
786   a) the <x:ref>304 (Not Modified)</x:ref> status code if the request method
787   is GET or HEAD; or,
788   b) the <x:ref>412 (Precondition Failed)</x:ref> status code for all other
789   request methods.
790</t>
791<t>
792   Requirements on cache handling of a received If-None-Match header field
793   are defined in &cache-validation-received;.
794</t>
795</section>
796
797<section title="If-Modified-Since" anchor="header.if-modified-since">
798  <iref primary="true" item="If-Modified-Since header field" x:for-anchor=""/>
799  <x:anchor-alias value="If-Modified-Since"/>
800<t>
801   The "If-Modified-Since" header field makes a GET or HEAD request method
802   conditional on the selected representation's modification date being more
803   recent than the date provided in the field-value. Transfer of the selected
804   representation's data is avoided if that data has not changed.
805</t>
806<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Modified-Since"/>
807  <x:ref>If-Modified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
808</artwork></figure>
809<t>
810   An example of the field is:
811</t>
812<figure><artwork type="example">
813  If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
814</artwork></figure>
815<t>
816   A recipient &MUST; ignore If-Modified-Since if the request contains an
817   <x:ref>If-None-Match</x:ref> header field; the condition in
818   <x:ref>If-None-Match</x:ref> is considered to be a more accurate
819   replacement for the condition in If-Modified-Since and the two are only
820   combined for the sake of interoperating with older intermediaries that
821   might not implement <x:ref>If-None-Match</x:ref>.
822</t>
823<t>
824   A recipient &MUST; ignore the If-Modified-Since header field if the
825   received field-value is not a valid HTTP-date, or if the request method
826   is neither GET nor HEAD.
827</t>
828<t>
829   A recipient &MUST; interpret an If-Modified-Since field-value's timestamp
830   in terms of the origin server's clock.
831</t>
832<t>
833   If-Modified-Since is typically used for two distinct purposes:
834   1) to allow efficient updates of a cached representation that does not
835   have an entity-tag; and,
836   2) to limit the scope of a web traversal to resources that have recently
837   changed.
838</t>
839<t>
840   When used for cache updates, a cache will typically use the value of the
841   cached message's <x:ref>Last-Modified</x:ref> field to generate the field
842   value of If-Modified-Since. This behavior is most interoperable for cases
843   where clocks are poorly synchronized or when the server has chosen to only
844   honor exact timestamp matches (due to a problem with Last-Modified dates
845   that appear to go "back in time" when the origin server's clock is
846   corrected or a representation is restored from an archived backup).
847   However, caches occasionally generate the field value based on other data,
848   such as the <x:ref>Date</x:ref> header field of the cached message or the
849   local clock time that the message was received, particularly when the
850   cached message does not contain a <x:ref>Last-Modified</x:ref> field.
851</t>
852<t>
853   When used for limiting the scope of retrieval to a recent time window, a
854   user agent will generate an If-Modified-Since field value based on either
855   its own local clock or a <x:ref>Date</x:ref> header field received from the
856   server in a prior response. Origin servers that choose an exact timestamp
857   match based on the selected representation's <x:ref>Last-Modified</x:ref>
858   field will not be able to help the user agent limit its data transfers to
859   only those changed during the specified window.
860</t>
861<t>
862   An origin server that receives an If-Modified-Since header field &SHOULD;
863   evaluate the condition prior to performing the method
864   (<xref target="evaluation"/>).
865   The origin server &SHOULD-NOT; perform the requested method if the selected
866   representation's last modification date is earlier than or equal to the
867   date provided in the field-value; instead, the origin server &SHOULD;
868   generate a <x:ref>304 (Not Modified)</x:ref> response, including only those
869   metadata that are useful for identifying or updating a previously cached
870   response.
871</t>
872<t>
873   Requirements on cache handling of a received If-Modified-Since header field
874   are defined in &cache-validation-received;.
875</t>
876</section>
877
878<section title="If-Unmodified-Since" anchor="header.if-unmodified-since">
879  <iref primary="true" item="If-Unmodified-Since header field" x:for-anchor=""/>
880  <x:anchor-alias value="If-Unmodified-Since"/>
881<t>
882   The "If-Unmodified-Since" header field makes the request method conditional
883   on the selected representation's last modification date being earlier than or
884   equal to the date provided in the field-value. This field accomplishes the
885   same purpose as <x:ref>If-Match</x:ref> for cases where the user agent does
886   not have an entity-tag for the representation.
887</t>
888<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Unmodified-Since"/>
889  <x:ref>If-Unmodified-Since</x:ref> = <x:ref>HTTP-date</x:ref>
890</artwork></figure>
891<t>
892   An example of the field is:
893</t>
894<figure><artwork type="example">
895  If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
896</artwork></figure>
897<t>
898   A recipient &MUST; ignore If-Unmodified-Since if the request contains an
899   <x:ref>If-Match</x:ref> header field; the condition in
900   <x:ref>If-Match</x:ref> is considered to be a more accurate replacement for
901   the condition in If-Unmodified-Since and the two are only combined for the
902   sake of interoperating with older intermediaries that might not implement
903   <x:ref>If-Match</x:ref>.
904</t>
905<t>
906   A recipient &MUST; ignore the If-Unmodified-Since header field if the
907   received field-value is not a valid HTTP-date.
908</t>
909<t>
910   A recipient &MUST; interpret an If-Unmodified-Since field-value's timestamp
911   in terms of the origin server's clock.
912</t>
913<t>
914   If-Unmodified-Since is most often used with state-changing methods
915   (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple
916   user agents might be acting in parallel on a resource that does
917   not supply entity-tags with its representations (i.e., to prevent the
918   "lost update" problem). It can also be used with safe methods to abort a
919   request if the <x:ref>selected representation</x:ref> does not match one
920   already stored (or partially stored) from a prior request.
921</t>
922<t>
923   An origin server that receives an If-Unmodified-Since header field &MUST;
924   evaluate the condition prior to performing the method
925   (<xref target="evaluation"/>).
926   The origin server &MUST-NOT; perform the requested method if the selected
927   representation's last modification date is more recent than the date
928   provided in the field-value; instead the
929   origin server &MUST; respond with either:
930   a) the <x:ref>412 (Precondition Failed)</x:ref> status code; or,
931   b) one of the <x:ref>2xx (Successful)</x:ref> status codes if the origin
932   server has verified that a state change is being requested and the final
933   state is already reflected in the current state of the target resource
934   (i.e., the change requested by the user agent has already succeeded, but
935   the user agent might not be aware of that because the prior response message
936   was lost or a compatible change was made by some other user agent).
937   In the latter case, the origin server &MUST-NOT; send a validator header
938   field in the response unless it can verify that the request is a duplicate
939   of an immediately prior change made by the same user agent.
940</t>
941<t>
942   The If-Unmodified-Since header field can be ignored by caches and
943   intermediaries because it is not applicable to a stored response.
944</t>
945</section>
946
947<section title="If-Range" anchor="header.if-range">
948<t>
949   The "If-Range" header field provides a special conditional request
950   mechanism that is similar to the <x:ref>If-Match</x:ref> and
951   <x:ref>If-Unmodified-Since</x:ref> header fields but instructs the
952   recipient to ignore the <x:ref>Range</x:ref> header field if the validator
953   doesn't match, resulting in transfer of the new selected representation
954   instead of a 412 response. If-Range is defined in &header-if-range;.
955</t>
956</section>
957</section>
958
959<section title="Status Code Definitions" anchor="status.code.definitions">
960<section title="304 Not Modified" anchor="status.304">
961  <iref primary="true" item="304 Not Modified (status code)" x:for-anchor=""/>
962  <x:anchor-alias value="304"/>
963  <x:anchor-alias value="304 (Not Modified)"/>
964<t>
965   The <x:dfn>304 (Not Modified)</x:dfn> status code indicates that a
966   conditional GET or HEAD request has been
967   received and would have resulted in a <x:ref>200 (OK)</x:ref> response
968   if it were not for the fact that the condition has evaluated to false.
969   In other words, there is no need for the server to transfer a
970   representation of the target resource because the request indicates that
971   the client, which made the request conditional, already has a valid
972   representation; the server is therefore redirecting the client to make
973   use of that stored representation as if it were the payload of a
974   <x:ref>200 (OK)</x:ref> response.
975</t>
976<t>
977   The server generating a 304 response &MUST; generate any of the following
978   header fields that would have been sent in a <x:ref>200 (OK)</x:ref>
979   response to the same request:
980   <x:ref>Cache-Control</x:ref>,
981   <x:ref>Content-Location</x:ref>,
982   <x:ref>Date</x:ref>,
983   <x:ref>ETag</x:ref>,
984   <x:ref>Expires</x:ref>, and
985   <x:ref>Vary</x:ref>.
986</t>
987<t>
988   Since the goal of a 304 response is to minimize information transfer
989   when the recipient already has one or more cached representations,
990   a sender &SHOULD-NOT; generate representation metadata other
991   than the above listed fields unless said metadata exists for the
992   purpose of guiding cache updates (e.g., <x:ref>Last-Modified</x:ref> might
993   be useful if the response does not have an <x:ref>ETag</x:ref> field).
994</t>
995<t>
996   Requirements on a cache that receives a 304 response are defined in
997   &freshening-responses;. If the conditional request originated with an
998   outbound client, such as a user agent with its own cache sending a
999   conditional GET to a shared proxy, then the proxy &SHOULD; forward the
1000   304 response to that client.
1001</t>
1002<t>
1003   A 304 response cannot contain a message-body; it is always
1004   terminated by the first empty line after the header fields.
1005</t>
1006</section>
1007
1008<section title="412 Precondition Failed" anchor="status.412">
1009  <iref primary="true" item="412 Precondition Failed (status code)" x:for-anchor=""/>
1010  <x:anchor-alias value="412 (Precondition Failed)"/>
1011<t>
1012   The <x:dfn>412 (Precondition Failed)</x:dfn> status code indicates that one
1013   or more conditions given in the request header fields evaluated to false
1014   when tested on the server. This response code allows the client to place
1015   preconditions on the current resource state (its current representations
1016   and metadata) and thus prevent the request method from being applied if the
1017   target resource is in an unexpected state.
1018</t>
1019</section>
1020</section>
1021
1022<section title="Evaluation" anchor="evaluation">
1023<t>
1024   Except when excluded below, a recipient cache or origin server &MUST;
1025   evaluate received request preconditions after it has successfully performed
1026   its normal request checks and just before it would perform the action
1027   associated with the request method.
1028   A server &MUST; ignore all received preconditions if its response to the
1029   same request without those conditions would have been a status code other
1030   than a <x:ref>2xx</x:ref> or <x:ref>412 (Precondition Failed)</x:ref>.
1031   In other words, redirects and failures take precedence over the evaluation
1032   of preconditions in conditional requests.
1033</t>
1034<t>
1035   A server that is not the origin server for the target resource and cannot
1036   act as a cache for requests on the target resource &MUST-NOT; evaluate the
1037   conditional request header fields defined by this specification, and
1038   &MUST; forward them if the request is forwarded, since the generating
1039   client intends that they be evaluated by a server that can provide a
1040   current representation.
1041   Likewise, a server &MUST; ignore the conditional request header fields
1042   defined by this specification when received with a request method that does
1043   not involve the selection or modification of a
1044   <x:ref>selected representation</x:ref>, such as CONNECT, OPTIONS, or TRACE.
1045</t>
1046<t>
1047   Conditional request header fields that are defined by extensions to HTTP
1048   might place conditions on all recipients, on the state of the target
1049   resource in general, or on a group of resources. For instance, the "If"
1050   header field in WebDAV can make a request conditional on various aspects
1051   of multiple resources, such as locks, if the recipient understands and
1052   implements that field (<xref target="RFC4918" x:fmt="," x:sec="10.4"/>).
1053</t>
1054<t>
1055   Although conditional request header fields are defined as being usable with
1056   the HEAD method (to keep HEAD's semantics consistent with those of GET),
1057   there is no point in sending a conditional HEAD because a successful
1058   response is around the same size as a <x:ref>304 (Not Modified)</x:ref>
1059   response and more useful than a <x:ref>412 (Precondition Failed)</x:ref>
1060   response.
1061</t>
1062</section>
1063
1064<section title="Precedence" anchor="precedence">
1065<t>
1066   When more than one conditional request header field is present in a request,
1067   the order in which the fields are evaluated becomes important. In practice,
1068   the fields defined in this document are consistently implemented in a
1069   single, logical order, since "lost update" preconditions have more strict
1070   requirements than cache validation, a validated cache is more efficient
1071   than a partial response, and entity tags are presumed to be more accurate
1072   than date validators.
1073</t>
1074<t>
1075   A recipient cache or origin server &MUST; evaluate the request
1076   preconditions defined by this specification in the following order:
1077   <list style="numbers">
1078     <t anchor="precedence1">When recipient is the origin server and
1079       <x:ref>If-Match</x:ref> is present,
1080       evaluate the <x:ref>If-Match</x:ref> precondition:
1081       <list style="symbols">
1082         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
1083         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref> unless
1084            it can be determined that the state-changing request has already
1085            succeeded (see <xref target="header.if-match"/>)</t>
1086       </list>
1087     </t>
1088     <t anchor="precedence2">When recipient is the origin server,
1089       <x:ref>If-Match</x:ref> is not present, and
1090       <x:ref>If-Unmodified-Since</x:ref> is present,
1091       evaluate the <x:ref>If-Unmodified-Since</x:ref> precondition:
1092       <list style="symbols">
1093         <t>if true, continue to step <xref target="precedence3" format="counter"/></t>
1094         <t>if false, respond <x:ref>412 (Precondition Failed)</x:ref> unless
1095            it can be determined that the state-changing request has already
1096            succeeded (see <xref target="header.if-unmodified-since"/>)</t>
1097       </list>
1098     </t>
1099     <t anchor="precedence3">When <x:ref>If-None-Match</x:ref> is present,
1100       evaluate the <x:ref>If-None-Match</x:ref> precondition:
1101       <list style="symbols">
1102         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
1103         <t>if false for GET/HEAD, respond <x:ref>304 (Not Modified)</x:ref></t>
1104         <t>if false for other methods, respond <x:ref>412 (Precondition Failed)</x:ref></t>
1105       </list>
1106     </t>
1107     <t anchor="precedence4">When the method is GET or HEAD,
1108       <x:ref>If-None-Match</x:ref> is not present, and
1109       <x:ref>If-Modified-Since</x:ref> is present,
1110       evaluate the <x:ref>If-Modified-Since</x:ref> precondition:
1111       <list style="symbols">
1112         <t>if true, continue to step <xref target="precedence5" format="counter"/></t>
1113         <t>if false, respond <x:ref>304 (Not Modified)</x:ref></t>
1114       </list>
1115     </t>
1116     <t anchor="precedence5">When the method is GET and both
1117       <x:ref>Range</x:ref> and <x:ref>If-Range</x:ref> are present,
1118       evaluate the <x:ref>If-Range</x:ref> precondition:
1119       <list style="symbols">
1120         <t>if the validator matches and the Range specification is
1121            applicable to the selected representation, respond
1122            <x:ref>206 (Partial Content)</x:ref> <xref target="Part5"/></t>
1123       </list>
1124     </t>
1125     <t anchor="precedencelast">Otherwise,
1126       <list style="symbols">
1127         <t>all conditions are met, so perform the requested action and
1128            respond according to its success or failure.</t>
1129       </list>
1130     </t>
1131   </list>
1132</t>
1133<t>
1134   Any extension to HTTP/1.1 that defines additional conditional request
1135   header fields ought to define its own expectations regarding the order
1136   for evaluating such fields in relation to those defined in this document
1137   and other conditionals that might be found in practice.
1138</t>
1139</section>
1140
1141<section title="IANA Considerations" anchor="IANA.considerations">
1142
1143<section title="Status Code Registration" anchor="status.code.registration">
1144<t>
1145   The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
1146   shall be updated with the registrations below:
1147</t>
1148<?BEGININC p4-conditional.iana-status-codes ?>
1149<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
1150<texttable align="left" suppress-title="true" anchor="iana.status.code.registration.table">
1151   <ttcol>Value</ttcol>
1152   <ttcol>Description</ttcol>
1153   <ttcol>Reference</ttcol>
1154   <c>304</c>
1155   <c>Not Modified</c>
1156   <c>
1157      <xref target="status.304"/>
1158   </c>
1159   <c>412</c>
1160   <c>Precondition Failed</c>
1161   <c>
1162      <xref target="status.412"/>
1163   </c>
1164</texttable>
1165<!--(END)-->
1166<?ENDINC p4-conditional.iana-status-codes ?>
1167</section>
1168
1169<section title="Header Field Registration" anchor="header.field.registration">
1170<t>
1171   HTTP header fields are registered within the Message Header Field Registry
1172   maintained at
1173   <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/>.
1174</t>
1175<t>
1176   This document defines the following HTTP header fields, so their
1177   associated registry entries shall be updated according to the permanent
1178   registrations below (see <xref target="BCP90"/>):
1179</t>
1180<?BEGININC p4-conditional.iana-headers ?>
1181<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
1182<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
1183   <ttcol>Header Field Name</ttcol>
1184   <ttcol>Protocol</ttcol>
1185   <ttcol>Status</ttcol>
1186   <ttcol>Reference</ttcol>
1187
1188   <c>ETag</c>
1189   <c>http</c>
1190   <c>standard</c>
1191   <c>
1192      <xref target="header.etag"/>
1193   </c>
1194   <c>If-Match</c>
1195   <c>http</c>
1196   <c>standard</c>
1197   <c>
1198      <xref target="header.if-match"/>
1199   </c>
1200   <c>If-Modified-Since</c>
1201   <c>http</c>
1202   <c>standard</c>
1203   <c>
1204      <xref target="header.if-modified-since"/>
1205   </c>
1206   <c>If-None-Match</c>
1207   <c>http</c>
1208   <c>standard</c>
1209   <c>
1210      <xref target="header.if-none-match"/>
1211   </c>
1212   <c>If-Unmodified-Since</c>
1213   <c>http</c>
1214   <c>standard</c>
1215   <c>
1216      <xref target="header.if-unmodified-since"/>
1217   </c>
1218   <c>Last-Modified</c>
1219   <c>http</c>
1220   <c>standard</c>
1221   <c>
1222      <xref target="header.last-modified"/>
1223   </c>
1224</texttable>
1225<!--(END)-->
1226<?ENDINC p4-conditional.iana-headers ?>
1227<t>
1228   The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
1229</t>
1230</section>
1231</section>
1232
1233<section title="Security Considerations" anchor="security.considerations">
1234<t>
1235   This section is meant to inform developers, information providers, and
1236   users of known security concerns specific to the HTTP/1.1 conditional
1237   request mechanisms. More general security considerations are addressed
1238   in HTTP messaging &messaging; and semantics &semantics;.
1239</t>
1240<t>
1241   The validators defined by this specification are not intended to ensure
1242   the validity of a representation, guard against malicious changes, or
1243   detect man-in-the-middle attacks. At best, they enable more efficient cache
1244   updates and optimistic concurrent writes when all participants are behaving
1245   nicely. At worst, the conditions will fail and the client will receive a
1246   response that is no more harmful than an HTTP exchange without conditional
1247   requests.
1248</t>
1249<t>
1250   An entity-tag can be abused in ways that create privacy risks. For example,
1251   a site might deliberately construct a semantically invalid entity-tag that
1252   is unique to the user or user agent, send it in a cacheable response with a
1253   long freshness time, and then read that entity-tag in later conditional
1254   requests as a means of re-identifying that user or user agent. Such an
1255   identifying tag would become a persistent identifier for as long as the
1256   user agent retained the original cache entry. User agents that cache
1257   representations ought to ensure that the cache is cleared or replaced
1258   whenever the user performs privacy-maintaining actions, such as clearing
1259   stored cookies or changing to a private browsing mode.
1260</t>
1261</section>
1262
1263<section title="Acknowledgments" anchor="acks">
1264<t>
1265  See &acks;.
1266</t>
1267</section>
1268</middle>
1269<back>
1270
1271<references title="Normative References">
1272
1273<reference anchor="Part1">
1274  <front>
1275    <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
1276    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1277      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1278      <address><email>fielding@gbiv.com</email></address>
1279    </author>
1280    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1281      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1282      <address><email>julian.reschke@greenbytes.de</email></address>
1283    </author>
1284    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1285  </front>
1286  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
1287  <x:source href="p1-messaging.xml" basename="p1-messaging"/>
1288</reference>
1289
1290<reference anchor="Part2">
1291  <front>
1292    <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
1293    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1294      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1295      <address><email>fielding@gbiv.com</email></address>
1296    </author>
1297    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1298      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1299      <address><email>julian.reschke@greenbytes.de</email></address>
1300    </author>
1301    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1302  </front>
1303  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;"/>
1304  <x:source href="p2-semantics.xml" basename="p2-semantics">
1305    <x:defines>2xx</x:defines>
1306    <x:defines>2xx (Successful)</x:defines>
1307    <x:defines>200 (OK)</x:defines>
1308    <x:defines>204 (No Content)</x:defines>
1309    <x:defines>Accept-Encoding</x:defines>
1310    <x:defines>Content-Location</x:defines>
1311    <x:defines>Content-Type</x:defines>
1312    <x:defines>Date</x:defines>
1313    <x:defines>Location</x:defines>
1314    <x:defines>Vary</x:defines>
1315    <x:defines>selected representation</x:defines>
1316  </x:source>
1317</reference>
1318
1319<reference anchor="Part5">
1320  <front>
1321    <title>Hypertext Transfer Protocol (HTTP/1.1): Range Requests</title>
1322    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1323      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1324      <address><email>fielding@gbiv.com</email></address>
1325    </author>
1326    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
1327      <organization abbrev="W3C">World Wide Web Consortium</organization>
1328      <address><email>ylafon@w3.org</email></address>
1329    </author>
1330    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1331      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1332      <address><email>julian.reschke@greenbytes.de</email></address>
1333    </author>
1334    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1335  </front>
1336  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;"/>
1337  <x:source href="p5-range.xml" basename="p5-range">
1338    <x:defines>If-Range</x:defines>
1339    <x:defines>Range</x:defines>
1340    <x:defines>206 (Partial Content)</x:defines>
1341  </x:source>
1342</reference>
1343
1344<reference anchor="Part6">
1345  <front>
1346    <title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
1347    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
1348      <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
1349      <address><email>fielding@gbiv.com</email></address>
1350    </author>
1351    <author initials="M." surname="Nottingham" fullname="Mark Nottingham" role="editor">
1352      <organization>Akamai</organization>
1353      <address><email>mnot@mnot.net</email></address>
1354    </author>
1355    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
1356      <organization abbrev="greenbytes">greenbytes GmbH</organization>
1357      <address><email>julian.reschke@greenbytes.de</email></address>
1358    </author>
1359    <date month="&ID-MONTH;" year="&ID-YEAR;"/>
1360  </front>
1361  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-&ID-VERSION;"/>
1362  <x:source href="p6-cache.xml" basename="p6-cache">
1363    <x:defines>Cache-Control</x:defines>
1364    <x:defines>Expires</x:defines>
1365  </x:source>
1366</reference>
1367
1368<reference anchor="RFC2119">
1369  <front>
1370    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
1371    <author initials="S." surname="Bradner" fullname="Scott Bradner">
1372      <organization>Harvard University</organization>
1373      <address><email>sob@harvard.edu</email></address>
1374    </author>
1375    <date month="March" year="1997"/>
1376  </front>
1377  <seriesInfo name="BCP" value="14"/>
1378  <seriesInfo name="RFC" value="2119"/>
1379</reference>
1380
1381<reference anchor="RFC5234">
1382  <front>
1383    <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
1384    <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor">
1385      <organization>Brandenburg InternetWorking</organization>
1386      <address>
1387        <email>dcrocker@bbiw.net</email>
1388      </address> 
1389    </author>
1390    <author initials="P." surname="Overell" fullname="Paul Overell">
1391      <organization>THUS plc.</organization>
1392      <address>
1393        <email>paul.overell@thus.net</email>
1394      </address>
1395    </author>
1396    <date month="January" year="2008"/>
1397  </front>
1398  <seriesInfo name="STD" value="68"/>
1399  <seriesInfo name="RFC" value="5234"/>
1400</reference>
1401
1402</references>
1403
1404<references title="Informative References">
1405
1406<reference anchor="RFC2616">
1407  <front>
1408    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
1409    <author initials="R." surname="Fielding" fullname="R. Fielding">
1410      <organization>University of California, Irvine</organization>
1411      <address><email>fielding@ics.uci.edu</email></address>
1412    </author>
1413    <author initials="J." surname="Gettys" fullname="J. Gettys">
1414      <organization>W3C</organization>
1415      <address><email>jg@w3.org</email></address>
1416    </author>
1417    <author initials="J." surname="Mogul" fullname="J. Mogul">
1418      <organization>Compaq Computer Corporation</organization>
1419      <address><email>mogul@wrl.dec.com</email></address>
1420    </author>
1421    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
1422      <organization>MIT Laboratory for Computer Science</organization>
1423      <address><email>frystyk@w3.org</email></address>
1424    </author>
1425    <author initials="L." surname="Masinter" fullname="L. Masinter">
1426      <organization>Xerox Corporation</organization>
1427      <address><email>masinter@parc.xerox.com</email></address>
1428    </author>
1429    <author initials="P." surname="Leach" fullname="P. Leach">
1430      <organization>Microsoft Corporation</organization>
1431      <address><email>paulle@microsoft.com</email></address>
1432    </author>
1433    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
1434      <organization>W3C</organization>
1435      <address><email>timbl@w3.org</email></address>
1436    </author>
1437    <date month="June" year="1999"/>
1438  </front>
1439  <seriesInfo name="RFC" value="2616"/>
1440</reference>
1441
1442<reference anchor='BCP90'>
1443  <front>
1444    <title>Registration Procedures for Message Header Fields</title>
1445    <author initials='G.' surname='Klyne' fullname='G. Klyne'>
1446      <organization>Nine by Nine</organization>
1447      <address><email>GK-IETF@ninebynine.org</email></address>
1448    </author>
1449    <author initials='M.' surname='Nottingham' fullname='M. Nottingham'>
1450      <organization>BEA Systems</organization>
1451      <address><email>mnot@pobox.com</email></address>
1452    </author>
1453    <author initials='J.' surname='Mogul' fullname='J. Mogul'>
1454      <organization>HP Labs</organization>
1455      <address><email>JeffMogul@acm.org</email></address>
1456    </author>
1457    <date year='2004' month='September' />
1458  </front>
1459  <seriesInfo name='BCP' value='90' />
1460  <seriesInfo name='RFC' value='3864' />
1461</reference>
1462
1463<reference anchor='RFC4918'>
1464  <front>
1465    <title>HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</title>
1466    <author initials="L.M." surname="Dusseault" fullname="Lisa Dusseault" role="editor" >
1467      <organization abbrev="CommerceNet">CommerceNet</organization>
1468      <address><email>ldusseault@commerce.net</email></address>
1469    </author>
1470    <date month="June" year="2007" />
1471  </front>
1472  <seriesInfo name='RFC' value='4918' />
1473</reference>
1474</references>
1475
1476<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
1477<t>
1478  The definition of validator weakness has been expanded and clarified.
1479  (<xref target="weak.and.strong.validators" />)
1480</t>
1481<t>
1482  Weak entity-tags are now allowed in all requests except range requests.
1483  (Sections <xref target="weak.and.strong.validators" format="counter"/> and
1484  <xref target="header.if-none-match" format="counter"/>)
1485</t>
1486<t>
1487  The <x:ref>ETag</x:ref> header field ABNF has been changed to not use
1488  quoted-string, thus avoiding escaping issues.
1489  (<xref target="header.etag" />)
1490</t>
1491<t>
1492  ETag is defined to provide an entity tag for the selected representation,
1493  thereby clarifying what it applies to in various situations (such as a
1494  PUT response).
1495  (<xref target="header.etag" />)
1496</t>
1497<t>
1498  The precedence for evaluation of conditional requests has been defined.
1499  (<xref target="precedence" />)
1500</t>
1501</section>
1502
1503<section title="Imported ABNF" anchor="imported.abnf">
1504  <x:anchor-alias value="ALPHA"/>
1505  <x:anchor-alias value="CR"/>
1506  <x:anchor-alias value="DIGIT"/>
1507  <x:anchor-alias value="DQUOTE"/>
1508  <x:anchor-alias value="LF"/>
1509  <x:anchor-alias value="OCTET"/>
1510  <x:anchor-alias value="VCHAR"/>
1511  <x:anchor-alias value="core.rules"/>
1512  <x:anchor-alias value="obs-text"/>
1513  <x:anchor-alias value="OWS"/>
1514  <x:anchor-alias value="HTTP-date"/>
1515<t>
1516  The following core rules are included by
1517  reference, as defined in <xref target="RFC5234" x:fmt="of" x:sec="B.1"/>:
1518  ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
1519  DIGIT (decimal 0-9), DQUOTE (double quote),
1520  HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
1521  OCTET (any 8-bit sequence of data), SP (space), and
1522  VCHAR (any visible US-ASCII character).
1523</t>
1524<t>
1525  The rules below are defined in <xref target="Part1"/>:
1526</t>
1527<figure><artwork type="abnf2616">
1528  <x:ref>OWS</x:ref>           = &lt;OWS, defined in &whitespace;&gt;
1529  <x:ref>obs-text</x:ref>      = &lt;obs-text, defined in &field-components;&gt;
1530</artwork></figure>
1531<t>
1532  The rules below are defined in other parts:
1533</t>
1534<figure><artwork type="abnf2616">
1535  <x:ref>HTTP-date</x:ref>     = &lt;HTTP-date, defined in &http-date;&gt;
1536</artwork></figure>
1537</section>
1538
1539<?BEGININC p4-conditional.abnf-appendix ?>
1540<section xmlns:x="http://purl.org/net/xml2rfc/ext" title="Collected ABNF" anchor="collected.abnf">
1541<t>
1542  In the collected ABNF below, list rules are expanded as per <xref target="Part1" x:rel="#notation"/>.
1543</t><figure>
1544<artwork type="abnf" name="p4-conditional.parsed-abnf">
1545<x:ref>ETag</x:ref> = entity-tag
1546
1547<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part2], Section 7.1.1.1&gt;
1548
1549<x:ref>If-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1550 entity-tag ] ) )
1551<x:ref>If-Modified-Since</x:ref> = HTTP-date
1552<x:ref>If-None-Match</x:ref> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1553 entity-tag ] ) )
1554<x:ref>If-Unmodified-Since</x:ref> = HTTP-date
1555
1556<x:ref>Last-Modified</x:ref> = HTTP-date
1557
1558<x:ref>OWS</x:ref> = &lt;OWS, defined in [Part1], Section 3.2.3&gt;
1559
1560<x:ref>entity-tag</x:ref> = [ weak ] opaque-tag
1561<x:ref>etagc</x:ref> = "!" / %x23-7E ; '#'-'~'
1562 / obs-text
1563
1564<x:ref>obs-text</x:ref> = &lt;obs-text, defined in [Part1], Section 3.2.6&gt;
1565<x:ref>opaque-tag</x:ref> = DQUOTE *etagc DQUOTE
1566
1567<x:ref>weak</x:ref> = %x57.2F ; W/
1568</artwork>
1569</figure>
1570</section>
1571<?ENDINC p4-conditional.abnf-appendix ?>
1572
1573<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
1574<t>
1575  Changes up to the IETF Last Call draft are summarized
1576  in <eref target="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-24#appendix-D"/>.
1577</t>
1578
1579<section title="Since draft-ietf-httpbis-p4-conditional-24" anchor="changes.since.24">
1580<t>
1581  Closed issues:
1582  <list style="symbols">
1583    <t>
1584      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/518"/>:
1585      "APPSDIR review of draft-ietf-httpbis-p4-conditional-24"
1586    </t>
1587  </list>
1588</t>
1589</section>
1590
1591<section title="Since draft-ietf-httpbis-p4-conditional-25" anchor="changes.since.25">
1592<t>
1593  Closed issues:
1594  <list style="symbols">
1595    <t>
1596      <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/542"/>:
1597      "improve introduction of list rule"
1598    </t>
1599  </list>
1600</t>
1601</section>
1602</section>
1603
1604</back>
1605</rfc>
Note: See TracBrowser for help on using the repository browser.