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2 HTTPbis Working Group R. Fielding, Ed.
3 Internet-Draft Adobe
4 Obsoletes: 2616 (if approved) J. Reschke, Ed.
5 Intended status: Standards Track greenbytes
6 Expires: August 10, 2014 February 6, 2014
8 Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
9 draft-ietf-httpbis-p4-conditional-26
11 Abstract
13 The Hypertext Transfer Protocol (HTTP) is a stateless application-
14 level protocol for distributed, collaborative, hypertext information
15 systems. This document defines HTTP/1.1 conditional requests,
16 including metadata header fields for indicating state changes,
17 request header fields for making preconditions on such state, and
18 rules for constructing the responses to a conditional request when
19 one or more preconditions evaluate to false.
21 Editorial Note (To be removed by RFC Editor)
23 Discussion of this draft takes place on the HTTPBIS working group
24 mailing list (ietf-http-wg@w3.org), which is archived at
25 .
27 The current issues list is at
28 and related
29 documents (including fancy diffs) can be found at
30 .
32 The changes in this draft are summarized in Appendix D.2.
34 Status of This Memo
36 This Internet-Draft is submitted in full conformance with the
37 provisions of BCP 78 and BCP 79.
39 Internet-Drafts are working documents of the Internet Engineering
40 Task Force (IETF). Note that other groups may also distribute
41 working documents as Internet-Drafts. The list of current Internet-
42 Drafts is at http://datatracker.ietf.org/drafts/current/.
44 Internet-Drafts are draft documents valid for a maximum of six months
45 and may be updated, replaced, or obsoleted by other documents at any
46 time. It is inappropriate to use Internet-Drafts as reference
47 material or to cite them other than as "work in progress."
48 This Internet-Draft will expire on August 10, 2014.
50 Copyright Notice
52 Copyright (c) 2014 IETF Trust and the persons identified as the
53 document authors. All rights reserved.
55 This document is subject to BCP 78 and the IETF Trust's Legal
56 Provisions Relating to IETF Documents
57 (http://trustee.ietf.org/license-info) in effect on the date of
58 publication of this document. Please review these documents
59 carefully, as they describe your rights and restrictions with respect
60 to this document. Code Components extracted from this document must
61 include Simplified BSD License text as described in Section 4.e of
62 the Trust Legal Provisions and are provided without warranty as
63 described in the Simplified BSD License.
65 This document may contain material from IETF Documents or IETF
66 Contributions published or made publicly available before November
67 10, 2008. The person(s) controlling the copyright in some of this
68 material may not have granted the IETF Trust the right to allow
69 modifications of such material outside the IETF Standards Process.
70 Without obtaining an adequate license from the person(s) controlling
71 the copyright in such materials, this document may not be modified
72 outside the IETF Standards Process, and derivative works of it may
73 not be created outside the IETF Standards Process, except to format
74 it for publication as an RFC or to translate it into languages other
75 than English.
77 Table of Contents
79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
80 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4
81 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
82 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5
83 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5
84 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7
85 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7
86 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8
87 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
88 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 10
89 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 10
90 2.3.3. Example: Entity-tags Varying on Content-Negotiated
91 Resources . . . . . . . . . . . . . . . . . . . . . . 11
92 2.4. When to Use Entity-tags and Last-Modified Dates . . . . . 12
93 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 13
94 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 13
95 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 14
96 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 15
97 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 16
98 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18
99 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18
100 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18
101 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 18
102 5. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 19
103 6. Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . 19
104 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
105 7.1. Status Code Registration . . . . . . . . . . . . . . . . . 21
106 7.2. Header Field Registration . . . . . . . . . . . . . . . . 21
107 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
108 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
109 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
110 10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
111 10.2. Informative References . . . . . . . . . . . . . . . . . . 23
112 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 23
113 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 24
114 Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 24
115 Appendix D. Change Log (to be removed by RFC Editor before
116 publication) . . . . . . . . . . . . . . . . . . . . 25
117 D.1. Since draft-ietf-httpbis-p4-conditional-24 . . . . . . . . 25
118 D.2. Since draft-ietf-httpbis-p4-conditional-25 . . . . . . . . 25
119 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
121 1. Introduction
123 Conditional requests are HTTP requests [Part2] that include one or
124 more header fields indicating a precondition to be tested before
125 applying the method semantics to the target resource. This document
126 defines the HTTP/1.1 conditional request mechanisms in terms of the
127 architecture, syntax notation, and conformance criteria defined in
128 [Part1].
130 Conditional GET requests are the most efficient mechanism for HTTP
131 cache updates [Part6]. Conditionals can also be applied to state-
132 changing methods, such as PUT and DELETE, to prevent the "lost
133 update" problem: one client accidentally overwriting the work of
134 another client that has been acting in parallel.
136 Conditional request preconditions are based on the state of the
137 target resource as a whole (its current value set) or the state as
138 observed in a previously obtained representation (one value in that
139 set). A resource might have multiple current representations, each
140 with its own observable state. The conditional request mechanisms
141 assume that the mapping of requests to a "selected representation"
142 (Section 3 of [Part2]) will be consistent over time if the server
143 intends to take advantage of conditionals. Regardless, if the
144 mapping is inconsistent and the server is unable to select the
145 appropriate representation, then no harm will result when the
146 precondition evaluates to false.
148 The conditional request preconditions defined by this specification
149 (Section 3) are evaluated when applicable to the recipient
150 (Section 5) according to their order of precedence (Section 6).
152 1.1. Conformance and Error Handling
154 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
155 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
156 document are to be interpreted as described in [RFC2119].
158 Conformance criteria and considerations regarding error handling are
159 defined in Section 2.5 of [Part1].
161 1.2. Syntax Notation
163 This specification uses the Augmented Backus-Naur Form (ABNF)
164 notation of [RFC5234] with a list extension, defined in Section 7 of
165 [Part1], that allows for compact definition of comma-separated lists
166 using a '#' operator (similar to how the '*' operator indicates
167 repetition). Appendix B describes rules imported from other
168 documents. Appendix C shows the collected grammar with all list
169 operators expanded to standard ABNF notation.
171 2. Validators
173 This specification defines two forms of metadata that are commonly
174 used to observe resource state and test for preconditions:
175 modification dates (Section 2.2) and opaque entity tags
176 (Section 2.3). Additional metadata that reflects resource state has
177 been defined by various extensions of HTTP, such as WebDAV [RFC4918],
178 that are beyond the scope of this specification. A resource metadata
179 value is referred to as a "validator" when it is used within a
180 precondition.
182 2.1. Weak versus Strong
184 Validators come in two flavors: strong or weak. Weak validators are
185 easy to generate but are far less useful for comparisons. Strong
186 validators are ideal for comparisons but can be very difficult (and
187 occasionally impossible) to generate efficiently. Rather than impose
188 that all forms of resource adhere to the same strength of validator,
189 HTTP exposes the type of validator in use and imposes restrictions on
190 when weak validators can be used as preconditions.
192 A "strong validator" is representation metadata that changes value
193 whenever a change occurs to the representation data that would be
194 observable in the payload body of a 200 (OK) response to GET.
196 A strong validator might change for reasons other than a change to
197 the representation data, such as when a semantically significant part
198 of the representation metadata is changed (e.g., Content-Type), but
199 it is in the best interests of the origin server to only change the
200 value when it is necessary to invalidate the stored responses held by
201 remote caches and authoring tools.
203 Cache entries might persist for arbitrarily long periods, regardless
204 of expiration times. Thus, a cache might attempt to validate an
205 entry using a validator that it obtained in the distant past. A
206 strong validator is unique across all versions of all representations
207 associated with a particular resource over time. However, there is
208 no implication of uniqueness across representations of different
209 resources (i.e., the same strong validator might be in use for
210 representations of multiple resources at the same time and does not
211 imply that those representations are equivalent).
213 There are a variety of strong validators used in practice. The best
214 are based on strict revision control, wherein each change to a
215 representation always results in a unique node name and revision
216 identifier being assigned before the representation is made
217 accessible to GET. A collision-resistant hash function applied to
218 the representation data is also sufficient if the data is available
219 prior to the response header fields being sent and the digest does
220 not need to be recalculated every time a validation request is
221 received. However, if a resource has distinct representations that
222 differ only in their metadata, such as might occur with content
223 negotiation over media types that happen to share the same data
224 format, then the origin server needs to incorporate additional
225 information in the validator to distinguish those representations.
227 In contrast, a "weak validator" is representation metadata that might
228 not change for every change to the representation data. This
229 weakness might be due to limitations in how the value is calculated,
230 such as clock resolution or an inability to ensure uniqueness for all
231 possible representations of the resource, or due to a desire by the
232 resource owner to group representations by some self-determined set
233 of equivalency rather than unique sequences of data. An origin
234 server SHOULD change a weak entity-tag whenever it considers prior
235 representations to be unacceptable as a substitute for the current
236 representation. In other words, a weak entity-tag ought to change
237 whenever the origin server wants caches to invalidate old responses.
239 For example, the representation of a weather report that changes in
240 content every second, based on dynamic measurements, might be grouped
241 into sets of equivalent representations (from the origin server's
242 perspective) with the same weak validator in order to allow cached
243 representations to be valid for a reasonable period of time (perhaps
244 adjusted dynamically based on server load or weather quality).
245 Likewise, a representation's modification time, if defined with only
246 one-second resolution, might be a weak validator if it is possible
247 for the representation to be modified twice during a single second
248 and retrieved between those modifications.
250 Likewise, a validator is weak if it is shared by two or more
251 representations of a given resource at the same time, unless those
252 representations have identical representation data. For example, if
253 the origin server sends the same validator for a representation with
254 a gzip content coding applied as it does for a representation with no
255 content coding, then that validator is weak. However, two
256 simultaneous representations might share the same strong validator if
257 they differ only in the representation metadata, such as when two
258 different media types are available for the same representation data.
260 Strong validators are usable for all conditional requests, including
261 cache validation, partial content ranges, and "lost update"
262 avoidance. Weak validators are only usable when the client does not
263 require exact equality with previously obtained representation data,
264 such as when validating a cache entry or limiting a web traversal to
265 recent changes.
267 2.2. Last-Modified
269 The "Last-Modified" header field in a response provides a timestamp
270 indicating the date and time at which the origin server believes the
271 selected representation was last modified, as determined at the
272 conclusion of handling the request.
274 Last-Modified = HTTP-date
276 An example of its use is
278 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
280 2.2.1. Generation
282 An origin server SHOULD send Last-Modified for any selected
283 representation for which a last modification date can be reasonably
284 and consistently determined, since its use in conditional requests
285 and evaluating cache freshness ([Part6]) results in a substantial
286 reduction of HTTP traffic on the Internet and can be a significant
287 factor in improving service scalability and reliability.
289 A representation is typically the sum of many parts behind the
290 resource interface. The last-modified time would usually be the most
291 recent time that any of those parts were changed. How that value is
292 determined for any given resource is an implementation detail beyond
293 the scope of this specification. What matters to HTTP is how
294 recipients of the Last-Modified header field can use its value to
295 make conditional requests and test the validity of locally cached
296 responses.
298 An origin server SHOULD obtain the Last-Modified value of the
299 representation as close as possible to the time that it generates the
300 Date field value for its response. This allows a recipient to make
301 an accurate assessment of the representation's modification time,
302 especially if the representation changes near the time that the
303 response is generated.
305 An origin server with a clock MUST NOT send a Last-Modified date that
306 is later than the server's time of message origination (Date). If
307 the last modification time is derived from implementation-specific
308 metadata that evaluates to some time in the future, according to the
309 origin server's clock, then the origin server MUST replace that value
310 with the message origination date. This prevents a future
311 modification date from having an adverse impact on cache validation.
313 An origin server without a clock MUST NOT assign Last-Modified values
314 to a response unless these values were associated with the resource
315 by some other system or user with a reliable clock.
317 2.2.2. Comparison
319 A Last-Modified time, when used as a validator in a request, is
320 implicitly weak unless it is possible to deduce that it is strong,
321 using the following rules:
323 o The validator is being compared by an origin server to the actual
324 current validator for the representation and,
326 o That origin server reliably knows that the associated
327 representation did not change twice during the second covered by
328 the presented validator.
330 or
332 o The validator is about to be used by a client in an If-Modified-
333 Since, If-Unmodified-Since header field, because the client has a
334 cache entry, or If-Range for the associated representation, and
336 o That cache entry includes a Date value, which gives the time when
337 the origin server sent the original response, and
339 o The presented Last-Modified time is at least 60 seconds before the
340 Date value.
342 or
344 o The validator is being compared by an intermediate cache to the
345 validator stored in its cache entry for the representation, and
347 o That cache entry includes a Date value, which gives the time when
348 the origin server sent the original response, and
350 o The presented Last-Modified time is at least 60 seconds before the
351 Date value.
353 This method relies on the fact that if two different responses were
354 sent by the origin server during the same second, but both had the
355 same Last-Modified time, then at least one of those responses would
356 have a Date value equal to its Last-Modified time. The arbitrary 60-
357 second limit guards against the possibility that the Date and Last-
358 Modified values are generated from different clocks, or at somewhat
359 different times during the preparation of the response. An
360 implementation MAY use a value larger than 60 seconds, if it is
361 believed that 60 seconds is too short.
363 2.3. ETag
365 The "ETag" header field in a response provides the current entity-tag
366 for the selected representation, as determined at the conclusion of
367 handling the request. An entity-tag is an opaque validator for
368 differentiating between multiple representations of the same
369 resource, regardless of whether those multiple representations are
370 due to resource state changes over time, content negotiation
371 resulting in multiple representations being valid at the same time,
372 or both. An entity-tag consists of an opaque quoted string, possibly
373 prefixed by a weakness indicator.
375 ETag = entity-tag
377 entity-tag = [ weak ] opaque-tag
378 weak = %x57.2F ; "W/", case-sensitive
379 opaque-tag = DQUOTE *etagc DQUOTE
380 etagc = %x21 / %x23-7E / obs-text
381 ; VCHAR except double quotes, plus obs-text
383 Note: Previously, opaque-tag was defined to be a quoted-string
384 ([RFC2616], Section 3.11), thus some recipients might perform
385 backslash unescaping. Servers therefore ought to avoid backslash
386 characters in entity tags.
388 An entity-tag can be more reliable for validation than a modification
389 date in situations where it is inconvenient to store modification
390 dates, where the one-second resolution of HTTP date values is not
391 sufficient, or where modification dates are not consistently
392 maintained.
394 Examples:
396 ETag: "xyzzy"
397 ETag: W/"xyzzy"
398 ETag: ""
400 An entity-tag can be either a weak or strong validator, with strong
401 being the default. If an origin server provides an entity-tag for a
402 representation and the generation of that entity-tag does not satisfy
403 all of the characteristics of a strong validator (Section 2.1), then
404 the origin server MUST mark the entity-tag as weak by prefixing its
405 opaque value with "W/" (case-sensitive).
407 2.3.1. Generation
409 The principle behind entity-tags is that only the service author
410 knows the implementation of a resource well enough to select the most
411 accurate and efficient validation mechanism for that resource, and
412 that any such mechanism can be mapped to a simple sequence of octets
413 for easy comparison. Since the value is opaque, there is no need for
414 the client to be aware of how each entity-tag is constructed.
416 For example, a resource that has implementation-specific versioning
417 applied to all changes might use an internal revision number, perhaps
418 combined with a variance identifier for content negotiation, to
419 accurately differentiate between representations. Other
420 implementations might use a collision-resistant hash of
421 representation content, a combination of various file attributes, or
422 a modification timestamp that has sub-second resolution.
424 An origin server SHOULD send ETag for any selected representation for
425 which detection of changes can be reasonably and consistently
426 determined, since the entity-tag's use in conditional requests and
427 evaluating cache freshness ([Part6]) can result in a substantial
428 reduction of HTTP network traffic and can be a significant factor in
429 improving service scalability and reliability.
431 2.3.2. Comparison
433 There are two entity-tag comparison functions, depending on whether
434 the comparison context allows the use of weak validators or not:
436 o Strong comparison: two entity-tags are equivalent if both are not
437 weak and their opaque-tags match character-by-character.
439 o Weak comparison: two entity-tags are equivalent if their opaque-
440 tags match character-by-character, regardless of either or both
441 being tagged as "weak".
443 The example below shows the results for a set of entity-tag pairs,
444 and both the weak and strong comparison function results:
446 +--------+--------+-------------------+-----------------+
447 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
448 +--------+--------+-------------------+-----------------+
449 | W/"1" | W/"1" | no match | match |
450 | W/"1" | W/"2" | no match | no match |
451 | W/"1" | "1" | no match | match |
452 | "1" | "1" | match | match |
453 +--------+--------+-------------------+-----------------+
455 2.3.3. Example: Entity-tags Varying on Content-Negotiated Resources
457 Consider a resource that is subject to content negotiation (Section
458 3.4 of [Part2]), and where the representations sent in response to a
459 GET request vary based on the Accept-Encoding request header field
460 (Section 5.3.4 of [Part2]):
462 >> Request:
464 GET /index HTTP/1.1
465 Host: www.example.com
466 Accept-Encoding: gzip
468 In this case, the response might or might not use the gzip content
469 coding. If it does not, the response might look like:
471 >> Response:
473 HTTP/1.1 200 OK
474 Date: Fri, 26 Mar 2010 00:05:00 GMT
475 ETag: "123-a"
476 Content-Length: 70
477 Vary: Accept-Encoding
478 Content-Type: text/plain
480 Hello World!
481 Hello World!
482 Hello World!
483 Hello World!
484 Hello World!
486 An alternative representation that does use gzip content coding would
487 be:
489 >> Response:
491 HTTP/1.1 200 OK
492 Date: Fri, 26 Mar 2010 00:05:00 GMT
493 ETag: "123-b"
494 Content-Length: 43
495 Vary: Accept-Encoding
496 Content-Type: text/plain
497 Content-Encoding: gzip
499 ...binary data...
501 Note: Content codings are a property of the representation data,
502 so a strong entity-tag for a content-encoded representation has to
503 be distinct from the entity tag of an unencoded representation to
504 prevent potential conflicts during cache updates and range
505 requests. In contrast, transfer codings (Section 4 of [Part1])
506 apply only during message transfer and do not result in distinct
507 entity-tags.
509 2.4. When to Use Entity-tags and Last-Modified Dates
511 In 200 (OK) responses to GET or HEAD, an origin server:
513 o SHOULD send an entity-tag validator unless it is not feasible to
514 generate one.
516 o MAY send a weak entity-tag instead of a strong entity-tag, if
517 performance considerations support the use of weak entity-tags, or
518 if it is unfeasible to send a strong entity-tag.
520 o SHOULD send a Last-Modified value if it is feasible to send one.
522 In other words, the preferred behavior for an origin server is to
523 send both a strong entity-tag and a Last-Modified value in successful
524 responses to a retrieval request.
526 A client:
528 o MUST send that entity-tag in any cache validation request (using
529 If-Match or If-None-Match) if an entity-tag has been provided by
530 the origin server.
532 o SHOULD send the Last-Modified value in non-subrange cache
533 validation requests (using If-Modified-Since) if only a Last-
534 Modified value has been provided by the origin server.
536 o MAY send the Last-Modified value in subrange cache validation
537 requests (using If-Unmodified-Since) if only a Last-Modified value
538 has been provided by an HTTP/1.0 origin server. The user agent
539 SHOULD provide a way to disable this, in case of difficulty.
541 o SHOULD send both validators in cache validation requests if both
542 an entity-tag and a Last-Modified value have been provided by the
543 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
544 respond appropriately.
546 3. Precondition Header Fields
548 This section defines the syntax and semantics of HTTP/1.1 header
549 fields for applying preconditions on requests. Section 5 defines
550 when the preconditions are applied. Section 6 defines the order of
551 evaluation when more than one precondition is present.
553 3.1. If-Match
555 The "If-Match" header field makes the request method conditional on
556 the recipient origin server either having at least one current
557 representation of the target resource, when the field-value is "*",
558 or having a current representation of the target resource that has an
559 entity-tag matching a member of the list of entity-tags provided in
560 the field-value.
562 An origin server MUST use the strong comparison function when
563 comparing entity-tags for If-Match (Section 2.3.2), since the client
564 intends this precondition to prevent the method from being applied if
565 there have been any changes to the representation data.
567 If-Match = "*" / 1#entity-tag
569 Examples:
571 If-Match: "xyzzy"
572 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
573 If-Match: *
575 If-Match is most often used with state-changing methods (e.g., POST,
576 PUT, DELETE) to prevent accidental overwrites when multiple user
577 agents might be acting in parallel on the same resource (i.e., to
578 prevent the "lost update" problem). It can also be used with safe
579 methods to abort a request if the selected representation does not
580 match one already stored (or partially stored) from a prior request.
582 An origin server that receives an If-Match header field MUST evaluate
583 the condition prior to performing the method (Section 5). If the
584 field-value is "*", the condition is false if the origin server does
585 not have a current representation for the target resource. If the
586 field-value is a list of entity-tags, the condition is false if none
587 of the listed tags match the entity-tag of the selected
588 representation.
590 An origin server MUST NOT perform the requested method if a received
591 If-Match condition evaluates to false; instead the origin server MUST
592 respond with either: a) the 412 (Precondition Failed) status code;
593 or, b) one of the 2xx (Successful) status codes if the origin server
594 has verified that a state change is being requested and the final
595 state is already reflected in the current state of the target
596 resource (i.e., the change requested by the user agent has already
597 succeeded, but the user agent might not be aware of it, perhaps
598 because the prior response was lost or a compatible change was made
599 by some other user agent). In the latter case, the origin server
600 MUST NOT send a validator header field in the response unless it can
601 verify that the request is a duplicate of an immediately prior change
602 made by the same user agent.
604 The If-Match header field can be ignored by caches and intermediaries
605 because it is not applicable to a stored response.
607 3.2. If-None-Match
609 The "If-None-Match" header field makes the request method conditional
610 on a recipient cache or origin server either not having any current
611 representation of the target resource, when the field-value is "*",
612 or having a selected representation with an entity-tag that does not
613 match any of those listed in the field-value.
615 A recipient MUST use the weak comparison function when comparing
616 entity-tags for If-None-Match (Section 2.3.2), since weak entity-tags
617 can be used for cache validation even if there have been changes to
618 the representation data.
620 If-None-Match = "*" / 1#entity-tag
622 Examples:
624 If-None-Match: "xyzzy"
625 If-None-Match: W/"xyzzy"
626 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
627 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
628 If-None-Match: *
630 If-None-Match is primarily used in conditional GET requests to enable
631 efficient updates of cached information with a minimum amount of
632 transaction overhead. When a client desires to update one or more
633 stored responses that have entity-tags, the client SHOULD generate an
634 If-None-Match header field containing a list of those entity-tags
635 when making a GET request; this allows recipient servers to send a
636 304 (Not Modified) response to indicate when one of those stored
637 responses matches the selected representation.
639 If-None-Match can also be used with a value of "*" to prevent an
640 unsafe request method (e.g., PUT) from inadvertently modifying an
641 existing representation of the target resource when the client
642 believes that the resource does not have a current representation
643 (Section 4.2.1 of [Part2]). This is a variation on the "lost update"
644 problem that might arise if more than one client attempts to create
645 an initial representation for the target resource.
647 An origin server that receives an If-None-Match header field MUST
648 evaluate the condition prior to performing the method (Section 5).
649 If the field-value is "*", the condition is false if the origin
650 server has a current representation for the target resource. If the
651 field-value is a list of entity-tags, the condition is false if one
652 of the listed tags match the entity-tag of the selected
653 representation.
655 An origin server MUST NOT perform the requested method if the
656 condition evaluates to false; instead, the origin server MUST respond
657 with either a) the 304 (Not Modified) status code if the request
658 method is GET or HEAD; or, b) the 412 (Precondition Failed) status
659 code for all other request methods.
661 Requirements on cache handling of a received If-None-Match header
662 field are defined in Section 4.3.2 of [Part6].
664 3.3. If-Modified-Since
666 The "If-Modified-Since" header field makes a GET or HEAD request
667 method conditional on the selected representation's modification date
668 being more recent than the date provided in the field-value.
669 Transfer of the selected representation's data is avoided if that
670 data has not changed.
672 If-Modified-Since = HTTP-date
674 An example of the field is:
676 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
678 A recipient MUST ignore If-Modified-Since if the request contains an
679 If-None-Match header field; the condition in If-None-Match is
680 considered to be a more accurate replacement for the condition in If-
681 Modified-Since and the two are only combined for the sake of
682 interoperating with older intermediaries that might not implement If-
683 None-Match.
685 A recipient MUST ignore the If-Modified-Since header field if the
686 received field-value is not a valid HTTP-date, or if the request
687 method is neither GET nor HEAD.
689 A recipient MUST interpret an If-Modified-Since field-value's
690 timestamp in terms of the origin server's clock.
692 If-Modified-Since is typically used for two distinct purposes: 1) to
693 allow efficient updates of a cached representation that does not have
694 an entity-tag; and, 2) to limit the scope of a web traversal to
695 resources that have recently changed.
697 When used for cache updates, a cache will typically use the value of
698 the cached message's Last-Modified field to generate the field value
699 of If-Modified-Since. This behavior is most interoperable for cases
700 where clocks are poorly synchronized or when the server has chosen to
701 only honor exact timestamp matches (due to a problem with Last-
702 Modified dates that appear to go "back in time" when the origin
703 server's clock is corrected or a representation is restored from an
704 archived backup). However, caches occasionally generate the field
705 value based on other data, such as the Date header field of the
706 cached message or the local clock time that the message was received,
707 particularly when the cached message does not contain a Last-Modified
708 field.
710 When used for limiting the scope of retrieval to a recent time
711 window, a user agent will generate an If-Modified-Since field value
712 based on either its own local clock or a Date header field received
713 from the server in a prior response. Origin servers that choose an
714 exact timestamp match based on the selected representation's Last-
715 Modified field will not be able to help the user agent limit its data
716 transfers to only those changed during the specified window.
718 An origin server that receives an If-Modified-Since header field
719 SHOULD evaluate the condition prior to performing the method
720 (Section 5). The origin server SHOULD NOT perform the requested
721 method if the selected representation's last modification date is
722 earlier than or equal to the date provided in the field-value;
723 instead, the origin server SHOULD generate a 304 (Not Modified)
724 response, including only those metadata that are useful for
725 identifying or updating a previously cached response.
727 Requirements on cache handling of a received If-Modified-Since header
728 field are defined in Section 4.3.2 of [Part6].
730 3.4. If-Unmodified-Since
732 The "If-Unmodified-Since" header field makes the request method
733 conditional on the selected representation's last modification date
734 being earlier than or equal to the date provided in the field-value.
735 This field accomplishes the same purpose as If-Match for cases where
736 the user agent does not have an entity-tag for the representation.
738 If-Unmodified-Since = HTTP-date
740 An example of the field is:
742 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
744 A recipient MUST ignore If-Unmodified-Since if the request contains
745 an If-Match header field; the condition in If-Match is considered to
746 be a more accurate replacement for the condition in If-Unmodified-
747 Since and the two are only combined for the sake of interoperating
748 with older intermediaries that might not implement If-Match.
750 A recipient MUST ignore the If-Unmodified-Since header field if the
751 received field-value is not a valid HTTP-date.
753 A recipient MUST interpret an If-Unmodified-Since field-value's
754 timestamp in terms of the origin server's clock.
756 If-Unmodified-Since is most often used with state-changing methods
757 (e.g., POST, PUT, DELETE) to prevent accidental overwrites when
758 multiple user agents might be acting in parallel on a resource that
759 does not supply entity-tags with its representations (i.e., to
760 prevent the "lost update" problem). It can also be used with safe
761 methods to abort a request if the selected representation does not
762 match one already stored (or partially stored) from a prior request.
764 An origin server that receives an If-Unmodified-Since header field
765 MUST evaluate the condition prior to performing the method
766 (Section 5). The origin server MUST NOT perform the requested method
767 if the selected representation's last modification date is more
768 recent than the date provided in the field-value; instead the origin
769 server MUST respond with either: a) the 412 (Precondition Failed)
770 status code; or, b) one of the 2xx (Successful) status codes if the
771 origin server has verified that a state change is being requested and
772 the final state is already reflected in the current state of the
773 target resource (i.e., the change requested by the user agent has
774 already succeeded, but the user agent might not be aware of that
775 because the prior response message was lost or a compatible change
776 was made by some other user agent). In the latter case, the origin
777 server MUST NOT send a validator header field in the response unless
778 it can verify that the request is a duplicate of an immediately prior
779 change made by the same user agent.
781 The If-Unmodified-Since header field can be ignored by caches and
782 intermediaries because it is not applicable to a stored response.
784 3.5. If-Range
786 The "If-Range" header field provides a special conditional request
787 mechanism that is similar to the If-Match and If-Unmodified-Since
788 header fields but instructs the recipient to ignore the Range header
789 field if the validator doesn't match, resulting in transfer of the
790 new selected representation instead of a 412 response. If-Range is
791 defined in Section 3.2 of [Part5].
793 4. Status Code Definitions
795 4.1. 304 Not Modified
797 The 304 (Not Modified) status code indicates that a conditional GET
798 or HEAD request has been received and would have resulted in a 200
799 (OK) response if it were not for the fact that the condition has
800 evaluated to false. In other words, there is no need for the server
801 to transfer a representation of the target resource because the
802 request indicates that the client, which made the request
803 conditional, already has a valid representation; the server is
804 therefore redirecting the client to make use of that stored
805 representation as if it were the payload of a 200 (OK) response.
807 The server generating a 304 response MUST generate any of the
808 following header fields that would have been sent in a 200 (OK)
809 response to the same request: Cache-Control, Content-Location, Date,
810 ETag, Expires, and Vary.
812 Since the goal of a 304 response is to minimize information transfer
813 when the recipient already has one or more cached representations, a
814 sender SHOULD NOT generate representation metadata other than the
815 above listed fields unless said metadata exists for the purpose of
816 guiding cache updates (e.g., Last-Modified might be useful if the
817 response does not have an ETag field).
819 Requirements on a cache that receives a 304 response are defined in
820 Section 4.3.4 of [Part6]. If the conditional request originated with
821 an outbound client, such as a user agent with its own cache sending a
822 conditional GET to a shared proxy, then the proxy SHOULD forward the
823 304 response to that client.
825 A 304 response cannot contain a message-body; it is always terminated
826 by the first empty line after the header fields.
828 4.2. 412 Precondition Failed
830 The 412 (Precondition Failed) status code indicates that one or more
831 conditions given in the request header fields evaluated to false when
832 tested on the server. This response code allows the client to place
833 preconditions on the current resource state (its current
834 representations and metadata) and thus prevent the request method
835 from being applied if the target resource is in an unexpected state.
837 5. Evaluation
839 Except when excluded below, a recipient cache or origin server MUST
840 evaluate received request preconditions after it has successfully
841 performed its normal request checks and just before it would perform
842 the action associated with the request method. A server MUST ignore
843 all received preconditions if its response to the same request
844 without those conditions would have been a status code other than a
845 2xx or 412 (Precondition Failed). In other words, redirects and
846 failures take precedence over the evaluation of preconditions in
847 conditional requests.
849 A server that is not the origin server for the target resource and
850 cannot act as a cache for requests on the target resource MUST NOT
851 evaluate the conditional request header fields defined by this
852 specification, and MUST forward them if the request is forwarded,
853 since the generating client intends that they be evaluated by a
854 server that can provide a current representation. Likewise, a server
855 MUST ignore the conditional request header fields defined by this
856 specification when received with a request method that does not
857 involve the selection or modification of a selected representation,
858 such as CONNECT, OPTIONS, or TRACE.
860 Conditional request header fields that are defined by extensions to
861 HTTP might place conditions on all recipients, on the state of the
862 target resource in general, or on a group of resources. For
863 instance, the "If" header field in WebDAV can make a request
864 conditional on various aspects of multiple resources, such as locks,
865 if the recipient understands and implements that field ([RFC4918],
866 Section 10.4).
868 Although conditional request header fields are defined as being
869 usable with the HEAD method (to keep HEAD's semantics consistent with
870 those of GET), there is no point in sending a conditional HEAD
871 because a successful response is around the same size as a 304 (Not
872 Modified) response and more useful than a 412 (Precondition Failed)
873 response.
875 6. Precedence
877 When more than one conditional request header field is present in a
878 request, the order in which the fields are evaluated becomes
879 important. In practice, the fields defined in this document are
880 consistently implemented in a single, logical order, since "lost
881 update" preconditions have more strict requirements than cache
882 validation, a validated cache is more efficient than a partial
883 response, and entity tags are presumed to be more accurate than date
884 validators.
886 A recipient cache or origin server MUST evaluate the request
887 preconditions defined by this specification in the following order:
889 1. When recipient is the origin server and If-Match is present,
890 evaluate the If-Match precondition:
892 * if true, continue to step 3
894 * if false, respond 412 (Precondition Failed) unless it can be
895 determined that the state-changing request has already
896 succeeded (see Section 3.1)
898 2. When recipient is the origin server, If-Match is not present, and
899 If-Unmodified-Since is present, evaluate the If-Unmodified-Since
900 precondition:
902 * if true, continue to step 3
904 * if false, respond 412 (Precondition Failed) unless it can be
905 determined that the state-changing request has already
906 succeeded (see Section 3.4)
908 3. When If-None-Match is present, evaluate the If-None-Match
909 precondition:
911 * if true, continue to step 5
913 * if false for GET/HEAD, respond 304 (Not Modified)
915 * if false for other methods, respond 412 (Precondition Failed)
917 4. When the method is GET or HEAD, If-None-Match is not present, and
918 If-Modified-Since is present, evaluate the If-Modified-Since
919 precondition:
921 * if true, continue to step 5
923 * if false, respond 304 (Not Modified)
925 5. When the method is GET and both Range and If-Range are present,
926 evaluate the If-Range precondition:
928 * if the validator matches and the Range specification is
929 applicable to the selected representation, respond 206
930 (Partial Content) [Part5]
932 6. Otherwise,
934 * all conditions are met, so perform the requested action and
935 respond according to its success or failure.
937 Any extension to HTTP/1.1 that defines additional conditional request
938 header fields ought to define its own expectations regarding the
939 order for evaluating such fields in relation to those defined in this
940 document and other conditionals that might be found in practice.
942 7. IANA Considerations
944 7.1. Status Code Registration
946 The HTTP Status Code Registry located at
947 shall be updated
948 with the registrations below:
950 +-------+---------------------+-------------+
951 | Value | Description | Reference |
952 +-------+---------------------+-------------+
953 | 304 | Not Modified | Section 4.1 |
954 | 412 | Precondition Failed | Section 4.2 |
955 +-------+---------------------+-------------+
957 7.2. Header Field Registration
959 HTTP header fields are registered within the Message Header Field
960 Registry maintained at .
963 This document defines the following HTTP header fields, so their
964 associated registry entries shall be updated according to the
965 permanent registrations below (see [BCP90]):
967 +---------------------+----------+----------+-------------+
968 | Header Field Name | Protocol | Status | Reference |
969 +---------------------+----------+----------+-------------+
970 | ETag | http | standard | Section 2.3 |
971 | If-Match | http | standard | Section 3.1 |
972 | If-Modified-Since | http | standard | Section 3.3 |
973 | If-None-Match | http | standard | Section 3.2 |
974 | If-Unmodified-Since | http | standard | Section 3.4 |
975 | Last-Modified | http | standard | Section 2.2 |
976 +---------------------+----------+----------+-------------+
978 The change controller is: "IETF (iesg@ietf.org) - Internet
979 Engineering Task Force".
981 8. Security Considerations
983 This section is meant to inform developers, information providers,
984 and users of known security concerns specific to the HTTP conditional
985 request mechanisms. More general security considerations are
986 addressed in HTTP messaging [Part1] and semantics [Part2].
988 The validators defined by this specification are not intended to
989 ensure the validity of a representation, guard against malicious
990 changes, or detect man-in-the-middle attacks. At best, they enable
991 more efficient cache updates and optimistic concurrent writes when
992 all participants are behaving nicely. At worst, the conditions will
993 fail and the client will receive a response that is no more harmful
994 than an HTTP exchange without conditional requests.
996 An entity-tag can be abused in ways that create privacy risks. For
997 example, a site might deliberately construct a semantically invalid
998 entity-tag that is unique to the user or user agent, send it in a
999 cacheable response with a long freshness time, and then read that
1000 entity-tag in later conditional requests as a means of re-identifying
1001 that user or user agent. Such an identifying tag would become a
1002 persistent identifier for as long as the user agent retained the
1003 original cache entry. User agents that cache representations ought
1004 to ensure that the cache is cleared or replaced whenever the user
1005 performs privacy-maintaining actions, such as clearing stored cookies
1006 or changing to a private browsing mode.
1008 9. Acknowledgments
1010 See Section 10 of [Part1].
1012 10. References
1013 10.1. Normative References
1015 [Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
1016 Protocol (HTTP/1.1): Message Syntax and Routing",
1017 draft-ietf-httpbis-p1-messaging-26 (work in progress),
1018 February 2014.
1020 [Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
1021 Protocol (HTTP/1.1): Semantics and Content",
1022 draft-ietf-httpbis-p2-semantics-26 (work in progress),
1023 February 2014.
1025 [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
1026 "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
1027 draft-ietf-httpbis-p5-range-26 (work in progress),
1028 February 2014.
1030 [Part6] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
1031 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
1032 draft-ietf-httpbis-p6-cache-26 (work in progress),
1033 February 2014.
1035 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1036 Requirement Levels", BCP 14, RFC 2119, March 1997.
1038 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
1039 Specifications: ABNF", STD 68, RFC 5234, January 2008.
1041 10.2. Informative References
1043 [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
1044 Procedures for Message Header Fields", BCP 90, RFC 3864,
1045 September 2004.
1047 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1048 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1049 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1051 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
1052 Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
1054 Appendix A. Changes from RFC 2616
1056 The definition of validator weakness has been expanded and clarified.
1057 (Section 2.1)
1059 Weak entity-tags are now allowed in all requests except range
1060 requests. (Sections 2.1 and 3.2)
1061 The ETag header field ABNF has been changed to not use quoted-string,
1062 thus avoiding escaping issues. (Section 2.3)
1064 ETag is defined to provide an entity tag for the selected
1065 representation, thereby clarifying what it applies to in various
1066 situations (such as a PUT response). (Section 2.3)
1068 The precedence for evaluation of conditional requests has been
1069 defined. (Section 6)
1071 Appendix B. Imported ABNF
1073 The following core rules are included by reference, as defined in
1074 Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
1075 CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
1076 quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
1077 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
1078 character).
1080 The rules below are defined in [Part1]:
1082 OWS =
1083 obs-text =
1085 The rules below are defined in other parts:
1087 HTTP-date =
1089 Appendix C. Collected ABNF
1091 In the collected ABNF below, list rules are expanded as per Section
1092 1.2 of [Part1].
1094 ETag = entity-tag
1096 HTTP-date =
1098 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1099 entity-tag ] ) )
1100 If-Modified-Since = HTTP-date
1101 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1102 entity-tag ] ) )
1103 If-Unmodified-Since = HTTP-date
1105 Last-Modified = HTTP-date
1107 OWS =
1109 entity-tag = [ weak ] opaque-tag
1110 etagc = "!" / %x23-7E ; '#'-'~'
1111 / obs-text
1113 obs-text =
1114 opaque-tag = DQUOTE *etagc DQUOTE
1116 weak = %x57.2F ; W/
1118 Appendix D. Change Log (to be removed by RFC Editor before publication)
1120 Changes up to the IETF Last Call draft are summarized in .
1123 D.1. Since draft-ietf-httpbis-p4-conditional-24
1125 Closed issues:
1127 o : "APPSDIR
1128 review of draft-ietf-httpbis-p4-conditional-24"
1130 D.2. Since draft-ietf-httpbis-p4-conditional-25
1132 Closed issues:
1134 o : "add
1135 'stateless' to Abstract"
1137 o : "improve
1138 introduction of list rule"
1140 o : "augment
1141 security considerations with pointers to current research"
1143 Index
1145 3
1146 304 Not Modified (status code) 18
1148 4
1149 412 Precondition Failed (status code) 18
1151 E
1152 ETag header field 9
1154 G
1155 Grammar
1156 entity-tag 9
1157 ETag 9
1158 etagc 9
1159 If-Match 13
1160 If-Modified-Since 15
1161 If-None-Match 14
1162 If-Unmodified-Since 16
1163 Last-Modified 7
1164 opaque-tag 9
1165 weak 9
1167 I
1168 If-Match header field 13
1169 If-Modified-Since header field 15
1170 If-None-Match header field 14
1171 If-Unmodified-Since header field 16
1173 L
1174 Last-Modified header field 7
1176 M
1177 metadata 5
1179 S
1180 selected representation 4
1182 V
1183 validator 5
1184 strong 5
1185 weak 5
1187 Authors' Addresses
1189 Roy T. Fielding (editor)
1190 Adobe Systems Incorporated
1191 345 Park Ave
1192 San Jose, CA 95110
1193 USA
1195 EMail: fielding@gbiv.com
1196 URI: http://roy.gbiv.com/
1198 Julian F. Reschke (editor)
1199 greenbytes GmbH
1200 Hafenweg 16
1201 Muenster, NW 48155
1202 Germany
1204 EMail: julian.reschke@greenbytes.de
1205 URI: http://greenbytes.de/tech/webdav/