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2 HTTPbis Working Group R. Fielding, Ed.
3 Internet-Draft Adobe
4 Obsoletes: 2616 (if approved) J. Gettys
5 Intended status: Standards Track Alcatel-Lucent
6 Expires: July 7, 2012 J. Mogul
7 HP
8 H. Frystyk
9 Microsoft
10 L. Masinter
11 Adobe
12 P. Leach
13 Microsoft
14 T. Berners-Lee
15 W3C/MIT
16 Y. Lafon, Ed.
17 W3C
18 J. Reschke, Ed.
19 greenbytes
20 January 4, 2012
22 HTTP/1.1, part 4: Conditional Requests
23 draft-ietf-httpbis-p4-conditional-18
25 Abstract
27 The Hypertext Transfer Protocol (HTTP) is an application-level
28 protocol for distributed, collaborative, hypertext information
29 systems. HTTP has been in use by the World Wide Web global
30 information initiative since 1990. This document is Part 4 of the
31 seven-part specification that defines the protocol referred to as
32 "HTTP/1.1" and, taken together, obsoletes RFC 2616.
34 Part 4 defines request header fields for indicating conditional
35 requests and the rules for constructing responses to those requests.
37 Editorial Note (To be removed by RFC Editor)
39 Discussion of this draft should take place on the HTTPBIS working
40 group mailing list (ietf-http-wg@w3.org), which is archived at
41 .
43 The current issues list is at
44 and related
45 documents (including fancy diffs) can be found at
46 .
48 The changes in this draft are summarized in Appendix C.19.
50 Status of This Memo
52 This Internet-Draft is submitted in full conformance with the
53 provisions of BCP 78 and BCP 79.
55 Internet-Drafts are working documents of the Internet Engineering
56 Task Force (IETF). Note that other groups may also distribute
57 working documents as Internet-Drafts. The list of current Internet-
58 Drafts is at http://datatracker.ietf.org/drafts/current/.
60 Internet-Drafts are draft documents valid for a maximum of six months
61 and may be updated, replaced, or obsoleted by other documents at any
62 time. It is inappropriate to use Internet-Drafts as reference
63 material or to cite them other than as "work in progress."
65 This Internet-Draft will expire on July 7, 2012.
67 Copyright Notice
69 Copyright (c) 2012 IETF Trust and the persons identified as the
70 document authors. All rights reserved.
72 This document is subject to BCP 78 and the IETF Trust's Legal
73 Provisions Relating to IETF Documents
74 (http://trustee.ietf.org/license-info) in effect on the date of
75 publication of this document. Please review these documents
76 carefully, as they describe your rights and restrictions with respect
77 to this document. Code Components extracted from this document must
78 include Simplified BSD License text as described in Section 4.e of
79 the Trust Legal Provisions and are provided without warranty as
80 described in the Simplified BSD License.
82 This document may contain material from IETF Documents or IETF
83 Contributions published or made publicly available before November
84 10, 2008. The person(s) controlling the copyright in some of this
85 material may not have granted the IETF Trust the right to allow
86 modifications of such material outside the IETF Standards Process.
87 Without obtaining an adequate license from the person(s) controlling
88 the copyright in such materials, this document may not be modified
89 outside the IETF Standards Process, and derivative works of it may
90 not be created outside the IETF Standards Process, except to format
91 it for publication as an RFC or to translate it into languages other
92 than English.
94 Table of Contents
96 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
97 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 5
98 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6
99 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 6
100 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 6
101 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 8
102 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 8
103 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 9
104 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
105 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 11
106 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 12
107 2.3.3. Example: Entity-tags varying on Content-Negotiated
108 Resources . . . . . . . . . . . . . . . . . . . . . . 12
109 2.4. Rules for When to Use Entity-tags and Last-Modified
110 Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 13
111 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 15
112 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 15
113 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 16
114 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 17
115 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 18
116 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 19
117 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 19
118 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 19
119 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 20
120 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
121 5.1. Status Code Registration . . . . . . . . . . . . . . . . . 20
122 5.2. Header Field Registration . . . . . . . . . . . . . . . . 20
123 6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
124 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
125 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
126 8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
127 8.2. Informative References . . . . . . . . . . . . . . . . . . 22
128 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
129 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 23
130 Appendix C. Change Log (to be removed by RFC Editor before
131 publication) . . . . . . . . . . . . . . . . . . . . 23
132 C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 23
133 C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23
134 C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 24
135 C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 24
136 C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 24
137 C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24
138 C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 25
139 C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 25
140 C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 25
141 C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 25
142 C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 25
143 C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25
144 C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 26
145 C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 26
146 C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 26
147 C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 26
148 C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 26
149 C.18. Since draft-ietf-httpbis-p4-conditional-16 . . . . . . . . 26
150 C.19. Since draft-ietf-httpbis-p4-conditional-17 . . . . . . . . 27
151 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
153 1. Introduction
155 This document defines the HTTP/1.1 conditional request mechanisms,
156 including both metadata for indicating/observing changes in resource
157 representations and request header fields that specify preconditions
158 on that metadata be checked before performing the request method.
159 Conditional GET requests are the most efficient mechanism for HTTP
160 cache updates [Part6]. Conditionals can also be applied to state-
161 changing methods, such as PUT and DELETE, to prevent the "lost
162 update" problem: one client accidentally overwriting the work of
163 another client that has been acting in parallel.
165 Conditional request preconditions are based on the state of the
166 target resource as a whole (its current value set) or the state as
167 observed in a previously obtained representation (one value in that
168 set). A resource might have multiple current representations, each
169 with its own observable state. The conditional request mechanisms
170 assume that the mapping of requests to corresponding representations
171 will be consistent over time if the server intends to take advantage
172 of conditionals. Regardless, if the mapping is inconsistent and the
173 server is unable to select the appropriate representation, then no
174 harm will result when the precondition evaluates to false.
176 We use the term "selected representation" to refer to the current
177 representation of the target resource that would have been selected
178 in a successful response if the same request had used the method GET
179 and had excluded all of the conditional request header fields. The
180 conditional request preconditions are evaluated by comparing the
181 values provided in the request header fields to the current metadata
182 for the selected representation.
184 1.1. Conformance and Error Handling
186 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
187 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
188 document are to be interpreted as described in [RFC2119].
190 This document defines conformance criteria for several roles in HTTP
191 communication, including Senders, Recipients, Clients, Servers, User-
192 Agents, Origin Servers, Intermediaries, Proxies and Gateways. See
193 Section 2 of [Part1] for definitions of these terms.
195 An implementation is considered conformant if it complies with all of
196 the requirements associated with its role(s). Note that SHOULD-level
197 requirements are relevant here, unless one of the documented
198 exceptions is applicable.
200 This document also uses ABNF to define valid protocol elements
201 (Section 1.2). In addition to the prose requirements placed upon
202 them, Senders MUST NOT generate protocol elements that are invalid.
204 Unless noted otherwise, Recipients MAY take steps to recover a usable
205 protocol element from an invalid construct. However, HTTP does not
206 define specific error handling mechanisms, except in cases where it
207 has direct impact on security. This is because different uses of the
208 protocol require different error handling strategies; for example, a
209 Web browser may wish to transparently recover from a response where
210 the Location header field doesn't parse according to the ABNF,
211 whereby in a systems control protocol using HTTP, this type of error
212 recovery could lead to dangerous consequences.
214 1.2. Syntax Notation
216 This specification uses the ABNF syntax defined in Section 1.2 of
217 [Part1] (which extends the syntax defined in [RFC5234] with a list
218 rule). Appendix B shows the collected ABNF, with the list rule
219 expanded.
221 The following core rules are included by reference, as defined in
222 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
223 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
224 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
225 sequence of data), SP (space), and VCHAR (any visible US-ASCII
226 character).
228 The ABNF rules below are defined in [Part1] and [Part2]:
230 OWS =
231 obs-text =
232 HTTP-date =
234 2. Validators
236 This specification defines two forms of metadata that are commonly
237 used to observe resource state and test for preconditions:
238 modification dates and opaque entity tags. Additional metadata that
239 reflects resource state has been defined by various extensions of
240 HTTP, such as WebDAV [RFC4918], that are beyond the scope of this
241 specification. A resource metadata value is referred to as a
242 "validator" when it is used within a precondition.
244 2.1. Weak versus Strong
246 Validators come in two flavors: strong or weak. Weak validators are
247 easy to generate but are far less useful for comparisons. Strong
248 validators are ideal for comparisons but can be very difficult (and
249 occasionally impossible) to generate efficiently. Rather than impose
250 that all forms of resource adhere to the same strength of validator,
251 HTTP exposes the type of validator in use and imposes restrictions on
252 when weak validators can be used as preconditions.
254 A "strong validator" is a representation metadata value that MUST be
255 changed to a new, previously unused or guaranteed unique, value
256 whenever a change occurs to the representation data such that a
257 change would be observable in the payload body of a 200 response to
258 GET. A strong validator MAY be changed for other reasons, such as
259 when a semantically significant part of the representation metadata
260 is changed (e.g., Content-Type), but it is in the best interests of
261 the origin server to only change the value when it is necessary to
262 invalidate the stored responses held by remote caches and authoring
263 tools. A strong validator MUST be unique across all representations
264 of a given resource, such that no two representations of that
265 resource share the same validator unless their payload body would be
266 identical.
268 Cache entries might persist for arbitrarily long periods, regardless
269 of expiration times. Thus, a cache might attempt to validate an
270 entry using a validator that it obtained in the distant past. A
271 strong validator MUST be unique across all versions of all
272 representations associated with a particular resource over time.
273 However, there is no implication of uniqueness across representations
274 of different resources (i.e., the same strong validator might be in
275 use for representations of multiple resources at the same time and
276 does not imply that those representations are equivalent).
278 There are a variety of strong validators used in practice. The best
279 are based on strict revision control, wherein each change to a
280 representation always results in a unique node name and revision
281 identifier being assigned before the representation is made
282 accessible to GET. A cryptographic hash function applied to the
283 representation data is also sufficient if the data is available prior
284 to the response header fields being sent and the digest does not need
285 to be recalculated every time a validation request is received.
286 However, if a resource has distinct representations that differ only
287 in their metadata, such as might occur with content negotiation over
288 media types that happen to share the same data format, then a server
289 SHOULD incorporate additional information in the validator to
290 distinguish those representations and avoid confusing cache behavior.
292 In contrast, a "weak validator" is a representation metadata value
293 that might not be changed for every change to the representation
294 data. This weakness might be due to limitations in how the value is
295 calculated, such as clock resolution or an inability to ensure
296 uniqueness for all possible representations of the resource, or due
297 to a desire by the resource owner to group representations by some
298 self-determined set of equivalency rather than unique sequences of
299 data. A weak entity-tag SHOULD change whenever the origin server
300 considers prior representations to be unacceptable as a substitute
301 for the current representation. In other words, a weak entity-tag
302 SHOULD change whenever the origin server wants caches to invalidate
303 old responses.
305 For example, the representation of a weather report that changes in
306 content every second, based on dynamic measurements, might be grouped
307 into sets of equivalent representations (from the origin server's
308 perspective) with the same weak validator in order to allow cached
309 representations to be valid for a reasonable period of time (perhaps
310 adjusted dynamically based on server load or weather quality).
311 Likewise, a representation's modification time, if defined with only
312 one-second resolution, might be a weak validator if it is possible
313 for the representation to be modified twice during a single second
314 and retrieved between those modifications.
316 A "use" of a validator occurs when either a client generates a
317 request and includes the validator in a precondition or when a server
318 compares two validators. Weak validators are only usable in contexts
319 that do not depend on exact equality of a representation's payload
320 body. Strong validators are usable and preferred for all conditional
321 requests, including cache validation, partial content ranges, and
322 "lost update" avoidance.
324 2.2. Last-Modified
326 The "Last-Modified" header field indicates the date and time at which
327 the origin server believes the selected representation was last
328 modified.
330 Last-Modified = HTTP-date
332 An example of its use is
334 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
336 2.2.1. Generation
338 Origin servers SHOULD send Last-Modified for any selected
339 representation for which a last modification date can be reasonably
340 and consistently determined, since its use in conditional requests
341 and evaluating cache freshness ([Part6]) results in a substantial
342 reduction of HTTP traffic on the Internet and can be a significant
343 factor in improving service scalability and reliability.
345 A representation is typically the sum of many parts behind the
346 resource interface. The last-modified time would usually be the most
347 recent time that any of those parts were changed. How that value is
348 determined for any given resource is an implementation detail beyond
349 the scope of this specification. What matters to HTTP is how
350 recipients of the Last-Modified header field can use its value to
351 make conditional requests and test the validity of locally cached
352 responses.
354 An origin server SHOULD obtain the Last-Modified value of the
355 representation as close as possible to the time that it generates the
356 Date field-value for its response. This allows a recipient to make
357 an accurate assessment of the representation's modification time,
358 especially if the representation changes near the time that the
359 response is generated.
361 An origin server with a clock MUST NOT send a Last-Modified date that
362 is later than the server's time of message origination (Date). If
363 the last modification time is derived from implementation-specific
364 metadata that evaluates to some time in the future, according to the
365 origin server's clock, then the origin server MUST replace that value
366 with the message origination date. This prevents a future
367 modification date from having an adverse impact on cache validation.
369 An origin server without a clock MUST NOT assign Last-Modified values
370 to a response unless these values were associated with the resource
371 by some other system or user with a reliable clock.
373 2.2.2. Comparison
375 A Last-Modified time, when used as a validator in a request, is
376 implicitly weak unless it is possible to deduce that it is strong,
377 using the following rules:
379 o The validator is being compared by an origin server to the actual
380 current validator for the representation and,
382 o That origin server reliably knows that the associated
383 representation did not change twice during the second covered by
384 the presented validator.
386 or
388 o The validator is about to be used by a client in an If-Modified-
389 Since, If-Unmodified-Since header field, because the client has a
390 cache entry, or If-Range for the associated representation, and
392 o That cache entry includes a Date value, which gives the time when
393 the origin server sent the original response, and
395 o The presented Last-Modified time is at least 60 seconds before the
396 Date value.
398 or
400 o The validator is being compared by an intermediate cache to the
401 validator stored in its cache entry for the representation, and
403 o That cache entry includes a Date value, which gives the time when
404 the origin server sent the original response, and
406 o The presented Last-Modified time is at least 60 seconds before the
407 Date value.
409 This method relies on the fact that if two different responses were
410 sent by the origin server during the same second, but both had the
411 same Last-Modified time, then at least one of those responses would
412 have a Date value equal to its Last-Modified time. The arbitrary 60-
413 second limit guards against the possibility that the Date and Last-
414 Modified values are generated from different clocks, or at somewhat
415 different times during the preparation of the response. An
416 implementation MAY use a value larger than 60 seconds, if it is
417 believed that 60 seconds is too short.
419 2.3. ETag
421 The ETag header field provides the current entity-tag for the
422 selected representation. An entity-tag is an opaque validator for
423 differentiating between multiple representations of the same
424 resource, regardless of whether those multiple representations are
425 due to resource state changes over time, content negotiation
426 resulting in multiple representations being valid at the same time,
427 or both. An entity-tag consists of an opaque quoted string, possibly
428 prefixed by a weakness indicator.
430 ETag = entity-tag
432 entity-tag = [ weak ] opaque-tag
433 weak = %x57.2F ; "W/", case-sensitive
434 opaque-tag = DQUOTE *etagc DQUOTE
435 etagc = %x21 / %x23-7E / obs-text
436 ; VCHAR except double quotes, plus obs-text
438 Note: Previously, opaque-tag was defined to be a quoted-string
439 ([RFC2616], Section 3.11), thus some recipients might perform
440 backslash unescaping. Servers therefore ought to avoid backslash
441 characters in entity tags.
443 An entity-tag can be more reliable for validation than a modification
444 date in situations where it is inconvenient to store modification
445 dates, where the one-second resolution of HTTP date values is not
446 sufficient, or where modification dates are not consistently
447 maintained.
449 Examples:
451 ETag: "xyzzy"
452 ETag: W/"xyzzy"
453 ETag: ""
455 An entity-tag can be either a weak or strong validator, with strong
456 being the default. If an origin server provides an entity-tag for a
457 representation and the generation of that entity-tag does not satisfy
458 the requirements for a strong validator (Section 2.1), then that
459 entity-tag MUST be marked as weak by prefixing its opaque value with
460 "W/" (case-sensitive).
462 2.3.1. Generation
464 The principle behind entity-tags is that only the service author
465 knows the implementation of a resource well enough to select the most
466 accurate and efficient validation mechanism for that resource, and
467 that any such mechanism can be mapped to a simple sequence of octets
468 for easy comparison. Since the value is opaque, there is no need for
469 the client to be aware of how each entity-tag is constructed.
471 For example, a resource that has implementation-specific versioning
472 applied to all changes might use an internal revision number, perhaps
473 combined with a variance identifier for content negotiation, to
474 accurately differentiate between representations. Other
475 implementations might use a stored hash of representation content, a
476 combination of various filesystem attributes, or a modification
477 timestamp that has sub-second resolution.
479 Origin servers SHOULD send ETag for any selected representation for
480 which detection of changes can be reasonably and consistently
481 determined, since the entity-tag's use in conditional requests and
482 evaluating cache freshness ([Part6]) can result in a substantial
483 reduction of HTTP network traffic and can be a significant factor in
484 improving service scalability and reliability.
486 2.3.2. Comparison
488 There are two entity-tag comparison functions, depending on whether
489 the comparison context allows the use of weak validators or not:
491 o The strong comparison function: in order to be considered equal,
492 both opaque-tags MUST be identical character-by-character, and
493 both MUST NOT be weak.
495 o The weak comparison function: in order to be considered equal,
496 both opaque-tags MUST be identical character-by-character, but
497 either or both of them MAY be tagged as "weak" without affecting
498 the result.
500 The example below shows the results for a set of entity-tag pairs,
501 and both the weak and strong comparison function results:
503 +--------+--------+-------------------+-----------------+
504 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
505 +--------+--------+-------------------+-----------------+
506 | W/"1" | W/"1" | no match | match |
507 | W/"1" | W/"2" | no match | no match |
508 | W/"1" | "1" | no match | match |
509 | "1" | "1" | match | match |
510 +--------+--------+-------------------+-----------------+
512 2.3.3. Example: Entity-tags varying on Content-Negotiated Resources
514 Consider a resource that is subject to content negotiation (Section 5
515 of [Part3]), and where the representations returned upon a GET
516 request vary based on the Accept-Encoding request header field
517 (Section 6.3 of [Part3]):
519 >> Request:
521 GET /index HTTP/1.1
522 Host: www.example.com
523 Accept-Encoding: gzip
525 In this case, the response might or might not use the gzip content
526 coding. If it does not, the response might look like:
528 >> Response:
530 HTTP/1.1 200 OK
531 Date: Thu, 26 Mar 2010 00:05:00 GMT
532 ETag: "123-a"
533 Content-Length: 70
534 Vary: Accept-Encoding
535 Content-Type: text/plain
537 Hello World!
538 Hello World!
539 Hello World!
540 Hello World!
541 Hello World!
543 An alternative representation that does use gzip content coding would
544 be:
546 >> Response:
548 HTTP/1.1 200 OK
549 Date: Thu, 26 Mar 2010 00:05:00 GMT
550 ETag: "123-b"
551 Content-Length: 43
552 Vary: Accept-Encoding
553 Content-Type: text/plain
554 Content-Encoding: gzip
556 ...binary data...
558 Note: Content codings are a property of the representation, so
559 therefore an entity-tag of an encoded representation must be
560 distinct from an unencoded representation to prevent conflicts
561 during cache updates and range requests. In contrast, transfer
562 codings (Section 5.1 of [Part1]) apply only during message
563 transfer and do not require distinct entity-tags.
565 2.4. Rules for When to Use Entity-tags and Last-Modified Dates
567 We adopt a set of rules and recommendations for origin servers,
568 clients, and caches regarding when various validator types ought to
569 be used, and for what purposes.
571 HTTP/1.1 origin servers:
573 o SHOULD send an entity-tag validator unless it is not feasible to
574 generate one.
576 o MAY send a weak entity-tag instead of a strong entity-tag, if
577 performance considerations support the use of weak entity-tags, or
578 if it is unfeasible to send a strong entity-tag.
580 o SHOULD send a Last-Modified value if it is feasible to send one.
582 In other words, the preferred behavior for an HTTP/1.1 origin server
583 is to send both a strong entity-tag and a Last-Modified value.
585 HTTP/1.1 clients:
587 o MUST use that entity-tag in any cache-conditional request (using
588 If-Match or If-None-Match) if an entity-tag has been provided by
589 the origin server.
591 o SHOULD use the Last-Modified value in non-subrange cache-
592 conditional requests (using If-Modified-Since) if only a Last-
593 Modified value has been provided by the origin server.
595 o MAY use the Last-Modified value in subrange cache-conditional
596 requests (using If-Unmodified-Since) if only a Last-Modified value
597 has been provided by an HTTP/1.0 origin server. The user agent
598 SHOULD provide a way to disable this, in case of difficulty.
600 o SHOULD use both validators in cache-conditional requests if both
601 an entity-tag and a Last-Modified value have been provided by the
602 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
603 respond appropriately.
605 An HTTP/1.1 origin server, upon receiving a conditional request that
606 includes both a Last-Modified date (e.g., in an If-Modified-Since or
607 If-Unmodified-Since header field) and one or more entity-tags (e.g.,
608 in an If-Match, If-None-Match, or If-Range header field) as cache
609 validators, MUST NOT return a response status code of 304 (Not
610 Modified) unless doing so is consistent with all of the conditional
611 header fields in the request.
613 An HTTP/1.1 caching proxy, upon receiving a conditional request that
614 includes both a Last-Modified date and one or more entity-tags as
615 cache validators, MUST NOT return a locally cached response to the
616 client unless that cached response is consistent with all of the
617 conditional header fields in the request.
619 Note: The general principle behind these rules is that HTTP/1.1
620 servers and clients ought to transmit as much non-redundant
621 information as is available in their responses and requests.
622 HTTP/1.1 systems receiving this information will make the most
623 conservative assumptions about the validators they receive.
625 HTTP/1.0 clients and caches might ignore entity-tags. Generally,
626 last-modified values received or used by these systems will
627 support transparent and efficient caching, and so HTTP/1.1 origin
628 servers should provide Last-Modified values. In those rare cases
629 where the use of a Last-Modified value as a validator by an
630 HTTP/1.0 system could result in a serious problem, then HTTP/1.1
631 origin servers should not provide one.
633 3. Precondition Header Fields
635 This section defines the syntax and semantics of HTTP/1.1 header
636 fields for applying preconditions on requests.
638 3.1. If-Match
640 The "If-Match" header field MAY be used to make a request method
641 conditional on the current existence or value of an entity-tag for
642 one or more representations of the target resource. If-Match is
643 generally useful for resource update requests, such as PUT requests,
644 as a means for protecting against accidental overwrites when multiple
645 clients are acting in parallel on the same resource (i.e., the "lost
646 update" problem). An If-Match field-value of "*" places the
647 precondition on the existence of any current representation for the
648 target resource.
650 If-Match = "*" / 1#entity-tag
652 If any of the entity-tags listed in the If-Match field value match
653 (as per Section 2.3.2) the entity-tag of the selected representation
654 for the target resource, or if "*" is given and any current
655 representation exists for the target resource, then the server MAY
656 perform the request method as if the If-Match header field was not
657 present.
659 If none of the entity-tags match, or if "*" is given and no current
660 representation exists, the server MUST NOT perform the requested
661 method. Instead, the server MUST respond with the 412 (Precondition
662 Failed) status code.
664 If the request would, without the If-Match header field, result in
665 anything other than a 2xx or 412 status code, then the If-Match
666 header field MUST be ignored.
668 Examples:
670 If-Match: "xyzzy"
671 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
672 If-Match: *
674 The result of a request having both an If-Match header field and
675 either an If-None-Match or an If-Modified-Since header fields is
676 undefined by this specification.
678 3.2. If-None-Match
680 The "If-None-Match" header field MAY be used to make a request method
681 conditional on not matching any of the current entity-tag values for
682 representations of the target resource. If-None-Match is primarily
683 used in conditional GET requests to enable efficient updates of
684 cached information with a minimum amount of transaction overhead. A
685 client that has one or more representations previously obtained from
686 the target resource can send If-None-Match with a list of the
687 associated entity-tags in the hope of receiving a 304 response if at
688 least one of those representations matches the selected
689 representation.
691 If-None-Match MAY also be used with a value of "*" to prevent an
692 unsafe request method (e.g., PUT) from inadvertently modifying an
693 existing representation of the target resource when the client
694 believes that the resource does not have a current representation.
695 This is a variation on the "lost update" problem that might arise if
696 more than one client attempts to create an initial representation for
697 the target resource.
699 If-None-Match = "*" / 1#entity-tag
701 If any of the entity-tags listed in the If-None-Match field-value
702 match (as per Section 2.3.2) the entity-tag of the selected
703 representation, or if "*" is given and any current representation
704 exists for that resource, then the server MUST NOT perform the
705 requested method. Instead, if the request method was GET or HEAD,
706 the server SHOULD respond with a 304 (Not Modified) status code,
707 including the cache-related header fields (particularly ETag) of the
708 selected representation that has a matching entity-tag. For all
709 other request methods, the server MUST respond with a 412
710 (Precondition Failed) status code.
712 If none of the entity-tags match, then the server MAY perform the
713 requested method as if the If-None-Match header field did not exist,
714 but MUST also ignore any If-Modified-Since header field(s) in the
715 request. That is, if no entity-tags match, then the server MUST NOT
716 return a 304 (Not Modified) response.
718 If the request would, without the If-None-Match header field, result
719 in anything other than a 2xx or 304 status code, then the If-None-
720 Match header field MUST be ignored. (See Section 2.4 for a
721 discussion of server behavior when both If-Modified-Since and If-
722 None-Match appear in the same request.)
724 Examples:
726 If-None-Match: "xyzzy"
727 If-None-Match: W/"xyzzy"
728 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
729 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
730 If-None-Match: *
732 The result of a request having both an If-None-Match header field and
733 either an If-Match or an If-Unmodified-Since header fields is
734 undefined by this specification.
736 3.3. If-Modified-Since
738 The "If-Modified-Since" header field MAY be used to make a request
739 method conditional by modification date: if the selected
740 representation has not been modified since the time specified in this
741 field, then do not perform the request method; instead, respond as
742 detailed below.
744 If-Modified-Since = HTTP-date
746 An example of the field is:
748 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
750 A GET method with an If-Modified-Since header field and no Range
751 header field requests that the selected representation be transferred
752 only if it has been modified since the date given by the If-Modified-
753 Since header field. The algorithm for determining this includes the
754 following cases:
756 1. If the request would normally result in anything other than a 200
757 (OK) status code, or if the passed If-Modified-Since date is
758 invalid, the response is exactly the same as for a normal GET. A
759 date which is later than the server's current time is invalid.
761 2. If the selected representation has been modified since the If-
762 Modified-Since date, the response is exactly the same as for a
763 normal GET.
765 3. If the selected representation has not been modified since a
766 valid If-Modified-Since date, the server SHOULD return a 304 (Not
767 Modified) response.
769 The purpose of this feature is to allow efficient updates of cached
770 information with a minimum amount of transaction overhead.
772 Note: The Range header field modifies the meaning of If-Modified-
773 Since; see Section 5.4 of [Part5] for full details.
775 Note: If-Modified-Since times are interpreted by the server, whose
776 clock might not be synchronized with the client.
778 Note: When handling an If-Modified-Since header field, some
779 servers will use an exact date comparison function, rather than a
780 less-than function, for deciding whether to send a 304 (Not
781 Modified) response. To get best results when sending an If-
782 Modified-Since header field for cache validation, clients are
783 advised to use the exact date string received in a previous Last-
784 Modified header field whenever possible.
786 Note: If a client uses an arbitrary date in the If-Modified-Since
787 header field instead of a date taken from the Last-Modified header
788 field for the same request, the client needs to be aware that this
789 date is interpreted in the server's understanding of time.
790 Unsynchronized clocks and rounding problems, due to the different
791 encodings of time between the client and server, are concerns.
792 This includes the possibility of race conditions if the document
793 has changed between the time it was first requested and the If-
794 Modified-Since date of a subsequent request, and the possibility
795 of clock-skew-related problems if the If-Modified-Since date is
796 derived from the client's clock without correction to the server's
797 clock. Corrections for different time bases between client and
798 server are at best approximate due to network latency.
800 The result of a request having both an If-Modified-Since header field
801 and either an If-Match or an If-Unmodified-Since header fields is
802 undefined by this specification.
804 3.4. If-Unmodified-Since
806 The "If-Unmodified-Since" header field MAY be used to make a request
807 method conditional by modification date: if the selected
808 representation has been modified since the time specified in this
809 field, then the server MUST NOT perform the requested operation and
810 MUST instead respond with the 412 (Precondition Failed) status code.
811 If the selected representation has not been modified since the time
812 specified in this field, the server SHOULD perform the request method
813 as if the If-Unmodified-Since header field were not present.
815 If-Unmodified-Since = HTTP-date
817 An example of the field is:
819 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
821 If the request normally (i.e., without the If-Unmodified-Since header
822 field) would result in anything other than a 2xx or 412 status code,
823 the If-Unmodified-Since header field SHOULD be ignored.
825 If the specified date is invalid, the header field MUST be ignored.
827 The result of a request having both an If-Unmodified-Since header
828 field and either an If-None-Match or an If-Modified-Since header
829 fields is undefined by this specification.
831 3.5. If-Range
833 The If-Range header field provides a special conditional request
834 mechanism that is similar to If-Match and If-Unmodified-Since but
835 specific to HTTP range requests. If-Range is defined in Section 5.3
836 of [Part5].
838 4. Status Code Definitions
840 4.1. 304 Not Modified
842 The 304 status code indicates that a conditional GET request has been
843 received and would have resulted in a 200 (OK) response if it were
844 not for the fact that the condition has evaluated to false. In other
845 words, there is no need for the server to transfer a representation
846 of the target resource because the client's request indicates that it
847 already has a valid representation, as indicated by the 304 response
848 header fields, and is therefore redirecting the client to make use of
849 that stored representation as if it were the payload of a 200
850 response. The 304 response MUST NOT contain a message-body, and thus
851 is always terminated by the first empty line after the header fields.
853 A 304 response MUST include a Date header field (Section 9.2 of
854 [Part2]) unless the origin server does not have a clock that can
855 provide a reasonable approximation of the current time. If a 200
856 response to the same request would have included any of the header
857 fields Cache-Control, Content-Location, ETag, Expires, Last-Modified,
858 or Vary, then those same header fields MUST be sent in a 304
859 response.
861 Since the goal of a 304 response is to minimize information transfer
862 when the recipient already has one or more cached representations,
863 the response SHOULD NOT include representation metadata other than
864 the above listed fields unless said metadata exists for the purpose
865 of guiding cache updates (e.g., future HTTP extensions).
867 If the recipient of a 304 response does not have a cached
868 representation corresponding to the entity-tag indicated by the 304
869 response, then the recipient MUST NOT use the 304 to update its own
870 cache. If this conditional request originated with an outbound
871 client, such as a user agent with its own cache sending a conditional
872 GET to a shared proxy, then the 304 response MAY be forwarded to the
873 outbound client. Otherwise, the recipient MUST disregard the 304
874 response and repeat the request without any preconditions.
876 If a cache uses a received 304 response to update a cache entry, the
877 cache MUST update the entry to reflect any new field values given in
878 the response.
880 4.2. 412 Precondition Failed
882 The 412 status code indicates that one or more preconditions given in
883 the request header fields evaluated to false when tested on the
884 server. This response code allows the client to place preconditions
885 on the current resource state (its current representations and
886 metadata) and thus prevent the request method from being applied if
887 the target resource is in an unexpected state.
889 5. IANA Considerations
891 5.1. Status Code Registration
893 The HTTP Status Code Registry located at
894 shall be updated
895 with the registrations below:
897 +-------+---------------------+-------------+
898 | Value | Description | Reference |
899 +-------+---------------------+-------------+
900 | 304 | Not Modified | Section 4.1 |
901 | 412 | Precondition Failed | Section 4.2 |
902 +-------+---------------------+-------------+
904 5.2. Header Field Registration
906 The Message Header Field Registry located at shall be
908 updated with the permanent registrations below (see [RFC3864]):
910 +---------------------+----------+----------+-------------+
911 | Header Field Name | Protocol | Status | Reference |
912 +---------------------+----------+----------+-------------+
913 | ETag | http | standard | Section 2.3 |
914 | If-Match | http | standard | Section 3.1 |
915 | If-Modified-Since | http | standard | Section 3.3 |
916 | If-None-Match | http | standard | Section 3.2 |
917 | If-Unmodified-Since | http | standard | Section 3.4 |
918 | Last-Modified | http | standard | Section 2.2 |
919 +---------------------+----------+----------+-------------+
921 The change controller is: "IETF (iesg@ietf.org) - Internet
922 Engineering Task Force".
924 6. Security Considerations
926 No additional security considerations have been identified beyond
927 those applicable to HTTP in general [Part1].
929 7. Acknowledgments
931 See Section 11 of [Part1].
933 8. References
935 8.1. Normative References
937 [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
938 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
939 and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
940 and Message Parsing", draft-ietf-httpbis-p1-messaging-18
941 (work in progress), January 2012.
943 [Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
944 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
945 and J. Reschke, Ed., "HTTP/1.1, part 2: Message
946 Semantics", draft-ietf-httpbis-p2-semantics-18 (work in
947 progress), January 2012.
949 [Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
950 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
951 and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
952 and Content Negotiation", draft-ietf-httpbis-p3-payload-18
953 (work in progress), January 2012.
955 [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
956 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
957 and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
958 Partial Responses", draft-ietf-httpbis-p5-range-18 (work
959 in progress), January 2012.
961 [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
962 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
963 Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
964 6: Caching", draft-ietf-httpbis-p6-cache-18 (work in
965 progress), January 2012.
967 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
968 Requirement Levels", BCP 14, RFC 2119, March 1997.
970 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
971 Specifications: ABNF", STD 68, RFC 5234, January 2008.
973 8.2. Informative References
975 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
976 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
977 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
979 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
980 Procedures for Message Header Fields", BCP 90, RFC 3864,
981 September 2004.
983 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
984 Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
986 Appendix A. Changes from RFC 2616
988 Allow weak entity-tags in all requests except range requests
989 (Sections 2.1 and 3.2).
991 Change ETag header field ABNF not to use quoted-string, thus avoiding
992 escaping issues. (Section 2.3)
994 Change ABNF productions for header fields to only define the field
995 value. (Section 3)
997 Appendix B. Collected ABNF
999 ETag = entity-tag
1001 HTTP-date =
1003 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1004 entity-tag ] ) )
1005 If-Modified-Since = HTTP-date
1006 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
1007 entity-tag ] ) )
1008 If-Unmodified-Since = HTTP-date
1010 Last-Modified = HTTP-date
1012 OWS =
1014 entity-tag = [ weak ] opaque-tag
1015 etagc = "!" / %x23-7E ; '#'-'~'
1016 / obs-text
1018 obs-text =
1019 opaque-tag = DQUOTE *etagc DQUOTE
1021 weak = %x57.2F ; W/
1023 ABNF diagnostics:
1025 ; ETag defined but not used
1026 ; If-Match defined but not used
1027 ; If-Modified-Since defined but not used
1028 ; If-None-Match defined but not used
1029 ; If-Unmodified-Since defined but not used
1030 ; Last-Modified defined but not used
1032 Appendix C. Change Log (to be removed by RFC Editor before publication)
1034 C.1. Since RFC 2616
1036 Extracted relevant partitions from [RFC2616].
1038 C.2. Since draft-ietf-httpbis-p4-conditional-00
1040 Closed issues:
1042 o : "Normative and
1043 Informative references"
1045 Other changes:
1047 o Move definitions of 304 and 412 condition codes from Part2.
1049 C.3. Since draft-ietf-httpbis-p4-conditional-01
1051 Ongoing work on ABNF conversion
1052 ():
1054 o Add explicit references to BNF syntax and rules imported from
1055 other parts of the specification.
1057 C.4. Since draft-ietf-httpbis-p4-conditional-02
1059 Closed issues:
1061 o : "Weak ETags on
1062 non-GET requests"
1064 Ongoing work on IANA Message Header Field Registration
1065 ():
1067 o Reference RFC 3984, and update header field registrations for
1068 header fields defined in this document.
1070 C.5. Since draft-ietf-httpbis-p4-conditional-03
1072 Closed issues:
1074 o : "Examples for
1075 ETag matching"
1077 o : "'entity
1078 value' undefined"
1080 o : "bogus 2068
1081 Date header reference"
1083 C.6. Since draft-ietf-httpbis-p4-conditional-04
1085 Ongoing work on ABNF conversion
1086 ():
1088 o Use "/" instead of "|" for alternatives.
1090 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1091 whitespace ("OWS") and required whitespace ("RWS").
1093 o Rewrite ABNFs to spell out whitespace rules, factor out header
1094 field value format definitions.
1096 C.7. Since draft-ietf-httpbis-p4-conditional-05
1098 Final work on ABNF conversion
1099 ():
1101 o Add appendix containing collected and expanded ABNF, reorganize
1102 ABNF introduction.
1104 C.8. Since draft-ietf-httpbis-p4-conditional-06
1106 Closed issues:
1108 o : "case-
1109 sensitivity of etag weakness indicator"
1111 C.9. Since draft-ietf-httpbis-p4-conditional-07
1113 Closed issues:
1115 o : "Weak ETags on
1116 non-GET requests" (If-Match still was defined to require strong
1117 matching)
1119 o : "move IANA
1120 registrations for optional status codes"
1122 C.10. Since draft-ietf-httpbis-p4-conditional-08
1124 No significant changes.
1126 C.11. Since draft-ietf-httpbis-p4-conditional-09
1128 No significant changes.
1130 C.12. Since draft-ietf-httpbis-p4-conditional-10
1132 Closed issues:
1134 o : "Clarify
1135 'Requested Variant'"
1137 o : "Clarify
1138 entity / representation / variant terminology"
1140 o : "consider
1141 removing the 'changes from 2068' sections"
1143 C.13. Since draft-ietf-httpbis-p4-conditional-11
1145 None.
1147 C.14. Since draft-ietf-httpbis-p4-conditional-12
1149 Closed issues:
1151 o : "Header
1152 Classification"
1154 C.15. Since draft-ietf-httpbis-p4-conditional-13
1156 Closed issues:
1158 o : "If-* and
1159 entities"
1161 o : "Definition of
1162 validator weakness"
1164 o : "untangle
1165 ABNFs for header fields"
1167 o : "ETags and
1168 Quotes"
1170 C.16. Since draft-ietf-httpbis-p4-conditional-14
1172 None.
1174 C.17. Since draft-ietf-httpbis-p4-conditional-15
1176 Closed issues:
1178 o : "If-Range
1179 should be listed when dicussing contexts where L-M can be
1180 considered strong"
1182 C.18. Since draft-ietf-httpbis-p4-conditional-16
1184 Closed issues:
1186 o : "Document
1187 HTTP's error-handling philosophy"
1189 C.19. Since draft-ietf-httpbis-p4-conditional-17
1191 Closed issues:
1193 o : "does etag
1194 value really use quoted-string"
1196 Index
1198 3
1199 304 Not Modified (status code) 19
1201 4
1202 412 Precondition Failed (status code) 20
1204 E
1205 ETag header field 10
1207 G
1208 Grammar
1209 entity-tag 10
1210 ETag 10
1211 etagc 10
1212 If-Match 15
1213 If-Modified-Since 17
1214 If-None-Match 16
1215 If-Unmodified-Since 18
1216 Last-Modified 8
1217 opaque-tag 10
1218 weak 10
1220 H
1221 Header Fields
1222 ETag 10
1223 If-Match 15
1224 If-Modified-Since 17
1225 If-None-Match 16
1226 If-Unmodified-Since 18
1227 Last-Modified 8
1229 I
1230 If-Match header field 15
1231 If-Modified-Since header field 17
1232 If-None-Match header field 16
1233 If-Unmodified-Since header field 18
1235 L
1236 Last-Modified header field 8
1238 M
1239 metadata 6
1241 S
1242 selected representation 5
1243 Status Codes
1244 304 Not Modified 19
1245 412 Precondition Failed 20
1247 V
1248 validator 6
1249 strong 6
1250 weak 6
1252 Authors' Addresses
1254 Roy T. Fielding (editor)
1255 Adobe Systems Incorporated
1256 345 Park Ave
1257 San Jose, CA 95110
1258 USA
1260 EMail: fielding@gbiv.com
1261 URI: http://roy.gbiv.com/
1263 Jim Gettys
1264 Alcatel-Lucent Bell Labs
1265 21 Oak Knoll Road
1266 Carlisle, MA 01741
1267 USA
1269 EMail: jg@freedesktop.org
1270 URI: http://gettys.wordpress.com/
1272 Jeffrey C. Mogul
1273 Hewlett-Packard Company
1274 HP Labs, Large Scale Systems Group
1275 1501 Page Mill Road, MS 1177
1276 Palo Alto, CA 94304
1277 USA
1279 EMail: JeffMogul@acm.org
1280 Henrik Frystyk Nielsen
1281 Microsoft Corporation
1282 1 Microsoft Way
1283 Redmond, WA 98052
1284 USA
1286 EMail: henrikn@microsoft.com
1288 Larry Masinter
1289 Adobe Systems Incorporated
1290 345 Park Ave
1291 San Jose, CA 95110
1292 USA
1294 EMail: LMM@acm.org
1295 URI: http://larry.masinter.net/
1297 Paul J. Leach
1298 Microsoft Corporation
1299 1 Microsoft Way
1300 Redmond, WA 98052
1302 EMail: paulle@microsoft.com
1304 Tim Berners-Lee
1305 World Wide Web Consortium
1306 MIT Computer Science and Artificial Intelligence Laboratory
1307 The Stata Center, Building 32
1308 32 Vassar Street
1309 Cambridge, MA 02139
1310 USA
1312 EMail: timbl@w3.org
1313 URI: http://www.w3.org/People/Berners-Lee/
1314 Yves Lafon (editor)
1315 World Wide Web Consortium
1316 W3C / ERCIM
1317 2004, rte des Lucioles
1318 Sophia-Antipolis, AM 06902
1319 France
1321 EMail: ylafon@w3.org
1322 URI: http://www.raubacapeu.net/people/yves/
1324 Julian F. Reschke (editor)
1325 greenbytes GmbH
1326 Hafenweg 16
1327 Muenster, NW 48155
1328 Germany
1330 Phone: +49 251 2807760
1331 Fax: +49 251 2807761
1332 EMail: julian.reschke@greenbytes.de
1333 URI: http://greenbytes.de/tech/webdav/