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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: May 3, 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 October 31, 2011
22 HTTP/1.1, part 4: Conditional Requests
23 draft-ietf-httpbis-p4-conditional-17
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.18.
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 May 3, 2012.
67 Copyright Notice
69 Copyright (c) 2011 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 . . . . . . . . . . . . . . . . . . . . . . 11
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 . . . . . . . . . . . . . . . . . . 14
112 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 15
113 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15
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 . . . . . . . . . . . . . . . . . . . 20
124 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
125 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
126 8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
127 8.2. Informative References . . . . . . . . . . . . . . . . . . 21
128 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
129 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 22
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 . . . . . . . . 23
135 C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23
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 . . . . . . . . 24
139 C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24
140 C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24
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 . . . . . . . . 25
145 C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25
146 C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25
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 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
152 1. Introduction
154 This document defines the HTTP/1.1 conditional request mechanisms,
155 including both metadata for indicating/observing changes in resource
156 representations and request header fields that specify preconditions
157 on that metadata be checked before performing the request method.
158 Conditional GET requests are the most efficient mechanism for HTTP
159 cache updates [Part6]. Conditionals can also be applied to state-
160 changing methods, such as PUT and DELETE, to prevent the "lost
161 update" problem: one client accidentally overwriting the work of
162 another client that has been acting in parallel.
164 Conditional request preconditions are based on the state of the
165 target resource as a whole (its current value set) or the state as
166 observed in a previously obtained representation (one value in that
167 set). A resource might have multiple current representations, each
168 with its own observable state. The conditional request mechanisms
169 assume that the mapping of requests to corresponding representations
170 will be consistent over time if the server intends to take advantage
171 of conditionals. Regardless, if the mapping is inconsistent and the
172 server is unable to select the appropriate representation, then no
173 harm will result when the precondition evaluates to false.
175 We use the term "selected representation" to refer to the current
176 representation of the target resource that would have been selected
177 in a successful response if the same request had used the method GET
178 and had excluded all of the conditional request header fields. The
179 conditional request preconditions are evaluated by comparing the
180 values provided in the request header fields to the current metadata
181 for the selected representation.
183 1.1. Conformance and Error Handling
185 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
186 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
187 document are to be interpreted as described in [RFC2119].
189 This document defines conformance criteria for several roles in HTTP
190 communication, including Senders, Recipients, Clients, Servers, User-
191 Agents, Origin Servers, Intermediaries, Proxies and Gateways. See
192 Section 2 of [Part1] for definitions of these terms.
194 An implementation is considered conformant if it complies with all of
195 the requirements associated with its role(s). Note that SHOULD-level
196 requirements are relevant here, unless one of the documented
197 exceptions is applicable.
199 This document also uses ABNF to define valid protocol elements
200 (Section 1.2). In addition to the prose requirements placed upon
201 them, Senders MUST NOT generate protocol elements that are invalid.
203 Unless noted otherwise, Recipients MAY take steps to recover a usable
204 protocol element from an invalid construct. However, HTTP does not
205 define specific error handling mechanisms, except in cases where it
206 has direct impact on security. This is because different uses of the
207 protocol require different error handling strategies; for example, a
208 Web browser may wish to transparently recover from a response where
209 the Location header field doesn't parse according to the ABNF,
210 whereby in a systems control protocol using HTTP, this type of error
211 recovery could lead to dangerous consequences.
213 1.2. Syntax Notation
215 This specification uses the ABNF syntax defined in Section 1.2 of
216 [Part1] (which extends the syntax defined in [RFC5234] with a list
217 rule). Appendix B shows the collected ABNF, with the list rule
218 expanded.
220 The following core rules are included by reference, as defined in
221 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
222 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
223 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
224 sequence of data), SP (space), and VCHAR (any visible US-ASCII
225 character).
227 The ABNF rules below are defined in [Part1] and [Part2]:
229 OWS =
230 quoted-string =
231 HTTP-date =
233 2. Validators
235 This specification defines two forms of metadata that are commonly
236 used to observe resource state and test for preconditions:
237 modification dates and opaque entity tags. Additional metadata that
238 reflects resource state has been defined by various extensions of
239 HTTP, such as WebDAV [RFC4918], that are beyond the scope of this
240 specification. A resource metadata value is referred to as a
241 "validator" when it is used within a precondition.
243 2.1. Weak versus Strong
245 Validators come in two flavors: strong or weak. Weak validators are
246 easy to generate but are far less useful for comparisons. Strong
247 validators are ideal for comparisons but can be very difficult (and
248 occasionally impossible) to generate efficiently. Rather than impose
249 that all forms of resource adhere to the same strength of validator,
250 HTTP exposes the type of validator in use and imposes restrictions on
251 when weak validators can be used as preconditions.
253 A "strong validator" is a representation metadata value that MUST be
254 changed to a new, previously unused or guaranteed unique, value
255 whenever a change occurs to the representation data such that a
256 change would be observable in the payload body of a 200 response to
257 GET. A strong validator MAY be changed for other reasons, such as
258 when a semantically significant part of the representation metadata
259 is changed (e.g., Content-Type), but it is in the best interests of
260 the origin server to only change the value when it is necessary to
261 invalidate the stored responses held by remote caches and authoring
262 tools. A strong validator MUST be unique across all representations
263 of a given resource, such that no two representations of that
264 resource share the same validator unless their payload body would be
265 identical.
267 Cache entries might persist for arbitrarily long periods, regardless
268 of expiration times. Thus, a cache might attempt to validate an
269 entry using a validator that it obtained in the distant past. A
270 strong validator MUST be unique across all versions of all
271 representations associated with a particular resource over time.
272 However, there is no implication of uniqueness across representations
273 of different resources (i.e., the same strong validator might be in
274 use for representations of multiple resources at the same time and
275 does not imply that those representations are equivalent).
277 There are a variety of strong validators used in practice. The best
278 are based on strict revision control, wherein each change to a
279 representation always results in a unique node name and revision
280 identifier being assigned before the representation is made
281 accessible to GET. A cryptographic hash function applied to the
282 representation data is also sufficient if the data is available prior
283 to the response header fields being sent and the digest does not need
284 to be recalculated every time a validation request is received.
285 However, if a resource has distinct representations that differ only
286 in their metadata, such as might occur with content negotiation over
287 media types that happen to share the same data format, then a server
288 SHOULD incorporate additional information in the validator to
289 distinguish those representations and avoid confusing cache behavior.
291 In contrast, a "weak validator" is a representation metadata value
292 that might not be changed for every change to the representation
293 data. This weakness might be due to limitations in how the value is
294 calculated, such as clock resolution or an inability to ensure
295 uniqueness for all possible representations of the resource, or due
296 to a desire by the resource owner to group representations by some
297 self-determined set of equivalency rather than unique sequences of
298 data. A weak entity-tag SHOULD change whenever the origin server
299 considers prior representations to be unacceptable as a substitute
300 for the current representation. In other words, a weak entity-tag
301 SHOULD change whenever the origin server wants caches to invalidate
302 old responses.
304 For example, the representation of a weather report that changes in
305 content every second, based on dynamic measurements, might be grouped
306 into sets of equivalent representations (from the origin server's
307 perspective) with the same weak validator in order to allow cached
308 representations to be valid for a reasonable period of time (perhaps
309 adjusted dynamically based on server load or weather quality).
310 Likewise, a representation's modification time, if defined with only
311 one-second resolution, might be a weak validator if it is possible
312 for the representation to be modified twice during a single second
313 and retrieved between those modifications.
315 A "use" of a validator occurs when either a client generates a
316 request and includes the validator in a precondition or when a server
317 compares two validators. Weak validators are only usable in contexts
318 that do not depend on exact equality of a representation's payload
319 body. Strong validators are usable and preferred for all conditional
320 requests, including cache validation, partial content ranges, and
321 "lost update" avoidance.
323 2.2. Last-Modified
325 The "Last-Modified" header field indicates the date and time at which
326 the origin server believes the selected representation was last
327 modified.
329 Last-Modified = HTTP-date
331 An example of its use is
333 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
335 2.2.1. Generation
337 Origin servers SHOULD send Last-Modified for any selected
338 representation for which a last modification date can be reasonably
339 and consistently determined, since its use in conditional requests
340 and evaluating cache freshness ([Part6]) results in a substantial
341 reduction of HTTP traffic on the Internet and can be a significant
342 factor in improving service scalability and reliability.
344 A representation is typically the sum of many parts behind the
345 resource interface. The last-modified time would usually be the most
346 recent time that any of those parts were changed. How that value is
347 determined for any given resource is an implementation detail beyond
348 the scope of this specification. What matters to HTTP is how
349 recipients of the Last-Modified header field can use its value to
350 make conditional requests and test the validity of locally cached
351 responses.
353 An origin server SHOULD obtain the Last-Modified value of the
354 representation as close as possible to the time that it generates the
355 Date field-value for its response. This allows a recipient to make
356 an accurate assessment of the representation's modification time,
357 especially if the representation changes near the time that the
358 response is generated.
360 An origin server with a clock MUST NOT send a Last-Modified date that
361 is later than the server's time of message origination (Date). If
362 the last modification time is derived from implementation-specific
363 metadata that evaluates to some time in the future, according to the
364 origin server's clock, then the origin server MUST replace that value
365 with the message origination date. This prevents a future
366 modification date from having an adverse impact on cache validation.
368 An origin server without a clock MUST NOT assign Last-Modified values
369 to a response unless these values were associated with the resource
370 by some other system or user with a reliable clock.
372 2.2.2. Comparison
374 A Last-Modified time, when used as a validator in a request, is
375 implicitly weak unless it is possible to deduce that it is strong,
376 using the following rules:
378 o The validator is being compared by an origin server to the actual
379 current validator for the representation and,
381 o That origin server reliably knows that the associated
382 representation did not change twice during the second covered by
383 the presented validator.
385 or
387 o The validator is about to be used by a client in an If-Modified-
388 Since, If-Unmodified-Since header field, because the client has a
389 cache entry, or If-Range for the associated representation, and
391 o That cache entry includes a Date value, which gives the time when
392 the origin server sent the original response, and
394 o The presented Last-Modified time is at least 60 seconds before the
395 Date value.
397 or
399 o The validator is being compared by an intermediate cache to the
400 validator stored in its cache entry for the representation, and
402 o That cache entry includes a Date value, which gives the time when
403 the origin server sent the original response, and
405 o The presented Last-Modified time is at least 60 seconds before the
406 Date value.
408 This method relies on the fact that if two different responses were
409 sent by the origin server during the same second, but both had the
410 same Last-Modified time, then at least one of those responses would
411 have a Date value equal to its Last-Modified time. The arbitrary 60-
412 second limit guards against the possibility that the Date and Last-
413 Modified values are generated from different clocks, or at somewhat
414 different times during the preparation of the response. An
415 implementation MAY use a value larger than 60 seconds, if it is
416 believed that 60 seconds is too short.
418 2.3. ETag
420 The ETag header field provides the current entity-tag for the
421 selected representation. An entity-tag is an opaque validator for
422 differentiating between multiple representations of the same
423 resource, regardless of whether those multiple representations are
424 due to resource state changes over time, content negotiation
425 resulting in multiple representations being valid at the same time,
426 or both. An entity-tag consists of an opaque quoted string, possibly
427 prefixed by a weakness indicator.
429 ETag = entity-tag
431 entity-tag = [ weak ] opaque-tag
432 weak = %x57.2F ; "W/", case-sensitive
433 opaque-tag = quoted-string
435 An entity-tag can be more reliable for validation than a modification
436 date in situations where it is inconvenient to store modification
437 dates, where the one-second resolution of HTTP date values is not
438 sufficient, or where modification dates are not consistently
439 maintained.
441 Examples:
443 ETag: "xyzzy"
444 ETag: W/"xyzzy"
445 ETag: ""
447 An entity-tag can be either a weak or strong validator, with strong
448 being the default. If an origin server provides an entity-tag for a
449 representation and the generation of that entity-tag does not satisfy
450 the requirements for a strong validator (Section 2.1), then that
451 entity-tag MUST be marked as weak by prefixing its opaque value with
452 "W/" (case-sensitive).
454 2.3.1. Generation
456 The principle behind entity-tags is that only the service author
457 knows the implementation of a resource well enough to select the most
458 accurate and efficient validation mechanism for that resource, and
459 that any such mechanism can be mapped to a simple sequence of octets
460 for easy comparison. Since the value is opaque, there is no need for
461 the client to be aware of how each entity-tag is constructed.
463 For example, a resource that has implementation-specific versioning
464 applied to all changes might use an internal revision number, perhaps
465 combined with a variance identifier for content negotiation, to
466 accurately differentiate between representations. Other
467 implementations might use a stored hash of representation content, a
468 combination of various filesystem attributes, or a modification
469 timestamp that has sub-second resolution.
471 Origin servers SHOULD send ETag for any selected representation for
472 which detection of changes can be reasonably and consistently
473 determined, since the entity-tag's use in conditional requests and
474 evaluating cache freshness ([Part6]) can result in a substantial
475 reduction of HTTP network traffic and can be a significant factor in
476 improving service scalability and reliability.
478 2.3.2. Comparison
480 There are two entity-tag comparison functions, depending on whether
481 the comparison context allows the use of weak validators or not:
483 o The strong comparison function: in order to be considered equal,
484 both opaque-tags MUST be identical character-by-character, and
485 both MUST NOT be weak.
487 o The weak comparison function: in order to be considered equal,
488 both opaque-tags MUST be identical character-by-character, but
489 either or both of them MAY be tagged as "weak" without affecting
490 the result.
492 The example below shows the results for a set of entity-tag pairs,
493 and both the weak and strong comparison function results:
495 +--------+--------+-------------------+-----------------+
496 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
497 +--------+--------+-------------------+-----------------+
498 | W/"1" | W/"1" | no match | match |
499 | W/"1" | W/"2" | no match | no match |
500 | W/"1" | "1" | no match | match |
501 | "1" | "1" | match | match |
502 +--------+--------+-------------------+-----------------+
504 2.3.3. Example: Entity-tags varying on Content-Negotiated Resources
506 Consider a resource that is subject to content negotiation (Section 5
507 of [Part3]), and where the representations returned upon a GET
508 request vary based on the Accept-Encoding request header field
509 (Section 6.3 of [Part3]):
511 >> Request:
513 GET /index HTTP/1.1
514 Host: www.example.com
515 Accept-Encoding: gzip
517 In this case, the response might or might not use the gzip content
518 coding. If it does not, the response might look like:
520 >> Response:
522 HTTP/1.1 200 OK
523 Date: Thu, 26 Mar 2010 00:05:00 GMT
524 ETag: "123-a"
525 Content-Length: 70
526 Vary: Accept-Encoding
527 Content-Type: text/plain
529 Hello World!
530 Hello World!
531 Hello World!
532 Hello World!
533 Hello World!
535 An alternative representation that does use gzip content coding would
536 be:
538 >> Response:
540 HTTP/1.1 200 OK
541 Date: Thu, 26 Mar 2010 00:05:00 GMT
542 ETag: "123-b"
543 Content-Length: 43
544 Vary: Accept-Encoding
545 Content-Type: text/plain
546 Content-Encoding: gzip
548 ...binary data...
550 Note: Content codings are a property of the representation, so
551 therefore an entity-tag of an encoded representation must be
552 distinct from an unencoded representation to prevent conflicts
553 during cache updates and range requests. In contrast, transfer
554 codings (Section 5.1 of [Part1]) apply only during message
555 transfer and do not require distinct entity-tags.
557 2.4. Rules for When to Use Entity-tags and Last-Modified Dates
559 We adopt a set of rules and recommendations for origin servers,
560 clients, and caches regarding when various validator types ought to
561 be used, and for what purposes.
563 HTTP/1.1 origin servers:
565 o SHOULD send an entity-tag validator unless it is not feasible to
566 generate one.
568 o MAY send a weak entity-tag instead of a strong entity-tag, if
569 performance considerations support the use of weak entity-tags, or
570 if it is unfeasible to send a strong entity-tag.
572 o SHOULD send a Last-Modified value if it is feasible to send one.
574 In other words, the preferred behavior for an HTTP/1.1 origin server
575 is to send both a strong entity-tag and a Last-Modified value.
577 HTTP/1.1 clients:
579 o MUST use that entity-tag in any cache-conditional request (using
580 If-Match or If-None-Match) if an entity-tag has been provided by
581 the origin server.
583 o SHOULD use the Last-Modified value in non-subrange cache-
584 conditional requests (using If-Modified-Since) if only a Last-
585 Modified value has been provided by the origin server.
587 o MAY use the Last-Modified value in subrange cache-conditional
588 requests (using If-Unmodified-Since) if only a Last-Modified value
589 has been provided by an HTTP/1.0 origin server. The user agent
590 SHOULD provide a way to disable this, in case of difficulty.
592 o SHOULD use both validators in cache-conditional requests if both
593 an entity-tag and a Last-Modified value have been provided by the
594 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
595 respond appropriately.
597 An HTTP/1.1 origin server, upon receiving a conditional request that
598 includes both a Last-Modified date (e.g., in an If-Modified-Since or
599 If-Unmodified-Since header field) and one or more entity-tags (e.g.,
600 in an If-Match, If-None-Match, or If-Range header field) as cache
601 validators, MUST NOT return a response status code of 304 (Not
602 Modified) unless doing so is consistent with all of the conditional
603 header fields in the request.
605 An HTTP/1.1 caching proxy, upon receiving a conditional request that
606 includes both a Last-Modified date and one or more entity-tags as
607 cache validators, MUST NOT return a locally cached response to the
608 client unless that cached response is consistent with all of the
609 conditional header fields in the request.
611 Note: The general principle behind these rules is that HTTP/1.1
612 servers and clients ought to transmit as much non-redundant
613 information as is available in their responses and requests.
614 HTTP/1.1 systems receiving this information will make the most
615 conservative assumptions about the validators they receive.
617 HTTP/1.0 clients and caches might ignore entity-tags. Generally,
618 last-modified values received or used by these systems will
619 support transparent and efficient caching, and so HTTP/1.1 origin
620 servers should provide Last-Modified values. In those rare cases
621 where the use of a Last-Modified value as a validator by an
622 HTTP/1.0 system could result in a serious problem, then HTTP/1.1
623 origin servers should not provide one.
625 3. Precondition Header Fields
627 This section defines the syntax and semantics of HTTP/1.1 header
628 fields for applying preconditions on requests.
630 3.1. If-Match
632 The "If-Match" header field MAY be used to make a request method
633 conditional on the current existence or value of an entity-tag for
634 one or more representations of the target resource. If-Match is
635 generally useful for resource update requests, such as PUT requests,
636 as a means for protecting against accidental overwrites when multiple
637 clients are acting in parallel on the same resource (i.e., the "lost
638 update" problem). An If-Match field-value of "*" places the
639 precondition on the existence of any current representation for the
640 target resource.
642 If-Match = "*" / 1#entity-tag
644 If any of the entity-tags listed in the If-Match field value match
645 (as per Section 2.3.2) the entity-tag of the selected representation
646 for the target resource, or if "*" is given and any current
647 representation exists for the target resource, then the server MAY
648 perform the request method as if the If-Match header field was not
649 present.
651 If none of the entity-tags match, or if "*" is given and no current
652 representation exists, the server MUST NOT perform the requested
653 method. Instead, the server MUST respond with the 412 (Precondition
654 Failed) status code.
656 If the request would, without the If-Match header field, result in
657 anything other than a 2xx or 412 status code, then the If-Match
658 header field MUST be ignored.
660 Examples:
662 If-Match: "xyzzy"
663 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
664 If-Match: *
666 The result of a request having both an If-Match header field and
667 either an If-None-Match or an If-Modified-Since header fields is
668 undefined by this specification.
670 3.2. If-None-Match
672 The "If-None-Match" header field MAY be used to make a request method
673 conditional on not matching any of the current entity-tag values for
674 representations of the target resource. If-None-Match is primarily
675 used in conditional GET requests to enable efficient updates of
676 cached information with a minimum amount of transaction overhead. A
677 client that has one or more representations previously obtained from
678 the target resource can send If-None-Match with a list of the
679 associated entity-tags in the hope of receiving a 304 response if at
680 least one of those representations matches the selected
681 representation.
683 If-None-Match MAY also be used with a value of "*" to prevent an
684 unsafe request method (e.g., PUT) from inadvertently modifying an
685 existing representation of the target resource when the client
686 believes that the resource does not have a current representation.
687 This is a variation on the "lost update" problem that might arise if
688 more than one client attempts to create an initial representation for
689 the target resource.
691 If-None-Match = "*" / 1#entity-tag
693 If any of the entity-tags listed in the If-None-Match field-value
694 match (as per Section 2.3.2) the entity-tag of the selected
695 representation, or if "*" is given and any current representation
696 exists for that resource, then the server MUST NOT perform the
697 requested method. Instead, if the request method was GET or HEAD,
698 the server SHOULD respond with a 304 (Not Modified) status code,
699 including the cache-related header fields (particularly ETag) of the
700 selected representation that has a matching entity-tag. For all
701 other request methods, the server MUST respond with a 412
702 (Precondition Failed) status code.
704 If none of the entity-tags match, then the server MAY perform the
705 requested method as if the If-None-Match header field did not exist,
706 but MUST also ignore any If-Modified-Since header field(s) in the
707 request. That is, if no entity-tags match, then the server MUST NOT
708 return a 304 (Not Modified) response.
710 If the request would, without the If-None-Match header field, result
711 in anything other than a 2xx or 304 status code, then the If-None-
712 Match header field MUST be ignored. (See Section 2.4 for a
713 discussion of server behavior when both If-Modified-Since and If-
714 None-Match appear in the same request.)
716 Examples:
718 If-None-Match: "xyzzy"
719 If-None-Match: W/"xyzzy"
720 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
721 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
722 If-None-Match: *
724 The result of a request having both an If-None-Match header field and
725 either an If-Match or an If-Unmodified-Since header fields is
726 undefined by this specification.
728 3.3. If-Modified-Since
730 The "If-Modified-Since" header field MAY be used to make a request
731 method conditional by modification date: if the selected
732 representation has not been modified since the time specified in this
733 field, then do not perform the request method; instead, respond as
734 detailed below.
736 If-Modified-Since = HTTP-date
738 An example of the field is:
740 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
742 A GET method with an If-Modified-Since header field and no Range
743 header field requests that the selected representation be transferred
744 only if it has been modified since the date given by the If-Modified-
745 Since header field. The algorithm for determining this includes the
746 following cases:
748 1. If the request would normally result in anything other than a 200
749 (OK) status code, or if the passed If-Modified-Since date is
750 invalid, the response is exactly the same as for a normal GET. A
751 date which is later than the server's current time is invalid.
753 2. If the selected representation has been modified since the If-
754 Modified-Since date, the response is exactly the same as for a
755 normal GET.
757 3. If the selected representation has not been modified since a
758 valid If-Modified-Since date, the server SHOULD return a 304 (Not
759 Modified) response.
761 The purpose of this feature is to allow efficient updates of cached
762 information with a minimum amount of transaction overhead.
764 Note: The Range header field modifies the meaning of If-Modified-
765 Since; see Section 5.4 of [Part5] for full details.
767 Note: If-Modified-Since times are interpreted by the server, whose
768 clock might not be synchronized with the client.
770 Note: When handling an If-Modified-Since header field, some
771 servers will use an exact date comparison function, rather than a
772 less-than function, for deciding whether to send a 304 (Not
773 Modified) response. To get best results when sending an If-
774 Modified-Since header field for cache validation, clients are
775 advised to use the exact date string received in a previous Last-
776 Modified header field whenever possible.
778 Note: If a client uses an arbitrary date in the If-Modified-Since
779 header field instead of a date taken from the Last-Modified header
780 field for the same request, the client needs to be aware that this
781 date is interpreted in the server's understanding of time.
782 Unsynchronized clocks and rounding problems, due to the different
783 encodings of time between the client and server, are concerns.
784 This includes the possibility of race conditions if the document
785 has changed between the time it was first requested and the If-
786 Modified-Since date of a subsequent request, and the possibility
787 of clock-skew-related problems if the If-Modified-Since date is
788 derived from the client's clock without correction to the server's
789 clock. Corrections for different time bases between client and
790 server are at best approximate due to network latency.
792 The result of a request having both an If-Modified-Since header field
793 and either an If-Match or an If-Unmodified-Since header fields is
794 undefined by this specification.
796 3.4. If-Unmodified-Since
798 The "If-Unmodified-Since" header field MAY be used to make a request
799 method conditional by modification date: if the selected
800 representation has been modified since the time specified in this
801 field, then the server MUST NOT perform the requested operation and
802 MUST instead respond with the 412 (Precondition Failed) status code.
803 If the selected representation has not been modified since the time
804 specified in this field, the server SHOULD perform the request method
805 as if the If-Unmodified-Since header field were not present.
807 If-Unmodified-Since = HTTP-date
809 An example of the field is:
811 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
813 If the request normally (i.e., without the If-Unmodified-Since header
814 field) would result in anything other than a 2xx or 412 status code,
815 the If-Unmodified-Since header field SHOULD be ignored.
817 If the specified date is invalid, the header field MUST be ignored.
819 The result of a request having both an If-Unmodified-Since header
820 field and either an If-None-Match or an If-Modified-Since header
821 fields is undefined by this specification.
823 3.5. If-Range
825 The If-Range header field provides a special conditional request
826 mechanism that is similar to If-Match and If-Unmodified-Since but
827 specific to HTTP range requests. If-Range is defined in Section 5.3
828 of [Part5].
830 4. Status Code Definitions
832 4.1. 304 Not Modified
834 The 304 status code indicates that a conditional GET request has been
835 received and would have resulted in a 200 (OK) response if it were
836 not for the fact that the condition has evaluated to false. In other
837 words, there is no need for the server to transfer a representation
838 of the target resource because the client's request indicates that it
839 already has a valid representation, as indicated by the 304 response
840 header fields, and is therefore redirecting the client to make use of
841 that stored representation as if it were the payload of a 200
842 response. The 304 response MUST NOT contain a message-body, and thus
843 is always terminated by the first empty line after the header fields.
845 A 304 response MUST include a Date header field (Section 9.2 of
846 [Part2]) unless the origin server does not have a clock that can
847 provide a reasonable approximation of the current time. If a 200
848 response to the same request would have included any of the header
849 fields Cache-Control, Content-Location, ETag, Expires, Last-Modified,
850 or Vary, then those same header fields MUST be sent in a 304
851 response.
853 Since the goal of a 304 response is to minimize information transfer
854 when the recipient already has one or more cached representations,
855 the response SHOULD NOT include representation metadata other than
856 the above listed fields unless said metadata exists for the purpose
857 of guiding cache updates (e.g., future HTTP extensions).
859 If the recipient of a 304 response does not have a cached
860 representation corresponding to the entity-tag indicated by the 304
861 response, then the recipient MUST NOT use the 304 to update its own
862 cache. If this conditional request originated with an outbound
863 client, such as a user agent with its own cache sending a conditional
864 GET to a shared proxy, then the 304 response MAY be forwarded to the
865 outbound client. Otherwise, the recipient MUST disregard the 304
866 response and repeat the request without any preconditions.
868 If a cache uses a received 304 response to update a cache entry, the
869 cache MUST update the entry to reflect any new field values given in
870 the response.
872 4.2. 412 Precondition Failed
874 The 412 status code indicates that one or more preconditions given in
875 the request header fields evaluated to false when tested on the
876 server. This response code allows the client to place preconditions
877 on the current resource state (its current representations and
878 metadata) and thus prevent the request method from being applied if
879 the target resource is in an unexpected state.
881 5. IANA Considerations
883 5.1. Status Code Registration
885 The HTTP Status Code Registry located at
886 shall be updated
887 with the registrations below:
889 +-------+---------------------+-------------+
890 | Value | Description | Reference |
891 +-------+---------------------+-------------+
892 | 304 | Not Modified | Section 4.1 |
893 | 412 | Precondition Failed | Section 4.2 |
894 +-------+---------------------+-------------+
896 5.2. Header Field Registration
898 The Message Header Field Registry located at shall be
900 updated with the permanent registrations below (see [RFC3864]):
902 +---------------------+----------+----------+-------------+
903 | Header Field Name | Protocol | Status | Reference |
904 +---------------------+----------+----------+-------------+
905 | ETag | http | standard | Section 2.3 |
906 | If-Match | http | standard | Section 3.1 |
907 | If-Modified-Since | http | standard | Section 3.3 |
908 | If-None-Match | http | standard | Section 3.2 |
909 | If-Unmodified-Since | http | standard | Section 3.4 |
910 | Last-Modified | http | standard | Section 2.2 |
911 +---------------------+----------+----------+-------------+
913 The change controller is: "IETF (iesg@ietf.org) - Internet
914 Engineering Task Force".
916 6. Security Considerations
918 No additional security considerations have been identified beyond
919 those applicable to HTTP in general [Part1].
921 7. Acknowledgments
923 See Section 11 of [Part1].
925 8. References
927 8.1. Normative References
929 [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
930 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
931 and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
932 and Message Parsing", draft-ietf-httpbis-p1-messaging-17
933 (work in progress), October 2011.
935 [Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
936 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
937 and J. Reschke, Ed., "HTTP/1.1, part 2: Message
938 Semantics", draft-ietf-httpbis-p2-semantics-17 (work in
939 progress), October 2011.
941 [Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
942 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
943 and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
944 and Content Negotiation", draft-ietf-httpbis-p3-payload-17
945 (work in progress), October 2011.
947 [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
948 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
949 and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
950 Partial Responses", draft-ietf-httpbis-p5-range-17 (work
951 in progress), October 2011.
953 [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
954 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
955 Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
956 6: Caching", draft-ietf-httpbis-p6-cache-17 (work in
957 progress), October 2011.
959 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
960 Requirement Levels", BCP 14, RFC 2119, March 1997.
962 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
963 Specifications: ABNF", STD 68, RFC 5234, January 2008.
965 8.2. Informative References
967 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
968 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
969 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
971 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
972 Procedures for Message Header Fields", BCP 90, RFC 3864,
973 September 2004.
975 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
976 Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
978 Appendix A. Changes from RFC 2616
980 Allow weak entity-tags in all requests except range requests
981 (Sections 2.1 and 3.2).
983 Change ABNF productions for header fields to only define the field
984 value. (Section 3)
986 Appendix B. Collected ABNF
988 ETag = entity-tag
990 HTTP-date =
992 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
993 entity-tag ] ) )
994 If-Modified-Since = HTTP-date
995 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
996 entity-tag ] ) )
997 If-Unmodified-Since = HTTP-date
999 Last-Modified = HTTP-date
1001 OWS =
1003 entity-tag = [ weak ] opaque-tag
1005 opaque-tag = quoted-string
1007 quoted-string =
1009 weak = %x57.2F ; W/
1010 ABNF diagnostics:
1012 ; ETag defined but not used
1013 ; If-Match defined but not used
1014 ; If-Modified-Since defined but not used
1015 ; If-None-Match defined but not used
1016 ; If-Unmodified-Since defined but not used
1017 ; Last-Modified defined but not used
1019 Appendix C. Change Log (to be removed by RFC Editor before publication)
1021 C.1. Since RFC 2616
1023 Extracted relevant partitions from [RFC2616].
1025 C.2. Since draft-ietf-httpbis-p4-conditional-00
1027 Closed issues:
1029 o : "Normative and
1030 Informative references"
1032 Other changes:
1034 o Move definitions of 304 and 412 condition codes from Part2.
1036 C.3. Since draft-ietf-httpbis-p4-conditional-01
1038 Ongoing work on ABNF conversion
1039 ():
1041 o Add explicit references to BNF syntax and rules imported from
1042 other parts of the specification.
1044 C.4. Since draft-ietf-httpbis-p4-conditional-02
1046 Closed issues:
1048 o : "Weak ETags on
1049 non-GET requests"
1051 Ongoing work on IANA Message Header Field Registration
1052 ():
1054 o Reference RFC 3984, and update header field registrations for
1055 header fields defined in this document.
1057 C.5. Since draft-ietf-httpbis-p4-conditional-03
1059 Closed issues:
1061 o : "Examples for
1062 ETag matching"
1064 o : "'entity
1065 value' undefined"
1067 o : "bogus 2068
1068 Date header reference"
1070 C.6. Since draft-ietf-httpbis-p4-conditional-04
1072 Ongoing work on ABNF conversion
1073 ():
1075 o Use "/" instead of "|" for alternatives.
1077 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1078 whitespace ("OWS") and required whitespace ("RWS").
1080 o Rewrite ABNFs to spell out whitespace rules, factor out header
1081 field value format definitions.
1083 C.7. Since draft-ietf-httpbis-p4-conditional-05
1085 Final work on ABNF conversion
1086 ():
1088 o Add appendix containing collected and expanded ABNF, reorganize
1089 ABNF introduction.
1091 C.8. Since draft-ietf-httpbis-p4-conditional-06
1093 Closed issues:
1095 o : "case-
1096 sensitivity of etag weakness indicator"
1098 C.9. Since draft-ietf-httpbis-p4-conditional-07
1100 Closed issues:
1102 o : "Weak ETags on
1103 non-GET requests" (If-Match still was defined to require strong
1104 matching)
1106 o : "move IANA
1107 registrations for optional status codes"
1109 C.10. Since draft-ietf-httpbis-p4-conditional-08
1111 No significant changes.
1113 C.11. Since draft-ietf-httpbis-p4-conditional-09
1115 No significant changes.
1117 C.12. Since draft-ietf-httpbis-p4-conditional-10
1119 Closed issues:
1121 o : "Clarify
1122 'Requested Variant'"
1124 o : "Clarify
1125 entity / representation / variant terminology"
1127 o : "consider
1128 removing the 'changes from 2068' sections"
1130 C.13. Since draft-ietf-httpbis-p4-conditional-11
1132 None.
1134 C.14. Since draft-ietf-httpbis-p4-conditional-12
1136 Closed issues:
1138 o : "Header
1139 Classification"
1141 C.15. Since draft-ietf-httpbis-p4-conditional-13
1143 Closed issues:
1145 o : "If-* and
1146 entities"
1148 o : "Definition of
1149 validator weakness"
1151 o : "untangle
1152 ABNFs for header fields"
1154 o : "ETags and
1155 Quotes"
1157 C.16. Since draft-ietf-httpbis-p4-conditional-14
1159 None.
1161 C.17. Since draft-ietf-httpbis-p4-conditional-15
1163 Closed issues:
1165 o : "If-Range
1166 should be listed when dicussing contexts where L-M can be
1167 considered strong"
1169 C.18. Since draft-ietf-httpbis-p4-conditional-16
1171 Closed issues:
1173 o : "Document
1174 HTTP's error-handling philosophy"
1176 Index
1178 3
1179 304 Not Modified (status code) 19
1181 4
1182 412 Precondition Failed (status code) 20
1184 E
1185 ETag header field 10
1187 G
1188 Grammar
1189 entity-tag 10
1190 ETag 10
1191 If-Match 15
1192 If-Modified-Since 17
1193 If-None-Match 16
1194 If-Unmodified-Since 18
1195 Last-Modified 8
1196 opaque-tag 10
1197 weak 10
1199 H
1200 Header Fields
1201 ETag 10
1202 If-Match 15
1203 If-Modified-Since 17
1204 If-None-Match 15
1205 If-Unmodified-Since 18
1206 Last-Modified 8
1208 I
1209 If-Match header field 15
1210 If-Modified-Since header field 17
1211 If-None-Match header field 15
1212 If-Unmodified-Since header field 18
1214 L
1215 Last-Modified header field 8
1217 M
1218 metadata 6
1220 S
1221 selected representation 5
1222 Status Codes
1223 304 Not Modified 19
1224 412 Precondition Failed 20
1226 V
1227 validator 6
1228 strong 6
1229 weak 6
1231 Authors' Addresses
1233 Roy T. Fielding (editor)
1234 Adobe Systems Incorporated
1235 345 Park Ave
1236 San Jose, CA 95110
1237 USA
1239 EMail: fielding@gbiv.com
1240 URI: http://roy.gbiv.com/
1241 Jim Gettys
1242 Alcatel-Lucent Bell Labs
1243 21 Oak Knoll Road
1244 Carlisle, MA 01741
1245 USA
1247 EMail: jg@freedesktop.org
1248 URI: http://gettys.wordpress.com/
1250 Jeffrey C. Mogul
1251 Hewlett-Packard Company
1252 HP Labs, Large Scale Systems Group
1253 1501 Page Mill Road, MS 1177
1254 Palo Alto, CA 94304
1255 USA
1257 EMail: JeffMogul@acm.org
1259 Henrik Frystyk Nielsen
1260 Microsoft Corporation
1261 1 Microsoft Way
1262 Redmond, WA 98052
1263 USA
1265 EMail: henrikn@microsoft.com
1267 Larry Masinter
1268 Adobe Systems Incorporated
1269 345 Park Ave
1270 San Jose, CA 95110
1271 USA
1273 EMail: LMM@acm.org
1274 URI: http://larry.masinter.net/
1276 Paul J. Leach
1277 Microsoft Corporation
1278 1 Microsoft Way
1279 Redmond, WA 98052
1281 EMail: paulle@microsoft.com
1282 Tim Berners-Lee
1283 World Wide Web Consortium
1284 MIT Computer Science and Artificial Intelligence Laboratory
1285 The Stata Center, Building 32
1286 32 Vassar Street
1287 Cambridge, MA 02139
1288 USA
1290 EMail: timbl@w3.org
1291 URI: http://www.w3.org/People/Berners-Lee/
1293 Yves Lafon (editor)
1294 World Wide Web Consortium
1295 W3C / ERCIM
1296 2004, rte des Lucioles
1297 Sophia-Antipolis, AM 06902
1298 France
1300 EMail: ylafon@w3.org
1301 URI: http://www.raubacapeu.net/people/yves/
1303 Julian F. Reschke (editor)
1304 greenbytes GmbH
1305 Hafenweg 16
1306 Muenster, NW 48155
1307 Germany
1309 Phone: +49 251 2807760
1310 Fax: +49 251 2807761
1311 EMail: julian.reschke@greenbytes.de
1312 URI: http://greenbytes.de/tech/webdav/