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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: February 25, 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 August 24, 2011
22 HTTP/1.1, part 4: Conditional Requests
23 draft-ietf-httpbis-p4-conditional-16
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.17.
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 February 25, 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. Requirements . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . 14
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 . . . . . . . . 23
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 . . . . . . . . 24
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 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
151 1. Introduction
153 This document defines the HTTP/1.1 conditional request mechanisms,
154 including both metadata for indicating/observing changes in resource
155 representations and request header fields that specify preconditions
156 on that metadata be checked before performing the request method.
157 Conditional GET requests are the most efficient mechanism for HTTP
158 cache updates [Part6]. Conditionals can also be applied to state-
159 changing methods, such as PUT and DELETE, to prevent the "lost
160 update" problem: one client accidentally overwriting the work of
161 another client that has been acting in parallel.
163 Conditional request preconditions are based on the state of the
164 target resource as a whole (its current value set) or the state as
165 observed in a previously obtained representation (one value in that
166 set). A resource might have multiple current representations, each
167 with its own observable state. The conditional request mechanisms
168 assume that the mapping of requests to corresponding representations
169 will be consistent over time if the server intends to take advantage
170 of conditionals. Regardless, if the mapping is inconsistent and the
171 server is unable to select the appropriate representation, then no
172 harm will result when the precondition evaluates to false.
174 We use the term "selected representation" to refer to the current
175 representation of the target resource that would have been selected
176 in a successful response if the same request had used the method GET
177 and had excluded all of the conditional request header fields. The
178 conditional request preconditions are evaluated by comparing the
179 values provided in the request header fields to the current metadata
180 for the selected representation.
182 1.1. Requirements
184 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
185 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
186 document are to be interpreted as described in [RFC2119].
188 An implementation is not compliant if it fails to satisfy one or more
189 of the "MUST" or "REQUIRED" level requirements for the protocols it
190 implements. An implementation that satisfies all the "MUST" or
191 "REQUIRED" level and all the "SHOULD" level requirements for its
192 protocols is said to be "unconditionally compliant"; one that
193 satisfies all the "MUST" level requirements but not all the "SHOULD"
194 level requirements for its protocols is said to be "conditionally
195 compliant".
197 1.2. Syntax Notation
199 This specification uses the ABNF syntax defined in Section 1.2 of
200 [Part1] (which extends the syntax defined in [RFC5234] with a list
201 rule). Appendix B shows the collected ABNF, with the list rule
202 expanded.
204 The following core rules are included by reference, as defined in
205 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
206 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
207 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
208 sequence of data), SP (space), VCHAR (any visible USASCII character),
209 and WSP (whitespace).
211 The ABNF rules below are defined in [Part1]:
213 OWS =
214 quoted-string =
215 HTTP-date =
217 2. Validators
219 This specification defines two forms of metadata that are commonly
220 used to observe resource state and test for preconditions:
221 modification dates and opaque entity tags. Additional metadata that
222 reflects resource state has been defined by various extensions of
223 HTTP, such as WebDAV [RFC4918], that are beyond the scope of this
224 specification. A resource metadata value is referred to as a
225 "validator" when it is used within a precondition.
227 2.1. Weak versus Strong
229 Validators come in two flavors: strong or weak. Weak validators are
230 easy to generate but are far less useful for comparisons. Strong
231 validators are ideal for comparisons but can be very difficult (and
232 occasionally impossible) to generate efficiently. Rather than impose
233 that all forms of resource adhere to the same strength of validator,
234 HTTP exposes the type of validator in use and imposes restrictions on
235 when weak validators can be used as preconditions.
237 A "strong validator" is a representation metadata value that MUST be
238 changed to a new, previously unused or guaranteed unique, value
239 whenever a change occurs to the representation data such that a
240 change would be observable in the payload body of a 200 response to
241 GET. A strong validator MAY be changed for other reasons, such as
242 when a semantically significant part of the representation metadata
243 is changed (e.g., Content-Type), but it is in the best interests of
244 the origin server to only change the value when it is necessary to
245 invalidate the stored responses held by remote caches and authoring
246 tools. A strong validator MUST be unique across all representations
247 of a given resource, such that no two representations of that
248 resource share the same validator unless their payload body would be
249 identical.
251 Cache entries might persist for arbitrarily long periods, regardless
252 of expiration times. Thus, a cache might attempt to validate an
253 entry using a validator that it obtained in the distant past. A
254 strong validator MUST be unique across all versions of all
255 representations associated with a particular resource over time.
256 However, there is no implication of uniqueness across representations
257 of different resources (i.e., the same strong validator might be in
258 use for representations of multiple resources at the same time and
259 does not imply that those representations are equivalent).
261 There are a variety of strong validators used in practice. The best
262 are based on strict revision control, wherein each change to a
263 representation always results in a unique node name and revision
264 identifier being assigned before the representation is made
265 accessible to GET. A cryptographic hash function applied to the
266 representation data is also sufficient if the data is available prior
267 to the response header fields being sent and the digest does not need
268 to be recalculated every time a validation request is received.
269 However, if a resource has distinct representations that differ only
270 in their metadata, such as might occur with content negotiation over
271 media types that happen to share the same data format, then a server
272 SHOULD incorporate additional information in the validator to
273 distinguish those representations and avoid confusing cache behavior.
275 In contrast, a "weak validator" is a representation metadata value
276 that might not be changed for every change to the representation
277 data. This weakness might be due to limitations in how the value is
278 calculated, such as clock resolution or an inability to ensure
279 uniqueness for all possible representations of the resource, or due
280 to a desire by the resource owner to group representations by some
281 self-determined set of equivalency rather than unique sequences of
282 data. A weak entity-tag SHOULD change whenever the origin server
283 considers prior representations to be unacceptable as a substitute
284 for the current representation. In other words, a weak entity-tag
285 SHOULD change whenever the origin server wants caches to invalidate
286 old responses.
288 For example, the representation of a weather report that changes in
289 content every second, based on dynamic measurements, might be grouped
290 into sets of equivalent representations (from the origin server's
291 perspective) with the same weak validator in order to allow cached
292 representations to be valid for a reasonable period of time (perhaps
293 adjusted dynamically based on server load or weather quality).
294 Likewise, a representation's modification time, if defined with only
295 one-second resolution, might be a weak validator if it is possible
296 for the representation to be modified twice during a single second
297 and retrieved between those modifications.
299 A "use" of a validator occurs when either a client generates a
300 request and includes the validator in a precondition or when a server
301 compares two validators. Weak validators are only usable in contexts
302 that do not depend on exact equality of a representation's payload
303 body. Strong validators are usable and preferred for all conditional
304 requests, including cache validation, partial content ranges, and
305 "lost update" avoidance.
307 2.2. Last-Modified
309 The "Last-Modified" header field indicates the date and time at which
310 the origin server believes the selected representation was last
311 modified.
313 Last-Modified = HTTP-date
315 An example of its use is
317 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
319 2.2.1. Generation
321 Origin servers SHOULD send Last-Modified for any selected
322 representation for which a last modification date can be reasonably
323 and consistently determined, since its use in conditional requests
324 and evaluating cache freshness ([Part6]) results in a substantial
325 reduction of HTTP traffic on the Internet and can be a significant
326 factor in improving service scalability and reliability.
328 A representation is typically the sum of many parts behind the
329 resource interface. The last-modified time would usually be the most
330 recent time that any of those parts were changed. How that value is
331 determined for any given resource is an implementation detail beyond
332 the scope of this specification. What matters to HTTP is how
333 recipients of the Last-Modified header field can use its value to
334 make conditional requests and test the validity of locally cached
335 responses.
337 An origin server SHOULD obtain the Last-Modified value of the
338 representation as close as possible to the time that it generates the
339 Date field-value for its response. This allows a recipient to make
340 an accurate assessment of the representation's modification time,
341 especially if the representation changes near the time that the
342 response is generated.
344 An origin server with a clock MUST NOT send a Last-Modified date that
345 is later than the server's time of message origination (Date). If
346 the last modification time is derived from implementation-specific
347 metadata that evaluates to some time in the future, according to the
348 origin server's clock, then the origin server MUST replace that value
349 with the message origination date. This prevents a future
350 modification date from having an adverse impact on cache validation.
352 2.2.2. Comparison
354 A Last-Modified time, when used as a validator in a request, is
355 implicitly weak unless it is possible to deduce that it is strong,
356 using the following rules:
358 o The validator is being compared by an origin server to the actual
359 current validator for the representation and,
361 o That origin server reliably knows that the associated
362 representation did not change twice during the second covered by
363 the presented validator.
365 or
367 o The validator is about to be used by a client in an If-Modified-
368 Since, If-Unmodified-Since header field, because the client has a
369 cache entry, or If-Range for the associated representation, and
371 o That cache entry includes a Date value, which gives the time when
372 the origin server sent the original response, and
374 o The presented Last-Modified time is at least 60 seconds before the
375 Date value.
377 or
379 o The validator is being compared by an intermediate cache to the
380 validator stored in its cache entry for the representation, and
382 o That cache entry includes a Date value, which gives the time when
383 the origin server sent the original response, and
385 o The presented Last-Modified time is at least 60 seconds before the
386 Date value.
388 This method relies on the fact that if two different responses were
389 sent by the origin server during the same second, but both had the
390 same Last-Modified time, then at least one of those responses would
391 have a Date value equal to its Last-Modified time. The arbitrary 60-
392 second limit guards against the possibility that the Date and Last-
393 Modified values are generated from different clocks, or at somewhat
394 different times during the preparation of the response. An
395 implementation MAY use a value larger than 60 seconds, if it is
396 believed that 60 seconds is too short.
398 2.3. ETag
400 The ETag header field provides the current entity-tag for the
401 selected representation. An entity-tag is an opaque validator for
402 differentiating between multiple representations of the same
403 resource, regardless of whether those multiple representations are
404 due to resource state changes over time, content negotiation
405 resulting in multiple representations being valid at the same time,
406 or both. An entity-tag consists of an opaque quoted string, possibly
407 prefixed by a weakness indicator.
409 ETag = entity-tag
411 entity-tag = [ weak ] opaque-tag
412 weak = %x57.2F ; "W/", case-sensitive
413 opaque-tag = quoted-string
415 An entity-tag can be more reliable for validation than a modification
416 date in situations where it is inconvenient to store modification
417 dates, where the one-second resolution of HTTP date values is not
418 sufficient, or where modification dates are not consistently
419 maintained.
421 Examples:
423 ETag: "xyzzy"
424 ETag: W/"xyzzy"
425 ETag: ""
427 An entity-tag can be either a weak or strong validator, with strong
428 being the default. If an origin server provides an entity-tag for a
429 representation and the generation of that entity-tag does not satisfy
430 the requirements for a strong validator (Section 2.1), then that
431 entity-tag MUST be marked as weak by prefixing its opaque value with
432 "W/" (case-sensitive).
434 2.3.1. Generation
436 The principle behind entity-tags is that only the service author
437 knows the implementation of a resource well enough to select the most
438 accurate and efficient validation mechanism for that resource, and
439 that any such mechanism can be mapped to a simple sequence of octets
440 for easy comparison. Since the value is opaque, there is no need for
441 the client to be aware of how each entity-tag is constructed.
443 For example, a resource that has implementation-specific versioning
444 applied to all changes might use an internal revision number, perhaps
445 combined with a variance identifier for content negotiation, to
446 accurately differentiate between representations. Other
447 implementations might use a stored hash of representation content, a
448 combination of various filesystem attributes, or a modification
449 timestamp that has sub-second resolution.
451 Origin servers SHOULD send ETag for any selected representation for
452 which detection of changes can be reasonably and consistently
453 determined, since the entity-tag's use in conditional requests and
454 evaluating cache freshness ([Part6]) can result in a substantial
455 reduction of HTTP network traffic and can be a significant factor in
456 improving service scalability and reliability.
458 2.3.2. Comparison
460 There are two entity-tag comparison functions, depending on whether
461 the comparison context allows the use of weak validators or not:
463 o The strong comparison function: in order to be considered equal,
464 both opaque-tags MUST be identical character-by-character, and
465 both MUST NOT be weak.
467 o The weak comparison function: in order to be considered equal,
468 both opaque-tags MUST be identical character-by-character, but
469 either or both of them MAY be tagged as "weak" without affecting
470 the result.
472 The example below shows the results for a set of entity-tag pairs,
473 and both the weak and strong comparison function results:
475 +--------+--------+-------------------+-----------------+
476 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
477 +--------+--------+-------------------+-----------------+
478 | W/"1" | W/"1" | no match | match |
479 | W/"1" | W/"2" | no match | no match |
480 | W/"1" | "1" | no match | match |
481 | "1" | "1" | match | match |
482 +--------+--------+-------------------+-----------------+
484 2.3.3. Example: Entity-tags varying on Content-Negotiated Resources
486 Consider a resource that is subject to content negotiation (Section 5
487 of [Part3]), and where the representations returned upon a GET
488 request vary based on the Accept-Encoding request header field
489 (Section 6.3 of [Part3]):
491 >> Request:
493 GET /index HTTP/1.1
494 Host: www.example.com
495 Accept-Encoding: gzip
497 In this case, the response might or might not use the gzip content
498 coding. If it does not, the response might look like:
500 >> Response:
502 HTTP/1.1 200 OK
503 Date: Thu, 26 Mar 2010 00:05:00 GMT
504 ETag: "123-a"
505 Content-Length: 70
506 Vary: Accept-Encoding
507 Content-Type: text/plain
509 Hello World!
510 Hello World!
511 Hello World!
512 Hello World!
513 Hello World!
515 An alternative representation that does use gzip content coding would
516 be:
518 >> Response:
520 HTTP/1.1 200 OK
521 Date: Thu, 26 Mar 2010 00:05:00 GMT
522 ETag: "123-b"
523 Content-Length: 43
524 Vary: Accept-Encoding
525 Content-Type: text/plain
526 Content-Encoding: gzip
528 ...binary data...
530 Note: Content codings are a property of the representation, so
531 therefore an entity-tag of an encoded representation must be
532 distinct from an unencoded representation to prevent conflicts
533 during cache updates and range requests. In contrast, transfer
534 codings (Section 6.2 of [Part1]) apply only during message
535 transfer and do not require distinct entity-tags.
537 2.4. Rules for When to Use Entity-tags and Last-Modified Dates
539 We adopt a set of rules and recommendations for origin servers,
540 clients, and caches regarding when various validator types ought to
541 be used, and for what purposes.
543 HTTP/1.1 origin servers:
545 o SHOULD send an entity-tag validator unless it is not feasible to
546 generate one.
548 o MAY send a weak entity-tag instead of a strong entity-tag, if
549 performance considerations support the use of weak entity-tags, or
550 if it is unfeasible to send a strong entity-tag.
552 o SHOULD send a Last-Modified value if it is feasible to send one.
554 In other words, the preferred behavior for an HTTP/1.1 origin server
555 is to send both a strong entity-tag and a Last-Modified value.
557 HTTP/1.1 clients:
559 o MUST use that entity-tag in any cache-conditional request (using
560 If-Match or If-None-Match) if an entity-tag has been provided by
561 the origin server.
563 o SHOULD use the Last-Modified value in non-subrange cache-
564 conditional requests (using If-Modified-Since) if only a Last-
565 Modified value has been provided by the origin server.
567 o MAY use the Last-Modified value in subrange cache-conditional
568 requests (using If-Unmodified-Since) if only a Last-Modified value
569 has been provided by an HTTP/1.0 origin server. The user agent
570 SHOULD provide a way to disable this, in case of difficulty.
572 o SHOULD use both validators in cache-conditional requests if both
573 an entity-tag and a Last-Modified value have been provided by the
574 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
575 respond appropriately.
577 An HTTP/1.1 origin server, upon receiving a conditional request that
578 includes both a Last-Modified date (e.g., in an If-Modified-Since or
579 If-Unmodified-Since header field) and one or more entity-tags (e.g.,
580 in an If-Match, If-None-Match, or If-Range header field) as cache
581 validators, MUST NOT return a response status code of 304 (Not
582 Modified) unless doing so is consistent with all of the conditional
583 header fields in the request.
585 An HTTP/1.1 caching proxy, upon receiving a conditional request that
586 includes both a Last-Modified date and one or more entity-tags as
587 cache validators, MUST NOT return a locally cached response to the
588 client unless that cached response is consistent with all of the
589 conditional header fields in the request.
591 Note: The general principle behind these rules is that HTTP/1.1
592 servers and clients ought to transmit as much non-redundant
593 information as is available in their responses and requests.
594 HTTP/1.1 systems receiving this information will make the most
595 conservative assumptions about the validators they receive.
597 HTTP/1.0 clients and caches might ignore entity-tags. Generally,
598 last-modified values received or used by these systems will
599 support transparent and efficient caching, and so HTTP/1.1 origin
600 servers should provide Last-Modified values. In those rare cases
601 where the use of a Last-Modified value as a validator by an
602 HTTP/1.0 system could result in a serious problem, then HTTP/1.1
603 origin servers should not provide one.
605 3. Precondition Header Fields
607 This section defines the syntax and semantics of HTTP/1.1 header
608 fields for applying preconditions on requests.
610 3.1. If-Match
612 The "If-Match" header field MAY be used to make a request method
613 conditional on the current existence or value of an entity-tag for
614 one or more representations of the target resource. If-Match is
615 generally useful for resource update requests, such as PUT requests,
616 as a means for protecting against accidental overwrites when multiple
617 clients are acting in parallel on the same resource (i.e., the "lost
618 update" problem). An If-Match field-value of "*" places the
619 precondition on the existence of any current representation for the
620 target resource.
622 If-Match = "*" / 1#entity-tag
624 If any of the entity-tags listed in the If-Match field value match
625 (as per Section 2.3.2) the entity-tag of the selected representation
626 for the target resource, or if "*" is given and any current
627 representation exists for the target resource, then the server MAY
628 perform the request method as if the If-Match header field was not
629 present.
631 If none of the entity-tags match, or if "*" is given and no current
632 representation exists, the server MUST NOT perform the requested
633 method. Instead, the server MUST respond with the 412 (Precondition
634 Failed) status code.
636 If the request would, without the If-Match header field, result in
637 anything other than a 2xx or 412 status code, then the If-Match
638 header field MUST be ignored.
640 Examples:
642 If-Match: "xyzzy"
643 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
644 If-Match: *
646 The result of a request having both an If-Match header field and
647 either an If-None-Match or an If-Modified-Since header fields is
648 undefined by this specification.
650 3.2. If-None-Match
652 The "If-None-Match" header field MAY be used to make a request method
653 conditional on not matching any of the current entity-tag values for
654 representations of the target resource. If-None-Match is primarily
655 used in conditional GET requests to enable efficient updates of
656 cached information with a minimum amount of transaction overhead. A
657 client that has one or more representations previously obtained from
658 the target resource can send If-None-Match with a list of the
659 associated entity-tags in the hope of receiving a 304 response if at
660 least one of those representations matches the selected
661 representation.
663 If-None-Match MAY also be used with a value of "*" to prevent an
664 unsafe request method (e.g., PUT) from inadvertently modifying an
665 existing representation of the target resource when the client
666 believes that the resource does not have a current representation.
667 This is a variation on the "lost update" problem that might arise if
668 more than one client attempts to create an initial representation for
669 the target resource.
671 If-None-Match = "*" / 1#entity-tag
673 If any of the entity-tags listed in the If-None-Match field-value
674 match (as per Section 2.3.2) the entity-tag of the selected
675 representation, or if "*" is given and any current representation
676 exists for that resource, then the server MUST NOT perform the
677 requested method. Instead, if the request method was GET or HEAD,
678 the server SHOULD respond with a 304 (Not Modified) status code,
679 including the cache-related header fields (particularly ETag) of the
680 selected representation that has a matching entity-tag. For all
681 other request methods, the server MUST respond with a 412
682 (Precondition Failed) status code.
684 If none of the entity-tags match, then the server MAY perform the
685 requested method as if the If-None-Match header field did not exist,
686 but MUST also ignore any If-Modified-Since header field(s) in the
687 request. That is, if no entity-tags match, then the server MUST NOT
688 return a 304 (Not Modified) response.
690 If the request would, without the If-None-Match header field, result
691 in anything other than a 2xx or 304 status code, then the If-None-
692 Match header field MUST be ignored. (See Section 2.4 for a
693 discussion of server behavior when both If-Modified-Since and If-
694 None-Match appear in the same request.)
696 Examples:
698 If-None-Match: "xyzzy"
699 If-None-Match: W/"xyzzy"
700 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
701 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
702 If-None-Match: *
704 The result of a request having both an If-None-Match header field and
705 either an If-Match or an If-Unmodified-Since header fields is
706 undefined by this specification.
708 3.3. If-Modified-Since
710 The "If-Modified-Since" header field MAY be used to make a request
711 method conditional by modification date: if the selected
712 representation has not been modified since the time specified in this
713 field, then do not perform the request method; instead, respond as
714 detailed below.
716 If-Modified-Since = HTTP-date
718 An example of the field is:
720 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
722 A GET method with an If-Modified-Since header field and no Range
723 header field requests that the selected representation be transferred
724 only if it has been modified since the date given by the If-Modified-
725 Since header field. The algorithm for determining this includes the
726 following cases:
728 1. If the request would normally result in anything other than a 200
729 (OK) status code, or if the passed If-Modified-Since date is
730 invalid, the response is exactly the same as for a normal GET. A
731 date which is later than the server's current time is invalid.
733 2. If the selected representation has been modified since the If-
734 Modified-Since date, the response is exactly the same as for a
735 normal GET.
737 3. If the selected representation has not been modified since a
738 valid If-Modified-Since date, the server SHOULD return a 304 (Not
739 Modified) response.
741 The purpose of this feature is to allow efficient updates of cached
742 information with a minimum amount of transaction overhead.
744 Note: The Range header field modifies the meaning of If-Modified-
745 Since; see Section 5.4 of [Part5] for full details.
747 Note: If-Modified-Since times are interpreted by the server, whose
748 clock might not be synchronized with the client.
750 Note: When handling an If-Modified-Since header field, some
751 servers will use an exact date comparison function, rather than a
752 less-than function, for deciding whether to send a 304 (Not
753 Modified) response. To get best results when sending an If-
754 Modified-Since header field for cache validation, clients are
755 advised to use the exact date string received in a previous Last-
756 Modified header field whenever possible.
758 Note: If a client uses an arbitrary date in the If-Modified-Since
759 header field instead of a date taken from the Last-Modified header
760 field for the same request, the client needs to be aware that this
761 date is interpreted in the server's understanding of time.
762 Unsynchronized clocks and rounding problems, due to the different
763 encodings of time between the client and server, are concerns.
764 This includes the possibility of race conditions if the document
765 has changed between the time it was first requested and the If-
766 Modified-Since date of a subsequent request, and the possibility
767 of clock-skew-related problems if the If-Modified-Since date is
768 derived from the client's clock without correction to the server's
769 clock. Corrections for different time bases between client and
770 server are at best approximate due to network latency.
772 The result of a request having both an If-Modified-Since header field
773 and either an If-Match or an If-Unmodified-Since header fields is
774 undefined by this specification.
776 3.4. If-Unmodified-Since
778 The "If-Unmodified-Since" header field MAY be used to make a request
779 method conditional by modification date: if the selected
780 representation has been modified since the time specified in this
781 field, then the server MUST NOT perform the requested operation and
782 MUST instead respond with the 412 (Precondition Failed) status code.
783 If the selected representation has not been modified since the time
784 specified in this field, the server SHOULD perform the request method
785 as if the If-Unmodified-Since header field were not present.
787 If-Unmodified-Since = HTTP-date
789 An example of the field is:
791 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
793 If the request normally (i.e., without the If-Unmodified-Since header
794 field) would result in anything other than a 2xx or 412 status code,
795 the If-Unmodified-Since header field SHOULD be ignored.
797 If the specified date is invalid, the header field MUST be ignored.
799 The result of a request having both an If-Unmodified-Since header
800 field and either an If-None-Match or an If-Modified-Since header
801 fields is undefined by this specification.
803 3.5. If-Range
805 The If-Range header field provides a special conditional request
806 mechanism that is similar to If-Match and If-Unmodified-Since but
807 specific to HTTP range requests. If-Range is defined in Section 5.3
808 of [Part5].
810 4. Status Code Definitions
812 4.1. 304 Not Modified
814 The 304 status code indicates that a conditional GET request has been
815 received and would have resulted in a 200 (OK) response if it were
816 not for the fact that the condition has evaluated to false. In other
817 words, there is no need for the server to transfer a representation
818 of the target resource because the client's request indicates that it
819 already has a valid representation, as indicated by the 304 response
820 header fields, and is therefore redirecting the client to make use of
821 that stored representation as if it were the payload of a 200
822 response. The 304 response MUST NOT contain a message-body, and thus
823 is always terminated by the first empty line after the header fields.
825 A 304 response MUST include a Date header field (Section 9.3 of
826 [Part1]) unless its omission is required by Section 9.3.1 of [Part1].
827 If a 200 response to the same request would have included any of the
828 header fields Cache-Control, Content-Location, ETag, Expires, Last-
829 Modified, or Vary, then those same header fields MUST be sent in a
830 304 response.
832 Since the goal of a 304 response is to minimize information transfer
833 when the recipient already has one or more cached representations,
834 the response SHOULD NOT include representation metadata other than
835 the above listed fields unless said metadata exists for the purpose
836 of guiding cache updates (e.g., future HTTP extensions).
838 If the recipient of a 304 response does not have a cached
839 representation corresponding to the entity-tag indicated by the 304
840 response, then the recipient MUST NOT use the 304 to update its own
841 cache. If this conditional request originated with an outbound
842 client, such as a user agent with its own cache sending a conditional
843 GET to a shared proxy, then the 304 response MAY be forwarded to the
844 outbound client. Otherwise, the recipient MUST disregard the 304
845 response and repeat the request without any preconditions.
847 If a cache uses a received 304 response to update a cache entry, the
848 cache MUST update the entry to reflect any new field values given in
849 the response.
851 4.2. 412 Precondition Failed
853 The 412 status code indicates that one or more preconditions given in
854 the request header fields evaluated to false when tested on the
855 server. This response code allows the client to place preconditions
856 on the current resource state (its current representations and
857 metadata) and thus prevent the request method from being applied if
858 the target resource is in an unexpected state.
860 5. IANA Considerations
862 5.1. Status Code Registration
864 The HTTP Status Code Registry located at
865 shall be updated
866 with the registrations below:
868 +-------+---------------------+-------------+
869 | Value | Description | Reference |
870 +-------+---------------------+-------------+
871 | 304 | Not Modified | Section 4.1 |
872 | 412 | Precondition Failed | Section 4.2 |
873 +-------+---------------------+-------------+
875 5.2. Header Field Registration
877 The Message Header Field Registry located at shall be
879 updated with the permanent registrations below (see [RFC3864]):
881 +---------------------+----------+----------+-------------+
882 | Header Field Name | Protocol | Status | Reference |
883 +---------------------+----------+----------+-------------+
884 | ETag | http | standard | Section 2.3 |
885 | If-Match | http | standard | Section 3.1 |
886 | If-Modified-Since | http | standard | Section 3.3 |
887 | If-None-Match | http | standard | Section 3.2 |
888 | If-Unmodified-Since | http | standard | Section 3.4 |
889 | Last-Modified | http | standard | Section 2.2 |
890 +---------------------+----------+----------+-------------+
892 The change controller is: "IETF (iesg@ietf.org) - Internet
893 Engineering Task Force".
895 6. Security Considerations
897 No additional security considerations have been identified beyond
898 those applicable to HTTP in general [Part1].
900 7. Acknowledgments
902 See Section 12 of [Part1].
904 8. References
906 8.1. Normative References
908 [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
909 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
910 and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
911 and Message Parsing", draft-ietf-httpbis-p1-messaging-16
912 (work in progress), August 2011.
914 [Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
915 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
916 and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
917 and Content Negotiation", draft-ietf-httpbis-p3-payload-16
918 (work in progress), August 2011.
920 [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
921 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
922 and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
923 Partial Responses", draft-ietf-httpbis-p5-range-16 (work
924 in progress), August 2011.
926 [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
927 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
928 Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
929 6: Caching", draft-ietf-httpbis-p6-cache-16 (work in
930 progress), August 2011.
932 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
933 Requirement Levels", BCP 14, RFC 2119, March 1997.
935 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
936 Specifications: ABNF", STD 68, RFC 5234, January 2008.
938 8.2. Informative References
940 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
941 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
942 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
944 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
945 Procedures for Message Header Fields", BCP 90, RFC 3864,
946 September 2004.
948 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
949 Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
951 Appendix A. Changes from RFC 2616
953 Allow weak entity-tags in all requests except range requests
954 (Sections 2.1 and 3.2).
956 Change ABNF productions for header fields to only define the field
957 value. (Section 3)
959 Appendix B. Collected ABNF
961 ETag = entity-tag
963 HTTP-date =
965 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
966 entity-tag ] ) )
967 If-Modified-Since = HTTP-date
968 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
969 entity-tag ] ) )
970 If-Unmodified-Since = HTTP-date
972 Last-Modified = HTTP-date
974 OWS =
976 entity-tag = [ weak ] opaque-tag
978 opaque-tag = quoted-string
980 quoted-string =
982 weak = %x57.2F ; W/
984 ABNF diagnostics:
986 ; ETag defined but not used
987 ; If-Match defined but not used
988 ; If-Modified-Since defined but not used
989 ; If-None-Match defined but not used
990 ; If-Unmodified-Since defined but not used
991 ; Last-Modified defined but not used
993 Appendix C. Change Log (to be removed by RFC Editor before publication)
995 C.1. Since RFC 2616
997 Extracted relevant partitions from [RFC2616].
999 C.2. Since draft-ietf-httpbis-p4-conditional-00
1001 Closed issues:
1003 o : "Normative and
1004 Informative references"
1006 Other changes:
1008 o Move definitions of 304 and 412 condition codes from Part2.
1010 C.3. Since draft-ietf-httpbis-p4-conditional-01
1012 Ongoing work on ABNF conversion
1013 ():
1015 o Add explicit references to BNF syntax and rules imported from
1016 other parts of the specification.
1018 C.4. Since draft-ietf-httpbis-p4-conditional-02
1020 Closed issues:
1022 o : "Weak ETags on
1023 non-GET requests"
1025 Ongoing work on IANA Message Header Field Registration
1026 ():
1028 o Reference RFC 3984, and update header field registrations for
1029 header fields defined in this document.
1031 C.5. Since draft-ietf-httpbis-p4-conditional-03
1033 Closed issues:
1035 o : "Examples for
1036 ETag matching"
1038 o : "'entity
1039 value' undefined"
1041 o : "bogus 2068
1042 Date header reference"
1044 C.6. Since draft-ietf-httpbis-p4-conditional-04
1046 Ongoing work on ABNF conversion
1047 ():
1049 o Use "/" instead of "|" for alternatives.
1051 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
1052 whitespace ("OWS") and required whitespace ("RWS").
1054 o Rewrite ABNFs to spell out whitespace rules, factor out header
1055 field value format definitions.
1057 C.7. Since draft-ietf-httpbis-p4-conditional-05
1059 Final work on ABNF conversion
1060 ():
1062 o Add appendix containing collected and expanded ABNF, reorganize
1063 ABNF introduction.
1065 C.8. Since draft-ietf-httpbis-p4-conditional-06
1067 Closed issues:
1069 o : "case-
1070 sensitivity of etag weakness indicator"
1072 C.9. Since draft-ietf-httpbis-p4-conditional-07
1074 Closed issues:
1076 o : "Weak ETags on
1077 non-GET requests" (If-Match still was defined to require strong
1078 matching)
1080 o : "move IANA
1081 registrations for optional status codes"
1083 C.10. Since draft-ietf-httpbis-p4-conditional-08
1085 No significant changes.
1087 C.11. Since draft-ietf-httpbis-p4-conditional-09
1089 No significant changes.
1091 C.12. Since draft-ietf-httpbis-p4-conditional-10
1093 Closed issues:
1095 o : "Clarify
1096 'Requested Variant'"
1098 o : "Clarify
1099 entity / representation / variant terminology"
1101 o : "consider
1102 removing the 'changes from 2068' sections"
1104 C.13. Since draft-ietf-httpbis-p4-conditional-11
1106 None.
1108 C.14. Since draft-ietf-httpbis-p4-conditional-12
1110 Closed issues:
1112 o : "Header
1113 Classification"
1115 C.15. Since draft-ietf-httpbis-p4-conditional-13
1117 Closed issues:
1119 o : "If-* and
1120 entities"
1122 o : "Definition of
1123 validator weakness"
1125 o : "untangle
1126 ABNFs for header fields"
1128 o : "ETags and
1129 Quotes"
1131 C.16. Since draft-ietf-httpbis-p4-conditional-14
1133 None.
1135 C.17. Since draft-ietf-httpbis-p4-conditional-15
1137 Closed issues:
1139 o : "If-Range
1140 should be listed when dicussing contexts where L-M can be
1141 considered strong"
1143 Index
1145 3
1146 304 Not Modified (status code) 19
1148 4
1149 412 Precondition Failed (status code) 20
1151 E
1152 ETag header field 10
1154 G
1155 Grammar
1156 entity-tag 10
1157 ETag 10
1158 If-Match 15
1159 If-Modified-Since 17
1160 If-None-Match 16
1161 If-Unmodified-Since 18
1162 Last-Modified 8
1163 opaque-tag 10
1164 weak 10
1166 H
1167 Header Fields
1168 ETag 10
1169 If-Match 14
1170 If-Modified-Since 17
1171 If-None-Match 15
1172 If-Unmodified-Since 18
1173 Last-Modified 8
1175 I
1176 If-Match header field 14
1177 If-Modified-Since header field 17
1178 If-None-Match header field 15
1179 If-Unmodified-Since header field 18
1181 L
1182 Last-Modified header field 8
1184 M
1185 metadata 6
1187 S
1188 selected representation 5
1189 Status Codes
1190 304 Not Modified 19
1191 412 Precondition Failed 20
1193 V
1194 validator 6
1195 strong 6
1196 weak 6
1198 Authors' Addresses
1200 Roy T. Fielding (editor)
1201 Adobe Systems Incorporated
1202 345 Park Ave
1203 San Jose, CA 95110
1204 USA
1206 EMail: fielding@gbiv.com
1207 URI: http://roy.gbiv.com/
1209 Jim Gettys
1210 Alcatel-Lucent Bell Labs
1211 21 Oak Knoll Road
1212 Carlisle, MA 01741
1213 USA
1215 EMail: jg@freedesktop.org
1216 URI: http://gettys.wordpress.com/
1217 Jeffrey C. Mogul
1218 Hewlett-Packard Company
1219 HP Labs, Large Scale Systems Group
1220 1501 Page Mill Road, MS 1177
1221 Palo Alto, CA 94304
1222 USA
1224 EMail: JeffMogul@acm.org
1226 Henrik Frystyk Nielsen
1227 Microsoft Corporation
1228 1 Microsoft Way
1229 Redmond, WA 98052
1230 USA
1232 EMail: henrikn@microsoft.com
1234 Larry Masinter
1235 Adobe Systems Incorporated
1236 345 Park Ave
1237 San Jose, CA 95110
1238 USA
1240 EMail: LMM@acm.org
1241 URI: http://larry.masinter.net/
1243 Paul J. Leach
1244 Microsoft Corporation
1245 1 Microsoft Way
1246 Redmond, WA 98052
1248 EMail: paulle@microsoft.com
1250 Tim Berners-Lee
1251 World Wide Web Consortium
1252 MIT Computer Science and Artificial Intelligence Laboratory
1253 The Stata Center, Building 32
1254 32 Vassar Street
1255 Cambridge, MA 02139
1256 USA
1258 EMail: timbl@w3.org
1259 URI: http://www.w3.org/People/Berners-Lee/
1260 Yves Lafon (editor)
1261 World Wide Web Consortium
1262 W3C / ERCIM
1263 2004, rte des Lucioles
1264 Sophia-Antipolis, AM 06902
1265 France
1267 EMail: ylafon@w3.org
1268 URI: http://www.raubacapeu.net/people/yves/
1270 Julian F. Reschke (editor)
1271 greenbytes GmbH
1272 Hafenweg 16
1273 Muenster, NW 48155
1274 Germany
1276 Phone: +49 251 2807760
1277 Fax: +49 251 2807761
1278 EMail: julian.reschke@greenbytes.de
1279 URI: http://greenbytes.de/tech/webdav/