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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTPbis Working Group R. Fielding, Ed. 3 Internet-Draft Adobe 4 Obsoletes: 2616 (if approved) Y. Lafon, Ed. 5 Intended status: Standards Track W3C 6 Expires: September 13, 2012 J. Reschke, Ed. 7 greenbytes 8 March 12, 2012 10 HTTP/1.1, part 4: Conditional Requests 11 draft-ietf-httpbis-p4-conditional-19 13 Abstract 15 The Hypertext Transfer Protocol (HTTP) is an application-level 16 protocol for distributed, collaborative, hypertext information 17 systems. HTTP has been in use by the World Wide Web global 18 information initiative since 1990. This document is Part 4 of the 19 seven-part specification that defines the protocol referred to as 20 "HTTP/1.1" and, taken together, obsoletes RFC 2616. 22 Part 4 defines request header fields for indicating conditional 23 requests and the rules for constructing responses to those requests. 25 Editorial Note (To be removed by RFC Editor) 27 Discussion of this draft should take place on the HTTPBIS working 28 group mailing list (ietf-http-wg@w3.org), which is archived at 29 . 31 The current issues list is at 32 and related 33 documents (including fancy diffs) can be found at 34 . 36 The changes in this draft are summarized in Appendix C.20. 38 Status of This Memo 40 This Internet-Draft is submitted in full conformance with the 41 provisions of BCP 78 and BCP 79. 43 Internet-Drafts are working documents of the Internet Engineering 44 Task Force (IETF). Note that other groups may also distribute 45 working documents as Internet-Drafts. The list of current Internet- 46 Drafts is at http://datatracker.ietf.org/drafts/current/. 48 Internet-Drafts are draft documents valid for a maximum of six months 49 and may be updated, replaced, or obsoleted by other documents at any 50 time. It is inappropriate to use Internet-Drafts as reference 51 material or to cite them other than as "work in progress." 53 This Internet-Draft will expire on September 13, 2012. 55 Copyright Notice 57 Copyright (c) 2012 IETF Trust and the persons identified as the 58 document authors. All rights reserved. 60 This document is subject to BCP 78 and the IETF Trust's Legal 61 Provisions Relating to IETF Documents 62 (http://trustee.ietf.org/license-info) in effect on the date of 63 publication of this document. Please review these documents 64 carefully, as they describe your rights and restrictions with respect 65 to this document. Code Components extracted from this document must 66 include Simplified BSD License text as described in Section 4.e of 67 the Trust Legal Provisions and are provided without warranty as 68 described in the Simplified BSD License. 70 This document may contain material from IETF Documents or IETF 71 Contributions published or made publicly available before November 72 10, 2008. The person(s) controlling the copyright in some of this 73 material may not have granted the IETF Trust the right to allow 74 modifications of such material outside the IETF Standards Process. 75 Without obtaining an adequate license from the person(s) controlling 76 the copyright in such materials, this document may not be modified 77 outside the IETF Standards Process, and derivative works of it may 78 not be created outside the IETF Standards Process, except to format 79 it for publication as an RFC or to translate it into languages other 80 than English. 82 Table of Contents 84 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 85 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4 86 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5 87 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5 88 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5 89 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7 90 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7 91 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8 92 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 93 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 10 94 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 11 95 2.3.3. Example: Entity-tags varying on Content-Negotiated 96 Resources . . . . . . . . . . . . . . . . . . . . . . 11 98 2.4. Rules for When to Use Entity-tags and Last-Modified 99 Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 12 100 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14 101 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14 102 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15 103 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 16 104 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 17 105 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18 106 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18 107 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18 108 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 19 109 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 110 5.1. Status Code Registration . . . . . . . . . . . . . . . . . 19 111 5.2. Header Field Registration . . . . . . . . . . . . . . . . 19 112 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 113 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 114 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 115 8.1. Normative References . . . . . . . . . . . . . . . . . . . 20 116 8.2. Informative References . . . . . . . . . . . . . . . . . . 21 117 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 21 118 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 22 119 Appendix C. Change Log (to be removed by RFC Editor before 120 publication) . . . . . . . . . . . . . . . . . . . . 22 121 C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 22 122 C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 22 123 C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23 124 C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23 125 C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23 126 C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 23 127 C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24 128 C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24 129 C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24 130 C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24 131 C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 24 132 C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 24 133 C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25 134 C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25 135 C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25 136 C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 25 137 C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 25 138 C.18. Since draft-ietf-httpbis-p4-conditional-16 . . . . . . . . 25 139 C.19. Since draft-ietf-httpbis-p4-conditional-17 . . . . . . . . 26 140 C.20. Since draft-ietf-httpbis-p4-conditional-18 . . . . . . . . 26 141 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 143 1. Introduction 145 This document defines the HTTP/1.1 conditional request mechanisms, 146 including both metadata for indicating/observing changes in resource 147 representations and request header fields that specify preconditions 148 on that metadata be checked before performing the request method. 149 Conditional GET requests are the most efficient mechanism for HTTP 150 cache updates [Part6]. Conditionals can also be applied to state- 151 changing methods, such as PUT and DELETE, to prevent the "lost 152 update" problem: one client accidentally overwriting the work of 153 another client that has been acting in parallel. 155 Conditional request preconditions are based on the state of the 156 target resource as a whole (its current value set) or the state as 157 observed in a previously obtained representation (one value in that 158 set). A resource might have multiple current representations, each 159 with its own observable state. The conditional request mechanisms 160 assume that the mapping of requests to corresponding representations 161 will be consistent over time if the server intends to take advantage 162 of conditionals. Regardless, if the mapping is inconsistent and the 163 server is unable to select the appropriate representation, then no 164 harm will result when the precondition evaluates to false. 166 We use the term "selected representation" to refer to the current 167 representation of the target resource that would have been selected 168 in a successful response if the same request had used the method GET 169 and had excluded all of the conditional request header fields. The 170 conditional request preconditions are evaluated by comparing the 171 values provided in the request header fields to the current metadata 172 for the selected representation. 174 1.1. Conformance and Error Handling 176 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 177 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 178 document are to be interpreted as described in [RFC2119]. 180 This document defines conformance criteria for several roles in HTTP 181 communication, including Senders, Recipients, Clients, Servers, User- 182 Agents, Origin Servers, Intermediaries, Proxies and Gateways. See 183 Section 2 of [Part1] for definitions of these terms. 185 An implementation is considered conformant if it complies with all of 186 the requirements associated with its role(s). Note that SHOULD-level 187 requirements are relevant here, unless one of the documented 188 exceptions is applicable. 190 This document also uses ABNF to define valid protocol elements 191 (Section 1.2). In addition to the prose requirements placed upon 192 them, Senders MUST NOT generate protocol elements that are invalid. 194 Unless noted otherwise, Recipients MAY take steps to recover a usable 195 protocol element from an invalid construct. However, HTTP does not 196 define specific error handling mechanisms, except in cases where it 197 has direct impact on security. This is because different uses of the 198 protocol require different error handling strategies; for example, a 199 Web browser may wish to transparently recover from a response where 200 the Location header field doesn't parse according to the ABNF, 201 whereby in a systems control protocol using HTTP, this type of error 202 recovery could lead to dangerous consequences. 204 1.2. Syntax Notation 206 This specification uses the Augmented Backus-Naur Form (ABNF) 207 notation of [RFC5234] with the list rule extension defined in Section 208 1.2 of [Part1]. Appendix B shows the collected ABNF with the list 209 rule expanded. 211 The following core rules are included by reference, as defined in 212 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF 213 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), 214 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit 215 sequence of data), SP (space), and VCHAR (any visible US-ASCII 216 character). 218 The ABNF rules below are defined in [Part1] and [Part2]: 220 OWS = 221 obs-text = 222 HTTP-date = 224 2. Validators 226 This specification defines two forms of metadata that are commonly 227 used to observe resource state and test for preconditions: 228 modification dates and opaque entity tags. Additional metadata that 229 reflects resource state has been defined by various extensions of 230 HTTP, such as WebDAV [RFC4918], that are beyond the scope of this 231 specification. A resource metadata value is referred to as a 232 "validator" when it is used within a precondition. 234 2.1. Weak versus Strong 236 Validators come in two flavors: strong or weak. Weak validators are 237 easy to generate but are far less useful for comparisons. Strong 238 validators are ideal for comparisons but can be very difficult (and 239 occasionally impossible) to generate efficiently. Rather than impose 240 that all forms of resource adhere to the same strength of validator, 241 HTTP exposes the type of validator in use and imposes restrictions on 242 when weak validators can be used as preconditions. 244 A "strong validator" is a representation metadata value that MUST be 245 changed to a new, previously unused or guaranteed unique, value 246 whenever a change occurs to the representation data such that a 247 change would be observable in the payload body of a 200 response to 248 GET. A strong validator MAY be changed for other reasons, such as 249 when a semantically significant part of the representation metadata 250 is changed (e.g., Content-Type), but it is in the best interests of 251 the origin server to only change the value when it is necessary to 252 invalidate the stored responses held by remote caches and authoring 253 tools. A strong validator MUST be unique across all representations 254 of a given resource, such that no two representations of that 255 resource share the same validator unless their payload body would be 256 identical. 258 Cache entries might persist for arbitrarily long periods, regardless 259 of expiration times. Thus, a cache might attempt to validate an 260 entry using a validator that it obtained in the distant past. A 261 strong validator MUST be unique across all versions of all 262 representations associated with a particular resource over time. 263 However, there is no implication of uniqueness across representations 264 of different resources (i.e., the same strong validator might be in 265 use for representations of multiple resources at the same time and 266 does not imply that those representations are equivalent). 268 There are a variety of strong validators used in practice. The best 269 are based on strict revision control, wherein each change to a 270 representation always results in a unique node name and revision 271 identifier being assigned before the representation is made 272 accessible to GET. A cryptographic hash function applied to the 273 representation data is also sufficient if the data is available prior 274 to the response header fields being sent and the digest does not need 275 to be recalculated every time a validation request is received. 276 However, if a resource has distinct representations that differ only 277 in their metadata, such as might occur with content negotiation over 278 media types that happen to share the same data format, then a server 279 SHOULD incorporate additional information in the validator to 280 distinguish those representations and avoid confusing cache behavior. 282 In contrast, a "weak validator" is a representation metadata value 283 that might not be changed for every change to the representation 284 data. This weakness might be due to limitations in how the value is 285 calculated, such as clock resolution or an inability to ensure 286 uniqueness for all possible representations of the resource, or due 287 to a desire by the resource owner to group representations by some 288 self-determined set of equivalency rather than unique sequences of 289 data. A weak entity-tag SHOULD change whenever the origin server 290 considers prior representations to be unacceptable as a substitute 291 for the current representation. In other words, a weak entity-tag 292 SHOULD change whenever the origin server wants caches to invalidate 293 old responses. 295 For example, the representation of a weather report that changes in 296 content every second, based on dynamic measurements, might be grouped 297 into sets of equivalent representations (from the origin server's 298 perspective) with the same weak validator in order to allow cached 299 representations to be valid for a reasonable period of time (perhaps 300 adjusted dynamically based on server load or weather quality). 301 Likewise, a representation's modification time, if defined with only 302 one-second resolution, might be a weak validator if it is possible 303 for the representation to be modified twice during a single second 304 and retrieved between those modifications. 306 A "use" of a validator occurs when either a client generates a 307 request and includes the validator in a precondition or when a server 308 compares two validators. Weak validators are only usable in contexts 309 that do not depend on exact equality of a representation's payload 310 body. Strong validators are usable and preferred for all conditional 311 requests, including cache validation, partial content ranges, and 312 "lost update" avoidance. 314 2.2. Last-Modified 316 The "Last-Modified" header field indicates the date and time at which 317 the origin server believes the selected representation was last 318 modified. 320 Last-Modified = HTTP-date 322 An example of its use is 324 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 326 2.2.1. Generation 328 Origin servers SHOULD send Last-Modified for any selected 329 representation for which a last modification date can be reasonably 330 and consistently determined, since its use in conditional requests 331 and evaluating cache freshness ([Part6]) results in a substantial 332 reduction of HTTP traffic on the Internet and can be a significant 333 factor in improving service scalability and reliability. 335 A representation is typically the sum of many parts behind the 336 resource interface. The last-modified time would usually be the most 337 recent time that any of those parts were changed. How that value is 338 determined for any given resource is an implementation detail beyond 339 the scope of this specification. What matters to HTTP is how 340 recipients of the Last-Modified header field can use its value to 341 make conditional requests and test the validity of locally cached 342 responses. 344 An origin server SHOULD obtain the Last-Modified value of the 345 representation as close as possible to the time that it generates the 346 Date field-value for its response. This allows a recipient to make 347 an accurate assessment of the representation's modification time, 348 especially if the representation changes near the time that the 349 response is generated. 351 An origin server with a clock MUST NOT send a Last-Modified date that 352 is later than the server's time of message origination (Date). If 353 the last modification time is derived from implementation-specific 354 metadata that evaluates to some time in the future, according to the 355 origin server's clock, then the origin server MUST replace that value 356 with the message origination date. This prevents a future 357 modification date from having an adverse impact on cache validation. 359 An origin server without a clock MUST NOT assign Last-Modified values 360 to a response unless these values were associated with the resource 361 by some other system or user with a reliable clock. 363 2.2.2. Comparison 365 A Last-Modified time, when used as a validator in a request, is 366 implicitly weak unless it is possible to deduce that it is strong, 367 using the following rules: 369 o The validator is being compared by an origin server to the actual 370 current validator for the representation and, 372 o That origin server reliably knows that the associated 373 representation did not change twice during the second covered by 374 the presented validator. 376 or 378 o The validator is about to be used by a client in an If-Modified- 379 Since, If-Unmodified-Since header field, because the client has a 380 cache entry, or If-Range for the associated 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 or 390 o The validator is being compared by an intermediate cache to the 391 validator stored in its cache entry for the representation, and 393 o That cache entry includes a Date value, which gives the time when 394 the origin server sent the original response, and 396 o The presented Last-Modified time is at least 60 seconds before the 397 Date value. 399 This method relies on the fact that if two different responses were 400 sent by the origin server during the same second, but both had the 401 same Last-Modified time, then at least one of those responses would 402 have a Date value equal to its Last-Modified time. The arbitrary 60- 403 second limit guards against the possibility that the Date and Last- 404 Modified values are generated from different clocks, or at somewhat 405 different times during the preparation of the response. An 406 implementation MAY use a value larger than 60 seconds, if it is 407 believed that 60 seconds is too short. 409 2.3. ETag 411 The ETag header field provides the current entity-tag for the 412 selected representation. An entity-tag is an opaque validator for 413 differentiating between multiple representations of the same 414 resource, regardless of whether those multiple representations are 415 due to resource state changes over time, content negotiation 416 resulting in multiple representations being valid at the same time, 417 or both. An entity-tag consists of an opaque quoted string, possibly 418 prefixed by a weakness indicator. 420 ETag = entity-tag 422 entity-tag = [ weak ] opaque-tag 423 weak = %x57.2F ; "W/", case-sensitive 424 opaque-tag = DQUOTE *etagc DQUOTE 425 etagc = %x21 / %x23-7E / obs-text 426 ; VCHAR except double quotes, plus obs-text 428 Note: Previously, opaque-tag was defined to be a quoted-string 429 ([RFC2616], Section 3.11), thus some recipients might perform 430 backslash unescaping. Servers therefore ought to avoid backslash 431 characters in entity tags. 433 An entity-tag can be more reliable for validation than a modification 434 date in situations where it is inconvenient to store modification 435 dates, where the one-second resolution of HTTP date values is not 436 sufficient, or where modification dates are not consistently 437 maintained. 439 Examples: 441 ETag: "xyzzy" 442 ETag: W/"xyzzy" 443 ETag: "" 445 An entity-tag can be either a weak or strong validator, with strong 446 being the default. If an origin server provides an entity-tag for a 447 representation and the generation of that entity-tag does not satisfy 448 the requirements for a strong validator (Section 2.1), then that 449 entity-tag MUST be marked as weak by prefixing its opaque value with 450 "W/" (case-sensitive). 452 2.3.1. Generation 454 The principle behind entity-tags is that only the service author 455 knows the implementation of a resource well enough to select the most 456 accurate and efficient validation mechanism for that resource, and 457 that any such mechanism can be mapped to a simple sequence of octets 458 for easy comparison. Since the value is opaque, there is no need for 459 the client to be aware of how each entity-tag is constructed. 461 For example, a resource that has implementation-specific versioning 462 applied to all changes might use an internal revision number, perhaps 463 combined with a variance identifier for content negotiation, to 464 accurately differentiate between representations. Other 465 implementations might use a stored hash of representation content, a 466 combination of various filesystem attributes, or a modification 467 timestamp that has sub-second resolution. 469 Origin servers SHOULD send ETag for any selected representation for 470 which detection of changes can be reasonably and consistently 471 determined, since the entity-tag's use in conditional requests and 472 evaluating cache freshness ([Part6]) can result in a substantial 473 reduction of HTTP network traffic and can be a significant factor in 474 improving service scalability and reliability. 476 2.3.2. Comparison 478 There are two entity-tag comparison functions, depending on whether 479 the comparison context allows the use of weak validators or not: 481 o The strong comparison function: in order to be considered equal, 482 both opaque-tags MUST be identical character-by-character, and 483 both MUST NOT be weak. 485 o The weak comparison function: in order to be considered equal, 486 both opaque-tags MUST be identical character-by-character, but 487 either or both of them MAY be tagged as "weak" without affecting 488 the result. 490 The example below shows the results for a set of entity-tag pairs, 491 and both the weak and strong comparison function results: 493 +--------+--------+-------------------+-----------------+ 494 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | 495 +--------+--------+-------------------+-----------------+ 496 | W/"1" | W/"1" | no match | match | 497 | W/"1" | W/"2" | no match | no match | 498 | W/"1" | "1" | no match | match | 499 | "1" | "1" | match | match | 500 +--------+--------+-------------------+-----------------+ 502 2.3.3. Example: Entity-tags varying on Content-Negotiated Resources 504 Consider a resource that is subject to content negotiation (Section 5 505 of [Part3]), and where the representations returned upon a GET 506 request vary based on the Accept-Encoding request header field 507 (Section 6.3 of [Part3]): 509 >> Request: 511 GET /index HTTP/1.1 512 Host: www.example.com 513 Accept-Encoding: gzip 515 In this case, the response might or might not use the gzip content 516 coding. If it does not, the response might look like: 518 >> Response: 520 HTTP/1.1 200 OK 521 Date: Thu, 26 Mar 2010 00:05:00 GMT 522 ETag: "123-a" 523 Content-Length: 70 524 Vary: Accept-Encoding 525 Content-Type: text/plain 527 Hello World! 528 Hello World! 529 Hello World! 530 Hello World! 531 Hello World! 533 An alternative representation that does use gzip content coding would 534 be: 536 >> Response: 538 HTTP/1.1 200 OK 539 Date: Thu, 26 Mar 2010 00:05:00 GMT 540 ETag: "123-b" 541 Content-Length: 43 542 Vary: Accept-Encoding 543 Content-Type: text/plain 544 Content-Encoding: gzip 546 ...binary data... 548 Note: Content codings are a property of the representation, so 549 therefore an entity-tag of an encoded representation must be 550 distinct from an unencoded representation to prevent conflicts 551 during cache updates and range requests. In contrast, transfer 552 codings (Section 4 of [Part1]) apply only during message transfer 553 and do not require distinct entity-tags. 555 2.4. Rules for When to Use Entity-tags and Last-Modified Dates 557 We adopt a set of rules and recommendations for origin servers, 558 clients, and caches regarding when various validator types ought to 559 be used, and for what purposes. 561 HTTP/1.1 origin servers: 563 o SHOULD send an entity-tag validator unless it is not feasible to 564 generate one. 566 o MAY send a weak entity-tag instead of a strong entity-tag, if 567 performance considerations support the use of weak entity-tags, or 568 if it is unfeasible to send a strong entity-tag. 570 o SHOULD send a Last-Modified value if it is feasible to send one. 572 In other words, the preferred behavior for an HTTP/1.1 origin server 573 is to send both a strong entity-tag and a Last-Modified value. 575 HTTP/1.1 clients: 577 o MUST use that entity-tag in any cache-conditional request (using 578 If-Match or If-None-Match) if an entity-tag has been provided by 579 the origin server. 581 o SHOULD use the Last-Modified value in non-subrange cache- 582 conditional requests (using If-Modified-Since) if only a Last- 583 Modified value has been provided by the origin server. 585 o MAY use the Last-Modified value in subrange cache-conditional 586 requests (using If-Unmodified-Since) if only a Last-Modified value 587 has been provided by an HTTP/1.0 origin server. The user agent 588 SHOULD provide a way to disable this, in case of difficulty. 590 o SHOULD use both validators in cache-conditional requests if both 591 an entity-tag and a Last-Modified value have been provided by the 592 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to 593 respond appropriately. 595 An HTTP/1.1 origin server, upon receiving a conditional request that 596 includes both a Last-Modified date (e.g., in an If-Modified-Since or 597 If-Unmodified-Since header field) and one or more entity-tags (e.g., 598 in an If-Match, If-None-Match, or If-Range header field) as cache 599 validators, MUST NOT return a response status code of 304 (Not 600 Modified) unless doing so is consistent with all of the conditional 601 header fields in the request. 603 An HTTP/1.1 caching proxy, upon receiving a conditional request that 604 includes both a Last-Modified date and one or more entity-tags as 605 cache validators, MUST NOT return a locally cached response to the 606 client unless that cached response is consistent with all of the 607 conditional header fields in the request. 609 Note: The general principle behind these rules is that HTTP/1.1 610 servers and clients ought to transmit as much non-redundant 611 information as is available in their responses and requests. 612 HTTP/1.1 systems receiving this information will make the most 613 conservative assumptions about the validators they receive. 615 HTTP/1.0 clients and caches might ignore entity-tags. Generally, 616 last-modified values received or used by these systems will 617 support transparent and efficient caching, and so HTTP/1.1 origin 618 servers should provide Last-Modified values. In those rare cases 619 where the use of a Last-Modified value as a validator by an 620 HTTP/1.0 system could result in a serious problem, then HTTP/1.1 621 origin servers should not provide one. 623 3. Precondition Header Fields 625 This section defines the syntax and semantics of HTTP/1.1 header 626 fields for applying preconditions on requests. 628 3.1. If-Match 630 The "If-Match" header field MAY be used to make a request method 631 conditional on the current existence or value of an entity-tag for 632 one or more representations of the target resource. If-Match is 633 generally useful for resource update requests, such as PUT requests, 634 as a means for protecting against accidental overwrites when multiple 635 clients are acting in parallel on the same resource (i.e., the "lost 636 update" problem). An If-Match field-value of "*" places the 637 precondition on the existence of any current representation for the 638 target resource. 640 If-Match = "*" / 1#entity-tag 642 If any of the entity-tags listed in the If-Match field value match 643 (as per Section 2.3.2) the entity-tag of the selected representation 644 for the target resource, or if "*" is given and any current 645 representation exists for the target resource, then the server MAY 646 perform the request method as if the If-Match header field was not 647 present. 649 If none of the entity-tags match, or if "*" is given and no current 650 representation exists, the server MUST NOT perform the requested 651 method. Instead, the server MUST respond with the 412 (Precondition 652 Failed) status code. 654 If the request would, without the If-Match header field, result in 655 anything other than a 2xx or 412 status code, then the If-Match 656 header field MUST be ignored. 658 Examples: 660 If-Match: "xyzzy" 661 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 662 If-Match: * 664 The result of a request having both an If-Match header field and 665 either an If-None-Match or an If-Modified-Since header field is 666 undefined by this specification. 668 3.2. If-None-Match 670 The "If-None-Match" header field MAY be used to make a request method 671 conditional on not matching any of the current entity-tag values for 672 representations of the target resource. If-None-Match is primarily 673 used in conditional GET requests to enable efficient updates of 674 cached information with a minimum amount of transaction overhead. A 675 client that has one or more representations previously obtained from 676 the target resource can send If-None-Match with a list of the 677 associated entity-tags in the hope of receiving a 304 response if at 678 least one of those representations matches the selected 679 representation. 681 If-None-Match MAY also be used with a value of "*" to prevent an 682 unsafe request method (e.g., PUT) from inadvertently modifying an 683 existing representation of the target resource when the client 684 believes that the resource does not have a current representation. 685 This is a variation on the "lost update" problem that might arise if 686 more than one client attempts to create an initial representation for 687 the target resource. 689 If-None-Match = "*" / 1#entity-tag 691 If any of the entity-tags listed in the If-None-Match field-value 692 match (as per Section 2.3.2) the entity-tag of the selected 693 representation, or if "*" is given and any current representation 694 exists for that resource, then the server MUST NOT perform the 695 requested method. Instead, if the request method was GET or HEAD, 696 the server SHOULD respond with a 304 (Not Modified) status code, 697 including the cache-related header fields (particularly ETag) of the 698 selected representation that has a matching entity-tag. For all 699 other request methods, the server MUST respond with a 412 700 (Precondition Failed) status code. 702 If none of the entity-tags match, then the server MAY perform the 703 requested method as if the If-None-Match header field did not exist, 704 but MUST also ignore any If-Modified-Since header field(s) in the 705 request. That is, if no entity-tags match, then the server MUST NOT 706 return a 304 (Not Modified) response. 708 If the request would, without the If-None-Match header field, result 709 in anything other than a 2xx or 304 status code, then the If-None- 710 Match header field MUST be ignored. (See Section 2.4 for a 711 discussion of server behavior when both If-Modified-Since and If- 712 None-Match appear in the same request.) 714 Examples: 716 If-None-Match: "xyzzy" 717 If-None-Match: W/"xyzzy" 718 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 719 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" 720 If-None-Match: * 722 The result of a request having both an If-None-Match header field and 723 either an If-Match or an If-Unmodified-Since header field is 724 undefined by this specification. 726 3.3. If-Modified-Since 728 The "If-Modified-Since" header field MAY be used to make a request 729 method conditional by modification date: if the selected 730 representation has not been modified since the time specified in this 731 field, then do not perform the request method; instead, respond as 732 detailed below. 734 If-Modified-Since = HTTP-date 736 An example of the field is: 738 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 740 A GET method with an If-Modified-Since header field and no Range 741 header field requests that the selected representation be transferred 742 only if it has been modified since the date given by the If-Modified- 743 Since header field. The algorithm for determining this includes the 744 following cases: 746 1. If the request would normally result in anything other than a 200 747 (OK) status code, or if the passed If-Modified-Since date is 748 invalid, the response is exactly the same as for a normal GET. A 749 date which is later than the server's current time is invalid. 751 2. If the selected representation has been modified since the If- 752 Modified-Since date, the response is exactly the same as for a 753 normal GET. 755 3. If the selected representation has not been modified since a 756 valid If-Modified-Since date, the server SHOULD return a 304 (Not 757 Modified) response. 759 The purpose of this feature is to allow efficient updates of cached 760 information with a minimum amount of transaction overhead. 762 Note: The Range header field modifies the meaning of If-Modified- 763 Since; see Section 5.4 of [Part5] for full details. 765 Note: If-Modified-Since times are interpreted by the server, whose 766 clock might not be synchronized with the client. 768 Note: When handling an If-Modified-Since header field, some 769 servers will use an exact date comparison function, rather than a 770 less-than function, for deciding whether to send a 304 (Not 771 Modified) response. To get best results when sending an If- 772 Modified-Since header field for cache validation, clients are 773 advised to use the exact date string received in a previous Last- 774 Modified header field whenever possible. 776 Note: If a client uses an arbitrary date in the If-Modified-Since 777 header field instead of a date taken from the Last-Modified header 778 field for the same request, the client needs to be aware that this 779 date is interpreted in the server's understanding of time. 780 Unsynchronized clocks and rounding problems, due to the different 781 encodings of time between the client and server, are concerns. 782 This includes the possibility of race conditions if the document 783 has changed between the time it was first requested and the If- 784 Modified-Since date of a subsequent request, and the possibility 785 of clock-skew-related problems if the If-Modified-Since date is 786 derived from the client's clock without correction to the server's 787 clock. Corrections for different time bases between client and 788 server are at best approximate due to network latency. 790 The result of a request having both an If-Modified-Since header field 791 and either an If-Match or an If-Unmodified-Since header field is 792 undefined by this specification. 794 3.4. If-Unmodified-Since 796 The "If-Unmodified-Since" header field MAY be used to make a request 797 method conditional by modification date: if the selected 798 representation has been modified since the time specified in this 799 field, then the server MUST NOT perform the requested operation and 800 MUST instead respond with the 412 (Precondition Failed) status code. 801 If the selected representation has not been modified since the time 802 specified in this field, the server SHOULD perform the request method 803 as if the If-Unmodified-Since header field were not present. 805 If-Unmodified-Since = HTTP-date 807 An example of the field is: 809 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT 811 If the request normally (i.e., without the If-Unmodified-Since header 812 field) would result in anything other than a 2xx or 412 status code, 813 the If-Unmodified-Since header field SHOULD be ignored. 815 If the specified date is invalid, the header field MUST be ignored. 817 The result of a request having both an If-Unmodified-Since header 818 field and either an If-None-Match or an If-Modified-Since header 819 field is undefined by this specification. 821 3.5. If-Range 823 The If-Range header field provides a special conditional request 824 mechanism that is similar to If-Match and If-Unmodified-Since but 825 specific to HTTP range requests. If-Range is defined in Section 5.3 826 of [Part5]. 828 4. Status Code Definitions 830 4.1. 304 Not Modified 832 The 304 status code indicates that a conditional GET request has been 833 received and would have resulted in a 200 (OK) response if it were 834 not for the fact that the condition has evaluated to false. In other 835 words, there is no need for the server to transfer a representation 836 of the target resource because the client's request indicates that it 837 already has a valid representation, as indicated by the 304 response 838 header fields, and is therefore redirecting the client to make use of 839 that stored representation as if it were the payload of a 200 840 response. The 304 response MUST NOT contain a message-body, and thus 841 is always terminated by the first empty line after the header fields. 843 A 304 response MUST include a Date header field (Section 10.2 of 844 [Part2]) unless the origin server does not have a clock that can 845 provide a reasonable approximation of the current time. If a 200 846 response to the same request would have included any of the header 847 fields Cache-Control, Content-Location, ETag, Expires, or Vary, then 848 those same header fields MUST be sent in a 304 response. 850 Since the goal of a 304 response is to minimize information transfer 851 when the recipient already has one or more cached representations, 852 the response SHOULD NOT include representation metadata other than 853 the above listed fields unless said metadata exists for the purpose 854 of guiding cache updates (e.g., future HTTP extensions). 856 If the recipient of a 304 response does not have a cached 857 representation corresponding to the entity-tag indicated by the 304 858 response, then the recipient MUST NOT use the 304 to update its own 859 cache. If this conditional request originated with an outbound 860 client, such as a user agent with its own cache sending a conditional 861 GET to a shared proxy, then the 304 response MAY be forwarded to the 862 outbound client. Otherwise, the recipient MUST disregard the 304 863 response and repeat the request without any preconditions. 865 If a cache uses a received 304 response to update a cache entry, the 866 cache MUST update the entry to reflect any new field values given in 867 the response. 869 4.2. 412 Precondition Failed 871 The 412 status code indicates that one or more preconditions given in 872 the request header fields evaluated to false when tested on the 873 server. This response code allows the client to place preconditions 874 on the current resource state (its current representations and 875 metadata) and thus prevent the request method from being applied if 876 the target resource is in an unexpected state. 878 5. IANA Considerations 880 5.1. Status Code Registration 882 The HTTP Status Code Registry located at 883 shall be updated 884 with the registrations below: 886 +-------+---------------------+-------------+ 887 | Value | Description | Reference | 888 +-------+---------------------+-------------+ 889 | 304 | Not Modified | Section 4.1 | 890 | 412 | Precondition Failed | Section 4.2 | 891 +-------+---------------------+-------------+ 893 5.2. Header Field Registration 895 The Message Header Field Registry located at shall be 897 updated with the permanent registrations below (see [RFC3864]): 899 +---------------------+----------+----------+-------------+ 900 | Header Field Name | Protocol | Status | Reference | 901 +---------------------+----------+----------+-------------+ 902 | ETag | http | standard | Section 2.3 | 903 | If-Match | http | standard | Section 3.1 | 904 | If-Modified-Since | http | standard | Section 3.3 | 905 | If-None-Match | http | standard | Section 3.2 | 906 | If-Unmodified-Since | http | standard | Section 3.4 | 907 | Last-Modified | http | standard | Section 2.2 | 908 +---------------------+----------+----------+-------------+ 910 The change controller is: "IETF (iesg@ietf.org) - Internet 911 Engineering Task Force". 913 6. Security Considerations 915 No additional security considerations have been identified beyond 916 those applicable to HTTP in general [Part1]. 918 7. Acknowledgments 920 See Section 9 of [Part1]. 922 8. References 924 8.1. Normative References 926 [Part1] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 927 "HTTP/1.1, part 1: URIs, Connections, and Message 928 Parsing", draft-ietf-httpbis-p1-messaging-19 (work in 929 progress), March 2012. 931 [Part2] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 932 "HTTP/1.1, part 2: Message Semantics", 933 draft-ietf-httpbis-p2-semantics-19 (work in progress), 934 March 2012. 936 [Part3] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 937 "HTTP/1.1, part 3: Message Payload and Content 938 Negotiation", draft-ietf-httpbis-p3-payload-19 (work in 939 progress), March 2012. 941 [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 942 "HTTP/1.1, part 5: Range Requests and Partial Responses", 943 draft-ietf-httpbis-p5-range-19 (work in progress), 944 March 2012. 946 [Part6] Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed., 947 and J. Reschke, Ed., "HTTP/1.1, part 6: Caching", 948 draft-ietf-httpbis-p6-cache-19 (work in progress), 949 March 2012. 951 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 952 Requirement Levels", BCP 14, RFC 2119, March 1997. 954 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 955 Specifications: ABNF", STD 68, RFC 5234, January 2008. 957 8.2. Informative References 959 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 960 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 961 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 963 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 964 Procedures for Message Header Fields", BCP 90, RFC 3864, 965 September 2004. 967 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed 968 Authoring and Versioning (WebDAV)", RFC 4918, June 2007. 970 Appendix A. Changes from RFC 2616 972 Allow weak entity-tags in all requests except range requests 973 (Sections 2.1 and 3.2). 975 Change ETag header field ABNF not to use quoted-string, thus avoiding 976 escaping issues. (Section 2.3) 978 Change ABNF productions for header fields to only define the field 979 value. (Section 3) 981 Appendix B. Collected ABNF 983 ETag = entity-tag 985 HTTP-date = 987 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 988 entity-tag ] ) ) 989 If-Modified-Since = HTTP-date 990 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 991 entity-tag ] ) ) 992 If-Unmodified-Since = HTTP-date 994 Last-Modified = HTTP-date 996 OWS = 998 entity-tag = [ weak ] opaque-tag 999 etagc = "!" / %x23-7E ; '#'-'~' 1000 / obs-text 1002 obs-text = 1003 opaque-tag = DQUOTE *etagc DQUOTE 1005 weak = %x57.2F ; W/ 1007 ABNF diagnostics: 1009 ; ETag defined but not used 1010 ; If-Match defined but not used 1011 ; If-Modified-Since defined but not used 1012 ; If-None-Match defined but not used 1013 ; If-Unmodified-Since defined but not used 1014 ; Last-Modified defined but not used 1016 Appendix C. Change Log (to be removed by RFC Editor before publication) 1018 C.1. Since RFC 2616 1020 Extracted relevant partitions from [RFC2616]. 1022 C.2. Since draft-ietf-httpbis-p4-conditional-00 1024 Closed issues: 1026 o : "Normative and 1027 Informative references" 1029 Other changes: 1031 o Move definitions of 304 and 412 condition codes from Part2. 1033 C.3. Since draft-ietf-httpbis-p4-conditional-01 1035 Ongoing work on ABNF conversion 1036 (): 1038 o Add explicit references to BNF syntax and rules imported from 1039 other parts of the specification. 1041 C.4. Since draft-ietf-httpbis-p4-conditional-02 1043 Closed issues: 1045 o : "Weak ETags on 1046 non-GET requests" 1048 Ongoing work on IANA Message Header Field Registration 1049 (): 1051 o Reference RFC 3984, and update header field registrations for 1052 header fields defined in this document. 1054 C.5. Since draft-ietf-httpbis-p4-conditional-03 1056 Closed issues: 1058 o : "Examples for 1059 ETag matching" 1061 o : "'entity 1062 value' undefined" 1064 o : "bogus 2068 1065 Date header reference" 1067 C.6. Since draft-ietf-httpbis-p4-conditional-04 1069 Ongoing work on ABNF conversion 1070 (): 1072 o Use "/" instead of "|" for alternatives. 1074 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional 1075 whitespace ("OWS") and required whitespace ("RWS"). 1077 o Rewrite ABNFs to spell out whitespace rules, factor out header 1078 field value format definitions. 1080 C.7. Since draft-ietf-httpbis-p4-conditional-05 1082 Final work on ABNF conversion 1083 (): 1085 o Add appendix containing collected and expanded ABNF, reorganize 1086 ABNF introduction. 1088 C.8. Since draft-ietf-httpbis-p4-conditional-06 1090 Closed issues: 1092 o : "case- 1093 sensitivity of etag weakness indicator" 1095 C.9. Since draft-ietf-httpbis-p4-conditional-07 1097 Closed issues: 1099 o : "Weak ETags on 1100 non-GET requests" (If-Match still was defined to require strong 1101 matching) 1103 o : "move IANA 1104 registrations for optional status codes" 1106 C.10. Since draft-ietf-httpbis-p4-conditional-08 1108 No significant changes. 1110 C.11. Since draft-ietf-httpbis-p4-conditional-09 1112 No significant changes. 1114 C.12. Since draft-ietf-httpbis-p4-conditional-10 1116 Closed issues: 1118 o : "Clarify 1119 'Requested Variant'" 1121 o : "Clarify 1122 entity / representation / variant terminology" 1124 o : "consider 1125 removing the 'changes from 2068' sections" 1127 C.13. Since draft-ietf-httpbis-p4-conditional-11 1129 None. 1131 C.14. Since draft-ietf-httpbis-p4-conditional-12 1133 Closed issues: 1135 o : "Header 1136 Classification" 1138 C.15. Since draft-ietf-httpbis-p4-conditional-13 1140 Closed issues: 1142 o : "If-* and 1143 entities" 1145 o : "Definition of 1146 validator weakness" 1148 o : "untangle 1149 ABNFs for header fields" 1151 o : "ETags and 1152 Quotes" 1154 C.16. Since draft-ietf-httpbis-p4-conditional-14 1156 None. 1158 C.17. Since draft-ietf-httpbis-p4-conditional-15 1160 Closed issues: 1162 o : "If-Range 1163 should be listed when dicussing contexts where L-M can be 1164 considered strong" 1166 C.18. Since draft-ietf-httpbis-p4-conditional-16 1168 Closed issues: 1170 o : "Document 1171 HTTP's error-handling philosophy" 1173 C.19. Since draft-ietf-httpbis-p4-conditional-17 1175 Closed issues: 1177 o : "does etag 1178 value really use quoted-string" 1180 C.20. Since draft-ietf-httpbis-p4-conditional-18 1182 Closed issues: 1184 o : "Required 1185 headers on 304 and 206" 1187 Index 1189 3 1190 304 Not Modified (status code) 18 1192 4 1193 412 Precondition Failed (status code) 19 1195 E 1196 ETag header field 9 1198 G 1199 Grammar 1200 entity-tag 9 1201 ETag 9 1202 etagc 9 1203 If-Match 14 1204 If-Modified-Since 16 1205 If-None-Match 15 1206 If-Unmodified-Since 17 1207 Last-Modified 7 1208 opaque-tag 9 1209 weak 9 1211 H 1212 Header Fields 1213 ETag 9 1214 If-Match 14 1215 If-Modified-Since 16 1216 If-None-Match 15 1217 If-Unmodified-Since 17 1218 Last-Modified 7 1220 I 1221 If-Match header field 14 1222 If-Modified-Since header field 16 1223 If-None-Match header field 15 1224 If-Unmodified-Since header field 17 1226 L 1227 Last-Modified header field 7 1229 M 1230 metadata 5 1232 S 1233 selected representation 4 1234 Status Codes 1235 304 Not Modified 18 1236 412 Precondition Failed 19 1238 V 1239 validator 5 1240 strong 5 1241 weak 5 1243 Authors' Addresses 1245 Roy T. Fielding (editor) 1246 Adobe Systems Incorporated 1247 345 Park Ave 1248 San Jose, CA 95110 1249 USA 1251 EMail: fielding@gbiv.com 1252 URI: http://roy.gbiv.com/ 1254 Yves Lafon (editor) 1255 World Wide Web Consortium 1256 W3C / ERCIM 1257 2004, rte des Lucioles 1258 Sophia-Antipolis, AM 06902 1259 France 1261 EMail: ylafon@w3.org 1262 URI: http://www.raubacapeu.net/people/yves/ 1263 Julian F. Reschke (editor) 1264 greenbytes GmbH 1265 Hafenweg 16 1266 Muenster, NW 48155 1267 Germany 1269 Phone: +49 251 2807760 1270 Fax: +49 251 2807761 1271 EMail: julian.reschke@greenbytes.de 1272 URI: http://greenbytes.de/tech/webdav/