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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) J. Gettys 5 Intended status: Standards Track Alcatel-Lucent 6 Expires: May 3, 2012 J. Mogul 7 HP 8 H. Frystyk 9 Microsoft 10 L. Masinter 11 Adobe 12 P. Leach 13 Microsoft 14 T. Berners-Lee 15 W3C/MIT 16 Y. Lafon, Ed. 17 W3C 18 J. Reschke, Ed. 19 greenbytes 20 October 31, 2011 22 HTTP/1.1, part 4: Conditional Requests 23 draft-ietf-httpbis-p4-conditional-17 25 Abstract 27 The Hypertext Transfer Protocol (HTTP) is an application-level 28 protocol for distributed, collaborative, hypertext information 29 systems. HTTP has been in use by the World Wide Web global 30 information initiative since 1990. This document is Part 4 of the 31 seven-part specification that defines the protocol referred to as 32 "HTTP/1.1" and, taken together, obsoletes RFC 2616. 34 Part 4 defines request header fields for indicating conditional 35 requests and the rules for constructing responses to those requests. 37 Editorial Note (To be removed by RFC Editor) 39 Discussion of this draft should take place on the HTTPBIS working 40 group mailing list (ietf-http-wg@w3.org), which is archived at 41 . 43 The current issues list is at 44 and related 45 documents (including fancy diffs) can be found at 46 . 48 The changes in this draft are summarized in Appendix C.18. 50 Status of This Memo 52 This Internet-Draft is submitted in full conformance with the 53 provisions of BCP 78 and BCP 79. 55 Internet-Drafts are working documents of the Internet Engineering 56 Task Force (IETF). Note that other groups may also distribute 57 working documents as Internet-Drafts. The list of current Internet- 58 Drafts is at http://datatracker.ietf.org/drafts/current/. 60 Internet-Drafts are draft documents valid for a maximum of six months 61 and may be updated, replaced, or obsoleted by other documents at any 62 time. It is inappropriate to use Internet-Drafts as reference 63 material or to cite them other than as "work in progress." 65 This Internet-Draft will expire on May 3, 2012. 67 Copyright Notice 69 Copyright (c) 2011 IETF Trust and the persons identified as the 70 document authors. All rights reserved. 72 This document is subject to BCP 78 and the IETF Trust's Legal 73 Provisions Relating to IETF Documents 74 (http://trustee.ietf.org/license-info) in effect on the date of 75 publication of this document. Please review these documents 76 carefully, as they describe your rights and restrictions with respect 77 to this document. Code Components extracted from this document must 78 include Simplified BSD License text as described in Section 4.e of 79 the Trust Legal Provisions and are provided without warranty as 80 described in the Simplified BSD License. 82 This document may contain material from IETF Documents or IETF 83 Contributions published or made publicly available before November 84 10, 2008. The person(s) controlling the copyright in some of this 85 material may not have granted the IETF Trust the right to allow 86 modifications of such material outside the IETF Standards Process. 87 Without obtaining an adequate license from the person(s) controlling 88 the copyright in such materials, this document may not be modified 89 outside the IETF Standards Process, and derivative works of it may 90 not be created outside the IETF Standards Process, except to format 91 it for publication as an RFC or to translate it into languages other 92 than English. 94 Table of Contents 96 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 97 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 5 98 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 99 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 6 100 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 6 101 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 8 102 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 8 103 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 9 104 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 105 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 11 106 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 11 107 2.3.3. Example: Entity-tags varying on Content-Negotiated 108 Resources . . . . . . . . . . . . . . . . . . . . . . 12 109 2.4. Rules for When to Use Entity-tags and Last-Modified 110 Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 13 111 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14 112 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 15 113 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15 114 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 17 115 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 18 116 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 19 117 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 19 118 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 19 119 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 20 120 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 121 5.1. Status Code Registration . . . . . . . . . . . . . . . . . 20 122 5.2. Header Field Registration . . . . . . . . . . . . . . . . 20 123 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 124 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 125 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 126 8.1. Normative References . . . . . . . . . . . . . . . . . . . 21 127 8.2. Informative References . . . . . . . . . . . . . . . . . . 21 128 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22 129 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 22 130 Appendix C. Change Log (to be removed by RFC Editor before 131 publication) . . . . . . . . . . . . . . . . . . . . 23 132 C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 23 133 C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23 134 C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23 135 C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23 136 C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 24 137 C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24 138 C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24 139 C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24 140 C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24 141 C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 25 142 C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 25 143 C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25 144 C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25 145 C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25 146 C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25 147 C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 26 148 C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 26 149 C.18. Since draft-ietf-httpbis-p4-conditional-16 . . . . . . . . 26 150 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 152 1. Introduction 154 This document defines the HTTP/1.1 conditional request mechanisms, 155 including both metadata for indicating/observing changes in resource 156 representations and request header fields that specify preconditions 157 on that metadata be checked before performing the request method. 158 Conditional GET requests are the most efficient mechanism for HTTP 159 cache updates [Part6]. Conditionals can also be applied to state- 160 changing methods, such as PUT and DELETE, to prevent the "lost 161 update" problem: one client accidentally overwriting the work of 162 another client that has been acting in parallel. 164 Conditional request preconditions are based on the state of the 165 target resource as a whole (its current value set) or the state as 166 observed in a previously obtained representation (one value in that 167 set). A resource might have multiple current representations, each 168 with its own observable state. The conditional request mechanisms 169 assume that the mapping of requests to corresponding representations 170 will be consistent over time if the server intends to take advantage 171 of conditionals. Regardless, if the mapping is inconsistent and the 172 server is unable to select the appropriate representation, then no 173 harm will result when the precondition evaluates to false. 175 We use the term "selected representation" to refer to the current 176 representation of the target resource that would have been selected 177 in a successful response if the same request had used the method GET 178 and had excluded all of the conditional request header fields. The 179 conditional request preconditions are evaluated by comparing the 180 values provided in the request header fields to the current metadata 181 for the selected representation. 183 1.1. Conformance and Error Handling 185 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 186 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 187 document are to be interpreted as described in [RFC2119]. 189 This document defines conformance criteria for several roles in HTTP 190 communication, including Senders, Recipients, Clients, Servers, User- 191 Agents, Origin Servers, Intermediaries, Proxies and Gateways. See 192 Section 2 of [Part1] for definitions of these terms. 194 An implementation is considered conformant if it complies with all of 195 the requirements associated with its role(s). Note that SHOULD-level 196 requirements are relevant here, unless one of the documented 197 exceptions is applicable. 199 This document also uses ABNF to define valid protocol elements 200 (Section 1.2). In addition to the prose requirements placed upon 201 them, Senders MUST NOT generate protocol elements that are invalid. 203 Unless noted otherwise, Recipients MAY take steps to recover a usable 204 protocol element from an invalid construct. However, HTTP does not 205 define specific error handling mechanisms, except in cases where it 206 has direct impact on security. This is because different uses of the 207 protocol require different error handling strategies; for example, a 208 Web browser may wish to transparently recover from a response where 209 the Location header field doesn't parse according to the ABNF, 210 whereby in a systems control protocol using HTTP, this type of error 211 recovery could lead to dangerous consequences. 213 1.2. Syntax Notation 215 This specification uses the ABNF syntax defined in Section 1.2 of 216 [Part1] (which extends the syntax defined in [RFC5234] with a list 217 rule). Appendix B shows the collected ABNF, with the list rule 218 expanded. 220 The following core rules are included by reference, as defined in 221 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF 222 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), 223 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit 224 sequence of data), SP (space), and VCHAR (any visible US-ASCII 225 character). 227 The ABNF rules below are defined in [Part1] and [Part2]: 229 OWS = 230 quoted-string = 231 HTTP-date = 233 2. Validators 235 This specification defines two forms of metadata that are commonly 236 used to observe resource state and test for preconditions: 237 modification dates and opaque entity tags. Additional metadata that 238 reflects resource state has been defined by various extensions of 239 HTTP, such as WebDAV [RFC4918], that are beyond the scope of this 240 specification. A resource metadata value is referred to as a 241 "validator" when it is used within a precondition. 243 2.1. Weak versus Strong 245 Validators come in two flavors: strong or weak. Weak validators are 246 easy to generate but are far less useful for comparisons. Strong 247 validators are ideal for comparisons but can be very difficult (and 248 occasionally impossible) to generate efficiently. Rather than impose 249 that all forms of resource adhere to the same strength of validator, 250 HTTP exposes the type of validator in use and imposes restrictions on 251 when weak validators can be used as preconditions. 253 A "strong validator" is a representation metadata value that MUST be 254 changed to a new, previously unused or guaranteed unique, value 255 whenever a change occurs to the representation data such that a 256 change would be observable in the payload body of a 200 response to 257 GET. A strong validator MAY be changed for other reasons, such as 258 when a semantically significant part of the representation metadata 259 is changed (e.g., Content-Type), but it is in the best interests of 260 the origin server to only change the value when it is necessary to 261 invalidate the stored responses held by remote caches and authoring 262 tools. A strong validator MUST be unique across all representations 263 of a given resource, such that no two representations of that 264 resource share the same validator unless their payload body would be 265 identical. 267 Cache entries might persist for arbitrarily long periods, regardless 268 of expiration times. Thus, a cache might attempt to validate an 269 entry using a validator that it obtained in the distant past. A 270 strong validator MUST be unique across all versions of all 271 representations associated with a particular resource over time. 272 However, there is no implication of uniqueness across representations 273 of different resources (i.e., the same strong validator might be in 274 use for representations of multiple resources at the same time and 275 does not imply that those representations are equivalent). 277 There are a variety of strong validators used in practice. The best 278 are based on strict revision control, wherein each change to a 279 representation always results in a unique node name and revision 280 identifier being assigned before the representation is made 281 accessible to GET. A cryptographic hash function applied to the 282 representation data is also sufficient if the data is available prior 283 to the response header fields being sent and the digest does not need 284 to be recalculated every time a validation request is received. 285 However, if a resource has distinct representations that differ only 286 in their metadata, such as might occur with content negotiation over 287 media types that happen to share the same data format, then a server 288 SHOULD incorporate additional information in the validator to 289 distinguish those representations and avoid confusing cache behavior. 291 In contrast, a "weak validator" is a representation metadata value 292 that might not be changed for every change to the representation 293 data. This weakness might be due to limitations in how the value is 294 calculated, such as clock resolution or an inability to ensure 295 uniqueness for all possible representations of the resource, or due 296 to a desire by the resource owner to group representations by some 297 self-determined set of equivalency rather than unique sequences of 298 data. A weak entity-tag SHOULD change whenever the origin server 299 considers prior representations to be unacceptable as a substitute 300 for the current representation. In other words, a weak entity-tag 301 SHOULD change whenever the origin server wants caches to invalidate 302 old responses. 304 For example, the representation of a weather report that changes in 305 content every second, based on dynamic measurements, might be grouped 306 into sets of equivalent representations (from the origin server's 307 perspective) with the same weak validator in order to allow cached 308 representations to be valid for a reasonable period of time (perhaps 309 adjusted dynamically based on server load or weather quality). 310 Likewise, a representation's modification time, if defined with only 311 one-second resolution, might be a weak validator if it is possible 312 for the representation to be modified twice during a single second 313 and retrieved between those modifications. 315 A "use" of a validator occurs when either a client generates a 316 request and includes the validator in a precondition or when a server 317 compares two validators. Weak validators are only usable in contexts 318 that do not depend on exact equality of a representation's payload 319 body. Strong validators are usable and preferred for all conditional 320 requests, including cache validation, partial content ranges, and 321 "lost update" avoidance. 323 2.2. Last-Modified 325 The "Last-Modified" header field indicates the date and time at which 326 the origin server believes the selected representation was last 327 modified. 329 Last-Modified = HTTP-date 331 An example of its use is 333 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 335 2.2.1. Generation 337 Origin servers SHOULD send Last-Modified for any selected 338 representation for which a last modification date can be reasonably 339 and consistently determined, since its use in conditional requests 340 and evaluating cache freshness ([Part6]) results in a substantial 341 reduction of HTTP traffic on the Internet and can be a significant 342 factor in improving service scalability and reliability. 344 A representation is typically the sum of many parts behind the 345 resource interface. The last-modified time would usually be the most 346 recent time that any of those parts were changed. How that value is 347 determined for any given resource is an implementation detail beyond 348 the scope of this specification. What matters to HTTP is how 349 recipients of the Last-Modified header field can use its value to 350 make conditional requests and test the validity of locally cached 351 responses. 353 An origin server SHOULD obtain the Last-Modified value of the 354 representation as close as possible to the time that it generates the 355 Date field-value for its response. This allows a recipient to make 356 an accurate assessment of the representation's modification time, 357 especially if the representation changes near the time that the 358 response is generated. 360 An origin server with a clock MUST NOT send a Last-Modified date that 361 is later than the server's time of message origination (Date). If 362 the last modification time is derived from implementation-specific 363 metadata that evaluates to some time in the future, according to the 364 origin server's clock, then the origin server MUST replace that value 365 with the message origination date. This prevents a future 366 modification date from having an adverse impact on cache validation. 368 An origin server without a clock MUST NOT assign Last-Modified values 369 to a response unless these values were associated with the resource 370 by some other system or user with a reliable clock. 372 2.2.2. Comparison 374 A Last-Modified time, when used as a validator in a request, is 375 implicitly weak unless it is possible to deduce that it is strong, 376 using the following rules: 378 o The validator is being compared by an origin server to the actual 379 current validator for the representation and, 381 o That origin server reliably knows that the associated 382 representation did not change twice during the second covered by 383 the presented validator. 385 or 387 o The validator is about to be used by a client in an If-Modified- 388 Since, If-Unmodified-Since header field, because the client has a 389 cache entry, or If-Range for the associated representation, and 391 o That cache entry includes a Date value, which gives the time when 392 the origin server sent the original response, and 394 o The presented Last-Modified time is at least 60 seconds before the 395 Date value. 397 or 399 o The validator is being compared by an intermediate cache to the 400 validator stored in its cache entry for the representation, and 402 o That cache entry includes a Date value, which gives the time when 403 the origin server sent the original response, and 405 o The presented Last-Modified time is at least 60 seconds before the 406 Date value. 408 This method relies on the fact that if two different responses were 409 sent by the origin server during the same second, but both had the 410 same Last-Modified time, then at least one of those responses would 411 have a Date value equal to its Last-Modified time. The arbitrary 60- 412 second limit guards against the possibility that the Date and Last- 413 Modified values are generated from different clocks, or at somewhat 414 different times during the preparation of the response. An 415 implementation MAY use a value larger than 60 seconds, if it is 416 believed that 60 seconds is too short. 418 2.3. ETag 420 The ETag header field provides the current entity-tag for the 421 selected representation. An entity-tag is an opaque validator for 422 differentiating between multiple representations of the same 423 resource, regardless of whether those multiple representations are 424 due to resource state changes over time, content negotiation 425 resulting in multiple representations being valid at the same time, 426 or both. An entity-tag consists of an opaque quoted string, possibly 427 prefixed by a weakness indicator. 429 ETag = entity-tag 431 entity-tag = [ weak ] opaque-tag 432 weak = %x57.2F ; "W/", case-sensitive 433 opaque-tag = quoted-string 435 An entity-tag can be more reliable for validation than a modification 436 date in situations where it is inconvenient to store modification 437 dates, where the one-second resolution of HTTP date values is not 438 sufficient, or where modification dates are not consistently 439 maintained. 441 Examples: 443 ETag: "xyzzy" 444 ETag: W/"xyzzy" 445 ETag: "" 447 An entity-tag can be either a weak or strong validator, with strong 448 being the default. If an origin server provides an entity-tag for a 449 representation and the generation of that entity-tag does not satisfy 450 the requirements for a strong validator (Section 2.1), then that 451 entity-tag MUST be marked as weak by prefixing its opaque value with 452 "W/" (case-sensitive). 454 2.3.1. Generation 456 The principle behind entity-tags is that only the service author 457 knows the implementation of a resource well enough to select the most 458 accurate and efficient validation mechanism for that resource, and 459 that any such mechanism can be mapped to a simple sequence of octets 460 for easy comparison. Since the value is opaque, there is no need for 461 the client to be aware of how each entity-tag is constructed. 463 For example, a resource that has implementation-specific versioning 464 applied to all changes might use an internal revision number, perhaps 465 combined with a variance identifier for content negotiation, to 466 accurately differentiate between representations. Other 467 implementations might use a stored hash of representation content, a 468 combination of various filesystem attributes, or a modification 469 timestamp that has sub-second resolution. 471 Origin servers SHOULD send ETag for any selected representation for 472 which detection of changes can be reasonably and consistently 473 determined, since the entity-tag's use in conditional requests and 474 evaluating cache freshness ([Part6]) can result in a substantial 475 reduction of HTTP network traffic and can be a significant factor in 476 improving service scalability and reliability. 478 2.3.2. Comparison 480 There are two entity-tag comparison functions, depending on whether 481 the comparison context allows the use of weak validators or not: 483 o The strong comparison function: in order to be considered equal, 484 both opaque-tags MUST be identical character-by-character, and 485 both MUST NOT be weak. 487 o The weak comparison function: in order to be considered equal, 488 both opaque-tags MUST be identical character-by-character, but 489 either or both of them MAY be tagged as "weak" without affecting 490 the result. 492 The example below shows the results for a set of entity-tag pairs, 493 and both the weak and strong comparison function results: 495 +--------+--------+-------------------+-----------------+ 496 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | 497 +--------+--------+-------------------+-----------------+ 498 | W/"1" | W/"1" | no match | match | 499 | W/"1" | W/"2" | no match | no match | 500 | W/"1" | "1" | no match | match | 501 | "1" | "1" | match | match | 502 +--------+--------+-------------------+-----------------+ 504 2.3.3. Example: Entity-tags varying on Content-Negotiated Resources 506 Consider a resource that is subject to content negotiation (Section 5 507 of [Part3]), and where the representations returned upon a GET 508 request vary based on the Accept-Encoding request header field 509 (Section 6.3 of [Part3]): 511 >> Request: 513 GET /index HTTP/1.1 514 Host: www.example.com 515 Accept-Encoding: gzip 517 In this case, the response might or might not use the gzip content 518 coding. If it does not, the response might look like: 520 >> Response: 522 HTTP/1.1 200 OK 523 Date: Thu, 26 Mar 2010 00:05:00 GMT 524 ETag: "123-a" 525 Content-Length: 70 526 Vary: Accept-Encoding 527 Content-Type: text/plain 529 Hello World! 530 Hello World! 531 Hello World! 532 Hello World! 533 Hello World! 535 An alternative representation that does use gzip content coding would 536 be: 538 >> Response: 540 HTTP/1.1 200 OK 541 Date: Thu, 26 Mar 2010 00:05:00 GMT 542 ETag: "123-b" 543 Content-Length: 43 544 Vary: Accept-Encoding 545 Content-Type: text/plain 546 Content-Encoding: gzip 548 ...binary data... 550 Note: Content codings are a property of the representation, so 551 therefore an entity-tag of an encoded representation must be 552 distinct from an unencoded representation to prevent conflicts 553 during cache updates and range requests. In contrast, transfer 554 codings (Section 5.1 of [Part1]) apply only during message 555 transfer and do not require distinct entity-tags. 557 2.4. Rules for When to Use Entity-tags and Last-Modified Dates 559 We adopt a set of rules and recommendations for origin servers, 560 clients, and caches regarding when various validator types ought to 561 be used, and for what purposes. 563 HTTP/1.1 origin servers: 565 o SHOULD send an entity-tag validator unless it is not feasible to 566 generate one. 568 o MAY send a weak entity-tag instead of a strong entity-tag, if 569 performance considerations support the use of weak entity-tags, or 570 if it is unfeasible to send a strong entity-tag. 572 o SHOULD send a Last-Modified value if it is feasible to send one. 574 In other words, the preferred behavior for an HTTP/1.1 origin server 575 is to send both a strong entity-tag and a Last-Modified value. 577 HTTP/1.1 clients: 579 o MUST use that entity-tag in any cache-conditional request (using 580 If-Match or If-None-Match) if an entity-tag has been provided by 581 the origin server. 583 o SHOULD use the Last-Modified value in non-subrange cache- 584 conditional requests (using If-Modified-Since) if only a Last- 585 Modified value has been provided by the origin server. 587 o MAY use the Last-Modified value in subrange cache-conditional 588 requests (using If-Unmodified-Since) if only a Last-Modified value 589 has been provided by an HTTP/1.0 origin server. The user agent 590 SHOULD provide a way to disable this, in case of difficulty. 592 o SHOULD use both validators in cache-conditional requests if both 593 an entity-tag and a Last-Modified value have been provided by the 594 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to 595 respond appropriately. 597 An HTTP/1.1 origin server, upon receiving a conditional request that 598 includes both a Last-Modified date (e.g., in an If-Modified-Since or 599 If-Unmodified-Since header field) and one or more entity-tags (e.g., 600 in an If-Match, If-None-Match, or If-Range header field) as cache 601 validators, MUST NOT return a response status code of 304 (Not 602 Modified) unless doing so is consistent with all of the conditional 603 header fields in the request. 605 An HTTP/1.1 caching proxy, upon receiving a conditional request that 606 includes both a Last-Modified date and one or more entity-tags as 607 cache validators, MUST NOT return a locally cached response to the 608 client unless that cached response is consistent with all of the 609 conditional header fields in the request. 611 Note: The general principle behind these rules is that HTTP/1.1 612 servers and clients ought to transmit as much non-redundant 613 information as is available in their responses and requests. 614 HTTP/1.1 systems receiving this information will make the most 615 conservative assumptions about the validators they receive. 617 HTTP/1.0 clients and caches might ignore entity-tags. Generally, 618 last-modified values received or used by these systems will 619 support transparent and efficient caching, and so HTTP/1.1 origin 620 servers should provide Last-Modified values. In those rare cases 621 where the use of a Last-Modified value as a validator by an 622 HTTP/1.0 system could result in a serious problem, then HTTP/1.1 623 origin servers should not provide one. 625 3. Precondition Header Fields 627 This section defines the syntax and semantics of HTTP/1.1 header 628 fields for applying preconditions on requests. 630 3.1. If-Match 632 The "If-Match" header field MAY be used to make a request method 633 conditional on the current existence or value of an entity-tag for 634 one or more representations of the target resource. If-Match is 635 generally useful for resource update requests, such as PUT requests, 636 as a means for protecting against accidental overwrites when multiple 637 clients are acting in parallel on the same resource (i.e., the "lost 638 update" problem). An If-Match field-value of "*" places the 639 precondition on the existence of any current representation for the 640 target resource. 642 If-Match = "*" / 1#entity-tag 644 If any of the entity-tags listed in the If-Match field value match 645 (as per Section 2.3.2) the entity-tag of the selected representation 646 for the target resource, or if "*" is given and any current 647 representation exists for the target resource, then the server MAY 648 perform the request method as if the If-Match header field was not 649 present. 651 If none of the entity-tags match, or if "*" is given and no current 652 representation exists, the server MUST NOT perform the requested 653 method. Instead, the server MUST respond with the 412 (Precondition 654 Failed) status code. 656 If the request would, without the If-Match header field, result in 657 anything other than a 2xx or 412 status code, then the If-Match 658 header field MUST be ignored. 660 Examples: 662 If-Match: "xyzzy" 663 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 664 If-Match: * 666 The result of a request having both an If-Match header field and 667 either an If-None-Match or an If-Modified-Since header fields is 668 undefined by this specification. 670 3.2. If-None-Match 672 The "If-None-Match" header field MAY be used to make a request method 673 conditional on not matching any of the current entity-tag values for 674 representations of the target resource. If-None-Match is primarily 675 used in conditional GET requests to enable efficient updates of 676 cached information with a minimum amount of transaction overhead. A 677 client that has one or more representations previously obtained from 678 the target resource can send If-None-Match with a list of the 679 associated entity-tags in the hope of receiving a 304 response if at 680 least one of those representations matches the selected 681 representation. 683 If-None-Match MAY also be used with a value of "*" to prevent an 684 unsafe request method (e.g., PUT) from inadvertently modifying an 685 existing representation of the target resource when the client 686 believes that the resource does not have a current representation. 687 This is a variation on the "lost update" problem that might arise if 688 more than one client attempts to create an initial representation for 689 the target resource. 691 If-None-Match = "*" / 1#entity-tag 693 If any of the entity-tags listed in the If-None-Match field-value 694 match (as per Section 2.3.2) the entity-tag of the selected 695 representation, or if "*" is given and any current representation 696 exists for that resource, then the server MUST NOT perform the 697 requested method. Instead, if the request method was GET or HEAD, 698 the server SHOULD respond with a 304 (Not Modified) status code, 699 including the cache-related header fields (particularly ETag) of the 700 selected representation that has a matching entity-tag. For all 701 other request methods, the server MUST respond with a 412 702 (Precondition Failed) status code. 704 If none of the entity-tags match, then the server MAY perform the 705 requested method as if the If-None-Match header field did not exist, 706 but MUST also ignore any If-Modified-Since header field(s) in the 707 request. That is, if no entity-tags match, then the server MUST NOT 708 return a 304 (Not Modified) response. 710 If the request would, without the If-None-Match header field, result 711 in anything other than a 2xx or 304 status code, then the If-None- 712 Match header field MUST be ignored. (See Section 2.4 for a 713 discussion of server behavior when both If-Modified-Since and If- 714 None-Match appear in the same request.) 716 Examples: 718 If-None-Match: "xyzzy" 719 If-None-Match: W/"xyzzy" 720 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 721 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" 722 If-None-Match: * 724 The result of a request having both an If-None-Match header field and 725 either an If-Match or an If-Unmodified-Since header fields is 726 undefined by this specification. 728 3.3. If-Modified-Since 730 The "If-Modified-Since" header field MAY be used to make a request 731 method conditional by modification date: if the selected 732 representation has not been modified since the time specified in this 733 field, then do not perform the request method; instead, respond as 734 detailed below. 736 If-Modified-Since = HTTP-date 738 An example of the field is: 740 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 742 A GET method with an If-Modified-Since header field and no Range 743 header field requests that the selected representation be transferred 744 only if it has been modified since the date given by the If-Modified- 745 Since header field. The algorithm for determining this includes the 746 following cases: 748 1. If the request would normally result in anything other than a 200 749 (OK) status code, or if the passed If-Modified-Since date is 750 invalid, the response is exactly the same as for a normal GET. A 751 date which is later than the server's current time is invalid. 753 2. If the selected representation has been modified since the If- 754 Modified-Since date, the response is exactly the same as for a 755 normal GET. 757 3. If the selected representation has not been modified since a 758 valid If-Modified-Since date, the server SHOULD return a 304 (Not 759 Modified) response. 761 The purpose of this feature is to allow efficient updates of cached 762 information with a minimum amount of transaction overhead. 764 Note: The Range header field modifies the meaning of If-Modified- 765 Since; see Section 5.4 of [Part5] for full details. 767 Note: If-Modified-Since times are interpreted by the server, whose 768 clock might not be synchronized with the client. 770 Note: When handling an If-Modified-Since header field, some 771 servers will use an exact date comparison function, rather than a 772 less-than function, for deciding whether to send a 304 (Not 773 Modified) response. To get best results when sending an If- 774 Modified-Since header field for cache validation, clients are 775 advised to use the exact date string received in a previous Last- 776 Modified header field whenever possible. 778 Note: If a client uses an arbitrary date in the If-Modified-Since 779 header field instead of a date taken from the Last-Modified header 780 field for the same request, the client needs to be aware that this 781 date is interpreted in the server's understanding of time. 782 Unsynchronized clocks and rounding problems, due to the different 783 encodings of time between the client and server, are concerns. 784 This includes the possibility of race conditions if the document 785 has changed between the time it was first requested and the If- 786 Modified-Since date of a subsequent request, and the possibility 787 of clock-skew-related problems if the If-Modified-Since date is 788 derived from the client's clock without correction to the server's 789 clock. Corrections for different time bases between client and 790 server are at best approximate due to network latency. 792 The result of a request having both an If-Modified-Since header field 793 and either an If-Match or an If-Unmodified-Since header fields is 794 undefined by this specification. 796 3.4. If-Unmodified-Since 798 The "If-Unmodified-Since" header field MAY be used to make a request 799 method conditional by modification date: if the selected 800 representation has been modified since the time specified in this 801 field, then the server MUST NOT perform the requested operation and 802 MUST instead respond with the 412 (Precondition Failed) status code. 803 If the selected representation has not been modified since the time 804 specified in this field, the server SHOULD perform the request method 805 as if the If-Unmodified-Since header field were not present. 807 If-Unmodified-Since = HTTP-date 809 An example of the field is: 811 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT 813 If the request normally (i.e., without the If-Unmodified-Since header 814 field) would result in anything other than a 2xx or 412 status code, 815 the If-Unmodified-Since header field SHOULD be ignored. 817 If the specified date is invalid, the header field MUST be ignored. 819 The result of a request having both an If-Unmodified-Since header 820 field and either an If-None-Match or an If-Modified-Since header 821 fields is undefined by this specification. 823 3.5. If-Range 825 The If-Range header field provides a special conditional request 826 mechanism that is similar to If-Match and If-Unmodified-Since but 827 specific to HTTP range requests. If-Range is defined in Section 5.3 828 of [Part5]. 830 4. Status Code Definitions 832 4.1. 304 Not Modified 834 The 304 status code indicates that a conditional GET request has been 835 received and would have resulted in a 200 (OK) response if it were 836 not for the fact that the condition has evaluated to false. In other 837 words, there is no need for the server to transfer a representation 838 of the target resource because the client's request indicates that it 839 already has a valid representation, as indicated by the 304 response 840 header fields, and is therefore redirecting the client to make use of 841 that stored representation as if it were the payload of a 200 842 response. The 304 response MUST NOT contain a message-body, and thus 843 is always terminated by the first empty line after the header fields. 845 A 304 response MUST include a Date header field (Section 9.2 of 846 [Part2]) unless the origin server does not have a clock that can 847 provide a reasonable approximation of the current time. If a 200 848 response to the same request would have included any of the header 849 fields Cache-Control, Content-Location, ETag, Expires, Last-Modified, 850 or Vary, then those same header fields MUST be sent in a 304 851 response. 853 Since the goal of a 304 response is to minimize information transfer 854 when the recipient already has one or more cached representations, 855 the response SHOULD NOT include representation metadata other than 856 the above listed fields unless said metadata exists for the purpose 857 of guiding cache updates (e.g., future HTTP extensions). 859 If the recipient of a 304 response does not have a cached 860 representation corresponding to the entity-tag indicated by the 304 861 response, then the recipient MUST NOT use the 304 to update its own 862 cache. If this conditional request originated with an outbound 863 client, such as a user agent with its own cache sending a conditional 864 GET to a shared proxy, then the 304 response MAY be forwarded to the 865 outbound client. Otherwise, the recipient MUST disregard the 304 866 response and repeat the request without any preconditions. 868 If a cache uses a received 304 response to update a cache entry, the 869 cache MUST update the entry to reflect any new field values given in 870 the response. 872 4.2. 412 Precondition Failed 874 The 412 status code indicates that one or more preconditions given in 875 the request header fields evaluated to false when tested on the 876 server. This response code allows the client to place preconditions 877 on the current resource state (its current representations and 878 metadata) and thus prevent the request method from being applied if 879 the target resource is in an unexpected state. 881 5. IANA Considerations 883 5.1. Status Code Registration 885 The HTTP Status Code Registry located at 886 shall be updated 887 with the registrations below: 889 +-------+---------------------+-------------+ 890 | Value | Description | Reference | 891 +-------+---------------------+-------------+ 892 | 304 | Not Modified | Section 4.1 | 893 | 412 | Precondition Failed | Section 4.2 | 894 +-------+---------------------+-------------+ 896 5.2. Header Field Registration 898 The Message Header Field Registry located at shall be 900 updated with the permanent registrations below (see [RFC3864]): 902 +---------------------+----------+----------+-------------+ 903 | Header Field Name | Protocol | Status | Reference | 904 +---------------------+----------+----------+-------------+ 905 | ETag | http | standard | Section 2.3 | 906 | If-Match | http | standard | Section 3.1 | 907 | If-Modified-Since | http | standard | Section 3.3 | 908 | If-None-Match | http | standard | Section 3.2 | 909 | If-Unmodified-Since | http | standard | Section 3.4 | 910 | Last-Modified | http | standard | Section 2.2 | 911 +---------------------+----------+----------+-------------+ 913 The change controller is: "IETF (iesg@ietf.org) - Internet 914 Engineering Task Force". 916 6. Security Considerations 918 No additional security considerations have been identified beyond 919 those applicable to HTTP in general [Part1]. 921 7. Acknowledgments 923 See Section 11 of [Part1]. 925 8. References 927 8.1. Normative References 929 [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 930 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 931 and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections, 932 and Message Parsing", draft-ietf-httpbis-p1-messaging-17 933 (work in progress), October 2011. 935 [Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 936 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 937 and J. Reschke, Ed., "HTTP/1.1, part 2: Message 938 Semantics", draft-ietf-httpbis-p2-semantics-17 (work in 939 progress), October 2011. 941 [Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 942 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 943 and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload 944 and Content Negotiation", draft-ietf-httpbis-p3-payload-17 945 (work in progress), October 2011. 947 [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 948 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 949 and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and 950 Partial Responses", draft-ietf-httpbis-p5-range-17 (work 951 in progress), October 2011. 953 [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 954 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 955 Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part 956 6: Caching", draft-ietf-httpbis-p6-cache-17 (work in 957 progress), October 2011. 959 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 960 Requirement Levels", BCP 14, RFC 2119, March 1997. 962 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 963 Specifications: ABNF", STD 68, RFC 5234, January 2008. 965 8.2. Informative References 967 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 968 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 969 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 971 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 972 Procedures for Message Header Fields", BCP 90, RFC 3864, 973 September 2004. 975 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed 976 Authoring and Versioning (WebDAV)", RFC 4918, June 2007. 978 Appendix A. Changes from RFC 2616 980 Allow weak entity-tags in all requests except range requests 981 (Sections 2.1 and 3.2). 983 Change ABNF productions for header fields to only define the field 984 value. (Section 3) 986 Appendix B. Collected ABNF 988 ETag = entity-tag 990 HTTP-date = 992 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 993 entity-tag ] ) ) 994 If-Modified-Since = HTTP-date 995 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 996 entity-tag ] ) ) 997 If-Unmodified-Since = HTTP-date 999 Last-Modified = HTTP-date 1001 OWS = 1003 entity-tag = [ weak ] opaque-tag 1005 opaque-tag = quoted-string 1007 quoted-string = 1009 weak = %x57.2F ; W/ 1010 ABNF diagnostics: 1012 ; ETag defined but not used 1013 ; If-Match defined but not used 1014 ; If-Modified-Since defined but not used 1015 ; If-None-Match defined but not used 1016 ; If-Unmodified-Since defined but not used 1017 ; Last-Modified defined but not used 1019 Appendix C. Change Log (to be removed by RFC Editor before publication) 1021 C.1. Since RFC 2616 1023 Extracted relevant partitions from [RFC2616]. 1025 C.2. Since draft-ietf-httpbis-p4-conditional-00 1027 Closed issues: 1029 o : "Normative and 1030 Informative references" 1032 Other changes: 1034 o Move definitions of 304 and 412 condition codes from Part2. 1036 C.3. Since draft-ietf-httpbis-p4-conditional-01 1038 Ongoing work on ABNF conversion 1039 (): 1041 o Add explicit references to BNF syntax and rules imported from 1042 other parts of the specification. 1044 C.4. Since draft-ietf-httpbis-p4-conditional-02 1046 Closed issues: 1048 o : "Weak ETags on 1049 non-GET requests" 1051 Ongoing work on IANA Message Header Field Registration 1052 (): 1054 o Reference RFC 3984, and update header field registrations for 1055 header fields defined in this document. 1057 C.5. Since draft-ietf-httpbis-p4-conditional-03 1059 Closed issues: 1061 o : "Examples for 1062 ETag matching" 1064 o : "'entity 1065 value' undefined" 1067 o : "bogus 2068 1068 Date header reference" 1070 C.6. Since draft-ietf-httpbis-p4-conditional-04 1072 Ongoing work on ABNF conversion 1073 (): 1075 o Use "/" instead of "|" for alternatives. 1077 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional 1078 whitespace ("OWS") and required whitespace ("RWS"). 1080 o Rewrite ABNFs to spell out whitespace rules, factor out header 1081 field value format definitions. 1083 C.7. Since draft-ietf-httpbis-p4-conditional-05 1085 Final work on ABNF conversion 1086 (): 1088 o Add appendix containing collected and expanded ABNF, reorganize 1089 ABNF introduction. 1091 C.8. Since draft-ietf-httpbis-p4-conditional-06 1093 Closed issues: 1095 o : "case- 1096 sensitivity of etag weakness indicator" 1098 C.9. Since draft-ietf-httpbis-p4-conditional-07 1100 Closed issues: 1102 o : "Weak ETags on 1103 non-GET requests" (If-Match still was defined to require strong 1104 matching) 1106 o : "move IANA 1107 registrations for optional status codes" 1109 C.10. Since draft-ietf-httpbis-p4-conditional-08 1111 No significant changes. 1113 C.11. Since draft-ietf-httpbis-p4-conditional-09 1115 No significant changes. 1117 C.12. Since draft-ietf-httpbis-p4-conditional-10 1119 Closed issues: 1121 o : "Clarify 1122 'Requested Variant'" 1124 o : "Clarify 1125 entity / representation / variant terminology" 1127 o : "consider 1128 removing the 'changes from 2068' sections" 1130 C.13. Since draft-ietf-httpbis-p4-conditional-11 1132 None. 1134 C.14. Since draft-ietf-httpbis-p4-conditional-12 1136 Closed issues: 1138 o : "Header 1139 Classification" 1141 C.15. Since draft-ietf-httpbis-p4-conditional-13 1143 Closed issues: 1145 o : "If-* and 1146 entities" 1148 o : "Definition of 1149 validator weakness" 1151 o : "untangle 1152 ABNFs for header fields" 1154 o : "ETags and 1155 Quotes" 1157 C.16. Since draft-ietf-httpbis-p4-conditional-14 1159 None. 1161 C.17. Since draft-ietf-httpbis-p4-conditional-15 1163 Closed issues: 1165 o : "If-Range 1166 should be listed when dicussing contexts where L-M can be 1167 considered strong" 1169 C.18. Since draft-ietf-httpbis-p4-conditional-16 1171 Closed issues: 1173 o : "Document 1174 HTTP's error-handling philosophy" 1176 Index 1178 3 1179 304 Not Modified (status code) 19 1181 4 1182 412 Precondition Failed (status code) 20 1184 E 1185 ETag header field 10 1187 G 1188 Grammar 1189 entity-tag 10 1190 ETag 10 1191 If-Match 15 1192 If-Modified-Since 17 1193 If-None-Match 16 1194 If-Unmodified-Since 18 1195 Last-Modified 8 1196 opaque-tag 10 1197 weak 10 1199 H 1200 Header Fields 1201 ETag 10 1202 If-Match 15 1203 If-Modified-Since 17 1204 If-None-Match 15 1205 If-Unmodified-Since 18 1206 Last-Modified 8 1208 I 1209 If-Match header field 15 1210 If-Modified-Since header field 17 1211 If-None-Match header field 15 1212 If-Unmodified-Since header field 18 1214 L 1215 Last-Modified header field 8 1217 M 1218 metadata 6 1220 S 1221 selected representation 5 1222 Status Codes 1223 304 Not Modified 19 1224 412 Precondition Failed 20 1226 V 1227 validator 6 1228 strong 6 1229 weak 6 1231 Authors' Addresses 1233 Roy T. Fielding (editor) 1234 Adobe Systems Incorporated 1235 345 Park Ave 1236 San Jose, CA 95110 1237 USA 1239 EMail: fielding@gbiv.com 1240 URI: http://roy.gbiv.com/ 1241 Jim Gettys 1242 Alcatel-Lucent Bell Labs 1243 21 Oak Knoll Road 1244 Carlisle, MA 01741 1245 USA 1247 EMail: jg@freedesktop.org 1248 URI: http://gettys.wordpress.com/ 1250 Jeffrey C. Mogul 1251 Hewlett-Packard Company 1252 HP Labs, Large Scale Systems Group 1253 1501 Page Mill Road, MS 1177 1254 Palo Alto, CA 94304 1255 USA 1257 EMail: JeffMogul@acm.org 1259 Henrik Frystyk Nielsen 1260 Microsoft Corporation 1261 1 Microsoft Way 1262 Redmond, WA 98052 1263 USA 1265 EMail: henrikn@microsoft.com 1267 Larry Masinter 1268 Adobe Systems Incorporated 1269 345 Park Ave 1270 San Jose, CA 95110 1271 USA 1273 EMail: LMM@acm.org 1274 URI: http://larry.masinter.net/ 1276 Paul J. Leach 1277 Microsoft Corporation 1278 1 Microsoft Way 1279 Redmond, WA 98052 1281 EMail: paulle@microsoft.com 1282 Tim Berners-Lee 1283 World Wide Web Consortium 1284 MIT Computer Science and Artificial Intelligence Laboratory 1285 The Stata Center, Building 32 1286 32 Vassar Street 1287 Cambridge, MA 02139 1288 USA 1290 EMail: timbl@w3.org 1291 URI: http://www.w3.org/People/Berners-Lee/ 1293 Yves Lafon (editor) 1294 World Wide Web Consortium 1295 W3C / ERCIM 1296 2004, rte des Lucioles 1297 Sophia-Antipolis, AM 06902 1298 France 1300 EMail: ylafon@w3.org 1301 URI: http://www.raubacapeu.net/people/yves/ 1303 Julian F. Reschke (editor) 1304 greenbytes GmbH 1305 Hafenweg 16 1306 Muenster, NW 48155 1307 Germany 1309 Phone: +49 251 2807760 1310 Fax: +49 251 2807761 1311 EMail: julian.reschke@greenbytes.de 1312 URI: http://greenbytes.de/tech/webdav/