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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. Reschke, Ed. 5 Intended status: Standards Track greenbytes 6 Expires: May 21, 2014 November 17, 2013 8 Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests 9 draft-ietf-httpbis-p4-conditional-25 11 Abstract 13 The Hypertext Transfer Protocol (HTTP) is an application-level 14 protocol for distributed, collaborative, hypertext information 15 systems. This document defines HTTP/1.1 conditional requests, 16 including metadata header fields for indicating state changes, 17 request header fields for making preconditions on such state, and 18 rules for constructing the responses to a conditional request when 19 one or more preconditions evaluate to false. 21 Editorial Note (To be removed by RFC Editor) 23 Discussion of this draft takes place on the HTTPBIS working group 24 mailing list (ietf-http-wg@w3.org), which is archived at 25 . 27 The current issues list is at 28 and related 29 documents (including fancy diffs) can be found at 30 . 32 The changes in this draft are summarized in Appendix D.1. 34 Status of This Memo 36 This Internet-Draft is submitted in full conformance with the 37 provisions of BCP 78 and BCP 79. 39 Internet-Drafts are working documents of the Internet Engineering 40 Task Force (IETF). Note that other groups may also distribute 41 working documents as Internet-Drafts. The list of current Internet- 42 Drafts is at http://datatracker.ietf.org/drafts/current/. 44 Internet-Drafts are draft documents valid for a maximum of six months 45 and may be updated, replaced, or obsoleted by other documents at any 46 time. It is inappropriate to use Internet-Drafts as reference 47 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on May 21, 2014. 50 Copyright Notice 52 Copyright (c) 2013 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (http://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 This document may contain material from IETF Documents or IETF 66 Contributions published or made publicly available before November 67 10, 2008. The person(s) controlling the copyright in some of this 68 material may not have granted the IETF Trust the right to allow 69 modifications of such material outside the IETF Standards Process. 70 Without obtaining an adequate license from the person(s) controlling 71 the copyright in such materials, this document may not be modified 72 outside the IETF Standards Process, and derivative works of it may 73 not be created outside the IETF Standards Process, except to format 74 it for publication as an RFC or to translate it into languages other 75 than English. 77 Table of Contents 79 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 80 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4 81 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4 82 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5 83 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5 84 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7 85 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7 86 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8 87 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 88 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 9 89 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 10 90 2.3.3. Example: Entity-tags Varying on Content-Negotiated 91 Resources . . . . . . . . . . . . . . . . . . . . . . 11 92 2.4. When to Use Entity-tags and Last-Modified Dates . . . . . 12 93 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 13 94 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 13 95 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 14 96 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 15 97 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 16 98 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18 99 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18 100 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18 101 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 18 102 5. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 19 103 6. Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . 19 104 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 105 7.1. Status Code Registration . . . . . . . . . . . . . . . . . 21 106 7.2. Header Field Registration . . . . . . . . . . . . . . . . 21 107 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 108 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 109 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 110 10.1. Normative References . . . . . . . . . . . . . . . . . . . 23 111 10.2. Informative References . . . . . . . . . . . . . . . . . . 23 112 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 23 113 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 24 114 Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 24 115 Appendix D. Change Log (to be removed by RFC Editor before 116 publication) . . . . . . . . . . . . . . . . . . . . 25 117 D.1. Since draft-ietf-httpbis-p4-conditional-24 . . . . . . . . 25 118 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 120 1. Introduction 122 Conditional requests are HTTP requests [Part2] that include one or 123 more header fields indicating a precondition to be tested before 124 applying the method semantics to the target resource. This document 125 defines the HTTP/1.1 conditional request mechanisms in terms of the 126 architecture, syntax notation, and conformance criteria defined in 127 [Part1]. 129 Conditional GET requests are the most efficient mechanism for HTTP 130 cache updates [Part6]. Conditionals can also be applied to state- 131 changing methods, such as PUT and DELETE, to prevent the "lost 132 update" problem: one client accidentally overwriting the work of 133 another client that has been acting in parallel. 135 Conditional request preconditions are based on the state of the 136 target resource as a whole (its current value set) or the state as 137 observed in a previously obtained representation (one value in that 138 set). A resource might have multiple current representations, each 139 with its own observable state. The conditional request mechanisms 140 assume that the mapping of requests to a "selected representation" 141 (Section 3 of [Part2]) will be consistent over time if the server 142 intends to take advantage of conditionals. Regardless, if the 143 mapping is inconsistent and the server is unable to select the 144 appropriate representation, then no harm will result when the 145 precondition evaluates to false. 147 The conditional request preconditions defined by this specification 148 (Section 3) are evaluated when applicable to the recipient 149 (Section 5) according to their order of precedence (Section 6). 151 1.1. Conformance and Error Handling 153 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 154 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 155 document are to be interpreted as described in [RFC2119]. 157 Conformance criteria and considerations regarding error handling are 158 defined in Section 2.5 of [Part1]. 160 1.2. Syntax Notation 162 This specification uses the Augmented Backus-Naur Form (ABNF) 163 notation of [RFC5234] with the list rule extension defined in Section 164 7 of [Part1]. Appendix B describes rules imported from other 165 documents. Appendix C shows the collected ABNF with the list rule 166 expanded. 168 2. Validators 170 This specification defines two forms of metadata that are commonly 171 used to observe resource state and test for preconditions: 172 modification dates (Section 2.2) and opaque entity tags 173 (Section 2.3). Additional metadata that reflects resource state has 174 been defined by various extensions of HTTP, such as WebDAV [RFC4918], 175 that are beyond the scope of this specification. A resource metadata 176 value is referred to as a "validator" when it is used within a 177 precondition. 179 2.1. Weak versus Strong 181 Validators come in two flavors: strong or weak. Weak validators are 182 easy to generate but are far less useful for comparisons. Strong 183 validators are ideal for comparisons but can be very difficult (and 184 occasionally impossible) to generate efficiently. Rather than impose 185 that all forms of resource adhere to the same strength of validator, 186 HTTP exposes the type of validator in use and imposes restrictions on 187 when weak validators can be used as preconditions. 189 A "strong validator" is representation metadata that changes value 190 whenever a change occurs to the representation data that would be 191 observable in the payload body of a 200 (OK) response to GET. 193 A strong validator might change for reasons other than a change to 194 the representation data, such as when a semantically significant part 195 of the representation metadata is changed (e.g., Content-Type), but 196 it is in the best interests of the origin server to only change the 197 value when it is necessary to invalidate the stored responses held by 198 remote caches and authoring tools. 200 Cache entries might persist for arbitrarily long periods, regardless 201 of expiration times. Thus, a cache might attempt to validate an 202 entry using a validator that it obtained in the distant past. A 203 strong validator is unique across all versions of all representations 204 associated with a particular resource over time. However, there is 205 no implication of uniqueness across representations of different 206 resources (i.e., the same strong validator might be in use for 207 representations of multiple resources at the same time and does not 208 imply that those representations are equivalent). 210 There are a variety of strong validators used in practice. The best 211 are based on strict revision control, wherein each change to a 212 representation always results in a unique node name and revision 213 identifier being assigned before the representation is made 214 accessible to GET. A collision-resistant hash function applied to 215 the representation data is also sufficient if the data is available 216 prior to the response header fields being sent and the digest does 217 not need to be recalculated every time a validation request is 218 received. However, if a resource has distinct representations that 219 differ only in their metadata, such as might occur with content 220 negotiation over media types that happen to share the same data 221 format, then the origin server needs to incorporate additional 222 information in the validator to distinguish those representations. 224 In contrast, a "weak validator" is representation metadata that might 225 not change for every change to the representation data. This 226 weakness might be due to limitations in how the value is calculated, 227 such as clock resolution or an inability to ensure uniqueness for all 228 possible representations of the resource, or due to a desire by the 229 resource owner to group representations by some self-determined set 230 of equivalency rather than unique sequences of data. An origin 231 server SHOULD change a weak entity-tag whenever it considers prior 232 representations to be unacceptable as a substitute for the current 233 representation. In other words, a weak entity-tag ought to change 234 whenever the origin server wants caches to invalidate old responses. 236 For example, the representation of a weather report that changes in 237 content every second, based on dynamic measurements, might be grouped 238 into sets of equivalent representations (from the origin server's 239 perspective) with the same weak validator in order to allow cached 240 representations to be valid for a reasonable period of time (perhaps 241 adjusted dynamically based on server load or weather quality). 242 Likewise, a representation's modification time, if defined with only 243 one-second resolution, might be a weak validator if it is possible 244 for the representation to be modified twice during a single second 245 and retrieved between those modifications. 247 Likewise, a validator is weak if it is shared by two or more 248 representations of a given resource at the same time, unless those 249 representations have identical representation data. For example, if 250 the origin server sends the same validator for a representation with 251 a gzip content coding applied as it does for a representation with no 252 content coding, then that validator is weak. However, two 253 simultaneous representations might share the same strong validator if 254 they differ only in the representation metadata, such as when two 255 different media types are available for the same representation data. 257 Strong validators are usable for all conditional requests, including 258 cache validation, partial content ranges, and "lost update" 259 avoidance. Weak validators are only usable when the client does not 260 require exact equality with previously obtained representation data, 261 such as when validating a cache entry or limiting a web traversal to 262 recent changes. 264 2.2. Last-Modified 266 The "Last-Modified" header field in a response provides a timestamp 267 indicating the date and time at which the origin server believes the 268 selected representation was last modified, as determined at the 269 conclusion of handling the request. 271 Last-Modified = HTTP-date 273 An example of its use is 275 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 277 2.2.1. Generation 279 An origin server SHOULD send Last-Modified for any selected 280 representation for which a last modification date can be reasonably 281 and consistently determined, since its use in conditional requests 282 and evaluating cache freshness ([Part6]) results in a substantial 283 reduction of HTTP traffic on the Internet and can be a significant 284 factor in improving service scalability and reliability. 286 A representation is typically the sum of many parts behind the 287 resource interface. The last-modified time would usually be the most 288 recent time that any of those parts were changed. How that value is 289 determined for any given resource is an implementation detail beyond 290 the scope of this specification. What matters to HTTP is how 291 recipients of the Last-Modified header field can use its value to 292 make conditional requests and test the validity of locally cached 293 responses. 295 An origin server SHOULD obtain the Last-Modified value of the 296 representation as close as possible to the time that it generates the 297 Date field value for its response. This allows a recipient to make 298 an accurate assessment of the representation's modification time, 299 especially if the representation changes near the time that the 300 response is generated. 302 An origin server with a clock MUST NOT send a Last-Modified date that 303 is later than the server's time of message origination (Date). If 304 the last modification time is derived from implementation-specific 305 metadata that evaluates to some time in the future, according to the 306 origin server's clock, then the origin server MUST replace that value 307 with the message origination date. This prevents a future 308 modification date from having an adverse impact on cache validation. 310 An origin server without a clock MUST NOT assign Last-Modified values 311 to a response unless these values were associated with the resource 312 by some other system or user with a reliable clock. 314 2.2.2. Comparison 316 A Last-Modified time, when used as a validator in a request, is 317 implicitly weak unless it is possible to deduce that it is strong, 318 using the following rules: 320 o The validator is being compared by an origin server to the actual 321 current validator for the representation and, 323 o That origin server reliably knows that the associated 324 representation did not change twice during the second covered by 325 the presented validator. 327 or 329 o The validator is about to be used by a client in an If-Modified- 330 Since, If-Unmodified-Since header field, because the client has a 331 cache entry, or If-Range for the associated representation, and 333 o That cache entry includes a Date value, which gives the time when 334 the origin server sent the original response, and 336 o The presented Last-Modified time is at least 60 seconds before the 337 Date value. 339 or 341 o The validator is being compared by an intermediate cache to the 342 validator stored in its cache entry for the representation, and 344 o That cache entry includes a Date value, which gives the time when 345 the origin server sent the original response, and 347 o The presented Last-Modified time is at least 60 seconds before the 348 Date value. 350 This method relies on the fact that if two different responses were 351 sent by the origin server during the same second, but both had the 352 same Last-Modified time, then at least one of those responses would 353 have a Date value equal to its Last-Modified time. The arbitrary 60- 354 second limit guards against the possibility that the Date and Last- 355 Modified values are generated from different clocks, or at somewhat 356 different times during the preparation of the response. An 357 implementation MAY use a value larger than 60 seconds, if it is 358 believed that 60 seconds is too short. 360 2.3. ETag 362 The "ETag" header field in a response provides the current entity-tag 363 for the selected representation, as determined at the conclusion of 364 handling the request. An entity-tag is an opaque validator for 365 differentiating between multiple representations of the same 366 resource, regardless of whether those multiple representations are 367 due to resource state changes over time, content negotiation 368 resulting in multiple representations being valid at the same time, 369 or both. An entity-tag consists of an opaque quoted string, possibly 370 prefixed by a weakness indicator. 372 ETag = entity-tag 374 entity-tag = [ weak ] opaque-tag 375 weak = %x57.2F ; "W/", case-sensitive 376 opaque-tag = DQUOTE *etagc DQUOTE 377 etagc = %x21 / %x23-7E / obs-text 378 ; VCHAR except double quotes, plus obs-text 380 Note: Previously, opaque-tag was defined to be a quoted-string 381 ([RFC2616], Section 3.11), thus some recipients might perform 382 backslash unescaping. Servers therefore ought to avoid backslash 383 characters in entity tags. 385 An entity-tag can be more reliable for validation than a modification 386 date in situations where it is inconvenient to store modification 387 dates, where the one-second resolution of HTTP date values is not 388 sufficient, or where modification dates are not consistently 389 maintained. 391 Examples: 393 ETag: "xyzzy" 394 ETag: W/"xyzzy" 395 ETag: "" 397 An entity-tag can be either a weak or strong validator, with strong 398 being the default. If an origin server provides an entity-tag for a 399 representation and the generation of that entity-tag does not satisfy 400 all of the characteristics of a strong validator (Section 2.1), then 401 the origin server MUST mark the entity-tag as weak by prefixing its 402 opaque value with "W/" (case-sensitive). 404 2.3.1. Generation 406 The principle behind entity-tags is that only the service author 407 knows the implementation of a resource well enough to select the most 408 accurate and efficient validation mechanism for that resource, and 409 that any such mechanism can be mapped to a simple sequence of octets 410 for easy comparison. Since the value is opaque, there is no need for 411 the client to be aware of how each entity-tag is constructed. 413 For example, a resource that has implementation-specific versioning 414 applied to all changes might use an internal revision number, perhaps 415 combined with a variance identifier for content negotiation, to 416 accurately differentiate between representations. Other 417 implementations might use a collision-resistant hash of 418 representation content, a combination of various filesystem 419 attributes, or a modification timestamp that has sub-second 420 resolution. 422 An origin server SHOULD send ETag for any selected representation for 423 which detection of changes can be reasonably and consistently 424 determined, since the entity-tag's use in conditional requests and 425 evaluating cache freshness ([Part6]) can result in a substantial 426 reduction of HTTP network traffic and can be a significant factor in 427 improving service scalability and reliability. 429 2.3.2. Comparison 431 There are two entity-tag comparison functions, depending on whether 432 the comparison context allows the use of weak validators or not: 434 o Strong comparison: two entity-tags are equivalent if both are not 435 weak and their opaque-tags match character-by-character. 437 o Weak comparison: two entity-tags are equivalent if their opaque- 438 tags match character-by-character, regardless of either or both 439 being tagged as "weak". 441 The example below shows the results for a set of entity-tag pairs, 442 and both the weak and strong comparison function results: 444 +--------+--------+-------------------+-----------------+ 445 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | 446 +--------+--------+-------------------+-----------------+ 447 | W/"1" | W/"1" | no match | match | 448 | W/"1" | W/"2" | no match | no match | 449 | W/"1" | "1" | no match | match | 450 | "1" | "1" | match | match | 451 +--------+--------+-------------------+-----------------+ 453 2.3.3. Example: Entity-tags Varying on Content-Negotiated Resources 455 Consider a resource that is subject to content negotiation (Section 456 3.4 of [Part2]), and where the representations sent in response to a 457 GET request vary based on the Accept-Encoding request header field 458 (Section 5.3.4 of [Part2]): 460 >> Request: 462 GET /index HTTP/1.1 463 Host: www.example.com 464 Accept-Encoding: gzip 466 In this case, the response might or might not use the gzip content 467 coding. If it does not, the response might look like: 469 >> Response: 471 HTTP/1.1 200 OK 472 Date: Fri, 26 Mar 2010 00:05:00 GMT 473 ETag: "123-a" 474 Content-Length: 70 475 Vary: Accept-Encoding 476 Content-Type: text/plain 478 Hello World! 479 Hello World! 480 Hello World! 481 Hello World! 482 Hello World! 484 An alternative representation that does use gzip content coding would 485 be: 487 >> Response: 489 HTTP/1.1 200 OK 490 Date: Fri, 26 Mar 2010 00:05:00 GMT 491 ETag: "123-b" 492 Content-Length: 43 493 Vary: Accept-Encoding 494 Content-Type: text/plain 495 Content-Encoding: gzip 497 ...binary data... 499 Note: Content codings are a property of the representation data, 500 so a strong entity-tag for a content-encoded representation has to 501 be distinct from the entity tag of an unencoded representation to 502 prevent potential conflicts during cache updates and range 503 requests. In contrast, transfer codings (Section 4 of [Part1]) 504 apply only during message transfer and do not result in distinct 505 entity-tags. 507 2.4. When to Use Entity-tags and Last-Modified Dates 509 We adopt a set of rules and recommendations for origin servers, 510 clients, and caches regarding when various validator types ought to 511 be used, and for what purposes. 513 In 200 (OK) responses to GET or HEAD, an origin server: 515 o SHOULD send an entity-tag validator unless it is not feasible to 516 generate one. 518 o MAY send a weak entity-tag instead of a strong entity-tag, if 519 performance considerations support the use of weak entity-tags, or 520 if it is unfeasible to send a strong entity-tag. 522 o SHOULD send a Last-Modified value if it is feasible to send one. 524 In other words, the preferred behavior for an origin server is to 525 send both a strong entity-tag and a Last-Modified value in successful 526 responses to a retrieval request. 528 A client: 530 o MUST send that entity-tag in any cache validation request (using 531 If-Match or If-None-Match) if an entity-tag has been provided by 532 the origin server. 534 o SHOULD send the Last-Modified value in non-subrange cache 535 validation requests (using If-Modified-Since) if only a Last- 536 Modified value has been provided by the origin server. 538 o MAY send the Last-Modified value in subrange cache validation 539 requests (using If-Unmodified-Since) if only a Last-Modified value 540 has been provided by an HTTP/1.0 origin server. The user agent 541 SHOULD provide a way to disable this, in case of difficulty. 543 o SHOULD send both validators in cache validation requests if both 544 an entity-tag and a Last-Modified value have been provided by the 545 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to 546 respond appropriately. 548 3. Precondition Header Fields 550 This section defines the syntax and semantics of HTTP/1.1 header 551 fields for applying preconditions on requests. Section 5 defines 552 when the preconditions are applied. Section 6 defines the order of 553 evaluation when more than one precondition is present. 555 3.1. If-Match 557 The "If-Match" header field makes the request method conditional on 558 the recipient origin server either having at least one current 559 representation of the target resource, when the field-value is "*", 560 or having a current representation of the target resource that has an 561 entity-tag matching a member of the list of entity-tags provided in 562 the field-value. 564 An origin server MUST use the strong comparison function when 565 comparing entity-tags for If-Match (Section 2.3.2), since the client 566 intends this precondition to prevent the method from being applied if 567 there have been any changes to the representation data. 569 If-Match = "*" / 1#entity-tag 571 Examples: 573 If-Match: "xyzzy" 574 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 575 If-Match: * 577 If-Match is most often used with state-changing methods (e.g., POST, 578 PUT, DELETE) to prevent accidental overwrites when multiple user 579 agents might be acting in parallel on the same resource (i.e., to 580 prevent the "lost update" problem). It can also be used with safe 581 methods to abort a request if the selected representation does not 582 match one already stored (or partially stored) from a prior request. 584 An origin server that receives an If-Match header field MUST evaluate 585 the condition prior to performing the method (Section 5). If the 586 field-value is "*", the condition is false if the origin server does 587 not have a current representation for the target resource. If the 588 field-value is a list of entity-tags, the condition is false if none 589 of the listed tags match the entity-tag of the selected 590 representation. 592 An origin server MUST NOT perform the requested method if a received 593 If-Match condition evaluates to false; instead the origin server MUST 594 respond with either: a) the 412 (Precondition Failed) status code; 595 or, b) one of the 2xx (Successful) status codes if the origin server 596 has verified that a state change is being requested and the final 597 state is already reflected in the current state of the target 598 resource (i.e., the change requested by the user agent has already 599 succeeded, but the user agent might not be aware of it, perhaps 600 because the prior response was lost or a compatible change was made 601 by some other user agent). In the latter case, the origin server 602 MUST NOT send a validator header field in the response unless it can 603 verify that the request is a duplicate of an immediately prior change 604 made by the same user agent. 606 The If-Match header field can be ignored by caches and intermediaries 607 because it is not applicable to a stored response. 609 3.2. If-None-Match 611 The "If-None-Match" header field makes the request method conditional 612 on a recipient cache or origin server either not having any current 613 representation of the target resource, when the field-value is "*", 614 or having a selected representation with an entity-tag that does not 615 match any of those listed in the field-value. 617 A recipient MUST use the weak comparison function when comparing 618 entity-tags for If-None-Match (Section 2.3.2), since weak entity-tags 619 can be used for cache validation even if there have been changes to 620 the representation data. 622 If-None-Match = "*" / 1#entity-tag 624 Examples: 626 If-None-Match: "xyzzy" 627 If-None-Match: W/"xyzzy" 628 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 629 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" 630 If-None-Match: * 632 If-None-Match is primarily used in conditional GET requests to enable 633 efficient updates of cached information with a minimum amount of 634 transaction overhead. When a client desires to update one or more 635 stored responses that have entity-tags, the client SHOULD generate an 636 If-None-Match header field containing a list of those entity-tags 637 when making a GET request; this allows recipient servers to send a 638 304 (Not Modified) response to indicate when one of those stored 639 responses matches the selected representation. 641 If-None-Match can also be used with a value of "*" to prevent an 642 unsafe request method (e.g., PUT) from inadvertently modifying an 643 existing representation of the target resource when the client 644 believes that the resource does not have a current representation 645 (Section 4.2.1 of [Part2]). This is a variation on the "lost update" 646 problem that might arise if more than one client attempts to create 647 an initial representation for the target resource. 649 An origin server that receives an If-None-Match header field MUST 650 evaluate the condition prior to performing the method (Section 5). 651 If the field-value is "*", the condition is false if the origin 652 server has a current representation for the target resource. If the 653 field-value is a list of entity-tags, the condition is false if one 654 of the listed tags match the entity-tag of the selected 655 representation. 657 An origin server MUST NOT perform the requested method if the 658 condition evaluates to false; instead, the origin server MUST respond 659 with either a) the 304 (Not Modified) status code if the request 660 method is GET or HEAD; or, b) the 412 (Precondition Failed) status 661 code for all other request methods. 663 Requirements on cache handling of a received If-None-Match header 664 field are defined in Section 4.3.2 of [Part6]. 666 3.3. If-Modified-Since 668 The "If-Modified-Since" header field makes a GET or HEAD request 669 method conditional on the selected representation's modification date 670 being more recent than the date provided in the field-value. 671 Transfer of the selected representation's data is avoided if that 672 data has not changed. 674 If-Modified-Since = HTTP-date 676 An example of the field is: 678 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 680 A recipient MUST ignore If-Modified-Since if the request contains an 681 If-None-Match header field; the condition in If-None-Match is 682 considered to be a more accurate replacement for the condition in If- 683 Modified-Since and the two are only combined for the sake of 684 interoperating with older intermediaries that might not implement If- 685 None-Match. 687 A recipient MUST ignore the If-Modified-Since header field if the 688 received field-value is not a valid HTTP-date, or if the request 689 method is neither GET nor HEAD. 691 A recipient MUST interpret an If-Modified-Since field-value's 692 timestamp in terms of the origin server's clock. 694 If-Modified-Since is typically used for two distinct purposes: 1) to 695 allow efficient updates of a cached representation that does not have 696 an entity-tag; and, 2) to limit the scope of a web traversal to 697 resources that have recently changed. 699 When used for cache updates, a cache will typically use the value of 700 the cached message's Last-Modified field to generate the field value 701 of If-Modified-Since. This behavior is most interoperable for cases 702 where clocks are poorly synchronized or when the server has chosen to 703 only honor exact timestamp matches (due to a problem with Last- 704 Modified dates that appear to go "back in time" when the origin 705 server's clock is corrected or a representation is restored from an 706 archived backup). However, caches occasionally generate the field 707 value based on other data, such as the Date header field of the 708 cached message or the local clock time that the message was received, 709 particularly when the cached message does not contain a Last-Modified 710 field. 712 When used for limiting the scope of retrieval to a recent time 713 window, a user agent will generate an If-Modified-Since field value 714 based on either its own local clock or a Date header field received 715 from the server in a prior response. Origin servers that choose an 716 exact timestamp match based on the selected representation's Last- 717 Modified field will not be able to help the user agent limit its data 718 transfers to only those changed during the specified window. 720 An origin server that receives an If-Modified-Since header field 721 SHOULD evaluate the condition prior to performing the method 722 (Section 5). The origin server SHOULD NOT perform the requested 723 method if the selected representation's last modification date is 724 earlier than or equal to the date provided in the field-value; 725 instead, the origin server SHOULD generate a 304 (Not Modified) 726 response, including only those metadata that are useful for 727 identifying or updating a previously cached response. 729 Requirements on cache handling of a received If-Modified-Since header 730 field are defined in Section 4.3.2 of [Part6]. 732 3.4. If-Unmodified-Since 734 The "If-Unmodified-Since" header field makes the request method 735 conditional on the selected representation's last modification date 736 being earlier than or equal to the date provided in the field-value. 737 This field accomplishes the same purpose as If-Match for cases where 738 the user agent does not have an entity-tag for the representation. 740 If-Unmodified-Since = HTTP-date 742 An example of the field is: 744 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT 746 A recipient MUST ignore If-Unmodified-Since if the request contains 747 an If-Match header field; the condition in If-Match is considered to 748 be a more accurate replacement for the condition in If-Unmodified- 749 Since and the two are only combined for the sake of interoperating 750 with older intermediaries that might not implement If-Match. 752 A recipient MUST ignore the If-Unmodified-Since header field if the 753 received field-value is not a valid HTTP-date. 755 A recipient MUST interpret an If-Unmodified-Since field-value's 756 timestamp in terms of the origin server's clock. 758 If-Unmodified-Since is most often used with state-changing methods 759 (e.g., POST, PUT, DELETE) to prevent accidental overwrites when 760 multiple user agents might be acting in parallel on a resource that 761 does not supply entity-tags with its representations (i.e., to 762 prevent the "lost update" problem). It can also be used with safe 763 methods to abort a request if the selected representation does not 764 match one already stored (or partially stored) from a prior request. 766 An origin server that receives an If-Unmodified-Since header field 767 MUST evaluate the condition prior to performing the method 768 (Section 5). The origin server MUST NOT perform the requested method 769 if the selected representation's last modification date is more 770 recent than the date provided in the field-value; instead the origin 771 server MUST respond with either: a) the 412 (Precondition Failed) 772 status code; or, b) one of the 2xx (Successful) status codes if the 773 origin server has verified that a state change is being requested and 774 the final state is already reflected in the current state of the 775 target resource (i.e., the change requested by the user agent has 776 already succeeded, but the user agent might not be aware of that 777 because the prior response message was lost or a compatible change 778 was made by some other user agent). In the latter case, the origin 779 server MUST NOT send a validator header field in the response unless 780 it can verify that the request is a duplicate of an immediately prior 781 change made by the same user agent. 783 The If-Unmodified-Since header field can be ignored by caches and 784 intermediaries because it is not applicable to a stored response. 786 3.5. If-Range 788 The "If-Range" header field provides a special conditional request 789 mechanism that is similar to the If-Match and If-Unmodified-Since 790 header fields but instructs the recipient to ignore the Range header 791 field if the validator doesn't match, resulting in transfer of the 792 new selected representation instead of a 412 response. If-Range is 793 defined in Section 3.2 of [Part5]. 795 4. Status Code Definitions 797 4.1. 304 Not Modified 799 The 304 (Not Modified) status code indicates that a conditional GET 800 or HEAD request has been received and would have resulted in a 200 801 (OK) response if it were not for the fact that the condition has 802 evaluated to false. In other words, there is no need for the server 803 to transfer a representation of the target resource because the 804 request indicates that the client, which made the request 805 conditional, already has a valid representation; the server is 806 therefore redirecting the client to make use of that stored 807 representation as if it were the payload of a 200 (OK) response. 809 The server generating a 304 response MUST generate any of the 810 following header fields that would have been sent in a 200 (OK) 811 response to the same request: Cache-Control, Content-Location, Date, 812 ETag, Expires, and Vary. 814 Since the goal of a 304 response is to minimize information transfer 815 when the recipient already has one or more cached representations, a 816 sender SHOULD NOT generate representation metadata other than the 817 above listed fields unless said metadata exists for the purpose of 818 guiding cache updates (e.g., Last-Modified might be useful if the 819 response does not have an ETag field). 821 Requirements on a cache that receives a 304 response are defined in 822 Section 4.3.4 of [Part6]. If the conditional request originated with 823 an outbound client, such as a user agent with its own cache sending a 824 conditional GET to a shared proxy, then the proxy SHOULD forward the 825 304 response to that client. 827 A 304 response cannot contain a message-body; it is always terminated 828 by the first empty line after the header fields. 830 4.2. 412 Precondition Failed 832 The 412 (Precondition Failed) status code indicates that one or more 833 conditions given in the request header fields evaluated to false when 834 tested on the server. This response code allows the client to place 835 preconditions on the current resource state (its current 836 representations and metadata) and thus prevent the request method 837 from being applied if the target resource is in an unexpected state. 839 5. Evaluation 841 Except when excluded below, a recipient cache or origin server MUST 842 evaluate received request preconditions after it has successfully 843 performed its normal request checks and just before it would perform 844 the action associated with the request method. A server MUST ignore 845 all received preconditions if its response to the same request 846 without those conditions would have been a status code other than a 847 2xx or 412 (Precondition Failed). In other words, redirects and 848 failures take precedence over the evaluation of preconditions in 849 conditional requests. 851 A server that is not the origin server for the target resource and 852 cannot act as a cache for requests on the target resource MUST NOT 853 evaluate the conditional request header fields defined by this 854 specification, and MUST forward them if the request is forwarded, 855 since the generating client intends that they be evaluated by a 856 server that can provide a current representation. Likewise, a server 857 MUST ignore the conditional request header fields defined by this 858 specification when received with a request method that does not 859 involve the selection or modification of a selected representation, 860 such as CONNECT, OPTIONS, or TRACE. 862 Conditional request header fields that are defined by extensions to 863 HTTP might place conditions on all recipients, on the state of the 864 target resource in general, or on a group of resources. For 865 instance, the "If" header field in WebDAV can make a request 866 conditional on various aspects of multiple resources, such as locks, 867 if the recipient understands and implements that field ([RFC4918], 868 Section 10.4). 870 Although conditional request header fields are defined as being 871 usable with the HEAD method (to keep HEAD's semantics consistent with 872 those of GET), there is no point in sending a conditional HEAD 873 because a successful response is around the same size as a 304 (Not 874 Modified) response and more useful than a 412 (Precondition Failed) 875 response. 877 6. Precedence 879 When more than one conditional request header field is present in a 880 request, the order in which the fields are evaluated becomes 881 important. In practice, the fields defined in this document are 882 consistently implemented in a single, logical order, since "lost 883 update" preconditions have more strict requirements than cache 884 validation, a validated cache is more efficient than a partial 885 response, and entity tags are presumed to be more accurate than date 886 validators. 888 A recipient cache or origin server MUST evaluate the request 889 preconditions defined by this specification in the following order: 891 1. When recipient is the origin server and If-Match is present, 892 evaluate the If-Match precondition: 894 * if true, continue to step 3 896 * if false, respond 412 (Precondition Failed) unless it can be 897 determined that the state-changing request has already 898 succeeded (see Section 3.1) 900 2. When recipient is the origin server, If-Match is not present, and 901 If-Unmodified-Since is present, evaluate the If-Unmodified-Since 902 precondition: 904 * if true, continue to step 3 906 * if false, respond 412 (Precondition Failed) unless it can be 907 determined that the state-changing request has already 908 succeeded (see Section 3.4) 910 3. When If-None-Match is present, evaluate the If-None-Match 911 precondition: 913 * if true, continue to step 5 915 * if false for GET/HEAD, respond 304 (Not Modified) 917 * if false for other methods, respond 412 (Precondition Failed) 919 4. When the method is GET or HEAD, If-None-Match is not present, and 920 If-Modified-Since is present, evaluate the If-Modified-Since 921 precondition: 923 * if true, continue to step 5 925 * if false, respond 304 (Not Modified) 927 5. When the method is GET and both Range and If-Range are present, 928 evaluate the If-Range precondition: 930 * if the validator matches and the Range specification is 931 applicable to the selected representation, respond 206 932 (Partial Content) [Part5] 934 6. Otherwise, 936 * all conditions are met, so perform the requested action and 937 respond according to its success or failure. 939 Any extension to HTTP/1.1 that defines additional conditional request 940 header fields ought to define its own expectations regarding the 941 order for evaluating such fields in relation to those defined in this 942 document and other conditionals that might be found in practice. 944 7. IANA Considerations 946 7.1. Status Code Registration 948 The HTTP Status Code Registry located at 949 shall be updated 950 with the registrations below: 952 +-------+---------------------+-------------+ 953 | Value | Description | Reference | 954 +-------+---------------------+-------------+ 955 | 304 | Not Modified | Section 4.1 | 956 | 412 | Precondition Failed | Section 4.2 | 957 +-------+---------------------+-------------+ 959 7.2. Header Field Registration 961 HTTP header fields are registered within the Message Header Field 962 Registry maintained at . 965 This document defines the following HTTP header fields, so their 966 associated registry entries shall be updated according to the 967 permanent registrations below (see [BCP90]): 969 +---------------------+----------+----------+-------------+ 970 | Header Field Name | Protocol | Status | Reference | 971 +---------------------+----------+----------+-------------+ 972 | ETag | http | standard | Section 2.3 | 973 | If-Match | http | standard | Section 3.1 | 974 | If-Modified-Since | http | standard | Section 3.3 | 975 | If-None-Match | http | standard | Section 3.2 | 976 | If-Unmodified-Since | http | standard | Section 3.4 | 977 | Last-Modified | http | standard | Section 2.2 | 978 +---------------------+----------+----------+-------------+ 980 The change controller is: "IETF (iesg@ietf.org) - Internet 981 Engineering Task Force". 983 8. Security Considerations 985 This section is meant to inform developers, information providers, 986 and users of known security concerns specific to the HTTP/1.1 987 conditional request mechanisms. More general security considerations 988 are addressed in HTTP messaging [Part1] and semantics [Part2]. 990 The validators defined by this specification are not intended to 991 ensure the validity of a representation, guard against malicious 992 changes, or detect man-in-the-middle attacks. At best, they enable 993 more efficient cache updates and optimistic concurrent writes when 994 all participants are behaving nicely. At worst, the conditions will 995 fail and the client will receive a response that is no more harmful 996 than an HTTP exchange without conditional requests. 998 An entity-tag can be abused in ways that create privacy risks. For 999 example, a site might deliberately construct a semantically invalid 1000 entity-tag that is unique to the user or user agent, send it in a 1001 cacheable response with a long freshness time, and then read that 1002 entity-tag in later conditional requests as a means of re-identifying 1003 that user or user agent. Such an identifying tag would become a 1004 persistent identifier for as long as the user agent retained the 1005 original cache entry. User agents that cache representations ought 1006 to ensure that the cache is cleared or replaced whenever the user 1007 performs privacy-maintaining actions, such as clearing stored cookies 1008 or changing to a private browsing mode. 1010 9. Acknowledgments 1012 See Section 10 of [Part1]. 1014 10. References 1015 10.1. Normative References 1017 [Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1018 Protocol (HTTP/1.1): Message Syntax and Routing", 1019 draft-ietf-httpbis-p1-messaging-25 (work in progress), 1020 November 2013. 1022 [Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1023 Protocol (HTTP/1.1): Semantics and Content", 1024 draft-ietf-httpbis-p2-semantics-25 (work in progress), 1025 November 2013. 1027 [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 1028 "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", 1029 draft-ietf-httpbis-p5-range-25 (work in progress), 1030 November 2013. 1032 [Part6] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 1033 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 1034 draft-ietf-httpbis-p6-cache-25 (work in progress), 1035 November 2013. 1037 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1038 Requirement Levels", BCP 14, RFC 2119, March 1997. 1040 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1041 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1043 10.2. Informative References 1045 [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration 1046 Procedures for Message Header Fields", BCP 90, RFC 3864, 1047 September 2004. 1049 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1050 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1051 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1053 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed 1054 Authoring and Versioning (WebDAV)", RFC 4918, June 2007. 1056 Appendix A. Changes from RFC 2616 1058 The definition of validator weakness has been expanded and clarified. 1059 (Section 2.1) 1061 Weak entity-tags are now allowed in all requests except range 1062 requests. (Sections 2.1 and 3.2) 1063 The ETag header field ABNF has been changed to not use quoted-string, 1064 thus avoiding escaping issues. (Section 2.3) 1066 ETag is defined to provide an entity tag for the selected 1067 representation, thereby clarifying what it applies to in various 1068 situations (such as a PUT response). (Section 2.3) 1070 The precedence for evaluation of conditional requests has been 1071 defined. (Section 6) 1073 Appendix B. Imported ABNF 1075 The following core rules are included by reference, as defined in 1076 Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), 1077 CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double 1078 quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 1079 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII 1080 character). 1082 The rules below are defined in [Part1]: 1084 OWS = 1085 obs-text = 1087 The rules below are defined in other parts: 1089 HTTP-date = 1091 Appendix C. Collected ABNF 1093 In the collected ABNF below, list rules are expanded as per Section 1094 1.2 of [Part1]. 1096 ETag = entity-tag 1098 HTTP-date = 1100 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1101 entity-tag ] ) ) 1102 If-Modified-Since = HTTP-date 1103 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1104 entity-tag ] ) ) 1105 If-Unmodified-Since = HTTP-date 1107 Last-Modified = HTTP-date 1109 OWS = 1111 entity-tag = [ weak ] opaque-tag 1112 etagc = "!" / %x23-7E ; '#'-'~' 1113 / obs-text 1115 obs-text = 1116 opaque-tag = DQUOTE *etagc DQUOTE 1118 weak = %x57.2F ; W/ 1120 Appendix D. Change Log (to be removed by RFC Editor before publication) 1122 Changes up to the IETF Last Call draft are summarized in . 1125 D.1. Since draft-ietf-httpbis-p4-conditional-24 1127 Closed issues: 1129 o : "APPSDIR 1130 review of draft-ietf-httpbis-p4-conditional-24" 1132 Index 1134 3 1135 304 Not Modified (status code) 18 1137 4 1138 412 Precondition Failed (status code) 18 1140 E 1141 ETag header field 9 1143 G 1144 Grammar 1145 entity-tag 9 1146 ETag 9 1147 etagc 9 1148 If-Match 13 1149 If-Modified-Since 15 1150 If-None-Match 14 1151 If-Unmodified-Since 16 1152 Last-Modified 7 1153 opaque-tag 9 1154 weak 9 1156 I 1157 If-Match header field 13 1158 If-Modified-Since header field 15 1159 If-None-Match header field 14 1160 If-Unmodified-Since header field 16 1162 L 1163 Last-Modified header field 7 1165 M 1166 metadata 5 1168 S 1169 selected representation 4 1171 V 1172 validator 5 1173 strong 5 1174 weak 5 1176 Authors' Addresses 1178 Roy T. Fielding (editor) 1179 Adobe Systems Incorporated 1180 345 Park Ave 1181 San Jose, CA 95110 1182 USA 1184 EMail: fielding@gbiv.com 1185 URI: http://roy.gbiv.com/ 1186 Julian F. Reschke (editor) 1187 greenbytes GmbH 1188 Hafenweg 16 1189 Muenster, NW 48155 1190 Germany 1192 EMail: julian.reschke@greenbytes.de 1193 URI: http://greenbytes.de/tech/webdav/