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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: January 16, 2014 July 15, 2013 8 Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests 9 draft-ietf-httpbis-p4-conditional-23 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.4. 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 January 16, 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 . . . . . . . . . . . . . . . . . . . . . . 10 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 . . . . . . . . . . . . . . . . . . . . . . . . . 16 99 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 17 100 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 17 101 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 17 102 5. Evaluation and Precedence . . . . . . . . . . . . . . . . . . 17 103 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 104 6.1. Status Code Registration . . . . . . . . . . . . . . . . . 20 105 6.2. Header Field Registration . . . . . . . . . . . . . . . . 20 106 7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 107 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 108 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 109 9.1. Normative References . . . . . . . . . . . . . . . . . . . 21 110 9.2. Informative References . . . . . . . . . . . . . . . . . . 22 111 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22 112 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 22 113 Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 23 114 Appendix D. Change Log (to be removed by RFC Editor before 115 publication) . . . . . . . . . . . . . . . . . . . . 23 116 D.1. Since draft-ietf-httpbis-p4-conditional-19 . . . . . . . . 23 117 D.2. Since draft-ietf-httpbis-p4-conditional-20 . . . . . . . . 24 118 D.3. Since draft-ietf-httpbis-p4-conditional-21 . . . . . . . . 24 119 D.4. Since draft-ietf-httpbis-p4-conditional-22 . . . . . . . . 25 120 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 122 1. Introduction 124 Conditional requests are HTTP requests [Part2] that include one or 125 more header fields indicating a precondition to be tested before 126 applying the method semantics to the target resource. This document 127 defines the HTTP/1.1 conditional request mechanisms in terms of the 128 architecture, syntax notation, and conformance criteria defined in 129 [Part1]. 131 Conditional GET requests are the most efficient mechanism for HTTP 132 cache updates [Part6]. Conditionals can also be applied to state- 133 changing methods, such as PUT and DELETE, to prevent the "lost 134 update" problem: one client accidentally overwriting the work of 135 another client that has been acting in parallel. 137 Conditional request preconditions are based on the state of the 138 target resource as a whole (its current value set) or the state as 139 observed in a previously obtained representation (one value in that 140 set). A resource might have multiple current representations, each 141 with its own observable state. The conditional request mechanisms 142 assume that the mapping of requests to a "selected representation" 143 (Section 3 of [Part2]) will be consistent over time if the server 144 intends to take advantage of conditionals. Regardless, if the 145 mapping is inconsistent and the server is unable to select the 146 appropriate representation, then no harm will result when the 147 precondition evaluates to false. 149 The conditional request preconditions defined by this specification 150 are evaluated by comparing the validators provided in the conditional 151 request header fields to the current validators for the selected 152 representation in the order defined by Section 5. 154 1.1. Conformance and Error Handling 156 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 157 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 158 document are to be interpreted as described in [RFC2119]. 160 Conformance criteria and considerations regarding error handling are 161 defined in Section 2.5 of [Part1]. 163 1.2. Syntax Notation 165 This specification uses the Augmented Backus-Naur Form (ABNF) 166 notation of [RFC5234] with the list rule extension defined in Section 167 1.2 of [Part1]. Appendix B describes rules imported from other 168 documents. Appendix C shows the collected ABNF with the list rule 169 expanded. 171 2. Validators 173 This specification defines two forms of metadata that are commonly 174 used to observe resource state and test for preconditions: 175 modification dates (Section 2.2) and opaque entity tags 176 (Section 2.3). Additional metadata that reflects resource state has 177 been defined by various extensions of HTTP, such as WebDAV [RFC4918], 178 that are beyond the scope of this specification. A resource metadata 179 value is referred to as a "validator" when it is used within a 180 precondition. 182 2.1. Weak versus Strong 184 Validators come in two flavors: strong or weak. Weak validators are 185 easy to generate but are far less useful for comparisons. Strong 186 validators are ideal for comparisons but can be very difficult (and 187 occasionally impossible) to generate efficiently. Rather than impose 188 that all forms of resource adhere to the same strength of validator, 189 HTTP exposes the type of validator in use and imposes restrictions on 190 when weak validators can be used as preconditions. 192 A "strong validator" is representation metadata that changes value 193 whenever a change occurs to the representation data that would be 194 observable in the payload body of a 200 (OK) response to GET. 196 A strong validator might change for other reasons, such as when a 197 semantically significant part of the representation metadata is 198 changed (e.g., Content-Type), but it is in the best interests of the 199 origin server to only change the value when it is necessary to 200 invalidate the stored responses held by remote caches and authoring 201 tools. A strong validator is unique across all representations of a 202 given resource, such that no two representations of that resource can 203 share the same validator unless their representation data is 204 identical. 206 Cache entries might persist for arbitrarily long periods, regardless 207 of expiration times. Thus, a cache might attempt to validate an 208 entry using a validator that it obtained in the distant past. A 209 strong validator is unique across all versions of all representations 210 associated with a particular resource over time. However, there is 211 no implication of uniqueness across representations of different 212 resources (i.e., the same strong validator might be in use for 213 representations of multiple resources at the same time and does not 214 imply that those representations are equivalent). 216 There are a variety of strong validators used in practice. The best 217 are based on strict revision control, wherein each change to a 218 representation always results in a unique node name and revision 219 identifier being assigned before the representation is made 220 accessible to GET. A collision-resistant hash function applied to 221 the representation data is also sufficient if the data is available 222 prior to the response header fields being sent and the digest does 223 not need to be recalculated every time a validation request is 224 received. However, if a resource has distinct representations that 225 differ only in their metadata, such as might occur with content 226 negotiation over media types that happen to share the same data 227 format, then the origin server SHOULD incorporate additional 228 information in the validator to distinguish those representations. 230 In contrast, a "weak validator" is representation metadata that might 231 not change for every change to the representation data. This 232 weakness might be due to limitations in how the value is calculated, 233 such as clock resolution or an inability to ensure uniqueness for all 234 possible representations of the resource, or due to a desire by the 235 resource owner to group representations by some self-determined set 236 of equivalency rather than unique sequences of data. An origin 237 server SHOULD change a weak entity-tag whenever it considers prior 238 representations to be unacceptable as a substitute for the current 239 representation. In other words, a weak entity-tag ought to change 240 whenever the origin server wants caches to invalidate old responses. 242 For example, the representation of a weather report that changes in 243 content every second, based on dynamic measurements, might be grouped 244 into sets of equivalent representations (from the origin server's 245 perspective) with the same weak validator in order to allow cached 246 representations to be valid for a reasonable period of time (perhaps 247 adjusted dynamically based on server load or weather quality). 248 Likewise, a representation's modification time, if defined with only 249 one-second resolution, might be a weak validator if it is possible 250 for the representation to be modified twice during a single second 251 and retrieved between those modifications. 253 Likewise, a validator is weak if it is shared by two or more 254 representations of a given resource at the same time, unless those 255 representations have identical representation data. For example, if 256 the origin server sends the same validator for a representation with 257 a gzip content coding applied as it does for a representation with no 258 content coding, then that validator is weak. However, two 259 simultaneous representations might share the same strong validator if 260 they differ only in the representation metadata, such as when two 261 different media types are available for the same representation data. 263 A "use" of a validator occurs when either a client generates a 264 request and includes the validator in a precondition or when a server 265 compares two validators. Weak validators are only usable in contexts 266 that do not depend on exact equality of the representation data. 268 Strong validators are usable and preferred for all conditional 269 requests, including cache validation, partial content ranges, and 270 "lost update" avoidance. 272 2.2. Last-Modified 274 The "Last-Modified" header field in a response provides a timestamp 275 indicating the date and time at which the origin server believes the 276 selected representation was last modified, as determined at the 277 conclusion of handling the request. 279 Last-Modified = HTTP-date 281 An example of its use is 283 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 285 2.2.1. Generation 287 Origin servers SHOULD send Last-Modified for any selected 288 representation for which a last modification date can be reasonably 289 and consistently determined, since its use in conditional requests 290 and evaluating cache freshness ([Part6]) results in a substantial 291 reduction of HTTP traffic on the Internet and can be a significant 292 factor in improving service scalability and reliability. 294 A representation is typically the sum of many parts behind the 295 resource interface. The last-modified time would usually be the most 296 recent time that any of those parts were changed. How that value is 297 determined for any given resource is an implementation detail beyond 298 the scope of this specification. What matters to HTTP is how 299 recipients of the Last-Modified header field can use its value to 300 make conditional requests and test the validity of locally cached 301 responses. 303 An origin server SHOULD obtain the Last-Modified value of the 304 representation as close as possible to the time that it generates the 305 Date field value for its response. This allows a recipient to make 306 an accurate assessment of the representation's modification time, 307 especially if the representation changes near the time that the 308 response is generated. 310 An origin server with a clock MUST NOT send a Last-Modified date that 311 is later than the server's time of message origination (Date). If 312 the last modification time is derived from implementation-specific 313 metadata that evaluates to some time in the future, according to the 314 origin server's clock, then the origin server MUST replace that value 315 with the message origination date. This prevents a future 316 modification date from having an adverse impact on cache validation. 318 An origin server without a clock MUST NOT assign Last-Modified values 319 to a response unless these values were associated with the resource 320 by some other system or user with a reliable clock. 322 2.2.2. Comparison 324 A Last-Modified time, when used as a validator in a request, is 325 implicitly weak unless it is possible to deduce that it is strong, 326 using the following rules: 328 o The validator is being compared by an origin server to the actual 329 current validator for the representation and, 331 o That origin server reliably knows that the associated 332 representation did not change twice during the second covered by 333 the presented validator. 335 or 337 o The validator is about to be used by a client in an If-Modified- 338 Since, If-Unmodified-Since header field, because the client has a 339 cache entry, or If-Range for the associated representation, and 341 o That cache entry includes a Date value, which gives the time when 342 the origin server sent the original response, and 344 o The presented Last-Modified time is at least 60 seconds before the 345 Date value. 347 or 349 o The validator is being compared by an intermediate cache to the 350 validator stored in its cache entry for the representation, and 352 o That cache entry includes a Date value, which gives the time when 353 the origin server sent the original response, and 355 o The presented Last-Modified time is at least 60 seconds before the 356 Date value. 358 This method relies on the fact that if two different responses were 359 sent by the origin server during the same second, but both had the 360 same Last-Modified time, then at least one of those responses would 361 have a Date value equal to its Last-Modified time. The arbitrary 60- 362 second limit guards against the possibility that the Date and Last- 363 Modified values are generated from different clocks, or at somewhat 364 different times during the preparation of the response. An 365 implementation MAY use a value larger than 60 seconds, if it is 366 believed that 60 seconds is too short. 368 2.3. ETag 370 The "ETag" header field in a response provides the current entity-tag 371 for the selected representation, as determined at the conclusion of 372 handling the request. An entity-tag is an opaque validator for 373 differentiating between multiple representations of the same 374 resource, regardless of whether those multiple representations are 375 due to resource state changes over time, content negotiation 376 resulting in multiple representations being valid at the same time, 377 or both. An entity-tag consists of an opaque quoted string, possibly 378 prefixed by a weakness indicator. 380 ETag = entity-tag 382 entity-tag = [ weak ] opaque-tag 383 weak = %x57.2F ; "W/", case-sensitive 384 opaque-tag = DQUOTE *etagc DQUOTE 385 etagc = %x21 / %x23-7E / obs-text 386 ; VCHAR except double quotes, plus obs-text 388 Note: Previously, opaque-tag was defined to be a quoted-string 389 ([RFC2616], Section 3.11), thus some recipients might perform 390 backslash unescaping. Servers therefore ought to avoid backslash 391 characters in entity tags. 393 An entity-tag can be more reliable for validation than a modification 394 date in situations where it is inconvenient to store modification 395 dates, where the one-second resolution of HTTP date values is not 396 sufficient, or where modification dates are not consistently 397 maintained. 399 Examples: 401 ETag: "xyzzy" 402 ETag: W/"xyzzy" 403 ETag: "" 405 An entity-tag can be either a weak or strong validator, with strong 406 being the default. If an origin server provides an entity-tag for a 407 representation and the generation of that entity-tag does not satisfy 408 all of the characteristics of a strong validator (Section 2.1), then 409 the origin server MUST mark the entity-tag as weak by prefixing its 410 opaque value with "W/" (case-sensitive). 412 2.3.1. Generation 414 The principle behind entity-tags is that only the service author 415 knows the implementation of a resource well enough to select the most 416 accurate and efficient validation mechanism for that resource, and 417 that any such mechanism can be mapped to a simple sequence of octets 418 for easy comparison. Since the value is opaque, there is no need for 419 the client to be aware of how each entity-tag is constructed. 421 For example, a resource that has implementation-specific versioning 422 applied to all changes might use an internal revision number, perhaps 423 combined with a variance identifier for content negotiation, to 424 accurately differentiate between representations. Other 425 implementations might use a collision-resistant hash of 426 representation content, a combination of various filesystem 427 attributes, or a modification timestamp that has sub-second 428 resolution. 430 Origin servers SHOULD send ETag for any selected representation for 431 which detection of changes can be reasonably and consistently 432 determined, since the entity-tag's use in conditional requests and 433 evaluating cache freshness ([Part6]) can result in a substantial 434 reduction of HTTP network traffic and can be a significant factor in 435 improving service scalability and reliability. 437 2.3.2. Comparison 439 There are two entity-tag comparison functions, depending on whether 440 the comparison context allows the use of weak validators or not: 442 o Strong comparison: two entity-tags are equivalent if both are not 443 weak and their opaque-tags match character-by-character. 445 o Weak comparison: two entity-tags are equivalent if their opaque- 446 tags match character-by-character, regardless of either or both 447 being tagged as "weak". 449 The example below shows the results for a set of entity-tag pairs, 450 and both the weak and strong comparison function results: 452 +--------+--------+-------------------+-----------------+ 453 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | 454 +--------+--------+-------------------+-----------------+ 455 | W/"1" | W/"1" | no match | match | 456 | W/"1" | W/"2" | no match | no match | 457 | W/"1" | "1" | no match | match | 458 | "1" | "1" | match | match | 459 +--------+--------+-------------------+-----------------+ 461 2.3.3. Example: Entity-tags Varying on Content-Negotiated Resources 463 Consider a resource that is subject to content negotiation (Section 464 3.4 of [Part2]), and where the representations sent in response to a 465 GET request vary based on the Accept-Encoding request header field 466 (Section 5.3.4 of [Part2]): 468 >> Request: 470 GET /index HTTP/1.1 471 Host: www.example.com 472 Accept-Encoding: gzip 474 In this case, the response might or might not use the gzip content 475 coding. If it does not, the response might look like: 477 >> Response: 479 HTTP/1.1 200 OK 480 Date: Fri, 26 Mar 2010 00:05:00 GMT 481 ETag: "123-a" 482 Content-Length: 70 483 Vary: Accept-Encoding 484 Content-Type: text/plain 486 Hello World! 487 Hello World! 488 Hello World! 489 Hello World! 490 Hello World! 492 An alternative representation that does use gzip content coding would 493 be: 495 >> Response: 497 HTTP/1.1 200 OK 498 Date: Fri, 26 Mar 2010 00:05:00 GMT 499 ETag: "123-b" 500 Content-Length: 43 501 Vary: Accept-Encoding 502 Content-Type: text/plain 503 Content-Encoding: gzip 505 ...binary data... 507 Note: Content codings are a property of the representation, so 508 therefore an entity-tag of an encoded representation has to be 509 distinct from an unencoded representation to prevent conflicts 510 during cache updates and range requests. In contrast, transfer 511 codings (Section 4 of [Part1]) apply only during message transfer 512 and do not require distinct entity-tags. 514 2.4. When to Use Entity-tags and Last-Modified Dates 516 We adopt a set of rules and recommendations for origin servers, 517 clients, and caches regarding when various validator types ought to 518 be used, and for what purposes. 520 In 200 (OK) responses to GET or HEAD, an origin server: 522 o SHOULD send an entity-tag validator unless it is not feasible to 523 generate one. 525 o MAY send a weak entity-tag instead of a strong entity-tag, if 526 performance considerations support the use of weak entity-tags, or 527 if it is unfeasible to send a strong entity-tag. 529 o SHOULD send a Last-Modified value if it is feasible to send one. 531 In other words, the preferred behavior for an origin server is to 532 send both a strong entity-tag and a Last-Modified value in successful 533 responses to a retrieval request. 535 A client: 537 o MUST use that entity-tag in any cache-conditional request (using 538 If-Match or If-None-Match) if an entity-tag has been provided by 539 the origin server. 541 o SHOULD use the Last-Modified value in non-subrange cache- 542 conditional requests (using If-Modified-Since) if only a Last- 543 Modified value has been provided by the origin server. 545 o MAY use the Last-Modified value in subrange cache-conditional 546 requests (using If-Unmodified-Since) if only a Last-Modified value 547 has been provided by an HTTP/1.0 origin server. The user agent 548 SHOULD provide a way to disable this, in case of difficulty. 550 o SHOULD use both validators in cache-conditional requests if both 551 an entity-tag and a Last-Modified value have been provided by the 552 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to 553 respond appropriately. 555 3. Precondition Header Fields 557 This section defines the syntax and semantics of HTTP/1.1 header 558 fields for applying preconditions on requests. Section 5 defines 559 when the preconditions are applied and the order of evaluation when 560 more than one precondition is present. 562 3.1. If-Match 564 The "If-Match" header field can be used to make a request method 565 conditional on the current existence or value of an entity-tag for 566 one or more representations of the target resource. 568 If-Match is generally useful for resource update requests, such as 569 PUT requests, as a means for protecting against accidental overwrites 570 when multiple clients are acting in parallel on the same resource 571 (i.e., the "lost update" problem). An If-Match field-value of "*" 572 places the precondition on the existence of any current 573 representation for the target resource. 575 If-Match = "*" / 1#entity-tag 577 The If-Match condition is met if and only if any of the entity-tags 578 listed in the If-Match field value match the entity-tag of the 579 selected representation using the weak comparison function (as per 580 Section 2.3.2), or if "*" is given and any current representation 581 exists for the target resource. 583 If the condition is met, the server MAY perform the request method. 585 Origin servers MUST NOT perform the requested method if the condition 586 is not met; instead they MUST respond with the 412 (Precondition 587 Failed) status code. 589 Proxy servers using a cached response as the selected representation 590 MUST NOT perform the requested method if the condition is not met; 591 instead, they MUST forward the request towards the origin server. 593 Examples: 595 If-Match: "xyzzy" 596 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 597 If-Match: * 599 3.2. If-None-Match 601 The "If-None-Match" header field can be used to make a request method 602 conditional on not matching any of the current entity-tag values for 603 representations of the target resource. 605 If-None-Match is primarily used in conditional GET requests to enable 606 efficient updates of cached information with a minimum amount of 607 transaction overhead. A client that has one or more representations 608 previously obtained from the target resource can send If-None-Match 609 with a list of the associated entity-tags in the hope of receiving a 610 304 (Not Modified) response if at least one of those representations 611 matches the selected representation. 613 If-None-Match can also be used with a value of "*" to prevent an 614 unsafe request method (e.g., PUT) from inadvertently modifying an 615 existing representation of the target resource when the client 616 believes that the resource does not have a current representation 617 (Section 4.2.1 of [Part2]). This is a variation on the "lost update" 618 problem that might arise if more than one client attempts to create 619 an initial representation for the target resource. 621 If-None-Match = "*" / 1#entity-tag 623 The If-None-Match condition is met if and only if none of the entity- 624 tags listed in the If-None-Match field value match the entity-tag of 625 the selected representation using the weak comparison function (as 626 per Section 2.3.2), or if "*" is given and no current representation 627 exists for that resource. 629 If the condition is not met, the server MUST NOT perform the 630 requested method. Instead, if the request method was GET or HEAD, 631 the server SHOULD respond with a 304 (Not Modified) status code, 632 including the cache-related header fields (particularly ETag) of the 633 selected representation that has a matching entity-tag. For all 634 other request methods, the server MUST respond with a 412 635 (Precondition Failed) status code when the condition is not met. 637 If the condition is met, the server MAY perform the requested method 638 and MUST ignore any If-Modified-Since header field(s) in the request. 639 That is, if no entity-tags match, then the server MUST NOT send a 304 640 (Not Modified) response. 642 Examples: 644 If-None-Match: "xyzzy" 645 If-None-Match: W/"xyzzy" 646 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 647 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" 648 If-None-Match: * 650 3.3. If-Modified-Since 652 The "If-Modified-Since" header field can be used with GET or HEAD to 653 make the method conditional by modification date: if the selected 654 representation has not been modified since the time specified in this 655 field, then do not perform the request method; instead, respond as 656 detailed below. 658 If-Modified-Since = HTTP-date 660 An example of the field is: 662 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 664 A GET method with an If-Modified-Since header field and no Range 665 header field requests that the selected representation be transferred 666 only if it has been modified since the date given by the If-Modified- 667 Since header field. The algorithm for determining this includes the 668 following cases: 670 1. If the request would normally result in anything other than a 200 671 (OK) status code, or if the passed If-Modified-Since date is 672 invalid, the response is exactly the same as for a normal GET. A 673 date that is later than the server's current time is invalid. 675 2. If the selected representation has been modified since the If- 676 Modified-Since date, the response is exactly the same as for a 677 normal GET. 679 3. If the selected representation has not been modified since a 680 valid If-Modified-Since date, the server SHOULD send a 304 (Not 681 Modified) response. 683 The two purposes of this feature are to allow efficient updates of 684 cached information, with a minimum amount of transaction overhead, 685 and to limit the scope of a web traversal to resources that have 686 recently changed. 688 When used for cache updates, a cache will typically use the value of 689 the cached message's Last-Modified field to generate the field value 690 of If-Modified-Since. This behavior is most interoperable for cases 691 where clocks are poorly synchronized or when the server has chosen to 692 only honor exact timestamp matches (due to a problem with Last- 693 Modified dates that appear to go "back in time" when the origin 694 server's clock is corrected or a representation is restored from an 695 archived backup). However, caches occasionally generate the field 696 value based on other data, such as the Date header field of the 697 cached message or the local clock time that the message was received, 698 particularly when the cached message does not contain a Last-Modified 699 field. 701 When used for limiting the scope of retrieval to a recent time 702 window, a user agent will generate an If-Modified-Since field value 703 based on either its own local clock or a Date header field received 704 from the server during a past run. Origin servers that choose an 705 exact timestamp match based on the selected representation's Last- 706 Modified field will not be able to help the user agent limit its data 707 transfers to only those changed during the specified window. 709 Note: If a client uses an arbitrary date in the If-Modified-Since 710 header field instead of a date taken from a Last-Modified or Date 711 header field from the origin server, the client ought to be aware 712 that its date will be interpreted according to the server's 713 understanding of time. 715 3.4. If-Unmodified-Since 717 The "If-Unmodified-Since" header field can be used to make a request 718 method conditional by modification date: if the selected 719 representation has been modified since the time specified in this 720 field, then the server MUST NOT perform the requested operation and 721 MUST instead respond with the 412 (Precondition Failed) status code. 722 If the selected representation has not been modified since the time 723 specified in this field, the server MAY perform the request. 725 If-Unmodified-Since = HTTP-date 727 An example of the field is: 729 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT 731 A server MUST ignore the If-Unmodified-Since header field if the 732 received value is not a valid HTTP-date. 734 3.5. If-Range 736 The "If-Range" header field provides a special conditional request 737 mechanism that is similar to If-Match and If-Unmodified-Since but 738 specific to range requests. If-Range is defined in Section 3.2 of 739 [Part5]. 741 4. Status Code Definitions 743 4.1. 304 Not Modified 745 The 304 (Not Modified) status code indicates that a conditional GET 746 or HEAD request has been received and would have resulted in a 200 747 (OK) response if it were not for the fact that the condition has 748 evaluated to false. In other words, there is no need for the server 749 to transfer a representation of the target resource because the 750 request indicates that the client, which made the request 751 conditional, already has a valid representation; the server is 752 therefore redirecting the client to make use of that stored 753 representation as if it were the payload of a 200 (OK) response. 755 The server generating a 304 response MUST generate any of the 756 following header fields that would have been sent in a 200 (OK) 757 response to the same request: Cache-Control, Content-Location, ETag, 758 Expires, and Vary. 760 Since the goal of a 304 response is to minimize information transfer 761 when the recipient already has one or more cached representations, a 762 sender SHOULD NOT generate representation metadata other than the 763 above listed fields unless said metadata exists for the purpose of 764 guiding cache updates (e.g., Last-Modified might be useful if the 765 response does not have an ETag field). 767 Requirements on a cache that receives a 304 response are defined in 768 Section 4.2.1 of [Part6]. If the conditional request originated with 769 an outbound client, such as a user agent with its own cache sending a 770 conditional GET to a shared proxy, then the proxy SHOULD forward the 771 304 response to that client. 773 A 304 response cannot contain a message-body; it is always terminated 774 by the first empty line after the header fields. 776 4.2. 412 Precondition Failed 778 The 412 (Precondition Failed) status code indicates that one or more 779 preconditions given in the request header fields evaluated to false 780 when tested on the server. This response code allows the client to 781 place preconditions on the current resource state (its current 782 representations and metadata) and thus prevent the request method 783 from being applied if the target resource is in an unexpected state. 785 5. Evaluation and Precedence 787 For each conditional request, a server MUST evaluate the request 788 preconditions after it has successfully performed its normal request 789 checks (i.e., just before it would perform the action associated with 790 the request method). Preconditions are ignored if the server 791 determines that an error or redirect response applies before they are 792 evaluated. Otherwise, the evaluation depends on both the method 793 semantics and the choice of conditional. 795 A conditional request header field that is designed specifically for 796 cache validation, which includes If-None-Match and If-Modified-Since 797 when used in a GET or HEAD request, allows cached representations to 798 be refreshed without repeatedly transferring data already held by the 799 client. Evaluating to false is thus an indication that the client 800 can continue to use its local copy of the selected representation, as 801 indicated by the server generating a 304 (Not Modified) response that 802 includes only those header fields useful for refreshing the cached 803 representation. 805 All other conditionals are intended to signal failure when the 806 precondition evaluates to false. For example, an If-Match 807 conditional sent with a state-changing method (e.g., POST, PUT, 808 DELETE) is intended to prevent the request from taking effect on the 809 target resource if the resource state does not match the expected 810 state. In other words, evaluating the condition to false means that 811 the resource has been changed by some other client, perhaps by 812 another user attempting to edit the same resource, and thus 813 preventing the request from being applied saves the client from 814 overwriting some other client's work. This result is indicated by 815 the server generating a 412 (Precondition Failed) response. 817 The conditional request header fields defined by this specification 818 are ignored for request methods that never involve the selection or 819 modification of a selected representation (e.g., CONNECT, OPTIONS, 820 and TRACE). Other conditional request header fields, defined by 821 extensions to HTTP, might place conditions on the state of the target 822 resource in general, or on a group of resources. For instance, the 823 If header field in WebDAV can make a request conditional on various 824 aspects (such as locks) of multiple resources ([RFC4918], Section 825 10.4). 827 When more than one conditional request header field is present in a 828 request, the order in which the fields are evaluated becomes 829 important. In practice, the fields defined in this document are 830 consistently implemented in a single, logical order, due to the fact 831 that entity tags are presumed to be more accurate than date 832 validators. For example, the only reason to send both If-Modified- 833 Since and If-None-Match in the same GET request is to support 834 intermediary caches that might not have implemented If-None-Match, so 835 it makes sense to ignore the If-Modified-Since when entity tags are 836 understood and available for the selected representation. 838 The general rule of conditional precedence is that exact match 839 conditions are evaluated before cache-validating conditions and, 840 within that order, last-modified conditions are only evaluated if the 841 corresponding entity tag condition is not present (or not applicable 842 because the selected representation does not have an entity tag). 844 Specifically, the fields defined by this specification are evaluated 845 as follows: 847 1. When If-Match is present, evaluate it: 849 * if true, continue to step 3 851 * if false, respond 412 (Precondition Failed) 853 2. When If-Match is not present and If-Unmodified-Since is present, 854 evaluate it: 856 * if true, continue to step 3 858 * if false, respond 412 (Precondition Failed) 860 3. When If-None-Match is present, evaluate it: 862 * if true, continue to step 5 864 * if false for GET/HEAD, respond 304 (Not Modified) 866 * if false for other methods, respond 412 (Precondition Failed) 868 4. When the method is GET or HEAD, If-None-Match is not present, and 869 If-Modified-Since is present, evaluate it: 871 * if true, continue to step 5 873 * if false, respond 304 (Not Modified) 875 5. When the method is GET and both Range and If-Range are present, 876 evaluate If-Range: 878 * if the validator matches and the Range specification is 879 applicable to the selected representation, respond 206 880 (Partial Content) [Part5] 882 6. Otherwise, 884 * all conditions are met, so perform the requested action and 885 respond according to its success or failure. 887 Any extension to HTTP/1.1 that defines additional conditional request 888 header fields ought to define its own expectations regarding the 889 order for evaluating such fields in relation to those defined in this 890 document and other conditionals that might be found in practice. 892 6. IANA Considerations 894 6.1. Status Code Registration 896 The HTTP Status Code Registry located at 897 shall be updated 898 with the registrations below: 900 +-------+---------------------+-------------+ 901 | Value | Description | Reference | 902 +-------+---------------------+-------------+ 903 | 304 | Not Modified | Section 4.1 | 904 | 412 | Precondition Failed | Section 4.2 | 905 +-------+---------------------+-------------+ 907 6.2. Header Field Registration 909 HTTP header fields are registered within the Message Header Field 910 Registry maintained at . 913 This document defines the following HTTP header fields, so their 914 associated registry entries shall be updated according to the 915 permanent registrations below (see [BCP90]): 917 +---------------------+----------+----------+-------------+ 918 | Header Field Name | Protocol | Status | Reference | 919 +---------------------+----------+----------+-------------+ 920 | ETag | http | standard | Section 2.3 | 921 | If-Match | http | standard | Section 3.1 | 922 | If-Modified-Since | http | standard | Section 3.3 | 923 | If-None-Match | http | standard | Section 3.2 | 924 | If-Unmodified-Since | http | standard | Section 3.4 | 925 | Last-Modified | http | standard | Section 2.2 | 926 +---------------------+----------+----------+-------------+ 928 The change controller is: "IETF (iesg@ietf.org) - Internet 929 Engineering Task Force". 931 7. Security Considerations 933 This section is meant to inform developers, information providers, 934 and users of known security concerns specific to the HTTP/1.1 935 conditional request mechanisms. More general security considerations 936 are addressed in HTTP messaging [Part1] and semantics [Part2]. 938 The validators defined by this specification are not intended to 939 ensure the validity of a representation, guard against malicious 940 changes, or detect man-in-the-middle attacks. At best, they enable 941 more efficient cache updates and optimistic concurrent writes when 942 all participants are behaving nicely. At worst, the conditions will 943 fail and the client will receive a response that is no more harmful 944 than an HTTP exchange without conditional requests. 946 An entity-tag can be abused in ways that create privacy risks. For 947 example, a site might deliberately construct a semantically invalid 948 entity-tag that is unique to the user or user agent, send it in a 949 cacheable response with a long freshness time, and then read that 950 entity-tag in later conditional requests as a means of re-identifying 951 that user or user agent. Such an identifying tag would become a 952 persistent identifier for as long as the user agent retained the 953 original cache entry. User agents that cache representations ought 954 to ensure that the cache is cleared or replaced whenever the user 955 performs privacy-maintaining actions, such as clearing stored cookies 956 or changing to a private browsing mode. 958 8. Acknowledgments 960 See Section 9 of [Part1]. 962 9. References 964 9.1. Normative References 966 [Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 967 Protocol (HTTP/1.1): Message Syntax and Routing", 968 draft-ietf-httpbis-p1-messaging-23 (work in progress), 969 July 2013. 971 [Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 972 Protocol (HTTP/1.1): Semantics and Content", 973 draft-ietf-httpbis-p2-semantics-23 (work in progress), 974 July 2013. 976 [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., 977 "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", 978 draft-ietf-httpbis-p5-range-23 (work in progress), 979 July 2013. 981 [Part6] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 982 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", 983 draft-ietf-httpbis-p6-cache-23 (work in progress), 984 July 2013. 986 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 987 Requirement Levels", BCP 14, RFC 2119, March 1997. 989 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 990 Specifications: ABNF", STD 68, RFC 5234, January 2008. 992 9.2. Informative References 994 [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration 995 Procedures for Message Header Fields", BCP 90, RFC 3864, 996 September 2004. 998 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 999 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1000 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1002 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed 1003 Authoring and Versioning (WebDAV)", RFC 4918, June 2007. 1005 Appendix A. Changes from RFC 2616 1007 The definition of validator weakness has been expanded and clarified. 1008 (Section 2.1) 1010 Weak entity-tags are now allowed in all requests except range 1011 requests (Sections 2.1 and 3.2). 1013 The ETag header field ABNF has been changed to not use quoted-string, 1014 thus avoiding escaping issues. (Section 2.3) 1016 ETag is defined to provide an entity tag for the selected 1017 representation, thereby clarifying what it applies to in various 1018 situations (such as a PUT response). (Section 2.3) 1020 The precedence for evaluation of conditional requests has been 1021 defined. (Section 5) 1023 Appendix B. Imported ABNF 1025 The following core rules are included by reference, as defined in 1026 Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), 1027 CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double 1028 quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 1029 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII 1030 character). 1032 The rules below are defined in [Part1]: 1034 OWS = 1035 obs-text = 1037 The rules below are defined in other parts: 1039 HTTP-date = 1041 Appendix C. Collected ABNF 1043 In the collected ABNF below, list rules are expanded as per Section 1044 1.2 of [Part1]. 1046 ETag = entity-tag 1048 HTTP-date = 1050 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1051 entity-tag ] ) ) 1052 If-Modified-Since = HTTP-date 1053 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1054 entity-tag ] ) ) 1055 If-Unmodified-Since = HTTP-date 1057 Last-Modified = HTTP-date 1059 OWS = 1061 entity-tag = [ weak ] opaque-tag 1062 etagc = "!" / %x23-7E ; '#'-'~' 1063 / obs-text 1065 obs-text = 1066 opaque-tag = DQUOTE *etagc DQUOTE 1068 weak = %x57.2F ; W/ 1070 Appendix D. Change Log (to be removed by RFC Editor before publication) 1072 Changes up to the first Working Group Last Call draft are summarized 1073 in . 1076 D.1. Since draft-ietf-httpbis-p4-conditional-19 1078 Closed issues: 1080 o : "Need to 1081 clarify eval order/interaction of conditional headers" 1083 o : "Required 1084 headers on 304 and 206" 1086 o : "Optionality 1087 of Conditional Request Support" 1089 o : "ETags and 1090 Conditional Requests" 1092 o : "ABNF 1093 requirements for recipients" 1095 o : "Rare cases" 1097 o : "Conditional 1098 Request Security Considerations" 1100 o : "If-Modified- 1101 Since lacks definition for method != GET" 1103 o : "refactor 1104 conditional header field descriptions" 1106 D.2. Since draft-ietf-httpbis-p4-conditional-20 1108 o Conformance criteria and considerations regarding error handling 1109 are now defined in Part 1. 1111 D.3. Since draft-ietf-httpbis-p4-conditional-21 1113 Closed issues: 1115 o : "Conditional 1116 GET text" 1118 o : "Optionality 1119 of Conditional Request Support" 1121 o : "unclear prose 1122 in definition of 304" 1124 o : "ETags and 1125 Conneg" 1127 o : "Comparison 1128 function for If-Match and If-None-Match" 1130 o : "304 without 1131 validator" 1133 o : "If-Match and 1134 428" 1136 D.4. Since draft-ietf-httpbis-p4-conditional-22 1138 Closed issues: 1140 o : "explain list 1141 expansion in ABNF appendices" 1143 o : "incorrect 1144 example dates" 1146 Partly resolved issues: 1148 o : "Editorial 1149 suggestions" 1151 Index 1153 3 1154 304 Not Modified (status code) 17 1156 4 1157 412 Precondition Failed (status code) 17 1159 E 1160 ETag header field 9 1162 G 1163 Grammar 1164 entity-tag 9 1165 ETag 9 1166 etagc 9 1167 If-Match 13 1168 If-Modified-Since 15 1169 If-None-Match 14 1170 If-Unmodified-Since 16 1171 Last-Modified 7 1172 opaque-tag 9 1173 weak 9 1175 I 1176 If-Match header field 13 1177 If-Modified-Since header field 15 1178 If-None-Match header field 14 1179 If-Unmodified-Since header field 16 1181 L 1182 Last-Modified header field 7 1184 M 1185 metadata 5 1187 S 1188 selected representation 4 1190 V 1191 validator 5 1192 strong 5 1193 weak 5 1195 Authors' Addresses 1197 Roy T. Fielding (editor) 1198 Adobe Systems Incorporated 1199 345 Park Ave 1200 San Jose, CA 95110 1201 USA 1203 EMail: fielding@gbiv.com 1204 URI: http://roy.gbiv.com/ 1206 Julian F. Reschke (editor) 1207 greenbytes GmbH 1208 Hafenweg 16 1209 Muenster, NW 48155 1210 Germany 1212 EMail: julian.reschke@greenbytes.de 1213 URI: http://greenbytes.de/tech/webdav/