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'SEMNTCS' -- Obsolete informational reference (is this intentional?): RFC 2616 (Obsoleted by RFC 7230, RFC 7231, RFC 7232, RFC 7233, RFC 7234, RFC 7235) -- Obsolete informational reference (is this intentional?): RFC 7232 (Obsoleted by RFC 9110) Summary: 0 errors (**), 0 flaws (~~), 7 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTP Working Group R. Fielding, Ed. 3 Internet-Draft Adobe 4 Obsoletes: 7232 (if approved) M. Nottingham, Ed. 5 Intended status: Standards Track Fastly 6 Expires: October 5, 2018 J. Reschke, Ed. 7 greenbytes 8 April 3, 2018 10 Hypertext Transfer Protocol (HTTP): Conditional Requests 11 draft-ietf-httpbis-conditional-00 13 Abstract 15 The Hypertext Transfer Protocol (HTTP) is a stateless application- 16 level protocol for distributed, collaborative, hypertext information 17 systems. This document defines HTTP/1.1 conditional requests, 18 including metadata header fields for indicating state changes, 19 request header fields for making preconditions on such state, and 20 rules for constructing the responses to a conditional request when 21 one or more preconditions evaluate to false. 23 This document obsoletes RFC 7232. 25 Editorial Note 27 This note is to be removed before publishing as an RFC. 29 Discussion of this draft takes place on the HTTP working group 30 mailing list (ietf-http-wg@w3.org), which is archived at 31 . 33 Working Group information can be found at ; 34 source code and issues list for this draft can be found at 35 . 37 The changes in this draft are summarized in Appendix D.1. 39 Status of This Memo 41 This Internet-Draft is submitted in full conformance with the 42 provisions of BCP 78 and BCP 79. 44 Internet-Drafts are working documents of the Internet Engineering 45 Task Force (IETF). Note that other groups may also distribute 46 working documents as Internet-Drafts. The list of current Internet- 47 Drafts is at https://datatracker.ietf.org/drafts/current/. 49 Internet-Drafts are draft documents valid for a maximum of six months 50 and may be updated, replaced, or obsoleted by other documents at any 51 time. It is inappropriate to use Internet-Drafts as reference 52 material or to cite them other than as "work in progress." 54 This Internet-Draft will expire on October 5, 2018. 56 Copyright Notice 58 Copyright (c) 2018 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents 63 (https://trustee.ietf.org/license-info) in effect on the date of 64 publication of this document. Please review these documents 65 carefully, as they describe your rights and restrictions with respect 66 to this document. Code Components extracted from this document must 67 include Simplified BSD License text as described in Section 4.e of 68 the Trust Legal Provisions and are provided without warranty as 69 described in the Simplified BSD License. 71 This document may contain material from IETF Documents or IETF 72 Contributions published or made publicly available before November 73 10, 2008. The person(s) controlling the copyright in some of this 74 material may not have granted the IETF Trust the right to allow 75 modifications of such material outside the IETF Standards Process. 76 Without obtaining an adequate license from the person(s) controlling 77 the copyright in such materials, this document may not be modified 78 outside the IETF Standards Process, and derivative works of it may 79 not be created outside the IETF Standards Process, except to format 80 it for publication as an RFC or to translate it into languages other 81 than English. 83 Table of Contents 85 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 86 1.1. Conformance and Error Handling . . . . . . . . . . . . . 4 87 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4 88 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . 4 89 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . 5 90 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 6 91 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . 7 92 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . 7 93 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . 8 94 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . 9 95 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . 10 96 2.3.3. Example: Entity-Tags Varying on Content-Negotiated 97 Resources . . . . . . . . . . . . . . . . . . . . . . 10 98 2.4. When to Use Entity-Tags and Last-Modified Dates . . . . . 11 99 3. Precondition Header Fields . . . . . . . . . . . . . . . . . 12 100 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . 12 101 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 13 102 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 15 103 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 16 104 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . 17 105 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 17 106 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . 17 107 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 18 108 5. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 18 109 6. Precedence . . . . . . . . . . . . . . . . . . . . . . . . . 19 110 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 111 7.1. Status Code Registration . . . . . . . . . . . . . . . . 20 112 7.2. Header Field Registration . . . . . . . . . . . . . . . . 21 113 8. Security Considerations . . . . . . . . . . . . . . . . . . . 21 114 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 115 9.1. Normative References . . . . . . . . . . . . . . . . . . 22 116 9.2. Informative References . . . . . . . . . . . . . . . . . 23 117 Appendix A. Changes from RFC 7232 . . . . . . . . . . . . . . . 24 118 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . 24 119 Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 24 120 Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 25 121 D.1. Since RFC 7232 . . . . . . . . . . . . . . . . . . . . . 25 122 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 123 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 27 124 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 126 1. Introduction 128 Conditional requests are HTTP requests [SEMNTCS] that include one or 129 more header fields indicating a precondition to be tested before 130 applying the method semantics to the target resource. This document 131 defines the HTTP/1.1 conditional request mechanisms in terms of the 132 architecture, syntax notation, and conformance criteria defined in 133 [MESSGNG]. 135 Conditional GET requests are the most efficient mechanism for HTTP 136 cache updates [CACHING]. Conditionals can also be applied to state- 137 changing methods, such as PUT and DELETE, to prevent the "lost 138 update" problem: one client accidentally overwriting the work of 139 another client that has been acting in parallel. 141 Conditional request preconditions are based on the state of the 142 target resource as a whole (its current value set) or the state as 143 observed in a previously obtained representation (one value in that 144 set). A resource might have multiple current representations, each 145 with its own observable state. The conditional request mechanisms 146 assume that the mapping of requests to a "selected representation" 147 (Section 3 of [SEMNTCS]) will be consistent over time if the server 148 intends to take advantage of conditionals. Regardless, if the 149 mapping is inconsistent and the server is unable to select the 150 appropriate representation, then no harm will result when the 151 precondition evaluates to false. 153 The conditional request preconditions defined by this specification 154 (Section 3) are evaluated when applicable to the recipient 155 (Section 5) according to their order of precedence (Section 6). 157 This specification obsoletes RFC 7232, with the changes being 158 summarized in Appendix A. 160 1.1. Conformance and Error Handling 162 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 163 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 164 document are to be interpreted as described in [RFC2119]. 166 Conformance criteria and considerations regarding error handling are 167 defined in Section 2.5 of [MESSGNG]. 169 1.2. Syntax Notation 171 This specification uses the Augmented Backus-Naur Form (ABNF) 172 notation of [RFC5234] with a list extension, defined in Section 7 of 173 [MESSGNG], that allows for compact definition of comma-separated 174 lists using a '#' operator (similar to how the '*' operator indicates 175 repetition). Appendix B describes rules imported from other 176 documents. Appendix C shows the collected grammar with all list 177 operators expanded to standard ABNF notation. 179 2. Validators 181 This specification defines two forms of metadata that are commonly 182 used to observe resource state and test for preconditions: 183 modification dates (Section 2.2) and opaque entity tags 184 (Section 2.3). Additional metadata that reflects resource state has 185 been defined by various extensions of HTTP, such as Web Distributed 186 Authoring and Versioning (WebDAV, [RFC4918]), that are beyond the 187 scope of this specification. A resource metadata value is referred 188 to as a "validator" when it is used within a precondition. 190 2.1. Weak versus Strong 192 Validators come in two flavors: strong or weak. Weak validators are 193 easy to generate but are far less useful for comparisons. Strong 194 validators are ideal for comparisons but can be very difficult (and 195 occasionally impossible) to generate efficiently. Rather than impose 196 that all forms of resource adhere to the same strength of validator, 197 HTTP exposes the type of validator in use and imposes restrictions on 198 when weak validators can be used as preconditions. 200 A "strong validator" is representation metadata that changes value 201 whenever a change occurs to the representation data that would be 202 observable in the payload body of a 200 (OK) response to GET. 204 A strong validator might change for reasons other than a change to 205 the representation data, such as when a semantically significant part 206 of the representation metadata is changed (e.g., Content-Type), but 207 it is in the best interests of the origin server to only change the 208 value when it is necessary to invalidate the stored responses held by 209 remote caches and authoring tools. 211 Cache entries might persist for arbitrarily long periods, regardless 212 of expiration times. Thus, a cache might attempt to validate an 213 entry using a validator that it obtained in the distant past. A 214 strong validator is unique across all versions of all representations 215 associated with a particular resource over time. However, there is 216 no implication of uniqueness across representations of different 217 resources (i.e., the same strong validator might be in use for 218 representations of multiple resources at the same time and does not 219 imply that those representations are equivalent). 221 There are a variety of strong validators used in practice. The best 222 are based on strict revision control, wherein each change to a 223 representation always results in a unique node name and revision 224 identifier being assigned before the representation is made 225 accessible to GET. A collision-resistant hash function applied to 226 the representation data is also sufficient if the data is available 227 prior to the response header fields being sent and the digest does 228 not need to be recalculated every time a validation request is 229 received. However, if a resource has distinct representations that 230 differ only in their metadata, such as might occur with content 231 negotiation over media types that happen to share the same data 232 format, then the origin server needs to incorporate additional 233 information in the validator to distinguish those representations. 235 In contrast, a "weak validator" is representation metadata that might 236 not change for every change to the representation data. This 237 weakness might be due to limitations in how the value is calculated, 238 such as clock resolution, an inability to ensure uniqueness for all 239 possible representations of the resource, or a desire of the resource 240 owner to group representations by some self-determined set of 241 equivalency rather than unique sequences of data. An origin server 242 SHOULD change a weak entity-tag whenever it considers prior 243 representations to be unacceptable as a substitute for the current 244 representation. In other words, a weak entity-tag ought to change 245 whenever the origin server wants caches to invalidate old responses. 247 For example, the representation of a weather report that changes in 248 content every second, based on dynamic measurements, might be grouped 249 into sets of equivalent representations (from the origin server's 250 perspective) with the same weak validator in order to allow cached 251 representations to be valid for a reasonable period of time (perhaps 252 adjusted dynamically based on server load or weather quality). 253 Likewise, a representation's modification time, if defined with only 254 one-second resolution, might be a weak validator if it is possible 255 for the representation to be modified twice during a single second 256 and retrieved between those modifications. 258 Likewise, a validator is weak if it is shared by two or more 259 representations of a given resource at the same time, unless those 260 representations have identical representation data. For example, if 261 the origin server sends the same validator for a representation with 262 a gzip content coding applied as it does for a representation with no 263 content coding, then that validator is weak. However, two 264 simultaneous representations might share the same strong validator if 265 they differ only in the representation metadata, such as when two 266 different media types are available for the same representation data. 268 Strong validators are usable for all conditional requests, including 269 cache validation, partial content ranges, and "lost update" 270 avoidance. Weak validators are only usable when the client does not 271 require exact equality with previously obtained representation data, 272 such as when validating a cache entry or limiting a web traversal to 273 recent changes. 275 2.2. Last-Modified 277 The "Last-Modified" header field in a response provides a timestamp 278 indicating the date and time at which the origin server believes the 279 selected representation was last modified, as determined at the 280 conclusion of handling the request. 282 Last-Modified = HTTP-date 284 An example of its use is 285 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 287 2.2.1. Generation 289 An origin server SHOULD send Last-Modified for any selected 290 representation for which a last modification date can be reasonably 291 and consistently determined, since its use in conditional requests 292 and evaluating cache freshness ([CACHING]) results in a substantial 293 reduction of HTTP traffic on the Internet and can be a significant 294 factor in improving service scalability and reliability. 296 A representation is typically the sum of many parts behind the 297 resource interface. The last-modified time would usually be the most 298 recent time that any of those parts were changed. How that value is 299 determined for any given resource is an implementation detail beyond 300 the scope of this specification. What matters to HTTP is how 301 recipients of the Last-Modified header field can use its value to 302 make conditional requests and test the validity of locally cached 303 responses. 305 An origin server SHOULD obtain the Last-Modified value of the 306 representation as close as possible to the time that it generates the 307 Date field value for its response. This allows a recipient to make 308 an accurate assessment of the representation's modification time, 309 especially if the representation changes near the time that the 310 response is generated. 312 An origin server with a clock MUST NOT send a Last-Modified date that 313 is later than the server's time of message origination (Date). If 314 the last modification time is derived from implementation-specific 315 metadata that evaluates to some time in the future, according to the 316 origin server's clock, then the origin server MUST replace that value 317 with the message origination date. This prevents a future 318 modification date from having an adverse impact on cache validation. 320 An origin server without a clock MUST NOT assign Last-Modified values 321 to a response unless these values were associated with the resource 322 by some other system or user with a reliable clock. 324 2.2.2. Comparison 326 A Last-Modified time, when used as a validator in a request, is 327 implicitly weak unless it is possible to deduce that it is strong, 328 using the following rules: 330 o The validator is being compared by an origin server to the actual 331 current validator for the representation and, 333 o That origin server reliably knows that the associated 334 representation did not change twice during the second covered by 335 the presented validator. 337 or 339 o The validator is about to be used by a client in an If-Modified- 340 Since, If-Unmodified-Since, or If-Range header field, because the 341 client has a cache entry for the associated representation, and 343 o That cache entry includes a Date value, which gives the time when 344 the origin server sent the original response, and 346 o The presented Last-Modified time is at least 60 seconds before the 347 Date value. 349 or 351 o The validator is being compared by an intermediate cache to the 352 validator stored in its cache entry for the representation, and 354 o That cache entry includes a Date value, which gives the time when 355 the origin server sent the original response, and 357 o The presented Last-Modified time is at least 60 seconds before the 358 Date value. 360 This method relies on the fact that if two different responses were 361 sent by the origin server during the same second, but both had the 362 same Last-Modified time, then at least one of those responses would 363 have a Date value equal to its Last-Modified time. The arbitrary 364 60-second limit guards against the possibility that the Date and 365 Last-Modified values are generated from different clocks or at 366 somewhat different times during the preparation of the response. An 367 implementation MAY use a value larger than 60 seconds, if it is 368 believed that 60 seconds is too short. 370 2.3. ETag 372 The "ETag" header field in a response provides the current entity-tag 373 for the selected representation, as determined at the conclusion of 374 handling the request. An entity-tag is an opaque validator for 375 differentiating between multiple representations of the same 376 resource, regardless of whether those multiple representations are 377 due to resource state changes over time, content negotiation 378 resulting in multiple representations being valid at the same time, 379 or both. An entity-tag consists of an opaque quoted string, possibly 380 prefixed by a weakness indicator. 382 ETag = entity-tag 384 entity-tag = [ weak ] opaque-tag 385 weak = %x57.2F ; "W/", case-sensitive 386 opaque-tag = DQUOTE *etagc DQUOTE 387 etagc = %x21 / %x23-7E / obs-text 388 ; VCHAR except double quotes, plus obs-text 390 Note: Previously, opaque-tag was defined to be a quoted-string 391 ([RFC2616], Section 3.11); thus, some recipients might perform 392 backslash unescaping. Servers therefore ought to avoid backslash 393 characters in entity tags. 395 An entity-tag can be more reliable for validation than a modification 396 date in situations where it is inconvenient to store modification 397 dates, where the one-second resolution of HTTP date values is not 398 sufficient, or where modification dates are not consistently 399 maintained. 401 Examples: 403 ETag: "xyzzy" 404 ETag: W/"xyzzy" 405 ETag: "" 407 An entity-tag can be either a weak or strong validator, with strong 408 being the default. If an origin server provides an entity-tag for a 409 representation and the generation of that entity-tag does not satisfy 410 all of the characteristics of a strong validator (Section 2.1), then 411 the origin server MUST mark the entity-tag as weak by prefixing its 412 opaque value with "W/" (case-sensitive). 414 2.3.1. Generation 416 The principle behind entity-tags is that only the service author 417 knows the implementation of a resource well enough to select the most 418 accurate and efficient validation mechanism for that resource, and 419 that any such mechanism can be mapped to a simple sequence of octets 420 for easy comparison. Since the value is opaque, there is no need for 421 the client to be aware of how each entity-tag is constructed. 423 For example, a resource that has implementation-specific versioning 424 applied to all changes might use an internal revision number, perhaps 425 combined with a variance identifier for content negotiation, to 426 accurately differentiate between representations. Other 427 implementations might use a collision-resistant hash of 428 representation content, a combination of various file attributes, or 429 a modification timestamp that has sub-second resolution. 431 An origin server SHOULD send an ETag for any selected representation 432 for which detection of changes can be reasonably and consistently 433 determined, since the entity-tag's use in conditional requests and 434 evaluating cache freshness ([CACHING]) can result in a substantial 435 reduction of HTTP network traffic and can be a significant factor in 436 improving service scalability and reliability. 438 2.3.2. Comparison 440 There are two entity-tag comparison functions, depending on whether 441 or not the comparison context allows the use of weak validators: 443 o Strong comparison: two entity-tags are equivalent if both are not 444 weak and their opaque-tags match character-by-character. 446 o Weak comparison: two entity-tags are equivalent if their opaque- 447 tags match character-by-character, regardless of either or both 448 being tagged as "weak". 450 The example below shows the results for a set of entity-tag pairs and 451 both the weak and strong comparison function results: 453 +--------+--------+-------------------+-----------------+ 454 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | 455 +--------+--------+-------------------+-----------------+ 456 | W/"1" | W/"1" | no match | match | 457 | W/"1" | W/"2" | no match | no match | 458 | W/"1" | "1" | no match | match | 459 | "1" | "1" | match | match | 460 +--------+--------+-------------------+-----------------+ 462 2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources 464 Consider a resource that is subject to content negotiation 465 (Section 3.4 of [SEMNTCS]), and where the representations sent in 466 response to a GET request vary based on the Accept-Encoding request 467 header field (Section 5.3.4 of [SEMNTCS]): 469 >> Request: 471 GET /index HTTP/1.1 472 Host: www.example.com 473 Accept-Encoding: gzip 475 In this case, the response might or might not use the gzip content 476 coding. If it does not, the response might look like: 478 >> Response: 480 HTTP/1.1 200 OK 481 Date: Fri, 26 Mar 2010 00:05:00 GMT 482 ETag: "123-a" 483 Content-Length: 70 484 Vary: Accept-Encoding 485 Content-Type: text/plain 487 Hello World! 488 Hello World! 489 Hello World! 490 Hello World! 491 Hello World! 493 An alternative representation that does use gzip content coding would 494 be: 496 >> Response: 498 HTTP/1.1 200 OK 499 Date: Fri, 26 Mar 2010 00:05:00 GMT 500 ETag: "123-b" 501 Content-Length: 43 502 Vary: Accept-Encoding 503 Content-Type: text/plain 504 Content-Encoding: gzip 506 ...binary data... 508 Note: Content codings are a property of the representation data, 509 so a strong entity-tag for a content-encoded representation has to 510 be distinct from the entity tag of an unencoded representation to 511 prevent potential conflicts during cache updates and range 512 requests. In contrast, transfer codings (Section 4 of [MESSGNG]) 513 apply only during message transfer and do not result in distinct 514 entity-tags. 516 2.4. When to Use Entity-Tags and Last-Modified Dates 518 In 200 (OK) responses to GET or HEAD, an origin server: 520 o SHOULD send an entity-tag validator unless it is not feasible to 521 generate one. 523 o MAY send a weak entity-tag instead of a strong entity-tag, if 524 performance considerations support the use of weak entity-tags, or 525 if it is unfeasible to send a strong entity-tag. 527 o SHOULD send a Last-Modified value if it is feasible to send one. 529 In other words, the preferred behavior for an origin server is to 530 send both a strong entity-tag and a Last-Modified value in successful 531 responses to a retrieval request. 533 A client: 535 o MUST send that entity-tag in any cache validation request (using 536 If-Match or If-None-Match) if an entity-tag has been provided by 537 the origin server. 539 o SHOULD send the Last-Modified value in non-subrange cache 540 validation requests (using If-Modified-Since) if only a Last- 541 Modified value has been provided by the origin server. 543 o MAY send the Last-Modified value in subrange cache validation 544 requests (using If-Unmodified-Since) if only a Last-Modified value 545 has been provided by an HTTP/1.0 origin server. The user agent 546 SHOULD provide a way to disable this, in case of difficulty. 548 o SHOULD send both validators in cache validation requests if both 549 an entity-tag and a Last-Modified value have been provided by the 550 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to 551 respond appropriately. 553 3. Precondition Header Fields 555 This section defines the syntax and semantics of HTTP/1.1 header 556 fields for applying preconditions on requests. Section 5 defines 557 when the preconditions are applied. Section 6 defines the order of 558 evaluation when more than one precondition is present. 560 3.1. If-Match 562 The "If-Match" header field makes the request method conditional on 563 the recipient origin server either having at least one current 564 representation of the target resource, when the field-value is "*", 565 or having a current representation of the target resource that has an 566 entity-tag matching a member of the list of entity-tags provided in 567 the field-value. 569 An origin server MUST use the strong comparison function when 570 comparing entity-tags for If-Match (Section 2.3.2), since the client 571 intends this precondition to prevent the method from being applied if 572 there have been any changes to the representation data. 574 If-Match = "*" / 1#entity-tag 576 Examples: 578 If-Match: "xyzzy" 579 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 580 If-Match: * 582 If-Match is most often used with state-changing methods (e.g., POST, 583 PUT, DELETE) to prevent accidental overwrites when multiple user 584 agents might be acting in parallel on the same resource (i.e., to 585 prevent the "lost update" problem). It can also be used with safe 586 methods to abort a request if the selected representation does not 587 match one already stored (or partially stored) from a prior request. 589 An origin server that receives an If-Match header field MUST evaluate 590 the condition prior to performing the method (Section 5). If the 591 field-value is "*", the condition is false if the origin server does 592 not have a current representation for the target resource. If the 593 field-value is a list of entity-tags, the condition is false if none 594 of the listed tags match the entity-tag of the selected 595 representation. 597 An origin server MUST NOT perform the requested method if a received 598 If-Match condition evaluates to false; instead, the origin server 599 MUST respond with either a) the 412 (Precondition Failed) status code 600 or b) one of the 2xx (Successful) status codes if the origin server 601 has verified that a state change is being requested and the final 602 state is already reflected in the current state of the target 603 resource (i.e., the change requested by the user agent has already 604 succeeded, but the user agent might not be aware of it, perhaps 605 because the prior response was lost or a compatible change was made 606 by some other user agent). In the latter case, the origin server 607 MUST NOT send a validator header field in the response unless it can 608 verify that the request is a duplicate of an immediately prior change 609 made by the same user agent. 611 The If-Match header field can be ignored by caches and intermediaries 612 because it is not applicable to a stored response. 614 3.2. If-None-Match 616 The "If-None-Match" header field makes the request method conditional 617 on a recipient cache or origin server either not having any current 618 representation of the target resource, when the field-value is "*", 619 or having a selected representation with an entity-tag that does not 620 match any of those listed in the field-value. 622 A recipient MUST use the weak comparison function when comparing 623 entity-tags for If-None-Match (Section 2.3.2), since weak entity-tags 624 can be used for cache validation even if there have been changes to 625 the representation data. 627 If-None-Match = "*" / 1#entity-tag 629 Examples: 631 If-None-Match: "xyzzy" 632 If-None-Match: W/"xyzzy" 633 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 634 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" 635 If-None-Match: * 637 If-None-Match is primarily used in conditional GET requests to enable 638 efficient updates of cached information with a minimum amount of 639 transaction overhead. When a client desires to update one or more 640 stored responses that have entity-tags, the client SHOULD generate an 641 If-None-Match header field containing a list of those entity-tags 642 when making a GET request; this allows recipient servers to send a 643 304 (Not Modified) response to indicate when one of those stored 644 responses matches the selected representation. 646 If-None-Match can also be used with a value of "*" to prevent an 647 unsafe request method (e.g., PUT) from inadvertently modifying an 648 existing representation of the target resource when the client 649 believes that the resource does not have a current representation 650 (Section 4.2.1 of [SEMNTCS]). This is a variation on the "lost 651 update" problem that might arise if more than one client attempts to 652 create an initial representation for the target resource. 654 An origin server that receives an If-None-Match header field MUST 655 evaluate the condition prior to performing the method (Section 5). 656 If the field-value is "*", the condition is false if the origin 657 server has a current representation for the target resource. If the 658 field-value is a list of entity-tags, the condition is false if one 659 of the listed tags match the entity-tag of the selected 660 representation. 662 An origin server MUST NOT perform the requested method if the 663 condition evaluates to false; instead, the origin server MUST respond 664 with either a) the 304 (Not Modified) status code if the request 665 method is GET or HEAD or b) the 412 (Precondition Failed) status code 666 for all other request methods. 668 Requirements on cache handling of a received If-None-Match header 669 field are defined in Section 4.3.2 of [CACHING]. 671 3.3. If-Modified-Since 673 The "If-Modified-Since" header field makes a GET or HEAD request 674 method conditional on the selected representation's modification date 675 being more recent than the date provided in the field-value. 676 Transfer of the selected representation's data is avoided if that 677 data has not changed. 679 If-Modified-Since = HTTP-date 681 An example of the field is: 683 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 685 A recipient MUST ignore If-Modified-Since if the request contains an 686 If-None-Match header field; the condition in If-None-Match is 687 considered to be a more accurate replacement for the condition in If- 688 Modified-Since, and the two are only combined for the sake of 689 interoperating with older intermediaries that might not implement If- 690 None-Match. 692 A recipient MUST ignore the If-Modified-Since header field if the 693 received field-value is not a valid HTTP-date, or if the request 694 method is neither GET nor HEAD. 696 A recipient MUST interpret an If-Modified-Since field-value's 697 timestamp in terms of the origin server's clock. 699 If-Modified-Since is typically used for two distinct purposes: 1) to 700 allow efficient updates of a cached representation that does not have 701 an entity-tag and 2) to limit the scope of a web traversal to 702 resources that have recently changed. 704 When used for cache updates, a cache will typically use the value of 705 the cached message's Last-Modified field to generate the field value 706 of If-Modified-Since. This behavior is most interoperable for cases 707 where clocks are poorly synchronized or when the server has chosen to 708 only honor exact timestamp matches (due to a problem with Last- 709 Modified dates that appear to go "back in time" when the origin 710 server's clock is corrected or a representation is restored from an 711 archived backup). However, caches occasionally generate the field 712 value based on other data, such as the Date header field of the 713 cached message or the local clock time that the message was received, 714 particularly when the cached message does not contain a Last-Modified 715 field. 717 When used for limiting the scope of retrieval to a recent time 718 window, a user agent will generate an If-Modified-Since field value 719 based on either its own local clock or a Date header field received 720 from the server in a prior response. Origin servers that choose an 721 exact timestamp match based on the selected representation's Last- 722 Modified field will not be able to help the user agent limit its data 723 transfers to only those changed during the specified window. 725 An origin server that receives an If-Modified-Since header field 726 SHOULD evaluate the condition prior to performing the method 727 (Section 5). The origin server SHOULD NOT perform the requested 728 method if the selected representation's last modification date is 729 earlier than or equal to the date provided in the field-value; 730 instead, the origin server SHOULD generate a 304 (Not Modified) 731 response, including only those metadata that are useful for 732 identifying or updating a previously cached response. 734 Requirements on cache handling of a received If-Modified-Since header 735 field are defined in Section 4.3.2 of [CACHING]. 737 3.4. If-Unmodified-Since 739 The "If-Unmodified-Since" header field makes the request method 740 conditional on the selected representation's last modification date 741 being earlier than or equal to the date provided in the field-value. 742 This field accomplishes the same purpose as If-Match for cases where 743 the user agent does not have an entity-tag for the representation. 745 If-Unmodified-Since = HTTP-date 747 An example of the field is: 749 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT 751 A recipient MUST ignore If-Unmodified-Since if the request contains 752 an If-Match header field; the condition in If-Match is considered to 753 be a more accurate replacement for the condition in If-Unmodified- 754 Since, and the two are only combined for the sake of interoperating 755 with older intermediaries that might not implement If-Match. 757 A recipient MUST ignore the If-Unmodified-Since header field if the 758 received field-value is not a valid HTTP-date. 760 A recipient MUST interpret an If-Unmodified-Since field-value's 761 timestamp in terms of the origin server's clock. 763 If-Unmodified-Since is most often used with state-changing methods 764 (e.g., POST, PUT, DELETE) to prevent accidental overwrites when 765 multiple user agents might be acting in parallel on a resource that 766 does not supply entity-tags with its representations (i.e., to 767 prevent the "lost update" problem). It can also be used with safe 768 methods to abort a request if the selected representation does not 769 match one already stored (or partially stored) from a prior request. 771 An origin server that receives an If-Unmodified-Since header field 772 MUST evaluate the condition prior to performing the method 773 (Section 5). The origin server MUST NOT perform the requested method 774 if the selected representation's last modification date is more 775 recent than the date provided in the field-value; instead the origin 776 server MUST respond with either a) the 412 (Precondition Failed) 777 status code or b) one of the 2xx (Successful) status codes if the 778 origin server has verified that a state change is being requested and 779 the final state is already reflected in the current state of the 780 target resource (i.e., the change requested by the user agent has 781 already succeeded, but the user agent might not be aware of that 782 because the prior response message was lost or a compatible change 783 was made by some other user agent). In the latter case, the origin 784 server MUST NOT send a validator header field in the response unless 785 it can verify that the request is a duplicate of an immediately prior 786 change made by the same user agent. 788 The If-Unmodified-Since header field can be ignored by caches and 789 intermediaries because it is not applicable to a stored response. 791 3.5. If-Range 793 The "If-Range" header field provides a special conditional request 794 mechanism that is similar to the If-Match and If-Unmodified-Since 795 header fields but that instructs the recipient to ignore the Range 796 header field if the validator doesn't match, resulting in transfer of 797 the new selected representation instead of a 412 (Precondition 798 Failed) response. If-Range is defined in Section 3.2 of [RANGERQ]. 800 4. Status Code Definitions 802 4.1. 304 Not Modified 804 The 304 (Not Modified) status code indicates that a conditional GET 805 or HEAD request has been received and would have resulted in a 200 806 (OK) response if it were not for the fact that the condition 807 evaluated to false. In other words, there is no need for the server 808 to transfer a representation of the target resource because the 809 request indicates that the client, which made the request 810 conditional, already has a valid representation; the server is 811 therefore redirecting the client to make use of that stored 812 representation as if it were the payload of a 200 (OK) response. 814 The server generating a 304 response MUST generate any of the 815 following header fields that would have been sent in a 200 (OK) 816 response to the same request: Cache-Control, Content-Location, Date, 817 ETag, Expires, and Vary. 819 Since the goal of a 304 response is to minimize information transfer 820 when the recipient already has one or more cached representations, a 821 sender SHOULD NOT generate representation metadata other than the 822 above listed fields unless said metadata exists for the purpose of 823 guiding cache updates (e.g., Last-Modified might be useful if the 824 response does not have an ETag field). 826 Requirements on a cache that receives a 304 response are defined in 827 Section 4.3.4 of [CACHING]. If the conditional request originated 828 with an outbound client, such as a user agent with its own cache 829 sending a conditional GET to a shared proxy, then the proxy SHOULD 830 forward the 304 response to that client. 832 A 304 response cannot contain a message-body; it is always terminated 833 by the first empty line after the header fields. 835 4.2. 412 Precondition Failed 837 The 412 (Precondition Failed) status code indicates that one or more 838 conditions given in the request header fields evaluated to false when 839 tested on the server. This response code allows the client to place 840 preconditions on the current resource state (its current 841 representations and metadata) and, thus, prevent the request method 842 from being applied if the target resource is in an unexpected state. 844 5. Evaluation 846 Except when excluded below, a recipient cache or origin server MUST 847 evaluate received request preconditions after it has successfully 848 performed its normal request checks and just before it would perform 849 the action associated with the request method. A server MUST ignore 850 all received preconditions if its response to the same request 851 without those conditions would have been a status code other than a 852 2xx (Successful) or 412 (Precondition Failed). In other words, 853 redirects and failures take precedence over the evaluation of 854 preconditions in conditional requests. 856 A server that is not the origin server for the target resource and 857 cannot act as a cache for requests on the target resource MUST NOT 858 evaluate the conditional request header fields defined by this 859 specification, and it MUST forward them if the request is forwarded, 860 since the generating client intends that they be evaluated by a 861 server that can provide a current representation. Likewise, a server 862 MUST ignore the conditional request header fields defined by this 863 specification when received with a request method that does not 864 involve the selection or modification of a selected representation, 865 such as CONNECT, OPTIONS, or TRACE. 867 Conditional request header fields that are defined by extensions to 868 HTTP might place conditions on all recipients, on the state of the 869 target resource in general, or on a group of resources. For 870 instance, the "If" header field in WebDAV can make a request 871 conditional on various aspects of multiple resources, such as locks, 872 if the recipient understands and implements that field ([RFC4918], 873 Section 10.4). 875 Although conditional request header fields are defined as being 876 usable with the HEAD method (to keep HEAD's semantics consistent with 877 those of GET), there is no point in sending a conditional HEAD 878 because a successful response is around the same size as a 304 (Not 879 Modified) response and more useful than a 412 (Precondition Failed) 880 response. 882 6. Precedence 884 When more than one conditional request header field is present in a 885 request, the order in which the fields are evaluated becomes 886 important. In practice, the fields defined in this document are 887 consistently implemented in a single, logical order, since "lost 888 update" preconditions have more strict requirements than cache 889 validation, a validated cache is more efficient than a partial 890 response, and entity tags are presumed to be more accurate than date 891 validators. 893 A recipient cache or origin server MUST evaluate the request 894 preconditions defined by this specification in the following order: 896 1. When recipient is the origin server and If-Match is present, 897 evaluate the If-Match precondition: 899 * if true, continue to step 3 901 * if false, respond 412 (Precondition Failed) unless it can be 902 determined that the state-changing request has already 903 succeeded (see Section 3.1) 905 2. When recipient is the origin server, If-Match is not present, and 906 If-Unmodified-Since is present, evaluate the If-Unmodified-Since 907 precondition: 909 * if true, continue to step 3 910 * if false, respond 412 (Precondition Failed) unless it can be 911 determined that the state-changing request has already 912 succeeded (see Section 3.4) 914 3. When If-None-Match is present, evaluate the If-None-Match 915 precondition: 917 * if true, continue to step 5 919 * if false for GET/HEAD, respond 304 (Not Modified) 921 * if false for other methods, respond 412 (Precondition Failed) 923 4. When the method is GET or HEAD, If-None-Match is not present, and 924 If-Modified-Since is present, evaluate the If-Modified-Since 925 precondition: 927 * if true, continue to step 5 929 * if false, respond 304 (Not Modified) 931 5. When the method is GET and both Range and If-Range are present, 932 evaluate the If-Range precondition: 934 * if the validator matches and the Range specification is 935 applicable to the selected representation, respond 206 936 (Partial Content) [RANGERQ] 938 6. Otherwise, 940 * all conditions are met, so perform the requested action and 941 respond according to its success or failure. 943 Any extension to HTTP/1.1 that defines additional conditional request 944 header fields ought to define its own expectations regarding the 945 order for evaluating such fields in relation to those defined in this 946 document and other conditionals that might be found in practice. 948 7. IANA Considerations 950 7.1. Status Code Registration 952 The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located 953 at has been 954 updated with the registrations below: 956 +-------+---------------------+--------------+ 957 | Value | Description | Reference | 958 +-------+---------------------+--------------+ 959 | 304 | Not Modified | Section 4.1 | 960 | 412 | Precondition Failed | Section 4.2 | 961 +-------+---------------------+--------------+ 963 7.2. Header Field Registration 965 HTTP header fields are registered within the "Message Headers" 966 registry maintained at . 969 This document defines the following HTTP header fields, so their 970 associated registry entries have been updated according to the 971 permanent registrations below (see [BCP90]): 973 +---------------------+----------+----------+--------------+ 974 | Header Field Name | Protocol | Status | Reference | 975 +---------------------+----------+----------+--------------+ 976 | ETag | http | standard | Section 2.3 | 977 | If-Match | http | standard | Section 3.1 | 978 | If-Modified-Since | http | standard | Section 3.3 | 979 | If-None-Match | http | standard | Section 3.2 | 980 | If-Unmodified-Since | http | standard | Section 3.4 | 981 | Last-Modified | http | standard | Section 2.2 | 982 +---------------------+----------+----------+--------------+ 984 The change controller is: "IETF (iesg@ietf.org) - Internet 985 Engineering Task Force". 987 8. Security Considerations 989 This section is meant to inform developers, information providers, 990 and users of known security concerns specific to the HTTP conditional 991 request mechanisms. More general security considerations are 992 addressed in HTTP "Message Syntax and Routing" [MESSGNG] and 993 "Semantics and Content" [SEMNTCS]. 995 The validators defined by this specification are not intended to 996 ensure the validity of a representation, guard against malicious 997 changes, or detect man-in-the-middle attacks. At best, they enable 998 more efficient cache updates and optimistic concurrent writes when 999 all participants are behaving nicely. At worst, the conditions will 1000 fail and the client will receive a response that is no more harmful 1001 than an HTTP exchange without conditional requests. 1003 An entity-tag can be abused in ways that create privacy risks. For 1004 example, a site might deliberately construct a semantically invalid 1005 entity-tag that is unique to the user or user agent, send it in a 1006 cacheable response with a long freshness time, and then read that 1007 entity-tag in later conditional requests as a means of re-identifying 1008 that user or user agent. Such an identifying tag would become a 1009 persistent identifier for as long as the user agent retained the 1010 original cache entry. User agents that cache representations ought 1011 to ensure that the cache is cleared or replaced whenever the user 1012 performs privacy-maintaining actions, such as clearing stored cookies 1013 or changing to a private browsing mode. 1015 9. References 1017 9.1. Normative References 1019 [CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 1020 Ed., "Hypertext Transfer Protocol (HTTP): Caching", draft- 1021 ietf-httpbis-cache-00 (work in progress), April 2018. 1023 [MESSGNG] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 1024 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message 1025 Syntax and Routing", draft-ietf-httpbis-messaging-00 (work 1026 in progress), April 2018. 1028 [RANGERQ] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 1029 Ed., "Hypertext Transfer Protocol (HTTP): Range Requests", 1030 draft-ietf-httpbis-range-00 (work in progress), April 1031 2018. 1033 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1034 Requirement Levels", BCP 14, RFC 2119, 1035 DOI 10.17487/RFC2119, March 1997, 1036 . 1038 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1039 Specifications: ABNF", STD 68, RFC 5234, 1040 DOI 10.17487/RFC5234, January 2008, 1041 . 1043 [SEMNTCS] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 1044 Ed., "Hypertext Transfer Protocol (HTTP): Semantics and 1045 Content", draft-ietf-httpbis-semantics-00 (work in 1046 progress), April 2018. 1048 9.2. Informative References 1050 [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration 1051 Procedures for Message Header Fields", BCP 90, RFC 3864, 1052 September 2004, . 1054 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1055 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1056 Transfer Protocol -- HTTP/1.1", RFC 2616, 1057 DOI 10.17487/RFC2616, June 1999, 1058 . 1060 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed 1061 Authoring and Versioning (WebDAV)", RFC 4918, 1062 DOI 10.17487/RFC4918, June 2007, 1063 . 1065 [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 1066 Protocol (HTTP/1.1): Conditional Requests", RFC 7232, 1067 DOI 10.17487/RFC7232, June 2014, 1068 . 1070 Appendix A. Changes from RFC 7232 1072 None yet. 1074 Appendix B. Imported ABNF 1076 The following core rules are included by reference, as defined in 1077 Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), 1078 CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double 1079 quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 1080 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII 1081 character). 1083 The rules below are defined in [MESSGNG]: 1085 OWS = 1086 obs-text = 1088 The rules below are defined in other parts: 1090 HTTP-date = 1092 Appendix C. Collected ABNF 1094 In the collected ABNF below, list rules are expanded as per 1095 Section 1.2 of [MESSGNG]. 1097 ETag = entity-tag 1099 HTTP-date = 1101 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1102 entity-tag ] ) ) 1103 If-Modified-Since = HTTP-date 1104 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1105 entity-tag ] ) ) 1106 If-Unmodified-Since = HTTP-date 1108 Last-Modified = HTTP-date 1110 OWS = 1112 entity-tag = [ weak ] opaque-tag 1113 etagc = "!" / %x23-7E ; '#'-'~' 1114 / obs-text 1116 obs-text = 1117 opaque-tag = DQUOTE *etagc DQUOTE 1119 weak = %x57.2F ; W/ 1121 Appendix D. Change Log 1123 This section is to be removed before publishing as an RFC. 1125 D.1. Since RFC 7232 1127 The changes in this draft are purely editorial: 1129 o Change boilerplate and abstract to indicate the "draft" status, 1130 and update references to ancestor specifications. 1132 o Remove version "1.1" from document title, indicating that this 1133 specification applies to all HTTP versions. 1135 o Adjust historical notes. 1137 o Update links to sibling specifications. 1139 o Replace sections listing changes from RFC 2616 by new empty 1140 sections referring to RFC 723x. 1142 o Remove acknowledgements specific to RFC 723x. 1144 o Move "Acknowledgements" to the very end and make them unnumbered. 1146 Index 1148 3 1149 304 Not Modified (status code) 17 1151 4 1152 412 Precondition Failed (status code) 18 1154 E 1155 ETag header field 8 1157 G 1158 Grammar 1159 entity-tag 9 1160 ETag 9 1161 etagc 9 1162 If-Match 12 1163 If-Modified-Since 15 1164 If-None-Match 14 1165 If-Unmodified-Since 16 1166 Last-Modified 6 1167 opaque-tag 9 1168 weak 9 1170 I 1171 If-Match header field 12 1172 If-Modified-Since header field 15 1173 If-None-Match header field 13 1174 If-Unmodified-Since header field 16 1176 L 1177 Last-Modified header field 6 1179 M 1180 metadata 4 1182 S 1183 selected representation 3 1185 V 1186 validator 4 1187 strong 5 1188 weak 5 1190 Acknowledgments 1192 See Appendix "Acknowledgments" of [MESSGNG]. 1194 Authors' Addresses 1196 Roy T. Fielding (editor) 1197 Adobe 1198 345 Park Ave 1199 San Jose, CA 95110 1200 USA 1202 EMail: fielding@gbiv.com 1203 URI: http://roy.gbiv.com/ 1205 Mark Nottingham (editor) 1206 Fastly 1208 EMail: mnot@mnot.net 1209 URI: https://www.mnot.net/ 1211 Julian F. Reschke (editor) 1212 greenbytes GmbH 1213 Hafenweg 16 1214 Muenster, NW 48155 1215 Germany 1217 EMail: julian.reschke@greenbytes.de 1218 URI: http://greenbytes.de/tech/webdav/