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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 HTTPbis Working Group R. Fielding, Ed. 3 Internet-Draft Adobe 4 Obsoletes: 2616 (if approved) J. Gettys 5 Intended status: Standards Track Alcatel-Lucent 6 Expires: July 7, 2012 J. Mogul 7 HP 8 H. Frystyk 9 Microsoft 10 L. Masinter 11 Adobe 12 P. Leach 13 Microsoft 14 T. Berners-Lee 15 W3C/MIT 16 Y. Lafon, Ed. 17 W3C 18 J. Reschke, Ed. 19 greenbytes 20 January 4, 2012 22 HTTP/1.1, part 4: Conditional Requests 23 draft-ietf-httpbis-p4-conditional-18 25 Abstract 27 The Hypertext Transfer Protocol (HTTP) is an application-level 28 protocol for distributed, collaborative, hypertext information 29 systems. HTTP has been in use by the World Wide Web global 30 information initiative since 1990. This document is Part 4 of the 31 seven-part specification that defines the protocol referred to as 32 "HTTP/1.1" and, taken together, obsoletes RFC 2616. 34 Part 4 defines request header fields for indicating conditional 35 requests and the rules for constructing responses to those requests. 37 Editorial Note (To be removed by RFC Editor) 39 Discussion of this draft should take place on the HTTPBIS working 40 group mailing list (ietf-http-wg@w3.org), which is archived at 41 . 43 The current issues list is at 44 and related 45 documents (including fancy diffs) can be found at 46 . 48 The changes in this draft are summarized in Appendix C.19. 50 Status of This Memo 52 This Internet-Draft is submitted in full conformance with the 53 provisions of BCP 78 and BCP 79. 55 Internet-Drafts are working documents of the Internet Engineering 56 Task Force (IETF). Note that other groups may also distribute 57 working documents as Internet-Drafts. The list of current Internet- 58 Drafts is at http://datatracker.ietf.org/drafts/current/. 60 Internet-Drafts are draft documents valid for a maximum of six months 61 and may be updated, replaced, or obsoleted by other documents at any 62 time. It is inappropriate to use Internet-Drafts as reference 63 material or to cite them other than as "work in progress." 65 This Internet-Draft will expire on July 7, 2012. 67 Copyright Notice 69 Copyright (c) 2012 IETF Trust and the persons identified as the 70 document authors. All rights reserved. 72 This document is subject to BCP 78 and the IETF Trust's Legal 73 Provisions Relating to IETF Documents 74 (http://trustee.ietf.org/license-info) in effect on the date of 75 publication of this document. Please review these documents 76 carefully, as they describe your rights and restrictions with respect 77 to this document. Code Components extracted from this document must 78 include Simplified BSD License text as described in Section 4.e of 79 the Trust Legal Provisions and are provided without warranty as 80 described in the Simplified BSD License. 82 This document may contain material from IETF Documents or IETF 83 Contributions published or made publicly available before November 84 10, 2008. The person(s) controlling the copyright in some of this 85 material may not have granted the IETF Trust the right to allow 86 modifications of such material outside the IETF Standards Process. 87 Without obtaining an adequate license from the person(s) controlling 88 the copyright in such materials, this document may not be modified 89 outside the IETF Standards Process, and derivative works of it may 90 not be created outside the IETF Standards Process, except to format 91 it for publication as an RFC or to translate it into languages other 92 than English. 94 Table of Contents 96 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 97 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 5 98 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 99 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 6 100 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 6 101 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 8 102 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 8 103 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 9 104 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 105 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 11 106 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 12 107 2.3.3. Example: Entity-tags varying on Content-Negotiated 108 Resources . . . . . . . . . . . . . . . . . . . . . . 12 109 2.4. Rules for When to Use Entity-tags and Last-Modified 110 Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 13 111 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 15 112 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 15 113 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 16 114 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 17 115 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 18 116 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 19 117 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 19 118 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 19 119 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 20 120 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 121 5.1. Status Code Registration . . . . . . . . . . . . . . . . . 20 122 5.2. Header Field Registration . . . . . . . . . . . . . . . . 20 123 6. Security Considerations . . . . . . . . . . . . . . . . . . . 21 124 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21 125 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 126 8.1. Normative References . . . . . . . . . . . . . . . . . . . 21 127 8.2. Informative References . . . . . . . . . . . . . . . . . . 22 128 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22 129 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 23 130 Appendix C. Change Log (to be removed by RFC Editor before 131 publication) . . . . . . . . . . . . . . . . . . . . 23 132 C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 23 133 C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23 134 C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 24 135 C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 24 136 C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 24 137 C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24 138 C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 25 139 C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 25 140 C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 25 141 C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 25 142 C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 25 143 C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25 144 C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 26 145 C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 26 146 C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 26 147 C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 26 148 C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 26 149 C.18. Since draft-ietf-httpbis-p4-conditional-16 . . . . . . . . 26 150 C.19. Since draft-ietf-httpbis-p4-conditional-17 . . . . . . . . 27 151 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 153 1. Introduction 155 This document defines the HTTP/1.1 conditional request mechanisms, 156 including both metadata for indicating/observing changes in resource 157 representations and request header fields that specify preconditions 158 on that metadata be checked before performing the request method. 159 Conditional GET requests are the most efficient mechanism for HTTP 160 cache updates [Part6]. Conditionals can also be applied to state- 161 changing methods, such as PUT and DELETE, to prevent the "lost 162 update" problem: one client accidentally overwriting the work of 163 another client that has been acting in parallel. 165 Conditional request preconditions are based on the state of the 166 target resource as a whole (its current value set) or the state as 167 observed in a previously obtained representation (one value in that 168 set). A resource might have multiple current representations, each 169 with its own observable state. The conditional request mechanisms 170 assume that the mapping of requests to corresponding representations 171 will be consistent over time if the server intends to take advantage 172 of conditionals. Regardless, if the mapping is inconsistent and the 173 server is unable to select the appropriate representation, then no 174 harm will result when the precondition evaluates to false. 176 We use the term "selected representation" to refer to the current 177 representation of the target resource that would have been selected 178 in a successful response if the same request had used the method GET 179 and had excluded all of the conditional request header fields. The 180 conditional request preconditions are evaluated by comparing the 181 values provided in the request header fields to the current metadata 182 for the selected representation. 184 1.1. Conformance and Error Handling 186 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 187 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 188 document are to be interpreted as described in [RFC2119]. 190 This document defines conformance criteria for several roles in HTTP 191 communication, including Senders, Recipients, Clients, Servers, User- 192 Agents, Origin Servers, Intermediaries, Proxies and Gateways. See 193 Section 2 of [Part1] for definitions of these terms. 195 An implementation is considered conformant if it complies with all of 196 the requirements associated with its role(s). Note that SHOULD-level 197 requirements are relevant here, unless one of the documented 198 exceptions is applicable. 200 This document also uses ABNF to define valid protocol elements 201 (Section 1.2). In addition to the prose requirements placed upon 202 them, Senders MUST NOT generate protocol elements that are invalid. 204 Unless noted otherwise, Recipients MAY take steps to recover a usable 205 protocol element from an invalid construct. However, HTTP does not 206 define specific error handling mechanisms, except in cases where it 207 has direct impact on security. This is because different uses of the 208 protocol require different error handling strategies; for example, a 209 Web browser may wish to transparently recover from a response where 210 the Location header field doesn't parse according to the ABNF, 211 whereby in a systems control protocol using HTTP, this type of error 212 recovery could lead to dangerous consequences. 214 1.2. Syntax Notation 216 This specification uses the ABNF syntax defined in Section 1.2 of 217 [Part1] (which extends the syntax defined in [RFC5234] with a list 218 rule). Appendix B shows the collected ABNF, with the list rule 219 expanded. 221 The following core rules are included by reference, as defined in 222 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF 223 (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), 224 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit 225 sequence of data), SP (space), and VCHAR (any visible US-ASCII 226 character). 228 The ABNF rules below are defined in [Part1] and [Part2]: 230 OWS = 231 obs-text = 232 HTTP-date = 234 2. Validators 236 This specification defines two forms of metadata that are commonly 237 used to observe resource state and test for preconditions: 238 modification dates and opaque entity tags. Additional metadata that 239 reflects resource state has been defined by various extensions of 240 HTTP, such as WebDAV [RFC4918], that are beyond the scope of this 241 specification. A resource metadata value is referred to as a 242 "validator" when it is used within a precondition. 244 2.1. Weak versus Strong 246 Validators come in two flavors: strong or weak. Weak validators are 247 easy to generate but are far less useful for comparisons. Strong 248 validators are ideal for comparisons but can be very difficult (and 249 occasionally impossible) to generate efficiently. Rather than impose 250 that all forms of resource adhere to the same strength of validator, 251 HTTP exposes the type of validator in use and imposes restrictions on 252 when weak validators can be used as preconditions. 254 A "strong validator" is a representation metadata value that MUST be 255 changed to a new, previously unused or guaranteed unique, value 256 whenever a change occurs to the representation data such that a 257 change would be observable in the payload body of a 200 response to 258 GET. A strong validator MAY be changed for other reasons, such as 259 when a semantically significant part of the representation metadata 260 is changed (e.g., Content-Type), but it is in the best interests of 261 the origin server to only change the value when it is necessary to 262 invalidate the stored responses held by remote caches and authoring 263 tools. A strong validator MUST be unique across all representations 264 of a given resource, such that no two representations of that 265 resource share the same validator unless their payload body would be 266 identical. 268 Cache entries might persist for arbitrarily long periods, regardless 269 of expiration times. Thus, a cache might attempt to validate an 270 entry using a validator that it obtained in the distant past. A 271 strong validator MUST be unique across all versions of all 272 representations associated with a particular resource over time. 273 However, there is no implication of uniqueness across representations 274 of different resources (i.e., the same strong validator might be in 275 use for representations of multiple resources at the same time and 276 does not imply that those representations are equivalent). 278 There are a variety of strong validators used in practice. The best 279 are based on strict revision control, wherein each change to a 280 representation always results in a unique node name and revision 281 identifier being assigned before the representation is made 282 accessible to GET. A cryptographic hash function applied to the 283 representation data is also sufficient if the data is available prior 284 to the response header fields being sent and the digest does not need 285 to be recalculated every time a validation request is received. 286 However, if a resource has distinct representations that differ only 287 in their metadata, such as might occur with content negotiation over 288 media types that happen to share the same data format, then a server 289 SHOULD incorporate additional information in the validator to 290 distinguish those representations and avoid confusing cache behavior. 292 In contrast, a "weak validator" is a representation metadata value 293 that might not be changed for every change to the representation 294 data. This weakness might be due to limitations in how the value is 295 calculated, such as clock resolution or an inability to ensure 296 uniqueness for all possible representations of the resource, or due 297 to a desire by the resource owner to group representations by some 298 self-determined set of equivalency rather than unique sequences of 299 data. A weak entity-tag SHOULD change whenever the origin server 300 considers prior representations to be unacceptable as a substitute 301 for the current representation. In other words, a weak entity-tag 302 SHOULD change whenever the origin server wants caches to invalidate 303 old responses. 305 For example, the representation of a weather report that changes in 306 content every second, based on dynamic measurements, might be grouped 307 into sets of equivalent representations (from the origin server's 308 perspective) with the same weak validator in order to allow cached 309 representations to be valid for a reasonable period of time (perhaps 310 adjusted dynamically based on server load or weather quality). 311 Likewise, a representation's modification time, if defined with only 312 one-second resolution, might be a weak validator if it is possible 313 for the representation to be modified twice during a single second 314 and retrieved between those modifications. 316 A "use" of a validator occurs when either a client generates a 317 request and includes the validator in a precondition or when a server 318 compares two validators. Weak validators are only usable in contexts 319 that do not depend on exact equality of a representation's payload 320 body. Strong validators are usable and preferred for all conditional 321 requests, including cache validation, partial content ranges, and 322 "lost update" avoidance. 324 2.2. Last-Modified 326 The "Last-Modified" header field indicates the date and time at which 327 the origin server believes the selected representation was last 328 modified. 330 Last-Modified = HTTP-date 332 An example of its use is 334 Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 336 2.2.1. Generation 338 Origin servers SHOULD send Last-Modified for any selected 339 representation for which a last modification date can be reasonably 340 and consistently determined, since its use in conditional requests 341 and evaluating cache freshness ([Part6]) results in a substantial 342 reduction of HTTP traffic on the Internet and can be a significant 343 factor in improving service scalability and reliability. 345 A representation is typically the sum of many parts behind the 346 resource interface. The last-modified time would usually be the most 347 recent time that any of those parts were changed. How that value is 348 determined for any given resource is an implementation detail beyond 349 the scope of this specification. What matters to HTTP is how 350 recipients of the Last-Modified header field can use its value to 351 make conditional requests and test the validity of locally cached 352 responses. 354 An origin server SHOULD obtain the Last-Modified value of the 355 representation as close as possible to the time that it generates the 356 Date field-value for its response. This allows a recipient to make 357 an accurate assessment of the representation's modification time, 358 especially if the representation changes near the time that the 359 response is generated. 361 An origin server with a clock MUST NOT send a Last-Modified date that 362 is later than the server's time of message origination (Date). If 363 the last modification time is derived from implementation-specific 364 metadata that evaluates to some time in the future, according to the 365 origin server's clock, then the origin server MUST replace that value 366 with the message origination date. This prevents a future 367 modification date from having an adverse impact on cache validation. 369 An origin server without a clock MUST NOT assign Last-Modified values 370 to a response unless these values were associated with the resource 371 by some other system or user with a reliable clock. 373 2.2.2. Comparison 375 A Last-Modified time, when used as a validator in a request, is 376 implicitly weak unless it is possible to deduce that it is strong, 377 using the following rules: 379 o The validator is being compared by an origin server to the actual 380 current validator for the representation and, 382 o That origin server reliably knows that the associated 383 representation did not change twice during the second covered by 384 the presented validator. 386 or 388 o The validator is about to be used by a client in an If-Modified- 389 Since, If-Unmodified-Since header field, because the client has a 390 cache entry, or If-Range for the associated representation, and 392 o That cache entry includes a Date value, which gives the time when 393 the origin server sent the original response, and 395 o The presented Last-Modified time is at least 60 seconds before the 396 Date value. 398 or 400 o The validator is being compared by an intermediate cache to the 401 validator stored in its cache entry for the representation, and 403 o That cache entry includes a Date value, which gives the time when 404 the origin server sent the original response, and 406 o The presented Last-Modified time is at least 60 seconds before the 407 Date value. 409 This method relies on the fact that if two different responses were 410 sent by the origin server during the same second, but both had the 411 same Last-Modified time, then at least one of those responses would 412 have a Date value equal to its Last-Modified time. The arbitrary 60- 413 second limit guards against the possibility that the Date and Last- 414 Modified values are generated from different clocks, or at somewhat 415 different times during the preparation of the response. An 416 implementation MAY use a value larger than 60 seconds, if it is 417 believed that 60 seconds is too short. 419 2.3. ETag 421 The ETag header field provides the current entity-tag for the 422 selected representation. An entity-tag is an opaque validator for 423 differentiating between multiple representations of the same 424 resource, regardless of whether those multiple representations are 425 due to resource state changes over time, content negotiation 426 resulting in multiple representations being valid at the same time, 427 or both. An entity-tag consists of an opaque quoted string, possibly 428 prefixed by a weakness indicator. 430 ETag = entity-tag 432 entity-tag = [ weak ] opaque-tag 433 weak = %x57.2F ; "W/", case-sensitive 434 opaque-tag = DQUOTE *etagc DQUOTE 435 etagc = %x21 / %x23-7E / obs-text 436 ; VCHAR except double quotes, plus obs-text 438 Note: Previously, opaque-tag was defined to be a quoted-string 439 ([RFC2616], Section 3.11), thus some recipients might perform 440 backslash unescaping. Servers therefore ought to avoid backslash 441 characters in entity tags. 443 An entity-tag can be more reliable for validation than a modification 444 date in situations where it is inconvenient to store modification 445 dates, where the one-second resolution of HTTP date values is not 446 sufficient, or where modification dates are not consistently 447 maintained. 449 Examples: 451 ETag: "xyzzy" 452 ETag: W/"xyzzy" 453 ETag: "" 455 An entity-tag can be either a weak or strong validator, with strong 456 being the default. If an origin server provides an entity-tag for a 457 representation and the generation of that entity-tag does not satisfy 458 the requirements for a strong validator (Section 2.1), then that 459 entity-tag MUST be marked as weak by prefixing its opaque value with 460 "W/" (case-sensitive). 462 2.3.1. Generation 464 The principle behind entity-tags is that only the service author 465 knows the implementation of a resource well enough to select the most 466 accurate and efficient validation mechanism for that resource, and 467 that any such mechanism can be mapped to a simple sequence of octets 468 for easy comparison. Since the value is opaque, there is no need for 469 the client to be aware of how each entity-tag is constructed. 471 For example, a resource that has implementation-specific versioning 472 applied to all changes might use an internal revision number, perhaps 473 combined with a variance identifier for content negotiation, to 474 accurately differentiate between representations. Other 475 implementations might use a stored hash of representation content, a 476 combination of various filesystem attributes, or a modification 477 timestamp that has sub-second resolution. 479 Origin servers SHOULD send ETag for any selected representation for 480 which detection of changes can be reasonably and consistently 481 determined, since the entity-tag's use in conditional requests and 482 evaluating cache freshness ([Part6]) can result in a substantial 483 reduction of HTTP network traffic and can be a significant factor in 484 improving service scalability and reliability. 486 2.3.2. Comparison 488 There are two entity-tag comparison functions, depending on whether 489 the comparison context allows the use of weak validators or not: 491 o The strong comparison function: in order to be considered equal, 492 both opaque-tags MUST be identical character-by-character, and 493 both MUST NOT be weak. 495 o The weak comparison function: in order to be considered equal, 496 both opaque-tags MUST be identical character-by-character, but 497 either or both of them MAY be tagged as "weak" without affecting 498 the result. 500 The example below shows the results for a set of entity-tag pairs, 501 and both the weak and strong comparison function results: 503 +--------+--------+-------------------+-----------------+ 504 | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | 505 +--------+--------+-------------------+-----------------+ 506 | W/"1" | W/"1" | no match | match | 507 | W/"1" | W/"2" | no match | no match | 508 | W/"1" | "1" | no match | match | 509 | "1" | "1" | match | match | 510 +--------+--------+-------------------+-----------------+ 512 2.3.3. Example: Entity-tags varying on Content-Negotiated Resources 514 Consider a resource that is subject to content negotiation (Section 5 515 of [Part3]), and where the representations returned upon a GET 516 request vary based on the Accept-Encoding request header field 517 (Section 6.3 of [Part3]): 519 >> Request: 521 GET /index HTTP/1.1 522 Host: www.example.com 523 Accept-Encoding: gzip 525 In this case, the response might or might not use the gzip content 526 coding. If it does not, the response might look like: 528 >> Response: 530 HTTP/1.1 200 OK 531 Date: Thu, 26 Mar 2010 00:05:00 GMT 532 ETag: "123-a" 533 Content-Length: 70 534 Vary: Accept-Encoding 535 Content-Type: text/plain 537 Hello World! 538 Hello World! 539 Hello World! 540 Hello World! 541 Hello World! 543 An alternative representation that does use gzip content coding would 544 be: 546 >> Response: 548 HTTP/1.1 200 OK 549 Date: Thu, 26 Mar 2010 00:05:00 GMT 550 ETag: "123-b" 551 Content-Length: 43 552 Vary: Accept-Encoding 553 Content-Type: text/plain 554 Content-Encoding: gzip 556 ...binary data... 558 Note: Content codings are a property of the representation, so 559 therefore an entity-tag of an encoded representation must be 560 distinct from an unencoded representation to prevent conflicts 561 during cache updates and range requests. In contrast, transfer 562 codings (Section 5.1 of [Part1]) apply only during message 563 transfer and do not require distinct entity-tags. 565 2.4. Rules for When to Use Entity-tags and Last-Modified Dates 567 We adopt a set of rules and recommendations for origin servers, 568 clients, and caches regarding when various validator types ought to 569 be used, and for what purposes. 571 HTTP/1.1 origin servers: 573 o SHOULD send an entity-tag validator unless it is not feasible to 574 generate one. 576 o MAY send a weak entity-tag instead of a strong entity-tag, if 577 performance considerations support the use of weak entity-tags, or 578 if it is unfeasible to send a strong entity-tag. 580 o SHOULD send a Last-Modified value if it is feasible to send one. 582 In other words, the preferred behavior for an HTTP/1.1 origin server 583 is to send both a strong entity-tag and a Last-Modified value. 585 HTTP/1.1 clients: 587 o MUST use that entity-tag in any cache-conditional request (using 588 If-Match or If-None-Match) if an entity-tag has been provided by 589 the origin server. 591 o SHOULD use the Last-Modified value in non-subrange cache- 592 conditional requests (using If-Modified-Since) if only a Last- 593 Modified value has been provided by the origin server. 595 o MAY use the Last-Modified value in subrange cache-conditional 596 requests (using If-Unmodified-Since) if only a Last-Modified value 597 has been provided by an HTTP/1.0 origin server. The user agent 598 SHOULD provide a way to disable this, in case of difficulty. 600 o SHOULD use both validators in cache-conditional requests if both 601 an entity-tag and a Last-Modified value have been provided by the 602 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to 603 respond appropriately. 605 An HTTP/1.1 origin server, upon receiving a conditional request that 606 includes both a Last-Modified date (e.g., in an If-Modified-Since or 607 If-Unmodified-Since header field) and one or more entity-tags (e.g., 608 in an If-Match, If-None-Match, or If-Range header field) as cache 609 validators, MUST NOT return a response status code of 304 (Not 610 Modified) unless doing so is consistent with all of the conditional 611 header fields in the request. 613 An HTTP/1.1 caching proxy, upon receiving a conditional request that 614 includes both a Last-Modified date and one or more entity-tags as 615 cache validators, MUST NOT return a locally cached response to the 616 client unless that cached response is consistent with all of the 617 conditional header fields in the request. 619 Note: The general principle behind these rules is that HTTP/1.1 620 servers and clients ought to transmit as much non-redundant 621 information as is available in their responses and requests. 622 HTTP/1.1 systems receiving this information will make the most 623 conservative assumptions about the validators they receive. 625 HTTP/1.0 clients and caches might ignore entity-tags. Generally, 626 last-modified values received or used by these systems will 627 support transparent and efficient caching, and so HTTP/1.1 origin 628 servers should provide Last-Modified values. In those rare cases 629 where the use of a Last-Modified value as a validator by an 630 HTTP/1.0 system could result in a serious problem, then HTTP/1.1 631 origin servers should not provide one. 633 3. Precondition Header Fields 635 This section defines the syntax and semantics of HTTP/1.1 header 636 fields for applying preconditions on requests. 638 3.1. If-Match 640 The "If-Match" header field MAY be used to make a request method 641 conditional on the current existence or value of an entity-tag for 642 one or more representations of the target resource. If-Match is 643 generally useful for resource update requests, such as PUT requests, 644 as a means for protecting against accidental overwrites when multiple 645 clients are acting in parallel on the same resource (i.e., the "lost 646 update" problem). An If-Match field-value of "*" places the 647 precondition on the existence of any current representation for the 648 target resource. 650 If-Match = "*" / 1#entity-tag 652 If any of the entity-tags listed in the If-Match field value match 653 (as per Section 2.3.2) the entity-tag of the selected representation 654 for the target resource, or if "*" is given and any current 655 representation exists for the target resource, then the server MAY 656 perform the request method as if the If-Match header field was not 657 present. 659 If none of the entity-tags match, or if "*" is given and no current 660 representation exists, the server MUST NOT perform the requested 661 method. Instead, the server MUST respond with the 412 (Precondition 662 Failed) status code. 664 If the request would, without the If-Match header field, result in 665 anything other than a 2xx or 412 status code, then the If-Match 666 header field MUST be ignored. 668 Examples: 670 If-Match: "xyzzy" 671 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 672 If-Match: * 674 The result of a request having both an If-Match header field and 675 either an If-None-Match or an If-Modified-Since header fields is 676 undefined by this specification. 678 3.2. If-None-Match 680 The "If-None-Match" header field MAY be used to make a request method 681 conditional on not matching any of the current entity-tag values for 682 representations of the target resource. If-None-Match is primarily 683 used in conditional GET requests to enable efficient updates of 684 cached information with a minimum amount of transaction overhead. A 685 client that has one or more representations previously obtained from 686 the target resource can send If-None-Match with a list of the 687 associated entity-tags in the hope of receiving a 304 response if at 688 least one of those representations matches the selected 689 representation. 691 If-None-Match MAY also be used with a value of "*" to prevent an 692 unsafe request method (e.g., PUT) from inadvertently modifying an 693 existing representation of the target resource when the client 694 believes that the resource does not have a current representation. 695 This is a variation on the "lost update" problem that might arise if 696 more than one client attempts to create an initial representation for 697 the target resource. 699 If-None-Match = "*" / 1#entity-tag 701 If any of the entity-tags listed in the If-None-Match field-value 702 match (as per Section 2.3.2) the entity-tag of the selected 703 representation, or if "*" is given and any current representation 704 exists for that resource, then the server MUST NOT perform the 705 requested method. Instead, if the request method was GET or HEAD, 706 the server SHOULD respond with a 304 (Not Modified) status code, 707 including the cache-related header fields (particularly ETag) of the 708 selected representation that has a matching entity-tag. For all 709 other request methods, the server MUST respond with a 412 710 (Precondition Failed) status code. 712 If none of the entity-tags match, then the server MAY perform the 713 requested method as if the If-None-Match header field did not exist, 714 but MUST also ignore any If-Modified-Since header field(s) in the 715 request. That is, if no entity-tags match, then the server MUST NOT 716 return a 304 (Not Modified) response. 718 If the request would, without the If-None-Match header field, result 719 in anything other than a 2xx or 304 status code, then the If-None- 720 Match header field MUST be ignored. (See Section 2.4 for a 721 discussion of server behavior when both If-Modified-Since and If- 722 None-Match appear in the same request.) 724 Examples: 726 If-None-Match: "xyzzy" 727 If-None-Match: W/"xyzzy" 728 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 729 If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" 730 If-None-Match: * 732 The result of a request having both an If-None-Match header field and 733 either an If-Match or an If-Unmodified-Since header fields is 734 undefined by this specification. 736 3.3. If-Modified-Since 738 The "If-Modified-Since" header field MAY be used to make a request 739 method conditional by modification date: if the selected 740 representation has not been modified since the time specified in this 741 field, then do not perform the request method; instead, respond as 742 detailed below. 744 If-Modified-Since = HTTP-date 746 An example of the field is: 748 If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 750 A GET method with an If-Modified-Since header field and no Range 751 header field requests that the selected representation be transferred 752 only if it has been modified since the date given by the If-Modified- 753 Since header field. The algorithm for determining this includes the 754 following cases: 756 1. If the request would normally result in anything other than a 200 757 (OK) status code, or if the passed If-Modified-Since date is 758 invalid, the response is exactly the same as for a normal GET. A 759 date which is later than the server's current time is invalid. 761 2. If the selected representation has been modified since the If- 762 Modified-Since date, the response is exactly the same as for a 763 normal GET. 765 3. If the selected representation has not been modified since a 766 valid If-Modified-Since date, the server SHOULD return a 304 (Not 767 Modified) response. 769 The purpose of this feature is to allow efficient updates of cached 770 information with a minimum amount of transaction overhead. 772 Note: The Range header field modifies the meaning of If-Modified- 773 Since; see Section 5.4 of [Part5] for full details. 775 Note: If-Modified-Since times are interpreted by the server, whose 776 clock might not be synchronized with the client. 778 Note: When handling an If-Modified-Since header field, some 779 servers will use an exact date comparison function, rather than a 780 less-than function, for deciding whether to send a 304 (Not 781 Modified) response. To get best results when sending an If- 782 Modified-Since header field for cache validation, clients are 783 advised to use the exact date string received in a previous Last- 784 Modified header field whenever possible. 786 Note: If a client uses an arbitrary date in the If-Modified-Since 787 header field instead of a date taken from the Last-Modified header 788 field for the same request, the client needs to be aware that this 789 date is interpreted in the server's understanding of time. 790 Unsynchronized clocks and rounding problems, due to the different 791 encodings of time between the client and server, are concerns. 792 This includes the possibility of race conditions if the document 793 has changed between the time it was first requested and the If- 794 Modified-Since date of a subsequent request, and the possibility 795 of clock-skew-related problems if the If-Modified-Since date is 796 derived from the client's clock without correction to the server's 797 clock. Corrections for different time bases between client and 798 server are at best approximate due to network latency. 800 The result of a request having both an If-Modified-Since header field 801 and either an If-Match or an If-Unmodified-Since header fields is 802 undefined by this specification. 804 3.4. If-Unmodified-Since 806 The "If-Unmodified-Since" header field MAY be used to make a request 807 method conditional by modification date: if the selected 808 representation has been modified since the time specified in this 809 field, then the server MUST NOT perform the requested operation and 810 MUST instead respond with the 412 (Precondition Failed) status code. 811 If the selected representation has not been modified since the time 812 specified in this field, the server SHOULD perform the request method 813 as if the If-Unmodified-Since header field were not present. 815 If-Unmodified-Since = HTTP-date 817 An example of the field is: 819 If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT 821 If the request normally (i.e., without the If-Unmodified-Since header 822 field) would result in anything other than a 2xx or 412 status code, 823 the If-Unmodified-Since header field SHOULD be ignored. 825 If the specified date is invalid, the header field MUST be ignored. 827 The result of a request having both an If-Unmodified-Since header 828 field and either an If-None-Match or an If-Modified-Since header 829 fields is undefined by this specification. 831 3.5. If-Range 833 The If-Range header field provides a special conditional request 834 mechanism that is similar to If-Match and If-Unmodified-Since but 835 specific to HTTP range requests. If-Range is defined in Section 5.3 836 of [Part5]. 838 4. Status Code Definitions 840 4.1. 304 Not Modified 842 The 304 status code indicates that a conditional GET request has been 843 received and would have resulted in a 200 (OK) response if it were 844 not for the fact that the condition has evaluated to false. In other 845 words, there is no need for the server to transfer a representation 846 of the target resource because the client's request indicates that it 847 already has a valid representation, as indicated by the 304 response 848 header fields, and is therefore redirecting the client to make use of 849 that stored representation as if it were the payload of a 200 850 response. The 304 response MUST NOT contain a message-body, and thus 851 is always terminated by the first empty line after the header fields. 853 A 304 response MUST include a Date header field (Section 9.2 of 854 [Part2]) unless the origin server does not have a clock that can 855 provide a reasonable approximation of the current time. If a 200 856 response to the same request would have included any of the header 857 fields Cache-Control, Content-Location, ETag, Expires, Last-Modified, 858 or Vary, then those same header fields MUST be sent in a 304 859 response. 861 Since the goal of a 304 response is to minimize information transfer 862 when the recipient already has one or more cached representations, 863 the response SHOULD NOT include representation metadata other than 864 the above listed fields unless said metadata exists for the purpose 865 of guiding cache updates (e.g., future HTTP extensions). 867 If the recipient of a 304 response does not have a cached 868 representation corresponding to the entity-tag indicated by the 304 869 response, then the recipient MUST NOT use the 304 to update its own 870 cache. If this conditional request originated with an outbound 871 client, such as a user agent with its own cache sending a conditional 872 GET to a shared proxy, then the 304 response MAY be forwarded to the 873 outbound client. Otherwise, the recipient MUST disregard the 304 874 response and repeat the request without any preconditions. 876 If a cache uses a received 304 response to update a cache entry, the 877 cache MUST update the entry to reflect any new field values given in 878 the response. 880 4.2. 412 Precondition Failed 882 The 412 status code indicates that one or more preconditions given in 883 the request header fields evaluated to false when tested on the 884 server. This response code allows the client to place preconditions 885 on the current resource state (its current representations and 886 metadata) and thus prevent the request method from being applied if 887 the target resource is in an unexpected state. 889 5. IANA Considerations 891 5.1. Status Code Registration 893 The HTTP Status Code Registry located at 894 shall be updated 895 with the registrations below: 897 +-------+---------------------+-------------+ 898 | Value | Description | Reference | 899 +-------+---------------------+-------------+ 900 | 304 | Not Modified | Section 4.1 | 901 | 412 | Precondition Failed | Section 4.2 | 902 +-------+---------------------+-------------+ 904 5.2. Header Field Registration 906 The Message Header Field Registry located at shall be 908 updated with the permanent registrations below (see [RFC3864]): 910 +---------------------+----------+----------+-------------+ 911 | Header Field Name | Protocol | Status | Reference | 912 +---------------------+----------+----------+-------------+ 913 | ETag | http | standard | Section 2.3 | 914 | If-Match | http | standard | Section 3.1 | 915 | If-Modified-Since | http | standard | Section 3.3 | 916 | If-None-Match | http | standard | Section 3.2 | 917 | If-Unmodified-Since | http | standard | Section 3.4 | 918 | Last-Modified | http | standard | Section 2.2 | 919 +---------------------+----------+----------+-------------+ 921 The change controller is: "IETF (iesg@ietf.org) - Internet 922 Engineering Task Force". 924 6. Security Considerations 926 No additional security considerations have been identified beyond 927 those applicable to HTTP in general [Part1]. 929 7. Acknowledgments 931 See Section 11 of [Part1]. 933 8. References 935 8.1. Normative References 937 [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 938 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 939 and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections, 940 and Message Parsing", draft-ietf-httpbis-p1-messaging-18 941 (work in progress), January 2012. 943 [Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 944 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 945 and J. Reschke, Ed., "HTTP/1.1, part 2: Message 946 Semantics", draft-ietf-httpbis-p2-semantics-18 (work in 947 progress), January 2012. 949 [Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 950 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 951 and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload 952 and Content Negotiation", draft-ietf-httpbis-p3-payload-18 953 (work in progress), January 2012. 955 [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 956 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 957 and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and 958 Partial Responses", draft-ietf-httpbis-p5-range-18 (work 959 in progress), January 2012. 961 [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 962 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 963 Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part 964 6: Caching", draft-ietf-httpbis-p6-cache-18 (work in 965 progress), January 2012. 967 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 968 Requirement Levels", BCP 14, RFC 2119, March 1997. 970 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 971 Specifications: ABNF", STD 68, RFC 5234, January 2008. 973 8.2. Informative References 975 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 976 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 977 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 979 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 980 Procedures for Message Header Fields", BCP 90, RFC 3864, 981 September 2004. 983 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed 984 Authoring and Versioning (WebDAV)", RFC 4918, June 2007. 986 Appendix A. Changes from RFC 2616 988 Allow weak entity-tags in all requests except range requests 989 (Sections 2.1 and 3.2). 991 Change ETag header field ABNF not to use quoted-string, thus avoiding 992 escaping issues. (Section 2.3) 994 Change ABNF productions for header fields to only define the field 995 value. (Section 3) 997 Appendix B. Collected ABNF 999 ETag = entity-tag 1001 HTTP-date = 1003 If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1004 entity-tag ] ) ) 1005 If-Modified-Since = HTTP-date 1006 If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS 1007 entity-tag ] ) ) 1008 If-Unmodified-Since = HTTP-date 1010 Last-Modified = HTTP-date 1012 OWS = 1014 entity-tag = [ weak ] opaque-tag 1015 etagc = "!" / %x23-7E ; '#'-'~' 1016 / obs-text 1018 obs-text = 1019 opaque-tag = DQUOTE *etagc DQUOTE 1021 weak = %x57.2F ; W/ 1023 ABNF diagnostics: 1025 ; ETag defined but not used 1026 ; If-Match defined but not used 1027 ; If-Modified-Since defined but not used 1028 ; If-None-Match defined but not used 1029 ; If-Unmodified-Since defined but not used 1030 ; Last-Modified defined but not used 1032 Appendix C. Change Log (to be removed by RFC Editor before publication) 1034 C.1. Since RFC 2616 1036 Extracted relevant partitions from [RFC2616]. 1038 C.2. Since draft-ietf-httpbis-p4-conditional-00 1040 Closed issues: 1042 o : "Normative and 1043 Informative references" 1045 Other changes: 1047 o Move definitions of 304 and 412 condition codes from Part2. 1049 C.3. Since draft-ietf-httpbis-p4-conditional-01 1051 Ongoing work on ABNF conversion 1052 (): 1054 o Add explicit references to BNF syntax and rules imported from 1055 other parts of the specification. 1057 C.4. Since draft-ietf-httpbis-p4-conditional-02 1059 Closed issues: 1061 o : "Weak ETags on 1062 non-GET requests" 1064 Ongoing work on IANA Message Header Field Registration 1065 (): 1067 o Reference RFC 3984, and update header field registrations for 1068 header fields defined in this document. 1070 C.5. Since draft-ietf-httpbis-p4-conditional-03 1072 Closed issues: 1074 o : "Examples for 1075 ETag matching" 1077 o : "'entity 1078 value' undefined" 1080 o : "bogus 2068 1081 Date header reference" 1083 C.6. Since draft-ietf-httpbis-p4-conditional-04 1085 Ongoing work on ABNF conversion 1086 (): 1088 o Use "/" instead of "|" for alternatives. 1090 o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional 1091 whitespace ("OWS") and required whitespace ("RWS"). 1093 o Rewrite ABNFs to spell out whitespace rules, factor out header 1094 field value format definitions. 1096 C.7. Since draft-ietf-httpbis-p4-conditional-05 1098 Final work on ABNF conversion 1099 (): 1101 o Add appendix containing collected and expanded ABNF, reorganize 1102 ABNF introduction. 1104 C.8. Since draft-ietf-httpbis-p4-conditional-06 1106 Closed issues: 1108 o : "case- 1109 sensitivity of etag weakness indicator" 1111 C.9. Since draft-ietf-httpbis-p4-conditional-07 1113 Closed issues: 1115 o : "Weak ETags on 1116 non-GET requests" (If-Match still was defined to require strong 1117 matching) 1119 o : "move IANA 1120 registrations for optional status codes" 1122 C.10. Since draft-ietf-httpbis-p4-conditional-08 1124 No significant changes. 1126 C.11. Since draft-ietf-httpbis-p4-conditional-09 1128 No significant changes. 1130 C.12. Since draft-ietf-httpbis-p4-conditional-10 1132 Closed issues: 1134 o : "Clarify 1135 'Requested Variant'" 1137 o : "Clarify 1138 entity / representation / variant terminology" 1140 o : "consider 1141 removing the 'changes from 2068' sections" 1143 C.13. Since draft-ietf-httpbis-p4-conditional-11 1145 None. 1147 C.14. Since draft-ietf-httpbis-p4-conditional-12 1149 Closed issues: 1151 o : "Header 1152 Classification" 1154 C.15. Since draft-ietf-httpbis-p4-conditional-13 1156 Closed issues: 1158 o : "If-* and 1159 entities" 1161 o : "Definition of 1162 validator weakness" 1164 o : "untangle 1165 ABNFs for header fields" 1167 o : "ETags and 1168 Quotes" 1170 C.16. Since draft-ietf-httpbis-p4-conditional-14 1172 None. 1174 C.17. Since draft-ietf-httpbis-p4-conditional-15 1176 Closed issues: 1178 o : "If-Range 1179 should be listed when dicussing contexts where L-M can be 1180 considered strong" 1182 C.18. Since draft-ietf-httpbis-p4-conditional-16 1184 Closed issues: 1186 o : "Document 1187 HTTP's error-handling philosophy" 1189 C.19. Since draft-ietf-httpbis-p4-conditional-17 1191 Closed issues: 1193 o : "does etag 1194 value really use quoted-string" 1196 Index 1198 3 1199 304 Not Modified (status code) 19 1201 4 1202 412 Precondition Failed (status code) 20 1204 E 1205 ETag header field 10 1207 G 1208 Grammar 1209 entity-tag 10 1210 ETag 10 1211 etagc 10 1212 If-Match 15 1213 If-Modified-Since 17 1214 If-None-Match 16 1215 If-Unmodified-Since 18 1216 Last-Modified 8 1217 opaque-tag 10 1218 weak 10 1220 H 1221 Header Fields 1222 ETag 10 1223 If-Match 15 1224 If-Modified-Since 17 1225 If-None-Match 16 1226 If-Unmodified-Since 18 1227 Last-Modified 8 1229 I 1230 If-Match header field 15 1231 If-Modified-Since header field 17 1232 If-None-Match header field 16 1233 If-Unmodified-Since header field 18 1235 L 1236 Last-Modified header field 8 1238 M 1239 metadata 6 1241 S 1242 selected representation 5 1243 Status Codes 1244 304 Not Modified 19 1245 412 Precondition Failed 20 1247 V 1248 validator 6 1249 strong 6 1250 weak 6 1252 Authors' Addresses 1254 Roy T. Fielding (editor) 1255 Adobe Systems Incorporated 1256 345 Park Ave 1257 San Jose, CA 95110 1258 USA 1260 EMail: fielding@gbiv.com 1261 URI: http://roy.gbiv.com/ 1263 Jim Gettys 1264 Alcatel-Lucent Bell Labs 1265 21 Oak Knoll Road 1266 Carlisle, MA 01741 1267 USA 1269 EMail: jg@freedesktop.org 1270 URI: http://gettys.wordpress.com/ 1272 Jeffrey C. Mogul 1273 Hewlett-Packard Company 1274 HP Labs, Large Scale Systems Group 1275 1501 Page Mill Road, MS 1177 1276 Palo Alto, CA 94304 1277 USA 1279 EMail: JeffMogul@acm.org 1280 Henrik Frystyk Nielsen 1281 Microsoft Corporation 1282 1 Microsoft Way 1283 Redmond, WA 98052 1284 USA 1286 EMail: henrikn@microsoft.com 1288 Larry Masinter 1289 Adobe Systems Incorporated 1290 345 Park Ave 1291 San Jose, CA 95110 1292 USA 1294 EMail: LMM@acm.org 1295 URI: http://larry.masinter.net/ 1297 Paul J. Leach 1298 Microsoft Corporation 1299 1 Microsoft Way 1300 Redmond, WA 98052 1302 EMail: paulle@microsoft.com 1304 Tim Berners-Lee 1305 World Wide Web Consortium 1306 MIT Computer Science and Artificial Intelligence Laboratory 1307 The Stata Center, Building 32 1308 32 Vassar Street 1309 Cambridge, MA 02139 1310 USA 1312 EMail: timbl@w3.org 1313 URI: http://www.w3.org/People/Berners-Lee/ 1314 Yves Lafon (editor) 1315 World Wide Web Consortium 1316 W3C / ERCIM 1317 2004, rte des Lucioles 1318 Sophia-Antipolis, AM 06902 1319 France 1321 EMail: ylafon@w3.org 1322 URI: http://www.raubacapeu.net/people/yves/ 1324 Julian F. Reschke (editor) 1325 greenbytes GmbH 1326 Hafenweg 16 1327 Muenster, NW 48155 1328 Germany 1330 Phone: +49 251 2807760 1331 Fax: +49 251 2807761 1332 EMail: julian.reschke@greenbytes.de 1333 URI: http://greenbytes.de/tech/webdav/