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'ISO-8859-1' == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p1-messaging-03 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p2-semantics-03 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p4-conditional-03 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p5-range-03 == Outdated reference: A later version (-26) exists of draft-ietf-httpbis-p6-cache-03 ** Obsolete normative reference: RFC 1766 (Obsoleted by RFC 3066, RFC 3282) ** Downref: Normative reference to an Informational RFC: RFC 1950 ** Downref: Normative reference to an Informational RFC: RFC 1951 ** Downref: Normative reference to an Informational RFC: RFC 1952 -- Obsolete informational reference (is this intentional?): RFC 1806 (Obsoleted by RFC 2183) -- Obsolete informational reference (is this intentional?): RFC 2068 (Obsoleted by RFC 2616) -- Obsolete informational reference (is this intentional?): RFC 2388 (Obsoleted by RFC 7578) -- 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 2822 (Obsoleted by RFC 5322) -- Obsolete informational reference (is this intentional?): RFC 4288 (Obsoleted by RFC 6838) Summary: 5 errors (**), 0 flaws (~~), 6 warnings (==), 15 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group R. Fielding, Ed. 3 Internet-Draft Day Software 4 Obsoletes: 2616 (if approved) J. Gettys 5 Intended status: Standards Track One Laptop per Child 6 Expires: December 19, 2008 J. Mogul 7 HP 8 H. Frystyk 9 Microsoft 10 L. Masinter 11 Adobe Systems 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 June 17, 2008 22 HTTP/1.1, part 3: Message Payload and Content Negotiation 23 draft-ietf-httpbis-p3-payload-03 25 Status of this Memo 27 By submitting this Internet-Draft, each author represents that any 28 applicable patent or other IPR claims of which he or she is aware 29 have been or will be disclosed, and any of which he or she becomes 30 aware will be disclosed, in accordance with Section 6 of BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF), its areas, and its working groups. Note that 34 other groups may also distribute working documents as Internet- 35 Drafts. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 The list of current Internet-Drafts can be accessed at 43 http://www.ietf.org/ietf/1id-abstracts.txt. 45 The list of Internet-Draft Shadow Directories can be accessed at 46 http://www.ietf.org/shadow.html. 48 This Internet-Draft will expire on December 19, 2008. 50 Abstract 52 The Hypertext Transfer Protocol (HTTP) is an application-level 53 protocol for distributed, collaborative, hypermedia information 54 systems. HTTP has been in use by the World Wide Web global 55 information initiative since 1990. This document is Part 3 of the 56 seven-part specification that defines the protocol referred to as 57 "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 3 defines 58 HTTP message content, metadata, and content negotiation. 60 Editorial Note (To be removed by RFC Editor) 62 Discussion of this draft should take place on the HTTPBIS working 63 group mailing list (ietf-http-wg@w3.org). The current issues list is 64 at and related 65 documents (including fancy diffs) can be found at 66 . 68 The changes in this draft are summarized in Appendix D.4. 70 Table of Contents 72 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 73 1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5 74 2. Notational Conventions and Generic Grammar . . . . . . . . . . 5 75 3. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 6 76 3.1. Character Sets . . . . . . . . . . . . . . . . . . . . . . 6 77 3.1.1. Missing Charset . . . . . . . . . . . . . . . . . . . 7 78 3.2. Content Codings . . . . . . . . . . . . . . . . . . . . . 7 79 3.3. Media Types . . . . . . . . . . . . . . . . . . . . . . . 8 80 3.3.1. Canonicalization and Text Defaults . . . . . . . . . . 9 81 3.3.2. Multipart Types . . . . . . . . . . . . . . . . . . . 10 82 3.4. Quality Values . . . . . . . . . . . . . . . . . . . . . . 11 83 3.5. Language Tags . . . . . . . . . . . . . . . . . . . . . . 11 84 4. Entity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 85 4.1. Entity Header Fields . . . . . . . . . . . . . . . . . . . 12 86 4.2. Entity Body . . . . . . . . . . . . . . . . . . . . . . . 12 87 4.2.1. Type . . . . . . . . . . . . . . . . . . . . . . . . . 13 88 4.2.2. Entity Length . . . . . . . . . . . . . . . . . . . . 13 89 5. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 13 90 5.1. Server-driven Negotiation . . . . . . . . . . . . . . . . 14 91 5.2. Agent-driven Negotiation . . . . . . . . . . . . . . . . . 15 92 5.3. Transparent Negotiation . . . . . . . . . . . . . . . . . 16 93 6. Header Field Definitions . . . . . . . . . . . . . . . . . . . 16 94 6.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . . 16 95 6.2. Accept-Charset . . . . . . . . . . . . . . . . . . . . . . 18 96 6.3. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 19 97 6.4. Accept-Language . . . . . . . . . . . . . . . . . . . . . 20 98 6.5. Content-Encoding . . . . . . . . . . . . . . . . . . . . . 22 99 6.6. Content-Language . . . . . . . . . . . . . . . . . . . . . 22 100 6.7. Content-Location . . . . . . . . . . . . . . . . . . . . . 23 101 6.8. Content-MD5 . . . . . . . . . . . . . . . . . . . . . . . 24 102 6.9. Content-Type . . . . . . . . . . . . . . . . . . . . . . . 25 103 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 104 7.1. Message Header Registration . . . . . . . . . . . . . . . 26 105 8. Security Considerations . . . . . . . . . . . . . . . . . . . 26 106 8.1. Privacy Issues Connected to Accept Headers . . . . . . . . 26 107 8.2. Content-Disposition Issues . . . . . . . . . . . . . . . . 27 108 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27 109 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27 110 10.1. Normative References . . . . . . . . . . . . . . . . . . . 27 111 10.2. Informative References . . . . . . . . . . . . . . . . . . 29 112 Appendix A. Differences Between HTTP Entities and RFC 2045 113 Entities . . . . . . . . . . . . . . . . . . . . . . 30 114 A.1. MIME-Version . . . . . . . . . . . . . . . . . . . . . . . 31 115 A.2. Conversion to Canonical Form . . . . . . . . . . . . . . . 31 116 A.3. Introduction of Content-Encoding . . . . . . . . . . . . . 31 117 A.4. No Content-Transfer-Encoding . . . . . . . . . . . . . . . 32 118 A.5. Introduction of Transfer-Encoding . . . . . . . . . . . . 32 119 A.6. MHTML and Line Length Limitations . . . . . . . . . . . . 32 120 Appendix B. Additional Features . . . . . . . . . . . . . . . . . 32 121 B.1. Content-Disposition . . . . . . . . . . . . . . . . . . . 33 122 Appendix C. Compatibility with Previous Versions . . . . . . . . 33 123 C.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 33 124 C.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 34 125 Appendix D. Change Log (to be removed by RFC Editor before 126 publication) . . . . . . . . . . . . . . . . . . . . 34 127 D.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 34 128 D.2. Since draft-ietf-httpbis-p3-payload-00 . . . . . . . . . . 34 129 D.3. Since draft-ietf-httpbis-p3-payload-01 . . . . . . . . . . 35 130 D.4. Since draft-ietf-httpbis-p3-payload-02 . . . . . . . . . . 35 131 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 132 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38 133 Intellectual Property and Copyright Statements . . . . . . . . . . 41 135 1. Introduction 137 This document defines HTTP/1.1 message payloads (a.k.a., content), 138 the associated metadata header fields that define how the payload is 139 intended to be interpreted by a recipient, the request header fields 140 that may influence content selection, and the various selection 141 algorithms that are collectively referred to as HTTP content 142 negotiation. 144 This document is currently disorganized in order to minimize the 145 changes between drafts and enable reviewers to see the smaller errata 146 changes. The next draft will reorganize the sections to better 147 reflect the content. In particular, the sections on entities will be 148 renamed payload and moved to the first half of the document, while 149 the sections on content negotiation and associated request header 150 fields will be moved to the second half. The current mess reflects 151 how widely dispersed these topics and associated requirements had 152 become in [RFC2616]. 154 1.1. Requirements 156 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 157 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 158 document are to be interpreted as described in [RFC2119]. 160 An implementation is not compliant if it fails to satisfy one or more 161 of the MUST or REQUIRED level requirements for the protocols it 162 implements. An implementation that satisfies all the MUST or 163 REQUIRED level and all the SHOULD level requirements for its 164 protocols is said to be "unconditionally compliant"; one that 165 satisfies all the MUST level requirements but not all the SHOULD 166 level requirements for its protocols is said to be "conditionally 167 compliant." 169 2. Notational Conventions and Generic Grammar 171 This specification uses the ABNF syntax defined in Section 2.1 of 172 [Part1] and the core rules defined in Section 2.2 of [Part1]: 173 [[abnf.dep: ABNF syntax and basic rules will be adopted from RFC 174 5234, see .]] 176 ALPHA = 177 DIGIT = 178 OCTET = 180 quoted-string = 181 token = 183 The ABNF rules below are defined in other parts: 185 absoluteURI = 186 Content-Length = 187 relativeURI = 188 message-header = 190 Last-Modified = 192 Content-Range = 194 Expires = 196 3. Protocol Parameters 198 3.1. Character Sets 200 HTTP uses the same definition of the term "character set" as that 201 described for MIME: 203 The term "character set" is used in this document to refer to a 204 method used with one or more tables to convert a sequence of octets 205 into a sequence of characters. Note that unconditional conversion in 206 the other direction is not required, in that not all characters may 207 be available in a given character set and a character set may provide 208 more than one sequence of octets to represent a particular character. 209 This definition is intended to allow various kinds of character 210 encoding, from simple single-table mappings such as US-ASCII to 211 complex table switching methods such as those that use ISO-2022's 212 techniques. However, the definition associated with a MIME character 213 set name MUST fully specify the mapping to be performed from octets 214 to characters. In particular, use of external profiling information 215 to determine the exact mapping is not permitted. 217 Note: This use of the term "character set" is more commonly 218 referred to as a "character encoding." However, since HTTP and 219 MIME share the same registry, it is important that the terminology 220 also be shared. 222 HTTP character sets are identified by case-insensitive tokens. The 223 complete set of tokens is defined by the IANA Character Set registry 224 (). 226 charset = token 228 Although HTTP allows an arbitrary token to be used as a charset 229 value, any token that has a predefined value within the IANA 230 Character Set registry MUST represent the character set defined by 231 that registry. Applications SHOULD limit their use of character sets 232 to those defined by the IANA registry. 234 HTTP uses charset in two contexts: within an Accept-Charset request 235 header (in which the charset value is an unquoted token) and as the 236 value of a parameter in a Content-Type header (within a request or 237 response), in which case the parameter value of the charset parameter 238 may be quoted. 240 Implementors should be aware of IETF character set requirements 241 [RFC3629] [RFC2277]. 243 3.1.1. Missing Charset 245 Some HTTP/1.0 software has interpreted a Content-Type header without 246 charset parameter incorrectly to mean "recipient should guess." 247 Senders wishing to defeat this behavior MAY include a charset 248 parameter even when the charset is ISO-8859-1 ([ISO-8859-1]) and 249 SHOULD do so when it is known that it will not confuse the recipient. 251 Unfortunately, some older HTTP/1.0 clients did not deal properly with 252 an explicit charset parameter. HTTP/1.1 recipients MUST respect the 253 charset label provided by the sender; and those user agents that have 254 a provision to "guess" a charset MUST use the charset from the 255 content-type field if they support that charset, rather than the 256 recipient's preference, when initially displaying a document. See 257 Section 3.3.1. 259 3.2. Content Codings 261 Content coding values indicate an encoding transformation that has 262 been or can be applied to an entity. Content codings are primarily 263 used to allow a document to be compressed or otherwise usefully 264 transformed without losing the identity of its underlying media type 265 and without loss of information. Frequently, the entity is stored in 266 coded form, transmitted directly, and only decoded by the recipient. 268 content-coding = token 270 All content-coding values are case-insensitive. HTTP/1.1 uses 271 content-coding values in the Accept-Encoding (Section 6.3) and 272 Content-Encoding (Section 6.5) header fields. Although the value 273 describes the content-coding, what is more important is that it 274 indicates what decoding mechanism will be required to remove the 275 encoding. 277 The Internet Assigned Numbers Authority (IANA) acts as a registry for 278 content-coding value tokens. Initially, the registry contains the 279 following tokens: 281 gzip 283 An encoding format produced by the file compression program "gzip" 284 (GNU zip) as described in [RFC1952]. This format is a Lempel-Ziv 285 coding (LZ77) with a 32 bit CRC. 287 compress 289 The encoding format produced by the common UNIX file compression 290 program "compress". This format is an adaptive Lempel-Ziv-Welch 291 coding (LZW). 293 Use of program names for the identification of encoding formats is 294 not desirable and is discouraged for future encodings. Their use 295 here is representative of historical practice, not good design. 296 For compatibility with previous implementations of HTTP, 297 applications SHOULD consider "x-gzip" and "x-compress" to be 298 equivalent to "gzip" and "compress" respectively. 300 deflate 302 The "zlib" format defined in [RFC1950] in combination with the 303 "deflate" compression mechanism described in [RFC1951]. 305 identity 307 The default (identity) encoding; the use of no transformation 308 whatsoever. This content-coding is used only in the Accept- 309 Encoding header, and SHOULD NOT be used in the Content-Encoding 310 header. 312 New content-coding value tokens SHOULD be registered; to allow 313 interoperability between clients and servers, specifications of the 314 content coding algorithms needed to implement a new value SHOULD be 315 publicly available and adequate for independent implementation, and 316 conform to the purpose of content coding defined in this section. 318 3.3. Media Types 320 HTTP uses Internet Media Types [RFC2046] in the Content-Type 321 (Section 6.9) and Accept (Section 6.1) header fields in order to 322 provide open and extensible data typing and type negotiation. 324 media-type = type "/" subtype *( ";" parameter ) 325 type = token 326 subtype = token 328 Parameters MAY follow the type/subtype in the form of attribute/value 329 pairs. 331 parameter = attribute "=" value 332 attribute = token 333 value = token | quoted-string 335 The type, subtype, and parameter attribute names are case- 336 insensitive. Parameter values might or might not be case-sensitive, 337 depending on the semantics of the parameter name. Linear white space 338 (LWS) MUST NOT be used between the type and subtype, nor between an 339 attribute and its value. The presence or absence of a parameter 340 might be significant to the processing of a media-type, depending on 341 its definition within the media type registry. 343 All parameters defined as a token are also allowed to occur as 344 quoted-string; both notations are equivalent. 346 Note that some older HTTP applications do not recognize media type 347 parameters. When sending data to older HTTP applications, 348 implementations SHOULD only use media type parameters when they are 349 required by that type/subtype definition. 351 Media-type values are registered with the Internet Assigned Number 352 Authority (IANA). The media type registration process is outlined in 353 [RFC4288]. Use of non-registered media types is discouraged. 355 3.3.1. Canonicalization and Text Defaults 357 Internet media types are registered with a canonical form. An 358 entity-body transferred via HTTP messages MUST be represented in the 359 appropriate canonical form prior to its transmission except for 360 "text" types, as defined in the next paragraph. 362 When in canonical form, media subtypes of the "text" type use CRLF as 363 the text line break. HTTP relaxes this requirement and allows the 364 transport of text media with plain CR or LF alone representing a line 365 break when it is done consistently for an entire entity-body. HTTP 366 applications MUST accept CRLF, bare CR, and bare LF as being 367 representative of a line break in text media received via HTTP. In 368 addition, if the text is represented in a character set that does not 369 use octets 13 and 10 for CR and LF respectively, as is the case for 370 some multi-byte character sets, HTTP allows the use of whatever octet 371 sequences are defined by that character set to represent the 372 equivalent of CR and LF for line breaks. This flexibility regarding 373 line breaks applies only to text media in the entity-body; a bare CR 374 or LF MUST NOT be substituted for CRLF within any of the HTTP control 375 structures (such as header fields and multipart boundaries). 377 If an entity-body is encoded with a content-coding, the underlying 378 data MUST be in a form defined above prior to being encoded. 380 The "charset" parameter is used with some media types to define the 381 character set (Section 3.1) of the data. When no explicit charset 382 parameter is provided by the sender, media subtypes of the "text" 383 type are defined to have a default charset value of "ISO-8859-1" when 384 received via HTTP. Data in character sets other than "ISO-8859-1" or 385 its subsets MUST be labeled with an appropriate charset value. See 386 Section 3.1.1 for compatibility problems. 388 3.3.2. Multipart Types 390 MIME provides for a number of "multipart" types -- encapsulations of 391 one or more entities within a single message-body. All multipart 392 types share a common syntax, as defined in Section 5.1.1 of 393 [RFC2046], and MUST include a boundary parameter as part of the media 394 type value. The message body is itself a protocol element and MUST 395 therefore use only CRLF to represent line breaks between body-parts. 396 Unlike in RFC 2046, the epilogue of any multipart message MUST be 397 empty; HTTP applications MUST NOT transmit the epilogue (even if the 398 original multipart contains an epilogue). These restrictions exist 399 in order to preserve the self-delimiting nature of a multipart 400 message-body, wherein the "end" of the message-body is indicated by 401 the ending multipart boundary. 403 In general, HTTP treats a multipart message-body no differently than 404 any other media type: strictly as payload. The one exception is the 405 "multipart/byteranges" type (Appendix A of [Part5]) when it appears 406 in a 206 (Partial Content) response. In all other cases, an HTTP 407 user agent SHOULD follow the same or similar behavior as a MIME user 408 agent would upon receipt of a multipart type. The MIME header fields 409 within each body-part of a multipart message-body do not have any 410 significance to HTTP beyond that defined by their MIME semantics. 412 In general, an HTTP user agent SHOULD follow the same or similar 413 behavior as a MIME user agent would upon receipt of a multipart type. 414 If an application receives an unrecognized multipart subtype, the 415 application MUST treat it as being equivalent to "multipart/mixed". 417 Note: The "multipart/form-data" type has been specifically defined 418 for carrying form data suitable for processing via the POST 419 request method, as described in [RFC2388]. 421 3.4. Quality Values 423 HTTP content negotiation (Section 5) uses short "floating point" 424 numbers to indicate the relative importance ("weight") of various 425 negotiable parameters. A weight is normalized to a real number in 426 the range 0 through 1, where 0 is the minimum and 1 the maximum 427 value. If a parameter has a quality value of 0, then content with 428 this parameter is `not acceptable' for the client. HTTP/1.1 429 applications MUST NOT generate more than three digits after the 430 decimal point. User configuration of these values SHOULD also be 431 limited in this fashion. 433 qvalue = ( "0" [ "." 0*3DIGIT ] ) 434 | ( "1" [ "." 0*3("0") ] ) 436 "Quality values" is a misnomer, since these values merely represent 437 relative degradation in desired quality. 439 3.5. Language Tags 441 A language tag identifies a natural language spoken, written, or 442 otherwise conveyed by human beings for communication of information 443 to other human beings. Computer languages are explicitly excluded. 444 HTTP uses language tags within the Accept-Language and Content- 445 Language fields. 447 The syntax and registry of HTTP language tags is the same as that 448 defined by [RFC1766]. In summary, a language tag is composed of 1 or 449 more parts: A primary language tag and a possibly empty series of 450 subtags: 452 language-tag = primary-tag *( "-" subtag ) 453 primary-tag = 1*8ALPHA 454 subtag = 1*8ALPHA 456 White space is not allowed within the tag and all tags are case- 457 insensitive. The name space of language tags is administered by the 458 IANA. Example tags include: 460 en, en-US, en-cockney, i-cherokee, x-pig-latin 462 where any two-letter primary-tag is an ISO-639 language abbreviation 463 and any two-letter initial subtag is an ISO-3166 country code. (The 464 last three tags above are not registered tags; all but the last are 465 examples of tags which could be registered in future.) 467 4. Entity 469 Request and Response messages MAY transfer an entity if not otherwise 470 restricted by the request method or response status code. An entity 471 consists of entity-header fields and an entity-body, although some 472 responses will only include the entity-headers. 474 In this section, both sender and recipient refer to either the client 475 or the server, depending on who sends and who receives the entity. 477 4.1. Entity Header Fields 479 Entity-header fields define metainformation about the entity-body or, 480 if no body is present, about the resource identified by the request. 482 entity-header = Content-Encoding ; Section 6.5 483 | Content-Language ; Section 6.6 484 | Content-Length ; [Part1], Section 8.2 485 | Content-Location ; Section 6.7 486 | Content-MD5 ; Section 6.8 487 | Content-Range ; [Part5], Section 6.2 488 | Content-Type ; Section 6.9 489 | Expires ; [Part6], Section 16.3 490 | Last-Modified ; [Part4], Section 7.6 491 | extension-header 493 extension-header = message-header 495 The extension-header mechanism allows additional entity-header fields 496 to be defined without changing the protocol, but these fields cannot 497 be assumed to be recognizable by the recipient. Unrecognized header 498 fields SHOULD be ignored by the recipient and MUST be forwarded by 499 transparent proxies. 501 4.2. Entity Body 503 The entity-body (if any) sent with an HTTP request or response is in 504 a format and encoding defined by the entity-header fields. 506 entity-body = *OCTET 508 An entity-body is only present in a message when a message-body is 509 present, as described in Section 4.3 of [Part1]. The entity-body is 510 obtained from the message-body by decoding any Transfer-Encoding that 511 might have been applied to ensure safe and proper transfer of the 512 message. 514 4.2.1. Type 516 When an entity-body is included with a message, the data type of that 517 body is determined via the header fields Content-Type and Content- 518 Encoding. These define a two-layer, ordered encoding model: 520 entity-body := Content-Encoding( Content-Type( data ) ) 522 Content-Type specifies the media type of the underlying data. 523 Content-Encoding may be used to indicate any additional content 524 codings applied to the data, usually for the purpose of data 525 compression, that are a property of the requested resource. There is 526 no default encoding. 528 Any HTTP/1.1 message containing an entity-body SHOULD include a 529 Content-Type header field defining the media type of that body. If 530 and only if the media type is not given by a Content-Type field, the 531 recipient MAY attempt to guess the media type via inspection of its 532 content and/or the name extension(s) of the URI used to identify the 533 resource. If the media type remains unknown, the recipient SHOULD 534 treat it as type "application/octet-stream". 536 4.2.2. Entity Length 538 The entity-length of a message is the length of the message-body 539 before any transfer-codings have been applied. Section 4.4 of 540 [Part1] defines how the transfer-length of a message-body is 541 determined. 543 5. Content Negotiation 545 Most HTTP responses include an entity which contains information for 546 interpretation by a human user. Naturally, it is desirable to supply 547 the user with the "best available" entity corresponding to the 548 request. Unfortunately for servers and caches, not all users have 549 the same preferences for what is "best," and not all user agents are 550 equally capable of rendering all entity types. For that reason, HTTP 551 has provisions for several mechanisms for "content negotiation" -- 552 the process of selecting the best representation for a given response 553 when there are multiple representations available. 555 Note: This is not called "format negotiation" because the 556 alternate representations may be of the same media type, but use 557 different capabilities of that type, be in different languages, 558 etc. 560 Any response containing an entity-body MAY be subject to negotiation, 561 including error responses. 563 There are two kinds of content negotiation which are possible in 564 HTTP: server-driven and agent-driven negotiation. These two kinds of 565 negotiation are orthogonal and thus may be used separately or in 566 combination. One method of combination, referred to as transparent 567 negotiation, occurs when a cache uses the agent-driven negotiation 568 information provided by the origin server in order to provide server- 569 driven negotiation for subsequent requests. 571 5.1. Server-driven Negotiation 573 If the selection of the best representation for a response is made by 574 an algorithm located at the server, it is called server-driven 575 negotiation. Selection is based on the available representations of 576 the response (the dimensions over which it can vary; e.g. language, 577 content-coding, etc.) and the contents of particular header fields in 578 the request message or on other information pertaining to the request 579 (such as the network address of the client). 581 Server-driven negotiation is advantageous when the algorithm for 582 selecting from among the available representations is difficult to 583 describe to the user agent, or when the server desires to send its 584 "best guess" to the client along with the first response (hoping to 585 avoid the round-trip delay of a subsequent request if the "best 586 guess" is good enough for the user). In order to improve the 587 server's guess, the user agent MAY include request header fields 588 (Accept, Accept-Language, Accept-Encoding, etc.) which describe its 589 preferences for such a response. 591 Server-driven negotiation has disadvantages: 593 1. It is impossible for the server to accurately determine what 594 might be "best" for any given user, since that would require 595 complete knowledge of both the capabilities of the user agent and 596 the intended use for the response (e.g., does the user want to 597 view it on screen or print it on paper?). 599 2. Having the user agent describe its capabilities in every request 600 can be both very inefficient (given that only a small percentage 601 of responses have multiple representations) and a potential 602 violation of the user's privacy. 604 3. It complicates the implementation of an origin server and the 605 algorithms for generating responses to a request. 607 4. It may limit a public cache's ability to use the same response 608 for multiple user's requests. 610 HTTP/1.1 includes the following request-header fields for enabling 611 server-driven negotiation through description of user agent 612 capabilities and user preferences: Accept (Section 6.1), Accept- 613 Charset (Section 6.2), Accept-Encoding (Section 6.3), Accept-Language 614 (Section 6.4), and User-Agent (Section 10.9 of [Part2]). However, an 615 origin server is not limited to these dimensions and MAY vary the 616 response based on any aspect of the request, including information 617 outside the request-header fields or within extension header fields 618 not defined by this specification. 620 The Vary header field (Section 16.5 of [Part6]) can be used to 621 express the parameters the server uses to select a representation 622 that is subject to server-driven negotiation. 624 5.2. Agent-driven Negotiation 626 With agent-driven negotiation, selection of the best representation 627 for a response is performed by the user agent after receiving an 628 initial response from the origin server. Selection is based on a 629 list of the available representations of the response included within 630 the header fields or entity-body of the initial response, with each 631 representation identified by its own URI. Selection from among the 632 representations may be performed automatically (if the user agent is 633 capable of doing so) or manually by the user selecting from a 634 generated (possibly hypertext) menu. 636 Agent-driven negotiation is advantageous when the response would vary 637 over commonly-used dimensions (such as type, language, or encoding), 638 when the origin server is unable to determine a user agent's 639 capabilities from examining the request, and generally when public 640 caches are used to distribute server load and reduce network usage. 642 Agent-driven negotiation suffers from the disadvantage of needing a 643 second request to obtain the best alternate representation. This 644 second request is only efficient when caching is used. In addition, 645 this specification does not define any mechanism for supporting 646 automatic selection, though it also does not prevent any such 647 mechanism from being developed as an extension and used within 648 HTTP/1.1. 650 HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable) 651 status codes for enabling agent-driven negotiation when the server is 652 unwilling or unable to provide a varying response using server-driven 653 negotiation. 655 5.3. Transparent Negotiation 657 Transparent negotiation is a combination of both server-driven and 658 agent-driven negotiation. When a cache is supplied with a form of 659 the list of available representations of the response (as in agent- 660 driven negotiation) and the dimensions of variance are completely 661 understood by the cache, then the cache becomes capable of performing 662 server-driven negotiation on behalf of the origin server for 663 subsequent requests on that resource. 665 Transparent negotiation has the advantage of distributing the 666 negotiation work that would otherwise be required of the origin 667 server and also removing the second request delay of agent-driven 668 negotiation when the cache is able to correctly guess the right 669 response. 671 This specification does not define any mechanism for transparent 672 negotiation, though it also does not prevent any such mechanism from 673 being developed as an extension that could be used within HTTP/1.1. 675 6. Header Field Definitions 677 This section defines the syntax and semantics of HTTP/1.1 header 678 fields related to the payload of messages. 680 For entity-header fields, both sender and recipient refer to either 681 the client or the server, depending on who sends and who receives the 682 entity. 684 6.1. Accept 686 The Accept request-header field can be used to specify certain media 687 types which are acceptable for the response. Accept headers can be 688 used to indicate that the request is specifically limited to a small 689 set of desired types, as in the case of a request for an in-line 690 image. 692 Accept = "Accept" ":" 693 #( media-range [ accept-params ] ) 695 media-range = ( "*/*" 696 | ( type "/" "*" ) 697 | ( type "/" subtype ) 698 ) *( ";" parameter ) 699 accept-params = ";" "q" "=" qvalue *( accept-extension ) 700 accept-extension = ";" token [ "=" ( token | quoted-string ) ] 702 The asterisk "*" character is used to group media types into ranges, 703 with "*/*" indicating all media types and "type/*" indicating all 704 subtypes of that type. The media-range MAY include media type 705 parameters that are applicable to that range. 707 Each media-range MAY be followed by one or more accept-params, 708 beginning with the "q" parameter for indicating a relative quality 709 factor. The first "q" parameter (if any) separates the media-range 710 parameter(s) from the accept-params. Quality factors allow the user 711 or user agent to indicate the relative degree of preference for that 712 media-range, using the qvalue scale from 0 to 1 (Section 3.4). The 713 default value is q=1. 715 Note: Use of the "q" parameter name to separate media type 716 parameters from Accept extension parameters is due to historical 717 practice. Although this prevents any media type parameter named 718 "q" from being used with a media range, such an event is believed 719 to be unlikely given the lack of any "q" parameters in the IANA 720 media type registry and the rare usage of any media type 721 parameters in Accept. Future media types are discouraged from 722 registering any parameter named "q". 724 The example 726 Accept: audio/*; q=0.2, audio/basic 728 SHOULD be interpreted as "I prefer audio/basic, but send me any audio 729 type if it is the best available after an 80% mark-down in quality." 731 If no Accept header field is present, then it is assumed that the 732 client accepts all media types. If an Accept header field is 733 present, and if the server cannot send a response which is acceptable 734 according to the combined Accept field value, then the server SHOULD 735 send a 406 (Not Acceptable) response. 737 A more elaborate example is 739 Accept: text/plain; q=0.5, text/html, 740 text/x-dvi; q=0.8, text/x-c 742 Verbally, this would be interpreted as "text/html and text/x-c are 743 the preferred media types, but if they do not exist, then send the 744 text/x-dvi entity, and if that does not exist, send the text/plain 745 entity." 747 Media ranges can be overridden by more specific media ranges or 748 specific media types. If more than one media range applies to a 749 given type, the most specific reference has precedence. For example, 750 Accept: text/*, text/html, text/html;level=1, */* 752 have the following precedence: 754 1) text/html;level=1 755 2) text/html 756 3) text/* 757 4) */* 759 The media type quality factor associated with a given type is 760 determined by finding the media range with the highest precedence 761 which matches that type. For example, 763 Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1, 764 text/html;level=2;q=0.4, */*;q=0.5 766 would cause the following values to be associated: 768 text/html;level=1 = 1 769 text/html = 0.7 770 text/plain = 0.3 771 image/jpeg = 0.5 772 text/html;level=2 = 0.4 773 text/html;level=3 = 0.7 775 Note: A user agent might be provided with a default set of quality 776 values for certain media ranges. However, unless the user agent is a 777 closed system which cannot interact with other rendering agents, this 778 default set ought to be configurable by the user. 780 6.2. Accept-Charset 782 The Accept-Charset request-header field can be used to indicate what 783 character sets are acceptable for the response. This field allows 784 clients capable of understanding more comprehensive or special- 785 purpose character sets to signal that capability to a server which is 786 capable of representing documents in those character sets. 788 Accept-Charset = "Accept-Charset" ":" 789 1#( ( charset | "*" ) [ ";" "q" "=" qvalue ] ) 791 Character set values are described in Section 3.1. Each charset MAY 792 be given an associated quality value which represents the user's 793 preference for that charset. The default value is q=1. An example 794 is 796 Accept-Charset: iso-8859-5, unicode-1-1;q=0.8 798 The special value "*", if present in the Accept-Charset field, 799 matches every character set (including ISO-8859-1) which is not 800 mentioned elsewhere in the Accept-Charset field. If no "*" is 801 present in an Accept-Charset field, then all character sets not 802 explicitly mentioned get a quality value of 0, except for ISO-8859-1, 803 which gets a quality value of 1 if not explicitly mentioned. 805 If no Accept-Charset header is present, the default is that any 806 character set is acceptable. If an Accept-Charset header is present, 807 and if the server cannot send a response which is acceptable 808 according to the Accept-Charset header, then the server SHOULD send 809 an error response with the 406 (Not Acceptable) status code, though 810 the sending of an unacceptable response is also allowed. 812 6.3. Accept-Encoding 814 The Accept-Encoding request-header field is similar to Accept, but 815 restricts the content-codings (Section 3.2) that are acceptable in 816 the response. 818 Accept-Encoding = "Accept-Encoding" ":" 819 #( codings [ ";" "q" "=" qvalue ] ) 820 codings = ( content-coding | "*" ) 822 Each codings value MAY be given an associated quality value which 823 represents the preference for that encoding. The default value is 824 q=1. 826 Examples of its use are: 828 Accept-Encoding: compress, gzip 829 Accept-Encoding: 830 Accept-Encoding: * 831 Accept-Encoding: compress;q=0.5, gzip;q=1.0 832 Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0 834 A server tests whether a content-coding is acceptable, according to 835 an Accept-Encoding field, using these rules: 837 1. If the content-coding is one of the content-codings listed in the 838 Accept-Encoding field, then it is acceptable, unless it is 839 accompanied by a qvalue of 0. (As defined in Section 3.4, a 840 qvalue of 0 means "not acceptable.") 842 2. The special "*" symbol in an Accept-Encoding field matches any 843 available content-coding not explicitly listed in the header 844 field. 846 3. If multiple content-codings are acceptable, then the acceptable 847 content-coding with the highest non-zero qvalue is preferred. 849 4. The "identity" content-coding is always acceptable, unless 850 specifically refused because the Accept-Encoding field includes 851 "identity;q=0", or because the field includes "*;q=0" and does 852 not explicitly include the "identity" content-coding. If the 853 Accept-Encoding field-value is empty, then only the "identity" 854 encoding is acceptable. 856 If an Accept-Encoding field is present in a request, and if the 857 server cannot send a response which is acceptable according to the 858 Accept-Encoding header, then the server SHOULD send an error response 859 with the 406 (Not Acceptable) status code. 861 If no Accept-Encoding field is present in a request, the server MAY 862 assume that the client will accept any content coding. In this case, 863 if "identity" is one of the available content-codings, then the 864 server SHOULD use the "identity" content-coding, unless it has 865 additional information that a different content-coding is meaningful 866 to the client. 868 Note: If the request does not include an Accept-Encoding field, 869 and if the "identity" content-coding is unavailable, then content- 870 codings commonly understood by HTTP/1.0 clients (i.e., "gzip" and 871 "compress") are preferred; some older clients improperly display 872 messages sent with other content-codings. The server might also 873 make this decision based on information about the particular user- 874 agent or client. 876 Note: Most HTTP/1.0 applications do not recognize or obey qvalues 877 associated with content-codings. This means that qvalues will not 878 work and are not permitted with x-gzip or x-compress. 880 6.4. Accept-Language 882 The Accept-Language request-header field is similar to Accept, but 883 restricts the set of natural languages that are preferred as a 884 response to the request. Language tags are defined in Section 3.5. 886 Accept-Language = "Accept-Language" ":" 887 1#( language-range [ ";" "q" "=" qvalue ] ) 888 language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" ) 890 Each language-range MAY be given an associated quality value which 891 represents an estimate of the user's preference for the languages 892 specified by that range. The quality value defaults to "q=1". For 893 example, 894 Accept-Language: da, en-gb;q=0.8, en;q=0.7 896 would mean: "I prefer Danish, but will accept British English and 897 other types of English." A language-range matches a language-tag if 898 it exactly equals the tag, or if it exactly equals a prefix of the 899 tag such that the first tag character following the prefix is "-". 900 The special range "*", if present in the Accept-Language field, 901 matches every tag not matched by any other range present in the 902 Accept-Language field. 904 Note: This use of a prefix matching rule does not imply that 905 language tags are assigned to languages in such a way that it is 906 always true that if a user understands a language with a certain 907 tag, then this user will also understand all languages with tags 908 for which this tag is a prefix. The prefix rule simply allows the 909 use of prefix tags if this is the case. 911 The language quality factor assigned to a language-tag by the Accept- 912 Language field is the quality value of the longest language-range in 913 the field that matches the language-tag. If no language-range in the 914 field matches the tag, the language quality factor assigned is 0. If 915 no Accept-Language header is present in the request, the server 916 SHOULD assume that all languages are equally acceptable. If an 917 Accept-Language header is present, then all languages which are 918 assigned a quality factor greater than 0 are acceptable. 920 It might be contrary to the privacy expectations of the user to send 921 an Accept-Language header with the complete linguistic preferences of 922 the user in every request. For a discussion of this issue, see 923 Section 8.1. 925 As intelligibility is highly dependent on the individual user, it is 926 recommended that client applications make the choice of linguistic 927 preference available to the user. If the choice is not made 928 available, then the Accept-Language header field MUST NOT be given in 929 the request. 931 Note: When making the choice of linguistic preference available to 932 the user, we remind implementors of the fact that users are not 933 familiar with the details of language matching as described above, 934 and should provide appropriate guidance. As an example, users 935 might assume that on selecting "en-gb", they will be served any 936 kind of English document if British English is not available. A 937 user agent might suggest in such a case to add "en" to get the 938 best matching behavior. 940 6.5. Content-Encoding 942 The Content-Encoding entity-header field is used as a modifier to the 943 media-type. When present, its value indicates what additional 944 content codings have been applied to the entity-body, and thus what 945 decoding mechanisms must be applied in order to obtain the media-type 946 referenced by the Content-Type header field. Content-Encoding is 947 primarily used to allow a document to be compressed without losing 948 the identity of its underlying media type. 950 Content-Encoding = "Content-Encoding" ":" 1#content-coding 952 Content codings are defined in Section 3.2. An example of its use is 954 Content-Encoding: gzip 956 The content-coding is a characteristic of the entity identified by 957 the Request-URI. Typically, the entity-body is stored with this 958 encoding and is only decoded before rendering or analogous usage. 959 However, a non-transparent proxy MAY modify the content-coding if the 960 new coding is known to be acceptable to the recipient, unless the 961 "no-transform" cache-control directive is present in the message. 963 If the content-coding of an entity is not "identity", then the 964 response MUST include a Content-Encoding entity-header (Section 6.5) 965 that lists the non-identity content-coding(s) used. 967 If the content-coding of an entity in a request message is not 968 acceptable to the origin server, the server SHOULD respond with a 969 status code of 415 (Unsupported Media Type). 971 If multiple encodings have been applied to an entity, the content 972 codings MUST be listed in the order in which they were applied. 973 Additional information about the encoding parameters MAY be provided 974 by other entity-header fields not defined by this specification. 976 6.6. Content-Language 978 The Content-Language entity-header field describes the natural 979 language(s) of the intended audience for the enclosed entity. Note 980 that this might not be equivalent to all the languages used within 981 the entity-body. 983 Content-Language = "Content-Language" ":" 1#language-tag 985 Language tags are defined in Section 3.5. The primary purpose of 986 Content-Language is to allow a user to identify and differentiate 987 entities according to the user's own preferred language. Thus, if 988 the body content is intended only for a Danish-literate audience, the 989 appropriate field is 991 Content-Language: da 993 If no Content-Language is specified, the default is that the content 994 is intended for all language audiences. This might mean that the 995 sender does not consider it to be specific to any natural language, 996 or that the sender does not know for which language it is intended. 998 Multiple languages MAY be listed for content that is intended for 999 multiple audiences. For example, a rendition of the "Treaty of 1000 Waitangi," presented simultaneously in the original Maori and English 1001 versions, would call for 1003 Content-Language: mi, en 1005 However, just because multiple languages are present within an entity 1006 does not mean that it is intended for multiple linguistic audiences. 1007 An example would be a beginner's language primer, such as "A First 1008 Lesson in Latin," which is clearly intended to be used by an English- 1009 literate audience. In this case, the Content-Language would properly 1010 only include "en". 1012 Content-Language MAY be applied to any media type -- it is not 1013 limited to textual documents. 1015 6.7. Content-Location 1017 The Content-Location entity-header field MAY be used to supply the 1018 resource location for the entity enclosed in the message when that 1019 entity is accessible from a location separate from the requested 1020 resource's URI. A server SHOULD provide a Content-Location for the 1021 variant corresponding to the response entity; especially in the case 1022 where a resource has multiple entities associated with it, and those 1023 entities actually have separate locations by which they might be 1024 individually accessed, the server SHOULD provide a Content-Location 1025 for the particular variant which is returned. 1027 Content-Location = "Content-Location" ":" 1028 ( absoluteURI | relativeURI ) 1030 The value of Content-Location also defines the base URI for the 1031 entity. 1033 The Content-Location value is not a replacement for the original 1034 requested URI; it is only a statement of the location of the resource 1035 corresponding to this particular entity at the time of the request. 1037 Future requests MAY specify the Content-Location URI as the request- 1038 URI if the desire is to identify the source of that particular 1039 entity. 1041 A cache cannot assume that an entity with a Content-Location 1042 different from the URI used to retrieve it can be used to respond to 1043 later requests on that Content-Location URI. However, the Content- 1044 Location can be used to differentiate between multiple entities 1045 retrieved from a single requested resource, as described in Section 8 1046 of [Part6]. 1048 If the Content-Location is a relative URI, the relative URI is 1049 interpreted relative to the Request-URI. 1051 The meaning of the Content-Location header in PUT or POST requests is 1052 undefined; servers are free to ignore it in those cases. 1054 6.8. Content-MD5 1056 The Content-MD5 entity-header field, as defined in [RFC1864], is an 1057 MD5 digest of the entity-body for the purpose of providing an end-to- 1058 end message integrity check (MIC) of the entity-body. (Note: a MIC 1059 is good for detecting accidental modification of the entity-body in 1060 transit, but is not proof against malicious attacks.) 1062 Content-MD5 = "Content-MD5" ":" md5-digest 1063 md5-digest = 1065 The Content-MD5 header field MAY be generated by an origin server or 1066 client to function as an integrity check of the entity-body. Only 1067 origin servers or clients MAY generate the Content-MD5 header field; 1068 proxies and gateways MUST NOT generate it, as this would defeat its 1069 value as an end-to-end integrity check. Any recipient of the entity- 1070 body, including gateways and proxies, MAY check that the digest value 1071 in this header field matches that of the entity-body as received. 1073 The MD5 digest is computed based on the content of the entity-body, 1074 including any content-coding that has been applied, but not including 1075 any transfer-encoding applied to the message-body. If the message is 1076 received with a transfer-encoding, that encoding MUST be removed 1077 prior to checking the Content-MD5 value against the received entity. 1079 This has the result that the digest is computed on the octets of the 1080 entity-body exactly as, and in the order that, they would be sent if 1081 no transfer-encoding were being applied. 1083 HTTP extends RFC 1864 to permit the digest to be computed for MIME 1084 composite media-types (e.g., multipart/* and message/rfc822), but 1085 this does not change how the digest is computed as defined in the 1086 preceding paragraph. 1088 There are several consequences of this. The entity-body for 1089 composite types MAY contain many body-parts, each with its own MIME 1090 and HTTP headers (including Content-MD5, Content-Transfer-Encoding, 1091 and Content-Encoding headers). If a body-part has a Content- 1092 Transfer-Encoding or Content-Encoding header, it is assumed that the 1093 content of the body-part has had the encoding applied, and the body- 1094 part is included in the Content-MD5 digest as is -- i.e., after the 1095 application. The Transfer-Encoding header field is not allowed 1096 within body-parts. 1098 Conversion of all line breaks to CRLF MUST NOT be done before 1099 computing or checking the digest: the line break convention used in 1100 the text actually transmitted MUST be left unaltered when computing 1101 the digest. 1103 Note: while the definition of Content-MD5 is exactly the same for 1104 HTTP as in RFC 1864 for MIME entity-bodies, there are several ways 1105 in which the application of Content-MD5 to HTTP entity-bodies 1106 differs from its application to MIME entity-bodies. One is that 1107 HTTP, unlike MIME, does not use Content-Transfer-Encoding, and 1108 does use Transfer-Encoding and Content-Encoding. Another is that 1109 HTTP more frequently uses binary content types than MIME, so it is 1110 worth noting that, in such cases, the byte order used to compute 1111 the digest is the transmission byte order defined for the type. 1112 Lastly, HTTP allows transmission of text types with any of several 1113 line break conventions and not just the canonical form using CRLF. 1115 6.9. Content-Type 1117 The Content-Type entity-header field indicates the media type of the 1118 entity-body sent to the recipient or, in the case of the HEAD method, 1119 the media type that would have been sent had the request been a GET. 1121 Content-Type = "Content-Type" ":" media-type 1123 Media types are defined in Section 3.3. An example of the field is 1125 Content-Type: text/html; charset=ISO-8859-4 1127 Further discussion of methods for identifying the media type of an 1128 entity is provided in Section 4.2.1. 1130 7. IANA Considerations 1132 7.1. Message Header Registration 1134 The Message Header Registry located at should be 1136 updated with the permanent registrations below (see [RFC3864]): 1138 +---------------------+----------+----------+--------------+ 1139 | Header Field Name | Protocol | Status | Reference | 1140 +---------------------+----------+----------+--------------+ 1141 | Accept | http | standard | Section 6.1 | 1142 | Accept-Charset | http | standard | Section 6.2 | 1143 | Accept-Encoding | http | standard | Section 6.3 | 1144 | Accept-Language | http | standard | Section 6.4 | 1145 | Content-Disposition | http | | Appendix B.1 | 1146 | Content-Encoding | http | standard | Section 6.5 | 1147 | Content-Language | http | standard | Section 6.6 | 1148 | Content-Location | http | standard | Section 6.7 | 1149 | Content-MD5 | http | standard | Section 6.8 | 1150 | Content-Type | http | standard | Section 6.9 | 1151 +---------------------+----------+----------+--------------+ 1153 The change controller is: "IETF (iesg@ietf.org) - Internet 1154 Engineering Task Force". 1156 8. Security Considerations 1158 This section is meant to inform application developers, information 1159 providers, and users of the security limitations in HTTP/1.1 as 1160 described by this document. The discussion does not include 1161 definitive solutions to the problems revealed, though it does make 1162 some suggestions for reducing security risks. 1164 8.1. Privacy Issues Connected to Accept Headers 1166 Accept request-headers can reveal information about the user to all 1167 servers which are accessed. The Accept-Language header in particular 1168 can reveal information the user would consider to be of a private 1169 nature, because the understanding of particular languages is often 1170 strongly correlated to the membership of a particular ethnic group. 1171 User agents which offer the option to configure the contents of an 1172 Accept-Language header to be sent in every request are strongly 1173 encouraged to let the configuration process include a message which 1174 makes the user aware of the loss of privacy involved. 1176 An approach that limits the loss of privacy would be for a user agent 1177 to omit the sending of Accept-Language headers by default, and to ask 1178 the user whether or not to start sending Accept-Language headers to a 1179 server if it detects, by looking for any Vary response-header fields 1180 generated by the server, that such sending could improve the quality 1181 of service. 1183 Elaborate user-customized accept header fields sent in every request, 1184 in particular if these include quality values, can be used by servers 1185 as relatively reliable and long-lived user identifiers. Such user 1186 identifiers would allow content providers to do click-trail tracking, 1187 and would allow collaborating content providers to match cross-server 1188 click-trails or form submissions of individual users. Note that for 1189 many users not behind a proxy, the network address of the host 1190 running the user agent will also serve as a long-lived user 1191 identifier. In environments where proxies are used to enhance 1192 privacy, user agents ought to be conservative in offering accept 1193 header configuration options to end users. As an extreme privacy 1194 measure, proxies could filter the accept headers in relayed requests. 1195 General purpose user agents which provide a high degree of header 1196 configurability SHOULD warn users about the loss of privacy which can 1197 be involved. 1199 8.2. Content-Disposition Issues 1201 [RFC1806], from which the often implemented Content-Disposition (see 1202 Appendix B.1) header in HTTP is derived, has a number of very serious 1203 security considerations. Content-Disposition is not part of the HTTP 1204 standard, but since it is widely implemented, we are documenting its 1205 use and risks for implementors. See [RFC2183] (which updates 1206 [RFC1806]) for details. 1208 9. Acknowledgments 1210 10. References 1212 10.1. Normative References 1214 [ISO-8859-1] 1215 International Organization for Standardization, 1216 "Information technology -- 8-bit single-byte coded graphic 1217 character sets -- Part 1: Latin alphabet No. 1", ISO/ 1218 IEC 8859-1:1998, 1998. 1220 [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1221 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1222 and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections, 1223 and Message Parsing", draft-ietf-httpbis-p1-messaging-03 1224 (work in progress), June 2008. 1226 [Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1227 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1228 and J. Reschke, Ed., "HTTP/1.1, part 2: Message 1229 Semantics", draft-ietf-httpbis-p2-semantics-03 (work in 1230 progress), June 2008. 1232 [Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1233 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1234 and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional 1235 Requests", draft-ietf-httpbis-p4-conditional-03 (work in 1236 progress), June 2008. 1238 [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1239 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1240 and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and 1241 Partial Responses", draft-ietf-httpbis-p5-range-03 (work 1242 in progress), June 2008. 1244 [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., 1245 Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., 1246 and J. Reschke, Ed., "HTTP/1.1, part 6: Caching", 1247 draft-ietf-httpbis-p6-cache-03 (work in progress), 1248 June 2008. 1250 [RFC1766] Alvestrand, H., "Tags for the Identification of 1251 Languages", RFC 1766, March 1995. 1253 [RFC1864] Myers, J. and M. Rose, "The Content-MD5 Header Field", 1254 RFC 1864, October 1995. 1256 [RFC1950] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format 1257 Specification version 3.3", RFC 1950, May 1996. 1259 RFC1950 is an Informational RFC, thus it may be less 1260 stable than this specification. On the other hand, this 1261 downward reference was present since [RFC2068] (published 1262 in 1997), therefore it is unlikely to cause problems in 1263 practice. 1265 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification 1266 version 1.3", RFC 1951, May 1996. 1268 RFC1951 is an Informational RFC, thus it may be less 1269 stable than this specification. On the other hand, this 1270 downward reference was present since [RFC2068] (published 1271 in 1997), therefore it is unlikely to cause problems in 1272 practice. 1274 [RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. 1275 Randers-Pehrson, "GZIP file format specification version 1276 4.3", RFC 1952, May 1996. 1278 RFC1952 is an Informational RFC, thus it may be less 1279 stable than this specification. On the other hand, this 1280 downward reference was present since [RFC2068] (published 1281 in 1997), therefore it is unlikely to cause problems in 1282 practice. 1284 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1285 Extensions (MIME) Part One: Format of Internet Message 1286 Bodies", RFC 2045, November 1996. 1288 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1289 Extensions (MIME) Part Two: Media Types", RFC 2046, 1290 November 1996. 1292 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1293 Requirement Levels", BCP 14, RFC 2119, March 1997. 1295 10.2. Informative References 1297 [RFC1806] Troost, R. and S. Dorner, "Communicating Presentation 1298 Information in Internet Messages: The Content-Disposition 1299 Header", RFC 1806, June 1995. 1301 [RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext 1302 Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. 1304 [RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1305 Extensions (MIME) Part Five: Conformance Criteria and 1306 Examples", RFC 2049, November 1996. 1308 [RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. 1309 Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", 1310 RFC 2068, January 1997. 1312 [RFC2076] Palme, J., "Common Internet Message Headers", RFC 2076, 1313 February 1997. 1315 [RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating 1316 Presentation Information in Internet Messages: The 1317 Content-Disposition Header Field", RFC 2183, August 1997. 1319 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and 1320 Languages", BCP 18, RFC 2277, January 1998. 1322 [RFC2388] Masinter, L., "Returning Values from Forms: multipart/ 1323 form-data", RFC 2388, August 1998. 1325 [RFC2557] Palme, F., Hopmann, A., Shelness, N., and E. Stefferud, 1326 "MIME Encapsulation of Aggregate Documents, such as HTML 1327 (MHTML)", RFC 2557, March 1999. 1329 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 1330 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 1331 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1333 [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, 1334 April 2001. 1336 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 1337 10646", RFC 3629, STD 63, November 2003. 1339 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 1340 Procedures for Message Header Fields", BCP 90, RFC 3864, 1341 September 2004. 1343 [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and 1344 Registration Procedures", BCP 13, RFC 4288, December 2005. 1346 Appendix A. Differences Between HTTP Entities and RFC 2045 Entities 1348 HTTP/1.1 uses many of the constructs defined for Internet Mail 1349 ([RFC2822]) and the Multipurpose Internet Mail Extensions (MIME 1350 [RFC2045]) to allow entities to be transmitted in an open variety of 1351 representations and with extensible mechanisms. However, RFC 2045 1352 discusses mail, and HTTP has a few features that are different from 1353 those described in RFC 2045. These differences were carefully chosen 1354 to optimize performance over binary connections, to allow greater 1355 freedom in the use of new media types, to make date comparisons 1356 easier, and to acknowledge the practice of some early HTTP servers 1357 and clients. 1359 This appendix describes specific areas where HTTP differs from RFC 1360 2045. Proxies and gateways to strict MIME environments SHOULD be 1361 aware of these differences and provide the appropriate conversions 1362 where necessary. Proxies and gateways from MIME environments to HTTP 1363 also need to be aware of the differences because some conversions 1364 might be required. 1366 A.1. MIME-Version 1368 HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages 1369 MAY include a single MIME-Version general-header field to indicate 1370 what version of the MIME protocol was used to construct the message. 1371 Use of the MIME-Version header field indicates that the message is in 1372 full compliance with the MIME protocol (as defined in [RFC2045]). 1373 Proxies/gateways are responsible for ensuring full compliance (where 1374 possible) when exporting HTTP messages to strict MIME environments. 1376 MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT 1378 MIME version "1.0" is the default for use in HTTP/1.1. However, 1379 HTTP/1.1 message parsing and semantics are defined by this document 1380 and not the MIME specification. 1382 A.2. Conversion to Canonical Form 1384 [RFC2045] requires that an Internet mail entity be converted to 1385 canonical form prior to being transferred, as described in Section 4 1386 of [RFC2049]. Section 3.3.1 of this document describes the forms 1387 allowed for subtypes of the "text" media type when transmitted over 1388 HTTP. [RFC2046] requires that content with a type of "text" 1389 represent line breaks as CRLF and forbids the use of CR or LF outside 1390 of line break sequences. HTTP allows CRLF, bare CR, and bare LF to 1391 indicate a line break within text content when a message is 1392 transmitted over HTTP. 1394 Where it is possible, a proxy or gateway from HTTP to a strict MIME 1395 environment SHOULD translate all line breaks within the text media 1396 types described in Section 3.3.1 of this document to the RFC 2049 1397 canonical form of CRLF. Note, however, that this might be 1398 complicated by the presence of a Content-Encoding and by the fact 1399 that HTTP allows the use of some character sets which do not use 1400 octets 13 and 10 to represent CR and LF, as is the case for some 1401 multi-byte character sets. 1403 Implementors should note that conversion will break any cryptographic 1404 checksums applied to the original content unless the original content 1405 is already in canonical form. Therefore, the canonical form is 1406 recommended for any content that uses such checksums in HTTP. 1408 A.3. Introduction of Content-Encoding 1410 RFC 2045 does not include any concept equivalent to HTTP/1.1's 1411 Content-Encoding header field. Since this acts as a modifier on the 1412 media type, proxies and gateways from HTTP to MIME-compliant 1413 protocols MUST either change the value of the Content-Type header 1414 field or decode the entity-body before forwarding the message. (Some 1415 experimental applications of Content-Type for Internet mail have used 1416 a media-type parameter of ";conversions=" to perform 1417 a function equivalent to Content-Encoding. However, this parameter 1418 is not part of RFC 2045). 1420 A.4. No Content-Transfer-Encoding 1422 HTTP does not use the Content-Transfer-Encoding field of RFC 2045. 1423 Proxies and gateways from MIME-compliant protocols to HTTP MUST 1424 remove any Content-Transfer-Encoding prior to delivering the response 1425 message to an HTTP client. 1427 Proxies and gateways from HTTP to MIME-compliant protocols are 1428 responsible for ensuring that the message is in the correct format 1429 and encoding for safe transport on that protocol, where "safe 1430 transport" is defined by the limitations of the protocol being used. 1431 Such a proxy or gateway SHOULD label the data with an appropriate 1432 Content-Transfer-Encoding if doing so will improve the likelihood of 1433 safe transport over the destination protocol. 1435 A.5. Introduction of Transfer-Encoding 1437 HTTP/1.1 introduces the Transfer-Encoding header field (Section 8.7 1438 of [Part1]). Proxies/gateways MUST remove any transfer-coding prior 1439 to forwarding a message via a MIME-compliant protocol. 1441 A.6. MHTML and Line Length Limitations 1443 HTTP implementations which share code with MHTML [RFC2557] 1444 implementations need to be aware of MIME line length limitations. 1445 Since HTTP does not have this limitation, HTTP does not fold long 1446 lines. MHTML messages being transported by HTTP follow all 1447 conventions of MHTML, including line length limitations and folding, 1448 canonicalization, etc., since HTTP transports all message-bodies as 1449 payload (see Section 3.3.2) and does not interpret the content or any 1450 MIME header lines that might be contained therein. 1452 Appendix B. Additional Features 1454 [RFC1945] and [RFC2068] document protocol elements used by some 1455 existing HTTP implementations, but not consistently and correctly 1456 across most HTTP/1.1 applications. Implementors are advised to be 1457 aware of these features, but cannot rely upon their presence in, or 1458 interoperability with, other HTTP/1.1 applications. Some of these 1459 describe proposed experimental features, and some describe features 1460 that experimental deployment found lacking that are now addressed in 1461 the base HTTP/1.1 specification. 1463 A number of other headers, such as Content-Disposition and Title, 1464 from SMTP and MIME are also often implemented (see [RFC2076]). 1466 B.1. Content-Disposition 1468 The Content-Disposition response-header field has been proposed as a 1469 means for the origin server to suggest a default filename if the user 1470 requests that the content is saved to a file. This usage is derived 1471 from the definition of Content-Disposition in [RFC1806]. 1473 content-disposition = "Content-Disposition" ":" 1474 disposition-type *( ";" disposition-parm ) 1475 disposition-type = "attachment" | disp-extension-token 1476 disposition-parm = filename-parm | disp-extension-parm 1477 filename-parm = "filename" "=" quoted-string 1478 disp-extension-token = token 1479 disp-extension-parm = token "=" ( token | quoted-string ) 1481 An example is 1483 Content-Disposition: attachment; filename="fname.ext" 1485 The receiving user agent SHOULD NOT respect any directory path 1486 information present in the filename-parm parameter, which is the only 1487 parameter believed to apply to HTTP implementations at this time. 1488 The filename SHOULD be treated as a terminal component only. 1490 If this header is used in a response with the application/ 1491 octet-stream content-type, the implied suggestion is that the user 1492 agent should not display the response, but directly enter a `save 1493 response as...' dialog. 1495 See Section 8.2 for Content-Disposition security issues. 1497 Appendix C. Compatibility with Previous Versions 1499 C.1. Changes from RFC 2068 1501 Transfer-coding and message lengths all interact in ways that 1502 required fixing exactly when chunked encoding is used (to allow for 1503 transfer encoding that may not be self delimiting); it was important 1504 to straighten out exactly how message lengths are computed. 1505 (Section 4.2.2, see also [Part1], [Part5] and [Part6]). 1507 Charset wildcarding is introduced to avoid explosion of character set 1508 names in accept headers. (Section 6.2) 1510 Content-Base was deleted from the specification: it was not 1511 implemented widely, and there is no simple, safe way to introduce it 1512 without a robust extension mechanism. In addition, it is used in a 1513 similar, but not identical fashion in MHTML [RFC2557]. 1515 A content-coding of "identity" was introduced, to solve problems 1516 discovered in caching. (Section 3.2) 1518 Quality Values of zero should indicate that "I don't want something" 1519 to allow clients to refuse a representation. (Section 3.4) 1521 The Alternates, Content-Version, Derived-From, Link, URI, Public and 1522 Content-Base header fields were defined in previous versions of this 1523 specification, but not commonly implemented. See [RFC2068]. 1525 C.2. Changes from RFC 2616 1527 Clarify contexts that charset is used in. (Section 3.1) 1529 Remove reference to non-existant identity transfer-coding value 1530 tokens. (Appendix A.4) 1532 Appendix D. Change Log (to be removed by RFC Editor before publication) 1534 D.1. Since RFC2616 1536 Extracted relevant partitions from [RFC2616]. 1538 D.2. Since draft-ietf-httpbis-p3-payload-00 1540 Closed issues: 1542 o : "Media Type 1543 Registrations" () 1545 o : 1546 "Clarification regarding quoting of charset values" 1547 () 1549 o : "Remove 1550 'identity' token references" 1551 () 1553 o : "Accept- 1554 Encoding BNF" 1556 o : "Normative 1557 and Informative references" 1559 o : "RFC1700 1560 references" 1562 o : "Updating 1563 to RFC4288" 1565 o : 1566 "Informative references" 1568 o : 1569 "ISO-8859-1 Reference" 1571 o : "Encoding 1572 References Normative" 1574 o : "Normative 1575 up-to-date references" 1577 D.3. Since draft-ietf-httpbis-p3-payload-01 1579 Ongoing work on ABNF conversion 1580 (): 1582 o Add explicit references to BNF syntax and rules imported from 1583 other parts of the specification. 1585 D.4. Since draft-ietf-httpbis-p3-payload-02 1587 Closed issues: 1589 o : "Quoting 1590 Charsets" 1592 o : 1593 "Classification for Allow header" 1595 o : "missing 1596 default for qvalue in description of Accept-Encoding" 1598 Ongoing work on IANA Message Header Registration 1599 (): 1601 o Reference RFC 3984, and update header registrations for headers 1602 defined in this document. 1604 Index 1606 A 1607 Accept header 16 1608 Accept-Charset header 18 1609 Accept-Encoding header 19 1610 Accept-Language header 20 1611 Alternates header 34 1613 C 1614 compress 8 1615 Content-Base header 34 1616 Content-Disposition header 33 1617 Content-Encoding header 22 1618 Content-Language header 22 1619 Content-Location header 23 1620 Content-MD5 header 24 1621 Content-Type header 25 1622 Content-Version header 34 1624 D 1625 deflate 8 1626 Derived-From header 34 1628 G 1629 Grammar 1630 Accept 16 1631 Accept-Charset 18 1632 Accept-Encoding 19 1633 accept-extension 16 1634 Accept-Language 20 1635 accept-params 16 1636 attribute 9 1637 charset 7 1638 codings 19 1639 content-coding 7 1640 content-disposition 33 1641 Content-Encoding 22 1642 Content-Language 22 1643 Content-Location 23 1644 Content-MD5 24 1645 Content-Type 25 1646 disp-extension-parm 33 1647 disp-extension-token 33 1648 disposition-parm 33 1649 disposition-type 33 1650 entity-body 12 1651 entity-header 12 1652 extension-header 12 1653 filename-parm 33 1654 language-range 20 1655 language-tag 11 1656 md5-digest 24 1657 media-range 16 1658 media-type 9 1659 MIME-Version 31 1660 parameter 9 1661 primary-tag 11 1662 qvalue 11 1663 subtag 11 1664 subtype 9 1665 type 9 1666 value 9 1667 gzip 8 1669 H 1670 Headers 1671 Accept 16 1672 Accept-Charset 18 1673 Accept-Encoding 19 1674 Accept-Language 20 1675 Alternate 34 1676 Content-Base 34 1677 Content-Disposition 33 1678 Content-Encoding 22 1679 Content-Language 22 1680 Content-Location 23 1681 Content-MD5 24 1682 Content-Type 25 1683 Content-Version 34 1684 Derived-From 34 1685 Link 34 1686 Public 34 1687 URI 34 1689 I 1690 identity 8 1692 L 1693 Link header 34 1695 P 1696 Public header 34 1698 U 1699 URI header 34 1701 Authors' Addresses 1703 Roy T. Fielding (editor) 1704 Day Software 1705 23 Corporate Plaza DR, Suite 280 1706 Newport Beach, CA 92660 1707 USA 1709 Phone: +1-949-706-5300 1710 Fax: +1-949-706-5305 1711 Email: fielding@gbiv.com 1712 URI: http://roy.gbiv.com/ 1714 Jim Gettys 1715 One Laptop per Child 1716 21 Oak Knoll Road 1717 Carlisle, MA 01741 1718 USA 1720 Email: jg@laptop.org 1721 URI: http://www.laptop.org/ 1723 Jeffrey C. Mogul 1724 Hewlett-Packard Company 1725 HP Labs, Large Scale Systems Group 1726 1501 Page Mill Road, MS 1177 1727 Palo Alto, CA 94304 1728 USA 1730 Email: JeffMogul@acm.org 1732 Henrik Frystyk Nielsen 1733 Microsoft Corporation 1734 1 Microsoft Way 1735 Redmond, WA 98052 1736 USA 1738 Email: henrikn@microsoft.com 1739 Larry Masinter 1740 Adobe Systems, Incorporated 1741 345 Park Ave 1742 San Jose, CA 95110 1743 USA 1745 Email: LMM@acm.org 1746 URI: http://larry.masinter.net/ 1748 Paul J. Leach 1749 Microsoft Corporation 1750 1 Microsoft Way 1751 Redmond, WA 98052 1753 Email: paulle@microsoft.com 1755 Tim Berners-Lee 1756 World Wide Web Consortium 1757 MIT Computer Science and Artificial Intelligence Laboratory 1758 The Stata Center, Building 32 1759 32 Vassar Street 1760 Cambridge, MA 02139 1761 USA 1763 Email: timbl@w3.org 1764 URI: http://www.w3.org/People/Berners-Lee/ 1766 Yves Lafon (editor) 1767 World Wide Web Consortium 1768 W3C / ERCIM 1769 2004, rte des Lucioles 1770 Sophia-Antipolis, AM 06902 1771 France 1773 Email: ylafon@w3.org 1774 URI: http://www.raubacapeu.net/people/yves/ 1775 Julian F. Reschke (editor) 1776 greenbytes GmbH 1777 Hafenweg 16 1778 Muenster, NW 48155 1779 Germany 1781 Phone: +49 251 2807760 1782 Fax: +49 251 2807761 1783 Email: julian.reschke@greenbytes.de 1784 URI: http://greenbytes.de/tech/webdav/ 1786 Full Copyright Statement 1788 Copyright (C) The IETF Trust (2008). 1790 This document is subject to the rights, licenses and restrictions 1791 contained in BCP 78, and except as set forth therein, the authors 1792 retain all their rights. 1794 This document and the information contained herein are provided on an 1795 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1796 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 1797 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 1798 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 1799 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1800 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1802 Intellectual Property 1804 The IETF takes no position regarding the validity or scope of any 1805 Intellectual Property Rights or other rights that might be claimed to 1806 pertain to the implementation or use of the technology described in 1807 this document or the extent to which any license under such rights 1808 might or might not be available; nor does it represent that it has 1809 made any independent effort to identify any such rights. 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